[1] conducted an experiment to investigate the effect of phase behavior on the final recovery. Their experiment showed the manner in which the development of miscibility affects oil recovery for a relatively simple system. They used CO₂ as the injection solvent and a mixture of butane and decane (40 mol% and 60 mol%, respectively) as the displaced fluid. So they could use ternary diagram to study the miscibility. The porous medium consisted of an 8 feet long, 2 inch diameter Berea sandstone core. The core was fully saturated with oil sample (butane + decane) without any initial water saturation. The core was flooded at temperature of 160°F and with 3 different pressures: 1900 psi, 1700 psi and 1500 psi. For the first test at 1900 psi, the injected gas (CO₂) is miscible with the oil over all composition. Thus the displacement was first contact miscibility displacement.  As shown in Figure 5‑25.a the reduced liquid concentration followed the ideal mixing line from the original oil composition point ‘O’ to the pure CO₂. Oil recovery was 99% of the original oil in place (OOIP).

The same experiment was run at 1700 psi. All other parameters such as temperature and porous sample held constant. As Figure 5‑25.b shows at this pressure a small two phase region exists, but the oil composition lie to the right of the critical tie line. The produced oil composition was measured after detecting the CO₂ in the effluent fluid. The composition measurement of produced oil showed that the composition changed in essentially a linear manner from the initial oil composition to the vicinity of critical point. Composition then moved around the two phase envelop and finally to pure CO₂. Thus the concentration change followed the description of an MCM process. Oil recovery for this test was 90% of OOIP.

The third experiment was conducted at 1500 psi. According to the Figure 5‑25.c achievement of miscibility would not be expected, because both the oil and CO₂ composition points lie on the same side of the critical tie line. The produced fluid concentration changed almost linearly from the initial composition to a point on the two phases envelop. The effluent then become two phase, indicating an immiscible displacement. Recovery was 81% of OOIP.

Figure 5-25: Displacement of a C₄ / C₁₀ Mixture by Pure CO₂, Representation on a Ternary Diagram[1]

Almost all the oil was displaced during the first contact miscibility process. Although final recovery during MCM process is high but it is less than the first contact miscibility. There are factors that contribute to the efficiency reduction, such as

• Miscibility must be developed in-situ,
• Complete miscibility may not exist across the entire miscible zone,
• Dispersion may lead to a temporary or permanent loss of miscibility in some part of the medium.

Thus the recovery in a MCM process is usually less than in a FCM process.  The experiment showed that immiscible displacement has a much less efficiency than miscible displacement.

References

[1] “Effect of Phase Equilibria on the Co₂ Displacement Mechanism”, SPEJ, Aug. 1979, 242-52

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