Page 110 - Primer on Enhanced Oil Recovery

100 Primer on Enhanced Oil Recovery

Figure 10.4 Minimal miscible pressure determination.

zones are formed in the reservoir in the direction of oil displacement: carbon diox-
ide zone; transition zone containing both CO 2 and oil; residual oil zone. Laboratory
experiments on natural cores show that the displacement coefficient of the mixed
displacement of oil by carbon dioxide can reach 0.95.
Measuring minimal miscibility pressure. The pressure of complete miscibility
of carbon dioxide and the oil is determined experimentally. There are few methods
to do it but the most common are two gravitational-stable and slim tube tests.
In gravitational-stable test, a vertical formation rock filled column saturated with
the oil first. The carbon dioxide is injected from the top. The gas displaces the oil
vertically downward at a speed slow enough to maintain a gravitational-stable flow.
The experiment is carried out at various pressures, and the coefficient of oil dis-
placement is determined. A graph linking the oil displacement (recovery) with pres-
sure is drawn (see. Fig. 10.4). As can be seen, the minimum miscibility pressure
(MMP) is the pressure corresponding to the transition point to the maximum
achievable recovery.
The second method for determining the minimum mixing pressure is an experi-
ment in a slim tube. A spiral slim stainless steel tube with a diameter of 6 mm
(0.25 inches) and a length of 1.2 m is packed with sand, saturated with oil at a
given pressure and temperature. The sufficiently high value of the ratio of the
length of the tube to its diameter provides a stable front for displacing oil with car-
bon dioxide without fingering. The results of the experiments are plotted and pro-
cessed as above on the Fig. 10.4.
Immiscible displacement. In the case of immiscible displacement, the light
fractions of oil are dissolved in carbon dioxide and part of the carbon dioxide is dis-
solved in oil. At the same time, due to the fact that carbon dioxide enriched in light
fractions of oil displaces oil saturated with CO 2 , component separation of oil