U.S. patent application number 13/514593 was filed with the patent office on 2012-10-25 for synthesis of degradable polymers downhole for oilfield applications.
Invention is credited to Vadim Kamil'evich Khlestkin.
Application Number | 20120267111 13/514593 |
Document ID | / |
Family ID | 44226681 |
Filed Date | 2012-10-25 |
United States Patent
Application |
20120267111 |
Kind Code |
A1 |
Khlestkin; Vadim
Kamil'evich |
October 25, 2012 |
Synthesis of Degradable Polymers Downhole for Oilfield
Applications
Abstract
A method is given for polymerizing monomers downhole to create
well treatment polymers. The monomers each contain two
polymerizable groups and a degradable group or groups which allow
the polymer to degrade under downhole conditions, for example by
hydrolysis. Polymerization downhole allows easier, more precise,
placement of polymer. Polymer obtained from monomers pumped
downhole may be used, for example, for fluid diversion plugs,
isolation plugs, formation consolidation, flowback control, and
fluid loss control. Synthesized polymer may form gels, films,
solids or other structure in treated wellbores, fractures, and/or
formations. After the desired effect is achieved, the deposited
polymer degrades and the polymer degradation products dissolve,
leaving the wellbore, fracture and/or formation clean, with no
damage that might have decreased fluid flow.
Inventors: |
Khlestkin; Vadim Kamil'evich;
(Novosibirsk, RU) |
Family ID: |
44226681 |
Appl. No.: |
13/514593 |
Filed: |
December 30, 2009 |
PCT Filed: |
December 30, 2009 |
PCT NO: |
PCT/RU2009/000751 |
371 Date: |
July 1, 2012 |
Current U.S.
Class: |
166/305.1 |
Current CPC
Class: |
C09K 8/88 20130101; C09K
8/68 20130101; C09K 8/805 20130101; C09K 8/508 20130101; C09K 8/887
20130101 |
Class at
Publication: |
166/305.1 |
International
Class: |
E21B 43/25 20060101
E21B043/25 |
Claims
1. A method comprising pumping a fluid comprising a polymerizable
monomer downhole, initiating downhole polymerization, and allowing
decomposition of the resulting polymer under downhole
conditions.
2. The method of claim 1 used for downhole treatment.
3. The method of claim 1 or 2, wherein the monomer comprises at
least two polymerizable chemical groups and one or more degradable
chemical groups.
4. The method of claim 1 or 2, wherein the monomer comprises one or
more water-soluble groups linking the polymerizable and degradable
groups.
5. The method of claim 4, wherein the water soluble linking groups
comprise oligoethyleneglycol, glycerol, phosphate, or mixtures of
these.
6. The method of claim 1 or 2, wherein the monomer comprises one or
more precursors to water-soluble groups linking the polymerizable
and degradable groups.
7. The method of claim 1 or 2, wherein the polymerizable groups
comprise acrylic, methacrylic, aziridine, oxirane, isocyanate,
vinyl, and mixtures of these groups.
8. The method of claim 1 or 2, wherein the fluid comprises a
second, different, monomer comprising at least two polymerizable
chemical groups and one or more degradable chemical groups.
9. The method of claim 8, wherein the fluid comprises a
nucleophilic monomer and an electrophilic monomer.
10. The method of claim 1 or 2, wherein the fluid further comprises
acrylic acid, methacrylic acid, or both.
11. The method of claim 1 or 2, wherein the monomers for
polymerization are diacrylic ethers or anhydrides or
dimethylacrylic ethers or anhydrides.
12. The method of claim 1 or 2, wherein downhole polymerization of
monomer is emulsion polymerization.
13. The method of claim 1 or 2, wherein the fluid further comprises
a polymerization initiator.
14. The method of claim 1 or 2, wherein the fluid further comprises
a chain transfer agent.
15. The method of claim 1 or 2, wherein the polymerization is
nitroxide mediated free radical polymerization.
16. The method of claim 1 or 2, wherein the fluid further comprises
a free radical trap.
17. The method of claim 1 or 2, wherein the degradation of polymer
is caused by an increase in temperature.
18. The method of claim 1 or 2, wherein the degradation of polymer
is caused by a change in fluid pH.
19. The method of claim 1 or 2, wherein the degradation of polymer
is caused by a polymer degradation agent pumped downhole before or
after the fluid comprising monomer.
20. The method of claim 1 or 2, wherein the fluid further comprises
a polymer degradation agent.
21. The method of claim 1 or 2, wherein the fluid further comprises
a surfactant.
22. The method of claim 1 or 2, wherein the fluid further comprises
a fiber.
23. The method of claim 1 or 2, wherein the monomer comprises a
Mannich reaction product.
24. The method of claim 1 or 2, wherein the fluid is an
emulsion.
25. The method of claim 2, wherein the treatment includes at least
one of diversion, isolation, formation consolidation, flowback
control, or fluid loss control.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to well completion, stimulation and
remediation. More specifically, it relates to a method of
polymerization of monomers downhole to create responsive polymer
structures to provide a temporary or reversible treatment in a
wellbore or in a fracture. The method includes irreversible
polymerization of monomers under downhole conditions. The resulting
polymer slowly degrades after it has performed its function, such
as fluid diversion, wellbore isolation, proppant aggregation,
formation consolidation, and fluid loss prevention.
[0002] Degradable polymers have found many uses in the oilfield. As
non-limiting examples: They may be used as diverting or isolating
agents, for example in the form of ball-sealers, polymer fibers,
flakes, and fine powders, and as a coating on proppant particles.
They have been delivered downhole in the discontinuous (internal)
phase of emulsions. They have also been used as acid precursors,
and fluid loss additives in drill-in fluids. They have been used to
replace some of the proppant used so that, after they degrade, the
proppant placement is heterogeneous. Many materials have been used
for these purposes, such as hydrolysable polyesters and oil soluble
polyolefins. The degradation may be, for example, controlled
hydrolysis, or dissolution in water or oil. There are several
problems in common for most of the applications here. First of all,
pumping of solids increases the risk of bridging and plugging in
the wellbore, fracture, coiled tubing or surface equipment. Second,
there are always limitations on the concentrations of solids and on
the velocity that can be attained during slurry pumping. Third,
placement of solids is difficult to control. Fourth, fluid-solid
separation, for example by settling, during a job is possible.
Fifth, if the polymer is soluble, the viscosity of the fluid may
make pumping difficult and expensive. Formation of the polymer
downhole alleviates these problems.
[0003] Polymerization of monomers downhole has been used, but to
form polymers that are intended to remain permanently in place to
perform their functions (or that may degrade gradually but are not
required to degrade to perform their function). For example, most
water control treatments use polymers, such as polyacrylamides,
that are polymerized in place. Most curable resin proppants have a
coating containing monomers or oligomers of the final polymer.
Monomers or oligomers of epoxy, melamine, urethane, phenolic and
other resins have been polymerized downhole, for example to create
thermoset nanocomposite particulates. All of the methods mentioned
above for downhole polymerization assume permanent polymer
placement, and application of the methods is limited to situations
in which irreversible polymer placement is acceptable. However, for
a number of applications such as fluid diversion, well isolation,
multistage fracturing, fluid loss and formation control, there are
more sophisticated requirements. Namely, it is preferable to pump
monomers downhole to avoid incorrect placement, pumping power loss,
and plugging issues, but at the same time it would be preferable to
have a degradable polymer, which would disappear (degrade) after
the time needed for a given purpose.
[0004] There is a need for a method of synthesizing degradable
polymers downhole and for controlling their placement and the
timing of their formation and degradation.
SUMMARY OF THE INVENTION
[0005] One embodiment of the invention is a method of downhole
treatment that includes pumping a fluid containing a polymerizable
monomer downhole, initiating downhole polymerization, and allowing
decomposition of the resulting polymer under downhole conditions.
The monomer contains at least two polymerizable chemical groups and
one or more degradable chemical groups and, optionally, one or more
water-soluble groups linking the polymerizable and degradable
groups. Examples of the water soluble linking groups include
oligoethyleneglycol, glycerol, phosphate, and mixtures of these.
The monomer may also include one or more precursors to
water-soluble groups that link the polymerizable and degradable
groups. Examples of the polymerizable groups include acrylic,
methacrylic, aziridine, oxirane, isocyanate, vinyl, and mixtures of
these groups. The monomer may be a Mannich reaction product.
[0006] In another embodiment, the fluid containing a first such
monomer also includes a second, different, monomer that contains at
least two polymerizable chemical groups and one or more degradable
chemical groups. The fluid may contain acrylic acid, methacrylic
acid, or both. The two different monomers may be a nucleophilic
monomer and an electrophilic monomer
[0007] In a preferred embodiment, the monomers for polymerization
are diacrylic ethers or anhydrides or dimethylacrylic ethers or
anhydrides.
[0008] In other embodiments, the downhole polymerization of monomer
is emulsion polymerization; the fluid further contains a
polymerization initiator; the fluid further contains a chain
transfer agent; the polymerization is nitroxide mediated free
radical polymerization; the fluid further contains a free radical
trap; the fluid further contains a surfactant; and/or the fluid
further contains a fiber.
[0009] In yet other embodiments, the degradation of the polymer is
caused by an increase in temperature; a change in fluid pH; and/or
a polymer degradation agent pumped downhole before or after the
fluid containing monomer. The fluid may also contain a polymer
degradation agent.
[0010] In a further embodiment the fluid is an emulsion.
DETAILED DESCRIPTION OF THE INVENTION
[0011] Although the following discussion emphasizes fracturing, the
downhole polymerization method of the invention may be used in
other stimulation methods, such as gravel packing, and combined
fracturing and gravel packing in a single operation, and may be
used in other oilfield practices such as, but not limited to, water
control, well completion, scale control, acidizing, and drilling
fluid loss control. The invention will be described in terms of
treatment of vertical wells, but is equally applicable to wells of
any orientation. The invention will be described for hydrocarbon
production wells, but it is to be understood that the invention may
be used for wells for production of other fluids, such as water or
carbon dioxide, or, for example, for injection or storage wells. It
should also be understood that throughout this specification, when
a concentration or amount range is described as being useful, or
suitable, or the like, it is intended that any and every
concentration or amount within the range, including the end points,
is to be considered as having been stated. Furthermore, each
numerical value should be read once as modified by the term "about"
(unless already expressly so modified) and then read again as not
to be so modified unless otherwise stated in context. For example,
"a range of from 1 to 10" is to be read as indicating each and
every possible number along the continuum between about 1 and about
10. In other words, when a certain range is expressed, even if only
a few specific data points are explicitly identified or referred to
within the range, or even when no data points are referred to
within the range, it is to be understood that the inventors
appreciate and understand that any and all data points within the
range are to be considered to have been specified, and that the
inventors have possession of the entire range and all points within
the range.
[0012] The techniques of emulsion polymerization (for example batch
processing), including the choices of monomers, surfactants,
initiators, and chain transfer agents are well known in the art.
Surfactants having low critical micelle concentrations are favored;
the polymerization rate shows a dramatic increase when the
surfactant level is above the critical micelle concentration, and
minimization of the amount of surfactant used is preferred for
economic reasons. Mixtures of surfactants are often used, including
mixtures of anionic and nonionic surfactants. Examples of
surfactants commonly used in emulsion polymerization include fatty
acids, fatty acid salts, sodium lauryl sulfate, quaternary ammonium
salts, and alpha olefin sulfonates.
[0013] The monomers may be pumped downhole in an emulsion, in a
one-phase solution, in a dispersion, or in any other type of
mixture.
[0014] Monomers suitable for use in the invention have two types of
chemical groups: first, a polymerizable group or groups and second,
a degradable group or groups which allow the polymer to degrade
under downhole conditions via hydrolysis or another mechanism. This
approach is different from the approach which uses reversible
cross-linking of a polymer via a change in pH, or in ion
concentration, such as the way polysaccharides are crosslinked with
borate. In the present invention, the two chemical processes
(polymerization from monomers and degradation of the polymer) take
place almost independently and are controlled differently.
[0015] If the monomer is delivered to a downhole location in
emulsified form, the monomer emulsion may be stabilized with
surfactants (ionic or non-ionic) and stabilizers. Monomer may also
be diluted with organic solvent and/or mixed with other materials
such as proppants. Degradable inorganic or organic solid
particulates that form a composite material with the polymer under
bottomhole conditions may also be included.
[0016] Polymerization may be triggered, for example, by a radical
initiator and/or by an increase in temperature. The rate of the
downhole polymerization process and the length of the polymer
produced may be controlled with additives that are responsible for
the polymer chain length and branching.
[0017] Polymer obtained from monomers pumped downhole may be used,
by non-limiting example, for fluid diversion plugs, isolation
plugs, formation consolidation, flowback control, and fluid loss
control. Synthesized polymer may form gels, films, solids or any
other kind of structure in treated wellbores, fractures, and/or
formations. After the desired effect is achieved, the deposited
polymer degrades via hydrolysis or other mechanism and the polymer
degradation products dissolve, leaving the wellbore, fracture
and/or formation clean with no damage that might have decreased
fluid flow.
Monomers
[0018] Examples of suitable monomers include polymerizable acrylic
acid esters or anhydrides, which may subsequently be hydrolyzed to
produce water-soluble acrylic acid derivatives; the monomers
usually include an optional water-soluble linking group. A
generalized formula is as follows:
[0019] The two polymerizable groups and the two decomposable
(degradable) groups may each either be the same as one another or
different. There may be more than two polymerizable groups in a
single monomer. There may be only a single degradable group between
the polymerizable groups. The water-soluble linker or its precursor
may not be needed if the monomer is sufficiently water-soluble
without it. The monomer used may be a mixture of monomers of this
type. The polymerizable groups in the monomers may be, by
non-limiting example, acrylic, methacrylic, aziridine, oxirane,
isocyanate, vinyl, and mixtures of these. The water soluble linking
groups may include, by non-limiting example, mono-, di-, tri-,
tetraethyleneglycol (oligoethyleneglycol), glycerol, phosphate, and
other units. The following are commercially available examples of
suitable monomers; the first four, designated M1-M4, were used in
the examples described later. These are used as non-limiting
examples of suitable monomers in the Experimental section
below.
##STR00001##
[0020] Another example of monomers suitable for downhole formation
of degradable polymers is the product of certain Mannich reactions.
For example, the reaction product of a basic component (for example
urea, alkylamine, or melamine), formaldehyde (or a precursor, for
example paraformaldehyde or hexamine), and an acrylamide is used as
the monomer or as a co-monomer. For example, mixing, under basic
conditions, of urea, formaldehyde and acrylamide results in a
mixture of products, some of which are polymerizable-degradable
monomers, such as that shown below, for further polymerization. The
acrylic moieties are responsible for subsequent polymerization; the
structure formed from the urea and N--CH.sub.2--N fragments, is
responsible for hydrolysis.
##STR00002##
[0021] Yet other examples of polymerizable--degradable downhole
materials are those based on nucleophile--electrophile
polymerization reactions. In such a reaction, an electrophilic
monomer having two or more electrophilic groups reacts with a
nucleophilic monomer having two or more nucleophilic groups,
resulting in a cross-linked polymer. When one or both of those
monomers has one or more cleavable units (for example ester,
orthoester, etc.), the polymer slowly dissolves (for example, as a
result of hydrolysis).
[0022] Electrophilic groups may, by non-limiting example, be
aziridine, carbodiimide, oxirane, epoxide, and molecules containing
combinations of these groups; examples include ethyleneglycol
bis-(2,3-epoxybutyrate); trimethylolpropane
tri-[.beta.-(N-aziridinyl)-propionate], and 2,2-bishydroxymethyl
butanoltris[3-(1-aziridine)propionate].
[0023] Nucleophilic groups may, by non-limiting example, be amines,
hydroxyls, thiols, urethanes, and amides. Examples include
tetraethylenepentamine; ethylene diamine; cadaverine; sucrose;
glycerin; pentaerythritol; pentaerythritolethoxylate;
pentaerythritol propoxylate; ethylene glycol; diethylene glycol;
triethylene glycol; tetraethylene glycol; polyethylene glycol;
1,2,3-propanetriol; polyglycerol; propylene glycol;
1,2-propanediol; 1,3-propanediol; trimethylol propane;
diethanolamine; triethanolamine; sorbitol; and molecules containing
combinations of these groups. Examples of such reactions
include
##STR00003##
[0024] Also, combined mechanisms of downhole polymerizations are
suitable. For example, glycidyl acrylate contains both acrylic and
epoxide units; 2-isocyanatoethyl methacrylate contains an
isocyanate and an acrylic group. Such molecules undergo both
radical and electrophile-nucleophile polymerization. The ester
function is responsible for the subsequent hydrolysis.
[0025] Monomers are pumped downhole in liquid, encapsulated,
adsorbed or emulsified form and may be diluted with water or with
organic solvents.
[0026] Monomer emulsions may be stabilized by surfactants
(cationic, anionic, non-ionic, amphoteric, and zwitterionic).
Monomers may be emulsified in pure form or first dissolved in an
organic solvent.
[0027] Monomers may be pre-mixed with solid particulates (including
fibers) to give a composite plugging material after downhole
polymerization.
Initiators
[0028] After the monomers are pumped downhole, monomer
polymerization is induced, for example by temperature increase, or
by chemical initiators. Examples of initiators for radical
polymerization include persulfates, azo compounds (such as
2,2'-azobis(2-methylpropionitrile and 4,4-azobis(4-cyanovaleric
acid)), hydroxylamine derivatives, such as
(2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane, peroxide
derivatives such as benzoyl peroxide and peracids; these are all
well known chemical initiators for radical polymerization.
[0029] The initiator may be placed downhole before the monomer is
pumped; it may be added to the fluid as a solute, as solid
particles, adsorbed or absorbed by solid particulates,
encapsulated, or impregnated, or in a monomer emulsion. The
initiator may be pumped together with the monomer, or may be pumped
after monomer placement. Initiators may be activated by
temperature, shear, or chemical reaction, for example reaction on a
surface. For Mannich reactions, formaldehyde or its precursor
(hexamine, polyformaldehyde, etc) can be the initiator.
Polymerization
[0030] The initiator causes radical or another type (for example,
ionic) of monomer polymerization downhole, resulting in a
homogeneous or heterogeneous polymer mass, which provides bridging,
sealing, cross-linking, plugging, particulate aggregation, or other
effects where it is placed. As a result of these effects, the
method is used for example for downhole fluid diversion, fluid loss
or circulation loss control, proppant aggregation, wellbore
isolation, and flowback control.
[0031] Various additives may be added to the monomer to provide
polymerization control, for example chain length control. Chain
length may be critical for the rate of subsequent polymer
decomposition, as well as for precipitation with calcium or other
ions coming from the formation or from other fluids, and for fines
formation.
[0032] In the case of acrylates, chain length may be controlled by
increasing or decreasing the initiator concentration. The more
polymer chains are initiated simultaneously, the more chains are
synthesized, and fewer monomers are involved in each single chain.
Polymer chain transfer agents (such as ferric chloride, ethyl
acetate and others) may be used as additives for chain growth
control in radical polymerization reactions via transfer of
radicals from growing polymer chains to monomers, thus initializing
new chain formation and stopping another chain growth.
[0033] The effect of chain transfer agents on polymerization is
based on competition between chain propagation (reaction of the
chain with a monomer) (k.sub.p--rate constant of the propagation
reaction) and reaction of the chain with the chain transfer agent
(k.sub.tr--rate constant of the chain transfer). The higher the
ratio
C = k tr k p ##EQU00001##
the more favored is the chain transfer reaction. After reaction
with the polymer, the radical chain transfer agent can initiate a
new chain. That is why chain transfer agents do not stop the
polymerization reaction. Among others, iron chloride (III) and
ethyl acetate exhibit large chain transfer constants.
[0034] Radical traps (RT) scavenge polymer radicals and reduce the
number of growing polymer chains. Compounds such as
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) also can act as
mediators of radical polymerization (called nitroxide mediated
polymerization (NMP)). TEMPO reacts reversibly with carbon centered
polymer radicals, forming alkoxyamines. The latter can undergo
radical C--O bond homolysis, releasing active polymer chains and
the nitroxyl radical TEMPO. The polymer obtained in this case has
an alkoxyamine end-group. Thus it can act as a macromolecular
polymerization initiator for the preparation of suitable
co-polymers of the invention. Such polymers are called "living"
polymers. Among nitroxyl radicals available for NMP of acrylates
are TEMPO, DEPN
(N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl), and TIPNO
(N-tert-butyl-N-(2-methyl-1-phenylpropyl)-N-oxyl) are commonly
used. In some cases when the rate constant of homolysis of an
alkoxyamine is too low at a given temperature, nitroxyl radical
will stop the polymerization of monomer. Depending on the rate of
alkoxyamine homolysis and the recombination rate constant between
nitroxyl and C-centered radical for a particular monomer and
nitroxide, one can predict the behavior of polymerizations.
[0035] Radical traps or their precursors (tetramethylpipiridine
1-oxyl (TEMPO), TEMPO derivatives, other stable radicals;
phenyl-tert-butyl nitrone (PBN), PBN analogues, other nitrones;
alkoxyamines; and other chemicals) scavenge polymer radicals and
reduce the number of growing polymer chains, but they can also stop
monomer conversion. TEMPO also can act as a mediator of radical
polymerization (so called nitroxide mediated polymerization). It
reacts reversibly with carbon-centered polymer radicals, forming
alkoxyamines. The latter can undergo radical C--O bond homolysis,
releasing active polymer chains and nitroxyl radicals. The polymer
obtained in this case has an alkoxyamine end-group; thus it can
react as a macromolecular polymerization initiator for preparation
of additional polymers. Such polymers are called "living polymers".
Phenyl-tert-butyl nitrone (PBN) can also form nitroxyl radicals
after reaction with C-centered radicals. This macromolecular
nitroxyl radical can act as a polymerization mediator and control
the polymerization of monomers such as acrylates.
[0036] Preferred monomers, that give high polymerization yields
under typical downhole conditions, include but are not limited to
the monomers of the type named M1 and M2 above (derivatives of
acrylate esters), and monomers of the types named M3 and M4 above
(such as methacrylic anhydride). Monomers such as M3 and M4 and
their variants may be used as co-monomers with M1 and M2 monomers.
In this embodiment, the time of complete polymerization (typically
from about 10 minutes to several hours) is controlled by
polymerization initiators or other additives and the downhole
temperature. A nonlimiting example of an initiator is ammonium
persulfate in concentrations of from about 0.5 to about 20 percent
by weight of the monomer. Another example of a polymerization
initiator is phenyl-tert-butyl nitrone (PBN) in concentrations from
about 0.5 to about 20% percent by weight of the monomer. Other
examples of radical initiators for polymerization are well known in
the art. In another embodiment of the invention, the rate of
polymerization and the later stability of the polymer chain are
controlled with a chain transfer agent such as ferric chloride or
ethyl acetate.
[0037] In a preferred embodiment, monomers of the type of M1 and M2
(derivatives of acrylate esters) are pumped in the form of an
emulsion, produced by stirring with or without nonionic or ionic
surfactants. The stability of the monomer-surfactant emulsion is
checked easily in the laboratory before field operations. Monomers
that are miscible with or soluble in water may be pumped as water
solutions.
[0038] It has been found that emulsion polymerization in many cases
gives a number of advantages, as follows:
[0039] 1. Controlled polymer chain length;
[0040] 2. Greater homogeneity of the resulting polymer;
[0041] 3. Higher degradation rate of the resulting polymer;
[0042] 4. Better wellsite delivery (no plugging risk); and
[0043] 5. Lower viscosity (lower pump power required).
Degradation
[0044] The polymer formed should stay substantially intact downhole
while it is required for bridging, sealing, cross-linking,
plugging, particulate aggregation, or the like. After that,
decomposition occurs via thermolysis or another chemical
transformation, such as hydrolysis, with the formation of
oligomeric or small molecules. All decomposition products are
water- or oil-soluble. Thus the effect of the polymer is stopped or
reversed and after the polymer degradation, no damage to the
production is left. In one method, for example, hydrolysis of
gelled polymer occurs at high pH. The high pH is provided by
addition of alkaline-releasing material into the working fluid, by
addition of fine slowly-dissolving solid alkaline into the slurry,
or due to reaction of the polymer with a slightly alkaline (e.g.,
carbonate) formation. The amount of alkaline additive and the
resulting pH under downhole conditions depends upon the time
selected for removal of the emplaced polymer.
[0045] It has been found experimentally that diacrylic monomers
polymerized in water form polymers by free radical polymerization
that can easily be hydrolyzed in water. Polymers formed from
dimethacrylic monomers are not as readily hydrolyzed; not to be
limited by theory, but it is believed that these polymers are more
highly crosslinked. Four strategies may be used to produce polymers
that are more easily hydrolyzed under typical downhole conditions:
(i) perform polymerization in the presence of surfactants; (ii)
increase the amount of initiator; (iii) add a co-monomer; and (iv)
employ chain transfer agents and radical traps. These may be used
to produce polymers that can be degraded in a wellbore, fracture,
or formation in a suitable time at the temperature and pH of the
fluid.
[0046] For polymerization in the presence of surfactants the
stability of monomer emulsions was first studied, and it was found
that emulsions of acrylic monomers are stable during the time of
polymerization, but emulsions of methacrylic polymers are not. It
was then found that polymers obtained in the presence of
surfactants undergo hydrolysis more easily. The products of
hydrolysis produce relatively little precipitate with Ca.sup.2+.
Precipitation of degradation products by cations in the fluid will
usually, but not always, be detrimental to the use of the
degradable polymers.
[0047] Again not to be limited by theory, but it is believed that
polymerization with a higher concentration of initiator leads to
polymers in which more monomer units have both polymerizable groups
of the monomer incorporated into the chain backbone. This plus the
lower polymer molecular weight makes hydrolysis easier.
[0048] It was found that copolymers of monomers M1 and M4 can be
hydrolyzed under mild conditions, although the products of
hydrolysis form a precipitate with Ca.sup.2+. Thus copolymerization
is another way to control degradation rate.
[0049] Polymerization of M1 in the presence of chain transfer
agents (iron chloride (III) and ethyl acetate) produced polymers
that underwent hydrolysis relatively quickly; the products of
hydrolysis formed only a light precipitate with Ca.sup.2+.
[0050] Polymerization of one acrylic monomer (M1) in the presence
of TEMPO failed at low temperatures and was slow at higher
temperatures.
[0051] A particularly preferred polymer, based on the
polymerization time, hydrolysis rate, and low precipitation by
Ca.sup.2+, was a polymer of M1 obtained in the presence of
tert-butyl-phenylnitrone (PBN).
[0052] Polymerization of M1 in the presence of ammonium persulfate
at 50.degree. C. with PLA fiber resulted in soft well-consolidated
fiber flocs. The approach of downhole delivery of monomer of the
invention and fibers or other solid particles and subsequent
polymerization and consequent flocculation may be used for
diversion and/or isolation, for example in curing lost
circulation.
[0053] The present invention can be further understood from the
following examples.
General Polymerization Procedure:
[0054] A calculated amount of monomer (any one of M1-M4, or a 9:1
mixture of any two, or a 10:1 mixture of any one with acrylamide)
was added to a round-bottom two-necked flask to which de-ionized
water was then added. The concentration of monomer was 0.5 mol/l
and the total volume of the reaction mixture was 5 ml. Surfactants,
chain transfer agents, and radical traps were added to the mixture
when needed. The mixture was stirred on a magnetic stirrer (or an
Ika-Werke T 10 emulsifier) for 10 minutes and then 5 or 50 mM of a
polymerization initiator (for example ammonium persulfate) was
added. The reaction mixture was made oxygen-free by degassing by 10
minutes of argon bubbling and then heated in an oil bath to 60
C.degree.. The typical heating duration was from 20 min to 7 hours,
depending upon the monomer: fast polymerization typically occurred
for M1 and M2 (for example about 20 min for M1, 30 mM for M2), but
longer polymerization was required for monomers of type M3 and M4
(for example about 7 hours). After heating, the mixture was cooled
to room temperature; a portion was dried for polymer mass
determination and a portion was left un-dried for hydrolysis tests.
According to gravimetric analyses, at these times and temperatures,
polymerizations of polymers made only from M1 or M2 were typically
about 100%, from M3 about 30% and from M4 less than about 10%.
These results indicate that by appropriate choices of
concentration, initiator, inhibitor, temperature, co-monomers, and
other factors, the timing and location of polymerization can be
controlled.
General Hydrolysis Procedure:
[0055] A 0.002 g sample of the dry polymer was placed in a
chromatographic vial and 1 ml of alkaline solution (for example
solution of KOH in de-ionized water or buffer solution) was added.
The sample was heated in an oil bath for the desired temperature
and time. It was assumed that hydrolysis was finished after the
solution became transparent. It was found that polymers made from
only M1, M2 or M3 were relatively difficult to hydrolyze, commonly
requiring a pH of above about 12.5, but polymers made from only M4
hydrolyzed readily. These results indicate that by proper choices
of monomers, pH, co-monomer and other factors, the degradation of
the polymers can be controlled. For example, inclusion of some M4
in a polymer made of any of the other monomers accelerates the
degradation.
[0056] After hydrolysis, neutralized reaction mixtures were mixed
with Ca.sup.2+ solution (to provide final concentrations of 1%
Ca.sup.2+) to see whether Ca.sup.2+ complexes precipitated. It was
found that hydrolysis of polymers obtained with chain transfer
agents or PBN (nitron) result in soluble products that give no
precipitation with Ca.sup.2+ ions. The polymer hydrolysis products
do not cause formation damage.
Example 1
[0057] It is well known that methacrylates cannot undergo thermal
self-initiation of polymerization. Thus, for the polymerization of
these monomers, radical polymerization initiators were employed.
The number of polymeric chains in the polymerization is in direct
proportion to the initial concentration of initiator. As
polymerization proceeds up to a monomer conversion of 100%, then
for a given polymer concentration, the larger the initial initiator
concentration, the larger the number of growing polymer radicals
that will be formed and be able to grow; thus there is a smaller
number of monomeric units in each polymer chain. Thus, by varying
the initiator/polymer concentration ratio, one can control the
polymer molecular weight, and therefore the ease of polymer
hydrolysis.
[0058] Polymerization of M1 in the presence of 10% of ammonium
persulfate as the initiator was performed by the standard procedure
described above, yielding white flakes of dry polymer. Hydrolysis
of the polymer was performed with 0.1 M KOH solution (pH about
13.1) at 95.degree. C. The time for complete hydrolysis was
estimated to be about 18 hours. A "wet" sample, obtained from the
reaction mixture by simple vacuum filtration through a glass filter
without drying, was completely hydrolyzed under similar conditions
in about 10 hours.
Example 2
[0059] Samples containing 0.5 M monomer in 5 ml of water were
prepared with double the critical micelle concentration of
surfactant (0.015 M for SDS and 0.007 M for TTAB) and treated with
an Ika-Werke T 10 basic emulsifier for 5 minutes. The stability of
the emulsions vs. time was checked; the results are sown in Table
1:
TABLE-US-00001 TABLE 1 M1 M2 M4 time SDS TTAB SDS TTAB SDS TTAB 5
min Stable Stable Stable Stable Stable Stable 10 min Stable Stable
Stable Stable Stable Stable 20 min Stable Stable A A -- -- 30 min
-- -- -- -- B B 1 hour B B B B B B 2 hours B B B B C B 5 hours --
-- -- -- C B 6 hours -- -- -- -- D 24 hours E F C G G G A: White
precipitate formed as emulsion begins to separate and disappears
after mixing; emulsion was white. B: White precipitate which
disappeared after mixing; solution was white. C: White precipitate
which disappeared after mixing; solution was transparent. D:
Emulsion nearly completely separated. E: Emulsion started to
separate. F: Emulsion stared to separate; less stable than E (with
SDS). G: White precipitate which disappeared after mixing; solution
was transparent; emulsion was totally separated. (Note that all
these emulsions re-formed upon shaking or mixing.)
Example 3
[0060] Emulsion polymerization experiments were carried out as
follows: Samples containing 0.5 M monomer in 5 ml of de-ionized
water were prepared with double the critical micelle concentration
of surfactant (0.015 M (0.0217 g/5 ml) for SDS and 0.007 M 0.0118
g/5 ml) for TTAB). The mixture was stirred with an emulsifier for 5
mm. Ammonium persulfate initiator (2.5 10.sup.-3 mol/l (0.0029 g/5
ml)) was added. The mixture was degassed by argon bubbling for 10
min and then heated in an oil bath to 60.degree. C. for about 10
min to about 7 hours, depending on the monomer. The polymers
obtained were dried at 100.degree. C. The results are shown in
Table 2.
TABLE-US-00002 TABLE 2 M1 M2 M4 SDS TTAB SDS TTAB SDS TTAB Time of
polymerization 5 5 6.5 6.5 Rapid* Rapid* hours hours hours hours
Physical Clotted Clotted Cloudy Cloudy Cloudy Cloudy state of white
white solu- solution with solu- solution the polymer polymer tion
material tion with polymer insoluble, glass- in water like droplets
particles *Mixture polymerized during bubbling of argon in less
than 5 min.
Example 4
[0061] The hydrolysis of polymers prepared with surfactants was
carried out by the standard procedure. The results of the
hydrolysis experiments are presented in Table 3. It should be
mentioned that M2 TTAB polymer did not undergo hydrolysis in 0.1 M
KOH solution in 4 days at 95.degree. C.
TABLE-US-00003 TABLE 3 Initial Final Final # .degree. C.
description solution pH mass, g pH t, hours 1 95 M1 TTAB 0.1M 13.1
0.002 13.05 9.5 KOH 2 95 M1 SDS 0.1M 13.1 0.0018 13.05 12 KOH 3 70
M2 SDS 0.02M 12.3 0.0024 14* 95 KOH 8.4 7** *temperature was
increased for the sample **solution became cloudy-white without
solid particles.
[0062] Emulsions made with the surfactants and M1 or M2 were stable
during the polymerization time; not shown is that emulsions made
with the surfactants and M4 were not stable. The rates of
polymerization were high for M1 and M2 and lower for M4 compared to
the polymerizations in the absence of surfactants. Hydrolysis of
the polymer samples made with surfactants proceeded faster than for
the samples obtained without surfactants. This indicates that the
polymer particles were smaller. This demonstrates how surfactants
can be used to control the placement And degradation of polymers.
Not to be limited by theory, but degradation kinetics and
degradation product molecular weight examination suggest that the
surfactant affected the amount of polymer in which only one monomer
polymerizable group was included in the polymer backbone as opposed
to the amount of polymer in which both monomer polymerizable groups
were included in the polymer backbone (the extent of
crosslinking).
Example 5
[0063] Water-soluble products formed after hydrolysis of polymers
made with M1 or M2, were checked for precipitation with Ca.sup.2+.
1% by weight of Ca.sup.2+ (in the form of CaCl.sub.2) was added to
neutralized or non-neutralized solutions of hydrolyzed polymers
with the results shown in Table 4.
TABLE-US-00004 TABLE 4 Solution used for pH (after neu- Sample
hydrolysis hydrolysis) tralized Result M1 SDS KOH 0.1M 12.9 No
cloudy non-viscous solution M1 SDS KOH 0.1M 12.9 Yes white flakes
M1 TTAB KOH 0.1M 12.9 No cloudy non-viscous solution M2 SDS KOH
0.1M 12.9 No white flakes KOH 0.1M 13.05 No cloudy non-viscous
solution De-ionized 7 No Transparent solution water
Example 6
[0064] Co-polymerization of M1 and M4 was performed using the
standard polymerization procedure and a M1 to M4 ratio of 1 to 9.
The conversion was about 100 percent and the dry polymer obtained
was a white powder that did not hydrolyze in 0.1 M KOH solution at
95.degree. C. in 4 days.
[0065] Co-polymerization of M1 with acrylamide (AA) was done with
the standard polymerization procedure using an M1 to AA ratio of 1
to 9. The wet polymer obtained was a transparent gel; when dried
this material was white particles. Part of the wet polymer was
precipitated in a 9/1 acetone/methanol solution, resulting in white
particles. Polymer samples were hydrolyzed in alkaline solutions,
with the results shown in Table 5. In the laboratory, sometimes dry
polymer samples were obtained, for example in determining yields or
solubilities. Wet samples, as would be found in downhole use,
degraded more rapidly than dry.
TABLE-US-00005 TABLE 5 time, # T polymer solution pH.sub.0 hours
pH.sub.t 1 60 M1 + AA wet phosphate 8.3 3.5 8.3 buffer, 1 ml 2 60
M1 + AA wet carbonate 10.35 1 10.35 buffer, 1 ml 3 80 M1 + AA wet
carbonate 10.35 0.5 10.35 buffer, 1 ml 4 80 M1 + AA carbonate 10.35
0.5 10.35 precipitated buffer, 1 ml 5 80 M1 + AA dry carbonate
10.35 1.25* 10.35 buffer, 1 ml 5.5** 6 80 M1 + AA dry phosphate 8.3
1.25* 8.3 buffer, 1 ml 5.5** *transparent viscous solution is
formed **transparent non viscous solution is formed pH.sub.0 is the
initial pH; pH.sub.t is the pH at time t; m.sub.0 is the initial
mass "precipitated" above means precipitated from water with an
acetone/methanol 9:1 mixture
[0066] Co-polymerization of M4 with AA was performed using the
standard polymerization procedure and an M4 to AA ratio of 1 to 9.
The wet polymer obtained was a suspension of white particles in
water; the dry polymer was transparent particles. Polymer samples
were hydrolyzed in alkaline solutions, and the results are shown in
Table 6. The polymers degraded at low temperatures in several
minutes at relatively low pH, typical for gels, which means that
degradation can be controlled by mixing these monomers and
acrylamide, and that the degradation time downhole is not months or
years and that the polymer disappears downhole after treatment.
TABLE-US-00006 TABLE 6 # T Polymer solution pH.sub.0 Time pH.sub.t
1 RT M4 + AA wet phosphate 8.4 5 min 7.7 buffer, 0.8 ml 2 RT M4 +
AA wet carbonate 10.35 5 min 10.0 buffer, 0.8 ml 3 40.degree. C. M4
+ AA dry phosphate 8.4 20 min 8.3 buffer, 0.5 ml pH.sub.0 is the
initial pH; pH.sub.t is the pH at time t
[0067] In the presence of co-monomer, polymerization of M4
proceeded faster and with higher conversion. Preparation of
co-polymers provided polymers having low crosslinking. That favored
the hydrolysis of the polymeric samples under mild conditions.
Example 7
[0068] Water-soluble polymers formed after hydrolysis of copolymers
were checked for precipitation with Ca.sup.2+. 1% weight percent of
Ca.sup.2+ (in the form of CaCl.sub.2) was added to non-neutralized
solutions of hydrolyzed polymers and the results are shown in Table
7. Note that the same amount of Ca.sup.2+ dissolved in a carbonate
buffer formed a white non-viscous solution without formation of
flakes.
TABLE-US-00007 TABLE 7 polymer solution neutralization result M 10%
ammonium KOH 0.1M No cloudy non-viscous persulfate solution M1-AA
Phosphate No white flakes buffer M4-AA Phosphate No white flakes
buffer
[0069] These copolymers of AA-M1 and AA-M4 hydrolyzed easily, and
produced water-soluble polymers that precipitate with Ca.sup.2+.
The sample of M1 (made with 10 percent ammonium persulfate)
hydrolyzed in an alkaline solution formed a non-viscous solution
with Ca.sup.2+; this was mainly due to the high concentration of
alkaline.
Example 8
[0070] Polymerization of M1 was carried out in the presence of
either iron (III) chloride or ethyl acetate as transfer agents and
TEMPO or PBN as radical traps. A calculated amount of polymer and
sodium dodecyl sulfate (SDS) surfactant was put into a round
bottomed flask and de-ionized water was added to give a volume of 5
ml. The mixture was mixed by an emulsifier for 5 min. A chain
transfer agent (CTA) or radical trap (RT) was added and the mixture
was stirred for 1 more min. The initiator was added afterwards. The
mixture was bubbled with Ar for 10 min and heated in an oil bath at
80.degree. C. (for samples with RT). For polymerization with CTA,
no heating was applied; polymerization proceeded rapidly at room
temperature. If heated, the mixture was then cooled to room
temperature, and the polymer was dried at 100.degree. C. to
constant mass. [0071] A typical mixture for polymerization with CTA
was prepared as following: [0072] [M1]=0.5 M [0073] [APS]=2.5
10.sup.-3M (polymerization initiator) [0074] [SDS]=0.015 M [0075]
[CTA]=5.times.10.sup.-3M ethyl acetate or 5.times.10.sup.-2M
FeCl.sub.3 [0076] Water up to 5 ml [0077] A mixture for
polymerization of M1 with TEMPO was prepared as following: [0078]
[M1]=0.5 M [0079] [APS]=2.5.times.10.sup.-3M (polymerization
initiator) [0080] [SDS]=0.015 M [0081] [TEMPO]=5.times.10.sup.-3M
[0082] A mixture for polymerization with PBN was prepared according
to: [0083] [M1]=0.5 M [0084] [APS]=1.25.times.10.sup.-3M
(polymerization initiator) [0085] [SDS]=0.015 M [0086]
[PBN]=2.63.times.10.sup.-3M
[0087] Polymerization of M1 in the presence of CTA proceeded under
more mild conditions in comparison to polymerization without CTA.
Conversions were high in both cases. Polymerization of acrylic
monomer M1 in the presence of TEMPO stopped at low conversion due
to the low homolysis rate of dormant species (intermediate
alkoxyamines, formed when TEMPO quenched a polymer chain).
Polymerization in the presence of PBN produced did not reduce the
conversion; PBN is particularly suitable for polymerization of
monomers like M1.
Example 9
[0088] "Wet" and "dry" samples of polymers obtained in the presence
of chain transfer agents and radical traps were hydrolyzed in
alkaline solutions at 95.degree. C. The mass of "wet" samples was
calculated from the monomer/water ratio in the polymerization
mixture (0.011 g/ml of alkaline solution). The results are
summarized in Tables 8 (for CTA) and 9 (for RT).
TABLE-US-00008 TABLE 12 # description m.sub.0 Solution t, hours 1
M1 FeCl.sub.3 wet 0.0129 0.1M KOH 5 2 M1 EtOAc wet 0.0102 0.1M KOH
7.5 3 M1 FeCl.sub.3 dry 0.0021 0.1M KOH 7.5 4 M1 EtOAc dry 0.0017
0.1M KOH 8 5 M1 FeCl.sub.3 wet 0.0104 0.02M KOH * 6 M1 EtOAc wet
0.01 0.02M KOH * 7 M1 FeCl.sub.3 wet 0.0123 carbonate buffer * 8 M1
EtOAc wet 0.0116 carbonate buffer * 9 M1 FeCl.sub.3 10% 0.0020 0.1M
KOH 10 * not finished in 5 days
TABLE-US-00009 TABLE 13 # description m.sub.0 solution results 1 M1
PBN wet 0.0127 0.1M KOH 2 hours 2 M1 PBN wet 0.0114 0.02M KOH
formed white solution in 5 hours, totally hydrolyzed in 3 days 3 M1
PBN wet 0.0107 carbonate buffer formed white solution in 5 hours,
totally hydrolyzed in 3 days 4 M1 PBN wet 0.0106 phosphate buffer
not finished in 3 days 5 M1 PBN dry 0.0023 0.02M KOH formed
solution with white-transparent flakes in 1 day 6 M1 PBN dry 0.0022
phosphate buffer not finished in 3 days
[0089] It can be seen that addition of CTA to these emulsion
polymerizations produced polymers that underwent hydrolysis faster
that the ones prepared in the presence of only surfactants. From
GPC of hydrolyzed samples it can be seen that low molecular weight
polymer molecules (10.sup.3 g/mol) were formed. This may be due to
the reaction of polymer radicals with CTA. GPC also shows material
at high molecular weight (>10.sup.6) which is believed to have
been from polymerization after all the CTA had reacted. An increase
in the amount of chain transfer agent (FeCl.sub.3) resulted in
formation of polymer with a smaller amount of high molecular weight
material.
[0090] Polymer obtained in the presence of PBN underwent hydrolysis
under the most mild conditions and much faster than all other
polymer samples obtained. The amount of water-soluble material
obtained after hydrolysis depended strongly on the time of heating.
In the case of hydrolysis in 0.02 M KOH, the polymer was heated for
a longer time, so that not only could ether function react with
alkaline but also alkoxyamine functions could cleave. This resulted
in polymers with lower molecular weights than in the case of
heating for a short time.
[0091] For both the CTA and PBN samples, the differences in the
time of hydrolysis of "wet" and "dry" samples showed that for dry
samples some additional time was required for the alkaline ions to
reach the reaction centers in the polymer molecules.
Example 10
[0092] The precipitation of hydrolysis products with Ca.sup.2+ was
studied by adding 1% by weight of Ca.sup.2+ in the form of
CaCl.sub.2 to the transparent solutions obtained after hydrolysis
of M1 made with FeCl.sub.3 or M1 made with ethyl acetate.
[0093] For M1 made with FeCl.sub.3, a cloudy non-viscous solution
was formed immediately after adding Ca.sup.2+. For M1 made with 1%
EtOAc, a cloudy viscous solution containing several flakes was
formed. For M1 made with 10% FeCl.sub.3, less cloudy solution was
formed than for M1 made with 1% FeCl.sub.3. For M1 made with PBN in
0.02 M KOH a cloudy non-viscous solution looking much the same as
the initial 1 percent Ca.sup.2+ in 0.1 M KOH. The cloudiness of the
solution of M1 made with FeCl3 and of the M1 made with EtOAc was
roughly the same as for solutions of M1 containing 10 percent
sodium dodecyl sulfate or M1 containing TTAB. Of all the polymer
samples obtained in the presence of a CTA or RT, M1 made with PBN
and hydrolyzed in 0.02 M KOH gave the least cloudy solution with
Ca.sup.2+.
Example 11
[0094] A bottle with 100 ppt of PLA fiber mixed with 0.5 g of
ammonium persulfate and 5 g of diacrylic monomer M1 in 100 ml of DI
water, and another bottle with 100 ppt of PLA fibers mixed with 5 g
of diacrylic monomer M1 in 100 ml of DI water were placed in an
oven at 50.degree. C. Two bottles with similar contents were left
at ambient temperature. After 30 minutes, traces of polymerization
were found in the ambient temperature bottle that included ammonium
persulfate, and well-developed polymerization was seen in the
bottle that included ammonium persulfate at 50.degree. C. After 2
hours, all the fibers inside the bottle with persulfate kept at
50.degree. C. were flocculated, giving white, soft, but
well-consolidated flocs. When the fiber was mixed with the monomer
alone, there was no organic/water phase separation. The monomer
covered all the fibers with an even layer, showing that this is a
suitable method for downhole delivery of the slurry.
* * * * *