Effects of low salinity water on calcite/brine interface: A molecular dynamics simulation study

Calcite is among the most abundant minerals organizing the oil reservoir formation and therefore its surface properties play a central role in the increase of the oil recovery efficiency. The effect of low-salinity water in carbonate rocks reveals that brine composition and salinity can improve the oil recovery in carbonates through wettability alteration. However, the specific mechanism for wettability changes that leads to improved oil recovery in calcite is not well understood. To obtain deeper insights at atomic level into the understanding the characteristics of the calcite-water interface, we performed classical molecular dynamics simulations in the presence of different ions in brine solutions in the oil reservoirs. The results showed highly ordered water layer in the vicinity of calcite surface, followed by subsequent layers rich in ions of brine. These layers form an electric double layer, with monovalent Na⁺ ions in the Stern layer and anions in the diffuse layer. The results also indicated that divalent cations do not appear in the electric double layer. The appearance of ions at calcite/brine interface is more effective in wettability alteration for low saline brines such as seawater than high saline ones like formation water validating the applicability of low saline water injection method to improve oil recovery efficiency.