Interactions between Rock/Brine and Oil/Brine Interfaces within Thin Brine Film Wetting Carbonates: A Molecular Dynamics Simulation Study

The thin brine film that wets rock surfaces governs the wettability of underground reservoirs. The ionic composition and salinity of this nanosized brine film influence the wetting preference of the rock pore space occupied by hydrocarbons. Despite numerous investigations over the last decades, a unanimous fundamental understanding that concerns the contribution of ions in the original wetting state of the reservoir is lacking and hence the mechanisms responsible for the wettability reversal of the mineral are still unclear. This wettability reversal is the main consequence of ion-tuned waterflooding. Although the method is widely accepted in practice, there is no universal consensus on the underlying mechanisms involved. Molecular dynamics simulation is an excellent choice to remove such ambiguities. This method can capture an atomic-level picture of the phenomena that affect reservoir wettability upon injecting low-salinity water. For the purpose, we performed simulations of brine films confined between a calcite substrate and a layer of an oil model. The films were at two different salinities to represent the initial state of high-salinity connate water and low-salinity brine. We found the development of ionic aggregates, mainly comprising Na+ and Cl-, within the high-salinity thin brine film. These aggregates act as pinning entities to localize polar oil components within oil/brine interface and connect the hydrocarbon phase to the calcite surface. This results in the adhesion of oil components to the rock surface though a high-salinity thin brine film. Simulation results suggest that the aggregates do not form after the change of the brine content to low salinity. From these observations, we concluded that diluting the brine content of the reservoir leads to the disintegration of such ionic aggregates. As a consequence, electrical double layers (EDLs) form at both rock/brine and brine/oil interfaces, which is supposed to be reflected by additional oil recovery at the macroscopic scale. Furthermore, we pointed out that EDL at an oil interface is established by negatively charged oleic polar species and cations around those compounds. Likewise, the EDL in proximity to calcite is composed of a positive Stern layer of Na+ cations and a negative diffuse layer of Cl- anions beyond that.