Proppant Transport in Slickwater Fracturing of Shale-Gas Formations
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 125068, ’Proppant Transport in Slickwater Fracturing of Shale-Gas Formations,’ by Adam Dayan, Shaun M. Stracener, and Peter E. Clark, SPE, University of Alabama, prepared for the 2009 SPE Annual Technical Conference and Exhibition, New Orleans, 4-7 October. The paper has not been peer reviewed.

Predicting proppant transport is important in both treatment design and post-treatment analysis. The transport process has been studied since the late 1950s, and much has been learned from those simple laboratory models. There are indications that fracturing in gas-shale plays can produce complex fracture networks. To study how the slurry travels through the network, a small 1D fracture model with bifurcation was constructed along with a larger 3D slot model.  

Introduction

It has been shown that fracture-width variation has a larger role than convection with respect to proppant placement. In nonsettling slurries, the motion of proppant-laden fluids into a fracture is influenced strongly by fracture-width variations. Therefore, width variations should influence settling slurries in the same manner. Also, the trajectories of both types of fluids will be influenced strongly by height growth and high-fluid-loss zones that will attract fluid flow. In gas shales, fracture bifurcation hinders understanding of and predicting proppant transport.

Noncrosslinked Fluids

Low-viscosity fluids can be pumped in laminar or turbulent flow, although turbulent flow may not persist away from the wellbore. The viscosity and structure of these fluids allow particle settling.  Convection and particle settling act in the same direction, so the process is considerably more complex than simple settling.

Particle Settling. Several factors influence particle settling in Newtonian and non-Newtonian fluids. The density difference between the particle and the carrier fluid, the square of the particle diameter, and the fluid viscosity are important. In addition, the presence of boundaries (walls) and particle/particle interaction (hindered settling) can affect particle settling in fracturing fluids. With non-Newtonian fluids, viscosity and viscoelasticity make the problem more complex.

Non-Newtonian Fluids. Most of the polymeric thickening agents used in the oil field exhibit shear thinning over some range of shear rates. As an additional complication, noncrosslinked polymeric fluids can exhibit viscoelasticity, for which the influence on the settling velocity is not predicted easily. Three distinct regions of viscosity behavior exist with respect to shear rate that most, if not all, non-Newtonian fluids exhibit: a power-law region bounded by upper and lower Newtonian regions.

The range of the power-law region can vary by orders of magnitude depending upon the fluid. While the power-law region is most often assumed to occur within the boundaries of this region, the shear rate caused by particle settling falls outside this region. The small particles and relatively low shear rates in the fracture allow the high-shear-rate upper Newtonian region to be ignored safely. However, fluid behavior in the low-shear-rate portion of the power-law region and in the lower Newtonian region is likely to be significant in determining the particle-settling velocity.

The velocity profile for a non-Newtonian fluid tends to be flatter in the center with a near-zero shear rate. The zero-shear viscosity of the fluid is even more important in the settling of particles traveling in the center of the flow.