The stabilization of oil-bound thin brine films over a fixed substrate with electrically charged surfactants subject to van der Waals and electrostatic forces

Abstract

Enhanced oil recovery through low salinity water flooding has been attributed to increased rock-to-water wettability due to the stabilization of a thin brine film between the rock and the oil phase resulting from the expansion of the electric double layer. However, the underlying physicochemical mechanisms are not well understood and an electrohydrodynamic approach to the thin film dynamics helps to better understand its evolution to equilibrium states. Lubrication theory is applied to a thin brine film bounded by a viscous phase with diffusing charged surfactants at the oil–brine interface under van der Waals and electrostatic interactions. Linear analysis and numerical solutions of the nonlinear governing equations are carried out and linear stability diagrams indicate that disturbances evolve in oscillatory or non-oscillatory modes. In addition to the expansion of the electric double layer, other physicochemical mechanisms affect the stabilization of the brine film. Among them are the (secondary) ionic hydration within the brine, surface charge concentrations, diffusion of surface-active species, as well as Marangoni, tangential and normal stresses. In the nonlinear regime, the van der Waals attraction leads to rupture in regions where the film is narrower since primary hydration is not considered. The oil–brine interface may be negatively or positively charged and different mechanisms can change the wettability of rock formations. Including hydration forces led to nonlinear metastable states, in addition to linearly stable states. The findings help design ’smart waters’ based on modifying the properties of surfaces and adjacent fluids of thin liquid films in order to drive positively or negatively charged rock formations into more water-wet states.