Note: Descriptions are shown in the official language in which they were submitted.
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DEGRADABLE FLUID SEALING COMPOSITIONS HAVING AN ADJUSTABLE
DEGRADATION RATE AND METHODS FOR USE THEREOF
BACKGROUND
[0001] The present disclosure relates to methods and compositions for
fluid blocking and diversion in subterranean formations, and, more
specifically,
to treatment operations that form a temporary fluid seal in a subterranean
formation.
[0002] Treatment fluids can be used in a variety of subterranean
operations. Such subterranean operations can include, without limitation,
drilling operations, stimulation operations, production operations,
remediation
operations, sand control treatments and the like. As used herein, the terms
"treat," "treatment," and "treating" refer to any subterranean operation that
uses a fluid in conjunction with achieving a desired function and/or for a
desired
purpose. Use of these terms does not imply any particular action by the
treatment fluid. Illustrative treatment operations can include, for example,
fracturing operations, gravel packing operations, acidizing treatments, scale
dissolution and removal, consolidation treatments, and the like.
[0003] When performing these or other subterranean treatment
operations, it can sometimes be desirable to temporarily or permanently block
or
divert the flow of a fluid within at least a portion of the subterranean
formation.
The blocking or diversion of the fluid can itself be considered a treatment
operation.
Illustrative fluid blocking and diversion operations can include,
without limitation, fluid loss control operations, kill operations,
conformance
control operations, and the like. The fluid that is blocked or diverted can be
a
formation fluid that is natively present in the subterranean formation, such
as
petroleum, gas, or water. In other cases, the fluid that is blocked or
diverted
can be a subterranean treatment fluid, including the types mentioned above. In
some cases, treatment fluids can be made to be self-diverting, such that they
are directed to a desired location within the subterranean formation.
[0004] Providing effective fluid loss control during subterranean
treatment operations can be highly desirable. "Fluid loss," as used herein,
refers
to the undesired migration or loss of fluids into a subterranean formation
and/or
a particulate pack. Fluid loss can be problematic in a number of subterranean
operations including, for example, drilling operations, fracturing operations,
acidizing operations, gravel-packing operations, workover operations, chemical
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treatment operations, wellbore clean-out operations, and the like. In
fracturing
operations, for example, fluid loss into the formation matrix can sometimes
result in incomplete fracture propagation.
[0005] Diverting agents can function similarly to fluid loss control
agents, but may involve a somewhat different approach. Diverting agents can
be used to seal off a portion of the subterranean formation. By sealing off a
portion of the subterranean formation, a treatment fluid can be diverted from
a
highly permeable portion of the subterranean formation to a lower permeability
portion, for example.
Plugging or sealing agents can be used similarly to
diverting agents, except they are generally used to seal off the wellbore to
provide zonal isolation.
[0006] When only a temporary blocking or diversion of fluid is desired,
a fluid seal within a subterranean formation can be removed to allow fluid
flow to
resume. In some cases, an external degradant can be introduced to the
subterranean formation to remove the fluid seal. The external degradant agent
may be introduced to the subterranean formation after the fluid seal is no
longer
necessary (e.g., after performing a treatment operation). Use of an external
degradant in a separate cleanup operation can add to the time and expense
needed to produce a fluid from the subterranean formation. In other cases, a
fluid seal may comprise a substance that is natively unstable, such that the
fluid
seal degrades and/or dissolves over time to allow fluid flow to resume. When
relying on the native degradation rate of a fluid seal, undesirably slow
degradation can again add to the time and expense of production operations.
[0007] Gelled polymers can be used to form a fluid seal in subterranean
operations. As used herein, a "gelled polymer" refers to a polymer in semi-
solid
form that has at least a portion of its polymer chains crosslinked with one
another via a crosslinking agent. A gelled polymer has a rheological yield
point.
It is to be understood in the description that follows that any reference to a
gelled polymer refers to a polymer that is crosslinked.
Crosslinked
polyacrylamide, other acrylamide-containing polymers, and hydrolyzed or
partially hydrolyzed variants thereof are illustrative examples of gelled
polymers
that can be used in subterranean operations.
[0008] Various modes of crosslinking can be used to form the crosslinks
in a gelled polymer. The crosslinks can be in the form of a covalent bond or a
non-covalent bonding interaction. They can be temporary or permanent.
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Chromium and other transition metal ions can be used to crosslink acrylamide-
containing polymers. Polymer gels formed using such crosslinking agents have
proven unsuitable at higher temperatures (e.g., above about 80 C) due to
uncontrolled crosslinking rates (e.g., short gel-times), crosslinking agent
precipitation, polymer degradation, and the like. In addition, chromium and
certain other transition metal ions can have an undesirable environmental
impact. Acrylamide-containing polymers can also be crosslinked with
polyalkyleneimines and polyalkylenepolyamines. Depending on the type and
concentration of crosslinking agent used, the gel-times and gel strengths of
the
gelled polymers can be impacted.
[0009] Gelled acrylamide-containing polymers can be particularly
effective for fluid blocking and diversion in lower permeability formations
(e.g.,
formations having a permeability of about 0.1 darcy (D) or lower). As the
formation permeability increases, gelled acrylamide-containing polymers can
become less effective due to their reduced ability to block larger pore
throats
that can be characteristic of higher permeability formations. For example,
above
about 0.1 D, and particularly above about 0.5 D, gelled acrylamide-containing
polymers may be less effective for blocking or diverting fluid flow at normal
formation operating temperatures and pressures. To block these larger pore
throats in higher permeability formations, particulate matter may be added to
gelled acrylamide-containing polymers as a bridging agent.
SUMMARY OF THE INVENTION
[0010] The present disclosure relates to methods and compositions for
fluid blocking and diversion in subterranean formations, and, more
specifically,
to treatment operations that form a temporary fluid seal in a subterranean
formation.
[0011] In some embodiments, the present disclosure provides a method
comprising: providing a sealing composition comprising: a degradable polymer,
and a water-soluble material comprising a first portion of rigid particulates
and a
second portion of rigid particulates, each portion of rigid particulates
having a
sealing time and a particulate size distribution associated therewith, the
particulate size distributions of the first portion of rigid particulates and
the
second portion of rigid particulates differing from one another; determining
an
amount of the first portion of rigid particulates relative to the second
portion of
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rigid particulates in the sealing composition needed to produce a degradable
fluid seal having a desired sealing time that is different than that of the
sealing
time of either the first portion of rigid particulates or the second portion
of rigid
particulates; introducing the sealing composition into a subterranean
formation;
and allowing the sealing composition to form a degradable fluid seal in the
subterranean formation.
[0012] In other embodiments, the present disclosure provides a method
comprising: providing a sealing composition comprising: particulates of a
gelled
degradable polymer, and a water-soluble material comprising rigid particulates
having a sealing time and a particulate size distribution associated
therewith, the
particulate size distribution of the water-soluble material differing from
that of a
like unsized water-soluble material; determining a particulate size
distribution of
the rigid particulates needed to produce a degradable fluid seal having a
desired
sealing time; introducing the sealing composition into a subterranean
formation;
forming a degradable fluid seal in the subterranean formation from the sealing
composition; performing a treatment operation in the subterranean formation
while the degradable fluid seal is intact; and allowing the degradable fluid
seal to
degrade.
[0013] In still other embodiments, the present disclosure provides a
method comprising:
providing a plurality of gelled degradable polymer
particulates; providing a first portion of a water-soluble material and a
second
portion of a water-soluble material, each portion comprising rigid
particulates
and each portion having a sealing time and a particulate size distribution
associated therewith, the particulate size distributions differing from one
another; mixing the first portion of the water-soluble material and the second
portion of the water-soluble material with the plurality of gelled degradable
polymer particulates, thereby forming a sealing composition; determining an
amount of the first portion of the water-soluble material relative to the
second
portion of the water-soluble material in the sealing composition needed to
produce a degradable fluid seal having a desired sealing time that is
different
than that of the sealing time of either the first portion of the water-soluble
material or the second portion of the water-soluble material; and introducing
the
sealing composition into a subterranean formation to form a degradable fluid
seal therein.
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[0014] The features and advantages of the present disclosure will be
readily apparent to one having ordinary skill in the art upon a reading of the
description of the preferred embodiments that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The following figures are included to illustrate certain aspects of
the present invention, and should not be viewed as exclusive embodiments. The
subject matter disclosed is capable of considerable modifications,
alterations,
combinations, and equivalents in form and function, as will occur to one
having
ordinary skill in the art and having the benefit of this disclosure.
[0016] FIGURE 1 shows an illustrative fluid breakthrough plot of a
treatment fluid containing crosslinked polyacrylamide particulates and
polyvinyl
alcohol particulates in which 14.3 wt. % of the polyvinyl alcohol particulates
had
a particle size <125 microns and 85.7 wt. % of the polyvinyl alcohol
particulates
had a particle size between 125 microns and 355 microns.
[0017] FIGURE 2 shows an illustrative fluid breakthrough plot of a
treatment fluid containing crosslinked polyacrylamide particulates and
polyvinyl
alcohol particulates in which 18.6 wt. % of the polyvinyl alcohol particulates
had
a particle size <125 microns and 81.4 wt. % of the polyvinyl alcohol
particulates
had a particle size between 125 microns and 355 microns.
[0018] FIGURE 3 shows an illustrative fluid breakthrough plot of a
treatment fluid containing crosslinked polyacrylamide particulates and
polyvinyl
alcohol particulates in which 21.4 wt. % of the polyvinyl alcohol particulates
had
a particle size <125 microns and 78.6 wt. % of the polyvinyl alcohol
particulates
had a particle size between 125 microns and 355 microns.
[0019] FIGURE 4 shows an illustrative fluid breakthrough plot of a
treatment fluid containing crosslinked polyacrylamide particulates and
polyvinyl
alcohol particulates in which 28.6 wt. % of the polyvinyl alcohol particulates
had
a particle size <125 microns and 71.4 wt. % of the polyvinyl alcohol
particulates
had a particle size between 125 microns and 355 microns.
DETAILED DESCRIPTION
[0020] The present disclosure relates to methods and compositions for
fluid blocking and diversion in subterranean formations, and, more
specifically,
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to treatment operations that form a temporary fluid seal in a subterranean
formation.
[0021] As previously described, gelled degradable polymers can be used
for fluid blocking and diversion in a subterranean formation. In low
permeability
formations, the gelled polymer alone may be suitable for this purpose. In
higher
permeability formations, however, particulates of a bridging agent may be used
in combination with the gelled polymer so that wider pore throats in the
formation can be sealed. The bridging agent particulates may comprise a rigid
material. As used herein, the term "rigid" refers to a particulate form that
is
substantially non-pliable and substantially retains it shape when subjected to
stress. Rigid particulates suitable for use in the present embodiments are
described in more detail hereinafter.
[0022] Not only can rigid bridging particulates desirably facilitate the
use of gelled polymers for fluid sealing and diversion applications in higher
permeability subterranean formations, but the presence of the bridging
particulates themselves may alter the degradation rate of a degradable fluid
seal
formed therefrom. In the case of crosslinked polyacrylamide as the gelled
polymer and polyvinyl alcohol as the rigid bridging particulates, this result
is
particularly surprising, since an aqueous polyvinyl alcohol solution is non-
buffering and produces a pH range (e.g., ¨5.5 - 7.5) that does not by itself
appreciably impact the degradation rate of crosslinked polyacrylamide. Unless
otherwise specified herein, a fluid seal will be considered to be degraded
when it
is no longer impermeable to a fluid of interest. After failure of the fluid
seal, the
seal may undergo further degradation and/or dissolution, as described
hereinafter. A sealing time of the fluid seal may be a function of its
degradation
rate, which may be a function of, among other things, the degradation rate of
the gelled polymer, the dissolution or degradation rate of the rigid bridging
particulates, hydrophobicity or hydrophilicity of the rigid bridging
particulates
and/or the gelled polymer, pH and temperature conditions, and the presence of
other materials that can accelerate or slow the degradation rate. As used
herein
the term "sealing time" refers to the period of time over which a seal is
substantially impermeable to a fluid.
[0023] We have surprisingly discovered that the degradation rate of a
fluid seal comprising a gelled degradable polymer can be further altered by
using
rigid bridging agent particulates that have a different particulate size
distribution
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than does a like unsized bridging agent. As used herein, the term "unsized"
refers to the native particulate size distribution obtained when synthesizing
a
material. By sizing a sample of rigid bridging agent particulates, the
degradation
rate of a fluid seal formed therefrom can be desirably altered compared to
that
obtained using a comparable quantity of unsized rigid bridging agent
particulates. Rigid bridging agent particulates having two or more different
particulate size ranges can also be combined to create a custom particulate
size
distribution suitable for producing a desired degradation rate. By adjusting
the
relative amounts of the two or more different particulate size ranges relative
to
one another, the degradation rate of the fluid seal may be further altered.
Although the use of rigid bridging agent particulates in a degradable fluid
seal
may be particularly advantageous in higher permeability subterranean
formations (e.g., about 0.5 D or greater), it is to be recognized that the
foregoing benefits may also be realized in subterranean formations having a
lower permeability.
[0024] The ability to alter the degradation rate of the fluid seal may be
especially beneficial when it is desired to resume fluid flow or terminate
fluid
diversion before the native degradation of the fluid seal occurs. Further, by
altering the degradation rate of the fluid seal from within, as opposed to
using
an external degradant in a cleanup fluid, the cost of goods can be minimized
and
lost time during cleanup operations can be avoided. Thus, by keeping the fluid
seal intact only for as long as functionally necessary, the formation may be
returned to production more quickly, thereby allowing beneficial cost savings
to
be realized.
[0025] More specifically, we have discovered that the combination of
gelled degradable polymer particulates and rigid particulates of a water-
soluble
material may form degradable fluid seals whose degradation rate may be altered
by varying the size distribution of the rigid particulates. As used herein,
the
term "water-soluble" refers to a material that is by itself water-soluble or
becomes water-soluble upon undergoing a chemical transformation.
No
particular degree of water solubility is implied by the term "water-soluble."
Degradable fluid seals formed from the combination of gelled degradable
polymer particulates and rigid particulates of a water-soluble material may be
particularly advantageous for subterranean operations, since the fluid seals
may
be self-cleaning. That is, the fluid seals are not believed to leave a residue
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(damage) in the subterranean formation over the long term. Without being
bound by theory or mechanism, it is believed that the degradable fluid seals
described herein may begin to fail due to dissolution of the water-soluble
material. Either concurrently with or subsequent to the dissolution of the
water-
soluble material, the degradable polymer particulates may degrade to form a
substantially water-soluble material. Thus, both components of the degradable
fluid seal may become soluble over time.
It should be noted that the
degradation of the gelled degradable polymer need not necessarily take place
by
chemical degradation. For example, in some cases, the degradation can take
place by physical or enzymatic (biological) transformations. In some cases,
the
gelled degradable polymer may simply become soluble in a fluid or erode over
time such that the seal is gradually removed. Unless otherwise specified
herein,
the mechanism by which the gelled degradable polymer degrades may take
place in any manner.
In the case of gelled polyacrylamide particulates,
degradation may take place more rapidly at alkaline pH values, but there also
may be a solubilization or erosion component to their degradation as well.
[0026] Particularly in embodiments in which gelled polyacrylamide
particulates are used, further control over the degradation rate of the
degradable
fluid seal may be realized by adjusting the pH of a treatment fluid used to
introduce the particulates into a subterranean formation. For example, if more
rapid degradation of the degradable fluid seal is desired than can be realized
through altering the size distribution of the water-soluble particulates, the
pH of
the treatment fluid may be increased. Specifically, in some embodiments, the
pH of the treatment fluid may be raised using calcium carbonate or a
comparable base, which may result in more rapid degradation of the
polyacrylamide particulates. Conversely, in embodiments in which gelled
polyacrylamide particulates are used, if slower degradation of the degradable
fluid seal is desired, a treatment fluid having a lower pH may be used. Other
types of materials may have different degradation characteristics. For
example,
esters may degrade more rapidly at either high or low pH, but degrade very
slowly at intermediate pH (e.g., a pH of about 3 to about 6). In some
embodiments, particulates of calcium carbonate or a comparable base may be
included in the degradable fluid seal, such that a localized alkaline
environment
is formed to promote degradation, particularly for a fluid seal comprising
gelled
polyacrylamide particulates. In some embodiments, the base particulates can
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comprise the water-soluble material. In other embodiments, base particulates
can be used in conjunction with another water-soluble material that can be
degraded with a base.
[0027] In various embodiments, sealing compositions described herein
may comprise a degradable polymer and a water-soluble material comprising
rigid particulates, where the size distribution of the water-soluble material
differs
from that of a like unsized water-soluble material. As used herein, a "like
unsized water-soluble material" refers to a water-soluble material that has a
substantially identical chemical composition, but a size distribution that
differs
from that of a sized water-soluble material. That is, a sized water-soluble
material has a size distribution that measurably differs from that of an as-
synthesized water-soluble material. In some embodiments, rigid particulates
having two or more different particulate size ranges may be combined to form a
water-soluble material having a particulate size distribution that differs
from that
of the comparable unsized water-soluble material.
That is, in some
embodiments, two or more portions of rigid particulates, each having
particulate
size distributions that differ from one another, may comprise the water-
soluble
material.
[0028] As one of ordinary skill in the art will recognize, there can be a
distribution of particulate sizes in a material, where the particulate sizes
may be
clustered around a most likely value (i.e., the mode value). In the absence of
intervening factors, the distribution of particulate sizes may approximate a
Gaussian distribution. However, as one of ordinary skill in the art will
further
recognize, the distribution of particulate sizes in a material is most often
non-
Gaussian, with the mode value being skewed to one side of the median value,
often with a tail in the distribution curve favoring higher particulate sizes.
Various techniques that will be familiar to one having ordinary skill in the
art
may be used to separate particulates having various size ranges from one
another (e.g., sieving). In general, sizing of the water-soluble material used
in
the present embodiments may take place through any particulate size separation
technique, known or presently unknown.
[0029] When using sized particulates, the sized particulates may
themselves have a distribution of particulate sizes. Particulates having two
or
more different particulate size ranges may be combined with one another to
produce a sample having yet another particulate size distribution. In general,
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when combining particulates having different particulate size ranges with one
another, the resulting sample will have a different particulate size
distribution
than that of a like unsized material. Further, the particulate size
distribution of
the combined sample may be additionally altered by using different quantities
of
[0030] In some embodiments, the water-soluble material used herein
30 components.
[0031] In some embodiments, the degradable polymer may comprise
particulates of a gelled degradable polymer.
In some embodiments, the
degradable polymer may be crosslinked. For example, in some embodiments,
the gelled degradable polymer may comprise at least one crosslinked polymer
35 such as, for example, a crosslinked polyacrylamide, a crosslinked
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polymethacrylamide, any hydrolyzed or partially hydrolyzed variant thereof,
any
copolymer thereof, any derivative thereof, and any combination thereof. As
used herein, a partially hydrolyzed poly(meth)acrylamide will have at least a
portion of its (meth)acrylamide monomer units hydrolyzed to (meth)acrylic
acid.
Any partially hydrolyzed polymer variant that remains gellable may be used in
the present embodiments. In some embodiments, between about 1% and about
30% of the (meth)acrylamide monomer units may be hydrolyzed. In alternative
embodiments, non-particulate versions of these degradable polymers may be
used as well. Techniques for preparing particulates of gelled polyacrylamide
and
other gellable polymers are described in detail in commonly owned United
States
Patent Application 13/190,509, filed July 26, 2011, which is incorporated
herein
by reference in its entirety. In some embodiments, the particulates may be
produced by any combination of techniques including, for example, chopping;
extruding through a die, a filter, or the like; high speed mixing;
homogenizing;
blending; emulsifying; and the like.
[0032] Examples of acrylamide- and methacrylamide-containing
polymers suitable for use in the present embodiments are described in
commonly owned United States Patent 6,176,315, which is incorporated herein
by reference in its entirety. In some or other embodiments, suitable gelled
degradable polymers may include stimuli-degradable gelled polymers such as
those described in commonly owned United States Patent 7,306,040, which is
incorporated herein by reference in its entirety.
[0033] In some embodiments, the gelled degradable polymers used
herein may include ethylenically unsaturated monomers such as, for example,
ionizable monomers (e.g., 1-N,N-diethylaminoethylmethacrylate, and the like);
diallyldimethylammonium chloride; 2-acrylamido-2-methyl propane sulfonate;
acrylic acid; allylic monomers (e.g., diallyl phthalate; diallyl maleate;
allyldiglycol carbonate; and the like); vinyl formate; vinyl acetate; vinyl
propionate; vinyl butyrate; crotonic acid; itaconic acid; acrylamide;
methacrylamide; methacrylonitrile; acrolein; methyl vinyl ether; ethyl vinyl
ether; vinyl ketone; ethyl vinyl ketone; allyl acetate; allyl propionate;
diethyl
maleate; any derivative thereof; and any copolymer thereof.
[0034] In some embodiments, a crosslinking agent used to form the
gelled degradable polymer may comprise an organic crosslinking agent. In some
embodiments, the degradation rate of the gelled degradable polymer may be
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altered by changing the identity and/or concentration of the crosslinking
agent.
Accordingly, the degradation rate of the degradable fluid seal may be further
adjusted by altering the identity and/or concentration of the crosslinking
agent
in the gelled degradable polymer. In some embodiments, the crosslinking agent
itself may be degradable. Suitable degradable crosslinking agents may comprise
degradable functional groups such as, for example, esters, phosphate esters,
amides, acetals, ketals, orthoesters, carbonates, anhydrides, silyl ethers,
alkene
oxides, ethers, imines, ether esters, ester amides, ester urethanes, carbonate
urethanes, amino acids, any derivative thereof, or any combination thereof.
The
choice of the degradable functional group(s) used in the degradable
crosslinking
agent may be determined by the pH and temperature conditions under which the
degradable fluid seal will be used, for example.
[0035] The size of the gelled degradable polymer particulates is not
believed to be particularly limited. In some embodiments, the gelled
degradable
polymer particulates may range between about 1 micron and about 10 mm in
size. In other embodiments, the gelled degradable polymer particulates may
range between about 10 microns and about 1 mm in size. In still other
embodiments, the gelled degradable polymer particulates may range between
about 50 microns and about 500 microns in size. In some embodiments, the
gelled degradable polymer particulates may be at least about 50 microns in
size,
or at least about 100 microns in size in other embodiments.
[0036] In general, rigid particulates of any water-soluble material may
be used in the present embodiments. Both inorganic and organic water-soluble
materials may be used. The rigid particulates are not particularly limited in
shape, which may include various non-limiting forms such as, for example,
platelets, shavings, flakes, ribbons, rods, strips, spheroids, toroids,
pellets,
tablets, needles, powders and/or the like. The choice of a suitable water-
soluble
material may be dictated by operational needs including, for example,
dissolution rate, subterranean formation temperature and pH, availability of
different particulate size ranges, chemical compatibility, environmental
concerns,
and the like. For example, if the water solubility of the water-soluble
material is
too great, premature failure of the degradable fluid seal may occur. Likewise,
if
the water solubility is too low, the water-soluble material may not
sufficiently
alter the degradation rate of the degradable fluid seal over that of its
native
degradation rate, even if sized particulates are used.
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[0037] In some embodiments, a suitable water-soluble material may
comprise a water-soluble polymer. Water-soluble polymers that may form rigid
particulates may include, for example, polyvinyl alcohol, methyl cellulose,
ethyl
cellulose, carboxymethyl cellulose, carboxyethyl cellulose, acetyl cellulose,
hydroxyethyl cellulose, shellac, chitosan, chitin, dextran, guar, xanthan,
starch,
scleroglucans, a diutan, poly(vinyl pyrollidone), polyacrylamide, polyacrylic
acid,
poly(diallyldimethylammonium chloride), poly(ethylene glycol), poly(ethylene
oxide), polylysine, polymethacrylamide, polymethacrylic acid,
poly(vinylamine),
any derivative thereof, any copolymer thereof, and any combination thereof. In
some embodiments, the foregoing polymers may be crosslinked to alter their
dissolution rate and/or their hydrophobicity. Crosslinking may also improve
the
rigidity of particulates formed therefrom. In other embodiments, fish eyes may
be formed from the foregoing polymers to alter their dissolution rate.
Derivatization and the degree of functionalization may also impact the water
solubility and polymer dissolution rate in some cases. In some embodiments,
the polymer may not become water soluble until after hydrolysis of at least a
portion of its functional groups. This behavior may be seen in methyl
cellulose,
ethyl cellulose, and acetyl cellulose, for example.
[0038] In some embodiments, the water-soluble material may comprise
polyvinyl alcohol. Polyvinyl alcohol may be particularly advantageous for use
in
the present embodiments. Polyvinyl alcohol may be produced by at least partial
hydrolysis of polyvinyl acetate or a like acylated polymer. In some or other
embodiments, polyvinyl acetate having a sufficient degree of hydrolysis to be
at
least partially water soluble may be used. A chief advantage of polyvinyl
alcohol
for use in the present embodiments is that it is non-toxic and biodegradable,
which facilitates its use in the medical and textile industries, for example.
Polyvinyl alcohol may be obtained in many forms including, for example,
fibers,
sheets, granules, beads, powders, and the like. Further, polyvinyl alcohol may
exist as an amorphous solid in an aqueous environment or become completely
soluble depending upon the solution conditions. Among factors that can affect
the dissolution rate include the degree of hydrolysis, the polymer molecular
weight, crystallinity, polymer concentration, ionic strength, and the like.
[0039] In alternative embodiments, a degradable polymer may form
rigid particulates that may be used in place of water-soluble rigid
particulates.
Degradation may take place by chemical, physical, or enzymatic (biological)
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means, for example. Degradable polymers that may form rigid particulates
suitable for use in these alternative embodiments include, for example,
polysaccharides (e.g., dextran, cellulose, guar, and derivatives thereof),
chitin,
proteins, aliphatic polyesters [e.g., poly(hydroxy alkanoates)], polyglycolic
acid
and other poly(glycolides), polylactic acid and other poly(lactides),
polyacrylamide and other polyacrylates, polymethacrylamide and other
polymethacrylates, polyvinyl alcohol, poly(13-hydroxy alkanoates) [e.g.,
poly(13-
hydroxy butyrate) and poly(13-hydroxybutyrates-co-13-hydroxyvalerate)],
poly(hydroxybutyrates), poly(co-hydroxy alkanoates) [e.g., poly(13-
propiolactone)
and poly(c-caprolactone], poly(alkylene dicarboxylates) [e.g., poly(ethylene
succinate) and poly(butylene succinate)], poly(hydroxy ester ethers),
poly(anhydrides) [e.g., poly(adipic anhydride), poly(suberic anhydride),
poly(sebacic anhydride), poly(dodecanedioic anhydride), poly(maleic anhydride)
and poly(benzoic anhydride)], polycarbonates (e.g., trimethylenecarbonate),
poly(orthoesters), poly(amino acids), poly(ethylene oxides),
poly(etheresters),
polyester amides, polyamides, poly(dioxepan-2-one), and polyphosphazenes.
Combinations of these polymers and others may also be used in various
embodiments. In various embodiments, homopolymers or copolymers of these
various polymers may be used. Copolymers may include random, block, graft,
and/or star copolymers in various embodiments.
[0040] The degradation rate of a degradable polymer may depend at
least in part on its backbone structure. The degradability of a degradable
polymer may be due to a chemical change, for example, that destroys the
polymer structure or that changes the solubility of the polymer such that it
becomes more soluble than the parent polymer. For example, the presence of
hydrolysable and/or oxidizable linkages in the backbone may make a polymer
degradable in one or more of the foregoing manners. The rates at which
polymers degrade may be dependent on factors such as, for example, the repeat
unit, composition, sequence, length, molecular geometry, molecular weight,
morphology (e.g., crystallinity, particle size, and the like),
hydrophilicity/hydrophobicity, and surface area.
In addition, exposure to
conditions such as for example, temperature, moisture, oxygen,
microorganisms, enzymes, pH, and the like may alter the degradation rate.
Knowing how the degradation rate is influenced by the polymer structure, one
of
ordinary skill in the art will be able to choose an appropriate degradable
polymer
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for a given application. It is to be noted that the foregoing factors may also
influence the degradation rate of the gelled degradable polymers used in the
present embodiments.
[0041] In still other alternative embodiments, a dehydrated compound
may comprise the rigid particulates. A dehydrated compound, particularly a
dehydrated borate, may degrade over time as the dehydrated compound
rehydrates and becomes soluble. Illustrative dehydrated borates may include,
for example, anhydrous sodium tetraborate (anhydrous borax) and anhydrous
boric acid. These anhydrous borates and others are only slightly soluble in
water. However, upon exposure to subterranean temperatures, they may slowly
rehydrate over time and become considerably more soluble. As a result of the
increased solubility, anhydrous borate particulates may degrade by becoming
soluble. The time required for anhydrous borates to become soluble may range
from between about 8 hours and about 72 hours, depending upon the
temperature of the subterranean zone in which they are placed.
[0042] In embodiments in a non-native particulate size distribution of
rigid particulates is used, the mean particulate size may range between about
1
micron and about 5 mm. In some embodiments, the mean particulate size of
the rigid particulates may range between about 10 microns and about 1 mm. In
some embodiments, the mean particulate size of the rigid particulates may
range between about 50 microns and about 750 microns.
In some
embodiments, the mean particulate size of the rigid particulates may range
between about 100 microns and about 500 microns. In some embodiments, the
particulate size distribution of the rigid particulates may fall within about
5% of
the mean particulate size.
In some embodiments, the particulate size
distribution of the rigid particulates may fall within about 10% of the mean
particulate size. In some embodiments, the particulate size distribution of
the
rigid particulates may fall within about 15% of the mean particulate size. In
some embodiments, the particulate size distribution of the rigid particulates
may
fall within about 20% of the mean particulate size. In some embodiments, the
particulate size distribution of the rigid particulates may fall within about
25% of
the mean particulate size.
In some embodiments, the particulate size
distribution of the rigid particulates may fall within about 30% of the mean
particulate size.
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[0043] In embodiments in which two or more portions of rigid
particulates are combined to produce a "custom" particulate size distribution,
the
particulates may again range between about 1 micron and about 5 mm in size.
In some embodiments, a mean size of a first portion of rigid particulates may
be
about 50 microns in size or less, and a second portion of rigid particulates
may
be about 50 microns in size or greater. In some embodiments, a mean size of a
first portion of rigid particulates may be about 50 microns in size or less,
and a
second portion of rigid particulates may be about 100 microns in size or
greater.
In some embodiments, a mean size of a first portion of rigid particulates may
be
about 50 microns in size or less, and a second portion of rigid particulates
may
be about 150 microns in size or greater. In some embodiments, a mean size of
a first portion of rigid particulates may be about 50 microns in size or less,
and a
second portion of rigid particulates may be about 200 microns in size or
greater.
In some embodiments, a mean size of a first portion of rigid particulates may
be
about 100 microns in size or less, and a second portion of rigid particulates
may
be about 100 microns in size or greater. In some embodiments, a mean size of
a first portion of rigid particulates may be about 100 microns in size or
less, and
a second portion of rigid particulates may be about 150 microns in size or
greater. In some embodiments, a mean size of a first portion of rigid
particulates may be about 100 microns in size or less, and a second portion of
rigid particulates may be about 200 microns in size or greater. In some
embodiments, a mean size of a first portion of rigid particulates may be about
100 microns in size or less, and a second portion of rigid particulates may be
about 250 microns in size or greater. In some embodiments, a mean size of a
first portion of rigid particulates may be about 150 microns in size or less,
and a
second portion of rigid particulates may be about 150 microns in size or
greater.
In some embodiments, a mean size of a first portion of rigid particulates may
be
about 150 microns in size or less, and a second portion of rigid particulates
may
be about 200 microns in size or greater. In some embodiments, a mean size of
a first portion of rigid particulates may be about 150 microns in size or
less, and
a second portion of rigid particulates may be about 250 microns in size or
greater. In some embodiments, a mean size of a first portion of rigid
particulates may be about 150 microns in size or less, and a second portion of
rigid particulates may be about 300 microns in size or greater.
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[0044] In some embodiments, the first portion of rigid particulates may
have a smaller mean particulate size than does the second portion of rigid
particulates. In some embodiments, the first portion of rigid particulates may
have no particular lower limit in particulate size, while the second portion
of rigid
particulates may have a distinct upper and lower particulate size. In some
embodiments, the first portion of rigid particulates may be about 50 microns
in
size or less. In some embodiments, the first portion of rigid particulates may
be
about 75 microns in size or less. In some embodiments, the first portion of
rigid
particulates may be about 100 microns in size or less. In some embodiments,
the first portion of rigid particulates may be about 125 microns in size or
less.
In some embodiments, the first portion of rigid particulates may be about 150
microns in size or less.
In some embodiments, the first portion of rigid
particulates may be about 175 microns in size or less. In some embodiments,
the first portion of rigid particulates may be about 200 microns in size or
less.
In some embodiments, the second portion of rigid particulates may range
between about 100 microns and about 500 microns in size. In some
embodiments, the second portion of rigid particulates may range between about
150 microns and about 500 microns in size. In some embodiments, the second
portion of rigid particulates may range between about 200 microns and about
500 microns in size. In some embodiments, the second portion of rigid
particulates may range between about 250 microns and about 500 microns in
size.
[0045] In various embodiments, the first portion of rigid particulates
and the second portion of rigid particulates may be present in a ratio ranging
between about 1:19 and about 19:1. In some embodiments, the first portion of
rigid particulates may comprise at least about 10% of the total rigid
particulates.
In some embodiments, the first portion of rigid particulates may comprise at
least about 15% of the total rigid particulates. In some embodiments, the
first
portion of rigid particulates may comprise at least about 20% of the total
rigid
particulates. In some embodiments, the first portion of rigid particulates may
comprise at least about 25% of the total rigid particulates.
In some
embodiments, the first portion of rigid particulates may comprise at least
about
30% of the total rigid particulates. In some embodiments, the first portion of
rigid particulates may comprise at least about 35% of the total rigid
particulates.
In some embodiments, the first portion of rigid particulates may comprise at
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least about 40% of the total rigid particulates. In some embodiments, the
first
portion of rigid particulates may comprise at least about 45% of the total
rigid
particulates. In some embodiments, the first portion of rigid particulates may
comprise at least about 50% of the total rigid particulates.
[0046] In some embodiments, the sealing compositions described
hereinabove may be used in various subterranean treatment operations. Such
operations may vary without limitation. Functions performed by the sealing
compositions in subterranean operations may include, for example, fluid loss
control, fluid diversion, conformance control, and the like.
[0047] In some embodiments, the methods can comprise: providing a
sealing composition comprising: a degradable polymer, and a water-soluble
material comprising a first portion of rigid particulates and a second portion
of
rigid particulates, each portion of rigid particulates having a sealing time
and a
particulate size distribution associated therewith, the particulate size
distributions of the first portion of rigid particulates and the second
portion of
rigid particulates differing from one another; determining an amount of the
first
portion of rigid particulates relative to the second portion of rigid
particulates in
the sealing composition needed to produce a degradable fluid seal having a
desired sealing time that is different than that of the sealing time of either
the
first portion of rigid particulates or the second portion of rigid
particulates;
introducing the sealing composition into a subterranean formation; and
allowing
the sealing composition to form a degradable fluid seal in the subterranean
formation.
In some embodiments, the methods may further comprise:
performing a treatment operation in the subterranean formation while the
degradable fluid seal is intact; and allowing the degradable fluid seal to
degrade.
[0048] In some embodiments, the methods can comprise: providing a
sealing composition comprising: particulates of a gelled degradable polymer,
and a water-soluble material comprising rigid particulates having a sealing
time
and a particulate size distribution associated therewith, the particulate size
distribution of the water-soluble material differing from that of a like
unsized
water-soluble material; determining a particulate size distribution of the
rigid
particulates needed to produce a degradable fluid seal having a desired
sealing
time; introducing the sealing composition into a subterranean formation;
forming a degradable fluid seal in the subterranean formation from the sealing
composition; performing a treatment operation in the subterranean formation
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while the degradable fluid seal is intact; and allowing the degradable fluid
seal to
degrade.
[0049] In alternative embodiments of the foregoing methods, a
degradable polymer or an anhydrous borate may comprise the rigid particulates
instead of a water-soluble material.
[0050] In some embodiments, the sealing compositions may be
introduced into a subterranean formation in a treatment fluid.
In some
embodiments, the treatment fluids may comprise an aqueous carrier fluid or an
oil-based carrier fluid. Suitable aqueous carrier fluids may include, for
example,
fresh water, salt water, brine (saturated salt water), seawater, produced
water
(i.e., subterranean formation water brought to the surface), surface water
(e.g.,
lake or river water), and flow back water (i.e., water placed into a
subterranean
formation and then brought back to the surface). In some embodiments, the
treatment fluid may be gelled so as to better support the transport of the
particulates into the subterranean formation.
[0051] Depending upon the type of subterranean formation being
treated and the intended type of treatment operation being conducted, other
components may be optionally included in the treatment fluid.
Such
components may include, for example, salts, pH control additives, surfactants,
foaming agents, antifoaming agents, breakers, biocides, crosslinkers,
additional
fluid loss control agents, stabilizers, chelating agents, scale inhibitors,
gases,
mutual solvents, particulates, corrosion inhibitors, oxidizing agents,
reducing
agents, antioxidants, relative permeability modifiers, viscosifying agents,
proppant particulates, gravel particulates, scale inhibitors, emulsifying
agents,
de-emulsifying agents, iron control agents, clay control agents, flocculants,
scavengers, lubricants, friction reducers, viscosifiers, weighting agents,
hydrate
inhibitors, consolidating agents, any combination thereof, and the like. A
person having ordinary skill in the art, with the benefit of this disclosure,
will
recognize when such optional additives should be included in a treatment
fluid,
as well as the appropriate amounts to include.
[0052] The methods described herein may be used in many different
types of subterranean treatment operations. Such operations may include, but
are not limited to, acidizing operations, scale inhibiting operations, water
blocking operations, clay stabilizer operations, biocide operations,
fracturing
operations, frac-packing operations, and gravel packing operations.
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[0053] In some embodiments, a treatment fluid used to introduce the
sealing compositions into a subterranean formation may have a basic pH. For
example, a basic compound may be present in the treatment fluid. As one of
ordinary skill in the art will recognize, inclusion of a basic compound in the
treatment fluid may promote the degradation of a degradable polymer,
particularly a poly(meth)acrylamide. Suitable basic compounds that may be
used to accelerate the degradation rate of a poly(meth)acrylamide may include,
for example, calcium carbonate, calcium bicarbonate, calcium oxide, magnesium
oxide, magnesium hydroxide, and the like. In some or other embodiments, the
treatment fluid may contain an oxidant, which may also accelerate the
degradation rate of a poly(meth)acrylamide.
[0054] Likewise, in some embodiments, a treatment fluid used to
introduce the sealing compositions into a subterranean formation may also
contain an additive that accelerates the solubilization or degradation of the
rigid
particulates. Suitable additives may include acids, acid-generating compounds
(e.g., esters and orthoesters), bases, base-generating compounds, enzymes,
oxidants, solvents, oil, chelating agents, surfactants, azo compounds,
buffers,
catalysts, solubility-enhancing compounds, and the like. In other embodiments,
the treatment fluids may also contain an additive that decelerates the
solubilization or degradation of the rigid particulates. For example, for
rigid
particulates that oxidatively degrade, the treatment fluid may contain an
antioxidant.
[0055] In some embodiments, the sealing compositions themselves
may contain additional solid particulates that accelerate the degradation rate
of
the gelled degradable polymer and/or the solubilization or degradation of the
rigid particulates. As previously described, inclusion of other solid
particulates
within the degradable fluid seal formed from the sealing compositions may
create a localized chemical and/or physical environment that accelerate
degradation or solubilization. Any of the foregoing additives may be included
as
additional solid particulates in the degradable fluid seals described herein.
[0056] Generally, the sealing compositions may be introduced into any
type of subterranean formation. Further, the subterranean formation may have
any permeability. However, as noted above, the sealing compositions may be
particularly useful in high permeability formations. In some embodiments, the
subterranean formation may have a permeability of at least about 0.5 darcy
(D).
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In other embodiments, the subterranean formation may have a permeability of
at least about 1 D, or at least about 5 D, or at least about 10 D, or at least
about
50 D, or at least about 100 D. In some embodiments, a high permeability
subterranean formation may have at least some pore throats therein that have a
nominal opening size of at least about 20 m.
[0057] In some embodiments, the degradable fluid seal formed in the
subterranean formation may dissolve in a fluid present therein. Dissolution of
the fluid seal may take place during or after degradation of the degradable
fluid
seal takes place. For example, partial dissolution of the degradable fluid
seal
may take place up through the time when the fluid seal fails. After fluid flow
resumes in the formation, the failed seal may persist for some period of time
but
become soluble thereafter. In some embodiments, the degradable polymer of
the fluid seal may become soluble, thereby leading to failure of the fluid
seal,
followed by solubilization of the rigid particulates thereafter.
In other
embodiments, the rigid particulates may be at least partially solubilized or
degraded, thereby leading to failure of the fluid seal, followed by
degradation
and solubilization of the gelled degradable polymer and solubilization of the
remaining rigid particulates thereafter. In some embodiments, the foregoing
degradation and dissolution processes may take place concurrently.
[0058] In some embodiments, a sealing time of the degradable fluid
seal may be altered by changing the amount of the rigid particulates having at
least two different particulate size ranges relative to one another. That is,
the
degradation rate of the degradable fluid seal may be altered by changing the
size distribution of the rigid particulates, which may be performed by mixing
two
or more differentially sized rigid particulates with one another. Given the
benefit
of the present disclosure, one having ordinary skill in the art will be able
to
determine a suitable combination of sized rigid particulates to produce a
desired
sealing time for a degradable fluid seal within a subterranean formation.
[0059] In some embodiments, the methods may comprise: providing a
plurality of gelled degradable polymer particulates; providing a first portion
of a
water-soluble material and a second portion of a water-soluble material, each
portion comprising rigid particulates and each portion having a sealing time
and
a particulate size distribution associated therewith, the particulate size
distributions differing from one another; mixing the first portion of the
water-
soluble material and the second portion of the water-soluble material with the
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plurality of gelled degradable polymer particulates, thereby forming a sealing
composition; determining an amount of the first portion of the water-soluble
material relative to the second portion of the water-soluble material in the
sealing composition needed to produce a degradable fluid seal having a desired
sealing time that is different than that of the sealing time of either the
first
portion of the water-soluble material or the second portion of the water-
soluble
material; and introducing the sealing composition into a subterranean
formation
to form a degradable fluid seal therein. In some embodiments, the methods
may further comprise: choosing the particulate size distributions of the first
portion of the water-soluble material and the second portion of the water-
soluble
material needed to produce a degradable fluid seal having a desired sealing
time.
[0060] To facilitate a better understanding of the present invention, the
following examples of preferred embodiments are given. In no way should the
following examples be read to limit, or to define, the scope of the invention.
EXAMPLES
[0061] EXAMPLE 1: Degradation rates of degradable fluid seals
containing chopped crosslinked polyacrylamide particulates and
polyvinyl alcohol particulates.
A combination of chopped crosslinked
polyacrylamide particulates and polyvinyl alcohol particulates was used to
form a
degradable fluid seal on a 90 micron Aloxite disc (2.5" diameter). The fluid
breakthrough time of the degradable fluid seal was evaluated in a high
pressure,
high temperature flow cell. Testing was conducted at a differential pressure
of
500 psi and a temperature of 200 F. In each case, a degradable fluid seal
(filter
cake) was generated over 36 minutes at room temperature under a 500 psi
differential pressure by flowing a treatment fluid containing the particulates
through the Aloxite disc. Thereafter, fluid loss was recorded with a graduated
cylinder until the filter cake was blown through.
[0062] The polyacrylamide was made degradable by incorporating a
cleavable linkage in the polymer backbone in the form of a degradable
crosslinking agent. The degradable polyacrylamide was prepared by
polymerizing acrylamide with a polyethylene oxide/diacrylate oligomer having a
molecular weight of 258. The polymerization reaction was conducted at ambient
temperature using potassium persulfate as an initiator in the presence of
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N,N,N'N'-tetramethylethylenediamine.
The resulting polymer gel was then
chopped in 40 ppt PAC-RTM filtration control agent (carboxylmethyl cellulose,
commercially available from Halliburton Energy Services) by using a SiIverson
blender operating at 6000 RPM for 1 min. The average size of the chopped
particulates was about 100 microns. BARRACARB 150 bridging agent (CaCO3,
commercially available from Halliburton Energy Services) was added in the
chopped gels to increase the degradation rate.
[0063] The polyvinyl alcohol particulates were CELVOLTM 125, a highly
hydrolyzed polyvinyl alcohol (99.3%) available from Celanese Corp. Two
different particulate size ranges were used, as obtained from grinding and
sieve-
based size separation: 1) <125 microns and 2) 125 microns - 355 microns.
Various compositional ratios of these sized polyvinyl alcohol particulates
were
used as set forth below.
[0064] A first treatment fluid was prepared at the following
composition: 10% degradable polyacrylamide gel particulates in 400 mL of 40
ppt PAC-RTM filtration control additive, also containing 0.05 wt. % BARRACARB
150C) and 1.75 wt. % CELVOLTM 125 additive. In this case, 14.3 wt. % of the
CELVOLTM 125 additive had a particle size <125 microns and 85.7 wt. % had a
particle size between 125 microns and 355 microns. FIGURE 1 shows an
illustrative fluid breakthrough plot of a treatment fluid containing
crosslinked
polyacrylamide particulates and polyvinyl alcohol particulates in which 14.3
wt.
% of the polyvinyl alcohol particulates had a particle size <125 microns and
85.7
wt. % of the polyvinyl alcohol particulates had a particle size between 125
microns and 355 microns. As shown in FIGURE 1, after the initial formation
period at room temperature, the degradable fluid seal remained intact for more
than 5 hours at 200 F under a 500 psi differential pressure.
[0065] A second treatment fluid was prepared at the following
composition: 10% degradable polyacrylamide gel particulates in 400 mL of 40
ppt PAC-RTM filtration control additive also containing 0.05 wt. % BARRACARB
150C) and 1.75 wt. % CELVOLTM 125 additive. In this case, 18.6 wt. % of the
CELVOLTM 125 additive had a particle size <125 microns and 81.4 wt. % had a
particle size between 125 microns and 355 microns. FIGURE 2 shows an
illustrative fluid breakthrough plot of a treatment fluid containing
crosslinked
polyacrylamide particulates and polyvinyl alcohol particulates in which 18.6
wt.
% of the polyvinyl alcohol particulates had a particle size <125 microns and
81.4
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wt. % of the polyvinyl alcohol particulates had a particle size between 125
microns and 355 microns. As shown in FIGURE 2, after the initial formation
period at room temperature, the degradable fluid seal remained intact for
about
140 minutes at 200 F under a 500 psi differential pressure.
After fluid
breakthrough occurred, the remaining degradable fluid seal was removed from
the flow cell and was placed in a glass jar with 250 mL of 40 ppt PAC-RTM
filtration control additive, which was subsequently heated to 200 F. Within 15
hours, the remaining degradable fluid seal had been completely removed as the
remaining polyacrylamide gel particulates were degraded and the polyvinyl
alcohol particulates were solubilized.
[0066] A third treatment fluid was prepared at the following
composition: 10% polyacrylamide gel particulates in 400 mL of 40 ppt PAC-RTM
filtration control additive, also containing 0.05 wt. % BARRACARB 150C) and
1.75 wt. % CELVOLTM 125 additive. In this case, 21.4 wt. % of the CELVOLTM
125 additive had a particle size <125 microns and 78.6 wt. % had a particle
size
between 125 microns and 355 microns. FIGURE 3 shows an illustrative fluid
breakthrough plot of a treatment fluid containing crosslinked polyacrylamide
particulates and polyvinyl alcohol particulates in which 21.4 wt. % of the
polyvinyl alcohol particulates had a particle size <125 microns and 78.6 wt. %
of
the polyvinyl alcohol particulates had a particle size between 125 microns and
355 microns. As shown in FIGURE 3, after the initial formation period at room
temperature, the degradable fluid seal remained intact for about 80 minutes at
200 F under a 500 psi differential pressure.
[0067] A fourth treatment fluid was prepared at the following
composition: 10% polyacrylamide gel particulates in 400 mL of 40 ppt PAC-RTM
filtration control additive, also containing 0.05 wt. % BARRACARB 150C) and
1.75 wt. % CELVOLTM 125 additive. In this case, 28.6 wt. % of the CELVOLTM
125 additive had a particle size <125 microns and 71.4 wt. % had a particle
size
between 125 microns and 355 microns. FIGURE 4 shows an illustrative fluid
breakthrough plot of a treatment fluid containing crosslinked polyacrylamide
particulates and polyvinyl alcohol particulates in which 28.6 wt. % of the
polyvinyl alcohol particulates had a particle size <125 microns and 71.4 wt. %
of
the polyvinyl alcohol particulates had a particle size between 125 microns and
355 microns. As shown in FIGURE 4, after the initial formation period at room
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temperature, the degradable fluid seal remained intact for about 30 minutes at
200 F under a 500 psi differential pressure.
[0068] The foregoing series of tests illustrate that a combination of
chopped degradable polyacrylamide gel particulates and polyvinyl alcohol
particulates can be used to produce a degradable fluid seal. The rate at which
the fluid seal fails and permits fluid flow to resume can be altered by
combining
sized polyvinyl alcohol particulates in various ratios with one another.
[0069] Therefore, the present invention is well adapted to attain the
ends and advantages mentioned as well as those that are inherent therein. The
particular embodiments disclosed above are illustrative only, as the present
invention may be modified and practiced in different but equivalent manners
apparent to those skilled in the art having the benefit of the teachings
herein.
Furthermore, no limitations are intended to the details of construction or
design
herein shown, other than as described in the claims below. It is therefore
evident that the particular illustrative embodiments disclosed above may be
altered, combined, or modified and all such variations are considered within
the
scope and spirit of the present invention. The invention illustratively
disclosed
herein suitably may be practiced in the absence of any element that is not
specifically disclosed herein and/or any optional element disclosed herein.
While
compositions and methods are described in terms of "comprising," "containing,"
or "including" various components or steps, the compositions and methods can
also "consist essentially of" or "consist of" the various components and
steps.
All numbers and ranges disclosed above may vary by some amount. Whenever
a numerical range with a lower limit and an upper limit is disclosed, any
number
and any included range falling within the range is specifically disclosed. In
particular, every range of values (of the form, "from about a to about b," or,
equivalently, "from approximately a to b," or, equivalently, "from
approximately
a-b") disclosed herein is to be understood to set forth every number and range
encompassed within the broader range of values. Also, the terms in the claims
have their plain, ordinary meaning unless otherwise explicitly and clearly
defined
by the patentee. Moreover, the indefinite articles "a" or "an," as used in the
claims, are defined herein to mean one or more than one of the element that it
introduces. If there is any conflict in the usages of a word or term in this
specification and one or more patent or other documents that may be
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incorporated herein by reference, the definitions that are consistent with
this
specification should be adopted.
26
