Scaling in a Geothermal Heat Exchanger at Soultz-Sous-Forêts (Upper Rhine Graben, France): A XRD and SEM-EDS Characterization of Sulfide Precipitates

Abstract

The Soultz-Sous-Forêts geothermal site (France) operates three deep wells for electricity production. During operation, scales precipitate within the surface installation as (Ba, Sr) sulfate and (Pb, As, Sb) sulfide types. Scales have an impact on lowering energy production and inducing specific waste management issues. Thus scaling needs to be reduced for which a thorough characterization of the scales has to be performed. The geothermal brine is produced at 160 °C and reinjected at 70 °C during normal operation. In the frame of the H2020 MEET project, a small heat exchanger was tested in order to allow higher energy production, by reinjecting the geothermal fluid at 40 °C. Samples of scales were analyzed by XRD and SEM-EDS, highlighting that mostly galena precipitates and shows various crystal shapes. These shapes can be related to the turbulence of the flow and the speed of crystal growth. Where the flow is turbulent (entrance, water box, exit), crystals grow quickly and mainly show dendritic shape. In the tubes, where the flow is laminar, crystals grow more slowly and some of them are characterized by well-developed faces leading to cubes and derived shapes. The major consequence of the temperature decrease is the increased scaling phenomenon.

Corrosion and Scaling in Geothermal Heat Exchangers

“…Notably, the presence of Ra 226 and Pb 210 radionuclides in some scale products classifies them as Naturally Occurring Radioactive Material waste (NORM classification), increasing disposal costs [55]. Additionally, scale formation significantly impacts the corrosion of surface installations in geothermal power plants [11]. Regular maintenance operations are necessary to remove scales, often requiring manual hydroblasting to clean tubes [54].…”

A Review on Geothermal Heat Exchangers: Challenges, Coating Methods, and Coating Materials

“…A study conducted on the small tubular heat exchangers of Soultz-sous-forêts by Ledésert, B. A. et al mentioned the occurrence of PbS (lead sulphide) scaling alongside As and Sb sulphides and halite [24]. One of the impacts of silica scaling includes the roughening of the heat exchanger surface.…”

Editorial of Special Issue “Enhanced Geothermal Systems and Other Deep Geothermal Applications throughout Europe: The MEET Project”

“…Scales have a negative impact on the power plant in lowering the electricity production and inducing specific waste management issues. Thus, scales have to be thoroughly characterized [1] and their deposition process modelled [2] to control scaling processes in surface installations. In order to increase energy output, a small-scale heat exchanger prototype, called SHEx in the following, has been tested for 3 months in order to lower the reinjection temperature down to 40 • C. The SHEx is of tubular type, made of an entrance, an exit, water boxes at each end, and tubes made of several alloys and metals in between.…”

“…During the four years of the project, two main types of exploitations were investigated: Enhanced Geothermal Systems (EGS) and oil-to-geothermal conversion or co-production. The following topics were addressed: the upscaling of thermal power production and optimized operation of EGS plants (papers [1][2][3][4][5]); variscan geothermal reservoirs in granitic and metamorphic rocks (papers [6][7][8][9][10][11][12][13][14][15][16][17]); and technical, economic and environmental assessment for oil-to-geothermal fields and EGS integration into energy systems (papers [18][19][20]). These 20 papers give an overview of some of the work performed in the MEET project, but they are not exhaustive of all the results obtained in this frame.…”