Introduction The viscosity of oil-base muds can be determined over a wide range of temperature and pressure using the BHC Viscometer. This instrument consists of two concentric cylinders mounted in a 20,000-psig autoclave that has an upper operating-temperature limit of 650 degrees F. The inner cylinder, or rotor, is connected by a magnetic couple to a Haake Rotovisko. A detailed description of the instrument has been published previously. In operating the BHC Viscometer, the autoclave is completely filled with mud and sealed. The rotor is rotated at a constant rate and the sample is allowed to reach an equilibrium temperature. The pressure is then adjusted with an auxiliary pump and the shear stress is recorded for each shear rate. The instrument was calibrated by first calculating the shear rates from the viscometer dimensions. This method gave shear rates of 11, 21, 32, 64, 96, 191, 286, 573, and 860 sec-1. The mean shear-stress constant was then determined experimentally to be 8.1 dynes/cm2/scale part (1.6 lb/100 ft2/scale part). Discussion The best mathematical description of the viscosity of an oil-base mud at constant temperature and pressure is the power-law model. The linear form of this model is power-law model. The linear form of this model is(1)ln = ln K + n ln y However, two sets of constants are required, one set for use at shear rates below ~ 200 sec and the other set for the higher shear rates. The analysis of the pressure and temperature effects shows that the logarithm of the shear stress is directly proportional to the pressure and inversely proportional to proportional to the pressure and inversely proportional to the temperature. These relationships can be expressed by the following equations:(2)ln p, JPT P. 884

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