Question 2 of 4: CO 2 Slim-Tube Displacement — Pseudo-Ternary Diagram

Question 2: CO₂ Slim-Tube Displacement — Pseudo-Ternary Diagram (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.

This is a classic multi-contact (dynamic) miscibility problem, not a first-contact one. Reading the diagram: the reservoir oil plots on the C1–(C2-C6) base edge (it contains no CO₂), inside the region bounded by the two-phase envelope's liquid branch, while both candidate injection gases plot on the opposite CO₂–(C2-C6) edge, close to the CO₂ apex. The straight mixing (dilution) line joining fresh oil to fresh gas passes through the two-phase envelope for both cases — so the very first contact between injected gas and virgin oil is always two-phase (immiscible) here, regardless of which gas is used. What decides whether the flood as a whole ever becomes miscible is the limiting (critical) tie line: extend the tie line that passes through the oil composition and the plait point onward until it crosses the CO₂–(C2-C6) edge. Any injection-gas composition that plots beyond that crossing point (i.e. richer in CO₂, further from C1) drives a vaporizing-gas-drive mechanism that develops miscibility dynamically after many contacts, even though no single contact is miscible; any composition short of that crossing point never develops miscibility no matter how many contacts occur.

(a) 96% CO₂ / 4% C2-C6. This point lies very close to the CO₂ apex, clearly beyond (more CO₂-rich than) the limiting-tie-line crossing on the injection edge — on the "first contact miscible" side of the diagram's own labelling. Yes, miscibility is achieved — not on the first contact (that mixture is still two-phase), but dynamically: as fresh, CO₂-rich gas repeatedly contacts fresh oil ahead of it, it vaporizes (strips) the C2-C6 intermediates out of that oil into the gas phase. Each new contact enriches the leading edge of the gas bank further, moving its local equilibrium composition along successive tie lines that rotate toward the limiting tie line/plait point; because the injection gas composition sits beyond that limiting tie line's crossing, the front's own composition eventually becomes tangent to (miscible with) the oil immediately ahead of it before it exits the tube. This is a vaporizing-gas-drive dynamic miscibility, the mechanism CO₂ floods normally rely on.

(b) Composition at breakthrough (exit end). The leading tip of the gas bank is the material that has travelled the full length of the slim tube, and by the vaporizing-gas-drive mechanism it has continuously re-equilibrated against fresh oil the entire way — each contact strips a further increment of C2-C6 into it. It therefore arrives at the exit far more enriched in intermediates than the 96%CO₂/4%C2-C6 gas that was originally injected: its composition should plot on the vapour (upper) branch of the two-phase envelope, close to the plait point (marked "gas @ breakthrough" on Fig. 1) — i.e. it approaches, but does not necessarily exactly reach, the critical composition, since the number of contacts is finite over a finite tube length. It is emphatically not the injected composition; a common error is to assume the produced gas equals the injected gas because "no dispersion" is specified — that instruction only removes physical mixing/smearing of the front, it does not prevent the vaporizing mass transfer that is the whole mechanism being tested.

(c) Oil composition near the entrance after many PV's. The rock just downstream of the injector has been contacted by a very large number of pore volumes of fresh, unspent CO₂-rich gas over the course of the run, so essentially all of the extractable C2-C6 intermediates have been leached out of the oil there. The residual oil at that location approaches the liquid (lower) branch of the two-phase envelope at the limiting tie line — i.e. the leanest, most C1-rich oil composition the local vaporization process can reach (marked "stripped oil @ entrance" on Fig. 1), a residual composition far poorer in intermediates than the virgin reservoir oil. Physically this is the near-wellbore oil that a real CO₂ flood leaves behind once its miscible "front" has swept past: a lean, largely non-extractable residue.

(d) 70% CO₂ / 30% C2-C6. This composition plots further from the CO₂ apex than the 96%/4% gas — short of (on the C1 side of) the limiting-tie-line crossing, i.e. on the diagram's own "immiscible" side. Miscibility will not be achieved, at first contact or dynamically: the injection gas is not CO₂-rich enough for the vaporizing-gas-drive enrichment path to ever reach a tie line tangent to the oil ahead of it, so the displacement remains a two-phase, immiscible gas flood throughout (lower ultimate recovery than case (a), governed by relative-permeability/residual-saturation trapping rather than a miscible piston-like sweep). Achieving miscibility with this leaner gas would require raising the process pressure enough to shrink the two-phase envelope until the limiting tie line's crossing point moves past 70% CO₂ — i.e. operating above this gas's own (higher) minimum miscibility pressure.