Characteristics and inhibition of microbial induced corrosion and biofilm formation by thermal shocks under in-situ conditions in a geothermal heat-storage plant in Neubrandenburg (Germany)

Geothermal facilities in the North German Basin are frequently affected by corrosion and scaling due to high salinity as well as microbial induced corrosion. To study biofilm formation, corrosion processes and countermeasures, a mobile bypass system was installed at the Neubrandenburg geothermal heat storage plant. The reservoir was located in a depth of 1300m in a saline aquifer (~130g/L NaCl). Operation was conducted in two different seasonal modes. In the warm months, heat was stored, while in the winter months the direction of flow was reversed and heat was recovered. At temperatures lower than 60°C, at the cold side of the aquifer, corrosion was promoted by microbial activity, which led to the formation of biofilms on plant components and pipes. Biofilms consisted mainly of various genera of fermentative, sulfate reducing and hydrogen consuming bacteria. Longer incubation time as well as inoculation with fresh biofilm showed an independency of seasonal mode and enrichment of highly adapted community composition with the dominating sulfate-reducing genus Desulfallas.

As a countermeasure to corrosion, heat shocks were evaluated in the bypass system and tested also two times at the large scale plant. Heat shocks led to significantly reduced biofilm formation on corrosion coupons and correspondingly reduced iron sulfide precipitates and corrosion rates from 0.538mm/a to 0.170mm/a over an observation period of 48 days. The impact of one heat shock lasted for more than four weeks. Overall the use of periodic heat shocks showed its preventative measure against microbial induced corrosion and scaling in geothermal plants.