Terms with Russian and English names and abbreviations. The filter matches any of them.
The average reservoir pressure at which further gas production becomes impossible or uneconomic: the pressure is not sufficient to deliver gas into the pipeline at an acceptable rate and compression cost. It determines the recoverable reserves on the p/Z plot.
The flow rate that a well would give at zero bottom-hole pressure; a conventional measure of its potential, obtained by extrapolating the inflow performance curve.
The pressure measured from a vacuum; it exceeds the gauge pressure by the atmospheric pressure. The gas laws, formation volume factors and the properties of reservoir fluids depend on it.
An investment that adds no reserves, or adds few, but allows the same barrels to be produced sooner: infill wells, well stimulation, a more powerful pump. The incremental cash flow is an outlay, a gain, then a late shortfall; pure acceleration is loss-making without discounting, the NPV profile is a hump, and there are often two IRRs.
A constant of the substance, at : it shows how much lower the vapour pressure is than for a substance with small spherical molecules. It is 0.011 for methane and 0.49 for n-decane; it enters the temperature function of the Soave–Redlich–Kwong and Peng–Robinson equations.
Techniques that make the acid enter all the layers of the interval, not only the most permeable one: packers and coiled tubing, ball sealers, viscous gels and foams, diverting particles. Slugs of diverter temporarily raise the injection pressure.
Hydraulic fracturing of a carbonate formation with acid and no proppant: the acid dissolves the fracture walls unevenly, and when the pressure is released the remaining asperities prevent them from closing. It is used in strong, heterogeneous carbonates; it is not applicable in sandstones.
The product of the density of a rock and the velocity of the compressional wave in it.
A scheme in which some cells are treated fully implicitly and the rest by the IMPES scheme; the choice is made at each step from the local Courant number. Only a few cells become implicit (those near wells and in the front zone), so the system of equations is smaller than in the fully implicit scheme, while the step is not limited by the stability of the “fastest” cells.
Relationships for a centrifugal pump when the rotational speed is changed: the flow rate is proportional to the speed, the head to its square, the power to its cube, and the efficiency at corresponding points does not change. They relate corresponding points on the performance curves, not the operating points of the pump in the well.
The continuing inflow from the reservoir into the wellbore after the well is closed at the surface, while the fluid in the wellbore is being compressed. A manifestation of wellbore storage: because of it, the early points of the build-up lie below the Horner line.
The recalculated time after shut-in, , where is the producing time before shut-in: the pressure rise on the build-up at time equals the pressure decline on the drawdown at time . It makes it possible to process a build-up after a short producing period by drawdown analysis methods and with drawdown type curves; it is valid for infinite-acting radial flow and is always less than .
A projection of the layers of both fault walls onto the fault plane. It shows the windows where the reservoir of one wall abuts the reservoir of the other, and the areas where impermeable rocks stand opposite the reservoir.
Removal of hydrogen sulfide and carbon dioxide with aqueous solutions of amines (MEA, DEA, MDEA): in the absorber they bind the acid gases, and in the regenerator they release them on heating. The separated acid gas is converted into sulfur by the Claus process or injected into a formation.
A wettability index from the Amott test: , where and are the fractions of oil and water displaced by spontaneous imbibition of water and of oil respectively, of the total volume displaced. It varies from −1 (hydrophobic rock) to +1 (hydrophilic).
The change in reflection amplitude with the angle at which the wave strikes the boundary, that is, with the distance of the receiver from the source. It is described by the intercept (the reflection at normal incidence) and the gradient; these are used to distinguish, for example, soft gas-bearing sands from water-bearing ones, but an AVO anomaly is an indicator, not proof of gas.
The equal annual sum that over the service life of an asset recovers the invested capital with a return equal to the discount rate: the investment multiplied by the capital recovery factor. Each payment is the return on capital not yet recovered plus a repayment of capital; it is used to calculate minimum tariffs and to compare buying with leasing.
A series of equal payments at equal intervals of time. If the payments arrive at the end of each period, it is an ordinary annuity (payments in arrears); if at the beginning, an annuity due (payments in advance). The present value of the series equals the payment multiplied by the annuity factor.
The present value of one unit received at the end of each of periods: , the sum of the discount factors. It is always less than and tends to as the term grows. The value of the same series accumulated to the date of the last payment is given by the factor .
Measurement of the pressure in the annulus by a sensor in the bottom hole assembly (BHA) while drilling. It is used to monitor in real time the equivalent circulating density, hole cleaning, the surge and swab pressures during tripping, and the signs of an influx or of losses.
A preventer with an annular rubber packing element that a piston squeezes around a pipe of any shape and size or, as a last resort, around an empty hole. It allows the string to be stripped under pressure; its working pressure is often lower than that of the ram preventers in the same stack.
A fold that is convex upwards, with older rocks in its core. It is the most common type of structural trap: it is closed by the dip of the beds in all directions.
The unit of the gamma ray log scale, defined by a reference formation model: the difference between the readings in its zones of high and low radioactivity is taken as 200 API units. Approximate values: less than 10–15 in salt and anhydrite, 15–35 in clean sandstones, 80–150 in shales.
A conventional inverse scale of stock-tank oil density: , where is the specific gravity relative to water at 60 °F (15.6 °C). Water is 10 °API; the lighter the oil, the more degrees.
The water influx from the material balance on a date, divided by the influx function of the chosen aquifer model on the same date. If the model is correct in form, the value does not depend on the date; a drift indicates a wrong size, time scale or geometry of the aquifer.
The angle of inclination of a bed on a vertical section that is not perpendicular to the strike; it is always smaller than the true dip: , where is the angle between the section and the strike. On a section with a distorted vertical scale the inclination is, moreover, exaggerated.
The density with which gas enters the density log equations: the tool counts electrons and is calibrated on water-filled limestone, so for methane g/cm³. It is much lower than the true density of the gas, and for a light gas it is formally negative.
Hydrocarbons in place calculated from the material balance for a single date on the assumption that there are no sources of energy other than those accounted for. For a closed accumulation they do not depend on the date; growth from one measurement to the next indicates water influx.
The matrix density calculated from the bulk density and the apparent porosity on the assumption of a liquid-filled bed: . For a clean liquid-filled bed it equals the density of the mixture of minerals; gas lowers it, and an error of one porosity unit changes it by about 0.02 g/cm³.
The transit time of the matrix calculated from the sonic log and the apparent porosity with the same transform that was used to construct the mineral points on the chart: the time-average equation or an empirical one. Vugs lower it, gas usually raises it.
The volumetric photoelectric cross section of the rock converted to zero porosity using the apparent porosity: . Together with the apparent matrix density it forms a crossplot on which mixtures of quartz, calcite and dolomite lie within a triangle and are read in volume fractions.
The compressibility of oil below the bubble point pressure together with the gas coming out of solution: . It is tens of times greater than the compressibility of undersaturated oil; it enters the total compressibility in well tests below the bubble point pressure.
The resistivity of the homogeneous medium in which the tool would give the same reading as it does in the well. It mixes the contributions of the mud, the invaded zone, the formation and the enclosing rocks, so the formation resistivity is obtained from it after corrections.
The porosity read from a crossplot of two porosity methods (“neutron–density” or “sonic–neutron”) on the assumption of a clean liquid-filled bed; it is almost independent of the matrix minerals. It is used to calculate the apparent matrix parameters; for points above the sandstone line it is taken from the sandstone line at the same neutron reading.
The ratio of shear stress to shear rate at a given point on the flow curve of a non-Newtonian fluid; it depends on the shear rate. From a viscometer reading at 600 rpm it equals mPa·s. It is distinguished from the effective viscosity — the viscosity of a Newtonian fluid that would give the same laminar pressure loss in the same channel.
The water resistivity calculated from the formation resistivity and its formation resistivity factor as if the formation were fully water-bearing: . In a clean water-bearing formation it equals ; in a formation containing hydrocarbons it is greater: .
The work between discovery and the decision to develop: appraisal wells, tests, seismic surveys and studies that refine the volume, productivity and structure of the accumulation. The aim is not to increase reserves but to narrow their range enough to decide on a sound basis whether to develop the accumulation and with what project.
The water-saturated part of a formation (or of the formations connected to it) that is in hydraulic communication with the accumulation. When the pressure at the contact falls, the water and the pores of the aquifer expand elastically and water enters the accumulation; occasionally the aquifer is also recharged from the surface.
The multiplier of the unsteady-state influx model, , m³/MPa (bbl/psi): the influx at a constant pressure decline at the contact equals . It depends on the porosity, compressibility and thickness of the aquifer, the radius of the accumulation and the fraction of the perimeter , but not on permeability, which enters only the dimensionless time.
The dimensionless pressure decline at the contact of the accumulation with the aquifer at a constant water influx rate — the constant-rate solution in which the accumulation plays the role of the well. It is used in the Carter–Tracy method; for a closed aquifer, after the pressure decline reaches the boundary, it grows linearly with time.
A parameter of an aquifer model: the water influx at reservoir conditions per day per 1 MPa of pressure difference between the aquifer and its contact with the accumulation. In the steady-state model this difference is the pressure decline at the contact from the initial value, and the index itself reflects the flow capacity of the aquifer and the distance to the region of constant pressure.
An empirical relationship between the resistivity of a clean reservoir rock and its porosity and water saturation.
A graph of the area enclosed by each structure contour on the top (or base) of a bed against its depth. The area under the top curve from the crest to the contact is the volume of rock above the contact for a bed thicker than the accumulation; for a thin bed the area under the base curve is subtracted from it. The volume is calculated by layers between contours using the trapezoidal or pyramidal rules.
A sandstone rich in feldspars: in one widely used scheme, quartz makes up less than 75% of the framework and feldspars outnumber rock fragments. It is mineralogically immature: on burial the feldspars partly dissolve and turn into kaolinite, and potassium feldspars raise the gamma ray readings.
An empirical conversion of the resistivity of an aqueous solution to another temperature: with temperatures in °C. On heating from 20 to 100 °C the resistivity falls by almost a factor of three.
A man-made fill base in shallow water on which wells are drilled and equipment is installed almost as on land; it protects them from waves and drifting ice. The Kashagan field in the northern Caspian, where the water depth is 3–4 m, is developed in this way.
Methods of production in which the energy of the reservoir is supplemented by a pump or by injection of gas into the well.
The pressure, above the bubble point pressure, at which the first particles of asphaltenes appear in the reservoir oil as pressure falls. It is measured on a reservoir sample from the attenuation of transmitted light; below the bubble point pressure the asphaltenes partly redissolve.
History matching in which an algorithm searches the parameters within ranges set by the engineer, minimising the misfit function; the result is usually a set of models consistent with the history.
A weak, hot layer of the upper mantle beneath the lithosphere, from roughly 80–100 km to 200–300 km, where seismic wave velocities are reduced. It is solid, since it transmits shear waves, but over thousands and millions of years it flows, and the lithospheric plates move over it.
The quantities that fix the position of a plane in space: the strike azimuth (of a horizontal line on the plane), the dip azimuth (the direction of steepest downward inclination) and the dip angle. The Russian-language convention gives the dip azimuth and the dip angle, “dip az. 130° ∠ 35°”, while the English-language convention often gives the strike by the right-hand rule and the dip angle, 040/35.
Clay minerals that grew in the pores after burial: coatings on grains (chlorite, smectite), pore fill (kaolinite booklets), bridges across pores and pore throats (fibrous illite). They reduce permeability more than porosity and increase the irreducible water saturation; continuous chlorite coatings prevent quartz cement from growing.
The approved budget for a well or other work item. In joint ventures it is approved by the partners; if it is overspent by more than an agreed percentage, the operator prepares a supplementary AFE. The estimate is divided into costs by time, costs by depth and materials, and one-off costs, and also into tangible and intangible costs.
The mean of the absolute values of the relative errors of a calculation compared with measurements; together with the mean signed error (the bias) it characterises the accuracy of a correlation on a data set.
The pore-volume-averaged pressure in the drainage area of a well or in the accumulation. It is found from pressure build-up with corrections and from the material balance; the productivity index and the inflow performance relationship are written in terms of it.
The angle in the horizontal plane between the direction to north and the horizontal projection of the wellbore, measured clockwise from 0 to 360°. Always state which north it is measured from: true, grid or magnetic.
Gamma ray logging while drilling with a detector that looks in one direction: as the string rotates, the counts are divided into sectors of the wellbore — top, bottom and sides. The difference between the readings from above and below shows which boundary of the formation — top or base — the wellbore is approaching, which is needed for geosteering.
The angle through which a PDC cutter is tilted backwards: by the same angle its axis is deflected from the direction of motion and its cutting face from the perpendicular to the bottom of the hole. A small angle gives a thick cut and fast penetration, but the cutter chips more easily; a large one is slower, but the cutter is stronger. It is usually about 15–20°, and larger at the gauge and in bits for hard rocks.
The rotation of a pump in the reverse direction after shutdown: in an electric submersible pump, under the action of the liquid draining from the tubing through the pump (turbine rotation); in a progressing cavity pump with a rod drive, also from the unwinding of the twisted rod string. Starting during backspin damages the motor and the shaft, so the drives of progressing cavity pumps are fitted with a backspin brake.
An obstruction in a reservoir that slows the flow or forces it to detour around the obstruction, but does not stop it: discontinuous clay layers, clay drapes at the boundaries of stacked bodies, tight concretions, partially conductive faults. Unlike a barrier (an obstruction that is impermeable on the time scale of development), a baffle allows pressure on its two sides to equalise over geological time.
Balls of rubber or of a degradable material that are pumped with the treatment fluid: the flow presses them against the holes that are taking fluid and closes them, diverting the fluid into the others. They work in cased and perforated wells at a sufficient flow rate.
A unit of oil volume: 42 US gallons, exactly 0.158987294928 m³, or 5.6146 cubic feet; one cubic metre contains 6.2898 barrels. The number of barrels per tonne depends on the density of the oil; STB is a barrel at standard conditions, rb a barrel at reservoir conditions.
A narrow sand ridge along a coast, separated from the mainland by a lagoon and cut by tidal inlets. Its sandstone, a lens elongated along the ancient shoreline and enclosed by lagoonal and shelf clays, is a common stratigraphic trap. On coasts with tides higher than 4 m barriers do not, as a rule, form.
A run of the project model with the best estimates of all the input parameters. It serves as the reference point for sensitivity analysis and scenarios; it need not coincide with the average expected result.
A production forecast without the intervention being evaluated; the increment from the intervention is the difference between production with it and the baseline. Baseline production is calculated analytically (from decline curves or displacement characteristics) or from a reservoir simulation model that has been history-matched.
Compression of a basin formed by extension: normal faults are reactivated as reverse faults, and the sequence that accumulated in the former depressions is squeezed upwards into anticlines. Below the null point the displacement on such a fault remains normal; above it, it becomes reverse.
A rule for updating a probability with new data: the posterior probability of an event equals its prior probability multiplied by the probability of the observed result given that event and divided by the total probability of the result. It requires knowing how the data respond both when the event is present and when it is absent.
Interpretation based on readings taken opposite each picked bed, as opposed to sample-by-sample interpretation, in which the properties are calculated at every recorded depth and the intervals that pass the cut-offs are then summed. Bed-by-bed interpretation is faster and clearer; sample-by-sample interpretation conveys changes within a bed better.
The ridges that a current builds on a mobile sandy bed (ripples, dunes, antidunes) and also the flat bed. Which form appears depends on the flow velocity and depth and on the grain size: as the velocity increases in fine and medium sand, ripples give way to dunes, then to the upper-regime plane bed and antidunes.
A grade of crude oil whose price serves as the reference for the prices of other grades: they are sold on the formula “benchmark price plus or minus an adjustment” for quality and delivery. The main benchmark crudes are Brent (North Sea), WTI (USA, delivery at Cushing, Oklahoma) and Dubai (Middle East, sales to Asia).
A sub whose upper and lower thread axes intersect at a small angle: placed above a downhole motor, it tilts the bit relative to the string. In steerable motors this bend is built into the motor housing, closer to the bit, and the assembly can be rotated.
A clay consisting mainly of montmorillonite (smectite). Sodium bentonite swells in fresh water and breaks up into extremely thin platelets, so even a few per cent by mass give the fluid viscosity, a gel structure and a thin, low-permeability filter cake; in salt water it hardly swells.
The simultaneous action of pressure and axial force on a pipe. Axial tension reduces the collapse resistance: the collapse pressure is recalculated with a yield strength reduced according to the axial stress. A full calculation with three stresses — axial, hoop and radial — is made using the von Mises equivalent stress.
The early flow regime towards a hydraulic fracture of finite conductivity: fluid flows at the same time along the fracture to the wellbore and from the formation to the part of the fracture nearest the wellbore. The pressure drop grows as the fourth root of time, a slope of ¼ on the diagnostic plot; this section is used to find the fracture conductivity.
An empirical correction to the attraction between unlike molecules in the mixing rules of an equation of state: the attraction of a pair is taken as . It is nearly zero for hydrocarbons of similar size and noticeable for pairs involving carbon dioxide, nitrogen or hydrogen sulfide, and for methane with heavy fractions; each equation has its own, and it is fitted to experiments.
The rheological model : the fluid does not flow until the stress exceeds the yield point , and beyond that the stress rises linearly with a slope equal to the plastic viscosity . The parameters are found from viscometer readings at 600 and 300 rpm; at low shear rates the model overestimates the stresses.
Subdivision and correlation of sections using the remains of organisms: biostratigraphic zones are defined by the levels of first and last appearance of species, and their time ranges have been calibrated by dating. In wells, microfossils from cuttings and core are studied.
The fraction of the pore pressure that takes load off the rock frame: the effective stress equals . It equals , where is the grain compressibility and is the bulk compressibility of the rock as a whole; for loose, weak rocks it is close to unity, and for tight, cemented rocks it is 0.6–0.9.
The mixing of sediment by burrowing and crawling organisms; their trails and burrows are traces of biological activity (trace fossils). The degree of reworking is assessed on an index from 0 (none) to 6 (sediment completely mixed). Bioturbation can degrade a reservoir by mixing clay into sand, or can raise vertical permeability by breaking up clay interlayers.
The power dissipated in the bit nozzles: the pressure drop across the bit multiplied by the flow rate, (MPa × L/s = kW). It is a measure of the energy the jets bring to the bottom of the hole; when the pumps are working at their pressure limit, it is greatest when the share of pressure drop at the bit is , where is the loss exponent of the system.
The pressure spent on accelerating the drilling fluid in the bit nozzles: , where is the discharge coefficient, about 0.95. It does not depend on the rheology of the fluid and rises as the square of the flow rate; the energy of the jets cleans the bottom of the hole.
A record of a bit and its run: diameter, model and IADC code, nozzles, depths in and out, footage drilled, hours and rate of penetration, weight on bit, rotary speed, flow rate and drilling fluid, dull grade and reason for pulling. Bits and operating parameters for new wells are selected on the basis of the records of offset wells.
A type of reservoir simulation model with three phases (oil, gas and water) in which the exchange of gas between the oil and gas phases is described by the solution gas–oil ratio and formation volume factors.
The solution gas–oil ratio, the oil and gas formation volume factors and the viscosities against pressure, on one basis and for one separation scheme, from a pressure above the initial pressure down to atmospheric; input data for material balance and for the reservoir simulation model. Below the bubble point pressure the gas that comes out of solution must occupy more volume than the oil loses.
The ratio of the load carried by the lines to the fast-line tension multiplied by the number of working lines: . It accounts for the friction and bending of the line in the sheaves and falls as the number of lines increases: per the API RP 9B calculation factor for sheaves on roller bearings, 0.874 with 6 lines and 0.811 with 10.
An uncontrolled flow of formation fluid out of the well through the wellhead. An uncontrolled flow of fluid along the wellbore from one formation into another, without reaching the surface, is called an underground blowout (cross-flow between formations).
An assembly of preventers and manifolds for sealing the wellhead and controlling the well during a kick.
The combination of distance and time on which the pressure in an infinite reservoir depends at a constant well rate. It equals the square of the ratio of the distance to the radius of investigation; changing to it turns the diffusivity equation into an ordinary differential equation.
The ratio of the attenuation that the cement adds to the casing wave in the interval under study to the same attenuation in an interval of full bonding of the same cement; in terms of amplitude, . It equals 0 in free pipe and 1 with full bonding; it is not a ratio of amplitudes.
The amount at which an asset is carried in the balance sheet: the cost of creating or buying it less accumulated depreciation and impairment losses. It shows costs not yet charged to expense, not what the asset will earn; the value of oil reserves in the ground is not reflected in it.
Accumulator bottles in which the working fluid is stored under the pressure of compressed nitrogen, together with pumps, pressure regulators and control valves. The stored energy closes the preventers and valves in seconds or tens of seconds, independently of the rig's electricity supply.
Sections of the borehole wall that have caved in during drilling where the hoop stress is greatest. In a vertical well the breakouts are elongated in the direction of the least horizontal stress, , and are visible on image logs and on a multi-arm caliper; they are used to find the direction of the stresses.
An unrolled image of the borehole wall made from resistivity (electrical micro-imagers) or from the reflection of ultrasound (acoustic scanners). Inclined planes (bed boundaries, cross-laminae, fractures) appear on it as sinusoids: the amplitude gives the dip angle and the lowest point gives the azimuth. The predecessor of imagers, the dipmeter, calculated only dips from several micro-resistivity curves.
The lower part of the drill string above the bit: drill collars, stabilisers, subs, a downhole motor or a rotary steerable system, and MWD and LWD sensors. Its composition and the placement of the stabilisers determine whether the hole builds, holds or drops inclination.
A sample of reservoir fluid taken in the wellbore opposite the formation by a sampler run on a cable or wire, when the pressure at the sampling point is above the saturation pressure. An ordinary sample cools as it is brought up and becomes two-phase, and the laboratory restores it by heating and compression.
The temperature measured by a tool at the bottom of the hole during logging. Because the bottom of the hole is cooled by mud circulation, it is usually lower than the formation temperature; the formation temperature is extrapolated from several measurements made at different times using the Horner method.
The pressure at the bottom of a producing well, usually referred to the depth of the middle of the perforated interval.
The gravity anomaly with corrections for latitude, station elevation (0.3086 mGal/m, the free-air or Faye correction), the attraction of the intermediate layer of rock between the station and the reference level (, the Bouguer correction) and terrain. It reflects density inhomogeneities below the reference level; on land it is the main map of gravity surveys.
Side-pocket mandrels below the operating gas lift valve, usually down to the packer itself, fitted with blanking plugs. When the reservoir pressure falls and more gas is supplied, the gas injection point is moved into them using wireline equipment, without pulling the tubing — provided the unloading valves can bring the gas to the new mandrel.
A river with many shallow, mobile channels separated by bars; it carries a lot of sand and gravel, flows on a comparatively steep gradient and its discharge varies widely. Its deposits are broad multistorey sand sheets with a high net-to-gross ratio and thin, flat clay interlayers.
The product price at which the chosen project indicator, usually the NPV at a given discount rate, equals zero with the other parameters at their base values. It depends on the indicator and the rate, so these are always stated. Break-even values of production, costs and timing are determined in the same way.
The pressure in the well at which the intact wall of the wellbore splits. It is usually higher than the minimum stress and depends on both horizontal stresses, the tensile strength of the rock and whether the fluid penetrates the rock, so it is not a measure of stress.
An additive to the fracturing fluid that breaks down the polymer after pumping and removes the viscosity, so that the fluid flows out of the fracture and does not plug the proppant pack: oxidisers (persulfates, including encapsulated slow-release types), enzymes and acids. Too much breaker destroys the gel before the fracture closes.
The time until water or gas, arriving as a cone or through the formation, appears in the production of a well. For a cone it is counted from the start of production and estimated from dimensionless correlations or by simulation, to an accuracy of about a factor of two; in waterflooding it is counted from the start of injection, in the simplest case using the Buckley–Leverett method.
Particles of a selected size — calcium carbonate, salt, fibres — in a drilling fluid or kill fluid that quickly bridge the pore channels at the wall and form a thin external filter cake. As a rule of thumb they should be at least one third of the diameter of the pore channels.
The conditions, primarily depth and temperature, at which rocks change from brittle failure, with the formation of fractures and faults, to ductile flow. In the continental crust it is usually at a depth of 10–20 km, about 300 °C for quartz-rich rocks; salt and undercompacted clays are ductile even near the surface.
The pressure at which the first bubbles of gas come out of the oil as the pressure is reduced.
The equation of one-dimensional displacement of immiscible phases: each water saturation moves at a velocity , proportional to the slope of the fractional flow curve. It is the basis for calculating the displacement front and water breakthrough.
The change of inclination per unit of wellbore length in a build (or drop) section, in degrees per 30 m, per 100 ft or per 10 m. At a constant rate the section is a circular arc of radius , where is the reference length.
The relative change in the external (bulk) volume of the rock together with its pores per unit change in load. It governs compaction of the formation and subsidence of the surface; for incompressible grains it equals the pore compressibility multiplied by porosity.
The mass of a unit volume of rock together with the fluid in its pores; for a clean rock . It is measured by the density log, in g/cm³; it is used to calculate porosity, overburden pressure and acoustic impedance.
The fraction of the rock volume occupied by water, the product of porosity and water saturation. In the zone of irreducible water saturation of one rock type it is almost constant, so the same in beds of different porosity is a sign that the water in them is immobile.
Pumping into a shut-in well to force the influx fluid back into the formation. It is used when the influx cannot be brought to the surface, or doing so is dangerous (for example, gas containing hydrogen sulfide), or when circulation is impossible; the pumping pressure is limited by the strength of the casing, the equipment and the rocks below the shoe.
The ratio of the weight of a pipe string in fluid to its weight in air: , where is the density of the fluid and is the density of steel (7.85 g/cm³).
A graph of the depth of a bed, usually a source rock, against time, taking into account compaction, hiatuses and uplift. Together with the geothermal gradient, it shows when the rock was in the oil and gas generation zone.
The difference between the pressures inside and outside the casing string at the same depth; for the surface and intermediate casing it is greatest at the wellhead of a shut-in well filled with gas. It is compared with the internal yield pressure of the pipe.
Oil in the parts of a reservoir that the displacing water or gas has not reached: the oil saturation there is close to the initial value, and the oil is mobile. It is recovered by new wells, injection management and methods that improve sweep. Unlike residual oil trapped in swept pores, it does not require the residual saturation to be reduced; it is enough to bring the displacing agent to it.
A record of the borehole diameter against depth. Comparison with the bit size reveals washouts and mudcake and serves as the first check of log quality.
The volume of fluid that a reservoir or part of it yields when the pressure falls by 1 MPa: the product of total compressibility and pore volume, , m³/MPa. Divided by the transmissibility of the connection, it sets the pressure equalisation time.
The accumulation of the wetting phase at the outlet end of a core sample, where the capillary pressure is zero. It overstates the irreducible water saturation in drainage and, in a rock of mixed wettability, the residual oil as well, delays breakthrough and distorts the relative permeabilities; it weakens with increasing flow rate and sample length.
The ratio of viscous forces to capillary forces, . In waterflooding it is of the order of ; residual oil begins to decrease at about , so to displace it the interfacial tension is lowered with surfactants.
The difference between the pressures of the non-wetting and wetting phases across the interface between them in the pores.
Water in fine pores and in the corners of large ones, which capillary forces retain at reservoir pressure differentials. From NMR it is the part of the distribution from the clay-bound water boundary (about 3 ms) to the T2 cutoff; its volume as a fraction of the rock volume is denoted BVI. Together with the clay-bound water it makes up all the irreducible water: is the irreducible water saturation; together with the free fluid it makes up the NMR effective porosity, .
Expenditure on creating or expanding assets that work for years: wells, facilities, pipelines. For tax and reporting purposes it is written off gradually, through depreciation.
Capital expenditure per unit of daily production or capacity, for example in dollars per 1 bbl/d. It depends on which production rate is used (plateau, average or equivalent flat) and says nothing about the duration of production.
A situation in which there is not enough money for all projects with a positive NPV. The task is then to find the set of projects with the greatest total NPV for the given budget: divisible projects are taken in descending order of profitability index, indivisible ones are tested by trying combinations.
The fraction of an investment that must be received at the end of each of years in order to recover it with a return : . It equals for a single year and tends to for an infinite term; it is made up of the return on capital and the contribution to a sinking fund that returns the capital.
A body of carbonate rock built up by organisms above the surrounding sea floor. A reef is a buildup with a rigid framework that withstands waves; an atoll is a ring-shaped buildup around a lagoon. The subsalt buildups of the Pre-Caspian Basin are the reservoirs of Tengiz, Kashagan and Karachaganak.
The community of organisms and the conditions in which carbonate sediment is produced: the skeletons of corals, molluscs, algae and foraminifera, and lime mud. The output of a shallow-water factory depends on light, and therefore on water depth and clarity, and on temperature, salinity and the amount of clastic material; it is greatest in the upper tens of metres of a warm sea.
A carbonate platform whose floor slopes gently, usually by less than a degree, from the shore into the basin without an abrupt break. Its belts are distinguished by the fair-weather and storm wave bases; they are wide, so facies can be traced over kilometres.
Limits of the equivalent permeability of a block with impermeable lateral faces or periodic conditions for flow along an axis, narrower than the Wiener bounds: the lower is the arithmetic mean of the harmonic means along the lines of cells in the direction of flow, the upper is the harmonic mean of the arithmetic means over the cross-sections perpendicular to the flow. They correspond to imaginary impermeable partitions along the flow and perfectly conducting ones across it.
A working interest whose costs at a certain stage, for example an exploration well, are paid by another participant. The owner of such an interest takes part in the result without paying anything at that stage; under the terms of some deals the costs paid on its behalf are later recovered from its share of production.
A step-by-step calculation of water influx from an aquifer without superposition: the influx at a new step is found from the influx at the previous step, the total pressure decline at the contact and the influence function with its derivative. It is exact at a constant influx rate; in other cases the error is a few per cent, and it is also inaccurate at very short steps; it is the standard analytical aquifer of reservoir simulation models.
A bottom-hole design in which the formation is covered by a cemented casing string or liner and opened by perforating in selected intervals. It makes it possible to produce from, inject into and treat the formation interval by interval, and to isolate intervals that have watered out; it depends on the quality of the cement sheath.
The receipts and payments of money caused by a project, with the amount, the sign (inflow “+”, outflow “−”) and the moment of actual payment.
A string of steel pipes that is run into the well and cemented to support the wellbore and isolate the formations.
A downhole tool made up of a magnet and a coil: opposite a collar of the casing or tubing there is more metal, and when the tool moves a pulse is induced in the coil. The collar pattern and the pipe tally, together with the gamma ray log, are used to tie the perforating guns and tools in depth.
The steel grade of casing and tubing; the number in the API 5CT designation is the minimum yield strength in thousands of psi (for N-80, 552 MPa). For environments containing hydrogen sulfide, grades with limited hardness (L-80, C-90, T-95) are used.
The component on which a casing string hangs in the bowl of the casing head. A mandrel (threaded) hanger is screwed onto the top joint, and the string has to be adjusted to length; a slip hanger grips the pipe with slips at any point, so the string can be hung at any depth and under tension.
The flanged wellhead assembly in whose bowl a casing string is hung and which closes off the annulus between the strings. The lowest head, single-flanged, is installed on the surface casing; for each following string a double-flanged casing spool is used; the blowout preventers for the next interval are installed on the upper flange.
The number of casing strings, their diameters, setting depths and cemented intervals, together with the diameters of the hole sections. The strings, in the order in which they are run, are the conductor, the surface casing, the intermediate (technical) strings, and the production casing or liner.
The lower end of a casing string, fitted with a guide nose that leads the string down the hole, often with a built-in check valve. The term “shoe” is also used for the depth at which the bottom of the string is set: the rock below it is usually the weakest point of the next open-hole section.
The quantity of cations that a clay holds on its surface and can exchange for others, in milliequivalents per 100 g of dry rock. It arises from the negative charge of the layers: 80–150 for smectite, 10–40 for illite and chlorite, 3–15 meq/100 g for kaolinite. It is measured, in particular, by the methylene blue test.
The formation of vapour bubbles in a liquid where the pressure falls to the vapour pressure, and their collapse downstream, where the pressure is higher. It limits the delivery of pumps and destroys metal by the impacts of collapsing bubbles; in a jet pump it occurs at the entrance to the mixing chamber.
A record of the sound wave that has travelled along the casing string: the amplitude of the casing wave at the near receiver shows whether the cement is bonded to the pipe, and the waveform display of the far receiver — the variable density log (VDL) — whether it is bonded to the rock. In free pipe the amplitude is greatest; with full bonding it is tens of times smaller.
A plug of set cement that closes a cased or open hole over a specified length: for abandonment or suspension, against lost circulation, for kicking off a sidetrack, or for isolation during tests. It is most often placed by the balanced method: the cement levels inside the pipe and in the annulus are equalised, then the pipe is pulled out.
The exponent in the relationship , the slope of the straight line on a log–log plot. It is about 2 in cemented sandstones, lower in fractured rocks and higher in vuggy rocks.
In Kazakhstan, the commission that, with the involvement of independent experts, carries out the state expert review of base project documents for hydrocarbons and of development analyses: addenda to the exploration project that provide for appraisal, offshore exploration or an increase in the area, and trial production and development projects. The review period is 2 months for an exploration project and 3 months, extendable by up to 3 months, for the others.
A field facility where oil separation is completed and the oil is dehydrated, desalted and stabilised, stored in tanks, measured for quantity and quality at the custody transfer metering unit and delivered into the main oil pipeline; gas and water are usually treated there as well.
A device on the body of a casing pipe that holds the string in the middle of the hole so that the cement surrounds it on all sides. Bow-spring types pass constrictions and expand in washouts, rigid types withstand large lateral forces; the spacing is calculated from the lateral forces and the deflection of the pipe.
The redistribution of pressure by an isolated, dipping, permeable bed enclosed in clays with abnormally high pressure: the pressure in the bed equalises along the water gradient and equals the pressure in the clays only at the depth of the centroid. Above that depth, at the crest, it is higher than in the clays at the same depth, and below it is lower; for this reason wells at the crest encounter a pressure higher than predicted from the clays.
The guaranteed amount that a decision-maker regards as equivalent in value to a risky option. For a risk-averse decision-maker it is less than the expected monetary value; the difference is called the risk premium.
The strip over which a river wandered before moving to a new position; the sand body of a meandering river. The width of the belt is estimated from the channel width by empirical formulae (for meandering rivers it is about 6–10 times the channel width), and the channel width from the depth, which is close to the thickness of a complete channel cycle; the estimates are rough.
The constant in the linear flow equation for a well in a channel of width located a distance from one of the walls: the additional pressure drop for the transition from linear flow to radial flow at the wellbore. It increases as the well approaches a wall.
A narrow formation between two parallel no-flow boundaries: a graben, a block between parallel faults, or a channel sand body. In the late part of a well test the flow runs along the channel (linear flow): the pressure change grows as , the derivative has a slope of ½, and the channel width is found from the slope of the straight line of against .
Measurement of the arrival time of the direct wave from a source at the wellhead to a geophone lowered in the borehole to a series of depths. It gives the time–depth relationship and the average velocities down to each depth; it is used to calibrate the sonic log and to tie reflections to formations.
Injection of a resin into the formation near the well which, after displacement, remains at the grain contacts and, when it hardens, binds the grains together, leaving the pores open. It leaves no equipment in the wellbore, but it is applicable to short intervals of clean sandstones.
A restriction in the flow path in the Christmas tree or in the well that creates a set pressure drop: it is used to limit the flow rate and drawdown, to protect equipment from high pressure and to distribute production. A fixed choke is a replaceable bean with a calibrated orifice; an adjustable choke changes its opening without being dismantled.
An assembly of valves, piping and adjustable chokes to which fluid from the shut-in well flows through the choke line. The choke sets the pressure in the annulus, and through it the bottom-hole pressure. The working pressure of the manifold up to and including the chokes is the same as that of the ram preventers of the stack.
A classification of pores in carbonate rocks by their relation to the rock fabric: fabric-selective (interparticle, intraparticle, intercrystal, moldic, fenestral, shelter, framework), non-fabric-selective (fractures, channels, vugs, caverns) and mixed. Fabric-selective pores are predicted from facies, non-fabric-selective ones from the structural and diagenetic history.
Layered aluminosilicates of micrometre size: kaolinite, illite, smectite (montmorillonite), chlorite and mixed-layer clays. They differ in potassium and thorium content, bound water, swelling capacity and photoelectric factor, and they affect permeability, neutron porosity and resistivity in different ways.
A layer of clayey material smeared over the fault surface as the fault walls move past clay beds. It can make a fault sealing even where permeable rocks are in contact across it.
The fraction of the rock volume occupied by the clay minerals proper: illite, kaolinite, smectite and chlorite. It is usually smaller than the shale volume, which also includes silt and bound water; on core it is determined by X-ray analysis.
Water in the electrical double layer at the surface of clay particles. It is part of the total porosity but is not displaced by oil and gas; its fraction of the total pore volume is denoted . In models of shaly reservoirs the effective porosity is the total porosity minus this water.
The ability of a crystal to split along definite planes of the lattice, leaving smooth, shiny surfaces. Micas have one perfect cleavage, feldspars two, and calcite, halite and anhydrite three; quartz has no cleavage and breaks with a conchoidal fracture.
A formation between two parallel no-flow boundaries blocked by a third one on one side: a block between faults with a transverse fault, or a channel body that pinches out at one end. The late linear flow in it is twice as strong as in an open channel of the same width: the slope of the straight line of against and the derivative are doubled.
The vertical distance from the crest of a trap to its spill point, which is the greatest possible height of an accumulation in it.
The pressure in the fracture at which its walls come together as the pressure declines after injection. In a homogeneous formation it equals the minimum principal stress; it is found from the change in the character of the pressure decline after a calibration injection and is used to calibrate the stress model.
The standard deviation divided by the arithmetic mean: the spread as a fraction of the mean. For permeability it serves as a measure of heterogeneity: below 0.5 the rock is homogeneous, from 0.5 to 1 it is heterogeneous, above 1 it is highly heterogeneous. The number of samples needed for a reliable mean also depends on it.
Continuous steel pipe, usually 25 to 89 mm in diameter, wound on a reel and run into the well without threaded connections through a wellhead stripper, including under pressure. It allows acid or another fluid to be delivered precisely opposite the required interval, the wellbore to be washed, and work to be done in a horizontal well without killing it.
The difference between the pressures outside and inside the casing string at the same depth; it is greatest at the bottom of an emptied string. It is compared with the collapse pressure of the pipe.
The external pressure at which a casing pipe loses its shape. It falls sharply as the ratio of outside diameter to wall thickness, , increases: thick-walled pipes collapse at the yield of the steel, thin-walled ones by loss of stability. Axial tension reduces the collapse resistance.
A screening indicator of the tendency of an oil to precipitate asphaltenes, based on the group composition (SARA): the ratio of the sum of saturated hydrocarbons and asphaltenes to the sum of aromatic hydrocarbons and resins. Below 0.7 the asphaltenes are stable, 0.7–0.9 is uncertain, above 0.9 they are unstable.
The probability that an exploration well will discover an accumulation no smaller than the minimum economic field size: the geological chance of success multiplied by the fraction of the success-case volume distribution that lies above this threshold. It is no greater than the geological chance of success and is usually noticeably smaller.
The footage drilled per rig-month (720 h) of calendar time spent drilling and casing the well, including non-productive time: . The technical speed is calculated in the same way from the productive time; their ratio is equal to the share of productive time.
Production from several formations into a common wellbore, in which their output is mixed at the bottom of the well and rises through a single channel. It saves wells, but does not allow the formations to be controlled, measured and tested separately, and, where the formation pressures differ, causes crossflow through the wellbore.
The midpoint between a source and a receiver, shared by many source–receiver pairs. For horizontal boundaries all such pairs give a reflection from one point of the boundary beneath that midpoint; their traces are stacked after the normal moveout correction, strengthening the reflections and weakening random noise. The common midpoint (CMP) method is based on this and is the main method of 2D and 3D seismic surveying.
A part of a play within which the elements common to all traps (charge, regional seal, reservoir distribution) are assessed uniformly. Segments are obtained by overlaying maps of the individual elements; all the prospects in a segment inherit its common play risk.
A number of one or more by which the porosity from the time-average equation is divided in undercompacted sands. It is roughly estimated as the interval transit time of the adjacent shales in µs/m divided by 328 (in µs/ft, by 100); more reliably, it is determined by comparison with an independent porosity estimate in a clean water-bearing bed.
Displacement of fluids by the reduction of pore volume, when, as the reservoir pressure declines, the effective stress rises and weak rock — unconsolidated sand, high-porosity chalk — is irreversibly compacted. It supplies a significant share of the reservoir energy but is accompanied by subsidence of the formation and of the ground surface.
A sonde with two detectors or receivers at different distances from the source. The borehole, the mudcake and the mud affect them differently, and a comparison of their readings eliminates this influence; this is how the compensated density and neutron tools are built. In a borehole-compensated acoustic sonde there are two transmitters, one above and one below the receivers, and the average of the two opposing measurements eliminates the influence of borehole enlargements and of tool tilt.
The central state body that, under Kazakhstan's Subsoil Code, represents the interests of the Republic in the field of subsoil use: it grants and terminates subsoil use rights, concludes contracts and monitors their performance. For hydrocarbons, it is the Ministry of Energy.
The ratio of the productivity of a well to that of an undamaged open hole of the same radius in the same drainage area. It shows what part of the productivity is taken away by the bottomhole design: perforations, the crushed zone and the gravel pack.
A regular change in the composition of the reservoir fluid with depth in tall accumulations: under gravity the heavy components concentrate at the bottom, and the gas–oil ratio and saturation pressure vary through the section. The fluid density increases with depth, so its pressure line is curved, and near the critical point a gas condensate can pass into oil without a clear contact.
A type of reservoir simulation model in which each component (or pseudo-component) of the mixture is conserved, and the distribution of components between the oil and gas phases is calculated in each cell from an equation of state. It is needed when phase compositions change appreciably: near-critical fluids, gas cycling, miscible displacement.
Accrual in which interest is added to the amount and itself earns interest in subsequent periods; the amount grows exponentially: .
A drive mechanism in which oil is driven towards the wells by the expansion of undersaturated oil and connate water and by the reduction of the pore volume as the pressure falls above the bubble point pressure. The pressure falls rapidly and the gas–oil ratio is constant; usually only a few per cent of the oil in place is recovered before the bubble point pressure is reached.
A collision of a gamma ray photon with an electron in which the photon gives up part of its energy and changes direction. The probability of scattering is proportional to the number of electrons per unit volume, which is the basis of density logging; the weakened photons are then absorbed by the photoelectric effect.
A gamma ray curve calculated from the spectrometry data using only potassium and thorium, approximately in API units. Uranium is mobile and is often unrelated to clays, so this curve reflects shale volume more reliably than the total gamma ray.
A contract or licence under which a company obtains the exclusive right to explore for and produce oil and gas in an area and becomes the owner of the production, while the state receives its share through payments: bonuses, royalties and taxes. In Kazakhstan, hydrocarbon licences are not issued: the right arises from a contract.
The accumulation of condensate around a gas-condensate well when the bottomhole flowing pressure is below the dew point. All the gas of the well passes through this zone, and the saturation rises to values at which the condensate flows, much higher than in depletion of the reservoir; the gas permeability and productivity fall several-fold.
The amount of stable condensate obtained from a unit volume of gas, in cm³/m³ or g/m³; the reciprocal of the gas–oil ratio. It falls as the pressure drops below the dew point: the condensate stays in the reservoir.
The ratio of the largest eigenvalue of a symmetric positive definite matrix to the smallest. It shows by how many times the relative error of the solution can exceed the relative residual, and it sets the speed of iterative methods; in the pressure system it grows with the time step and with contrasts in transmissibility.
The horizontal distance between the axis of the bit and the vertical plane passing through the axis of the cone journal. In bits for soft rocks it reaches about 9–10 mm (3/8 in), in bits for hard rocks about a millimetre or zero: the offset makes the cone slip, and scraping of the rock is added to indentation.
A range around an estimate that covers the true value of the parameter with a stated confidence level (usually 95%). For the mean: , where Student's coefficient is close to 2 for a large number of samples and greater for a small number.
The compressive strength of a rock under lateral confinement. For a bit the confinement is provided by the overbalance: according to the Mohr–Coulomb criterion it is , where is the unconfined (uniaxial) strength and . The mechanical specific energy of drilling is compared with it.
The drawing up of bottom water from below, or of gas from the gas cap from above, towards the interval of a producing well: under the influence of the drawdown cone, the interface between the phases bulges towards the interval. Above the critical rate, water or gas breaks through into the well.
The pore volume in hydraulic communication with the well — the one from which it draws fluid. In a reservoir limit test it is found from the rate of pseudo-steady-state pressure decline, ; it may be smaller than the volume of the accumulation on the map if the accumulation is divided by sealing faults.
A solution of the diffusivity equation in which a constant pressure is specified at the inner boundary — the wellbore wall or the contact of the accumulation with the aquifer — and the flow rate, which declines with time, and its cumulative value are calculated. The van Everdingen–Hurst method is built on it; its counterpart is the constant-rate solution.
A solution of the diffusivity equation in which a constant flow rate is specified at the well or at the inner boundary of the aquifer and the pressure is calculated. Well test interpretation is built on it. Its counterpart is the solution at constant pressure on the inner boundary (constant terminal pressure), in which the declining flow rate and the cumulative influx are calculated.
A test on a gas condensate: the pressure is reduced in steps and gas is released so that the volume of the cell remains equal to its volume at the dew point. It gives the condensate dropout curve, the quantity and composition of the gas withdrawn, and the two-phase gas deviation factor.
A test in which the pressure of a sample at reservoir temperature is reduced in steps without releasing gas from the cell. It gives the bubble point pressure from the break in the pressure–volume curve, the relative volume and the compressibility of the oil above the bubble point pressure.
The angle between the mineral surface and the interface between two phases, measured through the denser phase, water. Below 90° the surface is hydrophilic; above 90° it is hydrophobic.
Under PRMS, the potentially recoverable quantities from discovered accumulations whose development project is not yet commercial because of one or more contingencies: no market or technology, the accumulation has not been sufficiently appraised, or there is no decision or approval. The categories are 1C, 2C and 3C; the chance of commerciality equals the chance of development.
A statement of conservation of mass for an element of the formation: the difference between the mass of fluid that has entered the element and the mass that has left it equals the change of mass in its pores. Together with Darcy's law and an equation of state it gives the diffusivity equation.
The limiting case of infinitely frequent compounding: the sum grows as , and the equivalent effective rate is .
The difference in value between adjacent contour lines on a map, for example 10 m between the structure contours −2410 and −2420 m; it is constant on one map. The interval is chosen to be no finer than the accuracy of the data and so that the contours do not merge on the steepest parts.
A selected volume of the reservoir, usually a grid cell, for which the mass of fluid entering, leaving and accumulating is counted. The balance in a control volume is exact whatever its size; the cell equations of a reservoir simulator are built on it.
The pressure at which the equilibrium ratios of all the components of a mixture at a given temperature would converge to one if their curves were extended beyond the saturation pressure. It depends on composition: the heavier the liquid, the higher it is; for a mixture at its critical temperature it equals the critical pressure. It served as a parameter of equilibrium ratio charts and enters the Whitson–Torp formula.
The number of throats meeting at a pore, usually its average over the rock or network. The larger it is, the more bypass paths the phases have: the percolation threshold is lower and trapping is smaller.
A column of rock drilled out of the formation by a core bit with a hole in the centre and brought to the surface in a core barrel. It is the main direct source of information about the reservoir rock; the logs are calibrated against it.
A cylinder 25–38 mm in diameter drilled out of the core along the bedding (horizontal) or across it (vertical). Porosity, permeability and special properties are measured on such samples.
The ratio of the length of core recovered to the length of the interval drilled with coring, as a percentage. When recovery is low, it is usually the weakest and often the most permeable rocks that are lost, and the remaining core no longer represents the formation.
Selection of interpretation parameters, for example the matrix and fluid densities, so that the calculation from logs reproduces the properties measured on core. It requires depth matching, bringing the core to reservoir conditions and averaging the core data to the scale of the tool.
A power-law approximation of relative permeability curves: and , where is the normalised water saturation. It is used in processing experiments and in reservoir simulation models.
A grid whose cells keep their (i, j, k) indices but whose position is defined by the coordinates of eight corners on pillars. The cells follow the top, the base and the faults; connections across faults are given by non-neighbour connections (NNC).
A measure of the linear relationship between two quantities, from −1 to +1: with positive correlation, large values of one more often go with large values of the other; with negative correlation, with smaller ones; with zero correlation there is no linear relationship. In probabilistic calculations the relationship between parameters is specified by correlation, so that related quantities are not sampled independently.
The distance over which the values of a reservoir property are still similar to one another: centimetres to metres vertically, and tens of metres to kilometres horizontally. Comparing it with the well spacing shows whether the reservoir behaves as layered or as a fine mosaic.
Correction of a correlation using measurements, for example by multiplying its result by an adjustment factor found from a sample. It removes the systematic bias for oils of the same system, but not the scatter of errors.
The return that the company's lenders and shareholders expect from their investment; for the company as a whole it is usually estimated as the weighted average cost of equity and debt capital (WACC). It is the lower bound of the discount rate for projects of ordinary risk.
In a production sharing agreement, the part of production that the contractor receives in recovery of its costs, usually no more than a set share of production in the period; unrecovered costs are carried forward to later periods.
The cost of a bit run divided by the metres drilled: the price of the bit plus the cost of rig time for drilling, connections and tripping. It is the main criterion for comparing bits and operating parameters: it takes account of both speed and run length. At the minimum of the cost per metre, the cost of the next metre equals the average cost.
Flow of two phases in opposite directions with no net total flow: gas rises while oil drains downwards under gravity, or water imbibes into a block of rock, displacing oil from it. Unlike the cocurrent flow of displacement, the rate here is set by the density difference or the capillary forces and the phase mobilities.
The stability condition of explicit transport schemes: in one step the transport must not travel more than one cell, that is, the Courant number must not exceed one. For saturation in the IMPES scheme, : the time step multiplied by the flow rate into the cell and by the slope of the water fractional flow curve and divided by the pore volume of the cell.
A sequence of NMR radio-frequency pulses: a pulse that rotates the magnetisation by 90°, followed by 180° pulses at equal intervals, after each of which the dephased protons come back into phase and a spin echo appears. The echo amplitudes, separated by the echo spacing , form a decay curve from which the distribution is found.
A time-stepping scheme in which the flows during the step are taken as the average of the flows computed from the pressures at the start and at the end of the step. It is second-order in time and stable at any step size, but with a large step the fast components of the solution decay slowly and change sign, producing oscillations after abrupt changes in well operating conditions.
The highest pressure at which a mixture can exist in two phases; above it the mixture is single-phase at any temperature.
The highest temperature at which a mixture can exist in two phases. If the reservoir temperature is above it, the fluid remains gas in the reservoir at any pressure.
The compressive force at which a pipe lying on the low side of an inclined wellbore begins to bend in a wave: , where is the bending stiffness of the pipe, is the weight of one metre of it in the drilling fluid, is the inclination and is the radial clearance between the pipe and the wall. At a larger force the pipe lies in a helix and presses against the walls; in a vertical hole there is no support, and compressed drill pipe buckles almost at once.
The regime in which the velocity in the throat of the choke reaches the speed of sound: the flow rate is determined only by the pressure and composition of the flow upstream of the choke and does not increase when the pressure downstream falls. For gas it occurs when the downstream pressure is less than about 0.53–0.55 of the upstream pressure; for a gas–liquid mixture the limit is about 0.5–0.6.
The largest drawdown on the formation at which the rock at the wall of the wellbore or perforation tunnel does not yet fail and the well produces without sand. It decreases with depletion and increasing water cut; it is estimated by calculating the stresses around the cavity and refined from the behaviour of wells.
The gas saturation below which gas liberated from the oil remains as immobile bubbles; usually a few per cent of the pore volume.
The time that best describes the generation, migration and accumulation of the bulk of the hydrocarbons of a petroleum system. By then, traps and seals must already exist.
The state at which the properties of the liquid and the gas become identical and the boundary between the phases disappears. For a pure substance it is the end of the vapour pressure curve, with coordinates and ; above a separate liquid phase cannot be obtained by compression.
The highest flow rate at which a stationary cone of water (gas) still does not reach the open interval. For vertical wells it is usually small, and different calculation methods give values that differ several-fold.
The highest flow rate, at reservoir conditions, through the cross-section of a dipping formation at which the interface between the displacing phase and the oil remains stable, when gravity holds back the displacing phase (water displacing oil updip, gas displacing it downdip): for a mobility ratio , where is the flow rate at which the gravity number equals one. Above it, water runs ahead as a tongue along the base and gas along the top. Not to be confused with the critical water-free rate of a well in coning.
The salinity of the pore water below which electrical repulsion detaches clay particles from the grains, and they migrate and plug the channels. It exists for monovalent cations — sodium and potassium; the divalent calcium and magnesium hold the particles in place even at a much lower salinity. A sharp reduction in salinity is more dangerous than a gradual one.
The filtration velocity of the wetting phase — usually water — above which fine particles are torn from the grains and begin to migrate. It is measured on core by increasing the flow rate in steps until the permeability begins to fall, and is transferred to the well through the inflow area: the wall of the open hole or the walls of the perforation tunnels.
A fracture on which, under the present-day stresses, the ratio of shear stress to effective normal stress is close to the coefficient of friction. Such fractures more often remain conductive, and an increase in pore pressure can make them slip.
Laminae inclined to the bed boundaries: the preserved lee slopes of ripples and dunes that migrated across the bed. The laminae dip downcurrent, so they are used to find the palaeocurrent direction; sets thinner than 4–5 cm are left by ripples, thicker ones by dunes. Straight crests produce planar (tabular) cross-bedding, curved crests produce trough cross-bedding.
The movement of water and oil between adjacent communicating layers across the bedding. It equalises the pressure across the thickness; if gravity is small, during waterflooding it improves the vertical sweep when the mobility ratio is less than one and worsens it when it is greater.
A polymer solution, usually of guar or its derivatives, whose molecules are linked by a crosslinker: borates or zirconium and titanium salts. Its viscosity is tens of times higher than that of a linear gel, and the gel holds proppant well. Crosslinking is often delayed so that the gel thickens closer to the bottom of the well, and after pumping the gel is broken down by a breaker.
A graph on which the readings of two porosity methods in the same beds are plotted against each other; lines of clean minerals with porosity marks and points for evaporites are drawn on it. The position of a bed's point gives the porosity and the mineral composition of the matrix, and its displacement gives the effect of shales, gas and vugs.
Reduction of the chloride salt content of oil: the oil is mixed with fresh wash water, which dilutes the residual formation water, and the water is separated again, usually in an electric field. It is needed because the salts are dissolved in the water, and even tenths of a per cent of brine give hundreds of milligrams of salts per cubic decimetre of oil.
Removal of dissolved light hydrocarbons — methane, ethane, propane, butanes — from oil by hot separation or in a stabilisation column, in order to reduce the saturated vapour pressure and the evaporation losses during storage and transport. Under the technical regulation of the Eurasian Economic Union (EAEU), the saturated vapour pressure of marketable oil must not exceed 66.7 kPa.
A ring of rock about a centimetre thick around a perforation tunnel, crushed and compacted by the shaped-charge jet; its permeability is several times lower than that of the formation. It often contributes more skin than the perforation geometry; underbalanced perforating and clean-up of the tunnels reduce its contribution.
The sum of a project's net cash flows from its start to a given moment. Its curve gives the maximum cash outlay, the payback period and the final result of the project; the flows are added up in real terms.
The ratio of all the gas produced to all the oil produced since the start of development, both at standard conditions. Unlike the current gas–oil ratio, it describes the whole history of withdrawal; it is this ratio that enters the material balance equation.
The position of the oil–water or gas–oil contact (free water level) in a producing accumulation on the date of the measurement: with bottom water it rises, with the expansion of the gas cap it descends. From pressure data it is found as the intersection of the phase lines in a new well; below the risen contact a swept zone with immobile residual oil remains, which logs can see.
The particle size at which a cleaning device — a screen, hydrocyclone or centrifuge — sends half of the particles to waste: particles noticeably larger are almost all removed, and noticeably smaller ones mostly stay in the fluid. For barite the cut point of the same device is smaller than for drilled solids — by a factor of about in water.
Threshold values of shale volume, porosity and saturation (sometimes permeability) by which the section is divided into reservoir and non-reservoir when reserves are estimated.
A failure of sonic logging: an attenuated signal does not reach the detection threshold on its first cycle, the far receiver triggers a cycle later, and the interval transit time jumps too high, by tens of µs/m on a compensated curve. The causes are gas, fractures, loose rocks, enlargements and irregularities of the borehole.
An index of the drillability of a rock, derived from the rate of penetration corrected for weight on bit, rotary speed and bit diameter; the corrected exponent also accounts for the drilling fluid density. In uniform clays at normal pressure increases with depth; a deviation from the trend to lower values is a sign of undercompaction and overpressure.
The component of the skin factor caused by the reduced permeability of the near-wellbore zone; for a zone of radius with permeability it equals . It is the only component of skin that can be removed by treatment or bypassed.
The dimensionless ratio of the rate of reaction to the rate at which the flow supplies the reagent. For a wormhole in a carbonate, it is the ratio of the amount of acid its walls are able to consume to the amount that the flow delivers; the least acid is needed to break through at about .
A unit of permeability: 1 D ≈ m²; in practice the millidarcy is used (1 mD = 0.001 D).
The depth — usually the mid-height of the accumulation or the initial oil–water contact — to which pressure measurements are converted using the fluid gradient so that they can be compared between wells and over time.
A graph of well construction: days from the start of drilling along the horizontal axis, the measured depth of the hole bottom, increasing downwards, along the vertical axis. Sloping sections correspond to drilling ahead, flat sections to time without progress in depth: trips, casing and cementing, logging, pressure tests, drilling problems. The planned and actual lines are compared on the same graph.
The viscosity of oil without dissolved gas at reservoir temperature and atmospheric pressure, the basis for calculating the viscosity of reservoir oil by correlations. It depends strongly on temperature and composition; correlations from density and temperature can be out by a factor of several for individual oils, so it is measured.
Removal of dissolved oxygen from water by reducing its partial pressure above the water — by vacuum or by stripping with an oxygen-free gas in a column with packing or trays. According to Henry's law the equilibrium concentration is proportional to the partial pressure; the residual oxygen is removed with a scavenger.
The cash flow available for servicing debt (revenue less operating costs and taxes) divided by the interest and principal repayments of the same period. The lender requires it to be kept above an agreed level, the covenant; if it is breached, the lender may prohibit dividends or demand early repayment.
A horizontal rotating bowl with a screw conveyor: particles settle against the wall under accelerations of hundreds and thousands of , the conveyor carries the settled solids out, and the liquid with the fines overflows at the other end. With a weighted fluid it returns the barite and discards the liquid with the colloids; with an unweighted or oil-based fluid it removes fine drilled solids and returns the liquid.
A diagram of a sequential decision problem: decision nodes (squares) alternate with chance nodes (circles), the branches carry cash flows and conditional probabilities, and the ends of the paths carry the outcomes. Folding back from right to left, taking the EMV at chance nodes and the best branch at decision nodes, gives the expected value of the best strategy and the strategy itself.
Forecasting of production by extending the observed decline in rate (along an exponential, hyperbolic or harmonic Arps curve) down to the economic limit. It is quick and needs no reservoir model, but it knows nothing of the causes of the decline: from a few points the shape of the curve is ambiguous, and a change in well operating conditions makes the forecast invalid.
A protrusion of the shoreline at a river mouth, built of the river's sediments where the river delivers more sediment than waves and tides can carry away: a delta plain with distributary channels and interdistributary bays, a delta front with mouth bars, and prodelta clays. By the dominant process, deltas are divided into river-dominated, wave-dominated and tide-dominated: in the first the sand is in distributary channels and bars extending away from the shore, in the second in shore ridges parallel to the shore, and in the third in ridges perpendicular to the shore.
The correction for mudcake and wall roughness, which a two-detector density tool derives from the difference between the detector readings and records as a separate curve. It is positive for a light mudcake and negative for a mudcake containing barite; a correction that is large in absolute value, together with a washout, means that the density is unreliable.
An active nuclear method: a gamma ray source in a pad pressed against the wall irradiates the rock, and two detectors count the scattered photons; the count rate falls as the electron density of the rock increases. It gives the bulk density and a mudcake correction, and the lithodensity version also gives the photoelectric factor.
The porosity calculated from the bulk density of the rock and the assumed matrix and fluid densities: . It is close to the total porosity; it is overestimated in gas-bearing beds and depends on the correct choice of the matrix density.
A narrow conical device with no moving parts: water with oil droplets enters tangentially and is swirled, a centrifugal acceleration hundreds or thousands of times greater than the acceleration of gravity throws the light oil towards the axis, and it leaves through the overflow. It captures droplets of the order of ten micrometres; a pressure differential and a continuous water phase are needed.
The allocation of capital costs over the years for reporting and for calculating corporate income tax. It is not a payment in itself: it is not part of the cash flow and affects it only through tax.
Construction of structure maps in depth from seismic time maps and a velocity model: using a single average velocity, layer by layer using the interval velocities of the layers, using a law of velocity increase with depth, or by depth migration; in every case the result must be tied to the wells. An error in velocity δV/V gives an error in depth of about z·δV/V.
Bringing the logs of different runs, as well as LWD data and core, to a single reference depth, usually using tie points on the gamma ray log, which is recorded in every tool string. The shift is determined at several tie points and interpolated between them, because it changes with depth.
The distance from the borehole wall from which a logging tool receives the main part of its signal. It grows with the spacing between the source and the receiver and determines which zone around the wellbore the tool sees.
The horizontal section of the logarithmic pressure derivative, the signature of radial flow. Its level gives the flow capacity: in metric oilfield units; in dimensionless form the level is 0.5.
The force carried by the derrick or mast: the hook load plus the tensions in the fast line and the dead line, ; with 10 lines it is about 22% more than the hook load. It is distributed unevenly among the legs of the derrick, so the allowable hook load is specified for a particular reeving.
A number not less than one by which the design load on a casing string — collapse, internal pressure, tension — is multiplied before it is compared with the rated strength of the pipe. It covers the inaccuracy of pressure predictions, wear and corrosion; the values are set by regulatory documents and company standards.
The tubing pressure against depth under the design conditions of a gas lift well — the design flow rate and gas rate with injection through the operating valve. When the unloading valves are spaced and set, the tubing pressure at the valve that is passing gas is taken from this line; while the gas enters higher up the string, the tubing pressure is usually below the line, and the calculation therefore leaves a margin.
Reconstruction of the drawdown curve from the build-up: to the pressure rise after shut-in is added the continuation of the pressure decline that the producing well would have given over the same time — for radial flow, . The slope is refined by iteration; the result is the same as that of Horner analysis and of analysis by equivalent time.
A gently dipping slip surface along a weak layer — salt, clays, often with abnormally high pressure — above which the sequence is crumpled into folds and thrusts or stretched by normal faults, and below which it is hardly deformed. The depth of the detachment below a horizon is estimated from the excess area of that horizon above the regional level and from the shortening: .
The basic decision on how the field will produce and deliver its output: where wells are drilled from, where oil, gas and water are separated and treated, by what route the marketable product is exported, by what means water or gas is injected, and where the energy comes from. It is chosen before detailed design by comparing the options by NPV across the whole range of reserves.
A formation or group of formations developed by one system of wells. Formations with similar oil and reservoir properties and similar reservoir pressures are combined into one development unit; different units are separated by laterally continuous impermeable beds.
The pressure at which the first drops of liquid condense out of the gas.
The physical and chemical changes in sediment and sedimentary rock after deposition and before metamorphism: compaction, pressure solution, cementation, growth of authigenic minerals, dissolution and replacement. In Russian-language lithology the transformation of sediment into rock is called diagenesis and the changes in the rock during burial are called catagenesis; in the English-language literature all of this is diagenesis.
A plot of the pressure change and its logarithmic derivative against time on a log–log scale. The flow regimes are recognised from the shape of the curves and the reservoir model is chosen; well test interpretation begins with it.
Determination of the average reservoir pressure from the build-up of a well that was producing in the pseudo-steady-state regime before shut-in: the MDH line is extended to the time and the average pressure is read from it. This is not the time at which the pressure actually equalises, but a point on the extrapolation of the line.
A dimensionless coefficient in the pseudo-steady-state inflow formula that accounts for the shape of the drainage area and the position of the well in it: the logarithmic term is written as . For a well at the centre of a circle it is 31.62; for elongated areas and wells near a boundary it is smaller, and productivity is lower.
A decline in the pressures of the zones of one reservoir at different rates because of differences in withdrawals, injection and pore volumes. A pressure difference arises between zones separated by a tight barrier and is sustained by flow across the barrier; if withdrawals are proportional to the pore volumes of the zones, there is no difference even with an impermeable barrier.
A test in which the pressure of an oil at reservoir temperature is reduced in steps and at each step all the gas that has come out of solution is released. It gives the solution gas–oil ratio and the formation volume factor per 1 m³ of residual oil, as well as the formation volume factor, gas deviation factor and density of the gas at each step.
The distance, in fractions of a logarithmic cycle of time, to the measurements from which the pressure derivative at a given point is calculated; usually 0.1–0.3 of a cycle. A narrow interval amplifies gauge noise; a wide one smooths and distorts the shapes — it cuts off the hump and stretches the transitions.
A binary, self-describing format for well log data, first issued by the American Petroleum Institute in 1991 as Recommended Practice RP 66. Together with the curves it stores the description of the well and of the tools, processing parameters, calibrations, sets of curves with different sampling steps, and arrays such as waveforms and images; it is the main format of the field data of service companies.
Calculation of rock properties from a three-dimensional image of the rock, usually an X-ray micro-tomogram: the image is segmented into pores and grains and flow is computed on an extracted pore network or directly on the voxels. Pores smaller than a voxel and wettability are not visible in the image, so the calculations are checked against laboratory data.
The cumulative water influx from a radial aquifer after a unit step change in pressure at the contact, in units of the aquifer constant; a function of the dimensionless time and the radius ratio . Initially ; for a closed aquifer it tends to , its elastic storage.
The ratio of the transmissibility of the fault zone, , to the transmissibility of the formation along the path from the well to the fault, : . Zero is a sealing fault; the larger is, the lower and earlier the hump of the pressure derivative.
The ratio of the fracture conductivity to the product of the reservoir permeability and the half-length: . It compares the ability of the fracture to conduct fluid with the ability of the formation to deliver it; at values of 10 and above the fracture behaves almost as an infinite-conductivity one.
The productivity index divided by . It allows wells and fractures in reservoirs with different properties to be compared: for a vertical well with no skin in a square drainage area it is about 0.14–0.15, and for a fracture of infinite conductivity across the full width of the square it reaches the limit .
Distance in wellbore radii, , time, , and pressure decline, , in units of . In these variables the diffusivity equation contains no reservoir parameters, so one solution can be scaled to any reservoir. In metric oilfield units the factor multiplies (time in hours).
The wellbore storage coefficient divided by — twice the elastic storage of a cylinder of rock the size of the well. Together with the skin factor it sets the shape of the early part of the pressure record; for oil wells shut in at the surface it is often measured in thousands.
Signs of a hydrocarbon accumulation in the seismic amplitudes: bright spot, dim spot and flat spot, polarity reversal, pull-down of reflections beneath gas. They are convincing when the edge of the anomaly lies at one depth along the whole contour of the structure and coincides with a flat spot; similar anomalies are produced by coals, tuning of thin beds and low-saturation gas.
Measurement of the inclination and azimuth of the wellbore at known depths along the wellbore — at survey stations. The coordinates of the wellbore are calculated from the measurements by assuming its shape between the stations according to a model: one straight segment, two segments or an arc.
The multiplier that converts an amount received in years into present value at rate .
The interest rate at which future cash flows are brought to the present day; it reflects the cost of capital and risk.
The cash flow of a period multiplied by the discount factor; the term is also used for the valuation method based on bringing all the cash flows of a project to a single date.
The time in which a project's cumulative discounted cash flow becomes positive for good: the project has recovered its investment together with a return equal to the discount rate. It is longer than the simple payback period; for a conventional cash flow it exists only if the NPV at that rate is not negative.
The sum of production by year multiplied by the discount factors, . It shows what the reserves are worth given when they will be produced: the same reserves produced more slowly give a smaller discounted production and a smaller NPV.
The fraction of the initial oil displaced from a completely swept volume of the reservoir: . The recovery factor is the product of displacement efficiency and sweep efficiency.
The amount of mineral that an acid dissolves: gravimetric — the mass of mineral per unit mass of pure acid, ; volumetric — the volume of mineral per unit volume of solution, . A cubic metre of 15% hydrochloric acid dissolves about 0.08 m³ of calcite.
The spreading of money across projects whose outcomes are unrelated: the successes of some cover the failures of others, and the overall result becomes more predictable at the same expected value. For identical independent stakes the spread relative to the mean is smaller by a factor of ; a risk common to all the projects, such as the risk of a single play, is not removed by diversification.
A low-pressure preventer with large-diameter vent lines. In the event of a kick at shallow depth, when the rock below the conductor shoe would not withstand the pressure of a shut-in well, it does not shut in the well but diverts the flow in a safe direction.
The angle between the directions of the wellbore axis at two survey stations; it takes account of the change in both inclination and azimuth: . Divided by the wellbore length, it gives the dogleg severity.
The change in direction of the wellbore per unit length, usually in degrees per 30 m.
Replacement of calcite (and aragonite) by dolomite under the action of solutions that bring in magnesium. Mole-for-mole replacement reduces the volume of the solid phase by almost 13%, but requires hundreds to thousands of pore volumes of solution to be pumped through; dolomite often replaces limestone volume-for-volume, and the porosity is largely inherited.
Shutting in a well with a valve set near the formation instead of a valve at the wellhead. Only a small volume of fluid remains below the valve, so wellbore storage is reduced by tens or hundreds of times and radial flow on the pressure build-up curve begins within minutes.
The transmission of commands from the surface to the downhole MWD/LWD tools and the rotary steerable system, usually as a coded sequence of changes in mud flow rate or string rotation speed. This is how the data transmission mode and sequence are changed, or the setting of the steering unit.
A process in which the non-wetting phase displaces the wetting phase, for example oil or gas displacing water from a hydrophilic rock as an accumulation forms. The capillary pressure and relative permeability curves for drainage differ from those for imbibition.
The part of the reservoir from which fluid flows to a given well; its boundaries are set by neighbouring wells, faults and the outline of the accumulation. In inflow formulas it is usually replaced by a circle of the same area with radius . In Russian-language practice is sometimes also called the drainage radius, but in well testing the radius of drainage is a different quantity that grows with time (radius of drainage).
The difference between the average reservoir pressure and the bottom-hole pressure of a producing well, ; the flow rate depends on it.
The ratio of permeability to viscosity found from the steady drawdown of a pretest for spherical flow to the probe: , where is the probe constant. It characterises the drilling-mud filtrate in the few centimetres of rock next to the wall and serves as a relative indicator of permeability.
The decline in bottomhole pressure after a well is put on production at a constant rate.
The machinery of the rig's hoisting system: a drum, driven by motors and fitted with brakes, onto which the fast line of the drilling line is wound. Through the block-and-tackle system it raises and lowers the strings; when lowering, the energy of the load is absorbed by the main and auxiliary brakes or by the electric motors themselves. Its power determines the hoisting speed, not the maximum load.
The guaranteed passage diameter of a casing pipe — the diameter of the drift mandrel that is pulled through the pipe during inspection. It is slightly smaller than the nominal inside diameter (by 3.18 mm for strings smaller than 244.5 mm); the bit and other tools must be smaller than it.
A test of a new well with an assembly run on pipe, with a packer, a test valve near the bottom and downhole gauges, usually with two flow periods and two pressure build-ups. It gives the fluid composition and samples, initial pressure, permeability, skin and productivity.
The string of pipe from the top drive or rotary table to the bit: drill pipe and, in the lower part, drill collars, whose weight provides the weight on bit. It carries the drilling fluid to the bit and transmits rotation.
A test for choosing the weight on bit: the bit is loaded, the drawworks brake is locked and the time in which the load falls by each step is recorded while the bit advances owing to the stretch of the string. The advance per step is the reduction in load multiplied by the compliance of the string, ; from it and the time, the rate of penetration at each load, the threshold and the founder point are found.
The fraction of drilled solids that the equipment removes from the drilling fluid; the rest has to be diluted away, and the volume of fluid needed for dilution is proportional to . It is estimated from the actual dilution: the volume of fluid made up is found from the water consumption, and the drilled-solids fraction in it from the retort and the methylene blue test.
A well-killing method in two circulations at constant bottom-hole pressure: first the influx is circulated out with the current drilling fluid while the standpipe pressure is held at the initial circulating pressure, then the fluid is replaced with kill mud.
A liquid, less often a gas or foam, circulated through the drill string and the annulus during drilling: it carries cuttings out, holds back the formations and the borehole wall by its pressure, and cools the bit. It can be water-based, oil-based (hydrocarbon-based) or synthetic-based.
An impact tool in the bottom hole assembly for freeing a stuck string: it stores the energy of stretch or of the weight of the pipe and releases it as a blow upwards or downwards. A hydraulic jar fires after a time delay, a mechanical jar at a preset load.
The reduction in well duration from the first well to the following ones in a series of similar wells in the same area. In the Brett–Millheim model, , the time tends to the limit , and each well reduces the excess over it by a factor of .
A stranded steel wire rope — usually 6 strands of 19–25 wires around a steel core — reeved several times between the crown block and the travelling block. It is checked by its design factor — under API RP 9B, not less than 3 when drilling and 2 when running casing and working stuck pipe; to spread the wear, the line is slipped through the system and the worn end is cut off according to a programme.
The complex of machinery for constructing a well: the hoisting system (derrick, drawworks, block and tackle), the rotating system (top drive or rotary table), the circulating system (pumps, fluid cleaning), and the power and blowout prevention systems.
A self-propelled ship with a derrick over a shaft in the hull (the moonpool). It is usually held in position by dynamic positioning, carries large loads and moves quickly between locations; it drills through a subsea wellhead and riser, including in the greatest water depths.
The share of the reservoir volume of oil and gas withdrawn that is supplied by a single source of energy: the expansion of oil with dissolved gas, the gas cap, the compressibility of water and pores, or the influx of aquifer water. The indices sum to one.
The main source of reservoir energy that drives fluids towards the wells: solution gas, gas cap, aquifer water, the compressibility of rocks and fluids, and gravity.
Gas consisting almost entirely of methane, which forms no hydrocarbon liquid either in the reservoir or in the separator.
A description of a fractured reservoir by two overlapping media: the matrix stores most of the fluids, the fractures conduct them, and there is exchange between a matrix cell and its fracture cell. If fluid also flows between matrix blocks, the model is called a dual-permeability model.
A wireline formation tester test in which two inflatable packers isolate a section of the borehole about a metre long and a pump withdraws fluid from the whole interval. It creates radial flow to the interval, allows its to be estimated, makes it possible to test tight and fractured rocks and to take samples at a lower drawdown.
A model of a shaly reservoir in which the pore water consists of clay-bound water (a fraction of the total pore volume, with conductivity ) and free formation water; hydrocarbons displace only the free water. It works with total porosity; in log analysis the parameters of the bound water are taken from the adjacent shales.
The standard IADC record of the condition of a bit that has been pulled, in eight positions: wear of the inner and outer rows of the cutting structure on a scale of 0–8, the main wear characteristic and its location, the bearing condition, the loss of diameter in sixteenths of an inch, another wear characteristic and the reason for pulling — for example, 1-4-WT-S-X-I-BT-PR.
A classification of carbonate rocks by depositional texture: what supports the framework, lime mud or grains, and whether the grains were bound together by organisms. Mudstone and wackestone are mud with grains (less than and more than 10%, respectively), packstone is a grain-supported framework with mud, grainstone has no mud, and boundstone is a structure bound together by organisms; the name indicates the energy of the depositional environment.
The flow rate of steady-state radial inflow to a well: in consistent units, where is the flow rate at reservoir conditions and the logarithm is natural.
A measure of permeability heterogeneity from 0 to 1: read from the straight line on a probability plot, and for log-normal permeability . It describes the spread of the values, but not their arrangement in the reservoir.
A calculation of waterflooding of layers without crossflow, with piston-like displacement and a common pressure drop, that takes the mobility ratio into account: for the flooded layers take an ever greater share of the injection. It gives the positions of the fronts, the vertical sweep, the water cut and the injection rate; for a log-normal section it gives generalised dependences of the sweep on and .
The level of liquid in the annulus of a producing well with a pump. It is measured with an echometer and used to estimate the pump intake pressure and the bottom-hole pressure; in a shut-in well the liquid rises to the static level.
A graph of the load on the polished rod of a sucker rod pump against its position over one pumping cycle. For a sound pump under static loading it is a parallelogram; distortions of the shape reveal gas in the pump barrel, incomplete filling, valve leaks and parted rods.
The production level below which a year's revenue, after the payments levied on it, does not cover the costs that could be avoided by shutting down: , where is the year's fixed costs, is the price at the field, is the royalty share of revenue and is the variable cost per barrel. It depends on price, costs and payment terms, so recoverable reserves depend on them too.
The part of an accumulation between the outer and inner pool outlines, where there is water beneath the oil in the same bed and the oil-saturated thickness decreases towards the outer outline. If the bed is thicker than the accumulation, water underlies the whole accumulation (bottom water); if it is thinner, water comes in from the edges (edge water).
The rate, compounded once a year, that gives the same growth as the given compounding scheme: for a nominal rate compounded times a year. Rates with different compounding frequencies are compared only as effective rates.
The difference between the fracture closure pressure and the fluid pressure in the fracture; at the wellbore it is the closure pressure minus the bottomhole flowing pressure. It rises as the bottomhole flowing pressure falls, so the proppant is selected for the highest value of this stress over the life of the well.
The permeability of a rock to one phase when several immiscible fluids are present in the pores at the same time; it depends on saturation and is always lower than the absolute permeability.
The fraction of the rock volume occupied by interconnected pores that are accessible to fluid. In English-language usage, effective porosity from core corresponds to open porosity, whereas in log interpretation (PHIE) it is total porosity minus clay-bound water; in Russian-language petrophysics, “effective” porosity means open porosity minus the volume of residual water.
The part of the overburden pressure that is carried by the mineral framework of the rock: the difference between the overburden and pore pressures. It is this stress that compacts the rock and determines its strength.
The radius of an ideal well with no skin that has the same productivity as the real one: . A negative skin “widens” the well, a positive one “narrows” it.
A multistage centrifugal pump with a submersible electric motor on the lower end of the tubing, powered by a cable from the surface. It delivers the highest liquid flow rates and works in deep and inclined wells, but tolerates free gas, sand and viscous oil poorly.
The charged surface of a particle together with the cloud of counter-ions around it in the water. Its thickness is of the order of the Debye length, nm (water, 25 °C, ionic strength in mol/L): salt compresses the layer, the repulsion between particles weakens, and the clays flocculate.
The voltage at which a sample of oil-based drilling fluid between the electrodes of a standard probe begins to conduct current (at 50 °C). It is an indicator of the stability of the emulsion and of the oil-wetting of the solids; it is judged by the trend, not by a single value.
The main component of the SP: the sum of the membrane potential across shales, which let cations through and hold back anions, and the liquid-junction (diffusion) potential at the contact between the filtrate and the formation water, which arises because chloride ions are more mobile. The membrane part is about five times larger than the diffusion part.
Transmission of MWD data by low-frequency currents through the rock: an insulating sub in the string creates an alternating voltage, which is received at the surface between the string at the wellhead and an electrode grounded to one side. It does not require mud circulation and works in air drilling and with aerated fluids; its range is limited by attenuation in low-resistivity rocks.
The number of electrons per unit volume of a substance, expressed in g/cm³: . This is exactly what the density tool senses; for most minerals it is almost equal to the bulk density, for water it is 11% greater, and for rock salt 4% less.
The region in which the true point of the wellbore lies with a given probability, given the known errors of the survey: an ellipsoid in space, an ellipse in plan view. It is calculated from a model of tool errors (the ISCWSA model is widely used); in inclined wells the ellipse is usually elongated across the wellbore direction because of the azimuth error.
Hydrochloric acid dispersed as droplets in an external hydrocarbon phase with an emulsifier. It reaches the rock more slowly, so its optimum injection rate is lower and the wormholes are deeper at low rates; it is viscous, with increased friction in the tubing.
The relative permeability to oil at irreducible water saturation and to water at residual oil saturation. Together with these saturations, they set the limits of the relative permeability curves and the mobility ratio.
Transfer of a rock type's relative permeability table to a cell with different end-point saturations and relative permeabilities: the saturation of the cell is mapped linearly onto the interval of the table (two-point or three-point scaling), and the values are, if necessary, multiplied by the ratio of the end-point relative permeabilities.
Treatments of the reservoir that change the properties of the fluids or their interaction with the rock in order to recover oil left after natural drive and waterflooding: thermal, miscible gas, chemical and microbial methods. A broader concept is the increase of oil recovery by any means, including new wells and injection management (improved oil recovery, IOR).
The value of a company's business to everyone who has provided it with capital: the market value of its shares plus debt minus cash. It is divided by EBITDA, reserves or production in order to compare companies of different sizes and with different proportions of debt.
The capillary pressure at which the non-wetting phase begins to enter the largest connected pore throats of the rock. It determines the height of the oil–water contact above the free water level and the sealing capacity of the cap rock.
Corrections to log readings for the difference between the conditions in the well and the conditions of tool calibration: borehole diameter, mudcake, standoff, mud properties, temperature, pressure and water salinity. They are applied before any quantitative interpretation and are not duplicated if they have already been applied during recording.
A formula relating the pressure, temperature and molar volume of a substance or mixture. Cubic equations — van der Waals, Soave–Redlich–Kwong and Peng–Robinson — describe both gas and liquid with two parameters per component, so they are used to calculate phase densities, saturation pressures and the compositions of equilibrium phases.
Calculation of the initial pressures and saturations of the model cells from the pressure at a reference depth, the phase densities, the contact depths and the capillary pressure tables. The contact depth in the input data is the level where the capillary pressure equals the specified value (usually zero), that is, the free water level.
The ratio of the mole fractions of a component in the equilibrium gas and liquid, . It is greater than one for light components and less than one for heavy ones; at low pressure it is close to , while at reservoir pressures it also depends on the phase compositions and is calculated from an equation of state.
All the costs of an option over its service life (investment, operation, salvage value) converted into an equal annual payment. It is used to compare options with different service lives if each can be renewed as many times as needed; if the asset is needed for less than its service life, the costs over the actual period are compared.
The density of the mud that would create the same bottomhole pressure at rest as is created during circulation, taking the pressure losses in the annulus into account.
A pressure at depth expressed as the density of the fluid whose column from the zero datum would create that pressure. It makes it possible to compare pore pressure, fracture pressure and mud density in the same units.
The concentration of a sodium chloride solution with the same electrical conductivity as a water of complex composition. It is obtained by multiplying the ion concentrations by weighting factors that depend on the total salinity, and it is used in charts and formulas constructed for NaCl.
The permeability of a homogeneous reservoir that under the same boundary conditions gives the same flow as the heterogeneous one. It lies between the harmonic and arithmetic means; where exactly depends on the arrangement of the areas of different permeability and on the direction of flow.
The notional duration of production at the last rate over which the well would have produced as much as over its whole history: , where is cumulative production. It is substituted into the Horner function in place of the rate history; the device is accurate for the late part of the build-up and is in error if the rate changed greatly shortly before shut-in.
A resistivity that is inversely proportional to the chemical activity of a solution; it is this quantity that enters the formula for the static spontaneous potential. For waters of moderate salinity it is close to the true resistivity, for brines it is several times smaller, so the conversion to is made with a chart or an approximation of it.
A large area of desert covered by dunes and megadunes (draa) with depressions between them. Wind sorts and rounds sand well and carries clay away, so the aeolian sandstones of ergs are thick, clean reservoirs, but anisotropic: grainflow laminae are more permeable than wind-ripple laminae.
The rise over time in the price of an individual commodity or cost item; it can be faster or slower than general inflation. Escalation in excess of general inflation is real escalation: .
Rocks that precipitated from evaporating sea or lake water: gypsum and anhydrite, rock salt (halite), and potassium and magnesium salts. The minerals precipitate as the brine becomes concentrated: gypsum at about a fourfold concentration, halite at about tenfold; a 1 km column of seawater yields only about 13 m of rock salt. Evaporites are the best seals, and salt under load flows and forms domes.
An additional underestimation of neutron porosity in a gas-bearing formation over and above what the low hydrogen index of gas produces: the pores occupied by gas contain almost no substance to slow down neutrons. The correction grows with the porosity and the gas saturation of the zone of investigation and reaches several porosity units.
A graph of the probability that a quantity will be no less than a given value; it falls from one to zero. It is used to read the reserves estimates P90, P50 and P10, the values that are exceeded with a probability of 90, 50 and 10%.
Depth plotted against the difference between the measured pressure and the pressure on a reference line (usually the water line) at the same depth. Water falls on the vertical zero line, oil and gas on inclined straight lines that reach zero at the free water level; offsets of a few kilopascals, invisible on a plot in megapascals, become visible. When wells are compared, it shows whether both phases are shifted in a well or only one.
A Kazakhstani tax on the part of a subsoil use contract's annual net income that exceeds 25% of deductions; the rates, from 10 to 60%, rise with the ratio of net income to deductions, and each rate applies only to its own slice of income.
The material balance quantities , and : the increase in the reservoir volume of oil with dissolved gas, of the gas cap, and of connate water with the pores, per 1 m³ of oil in place as the pressure falls from the initial value to the current one.
The sum of the values of the possible outcomes, weighted by their probabilities.
The increase in EMV if the outcome of an uncertain event were known without error before the decision; it equals the expected regret of the best decision taken without information. It is the upper limit of the value of any data about this event. For a drilling decision it is the smaller of and .
A way of stepping in time in which the flows and withdrawals during the step are computed from the values at the start of the step: each cell is recalculated separately, but the step is limited by the stability condition; for the pressure equation it is of the order of the characteristic time of the “fastest” cell.
A decline in the production rate at a constant fractional rate: , where is the nominal decline rate, 1/year; over a year the production rate falls by the fraction . Cumulative production down to rate equals if the rates are expressed as volumes per year.
The function , the integral of over from to infinity. The pressure decline around a well in an infinite reservoir is expressed through it. For it is replaced by the logarithm ; for it is practically zero.
Prolonged production, from weeks to months, of an exploration or appraisal well to study the reservoir: its connected volume, boundaries, pressure behaviour and the composition of the production. Its pressure record is used for a reservoir limit test. In Kazakhstan's project documents the term also refers to a longer stage — production from a discovered accumulation under a trial production project, for up to three years (trial production project).
The pressure on the extrapolation of the Horner line at the ratio . In a new reservoir with little production it is close to the initial reservoir pressure; in a producing accumulation it is higher than the average pressure in the drainage area.
A bed or package of rock with a consistent set of characteristics (composition and grain size, sedimentary structures, colour, fossils) that reflect the process of deposition. In the descriptive sense a facies is an observation (lithofacies); in the interpretive sense it is an environment (“beach facies”); it is useful not to confuse the two.
Facies that occur together and are linked by a single environment, for example the channel sands, rippled sands and floodplain clays of a meandering river. The environment is identified from the association and order of the facies, by comparing them with facies models.
A regular change of facies from bottom to top of a section. Fining upwards is typical of channel fills, coarsening upwards of prograding shorelines, bars and deltas; the order of the facies also sets the distribution of permeability through the thickness of the reservoir.
A record of the decline in bottomhole pressure after injection is stopped. In meaning it is the analogue of the pressure build-up curve of a producing well; it is processed by the same methods, taking into account the zone of injected water around the wellbore.
The dimensionless ratio of the wall shear stress to the dynamic pressure of the flow, ; the friction loss in a pipe is . In laminar flow ; in turbulent flow it is taken from the Blasius, Colebrook or Dodge–Metzner correlations. In Russian-language handbooks the hydraulic resistance coefficient is more often used.
A transaction in which the owner of a block assigns part of its interest to a partner, who in exchange pays for part of the work, usually more than corresponds to the interest received. For the partner this is entry into the project (farm-in); the excess of its share of costs over its working interest is called the promote.
The fast line runs from the crown block to the drawworks drum and, when hoisting, is under greater tension than any other line: ; the safety factor of the line is checked against it. The dead line is anchored at the base of the derrick; at rest it is under a tension of , and the weight indicator shows the hook load from its tension.
A surface or narrow zone in rocks along which the adjacent blocks — the walls — have moved relative to each other. By the direction of displacement, normal faults, reverse faults and thrusts, strike-slip faults and oblique-slip faults are distinguished; a fault can act as a seal for an accumulation or as a migration path.
The line along which the surface of a bed meets the fault plane in one of the fault walls; on a structure map it is the edge of the fault polygon. At every point on it the bed and the fault plane are at the same depth, so a structure contour of the bed terminates on the structure contour of the fault plane with the same value.
The area on a structure map between the lines where a horizon meets the fault plane in the footwall and in the hanging wall. For a normal fault the horizon is absent inside the polygon; its width measured normal to the fault equals the horizontal offset (heave), and the difference in depth between its edges equals the vertical offset (throw). Each horizon has its own polygon: for a deeper horizon it is shifted down along the dip of the fault plane.
A structure map of the fault surface, made from the points where wells have intersected it and from seismic data. For a planar fault the structure contours are parallel straight lines at intervals of . Overlaying it on the maps of a bed gives the fault cutoff lines and the fault polygons. Not to be confused with the Allan diagram, a projection of beds onto the fault plane.
The vertical component of the displacement along a fault, ; the horizontal component across the strike is called the heave. Along the fault the throw varies: from zero at its tips to a maximum in the middle.
The ratio of the permeability of the fault zone to its width, mD/m. It governs the flow across the fault for a given pressure jump across it; a well test gives only this ratio, not the permeability and width of the zone separately. In a model it is converted into a transmissibility multiplier that depends on the cell size.
A comprehensive study of the results of well and reservoir surveys and of the dynamics of development indicators, to understand where reserves remain and whether the development system needs improving. In Kazakhstan it is carried out by a licensed design organisation at least once every three years, and the subsoil user sends it to the ministry by notification; where actual results differ from the project by more than 10% and there is a justified need to change the project, the analysis undergoes expert review by the central commission (TsKRR).
The main design document for development: the development system, the number and placement of wells, pressure maintenance, production profiles. It is drawn up on the basis of the estimated reserves and is revised when actual performance departs materially from the design figures.
The volume of 0.02 N sulfuric acid consumed per 1 cm³ of filtrate to reach pH 8.3 () and pH 4.3 (); from their ratio the hydroxide, carbonate and bicarbonate ions are determined. The alkalinity of the drilling fluid itself, , together with , gives the excess of undissolved lime.
A company's costs of exploration, appraisal and development in a period divided by the reserves added in the same period. Spending and reserve additions are poorly matched in time, so the indicator is averaged over several years and compared only when the same methodology is used.
The detachment of fine particles — clay minerals, quartz — from the rock grains by the flow of fluid, and the plugging of pore channels by them. It is intensified by high velocities near the wellbore, water inflow and changes in water salinity.
A solution of the diffusivity equation in which a constant flow rate is specified at the wall of a well of radius , rather than on its axis as for a line source. It differs from the line-source solution only in the first moments after start-up (–100) and right at the well; it matters for a strongly negative skin and for “large wells” — for example, an accumulation in an aquifer.
A numerical method in which the derivatives in a differential equation are replaced by differences of values at neighbouring grid nodes and at neighbouring moments of time; the equation becomes a system of algebraic equations, a finite-difference scheme. For the pressure equation on a grid of identical cells it coincides with the mass balance in the cells.
A device on a tubing-conveyed perforating gun that detonates the charges on a signal: from the impact of a bar dropped into the tubing, from pressure in the tubing or in the annulus, or through a cable connected to the head in the well.
The recovery from a well of tools, instruments, broken wire or pipe left in it, using special tools. The work starts by finding out what is lying in the well and how, for example with an impression block; the object is caught by its fishing neck, and wire by a wire grab.
The calculation of the gas-phase fraction and of the compositions of the equilibrium gas and liquid for a mixture of known composition at a given pressure and temperature; with known equilibrium ratios it reduces to the Rachford–Rice equation. It is performed in every cell of a compositional model and at every stage of separation.
Casing string equipment placed one or two joints above the shoe: a check valve prevents the heavy cement from returning from the annulus into the string, and the cementing plugs land on the ring — the landing of the top plug (the “bump”) ends the displacement. The joints below the valve (the shoe track) retain contaminated cement.
A ship or floating structure with a processing plant and storage tanks: it receives the production of subsea wells through flexible risers, processes it, stores the oil and offloads it to shuttle tankers. It is used in medium and great water depths, especially where there are no pipelines.
The sticking together of clay particles edge to face or edge to edge into a loose structure (a “house of cards”). It raises the yield point, gel strength and fluid loss; it is caused by salt, calcium, low pH and high temperature. The reverse process, deflocculation, is produced by thinners and high pH.
The arrangement of injection wells relative to the producing wells and the outline of the accumulation: peripheral, near-contour and intra-contour flooding — line-drive (block), areal (five-spot, seven-spot, nine-spot), selective, focal and barrier.
The product of permeability and the net thickness of the formation, (mD·m). At a given drawdown the flow rate is proportional to , the transmissibility; it is that well tests determine.
The cumulative fraction of flow plotted against the cumulative fraction of pore volume when the parts of a reservoir are ordered from the fastest to the slowest: layers in descending order of , model cells by travel time from the injection well to the producing well. The steeper the beginning of the curve, the earlier the water arrives; comparing the curves of the fine and coarse models shows whether the grid has preserved the fast paths.
The ratio of the actual productivity of a well to the productivity of the same well without skin; it is less than one for damaged wells and greater than one for stimulated wells.
The characteristic distribution of gas and liquid in a pipe: bubble, slug (plug), churn and annular flow, and, in horizontal pipes, also stratified flow. It depends on the velocities and properties of the phases and on the diameter and inclination of the pipe.
A part of the reservoir that can be traced between wells, with similar porosity and permeability and common flow behaviour, separated from its neighbours by baffles or by a contrast in properties; it includes both reservoir and non-reservoir rock. Not to be confused with the hydraulic flow unit, a rock type defined by FZI whose samples occur throughout the section.
The reservoir quality index RQI divided by the ratio of pore volume to grain volume, , in µm. According to the Kozeny–Carman model it depends only on the pore geometry (specific surface area, tortuosity, channel shape) and is therefore almost constant within a rock type; the relationship for a type is .
A test of a well at several successive rates, each of which is continued until the bottomhole pressure stabilises; an inflow performance line is plotted from the measurements. For gas wells it gives the coefficients of the two-term inflow formula.
Calculation of the equivalent permeability of a coarse cell by solving a single-phase steady-state flow problem on a fine grid inside it with given boundary conditions: the total flow rate and pressure drop are substituted into Darcy's law for the block.
A fan of faults diverging upwards from a steep strike-slip fault. In transpression the faults of the fan are reverse and push a wedge of rock upwards — a positive, or palm-tree, structure; in transtension they are normal and lower the wedge — a negative structure.
Running a downhole tool with pressure and temperature sensors into a producing well with stops at specified depths. In a gas lift well, the kink in the pressure profile and the steps in temperature show where the gas enters the tubing; the profiles are also used to check the model of flow in the tubing.
The pipeline from the Christmas tree of a well to the gathering point — a metering station, a manifold or a separator. The pressure losses in it rise with the flow rate, particularly fast at low pressure, and act on the well as back pressure; they are reduced by a parallel line (looping) or by lowering the separator pressure.
The boundary between zones of a formation saturated with different fluids: oil–water (OWC), gas–oil (GOC) or gas–water (GWC). It bounds the accumulation from below and defines the rock volume when reserves are estimated.
The average density of what fills the pores in the zone investigated by the density tool: mud filtrate with residual hydrocarbons. It depends on the salinity of the filtrate, the temperature and the pressure; residual gas lowers it sharply.
The fraction of the pumped volume that is still in the fracture at the end of pumping; the rest has leaked off through the fracture walls into the formation. It falls as leak-off and pumping time increase; it is estimated from the fracture closure time after a mini-frac, and the pad is calculated from it.
The volume of filtrate that passes through filter paper in 30 min in the standard filter-press test. Together with the filter-cake thickness, it characterises the ability of the fluid to seal off permeable rocks.
The impact of the plunger of a sucker rod pump on the surface of the liquid in a barrel that is not completely full — usually because the level has been pumped down to the intake and the inflow is less than the pump delivery. On the dynamometer card the load holds for part of the downstroke and then drops almost vertically; the impacts damage the pump, the rods and the gearbox.
The part of the invaded zone nearest the wellbore, where the filtrate has displaced almost all of the formation water and the moveable hydrocarbons. Its resistivity is measured with microresistivity tools.
A function of the ratio of neighbouring differences of the solution which, in TVD schemes, reduces the slope of the reconstruction to zero at jumps and extrema and preserves it on smooth stretches; well-known variants are minmod, van Leer and superbee.
The number of traces stacked at one common midpoint (in a 3D survey, in one bin). For a regular 2D line ; stacking traces improves the signal-to-random-noise ratio by a factor of about , but does not by itself remove coherent noise.
The line of greatest curvature of a layer in a fold. The hinges of all the layers lie on the axial surface; the angle between the hinge and the horizontal is the plunge of the hinge: at the ends of an anticline the hinge plunges and closes it.
A zone of shortening at the edge of a mountain belt, where the sedimentary sequence is folded and cut by thrusts that usually detach from a weak layer — salt or clays. The thrusts develop progressively towards the foreland basin, forming imbricate slices and duplexes.
A classification of carbonate rocks by their main grains (intraclasts, ooids, bioclasts, peloids) and by the material between them: sparite cement (clear crystalline calcite) or micrite (lime mud made of crystals a few micrometres in size). The name is made up of a prefix and an ending: oosparite, biomicrite.
Drawing a fluid pressure line on a pressure–depth plot with a slope set by the fluid density from a sample or analysis rather than fitted to the points; only the level of the line is found from the formation-tester measurements. It is used when there are few points, the interval is short or the pretests are supercharged; in a tight rock the line is drawn along the left-hand boundary of the reliable points.
A law of filtration with an inertial term: . At low velocities the second term vanishes and the equation reduces to Darcy's law.
An asymmetric downwarp of the lithosphere in front of the thrusts of a mountain belt as plates converge: the weight of the thrusts and of the sediments bends the elastic lithosphere. The basin is deepest next to the mountains, its width is set by the flexural rigidity of the lithosphere, and beyond it the plate bulges up in a gentle peripheral bulge. An example is the Pre-Ural foredeep.
A reduction in the permeability of the near-wellbore zone during the drilling, completion, production or workover of a well.
The ratio of the resistivity of a rock fully saturated with water to the resistivity of that water: . It depends on the porosity and the tortuosity of the pore space; usually .
The ratio of the volume of a fluid at reservoir conditions to its volume at standard conditions.
The electrical resistivity of formation water at reservoir temperature.
The prices for delivery of oil or gas on future dates that can be agreed today, taken from futures and forwards. If future prices are above spot, the market is in contango (limited by the cost of storage and of money); if below, in backwardation. A forward price is not a forecast but the price of a deal made today.
The weight on bit above which the rate of penetration stops increasing linearly: the added weight is “eaten up” by balling, poor hole-bottom cleaning, vibration or full embedment of the cutters in the rock. Below it the bit works efficiently; it is found by a drill-off test or from the rise in mechanical specific energy.
Creation, in a weak permeable formation, of a short, wide fracture filled with gravel-size proppant, together with a gravel pack around the screen, in a single operation. It protects against sand and bypasses the damaged zone; the fracture width is built up by stopping its growth in length.
The fraction of water in the total fluid flow at a given point in the reservoir; it depends on saturation, viscosities and relative permeabilities.
The distance between the walls of a fracture. The flow rate of fluid through a fracture is proportional to the cube of the aperture — the cubic law — so the conductivity of the system is governed by the widest fractures; the hydraulic aperture of a rough fracture is smaller than the mechanical one.
The permeability of the proppant pack multiplied by the width of the fracture, in mD·m (mD·ft). It decreases under the closure pressure, because of gel residue and because of plugging by fines.
The pressure per unit depth at which a fracture forms and propagates in the rock.
The distance from the wellbore to the tip of one wing of a vertical hydraulic fracture. What matters for inflow is the propped half-length — the part of the fracture that is held open by proppant.
A measure of the quantity of fractures: the linear density is the number of fractures per metre of scan line; along a line normal to the set, its reciprocal is the spacing of the set; is the fracture area per unit volume. The number of fractures along a wellbore is converted into the true density with the Terzaghi correction — by dividing by the cosine of the angle between the wellbore and the normal to the fractures.
Fractures of one orientation and origin. On a stereographic projection the poles of the fractures of a set form a cluster; several sets that formed together make up conjugate pairs or networks.
The additional resistance of a thin layer of the formation along the surface of a hydraulic fracture, whose permeability has been reduced by the filtrate and polymer of the fracturing fluid. Because of the huge area of the fracture walls it is usually small; it is noticeable for short fractures in permeable formations and in the linear flow regime just after start-up.
The fluid used to open and grow a hydraulic fracture and to carry proppant into it: slickwater, linear and crosslinked gels, viscoelastic surfactants, foams and gelled hydrocarbons. Its viscosity at different shear rates, its leak-off, its ability to hold proppant and to break down after pumping determine the fracture dimensions, the fracture conductivity and the formation damage.
A company's operating cash flow minus capital expenditure. It is the source of dividends, share buybacks and debt repayment; a negative free cash flow is covered by borrowing or by selling assets. It is important to state whether all capital expenditure or only maintenance capital expenditure has been deducted.
The fraction of the rock volume occupied by fluid with times longer than the cutoff, that is, the moveable part of the porosity according to NMR. Together with the capillary-bound water it makes up the NMR effective porosity; in meaning it is close to the effective porosity used in the national practice of reserves estimation: open porosity minus residual water.
The level in an accumulation at which the hydrocarbon and water pressures are equal and the capillary pressure is zero. The oil–water contact lies above it by a height corresponding to the threshold capillary pressure.
The area on a reflecting boundary from all points of which the reflections arrive at the receiver nearly in phase: the paths to them are longer than the shortest by less than a quarter of a wavelength. Its radius is about , hundreds of metres at a depth of several kilometres; details smaller than it are blurred before migration, while migration shrinks it to a fraction of a wavelength.
The “corrected” partial pressure of a component, a measure of its tendency to leave a phase: , where is the fugacity coefficient (equal to 1 for an ideal gas). Phases are in equilibrium when the fugacity of each component is the same in all phases.
Unit cost or break-even price taking into account all the project's costs from the start of exploration: exploration and appraisal, development, operations, overheads and taxes. The opposite approach is to calculate from the current point, excluding costs already incurred; the next decision is taken on that basis.
A time-stepping scheme in which the flows, mobilities and well withdrawals are taken at the unknown values at the end of the step and the equations of all phases in all cells are solved together, usually by Newton's method. It is stable at any step size; the step is limited by accuracy and by the convergence of the iterations, and each step costs more than in the IMPES scheme.
The time taken by a drilling fluid to flow out of a funnel of standard dimensions, in seconds; for the Marsh funnel, the time to discharge 946 cm³ (fresh water takes about 26 s). It is convenient for quick monitoring of changes, but cannot be converted into either plastic viscosity or yield point.
The loss of all funds through a run of failures before the positive average has time to show itself. The probability that independent attempts, each with success probability , all fail is . For a company whose capital is enough for attempts, this is a lower bound on the probability of ruin: after a success the bets continue, and ruin can come later.
The position of a gamma ray reading between the clean-formation baseline and the shale baseline: the difference between the reading and the clean-formation baseline, divided by the difference between the shale baseline and the clean-formation baseline. It is not yet the shale volume: that is obtained from the index by a linear relationship or by empirical non-linear ones.
A recording of the natural radioactivity of rocks; the main indicator of shale volume.
The shape of the gamma-ray (or SP) curve in a sand unit: a bell means shale content increases upwards, a funnel means it decreases upwards, a cylinder is a uniform unit with sharp boundaries, a serrated curve is interbedding. It suggests the environment but is ambiguous and is checked against core.
A separator at the intake of a sucker rod pump: the flow is turned downwards, the liquid goes to the intake, and the gas bubbles rise into the annulus and are carried to the surface, bypassing the pump.
A drive mechanism in which oil is displaced by an expanding gas cap. The pressure falls slowly, the gas–oil ratio rises gradually and increases sharply when gas breaks through to the wells; oil recovery is usually 20–40%.
The ratio of the initial reservoir volume of free gas in the gas cap to the initial reservoir volume of oil in the accumulation. The larger it is, the more slowly the pressure falls during production.
A reservoir mixture whose reservoir temperature lies between its critical temperature and the cricondentherm: under initial conditions it is a gas, but when the pressure falls below the dew point, a liquid — condensate — drops out in the reservoir.
The volume of reservoir gas from which 1 m³ of stabilised condensate is obtained at the surface: . Its reciprocal is the condensate content per cubic metre of pore volume occupied by gas.
Production of a gas-condensate accumulation with injection of dry gas, from which the condensate has been separated, back into the reservoir, so that the pressure stays above the dew point and the condensate does not drop out in the reservoir.
The ratio of the volume of a real gas to the volume of an ideal gas at the same pressure and temperature.
The volume of gas at standard conditions that in the reservoir corresponded to a unit volume of the condensate (or water) produced: m³/m³ with density in kg/m³ and molar mass in kg/kmol. It is added to the gas production in the material balance of a gas accumulation.
The reciprocal of the gas formation volume factor: how many standard cubic metres of gas are contained in one cubic metre of gas in the reservoir. At reservoir pressures of 10–30 MPa it is usually from 80 to 300.
The burning of associated or other raw gas that is not used, processed or injected into the reservoir. In Kazakhstan it is prohibited, except for emergencies (without a permit, with notification within ten days) and, by permit of the Ministry of Energy and within calculated norms, for well-object testing, trial production and technologically unavoidable flaring.
Water cleaning with gas bubbles: the bubbles attach to oil droplets and particles and carry them into a froth, which is skimmed off the top. The gas is introduced by an impeller or an eductor, or dissolved under pressure and released when the pressure is reduced; at oilfields, associated gas or nitrogen is used rather than air.
Solid crystalline compounds of water with molecules of light gases (methane, ethane, propane, carbon dioxide, hydrogen sulfide), stable at high pressure and low temperature. They plug chokes, flowlines and trees, particularly downstream of the chokes of gas wells; they are prevented by heating, inhibitors (methanol, glycols), gas dehydration and the choice of throttling point.
The quantity of free gas in the accumulation before the start of development, referred to standard conditions.
Injection of gas into an oil or gas-condensate accumulation to maintain reservoir pressure, to displace oil, or to keep the produced gas in the reservoir. Displacement is most stable when gas is injected into the crest or the gas cap at a rate below the critical one; gas injected along a thin formation quickly breaks through to the wells.
A method of artificial lift in which gas injected into the tubing lightens the column of liquid.
The dependence of the flow rate of a gas lift well on the injected gas rate at a given injection depth. It first rises steeply, then flattens out and passes through a maximum — the technical optimum, beyond which additional gas increases friction more than it lightens the column.
A condition of a sucker rod pump in which the barrel is filled with gas and the plunger merely compresses and expands it: the pressure below the plunger does not reach the discharge pressure and does not fall to the intake pressure, the valves do not open, and delivery stops. It occurs when the compression ratio of the pump is less than the ratio of the absolute discharge and intake pressures.
The fraction of the gas in place withdrawn by the end of development. Under volumetric depletion drive it equals ; with active water influx it is lower, because the water traps gas at high pressure.
The ratio of the density of the gas to the density of air at the same pressure and temperature; for ideal gases it equals the ratio of the molar masses, .
The resistance of gas to flow, usually 0.01–0.03 mPa·s (cP). At low pressure it increases with temperature and is almost independent of pressure; at reservoir pressures it increases with pressure and decreases with temperature. Kinematic viscosity is dynamic viscosity divided by density.
Drilling fluid containing gas bubbles. Its density at the well outlet is greatly reduced, but the bottom-hole pressure falls much less, because the gas expands mainly near the wellhead.
Periodic fluctuations in the pressures and flow rate of a gas lift well. Tubing heading occurs at a low gas rate, when liquid accumulates in the tubing and is ejected in slugs; casing (annulus) heading arises from feedback between the annulus, which stores gas, and the operating valve, the flow through which in subcritical flow increases when the tubing pressure falls.
Displacement of the kill fluid from the annulus and tubing by injected gas in stages, through the unloading valves from top to bottom, until the gas reaches the operating valve. When the gas reaches the next valve, the annulus pressure falls and the upper valve closes.
Calculation of the depths of the unloading valves from top to bottom: the first is placed where the gas at the kick-off pressure can still displace the kill fluid, and each following one where the gas displaces the liquid remaining below the previous valve, with the tubing pressure taken from the design line. Each following valve is set to open at a gas pressure lower by one step and is placed no closer than the minimum interval.
The volume of gas at standard conditions per unit volume of produced liquid, oil and water. It is related to the producing gas–oil ratio and the water cut by the expression ; in gas lift the injected gas is included in it too.
The ratio of the relative permeabilities to gas and to oil at a given gas saturation. Together with the viscosities and formation volume factors, it determines the share of free gas in the production; the field ratio, obtained from the gas–oil ratio history, is usually higher than the laboratory one.
The interval of an accumulation below the bubble point pressure, between the secondary gas cap and the oil zone, where the gas saturation is above the critical value: the gas that has come out of solution rises and the oil drains downwards to meet it. The pressure gradient in it is weighted by the phase mobilities: it goes half-way to the gas gradient as soon as the gas saturation exceeds the critical by a few per cent and becomes almost the gas gradient well before the top of the zone, so only the base of the zone is visible on pressure data.
The pressure measured relative to atmospheric pressure, which is what an ordinary pressure gauge reads. In the gas laws and in fluid property calculations the absolute pressure is used instead: atmospheric pressure, 0.101325 MPa (14.696 psi), is added to the gauge pressure.
A direct method of solving a system of linear equations: in forward elimination the unknowns are eliminated one by one and the matrix is reduced to triangular form; in back substitution they are found by substitution. The elimination multipliers form the LU decomposition of the matrix; in a sparse matrix elimination creates fill-in, and the work grows as the square of the bandwidth.
The strength of the structure (gel) that a drilling fluid builds up at rest; it is measured after 10 s and 10 min at rest. The gel holds cuttings in suspension when circulation stops, but a gel that is too strong makes it harder to restore circulation.
The sequence of divisions of Earth's history (eons, eras, periods, epochs and ages) with the numerical ages of their boundaries. The international scale is maintained by the International Commission on Stratigraphy and is revised periodically, so reports state its version.
The probability that an exploration well will discover moveable hydrocarbons. With independent elements, it is the product of the probabilities of source and migration, reservoir, seal, trap, and timing.
A three-dimensional digital description of the structure, lithology and properties of a reservoir (porosity, permeability, saturation) without flow calculation; it is the basis of the reservoir simulation model.
The ratio of the rock volume of an accumulation to the “slab”, the area inside the outer pool outline multiplied by the bed thickness. It is less than one; it depends on the ratio of the bed thickness to the height of the accumulation and on the shape of the trap: for a paraboloid dome it is when the thickness is less than the height of the accumulation.
The th root of the product of values, or the exponential of the arithmetic mean of their logarithms. For permeability it describes randomly mixed areas and is close to the median; it always lies between the harmonic mean and the arithmetic mean.
Transfer of reservoir properties in several steps across geologically meaningful scales: laminae to a bed or bed set, bed to a cell of the geological model, then to a cell of the simulation model. At each step the equivalent permeability and pseudo functions are calculated from a model of the structure, choosing the method according to the balance of forces at that scale.
One of many equally probable versions of the distribution of reservoir properties between wells, generated by geostatistical simulation and consistent with the well data. The differences between realisations reflect the uncertainty that the wells leave unresolved.
Control of the wellbore trajectory within the formation during drilling on the basis of logging-while-drilling data and cuttings: the wellbore is kept at a specified distance from the top of the formation, a shale layer or a fluid contact.
The increase in rock temperature with depth, usually 2–4 °C per 100 m in sedimentary basins; the reciprocal is the geothermal step, about 33 m per degree at 3 °C per 100 m. It equals the heat flow divided by the thermal conductivity of the rocks, so it is smaller in salt than in clays.
Removal of water vapour from gas with concentrated glycol, usually triethylene glycol: in the absorber the gas rises counter-current to the glycol; the glycol loaded with water has the water boiled off in a regenerator with a reboiler and is returned. The depth of dehydration is set by the concentration of the regenerated glycol and the temperature in the absorber; lower dew points are given by solid adsorbents.
The share of the project's cash flow before payments to the government (revenue minus all costs) that is taken by royalties, taxes and other payments. It is calculated without discounting or on discounted flows; the method of calculation should be stated.
A gradual change in grain size from bottom to top within a bed: in normal grading the coarse grains are at the bottom and the fine ones at the top (the bed was deposited by a waning current, for example a turbidity current); in reverse grading it is the other way round. Normal grading shows which way up the bed originally was.
The density of the solid phase of the rock, without pores: about 2.65 g/cm³ for quartz, 2.71 for calcite and 2.87 for dolomite. It is needed to calculate porosity from the density log and serves as a check on laboratory measurements.
Comparison of two sections on a graph of the depths of the same events: the tops and bases of species ranges, ash beds, log markers. The points fall near a line of correlation: its slope is the ratio of the accumulation rates, and a horizontal step is a hiatus in one of the sections.
A drawing of a section made from the description of a core or outcrop: depth, lithology, grain-size profile (the width of the column), sedimentary structures, bed boundaries, fossils and interpretation. Core is described at a scale of 1:20–1:50 and compared with logs at 1:200 after depth matching.
A layer of sorted gravel between the sand screen and the formation that holds back the sand; the gravel size is chosen to match the size of the formation grains. In a cased hole the gravel also fills the perforation tunnels, and the viscous and inertial losses in them noticeably reduce productivity; in an open hole the pack hardly impedes inflow.
The deviation of the measured gravity from the normal value after corrections for latitude, station elevation and the attraction of the rocks above the reference level. It is produced by bodies whose density differs from that of the host rocks; it is measured in milligals: 1 mGal m/s², about a millionth of the acceleration due to gravity.
A drive mechanism in which oil drains under gravity towards the wells in the lower part of the formation while gas rises to the top. It is slow, but in thick or steeply dipping formations with good vertical permeability it can give a high oil recovery; it usually shows itself at a late stage of development.
The dimensionless ratio of the gravity force acting along a dipping formation on phases of different density to the viscous resistance of the oil at a given Darcy velocity: . The larger it is, the more strongly gravity changes the fractional flow of water; it grows as the injection rate is reduced.
A thin layer of the displacing phase — water along the base or gas along the top — that runs ahead of the main mass when the rate is above the critical one or gravity drives the displacing phase on. It leads to early breakthrough of water or gas to the producing wells.
An elementary volume of the computational grid of a reservoir simulation model; within it the reservoir properties, pressure and saturations are taken to be uniform.
The angle between the true meridian at a point and the vertical line of the coordinate grid. The grid azimuth equals the true azimuth minus the grid convergence; in the northern hemisphere, east of the central meridian of the zone, the convergence is positive.
The dependence of the calculated result on whether the flow runs along the grid lines or diagonally; it is strongest at an unfavourable mobility ratio, for example in gas injection. It is reduced by nine-point schemes and by grid refinement.
Calculation of the values of a surface at the nodes of a regular grid from scattered data points, such as wells and seismic data; contours are then drawn and volumes calculated from the grid. The result depends on the interpolation method, the cell size, the search radius and the smoothing.
The volume of rock between the top and the base of the productive formation within the outline of the accumulation, above the fluid contact, including non-reservoir rock.
A series of payments each of which is larger than the previous one by the fraction ; for a declining series, . Its present value reduces to a single formula, and for it equals . This is how the income of a well with a constant decline rate is valued.
Layers deposited while a structure was growing: they thicken towards the downthrown wall of a normal fault and into a syncline, and thin towards the crest of a growing fold. Their thicknesses are used to date the movements; the ratio of the thicknesses of a unit in the hanging wall and footwall of a normal fault is called the expansion index.
Measurement of the trajectory by tools that determine azimuth not from the magnetic field but with gyroscopes: from the initial setting of the axis (free gyroscope) or from the rotation of the Earth (gyrocompass). It works inside casing and near the steel of neighbouring wells; the accuracy of gyrocompassing decreases towards high latitudes and in inclined wellbores running east or west.
A graph of the cumulative sum of the excesses of the injection bottom-hole pressure over the reservoir pressure, multiplied by time, against cumulative injection. With steady injection the points lie on a straight line whose slope is the reciprocal of the injectivity: an increasing slope indicates plugging of the near-wellbore zone, a decreasing one indicates cleaning-up or the opening of a fracture.
The number of values divided by the sum of their reciprocals. For permeability it corresponds to flow across the layers, when the layers act in series; it is controlled by the tightest layers.
The material balance equation written as the equation of a straight line: each date of a reservoir pressure measurement gives a point, and the slope and intercept of the line give the oil in place, the size of the gas cap or an aquifer parameter. A departure of the points from the line shows that the model of the accumulation is incomplete or that the data are wrong.
The heat passing from the Earth's interior to the surface through a unit area in a unit of time, in mW/m². It equals the thermal conductivity of the rocks multiplied by the geothermal gradient; on average about 65 mW/m² on the continents and about 100 mW/m² in the oceans, and usually 40–80 mW/m² in sedimentary basins.
A drill pipe with a thick wall, extended tool joints and a central upset. It is placed between the drill collars and the drill pipe to give a gradual transition in stiffness, and in inclined wellbores it provides part of the weight on bit.
The transfer of price risk to market participants: a forward or futures contract fixes the price of a future sale, an option gives the right to sell or buy at the strike price for a premium, and a swap exchanges a floating price for a fixed one. On average hedging reduces income by the amount paid to the other side, but it removes the worst outcomes.
Uneven inflow along a horizontal wellbore due to frictional pressure losses: the pressure in the wellbore is lower at the heel, so the drawdown and inflow there are greater, and water or gas breaks through there earlier.
Radial flow to a well located right at a no-flow boundary: fluid arrives from one side only, and the slope of the semilog straight line is twice as large as in an infinite reservoir. If it is taken for ordinary radial flow, the permeability will come out at half its true value and the skin falsely small.
Well and reservoir conditions with higher-than-usual pressure and temperature. There is no generally accepted boundary: often it is a bottomhole temperature above 149 °C with a pore-pressure gradient of 18.1 kPa/m or more, or equipment rated for a pressure above 69 MPa (10,000 psi); according to other documents, a pressure above 103 MPa or a temperature above 177 °C. At such pressures gas is dense and its gas deviation factor is greater than one.
A test of a model that has been matched to part of the history: the remainder is predicted and compared with measurements that were not used in the matching. It is the most convincing test that the model does more than merely repeat the past.
A crossplot of the reading of a porosity tool (density, interval transit time) against the quantity for the beds of one interval. Lines of equal water saturation are straight lines radiating from the matrix point on the axis of infinite resistivity; the line of water-bearing beds gives the matrix parameter and for a previously assumed .
Adjustment of the parameters of a reservoir simulation model so that the calculation reproduces the actual development history.
A plot of the logarithm of the product of the equilibrium ratio and the pressure against the component characterisation factor . At low pressures the points for the components fall on a straight line; correlations of equilibrium ratios for separators and the consistency check of the analyses of separator gas and liquid samples are based on this.
The force shown by the weight indicator of the rig: the weight of the string in the drilling fluid less the share carried by the wall in an inclined well (plus the weight of the travelling equipment, if it has not been deducted), plus the drag when pulling out or minus the drag when running in. The weight on bit is determined from the decrease in hook load when the bit is set down on the bottom of the hole, and problems in the wellbore from the deviation of the measurements from the calculated lines.
The horizontal distance from the wellhead to the projection of a point on the wellbore, or of the target, onto the horizontal plane: , where and are the displacements to the north and to the east.
A well whose wellbore in the productive formation is drilled almost parallel to the bedding for hundreds of metres. For the same drawdown it gives a flow rate several times that of a vertical well; the gain falls as the vertical permeability decreases.
A plot of the pressure in a shut-in well against the logarithm of the ratio . In the radial flow period the points fall on a straight line: its slope gives the flow capacity , its extrapolation gives the pressure , and comparison with the last flowing bottomhole pressure gives the skin.
The minimum return a project must deliver to be accepted; its IRR is compared with this rate. It is usually equal to the company's discount rate, sometimes with a risk premium added.
A combination of two methods of lift in one well: for example, gas lift lightening the column of liquid above an electric submersible or progressing cavity pump, or a progressing cavity pump with a submersible electric motor instead of rods. It removes the limitations of each method at the cost of more complex equipment.
A substance that prevents hydrate plugs: methanol and glycols, dissolving in water, lower the hydrate formation temperature — according to the Hammerschmidt formula, the more so the higher their concentration in the water and the lower their molar mass; low-dosage inhibitors slow the growth of crystals or prevent them from agglomerating.
Four times the cross-sectional area of the channel divided by the wetted perimeter; for an annulus it equals . It is substituted into the Reynolds number and into the turbulent formulas for non-circular channels; the laminar losses in the annulus are represented more accurately by a diameter of .
The ratio of the fluid mobility to the elastic storage capacity of the formation, , m²/s. It sets the speed at which pressure disturbances propagate: in time a disturbance noticeably covers an area of radius of the order of . In Russian-language literature it is also denoted or .
Creation of a fracture in a formation by injecting fluid at a pressure above the fracture pressure and holding it open with proppant.
The bit hydraulic power divided by the area of the hole bottom, in W/mm² or hp/in² (1 hp/in² ≈ 1.156 W/mm²). It allows the hydraulics of bits of different diameters to be compared; guideline values for it are given by bit manufacturers and by the experience of offset wells.
A downhole unit consisting of a reciprocating hydraulic engine and a piston pump on one rod: pressurised power fluid drives the engine piston, a spool valve at the end of the stroke reverses the direction, and a double-acting pump delivers production on both the upstroke and the downstroke. The delivery is proportional to the number of double strokes.
The temperature at which hydrocarbon liquid begins to condense from gas at a given pressure. It is set by the heavy components — pentanes and heavier; to lower it, the gas is cooled and the condensed liquid is separated.
The movement of oil and gas from where they form to a trap. Primary migration (expulsion) is their exit from the source rock, usually along microfractures as the generation pressure rises; secondary migration is their rise through permeable beds and faults to the traps under the action of buoyancy against capillary forces.
The part of the pore volume of the accumulation occupied by oil or gas, at reservoir conditions: . Per unit area it is expressed as the thickness .
The shift in the density and neutron log readings caused by residual oil or gas in the flushed zone: the density porosity increases and the neutron porosity decreases, in proportion to the hydrocarbon volume . For gas the shifts are large and opposite in sign, for oil they are small; they are allowed for before porosity and lithology are read.
A conical device with no moving parts: the fluid is pumped in tangentially, and the swirling flow throws the coarse and heavy particles towards the wall and downwards to the underflow, while the liquid with the fines leaves upwards through the overflow. Desanders (cones of 150–300 mm) remove drilled solids of about 40 µm and larger, desilters (about 100 mm) those of about 20 µm and larger; with a weighted fluid, hydrocyclones discard barite too.
The amount of hydrogen per unit volume of a substance relative to fresh water at surface conditions. The value is about 1 for oil, slightly below 1 for saline water and 0.1–0.5 for gas, depending on pressure; for a hydrocarbon of density (g/cm³), .
The pressure of a column of liquid: the product of its density, the acceleration due to gravity and the vertical height of the column.
A system of codes of the International Association of Drilling Contractors (IADC) for comparing bits from different manufacturers. A roller-cone bit is described by three digits and a letter: the series (1–3 steel teeth, 4–8 tungsten carbide inserts, by rock hardness), the type within the series, the bearing with gauge protection, and an additional feature. A fixed-cutter bit is described by a body letter (M or S) and three digits: cutter density, cutter size, profile.
Rocks that crystallised from a melt: intrusive, or plutonic (granite, gabbro), with large crystals, and extrusive, or volcanic (rhyolite, basalt), with small crystals and glass. Plutonic rocks have almost no porosity of their own; reservoirs can be fractured and weathered varieties and, occasionally, vesicular lavas.
An imaginary well placed as a mirror image of the real one across the boundary of the formation so that the solution for an infinite reservoir satisfies the boundary condition. With the same rate it reproduces a no-flow boundary, for example a fault; with the opposite sign it reproduces a constant-pressure boundary.
A process in which the wetting phase displaces the non-wetting phase, for example water displacing oil from a hydrophilic rock. Spontaneous imbibition is driven by capillary forces; forced imbibition takes place at negative capillary pressure.
A reduction of an asset's book value to its recoverable amount, the greater of its selling price less costs of disposal and the present value of the cash flows from its use. It is recognised when prices, reserves or costs have changed so that the investment will not be fully recovered; it does not change the year's cash flow.
A time-stepping scheme for a multiphase model: the pressures of all cells are found implicitly, by one linear system with the phase mobilities taken at the start of the step, and the saturations are then updated explicitly in each cell from the flows found. The step is limited by the Courant condition for saturation, which is most severe in small cells with a high flow rate.
A way of stepping in time in which the flows and withdrawals during the step are computed from the still unknown values at the end of the step, so a system of equations for all the cells is solved at every step. The scheme is stable at any step size, but a large step smooths the solution.
A bit with small diamond grains embedded in its matrix; as the matrix wears, new grains are exposed. It grinds very hard, abrasive rocks and requires a high rotary speed — it usually runs with a turbodrill or a high-speed motor.
The share of Kazakhstani content in the goods, works, services and personnel used by a subsoil user, calculated under unified methodologies. In the works and services bought in a calendar year it must be at least 70% of their value (except for complex projects in the Kazakhstan sector of the Caspian), and in tenders the price of Kazakhstani producers of works and services is conditionally reduced by 20%.
A valley cut by a river into an exposed shelf or coastal plain when relative sea level falls, and filled with fluvial, estuarine and marine sediments when it rises. The fill rests on a sharp erosional base, can be thick and, enclosed by marine clays, often serves as a stratigraphic trap.
The angle between the axis of the wellbore and the vertical: 0° is a vertical hole, 90° a horizontal one. In ordinary wells the inclination of the straight inclined (tangent) section is normally held between 10 and 60°.
A state of the flushed zone in which part of the formation water remains in addition to the filtrate: the resistivity of the water in the zone is lower than , is too low, and by the resistivity ratio a water-bearing bed looks like a bed with moveable hydrocarbons, the more so the greater the contrast between the waters.
The difference between the cash flows of two forecasts, “with the project” and “without the project”, on every line, including tax and decommissioning; its NPV equals the difference between the NPVs of the two cases. Comparing “before” and “after” instead of “with” and “without” distorts the result.
The IRR of the difference between the cash flows of two mutually exclusive options; it equals the rate at which their NPVs are the same. If it is above the required rate, the additional investment in the larger option is justified.
A method that measures the conductivity of rocks from the eddy currents induced in the rock by the alternating magnetic field of coils; a conductive mud is not needed. It is the best choice with fresh mud and oil-based mud; it loses accuracy in very high-resistivity rocks and with very saline mud.
A property of the rock with the dimension of inverse length (m⁻¹) that sets the inertial term of the Forchheimer equation. It increases as permeability decreases, roughly as ; it is measured on core at several gas flow rates.
The drilling of new wells between existing ones to bring into production oil that the waterflood bypasses, or to accelerate production. The effect is assessed from the increase in production of the area in calculations with and without the new well, not from the production of the well itself.
The section of a pressure record in which wellbore storage has already ended and the disturbance has not yet reached the boundaries: the pressure depends linearly on the logarithm of time, and its logarithmic derivative is constant. Flow capacity and skin are found from this section alone.
A sustained rise in the general price level, that is, a fall in the purchasing power of money; it is measured by price indices. A fall in the general price level is called deflation.
A calibrated restriction (nozzle, channel, tube) between the formation and the screen pipe of a horizontal wellbore. It equalises inflow along the length and restricts inflow from highly permeable or watered-out sections at the cost of an additional pressure drop; it works only when the annulus is divided by packers.
The dependence of the well's flow rate on the bottom-hole pressure.
The part of an aquifer model that is calculated from the pressure history at the contact: the influx equals the aquifer parameter multiplied by this function, . For an unsteady-state aquifer it is the sum of the pressure steps weighted by the dimensionless influx; it depends on the size and permeability of the aquifer, but not on its constant.
The pressure of the fluids in an accumulation before the start of development. Together with the reservoir temperature it determines the initial state of the fluids and the volumes in place; it is compared with the bubble point or dew point pressure and with the normal hydrostatic pressure.
The volume of injected gas at standard conditions per cubic metre (or tonne) of liquid lifted by gas lift. It increases as the technical optimum is approached; at low rates of continuous gas lift, because of gas slippage, it reaches thousands of cubic metres per cubic metre.
A gas lift valve that is opened by the pressure of the injected gas in the annulus, acting on the bellows against the nitrogen pressure in the dome. The tubing pressure acts only on the seat area and lowers the opening pressure only slightly; the valve closes when the gas pressure falls to the nitrogen pressure.
The rate that an injection well accepts at a given pressure difference; the injectivity index is the rate per unit of pressure difference between the bottomhole and the reservoir. It increases when the mobility ratio is unfavourable and when fractures open, and decreases when the near-wellbore zone becomes plugged.
A completion in which formations or intervals are isolated by packers, an interval control valve operated from the surface sits at the inlet of each, and permanent downhole gauges measure the pressure and temperature of each. It makes it possible to open, throttle, shut in and test formations without well interventions.
Deposits of the depressions between dunes. In dry interdunes these are wind ripples and lag; in damp ones, adhesion laminae; in wet ones, clay layers, salts and root traces. Damp and wet layers are tighter than dune sands and divide the aeolian sequence vertically.
The work of forming a unit area of the interface between two immiscible phases, N/m (in practice mN/m, the same as dyn/cm); for the interface between a liquid and a gas it is called surface tension. It sets the scale of capillary forces: about 72 mN/m for water–air and usually 10–35 mN/m for oil–water.
The excess of the size of the rotor of a progressing cavity pump over the size of the stator cavity: the elastomer fits tightly around the rotor and creates sealing lines. A larger interference reduces slippage, but increases the friction torque and the heating of the elastomer; in the well the interference grows through swelling of the elastomer and temperature.
A test of communication between wells: the rate is changed in the active well and the pressure response is recorded in the observation well. The delay and magnitude of the response are used to estimate the flow capacity and storativity of the formation between the wells and the presence of barriers.
The fraction of the rock volume between the framework grains (intergranular pores, cement and matrix), found by point counting in thin section; a measure of compaction. For fresh, well-sorted sand it is 40–45%; in well-sorted sandstones of rigid grains mechanical compaction reduces it to about 26%.
Gas lift in cycles: liquid accumulates in the tubing above the operating valve, then gas is admitted quickly into the tubing and ejects the liquid to the surface as a slug. It is used at low rates, when continuous gas lift wastes gas on slippage; a plunger reduces the fallback of liquid, and an accumulation chamber reduces the pressure on the formation.
An actuated valve on the gas line of an intermittent gas lift well that opens on a timer for the set injection time and closes until the next cycle. The injection time and the number of cycles are selected at the well from pressure records and production.
The discount rate at which the net present value of a project is zero.
The excess internal pressure at which the hoop stress in the wall of a casing pipe reaches the yield strength: (the Barlow formula; the factor allows for the wall thickness tolerance).
A private oil company whose shares are traded on stock exchanges and which operates in many countries, usually at every link from production to marketing. The largest of them (ExxonMobil, Chevron, Shell, BP and TotalEnergies) are called the majors. According to the IEA's estimate for 2023, the seven largest international companies account for less than 13% of world oil and gas production and reserves.
A graph or nomogram with a pre-computed solution of a tool or formation model for a range of parameter values: borehole corrections, invasion and conversion charts, porosity and lithology crossplots, SP and saturation charts. Each one applies to its own tool, processing and conditions: borehole diameter, mud, scale units.
The moment at which the extrapolations of the first and second semilog straight lines of the record of a well near a sealing fault intersect: , . It is used to find the distance to the fault; if the second line has not yet formed, the distance comes out underestimated.
The travel time of an elastic wave per unit length of rock, the reciprocal of velocity: in µs/m for a velocity in m/s. To convert from µs/ft to µs/m, divide the value by 0.3048. For a sandstone matrix it is about 182 µs/m, for water about 620 µs/m.
The velocity of a P-wave in an individual layer: its thickness divided by half the two-way travel time in it. Unlike the average velocity to a horizon (depth divided by half the time) it describes a single layer; it is obtained from CMP velocities by the Dix formula, and errors are then greatly amplified.
The part of the formation around the wellbore into which mud filtrate has penetrated during drilling; it includes the flushed and transition zones. Its diameter grows with time and, for the same volume of filtrate, is larger in low-porosity rocks.
A grid of lines of equal invasion diameter and equal in the coordinates of the ratios of the readings of three sondes of different depths of investigation, calculated from the step-profile invasion model. From a bed's point on the grid, , and the correction to the deep sonde are read; because of the shape of its grid it is called a “tornado” chart.
A model of slow drainage: the non-wetting phase occupies throats in order of their entry pressures, but only accessible ones, that is, those connected to the inlet through pores already occupied. The front advances in fingers along the widest accessible paths, and a through path appears when the open throats reach the percolation threshold.
An estimate of the value at a point as the average of the values at the data points with weights , where is the distance and usually . The estimate never goes outside the range of the data values; the method draws closed contours around the points and tends towards their average far from the data.
Determining the parameters of a system from its observed response, for example reservoir properties from the pressure and production history. It usually has many solutions: different sets of parameters explain the data equally well.
An emulsion in which the continuous phase is a hydrocarbon liquid and the water (usually calcium chloride brine) is broken up into droplets protected by a film of emulsifier. It is the basis of most oil-based drilling fluids: the water in it does not touch the borehole wall or the cuttings.
An additive to acid that keeps dissolved iron in solution: chelating agents (citric acid, EDTA) or reducing agents that convert ferric iron to ferrous iron. Without it, as the acid spends, ferric hydroxide precipitates — already at a pH of about 2–3 — and ferric iron also causes sludge with asphaltenic oils.
The minimum water saturation that can be reached by displacing water with oil or gas: the water remains as films on the grains and in the small pores. It is immobile, so inflow from a part of the reservoir at this saturation is water-free.
A test of a gas well at several rates of equal duration with full pressure build-up between them, plus one long rate. It gives the coefficients of the two-term inflow formula when stabilisation of each rate would take too long.
A map of equal true stratigraphic thickness of a bed, measured along the normal to the bedding; an isochore map shows vertical thicknesses. The zero contour runs along the limit of the extent of the bed, between the wells where it is present and the wells where it is absent.
A segment on a crossplot joining identical porosity marks on the lines of two minerals. Beds of the same porosity with a matrix made of a mixture of these minerals lie on it, and the position of a point on the segment gives the mineral fractions.
The equilibrium of the light lithosphere resting on the denser, more yielding mantle: at the depth of compensation the pressure beneath any region is the same. This is why high mountains have deep roots, a basin subsides under the weight of sediments, and an area rises as it is eroded.
The relative decrease in gas volume per unit increase in pressure at constant temperature, MPa⁻¹: . For an ideal gas it equals . Not to be confused with the gas deviation factor .
The relative decrease in the volume of single-phase oil per unit increase in pressure at constant temperature, MPa⁻¹: . For undersaturated oil it is usually MPa⁻¹; not to be confused with the gas deviation factor .
A method that refines an approximate solution of a system until the residual becomes small. The simplest are the Jacobi, Gauss–Seidel and successive over-relaxation (SOR) methods: each unknown is recalculated from its own equation. On large grids they converge slowly, so simulators use Krylov methods with preconditioning.
An offshore drilling rig with a floating hull and legs: at the location the legs are lowered to the seabed and the hull is jacked up on them above the water. It is stationary relative to the seabed, and therefore drills with surface preventers; most such rigs are designed for water depths of up to 120 m.
The matrix of partial derivatives of the equation residuals with respect to the unknowns, . In a reservoir simulation model it is sparse (the equations of a cell depend only on the unknowns of the cell itself and of its neighbours), and with upstream weighting it is non-symmetric.
The momentum that the jets from the bit nozzles bring to the bottom of the hole per second: . For the same hydraulic power it is greater with slower jets and a higher flow rate; when the pumps work at their pressure limit it is greatest when the share of pressure drop at the bit is .
A hydraulic pump with no moving parts: power fluid under high pressure from the surface passes through a nozzle, the jet entrains the production of the well, and in the diffuser the velocity of the mixture is converted into pressure. It tolerates gas and solids, but has low efficiency and cannot draw a well down to a low pressure.
The exit area of the nozzle divided by the area of the mixing chamber (throat) of a jet pump. Large values give a high head at a low rate of withdrawal, small values a high rate of withdrawal at a low head; it also determines the production inlet area, which limits the withdrawal because of cavitation.
Processing of an unsteady-state displacement test on core: from the derivatives of oil production and of relative injectivity after breakthrough, the relative permeabilities of both phases are found at the saturation at the outlet end. It rests on the Buckley–Leverett assumptions and is sensitive to the capillary end effect.
A fracture whose walls have moved apart perpendicular to its surface with no appreciable displacement along it. It opens in the direction of the least principal stress; at depth it forms when the pore pressure exceeds the least stress by the tensile strength of the rock.
The change in temperature on throttling without heat exchange, that is, at constant enthalpy. Natural gas cools as its pressure falls — methane at pressures of 5–13 MPa by about 2.5–4.5 °C per megapascal of pressure drop, heavier gases more strongly; liquids warm up slightly when throttled.
The channel between the blades of a fixed-cutter bit through which the drilling fluid with cuttings leaves the bottom of the hole past the gauge into the annulus. The total area of the slots limits the removal of cuttings: in bits for soft rocks the slots are large, in densely set bits for hard rocks they are smaller; with narrow slots in sticky clays the bit balls up.
A square or hexagonal pipe at the top of the string, rotated by the rotary table; it slides down through the drive bushing, and drilling proceeds for its length until the next connection. Its upper thread is left-handed; ball valves are installed above and below the kelly.
The part of the organic matter of sedimentary rocks that is insoluble in organic solvents, which makes up the bulk of that matter. On heating it decomposes to form oil and gas; by hydrogen content, types I–IV are distinguished: I and II are oil-prone, III is mainly gas-prone, and IV is almost inert.
The entry of formation fluid into the well when the pressure in the wellbore falls below the formation pressure.
The largest volume of influx, usually gas, that can be shut in and circulated out without exceeding the leak-off pressure at the casing shoe, for a given excess of the pore pressure over the drilling fluid pressure. It is determined by the margin between the leak-off pressure at the shoe and the pressure in the well.
The depth at which the wellbore starts to be deviated from the vertical and directed along a given azimuth. The farther the target is from the wellhead, the shallower the build-up is started, so that the inclination of the straight inclined section stays moderate.
The gas pressure at the annulus wellhead that the gas system can deliver when a gas lift well is started up. It determines the depth of the uppermost unloading valve; if it is higher than the operating pressure (a start-up compressor, nitrogen), fewer unloading valves are needed, but such a source will be needed at every start-up.
Drilling fluid whose column balances the pressure of the formation that has kicked: , where is the density of the current fluid, is the drill pipe pressure after the well is shut in, and is the true vertical depth. The volume of the influx and the pressure in the annulus have no effect on it.
Dynamic viscosity divided by density, , in mm²/s, numerically equal to centistokes. It is measured with glass capillary viscometers and given in analyses of stock-tank oil; Darcy's law involves the dynamic viscosity.
Conversion of the permeability of a sample measured with gas into the equivalent liquid permeability . At low pressure gas slips along the walls of the pore channels, and the gas permeability is overstated, the more so the tighter the rock and the lower the mean pressure of the test. The gas permeability is measured at several mean pressures and the line in the coordinates versus is extrapolated to .
Estimation of a property at a point without data as a weighted average of the data, with weights found from the variogram so that the estimate is unbiased and has the smallest expected error. It takes into account the distances both to the point and between the data, so it down-weights clusters of data; it gives the error variance, and the resulting map is smoothed.
Iterative methods that seek the solution as the best combination of the vectors , , , …: the conjugate gradient method for symmetric positive definite matrices, and GMRES, ORTHOMIN and BiCGStab for non-symmetric ones. For the conjugate gradient method the number of iterations grows as the square root of the condition number; preconditioning greatly reduces it.
A dynamic method of calculating pseudo relative permeabilities of coarse cells from a displacement calculation on a fine grid: the phase flow rates are taken at the outlet face of the block, the phase pressures are averaged over the central column of the block with the weight , and the pseudo relative permeabilities are found from Darcy's law between block centres. It compensates for numerical dispersion but creates a spurious pseudo capillary pressure; the variant that averages pressure over the pore volume of the block (PVW) does not.
The time it takes the drilling fluid to rise from the bottom of the hole up the annulus to the surface: the annulus volume divided by the pump output. It is used to tie samples of cuttings and gas to depth.
An empirical relation between the trapped saturation of the non-wetting phase and its initial saturation: , in fractions of the pore volume excluding irreducible water. It sets the residual oil in the transition zone and the trapped gas in hysteresis models.
A short sub in the tubing string with an internal profile for a lock and a polished sealing bore. A lock with a plug, a gauge, a choke or a valve is run into it on wireline. Selective nipples in a string have the same size, while the lowest one is usually a no-go nipple, with a reduced bore through which the lock does not pass.
An empirical relationship between shale volume and the gamma ray index : , where for Tertiary rocks and for older rocks. It gives a lower shale volume than the linear estimate; the coefficient is refined using core data.
An electrode method of measuring resistivity in which guard electrodes focus the measuring current into the formation in a thin sheet. It is accurate in saline muds and in high-resistivity and thin beds; it does not work in oil-based muds.
A way of drawing values in the Monte Carlo method: the probability scale of each parameter is divided into equally probable strata, exactly one value is taken from each stratum, and the strata of different parameters are combined in random order. The mean of the result is more accurate than with simple random sampling with the same number of trials.
Different substances at the same reduced temperature and pressure have similar properties, in particular the gas deviation factor. The generalised chart of for natural gases is built on this principle.
In undisturbed bedding each layer of sedimentary rock is younger than the one beneath it and older than the one above it. Together with the principles of original horizontality and lateral continuity, it was formulated by Nicolas Steno in 1669.
A possible trap identified on insufficient data. Before drilling, it is studied further, either to turn it into a prospect or to drop it.
The constant in Carter's law: the rate at which fracturing fluid leaks off through a unit area of fracture wall equals , where is the moment when that part of the wall opened. It is measured in m/s; it depends on the formation and the fluid and is calibrated from a mini-frac.
Pressure testing of the open hole below the casing shoe to determine the pressure at which the rock begins to take drilling fluid. In a test to a specified pressure (FIT, formation integrity test), pumping is stopped at a pre-selected pressure: such a test confirms that the rock will withstand the required pressure but does not show its limit.
The present value of costs divided by discounted production at the same rate: the constant price at which the project recovers its costs with a return equal to the rate. It does not depend on the choice of the reference date, but depends strongly on the rate.
Dimensionless capillary pressure, . It collapses the capillary pressure curves of samples of one rock type with different permeability and porosity onto a single curve.
The sharing of a limited gas supply between gas lift wells so as to produce the most oil: gas is given to the wells where the next increment yields more oil, until the marginal returns of the wells are equal; wells that require a minimum of gas are assessed together with that minimum; limits on water, gas and system pressures are taken into account.
A year's operating costs divided by the same year's production, usually excluding taxes, depreciation and transport to market; the composition of the indicator should be specified. It rises towards the end of the field's life, when fixed costs are spread over falling production, and as water cut increases.
A neutron log scale on which the reading equals the porosity of a clean water-filled limestone under standard calibration conditions. In sandstone such a reading is lower than the true porosity, in dolomite higher.
A way of distributing the injection during hydraulic fracturing among several clusters or intervals: so few perforations are made that the pressure loss across them at the design rate is greater than the difference in closure pressures, and the flow rate into a cluster is determined mainly by the number of its perforations. Erosion of the perforations by proppant weakens the effect.
A solution of the diffusivity equation for a well producing at a constant rate in an infinite reservoir, in which the well is replaced by a line of zero radius: , where . It gives the pressure at any point of the reservoir until the disturbance reaches the boundaries.
Flow along parallel streamlines, either towards the plane of a high-conductivity hydraulic fracture or along a narrow reservoir between parallel boundaries. The pressure drop grows as the square root of time, a slope of ½ on the diagnostic plot.
The part of a reservoir simulator that solves the systems of linear equations of implicit steps and Newton iterations, either by direct methods (Gaussian elimination) or by iterative ones (Krylov methods with preconditioning). In large models the solver usually takes most of the run time.
A casing string that does not reach the surface: it is hung on a liner hanger inside the previous string with an overlap of tens of metres and cemented. It may be a drilling liner (covering a problem interval) or a production liner; if necessary it is extended to the surface with a tie-back string.
The fraction of the cross-section (volume) of the pipe occupied by liquid when it flows together with gas. Because of gas slip in upward flow, it is greater than the no-slip liquid fraction ; the weight of the column of mixture depends on it.
The condition in which the gas velocity in the tubing is too low to carry the liquid out: the gas slips past, the liquid accumulates in the tubing and at the bottom of the well, the column becomes heavier, and the well flows in pulses or stops. It is typical of gas wells and of oil wells with a low flow rate in tubing that is too wide.
The positions on a crossplot of the points of a clean bed of a single mineral with liquid in the pores, for porosity from zero to 40%, with porosity marks. The positions of the lines depend on the tool, the processing method and the filtrate, so a chart is chosen to suit the data in hand.
The rigid outer shell of the Earth: the crust together with the upper part of the mantle. Its thickness ranges from a few kilometres at mid-ocean ridges to 200–250 km beneath ancient shields; its base is associated with a temperature of about 1300 °C. The lithosphere is broken into plates that slowly move over the asthenosphere.
A situation in the life of a casing string — running, cementing, a kick, lost circulation, production — for which consistent pressures inside and outside are specified. The string is checked against the most severe plausible case for each load.
Replacing selected coarse cells with a nested fine grid, usually near wells and at fronts, where pressure and saturation change rapidly; the rest of the model remains coarse.
Upscaling in which the boundary conditions of the local calculations for cells and connections are taken from a calculation of the whole model with its wells, rather than being set by assumption. In the global method this is a single-phase calculation of the fine model; in the local–global method it is a calculation of the coarse model, alternated with local calculations until the transmissibilities stop changing. It is more accurate than the local method, but it is tied to the well configuration for which it was carried out.
A text format for exchanging well log curves, introduced by the Canadian Well Logging Society (CWLS) in 1989; the most widespread version is 2.0 (1992). A file consists of sections that begin with the character “~”: version, well, curve descriptions, parameters, other information, and a table of values whose first column is depth or time; missing values are written as a conventional number, usually −999.25.
The recording conditions at the start of the log and in the file parameters: the well and the depth reference, the run and the recorded interval, the bit size, the type and properties of the mud, the resistivities of the mud, filtrate and mudcake with the temperature of measurement, the bottom-hole temperature and the time since circulation stopped, the tool string configuration and the processing applied. Without them the curves cannot be corrected and interpreted.
The distribution of a quantity whose logarithm is normally distributed. It is skewed to the right: the mode is smaller than the median, and the median (which is also the geometric mean) is smaller than the arithmetic mean. It is characteristic of permeability, grain sizes and quantities obtained by multiplying many factors, including reserves.
Replacement of the exponential integral by a logarithm for a small argument: for . At the wellbore wall it gives and is valid within seconds of start-up; far from the well the condition is satisfied only after a long time.
A downhole tool or its measuring part: a source of a field or radiation and one or several receivers at set distances. The distance between the source and the receiver, known as the sonde length, determines how deep into the formation the sonde sees and how thin a bed it can resolve.
A sealed capsule containing a radionuclide in active nuclear tools: caesium-137 emits 662 keV gamma rays for density logging, and a mixture of americium-241 with beryllium emits fast neutrons for neutron logging. The source emits radiation continuously; its storage, transport and handling are regulated by radiation safety standards, and only trained specialists carry out such work.
The time constant with which hydrogen nuclei become magnetised in a static field: in time the fraction of the equilibrium magnetisation is reached. It determines how long the formation must be polarised before an NMR measurement: about of the slowest fluid, and for gas and light oil is measured in seconds.
A comparison of the forecast on which a decision was made with the actual production several years later. An outcome outside the forecast range is a reason to revisit the model and the decisions; systematic deviations across many projects are taken into account in later forecasts.
Twice the area between the curve of cumulative flow capacity against cumulative storage capacity (layers ordered by decreasing ) and the diagonal; it ranges from 0 for a homogeneous reservoir to 1. For log-normal permeability at constant porosity it equals .
Particles in drilling fluid with a density of about 2.6 g/cm³ — bentonite, drilled clay and sand, carbonates — as opposed to the weighting material (barite, 4.2 g/cm³). Their fraction is found from the fluid density and the retort and is kept as low as possible: they make the fluid more viscous and the filter cake thicker.
A formation that yields oil or gas at a resistivity close to that of water-bearing formations. The causes are the conductivity of dispersed clays, thin interbedding of sandstones and shales, fine pores holding a large amount of immobile water, and conductive minerals. Archie's equation gives a high water saturation for such a formation.
Gas treatment by cooling: high-pressure gas is cooled in a recuperative heat exchanger and throttled, and the condensed hydrocarbons and water are separated in a cold separator. It works on the pressure drop available from the reservoir; when this is not enough, refrigeration units, turboexpanders or booster compressors are added.
A pipe, or several pipes with quick-connect unions, above the Christmas tree and rated for the well pressure: a pressure lock through which a tool on wireline or cable is introduced into the well under pressure and withdrawn from it. The length of the lubricator limits the length of the tool string and of the perforating gun.
A crossplot of two parameters calculated from the density, neutron and sonic logs: and are the slopes of the rays from the fluid point to the bed point on the “sonic–density” and “neutron–density” planes. They are almost independent of porosity, so minerals appear on it as points and mixtures of three minerals fall inside triangles.
The angle between the directions to true (geographic) north and to magnetic north at a given point; east declination is taken as positive, and true azimuth equals magnetic azimuth plus declination. It varies with place and time, so it is taken from a geomagnetic model for the date of the survey.
The angle between the vector of the Earth's magnetic field and the horizontal plane; in the northern hemisphere the field points downwards. In Kazakhstan the dip is about 61–74°, and the horizontal component of the field, by which a magnetic tool finds north, is only 28–48% of the total field.
The ratio of the induced magnetisation of a rock to the strength of the magnetising field; in SI it is dimensionless. It is determined almost entirely by the magnetite content: for sedimentary rocks it is usually less than SI units, for mafic igneous rocks up to 0.1–0.2.
A passive electrical survey method: from the natural variations of the Earth's electric and magnetic fields, with periods from fractions of a second to hours, the variation of rock resistivity with depth is found, at depths from hundreds of metres to tens of kilometres. It is used where seismic is difficult: beneath salt and basalts, in thrust zones.
The torque with which a drill string tool joint is made up to press the shoulders together. It is chosen so that the stress in the weaker element of the connection is about 60% of the yield strength; the working torque of the string is kept below the make-up torque.
The force that can be applied to the string in excess of its weight (in an inclined well, in excess of the weight when pulling up) without exceeding the allowable load on the pipe, — 90% of the tensile strength of the pipe body. The required margin is usually set at 50–100 thousand lbf (0.22–0.44 MN).
The increase in oil rate per additional thousand cubic metres of injected gas — the slope of the performance curve of a gas lift well converted to oil. With the best allocation of a limited supply of gas it is the same for all the wells receiving gas.
A string of large-diameter pipe between the subsea blowout preventer stack and the floating drilling rig: the drilling fluid returns to the rig through it, tools and casing strings are run through it, and the choke and kill lines run along it. Tensioners keep it in tension, and a telescopic joint decouples it from the heave of the rig.
A thin, laterally persistent layer or surface that can be recognised in all wells and was deposited at practically the same time over the whole area: a layer of volcanic ash, the clays of a maximum flooding surface, a persistent limestone or anhydrite. It is the backbone of well correlation and the reference surface for flattening correlation panels.
The estimated amount for which an asset would change hands on the valuation date in a commercial transaction between knowledgeable parties acting prudently and without compulsion. It is derived from the prices of transactions in similar assets; a related concept in IFRS reporting is fair value.
A sedimentary rock made of a mixture of carbonates (calcite or dolomite) and clayey material, each making up from 25 to 75% (on the widely used Russian scale; other schemes put the limits slightly differently). With a smaller share of clayey material the rock is called a clayey limestone, with a larger share a calcareous clay. Marls are common seals and source rocks.
Any point whose coordinates, after the record has been overlaid on the type curves, are read in both systems: the dimensionless and and the dimensional and . The ratios and are called the pressure match and the time match.
The equality between the quantity of fluids withdrawn from an accumulation and the expansion of the remaining fluids and rocks; it is used to estimate the hydrocarbons in place and the drive mechanism of the accumulation.
The ratio of cumulative production to the current flow rate, . If the rate has varied, the rate-normalised pressure in the pseudo-steady-state regime falls on a straight line of slope when plotted against it — this is how the pore volume is found from a record with variable rate.
Injection of acid into a formation at a pressure below the fracture pressure to dissolve damaging materials and part of the rock near the well.
A crossplot of the apparent matrix transit time against the apparent matrix density; in the version using the lithodensity log, the apparent matrix volumetric cross section is used instead of the transit time. Minerals appear on it as points with their own parameters, while gas, vugs and shales shift the point of a bed in characteristic directions.
Determination of the average reservoir pressure of the drainage area from a pressure build-up: a correction , calculated in advance for the shape of the area and the position of the well, is subtracted from the pressure on the extrapolation of the Horner line. If the well had reached the pseudo-steady-state regime before shut-in, .
The surface pressure in the closed annulus at which the pressure below the last casing shoe reaches the leak-off pressure from the leak-off test (LOT): , where is the density of the fluid in the annulus and is the true vertical depth of the shoe. It decreases as the fluid is weighted up, so it is recalculated after every change in density.
The minimum of a project's cumulative cash flow: the amount that has to be found outside the project before it starts to support itself. It is usually smaller than total capital expenditure, because late costs are paid from revenue, and it grows if the start of production is delayed.
The normalising quantity of the Fetkovich method: — the influx that a closed aquifer of pore volume would give if the pressure in it fell from the initial value to zero. It is unattainable; the real limit for a pressure decline of is the elastic storage .
A price model in which a deviation from the long-term level partly disappears each year: a high price prompts higher production and more economical use, a low price prompts cuts in investment. Unlike a random walk, the spread of the price grows only up to a limit.
A river with a single sinuous channel (the sinuosity is usually 1.5 or more): it erodes the concave bank and builds a point bar sideways on the convex one. Its sand body is a meander belt made of point bars with inclined clay drapes, cut by the clay plugs of oxbow lakes and surrounded by floodplain clays; permeability within it decreases upwards.
Downhole systems that transmit data on the trajectory and rock properties to the surface directly while drilling is under way.
The energy the bit expends to break a unit volume of rock: the work done by the weight and by rotation per unit time, divided by the volume drilled in the same time, . It is measured in units of pressure; when the bit is working well it is roughly three times the strength of the rock under the bit, and a rise in it at the same operating parameters points to balling, vibration or wear.
The value that divides an ordered set of measurements in half: half of the values are smaller than it, half are greater. Unlike the arithmetic mean it does not depend on the size of the extreme values, and so it describes the “typical” sample of skewed data, for example permeability.
A laboratory method: mercury, the non-wetting phase, is injected into an evacuated sample in pressure steps, and the capillary pressure curve and the pore-throat size distribution are constructed from the volume of mercury that has entered. The result is converted to reservoir fluids by the ratio.
The dimensionless ratio : the time step expressed in times of pressure equalisation across one cell. It determines the stability and accuracy of schemes for the pressure equation; the explicit scheme is stable for .
Rocks recrystallised in the solid state under the action of high temperature and pressure: slates and schists, gneisses, quartzites and marbles. Their porosity is almost zero; they can be reservoirs only thanks to fractures and weathering.
Titration of a sample of drilling fluid or rock with methylene blue dye, which clays absorb by cation exchange. For drilling fluid the result is expressed as the bentonite equivalent — 14.25 kg/m³ for each 1 cm³ of dye per 1 cm³ of fluid; it includes the active drilled clay as well.
A thin gap — micrometres or tens of micrometres — between the casing string and the set cement. It appears when the string contracts after the cement has set: the pressure or temperature inside has fallen. It overstates the amplitude on the cement bond log and can let gas through; logging under pressure closes it.
A record made with a focused pad microresistivity tool: a guard current forces the measuring current through the mudcake into the flushed zone, so the reading is determined mainly by the resistivity of that zone. The depth of investigation is a few centimetres; a thick mudcake and shallow invasion require corrections.
A record made with two unfocused micro-electrode arrays on a pad: the micro-inverse (microgradient) array A0.025M0.025N and the micro-normal (micropotential) array A0.05M. Opposite permeable beds with a mudcake the deeper micro-normal array reads a higher resistivity; the microlog is used to identify reservoirs and thin beds, and the resistivity of the flushed zone can be found only in favourable conditions.
The excess of the micro-normal (micropotential) reading over the micro-inverse (microgradient) reading opposite a permeable bed: the shallow sonde sees mainly the low-resistivity mudcake, the deeper one also sees the flushed zone. In tight rocks there is no mudcake, and the separation is absent or negative. The separation says nothing about the magnitude of the permeability.
A convention under which the annual cash flow is regarded as received in the middle of the year; when counting from the start of the first year, the exponent for year is .
A simplified analysis of a pressure build-up curve: the pressure is plotted against the logarithm of the shut-in time alone. It is valid when the shut-in time is much shorter than the producing time of the well.
A naturally occurring solid substance of definite chemical composition with an ordered crystal lattice. Minerals are identified by hardness, cleavage, lustre, streak colour, density and reaction with acid, and in the laboratory by thin sections and X-ray diffraction; a rock is an aggregate of minerals.
A tax on the value of the raw material produced, paid regardless of profit. In Kazakhstan the MET rate on oil in 2026 depends on the contract's annual production (5–18%), and from 2027 on production and the world price (19–67%).
In the USA, the right of ownership of oil, gas and other minerals beneath a tract of land. It usually belongs to the landowner but can be separated from the land and held by another person or by the state (a split estate); the owner leases the rights in return for a bonus and royalty. In Kazakhstan there are no private rights to the subsoil: it belongs to the people, and a company obtains a subsoil use right.
Injection of a small volume of fluid without proppant that creates a fracture before the main hydraulic fracturing treatment. The pressure record during injection and after shut-in gives the closure pressure (the minimum stress), the net pressure and the rate at which fluid leaks off into the formation, and the part of the record after closure gives the reservoir pressure.
A depression between salt domes from which the salt has moved into the domes and in which a thick sequence of sediments has accumulated; its growth is recorded in the thickening of the layers towards the centre of the minibasin.
Construction of the smoothest surface that passes through the data points: the total squared curvature is a minimum, as for a thin elastic plate pinned to the points. It preserves slopes and can reproduce a crest between wells, but beyond the data it extends the slopes and produces overshoots. Not to be confused with the wellbore trajectory calculation method of the same name.
The industry standard for calculating a trajectory from survey data: the wellbore between survey stations is taken to be an arc of a circle tangent to both measured directions. The coordinate increments equal the result of the balanced tangential method multiplied by .
The smallest recoverable volume of a discovery whose development pays back at the assumed price, costs, taxes and discount rate, that is, the NPV of the development is not less than zero. It depends on the project conditions and changes over time.
The smaller of the two horizontal principal stresses in the rock. In most sedimentary basins it is also the least principal stress: a hydraulic fracture opens against it and grows perpendicular to it, and the fracture closure pressure is approximately equal to it.
The lowest pressure at which the injected gas, at reservoir temperature, becomes miscible with the given oil — immediately on contact or after repeated exchange of components. Above it the interface between the phases disappears and the residual oil in the swept zone is small; it is determined in a slim-tube apparatus (a long, thin tube) or by calculation with an equation of state.
A measure of the discrepancy between the model's calculation and the observed data, usually the sum of squared differences normalised by the measurement error. It is minimised during history matching; when normalised by the number of measurements, a value close to 1 means discrepancies at the level of the measurement error.
Non-uniform wettability in which the large pores are hydrophobic and the small ones hydrophilic; it arises when oil remains in contact with the walls of the large pores for a long time. In spotted (fractional) wettability, hydrophobic patches occur in pores of any size.
The ratio of the effective permeability to a phase to its viscosity, . It shows which phase moves more easily; the ratio of the mobilities of the displacing and displaced phases is called the mobility ratio.
The ratio of the mobility of the displacing phase to the mobility of the displaced phase; at values greater than one the displacement is less stable.
The equation that a finite-difference scheme solves more exactly than the original one: the original equation plus the leading truncation-error terms obtained from the Taylor series. For an upwind scheme applied to the saturation transport equation, such a term is diffusive, with the coefficient ; this is numerical diffusion.
The return on a project under explicit assumptions: outlays are discounted to the start at the financing rate, and receipts are compounded to the end of the project at the reinvestment rate. Unlike IRR, it is always unambiguous.
The boundary between the Earth's crust and the mantle, across which the velocity of compressional seismic waves jumps from roughly 6.5–7 to 8 km/s. It lies at a depth of 5–10 km beneath the ocean floor and on average about 40 km beneath the continents, reaching 70 km or more under high mountains.
The condition for shear failure of a rock: the shear stress on a plane reaches , where is the cohesion and is the angle of internal friction. In terms of principal stresses, , where : confining stress increases the strength.
An ordinal scale of hardness based on ten reference minerals, from talc (1) to diamond (10): each scratches the preceding ones. The steps are unequal: diamond is almost four times harder than corundum. In the field, hardness is compared with a fingernail (about 2.5), a copper coin (about 3) and glass (5.5).
Amounts in the money of the year in which they are paid or received, that is, including inflation up to that year. Nominal amounts from different years have different purchasing power, so their simple sum does not measure value.
A calculation in which a model is recomputed many times with the values of uncertain parameters drawn at random from their distributions, and the distribution of the final quantity (reserves or NPV) is built from the set of results. The result is no more reliable than the distributions and the relationships between parameters that are built into it.
The part of the hydrocarbons that the filtrate has displaced from the flushed zone: a fraction of the pore volume, a volume in the rock of ; the remainder is residual hydrocarbons. A ratio of about 1 indicates that the hydrocarbons have not moved; by an empirical rule, a value of about 0.7 or less indicates that they are moveable.
A mixture of hydrochloric and hydrofluoric acids for treating sandstones: it dissolves clays, feldspars, fines and particles of drilling fluid, hardly affecting quartz. It is pumped after hydrochloric acid, which removes carbonates; otherwise secondary precipitates form from the solution.
A battery of hydrocyclones whose underflow is fed onto a fine vibrating screen: barite and small particles pass through the screen and return to the drilling fluid, while drilled particles larger than the screen openings go to waste. It is used on weighted fluids up to about 1.8 g/cm³.
The liquid part of the drilling fluid that has passed through the mudcake into the formation. The contrast between its resistivity and the resistivity of the formation water determines how invasion shows up on the resistivity curves.
Real-time observation of the geological section and the drilling process: description of the drilled rock (cuttings), measurement of gas in the drilling fluid, and recording of rate of penetration, weight on bit, pump pressure and fluid volumes.
A positive-displacement reciprocating pump that delivers drilling fluid into the well: either a single-acting three-piston (triplex) pump (per stroke — one revolution of the shaft — the three cylinders each displace ) or a double-acting two-piston (duplex) pump. The actual output is smaller than the theoretical one by a fill factor (volumetric efficiency) of about 0.9–1.0; the pumped volumes are calculated from the number of strokes.
The resistivity of the drilling fluid. Together with the resistivities of the filtrate and the mudcake it is measured on a sample before logging and recorded in the log heading with the temperature of measurement; before calculations all three are converted to formation temperature.
Transmission of LWD and MWD data from the bottom of the hole to the surface by pressure pulses in the flow of drilling fluid. The rate is a few bits per second, rarely tens, so only the main curves are sent in real time, and the full data are read from the tool memory after it has been pulled out of the hole.
A layer of drilling-fluid solids on the borehole wall opposite permeable formations. Its permeability is orders of magnitude lower than that of the reservoir, so the cake limits the penetration of filtrate; on a caliper log it shows up as a reduction in diameter.
A model of an accumulation made of several tanks (blocks or zones with uniform pressure inside) connected by transmissibilities across faults and barriers. A material balance is written for each tank, with flows to its neighbours; the transmissibilities and capacities are matched to the pressures of the blocks over time.
The place where a branch of a multilateral well leaves the main bore. Depending on its design, it provides mechanical support for the rock, hydraulic isolation of the bores and access for tools to each bore; the level of complexity of the junction is defined by the TAML classification.
A well with one wellhead and a main bore from which one or more branches, usually horizontal in the formation, are drilled; all the bores operate simultaneously. A well that branches within the formation itself is often called a multi-branch well. Branches in one formation share the drainage area, so each additional one adds less than the previous one.
Operation of several formations by one well, the formations being separated by packers, in which the production or injection of each formation is controlled, measured and tested separately — through its own tubing string, through the annulus or through downhole regulators. In Kazakhstan, simultaneous separate operation of two or more development units by one well is permitted only with equipment for separate metering of the production and for testing each unit.
Calculation of the flow across a face from the pressures of several cells around it. It accounts for the off-diagonal components of the permeability tensor and for skewed cells, on which the two-point formula gives wrong results; the price is more connections in each row of the system of equations.
Gas enters the tubing of a gas lift well through several valves at once — usually because the gas pressure in the annulus has risen above the opening pressures of the lower unloading valves (the operating valve is plugged or more gas is supplied than designed) or because the valves are set without the necessary margins. The gas lightens the column less effectively than when it is injected through the operating valve, and the flow rate falls.
A forecast of solution gas drive in differential form: an equation for the change of average oil saturation with pressure, obtained from the material balance and the instantaneous gas–oil ratio; it is integrated numerically downwards from the bubble point pressure.
Under Kazakhstan's Subsoil Code, a joint-stock company created by decision of the Government in which the controlling stake belongs to the state or the national managing holding. Only such a company can obtain hydrocarbon rights without an auction, by direct negotiations; for large fields its share in such a contract is at least 50%.
The combination of reservoir pressure, water cut and other conditions at which the inflow and outflow curves of a well cease to intersect and it can no longer produce without a pump or gas lift. It is found by calculation and compared with the forecast so that artificial lift can be prepared in advance.
A formation in which natural open fractures noticeably affect flow and production. By the role of the fractures, reservoirs are distinguished in which the fractures provide both storage and permeability; in which they provide permeability while the storage is in the matrix; in which they merely enhance permeability; and in which they create only anisotropy.
A reservoir mixture whose reservoir temperature is close to its critical temperature: a small change in pressure, temperature or composition turns a dew point into a bubble point, the densities and compositions of the phases are close, and the interfacial tension is low. It requires experiments in a cell with a window and compositional calculations.
Calculation of the well index of a coarse cell, and where necessary of the transmissibilities around it, by solving the inflow problem on a fine grid: the index is chosen so that the coarse model gives the same rate at the same pressures. It is needed because the flow near a well is radial, whereas the equivalent permeability of the cell is calculated for linear flow.
The part of the formation around the wellbore, usually extending a few metres, whose properties have been changed by drilling, completion, production or treatment. Because the flow converges, its condition strongly affects the production rate.
A sum-of-the-parts valuation of the equity of an exploration and production company: the value of each asset from its risk-adjusted cash flow, minus the present value of general expenses and net debt.
The sum of all inflows and outflows of a project over a period: revenue minus capital and operating expenditure and taxes.
The difference between the pressure with which a core sample is squeezed in the core holder and the pressure in its pores. It is set equal to the mean effective stress in the reservoir so that porosity and permeability are measured at reservoir conditions.
The total thickness of the intervals that pass the cut-offs for shale volume, porosity and water saturation: reservoir rock containing hydrocarbons above the contact. Unlike net reservoir (net sand) thickness, it does not include water-bearing reservoir; of all the interpretation results it depends most on the choice of the saturation model and of the cut-offs.
The sum of the discounted cash flows of a project; a positive value means that the project creates value at the chosen rate.
The excess of the fluid pressure in a fracture over the closure pressure. It creates the fracture width, which is proportional to the fracture height and inversely proportional to the elastic modulus of the rock; how it changes during pumping shows whether the fracture is growing in length or breaking through in height.
The ratio of net thickness — the sum of the layers classed as reservoir by cut-off values of shale volume and porosity (sometimes permeability) — to the gross thickness of the formation. Sometimes the net oil- and gas-saturated thickness (net pay) is used in the numerator, so the cut-offs applied are stated together with the NTG.
The price of the product at the field: the market price less the cost of transport to market and a quality discount. It shows how much the project receives per barrel and can be compared directly with the cost per barrel.
The cross-section of the drill string at which the effective axial force is zero: below it the pipes are compressed by the weight on bit, above it they are in tension from the weight. Its height above the bit is the weight on bit divided by the weight per metre of the lower pipes in the drilling fluid, measured along the wellbore; in a vertical hole it is kept within the drill collars.
An active nuclear method: fast neutrons from the source are slowed down in the rock, mainly by hydrogen nuclei, and the detectors count thermal or epithermal neutrons (neutron–neutron tools) or the capture gamma rays (neutron–gamma tools). The reading reflects the hydrogen index of the rock and is expressed as neutron porosity.
Conversion from a reading in limestone units to the porosity of sandstone or dolomite using the response characteristic of the tool. The conversion is non-linear and depends on the tool type: for compensated thermal-neutron tools the sandstone value is 2–5 porosity units greater than the reading (2–3 near zero, 4–5 at medium porosity), and the dolomite value is 2–8 units smaller.
The neutron log reading in porosity units, usually limestone units: the porosity of a clean water-filled limestone that would give the same reading. In sandstone it is lower than the true porosity, in dolomite higher; shales raise the reading and gas lowers it.
The pattern, on compatible scales, in which the density porosity is greater than the neutron porosity: residual gas in the flushed zone lowers both the bulk density and the hydrogen index. It is the main indicator of gas, but rock salt, and sandstone on limestone scales, also produce it, and shales can mask it.
An iterative method for solving nonlinear equations: at each iteration the residuals are replaced by their linear part and a linear system with the Jacobian matrix is solved for the correction to the unknowns. Near the solution it converges quadratically (the number of correct digits roughly doubles at each iteration); from a distant initial guess it may fail to converge, so the corrections are limited.
A boundary of the drainage area across which fluid does not flow: a sealing fault, a pinch-out of the formation, or a flow divide between neighbouring wells. In the pseudo-steady-state regime the divides behave like genuine no-flow boundaries, and each well drains a pore volume proportional to its rate.
A calculation of the performance of a production system by matching the inflow and outflow curves at a chosen point — the node.
The annual rate quoted without regard to the frequency of compounding: the rate per compounding period multiplied by the number of periods in a year. When interest is compounded more often than once a year, the effective rate is higher than the nominal rate.
A chart of graduated scales on which the answer is given by a straight line drawn through the marks of the known quantities. A nomogram with parallel scales solves equations of the form , for example Archie's equation in logarithms.
A cash flow whose sign changes more than once, for example because of large late costs or accelerated production. Such a flow may have several IRRs or none, so the decision is made on NPV.
The inertial component of skin per unit flow rate: the skin measured at a single rate equals . It is found from tests at several rates; it increases with damage to the near-wellbore zone, with penetration of only part of the thickness and with the constriction of flow towards the perforations.
Flow in a porous medium at high velocities, when an inertial resistance proportional to the square of the velocity is added to the viscous resistance of Darcy's law. It is noticeable primarily in gas wells; at a well it is concentrated in the first few metres around the wellbore.
A drill collar made of a non-magnetic alloy, inside which magnetic directional sensors are placed to keep them away from the magnetised steel of the string, the motor and the bit. The required length of the non-magnetic section grows with inclination, with the departure of the wellbore from the north–south direction and with the decrease in the horizontal component of the magnetic field.
A connection between grid cells that are not neighbours by their indices : across a fault with displaced layers, across a pinched-out layer, between the main grid and a nested fine grid. Its transmissibility is calculated from the area of overlap of the cells, as for an ordinary connection between two half-cells.
Time on the well that was not in the plan and did not advance construction: equipment repairs, dealing with problems and accidents, standby waiting for materials, transport, weather or a decision. The term hidden losses (hidden NPT, invisible lost time) is used for the slow execution of productive operations compared with the best performance or the technical limit.
Wellbore storage with a coefficient that changes during the pressure record — because of free gas in the wellbore, redistribution of gas and liquid, or movement of the liquid level. The early unit slope gives only the initial, apparent coefficient; the record is described by empirical models with a redistribution pressure, and the effect is removed by downhole shut-in.
The smooth change in the properties of shales with depth at normal pore pressure: density increases, while porosity and interval transit time decrease. A deviation from the trend towards lower density and longer interval transit time is a sign of undercompaction and abnormally high pore pressure.
A pressure of 101.325 kPa and a temperature of 0 °C, adopted in physics and chemistry; the same gas in “normal cubic metres” occupies a volume 6.8% smaller than in standard cubic metres at 20 °C. Not to be confused with the standard conditions used for gas accounting.
A symmetric bell-shaped distribution defined by the mean and the standard deviation : 68% of the values lie within and 95% within . It arises when many independent influences add up; measurement errors, the porosity of a single rock type and the means of large samples are close to it.
A fault along which the hanging wall has moved down relative to the footwall along the dip of the fault plane; it forms in extension. In a vertical well a normal fault usually “cuts out” part of the section.
A correction that removes the dependence of the reflection arrival time on offset: the time at offset is reduced by , where the traveltime curve is a hyperbola. The velocity at which the traveltime curve is flattened best is the stacking velocity (CMP velocity); it is picked by velocity analysis, and for flat layers it is close to the root-mean-square velocity of the overburden.
The NPV at the company's rate divided by the maximum discounted cash outlay: the value created per unit of money “frozen” in the project at its worst moment. For projects with the investment at the start it is close to the profitability index minus one.
A graph of the NPV of a project against the discount rate; at a zero rate it equals the undiscounted total of the project, and it crosses the rate axis at the IRR.
A logging method that polarises the hydrogen nuclei of the fluids in the pores with a static magnetic field and records their signal, a train of spin echoes, after radio-frequency pulses. The initial amplitude of the signal is proportional to the fluid-filled porosity, the rate of decay reflects the pore sizes and the fluid properties; the matrix minerals have almost no effect on the signal.
The jump of the variogram at the origin: variability that is uncorrelated at the scale of the data spacing, that is, measurement errors and heterogeneity at distances smaller than the spacing. The larger it is, the less the neighbouring data help the estimate between them.
The smearing of the displacement front in a model owing to the finite size of the cells: the displacing agent reaches the producing well earlier than in reality. For a tracer or a miscible agent the smearing decreases with grid refinement only as the square root of the cell size.
The property of a scheme whereby errors (in the initial data, rounding errors, errors in solving the step equations) are not amplified as the calculation proceeds. For linear problems, approximation and stability together are equivalent to convergence: the solution of the scheme tends to the exact one as the grid and the time step are refined (the Lax equivalence theorem).
An admixture of oil-based mud filtrate in a sample of oil or gas condensate. The filtrate dissolves in the oil and lowers the gas–oil ratio and the saturation pressure; its fraction is monitored during pump-out with a formation tester and determined in the laboratory, and the properties are recalculated to the clean fluid.
A well in which measurements are used to monitor the state of the reservoir: pressure, contact positions, fluid composition. A new well drilled into a producing accumulation is also an observation well for the duration of an open-hole formation-tester survey: the formations are not in communication through a mud-filled borehole, and the measurements show the pressure of each of them.
The deepest level down to which oil has been encountered in wells, and the shallowest level from which water has been encountered. As long as no well has crossed the contact, it lies between them (or between the “oil down to” level and the spill point); its position within this range is refined by pressure measurements, and in reserves estimation a conventional oil–water contact (OWC) is adopted.
The concentration of oil in water, usually in mg/L, is the main indicator of the cleaning of produced water and water for injection. Gravimetry, infrared spectrometry, fluorescence and gas chromatography measure different things, so a limit is always specified together with the measurement method.
The oil-saturated part of an accumulation sandwiched between the gas cap and the water. Wells in an oil rim are threatened by two cones at once, so the production rate and the position of the interval or horizontal wellbore are chosen with both contacts in mind.
The relative increase in the volume of oil when heated by 1 °C at constant pressure, about 1/°C. It is needed to refer densities and volumes to standard temperature; trapped oil gains a pressure of per degree of heating.
The maturity interval in which a source rock generates the bulk of its oil: roughly 0.6–1.3%, most often at 100–150 °C. Its temperatures depend on the kerogen type and the heating rate; at higher maturity wet gas and then dry gas are formed.
The fraction of the hydrocarbon base in the liquid phase of an oil-based drilling fluid, from the retort volumes, ; it is written as a pair of numbers, for example 80/20. The higher it is, the more stable the emulsion and the lower the viscosity, but the more expensive the fluid.
A contractor that carries out work and supplies equipment for exploration and production (drilling, logging, cementing, hydraulic fracturing, well workovers) but does not produce oil itself. Its revenue follows the investment of subsoil users.
The system of units of American oil practice: feet, barrels, pounds-force per square inch (psi), centipoise, millidarcies, degrees Fahrenheit. Formulae in it contain conversion constants, for example in Darcy's law and 141.2 in the well inflow equation.
Portland cement of a special, stable composition for cementing wells; it is mixed with water and additives to make a cement slurry, which hardens into set cement. The main cements are classes G and H under API Spec 10A, and in the CIS standards types I-G and I-H under GOST 1581.
The pressure read from the semilog straight line, or its extrapolation, at 1 h after the well is started or shut in. It fixes the position of the line and enters the skin factor formula; because of wellbore storage it may differ from the actual measurement at one hour.
The OPEC countries together with other oil producers that signed the “Declaration of Cooperation” on 10 December 2016, among them Russia and Kazakhstan. The participants agree on oil production levels expressed in barrels per day.
A bottom-hole design in which the production casing is run to the top of the formation and cemented, and the productive interval is left uncased. It is cheap and does not impede inflow, but it does not allow individual intervals to be controlled and is suitable only for stable rocks.
Expenditure on keeping existing facilities running: personnel, energy, chemicals, routine repairs. It normally reduces taxable profit in the same year.
The intersection of the inflow and outflow curves at the node: the flow rate and pressure at which the reservoir delivers as much as the pipes and surface equipment can pass.
The return on the best available alternative that is given up by choosing this project. Discounting at that rate shows at once whether the project beats the alternative.
The pressure of the first separation stage (with several separators, a set of pressures) at which the most stock-tank oil is obtained from the reservoir oil and the solution gas–oil ratio and formation volume factor are lowest. It exists because at low pressure the intermediate components leave with the separator gas, and at high pressure with the stock-tank gas. The minimum is flat and shifts to higher pressures as the separation temperature rises.
An organisation founded in Baghdad in September 1960 by Iran, Iraq, Kuwait, Saudi Arabia and Venezuela after the international companies cut the posted prices of oil. The member countries agree production levels; as of October 2026 there are 11 of them, the United Arab Emirates having left the organisation with effect from 1 May 2026. Kazakhstan is not a member of OPEC but takes part in the OPEC+ agreements.
Bentonite or hectorite whose exchangeable cations have been replaced by large organic cations (amines): such a clay does not swell in water but swells in oil, and provides the viscosity and gel of an oil-based drilling fluid.
The lines on a map where the fluid contact intersects the top (outer outline) and the base (inner outline) of the bed. Inside the inner outline, oil occupies the full thickness of the bed; between the outlines lies the edge-water zone.
The pressure at the node required to pass a given flow rate through all the elements of the system downstream of the node to the separator. For a node at the bottom of the well, it is the bottom-hole pressure needed to lift the fluid up the tubing.
The excess of the wellbore pressure over the formation pore pressure at the same depth. If the wellbore pressure is lower than the pore pressure (underbalance), the formation may flow into the well.
The pressure created by the weight of the overlying rocks and the fluids filling them.
The stage of an acid treatment after the mud acid: an ammonium chloride solution, weak hydrochloric acid or a hydrocarbon liquid pushes the spent acid 1–1.5 m away from the wellbore, where secondary precipitates from it have little effect on inflow.
Pore pressure higher than the normal hydrostatic pressure for the given depth. It is retained by a low-permeability seal; it most often arises when water has not had time to escape from rapidly subsiding clays (undercompaction), and also from hydrocarbon generation, tectonic compression and the transfer of pressure from deep horizons.
A reagent that chemically binds the oxygen remaining in water after deaeration — most often a bisulfite: according to the reaction equation, about 6.5 mg of sodium bisulfite per 1 mg of oxygen. It is converted into sulfate and reacts with chlorine, so it is dosed after the deaerator and separately from chlorination.
The dependence of the ratio of average reservoir pressure to the gas deviation factor on cumulative production. For a closed gas accumulation it is a straight line: it cuts off the gas in place on the production axis. With water influx the points lie above the line, and the gas in place estimated from it is overstated.
The primary seal, in the hanger, closes off the annulus below it; the secondary seal, in the lower flange of the next spool, surrounds the cut-off end of the string and isolates the well pressure from the lower flange connection. Because of it, the upper flange of the spool may have a higher pressure rating than the lower one, usually by one step.
A device with a rubber sealing element on the tubing string that, when set, closes off the annulus between the tubing and the production casing. It isolates the casing from the pressure and fluids of the formation and makes the flow up the tubing stable.
The portion of fracturing fluid without proppant pumped at the start of the treatment. It creates a fracture length and width sufficient for the proppant grains and takes up the leak-off at the fracture tip; its share is calculated from the fluid efficiency.
A tool whose sensors are mounted in a pad pressed against the borehole wall by an arm, so that there is as little mud as possible between them and the rock; the arm also serves as a caliper. The density tool, the microresistivity tools and borehole imagers are built this way; they are sensitive to mudcake and to wall roughness and lose contact in washouts.
The direction of the current that deposited a sediment, reconstructed from cross-bedding, sole marks and the imbrication of pebbles. The azimuths of individual measurements are averaged as vectors; the palaeocurrent is used to judge where the material came from and how the sand bodies are elongated.
A former position of the oil–water or gas–water contact, preserved as the lower limit of residual oil or gas saturation below the present free water level: after the water rose, the structure tilted or blocks moved, the hydrocarbons were left in the pores as isolated droplets. Logs see hydrocarbons in this interval, while pressure measurements see the water line: the hydrocarbons are immobile.
A constant of a substance relating surface tension to the difference between the molar densities of liquid and vapour: . For mixtures the parachors of the components are added according to the mole fractions in each phase; this is how the interfacial tension between gas and oil is calculated.
The order in which compaction, cementation, dissolution and replacement took place in a rock, reconstructed from the relationships between minerals in thin sections and under the electron microscope; it is tied to time and temperature using fluid inclusions, isotopes and the age of authigenic illite.
All of the standpipe pressure except the pressure drop across the bit: friction in the surface equipment, the drill string and the annulus, and the pressure drops across the downhole motor and the MWD tool. It rises with the flow rate approximately as ; the exponent is found from standpipe pressure measurements at two or three flow rates, after subtracting the pressure drop across the nozzles.
Communication of the well with the formation over only part of its thickness. The flow converges on the open interval and a positive pseudo-skin arises, which is larger the lower the degree of penetration and the vertical permeability.
A fault across which fluid flows with additional resistance: the fault zone is thin and poorly permeable. In a well test the pressure derivative rises, does not reach the doubled level of a sealing fault and returns to the radial flow plateau.
The edge of a continent within a lithospheric plate, formed when a continent broke up and an ocean opened. The stretched lithosphere of the margin cools and subsides for tens of millions of years, and wedges of sediments 10 km or more thick accumulate on it: deltas, submarine fans, carbonate platforms and sometimes salt. An example is the margins of the Atlantic.
The time in which the accumulated income from a project or intervention repays the costs invested in it. It is a simple and clear criterion, but it ignores receipts after payback and, without discounting, the time value of money.
A bit with fixed cutters of polycrystalline diamond on a tungsten carbide substrate, mounted on the blades of a steel or matrix body. The cutters shear the rock; there are no moving parts, so the runs are long, but the cutters are sensitive to impact, vibration and overheating.
The distance from the well at which the pressure of steady-state radial flow equals the calculated pressure of the well block; for a square cell it is about .
An unstructured grid of polygons whose faces are the perpendicular bisectors of the segments between neighbouring computational points. It is easily refined towards wells and follows faults and reservoir boundaries.
The ratio of the length of a hydraulic fracture (both wings, ) to the side of the drainage area parallel to the fracture. At small values the fracture can be replaced by an effective wellbore radius; at large values a calculation for a bounded area is needed. Not to be confused with the degree of penetration of the formation across its thickness.
The value below which a given fraction of the data lies; the 50th percentile is the median. The notations P10 and P90 are read in two ways: as a cumulative fraction (then P90 is a large value) or, in reserves estimation, as a probability of exceedance (then P90 is a small, conservative value). The convention should always be stated.
Water trapped beneath an accumulation in a closed volume that does not communicate with the main aquifer. Its free water level lies above the general level of the accumulation (usually at the depth of the lowest point through which the block communicated with its neighbour), and its pressure is higher than that of the main aquifer at the same depth.
The fraction of conducting elements (for example, sand bodies in the volume of a reservoir) at which a connected cluster of them first extends across the whole system. For identical, randomly placed, overlapping bodies it is about 0.28–0.29 in three dimensions and about 0.67–0.68 in two; near the threshold connectivity changes abruptly.
A device in which charges are run into the well and fired at the required depth and in the required directions. A carrier gun holds the charges in a sealed tube and takes the debris out with it; an expendable gun accommodates larger charges but leaves debris in the well; a through-tubing gun passes through the tubing.
The depth of the tunnel punched by a charge. In the manufacturer's data sheet it is given from firing into a concrete target at the surface, and it includes the pipe wall and the cement; in the formation the tunnel is shorter because of the rock strength, the stresses, the casing thickness and the clearance. For the skin calculation, the length of the tunnel in the rock beyond the wellbore wall is used.
The angle between the directions of adjacent charges of a perforating gun, seen in plan: 0° means all the tunnels point one way, 180° two opposite ways, and 60–120° a spiral. The phasing determines how far the flow to the tunnels has to go around the casing and how far apart vertically the tunnels pointing in the same direction are.
The additional resistance to inflow into a perforated well compared with an undamaged open hole of the same radius: the flow converges on individual tunnels in the horizontal plane and vertically, and flows around the casing. It depends on the length, density and phasing of the tunnels and on the anisotropy of the formation; with dense, deep perforations it can be negative.
The ability of a rock to transmit fluid under a pressure difference; it is measured in darcies, millidarcies or square metres.
The dependence of permeability on direction. In layered formations the vertical permeability is usually several times, or even an order of magnitude, lower than the horizontal permeability ; the pseudo-skin of partial penetration, the gain from wellbore deviation and coning depend on their ratio.
The dependence of the measured permeability on the volume of rock that the measurement covers: a core plug, a formation-tester pretest and a well test average the properties over volumes that differ by many orders of magnitude. More often permeability increases with volume, because fractures and continuous high-permeability layers fall within the larger volume, but discontinuous permeable bodies produce the opposite effect.
The matrix relating the vector of Darcy velocity to the pressure gradient in Darcy's law. In axes that coincide with the principal directions it is diagonal (, , ); off-diagonal components appear when layers or conducting bodies lie obliquely to the axes, and then a pressure difference along one axis causes flow along another axis as well.
An infinite series of equal payments; at rate it is worth today. A series growing at rate is worth , but only when ; when is close to , this estimate is unstable.
An international system for classifying oil and gas reserves and resources, adopted in 2007 by the SPE together with the WPC, AAPG and SPEE and updated in 2018. It divides recoverable quantities into classes by the chance of commerciality (reserves, contingent resources and prospective resources) and into categories by uncertainty: 1P, 2P, 3P, 1C, 2C, 3C, 1U, 2U, 3U.
The combination of a source rock, migration pathways, a reservoir, a seal and a trap, formed in the right sequence.
The boundary of the two-phase region of a mixture on the pressure–temperature diagram: the bubble point and dew point lines, which meet at the critical point of the mixture. Inside the envelope the mixture separates into gas and liquid.
The pressure of a phase referred to zero depth, allowing for the weight of its column: , where the depth is measured downwards. A phase flows from higher potential to lower; in a reservoir at rest the potential of each phase is the same at all heights.
A check of whether a mixture at a given pressure and temperature can form a new phase with a decrease in Gibbs energy. A trial phase is searched for numerically, with the same component fugacities as the mixture; if its total amount is greater than one, the mixture is unstable and splits into two phases.
A logarithmic scale of grain sizes: , where is the diameter in millimetres. Each class of the Udden–Wentworth scale occupies exactly one φ unit; the larger the φ, the finer the grain (sand spans −1 to 4 φ). In these units the size distribution is close to normal and can be described by ordinary statistics.
The cross section for photoelectric absorption of gamma rays per electron, in barns per electron; for an element it is approximately . An indicator of mineral composition: quartz 1.8, dolomite 3.1, calcite 5.1; it is almost independent of porosity and fluid, but is unusable when there is barite in the mud.
A crossplot of resistivity against porosity for the beds of one interval on logarithmic axes. Water-bearing beds fall on a straight line whose slope is set by the cementation exponent and whose position is set by the quantity ; lines of equal water saturation are parallel to it and shifted by a factor of along the resistivity axis.
Idealised displacement in which only residual oil is left behind the front and, ahead of it, oil with immobile water: all the mobile oil is displaced before water breaks through. Displacement at a mobility ratio much less than one comes close to it.
The slope of the Bingham line for a drilling fluid; from rotational viscometer readings, , in mPa·s. It rises, above all, with the solids content.
A settling vessel with a pack of inclined parallel plates: each plate works as the bottom of a shallow settling tank, and the settling area equals the sum of the horizontal projections of the plates. For this reason, in a volume of a few cubic metres it retains oil droplets of a few tens of micrometres and larger — as a tank of hundreds of cubic metres does.
The theory that the lithosphere is divided into seven or eight large and many small rigid plates that move over the asthenosphere at rates from 1–2 to 15 cm per year. Oceanic crust is born at divergent boundaries; at convergent boundaries one plate descends beneath another or continents collide; at transform boundaries plates slide past each other. Earthquakes, volcanoes and mountain building are concentrated along these boundaries.
An exploration model: a group of known and postulated accumulations and traps at one stratigraphic level with a common reservoir, seal, hydrocarbon source and type of trap. A similar concept in domestic (Russian-language) practice is the oil-and-gas-bearing complex.
The final withdrawal of a well from service: the equipment is recovered, the fluid-bearing formations, the perforated intervals and the casing shoes are sealed with cement plugs, the plugs are tested, and the wellhead is equipped. Unlike suspension — a temporary shutdown — the isolation must last indefinitely. In Kazakhstan, the requirements for the plugs are set by the Rules for conservation and liquidation in the exploration and production of hydrocarbons.
A method of lifting liquid using the energy of the gas of the well itself: a free plunger, driven by the gas accumulated while the well is shut in, carries a slug of liquid to the surface. It is used in gas wells in which liquid accumulates and in low-rate oil wells with a high gas–oil ratio.
All the components of the reservoir fluid heavier than hexane (or another chosen cut-off), lumped into a single component. It is characterised by its measured molar mass and specific gravity, and for equation-of-state calculations it is split into pseudo-components.
A description of the C7+ fraction for equation-of-state calculations: splitting it into pseudo-components and calculating their critical temperature and pressure, acentric factor and interaction coefficients from molar mass and density with the help of correlations. The properties are then refined by tuning to laboratory experiments.
The greatest depth at which gas, for a given annulus wellhead pressure and gas rate, can still enter the tubing under operating conditions: the gas pressure there equals the tubing pressure plus the pressure drop across the operating valve. With more gas or a higher pressure the point of balance is deeper; the operating valve is set a little above it, with a pressure margin.
The depth at which the injected gas enters the tubing of a gas lift well — usually the depth of the operating valve. It is limited by the gas pressure: at the valve it must exceed the pressure in the tubing by the pressure drop across the valve. The deeper the injection, the lower the bottom-hole pressure for the same gas rate.
The ratio of the lateral expansion of an elastic body to its axial compression under uniaxial loading. For reservoir rocks it is usually 0.15–0.35, and for clays it is higher than for clean sandstones; it sets the share of the vertical load that a laterally confined layer transmits sideways, . The dynamic value is determined from the compressional and shear wave velocities, the static value from core tests.
The top member of the rod string, with a smooth surface: it moves through the stuffing box at the wellhead and connects the rods to the wire-rope hanger of the pumping unit. The load and stroke are measured on it when a dynamometer card is recorded.
A model in which the pore space is represented by pore bodies as nodes and the throats, treated as tubes, that connect them. Single-phase flow in it is a system of flow-balance equations in the pores, and displacement is the filling of pores and throats in the order of their capillary thresholds; it is used to calculate permeability, capillary pressure and relative permeabilities.
The pressure of the fluid in the pores of the rock. Normal pore pressure equals the pressure of a column of formation water from the surface; above normal it is overpressure, below normal subnormal pressure.
The relative change in pore volume per unit change in pore pressure at constant overburden pressure. For cemented rocks it is usually MPa⁻¹; for weak and loose rocks it is several times greater. It enters the total compressibility and the material balance.
The cumulative injection of the displacing agent, converted to reservoir conditions (for water, ) and divided by the pore volume of the reservoir element or sample. In these units the solution of the displacement problem does not depend on the size of the element or the injection rate.
The volume of acid injected into a carbonate sample until the wormhole breaks through at the other end, divided by the pore volume of the sample. Its dependence on the velocity in the pores — the acid efficiency curve — has a minimum at the optimum injection rate.
The fraction of the rock volume occupied by pores. It determines how much fluid the rock can hold.
A check of the resistivity ratio method: from the saturation obtained from the ratio, and Archie's equation, the formation resistivity factor and the porosity are calculated, and this porosity is compared with an independent one from porosity tools or core. A discrepancy points to a wrong invasion profile, an annulus, or an unsuitable closing relationship.
A downhole motor in which drilling fluid, forced through the cavities between a steel helical rotor and a rubber-lined stator, turns the rotor, and through it the bit. The flow rate sets the rotational speed and the pressure drop across the motor sets the torque, so the load can be read from the standpipe pressure.
A model in which the pressure throughout the aquifer at every moment is equal to the pressure at the contact with the accumulation: the influx depends only on the pressure decline. It suits small, highly permeable aquifers.
The surface formed by the elevations of the levels to which water would rise in wells penetrating the formation; the height of this level above a point in the formation is the hydraulic head. The slope of the surface sets the direction and strength of the flow of formation water; the difference in head between two wells is the difference in their reduced water pressures divided by the water gradient.
The temperature at which oil, cooled by a standard procedure, ceases to flow; the closely related pour point is the lowest temperature at which it still flows. It is usually considerably below the wax appearance temperature; it matters for pipeline shutdowns and for the storage of high-wax oils.
Oil or water that a power pump at the surface injects into the well at high pressure to drive a hydraulic piston, jet or hydraulic turbine pump. In an open system the spent power fluid returns to the surface with the production, in a closed system it returns through a separate channel.
The rheological model with consistency index and flow behaviour index ; for the fluid is shear-thinning. Adding a yield stress gives the Herschel–Bulkley model , which describes most drilling fluids over the whole range of shear rates.
An easily invertible approximation of the system matrix, , applied at each iteration of a Krylov method to reduce the number of iterations: incomplete LU decomposition (ILU), nested factorisation, two-stage CPR with a separate solution of the pressure equation, algebraic multigrid methods (AMG).
The first stage of a sandstone acid treatment: 5–15% hydrochloric acid or an organic acid dissolves the carbonates in the zone that the mud acid will reach later, and pushes back the formation water with its sodium, potassium and calcium. Without it, calcium fluoride and fluorosilicates precipitate from the hydrofluoric acid.
The present-day equivalent of a future amount: the amount that, if invested at the discount rate, will grow to that future amount by the due date.
The rise in bottomhole pressure after a producing well is shut in; it is used to determine permeability, skin factor and reservoir pressure.
The derivative of the change in bottomhole pressure with respect to the natural logarithm of time, — the slope of the semilog plot at each point, in pressure units. It is plotted together with on the diagnostic plot: a horizontal section corresponds to radial flow, a slope of 1 to wellbore storage or a closed area, ½ to linear flow, and −½ to spherical flow.
The time in which the pressure difference between two communicating volumes decreases by a factor of : , where are the elastic capacities and is the transmissibility of the connection. If it is much longer than the development period, the volumes behave separately, although their pressures equalised over geological time.
The flow equation in terms of pressure, obtained from mass balance, Darcy's law and the compressibility of the fluid and the pores. In a reservoir simulator it is written for each cell: the change in the amount of fluid stored equals the flows from neighbouring cells minus withdrawal by a well.
The increase in pressure per unit of true vertical depth, kPa/m or psi/ft. In a static, continuous phase it equals : about 10–11 kPa/m in water, 6–8.5 in oil and 1–3 in gas. The pressure lines of different phases intersect at the contacts.
Replacement of withdrawals by injection of water or gas so that reservoir pressure does not decline — usually so that it remains above the bubble point pressure. “Pressure maintenance” refers both to the injection itself and to its system: water intake, water treatment, pumping stations, water lines and injection wells.
Dissolution of grains at stressed contacts and along stylolites, with the dissolved material reprecipitated nearby as cement. It is accelerated by clay particles and mica at the contacts; in quartz sandstones it is the main source of silica for quartz cement, and in carbonates it begins at shallower depths.
The dependence of pressure on depth in a flowing well for a given rate, gas–liquid ratio, water cut and tubing diameter. A family of such curves is used to construct the outflow performance curve.
The pressure that a well would show after shut-in had it continued to produce: it is made up of its own production, the production of neighbours and the influence of boundaries. In a block that has reached the pseudo-steady-state regime at constant rates, it is a straight line with slope , where .
The withdrawal of a few cubic centimetres of fluid from the formation by the piston of a formation tester, with recording of the drawdown and pressure build-up. The stabilised build-up pressure is the formation pressure at the measurement depth; the steady drawdown is a measure of the mobility of the fluid near the wall.
The ratio of the relative change in demand or supply to the relative change in price: . For oil demand over a period of months it is small in absolute value, so a small loss of supply raises the price sharply; over years the response is several times stronger.
The cost of a fixed basket of goods and services relative to a base period, which is taken as 100. The consumer price index (CPI) is the main measure of inflation; for industry costs, producer price indices and industry-specific indices are also used.
A measure of how much a price varies: the standard deviation of the logarithmic returns over a period, annualised by multiplying by the square root of the number of periods in a year. For oil it is about 20–30% a year in calm years and several times higher in crises.
Keeping the formations contained by the pressure of the drilling fluid column, which exceeds the pore pressure. If primary control is lost and an influx begins, secondary control comes into play — sealing the wellhead with blowout prevention equipment.
A graph of the ordered values against the quantiles of the normal distribution. If the data are normally distributed (or, on a logarithmic axis, log-normally distributed), the points fall on a straight line. It is used to check the form of the distribution, to find the median and the spread, and to reveal mixtures of rock types and outliers.
Water produced by wells together with oil and gas: formation water and, after breakthrough, also injected water. It is separated from the oil, cleaned of oil droplets, suspended solids and gases, and injected into a formation or disposed of; under Kazakhstan law it is classed as wastewater. At a field with a high water cut, several times as much of it is produced as oil.
The ratio of the volume of gas produced to the volume of stock-tank oil produced, both at standard conditions. Above the bubble point pressure it equals the solution gas–oil ratio; below it, it includes free gas from the reservoir and can be several times larger.
Distortion of the pressure build-up curve because the producing time of the well before shut-in is limited: the pressure rise lags behind the pressure decline after start-up and cannot exceed , the slope against the logarithm of shut-in time is reduced by a factor of , and the derivative falls, resembling a constant-pressure boundary. It is accounted for by the Horner function, the equivalent time or superposition of the whole rate history.
A property of the solutions for rectangular boundary systems: the pressure derivative of the well, normalised to the radial flow plateau, equals the product of factors for two perpendicular directions — “open”, “one boundary”, “two boundaries”. The corner, the channel, the closed-end channel and the closed rectangle are built up from such factors.
Determination of the share of each formation in the production of a commingled well — from production logging, sequential measurements, the inflow performance curves of the formations, or the composition of the oil. It is needed for accounting of production and reserves, for material balance and for managing development unit by unit.
Surveys of operating production and injection wells with tools run into the wellbore (flowmeters, thermometers, fluid-composition and pressure sensors) to determine the inflow and injection profiles by reservoir and interval. Together with pressure measurements in the zones they give the distribution of withdrawals, without which the transmissibility of the barriers between the zones cannot be assessed.
The period of highest, roughly constant production, which is limited not by the reservoir but by the capacity of the facilities and the number of wells. The height and duration of the plateau are chosen at the design stage by comparing options on economic indicators.
The ratio , where is the ratio of the seat area to the bellows area of the gas lift valve: this is by how much the gas pressure that opens the valve falls when the tubing pressure at the valve rises by one unit. For valves with a small seat it is a few hundredths.
Oil and gas production by year over the life of a field: build-up, plateau, decline and shutdown at the economic limit. It is the first row of a project's cash flow model.
A contract under which the production, after the investor's costs have been recovered, is shared between the state and the investor.
A gas lift valve in which the tubing pressure acts on the bellows and the gas pressure acts only on the seat area: it opens when the pressure in the tubing at the valve rises. It is installed in a side-pocket mandrel with a different flow-path arrangement from that for an injection-pressure-operated valve.
The ratio of the productivity index of a horizontal (or otherwise configured) well to the productivity of a vertical well in the same formation and with the same drainage area.
The ratio of the well's flow rate to the drawdown on the formation.
In a production sharing agreement, production less cost oil; it is divided between the state and the contractor in agreed shares, which often depend on the production volume or the project's return.
A project's terminal cash surplus divided by its maximum cash outlay: how much net surplus one unit of investment brings. It ignores the timing of the money; its discounted counterpart is the NPV index.
The present value of the project's cash flow excluding the capital investment, divided by the present value of the investment: . It is greater than one when NPV is positive; with a limited budget, projects are selected in descending order of the index.
A positive-displacement pump consisting of a metal helical rotor turning in an elastomer stator; the cavities between them move the fluid. It is driven by rotating rods from the surface or by a submersible motor. It lifts viscous oil and fluid containing sand better than other pumps; it is limited by temperature and the resistance of the elastomer.
A loan secured on the cash flow of a stand-alone project, in which the claims of the lenders are limited to the assets and revenues of the project. Part of the project risk passes to the lenders, so they check the reserves, the model and the cost estimate themselves and charge a higher rate.
Solid particles — sand or ceramic — that hold a hydraulic fracture open after the pressure is released.
A dimensionless measure of the amount of proppant in the formation: twice the product of the pack permeability and the pack volume within the pay zone, divided by the product of the reservoir permeability and the volume of the drainage area; it equals . It gives the optimum dimensionless fracture conductivity and the maximum productivity of the well.
A mapped trap for which the possible volumes of hydrocarbons and the geological chance of success have been estimated; it is ready for drilling an exploration well.
Under PRMS, the quantities that could be recovered from accumulations not yet discovered; they are estimated together with the chances of geological discovery and of development, and the categories are 1U, 2U and 3U. In the Kazakhstan classification the term “prospective” is applied to resources of category C3, those of areas prepared for exploration drilling.
The component of an electric submersible pump installation between the motor and the pump: it transmits the rotation, separates the motor oil from the formation fluid, compensates for the thermal expansion of the oil, equalises the pressure inside the motor with the well pressure and takes up the axial thrust of the pump.
Under PRMS, reserves that will be commercially recovered with reasonable certainty; in a probabilistic estimate, the quantity actually recovered will reach them with a probability of at least 90%. In the Kazakhstan classification the term “proved” is applied to reserves of categories A, B and C1, which is a measure of how thoroughly they have been studied, not of probability.
The capillary pressure of a coarse cell as a function of its average saturation: in the capillary equilibrium limit it is the common pressure of the cell, in dynamic methods the difference between the averaged oil and water pressures. With different averaging weights for the phases it can be non-zero even without capillary forces on the fine grid.
A relative permeability averaged, together with saturation, over the thickness of a section or over a large model cell: a property of a notional homogeneous reservoir that replaces a heterogeneous section and reproduces its behaviour. For a layered section under vertical equilibrium it is obtained by adding the flow capacities of the flooded and unflooded layers.
A notional substance with average properties that replaces a group of similar components or fractions in calculations; it makes it possible to describe a reservoir fluid with 8–15 components instead of hundreds.
Notional critical temperature and pressure of a gas mixture: averages of the critical parameters of the components weighted by mole fractions (Kay's rule), or estimated from the specific gravity of the gas. They are used to find the pseudo-reduced parameters and , and from those the gas deviation factor; they do not coincide with the true critical point of the mixture.
The share of a zone (usually the flushed zone) in the reading of an electrode sonde with step-profile invasion: . It grows with the invasion diameter, faster for shallow sondes; for induction sondes the same role is played by the geometric factor, which weights the conductivities that are added together.
The late flow regime towards a fractured well: the pressure disturbance has moved away from the fracture by several fracture lengths, and the flow converges on it as on an ordinary well with a large effective wellbore radius. On the diagnostic plot it is the derivative plateau, from which and the negative skin of the fracture are found.
Numbers calculated by a deterministic algorithm that nevertheless resemble random numbers in their statistical properties. The sequence is fully determined by the starting number (seed), so a calculation with the same seed is reproduced exactly.
The component of skin caused not by a change in permeability but by the geometry of the flow: partial penetration of the formation, wellbore deviation, perforation. It is not removed by an acid treatment.
A model of a finite aquifer as a tank behind a resistance: water enters the accumulation at a rate proportional to the difference between the average aquifer pressure and the pressure at the contact, while the aquifer is depleted according to its own balance and releases its elastic storage exponentially.
A regime of inflow from a closed area without recharge, in which the pressure at all points declines at the same rate, , while the shape of its distribution and the drawdown do not change. It sets in when the disturbance from the start of the well has reached all the boundaries of the area.
A vessel on the discharge line of a mud pump in which nitrogen behind an elastic diaphragm smooths the pressure fluctuations caused by the piston strokes. Without it, the pulsations shake the lines loose and interfere with mud-pulse telemetry.
The replaceable cylinder sleeve of a mud pump. The smaller its diameter, the lower the output per stroke and the higher the allowable pressure: the load on the rod is produced by the pressure acting on the piston area, and it is limited by the pump rating. Liners are changed when moving to intervals needing a different flow rate and pressure.
The dependence of the head, shaft power and efficiency of a centrifugal pump, or of a single stage of it, on the flow rate, measured on water at the nominal rotational speed. The head is expressed as the height of a column of the pumped liquid in metres; the pump should operate in its working range near the maximum efficiency.
The ratio of the actual delivery of a sucker rod pump to the theoretical delivery — the volume that the plunger would sweep with a stroke equal to the stroke of the polished rod. It accounts for the loss of stroke to the stretch of the rods and tubing, incomplete filling of the barrel, leaks and oil shrinkage; for an installation in good order it is usually 0.6–0.8.
The ratio of the working volumes (effective areas) of the pump end and of the hydraulic engine of a hydraulic piston unit. The pressure drop of the power fluid across the engine equals the pressure rise that the pump gives the production multiplied by P/E, plus friction; the power fluid rate is inversely proportional to P/E.
An empirical formula that estimates the bubble point pressure, the solution gas–oil ratio or the oil formation volume factor from the gas–oil ratio, the gas and oil densities and the reservoir temperature. It is fitted to laboratory data for a particular set of oils and is reliable only within the range of that set.
The dependence of the volume and phase state of reservoir fluids on pressure and temperature; it is determined in the laboratory or from correlations.
A laboratory report on the study of reservoir fluid samples: information on the sampling and validation of the samples, composition, constant-composition expansion, differential liberation, separator tests and viscosity for an oil; recombination, dew point and constant-volume depletion for a gas condensate. Before any calculations the basis of each number and the consistency of the tables are checked.
Quartz cement that grows on detrital quartz grains as a continuation of their crystal lattice. The growth rate increases rapidly with temperature (it becomes appreciable above about 70–80 °C), so the volume of cement is determined by the time spent in the hot zone, the area of clean quartz surface and the coatings on the grains.
Rapid interpretation of a log right after recording, at the rig or within the first hours, using simplified models and readings taken opposite the beds, in order to decide whether to test the zones, whether to run casing and where to perforate. The steps are the same as in a full interpretation, but the parameters are preliminary.
The ratio of the investor's cumulative receipts to its cumulative costs. The scales of production sharing and of some taxes depend on it: the larger R is, the higher the state's share.
An equation for the mole fraction of the gas phase given known equilibrium ratios: the difference between the sums of the mole fractions of the components in the gas and in the liquid, expressed through the composition of the mixture, the equilibrium ratios and the gas fraction, is equal to zero. Its left-hand side decreases monotonically, so the root is unique; a root between 0 and 1 means a two-phase mixture.
A model of the reservoir in which around the well there is a zone of different permeability or mobility with a finite radius: a damaged or stimulated zone, water near an injection well, condensate that has dropped out. As long as the disturbance has not travelled far beyond the zone, the pressure derivative reflects its permeability; when the radius of investigation is about ten times the radius of the zone (for a zone with permeability 3–5 times lower than that of the reservoir; later if the damage is more severe), the zone acts as a skin.
Determination of the age of a mineral or rock in years from the accumulation of a radiogenic daughter isotope: , where is the decay constant. The age of sedimentary sequences is obtained from volcanic ash layers and cross-cutting intrusions, most often by the uranium–lead method on zircon.
The radius at which the logarithmic pressure profile near the well, extended from the bottomhole, reaches the initial reservoir pressure: the Dupuit equation with this radius gives the same bottomhole pressure as transient flow. It grows as , that is, it is 0.75 of the radius of investigation. Not to be confused with the radius of the drainage area.
The distance from the well to which a noticeable pressure disturbance has propagated during the test; usually — at this distance the rate of pressure decline is just now passing through its maximum: closer to the well it is already decreasing, farther away it is still increasing. There is no sharp boundary: a weak disturbance travels farther.
A preventer with two opposed rams. Pipe rams seal the annulus around a pipe of their own size, blind rams close an empty hole, shear rams cut the pipe, and blind shear rams then also seal the hole. It holds pressure from below.
The rate at which the well is deepened during drilling, usually in metres per hour.
The change in bottomhole pressure divided by the current flow rate, . It makes it possible to process together sections of a record with different rates.
The highest pressure at which the equipment is permitted to operate. For blowout prevention and wellhead equipment it is taken from the standard series (14, 21, 35, 70, 105 MPa) and is not less than the highest expected pressure at the wellhead — the formation pressure less the gas column.
In Kazakhstan, a mandatory programme of the subsoil user setting out how the raw gas produced will be processed or utilised. It is drawn up on the basis of an approved base project document or development analysis and approved by the authorised body for hydrocarbons, taking into account the recommendations of a working group; the programme is updated every three years and progress on it is reported annually.
The function , which accounts for the dependence of gas viscosity and compressibility on pressure. With it the gas inflow equations are written in the same way as for a liquid, over the whole pressure range.
The rate adjusted for inflation, that is, the growth in the purchasing power of money: , where is the rate in nominal terms and is the inflation rate.
The right, but not the obligation, to change a project later, when more is known: to defer development, expand it, stop it or change its operating mode. Its value rises with uncertainty: it makes it possible not to invest money in a bad outcome.
Expressing amounts in money of a single purchasing power, that of the base year: a nominal amount is divided by the growth of the price index since the base year. A cash flow in real terms is discounted at the real rate.
Mixing of separator gas and oil samples in the proportion in which they left the separator, in order to restore the reservoir mixture. The proportion is taken from the gas–oil ratio, corrected for the actual gas density and recalculated to separator oil.
The fraction of the oil in place that is, or is expected to be, recovered; it equals the product of the displacement efficiency and the sweep efficiency.
A thick, clean bed known to be water-saturated, close to the interval being studied and containing the same formation water. It is used to determine the static spontaneous potential and the apparent water resistivity, with which the saturation calculation is calibrated.
The ratio of the amplitude of the reflected wave to that of the incident wave. For normal incidence of a P-wave, , where and are the acoustic impedances of the rocks above and below the boundary; the sign shows whether the reflected wave keeps the polarity of the incident one. The fraction of energy reflected is .
The study of the structure of the subsurface using elastic waves reflected from boundaries between layers of different acoustic impedance.
The ratio of the effective permeability to a phase to a base (usually absolute) permeability; it depends on saturation.
The magnetisation fixed in a rock when it formed (for example, when basalt cools in the Earth's field) and retained independently of the present field; it may point in a different direction from it. The ratio of remanent to induced magnetisation is the Koenigsberger ratio ; when the remanent component dominates.
A Kazakhstani tax on the value of exported crude oil: a single rate applies to the whole value, ranging from 0% at a world price of up to $40/bbl to 32% at a price above $180. From 2027, oil produced by MET payers is exempt from it.
An interval, usually several tens of metres long, recorded a second time with the same tool. Agreement with the main log to within the statistical error confirms repeatability; a systematic discrepancy points to tool drift, or a calibration or depth error.
The smallest volume of rock above which an averaged property, such as porosity or permeability, almost ceases to depend on the size of the volume. For a homogeneous sandstone it can be as small as a core plug; for a layered, vuggy or fractured rock it is larger than the volume seen by a formation-tester probe, and sometimes larger than a full-diameter core.
Reserves added in a period divided by production in the same period. The organic ratio counts discoveries, extensions, revisions and improved recovery; the full ratio also counts acquisitions less disposals. Above 100%, reserves are growing; the average over several years is the one to watch.
Under PRMS, the quantities of hydrocarbons anticipated to be commercially recoverable by development projects on known accumulations from a given date under defined conditions: discovered, recoverable, commercial and not yet produced at the date of evaluation. By uncertainty they are divided into the categories 1P, 2P and 3P; in a probabilistic assessment these are P90, P50 and P10. In Kazakhstan's legal acts, “reserves” means both the quantity in the subsurface (geological reserves) and its recoverable part.
Reserves divided by annual production: how many years the reserves would last at the present rate of production. In practice production declines, so the reserves are produced over a longer time, and the plateau is shorter.
The shape, size, internal structure and mutual arrangement of the reservoir bodies and the non-reservoir rocks that separate them, from layers within a body to the horizons and blocks of an accumulation. In Russian-language practice it is described as macroheterogeneity, by the coefficients of sand content, dissection, lithological connectivity and continuity; by the relation between the size of the bodies and the well spacing, “layer-cake”, “jigsaw-puzzle” and “labyrinth” architectures are distinguished.
A part of an accumulation that does not communicate with the rest during development, because of isolated bodies, pinch-outs, barriers within the section or sealing faults. It is revealed by different contacts and pressure lines, differences in fluid composition, rapid pressure decline and a small connected volume; equal initial pressures do not prove communication during production.
The fraction of the reservoir volume that is joined into a single whole, or the fraction connected to the wells of a given well pattern. It depends on the net-to-gross ratio and on the size and arrangement of the bodies, and changes abruptly near the percolation threshold. Static connectivity takes into account only the geometry of the bodies; dynamic connectivity also takes into account the conductivity of their contacts.
The engineering discipline concerned with how much oil and gas an accumulation contains, what part of it, within what time and by what method it is profitable to recover. It relies on the properties of rocks and fluids, the laws of fluid flow in porous media and the sources of reservoir energy; its problems are solved by field development specialists.
A long record of bottomhole pressure at a constant rate until the pseudo-steady-state regime is reached. The rate of linear pressure decline gives the pore volume of the drainage area, and the position of the line gives the shape factor.
The mass per unit volume of oil with dissolved gas at reservoir pressure and temperature. From the mass balance, ; it is at a minimum at the bubble point pressure and sets the pressure gradient in the oil zone.
The quantity in µm ( in mD, as a fraction), the characteristic size of the rock's pore channels. It is the basis of the flow zone indicator, FZI.
A numerical calculation, in time steps, of the flow of oil, gas and water in a reservoir divided into cells; it is used to forecast production under different development options.
The systematic collection and analysis of data on the performance of an accumulation and its wells (rates, gas–oil ratio, water cut, reservoir pressure, inflow and injection profiles, contact positions) to compare with the forecast and to select interventions.
The fraction of the pore volume in which gas remains trapped after water has swept through the formation; roughly 20–40%. The higher the pressure at the moment of trapping, the more gas, in standard cubic metres, remains in these pores.
The fraction of the pore volume occupied by the oil left in the swept part of the reservoir after displacement by water or gas; it is held by capillary forces as disconnected droplets.
The fraction of the pore volume that oil occupies after displacement by gas in the presence of connate water. In a hydrophilic rock it is often lower than the residual oil saturation after waterflooding: the oil spreads as films and continues to drain, especially during slow gravity drainage.
The ratio of the resistivity of a rock at a given water saturation to its resistivity at full water saturation: .
An estimate of water saturation and hydrocarbon mobility without porosity: the ratio of the resistivities of the flushed zone and the uninvaded formation, , is divided by the water contrast , which gives . itself is found with a closing relationship, either the empirical or the residual oil saturation from core.
The value of the oil or gas produced over and above the recovery of all costs together with the return the investor requires; for a project, its NPV before payments to the government at the investor's rate. Subsoil use terms divide the rent between the owner of the subsoil and the investor but do not create it.
A tax on the positive balance of a project's cash flow, in which past negative balances are carried forward with an uplift, the threshold rate of return. It is paid only after the investment has been recovered with that return; if the threshold equals the investor's rate, it does not change the investor's decisions.
Continuation of a reservoir simulation run from the saved state of another run (the pressures and saturations at a chosen date). This is how forecasts are started from the end of the history run.
The volume that a phase occupies in a separator or settling tank divided by its flow rate — how long, on average, the liquid stays in the vessel. It determines whether gas bubbles have time to separate and water droplets to settle; in three-phase separators it is usually minutes, and for heavy and cold oil tens of minutes.
Distillation of a drilling fluid sample: the fractions of water and oil are found from the volumes of condensed water and oil, and the solids fraction by difference. Dissolved salts stay in the retort cup and are counted as “solids”, so in saline fluids the result is corrected using the chloride content.
The dropout of liquid from gas as the pressure is reduced, at a temperature between the critical temperature of the mixture and the cricondentherm. Because of it, when a gas-condensate accumulation is produced without pressure maintenance, part of the condensate is left in the reservoir.
Accounting profit divided by the book value of the investment. With straight-line depreciation it rises as the asset ages, even if its return (IRR) is constant, so for investment decisions it is no substitute for NPV and IRR.
The ratio of the permeability of a core sample after exposure to drilling fluid or another fluid to its initial permeability measured with the same fluid. It is used to compare fluids before they are used in a well.
A fault along which the hanging wall has moved up relative to the footwall; it forms in compression. A reverse fault with a gently dipping fault plane is called a thrust. In a vertical well a reverse fault usually repeats part of the section.
The dimensionless ratio of inertial to viscous forces in a flow, . For a Newtonian fluid in a pipe the flow is laminar at Re below about 2100 and turbulent at Re above about 4000; for drilling fluid the effective viscosity is substituted, and for the annulus the hydraulic diameter .
The dependence of the shear stress in a drilling fluid on the shear rate, and the models that describe it: Bingham, power-law and Herschel–Bulkley. It governs the pressure losses during circulation, the equivalent circulating density, cuttings transport and the pressure fluctuations when running in and pulling out pipe.
An elongated depression formed by extension of the lithosphere: the crust is broken by normal faults into tilted blocks and thinned, and the surface subsides; after the extension the basin continues to subside as the lithosphere cools. Young rifts are hot; lacustrine and marine source rocks accumulate in their grabens.
A carbonate platform with a rim of reefs or oolite shoals at the break in the sea floor: behind the rim lie a calm lagoon and tidal flats, and in front of it a steep slope into the basin. The best reservoirs of such a platform form a narrow strip along the rim.
A rule under which a project or contract is taxed separately from the company's other activities: its losses cannot be offset against the profits of other projects. In Kazakhstan, losses on a subsoil use contract are covered only from the future income of the same contract.
The excess of the drilling fluid density over the pore-pressure equivalent density at which a well with a subsea wellhead stays controlled even after the riser is disconnected, when the fluid above the seabed is replaced by seawater. It increases with water depth; in deep water it is often unattainable because of the narrow window to the fracture pressure, and then the well is held by the preventers when the riser is disconnected.
An attitude to risk in which a guaranteed amount is valued more highly than a risky option with the same expected value. With a risk-neutral attitude, options are compared by expected value alone; with a risk-seeking attitude, the risky option is valued more highly.
The parameter of the exponential utility function, expressed in money: the larger it is, the closer the behaviour is to risk-neutral. A rule of thumb: a bet to win or lose with equal chances is almost a matter of indifference to such a company.
The return on investments with practically no risk of non-payment, usually government bonds of a reliable borrower in the same currency. It is the base to which a risk premium is added.
The mean volume of a prospect over all outcomes, including failure: the geological chance of success multiplied by the mean success-case volume. It is not a possible outcome; it is used to compare prospects and to sum the volumes of a portfolio.
The continuous transformation of rocks of one class into another: weathering and erosion produce sediments; on burial the sediments become sedimentary rocks, with further burial and heating metamorphic rocks, and on melting magma, from which igneous rocks crystallise. It is driven by the Earth's internal heat, plate tectonics and the energy of the Sun.
A group of rocks with a similar pore structure (similar sizes of pores and pore throats) and therefore with its own porosity–permeability relationship, its own capillary pressure curves and its own relative permeabilities. It is identified from core (thin sections, FZI, mercury curves); a useful type can be explained geologically and recognised from logs.
Heating of crushed rock in the absence of oxygen according to a standard temperature programme. It gives the free hydrocarbons , the residual potential of the kerogen , the carbon dioxide and the temperature ; from these the kerogen hydrogen index and the production index are calculated.
A bit whose cutting structure — milled steel teeth or tungsten carbide inserts — sits on cones that rotate on the journals of the bit legs. The cones roll over the bottom of the hole and break the rock by indentation and chipping and, where their axes are offset, by scraping as well; the life of the bit is often limited by the cone bearings.
A module at the intake of an electric submersible pump that spins the flow: the liquid is thrown to the wall and goes into the pump, while the separated free gas is vented into the annulus. It reduces the gas fraction at the intake, but is worn by sand.
A steering device above the bit that changes the direction of the wellbore without stopping the rotation of the drill string, either by extendable pads that push against the wall or by bending the shaft so that it tilts the bit axis. It is controlled by commands from the surface.
A rotating table at the centre of the rig floor: through the bushings and the kelly drive bushing it rotates the string, while slips in its opening hold the string during connections and trips. With a top drive, the rotary table serves mainly as a support for the slips.
The height of the rotary table of the drilling rig, the zero point for measured depths along the hole, above sea level. To compare the depths of different wells, true vertical depths are converted into subsea depths by subtracting the elevation; an error in it shifts all the depths of the well by the same amount.
The degree to which the corners and edges of a grain are smoothed: the ratio of the average radius of curvature of the corners to the radius of the inscribed circle, from 0.12–0.17 for very angular grains to 0.7–1 for well-rounded ones. Well-rounded quartz indicates long transport or aeolian reworking; angular grains indicate a nearby source and rapid burial.
Measurements on all the core samples, usually taken every 25–30 cm: porosity, gas permeability, grain density and saturation. They give a profile of properties through the section and a basis for calibrating the logs.
A payment to the state as a share of production or revenue that does not depend on the profitability of the project.
The curve , overlaid on the SP in a water-bearing bed. In water-bearing beds the curves coincide; opposite beds with moveable hydrocarbons the calculated curve moves towards the shale baseline; a separation of about 25 mV corresponds to at mV.
The total content of salts dissolved in water; it is expressed in grams per litre of solution or in ppm (milligrams per kilogram of solution), and the two units differ by the density of the solution. Together with temperature, it determines the resistivity of the water.
A body of rock salt that has risen from a salt layer and pierced the overlying rocks; the salt flows under the action of uneven loading. Numerous traps form above salt pillows and domes and against the walls of diapirs, and the salt itself acts as a seal.
A place where the salt has been squeezed out completely and the suprasalt rocks rest on the subsalt ones: a “window” in the salt seal. Through such areas — in the Pre-Caspian Basin they are called salt-free troughs — fluids pass from the subsalt tier of the section into the suprasalt one.
A systematic difference between a sample and the population, because of which the estimate converges to a wrong value however many samples are taken. In core data it arises when plugs are drilled only from competent rock, when sampling is concentrated in the best layers, or when core is taken only in crestal wells.
The entry into the well of load-bearing grains of the reservoir rock after the rock at the wall of the wellbore or perforation tunnel has failed and been carried away by the flow. It intensifies with rising drawdown, depletion and increasing water cut; it can be transient, continuous or catastrophic.
A pipe with a permeable wall (slots, wire wrapped with a gap, or layers of metal mesh) that lets fluid through and holds back gravel or formation sand. It is set opposite the producing interval, alone or together with a gravel pack.
The flow rate of fluid entering the wellbore from the formation, referred to standard conditions. After the well is opened or shut in at the surface, it differs from the surface rate while the contents of the wellbore expand or are compressed.
The fraction of the pore volume occupied by a particular fluid — water, oil or gas; the saturations add up to one.
The balance of one of the phases (usually water), written in terms of its saturation for a known pressure field or total velocity. It describes the transport of saturation by the flow, gravity segregation of the phases and capillary smearing of the front.
The exponent in the relationship . It is about 2 in water-wet rocks and markedly higher in oil-wet rocks, where the water forms separate droplets.
The decimal logarithm of the ratio of the product of the activities of the ions of a salt in water to its solubility product. At SI > 0 the water is supersaturated and the salt may precipitate, at SI < 0 it is undersaturated. The index indicates the possibility of precipitation, but not its rate.
The precipitation of inorganic salts — calcium carbonate, barium, strontium and calcium sulfates, halite, iron sulfides — from supersaturated water in the pores near the wellbore, in perforation tunnels, pipes and equipment. The causes are the fall in pressure and temperature, evaporation and the mixing of incompatible waters.
A substance that, at concentrations of the order of a few to tens of parts per million, retards the nucleation and growth of salt crystals. It is squeezed into the formation, from which it is slowly carried out with the water, or is dosed into the well. The inhibitor changes the rate of precipitation, not the equilibrium, so it is applied before deposition begins.
The condition under which a model cell has an equivalent permeability of its own: the heterogeneity is much finer than the cell, and the flow changes over distances much greater than the cell. When the cell is comparable to the correlation length, its equivalent permeability depends on the boundary conditions of the calculation, that is, on unknown neighbours.
A consistent set of values of several of a project's input parameters changed at the same time, for example the “low”, “base” and “high” cases. It describes one possible picture of the future, not a mechanical mixture of independently worsened numbers. Unlike sensitivity analysis, it shows the combined effect of the parameters, but a scenario has no probability.
A sharp rise in injection pressure when proppant blocks the fracture or the near-wellbore zone and pumping has to be stopped ahead of schedule. Deliberately stopping the growth of a fracture at its tip is a separate technology for creating short, wide fractures (tip screen-out).
A low-permeability rock above a reservoir (clay, mudstone, salt, anhydrite) that holds back rising oil and gas by a capillary barrier as long as their excess pressure is lower than its entry pressure.
A set of seals on the lower end of the tubing, entering the polished bore of a permanent packer. An anchor seal assembly (with a latch) fixes the tubing, so that changes in its length turn into forces; a locator seal assembly (with a stop) allows the tubing to move upwards; large movements are taken up by an expansion (telescopic) joint.
The greatest height of an oil or gas column above the contact that a seal can hold capillarily. It is greater the narrower the pore throats of the seal and the smaller the density difference between water and hydrocarbon; for gas it is usually less than for oil.
A gas cap formed by gas that has come out of the oil below the bubble point pressure and risen to the top of the formation. It lowers the gas–oil ratio of the wells and increases oil recovery; it forms when vertical permeability is good and the withdrawal rate is moderate.
Voids formed after the sediment was deposited and compacted: solution vugs, fractures and dolomitisation pores. It is especially characteristic of carbonates and usually gives disproportionately high permeability.
The difference between the total porosity from the density and neutron logs and the sonic porosity, , because the sonic log hardly detects vugs. It estimates their share in clean carbonates with liquid in the pores, provided that the calculation parameters have been checked in a bed without vugs.
Precipitates that form from spent mud acid as it continues to react with clays and feldspars: hydrated silica (a gel) from fluorosilicates, aluminium fluorides and hydroxides, and, as the pH rises, iron hydroxides. They cannot be avoided completely, so the spent acid is pushed away from the wellbore and flushed out quickly.
A check that a structural cross-section is geometrically possible: the deformation is “restored” backwards to see whether a plausible pre-deformation configuration results. It relies on the conservation of the area of units in a section along the direction of transport and, for flexural slip folding, of the bed lengths as well. A balanced section is possible, but not necessarily correct.
A reservoir simulation model of part of an accumulation (a block, an area or an element of the development system) with a finer grid than a full-field model allows. It requires boundary conditions: the flows across the boundaries are taken from the full-field model, or the boundaries are drawn along faults and lines of symmetry.
An area where the surface of the lithosphere subsided for a long time and accumulated a sequence of sediments from a few kilometres to 20 km thick. The subsidence is caused by extension and subsequent cooling of the lithosphere (rifts, passive margins), by its flexure under a load (foreland basins) and by other causes; the weight of the sediments amplifies it by a factor of 2–3. Almost all oil and gas accumulations are found in sedimentary basins.
The sequence of sedimentary rocks lying on the basement, which consists of igneous and metamorphic rocks. Its thickness ranges from zero on the shields to 10–20 km in deep basins (in the centre of the Pre-Caspian Basin the basement lies 20 km or more deep); almost all oil and gas accumulations lie within the cover.
Displacement in which gravity separates the phases across the thickness of the formation: water moves along the base and gas along the top, and the interface with the oil is sharp. It is calculated on the assumption of vertical equilibrium, with relative permeabilities averaged over the thickness.
Processing that moves reflections from the position beneath the observation point to where the reflecting boundary actually lies, and collapses diffracted waves to the points from which they emanated. Without it, dipping boundaries are displaced down-dip and look gentler, and the horizontal resolution is limited by the Fresnel zone. Time migration produces an image in time, depth migration an image in depth, using a model of interval velocities.
A short oscillation lasting tens of milliseconds that each reflecting boundary returns to the surface; its shape is determined by the source, absorption in the earth, the receivers and processing. A zero-phase wavelet is symmetric about the reflection time, a minimum-phase wavelet starts at it.
A model of a reservoir bounded by no-flow boundaries on some sides only: a half-plane beyond a fault, a right-angle corner, a channel, a channel with one closed end. It has no pseudo-steady-state regime; the late derivative reaches a new plateau or follows a slope of ½, and after a long shut-in the pressure recovers to its initial value.
A floating drilling rig on submerged pontoons and columns: the small waterplane area reduces the motion in waves. It is held in position by anchors or dynamic positioning; it drills through a subsea wellhead and a subsea blowout preventer stack, connected to the rig by a riser.
A plot of well pressure against the logarithm of time. In the period of infinite-acting radial flow the points fall on a straight line: its slope gives the flow capacity , and its position gives the skin factor.
The change in pressure along the radial flow line per one log cycle of time. It is inversely proportional to the flow capacity: in metric oilfield units (m³/d, mPa·s, mD, m, MPa); in oilfield units the constant is 162.6. The slope per unit of natural logarithm is 2.303 times smaller.
A check of how a project indicator, usually NPV, changes when one input parameter is varied and the others are held constant. It shows which parameters matter most, but says nothing about the probability of the deviations and does not account for their combined effect.
A division of carbonate voids that is important for petrophysics: separate vugs (grain moulds, shell cavities) are connected to one another only through interparticle pores and add porosity with almost no permeability, whereas touching vugs (fractures, solution channels and vugs, caverns) form a network of their own and give high permeability at low porosity.
The ratio of the distance between the centres of two wellbores to the sum of the sizes of their uncertainty regions in the direction towards each other. At a value of less than one the regions overlap; the permissible values are set by the company's standards for collision avoidance.
A test in which oil at the bubble point pressure is passed through one or more stages of separation at set pressures and temperatures. It gives the solution gas–oil ratio and the formation volume factor at the bubble point pressure for that separation scheme, and the density of the stock-tank oil.
Subdivision of a section into sequences (successions bounded by unconformities and their correlative conformities) and into systems tracts related to cycles of relative sea level. It helps to predict the position of reservoirs and seals and to correlate bodies of the same age.
A way of building realisations of the distribution of a property between wells: the grid nodes are visited in a random order, at each node the mean and variance are found by simple kriging from the data and the nodes already filled, and a value is drawn from the normal distribution. The realisations honour the data, reproduce the histogram and variogram, and differ between wells.
An agreement under which a contractor explores for or produces oil for the state or a national company for a fee, without acquiring ownership of the production. In a pure service contract the contractor bears almost no risk and does not book reserves; in a risk service contract it finances the work and is paid only if it succeeds.
The reference level of the SP or gamma ray curve opposite thick shales, from which the readings in other beds are measured. On the SP it can drift smoothly with depth and shift abruptly if the shales separate beds containing waters of different salinity.
The ways in which shaly material is distributed in a formation: as layers between layers of clean sandstone (laminated), in the pores as films and pore fill (dispersed), or as grains replacing part of the framework (structural). For the same shale volume they affect the effective porosity, permeability and resistivity in different ways; they are distinguished on a plot of total porosity against shale volume.
The proportion of clay material in the layers that have slipped past a point on the fault plane: . A measure of the sealing capacity of the fault zone rock; according to field data, appreciable sealing begins at an SGR of about 15–20%.
Suppression of the hydration, swelling and dispersion of clays and shales by the drilling fluid: by cation exchange (potassium, calcium), by lowering the water activity, by encapsulation with polymers and by plugging of micropores. Inhibitive water-based fluids are potassium, lime, gypsum, glycol, silicate and amine systems.
The readings of the adjacent shale units plotted on a crossplot. The points of laminated shaly beds lie between the point of their clean rock and the shale point, so the shale point is used to estimate shale volume and to correct porosity; for dispersed clay this is only an approximation.
The first stage of drilling fluid cleaning: inclined vibrating screens through which the whole flow from the well passes; the liquid and fine particles pass through the screen, while the coarse cuttings come off it as waste. Screens are designated by an API number, according to the size of the particles they retain almost completely (API 200 — about 75 µm).
The fraction of the rock volume occupied by shaly material: clay minerals together with bound water and a silt admixture. It is estimated from the gamma ray log, the spontaneous potential (SP) log and the neutron–density logs, and is used to identify reservoirs, to correct porosity and to calculate the saturation of shaly rocks.
An explosive charge with a metal liner in a conical cavity. The detonation collapses the liner into a thin, fast jet that pierces the casing, cement and rock by the pressure of impact; the slow, massive part of the liner is called the slug. Deep-penetrating charges produce a long, narrow tunnel, and big-hole (large-diameter) charges a wide, short hole.
The rate at which the flow velocity changes across the flow, in s⁻¹. In a rotational viscometer of standard geometry it equals s⁻¹ at a rotational speed of rpm; in the well annulus it is usually tens and hundreds of s⁻¹, in drill pipe hundreds and thousands.
The plane at which water saturation drops abruptly from the saturation behind the front to the initial one; only oil flows ahead of it. The saturation behind the front is found by drawing a tangent to the fractional flow curve from the point of initial saturation.
The strip of sea floor from the low-water level to the fair-weather wave base, where sand is in constant motion. Deeper, down to the storm wave base, lies the offshore transition zone, where storm sands alternate with clays. As the shoreline progrades, the shoreface and the beach leave behind a sand wedge that coarsens upwards, a coastal sand sheet elongated along the shore; its thickness is set by the depth of the base of the shoreface, and its width by the sand supply and time.
The number of perforations per metre of the interval being perforated; its reciprocal is the vertical distance between shots. The higher the density, the smaller the vertical component of the perforation skin and the lower the flow velocity in each tunnel.
The reciprocal of the oil formation volume factor, : the fraction of the reservoir volume of oil that remains oil in the stock tank. The loss of volume, , is called shrinkage.
The excess pressures at the wellhead, in the drill pipe (SIDPP) and in the annulus (SICP), after the well has been shut in on a kick and the pressures have stabilised. The drill pipe pressure gives the formation pressure and the kill mud density; the annulus pressure is higher if the influx fluid is lighter than the drilling fluid.
A sub in the tubing string with a pocket offset to the side of the full-bore main channel and with ports to the annulus. Gas lift valves, chemical injection valves, circulating valves or blanking plugs are installed in the pocket, and retrieved from it, on wireline without pulling the tubing.
A small rock sample taken by a wireline tool from the wall of a well that has already been drilled. A percussion (gun-fired) sidewall corer damages the rock by impact, whereas a rotary (drilling) corer hardly damages it; the samples come from the zone invaded by filtrate.
Sieving of a sand sample that has been broken down into individual grains through a set of sieves with decreasing mesh size, and construction of a cumulative curve: what fraction of the mass is coarser than a given size. The curve gives the characteristic grain sizes and the sorting, and from these the gravel and screens are selected.
A one-off payment for acquiring a subsoil use right. In Kazakhstan the Tax Code sets its starting amount, either in monthly calculation indices (MCI) or based on the value of approved reserves, while the final amount is determined by an auction or direct negotiations; in a hydrocarbon auction the highest bonus wins.
The level that the variogram reaches at large distances; under stationarity it equals the variance of the property. It is made up of the nugget effect and the partial sill, the variability that is related to distance.
A model of a shaly reservoir: a shale term is added to the Archie term with effective porosity. In the modified version the water term is divided by . Different formulas appear in the literature and in software under these names, so the formula itself needs to be checked.
Accrual in which the interest of each period is calculated only on the original amount, and the interest already accrued earns no income; the amount grows linearly: .
A text description of a model for a simulator: sections in a strict order (problem description, grid, properties, regions, initial state, output, well schedule) and included files with arrays and tables. The simulator reads it from top to bottom.
A number assigned to the cells of a reservoir simulation model to determine which relative permeability or PVT table they use, which equilibrium conditions apply at initialisation, and for which areas reserves and balances are reported.
A sampler in which high-pressure nitrogen sits behind the piston: it keeps the sample compressed above reservoir pressure and prevents it from becoming two-phase as it is brought up and cooled. It is needed for asphaltenes and other properties that are irreversibly spoiled by degassing.
Money set aside in advance from the project's receipts at an explicit rate to cover future costs, for example decommissioning. An IRR calculation that includes such a fund does not assume that the money set aside grows at the IRR.
A dimensionless quantity describing the additional resistance (positive skin) or the easing (negative skin) of inflow in the near-wellbore zone.
The part of the drawdown that is spent in the near-wellbore zone in addition to the losses in a homogeneous formation: . It is proportional to the flow rate.
The skin factor calculated at each point of a drawdown curve using the permeability and the pressure found from the build-up curve that follows it. It shows how the clean-up of the well is going or, conversely, how the damage to the near-wellbore zone is growing.
The negative component of the skin of a well that crosses the formation at an angle to the normal to the bedding: the wellbore is longer than the thickness of the formation, and inflow is eased. Permeability anisotropy reduces its magnitude.
A smooth single-strand steel wire, usually 1.8–3.2 mm in diameter, on which tools and instruments with their own power supply are run. It is cheaper and simpler than cable and transmits jar blows well, but provides no communication with the tool.
Water with a small addition of polymer, usually polyacrylamide, that reduces friction in turbulent flow in pipes. It makes it possible to pump large volumes at high rates and leaves little residue, but it hardly holds any proppant and does not limit leak-off; it is used in tight and shale rocks.
Processing of the pressure build-up curve of a well in a fully drilled, depleting accumulation: the pressure decline recorded before shut-in is extended as a straight line and added to the pressure rise, . The result is processed as a drawdown curve: by the slope, by the one-hour point and by the derivative.
A coupling in the tubing string made of two concentric sleeves with ports; the inner sleeve is moved by a wireline tool, opening or closing communication between the tubing and the annulus. It is needed for circulating to wash and kill the well, and for access to the formations in multi-zone wells.
A fluid whose compressibility is small and independent of pressure, so that the product of compressibility and pressure change is much less than one. Water and oil above the bubble point pressure are classed as such; for them the diffusivity equation is linear. Gas does not belong to this class.
The movement of gas faster than the liquid in a combined flow; the slip velocity is the difference between the actual velocities of the gas and the liquid. Because of slip the liquid is held back in the pipe, and the mixture is heavier than the flow-rate fractions of the phases would give.
Damage to the body of a drill pipe by the squeezing action of the slips on which the string hangs: the taper of the slips converts the weight into radial pressure, which together with tension can crush the pipe at a weight lower than the allowable one. The weight carried by the slips is multiplied by the ratio (for a 127 mm pipe in 406 mm long slips, 1.42) and compared with the allowable load.
Liquid that flows back under the pressure differential through the sealing lines between the rotor and the stator; the difference between the theoretical and the actual delivery. It increases with the differential pressure and wear, decreases with viscosity and interference, and is almost independent of the rotational speed.
An uncemented liner with slots, wire wrap or a mesh screen opposite the productive interval. It supports the wellbore wall and retains sand while passing fluid; it does not separate the intervals of the formation.
The volume of slurry obtained from a unit mass of cement together with water and additives: m³ per tonne or, in English-language practice, cubic feet per 94 lb sack. It is used to find how much cement is needed for the required volume of slurry.
A group of clay minerals with a 2:1 layer and a low layer charge; in montmorillonite, the principal one, the charge arises mainly in the octahedral sheet. Exchangeable cations and water enter between the layers, and the mineral swells. It is the main mineral of bentonite and of “active” clays; its cation exchange capacity is 80–150 meq/100 g.
An imbibition mechanism: the wetting phase swells in the corners of a throat, closes up and cuts off the non-wetting phase in the throat without involvement of a neighbouring pore. The oil that is cut off remains in the pores as droplets; snap-off is favoured by a small advancing contact angle and by pores much wider than the throats.
A drive mechanism below the bubble point pressure in which oil is displaced by the gas that comes out of it and expands. After a brief decline, the gas–oil ratio rises sharply and the pressure falls rapidly; oil recovery is usually 5–30%.
The volume of gas, measured at standard conditions, dissolved in a unit volume of oil at a given pressure. Not to be confused with the producing (field) gas–oil ratio, the ratio of gas produced to oil produced.
An active method: a transmitter sends out elastic pulses, and receivers record the arrival of the wave that has travelled through the rock along the borehole wall. It gives the interval transit time of the compressional wave, and tools with an array of receivers and dipole transmitters also give those of the shear wave and the Stoneley wave. It is used to estimate porosity, to tie wells to seismic data, to calculate elastic moduli and to check cementing.
The porosity calculated from the interval transit time, using the time-average equation or an empirical transform. It mainly reflects intergranular pores: it almost ignores vugs, and in loose sands without a correction for undercompaction it is overestimated.
The degree of similarity in the sizes of the grains of a sediment. It is measured by the standard deviation of sizes in φ units, for example the graphic of Folk and Ward: below 0.5 is good sorting, 0.5–1 moderate, above 1 poor. A well-sorted sand, other things being equal, is more porous and more permeable.
The allowable gas velocity in a separator, , at which liquid droplets still have time to settle and are not carried over with the gas. The empirical coefficient depends on the mist eliminator, the orientation of the vessel and the pressure; for estimates it is about 0.05–0.15 m/s.
Gas with a noticeable content of hydrogen sulfide; hydrogen sulfide, together with carbon dioxide, is classed as an acid gas. Such gas requires sour-service materials and special safety measures; gas without hydrogen sulfide is called “sweet”.
A fine-grained rock rich in organic matter that generates oil and gas as it is buried and heated. The area where it lies within the generation zone is called a generation centre (“kitchen”).
A fluid pumped between the drilling fluid and the cement slurry: a low-viscosity wash erodes the drilling fluid and the filter cake in turbulent flow, while a weighted spacer separates the cement from the drilling fluid and displaces it like a piston — for this it is heavier and “thicker” than the fluid being displaced.
Production that can be brought on stream within 30 days and sustained for at least 90 days (this is how the EIA defines it). Most of the world's spare capacity is in OPEC countries; it acts as a buffer against supply disruptions and as a measure of how tight the market is.
A matrix almost all of whose elements are zeros: in a row of the system of a reservoir simulation model the number of non-zero entries equals the number of connections of the cell plus the diagonal. Elimination creates new non-zero elements in it, known as fill-in; with natural ordering they occupy a band whose width is a row or a layer of the grid.
Studies on a small number of selected samples: capillary pressure, relative permeabilities, wettability, electrical, acoustic and mechanical properties, and pore compressibility. They are carried out on core with reservoir wettability and, where possible, at reservoir conditions.
Gamma ray logging that measures the energy of the photons, separates the contributions of potassium, uranium and thorium, and gives their concentrations (potassium in %, uranium and thorium in ppm). It helps to distinguish shales from radioactive sandstones and uranium-bearing carbonates and to estimate the type of clay minerals.
Flow to a short inflow interval, when the pressure disturbance has extended beyond it vertically but has not yet reached the top and base. The pressure drop tends to a constant value, and the derivative declines with a slope of −½; for an interval at the top or base of the formation the flow is hemispherical.
The permeability determined from the spherical flow period: . Together with the horizontal permeability from late-time radial flow, it gives the vertical permeability: .
The time function of pressure build-up after a pretest, , where is the duration of withdrawal. In spherical flow the pressure is linear in : the slope of the line gives the spherical permeability and the intercept at gives the pressure at the wall, .
A measure, from 0 to 1, of how close the shape of a grain is to a sphere, based on its long, intermediate and short axes: by Krumbein's formula , by the maximum projection formula . It is unrelated to roundness: a flat pebble can be well rounded.
A graph of a project indicator against the relative change in each parameter. All the lines pass through the point of the base case; the slope of a line shows the sensitivity and its length shows the range assumed for the parameter.
The lowest point of a trap down to which it retains hydrocarbons; the excess escapes through it further up the bed. The structure contour of the top that passes through the spill point is the closing contour.
The natural electrical potentials that arise in a borehole at the contacts between mud filtrate, formation water and shales, and the method of recording them along the wellbore. Deflections of the curve from the shale baseline identify permeable beds, and the static spontaneous potential is used to estimate the resistivity of the formation water.
The sum of all costs charged per day (or hour) of work on the well: the day rate of the drilling rig with its crew and the daily services — drilling fluid, mud logging, rental tools, supervisors, transport, fuel. Multiplied by the number of days, it gives the time-dependent costs in the cost estimate; it is also used to convert non-productive time into money.
The volume of filtrate that passes through the filter, or into the formation, before a filter cake forms; it is the intercept on the volume axis of the straight line of filtrate volume against the square root of time. From two readings of the standard test, .
A plot of the pressure change against the square root of time. In the period of linear flow — along a channel or to a hydraulic fracture — the points fall on a straight line: its slope gives the channel width or the fracture length (for a known permeability), and its intercept gives the skin.
Forcing cement slurry under pressure through perforations or special holes, through leaks in the casing, or into channels behind it — to repair the cement sheath, shut off an unwanted interval or stop a leak. Without fracturing the formation, the slurry loses filtrate and leaves a dense cake of dehydrated cement in the holes.
A provision in a contract or law that protects the investor from adverse changes in the rules after the investment is made: it freezes the applicable law at the date of the contract or obliges the state to restore the investor's economic position. It does not usually extend to safety or to environmental and health protection; in Kazakhstan the Subsoil Code does not extend it to taxation either.
A short section near the displacement front where capillary forces smear out the saturation jump; at a constant rate its shape does not change as the front advances. Its length is of the order of : in a reservoir from tens of centimetres to metres, in a core experiment comparable to the length of the sample.
The separation of gas from oil in several stages at successively lower pressures: in one or several separators and then in the stock tank. Each stage is calculated as a flash of the liquid from the previous stage. Compared with degassing directly to atmospheric pressure, more propane, butanes and pentanes remain in the oil, and more stock-tank oil is obtained.
The pressure and temperature to which volumes of gas and oil at the surface are referred: in Russia and Kazakhstan, 0.101325 MPa and 20 °C; in ISO, 0.101325 MPa and 15 °C; in US oilfield units, 14.696 psia and 60 °F; normal conditions are at 0 °C. For gas, the choice of conditions changes the volume by 1.5–7%, so they are stated next to the number.
A measure of the spread of data around the mean: the square root of the variance, the mean of the squared deviations (for a sample, with the divisor ). It is expressed in the units of the quantity itself; for a normal distribution about 68% of the values lie within one deviation of the mean.
The standard deviation that the sample mean would have if sampling were repeated many times: for independent measurements. It describes the uncertainty of the mean, not the spread of the data, and decreases only as the square root of the number of samples.
Empirical formulas for the equilibrium ratios of oil and natural gas at pressures up to 6.9 MPa (1000 psia) and temperatures up to 93 °C (200 °F), the range of the data from which they were fitted: the intercept and slope of the Hoffman line are functions of pressure; the component parameters and are tabulated, and for the C7+ fraction they are found from the effective number of carbon atoms. They are used for separator calculations.
The distance between the body or pad of a tool and the borehole wall. The mud filling it enters the zone of investigation: because of it, neutron logging overestimates porosity and density logging underestimates density.
The smallest clearance between the string and the wall of the hole divided by the clearance with ideal centring, as a percentage: 100% means the string is in the middle, 0% means it touches the wall. On the narrow side of an eccentric annulus the fluid moves much more slowly, and channels of drilling fluid are left there.
The pressure on the standpipe gauge during circulation — the sum of the pressure losses in the surface equipment, the drill string, across the bit and in the annulus, plus the pressure drops across the downhole motor and the MWD tool, if there are any. A change in it at constant flow rate reveals a plugged nozzle, a washout in the string or a kick.
Under Kazakhstan's Subsoil Code, the state's right to acquire, ahead of any other person, a subsoil use right being transferred, a share in it, or related facilities in a strategic subsoil area. It is exercised on the decision of the competent authority through the national managing holding or a national company, at a price and on payment terms no worse than those stated in the application.
In Kazakhstan, the commission that carries out the state expert review of reports on the estimation and the operational estimation of hydrocarbon reserves (under the transitional provisions of the Subsoil Code, until 1 January 2030). A positive conclusion is the basis for entering the reserves in the state register; without it, trial production cannot begin and the production period cannot be established.
A time shift of all the traces of one source or receiver point that brings them to a horizontal datum: it removes the delay of the waves in the loose near-surface low-velocity zone (weathering layer) and the effect of relief. Errors in the statics break the reflections into steps, weaken them in stacking and create false low-amplitude structures.
The total electrochemical electromotive force (EMF) of a clean water-bearing bed: the deflection of the SP from the shale baseline that there would be if no currents flowed. It is proportional to the logarithm of the ratio of the equivalent resistivities of the filtrate and the formation water: about 71 mV for a tenfold contrast at 25 °C.
The rubber-like material in which the helical cavity of the stator of a progressing cavity pump is formed. It is chosen according to the temperature and the composition of the fluid — aromatic hydrocarbons, gas, hydrogen sulfide and carbon dioxide; swelling, blistering by gas, ageing and running without liquid are the main causes of stator failure.
A model in which the influx rate is proportional to the pressure decline at the contact, , because the pressure at the remote boundary of the aquifer is constant. It suits aquifers with recharge and serves as an empirical description over a limited stretch of the history.
A regime in which the pressure and flow rate at every point of the formation do not change with time: the withdrawal is replaced across the outer boundary by injection or by an aquifer. Inflow to the well in this regime is described by the Dupuit equation.
Calculation of pseudo relative permeabilities on the assumption of steady-state flow in a coarse cell: the saturation of the fine cells is set by the capillary equilibrium limit (the capillary pressure is the same) or the viscous limit (the water fraction in the flow is the same), and the phase permeabilities are upscaled as single-phase ones. The capillary equilibrium limit suits thin layers and slow flow, the viscous limit suits cases where capillary forces can be neglected.
A way of numerically finding the value of a function of time from its Laplace transform: a weighted sum of the values of the transform at several points on the real axis. It is used to calculate well pressure curves for which solutions are known only as Laplace transforms.
A calculation of waterflooding of a layered reservoir without crossflow: displacement in the layers is piston-like, the fronts move at velocities proportional to (as for a mobility ratio of 1), and the water cut accounts for the mobility of water in the flooded layers and of oil in the others. It gives the vertical sweep and the water cut at the breakthrough of each layer.
The quantity of oil in the reservoir before the start of development, referred to surface conditions.
The gas liberated in the stock tank from the oil of the last separator at a pressure close to atmospheric. There is little of it by volume, but it is heavy — rich in propane and butanes — and it is included in the solution gas–oil ratio of the oil. It is compressed or captured; when it is flared, both the gas and its valuable components are lost.
Oil from which the dissolved gas has been liberated under the given separation conditions, brought to standard conditions. The solution gas–oil ratio and the oil formation volume factor are referred to its volume.
Schemes for calculating the oil relative permeability with three mobile phases from the two-phase water–oil and gas–oil tables. The second model: .
The change in the size and shape of a body, expressed dimensionlessly: as the relative elongation , the shear strain , or the change in volume. The shape of a rock is changed by the difference between the principal stresses; the strain rate is measured in s⁻¹, and in tectonics it is of the order of s⁻¹.
Under Kazakhstan's Subsoil Code, an area with geological reserves of more than 50 million t of oil or 15 billion m³ of natural gas, any area in the Kazakhstan sector of the Caspian Sea, or an area with a uranium deposit. The state priority right applies to such areas.
The marker bed to which a correlation panel is flattened: the wells are shifted vertically so that it lies horizontal. If the marker was deposited horizontally, the panel removes later deformation and shows the relationships of the beds at the time of its deposition. Not to be confused with the datum depth for pressures.
A trap whose form is determined by the conditions of deposition or by changes in the rocks (pinch-out, facies change, cementation, an unconformity, a reef) rather than by deformation. Pinch-out traps and traps beneath an unconformity also need the beds to be tilted.
The electromotive force that arises when a solution flows through a porous medium, for example filtrate through a mudcake. It is usually small, but with a high overbalance and in tight beds it distorts the SP curve and makes the estimate of from it unreliable.
A calculation method in which the pressure is found on a grid and the transport of saturation is calculated as a one-dimensional problem along streamlines from injection wells to producing wells. It is fast and smears the front little in waterflooding, but handles gravity, capillary forces and compressibility less well.
The loss of strength and increase in permeability of set Portland cement at temperatures above about 110 °C, when the calcium silicate hydrate gel recrystallises into α-C₂SH. It is prevented by adding 35–40% of silica by mass of cement.
The decrease in rock permeability as effective stress increases, owing to the narrowing of pore throats and the closing of fractures. It is small in strong cemented rocks and large in weak, tight clayey and fractured ones; it is often irreversible.
A characteristic set of folds, faults and other structures that arises under a single type of deformation — extension, compression, strike-slip, inversion or salt flow. Once the style has been recognised from few data, structures are predicted where there are no data.
A trap created by deformation of the rocks after their deposition: a fold, a fault or salt movement. It is younger than the enclosing rocks and may turn out to be younger than the migration of the hydrocarbons.
A line of equal absolute elevation (or depth) of a geological surface: the top or base of a bed, or a fault. Structure contours run along the strike, and closer spacing of contours means a steeper dip: . A map of structure contours is a structure map.
A seal on the wire where it leaves the lubricator: packing that is tightened by hand or hydraulically. Braided line and cable are sealed differently, with thick grease injected into tubes with a small clearance around the line.
A serrated pressure-solution seam in carbonate rocks, less often in sandstones, with a film of insoluble residue. A seam lined with clay residue can have low permeability across it, and the dissolved material is redeposited as cement in neighbouring pores; stylolites are a common cause of vertical heterogeneity in carbonate reservoirs.
The descent of an oceanic lithospheric plate beneath another plate into the mantle at a convergent boundary. Above the subduction zone are a deep-sea trench and a chain of volcanoes (an island arc or a volcanic belt at the edge of a continent); earthquakes can be traced along the descending plate to a depth of about 700 km.
A body at the foot of the continental slope, built by gravity flows: channels with levees on the slope and lobes at their mouths. In the channel axis the sand beds amalgamate (the amalgamation ratio is about 0.6) and at the margins they are separated by clays; the lobes are sand sheets kilometres to tens of kilometres wide.
An oil-filled electric motor operating in the well below an electric submersible or progressing cavity pump; usually a three-phase induction motor, increasingly a brushless permanent-magnet motor. It is cooled by the flow of fluid going to the pump intake.
Pore pressure lower than the normal hydrostatic pressure for the given depth. Most often these are accumulations depleted by production; in nature it arises from the uplift and erosion of rock sequences and in areas with a deep groundwater level. In drilling it brings the risk of lost circulation and differential sticking.
A development in which the wellheads stand on the seabed and the production flows through a subsea pipeline to a neighbouring platform, a floating unit or the shore. It saves the cost of a dedicated platform, but requires spare capacity at the receiving facility and protection of the long line from hydrates, wax and liquid slugs.
The wellhead equipment of a subsea well on the seabed: a low-pressure housing on the conductor and a high-pressure housing inserted in it on the surface casing, in which all the following strings are hung and sealed. The connector of the subsea blowout preventer stack latches onto the housing, and after completion the Christmas tree does too.
The right to use the subsoil for a fee, for business purposes, within an allocated area and for a defined period. In Kazakhstan it is a real right (a right in rem) and is indivisible; for hydrocarbons it arises from a contract with the competent authority. The subsoil remains the property of the people, and the minerals produced belong to the subsoil user unless the Subsoil Code provides otherwise.
A valve in the tubing string below the wellhead that shuts off the flow if the wellhead equipment is damaged or inaccessible. A surface-controlled valve is held open by pressure in a hydraulic line and closes when this pressure is bled off.
A plunger pump at depth that is driven by a string of rods from a drive at the surface — most often a pumping unit (beam pump). It is the most widespread artificial lift method onshore by number of wells: simple and cheap, it pumps the well down effectively, but it is limited in the combination of depth and flow rate and is sensitive to gas and to wellbore curvature.
Bacteria that live without oxygen and reduce the sulfate in water to hydrogen sulfide. In produced and injected water systems they grow under deposits and cause pitting corrosion; carried into the formation, they cause hydrogen sulfide to appear in the production. Biocides and cleaning of the equipment are used against them.
A quantity that the simulator records at every report step, such as rate, cumulative volume, pressure or water cut, for the field, a group, a well or a region. In a widely used format the name of the series is composed of letters denoting the object, the phase and the kind of quantity: FOPR is the oil production rate of the field.
Costs that have already been incurred and do not depend on the decision being made, and so are not included in its evaluation.
The excess of pressure at the borehole wall over the formation pressure caused by drilling-mud filtrate, which passes through the mud cake faster than a tight formation can carry it away. A formation tester in such a bed shows an overstated pressure; the signs are low mobility and a point to the right of the line of valid measurements.
The volumetric flow rate of a phase at pipe conditions divided by the whole cross-sectional area of the pipe: the velocity at which the phase would flow if it occupied the pipe alone. The sum of the superficial velocities of the phases is the mixture velocity.
The logarithmic pressure derivative taken not with respect to time but with respect to the superposition function of the whole rate history. For radial flow it equals the plateau with the last rate whatever the history, so changes in rate do not create false boundaries on the diagnostic plot; for a single rate it coincides with the derivative with respect to equivalent time.
A property of the linear diffusivity equation: the pressure change caused by several wells or several rate changes equals the sum of the changes caused by each event, calculated as if the others were absent. Pressure build-up analysis, the processing of records with variable rate, interference testing and the method of images are built on it.
The weighted sum of the base-10 logarithms of the times elapsed since each change of rate, with weights . The pressure decline divided by the current rate depends linearly on it, so a record with several rates is processed as a single semilog straight line; for one rate it equals .
The addition to the hydrostatic pressure in the wellbore when the string is lowered: the displaced drilling fluid moves up the annulus, and the friction in it acts on the wellbore wall. It increases with the running speed, with a smaller clearance and with the displaced volume (a string with a closed bottom); when the string is pulled, the same mechanism reduces the pressure — this is swabbing.
A drop in pressure below the bit when the drill string is pulled: the drilling fluid cannot flow under the bit fast enough, and the formation may “suck” fluid into the well. Running in has the opposite effect — a rise in pressure (surge).
An approximate mean of a distribution from three estimates: 0.3 × P90 + 0.4 × P50 + 0.3 × P10. It is exact for symmetric distributions and works well for moderate skew; for very wide skewed distributions it underestimates the mean.
The fraction of the reservoir volume through which the displacing agent has passed: the product of the areal sweep and the vertical sweep . Together with the displacement efficiency it gives the oil recovery factor; it increases with the volume of fluid pumped through the reservoir and is reduced by reservoir heterogeneity and an unfavourable mobility ratio.
A device suspended from the hook above the kelly: it carries the weight of the string, allows it to rotate, and through the gooseneck and a packed washpipe passes drilling fluid from the rotary hose into the rotating string. In a top drive the swivel is built into the drive.
A fold that is convex downwards, with younger rocks in its core.
The producing time that is substituted into the Horner function after a well has produced for a long time in a bounded area or among producing neighbours: over such a period a well started in a reservoir at a pressure equal to the pressure at the boundary of its drainage area would have created nearly the same pressure distribution as existed at the moment of shut-in. With it, the extrapolation of the Horner line, , equals the pressure at the boundary of the area; for a well at the centre of the area it is about half the time needed to reach the pseudo-steady-state regime.
A seismic trace calculated from the sonic and density logs: the reflection coefficients of the boundaries, placed on a two-way-time axis, are convolved with a wavelet. It is used to find out which reflection on the section corresponds to which formation, and to check the polarity and phase of the data.
The value in a distribution that separates capillary-bound water (shorter times) from free fluid (longer times). Typical values are about 33 ms in sandstones and 90–100 ms in carbonates; for each rock type it is refined using core that has been centrifuged to irreducible water saturation. Clay-bound water is separated off by a boundary at about 3 ms.
The level of complexity of a multilateral well junction according to the classification adopted in 1997 by the TAML group of operators and service companies: from 1 — an open hole without support — to 6 — a junction that is pressure-tight by virtue of the casing itself. At level 5 the pressure integrity is provided by the packers and the tubing of the completion.
A representation of an accumulation as a single volume with one average pressure and phase equilibrium, without any spatial distribution of properties. Material balance is based on it.
A step-by-step forecast of the performance of an accumulation with a solution gas drive: at each pressure step the oil production is chosen so that the gas required by the material balance coincides with the gas produced at the instantaneous gas–oil ratio. Tracy's form is the same calculation expressed through pressure functions.
Imprecise knowledge of what is determined by the subsurface, the wells and the surface facilities: volumes, rates, recovery factor, costs, schedules and the reliability of equipment. Unlike market uncertainty, it can be reduced by data: appraisal work, studies and development experience.
The end point of the cumulative cash flow curve: all the project's receipts less all costs and payments, including decommissioning. It equals the NPV at a zero discount rate and does not take into account when the money was received.
The gauge gas pressure at which a gas lift valve opens on the test rack at standard temperature (usually 15.6 °C, 60 °F) and atmospheric pressure on the tubing side: , where is the nitrogen pressure in the dome at the temperature of the test rack. The dome is charged with nitrogen with a correction for the temperature of the valve in the well.
A reservoir simulation model in which temperature is an unknown: an energy balance, with heat transport, phase changes of water and heat losses to the adjacent rocks, is added to the mass balances. It is used for steam injection and in-situ combustion in heavy-oil accumulations.
A test of a rock sample with an axial hole, which is squeezed from outside by pressure until the wall of the hole fails. The wall is loaded like the wall of a perforation tunnel or a wellbore, with zero radial stress on it, which is why the result is used to predict sand production.
The time in which a cement slurry at well pressure and temperature becomes unpumpable — in a consistometer, reaching 100 consistency units (Bc). It must exceed the pumping and displacement time with a margin; it is controlled with accelerators and retarders.
A thin layer with permeability much higher than its neighbours, through which most of the injected water or gas travels: it causes the producing wells to water out early and leaves oil in the other layers. Isolating it at the well helps only if the layer is not in communication with the adjacent layers.
The ratio of the thorium content (ppm) to the potassium content (%) from spectral gamma ray logging. It indicates the type of radioactive minerals: less than 0.5 for potassium feldspars, about 1–4 for micas and illite, more than 10 for kaolinite and chlorite. The fields overlap, and in the ratio for a mixture the mineral rich in potassium predominates.
A pressure vessel in which well production is separated into gas, oil and water: the gas leaves from the top through a mist eliminator, water droplets settle through the oil layer, the oil spills over a weir, and the water is drawn off from the bottom under an interface level controller. The size of such a vessel is usually governed by the retention time of the liquid, not by the gas.
A graph of the vertical throw along the strike of a fault, taken from a map or from seismic data. For a single fault it is smooth, without a change of sign, with its largest value near the middle and zeros at the tips; jumps point to a mapping error, a linkage of segments or an intersection with another fault.
Oil from low-permeability rocks, produced by horizontal wells with multistage hydraulic fracturing; it is more often called shale oil (not to be confused with synthetic oil from oil shale). The wells are brought on stream quickly and depleted quickly, so this production responds to price faster than other kinds.
An oil–water or gas–water contact tilted by the flow of formation water: it dips in the direction of flow times more steeply than the potentiometric surface (for a gas–water contact, times). A tilt can also exist without flow, if equilibrium has not yet been established after late tectonic movements.
A map of a reflecting horizon in time: contours of equal two-way time . It is not yet a structure map: its relief is distorted by changes in the velocities above the horizon, and the trap is checked on a map in depth.
The principle that an amount received earlier is worth more than the same amount received later: money received earlier can be invested to earn a return.
An empirical relationship between the interval transit time and the porosity of clean, compacted rocks: . The travel times in the matrix and in the fluid add up in proportion to their volume fractions.
A hydraulic fracturing technique in which a small pad leaks off into the formation just as the fracture reaches the required length, and the first proppant-laden slurry blocks the fracture tip. The length stops growing, further pumping widens the fracture and packs it with proppant, and the net pressure rises steeply. This produces short, wide fractures, including those for frac-packs.
A measure of drilling line wear — the sum of the products of load and distance travelled during trips, drilling, running casing and other operations; under API RP 9B it is calculated in ton-miles (1 ton-mile ≈ 1.46 t·km). The line is slipped and cut according to a programme based on the accumulated work; inspection of the line supplements the programme.
The thickened ends of a drill pipe with coarse tapered threads — a pin at the bottom and a box at the top — welded to the upset ends of the pipe. The connection is sealed by the shoulders, pressed together by the make-up torque, not by the threads.
The volume of liquid in the flowline and pretest chamber of a formation tester multiplied by its compressibility: how much fluid must come from the formation for the pressure at the gauge to rise by one unit. It sets the time constant of the pretest, ; until its influence has ended, the drawdown and build-up are distorted.
The direction of the bend of a deflecting assembly, measured clockwise from the high side of the hole (gravity toolface) or, in a nearly vertical hole, from north (magnetic toolface). 0° builds inclination, 180° drops it, and 90° and 270° turn the well to the right and to the left.
A motor suspended from the travelling block that rotates the drill string through its own shaft and feeds drilling fluid into it. It makes it possible to drill with stands, and to rotate and circulate the string while pulling out; ball valves are built into it. When drilling with a rotary table, the string is rotated by the kelly.
The depth to which the cement slurry has been raised behind the casing string. The conductor and surface casing are cemented to the surface; the other strings are cemented up so as to overlap the shoe of the previous string, or to above all the formations that need to be isolated.
Horizontal bars showing the swing in a project indicator as each parameter is varied from one end of its range to the other; the parameters are arranged in descending order of swing. It ranks the parameters by influence but does not show probabilities.
An anchor below the stator of a progressing cavity pump that bears against the casing and prevents the reaction torque from turning the stator and unscrewing the tubing threads.
The sum of the fluid compressibilities weighted by saturation and the pore compressibility: . It shows how much fluid a unit of pore volume releases when the pressure drops by one unit; the product is called the elastic storage capacity coefficient of the formation in Russian-language literature.
The sum of the cross-sectional areas of all the bit nozzles. For a given flow rate it determines the jet velocity and the pressure drop across the bit; nozzle diameters are usually stated in 32nds of an inch.
The volume of the oil together with the gas liberated from it, per unit volume of stock-tank oil: . Above the bubble point pressure it equals ; it is used in material balance.
The sum of the phase mobilities, : the coefficient of the pressure equation for multiphase flow. It depends on saturation and is usually at a minimum in the two-phase zone, where the phases impede each other.
The mass fraction of organic carbon in a rock, in %. Rocks with a TOC of at least about 0.5% are regarded as source rocks, and those with 1–2% or more as good ones. It is measured by combusting a sample after the carbonates have been removed with acid.
The fraction of the rock volume occupied by all voids, both interconnected and isolated. On core it is measured on a crushed sample; it is total porosity that the density log gives.
A nonlinear finite-difference scheme that does not increase the total variation of the profile : it is second-order on smooth stretches, while at jumps and extrema a limiter switches it to an upwind scheme. It reduces numerical dispersion without oscillations or unphysical saturations at the front.
The sum of the phase Darcy velocities, . In incompressible flow its divergence is determined only by the wells; the pressure equation is written in terms of it, and the water flux as the fraction of the total flux plus gravity and capillary fluxes.
A regime in which the reservoir pressure changes with time differently at different points: the drawdown cone around the well is still expanding and has not reached the boundaries, so the reservoir behaves as an infinite one. It occurs after a start-up, shut-in or change of rate; well testing is based on it.
The ratio of the flow rate to the difference between the initial reservoir pressure and the bottomhole pressure in transient flow. It falls with time as the radius of drainage grows, so the productivity from a short test is higher than that at which the well will operate.
The interval above the oil–water (gas–water) contact in which water saturation decreases with height from unity to the irreducible value; both phases are mobile here, and wells produce hydrocarbons together with water.
The coefficient relating the flow between adjacent model cells to the pressure difference between them; it is determined by the cell dimensions and their permeability, averaged in series (harmonically).
A coefficient by which the calculated transmissibility between model cells is multiplied: 0 makes a fault or thin layer impermeable, values between 0 and 1 make it partly conductive. It is the main way of testing hypotheses of reservoir compartmentalisation during history matching.
Calculation of the transmissibility of the connection between two neighbouring coarse cells directly from the detailed model, that is, from the flow rate between them and the difference of their pressures in a fine-grid calculation, without an intermediate equivalent permeability of the cells. It preserves the position of barriers and channels within the cells.
The time constant of decay of the NMR signal, that is, of the amplitudes of the train of spin echoes. In the wetting phase it is determined mainly by the ratio of the pore surface to the pore volume, so the distribution reflects the pore size distribution; bulk relaxation of the fluid and diffusion in a field gradient shorten it.
A part of a reservoir where hydrocarbons cannot migrate any further and accumulate.
Closed bodies of mobile water inside an accumulation (sand lenses, blocks, depressions) from which oil or gas was unable to displace the water as the trap filled. Its pressure does not fall on the oil line, and the contact of the accumulation is not drawn from it; it should not be confused with the residual water in the pores.
In Kazakhstan, the base project document under which a discovered accumulation is put on trial production during the exploration period: temporary operation of wells and production for research purposes, to obtain data for the development project. It is prepared on the basis of the report on the operational estimate of reserves within three months of the decision to carry out trial production; the trial production itself lasts no more than three years.
A distribution defined by the minimum, the most likely and the maximum values: the probability density rises linearly from the minimum to the most likely value and falls linearly to the maximum. The mean equals one third of the sum of the three values. It is convenient when only three expert estimates of a parameter are known.
A narrow, tall tank connected to the well for accurate accounting of drilling fluid volumes during trips: how much is added when pulling out and how much is displaced when running in.
The footage drilled in a bit run divided by the drilling time together with the trip time: . Its maximum gives the moment at which to pull the bit, when the price of the bit is small compared with the cost of rig time.
The electrical resistivity of the formation beyond the invaded zone, the basis for calculating water saturation. It is not measured directly: it is obtained from deep-reading resistivity tools with corrections.
The thickness of a bed measured along the normal to the bedding. In a well it is found from the length of the wellbore in the bed and the angle between the wellbore and the normal to the bed: . A map of true stratigraphic thicknesses is an isopach map.
The vertical distance from the zero datum, usually the rotary table, to a point on the wellbore. Hydrostatic pressure is calculated from true vertical depth, while friction losses and volumes are calculated from the length along the wellbore (measured depth, MD).
The thickness of a bed measured vertically between its top and base at a given point; in a vertical well it equals the difference between the depths of the top and the base. It is related to the true stratigraphic thickness through the dip angle: . In a deviated well the difference in true vertical depth between the top and the base equals the TVT only if the wellbore runs along the strike of the bed.
The residual left when the exact solution is substituted into the difference equation. Its leading term is proportional to a power of the step, ; the exponent is called the order of approximation: when the step is halved, the error falls by a factor of about . A scheme approximates the equation if the residual tends to zero as the grid and the time step are refined.
A string of pipe inside the casing string through which production flows from the bottom of the well to the surface or through which fluids are injected into the formation.
The component on which the tubing string hangs in the tubing head at the wellhead. It seals the annulus and passes the control line of the safety valve or the pump cable; the Christmas tree is installed on top of the tubing head.
The lengthening and shortening of tubing sealed in a packer when pressures and temperature change: due to the pressure force on the seals, ballooning (the Poisson effect), temperature and helical buckling. If the seals are anchored, the change in length turns into a force that is taken by the tubing, the packer and the hanger.
The thickness of a bed at which the reflections from its top and base add up to the greatest amplitude; for a Ricker wavelet it is about in time, roughly a quarter of the wavelength at the dominant frequency. For thinner beds the time between the reflections ceases to decrease, and the amplitude falls almost in proportion to the thickness.
The deposit of a turbidity current (a cloud of suspended sediment flowing down a slope): a graded bed with an erosional base, ideally with the Bouma sequence of intervals. Each bed is one event; packages of turbidites make up the channels and lobes of submarine fans.
A generator with a turbine driven by the flow of drilling fluid, serving as the power source of the downhole MWD/LWD module instead of lithium batteries or together with them. It provides more power and is not limited in time, but it works only while fluid is circulating within a given range of flow rates and is vulnerable to debris and erosion.
A downhole motor in the form of a multistage turbine driven by the flow of drilling fluid. It gives a high rotational speed at moderate torque; at a constant flow rate the torque falls as the speed rises, while the pressure drop hardly depends on the load. It has no rubber parts and withstands high temperature.
Calculation of the flow across a face from the pressures of only the two neighbouring cells, using the transmissibility of the two half-cells. It is exact if the line joining the cell centres is perpendicular to the face and the axes of anisotropy coincide with the grid axes (a K-orthogonal grid); otherwise its error does not vanish with grid refinement.
The time taken by a reflected wave to travel from the surface to the reflecting boundary and back, reduced to the vertical; time sections of seismic data are built from it. Depth is found from the interval velocities of the layers: in each layer .
A calculated relationship between dimensionless pressure change and dimensionless time for a chosen model of well and reservoir; usually a family of curves with a parameter on a log–log scale. For a well with wellbore storage and skin, is plotted against with the parameter .
Overlaying the pressure record on a log–log scale on a family of type curves, shifting it parallel to the axes. The pressure shift gives the flow capacity , the time shift gives the wellbore storage coefficient, and the parameter of the matching curve gives the skin. The result is reliable if the record has reached radial flow.
When the string contains heavy cement slurry while the annulus still contains drilling fluid, the fluid in the string falls faster than the pumps deliver: the pump pressure drops to zero, a void forms at the top of the string, and the return from the annulus is greater than the pump rate. Then the pumps fill the void, and the return becomes smaller than the pump rate.
A scale of clast sizes in which each boundary is twice the previous one: clay is finer than 1/256 mm, silt goes up to 1/16 mm, sand up to 2 mm (very fine, fine, medium, coarse and very coarse), then gravel, pebbles, cobbles and boulders. The decimal scales of Russian-language practice draw some of the boundaries differently, at 0.1, 0.05 and 0.01 mm.
The highest stress that a cylindrical rock sample withstands when compressed along its axis without lateral confinement. For weakly cemented sandstones it is a few megapascals, for strong rocks tens or hundreds; it is the main strength parameter in assessments of sand production and wellbore stability.
A surface separating sequences between the deposition of which there was a break: with no accumulation of sediments, or with erosion. In an angular unconformity young beds lie on truncated, tilted beds; in a parallel unconformity the beds above and below are parallel; the time missing at the unconformity is estimated from the ages of the rocks above and below it.
Perforating with the pressure in the well lower than the reservoir pressure. The instantaneous surge of inflow immediately after the shot carries debris and part of the crushed zone out of the tunnels; in overbalanced perforating, by contrast, the tunnels become plugged with well fluid. Excessive drawdown in weak rocks causes sand production.
The state of rapidly buried clays from which the water has not had time to be squeezed out: the fluid carries part of the weight of the rocks, the pore pressure is higher than normal, and the porosity is higher and the density and sonic velocity lower than under normal compaction at the same depth. It is the most common mechanism of overpressure.
The volume of oil, gas and water produced, referred to reservoir conditions at the current pressure: oil with dissolved gas, free gas and water. The left-hand side of the material balance equation.
Oil at a pressure above the bubble point pressure: it could dissolve more gas, and there is no free gas in the reservoir. Oil at or below the bubble point pressure is called saturated.
Deformation in one direction only, with the lateral dimensions unchanged. A laterally extensive formation is in this state as pressure declines: it is compressed vertically, while the surrounding rocks hold it at the sides. The pore compressibility is then of that measured in hydrostatic compression.
The ratio of grain sizes , where is the grain size such that % of the sand mass is coarser; in soil mechanics the same number is written as , using the fraction that passes through the sieve. Up to 3 the sand is well sorted, above 5 it is poorly sorted; the value determines whether a screen without gravel will hold the sand.
Capital and operating costs and decommissioning, divided by production over the life of the project, excluding payments to the government. It is calculated without discounting or on discounted values, at one rate for both costs and production.
Writing off the cost of a field in the same proportion as its reserves are produced during the year: the year's depreciation equals the cost multiplied by the ratio of the year's production to reserves. It is the main method of depreciating oil and gas assets; a revision of reserves changes only future years.
The section of a record on a log–log plot where the pressure change is proportional to time and the points lie on a straight line of slope 1. At the start of the record it signifies pure wellbore storage and makes it possible to find its coefficient.
A model in which the influx is calculated by solving the diffusivity equation for the aquifer: the pressure decline propagates gradually from the contact, and the influx depends on the entire pressure history at the contact. It is the general case; the simple models are its limiting forms.
Transfer of reservoir properties from a detailed grid to a coarser one while preserving its flow behaviour.
An assumption about how a block cut out of the reservoir exchanges fluid with its neighbours when its equivalent permeability is calculated: impermeable lateral faces, periodic conditions (the block repeats like a tile) or linear pressure on all faces. Their influence is reduced by a buffer zone, a rim of neighbouring cells around the block.
The first link in the oil business: exploration and appraisal, development of fields, and production. The following links are transportation and storage (midstream) and refining and marketing (downstream). The greatest risk and the greatest rent lie in exploration and production.
A rule of the finite-difference scheme: the phase mobility at the face between cells is taken in the cell from which the phase flows. It allows the displacing phase to enter cells where it is still immobile and keeps the saturations within physical limits; the price is numerical smearing of the front.
The base-10 logarithm of the ratio of the areas under the capillary pressure curves for displacement of water by oil and of oil by water, . It is positive for a hydrophilic rock and negative for a hydrophobic one; it usually lies between −1 and +1.
The category of a drill pipe according to inspection results: “premium” — remaining body wall thickness of at least 80% (two white bands), class 2 — at least 70% (a yellow band), class 3 — more wear; pipes with cracks and washouts are rejected (a red band).
The relationship between the “value” of an amount of money to a decision-maker and the amount itself. Options are compared by expected utility, the mean utility of the outcomes weighted by their probabilities. For a risk-averse decision-maker the function is concave: each successive million is valued less than the previous one.
The increase in the expected value of a decision from being able to obtain data before it is made: the EMV of the best decision after the data minus the EMV of the best decision without them, in both cases with the right to abandon the project. It is the largest price worth paying for the data; it is zero if no result of the data changes the decision.
The difference between the gas pressures at which a bellows-type injection-pressure-operated valve opens and closes: , where . For unloading valves it is made small so that they close reliably in turn.
The pressure at which the liquid and gas of a pure substance are in equilibrium at a given temperature. It rises rapidly with temperature; the vapour pressure curve ends at the critical point.
A device that changes the frequency and voltage of the current supplying the motor, and thereby the rotational speed of the pump. It allows a submersible motor to be started smoothly and the delivery of an electric submersible or progressing cavity pump to be adjusted to the inflow without pulling the equipment.
Half the mean squared difference between the values of a property at pairs of points separated by a distance , as a function of that distance and of direction. It describes spatial correlation: it rises from the nugget effect to the sill, which it reaches at the correlation range; it is the basis of kriging and stochastic modelling.
A tubing string of smaller diameter run late in the life of a well to increase the flow velocity, reduce gas slippage and liquid accumulation, and prolong natural flow. It gives a delay but does not replace artificial lift.
The assumption that the pressure of each phase in any cross-section of a reservoir is distributed hydrostatically over the thickness: vertical crossflows equalise it faster than the front advances between wells. It is justified when vertical equilibration is much faster than advance along the reservoir: at a large effective length-to-thickness ratio , and, for gravity segregation of the phases, at a large ratio of the time to traverse the reservoir to the time of gravity segregation. It allows the section to be reduced to a one-dimensional problem with thickness-averaged functions.
The factor by which the vertical scale of a section is larger than the horizontal scale. On such a section the tangent of every inclination is multiplied by : ; gentle folds look steep and faults look almost vertical, so angles are measured from a section without distortion.
A test with a multi-probe formation tester: a pump withdraws fluid through one probe, while an observation probe at some vertical distance records the pressure disturbance that arrives. The level of the response gives the horizontal permeability and the speed with which it is established gives the vertical permeability; damage and storage at the sink probe do not affect the response.
The smallest bed thickness for which a logging tool shows the true value of the bed. In thinner beds the reading is mixed with the readings of the enclosing rocks, and the bed looks thicker and “paler” than it is.
The projection of the horizontal displacement of a point of the wellbore onto a vertical plane passing through the wellhead along the planned azimuth — usually towards the target. It and the true vertical depth are used to plot the well profile; the departure of the wellbore from this plane is shown separately.
Recording of the full wavefield by receivers placed in the well at intervals of 10–20 m, with a source at the surface. The downgoing and upgoing waves are separated: they are used to find the wavelet and velocities at the well, to distinguish primary reflections from multiples, to obtain a trace for the tie and to predict the section below the bottom of the hole.
Surfactants whose molecules in a salt solution assemble into long wormlike micelles, so that the solution becomes viscous and elastic without any polymer. They are the basis of fracturing fluids with no polymer residue and of self-diverting acids: the viscosity rises as the acid is neutralised and disappears on contact with hydrocarbons.
The fraction of light reflected by polished vitrinite particles in immersion oil, in %. It increases irreversibly with heating and is the main indicator of the maturity of organic matter: the oil window is roughly 0.6–1.3%, and gas generation lies above that.
An empirical dimensionless inflow performance curve of a well in a solution-gas drive reservoir: , where .
The ratio of the volume of water (or gas) injected to the volume of oil, gas and water produced, referred to reservoir conditions; at 100% the withdrawal is fully replaced and the reservoir pressure is maintained.
Oil with a high content of light and intermediate components: the reservoir temperature is close to the critical temperature, and a large amount of gas is liberated quickly below the bubble point pressure. The gas–oil ratio is about 310–570 m³/m³, and the formation volume factor is greater than 2.
A correction to the molar volume calculated from a cubic equation of state: a constant , specific to each component, is subtracted from it. It corrects the systematic error in liquid density and does not change equilibrium ratios, saturation pressures or phase compositions.
The drive mechanism of a gas accumulation without water influx: gas is produced only through its own expansion, and the pore volume occupied by gas remains practically constant. Gas recovery is determined by the abandonment pressure and is usually 70–90%.
A way of managing migrating gas in a shut-in well without circulation: the annulus pressure is allowed to rise by a set step and a portion of fluid corresponding to that step is bled off, so that the bottom-hole pressure stays no lower than the formation pressure. When the gas reaches the wellhead, it is replaced with fluid: portions of fluid are pumped in and the gas is bled off.
The product of the photoelectric factor and the electron density, , in barns/cm³: the probability of photoelectric absorption of gamma rays per unit path length. Unlike , it adds up in proportion to the volume fractions of the minerals and the fluid, so it is used to calculate of mixtures and the apparent matrix volumetric cross section.
A stability check of a linear finite-difference scheme: the error is decomposed into harmonics and for each one the amplification factor is found, that is, the factor by which its amplitude changes in one step. The scheme is stable if for all harmonics; for the explicit scheme for the pressure equation this gives .
A sandstone with a large proportion of fine clay–silt matrix, from 15 to 75% (arenites have less than 15%). The matrix plugs the pores, so wackes are usually poor reservoirs. In the English-language literature wackes are also called greywackes; in the Russian-language literature a greywacke is a dark sandstone in which rock fragments predominate.
A well-killing method in one circulation: the kill mud is prepared first, and then the influx is circulated out directly with it, the standpipe pressure being reduced on a schedule from the initial to the final circulating pressure. Its peak annulus pressure is usually lower than that of the driller's method, but time is needed to weight up the fluid.
The time after cementing while the set cement gains enough strength to continue operations — drilling out the float equipment and the shoe, pressure testing. It depends on the temperature and the slurry formulation; accelerators shorten it.
Only deposits of environments that were adjacent to one another laterally at the time of accumulation can lie on top of one another without a break. At erosion surfaces, unconformities and faults the law does not apply.
The effective concentration of water in a solution: 1 for pure water and less in salt solutions; it equals the relative humidity of the air above the solution at equilibrium. Water moves across a semipermeable membrane towards where its activity is lower; for this reason the water activity of an oil-based drilling fluid is balanced with that of the clays.
A reduction in the phase permeability to oil or gas caused by increased water saturation near the wellbore after filtrate or kill fluid has invaded. It is especially dangerous in low-permeability and gas formations, where capillary forces hold the water for months.
The ability of a water to mix with other waters and to contact the formation rock without precipitation of salts — barium and strontium sulfates, calcium carbonate — and without swelling and migration of clays. It is checked by calculating saturation indices, by mixing samples at operating temperature and by flowing the water through core.
The fraction of water in the produced liquid, usually at standard conditions. It is determined by the fraction of water in the flow in the reservoir, which depends on saturation, relative permeabilities and viscosities.
The temperature at which water begins to drop out of gas at a given pressure. The less water there is in the gas, the lower it is; as pressure rises, the same water drops out at a higher temperature, so the dew point is always quoted together with the pressure — in the technical requirements for gas for main gas pipelines, at 3.92 MPa.
A drive mechanism in which the oil withdrawn is replaced by water from the aquifer — through the elastic expansion of the water and rock of the aquifer (elastic water drive) or through its recharge from outside (rigid water drive). The water reaches the accumulation through the formation from the flanks of the structure (edge water) or underlies it over the whole area (bottom water). The pressure remains high, the gas–oil ratio is constant and the water cut rises early; oil recovery is usually 35–60%.
The volume of water that has entered an accumulation from the aquifer in response to the decline in reservoir pressure.
The pumping station of a pressure maintenance system: multistage centrifugal pumps raise the pressure of the water and deliver it through high-pressure water lines to the water distribution units of a group of injection wells. In the packaged (modular) version it is assembled from factory-made blocks.
Fine droplets of formation water suspended in oil and protected by a film of natural emulsifiers — asphaltenes, resins and fine particles. It forms when the flow is broken up in chokes and pumps; it is broken by heating, demulsifiers and an electric field, after which the water is separated by settling.
The mass fraction of salt (usually calcium chloride) in the brine of an oil-based drilling fluid; it is found from the chlorides of the whole fluid and the water fraction from the retort. It determines the density of the brine and the water activity of the fluid.
The mass of mixing water per unit mass of cement in a cement slurry; for classes G and H in testing, 0.44 and 0.38. The higher it is, the lighter the slurry and the weaker and more permeable the set cement, unless a filler binds the excess water.
Injection of water into the reservoir through injection wells to maintain reservoir pressure and displace oil towards the producing wells; the most widespread method of developing oil fields.
The temperature below which wax crystals appear in oil on cooling; it is determined in the laboratory. Where the flow or the pipe wall is colder than this temperature, wax deposits grow; the temperature profile of the well is used to find the depth above which they should be expected.
A model of the conductivity of a shaly sandstone in which the exchangeable cations of the clays conduct current in parallel with the pore water: . is the cation exchange capacity per unit pore volume (from core), and is the equivalent conductance of the cations, which grows with temperature and falls in fresh water.
The part of the formation between two intersecting sealing faults. In the late part of a test the pressure derivative reaches a level times higher than in radial flow ( is the angle between the faults); the angle is found from it.
The cost of the company's equity and debt weighted by their market shares: . The cost of debt is taken after tax because interest reduces corporate income tax.
A finely ground heavy mineral added to drilling fluid to raise its density; the main one is barite (about 4.2 g/cm³). The denser the weighting material, the smaller the amount of solids that has to be added to reach the same fluid density.
A displacement calculation from the fractional flow curve: the tangent from the point of initial saturation gives the saturation at the front, and its intersection with the line gives the average saturation behind the front. After breakthrough, the same formula for the saturation at the outlet gives oil production as a function of water injected.
The removal from the near-wellbore zone of the filtrate and solids of the drilling fluid, perforation debris, acid or fracturing fluid during the first hours and days of production of a new or treated well; the skin decreases as this happens. While clean-up is under way, the permeability and skin obtained from the drawdown curve come out too high, and the pressure may “stabilise” or rise at a constant flow rate.
The operations and equipment that turn a drilled hole into a working well: the bottom-hole design (open hole, a screen or a cemented, perforated casing), the flow path, and the tubing string with the equipment for controlling the flow and safely shutting in the well.
Cleaning the well of filtrate and process fluids and operating it at a low steady rate at which the bottomhole flowing pressure is above the saturation pressure and the gas–oil ratio, pressures and water cut are stable.
Comparison of well sections: which beds, units and surfaces in one well correspond to which in another. Lithostratigraphic correlation connects similar rocks, chronostratigraphic correlation connects surfaces of the same age; the result is a correlation panel and consistent picks for maps and models.
The coefficient of the well model in a reservoir simulator, : it relates the well's rate to the difference between the cell pressure and the bottomhole flowing pressure; is the equivalent radius of the cell.
A change in pressure in a well caused by the operation of neighbouring wells in the same formation. The disturbance from a change in a neighbour's rate arrives with a delay of the order of ; as long as the neighbours' rates are constant, their influence on the well under test is included in the pressure trend, while changes in rates distort the analysis of its tests.
Measurements of the physical properties of rocks along the wellbore, made with tools run on a cable or incorporated in the drill string assembly.
A site from which several, and sometimes dozens of, inclined and horizontal wells are drilled to different points of the reservoir. It reduces the number of roads, power lines and flowlines and the area of disturbed land; the departure of the bottom-hole locations from the wellhead is limited by the capabilities of directional drilling.
A plot of bottomhole pressure against flow rate on which the inflow line from the reservoir and the lift (outflow) curve are superimposed; their intersection is the operating point of the well.
The planned trajectory of the wellbore in the vertical plane, made up of vertical sections, sections that build and drop inclination (circular arcs) and straight inclined sections. The most common profiles are build-and-hold and S-shaped.
Measures that increase the productivity of a well: removal of damage by acid and other chemical treatments, bypassing the damage and increasing the effective wellbore radius by hydraulic fracturing, and re-perforation. “Near-wellbore treatment” usually means chemical treatment of that zone.
Measurements of bottomhole pressure and flow rate during a deliberate change in the operating regime of a well. The pressure response is used to determine the flow capacity of the formation, skin factor, reservoir pressure, and the boundaries and volume of the drainage area.
The pressure calculated by the simulator for the cell through which the well passes. It exceeds the bottomhole flowing pressure by the pressure drop in the vicinity of the well and is not equal to the average pressure over the area of the cell; the well model links these pressures.
The flow of fluid from a formation at higher pressure into a formation at lower pressure through a shared wellbore — above all in a shut-in well or at a low withdrawal rate. It carries foreign oil, gas and water into the formation, and salts and particles with them.
The rising of gas and the settling of liquid in a well closed at the surface. The gas volume hardly changes, so the gas carries pressure from depth to the surface, and the pressure in the wellbore rises; if it exceeds the reservoir pressure at the well, part of the liquid goes back into the formation, and the pressure on the build-up curve passes through a maximum — a “hump” — and falls for some time.
An effect in which, at the beginning of a test, the pressure change is determined by the compression or expansion of the fluid in the wellbore rather than by inflow from the reservoir.
The volume of fluid that the wellbore gives up or takes in when the bottomhole pressure changes by one unit, m³/MPa or bbl/psi. For a wellbore filled with a single fluid it equals the product of the fluid compressibility and the volume of the wellbore below the shut-in valve; free gas in the wellbore increases it by a factor of tens.
The pressure in the Christmas tree above the tubing of a producing well (tubing head pressure); it is maintained by a choke or set by the pressure in the gathering system.
Gas that remains single-phase in the reservoir at any pressure but yields a hydrocarbon liquid, condensate, in the separator. “Wet” refers to the hydrocarbon liquid, not to water.
The tendency of a rock surface to be in preferential contact with one of the liquids; it determines the distribution of the phases in the pores.
The phase preferred by the rock surface: it coats the grains with a film, occupies the small pores and constrictions, and is imbibed spontaneously. The non-wetting phase collects in the centres of the large pores.
A long steel wedge with an inclined surface that is set in the wellbore at the kick-off depth: the mills cut a window in the casing along it, and the bit then goes off into the new wellbore. It is needed for kicking off sidetracks and the branches of multilateral wells.
The volume-weighted harmonic mean and arithmetic mean of the permeabilities of the parts, which are the lower and upper limits of the equivalent permeability for any arrangement of the parts. They are reached only for layers connected in series and in parallel respectively.
Drill pipes with a built-in cable and inductive coils in the ends of the tool joints, forming a communication channel with the bottom hole at tens of thousands of bits per second in both directions; booster subs are installed every few hundred metres. They are expensive and require careful handling of every tool joint, so they are used less often than mud-pulse telemetry.
A tool run on a logging cable into an open hole and pressed against the borehole wall by a probe with a rubber packer. It draws a few cubic centimetres of fluid from the formation, records the drawdown and the pressure build-up, and gives the formation pressure and fluid mobility at the depth of the measurement; modular tools also take samples.
The part of the tool string that converts the movement of the line into a blow: a mechanical jar strikes up and down when the accelerated sinker bars travel through its free stroke; a hydraulic jar strikes upward after a delay, releasing the energy stored in the stretched line.
Recording of rock properties with tools lowered into the well on an armoured logging cable after an interval has been drilled; the cable holds the tool, powers it and transmits the data. The main log is usually recorded while the tool is being pulled up.
The elements at the end of the slickline, wireline or cable: the rope socket, sinker bars, jar, knuckle joint and the working tool for the specific operation. The bars supply weight to overcome the upward force of the well pressure, and mass for jarring.
The share of the rights and obligations under the contract that belongs to a participant in a joint project: in the same share it pays the costs and receives the production. The work on behalf of all is carried out by one of the participants, the operator, under a joint operating agreement.
A fluid — usually a solids-free brine — whose column balances the formation pressure during the workover or completion of a well. When it goes into the formation it damages it if it carries suspended solids and polymers or is incompatible with the clays and formation water; it is therefore filtered and selected by testing.
A thin, branching channel of high conductivity that hydrochloric acid cuts in carbonate rock by dissolving it along the most conductive pores. Wormholes pass through the damaged zone, so an acid treatment of a carbonate can give a negative skin.
A dimensionless function of the two-phase measurements of a constant-composition expansion test, . For black oils it is almost linear in pressure, so it is used to check and smooth the measurements and to refine the bubble point pressure.
The intercept of the Bingham line on the stress axis: , in lb/100 ft² (1 lb/100 ft² ≈ 0.479 Pa). It reflects the attractive forces between the particles of the fluid and affects cuttings transport.