Process for the preparation of a water-soluble polymer in powder form

Abstract

The present invention relates to a process for preparing a water-soluble polymer with a weight-average molecular weight greater than 1 million daltons. This process comprises the following successive steps: a) Preparing a polymer, in the form of a gel, by polymerization, by radical route in aqueous solution at an initiation temperature of between -20°C and +50°C, of at least one hydrophilic monounsaturated ethylenic monomer, the concentration by total weight of monomer(s) relative to the polymerization charge being 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 polymer gel thus obtained; c) Drying the polymer gel to obtain a polymer in powder form; d) Grinding and sieving the powder.

Description

Title of the invention: Process for preparing a water-soluble polymer in powder form Field of invention

[0001] The present invention relates to a process for preparing a water-soluble polymer in powder form whose dissolution 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 art

[0002] Water-soluble polymers are commonly used in enhanced oil recovery (EOR) or hydraulic fracturing operations. These operations often involve very salty waters which contain divalent ions in particular.

[0003] The dissolution of water-soluble polymers in powder form in these very saline waters is complex, which results in very long dissolution times and / or the presence of polymers that are not completely hydrated. It involves in particular the use of specific equipment known as polymer particle hydration equipment, such as the PSU: Polymer Slicing Unit (document WO 2008 / 071808). These systems are expensive, energy-intensive and have a significant footprint, especially since the solutions 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 complete dissolution in very saline waters. Generally speaking, the hydration of water-soluble polymers in powder form corresponds to the aqueous solution of these polymers.

[0004] The existing alternative to reduce the residence time in the maturation tanks is to use, before the hydration system, very small polymer particles. However, handling fine particles is restrictive for operators. Indeed, fine polymer particles tend to agglomerate. This agglomeration phenomenon proves problematic for good dissolution of the polymer and gives rise to the formation of insoluble polymer clusters.

[0005] The addition of water-soluble polymers to an aqueous solution containing at least one surfactant may be a solution to enable better dissolution of water-soluble polymers in powder form in an aqueous solution. However, the effectiveness of this addition of surfactant on the dissolution of the polymer decreases. when the salinity of the water increases.

[0006] Hydraulic fracturing or RAP 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 less sensitive to degradation in very saline aqueous solutions. This monomer is however less available and much more expensive than sodium acrylate.

[0007] For hydraulic fracturing or RAP operations, when the salinity of the water is high and in particular when it contains 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. Statement of the invention

[0008] The invention relates to a process for preparing a water-soluble polymer which has improved solubility (shorter dissolution time, absence of gel points) in high salinity brines and which contain in particular divalent ions (in particular divalent cations). In addition, the polymer resulting from said process makes it possible to reduce the footprint of the dissolution equipment (size and number of maturation tanks) and can contain a lower quantity of sodium 2-acrylamido-2-methylpropane sulfonate monomers in its monomeric composition. The invention relates to the preparation of saline solutions from the polymers resulting from this process and their use for applications in oil and gas fields.

[0009] More specifically, the first aspect of the invention relates to a process for preparing a water-soluble polymer with a weight-average molecular weight greater than 1 million daltons, the process comprising at least the following successive steps: a) Prepare a polymer, in the form of a gel, by polymerization, by radical route in aqueous solution at an initiation temperature 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) Granulate the polymer gel thus obtained; c) Dry the granulated polymer gel to obtain a polymer in powder form; d) Grind and sieve the powder.

[0010] According to the invention, “X and / or Y” means “X”, or “Y”, or “X and Y”.

[0011] Also part of the invention are all possible combinations between the various embodiments disclosed, whether preferred or exemplary. Furthermore, when ranges of values are indicated, the limits are part of these ranges. The disclosure also includes all combinations between the limits of these ranges of values. For example, the ranges of values "1-20, preferably 5-15", imply the disclosure of the ranges "1-5", "1-15", "5-20" and "15-20" and the values 1, 5, 15 and 20.

[0012] By "polymer" is meant a homopolymer prepared from a monomer or a copolymer prepared from at least two different monomers; it may therefore be a copolymer of at least two monomers chosen from anionic hydrophilic monomers, cationic hydrophilic monomers, non-ionic hydrophilic monomers, zwitterionic hydrophilic monomers, hydrophobic monomers and mixtures thereof.

[0013] 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. The molecular weight is understood to be the weight average molecular weight.

[0014] The molecular weight is advantageously determined by the intrinsic viscosity of the polymer. The intrinsic viscosity can be measured by methods known to those skilled in the art and can be calculated from the reduced viscosity values for different polymer concentrations by a graphical method consisting of plotting the reduced viscosity values (y-axis) on the concentration (x-axis) and extrapolating the curve to zero concentration. The intrinsic viscosity value is plotted on the y-axis or by using the least squares method. The molecular weight can then be determined by the Mark-Houwink equation: [q] = KM" [q] represents the intrinsic viscosity of the polymer determined by the solution viscosity measurement method. K represents an empirical constant. M represents the molecular weight of the polymer, a represents the Mark-Houwink coefficient. K and a depend on the particular polymer-solvent system.

[0015] By "water-soluble polymer" is meant a polymer which gives a solution aqueous without insoluble particles when dissolved under stirring at 25°C and with a concentration of 10 gL 1 in deionized water.

[0016] By "hydrophobic monomer" is meant a monomer which has an octanol / water partition coefficient, Kow, greater than 1, in which the Kow partition coefficient is determined at 25°C in an octanol / water mixture having a volume ratio of 1 / 1, at a pH between 6 and 8.

[0017] By “hydrophilic monomer” is meant a monomer which has an octanol / water partition coefficient, Kow, less than or equal to 1, in which the Kow partition coefficient is determined at 25°C in an octanol / water mixture having a volume ratio of 1 / 1, at a pH between 6 and 8.

[0018] The polymer may be non-ionic, cationic, anionic or amphoteric.

[0019] By “non-ionic polymer” is meant a polymer which comprises only non-ionic hydrophilic monomers and optionally zwitterionic and / or hydrophobic hydrophilic monomers.

[0020] By “cationic polymer” is meant a polymer which comprises only cationic hydrophilic monomers and optionally non-ionic hydrophilic monomers and / or zwitterionic and / or hydrophobic hydrophilic monomers.

[0021] By "anionic polymer" is meant a polymer which comprises only anionic hydrophilic monomers and optionally non-ionic hydrophilic monomers and / or zwitterionic and / or hydrophobic hydrophilic monomers.

[0022] By "amphoteric polymer" is meant a polymer which comprises cationic hydrophilic monomers and anionic hydrophilic monomers and optionally non-ionic hydrophilic monomers and / or zwitterionic and / or hydrophobic hydrophilic monomers.

[0023] By "semi-synthetic polymer" is meant a natural polymer, such as for example compounds of the polysaccharide family, having undergone chemical grafting reactions of different synthetic substituents. Those skilled in the art know this type of reaction which remains classic chemical reactions applied to natural polymers.

[0024] Advantageously, the polymerization feedstock does not comprise a polymer before the start of the polymerization of step a). In other words, advantageously, step a) is carried out by polymerization of at least one hydrophilic monounsaturated ethylenic monomer in the absence of a host polymer.

[0025] Preferably, the at least one monounsaturated ethylenic monomer polymerized in step a) of the process of the invention is chosen from the group consisting of non-ionic monomers, anionic monomers, cationic monomers and their mixtures, - the non-ionic hydrophilic monomer(s) being chosen from the group including water-soluble vinyl monomers, such as acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N,N-dialkyl acrylamides (e.g., 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-vinyl caprolactam, N-vinylformamide (NVF), N-vinyl acetamide, N-vinyl imidazole, N-vinyl succinimide, acryloyl morpholine (ACMO), glycidyl methacrylate, glyceryl methacrylate, diacetone acrylamide, methacrylic anhydride, acrylonitrile, maleic anhydride, itaconic anhydride, itaconamide, vinylpyridine, hydroxyalkyl (meth)acrylate, thioalkyl (meth)acrylate, isoprenol and its alkoxylated derivatives, hydroxyethyl (meth)acrylates and their alkoxylated derivatives, hydroxypropylacrylate and its alkoxylated derivatives,vinyl acetate, and mixtures thereof, the alkyl groups being C1-C3 hydrocarbon chains; - the anionic hydrophilic monomers being chosen from the monomers acrylic acid, methacrylic acid, dimethylacrylic acid, itaconic acid, the hemi-esters of itaconic acid in C1-C3> 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-methylpropane sulfonic acid (ATBS), 2-acrylamido-2-methylpropane disulfonic acid, 3-allyloxy-2-hydroxypropane sulfonic acid, diethylallylphosphonate, carboxyethyl acrylate;the water-soluble salts of all of these monomers, such as their alkali metal, alkaline earth metal or ammonium salts; 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 dialkyl-aminoalkyl(meth)acrylamides, such as for example methacrylamido-propyl trimethyl ammonium chloride (MAPTAC), acrylamido-propyl trimethyl ammonium chloride (APTAC), acidified or quaternized salts of dialkyl-aminoalkyl acrylate such as quaternized or salified dimethylaminoethyl acrylate (ADAME), acidified or quaternized salts of dialkyl aminoalkyl methacrylate such as quaternized or salified dimethylaminoethyl methacrylate (MADAME), and mixtures thereof.

[0026] Optionally, zwitterionic hydrophilic monomers may be polymerized during step a) of the process, these monomers being chosen from the derivatives dimethylaminoethyl acrylate, such as 2 - ((2-9 (acryloyloxy) ethyl) dimethyl-ammonio) ethane-l-sulfonate, 3 - ((2- (acryloyloxy) ethyl) dimethylammonio) propane-1-sulfonate, 4 - ((2- (acryloyloxy) ethyl) dimethylammonio) butane-1-sulfonate, [2- (acryloyloxy) ethyl] (dimethylammonio) acetate, dimethylaminoethyl methacrylate derivatives such as 2 - ((2- (methacryloyloxy) ethyl) dimethylammonio) ethane-l-sulfonate, 3 - ((2- (methacryloyloxy) ethyl) dimethylammonio) propane-1-sulfonate, 4 - ((2 - (methacryloyloxy) ethyl) dimethylammonio) butane-1-sulfonate, [2-(methacryloyloxy)ethyl](dimethylammonio)acetate, dimethylamino propylacrylamide derivatives 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, propyl [3-(methacryloyloxy)] (dimethylammonio) acetate; and mixtures thereof.

[0027] 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%.

[0028] Optionally, hydrophobic monomers may be polymerized during step a) of the process, these hydrophobic monomers being chosen from (meth)acrylic acid esters having a (i) C4-C30 alkyl, or (ii) arylalkyl (C4-C30 alkyl, C4-C30 aryl), or (iii) propoxylated, or (iv) ethoxylated, or (v) ethoxylated and propoxylated chain; alkyl aryl sulfonates (C4-C30 alkyl, C4-C30 aryl); mono- or di-substituted amides of (meth)acrylamide having a (i) C4-C30 alkyl, or (ii) arylalkyl (C4-C30 alkyl, C4-C30 aryl), or (iii) propoxylated, or (iv) ethoxylated, or (v) ethoxylated and propoxylated chain; anionic or cationic monomeric derivatives of (meth)acrylamide or (meth)acrylic acid bearing a hydrophobic chain; and mixtures thereof. The hydrophobic monomers may comprise halogen atoms, for example chlorine.

[0029] Among these hydrophobic monomers: - the alkyl groups are advantageously C4-C2o, more preferably C4-C8. The C6-C2o alkyls are advantageously linear alkyls while the C4-C5 alkyls are advantageously branched, - the arylalkyl groups are C7-C25, more preferably C7-C15, - the ethoxylated chains comprise between 6 and 100 -CH2-CH2-O- groups, more preferably between 10 and 40, - the propoxylated chains advantageously comprise between 1 and 50 -CH2 - groups CH2-CH2-O-, more preferably between 1 and 20.

[0030] Preferred 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, vinyl pyridine, 2-ethylhexyl acrylate, C4-C22 itaconic acid hemi-esters, acidified salts or quaternized salts of C4-C22 dialkylaminoalkyl (meth)acrylate, acidified or quaternized salts of C4-C22 dialkylaminoalkyl(meth)acrylamides, acrylamido undecanoic acid, and mixtures thereof.

[0031] The water-soluble polymer generally comprises less than 1 mol% of hydrophobic monomers. Preferably, this polymer is free of hydrophobic monomers.

[0032] When the polymer comprises one or more hydrophobic monomers, they are present in such an amount that the polymer remains soluble in water.

[0033] In a particular embodiment, the polymer may comprise at least one LCST group, preferably 0 to 1 mol%.

[0034] According to the general knowledge of a person skilled in the art, an LCST group corresponds to a group whose solubility in water for a given concentration is modified beyond a certain temperature and as a function of the salinity. This is a group having a transition temperature upon heating defining its lack of affinity with the solvent medium. The lack of affinity with the solvent results in opacification or a loss of transparency which may be due to precipitation, aggregation, gelation or viscosification of the medium. The minimum transition temperature is called “LCST” (lower critical solubility temperature). For each concentration of LCST group, a transition temperature upon heating is observed. It is higher than the LCST which is the minimum point of the curve.Below this temperature the polymer is soluble in water, above this temperature the polymer loses its solubility in water.

[0035] In a particular embodiment, the polymer may comprise at least one UCST group, preferably 0 to 1 mol%.

[0036] According to the general knowledge of a person skilled in the art, a UCST group corresponds to a group whose solubility in water for a given concentration is modified below a certain temperature and as a function of the salinity. This is a group having a cooling transition temperature deposition defining its lack of affinity with the solvent medium. The lack of affinity with the solvent results in opacification or loss of transparency which can be due to precipitation, aggregation, gelation or viscosification of the medium. The maximum transition temperature is called "UCST" (Upper Critical Solution Temperature). For each group concentration at UCST, a cooling transition temperature is observed. It is lower than the UCST which is the maximum point of the curve. Above this temperature, the polymer is soluble in water, below this temperature, the polymer loses its solubility in water.

[0037] The quantities of the different monomers will be adjusted by those skilled in the art so as not to exceed 100 mol% during the preparation of the 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 LCST groups, monomers with UCST groups, and mixtures thereof.

[0038] 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 mixtures thereof.

[0039] The polymer may be partially or completely post-hydrolyzed.

[0040] Post hydrolysis is the hydrolysis reaction of the polymer after its formation by polymerization of monomer(s). This step, which can be carried out between steps b) and c) of the process of the invention, consists of the reaction of hydrolyzable functional groups of advantageously non-ionic monomers, more advantageously the amide or ester functions, with a hydrolysis agent. This hydrolysis agent can, for example, be an enzyme, an ion exchange resin, or a Brpnsted acid (for example a hydrohalic acid) or a Brpnsted base (for example an alkali hydroxide or an alkaline earth hydroxide). Preferably, the hydrolysis agent is a Brpnsted base. During this post-hydrolysis step of 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.

[0041] According to the invention, the polymer may have a linear, branched, star-shaped or comb-shaped structure. This structure may 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, or the polymerization technique. polymerization such as reversible addition-fragmentation chain transfer polymerization (RAFT), nitroxide-mediated polymerization (NMP) or atom transfer radical polymerization (ATRP), incorporation of structural monomers, or concentration.

[0042] The polymer may further be structured by a branching agent. A structured polymer means a non-linear polymer which has side chains.

[0043] The branching agent is advantageously chosen from: - structural agents advantageously chosen from the group comprising monomers with polyethylene unsaturation (having at least two unsaturated functions), such as for example vinyl functions, in particular allylic or acrylic, and we can cite for example methylene bis acrylamide (MBA), triallyamine, tetraallylammonium chloride or 1,2 dihydroxyethylene bis-(N-acrylamide), - monomers having at least two epoxy functions, - monomers having at least one unsaturated function and one epoxy function, - macroinitiators such as polyperoxides, polyazos and polytransfer agents such as polymercaptant polymers and polyols, - functionalized polysaccharides, - water-soluble metal complexes composed of: * of a metal with a valence greater than 3 such as, by way of example and without limitation, aluminum, boron, zirconium or titanium, and * of a ligand carrying a hydroxyl function.

[0044] The amount 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, more preferably less than 10 ppm by weight.

[0045] When the polymer is water-soluble and comprises at least one branching agent, it remains water-soluble. Those skilled in the art will know how to adjust the amount of branching agent, and possibly the amount of transfer agent, in order to achieve this result.

[0046] In a particular embodiment, the polymer does not comprise a branching agent.

[0047] The polymerization for step a) of the process of the invention is carried out by free radical route. It includes free radical polymerization using UV, azo, redox or thermal initiators as well as controlled radical polymerization (CRP) techniques or more particularly RAFT (Reversible Addition Frag- Nutrition Chain Transfer).

[0048] The polymerization charge corresponds to the solution of water-soluble monounsaturated ethylenic monomers optionally supplemented with usual polymerization regulators before the start of the polymerization. The usual polymerization regulators are, for example, sulfur compounds such as thioglycolic acid, mercapto 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, can be used. We can notably cite these compounds from 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-(l-methoxy carbonyl ethyl) xanthate is widely used for its compatibility with acrylic monomers.

[0049] The polymerization initiators used may be any compounds which dissociate into radicals under the 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.

[0050] 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 pemeohexanoate, tert-butyl perbuto-butylate, -ethylhexanoate, tert-butyl perisononanoate, tert-butyl permaleate, tert-butyl perbenzoate, tert-butyl per-3,5,5-trimethylhexanoate and tert-amyl perneodecanoate.

[0051] Suitable persulfates may be chosen from alkali metal persulfates such as sodium persulfate.

[0052] Suitable azo initiators are advantageously water-soluble and selected from the following list: 2,2'-azobis-(2-amidinopropane) dihydrochloride, 2,2'-azobis(N,N'-dimethylene)isobutyramidine dihydrochloride, 2-(azo(l-cyano-l-methylethyl))-2-methylpropane nitrile, 2,2'-azobis[2-(2'-dimidazolin-2-yl)propane] dihydrochloride and 4,4'-azobis(4-cyanovaleric acid).

[0053] Said polymerization initiators are used in usual amounts, for example in amounts of 0.001 to 2%, preferably of 0.01 to 1% by weight, relative to to the monomers to be polymerized.

[0054] As an oxidizing component, the redox catalysts contain at least one of the above-mentioned 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 hydroxymethyl sulfoxylate. The reducing component of the redox catalyst preferably used is Mohr's salt (NH^Fe^O^, 6 H2O. For example, based on the amount of monomers used in the polymerization, from 5 x 106 to 1 mole % of the reducing component of the redox catalyst system and from 5 x 105 to 2 mole % of the oxidizing component of the redox catalyst may be used. Instead of the oxidizing component of the redox catalyst, one or more water-soluble azo initiators may also be used.

[0055] For step a) of the process of the invention, the concentration by total 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%.

[0056] For step a) of this process, the at least one hydrophilic monounsaturated ethylenic monomer and the various polymerization additives are dissolved, for example, in stirred containers in the aqueous medium to be polymerized. This solution, also called charge to be polymerized, is adjusted to an initiation temperature of between -20°C and 50°C. Advantageously, this initiation temperature is adjusted between -5°C and 30°C and even more advantageously between 0 and 20°C.

[0057] The addition of surfactant during step a) of the process of the invention can be carried out during the dissolution of the polymerization monomers and additives. Thus, it is mixed into the polymerization charge by means of a stirring blade in order to disperse the latter in the polymerization charge. It is also possible to pass the mixture through a rotor, rotor / stator type homogenizer.

[0058] Another way of adding the surfactant to the polymerization feed is to inject it into the polymerization feed stream going to the polymerization reactor, a static mixer being inserted between the surfactant injection point and the reactor.

[0059] 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 process.

[0060] Preferably, the surfactant added in step a) of the process is anionic or non-ionic with an HLB of between 8 and 20, more preferably between 12 and 20.

[0061] The hydrophilic-lipophilic balance (HLB) of a chemical compound is a measure of its hydrophilic and / or lipophilic properties, determined by calculating the values for the different regions of the molecule, as described by Griffin in 1949.

[0062] In the present invention, we have adopted Griffin's method based on calculating a value based on the chemical groups of the molecule. Griffin assigned a dimensionless number between 0 and 20 to give information on the solubility of water and oil.

[0063] 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).

[0064] Advantageously, 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, polyalkoxy alkyl alkyl sulfonates, polyalkoxy alkyl alkyl aryl sulfonates, branched alkyl benzene sulfonates, sodium docusate, ethoxylated glycidyl sulfonates, propoxylated glycidyl sulfonates, alkyl ether sulfates, alkyl sulfates Guerbet, internal olefin sulfonates, sulfonated ethoxylated alcohols, sulfonated alkyl phenols, sodium petroleum sulfonate, propoxylated alkyl alcohol sulfates, carboxylated ethoxylated alkyls, polyglycoside alkyls, ethoxylated alcohols,propoxylated alcohols, propoxylated alcohol sulfates, ethoxylated alkylphenols, polyethoxylated alkylphenols, poly(ethylene / propylene) glycol ethers, iV-ethoxylated secondary alcohols, iV-propoxylated secondary alcohols, ethoxylated tridecyl alcohols, triphenylmethane, and mixtures thereof, with these surfactants: - the alkyl groups being C4-C2o, - ethoxylated chains comprising between 6 and 100 -CH2-CH2-O- groups, - propoxylated chains comprising between 6 and 100 -CH2-CH2-CH2-O- groups.

[0065] The surfactant preferably used in the process 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 ethoxylated alkyls, ethoxylated alcohols, propoxylated alcohols, propoxylated alcohol sulfates, poly(ethylene / propylene) glycol ethers, iV-ethoxylated secondary alcohols, iV-propoxylated secondary alcohols, ethoxylated tridecyl alcohols.

[0066] Among these surfactants: - the alkyl groups are advantageously C4-C2o, - the ethoxylated chains advantageously comprise between 6 and 100 -CH2 -CH2-0- groups, more preferably 10 to 40, - the propoxylated chains advantageously comprise between 6 and 100 -CH2 -CH2-CH2-O- groups, more preferably 10 to 40.

[0067] In order to remove residual oxygen from the polymerization charge (whether or not with added 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.

[0068] Enzymatic methods can also be used to remove residual oxygen, such as the use of glucose oxidase.

[0069] The polymerization is advantageously 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 solid form in an aqueous medium, or in the form of a solution in an organic solvent.

[0070] The polymerization can be carried out batchwise or continuously. In a batch procedure, a reactor is filled with a monomer solution and then with a solution of the initiator. As soon as the polymerization begins, the reaction mixture heats up depending on the starting conditions chosen, such as the concentration of the monomers in the aqueous solution and the nature of the monomers. Due to the released heat of polymerization, the temperature of the reaction mixture rises, for example, from 30 to 180 °C, preferably from 40 °C to 130 °C. The polymerization can be carried out at normal pressure, reduced pressure or even elevated pressure. Working at elevated pressure can be advantageous in cases where the maximum temperature expected in the polymerization is higher than the boiling point of the solvent mixture used.On the other hand, it can be advantageous, especially when preparing very high molecular weight products, to lower the maximum temperature by means of cooling, for example with a cooling fluid. The reactor is in most cases jacketed so that the reaction mixture can be cooled or heated as required. Once the polymerization reaction is complete, the resulting polymer gel can be quickly cooled, for example by cooling the reactor wall.

[0071] 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 with a side of 2.5 cm substantially maintains its shape when placed on a flat surface). The gel thus obtained is a viscoelastic gel.

[0072] 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 the form of an inverted conical tubular shape (cone downwards) in order to discharge the gel downwards by applying inert gas or air pressure to the surface of the gel or in the form of a rocker in order to discharge the mass of gel by tilting the reactor.

[0073] Step b) of the process of the invention consists of 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 8 mm. Those skilled in the art will know how to choose the appropriate means for optimal granulation.

[0074] Optionally, a surfactant, identical or not to that of step a), chosen from the list of surfactants described for step a) can be added during the granulation step b) of the process of the invention. It can be added by spraying onto the surface of the gel pieces. This optional surfactant is advantageously chosen from the same lists as those of the surfactant of step a).

[0075] 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)).

[0076] Step c) of the process consists of drying the granulated polymer. The drying means is a routine choice 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 advantageously 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. At the end of the drying, the water-soluble polymer is physically in powder form.

[0077] For step d) of the process, the powder is ground and sieved.

[0078] The grinding step consists of breaking the large polymer particles into smaller particles. This can be done by shearing or by mechanically crushing the particle between two hard surfaces. Different types of equipment known to those skilled in the art can be used for this purpose. Examples include rotor mills, where the particle is crushed using the rotating part on a compression blade, or the roller mill, where the particle is crushed between two rotating rollers.

[0079] The purpose of sieving is then to eliminate, depending on the specifications, the medium-sized particles that are too small or too large.

[0080] Advantageously, the water-soluble polymer obtained according to the process of the invention is in the form of a powder having an average particle size of between 100 and 1500 pm, even more preferably between 150 and 800 pm.

[0081] The average size of polymer particles, also called median size (D50) in number of particles is defined as the largest dimension (the diameter in the case of spherical particles) particles for which half of the population (half of the particles) is below this value.

[0082] In the case of spherical particles, particle size refers to the average diameter preferably measured using a laser diffraction particle analyzer according to conventional techniques of the person skilled in the art. An example of an apparatus for measuring particle size is the Mastersizer from Malvem Instruments.

[0083] Preferably, the water-soluble polymer in powder form obtained by the process of the invention is a copolymer of acrylamide and sodium acrylate or a terpolymer of acrylamide, sodium acrylate and sodium 2-acrylamido-2-methylpropane sulfonate containing less than 50 mol% of sodium 2-acrylamido-2-methylpropane sulfonate, and more preferably less than 25 mol% of sodium 2-acrylamido-2-methylpropane sulfonate.

[0084] Another aspect of the invention relates to a method for preparing an aqueous saline solution of water-soluble polymer, said water-soluble polymer being obtained according to the process described above, said method comprising the following successive steps: - Prepare a brine comprising more than 50,000 ppm of salts, with a ratio R+ > 0.03, R+ = mass ratio: divalent salts / total salts; - Dissolve completely in the brine between 0 and 2% by weight of water-soluble polymer in less than 120 minutes at a temperature between 15 and 60°C.

[0085] 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, aluminum salts, ammonium salts, phosphate salts, sulfate salts, chloride salts, fluoride salts, citrate salts, acetate salts, tartrate salts, hydrogen phosphate salts, and mixtures thereof.

[0086] 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.

[0087] The brine advantageously contains more than 30,000 ppm of salts, preferably more than 50,000 ppm of salts, even more preferably more than 75,000 ppm of salts.

[0088] 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.

[0089] A person skilled in the art will know how to choose the means of 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.

[0090] Finally, a last aspect of the invention concerns the use of the aqueous saline polymer solution prepared according to the method previously described for applications in oil and gas fields such as the enhanced recovery of hydrocarbons (in particular oil) and hydraulic fracturing operations.

[0091] The invention and the advantages resulting therefrom will become more apparent from the following figures and examples given to illustrate the invention, in a non-limiting manner. Figures

[0092] [Fig.l] [Fig.l] illustrates the appearance of filters for non-total (A, B and C) and optimal (D) dissolutions.

[0093] [Fig.2] [Fig.2] represents the evolution of the quantity of mother solution filtered over time.

[0094] [Fig.3] [Fig.3] represents the evolution of the optimal dissolution time of a powder with a particle size between 300 and 500 pm in different brines.

[0095] [Fig.4] [Fig.4] represents the evolution of the optimal dissolution time of a powder with a particle size between 300 and 500 pm in different brines at 75g / L of salts of varying hardness. Examples

[0096] Example 1: Synthesis of polymers in powder form

[0097] CEI (Counter-example): Gel synthesis of an acrylamide / acrylic acid copolymer PI

[0098] In a 2 L beaker, an aqueous charge comprising 580 g of acrylamide at 50% by weight 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, while adding sodium hydroxide 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, then placed in a Dewar. An azo catalyst, namely 2,2' azo bis (2-amidinopropane) dihydrochloride (V50) is added at a rate of 3.7 mg.

[0099] The homogenization of the charge is carried out using a hand mixer at a speed of 500 rpm for 20 seconds. This charge is then deoxygenated under nitrogen bubbling for at least 10 minutes. Once the deoxygenation of the reaction medium is complete, the polymerization is initiated by successive addition to the charge of 1.8 mg of tert-butyl hydroperoxide (TBHP) and 2.3 mg of Mohr's salt.

[0100] The polymerization reaction is complete after one and a half hours and the polymer obtained in the form of a gel reaches a final temperature of 80°C. The gel is aged as is, in the Dewar, overnight. Subsequently, the gel is ground and then dried. in an oven at 50°C for twenty hours. The dry polymer particles are then ground and sieved to obtain a powder of predefined particle size. The resulting PI polymer has a weight-average molecular weight of between 17 and 22 million daltons.

[0101] El (Example according to the invention): Synthesis by gel route of an acrylamide / acrylic acid copolymer P2 in the presence of 0.5% by weight of surfactant (ethoxylated alcohol).

[0102] The synthesis is carried out according to the same method as for the polymer PI except that the initial aqueous charge comprises 580 of acrylamide at 50% by weight in water, 115g of acrylic acid, 665g of deionized water and 2.2g of isotridecanol, branched, ethoxylated (approximately 12 EO units). The polymer P2 obtained has a weight average molecular weight between 17 and 22 million daltons.

[0103] CE2 (Counter-example): Gel synthesis of an acrylamide / acrylic acid copolymer P3 in the presence of 0.05% by weight of surfactant (ethoxylated alcohol)

[0104] The synthesis is carried out according to the same method as for the polymer PI except 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 0.2 g of isotridecanol, branched, ethoxylated (approximately 12 EO units). The polymer P3 obtained has a weight average molecular weight between 17 and 22 million daltons.

[0105] CE3 (Counter-example): Gel synthesis of an acrylamide / acrylic acid copolymer in the presence of 6% of ethoxylated alcohol surfactant (counter-example 2)

[0106] The synthesis is carried out according to the same method as for the polymer PI except that the initial aqueous charge comprises 580 of acrylamide at 50% by weight in water, 115g of acrylic acid, 665g of deionized water and 24g of isotridecanol, branched, ethoxylated (approximately 12 EO units). Note that the polymerization reaction is only completed after about twelve hours. The polymer P4 obtained has a weight average molecular weight between 17 and 22 million daltons.

[0107] Example 2: Dissolution in brine of polymers PI to P4.

[0108] The powder dissolution monitoring presented in [Fig.2], corresponds to the preparation of a mother solution of polymer 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 (MgCl2,6 H2O). For this analysis, a narrow particle size cut, namely 300-500 microns, was chosen in order to limit the uncertainties on the particle size distribution of each polymer generally observed for particle size cuts. wider metrics (100-1500 microns). Note, however, that these tests were also carried out with such particle size distributions and their trend is identical, except that the dissolution times are longer.

[0109] Dissolution is considered complete when the maximum viscosity is obtained and the mother solution is completely homogeneous. This means that the entire solution has passed through a 200 micron filter without leaving any non-fully hydrated polymer particles.

[0110] [Fig. 1] illustrates the appearance of the filters obtained once the maximum viscosity has been reached. The appearance of the filter on the left (case A) corresponds to a mother solution for which the hydration / solubilization time was not sufficient for the study conditions. By increasing the dissolution time, it is possible to achieve a completely homogeneous solution, which results in the absence of deposits on the filter (case D, photo on the right).

[0111] The graph below ([Fig.2]) represents the proportion of mother solution filtered through a 200 micron filter for solutions prepared from polymers PI (CEI), P2 (El), P3 (CE2), P4 (CE3) and also for a solution prepared with polymer PI in a brine to which an isomolar quantity of surfactant (ethoxylated alcohol) in example El has been added beforehand (counter-example CE4).

[0112] The stock solutions prepared from polymers P2 and P4 pass almost completely through the filter after 60 minutes, whereas a hydration time twice as long is required for polymers PI, P3 and for the polymer PI + surfactant mixture (CE4). 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 [Fig.l].

[0113] [Tables 1] Hydration time (mm) Polymer 60 90 120 150 180 240 PI ndnd ABCD P2 ACDDDD P3 ndnd ABCD P4 ABCDDD Pi + surfactant md. nd AB Ç D

[0114] Table 1: Summary table of the appearance of the filters after passing the stock solutions at different hydration times

[0115] The optimum hydration time under the conditions tested for the PI polymer is 240 minutes while only 120 minutes are required for the polymer obtained according to the process of the invention (polymer P2). Based solely on dissolution, polymer P4 is also effective, but several parameters are restrictive regarding 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.

[0116] 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 PI and P2 ([Fig.3]).

[0117] A notable gain in terms of dissolution speed of the powders is observed as soon as the salinity is higher than that of synthetic seawater (30 g / L). Indeed, the mother solutions are completely homogeneous in half the time when the P2 polymer is used.

[0118] It should also be noted that, above the solubility limit of the polymer with very high salinity (> 250 g / L), it is possible to obtain a homogeneous mother solution only with the polymer P2.

[0119] Similarly, if the overall salinity of a brine is set at 75g / L, for which the hardness (R+ mass) is adjusted by adjusting the proportions of calcium chloride (NaCl) and calcium chloride dihydrate (CaCl2.2H2O), optimum dissolution times are obtained which are significantly lower for the polymer of the invention P2 compared to the polymer PI as soon as R+ is greater than or equal to 0.03.

Claims

Claims

1. A process for preparing a water-soluble polymer with a weight-average molecular weight greater than 1 million daltons, the process comprising at least the following successive steps: a) Preparing a polymer, in the form of a gel, by polymerization, by radical means 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; b) Granulating the polymer gel thus obtained; c) Drying the granulated polymer gel to obtain a polymer in powder form; d) Grinding and sieving the powder.

2. Process 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.

3. Process according to claim 1 or 2, characterized in that the surfactant of step a) is anionic or non-ionic and has an HLB of between 8 and 20.

4. Process according to the preceding claims, characterized in that 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, polyalkoxy alkyl alkyl sulfonates, polyalkoxy alkyl aryl sulfonates, branched alkyl benzene sulfonates, sodium docusate, ethoxylated glycidyl sulfonates, propoxylated glycidyl sulfonates, alkyl sulfates ether, Guerbet alkyl sulfates, internal olefin sulfonates, sulfonated ethoxylated alcohols, alkyl sulfonated phenol, sodium petroleum sulfonate, propoxylated alkyl alcohol sulfates, carboxylated ethoxylated alkyls, polyglycoside alkyls, ethoxylated alcohols, propoxylated alcohols, propoxylated alcohol sulfates, ethoxylated alkylphenols, polyethoxylated phenol alkyls, poly(ethylene / propylene) glycol ethers, iV-ethoxylated secondary alcohols, iV-propoxylated secondary alcohols, ethoxylated tridecyl alcohols, triphenylmethane and mixtures thereof, with for these surfactants: - the alkyl groups being C4-C2o, - ethoxylated chains comprising between 6 and 100 -CH2-CH2 -O- groups, - propoxylated chains comprising between 6 and 100 -CH2-CH2 groups - ch2-o-,

5. Method according to claim 4, characterized in that the added surfactant 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 ethoxylated alkyls, ethoxylated alcohols, propoxylated alcohols, propoxylated alcohol sulfates, poly(ethylene / propylene) glycol ethers, iV-ethoxylated secondary alcohols, iV-propoxylated secondary alcohols, ethoxylated tridecyl alcohols, with for these surfactant: - the alkyl groups being C4-C20, - the ethoxylated chains comprising between 6 and 100 -CH2-CH2 -O- groups, - propoxylated chains comprising between 6 and 100 -CH2-CH2 groups - ch2-o-,

6. Process according to the preceding claims, characterized in that the at least one monounsaturated ethylenic monomer of step a) is chosen from the group consisting of non-ionic monomers, anionic monomers, cationic monomers and mixtures thereof, - the non-ionic hydrophilic monomer(s) being chosen from the group comprising water-soluble vinyl monomers, such as acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N,N-dialkyl acrylamides (for example N,N-dimethylacrylamide or N,N-diethylacrylamide), N,N-dialkylmethacrylamides, acrylic acid alkoxylated esters, methacrylic acid alkoxylated esters, N-vinylpyrrolidone, N-methylol(meth)acrylamide, N-vinyl caprolactam, N-vinylformamide (NVF), N-vinyl acetamide, N-vinyl imidazole, N-vinyl succinimide, acryloyl morpholine (ACMO), glycidyl methacrylate, glyceryl methacrylate, diacetone acrylamide, methacrylic anhydride, acrylonitrile, maleic anhydride, itaconic anhydride, itaconamide, vinylpyridine, hydroxyalkyl (meth)acrylate, thioalkyl (meth)acrylate, isoprenol and its alkoxylated derivatives, hydroxyethyl (meth)acrylates and their alkoxylated derivatives, hydroxypropylacrylate and its alkoxylated derivatives, vinyl acetate, and mixtures thereof, the alkyl groups being CrC3 hydrocarbon chains; - the anionic hydrophilic monomers being chosen from the monomers acrylic acid, methacrylic acid, dimethylacrylic acid, itaconic acid, the hemi-esters of itaconic acid in C1-C3> acryloyl chloride, crotonic acid, maleic acid, fumaric acid, 3-acrylamido 3-methylbutanoic acid vinyl sulfonic 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-methylpropane sulfonic acid (ATBS), 2-acrylamido-2-methylpropane disulfonic acid, 3-allyloxy-2-hydroxypropane sulfonic acid, diethylallylphosphonate, carboxyethyl acrylate;the water-soluble salts of all of these monomers, such as their alkali metal, alkaline earth metal or ammonium salts; 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 dialkyl-aminoalkyl(meth)acrylamides, such as for example me-thacrylamido-propyl trimethyl ammonium chloride (MAPTAC), acrylamido-propyl trimethyl ammonium chloride (APTAC), acidified or quaternized salts of dialkyl-aminoalkyl acrylate such as quaternized or salified di-methylaminoethyl acrylate (ADAME), acidified salts or quaternized dialkyl aminoalkyl methacrylate such as quaternized or salified dimethylaminoethyl methacrylate (MADAME), and mixtures thereof.

7. Water-soluble polymer in powder form obtained according to the process of one of the preceding claims, the powder having an average particle size of between 100 and 1500 pm.

8. Water-soluble polymer according to claim 7, characterized in that the powder has an average particle size of between 150 and 800 pm.

9. Water-soluble polymer according to one of claims 7 to 8, characterized in that the water-soluble polymer is a copolymer of acrylamide and sodium acrylate or a terpolymer of acrylamide, sodium acrylate and sodium 2-acrylamido-2-methylpropane sulfonate containing less than 50 mol% of sodium 2-acrylamido-2-methylpropane sulfonate.

10. Water-soluble polymer according to one of claims 7 to 9, characterized in that the water-soluble polymer is a terpolymer of acrylamide, sodium acrylate and sodium 2-acrylamido-2-methylpropane sulfonate containing less than 25 mol% of sodium 2-acrylamido-2-methylpropane sulfonate.

11. Method for preparing an aqueous saline solution of water-soluble polymer, said water-soluble polymer being obtained according to the process of claims 1 to 6 or being the water-soluble polymer according to one of claims 7 to 10, said method comprising the following successive steps: - Prepare a brine comprising more than 50,000 ppm of salts, with a ratio R+> 0.03, R+= mass ratio: divalent salts / total salts; - Completely dissolve between 0 and 2% by weight of water-soluble polymer in less than 120 minutes at a temperature between 15 and 60°C.

12. Use of the aqueous saline solution of water-soluble polymer prepared according to the method of claim 11 in the enhanced recovery of hydrocarbons or in hydraulic fracturing operations.

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