• Energy Research
• 2021-2025close

Abstract. Dissolved organic matter and microorganisms were analyzed along the flow path of a geothermal facility in Austria. Various analytical methods were used to characterize and differentiate between natural and synthetic organic matter, characterize the microbial community composition, and determine the implications of microorganisms in an operating a geothermal site. Dissolved organic carbon (DOC) concentrations were in the range of 8.4–10.3 mg C L−1 and typically decreased from the production to the injection side. Carbonate scalings are avoided in the facility by the injection of a chemical scaling inhibitor within the production well at 500 m depth. It was calculated that the inhibitor contributes approximately 1 mg C L−1 DOC to the produced fluids. Ion chromatography (IC), liquid chromatography – organic carbon detection (LC-OCD) and Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in negative electrospray ionization (ESI(−)) and positive atmospheric pressure photoionization (APPI(+)) mode were applied to the fluid samples to characterize the dissolved organic matter (DOM) composition and distinguish between the inhibitor and the natural DOM. Depending on the applied ionization mode, FT-ICR-MS results show that between 31 % and 65 % of the macromolecular formulas detected in the fluid samples seem to originate from the inhibitor. However, the DOM is mainly composed of low molecular weight acids (LMWA), especially acetate with up to 7.4 mg C L−1. The microbial community composition varied along the flowpath with dominant phyla being Firmicutes, Proteobacteria, and Thermotogae. Based on the microorganisms found in the sample, the metabolic pathways have been assessed. Acetate might be produced by microorganisms through various fermentation processes (e.g. from lysine, pyruvate and hexitol). Assessing the presence and interaction of organic compounds and microorganisms in geothermal fluids provides a broader understanding of processes within the geothermal facility. This understanding could be beneficial for the efficient use of a geothermal power plant.

The evaluation of the effect of organic compounds and microorganisms in formation and precipitation of colloids using artificial brines was performed by TNO using selected organic compounds based on the analysis of sampled fluids corresponding to the information gathered on the sites by GFZ. The same was done with biofilms prepared with microorganisms (Thermaerobacter sp., Penicilium citrinum) isolated from geothermal stations by UNINE. All carboxylic acids tested had an inhibiting effect on the precipitation of calcium carbonate. The biofilm components seem to develop intense interaction with the ions, nuclei and/or crystals formed during the executed experiments. In the presence of biofilms, the transformation of the intrinsically formed vaterite morphology to equilibrium calcite morphologies is delayed or hindered and scaling was inhibited.