The economic efficiency of geothermal plants is often impaired by depositions of secondary minerals or corrosion damage. A countermeasure with high potential to improve the sustainability and economic use of geothermal plants is the application of inhibitors. Geothermal plants in the North German Basin extract highly saline thermal water from porous aquifers. Flow parameters of such aquifers are sensitive to secondary mineral formation. Mineral deposition near the injection well may decrease the injectivity. To date no suitable inhibitors are available for areas in Germany with (planned) geothermal use of porous aquifers. In geothermal plants, which extract thermal water from fractured aquifers - such as in the Upper Rhine Graben - commercially available inhibitors are currently used in testing operation. In the future, the innoxiousness to water management needs to be ensured and the applicability needs to be extended to lower injection temperatures, in order to enable sustainable and economic long-term operation. An overview over investigations performed within the collaborative project

The economic and technical efficiency of geothermal plants is often impaired by corrosion, scaling and biological fouling. In Germany, the highly saline fluid of the North German Basin is known to cause severe corrosion. Meanwhile geothermal plants in the southern Molasse Basin, one of the most extensively exploited geothermal regions in Germany, are troubled by carbonate scaling. One possible solution is the employment of a scale inhibitor. A novel scaling inhibitor is evaluated in field- and laboratory tests. This inhibitor consists of a polysaccharide backbone structure and branches of polyacrylic- and maleic acid copolymer. The laboratory tests with different scaling inhibitor concentrations were designed to observe the biodegradation of the scaling inhibitor in an anaerobic environment similar to the conditions found in heat exchangers of geothermal plants. The concentration of inhibitor was quantified by UV/VIS and liquid chromatography (LC). Molecular biological techniques (PCR, DGGE, Microbiome analysis) were used to characterize the biocenosis on metal surfaces and in fluids of the experiments. During the experiment the concentration of inhibitor decreased up to 3 % of the initial concentration. The formation of methane and acetate was observed which indicates a biological degradation by acetoclastic methanogenesis. Hydrogen formation was observed in setups containing steel coupons. This implies that hydrogen is primarily formed by corrosion processes and in tests with active microorganisms hydrogen was consumed completely. Various fermentative bacteria classified as Clostridia and Firmicutes as well as methanogenic archaea were identified. In some experiments sulfate reducing bacteria were found. Those are well known to catalyze corrosion processes. Results of field experiments in a bypass system as well as microbiological monitoring of the inhibitor application in geothermal plant located in the molasse basin will be presented.

At geothermal plants, process failures often occur due to corrosion and scaling processes. Especially after heat extraction, sulfate reducing bacteria contribute to corrosion processes by producing reduced sulfur compounds. In biofilms containing scales such as iron sulfides, corrosion processes are enhanced. In a mobile bypass system located at the geothermal plant in Neubrandenburg (North German Basin), the influence of biofilm formation on corrosion and scaling was investigated. Short-term heat shocks were successfully tested in the bypass system in order to reduce biofilm formation and thus to diminish corrosion and scaling processes.

(1) GFZ German Research Centre for Geosciences, Section 5.3 Geomicrobiology, Telegrafenberg, 14473 Potsdam, Germany (anne.kleyboecker@gfz-potsdam.de), (2) Jagiellonian University, Institute of Geological Sciences, Oleandry 21, 30-063 Krakow, Poland (monika.kasina@uj.edu.pl), (3) Hydroisotop GmbH, Woelkestr. 9, 85301 Schweitenkirchen, Germany (fe@hydroisotop.de), (4) BWG Geochemische Beratung GmbH, Seestr. 7A, 17033 Neubrandenburg, Germany (aseibt@bwg-geochemie.de), (5) GTN Geothermie Neubrandenburg GmbH, Seestr. 7A, 17033 Neubrandenburg, Germany (Markus.Wolfgramm@gtn-online.de), (6) Merseburg University of Applied Sciences, Eberhard-Leibnitz-Str. 2, 06217 Merseburg, Germany (hilke.wuerdemann@hs-merseburg.de)

The microbial biocenosis in highly saline fluids produced from the cold well of a deep geothermal heat store located in the North German Basin was characterized during regular plant operation and immediately after plant downtime phases. Genetic fingerprinting revealed the dominance of sulfate-reducing bacteria (SRB) and fermentative Halanaerobiaceae during regular plant operation, whereas after shutdown phases, sequences of sulfur-oxidizing bacteria (SOB) were also detected. The detection of SOB indicated oxygen ingress into the well during the downtime phase. High 16S ribosomal RNA (rRNA) and dsrA gene copy numbers at the beginning of the restart process showed an enrichment of bacteria, SRB, and SOB during stagnant conditions consistent with higher concentrations of dissolved organic carbon (DOC), sulfate, and hydrogen sulfide in the produced fluids. The interaction of SRB and SOB during plant downtimes might have enhanced the corrosion processes occurring in the well. It was shown that scale content of fluids was significantly increased after stagnant phases. Moreover, the sulfur isotopic signature of the mineral scales indicated microbial influence on scale formation.

In geothermischen Anlagen können Biofilme die Mineralbildung und die Injektivität von Bohrungen sowie die Materialbeständigkeit beeinträchtigen. In drei bezüglich Temperatur und Salinität sehr unterschiedlichen Anlagen waren Organismen des Schwefelkreislaufs an Betriebsstörungen beteiligt: Die erhöhte Abundanz von Sulfat-reduzierenden Bakterien (SRB) auf der kalten Seite eines Wärmespeichers wies auf deren Beteiligung an der Korrosion und der Abnahme der Injektivität hin. In allen Anlagen führte der Zutritt von Sauerstoff bzw. der Eintrag von Nitrat zu einer temporären Zunahme Schwefel-oxidierender Bakterien (SOB) und hat vermutlich Korrosionsprozesse beschleunigt. Außerdem hatte in einem Kältespeicher die temporäre Zunahme der SOB ein Filterclogging zur Folge. Aufgrund ihrer entscheidenden Rolle bei mikrobiell induzierter Korrosion (MIC) weisen Änderungen in der Abundanz von SOB und SRB auf die Ursachen mikrobiell bedingter Störungen hin. Zur Beseitigung der Störungen wurden temporäre Erhöhungen der Temperatur, Säuerungen sowie die Zugabe von Wasserstoffperoxid (H2O2) oder Nitrat in den Anlagen getestet und aus mikrobiologischer Sicht bewertet.

This paper describes microbial metabolic processes that are considered to be relevant for the technical reliability of a geothermal heat store. The study reports on changes of the microbial community composition in geothermal well fluids of different temperatures and after plant downtimes monitored by genetic fingerprinting. Stagnant conditions favored the enrichment of bacteria, sulfate reducers (SRB), and sulfur oxidizers (SOB) in the well. Furthermore higher concentrations of DOC, SO42-, H2S, and H2 were detected in the first fluids produced after plant downtime. The increased abundance of SOB indicated oxygen ingress during plant downtime. The interaction of SRB and SOB might have further enhanced corrosion and scaling processes. A mobile bypass system installed at the site will help to understand the processes occurring in the well and to study biofilm formation and corrosion rates at different temperatures.

The microbial diversity of a deep saline aquifer used for geothermal heat storage in the North German Basin was investigated. Genetic fingerprinting analyses revealed distinct microbial communities in fluids produced from the cold and warm side of the aquifer. Direct cell counting and quantification of 16S rRNA genes and dissimilatory sulfite reductase (dsrA) genes by real-time PCR proved different population sizes in fluids, showing higher abundance of bacteria and sulfate reducing bacteria (SRB) in cold fluids compared with warm fluids. The operation-dependent temperature increase at the warm well probably enhanced organic matter availability, favoring the growth of fermentative bacteria and SRB in the topside facility after the reduction of fluid temperature. In the cold well, SRB predominated and probably accounted for corrosion damage to the submersible well pump and iron sulfide precipitates in the near wellbore area and topside facility filters. This corresponded to lower sulfate content in fluids produced from the cold well as well as higher content of hydrogen gas that was probably released from corrosion, and maybe favored growth of hydrogenotrophic SRB. This study reflects the high influence of microbial populations for geothermal plant operation, because microbiologically induced precipitative and corrosive processes adversely affect plant reliability.

The efficient use of energy is an important issue of public interest. In the Neubrandenburg heat storage surplus heat from a gas and steam cogeneration plant is stored in an aquifer system for use of the stored energy during times of high heat demand. The reliability of such a plant can strongly be influenced by microbial communities. Therefore, biogeochemical monitoring of the heat storage of Neubrandenburg was conducted from March 2006 to January 2010 to characterize the natural variability of prokaryotic life by way of phospholipid fatty acid (PLFA) analysis and the availability of electron acceptors (e.g. sulfate) and donors [e.g. dissolved organic carbon (DOC)] under different operating modes of the plant. Analysis of the fluid chemistry showed that a sufficient amount of electron acceptors (sulfate ca. 1g/l) and donors (DOC up to 19mg/l) for potential microbial respiration and energy consumption is present. Phospholipid analysis of filter samples from the heat storage revealed a viable microbial community in the plant with adaptation to changes in the operating mode (charge/discharge) and associated variation in temperature (45–73°C). The PLFAs mainly influenced were saturated and branched FAs, most likely reflecting temperature adaptation by a variable microbial community in different parts of the heat storage. Furthermore, branched monoenoic FAs indicated the presence of sulfate-reducing bacteria within the plant.

Abstract Enhanced process understanding of engineered geothermal systems is a prerequisite to optimize plant reliability and economy. We investigated microbial, geochemical and mineralogical aspects of a geothermal groundwater system located in the Molasse Basin by fluid analysis. Fluids are characterized by temperatures ranging from 61°C to 103°C, salinities from 600 to 900 mg/l and a dissolved organic carbon content (DOC) between 6.4 to 19.3 mg C/l. The microbial population of fluid samples was analyzed by genetic fingerprinting techniques based on PCR-amplified 16S rRNA- and dissimilatory sulfite reductase genes. Despite of the high temperatures, microbes were detected in all investigated fluids. Fingerprinting and DNA sequencing enabled a correlation to metabolic classes and biogeochemical processes. The analysis revealed a broad diversity of sulfate-reducing bacteria. Overall, the detection of microbes known to be involved in biocorrosion and mineral precipitation indicates that microorganisms could play an important role for the understanding of processes in engineered geothermal systems.

In this study, the operation of a cold store, located in 30–60 m depth in the North German Basin, was investigated by direct counting of bacteria and genetic fingerprinting analysis. Quantification of microbes accounted for 1 to 10⋅105 cells per ml fluid with minor differences in the microbial community composition between well and process fluids. The detected microorganisms belong to versatile phyla Proteobacteria and Flavobacteria. In addition to routine plant operation, a phase of plant malfunction caused by filter clogging was monitored. Increased abundance of sulfur-oxidizing bacteria indicated a change in the supply of electron acceptors, however, no changes in the availability of electron acceptors like nitrate or oxygen were detected. Sulfur- and iron-oxidizing bacteria played essential roles for the filter lifetimes at the topside facility and the injectivity of the wells due to the formation of biofilms and induced mineral precipitations. In particular, sulfur-oxidizing Thiothrix generated filamentous biofilms were involved in the filter clogging.

Abstract Enhanced process understanding of engineered geothermal systems is a prerequisite to optimize plant reliability and economy. We investigated microbial, geochemical and mineralogical aspects of a geothermal groundwater system located in the Molasse Basin by fluid analysis. Fluids are characterized by temperatures ranging from 61°C to 103°C, salinities from 600 to 900 mg/l and a dissolved organic carbon content (DOC) between 6.4 to 19.3 mg C/l. The microbial population of fluid samples was analyzed by genetic fingerprinting techniques based on PCR-amplified 16S rRNA- and dissimilatory sulfite reductase genes. Despite of the high temperatures, microbes were detected in all investigated fluids. Fingerprinting and DNA sequencing enabled a correlation to metabolic classes and biogeochemical processes. The analysis revealed a broad diversity of sulfate-reducing bacteria. Overall, the detection of microbes known to be involved in biocorrosion and mineral precipitation indicates that microorganisms could play an important role for the understanding of processes in engineered geothermal systems.

A multidisciplinary approach to make feasible the generation of geothermal electricity in the North-German Basin (NGB) was initiated in 2000. To attain this goal, formation fluids from the 4,000 m deep Rotliegend rocks (a well-known gas reservoir) in the NGB needed to be extracted and their geochemistry determined. An in-situ laboratory was established by opening and deepening the former gas well Groß Schönebeck. Subsequent hydraulic and stimulation tests focused on the aeolian sandstones. The in-situ downhole samples of the 150°C hot fluids contain high amounts of dissolved solids (with salinities of up to 265 g/l). To increase permeabilities, rocks were stimulated hydraulically. Temperature and chemistry of injected fluids and the condition of the applied equipment have an important influence, as these control the solution and precipitation of different minerals such as iron hydroxides, sulphates, and carbonates.

World-wide experience in high-enthalpy geothermics showed that, while geothermal fluids are exploited, scaling as well as corrosion cause the main problems. Their character is affected by the chemical composition of reservoir fluids and their interaction with reservoir rocks or with injection fluids. During the stimulation and hydraulic tests in the well Groß Schönebeck E Gr Sk 3/90, geochemical investigations on more than 80 drilling fluids, produced fluids, down-hole samples, and wellhead gas samples were conducted since December 2000. In parallel, the diagenetic evolution was reconstructed. The characterisation of pore space filling minerals is based on petrographic determinations of thin sections from approx. 200 drill cores of Permo-Carboniferous rocks from the NEGB. Secondarily formed minerals mainly quartz, carbonates, anhydrite, hematite, and illite correspond with the pore fluid. This reservoir fluid, represented by the deep well, is characterised by high temperature (150°C), high salinity (260 g/l), and high contents of heavy metals such as iron. The latter may be caused by the missed geochemical barrier of sulfide. There is a distinct tendency to precipitate iron minerals, especially when contaminated with O2. The influx of O2 is connected with the supply of injection fluids during stimulation tests. Geochemical models show the risk of precipitation of baryte, anhydrite and amorphous silica, conditionally of calcite or aragonite, respectively. Beside chemical interactions among the involved fluids and rock composition, changes of temperature, pH and Eh are responsible for the scale precipitations.

A multidisciplinary approach to make feasible the generation of geothermal electricity in the North-German Basin (NGB) was initiated in 2000. To attain this goal, formation fluids from the 4,000 m deep Rotliegend rocks (a well-known gas reservoir) in the NGB needed to be extracted and their geochemistry determined. An in-situ laboratory was established by opening and deepening the former gas well Gros Schonebeck. Subsequent hydraulic and stimulation tests focused on the aeolian sandstones. The in-situ downhole samples of the 150°C hot fluids contain high amounts of dissolved solids (with salinities of up to 265 g/l). To increase permeabilities, rocks were stimulated hydraulically. Temperature and chemistry of injected fluids and the condition of the applied equipment have an important influence, as these control the solution and precipitation of different minerals such as iron hydroxides, sulphates, and carbonates.