Development of sulfate-based smart water for improving the oil recovery from the calcite formation: New insights from molecular simulation

Wettability plays a huge role in the process of oil displacements, particularly in highly complex oil-wet carbonate reservoirs. Recently, sulfate ions were witnessed to have a great potency to alter the wetting state of calcite. Though the role of sulfate ion studied extensively, still the mechanism of their wettability modification is yet to be fully understood. It is expected due to the complex surface chemistry of rock in the presence of monovalent and divalent ions. In this study, we constructed a rock-oil-brine model at a molecular level and run the simulation on the wettability dynamics with various ions. The most abundant surface structure of calcite {1014}, was considered for this atomistic simulations. The model oil consisted of n-heptane and 10% octanoate ion as a polar component and sulfate-based salts were used as smart water. Two distinct hydration layers were found on the calcite surface. The contact angle measurements were made to find the effectiveness of smart water solutions in altering the wettability of the surface. Monovalent cations were observed to form a strong electric-double-layer while the divalent cations were preferred to get distributed in the bulk; most of them were attached to polar oil ions removing them from the calcite surface thus changing the wettability.