Successful application of both nitrate and nitrite to combat souring in oil fields has been reported. The effect of these treatments on corrosion is not well documented. Using up-flow, packed-bed bioreactors simulating an oil field we have found that both nitrate and nitrite are effective sulfide removers. The required dose depended on the concentration of oil organics used as the energy source by the microbial community. Because of its higher oxidative power, nitrate can remove more oxidizable oil organics than nitrite. However, nitrite is a strong SRB inhibitor. Nitrate gives less SRB inhibition, because it is only partially converted to nitrite. Because iron corrosion is either not affected or increased by the presence of nitrate, but strongly inhibited by nitrite under our experimental conditions we conclude that use of nitrite is on balance more favorable than use of nitrate.
Oil fields harbor a wide variety of bacteria, catalyzing reactions that affect the oil production process ~. Microbial growth in oil fields is limited by the concentration of available electron acceptors (02, nitrate, F e 3+ o r sulfate). The conditions for microbial growth are vastly improved when electron acceptors are provided, e.g. during offshore secondary oil recovery when seawater is injected for oil production. The high sulfate concentration in seawater (~30 mM) boosts the growth of sulfate-reducing bacteria (SRB), leading to souring. As an example in 1994 total H2S production from the Skjold field in the Danish sector of the North Sea was 100 kg/day, whereas in 2000 this had increased to 700 kg/day, with occasional surges to 1100 kg/day 2. The presence of sulfide is undesirable, because of its toxic and corrosive properties. Sulfide can be removed topsides by chemical treatment (e.g. amine scrubbing) of the oil-produced water mixture, or in the oil-bearing subsurface by amending injection waters with nitrate or nitrite. Addition of biocides to injection waters or topsides equipment can also prevent sulfide formation by SRB. Tetrakishydroxymethylphosphonium sulfate (THPS) and glutaraldehyde have been used in the Skjold and Velsefrikk fields, respectively 2' 3. In several Alberta oil reservoirs (Wildmere, Wainwright) diamines have been used routinely to reduce corrosion associated with SRB growth in water separators and oil storage tanks 4.
Use of nitrate has been advocated as a more environmentally friendly option with reduced health risks to oil field personnel. Injection of 400 ppm nitrate reduced sulfide concentrations in injector and producing wells of the Coleville field (Saskatchewan, Canada) by 73% 5. Repacement of pulsed application of glutaraldehyde in the Velsefrikk field by continuous injection of 30 ppm nitrate resulted in decreased souring and corrosion 3. Injection of 150 to 250 ppm nitrate for 3 months in the Skjold field was partially successful with 80% sulfide removal from fractured areas and lower levels of sulfide removal from areas of reduced permeability 2. Use of nitrite, instead of nitrate, gave successful souring control of gas wells for up to 7 months and of oil wells for up to 1 month following treatment 6. In this paper we will review microbial activities relevant to the use of nitrate or nitrite to contain souring. The effect of these agents on corrosion of coupons embedded in continuous up-flow, packed-bed bioreactors 7 will also be assessed in order to determine whether use of nitrate or nitrite is preferable.
MATERIALS AND METHODS
B ioreactors (glass columns of 64 cm, O = 4.5 cm; five sampling ports 14 cm apart) were packed with sand (average size 225 ~tm). Liquid medium, made anaerobic by purgin