Experimental and theoretical investigation on permeability evolution of fractures in anthracite with supercritical CO2 immersion

The strong adsorption capability of carbon dioxide (CO2) by coal matrix allows injecting CO2 into deep coal reservoirs a viable method for large-scale carbon sequestration. Typically, CO2 exists in a supercritical state (ScCO2) within deep geological environments. The long-term ScCO2 immersion would affect the permeability of coal fractures, potentially threaten the sustainable injection capacity of the system. To address this concern, a series of permeability tests on fractures in anthracite were conducted, involved ScCO2 injection time ranging from 0 to 240 h and confining stress from 20 to 40 MPa. The fracture surface morphology before and after ScCO2 immersion was measured and characterized to observe surface degradation. Additionally, the evolution of fracture normal stiffness induced by ScCO2 immersion was measured through triaxial adsorption tests. The results show that after 240 h of ScCO2 immersion, the permeability of anthracite fractures decreased by 72 %-85 %. Notably, the change in permeability exhibited a nonlinear decreasing trend with injection time. The long-term ScCO2 immersion caused the decrease of surface asperities height, and this degradation appeared to be closely linked to the applied confining stress. The relationship between fracture normal stiffness and ScCO2 injection time followed an exponential trend. Based on these, a hydro-mechanical coupling model was proposed, considering ScCO2 adsorption and fracture normal stiffness weakening, to characterize the anthracite fracture permeability. It was observed that the weakening of normal stiffness significantly influenced permeability compared to the Young's modulus of the coal mass. Furthermore, higher confinement stress restricts the weakening of fracture normal stiffness. The theoretical model established for permeability variation with ScCO2 injection time was in good agreement with experimental data. Moreover, this model was validated to effectively characterize the changes in anthracite fracture permeability concerning injection pressure. The study is significant to evaluate the sustainable injection capacity during CO2 sequestration within deep coal seams.