Australian Decommissioned Mines Gas Prediction

A previous ACARP project C13007 Gas Emission Curves For Sealed Goaves Or Abandoned Mines established gas emission decay curves for dry coal mines. The gas emission characteristics following cessation of coal production fall into three categories:

• Production gas rapid decline phase
• Background gas stabilisation phase, and
• Background gas long term decline phase.

Background gas is the emission from worked out areas and mature goaves.  Production gas is the release of gas from sources freshly disturbed by mining activity. During mining both production and background gas is emitted but about three months after coal production ceases, the latter becomes the dominant process. By the time 12 months have elapsed the long term decline phase is usually established and background gas flow can be represented by an exponential of the form

where ‘x’ is time, ‘a’ is initial emission and ‘b’, a decay constant.

This project is a continuation which updates the formulae (curves) for the dry mine case and extends the prediction process to take account of groundwater recovery which occurs when a mine is closed and water pumping is halted. The research shows that in Australian conditions, when estimating the total decommissioned mine methane, account should be taken of the gas remaining in both the coal seams and the strata disturbed by longwall extraction, plus gas remaining in coal pillars where there has been extensive room­ and­ pillar extraction. As the workings flood, gas sources are progressively isolated and gas flows decay more rapidly than with the dry situation.

The effect of flooding is to progressively remove gas sources and this process can be simulated using the “Wetsim” decay prediction method by setting emissions from progressively shallower longwall goaf areas to zero as water level rises. The time taken for the workings to flood is calculated using a void model and inferred water inflow data. Two simple, empirical methods for rapidly assessing flooding effects on gas emission have also been developed.

The integral of an emission decay curve provides a first order estimate of potentially accessible DMM resource. The magnitudes of the individual gas source components can be back calculated using a geological resource model and the extraneous strata gas found by difference. The original total quantities of gas ­ in ­place (GIP) before mining can also be estimated; for the two study mine sites these are substantially greater than previously thought which may have implications for gas emission prediction models applied to working coal mines. Thus, a methodology has been established for assessing greenhouse gas emissions from closed mines and estimating methane availability for utilisation for dry and flooding scenarios.

A package encapsulating all the key data will be supplied to ACARP to be used for future research subject to the written approval of the data owners.

The accuracy of DMM predictions depend largely on the quality and quantity of data available. Determined efforts are needed to gather relevant data during the mine closure process when key staff members are being reassigned and priorities have been refocused on the next mining project.