Long-term assessment of geochemical reactivity of CO2 storage in highly saline aquifers: Application to Ketzin, In Salah and Snøhvit storage sites

Abstract

Saline aquifers are choice targets for geological storage of CO2 because of their storage potential and because these formations are not suitable for other uses. Geochemical modeling is an interesting tool to assess the geochemical behavior of CO2 in the saline aquifer, including its dissolution in the brine and its interactions with minerals. Two key parameters which determine the confidence one can have in the results of geochemical modeling are tested in this paper: (i) the establishment of the conceptual model, including the selection of the primary and secondary minerals expected to react; and (ii) the activity model and the associated thermodynamic databases to calculate the interaction energies within the saline solution. In this study, we performed an analysis of a large set of CO2 storage natural analogs, which makes it possible to identify the minerals that are likely to precipitate and dissolve during CO2–brine–rock interactions. Interestingly, this analysis indicates a strong dependence of Dawsonite precipitation on the initial sandstone mineralogy. Dawsonite can precipitate in lithic and feldspar rich sandstones but was not observed in quartz rich sandstones. These observations on mineral reactivity are used to establish reactivity conceptual models for three CO2 storage case-studies in saline sandstone aquifers (Ketzin, In Salah and Snøhvit) and a methodology is proposed to evaluate the long-term geochemical reactivity of these saline aquifers as a result of CO2 injection. Noticeable differences are obtained between the case-studies as a function of the initial mineralogy and chemical conditions in the sandstones, which highlight that CO2 mineral trapping can take place in a given storage site but can be almost absent in other storage sites. Regarding the activity model and the database, the Pitzer interaction model is rarely used for simulating CO2 geochemical behavior in saline aquifers despite the fact that more conventionally used activity models are not valid for such salinities. A comparison between calculated mineral solubility evolution with salinity versus experimental data is performed here using both B-dot and Pitzer activity models as well as six different databases. This comparison exercise shows that chemical interactions within saline solutions can only be reproduced using the Pitzer model, even though Pitzer databases are still incomplete or are not coherent for a wide range of chemical species and temperatures. The geochemical simulations of CO2 injection in Ketzin, In Salah and Snøhvit saline aquifers give divergent results using different activity models and databases. A high uncertainty on the simulation results is then linked to the database choice and this study clearly stresses the need for a Pitzer database that can be confidently used in all physical/chemical conditions found in deep sedimentary aquifers.