Enormous research work has been performed in order to retain the desired rheological, filtration, and friction properties of water-based drilling fluids at elevated temperature and pressure. In the current work, polyester copolymer (PES) was synthesized by esterification of poly isobutylene maleic anhydride (PIBMA) with poly ethylene glycol (PEG). Copper oxide nanoparticles (CuO NPs) were added during the copolymerization in different ratios, 0.5, 1, and 2 Wt. %, so that new nanocomposites were obtained. The prepared PES and its nanocomposites were characterized by nuclear magnetic resonance (NMR), X-ray diffraction (XRD), transmission electron microscope (TEM), fourier infrared spectroscopy (FTIR), and thermal gravimetric analysis (TGA). Moreover, case studies were conducted through analyzing the samples composed of water-based muds (WBM), developed with the copolymer and its CuO nanocomposites, compared with the blank ones. The results showed great enhancement in rheological, filtration, and friction properties of WBM developed by 3.5 ppb of PES copolymer (PIBMA-PEG) that can withstand high temperature beyond〖 400〗^⁰ F. On the other hand, the prepared nanocomposites showed an adverse effect on mud rheology at temperature greater than 300⁰F. The results appear a promising development in copolymerization and nanotechnology that plays a vital role to enhance drilling fluids performance at high temperature environment and minimize drilling cost together with the non-productive time (NPT). Considering that the best result is approached by adding 7 ppb PES to NaCl polymer mud that sustain rheological properties especially yield point (YP = 23 Ib/100 〖ft〗^2) accompanied by controlling both filtrate volume and induced friction.