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.