Abstract

The application of molecular methods, like pyrosequencing of 16S rRNA genes, has now accelerated to the point where identification of microbial communities involved in microbiologically influenced corrosion (MIC) may become routine. We have used this technology to characterize samples obtained from North Sea oil-producing platforms.

Pigging solids were collected from a pipeline that carried a high water content, had a low velocity and cooled rapidly (mesophilic growth conditions), causing high corrosion rates to be expected. A second set of samples was obtained from a platform subjected to produced water reinjection (PWRI). The samples included produced water and PWRI water, a mixture of produced water and seawater. Historically, both locations showed high sulfate-reducing bacteria (SRB) numbers and the system was deemed to be under corrosion threat.

Sulfate concentrations were 2600 ppm in the PWRI water and 170 ppm in produced water, suggesting that sulfate reduction occurred. The pigging solids were found to have high counts of SRB, identified by pyrosequencing to be mostly Desulfovibrio. However, high fractions (13–50%) of sulfur-reducing bacteria (SuRB), of the order Desulfuromonadales were also found. Pyrosequencing indicated that produced water also had more SuRB (48%) than SRB (18%). The PWRI water had a high fraction of the sulfide-oxidizing bacterium (SOB) Arcobacter (25%).

The data suggest that mixing of produced water and oxygenated seawater gives rise to chemical and microbial (Arcobacter) oxidation of sulfide to sulfur, which is then reduced to sulfide by the SuRB using oil organics as electron donor. The predominance of SOB and SuRB indicates that sulfur is prevalent in the system, which indicates that MIC may be involved in this environment. Application of molecular methods greatly facilitates the collection of this information and is thus an important tool in assessing MIC threat.

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