Pore-scale investigation on the effect of capillary barrier on two-phase displacement in dual-structure porous media

Although immiscible fluid-fluid displacement in porous media has received extensive attention, understanding the dynamics behavior within complex structures remains elusive. This study utilizes the direct numerical simulation by solving the Navier-Stokes equations and coupling with the volume of fluid method to examine oil-water flow in porous media across various contact angles theta and capillary number Ca. Three kinds of artificial porous media were generated with designed opening angle beta, including single-structure and dual-structure models. A theoretical analysis of the capillary barrier phenomenon, as well as its occurrence conditions, is identified under water-wet conditions. Generally, when theta + beta < 90 degrees, the capillary force consistently drives oil displacement from throats to pores. Conversely, if theta + beta > 90 degrees, the direction of the capillary force can move toward the water phase side and prevent the fluid interface from continuing to move. For a single-structure porous medium, the dynamics behavior of fluids is controlled by the capillarity, wettability, and geometric structures. The greatest efficiency occurs when the condition theta + beta = 90 degrees is met, particularly at an intermediate Ca. For a dual-structure porous medium with smaller opening angles inside, the water phase tends to infiltrate the embedded pore structure due to weaker capillary barrier effects. Conversely, larger opening angles within the embedded structure lead to stronger capillary barrier effects, hindering water entry into the interior porous medium. This obstruction forces the water phase to bypass and traverse longer flow paths, resulting in the formation of a large amount of residual oil.