EVALUATION OF FLUID-ROCK INTERACTION BETWEEN HYDRAULIC FRACTURING FLUIDS AND UNCONVENTIONAL RESERVOIRS OF THE POWDER RIVER BASIN, WYOMING

Hydraulic fracturing fluid is engineered to maximize hydrocarbon production in unconventional reservoirs; however, impacts due to reactions between fluids and reservoir rocks are poorly understood. This research examines 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 above and below into 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 used in the wells. Four to six chemically-differing fluids are mixed and injected at ~13 different frac stages. Hydrochloric acid (15%) 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.

This field data is used to design hydrothermal experiments that evaluate fluid-rock interactions between both formations and hydraulic fracturing fluids. The experiments use one representative, simplified hydraulic fracturing fluid (pH ~2.35); gels and bactericides are not incorporated due to limitations of the experimental apparatus and lab safety. The fluid reacts with rocks at 115°C and 350 bars pressure for at least 28 days to replicate in-situ conditions. 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. Initial geochemical models of fluid-rock interaction predict carbonate mineral dissolution and clay alteration. Experimental results are coupled with geochemical modeling to examine interactions between hydraulic fracturing fluids and reservoir rocks and to predict the geochemistry of produced waters.