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| Titel |
Assessing the hydraulic connection between fresh water aquifers and unconventional gas production using methane and stable isotopes |
| VerfasserIn |
Charlotte P. Iverach, Dioni I. Cendón, Stuart I. Hankin, Dave Lowry, Rebecca E. Fisher, James L. France, Euan G. Nisbet, Andy Baker, Bryce F. J. Kelly |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250103864
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| Publikation (Nr.) |
 EGU/EGU2015-3284.pdf |
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| Zusammenfassung |
| Unconventional gas developments pose a risk to groundwater quality and quantity in
adjacent or overlying aquifers. To manage these risks there is a need to measure the
background concentration of indicator groundwater chemicals and to map pathways
of hydraulic connectivity between aquifers. This study presents methane (CH4)
concentration and isotopic composition, dissolved organic carbon concentration
([DOC]) and tritium (3H) activity data from an area of expanding coal seam gas
(CSG) exploration and production (Condamine Catchment, south-east Queensland,
Australia).
The target formation for gas production within the Condamine Catchment is the Walloon
Coal Measures (WCM). This is a 700 m thick, low-rank CSG resource, which consists of
numerous thin discontinuous lenses of coal separated by very fine-to medium-grained
sandstone, siltstone, and mudstone, with minor calcareous sandstone, impure limestone and
ironstone. The thickness of the coal makes up less than 10% of the total thickness of the unit.
The WCM are overlain by sandstone formations, which form part of the Great Artesian Basin
(GAB). The Condamine Alluvium fills a paleo-valley carved through the above
formations.
A combination of groundwater and degassing air samples were collected from irrigation
bores and government groundwater monitoring boreholes. Degassing air samples were
collected using an SKC 222-2301 air pump, which pumped the gas into 3 L Tedlar bags. The
groundwater was analysed for 3H and [DOC].
A mobile CH4 survey was undertaken to continuously sample air in and around areas of
agricultural and unconventional gas production. The isotopic signature of gas from the
WCM was determined by sampling gas that was off-gassing from a co-produced
water holding pond as it was the largest emission that could be directly linked to
the WCM. This was used to determine the source signature of the CH4 from the
WCM.
We used Keeling plots to identify the source signature of the gas sampled. For the borehole
samples these plots assume that there are only two sources of CH4, each with a unique
isotopic signature. When the two sources mix in varying proportions they will plot along a
straight line in the Keeling plot. Geometric mean displacement was used to fit a regression
line and determine the intercept value.
Within the Keeling plot, samples clustered according to their 3H and [DOC] values.
One cluster is associated with near surface biological processes, while the other
cluster can be attributed to gas sourced from the WCM. This indicates that in places
there is hydraulic connectivity between the WCM and the overlying Condamine
Alluvium.
The results from this case study demonstrate that measuring 3H activity, [DOC] and
CH4 concentrations in combination with CH4 isotopic analysis can provide an
early indicator of hydraulic connectivity in areas of expanding unconventional gas
development. |
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