Mine Site Greenhouse Gas Mitigation » Mine Site Greenhouse Gas Mitigation
A database of some 2000 boreholes with confidential measurements of gas reservoir parameters of various seams from mines across the Hunter Valley and Central Bowen Basin coalfield were compiled to examine the inherent variability of parameters used to estimate gas emissions from coal. The study was prompted by a need to understand this variability and its impact on estimating fugitive greenhouse gas emissions from open cut and underground mines using global emissions factors.
The results demonstrate that only one gas reservoir parameter (gas sorption capacity) follows measurable coal properties such as rank and type for differing pressures and temperatures. Actual gas contents do not always match theoretical capacity and all reservoir properties, in particular gas content, seam thickness and permeability, vary greatly at the mine scale. This in situ variability at the mine scale, coupled with varying approaches to mine gas management makes it almost impossible to develop emissions factors for coalfield or basin scales with a reasonable level of certainty, much more be extrapolotated to state and country scales. Current default fugitive emissions factors per tonne of coal mined can be considered ballpark relative to actual in-ground gas contents, which introduces high levels of uncertainty relative to the tonnages mined.
As has been suggested by various guidelines for the estimation of fugitive greenhouse gas emissions, the most reliable estimations are made at the mine scale from measurement and monitoring. Mine scale estimations can be improved by taking a domain approach, where depth and pressure relationships are relatively stable and within which the variability of reservoir parameters is reduced to some degree.
Although it is outside the scope of this study, the next logical steps are to conduct a series of Tier 3 mine scale studies and then compare them to estimations made by applying Tier 2 formulae and/or to develop sets of calibrated emission models that take account of both the varying reservoir properties and the mining process. For underground mining, the data are readily available from current monitoring systems. For open cut mines, this requires the development of efficient and effective measurement and monitoring systems. These types of projects are already underway.
The authors are indebted to the coal companies who provided confidential and sensitive information for use in this study and thank ACARP and its monitors for their support and patience in delivery of this project.