Underground » Ventilation, Gas Drainage and Monitoring
Outburst risk management
A gas outburst is an essentially dynamic event in which coal and/or rock is projected into a mine opening, accompanied by a rapid release of gas. In Australian experience, the mass of coal/rock may range from small (less than 1 t) to very large (greater than 400 t), and volumes of gas released may be very large (greater than 5000 m3 at atmospheric pressure). Although frequently characterised as sudden or instantaneous, the time scale may span many seconds for a large outburst, suggesting a progressive event. It appears that a key element is the presence and role of compressed gas in the initiation of the burst and in the transport of material. This distinguishes a gas outburst from rock bursts which are dynamic failures resulting from mechanical stress alone.
In the last decade outburst risk has been brought under control in Australia by the introduction of in-seam gas drainage ahead of mine development and production. Under strict government regulation, drainage to meet safe gas content threshold values (THV's), is carried out in all mines assessed as at risk. The regulations also mandate the use of outburst management plans, prepared and implemented at each mine. These are based on the recognition that sound management has a major and essential role to play, in conjunction with technical and operational procedures.
In the management of outburst risk, the two prime issues are safety and productivity. Worker safety is a very high level requirement with which there is no compromise. This underpins the setting of THV levels, and provides a fixed reference for discussion of mine productivity issues. Application of the THV criteria has been successful in virtually eliminating outbursts. However, the criteria are limited to the factors of gas content and gas composition (CH4 and CO2), with modified mining methods required in the close presence of coal structures.
It is widely recognised that other physical factors have the potential to modify the risk, but a much improved understanding of how these factors interact and contribute to the evolution of outburst conditions is required before they could be taken into account.
Model development
This project was undertaken to develop an improved understanding of outburst mechanisms and to develop methods to quantitatively model the impact of a range of factors that interact to produce outburst conditions. The new model would be applied to investigate the potential for:
- improved application of outburst risk criteria under variable mining conditions/li>
- improved application of outburst control measures
The method adopted was to couple modelling approaches developed for the solution of geomechanical - fluid flow problems in the mining and civil engineering fields, with those from the petroleum industry developed for coal seam methane reservoir characterisation and engineering. Two existing computer codes were used as a basis for the work; SIMED and FLOMEC.
Model development and applications were matched where possible to in situ data, observations and operations, taking into account broad natural variability. This is important for quantitative evaluation and interpretation, and to provide feedback for further model development.
The model is demonstrated generically using a combination of data and observations from three operating mines that are subject to THV criteria; West Cliff, Appin and Dartbrook. Information from Leichhardt Colliery, closed since 1983 has also been used. With the exception of Dartbrook, these mines have well documented experience with outbursts. For all four mines, a range of quantitative geomechanical and reservoir data is available. These data represent the outcomes of much work carried out by a number of workers over more than two decades.
Preceded by a review of the experience and understanding of outburst gained from the work of others, and a summary of theoretical aspects underpinning the modelling techniques used, the report examines the required input data, and the outcomes of parametric studies. These outcomes are then assessed in terms of the development and application of broader based outburst risk criteria tailored more specifically to individual mine seam conditions.
The model findings strongly support the importance of gas content and geological structures in determining THVs for outburst risk. However, in a mechanistic sense, pore fluid pressure and its gradient rather than gas content is a key determinant of outburst initiation risk. The pressure gradient field is in turn a function of the:
- reservoir pressure,
- desorption pressure,
- permeability components,
- relative permeabilities,
- desorption isotherm.