Atomistic Molecular Dynamics Simulations of Surfactant-Induced Wettability Alteration in Crevices of Calcite Nanopores

Abstract

Wettability alteration is an important mechanism in surfactant-facilitated enhanced oil recovery (EOR) in oil-wet carbonate reservoirs. An in-depth understanding of how surfactants influence the wettability of calcite surfaces is of crucial importance. In this study, atomistic molecular dynamics simulations were used to elucidate how the nonionic and anionic surfactants influence the wettability at the corners of angular calcite pores. A multicomponent oil model was constructed based on a chemical analysis of an unconventional reservoir crude oil. It was found that the inclusion of the calcite surface water layer caused a significant decrease in the attraction between the surface and the surfactant molecules from hundreds of kJ/mol to essentially zero. The surface water layer is therefore critical for an accurate description of calcite wettability. Simulated flooding of the oil-filled calcite pore with both types of surfactants was compared to that with blank brine. It was found that the tested surfactants alter the wettability of the calcite surface mainly by hydrophobic interactions with the adsorbed polar oil components and by weakening the interactions between polar and nonpolar oil components. The wettability alteration resulted in a 20–30° reduction in contact angles, while the ratio of the polar-to-nonpolar oil components within the pore decreased by approximately 50% by the surfactants as compared to just brine. The results presented in this work can provide guidance for selection of the most effective surfactants for oilfield applications.