| Title | In-Situ Material and Corrosion Studies at the Soultz-sous-Forêts (France) EGS Site |
|---|---|
| Authors | Julia SCHEIBER, Guillaume RAVIER, Nicolas CUENOT, Albert GENTER |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | metals and alloys, chemical and mechanical resistance, erosion, corrosion, coating stability, EGS |
| Abstract | At the Soultz EGS site a scientific and technical monitoring is associated with power plant operation. Material selection and testing of suitable materials became an essential part of the Soultz research program. Intensive in-situ material studies, involving scaling issues as well as corrosion and coating tests, were conducted. Saline Na-Ca-Cl brine of about 100 g/l Total Dissolved Solids is produced at 160°C/20 bars from a naturally fractured granitic reservoir. Measurements of the redox potential show reducing conditions and brine pH of 4.8 is slightly acidic. Gas Brine Volume Ratio is close to 1 with CO2 concentration more than 85%. In consequence, a pressure of 20 bars is maintained in the surface installations in order to avoid degassing and calcite precipitation. Selection of suitable materials for this aggressive environment focuses on stress resistance against chemical and mechanical attack, on durability and on cost effectiveness. It was recognized during those studies that material assessment needs to consider production and injection conditions as well as static and dynamic conditions. Volume of scale deposition and intensity of scale adherence as a function of surface roughness and electrochemical properties of applied metals turned out to be a crucial issue in Soultz especially for the heat exchanger unit and the re-injection side. Soultz scales are classified as NORM (Naturally Occuring Radioactive Material). Therefore, scale formation and scale deposition needs to be reduced not only by chemical scale inhibition but also by appropriate material selection. Static conditions seem to increase electrochemical corrosion for low alloyed metals contrary to dynamic conditions where a relatively low corrosion rate of 0.2 mm/year was observed at the re-injection side, 70°C/18 bars. Polymer coatings were found to be a promising option for metal surface protection but careful selection for specific power plant equipment is required and wrong selection can result in fatal consequences as observed for the heat exchanger unit at Soultz. Besides corrosion and material studies at operating power plant equipment, in-situ material test were conducted using bypass systems installed at the production and re-injection side, the High Temperature Skid (HTS) and the Low Temperature Skid (LTS). Metal coupons, straight pipes, T-shaped pipes and single heat exchanger pipes were installed at in-situ conditions and tested during circulation. In-situ experiments at the high and low temperature side of the Soultz geothermal loop provide unique opportunities to test and select materials based on their durability and cost effectiveness for geothermal applications with Upper Rhine Valley type fluids. |