Question 6 of 10: Characteristics Affecting Log Quality
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Examinations, 17-Pet-B1, Well Logging and Formation Evaluation — May 2019, 3 hours, closed book (Sharp or Casio approved calculators permitted), 10 questions, all marked. Every question on this paper is Well Logging & Formation Evaluation content, solved to the paper as printed. Every datum here was read from the paper: the Question 9 SP track prints "SSP −80 mV" and "PSP 47 mV", its gamma-ray track prints 28, 92 and 44 API against Zones B, C and A, and the attachments supply the gas-sand chart and SP departure chart used in Questions 7 and 10.
Reference texts: Bassiouni, Theory, Measurement, and Interpretation of Well Logs (SPE Textbook Series Vol. 4); Asquith & Krygowski, Basic Well Log Analysis, 2nd ed. (AAPG); Ellis & Singer, Well Logging for Earth Scientists, 2nd ed.; Schlumberger, Log Interpretation Charts / Log Interpretation Principles and Applications.
Borehole condition (rugosity, washout, and out-of-gauge hole). A rough, washed-out, or ovalized hole (Question 2) increases the standoff between a pad-contact tool (density, microresistivity) and the formation, degrading its reading and requiring a borehole-size correction; severe washout can make some pad tools lose contact entirely.
Mud-system properties (mud weight, salinity/Rm, and filtration). Mud weight and type set the invasion rate and depth (Question 3–4), while Rm/Rmf control the resistivity-tool response contrast and the achievable SP deflection (Questions 9–10); a heavily-weighted, high-solids mud also attenuates radioactive-source (density, neutron) tool counts.
Logging speed and depth-control (cable tension/stretch, tool centralization). Running a tool too fast for its designed sampling/count-rate statistics degrades vertical resolution and introduces statistical (shot) noise on nuclear logs; poor depth control (cable stretch, yo-yo motion) misregisters different logging runs against each other, corrupting any crossplot (Question 8) that combines curves from separate runs.
Environmental (temperature/pressure) corrections. Every raw log reading requires borehole-temperature, pressure, and (for resistivity) mud-filtrate-invasion corrections applied via the service company's own chart book before it is a valid formation reading; omitting or mis-applying a correction (e.g., the wrong BHT for an SP calculation, as in Questions 9–10) silently degrades apparent log quality even when the raw tool response was fine.
None of these four categories shows up as an obvious "error flag" on the log print itself — a rugose hole, a slightly-too-fast logging speed, or an unapplied temperature correction all still produce a smooth-looking, plausible curve. That is precisely why formal log-quality control is a repeat-section and calibration-check discipline rather than a visual one: a short repeat run over a known interval, compared curve-by-curve against the main pass, is the standard way of catching a borehole-, speed-, or calibration-driven quality problem before it is mistaken for a real formation feature during interpretation of Questions 7 through 10.
