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.
The rheological behaviour of water-based drilling fluids thickened with bentonite are of crucial importance for the success of a drill operation . Among different properties that a potential mud system should posses to be nominated for use in drill industry, shear-thinning property is perhaps the most important one. As a result of this property, viscosity would be low near the tip of the drill bit (where shear rates are high) with a subsequent drop in the driving torque. At the same time, viscosity would be high in regions far from the drill bit (where shear rates are lower) so that sedimentation of formation cuttings to the bottom of the bore well is significantly inhibited. Because of the importance of the viscous behaviour of drilling fluids, viscometers of robust design are often used for measuring their flow curves (i.e., shear stress vs. shear rate profile). Concentric cylinder viscometers are widely used used for this purpose. This viscometer benefit from such advantages as the simplicity of design, possibility of generating large torques at low shear rates, flow stability (by rotating the outer cylinder), lack of edge effects, and easy analysis of the end effects. However, the equations which are used in this instrument to relate stress field to the flow field is based on the assumption of the no-slip condition , therefore, the viscosity data can be seriously in error if the fluid exhibits wall slip.
Wall slip is a well established phenomenon for non- Newtonian fluids such as gels, foams, polymeric liquids, emulsions and suspensions. Although it should be conceded that true slip (i.e., a discontinuity in the velocity at the wall) has been observed only for polymer melts , for other fluid systems (e.g., suspensions), slip is in the form of a region of a higher velocity gradient adjacent to the wall, resulting in an apparent slip . That is, for suspensions, the local concentration of the particles is lower at the wall than in the bulk. When the fluid is sheared, large velocity gradients are produced in this low viscosity layer, resulting in apparent slippage of the bulk fluid. Obviously, when slip occurs in a viscometer, the flow field is no longer known and the rheological data may not be very reliable.
In spite of the fact that drilling muds are among suspensions and suspensions are long known to be vulnerable to wall slip, quite surprisingly, the slip phenomenon of drilling fluids and its possible effects on a typical drill operation has rarely been addressed. To the best of our knowledge, the only work cited in this area is by Yoshimura and Prud'homme . They investigated wall slip effects of a 1.96% bentonite clay suspension using a concentric-cylinder geometry and realized that their mud indeed slipped at the wall. The slip velocity increased with wall shear stress but no simple relation could be figured out between these two parameters. Also, because the above work was done at room conditions it cannot be applied to real site conditions where elevated pressures and temperatures are often encountered.