On the Sources of Errors in Rheological Measurements of Non-Newtonian Fluids and the Interpretation of Yield-Power Law Rheological Model Parameters

Drilling fluids are typically non-Newtonian and shear-thinning, often characterized by the Yield-Power Law (YPL) rheological model. Accurate measurement of rheological properties is essential for calculating pressure losses and estimating the fluid’s capability to suspend solid particles. Traditional concentric cylindrical (Couette) rheometers assume fluids are Newtonian, which can lead to inaccuracies when applied to non-Newtonian fluids. This paper presents a method to correct shear rates and stresses for YPL fluids. Additionally, an accurate solution for fitting the YPL model parameters to a series of shear rates and stresses is described. A similar approach to calibrating pressure gradients measured at different flow rates can be used to determine the optimal YPL model parameters for a pipe rheometer. The YPL model, expressed in consistent physical terms, simplifies the interpretation of parameters. However, significant variations in YPL model parameters can occur even for similar measurements. This is due to the shear-thinning effect being described in two different ways by the YPL model, leading to multiple possible convergences of the model parameters. Extensions to incorporate the time dependence of the rheological behavior are also presented. Thixotropy can introduce errors during measurements and must be taken into account during rheometer operation. Open-source tools for correcting rheometer data and calibrating YPL models are available, along with a web-based reference implementation, offering researchers accessible resources to streamline the calibration of rheological models for non-Newtonian drilling fluids.