24-Pet-B1 Natural Gas Engineering · December 2016

Question 6 of 11: Spectral GR, reflected-wave amplitude, and attenuation

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exams — December 2016. 98-Pet-B1, Well Logging and Formation Evaluation (every question is log-interpretation content, not gas-engineering material). 3-hour closed-book exam, 11 questions, all marked, calculators and attached graphs/formula sheet permitted.

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.

Check: Q3, Q7, Q8, Q9(b), Q10 and Q11 are built on the paper's printed logs and attached charts. Values printed as annotations on the logs (Q11's SSP, PSP and GR labels) are used exactly as printed. Values read off a curve or a chart (Q9(b) and the Q8 chart check, Q10's track readings) are read from the printed figure and flagged inline with their precision. All arithmetic that follows is exact.

Question 6: Spectral GR, reflected-wave amplitude, and attenuation (8 marks)

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.

(i) Three radioisotopes (3 pts). The spectral (natural) gamma-ray tool resolves the total gamma count into three independent energy-window contributions from: Potassium-40 (K-40), reported as weight-% K; Uranium-238 (via its Bismuth-214 decay-series peak), reported as ppm U (eU); and Thorium-232 (via its Thallium-208 decay-series peak), reported as ppm Th (eTh).

(ii) Amplitude of reflected waves (3 pts). As in Q1(c): cement-bond evaluation (CBL amplitude ↔ bond quality), acoustic-impedance-contrast identification of bed/fluid boundaries for synthetic-seismogram tie, and borehole-wall imaging (acoustic televiewer amplitude maps fractures, vugs and breakouts).

(iii) Compressional and shear wave attenuation (2 pts). As in Q1(b): fracture detection (attenuation rises across fractures ahead of any velocity change), permeability/fluid-mobility indication via Biot flow attenuation, and gas-zone / poor-cement-bond flagging (both attenuate elastic waves strongly).

Sub-part	Answer
(i) Radioisotopes	K-40, U-238 (via Bi-214), Th-232 (via Tl-208)
(ii) Reflected-amplitude use	Cement-bond QC; impedance-contrast/bed-boundary ID; acoustic borehole imaging
(iii) Attenuation use	Fracture detection; permeability/fluid-mobility; gas/poor-cement flagging

Topic: Spectral gamma-ray isotopes and acoustic-log applications (companion to Q1).

Key relations: Total GR $\propto$ K + Th + U contributions; $CGR$ (computed GR) $=$ Th + K contributions only (U excluded).

Why this works: K, Th and U are the three naturally occurring radioactive series/isotopes present in crustal rock at geologically significant concentrations; resolving their separate energy windows (rather than the total count alone) is what lets spectral GR distinguish clay type and flag non-clay-related uranium (see Q10).

Common pitfall: Assuming all three isotopes are clay-hosted — U is frequently associated with organic matter, phosphate or fracture-controlled mineralization rather than clay, which is exactly the distinction exploited in Q10.

Source: Ellis & Singer Ch. 6 (Natural Radioactivity); Schlumberger Log Interpretation Principles.

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