51313-02606-Corrosion Processes at the Geothermal Site Gross Schönebeck (North German Basin)

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Product Number: 51313-02606-SG

ISBN: 02606 2013 CP

Author: Simona Regenspurg

Publication Date: 2013

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SHighly saline geothermal fluid is produced from a Permian sandstone reservoir (4.4 km depth; 150 °C) at the geothermal in situ laboratory in Groß Schönebeck (North German Basin). The brine is of Na-Ca-Cl type and contains about 265 g/L total dissolved solids. The gas phase is predominantly marked by N2 (85 %) and CH4 (14 %) and the volume ratio of liquid to gas is at amospheric conditions about 1:1.
During the production of the fluid an intensive monitoring of the different phases of the fluid was performed in 2011 and 2012. An on-line gas monitoring allowed to measure the amount of N2 CH4 H2 CO2 O2 He and H2S in the gas flux which was seperated from the fluid in the above ground installations. A complex fluid monitoring system allowed to measure fluid physicochemical properties such as pH redox and oxygen in the solution. Additionally liquid samples have been collected at the production well and analyzed by ion chromatography and ICP-OES for their chemical composition. Before fluid re-injection the brine passes two filter units (< 20 ?m). The filter bags have been analyzed for their chemical and mineralogical content by X-ray diffraction (XRD) and scanning electron microscopy (SEM).
Results from filter residue- gas- and brine analysis strongly indicated anaerobic corrosion – presumably of the production well casing. The corrosion products are H2 in the gas phase (up to 8 vol-% during downtime) magnetite (Fe3O4) in the solid phase and a decreased Fe concentration in the liquid phase (80 mg/L) during fluid production as compared with 245 mg/L Fe in deep fluid samples.
These finding suggest that the Schikorr Reaction is taking place (3Fe + 4 H2O ? Fe3O4 + 4 H2) according to which zerovalent iron from steel casing or divalent iron ions from the solution are oxidized by H2O to form the Fe(II) / Fe(III) mineral magnetite (Fe3O4) and hydrogen gas.