Patent Publication Number: US-7897546-B2

Title: Composition and method for recovering hydrocarbon fluids from a subterranean reservoir

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 61/046,728, filed Apr. 21, 2008. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable. 
     FIELD OF THE INVENTION 
     This invention relates to compositions and methods for the recovery of hydrocarbon fluids from a subterranean reservoir and more particularly to an encapsulated expandable polymeric microparticle composition, which modifies the permeability of subterranean formations, thereby increasing the mobilization and/or recovery rate of hydrocarbon fluids present in the formations. 
     BACKGROUND 
     In the first stage of hydrocarbon recovery an energy source present in a reservoir is allowed to move to a producing wells(s) where the hydrocarbon can flow or be pumped to a surface handling facility. Typically a relatively small proportion of reservoir hydrocarbon can be recovered by this means. To increase production fluids are injected down adjacent wells to force an additional amount of hydrocarbon to the surface. This is commonly known as secondary recovery. The fluids normally used are water (such as aquifer water, river water, sea water, or produced water), or gas (such as produced gas, carbon dioxide, flue gas and various others). Additionally, if the fluid encourages movement of normally immobile residual oil or other hydrocarbons, such a process is termed tertiary recovery. 
     A prevalent problem with secondary and tertiary recovery projects relates to the heterogeneity of the reservoir rock strata. The mobility of the injected fluid typically is different from the hydrocarbon. For instance, when the fluid is more mobile various mobility control processes are required to make the sweep of the reservoir more uniform and the consequent hydrocarbon recovery more efficient. Unfortunately such processes have limited value when high permeability zones, commonly called thief zones or streaks, exist within the reservoir rock. The injected fluid follows a low resistance route from the injection well to the production well. In such cases the injected fluid does not effectively sweep the hydrocarbon from adjacent, lower permeability zones. Further, when the produced fluid is re-used this can lead to fluid cycling through the thief zone with little resulting benefit and at great cost in terms of fuel and maintenance of the pumping system. 
     Numerous physical and chemical methods have been used to divert injected fluids out of the thief zones and into or near production and injection wells. When the treatment is applied to a producing well it is usually termed a water (or gas etc.) shut-off treatment. When it is applied to an injection well it is termed a profile control or conformance control treatment. 
     In cases where the thief zone(s) are isolated from the lower permeability adjacent zones, mechanical seals or “plugs” can be set in the well to block the entrance of the injected fluid. If the fluid enters or leaves the formation from the bottom of the well, cement can also be used to fill the well bore to above the zone of ingress. 
     When the completion of the well allows the injected fluid to enter both the thief and the adjacent zones, a cement squeeze is often a suitable means of isolating the watered out zone. Certain cases, however, are not amenable to such methods because communication exists between layers of the reservoir rock outside the reach of cement. Typical examples of this are when fractures or rubble zones or washed out caverns exist behind the casing. In such instances chemical gels, capable of moving through pores in reservoir rock have been applied to seal the swept out zones. 
     When such methods fail the only remaining alternatives are to produce the well with poor recovery rate, sidetrack the well away from the prematurely swept zone, or abandon the well. Occasionally the producing well is converted to a fluid injector to increase the field injection rate above the net hydrocarbon extraction rate and increase the pressure in the reservoir. This can lead to improved overall recovery, but it is worth noting that the injected fluid will mostly enter the thief zone at the new injector and is likely to cause similar problems in nearby wells. Further, all of these methods are expensive. 
     Near wellbore conformance control methods always fail when the thief zone is in widespread contact with the adjacent, hydrocarbon containing, lower permeability zones. The reason for this is that the injected fluids can bypass the treatment and re-enter the thief zone having only contacted little or none of the remaining hydrocarbon. It is commonly known amongst those skilled in the art, that such near wellbore treatments do not succeed in significantly improving recovery in reservoirs having crossflow of the injected fluids between zones. 
     A few processes have been developed with the aim of reducing the permeability in a substantial proportion of the thief zone and, or at a significant distance from the injection and production wells. One example of this is the Deep Diverting Gel process patented by Morgan et al (1). This has been used in the field and suffered from sensitivity to unavoidable variations in quality of the reagents, which resulted in poor propagation. The gelant mixture is a two-component formulation and it is believed that this contributed to poor propagation of the treatment into the formation. 
     The use of swellable cross linked superabsorbent polymer microparticles for modifying the permeability of subterranean formations is disclosed in U.S. Pat. Nos. 5,465,792 and 5,735,349. However, swelling of the superabsorbent microparticles described therein is induced by changes of the carrier fluid from hydrocarbon to aqueous or from water of high salinity to water of low salinity. 
     Crosslinked, expandable polymeric microparticles and their use for modifying the permeability of subterranean formations and increasing the mobilization and/or recovery rate of hydrocarbon fluids present in the formation are disclosed in U.S. Pat. Nos. 6,454,003 B1, 6,709,402 B2, 6,984,705 B2 and 7,300,973 B2 and in published U.S. Patent Application No. 2007/0204989 A1. 
     SUMMARY 
     We have discovered novel expandable polymeric microparticles including an expandable polymeric base microparticle and a shell comprising at least one layer of an encapsulation material. The encapsulation material is a labile or degradable material. The unexpanded size of the base microparticle is maintained by the shell of encapsulation material until a desired activating event removes the shell and allows for overall expansion of the base microparticle. The unexpanded microparticle properties, such as average particle size distribution and density, of the encapsulated microparticle allow for efficient propagation through the pore structure of hydrocarbon reservoir matrix rock, such as sandstone. On exposing the encapsulated microparticles to an activating event such as a change in temperature and/or at a predetermined pH, the shell is removed thereby allow the expandable base microparticle to expand by absorbing the injection fluid (normally water). 
     The ability of the particle to expand from its original size (at the point of injection) depends on the presence of conditions that induce the removal of the encapsulation material. The particles of this invention can propagate through the porous structure of the reservoir without using a designated fluid or fluid with salinity higher than the reservoir fluid. 
     The released, expanded polymeric base microparticle is engineered to have a particle size distribution and physical characteristics, for example, particle rheology, that allow it to impede the flow of injected fluid in the pore structure. In doing so it is capable of diverting chase fluid into less thoroughly swept zones of the reservoir. 
     The rheology and expanded particle size of the particle can be designed to suit the reservoir target. For example, characteristics of a microparticle for use in a particular reservoir are influenced by selection of the encapsulation material. For example, the choice may be made to use a particular backbone monomer or comonomer ratio in a labile polymer constituting the shell. Another way to influence the characteristics of the microparticle is the degree of reversible (labile) and irreversible crosslinking introduced during manufacture of the primary polymers in the underlying base microparticle or during manufacture of a labile polymer constituting the shell. 
     Accordingly, in its principal embodiment, this invention is directed to an encapsulated polymeric microparticle composition comprising cross-linked expandable polymeric microparticles and a shell encapsulating the cross-linked expandable polymeric microparticles, the shell comprising at least one layer of a labile or degradable encapsulation material. The invention is further directed to a method for modifying the permeability to water of a subterranean formation by injecting into the subterranean formation the encapsulated polymeric microparticle composition. 
    
    
     DETAILED DESCRIPTION 
     Definitions of Terms 
     “Amphoteric polymeric microparticle” means a cross-linked polymeric microparticle containing both cationic substituents and anionic substitutents, although not necessarily in the same stoichiometric proportions. Representative amphoteric polymeric microparticles include terpolymers of nonionic monomers, anionic monomers and cationic monomers as defined herein. Preferred amphoteric polymeric microparticles have a higher than 1:1 anionic monomericationic monomer mole ratio. 
     “Ampholytic ion pair monomer: means the acid-base salt of basic, nitrogen containing monomers such as dimethylaminoethylacrylate (DMAEA), dimethylaminoethyl methacrylate (DMAEM), 2-methacryloyloxyethyidiethylamine, and the like 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, styrene sulfonic acid, and the like. 
     “Anionic monomer” means a monomer as defined herein which possesses an acidic functional group and the base addition salts thereof. Representative anionic monomers include acrylic acid, methacrylic acid, maleic acid, itaconic acid, 2-propenoic acid, 2-methyl-2-propenoic acid, 2-acrylamido-2-methyl propane sulfonic acid, sulfopropyl acrylic acid and other water-soluble forms of these or other polymerizable carboxylic or sulphonic acids, sulphomethylated acrylamide, allyl sulphonic acid, vinyl sulphonic acid, the quaternary salts of acrylic acid and methacrylic acid such as ammonium acrylate and ammonium methacrylate, and the like. Preferred anionic monomers include 2-acrylamido-2-methyl propanesulfonic acid sodium salt, vinyl sulfonic acid sodium salt and styrene sulfonic acid sodium salt. 2-Acrylamido-2-methyl propanesulfonic acid sodium salt is more preferred. 
     “Anionic polymeric microparticle” means a cross-linked polymeric microparticle containing a net negative charge. Representative anionic polymeric microparticles include copolymers of acrylamide and 2-acrylamido-2-methyl propane sulfonic acid, copolymers of acrylamide and sodium acrylate, terpolymers of acrylamide, 2-acrylamido-2-methyl propane sulfonic acid and sodium acrylate and homopolymers of 2-acrylamido-2-methyl propane sulfonic acid. Preferred anionic polymeric microparticles are prepared from about 95 to about 10 mole percent of nonionic monomers and from about 5 to about 90 mole percent anionic monomers. More preferred anionic polymeric microparticles are prepared from about 95 to about 10 mole percent acrylamide and from about 5 to about 90 mole percent 2-acrylamido-2-methyl propane sulfonic acid. 
     Betaine-containing polymeric microparticle” means a cross-linked polymeric microparticle prepared by polymerizing a betaine monomer and one or more nonionic monomers. 
     “Betaine monomer” means a monomer containing cationically and anionically charged functionality in equal proportions, such that the monomer is net neutral overall. Representative betaine monomers include N,N-dimethyl-N-acryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine, N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine, N,N-dimethyl-N-acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine, N,N-dimethyl-N-acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine, N,N-dimethyl-N-acryloxyethyl-N-(3-sulfopropyl)-ammonium betaine, N,N-dimethyl-N-acrylamidopropyl-N-(2-carboxymethyl)-ammonium betaine, N-3-sulfopropylvinylpyridine ammonium betaine, 2-(methylthio)ethyl methacryloyl-S-(sulfopropyl)-sulfonium betaine, 1-(3-sulfopropyl)-2-vinylpyridinium betaine, N-(4-sulfobutyl)-N-methyldiallylamine ammonium betaine (MDABS), N,N-diallyl-N-methyl-N-(2-sulfoethyl) ammonium betaine, and the like. A preferred betaine monomer is N,N-dimethyl-N-methacryloyloxyethyl-N-(3-sulfopropyl)-ammonium betaine. 
     “Cationic Monomer” means a monomer unit as defined herein which possesses a net positive charge. Representative cationic monomers include the quaternary or acid salts of dialkylaminoalkyl acrylates and methacrylates such as dimethylaminoethylacrylate methyl chloride quaternary salt (DMAEA.MCQ), dimethylaminoethylmethacrylate methyl chloride quaternary salt (DMAEM.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 or acid salts of dialkylaminoalkylacrylamides 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 ammonium chloride and acrylamidopropyl trimethyl ammonium chloride; and N,N-diallyldialkyl ammonium halides such as diallyldimethyl ammonium chloride (DADMAC). Preferred cationic monomers include dimethylaminoethylacrylate methyl chloride quaternary salt, dimethylaminoethylmethacrylate methyl chloride quaternary salt and diallyldimethyl ammonium chloride. Diallyldimethyl ammonium chloride is more preferred. 
     “Cross linking monomer” means an ethylenically unsaturated monomer containing at least two sites of ethylenic unsaturation which is added to constrain the microparticle conformation of the polymeric microparticles of this invention. The level of cross linking used in these polymer microparticles is selected to maintain a rigid non-expandable microparticle configuration. Cross linking monomers according to this invention include both labile cross linking monomers and non-labile cross linking monomers. 
     “Encapsulation” means the condition of being substantially enclosed as in a capsule or shell, in this case a capsule or shell of at least one layer of a labile encapsulation material. The encapsulation material may bond to polymeric base microparticles with non-covalent, for example ionic bonds or other bonds that break upon exposure to activating conditions or agents such as water or heat. 
     “Emulsion”, “microemulsion” and “inverse emulsion” mean a water-in-oil polymer emulsion comprising a polymeric microparticle according to this invention in the aqueous phase, a hydrocarbon oil for the oil phase and one or more water-in-oil emulsifying agents. Emulsion polymers are hydrocarbon continuous with the water-soluble polymers dispersed within the hydrocarbon matrix. The emulsion polymer are optionally “inverted” or converted into water-continuous form using shear, dilution, and, generally an inverting surfactant. See U.S. Pat. No. 3,734,873, incorporated herein by reference. 
     “Fluid mobility” means a ratio that defines how readily a fluid moves through a porous medium. This ratio is known as the mobility and is expressed as the ratio of the permeability of the porous medium to the viscosity for a given fluid. 
     1. EQUATION 1 for a single fluid x flowing in a porous medium. 
     
       
         
           
             λ 
             = 
             
               
                 k 
                 x 
               
               
                 η 
                 x 
               
             
           
         
       
     
     When more than one fluid is flowing the end point relative permeabilities must be substituted for the absolute permeability used in equation 1. 
     2. EQUATION 2 for a fluid x flowing in a porous medium in the presence of one or more other fluids. 
     
       
         
           
             
               λ 
               x 
             
             = 
             
               
                 k 
                 rx 
               
               
                 η 
                 x 
               
             
           
         
       
     
     When two or more fluids are flowing the fluid mobilities may be used to define a Mobility ratio: 
     3. EQUATION 3 
     
       
         
           
             M 
             = 
             
               
                 
                   λ 
                   x 
                 
                 
                   λ 
                   y 
                 
               
               = 
               
                 
                   
                     η 
                     y 
                   
                   ⁢ 
                   
                     k 
                     rx 
                   
                 
                 
                   
                     η 
                     x 
                   
                   ⁢ 
                   
                     k 
                     ry 
                   
                 
               
             
           
         
       
     
     The mobility ratio is used in the study of fluid displacement, for example in water flooding of an oil reservoir where x is water and y is oil, because the efficiency of the displacement process can be related to it. 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. 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. 
     “Ion-pair polymeric microparticle” means a cross-linked polymeric microparticle prepared by polymerizing an ampholytic ion pair monomer and one more anionic or nonionic monomers. 
     “Labile cross linking monomer” means a cross linking monomer which can be degraded by certain conditions of heat and/or pH, after it has been incorporated into the polymer structure, to reduce the degree of crosslinking in the polymeric microparticle of this invention. The aforementioned conditions are such that they can cleave bonds in the “cross linking monomer” without substantially degrading the rest of the polymer backbone. Representative labile cross linking monomers include diacrylamides 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, trimethylopropane trimethacrylate, ethoxylated trimethylol triacrylate, ethoxylated pentaerythritol tetracrylate, and the like; divinyl or diallyl compounds separated by an azo such as the diallylamide of 2,2′-Azobis(isbutyric acid) and the vinyl or allyl esters of di or tri functional acids. Preferred labile cross linking monomers include water soluble diacrylates such as PEG 200 diacrylate and PEG 400 diacrylate and polyfunctional vinyl derivatives of a polyalcohol such as ethoxylated (9-20) trimethylol triacrylate. The labile cross linker may be present in an amount of from about 100 to about 200,000 ppm. In alternative aspects, the labile cross linker is present in the amount of about 1,000 to about 200,000 ppm, about 9,000 to about 200,000 ppm, about 9,000 to about 100,000 ppm, about 20,000 to about 60,000, about 1,000 to about 20,000 ppm or about 500 to about 50,000 ppm based on total moles of monomer. 
     “Monomer” means a polymerizable allylic, vinylic or acrylic compound. The monomer may be anionic, cationic, nonionic or betaine. Vinyl monomers are preferred, acrylic monomers are more preferred. 
     “Nonionic monomer” means a monomer as defined herein which is electrically neutral. Representative nonionic monomers include N-isopropylacrylamide, N,N-dimethylacrylamide, 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-vinyl pyrrolidone, vinyl acetate and N-vinyl formamide. Preferred nonionic monomers include acrylamide, N-methylacrylamide, N,N-dimethylacrylamide and methacrylamide. Acrylamide is more preferred. 
     “Non-labile cross linking monomer” means a cross linking monomer which is not degraded under the conditions of temperature and/or pH which would cause incorporated labile cross linking monomer to disintegrate. Non-labile cross linking monomer is added, in addition to the labile cross linking monomer, to control the expanded conformation of the polymeric microparticle. Representative non-labile cross linking monomers include methylene bisacrylamide, diallylamine, triallylamine, divinyl sulfone, diethyleneglycol diallyl ether, and the like. A preferred non-labile cross linking monomer is methylene bisacrylamide. 
     In one aspect, the encapsulated base microparticle is made of cross-linked expandable polymers with labile and non-labile cross-linkers such as, but not limited to, those described in U.S. Pat. Nos. 6,454,003 B1, 6,709,402 B2, 6,984,705 B2 and 7,300,973 B2 and in published U.S. Patent Application No. 2007/0204989 A1. For example, in one aspect the non-labile cross linker is present in the expandable microparticles in an amount of from 0 to about 300 ppm, in one aspect from about 2 to about 300 ppm, in another aspect from about 0 to about 200 ppm, in another aspect from about 0 to about 100 ppm, in another aspect from about 01 to about 300 ppm, in another aspect from about 2 to about 300 ppm and in another aspect from about 5 to about 300 ppm based on total moles of monomer. In the absence of a non-labile cross linker, the polymer particle, upon complete scission of labile cross linker, is converted into a mixture of linear polymer strands. The particle dispersion is thereby changed into a polymer solution. This polymer solution, due to its viscosity, changes the mobility of the fluid in a porous medium. In the presence of a small amount of non-labile cross linker, the conversion from particles to linear molecules is incomplete. The particles become a loosely linked network but retain certain ‘structure’. Such ‘structured’ particles can block the pore throats of porous media and create a blockage of flow. 
     In another aspect, the encapsulated base microparticle is made of any expandable polymers with an unexpanded particle size of about 5,000 microns or less. Suitable expandable polymers include for example polyvinyl pyrrolidone, (hydroxyethyl)methacrylate, and polymers with polyacrylate backbones comprising anionic, cationic, nonionic and/or betaine monomers and optionally further comprising labile and non-labile cross-linkers as described herein. 
     The expandable polymeric base microparticles are encapsulated and may be used advantageously in recovery of hydrocarbon from the subterranean formation. The encapsulated microparticle has a particle size of about 5,000 microns or less. The encapsulation material forms a shell around the expandable polymeric microparticles and bonds to the polymeric microparticles with non-covalent, for example ionic bonds or other bonds that break upon exposure to activating conditions or agents such as water or heat. A suitable material for the encapsulation material is, for example, a water-soluble hydrolyzed polysaccharide such as that described, for example in WO/1994/027647, which is a water-soluble non-crosslinked polysaccharide prepared with a cyanogen halide under alkaline conditions so that crosslinking occurs, in the aqueous phase of a water-in-oil dispersion. Another suitable encapsulation material is a copolymer of N-isopropyl acrylamide (NIPAAm) and acrylamide which becomes soluble under increasing temperature conditions. Other suitable encapsulation materials or additives to the encapsulation materials include natural or synthetic cross-linked polymers, emulsifiers including for example those that form a hydrophobic shell and thus inverse emulsions, thermally responsive polymers, ethylenically unsaturated monomers, silicone elasteromers, gelatin, siloxanes, starches, gum agar, derivated cellulose, sulfonated polysaccharides, silica, colloidal clays such as bentonite, gum arabic, dextrins, arabinogalactan, gum acacia, casein, carboxymethyl cellulose, tragacanth, karaya, sodium alginate, tannin, salts or a combination thereof. water-soluble surfactants, surfactants, salts (e.g., sodium chloride, calcium chloride), polymer salts, polyvinyl alcohols, waxes (e.g., paraffin, carnauba), photo-reactive materials, degradable materials, biodegradable materials, ethoxylated acetylenic diols, and any other suitable substances. Specific examples of other encapsulation materials are water soluble surfactants such as the Surfynol 485W, 485, 2502, and 465 water soluble surfactants, sold by Air Products and Chemicals Corporation, of Allentown, Pa., waxes sold as Textile Wax-W and Size SF-2, by BASF Corporation, of Charlotte, N.C., and waxes sold as model numbers Kinco 878-S and Kinco 778-H by Kindt-Collins Company, of Cleveland, Ohio. Examples of methods encapsulation include but are not limited to: emulsion-based techniques, spray drying, spray coacervation, Wurster process, interfacial polymerization, in situ polymerization, desolvation, or a combination of the different methods. 
     Examples of labile polymers that may be used for the encapsulation material include for example the labile polymers described in U.S. Pat. No. 6,616,946, including temperature responsive polymers (such as NIPAAm), pH responsive polymers, light responsive polymers, ion responsive polymers and polymers with multi-stimulus sensitivity. pH sensitive polymers include those based on pH sensitive vinyl monomers, such as acrylic acid (AAc), methacrylic acid (MAAc), maleic anhydride (MAnh), maleic acid (MAc), 2-Acrylamido-2-Methyl-1-Propanesulfonic Acid (AMPS), N-vinyl formamide (NVA), N-vinyl acetamide (NVA), aminoethyl methacrylate (AEMA), phosphoryl ethyl acrylate (PEA), or methacrylate (PEMA). pH sensitive polymers may also be synthesized as polypeptides from amino acids (e.g. polylysine or polyglutamic acid) or derived from naturally occurring polymers such as proteins (e.g. lysozyme, albumin, casein, etc.), or polysaccharides (e.g. alginic acid, hyaluronic acid, carrageenan, chitosan, carboxymethyl cellulose, etc.) or nucleic acids, such as DNA. Specific examples of pH-sensitive polymers include: copolymer of derivatized 4-amino-N-[4,6-dimethyl-2-pyrimidinyl]benzene sulfonamide and N,N-dimethylacrylamide, poly(hydroxyethyl)methacrylate-co-methacrylic acid), and copolymer of M,N,dimethylaminoethyl methacrylate and divinyl benzene. Light responsive polymers usually include chromophoric groups pendant to or along the main chain of the polymer. When exposed to light of a select wavelength, the polymer is isomerized from the trans to the cis form, which is dipolar, more hydrophilic and can cause reversible polymer conformational changes. Light sensitive polymers and copolymers thereof are more typically synthesized from vinyl monomers that contain light sensitive pendant groups. Copolymers of these types of monomers are prepared with “normal” water-soluble comonomers such as acrylamide, or with temperature or pH sensitive comonomers such as NIPAAm or AAc. Ion responsive polymers include polysaccharides such as carrageenan that change their conformation, for example, from a random to an ordered conformation, as a function of exposure to a specific ions, or polymers such as those with pendant ion chelating groups, such as histidine or EDTA, for example. Dual or multi stimuli responsive polymers may also be used. 
     For example, expandable polymeric microparticles are encapsulated with ethylenically unsaturated monomers in a preferably aqueous suspension to yield microparticles comprising a labile polymer shell encapsulating the expandable polymer polymeric microparticles. Production using other encapsulation materials may follow the same principles as described for example in WO2007/091961, and U.S. Pat. Nos. 3,615,972, 3,945,956, 4,287,308, 5,536,756, and 6,235,800. The encapsulation material is for example externally applied to the polymeric base microparticle. Application is achieved by any suitable method such as, for example, introducing the base microparticles to a bath of the encapsulation material, spraying the encapsulation material on the microparticles, or mixing the microparticles in a solution or aqueous suspension of the encapsulation material and drying in a dryer or air-drying. 
     An exemplary microparticle is an expandable polymeric base microparticle encapsulated with a labile polymeric shell of NIPAAm which comprises about 0.1 to 5 wt % of the encapsulated microparticle. 
     The expandable polymeric microparticles may be, in one aspect, encapsulated by more than one layer of encapsulation material. Multiple layers may be composed of the same or different encapsulation materials. The layer or layers of encapsulation material are configured to degrade upon exposure to an activating event so that the cross-linked expandable polymeric microparticles are released and free to expand. The activating event is, for example, a change in temperature, pressure, pH, salinity, shear, dilution, exposure to high HLB inverter, or a biodegradation agent. The activating event may be for example exposure to an activating agent such as exposure to an oxidant, a reductant, an acid, a base, a biological agent, an organic or inorganic cross-linking agent, or a salt, or to a combination thereof. Upon exposure to the activating event and consequent degradation of the encapsulation material, the cross-linked expandable polymeric microparticles are free to expand to several times the original size of the microparticle. 
     In one aspect, an activating agent is also encapsulated with encapsulation material. The encapsulated activating agent can be delivered into the formation together with the encapsulated microparticles whereupon exposure to an activating event for the encapsulated activating agent, the activating agent is released from its encapsulation material and thereby induces the degradation of the encapsulation material of nearby encapsulated expandable microparticles. 
     In another aspect, the encapsulation material is a thermally responsive polymer or elasteromer with elastomeric properties, and the cross-linked expandable polymeric microparticles are composed of labile cross-linked polymers with at least a portion of cross-links being labile. The activating event, for example, causes the labile cross-links in the polymers to degrade so that the cross-linked expandable polymeric microparticles swell yet leave the encapsulation material intact. 
     Preferred Embodiments 
     In one aspect, the encapsulated polymeric microparticles are composed of cross-linked expandable polymeric microparticles that are prepared using an inverse emulsion or microemulsion process to assure certain particle size range. In an embodiment, the unexpanded volume average particle size diameter of the polymeric microparticle is about 0.05 to about 5,000 microns. In another embodiment the particle size is about 0.05 to about 50 microns. In another embodiment the particle size is about 0.1 to about 3 microns. In another embodiment, the particle size is about 0.1 to about 1 microns, including the encapsulation material. 
     Representative preparations of cross-linked expandable polymeric microparticles using a microemulsion process are described in U.S. Pat. Nos. 4,956,400, 4,968,435, 5,171,808, 5,465,792, 5,737,349, 6,454,003 B1, 6,709,402 B2, 6,984,705 B2, and 7,300,973 B2. 
     In an inverse emulsion or microemulsion process, an aqueous solution of monomers and cross linkers is added to a hydrocarbon liquid containing an appropriate surfactant or surfactant mixture to form an inverse monomer microemulsion consisting of small aqueous droplets dispersed in the continuous hydrocarbon liquid phase and subjecting the monomer microemulsion to free radical polymerization. 
     In addition to the monomers and cross linkers, the aqueous solution may also contain other conventional additives including chelating agents to remove polymerization inhibitors, pH adjusters, initiators and other conventional additives. 
     The hydrocarbon liquid phase comprises a hydrocarbon liquid or mixture of hydrocarbon liquids. Saturated hydrocarbons or mixtures thereof are preferred. Typically, the hydrocarbon liquid phase comprises benzene, toluene, fuel oil, kerosene, odorless mineral spirits and mixtures of any of the foregoing. 
     Surfactants useful in the microemulsion polymerization process described herein include sorbitan esters of fatty acids, ethoxylated sorbitan esters of fatty acids, and the like or mixtures thereof. Preferred emulsifying agents include ethoxylated sorbitol oleate and sorbitan sesquioleate. Additional details on these agents may be found in McCutcheon&#39;s Detergents and Emulsifiers, North American Edition, 1980. 
     Polymerization of the emulsion may be carried out in any manner known to those skilled in the art. Initiation may be effected with a variety of thermal and redox free-radical initiators including azo compounds, such as azobisisobutyronitrile; peroxides, such as t-butyl peroxide; organic compounds, such as potassium persulfate and redox couples, such as sodium bisulfite/sodium bromate. Preparation of an aqueous product from the emulsion may be effected by inversion by adding it to water, which may contain an inverting surfactant. 
     Alternatively, the polymeric microparticles cross linked with labile cross links are prepared by internally cross linking polymer particles which contain polymers with pendant carboxylic acid and hydroxyl groups. The cross linking is achieved through the ester formation between the carboxylic acid and hydroxyl groups. The esterification can be accomplished by azeotropic distillation (U.S. Pat. No. 4,599,379) or thin film evaporation technique (U.S. Pat. No. 5,589,525) for water removal. For example, a polymer microparticle prepared from inverse emulsion polymerization process using acrylic acid, 2-hydroxyethylacrylate, acrylamide and 2-acrylamido-2-methylpropanesulfonate sodium as monomer is converted into cross linked polymer particles by the dehydration processes described above. 
     The polymeric microparticles are optionally prepared in dry form by adding the emulsion to a solvent which precipitates the polymer such as isopropanol, isopropanollacetone or methanol/acetone or other solvents or solvent mixtures that are miscible with both hydrocarbon and water and filtering off and drying the resulting solid. 
     An aqueous suspension of the polymeric microparticles is prepared by redispersing the dry polymer in water. 
     In another embodiment, this invention is directed to a method of modifying the permeability to water of a subterranean formation an encapsulated polymeric microparticle composition comprising cross-linked expandable polymeric microparticles and a shell encapsulating the cross-linked expandable polymeric microparticles, the shell comprising at least one layer of a labile or degradable encapsulation material wherein the encapsulated microparticles have a smaller diameter than the pores of the subterranean formation and wherein the labile cross linkers break under the conditions of temperature and pH in the subterranean formation to form expanded microparticles. 
     The composition then flows through a zone or zones of relatively high permeability in the subterranean formation under increasing temperature conditions, until the composition reaches a location where the temperature or pH is sufficiently high to promote expansion of the microparticles. 
     Unlike conventional blocking agents such as polymer solutions and polymer gels that cannot penetrate far and deep into the formation, the composition of this invention, due to the size of the particles and low viscosity, can propagate far from the injection point until it encounters the high temperature zone. 
     Also, the exemplary polymeric microparticles, due to their highly crosslinked nature, do not necessarily expand in solutions of different salinity. Consequently, the viscosity of the dispersion may not be affected by the salinity of the fluid encountered in the subterranean formation. Accordingly, no special carrier fluid is needed for treatment. Only after the particles encounter conditions sufficient to reduce the crosslinking density, is the fluid rheology changed to achieve the desired effect. 
     Among other factors, the reduction in crosslinking density is dependent on the rate of cleavage of the labile crosslinker. In particular, different labile crosslinkers, have different rates of bond cleavage at different temperatures. The temperature and mechanism depend on the nature of the cross-linking chemical bonds. For example, when the labile crosslinker is PEG diacrylate, hydrolysis of the ester linkage is the mechanism of de-crosslinking. Different alcohols have slightly different rates of hydrolysis. In general, methacrylate esters will hydrolyze at a slower rate than acrylate esters under similar conditions. With divinyl or diallyl compounds separated by an azo group such as the diallylamide of 2,2′-Azobis(isobutyric acid), the mechanism of de-crosslinking is elimination of a nitrogen molecule. As demonstrated by various azo initiators for free radical polymerization, different azo compounds indeed have different half-life temperatures for decomposition. 
     In addition to the rate of de-crosslinking, we believe that the rate of particle diameter expansion also depends on the total amount of remaining crosslinking. We have observed that the particle expands gradually initially as the amount of crosslinking decreases. After the total amount of crosslinking passes below a certain critical density, the viscosity increases explosively. Thus, by proper selection of the labile cross-linker, both temperature- and time-dependent expansion properties can be incorporated into the polymer particles. 
     The particle size of the polymer particles before release from the shell and expansion is selected based on the calculated pore size of the highest permeability thief zone. The crosslinker type and concentration, and hence the time delay before the injected particles begin to expand, is based on the temperature both near the injection well and deeper into the formation, the expected rate of movement of injected particles through the thief zone and the ease with which water can crossflow out of the thief zone into the adjacent, lower permeability, hydrocarbon containing zones. A polymer microparticle composition designed to incorporate the above considerations results in a better water block after particle expansion, and in a more optimum position in the formation. 
     In an exemplary aspect of the invention, the encapsulated cross-linked expandable microparticles have an unexpanded volume average particle size diameter of from about 0.05 to about 5,000 microns and comprise a cross linking agent content of from about 100 to about 200,000 ppm of labile cross linkers and from 0 to about 300 ppm of non-labile cross linkers. However, other expandable polymeric microparticles may be used for the base microparticle. 
     In one aspect, the encapsulation material for the shell is selected from the group consisting of cross-linked polymers, emulsifiers, thermally responsive polymers, silicone elastomers, siloxanes, starches, gum gar, derivated cellulose sulfonated polysaccharides, silica, colloidal clays, a salt, and combinations thereof. In one aspect, the shell comprises at least two layers of encapsulation material. In one aspect, the shell comprises a dissolving material, such as a water-soluble gelatin. In one aspect, the shell comprises a degradable material, such material degrading upon exposure to an activating event. The activating event is, for example, a change in one or more of the following: of temperature, pressure, pH, salinity, shear force, or dilution in the local environment of the microparticles. The activating event may be exposure to an activation agent. An activation agent can be, for example, a reducing agent, an oxidation agent, an acid, a base, a biological agent, a cross-linking agent, and a salt. 
     In one aspect, the invention provides a method of modifying the permeability to water of a subterranean formation, the method including injecting into the subterranean formation an encapsulated polymeric microparticle composition comprising cross-linked expandable polymeric microparticles and a shell encapsulating the cross-linked expandable polymeric microparticles, the shell comprising at least one layer of a labile or degradable encapsulation material. 
     In one aspect of the method, the cross-linked expandable polymeric microparticles have an unexpanded volume average particle size diameter of from about 0.05 to about 5,000 microns, wherein the encapsulated microparticles have a smaller diameter than the pores of the subterranean formation and wherein the labile encapsulation material degrades under a change in environmental conditions in the subterranean formation so that the expandable polymeric microparticles are free to expand. In one aspect, from about 100 ppm to about 100,000 ppm of the composition, based on polymer actives, is added to the subterranean formation. In another aspect, from about 500 ppm to about 10,000 ppm of the composition, based on polymer actives, is added to the subterranean formation. In another aspect, from about 500 ppm to about 1000 ppm of the composition, based on polymer actives, is added to the subterranean formation. In one aspect, the composition is added to injection water as part of a secondary or tertiary process for the recovery of hydrocarbon from the subterranean formation. The injection water can be added to the subterranean formation at a temperature lower than the temperature of the subterranean formation. In another aspect, the method further comprises introducing a change in the environmental conditions in the subterranean formation wherein the change causes the labile encapsulation material to degrade. The change in the environmental conditions can be a change in temperature, pressure, pH, salinity, shear force or dilution in the local environment of the microparticle. In one aspect, introducing the change in the environmental conditions includes introducing an activation agent into the formation. The activation agent may be a reducing agent, an oxidation agent, an acid, a base, a biological agent. In one aspect of the method, the composition is used in a carbon dioxide and water tertiary recovery project. When used in a tertiary oil recovery process, the recovery process may include water injection. In one aspect, the injection water is added to a producing well. The subterranean formation is, for example, a sandstone or carbonate hydrocarbon reservoir. 
     In another aspect, the invention provides a polymeric composition comprising encapsulated expandable polymeric microparticles. 
     In another aspect, the invention provides a method of making an expandable microparticle comprising providing microparticles comprising cross-linked expandable polymers and encapsulating the cross-linked expandable polymeric microparticles in a shell comprising at least one layer of a labile encapsulation material. 
     The foregoing may be better understood by reference to the following examples, which are presented for purposes of illustration and are not intended to limit the scope of this invention. 
     EXAMPLES 
     Examples 1-8 
     Preparation of Expandable Polymeric Microparticles 
     Exemplary expandable cross-linked polymeric microparticles for making the encapsulated expandable polymeric microparticles of this invention are readily prepared using inverse emulsion polymerization techniques as described below. 
     A representative emulsion polymer composition is prepared by polymerizing a monomer emulsion consisting of an aqueous mixture of 164.9 g of 50% acrylamide, 375.1 g of 58% sodium acrylamido methylpropane sulfonate (AMPS), 16.38 g water, 0.5 g 40% pentasodium diethylenetriaminepentaacetate, 3.2 g of 1% solution of methylenebisacrylamide (mba), and 36.24 g polyethyleneglycol (PEG) diacrylate as the dispersed phase and a mixture of 336 g petroleum distillate, 60 g ethoxylated sorbitol oleate and 4 g sorbitan sesquioleate as the continuous phase. 
     The monomer emulsion is prepared by mixing the aqueous phase and the oil phase, followed by homogenization using a Silverson Homogenizer. After deoxygenation with nitrogen for 30 minutes, polymerization is initiated thermally by heating the emulsion to 70° C. and maintaining at 70° C. for the next 4 hrs. 
     If desired, the polymeric microparticle can be isolated from the latex by precipitating, filtering, and washing with a mixture of acetone and isopropanol. After drying the oil and surfactant free particle can be redispersed in aqueous media. The average particle size of this latex particle measured in deionized water, using a Malvern Instruments&#39; Mastersizer E, is 0.28 micron. 
     Table 1 lists representative emulsion polymers prepared according to the method of Example 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Preparation of polymeric microparticles in emulsion form 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
                 Ex. 6 
                 Ex. 7 
                 Ex. 8 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 50% acrylamide 
                 164.9 
                 164.9 
                 82.45 
                 131.84 
                 82.5 
                 82.5 
                 82.5 
                 82.5 
               
               
                 58% Na AMPS 
                 375.1 
                 375.1 
                 187.5 
                 300 
                 187.5 
                 187.5 
                 187.5 
                 187.5 
               
               
                 DI water 
                 19.58 
                 19.58 
                 12.5 
                 36.8 
                 18 
                 22.9 
                 25.3 
                 26.5 
               
               
                 Methylene bisacrylamide 
                 0.032 
                 0.032 
                 0 
                 7.36 
                 0 
                 0 
                 0 
                 0 
               
               
                 PEG-200 diacrylate 
                 36.24 
                 18.12 
                 30.46 +1   
                 0 
                 9.75 
                 4.87 
                 2.44 
                 1.22 
               
               
                 Crosslinker/monomer 
                 56890 
                 28500 
                 28390 
                 2839 
                 18390 
                 9080 
                 4540 
                 2270 
               
               
                 Molar ratio (ppm) 
               
               
                 Petroleum distillate 
                 336 
                 336 
                 168 
                 268.8 
                 168 
                 168 
                 168 
                 168 
               
               
                 Ethoxylated sorbitol oleate 
                 60 
                 60 
                 30 
                 48 
                 30 
                 30 
                 30 
                 30 
               
               
                 Sorbitan sesquioleate 
                 4 
                 4 
                 2 
                 3.2 
                 2 
                 2 
                 2 
                 2 
               
               
                   
               
               
                   +1 PEG-400 diacrylate 
               
            
           
         
       
     
     Example 9 
     Preparation of Encapsulated Polymeric Microparticles 
     Polymeric microparticles as described in examples 1-8 are encapsulated with a shell of a copolymer of NIPAAm and acrylamide. Oligomers of NIPAAm having a reactive group at one end can be prepared by the radical polymerization of NIPAAm using AIBN as the radical initiator, plus a chain transfer agent with a thiol (H—SH) group at one end and the desired “reactive” group (e.g. —OH, —COOH, —NH.sub.2) at the other end. Chen et al., Bioconjugate Chem. 4: 509-514 (1993) and Chen et al., J. Biomaterials Sci. Polymer Ed. 5: 371-382 (1994) and as further described in U.S. Pat. No. 6,616,946 which provides that appropriate quantities of NIPAAm, AIBN and the chain transfer reagent in DMF are placed in a thick-walled polymerization tube and the mixtures, degassed by freezing and evacuating and then thawing (4 times). After cooling for the last time, tubes are evacuated and sealed prior to polymerization. The tubes are immersed in a water bath at 60° C. for 4 hours. The resulting polymer is isolated by precipitation into diethyl ether and weighed to determine yield. The resulting polymer is then redissolved in methylene chloride or other volatile organic solvent to which the microparticle of example 1 is introduced. Evaporation of the volatile solvent results in a thin polymer shell encapsulating the microparticle. 
     Example 10 
     Activation of the Polymeric Microparticle by Heat 
     As the particles expand in a medium of fixed volume, the volume fraction occupied by the particles increases. Consequently, the volume fraction of the continuous phase decreases. The decrease in free volume is reflected in an increase in the viscosity of the dispersion. Heat activation of the microparticles of this invention is demonstrated in a bottle test. 
     To carry out a bottle test, a dispersion containing 5000 ppm of the particles is prepared in an aqueous medium (e.g. a synthetic brine). Dispersing of particles is accomplished by vigorous stirring or by using a homogenizer. To prevent oxidative degradation of the expanding particles during monitoring, 1000 ppm sodium thiosulfate is added to the mixture as an oxygen scavenger. 
     The bottles are placed in a constant temperature oven to age. Then, at a predetermined time, a bottle is removed from the oven and cooled to 75° F. The viscosity is measured at 75° F. using Brookfield LV No. 1 spindle at 60 rpm (shear rate 13.2 sec-1). 
     Heat activation of the polymeric microparticles by heat is characterized by monitoring the viscosity change of aqueous dispersions of particles aged for increasing periods of time and at different temperatures. 
     Example 11 
     Sand Pack Test 
     This Example demonstrates that the polymeric microparticle of this invention can be propagated with a conformation constrained in part by built-in reversible (labile) elements (one or more of labile crosslinks, a labile core, or a labile shell) and will expand in size when these degrade, to give a particle of suitable size to produce a substantial effect. 
     In the sand pack test, a 40 foot long sand pack of 0.25 inches internal diameter, made from degreased and cleaned 316 stainless steel tubing, is constructed in eight sections, fitted with pressure transducers, flushed with carbon dioxide gas and then placed in an oven and flooded with synthetic oil field injection water. 
     A dispersion of a representative polymeric microparticle is prepared in the synthetic injection water and injected into the pack to fill the pore volume. Pressure drops across the tube sections are monitored for signs of conformation change of the polymer particle as labile elements in the microparticle composition (which may include one or more of labile crosslinks, a labile core, or a labile shell) are hydrolyzed. The “popping open” of the polymer particles is observed as a steep rise in the pressure drop. The sand pack test is described in detail in WO 01/96707. 
     The data for representative polymeric microparticles shows that the particles are able to travel through the first two sections of the sand pack without changing the RRF of the sections. However, particles in the last section, after accumulating a sufficient amount of residence time, have expanded and give a higher value of RRF. The higher RRF value is maintained after the injection fluid is changed from polymer dispersion to brine. 
     This example demonstrates that the polymeric microparticle composition with the microparticle conformation constrained by built-in labile elements can be propagated through a porous media. The microparticle will expand in size when labile elements such as crosslinks degrade, to give a particle of suitable size to produce a substantial effect, even in a high permeability porous medium. 
     It is to be understood that the specific embodiments of the present teachings as set forth herein are not intended as being exhaustive or limiting, and that many alternatives, modifications, and variations will be apparent to those of ordinary skill in the art in light of the foregoing examples and detailed description. Accordingly, the present teachings are intended to embrace all such alternatives, modifications, and variations that fall within the spirit and scope of the following claims. 
     All publications, patents, patent applications and other references cited in this application are herein incorporated by reference in their entirety as if each individual publication, patent, patent application or other reference were specifically and individually indicated to be incorporated by reference.