Experimental investigation of tertiary CO2 injection for enhanced heavy oil recovery

The recovery efficiency of primary production for heavy oil reservoirs is generally poor meaning that significant quantities of oil still remain in place. Waterflood has been, and continues to be, the first intervention method for oil recovery in many oil reservoirs. However, heavy oil is considerably more viscous than water and hence, a significant fraction of the oil is bypassed by waterflood. In this study, we present the results of two coreflood experiments which have been performed under reservoir conditions to investigate the potential of tertiary CO2 injection for heavy oil recovery. In addition, the mechanisms involved in this process and their impacts on the efficiency of oil recovery are identified and discussed. The same crude oil and gas were used to prepare live oil for both experiments. Despite the difference in the viscosity of oil samples, secondary waterflood showed relatively similar behavior in both experiments and 22% and 24% of the initial oil were recovered after 2 and 1.1 pore volumes (PVs) of water injection in the first and second experiments, respectively. The oil production results show that tertiary CO2 injection recovered 23% of the remaining oil after 6 PVs of injection in the first experiment and after 3 PVs of injection in the second experiment. Another waterflood followed the tertiary CO2 injection which recovered 14% of the remaining oil in both experiments. The main mechanisms contributing to oil recovery by CO2 injection are the oil viscosity reduction by CO2 dissolution, the liberation of methane from oil, oil swelling, and the extraction of hydrocarbons by CO2. The results imply that viscous fingering dominates the flow of water or CO2 for heavy oil recovery. The compositional analysis of the core effluent shows that tertiary injection of CO2 alters the physical properties of heavy oil in the core and a higher quality oil (e.g. lower density and viscosity) than the original oil in place is recovered by CO2 injection. It is also discussed that CO2 can diffuse into the oil surrounded by water. The transport of CO2 into the oil results in the oil being saturated and this leads to the nucleation of a CO2 rich phase among the trapped oil ganglia. By continuing CO2 injection, the nucleated CO2 rich phase grows further and causes oil swelling by the extraction of light and intermediate components of the oil. (C) 2016 Elsevier B.V. All rights reserved.