On the end-effect correction for Couette type oil-field direct-indicating viscometers for Newtonian and non-Newtonian fluids

Analysis and experimental results are presented which quantify the magnitude of end-effects for direct-indicating, narrow gap, rotating cylinder viscometers used extensively in oil-field, for Newtonian and non-Newtonian fluids. Such viscometers have already embedded at manufacturing stage an end-effect correction and give directly the shear stress which does not allow end-effect quantification in a straight forward manner. The presented analysis resolves this problem by estimating the embedded correction, which has been determined as 6.69% of the torque developed along the cylindrical part of the stationary bob of this viscometer.Experimental results with Newtonian and non-Newtonian fluids show that there is no additional end-effect from the bottom of the stationary bob. However, there is additional end-effect contribution from the top section of the bob, which has not been taken into account in the design and manufacturing of the instrument. Its magnitude ranges from 5 to 6% for the Newtonian fluid and the high shear rate range for the non-Newtonian fluid, to 12% for the low shear rate range for the non-Newtonian fluid tested, while the impact on drilling situations is also discussed.