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Lookup NU author(s): Kimberley Noke, Professor Stephen Larter
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Fields from which oil is produced by injection of sulfate-bearing water often exhibit an increase in sulfide concentration with time (souring). Nitrate added to the injection water lowers the sulfide concentration by the action of sulfide-oxidizing, nitrate-reducing bacteria (SO-NRB). However, the injected nitrate can also be reduced with oil organics by heterotrophic NRB (hNRB). Aqueous volatile fatty acids (VFAs; a mixture of acetate, propionate, and butyrate) are considered important electron donors in this regard. Injection and produced waters from a western Canadian oil field with a low in situ reservoir temperature (30 °C) had only 0.1−0.2 mM VFAs. Amendment of these waters with nitrate gave therefore only partial reduction. More nitrate was reduced when 2% (v/v) oil was added, with light oil giving more reduction than heavy oil. GC−MS analysis of in vitro degraded oils and electron balance considerations indicated that toluene served as the primary electron donor for nitrate reduction. The differences in the extent of nitrate reduction were thus related to the toluene content of the light and heavy oil (30 and 5 mM, respectively). Reduction of nitrate with sulfide by SO-NRB always preceded that with oil organics by hNRB, even though microbially catalyzed kinetics with either electron donor were similar. Inhibition of hNRB by sulfide is responsible for this phenomenon. Injected nitrate will thus initially be reduced with sulfide through the action of SO-NRB. However, once sulfide has been eliminated from the near-injection wellbore region, oil organics will be targeted by the action of hNRB. Hence, despite the kinetic advantage of SO-NRB, the nitrate dose required to eliminate sulfide from a reservoir depends on the concentration of hNRB-degradable oil organics, with toluene being the most important in the field under study. Because the toluene concentration is lower in heavy oil than in light oil, nitrate injection into a heavy-oil-producing field of low temperature is more likely to succeed in containing souring.
Author(s): Lambo AJ, Noke K, Larter SR, Voordouw G
Publication type: Article
Publication status: Published
Journal: Environmental Science & Technology
Year: 2008
Volume: 42
Issue: 23
Pages: 8941-8946
ISSN (print): 0013-936X
ISSN (electronic): 1520-5851
Publisher: American Chemical Society
URL: http://dx.doi.org/10.1021/es801832s
DOI: 10.1021/es801832s
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