Common Versus Herschel-Bulkley Drilling Fluid Models: Effect of Their Rheological Parameters on Dynamic Particle Settling Velocity

Abstract

Rheological modelling of drilling fluids in oil fields is usually described by Bingham plastic and Ostwald-deWaele models. These models gain popularity because their specific descriptive parameters are fairly easy to estimate. Standard methods use Fann VG meter dial reading at 600 and 300 rpm to determine these rheological parameters. Unfortunately, these points correspond to higher shear rates which seldom prevail during particle settling. Recently, many researchers pointed out that the non-Newtonian behavior of drilling fluids can be described well by the three parameters Herschle-Bulkley model. Again, the determination of its parameters using the standard API method make use of dial readings at 6 & 3 rpm to determine yield stress and 600 and 300 rpm to determine the other two parameters. Furthermore, the use of non-linear regression techniques to determine these parameters though deemed more accurate, sometimes give meaningless negative yield stress values. This work aims to investigate different techniques and shear rates to derive rheological parameters and show their influence on the magnitude of effective viscosity and hence settling velocity. It is demonstrated that very small differences among the values of the model parameters determined by different techniques/dial readings can lead to substantial differences in predicted settling velocities. Results of this work shows that the use of Herschle-Bulkley rheological parameters was by far the most accurate for representing the example muds rheograms as well as predicting the settling velocities, particularly when using non-linear regression values. Moreover, the simplified API method to determine Herschle-Bulkley rheological parameters may lead to considerable errors that may negate the usefulness of the model. It has been shown that Chien [10] correlation is sensitive to mud rheology and is suitable only for thin fluids. However, Chien [10] correlation was not affected by the annular fluid velocities.