ORGANIC COMPOSITION AND MICROBIOLOGY OF PRODUCED WATERS FROM PENNSYLVANIA SHALE GAS WELLS

During hydraulic fracturing (HF) for shale gas extraction a mixture of water and additives is injected into a rock formation under high pressure to fracture the rock. Additives to the injected water include proppants, biocides, friction reducers, and other chemicals (FracFocus.org). During gas production, both methane and water flow from the well. This produced water (PW) includes the injected fluids and natural water from the formation in proportions that vary over time due to incompletely understood physical/chemical interactions within the formation.

We characterized the organic compounds and microbial communities in PW samples from the separator tanks of 13 shale gas wells in Tioga and Lycoming counties, PA that had been in production for 5-38 months. Twelve wells produced gas from the M. Devonian Marcellus Shale and one from the U. Devonian Burket Shale. Microbial abundance was generally low, ranging from 66 to 1626 cells per ml. Cell abundance was neither associated with salinity (200,000 - >300,000 mg/L TDS) nor production duration. Despite the addition of biocides to the injected water, microbes capable of sulfate reduction, anaerobic fermentation, and methanogenesis were cultured from PW samples. The factors that affect these microbial communities are unknown, although variations in subsurface geology, differences in sources and compositions of the HF fluids, and extent of biocide use may be important.

Non-volatile dissolved organic carbon was high (7-31 mg/L) relative to typical shallow groundwater and the presence of organic acids (e.g., acetate, formate, and pyruvate) indicated microbial activity. Volatile organic compounds (VOCs) were detected in four samples at levels of ~1-2 ppb: benzene and toluene in the Burket sample, benzene in two Marcellus samples, and trichloroethane (TCE) in one Marcellus sample. The source of the VOCs is unclear, although the TCE is anthropogenic. Benzene and toluene can be naturally occurring, although they are also associated with industrial activity. Initial results of our study indicate complex mixing between HF fluids and formation waters and suggest that microorganisms are contributing to the degradation of organic compounds present in the PW.