Effect of nitrate and nitrite on sulfide production by two thermophilic,sulfate-reducing enrichments from an oil field in the North Sea

Thermophilic sulfate-reducing bacteria (tSRB) can be major contributors to the production of H₂S (souring) in oil reservoirs. Two tSRB enrichments from a North Sea oil field, NS-tSRB1 and NS-tSRB2, were obtained at 58°C with acetate-propionate-butyrate and with lactate as the electron donor, respectively. Analysis by rDNA sequencing indicated the presence of Thermodesulforhabdus norvegicus in NS-tSRB1 and of Archaeoglobus fulgidus in NS-tSRB2. Nitrate (10 mM) had no effect on H₂S production by mid-log phase cultures of NS-tSRB1 and NS-tSRB2, whereas nitrite (0.25 mM or higher) inhibited sulfate reduction. NS-tSRB1 did not recover from inhibition, whereas sulfate reduction activity of NS-tSRB2 recovered after 500 h. Nitrite was also effective in souring inhibition and H₂S removal in upflow bioreactors, whereas nitrate was similarly ineffective. Hence, nitrite may be preferable for souring prevention in some high-temperature oil fields because it reacts directly with sulfide and provides long-lasting inhibition of sulfate reduction.