Lattice Boltzmann study on oil–water flow in shale heterogeneous porous media

Unconventional shale oil is characterized by nanoscale pores, heterogeneous pore structures, diverse mineral compositions, non–uniform wettability, and multiple fluid types, resulting in complex multiphase flow behaviors in shale porous media that require further investigation. In this study, a nanoscale multicomponent and multiphase lattice Boltzmann method is employed to simulate oil–water two–phase flow in heterogeneous porous media with heterogeneous wettability and structure. The effects of transverse/longitudinal structural heterogeneity, capillary number and nanoscale effects on the oil–water flow and relative permeability are investigated. The results indicate that due to the higher capillary resistance in the transverse porous media, the relative permeability of the water phase in the transversely heterogeneous porous media is lower than that in the longitudinally heterogeneous porous media. As the capillary number decreases, capillary resistance becomes dominant, and the viscous driving force is insufficient to overcome the capillary forces, making it difficult for oil and water to flow. Consequently, the relative permeabilities of both oil and water phases decrease, and more fluid becomes trapped due to capillary resistance. The presence of an oil film on the solid wall induces liquid–liquid slip, which significantly enhances water flow. This enhancement effect outweighs the weakening effect caused by viscosity heterogeneity. As a result, when nanoscale effects are considered, the relative permeability of the water phase increases.