24-Pet-A2 Petroleum Reservoir Fluids · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
98-Pet-A2 — Petroleum Reservoir Fluids · National Exams, December 2016 · 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. (phase behaviour, black-oil PVT laboratory data); Ahmed, T., Reservoir Engineering Handbook, 5th ed. (reservoir fluid classification, 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) Basic classification of reservoir fluids. Reservoir fluids are classified by where the initial reservoir pressure and (fixed) reservoir temperature plot relative to the fluid's own $p$–$T$ phase envelope — specifically, whether reservoir temperature sits below the critical temperature $T_c$, between $T_c$ and the cricondentherm, or above the cricondentherm — together with the resulting producing GOR, stock-tank API gravity and liquid colour. On this basis, five types are recognised; any four are: black oil (low-to-moderate GOR, dark liquid, reservoir $T<T_c$), volatile oil (higher GOR and API, reservoir $T$ just below $T_c$), retrograde gas condensate (reservoir $T$ between $T_c$ and the cricondentherm), wet gas (reservoir $T$ above the cricondentherm but the produced gas still crosses into the two-phase region at surface conditions), and dry gas (reservoir and surface conditions both stay outside the envelope; no liquid forms anywhere in the process).
b) Non-hydrocarbon components. Nitrogen (N$_2$), carbon dioxide (CO$_2$), and hydrogen sulfide (H$_2$S) are the three non-hydrocarbon ("impurity") components routinely reported in a reservoir-fluid compositional analysis (water vapour is also present but is normally handled separately as a distinct produced phase).
c) Specific gravity from API 30. $\gamma_o=\dfrac{141.5}{131.5+API}=\dfrac{141.5}{131.5+30}=\dfrac{141.5}{161.5}$, so $\boxed{\gamma_o=0.876}$ (relative to water).
d) API. API stands for the American Petroleum Institute, the industry body whose gravity scale ($API=\frac{141.5}{\gamma_o}-131.5$) is the standard measure of stock-tank oil density used throughout the industry.
e) Black oil vs. the two samples. Of the two, the $API=15$ sample is the one classified as a black oil. Black oils are the heavier, less volatile crudes (typically API roughly 15–40, dark in colour, GOR usually below about 2000 SCF/STB); the $API=45$ sample is far lighter and more characteristic of a volatile oil (or, at even higher GOR, a gas condensate) — higher API correlates with a lighter, more compositionally complex, near-critical fluid, the opposite of black-oil behaviour.
f) LPG. Liquefied Petroleum Gas is propane and/or butane (C$_3$–C$_4$) kept in the liquid state under modest pressure at ambient temperature for storage, transport, and use as a fuel — these components liquefy at pressures of only a few hundred psi at surface temperature, unlike methane.
g) LNG vs. NGL. LNG (Liquefied Natural Gas) is essentially pure methane, liquefied by deep refrigeration to about $-162\,{}^{\circ}\text{C}$ at near-atmospheric pressure for ocean transport, because methane cannot be liquefied by pressure alone at ambient temperature (its critical temperature is $-116.6\,{}^{\circ}\text{F}$). NGL (Natural Gas Liquids) is the mixture of heavier components (ethane through pentanes-plus) recovered from a raw gas stream and kept liquid at ambient temperature using only moderate pressure, exactly like LPG but covering a broader component slate; LNG is a cryogenic single-component liquid, NGL is a pressurised multicomponent liquid.
h) Degrees of freedom, propane + butane, two phases. Gibbs' phase rule gives $F=C-P+2$. With $C=2$ components (propane, butane) and $P=2$ phases (liquid + vapour) in equilibrium: $F=2-2+2$, so $\boxed{F=2}$ — two independent intensive variables (e.g. temperature and the liquid-phase composition $x_{C_3}$) fix the entire state of the two-phase system; pressure then follows from those two rather than being independently choosable.
i) PT diagram of a pure component. A pure substance's phase diagram has three boundary curves meeting at one triple point: the sublimation curve (solid–gas, rising gently from very low $T,p$), the fusion curve (solid–liquid, a steep, nearly vertical line running upward from the triple point), and the vaporization curve (liquid–gas, running from the triple point up to the critical point, beyond which the liquid/gas distinction disappears). The solid region lies to the left of the fusion curve and above the sublimation curve (low $T$, and/or very high $p$); the liquid region lies to the right of the fusion curve and above/left of the vaporization curve (between the triple point and critical point in $T$, high $p$); the gas region lies below the sublimation and vaporization curves and to the right of (above) the critical temperature, covering all high-$T$/low-$p$ conditions including the entire region past $T_c$ where no vaporization curve exists at all.
j) Triple point. The triple point is the single, invariant $(T,p)$ at which solid, liquid, and vapour phases of a pure substance all coexist in equilibrium simultaneously — the point where the sublimation, fusion, and vaporization curves meet. By Gibbs' rule $F=C-P+2=1-3+2=0$: with zero degrees of freedom, the triple point is a fixed, unique condition for a given pure substance, not a range.
