Propping Agent Transport in Horizontal Fractures
Authors: Lowe D.K. , Huitt J.L. 
Published: December 2007

This laboratory flow study covers propping agent transport in horizontal fractures as influenced by the characteristics of the propping particles, fluid and fracture. Correlations are presented for the transport of particles, deposition and fingering through dunes. For particles moving individually, the particle velocity in some cases may be low compared to the bulk average fluid velocity; in other cases, the particle velocity may exceed the bulk average fluid velocity. At high propping agent concentration or low fluid velocity, the likelihood of particle deposition and dune formation increases. Fingering and channeling accompany dune formation. Dune formation can be limited to the outer region of the fracture by adjustment of pump rate, fluid viscosity and particle concentration. A method is presented by which pump rate and fluid viscosity may be selected to control dune formation for given propping agent concentrations.

Introduction

The area and flow capacity of a fracture predominantly control the stimulation derived from hydraulic fracturing. Generation of fracture area exposes the rock matrix to a flow channel; the flow capacity of the fracture controls the flow through the channel to the wellbore under a given pressure gradient. This paper pertains to an important factor that affects the flow capacity of a fracture: the transport of the propping particles in the fracturing during the hydraulic fracturing treatment. With a better understanding of particle transport, fracturing treatments may be designed to obtain more effective distribution of the propping agent.

RELATED STUDIES

In recent years, the approach to obtaining high fracture flow capacity has been partial monolayer propping. Three papers have dealt with various facets of this concept. However, in some papers dealing specifically with particle transport, the results generally indicate that packing the propping particles within a fracture is inevitable. In early work with a sand slurry flowing in a pie-shaped flow cell, qualitative results showed that particle deposition in a fracture during transport should be expected. In other studies correlations based on slurry flow in vertical linear flow cells showed particle deposition was primarily a function of fluid velocity. In a recent paper the rate of sand advance (movement of dunes) in a horizontal linear flow cell was discussed. Subsequently, a method was given for designing fracturing treatments, based on the extent of sand advance during a treatment. In this work, particles whose diameter was small relative to the fracture width (i.e., d/W less than 0.2) were primarily considered. The particle transport approaches that of sand transport in a stream bed. The industry trend. however, has been to use larger size propping agents. That is, particles with diameters approaching fracture widths predicted by Perkins and Kern. This trend has led to generally better fracturing treatments with particle relative sizes in the range 0.2 less than d/W less than 1.0. In all of the particle transport studies previously cited. the fluid characteristics were discussed. However, little or no attention was given to the particle characteristics. In this study the particles, as well as the fluid and fracture, are considered.

VARIABLES AFFECTING PARTICLE TRANSPORT

In the exploratory work to set up a program for this study, it was noted that particles within a batch moved at different velocities in a horizontal linear flow cell. In some cases the particles moved independently. In others there was particle interference. Also, particle deposition and fluid fingering occurred in some cases (Fig. 1). As a result of observing the easily distinguishable types of particle movement, the particle variables as well as the fluid variables were included. In addition, the fracture with was included because the fluid velocity profile surely must affect the movement of the larger particles differently than that of smaller particles. Particle variables were density, diameter (based on a sphere of equal volumetric displacement), long and short dimensions, drag coefficient and velocity, fluid variables were density, viscosity, yield shear strength and velocity. Fracture variables were the width and the angle between the plane of flow and a horizontal plane. Two factors which may prove to be important were omitted because of the complexity resulting from the parameters cited above. These are the fracture-surface roughness and gross irregularities that cause changes in the direction of flow.

EXPERIMENTAL EQUIPMENT AND PROCEDURE

A linear parallel-plate flow cell (Fig. 2) was used in the study. Lucite plates 1 in. thick formed the test section and permitted a viewing section 11 ft long and 10 in. wide. Spacing between the plates was adjusted with shims to give fracture widths of 1/16 to 1/2 in. The model could be tilted to an angle of 45 degrees with the horizontal plane. Fluid flow was maintained with a Moyno positive displacement pump. Single particles were placed in the flow stream with a particle injector.

JPT

P. 753ˆ