4.3 Subsurface flow paths

Hydraulic fracture growth into an aquifer, well or fault has a low likelihood but warranted further investigation to address community concerns. Hydraulic fracture modelling shows that natural barriers, such as the Nappamerri aquitard, protect overlying aquifers from contamination caused by hydraulic fracturing and gas production in the Cooper Basin. Compromised aquitard integrity is of ‘potential concern’ in less than 0.01% of the Cadna-owie – Hooray aquifer. This is where the Nappamerri aquitard is less than 155 m thick, the estimated distance a contaminant can travel along an open fracture in 250 years.

Compromised aquitard integrity describes changes in the integrity of low permeability rock layers (called strata) between gas reservoirs and aquifers. Low permeability rock layers have very slow groundwater flows. There are 2 main regional aquitards in the Cooper GBA region: the Nappamerri Group, which separates the Gidgealpa Group (the host for tight, shale and deep coal gas plays) from confined aquifers in the Eromanga Basin, and the Rolling Downs Group aquitard, which separates the Cadna-owie – Hooray aquifer from aquifers at the surface in the Winton-Mackunda and Cenozoic sediments (Evans et al., 2020).

Compromised aquitard integrity is of ‘potential concern’ in less than 0.01% of the Cadna-owie – Hooray aquifer. This is where the Nappamerri aquitard is less than 155 m thick, the estimated distance a contaminant can travel along an open fracture in 250 years. Aquifer contamination leading to pollution cannot be ruled out within 500 m of existing water bores due to modelling constraints, as dilution of contaminants may be insufficient over this distance (refer to the Cenozoic aquifer drawdown , Cadna-owie – Hooray aquifer drawdown and Winton-Mackunda aquifer drawdown node descriptions). The overlying Winton-Mackunda and Cenozoic aquifers are protected by the more than 550 m thick Rolling Downs Group and Nappamerri Group aquitards that form natural barriers.

Conservative modelling of vertical hydraulic fracture growth (Geological and Bioregional Assessment Program (2021i)) supports a high level of confidence in the cause-and-effect relationship and materiality threshold for any new fluid pathway created through the Nappamerri or Rolling Downs Group aquitards due to hydraulic fracturing activities ( Box 6 ). Furthermore, there is high confidence that existing controls outlined in environmental management plans (Santos, 2015; Beach Energy, 2019) can mitigate potential impacts on Cadna- owie – Hooray aquifer condition. This is based on the review of the findings of domestic and international inquiries (Pepper et al., 2018), Cooper GBA region monitoring data and petroleum industry compliance reports (refer to Section 2 of Stage 2 hydraulic fracturing technical appendix; (Kear and Kasperczyk, 2020)).

Box 6 Visualising and assessing risks to aquifers from hydraulic fracturing

To assess and visualise the potential for hydraulic fractures to propagate vertically through an aquitard and thus create a new fluid pathway to an aquifer, the output of a probability bounds analysis of potential hydraulic fracture growth (Pandurangan et al., 2018) was combined with spatial geological data for the Cooper GBA region (Owens et al., 2020).

The probability bounds analysis predicted vertical hydraulic fracture extents of 56 to 389 m, with a mean vertical hydraulic fracture extent of 151 m (Geological and Bioregional Assessment Program (2021i)). In areas where the predicted vertical fracture height exceeds the thickness of the Nappamerri Group aquitard, there is a slightly higher potential likelihood of a hydraulic fracture extending through an aquitard into an aquifer and greater care needs to be taken in the design, injection and monitoring of hydraulic fracturing treatments. The risk of hydraulic fractures intersecting an aquifer is able to be adequately mitigated by controls in existing regulation, sufficient understanding of the baseline geological and environmental systems, and accepted industry practices (Kear and Kasperczyk, 2020).

Aquifer contamination due to compromised well integrity is of ‘very low concern’ in the Cooper GBA region based on findings from domestic and international inquiries, as well as historical compliance reports for Cooper Basin petroleum wells.

Compromised well integrity refers to breaches of a well system that allow the unintended movement of fluids (gas, hydrocarbons and water) into, out of or along the outside of the well. A minimum of 2 independent well barrier elements are required to form a leak-tight seal between the well and the rock (International Organization for Standardization, 2017; Department of Natural Resources‚ Mines and Energy (Qld), 2019; Northern Territory Government, 2019). This provides redundancy such that a failure in one well barrier does not lead to unintended fluid infiltration into geological layers or to the surface.

There is high confidence in the effectiveness of mitigation strategies, materiality thresholds and cause-and-effect relationships associated with aquifer contamination due to compromised well integrity. This is based on a review of the 2,288 petroleum wells drilled in the Cooper GBA region (noting there were 3,000 bores drilled but only 2,288 were reviewed), which identified 2 reported instances of fluid flow between formations or to the surface that were subsequently detected and remediated (Kear and Kasperczyk, 2020). Findings from domestic and international inquiries (Pepper et al., 2018) (refer to Section 2 of Stage 2 hydraulic fracturing technical appendix; (Kear and Kasperczyk, 2020)) also support this analysis.

Aquifer reinjection of wastewater is subject to detailed technical assessment, which includes modelling and requires regulatory approval prior to commencement (Santos, 2015). This includes comprehensive evaluation against a range of criteria, including understanding of historical seismic events, local geology, regional stress fields and the nature of the proposed reinjection process (The Royal Society and The Royal Academy of Engineering, 2012). For this reason, fault reactivation and induced seismicity were not prioritised for further assessment in Stage 3 (Section 5.2 in baseline synthesis and gap analysis (Holland et al., 2020)).


Impact assessment summary for the Cooper GBA region
Causal network dataset (Geological and Bioregional Assessment Program, 2021c)

Fact sheets are available on the Geological and Bioregional Assessment website .

  • Fact sheet 2: Assessing hydraulic fracture risks to groundwater (Geological and Bioregional Assessment Program, 2021i)
  • Fact sheet 8: Characterising the connectivity between the Cooper Basin, Great Artesian Basin and shallow aquifers (Geological and Bioregional Assessment Program, 2021j)
  • Fact sheet 9: Application of the chemical screening framework (Geological and Bioregional Assessment Program, 2021l)
  • Fact sheet 10: Groundwater sampling in the Cooper, Eromanga and Lake Eyre basins (Geological and Bioregional Assessment Program, 2021s)
  • Fact sheet 11: Environmental fate of hydraulic fracturing chemicals (Geological and Bioregional Assessment Program, 2021t)
  • Fact sheet 13: Flood inundation modelling for Cooper Creek floodplain (Geological and Bioregional Assessment Program, 2021a)
  • Fact sheet 16: Has development impacted flood characteristics? (Geological and Bioregional Assessment Program, 2021f)
  • Fact sheet 21: Revising the geology of aquifers in the Cenozoic Lake Eyre Basin in the Cooper GBA region (Geological and Bioregional Assessment Program, 2021u
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