Performance analysis of vertical goaf gas drainage holes using gas indicators in Australian coal mines

This paper focuses on assessing gas drainage performance from vertical surface goaf holes in an operating mine with high specific gas emission rates (~20 m3/t). Field monitoring results of gas flow rate, suction pressure, and gas composition collected from individual gas drainage holes were analysed in detail. The major challenge of implementing the full potential of goaf gas drainage system is the perceived risk of spontaneous combustion in goaf using arbitrary trigger set points. Although the provenance on the introduction and reasons for the use of CO as a trigger of goaf spontaneous combustion is unknown, the paper highlights that the current use of CO based trigger action response plan (TARP) for sponcom management leads to the premature closure of goaf holes, which largely constrains goaf hole gas drainage performance and has a significant bearing on longwall gas management. The CO concentration in gas captured by goaf holes has been found to be positively correlated with O2 concentration and negatively correlated with CH4. The start-up period, completion period, slow retreat and stoppage of LW faces are the main causes for the increase of oxygen ingress and associated high CO levels. Building upon abundant field measurement data, goaf gas profiles for CO, O2, CH4 and CO2 concentration were established, which suggest that the increase trend of CO level behind the face is a normal behaviour of goaf closure and would recover to trivial concentration after 300 m behind the face. This also helps define the ‘inert zone’ and ‘active zone’ in the goaf, which has distinct goaf gas composition and requires different gas drainage strategies. A dynamic goaf hole operation strategy based on real-time gas monitoring and feedback control of valves would maximise gas drainage performance whilst maintaining the sponcom and gas explosion risk at the minimum. •Field monitoring results of gas production from 263 goaf holes were analysed.•Current CO triggers for goaf gas drainage TARPs were found to be flawed and need revision.•Main contributors for the rising of CO levels have been identified.•Goaf gas profiles for CO, CH4, O2 and CO2 have been proposed to guide drainage practices.•A dynamic goaf hole operation strategy to optimise drainage performance at lowest risk has been proposed.