Abstract

The addition of intermediate hydrocarbon solvents 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, higher recovery is obtained by simultaneous improvement in displacement and vertical sweepout. A compositional model for CO2-LPG-enhanced oil recovery (EOR) was applied to investigate the complicated phenomenon more accurately. The influences of LPG concentration and composition on the displacement and sweep efficiencies during CO2-LPG EOR were examined. Enhanced displacement efficiency was assessed through oil density, viscosity, and the interfacial tension (IFT) between oil and gas. Moreover, the miscible flooding induced by LPG addition resulted in increased solvent viscosity and a lower density difference between the injected fluid and reservoir oil. It also provided a smaller viscous gravity number and improved the sweep efficiency, alleviating the impact of solvent gravity override. As the mole fraction of the total injected LPG and the LPG butane contents increased, significant increments in oil recovery were obtained from the reduction of MMP, oil viscosity, and IFT. The oil viscosity was reduced to 0.65 cp by CO2 and 0.26 cp with the addition of 25% LPG leading to miscible condition. In CO2-swept regions, the IFT became non-zero due to the immiscible conditions. The addition of 25% LPG into the CO2 stream resulted in negligible IFT in the swept area, indicating miscible conditions. For CO2 flooding, 11.9% of the total reservoir area remained in unswept and immiscible conditions despite the long production time. In contrast, the unswept area was reduced to only 2.4% of the total area when 25% LPG was injected into the CO2 stream under miscible conditions, and the viscous gravity number decreased to 0.9 from 3.8 in the CO2 flooding case. 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 efficiency of the EOR process with the addition of LPG as evaluated in the compositional model provided more accurate prediction on the performance of CO2-LPG EOR.

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