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.
All four terms describe the same physical process — drilling with an overbalanced water- or oil-base mud column — viewed at successive stages moving away from the wellbore:
a) Main reason of mud invasion. Invasion happens because the drilling mud is deliberately maintained at a hydrostatic column pressure HIGHER than the formation pore pressure (an overbalanced condition, kept as a safety margin against a kick); that positive pressure differential physically forces the liquid phase of the mud (the filtrate) radially into any permeable formation exposed at the wellbore, displacing part of the native formation fluid away from the hole. Invasion stops advancing (for a given differential) once the growing mud cake's own resistance throttles the filtration rate to near zero.
b) Mud cake. The thin, low-permeability layer of solid mud particles (clay/weighting-agent solids too large to enter the pore throats) that is filtered out and plastered onto the borehole wall as the liquid filtrate passes into the formation; it progressively self-seals and throttles the invasion rate, and its measured thickness (from the caliper, as a hole diameter LESS than bit size — the opposite sense of the washouts diagnosed in Question 2) is itself a permeability indicator.
c) Mud filtrate. The liquid phase of the drilling mud (water- or oil-base, with dissolved salts) that physically passes through the mud cake and into the formation pore space during invasion; its resistivity, Rmf, replaces the native connate-water resistivity Rw in the immediately-invaded rock and is the quantity used directly in the Sxo and ESSP relations of Questions 7 and 10.
d) Flushed zone. The near-wellbore annulus in which mud filtrate has displaced essentially ALL of the movable native fluid, leaving only mud filtrate plus any IRREDUCIBLE/RESIDUAL hydrocarbon saturation (1−Sxo); it is the zone shallow, pad-contact tools (microresistivity, density, neutron) primarily read, and its resistivity Rxo and saturation Sxo are the target of the Sxo/Rmp relations on the attachment.
Radial invasion profile around the wellbore, driven by the overbalanced mud column: mud cake at the wall, the flushed zone (Rxo, Sxo) immediately behind it, an invaded/transition annulus (Ri), and the undisturbed formation (Rt, true Sw) beyond the invasion front — the same radial picture developed further in Question 4.
