DEVELOPING A NEW METHOD TO IDENTIFY THE SOURCE OF GAS EMISSIONS INTO LONGWALL AND GOAF FROM SURROUNDING STRATA

During coal mining, strata is fractured and gas trapped in the roof and floor of coal seams travels into the workings. Depending on the extent and shape of fractured zones suitable gas drainage patterns are required to maximise the gas capture from strata but also to minimise the cost of operations. In this paper a new method to identify gas emitting zones/seams in the embedding strata and gas migration pathways is presented. The developed method was used in a coal mine in the Southern Coalfield of the Sydney Basin. Geochemical properties of gas trapped in coal seams above and below the mining horizon were analysed and compared with similar properties of gas collected from goaf areas. This study shows that using this method it is possible to identify the source of gas in goaf areas and thus determine the extent of fracturing in the strata around the mined seam. In this study the properties of Coal Seam Gas (CSG) itself were used to identify the origin of goaf gas. For example, a component of the gas trapped in coal as an identifier of the coal seam containing that gas component can be used . An example is ethane (C2H6), which occurs naturally in coal. It can then be used to identify the emitting seams. However, this gas is only present in deeper seams. For this study a combination of molecular and carbon isotopic compositions of gas initially trapped in coal seams was used to identify the emitting zones/seams. The method makes it possible to identify the extent of the mining-influenced zone by comparing geochemical properties of gas accumulated in the goaf and coal face with gas desorbed from fresh cores from coal seams in the strata (delineation of mining influenced zone).