Experimental Study on Damage Pattern and Fracture Mechanism of Coal by Radial Jet Drilling Under Triaxial Stress Conditions

The exploration of coal bed methane (CBM) plays a pivotal role in mitigating the greenhouse effect and promoting clean energy supply on a global scale. The Radial Jet Drilling (RJD) technology can create several drainage holes to improve the recovery of CBM by creating fracture network in coal seam. However, the damage pattern and fracture mechanism of coal between radial branch boreholes (RBBs) remain unclear. In this study, we performed RJD experiments under triaxial stress using raw coal specimens. Our findings reveal that the triaxial stress state significantly inhibits coal damage induced by RJD. Specifically, as the stress differential increases, the damage effect becomes more pronounced, accompanied by crack coalescence between RBBs. The fundamental principle underlying RJD’s ability to enhance coal seam permeability lies in the initial deformation of RBBs under stress difference, followed by the interconnection of cracks induced by this deformation. Notably, the deformation behavior of RBBs in coal differs markedly from that observed in other geological materials. While boreholes in sandstone, shale, and similar materials exhibit deformation tips pointing toward the σ₃ direction, RBB deformation in coal aligns with the σ₁–σ₂ plane. This observation underscores the crack-inducing capacity of RBBs, allowing them to overcome the constraints imposed by in-situ stress within coal seams. The results presented herein serve as an experimental reference for designing RJD applications in coal seams.