Molecular Mechanisms of CTAB-Induced Oil Droplet Detachment in Shale Nanopores

Abstract The efficient recovery of shale oil is severely hindered by the strong oil–solid interactions and nanoscale confinement within shale reservoirs, which substantially restrict crude oil mobility. Although surfactant flooding has emerged as an effective enhanced oil recovery technique, the microscopic mechanisms by which cationic surfactants promote oil detachment in nanopores remain insufficiently understood. In this study, molecular dynamics simulations are performed to systematically investigate the interfacial mechanisms of cetyltrimethylammonium bromide (CTAB)-induced oil droplet detachment from nanopore surfaces. The effects of temperature and crude oil composition on the detachment behavior are further evaluated. The results demonstrate that CTAB exhibits the highest oil displacement efficiency among the four investigated surfactants, characterized by the lowest interaction energy and the formation of the most stable interfacial adsorption layer. The CTAB-induced oil detachment process proceeds through three successive stages. CTAB molecules initially migrate to the oil–water interface through hydrophobic interactions, followed by their directional migration toward the oil–solid interface driven by electrostatic interactions, where they induce the formation of hydration channels. Subsequently, hydrogen-bond networks stabilize and expand these hydration channels, ultimately leading to the complete detachment of oil droplets and the formation of stable oil-in-water emulsions. Increasing temperature significantly enhances the interfacial adsorption of CTAB and improves oil displacement efficiency. In addition, crude oil composition exerts a pronounced influence on the detachment process. CTAB exhibits the highest detachment efficiency for the medium crude oil. In contrast, the detachment of heavy oil follows a distinctive two-stage mechanism involving initial aggregate disintegration, followed by interfacial stabilization. This study provides molecular-level insights into the interfacial regulation mechanisms of CTAB in shale nanopores and offers a theoretical foundation for the rational design of high-performance surfactant systems for enhanced shale oil recovery.