Inhibition of microbial H₂S production in an oil reservoir model column by nitrate injection

Abstract.

The effect of nitrate addition on microbial H₂S production in a seawater-flooded oil reservoir model column with crude oil as carbon and energy source was investigated. Injection of 0.5 mM nitrate for 2.5–3.5 months led to complete elimination of H₂S (initially 0.45–0.67 mM). The major decline in H₂S level coincided with the first complete nitrate consumption and production of nitrite. When nitrate was excluded, H₂S production resumed after approximately 2.5 months and reached previous levels after approximately 5 months. Using a fluorescent antibody technique, three populations each of sulfate-reducing bacteria (SRB) and nitrate-reducing bacteria (NRB) were monitored. SRB dominated the anoxic zone prior to nitrate addition, comprising 64–93% of the total bacterial population. The monitored NRB constituted less than 6% and no increase was observed during nitrate addition (indicating that other, unidentified, NRB populations were present). After 1–3 months without significant H₂S production (3.5–5.5 months with nitrate), the SRB population collapsed, the fraction being reduced to 9–25%. The dominant SRB strain in the column, which constituted on average 94% of the monitored SRB population, was partly/completely inhibited by 50/75 µM nitrite in batch culture tests. Similar nitrite concentrations (50–150 µM) were detected in the column when the H₂S level declined, indicating that nitrite inhibition was the main cause of H₂S elimination. The results from this study indicate that nitrate/nitrite can be used to prevent detrimental SRB activity in oil reservoirs.