Failure Mechanism of High-Rank Coal with Borehole under a Bidirectional Stress State

Surface well extraction of coalbed methane (CBM) in high-rank coal reservoirs frequently encounters wellbore collapse due to stress redistribution associated with protective layer mining. Therefore, it is critical to investigate the mechanical failure mechanisms of high-rank coal with a borehole. In this study, the Coal Seam 16# in the Sijichun Coal Mine, Zhijin County, Guizhou Province, was taken as the research area. Through biaxial compression tests with digital image correlation (DIC) real-time deformation monitoring, the failure mechanisms under different bidirectional stress states were analyzed. The key findings indicate that (1) the compressive strength of high-rank coal specimens with boreholes demonstrated a positive correlation with lateral stress, exhibiting value ranges of 22.39–35.81 MPa, 25.64–34.81 MPa, and 26.07–44.58 MPa corresponding to borehole diameters of 6 mm, 8 mm, and 10 mm, respectively, while exhibiting a characteristically high strain energy storage capacity, with an average elastic energy percentage (PEE) of 90.7%; (2) the specimens exhibited a brittle symmetric failure mode characterized by the development of both tensile and shear cracks, with the dominant fracture mechanism showing a clear dependence on lateral stress─tensile cracking prevailed below 7.5 MPa, while shear-dominated failure occurred above this threshold; (3) the points of maximum tangential stress on the wellbore wall, located at circumferential angles θ = 0° and θ = 180°, serve as the initiation sites for wellbore instability. Furthermore, the magnitude of the wellbore stress demonstrates a positive correlation with increasing lateral stress coefficient. These results are of guiding significance to the optimization of CBM drainage engineering in Guizhou high-rank coal seams or similar reservoirs.