Sulfur Production Associated with Souring Control by Nitrate Injection: a Potential Corrosion Risk?

Injection of nitrate into an oil field can significantly reduce the concentration of sulfide produced by endogenous sulfate-reducing bacteria (SRB). Although much is known of the effects of nitrate in relatively high temperature reservoirs (60-80 °C), flooded with seawater, its effectiveness in lower temperature reservoirs (30-40 °C) subjected to produced water reinjection (PWRI) is less well understood. The nitrate-reducing, sulfide-oxidizing bacterium (NR-SOB) Thiomicrospira sp. strain CVO, was isolated from such a reservoir. This organism converts sulfide and nitrate into sulfate and nitrite or into sulfur and nitrogen, depending on whether the initial nitrate to sulfide (N/S) ratio is high or low, respectively. The presence of iron minerals in reservoir rock (e.g. siderite FeCO₃) can delay the onset of souring by immobilizing SRB-produced sulfide as FeS $(FeCO3+HS−→FeS+HCO3−)$⁠. Strain CVO appeared incapable of oxidizing ferrous sulfide (FeS) with nitrate, indicating that it does not mobilize precipitated sulfides. Interestingly, in ferrous iron-containing cocultures of sulfate-reducing Desulfovibrio spp. and strain CVO, FeS was transformed into greigite (Fe₃S₄). The mechanism for greigite formation is probably reaction of FeS with CVO-produced sulfur (3FeS + S₀ → Fe₃S₄). Hence, on the plus side CVO-mediated conversion of FeS to greigite increases the sulfur-binding capacity of reservoir rock by 33%. On the downside, sulfur is corrosive towards iron increasing corrosion risk if it emerges in production wells. A reservoir model, taking some of these features into account and indicating the formation of sulfur as a function of space and time by the combined action of SRB and NR-SOB such as strain CVO, is presented