Electrokinetic Mechanisms and Synergistic Effect on Ion-Tuned Wettability in Oil-Brine-Rock Systems

Abstract

Wettability alteration has been recognized as the dominant mechanism of ion-tuned waterflooding, where the ionic composition of injecting brine is modified to improve oil recovery. Yet mechanisms of ion-tuned wettability have not been fully understood, and there are ongoing debates over the effectiveness and relative contribution of each mechanism. In this paper, ion-tuned wettability in variable oil-brine-rock (OBR) systems over a wide range of ionic compositions is theoretically investigated, with focuses on two electrokinetic mechanisms, double layer expansion (DLE) and multicomponent ion exchange (MIE). Results indicate that the qualitative features of ion-tuned wettability in terms of DLE and MIE in various OBR systems with different surface properties can be divided into two typical types, depending on the electrical double layer (EDL) interaction being repulsive or attractive. With attractive EDL force, both DLE and MIE do not contribute to the water-wetness. While with repulsive EDL force, DLE only takes effect at relatively high concentration and MIE has a significantly larger impact at a low percentage of divalent cation. In addition, DLE and MIE can have a synergistic effect, where one mechanism alone has a negligible effect on wettability but two mechanisms combined have a big impact.