Hydrogeochemistry and indicators of flowback and produced water from wells that are hydraulically fractured with recycled wastewater: A case study of the Changning gas field in the Sichuan Basin

With the increasing number of shale gas wells being hydraulically fractured with recycled flowback and produced water to reduce the consumption of fresh water, geochemical indicators of hydraulic fracturing flowback fluids (HFFFs) from such wells need to be found to evaluate the potential pollutants caused by shale gas development. To fill this knowledge gap, we analysed 64 HFFF samples from 3 Changning wells (fracked with recycled wastewater) to form a time series and compared them to HFFF samples from 3 Weiyuan wells (fracked with fresh water). We also made comparisons of HFFFs from two types of wells in the Appalachian Basin with reported data. The results indicated that regardless of the fracturing fluid being used, the water -rock reaction caused by the hydraulic fracturing process resulted in the release of exchangeable phase elements, such as lithium, boron, with relatively depleted 87Li and 611B, and strontium with relatively enriched 87Sr/86Sr values on the shale surface, into the HFFFs. The composition of the injected fracturing fluid affected the strength of the water -rock reaction, but the effect is negligible, as the fracturing process can still form the unique Sr, Li, and B isotope ratios of the HFFF. Thus, the elemental signatures (B/Cl and Sr/Cl) and isotopic fingerprints (611B, 87Li, and 87Sr/86Sr) derived from HFFFs can be used to distinguish between HFFFs from wells hydraulically fractured by either fresh water or recycled wastewater and flowback water from conventional oil and gas wells. The results are important for the environmental evaluation of the large number of wells being fracked with recycled wastewater each year.