Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams, 98-Pet-B1, Well Logging and Formation Evaluation — December 2015, 3 hours, closed book (approved calculators permitted), 10 questions, all marked.
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) Mud invasion. The process by which drilling-mud FILTRATE (the liquid phase of the mud) is forced radially into a permeable formation because the hydrostatic mud-column pressure exceeds the formation pore pressure (overbalanced drilling); invasion displaces some of the native formation fluids away from the wellbore and is the root cause of the Rxo/Ri/Rt resistivity profiles logged by a multi-depth-of-investigation tool suite.
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) 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 SP and Sxo equations of Questions 9 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: 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.
