Chemical and microbiological transformations induced in oilfield produced water by amendment with nitrate as a souring control measure: Investigation of sulfur cycling, effects of co-amendment with acetate and phosphate, and evaluation of storage protoco

Chemical and microbiological responses to nitrate amendment were monitored in microcosms containing produced waters from three souring Canadian oilfields. For the first time, persistence of sulfide in nitrate-amended microcosms was shown to result from sulfur cycling between elemental sulfur-reducing and nitrate-reducing, sulfide-oxidizing bacteria. Molybdate or nitrite suppressed cycling and encouraged sulfide removal. Increased production of ammonium coincided with the recalcitrance of sulfide, suggesting that ammonifying bacteria may play a significant role in the success of nitrate-mediated sulfide control. Phosphate and acetate were not unequivocally beneficial to nitrate-mediated sulfide control. Although phosphate increased rates of bacterial metabolism, it led to the enrichment of sulfate-reducing, as well as nitrate-reducing bacteria. Added acetate did not lead to enrichment of nitrate-reducing bacteria via competitive exclusion, but enhanced sulfidogenesis in sulfur-cycling microcosms. Storage for 2 weeks significantly altered the chemistry and microbiology of produced water samples; refrigeration generally lessened the severity of the changes.