Effects of Dynamic Impact on Microstructure Evolution and Permeability Enhancement of Coal

Deep coalbed methane development is significantly influenced by instantaneous dynamic loads. To investigate the pore damage characteristics and gas permeability evolution in coal samples during different impact loads, a Split Hopkinson Pressure Bar (SHPB) test system was used to perform the impact testing. The T₂ spectrum and permeability of coal samples were systematically measured before and after impact loading through nuclear magnetic resonance (NMR) analysis and an automated permeability testing device. Pore-fracture structure evolution was analyzed through NMR imaging, and the permeability variations were discussed as the impact load increased. The results showed that when the impact pressure increased from 0.25 to 0.45 MPa, the strain rate and dynamic strength of coal samples increased linearly. However, the peak strain decreased exponentially. With increasing impact pressure, the maximum increment in T₂ spectrum area of micropores in coal samples was 21.10%. Additionally, the evolution of mesopores and macropores gradually dominated, in which the maximum increase in the T₂ spectrum area reached 30.57%. During the impact of dynamic loading, the damage area in coal samples presented the “point-line-surface” distribution morphology transformation as a whole. The fractal dimension of pores decreased linearly, which accelerated the transformation from internal micropores to macropores and microfractures. As the impact pressure increases, the energy dissipation density exhibits a linear increase, while the permeability of coal samples rises exponentially. The gas flow state transitioned from micropore flow to microfracture flow, causing the growth trend of permeability change rate to slow down initially and then accelerate, increasing from 13.45% to 37.17%. The findings have critical implications for enhancing the efficiency of coal seam gas extraction.