Rheological Properties of Low Gel-Loading Borate Frac Gels
Authors: Harris P.C. , Heath S.J. 
Published: December 2007

Abstract
This paper presents a study of hydration, crosslinking and rheological properties of 15- to 25-lb/Mgal optimized borate-crosslinked guar fluids. During the hydration and crosslinking process. fluids were studied in a new turbulent flow-loop viscometer. These studies allowed researchers to simulate both the field application of frac fluids in surface equipment and the pumping of the fluids down tubulars. Researchers then simulated fluid flow in a fracture by measuring the fluids in low-shear laminar flow with heat applied. Viscosity profiles of the fluids during heating were measured. The effects of preshear conditioning of crosslinked fluids in high shear-rate flow on laminar flow properties were observed. The rheological behavior of the new crosslinked fluids with lower gel concentrations was compared to the behavior of conventional borate-crosslinked fluids with higher gel concentrations.
Hydration rates were measured for guar at different levels of shear history. Optimum shear levels and times for hydration were observed for the time-dependent base-gel fluids. These base-gel fluids were crosslinked with a borate crosslinking agent. and corresponding friction pressures were observed. The properties of the crosslinked fluid were critically dependent upon the base gel fluid state of hydration before crosslinking Properly crosslinked fluids did not shear-thin back to base gel levels but maintained significant viscosity even at high shear levels.
Power-law index values were measured for low-polymer borate fluids with a three-viscometer method that uses constant shear rates. Lower values of n' were found for the new fluids compared to conventional borate fluids with similar polymer loadings. Proppant transport properties were qualitatively observed in a transparent slot model showing good transport for both an optimized 25-lb/Mgal borate fluid and a conventional 35-lb/Mgal borate fluid.

Introduction
Borate-crosslinked guar fluids are the most commonly used fluids in the fracturing stimulation industry. These fluids are inexpensive. provide the highest viscosity of any commonly used crosslinked guar and can be used over a broad range of temperatures. Less than a decade ago. 40- and 50-lb/Mgal crosslinked polymer fluids were commonplace. In recent years. attention has been given to reducing the guar polymer loading of fracturing fluids as a way of minimizing the damage potential to reservoirs and fracture conductivity, and borate-crosslinked fluids have helped this reduction to occur. Borate-crosslinked fluids have higher viscosity and greater proppant transport ability than metal-crosslinked fluids with similar polymer loadings. Fracture treatments requiring 40 lb/Mgal of polymer crosslinked with titanium or zirconium can be performed with 30 to 35 lb/Mgal of polymer crosslinked with borate.
When lower polymer loadings were successfully used and operators were comfortable with those fluids, the industry was ready for further reduction of polymer in an attempt to minimize fracture conductivity damage. More recently. borate-crosslinked fluids containing 15- to 25-lb/Mgal polymer have been used successfully. A guar concentration of about 17 lb/Mgal has been considered the minimum amount of hydrated polymer to occupy the volume of solvent. Guar fluids at or below this critical concentration should be very unstable and quickly show syneresis. Nevertheless, borate-crosslinked fluids can be made near or below this critical guar concentration with at least transient stability. If these fluids are made at near optimum condition, then stability can be obtained long enough for operators to complete a fracturing treatment. In fact, long-term instability may be a desirable feature for minimizing reservoir damage. The first step in understanding continuous crosslinked fluids was to measure the rate and degree of base gel hydration as a function of tubing shear rate. We knew that some level of shear was necessary for proper dispersal and hydration of the guar slurries, but we did not know the effects of a maximum shear level, and whether viscosity production might suffer. P. 545^