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Water-soluble polymers are used to improve drilling operations to ensure that oil and gas wells are drilled in less time and completed with minimized formation damage. However, excessive utilization of these polymers additives may have implication on the drilling outcomes. In this work, drilling fluids comprising of different compositions of polyanionic cellulose (PAC), and starch; and a constant amount of bentonite were formulated to examine the effects of excessive use of each additive or their combinations in a water-based drilling fluid (WBDF). The fluid properties were further investigated at low pressure and low temperature (LPLT) as well as high pressure and high temperature (HPHT). The results suggest that the viscosities of the fluid are not only dependent on the shear rate, shear stress, and composition but also on the temperature and pressure. Model plots of shear stress (Pa) against shear rate (s−1) using the Power law, Bingham plastic and Herschel-Bulley shows that all composition have low viscosity and low yield point. Increasing the amount of additive increases, the equivalent circulating density (ECD) which may increase differential sticking and consequent formation damage. [ABSTRACT FROM AUTHOR]