(216d) Novel Enzymes Boost Performance in High pH, High Temperature Hydraulic Fracturing

Introduction

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.

Experimental

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. coli^1-4.
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.

Enzyme Testing:

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.

Acknowledgments

The authors would
like to thank Baker Hughes management for the opportunity to write and present
this research.

References

Ma, Y., et al.: Extremophiles, 2004, 8, 447-454.
Armstrong, C.D., Patent No. US 8,096,360 B2, 2012.
Armstrong, C.D., Patent No. US 8,058,212.
Armstrong, C.D. Patent Application US 2012/0031618.
Armstrong, C.D. et al. Patent Application P210-1840-US.