Experimental Measurement of Settling Velocity of Spherical Particles in Unconfined and Confined Surfactant-based Shear Thinning Viscoelastic Fluids

Summary

This paper demonstrates the experimental procedure to measure terminal settling velocities of spherical particles in surfactant-based shear thinning viscoelastic fluids. Fluids over a wide range of rheological properties are prepared and settling velocities are measured for a range of particle sizes in unbounded fluids and fluids between parallel walls.

Abstract

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic (VES) fluids. The measurements are made for particles settling in unbounded fluids and fluids between parallel walls. VES fluids over a wide range of rheological properties are prepared and rheologically characterized. The rheological characterization involves steady shear-viscosity and dynamic oscillatory-shear measurements to quantify the viscous and elastic properties respectively. The settling velocities under unbounded conditions are measured in beakers having diameters at least 25x the diameter of particles. For measuring settling velocities between parallel walls, two experimental cells with different wall spacing are constructed. Spherical particles of varying sizes are gently dropped in the fluids and allowed to settle. The process is recorded with a high resolution video camera and the trajectory of the particle is recorded using image analysis software. Terminal settling velocities are calculated from the data.

The impact of elasticity on settling velocity in unbounded fluids is quantified by comparing the experimental settling velocity to the settling velocity calculated by the inelastic drag predictions of Renaud et al.¹ Results show that elasticity of fluids can increase or decrease the settling velocity. The magnitude of reduction/increase is a function of the rheological properties of the fluids and properties of particles. Confining walls are observed to cause a retardation effect on settling and the retardation is measured in terms of wall factors.

Introduction

Suspensions of particles in liquids are encountered in applications including pharmaceutical manufacturing, wastewater treatment, space propellant reinjection, semiconductor processing, and liquid detergent manufacturing. In the oil industry, viscoelastic fracturing fluids are used to transport proppants (typically sand) in hydraulic fractures. Upon the cessation of pumping the proppants keep the fracture open and provide a conductive pathway for hydrocarbons to flow back.

Settling of particles is governed by the rheology and density of fluid, size, shape and density of particles and effect of confining walls. For a spherical particle settl....

Protocol

1. Preparation of the Fluids

A polymer-free, viscoelastic, two-component, surfactant-based fluid system is used for this experimental study. This fluid system has been used in oil and gas wells in many producing fields for hydraulic fracturing treatments^44,45. This fluid system is used for this study because it is optically transparent and the rheology can be controlled by systematically varying the concentrations and proportions of the two components. The fluid system consists of a an anionic surfactant (such as sodium xylene sulfonate) as component A and a cationic surfactant (such as N,N,N-trimethyl-1-octadecamonium chloride) as....

Results

The experiments are performed for five different diameter particles in seven different fluid mixtures with unique K, n and λ values. Figure 1 shows the settling velocity as a function of particle diameter in one fluid. The error bars shows the variability in the three measurements. The room temperature measured during the experiment is 23 °C. It can be observed that the settling velocities in.......

Discussion

The experimental study focuses on measurement of settling velocities of spherical particles in shear thinning viscoelastic fluids under unconfined and confined conditions. Detailed experimental procedure to obtain repeatable measurements of settling velocities is presented. Results are presented to show that fluid elasticity can increase or decrease the settling velocity. Walls exert a retardation effect on settling and this effect is measured in terms of wall factors.

Prior to the experiments.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Glass Microspheres	Whitehouse Scientific	#GP1750	Available in different sieve fractions.
Rheometer	TA Instruments	ARES	Any standard rheometer capable of taking dynamic and static measurements
Anionic Surfactant (Component A)		Proprietary fluid	Used in oil field services for hydraulic fracturing. Sodium Xylene Sulfonate can be used as a substitute.
Cationic Surfactant (Component B)		Proprietary fluid	Used in oil field services for hydraulic fractuing. N,N,N-Trimethyl-1-Octadecamonium Chloride can be used as a substitute.