It is difficult to drill efficiently with bentonite (BN)-based mud (BN-WBM) or water-based muds (WBMs) in high salt electrolytes and deep wells. This is because the fluid's rheological parameters and filtration properties change in undesirable ways, affecting the well's production efficiency. To fix this, a high-salt and high-temperature-resistant nanohydroxyapatite (nanoHAp) additive was designed using sodium dodecyl sulphate (SDS). 0.1 to 0.5 wt% nanoHAp was added to WBMs, and a salt-resistant BN-WBM with nanoHAp was formulated with 4.8 wt% BN, 5.0 wt% sodium chloride (NaCl), and 0.5 wt% nanoHAp. At 25, 150, 180, and 210° C, the filtration and rheological characteristics of the drilling muds were evaluated. The findings revealed that between 25 and 210° C, nanoHAp increased the viscosity of the WBM by 15–139% at a 1021 s-1 shear rate. It also controlled the fluid loss of the WBM from 12.1-44.6 mL to 6.7-21.8 mL at all temperatures. It serves as an anti-salt agent by decreasing the NaCl-contaminated BN's viscosity by 57% compared to the reference value of 20.8 mPa. s at a shear rate of 1021 s-1. Further, it reduced the fluid loss by 56.8%, from 169 mL to 73 mL at 210° C. The nanoHAp surface has anionic sulphate head groups of SDS that efficiently attach to the BN surface. This keeps the Na+ ions from attacking the plate-like structure of the BN. This study reveals that nanoHAp has the capacity to inhibit BN coalescence and flocculation under saturated Na+ solutions and at high temperatures.