Coal Seam Gas Drainage – Improving performance
Gas drainage systems in underground coal mines often fail to deliver the planned / expected gas production rates and subsequently fail to reduce the gas content of the coal seam within the scheduled timeframe.
Underperforming and ineffective gas drainage can result in delays to mine production due to high residual gas content (above outburst threshold limits), or methane gas concentrations in the ventilation system that exceed statutory limits. When faced with such conditions it is a common response from mine operators to drill more boreholes in the area (over drilling the area) which not only increases operating cost but also affects the drilling schedule as drill rigs are diverted away from scheduled works. In extreme cases, mine plans have been altered to avoid ‘difficult-to drain’ areas which reduces coal reserves and shortens mine life.
It is important that those responsible for the design and management of gas drainage systems understand the geological factors that must be considered in the initial design and the operational factors that impact gas drainage performance. Geological conditions vary between mines and often change significantly over short distances within the bounds of a mining lease. It is necessary to collect sufficient detail to understand the geological setting and the potential effect on gas drainage design.
Prevailing geological conditions, whilst unable to be changed, should be considered to design gas drainage programs that maximise gas production potential within those set constraints. The operational factors, if understood and managed effectively, can significantly affect gas drainage performance within the limitations imposed by the prevailing geological conditions. Significant operational factors that should be considered in the design and ongoing monitoring and management of gas drainage systems are listed in the table below.
The control and ongoing maintenance of gas drainage boreholes and gas reticulation pipelines, specifically the removal of water and coal fines, has arguably the greatest impact on gas drainage performance. I often find mines have going to significant effort and expense to design and implement gas drainage however failing to remove water accumulations from the boreholes and the pipe range which significantly reduces gas production. The design of gas drainage boreholes and gas reticulation networks should provide for the effective separation and removal of both gas and water. The presence of water in boreholes impedes gas desorption, and water accumulation in gas reticulation pipes/hoses increases resistance and reduces the effective gas carrying capacity of the system. Downhole water removal, standpipe design and water separators should be incorporated into the gas drainage system. The performance of the system should be regularly checked and maintained, as often as necessary, to clear water and fines accumulation.
Black, D J and Aziz, N I, 2011. Actions to improve coal seam gas drainage performance, in Proceedings of the 2011 Underground Coal Operators Conference, University of Wollongong, Wollongong, 10-11 February, pp. 309-316.
Black, D J and Aziz, N I, 2010. Impact of coal properties and operational factors on mine gas drainage, in Proceedings of the 2010 Underground Coal Operators Conference, University of Wollongong, Wollongong, 11-12 February, pp 229-240.
Black, D J and Aziz, N I, 2008. Improving UIS gas drainage in underground coal mines, in Proceedings of the 16th Coal Congress of Turkey, Chamber of Mining Engineers of Turkey, Zonguldak, Turkey, 26-28 May, pp 157-170.
For further information or assistance with design, audit and improvement of coal mine gas drainage system performance, contact CoalGAS.