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Gas Drainability and Outburst Risk Assessment based on the Distribution of Micro-Markers in Coal Seams

Underground » Ventilation, Gas Drainage and Monitoring

Published: September 03Project Number: C11057

Get ReportAuthor: Andrew Gurba, Lila Gurba, Colin Ward, Jeff Wood | AGURBA, CCSD, University of New South Wales, BHP Billiton Illawarra Coal

Access to underground coal reserves in Australia and other coal producing countries is often conditional on the reduction of coal gas content to below specified regulatory outburst threshold limits. Reduction of gas content is generally achieved by pre-drainage of the coal seam using underground in-seam drainage. Several coal mines in New South Wales and Queensland are encountering areas in which drainage of gas has proved to be very difficult. In many operations local areas of low drainability are a major obstacle in achieving economic longwall gate road development rates.

A recent study funded by ACARP (project C10011) was undertaken to determine the nature and origin of the very low drainability areas within the Bulli coal seam of the southern Sydney Basin. The purpose of the project was to investigate the coal properties affecting gas drainage, and to develop a simple means of detecting areas of difficult drainage well in advance of mining. Detailed petrological study was undertaken on coal samples selected from the Bulli seam at Tower and Tahmoor Collieries in the Southern Coalfield.

The results of the project have shown that some differences in coal properties are directly related to gas drainage efficiency. These differences are related to the presence of some specific micro-markers identified in the difficult drainage areas, particularly to the development of the micro-cleat system and its mineralization, micro-mylonitisation of the coal, the mode of occurrence of particular minerals within coal macerals, the presence of oil and solid bitumen as fracture infills and in coal macerals, the presence of heat-affected coals and the presence of pyrolytic carbon possibly derived from more distant intrusion effects.

This project was undertaken to test applicability of the micro-markers outside the Southern Coalfield of the Sydney Basin, i.e. in the Hunter Coalfield and the Bowen Basin.

Petrographic studies of samples collected from selected coal mines in those areas have confirmed that it is possible to infer the drainability of a coal seam and its proneness to outburst, based on microscopic examination of coal samples. The results of the tests proved that the micro-markers and their influence on coal drainability are not unique to the Bulli seam in the Southern Coalfield. The universality of the micro-markers makes them a very powerful tool in assessing potential gas drainage conditions for future coal mining operations and/or in new coal extraction areas of existing mines.

Identification of the presence of these micro-markers in coal, well ahead of mining, i.e. in exploration and/or development stages of a coal mining operation, will provide a significant input to the effective planning/design/timing of the underground drainage system for any future underground coal mine operations.

The major benefits of the project are the ability to detect areas with difficult and slow drainage characteristics, and also potential outburst prone areas, in advance of development. Applying appropriate drainage conditions will make coal mining operations safer, less prone to outburst and more economically viable.

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