Experimental study on evolutionary characteristics of pore-fissure structure in coal seam under temperature impact

In order to study the change of microstructure of coal under temperature impact,the temperature impact ex- periment of coal was carried out by using high and low temperature test system. Scanning electron microscopy (SEM), industrial micro-CT,mercury injection experiments and cryogenic liquid nitrogen adsorption experiments were used to characterize the evolution of the pore fracture structure of coal before and after temperature shock. Based on digital im- age processing technology,the SEM images of coal reservoirs were binarized before and after temperature impact,and the fissure width variation of coal reservoirs was qualitatively and quantitatively analyzed. Also the specific surface area and pore size of pores in coal reservoirs were statistically analyzed before and after temperature shock. The results show that the temperature impact causes the large pores in the coal sample to penetrate each other and form macroscopic cracks,resulting in a corresponding decrease in the volume of macropores and an increase in the volume of mesopores and small pores. The maximum thermal stress generated during the temperature impact test is located in the tangential direction of the surface of the coal sample. The thermal stress generated by the temperature shock exceeds the tensile strength of the coal sample,which is the direct cause of crack initiation,expansion and mutual penetration. The re- search results can provide technical support for the efficient development of coalbed methane and the improvement of gas drainage rate in coal seams.