The diffusion of water through oil contributes to spontaneous emulsification during low salinity waterflooding

Abstract

Increases in oil recovery due to Low Salinity Waterfloods (LSW) have been widely investigated. Literature is abundant with studies attempting to pinpoint the relevant mechanisms involved in the LSW process. Recently, there was clear evidence of spontaneous formation of emulsions during LSW under certain conditions. This work examines the diffusion of water through oil as a contributing factor for spontaneous emulsification during LSW. Experimental observations are pore scale using etched-silicon micromodels. Observations are accompanied by pore-scale mass transfer calculations. Spontaneous emulsification is observed as a result of so-called “low salinity water” and crude oil phases contacting under non-equilibrium conditions. The experimental observations provide clear evidence that physical contact between oil and low salinity water phases is imperative for emulsification. In addition, it is concluded that a larger salinity gradient results in greater emulsion concentrations. In such cases, the crude oil is preequilibrated through exposure to a formation brine that has substantial salinity and divalent cations. Emulsification results as the oil phase comes into equilibrium with a brine of lower salinity and different composition. A key mechanism is the osmotic pressure imbalance between water and oil phases that drives water into the oil phase by diffusion. Water reaches a supersaturated state and spontaneous nucleation of the water phase results in emulsions, under appropriate conditions. The experimental and numerical modeling schemes presented here demonstrate and quantify time scales of water-in-oil emulsion formation.