Characteristics of Permeability Evolution and Pore Structure of Coal with High Gas

To study the influence of gas pressure on coal permeability evolution, we conducted experiments on coal samples from the No. 9 coal seam in Tangshan Coal Mine, Hebei Province, China. Different gas pressures (helium and nitrogen) were applied, and nitrogen-induced deformations were measured. We also analyzed the coal samples’ pore structure using mercury injection porosimetry, obtaining pore surface fractal dimensions. The increase in nitrogen pressure from 0.3 MPa to 3 MPa resulted in an elevation of adsorption strain from 0.168 × 10 −3 to 1.076 × 10 −3 , with a gradual decrease observed in the extent of this increase. However, the permeability of coal samples initially decreased from 16.05 × 10 −18 m 2 to 4.91 × 10 −18 m 2 and subsequently rose to 5.69 × 10 −18 m 2 . Helium showed similar trends to nitrogen, with average permeability 1.42–1.88 times higher under the same pressure. The lowest permeability occurred at 1.5 MPa for helium and 2.5 MPa for nitrogen. Gas absorptivity plays a crucial role in coal permeability evolution. Additionally, we observed coal’s compressibility to be 7.2 × 10 −11 m 2 /N and corrected porosity to be 53.8%, considering matrix compression. Seepage pores larger than 100 nm accounted for 80.4% of the total pore volume, facilitating gas seepage. Surface fractal dimension D s1 correlated positively with micropore volume, while D s2 and D s3 correlated negatively with pore volume and gas permeability.