Laboratory coreflooding experiments were performed to investigate the technical feasibility of the use of the CO2 flooding process under immiscible conditions for the recovery of viscous oil. A stock-tank oil of 22° API gravity was recombined with solution gas at a gas-oil ratio (GOR) of 48 scf/bbl. The CO2 flooding tests were conducted in a consolidated Berea sandstone core under simulated reservoir conditions. The viscosity of the recombined oil at reservoir temperature (152° F) and pressure (2,500 psig) was 50 cp. Experimental results show that the oil viscosity was reduced to 8 cp, and the oil volume was swelled to 1.2 times the original volume after saturation with CO2 at the testing condition.
Oil recovery from a 2-in.-diameter and 2-ft-long core by CO2 flooding for the testing oil was unusually high even under Immiscible conditions. The ultimate oil recovery from a secondary CO2 flood is as much as 60 % of the original oil in place (OOIP), which is higher than the average oil recovery from a secondary waterflood, 44 % OOIP. After waterflood, a continuous injection of 1.5 PV of CO2 recovered an additional 33 % OOIP. The CO2 utilization factor was 11.7 Mscf/bbl of additional oil recovered. The CO2-alternate-brine injection method significantly delayed the breakthrough of gas and provided a slightly higher oil recovery and more efficient CO2 utilization than the continuous CO2 injection method. Overall, the ultimate residual-oil-saturation (Sor) was reduced to 18% PV by all of the above CO2 injection methods.