Low thermal conductivity and unfavorable rheological properties of drilling cement slurry remain the main challenges in cementing operations of oil and gas wells under high-temperature and high-pressure conditions. In the present study, the effect of carbon nanotubes with a diameter of 0.4 nm and spherical nanosilica with a size of 5–15 nm on the thermophysical and rheological properties of Class G cement slurry was experimentally investigated. Four slurry samples were prepared including the base sample (without nanomaterials), a sample containing 1 wt% nanosilica, a sample containing carbon nanotubes, and a sample with the simultaneous combination of both nanomaterials. All samples were tested under simulated well conditions (temperature of 264 °F and pressure of 7000 psi). The results demonstrate that nanosilica increases the 24-hour compressive strength by 85%, carbon nanotubes by 125%, and their simultaneous combination by 230% compared to the base sample. Fluid loss decreased by 37%, 50%, and 63% for the nanosilica, carbon nanotube, and combined samples, respectively, while free water content was reduced to zero in all three samples containing nanomaterials. Thickening time also improved by 11%, 20%, and 25%, respectively. Rheological model analysis showed that the designed slurries closely follow the Bingham plastic model. In the numerical section, a novel regression model was developed to predict compressive strength as a function of nanoparticle volume fraction and curing time, with a coefficient of determination of 0.95. The findings indicate that each nanomaterial, especially their simultaneous combination, effectively improves drilling cement performance under harsh conditions.