Uncertainty in geochemical modelling of CO2 and calcite dissolution in NaCl solutions due to different modelling codes and thermodynamic databases

A prognosis of the geochemical effects of CO₂ storage induced by the injection of CO₂ into geologic reservoirs or by CO₂ leakage into the overlaying formations can be performed by numerical modelling (non-invasive) and field experiments. Until now the research has been focused on the geochemical processes of the CO₂ reacting with the minerals of the storage formation, which mostly consists of quartzitic sandstones. Regarding the safety assessment the reactions between the CO₂ and the overlaying formations in the case of a CO₂ leakage are of equal importance as the reactions in the storage formation. In particular, limestone formations can react very sensitively to CO₂ intrusion. The thermodynamic parameters necessary to model these reactions are not determined explicitly through experiments at the total range of temperature and pressure conditions and are thus extrapolated by the simulation code. The differences in the calculated results lead to different calcite and CO₂ solubilities and can influence the safety issues.This uncertainty study is performed by comparing the computed results, applying the geochemical modelling software codes The Geochemist’s Workbench, EQ3/6, PHREEQC and FactSage/ChemApp and their thermodynamic databases. The input parameters (1) total concentration of the solution, (2) temperature and (3) fugacity are varied within typical values for CO₂ reservoirs, overlaying formations and close-to-surface aquifers. The most sensitive input parameter in the system H₂O–CO₂–NaCl–CaCO₃ for the calculated range of dissolved calcite and CO₂ is the fugacity of CO₂. Hence, the largest range of dissolved calcite is calculated at high fugacities and is 210 mmol/kgw. The average deviation of the results using the databases phreeqc.dat and wateq4f.dat in combination with the code PHREEQC is lowest in comparison to the results of the specific model of Duan and Li, which represents the experimental values at best. Still, the solubility of CO₂ is overestimated in the formation water using these two databases. Therefore, the model results calculate a larger retention capacity, defined as the quantity of CO₂ dissolved in the formation water, than the Duan and Li model would do.