Abstract
The injection of nitrate is one of the most commonly used technologies to impact the sulfur cycle in subsurface oil fields. Nitrate injection enhances the activity of nitrate-reducing bacteria, which produce nitrite inhibiting sulfate-reducing bacteria (SRB). Subsequent reduction of nitrate to di-nitrogen (N2) alleviates the inhibition of SRB by nitrite. It has been shown for the Medicine Hat Glauconitic C (MHGC) field, that alkylbenzenes especially toluene are important electron donors for the reduction of nitrate to nitrite and N2. However, the rate and extent of reduction of nitrate to nitrite and of nitrite to nitrogen have not been studied for multiple oil fields. Samples of light oil (PNG, CPM, and Tundra), light/heavy oil (Gryphon and Obigbo), and of heavy oil (MHGC) were collected from locations around the world. The maximum concentration of nitrate in the aqueous phase, which could be reduced in microcosms inoculated with MHGC produced water, increased with the toluene concentration in the oil phase. PNG, Gryphon, CPM, Obigbo, MHGC, and Tundra oils had 77, 17, 5.9, 4.0, 2.6, and 0.8 mM toluene, respectively. In incubations with 49 ml of aqueous phase and 1 ml of oil these were able to reduce 22.2, 12.3, 7.9, 4.6, 4.0, and 1.4 mM of nitrate, respectively. Nitrate reduced increased to 35 ± 4 mM upon amendment of all these oils with 570 mM toluene prior to incubation. Souring control by nitrate injection requires that the nitrate is directed toward oxidation of sulfide, not toluene. Hence, the success of nitrate injections will be inversely proportional to the toluene content of the oil. Oil composition is therefore an important determinant of the success of nitrate injection to control souring in a particular field.
Highlights
Sulfide accumulation in oil reservoirs through the reduction of sulfate in injection waters by sulfate-reducing bacteria (SRB) is referred to as souring and is highly undesirable
PNG, CPM, and Tundra oils were light with American Petroleum Institute (API) gravities from 38◦ to 46◦ and viscosities from 9.4 to 11.8 cP
The Gryphon and Obigbo oils were light/heavy with API gravities of 31◦ and 21◦ and viscosities of 16.6 and 50.9 cP, respectively, whereas the Medicine Hat Glauconitic C (MHGC) oil was heavy with an API gravity of 16◦ and a viscosity of 2471 cP
Summary
Sulfide accumulation in oil reservoirs through the reduction of sulfate in injection waters by sulfate-reducing bacteria (SRB) is referred to as souring and is highly undesirable. Nitrate promotes the growth of sulfide-oxidizing and heterotrophic nitrate-reducing bacteria (soNRB and hNRB). Souring Control by NRB sulfide directly while hNRB exclude SRB growth by competitive utilization of oil organics for nitrate reduction (Sunde and Torsvik, 2005; Youssef et al, 2009; Gieg et al, 2011; Voordouw, 2011). Volatile fatty acids (VFA, a mixture of acetate, butyrate, and propionate) and low molecular weight hydrocarbons such as alkylbenzenes are preferred oil organics for nitrate reduction by hNRB. Nitrite accumulation in oil reservoirs is stimulated at high-temperatures (50–70◦C). Nitrite is reduced further to dinitrogen (N2) in low-temperature oil fields (below 50◦C) in view of the excess of electron donors, which is usually present in oil fields (Reinsel et al, 1996; Agrawal et al, 2012, 2014; Fida et al, 2016)
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