Novel Oxidizing Breaker for High-Temperature Fracturing

Abstract

Summary 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 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 article 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.1,2 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 by which 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 to 175°F. However, these materials react too quickly at higher temperatures.3–5 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. Encapsulation is basically coating each particle of oxidizing breaker with paint. The coating or "capsule" of paint delays the release of breaker into the fracturing fluid. (Parker and Laramay provide details of encapsulation technology.6) 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).4,7,8 Recent developments show that these limits can be expanded,9 but their use above 200°F has been very controversial.8–10 The use of enzymes in the higher pH borate systems is ineffective without a means to lower fluid pH.9 Encapsulation can improve the stability of enzymes for use in slightly higher pH fluid and at slightly higher temperatures.11 Enzymes and persulfates have an important role as breakers in fracturing treatments above 200°F. As the wellbore and fracture cool during a treatment, these breakers can be added to fracturing fluids. Designing fluids by temperature cooldown during the treatment allows the engineer to remain aggressive in degrading the viscous fluids, thus providing a rapid, more effective cleanup. However, miscalculation in breaker loadings or temperature profiles can be disastrous. A new, soluble oxidizing breaker system has been developed for use throughout the entire treatment-design phase in wells from 200 to 325°F. The application of this new oxidizing breaker system overcomes limitations of current breaker technologies. The breaker provides the following benefits:stability in fluids above 325°F;compatibility with gel stabilizers;ability to remain active when in contact with formation rock;controlled viscosity reduction of crosslinked fracturing fluids. In this article we will present laboratory data and field case histories on the successful use of this novel, high-temperature viscosity-controlling breaker (HT-VCB) in a variety of fracturing fluids.

Experimental Evaluation of Oxidizing Breakers for a Polyacrylamide-Based Re-Crosslinkable Preformed Particle Gel

“…According to the targets, they contain hemicellulose, cellulase, amylase, and pectinase, respectively. , For instance, a guar-linkage-specific enzyme breaker was developed particularly for the guar polymer and was proven to be effective for both short- and long-term guar breaking . Enzyme breakers are reported to be sensitive to temperature and pH, though modifications have been done to expand their limitations. , …”

“…9 Enzyme breakers are reported to be sensitive to temperature and pH, though modifications have been done to expand their limitations. 8,11 Oxidizing breakers can chemically cleave the polymer backbone into shorter fragments and thus make it less viscous and easier to flush out. Hypochlorites, including sodium hypochlorite, lithium hypochlorite, and calcium hypochlorite, have been successfully used as gel breakers.…”

Seawater-Based Fracturing Fluid: A Review

“…Unlike enzymes, which are pH-dependent, oxidizers are temperature- and salt-dependent. Oxidizing breakers based on chlorine (CBO) allow for temperature regulation between 200 and 300 °F . However, for temperatures <200 °F, such high-temperature oxidizing breakers need to be enhanced by chemical activators (CAs) and chemical intensifiers (CIs).…”

“…Oxidizing breakers based on chlorine (CBO) allow for temperature regulation between 200 and 300 °F. 125 However, for temperatures <200 °F, such high-temperature oxidizing breakers need to be enhanced by chemical activators (CAs) and chemical intensifiers (CIs). While activated, CBO may dissolve gels in fresh water containing 2% KCl.…”

Modeling Gel Break-Time in Gravel Pack Fluids as a Function of Breaker and Activator Concentrations with Temperature

“…Mostly, breakers are oxidizing agents [6] needed for frac, gravel pack and fracpack operations. Improved fluids technology has also provided cross-linked gels that can maintain viscosity at elevated temperatures [9]. These thermally stable fluids improve the proppant carrying ability of the fluid, however, the recovery of these fluids would be reduced during flowback and requires a suitable breaker.…”