Impure CO2 storage and potential groundwater leakage indicators

CO₂ geological storage generally involves the injection of a captured CO₂ stream into a suitable reservoir overlain by a low permeability seal or cap-rock formation.  The risk of significant leakage out of the storage site is expected to be very low in well characterised and managed operations.  However, a stakeholder perceived risk factor is the potential for leakage and contamination to overlying drinking water aquifers via faults, legacy well bores, or leaky seals.  A pilot injection site in Queensland, Australia was planned in a low salinity aquifer.  The CO₂ was sourced initially from a coal fired power plant containing SOx, NOx and O₂.  Overlying aquifers were part of the Great Artesian Basin in Australia, which is one of the largest artesian aquifer systems in the world. 

Drill core and cuttings from various Great Artesian Basin aquifer formations were characterised from potential CO₂ injection and monitoring bores in Queensland, Australia.  The Hutton Sandstone aquifer contains potentially reactive minerals including calcite, siderite, plagioclase and chlorite.  Sandstones and mudstones were reacted at subsurface conditions of 75°C and 15 MPa with synthetic Hutton Sandstone aquifer formation water and either a pure supercritical CO₂ stream or an impure CO₂ stream composition of CO₂-O₂.  While dissolved concentrations of elements such as Ca, Ba, Sr, Rb, REE varied with rock type.  Dissolved Fe was affected by the addition of O₂ in the gas stream with Fe-oxyhydroxide/Fe-oxide precipitation re-sequestering metals.  Dissolved lead concentrations remained favourably low, with arsenic showing a decreasing trend after CO₂ addition, likely through Fe-oxide precipitation and absorption.  Several potential isotope tracers including ⁸⁷Sr/⁸⁶Sr, δ¹³C-DIC, δ¹³C-CO₂, δ¹⁸O-H₂O and δ²H-H₂O were also analysed during CO₂-water-rock experiments.