Borate-crosslinked Fluids Research Articles

An Innovative Approach to Gel Breakers for Hydraulic Fracturing

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 178991, “An Innovative Approach to Gel Breakers for Hydraulic Fracturing,” by Michael J. Fuller, SPE, Chevron Energy Technology Company, prepared for the 2016 SPE International Conference and Exhibition on Formation Damage Control, Lafayette, Louisiana, USA, 24–26 February. The paper has not been peer reviewed. Recent studies have found that the viscosities of borate gels at actual downhole pressure conditions may be 80% less than those from standard high-pressure/high-temperature rheometer measurement (which uses 400-psi top pressure). A proposed mechanism for this phenomenon is a pressure-induced shifting of the crosslink/temperature stability near the “melt temperature” of borate-crosslinked gels, leading to a reversible thinning. This paper poses a novel approach to exploit the pressure effect by capturing the pressure-thinned fluid in a thin state and irreversibly breaking the gel viscosity. Introduction The chemistry of most crosslinked-gel fracturing fluids has been optimized carefully over the years, yielding a base chemistry that has remained mostly consistent for 20 years with incremental innovations. These fluids include either metal-crosslinked polymer solutions or borate-crosslinked guar, which is used for the largest percentage of crosslinked-gel fracturing treatments. While widely used in both crosslinked and hybrid fracture designs, several performance issues have been noted in optimization and execution of these fluids. Slow shear recovery has been identified in many nondelayed borate-crosslinked fluids, which varies on the basis of shear rate, gel salinity, and additives. In addition to unpredictable shear recovery, researchers have identified surprising effects of downhole pressures on the viscosity of borate-crosslinked-gel fluids. While the pressure for testing of crosslinked fluids is 400 psi, according to industry recommended practices, realistic downhole pressures often exceed 5,000 psi during fracturing operations. Borate-crosslinked fluids exhibit a significant reduction in viscosity under realistic downhole pressures when qualified in specialized ultrahigh-pressure rheometers. Functionally, the findings of those studies suggest that the actual downhole viscosity may be more than 80% lower than the optimized values measured with standard methods for low-pressure gel testing. This pressure-induced thinning is demonstrated in Fig 1. The nature of this pressure effect is still under evaluation. The thinning phenomenon demonstrated in Fig. 1b could be mistakenly labeled as breaking of the gel; however, that label would be inaccurate because chemical breakers of any nature cause an irreversible change to the chemistry of the crosslinked polymer, rendering it unable to regain viscosity. Breakers for crosslinked gels fall into two categories: enzyme breakers, comprising a number of enzyme mixtures designed to hydrolyze polysaccharide polymers by cleavage of the polymer backbone, and oxidative breakers. Oxidative breakers are the most commonly used type of chemical breakers for crosslinked gels because of their versatility and their effectiveness in breaking at low concentrations. The effectiveness of low concentrations of oxidizers is attributable to their high reactivity, but this same reactivity presents a number of hazards.

Next-Generation Boron-Crosslinked Fracturing Fluids: Breaking the Lower Limits on Polymer Loadings

Summary Hydraulic fracturing is a robust stimulation technique that has been used for more than 60 years to help increase the recovery rate of hydrocarbons from reservoirs. Hydraulic-fracturing fluids are key components of the process. Significant efforts have been made to refine the fluids and advance new technology to help improve the economics, efficiency, and safety of the systems. Specifically, the pursuit of fluids that provide reliable and consistent performance while using lower concentrations of polymer, usually guar or a guar derivative, has been a recurrent point of emphasis in fracturing-fluid advancement. There are many advantages of using lower polymer concentrations, including lower costs, improved logistics, and introducing less polymer with its associated residues into the fracture, among others. This paper presents a new fracturing fluid that combines a next-generation boron crosslinker with a new hydroxypropyl guar (HPG) to be crosslinked with 40 to 60% less polymer than used in conventional borate-crosslinked fluids. For this fluid, HPG loadings in the range of 8 to 12 lbm/1,000 gal were used to produce boron-crosslinked stimulation fluids stable up to 200°F. Fluid-viscosity testing showed stable fracturing fluids with controlled breaking profiles at temperatures up to 180°F. Dynamic proppant-suspension testing indicated that the new fracturing fluid exhibited proppant transport equal to or better than conventional borate fluids. Regained-permeability testing with Berea sandstone cores exceeded 75% at 160°F, 85% at 180°F, and 95% at 200°F. In addition, excellent fluid cleanup was measured by retained proppant-pack conductivity with 2 lbm/ft2 of 20/40-mesh lightweight ceramic proppant. This new boron-crosslinked fluid retains the “rehealable” property and flexibility of conventional borate-crosslinked fluids; however, the polymer is crosslinked at or near the minimum concentration at which the polymer chains can entangle (and are capable of crosslinking), which is an improvement compared with conventional borate fluids. This concentration is known as the critical overlap concentration, c*, of the polymer. The use of the new crosslinking technology coupled with the new HPG allows for a two-fold advantage in terms of residue reduction. The derivatized polymer requires additional processing, yielding a cleaner polymer with less residue, and the lower polymer dosage results in a further reduction of residue compared with conventional fluids.

Applications of a New, Efficient Hydraulic Fracturing Fluid System

Summary This paper discusses low-temperature applications for an optimized low-gel borate fluid system that does not rely on a separate component to maintain system pH. This fluid system's viscosity, fluid loss, static proppant settling, and dynamic proppant transport were tested; experimental procedures and results are described. Case studies from Argentina, Ecuador, the Texas Panhandle, and Permian Basin are also included. A new, borate-crosslinked hydraulic fracturing fluid system has been developed. This new, optimized fluid system provides higher viscosity with lower gelling-agent concentrations compared to conventional, borate-crosslinked fluids. Application of the optimized low-gel borate (OLGB) fluid system at low temperatures is discussed. Viscosity, proppant transport, and fluid-loss data of the OLGB fluid system are compared to conventional, borate- crosslinked fracturing fluids. Treatment designs are also presented.

Novel Oxidizing Breaker for High-Temperature Fracturing

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.

New pH-Buffered Low Polymer Borate Crosslinked Fluids for Hydraulic Fracturing

Summary The utilization of low concentrations (15 to 20 lbm/1000 gal) of borate-crosslinked guar has not been successful in the past because of the inability to effectively control the pH and B(OH)−4 concentration of the fluid. Today's new pH buffer technology allows the use of low concentrations of guar at temperatures up to 210°F. These new pH-buffered borate-crosslinked guar fluids can be effectively utilized at temperatures up to 240°F by increasing the polymer loading. Due to the lower achievable polymer loading and relatively low pH, these new pH-buffered borate fluids possess excellent cleanup and retained proppant conductivity. In field documented cases, well cleanup times have been reduced by more than half. Laboratory conductivity tests show that these pH-buffered fluids, utilizing reduced polymer loading, produce retained conductivity values of 10 to 25% (absolute) higher than alternative conventional borate-crosslinked fluids incorporating higher polymer loading.

Chemistry and Rheology of Borate-Crosslinked Fluids at Temperatures to 300F

Summary This paper explains the variables involved in borate chemistry as they affect crosslink formation, stability, and breaking properties. The borate-ion concentration in solution is a function of pH and temperature. A method for calculating borate-ion equilibria is used to help establish the required final pH value for application of a borate-crosslinked gel at a specific temperature. Introduction Many of the properties of borate-crosslinked fluids can be explained in terms of the shift in borate-ion equilibria with pH and temperature. Syneresis, the process of forming a dense polymer mass and excluding liquid from the structure, is easily understood as a product of increased borate-ion concentration when a fluid is cooled. Proppant-settling behavior that is pH-dependent also can be explained. As the equilibrium concentration of borate ion increases with increasing pH, the number of network crosslinks increases and hence the proppant-settling rate decreases. Rheological measurements from flow-loop and Fann Model 50 viscometers are used to help identify the chemical variables that control such fluid properties as crosslinking time, viscosity development, and the rehearing character of borate fluids. A new theological procedure using steady-state shear rate is presented for obtaining viscosity data for fluids used in fracture design programs. Background Borate-crosslinked fluids have been gaining popularity in the fracturing industry. Even though such fluids have been used for more than 20 years, new developments in borate-fluid technology are occurring rapidly, particularly with respect to high-temperature capability. For years, borate-crosslinked fluids were considered inexpensive, high-viscosity fluids limited to low temperature. More recently, wells up to 300F have been treated with borates. To understand the extension to high temperature, we should reconsider the chemistry of borate-crosslinked fluids. Borate Crosslinking Borate-crosslinked fluids used for fracturing consist of polysaccharide polymers crosslinked with borate salts. These gels are difficult to investigate with analytical instruments, so model diol chemical systems and model sugars have been used to study the formation of borate esters and crosslinking reactions. Borate crosslinks are viewed as rapid exchange equilibria of monoborate ion on the cis-diols of galactomannan polymers (guar).