Molecular Dynamics Simulation of Adsorption and Absorption Behavior of Shale Oil in Realistic Kerogen Slits

Exploring the occurrence of shale oil is of guiding significance to the exploration and recovery of shale oil. However, most studies focus on adsorption and neglect absorption or study it independently, which weakens the understanding of the coupling relationship between adsorption and absorption and the occurrence mechanism of shale oil in kerogen and its associated pores. Based on molecular dynamics (MD) simulation, the kerogen matrix with a smooth surface is constructed, and the general amber force field is adopted to calculate the adsorption and absorption of shale oil in a type II kerogen slit, and the influences of temperature, pressure, and pore diameter on the occurrence of shale oil are also discussed. Molecules absorbed in the kerogen matrix are mainly retained in the free space formed by the accumulation of aliphatic carbon. The effects of temperature on the distribution of shale oil in adsorption and absorption are opposite. High temperature improves the desorption of molecules from the adsorption sites and causes a lower adsorption and greater absorption. In contrast, pressure and pore diameter only slightly influence the distribution of shale oil. Unexpectedly, the result under aqueous conditions shows that the shale oil was absorbed quickly before the formation of water clusters. Once the water clusters are formed due to the strong hydrogen bonding and adsorbed near the N-, S-, and O-containing groups on the kerogen surface, the matrix gaps are blocked due to the size exclusion, and the absorption is suppressed. Additionally, we established the evolution of the ratio of free-adsorbed–absorbed oil at different temperatures and pressures, which provides a new idea and a new method for detecting the occurrence and mobility evaluation of shale oil under reservoir conditions.