4.3 Subsurface flow paths

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)).

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)).