WO2015059024A1

WO2015059024A1 - Hydrolyzable polymeric microparticles and their use in compositions and methods for recovering hydrocarbon fluids from a subterranean formation

Info

Publication number: WO2015059024A1
Application number: PCT/EP2014/072206
Authority: WO
Inventors: David Chappell; Harry Frampton; Stephen Rimmer; Christopher SAYWELL
Original assignee: Bp Exploration Operating Company Limited
Priority date: 2013-10-22
Filing date: 2014-10-16
Publication date: 2015-04-30
Also published as: UY35791A; AR098139A1; GB201318681D0

Abstract

Cross-linked polymeric microparticles having a volume average particle size diameter of from about 0.05 to about 10 μιη and comprising from about 0.01 to about 5 mol% of one or more hydrolytically labile, crystallisable cross-linking structural units based on the total structural unit content of the polymeric microparticles and wherein the cross-linking structural units are derived from one or more hydrolytically labile, crystallisable cross-linking monomers having a number average molecular weight in the range of from about 1,500 to about 40,000 Daltons and comprise at least one polyester chain having at least five -RC(0)0- ester groups in a linear arrangement wherein the R groups each represent an alkanediyl group or a substituted alkanediyl group and wherein the cross-linking monomers have at least two sites of ethylenic unsaturation. The microparticles also comprise structural units derived from a hydrophilic monomer. When a dispersion of the microparticles in an aqueous fluid is injected into a hydrocarbon-bearing formation, the labile cross-linking structural units hydrolyze thereby releasing free polymer chains that are soluble or dispersible in the aqueous fluid such that a viscosified aqueous solution is generated within the formation,

Description

HYDROLYZABLE POLYMERIC MICROPARTICLES AND THEIR USE IN COMPOSITIONS AND METHODS FOR RECOVERING HYDROCARBON FLUIDS FROM A SUBTERRANEAN FORMATION

This invention relates in general to the recovery of hydrocarbon fluids from subterranean formations. More specifically, the invention relates to cross-linked polymeric microparticles and compositions comprising the same. The cross-linked polymeric microparticles are modified under the conditions of a subterranean formation so as to increase the viscosity of the composition and thereby increase the mobilization and recovery of hydrocarbon fluids present in the subterranean formation.

In the first stage of hydrocarbon recovery from a subterranean formation (referred to as "primary recovery"), natural pressure in the formation forces hydrocarbon fluids towards production wells where they can flow or are pumped to a surface production facility. However, formation pressure is generally sufficient only to recover around 10 to 20 percent of the total oil present in a subterranean formation. Accordingly "secondary recovery" techniques are applied to recover oil from subterranean formations in which the hydrocarbon fluids no longer flow by natural forces.

Secondary recovery techniques rely on the supply of external energy to increase the pressure in a subterranean formation. One such technique involves the injection of water (such as aquifer water, river water, seawater or produced water) or a gas (such as carbon dioxide, flue gas, or produced gas) into the subterranean formation via a network of injection wells to drive the hydrocarbon fluids towards one or more production wells. The injection of water during secondary recovery is commonly referred to as water flooding.

The efficiency of water flooding techniques depends on a number of variables, including the permeability of the formation and the viscosity of the hydrocarbon fluids in the formation.

Where the hydrocarbon fluids remaining in the formation are of a relatively high viscosity, the injected water can channel through the high viscosity hydrocarbon front in a process termed viscous fingering, following a low-resistance route to a production well which bypasses much of the available hydrocarbon fluids.

In order to address this problem, it has been proposed to carry out water flooding operations using viscosified aqueous flooding media, in particular, aqueous flooding media, having mobilities which are comparable to the mobility of the hydrocarbon fluids to be displaced. For example, it has been proposed to use viscous aqueous solutions of organic polymers as injection fluids in water flooding operations. When the injected water and hydrocarbon fluids have comparable mobilities, the tendency of the injected water to bypass the hydrocarbon-containing zones of the formation, by fingering through the hydrocarbon front is significantly reduced.

A disadvantage of increasing the viscosity of injected water is that the injectivity of viscosified aqueous flooding media into subterranean formations is substantially reduced compared with that of the injection of water. The reduced injectivity of the viscosified water into the subterranean formations may require the drilling of additional injection wells in order for voidage replacement to be maintained. A further disadvantage associated with the injection of viscous polymer solutions is that the free polymers can be degraded en route to the hydrocarbon-bearing zone of the subterranean formation, for instance, due to shear forces, extensional forces, oxidative degradation or hydrolytic degradation.

There is therefore a need in the art for improved processes for the recovery of hydrocarbon fluids from subterranean formations by water flooding, which maintain the advantages of using injected fluids of increased viscosity, but which eliminate or mitigate the disadvantages associated with the reduced injectivity of fluids of high viscosity.

There is also a need in the art for improved processes in which the polymer is injected into the hydrocarbon-bearing formation in a form that is not influenced by the salinity of the injected aqueous fluid or the salinity of the aqueous fluids that may be resident in the subterranean formations.

The present inventors have addressed these problems by developing a composition comprising cross-linked polymeric microparticles that comprise water-soluble or water- dispersible polymeric chains that are cross-linked via hydrolytically labile, crystallisable cross-linkers thereby rendering the microparticles insoluble in an aqueous fluid. The polymeric microparticles may be injected into a hydrocarbon-bearing subterranean formation as a relatively low- viscosity aqueous dispersion or suspension of the

microparticles dispersed in an aqueous fluid. The microparticle properties, such as particle size distribution of the cross-linked microparticles, allow efficient propagation of the microparticle dispersion through the pore structure of a formation rock, such as sandstone. The cross-links within the polymeric microparticles undergo hydrolysis within the subterranean formation, thereby releasing free water-soluble or water-dispersible polymer chains from the microparticles. The increase in hydrodynamic volume of the free polymer chains as they are released from the microparticles provides an increase in viscosity of the injected aqueous fluid within the formation, thereby forming a viscosified aqueous fluid in situ, providing similar benefits to a conventional polymer flood, without the disadvantages associated with the reduced injectivity of viscosified aqueous flooding media and the degradation of dissolved polymers prior to or during injection of viscosified aqueous flooding media into hydrocarbon bearing zones of a subterranean formation.

The polymeric microparticles of the present invention provide the particular advantage that hydrolysis of the labile, crystallisable cross-linkers within the subterranean formation, so as to release free water-soluble or water-dispersible polymer chains from the microparticles, can be controlled so as to take place only once the microparticles encounter specific reservoir conditions. More specifically, the microparticles of the present invention have a chemical composition such that hydrolysis of the labile cross-linkers is highly temperature-dependent. For example, in some embodiments of the invention hydrolytic degradation of the labile cross-linkers is substantially retarded at temperatures below around 50 °C and greatly accelerated at higher temperatures.

In a first aspect, the present invention provides cross-linked polymeric microparticles having a volume average particle size diameter of from about 0.05 to about 10 μηι and comprising from about 0.01 to about 10 mol%, preferably, from about 0.01 to 5 mol% of one or more hydrolytically labile, crystallisable cross-linking structural units based on the total structural unit content of the polymeric microparticles and wherein the cross-liriking structural units are derived from one or more hydrolytically labile, crystallisable cross- linking monomers having a number average (M_n) molecular weight in the range of from about 1,500 to about 40,000 Daltons wherein the cross-linking monomers comprise at least one polyester chain having at least five -RC(0)0- ester groups in a linear arrangement (hereinafter "linear polyester chain") wherein the R groups each represent an alkanediyl group or a substituted alkanediyl group and wherein the cross-linking monomers have at least two sites of ethylenic unsaturation.

The person skilled in the art will understand that the mol% of hydrolytically labile, crystallisable cross-linking structural units (based on the total structural unit content of the polymeric microparticles) corresponds to the mol% of hydrolytically labile crystallisable cross-linking monomer (based on the total molar amount of monomers) used to prepare the microparticles.

As used herein, the term "structural unit" refers to a structural unit derived from a polymerisable allylic, vinylic or acrylic monomer having either a single site or at least two sites of ethylenic unsaturation. Preferred monomers are vinylic or acrylic compounds, and particularly preferred monomers are acrylic compounds.

It is preferred that at least a portion of the structural units are derived from

hydrophilic monomers such that upon hydrolysis of the labile cross-linkers, the free polymer chains that are released from the microparticles are either soluble or dispersible in an aqueous fluid. However, a small portion of the structural units may be derived from hydrophobic monomers provided that units derived from hydrophilic monomers predominate in the polymeric microparticles. Typically, less than 5 mol%, preferably, less than 2.5 mol%, in particular, less than 1 mol% of the structural units are derived from hydrophobic monomers, based on the total structural unit content of the polymeric microparticles.

As used herein, the term "cross-linking structural unit" refers to a structural unit derived from a "cross-linking monomer" containing at least two sites of ethylenic unsaturation which forms a covalent link between two polymer chains and/or between different regions of the same polymer chain. Cross-linking structural units are included in the polymeric microparticles of the invention to constrain the microparticle conformation. Preferably, the sites of ethylenic unsaturation in the cross-linking monomer are located at terminal positions in the cross-linking monomer.

As used herein, the term "hydrolytically labile, crystallisable cross-linking structural unit" refers to a structural unit which contains hydrolysable functional groups which can be hydrolytically degraded under specific conditions of temperature to cleave the cross-links between polymer chains or between different regions of individual polymer chains. The hydrolytically labile, crystallisable cross-linking structural units also contain linear polyester chains that form crystalline and/or semi-crystalline domains within the microparticle structure which undergo a melting transition at a temperature that is dependent on the chemical structure of the cross-linking structural unit. Once the melting transition has occurred, further specific conditions may accelerate hydrolytic cleavage of the cross-links. For example, hydrolysis of the cross-links may accelerate under acidic or basic conditions, in particular, at a pH of less than 5 or at a pH of greater than 9. The hydrolytically labile, crystallisable cross-linking structural units are derived from cross-linking monomers having at least one polyester chain having at least five, preferably, at least ten, in particular, at least twelve -RC(0)0- ester groups in a linear arrangement wherein the R groups each represent an alkanediyl group or a substituted alkanediyl group and wherein the cross-linking monomers have at least two sites of ethylenic unsaturation.

Preferably, the sites of ethylenic unsaturation are located at a terminal end of the linear polyester chain(s) of the cross-linking monomer. Thus, the free end(s) of the linear polyester chain(s) are capped with a suitable olefinic capping group, for instance selected from vinylic, allylic, acrylic and methacrylic groups.

By an "alkanediyl" group is meant a saturated straight-chain bivalent hydrocarbon radical. Preferably, the alkanediyl group has from two to twelve, more preferably, from three to ten, for example, from three to six carbon atoms in a linear arrangement. By a "substituted alkanediyl" group is meant an alkanediyl group as defined herein in which one or more hydrogen atoms have been replaced with a substituent group. Preferably, the substituent group(s) of the substituted alkanediyl group is selected from short chain alkyl groups (in particular, methyl and ethyl), hydroxyl groups and halo groups (fluoro, chloro, bromo and iodo groups, preferably fluro and chloro groups).

When the microparticles of the present invention are suspended in an aqueous fluid, access of water molecules to the hydrolytically labile functional groups within the tightly packed crystalline domains is substantially impeded. However, once the crystalline domains undergo a melting transition, mobility of the cross-linking structural units increases significantly, allowing water molecules to penetrate the domains and to access the hydrolytically labile functional groups of the cross-linking structural units. Without wishing to be bound by any theory, the microparticles initially swell due to absorption of water. As the functional groups of the cross-linking structural units are hydrolysed, free water-soluble or water-dispersible polymer chains are released from the microparticle into the surrounding aqueous fluid, thereby increasing its viscosity. The temperature at which the crystalline domains of the microparticle undergo a melting transition can be controlled such that it falls within the range of temperatures encountered in hydrocarbon-bearing subterranean formations, for instance in the range of from 25 to 125 °C.

Thus, the microparticles of the present invention may be injected into a subterranean formation as an aqueous dispersion at a temperature below the melting transition temperature of the crystalline domains of the microparticles. At low temperatures, the polymer chains of the microparticles remain tightly packed and thus the microparticle dispersion has a viscosity substantially equivalent to that of the aqueous fluid of the dispersion. The dispersion may therefore be injected into a subterranean formation as a low viscosity fluid. Once the aqueous dispersion encounters higher temperatures in the subterranean formation at which the crystalline domains of the microparticles melt, the cross-links are hydrolysed, thereby releasing free water-soluble or water-dispersible polymer chains into the surrounding fluid to provide a viscosified fluid in situ that facilitates improved recovery of hydrocarbon fluids from the formation.

In preferred embodiments, the microparticles of the present invention have a volume average particle size diameter of from about 0.1 to about 3 μπι, more preferably, a volume average particle size diameter of from about 0.1 to about 1 μπι.

In preferred embodiments, the microparticles of the present invention comprise from 0.01 to 5 mol% of hydrolytically labile, crystallisable cross-linking structural units, most preferably, from 0.05 to 5 mol%, in particular, from 0.1 to 2 mol% of hydrolytically labile, crystallisable cross-linking structural units, based on the total structural unit content of the polymeric microparticles.

In some embodiments, the one or more hydrolytically labile, crystallisable cross- linking structural units are derived from monomers having a single linear crystallisable polyester chain having number average molecular weight, M_n, in the range of from 1,500 to 10,000, for instance, from 2,000 to 4,000 Daltons. Preferably, the single linear polyester chain is capped at each end thereof with a suitable olefinic capping group, for instance, selected from vinylic, allylic, acrylic or methacrylic groups. Preferably, the single linear polyester chain has a hydroxy terminus and a carboxy terminus and is capped at the hydroxy terminus by a suitable acrylic ester group, such as an acrylate or methacrylate group, and at the carboxy terminus by a suitable vinylic ester group, such as a vinyl or allyl group. In accordance with these embodiments of the invention, the crystallisable cross- linking structural unit may be derived from a monomer suitably having the formula (A):

wherein each n independently represents an integer of from 1 to 6, p represents an integer selected so as to fulfill the condition that the monomer of formula (A) has a number average molecular weight in the range of from 1,500 to 10,000 Daltons, for instance 2,000 to 4,000 Daltons, m is 0 or 1, and R¹ represents hydrogen, methyl or an ethyl group, preferably, hydrogen or methyl.

In accordance with other embodiments of the present invention, the hydrolytically labile, crystallisable cross-linking structural units are derived from monomers that comprise a linking group having two linear polyester chains attached thereto wherein the terminal (free) end of each polyester chain is capped with an olefinic capping group, for instance, selected from vinylic, allylic, acrylic and methacrylic groups.

In some embodiments, the crystallisable cross-linking monomer, used to prepare the microparticles, may have a structure in which two linear polyester chains are attached to a dioxyalkylene linking group (i.e. a linking group of the formula -O-alkylene-0-) or a bis- dioxyalkylene linking group (i.e. a linking group of the formula

-O-alkylene-O-alkylene-0-) such that the ends of both of the linear polyester chains are hydroxy termini. The hydroxy termini are each capped with a suitable acrylic ester group, such as an acrylate or methacrylate group. In accordance with these embodiments of the invention, the crystallisable cross-linking structural units may be derived from a monomer suitably having the formula (B):

wherein each n independently represents an integer of from 1 to 6; q and r each independently represent an integer selected so as to fulfill the condition that the monomer of formula (B) has a number average molecular weight in the range of from 1,500 to 10,000 Daltons, for instance 2,000 to 4,000 Daltons; each R¹ independently represents hydrogen, methyl, or an ethyl group, preferably, hydrogen or methyl; and, R² represents a group having the formula -(CR'R' ')_y- or the formula -(CR'R") _y-0-(CR'R' ') , wherein the R' and R' ' groups of each (CR'R' ') group independently represent hydrogen, methyl or ethyl and each y independently represents an integer of from 2 to 10. Preferably the R' and R" groups represent hydrogen. Preferably, R² represents a group having the formula -CH₂C(CH₃)₂CH₂-, -CH₂CH₂CH₂C¾-, or -CH₂CH₂-0-CH₂CH₂-.

In accordance with other embodiments of the present invention, the hydrolytically labile, crystallisable cross-linking structural units are derived from monomers that preferably comprise a branching group having three or four linear polyester chains attached thereto wherein the terminal end of each of the linear polyester chains is capped with a suitable olefinic capping group, for instance selected from vinylic, allylic, acrylic and methacrylic groups.

In some embodiments, the branched crystallisable cross-linking monomer, used to prepare the microparticles, may have a structure in which three polyester chains are attached to a CH₃CH₂C(CH₂0-)₃ branching group or four polyester chains are attached to a C(CH₂0-)₄ branching group. In particular, the branched crystallisable cross-linking groups may be derived from a trimethylolpropane polyester triacrylic ester monomer having the formula (C):

wherein each n independently represents an integer of from 1 to 6; q, r and s each independently represent an integer selected so as to fulfill the condition that each polyester chain of formula (C) has a number average molecular weight in the range of from 1,500 to 10,000 Daltons, for instance 2,000 to 4,000 Daltons; and, each R¹ independently represents hydrogen, methyl, or an ethyl group, preferably, hydrogen or a methyl group.

The branched crystallisable cross-linking groups may also be derived from a pentaerythritol polyester tetracrylic ester monomer having the formula (D):

wherein each n independently represents an integer of from 1 to 6; q, r, s and t each independently represent an integer selected so as to fulfill the condition that each polyester chain of formula (D) have a number average molecular weight in the range of from 1,500 to 10,000 Daltons, for instance 2,000 to 4,000 Daltons; and, each R¹ independently represents hydrogen, methyl, or an ethyl group, preferably hydrogen or a methyl group.

In the compounds of formula (A), formula (B), formula (C), and formula (D), each n preferably independently represents an integer of from 2 to 6, more preferably from 4 to 6, still more preferably 4 or 5, and most preferably 4. Still more preferably, each n represents the same integer of from 2 to 6, more preferably from 4 to 6, still more preferably 4 or 5, and most preferably 4.

In the compounds of formula (A), formula (B), formula (C), and formula (D), R¹ preferably represents hydrogen.

In the compounds of formula (A), m preferably represents 0.

In the compounds of formula (B), y preferably independently represents an integer of from 2 to 6, more preferably from 2 to 4. In the case where R² represents a group having the formula -(CR'R' ') _y-0-(CR'R' ') _r, in particular, -(C¾) _y-0-(CH₂) _r, each y may be the same or different, but are preferably the same.

The polyester chains of the hydrolytically labile, crystallisable cross-linking structural units may suitably be obtained via the ring-opening polymerization of the corresponding lactones. For instance, the polyester chains may be derived from one or more lactone precursors having the formula (E):

wherein n represents an integer of from 1 to 6, preferably from 2 to 6, more preferably from 4 to 6, still more preferably 4 or 5, and most preferably 4.

In accordance with the present invention, the polymeric microparticles may comprise a mixture of different microparticles having different hydrolytically labile, crystallisable cross-linking structural units derived from different cross-linking monomers. However, it is preferred that the polymeric microparticles have hydrolytically labile, crystallisable cross-linking structural units derived from the same cross-linking monomer. By "the same" it is meant that the cross-linking monomers are formed from the same chemical precursors and by the same process. It is not excluded that there will be some variation in the length of the polyester portions of the cross-linking monomer. It is not excluded that the polyester portions of the cross-linking monomers may be formed from a mixture of precursors (e.g. where n has a mixture of values). However, a random distribution of different ester units within the polyester portions of the cross-linking monomers is preferably avoided owing to the risk that the cross-linking structural units may not be crystallisable. Thus, where the polyester portions of the cross-linking monomers are formed from a mixture of precursors, it is preferred that the polyester portions are either alternating or block copolymers.

In a particularly preferred embodiment, the polymeric microparticles of the invention comprise a hydrolytically labile, crystallisable cross-linking structural unit derived from a monomer having the formula (F):

wherein q, r, and R² are as defined above.

In the polymeric microparticles of the invention that comprise a hydrolytically labile, crystallisable cross-linking unit derived from a monomer having the formula (F), the crystallisable cross-linking units form crystalline domains within the microparticles which undergo a melting transition at a temperature in the range of about 50 to about 60 °C.

The polymeric microparticles of the present invention are preferably substantially free of non-labile cross-linking structural units derived from non-labile cross-linking monomers. As used herein, the term "non-labile cross-linking structural unit" refers to a cross-linking structural unit which is not readily chemically degraded under conditions of 2014/072206

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temperature and/or pH which cause the chemical degradation of the hydrolytically labile, crystallisable cross-linking structural units. Examples of non-labile cross-linking monomers include compounds such as methylene bisacrylamide and diallylamine.

Preferably, the polymeric microparticles of the invention comprise less than 100 ppm, more preferably less than 50 ppm, and most preferably less than 10 ppm of units derived from non-labile cross-linking monomers, based on the total moles of monomers employed in the preparation of the microparticles. In general, the polymeric microparticles of the invention are prepared without using any non-labile cross-linking monomers, and thus the amount of non-labile cross-linking structural units is effectively 0 ppm based on the total moles of monomers used in the preparation of the microparticles. Without wishing to be bound by any theory, microparticles comprising non-labile cross-linking structural units in amounts greater than those above, are not degraded to form polymeric solutions, but instead form a loosely linked network that retains at least some of the structure of the microparticle.

It is preferred that the polymeric microparticles comprise hydrophilic structural units and structural units derived from the hydrolytically labile, crystallisable cross-ljjiking monomers such that the free polymer chains that are released from the microparticles upon hydrolysis of the cross-linking structural units are either soluble or dispersible in an aqueous fluid. The hydrophilic structural units may be derived from non-ionic, anionic, cationic, amphoteric, ion-pair or betaine monomers, as defined herein.

In preferred embodiments, the polymeric microparticles of the present invention may be non-ionic, anionic, cationic, amphoteric, ion-pair or betaine-containing polymeric microparticles. Preferably, the polymeric microparticles are anionic polymeric microparticles.

As used herein, the term "non-ionic polymeric microparticle" refers to polymeric microparticles that do not comprise any structural units having anionic functional groups, cationic functional groups or salts thereof. Thus, non-ionic polymeric microparticles comprise structural units derived solely from non-ionic monomers and hydrolytically labile, crystallisable cross-linking monomers as defined herein.

As used herein, the term "non-ionic monomer" refers to a polymerisable allylic, vinylic or acrylic compound which is electrically neutral (excluding the anionic monomer salts discussed above and the cationic monomer salts discussed below). Examples of preferred non-ionic monomers for synthesizing the microparticles in accordance with the present invention include acrylamide, methacrylamide, N-methylacrylamide, N-methyl methacrylamide, N,N-dimethylacrylamide, N-isopropylacrylamide, N,N- diethylacrylamide, dimethylaminopropyl acrylamide, dimethylaminopropyl

methacrylamide, acryloyl morpholine, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, dimethylaminoethylacrylate (DMAEA), dimethylaminoethyl methacrylate (DMAEM), maleic anhydride, N- inyl pyrrolidone, vinyl acetate and N- vinyl formamide. In preferred embodiments, the at least one non-ionic monomer for synthesising the microparticles is selected from acrylamide, N- methylacrylamide, N,N-dimethylacrylamide and methacrylamide. More preferably, the at least one non-ionic monomer is acrylamide.

As used herein, the term "anionic polymeric microparticle" refers to polymeric microparticles comprising structural units having anionic functional groups or salts thereof. Suitable anionic polymeric microparticles in accordance with the present invention include copolymers of anionic monomers and hydrolytically labile, crystallisable cross-linking monomers as defined herein, and copolymers of anionic monomers, non-ionic monomers and hydrolytically labile, crystallisable cross-linking monomers as defined herein.

Preferably, the anionic microparticles comprise from about 0.01 to about 10 mol%, preferably, about 0.01 to about 5 mol% of hydrolytically labile, crystallisable cross-linking structural units; about 0.01 to about 85 mol%, preferably, about 5 to about 50 mol% of structural units derived from at least one anionic monomer; and from about 5 to about 99.98 mol%, preferably, from about 5 to about 90 mol% of structural units derived from at least one non-ionic monomer, based on the total molar amounts of monomers used to prepare the microparticles. More preferably, the anionic microparticles comprise from about 15 to about 50 mol% of structural units derived from at least one anionic monomer and from about 50 to about 85 mol% of structural units derived from at least one non-ionic monomer, based on the total molar amounts of the anionic and non-ionic monomers used to prepare the microparticles.

As used herein, the term "anionic monomer" refers to a polymerisable allylic, vinylic or acrylic compound having a negatively charged functional group or a salt thereof.

Preferably, anionic monomers are selected from polymerisable allylic, vinylic or acrylic compounds having an acidic functional group and salts thereof, preferably, the alkali metal, alkaline earth metal, ammonium or phosphonium salts thereof (including

ammonium or phosphonium salts wherein one or more of the hydrogen atoms of the ammonium (NRt^) cation or phosphonium (PlV^") cation are replaced by organic radical groups, for example, quaternary ammonium or quaternary phosphonium cations).

Examples of preferred anionic monomers for synthesizing the microparticles in accordance with the present invention include acrylic acid, methacrylic acid, maleic acid, itaconic acid, 2-propenoic acid, 2-methyl-2-propenoic acid, 2-acrylamido-2-methylpropane sulfonic acid, sulfopropyl acrylic acid, sulfomethylated acrylamide, allyl sulfonic acid, vinyl sulfonic acid, and alkali metal, alkaline earth metal and ammonium or phosphonium salts thereof.

In preferred embodiments, the at least one anionic monomer for synthesizing the microparticles is selected from 2-acrylamido-2-methylpropanesulfonic acid sodium salt, vinyl sulfonic acid sodium salt and styrene sulfonic acid sodium salt. More preferably, the at least one anionic monomer is 2-acrylamido-2-methylpropanesulfonic acid sodium salt.

Preferred anionic polymeric microparticles in accordance with the present invention include those comprising a polymer selected from:

(i) copolymers of acrylamide, 2-acrylamido-2-methylpropane sulfonic acid

(AMPS) (or the AMPS sodium salt), and a hydrolytically labile, crystallisable cross-linking monomer as defined above;

(ii) copolymers of acrylamide, acrylic acid (or sodium acrylate), and a

hydrolytically labile, crystallisable cross-linking monomer as defined above; and

(iii) copolymers of AMPS (or the AMPS sodium salt), acrylic acid (or sodium

acrylate), and a hydrolytically labile, crystallisable cross-linking monomer as defined above.

More preferred anionic polymeric microparticles in accordance with the present invention include those comprising a polymer selected from:

(i) copolymers of AMPS (or the AMPS sodium salt) and a hydrolytically labile, crystallisable cross-linking monomer of formula (F) as defined above;

(ii) copolymers of acrylamide, acrylic acid (or sodium acrylate), and a

hydrolytically labile, crystallisable cross-linking monomer of formula (F) as defined above; and

(iii) copolymers of AMPS (or the AMPS salt), acrylic acid (or sodium acrylate) and a hydrolytically labile, crystallisable cross-linking monomer of formula (F) as defined above.

Particularly preferred anionic polymeric microparticies in accordance with the present invention include those comprising copolymers of from about 5 to about 99.98 mol% acrylamide, from about 0.01 to about 85 mol% 2-acrylamido-2-methylpropane sulfonic acid, and from about 0.01 to about 5 mol% of a hydrolytically labile, crystallisable cross-linking monomer of formula (F) as defined above.

As used herein, the term "cationic polymeric microparticle" refers to polymeric microparticies comprising structural units having cationic functional groups. Suitable cationic polymeric microparticies in accordance with the present invention include copolymers of cationic monomers and hydrolytically labile, crystallisable cross-linking monomers as defined herein, and copolymers of cationic monomers, non-ionic monomers and hydrolytically labile, crystallisable cross-linking monomers as defined herein.

As used herein, the term "cationic monomer" refers to a polymerisable allylic, vinylic or acrylic compound having a cationic functional group or a salt thereof. Examples of preferred cationic monomers for preparing microparticies in accordance with the present invention include the quaternary ammonium salts or acid salts of dialkylaminoalkyl acrylates and methacrylates such as dimethylaminoethylacrylate methyl chloride quaternary salt (DMAEA.MCQ), dimethylaminoethylmethacrylate methyl chloride quaternary salt (OMAEM.MCQ), dimethylaminoethylacrylate hydrochloric acid salt, dimethylaminoethylacrylate sulfuric acid salt, dimethylaminoethyl acrylate benzyl chloride quaternary salt (DMAEA.BCQ) and dimethylaminoethylacrylate methyl sulfate quaternary salt; the quaternary ammonium or acid salts of didkylaminoalkylacrylamides and methacrylamides such as dimethylaminopropyl acrylamide hydrochloric acid salt, dimethylaminopropyl acrylamide sulfuric acid salt, dimethylaminopropyl methacrylamide hydrochloric acid salt and dimethylaminopropyl methacrylamide sulfuric acid salt, methacrylamidopropyl trimethyl quaternary ammonium chloride and acrylamidopropyl trimethyl quaternary ammonium chloride. In preferred embodiments, cationic monomers may be selected from dimethylaminoethylacrylate methyl chloride quaternary salt, and dimethylaminoethylmethacrylate methyl chloride quaternary salt.

As used herein, the term "amphoteric polymeric microparticle" refers to polymeric microparticies comprising structural units having anionic functional groups and structural units having cationic functional groups, although not necessarily in the same stoichiometric proportions. Preferably, the amphoteric polymeric microparticles comprise an excess of structural units having anionic functional groups to structural units having cationic functional groups. Suitable amphoteric polymeric microparticles in accordance with the present invention comprise copolymers of non-ionic monomers, anionic monomers, cationic monomers and hydrolytically labile, crystallisable cross-linking monomers as defined herein.

As used herein, the term "ampholytic ion-pair polymeric microparticle" refers to polymeric microparticles comprising structural units derived from ampholytic ion-pair monomers. Suitable ampholytic ion-pair polymeric microparticles in accordance with the present invention include copolymers of ampholytic ion pair monomers, one or more anionic or non-ionic monomers, and hydrolytically labile, crystallisable cross-linking monomers, as defined herein.

As used herein, the term "ampholytic ion-pair monomers" refers to the acid-base salt of basic, nitrogen-containing monomers such as dimemylaminoethylacrylate (DMAEA), dimethylaminoethyl methacrylate (DMAEM), 2-methacryloyloxyethyldiethylamine and acidic monomers such as acrylic acid and sulfonic acids such as 2-acrylamido-2- methylpropane sulfonic acid, 2-methacryloyloxyethane sulfonic acid, vinyl sulfonic acid, and styrene sulfonic acid.

As used herein, the term "betaine-containing polymeric microparticle" refers to polymeric microparticles comprising structural units derived from betaine monomers. Suitable betaine-containing polymeric microparticles in accordance with the present invention include copolymers of betaine monomers, non-ionic monomers, and

hydrolytically labile, crystallisable cross-linking monomers, as defined herein.

As used herein, the term "betaine monomer" refers to monomers containing cationic and anionic functional groups in equal proportions, such that the monomer has zero net charge. Examples of preferred betaine monomers in accordance with the present invention include N,N-dimethyl-N-acryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine; N,N- dimethyl-N-methacryloyloxyethyl-N-(3 -sulfopropyl)-ammonium betaine; N^-dimethyl-N- acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine; N,N-dimethyl-iV- acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine; N,N-dimethyl-iV- acryloxyethyl-N-(3-sulfopropyl)-ammonium betaine; N,N-dimethyl-N-acrylamidopropyl- N-(2-carboxymethyl)-ammonium betaine; N-3-sulfopropylvinylpyridine ammonium betaine; 2-(methylthio)ethyl memacryloyl-S^sulfopropyl)-sulfonium betaine; and l-(3- sulfopropyl)-2-vinylpyridinium betaine. Preferably, the betaine monomer is N,N-dimethyl- N-methacryloyloxyethyl-iV-(3 -sulfopropyl)-ammonium betaine.

In accordance with preferred embodiments of the invention, the polymeric microparticles are prepared by an emulsion, or microemulsion process or a dispersion process in order to control the particle size distribution of the microparticles. Preferably, the polymeric microparticles are prepared by an inverse emulsion or inverse

microemulsion process or a non-aqueous dispersion process.

Inverse emulsion or inverse microemulsion processes are polymerization processes in which an aqueous solution of monomers is added to an oil containing a surfactant or mixture of surfactants that stabilizes the emulsion or microemulsion. An inverse emulsion consists of a discontinuous phase (also referred to as "disperse phase") of small droplets of the aqueous solution of monomers dispersed in a continuous oil phase wherein the droplets typically have a diameter of greater than 100 nm (0.1 micron). An inverse microemulsion consists of droplets of the aqueous solution of monomers dispersed in an oil phase wherein the droplets typically have a diameter of less than 100 nm. The person skilled in the art will understand that the microparticles are formed within the droplets of the inverse emulsion or inverse microemulsion.

The oil phase of the emulsion or microemulsion preferably comprises a saturated liquid hydrocarbon or a mixture thereof. Suitable hydrocarbon liquids for use as the continuous hydrocarbon phase of the emulsion or microemulsion include benzene, toluene, kerosene, fuel oil, mineral oils (for example, Multipar M supplied by Brenntag UK Limited or ShellSol D80 supplied by Shell Chemicals), and mixtures thereof.

Suitable surfactants for forming the emulsion or microemulsion include sorbitan esters of fatty acids, ethoxylated sorbitan esters of fatty acids, or mixtures thereof.

Examples of preferred surfactants include ethoxylated sorbitol oleate, sorbitan sesquioleate and sodium dodecylsulfate.

The polymeric microparticles of the invention may be obtained in dry form by precipitation of the microparticles from the emulsion using a suitable solvent, such as isopropanol, acetone, isopropanol/acetone or methanol/acetone or other solvents or solvent mixtures that are miscible with both the hydrocarbon and water. The microparticles may be isolated from the supernatant by centrifugation and/or filtration and dried by

conventional procedures. An aqueous dispersion of the polymeric microparticles may subsequently be formed by dispersing the dry polymeric microparticles in water or another suitable aqueous fluid with an appropriate surfactant/dispersant. Suitable

surfactants/dispersants for dispersing the polymeric microparticles are well known to the person skilled in the art.

Suitable procedures for the preparation of cross-linked polymeric microparticles using emulsion or microemulsion polymerization processes are available in the art, and reference in this regard is made to US 4,956,400, US 4,968,435, US 5,171,808, US 5,465,792 and US 5,737,349.

Dispersion processes are polymerization processes in which monomers are added to a an organic liquid dispersion medium containing a dispersion stabilizer, or a mixture of two or more dispersion stabilizers to form a dispersion consisting of a discontinuous phase comprising droplets of monomers (or droplets of a solution of the monomers dissolved in a solvent which is immiscible with the organic liquid dispersion medium) dispersed in a continuous organic liquid phase. Polymerization of the monomers results in dispersed polymeric microparticles that are stabilized by the dispersion stabiliser.

The organic liquid dispersion medium is preferably a mineral oil (for example, a paraffin oil), a silicone oil having hydrocarbyl side chains, for example, poly(dimethyl siloxane) or poly(diethyl siloxane), or mixtures of mineral oils and silicone oils.

Suitable solvents which are immiscible with the organic liquid dispersion medium (hereinafter "immiscible solvent") and which can be used to dissolve monomers to form a discontinuous phase comprising a solution of monomers in the immiscible solvent include, but are not limited to, glycol ethers, alcohols, dimethylacetamide, dimethylformamide, formamide, dimethylsulf oxide, acetonitrile, methyl-2-pyrrolidone, 1,4 dioxane, acetone, and blends of these solvents with water, in particular, a blend of 1,4 dioxane and water. Where the immiscible solvent is not a blend with water, the dispersion is referred to as a non-aqueous dispersion.

Suitable dispersion stabilizers for forming dispersions have a lyophilic group that is solvatable by the organic liquid dispersion medium and a non-lyophilic group that is relatively non-solvatable in the organic liquid dispersion medium. The lyophilic group of the dispersion stabilizer is of such a size and conformation that it extends away from the surface of the microparticles and provides an effective steric barrier around the

microparticles. The non-lyophilic group of the steric stabilizer forms an integral part of the microparticles. For example, the non-lyophilic group may adhere to the surface of the microparticles or may become buried inside the growing microparticles as polymerization proceeds. Alternatively, the non-lyophilic group of the steric stabilizer may be attached to the growing microparticles by means of either covalent or ionic bonds.

Dispersion stabilizers for use in dispersion polymerization processes are well known to the person skilled in the art. Preferred dispersion stabilizers include polymeric dispersion stabilizers such as block copolymer and graft copolymer stabilizers.

Particularly preferred polymeric dispersion stabilizers include block copolymer stabilizers or graft copolymer stabilizers in which the copolymer contains between 30 to 70 % by weight of one or more lyophilic groups and between 30 to 70 % by weight of one or more non-lyophilic) groups (often referred to as anchoring groups).

Where the polymeric dispersion stabilizer is a block copolymer, the copolymer may comprise one or more lyophilic blocks and one or more non-lyophilic blocks. Where the polymeric dispersion stabilizer is a graft copolymer, the copolymer may comprise a lyophilic backbone having one or more non-lyophilic side chains or a non-lyophilic backbone having one or more lyophilic side chains.

A first polymeric dispersion stabilizer may be combined with a second polymeric dispersion stabilizer having a lower molecular weight than the first polymeric dispersion stabilizer in order to achieve optimal steric stabilisation of the microparticles within thedispersion. Preferably, the first polymeric dispersion stabilizer has a number average molecular weight of at least 20,000 Daltons, in particular, at least 25,000 Daltons.

Preferably, the second polymeric dispersion stabilizer has a number average molecular weight in the range of 1 ,000 to 4,000 Daltons, in particular, 1 ,000 to 2,000 Daltons. The first and second polymeric dispersion stabilizers may be independently selected from block copolymers or graft copolymers.

Where the first or second polymeric dispersion stabilizer is a block copolymer, the block copolymer preferably contains between 40 and 60% by weight, preferably, between 45 and 55% by weight, for example, about 50% by weight of one or more lyophilic blocks, and between 40 and 60% by weight, preferably, between 45 and 55% by weight, in particular, about 50% by weight of one or more non-lyophilic blocks. Preferably, the block copolymer is a diblock copolymer having a single lyophilic block and a single non- lyophilic block.

Where the first or second polymeric dispersion stabilizer is a graft copolymer, the graft copolymer preferably contains between 30 to 70% by weight of one or more non- lyophilic polymer chains and between 30 to 70% by weight of one or more lyophilic polymer chains.

Where the first polymeric dispersion stabilizer is a block copolymer, it is preferred that the lyophilic block(s) is selected from polypropylene oxide, polyisoprene,

polybutylene, a polyhydroxyalkanoate (for example, polyhydroxystearate), or from copolymers of two or more lyophilic monomers selected from propylene oxide, isoprene and butylene. Where the first polymeric dispersion stabilizer is a block copolymer, it is preferred that the non-lyophilic block(s) is selected from hydroxypropylmethylcellulose, poly(methyl methacrylate), polyethylene oxide or any other polymer block that is insoluble in the organic liquid phase of the dispersion.

Where the second polymeric dispersion stabilizer is a block copolymer, it is preferred that the lyophilic block(s) is selected from polypropylene oxide, polyisoprene,

polybutylene or a polyhydroxyalkanoate (for example, polyhydroxystearate) or from copolymers of two or more lyophilic monomers selected from propylene oxide, isoprene, and butylene. Preferably, the lyophilic block of the second polymeric dispersion stabilizer is a polyhydroxyalkanoate, in particular, polyhydroxystearate. The non-lyophilic block of the second block copolymer dispersion stabilizer may be any polymeric block having units derived from an anionic, cationic, or non-ionic monomer or mixtures thereof, preferably, from cationic monomers.

Where the first or second polymeric dispersion stabilizer is a graft copolymer, it is preferred that the graft copolymer comprises a non-lyophilic backbone selected from polyacrylonitrile, polymethylmethacrylate, polyhydroxyethylmethacrylate, polyethylene oxide, and polyacrylamide wherein the backbone has at least one lyophilic side chain selected from polylauryl methacrylate, a polyhydroxyalkanoate (for example,

polyhydroxystearate), or polyisobutylene pendant therefrom. Alternatively the graft copolymer may comprise a lyophilic backbone selected from polylauryl methacrylate, a polyhydroxyalkanoate (for example, polyhydroxystearate), and polyisobutylene wherein the backbone has at least one non-lyophilic side chain selected from polyacrylonitrile, polymethylmethacrylate, polyhydroxyethylmethacrylate, polyethylene oxide, and polyacrylamide pendant therefrom.

It is particularly preferred to combine a higher molecular weight block or graft copolymer dispersion stabilizer (for example, having a number average molecular weight of at least 20,000 Daltons, in particular, at least 25,000 Daltons) with a lower molecular weight block or graft copolymer (for example, having a number average molecular weight in the range of 1,000 to 4,000 Daltons) in order to achieve optimal stabilization of the microparticles within the dispersion.

The dispersion polymerization process may be carried out using any of the well- known methods in the art for the dispersion polymerization of ethylenically unsaturated compounds, and reference in this regard is made to US 3,095,388, US 3,317,635 and US 3,514,500.

The polymerization process (emulsion, microemulsion or dispersion polymerization process) may be initiated using a thermal or redox free-radical initiator. Suitable initiators include azo compounds, such as azobisisobutyronitrile (AIBN) and 4,4'-azobis(4- cyanovaleric acid) (ACVA); peroxides, such as di-t-butyl peroxide; inorganic compounds, such as potassium persulfate; and, redox couples, such as benzoyl

peroxide/dimethylaminopyridine and potassium persulfate/sodium metabisulfite.

In addition to the monomers, cross-linkers, polymerization initiator and stabilizer(s), the emulsion, microemulsion or dispersion may also contain other conventional additives, for instance pH adjusters, and chelating agents to remove polymerization inhibitors.

In some embodiments, the microparticles of the present invention may comprise randomly dispersed crystalline and/or semi-crystalline domains within the microparticle structure. Such microparticles may be obtained by adding the hydrolytically labile, crystallisable cross-linking monomers to the polymerization reaction mixture together with the non-cross-lmking monomers.

In other embodiments, the microparticles of the present invention may have a crystalline core encapsulated by a non-crystalline shell. Such microparticles may be obtained by initially adding only the hydrolytically labile, crystallisable cross-linking monomers (or a major portion of the hydrolytically labile, crystallisable cross-linking monomers and a minor portion of non-cross-linking monomers) to the polymerization reaction mixture and allowing the polymerization reaction to proceed for an initial period of time, followed by addition of the non-cross-linking monomers (or a major portion of the non-cross-linking monomers and a minor portion of the hydrolytically labile, crystallisable cross-linking monomers) and allowing the polymerization reaction to continue to completion. In this way, a nucleus is formed from the hydrolytically labile, crystallisable cross-linking monomers which is encapsulated by non-crystalline polymer chains as the non-cross-linking monomers continue to polymerize. The microparticles may also have a crystalline core encapsulated by a non-crystalline layer which is further encapsulated by a crystalline outer layer. The person skilled in the art would understand that such

microparticles may be prepared by adding additional crystallisable cross-linking monomers towards the end of the polymerization reaction.

In still further embodiments, the microparticles of the present invention may have a non-crystalline core encapsulated by a crystalline shell. Such microparticles may be obtained by initially adding only the non-cross-linking monomers (or a major portion of the non-cross-linking monomers and a minor portion of the hydrolytically labile, crystallisable cross-linking monomers) to the polymerization reaction mixture and allowing the polymerization reaction to proceed for an initial period of time, followed by addition of the hydrolytically labile, crystallisable cross-linking monomers (or a major portion of the hydrolytically labile, crystallisable cross-linking monomers and a minor portion of the non-cross-linking monomers) and allowing the polymerization reaction to continue to completion. In this way, a nucleus is formed from the non-cross-linking monomers which is encapsulated by a crystalline layer as the hydrolytically labile, crystallisable cross-linking monomers continue to polymerize. The microparticles may also have a non-crystalline core encapsulated by a crystalline layer which is further encapsulated by a non-crystalline outer layer. The person skilled in the art would understand that such microparticles may be prepared by adding additional non-cross- linking monomers towards the end of the polymerization reaction.

If desired, in order to mitigate the risk of swelling of the microparticles upon dispersal of the microparticles in an aqueous fluid, non-crystallisable cross-linking monomers may be added to the polymerization reaction mixture towards the end of the reaction period to provide additional surface cross-linking (hereinafter referred to as

"surface cross-linking monomers". These surface cross-linking monomers may have the same structure as any of the crystallisable cross-linking monomers, described herein, in particular, the same structure as formulae A to D and F, except that the monomers are of lower molecular weight, for example, have a number average molecular weight of less than 1,250, preferably, less than 1,000, in particular, less than 750, such that the monomers are non-crystalline. Alternatively, the surface cross-linking monomers may include di- acrylamides and methacrylamides of diamines such as the diacrylamide of piperazine, acrylate or methacrylate esters of di, tri, tetra hydroxy compounds including ethyleneglycol diacrylate, polyethyleneglycol diacrylate, trimethylolpropane trimethacrylate, ethyoxylated trimethylol triacrylate, ethoxylated pentaerythritol tetracrylate, and the like; divinyl or diallyl compounds separated by an azo such as the diallylamide of 2,2'-azobis(isobutyric acid) and the vinyl or allyl esters of di or trifunctional acids.

In another aspect, the present invention provides a composition comprising polymeric microparticles as defined above dispersed in an aqueous fluid.

In preferred embodiments, the polymeric microparticle composition comprises polymeric microparticles as defined above in an amount of from 0.01 to 10 % by weight based on the total weight of the composition, more preferably in an amount of from 0.02 to 5 % by weight based on the total weight of the composition, and most preferably from 0.05 to 1 % by weight based on the total weight of the composition.

Where the polymeric microparticle composition is formed by dispersing dried microparticles in an aqueous fluid, the microparticles are preferably dispersed in a water- miscible organic solvent to form a concentrated dispersion of the microparticles in the water-miscible organic solvent which is subsequently diluted into the aqueous fluid as this had been found to mitigate the risk of aggregation of the microparticles. Suitable water- miscible solvents include tetrahydrofuran, 1,3-butylene glycol, tetrahydrofurfuryl alcohol, ethylene glycol monobutyl ether, ethylene glycol methyl ether, mono ethylene glycol, and methyl ethyl ketone. Suitably, the amount of water-miscible solvent used to form the concentrated dispersion is in the range of 50 to 150 millilitres (ml), preferably, 75 to 125 ml, for example, about 100 ml per kilogram of dried polymeric microparticles. Optionally, the water-miscible solvent may be subsequently removed from the diluted dispersion via a cross-flow ultrafiltration process.

A surfactant dispersant may be used to assist in dispersing either the dried

microparticles or the concentrated dispersion of the microparticles in the water-miscible organic solvent in the aqueous fluid. Suitable dispersants for dispersing dried microparticles in an aqueous fluid are well known to the person skilled in the art and include sodium dodecylsulfate, polyoxyethylene-20-sorbitan monooleate and sodium laureth sulfate.

Suitably, one or more surfactants employed in surfactant polymer (SP) flooding may be added to the aqueous fluid so that when the dispersion is injected into a hydrocarbon- bearing formation, the viscosified aqueous fluid that is formed in situ acts as an SP flooding medium. Typically, the SP flooding medium that is formed in situ has an ultra- low interfacial tension (IFT) with the hydrocarbon that is present in the formation resulting in incremental hydrocarbon recovery from the formation. It is also believed that the SP flooding medium may alter the wettability of the formation rock thereby releasing incremental hydrocarbon from the formation. Suitably, in addition to the surfactant, an alkali may be added to the aqueous fluid so that the viscosified fluid that is formed in situ acts as an alkali surfactant polymer (ASP) flooding medium. The person skilled in the art will understand that the type of surfactant that is added to the aqueous fluid and the concentration of surfactant in the aqueous fluid will be dependent on the salinity and hardness of the aqueous fluid, the salinity and hardness of the formation water, the reservoir temperature and the rock mineralogy. The surfactant that is added to the aqueous fluid may be selected from an anionic, non-ionic or cationic surfactant or mixtures thereof. In addition, a co-surfactant or co-solvent may be added to the aqueous fluid. Examples of suitable surfactants include petroleum sulfonates, alkyl aryl sulfonates, alpha olefin sulfonates, internal olefin sulfonates (10 S), alkyl ether sulfates having ethoxylated or propoxylated groups, alkyl glyceryl ether sulfonates, guerbet alkoxy sulfates, guerbet alkoxy carboxylates, and tristyrylphenol alkoxy sulfate. Preferred surfactants for generating ASP or SP flooding media in situ are given below for different reservoir conditions: Reservoir Condition Flooding Preferred Surfactants

Medium

Low temperature (<60°C), Low ASP/SP Alcohol ethoxy/propoxy sulfate, Salinity (<3% by weight NaCl) alkylaromatic sulfonate, 10 S

Low Temperature (<60°C), High ASP/SP Alcohol ethoxy/propoxy sulfate, Salinity (>3% by weight NaCl) IOS

Low Temperature (<60°C) , High SP Alcohol ethoxy/propoxy sulfate Hardness (>1700 ppm of hardness

ions, sea water)

High Temperature (>60°C), Low ASP/SP Alcohol ethoxy/propoxy

Salinity (<3% by weight NaCl) sulfonate, alkylaromatic

sulfonate, IOS

High Temperature(>60°C), High ASP/SP IOS, Alcohol ethoxy/propoxy Salinity (>3% by weight NaCl) sulfonate

High Temperature (>60°C), High SP Alcohol ethoxy/propoxy

Hardness (>1700 ppm of hardness sulfonate

ions, sea water)

When an SP flooding medium is to be generated in situ, the concentration of surfactant in the aqueous fluid may be in the range of 0.1 to 15% by weight, for example, 0.25 to 10% by weight, in particular 0.25 to 7.5 % by weight. When an ASP flooding medium is to be generated in situ, the concentration of surfactant in the aqueous fluid is typically in the range of 0.1 to 0.5 weight%. The lower concentration of surfactant is because the alkali that is included in the aqueous fluid reacts with acidic components of crude oil to generate surfactant in situ. In addition, surfactant adsorption is decreased owing to the higher pH of the ASP flooding medium. The alkali that is added to the aqueous fluid may be selected from Na₂C0₃, NaOH, NaHC0₃, KOH, LiOH, NH₄OH, sodium metaborate, sodium silicate, sodium orthosilicate, sodium ethylene diamine tetracetate (sodium form of EDTA), other poly-carboxylates and mixture thereof. The amount of alkali added to the aqueous fluid may be in the range of 0.75 to 1.5% by weight.

In accordance with this aspect of the invention, the aqueous fluid may be any aqueous fluid suitable for injection into a subterranean formation via an injection well. For instance, the aqueous fluid may be fresh water, estuarine water, brackish water, seawater, aquifer water, desalinated water, produced water or mixtures thereof. Due to the cross- linked nature of the microparticles of the invention, the microparticles do not significantly expand or contract when brought into contact with aqueous fluids of different salinity. Accordingly, the salinity of the aqueous fluid or the salinity of fluids that may be encountered in subterranean formations such as connate water or previously injected water has little or no effect on the viscosity of the dispersion. Thus, no specific aqueous fluid is required to form the aqueous microparticle compositions of the invention. Only once the dispersed microparticles reach a temperature sufficient to melt the crystalline domains of the microparticles is the viscosity increased as required to improve hydrocarbon recovery from the formation. Whilst the viscosity of the aqueous microparticle dispersion (prior to dissolution of the microparticles) is not affected by the salinity of the aqueous fluid in which the microparticles are dispersed, it is recognized that the viscosities of the majority of polymers used for enhanced oil recovery have been shown to be a function of water salinity and divalent cation content (Lee et al, 2009). Accordingly, it is preferred to disperse the microparticles in a lower salinity aqueous fluid, in particular, a lower salinity aqueous fluid having a low divalent cation content so as to effect the maximum viscosity change upon hydrolysis of the labile cross-links of the polymeric microparticles. By low salinity aqueous fluid is meant a water having a total dissolved solids (TDS) content of less than 10,000 ppm (on a weight by volume basis), preferably, in the range of 200 to 8,000 ppm, in particular, 500 to 5,000 ppm. By low divalent cation content is meant a concentration of divalent cations that is preferably less than 100 ppm, in particular, less than 40 ppm.

In other embodiments, the aqueous fluid may take the form of an oil-in-water emulsion. The discontinuous phase of the oil-in-water emulsion may be, for example, a crude oil, a refined petroleum fraction, a mineral oil, a synthetic hydrocarbon or any non- hydrocarbon oil that is capable of forming a stable emulsion with the continuous aqueous phase. Preferably, such a non-hydrocarbon oil is biodegradable and is therefore not associated with ecotoxic problems. It is particularly preferred that the non-hydrocarbon oil has a solubility in water at room temperature of less than 2 % by weight, preferably, less than 1.0 % by weight, most preferably, less than 0.5 % by weight.

Where the aqueous fluid is an oil-in-water emulsion, the oil phase is for example dispersed in the continuous aqueous phase in an amount of from 0.01 to 10 % by volume, preferably 0.02 to 5% by volume, most preferably 0.05 to 2 % by volume based on the total volume of the aqueous and oil phases. Generally, the oil phase is distributed in the aqueous phase in the form of finely divided droplets.

Suitably, the discontinuous oil phase may be a hydrocarbon oil or a non-hydrocarbon oil selected from the group consisting of polyalkylene glycols, esters, acetals, ethers and alcohols.

Suitable polyalkylene glycols include polypropylene glycols (PPG), polybutylene glycols, polytetrahydrofurans, and polyalkylene glycols produced by the polycondensation of 1,3-propane diol or by the polymerization of trimethylene oxide. Preferably, the molecular weight of the polyalkylene glycol should be sufficiently high that the polyalkylene glycol has a solubility in water at room temperature of less than 2 % by weight. The person skilled in the art would be able to readily select polyalkylene glycols that exhibit the desired low-water solubility.

Suitable hydrocarbon oils include polyalphaolefins (as disclosed in EP 0325466, EP

0449257, WO 94/16030 and WO 95/09215); isomerized linear olefins (as disclosed in EP 0627481A, US 5,627,143, US 5,432,152 and WO 95/21225); n-paraffms, in particular n- alkanes (see, for example, US 4,508,628 and US 5,846,913); linear alkyl benzenes and alkylated cycloalkyl fluids (see GB 2,258,258 and GB 2,287,049 respectively).

Suitable esters include esters of unsaturated fatty acids and saturated fatty acids (as disclosed in EP 0374671 A and EP 0374672 respectively); esters of neo-acids (as disclosed in WO 93/23491); oleophilic carbonic acid diesters having a solubility of at most 1% by weight in water (as disclosed in US 5,461,028); triglyceride ester oils such as rapeseed oil (as disclosed in US 4,631,136 and WO 95/26386).

Suitable acetals include those described in WO 93/16145.

Suitable ethers include those described in EP 0391251 and US 5,990,050.

Suitable alcohols include oleophilic alcohol-based fluids as disclosed in EP

0391252A.

The aqueous dispersion of the microparticles in an oil-in- water emulsion may be formed by adding a concentrated dispersion of the microparticles in a water-in-oil emulsion to an injection water such that the water-in-oil emulsion is inverted into an oil-in- water emulsion before injection of the dispersion into the hydrocarbon-bearing formation. Suitably, the water-in-oil emulsion of the concentrated dispersion comprising between 20 and 40% by weight water, for example, 20 to 35% by weight water.

In another embodiment, the present invention provides a process for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation penetrated by at least one injection well and at least one production well, the process comprising

(i) injecting into said formation through said at least one injection well a

composition comprising polymeric microparticles as defined above dispersed in an aqueous fluid (hereinafter referred to as "dispersion");

(ii) propagating said composition through the subterranean formation towards said at least one production well; and

(iii) recovering hydrocarbons from said at least one production well;

wherein said subterranean hydrocarbon-bearing formation contains at least one zone between said at least one injection well and said at least one production well having a temperature at which the hydrolytically labile cross-linking structural units of the microparticles undergo a melting transition.

In accordance with the process of the present invention, the composition comprising polymeric microparticles dispersed in an aqueous fluid is of relatively low viscosity and can be injected into the subterranean formation at relatively low injection pressures, with the proviso that the injection pressure is above the pressure in the subterranean formation, and can propagate far from the injection point through low temperature zones of the subterranean formation substantially unimpeded and without degradation of the tightly- bound polymeric microparticles.

Once the composition reaches a zone of the subterranean formation having a temperature above the melting transition temperature of the crystalline domains of the microparticles, the crystalline domains melt and the cross-linking functional groups are hydrolysed by the surrounding aqueous fluid, releasing free polymer chains into the aqueous fluid to form a viscosified polymer solution in situ. Consequently, the mobility of the injected fluid is lowered thereby improving the sweep efficiency within the formation.

Suitably, the zone of the subterranean formation having a temperature above the melting transition temperature of the crystalline domains of the microparticles is not so close to the injection well as to reduce injectivity of the dispersion and not so close to the production well that only a minor portion of the hydrocarbon-bearing formation is swept by the viscosified polymer solution that is formed in situ. Typically, injected waters are at a lower temperature than the hydrocarbon bearing formation such that the injected water cools the formation giving rise to a temperature front in the formation at an increasing radial distance from the injection well. The zone of the formation that is above the melting transition of the crystalline domains of the microparticles is preferably beyond this temperature front. Preferably, the temperature front is located at a radial distance from the injection well fhat is between 1% to 45%, preferably, 1 to 25%, of the interwell distance between the injection well and the production well i.e. is closer to the injection well than the production well. Accordingly, about 55 to 99%, preferably about 75 to 99% of effective pore volume of the hydrocarbon-bearing formation is swept by the viscosified polymer solution.

The composition comprising polymeric microparticles dispersed in an aqueous fluid may be injected in an amount sufficient to form a viscosified polymer solution that does not disperse in the subterranean formation such that a front of the viscosified polymeric solution moves through the formation towards a production well.

Suitably, the composition comprising polymeric microparticles dispersed in an aqueous fluid is injected continuously into the subterranean formation. However, it is also envisaged that the composition may be injected into the subterranean formation in a pore volume (PV) amount in the range of 0.25 to 0.75 PV, preferably 0.3 to 0.5 PV.

The term "pore volume" is used herein to mean the "effective pore volume" between an injection well and a production well. The "effective pore volume" is the interconnected pore volume or void space in a rock that contributes to fluid flow or permeability in a reservoir. Effective pore volume excludes isolated pores and pore volume occupied by water adsorbed on clay minerals or other grains. Effective pore volume may be determined using techniques well known to the person skilled in the art such as from reservoir modelling or reservoir engineering calculations.

Once the composition comprising polymeric microparticles dispersed in an aqueous fluid has been injected into the subterranean formation in an amount sufficient to ensure a discrete slug of viscosified polymeric solution is formed in situ, an aqueous drive fluid may be injected behind the microparticle composition to advance the front of the viscosified polymeric solution through the formation and to sweep at least a portion of the hydrocarbons within the formation to the at least one production well. Suitably, this aqueous drive fluid may be seawater, estuarine water, brackish water, an aquifer water, a produced water or mixtures thereof. Preferably the aqueous drive fluid is the aqueous fluid that is used to prepare the dispersion of the microparticles according to the present invention.

Preferably, the concentration of polymeric microparticles in the dispersion is tapered off towards the end of the injection period, for example, when injecting the final 0.1 or 0.05 PV portion of the dispersion, such that the viscosity of the polymer solution that is formed in situ is gradually reduced to that of the aqueous drive fluid.

The dispersion of the microparticles in the aqueous fluid is particularly suitable for use in recovery of hydrocarbons from hydrocarbon-bearing formations where the hydrocarbon is an oil having an American Petroleum Institute (API) gravity of at least 15°, preferably at least 20° more preferably, at least 30°, for example an API gravity in the range of30 to 50°.

Preferably, the oil associated with the reservoir rock has an apparent viscosity in the range of 1 to 10,000 cP, preferably, 5 to 1000 cP.

Preferably, the amount of microparticles dispersed in the aqueous fluid that is injected into the hydrocarbon-bearing subterranean formation is sufficient to form a viscosified aqueous solution in situ having a mobility that closely matches the mobility of the oil, preferably achieving a mobility ratio approaching 1. Typically, the microparticles are dispersed in the aqueous fluid that is injected into the hydrocarbon-bearing

subterranean formation in an amount of at least 50 ppm by weight, preferably, at least 250 ppm by weight, for example, in an amount in the range of 500 to 20,000 ppm by weight, preferably, 500 to 5,000 ppm by weight.

Mobility ratio is defined herein as the mobility of the viscosified aqueous fluid (the displacing fluid) that is formed in situ divided by the mobility of the hydrocarbon, for example, oil it is displacing (the displaced fluid). Mobility, M, is a measure of the flow of fluid through a permeable formation. It is defined herein as the ratio of the relative permeability of the fluid moving through a porous medium divided by the apparent viscosity of the fluid.

As a general principle at a mobility ratio of 1, the fluid front moves almost in a "plug flow" manner and the sweep of the reservoir is good. In contrast, when the mobility of the water is ten times greater than the oil, viscous instabilities, known as fingering, develop and the sweep of the reservoir is poor. When the mobility of the oil is ten times greater than the water, the sweep of the reservoir is almost total, provided there are no high permeability channels between the injection well and production well.

Preferably, the composition is injected into the subterranean formation at a temperature below that at which the hydrolytically labile cross-linking structural groups of the microparticles undergo a melting transition. Suitably, the composition is injected into the subterranean formation at a temperature of 50 °C or less, preferably 40 °C or less, more preferably 30 °C or less, in particular, a temperature in the range of 4 to 50°C.

The process of the present invention is particularly suitable for the recovery of hydrocarbons, in particular, oil, from subterranean hydrocarbon-bearing formations containing at least one zone between said at least one injection well and said at least one production well having a temperature of greater than 50 °C. For instance, the subterranean hydrocarbon-bearing formation may contain at least one zone having a temperature of 60 °C or greater, for example 70 °C or greater. Preferably, the temperature in the reservoir beyond the temperature front is in the range of greater than 50 to 100 °C, preferably 60 to 90 °C.

In preferred embodiments, the process of the present invention may be used at commencement of oil production from the reservoir (omitting primary recovery), in secondary recovery mode (after primary recovery of oil under the natural pressure of the reservoir) or in tertiary recovery mode (for example, after a previous waterflood).

There may be one injection well and one production well, but preferably there may be more than one injection well and more than one production well. There may be many different spatial relations between the or each injection well and the or each production well. Injection wells may be located around a production well. Alternatively the injection wells may be in two or more rows between each of which are located production wells.

These configurations are termed "pattern flood", and the person skilled in the art will know how to operate the injection wells to achieve maximum oil recovery during the water flood treatment (secondary or tertiary recovery). The person skilled in the art will understand that depending on the spatial arrangement of the injection well and its associated production wells, the polymer flood may break-through into each production well at different times.

The composition comprising polymeric microparticles dispersed in an aqueous fluid may be injected into at least one of the injection wells, preferably into a plurality of injection wells. However, there is no requirement to inject the composition into all of the injection wells, especially where there is compartmentalization of the reservoir.

The present invention will now be illustrated by reference to the following examples. Example 1 -Preparation of a Polycaprolactone Diacrylate labile cross-linker

12.72g (6.0mmol) of a linear polyester diol (Capa™ 2200; supplied by Perstorp) derived from caprolactone monomer, initiated with neopentyl glycol, terminated by primary hydroxyl groups and possessing a number average molecular weight of approximately 2,000^ο1^_1 , was dissolved in 100ml of anhydrous toluene. 5.67 ml (40.5mmol) of triethylamine was added with stirring followed by the drop- wise addition of 3.33 ml (40.5mmol) of acryloyl chloride during which time the reaction mixture was maintained at room temperature. The reaction mixture was heated at a temperature of 70 °C for 3 hours before being cooled and filtered. The product was then re-dissolved in a minimum amount of toluene and was precipitated from solution by addition to n-hexane. The precipitated polycaprolactone (PCL) diacrylate product was dried under vacuum.

The identity of the polycaprolactone 2000 diacrylate product was confirmed by Matrix Assisted Laser Desorption/Ionization Time of Flight (MALDI-TOF) mass spectrometry utilising trans-2-[3-(4-t-butyl-phenyl)-2-methyl-2- propenylidene]malononitrile/ a+ (DCTB/Na+) as the matrix. Linear regression analysis confirmed the presence of two acrylate groups per molecule. The product was denoted as PCL-2000 Diacrylate.

Example 2

3.78g of a monomer solution was prepared containing 0.33 g of acrylamide monomer (AA), 0.71 g of 2-acrylamido-2-methyl-l-propanesulfonic acid monomer (AMPS), 0.24 g of PCL-2000 diacrylate monomer (as prepared in Example 1) and 2.5g of

dimethylacetamide solvent. 15.67g of a liquid dispersion medium was prepared containing 0.20g of a polybutylene oxide-b-polyethylene oxide diblock copolymer dispersion stabilizer (PB0₁₆oPE0₃oo, 53 wt% polyethylene oxide (PEO) block, molecular weight of about 24,000 gmol^"1), 0.15g of silicone oil (Dow Coming 200 Fluid) and 15. Og of Multipar® M paraffin oil.

The monomer solution (discontinuous phase) and liquid dispersion medium

(continuous phase) containing the single dispersion stabilizer were homogenized in a reaction vessel using a Silverson L4RT mixer at a stirring rate of 8000 revolutions per minute (rpm) for 5 minutes. The reaction mixture was deoxygenated with nitrogen gas for 30 minutes before addition of 2 weight% (based on the total weight of all monomers in the system) of 4,4 '-azobis(4-cyano valeric acid) (ACVA). The reaction mixture was heated at a temperature of 65 °C and stirred at 400 rpm for 4 hours.

It was observed that a significant amount of coagulum formed on the sides of the reaction vessel during the course of the reaction. The contents of the reaction vessel which remained in partial suspension were then isolated by centrifugation and washed with acetone. After drying, the isolated product could not be dispersed in aqueous media and only a viscous solution was formed.

Example 3 - Microparticle Synthesis

3.78g of a monomer solution was prepared containing 0.33 g of acrylamide monomer (AA), 0.71 g of 2-acrylamido-2-methyl-l-propanesulfonic acid monomer (AMPS), 0.24 g of PCL-2000 diacrylate monomer (as prepared in Example 1) and 2.5g of dimethylacetamide solvent. 15.67g of a liquid dispersion medium was prepared containing 0.20g of a polybutylene oxide-polyethylene oxide diblock copolymer dispersion stabilizer (PBO₁₆₀PEO₃₀₀, 53 wt% PEO, Mw of about 24,000 gmol^"1), 0.32 g of a polyethylene oxide-polyhydroxystearic acid diblock copolymer dispersion stabilizer supplied by Lubrizol under the trade name Solplus K240, 0.15g of silicone oil (Dow Corning 200 Fluid) and 15.0g of Multipar® M paraffin oil.

The monomer solution (discontinuous phase) and liquid dispersion medium

(continuous phase) containing the dispersion stabilizers were homogenized in a reaction vessel using a Silverson L4RT mixer at a stirring rate of 8000 rpm for 5 minutes. The reaction mixture was deoxygenated with nitrogen gas for 30 minutes before addition of 2wt% (based on the total weight of all monomers in the system) of 4,4'-azobis(4- cyanovaleric acid) (ACVA). The reaction mixture was heated at a temperature of 65 °C and stirred at 400 rpm for 4 hours.

The polymeric microparticles were analysed by optical microscopy using a microscope fitted with a graticule scale; particle sizes ranged from 1 to 4 microns.

The polymeric microparticles were isolated by centrifugation and washing with acetone. After drying, the oil free microparticles were capable of being re-dispersed in aqueous media. Differential Scanning Calorimetry (DSC) analysis of both the polycaprolactone 2000 diacrylate cross-linking monomer and the microparticles was performed using a Perkin Elmer Pyris 1 instrument. A heating cycle of 25°C to 100°C was employed with the temperature increasing at a rate of 10°C per minute. DSC analysis of the polycaprolactone 2000 diacrylate cross-linking monomer showed a peak at a temperature of about 50°C. Similarly, DSC analysis of a sample of the isolated polymeric microparticles indicated melting transitions at temperatures of about 38 °C and about 56 °C indicative of crystalline domains in the microparticles arising from structural units derived from the

polycaprolactone 2000 diacrylate cross-linking monomer.

Example 4 - Microparticle Synthesis

dimethylacetamide solvent. 15.67g of a liquid dispersion medium was prepared containing 0.20g of a polybutylene oxide-b-polyethylene oxide diblock copolymer dispersion stabilizer (PB0₁₆₀PE0₃₀o, 53 weight% PEO, Mw of about 24,000 gmol^"1), 0.32 g Solplus K240 dispersion stabilizer (Lubrizol), 0.15g of silicone oil (Dow Corning 200 Fluid) and 15.0g of Multipar® M paraffin oil.

The monomer solution (discontinuous phase) and liquid dispersion medium

(continuous phase) comprising the dispersion stabilizers were homogenized in a reaction vessel using a Silverson L4RT mixer at 8000 rpm for 5 minutes. The reaction mixture was deoxygenated with nitrogen gas for 30 minutes before addition of 0.4wt% (based on the total weight of all monomers in the system) of 4,4'-azobis(4-cyanovaleric acid) (ACVA). The reaction mixture was heated at a temperature of 65 °C and stirred at a rate of 400 rpm for 4 hours.

The polymeric microparticles were analysed by optical microscopy using a microscope fitted with a graticule scale; particle sizes ranged from 1 to 4 microns. The polymeric microparticles can be isolated by centrifugation and washing with acetone followed by drying. After drying, the oil free microparticles were capable of being re- dispersed in aqueous media.

Differential Scanning Calorimetry (DSC) analysis of a sample of the isolated polymeric microparticles using a Perkin Elmer Pyris 1 instrument, operated with a heating cycle of 25 to 100°C (at a rate of 10°C per minute), indicated a melting transition at a temperature of about 54 °C (compared with a melting transition at a temperature of about 50°C for the polycaprolactone 2000 cross-linking monomer) indicative of crystalline domains in the microparticles arising from structural units derived from the

polycaprolactone 2000 diacrylate cross-linking monomer.

Example 5 - Microparticle Synthesis

3.78g of a monomer solution was prepared containing 0.33 g of acrylamide monomer (AA), 0.71 g of 2-acrylamido-2 -methyl- 1-propanesulfonic acid monomer (AMPS), 0.24 g of PCL-2000 diacrylate monomer (as prepared in Example 1) and 2.5g of

dimethylacetamide solvent. 15.67g of a liquid dispersion medium was prepared containing 0.20g of a polybutylene oxide-b-polyethylene oxide diblock copolymer dispersion stabilizer (PB0₁₆oPE0₃₀₀, 53 weight% PEO, Mw of about 24,000 gmol^"1), 0.32 g Solplus K240 dispersion stabilizer (Lubrizol), 0.15g of silicone oil (Dow Corning 200 Fluid) and 15.0g of Multipar® M paraffin oil.

The monomer solution (discontinuous phase) and the liquid dispersion medium

(continuous phase) were homogenized in a reaction vessel using a Silverson L4RT mixer at 8000 rpm for 5 minutes. The reaction mixture was deoxygenated with nitrogen gas for 30 minutes before addition of 0.2wt% (based on the total weight of all monomers in the system) of 4,4'-azobis(4-cyanovaleric acid) (ACVA). The reaction mixture was heated at a temperature of 65 °C and stirred at a rate of 400 rpm for 4 hours.

The polymeric microparticles were analysed by optical microscopy using a microscope fitted with graticule scale; particle sizes ranged from 1 to 4 microns. The polymeric microparticles were isolated by centrifugation and washing with acetone followed by drying. After drying, the oil free microparticles were capable of being re- dispersed in aqueous media.

Differential Scanning Calorimetry (DSC) analysis of a sample of the isolated polymeric microparticles using a Perkin Elmer Pyris 1 instrument, operated with a heating cycle of 25 to 100°C (at a rate of 10°C per minute), indicated a melting transition at a temperature of about 52 °C (compared with a melting transition at a temperature of about 50°C for the polycaprolactone 2000 cross-linking monomer) indicative of crystalline domains in the microparticles arising from structural units derived from the

polycaprolactone 2000 diacrylate cross-linking monomer. Example 6 - Degradation Studies on Microparticles dispersed in Synthetic Seawater

Polymeric microparticles prepared and isolated as per Examples 4 and 5 were re- dispersed in synthetic seawater at 0.1 weight% (lOOOppm).

Ion Composition (mg/1) Compound Composition (g/1)

Na⁺ 10,890 NaCl 24.0738

Ca²⁺ 428 CaCl₂.6H20 2.3395

Mg²⁺ 1,368 MgCl₂.6H20 11.4362

K⁺ 460 KCl 0.8771

S0₄ ^2" 2,960 Na₂S0₄ 4.376

CI^" 19,766

The samples were placed in a thermostatically controlled oven maintained at a temperature of 65 °C. The samples were removed after 4 days and their viscosity analysed by steady shear rate profiling using a rheometer (supplied by TA Instruments). The samples were placed on a horizontal plate and a shallow acrylic cone having a widest diameter of 6 cm and an angle between the surface of the cone and the plate of the order of 1⁰ was placed into the sample. The plate was rotated and the force on the cone measured. A solvent trap cover was employed to minimise evaporation. Samples were exposed to a shear rate down sweep from 40s^"1 to 0.1s^"1. A steady state flow mode was employed to ensure samples had reached or approached equilibrium before commencing data collection. Control samples which were prepared at concentration of 0.1 wt% in synthetic seawater and stored for 4 days at a temperature of 23 °C were also analysed using an identical methodology.

Viscosity (cP)

Shear EXAMPLE 3 EXAMPLE 3 EXAMPLE 4

EXAMPLE 4

Rate 4 days @ 4 days @ 4 days @ Control

4 days @ 23°C

(1/s) 23°C 65°C 65°C Sample*

40 1.79 1.76 1.95 2.32 6.74

10 1.664 1.801 1.753 3.342 8.2

4 1.664 1.916 1.709 5.225 8.5

1 2.126 2.953 2.614 17.02 8.9

0.63 2.646 1.248 2.81 24.39 -

0.25 2.732 10.63 3.212 53.84 -

0.1 2.669 19.72 2.653 120.9 - * Control sample consists of a linear HP AM having a number average molecular weight of 16 x 10⁶ Daltons

Example 7 - Dispersion of Polymeric Microparticles with the aid of a Water Miscible Organic Solvent

The polymeric microparticles prepared and isolated as per Example 5 were redispersed in aqueous media with the aid of a water-miscible organic solvent such as tetrahydrofuran. It was noted that by first dispersing the isolated polymeric microparticles in tetrahydrofuran (approximately 1ml of tetrahydrofuran per lOg of isolated polymeric microparticles) that the ability to redisperse the polymeric microparticles within aqueous media such as synthetic seawater was greatly enhanced, additionally the colloidal stability of the microparticles was also found to be much improved. The water-miscible organic solvent mitigates the risk of the polymeric particles aggregating when dispersed in aqueous media, which may result in larger particle sizes (of about 4-5 micron) when characterised by dynamic light scattering using a Malvern Zetasizer instrument.

Optionally, the water miscible organic solvent can be removed via ultrafiltration techniques (for example by application of a Sartorius cross-flow filtration apparatus employing as the ultrafiltration membrane Hydrosart® stabilized cellulose cassettes having a 10 kDalton number average molecular weight cut-off).

The microparticles of Example 5 were successfully dispersed in synthetic seawater at a concentration of 0.1 wt% with the aid of the water miscible organic solvent

tetrahydrofuran, which was subsequently removed by ultrafiltration. The size of the dispersed microparticles was determined by Dynamic Light Scattering using a Malvern Zetasizer instrument. The dispersed microparticles were found to have a Z-average particle size of approximately 300 nanometers.

Other water miscible organic solvents may also be employed such as 1,3-butylene glycol, tetrahydrofuriuryl alcohol, ethylene glycol monobutyl ether, ethylene glycol methyl ether, mono ethylene glycol, methyl ethyl ketone, and the like.

Example 8 - Microparticle Synthesis

30.1048 g of a monomer solution was prepared containing 8.99 g of acrylamide (AA) monomer, 3.8492 g of 2-acrylamido-2-methyl-l-propanesulfonic acid monomer (AMPS), 3.8498 g of PCL-2000 diacrylate monomer (as prepared in Example 1) in 14.4069 g of dimethylacetamide (DM Ac) solvent. 118.1247 g of a liquid dispersion medium was prepared containing 1.508 g of a polybutylene oxide-b-polyethylene oxide diblock copolymer dispersion stabilizer (ΡΒ0₁₄₂ΡΕ0₃οο, 47 wt% polyethylene oxide (PEO) block, molecular weight of about 23,000 gmol^"1), 2.4867 g of a polyethylene oxide- polyhydroxystearic acid diblock copolymer dispersion stabilizer supplied by Lubrizol under the trade name Solplus K240, 1.13 g of silicone oil (Dow Corning 200 Fluid) and 113 g of Multipar® M paraffin oil.

The monomer solution (discontinuous phase) and the liquid dispersion medium (continuous phase) were homogenized in a reaction vessel using a Silverson L4RT mixer at a stirring rate of 8000 revolutions per minute (rpm) for 5 minutes. The reaction mixture was deoxygenated with nitrogen gas for 45 minutes before addition of 0.5 weight% (based on the total weight of all monomers in the system) of lauroyl peroxide (LP) in 1.5 g of Multipar® M paraffin oil. The reaction mixture was heated at a temperature of 65 °C and stirred at a rate of 500 rpm for 4 hours.

It was observed that the solution was viscous following polymerisation, but still flowed out of the reaction vessel. The microparticles were then isolated by centrifugation and washed with acetone.

Example 9 - Dispersed Microparticles

The microparticles prepared and isolated as per Example 8 were subsequently dispersed in synthetic sea water at a concentration of 0.1 wt% with the aid of the addition of 0.033wt% of an Ethoxylated Alkyl Sulfate surfactant; tradename: Nalco EC9360A. A stabilizer to prevent microbial and bacterial growth in the samples was added at 0.5wt%, namely Nipaguard SCP, manufactured by Clarient.

Example 10- Microparticle Preparation

30.1048 g of a monomer solution was prepared containing 8.99 g of acrylamide (AA), 3.8492 g of 2-acrylamido-2-methyl-l-propanesulfonic acid monomer (AMPS),

3.8498 g of PCL-2000 diacrylate monomer (as prepared in Example 1) and 14.4069 g of a 70/30 wt% blend of 1 ,4 dioxane and water solvent. 118.1247 g of a liquid dispersion medium was prepared containing 1.508 g of a polybutylene oxide-b-polyethylene oxide diblock copolymer dispersion stabilizer (PBOi₄₂PE0₃oo, 47 wt% polyethylene oxide (PEO) block, molecular weight of about 23,000 gmol^"1), 2.4867 g of a polyethylene oxide- polyhydroxystearic acid diblock copolymer dispersion stabilizer supplied by Lubrizol under the trade name Solplus K240, 1.13 g silicone oil (Dow Corning 200 Fluid) and 113 g of Multipar® M paraffin oil.

The monomer solution (discontinuous phase) and liquid dispersion medium

(continuous phase) were homogenized in a reaction vessel using a Silverson L4RT mixer at a stirring rate of 8000 revolutions per minute (rpm) for 5 minutes. The reaction mixture was deoxygenated with nitrogen gas for 45 minutes before addition of 0.5 weight% (based on the total weight of all monomers in the system) of lauroyl peroxide (LP) in 1.5 g of Multipar® M paraffin oil. The reaction mixture was heated at a temperature of 65 °C and stirred at a rate of 500 rpm for 4 hours.

The microparticles were then isolated by precipitation into acetone.

Example 11 - Microparticle Preparation

5.2991 g of a monomer solution was prepared containing 1.811 g of acrylamide (AA) monomer, 0.7701 g of 2-acrylamido-2-methyl-l-propanesulfonic acid monomer (AMPS) and 2.718 g of deionized water. 15.6918 g of a hydrocarbon dispersion medium was prepared containing 0.2004 g of a polybutylene oxide-b-polyethylene oxide diblock copolymer stabilizer (PB0₁₂₈PE0₃oo, 43 wt% polyethylene oxide (PEO) block, molecular weight of about 22,000 gmol^"1), 0.3310 g of a polyethylene oxide-polyhydroxystearic acid diblock copolymer stabilizer supplied by Lubrizol under the trade name Solplus K240, 0.1524 g of silicone oil (Dow Corning 200 Fluid) and 15.008 g of Multipar® M paraffin oil.

The monomer solution (discontinuous phase) and liquid dispersion medium

(continuous phase) were homogenized in a reaction vessel using a Silverson L4RT mixer at a stirring rate of 8000 revolutions per minute (rpm) for 5 minutes. The reaction mixture was deoxygenated with nitrogen gas for 45 minutes before addition of 0.5 weight% (based on the total weight of all monomers in the system) of 2,2'-Azobis(2-methylpropionitrile) (AIBN) in 0.3 g of acetone. The reaction mixture was heated at a temperature of 65 °C and stirred at a rate of 500 rpm. Following addition of the AIBN, 0.773 g of PCL-2000 diacrylate monomer (as prepared in Example 1) in 1.08 g of 1,4 Dioxane was added via a syringe pump at a rate of 1.3 mL hr.

The microparticles were then isolated by precipitation into acetone.

Example 12 - Microparticle Preparation

6.14 g of a monomer solution was prepared containing 1.811 g of acrylamide (AA), 0.7701 g of 2-acrylamido-2-methyl-l-propanesulfonic acid monomer (AMPS), 2.5 g of deionized water and 1.06g of acetone. 20.68g of a liquid dispersion medium was prepared containing 0.2004 g of a polybutylene oxide-b-polyethylene oxide diblock copolymer dispersion stabilizer (PBO₁2₈PEO₃₀₀, 43 wt% polyethylene oxide (PEO) block, molecular weight of about 22,000 gmol^"1), 0.3310 g of a polyethylene oxide-polyhydroxystearic acid diblock copolymer dispersion stabilizer supplied by Lubrizol under the trade name Solplus K240, 0.1524 g of silicone oil (Dow Corning 200 Fluid) and 20 g of Multipar® M paraffin oil.

The monomer solution (discontinuous phase) and liquid dispersion medium (continuous phase) were homogenized in a reaction vessel using a Silverson L4RT mixer at a stirring rate of 8000 revolutions per minute (rpm) for 5 minutes. The reaction mixture was deoxygenated with nitrogen gas for 45 minutes before addition of 0.013g of ammonium persulfate (APS) in 0.3 g of water, followed by the addition of 8μί of tetramethylethylenediamine (TEMED). The reaction mixture was heated at a temperature of 30 °C and stirred at a rate of 600 rpm. Following addition of the TEMED, 0.773 g of PCL-2000 diacrylate monomer (as prepared in Example 1) in 1.08 g of acetone was added via a syringe pump at a rate of 1.3 mL/hr. The reaction mixture was heated at 30°C for 24 hours.

The microparticles were then isolated by precipitation into acetone.

Example 13 - Porous Flow Testing

Microparticles of the present invention were tested under porous flow conditions by being injected into a sand pack which was heated to above a trigger temperature at which the hydrolytically labile cross-links hydrolyse to release the free polymer chains.

A coiled stainless steel tube (length = 151.3cm, OD = 3/8", ID = 7.0mm) was packed with RH110 dry sand obtained from Sibelco. The pack of sand was tapped down and agitated to ensure uniform packing which resulted in a pack permeability of about 6 Darcy. The pack was wrapped with heating tape to allow trace heating. A number of pressure taps (PTA, PTB, PTC, and PTD) were uniformly spaced apart along the length of the pack to allow measurement of differential pressures across portions of the sand pack and across the entire sandpack.

The sand pack was initially saturated with a synthetic brine before being saturated with a 1,000 ppm dispersion of microparticles in the synthetic brine wherein the microparticles were prepared according to Example 8. Saturation of the sandpack was achieved by passing the synthetic brine and the dispersion of microparticles through the sandpack at flow rates of l.Oml/min and O.lml/min respectively and at ambient

temperature. The composition of the synthetic brine is given below:

Ion Composition (ppm)

Na⁺ 10,890

Ca²⁺ 428

Mg²⁺ 1,368

K⁺ 460

S0₄ ^2" 2,960

CI^' 19,766

Saturation of the sand pack with the dispersion of the microparticles was indicated by constant differential pressures throughout the pack and was achieved after about 8 days. Following saturation of the sandpack with the dispersion of particles, the temperature of the pack was increased to 70 °C and the "degrading" polymer dispersion was re-circulated by being removed from the pack and re-injected. Pressure readings were recorded at regular time intervals at pressure gauges located at each of the pressure taps, over a period of 25 days from commencement of re-circulation of the microparticle dispersion. All pressure data was adjusted to take into account a line pressure of approximately 75psi.

Time Pressure Pressure Pressure

(days) difference difference difference

between between between

PTA and PTB and PTC and

PTB (psi) PTC (psi) PTD (psi)

0.0 0.4 0.2 -0.1

1.0 0.4 0.4 0.3

2.0 0.6 0.4 0.4

3.0 33.2 3.2 0.4

4.0 39.3 23.8 0.5

5.0 39.2 28.1 15.4

6.0 41.1 30.0 17.4

7.0 42.9 31.9 18.9

8.0 44.9 33.5 20.1

9.0 34.6 31.2 18.9

10.0 52.7 79.6 46.1

11.0 100.2 121.6 88.5

12.0 132.0 133.4 114.3

13.0 158.9 143.7 128.2

14.0 174.5 146.9 134.6

15.0 183.0 147.1 137.0

16.0 188.7 148.4 137.1

17.0 188.2 149.6 137.9

18.0 186.0 151.6 138.4

19.0 181.7 151.6 137.9

20.0 175.1 148.3 135.2

21.0 166.4 143.6 130.1

22.0 161.3 140.5 125.3

23.0 156.5 136.3 119.3

24.0 148.4 131.4 114.1

25.0 139.6 128.2 109.3

From the pressure readings in the above table, the propagation and subsequent dissolution of the microparticles could be monitored throughout the sand pack. There was an initial drop in differential pressure when the temperature was increased owing to a reduction in the brine viscosity. The subsequent rise in differential pressures across all sections of the pack indicated that the temperature increase had triggered a viscosity increase in the brine, believed to be a result of the PCL crosslinks within the microparticles hydrolysing thereby releasing water-soluble polymer that acted to viscosity the fluid.

Claims

1. Cross-linked polymeric microparticles having a volume average particle size diameter of from about 0.05 to about 10 μηι and comprising from about 0.01 to about 10 mol%, preferably about 0.01 to about 5 mol% of one or more hydrolytically labile, crystallisable cross-linking structural units based on the total structural unit content of the polymeric microparticles and wherein the cross-linking structural units are derived from one or hydrolytically labile, crystallisable cross-linking monomers having a number average molecular weight in the range of from about 1,500 to about 40,000 Daltons and comprise at least one polyester chain having at least five -RC(0)0- ester groups in a linear arrangement (hereinafter "linear polyester chain") wherein the R groups each represent an alkanediyl group or a substituted alkanediyl group and wherein the cross-linking monomers have at least two sites of ethylenic unsaturation.

2. Cross-linked polymeric microparticles according to Claim 1 , wherein the

hydrolytically labile, crystallisable structural units are derived from:

(a) cross-linking monomers comprising a single linear polyester chain which is capped at each end thereof with an olefmic capping group wherein the linear polyester chain has a molecular weight in the range of from 1,500 to 10,000 Daltons, for instance from 2,000 to 4,000 Daltons;

(b) cross-linking monomers comprising a linking group having two linear polyester chains attached thereto wherein the terminal end of each of the linear polyester chains is capped with an olefmic capping group and wherein the molecular weight of each of the linear polyester chains is selected so as to fulfill the condition that the monomers have a number average molecular weight in the range of 1.500 to 10,000 Daltons, for instance from 2,000 to 4,000 Daltons; or

(c) cross-linking comonomers comprising a branching group having three or four linear polyester chains attached thereto wherein the terminal end of each of the linear polyester chains is capped with an olefmic capping group and wherein each of the linear polyester chains has a molecular weight in the range of from 1 ,500 to 10,000 Daltons, for instance from 2,000 to 4,000 Daltons.

3. Cross-linked polymeric microparticles according to Claim 2 wherein the olefmic capping groups are selected from vinylic, allylic, acrylic and methacrylic groups.

4. Cross-linked polymeric microparticles according to Claims 2 or 3, wherein the hydrolytically labile, crystallisable structural units are derived from one or more cross- linking monomers of formulae:

wherein each n mdependently represents an integer of from 1 to 6, p represents an integer selected so as to fulfill the condition that the monomer of formula (A) has a number average molecular weight in the range of from 1,500 to 10,000 Daltons, m is 0 or 1, and R¹ represents hydrogen, methyl or an ethyl group, preferably, hydrogen or methyl;

wherein each n independently represents an integer of from 1 to 6, q and r each

independently represent an integer selected so as to fulfill the condition that the monomer of formula (B) has a number average molecular weight in the range of from 1,500 to 10,000 Daltons, each R¹ independently represents hydrogen, methyl, or an ethyl group, preferably, hydrogen or methyl, and, R² represents a group having the formula -(CR'R")_y- or the formula -(CR'R' ') _y-0-(CR'R' ') _r , wherein the R' and R' ' groups of each (CR'R' ') group independently represent hydrogen, methyl or ethyl and each y independently represents an integer of from 2 to 10;

wherein each n independently represents an integer of from 1 to 6, q, r and s each independently represent an integer selected so as to fulfill the condition that each polyester branching group of formula (C) has a number average molecular weight in the range of from 1,500 to 10,000 Daltons, and, each R¹ independently represents hydrogen, methyl, or an ethyl group, pre

wherein each n independently represents an integer of from 1 to 6; q, r, s and t each independently represent an integer selected so as to fulfill the condition that each polyester branching group of formula (D) have a number average molecular weight in the range of from 1,500 to 10,000 Daltons, and, each R¹ independently represents hydrogen, methyl, or an ethyl group, preferably hydrogen or a methyl group.

5. Cross-linked polymeric microparticles according to Claim 4, wherein the

hydrolytically labile, crystallisable structural units are derived from cross-linking monomers of formula:

wherein q, r and R² are as defined in Claim 4.

6. Cross-linked polymeric microparticles according to any one of the preceding claims, having a volume average particle size diameter of from about 0.1 to about 3 μιη, more preferably a volume average particle size diameter of from about 0.1 to about 1 μηι, and most preferably a volume average particle size diameter of from about 0.2 to about 0.4 μπι.

7. Cross-linked polymeric microparticles according to any one of the preceding claims comprising from 0.2 to 2.5 mol% of structural units derived from hydrolytically labile, crystallisable cross-linking monomers, more preferably from 0.5 to 2 mol% of structural units derived from hydrolytically labile, crystallisable cross-linking monomers, based on the total structural unit content of the polymeric microparticles.

8. Cross-linked polymeric microparticles according to any one of the preceding claims which are substantially free of structural units derived from non-labile cross-linking monomers.

9. Cross-linked polymeric microparticles according to any one of the preceding claims further comprising structural units derived from hydrophilic monomers selected from non- ionic monomers, anionic monomers, cationic monomers, ampholytic ion-pair monomers, or betaine monomers.

10. Cross-linked polymeric microparticles according to Claim 9 comprising structural units derived from at least one anionic monomer and structural units derived from at least one non-ionic monomer.

11. Cross-linked polymeric microparticles according to Claim 10, comprising from about 0.01 to about 10 mol%, preferably, about 0.01 to about 5 mol% of hydrolytically labile, crystallisable cross-linking structural units; about 0.01 to about 85 mol%, preferably, about 5 to about 50 mol% of anionic structural units derived from at least one anionic monomer; and from 5 to about 99.98 mol%, preferably, about 5 to about 90 mol% of non-ionic structural units derived from at least one non-ionic monomer, based on the total molar amounts of monomers used to prepare the microparticles.

12. Cross-linked polymeric microparticles according to Claims 10 or 11, wherein the anionic structural units are derived from at least one anionic monomer selected from acrylic acid, methacrylic acid, maleic acid, itaconic acid, 2-propenoic acid, 2-methyl-2- propenoic acid, 2-acrylamido-2-methylpropane sulfonic acid, sulfopropyl acrylic acid, sulfomethylated acrylamide, allyl sulfonic acid, vinyl sulfonic acid, and alkali metal, alkaline earth metal or ammonium salts thereof.

13. Cross-linked polymeric microparticles according to Claim 12, wherein the anionic structural units are derived from at least one anionic monomer selected from 2-acrylamido-

2-methylpropane sulfonic acid sodium salt, vinyl sulfonic acid sodium salt and styrene sulfonic acid sodium salt.

14. Cross-linked polymeric microparticles according to Claim 13, wherein the anionic structural units are derived from 2-acrylamido-2-methylpropane sulfonic acid sodium salt.

15. Cross-linked polymeric microparticles according to any one of Claims 10 to 14, wherein the non-ionic structural units are derived from at least one non-ionic monomer selected from acrylamide, methacrylamide, N-methylacrylamide, N-methyl methacrylamide, Ν,Ν-dimethylacrylamide, N- isopropylacrylamide, N,N- diethylacrylamide, dimethylaminopropyl acrylamide, dimemylaminopropyl

methacrylamide, acryloyl morpholine, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, dimethylaminoethylacrylate (DMAEA), dimethylaminoethyl methacrylate (DMAEM), maleic anhydride, N-vinyl pyrrolidone, vinyl acetate and N-vinyl formamide.

16. Cross-linked polymeric microparticles according to Claim 15, wherein the non-ionic structural units are derived from at least one non-ionic monomer selected from acrylamide, N-methylacrylamide, N,N-dimethylacrylamide and methacrylamide.

17. Cross-linked polymeric microparticles according to Claim 16, wherein the non-ionic structural units are derived from acrylamide.

18. Cross-linked polymeric microparticles according to Claim 17, comprising a polymer selected from :

(i) copolymers of acrylamide; 2-acrylamido-2-methylpropane sulfonic acid

(AMPS) or AMPS sodium salt; and a hydrolytically labile, crystallisable cross-linking monomer as defined in any one of Claims 1 to 5;

(ii) copolymers of acrylamide; acrylic acid or sodium acrylate; and a hydrolytically labile, crystallisable cross-linking monomer as defined in any one of Claims 1 to 5; and

(iii) copolymers of acrylamide; 2-acrylamido-2-methylpropane sulfonic acid

(AMPS) or AMPS sodium salt; acrylic acid or sodium acrylate; and a hydrolytically labile, crystallisable cross-linking monomer as defined in any one of Claims 1 to 5.

19. Cross-linked polymeric microparticles according to Claim 18, comprising:

anionic polymeric microparticles comprising a polymer selected from:

(i) copolymers of acrylamide; 2-acrylamido-2-methylpropane sulfonic acid

(AMPS) or AMPS sodium salt; and a hydrolytically labile, crystallisable cross-linking monomer of formula (F) as defined in Claim 5;

(ii) copolymers of acrylamide; acrylic acid or sodium acrylate; and a hydrolytically labile, crystallisable cross-linking monomer of formula (F) as defined in Claim 5; and

(iii) copolymers of acrylamide; 2-acrylamido-2-methylpropane sulfonic acid (AMPS) or AMPS sodium salt; acrylic acid or sodium acrylate; and a hydrolytically labile, crystallisable cross-linking monomer of formula (F) as defined in Claim 5.

20. Cross-linked polymeric microparticles according to Claim 17, comprising a polymer selected from copolymers of from about 5 to about 99.98 mol% acrylamide, from about 0.01 to about 85 mol% of an anionic monomer selected from 2-acrylamido-2- methylpropane sulfonic acid, acrylic acid, and alkali metal, alkaline earth metal or ammonium salts thereof, and from about 0.01 to about 5 mol% of a hydrolytically labile, crystallisable cross-linking monomer of formula (F) as defined in Claim 5.

21. Cross-linked polymeric microparticles according to any one of the preceding claims comprising a non-crystalline core encapsulated by a crystalline shell.

22. Cross-linked polymeric microparticles according to any one of the preceding claims comprising additional cross-linking units at or near the surface of the microparticles wherein the additional cross-linking units are derived from non-crystallisable cross-linking monomers.

23. A composition comprising cross-linked polymeric microparticles as defined in any one of Claims 1 to 22 dispersed in an aqueous fluid.

24. A composition according to Claim 23, comprising from 0.01 to 10 % by weight, more preferably from 0.02 to 5 % by weight and most preferably from 0.05 to 1 % by weight of cross-linked polymeric microparticles based on the total weight of the composition.

25. A composition according to Claim 21 or Claim 22, wherein the aqueous fluid is fresh water, low salinity water, brackish water, seawater or produced water.

26. A process for preparing cross-linked polymeric microparticles as defined in any one of Claims 1 to 22 comprising:

(a) adding to an organic liquid dispersion medium that contains at least a first and a second polymeric dispersion stabilizer: (i) a hydrolytically labile, crystallisable cross-linking monomer, and (ii) a non-cross-linking monomer selected from the group consisting of non- ionic monomers, anionic monomers, cationic monomers, ampholytic ion-pair monomers, betaine monomers, and mixtures thereof thereby forming a dispersion comprising droplets of the cross-linking and non-cross-linking monomers, dispersed in the organic liquid dispersion medium; (b) subjecting the resulting dispersion to a temperature at which the cross-linking monomers and the non-crosslinking monomers polymerize thereby forming cross-linked polymeric microparticles that are dispersed in the organic liquid dispersion medium;

wherein the first and second polymeric dispersion stabilizers are selected from block copolymer stabilizers comprising one or more lyophilic blocks and one or more non- lyophilic blocks or from graft copolymer stabilizers comprising a lyophilic backbone having one or more non-lyophilic side chains or a non-lyophilic backbone having one or more lyophilic side chains; and wherein the first polymeric dispersion stabilizer has a number average molecular weight of at least 20,000 Daltons, in particular, at least 25,000 Daltons and the second polymeric dispersion stabilizer has a number average molecular weight in the range of 1,000 to 4,000 Daltons, in particular, 1,000 to 2,000 Daltons.

27. A process as claimed in Claim 26 wherein the cross-linking monomer and non-crosslinking monomer are dissolved in a solvent that is immiscible with the organic liquid dispersion medium and the resulting solution of the monomers is added to the organic liquid dispersion medium thereby forming a dispersion comprising droplets of the solution of the monomers dispersed in the organic liquid dispersion medium.

28. A process for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation penetrated by at least one injection well and at least one production well, the process comprising

(i) injecting into said formation through said at least one injection well a

composition comprising cross-linked polymeric microparticles as defined in any one of Claims 23 to 25;

(iii) recovering hydrocarbons from said at least one production well;

wherein said subterranean hydrocarbon-bearing formation contains at least one zone between said at least one injection well and said at least one production well having a temperature at which the hydrolytically labile cross-links of the microparticles undergo a melting transition.

29. A process according to Claim 28, wherein the process is carried out during secondary or tertiary recovery of hydrocarbons from the subterranean formation.

30. A process according to Claims 28 or 29, wherein the composition is injected into the subterranean formation at a temperature of 50 °C or less, preferably 40 °C or less, more preferably 30 °C or less, in particular, a temperature in the range of 4 to 50 °C.

31. A process according to any one of Claims 28 to 30, wherein the subterranean hydrocarbon-bearing formation contains at least one zone between said at least one injection well and said at least one production well having a temperature of greater than 50 °C, preferably, in the range of greater than 55 to 100 °C, and more preferably in the range of 60 to 90°C.

32. A process according to any one of Claims 28 to 31 , wherein the composition that is injected into the formation comprises a surfactant such that a surfactant polymer (SP) flooding medium is formed within the formation.

33. A process according to Claim 32, wherein the composition that is injected into the formation comprises an alkali such that an alkali surfactant polymer (ASP) flooding medium is formed within the formation.