Nitrate-reducing microorganisms prevent souring of an oil field produced water storage pond.

Nitrate addition for mitigating sulfide production in oil field systems has been studied in laboratory settings and in some subsurface oil reservoirs. To promote water recycling and reuse associated with oil reservoirs produced by hydraulic fracturing, high-salinity produced waters are temporarily stored in surface ponds prior to subsequent reinjection into the subsurface. In this study, nitrate was added directly to a storage pond to prevent sulfide accumulation. DNA sequencing of pond water over a 4-week period revealed a decrease in the proportion of sulfate-reducing microorganisms following nitrate application. Sulfate levels remained stable during this period, whereas nitrate and nitrite fluctuated in the days following the nitrate addition. Metagenome-assembled genomes (MAGs) reconstructed from the pond water microbiome highlighted different organisms with genes for organoheterotrophic and lithoheterotrophic nitrate reduction, whereas genes associated with sulfide production via sulfate or thiosulfate reduction were barely detected. Within those MAGs, genes for acetate metabolism were observed, consistent with acetate decreasing substantially in the pond water in the presence of nitrate. After nitrate was consumed an increase in relative abundance of putative autotrophic microorganisms was observed (e.g. Arhodomonas, Guyparkeria, and Psychroflexus), corresponding to a drop in total inorganic carbon measurements in the storage pond. This trial offers an overview on microbial processes taking place in storage pond environments in response to nitrate addition.