Emerging Oil Field Biotechnologies: Prevention of Oil Field Souring by Nitrate Injection

Abstract

Much of the world’s oil is produced by water injection. Water injected through injection wells helps to maintain the reservoir pressure required to sweep the oil to the surface through production wells. When injected water breaks through, an oil-water mixture is produced. After separation from produced oil, the produced water can be reinjected, a production strategy referred to as produced water reinjection (PWRI). PWRI is commonly practiced in landlocked reservoirs where access to water may be limited. The fraction of water produced from oil fields subjected to water injection (the water cut) generally increases with time. Oil production by water injection often results in increased sulfide levels (souring), because sulfate-reducing bacteria (SRB) couple the oxidation of degradable oil organics to the reduction of sulfate to sulfide. Nitrate injection also stimulates heterotrophic nitrate-reducing bacteria (hNRB), which couple either incomplete oxidation of degradable oil organics (to acetate and CO2) or complete oxidation of oil organics (to CO2 only) to the reduction of nitrate to nitrite, nitrogen, or ammonia. The need to provide both a hydrocarbon substrate and a fermenting inoculum designed for high yields of biosurfactant adds to the complexity of traditional microbially enhanced oil recovery. Nitrate injection changes the microbial community downhole to one in which the activities of hNRB and in some cases nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB) are more prominent.