CO2 Phase-Change Fracturing for Own-Seam Permeability Enhancement

Technical Context and Significance

Own-seam gas drainage, where boreholes are drilled directly from within the coal seam, is the most direct method for controlling coal and gas outburst hazards. However, in low-permeability coal seams, the gas drainage efficiency is often insufficient to reduce the gas pressure below the critical threshold required for safe mining. The CO2 phase-change fracturing technology has been applied to own-seam boreholes to enhance the permeability of the surrounding coal, creating a network of fractures that facilitate gas flow into the borehole. This approach is particularly valuable in seams where the gas pressure is high but the coal permeability is extremely low, as it can create new flow pathways that are not present in the virgin coal mass. The technical challenge lies in achieving effective fracturing without causing excessive damage to the coal structure that could compromise the stability of the borehole or the surrounding roadway.

Fracturing Process and Parameter Optimization

The CO2 phase-change fracturing process for own-seam boreholes involves the following key steps. First, the borehole is drilled to the designated depth, typically 100 to 200 meters, at an angle of 0 to 15 degrees relative to the seam plane. The borehole is then cleaned and prepared for the insertion of the fracturing device. The device is positioned at the target depth, and the borehole is sealed with a high-strength plug to contain the fracturing energy. The device is charged with liquid CO2 to the target pressure, and the initiation is triggered remotely. The resulting phase-change explosion creates a network of fractures in the surrounding coal, enhancing the gas drainage efficiency.

Material and Fabrication Requirements for Fracturing Devices

The fracturing device used in own-seam applications must be designed to withstand the extreme conditions of the phase-change explosion. The device body is typically manufactured from high-strength steel with a minimum yield strength of 800 MPa, and the wall thickness is designed to withstand the maximum internal pressure with a safety factor of at least 2.0. The welding joints must be full-penetration butt welds, inspected by ultrasonic testing, and the weld metal must have a Charpy impact energy of at least 27 joules at the service temperature. The device should be hydrostatically tested at 1.5 times the maximum working pressure, with a hold time of 30 minutes, and should pass without any visible deformation or leakage. The quality documentation for each device should include the material certificate, welding procedure qualification, welder qualification, and non-destructive examination report.

Effectiveness Evaluation and Engineering Considerations

The effectiveness of CO2 phase-change fracturing in own-seam boreholes is evaluated by comparing the gas drainage volume, gas concentration, and pressure decline rate before and after fracturing. Typical results show an increase in gas drainage volume of 40 to 70 percent in the first week after fracturing, with the enhancement effect gradually declining over 2 to 4 weeks. The pressure decline rate in the borehole is also accelerated, indicating that the fracture network has created effective flow pathways for the gas. However, the long-term effectiveness is limited by the gradual sealing of the fractures by coal fines and water. The use of proppants to keep the fractures open is a potential solution, but the injection of proppants into the fracture network is technically challenging and requires further research.

Study Insights and Future Directions

The application of CO2 phase-change fracturing in own-seam boreholes is a promising technology for improving gas drainage in low-permeability coal seams. The key to successful implementation lies in the careful selection and optimization of the fracturing parameters, the rigorous quality control of the fracturing device, and the effective monitoring of the gas drainage performance. Future research should focus on developing more reliable and safer initiation systems, optimizing the fracture network through numerical simulation and field validation, and extending the effective drainage period through the use of proppants or other techniques to keep the fractures open. The integration of CO2 phase-change fracturing with other gas drainage technologies, such as pre-splitting and hydraulic fracturing, may also offer synergistic benefits that warrant further investigation.