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.
Highlights
Our results on microbial activities and community compositions with Bakken shale oil field samples suggest a high potential for souring at both low and high salinities
High salinity souring can be effectively controlled by nitrate injection because nitrite accumulates
Assuming souring to be a problem in shale oil fields (Yevhen et al, 2011) and that nitrate injection is promising technology to remedy this problem, decreases in salinity should be avoided
Summary
Hydraulic fracturing with subsequent production from horizontal wells is used both in shale gas and in shale oil fields (Caper, 2010; Daly et al, 2016; Shrestha et al, 2017). Aerobic freshwater microorganisms from the initial fracturing fluids dominated microbial communities in initial flowback waters These changed to anaerobic halophilic communities with Firmicutes (Halanaerobium), Bacteroidetes, Beta-, Gamma-, Delta- and Epsilonproteobacteria, along with methanogenic taxa in subsequent flowback waters (Davis et al, 2012; Struchtemeyer et al, 2012; Murali Mohan et al, 2013; Strong et al, 2013; Cluff et al, 2014; Daly et al, 2016; Liang et al, 2016). The microbial communities in shale gas fields, introduced through well drilling or indigenous, have been studied extensively over the past decade
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