Control and Management of Outburst Risk

Outburst of coal and gas is a risk that affects many Australian underground coal mines. While the mechanisms that cause an outburst are not fully understood, the factors known to affect outburst risk include seam gas pressure, geology, and stress. Varying combinations of these and other factors may create an outburst risk whereby there is sufficient energy present in the coal to break and expel both coal and gas from the working face.

This report provides details of the geological conditions and mining practices at each of the Australian underground coal mines that have experienced outburst of coal and gas. In most cases, outbursts occurred in geologically disturbed areas in gassy mines with limited or no gas drainage.

The use of gas drainage in Australian underground coal mines has proven to be an effective control in reducing outburst risk. Investigation of recorded outburst event data from Australian underground coal mines reveals that outburst typically occurs in geologically disturbed coal where the gas content is greater than 9.0 m³/t. The investigation also indicates that seam gas composition does not affect outburst propensity i.e. a coal with high CO₂ concentration is not more outburst prone than an equivalent coal with high CH₄ concentration, with the same gas content.

Gas drainage serves to remove gas from the coal seam and reduce the stored gas pressure from the seam in advance of mining. It is the stored gas pressure within the coal that is the primary source of damaging energy in an outburst event. Coal seams are variable and the propensity to outburst is affected by geology and mining conditions. Significant factors that affect outburst propensity include coal strength, permeability, geological and mining induced stress, and the toughness of the coal. If the coal has enough toughness to withstand the gas pressure contained within the pore space of the coal, and the permeability of the coal allows gas pressure to dissipate from the intact coal to prevent high pressure difference between the mine workings and the coal face, then the risk of outburst is low. The presence of geological structures that form a weakness and/or barrier to gas drainage and gas pressure reduction, are commonly associated with outburst events in Australia. Therefore, identifying geological structures and draining gas to reduce pressure from the coal seam in advance of mining are key actions in control and management of outburst risk.

This project has identified areas where the current accepted approach to outburst threshold limit value (TLV) determination is deficient and areas where TLVs have been increased successfully, without creating an unacceptable level of risk. This has enabled controlled mining in areas that would otherwise be stood down due to inability to drain gas to reduce the gas content below the circa 1994 TLVs. Further research is needed to better understand mining performance and outburst risk in areas with elevated gas content.

Areas requiring further research and development to support improvements in control and management of outburst in Australian underground coal mines are also presented and discussed in this report.