Monitoring groundwater flow and chemical and isotopic composition at a demonstration site for carbon dioxide storage in a depleted natural gas reservoir

Between March 2008 and August 2009, 65,445 tonnes of ∼75 mol% CO2 gas were injected in a depleted natural gas reservoir approximately 2000 m below surface at the Otway project site in Victoria, Australia. Groundwater flow and composition were monitored biannually in two overlying aquifers between June 2006 and March 2011, spanning the pre-, syn- and post-injection periods. The shallower (∼0–100 m), unconfined, porous and karstic aquifer of the Port Campbell Limestone and the deeper (∼600–900 m), confined and porous aquifer of the Dilwyn Formation contain valuable fresh to brackish water resources. Groundwater levels in either aquifer have not been affected by the drilling, pumping and injection activities that were taking place, or by the rainfall increase during the project. In terms of groundwater composition, the Port Campbell Limestone groundwater is brackish (electrical conductivity = 801–3900 μS cm−1), cool (temperature = 12.9–22.5 °C), and near-neutral (pH = 6.62–7.45), whilst the Dilwyn Aquifer groundwater is fresher (electrical conductivity = 505–1473 μS cm−1), warmer (temperature = 42.5–48.5 °C), and more alkaline (pH = 7.43–9.35). Carbonate dissolution, evapotranspiration and cation exchange control the composition of the groundwaters. Comparing the chemical and isotopic composition of the groundwaters collected before, during and after injection shows no statistically significant changes; even if they were statistically significant, they are mostly not consistent with those expected if CO2 addition had taken place. The monitoring program reveals no impact on the groundwater resources attributable to the C storage demonstration project.