Research and application on technology of increased permeability by liquid CO2 phase change directional jet fracturing in low-permeability coal seam

How to achieve an efficient extraction of deep coal seam gas has been a main challenge facing China’s en- ergy security while the technology to increase the permeability of coal bed gas has been the focus of recent studies. In order to solve the technical problem of high efficiency gas drainage in deep low permeability coal seam,in this study, the method of advantageous direction of jet fracturing under the condition of in-suit stress are put forward based on the- oretical analysis. A novel technology for cracking and increasing permeability in low permeability coal seam namely LCO2 phase change directional jet fracturing has been proposed and experimented. And a mesh drilling coal seam gas drainage method is proposed based on LCO2 phase change directional jet fracturing technology. The results show that the fracture pressure of the borehole wall is affected by the azimuth,inclination of borehole and has obvious directionality. The advantageous direction interval of the LCO2 phase change directional jet fracturing is determined in the test area. LCO2 phase change directional jet fracturing technology can effectively increase the total porosity,pore size,sur- face area,the ratio of visible pores of coal sample,improve the pore structure and seepage capacity within the coal ma- trix to effectively increase the pure flow rate of gas drainage in coal seams by 9 to 12 times,which reduces the flow at- tenuation coefficient of gas drainage in coal seam by 92% . The variation law of on-site gas drainage parameters and the results of PFC2D particle flow program show that the technology’s influence radius is 9-13 m. The mesh drilling coal seam gas drainage method of deep and low-permeability coal seam is based on LCO2 phase change directional jet frac- turing technology,which can effectively prevent gas over alarm value during the roadway drivage with low permeability and high outburst coal seam,and improve drivage speed by 4-5 times.