Reactive modelling of CO₂ intrusion into freshwater aquifers: current requirements, approaches and limitations to account for temperature and pressure effects

The modelling of CO₂ intrusion into virtual freshwater aquifers after a leakage from CO₂ storage formations is a well-established approach for the identification of monitoring parameters and for the risk assessment. At presence, standard or close-to-standard conditions in terms of temperature (T), i.e. 25 °C and pressure (P), i.e. 1–5 bar, are assumed. This approach neglects the fact that temperature and pressure conditions change with the depth of the freshwater aquifer. This study tests the accuracy of T–P corrections of the geochemical constants in the system gaseous CO₂–water–mineral which are performed by the simulators PhreeqC (Parkhurst and Appelo in User’s guide to phreeqc (version 2)—a computer program for speciation, batch reaction, one-dimensional transport, and inverse geochemical calculations. Technical report, US Department of the Interior, 1999) and TOUGHREACT (Xu et al. in Toughreact user’s guide: a simulation program for non-isothermal multiphase reactive geochemical transport in variably saturated geologic media. Technical report, Lawrence Berkeley National Laboratory, 2004). It further identifies the impact of T and P variations on the predicted concentrations of the monitoring parameters pH and total inorganic carbon (TIC) and on the predicted concentration of the trace metal lead (Pb) in 3D multiphase-multicomponent simulations of virtual aquifers. The results reveal a strong imprecision in the correction of kinetic rates of mineral dissolution and a lack of corrections of sorption equilibrium states. The predicted pH and concentrations of TIC and lead depend strongly on the assumed T and P conditions. It is concluded that a neglect of T and P effects results in inaccurate predictions of groundwater chemistry. The impact assessment and monitoring strategies based on currently available modelling results consequently require strong improvements.