Settling Velocity of Particles in Viscoelastic Fluids: A Comparison of the Shear Viscosity vs Elasticity Effect
Authors: Arnipally Sumanth Kumar , Kuru Ergun 
Published: October 2017

Abstract
The objective of this paper is to determine how the fluid shear viscosity and the elasticity would influence the particle settling velocity and even more so to answer the question of which one of these two rheological properties is more dominant in controlling the particle settling velocity when viscoelastic drilling fluids are used.
The settling velocities of the spherical particles (diameters: 1.18, 1.5, 2 and 3mm) in partially hydrolyzed polyacrylamide (HPAM) polymer fluids were measured by using Particle Image Shadowgraph (PIS) technique. Two sets of test fluids were formulated by mixing three different grades of HPAM (molecular weights of 500,000; 8,000,000; and 20,000,000) at polymer concentrations 0.09%, 0.05% and 0.03% wt. The shear viscosity and elasticity characteristics of test fluids were determined by performing shear viscosity and frequency sweep oscillatory measurements, respectively. The first set of fluids had almost identical shear viscosity characteristics while showing significantly different elastic properties (quantified in terms of relaxation time). The second set of fluids had similar elastic properties but different shear viscosity characteristics. In addition, the effect of the particle size on the settling velocities in these test fluids was also investigated.
Experimental results showed that: (i) When the fluids having similar shear viscosity profile were used, the settling velocity of spherical particles decreased significantly with the increasing fluid elasticity. The experimentally measured settling velocities were also compared against the values calculated from Shah et al. (2007) model developed for predicting the settling velocity of spherical particles in power law (visco-inelastic) fluids. Results have shown that the settling velocity values can be 14 to 50 times over-estimated if the effect of the elasticity is not considered; (ii) At constant elasticity, the settling velocity of spherical particles also decreased significantly when the fluid shear viscosity was increased; (iii) The spherical particles settling velocity increased pronouncedly as their diameter increased from 1.18mm to 3mm. But the magnitude of the increase in settling velocity with the increasing particle diameter is less for the samples having higher elasticity and similar shear viscosity characteristics. The fluid shear viscosity and the elasticity both seem to have significant effect on the particle settling velocity. However, from the field operational point of view, fluids with high shear viscosity values are not always practical to use as the high shear viscosity increases the parasitic pressure losses and potentially has a negative effect on the drilling rate. Hence, in such cases increasing the fluid elasticity can help to reduce the particle settling velocity even at lower shear viscosity values.
By conducting experiments under controlled conditions, for the first time in drilling literature, we were able to quantify the individual effects of fluid shear viscosity and elasticity on the particle settling velocity.