Molecular dynamics simulation of shale oil adsorption in kerogen and its implications

Investigating the adsorption behavior in organic matter and its associated pores holds critical significance for revealing the occurrence states and mechanisms of shale oil. Differing from the previous method of replacing the organic matter model with the graphene model, a realistic kerogen molecular model, the Type Ⅱ-C model, is employed to simulate the adsorption behavior of multi-component shale oil within organic pores based on the general Amber force field (GAFF). The results are as follows. (1) Unlike graphene, which can only be used to simulate surface adsorption, kerogen has the dual functions of both adsorption and absorption. Competitive shale oil adsorption transpires on kerogen walls, dominated by the adsorption of polarity and heavy components, while the kerogen skeleton is characterized by absorption of small molecules moving far away. The shale oil adsorption on the surface and absorption in the skeleton with migration are influenced by the interaction energy between shale oil and kerogen, as well as the molecular size. Specifically, heavy components of shale oil are subjected to strong adsorption but weak absorption, while its light components undergo weak adsorption but strong absorption; (2) The absorption of shale oil components leads to the deformation of the kerogen skeleton and pores, manifested as the formation of new pores and the expansion and partial collapse of original pores. The plasticity of kerogen plays a significant role in its shale oil absorption and further skeleton swelling. Highly plastic kerogen (with low maturity) is more prone to absorb shale oil and swell significantly in skeleton. In contrast, weakly plastic kerogen swells slightly with absorption; (3) An increase in temperature enhances the absorption of aromatic hydrocarbon molecules (like naphthalene) and non-polar molecules (e. g., formic acid, ethanol, and thiophene) in kerogen skeleton, which reduces the adsorption on kerogen surface, and is conducive to the desorption of saturated hydrocarbon molecules. Additionally, pressure produces insignificant effects on the shale oil adsorption and absorption in kerogen. In this study, the realistic kerogen molecular model is innovatively applied to simulate kerogen’s adsorption and absorption of shale oil components, which is of great help in objectively revealing the shale oil occurrence state and mechanism in kerogen.