Geochemical characterization of geothermal systems in western Anatolia (Turkey): implications for CO2 trapping mechanisms in prospective CO2†EGS sites

Geological storage of CO2 is currently regarded as one of the major strategies to mitigate the increasing CO2 concentrations in the atmosphere due to anthropogenic emissions from large†scale point sources. Enhanced geothermal systems (EGS) are a novel concept in which CO2 is used as a working fluid to increase energy recovery, combined with its subsurface storage. In this study, the geothermal systems of western Anatolia are considered as potential sites for EGS, and the published hydrogeochemical data relevant to these systems are compiled and evaluated in terms of potential water/CO2/rock interaction processes. The evaluation is performed with geochemical approaches including speciation†solubility calculations, and inverse and dedolomitization modelling. The results lead to the recognition of two different groups with respect to the effective processes: carbonate precipitation, and carbonate precipitation plus dissolution. In high†enthalpy fields, carbonate precipitation seems to be the major mechanism, while others show the effects of both precipitation and dissolution; dedolomitization is also identified as a potential mechanism for one of the fields. These processes are examined with regard to CO2†fixation; carbonate precipitation may lead to mineral trapping while carbonate dissolution provides additional cations to the system to react with CO2. Being relevant to CO2 storage in geothermal fields, the conclusions from this study point to the importance of temperature control for CO2 stabilization, as high temperatures seem to promote mineral trapping. The success of a CO2†EGS project depends on complete geochemical characterization of reservoir processes with further kinetic modelling accompanying the thermodynamic modelling exemplified by this study. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd.