Competitive, Microbially-Mediated Reduction of Nitrate with Sulfide and Aromatic Oil Components in a Low-Temperature, Western Canadian Oil Reservoir

Abstract

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 degrees 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 oilthan in light oil, nitrate injection into a heavy-oil-producing field of low temperature is more likely to succeed in containing souring.

Implications of Limited Thermophilicity of Nitrite Reduction for Control of Sulfide Production in Oil Reservoirs

“…S3 in the supplemental material). Although we did not detect nitrate reduction with xylenes, the use of m-xylene as an electron donor for nitrate reduction with accumulation of nitrite under mesophilic conditions has been reported previously (34,48).…”

“…This was confirmed by determination of MPNs of mNRB at 40°C (5 ϫ 10 5 ml Ϫ1 and 3 ϫ 10 7 ml Ϫ1 for 18PW and 14IW, respectively) and of tNRB at 60°C (45 ml Ϫ1 and 20 ml Ϫ1 for 18PW and 14IW, respectively). NRB in the MHGC field use low-molecular-weight oil components, especially toluene, as electron donors for nitrate reduction (33,34). Concentrated 18PW reduced 10 mM nitrate at 40°C with toluene and ethylbenzene, but not with benzene or xylenes, as the electron donor (see Fig.…”

Acetate Production from Oil under Sulfate-Reducing Conditions in Bioreactors Injected with Sulfate and Nitrate

“…Our experimental design did not allow reliable determination of toluene and other alkyl-benzenes, which were depleted in all three columns. However, microcosm studies of MHGC oil incubated with produced water only (31) or with a produced water inoculum and medium (30) had previously indicated that hNRB prefer toluene and other alkyl benzenes as electron donors for nitrate reduction, whereas SRB used both toluene and other alkylbenzenes, as well as alkanes (30,31,44). Partial or complete inhibition of sulfate reduction in bioreactors S-LN and S-HN was likely responsible for the decreased use of alkanes under nitrate-reducing conditions.…”

“…These migrate from injectors to producers, depending on the availability of toluene, the preferred electron donor of hNRB of the genus Thauera (30,31,44). In the absence of nitrate breakthrough, produced waters contain sulfide but no sulfate and are dominated by the presence of hydrogenotrophic methanogens (Methanoculleus, Methanolinea, and Methanocalculus) and the acetotrophic methanogen Methanosaeta (Table 2, 3-PW, 4-PW, and 10-PW; see Table S1 in the supplemental material).…”

“…In applying this biotechnology (25)(26)(27)(28), volatile fatty acids (VFA; e.g., acetate, propionate, and butyrate) are often considered to be the predominant electron donors for nitrate reduction and decisions with respect to the nitrate dose to be used take VFA concentrations in produced waters into account (29). Direct use of oil components, especially toluene, as electron donors for nitrate reduction has also been demonstrated (30,31,32). When VFA were used as electron donors in microcosm or bioreactor studies, production of acetate from propionate and butyrate was seen under sulfate-reducing, but not under nitrate-reducing, conditions (33,34).…”