A new method to determine the depth of the de-stressed gas-emitting zone in the underburden of a longwall coal mine

• A method of identifying of sources of gas emitted into goaf and workings areas in underground coal mining is proposed.
• In this method molecular and carbon isotopic composition of gas trapped in coal are used as identifiers of the coal seams.
• The method was applied to a coal mine and results show that the identifiers could identify emitting seams.
• The method, at its current development, can be applied to mines where no obvious pre-mining communications exist between coal seams.

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.

Mining produces de-stressed (gas emitting) zones above and below the mining horizon, leading to gas emissions from satellite coal seams and gas-bearing strata into the working face and goaf areas. A method of determining the extent of the de-stressed zone below the working seam is presented.Figure optionsDownload as PowerPoint slide