Knowledge Bank

This research addresses rock-fluid interactions that occur between hydraulic fracturing fluids and shale formations to analyze geochemical reactions that occur in the subsurface. Specifically, the fractionation of 13C isotopes in dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) can be used as a tracer to determine carbon sources. Flowback fluids from the Utica-Point Pleasant in eastern Ohio have high salinity and complex chemistry, thus posing challenges for analysis. Experimental solutions and samples were analyzed for DIC, DOC, and δ13C using an OI Analytical Aurora Model 1030 Carbon Analyzer (TICTOC) interfaced with a Picarro Cavity Ring-Down Spectrometer (CRDS). Methods for measuring DIC, DOC and δ13C are developed to investigate and resolve chloride interference of high salinity brines and to determine effectiveness or limitations of wet chemical oxidation. Results of DIC analysis on experimental solutions that cover a range of salinity and DIC concentrations expected for flowback fluids show relatively stable δ13C signals when CO2 concentrations are high and that there is little fractionation in the δ13C signals over a range of salinity. However, results of DOC analysis affirm that elevated salinity inhibits the oxidation reaction thereby hindering CO2 recovery. DOC results from the CRDS data give stable δ13C signals when CO2 concentrations are high, but measured δ13C changes significantly as a function of chloride concentration in solution. Laboratory experiments explored various efforts to maximize CO2 recovery in the wet chemical oxidation process, which include (1) extending DIC/DOC reaction and detection times, (2) adjusting the addition of sodium persulfate, (3) variability of recovery in the presence of NaCl or CaCl2 salts, and (4) two organic compounds (potassium hydrogen phthalate and acetic acid) that differ in chemical structure.