Abstract
The Bakken Shale and underlying Three Forks Formation is an important oil and gas reservoir in the United States. The hydrocarbon resources in this region are accessible using unconventional oil and gas extraction methods, including horizontal drilling and hydraulic fracturing. However, the geochemistry and microbiology of this region are not well understood, although they are known to have major implications for productivity and water management. In this study, we analyzed the produced water from 14 unconventional wells in the Bakken Shale using geochemical measurements, quantitative PCR (qPCR), and 16S rRNA gene sequencing with the overall goal of understanding the complex dynamics present in hydraulically fractured wells. Bakken Shale produced waters from this study exhibit high measurements of total dissolved solids (TDS). These conditions inhibit microbial growth, such that all samples had low microbial loads except for one sample (well 11), which had lower TDS concentrations and higher 16S rRNA gene copies. Our produced water samples had elevated chloride concentrations typical of other Bakken waters. However, they also contained a sulfate concentration trend that suggested higher occurrence of sulfate reduction, especially in wells 11 and 18. The unique geochemistry and microbial loads recorded for wells 11 and 18 suggest that the heterogeneous nature of the producing formation can provide environmental niches with conditions conducive for microbial growth. This was supported by strong correlations between the produced water microbial community and the associated geochemical parameters including sodium, chloride, and sulfate concentrations. The produced water microbial community was dominated by 19 bacterial families, all of which have previously been associated with hydrocarbon-reservoirs. These families include Halanaerobiaceae, Pseudomonadaceae, and Desulfohalobiaceae which are often associated with thiosulfate reduction, biofilm production, and sulfate reduction, respectively. Notably, well 11 was dominated by sulfate reducers. Our findings expand the current understanding of microbial life in the Bakken region and provide new insights into how the unique produced water conditions shape microbial communities. Finally, our analysis suggests that produced water chemistry is tightly linked with microbiota in the Bakken Shale and shows that additional research efforts that incorporate coupled microbial and geochemical datasets are necessary to understand this ecosystem.
Highlights
IntroductionFractured wells generate billions of gallons of produced water each year (Horner et al, 2016) which contain high concentrations of salt (Gregory et al, 2011; Barbot et al, 2013; Murali Mohan et al, 2013; Cluff et al, 2014; Lipus et al, 2018; Wang et al, 2019), metals (Gregory et al, 2011; Barbot et al, 2013), and organics (Strong et al, 2014; Akyon et al, 2019) making management, handling, and disposal of these produced waters difficult and expensive
Hydrocarbon resources represent an important global energy source, currently providing over 75% of primary energy in the United States (U.S Department of Energy et al, 2020b), and play an essential role in other countries including Canada and the United Kingdom (Stevens, 2013; Elliott et al, 2014; Rivard et al, 2014)
total organic carbon (TOC) was measured in the form of non-purgeable organic carbon (NPOC), which ranged from 63 to 543 mg as C/L (Table 1), and is similar to previously reported dissolved organic carbon (DOC) values for the Bakken region (Wang et al, 2019)
Summary
Fractured wells generate billions of gallons of produced water each year (Horner et al, 2016) which contain high concentrations of salt (Gregory et al, 2011; Barbot et al, 2013; Murali Mohan et al, 2013; Cluff et al, 2014; Lipus et al, 2018; Wang et al, 2019), metals (Gregory et al, 2011; Barbot et al, 2013), and organics (Strong et al, 2014; Akyon et al, 2019) making management, handling, and disposal of these produced waters difficult and expensive Another major challenge is the presence of various kinds of microorganisms, which may contribute to the corrosion of well-components, including casing and pipes, and reservoir souring (Struchtemeyer and Elshahed, 2012; Mohan et al, 2014; Stringfellow et al, 2014; Daly et al, 2016; Torres et al, 2016; Lipus et al, 2017, 2018). Microbial activities in hydraulic fracturing may result in operational interruptions, negatively impact hydrocarbon recovery, and create safety and environmental concerns
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