24-Pet-B1 Natural Gas Engineering · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Bassiouni, Theory, Measurement, and Interpretation of Well Logs (SPE Textbook Series Vol. 4); Asquith & Krygowski, Basic Well Log Analysis, 2nd ed.; Ellis & Singer, Well Logging for Earth Scientists, 2nd ed.; Schlumberger, Log Interpretation Charts.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
(a) Compressional (V_p) and shear (V_s) wave velocities (3 pts). The sonic (acoustic) log records the transit time of both wave types, from which velocity is derived as the reciprocal of interval transit time. Applications: (i) porosity estimation via the Wyllie time-average or Raymer–Hunt–Gardner transforms, calibrated against a known matrix and fluid transit time; (ii) lithology discrimination, since $V_p/V_s$ ratio and Poisson's ratio differ systematically between sandstone, limestone, dolomite and shale (carbonates generally run higher $V_p/V_s$ than clastics); (iii) fluid-type / gas detection, because gas in the pore space depresses $V_p$ far more than $V_s$, sharply lowering $V_p/V_s$ and Poisson's ratio — the basis of AVO (amplitude-versus-offset) gas indicators; (iv) rock mechanical properties for geomechanics and hydraulic-fracture design — dynamic Young's modulus and Poisson's ratio are computed directly from $V_p$, $V_s$ and bulk density; (v) synthetic seismogram generation and seismic-to-well ties, since $V_p$ combined with density gives acoustic impedance, and integrated transit time converts the well from depth to two-way time.
(b) Compressional and shear wave attenuation (2 pts). Attenuation (amplitude loss per unit travel distance, often expressed as $1/Q$) is far more sensitive than velocity to rock texture and fluid state. Applications: (i) fracture detection — both intrinsic (fluid-flow, squirt-flow) and scattering attenuation rise sharply across natural or induced fractures, well before any velocity change is resolvable; (ii) permeability and fluid-mobility indication — attenuation in partially saturated, permeable rock (Biot flow mechanism) correlates with permeability, useful where velocity alone is ambiguous; (iii) gas/fluid-contact and cement-bond quality — attenuation increases markedly in gas-charged rock and across a poor cement sheath, complementing the amplitude-based CBL.
(c) Amplitude of reflected waves (3 pts). Applications: (i) cement bond logging (CBL/VDL) — the amplitude of the casing signal reflected/attenuated by the cement sheath is the primary indicator of cement-to-casing bond quality (high amplitude = poor/no bond, free pipe; low amplitude = good bond); (ii) acoustic impedance contrast at bed boundaries — reflection amplitude scales with the impedance contrast ($Z=\rho V$) across an interface, so a strong reflection flags a lithology or fluid-contact change, the physical basis for synthetic-seismogram calibration; (iii) borehole imaging (acoustic televiewer) — reflected-pulse amplitude around the borehole wall maps fractures, vugs, breakouts and bedding dip, since these features locally scatter or absorb the reflected pulse.
