Study on the Dynamic and Static Fracture Characteristics of Coal Subject to Water Jets under High Temperature and High Geo-Stress Conditions

The extraction of clean deep coalbed methane (CBM) is crucial for sustainable energy development. Water jet technology offers a promising approach for deep CBM extraction, but its efficiency depends on understanding the coal fragmentation mechanism under geo-stress and temperature conditions. This study derives the spatiotemporal evolution equations of stress and displacement fields under jet impact by integrating in situ stress and temperature effects. The rock-breaking mechanism of coal under water-jet-induced stress waves is analyzed, and a failure criterion for coal under jet impact is established. The study clarifies the shear range and tensile fracture zones in coal, revealing that geo-stress and temperature are key factors influencing coal’s stress and deformation responses, leading to changes in tensile and shear failure modes. A fracture pattern and discrimination criterion for coal under the combined effects of static hole pressure, triaxial stress, and temperature are proposed. Additionally, the mechanisms of crack initiation and propagation in fracture pit walls caused by a water jet impact are investigated. The results identify the three-dimensional stress state, ground temperature, jet velocity, and fracture surface distribution as critical factors influencing quasi-static coal fracturing. Experimental findings demonstrate that water jets impact the coal hole wall, inducing fractures on the side of the maximum principal stress in deep coal seams with fracture angles decreasing as temperatures rise.