Mechanisms of Miscible Oil Recovery: Effects of Pressure on Miscible and Near-Miscible Displacements of Oil by Carbon Dioxide

Abstract

Abstract This paper presents results obtained in a laboratory study investigating incentives for near-miscible CO2 processes. Reservoir-condition corefloods were completed in both idealized and West Texas-type fluid/ rock systems at pressures above and below the minimum miscibility pressure (MMP). Coreflood oil recoveries and CO2 mobilities are presented to examine effects of pressure, initial saturation, and volume of CO2 injection. Results show that decreases in tertiary oil recovery at pressures below the MMP are not as great as suggested by slim tube tests. The presence of water in the core apparently reduces the pressure sensitivity. In addition, it was found that CO2 mobility is reduced by the presence of a miscible flood residual oil saturation (Sorm), which increases as the pressure decreases. Both observations suggest that lower pressure (near-miscible) CO2 EOR processes have potential to economically recover oil. Two oil recovery mechanisms, low interfacial tension (IFT) displacement and extraction of oil components, were found to be important. This is deduced from analyses of produced oil compositions. Lower IFT (higher pressure) and lower initial water saturation enhance displacement and reduce pore level bypassing. Extraction dominates after solvent breakthrough; extraction increases with pressure and decreases with water saturation. The work suggests that, whenever possible, slim tube tests be replaced with reservoir-condition corefloods which provide more realistic data not only on pressure sensitivity, but also on miscible flood residual oil saturation and solvent mobility.