AM / SA / AMPS / NVP Crosslinked Copolymer Gel for Lost Circulation Control at 140°C in High‐Salinity Fractured Formations

Conventional polymer gels may suffer from thermal degradation and salt‐induced changes under severe downhole conditions. In this study, an AM/SA/AMPS/NVP crosslinked copolymer gel was synthesized by aqueous free‐radical polymerization using MBA as the crosslinker. The formulation was optimized through a sequential single‐factor approach, and the resulting gel was characterized in terms of network morphology, thermal stability, swelling behavior, compressive mechanical properties, salt resistance, and plugging performance. The gel retained 89.4% of its volume after aging at 140°C for 48 h and showed a uniaxial compressive strength of 0.203 MPa, with nearly 100% recovery after 10 loading–unloading cycles at 50% strain. The pure gel sealed fractures with widths of 1–3 mm, while the addition of 10 wt% 20–40 mesh quartz sand increased the breakthrough pressure to 2.05 MPa for a 3 mm fracture. In 20–40 mesh sand beds at 140°C, the total fluid loss after 30 min was 26.8 mL. These results indicate that the crosslinked copolymer gel provides favorable thermal, mechanical, and plugging performance, while quartz sand bridging improves the pressure‐bearing capacity of the sealing structure in wide fractures.