Effect of microscopic properties of coal on adsorption characteristics under fault activation

The present study investigates the influence of micromechanical features and pore configurations of coal on methane adsorption during fault activation. Tectonic coal samples representing different magnitudes of fault throw were employed to simulate the activation process. Variations in mechanical performance, pore geometry, and adsorption behavior during activation were analyzed using nanoindentation (DSI), atomic force microscopy (AFM), mercury intrusion porosimetry (MIP), and high-pressure methane adsorption tests. The results reveal that the elastic modulus of tectonic coal initially decreases and subsequently rises with greater vertical displacement. Pore morphology evolves gradually from relatively circular forms to complex honeycomb structures with pronounced heterogeneity. Fault activation demonstrates a three-phase evolution. (1) Plastic deformation stage: nanohardness consistently weakens, the pore morphology factor ( σ ) decreases by 4.4%, and the maximum methane adsorption capacity declines by 8.3%. Sample DC3 represents a synergistic deterioration inflection point with a 4.9% reduction in nanohardness. At this stage, the elastic modulus, pore morphology factor, and pore volume ( σ ) fall to their lowest levels, whereas the Langmuir adsorption constant reaches its maximum of 2.23 MPa −1 . The adsorption capacity then drops steeply by 30.7%, indicating the transition into the partial healing stage. (2) Represented by DC4, the pore morphology factor increases to 0.7158 along with improvements in porosity and elastic modulus, while maximum methane adsorption capacity recovers to 31.48 cm 3 /g. (3) Compaction and reorganization stage: nanohardness continues to deteriorate. However, the elastic modulus rebounds to 5.51 GPa, and the pore morphology factor recovers to 0.7531. Adsorption capacity only partially restores to 32.72 cm 3 /g. The activation process drives pore structure evolution through sequential stages of plastic deformation, partial healing, and compaction with reorganization. These transformations contribute to the decline of mechanical strength and to progressive pore structure failure, intensifying heterogeneity and the likelihood of localized methane buildup. The study illuminates the coupling mechanism between weakening of mechanical properties and dynamic pore structure failure under fault activation, offering a basis for identifying critical fault throw thresholds and establishing quantitative indicators for early warning of gas outburst hazards in coal seams affected by minor faults.