When an underground coal mine ceases coal production, methane gas continues to flow into the underground workings through the process of desorption from residual coal within strata disturbed by mining activity.
For gassy mines this desorption process will continue for many years after closure and can resume when flooded mine workings are dewatered.
The coal mine operator is therefore faced with potential long-term liabilities including explosion risks on the surface and possible dangers to the public as well as continuing greenhouse gas emissions.
After stopping the ventilation system, pumping mine water and sealing mine outlets to the atmosphere (shafts, inclines and service boreholes) gas release from goaf areas will continue until underground workings and macro-fractures are flooded.
In New South Wales and Queensland there are more than 50 underground gassy coal mines, which have ceased coal mining operations since after 1954.
Responsible operators are now looking at ways to exploit methane from closed/sealed underground coal mines where practical to reduce environmental emissions, minimise public liability, to take advantage of an energy resource that would otherwise be wasted, provide some continuing local employment and to gain added value from the mine before total abandonment.
Whereas various methods for predicting gas emissions in working mines are available and in regular use, no complete methodology has been established for predicting the decay of emissions once coal production ceases and the coal mine is no longer operating.
However, extensive studies on gas emission from sealed goaves and abandoned coal mines have been undertaken by Lunagas using data predominantly from NSW and Queensland underground coal mines complemented by additional data from UK, Poland, Czech Republic, USA and Japan.
Thus, a method for forecasting long-term gas emission decay was developed for use by specialist industry practitioners in which predicted and measured data showed good correspondence.
The future development of coal mine methane utilisation from coal mines goaf in Australia will depend on the availability of suitable sites and their proximity to a customer.
Viability will also depend on the ability to predict both the quantity and decline rate of coal mine methane from those mines, to meet customer requirements whilst at the same time generating a profit for the operator.
The national and local energy market for gas and electricity use is continually changing and schemes must be flexible to accommodate these changes while maintaining commercial viability and customer satisfaction.
It is therefore in the interests of coal mine operators to monitor and gather gas emission data from goaf areas to accurately quantify current emissions and to predict future emission for exploitation potential assessment.
Lunagas has developed a practical and scientifically-based prediction tool for assessing the methane production potential of underground coal mines goaf areas ranging in size from a single panel to whole-of-mine.
The Coal Mine Goaf Gas Predictor (CMGGP) is simulation software for predicting the decline rate of gas make and calculating the gas reservoir capacity of coal mine goaves.
The method for estimating abandoned coal mine and goaf area methane potential is applicable to both dry and flooded mines and also takes into account coal mine methane utilisation options.
The software comprises three main sections.
The first is the “coal mine parameters,” in which data is entered to facilitate calculation of the decline curves for both dry and wet mines.
The second is “gas reservoir characteristics,” which allows entry of stratigraphical data for calculating the available gas reservoir.
The third is “methane decline curves and gas reservoir,” charts (decline curves) showing the results of calculations made based on data in the above two sections.
The software will run on a PC with windows XP or vista operating system and can be used by individual specialists, coal mines and other institutions involved in coal mine closure, coal mine methane utilisation and greenhouse gas emissions reduction.
Energy companies and city councils may find the software useful for determining methods to better utilise the majority of coal mine gases in order to protect the environment from harmful gasses.
*This is an edited version of a conference paper, ‘Coal mine goaf gas predictor (CMGGP)’, originally published by Les Lunarzewski for AusIMM Illawarra Branch’s 2010 Underground Coal Operators’ Conference.