Mechanism of wettability alteration of the calcite {101[combining macron]4} surface

To understand the mechanism of wettability alteration of calcite, a typical mineral in oil reservoirs, the interactions of deionized water and brine (with different compositions) with the calcite {101[combining macron]4} surface are investigated using a combination of molecular dynamics and first-principles simulations. We show that two distinct water adsorption layers are formed through hydrogen bonding and electrostatic interactions with the calcite {101[combining macron]4} surface as well as hydrogen bonding between the water molecules. These highly ordered water layers resist penetration of large stable Mg²⁺ and Ca²⁺ hydrates. As Na⁺ and Cl^- hydrates are less stable, Na⁺ and Cl^- ions may penetrate the ordered water layers to interact with the calcite {101[combining macron]4} surface. In contact with this surface, Na⁺ interacts significantly with water molecules, which increases the water-calcite interaction (wettability of calcite), in contrast to Cl^-. We propose that formation of Na⁺ hydrates plays an important role in the wettability alteration of the calcite {101[combining macron]4} surface.