Dynamics of Subcritical CO2/Brine Floods for Heavy-Oil Recovery

Abstract

Summary Immiscible CO2 flooding is an important, field-proven heavy-oil recovery method, particularly suited for thin, marginal, or otherwise poor heavy-oil reservoirs, where thermal recovery processes are likely to be uneconomical. This paper describes the dynamics of this recovery technique on the basis of experiments conducted in a scaled model. The experiments represent a medium-heavy oil (1032 mPa·s at 23°C [1,032 cp at 73°F]) occurring in a shallow, thin sand. CO2 was injected together with brine at subcritical conditions (5.5 MPa and 21 to 23°C [800 psi and 70 to 73°F]). The CO2 and brine superficial velocities were varied from 0.18 to 2.9 m/d [0.6 to 9.5 ft/D]. This broad range of velocities permitted the study of the effect of the viscous, diffusive, and gravitational forces on the CO2 slug (20% HCPV) process for heavy-oil recovery. It was found that viscous forces completely dominated CO2 injection. Also, the mass transfer between CO2 and oil had a stabilizing effect on the brine injection. Although the molecular diffusion of CO2 in oil was high, it was not high enough to mobilize appreciable amounts of oil from uninvaded zones. The scaled experiment results showed that oil recoveries at CO2 and brine breakthroughs were rate-dependent. While recovery at CO2 breakthrough decreased with increasing rate, recovery at brine breakthrough increased. Reduction of interfacial tension (IFT) between brine and oil, leading to the formation of brine-in-oil emulsions, was found to be an additional effective mechanism of heavy-oil recovery by CO2/brine injection.