Investigation on the breakage and seepage characteristics of layered coal with high temperatures impacted by water jets

The application of water jets in coalbed methane development is manifested by pressurizing and gathering energy from the liquid and then spraying it from the nozzle to achieve cutting grooves and drilling in the reservoir to enhance reservoir permeability. The breakage and permeability transformation of high-temperature layered coal using water jets needs to be carefully studied as it directly affects the commercial development of deep coalbed methane (CBM) resources using water jet technology. In this study, experiments on coal breakage by water jet were conducted using a special apparatus under controlled conditions (temperature: 30/75 °C; jet pressure: 20 MPa; bedding angle: 0–90°) to quantitatively analyze the macroscopic fragmentation characteristics and damage mechanisms of high-temperature layered coal. The results demonstrated that the fragmentation volume exhibited a clear peak value of 31.3 or 33 cm³ at a bedding angle of 45°. High-temperature coal undergoes thermal damage to connected bedding planes due to matrix shrinkage, and the thermal fracture between bedding planes converges and connects under water jets. Therefore, many complex gas permeation channels are formed, making them less susceptible to the influence of principal stress in a single direction when the angle of jet impact on coal bedding at around 45° or 60°, which helps to form a long-term stable coalbed methane seepage channel and achieve CBM production rising.