Role of the calcite-water interface in wettability alteration during low salinity waterflooding

Abstract

Wettability alteration has been widely accepted as the key process for enhanced oil recovery during controlled salinity waterflooding. However, the decisive mechanism causing wettability alteration, particularly for crude-oil-brine-carbonate-rock systems, is not fully understood. Zeta potential is a key electrochemical parameter for the mineral-brine interface and is sensitive to physicochemical changes at the interface. In this study, we conducted core-flooding tests integrated with zeta potential measurements to characterize changes in wettability, oil recovery and zeta potential on two carbonate rock samples, Carbonate-I and Carbonate-II, comprised of close to pure calcite. Following the same coreflooding protocol with the same crude oil and flooding brine compositions, the two samples showed different behaviour. Incremental oil recovery during controlled (low) salinity waterflooding was observed in the oil-brine-Carbonate-II system (8% incremental oil recovery) but there was no response for the Carbonate-I system. The zeta potential of the Carbonate-I and Carbonate-II samples also showed different behaviour in high salinity formation brine, with Carbonate-I returning a positive zeta potential, consistent with previous published measurements, but Carbonate-II returning an anomalously negative value. We suggest that the different zeta potential and EOR behaviour is most likely caused by the different proportions of crystal faces and edges in each sample that are exposed to brine. The low permeability Carbonate-II sample contains nanoscale pores and the edges of the calcite crystals are the dominant surface in contact with brine. Our results suggest that textural variations in natural carbonates control calcite-brine interfacial properties and the behaviour during controlled salinity waterflooding.