The evolution of pore structure and gas seepage patterns of coal body under dynamic load

This study explores the pore damage and gas seepage patterns of coal samples under different dynamic load through dynamic impact tests using the split Hopkinson pressure bar (SHPB) testing system. The T₂ spectra and permeability of coal samples before and after different impact velocities were tested using nuclear magnetic resonance (NMR) and coal core permeability testing devices. We also analyzed the distribution of coal body pores and their damage evolution patterns. Results indicate that: As the impact velocity gradually increased, micropores and mesopores dominated the pore evolution process of coal samples, with maximum increases of 21.10% and 30.57%. Under dynamic load, the overall damage area of coal pore structure exhibited a transition from a scattered point distribution to a complex interwoven line and plane distribution. The fractal dimensions of both permeable pores and total pores decreased linearly with increasing impact velocity, while the increment of porosity increased exponentially with increasing impact velocity. The permeability increment of coal samples showed an increasing trend with the increase of impact velocity, and the increment was most significant when the gas pressure was 0.30 MPa. The increment of coal sample permeability showed exponential decrease with the increase of gas pressure. When the impact velocity increased, the development of mesopores and macropores caused the internal gas flow state of the coal sample to shift from micropore flow to microcrack flow, resulting in a significant increase in permeability.