Enhancing coal seam gas using liquid CO2 phase-transition blasting with cross-measure borehole

The efficiency of gas drainage in deep coal seams is generally poor because of their typically low-gas permeability and high-geostress characteristics. Therefore, we conducted a test study on the permeability enhancement of cross-measure boreholes using liquid CO2 phase-transition blasting (LCPTB) to identify an effective method for enhancing coalbed methane reservoir. Through a numerical simulation of LCPTB, the fracture propagation in the coal seam after blasting was analysed. Subsequently, a field test arrangement for boreholes of LCPTB was designed, and the range of enhanced permeability after blasting as well as the efficiency of gas drainage were investigated. The results indicate a significant increase in the permeability of the coal seam and the efficiency of gas drainage following LCPTB. The amount of gas extracted from the blast holes was 1.8-8 times greater than that extracted from boreholes without LCPTB. The practical spacing between boreholes was deemed to be 2.5-3 m. In addition, with decreasing measuring distance from the blast hole, the efficiency of LCPTB was improved by increasing the permeability of the coal mass surrounding the observation hole. The observation holes arranged around the blasting holes could increase the efficiency of gas drainage. In summary, this technology uses the energy released by LCPTB to increase the permeability of the coal surrounding the blast hole, and to displace methane in the coal seam, thereby improving the efficiency of gas drainage and providing economic and environmental benefits.