THE IMPACT OF COAL PROPERTIES ON SEAM GAS DRAINAGE IN UNDERGROUND COAL MINES

The problem of outbursts and high gas emissions in underground coal mines must be controlled ahead of mining operations in line with established safety requirements. Gas content threshold values for safe mining of a coal seam are applied in Australia across the underground coal mining industry. Access to underground coal reserves is conditional on the reduction of coal gas content to below the regulatory outburst threshold limit. These values are achieved by pre-drainage of the seam using underground drainage systems. Several coal mines in the Sydney Basin (NSW, Australia) have recently encountered areas in which drainage of gas has proved to be very difficult. In order to determine the nature and origin of the low drainability areas within the coal seam, a petrological study has been undertaken as a part of a research project funded by the Australian Coal Association Research Program (ACARP C10011).

Several differences in optical properties have been identified between the coal of normal and difficult drainage areas. These differences are related to the presence of some specific micro-markers such as: mylonitization of the coal; development of micro-cleat and its mineralisation; the mode of occurrence of minerals in coal macerals, the presence of oil and solid bitumen in coal macerals; the presence of heat-affected coals and the presence of pyrolytic carbon. The extent and the rate of gas flow through the coal will be influenced by coal microstructures, especially the micro-cleat openings and infillings. Microscopic examination of coal samples from difficult drainage areas has shown the presence of mylonite (brecciated coal) in micro-cleats. As revealed by electron microprobe analysis the mylonite in micro-cleats is cemented by calcite, dolomite or kaolinite. In good drainage coal, the micro-cleats are mostly empty, or only partly mineralized. The mineralisation in micro-cleats and the presence of mylonite probably play a major role blocking or slowing down seam-gas drainage. The micro-cleat system can be a source of anisotropic permeability, as micro-cleat development and its mineralisation in some samples seem to follow preferential directions.