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DrillingThe rig and the drilling process Basic 17 min read

How a well is drilled: the process and the people involved

Well construction is the whole cycle of work that ends with a reliable channel from the surface to the formation (the rock layer that holds the oil or gas). It includes preparing the site and setting up the drilling rig, drilling the hole, and securing the hole with casing strings (the steel pipe that lines the well) fixed in place with cement. It also includes evaluating the rocks that the hole has passed through. Finally, the well is either completed for production or abandoned. Today oil and gas wells are drilled by the rotary drilling method. The bit, the cutting tool at the bottom of the pipe, breaks the rock under weight and rotation. The drilling fluid (mud) is pumped round all the time: it cleans the bottom of the hole and carries the drilled rock up to the surface.

Why it matters

A well is the only way to find out whether a trap (a rock structure that can hold oil or gas) really holds any. It is also the only route by which the oil or gas can be produced. Drilling takes a large share of the cost of exploration and development. A mistake in the design of the well or in the choice of mud can lead to accidents, a lost hole or a damaged formation. Not only drillers need to understand the process. Geologists and reservoir engineers set the goals for a well and receive its data. Economists estimate its cost and duration. Safety and speed depend on the operator, the drilling contractor and the service companies working well together.

Quick refresher

  • In rotary drilling the bit is pressed against the bottom of the hole and turned. Mud pumped down the drill pipe carries the cuttings (rock chips brought up by the mud) up the annulus, the gap between the pipe and the wall of the hole. The pressure of the mud also holds the formations in check.
  • A drilling rig is a set of systems: hoisting, rotating, circulating, power, well control and monitoring.
  • A well is drilled in sections. Each section is lined with a casing string (steel pipe) and cemented. Each new string runs inside the previous one, so the hole gets narrower with depth. The casing programme, the plan of strings and sizes, is designed from the bottom up.
  • Geologists and reservoir engineers set the goals of a well. Drilling engineers choose the design and the technology. The contractor drills with its own rig, service companies do the specialised jobs, and the operator's representative checks that the programme is followed.
  • We get data on the formation while the well is being built: cuttings and gas in the mud, core (a cylinder of rock cut from the formation), logs (measurements made along the hole) and the formation test. These data decide whether to complete the well or abandon it.
Key relationships
Va=π4∑i(Di2−di2)LiV_a = \dfrac{\pi}{4}\sum_i \left(D_i^2 - d_i^2\right) L_i

annular volume by sections

tlag=Va/Qt_{lag} = V_a / Q

lag time: how long the mud takes to lift cuttings from the bottom of the hole to the wellhead

Common pitfalls
  • Confusing the Russian konduktor with the English conductor. The conductor is the napravlenie; the konduktor corresponds to surface casing.
  • Thinking that the cuttings on the shaker (the vibrating screen that takes the cuttings out of the mud) were drilled just now. They have been rising for tens of minutes, and in that time the bit has moved deeper.
  • Treating the drilling programme as fixed. The engineers revise it to match the actual pressures, the rocks met and any complications (unexpected problems in the hole).
  • Assuming that a drilled well with signs of oil is already ready to produce. It still needs completion, perforating (shooting holes through the casing) and bringing the well in (starting the flow).

After this lesson you can

  • explain why exploration, appraisal and development wells are drilled and what data about the formation we get while the well is built
  • name the systems of a rotary drilling rig and explain how they work together
  • list the stages of well construction, from the conductor to completion, and explain why the hole gets narrower with depth
  • tell apart the roles of the operator, the drilling contractor and the service companies, and the main types of drilling contract
  • explain how the drilling proposal differs from the drilling programme and what each contains
  • calculate the annular volume and the time it takes cuttings to travel from the bottom of the hole to the surface

A well in the life of a field

In most countries the subsurface belongs to the state. In Kazakhstan, under the Constitution, it belongs to the people, and the state exercises the right of ownership on their behalf. A company gets the right to explore for and produce oil and gas in an area under a licence or a subsoil use contract. These documents usually also set work obligations: how much seismic survey (mapping the rocks with sound waves) to carry out and how many wells to drill within a given period. The costs and risks of exploration are so large that companies often team up in consortia. One partner then becomes the operator: it runs the work on behalf of everyone. The others share the costs and the production according to their shares. A company can also join someone else's project later — for example, by taking on part of the costs or the drilling of a well that the work obligations require (a farm-in).

The life of a field is divided into stages, and at each stage wells are drilled for different purposes.

  • Exploration. Geologists use seismic data and the geology of the region to pick out a promising trap. They estimate the chances of success and the economics of a possible discovery (Petroleum system and the play concept). But only an exploration well can show whether the trap holds oil or gas. It cuts through the sequence of rocks and gives core and logs. If there are shows (signs) of oil or gas, the crew tests the formation: it makes the formation flow, takes fluid samples and assesses productivity (how much the well can deliver).
  • Appraisal. If the discovery promises to pay off, appraisal wells are drilled. They show how large the accumulation (the body of oil or gas) is, in area and in thickness. They also refine the reservoir properties and the hydrocarbons in place, which is the amount of oil and gas the reservoir holds (Volumetric estimation of hydrocarbons in place). Engineers use these data to build a reservoir model and an economic model and to choose how to develop the field.
  • Development. The state approves the field development plan. Under the plan, development wells are drilled and the field facilities are built. The wells are producers, which bring oil and gas out, and injectors, which put fluid into the reservoir.
  • Production. Crews maintain and repair the wells and equipment. As the accumulation becomes better understood, they drill additional wells and infill wells (wells placed between existing ones) and cut sidetracks (new branches drilled out of an existing hole).
  • Abandonment. When the cost of production is higher than the income, the wells are abandoned. The crew seals the formations off with cement plugs, removes the equipment and restores the site.

A well is not only a channel for production. It is also the main source of direct data about the rocks underground. While the well is being built, we can learn about the formation in several ways.

What data a well provides during construction
Source
Mud logging (watching the mud and the cuttings at the surface)
What it provides
a description of the drilled rock (the cuttings), gas and signs of oil in the mud, and the drilling parameters
When
continuously while drilling
Source
Core
What it provides
a rock sample for laboratory measurements of porosity (the share of pore space), permeability (how easily fluid flows through the rock) and saturation (how much of the pore space each fluid fills)
When
in selected intervals; cut with a ring-shaped core bit and a core barrel (a tube that catches the core)
Source
Well logging
What it provides
continuous curves of rock properties against depth, such as electrical resistivity, density, natural radioactivity and others
When
after an interval has been drilled, with tools lowered on a cable (wireline), or while drilling, with tools in the bottomhole assembly (the lower part of the drill string)
Source
Drill stem test (DST)
What it provides
flow from the formation, reservoir pressure, fluid samples, an estimate of productivity
When
after a promising formation has been drilled into

For more on logging, see the lesson Open-hole logging: why, how and when. The lesson Borehole environment: invaded zone and mudcake covers the conditions in which logging tools work, and the lesson Well testing: purposes and types of test covers flow tests.

Who builds the well

Many organisations work on a well, and each has its own area of responsibility. The operator is the customer: the subsoil user or the operating company. It sets the goals, pays for the work and answers to the state for the well. As a rule it hands the drilling itself to a drilling contractor. The contractor has the rig and the crews, and is responsible for the condition of the equipment and the training of the people. Service companies do the work that needs special equipment and narrow specialists: drilling fluids, directional drilling (steering the well along a planned path) and telemetry (sending data up from the tools in the hole), cementing, logging, mud logging, formation testing, the supply of bits and the rental of tools.

Who works at the rig
Role
Drilling supervisor (the operator's representative, “company man”)
Employer
operator
What they do
is responsible to the operator for the progress of the work: checks that the programme is followed, approves day-to-day decisions and supplies, and sends daily reports to the office
Role
Toolpusher (rig manager)
Employer
contractor
What they do
directs the drilling crews and the running of the rig, and agrees work plans with the supervisor
Role
Driller
Employer
contractor
What they do
operates the drawworks (the winch that lifts and lowers the drill string, the pipe that carries the bit), the pumps and the rotation of the string; watches the drilling parameters and the volume of mud; is often the first to notice the signs of a kick (an unwanted inflow of fluid from the formation into the well)
Role
Assistant drillers (roughnecks) and the derrickman
Employer
contractor
What they do
screw pipe together and unscrew it on the rig floor (the working platform around the well); during trips (pulling the string out and running it in) the derrickman works on the monkeyboard, a working platform high up in the derrick (the tower over the well), and while drilling he often watches the pumps and the mud
Role
Mechanics, electricians, crane operator, labourers
Employer
contractor
What they do
maintain the equipment and move loads
Role
Mud engineer
Employer
service company
What they do
prepares the mud, tests its properties several times a day and decides how to treat it, for example what to add (Drilling fluids: functions and properties)
Role
Directional drilling and telemetry engineers, mud logging unit operators, cementers, logging engineers
Employer
service companies
What they do
some stay at the rig for the whole construction of the well, others come only for single operations

Job titles and the split of duties depend on the company and the contract. The table shows a typical arrangement.

The operator's office has the drilling superintendent (the head of drilling operations) and the drilling engineers, who design the well. Geologists and reservoir engineers work there too: they set the goals of the well and receive its data. The rig works round the clock, so crews take over from one another. At remote fields the staff usually work a rotation: several weeks at the rig, then the same time at home.

The drilling contract decides how duties and risks are shared. There are three main types.

Main types of drilling contract
Contract
Day rate
What is paid for
a fixed rate for every day the rig works with its crew
Who designs and supplies
operator: design, programme, bits, pipe, cement, chemicals, services
Who bears the risk of complications
operator: complications make the work longer, and the operator pays for every extra day
Contract
Footage
What is paid for
a fixed price for each metre drilled to a specified depth
Who designs and supplies
the operator sets the requirements for the well, and the contractor chooses how to drill
Who bears the risk of complications
contractor, for the speed of drilling; separate operations are often paid by the day
Contract
Turnkey
What is paid for
a fixed price for a finished well that meets the specified requirements
Who designs and supplies
contractor: design, materials, transport, services
Who bears the risk of complications
contractor; the operator sets the goals, the evaluation and the quality criteria, and pays a higher price for the risk it passes on

The day-rate contract is the most common. Under any contract the contractor is responsible for keeping the rig and its equipment in good order.

The goal of the well and the drilling programme

Work on a well begins with a document that the operator's geologists and reservoir engineers prepare: the drilling proposal (the geological brief). It states why the well is being drilled and what it has to drill into:

  • the objective of the well and the signs of success
  • the coordinates and depth of the target (the vertical depth, usually measured from sea level)
  • the expected geological section (the sequence of rocks): which rocks the well will meet and at what depths
  • a forecast of pore pressure (the pressure of the fluid in the pores of the rock) and temperature with depth (Pore pressure and overburden pressure), and the expected complications
  • the evaluation programme: where to cut core, what logs to run, which formations to test

From these data the drilling engineer draws up the drilling programme, the technical plan for building the well:

In Kazakhstan and other post-Soviet countries the main document is the well construction project. It is often a group project that covers several similar wells, and it is approved by the official procedure. The rig receives a geological and technical order (GTN in Russian-language practice): a table by depth interval that lists the expected rocks and pressures, possible complications, the casing programme, the drilling parameters and the mud properties. The crew works from it every day.

A well is drilled from the top down, but its casing programme is designed from the bottom up. First we decide what diameter is needed at the bottom. A production well needs room for the tubing (the pipe that carries the oil or gas up to the surface) and the production equipment. An exploration well needs a diameter large enough to evaluate the formation fully. Next we choose the casing string that the bit for this bottom section must fit inside. Then we choose the diameter of the hole above it, large enough to run that string. We repeat this step by step up to the surface. Pressures and unstable rocks decide the setting depths of the strings, the depths to which each string is run (see the advanced section).

The rotary drilling rig

The first oil wells were drilled by the cable-tool method. A heavy tool carrying a bit hung from a cable. The crew lifted it and dropped it on the bottom of the hole again and again, and from time to time bailed out the crushed rock with a bailer (a long bucket). In rotary drilling the bit sits on the end of a string of hollow pipes. It is pressed against the bottom and turned at the same time. Drilling fluid is pumped through the pipes all the time. It cleans the bottom of the hole and carries the rock fragments, the cuttings, up to the surface. Drilling does not stop to clean out the hole, so this method is much more productive. The column of fluid also lets the crew control the pressure in the well.

Rotary drilling rigcrown blockderricktravelling blocktop drivedrawworksrotary hosestandpipepumpspower unitBOPshale shakerpitscasingdrill stringbit

A simplified drawing of a land rig. A derrick with a crown block at the top stands over the rig floor. The travelling block and the top drive hang from the drilling line, and the drill string runs down from the top drive. On the right of the floor is the drawworks, with the drilling line running to it from the crown block. On the left, on the ground, are the power unit and the mud pumps. From the pumps the mud goes up the standpipe and through the rotary hose into the top drive. Under the floor, on the wellhead, stands the blowout preventer stack. From it the mud flows along the flowline to the shale shaker and into the pits on the right. Below ground is the well, with the casing, the drill string and the bit on the bottom. Arrows show the mud moving down the pipes and up the annulus.

SchematicDiagram of a rotary drilling rig with a top drive. The hoisting system is the derrick, crown block, travelling block and drawworks. The rotating system is the top drive. The circulating system is the pumps, the standpipe with its rotary hose, the drill string, the shale shaker and the pits. The power system is the power unit. The well control system is the blowout preventers (BOP) under the rig floor. The arrows show the path of the mud. Not to scale.

The drilling rig is a group of machines that work together. It helps to divide them into systems.

Systems of a drilling rig
System
Hoisting
Main elements
derrick or mast (the tower), drawworks (the winch), block and tackle (the pulley system): crown block, travelling block, drilling line (the steel rope)
Purpose
lowering and lifting the drill string and the casing, holding their weight, controlling the weight on the bit
System
Rotating
Main elements
top drive (a motor that hangs from the travelling block and turns the string) or rotary table (a turntable in the rig floor) with a kelly (the square or six-sided pipe that the table turns); downhole motor
Purpose
turning the string and the bit
System
Circulating
Main elements
mud pumps, standpipe, rotary hose, drill string, annulus, flowline, solids control equipment (shakers and other machines that clean the mud), pits (mud tanks)
Purpose
delivering mud to the bit, carrying out cuttings, cleaning and preparing the mud
System
Power
Main elements
diesel generators or grid power, electric motors, transmissions
Purpose
energy for all the other systems
System
Well control
Main elements
blowout preventers on the wellhead, choke and kill manifolds (valves and pipes for controlling the flow out of the well and for pumping mud in), control panel
Purpose
closing the wellhead when there is a kick (Blowout prevention equipment)
System
Monitoring
Main elements
hook load indicator (shows the weight that hangs on the hook), pressure gauges, flow and pit level sensors, gas detector
Purpose
show what is happening in a well that cannot be seen

The systems are covered in detail in the lessons Drilling rig: power and hoisting systems and Rig circulating and rotary systems.

Here is how it works while drilling. The drill string is made of drill pipe and, in its lower part, thick-walled drill collars. The string hangs from the hook of the block and tackle. The driller lowers it with the drawworks and so puts part of the weight of the collars on the bit. This is the weight on bit. The weight indicator shows how much the hook load (the weight that hangs on the hook) has dropped. That drop is the load put on the bottom of the hole. The top drive turns the string. The pumps push mud through the standpipe and the hose into the pipe. The mud leaves through the nozzles of the bit and rises up the annulus, the ring-shaped gap between the string and the wall of the hole. It flows along the flowline to the shale shakers, which take out the cuttings, and then into the pits. The pumps take it from the pits again (Drill string: pipe, tool joints, drill collars and heavy-weight drill pipe, Drill bits: types and design).

As the hole gets deeper, the crew lengthens the string. It adds a joint (one length of pipe) or a stand, which is two or three joints screwed together, at the top. When the bit is worn out or the assembly must be changed, the crew pulls the whole string out and runs it in again, stand by stand. This is tripping (a round trip). In deep wells it takes many hours.

How a well is built: from the conductor to completion

Almost never can a well be drilled from the surface to the formation in one go. On the way the well meets loose rocks that cave in, formations that take the mud away (loss zones), unstable shales (clay-rich rocks) and salts, and formations with different pressures that need different mud densities. So the well is drilled in sections. When a section is drilled, the crew secures it with a casing string, steel pipe that is run into the hole and cemented. Only then does it drill deeper, with a smaller bit that passes inside that string.

Well casing programme after completionConductorØ 426 mm, 30 mSurface casingØ 324 mm, 600 mIntermediate casingØ 245 mm, 1900 mProduction casingØ 168 mm, 2800 mChristmas treetubingpackercementperforationspay zonenot to scale

A vertical section of a well. Four casing strings of decreasing diameter start at the surface and are run to different depths: conductor 426 mm to 30 metres, surface casing 324 mm to 600 metres, intermediate casing 245 mm to 1900 metres, production casing 168 mm to 2800 metres. Cement is shown between the strings and the wall of the hole. Tubing is run inside the production casing, a packer stands above the pay zone, the casing is perforated opposite the zone, and a Christmas tree is on the wellhead.

SchematicThe well from the example below after completion. The casing strings are nested inside one another, and cement fills the space between the strings and the wall of the hole. Inside the production casing is the tubing, with a packer above the pay zone (the productive layer). The casing and the cement opposite the pay zone are perforated. A Christmas tree (the set of valves that controls the flow) stands on the wellhead. Not to scale.
Example of a casing programme for a well 2800 m deep
String
Conductor
Bit, mm
490
Casing, mm
426
Setting depth, m
30
String
Surface casing
Bit, mm
393.7
Casing, mm
324
Setting depth, m
600
String
Intermediate casing
Bit, mm
295.3
Casing, mm
245
Setting depth, m
1900
String
Production casing
Bit, mm
215.9
Casing, mm
168
Setting depth, m
2800

The data in this example are assumed for illustration: the diameters come from a series common in the post-Soviet countries, and the depths are chosen to illustrate. English-language practice commonly uses a different series: casing of 20″, 13⅜″, 9⅝″ and 7″ in holes of 26″, 17½″, 12¼″ and 8½″, with a 30″ conductor above them (1″ = 25.4 mm). The number of strings and their setting depths differ from region to region.

The sequence of construction
  1. 1
    Preparation and rig-up

    The crews prepare the site and the access roads, tanks or pits for the mud and cuttings, and a water supply. Then they bring in the drilling rig and erect it (rig-up).

  2. 2
    Conductor

    The first string is short and has a large diameter. It is set only tens of metres deep. The crew drives it into the ground, or runs it into a hole drilled for it, and cements it. It stops the loose rocks near the surface from caving in, and it guides the mud that comes out of the well to the flowline.

  3. 3
    Surface casing

    Drilling starts (the well is spudded) with a bit that fits inside the conductor and goes down to stable rock. The crew runs the surface casing and cements it all the way up to the surface. This string shuts off the fresh groundwater. It also carries the casing head (the fitting from which the strings hang), the blowout preventers and the weight of the following strings.

  4. 4
    Blowout preventers

    The crew installs the preventers on the casing head and tests them under pressure. Then it drills out the cement and the casing shoe (the reinforced bottom end of the string). A leak-off test usually follows. It shows what pressure the rock just below the shoe can withstand (Fracture gradient and leak-off testing).

  5. 5
    Intermediate casing

    The next section is drilled with a smaller bit. The crew covers it with an intermediate string in two cases: rocks that need a different mud are expected ahead, or the section just drilled has caused trouble — losses, unstable shales, salts, or formations with a different pressure. There may be several such strings, or none.

  6. 6
    Drilling into the pay zone

    The crew drills the pay zone and watches the cuttings and the gas in the mud closely. If needed, it cuts core. Then it runs logs and, if there are signs of oil or gas, tests the formation.

  7. 7
    Decision

    The results of the evaluation decide the next step: to complete the well for production, or to abandon it and seal the formations off with cement plugs. An exploration well that finds no oil or gas still gives a result: it improves the geological model of the region.

  8. 8
    Completion

    The crew runs and cements the production casing, or a liner. A liner is a string that hangs inside the previous one and does not reach the surface. Next it runs the tubing inside, with a packer, which seals the annulus above the formation. It replaces the blowout preventers with a Christmas tree. It perforates the casing and the cement opposite the formation with a perforating gun (a tool that shoots holes through them). Then it gets the well to flow. This is called bringing the well in (Well completion: opening the reservoir and the tubing string).

  9. 9
    Rig-down

    The crew takes the rig apart and moves it to the next location. On a well pad (a site with several wells) it often moves the rig to the neighbouring wellhead without taking it apart.

Why the hole narrows with depth. Each new string has to pass inside the previous one. The bit for the next section has to pass inside the string that was just run. There must be a gap between the casing and the wall of the hole, so that the pipe runs in freely and a continuous ring of cement can form around it. So each new string is clearly narrower than the previous one, usually by a quarter or a third of the diameter, and the well looks like an extended telescope. The number of strings is limited. If more strings are needed than the design allowed for, there may be no room left at the bottom for the diameter we need. So the casing programme is chosen with a margin for complications.

Cementing. The crew pumps cement slurry (liquid cement) down inside the casing and pushes it out through the shoe into the annulus. When the cement hardens, it holds the casing and protects it from corrosion. Above all, it separates the formations: fluids cannot flow along the annulus from one formation to another or up to the surface (Primary cementing).

What can be seen at the surface: cuttings, gas and lag time

While drilling goes on, the mud logging unit watches the rock section. Its operators and geologists collect cuttings at the shale shakers and describe the rock. They look for signs of oil, for example a sample that glows (fluoresces) under ultraviolet light. They also measure, all the time, the gas that comes out of the mud. The unit's sensors record the drilling parameters too: rate of penetration (how fast the bit drills), weight on bit, rotary speed, torque (the turning force), standpipe pressure (the pump pressure measured on the standpipe) and the volume of mud in the pits (Well control: primary barrier and kick indicators).

The cuttings that the geologist holds were not drilled just now. They need tens of minutes to rise up the annulus from the bit to the shaker, and in that time the bit has gone deeper. So we tie every sample to a depth with a correction for the lag time. This is the time the mud takes to travel from the bottom of the hole to the surface. We calculate it from the volume of the annulus and the pump rate.

Annular volume and lag time
Va=π4∑i(Di2−di2)Li,tlag=VaQV_a = \frac{\pi}{4}\sum_i \left(D_i^2 - d_i^2\right) L_i, \qquad t_{lag} = \frac{V_a}{Q}
where:
  • VaV_avolume of the annulus from the bottom of the hole to the surface, m³
  • DiD_idiameter of the hole in the ii-th interval. In open hole (the part with no casing), use the bit diameter, or the real diameter from the caliper log (a log that measures the hole diameter). In cased hole, use the inside diameter of the casing, m
  • did_ioutside diameter of the drill pipe or drill collars in that interval, m
  • LiL_ilength of the interval along the hole, m
  • QQpump rate, m³/s
  • tlagt_{lag}lag time, s

Units: SI. It is handy to calculate the annular capacity in litres per metre as π4(D2−d2)⋅10−3\frac{\pi}{4}(D^2 - d^2)\cdot 10^{-3} with the diameters in millimetres

Holds when
  • The lengths of the intervals are measured along the hole, not vertically, because the mud travels the whole path.
  • The mud and the cuttings move at the same average speed. In reality the cuttings lag behind the mud, especially in deviated holes.
  • The open hole has its nominal (design) diameter. Washouts (places where the hole is wider than the bit) add volume and time, so we check the calculation with a tracer.
  • The pump rate is constant and known. We calculate it from the number of pump strokes and the volume of one stroke, allowing for the volumetric efficiency (the share of the stroke volume that the pump really delivers).

The lag time is the time the pumps need to “pump out” the volume of the annulus. Double the pump rate gives half the time. Washouts and wide upper casing strings make it longer.

In other unit systems (1)
Oilfield units
Ca [bbl/ft]=D2−d21029.4,tlag [min]=∑iCa,i LiQ [bbl/min]C_a\,[\text{bbl/ft}] = \frac{D^2 - d^2}{1029.4}, \qquad t_{lag}\,[\text{min}] = \frac{\sum_i C_{a,i}\,L_i}{Q\,[\text{bbl/min}]}

DD and dd in inches, LL in feet, the pump rate in barrels per minute.

1029.4=9702/(12 π/4)1029.4 = 9702/(12\,\pi/4). A barrel holds 9702 cubic inches. A ring-shaped space with a cross-section of π4(D2−d2)\frac{\pi}{4}(D^2 - d^2) square inches and a length of 1 ft (12 inches) holds 12⋅π4(D2−d2)12\cdot\frac{\pi}{4}(D^2 - d^2) cubic inches.

Worked exampleHow many minutes until cuttings from the bottom reach the shaker
Given (assumed data)
Measured depth of the bottom of the hole
2600 m
Intermediate casing
to 1900 m, inside diameter 224.5 mm
Open hole below the shoe
700 m, bit 215.9 mm
Drill pipe
outside diameter 127 mm (for simplicity, along the whole length)
Pump rate QQ
28 L/s
Rate of penetration
12 m/h
Solution
  1. Capacity of the annulus (its volume per metre of length) in the casing: π4(224.52−1272)⋅10−3=26.92\frac{\pi}{4}(224.5^2 - 127^2)\cdot 10^{-3} = 26.92 L/m. In open hole: π4(215.92−1272)⋅10−3=23.94\frac{\pi}{4}(215.9^2 - 127^2)\cdot 10^{-3} = 23.94 L/m.
  2. Annular volume: 1900⋅26.92+700⋅23.94≈67,9001900\cdot 26.92 + 700\cdot 23.94 \approx 67{,}900 L, that is 67.9 m³.
  3. Lag time: tlag=67.9/0.028=2425t_{lag} = 67.9/0.028 = 2425 s, or about 40 min.
  4. In that time the bit will drill deeper by 12⋅40/60≈812\cdot 40/60 \approx 8 m.
Answer: Cuttings from the bottom reach the shaker in about 40 min; in that time the well gets about 8 m deeper.

What it means. The geologist records a sample not at the current depth of the bottom of the hole, but at the depth where the bit was 40 min ago. Otherwise a change of rock or a gas-bearing formation will look “shifted” downward by several metres. We check the real time with a tracer. For example, the crew drops calcium carbide into the pipe. When the mud has made the whole trip down the pipe and up the annulus, the gas detector registers the acetylene (a gas) that the carbide gave off. The travel time down the pipe is calculated from its inside volume. If the real time is longer than the calculated one, the hole is wider than nominal: it has washouts. The same time is needed to circulate the hole clean before the string is pulled out. The crew pumps the mud for at least one full cycle, from the bottom to the wellhead, to carry out the cuttings and gas. The data in this example are assumed for illustration.

Terms and symbols

Rotary drilling
A drilling method in which the bit turns and is pressed against the bottom of the hole, while the mud continuously cleans the hole and carries out the cuttings. The bit is turned from the surface, by a rotary table or a top drive (surface-driven rotary drilling), or by a downhole motor.
Drilling rig
The set of hoisting, rotating, circulating, power and well control systems used to build a well.
Drill string, drill collars
The string of pipe from the top drive to the bit. In its lower part are the drill collars, thick-walled pipes that put weight on the bit.
Annulus
The ring-shaped gap between the string of pipe and the wall of the hole or the casing. The mud rises up through it to the wellhead.
Casing programme
The set of casing strings with their diameters, setting depths and cemented intervals, together with the diameters of the hole sections.
Conductor, surface, intermediate and production casing
The casing strings in the order in which they are run. A liner is a string that hangs inside the previous one and does not reach the surface.
Tripping
Running the drill string into the hole and pulling it out, for example to change the bit.
Mud logging
Watching the cuttings, the gas in the mud and the drilling parameters during drilling.
Drill stem test (DST)
Testing a formation with a tool run on drill pipe. The formation flows for a short time while pressure is measured and samples are taken.
Well completion
The work that turns a drilled well into a producing or injection well: the production casing, tubing with a packer, the Christmas tree, perforating and bringing the well in.

Common mistakes and misconceptions

Wrong

“The konduktor is the English conductor.”

In fact

The conductor corresponds to the napravlenie. The konduktor is the second string, in English surface casing.

Wrong

“The casing programme is designed from the top down, in the order in which the well is drilled.”

In fact

It is chosen from the bottom up: we start from the diameter needed at the bottom for completion or evaluation and work up to the strings above. Pressures and troublesome intervals decide the setting depths of the strings.

Wrong

“The cuttings on the shaker come from the current depth of the bottom of the hole.”

In fact

They have been rising up the annulus for tens of minutes. We tie samples to depth with a correction for the lag time.

Wrong

“Under a day-rate contract the contractor decides how to drill and is responsible for the casing programme.”

In fact

Under a day-rate contract the operator prepares the casing programme and the drilling programme. The contractor provides the rig with its crew and is responsible for their work and for the condition of the equipment.

Wrong

“Oil in the cuttings means the well can already produce oil.”

In fact

First the formation is evaluated by logging and testing. Then the well is completed: the crew runs the production casing and tubing, installs the Christmas tree, perforates the casing and brings the well in.

In depth: advanced detail

How casing setting depths are chosen

The main tool is the pressure–depth diagram (Pore pressure and overburden pressure). On it we plot, against depth, the equivalent mud weights of the pore pressure and of the fracture pressure of the rocks (the mud density that gives the same pressure at that depth). An open-hole section can hold only one mud at a time. That mud must be heavy enough to hold back the highest-pressure permeable formation in the section, with a margin. At the same time it must be light enough for the weakest rock in the open hole to withstand it. This is usually the rock just below the shoe of the previous string. While the mud circulates, the pressure in the well is higher still, because of the pressure loss in the annulus.

The order of selection (from the bottom up)
  1. 1
    Start at the bottom

    Find the mud density needed at the bottom: the equivalent mud weight of the pore pressure plus a margin.

  2. 2
    Go up the fracture curve

    Move up the section until the equivalent mud weight of the fracture pressure (with a margin) becomes less than the density of this mud. Above this depth the open hole cannot withstand such a mud, so the shoe of the string above must be set here.

  3. 3
    Repeat for the next string

    For the section above this shoe, find the highest mud density needed. Move up the fracture curve again. This gives the depth of the next string. Repeat up to the surface.

  4. 4
    Check the other constraints

    Adjust the depths for unstable shales, salts, loss zones, fresh water that must be sealed off, the risk of differential sticking (the pipe being held against the wall of the hole by the pressure difference) and kick tolerance, which is how large a kick the well can take and still be controlled (Well control: primary barrier and kick indicators). Then select the diameters and design the strings for strength (Casing design).

This is how the pressures “draw” the casing programme. In regions with normal pressure, a surface casing and a production casing are sometimes enough. Where the section has zones of abnormally high pressure (Abnormal formation pressures: origin, prediction and recognition) and salts, there may be two or three intermediate strings.

Connections to other topics

Check yourself

  1. 1

    Which string in Russian terminology corresponds to the English conductor (conductor pipe)?

    1. A.Technical (intermediate) string
    2. B.Konduktor
    3. C.Napravlenie
    4. D.Production string
    Show the answer

    Correct answer: C. Napravlenie

    The conductor is the first short string of large diameter. In Russian it is called the napravlenie. The konduktor is the next string (surface casing), and the blowout preventers are usually installed on it.

  2. 2

    The operator has signed a day-rate contract with a drilling contractor. Who prepares the drilling programme and pays for the bits, cement and chemicals?

    1. A.The state
    2. B.The mud service company
    3. C.The contractor
    4. D.The operator
    Show the answer

    Correct answer: D. The operator

    Under a day-rate contract the operator designs the well, prepares the programme and provides the materials and services. The contractor provides the rig with its crew for a daily rate. So the risk of delays and complications lies with the operator.

  3. 3

    Why is a well drilled in sections with casing strings, rather than in a single hole from the surface to the formation?

    Show the answer

    On the way the well meets loose rocks, unstable rocks, rocks that take the mud away, and formations with different pressures. One mud cannot hold back the highest-pressure formation and, at the same time, avoid fracturing the weakest rock in the open hole. Troublesome intervals also have to be sealed off. So the section that has been drilled is secured with casing and cement, and drilling continues with a smaller bit.

  4. 4

    The annular volume from the bottom of the hole to the surface is 45 m³, the pump rate is 25 L/s and the rate of penetration is 15 m/h. After how many minutes will the cuttings from the bottom reach the shaker, and by how many metres will the well be deeper in that time?

    Show the answer

    tlag=45/0.025=1800t_{lag} = 45/0.025 = 1800 s, that is 30 min. In half an hour at 15 m/h the bit drills about 7.5 m. So we tie samples of cuttings to the depth at which the bit was half an hour earlier.

  5. 5

    What is installed on the wellhead in place of the blowout preventers when the well is completed?

    1. A.A larger-diameter preventer
    2. B.The top drive
    3. C.The Christmas tree
    4. D.The casing head
    Show the answer

    Correct answer: C. The Christmas tree

    After the tubing with a packer has been run, the crew removes the blowout preventers and installs a Christmas tree, a set of valves that control the flow from the well. The casing head stays in place, because the casing strings hang from it.

  6. 6

    An exploration well found no oil or gas and was abandoned. Was the money spent on it wasted?

    Show the answer

    It brings no income, but it gives data: the rock section, the pressures, the rock properties, and whether a reservoir rock and a seal (the rock that stops oil and gas from escaping upward) are present. These data improve the geological model of the region and lower the risk of the next wells. Or they show that the area is unpromising. The economic evaluation of exploration takes such an outcome into account in advance, through the probability of success.

Updated: 28 October 2026