Browsing by Author "Hayes, PJ"

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    Geochemical tracers associated with methane in aquifers overlying a coal seam gas reservoir
    (Elsevier, 2024-07-15) Pearce, JK; Hofmann, H; Baublys, KA; Cendón, DI; Golding, SD; Herbert, SJ; Bhebhe, Z; Nguyen, A; Hayes, PJ
    Understanding inter-aquifer connectivity or leakage of greenhouse gases and groundwater to aquifers overlying gas reservoirs is important for environmental protection and social licence to operate. Australia's Great Artesian Basin (GAB) is the largest artesian groundwater system in the world with groundwater extracted for agriculture, livestock, mines, energy, private or town water supply. Microbial coal seam gas (CSG) and production water are also extracted from the GAB. Here a range of groundwater tracers is used to investigate the potential for gas and groundwater connectivity between the CSG reservoir and aquifers. The GAB aquifer and alluvium contained a range of methane concentrations (0.001 to 2100 mg/L) that exhibit an increase with depth and δ13C-CH4. Aquifer and alluvium groundwater 87Sr/86Sr were in the range 0.7042 to 0.7082. CSG production waters however had non-radiogenic, distinctive 87Sr/86Sr signatures <0.7036, indicating a lack of significant groundwater leakage. One gassy aquifer bore with 160 mg/L methane conversely has 87Sr/86Sr, δ13C-CH4, δ2H-CH4 and δ13C-DIC values overlapping the CSG waters. In several aquifers δ34S-SO4 and δ18O-SO4 are sourced from windblown surface salts of inland Australian playa lakes in recharge waters. Bacterial sulphate reduction is additionally occurring in a regional aquifer. Cosmogenic isotopes and tritium show recent recharge and mixing with older groundwaters in several shallow aquifers. Groundwater and gas signatures indicate that leakage of groundwater and methane from the CSG reservoir was not occurring in the majority of areas investigated here. Methane was consistent with in situ generation in shallow GAB aquifers by primary microbial CO2 reduction or acetate fermentation. Connectivity of one alluvial bore and the underlying GAB aquifer could not be completely ruled out. Separately, one gassy Springbok GAB aquifer bore is either connected to the underlying CSG gas reservoir, or has in situ secondary microbial CO2 reduction producing methane from interbedded coal within the aquifer. This study is relevant to other basins in Australia and internationally where gas is observed in aquifers that overly conventional, unconventional or coal seam gas reservoirs. © 2024 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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    Multiple tracers for dis-connectivity of shallow aquifers, alluvium, and coal seam gas wells in the Great Artesian Basin
    (CSIRO Publishing, 2022-05-13) Pearce, JK; Golding, SD; Baublys, KA; Hofmann, H; Cendón, DI; Herbert, SJ; Hayes, PJ
    The potential for connectivity between water supply aquifers and gas reservoirs raises community, government, and scientific concerns. Methane can occur naturally, making it difficult to determine whether water bore methane levels are being influenced by nearby gas operations. This poses a challenge in the Surat Basin, where coal seam gas production operates alongside groundwater using industries (including feedlots, agriculture, mines). Water and gas samples were taken from water bores and coal seam gas (CSG) wells in the Walloon Coal Measures and from overlying aquifers (nominally, the Springbok, Gubberamunda, Orallo, and Mooga sandstones) and the Condamine Alluvium, for stable isotopes of gases, groundwater and dissolved inorganic carbon, as well as strontium isotopes. Most of the sampled water bores had isotopic signatures distinct from CSG wells, though a minority from gassy Springbok Sandstone and Walloon Coal Measure water bores could not be distinguished from CSG wells. In those few cases, neither connectivity or dis-connectivity could be confirmed. Alluvium and shallow aquifer samples have higher R36Cl values distinct from the older CSG production waters, as is the case with most 14C measurements. Waters from the Condamine River indicate potential surface water connectivity with the alluvium. The use of multiple tracers has shown that groundwater in some aquifers can be differentiated from groundwater in the coal seam gas reservoir and hence are useful tools in identifying where groundwater connectivity occurs. Understanding this connectivity forms another line of evidence to improve impact prediction models on a regional scale as well as providing information on connectivity in local groundwater investigations. © 2022 The Author(s) (or their employer(s)). Published by CSIRO Publishing on behalf of APPEA.