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 A degrees 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.