Study on the Nanopore Deformation Mechanisms in Shale Oil Reservoir: Insights from the Molecular Simulation

The stress and adsorption motivation deformation during shale oil production directly affect its development dynamics. First, a mathematical simulation of pore deformation in shale oil under stress motivation is established. We analyzed the impact of factors including the reservoir pressure, Biot coefficient, bulk modulus, and tortuosity on the deformation characteristics of nanopores. Second, a pore deformation model under multifactor synergistic effect is derived by combining the molecular dynamics, which takes into account the influence of adsorption deformation on the total adsorption of shale oil reservoir. Finally, a shale oil pore deformation model under multifactor synergistic effect is obtained. The results show that the current pore diameter shows a trend of decreasing with the decrease of current formation pressure and the difference with original reservoir pressure increases. The pore shows a trend of less deformation with a decrease in the effective stress coefficient. When the Biot coefficient is 0.8, the pore diameter under 5 MPa is 4.01 nm. When the Biot coefficient decreases to 0.3, the pore diameter under 5 MPa becomes 9.12 nm, an increase of 127.43% compared to 4.01 nm. The bulk modulus affects the magnitude of pore deformation under the same pressure, which means that the pore diameter shows a tendency to deform more easily as the bulk modulus increases. Meanwhile, the pore diameter decreases with increasing tortuosity. In addition, the pore deformation is subject to both stress and adsorption motivation deformation synergistically. The stress motivation deformation leads to a decrease of pore diameter with pressure decrease, while, in contrast, the adsorption motivation leads to an increase of pore diameter. The pore diameter under synergy is influenced by the coupling effect, and the deformation under synergy tends to decrease as the pore diameter decreases. When the rock mechanical parameters are changed so that the pores are not easily deformed by stress, the adsorption motivation deformation plays a dominant role in synergy. The amount of synergistic deformation decreases with increasing temperature, while the change in the component ratio of multicomponent fluid mainly affects the corresponding adsorption and the amount of synergistic deformation. Interestingly, when the proportion of CO₂ is the largest, the corresponding maximum deformation is higher than the other proportions (43.77%). It not only enhances the recovery rate of shale oil reservoirs by utilizing CO₂ but also provides the possibility of geological CO₂ burial.