Recovery of Viscous Oil Under High Pressure by CO2 Displacement: A Laboratory Study

Abstract

Abstract 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 (152F) 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 (S) was reduced to 18% PV by all of the above CO2 injection methods. Introduction For heavy crude oils having API gravities below 25, viscosities range from about 20 to 1,000 cp, depending on the content of high-molecular-weight compounds. Distillation analysis showed that these oils usually contain more than 40 wt% residuum, which is a complicated mixture of high-molecular-weight compounds, such as asphaltene, resins, and polymers. All of these compounds contribute to the high viscosity of heavy crude oils. High viscosity is a major constraint to oil recovery, transportation, and processing; therefore, reducing oil viscosity becomes critical. Besides thermal methods, carbon dioxide is an effective viscosity-reducing agent. Recently, interest in CO2 immiscible flooding for viscous oil recovery has increased because of the poor economics and pollution problems that can be encountered in steamflooding. Field tests, such as those in Arkansas fields (Ritchie field and Lick Creek field) - and in Wilmington field in California, have demonstrated the applicability of immiscible CO2 flooding for heavy oil recovery. Several large-scale field applications are in progress; for example, the application in the Bati Raman oil field in Turkey. In recent years, many coreflooding tests have been conducted in laboratories to determine oil recovery efficiencies under varying CO2 -injection schemes. P. 401^