High-temperature fracturing fluids based on guar degrade rapidly at temperatures above 300°F, complicating stimulation of high-temperature wells. A synthetic polymer was developed that overcomes the hydrolytic stability limitations of polysaccharides while maintaining the ability to crosslink and break the fluid.
This paper presents the initial laboratory development of a fracturing fluid using the synthetic polymer. Hydration is rapid compared to guar, eliminating the need for special hydration tanks or equipment when mixed on-the-fly. Crosslinking is achieved with conventional zirconium crosslinkers to yield a highly viscoelastic gel. Static and dynamic fluid-loss test data are presented and compared to guar-based fluids. Dynamic effects on fluid-loss rates were negligible compared to a borate-crosslinked guar fluid over a wide range of matrix permeabilities. Rheological characterization up to 400°F was performed with a Fann Model 50 viscometer and clearly demonstrated the synthetic frac fluid's high stability relative to guar. Additionally, effects of various breaker types are presented.
Hydraulic fracturing of high-temperature wells places severe demands on the fluid. Many approaches have been used to extend the application of hydraulic fracturing fluids to higher temperatures. Because guar is cost effective, it is still the fluid of choice for much of the fracturing market. For high-temperature applications, though, guar is limited by its tendency to degrade, resulting in loss of viscosity. Much work has been done to improve its performance under these conditions. Derivatization to form hydroxypropyl guar (HPG) or carboxymethylhydroxypropyl guar (CMHPG) provides improved thermal stability over plain guar,1 but it is still inadequate at high temperatures.
One of the mechanisms of guar degradation is through hydrolysis of the acetal linkages in the backbone. Raising the pH of the fluid can greatly reduce the susceptibility of the acetal linkages to hydrolysis.2 Additionally, gel stabilizers, such as methanol or sodium thiosulfate, are commonly used to minimize oxidative degradation. Chelating agents3 and various free-radical scavengers4 have also been investigated. Another approach to minimize degradation uses precrosslinked guar powder to delay hydration and extend the life of the fluid at elevated temperatures.5 Because of the difficulties encountered when polysaccharides are used under extreme conditions, others have investigated the application of synthetic polymers in high-temperature fracturing fluids. Various polymers have been reported, including acrylamide-acrylate copolymers6,7 and acrylamide-methacrylate copolymers8 that can be crosslinked with metal ions such as Cr3+ and Zr4+. Another approach relied on associative interactions of acrylamide-dodecyl acrylate copolymer with surfactant to produce a shear-stable gel.9
With the inherent stability problem of polysaccharides under extreme conditions, a synthetic polymer seemed like a more reasonable material on which a high-temperature fracturing fluid could be based. The polymer chosen for this study is a terpolymer based on 2-acrylamido-2-methylpropanesulfonic acid (AMPS), acrylamide, and acrylic acid. The polymers were neutralized with sodium hydroxide. AMPS-containing polymers have good thermal stability and tolerance to divalent ions.10 Consequently, by avoiding precipitation when mixed with formation water, AMPS-containing polymers should have easier cleanup than acrylamide-acrylate copolymers.
To verify the thermal stability of AMPS-containing polymers, Fann Model 50 tests were performed with a simple combination of polymer solution and zirconium crosslinker. The polymer compositions were 60% AMPS, 0.5 to 10% acrylic acid, and the remainder was acrylamide. The viscosities of 0.8% aqueous solutions of the polymers with 0.4% v/v zirconium crosslinker were measured at 81 sec–1 at 300°F. No appreciable degradation was indicated for any of the polymers, as shown in Fig. 1. Although all four polymers showed similar stability, the overall viscosity increased with decreasing acrylate content. Interestingly, this relationship between acrylate content and viscosity is opposite that observed in earlier work with acrylamide-potassium acrylate copolymers that produced an increase in viscosity with increasing acrylate up to an acrylate content of approximately 30%.7