Micromechanical response of multiphase CO2 injected into high-rank coal fractures in the Qinshui Basin via nanoindentation

The carbon dioxide-enhanced coalbed methane process enables CO2 sequestration and enhances coalbed methane (CBM) extraction efficiency, offering broad application potential in deep CBM development. The coal fracture surface is a significant medium for fluid transport, and its mechanical properties are significant factors for affecting the migration and embedment of proppants. The results show that as CO2 gradually transforms into the supercritical state, the surface indentation depth, creep distance, and residual depth increase gradually. Young's modulus, hardness, and fracture toughness decrease by 39.45%, 36%, and 31.5%, respectively, and are controlled by the indentation depth. The irreversible work ratio effectively reflects the mechanical weakening process and indirectly confirms that the plastic deformation of coal is an irreversible change. The macromolecular structure, surface free energy, and pore and fractures of coal are important factors contributing to the weakening of mechanical properties, mainly manifested in the destruction of the macromolecular structure, the reduction of surface free energy, and the expansion of fractures. In addition, the Burgers model better validates the mechanical properties of coal fracture surfaces during the creep stage. This greatly reduces the resistance to crack propagation in coal, thus decreasing the mechanical properties of coal. These findings provide important insights into the mechanical behavior of coal in the context of CO2 geological sequestration and related applications.