Settling behavior of spherical particles in fiber-containing drilling fluids

Fiber-containing fluids are utilized in many industrial applications. In the petroleum industry, fiber suspensions are used to transport rock cuttings from the bottom of the hole to the surface. Moreover, fibrous fluids are applied in fracturing operations to transport proppant particles to the fractured space. Solids transport performance of these fluids largely depends on the settling behavior of suspended particles. This article presents results of experimental and theoretical investigations conducted on the settling behavior of 2 to 8 mm spherical particles in fiber-containing fluids. Experiments were carried out in a 2-m long and 100-mm diameter transparent cylinder. Both Newtonian and non-Newtonian fluids were tested. A moving digital camera system was used to track a particle while it settled. The camera records were used to determine the settling velocity of each particle as a function of time. Fiber concentration was varied from 0.00% to 0.08% by weight. When a particle settles in the fibrous fluid, it experiences fiber drag in addition to conventional hydrodynamic resistance, i.e. viscous drag. Measured terminal velocity was used to compute the viscous component of the total drag. Subsequently, applying the momentum balance, the fiber drag component acting on the particle was determined from the total drag. Results show that the fiber drag is a function of the particle's projected area, settling velocity, fiber drag coefficient, and density difference between the fluid and particle. Using experimental data, a semi-empirical model was developed to predict terminal settling velocity of a particle in fiber-containing fluids. The correlation is valid for both Newtonian and non-Newtonian base fluids that have low concentrations of fully dispersed fibers with a length and diameter of 10 mm and 100 mu m, respectively. (C) 2012 Elsevier B.V. All rights reserved.