Study of Different Gas Floodings on Minimum Miscibility Pressure and Oil Recovery in a Low-Permeability Reservoir with Medium Viscous Oil

Although gas flooding has been successfully applied for enhanced oil recovery throughout the world, most of its applications are conducted in low, ultralow, and tight oil reservoirs with light oil. Studies focused on a low-permeability reservoir with medium viscous oil are rarely reported, and there is little experience in the development of this type of reservoir for reference. Based on the special characteristics of low-permeability formation coupled with medium viscous oil in Niuquanhu Block, China, the gas–oil interactions of different gases are first evaluated through the pressure–volume–temperature (PVT) analysis. Then, the minimum miscibility pressure (MMP) of different gases is compared using slim tube experiments and a numerical simulation method. The oil recovery of different gas floodings is studied using core flooding experiments, and then the output oil compositions of different gas floodings are analyzed using a total hydrocarbon chromatograph. The PVT analysis results show that the interactions between hydrocarbons and medium viscous oil are the strongest compared with other gases. Under formation conditions, about 70 mol % hydrocarbons is dissolved in the medium viscous oil, leading to a 69% reduction of oil viscosity and a 48% increment of oil swelling, which is 1.19 and 1.35 times those of CO₂. The reduction of oil viscosity and increment of oil swelling of hydrocarbons are 1.64 and 2.02 times those of CH₄. And the reduction of oil viscosity and increment of oil swelling of hydrocarbons are 2.23 and 2.67 times those of N₂. Slim tube experiments show that the MMPs of hydrocarbon flooding, CO₂ flooding, CH₄ flooding, and N₂ flooding are 28.5, 36.8, 46.4, and 61.6 MPa, respectively. Therefore, compared with the other three types of gases, hydrocarbon flooding has the lowest MMP for low-permeability reservoirs with medium viscous oil. The errors between the MMPs of hydrocarbon flooding, CO₂ flooding, CH₄ flooding, and N₂ flooding obtained by the numerical simulation method and the results obtained by the slim tube experiment method are 0.71, 0.82, 0.87, and 0.98%, respectively. The numerical simulation results of the slim tube experiment show that the flooding pressure and oil recovery curves of different gas floodings all have inflection points. Under the existing condition, the inflection point pressure value is unchanged by changing the PV values of different gas injections. According to the results of a series of core flooding experiments and the analysis of the output oil compositions of different gas floodings, the oil recoveries of hydrocarbon flooding, CO₂ flooding, CH₄ flooding, and N₂ flooding are 57.02, 51.11, 36.88, and 21.35%, respectively, and the extraction ability of different gases to light hydrocarbon components is ranked as hydrocarbons > CO₂ > CH₄ > N₂. The findings revealed in this paper can provide technical support for gas flooding development in similar low-permeability reservoirs with medium viscous oil.