Geochemical modeling of CO2 sequestration in deep, saline, dolomitic-limestone aquifers: Critical evaluation of thermodynamic sub-models

Geochemical modeling is a common tool to estimate the potential for storing supercritical CO2 in deep saline aquifers, and to predict the effects thereof. Geochemical models necessarily rely on thermodynamic relationships or “sub-models” to estimate quantities such as chemical solubility, activity, and fugacity. In this paper, we compare three proposed models for the fugacity of supercritical CO2, four proposed models for the activity coefficient of dissolved (aqueous) CO2, and four proposed models for the solubility of CO2 in aquifer brine. We examine the conditions under which estimates from these different proposed thermodynamic sub-models agree or disagree. Also, we develop a geochemical model that predicts the effects of injecting CO2 into a deep, saline, dolomitic-limestone aquifer; specifically, we predict the change in brine pH, the mineral precipitation/dissolution, and the potential solubility trapping of injected CO2 in response to the CO2 injection. We test nine different versions of the model, utilizing different combinations of the thermodynamic sub-models listed above. This allows us to quantify how key geochemical predictions depend upon the selection of thermodynamic sub-models. We find that different combinations of thermodynamic sub-models yield similar estimates for equilibrium pH and for quantities of mineral precipitation/dissolution, but that agreement is worse when predicting the equilibrium CO2 concentration in brine and/or the solubility trapping of CO2. These results indicate that the predicted equilibrium pH and mineral precipitation/dissolution are not very sensitive to the underlying thermodynamic sub-models, whereas estimates for the CO2 concentration in brine and potential solubility trapping are. Overall, the geochemical predictions are least sensitive to the choice of CO2 activity coefficient sub-model, are moderately sensitive to the choice of CO2 solubility sub-model, and are most sensitive to the choice of CO2 fugacity coefficient sub-model.