Molecular Insights into Sandstone Wettability Alteration by Polar Oil Components

Enhanced oil recovery is crucial for maximizing production from conventional reservoirs, with surfactant-induced wettability alteration being a key mechanism. Investigating the molecular-scale dynamic adsorption behavior of heteroatom-containing (N, S, O) polar molecules on sandstone surfaces contributes to surfactant-based EOR development. This study examines wettability alteration mechanisms in hydrophilic sandstone induced by polar molecules in crude oil. Experimental aging tests were conducted on Colton Sandstone using n-decane solutions containing various polar compounds (3-decylthiophene, 4-octylphenol, 4-decylpyridine, n-decanoic acid, and decyltrimethylammonium bromide). Contact angles were measured over 26 days on both dry and water-saturated samples. Molecular dynamics simulations on hydroxylated and nonhydroxylated α-quartz (011) surfaces elucidated molecular-level adsorption mechanisms. Experimental results show that on dry sandstone, 3-decylthiophene, 4-octylphenol, and n-decanoic acid induced oil-wet transition. On prewater-saturated sandstone, the preexisting water film inhibited adsorption, with only ionic DTAB and Ca2+-decanoic acid effectively causing oil-wetness. MD simulations revealed strong adsorption of n-decanoic acid and Ca2+-decanoate complexes on silica surfaces, preventing water spreading and confirming their role in oil-wetness induction. Adsorption was driven by hydrogen bonding and electrostatic interactions like Si–O coordination and Ca2+ bridging, with distinct mechanisms on hydroxylated versus nonhydroxylated surfaces. This combined approach provides detailed molecular-level understanding of adsorption sites and mechanisms, offering insights for targeted surfactant design to reverse oil-wetness and improve recovery efficiency in sandstone reservoirs.