Enhanced heavy oil recovery by intermittent CO₂ injection

The Objective of this study is to experimentally investigate the performance of dense (liquid and super-critical) CO2 injection as an Enhanced Oil Recovery (EOR) method for extra-heavy oil reservoirs. Since CO2 is not expected to develop miscibility with heavy crude oil, CO2 injection is perceived not to be a viable technology for heavy oil reservoirs. The high viscosity difference between CO2 and heavy oil is also expected to further limit the performance of CO2 injection in these reservoirs. In this study, we introduce a new CO2 injection strategy in which CO2 is injected in an intermittent fashion that cycles between a period of injection and a period of halt. The injection strategy is particularly suitable for heavy and viscous oil reservoirs as it enhances the contact between injected CO2 and the resident heavy oil. In this paper, we present the results of two coreflood experiments which have been performed to evaluate the potential of intermittent injection of CO2 for enhancing recovery of an extra-heavy crude oil under reservoir conditions. The injected CO2 was in liquid state under the conditions of the first experiment, and supercritical in the second experiment. The same crude oil and gas were used to prepare live oil for both experiments. The viscosity of the live-oil (oil saturated with gas) used under the conditions of the first experiment was around 7 times higher than that of the second experiment. After a period of continuous CO2 injection, the injection was halted (soaking period) for a period of 24 hours and then CO2 injection resumed and 0.3 pore volume of CO2 was injected in the core. This cyclic soak-alternating-CO2 injection was repeated a number of times in both experiments. The results show that the CO2 breakthrough occurred early in both experiments due to the adverse mobility ratio and hence, front instability. Production of oil with foamy nature and liberation of gas from produced oil was observed after the CO2 breakthrough as a result of the increase in pressure gradient within the core before the CO2 breakthrough. However, oil production rate dropped significantly after this period because of low residence time of CO2 in the porous medium. Continuous CO2 injection after 1.3 PV of injection resulted in 22% and 19% of oil recovery for the first and second experiments, respectively. The oil production results showed that soak-alternating-CO2 injection recovered 16% and 47% of the remaining oil after four cycles in the first experiment and after nine cycles in the second experiment. A tertiary waterflood followed the soakalternating-CO2 injection periods which recovered 23% and 14% of the remaining oil after 1 PV of injection in the first and second experiments, respectively.