Effect of water content on the growth of SRB in crude oil

Corrosion due to bacterial activities is referred to as Microbiologically Influenced Corrosion (MIC). MIC is aggressive and can lead to piping failure. MIC attack is local and leads to development of pits, due to interference in the cathodic process under oxygen free conditions by anaerobic bacteria. Sulfate reducing bacteria (SRB) may often be associated with this type of corrosion. In pipelines, transporting oil over long distances, maintenance and sufficient control of the steel is difficult. It is therefore of great interest to find out if MIC is a problem in transport systems with crude oil where the water content is low.
In our research we have studied the growth of SRB in oil with variable amounts of enrichment medium. Gullfaks, Snorre and Grane crude oil were used as carbon and energy source. Anaerobic cultures with 2 to 95 % v/v brine in Gullfaks crude oil were inoculated with a mixed culture of SRB and incubated at 30 C for 4 weeks. The cultures were amended with volatile fatty acids (VFA) to enhance growth of SRB. Cultures were analyzed for growth of SRB and H₂S production. Growth of SRB was observed in cultures with 6 % water and more, and H₂S production was observed in cultures with 10 % water and more. In cultures without VFA H₂S production was not observed.
Anaerobic cultures with 2 to 10 % v/v enrichment medium in Snorre and Grane crude oil were inoculated with a mixed culture of SRB and incubated at 30 C for 4 weeks. Half of the cultures were amended with VFA to enhance growth of SRB, the other half contained only crude oil as carbon and energy source. In cultures grown on crude oil from Snorre, with or without VFA, no growth of SRB or sulfide production was observed. In contrast, growth of SRB and H₂S production was observed in all cultures with crude oil from Grane, regardless the water amount and whether acids were added or not.
Chemical analysis of the crude oil showed major differences in oil composition with regards to the lighter oil components. Snorre oil contained 23 % Wt of oil compounds lighter than C10, while Gullfaks and Grane oil contained 9.7 and 2.9 % Wt respectively. The light oil fraction also contains the most water soluble components, including benzene and toluene known to be toxic to many bacteria, including SRB.
Since a typical export specification for crude oil is less than 0.5 % water, the results from Gullfaks and Snorre (light crude oil) showed that growth of SRB in transport pipelines was strongly inhibited under these conditions. In cultures with crude oil from Grane, a heavier crude oil, growth of SRB was observed even at 2 % water content.