Page 76 - Fundamentals of Enhanced Oil and Gas Recovery

64 Ramin Moghadasi et al.

a. Vaporizing gas drive (VGD): Miscibility is achieved through in situ vaporiza-
tion of the intermediate molecular weight hydrocarbons from the reservoir oil
into the CO 2 .
b. Condensing gas drive (CGD): Miscibility is developed by an in situ transfer of
CO 2 into the reservoir oil. In fact, CO 2 will be diffused into the crude oil.
When CO 2 interacts with reservoir oil, a dynamic miscibility zone would be devel-
oped. Therefore a CO 2 -enriched crude oil is produced from the producing wells.

3.2.1.1 First-Contact Miscibility
Normally, through the FCM process, a relatively small slug volume is first injected. It is
then followed by an injection of a larger and less expensive slug. These slugs are called the
primary and secondary slug, respectively. Economically, these slugs should be miscible.
Otherwise, a residual saturation of primary slug will be trapped within the reservoir.
In order to determine the miscibility conditions, or in other words, the possibility
of the FCM process, it is essential to accurately predict fluids phase behavior in con-
tact. Phase behavior can be shown on a ternary or pseudo-ternary diagram. Fig. 3.1
shows a typical pseudo-ternary diagram. Each of the vertices represents the pure com-
ponents, and side edges of this equilateral triangle are scaled to represent the binary
composition of the three possible pairs. A fluid system consisting of all three compo-
nents, such as the typical crude oil characterized in this figure, is represented by points
interior to the triangle.
As shown in this figure, C 1 and C 2 6 can make a single-phase mixture in all pro-
portions. The same is true for all mixtures of C 2 6 and C 71 .However,C 1 and C 71

C 1

D
Two-phase envelope
P = P 1
A
Range of solvent composition to have FCM
Two-phase envelope with reservoir oil at P = P >P
Two Phase B Range of solvent composition to have FCM
P = P > P 2 1
2 1
C with reservoir oil at P = P 1

Oil

C C –C
7+ 2 6
Figure 3.1 Phase behavior on ternary diagram for Methane and pseudo components [29].