Pore-Scale and Molecular Insights into Surfactant and CO2 Huff-and-Puff for Enhanced Oil Recovery in Shale Reservoirs

Abstract This study systematically investigates the molecular mechanisms underlying the enhanced oil recovery (EOR) efficacy of surfactant and CO2 huff-and-puff processes across different pore sizes in shale reservoirs. By integrating nuclear magnetic resonance (NMR) experiments with molecular dynamics (MD) simulations, the differential behaviors of these two EOR agents at the pore scale were elucidated. NMR T2 spectroscopy reveals that CO2 huff-and-puff achieved superior cumulative recovery rates (37.30%) compared to surfactants (24.01%), primarily due to its miscibility with crude oil and efficient mobilization of mesopores (53.1% ± 3.7% contribution). Conversely, surfactants exhibited higher efficacy in micropores (47.8% ± 4.3% contribution), attributed to multiple synergistic mechanisms, including interfacial slip induced by near-zero interactions with rock surfaces, wettability alteration, and solubilization of adsorbed oil films. MD simulations further confirmed that CO2 exhibits a stronger affinity for saturated hydrocarbons, while surfactants preferentially interact with polar components (nonanoic acid, nonanone) via Coulomb-dominated hydrogen bonding and hydration, enhancing light oil extraction. The results from experimental observations and molecular-level insights underscore the distinct advantages of CO2 and surfactants in huff-and-puff processes, providing a theoretical foundation for designing composite methods to optimize recovery efficiency. This research bridges macroscopic performance discrepancies with nanoscale mechanisms, advancing the understanding of carbon dioxide utilization in carbon capture, utilization, and storage (CCUS) frameworks.