Experimental investigation of the concomitant effect of potential determining ions Mg²⁺/SO₄²⁻ and Ca²⁺/SO₄²⁻ on the wettability alteration of oil-wet calcite surfaces

Modifying the concentration of potential determining ions (PDIs)in the brine can result in alteration of the surface wetting properties towards more water-wet state, which is the desired wetting state in fractured carbonate reservoirs. However, few studies have discussed the interactions between these PDIs and their mechanisms of wettability alteration. Accordingly, the aim of this study was to investigate the concomitant effect of PDIs at different molar ratios on the wettability of oil-wet calcite surfaces as well as the interactions between the ions. Two combinations of wettability modifying agents (Mg²⁺/SO₄²⁻)and (Ca²⁺/SO₄²⁻)were considered. Contact angle and zeta potential measurements were performed to evaluate the effect of these combined ions and explore possible wettability alteration mechanisms. Results revealed that both PDIs groups were able to reduce the contact angle significantly and expel carboxylate group from the surface. Besides, combinations of PDIs were more effective in wettability alteration compared to those ions alone. It was also observed that equal concentrations of magnesium and sulfate provide more water-wet state. However, for combinations of calcium and sulfate, the most water-wet state was achieved when lower concentrations of calcium and higher concentrations of sulfate were used. Zeta potential measurements showed that the performance of each PDI in changing surface wettability is critically affected by the concentration of the other ions present in the same solution. Results indicated that when low concentration of magnesium ions is used, the adsorption of sulfate ions on the rock surface and the interaction of magnesium ions with the carboxylate group is the main mechanism of wettability alteration by magnesium and sulfate ions. However, at high concentration of magnesium ions, no adsorption of sulfate ions was detected which means that sulfate serves as a catalyst for wettability alteration by Mg²⁺/SO₄²⁻ and the formation of a complex Mg-carboxylate group is the dominant mechanism. Moreover, the use of cationic surfactant along with the combined PDIs resulted in strong water-wet conditions.