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Microbiology and Chemistry of Waters Produced from Hydraulic Fracking—A Case Study

Three sample bottles with cultures of hydrogen sulfide-producing bacteria
USGS scientists did laboratory experiments to assess the microbial activity of produced waters from hydraulically fractured shale gas wells in north-central Pennsylvania. Shown are bottles containing cultures of hydrogen sulfide-producing bacteria enriched from shale gas-produced waters. The black color denotes the presence of insoluble iron sulfide minerals, an indicator of bacterial sulfide production. The clear culture on the right is an uninoculated control sample. Photo Credit: Darren S. Dunlap, USGS.

A new U.S. Geological Survey (USGS) study determined that the microbiology and organic chemistry of produced waters varied widely among hydraulically fractured shale gas wells in north-central Pennsylvania.

Hydraulically fractured shales are an increasingly important source of natural gas production in the United States. This process has been known to create up to 420,000 gallons of produced water per well per day, consisting of a mixture of injected fracturing fluids and natural formation waters, for Marcellus shale wells. These large volumes of water are a waste product that present water management challenges, and operators are reusing these fluids more often in order to reduce the amount of wastewater requiring disposal.

The USGS initiated a case study in 2012 to understand the chemical and microbiological composition of produced waters. Scientists measured inorganic compounds (for example, sodium, chloride, barium, bromide, calcium, and strontium) and volatile organic compounds (VOCs) and characterized the microbiology of produced waters from 13 hydraulically fractured shale gas wells in north-central Pennsylvania.

The scientists determined that inorganic compounds were broadly similar among the wells. In contrast, there were variations in VOCs detected and microbial populations present. VOCs (benzene, toluene, tetrachloroethylene) were detected in four produced water samples at concentrations ranging from less than 1 to 11.7 micrograms per liter, although the sources (natural or industrially derived) are not clear.

Some wells were hotspots for microbial activity, which seemed to be associated with concentrations of specific organic compounds (for example, benzene or acetate, which some microbes metabolize). The connection between the presence of organic compounds and the detection of microbes was not, in itself, surprising. Many organic compounds used as hydraulic fracturing fluid additives are biodegradable and thus could have supported microbial activity at depth during shale gas production.

Two graphs A total dissolved solids B volatile organic compounds concentration in produced water
The concentration of total dissolved solids in milligrams per liter (mg/L) (A) and concentration of three volatile organic compounds (benzene, toluene, and tetrachloroethylene) in micrograms per liter (µg/L) (B) in the produced waters USGS scientists collected from 13 shale gas wells in north central Pennsylvania. Graph modified from Akob and others, 2015.

Variations in microbial characteristics and VOCs of the wells followed no obvious spatial pattern but may be linked to the time during which a well was in production. The VOCs from the produced waters of the tested wells could play a role in the management of produced waters, particularly since VOCs, such as benzene, may be a health concern around the well or a holding pond.

Microbes could also serve to help mitigate the effects of organic contaminants during the disposal or accidental release of produced waters. Additional research is needed to fully assess how microbial activity can most effectively be utilized to biodegrade organic compounds found in produced waters.

This research was supported by the USGS Toxic Substances Hydrology, Hydrologic Research and Development, and Energy Resources Programs.

References

Organic and inorganic composition and microbiology of produced waters from Pennsylvania shale gas wells: Applied Geochemistry, doi:10.1016/j.apgeochem.2015.04.011.

USGS Investigations of Water Produced During Hydrocarbon Reservoir Development: USGS Fact Sheet 2014–3104

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