A novel active prevention technology for borehole instability under the influence of mining activities

Coal bed methane (CBM) drainage is an efficient method used to control gas hazards. Nevertheless, borehole instability is prone to occur under repeated disturbances by external stress from mining activities. Consequently, the collapsed coal blocks the gas-flow channel to further induce several gas drainage problems, including low gas concentration and flow, poor drainage result, etc. Due to these problems, screen pipes were installed in this work to provide active support in order to prevent borehole instability. Initially, the effect of mining activities on borehole stability was analysed. Subsequently, we explored the instability characteristics and corresponding acoustic emission (AE) properties of a non-supported borehole and a borehole stabilized by internal active support. These boreholes were drilled in the lab sample instead of the real coal sample. Moreover, the abilities of the holes to resist disturbances due to external stresses were compared. When the stress reached 10 MPa, the non-supported borehole was completely destroyed, whereas the actively supported borehole only slightly deformed. As the stress continuously increased to 14 MPa, a deformation of 15 mm was detected along the radial direction of the actively supported borehole. Additionally, an analysis of the dynamic changes in the strain on the screen pipe surface, the AE time-sequence parameter and location information indicated that the maximum AE event count and energy dissipation rate of the supported borehole at 14 MPa were 44% and 23% lower, respectively, than the corresponding values for the non-supported borehole at 10 MPa. Thus, the support increases the ability of the borehole to resist disturbance by at least 40%. Furthermore, the locating points and energy rate appeared to temporally cluster at coal-yielding or failure stage. Hence, the supporting effect of the screen pipe on the coal around boreholes at the crushing stage is significant. The internal support to the borehole provides a positive confining pressure for the coal in the fracture zone to improve the coal's residual strength. Depending on the support from the screen pipe, the coal can mesh to remain in place and thus prevent collapse, which improves the stability of the borehole.