Most geothermal installations exploiting the Dogger aquifer of the Paris Basin have encountered corrosion and scaling problems. The well casings are made of carbon steel and therefore do not resist geothermal water which is an anaerobic, slightly acidic medium characterized mainly by the presence of Cl-, SO42-, CO2/HCO3- and H2S/HS-. The implementation of anti-corrosion treatments by bottom hole injection of surface-active cationic agents (effective corrosion inhibitors at very low concentrations, 2.5 mg.l-1), has enabled operators of geothermal installations to resolve most of these corrosion and scaling problems. New problems are, however, now arising on some sites. This article presents the current state of knowledge of microbiological activity in this geothermal environment. It concerns identification of strains, and the localizing and evolution of bacterial activity in casings and reservoir. In order to study this evolution, two methods were tested based on monitoring the composition of the water at production wellheads, at variable flowrates. These methods enabled us to distinguish origin (and the respective amount) of the sulfide content in the water sampled at certain production wellheads: sulfide produced in the casings or sulfide coming from the reservoir. Since 1989, the evolution of total sulfide content, [S2-]T , has been observed in water sampled at certain production wellheads. For a constant flowrate, this content has increased regularly since the commissioning of these wells, and for a variable flowrate, it varies. It was suggested that the increase in [S2-]T with decreasing flowrate was due to current and increasing activity of sulfate-reducing bacteria (SRB) on well casings. The evolution of the isotopic composition of sulfur in sulfide, ƒÔ34S(S2-) as a function of the flowrate was used to verify and confirm the previous explanations. Then, at certain geothermal wells (those with low initial content in sulfide), [S2-]T progressively increases before reaching a plateau in the early 2000s. Studies in 1991 and 1996 revealed that the reservoir produced water increasingly rich in sulfide. This behavior, well-known for years, is due to the presence of SRB in the Dogger water and on the wells tubing. Sulfate reducing bacteria (SRB) strains were identified in the water and in the scale deposited on corrosion samples. After 2001, the high content of dissolved sulfide (and largely injected back into the reservoir) stay quasi stable at the production wellheads. The reservoir contribution to the [S2-]T is currently predominant, meaning that a major part of the Dogger reservoir is transformed and currently produces water in a more reduced state. Consequently, the operators of geothermal installations need currently higher concentrations of corrosion inhibitors to resolve most of their corrosion and scaling problems.

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