Fracturing fluid comprising a polymer of a crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid and hydraulic fracturing process

Abstract

The present invention relates to a fracturing fluid comprising at least one propping agent and at least one water-soluble polymer obtained from the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid. The invention also relates to a process for preparing said fluid, as well as a hydraulic fracturing process using said fluid.

Description

Title of the invention: Fracturing fluid comprising a polymer of a crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid and hydraulic fracturing process Scope of the invention

[0001] The invention relates to a fracturing fluid comprising at least one propping agent and at least one water-soluble polymer prepared from the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0002] The invention also relates to a hydraulic fracturing process for unconventional underground hydrocarbon (oil and / or gas) reservoirs using said fluid. Prior art

[0003] 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 for economical hydrocarbon production.

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

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

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

[0007] Hydraulic fracturing

[0008] Hydraulic fracturing aims to create additional permeability and generate larger gas or oil production areas. Indeed, low permeability, natural barriers of compact layers, and impermeability caused by drilling operations severely limit production. The gas or oil contained in the unconventional reservoir cannot easily migrate from the rock to the well without stimulation.

[0009] These hydraulic fracturing operations on horizontal wells began in 1960 in the Appalachians and, today, several tens of thousands of operations have taken place in the United States.

[0010] The technologies for studying, modelling the reservoir, drilling, cementing and stimulation have become increasingly sophisticated and implement equipment that allows these operations to be carried out in increasingly shorter times with precise analysis of the results.

[0011] Reservoir stimulation by hydraulic fracturing

[0012] These operations consist of injecting water at high pressure and a very high flow rate of This involves creating fractures distributed perpendicularly to the production wells. The process is generally carried out in several stages to create fractures along the entire length of the horizontal well, thus allowing for maximum reservoir volume coverage.

[0013] In order to keep these fractures open, a support agent (for example sand, plastics or calibrated ceramics) is added so as to prevent the closure of these fractures and to maintain the capillarity created once the injection has stopped.

[0014] Water alone is insufficient to achieve good placement efficiency of the support agent due to its low viscosity. This limits its ability to hold the support agent in place within fractures. To overcome this problem, fracturing fluids containing viscosifying compounds have been developed.

[0015] By definition, we consider (in the description of the prior art and the invention) that a compound is viscosifying when it increases the viscosity of the solution in which it is dissolved.

[0016] In addition to having viscosifying properties, the compound must have a specific rheological profile. Indeed, the compound must be able to generate low viscosity so as not to hinder the transport and pumping of the fluid containing the proppant during the high shear stresses experienced during the injection of the fracturing fluid. Once injected, this same compound must be able to generate sufficient viscosity when the shear stress decreases to support the proppant and retain it within the fractures.

[0017] The viscosifying compound, generally a polymer, must therefore provide shear-thinning properties to the solution in order to have a relatively low viscosity during injection (at high shear) and a high viscosity in order to keep the support agent in suspension at the fracture site when the shear decreases.

[0018] The viscoelastic properties of polymers in solution must also be taken into consideration. This viscoelasticity, and its importance in the application, is described in SPE 147206 (Fracturing Fluid Comprised of Components Sourced Solely from the Food Industry Provides Superior Proppant Transport - David Loveless, Jeremy Holtsclaw, Rajesh Saini, Phil Harris, and Jeff Fleming, SPE, Halliburton) through visual observations in static or dynamic experiments, or through rheological measurements, such as the measurement of viscous and elastic moduli (G' and G"), or the measurement of viscosity as a function of shear using a rheometer. Thus, elastic properties will be advantageous for ensuring the transport and suspension of the fracture support agent.

[0019] The choice of polymer is therefore not obvious and requires a thorough rheological study in order to obtain satisfactory results.

[0020] Among the prior art viscosifying compounds for aqueous solutions, one can cite natural substances such as guar gum and its derivatives such as hydroxypropyl guar (HPG) or carboxymethyl hydroxypropyl guar (CMHPG); cellulosic derivatives such as carboxymethyl cellulose or hydroxyethyl cellulose. These compounds are notably described in patents US4033415, US3888312 and US4801389. In the document SPE 152596 (Hydraulic Fracturing 101: What Every Representative, Environmentalist, Regulator, Reporter, Investor, University Researcher, Neighbor and Engineer Should Know About Estimating Frac Risk and Improving Frac Performance in Unconventional Gas and Oil Wells - George E. King, Apache Corporation), the latest advances relating to the performance of fracturing fluids are discussed in detail.

[0021] However, these natural substances, and in particular guar derivatives, are also useful in other applications, such as the food or textile industries, and the growth The exploitation of oil and gas resources from unconventional reservoirs competes with these other applications. This creates pressure on the availability of these products and leads to pricing problems.

[0022] Other petrochemical compounds can have viscosity-increasing properties. Synthetic polymers are a good example. Poly(meth)acrylamides, possibly partially hydrolyzed, and poly(meth)acrylates and their polymers are particularly well known. These polymers develop viscosity primarily due to their molar mass and interchain ionic repulsions. These polymers are described in patents GB 951147, US 3,727,689, US 3,841,402, and US 3,938,594. The mechanism governing viscosity is linked to an increase in hydrodynamic volume due to inter- and intrachain repulsions, interchain entanglements, etc.

[0023] However, in the presence of high salinity or high operating temperatures, these polymers do not develop strong entanglements and repulsions, resulting in a significant decrease in their viscosifying capacity, especially after undergoing the shear stress of the pumping step. Furthermore, these polymers generally do not exhibit sufficient viscoelastic properties to support the proppant in the fracture. The dosage of these polymers must be increased to high levels to achieve the necessary proppant suspension properties. However, such dosage levels are not economically viable.

[0024] Polymers used to have viscosifying properties can advantageously also be friction reducers allowing to reduce pressure loss in turbulent medium and greatly increase flow rate at identical power and pipe diameter.

[0025] Synthetic polymers based on sodium 2-acrylamido-2-methylpropane sulfonate exhibit interesting friction-reducing properties in aqueous solution. These polymers are also known for their resistance to shear and thermal degradation, particularly in saline solutions. However, obtaining very high molecular weight polymers based on sodium 2-acrylamido-2-methylpropane sulfonate is difficult, and these polymers also exhibit solubility problems as their molecular weight increases. To achieve optimal friction reduction and high viscosity generation, it is essential that the polymer dissolves rapidly, especially in saline solution, and that it has a very high molecular weight. Description of the invention

[0026] The Applicant has found and developed a fracturing fluid which allows for a very high friction reduction effect while guaranteeing an improved viscosifying effect in saline (brine) or non-saline (water) solution.

[0027] Furthermore, the good solubility of the fracturing fluid polymer, combined with its sulfonated anionic character, prevents its adsorption onto the rock, resulting in increased conductivity and therefore higher hydrocarbon (oil and / or gas) production yields. Oil (or oils) extracted from an underground reservoir is also called petroleum. It is generally a mixture of hydrocarbons.

[0028] According to the present invention, the improved performance of the fracturing fluid containing polymers, which are obtained from the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid, allows for a reduction in the quantity of product (in particular the polymer) required and therefore a reduction in overall water consumption and greenhouse gas emissions such as CO2.

[0029] A first aspect of the invention relates to a fracturing fluid comprising at least one propping agent and at least one water-soluble polymer prepared from the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0030] A second aspect of the invention relates to a method for manufacturing a fracturing fluid with at least one water-soluble polymer prepared from the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0031] A third aspect of the invention relates to a hydraulic fracturing process for an unconventional underground oil or gas reservoir using the fracturing fluid according to the invention.

[0032] A fourth aspect of the invention relates to a friction reduction method using a fracturing fluid in a hydraulic fracturing operation of an unconventional underground oil or gas reservoir using the fracturing fluid according to the invention.

[0033] The term "polymer" refers to a homopolymer or a copolymer. A copolymer is defined as a polymer obtained from at least two different monomers. It can therefore be a copolymer of at least two monomers chosen from anionic monomers, cationic monomers, nonionic monomers, zwitterionic monomers, hydrophobic monomers, and mixtures thereof.

[0034] By "hydrophilic monomer" is meant a monomer which has an octanol / water partition coefficient, Kow, of less than 1, in which the partition coefficient Kow is determined at 25°C in an octanol / water mixture having a volume ratio of 1 / 1, at a pH between 6 and 8.

[0035] By "hydrophobic monomer" is meant a monomer which has an octanol / water partition coefficient, Kow, greater than 1, in which the partition coefficient Kow is determined at 25°C in an octanol / water mixture having a volume ratio of 1 / 1, at a pH between 6 and 8.

[0036] The octanol / water partition coefficient, Kow, represents the ratio of the concentrations (g / L) of a monomer between the octanol phase and the aqueous phase. It is defined as follows:

[0037] [Math.l] _ [monomer] "■ow r [my name mother Jea]

[0038] By definition, a water-soluble polymer is a polymer which gives an aqueous solution without insoluble particles when dissolved under stirring at 25°C and with a concentration of 50 gL 1 in water.

[0039] By "X and / or Y" means "X", or "Y", or "X and Y".

[0040] Also part of the invention are all possible combinations between the The disclosure includes various embodiments, whether preferred or given by way of example. Furthermore, when ranges of values ​​are specified, the bounds are included within those ranges. The disclosure also encompasses all combinations of the bounds within those ranges. For example, the value ranges "1-20, preferably 5-15" imply the disclosure of the ranges "1-5", "1-15", "5-20", and "15-20", as well as the values ​​1, 5, 15, and 20.

[0041] Crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid

[0042] The crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid has a powder X-ray diffraction pattern comprising peaks at 11.7°; 12.2°; 13.2°; 13.5°; 15.6°; 16.8°; 17.8°; 18.5°; 19.1°; 20.6°; 21.4°; 23.3°; 25.1°; 25.8°; 26.9°; 29.1°; 29.5°; 31.0°; 33.0°; 33.6°; 34.4°; 35.2°; 35.9°; 37.1°; 38.4°; 39.6°; 41.1°; 42.9°; 45.1°; 46.0°; 47.2°; 47.6°. The uncertainty of these peaks is generally on the order of 0.1°.

[0043] X-ray crystallography, also known as X-ray diffractometry, is an analytical technique used to study the structure of crystalline matter at the atomic scale. It relies on the physical phenomenon of X-ray diffraction. A diffractometer with a copper source can be used.

[0044] A powder formed from a particular crystalline phase always exhibits diffraction peaks in the same directions. This diffraction pattern thus forms a true signature of the crystalline phase. It is therefore possible to determine the nature of each crystalline phase within a mixture or a pure product.

[0045] This signature is specific to each organic or inorganic crystalline compound, and takes the form of a list of position peaks at an angle of 20 (2-theta).

[0046] This technique is used to characterize matter, in particular the different crystalline forms that can exist for the same chemical molecule.

[0047] The crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid has a Fourier transform infrared spectrum comprising peaks at 3576 cm⁻¹, 3485 cm⁻¹, 3310 cm⁻¹, 3079 cm⁻¹, 2975 cm⁻¹, 1658 cm⁻¹, 1629 cm⁻¹, 1543 cm⁻¹*, 1403 cm⁻¹, 1321 cm⁻¹, 1301 cm⁻¹, 1205 cm⁻¹, 1187 cm⁻¹, 1163 cm⁻¹, 1046 cm⁻¹, 980 cm⁻¹, and 629 cm⁻¹'. The uncertainty of these peaks is generally on the order of + / - 8 cm⁻¹.

[0048] Infrared measurement is carried out by Fourier transform, for example by means of a Perkin Elmer Spectrum 100 type spectrometer equipped with a single reflection ATR Polarization accessory, the accuracy of which is 8cm1.

[0049] Fourier transform infrared spectroscopy is the analysis of vibrations emitted, absorbed, or scattered by molecules. This technique is sensitive to so-called short interactions (influence of the unit cell on the bonds). In most cases, the Fourier transform infrared spectra of different crystal systems differ significantly. The Fourier transform infrared spectrum therefore reflects the details of the crystal structure of a chemical compound.

[0050] Generally, and unless otherwise indicated, the X-ray diffraction pattern and the infrared spectrum are obtained at 20°C and at a pressure of 1 absolute atmosphere (101,325 Pa).

[0051] The crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid has a minimum ignition energy greater than 500 mJ, preferably greater than 1000 mJ (1 mJ = 103 joules).

[0052] The minimum ignition energy represents the minimum energy that must be supplied to a product (chemical compound) to cause it to ignite. The energy can be electrical or thermal. The minimum ignition energy is an essential factor for considering the risk of explosion during the handling of the product (transfer, storage, reaction, shaping, etc.).

[0053] The minimum ignition energy depends on the properties of the powder (composition) as well as its macromolecular structure (particle size, crystalline shape, specific surface area).

[0054] In the case of solids, this energy is the minimum energy of an electrical spark capable of igniting a dust cloud. The higher the value of the minimum ignition energy, the lower the risk posed by the solid during its use, handling, and storage.

[0055] The measurement of the minimum ignition energy is carried out according to the standard NF EN 13821.

[0056] The crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid exhibits 4 thermal phenomena with the differentiating calorimetry technique scanning rential, at 49.8°C; 144.8°C; 169.8°C and 254.3°C. The uncertainty in the observation of these phenomena is generally on the order of 10°C, advantageously 5°C or less.

[0057] Thermal phenomena are measured by differential scanning calorimetry (DSC). This technique exploits the measurement of the heat variation associated with the thermal denaturation of the compound when it is heated at a constant rate, for example with a heating ramp of 10°C / minute.

[0058] Process for manufacturing the crystalline form of sodium salt

[0059] The process for manufacturing the crystalline form of the sodium salt of the acid 2-Acrylamido-2-methylpropane sulfonic acid comprises at least the following successive steps: 1) mixing of 2-acrylamido-2-methylpropane sulfonic acid with an aqueous solution SAi and at least one sodium salt, advantageously for at least 1 minute, in order to form an aqueous solution or aqueous suspension SA2; 2) distillation at a pressure lower than atmospheric pressure of the aqueous solution or aqueous suspension SA2 in order to form a suspension Si; 3) solid / liquid separation of the Si suspension and isolation of the Si suspension crystals obtained at the end of step 2) in the form of a composition Ci. The crystals obtained are in crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0060] By sodium salt(s) in step 1), means at least one inorganic salt(s), for example sodium hydroxide, sodium carbonate, sodium bicarbonate or mixtures thereof.

[0061] The temperature and mixing time in step 1) may vary depending, in particular, on the concentration of 2-acrylamido-2-methylpropane sulfonic acid. Those skilled in the art will know how to adjust the temperature variation and mixing time to optimize crystal formation.

[0062] The process for manufacturing the crystalline form of the sodium salt can be carried out on any form of 2-acrylamido-2-methylpropane sulfonic acid, such as the needle form or the hydrated form.

[0063] The manufacturing process can be carried out on any degree of purity of 2-acrylamido-2-methylpropane sulfonic acid.

[0064] Thus, the process can be carried out downstream of any type of manufacturing process for 2-acrylamido-2-methylpropane sulfonic acid. It can also be carried out on crystals of 2-acrylamido-2-methylpropane sulfonic acid already obtained.

[0065] Step 1) of the process for manufacturing the crystalline form of sodium salt:

[0066] 2-Acrylamido-2-methylpropane sulfonic acid is produced by a manufacturing process as described above (acrylonitrile, fuming sulfuric acid and iso butylene). 2-Acrylamido-2-methylpropane sulfonic acid can be in the form of a fine powder or shaped in a controlled manner by a process such as compaction, granulation, or extrusion.

[0067] 2-Acrylamido-2-methylpropane sulfonic acid can be added to an aqueous solution S Ai before, after or in parallel with sodium salt, preferably the addition is done in parallel.

[0068] Advantageously, the concentration of the aqueous solution or aqueous suspension SA2 in sodium salt is between 1% by weight and saturation, preferably between 10% by weight and saturation, more preferably between 20% by weight and saturation, more preferably between 30% by weight and saturation, more preferably between 40% by weight and saturation, and even more preferably between 50% by weight and saturation, by weight relative to the weight of the aqueous solution or aqueous suspension SA2.

[0069] 2-Acrylamido-2-methylpropane sulfonic acid and the sodium salt can be added all at once or in several stages. Preferably, they are added in several stages.

[0070] When the addition is made in several stages, the 2-acrylamido-2-methylpropane sulfonic acid and the sodium salt are added in fractions.

[0071] When 2-acrylamido-2-methylpropane sulfonic acid and sodium salt are added in fractions, there is no limit to the number of fractions, advantageously there are at least two fractions, preferably at least three fractions.

[0072] There is no limitation as to the order of addition between 2-acrylamido-2-methylpropane sulfonic acid and the sodium salt. They can be added at the same time (i.e. in parallel), one after the other (2-acrylamido-2-methylpropane sulfonic acid first then the sodium salt, or vice versa), or alternately (a first fraction of 2-acrylamido-2-methylpropane sulfonic acid, then a first fraction of the sodium salt, followed by a second fraction of 2-acrylamido-2-methylpropane sulfonic acid then a second fraction of the sodium salt and so on), preferably they are added at the same time.

[0073] When adding one after the other or alternately, the start of the addition of the second compound (whether it be 2-acrylamido-2-methylpropane sulfonic acid or sodium salt) can start before the end of the addition of the first compound.

[0074] A first fraction Fl of 2-acrylamido-2-methylpropane sulfonic acid advantageously represents at least 1 mol% of the total 2-acrylamido-2-methylpropane sulfonic acid present in the aqueous solution or aqueous suspension SA2, preferably at least 5 mol%, more preferably at least 10 mol%, even more preferably at least 15 mol%, and even more preferably at least 20 mol%.

[0075] A second fraction F2 of 2-acrylamido-2-methylpropane sulfonic acid advantageously represents at least 1 mol% of the total 2-acrylamido-2-methylpropane sulfonic acid present in the aqueous solution or aqueous suspension SA2, preferably at least 5 mol%, more preferably at least 10 mol%, even more preferably at least 15 mol% and even more preferably at least 20 mol%.

[0076] A third fraction F3 of 2-acrylamido-2-methylpropane sulfonic acid advantageously represents at least 1 mol% of the total 2-acrylamido-2-methylpropane sulfonic acid present in the aqueous solution or aqueous suspension SA2, preferably at least 5 mol%, more preferably at least 10 mol%, even more preferably at least 15 mol% and even more preferably at least 20 mol%.

[0077] In a particular mode, the process is carried out continuously; in this case, 2-acrylamido-2-methylpropane sulfonic acid and sodium salt are added continuously.

[0078] The amount of 2-acrylamido-2-methylpropane sulfonic acid in the aqueous solution or aqueous suspension SA2 is advantageously between 10 and 90% by weight relative to the total weight of the aqueous solution or aqueous suspension SA2, preferably between 20 and 85% by weight, more preferably between 30 and 80% by weight.

[0079] The mixing in step 1) is advantageously carried out at a temperature between 0 and 90°C, preferably between 5 and 60°C, more preferably between 10 and 40°C, in order to obtain the aqueous solution or aqueous suspension SA2.

[0080] In a particular mode, the aqueous solution or aqueous suspension SA2 may comprise one or more organic solvents.

[0081] The amount of organic solvent can vary depending on the temperature and the amount of 2-acrylamido-2-methylpropanesulfonic acid or sodium salt. This amount is not limited as long as it does not prevent the formation of the crystalline form of the sodium salt of 2-acrylamido-2-methylpropanesulfonic acid. A person skilled in the art will be able to determine this limit, which is a routine task. Generally, the aqueous solution or aqueous suspension SA2 contains more water (by volume) than organic solvent.

[0082] The organic solvent(s) are advantageously chosen from the following compounds: - organic acids, advantageously carboxylic acids comprising 1 to 8 carbons; - amides advantageously comprising from 1 to 8 carbon atoms; - alcohols advantageously comprising from 1 to 8 carbon atoms; - ketones advantageously comprising 3 to 8 carbon atoms; - ethers advantageously comprising from 2 to 8 carbon atoms; - esters advantageously comprising from 2 to 8 carbon atoms; - alkanes advantageously comprising 4 to 8 carbon atoms; - halogenated hydrocarbon compounds advantageously comprising from 2 to 8 carbon atoms; - nitriles advantageously comprising from 1 to 8 carbon atoms; or - their mixtures.

[0083] When an organic solvent is used in the context of the invention, the temperature can be adjusted so that the solvent + water mixture remains in liquid form.

[0084] These compounds can be linear or branched. They can be saturated or include unsaturations, an unsaturation corresponding to a double or triple bond (for example C=C or C=C).

[0085] Preferably, the organic solvent is chosen from acrylonitrile, isopropanol, acrylic acid, acetic acid, or mixtures thereof. Preferably, the organic solvent is acrylonitrile.

[0086] The organic solvent is generally in liquid form at the temperature at which steps 2) and 3) are carried out. In addition, it is advantageously partially miscible in water, preferably completely miscible in water.

[0087] The organic solvent may, where appropriate, allow the solubilization of any impurities or by-products present with the 2-acrylamido-2-methylpropane sulfonic acid used to form the aqueous solution or aqueous suspension SA2. However, 2-acrylamido-2-methylpropane sulfonic acid is not necessarily soluble in the solvent.

[0088] In a preferred mode according to the invention, the aqueous solution or aqueous suspension SA2 does not contain any organic solvent.

[0089] The mixing time between the aqueous solution SAb and 2-acrylamido-2-methylpropane sulfonic acid is advantageously at least 1 minute, preferably between 1 minute and 600 minutes, more preferably between 5 minutes and 400 minutes, and even more preferably between 10 minutes and 240 minutes.

[0090] The mixing of the compounds in step 1) can be carried out by various technologies. By way of example and without limitation, we can mention reactors with agitators, loop reactors, static mixers, microreactors, plug flow reactors, agitated filter-dryer reactors, for example Nutsche, paddle mixers, twin-cone mixers, plowshare mixers, and disc mixers.

[0091] The pH of step 1) is advantageously controlled between 6 and 14, preferably between 8 and 14, more preferentially between 10 and 14, even more preferentially between 12 and 14, even more preferentially between 13 and 14.

[0092] Step 2) of the process for manufacturing the crystalline form of sodium salt:

[0093] The distillation of the aqueous solution or aqueous suspension SA2 is carried out at a pressure lower than atmospheric pressure. It is generally carried out in a vacuum distillation device, which is typically an evaporator. It is therefore also referred to here as "vacuum distillation".

[0094] When the aqueous solution or aqueous suspension SA2 is distilled, typically by passing through an evaporator, crystals of the sodium salt of acrylamido-2-methyl-2-propanesulfonic acid begin to form. There is then coexistence of the aqueous solution or aqueous suspension SA2 comprising acrylamido-2-methyl-2-propanesulfonic acid, at least one sodium salt, and crystalline solid particles of the sodium salt of acrylamido-2-methyl-2-propanesulfonic acid.

[0095] The distillation of the aqueous solution or aqueous suspension SA2 can be carried out using an evaporator. This can be a falling film evaporator, a rising film evaporator, a scraped thin-film evaporator, a short-path evaporator, a forced-circulation evaporator, a spiral-tube evaporator, or a flash-cooling evaporator. It can also be a continuously stirred reactor. Preferably, the distillation takes place in a scraped thin-film evaporator, a short-path evaporator, or a forced-circulation evaporator. Even more preferably, the distillation takes place in a scraped thin-film evaporator.

[0096] Generally, an evaporator is a device comprising an inlet for the solution to be treated (aqueous solution or aqueous suspension SA2), an outlet for removing the distilled solvent (water and any organic solvents), and an outlet for removing the Sp suspension.

[0097] The residence time of the aqueous solution or aqueous suspension SA2 in the distillation device (advantageously under vacuum), which is advantageously an evaporator, in other words, the distillation time at a pressure below atmospheric pressure, is advantageously between 1 second and 600 seconds, preferably between 3 seconds and 300 seconds, and more preferably between 30 seconds and 100 seconds. The residence time corresponds to the time required to carry out step 2), that is, the time required to prepare the suspension Si by distilling the aqueous solution or aqueous suspension SA2. In other words, in the case of an evaporator, it is the residence time of the acrylamido-2-methyl-2-propanesulfonic acid (and / or its crystalline form of sodium salt) between the inlet and outlet of the device.This residence time depends on the amount of water (and any organic solvents), 2-acrylamido-2-propane sulfonic acid, and . sodium salt present in the aqueous solution or aqueous suspension SA2. A person skilled in the art will know how to adjust this residence time in order to obtain 2-acrylamido-2-propanesulfonic acid in crystalline form as sodium salt depending on the quantity of the constituents of the aqueous solution or aqueous suspension SA2.

[0098] Distillation can be carried out in a vertical or horizontal evaporator. Preferably, it is carried out in a vertical evaporator.

[0099] The aqueous solution or aqueous suspension SA2 can flow co-currently or counter-currently with the vapors generated by evaporation. Preferably, it flows counter-currently with the vapors in the distillation apparatus. In other words, the aqueous solution or aqueous suspension SA2 is preferably introduced into the distillation apparatus, advantageously an evaporator, co-currently or counter-currently with respect to the distilled solvent.

[0100] The aqueous solution or aqueous suspension SA2 can flow through one or more evaporators in series before obtaining the suspension Si. Preferably, it flows through a single evaporator.

[0101] The pressure during distillation is advantageously between 1 and less than 1000 mbar absolute (1 mbar = 100 Pa). It is preferably less than 900 mbar absolute, more preferably less than 800 mbar absolute, more preferably less than 700 mbar absolute, more preferably less than 600 mbar absolute, more preferably less than 500 mbar absolute, more preferably less than 400 mbar absolute, more preferably less than 300 mbar absolute, more preferably less than 200 mbar absolute, more preferably less than 100 mbar absolute, and even more preferably less than 50 mbar absolute, and advantageously greater than 1 mbar absolute. The absolute pressure corresponds to the pressure relative to zero pressure (vacuum).

[0102] In a particular mode, step 2) includes an optional step 2') to facilitate solvent evaporation. Step 2') then consists of increasing the temperature of the aqueous solution or aqueous suspension SA2; in other words, the distillation according to step 2') is carried out at high temperature.

[0103] Heating during distillation can be achieved using various technologies. By way of example, and without limitation, we can mention heating with steam, hot water, electricity, vapor compression, or a heat pump. Thus, the distillation apparatus can be of the double-walled type, with a hot heat transfer fluid circulating between the two walls.

[0104] The aqueous solution or aqueous suspension SA2 is advantageously heated to a temperature between more than 5°C and 95°C, preferably between more than 10°C and 60°C, more preferably between more than 20°C and 40°C.

[0105] When the aqueous solution or aqueous suspension SA2 is heated, the temp- the temperature is advantageously higher than the temperature of step 1).

[0106] The temperature rise of the solution or aqueous suspension SA2 is advantageously carried out at a rate of between 0.1 and 10°C / hour, preferably between 0.2 and 9°C / hour, more preferably between 0.3 and 8°C / hour, and even more preferably between 0.5 and 5°C / hour.

[0107] The temperature rise may not be constant throughout the process. For example, the aqueous solution or aqueous suspension SA2 may be heated by 5°C per hour for the first three hours, and then heated at a rate of 10°C per hour until the final temperature is reached.

[0108] According to another particular embodiment of the invention, step 2) may include an optional step 2"), following or instead of step 2"), which increases the productivity and profitability of the process of the invention by accelerating the crystallization of acrylamido-2-methyl-2-propanesulfonic acid in crystalline sodium salt form. Step 2") then consists of lowering the temperature of the aqueous solution or aqueous suspension SA2.

[0109] The aqueous solution or aqueous suspension SA2 is advantageously cooled to a temperature between 5 and less than 95 °C, preferably between 10 and less than 60 °C, more preferably between 20 and less than 40 °C.

[0110] When the aqueous solution or aqueous suspension SA2 is cooled, the temperature is advantageously lower than the temperature of steps 1), 2) and optionally 2').

[0111] The temperature decrease of the aqueous solution or aqueous suspension SA2 is advantageously carried out at a rate of between 0.1 and 10°C / hour, preferably between 0.2 and 9°C / hour, more preferably between 0.3 and 8°C / hour, and even more preferably between 0.5 and 5°C / hour.

[0112] The temperature decrease may not be constant throughout the process. For example, the aqueous solution or aqueous suspension SA2 may be cooled by 5°C per hour for the first three hours, and then cooled at a rate of 10°C per hour until the final temperature is reached.

[0113] Upon cooling of the aqueous solution or aqueous suspension SA2, crystals of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid are formed and a Sp suspension is obtained

[0114] In a particular mode, previously obtained sodium salt crystals of 2-acrylamido-2-methylpropane sulfonic acid can be added during this step to modify the formation of the Si suspension. This crystallization seeding allows for better control of the crystallization temperature, crystal particle size, particle size distribution, purity of the final product, and potentially, yield. The sodium salt crystals of the acid 2-acrylamido-2-methylpropane sulfonic acid thus added have advantageously a powder X-ray diffraction pattern including peaks at 11.7°; 12.2°; 13.2°; 13.5°; 15.6°; 16.8°; 17.8°; 18.5°; 19.1°; 20.6°; 21.4°; 23.3°; 25.1°; 25.8°; 26.9°; 29.1°; 29.5°; 31.0°; 33.0°; 33.6°; 34.4°; 35.2°; 35.9°; 37.1°; 38.4°; 39.6°; 41.1°; 42.9°; 45.1°; 46.0°; 47.2°; 47.6° degrees 2-theta (+ / - 0.1°).

[0115] According to a particular embodiment of the invention, the solvent distilled in step 2) can be partially or totally recycled to form the aqueous solution SAi of step 1). In other words, the distilled solvent is advantageously recycled at least partially into the aqueous solution SAb

[0116] According to another particular embodiment of the invention, the distilled solvent can be recycled partially or totally, generally to wash the sodium salt crystals of acrylamido-2-methyl-2-propane sulfonic acid obtained after liquid / solid separation step 3), in an optional step 4), with or without a pretreatment step.

[0117] The suspension Si obtained advantageously comprises between 30 and 80% by weight of 2-acrylamido-2-methylpropane sulfonic acid in crystalline form of sodium salt, relative to the total weight of the suspension Si, preferably between 50 and 60% by weight.

[0118] During step 2), the pH is advantageously greater than 10, preferably greater than 11, more preferably greater than 12, even more preferably the pH is between 13 and 14.

[0119] Step 3 of the process for manufacturing the crystalline form of sodium salt:

[0120] The sodium salt crystals of 2-acrylamido-2-methylpropane sulfonic acid contained in the Si suspension obtained at the end of step 2) are isolated by a liquid / solid separation step and are in the form of a composition Cp

[0121] The liquid / solid separation step can be carried out using various technologies. By way of example, and without limitation, we can mention the use of a centrifuge, a decanter, a filter press, a stirred smoother filter, a belt filter, a disc filter, or a rotary drum filter. Preferably, the liquid / solid separation is carried out using a centrifuge. The liquid / solid separation can also be carried out by gravity settling.

[0122] Step 3) is advantageously carried out at a temperature between -20 and 40°C, preferably between -5 and 30°C.

[0123] Preferably after step 3) of liquid / solid separation, the sodium salt crystals of 2-acrylamido-2-methylpropane sulfonic acid are not dried.

[0124] The isolated composition Ci has a sodium salt crystal content of 2-acrylamido-2-methylpropane sulfonic acid advantageously between 40 and 99%, preferably between 60 and 99% by weight, more preferably between 60 and 98%, in weight relative to the weight of composition Cp. The remainder of composition Ci can be water and / or sodium salt of solubilized 2-acrylamido-2-methylpropane sulfonic acid, and / or sodium salt introduced in step 1).

[0125] At the end of this step 3), the crystals are characterized as being crystals of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0126] In a particular mode, the liquid phase obtained as a result of the liquid / solid separation is used totally or partially in the aqueous solution S Ai of step 1).

[0127] During step 4) the pH is advantageously controlled between 6 and 14, preferably between 8 and 14, more preferably between 10 and 14, even more preferably between 12 and 14, even more preferably between 13 and 14.

[0128] Step 4) of the process for manufacturing the crystalline form of sodium salt:

[0129] In an optional step 4), the composition Ci containing the crystals obtained at the product from step 3) is washed using a washing solution.

[0130] The washing solution may be water, an aqueous solution of sodium salt (saturated or unsaturated), or a solution (saturated or unsaturated) of sodium salt of 2-acrylamido-2-methylpropane sulfonic acid (advantageously in the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid), preferably it is a saturated solution of a sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0131] Examples of sodium salt solutions include a solution of sodium hydroxide, sodium carbonate, sodium bicarbonate, or mixtures thereof.

[0132] The washing solution may comprise one or more organic solvents.

[0133] The amount of organic solvent may vary depending on the temperature, the amount of sodium salt of 2-acrylamido-2-methylpropane sulfonic acid or the amount of sodium salt.

[0134] Advantageously, the washing solution does not include any organic solvent.

[0135] As already indicated in relation to step 1), the organic solvent is advantageously chosen from organic acids, amides, alcohols, ketones, ethers, esters, alkanes, halogenated hydrocarbon compounds, nitriles, or mixtures thereof. Preferably, the organic solvent is chosen from acrylonitrile, isopropanol, acetic acid, or mixtures thereof. More preferably, the organic solvent is acrylonitrile.

[0136] In a particular mode, the washing of the composition Ci obtained at the end of step 3) is carried out by spraying the washing solution onto said composition Ci.

[0137] In a particular mode, the washing of the composition Ci obtained at the end of step 3) is carried out by suspending the composition Ci in the washing solution.

[0138] The weight ratio between the aqueous washing solution and the composition Ci obtained at the outcome of step 3) is advantageously between 0.05:1 and 10:1 and more preferably between 0.1:1 and 5:1.

[0139] This washing step is advantageously carried out at a temperature between -5 and 40°C, preferably between 0 and 30°C. A person skilled in the art will know how to adjust the temperature so as not to solubilize the crystals of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0140] The sodium salt crystals of 2-acrylamido-2-methylpropane sulfonic acid obtained at the end of this optional step 4) can be isolated from the washing solution by a liquid / solid separation step, in the form of a C2 composition.

[0141] The liquid / solid separation step can be carried out using various technologies. By way of example, and without limitation, we can mention the use of a vertical or horizontal centrifuge, a decanter, a filter press, a belt filter, a disc filter, a pusher filter, or a rotary drum filter. Liquid / solid separation can also be carried out by gravity settling.

[0142] In a particular mode, the recovered washing solution can be used, totally or partially, again in step 4), with or without a prior treatment step.

[0143] In a particular mode, the recovered washing solution can be used, totally or partially, in the aqueous solution SAi in step 1), with or without a prior treatment step.

[0144] The pH of the washing solution in step 5 is advantageously controlled between 6 and 14, preferably between 8 and 14.

[0145] Step 5) of the process for manufacturing the crystalline form of sodium salt:

[0146] In an optional step 5), the composition Ci obtained at the end of step 3) or the composition C2 obtained at the end of step 4) is dried.

[0147] The drying stage can be carried out by various technologies. By way of example and without limitation, we can cite the use of all drying technologies by convection, conduction or radiation (fluidized bed dryer, flow bed dryer, conveyor belt drying, microwave drying, heated agitated smoothing filter drying, high frequency radiation drying, infrared, spray drying).

[0148] The drying operation can be carried out at atmospheric pressure or under vacuum.

[0149] The drying step can be carried out discontinuously (batch) or continuously.

[0150] Other steps in the process of manufacturing the crystalline form of sodium salt:

[0151] During the manufacturing process, i.e. during steps 1) to 5), and regardless of the step, it is possible to introduce at least one polymerization inhibitor so as to prevent the possible polymerization of 2-acrylamido-2-methylpropane acid sulfonic acid or its salt. This inhibitor may be chosen without limitation from hydroquinone, paramethoxyphenol, phenothiazine, 2,2,6,6-tetramethyl(piperidin-l-yl)oxyl, 4-hydroxy-2,2,6,6-tetramethyl(piperidin-l-yl)oxyl, phenylenediamine derivatives, or mixtures thereof.

[0152] Preferably, the inhibitor is paramethoxyphenol or 4-hydroxy-2,2,6,6-tetramethyl(piperidin-l-yl)oxyl.

[0153] The amount of inhibitor introduced relative to the amount of 2-acrylamido-2-methylpropane sulfonic acid introduced in step 1) is advantageously between 0.001% and 5% by weight, more preferably between 0.01% and 1% by weight.

[0154] The inhibitor can be introduced during any one or more steps of the process. Preferably, it is introduced in additional quantity during step 1). More preferably, the inhibitor is part of the aqueous solution SAi introduced in step 1).

[0155] The manufacturing process (steps 1) to 5)) can be carried out continuously or discontinuously (in batch). Composition of the water-soluble polymer

[0156] The water-soluble polymer is obtained from the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid, and advantageously from at least one other monomer which may be selected from hydrophilic nonionic monomers and / or hydrophilic anionic monomers and / or hydrophilic cationic monomers and / or hydrophilic zwitterionic monomers and / or hydrophobic monomers and mixtures thereof. It may be a polymer of several distinct monomers or a homopolymer.

[0157] Advantageously, the nonionic hydrophilic monomer(s) that may be used in the context of the invention are chosen, in particular, from the following group: acrylamide, methacrylamide, N-alkylacrylamides, N-alkylmethacrylamides, N,N-dialkylacrylamides, N,N-dialkylmethacrylamides, N-vinylpyrrolidone, hydroxyalkyl acrylates, hydroxyalkyl methacrylates, and mixtures thereof. Among these nonionic monomers, the alkyl groups are advantageously C1-C5, more advantageously C1-C3. They are preferably linear alkyls. Preferably, the nonionic hydrophilic monomer is acrylamide.

[0158] The water-soluble polymer advantageously comprises between 0 and 99 mol% of non-ionic hydrophilic monomer(s), preferably between 40 and 95 mol%, more preferably between 45 and 90 mol%.

[0159] Advantageously, the hydrophilic anionic monomer(s) that can be used in the context of the invention can be chosen from a wide range of sources. These monomers They may exhibit vinyl functionalities (advantageously acrylic, maleic, fumaric, malonic, itaconic, or allylic), and contain a carboxylate, phosphonate, phosphate, sulfate, sulfonate, or other anionically charged group. The anionic monomer may be in acidic form or as an alkaline earth metal salt, an alkali metal salt, or an ammonium salt.Examples of suitable monomers include acrylic acid; methacrylic acid; dimethylacrylic acid; itaconic acid; crotonic acid; maleic acid; fumaric acid; acrylamidoundecanoic acid; 3-acrylamido-3-methylbutanoic acid; maleic anhydride; 2-acrylamido-2-methylpropanesulfonic acid (ATBS); strong acid monomers having, for example, a sulfonic acid or phosphonic acid function such as vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, methallylsulfonic acid, 2-methylidenepropane-1,3-disulfonic acid, 2-sulfoethyl methacrylate, sulfopropyl methacrylate, sulfopropylacrylate, allylphosphonic acid, styrene sulfonic acid, 2-acrylamido-2-methylpropane disulfonic acid; water-soluble salts of these monomers such as their alkali metal, alkaline earth metal, or ammonium salts; and mixtures thereof.In this list, the monomers mentioned as strong acid type exhibiting a sulfonic acid type function do not include the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0160] The water-soluble polymer advantageously comprises between 1 and 100 mol% of hydrophilic anionic monomer(s), preferably between 2 and 60 mol%, more preferably between 5 and 30 mol%, even more preferably between 5 and 20 mol%, these percentages including the crystalline form monomer of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0161] In a particular mode according to the invention, the hydrophilic anionic monomer(s), other than 2-acrylamido-2-methylpropane sulfonic acid in crystalline form of the sodium salt, can be salified.

[0162] By "salified," it is understood that at least one acid function of the anionic monomer is replaced by a salt that neutralizes the negative charge of the acid function. In other words, the unneutralized (unsalified) form corresponds to the acidic form of the monomer, for example RC(=O)-OH in the case of the carboxylic acid function, while the neutralized form of the monomer corresponds to the form RC(=O)-O X+, where X+ is a positively charged counterion (for example, a metal salt or an ammonium compound). The neutralization of the acid functions of the water-soluble polymer can be partial or total.

[0163] The salt form is advantageously suited to the salts of alkali metals (Li, Na, K...), alkaline earth metals (Ca, Mg...) or ammonium (for example the ion ammonium or a tertiary ammonium). The preferred salt is sodium salt.

[0164] Salification can take place before, during or after polymerization.

[0165] In a particular mode according to the invention, the water-soluble polymer advantageously comprises between 1 and 100 mol% of anionic monomer(s) in salified form, preferably between 20 and 100 mol%, more preferably between 50 and 100 mol% and even more preferably between 80 and 100 mol%.

[0166] Advantageously, the cationic hydrophilic monomer(s) that may be used in the context of the invention are chosen, in particular, from monomers derived from vinyl-type motifs (advantageously acrylamide, acrylic, allylic, or maleic), these monomers having a quaternary phosphonium or ammonium function. Examples include, in particular and without limitation, quaternized dimethylaminoethyl acrylate (ADAME), quaternized dimethylaminoethyl methacrylate (MADAME), dimethyldiallylammonium chloride (DADMAC), acrylamido propyltrimethyl ammonium chloride (APTAC), and methacrylamido propyltrimethyl ammonium chloride (MAPTAC). The quaternizing agent may be chosen from alkyl chlorides, dialkyl sulfates, or alkyl halides. The alkyl groups are advantageously C1-C3 and linear. Preferably, the quaternizing agent is chosen from methyl chloride or diethyl sulfate.

[0167] A person skilled in the art will know how to prepare the quaternized monomers, in particular by protonation, for example using an alkyl halide of the type RX, where R is a C1-C3 alkyl group and X is a halogen (in particular methyl chloride). Furthermore, the present invention also covers DADMAC, APTAC, and MAPTAC type monomers in which the halide counterion is fluoride, bromide, or iodide instead of chloride.

[0168] In a preferred embodiment according to the invention, the cationic hydrophilic monomer is selected from diallyldialkyl ammonium salts such as diallyl dimethyl ammonium chloride (DADMAC); acidified or quaternized dialkylaminoalkylacrylamides; acidified or quaternized dialkylaminoalkylmethacrylamides, such as methacrylamidopropyl trimethyl ammonium chloride (MAPTAC), acrylamidopropyl trimethyl ammonium chloride (APTAC), and mixtures thereof. Advantageously, the alkyl groups are CrC3.

[0169] The water-soluble polymer advantageously comprises between 0 and 20 mol% of cationic monomer(s), preferably between 0 and 6 mol%.

[0170] Advantageously, the hydrophilic zwitterionic monomer(s) may be a derivative of a vinyl-type motif (advantageously acrylamide, acrylic, allyl, or maleic), this monomer having a quaternary amine or ammonium function and an acid function of the carboxylic (or carboxylate), sulfonic (or sulfonate), or phosphoric (or phosphate) type. Examples include, but are not limited to Dimethylaminoethyl acrylate derivatives, such as 2-((2-(acryloyloxy)ethyl) dimethylammonio)ethane-1-sulfonate, 3-((2-(acryloyloxy)ethyl) dimethylammonio) propane-1-sulfonate, 4-((2-(acryloyloxy)ethyl) dimethylammonio) butane-1-sulfonate, [2-(acryloyloxy)ethyl] (dimethylammonio) acetate, dimethylaminoethyl methacrylate derivatives such as 2-((2-(methacryloyloxy)ethyl) dimethylammonio)ethane-1-sulfonate, 3-((2-(methacryloyloxy)ethyl) dimethylammonio) propane-1-sulfonate, 4-((2-(methacryloyloxy)ethyl) dimethylammonio) butane-1-sulfonate, [2-(methacryloyloxy)ethyl] (dimethylammonio) acetate, dimethylamino propylacrylamide derivatives such as 2-((3-acrylamidopropyl) dimethylammonio) ethane-1-sulfonate, 3-((3-acrylamidopropyl) dimethylammonio) propane-1-sulfonate, 4-((3-acrylamidopropyl) dimethylammonio) butane-1-sulfonate, [3-(acryloyloxy) propyl] (dimethylammonio) acetate,Derivatives of dimethylaminopropyl methylacrylamide such as 2-((3-methacrylamidopropyl) dimethylammonio) ethane-1-sulfonate, 3-((3-methacrylamidopropyl) dimethylammonio) propane-1-sulfonate, 4-((3-methacrylamidopropyl) dimethylammonio) butane-1-sulfonate and [3-(methacryloyloxy)propyl] (dimethylammonio) acetate and mixtures thereof.

[0171] The water-soluble polymer advantageously comprises 0 and 20 mol% of zwitterionic monomer(s), more preferably between 0 and 10 mol%.

[0172] Hydrophobic monomers with a Kow partition coefficient greater than 1 may also be used in the preparation of the water-soluble polymer used in the process of the invention. They are preferably selected from the following list: alkyl, arylalkyl, and / or ethoxylated and / or propoxylated (meth)acryl acid esters; alkyl, arylalkyl, or dialkyl and / or ethoxylated and / or propoxylated (meth)acrylamide derivatives; cationic allyl derivatives having an alkyl, arylalkyl, or dialkyl chain and / or an ethoxylated and / or propoxylated chain; hydrophobic anionic or cationic (meth)acryloyl derivatives; and anionic or cationic monomeric (meth)acrylamide derivatives bearing a hydrophobic chain. The hydrophobic monomers may include halogen atoms, for example, chloride.

[0173] Among these hydrophobic monomers: - Alkyl groups are preferably at C3-C20. Alkyls at C6-C20 are preferably linear alkyls, while alkyls at C3-C5 are preferably branched. - the arylalkyl groups are preferably at C7-C25, more preferably at C7-C15, - the ethoxylated chains preferably comprise 6 to 100 -CH2-CH2-O- groups, more preferably 10 to 40, - Propoxylated chains preferably comprise 0 to 50 -CH2-CH2-CH2 groups - O-, more preferably 0 to 20.

[0174] The water-soluble polymer advantageously comprises less than 5 mol% of hydrophobic monomers.

[0175] When the water-soluble polymer includes hydrophobic monomers, they are present in such quantity that the polymer remains soluble in water.

[0176] Monomers exhibiting a fluorescent function can also be used within the scope of the invention. A monomer exhibiting a fluorescent function can be detected by any suitable method, for example, by fluorometry with a fixed-wavelength fluorometer. Generally, detection of the monomer exhibiting a fluorescent function is carried out at the excitation and emission maxima, which can be determined using a scanning fluorometer.

[0177] Monomers having a fluorescent function are chosen, for example, from monomers comprising sodium styrene sulfonate and styrene sulfonic acid, vinylimidazole and its derivatives, 9-vinyl anthracene and its derivatives, pyranine and its derivatives, coumarin and its derivatives, quinolaxin and its derivatives, pi-nacyanol and its derivatives, xanthydrol and its derivatives, luminol and its derivatives, dabsyl and its derivatives, 3-hydroxy-2-methylene-3-(l-naphthyl)propionic acid and its derivatives, rhodamine and its derivatives, N-dibenzosuberenylacrylamide and its derivatives, N-9-xanthenyllacrylamide and its derivatives, naphthalic derivatives, fluorescein and its derivatives, pyrene and its derivatives, carbostyril and its derivatives, pyrazoline and its derivatives, allyl dibenzosuberenol and its derivatives, chinconicine and its derivatives, quininone and its derivatives, luminol and its derivatives, and cinchoninone and its derivatives.

[0178] In a particular embodiment according to the invention, the water-soluble polymer may comprise at least one cyclic monomer having a hydrolyzable function. Advantageously, the cyclic monomer(s) having a hydrolyzable function are chosen from cyclic ketene acetals, thionolactones, and mixtures thereof.

[0179] The cyclic ketene acetal is advantageously chosen from: 2-methylene-1,3-dioxepane (MDO), 5,6-benzo-2-methylene-1,3-dioxepane (BMDO), 2-methylene-4-phenyl-1,3-dioxolane (MPDL), 2-methylene-1,3,6-trioxocane (MTC), and mixtures thereof. Preferably, it is 2-methylene-1,3-dioxepane (MDO).

[0180] The thionolactone is advantageously chosen from: Dibenzo[c,e]oxepine(7H)-5-thione (DOT), e-thionocaprolactone, 3,3-dimethyl-2,3-dihydro-5Hbenzo[e][l,4]dioxepine-5-thione (DBT) and mixtures thereof. Preferably, it is 3,3-dimethyl-2,3-dihydro-5Hbenzo[e][l,4]dioxepine-5-thione.

[0181] In a particular mode according to the invention, the water-soluble polymer may comprise at least one LCST group.

[0182] According to the general knowledge of those skilled in the art, a LCST group is a group whose solubility in water, for a given concentration, changes above a certain temperature and depending on the salinity. It is a group exhibiting a heating transition temperature that defines its lack of affinity for the solvent medium. This lack of affinity for the solvent results in opacification or loss of transparency, which can be due to precipitation, aggregation, gelation, or viscosification of the medium. The minimum transition temperature is called the "LCST" (Lower Critical Solution Temperature). For each concentration of an LCST group, a heating transition temperature is observed. This temperature is higher than the LCST, which is the minimum point on the curve.Below this temperature, the polymer is soluble in water; above this temperature, the polymer loses its solubility in water.

[0183] In a particular mode according to the invention, the water-soluble polymer may comprise at least one UCST group.

[0184] According to the general knowledge of those skilled in the art, a UCST group corresponds to a group whose solubility in water, for a given concentration, changes below a certain temperature and depending on the salinity. It is a group exhibiting a cooling transition temperature that defines its lack of affinity for the solvent medium. This lack of affinity for the solvent results in opacification or a loss of transparency, which can be due to precipitation, aggregation, gelation, or viscosification of the medium. The maximum transition temperature is called the "UCST" (Upper Critical Solution Temperature). For each concentration of a UCST group, a cooling transition temperature is observed. This temperature is 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.

[0185] The quantities of the different monomer(s) will be adjusted by a person skilled in the art so as not to exceed 100% molar during the preparation of the water-soluble polymer. Water-soluble polymer

[0186] The 2-acrylamido-2-methylpropane used to obtain the water-soluble polymer is advantageously at least 50 mol% in the crystalline form of the sodium salt before polymerization, preferably between 70 and 100 mol%. More preferably, 100 mol% of the 2-acrylamido-2-methylpropane sulfonic acid is in the crystalline form of the sodium salt.

[0187] In a particular embodiment according to the invention, the water-soluble polymer is advantageously obtained from between 1 and 99 mol% of hydrophilic monomer(s) anionic(s), preferably between 2 and 60 mol%, more preferably between 5 and 30 mol%, even more preferably between 5 and 20 mol%, these percentages including the crystalline form monomer of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0188] In a particular embodiment according to the invention, the water-soluble polymer may comprise between 1 and 100 mol% of 2-acrylamido-2-methylpropanesulfonic acid, preferably between 1 and 99 mol%, more preferably between 2 and 60 mol%, more preferably between 3 and 50 mol%, advantageously with at least 50 mol% of the 2-acrylamido-2-methylpropanesulfonic acid used in the crystalline form of the sodium salt, preferably between 70 and 100 mol%. More preferably, 100 mol% of the 2-acrylamido-2-methylpropanesulfonic acid used is in the crystalline form of the sodium salt.

[0189] In a particular embodiment according to the invention, the water-soluble polymer comprises between 1 and 99 mol% of non-ionic hydrophilic monomers, preferably between 40 and 95 mol%, more preferably between 45 and 90 mol%; and between 1 and 99 mol% of 2-acrylamido-2-methylpropanesulfonic acid, preferably between 5 and 60 mol%, more preferably between 10 and 55 mol%, advantageously with at least 50 mol% of the 2-acrylamido-2-methylpropanesulfonic acid used in the crystalline form of the sodium salt, preferably between 70 and 100 mol%. More preferably, 100% of the 2-acrylamido-2-methylpropanesulfonic acid used is in the crystalline form of the sodium salt.

[0190] In a preferred embodiment of the invention, the water-soluble polymer is a polymer based on acrylamide and 2-acrylamido-2-methylpropane sulfonic acid, with at least 50 mol% of the 2-acrylamido-2-methylpropane sulfonic acid used in the crystalline form of the sodium salt. Preferably, the water-soluble polymer is a polymer consisting of acrylamide, acrylic acid, and 2-acrylamido-2-methylpropane sulfonic acid, with at least 50 mol% of the 2-acrylamido-2-methylpropane sulfonic acid used in the crystalline form of the sodium salt.

[0191] The water-soluble polymer can be partially or totally post-hydrolyzed.

[0192] In a preferred mode according to the invention, the water-soluble polymer is a polymer of 2-acrylamido-2-methylpropane sulfonic acid in the crystalline form of the sodium salt and a salt of acrylic acid, or a polymer of 2-acrylamido-2-methylpropane sulfonic acid in the crystalline form of the sodium salt and a hydrolyzable nonionic monomer.

[0193] The hydrolyzable nonionic monomer is preferably chosen from acrylamide; methacrylamide; N-mono derivatives of acrylamide or methacrylamide; N,N derivatives of acrylamide or methacrylamide; and esters acrylics or methacrylics. Preferably acrylamide.

[0194] According to the invention, the water-soluble polymer can have a linear, branched, 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 selection of the initiator, the transfer agent, the polymerization technique such as RAFT (Reversible-Addition Fragmentation chain transfer), NMP (Nitroxide-Mediated Polymerization) or ATRP (Atom Transfer Radical Polymerization), the incorporation of structural monomers, or the concentration.

[0195] The water-soluble polymer can further be structured by a branching agent. A structured polymer is defined as a non-linear polymer that has side chains such that, when this polymer is dissolved in water, it exhibits a high degree of entanglement leading to very high low-gradient viscosities.

[0196] The branching agent is advantageously chosen from: - Structuring agents, which may be chosen from the group comprising monomers with polyethylene unsaturation (having at least two unsaturated functions), such as vinyl, allylic, acrylic and epoxy functions, and examples include methylene bisacrylamide (MBA), triallyamine, or tetraallylammonium chloride or 1,2-dihydroxyethylene bis-(N-acrylamide), and / or - monomers having at least two epoxy functions, - monomers having at least one unsaturated function and one epoxy function, - macroinitiators such as polyperoxides, polyazo compounds and polytransfer agents such as polymer-capturing polymers, and polyols, and / or - functionalized polysaccharides.

[0197] The amount of branching agent in the water-soluble polymer is advantageously less than 40,000 ppm relative to the total weight of monomers in the water-soluble polymer, preferably less than 10,000 ppm, more preferably less than 5,000 ppm.

[0198] In a particular mode, the amount of branching agent is at least equal to 0.1 ppm, relative to the total weight of the monomers of the water-soluble polymer, preferably at least 1 ppm, more preferably at least 10 ppm, more preferably at least 100 ppm and even more preferably at least 1,000 ppm.

[0199] When the water-soluble polymer includes a branching agent, the polymer remains soluble in water. A person skilled in the art will know how to adjust the amount of branching agent, and possibly the amount of transfer agent, to achieve this result.

[0200] In a preferred mode according to the invention, the water-soluble polymer does not include a branching agent.

[0201] In a particular mode according to the invention, the water-soluble polymer comprises a transfer agent.

[0202] The transfer agent is advantageously chosen from methanol, isopropyl alcohol, sodium, calcium, magnesium, or potassium hypophosphite; 2-mercaptoethanol; 3-mercaptopropanol; dithiopropylene glycol; thioglycerol; thioglycolic acid; thiohydracrylic acid; thiolactic acid; thiomalic acid; cysteine; aminoethanethiol; sodium, calcium, magnesium, or potassium methallysulfonate; and mixtures thereof. Preferably, it is sodium hypophosphite.

[0203] The amount of transfer agent in the water-soluble polymer is advantageously between 0 and 100,000 ppm relative to the total weight of the monomers in the polymer, preferably between 0 and 10,000 ppm, more preferably between 0 and 1,000 ppm, and even more preferably between 0 and 100 ppm. When present, the transfer agent represents at least 0.1 ppm relative to the total weight of the monomers in the water-soluble polymer, preferably at least 1 ppm.

[0204] In a particular mode according to the invention, the water-soluble polymer does not comprise a transfer agent.

[0205] In general, the water-soluble polymer does not require the development of a specific polymerization process. Indeed, it can be obtained using all polymerization techniques well known to those skilled in the art. These may include, in particular, solution polymerization; gel polymerization; precipitation polymerization; emulsion polymerization (aqueous or inverse); suspension polymerization; reactive extrusion polymerization; water-in-water polymerization; or micellar polymerization.

[0206] The polymerization is generally a radical polymerization, preferably by inverse emulsion polymerization or by gel polymerization. By radical polymerization, we include free radical polymerization using UV, azo, redox or thermal initiators as well as controlled radical polymerization (CRP) techniques or matrix polymerization techniques.

[0207] Examples of controlled radical polymerization techniques include, but are not limited to, iodine transfer polymerization (ITP), nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer polymerization (RAFT), of which the technology MADIX (Macromolecular Design by Interchange of Xanthates), various polymerization variations with organometallic compounds (Organometallic Mediated Radical Polymerization (OMRP)), and radical polymerization controlled by heteroatomic compounds (“OrganoHeteroatom-mediated Radical Polymerization” in English (OHRP)).

[0208] As already indicated, the water-soluble polymer can be post-hydrolyzed. Post-hydrolysis is the hydrolysis reaction of the polymer after its formation by monomer polymerization. This step consists of the reaction of hydrolyzable functional groups of advantageously non-ionic monomers, more advantageously amide or ester groups, with a hydrolyzing agent. This hydrolyzing agent can be an enzyme, an ion-exchange resin, or a Brpnsted acid metal (for example, a hydrohalic acid) or a Brpnsted base (for example, an alkali hydroxide or an alkaline earth hydroxide). Preferably, the hydrolyzing agent is a Brpnsted base. During this post-hydrolysis step of the water-soluble polymer, the number of carboxylic acid groups increases. Indeed, the reaction between the base and the amide or ester groups present in the water-soluble polymer produces carboxylate groups.

[0209] The water-soluble polymer can be in liquid, gel or solid form when its preparation includes a drying step such as spray drying, drum drying, radiation drying such as microwave drying, or fluidized bed drying.

[0210] The water-soluble polymer advantageously has a weight-average molecular weight of at least 0.5 million g / mol, preferably between 0.5 and 40 million g / mol, more preferably between 5 and 30 million g / mol. Molecular weight is understood to mean weight-average molecular weight.

[0211] The weight-average molecular weight is determined by the intrinsic viscosity of the polymer. The intrinsic viscosity can be measured by methods known to those skilled in the art and can be calculated from the reduced viscosity values ​​for different polymer concentrations by a graphical method consisting of plotting the reduced viscosity values ​​(ordinate axis) against the concentration (abscissa axis) and extrapolating the curve down to zero concentration. The intrinsic viscosity value is plotted on the ordinate axis or using the least squares method. The molecular weight can then be determined by the Mark-Houwink equation: [q] = KM" [q] represents the intrinsic viscosity of the polymer determined by the solution viscosity measurement method. K represents an empirical constant. M represents the molecular weight of the polymer. a represents the Mark-Houwink coefficient. K and a depend on the particular polymer-solvent system. Fracking fluid

[0212] The present invention relates to a fracturing fluid comprising an aqueous phase, at least one propping agent and at least one water-soluble polymer of 2-acrylamido-2-methylpropane sulfonic acid, the 2-acrylamido-2-methylpropane sulfonic acid being, prior to polymerization, in the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid described above.

[0213] The aqueous phase is advantageously chosen from: seawater, brine, fresh water, advantageously it is brine.

[0214] Brine is defined as 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.

[0215] The support agent may be chosen without restriction from sand, ceramics, bauxite, glass beads, and resin-impregnated sand.

[0216] Advantageously, the amount of propping agent in the fracturing fluid is between 0.5 and 40% by weight relative to the total weight of the fracturing fluid, preferably between 1 and 25%, more preferably between 1.5 and 20%.

[0217] Advantageously, the fracturing fluid comprises between 0.001% and 1% by weight of water-soluble polymer obtained from the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid relative to the total weight of the fracturing fluid, preferably between 0.002% and 0.2%.

[0218] The fracturing fluid may comprise other compounds known to those skilled in the art, such as those mentioned in document SPE 152596, for example: - Anti-swelling agents for clays such as potassium chloride, or choline chloride, and / or - Biocides to prevent the growth of bacteria, particularly sulfate-reducing bacteria, which can form viscous masses that reduce surface area. Examples include glutaraldehyde, the most commonly used, as well as formaldehyde, isothiazolinones, and / or - Oxygen reducers such as ammonium bisulfite to prevent the destruction of other components by oxidation and corrosion of the injection tubes, and / or - Anti-corrosion additives to protect the tubes against oxidation by residual amounts of oxygen, with N,N dimethylformamide being preferred, and / or - Lubricants such as oil distillates, and / or - Iron chelating agents such as citric acid, EDTA (ethylenediaminetetraacetic acid), phosphonates, and / or - Anti-scale products such as phosphates, phosphonates, polyacrylates or ethylene glycol.

[0219] Before its use in the fracturing fluid, the water-soluble polymer according to the invention may be in various solid or liquid forms. Preferably, it may be in the form of a powder, a water-in-oil inverse emulsion, an aqueous polyphasic particulate suspension, or an oily polyphasic particulate suspension.

[0220] Method for preparing a fracturing fluid

[0221] The present invention also relates to a method for preparing a fracturing fluid by adding, to water or brine, at least one water-soluble polymer of the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid (described above), and wherein the water-soluble polymer is, before formation of the fracturing fluid: - either in powder form; - either in the form of a reverse water-in-oil emulsion; - either in the form of a polyphasic aqueous or oily particulate suspension.

[0222] The process for preparing a fracturing fluid according to the invention preferably includes a step of adding to said fluid at least one propping agent as described above.

[0223] When the water-soluble polymer added to the fracturing fluid is, before the formation of the fracturing fluid, in powder form, the average size of the polymer particles is advantageously less than 1.5 millimeters, preferably less than 850 micrometers, and more preferably less than 200 micrometers. The average size of the water-soluble polymer particles is advantageously greater than 5 µm.

[0224] The average particle size of water-soluble polymer is the average size of the largest dimension, for example, the diameter for spherical particles. It is advantageously measured with a laser measuring device using conventional techniques that are part of the knowledge of those skilled in the art. For example, a Mastersizer-type device from Malvern, such as the MS2000, can be used for this purpose. This type of device allows the particle size distribution of particles in liquid or solid form to be measured by laser diffraction.

[0225] When the water-soluble polymer according to the invention is in solid form, it can be partially or totally dissolved in water using a polymer preparation unit such as the Polymer Slicing Unit (PSU) disclosed in document EP 2 203 245.

[0226] When the water-soluble polymer added to the fracturing fluid is, before formation of the fracturing fluid, in the form of a reverse water-in-oil emulsion, the concentration of water-soluble polymer in the emulsion is preferably between 5 and 60% by weight, more preferably between 15 and 40% by weight relative to the weight of the emulsion.

[0227] In a preferred embodiment according to the invention, the water-in-oil inverse emulsion may comprise between 0.01% and 70% by weight of an organic salt and / or an inorganic salt relative to the weight of the emulsion, preferably between 5% and 20%. The salts may be selected without restriction from sodium chloride, sodium sulfate, sodium bromide, ammonium sulfate, ammonium chloride, lithium chloride, lithium bromide, potassium chloride, potassium bromide, magnesium sulfate, aluminum sulfate, and mixtures thereof. Ammonium chloride and ammonium sulfate are preferred salts.

[0228] When the water-soluble polymer added to the fracturing fluid is, before formation of the fracturing fluid, in the form of an aqueous particulate polyphasic suspension comprising: - 15 to 60% by weight of at least one water-soluble polymer in the form of solid particles with an average size between 5 and 500 pm; - 15 to 45% by weight of at least one salt of an alkali metal and / or at least one salt of an alkaline earth metal; - at least one viscosifying agent other than the water-soluble polymer; - at least 10% water by weight; and said suspension having a Brookfield viscosity between 500 and 20,000 cps at a temperature of 20°C; and said suspension having a density between 1.1 and 2 kg.L *.

[0229] When the water-soluble polymer added to the fracturing fluid is, before formation of the fracturing fluid, in the form of an oily particulate polyphasic suspension, said suspension preferably comprises: - 15 to 60% by weight of at least one water-soluble polymer in the form of solid particles with an average size between 5 and 500 pm; - at least one viscosifying agent other than the water-soluble polymer; - at least 10% oil by weight; and said suspension having a Brookfield viscosity between 500 and 20,000 cps at a temperature of 20°C, and said suspension having a volumetric mass between 0.6 and 1.4 kg.L *.

[0230] Brookfield viscosity is measured with a Brookfield apparatus, fitted with an LV module, the module being able to rotate at a speed of 30 revolutions per minute for example, the measurement being advantageously carried out at 20°C. The density is measured at 20°C, at a pressure of 1 atm i.e. 101,325 Pa.

[0231] Unconventional underground oil or gas hydraulic fracturing process

[0232] The present invention also 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 the underground reservoir.

[0233] The injection is carried out under pressure so as to create fractures distributed throughout the production well.

[0234] Optionally, before, during or after the creation of fractures, at least one oxidizing compound and / or at least one surfactant compound is injected into the reservoir.

[0235] Surfactant injection eliminates the viscosity generated by the polymer by inhibiting interchain hydrophobic interactions, while oxidizing compound injection destroys the polymer. In both cases, injection restores a fluid viscosity close to that of water.

[0236] Examples of oxidizing compounds include bleach (aqueous solution of a hypochlorite salt), hydrogen peroxide, ozone, chloramines, persulfates, permanganates or perchlorates.

[0237] The chemical nature of the surfactant compound(s) is not critical. They may be anionic, nonionic, amphoteric, zwitterionic, and / or cationic. Preferably, the surfactant compound(s) of the invention carry anionic charges.

[0238] Preferably, the surfactant compounds used are chosen from anionic surfactants and their zwitterions selected from the group comprising derivatives of alkyl sulfates, alkyl ethersulfates, arylalkyl sulfates, arylalkyl ethersulfates, alkylsulfonates, alkyl etherssulfonates, arylalkylsulfonates, arylalkyl etherssulfonates, alkyl phosphates, alkyl ethersphosphates, arylalkyl phosphates, arylalkyl ethersphosphates, alkylphosphonates, alkyl ethersphosphonates, arylalkyl phosphonates, arylalkyl ethersphosphonates, alkyl carboxylates, alkyl ethers carboxylates, arylalkyl carboxylates, arylalkyl ethers carboxylates, alkyl polyethers, arylalkyl polyethers...

[0239] An alkyl chain is defined as a chain of 6 to 24 carbons, branched or unbranched, with or without multiple repeating units, which may optionally contain one or more heteroatoms (O, N, S). An arylalkyl chain is defined as a chain of 6 to 24 carbons, branched or unbranched, comprising one or more aromatic rings and possibly comprising one or more heteroatoms (O, N, S).

[0240] The most commonly used surfactants, for reasons of cost, stability, and availability, are of the sulfonate or sulfate type, presented in the form of alkali metal or ammonium salts.

[0241] Method for reducing friction of a fracturing fluid in a hydraulic fracturing operation of an unconventional underground oil or gas reservoir

[0242] The present invention also relates to a method for reducing friction of a fracturing fluid in a hydraulic fracturing operation 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 the underground reservoir.

[0243] Friction reduction makes it possible to decrease or eliminate losses (pressure) related to friction during the injection of the fracturing fluid.

[0244] The invention and the resulting advantages will become clear from the following embodiments. Figures

[0245] [Fig-1] The [Fig. 1] represents the X-ray diffraction pattern of 2-acrylamido-2-methylpropane sulfonic acid.

[0246] [Fig.2] The [Fig.2] represents the X-ray diffraction pattern of the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid.

[0247] [Fig.3] Fig.3 represents the percentage reduction in friction as a function of time for post-hydrolyzed polymers.

[0248] [Fig.4] Fig.4 represents the percentage reduction in friction over time for ter-polymers.

[0249] [Fig.5] Fig.5 represents the percentage reduction in friction over time for homopolymers.

[0250] [Fig.6] Fig.6 represents the percentage reduction in friction over time for polymers. Examples

[0251] Example 1: Synthesis of 2-acrylamido-2-methylpropane sulfonic acid

[0252] In a 2000 ml double-jacketed stirred reactor, 1522 grams of acrylonitrile containing 0.4% water by weight and 180 grams of fuming sulfuric acid containing 104% H2SO4 (18% oleum) are added. The mixture is stirred for 1 hour and cooled by the reactor's double jacket, which maintains the temperature of the sulfonant mixture at -20°C.

[0253] 97 grams of isobutylene are added to the previous sulfonant mixture, at a flow rate of 1.6 grams / minute.

[0254] The temperature of the mixture is controlled at 45°C during the introduction of isobutylene. The particles of 2-acrylamido-2-methylpropanesulfonic acid precipitate in the mixture, and the solids content is approximately 20% by weight. The reaction mixture is filtered through a Buchner filter and dried under vacuum at 50°C. The solid obtained is 2-acrylamido-2-methylpropanesulfonic acid and is in the form of a very fine white powder.

[0255] Example 2: Synthesis of the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid

[0256] In a 1000 mL double-jacketed stirred reactor, 439 grams of a 22% sodium hydroxide solution (by weight in water) are added. To the preceding mixture are added 452 grams of 2-acrylamido-2-methylpropanesulfonic acid (white powder obtained from Example 1).

[0257] The mixture is stirred for 30 minutes, at 10°C, to form an aqueous solution SA2.

[0258] The aqueous solution SA2 is heated to a temperature of 40°C under a vacuum of 50 mbar for 20 minutes, then the temperature is maintained at atmospheric pressure for 30 minutes, and then cooled to a temperature of 10°C. The cooling time between 40°C and 10°C is 6 hours. A suspension Si of sodium salt crystals of 2-acrylamido-2-methylpropanesulfonic acid is obtained. The suspension Si is filtered using a Robatel vertical centrifuge. A solid of composition Ci is obtained, containing 80% by weight of sodium salt crystals of 2-acrylamido-2-methylpropanesulfonic acid. Example 3: X-ray diffraction analysis

[0259] The solids obtained in examples 1 and 2 are first ground into powders and are analyzed by X-ray diffraction over an angular range of 10 to 90°. The equipment used is a Rigaku miniflex II diffractometer equipped with a copper source.

[0260] We can observe that the solid obtained from example 2 ([Fig.2]) has an X-ray diffraction pattern with the following characteristic peaks: 11.7°; 12.2°; 13.2°; 13.5°; 15.6°; 16.8°; 17.8°; 18.5°; 19.1°; 20.6°; 21.4°; 23.3°; 25.1°; 25.8°; 26.9°; 29.1°; 29.5°; 31.0°; 33.0°; 33.6°; 34.4°; 35.2°; 35.9°; 37.1°; 38.4°; 39.6°; 41.1°; 42.9°; 45.1°; 46.0°; 47.2°; 47.6° degrees 2- theta (+ / -0.1°). Example 3: Polymer Synthesis

[0261] Polymers 1 and 2 (post-hydrolyzed ATBS / acrylamide polymers)

[0262] Polymer 1 (ATBS in crystalline form of sodium salt): In a 2000 mL beaker are added 761.9 g of deionized water, 574.2 g of acrylamide (in a 50% by weight solution in water), 11.7 g of urea and 103.2 g of 2-acrylamido-2-methylpropane sulfonic acid crystals (crystalline form of the sodium salt) from example 2.

[0263] The solution thus obtained is cooled to between 0 and 5°C and transferred to an adiabatic polymerization reactor; nitrogen bubbling is carried out for 30 minutes to eliminate any trace of dissolved oxygen.

[0264] The following are then added to the reactor: - 0.45 g of 2,2'-azobisisobutyronitrile, - 1.5 mL of a 5 g / L solution of 2,2'-azobis dihydrochloride [2-(2-imidazolin-2-yl)propane], - 1.5 ml of a 1 g / L sodium hypophosphite solution, - 2.25 ml of a 1 g / L tert-butyl hydroperoxide solution, - 3.0 ml of a 1 g / L solution of ammonium sulfate and iron(II) hexahydrate (Mohr's salt).

[0265] After a few minutes, the nitrogen supply is closed and the reactor is shut down. The polymerization reaction proceeds for 2 to 5 hours until a temperature peak is reached. The resulting gel is chopped into particles with a size between 1 and 6 mm.

[0266] Post-hydrolysis of acrylamide: 500.0 g of previously chopped gel are then mixed with 22.5 g of 50% sodium hydroxide solution, the mixture is brought and maintained at a temperature of 90°C for a period of 90 minutes.

[0267] The gel is then dried and ground to obtain the polymer in powder form.

[0268] Polymer 2 (ATBS not in crystalline form of the sodium salt): Polymer 2 is obtained like polymer 1, but by replacing the 103.2 g of crystalline sodium salt 2-acrylamido-2-methylpropane sulfonic acid with 93.1 g of non-crystalline sodium salt 2-acrylamido-2-methylpropane sulfonic acid (example 1) and 36 g of sodium hydroxide at 50% weight concentration in water.

[0269] Polymers 3 and 4 (acrylic acid / ATBS / acrylamide terpolymers)

[0270] Polymer 3 (ATBS in crystalline form of sodium salt): In a 2000 mL beaker are added 1022g of deionized water, 558.7g of acrylamide in 50% solution, 89g of 50% sodium hydroxide solution, 80g of glacial acrylic acid, 15.3g of urea and 125g of 2-acrylamido-2-methylpropane sulfonic acid crystals (crystalline form of the sodium salt) from Example 2.

[0271] The solution thus obtained is cooled to between 0 and 5°C and transferred to an adiabatic polymerization reactor; nitrogen bubbling is carried out for 30 minutes in order to to eliminate all traces of dissolved oxygen.

[0272] The following are then added to the reactor: - 1.13 g of 2,2'-azobisisobutyronitrile, - 1.5 mL of a 15 g / L solution of 2,2'-azobis dihydrochloride [2-(2-imidazolin-2-yl)propane], - 1.5 ml of a 3 g / L sodium hypophosphite solution, - 0.75 ml of a 1 g / L tert-butyl hydroperoxide solution, - 2.25 ml of a 1 g / L sodium persulfate solution, - 1.5 ml of a 2 g / L solution of ammonium sulfate and iron(II) hexahydrate (Mohr's salt).

[0273] After a few minutes, the nitrogen supply is closed and the reactor is shut down. The polymerization reaction proceeds for 1 to 5 hours until a temperature peak is reached. The resulting gel is chopped into particles with a size between 1 and 6 mm.

[0274] The gel is then dried and ground to obtain the polymer in powder form.

[0275] Polymer 4 (ATBS not in crystalline form of the sodium salt): Polymer 4 is obtained like polymer 3, but by replacing the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid of sodium salt with 113 g of 2-acrylamido-2-methylpropane sulfonic acid (example 1) not being in crystalline form of sodium salt and 113.5 g of sodium hydroxide at 50% weight concentration in water. Polymers 5 and 6 (ATBS homopolymers)

[0276] Polymer 5 (ATBS in crystalline form of sodium salt): In a 2000 mL beaker, 800 g of deionized water and 500 g of 2-acrylamido-2-methylpropane sulfonic acid crystals (crystalline form of the sodium salt) from example 2 are added.

[0277] The solution thus obtained is cooled to between 5 and 10°C and transferred to an adiabatic polymerization reactor; nitrogen bubbling is carried out for 30 minutes to eliminate any trace of dissolved oxygen.

[0278] The following are then added to the reactor: - 0.45 g of 2,2'-azobisisobutyronitrile, - 1.5 ml of a 2.5 g / l solution of 2,2'-azobis dihydrochloride [2-(2-imidazolin-2-yl)propane], - 1.5 ml of a 1 g / L sodium hypophosphite solution, - 1.5 ml of a 1 g / L tert-butyl hydroperoxide solution, - 1.5 ml of a 1 g / 1 solution of ammonium sulfate and iron(II) hexahydrate (Mohr's salt).

[0279] After a few minutes, the nitrogen supply is closed and the reactor is shut down. The The polymerization reaction takes place over 2 to 5 hours until a temperature peak is reached. The resulting gel is chopped and dried to obtain a coarse powder, which is then ground and sieved to obtain the polymer in powder form.

[0280] Polymer 6 (ATBS not in crystalline form of the sodium salt): Polymer 6 is obtained like polymer 5, but by replacing the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid of sodium salt with 452 g of 2-acrylamido-2-methylpropane sulfonic acid (example 1) not being in crystalline form of sodium salt and 175 g of sodium hydroxide at 50% weight concentration in water. Polymers 7 and 8 (ATBS / acrylamide polymers)

[0281] Polymer 7 (ATBS in crystalline form of the sodium salt): In a 2000 mL beaker, 1035 g of deionized water, 520.5 g of acrylamide in a 50% solution, 16.2 g of urea and 285 g of 2-acrylamido-2-methylpropane sulfonic acid crystals (crystalline form of the sodium salt) from example 2 are added.

[0282] The solution thus obtained is cooled to between 0 and 5°C and transferred to an adiabatic polymerization reactor; nitrogen bubbling is carried out for 30 minutes to eliminate any trace of dissolved oxygen.

[0283] The following are then added to the reactor: - 0.75 g of 2,2'-azobisisobutyronitrile, - 1.5 ml of a 5 g / L solution of 2,2'-azobis dihydrochloride [2-(2-imidazolin-2-yl)propane], - 1.5 ml of a 3 g / L sodium hypophosphite solution, - 2.25 ml of a 1 g / L tert-butyl hydroperoxide solution, - 2.25 ml of a 1 g / L solution of ammonium sulfate and iron(II) hexahydrate (Mohr's salt).

[0284] After a few minutes, the nitrogen supply is closed and the reactor is shut down. The polymerization reaction proceeds for 1 to 5 hours until a temperature peak is reached. The resulting gel is chopped into particles with a size between 1 and 6 mm.

[0285] The gel is then dried and ground to obtain the polymer in powder form.

[0286] Polymer 8 (ATBS not in crystalline form of the sodium salt): Polymer 8 is obtained like polymer 7, but by replacing the crystalline form of 2-acrylamido-2-methylpropane sulfonic acid of sodium salt with 257 g of 2-acrylamido-2-methylpropane sulfonic acid (example 1) not being in crystalline form of sodium salt and 99 g of sodium hydroxide at 50% weight concentration in water.

[0287] Examples 4: Preparation of fracturing fluids

[0288] Polymers 1 to 8 in powder form are dissolved under stirring at a concentration of 10 OOppm by weight in a brine composed of water, 85 g of sodium chloride (NaCl) and 33.1 g of calcium chloride (CaCl2, 2H2O) per liter of brine.

[0289] The resulting polymer saline solutions are then injected at a concentration of 0.05 pptg (parts per thousand gallons) into the brine recirculated for the subsequent Flow Loop tests.

[0290] Example 5: Flow Loop Friction Reduction Tests

[0291] To evaluate the friction reduction of each of the polymers 1 to 8, the reservoir of the The Flow Loop was filled with 20 L of brine (brine as described in Example 4). The brine was then recirculated through the Flow Loop at a rate of 24 gallons per minute (1 gallon = 3.78541 liters). The polymer was added at a concentration of 0.5 pptg to the recirculating brine. The percentage of friction reduction was then determined by measuring the pressure variations within the Flow Loop. Figures 3 to 6:

[0292] Figures 3 to 6 are graphs showing the percentage reduction in friction over time for each type of polymer ([Fig. 3]: post-hydrolyzed polymers 1 and 2, [Fig. 4]: tert-polymers 3 and 4, [Fig. 5]: homopolymers 5 and 6, [Fig. 6]: polymers 7 and 8). Figures 1 and 2 illustrate the X-ray diffraction pattern of the crystals obtained according to Example 1 ([Fig. 1]) and according to Example 2 ([Fig. 2]).

[0293] These figures demonstrate that the injection fluids according to the invention allow for improved friction reduction. Indeed, when the polymers contain ATBS in the crystalline form of the sodium salt, the friction reduction is better.

Claims

Demands

1. Fracking fluid comprising at least one aqueous phase, a propping agent and at least one water-soluble polymer of 2-acrylamido-2-methylpropane sulfonic acid, 2-acrylamido-2-methylpropane sulfonic acid being, prior to polymerization, a crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid having a powder X-ray diffraction pattern comprising peaks at 11.7°; 12.2°; 13.2°; 13.5°; 15.6°; 16.8°; 17.8°; 18.5°; 19.1°; 20.6°; 21.4°; 23.3°; 25.1°; 25.8°; 26.9°; 29.1°; 29.5°; 31.0°; 33.0°; 33.6°; 34.4°; 35.2°; 35.9°; 37.1°; 38.4°; 39.6°; 41.1°; 42.9°; 45.1°; 46.0°; 47.2°; 47.6° degrees 2-theta.3

2. Fracturing fluid according to claim 1, characterized in that at least 50 mol% of the 2-acrylamido-2-methylpropane sulfonic acid of the water-soluble polymer is, prior to polymerization, in the crystalline form of the sodium salt.

3. Fracturing fluid according to claim 1, characterized in that the water-soluble polymer is a polymer of the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid, and of at least one hydrophilic monomer selected from non-ionic monomers, anionic monomers, cationic monomers, zwitterionic monomers and mixtures thereof.

4. Fracturing fluid according to claim 1 or 3, characterized in that the water-soluble polymer comprises between 1 and 100 mol% of 2-acrylamido-2-methylpropane sulfonic acid, at least 50 mol% of the 2-acrylamido-2-methylpropane sulfonic acid being, prior to polymerization, in the crystalline form of the sodium salt.

5. Fracturing fluid according to any one of the preceding claims, characterized in that the water-soluble polymer is a polymer of 2-acrylamido-2-methylpropane sulfonic acid in the crystalline form of the sodium salt and a salt of acrylic acid, or an anionic polymer of 2-acrylamido-2-methylpropane sulfonic acid in the crystalline form of the sodium salt and a hydrolyzable nonionic monomer.

6. A fracturing fluid according to any one of the preceding claims, characterized in that the fracturing fluid comprises between 0.001% and 1% by weight of water-soluble polymer relative to the total weight of the fracturing fluid.

7. A method for preparing a fracturing fluid according to any one of the preceding claims, by adding, in water or brine, at least one water-soluble polymer of the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid, and in which the water-soluble polymer is, before formation of the fracturing fluid: - either in powder form; - or in the form of a water-in-oil inverse emulsion; - or in the form of a polyphasic aqueous or oily particulate suspension of the crystalline form of the sodium salt of 2-acrylamido-2-methylpropane sulfonic acid having a powder X-ray diffraction pattern comprising peaks at 11.70°; 12.20°; 13.2°; 13.5°; 15.60°; 16.80°; 17.80°; 18.5°; 19.1°; 20.6°; 21.40°; 23.3°; 25.1°; 25.8°; 26.9°; 29.10°; 29.50°; 31.0°; 33°; 33.6°; 34.4°; 35.2°; 35.9°; 37.1°; 38.4°; 39.6°; 41.10°; 42.90°; 45.10°; 46.0°; 47.2°; 47.6° degrees 2-theta.

8. A process according to claim 7, characterized in that the water-soluble polymer is, before formation of the fracturing fluid, in powder form with an average polymer particle size of less than 1.5 millimeters.

9. A process according to claim 7, characterized in that the water-soluble polymer is, before formation of the fracturing fluid, in the form of a reverse water-in-oil emulsion, the concentration of water-soluble polymer in the emulsion being between 5 and 60% by weight relative to the weight of the emulsion.

10. A process according to claim 7 or 9, characterized in that the water-in-oil reverse emulsion comprises between 0.01% and 70% by weight of an organic salt and / or an inorganic salt relative to the weight of the emulsion.

11. A process according to claim 7 or 10, characterized in that the water-soluble polymer is, before formation of the fracturing fluid, in the form of an aqueous particulate polyphasic suspension comprising: - 15 to 60% by weight of at least one water-soluble polymer in the form of solid particles of average size between 5 and 500 pm; - 15 to 45% by weight of at least one alkali metal salt and / or at least one alkaline earth metal salt; - at least one viscosifying agent other than the water-soluble polymer according to the invention; - at least 10% by weight of water; and said suspension having a Brookfield viscosity of between 500 and 20000 cps at a temperature of 20°C, and said suspension having a density of between 1.1 and 2 kg.L1.

12. A process according to claim 7 or 10, characterized in that the water-soluble polymer is, before formation of the fracturing fluid, in the form of an oily, particulate, polyphasic suspension comprising: - 15 to 60% by weight of at least one water-soluble polymer in the form of solid particles of average size between 5 and 500 µm; - at least one viscosifying agent other than the water-soluble polymer according to the invention; - at least 10% by weight of oil; and said suspension having a Brookfield viscosity between 500 and 20,000 cps at a temperature of 20°C, and said suspension having a density between 0.6 and 1.4 kg.L1.

13. A method for hydraulically fracturing an unconventional underground oil or gas reservoir, comprising preparing a fracturing fluid according to any one of claims 1 to 6, and injecting said fracturing fluid into an underground reservoir.

14. A method for reducing friction of a fracturing fluid in a hydraulic fracturing operation of an unconventional underground oil or gas reservoir, comprising the preparation of a fracturing fluid according to any one of claims 1 to 6, and the injection of said fracturing fluid into an underground reservoir.

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