Failure mechanism of methane drainage borehole in soft coal seams: Insights from simulation, theoretical analysis and in-borehole imaging

Methane drainage borehole in soft coal seams is prone to instability. This could seriously compromise methane drainage efficiency, leading to risks of methane hazards at coal mines. In this paper, the failure of methane drainage boreholes in soft coal seams is studied through theoretical analysis, numerical simulation and field monitoring methods, which aims to guide the borehole protection and improve the methane drainage capacity. The functional relationship between effective radial normal stress and effective tangential shear stress at the unstable position of the borehole and the in-situ stress and pore pressure is constructed through the theoretical study. The equation to derive borehole wall collapse pressure is established, considering cohesion, internal friction angle, coal seam porosity, pore pressure, in-situ stress and other factors. The numerical modelling shows that the coal around the borehole undergoes tensile breakage towards the borehole in both single and multiple borehole conditions. The vertical displacement at the borehole top is the largest and the borehole turned into an elliptical shape under compression. Also, it shows the opening and the end of the borehole are at higher failure risks than middle sections. The horizontal and vertical displacements of multiple boreholes are larger than that of a single borehole. While the usage of multiple boreholes improves the methane drainage efficiency, it brings more challenges to the borehole stability. The creep deformation law of borehole is revealed based on new developed in-borehole imaging monitoring system, including four stages: first (transient), second (steady-state), tertiary (accelerating state) and closure. The field measurement shows that the methane drainage efficiency was relatively high in the first two stages, and then the efficiency dropped rapidly when borehole closing. The research results are critical for guiding methane drainage borehole protection, improving the methane drainage efficiency and enhancing safety in production.

Automated summary

This paper investigates the failure of methane drainage boreholes in soft coal seams using theoretical analysis, numerical simulation, and field monitoring methods, aiming to guide borehole protection and improve methane drainage capacity.