Abstract

Guar gum is a naturally occurring polysaccharide that is used extensively as a water-based viscosifier. It is the principle agent used to manufacture viscoelastic fracturing fluids. It is composed of the simple sugars galactose and mannose in a ratio of approximately 1:1.6.

Guar is a linear polymer with a backbone composed of mannose connected by ß–1,4 acetal linkages. This backbone has single-unit branches of galactose connected by a-1,6 acetal linkages. The ratio of mannose to galactose has been shown to influence the solubility of the guar gum. As the galactose content is reduced, the solubility of the polymer in water decreases dramatically. Low-galactose polymers, such as locust bean gum, have very low solubility in water.

The viscosity of guar gum-based fracturing fluids is reduced at the end of a fracturing treatment to promote fluid recovery rates. To reduce the viscosity, several materials are used as gel breakers. Most of these materials cause the degradation of the polymer by hydrolysis of the acetal linkage. However, two types of acetal linkages in guar are available for hydrolysis. Hydrolysis of the backbone ß–1,4 acetal linkage results in large changes in molecular weight and fluid viscosity. Competing hydrolysis of the side-chain a-1,6 acetal linkage results in the removal of galactose branches that have little impact on the molecular weight but significant impact on solubility.

This paper presents kinetic, viscosity, and solubility data that demonstrate the importance of acetal hydrolysis in the chemistry of guar degradation. Additionally, rate expressions that can be used to influence gel breaker design and well flowback programs are presented.

Introduction

Guar gum and its derivatives have become the preeminent materials for hydraulic fracturing operations over the past five decades owing to reliable and economical supply as well as flexible chemistry. Much effort has gone into understanding the chemistry that controls fluid rheology, both providing viscosity increase to generate fracture geometry and deliver proppant during a fracturing treatment, and decreasing the viscosity after treatment to permit rapid flowback of the fracturing fluid and well cleanup.

Many years ago, insoluble residue associated with guar gum (protein and cellulose) was recognized as partially responsible for the loss of conductivity within the proppant pack. Significant advancements to reduce the insoluble residue concentration in the guar gum through improvements in manufacturing processes and chemical modifications, such as propoxylation, have resulted in improvements in regained conductivity. Improvements in crosslinkers, the control of crosslinking rate, and reliable computer-controlled pumping equipment have permitted the development of low polymer-concentration fluid systems that further reduce conductivity damage caused by guar-based polymers. Regained conductivity of less than 15% was typical with the crosslinked guar fracturing fluids of the 1970s. With the improvements indicated above, regained conductivities have improved by a factor of two to three. Although regained conductivity values approaching 100% are often advertised, conductivity inflation should be considered a factor. When real-use fluid formulations are evaluated under realistic conditions, the regained conductivity values are usually much less than 100%.

Gel breakers are used to reduce viscosity at the conclusion of a fracturing treatment. The gel breaker should not work too quickly or the fracturing treatment may not be completed as designed. Yet, it should work as fast as possible to reduce viscosity so that the fracturing fluid can be produced back quickly and fracture closure can be achieved. Field results indicate that aggressive breaking packages result in improved well production.1

Recently, a new guar-based fluid system that uses conventional crosslinking chemistry has been reported.2,3 It is unique in that it does not require gel breakers. The guar polymer is extensively depolymerized under controlled conditions during the manufacturing process so that breakers are not required to reduce the polymer molecular weight and the resulting viscosity during use. The gel-breaking mechanism for this fluid system takes advantage of the pH reversibility derived from the use of conventional boron crosslinking chemistry. Most formations have been found to have adequate buffering capacity to reduce the pH of the nonbuffered fracturing fluid below that required to maintain a stable crosslinked gelled fluid.4

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