Permeability characteristics of gas-bearing coal specimens under cyclic loading-unloading of confining pressure

The comprehensive supposition of coal and gas outburst confirms the main role of stress,gas and the physical and mechanical properties of coal in the occurrence of coal and gas outburst. The comparative study of permeability re- sponse characteristics of reconstituted coal specimens and raw coal specimens under the influence of mining operation is of great significance to reveal the similarities and differences of micro structure damage evolution of the two coal specimens and to clarify whether the reconstituted coal specimens can replace the raw coal specimens for carrying out relevant experimental study. Some seepage experiments were conducted under a cyclic loading of confining pressure on raw coal specimens and reconstituted coal specimens to investigate the response characteristics of permeability to stress. The self-developed triaxial coal and rock permeability experimental system was used to carry out the test. The results show that the response characteristics of permeability to the stress of the two coal samples are similar in gener- al,but there are differences in details. For both types of coal specimens,the permeability decreases with the increase of confining pressure and increases with the decrease of confining pressure. The permeability of any one loading process is greater than the permeability of unloading process. The permeability of the raw coal specimen changes greatly in low con- fining pressure stage and changes slowly in high confining pressure stage. The permeability of the reconstituted coal spec- imen changes uniformly throughout loading and unloading processes. The permeability damage rates of the raw coal spec- imen and the reconstituted coal specimen decrease with the increasing cyclic times of confining pressure and the increas- ing gas pressure. The fracture compression coefficients of the raw coal specimen and the reconstituted coal specimen de- crease with the increase of cyclic times of confining pressure,and increase with the increase of gas pressure. The permea- bility of raw coal specimen increases with the increase of gas pressure,while that of the reconstituted coal specimen de- creases with the increase of gas pressure. It is feasible to use the reconstituted coal specimen instead of raw coal speci- men to study the seepage characteristics of gas bearing coal,and it has certain advantages in some specific occasions. However,it is also necessary to pay attention to the detail differences of permeability response characteristics of the two kinds of coal specimens,and the impact of these detailed differences on the reliability of test results.