16S rRNA gene sequencing was used to identify a sulfate-reducing bacterium (SRB) from a Danish North Sea oilfield water injection system. This species was cultivated, purified and subsequently identified as being >97% similar to Desulfovibrio gracilis.
Like some other Desulfovibrio species this SRB, strain OP102, could reduce nitrate as an electron acceptor and produce ammonia in the absence of sulfate. In addition, in the presence of sulfate, when nitrate was dosed at 100 mg/l it was again reduced by the bacterium, with some ammonium production. Therefore, this mechanism could be important in oilfield systems where nitrate is applied to prevent sulfide generation by SRB which leads to reservoir souring.
In static tests the influence of this Desulfovibrio on corrosion was assessed using carbon steel coupons, in the presence of sulfate and in the presence of sulfate with 100 mg/l nitrate. Corrosion rates were less than 1.5 mpy when coupons were incubated in the same water, with sulfate and with nitrate. Furthermore, the occurrence of pitting corrosion was fairly low under all circumstances.
Sulfate-reducing bacteria often thrive when seawater is injected into oilfields in order to increase pressure and drive more oil out (secondary recovery). These bacteria can be responsible for: H2S production, which causes ?souring? of the crude oil; microbiologically influenced corrosion (MIC); increased solids loading in water injection systems (iron sulfides, etc.); and can cause plugging of injection wells by sulfide or biofilm proliferation. In order to prevent their growth the industry spends millions of dollars on chemicals such as biocides. Recently, a novel treatment, using nitrate has been used under certain circumstances. Nitrate promotes the growth of nitrate-utilizing bacteria (indicated by the general acronym, NUB) to the detriment of sulfate-reducing bacteria (SRB). There are several potential mechanisms by which the nitrate acts1 but these are often not fully understood.
Previous field-testing has identified significant numbers of SRB growing in and around some water injectors, even where nitrate is being dosed to the injected water. However, conventional wisdom suggests that NUB populations, which are also high in number, should out-compete SRB for the available carbon source. A hypothesis for how SRB are able to survive nitrate application is that some species are able to switch from sulfate and respire nitrate/nitrite. Evidence in the literature suggests that some SRB can use nitrate as a terminal electron acceptor2. In the case of certain strains this has only been observed at very low sulfate concentrations3. However, for other species sulfate reduction is suppressed in favor of nitrate reduction when both sulfate and nitrate are present in non-limiting concentrations4. These results suggest that some SRB can compete successfully with specialized NUB. If a significant proportion of SRB are able to use nitrate it has been suggested that a larger mixed species biomass may result in the near well bore area of a reservoir. It is then possible that cessation of nitrate treatment may lead to worsened souring by the larger bacterial population.
Another potential drawback with nitrate application is that where biocides exert control by removal of bacteria, control by nitrate relies on a population of NUB being present in the system, or on a surface. The MIC potential of such NUB biofilms in topside equipment is not well documented. There have also been suggestions that nitrite, produced by some NUB, can be corrosive under certain conditions. In this work the potential for an SRB species to use nitrate is e