Method for enhanced oil recovery in a subterranean carbonate formation

Abstract

The present invention relates to a method for enhanced oil recovery in a subterranean carbonate formation, the method comprising injecting an aqueous composition comprising at least one water-soluble polymer containing acrylamidopropyl trimethylammonium chloride (APTAC).

Description

[0001] Enhanced oil recovery process in an underground carbonate formation

[0002] FIELD OF INVENTION

[0003] The present invention relates to the technical field of enhanced oil recovery in an underground carbonate formation. More specifically, the present invention relates to a method for enhanced oil recovery in an underground carbonate formation by means of the injection of an aqueous composition comprising a water-soluble polymer containing acrylamidopropyl trimethylammonium chloride (APTAC).

[0004] STATE OF THE ART

[0005] 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.

[0006] Generally, the recovery of crude oil contained in deposits is carried out in several stages.

[0007] 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.

[0008] 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 amount of water in the mixture produced by the wells is excessive. This secondary recovery thus allows for an additional recovery rate of approximately 10 to 20%. 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 of oil.Under the term enhanced oil recovery are known various thermal or non-thermal techniques, such as the so-called electrical, miscible, steam, or chemical techniques of "enhanced recovery of oil" remaining in place (see "Oil & gas science and technology" - IFP review, vol 63 (2008) no. 1, pp 9-19).

[0009] 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.

[0010] The present invention relates more specifically to the chemically assisted recovery of petroleum involving the injection, into an underground carbonate 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 petroleum out of the rock.

[0011] It is known to those skilled in the art that synthetic water-soluble polymers, and in particular those based on acrylamide, are very advantageous polymers for increasing the viscosity of aqueous solutions and are, in fact, predominantly used in enhanced oil recovery.

[0012] Subterranean carbonate formations represent approximately half of the world's oil reservoirs. They are particularly rich in calcite and / or dolomite. The unique characteristic of these formations lies in the fact that the rock surfaces tend to be wetted by oil rather than water. This results in difficulties in processing and exploiting these reservoirs with water-soluble polymers. Particularly complex phenomena related to rock wetting and the interfacial tensions of the chemical compounds in contact with the rock make the exploitation of these subterranean formations challenging.

[0013] In the specific case of underground carbonate formations, which generally have low permeability, acrylamide-based polymers offer limited performance. The document "Enhanced Oil Recovery Field Experiments in Carbonate Reservoirs in the United States," by E. Manrique, Center for Energy and Technology of the Americas (CET A), summarizes various enhanced oil recovery experiments in carbonate reservoirs. One of the technologies used successfully is "Carbon Dioxide Flooding," i.e., the injection of carbon dioxide into the reservoir. The other technique used is "Chemical Flooding," i.e., enhanced oil recovery by injecting an aqueous polymeric composition. Hydrolyzed polyacrylamides, as well as polysaccharides, were tested.

[0014] The problem that the present invention aims to solve is to improve the efficiency of enhanced oil recovery in underground carbonate formations using specific water-soluble polymers.

[0015] DESCRIPTION OF THE INVENTION

[0016] The Plaintiff surprisingly discovered that this was possible through the use of a water-soluble polymer containing acrylamido-propyl-trimethyl-ammonium chloride (APTAC).

[0017] More specifically, the present invention relates to a method for enhanced oil recovery in an underground carbonate formation comprising the injection of an aqueous composition (or injection fluid) comprising at least one water-soluble polymer containing acrylamido-propyl-trimethyl-ammonium chloride (APTAC).

[0018] The invention also relates to an aqueous composition, also called injection fluid, intended for use in an enhanced oil recovery process in an underground carbonate formation comprising said water-soluble polymer.

[0019] A water-soluble polymer is defined as a copolymer that yields an aqueous solution when dissolved under stirring at 25°C and with a concentration of 20 gL 1 in the water.

[0020] The quantity of acrylamidopropyl trimethylammonium chloride (APTAC) units in the water-soluble polymer according to the invention is preferably between 20 mol% and 100 mol% relative to the total number of moles of monomeric units of the water-soluble polymer. More preferably, it is between 20 mol% and 99.9 mol%, more preferably between 30 mol% and 99 mol%, more preferably between 40 and 95 mol%, and even more preferably between 50 and 90 mol%.

[0021] In a preferred embodiment of the invention, the water-soluble polymer further contains at least one non-ionic, anionic, and / or zwitterionic monomer. The amount of non-ionic, anionic, or zwitterionic monomer units in the water-soluble polymer is preferably between 0.1 mol% and 70 mol% relative to the total number of moles of monomeric units of the water-soluble polymer, preferably between 1 and 50 mol%, more preferably between 5 and 40 mol%, and even more preferably between 5 and 20 mol%.

[0022] Anionic monomers are preferentially chosen from acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, sulfonated monomers, phosphonated monomers, 2-acrylamido-2-methylpropanesulfonic acid (ATBS), vinyl sulfonic acid, vinyl phosphonic acid, allyl sulfonic acid, and allyl phosphonic acid, said anionic monomer being unsalified, partially or totally salified. The preferred anionic monomers are acrylic acid, sulfonated monomers, phosphonated monomers, 2-acrylamido-2-methylpropanesulfonic acid (ATBS), and their salts. The preferred anionic monomer is 2-acrylamido-2-methylpropanesulfonic acid (ATBS) and its salts.

[0023] Nonionic monomers are preferentially chosen from acrylamide, methacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide and N-methylolacrylamide, N-vinylformamide, N-vinyl acetamide, acryloyl morpholine (ACMO) and diacetone acrylamide. Acrylamide is a preferred nonionic monomer.

[0024] In a preferred embodiment of the invention, the water-soluble polymer is an amphoteric polymer containing acrylamidopropyl trimethylammonium chloride (APTAC) and at least one anionic monomer. Preferably, the anionic monomer is selected from the anionic monomers mentioned above. The same preferences apply. Advantageously, the anionic monomer is 2-acrylamido-2-methylpropanesulfonic acid (ATBS) and its salts.

[0025] Advantageously, such a water-soluble polymer exhibits very good stability at temperatures above 80°C, preferably between 90°C and 130°C. Thus, advantageously, the present invention enables the assisted recovery of petroleum from underground carbonate formations, particularly at temperatures above 80°C, preferably between 90°C and 130°C.

[0026] In a preferred embodiment of the invention, the water-soluble polymer further contains at least one non-ionic and / or zwitterionic monomer. The amount of non-ionic or zwitterionic monomer units in the water-soluble polymer is preferably between 0.1 mol% and 70 mol% relative to the total number of moles of monomeric units of the water-soluble polymer, preferably between 1 and 50 mol%, more preferably between 5 and 40 mol%, and even more preferably between 5 and 20 mol%.

[0027] The water-soluble polymer may also contain another cationic monomer, but it is highly preferable that the water-soluble polymer not contain any cationic monomers other than acrylamido-propyl-trimethyl-ammonium chloride (APTAC).

[0028] Other possible cationic monomers include quaternized dimethylaminoethyl acrylate (ADAME), quaternized dimethylaminoethyl methacrylate (MADAME), dimethyldiallylammonium chloride (DADMAC), and methacrylamido propyltrimethyl ammonium chloride (MAPTAC).

[0029] According to a particularly preferred method, the water-soluble polymers that offer the best performance in underground carbonate formations are the amphoteric polymers of acrylamidopropyltrimethylammonium chloride (APTAC) and 2-acrylamido-2-methylpropanesulfonic acid (ATBS) and / or one of its salts. The ATBS monohydrate described in document WO 2018 / 172676 can be advantageously used.

[0030] According to this method, the quantity of APTAC units is preferably between 30 mol% and 95 mol% relative to the total number of moles of monomeric units of the water-soluble polymer, preferably between 60 mol% and 80 mol%, and that of ATBS units and / or one of its salts is preferably between 5 and 70 mol%, preferably between 20 and 60 mol%, more preferably between 20 and 50 mol%, even more preferably between 25 and 40 mol%.

[0031] Throughout the invention, it is understood that the sum of the percentages of the polymer monomers equals 100%. According to this embodiment, and preferably, water-soluble polymers are polymers containing only APTAC and ATBS and / or one of its salts as monomeric units. In the present application, branching agents possessing at least two unsaturated ethylenic groups, such as methylene-bis-acrylamide, are not considered monomeric units; the latter are limited to monomers possessing only one unsaturated ethylenic group.

[0032] Preferably, the water-soluble polymer has a weight-average molecular weight between 1 million and 15 million g / mol.

[0033] The weight-average molecular weight is determined by the intrinsic viscosity of the water-soluble polymer. 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 concentrations of water-soluble polymer 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:

[0034] [h] = KM a

[0035] [h] represents the intrinsic viscosity of the water-soluble polymer determined by the viscosity in solution measurement method.

[0036] K represents an empirical constant (K = 3.73 x 10 -4 ),

[0037] M represents the molecular weight of the water-soluble polymer, a represents the Mark-Houwink coefficient (a = +0.66),

[0038] K and a depend on the particular water-soluble polymer-solvent system.

[0039] Water-soluble polymers 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 water-soluble polymers. The water-soluble polymer used can have a linear, branched, star, or comb structure. These structures can be obtained by selecting the appropriate initiator, transfer agent, and polymerization technique.

[0040] In general, water-soluble polymers do not require the development of a specific polymerization process. Indeed, they 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.

[0041] Polymerization is preferably carried out by reverse emulsion followed or not by a concentration step, or by gel method.

[0042] According to the invention, the water-soluble polymer 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.

[0043] Surprisingly, the Applicant discovered that the water-soluble polymer according to the invention is particularly suitable for use in enhanced oil recovery processes from underground carbonate formations, particularly rich in calcite and / or dolomite.

[0044] Preferably, in the enhanced oil recovery process according to the invention, the underground carbonate formation contains at least 20% carbonates, preferably at least 30% carbonates, and more preferably at least 50% carbonates. The percentage of carbonates corresponds to the proportion of carbonate rocks in the formation.

[0045] The presence of acrylamidopropyltrimethylammonium chloride (APTAC) monomeric units in the water-soluble polymer allows it to significantly improve enhanced oil recovery in these underground carbonate formations. The water-soluble polymer is particularly effective when it also includes 2-acrylamido-2-methylpropane sulfonic acid (ATBS) monomeric units and / or one of its salts.

[0046] Preferably, in the process according to the invention, the aqueous composition injected into the underground carbonate formation comprises between 50 and 50,000 ppm of water-soluble polymer, preferably between 100 and 30,000 ppm, more preferably between 500 and 10,000 ppm relative to the weight of the aqueous composition.

[0047] The implementation of the water-soluble polymer 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.

[0048] When the water-soluble polymer 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 composition.

[0049] Besides the water-soluble polymer, the aqueous composition may also include one or more chemical compounds useful for enhanced oil recovery and well known to the person skilled in the art.

[0050] Among these chemical compounds, we can mention the use of alkali agents, 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.

[0051] 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.

[0052] Surfactants can be cationic and chosen from quaternary ammonium salts, phosphonium salts, iodonium salts, and iodonium salts. They can be nonionic and chosen from glycerol esters, glycol esters, polyoxyethylene glycol esters, sorbitan esters and their derivatives, ethoxylated alcohols, alkoxylated alcohols, fatty alcohol ethers, and polyoxyethylene glycol.

[0053] When the aqueous composition includes at least one surfactant, said composition includes between 0.01 and 8% by weight of surfactants.

[0054] A skilled professional will know how to choose the appropriate surfactants, alkaline agents and bases depending on the nature of the underground formation to be treated.

[0055] The water or brine used to prepare the aqueous composition may be production water. "Production water" refers to any salt or non-salt 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.

[0056] Advantageously, the aqueous composition 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).

[0057] Preferably, the process according to the invention generally comprises the following steps:

[0058] - prepare an aqueous composition comprising at least one water-soluble polymer according to the invention,

[0059] - inject the aqueous composition into an underground carbonate formation,

[0060] - to sweep away the underground carbonate formation using the injected aqueous composition,

[0061] - to recover an aqueous and oily and / or gaseous mixture.

[0062] Preferably, the process can be an enhanced oil recovery process of the Surfactant Polymer (SP), Alkaline Polymer (AP), or Alkaline Surfactant Polymer (ASP) type, comprising the injection of surfactants and / or alkali agents. Indeed, the use of surfactants and / or alkali agents improves oil recovery, particularly in underground carbonate formations. These compounds can be added to the aqueous composition of the invention or injected separately, before or after.

[0063] The invention also relates to an aqueous composition, or injection fluid, for use in an enhanced oil recovery process in an underground carbonate formation, comprising a water-soluble polymer according to the invention. The injection fluid (or aqueous composition) comprises at least one water-soluble polymer according to the invention, and at least water or brine. Optionally, it comprises other components such as, for example, an alkali as described above, one or more surfactants as described above, co-solvents, or other water-soluble polymers.

[0064] 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.

[0065] EXAMPLES

[0066] Example 1: Obtaining polymers

[0067] Polymers are prepared by gel polymerization. Powders are obtained for each of the polymers whose monomeric composition is recorded in Table 1.

[0068] Table 1 - Monomeric composition of polymers in molar percentage.

[0069] ATBS: 2-acrylamido 2-methylpropane sulfonic acid

[0070] NVP: N-Vinyl Pyrrolidone

[0071] AM: Acrylamide

[0072] DADMAC: Dimethyldialylammonium chloride

[0073] APTAC: Acrylamidopropyl trimethylammonium chloride

[0074] Example 2: Evaluation of polymer properties

[0075] Polymers are subjected to various tests to evaluate their ability to be well injected into the underground formation (Filter Ratio), to provide viscosity to the injection fluid, including under conditions of high salinity and high temperature (Brookfield Viscosity), and finally to be effective in a carbonate field (adsoption).

[0076] The term filtration ratio (or filter ratio) refers to a test that determines the performance of a polymer solution under conditions closely resembling the permeability of the reservoir. It involves measuring the time it takes for given volumes / concentrations of the solution to pass through a filter. The filter ratio typically compares the filterability of the polymer solution for two consecutive equivalent volumes, indicating the solution's tendency to clog the filter. Lower filter ratios indicate better performance.

[0077] The test used to determine the FR involves measuring the time it takes for given volumes of a solution containing 1000 ppm active polymer to flow through a filter. The solution is contained in a cell pressurized to two bars, and the filter is 47 mm in diameter with a pore size of 1.2 µm.

[0078] The times required to obtain 100 ml (t100ml), 200 ml (t200ml) and 300 ml (t300ml) of filtrate are therefore measured, and a FR is then defined, expressed as:

[0079] FR = (t300ml - t200ml) / (t200ml - 1100ml)

[0080] Times are measured to the nearest 0.1 seconds.

[0081] The FR thus represents the ability of the polymer solution to clog the filter for two consecutive equivalent volumes.

[0082] Viscosity measurement at 25°C is performed with a Brookfield viscometer equipped with a UL module at 6 revolutions per minute (7.34 s 1) at 25°C on a 5g / litre polymer solution, the polymer being dissolved in water. The viscosity measurement is carried out under anaerobic conditions in a glove box.

[0083] The brine viscosity measurement is carried out under the same conditions, except that the measurement temperature is 70°C and the polymer is processed in a brine containing 10 g / L of NaCl and 1 g / L of CaCl2. The viscosity measurement is performed under anaerobic conditions in a glove box.

[0084] Viscosity results are expressed in centipoise (cps). The higher the viscosity, the better the oil sweeping performance in the subsurface formation.

[0085] Thermal stability corresponds to the percentage of viscosity retained at 25°C after 6 months of storage of a 4.5 g / L polymer solution in a brine containing 75.984 g NaCl per liter, 22.42 g CaCl2·2H2O per liter, and 13.475 g MgC6H2O per liter, at a temperature of 100°C or 120°C. Viscosity measurements are performed under the same conditions as the viscosity measurements at 25°C described previously. The higher the percentage, the better the sweeping performance under these temperature conditions.

[0086] Adsorption in carbonate media is measured according to the method described below. The test consists of injecting a polymer solution at 1000 ppm by mass into a core of carbonate rock and determining the amount of polymer adsorbed onto the surface of this carbonate rock by subtracting it from the proportion of polymer that has passed through the core. More specifically, two injections of the polymer solution to be evaluated are made into a core of the carbonate rock in question.

[0087] The carbonate rock on which the tests are carried out is a carbonate rock from Estaillades, a well-known reference rock, whose characteristics are as follows: permeability: 120 millidarcies, porosity (pore volume): 30.7%, and the following composition:

[0088] - Quartz: 0.3% by mass;

[0089] - Calcite: 99.4% by mass;

[0090] - Apatites: 0.2% by mass;

[0091] - Baryte: 0.1% by mass.

[0092] First, the carbonate rock core, 2.5 cm in diameter and 15 cm long, is saturated under vacuum with the same brine as previously described, previously filtered and deoxygenated, then placed in a porous injection cell, itself connected to the test device.

[0093] Next, an initial injection of the polymer solution is carried out at a constant flow rate of 5 ml / h. The volume of this injection is set at 5 times the porous volume of the rock in question.

[0094] Then, a volume of brine corresponding to 25 times the porous volume of the rock in question is injected at a constant flow rate of 5 ml / h to displace the non-adsorbed polymer out of the rock.

[0095] Finally, a second injection of the polymer solution is carried out at a constant flow rate of 5 ml / h. The volume of this front is fixed at 5 times the porous volume of the rock in question.

[0096] During each injection of the polymer solution, the effluent from the core sample is collected in 4 ml fractions. For each collected effluent fraction, the concentration of the polymer solution is determined to establish the breakthrough curves for each injection. The difference in breakthrough volume at 50% of the injected concentration between the two polymer injections is then used to determine the amount of polymer adsorbed in micrograms of polymer per gram of rock. Adsorption is expressed in pg / g, and a low value is advantageous as it indicates a lower propensity of the polymer to be adsorbed and thus immobilized by the carbonate rock. Lower adsorption therefore allows more polymer to participate in the formation sweep and consequently increases oil recovery performance in a carbonate field.

[0097] The results are recorded in Table 2.

[0098] Table 2 - Polymer performance These results show that the water-soluble polymers of invention 6, 7 and 8 are the only ones to offer at the same time good performance in carbonate rocks, and good sweeping properties including in high salinity conditions, while exhibiting very good stability at 100°C and 120°C.

[0099] Indeed, carbonate fields are often high-temperature fields. Therefore, the temperature resistance of the polymers was taken into account for these specific cases. Although polymers 1 and 2 (comparative) (and 9 and 10, to a lesser extent) offer excellent temperature resistance, the results show that they are not very effective in carbonate fields.

[0100] The examples according to the invention offer significantly better performance in carbonate fields without unduly compromising temperature resistance. The performance of polymer 5 in a carbonate field is very good, but its temperature stability is lower than that of polymers 6, 7, and 8. It would therefore be suitable for use in a carbonate field at a lower temperature.

Claims

A method for enhanced oil recovery in a subterranean carbonate formation comprising injecting an aqueous composition comprising at least one water-soluble polymer, wherein the water-soluble polymer is an amphoteric polymer containing acrylamidopropyltrimethylammonium chloride and at least one anionic monomer, wherein the number of acrylamidopropyltrimethylammonium chloride units in the water-soluble polymer is between 20 and 99.9 mol % relative to the total number of moles of monomer units of the water-soluble polymer. The method according to claim 1, characterised in that the number of anionic monomer units in the water-soluble polymer is between 0.1 and 70 mol % relative to the total number of moles of monomer units of the water-soluble polymer. The method according to claim 1, characterised in that the water-soluble polymer is an amphoteric polymer of acrylamidopropyltrimethylammonium chloride and of at least one anionic monomer, wherein the anionic monomer is selected from acrylic acid, methacrylic acid, itaconic acid, crotonic acid, maleic acid, fumaric acid, sulphonated monomers, phosphonated monomers, 2-acrylamido-2-methylpropanesulphonic acid, vinylsulphonic acid, vinylphosphonic acid, allylsulphonic acid, allylphosphonic acid, wherein the anionic monomer is not in salt form or partially or totally in salt form. The method according to claim 1, characterised in that the water-soluble polymer further contains at least one non-ionic monomer or al least one zwitterionic monomer, or at least one non-ionic monomer and one zwitterionic monomer. The method according to claim 1, characterised in that the water-soluble polymer is an amphoteric polymer of acrylamidopropyltrimethylammonium chloride and 2-acrylamido-2-methylpropanesulphonic acid, or is an amphoteric polymer of acrylamidopropyltrimethylammonium chloride and a salt of 2-acrylamido-2-methylpropanesulphonic acid.  The method according to claim 1, characterised in that the water-soluble polymer does not contain cationic monomers other than acrylamidopropyltrimethylammonium chloride.The method according to claim 1, characterised in that the weight-average molecular weight of the water-soluble polymer is between 1 and 15 million g / mol. The method according to claim 1, characterised in that the aqueous composition comprises between 50 and 50 000 ppm of water-soluble polymer.The method according to claim 1, characterised in that the subterranean carbonate formation contains at least 20 % carbonates. The method according to claim 1, characterised in that the method comprises the following steps:-               preparing an aqueous composition comprising at least one water-soluble polymer,-               injecting the aqueous composition into a subterranean carbonate formation,-               sweeping the subterranean carbonate formation by means of the injected aqueous composition,-               recovering a mixture of water and oil, or a mixture of water and gas or a mixture of water and oil and gas. The method according to claim 1, characterised in that the method is a Surfactant Polymer, Alkaline Polymer, or Alkaline Surfactant Polymer method of enhanced oil recovery comprising injecting surfactants or injecting alkaline agents or injecting surfactants and alkaline agents. An injection fluid for use in a method for enhanced oil recovery in a subterranean carbonate formation comprising the water-soluble polymer according to any one of claims 1 to 8.

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