Composition, stability, and relevance of organic compounds in geothermal fluids

Geothermal energy is a renewable resource that is becoming increasingly important for meeting global climate targets, particularly those of the European Union (EU), which aims to reduce net greenhouse gas emissions by 55% by 2030 and achieve climate neutrality by 2050. The effective use of geothermal systems for electricity generation, heating, and industrial processes contributes significantly to these goals. In geothermal operations, a carrier fluid, typically groundwater from aquifers, transports heat from the subsurface to surface facilities where thermal energy is extracted. The cooled fluid is then reinjected to maintain a closed loop. However, geothermal fluids often contain complex mixtures of dissolved minerals, gases, and organic compounds that create operational challenges. Corrosive substances such as hydrogen sulfide can promote mineral scaling, corrosion, and biofilm formation within infrastructure, reducing efficiency and increasing maintenance costs. These issues highlight the importance of effective fluid management. While the inorganic chemistry of geothermal fluids has been widely studied, organic compounds remain less well understood. They can influence fluid geochemistry by forming metal complexes, acting as microbial substrates, and contributing to corrosion, scaling, and biofilm growth. These processes intensify the challenges associated with the extreme conditions of geothermal systems and may reduce system performance and lifespan. This dissertation investigates the composition, stability and role of dissolved organic carbon (DOC, <0.45 μm) in geothermal fluids. Three key questions are addressed: (1) Which organic compounds commonly occur in geothermal fluids, and what are their sources? (2) Which factors control the concentration and composition of DOC? (3) How does organic matter affect geothermal operations, particularly with regard to scaling, corrosion, and microbial activity?