High Pressure Mixing Rheology of Drilling Fluids

Abstract

Drilling fluids are multicomponent emulsions and/or suspensions, which normally show non-Newtonian behavior, i.e. yield stress, shear-thinning and thixotropy, and strong thermal and pressure dependence. The rheological characterization of drilling fluids using conventional geometries can be a difficult task due to the inherently heterogeneous nature of these systems. These problems may be overcome using nonconventional geometries, such as helical ribbons and blade turbines, which maintain the homogeneity of the system during the measurement. The overall objective of this work was to evaluate the use of mixing geometries, such as helical ribbons and blade turbines, for characterizing the flow behavior of drilling fluids as a function of pressure. From the experimental results it may be concluded that, using the Metzner-Otto approach, both four-blade and helical ribbon type-geometries are suitable for this purpose. This study shows that the Metzner-Otto constant, for a four blade turbine geometry, is practically independent of the flow index at atmospheric pressure, which shows a linear dependence at higher pressures. On the contrary, for the helical ribbon geometry, an exponential dependence of the Metzner-Otto constant on the flow index is observed independently of the measured pressure. From the experimental results obtained, it can be concluded that both nonconventional geometries can be used to measure the influence of pressure on the rheological parameters of non-Newtonian fluids. These tools extend the experimental shear-rate window covered by the coaxial cylinders conventional geometry to lower values, allowing the measurement of important engineering parameters, such as, for instance, yield stress.