ANNUAL TRANSACTIONS OF THE NORDIC RHEOLOGY SOCIETY, VOL. 16, 2008

Rheology of drilling fluids affects the frictional pressure drop and the solids carrying capacity of the fluids during the drilling operation. Drilling fluid rheology is commonly controlled by using a variety of clay or polymeric materials, depending on the type of fluid used and the demands of the specific drilling operation. Most drilling fluids possess varying degrees of time- and shear-dependent thixotropic properties. Among them, water-based fluids containing clays, such as bentonite, exhibit a pronounced thixotropic behaviour. This characteristic can have a significant effect on the peaks and troughs of pressure that occur in the wellbore when the drillstring or the tool-string is moved in and out, or when pumping starts after a break in circulation. Major pressure fluctuations can lead to fracturing of the formation, loss of circulation, influx of formation fluids into the wellbore, or collapse of the wellbore. Thus, a practical means of accounting for thixotropy in hydraulics calculations would be of great value for maintaining safety as well as the integrity of the wellbore. As far as can be determined the thixotropic characteristics of drilling fluids have not been quantified for engineering applications. In this work, a simple model based on the concept of a structure parameter is used to describe drilling fluid thixotropy. Empirical relationships are devised that can predict, with good approximation, the time- and shear-dependence of rheological parameters within the range of shear rates encountered in the drillstring and in the annulus between the drillstring and the borehole wall. The model shows potential for use in calculation of drilling hydraulics