An Innovative Approach to Gel Breakers for Hydraulic Fracturing
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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.
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.
The disadvantages of oxidative breakers led to identification of the need to improve breaker chemistry and delivery for use in gelled fracturing fluids. The current study proposes an alternative family of breakers that could offer several advantages in crosslinked-gel breaking, including enhanced retained conductivity in crosslinked-gel/proppant packs. These alternative breakers are proposed to take advantage of the mechanism of the reversible thinning shown in Fig. 1b.
Experimental Methods and Materials
A variety of fluid formulations was used to validate the effects of ultrahigh pressures on viscosity stability. An oilfield-grade guar-gelling agent was used, as were borax and solutions of sodium hydroxide, potassium chloride, and other salts. Polymeric breakers, activated alumina, and activated carbon were all provided as laboratory grade. Hydrocalumite and ettringite were synthesized in the laboratory.
Rheometers capable of applying temperatures greater than 300°F and pressures greater than 20,000 psi were used. High pressure was applied to the test fluid by use of a high-pressure hydraulic pump to pressurize immiscible oil. The rheometers were designed to ensure minimal fluid contact between the pressurizing oil and test fluid, to reduce mixing between the fluids and reduce contamination.
Results and Discussion
Measurements were made to validate the feasibility of a new family of breakers for borate-crosslinked gels that uses a break mechanism that deactivates the crosslinking species. This proposed mechanism contrasts with that of conventional oxidizers, which act by cleaving the polysaccharide backbone of guar and its derivatives. While previous studies have presented breakers that inactivated metal crosslinkers in fracturing gels, the current study takes advantage of the reversible pressure-induced thinning illustrated in Fig. 1b. The application of high pressure and temperature to borate-crosslinked polymers may alter the borate anion, inactivating the crosslink and allowing the new form of boron to undergo further reaction. The subsequent proposed breaking reaction has the released crosslinker reacting irreversibly with a chemical with a high affinity for the released crosslinker. This breaking would allow the gel to maintain the low viscosity and encourage improvement in retained conductivity of the proppant pack.
Candidate-Material Screening. The current work identified a number of chemicals with the established potential to bind to the pressure-released crosslinker. Those chemicals fall into three categories that have known affinity for boron: inorganic metal oxides (including clays), activated solids, and polymeric solids.
The initial phase of the current qualification gauged the feasibility of the candidate breakers to react with the pressure-thinned borate gel. Each test was conducted using a previously optimized solution of 25 lbm/1,000 gal guar with borax crosslinker in a base brine of 2% potassium chloride.
Metal Oxide and Clay Materials. For the current study, ettringite and hydrocalumite were selected for qualification as potential pressure-reactive breakers because of their documented high affinity for boron compared with other metal oxides. Results from feasibility screening showed very different results with these two clay materials blended into the 25 lbm/1,000 gal guar/borate gel. At the intermediate temperature, the fluid remains largely stable near 350–400 cp, even when exposed to the pressure ramp to 10,000 psi. However, at 170°F, the gel/hydrocalumite experiences pressure-dependent thinning during the ramp to 10,000 psi similar to that demonstrated in Fig. 1b. The most notable aspect of this behavior is the full reversibility of this thinning, even in the presence of the candidate breaker. This reversibility indicates that, while the gel was stable at intermediate conditions blended with hydrocalumite, the latter was ultimately unable to maintain the pressure-thinned gel in the broken state.
Activated Solids. The current study assessed the feasibility of activated carbon and activated alumina for use as potential pressure-reactive breakers. The performance of each material was qualified using the borate/guar fluid with gel loading of 25 lbm/1,000 gal. When tested under ultrahigh-pressure conditions at 100 and 170°F with activated carbon, the gel demonstrates high stability at the low temperature with no effect from the pressure ramp to 10,000 psi. However, when qualified at the elevated temperature, the fluid demonstrates reversible thinning to less than 50 cp at 10,000 psi, which recovers to greater than 200 cp when the pressure is reduced to 500 psi. This behavior negates the potential to use activated carbon as a pressure-reactive breaker for borate gels.
Polymeric Materials. Dissolved polymers in crosslinked-gel fracturing fluids are tightly bound to borate. Consequently, a new material designed as a pressure-reactive breaker will need a strong bonding affinity toward boron that is able to exceed the stability of polysaccharide/borate bonds. The current study qualified three polymeric materials for potential pressure-reactive breaking behavior.
The first material qualified in the current study was PB2. At low temperature, the gel/PB2 solution showed minimal response to the pressure ramp; but, at 170°F, the fluid/breaker reversibly thins below 50 cp.
Because of the reversibility of the response, a follow-up measurement was conducted with a higher loading of PB2 breaker. Several observations can be made from the data in this test. Most notably, the gel/PB2 system undergoes an immediate thinning on heating to 170°F that irreversibly breaks the gel viscosity before the pressure ramp. The results illustrate that PB2 may have some potential as a pressure-active breaker; however, excessive reactivity toward the gel at temperature indicates either that PB2 is generally too active at these intermediate temperatures or that the material was highly sensitive to added concentration.
In additional tests, PB3 was qualified under conditions similar to those of PB2. Similar to the results for PB2, the gel/PB3 combination remained stable at low temperature but demonstrated reversible pressure-induced thinning at 170°F.
To complete the feasibility assessment of the polymeric materials, PB1 was examined in the optimized borate/guar-fluid system under conditions slightly varied from those of the previous tests. PB1 demonstrated stable viscosity in the gel/PB1 mixture for 2 hours at each temperature.
Having demonstrated stable viscosity under standard low-pressure conditions, testing further characterized the performance of PB1 as a candidate breaker at ultrahigh pressures. While the gel experiences an increase in viscosity at 100°F, there is no apparent response to the applied pressure ramp at the low temperature. More remarkable, however, is the response to the pressure ramp when heated further. At 170°F, the fluid shows high stability near 200 cp at initial pressure. However, application of 2,500- and 5,000-psi pressure leads to sequential thinning of the gel viscosity. Upon reduction of applied pressure, the gel viscosity remains broken, the desired ultimate performance of a pressure-reactive breaker.
An Innovative Approach to Gel Breakers for Hydraulic Fracturing
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