Drivers of Low Salinity Effect in Carbonate Reservoirs Using Molecular Dynamic Simulation

Abstract

• Molecular dynamic simulation of calcite-oil-brine system reveals that monovalent ions are settled at calcite-brine interface. • The time-dependent evolution of the mass density profile demonstrates oil detachment from the calcite mineral. • The extension of the charge density profile coincides to the detachment of oil from the calcite surface. • The MD results show that an electrical double layer (EDL) is formed at calcite-brine interface. • The extension of the charge density profile occure ofter the settling of the monovalent ions at calcite-brine interface. Low salinity water (LSW) flooding has yielded promising results in improving the enhanced oil recovery (EOR) techniques in sandstone and carbonate reservoirs. Despite various studies and advances, the main mechanism behind the oil displacement in the rock-brine-oil system is unclear. To get a better understanding of the governing mechanisms, we used the molecular dynamic (MD) method to simulate a system including calcite-brine-oil under two different scenarios. At T=300 K and T=360 K each scenario is completed during 40 ns using a canonical ensemble. The results show that all mono/divalent ions are hydrated initially. Furthermore, Na + , Cl − and H 2 O molecules reside close to the calcite surface and their positions show little change at different time steps. However, the position of divalent ions is dynamic depending on the simulation time steps. Ca 2+ , which is initially detected at 2.35 Å above the solid surface at T=300 K and T=360 K, migrates away by time. On the other hand, Mg 2+ and SO 4 2− which are invisible up to t = 16 ns even at the radius of 10 Å, move gradually toward the surface with time and consolidate their position after oil displacement. Namely, prior to oil displacement, a shift in Na + , Cl − and Ca 2+ locations is noticed, and the trend of double electric layer (EDL) is extended, which might be the cause of oil displacement. The findings of this study are applicable in the improvement of EOR technology, water resource treatment and contaminant transfer from geological systems, dealing with solid solutions such as ocean-soil systems.