Anisotropy of the effective porosity and stress sensitivity of coal permeability considering natural fractures

Abstract

The distribution of natural fractures in coal seams and their effect on the anisotropy of the stress sensitivity of permeability are key issues that are commonly encountered in the simultaneous extraction of coal and gas. To fully understand the law of anisotropy of the effective porosity and stress sensitivity of coal permeability, a modified permeability model and the corresponding anisotropic evaluation index of the stress sensitivity were established considering the influence of the natural fracture distribution of coal. Based on anisotropic coal samples obtained from the Tashan coal mine, the effective porosity and permeability in three orthogonal directions under different stress conditions were systematically evaluated, and the anisotropic evolution law of the stress sensitivity of the permeability was quantitatively analyzed. The results show that the natural fracture system of coal exhibits complex heterogeneity, and the compressibility of the fractures in different directions is different, resulting in significant anisotropic permeability and stress sensitivity. The effective porosity sensitivity of the coal permeability exhibits a power relation with the effective porosity, and a higher effective porosity corresponds to a more notable anisotropy of the effective porosity sensitivity. Under a low stress, the coal stress sensitivity in the direction perpendicular to the bedding planes is the largest. When the stress increases to 19 MPa, the dominant direction of the stress sensitivity changes, and the direction parallel to the face cleats experiences the maximum stress sensitivity of permeability at a high stress level. The power index of the correlation between the coal permeability and effective porosity in different directions of the coal samples in the Tashan coal mine ranges from 2.42 to 9.18, and a constant value of 3 is not observed. This result can be attributed to the influence of the pore size, fracture width and difference in the pore–fracture distribution.