Phase Diagrams – P-T Envelopes, P-V Behaviour, Ternary Systems: 24-Pet-A2 Petroleum Reservoir Fluids, Undated paper, Q2

Question 2: Phase Diagrams – P-T Envelopes, P-V Behaviour, Ternary 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) P-T envelope year relabelling (6 marks)

Given. A composite P-T diagram shows four nested phase envelopes for the same reservoir fluid at four different times during primary depletion, currently mislabelled “1998”, “1996”, “2016”, “2010” in order of increasing envelope size (smallest to largest).

Find. The correct year for each envelope, and the physical reason the sizes must be reassigned.

Envelopes reordered smallest-to-largest with their correctly reassigned years (see reasoning below). Sizes are schematic, following the source's own nesting order.

the ordering PRINCIPLE below is certain and is what is marked.

State the governing physical trend. During primary depletion of an oil reservoir, the lightest components (methane and other light ends) are preferentially produced as free gas evolves and is produced ahead of the remaining liquid. The fluid left behind therefore becomes progressively enriched in intermediate and heavy components over time.
Relate composition to envelope size. Heavier, more complex hydrocarbon mixtures have larger two-phase envelopes with higher critical temperatures than lighter mixtures (Ahmed, Ch.1). So as the remaining reservoir fluid gets heavier with time, its P-T envelope grows.
Reorder by size, earliest year to smallest envelope. Sorting the four printed year labels chronologically (1996, 1998, 2010, 2016) and assigning them smallest-to-largest envelope in that order: smallest envelope → 1996; next → 1998; next → 2010; largest envelope → 2016.

Question 2(a) – corrected year labels
Envelope (size rank)	Corrected year
Smallest	1996
2nd smallest	1998
2nd largest	2010
Largest	2016

Reason: the two-phase envelope of the produced reservoir fluid grows over time during primary depletion, because light components are preferentially removed as free gas, progressively enriching the remaining liquid in heavier fractions – so the smallest envelope must be the earliest year and the largest the latest.

(b) Correct P-V isotherm above T_c (1 mark)

Diagram (c). A pure-substance isotherm strictly above its critical temperature has no phase transition at all: no horizontal (constant-pressure) two-phase segment, and no inflection point (that inflection only occurs exactly at the critical isotherm). Of the four options, only the smooth, monotonically decreasing curve with no flat segment and no inflection – diagram (c) – is physically consistent with a supercritical isotherm. Diagram (a) shows the two-phase plateau of a sub-critical isotherm; (b) shows the critical isotherm's inflection point; (d) is not physically realistic (pressure rising with volume).

(ii) Freehand ternary diagram (6 marks)

Ternary composition diagram: methane (C₁) at the light-component vertex, C₂–C₆ intermediates at the top vertex, C₇₊ heavies at the right vertex. Non-hydrocarbons behave similarly to light HC species: N₂ and CO₂ plot near the C₁ corner, H₂S nearer the C₁–intermediates edge. The two-phase (liquid+vapor) envelope is skewed toward the light (C₁-rich) side of the diagram, the region typical reservoir fluids traverse.

Any mixture point inside the shaded dome splits into a liquid and a vapor phase joined by a tie line; points outside it are single-phase.

(iii) Effect of isobaric cooling on the two-phase envelope (1 mark)

The two-phase envelope expands. At fixed pressure, lowering temperature favours condensation (the liquid phase becomes relatively more stable), so a larger area of the composition triangle now splits into coexisting liquid and vapor – the envelope grows.

(iv) ABDE pressure–volume isotherm (constant-composition expansion, oil) (6 marks)

CCE isotherm for an oil: A is the highest-pressure state (starting point), E the lowest. The steep E–D segment is single-phase liquid; the much flatter D–B–A segment is the two-phase region below the bubble point, where a small pressure drop releases a large volume of evolving gas.

Question 2(iv) – region identification
Item	Answer
(a) Liquid Region	Segment E–D
(b) Vapor Region	Not present – a CCE test on oil never reaches a single-phase vapor state within the tested range; that only occurs for a gas-condensate CCE.
(c) Liquid + Vapor Region	Segment D–B–A
(d) Dew-Point Pressure Point	Not present, for the same reason as (b) – no dew point exists along an oil's CCE path.
(e) Bubble-Point Pressure Point	Point D (the kink where the slope breaks from steep to flat)

If the test temperature were raised, the B–D (two-phase, flat) region would shrink. Raising the isotherm's temperature moves it toward the fluid's critical isotherm; the flat two-phase segment of a CCE P-V isotherm narrows as the critical point is approached and vanishes entirely exactly at the critical isotherm (where the curve instead shows a single inflection point, as in Q2(b) diagram (b)).

Topic. P-T phase-envelope evolution during depletion, pure-substance P-V behaviour, ternary composition diagrams, and the CCE (constant-composition expansion) P-V isotherm for oil.

Key relations. Heavier mixtures → larger P-T envelope, higher T_c; supercritical isotherm has no phase transition; ternary two-phase region grows as T falls at constant P; CCE isotherm slope break = bubble point.

Why this works. All four sub-parts are variations on the same idea – a two-phase envelope (in P-T, P-V, or ternary-composition space) is a boundary set by intermolecular forces and composition, and it swells or shrinks predictably whenever composition (heavier vs lighter) or condition (T at fixed P) shifts stability toward the liquid.

Common pitfall. Assuming an oil's CCE isotherm has a full liquid→two-phase→vapor progression like a gas condensate's; an oil's CCE test never reaches single-phase vapor at any practically achievable expansion.

Source. Ahmed, Reservoir Engineering Handbook, Ch.1 (phase behaviour, ternary diagrams); McCain, Properties of Petroleum Fluids, Ch.2–3 (PVT tests).