Study on permeability model under the influence of gas pressure and stress on fracture

The permeability evolution of coal seam plays an important role in the study of mine gas drainage, coalbed methane extraction and drilling optimization layout. In order to study the influence of gas-fracture and stress-fracture coupling on the evolution model of coal and rock permeability, this paper discussed the influence of gas and stress on coal fracture deformation and permeability based on the perspective of strain.The theoretical models of coal permeability based on gas-fracture and stress-fracture coupling were established. Combined with the previous experimental data, the coal permeability models based on gas-fracture and stress-fracture coupling were compared and verified. The results of the study are as follows: ①The structural unit of coal body is simplified as a cube model, and the influence of fracture deformation caused by gas pressure and coal matrix adsorption deformation on coal permeability is analyzed. Based on the relationship between coal fracture opening and permeability, the permeability model of coal under gas-fracture coupling is deduced. ②The influence of lateral stress on fracture deformation is similar to the internal expansion deformation caused by coal adsorption, both of which affect the deformation of coal cracks by changing the coal skeleton expansion into the fracture; the experimental data verify that the influence mechanism of lateral stress and normal stress on coal permeability is the same, and the coal permeability model under three-dimensional stress-fracture coupling is constructed. ③Combined with the previous experimental data, the global optimization of nonlinear fitting is performed. Compared with the model based on effective stress, the model constructed in this paper is in good agreement with the experimental data, which verifies the reliability of the model established in this paper. It is found that the sensitivity of fractures to normal stress is much greater than that of lateral stress.