Experimental study on the influence of CO2 adsorption on the mechanical properties of anisotropic coal

In the project focused on CO₂-enhanced coalbed methane exploitation and geological storage, the seam network structure of coal seams serves as a conduit for gas migration, diffusion, displacement, and storage. The mechanical properties of these coal seams are intrinsically linked to the propagation and evolution of cracks. Prolonged exposure of coal seams to CO₂ adsorption environments inevitably alters their structure and mechanical properties. Consequently, experimental research has been conducted on the microstructure and mechanical properties of coal seams with potential for CO₂ geological storage in China. The results indicate that, under varying CO₂ adsorption pressures and durations: The relative contents of calcite, chlorite, and kaolinite in coal decrease, while the relative content of quartz increases significantly. Notably, the influence of supercritical CO₂ on mineral composition and relative content changes is the most pronounced. Long-term CO₂ adsorption accelerates mineral dissolution and ion exchange rates in coal, resulting in a rougher surface of coal mineral particles. Numerous secondary pores and fractures emerge and coalesce to form dissolution pits and grooves. Some mineral particle structures transition from intact to fragmented, severely weakening the skeleton particles and mineral bonding strength. Significant transformations occur in pores and fractures of different scales, with CO₂ adsorption causing a mutual transformation of mesopores and micropores in coal, albeit without altering the pore type. The uniaxial compressive strength, Brazilian splitting strength, and fracture toughness of coal exhibit a similar trend with increasing CO₂ pressure: an initial rapid decrease followed by a gradual, more gradual decrease. The mechanical strength/fracture toughness of coal samples with three different bedding types follows the order: Diverder type > Arrester type > Short transverse type. As CO₂ pressure increases, the destructive characteristics of coal transition from sudden instability to gradual instability, and then back to sudden instability. Under CO₂ adsorption, coal fracture trajectories can be classified into three types and 12 subtypes: single destruction, multi-source destruction, and fragmentation destruction trajectories. The interaction between CO₂ and coal alters the specific surface area, total pore volume, and uniformity of pore size distribution of coal, significantly impacting its composition. These microstructural changes underpin the macroscopic mechanical properties of coal, which in turn affect its mechanical properties and failure characteristics. The research findings have significant implications for evaluating the efficiency and stability of CO₂-enhanced coalbed methane mining and CO₂ geological storage.