The paper was presented at the 46th Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, California, February 15-17, 2021. In this study, the effectiveness of different additives was evaluated in maintaining drilling fluid rheology at HPHT(high pressure and high temperature) conditions. The additives considered in this investigation are bentonite, xanthan gum (XC), low-viscosity and regular polyanionic cellulose (PAC-L and PAC-R), hydroxyethyl cellulose (HEC), other synthetic polymers and clay such as THERMA-VIS. Fluid samples were prepared in various concentrations and left to hydrate for 20-24 hrs. The rheological analysis was performed under HPHT conditions using a rheometer. Different parameters were considered in the screening, such as temperature, concentration, shear rate, and aging time.