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Author index ►Vassilios C. Kelessidis, Vasiliki Hatzistamou, Roberto Maglione

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Wall slip phenomenon assessment of yield stress pseudoplastic fluids in Couette geometry

Appl. Rheol.20:5 (2010) 52656 (8 pages) ►

Results are presented aiming to determine whether wall slip occurs while performing rheological measurements of Carbopol solutions and bentonite dispersions at different concentrations using a standard oil-field Couette-type viscometer with two gap sizes. Yield stresses using a vane rheometer were also determined and compared to those obtained by extension of the experimentally-derived rheological curves at the Couette viscometer.The results show that, if preparation procedures are followed as suggested for Carbopol solutions and by API standard for drilling fluids, simulating either the pre-shearing in the rig mud pumps or in the bit nozzles during drilling operations, wall slip does not occur, with a good agreement of the rheograms and of the yield stresses determined for both gap sizes of the Couette viscometer and by the vane rheometer. No slip occurs also for CMC solutions which exhibited pseudoplastic power-law behavior.► Cite this publication as follows:

Kelessidis VC, Hatzistamou V, Maglione R: Wall slip phenomenon assessment of yield stress pseudoplastic fluids in Couette geometry, Appl. Rheol. 20 (2010) 52656.

A methodology is presented to invert the flow equation of a Herschel-Bulkley fluid in Couette concentric cylinder geometry, thus enabling simultaneous computation of the true shear rates, Γ_{HB}, and of the three Herschel-Bulkley rheological parameters. The errors made when these rheological parameters are computed using Newtonian shear rates, Γ_{N}, as it is normal practice by research and industry personnel, can then be estimated. Quantification of these errors has been performed using narrow gap viscometer data from literature, with most of them taken with oil-field rheometers. The results indicate that significant differences exist between the yield stress and the flow behavior index computed using Γ_{HB}versus the parameters obtained using Γ_{N}and this is an outcome of the higher Γ_{HB}values. Predicted true shear rates and rheological parameters are in very good agreement with results reported by other investigators, who have followed different approaches to invert the flow equation, both for yield-pseudoplastic and power-law fluids.► Cite this publication as follows:

Kelessidis V, Maglione R: Shear rate corrections for Herschel-Bulkley fluids in Couette geometry , Appl. Rheol. 18 (2008) 34482.

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