One of the main tasks of drilling fluids is to transport cuttings to the surface to clean the borehole. During this process the fluid is exposed to different flow regimes, shear rates and temperatures. The fluid properties affect the flow behavior, where viscosity, density and gel strength are among the key parameters. Field experience indicates that oil-based drilling fluids perform better in terms of hole cleaning than water-based fluids do, even if viscosity and density values are similar. The exact reasons for this are not entirely understood and increased knowledge will contribute towards more efficient drilling operations.

To deepen the understanding of the hole-cleaning abilities of drilling fluids, a comparative study with oil-based and water-based drilling fluids with similar viscosity profiles was executed. Flow-loop experiments in a 10 m long test section with a free-whirling rotating inner drill string are correlated to rheological investigations on a Fann 35 viscometer and an Anton Paar rheometer. The Fann 35 measurements follow the ISO/API standards and include viscosity and gel-strength measurements. With the Anton Paar rheometer flow curves and temperature dependence were obtained.

This paper presents selected rheological properties in correlation to flow-loop experiments in order to get a better understanding of the unequal behavior in solids transport of the two drilling fluid types. The presented results include yield point and plastic viscosity determination. In addition Herschel-Bulkley and power-law regressions were used to obtain comparable yield-point values. The rheometer measurements provide more detailed data due to extended functionality of the instrument, higher accuracy for each measuring point and a higher number of data points acquired compared to the Fann viscometer.

The yield point of the fluids is expected to influence mainly the static and the low-flow conditions in the wellbore, whereas the plastic viscosity may have more importance in the high-shear region of the flow. To draw back conclusions from the rheology measurements to the flow-loop results, these two points are highlighted in this study.

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