| Authors |
Olivier SEIBEL, Justine MOUCHOT, Guillaume RAVIER, Béatrice LEDESERT, Xavier SENGELEN, Ronan HEBERT, Kolbrún Ragna RAGNARSDOTTIR, Dagur Ingi ÓLAFSSON, Helen Ósk HARALDSDOTTIR |
| Abstract |
Several projects in the Upper Rhine Graben (URG), both in France and Germany, have been developed over the last decade thanks to knowledge gathered at the Soultz power plant. These plants produce very saline geothermal fluids (TDS 100 g/L) at 150-170°C with similar physico-chemical characteristics, and reinject them at about 70°C, a temperature equivalent to the production temperature in the Paris Basin. Growing knowledge surrounding EGS and reservoir exploitation suggests that higher energy use would be sustainable, by reinjecting the geothermal fluid at a lower temperature (less than 50°C), which is to be further investigated through parallel simulations of the impact that a colder reinjection would have on the reservoir. An objective of the H2020 MEET project is to identify the operating limitations of this colder reinjection at the Soultz-sous-Forêts geothermal power plant with regards to scaling and corrosion. These operating limitations are defined through a field test using a specifically designed tube heat exchanger composed of 6 different materials, including various stainless-steel grades, and titanium and nickel alloy, among others. This heat exchanger, operated for three months during the first semester of 2019, allows the scaling formation and corrosion processes to be investigated at five temperature levels. The geothermal fluid flowing through the equipment represents a fraction of the total treated production flow in order to limit the potential impacts on the equipment itself and protect the rest of the installation. After three months of continuous geothermal fluid circulation, the structure, mineral and chemical composition, and quantity of the scaling samples is analysed for each temperature level and material, enabling a better understanding of the relationship between scaling formation, corrosion processes and tube materials. These analyses are performed using X-Ray Diffraction and a Scanning Electronic Microscope. In the URG geochemical context, barium sulphate and metal-rich sulphides precipitation is triggered by thermal exchanges. This process is enhanced by gradually decreasing the temperature. This three-month test provides a better understanding of the efficiency at a lower temperature of the chemical treatments than currently used, and helps identify potential new interactions in this context, like silica precipitation. In addition, the heat exchanger is dismantled in order to analyse the integrity of each tube material at different temperature levels through corrosion analysis. Corrosion analyses are carried out in order to rank the different alloys tested in terms of resistance to general and pitting corrosion. The detection of severe localised corrosion would be a way to discard the use of certain tube materials in the detailed designs for future applications. The expected outcome of this field test is to optimise the geothermal fluid injection temperature in the URG, finding a compromise between scaling precipitation and corrosion processes with controlled costs, in order to allow harnessing up to 20-35 percent of additional heat. |