24-Pet-A3 Fundamental Reservoir Engineering · December 2018
Question 1 of 7: Reservoir Engineering Terminology
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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17-Pet-A3 — Fundamental Reservoir Engineering · National Exams, December 2018 · 3 hours, closed book, approved Casio/Sharp calculator only · five (5) questions constitute a complete exam paper (the first five as they appear in the answer book are marked), all questions equal value, all parts of a multipart question equal weight.
Reference texts: Craft, B.C. & Hawkins, M.F., Applied Petroleum Reservoir Engineering, 3rd ed. (material balance, well testing, relative permeability, Darcy flow); Ahmed, T., Reservoir Engineering Handbook, 5th ed. (material balance, transient well testing, gas PVT, decline-curve analysis); Lyons, W.C. (ed.), Standard Handbook of Petroleum and Natural Gas Engineering, 3rd ed.; McCain, W.D., The Properties of Petroleum Fluids, 3rd ed. (PVT properties, Z-factor correlations).
Find. A brief (one-to-two sentence) engineering definition of each of the ten terms (a)–(j).
Approach. Each term is answered directly from its standard reservoir-engineering definition; no calculation is required.
a) Critical gas saturation
The minimum gas saturation at which the gas phase becomes continuous through the pore network and first begins to flow; below this saturation, liberated gas remains as disconnected bubbles and has zero effective permeability.
b) Dry gas reservoir
A gas reservoir whose produced fluid stays entirely in the gas phase from reservoir to surface conditions — the produced gas-oil ratio is essentially infinite and no liquid condenses out in surface separators.
c) Reservoir rock
A porous and permeable rock (commonly sandstone, limestone, or dolomite) whose interconnected pore network can store and transmit oil, gas, and water in economically producible quantities.
d) Trap
A subsurface configuration — structural (e.g., an anticline or a fault juxtaposing rock against a seal) or stratigraphic (e.g., a facies pinch-out) — that is capped by an impermeable seal and halts the upward/lateral migration of hydrocarbons, allowing them to accumulate.
e) Volumetric reservoir
A reservoir with no significant water influx or pressure support from an aquifer, so that as fluids are produced the reservoir pressure declines purely as a result of fluid and rock expansion within a fixed pore volume.
f) Wet gas reservoir
A gas reservoir whose produced fluid remains single-phase gas in the reservoir but crosses into the two-phase envelope as it cools and depressurizes to surface conditions, condensing a small amount of liquid (condensate) in the surface separator.
g) Original oil in place
The total volume of oil, expressed in stock-tank barrels (STB), estimated to exist in a reservoir at discovery, before any production — denoted $N$ and computed volumetrically or from material balance.
h) Residual oil saturation
The fraction of pore volume still occupied by oil after a displacing fluid (water or gas) has swept through a zone as thoroughly as it can under the prevailing forces; below this saturation the oil is disconnected and immobile regardless of further flooding.
i) Infinite acting
The early-time flow regime during which a transient pressure disturbance has not yet reached any reservoir boundary, so the reservoir behaves, for pressure-response purposes, as if it were unbounded and the outer boundary condition is irrelevant.
j) Threshold pressure
The minimum (displacement) capillary pressure that a non-wetting phase must exceed before it can enter and begin to displace the wetting phase from the largest connected pore throats of a rock — the entry point of the capillary-pressure curve.
