Thixotropy and Yield Stress Behavior in Drilling Fluids

Abstract

Drilling fluids are commonly recognized as complex fluids which exhibit both yield stress behavior and varying degrees of thixotropy. Traditional models for yield stress behavior have been used extensively, with the Bingham plastic model remaining prevalent for description in the field and the Herschel-Bulkley model becoming the standard for computer simulations of fluid behavior. Neither of these models represents the full behavior of drilling fluids, either missing aspects of the shear-thinning behavior or inaccurately predicting the yield stress of the fluids. These errors are exacerbated through variations in measurement techniques by technicians who do not fully appreciate the thixotropic nature of these fluids. An improved understanding of the rheological behavior, in particular the yield and thixotropic nature, of drilling fluids would result in improved models and enhanced drilling performance, thereby reducing drilling costs. An examination of some typical drilling fluids will be presented. The intertwined effects of thixotropy and yield stress on rheological measurements will be highlighted. These fluids will also be evaluated with traditional yield stress models as well as with several recently proposed models.