Use of CO2 in Heavy-Oil Waterflooding

Abstract Waterflooding has been used in oil recovery for many years and is an important technique in conventional oil recovery. In the case of viscous heavy oils, due to the low mobility of heavy oil and high mobility ratio between the displacing fluid (water) and the displaced fluid (heavy oil), reported recoveries have been very low and have been associated with very high volumes of produced water. Use of CO2 in heavy oil waterflooding, as a solvent that might effectively reduce the viscosity of heavy oil and causes it to swell, is the focus of this study. This paper presents the results of eleven core-flooding experiments designed to study the effect of CO2 utilization in waterflooding of heavy oils. Injection strategies used in these experiments involved different combinations of CO2 and brine, including intermittent injection of separate slugs as well as injecting carbonated water. In reported experiments, following the completion of waterflooding tests, CO2 slugs of 10% and 25% pore volumes, or carbonated water was injected into the cores followed by a shut-in period. Water injection was resumed at the end of shut-in period, and any additional oil produced was collected. Heavy oil samples with viscosities of 1000 to 2000 cp were used and experiments were carried out at pressures of 500 and 1000 psi (3.45 and 6.9 MPa), temperature of 30°C, and water injection rates between 1 and 50 feet per day. Carbonated water used in these experiments was prepared by dissolving CO2 in brine (1% wt. NaCl) at 820 psi over 4 days. Results of this study indicate that the use of CO2 significantly improves recovery of heavy oil by waterflooding. Incremental recoveries in the range of 5 to 27.5% OOIP were achieved by CO2 in combination with waterflooding. It was also found that the increase in the operating pressure results in increased oil recovery. Furthermore, the injection of larger CO2 volume increased the oil recovery. It was also found that during the post CO2 waterflooding, greater recovery improvements are achieved from lower permeability systems. Comparison between the lower and the higher viscosity oils also showed that the use of CO2 results in greater recovery improvements for the higher viscosity oil system.