Use of a New Sodium/Lithium (Na/Li) Geothermometric Relationship for High-Temperature Dilute Geothermal Fluids from Iceland

This paper presents the study which has allowed determining the best adapted Na/Li thermometric relationship for the High-Temperature (HT) dilute waters collected from wells located in the Krafla geothermal area, North-east Iceland. This work was carried out in the framework of the European HITI project (HIgh Temperature Instruments for supercritical geothermal reservoir characterization and exploitation). This relationship, which can give estimations of temperature ranging from 200 to 325°C with an uncertainty of ± 20°C, was also successfully used on HT dilute fluids collected from wells located in other Icelandic geothermal areas (Namafjall, Nesjavellir and Hveragerdi). So, a more general Na/Li geothermometric relationship including these other geothermal areas was obtained. These new relationships determined for Icelandic HT dilute geothermal fluids, which are very different from that commonly used for the dilute world geothermal waters, will be tested on the fluids collected during the international Iceland Deep Drilling Project (IDDP) for which the main objective is to drill deep wells (> 5 km) in supercritical reservoir conditions (T > 375°C), located in the Krafla and Reykjanes geothermal areas. Another Na/Li relationship determined for HT saline geothermal fluids derived from sea water and basalt interaction processes will be tested on the Reykjanes supercritical reservoirs. The existence of several different Na/Li geothermometric relationships seems to show that the Na/Li ratios not only depend on the temperature but also on other parameters such as the fluid salinity and origin, or the nature of the geothermal reservoir rocks in contact with the deep hot fluids. Thermodynamic considerations and some case studies in the literature suggest that the Na/Li and Na/K ratios for the HT dilute geothermal waters from Iceland could be controlled by a full equilibrium reaction involving a mineral association constituted, at least, of Albite, K-Feldspar, Quartz and Clay minerals such as Kaolinite, Illite and Na-, Li-Micas.

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