Wettability alteration by Smart Water multi-ion exchange in carbonates: A molecular dynamics simulation study

Abstract

Smart Water ions (Ca2+, Mg2+, and SO42−) have been found to alter the wettability of oil-wet calcite surfaces, however, the widely proposed multi-ion exchange mechanism has never been understood on a molecular scale. In this work, a new model calcite surface with steps and locally charged sites was developed, and its properties were studied by quantum chemical calculations and molecular dynamics simulations to investigate the origin of the oil-wetness and its reversal by the Smart Water ions at temperatures ranging from 300 to 400 K. The simulation results using the proposed surface model are consistent with experimental results reported in the literature, including the Smart Water ions acting as the potential determining ions, and the oil carboxylate adsorption causing oil-wetness. The wettability alteration involves initial surface binding of Ca2+, which facilitates adsorption of SO42−. The latter enables the detachment of the Ca2+-carboxylate complexes at elevated temperatures (>350 K) where the ionic hydration shells are weakened as suggested by the increased interaction energy from less than −100 to over −400 kJ/mol between Ca2+ and SO42−, rationalizing the observed temperature dependence of the Smart Water effect. Overall, the presented simulations with the newly developed model calcite surface provide important new insights into wettability alteration effect of Smart Water ions, which has important implications for the optimization of the injection brines for enhanced oil recovery.