METHOD FOR IMPROVED OIL RECOVERY BY INJECTING AN AQUEOUS POLYMER COMPOSITION

Description

FIELD OF INVENTION

[0001] The present invention relates to the technical field of enhanced oil recovery in a reservoir. More specifically, the present invention relates to an improved method for enhanced oil recovery by sweeping an underground formation through the introduction into the reservoir of an injection fluid comprising at least one water-soluble polymer with LCST macromonomers. The invention also relates to an injection fluid comprising said polymer. PRIOR STATE OF TECHNOLOGY

[0002] Most of the oil fields currently in operation have reached maturity and have, in fact, begun to decline in production or are about to do so. The recovery rate of these fields is currently on average between 15 and 35% of the initial quantity of oil. They therefore still offer considerable production potential.

[0003] Generally, the recovery of crude oil contained in deposits is carried out in several stages.

[0004] Production initially results from the natural energy of the fluids and rock as they decompress. Following this depletion phase, the amount of oil recovered at the surface represents on average only 5 to 15% of the initial reserve. Therefore, it is then necessary to employ techniques aimed at increasing recovery efficiency while maintaining field pressure.

[0005] The most common method for achieving this is injecting water into the reservoir through dedicated injection wells. This is known as secondary recovery. This second phase ends when the water-to-oil ratio becomes too high, meaning when the water content in the mixture produced by the production wells is excessive. This secondary recovery thus allows for an additional recovery rate of approximately 10 to 20%.

[0006] Other usable techniques are grouped under the name enhanced oil recovery (EOR). Their goal is to recover between 10 and 35% more oil compared to the initial quantity. The term enhanced oil recovery encompasses various thermal and non-thermal techniques, such as electrical, miscible, steam, and chemical methods for "enhanced recovery of oil remaining in place" (see "Oil & gas science and technology" - IFP journal, vol. 63 (2008) no. 1, pp. 9-19).

[0007] The term "petroleum" refers to any type of oil, including light, heavy, and even bituminous oils. An oil generally results from the natural transformation of organic matter and is composed of a mixture of hydrocarbons. In the description of the prior art or the invention, the terms "petroleum" and "oil" are used interchangeably to refer to the same substance.

[0008] The present invention relates more specifically to chemically assisted oil recovery involving the injection, into an underground formation, of an aqueous injection fluid comprising at least one water-soluble polymer, said fluid being capable of sweeping through the underground formation, and thus of pushing the oil out of the rock.

[0009] The objective of the invention is not to gel the polymer to modify the permeability of the rock and plug the holes, a technique known as "profile control" or sealing of waterways.

[0010] The objective of the invention is to offer an improvement in the efficiency of sweeping in a chemically assisted oil recovery process through improved mobility control.

[0011] Enhanced oil recovery (EOR) techniques are distinguished from reservoir stimulation operations. The latter are characterized by limited-volume injections of polymer solutions to create a localized phenomenon within the reservoir. For conformance, this involves plugging areas of high permeability from a water-injection well, while for water shutoff, it involves plugging a conduit by direct injection into an oil-producing well. Injections are generally carried out either from an injection well or a production well over relatively short periods, typically a few days and usually less than a month, with injected volumes representing less than 5% of the reservoir's pore volume. The pore volume corresponds to the volume of the reservoir not occupied by rock, which is a permeable zone.

[0012] Conversely, enhanced oil recovery (EOR) techniques using polymers involve the continuous and prolonged injection of a polymer solution to sweep the reservoir from an injection well to a producing well. The goal is not to treat a specific area of ​​the reservoir but to sweep its entirety in order to recover as much oil as possible. This requires injecting a much larger volume of aqueous solution, generally between 30% and 500%, or even more, of the pore volume. At the producing well(s), a mixture of water, oil, and sometimes gas is recovered.

[0013] In all these techniques, the efficiency of water-injection sweeping is generally improved by the addition of water-soluble polymers. The expected and proven benefits of using these polymers, through their ability to viscosify the injection fluids, are improved sweeping and mobility control within the field in order to recover a maximum amount of oil quickly and efficiently.

[0014] It is well known to those skilled in the art that synthetic water-soluble polymers, particularly those based on acrylamide, are highly advantageous for increasing the viscosity of aqueous solutions and are therefore predominantly used in enhanced oil recovery. Recently, numerous solutions have been proposed to improve these processes, notably through the development of new-generation polymers.

[0015] Documents WO 95 / 26455 and WO 2018 / 172682 describe processes related to petroleum recovery. Document WO 02 / 055607 describes a dispersion containing an LCST polymer and its use in cosmetics.

[0016] Document WO 2014 / 166858 describes an enhanced oil recovery process using copolymers with an isomolar ratio of ATBS and NVP, offering improved temperature stability. Document US 2013 / 108173 describes the use of ACMO-based copolymers with very good temperature and salinity resistance. Document WO 2013 / 08174 describes specific polymers with very good iron resistance.

[0017] Unlike the aforementioned documents, WO 2016 / 162532 does not describe a sweep-assisted oil recovery process, but rather a process for diverting an underground formation using a polymer capable of forming temporary diversion gels in situ. These polymers are obtained from LCST (Lower Critical Solution Temperature) macromonomers, which allow the polymers to gel in situ when they encounter sufficiently high temperature conditions.

[0018] LCST technology is widely applied to polymers to give them the ability to transform into physical gels to block or seal conduits, pipes, or other porous materials. This technology is particularly used in construction.

[0019] LCST technology applied to polymers used in oil extraction makes it possible to temporarily or permanently block areas of underground formations through the gelation of the injection fluid containing a thermosensitive polymer which, under the action of heat, will transform into a gel.

[0020] Regarding enhanced oil recovery by sweeping, although significant progress has been made in recent years, water-soluble polymers do not yet offer optimal sweeping efficiency and mobility control, particularly under harsh conditions. This is typically the case when the polymers encounter high temperature and / or high salinity. A significant amount of residual oil then remains in the treated formations, which is currently difficult to recover.

[0021] The problem that the present invention aims to solve is to improve the efficiency of the sweeping and the mobility control provided by water-soluble polymers in the processes of enhanced oil recovery by sweeping of the underground formation. DESCRIPTION OF THE INVENTION

[0022] The Plaintiff surprisingly discovered that this was possible through the use of a water-soluble copolymer comprising a homeopathic dose of LCST macromonomers.

[0023] The term "homeopathic dose" refers to an extremely small quantity compared to the other constituents of the water-soluble copolymer.

[0024] It is thanks to a homeopathic dosage of the latter that it is possible to obtain water-soluble copolymers with a rheological profile such that the injection fluids containing them offer optimized sweeping and mobility control, particularly at high temperatures (thermosensitive) and / or high salinity (sensitive to salt(s)).

[0025] More specifically, the present invention relates to a method for enhanced oil recovery by sweeping an underground formation comprising the following steps: prepare an aqueous injection fluid comprising at least one water-soluble copolymer, inject the injection fluid into an underground formation, sweep the underground formation using the injected fluid, recover an aqueous and oily and / or gaseous mixture, the water-soluble copolymer containing monomeric units derived from: a) of at least one water-soluble monomer bearing at least one unsaturated function; and b) of at least one LCST macromonomer in an amount greater than 10⁻⁵ mol% and less than 10⁻¹ mol% relative to the total number of moles of water-soluble monomer(s) and LCST macromonomer(s).

[0026] The LCST macromonomer has a weight average molecular weight between 500 g / mol and 200,000 g / mol.

[0027] The LCST macromonomer is obtained from an LCST oligomer made from at least one LCST monomer and at least one water-soluble monomer selected from non-ionic monomers, anionic monomers, cationic monomers, and zwitterionic monomers.

[0028] The LCST monomer is chosen from N-isopropylacrylamide; N,N-dimethylacrylamide; acryloyl morpholine; N,N-diethyl acrylamide; N-tert butyl acrylamide; N-vinyl caprolactame; and diacetone acrylamide.

[0029] For clarity, the term "water-soluble copolymer" refers to the water-soluble copolymer used in the present invention. Furthermore, "water-soluble monomer(s)" and "macromomomer(s)" refer respectively to "at least one water-soluble monomer" and "at least one macromonomer".

[0030] A water-soluble copolymer is a polymer of at least one water-soluble monomer and at least one LCST macromonomer. It is therefore obtained from at least one water-soluble monomer and at least one LCST macromonomer. Thus, it contains monomeric units derived from the aforementioned monomer(s) and macromonomer(s). In other words, a water-soluble copolymer is a copolymer of at least one water-soluble monomer bearing at least one unsaturated functional group capable of being polymerized to form a water-soluble backbone, and at least one LCST macromonomer.

[0031] By "water-soluble copolymer" we mean a copolymer which makes it possible to obtain an aqueous solution when dissolved under stirring at 25°C and with a concentration of 50 gL -1 in water.

[0032] As defined by IUPAC, a macromonomer is a polymer or oligomer bearing a terminal group that acts as a monomer; thus, each polymer or oligomer corresponds to a monomer unit in the chain of the final polymer.

[0033] According to an advantageous embodiment, the molar percentage of motifs (monomer units) from LCST macromonomers in the copolymer is greater than 10⁻⁴ mol% relative to the total number of moles of monomeric units of LCST monomers and macromonomers, preferably greater than 10⁻³ mol%, and even more preferably greater than 5 × 10⁻³ mol%. The molar percentage of motifs from LCST macromonomers in the copolymer is preferably less than 9 × 10⁻² mol% relative to the total number of moles of LCST monomers and macromonomers, preferably less than 8 × 10⁻² mol%, more preferably less than 6 × 10⁻² mol%, even more preferably less than 5 × 10⁻² mol%, and even more preferably less than 4 × 10⁻² mol%.

[0034] Generally, the quantity of monomeric units of a monomer (monomer or macromonomer) corresponds to the quantity of that monomer used in the preparation of the polymer. This definition is applicable to the preparation of the water-soluble copolymer or the macromonomer, and therefore of the oligomer (see below). The invention also relates to an injection fluid intended for use in an enhanced oil recovery by sweeping process, comprising a water-soluble copolymer having monomeric units derived from: a) of at least one water-soluble monomer bearing at least one unsaturated function; and b) of at least one LCST macromonomer in an amount greater than 10⁻⁵ mol% and less than 10⁻¹ mol% relative to the total number of moles of water-soluble monomers and LCST macromonomers.

[0035] In the injection fluid: The LCST macromonomer has a weight-average molecular weight between 500 g / mol and 200,000 g / mol. The LCST macromonomer is obtained from an LCST oligomer made from at least one LCST monomer and at least one water-soluble monomer selected from non-ionic monomers, anionic monomers, cationic monomers, and zwitterionic monomers. The LCST monomer is selected from N-isopropylacrylamide; N,N-dimethylacrylamide; acryloyl morpholine; N,N-diethyl acrylamide; N-tert-butyl acrylamide; N-vinyl caprolactam; and diacetone acrylamide. The LCST macromonomer and its synthesis

[0036] Given the presence of extremely small quantities of macromonomer motifs at LCST, the copolymer used has the dual advantage of not turning into a gel in the underground formation, and of giving the injection fluid containing it improved sweeping and mobility control properties.

[0037] According to the general knowledge of those skilled in the art, LCST groups are those whose solubility in water, at a given concentration, changes above a certain temperature and depending on the salinity. These groups exhibit a heating transition temperature that defines their lack of affinity for the solvent. This lack of affinity for the solvent results in opacification or a loss of transparency.

[0038] The minimum transition temperature is called the LCST (Lower Critical Solution Temperature). For each group concentration at the LCST, a heating transition temperature is observed. This temperature is higher than the LCST, which is the minimum point on the curve. Below this temperature, the copolymer is soluble in water; above this temperature, the copolymer loses its solubility in water.

[0039] Typically, LCST can be measured visually: the temperature at which the lack of affinity for the solvent appears is determined, i.e., the cloud point. The cloud point corresponds to the opacification of the solution or loss of transparency.

[0040] LCST can also be determined according to the type of phase transition, for example by DSC (differential scanning calorimetry), by a transmittance measurement or by a viscosity measurement.

[0041] Preferably, the LCST is determined by determining the cloud point by transmittance according to the following protocol.

[0042] The transition temperature is measured for a compound by LCST in a solution having a mass concentration of 1 wt% of said compound in deionized water. The cloud point corresponds to the temperature at which the solution exhibits a transmittance equal to 85% of light rays with a wavelength between 400 and 800 nm.

[0043] In other words, the temperature at which the solution exhibits a transmittance of 85% corresponds to the minimum LCST transition temperature of the compound, in this case the LCST macromonomer.

[0044] Generally speaking, a transparent composition exhibits a maximum light transmittance value, regardless of the wavelength between 400 and 800 nm, through a 1 cm thick sample, of at least 85%, preferably at least 90%. This is why the cloud point corresponds to a transmittance of 85%.

[0045] In general, the LCST macromonomer is obtained by synthesizing an LCST oligomer possessing a functional end, and then grafting an ethylenic group onto this functional end.

[0046] One example is the synthesis of the LCST macromonomer from an LCST oligomer of controlled size and functionality, carried out using a radical or ionic initiator possessing the desired chemical function, and / or by introducing a transfer agent substituted by the desired chemical group and / or by polycondensation.

[0047] The LCST monomers that can be used to manufacture the LCST oligomer, which is used to obtain the LCST macromonomers, are chosen from N-isopropylacrylamide; N,N-dimethylacrylamide; acryloyl morpholine; N,N-diethyl acrylamide; N-tert butyl acrylamide; N-vinyl caprolactam; and diacetone acrylamide.

[0048] In the context of the invention, the LCST oligomer advantageously comprises between 10 mol% and 100 mol% of monomer(s) including an LCST motif, more advantageously between 40 mol% and 100 mol%, and even more advantageously between 50 mol% and 100 mol%, relative to the total number of moles of monomers in the oligomer. According to a particular embodiment, the LCST oligomer may, in particular, comprise 90 to 96 mol% of monomer(s) including an LCST motif.

[0049] In addition to LCST monomers, water-soluble monomers suitable for manufacturing the LCST oligomer are selected from non-ionic, anionic, cationic, and zwitterionic monomers. Preferably, they are selected from non-ionic and anionic monomers.

[0050] In the context of the invention, the LCST oligomer advantageously comprises between 0 mol% and 90 mol% of these monomer(s) (nonionic and / or anionic and / or cationic and / or zwitterionic), more advantageously between 0 mol% and 60 mol%, and even more advantageously between 0 mol% and 50 mol%, relative to the total number of moles of monomers in the oligomer. According to a particular embodiment, the LCST oligomer may, in particular, comprise 4 to 10 mol% of these monomer(s). These monomers may be hydrophilic or hydrophobic.

[0051] Thus, the LCST oligomer, and therefore the LCST macromonomer, is obtained from at least one LCST monomer and at least one water-soluble monomer. It therefore contains monomeric units derived from the aforementioned monomer(s) and macromonomer(s).

[0052] The water-soluble monomer can be a non-ionic monomer, particularly one chosen from the group comprising water-soluble vinyl monomers, and especially acrylamide. Thus, the LCST oligomer can comprise a non-ionic monomer advantageously chosen from the group including acrylamide, methacrylamide, N-vinylformamide, and N-vinylpyrrolidone.

[0053] The water-soluble monomer can also be anionic monomer. The anionic monomer(s) that can be used in the context of the invention can be selected from a wide range. These monomers can have acrylic, vinyl, maleic, fumaric, malonic, itaconic, or allylic functional groups and contain a carboxylate, phosphonate, phosphate, sulfate, sulfonate, or other anionically charged group. The anionic monomer can be in acidic form or as a salt of an alkali earth metal, an alkali metal, or an ammonium (advantageously quaternary).Examples of suitable monomers include acrylic acid; methacrylic acid; itaconic acid; crotonic acid; maleic acid; fumaric acid; strong acid-type monomers exhibiting, for example, a sulfonic acid or phosphonic acid function such as 2-acrylamido-2-methylpropanesulfonic acid, vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, allylphosphonic acid, styrene sulfonic acid; and water-soluble salts of these monomers such as their alkali metal, alkaline earth metal, or ammonium (advantageously quaternary) salts.

[0054] Optionally, the LCST oligomer may include at least one cationic monomer.

[0055] The water-soluble monomer may optionally be a cationic monomer of the acrylamide, acrylic, vinyl, allylic, or maleic type possessing a quaternary amine or ammonium function. Examples include, but are not limited to, quaternized or salified dimethylaminoethyl acrylate (ADAME) and dimethylaminoethyl methacrylate (MADAME), dimethyldiallylammonium chloride (DADMAC), acrylamido propyltrimethyl ammonium chloride (APTAC), and methacrylamido propyltrimethyl ammonium chloride (MAPTAC).

[0056] Hydrophobic monomers can also be used, chosen in particular from among acrylamide, acrylic, vinyl, allyl, or maleic monomers possessing a pendant hydrophobic function. One example is the butyl methacrylate monomer.

[0057] According to a preferred embodiment, the LCST oligomer is a polymer of an LCST monomer (preferably N-isopropylacrylamide), an anionic monomer (preferably acrylic acid) and a hydrophobic monomer (preferably butyl methacrylate).

[0058] Thus, according to another preferred embodiment, the LCST oligomer is a polymer of an LCST monomer (preferably N-isopropylacrylamide), a cationic monomer (preferably MADAME.MeCl) and a hydrophobic monomer (preferably butyl methacrylate).

[0059] Regarding the synthesis of the macromonomer at LCST, in a first step we can mention telomerization, which is a mode of synthesis of low molar masses LSCT oligomers (called telomeres).

[0060] According to the invention, the LCST macromonomer has a molecular weight between 500 g / mol and 200,000 g / mol, preferably between 1,000 g / mol and 100,000 g / mol, more preferably between 1,500 g / mol and 100,000 g / mol. The molecular weight is understood to be the weight average molecular weight.

[0061] Telogenic agents can be chosen from among others thiols, alcohols, disulfides, phosphorus, boron and halogenated derivatives. In particular, they can allow the introduction of specific functions at the ends of telomere chains, for example silanes, trialkoxysilanes, amines, epoxies, hydroxyl groups, phosphonates, or acids.

[0062] Once these LCST oligomers are formed, in a second step, a vinyl double bond (R1R2C=CR3-, R1, R2 and R3 being independently of each other a hydrogen atom or a hydrocarbon group or not which may include heteroatoms) can be introduced at the end of the chain so that they serve as LCST macromonomers which can in turn be polymerized.

[0063] According to another synthesis method, an LCST macromer is obtained by controlled radical polymerization, known as RAFT (reversible addition-fragmentation chain transfer), of LCST monomers in the presence of at least one crosslinking agent. The macromonomer thus obtained is structured and can serve as a core for the production of water-lubricable copolymers in a star-shaped form. The crosslinking agent can be chosen from the group comprising monomers with polyethylene unsaturation (having at least two unsaturated functional groups), such as vinyl, allylic, and acrylic groups; methylene bisacrylamide (MBA) is an example.

[0064] The LCST macromonomer can be obtained by the same technique but without the use of a crosslinking agent.

[0065] The reactions that can be implemented for couplings on monomers are numerous: alkylation, esterification, amidation, transesterification or transamidation.

[0066] In a preferred embodiment, the LCST macromonomer is prepared by a radical reaction between an LCST oligomer and a compound containing a double bond, the double bond still being present after said radical reaction. Advantageously, the LCST oligomer is characterized by having a nitrogen or oxygen atom at its end, such as an alcohol or amine group, which is functionalized via the compound containing the double bond. This compound containing the double bond is preferably chosen from acryloyl chloride, acrylic acid, methacryloyl chloride, methacrylic acid, maleic anhydride, methacrylic anhydride, unsaturated aliphatic isocyanates, allyl chloride, allyl bromide, glycidyl acrylate, and glycidyl methacrylate.

[0067] According to a particular embodiment, the LCST macromonomer can have the formula (I):

[0068] In which: m is an integer advantageously between 2 and 40.

[0069] In general, the LCST groups of the water-soluble copolymer have a heating transition temperature of 0°C to 180°C for a mass concentration in deionized water of 1% by weight of said LCST groups, preferably between 0°C and 100°C, even more preferably between 10°C and 60°C. Water-soluble copolymer and its synthesis

[0070] According to one embodiment of the invention, the water-soluble copolymer can be obtained by copolymerizing at least one type of water-soluble monomer bearing at least one unsaturated functional group and at least one LCST macromonomer. In other words, the water-soluble monomer(s) and the LCST macromonomer(s) are polymerized simultaneously in a reactor. The polymer chain forms progressively in the presence of the water-soluble monomer(s) and the LCST macromonomer(s).

[0071] According to another embodiment, a water-soluble copolymer (the backbone) is first obtained by polymerizing the water-soluble monomers, and then the LCST oligomers are grafted onto this copolymer. Those skilled in the art are familiar with the techniques for grafting LCST macromonomers onto a copolymer. For example, patent WO 2014 / 047243 describes this technique.

[0072] According to a third embodiment, a water-soluble copolymer is obtained by polymerizing water-soluble monomers onto the structured LCST macromonomer obtained in the presence of a radical crosslinking agent (RAFT). The copolymers thus obtained are therefore in the form of LCST-core stars.

[0073] The invention is not limited to these methods of obtaining the copolymer.

[0074] Water-soluble monomers bearing at least one unsaturated functional group, suitable for use in manufacturing the water-soluble copolymer, are preferably selected from non-ionic, anionic, cationic, and zwitterionic monomers. Preferably, they are selected from non-ionic and anionic monomers.

[0075] The water-soluble monomer can be a non-ionic monomer, particularly one chosen from the group comprising water-soluble vinyl monomers, and especially acrylamide, methacrylamide, N-vinylformamide, and N-vinylpyrrolidone. Advantageously, the non-ionic monomer is acrylamide.

[0076] The water-soluble monomer can also be anionic monomer. The anionic monomer(s) that can be used in the context of the invention can be selected from a wide range. These monomers can have acrylic, vinyl, maleic, fumaric, malonic, itaconic, or allylic functional groups and contain a carboxylate, phosphonate, phosphate, sulfate, sulfonate, or other anionically charged group. The anionic monomer can be in acidic form or as a salt of an alkali metal, an alkali metal, or an ammonium compound.Examples of monomers include acrylic acid; methacrylic acid; itaconic acid; crotonic acid; maleic acid; fumaric acid; strong acid-type monomers exhibiting, for example, a sulfonic acid or phosphonic acid function such as 2-acrylamido-2-methylpropane sulfonic acid, vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, allylphosphonic acid, styrene sulfonic acid; and water-soluble salts of these monomers such as their alkali metal, alkaline earth metal, or ammonium salts.

[0077] Optionally, the skeleton of the water-soluble copolymer may include at least one cationic monomer.

[0078] The water-soluble monomer may optionally be a cationic monomer of the acrylamide, acrylic, vinyl, allylic, or maleic type possessing an amine or ammonium function (for example, a quaternary ammonium). Examples include, in particular and without limitation, quaternized or salified dimethylaminoethyl acrylate (ADAME) and dimethylaminoethyl methacrylate (MADAME), dimethyldiallylammonium chloride (DADMAC), acrylamido propyltrimethyl ammonium chloride (APTAC), and methacrylamido propyltrimethyl ammonium chloride (MAPTAC).

[0079] According to a particular embodiment, the water-soluble copolymer advantageously comprises up to 99.99999 mol% of anionic monomer(s), relative to the total number of monomers. The copolymer advantageously comprises a significant amount of anionic monomer, preferably greater than 30 mol%, preferably greater than 50 mol%, and even more preferably greater than 70 mol%. This amount will be greater the higher the higher the temperature and / or salinity conditions encountered by the copolymer in the underground formation.

[0080] When the water-soluble copolymer includes at least one non-ionic monomer, this is preferentially present between 1 and 70 mol%, more preferably between 20 and 50 mol%, relative to the total number of monomers.

[0081] When the water-soluble copolymer includes at least one cationic monomer, this is preferentially present in an amount of less than 5 mol%.

[0082] The water-soluble copolymer can be obtained by radical polymerization. Polymerization techniques such as RAFT (reversible-addition-fragmentation chain transfer), NMP (nitroxide-mediated polymerization), or ATRP (atom transfer radical polymerization) can be used to obtain the water-soluble copolymer.

[0083] In general, the copolymer 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 include solution polymerization; gel polymerization; precipitation polymerization; emulsion polymerization (aqueous or inverse); suspension polymerization; reactive extrusion polymerization; water-in-water polymerization; and micellar polymerization.

[0084] Polymerization is preferably carried out by reverse emulsion followed or not by a concentration step, or by gel method.

[0085] According to the invention, the copolymer can be in liquid, gel or solid form (powder or bead) when its preparation includes a drying step such as spray drying, drum drying, radiation drying such as microwave drying, or fluidized bed drying.

[0086] According to the invention, the water-soluble copolymer advantageously has a molecular weight of at least 0.5 million g / mol, preferably between 0.5 million and 40 million g / mol, more preferably between 5 million and 30 million g / mol. Molecular weight is understood to be the average molecular weight by weight.

[0087] The molecular weight is determined by the intrinsic viscosity of the copolymer. Intrinsic viscosity can be measured using methods known to those skilled in the art and can be calculated from the reduced viscosity values ​​for different copolymer concentrations using a graphical method. This involves plotting the reduced viscosity values ​​(y-axis) against the concentration (x-axis) and extrapolating the curve down to zero concentration. The intrinsic viscosity value is then plotted on the y-axis or using the least squares method. The molecular weight can then be determined using the Mark-Houwink equation: η = K M α [η] represents the intrinsic viscosity of the copolymer determined by the solution viscosity measurement method, K represents an empirical constant (K = 3.73.10 -4< ), M represents the molecular weight of the copolymer, α represents the Mark-Houwink coefficient (α = +0.66), K and α depend on the particular copolymer-solvent system. Properties of the water-soluble copolymer

[0088] The water-soluble copolymer exhibits association properties for a given mass concentration in aqueous solution and above a given critical temperature.

[0089] These heating association properties observed beyond the transition temperature of the chains at LCST generate sticky points between the main chains and a slowing of the chain creep.

[0090] A slowing down of chain creep in porous media leads to an increase in mobility reduction, which is sought in enhanced oil recovery by sweeping ("polymer flooding").

[0091] In the process according to the invention, the underground formation is scanned by injecting an aqueous injection fluid containing the water-soluble copolymer into the formation. The gradual change in temperature until the underground formation reaches its optimal level allows the polymer chains to bond. This temperature is generally between 20°C and 200°C.

[0092] The associating effect of the water-soluble copolymer can be adapted to the nature of the underground formation (temperature) in particular depending on the nature of the macromonomer at LCST, the molar quantity of macromonomer at LCST, the salinity and / or pH of the water used to prepare the aqueous injection fluid to be injected, or the concentration of the copolymer.

[0093] According to an advantageous embodiment, the water-soluble copolymer has an association temperature advantageously between 25°C and 160°C, preferably above 40°C (40°C < temperature ≤ 160°C), more preferably above 60°C (60°C < temperature ≤ 160°C), even more preferably above 80°C (80°C < temperature ≤ 160°C), even more preferably above 90°C (90°C < temperature ≤ 160°C).

[0094] Advantageously, the association effect occurs instantaneously once the injection fluid has reached the copolymer association temperature, after injection into the underground formation to be treated. Enhanced oil recovery process by sweeping

[0095] Surprisingly, the Applicant discovered that the water-soluble copolymer is particularly well-suited for use in enhanced oil recovery processes by sweeping an underground formation.

[0096] The use of LCST macromonomers in extremely small quantities within the copolymer allows for a unique technical effect. The water-soluble copolymer yields a completely unexpected and particularly effective rheological profile for clearing underground formations.

[0097] As mentioned previously, and without being linked to any theory, it seems that the association properties by heating observed beyond the transition temperature of the chains at LCST generate sticky spots between the main chains of the copolymers according to the invention and a slowing of the creeping of the chain.

[0098] This slowing down of the creeping of the chains thus generates in porous media an increase in the reduction of mobility, and thus a better efficiency of sweeping the underground formation.

[0099] The water-soluble copolymer is particularly interesting and effective when field conditions are difficult, for example when the temperature of the underground formation is high and / or the salinity of the aqueous injection fluid in which the copolymer is located is high.

[0100] In a particular embodiment of the invention, the process comprises the treatment of an underground formation having a maximum temperature between 25°C and 160°C, preferably above 40°C, more preferably above 60°C, even more preferably above 80°C, even more preferably above 90°C.

[0101] The concentration of water-soluble copolymer in the aqueous injection fluid is advantageously between 50 and 50,000 ppm by weight, preferably between 100 and 30,000 ppm, more preferably between 500 and 10,000 ppm relative to the weight of the injection fluid.

[0102] The water or brine used to prepare the injection fluid may be production water. "Production water" refers to any saline or non-saline water, brine, seawater, or aquifer water originating from a hydrocarbon reservoir. This production water may be treated prior to the preparation of the injection fluid as described in patent application WO 2018 / 020175.

[0103] In a particular embodiment of the invention, the process comprises the treatment of an underground formation with an aqueous injection fluid comprising more than 5% by weight of salts, preferably more than 7%, even more preferably more than 10%.

[0104] The water-soluble copolymers according to the invention can be combined with stabilizing compounds. These stabilizing compounds (stabilizing agents) can be compounds that adequately protect the copolymers, for example, against thermal, chemical, and / or mechanical degradation. Examples of suitable stabilizing agents are provided in patent application WO 2010 / 133258.

[0105] SP (Surfactant Polymer) and ASP (Alkaline Surfactant Polymer) techniques or any other technique well known to those skilled in the art may be implemented with the same principle according to the invention.

[0106] Advantageously, the injection fluid has, at the time of its injection, a viscosity between 1 and 200 cps (centipoise) (viscosity measurements at 20°C with a Brookfield viscometer with a UL module and at a speed of 6 revolutions per minute).

[0107] The implementation of the water-soluble copolymer is advantageously carried out on-site, just upstream of its injection into the reservoir. Generally, all the components introduced into the water or brine to constitute the injection fluid are most often added to a circulation line of the aqueous solution or brine.

[0108] When the water-soluble copolymer is in granular form, it can be dissolved in an aqueous medium in a dispersion device. An example of a dispersion device is the polymer slicing unit (PSU) described in US patent 8,186,871, which allows the preparation of a concentrated aqueous polymer solution.

[0109] Besides the water-soluble copolymer, the injection fluid may also include one or more chemical compounds useful for enhanced oil recovery and well known to those skilled in the art.

[0110] Among these chemical compounds, we can mention the use of weak, strong, or super-strong bases, mineral or organic, capable of saponifying crude oil and forming in-situ petroleum-solubilizing surfactants. Examples include sodium carbonate, caustic soda, borate and metaborate compounds, amines, and basic polymeric species.

[0111] Another family of compounds widely injected with polymers is that of surfactants, often anionic, zwitterionic, cationic, and sometimes also nonionic. These compounds are rarely injected pure but with a co-surfactant and a co-solvent to improve their compatibility and efficiency in the reservoir.

[0112] Another aspect of the invention relates to an injection fluid comprising the water-soluble copolymer as described above. The injection fluid comprises at least one water-soluble copolymer in the proportions described above, and at least water or brine. Optionally, it comprises other components such as, for example, a base as described above, one or more surfactants as described above, co-solvents, or other water-soluble (co)polymers.

[0113] The invention and its benefits will be more clearly seen from the following examples given to illustrate the invention and not in a limiting way. EXAMPLES OF THE INVENTION'S IMPLEMENTATION 1 / Telomere synthesis (or LCST oligomers)

[0114] To produce a telomere called T1, the following process is carried out.

[0115] In a double-jacketed reactor: A hydroalcoholic solution (410 g) is loaded with the following monomers: N-isopropylacrylamide (NIPAM, 113 g, or 1 mol), butyl methacrylate (7.9 g, or 0.055 mol), and acrylic acid (4.44 g, or 0.055 mol). The mixture is stirred. The pH of the mixture is adjusted to between 4.0 and 5.0 using a 40% (wt) NaOH solution in water. The resulting mixture is heated to 50°C. It is deoxygenated by bubbling nitrogen for 40 minutes. Aminoethanethiol HCl (2.5 g) is added. 2,2'-Azobis(2-methylpropionamidine)dihydrochloride (0.22 g) is added to initiate telomerization. After the temperature has stabilized, the mixture is stirred for 2 hours and then cooled to 25°C.

[0116] A concentrated viscous solution is obtained containing 23% by weight of a telomere with a degree of polymerization of 50 monomeric units (DPn 50). The LCST of this telomere T1 was estimated at 38°C according to the method described previously.

[0117] To produce a telomere called T2, the following process is carried out.

[0118] In a double-jacketed reactor: In 445 g of a hydroalcoholic solution, the monomers N-isopropylacrylamide (NIPAM, 113 g, i.e., 1 mol), butyl methacrylate (4.44 g, i.e., 0.031 mol), and chloromethylaminoethyl dimethyl methacrylate (MADAME.MeCl, 2.16 g, i.e., 0.01 mol) are loaded. The mixture is stirred. The pH of the mixture is adjusted to between 4.0 and 5.0 using a 40 wt. NaOH solution in water. The resulting mixture is heated to 50 °C. Deoxygenated by bubbling nitrogen for 40 minutes. Aminoethanethiol HCl (2.35 g) is added. 2,2'-Azobis(2-methylpropionamidine)dihydrochloride (0.22 g) is added to initiate polymerization. After the temperature has stabilized, it is kept stirring for 2 hours and then cooled to 25°C.

[0119] A concentrated viscous solution containing 21% by weight of a telomere with a degree of polymerization of 50 monomeric units (DPn 50) is obtained. The LCST of this telomere T2 was estimated at 32°C according to the method described previously. Table 1: List and monomeric compositions of telomeres T1 and T2 Telomere LCST monomer (A), mol% Hydrophilic Monomer (B), mol% Hydrophobic Monomer (C), mol% LCST Telomer (°C) T1 NIPAM, 90 Acrylic acid, 5 Butyl methacrylate, 5 38 T2 NIPAM 96 MADAME.MeCl, 1 Butyl methacrylate, 3 32 2 / Synthesis of macromonomers

[0120] To manufacture a macromonomer called M1, the following process is carried out.

[0121] In a double-jacketed reactor: Four hundred grams of a 23% (wt) Telomer T1 (5581 g / mol) solution is loaded into water. The solution is stirred. The pH is adjusted to 7.5 using a 40% (wt) NaOH solution. The mixture is cooled to 5°C. Using a burette, 3.0 g of acryloyl chloride is added dropwise. The pH is continuously adjusted between 7 and 9 using a 40% (wt) NaOH solution. The temperature is maintained at 5°C throughout the reaction. The mixture is stirred for two hours after the reaction is complete, with continued pH monitoring.

[0122] A concentrated viscous solution containing 21.5% by weight of macromonomer is obtained at LCST M1 (5711 g / mol).

[0123] Macromonomer M2 is manufactured using the same process, with telomer T2 (5740 g / mol). A concentrated viscous solution containing 21.5% by weight of macromonomer at LCST M2 (5869 g / mol) is obtained. 3 / Polymer Synthesis A- Polymer in powder form obtained by gel polymerization

[0124] To manufacture a polymer called P1 from the macromonomer M1, the following process is carried out.

[0125] 173 g of acrylamide, 186 g of ATBS (2-acrylamido-2-methylpropanesulfonic acid), 1.4 g (0.0075 mol% by weight of monomer) of LCST M1 macromonomer, and 640 g of deionized water are loaded into a beaker to prepare the aqueous monomer solution. The pH of the monomer solution is adjusted to 7 using a 40 wt. NaOH solution. It is cooled to 5°C.

[0126] The following additives are then added: 30 ppm Versenex 80 (complexing agent), 250 ppm AZDN (azo-bis-isobutyronitrile) (azo initiator), 2 ppm TBHP (tert-butylhydroperoxide) (oxidant).

[0127] The solution is cooled to 10°C and then transferred to a heated reactor. It is deoxygenated with an inert gas for 15 minutes. Four ppm of Mohr's salts are added to initiate the polymerization reaction. This polymerization proceeds under adiabatic conditions until a temperature of 85°C is reached. Once polymerization is complete, the reaction mixture is left to stand for 12 hours. The resulting gel is then ground and dried in an oven, yielding a white powder.

[0128] The P2 polymer is manufactured using the same process with the M2 macromonomer. A white powder is obtained.

[0129] As a counter-example, polymers P3 and P4 are manufactured using the same process with macromonomer M1 and M2 respectively, with the difference that the quantity of macromonomer is 10 -6 < mol% compared to the total number of monomers.

[0130] Also as a counter-example, polymers P5 and P6 are manufactured using the same process with macromonomer M1 and M2 respectively, with the difference that the quantity of macromonomer is 2.10 -1 < mol% compared to the total number of monomers. B- Polymer in the form of an inverse emulsion

[0131] To manufacture a reverse emulsion called EM1, the following process is carried out.

[0132] 146 g (74.997 mol%) of acrylamide, 157 g (25 mol%) of ATBS (2-acrylamido-2-methylpropane sulfonic acid), 0.5 g (0.003 mol%) of LCST M1 macromonomer, and 370 g of water are loaded into a beaker to prepare the aqueous monomer solution. The pH of the monomer solution is adjusted to between 5 and 6 using NaOH.

[0133] The following additives are added: 0.37g of Versenex 80 (complexing agent), 1.29g of TBHP (tert-butylhydroperoxide) (oxidant).

[0134] 295g of Exxsol D100 and 30g of Span 80 are mixed before being transferred to a reactor along with the aqueous phase. The two-phase mixture is emulsified using a mixer, deoxygenated with an inert gas, and then cooled to a temperature of 15°C.

[0135] The synthesis begins with the addition of a solution of MBS (sodium metabisulfite, 1g / l) at a flow rate of (1ml / min). This yields a reverse emulsion with a polymer concentration of 30% by weight.

[0136] An EM2 inverse emulsion is manufactured using the same process, with the LCST macromonomer M2. An EM2 inverse emulsion with a polymer concentration of 30% by weight is obtained.

[0137] As a counter-example, EM3 and EM4 emulsions are manufactured using the same process with macromonomer M1 and M2 respectively, with the difference that the quantity of macromonomer is 10 -6 < mol% compared to the total number of monomers.

[0138] Also as a counter-example, EM5 and EM6 emulsions are manufactured using the same process with macromonomer M1 and M2 respectively, with the difference that the quantity of macromonomer is 2.10 -1 < mol% compared to the total number of monomers. 4 / Evaluation of polymer behavior in porous media a) Materials and methods

[0139] Several propagation experiments in porous media were carried out to evaluate the propagation profile and the reduction in mobility.

[0140] The polymer solutions are prepared from powders P1 to P6 and emulsions EM1 to EM6 in a 90,000 TDS (Total Dissolved Solids, 90,000 mg / L water) brine with the following composition: 86.23 g NaCl, 5 g CaCl₂, 2 H₂O in water for a total weight of 1000 g. The polymer concentration in these solutions is 900 ppm.

[0141] The rock used is a Bentheimer-type sandstone with an average water permeability of around 1.5 darcies. Our porous media are 1 inch (2.54 cm) in diameter and 3.5 inches (8.89 cm) long. The porous media were dried overnight in an oven before being saturated under vacuum in deoxygenated water. These porous media are placed in a Hassler cell under a confining pressure of 50 bar. This setup is equipped with a pressure sensor between the cell's inlet and outlet. b) Test

[0142] First, brine is injected at a flow rate of 12 cm³ / h to create a control. After signal stabilization, the polymer solution is injected. The pressure sensor measures the pressure associated with the viscosity development. The reduction in mobility is determined after signal stabilization using the following formula: Rm = Δ P solution de polym è re Δ P saumure

[0143] A high Rm value is sought because it corresponds to a good ability of the polymer to effectively sweep the rock. c) Results

[0144] During the injection of the polymer solution, a pressure increase is observed, corresponding to the increase in the viscosity of the polymer solution, until the signal stabilizes. The temperature during the tests is 80 °C. A- Polymer in powder form

[0145] Polymers P1 to P2 and counterexamples P3 to P6 are tested, and the results are shown in Table 2. Table 2: Results of the injections of polymer solutions P1 to P6. Polymer Macromonomer Macromonomer (mol%) Polymer concentration (ppm) Rm P1 M1 7.5 10 -3< 900 100 P2 M2 7.5 10 -3< 900 150 P3 M1 10 -6< 900 10 P4 M2 10 -6< 900 11 P5 M1 2.10 -1< 900 N / A P6 M2 2.10 -1< 900 N / A

[0146] NA: Not Applicable. The signal is not stabilized, and therefore no Rm value can be measured. In practice, injecting such solutions would risk damaging the rock by clogging the pores of the formation. These solutions are therefore completely unsuitable for enhanced oil recovery by sweeping.

[0147] It is observed that polymers P1 and P2, containing an appropriate amount of macromonomer at LCST (7.5 x 10⁻³ < mol%), offer a significantly greater reduction in mobility than polymers P3 and P4, which contain an insufficient amount of macromonomer (10⁻⁶ < mol%). Polymers P5 and P6, containing an excessive amount of macromonomer (2 x 10⁻¹ < mol%), are not suitable for enhanced oil recovery by sweeping. B- Polymer in the form of an inverse emulsion

[0148] EM1 to EM2 emulsions and EM3 to EM6 counterexamples are tested, and the results are shown in Table 3. Table 3: Results of the injections of polymer solutions from emulsions EM1 to EM6. Polymer Macromonomer Macromonomer (mol%) Polymer concentration (ppm) Rm EM1 M1 7.5 10 -3< 900 80 EM2 M2 7.5 10 -3< 900 140 EM3 M1 10 -6< 900 9 EM4 M2 10 -6< 900 12 EM5 M1 2.10 -1< 900 N / A EM6 M2 2.10 -1< 900 N / A

[0149] NA: Not Applicable. The signal is not stabilized, and therefore no Rm value can be measured. In practice, injecting such solutions would risk damaging the rock by clogging the pores of the formation. These solutions are therefore completely unsuitable for enhanced oil recovery by sweeping.

[0150] The same trend is observed as for polymers P1 to P6 in powder form. More precisely, the polymers in emulsions EM1 and EM2 containing an appropriate amount of macromonomer at LCST (7.5 x 10⁻³ < mol%) offer a significantly greater reduction in mobility than that obtained with the polymers in emulsions EM3 ​​and EM4 containing an insufficient amount of macromonomer (10⁻⁶ < mol%). The polymers in emulsions EM5 and EM6 containing an excessive amount of macromonomer (2 x 10⁻¹ < mol%) are not suitable for enhanced oil recovery by sweeping.

[0151] The polymers according to the invention therefore offer, thanks to the presence of an appropriate quantity of LCST macromonomers, very interesting properties in enhanced oil recovery processes by sweeping.

Claims

1. A process of enhanced oil recovery by sweeping an underground formation comprising the following steps: - preparing an aqueous injection fluid comprising at least one water-soluble copolymer, - injecting the injection fluid into an underground formation, - sweeping the underground formation using the injected fluid, - recovering an aqueous and oily and / or gaseous mixture, the water-soluble copolymer containing monomeric units resulting from: a) at least one water-soluble monomer bearing at least one unsaturated function; and b) at least one LCST macromonomer in an amount greater than 10-5 mol% and less than 10-1 mol% based on the total number of moles of water-soluble monomer(s) and LCST macromonomer(s), the LCST macromonomer having a weight-average molecular weight of between 500 g / mol and 200,000 g / mol, the LCST macromonomer being obtained from an LCST oligomer made from at least one LCST monomer and at least one water-soluble monomer selected from nonionic monomers, anionic monomers, cationic monomers, and zwitterionic monomers, the LCST monomer being selected from N-isopropylacrylamide; N,N-dimethylacrylamide; acryloyl morpholine; N,N-diethyl acrylamide; N-tert-butyl acrylamide; N-vinyl caprolactam; and diacetone acrylamide.

2. Process according to claim 1, characterized in that the molar percentage of LCST macromonomers in the copolymer is greater than 10-4 mol% based on the total number of moles of monomers and LCST macromonomers, preferably greater than 10-3 mol%, even more preferably greater than 5.10-3 mol%.

3. Process according to claims 1 or 2, characterized in that the molar percentage of LCST macromonomers in the copolymer is less than 9.10-2 mol% based on the total number of moles of monomers and LCST macromonomers, preferably less than 8.10-2 mol%, more preferably less than 6.10-2 mol%, even more preferably less than 4.10-2 mol%.

4. Process according to one of the preceding claims, characterized in that the LCST macromonomer is obtained by radical reaction between an LCST oligomer and a compound containing a double bond, the double bond still being present after said radical reaction.

5. Process according to claim 4, characterized in that the LCST oligomer has a nitrogen or oxygen atom at its end.

6. Process according to claim 4 or 5, characterized in that the compound containing a double bond is selected from acryloyl chloride, acrylic acid, methacryloyl chloride, methacrylic acid, maleic anhydride, methacrylic anhydride, unsaturated aliphatic isocyanates, allyl chloride, allyl bromide, glycidyl acrylate, and glycidyl methacrylate.

7. Process according to one of claims 4 to 6, characterized in that the LCST oligomer comprises water-soluble monomers selected from non-ionic monomers and anionic monomers.

8. Process according to one of the preceding claims, characterized in that the LCST macromonomer is of formula (I): wherein m is an integer advantageously between 2 and 40.

9. Process according to one of the preceding claims, characterized in that the water-soluble monomers of the water-soluble copolymer bearing at least one unsaturated function are selected from non-ionic monomers, anionic monomers, cationic monomers, and zwitterionic monomers.

10. Process according to one of claims 4 to 9, characterized in that the water-soluble copolymer comprises non-ionic monomers, and in that the LCST oligomer comprises non-ionic monomers, said non-ionic monomers being selected from the group comprising acrylamide; methacrylamide; N- vinylformamide; N-vinylpyrrolidone.

11. Process according to one of claims 4 to 10, characterized in that the water-soluble copolymer comprises anionic monomers, and in that the LCST oligomer comprises anionic monomers, the anionic monomers being selected from the group comprising acrylic acid; methacrylic acid; itaconic acid; crotonic acid; maleic acid; fumaric acid; 2-acrylamido 2-methylpropanesulfonic acid, vinylsulfonic acid, vinylphosphonic acid, allylsulfonic acid, allylphosphonic acid, styrene sulfonic acid; and the water-soluble salts of these monomers.

12. Process according to one of the preceding claims, characterized in that the water-soluble copolymer has an association temperature between 25°C and 160°C, preferably greater than 60°C and less than or equal to 160°C, more preferably greater than 90°C and less than or equal to 160°C.

13. Process according to one of the preceding claims, characterized in that the aqueous injection fluid comprises more than 5% by weight of salts, preferably more than 7%, even more preferably more than 10%.

14. Injection fluid intended for use in a process for enhanced oil recovery process by sweeping comprising a water-soluble copolymer containing monomeric units derived from: a) at least one water-soluble monomer bearing at least one unsaturated function; and b) at least one LCST macromonomer in an amount greater than 10-5 mol% and less than 10-1 mol% based on the total number of moles of water-soluble monomers and LCST macromonomers, the LCST macromonomer having a weight average molecular weight of between 500 g / mol and 200,000 g / mol, the LCST macromonomer being obtained from an LCST oligomer made from at least one LCST monomer and at least one water-soluble monomer selected from nonionic monomers, anionic monomers, cationic monomers, and zwitterionic monomers, the LCST monomer being selected from N-isopropylacrylamide; N,N-dimethylacrylamide; acryloyl morpholine; N,N-diethyl acrylamide; N-tert-butyl acrylamide; N-vinyl caprolactam; and diacetone acrylamide.