Hydrogeochemical modeling of a thermal system and lessons learned for CO2 geologic storage

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 CO2 geological storage, one of the main geological candidates for hosting injected CO2 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 CO2 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 CO2 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 CO2 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 CO2 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. CO2(g) mass transfer has been estimated to be negligible, which suggests that the main processes responsible for the variation in the TIC and the CO2(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 CO2 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 CO2 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 CO2 sequestration under certain conditions.