WO2025248142A1

WO2025248142A1 - Inverse emulsion for hydraulic fracturing

Info

Publication number: WO2025248142A1
Application number: PCT/EP2025/065221
Authority: WO
Inventors: Cédrick FAVERO; Hadrien CHOLLAT-NAMY; Rémi THIBAL
Original assignee: SNF Group
Current assignee: SNF Group
Priority date: 2024-05-31
Filing date: 2025-06-02
Publication date: 2025-12-04
Anticipated expiration: 2026-11-30
Also published as: FR3162751A1

Abstract

The present invention relates to an inverse emulsion and the use thereof in a method for the hydraulic fracturing of an unconventional underground hydrocarbon (oil and/or gas) reservoir.

Description

Title of the invention: Inverse emulsion for hydraulic fracturing

Technical field of the invention

The present invention relates to an inverse emulsion and the use thereof in a method for the hydraulic fracturing of an unconventional underground hydrocarbon reservoir (oil and/or gas).

Prior art

The production of hydrocarbons (oil and/or gas) contained in unconventional underground reservoirs has been developing for several years and requires opening fractures in the reservoir to extract hydrocarbons stuck in the rock.

In the remainder of the description of the prior art and of the invention, the term “unconventional underground reservoirs” or “unconventional reservoirs” designates deposits requiring particular extraction technologies because hydrocarbons do not exist in the form of an accumulation in a porous and permeable rock (see Les hydrocarbures de roche-mere en France Rapport provisoire - CGIET n° 2011-04-G Ministere de I’ecologie, du developpement durable, des transports et du logement - April 2011).

For unconventional gas reservoirs, mention may be made, in particular, of shale gas, coal bed methane or tight gas. For unconventional oil reservoirs, mention may be made, in particular, of heavy oil, shale oil or tight oil.

The reserves contained in unconventional reservoirs are substantial and extremely extensive in previously unworkable areas. In the United States, shale gas is widely exploited and now accounts for 80% of all natural gas produced in the United States, whereas it represented only 28% in 1998. The exploitation of tight reservoirs has been made possible by an evolution of drilling techniques.

Production methods have indeed evolved from vertical wells to horizontal wells, reducing the number of production wells required and their footprint on the ground, while making it possible to better cover the volume of the reservoir to recover a maximum of hydrocarbons. However, the permeability of the rocks is insufficient for the hydrocarbons to easily migrate from the source rock to the well. 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.

The purpose of hydraulic fracturing is to create additional permeability to generate larger gas or oil production volumes. These hydraulic fracturing operations on horizontal wells began in 1960 in the Appalachian Mountains and, today, several tens of thousands of operations have taken place throughout the United States. They consist of injecting water at high pressure and very high flow rate so as to create fractures distributed perpendicularly to the production wells. The procedure is generally carried out in several steps 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 (e.g. sand, plastic materials or calibrated ceramic particles) is added so as to prevent these fractures from closing and to maintain the capillarity created after the water injection is stopped.

Water alone is not enough to achieve good propping agents placement efficiency due to its low viscosity. This also limits its ability to hold the propping agent in place in fractures. To solve this problem, fracturing fluids containing viscosifying compounds have been developed.

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

The viscoelastic properties of polymers in solution should also be taken into consideration. This viscoelasticity, and its importance for the application of polymers in the recovery of hydrocarbons, are described in document 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 by rheology measurements, such as the measurement of viscous and elastic moduli (G’ and G”), or the rheometer measurement of viscosity as a function of shear. Thus, elastic properties are sought to ensure the transport and suspension of the propping agent.

Ideally, polymers with adequate viscosifying properties are also friction reducers. In other words, these polymers can advantageously make it possible to reduce the pressure drop in a turbulent medium and thus greatly increase the flow rate at the same power and pipe diameter. This results in a reduction in the energy required to inject the fracking fluid.

High molecular weight linear polymers are commonly used in these applications. The stretching of the polymer chains in solution makes it possible to delay the turbulent regime established during the transport of the fluid at high speed.

These polymers may be in various forms, such as in powder form, in solution or in inverse emulsion form.

When the polymer is in powder form, a dissolution step is necessary. This crucial step is difficult to implement, either because of the need for special logistical equipment, and therefore an additional cost, in addition to their maintenance, or because of the control of the dissolution of high molecular weight polymers in order to avoid the phenomena of agglomeration of the powder and the production of "fish eyes" which can, in addition to deteriorating the application performance, clog the pores during production.

When the polymer is in the form of a solution, it must be diluted before injection in order to avoid the injection of excessively viscous water and cause pore clogging. Furthermore, water transport is not economically and especially not environmentally viable.

The polymer in inverse emulsion form represents a compromise between a polymer in powder form and a polymer in solution form. The inverse emulsion has the advantage of being more concentrated than a solution and of being already ready for use without being viscous.

However, the presence of oils and surfactants in the composition of the emulsion causes an accentuation of the phenomena deteriorating the permeability of the fractures and the interstices between the propping agents, which results in a loss of production.

It is also possible to use dispersions of polymer powder in an oil, i.e. a ready-to-use product, because the powder is previously finely ground (< 300 pm) and dissolves almost instantaneously on contact with water. However, the form of dispersion is very expensive (ground powder + oil) and, because of the presence of oil, also causes risks of deterioration of the underground formation. FR 3 126 988 Al of the Applicant discloses an injection fluid for hydraulic fracturing comprising at least one synthetic water-soluble polymer prepared in inverse emulsion.

US 4 034 809 A relates to a water flooding process for recovering petroleum from subterranean formations comprising the use of a hydrolysed acrylamide polymer from an acrylamide polymer in water-in-oil emulsion prior to hydrolysis.

EP 2 920 270 Bl describes a process for enhanced oil recovery in particular for use in offshore oil production in which an aqueous injection fluid comprising at least a water soluble polyacrylamide-copolymer dissolved in the aqueous fluid is injected through at least one injection borehole into a mineral oil deposit, and crude oil is withdrawn from the deposit through at least one production borehole. The polyacrylamide copolymer is, prior to dissolution, dispersed in a hydrophobic liquid.

These fracture damage (deterioration) phenomena are characterised by an increase of conductivity. The increase in conductivity makes it possible to measure the evolution of the pressure in the fractures. The increase in conductivity makes it possible to determine whether the residues of the fracturing fluid, present in the gaps between the propping agents, are released and do not damage the pores that allow the flow of the hydrocarbons and therefore their recovery.

The Applicant has developed a novel inverse emulsion which makes it possible to significantly improve the increase in conductivity. In addition, this inverse emulsion has improved application performance, particularly with regard to the inversion time. Finally, the use of the inverse emulsion according to the invention makes it possible to reduce the amount of oil present in the reflux water. The reflux water corresponds to the water used to form the fractures and which rises to the surface once the fracturing process has been carried out. This water is difficult to treat by its nature but especially because it is difficult to collect. With less oil present in the inverse emulsion according to the invention, there is less oil in the reflux water and therefore less pollution.

Using the inverse emulsion according to the invention, falls under a principle of environmental awareness and of the impact of industries and man on the planet. By improving the increase in conductivity, the production of hydrocarbons is increased, thus reducing the quantity of greenhouse gases such as CO2 necessary for its extraction. The Applicant has also found that the inverse emulsions according to the invention can be used as thickener for preparing dye printing paste, that may be used in textile manufacturing.

Disclosure of the invention

The present invention relates to an inverse emulsion comprising: i) a hydrophilic phase comprising:

- at least one polymer representing at least 30% by weight based on the total weight of the inverse emulsion;

- at least one hydrophilic solvent representing between 32 and 59% by weight based on the total weight of the inverse emulsion; ii) a lipophilic phase comprising:

- an oil Hi, having a flash point of between 40 and 85°C, representing between 10 and 100% by weight based on the total weight of oil in the lipophilic phase,

- optionally an oil H2, different from the oil Hi, representing between 0 and 90% by weight with respect to the weight of oil in the lipophilic phase,

- the quantity of oil Hi and H2 represents between 10 and 20% by weight with respect to the total weight of the inverse emulsion;

- at least one water-in-oil emulsifying agent representing between 1 and 3% by weight with respect to the total weight of the inverse emulsion; and the inverse emulsion having a weight ratio between the at least one polymer and the oil Hi and optionally H2 of between 1.5 and 4.5.

The present invention also relates to a method for the hydraulic fracturing of an unconventional underground hydrocarbon reservoir (oil and/or gas) comprising the injection of a fracturing fluid comprising at least said inverse emulsion.

The present invention also relates to a fracturing fluid comprising at least said inverse emulsion.

The present invention also relates to a method of preparation of said inverse emulsion.

Finally, the present invention also relates to the use of said inverse emulsion in the fields of hydrocarbon recovery (oil and/or gas); drilling wells; cementing wells; stimulation of hydrocarbon wells (oil and/or gas), other than hydraulic fracturing; for example conformance or diversion; open, closed or semi-closed circuit water treatment; treatment of fermentation broth; sludge treatment; construction; paper or cardboard manufacture; batteries; wood treatment; treatment of hydraulic composition (concrete, cement, mortar and aggregates); mining; formulation of cosmetic products; formulation of detergents; textile manufacture; geothermal energy; manufacture of diapers; or agriculture.

Description of the invention

By "polymer", is meant a homopolymer prepared from one monomer or a copolymer prepared from at least two different monomers. Therefore this may be a polymer of at least one monomer selected from the group consisting of hydrophilic anionic monomers, hydrophilic cationic monomers, hydrophilic non-ionic monomers, hydrophilic zwitterionic monomers, hydrophobic monomers and the mixtures thereof.

The term “hydrophilic monomer” should be understood to mean a monomer that has an octanol -water partition coefficient, K_ow, of less than or equal to 1, in which the partition coefficient K_ow is determined at 25°C in an octanol-water mixture with a volume ratio of 1/1, at a pH of between 6 and 8.

The term “hydrophobic monomer” should be understood to mean a monomer that has an octanol -water partition coefficient, K_ow, greater than 1, in which the partition coefficient K_ow is determined at 25°C in an octanol-water mixture with a volume ratio of 1/1, at a pH of between 6 and 8.

The octanol-water partition coefficient, K_ow, represents the ratio of concentrations (g/L) of a monomer between the octanol phase and the aqueous phase. It is defined as follows:

[Math 2]

> [monomer _octanol o^w [monomer]_water

By "water-soluble polymer", is meant a polymer which gives an aqueous solution without insoluble particle, when it is dissolved under stirring at 25°C and with a concentration of 10 g.l’¹ in deionised water.

“X and/or Y” should be understood to mean “X”, or “Y”, or “X and Y”.

The invention also includes all possible combinations of the various embodiments disclosed, whether they are preferred embodiments or given by way of example. Furthermore, when ranges of values are indicated, the limit values are included in these ranges. The disclosure also includes all of the combinations between the limit values of these ranges of values. For example, the ranges of values “1-20, preferably 5-15” imply disclosure of the ranges “1-5”, “1-15”, “5-20” and “15-20” and the values 1, 5, 15 and 20.

The flash point of the oils is determined according to the standard method ASTM D93 (Pensky-Martens closed cup). This method provides a reliable measurement of the lowest temperature at which the oil vapors form a flammable mixture with air under controlled laboratory conditions.

Inverse emulsion

- an oil Hi, having a flash point of between 40 and 85°C, representing between 10 and 100% by weight based on the total weight of oil in the lipophilic phase;

- optionally an oil H2, different from the oil Hi, representing between 0 and 90% by weight based on the weight of oil in the lipophilic phase;

- the quantity of oil Hi and H2 represents between 10 and 20% by weight based on the total weight of the inverse emulsion;

- at least one water-in-oil emulsifying agent representing between 1 and 3% by weight based on the total weight of the inverse emulsion; and the inverse emulsion having a weight ratio between the at least one polymer and the oil Hi and optionally H2 of between 1.5 and 4.5.

Composition of the polymer

The polymer(s) of the hydrophilic phase of the inverse emulsion are obtained from at least one monomer selected from the group consisting of hydrophilic anionic monomers, hydrophilic cationic monomers, hydrophilic non-ionic monomers, hydrophilic zwitterionic monomers, and the mixtures thereof.

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

“Non-ionic polymer” means a polymer which comprises only hydrophilic non-ionic monomers and optionally hydrophilic zwitterionic and/or hydrophobic monomers.

“Cationic polymer” means a polymer which comprises only hydrophilic cationic monomers and optionally hydrophilic non-ionic monomers and/or hydrophilic zwitterionic and/or hydrophobic monomers.

“Anionic polymer” means a polymer which comprises only hydrophilic anionic monomers and optionally hydrophilic non-ionic monomers and/or hydrophilic zwitterionic and/or hydrophobic monomers.

“Amphoteric polymer” means a polymer which comprises hydrophilic cationic monomers and hydrophilic anionic monomers and optionally hydrophilic non-ionic monomers and/or hydrophilic zwitterionic and/or hydrophobic monomers.

The polymer may be a natural polymer, such as, for example, xanthan gums, guar gums or compounds of the polysaccharide family, or a synthetic or semi-synthetic polymer. Preferably, the polymer is a synthetic polymer, advantageously water-soluble.

“Semi -synthetic polymer” means a natural polymer which has undergone chemical reactions for grafting different synthetic substituents. A person skilled in the art knows this type of reactions, which remain conventional chemical reactions applied to natural polymers.

The polymer may be water-soluble, water-swelling, or a superabsorbent. Preferably, the polymer is water-soluble.

Advantageously, the hydrophilic anionic monomer(s) that may be used in the scope of the invention may be selected from a broad group. These monomers may have a vinyl function, in particular, acrylic, maleic, fumaric, malonic, itaconic, or allylic. They can also contain a carboxylate, phosphonate, phosphate, sulfonate, sulphate group, or another anionic chargecontaining group. Preferred monomers belonging to this class are, for example, acrylic acid; methacrylic acid; dimethylacrylic acid; crotonic acid; maleic acid; fumaric acid; 3-acrylamido 3 -methylbutanoic acid; strong acid-type monomers having, for example, a sulfonic acid- or phosphonic acid-type function, such as vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, methallylsulfonic acid, 2-methylidenepropane-l,3-disulfonic acid, 2- sulfoethylmethacrylate, sulfopropylmethacrylate, sulfopropylacrylate, allylphosphonic acid, ethylene glycol methacrylate phosphate, sulfonic styrene acid, 2-acrylamido-2-methylpropane sulfonic acid (ATBS), 2-acrylamido-2-methylpropane disulfonic acid, 3-allyloxy-2- hydroxypropane sulfonic acid, diethylallylphosphonate, carboxyethyle acrylate; water-soluble salts of these monomers such as the alkaline metalor alkaline earth metals or ammonium salts thereof; and the mixtures thereof. Preferably, the hydrophilic anionic monomer is acrylic acid.

Preferably, the hydrophilic anionic monomer is acrylic acid or 2-acrylamido-2- methylpropanesulphonic acid (ATBS).

The polymer advantageously comprises between 0 and 100 mol% of hydrophilic anionic monomer(s), preferably between 1 and 99 mol%, more preferably between 5 and 95 mol%, and even more preferably between 10 and 90 mol%.

In one particular embodiment, when the hydrophilic anionic monomer is 2-acrylamido-2- methylpropanesulphonic acid (ATBS), this is the hydrated form thereof. The hydrated form of ATBS is a particular form of ATBS which can be obtained by controlled crystallization of the ATBS monomer. The document U.S. Pat. No. 10,759,746 describes this hydrated form of the ATBS.

In a particular embodiment of the invention, the anionic monomer(s) can be salified. This may also be a mixture of monomers in acid form and salified form, for example, a mixture of acrylic acid and acrylate. By salified, is meant the substitution of a proton of at least one acid function of the -R^a(=O)-OH type (with R^a representing P, S or C) of the anionic monomer by a metal cation or organic cation to form a salt of the -R^a(=O)-OX type (X being a metal cation or an organic cation). In other words, the non-salified form corresponds to the acid form of the monomer, for example R^b-C(=O)-OH in the case of the carboxylic acid function, while the salified form of the monomer corresponds to the R^b-C(=O)-O' X⁺, X⁺ corresponding to a metal cation or an organic cation. The salification of the acid functions can be partial or total.

The metal cation is advantageously an alkaline metal salt (Li, Na, K, etc. . .) or an alkaline earth metal (Ca, Mg, etc. . .) salt, and the organic cation is advantageously the ammonium ion or a tertiary ammonium. The preferred salts are sodium salts. The salification may take place before or after polymerisation.

The polymer advantageously comprises between 0 and 100 mol% hydrophilic anionic monomer(s) in salified form, and preferably between 30 and 100 mol%.

Advantageously, the hydrophilic cationic monomer(s) being able to be used in the scope of the invention are chosen, in particular, from among the vinyl-type monomers, in particular acrylamide, acrylic, allylic or maleic having a protonable amine function or ammonium, advantageously quaternary ammonium. In particular, and in a non-limiting manner, diallyldialkyl ammonium salts such as dimethyldiallylammonium chloride (DADMAC); acidified or quaternised dialkyl- aminoalkyl(meth)acrylamide salts, like for example (3- methacrylamidopropyl)trimethylammonium chloride (MAPTAC), (3- acrylamidopropyl)trimethylammonium chloride (APTAC); acidified or quaternised dialkylaminoalkyl acrylate salts such as quaternised or salified dimethylaminoethyl acrylate (DMAEA); acidified or quaternised dialkyl aminoalkyl methacrylate salts such as quaternised or salified dimethylaminoethyl methacrylate (DMAEMA); acidified or quaternised N,N- dimethylallylamine salts; acidified or quaternised diallylmethylamine salts; acidified or quaternised diallylamine salts; vinylamine obtained by hydrolysis (basic or acid) of an amide group -N(R²)-CO-R¹ with R¹ and R² being, independently, a hydrogen atom or an alkyl chain of 1 to 6 carbons, for example vinylamine coming from the hydrolysis of vinylformamide; vinylamine obtained by Hofmann degradation; and their mixtures can be mentioned. Advantageously, the alkyl groups are C1-C7, preferably C1-C3 and can be linear, cyclic, saturated or unsaturated chains. Preferably, the hydrophilic cationic monomer is quaternised or salified dimethylaminoethyl acrylate (DMAEA).

The polymer advantageously comprises between 0 and 100 mol% of hydrophilic cationic monomer(s), preferably between 1 and 99 mol%, more preferably between 5 and 95 mol%, and even more preferably between 10 and 90 mol%.

The person skilled in the art will know how to prepare the quaternised monomers, for example, by means of an R-X-type quaternisation agent, R being an alkyl group and X being a halogen or a sulphate.

The term “quaternisation agent” means a molecule being able to alkylate a tertiary amine.

The quaternisation agent may be selected from among dialkyl sulphates comprising 1 to 6 carbon atoms or alkyl halides comprising 1 to 6 carbon atoms. Preferably, the quaternisation agent is selected from among methyl chloride, benzyl chloride, dimethyl sulphate or diethyl sulphate. Furthermore, the present invention also covers DADMAC, APTAC and MAPTAC monomers in which the counterion is sulphate, fluoride, bromide or iodide instead of chloride.

Advantageously, the hydrophilic non-ionic monomer(s) being able to be used in the scope of the invention are chosen, in particular, from among acrylamide, methacrylamide, N- alkylacrylamides, N-alkylmethacrylamides, N,N-dialkyl acrylamides (for example, N,N- dimethylacrylamide or N,N-di ethyl acrylamide), N,N-dialkylmethacrylamides, acrylic acid alkoxyl esters, methacrylic acid alkoxyl esters, N-vinylpyrrolidone, N- methylol(meth)acrylamide, N-vinyl caprolactame, N-vinylformamide (NVF), N-vinyl acetamide, N-vinyl imidazole, N-vinyl succinimide, acryloyl morpholine (ACMO), acryloyl chloride, glycidyl methacrylate, vinyl acetate, glyceryl methacrylate, diacetone acrylamide, methacrylic anhydride, acrylonitrile, maleic anhydride, itaconic anhydride, itaconamide, hydroxyalkyl (meth)acrylate, thioalkyl (meth)acrylate, isoprenol and its alcoxyl derivatives, hydroxyethyl(meth)acrylates and their alkoxyl derivatives, hydroxypropyl(meth)acrylate and its alkoxyl derivatives, and their mixtures. Of these non-ionic monomers, the alkyl groups are advantageously in C1-C5, and more advantageously in C1-C3. Preferably, the hydrophilic non- ionic monomer is acrylamide.

The polymer advantageously comprises between 0 and 100 mol% of hydrophilic non-ionic monomer(s), preferably between 1 and 99 mol%, more preferably between 5 and 95 mol%, and even more preferably between 10 and 90 mol%.

Advantageously, the hydrophilic zwitterionic monomer(s) being able to be used in the scope of the invention are chosen, in particular, from among the derivatives of a vinyl-type moeity (advantageously acrylamide, acrylic, allylic or maleic), this monomer having a quaternary amine or ammonium function and an acid function such as carboxylic- (or carboxylate)-, sulfonic- (or sulfonate)- or phosphoric- (or phosphate)-type acid function.

Preferably, this monomer includes a quaternary amine or ammonium function and a carboxylic- (or carboxylate-), sulfonic- (or sulfonate-) or phosphoric- (or phosphate-) type acid function.

Examples include, but are not limited to, dimethylaminoethyl acrylate derivatives, such as 2- ((2-9(acryloyloxy) ethyl)dimethylammonio) ethane- 1 -sulfonate, can be mentioned in particular, and in a non-limiting manner, 3-((2-(acryloyloxy)ethyl) dimethylammonio) propane- 1 -sulfonate, 4-((2-(acryloyloxy)ethyl) dimethylammonio) butane- 1 -sulfonate, [2- (acryloyloxy)ethyl](dimethylammonio)acetate, methacrylate dimethylaminoethyl derivatives, such as 2-((2-(methacryloyloxy) ethyl)dimethylammonio)ethane-l -sulfonate, 3-((2- (methacryloyloxy)ethyl) dimethylammonio)propane- 1 -sulfonate, 4-((2- (methacryloyloxy)ethyl) dimethylammonio)butane-l -sulfonate, [2- (methacryloyloxy)ethyl](dimethylammonio) acetate, propyl acrylamide dimethylamino derivatives, such as 2-((3-acrylamidopropyl) dimethylammonio)ethane-l -sulfonate, 3-((3- acrylamidopropyl)dimethylammonio) propane- 1 -sulfonate, 4-((3- acrylamidopropyl)dimethylammonio)butane-l -sulfonate, [3- (acryloyl)oxy)propyl](dimethylammonio)acetate, dimethylamino propyl methylacrylamide, or also derivatives, such as 2-((3-methacrylamidopropyl)dimethylammonio)ethane-l -sulfonate, 3 -(dimethylammonio)propane-l -sulfonate 4-((3- methacrylamidopropyl)dimethylammonio)butane-l -sulfonate and propyl[3- (methacryloyloxy)](dimethylammonio)acetate and their mixtures.

Other zwitterionic monomers are described by the Applicant in document WO2021/123599 Al.

The polymer advantageously comprises between 0 and 30 mol% of hydrophilic zwitterionic monomer(s), preferably between 1 and 20 mol%, and more preferably between 2 and 15 mol%.

The polymer according to the invention may furthermore comprise at least one hydrophobic monomer.

Advantageously, the monomer(s) having a hydrophobic character being able to be used in the scope of the invention can be chosen, in particular, from among the (meth)acrylic acid esters having (i) a C4-C30 alkyl chain or (ii) an arylalkyl (C4-C30 alkyl, C4-C30 aryl) chain, or (iii) propoxylated chain, or (iv) ethoxylated chain, or (v) ethoxylated and propoxylated chain; alkyl aryl sulfonates (C4-C30 alkyl, C4-C30 aryl); mono- or di-amides substituted by (meth)acrylamide having (i) a C4-C30 alkyl chain, or (ii) an arylalkyle (C4-C30 alkyl, C4-C30 aryl), or (iii) propoxylated chain, or (iv) ethoxylated chain, or (v) ethoxylated and propoxylated chain; anionic or cationic monomer derivatives of (meth)acrylamide or meth)acrylic acid (carrying a hydrophobic chain; and their mixtures. The hydrophobic monomers may comprise halogen atoms, for example chlorine.

Among these hydrophobic monomers:

- alkyl groups are preferably in C4-C20, more preferably in C4-C8 groups. C6-C20 alkyls are preferably linear alkyls, while C4-C5 alkyls are preferably branched,

- the arylalkyl groups are preferably in C7-C25, more preferably in C7-C15,

- ethoxyl chains advantageously comprise between 1 and 200 -CH2-CH2-O- groups, preferably between 6 and 100, more preferably between 10 and 40,

- propoxyl chains advantageously comprise between 1 and 50 -CH2-CH2-CH2-O- groups, more preferably between 1 and 20.

Preferred hydrophobic monomers belonging to these classes are, for example:

- n-hexyl (meth)acrylate, n-octyl (meth)acrylate, octyl (meth)acrylamide, lauryl (meth)acrylate, lauryl (meth)acrylamide, myristyl (meth)acrylate, myristyl (meth)acrylamide, pentadecyl (meth)acrylate, pentadecyl (meth)acrylamide, cetyl (meth)acrylate, cetyl (meth)acrylamide, oleyl (meth)acrylate, oleyl (meth)acrylamide, erucyl (meth)acrylate, erucyl (meth)acrylamide, N-tert-Butyl(meth)acrylamide, vinylpyridine, 2-ethylhexyl acrylate, C4-C22 itaconic acid hemiesters, acidified or quatemised C4-C22(meth)arcrylate dialkyl aminoalkyl salts, acidified or quaternised C4-C22 dialkyl-aminoalkyl(meth)acrylamide salts, undecanoic acrylamido acid, and their mixtures,

- cationic allyl derivatives having a formula (I) or (II): wherein:

Rs independently represents an alkyl chain containing 1 to 4 carbons;

Re represents an alkyl chain or arylalkyl comprising 8 to 30 carbons;

X represents a halide selected from the group consisting of bromides, chlorides, iodides, fluorides and of any negatively charged counter-ion; and, preferably, hydrophobic cationic derivatives of the methacryloyl type responding to formula (III): wherein:

- A represents O or N-R9 (preferably, A = N-R9),

- R7, Rs, R9, Rio, R11 independently represents a hydrogen or an alkyl chain comprising 1 to 4 carbons,

- Q represents an alkyl chain comprising 1 to 8 carbons,

- R12 represents an alkyl chain or arylalkyl comprising 8 to 30 carbons,

- X represents a halide selected from the group consisting of bromides, chlorides, iodides, fluorides, and any negatively charged counter-ion.

The polymer generally comprises less than 5 mol% of hydrophobic monomers. Preferably, this polymer is free of hydrophobic monomers.

When the polymer is water-soluble, the quantity of hydrophobic monomers present is adjusted so that the polymer remains soluble in water.

In one particular embodiment, the polymer may comprise at least one LCST group.

According to the general knowledge of the person skilled in the art, a LCST group corresponds to a group whose water solubility, for a given concentration, is modified above a certain temperature and as a function of salinity. It is a group with a heating transition temperature that defines its lack of affinity with the solvent medium. The lack of affinity with the solvent results in opacification or loss of transparency, which may be due to precipitation, aggregation, gelation or viscosification of the medium. The minimum transition temperature is known as the LCST (Lower Critical Solution Temperature). For each concentration of a LCST group, a heating transition temperature is observed. It is higher than the LCST, which is the minimum point on the curve. Below this temperature, the polymer is soluble in water; above this temperature, the polymer loses its solubility in water.

In one particular embodiment, the polymer may comprise at least one UCST group.

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

The quantities of the different monomers will be adjusted by a person skilled in the art in order not to exceed 100 mol% when preparing the polymer.

The polymer may be partially or totally post-hydrolyzed.

Post-hydrolysis is the hydrolysis reaction of the polymer after it has been formed by polymerisation of the monomer(s). This step consists of the reaction of hydrolyzable functional groups of advantageously non-ionic monomers, more advantageously amide or ester functions, with a hydrolysis agent. This hydrolysis agent may, for example, be an enzyme, an ion-exchange resin, or a Bronsted acid metal (for example a hydrohalogenic acid) or a Bronsted base (for example an alkali hydroxide or an alkaline-earth hydroxide). Preferably, the hydrolysis agent is a Bronsted base. During this step of post-hydrolysis of the polymer, the number of carboxylic acid functions increases. Indeed, the reaction between the base and the amide or ester functions present in the polymer produces carboxylate groups.

Structure of the polymer

According to the invention, the polymer may have a linear, branched, ramified, cross-linked, star-shaped or comb-shaped structure. This structure can be obtained, according to the general knowledge of a person skilled in the art, for example by selecting the initiator, the transfer agent, the polymerisation technique such as Reversible Addition Fragmentation chain Transfer Polymerisation (RAFT), Nitroxide Mediated Polymerisation (NMP) or Atom Transfer Radical Polymerisation (ATRP), the incorporation of structural monomers, or the concentration.

The polymer may be a statistical polymer, a block polymer, or a gradient polymer.

“Statistical polymer” means a polymer in which the organization of the monomers is random. A statistical polymer is obtained by putting all the monomers that compose the polymer at the start of the polymerisation.

By "block polymer", is meant di-blocks, tri-blocks or multi-blocks, grafted sequenced polymers, branched sequenced polymers (also known as star-shaped linear polymers).

Polymers having a block structure are polymers composed of at least two blocks of different monomer. Di-block polymers have two distinct blocks; tri-block polymers have three of them, and so on. They are advantageously obtained by successively polymerising different species of monomers.

In a particular embodiment, the polymer has an X-Y-type structure, when it is composed of two different monomers. In other words, the first fraction only comprises, as a monomer, monomers X. They are polymerised initially and when all the monomers X have reacted, then the second fraction comprising the monomers Y is added.

In a particular embodiment, the polymer has an X-Y-Z type structure, when it is composed of three different monomers. In other words, the first fraction only comprises, monomers X. They are polymerised initially and when all the monomers X have reacted, then the second fraction comprising the monomers Y is added. When the monomers Y have reacted, then the third fraction comprising the monomers Z is added.

This polymerisation system can be extended to obtain so-called multi-block polymers having a structure Xi-Yi-...-Xn-i-Y_n-i-Xn-Y_n, n being an integer greater than or equal to 2 representing the numbers of blocks.

“Gradient polymer” means a polymer which monomer composition varies in a controlled manner all along the polymer chain. Polymers having a gradient structure are polymers composed of at least two monomers in which the monomers composition change is gradual, unlike block polymers, which have a sudden composition change, and random polymers, which do not have a continuous composition change. In the gradient polymer, due to the gradual change of composition over the length of the polymer chain, less intra-chain and inter-chain repulsion is observed.

The gradient can be formed by a spontaneous or forced gradient. Spontaneous gradient polymerisation is due to a difference of monomer reactivity. Forced gradient polymerisation involves varying the monomer composition introduced all along the polymerisation time.

A forced gradient method comprises (1) the introduction of a first fraction of monomers in a reactor, (2) the addition of at least one supplemental fraction of monomers and advantageously different from the first fraction and (3) the polymerisation of the monomers introduced in the reactor. The polymerisation of monomers is initiated as soon as the first fraction is introduced.

The addition of the supplemental fraction of monomers can be done in parallel with the introduction of the first fraction of monomers in the reactor (the introduction of thefractions can therefore start and end at the same time). Alternatively, the start of the first provision in monomer (first fraction) in the reactor can precede the start of the addition of a second monomer fraction. Alternatively, a first and a second fraction can be introduced simultaneously, but the duration of addition of the second fraction can be longer than the duration of introduction of the first fraction in the reactor. This embodiment is also applicable to the methods implementing at least 3 fractions of monomers.

The polymer may further by structured by a branching agent. By structured polymer, is meant a nonlinear polymer, bearing side chains.

The branching agent is advantageously selected from the group consisting of :

- structural agents, which can be selected from the group comprising unsaturated polyethylene compounds (having at least two unsaturated functions), for example, vinyl functions, in particular allyl or acrylic, and examples include, methylene bis acrylamide (MBA), triallylamine, or tetraallylammonium chloride or 1,2 dihydroxyethylene bis-(N-acrylamide);

- compounds having at least two epoxy functions;

- compounds having at least one unsaturated function and one epoxy function;

- macroinitiators such as polyperoxides, polyazoics; - poly transfer agent, such as polymercaptan polymers, and polyols,

- functionalised polysaccharides;

- water-soluble metal complexes composed of:

* of a metal of valence greater than 3 such as, as an example and in a non-limiting manner, aluminum, boron, zirconium, or also titanium, and

* of a ligand carrying a hydroxyl function.

The quantity of branching agent in the polymer is advantageously less than 40,000 ppm by weight based on the total weight of monomers of the polymer, preferably less than 10,000 ppm by weight, and more preferably less than 5,000 ppm by weight.

In one particular embodiment, the quantity of branching agent is at least equal to 0.1 ppm by weight based on the total weight of monomers of the polymer, preferably at least 1 ppm by weight, more preferably at least 10 ppm by weight, more preferably at least 100 ppm by weight and even more preferably at least 1 000 ppm by weight.

When the polymer is water soluble and comprises at least one branching agent, it remains soluble in water. A person skilled in the art knows how to adjust the quantity of branching agent and, possibly, the quantity of transfer agent needed to obtain this result.

In one particular embodiment, the polymer is free of branching agent.

In one particular embodiment, the polymer may comprise a transfer agent.

The transfer agent is advantageously selected from the group consisting of methanol; isopropyl alcohol; sodium hypophosphite; calcium hypophosphite; magnesium hypophosphite; potassium hypophosphite; ammonium hypophosphite; formic acid; sodium formiate; calcium formiate; magnesium formate; potassium formate; ammonium formate; 2- mercaptoethanol; 3 -mercaptopropanol; dithiopropylene glycol; thioglycerol; thioglycolic acid; thiohydracrylic acid; thiolactic acid; thiomalic acid; cysteine; aminoethanethiol; thioglycolates; allyl phosphites; allyl mercaptans; such as w-dodecyl mercaptan; sodium methallysulfonate; calcium methallysulfonate; magnesium methallysulfonate; potassium methallysulfonate; ammonium methallysulfonate; alkyl phosphites like tri alkyl (C12-C15) phosphites, di-oleyl-hydrogenophosphites, dibutyl phosphite, dialkyldithiophosphates such as dioctyl phosphonate, tertiary nonylmercaptan, 2-ethylhexyl thioglycolate, n-octyl mercaptan, n-dodecyl mercaptan, ter-dodecyl mercaptan, iso-octylthioglycolate, 2-ethylhexyl thioglycolate, 2-ethylhexyl mercaptoacetate, polythiols, and their mixtures. Preferably, this is sodium hypophosphite or sodium formate.

The quantity of transfer agent in the polymer is advantageously of between 0 and 100,000 ppm by weight based on the total weight of monomers of the polymer, preferably of between 0 and 10,000 ppm by weight, more preferably between 0 and 1,000 ppm by weight, and even more preferably between 0 and 100 ppm by weight. When it is present, the transfer agent represents at least 0.1 ppm by weight based on the total weight of monomers of the polymer, and preferably at least 1 ppm by weight.

In one particular embodiment, the polymer is free of transfer agent.

The polymer has advantageously a molecular weight of at least 0.5 million g/mol, preferably between 0.5 and 40 million g/mol, more preferably between 1 and 30 million g/mol, most preferably between 2 and 20 millions g/mol et even most preferably between 3 and 15 millions g/mol. Molecular weight is defined as weight-average molecular weight. The polymer may also have a molecular weight of between 5,000 and 100,000 g/mol or between 100,000 and 500,000 g/mol.

The molecular weight is determined by the intrinsic viscosity of the polymer. The intrinsic viscosity can be measured by methods known to the person skilled in the art and can be calculated from the reduced viscosity values for different polymer concentrations by a graphical method consisting in plotting the reduced viscosity values (y-axis) against the concentration (x-axis) and extrapolating the curve to zero concentration. The intrinsic viscosity value is plotted on the y-axis or using the least-squares method. The molecular weight can then be determined using the Mark-Houwink equation: [r|] = K.M“

[q] represents the intrinsic viscosity of the polymer as determined by the solution viscosity method.

K represents an empirical constant.

M represents the molecular weight of the polymer, a represents the Mark-Houwink coefficient.

K and a depend on the particular polymer-solvent system.

The inverse emulsion comprises at least: i) a hydrophilic phase comprising: - at least one polymer representing at least 30 % by weight based on the total weight of the inverse emulsion;

- at least one water-in-oil emulsifying agent representing between 1 and 3% by weight based on the total weight of the inverse emulsion;

Advantageously the inverse emulsion comprises between 30% and 45% by weight of polymer based on the total weight of the inverse emulsion, most preferably between 32% and 42% by weight.

The hydrophilic phase comprises at least one hydrophilic solvent capable of solubilizing the at least one polymer. Advantageously, this is water.

The quantity of hydrophilic solvent(s) of the hydrophilic phase in the inverse emulsion is advantageously between 27 and 59% by weight with respect to the total weight of the inverse emulsion, preferably between 30 and 45% by weight.

The lipophilic phase comprises between 10 and 100% by weight of an oil Hi having a flash point between 40 and 85°C. based on the total weight of oil, preferably between 30 and 100% by weight, more preferably between 50 and 100% by weight, even more preferably between 70 and 100% by weight.

In a particularly preferred embodiment, the lipophilic phase consists of 100% by weight of an oil Hi having a flash point between 40 and 85°C.

It may also comprise between 0 and 90% by weight of an oil H2 based on the total weight of oil, preferably between 5 and 90% by weight, preferably between 7 and 70% by weight, more preferably between 10 and 50% by weight, even more preferably between 15 and 30% by weight.

It is understood that the total weight of oil corresponds to the weight of oil Hi and oil H2. Thus, the quantities of oils Hi and H2 will be adjusted by the person skilled in the art in order not to exceed 100% by weight based on the total weight of oil.

The lipophilic phase comprises a quantity of oil Hi and H2 representing between 10 and 20% by weight based on the weight of the inverse emulsion.

The oil Hi has a flash point of between 40 and 85°C, preferably between 45 and 85°C, more preferably between 50 and 85°C and even more preferably between 55 and 85°C and represents at least 10% by weight based on the total weight of the lipophilic phase.

Examples of oils Hi, having a flash point between 40 and 85°C, are, for example, oils containing saturated and unsaturated hydrocarbons comprising between 9 and 14 carbon atoms, in particular hydrocarbons having C9-C11, C10-C 13, C10-C 14 or C11-C14 carbon chains. By way of examples, mention may be made of the following commercial products: Exxsol™ D40, Exxsol™ D60, Exxsol ™ D70, Exxsol™ D80, Isopar™ G, Isopar™ J, Isopar™ K. Preferably, it is Exxsol™ D60

In a preferred embodiment, the oil Hi represents at least 30% by weight of the oils based on the total weight of oil in the lipophilic phase, preferably at least 50% by weight, more preferably at least 70% by weight, and even more preferably 100% by weight based on the total weight of oil in the lipophilic phase.

Oil H2 of the lipophilic phase may be a mineral oil, a vegetable oil, a synthetic oil, or a mixture of several of these oils.

Oil H2 has a different flash point than the flash point of oil Hi. In other terms, the flash point of oil H2 is outside the flash point range of oil Hi i.e. outside between 40°C and 85°C.

Advantageously, the oil H2 has a flash point below 40°C and higher than 85°C, preferably higher than 90°C, more preferably higher than 100°C. Preferably, the oil H2 has a flash point of at least 30°C, even more preferably of at least 35°C and not more than 200°C, preferably not more than 150°C.

Examples of mineral oil H2 are mineral oils containing saturated hydrocarbons of the aliphatic, naphthenic, paraffinic, isoparaffinic, cycloparaffinic or naphthyl type comprising between 15 and 30 carbon atoms. By way of example, mention may be made of Exxsol™ D100, Isopar™ L, Isopar™ M, Isopar™ N sold by ExxonMobil, and white oils.

Examples of vegetable oil are squalene, an ester- or triglyceride-type oil, such as coco capryl ate/caprate, octyldodecyl myristate, ethoxylated vegetable oils, jojoba oil, macadamia oil.

Examples of synthetic oil are hydrogenated polydecene or hydrogenated polyisobutene, esters such as octyl stearate or butyl oleate.

The weight ratio between the hydrophilic solvent (s) of the hydrophilic phase of the inverse emulsion and the oil(s) Hi and optionally H2 of the lipophilic phase in the inverse emulsion is advantageously of between 1.35 and 5.9, preferably between 1.5 and 4.5.

The weight ratio between the polymer (s) and the oil(s) Hi and optionally H2 of the lipophilic phase is of between 1.5 and 4.5, preferably between 1.8 and 3.

By "water-in-oil emulsifying agent", is meant a compound capable of emulsifying water in an oil and an "oil-in-water emulsifying agent" is a compound capable of emulsifying an oil in water. Generally, it is considered that an emulsifying agent of the water-in-oil type is a surfactant having an HLB strictly less than 8, and that an oil-in-water emulsifying agent is a surfactant having an HLB greater than or equal to 10. A surfactant having an HLB of between 8 and 10 is considered as a wetting agent. A person skilled in the art can refer to the document, " Handbook of Applied Surface and Colloid Chemistry" by K. Holmberg, Chapter 11, if needed.

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

In the present invention, the Griffin method has been adopted, based on calculating a value based on the chemical groups of the molecule. Griffin has attributed a dimensionless number between 0 and 20 to give information on the solubility of water and oil.

The HLB value of a substance having a total molecular mass M and a hydrophilic part of a molecular mass Mh is given by: HLB = 20 (Mh/M). The water-in-oil emulsifying agent is advantageously selected from the group consisting of the following list: polyesters having a molecular weight of between 1000 and 3000 g/mol; condensation products between a succinic poly(isobutenyl) acid or its anhydride and a glycol polyethylene; sequenced block polymers having a molecular weight of between 2500 and 3500 g/mol, such as for example those sold under Hypermer® names; sorbitan extracts, such as sorbitan monooleate or sorbitan polyoleates; sorbitan isostearate or sorbitan sesquioleate; polyethoxylated sorbitan esters; diethyoxylated oleocetyl alcohol; tetraethoxylated laurylacrylate; fatty alcohol condensation products greater than ethylene, such as oleic alcohol reaction product with 2 ethylene oxide units; alkylphenol and ethylene oxide condensation products, such as the nonyl phenol reaction product with 4 ethylene oxide units. Ethoxyl fatty amines such as Witcamide® 511, alkyl phosphate esters, betaine-based products and ethoxyl amine are also good candidates as emulsifying agents of the water-in-oil type.

The quantity of water-in-oil emulsifying agent in the inverse emulsion is advantageously between 1 and 3% by weight based on the total weight of the inverse emulsion, preferably between 1 and 2.5% by weight, more preferably between 1.5 and 2.5% by weight.

The method of the invention can comprise the addition of at least one oil-in-water emulsifying agent.

The oil-in-water emulsifying agent(s) are advantageously selected from the group consisting of ethoxylated nonylphenol, preferably having 4 to 10 ethoxylations (i.e. preferably having an ethoxylation degree going from 4 to 10); ethoxylated/propoxylated alcohols preferably having an ethoxylation/propoxylation comprising 12 to 25 carbon atoms; ethoxylated tridecyl alcohols; ethoxylated/propoxylated fatty alcohols; ethoxylated sorbitan esters (advantageously having 20 ethylene oxide molar equivalents); polyethoxylated sorbitan laurate (advantageously having 20 ethylene oxide molar equivalents); polyethoxylated castor oil (advantageously having 40 ethylene oxide molar equivalents); decaethoxylated oleodecyclic alcohol; hepta oxy ethylated lauric alcohol; polyethoxylated sorbitan monostearate (advantageously having 20 ethylene oxide molar equivalents); cetyl ether polyethoxylated phenol alkyls (advantageously having 10 ethylene oxide molar equivalents); aryl ether alkyl ethylene polyoxides; N-cetyl-N-ethyl morpholinium ethosulfate; sodium lauryl sulfate; fatty alcohol condensation products with ethylene oxide (advantageously having 10 ethylene oxide molar equivalents); alkylphenol and ethylene oxide condensation products (advantageously having 12 oxide ethylene molar equivalents); fatty amine condensation products with 5 molar equivalent or more ethylene oxide (advantageously 5 to 50 equivalents); ethoxylated phenol tristyryl, condensates of ethylene oxide with partially esterified polyhydric alcohols with fatty chains, as well as their anhydrous forms; amine oxides advantageously having alkyl polyglucosides; glucamide; phosphate esters; sulphonic alkylbenzene acids and their salts; and surfactant block polymers and their mixtures. The alkyl groups of these oil-in-water-type emulsifying agents mean linear or branched groups and advantageously having 1 to 20 carbon atoms, more advantageously 3 to 15 carbon atoms. Furthermore, the aryls of these oil-in- water-type emulsifying agents, advantageously comprise 6 to 20 carbon atoms, more advantageously 6 to 12 carbon atoms.

Generally, the inverse emulsion comprises between 1 and 6% by weight of oil-in-water emulsifying agent based on the total weight of the inverse emulsion.

The oil-in-water emulsifying agent(s) can be added before, during or after polymerisation. Preferably the oil-in-water emulsifying agent(s) are added after polymerisation.

Method for the hydraulic fracturing of an unconventional underground hydrocarbon (oil and/or gas) reservoir

The present invention also relates to a method for the hydraulic fracturing of an unconventional underground hydrocarbon (oil and/or gas) reservoir comprising the injection of a fracturing fluid comprising at least one inverse emulsion comprising: i) a hydrophilic phase comprising:

- at least one polymer representing at least 30 % by weight based on the total weight of the inverse emulsion;

- an oil Hi, having a flash point between 40 and 85°C, representing between 10 and 100% by weight based on the total weight of oil in the lipophilic phase;

Injection is carried out under pressure in order to create fractures distributed along the entire length of the production well.

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

Injecting a surfactant allows eliminating the viscosity generated by the polymer by inhibiting hydrophobic interchain interactions, while injecting the oxidising compound destroys the polymer. In both cases, injection allows a fluid viscosity close to that of water to be reestablished.

Oxidizing compound includes bleach (an aqueous solution of a hypochlorite salt), hydrogen peroxide, ozone, chloramines, persulphates, permanganates and perchlorates.

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 bear(s) anionic charges.

Preferably, the surfactant compounds used are selected from the group consisting of anionic surfactants and their zwitterionic form selected the group consisting of derivatives of alkyl sulphates, alkyl ether sulphates, arylalkyl sulphates, arylalkylether sulphates, alkyl sulphonates, alkyl ether sulphonates, arylalkylsulphonates, arylalkylether sulphonates, alkylphosphates, alkyletherphosphates, arylalkylphosphates, arylalkyletherphosphates, alkylphosphonates, alkyletherphosphonates, arylalkylphosphonates, arylalkyletherphosphonates, alkylcarboxylates, alkylethercarboxylates, arylalkylcarboxylates, arylalkylethercarboxylates, polyalkyl ethers, arylalkyl polyethers, and the like.

An alkyl chain is defined as a chain comprising from 6 to 24 carbons, branched or unbranched, with several or without motifs, which may optionally include one or more heteroatoms (O, N, S). An arylalkyl chain is defined as a chain comprising from 6 to 24 carbons, branched or unbranched, comprising one or more aromatic rings and possibly comprising one or more heteroatoms (O, N, S). The surfactant agents the most commonly used, for cost, stability and availability reasons are sulphonates or sulphates in the form of alkali metal or ammonium salts.

Fracturing fluid

The present invention relates to a fracturing fluid comprising at least one inverse emulsion according to the invention.

The fracturing fluid comprises an aqueous fluid, at least one propping agent and at least one inverse emulsion comprising: i) a hydrophilic phase comprising:

The aqueous fluid is advantageously selected from the group consisting of: sea water, brine and fresh water. Advantageously, this is brine.

Brine is a solution comprising water and organic or inorganic salts. The salts may include monovalent salts, divalent salts, trivalent salts and mixtures thereof. Advantageously, the brine comprises at least 1,000 mg/L of salts, preferably at least 5,000 mg/L, more preferably at least 10,000 mg/L, even more preferably at least 50,000 mg/L and even more preferably the brine is saturated with salts. The propping agent may be chosen in a non-restrictive manner from sand, ceramics, bauxite, glass beads and resin-impregnated sand.

Advantageously, the quantity of propping agent in the fracturing fluid is of between 0.5 and 40% by weight based on the total weight of the fracturing fluid, preferably between 1 and 25%, and more preferably between 1.5 and 20%.

Advantageously, the fracturing fluid comprises between 50 ppm and 50,000 ppm of emulsion according to the invention, preferably between 100 ppm and 20,000 ppm.

The fracturing fluid may comprise other compounds known to the person skilled in the art, such as those cited in document SPE 152596, for example:

- Clay anti-swelling agents such as potassium chloride or choline chloride, and/or

- Biocides for preventing the development of bacteria, in particular sulphate-reducing bacteria, which can form viscous masses reducing the passage surfaces. Examples include glutaraldehyde, which is the most used, or indeed formaldehyde or the isothiazolinones, and/or

- Oxygen reducers such as ammonium bisulphite for preventing the destruction of the other components by oxidation and corrosion of the injection tubes, and/or

- Anti-corrosion additives for protecting the tubes from oxidation by residual quantities of oxygen, with N,N dimethylformamide being preferred, and/or

- Lubricants such as oil distillates, and/or

- Iron chelating agents such as citric acid, EDTA (ethylene diamine tetra-acetic) acid, phosphonates, and/or

- Antiscaling agents such as phosphates, phosphonates, polyacrylates or ethylene glycol.

Method for obtaining the inverse emulsion

The invention also relates to a method for preparing an inverse emulsion comprising the following steps: a) Mixing with stirring: i) a hydrophilic phase comprising:

- at least one monomer representing at least 30% by weight based on the total weight of the inverse emulsion;

- at least one water-in-oil emulsifying agent representing between 1 and 3% by weight based on the total weight of the inverse emulsion; b) Polymerisation of the at least one monomer of the hydrophilic phase in order to obtain a polymer in the inverse emulsion; the inverse emulsion having a weight ratio between the at least one polymer and the oil Hi and optionally H2 of between 1.5 and 4.5.

Polymerisation is done in inverse emulsion. The expression "inverse emulsion " means both inverse emulsions and inverse microemulsions. These are water-in-oil emulsions, wherein the aqueous phase is dispersed in the lipid phase in the form of drops or droplets.

An inverse emulsion consists of a two-phase medium. It can be unstable in the absence of surfactant (the surfactants group together the water-in-oil emulsifying agents and the oil-in- water emulsifying agents). Under stirring, hydrophilic phase particles dispersed in a lipophilic phase are observed, having a wide size distribution around an average, which can be around one micrometer. During an inverse emulsion polymerisation, the monomer is dispersed in large droplets of the emulsion (diameter: around 50 nm to 10pm), as well as in small emulsifying micelles (diameter: around 5 to 10 nm).

Step a)

Mixing of the hydrophilic phase and the lipophilic phase is carried out under stirring, advantageously at a speed of between 10 and 10,000 rpm (rotations per minute), preferably between 100 and 1,000 rpm.

The stirring can be done by any system enabling a homogeneous mixture, as an example, a mixing foot, a homogeniser can be mentioned. Preferably, the mixing is carried out with a mixing foot. The stirring is advantageously maintained during step b).

Step b)

The polymerisation is a radical polymerisation. The radical polymerisation includes the polymerisation by means of UV, azoic, thermal primers, or redox salts.

In radical polymerisations, we include controlled radical polymerisation (CRP) or matrix polymerisation techniques.

Controlled radical polymerisation techniques include, but are not limited to, techniques such as Iodine Transfer polymerisation (ITP), Nitroxide Mediated Polymerisation (NMP), Atom Transfer Radical Polymerisation (ATRP), Reversible Addition Fragmentation chain Transfer (RAFT) Polymerisation, which includes MAD IX (MAcromolecular Design by Interchange of Xanthates) technology, various variations of Organometallic Mediated Radical Polymerisation (OMRP), and OrganoHeteroatom-mediated Radical Polymerisation (OHRP).

In a preferred embodiment, the polymerisation is done by reversible addition-fragmentation chain transfer polymerisation (RAFT).

RAFT is a reversible deactivation radical polymerisation (RDRP) technique, combine both the facility to implement conventional radical polymerisation and the living character of ionic polymerisation.

It is based on a reversible activation-deactivation balance between a dormant species and an active species (growing macro-radical). This activation-deactivation process enables the chains to grow at the same speed, and this, until the total consumption of the monomer, making controlling the molecular weights of the polymers possible and obtaining the narrow distributions of molecular weights. This will also make it possible to minimise the heterogeneity of the composition. The reversible deactivation of the growing chains lies at the origin of minimising irreversible end reactions. The large majority of polymer chains remains in dormant form and can therefore be reactivated. It is thus possible to functionalise the chain ends in view of priming other polymerisation modes or to make chain extensions. This is the key for accessing high molecular weights, controlled compositions and architectures.

Controlled radical polymerisation can therefore have the following distinctive aspects: 1. the number of polymer chains is fixed throughout the duration of the reaction, 2. the polymer chains all grow at the same speed, which is reflected by:

* a linear increase of the molecular weights,

* a narrowed molecular weight distribution,

3. the mean molecular weight is controlled by the monomer/precursor molar ratio.

The controlled character is all the more marked that the reactivation speed of the radical chains is high in front of the growth speed of the chains (propagation). However, in certain cases, the reactivation speed of the radical chains is greater than or equal to the propagation speed. In these cases, conditions 1 and 2 are not observed and, consequently, controlling the molecular weights is not possible.

Reversible addition-fragmentation chain transfer polymerisation requires the use of a control agent.

In the scope of the invention, the control agent is water-soluble of formula (I): wherein

- A represents an oxygen atom (O), a sulphur atom (S) or an amine (NR3);

- Ri and R2 and R3 , identical or different, represent:

* an optionally substituted alkyl, acyl, alcenyl or alcynyl group (i), or

* a carbon cycle (ii), saturated or not, optionally substituted or aromatic, or

* a heterocycle (iii), saturated or not, optionally substituted or aromatic, these groups and cycles (i), (ii) and (iii) may be substituted by substituted aromatic groups or by alkoxycarbonyl or aryloxycarbonyl (-COOR), carboxy (-COOH), acyloxy (-O2CR), carbamoyl (-C0N(R)2), cyano (-CN), alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl, arylalkylcarbonyl, phtalimido, maleimido, succinimido, amidino, guanidimo, hydroxy (-OH), amino (-N R)2), halogen, allyl, epoxy, alkoxy (-OR), S-alkyl, S-aryl groups, groups having a hydrophilic or ionic character, such as carboxylic acid alkaline salts, sulfonic acid alkaline salts, alkylene polyoxide chains (POE, POP), cationic substituents (quaternary ammonium salts);

R represents a C1-C20 alkyl or aryl group; - Rs can further represent a hydrogen atonr

- Q is a linear or structured polymeric chain comprising n identical or different monomers comprising at least one ethylenic function and n being between 4 and 500, preferably between 4 and 100;

- p is an integer equal to 0 or 1

The monomer(s) used to form Q are advantageously selected from the group consisting of the same monomers as those described to form the polymer of the hydrophilic phase. These are preferably hydrophilic monomers.

In the N(R)₂ functions, the two R groups can be identical or different from one another.

According to a preferred embodiment, the water-soluble control agent of formula (I), is a dithiocarbonate or xanthate derivative, wherein A represents an oxygen atom (O).

According to a preferred embodiment, the water-soluble control agent is of formula (I), wherein:

- A represents an oxygen atom (O);

- p = 1 and Q is a linear or structured polymer chain obtained from 4 to 100 monomers comprising at least one hydrophilic non-ionic monomer and/or at least one hydrophilic anionic monomer and/or at least one hydrophilic cationic monomer.

- A represents an oxygen atom (O);

- p = 1 and Q is a linear or structured polymer chain obtained from 4 to 100 monomers comprising at least one hydrophilic non-ionic monomer and/or at least one hydrophilic anionic monomer and/or at least one monomer having a LCST group.

In another embodiment, the water-soluble control agent is of formula (V): wherein n is an integer of between 4 and 100, preferably between 4 and 50.

In another embodiment, the control agent is of formula (VI):

Wherein: the R4 are identical or different, independently represent H or CH3 or a salt, the salt being advantageously selected from the group consisting of salts of alkaline metals (Li, Na, K etc. . .), alkaline earth metals (Ca, Mg, etc. . .) or ammonium ions (for example, ammonium ion or a tertiary ammonium) Preferably a sodium salt.

In another preferred embodiment, the control agent is a trithiocarb onate of following formula (VII): wherein

- the R4 are identical or different, independently represent an H or a CH3 or a monovalent or divalent cation, advantageously selected from the group consisting of cations of alkaline metals (Li, Na, K, etc. . .), alkaline earth metals (Ca, Mg, etc. . .) or ammonium ion (for example, ammonium ion or a tertiary ammonium). Preferably this is sodium;

- n and n’ are integers independent of one another, of between 4 and 100, preferably between 4 and 50.

In another embodiment, the control agent is of following formula (VIII): wherein: the R4 are identical or different, independently represent H, CH3 or a salt, the salt being advantageously selected from the group consisting of salts of alkaline metals (Li, Na, K, etc. . .), alkaline earth metals (Ca, Mg, etc. . .) or ammonium ions (for example, ammonium ion or a tertiary ammonium). Preferably, it is a sodium salt.

In another embodiment, the control agent is of formula (IX): wherein:

- the R4 are identical or different, independently represent an H, a CH3 or a monovalent or divalent cation, advantageously selected from the group consisting of cations of alkaline metals (Li, Na, K, etc. . .), alkaline earth metals (Ca, Mg, etc. . .) or ammonium ion (for example, ammonium ion or a tertiary ammonium), preferably this is sodium; and

- n is an integer, of between 4 and 100, preferably between 4 and 50.

The initiator(s) can be added to the hydrophilic phase before or after the formation of the inverse emulsion.

The polymerisation initiators used are advantageously selected from the group consisting of the compounds which dissociate into radicals under polymerisation conditions, for example: organic peroxides, hydroperoxides, hydrogen peroxide, persulfates, azoic compounds and redox salts. The use of initiators that are soluble in water is preferred. In certain cases, it is advantageous to use mixtures of various polymerisation initiators, for example mixtures of redox salts and of azoic compounds.

In a preferred embodiment, the polymerisation initiator is a redox salts couple.

The reducing agent of the redox salts couple is advantageously selected from the group consisting of the sulphites salts of alkaline metals (Li, Na, K, etc. . .), alkaline earth metals (Ca, Mg, etc. . .) or ammonium (for example, the ammonium ion or a tertiary ammonium), sulphur dioxide, the metabisulphites salts of alkaline metals (Li, Na, K, etc. . .), alkaline earth metals (Ca, Mg, and the like) or ammonium (for example, the ammonium ion or a tertiary ammonium). The oxidant of the redox salts couple is advantageously selected from the group consisting of peroxides such as tert-butyl hydroperoxide, perfulfates of alkaline metals (Li, Na, K, and the like), alkaline earth metals (Ca, Mg, etc. . .) or ammonium (for example, ammonium ion or a tertiary ammonium), hydrogen peroxide.

Advantageously, the quantity of initiator is comprised between 0,01 and 100,000 ppm by weight based on the total weight of the monomer(s) of the polymer, preferably between 0, 1 and 10,000 ppm by weight, more preferably between 1 and 1,000 ppm by weight, and even more preferably between 10 and 100 ppm by weight.

The initiator(s) can be added all at once, in several times or continuously, i. e. by casting. Preferably the initiator(s) are added continuously.

In the case of redox salts, the reducing agent and the oxidizing agent may be added continuously, i.e. by casting, in parallel or one after the other. Advantageously, at least the reducing agent or the oxidant is charged to the inverse emulsion, and the other compound of the redox couple is added continuously, i. e. by casting throughout the polymerisation.

The initial polymerisation temperature is advantageously of between more than 0°C and 50°C.

Once the polymerisation is initiated, the temperature is controlled to be advantageously of between 30°C and less than 100°C.

The polymerisation time is advantageously of between 15 minutes and 600 minutes, preferably between 30 minutes and 320 minutes.

The method of polymerisation according to the invention can be done in batch, in semi-batch or continuously, it is advantageously carried out in batch.

In a preferred embodiment, the polymerisation is carried out at a pressure of less than the atmospheric pressure, preferably at a pressure of between 20 mbar and less than 800 mbar, more preferably of between 30 and 500 mbar, more preferably between 40 and 400 mbar.

In a preferred embodiment, the polymerisation is carried out in a smooth-walled polymerisation reactor and does not comprise a cooling system. By using this type of polymerisation reactor, it is possible to avoid the formation of gel micro-points during polymerisation which could damage the fractures.

In a particular embodiment, the method of the invention comprises, following the polymerisation, a removal step of the residual monomers. The removal of the residual monomers can be done, for example, by the addition of an excess of initiator.

In one particular embodiment, once the inverse emulsion is obtained, it is possible to dilute it, in particular using a brine.

Use of the inverse emulsion

Finally, the present invention also relates to the use of said inverse emulsion above described in the fields of hydrocarbon recovery (oil and/or gas); drilling wells; cementing wells; stimulation of hydrocarbon wells (oil and/or gas), other than hydraulic fracturing; for example conformance or diversion; open, closed or semi-closed circuit water treatment; treatment of fermentation broth; sludge treatment; construction; paper or cardboard manufacture; batteries; wood treatment; treatment of hydraulic composition (concrete, cement, mortar and aggregates); mining; formulation of cosmetic products; formulation of detergents; textile manufacture; geothermal energy; manufacture of diapers; or agriculture.

The present invention also relates to a method comprising the use of the inverse emulsion according to the invention in the above cited fields.

Examples

Abbreviation list:

AM: Acrylamide

AA: Acrylic acid

ATBS: 2-Acrylamido-2-methylpropane sulfonic acid

DADMAC: Diallyldimethylammonium chloride

MAPTAC: (3-Methacrylamidopropyl)trimethylammonium chloride

IA: Itaconic acid

DMAEA.Me: Dimethylaminoethyl acrylate methyl quaternary ammonium salt DMA: N,N-Dimethylacrylamide

APTAC: (3-Acrylamidopropyl)trimethylammonium chloride

DMAEA.Bn: Dimethylaminoethyl acrylate benzyl quaternary ammonium salt NVF : N-Vinylformamide DMAEMA.Me: Dimethylaminoethyl methacrylate methyl quaternary ammonium salt ACMO: Acryloylmorpholine

MA: Maleic acid

Oils used:

Spirdane® D30, commercially supplied by TotalEnergies, flash point 35°C (H2);

Exxsol™ D40, commercially supplied by ExxonMobil, flash point 40°C (Hi);

Spirdane® D40, commercially supplied by TotalEnergies, flash point 44°C (Hi);

Isopar™ K, commercially supplied by ExxonMobil, flash point 50°C (Hi);

Exxsol™ D60, commercially supplied by ExxonMobil, flash point 62°C (Hi);

Escaid™ PathFrac™, commercially supplied by ExxonMobil, flash point 70°C (Hi);

Shellsol® D70, commercially supplied by Shell, flash point 78°C (Hi);

Isopar™ N, commercially supplied by ExxonMobil, flash point 87°C (H2);

Exxsol™ DI 00, commercially supplied by ExxonMobil, flash point 101°C (H2).

Example 1: Preparation of inverse emulsions

Preparation of the hydrophilic phase:

In a reactor equipped with a stirring system, are mixed, at ambient temperature:

- 138 g of acrylamide (50% by weight in water);

- 30 g of acrylic acid;

- 70 g of deionised water;

- 30 g of caustic soda (50% by weight in water)

- 50 ppm of tertio butyl hydroperoxide.

Preparation of the lipophilic phase:

1.5% by weight of sorbitan monooleate based on the weight of the inverse emulsion are mixed in 45 g of Exxsol DI 00 oil (H2) and 5 g of Exxsol D40 oil (Hi).

Emulsification and polymerisation:

The hydrophilic phase is mixed and emulsified in the lipophilic phase. The inverse emulsion EMI1 obtained is then degassed for 60 minutes before the polymerisation is initiated at room temperature by pouring 8 mL of an aqueous solution to 1 g/L of sodium metabisulfite.

The inverse emulsions, according to the invention, EMI2 to EMH 9 are prepared according to the above protocol, as are the comparative inverse emulsions, EMI-CE1 to EMI-CE14. Exxsol™ D40 was used as oil Hi to prepare EMU to EMI6, EMI13 to EMI15, EMI19 and comparative inverse emulsions EMI-CE1 to CE5 and EMI-CE8 to CE14.

Spirdane® D40 was used as oil Hi to prepare EMI7. Isopar™ K was used as oil Hi to prepare EMI8.

Exxsol™ D60 was used as oil Hi to prepare EMI9, EMI12 and EMI16.

Spirdane® D30 was used as oil H2 to prepare comparative example EMI-CE6.

Isopar™ N was used as oil H2 to prepare comparative example EMI-CE7.

Escaid™ PathFrac™ was used as oil Hi to prepare EMH 1. Shellsol® D70 was used as oil Hi to prepare EMH 8.

For all examples Exxsol™ DI 00 was used as oil H2 when the amount of oil Hi was less than 100% by weight of oil (Hi,+ H2) with the exception of EMI-CE6 where Spirdane® D30 was used and EMI-CE7 where Isopar™ N was used.

The composition of these EMI is presented in Table 1.

*Spirdane® D30 was used (100 wt% of oil H2). ** Isopar™ N was used (100 wt% of oil H2).

Example 2: Evaluation of the increase in conductivity of the inverse emulsions

Sand (200-400 pm) is placed between two laboratory presses at a pressure of 4,000 psi in order to simulate conditions in a fracture. Injection pressure is measured using Yokogawa™ EJA130E sensors measuring the differential pressure.

In a first step, a brine (2% KC1) is injected, at a rate of 3 ml/minute, in order to measure the initial pressure loss during fracturing, once stabilized, this value serves as a reference and corresponds to APwater initial injection.

A fracturing fluid is then injected for 4 hours at a flow rate of 3 ml/minute, comprising the emulsion according to the invention previously inversed (10 ppm based on the total weight of the fluid).

Finally, the brine is again injected at a rate of 3 ml/minute to measure the final pressure loss corresponding to APwater final injection.

A fracture damage factor FD is then calculated according to the formula:

AFinal injection skin FD = - - -

Alnitial injection skin

The value of the increase in conductivity RC % is calculated according to the formula:

RC% = 1 - FD

The more the polymer remains blocked in the fracture, the greater the FD value, and the less important will be the increase in conductivity RC%.

The values of the increase in conductivity of the various inverse emulsions and a linear guar solution, serving as a reference, are summarized in Table 2.

Table 2 - Conductivity recovery after a first cleaning phase for EMU to EMU 9 and EMI-CE1 to EMI-CE14

Example 3: Preparation of a disperse dye printing paste Inverse emulsions EMI20 to EMI25 and comparative inverse emulsion EMI-CE15 were prepared according to the protocol described in Example 1. The composition of these EMI is presented in Table 3.

Table 3 - Composition of inverse emulsions EMI20 to EMI25 and comparative emulsion EMI-CE15

The printing paste is prepared as follows:

A disperse dye printing paste is prepared in a beaker using a three-blade agitator.

285.6 g of water is added into the beaker.

While stirring at 400 rpm, the inverse emulsion is gradually added to the water. Once the inverse emulsion is fully dispersed, 3 g of disperse Navy Blue 3G H/C, commercially available from Huntsman, is added to have a concentration of 1 wt% of dye in water.

After the addition of the dye, the viscosity of the mixture is measured using a Brookfield RV viscometer, spindle 6 at 20 rpm. The target viscosity of the printing paste is 13.000 ± 500 cPs.

If the measured viscosity is below the target range, additional inverse emulsion is incrementally added under agitation until the desired viscosity is reached.

If the viscosity is above the target range, the mixture is diluted by adding a prepared 1 wt% dye solution until the target viscosity is achieved. Results are summarised in the table 4 below.

The lower the quantity of EMI required, the better are the performances.

Table 4 - EMI consumed for the preparation of disperse dye printing paste

Claims

1. Inverse emulsion comprising: i) a hydrophilic phase comprising:

- optionally an oil H2 representing between 0 and 90% by weight based on the weight of oil in the lipophilic phase;

2. The inverse emulsion according to claim 1, characterised in that the at least one polymer represents 30% to 45% by weight based on the total weight of the inverse emulsion.

3. The inverse emulsion according to claim 1 or 2, characterised in that the at least one polymer is a polymer of at least one monomer selected from the group consisting of hydrophilic anionic monomers, hydrophilic cationic monomers, hydrophilic non-ionic monomers, hydrophilic zwitterionic monomers, hydrophobic monomers and the mixtures thereof.

4. The inverse emulsion according to any one of claims 1 to 3, characterised in that the at least one polymer is a synthetic water-soluble polymer.

5. The inverse emulsion according to any one of claims 1 to 4, characterised in that the at least one polymer has a molecular weight of between 0.5 and 40 million g/mol.

6. The inverse emulsion according to any one of claims 1 to 5, characterised in that the oil Hi represents at least 30% by weight based on the total weight of oil in the lipophilic phase.

7. The inverse emulsion according to any one of claims 1 to 6, characterised in that the oil Hi is selected from the group consisting of saturated and unsaturated hydrocarbons comprising between 9 and 14 carbon atoms.

8. Hydraulic fracturing method of an unconventional underground hydrocarbon reservoir comprising the injection of a fracturing fluid comprising the inverse emulsion according to any one of claims 1 to 7.

9. Fracturing fluid comprising the inverse emulsion according to any one of claims 1 to 7.

10. A method for preparing an inverse emulsion comprising the following steps: a) Mixing with stirring: i) a hydrophilic phase comprising:

* at least one monomer representing at least 30% by weight based on the total weight of the inverse emulsion;

* at least one hydrophilic solvent representing between 32 and 59% by weight based on the total weight of the inverse emulsion, and capable of solubilizing the polymer;

- a lipophilic phase comprising:

11. Method according to claim 10, characterised in that the polymerisation is initiated by a redox salts couple.

12. Method according to claim 11, characterised in that at least the reducing agent or the oxidant of the redox salts couple is added to the charge of the inverse emulsion according to any one of claims 1 to 7, and the other compound of the redox salts couple is added continuously.

13. Method according to any one of claims 10 to 12, characterised in that the polymerisation is carried out at a pressure of less than the atmospheric pressure.

14. Method according to any one of claims 10 to 13, characterised in that the polymerisation is carried out in a polymerisation reactor with smooth walls and not comprising a cooling system.

15. Use of the inverse emulsion according to any one of claims 1 to 7, in the fields of hydrocarbon recovery, drilling wells; cementing wells; hydrocarbon wells stimulation other than hydraulic fracturing; open, closed or semi-closed circuit water treatment; treatment of fermentation broth; sludge treatment; construction; paper or cardboard manufacture; batteries; wood treatment; treatment of hydraulic compositions; mining; formulation of cosmetic products; formulation of detergents; textile manufacture; geothermal energy; manufacture of nappies; or agriculture.