Page 121 - Primer on Enhanced Oil Recovery

Gas flooding 111

Figure 10.15 Oil displacement scheme for miscible hydrocarbon gas injection.

the injected gas itself during propagation through the reservoir is enriched by oil
components;
the remaining oil slightly shrinks due to loss of the above components.
The effectiveness of the displacement process is significantly affected by the
pressure. The bigger the injected gas dissolution, the bigger is displacement. This is
mostly due to the oil swelling by the injected gas.
Displacement of oil with full mixing is demonstrated on Fig. 10.15. The
injected gas should have pressure higher than the pressure of full mixing. In this
case it is possible to distinguish four zones. The first zone contains injected gas.
Partially enriched by oil components gas is forming the second zone. After signifi-
cant component exchange oil and injected gas are undisguisable (Zone 3). The dis-
placed virgin oil forms the last, forth, zone.
˙ It is also possible to see how the concentration of intermediate carbons increases
from zone to zone. The concentration reaches the concentration of those hydrocar-
bons in the virgin oil on the border between third and fourth zones.
It is convenient to analyze the displacement process in pseudo-ternary diagram
when the process of oil displacement. The analysis can be done as described in the
previous chapter.
Fig. 10.16 shows a ternary phase diagram with boundary phase curves for three
pressures. As can be seen from the figure, as the pressure decreases, the mixing
zone decreases, and the two-phase region, on the contrary, increases. Considering
the process of displacing oil of composition O by gas with composition G, we find
that at 21 and 24.5 MPa, the miscibility of oil and gas is not achieved. With this
composition of oil and gas miscibility occurs only at a pressure of 28 MPa. Studies
have shown that to achieve mixing at a pressure of 28 MPa it is necessary that the
concentration of intermediate components in the oil be at least 35%, otherwise
higher pressure will be required.
Thus, the most important factor ensuring the miscibility is the composition of
the oil, namely the presence in it of a sufficient amount of intermediate hydrocar-
bons. Only presence of intermediate hydrocarbons and high enough pressure guar-
anty, complete gas mixing with oil is achieved (i.e., all mixtures will be single
phases).