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Choosing the best rheological model for water-bentonitesuspensions

Maglione Roberto, Kelesidis Vasilis

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Summary

Flow of water-bentonite dispersions is encountered in a varietyof situations in oil-well drilling, chemical, petroleum and wastetreatment industries and in complex geometries like pipe, concentricand eccentric annulus, and rectangular ducts. Most of the time, theflow of these dispersions is laminar and analytical solutions havebeen developed for a variety of rheological models like the Casson,the Robertson-Stiff, and the Herschel-Bulkley models. Couetteviscometers are often used to determine the applicable rheologicalmodel but most of the time the shear rates experienced by the fluidsare often computed as if the fluids were Newtonian or using anarrow gap approximation, giving thus only approximate valuesof the rheological parameters for the particular model. Recentadvances, though, enable the computation of the true shear ratesfor any of the three models mentioned. Using Couette viscometricdata from the literature, the three models are applied to obtain therheological parameters using Newtonian and true shear rates in thenarrow gap of the viscometer and the best fit model is determined.The flow parameters in laminar flow for pipes and annuli, suchas velocity profile, pressure drop gradient as well as the onset ofthe transition to turbulent flow are then predicted. Differencesin the rheological behaviour for all three models and from usingNewtonian or true shear rates, as well as on the prediction of theflow parameters are evaluated and discussed.

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