<p>Severe scaling and subsurface formation damage may occur during the reinjection of fracturing flowback water due to its incompatibility with formation water, posing challenges for water management in oilfield operations. In this study, actual fracturing flowback water from an oilfield and formation waters from multiple reservoir intervals were investigated to evaluate mixed-water compatibility, scaling risk, scale inhibitor performance, and associated formation damage. Compatibility experiments combined with ScaleChem simulations revealed a high scaling tendency in all fracturing flowback water-formation water mixtures. Among them, the mixture with formation water C exhibited the poorest compatibility and produced composite scales mainly composed of barium-strontium sulfate and calcium carbonate. Based on these findings, eight scale inhibitors were screened and further optimized through formulation. A blended inhibitor composed of polymaleic acid (PMA) and acrylic acid-maleic acid copolymer (AA-MA) showed the best performance at a mass ratio of 3:1 and a dosage of 10 mg L^− 1, achieving a scale inhibition efficiency of 95.4% against composite scales. High inhibition efficiencies ranging from 92.9% to 98.4% were maintained in other mixed-water systems, indicating good adaptability under varying water chemistry conditions. Scale characterization results demonstrated that the optimized inhibitor reduced scale crystallinity and promoted the formation of fine, disordered amorphous particles, thereby suppressing crystal growth and aggregation. Core-flooding experiments further confirmed that the addition of the inhibitor effectively mitigated formation damage induced by fracturing flowback water reinjection. These results provide practical insights into scaling control and subsurface formation protection during fracturing flowback water reinjection, contributing to environmentally sustainable water management in oilfield operations.</p> Graphical Abstract <p></p>