Thickening agent for improving recovery efficiency and preparation method thereof

Abstract

The invention relates to the technical field of oilfield exploitation, and provides a thickening agent for improving recovery efficiency and a preparation method thereof, the thickening agent is obtained by aqueous solution free radical polymerization reaction of a monomer A, a monomer B, a monomer C and a monomer D according to the mass ratio of 0.002-0.006: 1: 0.02-0.04: 0.01-0.05, the monomer A is trifluoroethyl acrylate, the monomer B is polyethylene glycol monoallyl ether, the monomer C is triethylene glycol monoallyl ether, the monomer D is triethylene glycol monoallyl ether, the monomer C is triethylene glycol monoallyl ether, and the monomer D is triethylene glycol monoallyl ether. The monomer C is prepared by taking 2-picolinic acid as a raw material and carrying out an alkylation reaction with fatty aldehyde containing carbon-carbon double bonds, and the monomer D is prepared by taking isocyanate ethyl acrylate as a raw material and carrying out a reaction with N, N-dimethylethylenediamine to generate an intermediate and then utilizing the intermediate and 1, 3-propane sultone to prepare the target product. The prepared thickening agent solves the problems that the existing synthesized thickening agent is poor in comprehensive performance and cannot meet the requirement of an oilfield exploitation process for high-quality fracturing fluid.

Description

Technical Field

[0001] The invention relates to the technical field of oilfield exploitation, and in particular to a thickener for improving oil recovery and a preparation method thereof. Background Art

[0002] As a powerful booster for industrial development, the demand for oil is increasing day by day. However, with the continuous development of the oil and gas industry at home and abroad, the proportion of high-porosity, high-permeability high-quality oil and gas resources is getting lower and lower, and they are replaced by some oil and gas reservoirs with relatively poor reservoir conditions and high requirements for exploration and development technology. As a non-renewable energy source, oil is facing severe challenges in exploration, development and production increase under the background of declining global reserves, the severe situation of global oil energy shortage and the rapid increase in my country's oil demand.

[0003] Fracturing is a process technology developed in the 1940s to transform the seepage characteristics of oil layers. It is an important measure to increase oil well production and water well injection. The performance of fracturing fluid in the fracturing process directly affects the fracturing effect. Fracturing fluid plays two main roles in the entire fracturing process: first, it provides a sufficiently high viscosity, forms cracks by hydraulic splitting, and enables the cracks to continue to extend, and fills fracturing proppants along the cracks; second, after fracturing, the fracturing fluid quickly breaks down and decomposes under chemical action, and the viscosity drops sharply, so that most of the fracturing fluid can be quickly returned to the ground to prevent contamination of oil and gas reservoirs. Faced with the severe challenges in the current oil and gas field development, the importance of fracturing fluid has become increasingly prominent, and it is urgent to develop a new fracturing fluid system that is suitable for the current complex reservoir transformation situation. Thickener is the main agent of fracturing fluid and is the decisive factor affecting the performance of fracturing fluid. Therefore, many people in the industry regard thickener as the main entry point for the development of excellent performance fracturing fluid, and have done a lot of research work and developed a large number of new thickeners.

[0004] For example, patent application number CN202410986690.6 discloses a thickener for improving oil recovery and a preparation method thereof, wherein the thickener for improving oil recovery is composed of the following raw materials in parts by weight: 40 to 70 parts of cage-type octa(3-ammonium chloride propyl)silsesquioxane / 2-acrylamido-2-methylpropane sulfonic acid / acrylamide polymer, 2 to 6 parts of clay stabilizer, 8 to 12 parts of emulsifier, 1 to 3 parts of buffer and 1 to 2 parts of preservative. The cage-type octapoly (3-ammonium chloride propyl) silsesquioxane / 2-acrylamido-2-methylpropane sulfonic acid / acrylamide polymer is prepared from the following raw materials in parts by weight: 2.5-25 parts of cage-type octapoly (3-ammonium chloride propyl) silsesquioxane, 10-100 parts of 2-acrylamido-2-methylpropane sulfonic acid, 1000 parts of acrylamide and 0.5-1.9 parts of azobisisobutylamidine hydrochloride; the cage-type octapoly (3-ammonium chloride propyl) silsesquioxane is prepared from the following raw materials in parts by weight: 0.94-1.88 parts of 3-aminopropyltriethoxysilane and 1.18-1.77 parts of concentrated hydrochloric acid. The thickener for improving oil recovery prepared by the present invention has good thickening and viscosity-enhancing and oil recovery-improving effects.

[0005] The thickening property, shear stability, thermal stability, and salt resistance of thickeners are the main indicators for evaluating their quality, and are also important factors affecting the performance of fracturing fluids and fracturing effects. However, existing synthetic thickeners often have excellent performance in only one aspect, while other performances are relatively average or even poor. Summary of the invention

[0006] Therefore, in view of the above, one of the purposes of the present invention is to provide a thickener for improving oil recovery, so as to solve the problem that the existing synthetic thickeners have poor comprehensive performance and cannot meet the demand for high-quality fracturing fluids in the oil field exploitation process.

[0007] Another object of the present invention is to provide a method for preparing the thickener.

[0008] To achieve the above object, the present invention adopts the following technical solutions: A thickener for improving oil recovery, the thickener is obtained by free radical polymerization of monomers A, B, C and D in a mass ratio of 0.002-0.006:1:0.02-0.04:0.01-0.05 in aqueous solution, wherein monomer A is trifluoroethyl acrylate, monomer B is polyethylene glycol monoallyl ether (molecular weight of 300-500), the structure of monomer C is shown in formula (I), and the structure of monomer D is shown in formula (II);

[0009] In formula (I), R1 is selected from methyl or hydrogen, and R2 is selected from methyl or n-propyl. When R1 is methyl, R2 is also methyl; when R1 is hydrogen, R2 is n-propyl.

[0010] A further improvement is that the preparation of the monomer C includes the following steps: adding 2-pyridinecarboxylic acid into a reactor filled with solvent I, stirring until completely dissolved, adding a Lewis acid catalyst and p-hydroxyanisole, heating to 60-90° C., and simultaneously starting to dropwise add reactant A, and continuing the reaction for 3-6 hours after the dropwise addition is completed; after the reaction is completed, cooling to room temperature, filtering, distilling under reduced pressure, and purifying to obtain monomer C.

[0011] A further improvement is that the Lewis acid catalyst is aluminum trichloride or tin tetrachloride.

[0012] A further improvement is that the reactant A is isopentenal or 2-hexenal.

[0013] A further improvement is that the molar ratio of the reactant A to 2-pyridinecarboxylic acid is 1.1-1.5:1.

[0014] A further improvement is that, based on 100% of the total mass of reactant A and 2-picolinic acid, the added amount of the Lewis acid catalyst is 0.08-0.24%, and the added amount of the p-hydroxyanisole is 0.3-1.2%.

[0015] The further improvement is that the preparation of the monomer D comprises the following steps: (1) adding ethyl isocyanate acrylate, an inhibitor and a catalyst into a reactor containing solvent II, heating to 50-80° C., then dropping N,N-dimethylethylenediamine, continuing the reaction for 4-12 hours after the dropping is completed, cooling and filtering, distilling under reduced pressure, and purifying to obtain an intermediate; (2) using ethanol as the reaction medium, uniformly mixing the intermediate prepared in step (1) and 1,3-propane sultone under an inert gas atmosphere, adding an appropriate amount of inhibitor and catalyst, reacting at 45-65° C. for 3-6 hours, cooling and filtering, distilling under reduced pressure, and purifying after the reaction is completed to obtain monomer D.

[0016] A further improvement is that, based on 100% of the total mass of ethyl isocyanate acrylate and N,N-dimethylethylenediamine, the amounts of the inhibitor and the catalyst added in step (1) are 0.8% to 2% and 0.2% to 0.4%, respectively.

[0017] A further improvement is that, based on 100% of the total mass of the intermediate and 1,3-propane sultone, the amounts of the inhibitor and the catalyst added in step (2) are 1% to 3% and 0.3% to 0.9%, respectively.

[0018] A further improvement is that the polymerization inhibitor is any one of a phenol polymerization inhibitor or a quinone polymerization inhibitor or a mixture of the two in any ratio, and the catalyst is any one of an organic tin catalyst or a titanate catalyst or a mixture of the two in any ratio.

[0019] The present invention also provides a method for preparing the thickener for enhancing oil recovery, comprising the following steps: S1. Weigh each raw material according to the predetermined weight and set aside; S2. Dissolve monomer B and monomer C completely with deionized water, then add monomer A and monomer D, the total monomer concentration is 20% of the system mass, and adjust the pH value of the solution to neutral, then add initiator, and heat the reaction under inert gas atmosphere for 5 to 10 hours, the reaction temperature is 55 to 75°C; S3. After the reaction is completed, the mixture is cooled to room temperature, and the obtained product is repeatedly washed and precipitated in anhydrous ethanol to remove the unreacted monomers, and the purified product is collected and dried to obtain the desired thickener.

[0020] A further improvement is that the added amount of the initiator is 0.1% to 0.3% of the total weight of the monomers.

[0021] A further improvement is that the initiator is any one of dibenzoyl peroxide, dilauroyl peroxide, azobisisobutyronitrile and azobisisoheptylnitrile, or a mixture of two or more of them in any ratio.

[0022] A further improvement is that the solvent I is any one of ethanol, tetrahydrofuran and toluene, and the solvent II is any one of tetrahydrofuran, toluene and N,N-dimethylformamide.

[0023] By adopting the above technical solution, the beneficial effects of the present invention are: 1. The poor salt resistance of polymers is a major pain point in the application of polymers in oil fields. The present invention introduces two new monomers (monomers C and D), which are used to carry out polymerization reactions with monomers A and B to design a thickener with a specific molecular structure. Through the interaction between molecules in water, the thickener molecules are entangled with each other to form a dense three-dimensional spatial network structure, which increases the hydrodynamic volume of the thickener in water, thereby increasing the apparent viscosity of the aqueous solution. The dense cross-linked structure makes the thickener molecular chain less likely to be destroyed under high-speed shearing by external force, thereby showing more excellent shear stability.

[0024] 2. The thickener synthesized by the present invention introduces a nitrogen-containing six-membered heterocyclic structure, a urea group and a CF group into the main chain of the molecule to improve the rigidity of the main chain of the molecule. The main chain of the molecule is not easy to break at high temperature, and the group does not undergo structural changes or group shedding at high temperature, so that the apparent viscosity decrease rate of the thickener solution at high temperature is low.

[0025] 3. The surface of the quartz sand proppant carries a negative charge, while the quaternary ammonium group in the thickener molecule makes the molecular chain carry a partial positive charge. In the solution, due to the mutual attraction between positive and negative charges, the thickener molecular chain is adsorbed on the surface of the quartz sand, thereby improving its sand suspension performance. Other active groups on the thickener molecular chain play a synergistic role, improving the adsorption of sand particles through hydrogen bonds, van der Waals forces and other forces, thereby controlling the sinking speed of quartz sand and meeting the requirements of thickener suspension.

[0026] 4. In a high-salt environment, the molecular chain of a conventional thickener rapidly changes from an extended state to a curled state, and the fluid mechanics volume is greatly reduced, which significantly reduces the apparent viscosity of the solution, exposing the problem of poor salt resistance. The various active groups in the molecular structure of the thickener synthesized in the present invention make the molecular chain extend more fully, and inhibit the molecular chain from changing from an extended state to a curled state, thus having good salt resistance.

[0027] In summary, the thickener prepared by the present invention has excellent comprehensive performance, good viscosity, shear stability, thermal stability, and salt resistance, and can be used to produce high-quality fracturing fluid, thereby improving the recovery rate of oil and gas. In addition, the research process also found that the thickener provided by the present invention can be used in fracturing fluid to effectively reduce the oil-water interfacial tension, increase the fracturing fluid's ability to strip and migrate oil, and thus improve the recovery rate of oil and gas. DETAILED DESCRIPTION

[0028] The present invention is further described below by way of examples, but the present invention is not limited to the scope of the examples. The reagents and raw materials used in the following examples are commercially available, and the experimental methods without specific conditions are usually carried out under conventional conditions or under conditions recommended by the manufacturers.

[0029] Example 1 A thickener for improving oil recovery, the thickener is obtained by free radical polymerization of monomers A, B, C and D in a mass ratio of 0.002:1:0.02:0.01 in aqueous solution, wherein monomer A is trifluoroethyl acrylate, and monomer B is polyethylene glycol monoallyl ether (APEG-300); the preparation of monomer C comprises the following steps: adding 2-pyridine carboxylic acid into a reactor containing ethanol, stirring and dissolving completely, adding aluminum chloride and p-hydroxyanisole, heating to 60°C, and simultaneously starting to drop isopentenal, and continuing the reaction for 6 hours after the drop addition is completed; after the reaction is completed, cooling to room temperature, filtering, vacuum distillation and purification are performed to obtain monomer C. The molar ratio of isopentenal to 2-pyridine carboxylic acid is 1.1:1, and based on 100% of the total mass of isopentenal and 2-pyridine carboxylic acid, the addition amount of aluminum chloride is 0.08%, and the addition amount of p-hydroxyanisole is 0.3%.

[0030] The preparation of the monomer D comprises the following steps: (1) adding ethyl isocyanate acrylate, 1,4-benzoquinone and dibutyltin dilaurate into a reactor containing tetrahydrofuran, heating to 50° C., then dropping N,N-dimethylethylenediamine, continuing the reaction for 12 hours after the dropping is completed, cooling and filtering, distilling under reduced pressure, and purifying to obtain an intermediate, wherein the addition amounts of 1,4-benzoquinone and dibutyltin dilaurate are 0.8% and 0.2% respectively, based on the total mass of ethyl isocyanate acrylate and N,N-dimethylethylenediamine being 100%; (2) using ethanol as a reaction medium, uniformly mixing the intermediate prepared in step (1) and 1,3-propane sultone under an inert gas atmosphere, and simultaneously adding appropriate amounts of 1,4-benzoquinone and dibutyltin dilaurate, reacting at 45° C. for 6 hours, cooling and filtering, distilling under reduced pressure, and purifying after the reaction is completed to obtain monomer D. Based on 100% of the total mass of the intermediate and 1,3-propane sultone, the added amounts of 1,4-benzoquinone and dibutyltin dilaurate are 1% and 0.3%, respectively.

[0031] A method for preparing a thickener for enhancing oil recovery comprises the following steps: S1. Weigh each raw material according to the predetermined weight and set aside; S2, dissolving monomer B and monomer C completely with deionized water, then adding monomer A and monomer D, adjusting the pH value of the solution to neutral, and then adding azobisisobutyronitrile in an amount of 0.1% of the total mass of the monomers, heating to 55° C. under an inert gas atmosphere, and heating the reaction for 10 hours; S3. After the reaction is completed, the mixture is cooled to room temperature, and the obtained product is repeatedly washed and precipitated in anhydrous ethanol to remove the unreacted monomers, and the purified product is collected and dried to obtain the desired thickener.

[0032] Example 2 A thickener for improving oil recovery, the thickener is obtained by free radical polymerization of monomers A, B, C and D in a mass ratio of 0.004:1:0.03:0.03, wherein monomer A is trifluoroethyl acrylate, and monomer B is polyethylene glycol monoallyl ether (APEG-400); the preparation of monomer C comprises the following steps: adding 2-pyridine carboxylic acid into a reactor filled with tetrahydrofuran, stirring and dissolving completely, adding aluminum chloride and p-hydroxyanisole, heating to 75°C, and simultaneously starting to drop 2-hexenal, and continuing the reaction for 5 hours after the drop addition is completed; after the reaction is completed, cooling to room temperature, filtering, vacuum distillation and purification are performed to obtain monomer C. The molar ratio of 2-hexenal to 2-pyridine carboxylic acid is 1.3:1, and based on 100% of the total mass of 2-hexenal and 2-pyridine carboxylic acid, the addition amount of aluminum chloride is 0.16%, and the addition amount of p-hydroxyanisole is 0.8%.

[0033] The preparation of the monomer D comprises the following steps: (1) adding ethyl isocyanate acrylate, 1,4-benzoquinone and dibutyltin dilaurate into a reactor containing tetrahydrofuran, heating to 65° C., then dropping N,N-dimethylethylenediamine, continuing the reaction for 8 hours after the dropping is completed, cooling and filtering, distilling under reduced pressure, and purifying to obtain an intermediate, wherein the addition amounts of 1,4-benzoquinone and dibutyltin dilaurate are 1.5% and 0.3%, respectively, based on the total mass of ethyl isocyanate acrylate and N,N-dimethylethylenediamine being 100%; (2) using ethanol as a reaction medium, uniformly mixing the intermediate prepared in step (1) and 1,3-propane sultone under an inert gas atmosphere, adding appropriate amounts of 1,4-benzoquinone and dibutyltin dilaurate, reacting at 55° C. for 4 hours, cooling and filtering, distilling under reduced pressure, and purifying after the reaction is completed to obtain monomer D. Based on 100% of the total mass of the intermediate and 1,3-propane sultone, the added amounts of 1,4-benzoquinone and dibutyltin dilaurate are 2% and 0.6%, respectively.

[0034] A method for preparing a thickener for enhancing oil recovery comprises the following steps: S1. Weigh each raw material according to the predetermined weight and set aside; S2, dissolving monomer B and monomer C completely with deionized water, then adding monomer A and monomer D, adjusting the pH value of the solution to neutral, and then adding azobisisobutyronitrile in an amount of 0.2% of the total mass of the monomers, heating to 65°C under an inert gas atmosphere, and heating to react for 8 hours; S3. After the reaction is completed, the mixture is cooled to room temperature, and the obtained product is repeatedly washed and precipitated in anhydrous ethanol to remove the unreacted monomers, and the purified product is collected and dried to obtain the desired thickener.

[0035] Example 3 A thickener for improving oil recovery, the thickener is obtained by free radical polymerization of monomers A, B, C and D in a mass ratio of 0.006:1:0.04:0.05 in aqueous solution, wherein monomer A is trifluoroethyl acrylate, and monomer B is polyethylene glycol monoallyl ether (APEG-500); the preparation of monomer C comprises the following steps: adding 2-pyridine carboxylic acid into a reactor containing toluene, stirring and dissolving completely, adding tin tetrachloride and p-hydroxyanisole, heating to 90°C, and simultaneously starting to drop isopentenal, and continuing to react for 3 hours after the drop addition is completed; after the reaction is completed, cooling to room temperature, filtering, vacuum distillation and purification are performed to obtain monomer C. The molar ratio of isopentenal to 2-pyridine carboxylic acid is 1.5:1, and based on 100% of the total mass of isopentenal and 2-pyridine carboxylic acid, the addition amount of tin tetrachloride is 0.24%, and the addition amount of p-hydroxyanisole is 1.2%.

[0036] The preparation of the monomer D comprises the following steps: (1) adding ethyl isocyanate acrylate, 2-tert-butylhydroquinone and tetrabutyl titanate into a reactor containing N,N-dimethylformamide, heating to 80°C, then dropping N,N-dimethylethylenediamine, continuing the reaction for 4 hours after the dropping is completed, cooling and filtering, distilling under reduced pressure, and purifying to obtain an intermediate, wherein the addition amounts of 2-tert-butylhydroquinone and tetrabutyl titanate are 2% and 0.4% respectively, based on the total mass of ethyl isocyanate acrylate and N,N-dimethylethylenediamine being 100%; (2) using ethanol as a reaction medium, uniformly mixing the intermediate prepared in step (1) and 1,3-propane sultone under an inert gas atmosphere, adding appropriate amounts of 2-tert-butylhydroquinone and tetrabutyl titanate, reacting at 65°C for 3 hours, cooling and filtering, distilling under reduced pressure, and purifying after the reaction is completed to obtain monomer D. Based on 100% of the total mass of the intermediate and 1,3-propane sultone, the added amounts of 2-tert-butylhydroquinone and tetrabutyl titanate are 3% and 0.9%, respectively.

[0037] A method for preparing a thickener for enhancing oil recovery comprises the following steps: S1. Weigh each raw material according to the predetermined weight and set aside; S2. Dissolve monomer B and monomer C completely with deionized water, then add monomer A and monomer D, adjust the pH value of the solution to neutral, add dibenzoyl peroxide in an amount of 0.3% of the total mass of the monomers, raise the temperature to 75°C under an inert gas atmosphere, and heat the reaction for 5 hours; S3. After the reaction is completed, the mixture is cooled to room temperature, and the obtained product is repeatedly washed and precipitated in anhydrous ethanol to remove the unreacted monomers, and the purified product is collected and dried to obtain the desired thickener.

[0038] Comparative Example 1 A thickener for enhancing oil recovery, which is different from Example 1 only in that monomer A is replaced by the other three monomers in proportion.

[0039] Comparative Example 2 A thickener for enhancing oil recovery, which is different from Example 1 only in that monomer B is replaced by the other three monomers in proportion.

[0040] Comparative Example 3 A thickener for enhancing oil recovery, which is different from Example 1 only in that monomer C is replaced by the other three monomers in proportion.

[0041] Comparative Example 4 A thickener for enhancing oil recovery, which is different from Example 1 only in that monomer D is replaced by the other three monomers in proportion.

[0042] Comparative Example 5 A thickener for enhancing oil recovery, which differs from Example 1 only in that monomer C is replaced with an equal mass of 2-vinylpyridine.

[0043] Comparative Example 6 A thickener for enhancing oil recovery, which differs from Example 1 only in that monomer C is replaced with an equal mass of 3-hydroxy-3-phenyl-propylene.

[0044] Comparative Example 7 A thickener for enhancing oil recovery, which is different from Example 1 only in that monomer D is replaced with an equal mass of methacryloyloxyethyltrimethylammonium chloride.

[0045] Comparative Example 8 A thickener for enhancing oil recovery, which differs from Example 1 only in that monomer D is replaced with an equal mass of an amphoteric sulfonate-type surfactant monomer (AMSC18DMAB). The preparation method of the amphoteric sulfonate-type surfactant monomer is described in the literature (Zhang Xuan. Synthesis and Solution Properties of Microblock Amphiphilic Polymers [D]. Chengdu: Southwest Petroleum University, 2015.).

[0046] The thickeners prepared in Examples 1 to 3 and Comparative Examples 1 to 8 were prepared into aqueous solutions with a concentration of 0.5%, and the apparent viscosity (170s -1 ). Temperature resistance: Starting from 20℃, the heating rate is 5℃ / min, and the apparent viscosity of 0.5% aqueous solution at 20℃ and 100℃ is recorded. Shear stability: Pour the aqueous solution into the closed system sample cup of RS600 rheometer for preheating, set the parameter starting temperature to 30℃, the heating rate to 3℃ / min±0.2℃ / min, and the shear rate to 170s -1 . When the temperature rises to 90°C, keep the shear rate and temperature constant, the shear time is 60 minutes, and read the shear viscosity value. Salt resistance: Take a certain amount of 0.5% aqueous solution in a container, then add NaCl to the container to make the salt concentration reach 1%, and test its apparent viscosity at room temperature. Suspended sand: First, prepare the fracturing fluid according to the following mass fractions: 0.6% thickener, 0.5% organic zirconium cross-linking agent, 1% potassium chloride, 0.1% sodium thiosulfate, and the balance is water. Then add the fracturing fluid to the stoppered measuring cylinder and mark the initial position; then gently place a grain of quartz sand on the surface of the fracturing fluid and start timing; after a period of time, observe its sedimentation, record the height of the sedimentation, and then calculate the sedimentation rate through the sedimentation time. According to the above method, the sedimentation at 25°C, 60°C, and 90°C was measured respectively. Specific test results are shown in Table 1-2.

[0047] Table 1

[0048] Table 2

[0049] It can be seen from Table 1 and Table 2 that the monomers have a synergistic effect and cooperate with each other to improve the comprehensive performance of the thickener.

[0050] Although the present invention has been specifically shown and described in conjunction with the preferred embodiments, it should be understood by those skilled in the art that various changes may be made to the present invention in form and details without departing from the spirit and scope of the present invention as defined by the appended claims, all of which are within the scope of protection of the present invention.

Claims

1. A thickener for enhancing oil recovery, characterized in that: The thickener is obtained by free radical polymerization of monomer A, monomer B, monomer C and monomer D in a mass ratio of 0.002-0.006:1:0.02-0.04:0.01-0.05 in aqueous solution, wherein monomer A is trifluoroethyl acrylate, monomer B is polyethylene glycol monoallyl ether, the structure of monomer C is shown in formula (I), and the structure of monomer D is shown in formula (II); In formula (I), R1 is selected from methyl or hydrogen, and R2 is selected from methyl or n-propyl.

2. The thickener for enhancing oil recovery according to claim 1, characterized in that: The preparation of the monomer C comprises the following steps: adding 2-pyridinecarboxylic acid into a reactor filled with solvent I, stirring to completely dissolve, adding Lewis acid catalyst and p-hydroxyanisole, heating to 60-90° C., and simultaneously starting to dropwise add reactant A, and continuing the reaction for 3-6 hours after the dropwise addition is completed; after the reaction is completed, cooling to room temperature, filtering, distilling under reduced pressure, and purifying to obtain monomer C.

3. The thickener for enhancing oil recovery according to claim 2, characterized in that: The reactant A is isopentenal or 2-hexenal.

4. The thickener for enhancing oil recovery according to claim 2, characterized in that: The molar ratio of the reactant A to 2-pyridinecarboxylic acid is 1.1-1.5:

1.

5. The thickener for enhancing oil recovery according to claim 2, characterized in that: Based on 100% of the total mass of reactant A and 2-picolinic acid, the added amount of the Lewis acid catalyst is 0.08-0.24%, and the added amount of the p-hydroxyanisole is 0.3-1.2%.

6. The thickener for enhancing oil recovery according to claim 1, characterized in that: The preparation of the monomer D comprises the following steps: (1) adding ethyl isocyanate acrylate, a polymerization inhibitor and a catalyst into a reactor containing a solvent II, heating to 50-80° C., then dropping N,N-dimethylethylenediamine, continuing the reaction for 4-12 hours after the dropping is completed, cooling and filtering, distilling under reduced pressure, and purifying to obtain an intermediate; (2) using ethanol as a reaction medium, uniformly mixing the intermediate prepared in step (1) and 1,3-propane sultone under an inert gas atmosphere, adding an appropriate amount of a polymerization inhibitor and a catalyst, reacting at 45-65° C. for 3-6 hours, cooling and filtering, distilling under reduced pressure, and purifying after the reaction is completed to obtain the monomer D.

7. The thickener for enhancing oil recovery according to claim 6, characterized in that: Based on 100% of the total mass of ethyl isocyanate acrylate and N,N-dimethylethylenediamine, the amounts of the inhibitor and the catalyst added in step (1) are 0.8% to 2% and 0.2% to 0.4%, respectively.

8. The thickener for enhancing oil recovery according to claim 6, characterized in that: Based on 100% of the total mass of the intermediate and 1,3-propane sultone, the amounts of the inhibitor and the catalyst added in step (2) are 1% to 3% and 0.3% to 0.9%, respectively.

9. A method for preparing a thickener for enhanced oil recovery according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1. Weigh each raw material according to the predetermined weight and set aside; S2, dissolving monomer B and monomer C completely with deionized water, then adding monomer A and monomer D, adjusting the pH value of the solution to neutral, then adding an initiator, and heating the reaction under an inert gas atmosphere for 5 to 10 hours; S3. After the reaction is completed, the mixture is cooled to room temperature, and the obtained product is repeatedly washed and precipitated in anhydrous ethanol to remove the unreacted monomers, and the purified product is collected and dried to obtain the desired thickener.

10. The method for preparing a thickener for enhancing oil recovery according to claim 9, characterized in that: The added amount of the initiator is 0.1% to 0.3% of the total weight of the monomers.

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