Investigation of Low-Density CO2 Injection for Enhanced Oil Recovery

The density of the CO2-rich phase in a reservoir would play a crucial role in the performance of an enahcned oil recovery (EOR) scheme. Many oil reservoirs are located in deep formations; hence, they have high temperatures. Moreover, the pressure of reservoirs decreases because of natural depletion. Under the conditions of those reservoirs, CO2 would be a low density gas. A series of coreflood experiments were performed to evaluate the potential of low-density CO2 EOR. The experiments are intermittent CO2 injection, continuous tertiary and secondary CO2 injection, and water alternating CO2 injection followed by the coinjection of a surfactant and CO2. The same oil and gas were mixed to prepare live oil for all the experiments. The initial rate of oil recovery during secondary waterflood was high, but the efficiency of the process decreased after the breakthrough. Three pore volumes (PVs) of secondary CO2 injection resulted in the recovery of around 50% of the initial oil in place, which was 27% higher than the oil recovered during 1 PV of water injection. It was also observed that CO2 injection can improve the recovery factor after waterflood. However, the performance of tertiary CO2 injection is reduced because of the presence of water in pore spaces, which likely makes the oil less accessible to CO2. Waterflood after a period of CO2 injection recovered 20% of initial oil in place mainly because of the dissolution of CO2 in the oil and the resultant oil viscosity reduction. The impact of the rate of CO2 injection on the efficiency of oil recovery was investigated, and it appears that the dissolution of CO2 in the oil is the main mechanism of enhanced recovery. The reduction of oil viscosity as a result of the dissolution of CO2 in the oil as well as the low density of CO2 improved the effect of gravity drainage on oil production. In addition, it was observed that the mechanism of solution gas drive plays an important role in the process of oil recovery. The analysis of the physical properties of the core effluent reveals that CO2 can also improve the quality of produced oil compared to that of the original oil in the rock. The results of this study provide experimental evidence of the potential of low-density CO2 EOR.