In the context of deep well drilling, the addition of functionalized additives into mud systems becomes imperative due to the adverse impact of elevated borehole temperatures and salts on conventional additives, causing them to compromise their intrinsic functionalities. Numerous biomaterials have undergone modifications and have been evaluated in drilling muds. However, the addition of dually modified tapioca starch in bentonite-free mud systems remains a notable gap within the existing literature. This study aims to examine the performance of dually modified carboxymethyl irradiated tapioca starch (CMITS) under high temperature and salt-containing conditions employing central composite design approach; the study evaluates the modified starch's impact on mud rheology, thermal stability, and salt resistance. The findings indicated that higher DS (0.66) and CMITS concentrations (8 ppb) improved plastic viscosity (PV), yield point (YP) and gel strength (GS), while increased salt and temperature decreased it, demonstrating the complex interplay of these factors on mud rheology. The developed empirical models suggested that DS 0.66 starch addition enhanced rheology, especially at elevated temperatures, demonstrating improved borehole cleaning potential, supported by quadratic model performance indicators in line with American Petroleum Institute (API) ranges. The optimized samples showed a non-Newtonian behavior, and Power-law model fitting yields promising results for improved cuttings transportation with starch additives. © 2023 The Royal Society of Chemistry.