Question 8 of 9: Geophysical Well-Logging Tools

Question 8: Geophysical Well-Logging Tools (16.66% of paper)

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Natural gamma-ray log. A passive scintillation (or Geiger) detector counts gamma photons emitted by the natural decay of potassium-40, and the uranium and thorium decay series, as the tool is lowered (or raised) through the borehole. Acquisition/interpretation: shale and clay minerals concentrate K, U and Th far more than clean sand, carbonate or evaporite, so the gamma-ray curve is the standard LITHOLOGY and shale-volume ($V_{sh}$) indicator, and is used to correlate stratigraphy between wells. Significance: works through casing (unlike most other logs) and in any mud type, so it is almost always run and is frequently the FIRST curve used to pick formation tops.

Spontaneous potential (SP) log. A passive electrode measures the natural DC electrochemical and electrokinetic potential that develops where drilling-mud filtrate (usually less saline than formation water) invades a permeable bed, driving an ionic diffusion/membrane potential across the shale-sand boundary. Acquisition/interpretation: the SP curve deflects (typically negative, in a fresh-mud/saline-formation-water case) opposite permeable, non-shale beds and reads a flat "shale baseline" opposite impermeable shale, giving both a lithology/permeability indicator and, from the magnitude of the deflection, an estimate of formation-water resistivity $R_w$. Significance: essential for the $R_w$ input to Archie's saturation equation, but only works in a WATER-BASED, conductive mud system with a genuine salinity contrast — it is unusable in oil-based mud or where mud and formation water salinity are similar.

Resistivity logs (induction and laterolog families). An induction tool induces AC eddy currents in the formation from a transmitter coil and measures the secondary field at a receiver coil (works well in low-conductivity, i.e. oil-based or resistive, mud); a laterolog instead injects a focused DC (or low-frequency AC) current directly into the formation through electrodes (works well in conductive, water-based mud, where induction signal is swamped). Interpretation: hydrocarbon-bearing rock is far more resistive than the same rock saturated with saline formation water, so resistivity, combined with porosity from Q8's other logs, is used via Archie's equation $S_w^n=\frac{aR_w}{\phi^mR_t}$ to compute water saturation $S_w$ (and hence hydrocarbon saturation $1-S_w$) directly. Significance: the single most important log for quantifying whether, and how much, hydrocarbon is present at a given depth.

Sonic (acoustic) log. A transmitter emits an acoustic pulse and two (or more) receivers at known spacing measure the compressional-wave travel time $\Delta t$ (interval transit time, µs/ft) through the formation. Interpretation: the Wyllie time-average relation $\phi=\frac{\Delta t_{log}-\Delta t_{ma}}{\Delta t_{fl}-\Delta t_{ma}}$ converts travel time to porosity; the sonic log is also the primary tie between well data and surface seismic, since integrating $\Delta t$ down the well gives a synthetic check-shot time-depth curve used to build synthetic seismograms and calibrate seismic interpretation. Significance: critical wherever seismic-to-well ties are needed, and as an independent porosity estimate that (unlike neutron/density) is relatively insensitive to borehole rugosity.

Density (gamma-gamma) log. A chemical or electronic gamma source irradiates the formation and a shielded detector counts back-scattered (Compton-scattered) gamma rays, whose count rate is inversely related to bulk electron density (closely tracking bulk density $\rho_b$). Interpretation: porosity from $\phi=\frac{\rho_{ma}-\rho_b}{\rho_{ma}-\rho_{fl}}$. Significance: when combined with the NEUTRON log (which measures hydrogen index, dominated by pore fluid, via the scattering/absorption of emitted neutrons), the two porosity estimates normally track each other in a liquid-filled formation, but a marked NEUTRON-DENSITY CROSSOVER (density porosity reading high, neutron porosity reading low) is the classic direct indicator of GAS in the pore space, since gas has a much lower hydrogen index than oil or water for a given density.

Caliper log. A mechanical (multi-arm) or acoustic tool measures actual borehole diameter versus depth. Significance: identifies washed-out (enlarged) or mud-caked (reduced) intervals where OTHER logs (density, neutron, and especially any pad-contact tool) are unreliable, so the caliper is used routinely as a data-quality/environmental-correction check on every other log run in the same hole, and it independently flags unstable shale or poorly consolidated zones important for drilling and completion.