An Innovative Approach to Gel Breakers for Hydraulic Fracturing
Authors: Wilson Adam 
Published: April 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 178991, “An Innovative Approach to Gel Breakers for Hydraulic Fracturing,” by Michael J. Fuller, SPE, Chevron Energy Technology Company, prepared for the 2016 SPE International Conference and Exhibition on Formation Damage Control, Lafayette, Louisiana, USA, 24–26 February. The paper has not been peer reviewed.

Recent studies have found that the viscosities of borate gels at actual downhole pressure conditions may be 80% less than those from standard high-pressure/high-temperature rheometer measurement (which uses 400-psi top pressure). A proposed mechanism for this phenomenon is a pressure-induced shifting of the crosslink/temperature stability near the “melt temperature” of borate-crosslinked gels, leading to a reversible thinning. This paper poses a novel approach to exploit the pressure effect by capturing the pressure-thinned fluid in a thin state and irreversibly breaking the gel viscosity.

Introduction

The chemistry of most crosslinked-gel fracturing fluids has been optimized carefully over the years, yielding a base chemistry that has remained mostly consistent for 20 years with incremental innovations. These fluids include either metal-crosslinked polymer solutions or borate-crosslinked guar, which is used for the largest percentage of crosslinked-gel fracturing treatments.

While widely used in both crosslinked and hybrid fracture designs, several performance issues have been noted in optimization and execution of these fluids. Slow shear recovery has been identified in many nondelayed borate-crosslinked fluids, which varies on the basis of shear rate, gel salinity, and additives.

In addition to unpredictable shear recovery, researchers have identified surprising effects of downhole pressures on the viscosity of borate-crosslinked-gel fluids. While the pressure for testing of crosslinked fluids is 400 psi, according to industry recommended practices, realistic downhole pressures often exceed 5,000 psi during fracturing operations. Borate-crosslinked fluids exhibit a significant reduction in viscosity under realistic downhole pressures when qualified in specialized ultrahigh-pressure rheometers. Functionally, the findings of those studies suggest that the actual downhole viscosity may be more than 80% lower than the optimized values measured with standard methods for low-pressure gel testing. This pressure-induced thinning is demonstrated in Fig 1.

The nature of this pressure effect is still under evaluation. The thinning phenomenon demonstrated in Fig. 1b could be mistakenly labeled as breaking of the gel; however, that label would be inaccurate because chemical breakers of any nature cause an irreversible change to the chemistry of the crosslinked polymer, rendering it unable to regain viscosity. Breakers for crosslinked gels fall into two categories: enzyme breakers, comprising a number of enzyme mixtures designed to hydrolyze polysaccharide polymers by cleavage of the polymer backbone, and oxidative breakers. Oxidative breakers are the most commonly used type of chemical breakers for crosslinked gels because of their versatility and their effectiveness in breaking at low concentrations. The effectiveness of low concentrations of oxidizers is attributable to their high reactivity, but this same reactivity presents a number of hazards.