Evaluation of the gas drainage efficiency at the grasstree mine

Abstract

Inappropriate coal mine gas management has the potential to cause catastrophic disastersresulting in multiple fatalities. Gas is naturally released into the mining work environmentduring production and development; pre- and post-drainage form the basis of gas management.The Grasstree Mine site located in the Bowen Basin is an underground coal mine that utilisesboth pre- and post-drainage techniques. Unique to Grasstree is the high gas content of themining and adjacent seams. This project studies the methane capture efficiency of the GC906longwall panel against critical events that affect the gas management and ventilation of themine. The aim is to identify the most effective gas management techniques for future longwallblocks with reduced ventilation at the Grasstree Mine.The data utilised in this project was post-drainage flows and concentrations provided frommeasurements and recordings taken for the full duration of GC906 mining. Methane captureefficiency was calculated for each hour for the duration of GC906 longwall mining;18/11/2016-21/06/2017. The critical events analysed were: • Initial longwall square-up period; • Transitioning from two to three return gateroads; • Full duration of a HGH well transitioning from long to short goaf gas drainage; • The restriction of a return roadway to improve goaf dynamics; and • Analysis of the effects of vertical post-drainage holes over the maingate side on the overall gas drainage.Analysis of the longwall square-up period provided the insight that the methane captureefficiency was unstable and averaged 71% (average for the GC906 duration was 90%). Thislow efficiency will strain a reduced ventilation system due to gas dilution requirements.Improvements to the UIS goaf drainage HGH methods are required. Although, there were twoin-seam post–drainage wells (9CT ad 17CT) for the GC906 panel. This will allow more gasextraction during maintenance periods of other post-drainage techniques.Extending the scope of this project to include capture efficiency of other major mining gasseswould be beneficial for understanding how gas management differs with gas compositions.Determining the pre-drainage extraction efficiency will determine where improvements can bemade prior to mining the longwall panel. This will affect post–drainage gas management.