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) Mud invasion. The process by which drilling-mud filtrate is forced into a permeable formation under the differential pressure between the (overbalanced) mud column and the formation pore pressure, displacing some or all of the original formation fluid near the wellbore. Invasion depth and profile depend on formation permeability/porosity, mud-cake quality, overbalance pressure and elapsed time since drilling.
(b) Mud cake. The thin layer of solid mud particles (clay/weighting-agent solids too large to pass through the formation pore throats) that filters out and builds up on the borehole wall opposite a permeable bed as the liquid phase (filtrate) invades. A low-permeability, self-limiting mud cake is desirable — it progressively slows further filtrate loss/invasion and is a diagnostic sign of permeability (a caliper log reads a reduced hole diameter opposite mud cake).
(c) Mud filtrate. The liquid phase of the drilling mud that passes through the mud cake and into the formation during invasion (the solids are screened out by the cake). Its resistivity, $R_{mf}$, is measured at the surface (or estimated from mud resistivity via a standard chart) and is the resistivity that saturates the flushed zone immediately around the wellbore.
(d) Flushed zone. The near-wellbore region where mud filtrate has displaced essentially all of the movable original formation fluid, leaving only mud filtrate (at saturation $S_{xo}$, generally $\lt 100\%$ once residual hydrocarbon is accounted for) plus any immovable (irreducible/residual) original fluid. It is the zone sampled by microresistivity devices (MSFL, microlog) and its resistivity is denoted $R_{xo}$.
