EP4402220A1

EP4402220A1 - Dispersion of water-soluble polymer for hydraulic fracturing

Info

Publication number: EP4402220A1
Application number: EP22789177.7A
Authority: EP
Inventors: Cédrick FAVERO; Bruno Tavernier; Sebastien Coccolo; Siham TELILEL
Original assignee: SNF Group
Current assignee: SNF Group
Priority date: 2021-09-15
Filing date: 2022-09-13
Publication date: 2024-07-24
Also published as: CN118043430A; FR3126988A1; WO2023041539A1; AR127048A1; CA3230987A1

Abstract

The present application relates to an 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, said fluid being prepared according to a method comprising the following successive steps: a) preparing, by radical polymerization, an inverse emulsion E comprising between 15% and 40% by weight of polymer P, between 20% and 60% by weight of water and at least one hydrocarbon-based solvent, the percentages being expressed by weight relative to the weight of the emulsion E, b) distilling the inverse emulsion E to obtain a dispersion D comprising between 40% and 60% by weight of particles of polymer P, less than 10% by weight of water and at least one hydrocarbon-based solvent, the percentages being expressed by weight relative to the weight of the dispersion D, c) diluting the dispersion D 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.

Description

TITLE: WATER SOLUBLE POLYMER DISPERSION FOR HYDRAULIC FRACTURING

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 diluting a previously distilled inverse emulsion of said polymer with brine. The invention also relates to a method for the hydraulic fracturing of underground reservoirs of unconventional oil and gas using said injection fluid.

PRIOR ART OF THE TECHNIQUE

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

In the rest of the description of the prior art and of the invention, by “unconventional underground reservoirs”, we designate deposits requiring particular extraction technologies because they do not exist in the form of an accumulation in a rock. porous and permeable (see Source rock hydrocarbons in France Provisional report - CGIET n° 2011-04-G - Ministry of ecology, sustainable development, transport and housing - April 2011). For unconventional gas, mention may be made of shale gas, coal bed methane or tight gas. For unconventional oil, we can cite heavy oil, shale oil or tight oil.

The reserves contained in unconventional reservoirs are enormous and extremely extensive in areas that were once unexploitable as source rock hydrocarbons such as shale, tight gas, and coal gas. In the United States, shale gas is widely exploited and today represents 46% of the total natural gas produced in the United States, whereas it represented only 28% in 1998. The very extensive basins are known as Barnett Shale, Ville Fayette Shale, Mowry Shale, Marcellus Shale, Utica Shale... The exploitation of compact reservoirs has been made possible by an evolution in drilling techniques.

Production techniques have in fact evolved from vertical wells to horizontal wells, reducing the number of production wells required and their footprint in ground and allowing better coverage of the volume of the tank to recover as much gas or oil as possible. However, the permeabilities are insufficient for the hydrocarbon to migrate easily from the source rock to the well, and thus allow the gas or oil to be produced economically and in quantity. It is therefore necessary to increase the permeability and the production surfaces by stimulation operations and in particular by hydraulic fracturing of the rock in contact with the well.

Fracturing

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

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.

Reservoir study, modeling, drilling, cementing and stimulation technologies have become more and more sophisticated and implement equipment allowing these operations to be carried out in increasingly shorter times with precise analysis. results.

Reservoir stimulation by fracturing

These operations consist of injecting water at high pressure and at a very high rate so as to create fractures distributed perpendicular to the production wells. One generally proceeds in several stages in order to create fractures along the entire length of the horizontal well, which makes it possible to cover a maximum volume of the reservoir.

In order to keep these fractures open, a propping agent is added (for example sand, plastic materials or calibrated ceramics) so as to prevent these fractures from closing and to maintain the capillarity created once the injection has stopped. Water alone is not sufficient to achieve good proppant placement efficiency due to its low viscosity. This limits its ability to hold the proppant in place in the fractures. To counter this problem, fracturing fluids containing viscosifying compounds have been developed.

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

In addition to having viscosifying properties, the compound must have a particular rheological profile. Indeed, the compound must be able to generate a low viscosity so as not to interfere with the transport and pumping of the fluid containing the proppant during the strong shears undergone during the injection of the fracturing fluid. Once injected, this same compound must be able to generate sufficient viscosity when the shear decreases to support the proppant in order to maintain it in the fractures.

Fracturing 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 maintain the proppant in suspension at the level of the fracture when the shear decreases.

The viscoelastic properties of polymers in solution are also to be considered. This viscoelasticity, and its importance in 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 even by rheology measurements, such as the measurement of the viscous and elastic moduli (G' and G"), or the measurement on a rheometer of the viscosity depending on the shear. Thus, elastic properties will be advantageous to ensure the transport and suspension of the fracture proppant.

The choice of polymer is therefore not obvious and requires an in-depth rheological study in order to obtain satisfactory results.

Among the viscosifying compounds of aqueous solutions belonging to the state of the art, mention may be made of natural substances such as guar gums and their derivatives such as hydroxypropylguar (H PG), or carboxymethylhydroxypropyl guar (CMHPG); cellulose derivatives such as carboxymethyl cellulose or hydroxyethyl cellulose. These compounds are described in particular in patents US4033415, US3888312 and US4801389. In 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 advancements in fracturing fluid performance are discussed in detail.

However, these natural substances, and in particular guar derivatives, are also useful in other applications, such as the food industry or the textile industry, and the development of the exploitation of unconventional oil and gas resources competes these other applications. This creates pressure on the availability of these products and causes price problems.

Other petrochemical compounds can have viscosifying properties. Mention may be made of synthetic polymers. Poly(meth)acrylamides, optionally partially hydrolyzed, and poly(meth)acrylates and their copolymers are particularly known. These polymers develop viscosity thanks to their molar mass and the inter-chain ionic repulsions. These polymers are described in patents GB951 147, US3727689, US3841402 or even US3938594. The mechanism governing viscosity is related to an increase in hydrodynamic volume through intra-chain repulsions, inter-chain entanglements, etc.

However, in the presence of high salinity or a high temperature of use, these polymers do not develop strong entanglements and repulsions, which results in a strong reduction in their viscosifying power, especially after having undergone the shearing of the step pumping. Furthermore, these polymers generally do not have sufficient viscoelastic properties to support the proppant in the fracture. The dosage of these polymers must be increased to levels too high to achieve the suspending properties of the proppant. However, the necessary dosage levels are not economically viable. The polymers used to have viscosifying properties can advantageously also be friction reducers making it possible to reduce the pressure drop in a turbulent medium and greatly increase the flow rate at identical power and pipe diameter.

Synthetic polymers based on 2-acrylamido-2-methylpropane sulfonic acid and/or its salts have interesting friction reduction properties in aqueous solution. These polymers are also known for their resistance to shear and thermal degradation, especially in salt solutions. However, obtaining very high molecular weight polymer based on 2-acrylamido-2-methylpropane sulfonic acid is difficult, not to mention that these polymers have solubility problems when their molecular weight increases. However, to have an optimal phenomenon of friction reduction and a strong generation of viscosity, it is essential that the polymer is dissolved quickly in particular in saline solution and that it has a very high molecular weight.

For questions of logistics, transport and supply of synthetic water-soluble polymer for hydraulic fracturing, the preferred physical form of these polymers is the powder because it makes it possible to have a high percentage by weight of active material. The physical powder form of these polymers can be obtained by drying, thermo-drying, spraying (spray drying in English) and drum drying (drum drying in English). However, their re-dissolution requires suitable equipment, for example a wet grinding unit for powders such as a PSU (Polymer Slicing Unit).

Inverse polymer emulsions are also interesting, but they require rigorous optimization of their formulation so that their inversion in an aqueous medium is the fastest and their stability (during storage and transport) is guaranteed (especially during freeze/thaw cycles) .

DISCLOSURE OF THE INVENTION

The Applicant has found and developed an injection fluid for hydraulic fracturing which makes it possible to have high friction reduction and viscosifying effects. This injection fluid is in the form of a water-soluble polymer dispersion and it is prepared by diluting a previously distilled inverse polymer emulsion with brine. Surprisingly, the prior dilution of the distilled polymer emulsion (polymer dispersion) with a brine significantly increases its ability to subsequently invert in the injection salt waters (during injection into Training underground), which implies effective (rapid and almost total) dissolving of the polymer to maximize its application effect.

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

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 with a 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) An inverse emulsion E 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 s percentages being expressed by weight relative to the weight of dispersion D.

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

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

As used herein, the term "water-soluble polymer" means a polymer which yields an aqueous solution without insoluble particles when dissolved with stirring for 4 hours at 25°C and with a concentration of 20 gL ^-1 in water .

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 in plotting the reduced viscosity values (on the ordinate axis) as a function of the concentrations (on the abscissa axis) and by extrapolating the curve at zero concentration. The intrinsic viscosity value is read on the ordinate axis or using the least squares method. Then the weight average molecular weight can be determined by the famous Mark-Houwink equation:

[q] = KM ^a

[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, 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.

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 comprised between 1 million daltons and 20 million daltons, even more advantageously comprised between 1.5 million daltons and 20 million daltons, even more advantageously comprised between 2 million daltons and 20 million daltons.

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

- a hydrophilic phase comprising at least one water-soluble structured polymer

- a lipophilic phase;

- at least one emulsifying agent;

The lipophilic phase can be a mineral oil, a vegetable oil, a synthetic oil or a mixture of several of these oils. Examples of mineral oil are mineral oils comprising saturated hydrocarbons of the aliphatic, naphthenic, paraffinic, isoparaffinic, cycloparaffinic or naphthyl type. Examples of synthetic oil are hydrogenated polydecene or hydrogenated polyisobutene, esters such as octyl stearate or butyl oleate. Exxon's Exxsol® product line is a perfect fit.

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

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

The hydrophilic-lipophilic balance (HLB) of a chemical compound is a measure of its degree of hydrophilicity or lipophilicity, determined by calculating the values of 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).

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 about water and oil solubility. Substances with an HLB value of 10 are distributed between the two phases, so that the hydrophilic group (molecular mass Mh) projects completely into the water while the hydrophobic hydrocarbon group (molecular mass Mp) is adsorbed in the non-phase. watery.

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

The inverse emulsion E according to the invention is prepared by radical polymerization. An aqueous solution comprising the monomer(s) making it possible to obtain the polymer P is emulsified in an oily phase comprising the emulsifying agent(s). Then, the polymerization is carried out by adding a free radical initiator. Reference may be made, as initiator, to redox couples, with cumene hydroperoxide, tertiary butylhydroxyperoxide or persulfates among the oxidizing agents, sodium sulfite, sodium metabisulfite and Mohr's salt among the agents reducers. Azo compounds such as 2,2'-azobis (isobutyronitrile) and 2,2'-azobis (2-amidinopropane) hydrochloride can also be used. Conventionally, the polymerization is generally carried out in an isothermal, adiabatic or temperature-controlled manner. That is, the temperature is kept constant, usually between 10 and 60°C (isothermal), or the temperature is allowed to rise naturally (adiabatic) and in this case the reaction is usually started at a lower temperature at 10°C and the final temperature is usually above 50°C or, finally, the temperature increase is controlled so as to have a temperature curve between the isothermal curve and the adiabatic curve (controlled temperature).

The polymerization can be carried out under a pressure below atmospheric pressure, optionally under conditions making it possible to evaporate part of the water and of the hydrocarbon solvent from the reaction medium and to preconcentrate the emulsion.

The synthetic water-soluble polymer P preferably results from the polymerization of monounsaturated ethylenic monomers which may be nonionic and/or anionic and/or cationic and/or zwiterrionic. These monomers are preferably the following:

- nonionic monomers chosen from the group comprising acrylamide, 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,

- anionic monomers 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 sulphonic acid function and their salts, including acrylamido tert-butyl sulphonic acid (ATBS), allyl sulphonic acid and methallyl sulphonic acid and their salts; monomers having a phosphonic acid function and their salts,

- cationic monomers chosen from the group comprising quaternized or salified dimethylaminoethyl acrylate (ADAME); dimethylaminoethyl methacrylate (MADAME) quaternized or salified; diallyldimethylammonium chloride (DADMAC); acrylamidopropyltrimethylammonium chloride (APT AC); methacrylamidopropyltrimethylammonium chloride (MAPTAC),

- zwitterionic monomers chosen from the group comprising sulfobetaine monomers such as sulfopropyl dimethylammonium ethyl methacrylate, sulfopropyl dimethylammonium propylmethacrylamide, sulfopropyl 2-vinylpyridinium; THE phosphobetaine monomers, such as phosphato ethyl trimethylammonium ethyl methacrylate; carboxybetaine monomers.

Optionally, the water-soluble polymer P can comprise at least one LCST or UCST group.

According to the general knowledge of those 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 according to salinity. This is a group with a transition temperature by heating defining its lack of affinity with the solvent medium. The lack of affinity with the solvent results in an opacification or a loss of transparency which may be due to precipitation, aggregation, gelling or viscosification of the medium. The minimum transition temperature is called "LCST" (lower critical solubility temperature, from the acronym "Lower Critical Solution Temperature"). For each group concentration at LCST, a heating transition temperature is observed. It is greater than the LCST which is the minimum point of the curve. Below this temperature, the (co)polymer is soluble in water, above this temperature, the (co)polymer loses its solubility in water.

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 is modified below a certain temperature and according to salinity. This is a group with a cooling transition temperature defining its lack of affinity with the solvent medium. The lack of affinity with the solvent results in an opacification or a loss of transparency which may be due to precipitation, aggregation, gelling or viscosification of the medium. The maximum transition temperature is called “UCST” (upper critical solubility temperature, from the acronym “Upper Critical Solution Temperature”). For each group concentration at UCST, a cooling transition temperature is observed. It is greater than the LCST which is the minimum point of the curve. Above this temperature, the (co)polymer is soluble in water, below this temperature, the (co)polymer loses its solubility in water.

The water-soluble polymer P in the inverse emulsion E can be linear or structured by at least one structural agent, which can be chosen from the group comprising polyethylenically unsaturated monomers (having at least two unsaturated functions), such as, for example, vinyl functions , allylics, acrylics and epoxies and one can cite for example methylene bis acrylamide (MBA), diallylamine, triallylamine, tetraallylammonium chloride, polyethylene glycol dimethacrylate or even by macroinitiators such as polyperoxides, polyazos and polytransfer agents such as polymercaptants polymers or even hydroxyalkylacrylates, epoxyvinyls.

The water-soluble polymer P can also be structured using techniques of controlled radical polymerization (CRP) or, and more particularly, of the RAFT (Reversible Addition Fragmentation Chain Transfer) type in inverse emulsion.

According to another preference, the inverse emulsion E of water-soluble polymer P may comprise: a hydrophilic phase comprising at least one water-soluble (co)polymer P, a lipophilic phase, at least one interfacial polymer composed of at least one monomer of formula ( I): [Chem 1]

Formula (I) in which,

Ri, R ₂ , R3 are independently selected from the group consisting of a hydrogen atom, a methyl group, a carboxylate group and ZX,

Z is selected from the group consisting of C(=0)-0; C(=O)-NH; O-C(=O); NH-C(=O)-NH; NH-C(=0)-0; and a carbon chain comprising from 1 to 20 unsaturated or unsaturated, substituted or unsubstituted carbon atoms which may comprise one or more heteroatoms, preferably 1, 2 or 3 heteroatoms, chosen from nitrogen and oxygen,

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

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. In general, the hydrophilic phase is in the form of micrometric droplets dispersed, advantageously emulsified, in the lipophilic phase. The average size of these droplets is advantageously between 0.01 and 30 μm, more advantageously between 0.05 and 3 μm. The interfacial polymer therefore comes to be placed at the interface between the hydrophilic phase and the lipophilic phase at the level of each droplet. The interfacial polymer partially or totally envelopes each of these droplets. The average size of the droplets is advantageously measured with a laser measuring device using conventional techniques which form part of the general knowledge of those skilled in the art. A Mastersizer type device from the Malvern company can be used for this purpose.

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%, by number of monomer of formula (I), relative to the number total monomers.

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

Preferably, 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 preferably chosen from esters of sorbitan, 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 a poly(isobutenyl) succinic acid or its anhydride and a polyethylene glycol , block copolymers with 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, stearyl methacrylate copolymers, and mixtures of these emulsifying agents. This emulsifying agent is added to the lipophilic phase prior to the radical polymerization reaction.

Optionally, a natural or synthetic polymer (described in particular in US Pat. No. 10,647,908) can be added at the end of the radical polymerization reaction of step a) of the process for obtaining the fluid F. Among the natural polymers there are, for example, the guar gums, and their derivatives such as hydroxypropylguar (HPG), or carboxymethylhydroxypropyl guar (CMHPG); cellulose derivatives such as carboxymethyl cellulose, hydroxyethyl cellulose or hydroxypropyl cellulose. Advantageously, the polymer P predominates with respect to the natural polymer, preferably the polymer P represents from 50 to 100% by weight, preferably from 70 to 100%, more preferably from 90 to 100%, relative to the total weight of the polymer P + natural polymer.

Step b) of the process for obtaining the injection fluid F consists in distilling the inverse emulsion E to obtain a dispersion D, the polymer P is therefore, after distillation, in the form of (solid) particles, 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. The distillation is carried out under reduced pressure , preferably at a pressure of between 20 and 250 mbar and at a temperature of between 10 and 110°C.

Preferably, in the injection fluid of the invention, the polymer P is in the form of (solid) particles.

Preferably, the particles of synthetic water-soluble polymer P in dispersion D have an average size of between 0.01 μm and 100 μm, even more preferably between 0.1 μm and 5 μm. By average size is meant in the present invention the average diameter of the particles. The particle size analysis is carried out according to conventional techniques known to those skilled in the art. An example of a device for measuring mean particle diameter is the Mastersizer from Malvern Instruments.

At the end of the distillation and therefore before step c) of the process for obtaining the injection fluid F of the invention, preferably added to the dispersion (D) 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. The reversing agent is preferably chosen from ethoxylated nonylphenol, 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; polyethylene oxide alkyl aryl ether; N-cetyl-N-ethyl morpholinium ethosulfate; sodium lauryl sulphate; 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 partially esterified polyhydric alcohols with fatty chains as well as their anhydrous forms; amine oxides; alkyl polyglucosides; glucamide; phosphate esters; alkylbenzene sulfonic acids and their salts; surfactant water-soluble polymers; and mixtures of several of these reversing agents.

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

Preferably at least a portion of the water of the solution S is extracted from the distillate of the emulsion E.

Advantageously, the salts of the aqueous solution S are alkaline or alkaline-earth or ammonium salts or organic salts or a mixture of these salts. More preferentially, the salts are chosen from sodium chloride, ammonium sulphate, ammonium thiosulphate, ammonium chloride, choline chloride, monosaccharide salts or a mixture of these salts.

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

Biocides to prevent the development of bacteria, in particular sulphate-reducing bacteria, which can form viscous masses that reduce passage surfaces. Mention may be made, for example, of glutaraldehyde, which is the most widely used, or else formaldehyde or isothiazolinones, and/or

Oxygen scavengers such as ammonium bisulphite 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 quantities of oxygen, such as N,N-dimethylformamide, and/or

Lubricants such as oil distillates, and/or

Iron chelators such as citric acid, EDTA (ethylene diamine tetraacetic acid), phosphonates, and/or

Scale inhibitors such as phosphates, phosphonates, polyacrylates or ethylene glycol.

The present invention therefore also relates to a process for the preparation of an injection fluid F comprising the following steps: a) An inverse emulsion E comprising between 15% and 40% by weight of a synthetic water-soluble polymer P of 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) 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 scatter d.

The process steps are as described above.

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

The injection is carried out under pressure so as to create fractures distributed all along the production well. Prior to its injection into the underground formation, the injection fluid F obtained by the method of the invention is dissolved in salt water to have a polymer concentration of between 0.01 and 10 g/L in this water. salty.

The salt waters are sea waters or can advantageously be prepared with monovalent and/or polyvalent salts or combinations thereof.

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

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

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.

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

Preferably, for the injection of the injection fluid F into the underground formation, at least one proppant is added before or after it is dissolved in salt water.

The proppant can be chosen without limitation from sand, ceramic, bauxite, glass beads, and sand impregnated with resin. It advantageously represents from 0.5 to 40%, more preferentially from 1 to 25% and even more preferentially from 1.5 to 20%, by weight relative to the total weight of the injection fluid F for hydraulic fracturing.

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. The injection of surfactant makes it possible to eliminate the wettability with the rock, while the injection of the oxidizing compound destroys the copolymer. In both cases, the injection restores a fluid viscosity close to that of water.

As oxidizing compound, mention may be made of bleach (aqueous solution of a hypochlorite salt), hydrogen peroxide, ozone, chloramines, persulphates, permanganates or perchlorates.

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

Preferably, the surfactant compounds used are chosen from anionic surfactants and their zwitterions chosen from the group comprising derivatives of alkyl sulphates, alkyl ether sulphates, arylalkyl sulphates, arylalkyl ether sulphates, alkyl sulphonates, alkyl ether sulphonates, d arylalkyl ether carboxylates, alkyl polyethers, arylalkyl polyethers...

The term alkyl chain is defined as a chain of 6 to 24 carbons, branched or not, with or without several units, possibly comprising one or more heteroatoms, for example O, N or S, preferably 1, 2 or 3 heteroatoms. Arylalkyl chain is defined as a chain of 6 to 24 carbons, branched or not, comprising one or more aromatic rings and possibly comprising one or more heteroatoms, for example 1, 2 or 3 heteroatoms, preferably O, N or S.

The most commonly used surfactants, for reasons of cost, stability, and availability, are of the sulphonate or sulphate type, presented in the form of alkali metal or ammonium salts.

Finally, a last aspect of the invention relates to a method for reducing friction during a hydraulic fracturing operation of an underground reservoir of oil or untreated gas. conventional, comprising the preparation of a fluid F as described previously, the dissolving in salt water and the injection of said fracturing fluid into an underground formation.

Friction reduction makes it possible to reduce or eliminate friction-related losses during the injection of fracturing fluid.

EXAMPLES OF REALIZATION OF THE INVENTION

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

The proportions of the various compounds are given in percentage by weight relative to the final weight of the emulsions. - Preparation of an inverse emulsion E1 of polymer P1

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 ( at 50% by weight in water), 0.49 g of a solution of diethylenetriaminepentaacetic acid (at 40% by weight in water), 0.02 g of a solution of tert-butyl hydroperoxide (at 70% by weight in water), 0.006 g of sodium hypophosphite and 9.134 g of water.

An organic phase is prepared by mixing 20.1 g of an oil (Exxsol® D120 S) with 2.3 g of sorbitan monooleate, 0.5 g of 5-fold ethoxylated sorbitan monooleate and 5 g of a surfactant polymer .

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. At the end of this time, 0.75 g of sodium metabisulphite in solution (at 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 bisulphite solution (at 40% by mass concentration).

An inverse emulsion E1 containing 33% by weight of a copolymer P1 of acrylamide and sodium acrylate is thus obtained. - Preparation of a dispersion D1 of polymer P1

The inverse emulsion E1 of polymer P1 obtained in Example 1 is heated to 90° C. in an atmosphere at reduced pressure (100 millibars) in order to evaporate the water and the lighter fractions of the oil. A dispersion D1 containing 55.5% by weight of polymer P1 is thus obtained. This dispersion contains less than 10% water. - Preparation of an injection fluid F1 of polymer P1

An inverting agent, ethoxylated fatty alcohol (Lutensol TO89®) is added at 10% by weight to dispersion D1. An injection fluid F1 containing 50% by weight of polymer P1 is thus obtained.

4 Preparation of an injection fluid F2 of polymer P1

Dispersion D1 obtained in Example 2 is diluted by adding 18% of a saturated aqueous solution of sodium chloride and 10% of inverting agent (Lutensol TO89®). An injection fluid F2 containing 40% by weight of polymer P1 is thus obtained. Preparation of an F3 injection fluid of polymer P1

Dispersion D1 obtained in example 2 is diluted by adding 18% by weight of a saturated aqueous solution of ammonium chloride and 10% of inverting agent (Lutensol TO89®). An injection fluid F3 containing 40% by weight of polymer P1 is thus obtained.

6 Preparation of an F4 injection fluid of polymer P1

Dispersion D1 obtained in example 2 is diluted by adding 18% by weight of a saturated aqueous solution of ammonium thiocyanate and 10% of inverting agent (Lutensol

TO89®). An injection fluid F4 containing 40% by weight of polymer P1 is thus obtained.

7 Preparation of an F5 injection fluid of polymer P1

Dispersion D1 obtained in Example 2 is diluted by adding 18% by weight of a saturated aqueous solution of ammonium sulphate and 10% of inverting agent (Lutensol TO89®). An injection fluid F5 containing 40% by weight of polymer P1 is thus obtained.

8 Preparation of an F6 injection fluid of polymer P1

Dispersion D1 obtained in example 2 is diluted by adding 18% by weight of a saturated aqueous solution of ammonium thiosulfate and 10% of inverting agent (Lutensol TO89®). An injection fluid F6 containing 40% by weight of polymer P1 is thus obtained. Reversal tests

The injection fluids F1 (comparative) and F2 to F6 (according to the invention) are dissolved by following two different protocols.

Protocol 1 (direct addition of injection fluids in brine)

Prepare a synthetic seawater by dissolving 30 g of NaCl and 3 g CaCls in 1000 mL of water.

With stirring (using three blades) - 500 rpm - inject X g of dispersion into Y mL of synthetic seawater at 20°C.

Where X is 5 and Y is 495 for dispersion F1.

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

Leave to stir for 20 minutes, a 5 g/L solution of polymer P1 is then obtained.

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

Protocol 2 (addition of injection fluids in water then addition of salts)

With stirring - 500 rpm - inject X g of dispersion into Y mL of deionized water at 20°C.

Where X is 5 and Y is 478.5 for dispersion F1.

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

Leave to stir for 20 minutes, then add 15 g of NaCl and 1.5 g of CaCls to the solution.

Leave to stir for 10 minutes, a 5 g/L solution of polymer P1 is then obtained.

Experimental results Table 1: Viscosity of solutions at 5 g/L of polymer P1 in synthetic seawater

By following protocol 2, whatever the injection fluid, the viscosity obtained is 240 centipoise +/-5. Polymer P1 releases rapidly in deionized water. The subsequent addition of salts no longer induces differences between the different viscosities. However, this protocol 2 does not apply to fields because the polymer solution is done directly in brines.

By following protocol 1, we observe that the injection fluid F1 (counterexample) generates very little viscosity (89 cP).

By using injection fluids F2 to F6, the viscosity of polymer solutions P1 in seawater is higher. The polymer P1 in these injection fluids is released more easily into the seawater. The process for preparing the injection fluids according to the invention therefore makes it possible to improve the release of the polymer into the brine.

Claims

22 CLAIMS

1 . 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, prepared according to a process comprising the following successive steps: a) A inverse emulsion E 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 per 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.

2. Injection fluid F according to claim 1, characterized in that the synthetic water-soluble polymer P is obtained from the following nonionic and/or anionic and/or cationic and/or zwiterrionic water-soluble monoethylenically unsaturated monomers:

- the nonionic monomers being chosen from the group comprising acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N,N-dialkylacrylamides, N,N-dialkylmethacrylamides, alkoxylated esters of acid acrylic, 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 sulphonic acid function and their salts, including acrylamido tert-butyl sulphonic acid (ATBS), allyl sulphonic acid and methallyl sulphonic 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); the methacrylate of dimethylaminoethyl (MADAME) quaternized or salified; 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.

3. 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 , 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 with 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.

4. 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 μm and 100 μm.

5. Injection fluid according to claim 1, characterized in that there is added to the dispersion D before step c) 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 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; alkyls polyethoxylated phenol cetyl ether; polyethylene oxide alkyl aryl ether; N-cetyl-N-ethyl morpholinium ethosulfate; sodium lauryl sulphate; 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 partially esterified polyhydric alcohols with fatty chains as well as their anhydrous forms; amine oxides; alkyl polyglucosides; glucamide; phosphate esters; alkylbenzene sulfonic acids and their salts; surfactant water-soluble polymers; and mixtures of several of these reversing agents.

6. 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.

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

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

9. Injection fluid F according to claim 1, characterized in that at least one propping 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.

10. Process for the preparation of an injection fluid F comprising the following steps: a) An inverse emulsion E comprising between 15% and 40% by weight of a synthetic water-soluble polymer P of average molecular weight in 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 emulsion is distilled inverse E 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 25 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 dispersion D.

11. Process for the hydraulic fracturing of an underground oil or unconventional gas reservoir, comprising the preparation of an injection fluid F according to one of claims 1 to 6, the dissolving in salt water and injecting said fracturing fluid into a subterranean formation.

12. Method for reducing friction in a hydraulic fracturing operation of an underground reservoir of unconventional oil or gas, comprising the preparation of a fluid F according to one of claims 1 to 6, the dissolution in salt water and injecting said fracturing fluid into a subterranean formation.