Effective quantification and management of risk associated with sudden gas release during mining (outburst) is reliant on coal and gas properties measurement bases being representative of the local geological and operational conditions. Contemporary gas emission calculation techniques often inappropriately generalise, or neglect, known site-specific extraction geometry, geological conditions, or heterogeneity and anisotropy within the working seam. Over 5000 coal core gas sample results, obtained from two Southern Sydney Basin Bulli Seam underground coal mines, plus coal samples collected from multiple locations at one studied mine, have been analysed in various geospatial and mining process-based context. Analysis has focused on the alignment of spatiotemporal relationships between; in-situ coal and gas reservoir character, stress regimes influenced by stratigraphic features and geological structure, and gas emission response to mining extraction processes. Sampling and experimental processes have been developed to facilitate the alignment of experimentally determined coal and gas characteristic properties and their response to applied stress, with datasets typically collected as part of mine gas and outburst risk management practice. Results demonstrate the high degree of intra-seam variability and anisotropy possible in Bulli Seam coal within relatively small lateral and vertical extents. Generalised relationships between increased applied confining stress and reduced gas permeability were found to be consistent with the literature. However, in specific cores featuring bright horizontal bands, bedding plies, or other visible cleat fractures, the permeability response to increased stress load was found to vary up to two orders of magnitude greater than those cores without such features. Furthermore, gas emission data from the assembled coal gas core results database, combined with density-based CT analysis and other characterisation of spatially referenced coal samples, confirmed laterally extensive uniform floor-to-roof intra-seam coal character and gas emission response patterns. These patterns, consistent with geochronological deposition sequence, may be used to reduce the apparent variability in observed gas emission behaviour. Results demonstrate the criticality of retaining both spatial and intra-seam context in the measurement of gas sorption capacity, content and emission behaviour. Measurement should therefore be undertaken at a resolution appropriate to the local in-situ conditions and degree of intra-seam vertical heterogeneity. This additional context may facilitate ongoing spatiotemporal alignment of observed gas emission response and hence improvement to outburst risk management processes as higher resolution information becomes available with mining advance.
Patrick Booth, Jan Nemcik, Ting Ren, The influence of intra-seam coal character variation on outburst risk potential, Proceedings of the 2020 Coal Operators' Conference, University of Wollongong - Mining Engineering, 12-14 February 2020, University of Wollongong, 242-263.