Experimental study on relationship between acoustic velocity and porosity of coal samples under tri-axial loading

The porosity of coal is an important indicator of the degree of gas accumulation in reservoir, and it is very important to evaluate CBM resources in the mining area. In order to improve the exploitation and utilization of CBM and better study the porosity of coal reservoirs, raw coal samples of standard coal were used for this research from Guhanshan Mine, the coal samples were processed into three directions parallel to the face cutting, perpendicular to the face cutting and perpendicular to the bedding using the gas geology reservoir fracturing simulation and physical property test system under normal temperature conditions, the ultrasound characteristics and stress-strain tests were performed simultaneously for each group of coal samples, and the measured sonic velocity, porosity, and axial pressure were fitted in pairs and the results were discussed. The results show that under constant confining pressure, the porosity of coal samples decreases linearly with increases of axial pressure, and influence coefficients of porosity pressure in different directions show obvious anisotropy. The acoustic velocity of coal samples increases linearly with the increase of axial pressure, and the ultrasonic wave velocity of coal rocks has a significant bedding effect. With the increase of axial pressure in the loading process, the acoustic velocity and porosity of coal samples satisfy a good linear relationship, which provides a theoretical basis for predicting the porosity of coal seams by using acoustic acoustic velocity.