Formation Mechanism and Semi-Quantitative Threshold Assessment of Metal-Induced Gel-like Agglomerate Blockages in Slickwater Fracturing Fluids

The reuse of flowback water in slickwater fracturing can introduce elevated concentrations of dissolved metal ions, intensifying the incompatibility between polyacrylamide-based friction reducers and multivalent metal ions and promoting the formation of rubbery gel-like agglomerates, commonly referred to as “gummy bears”. These agglomerates can adversely affect flowback, surface fluid handling, and well productivity. In this study, field sample characterization and laboratory simulation are combined to investigate the composition and formation mechanism of such gel-like agglomerates. The results show that these materials are not simple polymer residues, but polymer–inorganic composites composed of polyacrylamide components, metal ions, and inorganic particles, with Fe³⁺ and Al³⁺ playing a particularly important role in their formation. Furthermore, a molar-ratio-based semiquantitative method is introduced to describe the relationship between metal-ion loading and effective polymer reactive sites, and to predict the threshold for gel-like agglomeration. The results improve the understanding of polymer–metal incompatibility under flowback water conditions and provide practical guidance for risk evaluation, fluid design, and mitigation of gummy-bear formation in field operations.