Study on the permeability characteristics of coal containing coalbed methane under different loading paths

During the degassing process of coalbed methane, the external loads could change the permeability of the coal seam by means of transient and permanent deformations, which dramatically influence the production efficiency of coalbed gas. Due to the ignorance of irreversible deformation of coal permeability, the gas extraction efficiency and the prediction of gas disaster in underlying minimum need to be improved. Thus, it is better to clarify the permeability change characteristics with irreversible deformation of coal. In this paper, the influence of different loading paths on the permeability of gas-containing coal was studied, and six loading paths were studied by employing a self-designed triaxial cyclic loading system. The results of this study show that the permeability of gas-containing coal samples would significantly change with different loading paths. Corresponding to the variation of mining pressure or periodic geo-stress varying in the excavation of roadways, both an increase in the axial pressure and an increase in the confining pressure would cause a decrease in gas permeability. In addition, the permeability of gas-containing coal would he damaged even when the external loads are removed. When given a constant strain loading, the permeability shows a "V"-shaped change with increased strain, and the transversal point of permeability locates between the yield point and the peak intensity. Therefore, it can be concluded that external loads less than the maximum strength of coal would lead to permeability loss in the coal seam, vice versa, the permeability will be increased. Hence, research results contributes to improving the gas permeability and increasing the gas production from coalbed by imposing effective loads on original coal seams through mining speed control.