Underground » Ventilation, Gas Drainage and Monitoring
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 mines in the Southern Coalfield of New South Wales have encountered 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 study funded by the Australian Coal Association Research Program (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 coal properties affecting gas drainage, and 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 micro-cleat system and its mineralisation, micro-mylonitisation of the coal, the mode of occurrence of particular minerals within coal macerals, the presence of oil and solid bitumen as fracture infill and in coal macerals, the presence of heat-affected coals and the presence of pyrolytic carbon possibly derived from more distant intrusion effect.
The applicability of the micro-markers was also successfully tested in a petrological study of coal samples from an area in Central Colliery (Bowen Basin, Queensland) where features indicating difficult drainage were found near outburst zone. The results of the petrographic studies of these samples confirmed that, based on microscopic examination of coal samples, it is possible to infer the drainability of a coal seam and its proneness to outburst. The results of the test case investigation undertaken on the Central Colliery coal samples proves that the micro-markers and their influence on coal drainability are not unique to the Bulli seam in the Southern Coalfield. The universality of micro-markers makes them a very powerful tool in the assessment of 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 the coal in other areas, well ahead of mining, i.e. in exploration or/and development stages of a coal mining operation, should 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 detection, in advance of development, of areas with difficult and slow drainage characteristics and potential outburst prone areas. Applying appropriate drainage conditions will make coal mining operations safer, less prone to outburst and more economically viable.