FRACTURING FLUID AND METHOD FOR FRACTURING SUBTERRANEAN FORMATIONS

FLUIDE ET METHODE DE FRACTURATION DE FORMATIONS SOUTERRAINES

English Abstract

A fluid for fracturing subterranean formations and method of use are disclosed. The fracturing fluid is comprised of a polymer (guar gum or a guar derivative) as a gelling agent, a pH adjusting reagent, a delayed borate cross- linking agent (sparingly soluble borate minerals), and a high pH buffer. The fracturing fluid, upon mixture, initially has a low pH for optimal rapid hydration of the polymer (and the borate remains idle), allows for low tubular friction pressure due to the low viscosity. However, the fracturing fluid allows for a slow, continuous pH shift from the low pH to a higher pH, where the borate ion exists and is available to cross-link and cause gelling of the polymer.

French Abstract

On décrit un liquide pour fracturer des formations souterraines et une méthode dutilisation. Le liquide de fracturation est constitué dun polymère (gomme de guar ou un dérivé de guar) comme agent gélifiant, un tampon, un agent de réticulation différé au borate (minéraux de borate peu solubles) et un tampon à pH élevé. Lors du mélange, le liquide de fracturation possède un faible pH initial pour une hydratation rapide du polymère (et le borate ne réagit pas), permettant une faible pression de friction tubulaire en raison de la faible viscosité. Toutefois, le liquide de fracturation permet un lent changement du pH dun faible pH à un pH plus élevé, où lion borate existe et est disponible pour réticuler et causer la gélification du polymère.

Claims

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What is claimed is:
1. A fracturing fluid for fracturing subterranean formations, the
fracturing fluid
comprising:
an aqueous solution;
a polymer selected from the group consisting of: a guar gum polymer and a
guar derivative polymer;
a pH adjusting reagent;
at least one delayed borate cross-linking agent;
a high pH buffer;
wherein, the polymer and the delayed borate cross-linking agent are adapted
to be injected simultaneously into the subterranean formation without pre-
hydration
of the polymer; and
whereby, the fracturing fluid has an initial low pH that provides for the
rapid
hydration of the polymer, providing a low viscosity fluid;
whereby, the fracturing fluid provides for a slow, continuous increase in pH,
wherein a highly shear stable cross-linked fluid is formed at a pH of about 8
or
higher.
2. The fracturing fluid according to claim 1, wherein the polymer is
present in
the fracturing fluid at a concentration of about 1.2 kg/m3 to about 4.8 kg/m3.
3. The fracturing fluid according to claim 1, wherein the polymer is
present in
the fracturing fluid at a concentration of about 1.8 kg/m3 to about 3.0 kg/m3.
4. The fracturing fluid according to any one of claims 1 to 3, wherein the
polymer is hydroxypropyl guar.

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5. The fracturing fluid according to any one of claims 1 to 4, wherein the
pH
adjusting reagent adjusts the initial pH of the aqueous solution to be within
the
range of about pH 4 to about pH 7.
6. The fracturing fluid according to any one of claims 1 to 5, wherein the
pH
adjusting reagent is a weak organic acid.
7. The fracturing fluid according to claim 6, wherein the pH adjusting
reagent is
acetic acid.
8. The fracturing fluid according to claims 6, wherein the pH adjusting
reagent
is fumaric acid.
9. The fracturing fluid according to claims 6, wherein the pH adjusting
reagent
is formic acid.
10. The fracturing fluid according to any one of claims 1 to 9, wherein the
at least
one borate cross-linking agent consists of a sparingly soluble borate mineral.
11. The fracturing fluid according to claim 10, wherein the at least one
borate
cross-linking agent is present in the fracturing fluid at a concentration of
about
0.0025 to about 0.4 percent by weight of water.
12. The fracturing fluid according to any one of claims 1 to 11, wherein
the high
pH buffer adjusts the pH of the aqueous solution to be within the range of
about pH
8 to about pH 11.
13. The fracturing fluid according to any one of claims 1 to 11, wherein
the high
pH buffer adjusts the pH of the aqueous solution to be within the range of
about pH
8.5 to about pH 9.5.
14. The fracturing fluid according to any one of claims 1 to 13, wherein
the
fracturing fluid further comprises a clay control additive.

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,
15. The fracturing fluid according to any one of claims 1 to 14, wherein
the
fracturing fluid further comprises a delayed viscosity breaker, whereby the
delayed
viscosity breaker allows for controlled viscosity reduction.
16. The fracturing fluid according to claim 15, wherein the delayed
viscosity
breaker is an encapsulated oxidizer.
17. The fracturing fluid according to claim 16, wherein the delayed
viscosity
breaker is ammonium persulfate.
18. The fracturing fluid according to claim 16, wherein the delayed
viscosity
breaker is potassium persulfate.
19. The fracturing fluid according to claim 15, wherein the delayed
viscosity
breaker is a combination of an encapsulated oxidizer and a delayed release
acid.
20. A method for fracturing subterranean formations, comprising the steps
of:
blending the following to form a fracturing fluid:
an aqueous solution;
a polymer selected from the group consisting of: a guar gum polymer
and a guar derivative polymer;
a pH adjusting reagent;
at least one delayed borate cross-linking agent; and
a high pH buffer;
during the blending step, the polymer and the delayed borate cross-linking
agent are simultaneously injected into the subterranean formation without pre-
hydration of the polymer; and
whereby, the fracturing fluid has an initial low pH that provides for the
rapid
hydration of the polymer, providing a low viscosity fluid; and

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whereby, the fracturing fluid provides for a slow, continuous increase in pH,
wherein a highly shear stable cross-linked fluid is formed at a pH of about 8
or
higher.
21. The method according to claim 20, wherein the polymer is present in the
fracturing fluid at a concentration of about 1.2 kg/m3 to about 4.8 kg/m3.
22. The method according to claim 20, wherein the polymer is present in the
fracturing fluid at a concentration of about 1.8 kg/m3 to about 3.0 kg/m3.
23. The method according to any one of claims 20 to 22, wherein the polymer
is
hydroxypropyl guar.
24. The method according to any one of claims 20 to 23, wherein the pH
adjusting reagent adjusts the pH of the aqueous solution to be within the
range of
about pH 4 to about pH 7.
25. The method according to any one of claims 20 to 24, wherein the pH
adjusting reagent is a weak organic acid.
26. The method according to claim 25, wherein the pH adjusting reagent is
acetic
acid.
27. The method according to claim 25, wherein the pH adjusting reagent is
fumaric acid.
28. The method according to claim 25, wherein the pH adjusting reagent is
formic acid.
29. The method of any one of claims 20 to 28, wherein the at least one
borate
cross-linking agent consists of a sparingly soluble borate mineral.
30. The method according to claim 29, wherein the at least one borate cross-
linking agent is present in the fracturing fluid at a concentration of about
0.0025 to
about 0.4 percent by weight of water.

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31. The method according to any one of claims 20 to 30, wherein the high pH
buffer adjusts the pH of the aqueous solution to be within the range of about
pH 8 to
about pH 11.
32. The method according to any one of claims 20 to 30, wherein the high pH
buffer adjusts the pH of the aqueous solution to be within the range of about
pH 8.5
to about pH 9.5.
33. The method according to any one of claims 20 to 32, wherein the
fracturing
fluid further comprises a clay control additive.
34. The method according to any one of claims 20 to 33, wherein the
fracturing
fluid further comprises a delayed viscosity breaker, whereby the delayed
viscosity
breaker allows for controlled viscosity reduction.
35. The method according to claim 34, wherein the delayed viscosity breaker
is
an encapsulated oxidizer.
36. The method according to claim 34, wherein the delayed viscosity breaker
is
ammonium persulfate.
37. The method according to claim 34, wherein the delayed viscosity breaker
is
potassium persulfate.
38. The method according to claim 34, wherein the delayed viscosity breaker
is a
combination of an encapsulated oxidizer and a delayed release acid.

Description

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Fracturing Fluid and Method for Fracturing Subterranean Formations
Field of the Invention
This invention relates to a fracturing fluid and method for fracturing
subterranean formations.
Background of the Invention
In the production of hydrocarbons from subterranean formations, it is
common practice to hydraulically fracture the formation to improve hydrocarbon
recovery. A fracturing fluid is introduced into the subterranean formation via
the
well-bore at a rate and pressure sufficient to produce fractures in the
formation and
to extend the fractures so formed from the well-bore into the formation.
Fluids
employed to hydraulically fracture a subterranean formation will desirably
have
relatively low initial viscosities and low friction pressures when pumped, but
high
viscosities in the formation due to a cross-linking reaction between a gelling
agent
and a cross-linking agent.
While the use of high viscosity fracturing fluids is desirable in the
fracturing
of a subterranean formation, problems are nevertheless encountered in the use
of
such high viscosity fracturing fluids due to the high friction losses
encountered
during the introduction of the fluid into the subterranean formation. Since
pumping
equipment and auxiliary equipment used in the delivery of the fracturing
fluids to
the subterranean formation have limited capacity and operating pressure, the
viscosity of the fracturing fluid which can be pumped is limited accordingly.
In an effort to overcome these problems, numerous compositions and systems
have been proposed in the art to delay the cross-linking of the gelling agent
so that
low viscosity of the fracturing fluid can be maintained during pumping thus
minimizing excessive friction losses and high well head pumping pressures,
while
at the same time permitting the desired cross-linking to occur in the
subterranean
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formations so that the desired high viscosity of the fracturing fluid can be
achieved
in the formation.
It is well known that boric acid is a very weak, inorganic acid and the borate
ion does not exist as such until the pH is sufficiently high to react with
more firmly
bound second and third hydrogens. The borate ion complexes with many
compounds, for example certain polysaccharides like guar gum. At a high pH,
above pH 8, the borate ion exists and is available to cross-link and cause
gelling. At
lower pH, the borate is tied up by hydrogen and is not available for cross-
linking.
The rate of cross-linking can be controlled by suitable adjustment of one or
more of the following variables: initial pH of the aqueous solutions system,
relative
concentration of one or more of the sparingly soluble borates, the temperature
of the
borates, temperature of the aqueous system and particle size of the borate.
Summary of the Invention
In one aspect, the invention relates to a fracturing fluid for fracturing a
subterranean formation comprising a polymer (guar gum or a guar derivative) as
a
gelling agent, a pH adjusting reagent, a delayed borate cross-linking agent
(sparingly soluble borate minerals), and a high pH buffer. The fracturing
fluid,
upon blending, initially has a low pH for optimal rapid hydration of the
polymer
(with the borate remaining idle), allows for low tubular friction pressure due
to the
low viscosity. However, the fracturing fluid allows for a slow, continuous pH
shift
from the low pH to a higher pH, where the borate ion exists and is available
to cross-
link and cause gelling of the polymer.
The invention also relates to a method for fracturing subterranean formations
utilizing a fracturing fluid composition, as described above, that undergoes a
slow,
continuous pH transition. This provides for the ability to simultaneously
inject the
polymer (guar gum) and the cross-linking agent (sparingly soluble borate
minerals)
into the well-bore, without pre-hydration of the polymer.
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Detailed Description of the Preferred Embodiments
In one embodiment, a fracturing fluid according to the invention is comprised
of an aqueous solution, guar gum or a guar derivative as the polymer gelling
agent, a
pH adjusting reagent, a delayed borate cross-linking agent, a high pH buffer,
a clay
control additive, and a delayed viscosity breaker for controlled viscosity
reduction of
the fluid once placed in the formation.
The guar derivative in the fracturing fluid may be compounds such as
hydroxypropyl guar. The guar gum or guar derivative in the fracturing fluid is
high
purity, high yielding, and fast hydrating. It is used at concentrations of
about 1.2
kg/m3 to about 4.8 kg/m3, preferably at a relatively low concentrations,
ranging from
1.8 kg/m3 to 3.0 kg/m3, to minimize formation damage. The guar gum or guar
derivative may be mixed into an oil-based slurry to allow for accurate and
continuous injection during fracturing.
The pH adjusting reagent in the fracturing fluid may be reagents such as
acetic
acid, fumaric acid, formic acid, or other suitable weak organic acid. The pH
adjusting
reagent, is added to ensure the pH of the aqueous solution is in the optimum
range
for polymer hydration, namely, between about pH 4 and about pH 7 for the guar
gum or guar derivatives.
The borate cross-linking agent, or agents, is comprised of a sparingly soluble
borate mineral, or minerals. These minerals are added at concentrations of
about
0.025 to about 0.4 percent by weight of water. The slow solubility of these
borate
reagents, in conjunction with the high pH buffer, serves to delay cross-
linking. This
mechanism provides more effective and accurate use of the known cross-link
time
controls, such as: initial water temperature, initial water pH, and
borate/buffer
concentration.
Upon addition of the high pH buffer (i.e., a buffer that has the tendency
towards shifting the....pti_of the solution to a basic, pH 8
or higher) and the
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borate mineral to the aqueous solution, the aqueous solution undergoes a pH
shift
from low pH, for optimum polymer hydration (i.e. pH of about 4 to 7), to high
pH, of
about pH 8 to about pH 11, whereupon, the boron is able to cross-link the
polymer.
The preferred final pH is about 8.5 to about 9.5, which maximizes the shear
stability
resulting in the final gel.
The slow, continuous pH shift instils a unique advantage to this fluid system.
At the initial, low pH, rapid hydration of the polymer occurs, while at the
same time,
the boron remains idle. This initial stage provides low tubular friction
pressure due
to the low viscosity. As the pH increases from the initial range of about 4 to
7,
increasing to a pH of about 8.2 to 8.8, a highly shear stable cross-linked
fluid is
formed. This occurs ideally within the time it takes the fluid to travel from
the
surface to the bottom of the well-bore.
After entering the subterranean zone, the buffer continues to raise the pH to
about 9.0 to 10, resulting in a gel thermally stable to temperatures in excess
of 1200C
and also increasing the proppant suspension capability of the fluid.
Guar based polymers are normally pre-hydrated in the mix solution using an
hydration unit/retention tank before the cross-linking agent is added.
However, in
the slow pH transition of the above mentioned fracturing fluid allows
hydration of
the polymer to occur even after the cross-linking agent has been added. As a
result,
the polymer (guar gum or derivative) and borate cross-linking agent can be
injected
"on-the-fly" simultaneously without pre-hydration of the polymer.
In addition, the slow and controllable shift from low pH to high pH allows
high concentrations of the borate cross-linking minerals and high pH buffers
to be
added without risking over cross-linking the polymer.
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The fracturing fluid is slowly and controllably degraded using an
encapsulated oxidizer such as ammonium or potassium persulfate, or a
combination
of an encapsulated oxidizer and a delayed release acid.
In another aspect, this invention relates to a method for fracturing a
subterranean formation utilizing the disclosed fracturing fluid. Furthermore,
this
invention allows for a method of fracturing a subterranean formation utilizing
the
disclosed fracturing fluid by simultaneously injecting, "on-the-fly", the
polymer
(guar gum or derivative) and borate cross-linking agent, without requiring
pre-hydration of the polymer.
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