Target coal seams are depressurised during the CSG extraction process to release methane gas. CSG wells do not capture all of the methane released from the coal seams and as a result of depressurisation some methane has the potential to escape the capture zone of a production well. This fugitive gas has the potential to migrate up-dip and rise through porous saturated aquifer media towards overlying water resource aquifers. When investigating increased gas occurrences in water resource aquifers, it is important to establish if methane levels in aquifers/water bores have actually increased, and whether any increase in gas levels is related to the CSG industry. Since methane gas naturally occurs in geological formations in the Bowen Basin distinguishing between the methane from shallow Jurassic aquifers and deeper Permian CSG formations is a critical issue in determining its source.
In light of this a pilot study was conducted in the Surat\Bowen Basin involving sampling selected water bores and CSG production wells for stable isotopes, standard hydrochemical parameters, dissolved inorganic carbon (DIC) and dissolved C1-C6 hydrocarbon gasses. This study focussed on characterising the methane pathways in the groundwater systems in the study area in order to inform GIAT during gassy bore investigations.
The δ2H and δ18O compositions showed three distinguishable groundwater categories namely 1. Deep CSG water 2. Precipice Sandstone water with low dissolved methane levels and 3. Precipice Sandstone water with high dissolved methane levels.