Underground storage and technical facilities

Due to unexpected, critical pressure increases in the circuit of a geothermal plant, the material of a pump pre-filter was examined micro­bio­lo­gi­cally. High levels of hydrogen-utilizing, sulphate-reducing bacteria (SRB) were detected in liquid cultures and by PCR. Growth tests with the enriched bacterial cultures proved that the organisms only multiply at tempe­ra­tures below 40°C and therefore cannot originate from the heating circuit, which is over 55°C warm. Instead, they were bacteria intro­duced from the cold water circuit.

The risk of the organisms adapting to higher tempe­ra­tures and the associated conta­mi­nation of the warm water circuit was pointed out and sugges­tions for further measures were provided.

A drilled shaft with a 6 m diameter for a salt mine was to be sunk using a CMC drilling fluid (carboxy-methyl cellulose). Several 1,000 m³ of drilling fluid was stored in over ground basins. An intensive microbial CMC disin­te­gration set in during the summer months. As a result, the drilling fluid could not be stabi­lized even through conti­nuously high subse­quent doses of CMC. The driller threa­tened to remain stuck due to the swelling clay in the drilled hole.

The result of extensive biocide tests showed that in the short term no prepa­ration could be provided for this dimension.

Therefore an alkaline stabi­li­sation of the drilled fluid was tested and suggested at a pH value of 11–12. The bacteria were thus effec­tively destroyed and the drill could be lowered safely without any further loss of fluid.

 During a solution mining process for under­ground gas storage a heavy preci­pi­tation and floccu­lation with subse­quent plugging occurred.

In water samples high cell numbers of various physio­lo­gical groups of bacteria were detected. It was supposed that organic compounds (cellu­losic substances), which have been injected with water into cavern serve as nutrient source. Microbial degra­dation processes obviously resulted in a high conta­mi­nation, which has not been suppressed by high salt content of produced brine. Rather, it was demons­trated that during the solution mining process a salt resistant bacteria population estab­lished, which could grow on 245 g NaCl per litre.

It could be proven that slime-forming micro­or­ga­nisms were substan­tially involved in floccu­lation. As this process continues in simulated tests net-like struc­tures developed containing filamentous bacteria. With incre­asing salt concen­tra­tions more complex filaments were produced. This corre­lation could be explained with emergence of new bacterial cell morpho­logies charac­te­rized by reduced mobility and disturbed cell division under physio­lo­gical stress at high salinity.

As a result of micro­bio­lo­gical inves­ti­ga­tions case-specific measures were recom­mended to reduce nutrient discharge and finally suppress bacteria.

 Corrosion damages on the outer wall of a natural gas pipeline within a swampy area have been detected by the operator. The pipeline was equipped with a cathodic corrosion protection.

A key factor for corrosion was initial process of hydrogen sulfide generation by sulfate reducing bacteria (SRB) which utilized the hydrogen jacket as energy source generated by cathodic corrosion protection. Hydrogen sulfide (H₂S) produced by SRB reacted with iron ions dissolving from pipeline wall finally forming black iron sulfide (FeS). H₂S and FeS are oxidized by several groups of sulfur oxidizing bacteria (SOB) to sulfuric acid and iron(II) sulfate. Speci­fi­cally Acidi­thi­o­ba­cillusferro­oxidans oxidizes iron(II) sulfate to iron(III) salts. Starting from iron(II)sulfide (pyrite) substantial amounts of sulfuric acids will be produced in connection with water, which finally lead to an increased micro­bially influenced corrosion (MIC) of metal components.

As a result of extensive micro­bio­lo­gical inves­ti­ga­tions, the activity of bacteria involved and cause of the corrosion process have been determined.