An atomistic insight into interfacial properties of brine nanofilm confined between calcite substrate and hydrocarbon layer

Abstract

Abstract Molecular dynamics simulation was applied in this study to scrutinize the interfacial properties of water nano-film confined between calcite mineral and hydrocarbon layer, as two intrinsically different media. Such system resembles the environment experienced by water molecules in the pore spaces of underground carbonate reservoirs. The interplay between water film and confining phases, oil and mineral, strongly influences hydrocarbon production process; however, there is a lack of detailed understanding of the involved interactions. MD simulations indicate development of several layers with different water densities in the confined brine. Water molecules form well-ordered structure in three hydration layers neighboring calcite substrate through particular interactions with the mineral. Likewise, the development of hydrogen bonding results in a subtle arrangement of the molecules in oil/water interface. The extent of water self-hydrogen bonding differs in each layer and is lowest in interfacial regions as a result of interruption by water-calcite (or oil) interactions. It was pointed out that strong interactions are responsible for higher residence time of water within the interfacial layers. We also studied the contribution of mono- and divalent ions (Na+, Cl−, Mg2+ and SO42−) wandering in the thin brine film. While Na+ and Cl− create a double layer over substrate, their effect on the water structuring is quite insignificant. However, the dense population of ions in interfacial regions, particularly calcite/brine, results in larger residence time of water in those regions which is a sign of a more water-wetness of the substrate.