Petroleum Formation Evaluation: SP, Resistivity, Radioactive and Acoustic Logs, LWD, Cement and Casing Evaluation, Formation Testers, Production Logging, FMS and NMR, and Log Interpretation

The spontaneous potential (SP) log records the natural potential between an electrode in the borehole and one at surface. Opposite a permeable bed it deflects because of the electrochemical potential set up where mud filtrate of resistivity R_mf meets formation water of resistivity R_w across the shale membrane and the invaded zone. In a thick, clean, water-bearing bed the full deflection is the static SP, SSP = −K log(R_mf/R_w), with K ≈ 61 + 0.133T (T in °F), about 71 mV at 75 °F. Uses: identifying permeable beds and their boundaries, correlating wells, estimating R_w and indicating shaliness. It needs a conductive (water-based) mud, fails when R_mf ≈ R_w, and is suppressed by shale and hydrocarbons. Mud and water resistivities are corrected to formation temperature by Arps’ relation, R₂ = R₁(T₁ + 6.77)/(T₂ + 6.77) with T in °F.

Resistivity logs exploit the fact that rock grains are insulators, so a formation conducts only through its water. Laterologs (focused electrode tools) force current sideways into the formation and work best in salty, conductive muds and resistive formations; induction tools induce eddy currents with coils and work in oil-based or fresh muds and conductive formations; both are run as arrays of several depths of investigation to read the flushed zone (R_xo, also from micro-resistivity pads), the invaded zone and the true formation resistivity R_t. Invasion profiles show where hydrocarbons have been flushed. Archie’s equations connect resistivity to porosity and saturation in clean rock: the formation factor F = R_o/R_w = a/φ^m, and the water saturation S_w^n = aR_w/(φ^m R_t), with a the tortuosity factor, m the cementation exponent and n the saturation exponent (commonly a = 1, m = n = 2 unless the stem gives others). Hydrocarbon saturation is S_h = 1 − S_w.

The gamma-ray (GR) log counts natural radioactivity from potassium, thorium and uranium, which is concentrated in clay minerals, so shales read high and clean sands and carbonates low. The gamma-ray index I_GR = (GR − GR_clean)/(GR_shale − GR_clean) is the usual first estimate of shale volume (linear, or reduced by non-linear corrections for older rocks). It works in any mud and through casing, and it correlates wells; the spectral GR separates K, Th and U, identifying clay types and radioactive non-shale rocks such as uranium-rich carbonates. The density log fires gamma rays from a caesium source and counts those scattered back; the count rate falls as electron density rises, giving bulk density ρ_b and porosity φ_D = (ρ_ma − ρ_b)/(ρ_ma − ρ_f), with matrix densities of 2.65 g/cm³ for sandstone, 2.71 for limestone and 2.87 for dolomite; the photoelectric factor P_e adds a lithology reading. The neutron log fires fast neutrons that lose energy mainly in collisions with hydrogen, so it reads the hydrogen index — essentially the liquid-filled porosity, scaled to a limestone matrix. Gas, with little hydrogen per unit volume, makes neutron porosity read low while density porosity reads high: the gas crossover. Shale-bound water makes the neutron read high.

The acoustic (sonic) log measures the interval transit time Δt of a compressional wave, in μs/ft, between transmitters and receivers on the tool. Denser, better-cemented rock is faster (smaller Δt). The Wyllie time-average relation gives porosity φ_S = (Δt − Δt_ma)/(Δt_f − Δt_ma), with matrix and fluid transit times supplied (for example about 55.5 μs/ft for sandstone and 189 μs/ft for fresh mud filtrate); it overestimates porosity in uncompacted sands and misses vuggy and fracture porosity, because the first arrival travels through the fastest continuous path — so the difference between sonic and density-neutron porosity estimates secondary porosity. Array (dipole) sonic tools also measure shear and Stoneley waves for rock mechanics and permeability indication, and sonic data tie wells to seismic. Logging while drilling (LWD) puts resistivity, gamma-ray, density, neutron, sonic, imaging and even NMR sensors in drill collars, transmitting by mud-pulse telemetry: measurements are taken before invasion has progressed, in wells too deviated for wireline, and in real time for geosteering, at the cost of lower data rates and the constraints of the drilling assembly.

The cement bond log (CBL) fires a sonic pulse inside the casing and measures the amplitude of the casing arrival at a near receiver: free, unsupported pipe rings loudly (high amplitude), and pipe bonded to cement is damped (low amplitude). The variable density log (VDL) displays the full waveform at a far receiver: straight, strong casing arrivals mean free pipe, while weak casing arrivals with strong, wavy formation arrivals mean cement bonded both to casing and to formation. The CBL averages around the circumference, so a channel can hide in a good average. The ultrasonic imager (USIT) rotates a transducer that excites casing thickness resonance at many azimuths and maps the acoustic impedance of what lies behind the pipe — cement, liquid or gas — as an image, while also giving casing thickness and internal radius; segmented bond tools, of which the syllabus names the SFT, resolve the bond sector by sector for the same reason. Formation testers (the RFT and its successors) set a probe against the borehole wall, draw a small fluid sample and record pressure, repeatably at many depths in one trip: formation pressures, fluid gradients and contacts, permeability from the drawdown and build-up, and fluid samples.

Production logging diagnoses a flowing well: a spinner flowmeter measures fluid velocity and hence the flow profile, showing which perforations produce or take injection; a temperature log shows entry points and leaks by cooling (gas expansion) or warming anomalies; a fluid-density tool (gradiomanometer or nuclear) and capacitance or holdup tools identify which fluids enter where; a noise log hears flow behind casing; and pressure gauges give the flowing gradient. Limitations: multiphase and deviated flow segregates, so a centralised spinner can misread; low rates stall the spinner. Casing-inspection tools find corrosion and damage: multi-finger mechanical calipers map internal radius; electromagnetic tools (phase-shift thickness and magnetic flux leakage) sense metal loss on the inner and outer walls and through multiple strings; ultrasonic tools map thickness and internal surface. Among special tools are the dipmeter and borehole imagers, pulsed-neutron tools that measure saturation through casing (capture cross-section, carbon-oxygen ratio), and sidewall coring and seismic tools.

The formation micro-scanner (FMS) and its successors press pads carrying many small button electrodes against the wall and map micro-resistivity as an electrical image of the borehole. The images show bedding, dips, cross-bedding, fractures (open ones conductive in water-based mud), vugs, borehole breakouts (whose orientation gives the stress direction) and thin beds below the resolution of conventional logs; they need a conductive mud, with ultrasonic and oil-mud imagers as alternatives. NMR logging polarises the hydrogen nuclei in the pore fluids with a strong static magnet and measures the decay of their echo signal after radio-frequency pulses. The initial amplitude gives a porosity independent of lithology, and the distribution of transverse relaxation times T₂ reflects pore size: short T₂ is clay-bound and capillary-bound water, long T₂ is free fluid. From these the tool gives bound and free fluid volumes, a permeability estimate and, with diffusion and T₁ measurements, fluid typing.

Standard interpretation follows a fixed order: pick the permeable zones (SP, GR, caliper, mudcake); estimate shale volume (GR, SP, neutron-density); determine porosity (density, neutron, sonic, corrected for shale and gas); determine R_w (from the SP, from a water zone by R_wa = R_t φ^m/a, or from produced water); compute S_w by Archie, or by a shaly-sand equation in dirty rock; compare S_xo from R_xo with S_w — hydrocarbon that was moved by invasion is movable (S_w/S_xo well below 1); and set cut-offs for net pay. Cross-plots do several steps at once. The neutron-density cross-plot reads porosity and lithology together, with sandstone, limestone and dolomite lines, and shows gas as a shift toward the north-west. The Pickett plot of log R_t against log φ turns Archie into straight lines: water-bearing points fall on a line of slope −m, and lines of constant S_w run parallel to it. The Hingle plot and the M-N and MID plots are the other classical cross-plots for R_w, lithology and secondary porosity.