Abstract
In a complex water–rock system that contains clay (or/and organic matter) and carbonates, multiple factors can affect specific isotopic compositions or ratios of fluids, such as the adsorption–desorption, ion exchange, and dissolution-precipitation of the related minerals. In this situation, the Rayleigh model is inappropriate for simulating the pore fluid isotopic data, and the ultimate mechanism causing isotopic fractionation has not yet been resolved. Besides, the endmembers (including concentration and isotopic composition/ratio) related to geochemical processes have not been well identified. Shale is enriched in clay and organic matter and usually contains carbonates. Comprehensive adsorption–desorption and ion exchange occurs within a short time. Accordingly, the flowback water following hydraulic fracturing of shale gas reservoirs is a unique material for studying the mechanism affecting the specific isotopic composition/ratio (lithium, boron, and strontium). This study aims to investigate the mechanism of changes in the isotopic compositions/ratios following the hydraulic fracturing of a shale gas reservoir by obtaining high-quality flowback water, determining all potential endmembers (concentration and isotopic composition/ratio for water-soluble fraction, exchangeable fraction, and carbonate minerals), and conducting laboratory water–rock interactions to confirm the trends observed in the field. The results indicated that desorption-adsorption or/and ion change was a critical factor controlling the concentration and isotopic compositions (δ7Li and δ11B) and radiogenic 87Sr/86Sr ratios in this complex water–rock system. The contribution from the dissolution of carbonates to Li, B, and Sr in the flowback water beyond the mixing between fracturing water and formation water was estimated to be less than 16%. Desorption of B from shale would result in higher B/Cl ratios and changes in δ11B values of fluids rely on the exchangeable B in shale (δ11B values decreased in this study). Another new finding demonstrates that the increased 87Sr/86Sr values of flowback water beyond mixing was inferred from the re-equilibrium between the dissolved Sr and exchangeable Sr, rather than dissolution of silicates with a high 87Sr/86Sr value, which suggests a more complex process than previously expected. While clay minerals or/and organic matter are commonly distributed in groundwater systems, the findings are also beneficial for tracing water–rock interactions and the sources/sinks of a specific solute in a groundwater system that may be impacted by adsorption-desorption or/and ion exchange.
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