The shear stresses and frictional pressure losses that exist in a circulating system could be predicted through rheology models. This paper presents the experimental comparison of a relatively new rheology model called the Robertson-Stiff [1] model to widely used Ostwald-de Walle (Power Law) [2] and Bingham Plastic [3] models.

The models were used to predict shear stresses and frictional pressure losses in laboratory experiments using clay-water drilling fluids for flows through pipes and annuli. Two types of clay-water drilling fluids were used; one with a low yield stress and the other with a high yield stress.

The result of the research showed that the Robertson-Stiff [1] model predicted the best fit of the rheologic data for the two types of drilling fluids used. However, for fluid velocities greater than 2 ft/sce (0.61 m/sec) the frictional pressure losses predicted by the Robertson-Stiff [1] model differ from that of the Power Law [2] model by less than three percent.

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