Effect of the capillarity and viscosity on the change of flow paths during two-phase displacement in porous media

Pore-scale direct numerical simulations were conducted to explore the changes of flow paths and dynamic distribution of water-oil phases during two-phase displacement in porous media. Three contact angles, i.e., 45 degrees, 90 degrees, and 135 degrees, correspond to water-wet, intermediate-wet, and oil-wet rock surfaces, respectively. The medial axis within complex pore channels is obtained using a grid-based method. Subsequently, a novel particle tracking-based approach is proposed to extract the flow paths during two-phase displacement. The results show that when water enters a porous medium filled with oil, the distribution of oil phase flow paths is uniform, indicating that all oil phases are movable. As water continues to be injected, some water-oil interfaces stop at pores or throats due to capillary force, causing the flow paths through these locations to disappear. In oil-wet and intermediate porous media, capillary forces always act as resistance; therefore, the water-oil interfaces stop at the throat channels. Interestingly, under water-wet conditions, the water-oil interface can reverse, and the capillary force presents resistance, resulting in the interfaces being blocked at enlarging areas, usually from throats to pores. Once breakthrough occurs, preferential flow paths for pure water are established, resulting in a rapid decrease of flow paths with moving water-oil interfaces under oil-wet and intermediate-wet conditions, while movement continues in a water-wet medium. Due to the typically lower viscosity of water than oil, the viscous force decreases as water saturation increases. As water injection continues, the water in some channels gradually displaces oil, resulting in differences in flow rates between oil-filled and water-filled channels. This study provides a method for identifying the changes in flow paths during two-phase displacement in porous media and gives potential benefits for designing strategies to improve oil recovery.