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Formation evaluation from well logs

Well logging is the main source of continuous data on the formations along the wellbore. Petrophysics turns tool readings into porosity, shale volume and hydrocarbon saturation.

The section starts with the properties of rocks and fluids and the principles of how the tools work, then covers the determination of porosity, of lithology in complex sections and of saturation.

28 / 28 topicsWhere to start: Open-hole logging: why, how and when

Logging basics

  1. 01Open-hole logging: why, how and whenWhat logging measures and how physical quantities are turned into formation properties; wireline logging and logging while drilling (LWD), depth of investigation and vertical resolution, the layout of a log and the logging programme. Basic
  2. 02Log acquisition, processing and presentationThe acquisition chain from sensor to file; sampling interval and logging speed against noise and resolution; calibrations and repeat sections; depth tie-in and run-to-run depth matching on the gamma ray log; the log header and the formation temperature taken from it; the LAS and DLIS formats; tracks, logarithmic and compatible scales, backup curves and depth scales; LWD data in real time and from memory; quality control. Basic
  3. 03Borehole environment: invaded zone and mudcakeHow drilling changes the formation around the hole — mudcake, flushed zone, invaded zone — and why the reading of each tool has to be interpreted with this picture in mind. Basic
  4. 04Formation water and formation temperatureFormation water salinity in g/L, ppm and NaCl equivalent, water resistivity from salinity and its conversion to formation temperature, the geothermal gradient, and formation temperature from bottomhole measurements corrected for the cooling effect of circulation. Intermediate
  5. 05Log interpretation workflowA step-by-step map of well-log interpretation — data checks and depth matching, zonation, shale volume, porosity, formation water resistivity, saturation, cut-offs, net thickness, averaging and uncertainty — and a quick-look interpretation of a hypothetical well from log readings to hydrocarbon pore thickness. Basic

Tools and measurements

  1. 06Overview of logging toolsPassive and active methods and what each tool measures, tool spacing, depth of investigation and vertical resolution, pad-type and compensated tools, typical readings in the main rock types, choosing a logging suite for the conditions in the well, and radioactive sources. Basic
  2. 07Gamma ray log and natural gamma ray spectrometryThe natural radioactivity of rocks (potassium, uranium, thorium), the gamma ray tool and API units, lithology, correlation and the gamma ray index, the pitfalls of radioactive sandstones and uranium-rich carbonates, spectrometry and the uranium-free gamma ray log. Basic
  3. 08Spontaneous potential (SP) logMembrane and liquid-junction potentials, the shale baseline and the static SP, what reduces the SP deflection, picking permeable beds and calculating formation water resistivity from the static SP. Intermediate
  4. 09Sonic logElastic waves in the borehole and the interval transit time Δt in µs/m and µs/ft, what it depends on, measurement with two receivers and the borehole-compensated tool, long-spaced tools, receiver arrays and dipoles, cycle skipping and record quality control, tie to seismic, dynamic elastic moduli and cement evaluation. Intermediate
  5. 10Density log and photoelectric factor (Pe)Compton scattering and the photoelectric effect, electron, apparent and bulk density, the two-detector tool and the Δρ correction, density porosity with allowance for mud filtrate, the photoelectric factor Pe and lithology, signs of unreliable readings and the effect of barite. Intermediate
  6. 11Neutron logSlowing-down of neutrons by hydrogen, the hydrogen index of fluids, thermal and epithermal neutrons, compensated tools and limestone units, the effects of shale, gas and lithology, and the crossover of the neutron and density curves as an indicator of gas. Intermediate
  7. 12Nuclear magnetic resonance (NMR) loggingWhat NMR logging measures: proton polarisation, the T1 and T2 times, the echo train and the T2 distribution; total and effective porosity, bound and free fluid from cut-off times, permeability from the Coates and SDR models, gas and oil in the signal, choice of acquisition mode. Intermediate
  8. 13Resistivity logs: electrical and induction toolsResistivity of rocks and fluids, potential (normal) and gradient (lateral) arrays, laterolog and induction logging, choosing the method according to the mud and the section, skin effect and the determination of Rt from three curves of different depths of investigation. Intermediate
  9. 14Microresistivity tools and flushed-zone resistivityMicrolog and microlaterolog: design and depth of investigation of microresistivity tools, positive separation, corrections for mudcake and shallow invasion, flushed-zone resistivity and saturation, movable and residual oil, thin beds. Intermediate

Porosity and lithology

  1. 15Porosity from the sonic logThe time-average equation and its assumptions, choice of the matrix and fluid transit times, the correction for undercompaction and the empirical transform, calibration on a reference bed and on core, allowance for shale, oil and gas, and estimation of secondary porosity by comparison with the density and neutron logs. Intermediate
  2. 16Porosity from the density logRock density as a mass balance: choice of matrix and fluid density, the error in porosity, corrections for shale volume and residual gas, total and effective porosity, calibration of the calculation on core. Intermediate
  3. 17Porosity from the neutron logFrom a reading in limestone units to the porosity of the formation: standard conditions and environmental corrections, conversion to a sandstone or dolomite matrix, allowance for shale, gas and the excavation effect, comparison with density porosity. Intermediate
  4. 18Gas, shale, carbonate and pressure effects on porosity logsWhy the density, neutron and sonic logs disagree in gas, shale and carbonates and under abnormal pressure, and how to use that: hydrogen index and apparent density of oil and gas, porosity and gas saturation of the flushed zone from two curves with an excavation correction, shale corrections before the gas correction, dolomite, sulfates and vugs, the gas effect with depth and the density trend of shales. Advanced
  5. 19Lithology and porosity from crossplotsThe neutron–density, sonic–density and sonic–neutron crossplots: mineral lines and isoporosity lines, porosity and the fractions of two minerals and why porosity is more reliable than composition, which rocks each pair can distinguish, the shale point, vugs and the shift due to gas. Intermediate
  6. 20Lithology from the photoelectric factor and spectral gamma rayThe photoelectric factor as an average per electron, and the volumetric cross section U, which mixes by volume; the density–Pe crossplot and the apparent matrix parameters ρmaa and Umaa for two or three minerals; gas, shale, heavy minerals, barite and saline filtrate; spectrometry of potassium, thorium and uranium: the Th/K ratio, clay minerals, feldspars and micas, uranium and organic matter. Advanced
  7. 21M–N and MID plots for complex lithologiesThe parameters M and N, which are slopes of lines drawn from the fluid point and do not depend on porosity; mineral points, mixing triangles and the ambiguity of composition; apparent matrix density and interval transit time and the matrix identification (MID) plot; vugs, gas, shale and evaporites on both plots, secondary porosity from the shift in M, sensitivity to apparent porosity and the limits of applicability. Advanced

Saturation and interpretation

  1. 22Archie's equation and saturation of clean reservoirsHow the resistivity of a clean rock depends on porosity and water saturation: the formation resistivity factor and resistivity index, the coefficients a, m and n, calculating water saturation, its sensitivity to the input data and the limits of the equation. Intermediate
  2. 23Determining formation water resistivityWhere formation water resistivity comes from: samples and regional data, the full calculation from the SP log with a bed-thickness correction and the conversion from the equivalent to the true water resistivity, apparent water resistivity in water-bearing beds, reconciling the estimates and the cost of an error for water saturation. Intermediate
  3. 24Resistivity–porosity crossplotsPickett and Hingle plots: the water line, the exponent m, aRw and matrix density from a series of beds, water saturation lines and the role of the exponent n, picking water-bearing points, how shale, hydrocarbons, gas and invasion shift the points, lines of constant bulk volume water. Advanced
  4. 25Resistivity ratio methods and movable oilSaturation without knowing the porosity: the ratio of flushed-zone to formation resistivity against the water contrast, the movability index Sw/Sxo, closure relationships (empirical, and from the residual oil in core), a quick comparison with the SP and a pair of apparent water resistivities, the invasion window, incomplete flushing, shale and the annulus, porosity balance. Advanced
  5. 26Determining shale volumeShale volume and clay mineral content, the clean and shale baselines, the gamma ray index, the linear and non-linear relationships (Larionov, Stieber, Clavier), shale volume from the SP and neutron–density logs and the choice of the final estimate. Intermediate
  6. 27Water saturation in shaly sandsWhy shale conducts current and depresses the resistivity of pay zones, laminated, dispersed and structural shale, total and effective porosity and saturation, laminated-sand models, the Simandoux, Indonesia, Waxman–Smits and dual-water models and a comparison of them on one formation. Advanced
  7. 28Interpretation charts: how to read themA chart as an equation solved in advance, and its conditions; how a correction chart is constructed (induction tool and borehole) and the “tornado” invasion chart based on the step-profile invasion model, and how it is used to find true formation resistivity, flushed-zone resistivity and invasion diameter; porosity, lithology, SP and saturation charts, nomograms; reading accuracy on logarithmic scales and typical mistakes — the wrong tool, mud or units. Intermediate

This section is growing: new topics are published after a technical review.