Abstract

A novel oxidizing breaker system has been developed for fracturing fluids at high temperatures. Below 200 F, the system is not active, but above 200 F, the oxidizing system aggressively attacks the polysaccharide backbone of the fracturing fluids, resulting in a complete break of the crosslinked fluids. In the presence of a gel stabilizer, an intermediate, reactive oxidizing species is formed. The result of this formation is a delayed, soluble, high-temperature oxidizing system.

Controlled viscosity reduction at 200 F to 300 F in crosslinked gelled fluids with and without a gel stabilizer will be demonstrated. Testing included Model 50 viscosity profiles, high-temperature static break tests, and conductivity testing. Results from all testing showed the effect of oxidant concentration in producing a predictable, controlled break of the thermally stabilized crosslinked systems. Data were obtained in low-pH and high-pH Zr-crosslinked fluids as well as in borate-crosslinked fluids. The delayed mechanism of the new breaker system provides fluids with excellent crosslinked viscosity properties at early times with predictable, long-term viscosity reductions. Case histories show that the breaker system can be used throughout the treatment in the pad fluid, proppant-laden fluid, and flush.

This paper provides data that allow significant improvements in job design. The operations engineer can obtain predictable, controlled gel degradation by using the data provided for temperature, gel type, gel stabilizers, and breaker concentration. The results are optimized treatment designs with rapid fluid recovery, improved proppant-bed conductivity, and increased well productivity.

Introduction

Breakers are an essential component of fracturing fluids. Ideally, a breaker should maintain high viscosity throughout the pumping of the fluid and sand. Once pumping is complete, the fluid should immediately break back to the viscosity of water. An ideal viscosity profile is shown in Fig. 1. In most cases, current technology provides a quick initial drop in viscosity followed by a slow, gradual decline in viscosity until the fluid is completely broken. Encapsulation helps achieve an improved break profile at low to moderate temperature, but above about 175 F, diffusion from the capsules becomes the determining factor because the breakers are only briefly stable at those temperatures.

Improved fluids technology has provided crosslinked gels that can maintain viscosity at elevated temperatures for long periods of time. These thermally stable fluids improved overall gel viscosity and the ability of the fluid to carry proppant. However, the advancement in fluids technology to provide more stable fracturing gels limited the recovery of the fluid and ultimately the fracture conductivity. The use of breakers in high-temperature fracturing applications would provide a method to efficiently recover these thermally stable fluids. Even today, the need for breakers throughout an entire fracturing treatment above 200 F is not a generally accepted concept because of the lack of controllable breaker systems at these high temperatures.

Oxidizing breakers, such as persulfate, are effective from about 120 F to about 175 F. However, these materials react too quickly at higher temperatures. The rapid oxidation causes uncontrolled breaks and premature gel degradation, which lead to the following:

  • poor proppant transport

  • insufficient fluid leakoff control

  • limited ability of the fluid to maintain fracture geometry

Encapsulation technology can provide slow release of oxidant, providing a delay in the breaking process. However, these methods offer only limited control above 175 F, especially above 200 F and when gel stabilizers are required.

Enzymes are the other major class of gel breakers. Typically, the application of enzymes is limited to lower temperatures (150 F or lower) and an optimized pH range (5 to 8).

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