Experimental results of the settling of spherical particles in flowing non-Newtonian fracturing fluids show that Stokes Law based on 'Power Law' viscosities is insufficient to predict particle fall rates in both flowing and quiescent fluids.

In a stagnant fluid the experimental settling velocities are more than an order of magnitude higher than those calculated, while in a flowing fluid, settling is lower than that calculated. These phenomena can be explained by extending the 'Power Law' model with a zero shear viscosity and by assuming an anisotropic viscosity in a flowing fluid.

Anisotropy in the viscosity only becomes important above shear rates of, say, 25 s-1, and so will not play a role in the majority of fracturing treatments where average shear rates in the fracture will be below this value.

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