VGP Technical Report 60 - Site scale groundwater impact assessment, Onshore Otway Basin.
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Product description:About the Victorian Gas Program: The Victorian Gas Program (VGP) is a comprehensive science-led program, incorporating geoscientific and environmental research to assess the risks, benefits and impacts of potential onshore conventional gas exploration and production. The program is also investigating the potential for further discoveries of onshore conventional and offshore gas in the Otway and Gippsland geological basins and assessing the feasibility of additional onshore underground gas storage in depleted reservoirs around the Port Campbell area. The VGP includes an extensive, proactive and phased community and stakeholder engagement program, through which the results of the scientific studies are being communicated. Executive summary: The potential for environmental impacts in the onshore Otway Basin were assessed at a site scale using groundwater impact modelling as part of the Victorian Gas Program’s (VGP) environmental studies. Through the modelling, potential mixing between aquifers and reservoirs was assessed. The modelling assumed that the aquifer and reservoir were in communication via an open borehole. This method was used to gauge the maximum potential interaction and environmental impact, noting that an open borehole could never be used for petroleum exploration in Victoria due to the regulatory requirement for wells to be cased. Existing pressure differentials are due to low pressure within the Dilwyn aquifer and variable pressure within the Waarre Formation. The volume of groundwater that could flow between aquifers was limited by the hydraulic conductivity of aquifers to a greater extent than the radius of the borehole. Movement of groundwater from the Port Campbell Limestone aquifer to the Dilwyn aquifer would be likely to dominate open borehole flux with a minor (two per cent) contribution moving from the Waarre Formation into the Dilwyn aquifer. The impacts on groundwater quantity and quality observed from the modelling simulation were localised: groundwater quantity impacts were within 1.1 kilometres of the well and groundwater quality impacts within 2.8 kilometres of the well. Groundwater quality impacts associated with solutes in groundwater were likely to be concentrated within the Dilwyn aquifer. The maximum impact of an open borehole on groundwater quality in the Dilwyn aquifer was modelled as an increase in salinity of 0.05 mg/L/yr over a one-kilometre impact radius. This compares with a current salinity range within the Dilwyn aquifer of 320–620 mg/L. The results from the simulation suggest that the optimum distance for monitoring a well to detect localised changes in groundwater quality and quantity is likely to be less than 1.1 kilometres but that the level of change in groundwater quality would be difficult to detect through routine monitoring. Bibliographic reference: O’Neill, C.D., Grover, J.D., Bold, T.A., Iverach, C.P. & Hocking, M.J. 2021. Site scale groundwater impact assessment, Onshore Otway Basin. VGP Technical Report 60. Geological Survey of Victoria, Department of Jobs, Precincts and Regions. Download: The downloadable version of this report is supplied in PDF format (8 MB). Related products:
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