Mechanisms of CO2 geological storage in shale organic nano-slits: A molecular simulation study

CO2 geological storage technology is a promising method for effectively reducing greenhouse gas emissions. Currently, the microscopic mechanisms of CO2 in organic shale reservoirs have not been clarified. The molecular simulation method is used to analyze the CO2 storage mechanisms under different formation pressures and nano-slit widths by constructing an organic shale nano-slit structure with three-layer graphene. It is found that the CO2 molecule preferentially absorbs on the surface of organic shale nano-slits compared with the H2O molecule. Due to the stronger adsorption between CO2 molecules and the surface of shale nano-slits, H2O molecules on the surface are first displaced by the flow front of CO2 gas. During the CO2 displacement process, the flow front of CO2 gas forms a type with “protrusions” at both ends and a “depression” in the middle. Organic shale is primarily hydrophobic, typically exhibiting a water contact angle exceeding 90°. Under CO2 injection, the contact angle of the H2O–CO2–organic shale nano-slit system approaches 115°. The maximum amount of CO2 storage capacity is related to the properties of the organic shale nano-slits. For a given nano-slit width, a higher initial CO2 injection pressure accelerates the displacement rate of H2O. Under constant pressure conditions, wider nano-slits result in faster gas filling, more rapid H2O displacement, and a stronger overall geological CO2 storage capacity. There exists a saturation pressure for CO2 storage, when the pressure exceeds this value, the final amount of CO2 stored decreases, which is related to the diffusion rate of CO2 and the capture capacity of organic shale.