--> Temporal Changes in Fluid Biogeochemistry and Microbial Cell Abundance after Hydraulic Fracturing in Marcellus Shale

AAPG Eastern Section Meeting

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Temporal Changes in Fluid Biogeochemistry and Microbial Cell Abundance after Hydraulic Fracturing in Marcellus Shale

Abstract

Hydraulic fracturing methods used to enhance gas production in shales result in biogeochemical changes that are poorly understood. Here, temporal trends in macronutrients and redox active elements important for supporting microbial life in shale wells are reported. In this paired study at the Marcellus Shale Energy and Environment Laboratory (MSEEL) well pad near Morgantown, WV, one well was fractured using a traditional geometric design (5H) while a second used a non-traditional engineered design (3H). Source water, fracturing fluids, and produced fluids were sampled over a period of more than six months and analyzed for biogeochemical parameters and the abundance of microbial cells. Dissolved organic carbon decreased through time from 69 to 42 mg/L in the traditional design (5H) and 312 to 68 mg/L in the engineered design (3H). Total dissolved nitrogen (TDN) concentrations decreased dramatically in initial flowback, and then show a rapid increase to approximately 90 mg/L in both wells. Trends in ammonia parallel TDN through time, with NH3 consistently comprising 93% TDN by mass. Dissolved manganese (Mn) and ferrous iron (Fe(II)), both potential products of microbial respiration under anaerobic conditions, were below detection in the source water and fracturing fluids, but increased during flowback. Fe(II) levels in the geometric design (5H) were consistently higher than the engineered design (3H). Mn reached a maximum of 0.3 and 0.7 mg/L for 3H and 5H, respectively, then decreased below detection limits in both wells one month after the start of flowback. Concurrent with a decrease in sulfate concentrations to levels below detection, sulfide increased with time into flowback, leveling off at more than 20 mg/L in both wells. The number of microbial cells decreased dramatically after hydraulic fracturing in the engineered design well (3H), dropping from 1×106 cells/mL to 2 × 105 cells/mL after four months. In contrast, cells in the geometric design (5H) were initially level (0.7×10^5 to 5×10^5 cells/mL) over the first two months of flowback, then decreased an order of magnitude (0.6×10^4 cells/mL). These data provide initial insight into available nutrients and changes in redox-active elements potentially supporting microbial life in deep fractured shale.