A phase transition-induced fracturing permeability enhancement technology using carbon dioxide capsules for low-permeability coal seams and its application

Objective and Methods To address engineering challenges like high gas content, high pressure, and low drainage efficiency in the mining of low-permeability coal seams, this study developed a phase transition-induced fracturing and permeability enhancement technology using carbon dioxide (CO2) capsules for low-permeability coal seams. This technology leverages the feature that CO2 transitions into the supercritical state under high pressure. A capsule encapsulating critical CO2 is placed into a conduit, which is then placed within a coal seam. Subsequently, phase transition-induced blasting is triggered using a heating blasting start device. Consequently, liquid CO2 vaporizes rapidly and expands, generating high-pressure shock waves, which induce pre-fracturing of the coal seam. The fracturing process, producing no sparks, is prone to form fractures in coal seams, thus enhancing coal seam permeability. Results and Conclusions The results indicate that the technology developed in this study can effectively increase the quantity of fracture networks in coal seams, thereby significantly enhancing the coal seam permeability. Calculations reveal that during phase transition-induced fracturing, each capsule containing 1.4 kg of liquid CO2 can release explosive power equivalent to 243 g of trinitrotoluene (TNT). After secondary fracturing, the gas drainage influence radius gradually expands, indicating high timeliness. The field application effects demonstrate that this technology increased the pure gas drainage by 2 to 4 times, the volume fraction of methane by 2 to 3 times, and the gas drainage influence radius by 1.5 to 2.0 times. The field monitoring of the gas drainage effects of boreholes at 18, 30, 60, 90, and 120 d reveals that the gas drainage influence radius increased with time. Specifically, the influence radius increased rapidly in the initial stage, followed by a gradual decrease in the increasing amplitude. With time, the increasing amplitude of the gas drainage influence radius tended to level off, presenting a gradually stabilizing relationship with the drainage time. The effective drainage radius after fracturing measured from 2.45 m to 4.95 m. All these demonstrate that the fracturing technology developed in this study enjoys high timeliness and sustainability in gas drainage from coal seams. After fracturing, the permeability coefficient of the coal seam in the study area increased significantly from 0.009 6 m2/(MPa2·d) to 0.577 m2/(MPa2·d). Additionally, since CO2, as the medium inducing fracturing, can be sourced from industrial emissions or stored CO2, the fracturing technology proposed in this study serves as a green permeability enhancement method, enjoying significant environmental and social benefits.