High shear viscosity at the bit nozzle is an important parameter in drilling optimization. Four rheological models were used to calculate high shear viscosity from rheometer data and the calculated viscosities were compared with viscosities measured with high pressure capillary viscometers for five types of 16 different kinds of water base non-weighted muds. The Casson model gives the best fit and rheometers can be used in the field to predict nozzle viscosities. Regression equations and experimental relationship between drilling rate and high shear viscosity were obtained from full scale laboratory drilling rig tests. The high shear viscosity has a considerable influence on drilling rate in sandstone and limestone. A lower nozzle viscosity is preferred in optimized drilling.

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