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• A pore-level numerical investigation and a subgrid scale lubrication model for studying thin-films is presented. • The water salinity and solid roughness effects are studied for the thin-film stability. • A physical map quantifying the dependency of thin-film stability on salinity and roughness is presented. • Novelty of the work is at capturing the electric double layer effects at the pore-scale. • This work can well serve as a physical basis for studying water salinity effects at larger scales. The interfacial dynamics in natural porous media are affected not only by the interplay between viscous and capillary forces but also the solid surface wettability. It has been hypothesized that the wettability alteration induced by changes in the water salinity is primarily caused by electric double-layer force expansion, which strongly affects the multiphase flow dynamics. We investigate the effect of water ionic composition and surface roughness on pore-scale wettability alteration. Multiphase hydrodynamics is numerically captured by a lubrication approximation describing the evolution of thin-films coupled with a multiscale level-set approach. An oil blob mobilized by water within a single pore is considered as a case study. The effect of brine ionic composition is accounted for by an electric double-layer through the water ionic strength and zeta-potential parameters. We demonstrate that high-salinity water thin-films collapse to an adsorbed nanometer layer, leading to a large pressure drop during mobilization of the blob induced by the attractive surface forces. However, low-salinity water thin-films are stable due to the repulsive electric double-layer forces, leading to less pressure drop during mobilization of the blob. The novelty of this work lies in efficiently capturing the nanoscale effects of the electric double-layer in pore-scale multiphase flow displacements. Our quantitative investigations provide fundamental insights into the efficiency of low-salinity waterflooding. [ABSTRACT FROM AUTHOR]

• A multiscale numerical framework has been developed to simulate the multiphase flow in complex porous media. • This model includes a subgrid lubrication approximation for capturing the evolution of thin films on curved solid surfaces. • A Navier-Stokes Level-set solver is coupled with an immersed boundary approach for resolving the multiphase interactions. Complex solid geometries, subgrid thin films, and interfacial deformations are critical for studying multiphase flows in porous media. However, it is challenging to accurately predict the flow evolution in porous media because of the difficulties in capturing the fluid-solid and fluid-fluid interactions in complex-shaped confinements at the pore-scale. In this work, we propose a multiscale continuum framework enabling the accurate simulation of the multiphase flow physics in complex porous media. Within this framework, we couple an efficient and accurate incompressible Navier-Stokes solver developed on a fixed Cartesian grid with a level-set method designed for capturing the interface between immiscible two-phase flows at low Capillary numbers. To capture the effects of complex solid boundaries on the flow dynamics, we employ an immersed boundary method based on a direct forcing approach. Most importantly, we develop a subgrid-scale thin-film model on a cylindrical coordinate around the curved solid surfaces to resolve the thin liquid films below the grid resolution and capture their effects on the fluid-fluid interfaces. The proposed multiscale framework illustrates how the embedded subroutines such as the level-set approach, the immersed boundary method, and the thin-film model can be coupled in order to capture the crucial physics involved in multiphase flow within porous media at different length scales. [ABSTRACT FROM AUTHOR]