The effects of wall slip on the reliability of viscosity data obtained using concentric-cylinder viscometers was investigated for water-based Bentonite drilling muds. Viscosity data for a typical mud system (solid content = 8%) were found to depend on the gap width demonstrating that wall slip was occurring. The slip velocity was found to increase with wall shear stress implying that stress-induced migration might be among the mechanism involved in driving the particles away from the wall. At any given wall shear stress, an increase in pressure was realized to reduce wall slip effects but temperature rise was found to make slip effects even more pronounced.

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