Geochemical interactions between fracturing fluid, brine, and reservoir rock can influence hydrocarbon productivity from unconventional reservoirs. These reactions also affect the composition of produced water returned to the surface that requires treatment prior to beneficial use or disposal. Here we report results from both laboratory-based experiments and field produced water characterization associated with the Marcellus Shale Energy and Environmental Laboratory (MSEEL), a U.S. Department of Energy oil and gas field research laboratory. MSEEL core flood experiments designed to simulate geochemistry during hydraulic fracturing resulted in both generation of porosity through mineral dissolution and coating of fracture faces with barite. Barite precipitates on the fracture surface may inhibit transport pathways in fluid-shale interface layer and reduce oxidation reactions there. Calcite dissolution and pyrite oxidation were both observed during core flood experiments, as confirmed through analysis of experimental core flood effluent chemistry showing increases in Ca2+, SO42-, thiosulfate, and total Fe. Changes in 87Sr/86Sr in early core flood effluent samples are higher than the influent, indicating contributions of more radiogenic Sr from the exchangeable and carbonate components from shale. The carbon isotope composition of the effluent samples was 1-4.5‰ V-PDB enriched compared to the injection fluid. The two end member isotope mixing models suggest 23 to 92% contribution from dissolution of carbonate rich zones in the rock. Analysis of produced water returned within 10 days of MSEEL MIP 3H and 5H well production showed similar evidence for carbonate dissolution through changes in 87Sr/86Sr, and the occurrence of oxidation-reduction reactions through non-conservative changes in SO42- and redox-active metals (Fe, Mn, Mo, Co). Coupling laboratory experimentation with analysis of produced water chemical changes can provide important insights on which geochemical reactions influence unconventional oil and gas reservoirs and will inform the design of reservoir management approaches to enhance production while minimizing environmental impacts of development.

Characterizing geochemical reactions that influence unconventional oil and gas reservoir productivity and produced water composition

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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