Enhanced Solubility and Miscibility of CO 2 -Oil Mixture in the Presence of Propane under Reservoir Conditions to Improve Recovery Efficiency

The existence of propane (C 3 H 8 ) in a CO 2 -oil mixture has great potential for increasing CO 2 solubility and decreasing minimum miscibility pressure (MMP). In this study, the enhanced solubility, reduced viscosity, and lowered MMP of CO 2 -saturated crude oil in the presence of various amounts of C 3 H 8 have been systematically examined at the reservoir conditions. Experimentally, a piston-equipped pressure/volume/temperature (PVT) cell is first validated by accurately reproducing the bubble-point pressures of the pure component of C 3 H 8 at temperatures of 30, 40, and 50 °C with both continuous and stepwise depressurization methods. The validated cell is well utilized to measure the saturation pressures of the CO 2 -C 3 H 8 -oil systems by identifying the turning point on a P-V diagram at a given temperature. Accordingly, the gas solubilities of a CO 2 , C 3 H 8 , and CO 2 -C 3 H 8 mixture in crude oil at pressures up to 1600 psi and a temperature range of 25–50 °C are measured. In addition, the viscosity of gas-saturated crude oil in a single liquid phase is measured using an in-line viscometer, where the pressure is maintained to be higher than its saturation pressure. Theoretically, a modified Peng–Robinson equation of state (PR EOS) is utilized as the primary thermodynamic model in this work. The crude oil is characterized as both a single and multiple pseudo-component(s). An exponential distribution function, together with a logarithm-type lumping method, is applied to characterize the crude oil. Two linear binary interaction parameters (BIP) correlations have been developed for CO 2 -oil binaries and C 3 H 8 -oil binaries to accurately reproduce the measured saturation pressures. Moreover, the MMPs of the CO 2 -oil mixture in the presence and absence of C 3 H 8 have been determined with the assistance of the tie-line method. It has been found that the developed mathematical model can accurately calculate the saturation pressures of C 3 H 8 and/or CO 2 -oil systems with an absolute average relative deviation (AARD) of 2.39% for 12 feed experiments. Compared to CO 2 , it is demonstrated that C 3 H 8 is more soluble in the crude oil at the given pressure and temperature. The viscosity of gas-saturated crude oil can decrease from 9.50 cP to 1.89 cP and the averaged MMP from 1490 psi to 1160 psi at 50 °C with the addition of an average 16.02 mol% C 3 H 8 in the CO 2 -oil mixture.