Page 118 - Primer on Enhanced Oil Recovery

108 Primer on Enhanced Oil Recovery

carbon dioxide slug which actively mixes with oil. The oil, carbon dioxide and
water are moving towards production well. In zone 3 carbon dioxide is fully mixed
with oil, oil is swollen and has high mobility. This oil is pushed towards production
well by advancing injected water in zone 4. Some carbon dioxide is transferred to
the advancing water. In zone 4 we have movement of highly mobile oil and carbon-
ated water. Somewhere in this zone all mobile oil is already displaced and only hea-
vy, stripped of light hydrocarbons, oil is left behind. At the same time carbon
dioxide is getting diluted by the advancing fresh water and at certain point zone 5
develops. Zone 5 has low mobility heavy oil, carbonated and modified towards
fresh connate water.
There are different scenarios of the painted picture developing in time. What is
clear that zones 1 and 2 will disappear and the rest of zones will spread in the for-
mation volume with highly mobile liquids being displaced towards production well.
It has been shown that the optimal results are usually achieved when the CO 2
slug volume is at 20 30% of the pore volume. At the same time, the field produc-
tion lifetime significantly increases, the volume of injected water, as compared to
the simple water flooding, is reduced and high oil recovery factor is achieved. An
important influence on the process has gravitational effect. In this regard, in the for-
mations with high vertical permeability, the oil recovery factor would be signifi-
cantly reduced (by a factor of 2 3).
Alternating injection of carbon dioxide and water is but a further method
development (see Fig. 10.13) and according to the numerous studies has the highest
efficiency.
In this modification effectiveness depends on the correct choice of the volume of
the injected slugs and gas-water ratio. Injection of CO 2 and water can be done
sequentially or simultaneously.
The implemented projects show that the optimal ratio of slugs of carbon dioxide
and water lies in the range between 0.25 and 1. Low CO 2 volumes reduce the
method efficiency and the effect of the implementation it closer to the efficiency

Figure 10.13 Alternating CO 2 water injection.