UK Corrosion and Eurocorr
Oil Plus Ltd
Corrosion Implications of Thermophilic Sulphate- Reducing Bacterial Growth in Oil Reservoirs
Sulphate-reducing bacteria (SRB) are well known as problem organisms in industrial systems and are involved in microbially influenced corrosion (MIC) of, for example, cold water injection pipe work in oilfield process plant. This is due to the formation of an anaerobic biofilm containing an active mesophilic SRB (mSRB) population, in consortia with other heterotrophic bacteria.
Recent discoveries of thermophilic SRB (tSRB) from oilfield reservoirs and production systems have significant importance for biofouling and corrosion of hot oil production pipe work and associated plant. tSRB have been detected in many hot production systems, as well as in injection water and open seawater. In addition, other (non sulphide-producing) thermophilic heterotrophic bacteria have been isolated along with the tSRB, indicating the presence of widespread consortia of thermophilic bacteria in these hot, nutrient-rich environments. Actively growing tSRB readily attach to surfaces, assisted by the production of large amounts of exopolysaccharide. tSRB survive adverse conditions by the formation of dormant cells which have been shown to retain viability for long periods of time. These dormant cells may pass through the reservoir rock since they are small and produce less exopolysaccharide. They thus contaminate both the deep reservoir and the production system. In some situations, even starved cells may attach to surfaces and form part of a developing biofilm. Biofilms, containing tSRB and other thermophilic bacteria, thus develop in both rock pores and on metal surfaces. When such a biofilm develops on exposed steel, corrosion can be initiated by the production of sulphide or organic acids.
The microbiology of hot production systems is thus directly analogous to that in a cold water injection system, with mSRB being involved in MIC of injection systems and tSRB being involved in MIC of production systems. Biocides used to control tSRB growth must be selected with care as some active ingredients are inactivated by high temperatures or the presence of sulphide. Laboratory trials, using a range of active ingredients, have been used to determine the performance of biocides under simulated field conditions, prior to their use in field trials designed to control MIC mediated by tSRB in hot production systems.