Due to the explosibility of methane gas and its abundant presence in coal seams, underground coal mines deal with the hazards associated with seam gas by implementing gas drainage. The aim of this thesis is to present an analysis of pre-drainage, its design, implementation and effective drainage, focussing on the potential of improving this type of gas drainage technology as applied at Moranbah North Coal (MNC) mine.
Despite in-seam drilling for years being the only trialled and proven method of gas drainage, surface to in-seam methods provide exciting new technologies that involve drilling from the surface, and focus on pumping water from the seam to allow for maximum gas extraction. Of the methods available, Medium Radius Drilling (MRD) is used in a number of non-commercial gas producing underground coal mines, due to its extensive gas and water drainage zone (boreholes up to 2km in length), and the flexibility of uninhibited lead time for drainage.
The gas drainage program at MNC currently operates both MRD and in-seam drilling techniques. To date, the MRD wells have produced a lot of water and little gas, contrary to the in-seam wells that have produced significant amounts of gas. Geological reporting shows no anomalies, and questions have been raised as to the effectiveness and efficiency of drainage of the MRD wells.
Results seen from past and current gas drainage wells indicate that the Goonyella Middle Seam mined at MNC, having such high permeability will persist in rejuvenating its water table. Without a buffer, the MRD wells, orientated near parallel to the main cleat direction, cannot dewater a large enough region to allow the gas pressure to overcome the total water head. It is recommended in this thesis that boreholes are used together to flank the gateroads, such that one MRD well is ‘sacrificial’ performing mainly water drainage, thus allowing the other to produce gas from the locally and effectively dewatered region. Alternatively, by increasing the effective permeability of the well (eliminating skin and orientating the wells perpendicular to the main cleat), the dewatering capability may be sufficiently increased such that it can overcome the effect of recharge.