Effects of B, Ca 2+ , and Mg 2+ on hydroxypropyl guar dispersion–hydration and borate-crosslinked gel rheology: formulation-specific reuse boundaries for fracturing flowback water

Mid-stage fracturing flowback water from the Jingbian oil and gas field, northern Shaanxi, China, was investigated to determine how residual B, Ca2+, and Mg2+ affect hydroxypropyl guar (HPG) dispersion–hydration and borate-crosslinked gel rheology. The formulation contained 0.45 wt% HPG; crosslinking pH was 8.0–8.5; performance was evaluated at 60 °C and 170 s−1. Base-fluid hydration, crosslinking kinetics, dynamic viscoelasticity, major-ion concentration responses, B–Ca2+–Mg2+ coexistence, thermal/shear stability, proppant carrying, gel breaking, and fluid loss were assessed. In distilled-water controlled ion-addition experiments, 5 mg·L−1 elemental B was an upper boundary for extended-hydration screening: the base fluid plateaued only after approximately 8 h and failed the standard 4-h hydration window. A separate post-hydration benchmark showed the strongest combined elastic and viscosity response at 5 mg·L−1 total B; this value is not interchangeable with operational crosslinker dosage. When tested separately, Ca2+ and Mg2+ at 300 mg·L−1 retained endpoint viscosity above 50 mPa·s. Under coexistence at B = 5 mg·L−1 and Ca2+ = Mg2+ = 300 mg·L−1, endpoint viscosity decreased to 41.5 ± 4.5 mPa·s, with a significant Ca2+ × Mg2+ interaction. HCO3- and iron were retained as auxiliary risk indicators. Untreated field water was unsuitable for direct preparation. A tiered framework combining water-quality screening, formulation-envelope confirmation, performance verification, and reuse-route selection is proposed. The B, Ca2+, and Mg2+ values are preliminary, laboratory-scale, formulation-specific screening values, not universal chemical limits.