Pore-Scale Simulation of the Effect of Wettability Alteration on Flow Transport Kinetics in 3D Natural Porous Media

Summary Wettability alteration commonly occurs in subsurface two-phase displacements, such as enhanced hydrocarbon recovery, hydrogen storage, and carbon dioxide sequestration. A comprehensive understanding of two-phase flow transport kinetics during wettability alteration in natural rocks is essential for optimizing these processes. To address this, a wettability alteration model induced by low-salinity waterflooding (LSWF) was implemented based on the volume of fluid (VOF) method and the compressive continuous species transfer (C-CST) method in the OpenFOAM platform, which integrates the pore-scale two-phase fluid flow and the advection-diffusion of species. Following validation against experimental data from existing literature, extensive direct numerical simulations (DNSs) were conducted in an actual 3D sandstone sample obtained by microcomputed tomography (micro-CT) images. The effects of the wettability alteration degree, wettability alteration model, and capillary number on dynamic salt dispersion and fluid redistribution are considered in simulation works. The findings indicate that a higher wettability alteration degree facilitates the release of more oil trapped in smaller pores toward the outlet, while the mobilized oil might become trapped again due to snap-off in larger downstream pores. Moreover, due to the presence of alternative flow pathways in the system, the backflowed oil induced by heterogeneous salinity distribution might not be effectively recovered. A faster wettability alteration rate enhances the performance of LSWF because of the rapid reduction of entry capillary pressure and the delayed negative effect of salt dispersion. In terms of the capillary number, a higher capillary number accelerates the diffusion of species to the three-phase contact line and reduces the occurrence of snap-off retrapping, thereby increasing ultimate oil recovery. This study contributes to a deeper understanding of the microscopic displacement mechanism during the wettability alteration processes, especially for LSWF, in 3D heterogeneous porous media.