Experimental analysis of coal permeability evolution under cyclic loading LIU Yongqian

The pore structure and the permeability of coal matrix can be changed complexly under the multi-stage load-ing. However,the evolution of coal permeability and its stress sensitivity are determined by the different loading / unloa-ding rates and the cyclic cycles. It is of theoretical significance for interpreting the anisotropic characteristics of coal seam permeability under complex stress field by researching the evolution law of coal permeability under cyclic loading. With the aid of the stress sensitive model of coal seam permeability,the key characterization parameters and their functional relationships affecting permeability development were proposed in this study. As for the purpose of verifying the control effect of the key parameters on coal permeability,a triaxial cyclic variable-load gas seepage experiment was carried out by preset axial pressure,gas pressure and loading-unloading confining pressure,and the evolution laws of coal permeability and coal volume strain under different confining pressures (2. 0-12. 0 MPa) were analyzed. As for the influence of pore structure on permeability,the comparison of pore structure change before and after cyclic loading and unloading was completed by the low temperature nitrogen adsorption experiment and the fluorescence microscope observation. The results showed that the change trend of permeability was negatively correlated with the confining pres-sure in the process of loading / unloading of coal and it was divided into three stages for the whole process:linear stage, exponential stage and stable stage. With the increase of cyclic loading times,the volumetric strain of coal increased gradually,but the permeability decreased. Under the same experimental conditions,the permeability of coal increased with the increase of volumetric strain,increasing by more than 16. 79% ,while the permeability recovery rate decreased gradually,and was negatively correlated with the confining pressure. After three cyclic loading and unloading experi-ments,the pore structure of coal matrix was changed significantly,with the micro-pore volume increased by 71. 79% , the specific surface area increased by 52. 19% ,and the average pore size decreased by 32. 06% ,but the most proba-ble aperture of coal matrix was not affected by the cyclic stress. The data of pore structure change showed that the in-crease of micro pore volume was one of the key indicators of coal permeability deterioration. Comparing the experimen-tal data of pore structure before and after cyclic loading,it was found that the change degree of pore structure affected gas adsorption-desorption,determined the area of “hysteresis loop”,and the key to determine the shape of “hysteresis loop” was the sudden change pressure controlled by the most probable pore size of coal. In addition,the strain of coal body included the change of fracture volume and pore volume,the important index of fracture influence ( χ) showed the weight relationship of the change of fracture volume in the strain of coal body,and decreased with the increase of confining pressure.