Potential of nitrate addition to control the activity of sulfate-reducing prokaryotes in high-temperature oil production systems - a comparative study on a nitrate-treated and an untreated system

Sulfate-reducing prokaryotes (SRP) cause severe problems like microbial corrosion and reservoir souring in seawater-injected oil production systems. Adding nitrate to the injection water is applied to control SRP activity by favoring the growth of heterotrophic, nitrate-reducing bacteria (hNRB) and nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB). Microbial diversity, abundance of Bacteria, Archaea and sulfate-reducing prokaryotes (SRP) and the potential activity of SRP were studied in production water samples from a nitrate-treated and an untreated system. The reservoirs and the produced water share similar physicochemical characteristics. At both sites, Archaea and Archaeoglobus-related SRP dominated the total prokaryotic and the sulfate-reducing community, respectively. It was however indicated from clone libraries and the quantification of 16S rRNA and dsrAB gene copies that Archaeoglobus-related SRP were less prominent at the nitrate-treated site than at the untreated site. In return, thermophilic bacterial SRP appeared to be more abundant (2 and 8 % of all SRP, respectively). They were related to members of the genera Desulfacinum and Desulfoglaeba (system without nitrate) and Desulfotomaculum (system with nitrate). In samples from the untreated site, the presence of active SRP was supported by demonstrating their activity (incubations with 35S-sulfate) and growth in batch cultures at pipeline temperature. No SRP activity was detected at reservoir temperature and in samples from the nitrate-treated site. In addition, potential competitive nitrate reducers of the genus Sulfurospirillum (NR-SOB) and the order Deferribacterales (hNRB) were exclusively detected at the nitrate-treated site. It is therefore indicated that nitrate addition resulted in decreased SRP activity, an increase in diversity of Bacteria and bacterial SRP and a stimulation of nitrate-reducing competitors.