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 pure component's vapour-pressure curve on a $p$–$T$ plot is a single line running from its triple point to its critical point — bubble point and dew point coincide there is no "envelope," just a boundary between liquid and vapour. Mixing two pure components opens that line into a genuine two-phase region bounded by separate bubble-point and dew-point curves that meet at a mixture critical point $C_m$; unlike a pure substance, $C_m$ is not the highest point of the mixture's envelope — the cricondenbar (max pressure) sits above and to the right of $C_m$, and the cricondentherm (max temperature) sits to the right of and below $C_m$.
Part (a) — 80% C1 / 20% n-C4. Being methane-rich, this mixture's envelope sits close to and just to the right of pure methane's own curve, but a small amount of a much heavier component produces a disproportionately large upward bulge in the critical-pressure locus — this is the classic light/heavy binary effect (McCain, Fig. 1-4 type construction) in which the mixture cricondenbar exceeds BOTH pure components' critical pressures, here drawn approaching the hinted magnitude (roughly 1800–1900 psia) well above methane's own 667 psia. Its critical temperature sits only modestly above methane's $T_c=-116.6\,{}^{\circ}\text{F}$, and its cricondentherm extends further right into the retrograde region.
Part (b) — 20% C1 / 80% n-C4. Being butane-rich, this mixture's envelope sits close to pure n-butane's curve, shifted left in temperature by the more volatile methane and with a more modest cricondenbar boost above n-C4's own $p_c=550.6$ psia (drawn here around 800–850 psia) — the light/heavy bulge effect is present but far smaller than in Part (a), because only a minority mole fraction is the light component. Its critical temperature sits noticeably below pure n-C4's $T_c\approx306\,{}^{\circ}\text{F}$.
Any reservoir whose temperature falls between a mixture's $T_c$ and its cricondentherm exhibits retrograde condensation as pressure declines isothermally — liquid drops out of the single-phase gas even though pressure is falling, exactly the mechanism described in Question 1(e). Because both drawn mixtures have their cricondentherm well to the right of their own critical point, a reservoir at, say, 40–150°F producing either blend would be a retrograde gas condensate rather than a simple dry or wet gas.
