Assessment of coal mine methane (CMM) and abandoned mine methane (AMM) resource potential of longwall mine panels: Example from Northern Appalachian Basin, USA

Abstract

Coal mine methane (CMM) and abandoned mine methane (AMM) are by-products of underground mining of gassy coal beds. The quantity and the emission rate of CMM and AMM may vary depending on the type of mine, gas content of the mined coal bed, and gas sourced from strata and coal beds in overlying and underlying formations affected by mining. Therefore, if a mine has the potential of accumulating gas after being abandoned and sealed properly, that methane may be produced and used as an energy source to serve local communities in the area. Producing AMM also prevents methane, which is a potent greenhouse gas, from leaking to the atmosphere through seals, shaft plugs or surface cracks.A technical barrier to economical utilization of CMM and AMM is the uncertainty about how much methane may be available in the gas emission zone (GEZ) as a resource during mining, as well as after the panels are sealed and the mine is abandoned. Another difficulty is estimating how much of the potential methane resource can be produced from gob gas ventholes (GGV) converted to capture AMM.In this study, a comparative assessment is presented to address the issues stated above. The assessment was conducted on two adjacent panels of a longwall mine that operated in Pennsylvania until 2016 in the Northern Appalachian Basin. The study is based on two approaches that might be used depending on the availability of data. The first approach uses an extensive geological data set, geostatistics, and measured shaft gas emission and GGV production values that were collected while the panel (s) were active to assess the AMM resource. The second approach uses a minimal amount of geologic data, uncertainty of the data as probabilistic distributions, and the use of publicly available software to predict during-mining emissions. Results showed that both approaches provide relatively comparable estimates of AMM resources and AMM recovery potential using wellbores. The differences in assessed quantities are mostly due to the characteristics of the two methods. In that regard, this paper can be considered as guidance to choose the assessment approach based on data availability.