Guar Gum Stimulates Biogenic Sulfide Production in Microbial Communities Derived from UK Fractured Shale Production Fluids

Abstract

Shale gas production fluids offer a window into the engineered deep biosphere. Here, for the first time, we report on the geochemistry and microbiology of production fluids from a UK shale gas well in the Bowland shale formation. The composition of input fluids used to fracture this well were comparatively lean, consisting only of water, sand, and polyacrylamide. This formation therefore represents an interesting comparison to previously explored fractured shales in which more additives were used in the input fluids. Here, we combine cultivation and molecular ecology techniques to explore the microbial community composition of hydraulic fracturing production fluids, with a focus on the potential for common viscosity modifiers to stimulate microbial growth and biogenic sulfide production. Production fluids from a Bowland Shale exploratory well were used as inocula in substrate utilization experiments to test the potential for polyacrylamide and guar gum to stimulate microbial metabolism. We identified a consortium of thiosulfate-reducing bacteria capable of utilizing guar gum (but not polyacrylamide), resulting in the production of corrosive and toxic hydrogen sulfide. Results from this study indicate polyacrylamide is less likely than guar gum to stimulate biogenic sulfide production during shale gas extraction and may guide planning of future hydraulic fracturing operations. IMPORTANCE Shale gas exploitation relies on hydraulic fracturing, which often involves a range of chemical additives in the injection fluid. However, relatively little is known about how these additives influence fractured shale microbial communities. This work offers a first look into the microbial community composition of shale gas production fluids obtained from an exploratory well in the Bowland Shale, United Kingdom. It also seeks to establish the impact of two commonly used viscosity modifiers, polyacrylamide and guar gum, on microbial community dynamics and the potential for microbial sulfide production. Not only does this work offer fascinating insights into the engineered deep biosphere, it could also help guide future hydraulic fracturing operations that seek to minimize the risk of biogenic sulfide production, which could reduce efficiency and increase environmental impacts of shale gas extraction.

Keywords: Bowland; bacteria; hydraulic fracturing; shale; sulfidogenic; thiosulfate-reducing.

FIG 1

Bacterial community composition of UK-produced waters based on 16S rRNA gene sequencing. Taxa displayed at genus level or the next highest resolved phylogeny in brackets. (a) All taxa, where “other” representing species with a combined relative abundance (RA) below 1%. (b) Commonly detected fractured shale taxa based on previous literature. *, potential sulfidogenic taxa identified here and in previous shale gas-produced water studies. (c) Putative sulfidogenic taxa.

FIG 2

Average change in sulfide (mM) detected in cultures amended with guar gum and thiosulfate. After 150 days (control and gen 1) and 119 days (gen 2). Control shown here is noninoculated minimal medium amended with guar gum with thiosulfate. Error bars show the standard error of the mean. *, statistically significant change (P < 0.005).

FIG 3

Change in concentration of volatile fatty acids (VFAs) in cultures supplied with guar gum and thiosulfate, based on mean of triplicate data sets. Error bars display standard deviation. Acetate concentrations are displayed in green, propionate in blue, and formate in red. No VFAs were detected in controls. Solid line represents first-generation enrichments, dashed line represents second generation.

FIG 4

Bacterial community composition of viscosity modifier enrichments based on 16S rRNA gene sequencing of second-generation cultures. Results were taken after 41 days of incubation at 30°C. Shannon’s H index is displayed above each bar. Most dominant genera are indicated in the key.