Note: Descriptions are shown in the official language in which they were submitted.
CA 02579970 2007-02-28
APPLICATION FOR PATENT
TITLE: METHOD OF USING AND TRANSPORTING A NON-
EMULSIFIED ORGANIC PEROXIDE-CONTAINING
COMPOSITION
SPECIFICATION
Field of the Invention
The invention relates to a method of using and transporting a non-emulsified
well
treating composition.
Background of the Invention
In hydraulic fracturing, a fracturing fluid is injected into a wellbore under
high
pressure. Once the natural reservoir pressures are exceeded, the fracturing
fluid initiates
a fracture in the formation. The fracture usually continues to grow during
pumping.
Typically, treatment design requires the fracturing fluid to reach maximum
viscosity as it
enters the fracture since this affects fracture length and width. The
fracturing fluid may
include a proppant; the proppant being placed within the produced fracture.
The
proppant remains in the produced fracture to prevent the complete closure of
the fracture
and to form a conductive channel extending from the wellbore into the
formation.
Viscosity affects the fluid's ability to place proppant within the produced
fracture.
In addition, fluid viscosity influences fracture geometry and minimizes fluid
loss. The
fracturing fluid's viscosity may principally be attributed to the presence of
polymers, such
as polysaccharides, in the fracturing fluid. To further enhance the viscosity,
a
crosslinking agent is frequently added to the fracturing fluid to gel the
polymer.
The recovery of fracturing fluid from the formation is accomplished by
reducing
the viscosity of the fluid. Such reduction in viscosity should further be
instrumental in
retention of proppant within the fracture. Viscosity reduction may be
accomplished by
incorporating breakers into the initial fracturing fluid.
Many chemical agents have been reported in the literature for use as breakers.
Conventional breakers have included oxidizers, acids and enzymes. In light of
their
reactivity and oxidative capacity, peroxides have been used as breakers for
many years.
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For instance, U.S. Patent No. 5,447,199 reports the use of organic peroxides,
slightly
soluble in water (less than about 5% solubility), with water immiscible non-
oxidizable
organic solvents. Such peroxides render controlled viscosity reduction rate to
the
fracturing fluid after the fluid is pumped into the formation. The viscosity
is controlled
by the diffusion of the peroxide out of the oil phase and by the slow reaction
of the
peroxide and polymer. This allows for efficient recovery of the fracturing
fluid from the
formation.
In many instances, however, it is more desirable to use a water-soluble
peroxide
as the breaker composition. US Patent No. 3,922,173 discloses the use of t-
butyl
hydroperoxide in a gelled fluid as a breaker. However, no teaching is provided
on the
transport, storage and/or use of the product in cold weather environments.
In order for peroxides to be suitable at sub-freezing conditions, the breaker
composition must exhibit a low freeze point. Dilution of water soluble organic
peroxides
with only water as diluent is unsuitable since the resulting composition is
unacceptable
for use at sub-freezing conditions.
Further, the difficulty in using organic peroxides in breaker compositions is
compounded by stringent storage and transport regulations. Peroxides are
typically prone
to violent decomposition initiated by such external factors as mechanical
agitation,
friction and/or heat. Thus, handling and storage regulations as well as limits
on container
size have evolved in regards to the transport of such compounds. Such
regulations have
been implemented to reduce the risk of explosion which may, in turn, be caused
by
oxidation of peroxides in the presence of atmospheric oxygen.
A review of governmental transport and storage regulations appears in
"Recommendations on the Transport of Dangerous Goods" Vol. 1, 14th revised
edition,
United Nations, New York and Geneva, 2005. While the regulations state that
select
organic peroxides may be diluted with a "suitable solvent", defined as a "Type
A
solvent" having a boiling point greater than 150 C and which is compatible
with the
organic peroxide, no identification of a "suitable solvent" or a "Type A" is
made. Thus,
extensive testing is required in order to identify an acceptable solvent.
Further, the
regulations mandate a restricted container size and extensive explosion
testing must be
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undertaken in such containers to satisfy the stated criteria. Further, the
diluted product
must be stored in an isolated and controlled area.
It therefore is desired to develop peroxide-containing breaker compositions
having a freeze point which is acceptable for use in cold environments. It is
also desired
to develop peroxide-containing breaker compositions which may be stored and
transported to their destination of use and thus used on-the-fly, wherein such
breaker
compositions exhibits a low decomposition rate. Furthermore, it is desired to
develop
peroxide-containing breaker compositions which would render unnecessary the
need for
explosion testing of shipping containers containing the compositions and allow
the
transport of the breaker without restrictions on container size.
Summary of the Invention
The organic peroxide containing composition used herein is outside of current
governmental regulations relating to the transportation and storage of organic
peroxides.
The organic peroxide containing composition is a dilute composition of water
and at least
one water miscible solvent. The available oxygen content in the composition is
less than
1 weight percent. Reduced safety hazards make the dilution process a very
simple and
convenient method of using organic peroxides as breakers for water based
fracturing
fluids. [Even in its diluted state (less than 1 wt% oxygen), the rate of
addition of the
diluted product to the fracturing fluid is typically less than 2%.]
Since the organic peroxide containing composition is typically used in
locations
where ambient temperature conditions are less than 0 C, the diluent typically
contains a
suitable freeze point depressing solvent. The choice of freeze point
depressing solvent
may be made based on such factors as cost, effect on degradation rate of the
peroxide,
anticipated shelf life, degree of freeze point depression required and the
effect of the
solvent on the overall safety hazard of the diluted product. Typically, the
breaker
composition requires a freeze point which is at least less than or equal to -
10 C. In
preferred embodiments, the freeze point may be less than or equal to -20 C
and in many
cases less than or equal to -40 C. The breaker composition exhibits an
available oxygen
content of less than 1 weight percent and principally exists in a non-
emulsified state.
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The organic peroxide is substantially soluble in water. The diluent contains
water
and a water miscible solvent selected from ethylene glycol, methanol, acetone,
ethanol
and 1-propanol.
The amount of organic solvent in the breaker composition is between from about
0 to about 95 volume percent, preferably between from about 30 to about 70
volume
percent.
In a preferred embodiment, the organic peroxide is t-butyl hydroperoxide and
the
organic solvent is ethylene glycol.
The concentration of organic peroxide in the breaker composition is such that
the
amount of available oxygen in the breaker composition is less than or equal to
1 weight
percent. At such concentrations, explosion testing of shipping containers
holding the
compositions is unnecessary under regulations of certain jurisdictions.
The breaker compositions have a degradation of less than 20% over the expected
storage life and storage temperature of the product.
The breaker compositions are extremely useful in the degradation of gels in
subterranean formations. As such, the breaker compositions may be introduced
into the
formation with a fracturing fluid of an aqueous fluid and hydratable polymer
and then
pumped to a desired location within the wellbore under pressure sufficient to
fracture the
subterranean formation. The breaker is then capable of degrading the polymer
to render a
pumpable fluid.
Detailed Description of the Preferred Embodiments
The breaker compositions for use in the method defined herein contain an
organic
peroxide and a diluent. The peroxide in the composition is dilutes such that
less than 1
weight percent active oxygen is in the breaker composition.
The diluent in the breaker composition remains liquid and is stable at
subfreezing
conditions. As such, the breaker compositions have particular applicability
for use in
well treatment servicing when conditions are below freezing.
In particular, the use of the organic solvent provides excellent shelf-life
freeze-
proofing to the composition. For instance, the freeze point temperature of the
breaker
composition may be less than or equal to -10 C. In some cases, the freeze
point of the
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breaker composition is less than or equal to -20 C. In other cases, the
freeze point of the
breaker composition is less than or equal to -30 C. Still, in other cases,
the freeze point
of the breaker composition is less than or equal to -40 C. As such, the
breaker
composition exhibits excellent storage capabilities at very low temperatures.
The shelf-life of the breaker compositions defined herein is typically in
excess of
three months when stored or transported at a temperature of 25 C or lower.
The breaker composition has an active oxygen content equal of less than 1
weight
% and principally exists in a non-emulsified state.
The diluent contains water and/or an organic solvent acceptable for freeze-
proofing. The organic solvent must be compatible with the organic peroxide.
Compatibility may be measured by assaying the activity of the peroxide during
storage..
Preferably, the activity of the peroxide will not deteriorate by less than 20
% over a
storage period of three months. Preferred organic solvents are those selected
from the
group consisting of ethylene glycol, methanol, ethanol, 1-propanol and
acetone. Ethylene
glycol is most preferred since it has been illustrated to provide very high
stability to the
resulting composition. Further, low molecular weight alcohols, as well as
acetones, are
generally less preferred in light of domestic and/or international shipping
regulations.
The organic peroxide is substantially soluble in water so as to remain as a
single
homogeneous phase in the diluent. "Substantially soluble" is meant to mean up
to a
solubility of at least 10 weight %. Further, in order to conserve costs, the
peroxide should
be of relatively low molecular weight, typically less than or equal to
approximately 200,
in order to maximize available oxygen content.
Exemplary organic peroxides include t-butyl hydroperoxide, disuccinic acid
peroxide, dipropionyl peroxide, diacetone alcohol peroxides, di-(2-
methylbenzoyl)
peroxide, and 3-chloroperoxybenzoic acid. Of these, t-butyl hydroperoxide is
especially
preferred. Commercially available t-butyl hydroperoxides are typically about
70% active
in water and can be readily diluted in the organic solvents described herein.
Such
commercially available hydroperoxides include Trigonox A-W 70 of AKZO Nobel.
The amount of organic solvent in the breaker composition is between from about
0 to about 95 volume percent, preferably between from about 30 to about 70
volume
percent.
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In a preferred embodiment, the organic peroxide is t-butyl hydroperoxide and
the
organic solvent is ethylene glycol.
The maximum amount of organic peroxide employed in the breaker composition
is that required to provide an available oxygen content of 1 weight percent.
As such, the
breaker compositions of the invention are preferably non-emulsified
compositions At
such concentrations, the breaker composition may be transported in shipping
containers
without the need for conducting explosion tests on representative containers,
under the
regulations of some jurisdictions.
In addition to having low freeze points, the breaker compositions defined
herein
exhibit high flash points. Typically, the flash point of the breaker
compositions is in
excess of 37.8
Further, the breaker compositions display excellent compatibility with
elastomers
in conventionally employed pumping equipment, are readily miscible in water
and
demonstrate no adverse facts on crosslinked water gels.
In addition to exhibiting an acceptable freeze point, the breaker compositions
for
use herein do not exhibit separation into liquid phases nor does the
precipitation of
peroxide crystals occur at subfreezing conditions.
The breaker compositions are extremely useful in the degradation of gels in
subterranean formations. As such, the breaker composition may be introduced
into the
formation with a fracturing fluid of an aqueous fluid, hydratable polymer and
optional
crosslinking agent. The aqueous fluid could be, for example, water, brine,
aqueous based
foams as well as mixtures of water and alcohol. Alternatively, the fracturing
fluid
introduced into the formation may by prepared by combining hydratable polymer,
aqueous fluid, optional crosslinking agent with the organic peroxide and
diluent
described herein. Any suitable mixing apparatus may be used for this
procedure.
The fluid may then be pumped to a desired location within the wellbore under
pressure sufficient to fracture the surrounding subterranean formation. The
breaker, as
defined herein, degrades the polymer, whereby the fluid may be pumped from the
subterranean formation to the well surface.
The hydratable polymer may be any of the hydratable polysaccharides that are
conventionally employed in the well service industry. These polysaccharides
may be
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capable of gelling in the presence of a crosslinking agent to form a gelled
based fluid.
For instance, suitable hydratable polysaccharides are the galactomannan gums,
guars,
derivatized guars, cellulose and cellulose derivatives. Specific examples are
guar gum,
locust bean gum, fenugreek gum, caraya gum, xanthan gum, cellulose, and
derivatives of
these gums. The preferred gelling agents are guar gum, carboxymethyl guar,
hydroxypropyl guar, carboxymethyl hydroxypropyl guar, cellulose, carboxymethyl
cellulose, carboxymethyl hydroxyethyl cellulose and hydroxyethyl cellulose.
The most
preferred gelling agents are guar gum, hydroxypropyl guar, carboxymethyl
hydroxypropyl guar, hydroxyethyl cellulose and carboxymethyl hydroxyethyl
cellulose.
The polysaccharide of the fracturing fluid may further not be crosslinked. For
instance, polysaccharides not requiring a crosslinking agent, such as starch,
derivatized
starch, xanthans and xanthan gums, may be used.
The fracturing fluids of the invention often include the crosslinking agent.
The
crosslinking agent can be any of the conventionally used crosslinking agents
which are
known to those skilled in the art. For instance, in recent years, gellation of
the hydratable
polymer has been achieved by crosslinking these polymers with metal ions
including
aluminum, antimony, zirconium, for example, zirconium chelates such as
zirconium
acetate, zirconium lactate, zirconium lactate triethanolamine and titanium
containing
compounds including organotitinates, for example, the titanium chelates such
as
triethanolamine titanates, titanium acetylacetonate and titanium lactate. See,
for instance,
U.S. Pat. No. 4,514,309.
Borate crosslinkers are further typically used. Such crosslinking agents are
those
capable of supplying borate ions in solution. For instance, such crosslinkers
include
those which are a convenient source of borate ions, for instance the alkali
metal and the
alkaline earth metal borates and boric acid. One such crosslinking additive is
sodium
borate decahydrate, the crosslinking agent described in Pat. No. 5,160,643. In
a preferred
embodiment of the invention, the fracturing fluid contains guar and a borate
crosslinking
agent. In such guar gels, the crosslinking additive is preferably present in
the range from
about 0.024% to in excess of 0.18% by weight of the fracturing fluid.
Preferably, the
concentration of crosslinking agent is in the range from about 0.024% to about
0.09% by
weight of the fracturing fluid.
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Any proppant known in the art may be added to the fracturing fluid. If a
crosslinking agent is present in the fracturing fluid, the proppant is
typically added to the
fluid prior to the addition of the crosslinking agent. Suitable proppants
include quartz
sand grains, glass, ceramics, walnut shell fragments, aluminum pellets, nylon
pellets and
the like. The proppants are normally used in concentrations between about 1 to
18
pounds per gallon of fracturing fluid composition, but higher or lower
concentrations can
be used as required.
The fracturing fluid may also contain other conventional additives common to
the
well service industry such as surfactants and the like.
The following example illustrates the practice of the present invention in a
preferred embodiment. Other embodiments within the scope of the claims herein
will be
apparent to one skilled in the art from consideration of the specification and
practice of
the invention as disclosed herein. It is intended that the specification,
together with the
example, be considered exemplary only, with the scope and spirit of the
invention being
indicated by the claims which follow.
EXAMPLES
Exam lp e 1. A solution of 8.5 volume % t-butyl hydroperoxide (70% in water)
was
prepared in a blend of 49.0 volume percent water and 43.5 volume percent
ethylene
glycol. The solution exhibited a flash point of 69.4 C. and a freeze point of
-40 C.
After storage at room temperature for 4 months the t-butyl hydroperoxide
content was
analyzed by a gas chromatograph equipped with an mass selective detector and
the
concentration was found to have degraded less than 5% compared to a new
solution. This
demonstrates the suitability of the ethylene glycol as a diluent in this
invention.
From the foregoing, it will be observed that numerous variations and
modifications may be effected without departing from the true spirit and scope
of the
novel concepts of the invention.
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