Molecular Dynamics Investigation of Surfactant–Oil Interactions within Quartz Slit Pores during Spontaneous Imbibition

During the spontaneous imbibition stage of tight reservoirs, surfactant-based displacement agents play a crucial role in regulating the interfacial wettability and enhancing oil recovery. However, the microscopic mechanism governing the interaction among surfactants, oil, and mineral surfaces remains elusive. The interfacial properties of oil-surfactant systems in hydroxylated SiO₂ slit pores were studied via molecular dynamics simulations. Three representative surfactantsSDS, CTAB, and HGMEwere selected to elucidate how their polarity and molecular architecture affect adsorption configuration, diffusion, and wettability at the solid-liquid interface. The results reveal distinct interfacial morphologies: in the SDS system, oil forms two asymmetric clusters with contact angles ranging from 93° to 118°, while CTAB induces more spherical or vortex-shaped aggregates with contact angles between 97° and 145°. In contrast, hexaethylene glycol monododecyl ether (HGME) promotes extensive spreading of the oil phase, with contact angles of approximately 84°-86°, indicating enhanced surface wetting. Interaction energy analyses further show that van der Waals forces dominate interfacial stabilization (LJ potentials of -2000 to -3500 kJ·mol^-1), while Coulombic interactions mainly dictate molecular orientation and surface affinity. Nonionic HGME exhibits strong adsorption through hydrogen bonding and dispersion forces, whereas ionic surfactants (SDS and CTAB) anchor primarily via electrostatic interactions. Overall, this work provides molecular-level insights into surfactant-assisted oil displacement mechanisms during the imbibition process and offers theoretical guidance for optimizing wettability control in silica-rich tight reservoirs.