Over the past 20 years, the Soultz experimental geothermal site in Alsace, France, has been explored in detail by the drilling of five boreholes, three of which extend to 5 km depth. Data on geology, fluid geochemistry, temperature, microseismicity, hydraulics and geomechanics have been collected and interpreted by the various teams from the participating European countries and their international collaborators. Two reservoirs have been developed within granite at depths of 3.5 and 5 km. The reservoir at 3.5 km was formed from two wells, 450 m apart, both of which were subjected to hydraulic stimulation injections. The system was circulated continuously for 4 months at 25 kg/s in 1997 using a downhole pump, and yielded results that were extremely encouraging. The impedance reduced to 0.1 MPa/l/s, the first time this long-standing target had been attained. Construction of a deeper system began shortly afterwards with the drilling of 3 deviated wells to 5 km true vertical depth, where the temperature was 200 8C. The wells were drilled in a line, 600 m apart at reservoir depth, and all were hydraulically stimulated and subjected to acidization injections. The 3-well system was circulated under buoyancy drive for 5 months in 2005 with injection in the central well, GPK-3, and production from the two outer wells, GPK-2 and GPK-4. This showed good linkage between one doublet pair, but not the other. Further acidization operations on the low-productivity well led to its productivity increasing to almost the same level as the other wells. Construction of a power plant at the site was completed in 2008 and a trial circulation with a production pump in one well and the other shut-in was conducted with power production. Downhole pumps are now installed in both production wells in preparation for long-term circulation of the system. In this article we present an overview of the principal accomplishments at Soultz over the past two decades, and highlight the main results, issues identified, and lessons learnt.

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. Lithium isotopes in island arc geothermal systems: Guadeloupe, Martinique (French West Indies) and experimental approach. Geochimica et Cosmochimica Acta, Elsevier, 2010, 74 (6) AbstractWe report Li isotopic measurements in seawater derived waters discharged from geothermal wells, thermal and submarine springs located in volcanic island arc areas in Guadeloupe (the Bouillante geothermal field) and Martinique (Lamentin plain and the Diamant areas). The lithium isotopic signatures of the geothermal fluids collected from deep reservoirs are homogeneous for a given site.However, the δ 7 Li signatures of each of these reservoirs are significantly different. We provide the first low temperature (25-250°C) experiments of Li isotope exchange during seawater/basalt interaction), which confirm that Li isotopic exchange is strongly temperature dependent, as previously inferred from natural studies. Li isotopic fractionation ranges from +19.4‰ (Δ solution -solid ) at 25°C to +6.7‰ at 250°C.The experiments also evidence the importance of Li isotopic fractionation during formation of Libearing secondary minerals by the uptake of lithium into the alteration minerals. Application of experimental results to the Bouillante area suggests that the geothermal water is in equilibrium at 250-260°C with a deep and large reservoir located in the transition zone possibly at the contact between volcanic flows and basaltic dikes. For the Lamentin and Diamant areas, the geothermal fluid appear to be partially in equilibrium at 90-120°C and 180°C, respectively, with reservoir sedimentary rocks. Our study highlights that lithium isotopic systematics is a powerful tool for the characterization of the origin of geothermal waters as well as the nature of their reservoir rocks.

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