24-Pet-A2 Petroleum Reservoir Fluids · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
17-Pet-A2 — Petroleum Reservoir Fluids · National Exams, May 2018 · 3 hours, closed book, Casio/Sharp approved calculators only · a formula sheet is provided; FIVE (5) questions constitute a complete exam paper (the first five as submitted are marked); all questions equal value, all parts of a multipart question equal weight; oilfield-unit questions must be answered in field units.
Reference texts: Craft, B.C. & Hawkins, M.F., Applied Petroleum Reservoir Engineering, 3rd ed. (Ch. 1–2, PVT properties, reservoir/well-stream classification); 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, gas hydrates/waxes/asphaltenes); Ahmed, T., Reservoir Engineering Handbook, 5th ed. (material balance, well-stream gravity, pseudo-critical property correlations); Danesh, A., PVT and Phase Behaviour of Petroleum Reservoir Fluids (equilibrium K-value flash calculations, Gibbs' phase rule).
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) Non-hydrocarbon impurities. The major non-hydrocarbon components routinely found in oil and gas reservoir fluids are nitrogen (N$_2$), carbon dioxide (CO$_2$), and hydrogen sulfide (H$_2$S), together with water vapour (H$_2$O) as a separate produced phase; trace helium, mercury, and oxygen are occasionally reported as well.
b) Definitions.
Gas hydrate — an ice-like crystalline solid formed when water molecules hydrogen-bond into a cage-like lattice that physically traps small gas molecules (methane, ethane, CO$_2$, H$_2$S) inside; hydrates form at the high-pressure, low-temperature conditions typical of subsea flowlines and can grow into solid plugs that block production tubing and pipelines.
Waxes — high-molecular-weight normal paraffins (roughly C$_{18}$ and heavier) that are fully dissolved in the crude oil at reservoir temperature but crystallize out as the oil cools below its wax-appearance temperature (e.g. in a wellbore or subsea pipeline), depositing as a solid paraffin layer that restricts flow.
Asphaltene — a class of large, polar, polycyclic aromatic hydrocarbon molecules (containing sulfur, nitrogen, oxygen, and trace metals) that are colloidally suspended/dissolved in the oil under reservoir conditions but can precipitate irreversibly as a hard, black, non-volatile solid when pressure drops below the asphaltene-onset pressure or when the oil's composition changes (e.g. CO$_2$ or light-hydrocarbon injection).
Bitumen — an extremely heavy, viscous crude oil (typically API gravity below about 10°, essentially immobile at native reservoir temperature without added heat or diluent) that is rich in asphaltenes and resins and depleted in light ends; the Athabasca oil-sands deposits referenced in Question 3 are the classic Canadian example.
c) PT diagram of a pure component. A pure substance has exactly three phase-boundary curves meeting at one triple point: the sublimation curve (solid–vapour, rising gently from very low $T,p$), the melting (fusion) curve (solid–liquid, steep and nearly vertical, rising from the triple point), and the vaporization curve (liquid–vapour, running from the triple point up to the critical point, beyond which no liquid/vapour distinction exists at all).
d) PV diagram — pure vs. multi-component. On a pressure–volume plot, a pure-component isotherm below $T_c$ enters the two-phase (liquid+vapour) dome from the compressed-liquid side, runs perfectly horizontal across the dome at the fixed saturation pressure $p_{sat}(T)$ while liquid converts to vapour at constant $p$, then exits into the superheated-vapour region. At $T=T_c$ the isotherm just touches the top of the dome at a single point — the critical point — with a horizontal inflection (zero slope, zero curvature) and no flat segment at all. Above $T_c$ the isotherm never enters the dome; it decreases smoothly and monotonically with no flat region, because no liquid/vapour distinction exists at that temperature. A multi-component mixture has the same three-region shape, but because the liquid and vapour compositions both change continuously as the mixture boils off (unlike a pure substance, whose composition can't change), the isotherm inside the two-phase region is not flat — pressure declines slightly and smoothly as volume increases across the dome, only becoming perfectly horizontal again in the pure-component limit.
e) PT diagram of a multi-component system. A multi-component mixture's phase envelope is a single closed loop (not three separate curves): the bubble-point curve runs from low pressure up to the critical point (where bubble- and dew-point curves meet and the liquid/vapour distinction vanishes), and the dew-point curve continues from the critical point back down to low pressure on the other side, enclosing the two-phase region. The cricondenbar is the highest pressure anywhere on the envelope (found on the dew-point branch, above and to the right of the critical point); the cricondentherm is the highest temperature anywhere on the envelope (also on the dew-point branch, to the right of the critical point). Outside the loop the system is single-phase liquid (left of the bubble-point curve) or single-phase gas (right of the dew-point curve / cricondentherm); inside the loop, quality lines mark the locus of constant liquid volume fraction.
