Cation-Dependent Oil-Rock Interactions in Nanopores: Insights from Core Flooding Experiments and Molecular Dynamics Simulations

A large amount of crude oil still adheres to the rock surface after water flooding in low-permeability reservoirs. Cations in the formation water were assumed to play a crucial role in crude oil adhesion. However, it lacks systematic investigation, and the mechanism remains unclear. In this work, the role of cations in these oil-rock interactions is investigated by performing water flooding experiments and molecular dynamics simulations. Results show that Ca2+ has a significant impact on the injection pressure and the oil recovery than Na+. The oil recovery of cores initially saturated by CaCl2 solution (39.02-47.48%) was lower than those saturated by NaCl solution (51.78-54.96%). It was found that the hydrated cations accumulate on the negatively charged rock surface to form a brine layer. The interaction energy between crude oil and rock surface was 1.7 times 0.5 M NaCl (-14.2 x 10(3) kJ.mol(-1)) when the brine layer contained 0.5 M CaCl2 (-24.2 x 10(3) kJ.mol(-1)), leading to the strong adhesion of crude oil. Thus, the amount of residual oil was higher in the presence of Ca2+ after water flooding. With the increase in Ca2+ concentration, the adhesion of crude oil became stronger, significantly reducing the efficiency of water flooding.