Modification characteristics of supercritical carbon dioxide temperature and action time on coal mesostructure

In using supercritical carbon dioxide to intensify the mining of coalbed methane, the structural characteristics of coal will change with the injection of supercritical carbon dioxide. It is particularly necessary to systematically study the modification characteristics of coal mesostructure by supercritical carbon dioxide of different conditions. Therefore, this study designs the experiments of soaking coal with supercritical carbon dioxide at different times and temperatures. The coal before and after the supercritical carbon dioxide effect is tested by ultrasonic detection and computed tomography scanning. The results show that the action of supercritical carbon dioxide causes the generation of new fractures in the coal body, extension of primary fractures, dissolution of hard minerals, and corrosion holes left after the organic matter extraction. Coal porosity gradually increases with the extension of soaking time at a constant pressure of 8 MPa and temperature of 40 °C. Generally, the development of pores and fractures is concentrated in the first 3 days of supercritical carbon dioxide soaking treatment. Then, the change in pores and fractures tends to be complete. However, at a constant pressure of 8 MPa with 3 days of soaking time, the development of pores and fractures in the sample is restrained as the supercritical carbon dioxide temperature increases. Meanwhile, its porosity is substantially higher than that of the original sample. After soaking in supercritical carbon dioxide, the ultrasonic wave velocity of coal considerably decreases, indicating a good corresponding response relationship between wave velocity and coal porosity.