24-Pet-A2 Petroleum Reservoir Fluids · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
98-Pet-A2 — Petroleum Reservoir Fluids · National Exams, December 2015 · 3 hours, closed book, non-communicating calculator only · first five questions 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. (Ch. 1–2, PVT properties, reservoir/well-stream classification, material balance); Lyons, W.C. (ed.), Standard Handbook of Petroleum and Natural Gas Engineering, 3rd ed. (Standing-Katz Z-factor correlation, gas properties); McCain, W.D., The Properties of Petroleum Fluids, 3rd ed. (black-oil PVT laboratory data, well-stream recombination); Ahmed, T., Reservoir Engineering Handbook, 5th ed. (p/Z material balance, well-stream gravity); Danesh, A., PVT and Phase Behaviour of Petroleum Reservoir Fluids (equilibrium K-value flash calculations).
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) Flash calculations (the source prints this as "Flsah calculations," a typographical rendering of "Flash calculations"). A flash calculation determines the amounts and compositions of the liquid and vapour phases in equilibrium at a specified pressure and temperature for a fluid of known overall (feed) composition $z_i$, using equilibrium ratios $K_i=y_i/x_i$ in the Rachford–Rice mass balance $\sum_i \dfrac{z_i(K_i-1)}{1+V(K_i-1)}=0$; unlike a differential test, neither phase is removed from the cell — only the phase split is computed.
b) Black-oil. A black oil is a crude whose reservoir behaviour can be represented with only two pseudo-components, stock-tank oil and total surface gas, because the reservoir temperature is well below the fluid's critical temperature; its properties ($B_o$, $R_s$, $\mu_o$) vary smoothly and monotonically with pressure, with GOR typically below about 2000 SCF/STB and no compositional (near-critical) behaviour.
c) Bitumen. Bitumen is an extremely viscous, essentially immobile hydrocarbon at reservoir conditions (API gravity typically below 10, viscosity often exceeding 10,000 cP) that will not flow to a wellbore under primary depletion; it requires thermal (SAGD, CSS) or solvent-based recovery methods and is the resource type found in oil-sand deposits such as the Athabasca.
d) Dry gas. A dry gas reservoir's temperature lies well to the right of the cricondentherm on the $p$–$T$ phase envelope, so the fluid remains single-phase gas along both the reservoir depletion path and the production path to standard conditions — no liquid hydrocarbon ever forms, in the reservoir or at surface (composition is almost entirely methane).
e) Retrograde condensation. Retrograde condensation is the formation of liquid from an initially single-phase gas as pressure decreases isothermally — the reverse of ordinary vaporization behaviour — and occurs only when the system temperature lies between the critical temperature $T_c$ and the cricondentherm, i.e. within the retrograde region of the phase envelope.
f) Dead oil. Dead oil is oil that carries no solution gas ($R_s\approx0$) at the stated conditions, typically the stock-tank oil remaining after all associated gas has flashed off at atmospheric pressure; the term is also used for surface oil that has lost its light ends through weathering.
g) Live oil. Live oil is reservoir (or PVT-cell) oil that still holds its original solution gas in solution at reservoir or bubble-point pressure — i.e. undegassed oil, the opposite of dead oil, and the fluid state that must be reconstituted by recombination when a well is sampled as separated surface streams.
h) Kay's mixing rule. Kay's rule sets a gas mixture's pseudo-critical properties as the mole-fraction-weighted average of the pure-component critical properties, $T_{pc}=\sum_i y_iT_{ci}$ and $p_{pc}=\sum_i y_ip_{ci}$, collapsing a multicomponent mixture onto one $(T_r,p_r)$ pair so a single corresponding-states $Z$-chart (Standing–Katz) applies to the whole mixture.
i) Dew point pressure. The dew-point pressure, at a fixed temperature, is the pressure at which the first drop of liquid forms from a system that starts entirely as vapour; on the $p$–$T$ diagram it is the locus forming the high-temperature (right-hand) boundary of the two-phase envelope.
j) Bubble point pressure. The bubble-point pressure, at a fixed temperature, is the pressure at which the first bubble of gas forms from a system that starts entirely as liquid; on the $p$–$T$ diagram it is the locus forming the low-temperature (left-hand) boundary of the envelope, and below it the oil is saturated (gas evolves as pressure falls further).
