Use of Nitrate or Nitrite for the Management of the Sulfur Cycle in Oil and Gas Fields

Abstract

Abstract The production of sulfide by sulfate-reducing bacteria (SRB) in oil and gas fields causes problems including enhanced corrosion risk, reservoir plugging and deterioration of product quality. Injection of nitrate or nitrite stimulates heterotrophic nitrate-reducing bacteria (hNRB), which compete with SRB for oil organics, such as volatile fatty acids (VFA). Nitrate also stimulates nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB), which lower sulfide levels. Nitrite is a strong and specific inhibitor of the SRB enzyme responsible for sulfide production, whereas nitrate does not inhibit SRB. Hence, injection of nitrate or nitrite can prevent or remediate problems in the oil and gas industry caused by SRB activity, provided hNRB and NR-SOB are present. A survey of 8 oil fields, 2 gas storage reservoirs and an oil storage tank indicated that SRB and hNRB were widely distributed, whereas the distribution of NR-SOB appeared more limited. The SRB and hNRB were able to use lactate, as well as VFA as electron donor for sulfate or nitrate reduction. However, the order of use of VFA components appeared to differ with acetate being used preferentially by hNRB and propionate and butyrate being used preferentially by SRB. The production of nitrite by hNRB and NR-SOB varied greatly with quantitative conversion of nitrate to nitrite (up to 30 mM) being observed in one case; the nitrite formed was then reduced further. Samples from a high temperature North Sea oil field had thermophilic SRB, but no hNRB or NR-SOB activity, causing sulfide production to be inhibited by nitrite only. Although nitrite appeared to react chemically with sulfide under these conditions, causing all nitrite to disappear within 100 h, the observed inhibition was long-lasting (>500-1500 h). Hence, nitrite can be used successfully to control SRB activity in fields where hNRB and NR-SOB are absent. In summary, it appears that many oil and gas fields contain hNRB and NR-SOB populations, which are activated upon injection of nitrate or nitrite, allowing sulfide remediation in situ. Characterization of these populations as described in this paper may allow prediction whether these injections will be successful and whether use of nitrate or of nitrite is preferred.