Geochemical and Mineralogical Monitoring of the Geothermal Power Plant in Soultz-sous-Forêts (France)

Abstract

At the Soultz-sous-Forêts EGS site, a three year programm, dealing with a scientific and technical monitoring of the Soultz power plant, is ongoing between 2010 and 2012. Several hydraulic circulation tests have been running that have involved the production well, GPK-2, and two injection wells, GPK-1 and GPK-3: a long term circulation test for about 11 months in 2010 and two short-term tests in 2011. Within this research programm, a geochemical and mineralogical monitoring concept is included: Physico-chemical parameters of the geothermal fluid, sampled from the production well and the injection wells, are determined regularly. Additionally, deposits in the surface installations of the plant, inorganic and organic, are also sampled regularly and characterized in detail. At the Soultz-site, scales are formed mainly at the cold part of the power plant due to the significant temperature decrease of 90˚C by passing the heat exchanger. These scales consist mainly of barium/strontium sulfates, lead sulfide and trace amounts of mixed sulfides (Fe, Cu, Sb and As). They are known to accumulate radionuclides by chemical substitution during the crystal growth: 226Ra in barium/strontium sulfates and 210Pb in lead sulfide. Therefore, the equivalent dose and the surface activity of the ORC heat exchanger scales were determined. A correlation between the mineralogical composition (sulfates and sulfides) and the incorporated radionuclides (226Ra, 210Pb) was found. To assure safety at work and reduce the costs of the cleaning procedure and the scale disposal, the setup of an inhibitor system for scale avoidance has been defined. Therefore, several phosphonic acids were tested in laboratory experiments for their ability to suppress scale formation in the Soultz brine. Based on these experiments, the product with the best performance for the Soultz site was chosen. The inhibitor will be injected continuously during circulation of the brine. For testing the effectiveness of the inhibitor injection on-site, the geochemical and mineralogical monitoring concept of Soultz includes the characterization of brine and deposits in the surface installations of the power plant before and during the continuously addition of the inhibitor.