The impact of salinity on ionic characteristics of thin brine film wetting carbonate minerals: An atomistic insight

Abstract

Connate water has been coexisting with oil and mineral for centuries within underground reservoirs. The oil recovery techniques, such as low salinity water injection, disturb this prolonged equilibrium state of oil/brine/rock system. However, a thorough understanding of this complex equilibrium in the reservoir is still lacking. In this study, we performed molecular dynamics simulations to provide quantitative comprehension of the thin brine film characteristics that wets carbonate reservoir rocks at molecular level. While an electric double layer is formed at the interface of calcite/low salinity water, the ions in the high saline water form several aggregates of ions. We found that these aggregates persist inside the nano-layer brine separating oil film from carbonate surface and act as anchors that hold oil components in the vicinity of the substrate. This phenomenon is suggested to be responsible for oil attachment to the carbonate reservoirs through a persisting thin water film. The injection of low salinity water is expected to breakdown such ionic aggregates and release oil components in favor of more water-wet state. This observation validates the hypothesis of pinning points previously suggested in the literature.