Dissecting The Salinity-Dependence Of Wettability In Oil/Brine/Calcite System Using Molecular Simulations

Low salinity water flooding has shown great promise due to its cost-effectiveness and low environmental impact for improving and sustaining oil production. It is believed that injecting water with ionic strength lower than that of the reservoir changes the reservoir from less to more water-wet and enhances oil recovery. This alteration phenomenon is not well understood, due to complex interactions between oil, water, and rock. Here we use molecular simulations to characterize the wettability of the 10.4-face of calcite in a calcite/brine/oil system, and address how wettability is altered by changing ionic strength and salt type (NaCl vs. CaCl₂). Using the test area method we calculate the superficial tension of the fluids against the solid and the surface tension between the two fluid phases. As the salinity is decreased, the wetting of calcite by brine is progressively less favored, contrary to what might be expected based on low salinity flooding. However, as salinity is decreased, forming the oil-brine interface is more favored. On balance, it is the latter effect that leads to the enhanced wetting of calcite by brine in the oil-brine-calcite system, and it is suggested as an important element in the physics underlying low-salinity flooding.