Effects of hydrocarbon solvents on simultaneous improvement in displacement and sweep efficiencies during CO2-enhanced oil recovery

The addition of hydrocarbon solvent such as liquefied petroleum gas (LPG) to the CO2 stream leads to miscible conditions in reservoirs at lower pressures by reducing the minimum miscibility pressure (MMP). Under miscible conditions, improved displacement and vertical sweepout occur simultaneously. The influences of LPG concentration and composition on the displacement and sweep efficiencies during CO2-LPG enhanced oil recovery (EOR) were investigated. Enhanced displacement efficiency was assessed through oil viscosity reduction and oil saturation change. Moreover, the miscible flooding induced by LPG addition, which resulted in increased solvent viscosity and a lower density difference between the injected fluid and reservoir oil, provided a smaller viscous gravity number, and improved the sweep efficiency, alleviating the impact of solvent gravity override. For CO2-LPG EOR, oil recovery increased up to 52% as compared with that for CO2 flooding. The amount of incremental oil recovery with 100% butane in the LPG was 16%, as compared with the 100% propane case. Mitigated gravity override enabled CO2-LPG EOR to enhance sweep efficiency. Results indicated that the compositional modeling of the EOR process with the addition of LPG provided more accurate prediction on the performance of CO2-LPG EOR.