A new method to determine the depth of the de-stressed gas-emitting zone in the underburden of a longwall coal mine
Underground coal mining induces de-stressing and fracturing of strata above and below the targeted seams. This creates a gas-emission zone, which contains gas-bearing coal seams and strata in the roof and floor of the mined (working) seam. In longwall mining, most of the gas released from the emission zone escapes into the coal face and the goaf (caved-in area) behind the coal face, where it presents a safety issue. Depending on the extent and shape of the emission zone, various gas drainage strategies could be applied to maximise capture of the gas from the emitting seams. We developed and trialled a new method to identify the gas emission zone in the underburden of an underground mine in the Sydney Basin, Australia. In this operation, all coal seams are located below the major targeted seam for mining. By measuring the isotopic and molecular composition of gas desorbed from coal cores from exploration drilling and gas collected from the goaf, we identified the source of gas and quantified the limit of the emission zone in the underburden of the working coal seam. This has allowed drainage to be focused and limited to the required depth. Our study will assist others to plan the required depth of gas drainage drilling below the floor of mined seams.