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 β-1,4 acetal linkages. This backbone has single-unit branches of galactose connected by α-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 β-1,4 acetal linkage results in large changes in molecular weight and fluid viscosity. Competing hydrolysis of the side-chain α-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.