Novel Application of Synergistic Guar/Non-Acetylated Xanthan Gum Mixtures in Hydraulic Fracturing

Abstract

Abstract Fracturing fluids have traditionally been viscosified with guar and guar derivatives. Non-acetylated xanthan is a variant of xanthan gum which when combined with guar in solution develops a synergistic interaction that generates superior viscosity and particle transport at low polymer concentrations. These water-base linear fluids have improved low shear viscosity at concentrations at or below 25 lb/1,000gal when compared to fluids viscosified using a single viscosifier such as guar or xanthan gum. The polymer mixtures can be crosslinked to provide enhanced viscosity at higher temperatures. Introduction The main function of fracturing fluids is to open the fracture and transport proppant along the length and height of the fracture. The rheological properties of the fluid are considered basic for these functions. Water-based hydraulic fracturing fluids have been generally viscosified with guar and guar derivatives. The guar molecules are normally crosslinked with different crosslinkers.1 However, optimum hydraulic fracturing treatments require that the fluid cleans out of the fracture, leaving minimum damage to the proppant pack. The damage or reduction in permeability to the proppant pack is due to polymer residue leftover from the fluid. In order to reduce the damage leftover by the polymer, oxidizers and enzymes are used to break down the molecular weight of the polymer.1 However, the effect of the breakers is still not enough to eliminate the damage to the proppant pack. Usually the majority of the polymer remains in the fracture and does not flow back.2,3 As a consequence, polymer loadings were reduced from the 30–40 lb/1,000gal to 15–25 lb/1,000gal.4,5 Fluid formulations were adjusted to still yield acceptable rheology for proppant transport after crosslinking the polymer. No adjustments were made for the linear gel at the surface, where the viscosity of low-polymer linear gels is very low. Alternatives to polymers are surfactants, which form worm-like micelles in solution.6 These micelles have a gross structure similar to polymer chains, which increase the viscosity of the fluid. The complete breakup of the fluid occurs when the polymer enters in contact with hydrocarbons. However, emulsions have been known to occur with some crude and the cost of these fluids is high in comparison to polymer-based fluids. In this paper a new technology is presented which enhances the viscosity of linear gels using a synergy between guar and Non-Acetylated Xanthan (NAX), a variant of xanthan gum. The synergy develops when NAX is combined with guar in solution yielding higher viscosities than guar or xanthan alone at the same total polymer concentration. The effect of different parameters on the rheology of the mixtures is presented, such as the ratio of guar to non-acetylated xanthan, the effect of salts, temperature and shear history. Large scale proppant transport tests were performed to evaluate proppant transport of the mixtures with respect to pure guar fluids. Background Guar gum is a non-ionic galactomannan, which is extracted form the endosperm of guar beans. The monomer unit of guar gum is composed of linear chains of ß-D-mannopyranosyl units linked to each other by 1–4 bonds. Also a single a-D-galactopyranosyl unit is linked to the mannose by a 1–6 bond. The ratio of mannose to galactose varies from 1.6:1 to 1.8:1, as opposed to 2:1, indicating some unsubstituted backbone.1