Law and Application of Permeability Improvement in Low Permeability Coal Seam due to CO₂ Phase Transition Cracking

In the process of M9 coal seam mining in Jiaozishan Coal Mine, the high gas content, high pressure, low extraction efficiency and long gas treatment cycle have been put forward, the permeability and gas extraction efficiency of coal seam can be improved by applying CO₂ phase transition cracking technology with the dual effect of "increasing penetration" and "displacement". The CO₂ phase transition cracking test was carried out in 9807 transport lane of Jiaozishan Coal Mine, and the effect of gas extraction with different cracking mode and different way of distribution hole was emphatically investigated. The results show that compared with the pure flow rate of gas extraction, the cracking hole is 2.0 times to 2.8 times higher than that of the normal one, the gas extraction concentration is increased by 1.7 times to 2.4 times. Under the similar conditions of coal seam occurrence and drilling conditions, the drilling of delayed (two) cracking mode is beneficial to crack extension and increase the crack range; when the auxiliary holes are constructed on both sides of the cracking hole, it provides a free surface for high pressure CO₂ gas, reduces its propagation resistance to the periphery, and can increase the influence range of the cracking hole, thus improving gas extraction efficiency.