Molecular Simulation Study of Crude Oil Desorption Behavior during Low-Salinity Waterflooding

To address the unresolved mechanisms and debated microscale interactions governing crude oil desorption during low-salinity waterflooding in tight oil reservoirs, this study focuses on molecular-level interactions within the waterflooding process. Using molecular dynamics (MD) simulations, we constructed a multicomponent crude oil model comprising n-dodecane, toluene, and pyridine, along with waterflooding systems at two salinities (5000 ppm and 50,000 ppm). The microscale desorption behavior of individual crude oil components from a hydroxylated quartz surface during low-salinity waterflooding was systematically investigated, and the mechanisms by which reduced salinity modulates desorption efficiency were elucidated. The results reveal significant differences in the adsorption behavior of crude oil components on the quartz surface. Toluene exhibits the strongest adsorption energy of -221.5 kcal/mol, attributed to the formation of OH-π hydrogen bonds via a planar benzene ring configuration. n-Dodecane shows an intermediate adsorption energy of -143.2 kcal/mol, while pyridine exhibits the weakest at -31.5 kcal/mol. Reducing the salinity of the injected water effectively induces wettability alteration. At low salinity, the water-rock interaction energy decreases from a positive value to -950.1 kcal/mol, while the oil-rock interaction energy approaches zero, enabling complete desorption of the oil film. In contrast, at high salinity, only partial shrinkage of the oil film is achieved. Low-salinity waterflooding exhibits component-specific desorption efficiency: the strongly polar pyridine is completely detached, the aromatic toluene forms nanoaggregates upon desorption, and the n-dodecane is fully released, which relies on the traction effect of polar components. These findings provide important theoretical guidance for enhanced oil recovery via waterflooding in tight oil reservoirs.