Permeability Increasing Technology by Using Liquid CO₂ Phase Transition Multi-Discharge Crack in Soft Thick Coal Seam

In view of the problem in soft thick coal seam with low permeability and high gas content outburst, such as the gas extraction efficiency is low, the gas concentration of extraction borehole decreases quickly, by using the optimized liquid CO₂ phase transition fracturing system, the multi-point fracturing test of perforating borehole (floor) in 4306 mining area of Changping Coal Mine was carried out. The experimental results show that the effective influence radius of multi-discharge crack of CO₂ phase transition is about 12.5 m, and it is independent of the number of cracking points; the gas concentration increases linearly with the number of cracking points; the concentration of gas extraction is increased by CO₂ phase transition cracking, and decreases with time going on. Therefore, it is an alternative to maintain or improve the gas extraction efficiency of coal seam to crack the coal seam at different locations and different times.