Mechanical Properties and Microstructural Damage Behavior of Coal under CO ₂ Injection

CO2 sequestration in coal seams offers significant potential for CO2 emission reduction. However, CO2 injection impacts the physical and chemical properties of coal, weakening its mechanical characteristics and consequently affecting the long-term stability of coal seams. Through a series of experiments, this study investigated the effect of carbon dioxide pressure on the mechanical behavior of coal and elucidated its microstructural damage law. Results show that rising CO2 pressure significantly reduces uniaxial and triaxial peak strength and elastic modulus, with failure modes shifting to axial splitting under uniaxial compression and shear failure under triaxial conditions. SEM and CT scans reveal that CO2 adsorption accelerates fracture propagation and connectivity, quantified by damage variables that increase quadratically with CO2 pressure. Integrating theories of thermodynamics and fracture mechanics, this study further explores potential microstructural mechanisms underlying the degradation of coal’s macroscopic mechanical properties. These findings provide theoretical support for the safe CO2 sequestration in coal seams.