CO2 flooding is a promising technique for improving oil recovery in tight oil reservoirs. Because of existence of nanoscale pores in tight oil reservoirs, which can affect phase equilibrium, there will be some variations between CO2 immiscible and miscible processes in conventional and tight oil reservoirs. In this paper, the effects of nanoscale pore confinement on CO2 immiscible and miscible processes are studied.
On the basis of Cardium crude oil composition, phase equilibrium at various nanoscale pore radii is investigated. Since the interfacial tension is related to capillary pressure, it can be scaled in normalized oil recovery calculations. Furthermore, a reservoir can be assumed an immiscible zone, a miscible zone, or an unswept zone during CO2 injection. In this case, nanoscale pore confinement on CO2 immiscible and miscible processes can be evaluated.
Taking the nanoscale pore confinement into consideration, the tie line length and interfacial tension can be reduced, which contribute to a lower CO2 minimum miscible pressure (MMP). In this way, the efficiency of CO2 immiscible flooding is improved, while the efficiency of miscible flooding is not affected by a confinement effect. Furthermore, the length of a CO2 immiscible zone can be decreased by nanoscale pore confinement.