Abstract
Water-rock interactions occurring during fracturing and flowback in shale gas reservoirs may dominate some chemical and isotopic compositions of hydraulic fracturing flowback fluids (HFFF) by imparting characteristic compositions of the continental shale to the HFFF. In this paper, a simple conservative mixing model (SCMM) and a progressive conservative mixing model (PCMM) were developed to evaluate and understand the impacts of geochemical processes on forensic indicators (δ11B, 87Sr/86Sr, and Ba/Sr) of HFFF produced from the Dameigou Shale. Application of the paired sample T-test showed that the measured values of δ11B, 87Sr/86Sr, Ba, and Sr in HFFF produced from the Dameigou Shale were all significantly (p < 0.05) different from SCMM-simulated values in the HFFF, suggesting fluid-shale interactions that occurred during hydraulic fracturing and flowback were critical in the hydrochemical evolution of these HFFF indicators. The comparison between the measured and PCMM-simulated values shows that extensive dissolution of feldspar occurring during hydraulic fracturing can impart a unique B isotopic fingerprint of continental shale (δ11B: −30.1 to 10.2‰) to HFFF, and define the B isotope signature of HFFF produced in the following flowback. Thus, the B isotope can be reliably used to delineate the release of HFFF to rapidly-circulating shallow groundwater mainly recharged by rainwater in North America and Asia (δ11B: −1.5 to 34.7‰). In contrast, Sr compositions with low 87Sr/86Sr values and high elemental concentrations released from substantial dissolution of carbonate minerals during hydraulic fracturing can obscure the Sr signatures associated with feldspar, which is characterized by high 87Sr/86Sr values and low elemental concentrations, thereby excluding silicate-rich shale as the source of the distinctive fingerprints of 87Sr/86Sr (~0.7200) and Ba/Sr (>1.45) in HFFF. Therefore, 87Sr/86Sr and Ba/Sr show potential as indicators of continental HFFF produced from carbonate-poor shales and provide a clear distinction from rapidly-circulating shallow groundwater mainly recharged by rainwater (87Sr/86Sr: 0.70903–0.70914 and Ba/Sr: <0.001).
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