Study on the Microscopic Seepage Mechanism of Different Injected Gas and Crude Oil in Complex Pore Scale

Gas injection is widely regarded as an effective method for enhancing oil recovery from tight reservoirs, with the displacement behavior primarily influenced by the seepage mechanisms of injected gases and crude oil within the reservoir. Previous studies on gas injection have mostly focused on miscibility behavior, minimum miscibility pressure (MMP) testing, and the response of oil recovery to reservoir conditions and injection parameters under single gas injection conditions. However, systematic comparisons and analyses of the seepage mechanisms of different injected gases and crude oil at the complex pore scale are still lacking. In this study, a three-dimensional pore structure model was constructed using the digital rock method, and computational fluid dynamics (CFD) simulations were conducted to investigate the seepage behavior of CO2, N2, CH4, and crude oil at the core scale. The simulation results indicate that during CO2 injection, the pressure and velocity changes are more uniform, allowing CO2 to invade the pore throats more evenly and effectively displace crude oil, resulting in higher swept volume and oil-washing capacity. In contrast, N2 and CH4 tend to flow along high-permeability channels, forming distinct preferential flow paths. Furthermore, CO2 forms a stronger dissolution effect with crude oil, significantly reducing the oil's density and viscosity, thereby greatly improving displacement efficiency. In comparison, N2 and CH4 exhibit weaker interactions with crude oil, with lower dissolution rates, resulting in a smaller effect on oil density and viscosity. The final recovery rates were as follows: CO2 injection (79.53%) > N2 injection (62.47%) > CH4 injection (53.59%). These simulation results reveal the seepage behavior of different injected gases and crude oil at the pore scale and further analyze factors such as flow field distribution, displacement pressure, and fluid interactions, shedding light on the microscopic reasons behind the differences in recovery rates. The findings provide new theoretical insights for optimizing gas injection strategies and improving oil recovery from tight reservoirs.