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Fluid-Rock Interaction in the Powder River Basin, Wyoming: An Investigation of Reactions Between Hydraulic Fracturing Fluid and Unconventional Reservoirs


Hydraulic fracturing fluid is engineered to maximize hydrocarbon production in unconventional reservoirs; however, reactions between fluids and reservoir rocks are poorly understood. We examine geochemical and mineralogical interactions in the Niobrara and Frontier Formations, two unconventional reservoirs within the Powder River Basin of NE Wyoming. In the Niobrara Formation, the horizontal leg of the well is drilled through a marl, and fractures extend into argillaceous, source shale. In the Frontier Formation, the horizontal leg extends into fine-grained sandstone, and fractures extend through interbedded shale and sandstone. Fresh water (I = 0.045M) from the Cheyenne River is used as mixing water for hydraulic fracturing fluids injected into these formations. Four to six chemically-differing hydraulic fracturing fluids are mixed and injected at ∼13 different fracturing stages. Hydrochloric acid (15 wt%) is initially injected to clean the wellbore and fractures. Fluids containing varying amounts of viscosity adjusters (guar gum, ethoxylates, boric acid), proppants (bauxite), clay stabilizers (tetramethylammonium chloride), iron stabilizers (sodium erythorbate), and bactericides are then injected along the horizontal leg of the well during each stage of hydraulic fracturing. Preliminary geochemical models of fluid-rock interaction predict carbonate mineral dissolution and clay alteration. Hydrothermal experiments that emulate these field conditions are being conducted to evaluate reactions between fluids and reservoir rocks. Individual experiments react samples from each formation with hydraulic fracturing fluids at 115°C and 350 bars for at least 28 days. The experiments use one representative, chemically-simplified hydraulic fracturing fluid (pH ∼2.35); gels and bactericides are not incorporated due to limitations of the experimental apparatus and lab safety. Fluids collected from extensive, in-situ sampling are analyzed for total dissolved carbon by coulometric titration, anions by IC, and major, minor, and trace cations by ICP-OES. Minerals are evaluated by SEM-EDS, XRD and XRF before and after each experiment. Experimental results are coupled with geochemical modeling to examine interactions between hydraulic fracturing fluids and reservoir rocks, evaluate the veracity of model predictions, and constrain the geochemistry of produced waters.