Mechanical Properties and Damage Constitutive Model of Coal Under Supercritical CO₂ Conditions

Deep coal seams offer considerable potential for simultaneous carbon sequestration and methane extraction. When injected into reservoirs, such as coal seams, CO₂ transitions into a supercritical state (supercritical CO₂ or Sc-CO₂), altering the internal structure and mechanical properties of coal. This study investigates the effects of the Sc-CO₂ soaking duration on coal strength via triaxial compression tests. The strength and elastic modulus of coal both decreased with increasing Sc-CO₂ soaking time, and the most prominent reduction occurs within the initial 1 day. A constitutive model is developed to characterize the mechanical properties of coal. This model simultaneously incorporates the degradation of the elastic modulus of coal owing to Sc-CO₂ exposure and the failure evolution based on a Weibull statistical distribution of microelement strength. This model successfully reproduces the stress-strain responses for different treatment durations, exhibiting high consistency with the experimental data. Sc-CO₂ damages the pore structure of coal through coupled physicochemical interactions, progressively deteriorating the macroscopic mechanical properties of coal. These findings provide valuable insights into the evolution of the mechanical properties of coal during carbon sequestration and aid the assessment of the long-term stability of CO₂-sequestered coal seams.