Abstarct: Drilling fluids serve as one of the fundamental components in drilling operations and are specifically formulated to accomplish a series of essential functions throughout the drilling process. Oil and gas reservoirs, due to continuous production and reservoir development, gradually experience pressure depletion. This phenomenon has a significant and direct impact on the selection and performance of drilling fluids. Under depleted reservoir conditions, controlling fluid loss and preventing formation damage become major operational challenges during drilling and well-completion activities. In this study, calcium carbonate nanoparticles were surface-modified and subsequently incorporated into a zwitterionic copolymer chain through aqueous radical polymerization and inverse mini-emulsion techniques. The performance of the synthesized nanocomposite as a fluid-loss control additive in a water-baxsed drilling fluid system was evaluated for the target field. The results demonstrated a 20% reduction in fluid loss at 160 °C, a 12.5% decrease in filter-cake thickness, and a notable improvement in the rheological behavior of the drilling fluid, particularly in the Herschel–Bulkley model under high-temperature and high-pressure conditions. Structural characterization of the produced nanocomposite using XRD, SEM, TGA, FTIR, DSC, and zeta potential analyses confirmed the uniform dispersion of nanoparticles, thermal stability, and successful polymer grafting. The overall findings indicate that the developed nanocomposite can deliver enhanced performance under variable operational conditions, including elevated temperatures and pressures, making it a promising additive for high-performance water-baxsed drilling fluids.