Mechanism and Dominant Controlling Factors of Borehole Failure in Soft Coal Seams

Borehole failure can cause difficulties in borehole sealing and restrict the methane extraction efficiency, thereby directly affecting the development effect of coal-bed methane. For the phenomenon of borehole spallation failure in soft coal seams, considering the influences of three dominant controlling factors of stress, methane, and coal strength on the formation of spallation, the multifield coupling model of stress, methane flow, and strength field is established. The mechanical condition and mechanism of borehole spallation failure are proposed. Results show that borehole spallation failure is the combined effect of a series of physical processes, including stress transfer, methane diffusion-seepage response, stress-relieved methane flow, and coal strength evolution. Under the benchmark working condition of this study, the plastic damage within the stress concentration zone significantly enhanced methane seepage and diffusion capabilities, with an increase of more than 3 orders of magnitude. Formation of a high methane pressure gradient within 0.12 m of the borehole wall is induced by significant differences in the methane flow rate among various stress zones. The tensile strength of coal around the borehole is reduced by plastic damage by about 14.86%. Eventually, the differential of methane pressure around the borehole can overcome tensile strength, which leads to the spallation structure being formed by the tensile damage of coal. The stress dominates plastic damage during the borehole spallation failure. Methane, as the driving force for spallation, dominates the tensile damage. Coal strength can resist the formation of spallation. This study reveals the borehole failure mechanism in soft coal seams and provides theoretical support for quantitatively regulating the negative pressure of methane extraction to improve its efficiency.