Impact of nitrate-mediated microbial control of souring in oil reservoirs on the extent of corrosion

The effect of microbial control of souring on the extent of corrosion was s tudied in a model system consisting of pure cultures of the nitrate-reducin g, sulfide-oxidizing bacterium (NR-SOB) Thiomicrospira sp. strain CVO and t he sulfate-reducing bacterium (SRB) Desulfovibrio sp. strain Lac6, as well as in an SRB consortium enriched from produced water from a Canadian oil re servoir. The average corrosion rate induced by the SRB consortium (1.4 g m( -2) day(-1)) was faster than that observed in the presence of strain Lac6 ( 0.2 g m(-2) day(-1)). Examination of the metallic coupons at the end of the tests indicated a uniform corrosion in both cases. Addition of CVO and 10 mM nitrate to a fully grown culture of Lac6 or the SRB consortium led to co mplete removal of sulfide from the system and a significant increase in the population of CVO, as determined by reverse sample genome probing. In the case of the SRB consortium addition of just nitrate (10 mM) had a similar e ffect. When grown in the absence of nitrate, the consortium was dominated b y Desulfovibrio sp. strains Lac15 and Lac29, while growth in the presence o f nitrate led to dominance of Desulfovibrio sp. strain Lac3 The addition of CVO and nitrate to the Lac6 culture or nitrate to the SRB consortium accel erated the average corrosion rate to 1.5 and 2.9 g m(-2) day(-1), respectiv ely. Localized corrosion and the occurrence of pitting were apparent in bot h cases. Although the sulfide concentration (0.5-7 mM) had little effect on corrosion rates, a clear increase of the corrosion rate with increasing ni trate concentration was observed in experiments conducted with consortia en riched from produced water.