Fluid–Solid Coupling Model with the Multiple Flow Mechanism for CO2-Enhanced Shale Gas Recovery and CO2 Sequestration

Injecting CO2 into shale gas reservoirs to improve shale gas recovery is a promising technique, with the dual benefit of CO2 sequestration. To reveal the coupling effects caused by adsorption–desorption properties on shale gas exploitation and the physical properties of the reservoir, a fluid–solid coupling model of fluid seepage and deformation of the shale is constructed. First, the model was validated, and then, it was used to study the effect of injection of CO2 on shale gas exploitation and reservoir physical properties changes. The results demonstrate that CO2 injection improves shale gas recovery mainly through its higher competitive adsorption capacity, replacing CH4, increasing CH4 partial pressure, enhancing CH4 diffusion, and the flooding effect on CH4. Increasing the CO2 injection pressure has a positive effect on CH4 production and CO2 sequestration, but CO2 transport is accelerated, and breakthrough occurs early. Lower production pressure is beneficial to CH4 production but detrimental to CO2 sequestration, so it is necessary to determine the injection and production pressure by combining CO2 flooding in CH4 and CO2 sequestration. In addition, the coupled effect of effective stress change and gas adsorption/desorption-induced matrix expansion/contraction controls the evolution of reservoir permeability. However, the influencing factors that play a dominant role in different locations of the reservoir change with the exploitation period, so the change law of permeability is more complex. This study is of great theoretical and practical significance for elaborating and summarizing the relationship between adsorption–desorption, stress–strain, and gas transport in shale, predicting the effects of enhanced shale gas recovery and CO2 sequestration.