Impact of Nitrate on the Sulfur Cycle in Oil Fields

Production of oil from subsurface reservoirs requires injection of water or gas to maintain reservoir pressure. Seawater is usually injected on offshore platforms (as in the North Sea). The combination of abundant electron donors (selected oil components) and electron acceptors (30 mM sulfate in sea water) can lead to significant production of sulfide in the subsurface through action of resident or injected sulfate-reducing bacteria (SRB). Lowering sulfide concentrations in the produced oil–water mixture is desirable to reduce corrosion risk. Injection of nitrate has recently emerged as a new technology that can reduce sulfide levels reliably. Adding low concentrations (50–100 ppm) of nitrate continuously to all injected water can eliminate sulfide from produced water and oil. The mechanism underlying this technology appears to be largely microbial. Nitrate-reducing, sulfide-oxidizing bacteria remove sulfide with production of nitrite and other reactive nitrogen species. Nitrite is a powerful SRB inhibitor that specifically affects dissimilatory sulfite reductase, the enzyme that produces the sulfide. Heterotrophic nitrate-reducing bacteria can directly oxidize oil components with the injected nitrate, outcompeting SRB. This results in a desirable subsurface microbial community change that prevents the formation of sulfide, improving oil quality. Nitrate injection is one of the first reliable, microbe-based processes that is becoming widely used in oil production to control the oil field sulfur cycle, making microbiologists partners in discovering how we can continue to produce the world’s most significant energy resource.