Experimental Investigation of Miscibility and Enhanced Oil Recovery Mechanisms in Tight Reservoirs Using Gas Flooding

China has significant tight oil reserves, particularly in the Mahu sag tight sandstone reservoir in the Junggar Basin, Xinjiang, China. Despite the effective use of volumetric fracturing technology, challenges such as a low recovery factor and rapid decline in production capacity persist. This study conducted a series of experiments, including minimum miscibility pressure tests, gas injection expansion tests, and mass transfer experiments, to investigate miscibility and enhanced oil recovery mechanisms in Block Y of the Mahu sag using gas flooding. The results reveal that the dissolution of injected gas improves crude oil properties, reduces viscosity, enhances flow capacity, and causes volumetric expansion, effectively boosting the elastic driving energy. The minimum miscibility pressures of the crude oil with dry gas and CO2 are 37.36 and 27.76 MPa, respectively. Under these conditions, oil recovery efficiencies for both gases reach 95%. Notably, the gas injection expansion experiments show significant increases in the saturation pressure, volume expansion coefficient, and gas-to-oil ratio for dry gas and CO2. The viscosity and density of the crude oil decreased by 44 and 47.6% for dry gas, and by 49 and 22.9% for CO2. The mass transfer mechanisms differ between dry gas and CO2: dry gas primarily undergoes condensation, while CO2 undergoes extraction. High-pressure pressure–volume–temperature tests provided insights into the volumetric and phase behavior of reservoir oil when mixed with various gases. This investigation into miscibility and EOR mechanisms reveals that gas flooding can effectively alter oil-phase behavior, reduce interfacial tension, and improve displacement efficiency. These findings offer valuable insights for future field applications and guide subsequent research in enhanced oil recovery, highlighting the promising role of gas flooding in optimizing resource extraction from tight formations.