The Stress Sensitivity and Porosity Sensitivity of Coal Permeability at Different Depths: A Case Study in the Pingdingshan Mining Area

Determining the relationships among the stress, porosity and permeability of coal masses at different depths is a key problem in coalbed methane extraction and greenhouse gas storage. In this study, we successfully conducted porosity and permeability experiments under different depth-dependent hydrostatic stress conditions using coal samples collected from the same coal seam group but at five different depths in the Pingdingshan mining area. The porosity, permeability, stress sensitivity and porosity sensitivity of the coal at different depths were analyzed, and the results showed that the coal porosity at different depths (300–1050 m) ranged from 0.15 to 0.50% and that the coal permeability ranged from 5 × 10⁻⁴ to 3 × 10⁻³ mD. The porosities and permeabilities of the coal samples collected from different depths decreased according to negative exponential functions as the effective stress increased. The porosity, permeability, porosity sensitivity and stress sensitivity of the coal mass at depths from 300 to 1050 m initially increased with depth and then decreased at depths greater than 600–700 m; this behavior is different from the conventional understanding in which the porosity and permeability of a coal mass decrease continuously with the depth. The stress sensitivity of the coal is also stress-dependent and clearly increases when the effective stress in the coal mass is less than 10 MPa. The porosity and stress sensitivities of the coal are directly associated with the initial porosity and physical properties of the coal. The depth effect on the porosity and stress sensitivities of the coal permeability is closely related to specific coal fracture compression properties at different depths. A coal mass with a high porosity and a correspondingly low density typically has high porosity and stress sensitivities, as supported by the experimental results and theoretical analyses.