Being able to estimate what is happening with gas emission trends, in particular the existence of methane-air explosive mixture in goaves, and the possibility of spontaneous combustion is crucial for mines.

Gas emission predictions from operational mines are calculated using various models and techniques on the basis of coal seams gas content, thickness and distances, gas reservoir other parameters such as: in situ pressure, permeability, sorption isotherms, strata relaxation zones, local lithology and mining system used. Some specialists estimate remaining gas quantity, which will be available (after the coal extraction is completed) as the difference between total quantity predicted and that measured during mining activities.

Depending on the situation, sealed goaf areas and abandoned underground coal mines may represent producible gas sources, where gas release and exploitation can be planned and controlled from both a time and quantity perspective. Mine operations also benefit from predicting gas emissions and being able to design ventilation circuits with this in mind.

Managing director of mine gas consulting company Lunagas, Les Lunarzewski said he has conducted extensive studies regarding gas emission from sealed goaves and abandoned coal mines.

Specific gas information and data have been collected and analysed from selected coal mines in Australia and other countries since 1983. The data enabled Lunagas to develop PC-based modelling tools called “Gas emission decline curves”

Using the modelling tools, Lunagas has found that gas emission rate changes with time for individual longwalls, districts and abandoned coal mines. Using a multi longwall database Lunagas has been able to predict whole mine gas emission rate, and the contribution of an individual longwall and its goaf to the total mine gassiness.

“Long term research and application of the original decline curves allows for utilising the existing model at a number of sites, which has confirmed that a strong correlation appears to exist between gas emission quantity predicted and measured in practice,” Lunarzewski said.

Lunagas has established various phases of gas emission. During production gas emission changes are related to the gas source type, coal production level, advance- retreat rate and time. Post production gas emission decay is related to the final gassiness’ time, gas sources characteristics, volume of gas reservoir and barometric pressure changes.

During the post production period, there are two physical processes/stages controlling gas emission versus time; rapid gas decay – ‘Stage 1’ and slow gas decay – ‘Stage 2’. During stage 1 there are three initial periods of gas decay from the cessation of longwall. During months 1& 2 rapid decay gas emission decline occur - up to 50% of final emission and after that is a stabilised period.

But, according to Lunarzewski, no comprehensive study on underground gas emission from sealed goaf areas and abandoned coal mines in Australia has been undertaken in relation to local mining, gassy and geological conditions, as well as underground safety such as goaf explosive air-gas mixtures and coal spontaneous combustion problems.

Lunarzewski said using the Lunagas programs will lead to long term improvements in underground safety and performance. He said it could reduce the costs of gas drainage and ventilation networks which directly affect coal production continuity.

In December 2003 Lunagas was granted 2004 ACARP funding for the project “Gas emission curves for sealed goaf area or abandoned coal mine” under which the final software for the Australian coal mining industry will be developed.