Experimental Study on the Porosity and Permeability Change of High-Rank Coal under Cyclic Loading and Unloading

It is of great significance to study the evolution law and change characteristics of the porosity and permeability of coal under cyclic disturbance to efficiently exploit coal-bed methane (CBM). In this paper, the GCTS rock mechanics experimental system is applied, the unsteady gas seepage test under cyclic loading and unloading is carried out with high-grade coal samples from Zhijin County, Guizhou Province, as the research object, and the mathematical formula of the dynamic change of porosity during loading and unloading is established. The results show that (1) the porosity and permeability parameters of coal are negatively correlated with confining pressure, and all decrease with the increase in confining pressure; (2) the change rate of the porosity and permeability of bedding coal decreases gradually with the increase in stress, and the porosity and permeability under high stress decrease gradually with the increase in cycle times; (3) during cyclic loading and unloading, the permeability loss (Dk) of coal mainly occurs in the first loading (>90%), and Dk gradually decreases with the increase in cyclic times; (4) under the same test conditions, the evolution law of the porosity and permeability of parallel bedding coal and vertical bedding coal is similar but the stress sensitivity of vertical bedding coal is higher; (5) the circumferential strain of parallel bedding coal is higher than that of vertical bedding coal, and the porosity change and permeability loss show a specific bedding effect; and (6) under the action of cyclic stress, the change of strain–permeability of coal is abnormal after it is destroyed. With the increase in strain before the sample is destroyed, the permeability drops sharply, and after the sample is destroyed, the strain decreases and the permeability never recovers. The research results can provide favorable theoretical guidance and technical support for pressure relief exploitation of coal-bed methane, such as multibranch horizontal wells.