Abstract: he Enping 21-4 ultra-deep and large-displacement well, operated by CNOOC, represents the deepest ϕ244.55 mm casing run in China, with a casing depth of 8,125 meters, a barehole length of 5,125 meters, and a horizontal displacement of 5,093.17 meters in the barehole section. Faced with extreme depth and displacement challenges, along with three actual drilling-induced loss zones encountered during construction that heightened risks of cementing loss and compromised cement quality assurance, the project team developed and implemented four key technologies to ensure successful casing deployment and cementing operations. Specifically, a dedicated casing accessory wear evaluation system was developed for in-hole accessory performance assessment and optimization, providing technical guarantees for safe casing running; circulation friction precision calculation technology enabled real-time pressure control and slurry structure optimization during operations, effectively avoiding cementing loss risks; a dual-suspended slurry system combining an ultra-low-density low-friction lead slurry with a high-solid content low-viscosity tail slurry ensured rheological compatibility of wellbore fluids, significantly improving cement displacement efficiency; and temperature-pressure coupled simulation accurately evaluated bottomhole circulating temperature, offering rational temperature parameters for cement slurry formulation. Through the integrated application of these technologies, the Enping 21-4 well achieved smooth cementing operations despite its extreme conditions. This case demonstrates that systematic technological innovation can effectively manage ultra-deep large-displacement well challenges, offering valuable reference for offshore oilfield development in deepwater and ultra-deep environments with high displacement and complex geological features. The successful implementation not only confirms CNOOC's technical leadership in global deepwater drilling engineering for frontier reservoirs but also underscores its capability to deliver engineering solutions for such extreme wells, reinforcing its position in international industry leadership.