Hydrogeochemical modeling of a thermal system and lessons learned for CO 2 geologic storage L.F. Auqué ⁎, P. Acero, M.J. Gimeno, J.B. Gómez, M.P. Asta Geochemical Modeling Group, Petrology and Geochemistry Area, Earth Sciences Department, University of Zaragoza, Spain Faculty of Geology, C/ Pedro Cerbuna 12, 50009 Zaragoza, Spain abstract article info Article history: Received 14 July 2009 Received in revised form 10 September 2009 Accepted 10 September 2009 Editor: J. Fein Keywords: Natural analogues Dedolomitization Thermal waters Geochemical modeling Geological storage of carbon dioxide is presently considered to be one of the main strategies to mitigate the impact of the emissions of this gas on global warming. Among the various alternatives considered for CO 2 geological storage, one of the main geological candidates for hosting injected CO 2 in the long term are deep porous reservoir rock formations saturated with brackish or saline solutions. Although valuable information on the expected hydrogeochemical processes involved in the CO 2 storage in such deep saline aquifers can be obtained in laboratory or modeling studies, the only direct source of information about the long-term behavior of geological storages for CO 2 in deep aquifers is natural analogues. In this work, a classical and simple geochemical methodology is successfully applied to the study of the features and hydrogeochemical processes determining the evolution of a Spanish thermal system (the Alhama–Jaraba complex), which can be considered as a natural analogue for deep geological CO 2 storage in carbonate rocks. The geology, structure, depth and hydrogeochemistry of the Alhama–Jaraba thermal system are very similar to the expected features of a potential CO 2 reservoir in carbonate materials. The processes determining the hydrogeochemical evolution in the Alhama–Jaraba thermal system have been successfully identified and quantified with the assistance of ion–ion plots, speciation–solubility calculations and mass-balance calculations. Furthermore, the feasibility of the proposed conceptual hydrogeochemical model for this system has been verified by using reaction-path calculations. Mass-balance calculation results have indicated that the observed hydrogeochemical evolution between springs is mainly due to halite dissolution and dedolomitization triggered by gypsum or anhydrite dissolution. CO 2 (g) mass transfer has been estimated to be negligible, which suggests that the main processes responsible for the variation in the TIC and the CO 2 (g) pressure during deep circulation are dissolution and precipitation reactions for carbonate minerals. All the processes identified in the Alhama–Jaraba thermal system are relevant for the long-term evolution of a deep CO 2 storage site hosted by carbonate rocks. As shown in this study, the application of classical geochemical tools provides an excellent starting point for understanding the behavior of prospective storage systems. Moreover, the existence of dedolomitization is very relevant for the hydraulic properties of carbonate aquifers potentially used for CO 2 geological storage because of the effects on porosity and, therefore, permeability during the long-term evolution of such systems. Furthermore, dedolomitization may represent a mechanism of mineral trapping for CO 2 sequestration under certain conditions. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Geological disposal and storage of carbon dioxide is at present considered to be one of the main strategies to mitigate the impact of the emissions of this gas on global warming (Metz et al., 2005 and references therein). Among the various alternatives considered for CO 2 geological storage, the main geological candidates for hosting injected CO 2 are: oil and gas reservoirs, deep saline aquifers, coal beds, caverns and mines (Hitchon et al., 1999; Holloway et al., 1999; Bachu, 2000; Metz et al., 2005; Wildenborg and Lokhorst, 2005). Owing to their larger storage capacity at a global scale, deep porous reservoir rock formations saturated with brackish or saline solutions are generally regarded as the most effective geological reservoirs for the long-term storage of CO 2 (Metz et al., 2005 and references therein). Although other types of hosting rock formations have also been considered (evaporites, basalts, etc.), most of the earlier studies have focused on the geological storage of CO 2 in saline aquifers hosted by sedimentary rocks, especially in carbonate (Gunter et al., 2000; Shiraki and Dunn, 2000; Christensen and Holloway, 2004; Chemical Geology 268 (2009) 324–336 ⁎ Corresponding author. Geochemical Modeling Group, Petrology and Geochemistry Area, Earth Sciences Department, University of Zaragoza, Spain. Tel.: +34 976761067; fax: +34 976761106. E-mail address: lauque@unizar.es (L.F. Auqué). 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.09.011 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo