U.S. patent application number 11/048489 was filed with the patent office on 2005-06-16 for compositions and methods for improving fracture conductivity in a subterranean well.
This patent application is currently assigned to Halliburton Energy Services, Inc.. Invention is credited to Mang, Michael N., Todd, Bradley L..
Application Number | 20050130848 11/048489 |
Document ID | / |
Family ID | 46205463 |
Filed Date | 2005-06-16 |
United States Patent
Application |
20050130848 |
Kind Code |
A1 |
Todd, Bradley L. ; et
al. |
June 16, 2005 |
Compositions and methods for improving fracture conductivity in a
subterranean well
Abstract
The invention relates to enhancing fluid flow from subterranean
formations, and more particularly, to enhancing the conductivity of
fractures in a subterranean formation so as to enhance fluid flow
therethrough. In one embodiment, the present invention provides a
proppant matrix composition comprising at least a plurality of
proppant particulates and at least a plurality of composite
particles, the composite particles comprising a degradable material
and a filler material.
Inventors: |
Todd, Bradley L.; (Duncan,
OK) ; Mang, Michael N.; (Eden Prairie, MN) |
Correspondence
Address: |
Robert A. Kent
2600 S. 2nd Street
Duncan
OK
73536-0440
US
|
Assignee: |
Halliburton Energy Services,
Inc.
|
Family ID: |
46205463 |
Appl. No.: |
11/048489 |
Filed: |
February 1, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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11048489 |
Feb 1, 2005 |
|
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10608291 |
Jun 27, 2003 |
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Current U.S.
Class: |
507/200 |
Current CPC
Class: |
E21B 43/267 20130101;
C09K 8/80 20130101; C09K 8/805 20130101 |
Class at
Publication: |
507/200 |
International
Class: |
E21B 043/00 |
Claims
What is claimed is:
1. A proppant matrix composition comprising at least a plurality of
proppant particulates and at least a plurality of composite
particles, the composite particles comprising a degradable material
and a filler material.
2. The composition of claim 1 wherein at least a portion of the
proppant particulates comprises at least one of the following:
interlocking proppant particulates, or proppant particulates coated
with a curable resin and/or a tackifying agent.
3. The composition of claim 1 wherein at least a portion of the
proppant particulates comprise at least one of the following:
natural sand, a ground nut hull, a man-made proppant particulate,
bauxite, a ceramic, a polymeric particulate, or a low density
particulate.
4. The composition of claim 1 wherein at least one of the composite
particles has a physical shape of a platelet, shaving, fiber,
flake, ribbon, rod, strip, spheroid, toroid, pellet, or tablet.
5. The composition of claim 1 wherein the proppant matrix
composition comprises at least one of the following: an acid
reactive material or an inert filler.
6. The composition of claim 1 wherein the degradable material is
capable of undergoing an irreversible degradation downhole.
7. The composition of claim 1 wherein the composite particles are
present in an amount of from about 0.1% to about 30% based on the
weight of the proppant particulates in the proppant matrix
composition.
8. The composition of claim 1 wherein the degradable material
comprises a degradable polymer.
9. The composition of claim 1 wherein at least one of the composite
particles comprises at least one of the following: a
polysaccharide; a chitin; a chitosan; a protein; an aliphatic
polyester; a poly(lactide); a poly(glycolide); a
poly(.epsilon.-caprolactone); a poly(hydroxy ester ether); a
poly(hydroxybutyrate); a polyanhydride; a polycarbonate; a
poly(orthoester); a poly(acetal); a poly(acrylate); a
poly(alkylacrylate); a poly(amino acid); a poly(ethylene oxide); a
poly ether ester; a polyester amide; a polyamide; a
polyphosphazene; or a copolymer or blend thereof.
10. The composition of claim 1 wherein the filler comprises at
least one of the following: an anhydrous salt, glass, talc, calcium
carbonate, mica, magnesium oxide, miheral filler, barite, silica,
ethylenediaminetetraacetic acid, an oxidizer, a breaker, sodium
persulfate, magnesium peroxide, an inorganic compound, an organic
compound, or a derivative thereof.
11. The composition of claim 1 wherein the filler comprises from
about 0.5% to about 60% of one of the composite particles.
12. The composition of claim 1 wherein at least one of the
composite particles comprises at least one of the following: a
processing aid, a lubricant, an antistat, an antiblock agent, a
pigment, or a derivative thereof.
13. The composition of claim 1 wherein at least one of the
composite particles is formed by a melt extrusion process.
14. A treatment fluid for use in a subterranean application
comprising a proppant matrix composition, the proppant matrix
composition comprising at least a plurality of proppant
particulates and at least a plurality of composite particles, the
composite particles comprising a degradable material and a filler
material.
15. The treatment fluid of claim 13 wherein the treatment fluid is
formed at a drill site.
16. The treatment fluid of claim 13 wherein the degradable material
comprises at least one of the following: a polysaccharide, a
chitin, a chitosan, a protein, an aliphatic polyester, a
poly(lactide), a poly(glycolide), a poly(.epsilon.-caprolactone), a
poly(hydroxybutyrate), a polyanhydride, an aliphatic polycarbonate,
an aromatic polycarbonate, a poly(orthoester), a poly(acetal), a
poly(acrylate), a poly(alkylacrylate), a poly(amino acid), a
poly(ethylene oxide), or a polyphosphazene.
17. The treatment fluid of claim 13 wherein the degradable material
produces an acid upon degradation.
18. A proppant matrix having at least one void therein, the void
resulting from the degradation of a degradable material in a
composite particle.
19. The proppant matrix of claim 18 wherein the degradable material
did not completely degrade until the proppant matrix developed some
compressive strength.
20. The proppant matrix of claim 18 wherein the degradable material
comprises at least one of the following: a polysaccharide, a
chitin, a chitosan, a protein, an aliphatic polyester, a
poly(lactide), a poly(glycolide), a poly(.epsilon.-caprolactone), a
poly(hydroxybutyrate), a polyanhydride, an aliphatic polycarbonate,
an aromatic polycarbonate, a poly(orthoester), a poly(acetal), a
poly(acrylate), a poly(alkylacrylate), a poly(amino acid), a
poly(ethylene oxide), or a polyphosphazene.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This Application is a continuation-in-part of U.S.
application Ser. No. 10/608,291, entitled "Compositions and Methods
for Improving Proppant Pack Permeability in a Subterranean Well,"
filed on Jun. 27, 2003.
BACKGROUND
[0002] The present invention relates to enhancing fluid flow from
subterranean formations, and more particularly, to enhancing the
conductivity of fractures in a subterranean formation so as to
enhance fluid flow therethrough.
[0003] Hydraulic fracturing is a technique for stimulating the
production of desirable fluids from a subterranean formation. The
technique normally involves introducing a viscous liquid through a
well bore into a formation at achosen rate and pressure to enhance
and/or create a fracture in a portion of the formation, and placing
proppant particulates in the resultant fracture to, inter alia,
maintain the fracture in a propped condition when the pressure is
released. The resultant propped fracture provides a conductive
channel in the formation for fluids to flow to the well bore.
[0004] The degree of stimulation afforded by the hydraulic
fracturing treatment is largely dependent on the conductivity and
width of the propped fracture. Thus, the productivity of the well
in effect becomes a function of fracture conductivity, which is
commonly defined as proppant permeability times fracture width. To
enhance well productivity, it may be beneficial to enhance fracture
conductivity.
[0005] Oftentimes, to effectively prop open the fractures as well
as prevent proppant particulate flowback, the proppant particulates
are caused or allowed to consolidate into proppant matrixes within
the fractures. One conventional means of doing this is to use
resin-coated proppant particulates so that when the resin cures
downhole, the proppant particulates can consolidate to form a
relatively stable proppant matrix within the fracture. Other
methods also have been used to facilitate the consolidation of the
proppant particulates within the fractures.
[0006] Although consolidating the proppant particulates within a
fracture may have some benefits, for example preventing proppant
flowback, such methods may adversely affect the conductivity of the
fracture. That is, some methods of consolidating proppant
particulates themselves may introduce a barrier to the free flow of
desirable fluids from the subterranean formation to the well bore
for subsequent production. Fracture conductivity may suffer as a
result. This is undesirable as this may affect overall well
productivity.
[0007] To counteract this potential problem, many different
techniques have been developed. One technique involves adding
calcium carbonate or salt to the proppant matrix composition. When
the proppant particulates consolidate, after a subsequent fluid is
added to the well bore, the calcium carbonate or salt is dissolved
out of the matrix. At least one problem associated with this method
is the incomplete removal of the calcium carbonate or salt if not
adequately contacted with the subsequent fluid. Another method has
been to add wax beads to the proppant matrix composition. Once
incorporated into the consolidated proppant particulates, the wax
beads melt as a result of the temperature of the formation. A
problem with this method is that the wax may re-solidify in the
well, causing countless problems. Another method that has been used
is to add an oil-soluble resin to the proppant matrix composition;
however, this method has not been successful because of, inter
alia, nonuniform removal of the particles.
[0008] Another way to address fracture conductivity has been to use
bigger proppant particulates. However, there are practical limits
to the size of the proppant particulates that may be used. For
instance, if the particles used are too large, premature screenout
at the perforations and/or fractures during the proppant stage of
fracturing treatment often occurs as a large amount of proppant
particulates is being injected into the fractures. In addition, by
using proppant particulates that are too large, the ability to
control formation sand is lost as the formation sand or fines tend
to invade or penetrate the large pore space of the proppant matrix
during production of hydrocarbons, thus potentially choking the
flow paths of the fluids.
SUMMARY
[0009] The present invention relates to enhancing fluid flow from
subterranean formations, and more particularly, to enhancing the
conductivity of fractures in a subterranean formation so as to
enhance fluid flow therethrough.
[0010] In one embodiment, the present invention provides a proppant
matrix composition comprising at least a plurality of proppant
particulates and at least a plurality of composite particles, the
composite particles comprising a degradable material and a filler
material.
[0011] In another embodiment, the present invention provides a
treatment fluid for use in a subterranean application comprising a
proppant matrix composition, the proppant matrix composition
comprising at least a plurality of proppant particulates and at
least a plurality of composite particles, the composite particles
comprising a degradable material and a filler material.
[0012] In another embodiment, the present invention provides a
proppant matrix having at least one void therein, the void
resulting from the degradation of a degradable material in a
composite particle.
[0013] The objects, features and advantages of the present
invention will be readily apparent to those skilled in the art upon
a reading of the description of the preferred embodiments that
follows.
DESCRIPTION
[0014] The present invention relates to enhancing fluid flow from
subterranean formations, and more particularly, to enhancing the
conductivity of fractures in a subterranean formation so as to
enhance fluid flow therethrough.
[0015] In preferred embodiments, the present invention provides
compositions and methods for enhancing subterranean well
productivity by enhancing fracture conductivity. The compositions
and methods of the present invention may be used to enhance the
conductivity of proppant matrixes within fractures so that fluids
from the subterranean formation may flow more freely to the well
bore. The compositions and methods may be used without negatively
affecting the ability of the proppant matrix to perform other
desired functions within the fracture, e.g., propping the fracture
open or providing some degree of sand control. The compositions and
methods also may be used to avoid the production of undesirable
acids within the matrix that may result from the degradation of
degradable materials within the matrix.
[0016] In the compositions and methods of this invention, a
proppant matrix composition may be made to consolidate within a
fracture to form a proppant matrix, e.g., a substantially stable
proppant matrix. The term "proppant matrix," as used herein, refers
to a consolidation of proppant particulates within a fracture that
may be adjacent to a well bore in a subterranean formation. The
mechanism by which the proppant matrix consolidates within the
fracture is not important and so any suitable method can be used in
conjunction with the present invention, e.g., through the use of
curable resins, tackifying agents, and/or a mechanical method such
as interlocking proppant particulates. The use of curable resins
may be preferred.
[0017] The proppant matrix compositions of the present invention
comprise proppant particulates and at least one "composite
particle," at least a portion of which is capable of undergoing an
irreversible degradation downhole. The composite particles used in
the methods and compositions of this invention comprise, a
degradable material and a filler material. As used herein, the term
"particle" or "particles" refers to a particle or particles that
may have a physical shape of platelets, shavings, fibers, flakes,
ribbons, rods, strips, spheroids, toroids, pellets, tablets, or any
other suitable shape. The composite particles generally are more
resistant to crushing forces within a fracture (as compared to
degradable materials by themselves) and, therefore, may help
support the fracture and maintain the integrity of the proppant
matrix. Also, when the composite particle degrades from the
proppant matrix, voids that have a desirable degree of integrity
are formed, at least in part due to the high crush strength of the
composite particles and the consolidation of the proppant matrix.
Suitable degradable materials and filler materials will be
discussed below. The proppant matrix compositions of the present
invention also may comprise an acid reactive material. Each element
of the proppant matrix compositions of this invention is discussed
below.
[0018] The concentration of composite particles in the proppant
matrix compositions of this invention may range from about 0.1% to
about 30%, based on the weight of the proppant particulates in a
particular composition. A concentration of composite particles
between about 1% and about 5% by weight of the proppant
particulates is preferred. At higher concentrations, there may be a
point of diminishing returns, but this will be dependent on the
particular factors, e.g., temperature, stress, how much filler is
in the composite particle, etc. Additionally, one should note that
the relative amounts of the composite particles in the proppant
matrix composition should not be such that when degraded, an
undesirable percentage of voids result in the proppant matrix that
could potentially make the proppant matrix ineffective in
maintaining the integrity of the fracture. One of ordinary skill in
the art, with the benefit of this disclosure, will recognize an
optimum concentration of composite particles that provides
desirable values in terms of enhanced conductivity or permeability
without undermining the stability of the proppant matrix
itself.
[0019] Any proppant particulates suitable for use in subterranean
applications are suitable for use in the compositions and methods
of the present invention. For instance, natural sand, ground nut
hulls, man-made proppant particulates, bauxite, ceramics, polymeric
particulate materials, low-density proppant particulates, or the
like are suitable. Ceramic proppant particulates, in certain
embodiments, are preferred because of their strength. Natural sand
is also a preferred material, especially when cost may be a
concern. Note that the term "particulate" implies no particular
shape or size of proppant particulates. Preferred particulates
should conform generally to API RP-56 and/or RP-60. Suitable sizes
for such proppant particulates may range from 4 to 100 U.S. mesh,
but are preferably in the range of 10 to 60 U.S. mesh.
[0020] In some embodiments, the proppant particulates may be used
in conjunction with a curable resin, e.g., the resin may be coated
on the proppant particulates, such that the resin cures when
downhole, resulting in a consolidation of the proppant particulates
into a proppant matrix. If a resin is used, the proppant
particulates can either be pre-coated or coated on the fly with a
suitable curable resin. Any type of curable resin that will allow
the proppant particulates to consolidate to form a proppant matrix
is suitable for use in the present invention. Examples include, but
are not limited to, epoxies, furans, phenolics, furfuryl aldehyde,
furfuryl alcohol, or derivatives thereof, or a mixture thereof. If
a curable resin is utilized, a better result may be achieved if the
proppant particulates are coated with a suitable curable resin
prior to being mixed with the composite particles.
[0021] Additionally, suitable tackifying agents may be used as an
alternative to or in conjunction with curable resins. If used, the
tackifying agent is preferably incorporated with the proppant
particulates before it is mixed with the composite particles.
[0022] In some embodiments, it may be desirable to coat the
proppant particulates with a tackifying agent rather than a curable
resin. Such a tackifying agent is preferably incorporated with the
proppant particulates before they are mixed with the degradable
material. The tackifying agent, among other things, helps
distribute the composite particle within the proppant matrix
composition and helps keep it in place within the proppant matrix.
Using a tackifying agent as opposed to a curable resin may be
particularly useful if the composite particles used have a low
density or specific gravity, or have a substantially different
particle size than the proppant particulates. The use of a
tackifying agent may help to reduce or eliminate the negative
effects of segregation between the proppant particulates and the
degradable material. Often, the composite particles will exhibit a
significantly different density from the proppant particulates. In
such cases, when the particulates are slurried into a carrier fluid
to be sent to a portion of a subterranean formation, the composite
particles may separate from the denser proppant particulates. Since
the methods of the present invention preferably create a relatively
uniform matrix of proppant particulates mixed with degradable
material, that separation may cause the job to be less successful.
The tacky nature of a chosen tackifying agent may help the chosen
composite particles to at least temporarily attach to the proppant
particulates. By becoming so attached, the negative effects of
segregation may be reduced or eliminated.
[0023] In one embodiment of the present invention, the tackifying
agent is coated onto the proppant particulates early in the
proppant stage of the fracturing treatment. Then, resin-coated
proppant particulates are used during the tail-in stage of the
fracturing treatment. In another embodiment, the tackifying agent
and the curable resin are coated on the proppant particulates
intermittently.
[0024] Compositions suitable for use as tackifying agents in the
present invention comprise any compound that, when in liquid form
or in a solvent solution, will form a non-hardening coating upon a
particulate. Some examples of suitable tackifying agents include
non-aqueous tackifying agents, aqueous tackifying agents, and silyl
modified polyamides.
[0025] Suitable non-aqueous tackifying agents generally comprise
polyamides that are liquids or in solution at the temperature of
the subterranean formation such that they are, by themselves,
non-hardening when introduced into the subterranean formation. One
suitable such tackifying agent comprises a condensation reaction
product comprised of a polyacid and a polyamine. Such commercial
products include compounds such as mixtures of C36 dibasic acids
containing some trimer and higher oligomers and also small amounts
of monomer acids that are reacted with polyamines. Other polyacids
include trimer acids, synthetic acids produced from fatty acids,
maleic anhydride, acrylic acid, and the like. Such acid compounds
are commercially available from companies such as Witco
Corporation, Union Camp, Chemtall, and Emery Industries. The
reaction products are available from, for example, Champion
Technologies, Inc. and Witco Corporation. Additional compounds
which may be used as tackifying compounds include liquids and
solutions of, for example, polyesters, polycarbonates and
polycarbamates, and natural resins such as shellac and the like.
Other suitable tackifying agents are described in U.S. Pat. No.
5,853,048, issued to Weaver, et al., and U.S. Pat. No. 5,833,000,
issued to Weaver, et al., the relevant disclosures of which are
herein incorporated by reference.
[0026] Such non-aqueous tackifying agents may be either used such
that they form non-hardening coating or they may be combined with a
multifunctional material capable of reacting with the tackifying
compound to form a hardened coating. A "hardened coating," as used
herein, means that the reaction of the tackifying compound with the
multifunctional material will result in a substantially
non-flowable reaction product that exhibits a higher compressive
strength in a consolidated agglomerate than the tackifying compound
alone with the particulates. In this instance, the tackifying agent
may function similarly to a hardenable resin. Multifunctional
materials suitable for use in the present invention include, but
are not limited to, aldehydes such as formaldehyde, dialdehydes
such as glutaraldehyde, hemiacetals or aldehyde, releasing
compounds, diacid halides, dihalides such as dichlorides and
dibromides, polyacid anhydrides such as citric acid, epoxides,
furfuraldehyde, glutaraldehyde or aldehyde condensates and the
like, and combinations thereof. In some embodiments of the present
invention, the multifunctional material may be admixed with the
tackifying compound in an amount of from about 0.01% to about 50%
by weight of the tackifying compound to effect formation of the
reaction product. In some preferred embodiments, the compound is
present in an amount of from about 0.5% to about 1% by weight of
the tackifying compound. Suitable multifunctional materials are
described in U.S. Pat. No. 5,839,510, issued to Weaver, et al., the
relevant disclosure of which is herein incorporated by
reference.
[0027] Another suitable group of tackifying agents is aqueous
tackifying agents; that is, tackifying agents that are soluble in
aqueous fluids. Suitable aqueous tackifying agents are capable of
forming at least a partial coating upon the surface of a
particulate (such as proppant particulates). Generally, suitable
aqueous tackifying agents are not significantly tacky when placed
onto a particulate, but are capable of being "activated" (that is
destabilized, coalesced and/or reacted) to transform the compound
into a sticky, tackifying compound at a desirable time. Such
activation may occur before, during, or after the aqueous
tackifying agent is placed in the subterranean formation. In some
embodiments, a pretreatment may be first contacted with the surface
of a particulate to prepare it to be coated with an aqueous
tackifying agent. Suitable aqueous tackifying agents are generally
charged polymers that comprise compounds that, when in an aqueous
solvent or solution, will form a non-hardening coating (by itself
or with an activator) and, when placed on a particulate, will
increase the continuous critical resuspension velocity of the
particulate when contacted by a stream of water. The aqueous
tackifying agents enhance the grain-to-grain contact between the
individual particulates within the formation (be they proppant
particulates, formation fines, or other particulates), helping to
bring about the consolidation of the particulates into a cohesive,
flexible, and permeable mass.
[0028] Examples of aqueous tackifier compounds suitable for use in
the present invention include, but are not limited to, acrylic acid
polymers, acrylic acid ester polymers, acrylic acid derivative
polymers, acrylic acid homopolymers, acrylic acid ester
homopolymers (such as poly(methyl acrylate), poly (butyl acrylate),
and poly(2-ethylhexyl acrylate)), acrylic acid ester co-polymers,
methacrylic acid derivative polymers, methacrylic acid
homopolymers, methacrylic acid ester homopolymers (such as
poly(methyl methacrylate), poly(butyl methacrylate), and
poly(2-ethylhexyl methacrylate)), acrylamido-methyl-propane
sulfonate polymers, acrylamido-methyl-propane sulfonate derivative
polymers, acrylamido-methyl-propane sulfonate co-polymers, and
acrylic acid/acrylamido-methyl-propane sulfonate co-polymers and
combinations thereof. These and other suitable aqueous tackifying
agents are described in U.S. application Ser. No. 10/864,061, filed
on Jun. 9, 2004, and U.S. application Ser. No. 10/864,618, filed on
Jun. 9, 2004, the relevant disclosures of which are herein
incorporated by reference.
[0029] Silyl-modified polyamide compounds suitable for use as a
tackifying agent in the present invention may be described as
substantially self-hardening compositions that are capable of at
least partially adhering to particulates in the unhardened state,
and that are further capable of self-hardening themselves to a
substantially non-tacky state to which individual particulates such
as formation fines will not adhere to, for example, in formation or
proppant matrix pore throats. Such silyl-modified polyamides may be
based, for example, on the reaction product of a silating compound
with a polyamide or a mixture of polyamides. The polyamide or
mixture of polyamides may be one or more polyamide intermediate
compounds obtained, for example, from the reaction of a polyacid
(e.g., diacid or higher) with a polyamine (e.g., diamine or higher)
to form a polyamide polymer with the elimination of water. Other
suitable silyl-modified polyamides and methods of making such
compounds are described in U.S. Pat. No. 6,439,309, issued to
Matherly, et al., the relevant disclosure of which is herein
incorporated by reference.
[0030] Where the proppant particulates are coated with a resin
and/or a tackifying agent, the matrix may consolidate and avoid
migration of the proppant particulates. As a proppant matrix is
formed within a fracture, the composite particles should be
distributed within the proppant matrix. In certain preferred
embodiments, the distribution of the composite particles in the
proppant matrix should be relatively uniform. In a preferred
embodiment, the removal of the degradable material in the composite
particles occurs after the proppant matrix has significantly
developed and become relatively stable to minimize shifting or
rearrangement of proppant particulates within the matrix.
[0031] Suitable degradable materials in the composite particles
used in the proppant matrix compositions of this invention should
be capable of undergoing an irreversible degradation downhole. The
term "irreversible," as used herein, means that the degradable
material should not reform a solid or reconsolidate while downhole,
e.g., the degradable material should degrade in situ but should not
recrystallize or reconsolidate in situ after degradation. The term
"degradation" or "degradable" refers to at least the partial
decomposition of the degradable material, and includes both
homogeneous and heterogeneous forms of degradation. This
degradation can be a result of, for example, a chemical or thermal
reaction, or a reaction induced by radiation.
[0032] After the requisite time period dictated by the
characteristics of the particular degradable material utilized in
the composite particles of this invention, voids are created in the
proppant matrix. Additionally, this degradation may result in the
production of an acid, e.g., to perform a desired function like
breaking a filter cake (for example, a filter cake in or near the
fracture), breaking a viscosified fluid, and curing a resin in a
fracture (for instance, resin coated on proppant particulates or on
the faces of a fracture). The filler used in the composite
particles in the proppant matrix composition can enhance either
effect, i.e., the creation of voids or the production of an acid.
The filler also may enhance the mechanical properties of the
composite particles, and may be selected so as to not impair the
mechanical properties of the proppant matrix. The resultant voids
enhance the permeability of the matrix, which may result in, inter
alia, enhanced fracture conductivity, which should lead to an
enhancement in the productivity of the well. Enhanced fracture
conductivity generally enhances well productivity as well
productivity is a function of, inter alia, fracture conductivity.
In a preferred embodiment, the degradation of the degradable
material takes place after the proppant particulates consolidate to
form a matrix inside a fracture or in place to minimize shifting or
rearrangement of proppant particulates within the proppant
matrix.
[0033] Nonlimiting examples of degradable materials that may be
used in conjunction with the composite particles and the proppant
matrix compositions and methods of the present invention include,
but are not limited to, degradable polymers. The differing
molecular structures of the degradable materials that are suitable
for the present invention give a wide range of possibilities
regarding regulating the degradation rate of the degradable
material. In choosing the appropriate degradable material, one
should consider the degradation products that will result. For
instance, some may form an acid upon degradation, and the presence
of the acid may be undesirable; others may form degradation
products that would be insoluble, and these may be undesirable.
Moreover, these degradation products should not adversely affect
other operations or components.
[0034] The degradability of a polymer depends at least in part on
its backbone structure. One of the more common structural
characteristics is the presence of hydrolyzable and/or oxidizable
linkages in the backbone. The rates of degradation of, for example,
polyesters, are dependent on the type of repeat unit, composition,
sequence, length, molecular geometry, molecular weight, morphology
(e.g., crystallinity, size of spherulites, and orientation),
hydrophilicity, surface area, and additives. Also, the environment
to which the polymer is subjected may affect how the polymer
degrades, e.g., temperature, presence of moisture, oxygen,
microorganisms, enzymes, pH, and the like. One of ordinary skill in
the art, with the benefit of this disclosure, will be able to
determine what the optimum polymer would be for a given application
considering the characteristics of the polymer utilized and the
environment to which it will be subjected.
[0035] Suitable examples of polymers that may be used in accordance
with the present invention include, but are not limited to,
homopolymers, random aliphatic polyester copolymers, block
aliphatic polyester copolymers, star aliphatic polyester
copolymers, or hyperbranched aliphatic polyester copolymers. Such
suitable polymers may be prepared by polycondensation reactions,
ring-opening polymerizations, free radical polymerizations, anionic
polymerizations, carbocationic polymerizations, coordinative
ring-opening polymerization for, such as, lactones, and any other
suitable process. Specific examples of suitable polymers include
polysaccharides such as dextran or cellulose; chitins; chitosans;
proteins; aliphatic polyesters; poly(lactides); poly(glycolides);
poly(.epsilon.-caprolactones); poly(hydroxy ester ethers);
poly(hydroxybutyrates); polyanhydrides; polycarbonates;
poly(orthoesters); poly(acetals); poly(acrylates);
poly(alkylacrylates); poly(amino acids); poly(ethylene oxide); poly
ether esters; polyester amides; polyamides; polyphosphazenes; and
copolymers or blends thereof. Other degradable polymers that are
subject to hydrolytic degradation also may be suitable. One
guideline for choosing which composite particles to use in a
particular application is what degradation products will result.
Another guideline is the conditions surrounding a particular
application.
[0036] Of these suitable polymers, aliphatic polyesters are
preferred. Of the suitable aliphatic polyesters, polyesters of
.alpha. or .beta. hydroxy acids are preferred. Poly(lactide) is
most preferred. Poly(lactide) is synthesized either from lactic
acid by a condensation reaction or more commonly by ring-opening
polymerization of cyclic lactide monomer. The lactide monomer
exists generally in three different forms: two stereoisomers L- and
D-lactide; and D,L-lactide (meso-lactide). The chirality of the
lactide units provides a means to adjust, inter alia, degradation
rates, as well as the physical and mechanical properties after the
lactide is polymerized. Poly(L-lactide), for instance, is a
semicrystalline polymer with a relatively slow hydrolysis rate.
This could be desirable in applications of the present invention
where slow degradation of the degradable material is desired.
Poly(D,L-lactide) is an amorphous polymer with a much faster
hydrolysis rate. This may be suitable for other applications of the
methods and compositions of the present invention. The
stereoisomers of lactic acid may be used individually or combined
for use in the compositions and methods of the present invention.
Additionally, they may be copolymerized with, for example,
glycolide or other monomers like .epsilon.-caprolactone,
1,5-dioxepan-2-one, trimethylene carbonate, or other suitable
monomers to obtain polymers with different properties or
degradation times. Additionally, the lactic acid stereoisomers can
be modified by blending high and low molecular weight polylactide
or by blending polylactide with other aliphatic polyesters. For
example, the degradation rate of polylactic acid may be affected by
blending, for example, high and low molecular weight polylactides;
mixtures of polylactide and lactide monomer; or by blending
polylactide with other aliphatic polyesters.
[0037] The physical properties of degradable polymers may depend on
several factors such as the composition of the repeat units,
flexibility of the chain, presence of polar groups, molecular mass,
degree of branching, crystallinity, orientation, etc. For example,
short chain branches reduce the degree of crystallinity of polymers
while long chain branches lower the melt viscosity and impart,
inter alia, extensional viscosity with tension-stiffening behavior.
The properties of the material utilized can be further tailored by
blending, and copolymerizing it with another polymer, or by a
change in the macromolecular architecture (e.g., hyper-branched
polymers, star-shaped, or dendrimers, etc.). The properties of any
such suitable degradable polymers (such as hydrophilicity, rate of
biodegration, etc.) can be tailored by introducing functional
groups along the polymer chains. One of ordinary skill in the art,
with the benefit of this disclosure, will be able to determine the
appropriate functional groups to introduce to the polymer chains to
achieve the desired effect.
[0038] Also, we have found that a preferable result is achieved if
the degradable material degrades slowly over time as opposed to
instantaneously. Even more preferable results have been obtained
when the degradable material does not begin to degrade until after
the proppant matrix has developed some compressive strength. The
slow degradation of the degradable material, inter alia, helps to
maintain the stability of the proppant matrix.
[0039] The filler material chosen for the composite particles of
this invention, inter alia, may enhance the mechanical properties
of the composite particles (e.g., to enhance the crush strength of
the composite particles), or may react with any degradation
products that result from the degradation of the degradable
material. In certain embodiments, the filler material may comprise
from about 0.5% to about 60% of the composition of a composite
particle. In preferred embodiments, the filler will comprise about
10% to about 40% of the composition of a composite particle. Also,
the filler material may improve the modulus between the T.sub.g
(the glass transition temperature) and the melting point of the
degradable material. In other embodiments, the filler material
chosen can interact with the degradation products produced when the
degradable material degrades. Examples of such filler materials
include anhydrous salts (see below), glass, talc, calcium
carbonate, mica, magnesium oxide, mineral filler, barite, silica,
materials that may be used as conventional bridging agents,
derivatives thereof, and combinations thereof. For instance, if the
degradation products include an acid, the filler may neutralize or
enhance that acid. An example of a filler material that could
neutralize the acid includes a base; an example of a filler
material that could enhance the acid includes another complimentary
acid. In still other embodiments, the filler material may release a
second chemical. For instance, the filler material may comprise
ethylenediaminetetraacetic acid ("EDTA"), an oxidizer, a breaker,
sodium persulfate, or magnesium peroxide. In such embodiments the
filler material should not negatively impact the degradable
material; preferably, the filler material and the degradable
material should compliment one another.
[0040] Examples of preferred nonreactive filler materials include
anhydrous salts. An anhydrous salt is suitable for use in the
present invention if it will degrade over time as it hydrates. For
example, a particulate solid anhydrous borate material that
degrades over time may be suitable. Specific examples of
particulate solid anhydrous borate materials that may be used
include, but are not limited to, anhydrous sodium tetraborate (also
known as anhydrous borax), and anhydrous boric acid. These
anhydrous borate materials are only slightly soluble in water.
However, with time and heat in a subterranean environment, the
anhydrous borate materials react with the surrounding aqueous fluid
and may become hydrated. The resulting hydrated borate materials
are highly soluble in water as compared to anhydrous borate
materials and as a result degrade in the aqueous fluid. In some
instances, the total time required for the anhydrous borate
materials to degrade in an aqueous fluid is in the range of from
about 8 hours to about 72 hours depending upon the temperature of
the subterranean zone in which they are placed. Other examples
include organic or inorganic salts like sodium acetate
trihydrate.
[0041] Optionally, the composite particles of the present invention
may comprise additional additives such as processing aids,
lubricants, antistats, antiblock agents, pigments, derivatives
thereof, or combinations thereof.
[0042] The composite particles used in conjunction with the present
invention can be prepared by any suitable process for example, by
bringing the components in solid form and dry-blending using
conventional means such as a barrel mixer, a tumble mixer, and the
like, followed by fluxing or melting in an appropriate apparatus,
such as a Banbury type internal mixer, rubber mill, single or twin
screw extruder or compounder, or the like. Preferably, the two
components are brought together and processed in an appropriate
melt extruder, from which the blend is extruded in the form of
strands, which are pelletized for fabrication purposes. Other
suitable techniques well known to those skilled in the art can be
used as well.
[0043] If the application in which the composite particles will be
used does not contain a component that will enable the degradable
material to degrade, e.g., in a dry gas hole, then in alternative
embodiments of the present invention, the degradable material can
be mixed with an inorganic or organic compound in addition to or as
a filler material. In preferred alternative embodiments, the
inorganic or organic compound in the composite is hydrated.
Examples of the hydrated organic or inorganic solid compounds that
can be utilized include, but are not limited to, hydrates of
organic acids or their salts such as sodium acetate trihydrate,
L-tartaric acid disodium salt dihydrate, sodium citrate dihydrate,
hydrates of inorganic acids or their salts such as sodium
tetraborate decahydrate, sodium hydrogen phosphate heptahydrate,
sodium phosphate dodecahydrate, amylose, starch-based hydrophilic
polymers, and cellulose-based hydrophilic polymers. Of these,
sodium acetate trihydrate is preferred. Additionally, if the
degradable material is susceptible to hydrolysis, it is preferred
that a sufficient amount of water is produced to effect hydrolytic
degradation of the degradable material. The degradable material is
then in a sense self-degradable, in that the degradable material
should at least partially degrade in the releasable water provided
by the hydrated organic or inorganic compound which dehydrates over
time when heated in the subterranean zone.
[0044] The specific features of the composite particles may be
chosen or modified to provide the proppant matrix with optimum
conductivity while maintaining its desirable filtering capability.
Preferably, the composite particles are selected to have a size and
shape similar to the size and shape of the proppant particulates to
help maintain substantial uniformity within the mixture. It is
preferable if the proppant particulates and the composite particles
do not segregate within the proppant matrix composition. Whichever
composite particles are utilized, the composite particles may have
any shape, depending on the desired characteristics of the
resultant voids in the proppant matrix including, but not limited
to, particles having the physical shape of platelets, shavings,
flakes, ribbons, rods, strips, spheroids, toroids, pellets,
tablets, or any other physical shape. The physical shape of the
composite particles should be chosen so as to enhance the desired
shape and relative composition of the resultant voids within the
proppant matrix. For example, a rod-like particle shape may be
suitable in applications wherein channel-like voids in the proppant
matrix are desired. One of ordinary skill in the art with the
benefit of this disclosure will recognize the specific degradable
material and the preferred size and shape for a given
application.
[0045] Additional materials may be incorporated in the proppant
matrix, if desired, including, but not limited to, acid-reactive
materials. Such acid-reactive materials in the proppant matrix
compositions may comprise any material that reacts with an acid so
that the acid is at least partially neutralized. Suitable examples
include, but are not limited to, materials such as calcium
carbonate, magnesium oxide, and calcium hydroxide. The
acid-reactive material may react with the degradation products of
the degradable material. This may be beneficial when the
degradation products comprise an undesirable acid. When included in
a proppant matrix composition, the acid-reactive material should be
included in an amount sufficient to control the pH of any fluid in
the proppant matrix and/or neutralize any acid present.
Considerations that may be taken into account when considering the
type and amount of acid-soluble component to include are, among
others, the solubility of the reaction products, corrosion of any
metals, and the control of scale formation.
[0046] An inert filler may be included in the proppant matrix
compositions. Suitable inert fillers are materials that, inter
alia, enhance the compressive strength of a proppant matrix.
Suitable fillers include, but are not limited to, calcium
carbonate, talc, mica, glass, silica, silica flour, or other
similar mineral fillers.
[0047] The proppant matrix composition can either be pre-blended
and then transported to the drill site, or it can be prepared on
the fly at the drill site and then introduced downhole within a
relatively short period of time. The term drill site, as used
herein, refers to the workplace at the site of a drill hole.
Preferably, the proppant particulates and the composite particles
should be mixed so as to form a mixture in a fracturing treatment
fluid. Any conventional fracturing treatment fluid may be used in
accordance with the present invention.
[0048] The concentration of the composite particles in the proppant
matrix composition ranges from about 0.1% to about 30%, based on
the weight of the proppant particulates in the mixture. In certain
preferred embodiments of the proppant matrix compositions of the
present invention, the composite particles make up about 1% to
about 5% of the proppant matrix composition. Additionally, the
relative amounts in the proppant matrix composition should not be
such that when degraded, an undesirable percentage of voids results
in the proppant matrix making the proppant matrix potentially
ineffective in maintaining the integrity of the fracture. One of
ordinary skill in the art with the benefit of this disclosure will
recognize an optimum concentration of composite particles that
provides desirable values in terms of enhanced conductivity or
permeability without undermining the stability of the proppant
matrix itself.
[0049] Thus, the present invention is well adapted to carry out the
objects and attain the ends and advantages mentioned as well as
those which are inherent therein. While numerous changes may be
made by those skilled in the art, such changes are encompassed
within the spirit and scope of this invention as defined by the
appended claims.
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