Analysis of longwall goaf gas drainage trials with surface directional boreholes

Surface directional boreholes (SDB) were recently trialled at two Australian coal mines as a new application to longwall goaf gas drainage. This method has become increasingly attractive for use at coal mines where surface access to drill large numbers of vertical goaf wells are limited or restricted. To demonstrate the performance and effectiveness of the SDB and help improve future design and implementation, results of the field trials at the two Australia coal mines are analysed and presented in this paper. The SDBs, drilled with medium-radius-drilling techniques, had different configuration parameters, including number of branches, diameter, and relative locations above the mining seam and from the tailgate. No borehole stability issues were encountered during the gas drainage operation. The drainage methane flow rate varied in all SDBs; however, if applied properly, they can gain a consistent high flow rate of methane at about 1000 L/s. It was observed that the drainage effectiveness and efficiency were significantly influenced by local geological settings, borehole horizontal and vertical placement, branching, and water filling. The key to having high and consistent gas flow in a SDB is to maintain a sufficient connection with the mining-induced fracture zone. In addition, drilling quality control for such boreholes is also crucial, to avoid any concave sections that might fill with groundwater and block the borehole. The trial results demonstrate that SDBs have the potential to become a common method of longwall goaf gas drainage.