Atomistic insights into role of low salinity water on montmorillonite-brine interface: Implications for EOR from clay-bearing sandstone reservoirs

Low salinity waterflooding (LSWF) is considered as a potentially promising approach to improve oil production from petroleum reservoirs. Nevertheless, ambiguities about dominant mechanisms and influential factors controlling the wettability alteration on rock surface still remain elusive. To date, the main attention has been given to securitizing the impact of salinity, oil composition, rock surface characteristics, and cationic part of the brine, largely overlooking the impact of anionic components of the brine. However, this is of great importance and interest to clarify how the anionic component of salt can contribute to rock-brine interface characteristics. In this paper, we used Molecular Dynamics (MD) simulation technique to characterize the effect of anionic components of brine solution on intermolecular interactions at the montmorillonite-brine interface. Here, the emphasis was put on NaCl and Na₂SO₄ brine solution to compare the ionic behavior of Cl^- and SO₄^2- in terms of distribution adjacent to the clay surface, potential for ionic aggregation formation, and pairwise interactions over a salinity range of 2000 ppm to 70000 ppm. Results obtained from this research work show that anionic SO₄^2- accumulate nearby the clay surface in the form of ionic aggregations with Na⁺ cations, compared to the Cl^- anions, which are uniformly distribute over the clay surface. The strong propensity of SO₄^2- towards Na⁺ reduces the possibility of hydration by water molecules, disintegrating the bridging role of Na⁺ to adsorb oil constituents onto the clay surface. Furthermore, the disparate adsorption of SO₄^2--Na⁺ aggregation onto the clay slab hinders oil diffusion toward the surface. Finally, besides salinity, anionic components of brine solution have no effect on expansion of the electrical double layer (EDL), casting a shadow on this phenomenon as a driving factor of LSWI-EOR. The results obtained from this research work can further our understanding of clay-brine interaction which is a major mechanism behind low salinity effect in clay-bearing petroleum reservoirs.