Mechanistic study of microbial control of hydrogen sulfide production in oil reservoirs

Microbial control of biogenic production of hydrogen sulfide in oil fields was studied in a model system consisting of pure cultures of the nitrate-re ducing, sulfide-oxidizing bacterium (NR-SOB) Thiomicrospira sp. strain CVO and the sulfate-reducing bacterium (SRB) Desulfovibrio sp. strain Lac6, as well as in microbial cultures enriched from produced water of a Canadian oi l reservoir. The presence of nitrate at concentrations up to 20 mM had litt le effect on the rate of sulfate reduction by a pure culture of Lac6. Addit ion of CVO imposed a strong inhibition effect on production of sulfide. In the absence of added nitrate SRB we were able to overcome this effect after an extended lag phase. Simultaneous addition of CVO and nitrate stopped th e production of H2S immediately. The concentration of sulfide decreased to a negligible level due to nitrate-dependent sulfide oxidation activity of C VO. This was not prevented by raising the concentration of Na-lactate, the electron donor for sulfate reduction. Similar results were obtained with en richment cultures. Enrichments of produced water with sulfide and nitrate w ere dominated by CVO, whereas enrichments with sulfate and Na-lactate were dominated by SRB. Addition of an NR-SOB enrichment to an SRB enrichment inh ibited the production of sulfide. Subsequent addition of sufficient nitrate caused the sulfide concentration to drop to zero. A similar response was s een in the presence of nitrate alone, although after a pronounced lag time, it was needed for emergence of a sizable CVO population. The results of th e present study show that two mechanisms are involved in microbial control of biogenic sulfide production. First, addition of NR-SOB imposes an inhibi tion effect, possibly by increasing the environmental redox potential to le vels which are inhibitory for SRB. Second, in the presence of sufficient ni trate, NR-SOB oxidize sulfide, leading to its complete removal from the env ironment. Successful microbial control of H2S in an oil reservoir is crucia lly dependent on the simultaneous presence of NR-SOB (either indigenous pop ulation or injected) and nitrate in the environment. (C) 2001 John Wiley & Sons, Inc.