Theory and technology of goaf gas drainage with large-diameter surface boreholes

Coal mine goaf drainage with surface boreholes is increasingly used in coal mines worldwide.However,poor performance of some of these surface gas boreholes is often experienced in terms of the gas concentration,flowrate and borehole stability.In order to improve performance,theoretical studies and field investigations were carried out to develop optimal surface borehole designs based on the experience from Huainan and Australia in recent years.Through the extensive numerical modelling of mining impact in the surrounding strata,and performance of various gas drainage configurations,and by applying the concept of the 3D annular-shaped overlying zone for optimal methane drainage,the trials using large diameter boreholes with diameters of 244.5 mm and 177.8 mm were designed and implemented.The trial results demonstrate that the large diameter boreholes perform significantly better than conventional boreholes in terms of total gas captured,longwall gas control effectiveness,and borehole stability.The total gas captured from a single large borehole can reach 3.167 million m3 with 1.7 million m3 of pure methane,and the borehole productive life is 106 days.The methane concentration in the longwall return gateroad is effectively controlled during the operation of the large boreholes.The trial results also show that the surface borehole located on the longwall return side performes better than that on the intake side.The total gas captured from the return side borehole is 33.7% more than that from the intake side and the methane concentration is 75.3% higher.