Meso-statistical damage constitutive model and validation of coal under CO₂ - load coupling

CO₂ adsorption causes damage degradation to the coal body and thus reduces its stability, challenging the long-term safety of CO₂ sequestration, and it is important to clarify the role of CO₂ degradation and establish an ontological model. The damage mechanics theory and statistical theory were used to derive the calculation formula that can comprehensively reflect the total damage variables of the coal body under CO₂ adsorption and load coupling, and focusing on the influence of the pressure-tight section, the fine statistical damage ontological equation of the coal body under the action of CO₂ was established by segmentation, and the method of determining each parameter of the model was clarified. Finally, the parameters of the constitutive model were determined by CT scanning experiment system and MTS 816 experiment system, and the uniaxial compression experiments of coal body under different CO₂ pressures were carried out by the self-developed gas-solid coupling experiment system to verify the rationality of the model. The results show that: ① The damage variables under adsorption and loading were defined based on the fracture rate obtained from CT scanning and by applying the Weibull distribution theory, respectively, and the total damage variable under the coupling of the two was further obtained by combining the damage theory, and a fine-scale statistical damage constitutive model was established; ② Three-dimensional reconstruction of the fracture based on CT scanning technology realistically reflects the fracture extension characteristics before and after CO₂ interaction, the higher the CO₂ pressure, the fuller the fracture expansion, the greater the three-dimensional fracture parameters and damage variables of the coal samples, and the more complex the spatial fracture network formed; ③ CO₂ had a significant deterioration effect on the physical properties of the coal body, and the compressive strength and modulus of elasticity of the coal body were both reduced by 49.78% and 22.63%, respectively, with the increase of CO₂ pressure, and the combined effect of dissolution, plasticization and air wedge effect of CO₂ on the coal body led to the reduction of the mechanical parameters; ④ The theoretical curves of the fine statistical damage model of the coal body under the action of CO₂ have a high degree of agreement with the uniaxial experimental curves, which indicates that the present structural model can better reflect the damage degradation effect of CO₂ on the physical properties of the coal body, and embodies the reasonableness and applicability of the constitutive model and the method for determining the model parameters.