Wettability Reversal on Dolomite Surfaces by Divalent Ions and Surfactants: An Experimental and Molecular Dynamics...

Due to the importance of the dolomite mineral in carbonate reservoirs, the wettability characteristics of dolomite surfaces were studied with both experiments and molecular dynamics simulations. Contact angle measurements confirm that the dolomite surface can be rendered oil-wet by carboxylates (acidic components of crude oil) and that the cationic surfactant can reverse the oil-wetness more effectively than the anionic surfactant used in this study. The oil-wetness of an aged dolomite chip was reduced when treated with MgSO₄ solution at 80 °C, while CaCl₂, MgCl₂, and Na₂SO₄ solutions did not produce any significant wettability alteration. The effects of surfactants and divalent ions, Ca²⁺, Mg²⁺, and SO₄^2- (also referred to as Smart Water ions), were simulated with two model dolomite surfaces containing point defects and step vacancies, respectively. The results indicate that the cationic surfactant can weaken the attraction between the oil phase and the carboxylates, while the anionic surfactant tends to maintain the oil-wetness of the dolomite surface by replacing the carboxylates through competitive adsorption. All Ca²⁺, Mg²⁺, and SO₄^2- ions can act as potential determining ions, and the detachment of carboxylates is due to the repulsion from SO₄^2- ions drawn close to the surface in the presence of adsorbed Mg²⁺.