Probing of the hydrated cation bridges in the oil/brine/silica system via atomic force microscopy and molecular dynamics simulation

Abstract Strong interaction between oil and rock in oil/brine/silica system can not only lay a theoretical foundation for improving the crude oil recovery of unconventional reservoirs, but also provide ideas for the remediation of oil-contaminated soil and the reduction of energy consumption in the process of oil pipeline transportation. Cations have a substantial impact on the interaction between oil and rock, and low salinity water flooding can enhance oil recovery by 4%~38%. However, the existence form and functional mechanism of cations between oil and rock remain to be elucidated. In this paper, the interaction between oil and rock is quantitatively characterized via atomic force microscopy (AFM). The polar components of oil have a strong interaction with rock, which is 2–10 times stronger than that between the nonpolar components of oil and the quartz glass in the same cationic environment. Cations have readily observable impacts on the interactions between the polar components of oil and rock. The interactions are strong in brine with high valence cations and a high cation concentration. Molecular dynamics simulation (MD) was used to investigate the micro conformation between oil and rock and to identify the existence form and functional mechanism of cations. The results demonstrate that cations act as bridges that connect polar components in oil to rock surfaces. Besides, we also found that cations are present as hydrated cations and the water molecules that form the hydrated cations connect oil to the rock surface via hydrogen bonds. Hydrogen bonds are essential for the strong interaction between oil and rock. Therefore, the strength of the hydrogen bonds that connect the oil to the rock surface in systems with Na+, Ca2+ and Al3+ was calculated using density functional theory (DFT), and the result is consistent with that of AFM experiments. This study identifies the key action site of the strong interaction between the oil film and rock and provides a theoretical basis for the study of peeling oil films and renovating oil-contaminated soil.