Question 7 of 7: Pressure–Temperature Diagrams of Hydrocarbon Systems

Question 7: Pressure–Temperature Diagrams of Hydrocarbon Systems (20 marks)

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 single two-phase envelope on a pressure–temperature plot, bounded by the bubble-point curve on its low-temperature side and the dew-point curve on its high-temperature side, classifies every hydrocarbon accumulation according to where the reservoir's own temperature and initial pressure sit relative to that envelope, and how the depletion path then crosses (or does not cross) its boundary.

Part (b) — Critical point, cricondenbar, cricondentherm. The critical point $C$ is where the bubble-point and dew-point curves meet and the liquid and vapour phases become identical (same density, composition, and properties) — unlike a pure substance, for a multicomponent mixture $C$ is not the highest point on the envelope. The cricondenbar is the maximum pressure at which two phases can coexist (the highest point on the whole envelope, at a temperature slightly above $T_c$); the cricondentherm is the maximum temperature at which two phases can coexist (the rightmost point on the envelope, at a pressure below $p_c$). Above the cricondentherm no liquid can ever form no matter how high the pressure is raised, and above the cricondenbar no gas can ever form no matter how the temperature is changed.

Part (c) — Quality lines, dew point, bubble point. The bubble-point curve (left/lower boundary) is the locus where the first bubble of gas appears from a saturated liquid; the dew-point curve (right/upper boundary) is the locus where the first drop of liquid appears from a saturated vapour. Inside the envelope, quality lines are constant-liquid-fraction contours (e.g. 10%, 30%, 50%, 70%, 90% liquid by volume) that fan out from the critical point across the two-phase region, lying between the bubble-point curve (the 100% liquid line) and the dew-point curve (the 0% liquid line); they show how much liquid is present at any interior $(T,p)$ point.

Part (a) — Reservoir and production paths for gas systems. A dry gas reservoir's temperature lies well to the right of the cricondentherm; both the reservoir depletion path (vertical, at constant reservoir $T$) and the production path to surface separator conditions stay entirely outside (to the right of / below) the envelope, so the fluid is single-phase gas everywhere, reservoir to stock tank — no liquid forms anywhere in the system. A wet gas reservoir is also single-phase gas at reservoir conditions (reservoir point still right of the cricondentherm), so its reservoir depletion path never crosses into the envelope either; but the production path, moving toward the much lower temperature and pressure at the surface separator, swings left and crosses into the envelope near the surface end, condensing liquid hydrocarbons only in the separator/pipeline, never in the reservoir. A retrograde gas condensate reservoir has its temperature between the critical temperature and the cricondentherm; the initial reservoir point sits just above the dew-point curve (single-phase gas), but as the reservoir depletes isothermally, the vertical depletion path crosses the dew-point curve and enters the two-phase region — liquid condenses inside the reservoir even though pressure is falling, which is the retrograde behaviour that gives the fluid its name, because ordinary vaporization behaviour would require liquid to disappear, not appear, as pressure drops. If depletion continues far enough to cross the lower dew-point curve, the liquid re-vaporizes, although in practice most of the condensed liquid is left behind in the pores. Its production path to the separator also ends inside the envelope, so condensate is recovered at the surface as well.

Part (d) — Oil reservoirs and the retrograde region. An undersaturated oil reservoir's initial pressure lies above the bubble-point curve at reservoir temperature (temperature to the left of $T_c$): the fluid is a single liquid phase, and the depletion path travels straight down through single-phase liquid until it reaches the bubble-point curve, at which point free gas first appears. A saturated oil reservoir starts with its initial pressure on the bubble-point curve, so free gas begins evolving from the very first pressure decline — there is no undersaturated period at all (if the initial point lies inside the envelope, the saturated oil already coexists with a free gas cap). The retrograde region is the part of the two-phase envelope between the critical temperature $T_c$ and the cricondentherm, bounded by the dew-point curve (from $C$ through the cricondenbar to the cricondentherm) and by the locus of maximum liquid dropout joining $C$ to the cricondentherm (shaded in the figure) — it is the zone in which isothermal pressure reduction causes liquid to condense (rather than vaporize) from an initially single-phase gas, exactly the mechanism described for retrograde gas condensates in Part (a).