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 Mg and Ca 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.