Control of Sulfide Production in High Salinity Bakken Shale Oil Reservoirs by Halophilic Bacteria Reducing Nitrate to Nitrite

Abstract

Microbial communities in shale oil fields are still poorly known. We obtained samples of injection, produced and facility waters from a Bakken shale oil field in Saskatchewan, Canada with a resident temperature of 60°C. The injection water had a lower salinity (0.7 Meq of NaCl) than produced or facility waters (0.6-3.6 Meq of NaCl). Salinities of the latter decreased with time, likely due to injection of low salinity water, which had 15-30 mM sulfate. Batch cultures of field samples showed sulfate-reducing and nitrate-reducing bacteria activities at different salinities (0, 0.5, 0.75, 1.0, 1.5, and 2.5 M NaCl). Notably, at high salinity nitrite accumulated, which was not observed at low salinity, indicating potential for nitrate-mediated souring control at high salinity. Continuous culture chemostats were established in media with volatile fatty acids (a mixture of acetate, propionate and butyrate) or lactate as electron donor and nitrate or sulfate as electron acceptor at 0.5 to 2.5 M NaCl. Microbial community analyses of these cultures indicated high proportions of Halanaerobium, Desulfovermiculus, Halomonas, and Marinobacter in cultures at 2.5 M NaCl, whereas Desulfovibrio, Geoalkalibacter, and Dethiosulfatibacter were dominant at 0.5 M NaCl. Use of bioreactors to study the effect of nitrate injection on sulfate reduction showed that accumulation of nitrite inhibited SRB activity at 2.5 M but not at 0.5 M NaCl. High proportions of Halanaerobium and Desulfovermiculus were found at 2.5 M NaCl in the absence of nitrate, whereas high proportions of Halomonas and no SRB were found in the presence of nitrate. A diverse microbial community dominated by the SRB Desulfovibrio was observed at 0.5 M NaCl both in the presence and absence of nitrate. Our results suggest that nitrate injection can prevent souring provided that the salinity is maintained at a high level. Thus, reinjection of high salinity produced water amended with nitrate maybe be a cost effective method for souring control.

Keywords: Bakken; halophilic; nitrate; nitrate-reducing bacteria; nitrite; shale oil; souring control; sulfate-reducing bacteria.

Figure 1

Schematic diagram of shale oil production from a field in the Bakken formation. Mannville formation source water (SW) is injected at the injection well (IW) to pressurize multiple producing wells (PW). Oil-water emulsions from producing wells enter free water knockout (FW) facility first, then treater facility (TW) to separate oil from water. The produced water is not reinjected, but is disposed. Circles indicate the sampling points.

Figure 2

Schematic of up-flow sand packed bioreactor system. The arrows indicate direction of media flow; circle S indicates the effluent sampling point. Pressure in both media and effluent containers was maintained using 30 mL plastic syringes of which the left one was filled with N₂-CO₂ gas.

Figure 3

Dendrogram of the 2015 Bakken field samples. The relational tree of microbial community compositions of 2015 samples indicates clades I to V (A). Distribution of phyla other than Proteobacteria in the community compositions (B). Distribution of classes from the phylum Proteobacteria in the community compositions (C). The scale indicates 10% sequence divergence.

Figure 4

Primary enrichments for November 2013 field samples in media with 2.5 M NaCl. Media contained 10 mM sulfate and 20 mM lactate (A,B), 10 mM sulfate and 3 mM VFA (C,D) or 10 mM nitrate and 3 mM VFA (E,F). Media were inoculated with 2PW_11/13, 4PW_11/13, 6PW_11/13, 8SW_11/13, 9FW_11/13, or 10TW_11/13, as indicated; the controls were not inoculated. The average concentrations of sulfide (B,D), sulfate (A,C), nitrate (E), and nitrite (F) are shown as a function of time ± SD.

Figure 5

Effect of nitrate on sulfide production in the effluent of bioreactors at low and high salinity. High salinity bioreactors were inoculated with high salinity chemostat cultures and were injected with LSN_2.5 or with LS_2.5. Low salinity bioreactors were inoculated with low salinity chemostat cultures and were injected with LSN_0.5 or with LS_0.5. The concentrations ± SD of sulfide (A) and nitrite (B) are shown as a function of time. Bioreactors were run at a flow rate of 0.6 PV/day.