Method for preparing water-soluble polymers in powder form

Abstract

The present invention relates to a method for preparing a water-soluble polymer having an average molecular weight by weight greater than 1 million Daltons. This method comprises the following successive steps: a) Preparing a polymer, in the form of a gel, by free-radical polymerization in aqueous solution at an initiation temperature of between -20°C and +50°C, of at least one hydrophilic monounsaturated ethylenic monomer, the at least one hydrophilic monounsaturated ethylenic monomer having, relative to the weight of the polymerization charge, a concentration of between 10 and 60%, and in the presence of 0.01 to 5% by weight of at least one surfactant, relative to the total weight of the at least one hydrophilic monounsaturated ethylenic monomer used; b) Granulating the resulting polymer gel; c) Drying the granulated polymer gel to obtain a polymer in powder form; d) Grinding and sifting the powder.

Description

METHOD FOR PREPARING WATER-SOLUBLE POLYMERS IN POWDER FORMField of the inventionThe present invention relates to a method for the preparation of a water-soluble polymer in powder form, the dissolution of which is improved in brines. The invention also relates to the preparation of brines containing these polymers and the use of these brines for the enhanced recovery of hydrocarbons (oil and / or gas) and hydraulic fracturing operations.Prior artWater-soluble polymers are commonly used in enhanced oil recovery (EOR) or hydraulic fracturing operations. These operations often involve very saline waters that contain divalent ions.The dissolution of water-soluble polymers in powder form in these very salty waters is complex, which is reflected by very long dissolution times and / or the presence of polymers not totally hydrated. In particular, it involves the use of specific equipment called polymer particle hydration equipment, such as the PSU: Polymer Slicing Unit (document WO 2008 / 071808). These systems are expensive, energy-consuming and have a significant footprint on the ground, all the more so the solutions resulting from this equipment must subsequently reside in one or more maturation tanks, to ensure complete dissolution of the polymer particles. The residence time in the maturation tanks can be long to ensure total dissolution in very saline waters. In general, the hydration of water-soluble polymers in powder form corresponds to the dissolution of these polymers.The existing alternative for reducing the residence time in the maturation tanks is to use very small polymer particles before the hydration system. However, the handling of fine particles is constraining for operators. In fact, the fine polymer particles tend to agglomerate. This agglomeration phenomenon proves to be problematic for a good dissolution of the polymer and gives rise to the formation of a mass of insoluble polymers.The addition of the water-soluble polymers to an aqueous solution containing at least one surfactant may be a solution to allow better dissolution of water-soluble polymers in powder form in an aqueous solution. However, the effectiveness of this surfactant addition on the dissolution of the polymer decreases when the salinity of the waters increases.Patent application FR 3 096 963 discloses a gel method for obtaining structured water-soluble synthetic polymers of high molecular weight in powder form.Hydraulic fracturing or EOR operations often require the use of water-soluble polymers such as copolymers of acrylamide and sodium acrylate. When the salinity of aqueous solutions is high, sodium acrylate is partially or totally substituted by sodium 2-acrylamido-2-methylpropane sulfonate. The incorporation of this monomer makes it possible to have polymers that are less sensitive to degradation in highly saline aqueous solutions. This monomer is however less available and much more expensive than sodium acrylate.For hydraulic fracturing or EOR operations, when the salinity of the waters is high and in particular when they contain divalent ions (in particular divalent cations), in order to reduce the energy and financial impact for the dissolution of the powders, but also the footprint on the ground of the dissolution equipment (large maturation tanks), it is necessary to have water-soluble polymers with improved dissolution.Disclosure of the inventionThe invention relates to a method for preparing a water-soluble polymer which has improved solubility (shorter dissolution time, absence of gel points) in high-salinity brines and which in particular contain divalent ions (in particular divalent cations). In addition, the polymer resulting from said method makes it possible to reduce the footprint on the ground of the dissolution equipment (size and number of maturation tanks) and may contain a lesser amount of sodium 2-acrylamido-2-methylpropane sulfonate type monomers in its monomer composition. The invention relates to the preparation of saline solutions from the polymers resulting from this method and their use for applications in oil and gas fields.More specifically, the first aspect of the invention relates to a method for preparing a water-soluble polymer having an average molecular weight by weight greater than 1 million Daltons, the method comprising at least the following successive steps:a) Preparing a polymer, in the form of a gel, by free-radical polymerization in aqueous solution at an initiation temperature of between -20°C and +50°C, of at least one hydrophilic monounsaturated ethylenic monomer,the at least one hydrophilic monounsaturated ethylenic monomer having, relative to the weight of the polymerization charge, a concentration of between 10 and 60%,in the presence of 0.01 to 5% by weight of at least one surfactant, relative to the total weight of the at least one hydrophilic monounsaturated ethylenic monomer;b) Granulating the resulting polymer gel;c) Drying the granulated polymer gel to obtain a polymer in powder form;d) Grinding and sifting the powder,wherein the surfactant of step a) is selected from the group consisting of: alkyl aryl sulfonates, alkyl benzene sulfonates, alkyl sulfates, N-ethoxy sulfonates, sodium dodecyl sulfonate, propoxylated alcohol sulfates, ethoxylated alkyl sulfates, ethoxylated alcohol sulfates, alpha-olefin sulfonates, alpha-olefin sulfates, alkyl polyalkoxy alkyl sulfonates, alkyl aryl polyalkoxy alkyl sulfonates, branched alkyl benzene sulfonates, sodium docusate, ethoxylated glycidyl sulfonates, propoxylated glycidyl sulfonates, alkyl ether sulfates, Guerbet alkyl sulfates, internal olefin sulfonates, sulfonated ethoxylated alcohols, sulfonated alkyl phenols, sodium petroleum sulfonate, propoxylated alkyl alcohol sulfates, carboxylated ethoxy alkyls, polyglycoside alkyls, ethoxylated alcohols, propoxylated alcohols, propoxylated alcohol sulfates, ethoxylated alkyl phenols, polyethoxylated phenol alkyls, poly(ethylene / propylene) glycol ethers, N -ethoxylated secondary alcohols, N -propoxylated secondary alcohols, tridecyl ethoxylated alcohols, triphenylmethane, and mixtures thereof,with, for these surfactants:- the alkyl groups being in C4-C20,- ethoxylated chains comprising between 6 and 100 -CH2-CH2-O- groups,- propoxylated chains comprising between 6 and 100 -CH2 -CH2-CH2-O- groupsBy “X and / or Y", this means, according to the invention, “X”, or “Y”, or “X and Y”.The invention also includes all possible combinations of the various embodiments disclosed, whether they are preferred embodiments or given by way of example. Furthermore, when ranges of values are indicated, the limit values are included in these ranges. The disclosure also includes all of the combinations between the limit values of these ranges of values. For example, the ranges of values “1-20, preferably 5-15” imply disclosure of the ranges “1-5”, “1-15”, “5-20” and “15-20” and the values 1, 5, 15 and 20.“Polymer” means a homopolymer prepared from one monomer or a copolymer prepared from at least two different monomers, it may therefore be a copolymer of at least two monomers chosen from among: anionic hydrophilic monomers, cationic hydrophilic monomers, non-ionic hydrophilic monomers, zwitterionic hydrophilic monomers, hydrophobic monomers, and their mixtures.The polymer has a molecular weight of at least 1 million Daltons, preferably between 1 and 40 million Daltons, more preferably between 1 and 30 million Daltons, more preferably between 2 and 20 million Daltons, and even more preferably between 3 and 15 million Daltons. Molecular weight refers to the average molecular weight by weight.The molecular weight is advantageously determined by the intrinsic viscosity of the polymer. The intrinsic viscosity can be measured by methods known to a person skilled in the art and can be calculated from the reduced viscosity values for different polymer concentrations by a graphical method consisting in plotting the reduced viscosity values (y-axis) against the concentration (x-axis) and extrapolating the curve to zero concentration. The intrinsic viscosity value is plotted on the y-axis or using the least-squares method. The molecular weight can then be determined using the Mark-Houwink equation:[η] = K.Mα[η] represents the intrinsic viscosity of the polymer as determined by the solution viscosity method.K represents an empirical constant.M represents the molecular weight of the polymer.α represents the Mark-Houwink coefficient.K and α depend on the particular polymer-solvent system.The term “water-soluble polymer” should be understood to mean a polymer that gives an aqueous solution without insoluble particle, when it is dissolved under stirring at 25°C and with a concentration of 10 g.L-1 in deionised water.The term “hydrophobic monomer” should be understood to mean a monomer that has an octanol-water partition coefficient, Kow, greater than 1, in which the partition coefficient Kow is determined at 25°C in an octanol-water mixture with a volume ratio of 1 / 1, at a pH of between 6 and 8.The term “hydrophilic monomer” should be understood to mean a monomer that has an octanol-water partition coefficient, Kow, less than or equal than 1, in which the partition coefficient Kow is determined at 25°C in an octanol-water mixture with a volume ratio of 1 / 1, at a pH of between 6 and 8.  The polymer may be non-ionic, cationic, anionic, or amphoteric.“Non-ionic polymer” means a polymer which comprises only non-ionic hydrophilic monomers and optionally zwitterionic hydrophilic and / or hydrophobic monomers.“Cationic polymer” means a polymer which comprises only cationic hydrophilic monomers and optionally non-ionic hydrophilic monomers and / or zwitterionic hydrophilic and / or hydrophobic monomers, more preferably only cationic hydrophilic monomers and optionally non-ionic hydrophilic and / or hydrophobic monomers, even more preferably only cationic hydrophilic monomers and optionally non-ionic hydrophilic monomers.“Anionic polymer” means a polymer which comprises only anionic hydrophilic monomers and optionally non-ionic hydrophilic monomers and / or zwitterionic hydrophilic and / or hydrophobic monomers, more preferably only anionic hydrophilic monomers and optionally non-ionic hydrophilic and / or hydrophobic monomers, even more preferably only anionic hydrophilic monomers and optionally non-ionic hydrophilic monomers.“Amphoteric polymer” means a polymer which comprises cationic hydrophilic monomers and anionic hydrophilic monomers and optionally non-ionic hydrophilic monomers and / or zwitterionic hydrophilic and / or hydrophobic monomers.“Semi-synthetic polymer” means a natural polymer, such as, for example, compounds of the polysaccharide family, which has undergone chemical reactions for grafting different synthetic substituents. A person skilled in the art knows this type of reactions, which remains conventional chemical reactions applied to natural polymers.Advantageously, the polymerization charge does not comprise a polymer before the start of the polymerization of step a). In other words, advantageously, step a) is carried out by polymerizing at least one hydrophilic monounsaturated ethylenic monomer in the absence of a host polymer.Preferably, the at least one monounsaturated ethylenic monomer polymerized in step a) of the method of the invention is chosen from the group consisting of non-ionic monomers, anionic monomers, cationic monomers and mixtures thereof,- the hydrophilic non-ionic monomer or monomers being chosen from the group comprising water-soluble vinyl monomers, such as acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N,N-dialkylacrylamides (for example N,N-dimethylacrylamide or N,N-diethylacrylamide), N,N-dialkylmethacrylamides, alkoxylated esters of acrylic acid, alkoxylated esters of methacrylic acid, N-vinylpyrrolidone, N-methylol(meth)acrylamide, N-vinylcaprolactam, N-vinylformamide (NVF), N-vinylacetamide, N-vinylimidazole, N-vinyl succinimide, acryloyl morpholine (ACMO), glycidyl methacrylate, glyceryl methacrylate, diacetone acrylamide, methacrylic anyhydride, acrylonitrile, maleic anydride, itaconic anhydride, itaconamide, vinylpyridine, hydroxyalkyl(meth)acrylate, thioalkyl(meth)acrylate, isoprenol and its alkoxylated derivatives, hydroxyethyl(meth)acrylates and their alkoxylated derivatives, hydroxypropyl acrylate and its alkoxylated derivatives, alkyl acetate and mixtures of their mixtures, the alkyl groups being C1-C3 alkylated hydrocarbon chains;- the anionic hydrophilic monomers being chosen from the acrylic acid, methacrylic acid, dimethylacrylic acid, itaconic acid, C1-C 3 itaconic acid semi-ester, acryloyl chloride, crotonic acid, maleic acid, fumaric acid, 3-acrylamido-3-methylbutanoic acid, vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, methallylsulfonic acid, 2-methylidenepropane-1,3-disulfonic acid, 2-sulfoethylmethacrylate, sulfopropylmethacrylate, sulfopropylacrylate, allylphosphonic acid, ethylene glycol methacrylate phosphate, styrene sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid (ATBS), 2-acrylamido-2-methylpropane disulfonic acid, 3-allyloxy-2-hydroxypropanesulfonic acid, diethylallylphosphonate, carboxyethyl acrylate; the water-soluble salts of all these monomers, such as their alkali metal salts, alkaline-earth metal salts or ammonium salts; and mixtures thereof; and mixtures thereof;- the cationic hydrophilic monomers being chosen from diallyldialkyl ammonium salts such as diallyl dimethyl ammonium chloride (DADMAC); acidified or quaternized salts of dialkylaminoalkyl(meth)acrylamides, such as, for example, methacrylamidopropyl trimethyl ammonium chloride (MAPTAC), acrylamidopropyl trimethyl ammonium chloride (APTAC), acidified or quaternized salts of dialkylaminoalkyl acrylate such as quaternized or salified dimethylaminoethyl acrylate (ADAME), acidified or quaternized salts of dialkylaminoalkyl methacrylate such as quaternized or salified dimethylaminoethyl methacrylate (MADAME), and mixtures thereof.Preferably, the water-soluble polymer contains between 5 mol% and 50 mol% of anionic hydrophilic monomer(s), preferably between 5 and 45 mol% and even more preferably between 5 and 40 mol%.Preferably, the water-soluble polymer contains between 50 mol% and 95 mol% of non-ionic hydrophilic monomer(s), preferably between 55 and 95 mol% and even more preferably between 60 and 95 mol%.Optionally, zwitterionic hydrophilic monomers can be polymerized during step a) of the method, these monomers being chosen from dimethylaminoethyl acrylate derivatives, such as 2-((2-9 acryloyloxy)ethyl)dimethylammonio) thane-1-sulfonate, 3-((2-(acryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate, 4-((2-(acryloyloxy)ethyl)dimethylammonio)butane-1-sulfonate, [2-(acryloyloxy)ethyl](dimethylammonio)acetate, derivatives of dimethylaminoethyl methacrylate such as 2-((2-(methacryloyloxy) ethyl)dimethylammonio)ethane-1-sulfonate, 3-((2-(methacryloyloxy)ethyl)dimethylammonio)propane-1-sulfonate, 4-((2-(methacryloyloxy)ethyl)dimethylammonio)butane-1-sulfonate, [2-(methacryloyloxy)ethyl](dimethylammonio)acetate, derivatives of dimethylamino propylacrylamide such as 2-((3-acrylamidopropyl)dimethylammonio)ethane-1-sulfonate, 3-((3-acrylamidopropyl)dimethylammonio)propane-1-sulfonate, 4-((3-acrylamidopropyl)dimethylammonio)butane-1-sulfonate, [3-(acryloyl)oxy)propyl](dimethylammonio)acetate, dimethylamino propyl methylacrylamide, 2-((3-methacrylamidopropyl)dimethylammonio)ethane-1-sulfonate, 3-(dimethylammonio)propane-1-sulfonate, 4-((3-methacrylamidopropyl)dimethylammonio)butane-1-sulfonate and propyl [3-(methacryloyloxy)](dimethylammonio)acetate and their mixtures.Preferably, the water-soluble polymer contains less than 5 mol% of zwitterionic monomers, more preferably less than 2 mol% and even more preferably less than 1 mol%.Optionally, hydrophobic monomers can be polymerized during step a) of the method, these hydrophobic monomers being chosen from among the (meth)acrylic acid esters having a C4-C30 alkyl (i) or an arylalkyl(C4-C30 alkyl, C4-C30 aryl) (ii) chain, or a propoxylated chain (iii), or an ethoxylated chain (iv), or ethoxylated and propoxylated chain (v); alkyl aryl sulfonates (C4-C30alkyl, C4-C30aryl); mono- or diamides substituted by (meth)acrylamide having a C4-C30 alkyl chain (i), or (ii) an arylalkyl(C4-C30 alkyl, C4-C30 aryl) chain (ii), or propoxylated chain (iii), or ethoxylated chain (iv), or ethoxylated and propoxylated chain (v); anionic or cationic monomer derivatives of (meth)acrylamide or of (meth)acrylic acid carrying a hydrophobic chain; and their mixtures. The hydrophobic monomers may comprise halogen atoms, for example chlorine.Of these hydrophobic monomers:- the alkyl groups are advantageously C4-C20, and more preferably C4-C8. The C6-C20 alkyls are advantageously linear alkyls whereas the C4-C5 alkyls are preferably branched,- the arylalkyl groups are C7-C25, and more preferably C7-C15,- the ethoxylated chains comprise 6 to 100 -CH2-CH2-O- groups, and more preferably 10 to 40,- the propoxylated chain advantageously comprise 1 to 50 -CH2-CH2-CH2-O- groups, and more preferably 1 to 20.Preferred hydrophobic monomers belonging to these classes are, for example:- n-hexyl (meth)acrylate, n-octyl (meth)acrylate, octyl (meth)acrylamide, lauryl (meth)acrylate, lauryl (meth)acrylamide, myristyl (meth)acrylate, myristyl (meth)acrylamide, pentadecyl (meth)acrylate, pentadecyl (meth)acrylamide, cetyl (meth)acrylate, cetyl (meth)acrylamide, oleyl (meth)acrylate, oleyl (meth)acrylamide, erucyl (meth)acrylate, erucyl (meth)acrylamide, N-tert-butyl(meth)acrylamide, vinylpyridine, 2-ethylhexyl acrylate, C4-C22 itaconic acid hemi-esters, acidified or quaternized C4-C22 dialkylaminoalkyl (meth)acrylate salts, acidified or quaternized C4-C22 dialkyl-aminoalkyl(meth)acrylamides salts, undecanoic acrylamido acid, and their mixtures.The water-soluble polymer generally comprises less than 1 mol% hydrophobic monomers. Preferably, this polymer is devoid of hydrophobic monomers.When the water-soluble polymer comprises one or more hydrophobic monomers, they are present in an amount such that the polymer remains soluble in water.In one particular embodiment, the polymer may comprise at least one group with an LCST, preferably 0 to 1 mol%.According to the general knowledge of a person skilled in the art, a group with an LCST corresponds to a group whose water solubility, for a given concentration, is modified above a certain temperature and as a function of salinity. It is a group with a heating transition temperature that defines its lack of affinity with the solvent medium. The lack of affinity with the solvent results in opacification or loss of transparency, which may be due to precipitation, aggregation, gelation or viscosification of the medium. The minimum transition temperature is known as the LCST (Lower Critical Solution Temperature). For each concentration a group with an LCST, a heating transition temperature is observed. It is higher than the LCST, which is the minimum point on the curve. Below this temperature, the polymer is soluble in water; above this temperature, the polymer loses its solubility in water.In one particular embodiment, the polymer may comprise at least one group with an UCST, preferably 0 to 1 mol%.According to the general knowledge of a person skilled in the art, a group with a UCST corresponds to a group whose water solubility, for a given concentration, is modified below a certain temperature and as a function of salinity. It is a group with a cooling transition temperature that defines its lack of affinity with the solvent medium. The lack of affinity with the solvent results in opacification or loss of transparency, which may be due to precipitation, aggregation, gelation or viscosification of the medium. The maximum transition temperature is known as the UCST (Upper Critical Solution Temperature). For each concentration a group with a UCST, a cooling transition temperature is observed. It is lower than the UCST, which is the maximum point on the curve. Above this temperature, the polymer is soluble in water; below this temperature, the polymer loses its solubility in water.The quantities of the different monomer(s) will be adjusted by a person skilled in the art in order not to exceed 100 mol% when preparing the water-soluble polymer:- at least one hydrophilic monounsaturated ethylenic monomer (non-ionic, cationic or anionic),- optionally at least one monomer chosen from zwitterionic monomers, hydrophobic monomers, monomers with an LCST group, monomers with a UCST group, and mixtures thereof.According to a particular embodiment, the polymer consists of at least one non-ionic, cationic or anionic hydrophilic monounsaturated ethylenic monomer. In this case, the polymer does not comprise a monomer chosen from zwitterionic hydrophilic monomers, hydrophobic monomers, monomers with an LCST group, monomers with a UCST group, and the mixtures thereof.The polymer may be partially or totally post-hydrolyzed.Post-hydrolysis is the hydrolysis reaction of the polymer after it has been formed by polymerization of the monomer(s). This step, which can be carried out between b) and c) steps, consists in reacting hydrolysable functional groups of monomers, advantageously non-ionic functional groups, more advantageously amide or ester functions, with a hydrolysis agent. This hydrolysis agent may, for example, be an enzyme, an ion-exchange resin, or a Brønsted acid metal (for example a hydrohalogenic acid) or a Brønsted base (for example an alkali hydroxide or an alkaline-earth hydroxide). Preferably, the hydrolysis agent is a Brønsted base. During this step of post-hydrolyzing the polymer, the number of carboxylic acid functions increases. Indeed, the reaction between the hydrolysis agent and the amide or ester functions present in the polymer produces carboxylate groups.According to the invention, the polymer may have a linear, branched, star-shaped or comb-shaped structure. This structure can be obtained, according to the general knowledge of a person skilled in the art, for example by selecting the initiator, the transfer agent, the polymerization technique such as Reversible Addition Fragmentation chain Transfer (RAFT) polymerization, Nitroxide Mediated Polymerization (NMP) or Atom Transfer Radical Polymerization (ATRP), the incorporation of structural monomers, or the concentration.The polymer may further by structured by a branching agent. By structured polymer, this means a non-linear polymer, which has side chains.The branching agent is advantageously chosen from:- structure agents advantageously chosen from the group comprising polyethylenically unsaturated monomers (having, as a minimum, two unsaturated functions), like for example, vinyl functions, in particular, allylic or acrylic, and, for example, methylene bis acrylamide (MBA), triallyamine, tetraallylammonium chloride or 1,2 dihydroxyethylene bis-(N-acrylamide) can be cited,- monomers having at least two epoxy functions,- monomers having at least one unsaturated function and an epoxy function,- macroprimers such as polyperoxides, polyazoics and transfer polyagents, such as polymercaptan polymers, and polyols,- functionalized polysaccharides,- water-soluble metal complexes composed:* of a metal of valence greater than 3 such as, as an example and in a non-limiting manner, aluminium, boron, zirconium, or also titanium, and* of a ligand carrying a hydroxyl function.The quantity of branching agent in the polymer is advantageously less than 1000 ppm by weight relative to the total weight of the monomers of the polymer, preferably less than 100 ppm by weight, and more preferably less than 10 ppm by weight.When the polymer is water soluble and comprises at least one branching agent, it remains water soluble. A person skilled in the art knows how to adjust the quantity of branching agent and, possibly, the quantity of transfer agent needed to obtain this result.In one particular embodiment, the polymer comprises no branching agent.The polymerization for step a) of the method of the invention is carried out by radical route. It includes polymerization by free radicals by means of UV, azo, redox or thermal initiators as well as controlled radical polymerization (CRP) techniques or more particularly using the RAFT (Reversible Addition Fragmentation chain Transfer) type.The polymerization charge is a solution of water-soluble monounsaturated ethylenic monomers optionally supplemented with conventional polymerization regulators before the polymerization starts. The usual polymerization regulators are, for example, sulphur compounds such as thioglycolic acid, mercaptan alcohols, dodecyl mercaptan, amines such as ethanolamine, diethanolamine, morpholine and phosphites such as sodium hypophosphites. In the case of a RAFT-type polymerization, specific polymerization regulators such as those comprising a transfer group comprising the -S-CS- function, may be used. Mention may in particular be made of these compounds of the family of xanthates (-S-CS-O-), dithioesters (-S-CS-C), trithiocarbonates (-S-CS-S-), or dithiocarbamates (-S-CS-N). Among the compounds of the xanthate family, O-ethyl-S-(1-methoxy carbonyl ethyl)xanthate is widely used for its compatibility with monomers of acrylic nature.The polymerization initiators used may be any compound which dissociates into radicals under polymerization conditions, for example: organic peroxides, hydroperoxides, hydrogen peroxide, persulfates, azo compounds, and redox catalysts. The use of water-soluble initiators is preferred. In some cases, it is advantageous to use mixtures of various polymerization initiators, for example, mixtures of redox catalysts and azo compounds.Suitable organic peroxides and hydroperoxides are, for example, sodium or potassium peroxodisulfate, acetylacetone peroxide, methyl ethyl ketone peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, tert-amyl perpivalate, tert-butyl perpivalate, tert-butyl perneohexanoate, tert-butyl perbuto-butylate, -ethyl hexanoate, tert-butyl perisononanoate, tert-butyl permaleate, tert-butyl perbenzoate, tert-butyl per-3,5,5-trimethylhexanoate and tert-amyl per neodecanoate.Appropriate persulphates may be selected from alkali metal persulphates such as sodium persulphate.Suitable azo initiators are advantageously water-soluble and chosen from the following list: 2,2'-azobis-(2-amidinopropane) dihydrochloride, isobutyramidine 2,2'-azobis N,N'-dimethylene)dihydrochloride, 2-(azo(1-cyano-1-methylethyl))-2-methylpropane nitrile, 2,2'-azobis[2-(2'-dimidazolin-2-yl)propane] dihydrochloride and 4,4'-azobis (4-cyanovaleric acid).Said polymerization initiators are used in usual amounts, for example in amounts of 0.001 to 2%, preferably 0.01 to 1% by weight, relative to the monomers to be polymerized.As an oxidizing component, the redox catalysts contain at least one of the above compounds and, as a reducing component, for example ascorbic acid, glucose, sorbose, hydrogen sulfite, sulfite, thiosulfate, hyposulfite, pyrosulfite or an alkali metal, metal salts, such as in the form of iron (II) ions or silver ions or sodium hydroxy methyl sulfoxylate. The reducing component of the redox catalyst preferably used is the Mohr's salt (NH4)2Fe(SO4)2, 6 H2O. Based on the number of monomers used in the polymerization, 5 x 10-6 to 1 mole% of the reducing component of the redox catalyst system and 5 x 10-5 to 2 moles% of the oxidizing component of the redox catalyst can, as an example, be used. Instead of the oxidizing component of the redox catalyst, one or more water-soluble azo initiators can also be used.For step a) of the method of the invention, the total concentration by weight of hydrophilic monounsaturated ethylenic monomer relative to the polymerization charge is between 10 and 60%, advantageously between 20 and 55% and even more advantageously between 25 and 50%.For step a) of this method, the at least one monounsaturated ethylenic monomer and the various polymerization additives are dissolved, for example, in vessels with stirring in the aqueous medium to be polymerized. This solution, also called the charge to be polymerized, is adjusted to an initiation temperature of between -20°C to 50°C. Advantageously, this initiation temperature is adjusted between -5°C and 30°C and even more advantageously between 0 and 20°C.The addition of surfactant during step a) of the method of the invention can be carried out during the dissolution of the monomers and polymerization additives. Thus, it is mixed into the polymerization charge by means of a stirring paddle for the purpose of finely dispersing it in the polymerization charge. It is also possible to cause the mixture to pass through a homogenizer of the rotor, rotor / stator type.Another means of adding the surfactant to the polymerization charge is to inject it into the polymerization charge stream going to the polymerization reactor, with a static mixer inserted between the point of injection of the surfactant and the reactor.Preferably, between 0.1 and 2% by weight, and even more preferably between 0.2 and 1% by weight of at least one surfactant, relative to the total weight of the hydrophilic monounsaturated ethylenic monomer(s), is used in step a) of the method.Preferably, the surfactant added in step a) of the method is anionic or non-ionic with an HLB of between 8 and 20, more preferably between 12 and 20.The hydrophilic-lipophilic balance (HLB) of a chemical compound is a measurement of its hydrophilic and / or lipophilic properties, determined by calculating the values for the different regions of the molecule, such as described by Griffin in 1949.In the present invention, the Griffin method has been adopted, based on calculating a value based on the chemical groups of the molecule. Griffin has attributed a dimensionless number between 0 and 20 to give information on the solubility of water and oil.The HLB value of a substance having a total molecular mass M and a hydrophilic part of a molecular mass Mh is given by:HLB = 20 (Mh / M).The surfactant of step a) is chosen from the group consisting of the following compounds: alkyl aryl sulfonates, alkyl benzene sulfonates, alkyl sulfates, N-ethoxy sulfonates, sodium dodecyl sulfonate, propoxylated alcohol sulfates, ethoxylated alkyl sulfates, ethoxylated alcohol sulfates, alpha-olefin sulfonates, alpha-olefin sulfates, alkyl polyalkoxy alkyl sulfonates, alkyl aryl polyalkoxy alkyl sulfonates, branched alkyl benzene sulfonates, sodium docusate, ethoxylated glycidyl sulfonates, propoxylated glycidyl sulfonates, alkyl ether sulfates, Guerbet alkyl sulfates, internal olefin sulfonates, sulfonated ethoxylated alcohols, sulfonated alkyl phenols, sodium petroleum sulfonate, propoxylated alkyl alcohol sulfates, carboxylated ethoxy alkyls, polyglycoside alkyls, ethoxylated alcohols, propoxylated alcohols, propoxylated alcohol sulfates, ethoxylated alkyl phenols, polyethoxylated phenol alkyls, poly(ethylene / propylene) glycol ethers, N -ethoxylated secondary alcohols, N -propoxylated secondary alcohols, tridecyl ethoxylated alcohols, triphenylmethane, and mixtures thereof,with, for these surfactants:- the alkyl groups being C4-C20,- ethoxylated chains comprising between 6 and 100 -CH2-CH2-O- groups,- propoxylated chains comprising between 6 and 100 -CH2 -CH 2-CH2-O- groups.The surfactant preferably used in the method is chosen from the group comprising the following compounds: alkyl sulfates, N-ethoxy sulfonates, propoxylated alcohol sulfates, ethoxylated alkyl sulfates, ethoxylated alcohol sulfates, ethoxylated glycidyl sulfonates, propoxylated glycidyl sulfonates, alkyl ether sulfates, sulfonated ethoxylated alcohols, propoxylated alkyl alcohol sulfates, carboxylated ethoxy alkyls, ethoxylated alcohols, propoxylated alcohols, propoxylated alcohol sulfates, poly(ethylene / propylene) glycol ethers, N-ethoxylated secondary alcohols, N-propoxylated secondary alcohols, tridecyl ethoxylated alcohols.Of these surfactants:- the alkyl groups are advantageously C4-C20,- ethoxylated chains advantageously comprise between 6 and 100 -CH2-CH2-O- groups,- propoxylated chains advantageously comprise between 6 and 100 -CH2 -CH 2-CH2-O- groups.In order to remove residual oxygen from the polymerization charge (with or without the addition of the surfactant), an inert gas is advantageously passed through it. Suitable inert gases for this purpose are, for example, nitrogen, carbon dioxide or rare gases such as neon, argon or helium.Enzymatic methods can also be used to remove residual oxygen, such as the use of glucose oxidase.The polymerization is carried out in the absence of oxygen, by introducing the initiators in the appropriate order, known to those skilled in the art, into the solution to be polymerized. The initiators are advantageously introduced either in soluble form in aqueous medium or in the form of a solution in an organic solvent.The polymerization may be carried out batchwise or continuously. In a batch procedure, a reactor is filled with a monomer solution and then with an initiator solution. As soon as the polymerization begins, the reaction mixture heats up depending on the starting conditions selected, such as the concentration of the monomers in the aqueous solution and the nature of the monomers. Due to the heat of polymerization released, the temperature of the reaction mixture rises, for example, from 30 to 180°C, preferably from 40°C to 130°C. The polymerization may be carried out at normal pressure, under reduced pressure or even at high pressure. Working at elevated pressure may be advantageous in cases where the maximum temperature expected in the polymerization is above the boiling point of the mixture of solvents used. On the other hand, it may be advantageous, in particular during the preparation of products of very high molecular weight, to lower the maximum temperature by means of cooling, for example with a cooling fluid. In most cases, the reactor is jacketed so that the reaction mixture may be cooled or heated as needed. Once the polymerization reaction is complete, the obtained polymer gel may be quickly cooled, for example by cooling the wall of the reactor.At the end of the reaction, the product resulting from the polymerization is a hydrated gel so viscous that it is self-supporting (thus a cube of gel of 2.5cm per side substantially maintains its shape when placed on a flat surface). The gel thus obtained is a viscoelastic gel.Note that when the reaction is carried out in a reactor, in order to facilitate the discharge of the gel at the end of the reaction, the reactor is advantageously in inverted conical tubular form (cone downwards) in order to discharge the gel downwards by application of an inert gas or air pressure at the surface of the gel or in the form of a rocker in order to discharge the mass of gel by rocking the reactor.Step b) of the method of the invention consists in granulating the water-soluble polymer gel obtained in step a). Granulation consists of cutting the gel into small pieces. Advantageously, the average size of these pieces of gel is less than 1 cm, more advantageously it is between 4 and 8mm. Those skilled in the art will know how to choose the means suitable for optimum granulation.Optionally, a surfactant, identical or not to that of step a), chosen from the list of surfactants described for step a) may be added during granulation step b) of the method of the invention. It can be added by spraying to the surface of the gel pieces. This optional surfactant is advantageously chosen from the same lists as those of the surfactant of step a).Advantageously, between 0.01% and 5.0% of surfactant in liquid form can be sprayed during step b) (% by weight relative to the total weight of the at least one hydrophilic monounsaturated ethylenic monomer used in step a)).Step c) of the method consists in drying the polymer. The choice of drying means is routine for those skilled in the art. Industrially, the drying is advantageously carried out by a fluidized bed or rotor dryer, using air heated to a temperature between 70°C and 200°C, the air temperature being a function of the nature of the product as well as the drying time applied. After drying, the water-soluble polymer is physically in powder form.For step d) of the method, the powder is crushed and sifted.The grinding step involves breaking up the large polymer particles into smaller sized particles. This may be done by shearing or by mechanical crushing of the particle between two hard surfaces. Different types of equipment known to those skilled in the art may be used for this purpose. For example, we may reference mills with rotors, where one crushes the particle assisted by the rotating part on a compression blade or the roller mill, where the particle is crushed between two rotating cylinders.The purpose of sifting is to then remove, depending on the specifications, the medium-sized particles that are too small or too large.Advantageously, the water-soluble polymer obtained according to the method of the invention is in the form of a powder having an average particle size of between 100 and 1500 mm, even more preferably between 150 and 800mm.The mean size of the polymer particles, also called the median size (D50) in number of the particles is defined as the largest dimension (the diameter in the case of spherical particles) of the particles for which half of the population (half of the particles) is below this value.In the case of spherical particles, the particle size refers to the average diameter preferably measured using a laser diffraction particle analyzer according to the conventional techniques of the person skilled in the art. An example of an apparatus for measuring the particle size is the Mastersizer from Malvern Instruments.Preferably, the water-soluble polymer is a copolymer of at least a hydrophilic non-ionic monomer and a hydrophilic anionic monomer.The copolymer advantageously comprises between 5 mol% and 50 mol% of hydrophilic anionic monomer(s) and between 50 mol% and 95 mol% of hydrophilic non-ionic monomer(s).The hydrophilic anionic monomer is selected from the group consisting of 2-acrylamido-2-methylpropane sulfonic acid or a salt thereof or acrylic acid or a salt thereof and mixtures thereof.The water-soluble polymer preferably contains less than 40 mol% of hydrophilic anionic monomer.The water-soluble polymer preferably comprises, as anionic monomer, 2-acrylamido-2-methylpropane sulfonic acid or a salt thereof.The water-soluble polymer preferably contains less than 40 mol% of 2-acrylamido-2-methylpropane sulfonic acid or a salt thereof.Preferably, the water-soluble polymer in powder form obtained by the method of the invention is a copolymer of acrylamide and sodium acrylate or a terpolymer of acrylamide, sodium acrylate and sodium 2-acrylamido-2-methylpropanesulfonate containing less than 40 mol% of sodium 2-acrylamido-2-methylpropane sulfonate, and more preferably less than 25 mol% of sodium 2-acrylamido-2-methylpropanesulfonate.Another aspect of the invention relates to a method for preparing a saline aqueous solution of water-soluble polymer, said water-soluble polymer being obtained according to the method described above, said method comprising the following successive steps:- Preparing a brine comprising more than 50,000ppm of salts, with a ratio R+ ≥ 0.03, R+= mass ratio: divalent salts / total salts;- Preparing a saline aqueous solution by dissolving 0.1 to 2% by weight of said water-soluble polymer in said brine at a temperature of between 15 and 60°C.Preferably, preparing the saline aqueous solution takes less than 120minutes.The brine may be prepared from monovalent salts, polyvalent salts, or combinations thereof. Examples of salts include, but are not limited to, water-soluble inorganic salts, other inorganic salts, and mixtures thereof, for example: sodium salts, lithium salts, potassium salts, magnesium salts, aluminium salts, ammonium salts, phosphate salts, sulfate salts, chloride salts, fluoride salts, citrate salts, acetate salts, tartrate salts, hydrogen phosphate salts, and mixtures thereof.The brine preferably contains at least one of the following: sodium chloride, calcium chloride, sodium bromide, calcium bromide, zinc bromide, sodium formate and potassium formate.The brine advantageously contains more than 30,000ppm of salts, preferably more than 50,000 ppm of salts, and even more preferably more than 75,000 of salts.The ratio R+= mass ratio: divalent salts / total salts is greater than or equal to 0.03, preferably greater than or equal to 0.06 and even more preferably greater than or equal to 0.1.A person skilled in the art knows how to choose the means for hydrating the polymer powder and stirring the resulting solution necessary to completely dissolve the water-soluble polymer. The PSU system: Polymer Slicing Unit (document WO 2008 / 071808) with a maturation tank could be used.The invention also relates to a method for enhanced hydrocarbon recovery comprising the following steps:a) Preparing an injection fluid comprising at least one water-soluble polymer according to the invention and water or brine;b) Injecting the injection fluid into a subterranean formation;c) Sweeping the subterranean formation using the injected fluid;d) Recovering an aqueous hydrocarbon mixture.Another aspect of the invention relates to a fracturing fluid comprising at least an aqueous phase, a propping agent and at least one water-soluble polymer according to the invention.Another aspect of the invention relates to a method for the hydraulic fracturing of an underground hydrocarbon (oil and / or gas) reservoir that comprises the following steps:- preparing a fracturing fluid as previously described,- injecting said fracturing fluid into the underground hydrocarbon reservoir and- recovering hydrocarbonFinally, a final aspect of the invention relates to the use of the saline aqueous polymer solution prepared according to the method described above for applications in oil and gas fields such as the enhanced recovery of hydrocarbons (in particular oil) and hydraulic fracturing operations.The invention and its advantages will be better understood in the light of the following figures and examples provided in order to illustrate the invention in a non-limiting manner.FiguresFigure 1 illustrates the appearance of filters for non-total (A, B and C) and optimal (D) dissolutions.Figure 2 shows the change in the amount of stock solution filtered over time.Figure 3 shows the change in the optimal dissolution time for a powder with a particle size between 300 and 500 µm in different brines.Figure 4 shows the evolution of the optimal dissolution time of a powder with a particle size of between 300 and 500 µm in different brines at 75g / L of salts of variable hardness.ExamplesExample 1: Synthesis of polymers in powder formCE1 (Comparative example): Gel synthesis of a copolymer of acrylamide / acrylic acid P1 in the absence of surfactantIn a 2 L beaker, an aqueous charge comprising 580g of a 50 % by weight acrylamide solution in water, 115 g of acrylic acid and 665 g of deionized water is prepared at room temperature. The charge is cooled to between -2 and 2°C, during the addition of soda in order to adjust the pH of the charge to between 7 and 8. When neutralization is complete, the charge is cooled to between -2 and 4°C and then placed in a Dewar. An azo catalyst, namely 2,2’-azobis-(2-amidinopropane)dihydrochloride (V50) is added at a level of 3.7mg.The homogenization of the charge is carried out using a hand mixer at a speed of 500 rpm for 20 seconds. This charge was then deoxygenated under nitrogen bubbling for at least 10 minutes. Once the deoxygenation of the reaction medium is completed, the polymerization is initiated by successive addition to the charge of 1.8 mg of tert-butyl hydroperoxide (TBHP) and 2.3mg of Mohr's salt.The polymerization reaction was completed after one and a half hours and the polymer obtained in the form of a gel reached a final temperature of 80°C. The gel is aged as it is, in Dewar, overnight. The gel is then ground and dried in an oven at 50°C for twenty hours. The dry polymer particles are then ground and then sifted to obtain a powder of predefined particle size. The polymer P1 obtained has a weight average molecular by weight of between 17 and 22 million Daltons.E1 (Example according to the invention): Gel synthesis of a copolymer of acrylamide / acrylic acid copolymer P2 in the presence of 0.5% by weight of surfactant (ethoxylated alcohol)The synthesis is carried out according to the same method as for polymer P1 with the exception that the initial aqueous charge comprises 580 g of acrylamide at 50% by weight in water, 115 g of acrylic acid, 665 g of deionized water and 2.2 g of branched, ethoxylated isotridecanol (approximately 12 EO units). The polymer P2 obtained has a weight average molecular by weight of between 17 and 22 million Daltons.CE2 (Comparative example): Gel synthesis of acopolymer of acrylamide / acrylic acid P3 in the presence of 0.005% by weight of surfactant (ethoxylated alcohol)The synthesis is carried out according to the same method as for polymer P1 with the exception that the initial aqueous charge comprises 580 g of acrylamide at 50% by weight in water, 115 g of acrylic acid, 665g of deionized water and 0.2 g ofbranched, ethoxylated isotridecanol (approximately 12 EO units). The polymer P3 obtained has a weight average molecular by weight of between 17 and 22 million Daltons.CE3 (Comparative example): Gel synthesis of acopolymer of acrylamide / acrylic acid P4 in the presence of 6% by weight of surfactant of ethoxylated alcohol typeThe synthesis is carried out according to the same method as for polymer P1 with the exception that the initial aqueous charge comprises 580 g of acrylamide at 50% by weight in water, 115 g of acrylic acid, 665 g of deionized water and 24 g ofbranched ethoxylated isotridecanol (approximately 12 EO units). It should be noted that the polymerization reaction is only completed after about twelve hours. The polymer P4 obtained has a weight average molecular by weight of between 17 and 22 million Daltons.E2 (Example according to the invention) : Gel synthesis of acopolymer of acrylamide / acrylic acid P5 in presence of 3% by weightof a surfactant (ethoxylated alcohol)The synthesis is carried out according to the procedure described for polymer P1 using an aqueous charge comprising 580 g of an acrylamide 50 wt% solution in water, 115 g of acrylic acid, 665 g of deionized water and 13.2 g of branched ethoxylated isotridecanol (approximately 12 EO units). The polymer P5 obtained has a weight average molecular by weight of between 17 and 22 million of daltons.E3 (Example according to the invention) : Gel synthesis of a copolymer ofacrylamide / acrylic acid P6 in presence of0.5% by weightof a surfactant (propoxylated alcoholsulfate)The synthesis is carried out according to the procedure described for polymer P1 using an aqueous charge comprising 580 g of an acrylamide 50% w / v solution in water, 115 g of acrylic acid, 665 g of deionized water and 2.2 g of a C13-propoxylated alcohol sulfate (approximately 7 PO units). The polymer P6 obtained has a weight average molecular by weight of between 17 and 22 million of daltons.CE4 (Comparative example) : Gel synthesis of a copolymer ofacrylamide / acrylic acid P7 in presence of 0.5% by weight of a castor oil ethoxylated type surfactantThe synthesis is carried out according to the procedure described for polymer P1 using an aqueous charge comprising 580 g of an acrylamide 50% wt% solution in water, 115 g of acrylic acid, 665 g of deionized water and 2.2 g of castor oil ethoxylated (approximately 40 EO units). The polymer P7 obtained is insoluble. Therefore its molecular weight cannot be determined by rheological measurements.CE5 (Comparative example) : Gel synthesis of aterpolymer ofacrylamide / acrylic acid / sodium 2-acrylamido-2-methylpropane sulfonate P8 in the absence of surfactantIn a 2 L beaker, an aqueous charge comprising 588 g of a 50 wt% acrylamide solution in water, 60 g of acrylic acid, 518 g of deionized water and 250 g of sodium 2-acrylamido-2-methylpropane sulfonate is prepared at room temperature. The charge is cooled to between -2 and 2°C, during the addition of soda in order to adjust the pH of the charge to between 6 and 7. When neutralization is complete, the charge is cooled to between -2 and 4°C and then placed in a Dewar. An azo catalyst, namely 2,2’-azobis-(2-amidinopropane)dihydrochloride (V50) is added at a level of 3.7mg.The homogenization of the charge is carried out using a hand mixer at a speed of 500 rpm for 20 seconds. This charge was then deoxygenated under nitrogen bubbling for at least 10 minutes. Once the deoxygenation of the reaction medium is completed, the polymerization is initiated by successive addition to the charge of 1.2 mg of tert-butyl hydroperoxide (TBHP) and 1.5 mg of Mohr's salt.The polymerization reaction was complete after 1h20 and the polymer obtained in the form of a gel reached a final temperature of 80°C. The gel is aged as it is, in a Dewar overnight. The gel is then ground and dried in an oven at 50°C for twenty hours. The dry polymer particles are then ground and then sifted to obtain a powder of predefined particle size. The polymer P8 obtained has a weight average molecular by weight of between 16 and 20 million of daltons.E4 (Example according to the invention) : Gel synthesis of a terpolymer ofacrylamide / acrylic acid / sodium 2-acrylamido-2-methylpropane sulfonateP9 in presence of0.5% by weightof a surfactant (ethoxylated alcohol)The synthesis is carried out according to the procedure described for polymer P8 using an aqueous charge comprising 588 g of a 50 %w / v acrylamide solution in water, 60 g of acrylic acid, 518 g of deionized water and 250 g of sodium 2-acrylamido-2-methylpropane sulfonate and 2.2 g 2.2 g of branched, ethoxylated isotridecanol (approximately 12 EO units). The polymer P9 obtained has a weight average molecular by weight of between 16 and 20 million of daltons.Example 2: Dissolution in a brine of polymers P1 to P9.The powder dissolution monitoring presented in Figure 2 corresponds to the preparation of a polymer stock solution at a mass concentration of 5 g / L in a brine comprising 100 g / L of sodium chloride (NaCl), 17 g / L of calcium chloride dihydrate (CaCl2.2H2O) and 17 g / L of magnesium chloride hexahydrate (MgCl2,6H2O). For this analysis, a narrow particle size cut, namely 300-500 µm, was chosen in order to limit the uncertainties on the particle size distribution of each polymer generally observed for larger particle size cuts (100-1500 µm). However, it should be noted that these tests were also carried out with such particle size distributions and their tendency is identical, with the exception that the dissolution times are more consistent.Dissolution is considered complete when the maximum viscosity is obtained and the stock solution is completely homogeneous. This means that the entire solution was passed through a 200 µm filter without leaving any polymer particles that were not totally hydrated.Figure 1 illustrates the appearance of the filters that are obtained once the maximum viscosity has been reached. The appearance of the left filter (case A) corresponds to a stock solution for which the hydration / solubilization time was not sufficient for the conditions of the study. By increasing the dissolution time, it is possible to access a totally homogeneous solution, which results in the absence of deposit on the filter (case D, right photo).The graph below (Figure 2) shows the proportion of stock solution filtered through a 200 µm filter for solutions prepared from polymers P1 (CE1), P2 (E1), P3 (CE2), P4 (CE3) and also for a solution prepared with polymer P1 in a brine to which an equimolar amount of surfactant (SF) (ethoxylated alcohol added so as to be present in 0.5 % by weight relative to the total weight of polymer in solution) in example E1 was previously added (comparative example CE6 = P1 + SF).The stock solutions prepared using the polymers P2 and P4 pass almost completely through the filter after 60 minutes, whereas a hydration time twice as long is required for the polymers P1, P3 and for the polymer mixture P1 + surfactant (SF) (CE6). The fact that the surfactant is incorporated into the polymer particles is essential to improve their dissolution. The appearance of the filters for which 99 to 99.999% of stock solutions have passed through the filter are recorded in Table 1 below using the identification (A, B, C and D) of Figure 1. Figure 1 illustrates the appearance of filters for non-total dissolution (deposits on the filter: A, B and C) and optimal dissolution (no deposits on the filter: D).    Hydratation time (min)PolymerAM / AA / ATBS6090120150180240P1 (CE1)70 / 30 / 0n.d.n.d.ABCDP2 (E1)70 / 30 / 0ACDDDDP3 (CE2)70 / 30 / 0n.d.n.d.ABCDP4 (CE3)70 / 30 / 0ABCDDDP5 (E2)70 / 30 / 0ACDDDDP6 (E3)70 / 30 / 0BCDDDDP7 (CE4)70 / 30 / 0n.d.n.d.n.d.n.d.n.d.n.d.P8 (CE5)75 / 15 / 10ABBCDDP9 (E4)75 / 15 / 10BCDDDDP1 + SF (CE6)70 / 30 / 0n.d.n.d.ABCD      Table 1: Summary table of the appearance of the filters after the passage of the stock solutions at different hydration times (n.d.: not determined)The optimal hydration time, that corresponds to the dissolution time, under the conditions tested for the polymer P1 is 240 minutes whereas only 120 minutes are required for the polymer obtained according to the method of the invention (polymer P2). By reference only to the solubilization, the polymer P4 is also effective but several parameters are restrictive with regard to the preparation and use of this polymer, namely: the significantly longer polymerization kinetics (see CE3) and the poor flow of the polymer in powder form.Dissolution times comparable to P2 are obtained with P5 and P6. This clearly illustrates that the dosage and nature of the surfactant are responsible for this significant improvement in dissolution speed. In the case where the surfactant used during the synthesis process is not suitable (P7), cross-linked polymers can be obtained and it becomes impossible to dissolve the polymer obtained.This gain in dissolution time is also observed with terpolymers composed of sodium 2-acrylamido-2-methylpropane sulfonate. This monomer is traditionally used to improve the thermal stability of polymers obtained from the latter, but also to make them easier to dissolve in salt solutions. This results in a shorter optimum dissolution time for the terpolymer (P8) than for the copolymers (P1). The addition of surfactant, as described in the invention, also significantly reduces the dissolution time for this type of terpolymer (P9).In the same way, the dissolution of powders in brines more or less loaded with salts, in the presence of mono- and / or divalent cations, was evaluated for the polymers P1 and P2 (Figure 3).A notable gain in terms of speed of dissolution of the powders is observed as soon as the salinity is greater than that of synthetic seawater (30 g / L). In fact, the stock solutions are completely homogeneous in half the time when the polymer P2 is used.It should also be noted that, above the solubility limit of the polymer with a very high salinity (≥ 250 g / L), it is possible to obtain a homogeneous stock solution only with polymer P2.Similarly, if the overall salinity of a brine is set at 75 g / L, for which the hardness (R+ by mass) is adjusted by adjusting the proportions of calcium chloride (NaCl) and calcium chloride dihydrate (CaCl2,2H2O), significantly lower optimum dissolution times are obtained for the polymer of the invention P2 compared to the polymer P1 as soon as R+ is greater than or equal to 0.03.   

Claims

1. Method for preparing a water-soluble polymer having an average molecular weight by weight greater than 1 million Daltons, the method comprising at least the following successive steps:a) Preparing a polymer, in the form of a gel, by free-radical polymerization in aqueous solution at an initiation temperature of between -20°C and +50°C, of at least one hydrophilic monounsaturated ethylenic monomer,the at least one hydrophilic monounsaturated ethylenic monomer having, relative to the weight of the polymerization charge, a concentration of between 10 and 60%, andin the presence of 0.01 to 5% by weight of at least one surfactant, relative to the total weight of the at least one hydrophilic monounsaturated ethylenic monomer;b) Granulating the resulting polymer gel;c) Drying the granulated polymer gel to obtain a polymer in powder form;d) Grinding and sifting the powder,the surfactant of step a) is selected from the group consisting of: alkyl aryl sulfonates, alkyl benzene sulfonates, alkyl sulfates, N-ethoxy sulfonates, sodium dodecyl sulfonate, propoxylated alcohol sulfates, ethoxylated alkyl sulfates, ethoxylated alcohol sulfates, alpha-olefin sulfonates, alpha-olefin sulfates, alkyl polyalkoxy alkyl sulfonates, alkyl aryl polyalkoxy alkyl sulfonates, branched alkyl benzene sulfonates, sodium docusate, ethoxylated glycidyl sulfonates, propoxylated glycidyl sulfonates, alkyl ether sulfates, Guerbet alkyl sulfates, internal olefin sulfonates, sulfonated ethoxylated alcohols, sulfonated alkyl phenols, sodium petroleum sulfonate, propoxylated alkyl alcohol sulfates, carboxylated ethoxy alkyls, polyglycoside alkyls, ethoxylated alcohols, propoxylated alcohols, propoxylated alcohol sulfates, ethoxylated alkyl phenols, polyethoxylated phenol alkyls, poly(ethylene / propylene) glycol ethers, N -ethoxylated secondary alcohols, N -propoxylated secondary alcohols, tridecyl ethoxylated alcohols, triphenylmethane, and mixtures thereof,with, for these surfactants:- the alkyl groups being in C4-C20< / sub>,- ethoxylated chains comprising between 6 and 100 -CH2-CH2-O- groups,- propoxylated chains comprising between 6 and 100 -CH2 -CH2-CH2-O- groups.< / p>2. The method according to claim 1, characterized in that the polymerization of step a) is carried out in the presence of 0.2 to 1% by weight of at least one surfactant, relative to the total weight of the at least one hydrophilic monounsaturated ethylenic monomer.< / p>3. The method according to claim 1 or 2, characterized in that the surfactant is chosen from the group consisting of the following compounds: alkyl sulfates, N-ethoxy sulfonates, propoxylated alcohol sulfates, ethoxylated alkyl sulfates, ethoxylated alcohol sulfates, ethoxylated glycidyl sulfonates, propoxylated glycidyl sulfonates, alkyl ether sulfates, sulfonated ethoxylated alcohols, propoxylated alkyl alcohol sulfates, carboxylated ethoxy alkyls, ethoxylated alcohols, propoxylated alcohols, propoxylated alcohol sulfates, poly(ethylene / propylene) glycol ethers, N-ethoxylated secondary alcohols, N-propoxylated secondary alcohols, tridecyl ethoxylated alcoholswith, for these surfactants:- the alkyl groups being in C4-C20< / sub>,- ethoxylated chains comprising between 6 and 100 -CH2-CH2-O- groups,- propoxylated chains comprising between 6 and 100 -CH2 -CH 2-CH2-O- groups.< / p>4. The method according to any one of the preceding claims, characterized in that the at least one monounsaturated ethylenic monomer in step a) is chosen from the group consisting of non-ionic monomers, anionic monomers, cationic monomers and mixtures thereof,- the hydrophilic non-ionic monomer or monomers being chosen from the group consisting of water-soluble vinyl monomers, such as acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N,N-dialkylacrylamides (for example N,N-dimethylacrylamide or N,N-diethylacrylamide), N,N-dialkylmethacrylamides, alkoxylated esters of acrylic acid, alkoxylated esters of methacrylic acid, N-vinylpyrrolidone, N-methylol(meth)acrylamide, N-vinylcaprolactam, N-vinylformamide (NVF), N-vinylacetamide, N-vinylimidazole, N-vinyl succinimide, acryloyl morpholine (ACMO), glycidyl methacrylate, glyceryl methacrylate, diacetone acrylamide, methacrylic anyhydride, acrylonitrile, maleic anydride, itaconic anhydride, itaconamide, vinylpyridine, hydroxyalkyl(meth)acrylate, thioalkyl(meth)acrylate, isoprenol and its alkoxylated derivatives, hydroxyethyl(meth)acrylates and their alkoxylated derivatives, hydroxypropyl acrylate and its alkoxylated derivatives, alkyl acetate and mixtures of their mixtures, the alkyl groups being C1-C3 alkylated hydrocarbon chains;- the anionic hydrophilic monomers being chosen from the monomers of acrylic acid, methacrylic acid, dimethylacrylic acid, itaconic acid, C1-C3 itaconic acid semi-ester, acryloyl chloride, crotonic acid, maleic acid, fumaric acid, 3-acrylamido-3-methylbutanoic acid, vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, methallylsulfonic acid, 2-methylidenepropane-1,3-disulfonic acid, 2-sulfoethylmethacrylate, sulfopropylmethacrylate, sulfopropylacrylate, allylphosphonic acid, ethylene glycol methacrylate phosphate, styrene sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid (AMPS), 2-acrylamido-2-methylpropane disulfonic acid, 3-allyloxy-2-hydroxypropanesulfonic acid, diethylallylphosphonate, carboxyethyl acrylate; the water-soluble salts of all these monomers, such as their alkali metal salts, alkaline-earth metal salts or ammonium salts; and mixtures thereof; and mixtures thereof;- the cationic hydrophilic monomers being chosen from diallyldialkyl ammonium salts such as diallyl dimethyl ammonium chloride (DADMAC); acidified or quaternized salts of dialkylaminoalkyl(meth)acrylamides, such as, for example, methacrylamidopropyl trimethyl ammonium chloride (MAPTAC), acrylamidopropyl trimethyl ammonium chloride (APTAC), acidified or quaternized salts of dialkylaminoalkyl acrylate such as quaternized or salified dimethylaminoethyl acrylate (ADAME), acidified or quaternized salts of dialkylaminoalkyl methacrylate such as quaternized or salified dimethylaminoethyl methacrylate (MADAME), and mixtures thereof.< / p>5. The method according to any one of the preceding claims, characterized in that the water-soluble polymer is a copolymer of at least a hydrophilic non-ionic monomer and a hydrophilic anionic monomer.6< / b>. The method according to claim 5, characterized in that the copolymer comprises between 5 mol% and 50 mol% of hydrophilic anionic monomer(s) and between 50 mol% and 95 mol% of hydrophilic non-ionic monomer(s).7< / b>. The method according to any one of claims 4 to 6, characterized in that the hydrophilic anionic monomer is selected from the group consisting of 2-acrylamido-2-methylpropane sulfonic acid or a salt thereof or acrylic acid or a salt thereof and mixtures thereof.8< / b>. The method according to any one of the preceding claims, characterized in that the polymer contains less than 40 mol% of hydrophilic anionic monomer.9< / b>. The water-soluble polymer in powder form obtained according to the method of any one of the preceding claims, the powder having an average particle size of between 100 and 1500 mm.< / p>10. The water-soluble polymer according to claim 9, characterized in that the powder has an average particle size of between 150 and 800 mm.

11. A method for preparing a saline aqueous solution of water-soluble polymer, said water-soluble polymer being obtained according to the method of claims 1 to 8 or being the water-soluble polymer according to one of claims 9 to 10, said method comprising the following successive steps:- Preparing a brine comprising more than 50,000 ppm of salts, with a ratio R+ ≥ 0.03, R+= mass ratio: divalent salts / total salts;- Preparing a saline aqueous solution by dissolving 0.1 to 2% by weight of said water-soluble polymer in said brine at a temperature of between 15 and 60°C.12< / b>. Method for enhanced hydrocarbon recovery comprising the following steps:a) Preparing an injection fluid comprising at least one water-soluble polymer obtained according to any of claims 1 to 8 or a water-soluble polymer according to any of claims 9 to 10, with water or brine;b) Injecting the injection fluid into a subterranean formation;c) Sweeping the subterranean formation using the injected fluid;d) Recovering an aqueous hydrocarbon mixture.

13. Fracturing fluid comprising at least an aqueous phase, a propping agent and at least one water-soluble polymer obtained according to any of claims 1 to 8 or a water-soluble polymer according to any of claims 9 to 10.14< / b>. Method for the hydraulic fracturing of an underground hydrocarbon (oil and / or gas) reservoir that comprises the following steps:- preparing a fracturing fluid according to claim 13,injecting said fracturing fluid into the underground hydrocarbon reservoir and- recovering hydrocarbon

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