Dynamics of electrostatic interaction and electrodiffusion in a charged thin film with nanoscale physicochemical heterogeneity: implications for low-salinity waterflooding

The slow kinetics of wettability alteration toward a more water-wetting state by low-salinity waterflooding (LSWF) in oil-brine-rock (OBR) systems is conjectured to be pertinent to the electrokinetic phenomena in the thin brine film. We hypothesize that the nanoscale physicochemical heterogeneities such as surface roughness and surface charge heterogeneity at the rock/brine interface control further the dynamics of electrodiffusion and electrostatic disjoining pressure (Πel), thus the time-scale and the magnitude of the low salinity effect (LSE). Film-scale computational fluid dynamics (CFD) simulations were performed to demonstrate this. The coupled Poisson-Nernst-Planck (PNP) equations were solved numerically in a thin water film confined between a solid surface and oil, both negatively charged. The solid surface is representative of quartz/kaolinite with patchwise physicochemical heterogeneity. The electrical properties of the oil are representative of a crude-oil sample. The OBR system is initially under chemical equilibrium with high salinity (HS) brine, then is exposed to low salinity (LS) brine. The time-scale of reaching chemical equilibrium under LS, and the evolution of electric potential were investigated. We find that surface roughness increases the diffusion time up to 3-fold due to increased tortuosity. Also, the effect of surface roughness and surface charge heterogeneity on the effective diffusioncoefficient (Deff) is minor. While surface roughness and surface charge 1 heterogeneity affect the disjoining pressure (Πel) significantly, the influence of surface roughness on Πel is more pronounced under HS than LS conditions. In contrast, the effect of surface charge heterogeneity (introduced by kaolinite patches on quartz) is more appreciable under LS than HS. Our findings imply that the LS effect can be enhanced in rough, heterogeneously charged systems like clayey sandstone, although its magnitude depends on the charge density of the roughness. We introduce two scaling factors, namely the effective diffusion coefficient (Deff) and the retardation coefficient (ω), to upscale the nanoscale results to pore-scale and beyond.