Carbon dioxide flooding has been proven to be one of the most effective and viable enhanced oil recovery (EOR) processes for light and medium oil reservoirs. In the past, an extremely large number of laboratory experiments and numerical simulations have been conducted to study the CO 2 EOR process. However, the specific effects of viscous and capillary forces on this tertiary oil recovery process are neither thoroughly studied nor well understood yet. In this paper, an experimental study is carried out to examine the detailed effects of viscous and capillary forces on the CO 2 EOR under the actual reservoir conditions. First, the equilibrium interfacial tensions between a light crude oil and CO 2 are measured at different equilibrium pressures. Second, a series of CO 2 coreflood tests are performed to measure the CO 2 EOR at different CO 2 injection pore volumes, pressures, and rates. Each CO 2 coreflood test is terminated after a total of 1.5 pore volume of CO 2 is injected. The detailed experimental results show that, in general, the measured equilibrium interfacial tension is reduced with the equilibrium pressure but the measured CO 2 EOR at 1.5 pore volume of CO 2 is increased with the CO 2 injection pressure and rate. Finally, the measured CO 2 EOR at 1.5 pore volume versus injection pressure data at different CO 2 injection rates are related to the measured equilibrium interfacial tension versus equilibrium pressure data in terms of the complete capillary number, which is defined as the ratio of the viscous force to the capillary force for each CO 2 coreflood test. This study shows that if the complete capillary number is in an intermediate range, the CO 2 EOR increases quickly with the complete capillary number. Otherwise, the CO 2 EOR is lower and remains almost constant for a smaller complete capillary number, or it is higher and remains unchanged for a larger complete capillary number.
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Nobakht et al. (2007) studied this question.