Effect of Thermophilic Nitrate Reduction on Sulfide Production in High Temperature Oil Reservoir Samples

Oil fields can experience souring, the reduction of sulfate to sulfide by sulfate-reducing microorganisms. At the Terra Nova oil field near Canada's east coast, with a reservoir temperature of 95 degrees C, souring was indicated by increased hydrogen sulfide in produced waters (PW). Microbial community analysis by 16S rRNA gene sequencing showed the hyperthermophilic sulfate-reducing archaeon Archaeoglobus in Terra Nova PWs. Growth enrichments in sulfate-containing media at 55-70 degrees C with lactate or volatile fatty acids yielded the thermophilic sulfate-reducing bacterium (SRB) Desulfotomaculum. Enrichments at 30-45 degrees C in nitrate-containing media indicated the presence of mesophilic nitrate-reducing bacteria (NRB), which reduce nitrate without accumulation of nitrite, likely to N-2. Thermophilic NRB (tNRB) of the genera Marinobacter and Geobacillus were detected and isolated at 30-50 degrees C and 40-65 degrees C, respectively, and only reduced nitrate to nitrite. Added nitrite strongly inhibited the isolated thermophilic SRB (tSRB) and tNRB and SRB could not be maintained in co-culture. Inhibition of tSRB by nitrate in batch and continuous cultures required inoculation with tNRB. The results suggest that nitrate injected into Terra Nova is reduced to N-2 at temperatures up to 45 degrees C but to nitrite only in zones from 45 to 65 degrees C. Since the hotter zones of the reservoir (65-80 degrees C) are inhabited by thermophilic and hyperthermophilic sulfate reducers, souring at these temperatures might be prevented by nitrite production if nitrate-reducing zones of the system could be maintained at 45-65 degrees C.