EP3947558A1

EP3947558A1 - Reverse emulsion for hydraulic fracturing

Info

Publication number: EP3947558A1
Application number: EP20713647.4A
Authority: EP
Inventors: Cédrick FAVERO; Olivier Braun; Bruno Tavernier
Original assignee: SPCM SA
Current assignee: SNF Group
Priority date: 2019-03-29
Filing date: 2020-03-27
Publication date: 2022-02-09
Also published as: WO2020201081A1; FR3094373B1; US20220177773A1; CN113646381B; AR118506A1; AU2020255203A1; CN113646381A; CA3130959A1; MX2021011649A; FR3094373A1

Abstract

The present application relates to a water-in-oil reverse emulsion comprising - an oil; - water; - at least one water-soluble cationic copolymer with an average molar mass of more than 3 million daltons, containing between 18 and 32 mol% of cationic monomers and 68 and 82 mol% of non-ionic monomers; - at least one inversion agent and at least one emulsifier, the weight ratio R of the total amount of inversion agent to the total amount of emulsifier being greater than 1.8, - the inversion agent being chosen from an ethoxylated nonylphenol, preferably having between 4 and 10 ethoxylations; an ethoxylated/propoxylated alcohol, preferably having ethoxylations/propoxylations so as to have a total carbon number between C12 and C25, an ethoxylated tridecyl alcohol and an ethoxylated/propoxylated fatty alcohol; - the emulsifying agent being chosen from sorbitan monooleate, polyethoxylated sorbitan esters or diethanolamide of tall oil fatty acids, and the use thereof in hydraulic fracturing.

Description

REVERSE EMULSION FOR HYDRAULIC FRACTURING

The present invention relates to the technical field of polymers in the form of a water-in-oil emulsion, otherwise known as an inverse emulsion. More specifically, the subject of the invention is an inverse emulsion containing a cationic polymer which is stable under conditions of very high salinity.

Other aspects of the invention relate to a process for preparing a fracturing fluid and to a process for hydraulic fracturing of unconventional underground oil and gas reservoirs using said inverse emulsion and finally the last aspect of the invention relates to a method of reducing friction of a fracturing fluid in a hydraulic fracturing operation.

PRIOR STATE OF THE ART

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

In the remainder of the description of the prior art and of the invention, the term “unconventional underground reservoirs” denotes 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 - CGI ET n ° 2011-04-G - Ministry of ecology, sustainable development, transport and housing - April 2011). For unconventional gas, we can cite shale gas (or shale gas in English), coal bed methane or gas from compact reservoirs (or tight gas in English). For unconventional oil, we can cite heavy oils (or heavy oil in English), shale oil (or shale oil in English) or compact reservoir oils (or tight oil in English).

Reserves in unconventional reservoirs are enormous and extremely extensive in once-unusable areas such as bedrock hydrocarbons such as shales, compact reservoir gas, and coal gas. In the United States, shale gas is widely exploited and represents today 46% of the total natural gas produced in the United States compared to only 28% in 1998. The very large 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 on the ground and making it possible to better cover the volume of the reservoir in order to recover the gas as much as possible. However, the permeabilities are insufficient for the gas to migrate from the source rock to the well easily, and thus to make it possible to produce the gas or oil 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.

The purpose of hydraulic fracturing is to create additional permeability and generate larger gas or oil production areas. Indeed, the low permeability, the natural barriers of compact layers and waterproofing by drilling operations severely limit production. The gas or oil 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 tens of thousands of operations have taken place in the United States.

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

Stimulation of the reservoir by hydraulic fracturing These operations consist in injecting water at high pressure and at a very high flow rate so as to create fractures distributed perpendicularly to the production wells. This is usually done in several stages in order to create fractures along the entire length of the horizontal well, which allows the maximum volume of the reservoir to be covered.

In order to keep these fractures open, a propping agent (eg sand, plastics or calibrated ceramics) is added to prevent these fractures from closing and to maintain the capillary action created after the injection is stopped.

In order to reduce the hydraulic power required to quickly inject water or brine into the underground formation polymers known as friction reducers are used. The use of such polymers reduces pressure losses due to internal friction in the fluid by up to 70%.

Polymers in reverse emulsion form are commonly used for their ease of processing. Their use is based on dissolving the polymer in water or in brine. To do this, the reverse emulsion is reversed, so as to release the polymer contained in the water phase of the reverse emulsion. After release, the polymer is in water or brine to which the reverse emulsion has been added.

Fracturing fluids are increasingly based on waters containing significant amounts of dissolved salts. In this context, the industry requires friction reducers that work effectively in high brines (brine with a high concentration of dissolved salts), some of which may contain more than 30,000 mg.L ^-1 of dissolved salts, or even more than 100 000 mg.L ^-1 with in particular high contents of divalent salts.

DISCLOSURE OF THE INVENTION

The Applicant has surprisingly discovered that a water-in-oil inverse emulsion of specific composition gives superior performance in terms of friction reduction under conditions of very high salinity with high contents of divalent salts.

The invention also relates to a process for preparing a fracturing fluid using the emulsion of the invention. A third aspect of the invention relates to a hydraulic fracturing process, the injection fluid of which has been prepared according to the method of the preceding invention.

Finally, a last aspect of the invention relates to a method of reducing friction of a fracturing fluid in a hydraulic fracturing operation using the emulsion of the invention.

More specifically, the invention relates first of all to an inverse water-in-oil emulsion comprising:

an oil;

- some water ;

- at least one water-soluble cationic copolymer with an average molecular mass greater than 3 million daltons, containing between 18 and 32 mol% of cationic monomers and 68 and 82 mol% of nonionic monomers;

- at least one inversion agent and at least one emulsifying agent, the mass ratio R of the total amount of inversion agent to the total amount of emulsifying agent being greater than 1.8,

o the inversion agent (or inverting agent) being chosen from an ethoxylated nonylphenol, preferably having between 4 and 10 ethoxylations; an ethoxylated / propoxylated alcohol, preferably having ethoxylations / propoxylations so as to have a total number of carbon between C12 and C25, an ethoxylated tridecyl alcohol and an ethoxylated / propoxylated fatty alcohol.

o the emulsifying agent being chosen from sorbitan monooleate, polyethoxylated sorbitan esters or diethanolamide of tall oil fatty acids.

The oil used to prepare the water-in-oil emulsion of the invention 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 containing saturated hydrocarbons of the aliphatic, naphthenic, paraffinic, isoparaffinic, cycloparaffinic or naphthyl type. Examples of synthetic oil are hydrogenated polydecene or hydrogenated polyisobutene, an ester such as octyl stearate or butyl oleate. Exxsol ^® line of products from Exxon is ideal. In general, the weight ratio of the aqueous phase to the oily phase in the reverse emulsion is preferably from 50/50 to 90/10, and preferably from 70/30 to 80/20.

The water-in-oil emulsion preferably comprises 12 to 24% by weight of oil, more preferably 15 to 22% by weight.

The water in oil emulsion advantageously comprises 30 to 55% by weight of water, more preferably 35 to 48% by weight.

As used herein, the term "water soluble polymer" refers to a polymer which gives 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. .

In the present invention, the term "emulsifying agent" denotes an agent capable of emulsifying water in an oil and an "inverting agent" is an agent capable of emulsifying an oil in water. More specifically, it is considered that an inverting agent is a surfactant having an HLB greater than or equal to 10, and an emulsifying agent is a surfactant having 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 31 1 - 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 provide information on 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) is fully projected into the water while the hydrophobic hydrocarbon group (molecular mass Mp) is adsorbed in the non-phase. watery.

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

The water-in-oil emulsion according to the invention can be prepared according to any method known to those skilled in the art. Generally, an aqueous solution comprising the monomer (s) and the emulsifying agent (s) is emulsified in an oily phase. Then, the polymerization is carried out by adding a free radical initiator. Reference may be made to redox couples, with cumene hydroperoxide, tertiary butylhydroxyperoxide or persulphates from among the oxidizing agents, sodium sulphite, sodium metabisulphite and Mohr's salt from among the reducing agents. 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 isothermally, adiabatically or at controlled temperature. 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 generally higher than 50 ° C or, finally, the increase in temperature is controlled so as to have a temperature curve between the isothermal curve and the adiabatic curve.

Usually, the inverting agent (s) are added at the end of the polymerization reaction, preferably at a temperature below 50 ° C.

Preferably, the emulsion of the invention contains between 12 and 50% by weight of water-soluble polymer (by dry weight), preferably between 12 to 40% by weight and even more preferably between 12 and 30% by weight.

According to another preference, for the emulsion of the invention, the mass ratio R of the total amount of inversion agent to the total amount of emulsifying agent is greater than 1.8, preferably greater than 2, again more preferably greater than 2.5, even more preferably greater than 3, still more preferably greater than 3.5, even more preferably greater than 4.

The water-soluble cationic polymer contained in the emulsion of the invention is a copolymer of nonionic and cationic monomers. The nonionic monomers are preferably chosen from acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N, N dialkylacrylamides, N, N dialkylmethacrylamides, acrylic esters; methacrylic esters. The preferred nonionic monomer is acrylamide.

The cationic monomers are preferably chosen from dimethylaminoethyl acrylate (ADAME) or its quaternized ammonium salts, dimethylaminoethyl methacrylate (MADAME) or its quaternized ammonium salts, dimethyldiallylammonium chloride (DADMAC), chloride d ' acrylamido propyltrimethyl ammonium (APTAC), and methacrylamido propyltrimethyl ammonium chloride (MAPTAC). Preferably, the quaternized ammonium salts of the ADAME or MADAME monomers are obtained by quaternization with alkyl chlorides, preferably methyl chloride. The preferred cationic monomer is dimethylaminoethyl acrylate quaternized with methyl chloride.

Several nonionic and cationic monomers can be selected to constitute the cationic copolymer. Advantageously, the water-soluble cationic polymer is a copolymer of acrylamide and dimethylaminoethyl acrylate quaternized with methyl chloride.

The water soluble cationic polymer has an average molecular weight greater than 3 million daltons. Preferably, this average molecular mass is between 3 and 30 million daltons and even more preferably between 8 and 18 million daltons.

The "average molecular weight" according to the present invention is determined by 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 values of reduced viscosity for different concentrations by a graphic method consisting in plotting the values of reduced viscosity (on the y-axis ) as a function of the concentrations (on the abscissa axis) and by extrapolating the curve to zero concentration. The intrinsic viscosity value is read on the y-axis or using the least squares method. Then the weight average molecular weight can be determined by the famous Mark-Houwink equation:

[h] = KM ^a [h] represents the intrinsic viscosity of the polymer determined by the method of measuring the viscosity in solution,

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.

The emulsion of the invention preferably contains between 0.5 and 10% by weight of inversion agent and between 0.5 and 16% by weight of emulsifying agent.

The water-in-oil emulsion advantageously comprises from 0.8 to 2% by weight of at least one emulsifying agent.

The water-in-oil emulsion preferably comprises from 3 to 6% by weight of at least one inverting agent.

Optionally the water-in-oil emulsion comprises from 1 to 40% by weight of salts, preferably from 3 to 30% by weight, even more preferably from 5 to 25% by weight and even more preferably from 7 to 17% by weight of salts .

The salts present in the water-in-oil emulsion can be, for example, sodium salts, lithium salts, potassium salts, magnesium salts, aluminum salts, ammonium salts, phosphate salts. , sulfate salts, chloride salts, citrate salts, acetate salts, hydrogen phosphate tartrate salts, water soluble inorganic salts or other inorganic salts and mixtures thereof. These salts include sodium chloride, sodium sulfate, sodium bromide, calcium chloride, ammonium sulfate, ammonium chloride, lithium chloride, lithium bromide, potassium chloride, potassium bromide, magnesium sulfate, aluminum sulfate, sodium hydrogen phosphate, potassium hydrogen phosphate, and mixtures thereof. Sodium chloride, calcium chloride, ammonium chloride, ammonium sulfate are preferred, and mixtures thereof are more preferred.

Another aspect of the invention relates to a process for preparing a fracturing fluid comprising:

a) The supply of an inverse emulsion according to the invention, b) The inversion of the inverse emulsion by adding it to a brine, containing more than 30,000 ppm of salts and with a divalent ratio R ⁺ ³0.15, R ⁺ = mass ratio: divalent salts / total salts,

c) Optionally, adding at least one proppant.

By total salts is meant the total amount of salt in the brine.

The brine can contain 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.

The brine preferably contains at least one of the following: sodium chloride, calcium chloride, sodium bromide, calcium bromide, barium chloride, magnesium chloride, zinc bromide, sodium formate and potassium formate.

Preferably the brine used for the preparation of the fracturing fluid contains more than 70,000 ppm of salts and preferentially more than 100,000 ppm of salts, preferably the brine contains from 70,000 to 350,000 ppm of salts, preferably 100,000 at 350,000 ppm.

According to an advantageous embodiment of the process for preparing the fracturing fluid:

- when the brine comprises 30,000 ppm to 70,000 ppm (upper limit excluded) of salts (step b), the ratio R of the emulsion (step a) is preferably greater than 1.8,

- when the brine comprises from 70,000 ppm to 100,000 ppm (upper limit excluded), the ratio R of the emulsion is preferably greater than 2,

- when the brine comprises 100,000 ppm to 150,000 ppm (upper limit excluded) of salts, the ratio R of the emulsion is preferably greater than 2.5,

- when the brine comprises 150,000 ppm to 200,000 ppm (upper limit excluded) of salts, the ratio R of the emulsion is preferably greater than 3,

- when the brine comprises 200,000 ppm to 250,000 ppm (upper limit excluded) of salts, the ratio R of the emulsion is preferably greater than 3.5, and

- When the brine comprises more than 250,000 ppm (upper limit excluded) of salts, the ratio R of the emulsion is preferably greater than 4. Preferably, the divalent ratio R ⁺ = mass ratio: divalent salts / total salts is greater than or equal to 0.20 and even more preferably R ⁺ > 0.25.

The inversion of the emulsion of the invention in the brine can advantageously be carried out with the device and the method of document US Pat. No. 8,383,560 where the emulsion is dissolved continuously with a multiple static mixer arrangement.

The present invention also relates to the fracturing fluid obtained by the method of the invention, in particular a fracturing fluid comprising:

- A brine solution;

- A water-soluble cationic (co) polymer according to the invention;

The oil of the reverse emulsion of the invention;

- Some water.

The proppant can be selected without limitation from sand, ceramic, bauxite, glass beads, and resin impregnated sand. It preferably represents from 0.5 to 40%, more preferably from 1 to 25% and even more preferably from 1.5 to 20%, by weight of the fracturing fluid.

The fracturing fluid according to the invention preferably comprises between 0.01% and 3% by weight of water-soluble cationic (co) polymer of the invention (added in the form of an emulsion), and even more preferably between 0.05% and 1%, by weight.

The brine that makes up the fracturing fluid may include other compounds known to those skilled in the art, such as those cited in SPE 152596, for example:

Anti-swelling agents for clays such as potassium chloride, or choline chloride, and / or

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

Oxygen reducers such as ammonium bisulfite to prevent the destruction of other components by oxidation and corrosion of the injection tubes, and / or Anticorrosion additives to protect the tubes against oxidation by residual amounts of oxygen, N, N dimethylformamide being preferred, and / or

Lubricants such as oil distillates, and / or

- Chelating agents for iron such as citric acid, EDTA (ethylenediaminetetraacetic acid), phosphonates, and / or

Anti-scale products such as phosphates, phosphonates, polyacrylates or ethylene glycol.

According to a preferred embodiment, the process for preparing a fracturing fluid comprises:

a) The supply of an inverse emulsion according to the invention containing at least between 12 and 30% by mass of a water-soluble cationic copolymer containing between 18 and 32 mol% of dimethylaminoethyl acrylate quaternized with methyl chloride and 68 and 82 mol% acrylamide; at least one inversion agent and at least one emulsifying agent, the mass ratio R of the total amount of inversion agent to the total amount of emulsifying agent being greater than 2.5,

b) The inversion of the reverse emulsion by adding it to a brine containing more than 100,000 ppm of salts and with a divalent ratio R ⁺ > 0.2, R ⁺ = mass ratio: divalent salts / total salts, in order to obtain a mass concentration of water-soluble cationic copolymer in the injection fluid of between 0.05 and 1%.

c) Optionally, adding at least one proppant.

A third aspect of the invention relates to a method of hydraulic fracturing of an unconventional underground oil or gas reservoir comprising the preparation of a fracturing fluid as described above, and the injection of said fracturing fluid into an underground formation. .

More specifically, the invention relates to a method of fracturing an underground formation comprising:

aa) the supply of a fracturing fluid obtained according to the preparation method described above,

bb) the introduction of the injection fluid into a part of the underground formation, cc) the fracturing of the underground formation with the injection fluid, dd) the recovery of a mixture of gas, oil and fluid aqueous. The injection is carried out under pressure so as to create fractures distributed all along 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.

The injection of these compounds makes it possible to restore 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 surface-active 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 alkylsulphates, of alkylethersulphates, of arylalkylsulphates, of arylalkylethersulphates, of alkylsulphonates, of alkylethersulphonates, d arylalkylsulfonates, of arylalkylethersulfonates, of alkylphosphates, of alkyletherphosphates, of arylalkylphosphates, of arylalkyletherphosphates, of alkylphosphonates, of alkyletherphosphonates, of arylalkylphosphonates, of alkyletherphosphonates, of arylalkylcarboxylcarboxylcarboxylates, of arylalkylcarboxylcarboxylcarboxylcarboxylarboxylcarboxylcarboxylates, 'arylalkylethercarboxylates, of polyalkyl ethers, of arylalkyl polyethers.

Finally, a fourth and last aspect of the invention relates to a method of reducing the friction of a fracturing fluid in a hydraulic fracturing operation of an underground reservoir of unconventional oil or gas, comprising the preparation of a fluid fracturing process as described above, and injecting said fracturing fluid into a subterranean formation. The friction reduction makes it possible to reduce or eliminate the losses linked to friction during the injection of the fracturing fluid.

For hydraulic fracturing, the reduction of friction implies that the polymer of the fracturing fluid brings 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 l proppant suspended at the fracture as the shear decreases. The invention and the advantages resulting therefrom will become apparent from the following exemplary embodiments.

EXAMPLES Example 1 (counter example): Emulsion containing 20% by weight of a polymer comprising 15 mol% of cationic monomers

An aqueous phase is prepared with 27.00% by weight of an acrylamide solution (50% by weight in water), 8.12% by weight of solution of ADAME-MC (dimethylaminoethyl quaternized with chloride of methyl, 80% by weight in water), 39.87% by weight of deionized water and 0.02% by weight of Versenex 80.

An oily phase is prepared from 23.45% by weight of oil (Exxsol® D100 S) and the following emulsifying agents: 1.16% by weight of Witcamide®51 1 (diethanolamine of tall oil fatty acids ), 0.16% by weight of Span® 80 (sorbitan monooleate) and 0.23% by weight of Tween® 81 (5EO sorbitan monooleate).

The water phase is added to the oil phase while mixing to form an emulsion. The resulting dispersion is bubbled with nitrogen for 30 minutes while the temperature is stabilized at 25 ° C, at which time 0.002% by weight of peroxide is added to the emulsion and a 0.075% by weight solution of sodium metabisulfite. (MBS) is introduced into the dispersion at a rate of 0.1 milliliter per minute. The polymerization temperature is controlled between 38 ° C and 42 ° C for about 90 minutes. The residual monomers are trapped by introducing a 0.03% by weight solution of sodium metabisulphite (MBS) at a flow rate of 1.0 milliliter per minute. A water-in-oil polymer emulsion is obtained containing 20% of active copolymer of acrylamide and ADC. 1.75% by weight of an inverting agent (Marlophen® NP 8, nonylphenol and polyethylene glycol 8 EO ethers) is added to the water-in-oil polymer emulsion to facilitate focusing during the process. 'use. The mass ratio R is equal to 1.5.

An aqueous phase is prepared with 23.78% by weight of an acrylamide solution (50% by weight in water), 10.14% by weight of solution of ADAME-MC (dimethylaminoethyl quaternized with chloride of methyl, 80% by weight in water), 41.08% by weight of deionized water and 0.02% by weight of Versenex 80.

An oily phase is prepared from 23.45% by weight of oil (Exxsol® D100 S) and the following emulsifying agents; 1.16% by weight of Witcamide®51 1 (diethanolamine of tall oil fatty acids), 0.16% by weight of Span® 80 (sorbitan monooleate) and 0.23% by weight of Tween® 81 (monooleate sorbitan 5EO).

The water phase is added to the oil phase while mixing to form an emulsion. The resulting dispersion is bubbled with nitrogen for 30 minutes while the temperature is stabilized at 25 ° C, at which time 0.002% by weight of peroxide is added to the emulsion and a 0.075% by weight solution of sodium metabisulphite. (SMBS) is introduced into the dispersion at a rate of 0.1 milliliters per minute. The polymerization temperature is controlled between 38 ° C and 42 ° C for about 90 minutes. The residual monomers are trapped by introducing a 0.03% by weight solution of sodium metabisulfite (SMBS) at a flow rate of 1.0 milliliter per minute. A water-in-oil polymer emulsion is obtained containing 20% of active copolymer of acrylamide and ADAME-MC.

1.75% by weight of an inverting agent (Marlophen® NP 8, ethers of nonylphenol and polyethylene glycol 8 EO) is added to the water-in-oil polymer emulsion to facilitate focusing during the process. 'use. The mass ratio R is equal to 1.5.

An aqueous phase is prepared with 16.20% by weight of an acrylamide solution (50% by weight in water), 14.87% by weight of solution of ADAME-MC (dimethylaminoethyl quaternized with chloride of methyl, 80% by weight in water), 43.92% by weight of deionized water and 0.02% by weight of Versenex 80. An oily phase is prepared from 23.45% by weight of oil (Exxsol® D100 S) and the following emulsifying agents: 1.16% by weight of Witcamide®51 1 (diethanolamine of tall oil fatty acids) , 0.16% by weight of Span® 80 (sorbitan monooleate) and 0.23% by weight of Tween® 81 (5EO sorbitan monooleate).

The water phase is added to the oil phase while mixing to form an emulsion. The resulting dispersion is bubbled with nitrogen for 30 minutes while the temperature is stabilized at 25 ° C, at which time 0.002% by weight of peroxide is added to the emulsion and a 0.075% by weight solution of sodium metabisulfite. (SMBS) is introduced into the dispersion at a rate of 0.1 milliliters per minute. The polymerization temperature is controlled between 38 ° C and 42 ° C for about 90 minutes. The residual monomers are trapped by introducing a 0.03% by weight solution of sodium metabisulfite (SMBS) at a flow rate of 1.0 milliliter per minute. This gives a water-in-oil polymer emulsion containing 20% of active acrylamide / ADC copolymer.

1.75% by weight of an inverting agent (Marlophen® NP 8, nonylphenol and polyethylene glycol 8 EO ethers) is added to the water-in-oil polymer emulsion to facilitate focusing during the process. 'use. The mass ratio R is equal to 1.5.

The following examples are carried out with a mass ratio R according to the invention. Examples 4 and 7, then 5 and 8 and finally 6 and 9 are produced respectively according to the same process as Examples 1, 2 and 3 but with higher amounts of Marlophen® NP 8 (inverting agent). Table 1 describes the mass ratio R for each example.

[Table 1]

Table 1 Mass ratios R of water-in-oil emulsions

Friction flow loop test A friction flow loop was constructed from stainless steel tubing with a 1/4 "outside diameter and a total length of 20 feet. The test solutions are pumped to the bottom of the pipe. a 5 liter conical reservoir The solution passes through the tubing and is returned to the reservoir The flow is obtained using a triplex pump fitted with a variable speed drive.

4 liters of 9% CaCh or API or 2xAPI brine are prepared in the sample reservoir and the pump is started and set to deliver a flow rate of 1.5 gai / min. The 9% CaCh brine corresponds to 9 g of CaCh in 100 ml of water, its R ⁺ is equal to 1.00. API brine is defined as being 8.5 g of NaCl + 2.5 g of CaCh in 100 ml of water, its R + being equal to 0.20. The 2 x API brine corresponds to 17 g of NaCl + 5 g of CaCl2 in 100 ml of water, its R ⁺ is equal to 0.20. The saline solution is recirculated until the temperature equilibrates to 25 ° C and a stabilized pressure differential is reached. This pressure is recorded as the "initial pressure" of the 9% CaCl2 or API or 2x API brine.

The test amount of pure water-in-oil emulsion polymer is quickly injected with a syringe into the sample reservoir containing the 9% CaCh or API or 2xAPI brine and a timer is started. The dose is recorded in gallons of water-in-oil emulsion per thousand gallons of 9% CaCh or API brine or 2 x API (gpt). The pressure is recorded every second for 5 minutes. The percentage of friction reduction (% FRt) at a given time ΐ is calculated from the initial pressure drop DRί and the pressure drop at time t, APt, using the equation:

Results

In Table 2, all emulsions contain 20% by weight cationic polymer. [Table 2]

The results show that the friction reduction performance is improved when the mass ratio R is increased. As the salt concentrations increase, the friction reduction performance decreases. But when the mass ratio R is chosen and adapted (according to the scope of the invention), it becomes possible to obtain very good friction performance in brines and even in high brines.

The friction reduction performance is improved when the cationicity of the polymer is 20 mol%. Lower cationicity (15%) and higher cationicity (35%) provide lower performance.

Claims

1. Water-in-oil inverse emulsion comprising:

- an oil;

- some water ;

- at least one water-soluble cationic copolymer of average molar mass, preferably average molar mass by weight, greater than 3 million daltons, containing between 18 and 32 mol% of cationic monomers and 68 and 82 mol% of nonionic monomers;

- the inverting agent being chosen from an ethoxylated nonylphenol, preferably having between 4 and 10 ethoxylations; an ethoxylated / propoxylated alcohol, preferably having ethoxylations / propoxylations so as to have a total number of carbon between C12 and C25, an ethoxylated tridecyl alcohol and an ethoxylated / propoxylated fatty alcohol.

- the emulsifying agent being chosen from sorbitan monooleate, polyethoxylated sorbitan esters or diethanolamide of tall oil fatty acids.

2. Emulsion according to claim 1, characterized in that it comprises between 12 and 50% by weight of at least one water-soluble polymer, preferably between 12 to 40% by weight and even more preferably between 12 and 30% by weight.

3. Emulsion according to claim 1 or 2 in that the mass ratio R of the total amount of inversion agent to the total amount of emulsifying agent is greater than 2, even more preferably greater than 2.5, even more. preferably greater than 3, even more preferably greater than 3.5, still more preferably greater than 4.

4. Emulsion according to any one of claims 1 to 3 characterized in that the nonionic monomers of the water-soluble cationic copolymer are chosen from acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N, N dialkylacrylamides , the N, N dialkylmethacrylamides, acrylic esters; methacrylic esters; with acrylamide as preferred monomer.

5. Emulsion according to any one of claims 1 to 4 characterized in that the cationic monomers of the water-soluble cationic copolymer are chosen from dimethylaminoethyl acrylate (ADAME) or its quaternized ammonium salts, dimethylaminoethyl methacrylate (MADAME) or its quaternized ammonium salts, dimethyldiallylammonium chloride (DADMAC), acrylamido propyltrimethyl ammonium chloride (APTAC), and methacrylamido propyltrimethyl ammonium chloride (MAPTAC), preferably the quaternized ammonium salts of ADAME monomers or MADAME are obtained by quaternization with alkyl chlorides, preferably methyl chloride, the preferred cationic monomer is dimethylaminoethyl acrylate quaternized with methyl chloride.

6. Emulsion according to any one of claims 1 to 5 characterized in that the water-soluble cationic polymer has an average molar mass of between 3 and 30 million daltons and preferably between 8 and 18 million daltons.

7. Emulsion according to any one of claims 1 to 6, characterized in that it contains between 0.5 and 10% by weight of inverting agent and 0.5 and 16% by weight of emulsifying agent.

8. A method of preparing a fracturing fluid comprising:

a) The provision of an inverse emulsion according to any one of claims 1 to 7,

b) The inversion of the inverse emulsion by adding it to a brine, containing more than 30,000 ppm of salts and with a divalent ratio R ⁺ > 0.15, R ⁺ = mass ratio: divalent salts / total salts,

c) Optionally, adding at least one proppant.

9. Process for preparing a fracturing fluid according to claim 8, characterized in that for step b) the brine contains more than 70,000 ppm of salts and preferably more than 100,000 ppm of salts.

10. Process for preparing a fracturing fluid according to claims 8 and 9 characterized in that for step b) the brine has a divalent ratio R ⁺ > 0.20 and preferably R ⁺ ³0.25.

11. A method of preparing a fracturing fluid according to claims 8 to 10 comprising:

a) The supply of an inverse emulsion according to the invention containing at least between 12 and 30% by mass of a water-soluble cationic copolymer containing between 18 and 32 mol% of dimethylaminoethyl acrylate quaternized with methyl chloride and between 68 and 82 mol% acrylamide; at least one inversion agent and at least one emulsifying agent, the mass ratio R of the total amount of inversion agent to the total amount of emulsifying agent being greater than 2.5,

b) The inversion of the reverse emulsion by adding it to a brine containing more than 100,000 ppm of salts and with a divalent ratio R ⁺ > 0.20, R ⁺ = mass ratio: divalent salts / total salts, in order to d 'obtain a mass concentration of water-soluble cationic copolymer in the injection fluid of between 0.05 and 1%.

c) Optionally, adding at least one proppant.

12. A method of fracturing an underground formation comprising:

aa) the supply of a fracturing fluid obtained according to the preparation method of claims 8 to 11,

bb) the introduction of the injection fluid into a part of the underground formation, cc) the fracturing of the underground formation with the injection fluid,

dd) recovering a mixture of gas, oil and aqueous fluid.

13. A method of reducing friction of a fracturing fluid in a hydraulic fracturing operation of an underground oil or unconventional gas reservoir comprising the preparation of a fracturing fluid according to claims 8 and 1 1 and injection. of said fracturing fluid in a subterranean formation.