Page 77 - Fundamentals of Enhanced Oil and Gas Recovery

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Enhanced Oil Recovery Using CO 2

could only make a one-phase mixture in a certain range of mixing ratios. The red
(gray in print version) line on the edge side of C 1 C 71 represents the composition of
mixed solutions, which will not make a single phase at a certain pressure of P 1 .For
pressures equal or more than P 1 , the compositional ratio of the crude oil lies within
the two-phase region.
Here in this typical representation, reservoir oil consists of all three components. In
general, it includes intermediate to heavy compounds. As an option, we are planning to
flood the reservoir with a mixture of C 1 1 C 2 6 . The question is: “What composition
of this mixture will lead to a miscible process?” Considering that reservoir stays at pres-
sure P 1 , injection of pure C 1 will not make a miscible process. On the other hand,
intermediate components are miscible with reservoir oil in all proportions. One could
find this by drawing a line from reservoir oil to pure C 1 and C 2 6 . For the case of pure
C 1 , the line that passes through the two-phase region, indicates an immiscible process.
So the question still remains. In order to find the maximum mixture concentration of
pure C 1 for the injection solution to be miscible with crude oil, a tangent line from res-
ervoir oil to the two-phase curve is drawn. Its intersection with the side edge of
C 1 C 2 6 indicates the maximum concentration of C 1 to be added to the injection slug
without altering the miscibility between crude oil and injection fluid. Any further addi-
tion of C 1 to the injection solution will lead to an immiscible process. As a result, a
mixture of C 1 1 C 2 6 is only a FCM process within a certain range of composition.
As shown, the pressure affects the two-phase region size. An increase in pressure
will lead to a reduction in the two-phase region size. Therefore in this specific exam-
ple, a higher concentration of C 1 could be used if higher pressure was set. Although
pressure could modify the phase behavior toward a FCM process, it is not always pos-
sible to increase the pressure, as it may lead to formation fracture. CO 2 -enriched mix-
tures have smaller two-phase regions compared with other gases at the fixed pressure.
Thus CO 2 has been much in use for miscible injections.
As the reservoir pressure depletes, the two-phase region will be developed.
Therefore CO 2 will not be totally miscible with reservoir oil anymore. A slug of C 4
enriched with CO 2 will possibly make a miscible solution with the crude oil at pres-
sure of 1700 psi. Reservoir depletion occurs naturally, thus development of a two-
phase region is inevitable. On the other hand, increasing the injection pressure is not
always possible due to the operation costs and safety issues. As a result, CO 2 is not
normally miscible with crude oil at first contact. Miscibility, however, could be
assisted through multiple contacts.

3.2.1.2 Multiple-Contact Miscibility
Miscibility in multiple contacts occurs through two kind of mechanisms. Fig. 3.2
shows a ternary diagram, which depicts the process of VGD. Obviously, the injection
solvent and reservoir oil are not miscible, as the line connecting them passes through