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Numerical Simulation of Potential Groundwater Contaminant Pathways from Hydraulically Fractured Oil Shale in the Nevada Basin and Range Province
AdvisorPohll, Greg M.
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In recent years, hydraulic fracturing (fracking) has become an increasingly popular method for extraction of oil and natural gas from tight formations. Concerns have been raised over a number of environmental risks associated with fracking, including contamination of groundwater by fracking fluids, upwelling of deep subsurface brines, and methane migration. Given the potentially long time scale for contaminant transport associated with hydraulic fracturing, numerical modeling remains the best practice for the assessment of migration of fracking fluids. Oil shale in the upper Humboldt Basin of northeastern Nevada has now become a target for hydraulic fracturing operations. Analysis of regional groundwater flow is used to assess several potential migration pathways specific to the geology and hydrogeology of this basin. The model domain in all simulations is defined by the geologic structure of the basin as determined by deep oil and gas well bores and formation outcrops. Vertical transport of gaseous methane along a density gradient is simulated in TOUGH2, while fluid transport along faults and/or hydraulic fractures and lateral flow through more permeable units adjacent to the targeted shale are modeled in FEFLOW. Sensitivity analysis considers basin, fault, and hydraulic fracturing parameters, and results highlight key processes that control fracking fluid and methane migration. Results indicate that migration of detectable concentrations is unlikely, and only one of the three migration scenarios tested showed potential for any transport to a shallow water aquifer. Any migration would be of concentrations several orders of magnitude below detection limits, and would be dependent on a number of very specific conditions. For migration of fracking fluids, these conditions include 1) hydraulic fractures connecting the Elko shale with the adjacent carbonate formation, 2) the presence of a continuous, conductive fault connecting the adjacent carbonate formation with an overlying shallow water aquifer, 3) significant mountain block recharge in the Ruby Mountains, 4) carbonate hydraulic conductivity at a minimum of 0.1-1 m/d, 5) metamorphic core complex hydraulic conductivity on the order of 0.005 m/d or higher, and 6) fracking fluid constituents with very low to moderate sorption parameters.