286915 Novel Enzymes Boost Performance in High pH, High Temperature Hydraulic Fracturing
Hydraulic fracturing is used to create subterranean fractures that extend from the borehole into rock formations in order to increase the rate at which fluids can be produced by the formation. Generally, a high-viscosity fracturing fluid is pumped into the well at sufficient pressure to fracture the formation. In order to maintain the increased formation exposure, a solid proppant is added to the fracturing fluid which is carried into the fracture by the high pressure applied to the fluid. Once the proppant is in place, chemical breakers are used to reduce the fluid's viscosity. This allows the proppant to settle into the fracture and thereby increase the formation's exposure to the well. Breakers work by reducing the molecular weight of the polymers, thus “breaking” the polymer and allowing the fracture to become a high permeability conduit for the production of fluids and gas to the well-bore.
More than 65% of conventional fracturing fluids are made of guar gum (galactomannans) or guar gum derivatives such as hydroxypropyl guar (HPG), carboxymethyl guar (CMG) or carboxymethylhydroxypropyl guar (CMHPG). These polymers can be crosslinked together to increase their viscosities and increase their proppant transport capabilities.
Chemical oxidizers and enzymes are most commonly used as breakers. The oxidizer produces a radical which then degrades the polymer. This reaction is limited by the fact that oxidizers are stoichiometric—they will attack not only the polymer but any molecule that is prone to oxidation. Enzymes, on the other hand, are catalytic and substrate specific and will catalyze the hydrolysis of specific bonds on the polymer. An enzyme will degrade many polymer bonds in the course of its useful lifetime. Unfortunately, enzymes typically operate under narrow pH and temperature ranges.
Enzymes isolated from extremophilic sources, however, have shown great promise as breakers and can be used under most fracturing conditions. Additionally, extremophilic enzymes are generally more robust than their mesophilic counterparts. They are much more able to withstand the various conditions of subterranean formations and the surfactant-based additives often used in fracturing fluid formulations.
This work details the use of several novel enzymes used in hydraulic fracturing. Since the genetic sequences for these enzymes are from thermo-tolerant and extremeophilic sources, the translated enzymes are more robust than conventional products. They are capable of degrading hydraulic fracturing biopolymers at elevated pH ranges (7-12), elevated temperatures (≥ 80°C), and do not generally denature in the presence of common fracturing fluid additives.
Enzyme production: Gene coding for the enzymes discussed here were isolated from thermo-tolerant and/or extremophilic sources and codon-optomized for expression in E. coli1-4. E. coli was transformed with a plasmid containing the gene for the enzyme breaker and incubated in LB-Miller for 18 hours at 100°F and 200 rpm agitation. Overexpression of enzyme was confirmed by SDS-PAGE and a cell-free lysate used as the enzyme solution in all tests. Rheology was used to determine enzyme activity and functionality against field-specific hydraulic fracturing fluid formulations.
Rheology: The reduction in viscosity of crosslinked guar polymer by the enzyme was measured across a range of temperatures (40 to 150°F using 17, 25 and 30 ppt crosslinked guar at pH 10.5. Rheology measurements were carried out in a Chandler model 5550 rheometer at a shear rate of 100 sec-1 and a pressure of 500 psi.
Effect of Allosteric Effectors: The effects of different anions and cations on the enzyme were determined by rheology. For the purpose of this paper, the mannanohydrolase enzyme was tested with and without a small concentration of allosteric effector at 150°F and pH 9.5. The effector did not change the pH of the fluid and did not, by itself, break the viscosity of the crosslinked fluid. Results are shown in Figure 3.
Assay of Oxidase-Type Enzyme: A solution of 6.25 ug/mL of oxidase-type enzyme was prepared in 100 mL of 25 ppt crosslinked guar fracturing fluid. Samples were prepared with either no glucose or with 0.3 mM glucose. Fluid viscosity was measured at 140°F on a Chandler rheometer. Results are displayed in Figure 4.
Results and Discussion
High-Temperature, High-pH Enzyme Breaker: A mannanohydrolase enzyme from a thermophilic source was tested for its ability to break the viscosity of a crosslinked guar polymer at 150°F and pH 9.5 (Figure 1). The enzyme functioned remarkably well under these conditions. Further testing was performed with this enzyme under temperatures ranging from 75°F to 250°F and pH ranges from 9.5-10.5 (data not shown). Under all conditions, the enzyme performed as expected with lower activity at 75°F and approaching its maximum at 150°F. The activity of the enzyme would begin to decline as the temperature approached 250°F. However, lowering the enzyme dilution to 1:50 or increasing the loading in the fluid formulation would provide the necessary activity for breaking the viscosity of the guar polymer. In contrast to previous enzyme breaker packages, this new enzyme does not need an associated pH modifier nor does it need the assistance of an oxidative breaker to perform at its peak limits.
Figure 1. Activity of the enzyme breaker at 150°F and pH 9.5. The enzyme effectively breaks the viscosity of the crosslinked 17 ppt guar polymer. The figure shows the activity of two dilutions of enzyme stock, 1:75 and 1:100.
High-pH, Low-Temperature Enzyme Breaker: A β-mannanase enzyme from an alkaliphilic source was examined for its ability to break the viscosity of guar at elevated pH ranges that are often found in fracturing conditions (Figure 2). The enzyme functioned amazingly well at pH 11 and 12. At pH 13, the activity of the enzyme was noticeably lessened. However, after 18 hours, the viscosity of the fluid had been greatly reduced signifying that the enzyme retained at least some activity at this point. The upper temperature limit for this enzyme was found to be approximately 150°F (data not shown). Increasing the enzyme concentration in the fluid formulation would extend the activity past this temperature at the cost of severely reducing the initial viscosity of the formulation, possibly resulting in a screen-out. However, the enzyme functions well at lower temperatures and elevated pH ranges.
Figure 2. β-mannanase activity at pH 11, 12 and 13. The enzyme was incubated in an 18 ppt crosslinked guar fracturing fluid for 1 hour (dashed lines) or 18 hours (solid lines). The enzyme still retained activity at pH 13. The pH of the control sample was 11. Samples were incubated at room temperature.
Allosteric Effectors Improve Catalytic Rates: It has been found that certain anions and cations can improve the reaction rates and/or stabilities of the β-mannanases and mannanohydrolases used in this study5. As seen in Figure 3, a small amount of allosteric effector greatly improves the catalytic activity of the enzyme. The identity of the effector will depend on which enzyme is used, with different effectors influencing the enzyme in different ways.
Figure 3. Ability of an allosteric effector to improve the catalytic ability of the enzyme against hydraulic fracturing fluid. The enzyme and effector was tested against 17 pound crosslinked guar fluid at pH 9.5 and 150 °F.
Universal Enzyme Breakers: An oxidase-type enzyme was tested for its ability to degrade crosslinked guar. On its own, the enzyme had no activity against the guar polymer. However, when the reaction was “seeded” with a monosaccharide (glucose, mannose or galactose) the reaction was initiated and the enzyme effectively “broke” the viscosity of the crosslinked polymer (Figure 4). The enzyme's ability to reduce the fracturing fluid's viscosity occurs by two pathways. The first is production of a carboxylic acid which lowers the fluid pH. This results in a reduction of the efficacy of the crosslinking reaction. Second, the enzyme produces the oxidizer, H2O2, which is extremely effective in degrading the polymer's molecular weight. Because the oxidizer is produced in situ, the hazard to operators and the environment at the surface of the well is greatly reduced, if not eliminated.
Once the enzyme reaction is seeded with a monosaccharide, the guar is broken down by the oxidizer, which releases additional monosaccharides enabling the process to continue without further seeding by the operator.
Figure 4. Rheology profile of the oxidase-type enzyme. When the enzyme is “seeded” with a monosaccharide, the viscosity of the polymer is broken.
In summary, enzymes are effective in hydraulic fracturing applications. The wide variety of enzymes available and the ability to modify their activity with different effectors makes it possible to customize a treatment fluid for a particular situation. They are also a more environmentally friendly alternative to traditional oxidative breaker chemicals and provide a safer working environment for the field operator.
The authors would like to thank Baker Hughes management for the opportunity to write and present this research.
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