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24-Pet-B4 Well Testing · December 2016

Question 10 of 22: Bitumen-saturated estuarine point-bar – well placement, completion, and reservoir controls

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

Notes on this paper

EGBC National Exam — Petroleum Engineering, 2016-Dec. 3 hours duration; closed book. This sitting's own cover page reads “98-Pet-B4, Petroleum Geology” and every question is descriptive/interpretive petroleum geology (source rocks, hydrocarbon chemistry, oil sands and tight-oil development, shale gas and coalbed methane, carbonate traps, siliciclastic/deltaic traps) – no well-test pressure-transient content anywhere. Five (5) of the paper's six 20-mark sections are marked (NOTES item 5); all six are solved in full below so this set also serves as a complete study reference. This sitting's Section 6 is a Siliciclastic Traps section (wave-dominated delta, grain-size–permeability–porosity relations, sandstone diagenesis). The paper is entirely qualitative (draw/describe/define/list), with no numeric given data anywhere.

Reference texts: Selley, R.C. & Sonnenberg, S., Elements of Petroleum Geology, 3rd ed., Academic Press (source rocks, migration, traps, carbonate systems, oil sands); Tissot, B.P. & Welte, D.H., Petroleum Formation and Occurrence, 2nd ed., Springer (kerogen typing, maceral groups, catagenesis); Boggs, S. Jr., Petrology of Sedimentary Rocks, 2nd ed., Cambridge (source-rock, deltaic and carbonate lithofacies; sandstone diagenesis); James, N.P. & Jones, B., Origin of Carbonate Sedimentary Rocks, Wiley-Blackwell (carbonate platform/ramp/sabkha facies models); Butler, R.M., Thermal Recovery of Oil and Gas, Prentice Hall, 1991 (SAGD, CSS, oil sands thermal recovery); Green, D.W. & Willhite, G.P., Enhanced Oil Recovery, SPE Textbook Series Vol. 6 (thermal EOR mechanisms); Law, B.E. & Curtis, J.B., “Introduction to Unconventional Petroleum Systems,” AAPG Bulletin 86, 2002 (shale gas, tight gas, coalbed methane); Lee, W.J. & Wattenbarger, R.A., Gas Reservoir Engineering, SPE Textbook Vol. 5 (unconventional gas reservoir characterization).

Section 3, Q3-4: Bitumen-saturated estuarine point-bar – well placement, completion, and reservoir controls (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.

Estuarine point-bar cross section: lithology, IHS, and SAGD well pair placement basal shale / mud drape (regional seal below) clean, well-sorted sand (point-bar core) inclined heterolithic strata (IHS): sand/mud couplets thick mud drape (baffle/barrier) estuarine/tidal mud cap (top seal) injector (landed in cleanest sand) producer channel margin channel thalweg ≈ 100 m
Estuarine point-bar cross section: inclined heterolithic strata (IHS) – alternating clean sand and mud drapes – accreting laterally from the channel margin toward the thalweg, capped by an estuarine/tidal mud seal and underlain by a basal shale. A SAGD-style horizontal well pair is landed within the thickest, cleanest, most laterally continuous sand package, oriented along the point bar's accretion trend to maximize contact with connected pay and to avoid crossing major mud-drape baffles perpendicular to flow.

An estuarine point bar accretes laterally as the meandering channel migrates, building a body of inclined heterolithic strata (IHS): alternating beds of clean, well-sorted, bitumen-saturated sand and thin, laterally extensive mud drapes deposited during slack-water (tidal) intervals, dipping gently toward the channel thalweg. The well pair (or single well for CSS) is best landed within the thickest, most continuous clean-sand package, oriented parallel to the dominant accretion/paleoflow direction so the horizontal lateral stays within connected pay and does not repeatedly cross the mud-drape baffles at a steep angle; landing depth is chosen to maximize standoff from the basal shale (avoiding early steam/gas breakthrough into an underlying water leg or shale) while staying below the overlying tidal mud cap that provides the top seal.

Reservoir characteristics affecting recovery efficiency and economics in this setting: (1) mud-drape frequency, thickness and lateral continuity – frequent, laterally continuous mud drapes act as vertical permeability baffles/barriers that compartmentalize the steam chamber, slowing or blocking its vertical growth in SAGD and reducing swept volume; (2) net-to-gross (sand fraction) – a lower sand fraction directly reduces both connected pay thickness and ultimate recoverable volume; (3) vertical and lateral heterogeneity/anisotropy (kv/kh) – IHS is a classic low-kv/kh facies, which specifically slows SAGD chamber rise (a process that depends on vertical steam/gravity communication); (4) bitumen saturation and viscosity – higher initial water saturation or a more viscous (lower-API) bitumen both reduce initial mobility and raise the steam-oil ratio (energy cost per barrel produced); and (5) presence and continuity of a competent top seal (the tidal mud cap) – a breached or discontinuous cap risks steam/fluid loss out of zone (thief zones), reducing chamber conformance and process efficiency.