Page 117 - Primer on Enhanced Oil Recovery

Gas flooding 107

permeability for oil increases accordingly. A significant lag in the front of carbon
dioxide concentration from the displacement front lengthens the time for obtaining
the CO 2 effect. Despite a slight increase in the sweep efficiency, as compared to
the ordinary water flooding, the reduction of capillary forces and the improvement
of the wettability, the final field recovery does not usually increase very
significantly.
Injection of carbon dioxide slug is more effective and more technologically
demanding method. The slug volume should be at around 10 30% of the pore vol-
ume. The CO 2 slug is then followed by a water slug as shown on Fig. 10.11.
The processes in the reservoir during injection are complex, especially in mixed
displacement volume. Somewhere in time after the carbon dioxide and water injec-
tions it is possible to try to artificially split the propagating displacement into num-
ber of zones and processes as it is shown on Fig. 10.12. The simplest zones are 1
and 6. In zone 1 oil in the virgin state is displaced by the injected liquids pressure
towards the production well. Some connate water will be pumped out as well, but
irreducible water saturation will remain. In zone 6 we have injected water and hea-
vy oil which was stripped of light hydrocarbon fractions. Zone 2 contains moving

Figure 10.11 CO 2 slug injection method.

Figure 10.12 Artificial zoning and processes during CO 2 /water slug injections.