WATER-SOLUBLE POLYMER DISPERSION FOR HYDRAULIC FRACTURING

Abstract

WATER-SOLUBLE POLYMER DISPERSION FOR HYDRAULIC FRACTURING This application relates to an injection fluid F for hydraulic fracturing comprising at least one synthetic water-soluble polymer P of weight-average molecular weight greater than or equal to 1 million daltons, said fluid being prepared according to a process comprising the following successive steps: A reverse emulsion E is prepared by radical polymerization comprising between 15% and 40% by weight of polymer P, between 20% and 60% by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the emulsion E. The reverse emulsion E is distilled to obtain a dispersion D comprising between 40% and 60% by weight of polymer P particles, less than 10% by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the dispersion D.The dispersion D is diluted with 1% to 15% by weight of an aqueous solution S comprising between 20% and 60% by weight of salts, the percentages being expressed as a percentage of the weight of the dispersion D. Figure for the abstract: None.

Description

Title of the invention: WATER-SOLUBLE POLYMER DISPERSION FOR HYDRAULIC FRACTURING

[0001] The invention relates to an injection fluid for hydraulic fracturing comprising at least one synthetic water-soluble polymer, said fluid being a dispersion prepared by dilution with brine of an inverse emulsion of said polymer previously distilled.

[0002] The invention also relates to a method of hydraulic fracturing of underground reservoirs of unconventional oil and gas using said injection fluid. prior art of the technique

[0003] The production of oil (oils, hydrocarbons) and gas contained in unconventional underground reservoirs has been developing for several years and requires opening fractures in the reservoir for economical production of oil and gas.

[0004] In the following description of the prior art and of the invention, "unconventional underground reservoirs" refers to deposits requiring specific extraction technologies because they do not exist as an accumulation in porous and permeable rock (see Shale Hydrocarbons in France, Provisional Report - CGIET No. 2011-04-G - Ministry of Ecology, Sustainable Development, Transport and Housing - April 2011). Examples of unconventional gas include shale gas, coalbed methane, and tight gas. Examples of unconventional oil include heavy oil, shale oil, and tight oil.

[0005] The reserves contained in unconventional reservoirs are enormous and extremely widespread in areas previously unexploitable, such as shale hydrocarbons like shale, tight gas, and coalbed methane. In the United States, shale gas is widely exploited and now accounts for 46% of total US natural gas production, compared to only 28% in 1998. The very large basins are known as the Bamett Shale, Ville Fayette Shale, Mowry Shale, Marcellus Shale, Utica Shale, etc. The exploitation of tight reservoirs has been made possible by advances in drilling techniques.

[0006] Production techniques have indeed evolved from vertical wells to wells Horizontal wells reduce the number of production wells required and their footprint, allowing for better coverage of the reservoir volume to maximize gas or oil recovery. However, the permeability is insufficient for hydrocarbons to migrate easily from the source rock to the well, thus hindering economical and significant gas or oil production. Therefore, it is necessary to increase permeability and production area through stimulation operations, particularly hydraulic fracturing of the rock in contact with the well. Hydraulic fracturing

[0007] Hydraulic fracturing aims to create additional permeability and generate larger gas or oil production areas. Indeed, low permeability, natural barriers of compact layers, and impermeability caused by drilling operations severely limit production. The gas or oil contained in the unconventional reservoir cannot easily migrate from the rock to the well without stimulation.

[0008] These hydraulic fracturing operations on horizontal wells began in 1960 in the Appalachians and, today, several tens of thousands of operations have taken place in the United States.

[0009] The technologies for studying, modelling the reservoir, drilling, cementing and stimulation have become increasingly sophisticated and implement equipment that allows these operations to be carried out in increasingly shorter times with precise analysis of the results.

[0010] Reservoir stimulation by hydraulic fracturing

[0011] These operations consist of injecting water at high pressure and very high flow rates to create fractures distributed perpendicularly to the production wells. This is generally done in several stages to create fractures along the entire length of the horizontal well, thus allowing maximum reservoir volume to be covered.

[0012] In order to keep these fractures open, a support agent (for example sand, plastics or calibrated ceramics) is added so as to prevent the closure of these fractures and to maintain the capillarity created once the injection has stopped.

[0013] Water alone is insufficient to achieve good placement efficiency of the support agent due to its low viscosity. This limits its ability to hold the support agent in place within fractures. To overcome this problem, fracturing fluids containing viscosifying compounds have been developed.

[0014] By definition, a compound is said to be viscosifying when it increases the viscosity of the solutions in which it is dissolved.

[0015] In addition to having viscosifying properties, the compound must have a specific rheological profile. Indeed, the compound must be able to generate low viscosity so as not to hinder the transport and pumping of the fluid containing the proppant during the high shear stresses experienced during the injection of the fracturing fluid. Once injected, this same compound must be able to generate sufficient viscosity when the shear stress decreases to support the proppant and retain it within the fractures.

[0016] Fracture fluids generally include a polymer which must therefore provide shear-thinning properties to the solution in order to have a relatively low viscosity during injection (at high shear) and a high viscosity in order to keep the support agent in suspension at the fracture site when the shear decreases.

[0017] The viscoelastic properties of polymers in solution must also be taken into consideration. This viscoelasticity, and its importance in the application, is described in SPE 147206 (Fracturing Fluid Comprised of Components Sourced Solely from the Food Industry Provides Superior Proppant Transport - David Loveless, Jeremy Holtsclaw, Rajesh Saini, Phil Harris, and Jeff Fleming, SPE, Halliburton) through visual observations in static or dynamic experiments, or through rheological measurements, such as the measurement of viscous and elastic moduli (G' and G"), or the measurement of viscosity as a function of shear using a rheometer. Thus, elastic properties will be advantageous for ensuring the transport and suspension of the fracture support agent.

[0018] The choice of polymer is therefore not obvious and requires a thorough rheological study in order to obtain satisfactory results.

[0019] Among the prior art viscosifying compounds for aqueous solutions, one can cite natural substances such as guar gum and its derivatives such as hydroxypropyl guar (HPG) or carboxymethyl hydroxypropyl guar (CMHPG); cellulosic derivatives such as carboxymethyl cellulose or hydroxyethyl cellulose. These compounds are notably described in patents US4033415, US3888312 and US4801389. In the document SPE 152596 (Hydraulic Fracturing 101: What Every Representative, Environmentalist, Regulator, Reporter, Investor, University Researcher, Neighbor and Engineer Should Know About Estimating Frac Risk and Improving Frac Performance in Unconventional Gas and Oil Wells - George E. King, Apache Corporation), the latest advances relating to the performance of fracturing fluids are discussed in detail.

[0020] However, these natural substances, and in particular guar derivatives, are also useful in other applications, such as the food or textile industries, and the growth of the exploitation of unconventional oil and gas resources is competing with these other applications. This creates pressure on the availability of these products and leads to pricing problems.

[0021] Other petrochemical compounds can have viscosity-increasing properties. Synthetic polymers are a good example. Poly(meth)acrylamides, possibly partially hydrolyzed, and poly(meth)acrylates and their copolymers are particularly well known. These polymers develop viscosity due to their molar mass and interchain ionic repulsions. These polymers are described in patents GB951147, US3727689, US3841402, and US3938594. The mechanism governing viscosity is linked to an increase in hydrodynamic volume due to intrachain repulsions, interchain entanglements, etc.

[0022] However, in the presence of high salinity or high operating temperatures, these polymers do not develop strong entanglements and repulsions, resulting in a significant decrease in their viscosifying capacity, especially after undergoing the shear stress of the pumping step. Furthermore, these polymers generally do not exhibit sufficient viscoelastic properties to support the proppant in the fracture. The dosage of these polymers must be increased to excessively high levels to achieve the necessary proppant suspension properties. However, the required dosage levels are not economically viable.

[0023] Polymers used to have viscosifying properties can advantageously also be friction reducers allowing to reduce pressure loss in turbulent medium and greatly increase flow rate at identical power and pipe diameter.

[0024] Synthetic polymers based on 2-acrylamido-2-methylpropanesulfonic acid and / or its salts exhibit interesting friction-reducing properties in aqueous solution. These polymers are also known for their resistance to shear and thermal degradation, particularly in saline solutions. However, obtaining very high molecular weight polymers based on 2-acrylamido-2-methylpropanesulfonic acid is difficult, and these polymers also exhibit solubility problems as their molecular weight increases. For optimal friction reduction and high viscosity generation, it is essential that the polymer dissolves rapidly, especially in saline solution, and that it has a very high molecular weight.

[0025] For logistical, transport, and supply reasons related to synthetic water-soluble polymers for hydraulic fracturing, the preferred physical form of these polymers is powder, as it allows for a high percentage by weight of active material. The powder form of these polymers can be obtained by drying, thermal drying, spray drying, and drum drying. However, their reconstitution requires suitable equipment, for example a wet grinding unit for powders such as a PSU (Polymer Slicing Unit).

[0026] Polymer inverse emulsions are also interesting but they require rigorous optimization of their formulation so that their inversion in aqueous media is as rapid as possible and that their stability (during storage and transport) is guaranteed (especially during freeze / thaw cycles). Description of the invention

[0027] The Applicant has discovered and developed an injection fluid for hydraulic fracturing that provides high friction reduction and viscosity-increasing effects. This injection fluid is in the form of a water-soluble polymer dispersion and is prepared by dilution with brine of a previously distilled inverse emulsion of the polymer.

[0028] Surprisingly, prior dilution of the distilled polymer emulsion (polymer dispersion) by brine significantly increases its ability to reverse in the subsequent salt water injection (during injection into the underground formation), which implies efficient (rapid and almost total) dissolution of the polymer to maximize its application effect.

[0029] The carbon footprint of this fluid is reduced because its concentrated polymer form implies lower transport costs, not to mention the absence of specific dissolution equipment for its use.

[0030] A first aspect of the invention therefore relates to an injection fluid F for hydraulic fracturing comprising at least one synthetic water-soluble polymer P of weight average molecular weight greater than or equal to 1 million daltons, said fluid being prepared according to a process comprising the following successive steps:

[0031] a. A reverse emulsion E is prepared by radical polymerization comprising between 15% and 40% by weight of polymer P, between 20% and 60% by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the emulsion E, b. The inverse emulsion E is distilled to obtain a dispersion D comprising between 40 and 60% by weight of polymer particles P, less than 10% by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the dispersion D, c. The dispersion D is diluted with 1% to 15% by weight of an aqueous solution S comprising between 20 and 60% by weight of salts, the percentages being expressed by weight relative to the weight of the dispersion D.

[0032] A second aspect of the invention relates to a hydraulic fracturing process of an underground reservoir of unconventional oil or gas using the injection fluid F according to the invention.

[0033] A third aspect of the invention relates to a friction reduction method using an injection fluid F in a hydraulic fracturing operation of an underground oil or unconventional gas reservoir using the injection fluid F according to the invention.

[0034] As used herein, the term "water-soluble polymer" refers to a polymer which gives an aqueous solution free of insoluble particles when dissolved under stirring for 4 hours at 25°C and with a concentration of 20 gL 1 in water.

[0035] According to the present invention, the "weight-average molecular weight" of the synthetic water-soluble polymer P is determined by measuring the intrinsic viscosity. The intrinsic viscosity can be measured by methods known to those skilled in the art and can, in particular, be calculated from the reduced viscosity values ​​for different concentrations by a graphical method consisting of plotting the reduced viscosity values ​​(on the y-axis) as a function of the concentrations (on the x-axis) and extrapolating the curve to zero concentration. The intrinsic viscosity value is read from the y-axis or using the least squares method. Then, the weight-average molecular weight can be determined by the well-known Mark-Houwink equation:

[0036] [q] = KM" [q] represents the intrinsic viscosity of the polymer determined by the viscosity in solution measurement method, K represents an empirical constant, M represents the molecular weight of the polymer, a represents the Mark-Houwink coefficient, and a and K depend on the particular polymer-solvent system. Tables known to those skilled in the art give the values ​​of a and K according to the polymer-solvent system.

[0037] The synthetic water-soluble polymer P of the invention has an average molecular weight advantageously greater than or equal to 1 million daltons, even more advantageously greater than or equal to 1.5 million daltons, and even more advantageously greater than or equal to 2 million daltons. It is advantageously less than or equal to 20 million daltons. Preferably, the synthetic water-soluble polymer P of the invention has an average molecular weight advantageously between 1 million daltons and 20 million daltons, even more advantageously between 1.5 million daltons and 20 million daltons, and even more advantageously between 2 million daltons and 20 million daltons.

[0038] The inverse emulsion E comprising the synthetic water-soluble polymer P obtained by radical polymerization in step a) of the process for obtaining the fluid F contains :

[0039] - a hydrophilic phase comprising at least one water-soluble structured polymer - a lipophilic phase; - at least one emulsifying agent;

[0040] The lipophilic phase may be a mineral oil, a vegetable oil, a synthetic oil, or a mixture of several of these oils. Examples of mineral oils include mineral oils containing saturated hydrocarbons of the aliphatic, naphthenic, paraffinic, isoparaffinic, cycloparaffinic, or naphthyl type. Examples of synthetic oils include hydrogenated polydecene or hydrogenated polyisobutene, and esters such as octyl stearate or butyl oleate. Exxon's Exxsol® product line is ideally suited.

[0041] In general, the weight ratio of the hydrophilic phase to the lipophilic phase in the reverse emulsion is preferably from 50 / 50 to 90 / 10.

[0042] In the present invention, the term "emulsifying agent" refers to an agent capable of emulsifying water in an oil, and an "inverting agent" is an agent capable of emulsifying oil in water. Generally, an inverting agent is considered to be a surfactant having an HLB greater than or equal to 10, and an emulsifying agent is considered to be a surfactant having an HLB strictly less than 10.

[0043] The hydrophilic-lipophilic equilibrium (HLB) of a chemical compound is a measure of its degree of hydrophilicity or lipophilicity, determined by calculating the values ​​of the different regions of the molecule, as described by Griffin in 1949 (Griffin WC, Classification of Surface-Active Agents by HLB, Journal of the Society of Cosmetic Chemists, 1949, 1, pages 311-326).

[0044] 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 provide information on solubility in water and oil. Substances with an HLB value of 10 are distributed between the two phases, such that the hydrophilic group (molecular mass Mh) is completely projected into the water while the hydrophobic hydrocarbon group (molecular mass Mp) is adsorbed into the non-aqueous phase.

[0045] The HLB value of a substance with a total molecular mass M, whose hydrophilic part has a molecular mass Mh, is: HLB = 20 (Mh / M)

[0046] The inverse emulsion E according to the invention is prepared by radical polymerization. An aqueous solution comprising the monomer(s) for obtaining polymer P is emulsified in an oily phase comprising the emulsifying agent(s). Then, polymerization is carried out by adding a free radical initiator. This initiator may be a redox couple, with Cumene hydroperoxide, tertiary butylhydroxyperoxide, or persulfates are among the oxidizing agents; sodium sulfite, sodium metabisulfite, and Mohr's salt are among the reducing agents. Azo compounds such as 2,2'-azobis hydrochloride (isobutyronitrile) and 2,2'-azobis hydrochloride (2-amidinopropane) may also be used.

[0047] Classically, polymerization is generally carried out isothermally, adiabatically, or at controlled temperature. That is to say, the temperature is kept constant, generally between 10 and 60 °C (isothermal), or the temperature is allowed to increase naturally (adiabatic), in which case the reaction is generally started at a temperature below 10 °C and the final temperature is generally above 50 °C, or finally, the temperature increase is controlled so as to have a temperature curve between the isothermal and adiabatic curves (controlled temperature).

[0048] Polymerization can be carried out under a pressure lower than atmospheric pressure, optionally under conditions allowing some of the water and hydrocarbon solvent from the reaction medium to evaporate and pre-concentrate the emulsion.

[0049] The synthetic water-soluble polymer P is preferably obtained from the polymerization of monounsaturated ethylenic monomers, which may be non-ionic and / or anionic and / or cationic and / or zwitterionic. These monomers are preferably the following:

[0050] - non-ionic monomers selected from the group comprising acrylamide, the methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N,N-dialkylacrylamides, N,N-dialkylmethacrylamides, alkoxylated esters of acrylic acid, alkoxylated esters of methacrylic acid, N-vinylpyridine, N-vinylpyrrolidone, hydroxyalkylacrylates, hydroxyalkyl methacrylates,

[0051] - the anionic monomers selected from the group comprising the monomers monomers possessing a carboxylic acid function and their salts, including acrylic acid, methacrylic acid, itaconic acid, and maleic acid; monomers possessing a sulfonic acid function and their salts, including acrylamido tert-butylsulfonic acid (ATBS), allylsulfonic acid, and methallylsulfonic acid and their salts; monomers having a phosphonic acid function and their salts,

[0052] - the cationic monomers selected from the group comprising dimethacrylate- quaternized or salified thylaminoethyl (ADAME); quaternized or salified dimethylaminoethyl methacrylate (MADAME); diallyldimethylammonium chloride (DADMAC); acrylamidopropyltrimethylammonium chloride (APTAC); methacrylamidopropyltrimethylammonium chloride (MAPTAC),

[0053] - zwitterionic monomers selected from the group comprising the monomers sulfobetaines such as sulfopropyl dimethylammonium ethyl methacrylate, sul- fopropyl dimethylammonium propylmethacrylamide, sulfopropyl 2-vinylpyridinium; phosphobetaine monomers, such as phosphato ethyl trimethylammonium ethyl methacrylate; carboxybetaine monomers.

[0054] Optionally, the water-soluble polymer P may comprise at least one LCST or UCST group.

[0055] According to the general knowledge of those skilled in the art, a group exhibiting LCST corresponds to a group whose solubility in water, for a given concentration, changes above a certain temperature and depending on the salinity. It is a group exhibiting a heating transition temperature that defines its lack of affinity for the solvent medium. This lack of affinity for the solvent results in opacification or loss of transparency, which can be due to precipitation, aggregation, gelation, or viscosification of the medium. The minimum transition temperature is called the "LCST" (Lower Critical Solution Temperature). For each concentration of a group exhibiting LCST, a heating transition temperature is observed. This temperature is higher than the LCST, which is the minimum point on the curve.Below this temperature, the (co)polymer is soluble in water; above this temperature, the (co)polymer loses its solubility in water.

[0056] According to the general knowledge of those skilled in the art, a UCST group corresponds to a group whose solubility in water, for a given concentration, changes below a certain temperature and depending on the salinity. It is a group exhibiting a cooling transition temperature that defines its lack of affinity for the solvent medium. This lack of affinity for the solvent results in opacification or a loss of transparency, which can be due to precipitation, aggregation, gelation, or viscosification of the medium. The maximum transition temperature is called the "UCST" (Upper Critical Solution Temperature). For each concentration of a UCST group, a cooling transition temperature is observed. This temperature is higher than the LCST, which is the minimum point on the curve.Above this temperature, the (co)polymer is soluble in water; below this temperature, the (co)polymer loses its solubility in water.

[0057] The water-soluble polymer P in the inverse emulsion E can be linear or structured by at least one structuring agent, which can be selected from the group comprising polyethylene unsaturated monomers (having at least two unsaturated functional groups), such as vinyl, allylic, acrylic, and epoxy functional groups, and examples include methylene bisacrylamide (MBA), diallylamine, triallylamine, tetraallylammonium chloride, and polyethylene dimethacrylate. glycol or by macroinitiators such as polyperoxides, polyazoids and polytransfer agents such as polymer-capturing polymers or hydroxyalkylacrylates, epoxyvinyls.

[0058] The water-soluble polymer P can also be structured using controlled radical polymerization (CRP) techniques or, more particularly, RAFT (Reversible Addition Fragmentation Chain Transfer) type reverse emulsion.

[0059] According to another preference, the water-soluble polymer P inverse emulsion E may comprise:

[0060] - a hydrophilic phase comprising at least one water-soluble (co)polymer P,

[0061] - a lipophilic phase,

[0062] - at least one interfacial polymer composed of at least one monomer of formula (I):

[0063] [Chem.l]

[0064] Formula (I) in which,

[0065] - Ri, R2, Ri are independently chosen from the group consisting of one atom hydrogen, a methyl group, a carboxylate group and ZX,

[0066] - Z is chosen from the group consisting of C(=0)-0 ; C(=O)-NH ; OC(=O) ; NH- C(=O)-NH; NH-C(=0)-O; and a carbon chain comprising 1 to 20 carbon atoms, unsaturated or not, substituted or not, which may comprise one or more heteroatoms, preferably 1, 2 or 3 heteroatoms, selected from nitrogen and oxygen,

[0067] - X is a group selected from alkanolamides, sorbitan esters, esters of ethoxylated sorbitan, glyceryl esters, and polyglycosides; X comprising a hydrocarbon chain preferably comprising 6 to 24 carbon atoms, saturated or unsaturated, linear, branched or cyclic, optionally aromatic.

[0068] Without wishing to be bound by any theory, the interfacial polymer obtained by polymerization of at least one monomer of formula (I) forms an envelope at the interface of the hydrophilic phase and the lipophilic phase.

[0069] Generally, the hydrophilic phase is in the form of dispersed micrometric droplets, advantageously emulsified in the lipophilic phase. The average size of these droplets is advantageously between 0.01 and 30 pm, more advantageously between 0.05 and 3 pm. The interfacial polymer is thus positioned at the interface between the hydrophilic and lipophilic phases at each droplet. The interfacial polymer partially or completely envelops each of these droplets. The average droplet size is advantageously measured with a laser measuring device using conventional techniques that are part of the general knowledge of those skilled in the art. A Mastersizer-type device from Malvern can be used for this purpose.

[0070] Advantageously, the interfacial polymer comprises between 0.0001 and 10%, more advantageously between 0.0001 and 5%, even more advantageously from 0.0001 to 1%, in number of monomers of formula (I), relative to the total number of monomers.

[0071] The interfacial polymer forms an envelope around the droplets forming the hydrophilic phase. In addition to the monomers mentioned above, the interfacial polymer may comprise at least one structuring agent. The structuring agent is advantageously chosen from among diacrylamides or methacrylamides of diamines; acrylic esters of di, tri, or tetrahydroxy compounds; methacrylic esters of di, tri, or tetrahydroxy compounds; divinyl compounds preferably separated by an azo group; diallylic compounds preferably separated by an azo group; vinyl esters of di or trifunctional acids; allylic esters of di or trifunctional acids; methylenebisacrylamide; diallylamine; triallylamine; tetraallylammonium chloride; divinylsulfone; polyethylene glycol dimethacrylate; and diethylene glycol diallyl ether.

[0072] Preferably, the inverse emulsion E comprises from 0.5% to 5.0% by weight, the percentages being expressed as weight relative to the weight of the emulsion E, of at least one emulsifying agent preferably selected from sorbitan esters, polyethoxylated sorbitan esters, polyethoxylated fatty acids, polyethoxylated fatty alcohols, polyesters having an average molecular weight between 1000 and 3000 daltons resulting from the condensation between poly(isobutenyl)succinic acid or its anhydride and polyethylene glycol, block copolymers having an average molecular weight between 2500 and 3500 daltons resulting from the condensation between hydroxystearic acid and polyethylene glycol, ethoxylated fatty amines, di-alkanol amide derivatives, stearyl methacrylate copolymers, and mixtures of these emulsifying agents.

[0073] This emulsifying agent is added to the lipophilic phase prior to the radical polymerization reaction.

[0074] Optionally, a natural or synthetic polymer (notably described in US 10,647,908) may be added at the end of the radical polymerization reaction in step a) of the process for obtaining fluid F. Examples of natural polymers include guar gum and its derivatives such as hydroxypropyl guar (HPG) or carboxymethyl hydroxypropyl guar (CMHPG); and cellulosic derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose, or hydroxypropyl cellulose. preferably polymer P is predominant in relation to the natural polymer, preferably polymer P represents 50 to 100% by weight, preferably 70 to 100%, more preferably 90 to 100%, in relation to the total weight of polymer P + natural polymer.

[0075] Step b) of the process for obtaining the injection fluid F consists of distilling the inverse emulsion E to obtain a dispersion D, the polymer P is therefore, after distillation, in the form of particles (solids), comprising between 40 and 60% by weight of polymer P particles, less than 10% by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the dispersion D. The distillation is carried out under reduced pressure, preferably at a pressure between 20 and 250 mbar and at a temperature between 10 and 110°C.

[0076] Preferably, in the injection fluid of the invention, the polymer P is in the form of particles (solids).

[0077] Preferably, the synthetic water-soluble polymer particles P in the dispersion D have an average size between 0.01 pm and 100 pm, and even more preferably between 0.1 pm and 5 pm. In the present invention, average size means the average diameter of the particles. Particle size analysis is performed according to conventional techniques known to those skilled in the art. An example of an apparatus for measuring the average particle diameter is the Mastersizer from Malvem Instruments.

[0078] At the end of the distillation and therefore before step c) of the process for obtaining the injection fluid F of the invention, between 0.2 and 10% by weight, the percentages being expressed as weight relative to the weight of the dispersion D, is preferably added to the dispersion (D) of at least one reversing agent. The reversing agent is preferably chosen from among ethoxylated nonylphenols, preferably having 4 to 10 ethoxylations; ethoxylated / propoxylated alcohols, preferably having an ethoxylation / propoxylation comprising between 12 and 25 carbon atoms; ethoxylated tridecyl alcohols; polyethoxylated fatty acids; poly(ethoxylated / propoxylated) fatty alcohols; ethoxylated sorbitan esters; polyethoxylated sorbitan laurate; polyethoxylated castor oil; heptaoxyethyl lauryl alcohol; polyethoxylated sorbitan monostearate; polyethoxylated alkyl phenol cetyl ether; alkyl aryl polyethylene ether;N-Cetyl-N-ethyl morpholinium ethosulfate; sodium lauryl sulfate; condensation products of fatty alcohols with ethylene oxide; condensation products of alkylphenols and ethylene oxide; condensation products of fatty amines with 5 or more molar equivalents of ethylene oxide; ethoxylated tristyrylphenol; condensates of ethylene oxide with polyhydric alcohols partially esterified with fatty chains, as well as their anhydrous forms; the; amine oxides; alkyl polyglucosides; glucamide; phosphate esters; alkylbenzene sulfonic acids and their salts; water-soluble surfactant polymers; and mixtures of several of these reversing agents.

[0079] Step c) of the process of obtaining the injection fluid F consists of diluting the dispersion D with 1% to 15% by weight of an aqueous solution S containing between 20 and 60% of salts, preferably between 25 and 45%, the percentages being expressed by weight relative to the weight of the dispersion D. The solution S is, preferably, a brine solution.

[0080] Preferably at least a portion of the water from solution 5 is extracted from the distillate of emulsion E.

[0081] Advantageously, the salts of the aqueous solution S are alkali or alkaline earth or ammonium salts, or organic salts, or a mixture of these salts. More preferably, the salts are selected from sodium chloride, ammonium sulfate, ammonium thiosulfate, ammonium chloride, choline chloride, monosaccharide salts, or a mixture of these salts.

[0082] After step c) of the process for obtaining fluid F, other compounds known to those skilled in the art may be added, such as those mentioned in document SPE 152596, for example:

[0083] • Biocides to prevent the growth of bacteria, particularly sulfate-reducing bacteria, which can form viscous masses that reduce the surface area. Examples include glutaraldehyde, which is the most commonly used, as well as formaldehyde or isothiazolinones, and / or • Oxygen reducers such as ammonium bisulfite to prevent the destruction of other components by oxidation and corrosion of the injection tubes, and / or • Anti-corrosion additives to protect the tubes against oxidation by residual amounts of oxygen, such as N,N-dimethylformamide, and / or • Lubricants such as oil distillates, and / or • Iron chelating agents such as citric acid, EDTA (ethylenediamine) tetraacetic acid), phosphonates, and / or • Anti-scale products such as phosphates, phosphonates, polyacrylates or ethylene glycol.

[0084] The present invention therefore also relates to a method for preparing an injection fluid F comprising the following steps:

[0085] a. A reverse emulsion E is prepared by radical polymerization comprising between 15% and 40% by weight of a synthetic water-soluble polymer P of average molecular weight by weight greater than or equal to 1 million daltons, between 20% and 60% by weight of water and at least one hydrocarbon solvent, the pro- Percentages being expressed by weight relative to the weight of emulsion E, b. The inverse emulsion E is distilled to obtain a dispersion D comprising between 40 and 60% by weight of polymer particles P, less than 10% by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the dispersion D, c. The dispersion D is diluted with 1% to 15% by weight of an aqueous solution S comprising between 20 and 60% by weight of salts, the percentages being expressed by weight relative to the weight of the dispersion D.

[0086] The steps of the process are as described above.

[0087] A second aspect of the invention also relates to a hydraulic fracturing process of an underground reservoir of unconventional oil or gas, comprising the preparation of an injection fluid F as described above, dissolving it in salt water and injecting said fracturing fluid F into an underground formation.

[0088] The injection is carried out under pressure so as to create fractures distributed throughout the production well.

[0089] Prior to its injection into the underground formation, the injection fluid F obtained by the process of the invention is dissolved in salt water to have a polymer concentration between 0.01 and 10 g / L in this salt water.

[0090] Salt waters are sea waters or can be advantageously prepared with monovalent and / or polyvalent salts or their combinations.

[0091] Examples of salts include, without limitation, sodium, lithium, potassium, aluminum, ammonium, phosphate, sulfate, magnesium, barium, nitrate and other inorganic salts and mixtures thereof.

[0092] Salt waters preferably contain at least one of the following: sodium chloride, calcium chloride, sodium bromide, calcium bromide, barium chloride, magnesium chloride, zinc bromide, sodium formate and potassium formate.

[0093] Preferably the salt water used for dissolving the injection fluid F contains more than 70,000 ppm of salts and preferably more than 100,000 ppm of salts, preferably the brine contains from 70,000 to 350,000 ppm of salts, preferably from 100,000 to 350,000 ppm.

[0094] Preferably the divalent ratio R+ = mass ratio: divalent salts / total salts of salt water is greater than or equal to 0.20 and even more preferably R+ > 0.25.

[0095] Preferably, for the injection of the injection fluid F into the underground formation at least one propping agent is added before or after its dissolution in salt water.

[0096] Preferably, at least one supporting agent is added before step a), and / or between step a) and step b), and / or between step b) and step c), and / or after step c) of the injection fluid preparation process.

[0097] The proppant can be chosen without restriction from sand, ceramics, bauxite, glass beads, and resin-impregnated sand. It advantageously represents from 0.5 to 40%, more preferably from 1 to 25%, and even more preferably from 1.5 to 20%, by weight relative to the total weight of the injection fluid F for hydraulic fracturing.

[0098] Optionally, before, during or after the creation of the fractures, at least one oxidizing compound and / or at least one surfactant compound is injected into the reservoir.

[0099] Surfactant injection eliminates wettability with the rock, while oxidizing compound injection destroys the copolymer. In both cases, injection restores a fluid viscosity close to that of water.

[0100] Examples of oxidizing compounds include bleach (aqueous solution of a hypochlorite salt), hydrogen peroxide, ozone, chloramines, persulfates, permanganates or perchlorates.

[0101] The chemical nature of the surfactant compound(s) is not critical. They may be anionic, nonionic, amphoteric, zwitterionic, and / or cationic. Preferably, the surfactant compound(s) of the invention carry anionic charges.

[0102] Preferably, the surfactant compounds used are chosen from anionic surfactants and their zwitterions selected from the group comprising derivatives of alkyl sulfates, alkyl ethersulfates, arylalkyl sulfates, arylalkyl ethersulfates, alkylsulfonates, alkyl etherssulfonates, arylalkylsulfonates, arylalkyl etherssulfonates, alkyl phosphates, alkyl ethersphosphates, arylalkyl phosphates, arylalkyl ethersphosphates, alkylphosphonates, alkyl ethersphosphonates, arylalkyl phosphonates, arylalkyl ethersphosphonates, alkyl carboxylates, alkyl ethers carboxylates, arylalkyl carboxylates, arylalkyl ethers carboxylates, alkyl polyethers, arylalkyl polyethers...

[0103] An alkyl chain is defined as a chain of 6 to 24 carbons, branched or unbranched, with or without multiple repeating units, which may optionally include one or more heteroatoms, for example O, N, or S, preferably 1, 2, or 3 heteroatoms. An arylalkyl chain is defined as a chain of 6 to 24 carbons, branched or unbranched, comprising one or more aromatic rings and which may optionally include one or more heteroatoms, for example 1, 2, or 3 heteroatoms, preferably O, N, or S.

[0104] The most commonly used surfactants, for reasons of cost, stability, and availability, are of the sulfonate or sulfate type, presented in the form of alkali metal or ammonium salts.

[0105] Finally, a last aspect of the invention relates to a friction reduction method during hydraulic fracturing operations of an underground reservoir of unconventional oil or gas, including the preparation of a fluid F as described above, dissolving it in salt water and injecting said fracturing fluid into an underground formation.

[0106] Friction reduction makes it possible to decrease or eliminate friction-related losses during the injection of the fracturing fluid. EXAMPLES OF THE INVENTION'S IMPLEMENTATION

[0107] The following examples illustrate the invention without however limiting its scope.

[0108] The proportions of the different compounds are given as a percentage by weight relative to the final weight of the emulsions.

[0109] Example 1 - Preparation of an inverse emulsion El of polymer PI

[0110] An aqueous phase is prepared with 42.1 g of an acrylamide solution (50% by weight in water), 9.1 g of acrylic acid, 10.1 g of a sodium hydroxide solution (50% by weight in water), 0.49 g of a diethylenetriaminepentaacetic acid solution (40% by weight in water), 0.02 g of a tert-butyl hydroperoxide solution (70% by weight in water), 0.006 g of sodium hypophosphite and 9.134 g of water.

[0111] An organic phase is prepared by mixing 20.1 g of an oil (Exxsol® DI20 S) with 2.3 g of sorbitan monooleate, 0.5 g of sorbitan monooleate ethoxylated 5 times and 5 g of a surfactant polymer.

[0112] The aqueous phase is added to the organic phase under shear to form an emulsion. The emulsion is then degassed with a stream of nitrogen for 30 minutes while maintaining the temperature at 20°C. After this time, 0.75 g of sodium metabisulfite solution (0.01% by weight in water) is injected for 90 minutes. The polymerization temperature is maintained between 40 and 55°C. The residual monomers are reacted by adding 0.4 g of a sodium bisulfite solution (40% by mass concentration).

[0113] An inverse emulsion El containing 33% by weight of a PI copolymer of acrylamide and sodium acrylate is thus obtained.

[0114] Example 2 - Preparation of a DI dispersion of PI polymer

[0115] The inverse emulsion El of PI polymer obtained in Example 1 is heated to 90°C in a reduced-pressure atmosphere (100 millibars) to evaporate the water and the lighter fractions of the oil. A dispersion DI containing 55.5% by weight of PI polymer is thus obtained. This dispersion contains less than 10% water.

[0116] Example 3 (counterexample) - Preparation of a PI polymer injection fluid Fl

[0117] An inverting agent, ethoxylated fatty alcohol (Lutensol TO89®) is added at a rate of 10% by weight to the dispersion Dl. An injection fluid Fl containing 50% by weight of PI polymer is thus obtained.

[0118] Example 4 - Preparation of an F2 injection fluid of PI polymer

[0119] The DI dispersion obtained in Example 2 is diluted by adding 18% of a solution aqueous solution saturated with sodium chloride and 10% reversing agent (Lutensol TO89®). An F2 injection fluid containing 40% by weight of PI polymer is thus obtained.

[0120] Example 5 - Preparation of an F3 injection fluid of PI polymer

[0121] The DI dispersion obtained in Example 2 is diluted by adding 18% by weight from a saturated aqueous solution of ammonium chloride and 10% of reversing agent (Lutensol TO89®). An F3 injection fluid containing 40% by weight of PI polymer is thus obtained.

[0122] Example 6 - Preparation of an F4 injection fluid of PI polymer

[0123] The DI dispersion obtained in Example 2 is diluted by adding 18% by weight from a saturated aqueous solution of ammonium thiocyanate and 10% of reversing agent (Lutensol TO89®). An F4 injection fluid containing 40% by weight of PI polymer is thus obtained.

[0124] Example 7 - Preparation of an F5 injection fluid of PI polymer

[0125] The DI dispersion obtained in Example 2 is diluted by adding 18% by weight from a saturated aqueous solution of ammonium sulfate and 10% of reversing agent (Lutensol TO89®). An F5 injection fluid containing 40% by weight of PI polymer is thus obtained.

[0126] Example 8 - Preparation of an F6 injection fluid of PI polymer

[0127] The DI dispersion obtained in Example 2 is diluted by adding 18% by weight from an aqueous solution saturated with ammonium thiosulfate and 10% of an inverting agent (Lutensol TO89 ®). An F6 injection fluid containing 40% by weight of PI polymer is thus obtained. Inversion tests

[0128] The injection fluids Fl (comparative) and F2 to F6 (according to the invention) are dissolved following two different protocols.

[0129] Protocol 1 (direct addition of injection fluids to a brine)

[0130] Prepare synthetic seawater by dissolving 30 g of NaCl and 3 g of CaCl2 in 1000 mL of water.

[0131] Under agitation (using three blades) - 500 rpm - inject X g of dispersion into Y mL of synthetic seawater at 20°C.

[0132] Where X equals 5 and Y equals 495 for the Fl dispersion.

[0133] Where X is 6.25 and Y is 493.75 for dispersions F2 to F6.

[0134] Allow to stir for 20 minutes, a solution of 5 g / L in PI polymer is then obtained.

[0135] The viscosity of this solution is then measured using a Brookfield viscometer at 25 °C with an LVT type module at 30 rpm.

[0136] Protocol 2 (addition of injection fluids to water then addition of salts)

[0137] Under stirring - 500 rpm - inject X g of dispersion into Y mL of deionized water at 20°C.

[0138] Where X equals 5 and Y equals 478.5 for the Fl dispersion.

[0139] Where X is 6.25 and Y is 477.25 for dispersions F2 to F6.

[0140] Let it stir for 20 minutes, then add 15 g of NaCl and 1.5 g of CaCl2 to the solution.

[0141] Allow to stir for 10 minutes, a solution of 5 g / L in PI polymer is then obtained.

[0142] The viscosity of this solution is then measured using a Brookfield viscometer at 25 °C with an LVT type module at 30 rpm. Experimental results Injection fluid Concentration PI by weight (%) Salt in brine (S) Viscosity (cP) protocol 1 Viscosity (cP) protocol 2 Fl 50 - 89 235 F2 40 NaCl 180 245 F3 40 NH4Cl 160 240 F4 40 NH4SCN 100 235 F5 40 (NH4)2SO4 165 240 F6 40 (NH4)2S2O3 195 245

[0144] Table 1: Viscosity of 5 g / L PI polymer solutions in synthetic seawater

[0145] Following protocol 2, regardless of the injection fluid, the resulting viscosity is 240 centipoise + / -5. The PI polymer is rapidly released in deionized water. The subsequent addition of salts no longer induces differences between the various viscosities. However, this protocol 2 is not applicable in the field because the polymer is dissolved directly in brines.

[0146] Following protocol 1, it is observed that the injection fluid Fl (counter-example) generates very little viscosity (89 cP).

[0147] By using F2 to F6 injection fluids, the viscosity of the PI polymer solutions in seawater is higher. The PI polymer in these injection fluids is released more readily into the seawater. The injection fluid preparation process according to the invention therefore improves the release of the polymer into the brine.

Claims

1. Claims Injection fluid F for hydraulic fracturing comprising at least one synthetic water-soluble polymer P with a weight-average molecular weight greater than or equal to 1 million daltons, obtained from the following water-soluble monoethylenically unsaturated non-ionic and / or anionic and / or cationic and / or zwitterionic monomers: - the non-ionic monomers being chosen from the group comprising acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N,N-dialkyl acrylamides, N,N-dialkylmethacrylamides, alkoxylated esters of acrylic acid, alkoxylated esters of methacrylic acid, N-vinylpyridine, N-vinylpyrrolidone, hydroxyalkylacrylates, hydroxyalkyl methacrylates, - the anionic monomers being chosen from the group comprising monomers having a carboxylic function and their salts, including acrylic acid, methacrylic acid, itaconic acid, maleic acid; monomers having a sulfonic acid function and their salts, including acrylamido tert-butyl sulfonic acid (ATBS), allyl sulfonic acid and methallyl sulfonic acid and their salts; monomers having a phosphonic acid function and their salts, - the cationic monomers being chosen from the group comprising quaternized or salified dimethylaminoethyl acrylate (ADAME); quaternized or salified dimethylaminoethyl methacrylate (MADAME); diallyldimethylammonium chloride (DADMAC); acrylamidopropyltrimethylammonium chloride (APTAC);methacrylamidopropyltrimethylammonium chloride (MAPTAC), - the zwitterionic monomers being chosen from the group comprising sulfobetaine monomers such as sulfopropyl dimethylammonium ethyl methacrylate, sulfopropyl dimethylammonium propylmethacrylamide, sulfopropyl 2-vinylpyridinium; phosphobetaine monomers, such as phosphato ethyl trimethylammonium ethyl methacrylate; carboxybetaine monomers, said injection fluid being prepared according to a process comprising the following successive steps:; a) An inverse emulsion E is prepared by radical polymerization, comprising between 15% and 40% by weight of polymer P, between 20 and 60%

2.

3.

4. by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the emulsion E, b) The inverse emulsion E is distilled to obtain a dispersion D comprising between 40 and 60% by weight of polymer particles P, less than 10% by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the dispersion D, c) The dispersion D is diluted with 1% to 15% by weight of an aqueous solution S comprising between 20 and 60% of salts, the percentages being expressed by weight relative to the weight of the dispersion D. Injection fluid F according to claim 1, characterized in that the inverse emulsion E comprises from 0.5% to 5.0% by weight, the percentages being expressed by weight relative to the weight of the emulsion E, of at least one emulsifying agent chosen from sorbitan esters, polyethoxylated sorbitan esters, polyethoxylated fatty acids, polyethoxylated fatty alcohols, polyesters having an average molecular weight of between 1000 and 3000 daltons resulting from the condensation between a poly(isobutenyl) succinic acid or its anhydride and a polyethylene glycol, block copolymers having an average molecular weight of between 2500 and 3500 daltons resulting from the condensation between hydroxystearic acid and a polyethylene glycol, ethoxylated fatty amines, derivatives of di-alkanol amides, copolymers of stearyl methacrylate, and mixtures of these emulsifying agents. Injection fluid F according to claim 1, characterized in that the particles of synthetic water-soluble polymer P in the dispersion D have an average size of between 0.01 pm and 100 pm. Injection fluid according to claim 1, characterized in that between 0.2 and 10% by weight, the percentages being expressed by weight relative to the weight of the dispersion D, of at least one reversing agent, chosen from ethoxylated non-nylphenols, preferably having 4 to 10 ethoxylations; ethoxylated / propoxylated alcohols preferably having an ethoxylation / propoxylation comprising between 12 and 25 carbon atoms; ethoxylated tridecyl alcohols; polyethoxylated fatty acids, poly(ethoxylated / propoxylated) fatty alcohols; ethoxylated sorbitan esters; polyethoxylated sorbitan laurate; polyethoxylated castor oil; heptaoxyethylated lauryl alcohol; polyethoxylated sorbitan monostearate; alkyl phenol polyethoxylated cetyl ether; polyoxide ethylene alkyl aryl ether; N-cetyl-N-ethyl morpholinium ethosulfate; sodium lauryl sulfate; condensation products of fatty alcohols with ethylene oxide; condensation products of alkylphenols and ethylene oxide; condensation products of fatty amines with 5 or more molar equivalents of ethylene oxide; ethoxylated tristyryl phenols; condensates of ethylene oxide with polyhydric alcohols partially esterified with fatty chains and their anhydrous forms; amine oxides; alkyl polyglucosides; glucamide; phosphate esters; alkylbenzene sulfonic acids and their salts; water-soluble surfactant polymers; and mixtures of several of these reversing agents.

5. Injection fluid F according to claim 1, characterized in that at least a portion of the water of the solution S is extracted from the distillate of the emulsion E.

6. Injection fluid F according to claim 1, characterized in that the salts of the aqueous solution S are alkali or alkaline-earth or ammonium or organic salts or a mixture of these salts.

7. Injection fluid F according to claim 6, characterized in that the salts are chosen from sodium chloride, ammonium sulfate, ammonium thiosulfate, ammonium chloride, choline chloride, monosaccharides or a mixture of these salts.

8. Injection fluid F according to claim 1, characterized in that at least one supporting agent is added before step a), and / or between step a) and step b), and / or between step b) and step c), and / or after step c) of its preparation process.

9. Process for preparing an injection fluid F comprising the following steps: a) An inverse emulsion E is prepared by radical polymerization comprising between 15% and 40% by weight of a synthetic water-soluble polymer P with a weight-average molecular weight greater than or equal to 1 million daltons, between 20 and 60% by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the emulsion E, b) The inverse emulsion E is distilled to obtain a dispersion D comprising between 40 and 60% by weight of polymer particles P, less than 10% by weight of water and at least one hydrocarbon solvent, the percentages being expressed by weight relative to the weight of the dispersion D, c) Dispersion D is diluted with 1% to 15% by weight of an aqueous solution S comprising between 20 and 60% by weight of salts, the percentages being expressed by weight relative to the weight of dispersion D

10. A method of hydraulic fracturing an unconventional underground oil or gas reservoir, comprising preparing an injection fluid F according to one of claims 1 to 5, dissolving it in salt water and injecting said fracturing fluid into an underground formation.

11. A method of reducing friction in a hydraulic fracturing operation of an unconventional underground oil or gas reservoir, comprising preparing a fluid F according to one of claims 1 to 5, dissolving it in salt water and injecting said fracturing fluid into an underground formation.