U.S. patent application number 11/446286 was filed with the patent office on 2007-12-06 for stimuli-degradable gels.
This patent application is currently assigned to Halliburton Energy Services, Inc.. Invention is credited to James M. Griffin, Ian D. Robb, Rajesh K. Saini, Diptabhas Sarkar, Bradley L. Todd.
Application Number | 20070277981 11/446286 |
Document ID | / |
Family ID | 38788776 |
Filed Date | 2007-12-06 |
United States Patent
Application |
20070277981 |
Kind Code |
A1 |
Robb; Ian D. ; et
al. |
December 6, 2007 |
STIMULI-DEGRADABLE GELS
Abstract
Some methods are provided that comprise: providing a treatment
fluid comprising an aqueous fluid, and a stimuli-degradable gel
formed by a reaction comprising a gelling agent, and a
stimuli-degradable cross linking agent that includes at least one
degradable group and two unsaturated terminal groups; placing the
treatment fluid into a subterranean formation; and allowing the
stimuli-degradable gel to degrade.
Inventors: |
Robb; Ian D.; (Duncan,
OK) ; Saini; Rajesh K.; (Duncan, OK) ; Sarkar;
Diptabhas; (Houston, TX) ; Todd; Bradley L.;
(Duncan, OK) ; Griffin; James M.; (Hessle,
GB) |
Correspondence
Address: |
Halliburton Energy Services, Inc.
2600 S. 2nd Street
Duncan
OK
73536-0440
US
|
Assignee: |
Halliburton Energy Services,
Inc.
|
Family ID: |
38788776 |
Appl. No.: |
11/446286 |
Filed: |
June 2, 2006 |
Current U.S.
Class: |
166/300 |
Current CPC
Class: |
C09K 8/685 20130101;
C09K 8/035 20130101; C09K 8/512 20130101 |
Class at
Publication: |
166/300 |
International
Class: |
E21B 43/22 20060101
E21B043/22 |
Claims
1. A method comprising: providing a treatment fluid comprising an
aqueous base fluid, and a stimuli-degradable gel formed by a
combination of a gelling agent, and a stimuli-degradable cross
linking agent described by the following formula:
R.sup.1-[A]-[R.sup.3]--[B]--R.sup.2 wherein R.sup.1 and R.sup.2 may
be the same or different, and comprise at least one group selected
from the group consisting of: a substituted or unsubstituted
ethylenically unsaturated group, N-acryl, O-acryl, acryloyl, vinyl,
allyl, melamide, a derivative thereof, and a combination thereof; A
and B comprise optional bridging units; and R.sup.3 comprises a
degradable group or polymer; and placing the treatment fluid into a
subterranean formation.
2. The method of claim 1 wherein the gelling agent comprises at
least one gelling agent selected from the group consisting of: an
ethylenically unsaturated monomer of the general formula
CH.sub.2.dbd.CXY, wherein X and Y may be hydrogen, an alkoxy amide
group, or an acetamides group; ethylene; propylene; butene-1; vinyl
cyclohexane; vinyl cyclohexene; styrene; vinyl toluene; an
ionizable monomer; 1-N,N-diethylaminoethylmethacrylate;
diallyldimethylammonium chloride; 2-acrylamido-2-methyl propane
sulfonate; acrylic acid; 2-acrylamido-2-methyl propane sulfonate;
acrylic acid; an allylic monomer; di-allyl phthalate; di-allyl
maleate; allyl diglycol carbonate; vinyl formate; vinyl acetate;
vinyl propionate; vinyl butyrate; crotonic acid; itaconic acid;
vinyl fluoride; vinyl chloride; vinylidine; fluoride;
tetrafluoroethylene; acrylamide; methacrylamide; methacrylonitrile;
acrolein; methyl vinyl ether; ethyl vinyl ether; vinyl ketone;
ethyl vinyl ketone; allyl acetate; allyl propionate; diethyl
maleate; a diene monomer; butadiene; isoprene; chloroprene; a
derivative thereof; and a combination thereof.
3. The method of claim 1 wherein R.sup.3 comprises at least one
group selected from the group consisting of: an esters, a phosphate
ester, an amide, an acetal, a ketal, an orthoester, a carbonate, an
anhydride, a silyl ether, an alkene oxide, an ether, an imine, an
ether ester, an ester amide, an ester urethane, a carbonate
urethane, an amino acid, a derivative thereof, and a combination
thereof.
4. The method of claim 1 wherein a G' of the treatment fluid at a
first time is greater than the G' of the treatment fluid at a later
second time because of the degradation of the stimuli-degradable
gel.
5. The method of claim 1 wherein A or B comprises at least one
group selected from the group consisting of: a peptide chain, an
aromatic substituent, an alkyl group, an alkylene group, a polar
group, and a derivative thereof.
6. The method of claim 1 wherein the cross linking agent is
synthesized from diketene acetals or multiketene acetals by the
addition of monohydric alcohol having acrylic, vinylic, or allylic
groups.
7. The method of claim 6 wherein the monohydric alcohol comprises
at least one monohydric alcohol selected from the group consisting
of: hydroxyethylacrylate; hydroxypropyl methacrylamide;
hydroxybutyl methacrylate; and glycerol monomethacrylate.
8. The method of claim 1 wherein the cross linking agent is formed
by a reaction comprising a monohydric alcohol that contains a
vinyl, allyl, or allylic group and a low molecular weight
orthoester of Formula I: ##STR00008## wherein R is H, CH.sub.3, or
C.sub.2H.sub.5, and R.sub.4 is an alkyl group having from about 1
to about 6 carbon atoms.
9. The method of claim 1 wherein the cross linking agent is water
soluble.
10. The method of claim 1 wherein the treatment fluid is foamed and
comprises a surfactant.
11. A method comprising: providing a treatment fluid comprising an
aqueous fluid, and a stimuli- degradable gel formed by a reaction
comprising a gelling agent, and a stimuli-degradable cross linking
agent that includes at least one degradable group and two
unsaturated terminal groups; placing the treatment fluid into a
subterranean formation; and allowing the stimuli-degradable gel to
degrade.
12. The method of claim 11 further comprising reducing the pH of
the treatment fluid before allowing the stimuli-degradable gel to
degrade.
13. The method of claim 11 further comprising subjecting the
treatment fluid to a temperature change before allowing the
stimuli-degradable gel to degrade.
14. The method of claim 11 wherein the degradable group comprises
at least one degradable group selected from the group consisting
of: an ester, a phosphate ester, an amide, an acetal, a ketal, an
orthoester, a carbonate, an anhydride, a silyl ether, an alkene
oxide, an ether, an imine, an ether ester, an ester amide, an ester
urethane, a carbonate urethane, an amino acid, a derivative
thereof, and a combination thereof.
15. The method of claim 11 wherein a G' of the treatment fluid is
reduced upon degradation of the stimuli-degradable gel.
16. The method of claim 11 wherein at least one of the unsaturated
terminal groups comprises at least one group selected from the
group consisting of: a substituted or unsubstituted ethylenically
unsaturated group, a vinyl group, an allyl group, an acryl group,
an unsaturated ester, an acrylate, a methacrylate, a butyl
acrylate, an amide, an acrylamide, an ether, a vinyl ether, a
combination thereof, and a derivative thereof.
17. The method of claim 11 wherein the gelling agent comprises at
least one gelling agent selected from the group consisting of: an
ethylenically unsaturated monomer of the general formula
CH.sub.2.dbd.CXY, wherein X and Y may be hydrogen, an alkyl group,
an aryl group, an alkoxy group, a carboxylic acid group, an amide
group, an alkoxy amide group, an acetamide group, an ester, or an
ether; ethylene; propylene; butene-1; vinyl cyclohexane; styrene;
vinyl toluene; an ionizable monomer;
1-N,N-diethylaminoethylmethacrylate; diallyldimethylammonium
chloride; 2-acrylamido-2-methyl propane sulfonate; acrylic acid;
2-acrylamido-2-methyl propane sulfonate; acrylic acid; an allylic
monomer; di-allyl phthalate; 2-acrylamido 2-methyl propane sulfonic
acid, acrylic acid, di-allyl maleate; allyl diglycol carbonate;
vinyl formate; vinyl acetate; vinyl propionate; vinyl butyrate;
crotonic acid; itaconic acid; vinyl fluoride; vinyl chloride;
vinylidine; fluoride; tetrafluoroethylene; acrylamide;
methacrylamide; methacrylonitrile; acrolein; methyl vinyl ether;
ethyl vinyl ether; vinyl ketone; ethyl vinyl ketone; allyl acetate;
allyl propionate; diethyl maleate; a diene monomer; butadiene;
isoprene; chloroprene; a derivative thereof; and a combination
thereof.
18. A method of providing fluid loss control in a subterranean
application comprising the steps of: providing degradable
crosslinked gelled particles that are formed by a reaction
comprising a gelling agent, and a stimuli-degradable cross linking
agent that includes at least one degradable group and two
unsaturated terminal groups; introducing the degradable crosslinked
gelled particles into a subterranean formation; and allowing the
degradable crosslinked gelled particles to reduce the loss of fluid
to a portion of the subterranean formation.
19. The method of claim 17 wherein at least one of the unsaturated
terminal groups comprises at least one group selected from the
group consisting of: a substituted or unsubstituted ethylenically
unsaturated group, a vinyl group, an allyl group, an acryl group,
an unsaturated ester, an acrylate, a methacrylate, a butyl
acrylate, an amide, an acrylamide, an ether, a vinyl ether, a
combination thereof, and a derivative thereof.
20. The method of claim 17 wherein the gelling agent comprises at
least one gelling agent selected from the group consisting of: an
ethylenically unsaturated monomer of the general formula
CH.sub.2.dbd.CXY, wherein X and Y may be hydrogen, an alkyl group,
an aryl group, an alkoxy group, a carboxylic acid group, an amide
group, an ester, an ether, an alkoxy amide group, or an acetamide
group; ethylene; propylene; butene-1; vinyl cyclohexane; vinyl
cyclohexene; styrene; vinyl toluene; an ionizable monomer;
1-N,N-diethylaminoethylmethacrylate; diallyldimethylammonium
chloride; 2-acrylamido-2-methyl propane sulfonate; acrylic acid; an
allylic monomer; di-allyl phthalate; di-allyl maleate; allyl
diglycol carbonate; vinyl formate; vinyl acetate; vinyl propionate;
vinyl butyrate; crotonic acid; itaconic acid; vinyl fluoride; vinyl
chloride; vinylidine fluoride; tetrafluoroethylene; acrylamide;
methacrylamide; methacrylonitrile; acrolein; methyl vinyl ether;
ethyl vinyl ether; vinyl ketone; ethyl vinyl ketone; allyl acetate;
allyl propionate; diethyl maleate; a diene monomer; butadiene;
isoprene; chloroprene; a derivative thereof; and a combination
thereof.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to methods and compositions
useful in subterranean applications, and, more specifically, to
stimuli-degradable gels.
[0002] Viscosified treatment fluids that are used in subterranean
operations generally are often aqueous-based fluids that comprise
gelling agents. Viscosified treatment fluids are often referred to
in the oilfield industry as "gels." The term "gel" as used herein
refers to a semi-solid, jelly-like state assumed by some colloidal
dispersions. The term "colloidal dispersion" as used herein refers
to a system in which finely divided particles are dispersed within
a continuous medium. The gelling agents used to form gels often
comprise macromolecules such as biopolymers or synthetic polymers.
Common gelling agents include, e.g., galactomannan gums, cellulosic
polymers, and other polysaccharides. As used herein, the term
"treatment fluid" refers to any fluid that may be used in a
subterranean application in conjunction with a desired function
and/or for a desired purpose. The term "treatment fluid" does not
imply any particular action by the fluid or any component
thereof.
[0003] Most viscosified treatment fluids include cross-linked
gelling agents that are cross-linked through a cross linking
reaction between gelling agent molecules and a suitable cross
linking agent. These cross linking agents may comprise a metal, a
metal complex, or a metalloid, collectively referred to herein as
"metal(s)." Examples include compounds containing boron, aluminum,
antimony, zirconium, magnesium, or titanium. Generally, the metal
of a cross linking agent interacts with at least two gelling agent
molecules to form a crosslink between them, thereby forming a
cross-linked gelling agent. The term "cross-linked gelling agent"
as used herein refers to a gelling agent that contains, on average,
at least one crosslink per molecule. This may be indicated when G'
>G'' at certain frequencies. The elastic modulus (or G') of a
gel is an accepted standard measure of a gel's elasticity.
[0004] Pills are often used in subterranean applications. The term
"pill" as used herein refers to a relatively small volume of
specially prepared fluid placed or circulated in the well bore.
Fluid pills are commonly prepared for a variety of special
functions, such as a sweep pill prepared at high viscosity to
circulate around the well bore and pick up debris or well bore
fill. In counteracting lost-circulation problems, a
lost-circulation pill prepared with flaked or fibrous material is
designed to plug the perforations or formation interval losing the
fluid. A "fluid-loss control pill" is a gelled fluid that is
designed or used to provide some degree of fluid-loss control.
Through a combination of viscosity, solids bridging, and cake
buildup on the porous rock, these pills oftentimes are thought to
seal off portions of the formation from fluid loss. They may also
generally enhance filter-cake buildup on the face of the formation
to inhibit fluid flow into the formation from the well bore. Pills
often may involve a relatively small quantity (less than 200 bbl)
of a special blend of a drilling fluid to accomplish a specific
task that a regular drilling fluid cannot perform. Examples include
high-viscosity pills to help lift cuttings out of a vertical well
bore; freshwater pills to dissolve encroaching salt formations;
pipe-freeing pills to destroy filter cake and relieve differential
sticking forces; and lost circulation material pills to plug a
thief zone.
[0005] Typically, pills comprise an aqueous base fluid and a high
concentration of a gelling agent polymer, and, sometimes, bridging
particles, like graded sand, potassium salts, or sized calcium
carbonate particles. An example of a commonly used pill contains
high concentrations (100 to 150 lbs/1000 gal) of a modified
hydroxyethylcellulose ("HEC"). Some other gelling agent polymers
that have been used include guar, guar derivatives,
carboxymethylhydroxyethylcellulose ("CMHEC"), and even starch.
[0006] As an alternative to linear polymeric gels for pills,
cross-linked gels often are used. Cross linking the gelling agent
polymer is thought to create a gel structure that is better able to
support solids and possibly, e.g., provide fluid-loss control.
Further, cross-linked pills are thought to invade the formation
face to a lesser extent to be desirably effective. To crosslink
these gelling agents, a suitable cross linking agent that comprises
polyvalent metal ions is often used. Complexes of aluminum,
titanium, boron, and zirconium are common examples.
[0007] A disadvantage associated with conventional cross-linked
gelling agents is that the resultant gel residue is often difficult
to remove from the subterranean formation once the treatment has
been completed. For example, in fracturing treatments, the
cross-linked gels used are thought to be difficult to completely
clean up with conventional breakers, such as oxidizers or enzymes.
Similarly, the gel residue can be difficult and time-consuming to
remove from the subterranean formation. The gel residue, at some
point in the completion operation, usually should be removed to
restore the formation's permeability, preferably to at least its
original level. If the formation permeability is not restored to
its original level, production levels can be significantly reduced.
This gel residue often requires long cleanup periods. Moreover, an
effective cleanup usually requires fluid circulation to provide
high driving force, which is thought to allow diffusion to take
place to help dissolve the concentrated buildup of the gel residue.
Such fluid circulation, however, may not be feasible. Additionally,
in lower temperature wells (i.e., those below about 80.degree. F.),
it is often difficult to find an internal breaker for the
viscosified treatment fluids that will break the gel residue
effectively. The term "break" (and its derivatives) as used herein
refers to a reduction in the viscosity of the viscosified treatment
fluid, e.g., by the breaking or reversing of the crosslinks between
polymer molecules or some reduction of the size of the gelling
agent polymers. No particular mechanism is implied by the term.
Another conventional method of cleaning up gel residue is to add a
spot of a strong acid (e.g., 10% to 15% hydrochloric acid) with
coiled tubing, which is expensive and can result in hazardous
conditions.
[0008] New developments in cleaning and removing filter cakes left
by fluid loss control additives and pills include materials that
degrade under acidic conditions such as calcite. While such
techniques can be effective, they require good contact between the
acid generating compound and the acid soluble compound, which is
not always easily achieved.
[0009] Another problem presented by today's cross-linked gelling
agent systems with respect to cleanup is that the high temperature
of the formations (e.g., bottom hole temperatures of about
200.degree. F. or greater) often require cross linking agents that
are more permanent, and thus harder to break. Examples include
transition metal cross linking agents. These more permanent cross
linking agents can make cleanup of the resulting gel residue more
difficult.
SUMMARY OF THE INVENTION
[0010] The present invention relates to methods and compositions
useful in subterranean applications, and more specifically, to
stimuli-degradable.
[0011] In one embodiment, the present invention provides a method
comprising: providing a treatment fluid comprising an aqueous base
fluid, and a stimuli-degradable gel formed by a combination of a
gelling agent, and a stimuli-degradable cross linking agent
described by the following formula:
R.sup.1-[A]-[R.sup.3]--[B]--R.sup.2
wherein R.sup.1 and R.sup.2 may be the same or different, and are
selected from the group consisting of: substituted or unsubstituted
ethylenically unsaturated groups, N-acryl, O-acryl, acrylol, vinyl,
allyl, melamide, and derivatives or combinations thereof; A and B
are optional bridging units; and R.sup.3 is a degradable group or
polymer; and placing the treatment fluid into a subterranean
formation.
[0012] In one embodiment, the present invention provides a method
comprising: providing a treatment fluid comprising an aqueous
fluid, and a stimuli-degradable gel formed by a reaction comprising
a gelling agent, and a stimuli-degradable cross linking agent that
includes at least one degradable group and two unsaturated terminal
groups; placing the treatment fluid into a subterranean formation;
and allowing the stimuli-degradable gel to degrade.
[0013] In one embodiment, the present invention provides a method
of providing fluid loss control in a subterranean application
comprising the steps of: providing degradable crosslinked gelled
particles that are formed by a reaction comprising a gelling agent,
and a stimuli-degradable cross linking agent that includes at least
one degradable group and two unsaturated terminal groups;
introducing the degradable crosslinked gelled particles into a
subterranean formation; and allowing the degradable crosslinked
gelled particles to reduce the loss of fluid to a portion of the
subterranean formation.
[0014] The features and advantages of the present invention will be
apparent to those skilled in the art. While numerous changes may be
made by those skilled in the art, such changes are within the
spirit of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] These drawings illustrate certain aspects of some of the
embodiments of the present invention, and should not be used to
limit or define the invention.
[0016] FIG. 1 illustrates a graph of data referenced in the
Examples section, which shows fluid loss for a system of starch
(1.0%) and xanthan (0.5%) with and without gel balls at 500 psi
through rock of permeability of approximately 30 mD. Lines 1 and 1
repeat show data without gel balls. Lines 2 and 2 repeat: show data
with gel balls.
[0017] FIG. 2 illustrates a graph of data referenced in the
Examples section, which shows fluid loss of polyacrylamide(PAm)
gels of various gel strengths and compositions through calcite
filter cake, measured at 500 psi for 5 minutes followed at 1000 psi
for a further 5 minutes. Line 1 shows 30% acrylamide in gel with 5%
cross linker on PAm and 5% volume fraction of gel in fluid,
measured at 500 psi only. Line 2 shows 10% acrylamide in gel, 5%
cross linker on PAm, and 5% volume fraction of gel in fluid at 500
psi only. Line 3 shows 3% acrylamide in gel, 5% cross linker on
PAm, and 5% volume fraction of gel in fluid, measured at 500 and
1000 psi for 5 minutes each. Line 4 is a repeat of line 3.
[0018] FIG. 3 illustrates a graph of data referenced in the
Examples section, which shows a comparison of fluid loss from
systems of gel particles (of 5% PAm-acrylic acid gel) at various
volume fractions with a borate crosslinked guar system. These were
measured at 500 psi for 5 minutes, followed by 1000 psi for a
further 5 minutes. Line 1 shows 20% PAm-AA phase volume of gel
particles. Line 2 shows a borate cross linked guar (0.5%) system.
Line 3 shows 10% PAm-AA phase volume of gel particles.
[0019] FIG. 4 illustrates a graph of data referenced in the
Examples section, which illustrates an effect of size of
poly(acrylamide-acrylic acid) gel particles on fluid loss through a
calcite filter cake when measured at 500 psi for 5 minutes followed
by 1000 psi for a further 5 minutes. Line 1 shows PAm-AA gel
particles of 100-200 microns average size. Line 2 shows a repeat of
Line 1. Line 3 shows PAm-AA gel particles of 50-100 microns average
size. Line 4 shows a repeat of Line 3
[0020] FIG. 5 illustrates a graph of data referenced in the
Examples section, which illustrates an effect of cross link density
of polyacrylamide gel particles on the fluid loss of these
particles through calcite filter cakes measured at 500 psi for 5
minutes followed by 1000 psi for a further 5 minutes. Line 1 shows
10% acrylamide in the gel, 5% cross linker, and 5% phase volume of
gel in fluid. Line 2 shows 10% acrylamide in gel, 3% cross linker,
and 5% phase volume of gel in fluid. Line 3 shows 10% acrylamide in
gel, 1% cross linker, and 5% phase volume of gel in fluid.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] The present invention relates to methods and compositions
useful in subterranean applications, and, more specifically, to
stimuli-degradable gels.
[0022] The stimuli-degradable gels of the present invention can be
used in any application in which it is desirable to have a
stimuli-degradable gel. Suitable subterranean applications in which
these stimuli-degradable gels can be used include pills (such as
fluid loss control pills), fracturing fluids, temporary plugs (for
example, in tubing), temporary sealing materials (e.g., in
screens), drilling fluids, and drill-in fluids. They may be also
used as fluid loss control agents when made in smaller forms.
[0023] In a subterranean application context, one of the desirable
features of the stimuli-degradable gels of the present invention is
that after a delay period, they degrade as a result of the
degradation of their acid-degradable crosslinks, which allows the
gel to break up into smaller components that should not negatively
impact well productivity or the flow of fluids through the rock. In
preferred embodiments, the smaller components should not impact
well productivity to an appreciable extent. This time-triggered
self-degradation may allow for the use of some equipment to be
avoided (e.g., coiled tubing to acid spot for gel residue cleanup),
thus reducing the overall cost of a well treatment.
[0024] Stimuli that may lead to the degradation of the gels of the
present invention include any change in the condition or properties
of the gel, such as a change in pH (e.g., caused by the buffering
action of the rock or the decomposition of materials that release
chemicals such as acids), or a change in the temperature of the
fluid (e.g., caused by the contact of the fluid with the rock
formation). In some respects, the stimuli can be considered a
function of the rock formation; in a sense, at least in some
circumstances, the rock formation can affect the degradation of the
gel, thus, increasing the reliability of the application. The
continuous rate of degradation of the stimuli-degradable gels may
be affected by pH and temperature. For instance, their acid
degradable crosslinks will degrade more rapidly as their
environment becomes more acidic, being relatively stable at higher
pHs (e.g., a pH of above about 10) but relatively unstable at lower
pHs (e.g., a pH of less than about 9) at ambient temperatures. At
higher temperatures, the crosslinks may degrade more quickly; at
lower temperatures, less quickly. (Also, in some embodiments, at a
pH of about 13 or greater with heat, the crosslinks may degrade at
an appreciable level.) Thus, a pH change in the treatment fluid can
trigger the degradable crosslinks in the stimuli-degradable gels to
degrade. Once the degradable crosslinks degrade, the
stimuli-degradable gel breaks up into smaller molecules that, in
preferred embodiments, should be water soluble or, at least, water
dispersible. In subterranean applications, these smaller molecules
should not be in situ impediments to produced fluids. The terms
"degrade" and "degradation" (and their derivatives) as used herein
refer to the continuous loss of gel properties, characterized by a
decrease in the elastic modulus (G') of the gelled system.
[0025] To form the stimuli-degradable gels of the present
invention, degradable cross linking agents may be used to crosslink
gelling agents that are formed from reactions comprising
"ethylenically unsaturated monomers" that include substituted or
unsubstituted ethylenically unsaturated monomer groups, vinyl
groups, allyl groups, acryl groups, melamide groups, and acryloyl
groups, and mixtures thereof. In certain embodiments, suitable
gelling agents that may be used in conjunction with the
stimuli-degradable cross linking agents of the present invention
are made from reactions comprising ethylenically unsaturated
monomers of the general formula CH.sub.2.dbd.CXY, wherein X and Y
may be hydrogen, alkyls, aryls, alkoxy, carboxylic acids, amides,
acetamides, esters, ethers, and the like. Suitable examples
include, but are not limited to, ethylene, propylene, butene-1,
vinyl cyclohexane, styrene, vinyl toluene, ionizable monomers (such
as 1-N,N-diethylaminoethylmethacrylate), diallyldimethylammonium
chloride, 2-acrylamido-2-methyl propane sulfonate, and acrylic
acid, and mixtures or derivatives thereof; allylic monomers (such
as di-allyl phthalate, di-allyl maleate, allyl diglycol carbonate,
and the like); vinyl formate, vinyl acetate, vinyl propionate,
vinyl butyrate, crotonic acid, itaconic acid, vinyl fluoride, vinyl
chloride, vinylidine fluoride, tetrafluoroethylene, acrylamide and
its derivatives, methacrylamide, methacrylonitrile, acrolein,
methyl vinyl ether, ethyl vinyl ether, vinyl ketone, ethyl vinyl
ketone, allyl acetate, allyl propionate, and diethyl maleate; and
diene monomers (such as butadiene, isoprene, and chloroprene,
etc.); and mixtures or derivatives thereof. The term "group" as
used herein refers to a combination of bonded atoms.
[0026] The cross linking reactions can be through a
copolymerization reaction. The stimuli-degradable gels should be
suitable for use at temperatures that they will encounter during
subterranean operations. One of ordinary skill in the art, with the
benefit of this disclosure, should be able to determine the
appropriate degradable cross linking agent to use to form the
stimuli-degradable gel based on, among other things, bottom hole
temperatures that may be encountered. For instance, under
moderately acidic conditions (pH of about 3), the stability of
amides, ketals, and orthoesters is thought to decrease, in the
order of amides>ketals>orthoesters.
[0027] The polymerization of the monomers can be done by any known
methods such as free radical polymerization, cationic
polymerization, anionic polymerization, condensation
polymerization, coordination catalyst polymerization, and hydrogen
transfer polymerization. The polymerization can be done in any
manner, e.g., solution polymerization, precipitation
polymerization, suspension polymerization, emulsion polymerization,
and bulk polymerization; these are known methods described in the
literature. Which particular method to use may depend on, inter
alia, the gelling agent monomer and the cross linking agent used,
and also the application for the resultant gel. In preferred
embodiments, the stimuli-degradable cross linking agent is added to
the gelling agent at the time of polymerization of the gelling
agent monomers. This polymerization can be conducted in any manner
suitable. Suitable temperatures and other conditions are well
known.
[0028] Gelling agent monomers may be present in an amount of from
about 1% to about 50% of the solution, and the cross linking agent
may be present in an amount of from about 0.1% to about 15% of the
monomer concentration. A preferred amount of the cross linking
agent may be from about 0.5% to about 10% of the monomer
concentration. In other embodiments, a stimuli-degradable cross
linking agent may be added to the gelling agent after
polymerization.
[0029] The stimuli-degradable cross linking agents include at least
one degradable group, and two unsaturated terminal groups. In some
embodiments, the cross linking agents of the present invention can
be described by the following general formula:
R.sup.1-[A]-[R.sup.3]--[B]--R.sup.2
wherein R.sup.1 and R.sup.2 represent two groups which may be the
same or different, and are selected from substituted or
unsubstituted ethylenically unsaturated groups, N-acryl, O-acryl,
acryloyl, vinyl, allyl, and maleimide, and derivatives or
combinations thereof, that are capable of polymerizing with the
monomers of the gelling agents. A and B optionally are extra groups
to aid compatibility of the cross linking groups with the reaction
solvent. A and B are bridging units that are relatively unreactive
with the other molecules to be cross-linked, and have
functionalities that are compatible with the terminal groups. A and
B may include peptide chains, aromatic substituents, alkyl chains,
or polar groups to make the cross linking agent compatible with the
reaction solvent and monomers forming the gelling agent. A and B
may be tailored to change the properties of a particular embodiment
of the cross linking agents of the present invention, e.g., to make
it soluble in water or organic solvents, which may be important
depending on the polymerization medium. R.sup.3 can be a degradable
group or a polymer.
[0030] In other embodiments, the degradable group may include any
degradable group or plurality of groups including, but not limited
to, esters, phosphate esters, amides, acetals, ketals, orthoesters,
carbonates, anhydrides, silyl ethers, alkene oxides, ethers,
imines, ether esters, ester amides, ester urethanes, carbonate
urethanes, and amino acids, and derivatives or combinations
thereof. The choice of the degradable group may be determined by
pHs and temperatures, the details of which are available in known
literature sources. The unsaturated terminal group may include
substituted or unsubstituted ethylenically unsaturated groups,
vinyl groups, allyl groups, acryl groups, or acryloyl groups, which
are capable of undergoing polymerization with the above-mentioned
gelling agents to form cross-linked stimuli-degradable gels.
Examples include, but are not limited to, unsaturated esters such
as acrylates, methacrylates, and butyl acrylates; amides such as
acrylamide; and ethers such as vinyl ether; and combinations
thereof. In one embodiment, a stimuli-degradable cross linking
agent comprises a degradable crosslink and two vinyl groups. Some
embodiments of these cross linking agents of the present invention
are sensitive to changes in pH, such as ortho ester-based
embodiments, acetal-based embodiments, ketal-based embodiments, and
silicon-based embodiments. Generally speaking, at room temperature,
the ortho ester-based embodiments should be stable at pHs of above
10, and should degrade at a pH below about 9; the acetal-based
embodiments should be stable at pHs above about 8 and should
degrade at pH below about 6; the ketal-based embodiments should be
stable at pHs of about 7 and should degrade at a pH below 7; and
the silicon-based embodiments should be stable at pHs above about 7
and should degrade faster in acidic media. Thus, under moderately
acidic conditions (pH of around 3), the relative stability of these
groups should decrease in the following order:
amides>ketals>orthoester. At higher well bore temperatures,
the more stable cross linking groups contain amides or ethers and
would be preferred over other choices including esters, acetals,
and ketals.
[0031] Also, some embodiments of the cross linking agents are
sensitive to changes in temperature. Thus, where R.sup.3 (in the
formula above) is an ester group, the cross-linking agent may
degrade at 170.degree. F. in about 10 hours at pH 10.8, whereas
when R.sup.3 is an amide, the cross-linking agent may be stable for
several days at pH 10.8 and 185.degree. F.
[0032] The ester embodiments of the cross linking agents can be
described as formed when any di, tri, or more functional alcohols
react with unsaturated acids or acid chlorides. Examples include:
poly(ethylene glycol) diacrylate, poly(ethylene glycol)
dimethacrylate, poly(propylene glycol) diacrylate, and hexanediol
acrylate. Some ether embodiments include: poly(ethylene glycol)
divinyl ether, and 1,4-cyclohexane dimethanol divinyl ether; some
amide embodiments include poly(ethylene glycol) bisacrylamide, and
N,N'-(1,2dihydroxyethylene) bisacrylamide.
N,O-dimethacryloylhydroxylamine is a relatively acid stable
cross-linking agent that should decompose more rapidly above pH
6.5, when formed as described U.S. Pat. No. 5,124,421.
[0033] An example of a cross linking agent suitable for use in the
present invention is a short chain poly(lactic acid) substituted
with an acrylate group on the two ends of the chain.
[0034] In certain embodiments, suitable orthoester cross linking
agents may be synthesized from diketene acetals or multiketene
acetals by the addition of two (in the case of a diketene acetal)
or more mole equivalents (in the case of a multiketene acetal) of a
monohydric alcohol containing ethylenically unsaturated monomers,
acrylic groups, vinylic groups, or allylic groups that are suitable
for polymerization with the monomers already described.
[0035] Examples of suitable diketene, or multiketene, acetals may
be synthesized as described in U.S. Pat. No. 4,304,767, U.S. Pat.
No. 6,822,000, and United States Patent Application Publication No.
2004/0096506, the relevant disclosures of which are incorporated
herein by reference. In one embodiment, as illustrated below in
Reaction Scheme 1, a diketene acetal may be synthesized by reacting
pentaerythritol and chloroacetaldehyde dimethyl acetal in the
presence of p-toluenesulfonic acid or methanesulfonic acid to
afford 2, which on dehydrohalogenation in presence of t-butoxide in
t-butanol afford diketene acetal 3, and a suitable orthoester cross
linking agent 4 may be synthesized by reacting the resultant
diketene acetal with two mole equivalent of the
N-methylolacrylamide in the presence of a small amount of iodine
dissolved in pyridine. In some embodiments, the orthoester cross
linking agent may be synthesized by mixing the monohydric alcohol
containing ethylenically unsaturated groups with the diketene
acetal, without the aid of an iodine/pyridine catalyst, provided
the alcohols and diketene acetals are extremely pure.
##STR00001##
[0036] Suitable degradable cross linking agents may be made to have
a balance between hydrophobic or hydrophilic characteristics by
using various kinds of mono alcohols. A water soluble degradable
cross linking agent may be desirable, for instance, in a reaction
of a gelling agent polymer in an aqueous medium and an organic
solvent soluble cross linking agent for the polymerization reaction
in a nonaqueous medium. In the design of the bisacrylamide
orthoester cross linking agent, for example the acrylol alcohol,
can be chosen based on certain factors, such as ease of synthesis,
solubility, and the type of hydrogel or microparticle desired. The
addition of N-methyloacrylamide to diketene acetal 3 should produce
a water soluble cross linking agent, which may be more useful in an
aqueous polymerization reaction. The cross linking agent can also
be made to be soluble in organic solvents by incorporation of
additional alkyl or methylene groups in the chain of the molecule.
An orthoester cross linking agent prepared by the addition of
2-hydroxyethyl methacrylate to diketene acetal 3 resulted in the
formation of a water insoluble cross linking agent 5 (Scheme 2).
The cross linking agent 5 contains an ester group which should
undergo hydrolysis at higher pH, and may be more suitable for lower
temperature applications.
##STR00002##
[0037] Monohydric alcohols that contain ethylenically unsaturated
groups can be any alcohol capable of reacting with the diketene
acetal or multiketene acetal to form an orthoester cross linking
agent. Exemplary alcohols suitable as reactants include
hydroxyethylacrylate; hydroxypropyl methacrylamide; hydroxybutyl
methacrylate; and glycerol monomethacrylate.
[0038] An example of a bisacrylamide orthoester cross linking agent
is shown in Reaction Scheme 3 which may be used to form (e.g., by
free radical polymerization reaction) an acrylamide cross-linked
polymer, which may then degrade according to the reaction sequence
shown in Reaction Scheme 3, (however, one should note that at
higher pHs (e.g., about 13) the bisacrylamide orthoester cross
linking agent may degrade by another mechanism, e.g., through amide
bond cleavage):
##STR00003##
[0039] In certain embodiments, suitable orthoester cross linking
agents may be synthesized by reacting, in one or more steps, a low
molecular weight orthoester of Formula I, with a monohydric alcohol
that contains ethylenically unsaturated groups in accordance with
the scheme illustrated in Scheme 4.
##STR00004##
wherein R is H, CH.sub.3, or C.sub.2H.sub.5, and R.sub.4 is an
alkyl group having from about 1 to about 6 carbon atoms. Examples
of suitable low molecular weight orthoesters of Formula I include,
but may not be limited to, trimethyl orthoformate, trimethyl
orthoacetate, triethyl orthoformate, triethyl orthoacetate,
tripropyl orthoformate, and tripropyl orthoacetate. Low molecular
weight orthoesters may be used due to the ease of
transesterification undergone by these molecules with high
molecular weight alcohols. Because the trimethyl orthoformate
molecule has three positions that may be substituted by the
reactants, the product of the reaction depicted in Reaction Scheme
4 can be made by either attaching two groups or three groups.
##STR00005##
[0040] Suitable cross linking agents also may be silicon-based. An
example is an acid labile dimethacrylate cross linking agent shown
in Reaction Scheme 5. Dimethyldi(methacryloyloxy-1-ethoxy)silane
may be synthesized by reaction of 2-hydroxyethyl methacrylate
(HEMA) and dichlorodiethyl silane in the presence of triethylamine,
which can be copolymerized with the gelling agents of the present
invention to form cross-linked stimuli-degradable gels. These
cross-linked gels can be easily broken in acidic media.
##STR00006##
[0041] While the stimuli-degradable gels of the present invention
included in the treatment fluids of the present invention are
generally degradable, it may be desired, in some embodiments, for a
faster degradation. Therefore, in some embodiments, to facilitate
the degradation of the cross-linked polymer, and thus degrade the
gel or gel particles, the pH of the treatment fluid may be
decreased at a desired time. For example, in an orthoester
cross-linked embodiment at a pH of about 8 or less, the orthoester
crosslinks should degrade at reasonable rates. In subterranean
applications, the buffering action of the formation together with
temperature may, in some embodiments, provide the desired
degradation.
[0042] Acetal cross linking agents can be made in many ways
suitable for cross linking with the gelling agents that can be
hydrolyzed in mild acidic conditions. Suitable cross linking agents
based on bisacryloyl acetal moiety are described in United States
Patent Application Publication No. 2003/0211158, the disclosure of
which is herein incorporated by reference. These cross linking
agents can be tuned to be water-soluble or -insoluble, depending on
bridging substituents and attached groups in the molecule. A
general procedure to synthesize an acetal is to react an aldehyde
with alcohol. For synthesizing an embodiment of an acetal cross
linking agent of the present invention, we can react an aromatic
aldehyde with a monohydric alcohol containing ethylenically
unsaturated groups in the presence of an acid catalyst. In some
cases the ethylenically unsaturated groups can be added after the
reaction of the alcohol with the aldehyde, as shown in United
States Patent Application Publication No. 2003/0211158. In addition
to the acetals already described above as being suitable,
bisacrylamide acetals, others are also suitable, including diketene
acetals that have a functionality of two or more (i.e., two or more
unsaturated groups), as described in U.S. Pat. No. 4,304,767 and
United States Patent Application Publication No. 2003/0211158 A1,
the disclosures of which are incorporated herein by reference.
[0043] Suitable ketal cross linking agents are described in U.S.
Pat. No. 5,191,015, and are described in Reaction Scheme 6.
##STR00007##
[0044] Because the degradable cross linking agents have a
degradable group, degradation of this bond in the
stimuli-degradable gel once formed should at least partially result
in a degradation of the gel. The degradable group is capable of
undergoing an irreversible degradation. The term "irreversible," as
used herein, means that a degradable cross linking agent or a
stimuli-degradable gel of the present invention should degrade in
situ (e.g., within a well bore) but should not reform in situ after
degradation. The terms "degradation" and/or "degradable," as used
herein, refer to the conversion of materials into smaller
components, intermediates, or end products by chemical processes
such as hydrolytic degradation or by the action of biological
entities, such as bacteria or enzymes. It refers to both
heterogeneous (or bulk erosion) and homogenous (or surface
erosion), and any stage of degradation between these two by action
of water on the degradable group. This degradation may be the
result of, inter alia, a chemical reaction, a thermal reaction, an
enzymatic reaction, or a reaction induced by radiation. The
degradability of the stimuli-degradable gel used in the methods of
the present invention depends, at least in part, on the backbone
structure of the cross linking agent. For instance, the presence of
hydrolysable and/or oxidizable linkages in the backbone often
yields a degradable cross linking agent that will degrade as
described herein. The rates at which such cross linking agents
degrade are dependent on the environment to which the degradable
cross linking agent and/or stimuli-degradable gel is subjected,
e.g., temperature, the presence of moisture, oxygen,
microorganisms, enzymes, pH, and the like may affect the rate of
degradation.
[0045] Among other things, as stated above, degradation of the
cross linking agent may be sensitive to pH and temperature.
Generally speaking, with an increase in temperature, the hydrolysis
of the degradable group should be faster. To reduce the pH of the
treatment fluid at a desired time, a number of methods may be
employed. In some embodiments, the treatment fluid may be contacted
by an acid after introduction of the treatment fluid into the
subterranean formation. Examples of suitable acids include, but are
not limited to, hydrochloric acid, hydrofluoric acid, formic acid,
phosphoric acid, sulfamic acid, and acetic acid, and derivatives
thereof, and mixtures thereof. In other embodiments, a
delayed-release acid, such as an acid-releasing degradable material
or an encapsulated acid, may be included in the treatment fluid so
as to reduce the pH of the treatment fluid at a desired time, for
example, after introduction of the treatment fluid into the
subterranean formation. Suitable encapsulated acids that may be
included in the treatment fluids of the present invention include,
but are not limited to, fumaric acid, formic acid, acetic acid,
acetic anhydride, anhydrides, hydrochloric acid, and hydrofluoric
acid, and combinations thereof, and the like. Exemplary
encapsulation methodology is described in U.S. Pat. Nos. 5,373,901;
6,444,316; 6,527,051; and 6,554,071, the relevant disclosures of
which are incorporated herein by reference. Acid-releasing
degradable materials also may be included in the treatment fluids
of the present invention to decrease the pH of the fluid. Suitable
acid-releasing degradable materials that may be used in conjunction
with the present invention are those materials that are
substantially water-insoluble such that they degrade over time,
rather than instantaneously, to produce an acid. Examples of
suitable acid-releasing degradable materials include esters,
polyesters, orthoesters, polyorthoesters, lactides, polylactides,
glycolides, polyglycolides, substituted lactides wherein the
substituted group comprises hydrogen, alkyl, aryl, alkylaryl, and
acetyl, and mixtures thereof, substantially water- insoluble
anhydrides, and poly(anhydrides), and mixtures and copolymers
thereof. Materials suitable for use as an acid-releasing degradable
material of the present invention may be considered degradable if
the degradation is due, inter alia, to chemical processes, such as
hydrolysis, oxidation, or enzymatic decomposition. The appropriate
pH-adjusting agent or acid-releasing material and amount thereof
may depend upon the formation characteristics and conditions, the
particular orthoester-based surfactant chosen, and other factors
known to individuals skilled in the art, with the benefit of this
disclosure.
[0046] In most embodiments, the stimuli-degradable cross linking
agents suitable for use in the present invention should be
relatively easy to synthesize in large amounts, and should have
good stability for long-term storage, especially in anhydrous
conditions.
[0047] In one embodiment, a cross-linked gelling agent that has
been cross-linked with a cross linking reaction comprising a
stimuli-degradable cross linking agent may be added to an aqueous
treatment fluid (e.g., a pill, a fracturing fluid, or a gravel pack
fluid), and then introduced into a subterranean formation. Suitable
aqueous treatment fluids include freshwater, salt water, brine,
seawater, or any other aqueous liquid that does not adversely react
with the other components used in accordance with this invention or
with the subterranean formation.
[0048] In some embodiments, the treatment fluid may be foamed. One
advantage of using a foamed treatment fluid over a non-foamed
version is that less of the aqueous fluid is used, relatively
speaking. This may be important in subterranean formations that are
water-sensitive or under pressure. In some embodiments, the foamed
treatment fluids have a foam quality of about 30% or above. These
may include commingled fluids. A preferred foam quality level is
about 50% or above.
[0049] In some embodiments wherein the treatment fluid is foamed,
the treatment fluid may comprise a surfactant. The choice of
whether to use a surfactant will be governed at least in part by
the mineralogy of the formation. As will be understood by those
skilled in the art, anionic, cationic, nonionic, or amphoteric
surfactants also may be used so long as the conditions they are
exposed to during use are such that they display the desired
foaming properties. For example, in particular embodiments,
mixtures of cationic and amphoteric surfactants may be used. When
used in treatment fluid embodiments, the surfactant is present in
an amount of from about 0.01% to about 5% by volume. When foamed,
the base fluid may comprise a gas. While various gases can be
utilized for foaming the treatment fluids of this invention,
nitrogen, carbon dioxide, and mixtures thereof are preferred. In
examples of such embodiments, the gas may be present in a base
fluid and/or a delayed tackifying composition in an amount in the
range of from about 5% to about 95% by volume, and more preferably
in the range of from about 20% to about 80%. The amount of gas to
incorporate into the fluid may be affected by factors including the
viscosity of the fluid and bottomhole pressures involved in a
particular application. Examples of preferred foaming agents that
can be utilized to foam the base fluid and/or the delayed
tackifying composition of this invention include, but are not
limited to, alkylamidobetaines such as cocoamidopropyl betaine,
alpha-olefin sulfonate, trimethyltallowammonium chloride, C.sub.8
to C.sub.22 alkylethoxylate sulfate and trimethylcocoammonium
chloride. Cocoamidopropyl betaine is especially preferred. Other
suitable surfactants available from Halliburton Energy Services
include: "19N.TM.," "G-Sperse.TM. dispersant," "Morflo III.RTM."
surfactant, "Hyflo.RTM. IV M" surfactant, "Pen-88M.TM." surfactant,
"HC-2.TM. Agent," "Pen-88 HT.RTM." surfactant, "SEM-7.TM."
emulsifier, "Howco-Suds.TM." foaming agent, "Howco Sticks.TM."
surfactant, "A-Sperse.TM." dispersing aid for acid additives,
"SSO-21E.TM." surfactant, and "SSO-21MW.TM." surfactant. Other
suitable foaming agents and foam-stabilizing agents may be included
as well, which will be known to those skilled in the art with the
benefit of this disclosure. The foaming agent is generally present
in a treatment fluid of the present invention in an amount in the
range of from about 0.01% to about 5%, by volume, more preferably
in the amount of from about 0.2% to about 1%, and most preferably
about 0.6% by volume.
[0050] Optionally, the treatment fluid may comprise a second
gelling agent. Any gelling agent suitable for use in subterranean
applications may be used in these treatment fluids, including, but
not limited to, natural biopolymers, synthetic polymers,
cross-linked gelling agents, viscoelastic surfactants, and the
like. Guar and xanthan are examples of suitable gelling agents. A
variety of gelling agents may be used, including hydratable
polymers that contain one or more functional groups such as
hydroxyl, carboxyl, sulfate, sulfonate, amino, or amide groups.
Suitable gelling agents typically comprise polysaccharides,
biopolymers, or synthetic polymers, or a combination thereof.
Examples of suitable polymers include, but are not limited to, guar
gum and derivatives thereof, such as hydroxypropyl guar and
carboxymethylhydroxypropyl guar, cellulose derivatives, such as
hydroxyethyl cellulose, locust bean gum, tara, konjak, tamarind,
starch, karaya, diutan, scleroglucan, wellan, gellan, xanthan,
tragacanth, and carrageenan, and derivatives of all of the above.
Additionally, synthetic polymers and copolymers may be used.
Examples of such synthetic polymers include, but are not limited
to, polyacrylate, polymethacrylate, polyacrylamide, polyvinyl
alcohol, and polyvinylpyrrolidone. In other exemplary embodiments,
the gelling agent molecule may be depolymerized. The term
"depolymerized," as used herein, generally refers to a decrease in
the molecular weight of the gelling agent molecule. Depolymerized
gelling agent molecules are described in U.S. Pat. No. 6,488,091,
issued Dec. 3, 2002 to Weaver, et al., the relevant disclosure of
which is incorporated herein by reference. Suitable gelling agents
generally may be present in the compositions of the present
invention in an amount in the range of from about 0.1% to about 5%
by weight of the water therein.
[0051] Combinations of surfactants may be used in the present
invention so that they form elongated or rod-like micelles or
structures that can control the viscosity of a well bore treatment
fluid. While these systems may lead to good filter cake cleanup,
their fluid loss control power may be considered poor. However,
addition of stimuli-degradable gel particles to viscoelastic
surfactants gives much improved fluid loss control while
maintaining good filter cake removal. Combinations of surfactants
that have an average packing factor of between about 1/3 to 1/2 are
thought to give good viscosity control. Examples include
combinations of betaines and fatty acids.
[0052] If a second gelling agent is used, a suitable breaker may be
necessary to ultimately reduce the viscosity of the fluid to a
desirable extent or any undesirable resulting gel residue. Any
breaker suitable for use in the subterranean formation and with the
gelling agent may be used. The amount of a breaker to include will
depend, inter alia, on the amount of gelling agent present in the
treatment fluid or the amount of gel residue present in the
formation. Other considerations regarding the breaker are known to
one skilled in the art with the benefit of this disclosure.
[0053] A treatment fluid may comprise proppant or gravel
particulates, as needed. Any proppant or gravel particulates that
are suitable for use in subterranean applications may be used in
the treatment fluids of the present invention.
[0054] To delay the degradation of a degradable crosslink or a
stimuli-degradable gel, an inhibitor may be included in the gel.
Suitable inhibitors include bases. Examples of some preferred
inhibitors may include metal hydroxides, potassium hydroxide,
amines such as hexamethylenetetramine, and sodium carbonate, and
combinations thereof. In certain embodiments, a small amount of a
strong base as opposed to a large amount of a relatively weak base
is preferred to achieve the delayed degradation.
[0055] In some embodiments, the stimuli-degradable gels of the
present invention may be used as or in conjunction with fluid loss
control pills. A "fluid-loss control pill" is a gelled fluid that
is designed or used to provide some degree of fluid-loss control.
Through a combination of viscosity, solids bridging, and cake
buildup on the porous rock, these pills oftentimes are able to
reduce fluid loss from portions of a formation. They also generally
enhance filter-cake buildup on the face of the formation to inhibit
fluid flow into the formation from the well bore.
[0056] In some embodiments, stimuli-degradable cross-linking agents
may be used to form degradable cross-linked fluid loss control
agents that comprise stimuli- degradable gel particles. These fluid
loss control agents can be added to any treatment fluid wherein it
is desirable to control fluid loss.
[0057] Two methods of making stimuli-degradable gel particles
include forming an emulsion, or forming a large gel and chopping up
the cross-linked polymer. An embodiment of the emulsion method
consists of forming a water-in-oil emulsion with appropriate
surfactants, an appropriate initiator, a stimuli-degradable cross
linking agent, and chosen monomers to form the gelling agent. This
emulsion can be heated to initiate polymerization in the water
phase. When polymer formation is complete, the gel particles can be
recovered by inversion of the emulsion. Alternatively, a
macroscopic gel can be formed by copolymerizing the cross linking
agent with the gelling agent monomers in an appropriate solvent.
The resultant gel can then be chopped up into smaller particles as
desired using a high speed shearing device such as a Waring
blender. A pourable dispersion of gel particles should result.
[0058] Although this invention has been described in terms of some
specific uses of the stimuli-degradable cross linking agents and
stimuli-degradable gels of the present invention, these may be used
in other applications, as recognized by one of skill in the art
with the benefit of this disclosure.
[0059] To facilitate a better understanding of the present
invention, the following examples of certain aspects of some
embodiments are given. In no way should the following examples be
read to limit, or define, the scope of the invention.
EXAMPLES
Example 1
[0060] The fluid loss efficiency of the stimuli-degradable gel
particles of the present invention was tested by comparing the
fluid loss of mixtures of starch and xanthan, with and without the
stimuli-degradable gel particles. The stimuli-degradable gel
particles were made from polyacrylamide and a bisacrylamide
stimuli-degradable cross linking agent. The particles were prepared
by making a water-in-oil emulsion, i.e., dispersing an aqueous
solution of acrylamide, the cross linking agent, and an initiator
in an oil phase, followed by polymerization within the dispersed
phase. The composition of the aqueous solution was as follows:
water (200 g), sodium chloride (15 g), acrylamide (50 g),
bisacrylamide cross linking agent (5 g), sodium carbonate (1 g),
and potassium persulfate (1 g). The oil phase was made up of
Norpar.TM. 12 oil (100 g) available from ExxonMobil at various
locations, xylene (100 g), and Hypermer.TM. surfactant (B246SF)
available from ICI Chemicals at various locations (2 g). A
water-in-oil emulsion was formed by shearing the mixture in a
Silverson emulsifier at 5000 rpm for 5 minutes. The ensuing water
droplets were approximately 10 microns in diameter. The
water-in-oil emulsion was then kept overnight at 60.degree. C. to
complete the polymerization process. Excess oil was removed by
centrifugation.
[0061] A starch component was then prepared. This starch component
was prepared by forming a dispersion of 100 g of WLC-4, which is a
modified starch fluid loss control component available from
Halliburton Energy Services, Duncan, Okla., in 500 g of water, and
then boiling the dispersion for 2 minutes to gelatinize the starch.
The thick solution was then freeze dried.
[0062] A xanthan component was then prepared. It was prepared by
dissolving food grade xanthan (available from Kelco at various
locations) in an aqueous solution at ambient temperature with
vigorous stirring.
[0063] The filtration mixture was prepared by first dispersing 5 g
of the stimuli-degradable gel particles in 500 g of water using the
surfactant NP(EO).sub.10.5, followed by the addition of 30 g of
potassium chloride, 5 g of pre-gelatinized WLC-4, and 2.5 g of food
grade xanthan.
[0064] Aqueous dispersions, with and without the stimuli-degradable
gel particles, were passed through discs of Berea sandstone (having
a permeability of 30 to 50 mD) under an applied pressure of 500 psi
and the rate of fluid loss measured. The resulting data is shown in
FIG. 1.
[0065] As can be seen from FIG. 1, it appears, inter alia, that the
addition of the stimuli-degradable gel particles reduces fluid loss
of the aqueous dispersion.
Example 2
[0066] To demonstrate the degradation of the stimuli-degradable gel
particles, samples of poly(acrylamide) [10% in water] cross-linked
with a bisacrylamide orthoester (4) stimuli-degradable cross
linking agent [10% w/w on the acrylamide monomer] were prepared
using potassium persulfate as the initiator. Small samples of these
gels were placed in 10 ml of buffer solutions having pH of 4, 7 and
10. These solutions were placed in a thermostat at 75.degree. C.
for 1 hour. The stimuli-degradable gel particles at pH 4 and 7 had
degraded, whereas the one at pH 10 remained intact, as would be
expected.
Example 3
[0067] Examples of application of stimuli-degradable gel particles
of the present invention as fluid loss agents.
[0068] Gels were prepared by forming a homogeneous solution of the
following: acrylamide monomer (M gm in 100 g water) with water (100
gm); ammonium persulfate (0.6 g); N,N,N'N' tetraethyl ethylene
diamine (TEED) (0.4 ml) and bisacrylamide cross linking agent (X%
of M). A known weight (V gm) of the cross-linked polyacrylamide
(PAm) gel that was formed was then added to (100-V) gm of water and
mechanically chopped, initially in a Waring blender at 2000 rpm for
1 min; then in a Silverson emulsifier at speeds varying from 4,000
to 10,000 rpm for 2 minutes. The resulting gel dispersion was then
flowed through a filter cake of solid particles such as silica or
calcite, prepared in the following way: calcite (10 g of 200 mesh)
particles were dispersed in 150 ml of water in a Waring blender and
then filtered at 30 psi pressure through a Whatman 42 filter paper
held in a standard high pressure, high temperature fluid loss cell.
About 5 mls of water were initially left covering the resulting
calcite filter cake, and these were removed by a pipette to avoid
cracking of the cake. From the flow rates of water through the
calcite cake, the permeability was found to be 11 mD. Filter cakes
prepared in this way are thought to resemble the formation of
sandstone rocks, and have been found to be much more reproducible
than natural or synthetic rocks.
[0069] The gel dispersion was then placed on a calcite cake and the
fluid loss was measured at pressures of 500 or 1000 psi and ambient
temperature. The results are shown in FIG. 2, which illustrates the
fluid loss of polyacrylamide gels through 200 mesh calcite filter
cakes at 500 and 1000 psi; 30M means that each gel particle, on
average, comprised 30% polyacrylamide; 5.times. means that the
cross linking agent was 5% by weight of the polymer in each gel
particle; 5% V means that the gel dispersion was 5% by volume of
the total dispersion. It is believed that for this particular
system of polyacrylamide gel particles (having various monomer
levels, 5% of the monomer as cross linking agent and 5% by volume
gel particles in the fluid), that fluid loss was reduced as the
monomer concentration in the gel was reduced.
Example 4
[0070] The fluid loss of the polymer gels was compared with a
standard borate cross-linked guar system, using 0.5% guar, a
calcite filter cake as in Example 3, and gels prepared from
mixtures of acrylamide ("AM") 80% by weight and acrylic acid ("AA")
20% by weight. The gels were prepared as in Example 3 and the fluid
loss measured in an HPHT cell at 500 and 1000 psi. The cross-linked
guar system was prepared by dissolving standard guar (0.5 g)
(obtained from Rhodia) in water (100 g), and adding sodium borate
as a cross linking agent at a pH of 10.5. The polymer gel was
chopped as in Example 3, though the guar gel was used without
chopping. The results are shown in FIG. 3, which illustrates a
comparison of the fluid loss of polymer gel particles with
cross-linked guar through calcite filter cakes at 500 and 1000 psi.
Pressure was increased to 1000 psi after 5 min. The results in FIG.
3(comparing 10% and 20% PAm-AA) show, as expected, that fluid loss
is reduced by having more gel particles in the fluid and that 20%
phase volume PAm-AA gave comparable fluid loss to a standard borate
cross-linked guar.
Example 5
[0071] To study the effect of particle size on fluid loss, a PAm
(80%)-AA(20%) gel was made having 10% M; 2% X. A 5% dispersion of
this gel in water was made and mechanically chopped in the
Silverson stirrer at speeds of 6000 rpm and 10,000 rpm, producing
particles in the range of 100-200 g and 50-100 a, respectively. The
fluid loss of these gel dispersions were measured on the calcite
filter cake as in Example 3 and the results are shown in FIG. 4,
where the pressure was increased from 500 to 1000 psi after 5
minutes. The sizes of the gel particles were measured by
microscopy, comparing the particles with a standard length on a
graticule. As shown in FIG. 4, the chopped PAm-AA gel particles
appeared to reduce fluid loss through the calcite filter cake (11
mD permeability) the fluid loss without the gel particles are about
200 mls after only 10 seconds at 500 psi. The results in FIG. 4
appear to indicate that the size of the gel particles may affect
the amount of fluid loss, with the smaller (hence more numerous)
particles appearing to give less fluid loss.
Example 6
[0072] The effect of cross-linking density of the PAm gel on the
fluid loss of a dispersion of it filtered through calcite filter
cake at 500/1000 psi was measured. The gel was chopped in a Waring
blender at 2000 rpm, and then through a Silverson at 6000 rpm for 2
min. The various cross linking densities are shown as values of X.
The effect of cross-linking density on the fluid loss of a
dispersion of PAm gel particles was measured when filtered through
a calcite filter cake. The results in FIG. 5 appear to show that
the cross-linking density has an effect on the fluid loss of a gel
dispersion, with the least fluid loss occurring with lower levels
of cross linking agent.
[0073] 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 or
modified, and all such variations are considered within the scope
and spirit of the present invention. 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 as referring to the
power set (the set of all subsets) of the respective range of
values, and set forth every 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.
* * * * *