Nitrate-and nitrite-mediated containment of souring was studied in continuous up-flow packed-bed bioreactors. Produced water of the Coleville oil field in Saskatchewan, Canada, was used as an inoculum for establishing biofilms of sulfate-reducing bacteria (SRB). With a medium containing 12 mM sulfate and 25 mM Nalactate (10–11 mM produced sulfide), 17.5 mM nitrate or 20 mM nitrite were required for complete removal of sulfide at a retention time of 24 h. During nitrate or nitrite addition environmental redox potential increased gradually from -700 mV to -400 mV, with final values still being in the optimum range for SRB activity Prior to addition of nitrate or nitrite microbial communities in the liquid phase were dominated by SRB. Addition of nitrate or nitrite shifted the composition of community to heterotrophic and nitrate-reducing (NRB) bacteria.
Souring, the production of hydrogen sulfide (H2S) in oil reservoirs subjected to water flooding for secondary oil recovery, is caused at least partly by sulfate-reducing bacteria (SRB). These bacteria use electron donors present in the oil and formation water (organic acids and hydrocarbons) to reduce sulfate to sulfide. The activity of SRB is also associated with increased corrosion risk. Strategies for mitigation of biogenic sulfide production include:
removal of sulfate from water prior to injection,
application of biocides to inhibit or kill SRB, or
manipulation of microbial metabolism through addition of nitrate or nitrite.
The latter approach has been tested in the Coleville oil field, located in Saskatchewan, Canada. Injection of 400 ppm ammonium nitrate for 50 days gave on average a 73% decrease in sulfide concentration in injection and producer wells (1). The control of biogenic sulfide production by addition of nitrite, nitrate, and nitrate-reducing, sulfide-oxidizing bacteria (NR-SOB) and the involved mechanisms have been studied in our recent work(2,3). NR-SOB oxidize sulfide (to sulfur or sulfate) when reducing nitrate to nitrite or nitrogen. To better simulate conditions prevailing in oil reservoirs, a study on the containment of souring was conducted in continuous packed-bed bioreactors with established SRB biofilms. The present paper describes the preliminary results of this study.
The containment of biogenic H2S production was studied in two up-flow packed-bed bioreactors, made of a glass column (D: 4.5 cm and H: 64 cm), with 5 sampling ports at 14 cm intervals. Sand was used as a matrix for biofilm establishment. Modified CSB medium (2) was introduced at the bottom of the bioreactors, using tygon tubing and a peristaltic pump. The effluent stream left the top of the bioreactor through a rubber stopper and tygon tubing connected to an effluent vessel. The immobilization of bacterial consortia (mainly SRB) was achieved by initial operation of the bioreactors in batch mode, followed by operation in continuous mode. The bioreactors, containing modified CSB medium were inoculated by injecting 10 mL Coleville produced water at each sampling port. The bioreactors were maintained at room temperature (22 °C). Following complete sulfate reduction, the medium was pumped into the bioreactor at an increasing flow rate (final value 9 mL/h, corresponding to a residence time of 24 h).