Salt-resistant emulsified high-viscosity tetra-copolymer, preparation method, polymer oil displacement agent and application thereof

Abstract

This invention relates to the field of oilfield chemical reagents, and particularly to a salt-resistant emulsifying high-viscosity quaternary copolymer, its preparation method, a polymer flooding agent, and its applications. The salt-resistant emulsifying high-viscosity quaternary copolymer of this invention contains amide structural units, lactam ring structural units, pyridine sulfonate structural units, and dodecylstyrene hydrophobic associative structural units in its molecular chain. This polymer can both increase the viscosity of the aqueous phase and, relying on the salt-resistant and hydrophobic associative structures in its molecular chain, enhance the viscosity retention capacity and oil-water interface emulsification stability under high-salt environments, thus possessing multifunctional effects of salt resistance, viscosity enhancement, and emulsification. The salt-resistant emulsifying high-viscosity quaternary copolymer provided by this invention, when used in polymer flooding of high-salt, low-permeability, or heterogeneous reservoirs, can increase the viscosity of the injected fluid, improve the mobility ratio, expand the swept volume, and improve oil recovery.

Description

Technical Field

[0001] This invention relates to the field of oilfield chemical reagent technology, and in particular to a salt-resistant emulsifying high-viscosity quaternary copolymer, its preparation method, a polymer flooding agent, and its application. Background Technology

[0002] Polymer flooding is a commonly used enhanced oil recovery (EOR) technology in oilfield development. Its main function is to increase the viscosity of injected water, thereby improving the oil-water mobility ratio and reducing viscous fingering and water channeling during waterflooding. In heterogeneous reservoirs, conventional waterflooding often preferentially injects water into high-permeability layers, large pores, or dominant channels, leaving low-to-medium permeability areas and residual oil between wells difficult to fully utilize. After entering the formation, polymer solutions increase flow resistance in high-permeability channels, causing subsequent injected fluid to gradually divert to low-to-medium permeability areas, thus expanding the swept volume and improving the utilization of residual oil after waterflooding.

[0003] Currently, the most widely used polymers in oilfields are partially hydrolyzed polyacrylamide polymers. These polymers are widely available and relatively inexpensive, and they play a role in increasing viscosity and adjusting mobility ratios in conventional reservoirs. However, as oilfield development targets increasingly shift towards high-salinity, low-permeability, and highly heterogeneous reservoirs, the adaptability of conventional polyacrylamide is gradually being limited. In highly saline formation water, sodium, calcium, and magnesium ions compress the hydration layer surrounding the polymer molecular chains, causing the molecular chains to coil, resulting in a decrease in hydrodynamic volume and a significant drop in solution viscosity. Especially in formation water with high divalent cation content, the carboxylate groups in the polyacrylamide molecules may also interact with calcium and magnesium ions, causing molecular chain contraction, aggregation, or even precipitation, thereby affecting the polymer's stability and injection performance.

[0004] To address the insufficient salt resistance and stability of conventional polyacrylamide, existing technologies typically modify the polymer by introducing salt-resistant monomers, rigid monomers, or hydrophobic monomers. For example, introducing sulfonate structures can improve the hydration capacity of the molecular chain and enhance the polymer's viscosity retention under high-salt conditions; introducing cyclic monomers can improve the rigidity of the molecular chain and its temperature stability; introducing a small amount of hydrophobic monomers can form a certain physical cross-linking network in aqueous solution through hydrophobic association, thereby improving solution viscosity and shear resistance. While these modification methods can improve polymer properties to some extent, most systems still focus on improving a single property and cannot simultaneously meet the needs of salt resistance, viscosity enhancement, emulsification, and interfacial adsorption. In addition, for late-stage high-water-cut reservoirs, some residual oil exists in the form of oil films, oil droplets, or pore-retained oil, and simply increasing the viscosity of the injected fluid cannot completely solve the problem of utilizing the residual oil.

[0005] Patent CN118063688A discloses an oil displacement agent. This patent describes a process involving the reaction of monooctyl maleate and phosphorus trichloride to prepare a functional monomer with a molecular structure containing carbon-carbon double bonds and long side-chain groups. This functional monomer is then copolymerized with acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, a temperature- and salt-resistant monomer, a shear-resistant monomer, a complexing agent, a structure modifier, and a surfactant under the action of an initiator to prepare an oil displacement agent with thickening, temperature resistance, salt resistance, and shear resistance properties. However, the preparation process of this oil displacement agent is relatively cumbersome, and its structure is not clearly defined.

[0006] Therefore, developing a multifunctional polymer flooding agent that combines salt resistance, viscosity enhancement, and emulsification is of practical significance for improving the development effect of complex oil reservoirs. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention provides a salt-resistant emulsifying high-viscosity quaternary copolymer, its preparation method, a polymer flooding agent, and its applications. The salt-resistant emulsifying high-viscosity quaternary copolymer provided by this invention simultaneously possesses thickening, salt resistance, emulsification, and oil-water interface adsorption capabilities. When used in polymer flooding of high-salt, low-permeability, or heterogeneous reservoirs, it can increase the viscosity of the injected fluid, improve the mobility ratio, expand the swept volume, and enhance oil recovery.

[0008] Specifically, the technical solution of the present invention is as follows:

[0009] In a first aspect, the present invention provides a salt-resistant, emulsifying, high-viscosity quaternary copolymer, the structural formula of which is shown in Formula I:

[0010] Formula I.

[0011] Secondly, the present invention provides a method for preparing the salt-resistant emulsifying high-viscosity quaternary copolymer described above, comprising the following steps:

[0012] 1) Add acrylamide, N-vinylpyrrolidone, and 4-vinylpyridine propanesulfonate to deionized water and stir at room temperature until completely dissolved to obtain a hydrophilic monomer mixture;

[0013] 2) Add p-dodecylstyrene to an aqueous solution containing sodium dodecyl sulfate and disperse it to obtain a hydrophobic monomer micelle dispersion;

[0014] 3) Under stirring, the hydrophobic monomer micelle dispersion is added dropwise to the hydrophilic monomer mixture. After the addition is complete, stirring is continued until the four monomers are evenly dispersed in the aqueous phase to obtain the monomer reaction solution.

[0015] 4) While stirring, adjust the pH of the monomer reaction solution system to 6.5-7.5, continue stirring and introduce nitrogen into the system, add ammonium persulfate / sodium bisulfite to initiate the free radical copolymerization reaction, and obtain salt-resistant emulsified high-viscosity quaternary copolymer.

[0016] Steps 1) and 2) are not subject to any time order.

[0017] Preferably, based on the total amount of monomers participating in the copolymerization reaction as 100%, the molar percentage of N-vinylpyrrolidone is 3%-8%, the molar percentage of 4-vinylpyridinepropanesulfonate is 3%-10%, the molar percentage of p-dodecylstyrene is 1%-2%, and the balance is acrylamide.

[0018] Preferably, the mass ratio of sodium dodecyl sulfate to p-dodecylstyrene in step 2) is 2:1.

[0019] Preferably, the reaction temperature in step 4) is 50°C and the reaction time is 8 hours.

[0020] Preferably, in step 4), the total amount of ammonium persulfate and sodium bisulfite added is 0.30% of the total mass of the monomers participating in the copolymerization reaction, and the molar ratio of ammonium persulfate to sodium bisulfite is 1:1.

[0021] Preferably, in step 4), nitrogen gas is introduced into the system for 25-35 minutes before adding ammonium persulfate / sodium bisulfite.

[0022] Preferably, after the reaction in step 4) is completed, purification is further included; the purification includes the following steps:

[0023] a. Cool the resulting polymer colloid to room temperature and cut it into small pieces;

[0024] b. Add anhydrous ethanol to the polymer colloid cut into small pieces to precipitate it, and obtain the precipitated colloid;

[0025] c. The precipitated colloid was washed sequentially with anhydrous ethanol and acetone, and then dried, pulverized and sieved to obtain the target quaternary copolymer.

[0026] Thirdly, the present invention provides a polymer flooding agent comprising the salt-resistant emulsifying high-viscosity quaternary copolymer described above and injection water, wherein the mass concentration of the salt-resistant emulsifying high-viscosity quaternary copolymer in the injection water is 500-3000 mg / L.

[0027] Fourthly, the present invention provides the application of the polymer flooding agent described above in low-permeability reservoirs, high-salinity reservoirs, or heterogeneous reservoirs.

[0028] The beneficial effects achieved by this invention are as follows:

[0029] The salt-resistant emulsifying high-viscosity quaternary copolymer molecular chain provided by this invention contains hydrophilic thickening structure, salt-resistant structure, rigid stabilizing structure and hydrophobic associative structure, making it more comprehensive in function; the resulting polymer still has good viscosity retention under high salinity conditions, and can improve oil displacement effect through oil-water interface adsorption and emulsification, making it suitable for improving oil recovery in complex reservoirs.

[0030] The present invention provides a method for preparing a salt-resistant, high-viscosity, hydrophobic associative quaternary copolymer by free radical micellar copolymerization of acrylamide, N-vinyl-2-pyrrolidone, 4-vinylpyridine propanesulfonate, and p-dodecylstyrene. To address the strong hydrophobicity of p-dodecylstyrene, sodium dodecyl sulfate micelles are used for solubilization, combined with mechanical stirring, high-speed shearing, and / or ultrasonic dispersion, to improve the uniformity of the hydrophobic monomer dispersion in the aqueous phase. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in this invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be described below.

[0032] Figure 1 A synthetic route diagram of the salt-resistant emulsifying high-viscosity quaternary copolymer provided by the present invention;

[0033] Figure 2 The hydrogen nuclear magnetic resonance spectrum of the salt-resistant emulsified high-viscosity quaternary copolymer provided by the present invention. Detailed Implementation

[0034] The technical solutions in specific embodiments of the present invention will be described in detail and completely below. Obviously, the described embodiments are only some specific implementations of the overall technical solution of the present invention, and not all implementations. Based on the overall concept of the present invention, all other embodiments obtained by those skilled in the art fall within the protection scope of the present invention.

[0035] This invention provides a salt-resistant, emulsifying, high-viscosity quaternary copolymer, the structural formula of which is shown in Formula I:

[0036] Formula I.

[0037] The salt-resistant emulsifying high-viscosity quaternary copolymer provided by this invention contains amide structural units, lactam ring structural units, pyridine sulfonate structural units, and p-dodecylstyrene hydrophobically associated structural units. The amide provides the basic hydrophilic framework and thickening ability; the lactam ring structure improves the rigidity of the polymer molecular chain and solution stability; the strong hydration structure of the sulfonate and the rigid structure of the pyridine ring improve the salt resistance of the polymer under high salinity conditions; and the p-dodecylstyrene hydrophobically associated structure introduces long-chain alkyl and benzene rings, which enhance the emulsifying and thickening effect through hydrophobic association and oil-water interface adsorption. This polymer can both increase the viscosity of the aqueous phase and, relying on the salt-resistant and hydrophobically associated structures in the molecular chain, enhance the viscosity retention ability and oil-water interface emulsification stability under high salinity conditions, preventing molecular chain curling. It possesses multiple functions of salt resistance, thickening, and emulsification, effectively increasing the viscosity of the injected fluid, improving the mobility ratio, expanding the swept volume, and improving oil recovery.

[0038] Secondly, the present invention provides a method for preparing the salt-resistant emulsifying high-viscosity quaternary copolymer described above, comprising the following steps:

[0039] 1) Add acrylamide, N-vinylpyrrolidone, and 4-vinylpyridine propanesulfonate to deionized water and stir at room temperature until completely dissolved to obtain a hydrophilic monomer mixture;

[0040] 2) Add p-dodecylstyrene to an aqueous solution containing sodium dodecyl sulfate and disperse it to obtain a hydrophobic monomer micelle dispersion;

[0041] 3) Under stirring, the hydrophobic monomer micelle dispersion is added dropwise to the hydrophilic monomer mixture. After the addition is complete, stirring is continued until the four monomers are evenly dispersed in the aqueous phase to obtain the monomer reaction solution.

[0042] 4) While stirring, adjust the pH of the monomer reaction solution system to 6.5-7.5, continue stirring and introduce nitrogen into the system, add ammonium persulfate / sodium bisulfite to initiate the free radical copolymerization reaction, and obtain salt-resistant emulsified high-viscosity quaternary copolymer.

[0043] Steps 1) and 2) are not subject to any time order.

[0044] In this invention, acrylamide, N-vinylpyrrolidone, and 4-vinylpyridine propanesulfonate are added to deionized water and stirred at room temperature until completely dissolved to obtain a hydrophilic monomer mixture. p-Dodecylstyrene is then added to an aqueous solution containing sodium dodecyl sulfate and dispersed to obtain a hydrophobic monomer micelle dispersion. In this invention, because p-dodecylstyrene contains both a benzene ring and a C12 long-chain alkyl group, direct addition to water easily leads to stratification or aggregation. Therefore, sodium dodecyl sulfate is used to assist dispersion, which helps improve its dispersion uniformity in the aqueous phase. In this invention, the preferred mass ratio of sodium dodecyl sulfate to p-dodecylstyrene is 2:1.

[0045] In this invention, acrylamide is used to provide the basic framework and thickening ability. 4-Vinylpyridine propanesulfonate introduces a strong hydration structure of sulfonate and a rigid pyridine ring structure into the polymer molecular chain. The sulfonate structure enhances the polymer's hydration ability in highly saline water, reducing the compressive effect of salt ions on the polymer molecular chain; the pyridine ring structure helps improve the rigidity of the molecular chain, allowing the polymer to maintain good chain extension ability and salt resistance under high salinity conditions. N-Vinylpyrrolidone introduces a lactam ring structure into the polymer molecular chain to improve the polymer's chain rigidity, temperature stability, and solution stability under high salinity conditions. Compared to acrylamide polymers alone, the introduction of N-vinylpyrrolidone makes the polymer molecular chain less prone to rapid coiling in complex formation water, which helps maintain solution viscosity. p-Dodecylstyrene is an aromatic long-chain hydrophobic monomer containing polymerizable vinyl groups, benzene rings, and C12 long-chain alkyl groups. Its vinyl groups participate in free radical copolymerization, while the benzene rings and dodecyl side chains are retained on the polymer chain as hydrophobic structures to form intra- and inter-chain hydrophobic association microdomains, thereby enhancing solution viscosity, viscoelasticity, and structural recovery ability after shearing.

[0046] In this invention, the p-dodecylstyrene is preferably 4-dodecylstyrene, wherein the vinyl group and the dodecyl group are located at the para position of the benzene ring.

[0047] In this invention, based on 100% of the total amount of monomers participating in the copolymerization reaction, the preferred molar percentage of N-vinylpyrrolidone is 3%-8%, the preferred molar percentage of 4-vinylpyridine propanesulfonate is 3%-10%, the preferred molar percentage of p-dodecylstyrene is 1%-2%, and the balance is acrylamide. In this invention, using the above proportions, acrylamide remains the main monomer, and the content of functional monomers is relatively moderate, ensuring both the water solubility of the polymer and the salt resistance and emulsifying and thickening effects.

[0048] After obtaining the hydrophilic monomer mixture and the hydrophobic monomer micelle dispersion, the present invention, under stirring, adds the hydrophobic monomer micelle dispersion dropwise to the hydrophilic monomer mixture. After the addition is complete, stirring continues until the four monomers are uniformly dispersed in the aqueous phase, resulting in a monomer reaction solution. The main purpose of this step is to ensure that the hydrophilic and hydrophobic functional monomers are uniformly distributed in the same reaction system, providing a stable reaction environment for the subsequent free radical copolymerization reaction. In this invention, when adding the hydrophobic monomer micelle dispersion to the hydrophilic monomer mixture, it is necessary to add it slowly dropwise under stirring to avoid local aggregation of the hydrophobic monomers.

[0049] After obtaining the monomer reaction solution, the pH of the monomer reaction solution system is adjusted to 6.5-7.5 under stirring. Nitrogen gas is continuously introduced into the system, and ammonium persulfate / sodium bisulfite is added to initiate the redox reaction, resulting in a salt-resistant, emulsified, high-viscosity quaternary copolymer. In this invention, to fully remove dissolved oxygen from the solution, nitrogen gas is preferably introduced into the system for 25-35 minutes before adding ammonium persulfate / sodium bisulfite.

[0050] In this invention, ammonium persulfate and sodium bisulfite are used as redox initiation systems. They undergo a redox reaction in aqueous solution, generating free radicals. These free radicals, under nitrogen protection, initiate the opening of carbon-carbon double bonds in four monomers, leading to a free radical copolymerization reaction that gradually integrates them into the polymer molecular chain, forming an AM / NVP / 4-VPPS / 4-DS quaternary copolymer. In this invention, it is preferable to prepare the ammonium persulfate / sodium bisulfite mixture separately before use to reduce the effective concentration decrease caused by the decomposition of ammonium persulfate and the oxidation of sodium bisulfite by air, thus maintaining stable redox initiation efficiency and free radical generation rate. The total amount of ammonium persulfate and sodium bisulfite added is preferably 0.30% of the total mass of the monomers participating in the copolymerization reaction, and their molar ratio is preferably 1:1. In this invention, the preferred reaction temperature is 50°C, and the preferred reaction time is 8 hours. The specific synthetic route diagram of the salt-resistant emulsifying high-viscosity quaternary copolymer provided by this invention is as follows: Figure 1 As shown.

[0051] In this invention, after the reaction is complete, purification is preferably performed to remove unreacted monomers, uncopolymerized p-dodecylstyrene from sodium dodecyl sulfate, and other small molecule impurities. In this invention, the purification preferably includes the following steps:

[0052] a. Cool the resulting polymer colloid to room temperature and cut it into small pieces;

[0053] b. Add anhydrous ethanol to the polymer colloid cut into small pieces to precipitate it, and obtain the precipitated colloid;

[0054] c. The precipitated colloid was washed sequentially with anhydrous ethanol and acetone, and then dried, pulverized and sieved to obtain the target quaternary copolymer.

[0055] In this invention, the drying method is preferably vacuum drying.

[0056] Thirdly, the present invention provides a polymer flooding agent comprising the salt-resistant emulsifying high-viscosity quaternary copolymer described above and injection water, wherein the mass concentration of the salt-resistant emulsifying high-viscosity quaternary copolymer in the injection water is 500-3000 mg / L.

[0057] In this invention, when preparing the polymer flooding agent, the powdered polymer is slowly added to the injection water or simulated formation water while stirring, allowing it to gradually swell and dissolve. During preparation, a large amount of polymer powder should be avoided from being added rapidly at once to prevent polymer agglomeration. After the polymer is completely dissolved, a salt-resistant, emulsified, high-viscosity polymer flooding agent is obtained. Once this agent enters the reservoir, it can, on the one hand, increase the viscosity of the injection fluid, reduce the water phase mobility, and improve the oil-water mobility ratio; on the other hand, it can enhance the emulsification and viscosity-increasing effect through hydrophobic association and oil-water interface adsorption, thereby expanding the swept volume and improving the utilization of residual oil after waterflooding.

[0058] Fourthly, this invention provides the application of the polymer flooding agent described above in low-permeability reservoirs, high-salinity reservoirs, or heterogeneous reservoirs. It should be noted that the specific concentration used can be adjusted according to the target reservoir temperature, formation water salinity, permeability, crude oil viscosity, and injection pressure conditions.

[0059] Example 1

[0060] In this embodiment, based on a total amount of monomers participating in the copolymerization reaction of 100 mol, 86 mol of acrylamide, 5 mol of N-vinylpyrrolidone, 8 mol of 4-vinylpyridine propanesulfonate and 1 mol of p-dodecylstyrene were weighed (i.e., approximately 6.11 g of acrylamide, approximately 0.56 g of N-vinylpyrrolidone, approximately 1.82 g of 4-vinylpyridine propanesulfonate and approximately 0.27 g of p-dodecylstyrene were weighed).

[0061] 1) Add acrylamide, N-vinylpyrrolidone and 4-vinylpyridine propanesulfonate to 60 mL of deionized water and stir at room temperature until completely dissolved to obtain a hydrophilic monomer mixture.

[0062] 2) Add 0.55g of sodium dodecyl sulfate to 20mL of deionized water and stir until completely dissolved to obtain an aqueous solution containing sodium dodecyl sulfate. Then add 0.27g of p-dodecylstyrene to the above aqueous solution containing sodium dodecyl sulfate and stir magnetically to form a hydrophobic monomer micelle dispersion in the aqueous phase.

[0063] 3) Add the hydrophobic monomer micelle dispersion obtained in step 2) dropwise to the hydrophilic monomer mixture obtained in step 1), stirring continuously during the addition process to prevent local aggregation of the hydrophobic monomers. After the addition is complete, continue stirring to ensure that acrylamide, N-vinylpyrrolidone, 4-vinylpyridine propanesulfonate, and p-dodecylstyrene are fully mixed in the aqueous phase to obtain the monomer reaction solution.

[0064] 4) Transfer the monomer reaction solution obtained in step 3) into a three-necked flask equipped with a mechanical stirrer, a condenser, and a nitrogen inlet tube, and place the three-necked flask in a constant temperature water bath. Adjust the pH of the system to 7, keeping the mixture stirred during the adjustment process to avoid localized excessive acidity or alkalinity affecting the stability of the monomer.

[0065] 5) Under stirring, nitrogen gas is introduced into the reaction system for 30 minutes to remove dissolved oxygen. After deoxygenation, the temperature of the constant temperature water bath is controlled at 50℃. Then, freshly prepared sodium bisulfite aqueous solution is added first, followed by freshly prepared ammonium persulfate aqueous solution slowly added dropwise over 10 minutes (the amount of initiator added is approximately 0.30% of the total mass of the monomers in the tool reaction, and the molar ratio of ammonium persulfate to sodium bisulfite is 1:1). The free radical copolymerization reaction of the four monomers is initiated under nitrogen protection, and the reaction time is controlled at 8 hours.

[0066] 6) After the reaction is complete,

[0067] Heating was stopped, and the resulting polymer colloid was cooled to room temperature and cut into small pieces. The polymer colloid was then added to 500 mL of anhydrous ethanol for precipitation, causing the polymer to separate from the reaction system. The precipitated polymer was washed repeatedly with anhydrous ethanol and acetone to remove unreacted monomers, sodium dodecyl sulfate, initiator residues, and other small molecule impurities. The washed product was dried in a vacuum drying oven at 40 °C to constant weight, then pulverized and sieved to obtain a powdered target quaternary copolymer salt-resistant emulsifying high-viscosity polymer. The 1H NMR spectrum of the prepared salt-resistant emulsifying high-viscosity quaternary copolymer is shown below. Figure 2 As shown.

[0068] In the polymer obtained in this embodiment, the acrylamide structural unit provides the polymer backbone and basic thickening ability; the N-vinylpyrrolidone structural unit introduces a lactam ring structure into the polymer, improving the rigidity of the molecular chain, temperature stability, and solution stability under high salt conditions; the 4-vinylpyridine propanesulfonate structural unit introduces a sulfonate strong hydration structure and a pyridine ring rigid structure into the polymer, improving the polymer's hydration ability, molecular chain extension ability, and salt resistance in highly saline water; and the dodecylstyrene structural unit introduces long-chain alkyl groups and benzene rings into the polymer, wherein the long-chain alkyl groups are used to form a hydrophobic association structure, and the benzene rings are used to enhance the oil phase affinity, thereby enhancing the emulsification performance and thickening effect of the polymer solution.

[0069] Example 2

[0070] Based on a total amount of monomers participating in the copolymerization reaction of 100 mol, weigh out 82 mol of acrylamide, 8 mol of N-vinylpyrrolidone, 8 mol of 4-vinylpyridine propanesulfonate and 2 mol of p-dodecylstyrene (weigh out approximately 5.83 g of acrylamide, approximately 0.89 g of N-vinylpyrrolidone, approximately 1.82 g of 4-vinylpyridine propanesulfonate and approximately 0.55 g of p-dodecylstyrene).

[0071] 1) Add acrylamide, N-vinylpyrrolidone and 4-vinylpyridine propanesulfonate to 60 mL of deionized water and stir at room temperature until completely dissolved to obtain a hydrophilic monomer mixture.

[0072] 2) Add 1.09 g of sodium dodecyl sulfate to 20 mL of deionized water and stir until completely dissolved to obtain an aqueous solution containing sodium dodecyl sulfate. Then add 0.55 g of p-dodecylstyrene to the above aqueous solution containing sodium dodecyl sulfate and stir magnetically to form a hydrophobic monomer micelle dispersion in the aqueous phase.

[0073] 3) Add the hydrophobic monomer micelle dispersion obtained in step 2) dropwise to the hydrophilic monomer mixture obtained in step 1), stirring continuously during the addition process to prevent local aggregation of the hydrophobic monomers. After the addition is complete, continue stirring to ensure that acrylamide, N-vinylpyrrolidone, 4-vinylpyridine propanesulfonate, and p-dodecylstyrene are fully mixed in the aqueous phase to obtain the monomer reaction solution.

[0074] 4) Transfer the monomer reaction solution obtained in step 3) into a three-necked flask equipped with a mechanical stirrer, a condenser, and a nitrogen inlet tube, and place the three-necked flask in a constant temperature water bath. Adjust the pH of the system to 7, keeping the mixture stirred during the adjustment process to avoid localized excessive acidity or alkalinity affecting the stability of the monomer.

[0075] 5) Under stirring, nitrogen gas is introduced into the reaction system for 30 minutes to remove dissolved oxygen. After deoxygenation, the temperature of the constant temperature water bath is controlled at 50℃. Then, freshly prepared sodium bisulfite aqueous solution is added first, followed by freshly prepared ammonium persulfate aqueous solution slowly added dropwise over 10 minutes (the total amount of initiator added is approximately 0.30% of the total mass of monomers in the copolymerization reaction, and the molar ratio of ammonium persulfate to sodium bisulfite oxidation is 1:1). The free radical copolymerization reaction of the four monomers is initiated under nitrogen protection, and the reaction time is controlled at 8 hours.

[0076] 6) After the reaction is complete, heating is stopped, the resulting polymer colloid is cooled to room temperature, and sheared into small pieces. The polymer colloid is then added to 500 mL of anhydrous ethanol for precipitation, causing the polymer to separate from the reaction system. The precipitated polymer is then washed repeatedly with anhydrous ethanol and acetone to remove unreacted monomers, sodium dodecyl sulfate, initiator residues, and other small molecule impurities. The washed product is placed in a vacuum drying oven and dried at 40°C to constant weight. After pulverization and sieving, a powdered target quaternary copolymer salt-resistant emulsified high-viscosity polymer is obtained.

[0077] Comparative Example 1

[0078] To investigate the effects of N-vinyl-2-pyrrolidone on molecular chain rigidity, temperature stability, and viscosity retention under high salt conditions, the molar fraction of NVP (which was eliminated) was replenished with acrylamide while maintaining the same feed ratio of 4-VPPS and 4-DS as in Example 1. Based on a total monomer content of 100 mmol, 91 mmol (6.47 g) of acrylamide, 8 mmol (1.82 g) of 4-vinylpyridine propanesulfonate, and 1 mmol (0.27 g) of p-dodecylstyrene were weighed, resulting in a total monomer mass of approximately 8.56 g.

[0079] 1) Add acrylamide and 4-vinylpyridine propanesulfonate to 60 mL of deionized water and stir at room temperature until completely dissolved to obtain a hydrophilic monomer mixture.

[0080] 2) Add 0.55 g sodium dodecyl sulfate to 20 mL of deionized water and stir until completely dissolved; then add 0.27 g p-dodecylstyrene, stir mechanically for 10 min, and then sonicate for 15 min to obtain a uniform hydrophobic monomer micelle dispersion.

[0081] 3) Under continuous mechanical stirring, the hydrophobic monomer micelle dispersion is slowly added dropwise to the hydrophilic monomer mixture over 20-30 min. After the addition is complete, stirring is continued for 30 min to obtain the monomer reaction solution.

[0082] 4) Transfer the monomer reaction solution into a three-necked flask equipped with a mechanical stirrer, a condenser and a nitrogen inlet tube. Adjust the pH of the system to 7.0, purge with nitrogen for 30 min to remove dissolved oxygen, and raise the reaction temperature to 50 °C.

[0083] 5) Prepare fresh 1.0% ammonium persulfate aqueous solution and 1.0% sodium bisulfite aqueous solution. First, add 0.80 mL of sodium bisulfite aqueous solution, and then slowly add 1.76 mL of ammonium persulfate aqueous solution over 10 min, so that the total mass of the initiator is about 0.30% of the total mass of the monomer, and the molar ratio of ammonium persulfate to sodium bisulfite is 1:1. After the addition is complete, react for 8 h under nitrogen protection.

[0084] 6) After the reaction is complete and cooled to room temperature, the polymer colloid is sheared into small pieces, and anhydrous ethanol equivalent to 5 times the volume of the colloid is added for precipitation. The precipitate is washed three times with anhydrous ethanol and acetone, dried under vacuum at 40 °C to constant weight, pulverized, and sieved to obtain a control copolymer without NVP. The theoretical monomer feed mass is 8.56 g. The actual conversion rate and product yield are based on the weighing and residual monomer analysis results.

[0085] To evaluate the effect of NVP structural units on the polymer's salt resistance, temperature resistance, and shear strength, polymers obtained in Example 1 and Comparative Example 1 were tested under the following conditions:

[0086] Polymer mass concentration: 1500 mg / L;

[0087] Simulated total mineralization of highly mineralized formation water: 50,000 mg / L;

[0088] Viscosity test temperature: 70 ℃; Shear rate: 7.34 s -1 ;

[0089] Thermal aging conditions: Sealed aging at 70℃ for 30 days;

[0090] Mechanical shearing conditions: 1000s -1 Cut for 10 minutes.

[0091] The viscosity retention rate of brine is calculated according to formula (1):

[0092] R salt =η salt / η0×100%

[0093] In the formula, η0 is the initial apparent viscosity of deionized water, η salt The apparent viscosity under high salt conditions.

[0094] The viscosity retention rate during heat aging is calculated according to formula (2):

[0095] R t =η t / η salt ×100%

[0096] In the formula, η tThe apparent viscosity of a high-salt solution after aging at 70°C for 30 days.

[0097] Shear viscosity retention is calculated according to formula (3):

[0098] R s =η s / η salt ×100%

[0099] In the formula, η s It represents the apparent viscosity of a high-salt polymer solution after mechanical shearing.

[0100] Table 1 Performance comparison between Example 1 and Comparative Example 1

[0101] As shown in Table 1, the initial apparent viscosity of the quaternary copolymer obtained in Example 1 in deionized water was 70.4 mPa·s, which was higher than that of Comparative Example 1 (64.1 mPa·s). In simulated high-salinity formation water, the apparent viscosity of Example 1 was 52.9 mPa·s, with a brine viscosity retention rate of 75.1%; while the apparent viscosity of Comparative Example 1 in high-salinity water was 41.8 mPa·s, with a brine viscosity retention rate of 65.2%. These trends indicate that the NVP structural unit is beneficial for improving the molecular chain stability and viscosity retention of the polymer under high-salinity conditions.

[0102] After being sealed and aged at 70 °C for 30 days, the apparent viscosity of the polymer solution in Example 1 was 45.7 mPa·s, with a viscosity retention rate of 86.3% after thermal aging; the apparent viscosity of the polymer solution in Comparative Example 1 was 31.4 mPa·s, with a viscosity retention rate of 75.1% after thermal aging. This trend indicates that the lactam ring structure introduced by NVP may improve the rigidity and hydration stability of the molecular chain, and slow down the molecular chain coiling and hydrodynamic volume decrease under high temperature and high salt conditions.

[0103] After mechanical shearing, the apparent viscosity of Example 1 was 42.6 mPa·s, with a shear viscosity retention rate of 80.5%, which was higher than that of Comparative Example 1 (75.8%). This trend suggests that there may be a synergistic effect between the NVP structural unit and the hydrophobic structure of 4-VPPS and p-dodecylstyrene, which is beneficial to maintaining the stability of the polymer molecular chain and hydrophobic association network under high salt, high temperature and shear conditions.

[0104] Performance testing

[0105] To verify the thickening, salt resistance, hydrophobic association, emulsification stability, and injection displacement properties of the salt-resistant emulsified high-viscosity quaternaries obtained in each example, the polymers obtained in Examples 1 and 2 were evaluated for apparent viscosity, salt sensitivity, thermal aging, shear resistance, emulsification thickening, and core injection displacement properties. Unless otherwise specified, the polymer solution was prepared using simulated high-salinity formation water, the test temperature was 70 °C, and the apparent viscosity test shear rate was 7.34 s⁻¹. -1 .

[0106] 1. Evaluation of apparent viscosity and salt sensitivity

[0107] The polymers obtained in Examples 1 and 2 were prepared into solutions with a mass concentration of 1500 mg / L. The apparent viscosity was measured under target temperature conditions using deionized water and simulated high-mineralization formation water as solvents, respectively, and the brine viscosity retention rate was calculated according to formula (4).

[0108] Rs = ηhighsalt / ηdeionized water × 100% (4)

[0109] In the formula, Rs is the brine viscosity retention rate (%), ηhigh-salt is the apparent viscosity of the polymer solution in high-salinity formation water (mPa·s), and ηdeionized water is the apparent viscosity of the polymer solution in deionized water (mPa·s).

[0110] Table 2. Results of Apparent Viscosity and Salt Sensitivity Evaluation

[0111] As shown in Table 2, the initial apparent viscosity of the polymer in Example 1 in deionized water was 70.4 mPa·s, and it remained at 52.9 mPa·s under high salinity conditions, with a viscosity retention rate of 75.1% in brine. The initial apparent viscosity of the polymer in Example 2 in deionized water was 78.2 mPa·s, and it remained at 61.5 mPa·s under high salinity conditions, with a viscosity retention rate of 78.6% in brine. These results demonstrate that the polymer provided by this invention exhibits good thickening and viscosity retention capabilities even in high-salt environments.

[0112] 2. Evaluation of thermal aging and shear resistance properties

[0113] The polymer solutions of Examples 1 and 2 were sealed and aged at 70 °C for 30 days. The apparent viscosity after aging was measured and the heat aging viscosity retention rate was calculated. The polymer solutions of Examples 1 and 2 were also subjected to mechanical shearing treatment, and the apparent viscosity after shearing was measured and the shear viscosity retention rate was calculated. The heat aging viscosity retention rate and the shear viscosity retention rate were calculated according to Equations (5) and (6), respectively.

[0114] R T = η after aging / η high salt × 100% (5)

[0115] R S = η after shearing / η high salt × 100% (6)

[0116] Table 3. Evaluation results of thermal aging and shear resistance

[0117] As shown in Table 3, after aging at 70 °C for 30 days, the apparent viscosity of the polymer in Example 1 decreased from 52.9 mPa·s under high-salt conditions to 45.7 mPa·s, with a viscosity retention rate of 86.3% after thermal aging. After mechanical shearing, the apparent viscosity was 42.6 mPa·s, with a shear viscosity retention rate of 80.5%. After aging at 70 °C for 30 days, the apparent viscosity of the polymer in Example 2 decreased from 61.5 mPa·s under high-salt conditions to 55 mPa·s, with a viscosity retention rate of 89.4% after thermal aging. After mechanical shearing, the apparent viscosity was 51.2 mPa·s, with a shear viscosity retention rate of 83.3%. This indicates that the polymer provided by the present invention has good thermal stability, shear resistance, and the ability to retain its hydrophobic association structure.

[0118] 3. Evaluation of viscosity and emulsion stability before and after emulsification

[0119] The polymers from Examples 1 and 2 were each prepared into 1500 mg / L solutions, and the apparent viscosity of the polymer solutions before emulsification was measured and denoted as η0. Subsequently, the crude oil and polymer solutions were mixed at a volume ratio of 1:1 and subjected to high-speed shearing at 8000 r / min for 5 min to obtain a crude oil-polymer emulsion. After emulsification, the emulsion was allowed to stand for 10 min, and the initial apparent viscosity of the emulsion was measured under the same temperature and shear rate conditions and denoted as ηE. The emulsion was then allowed to stand at 70 °C, and the viscosity and water separation rate of the emulsion were measured after 24 h and 72 h, respectively.

[0120] I η = (η E -η0) / η0 × 100% (7)

[0121] K η = η E / η0 (8)

[0122] Table 4. Evaluation results of viscosity and emulsion stability before and after emulsification

[0123] As shown in Table 4, the apparent viscosities of the polymer solutions in Examples 1 and 2 before emulsification were 52.9 mPa·s and 61.5 mPa·s, respectively. After emulsification, the initial apparent viscosities of the emulsions increased to 92.0 mPa·s and 107.0 mPa·s, respectively, with viscosity increases of 73.9% and 74.0%, and a viscosity increase factor of 1.74 for both. After standing for 72 hours, the viscosity retention rates of the emulsions were 85.9% and 99.5%, respectively, and the water separation rates were 35.5% and 31.2%. This indicates that the polymer provided by the present invention has significant emulsifying, viscosity-increasing, and emulsion-stabilizing effects after forming an emulsion system with crude oil.

[0124] 4. Evaluation of Injection and Core Displacement Effects

[0125] To evaluate the reservoir suitability of the polymer from Example 1, long core injection and displacement experiments were conducted under target temperature and simulated formation water conditions.

[0126] Table 5. Results of Polymer Recovery Evaluation

[0127] Under a core permeability of 100 mD, the polymer stages in Examples 1 and 2 enhanced oil recovery by 21.17% and 22.36%, respectively, with final recovery rates reaching 61.38% and 62.41%. The results indicate that the polymer provided by this invention can improve the oil-water mobility ratio and increase the utilization of remaining oil through viscosity enhancement, salt resistance stabilization, and hydrophobic association.

[0128] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A salt-resistant, emulsifying, high-viscosity quaternary copolymer, characterized in that, Its structural formula is shown in Formula I: Formula I.

2. The method for preparing the salt-resistant emulsifying high-viscosity quaternary copolymer according to claim 1, characterized in that, Includes the following steps: 1) Add acrylamide, N-vinylpyrrolidone, and 4-vinylpyridine propanesulfonate to deionized water and stir at room temperature until completely dissolved to obtain a hydrophilic monomer mixture; 2) Add p-dodecylstyrene to an aqueous solution containing sodium dodecyl sulfate and disperse it to obtain a hydrophobic monomer micelle dispersion; 3) Under stirring, the hydrophobic monomer micelle dispersion is added dropwise to the hydrophilic monomer mixture. After the addition is complete, stirring is continued until the four monomers are evenly dispersed in the aqueous phase to obtain the monomer reaction solution. 4) While stirring, adjust the pH of the monomer reaction solution system to 6.5-7.5, continue stirring and introduce nitrogen into the system, add ammonium persulfate / sodium bisulfite to initiate the free radical copolymerization reaction, and obtain salt-resistant emulsified high-viscosity quaternary copolymer. Steps 1) and 2) are not subject to any time order requirement.

3. The preparation method according to claim 2, characterized in that, Based on the total amount of monomers participating in the copolymerization reaction as 100%, the molar percentage of N-vinylpyrrolidone is 3%-8%, the molar percentage of 4-vinylpyridinepropanesulfonate is 3%-10%, the molar percentage of p-dodecylstyrene is 1%-2%, and the balance is acrylamide.

4. The preparation method according to claim 2, characterized in that, In step 2), the mass ratio of sodium dodecyl sulfate to p-dodecylstyrene is 2:

1.

5. The preparation method according to claim 2, characterized in that, The reaction temperature in step 4) is 50℃ and the reaction time is 8h.

6. The preparation method according to claim 2, characterized in that, In step 4), the amount of ammonium persulfate / sodium bisulfite added is 0.30% of the total mass of the monomers participating in the copolymerization reaction, and the molar ratio of ammonium persulfate to sodium bisulfite is 1:

1.

7. The preparation method according to claim 2, characterized in that, In step 4), nitrogen gas is introduced into the system for 25-35 minutes before adding ammonium persulfate / sodium bisulfite.

8. The preparation method according to claim 2, characterized in that, After the reaction in step 4) is completed, purification is also included; the purification includes the following steps: a. Cool the resulting polymer colloid to room temperature and cut it into small pieces; b. Add anhydrous ethanol to the polymer colloid cut into small pieces to precipitate it, and obtain the precipitated colloid; c. The precipitated colloid was washed sequentially with anhydrous ethanol and acetone, and then dried, pulverized and sieved to obtain the target quaternary copolymer.

9. A polymer-based oil displacement agent, characterized in that: It includes the salt-resistant emulsifying high-viscosity quaternary copolymer as described in claim 1 and injected water, wherein the mass concentration of the salt-resistant emulsifying high-viscosity quaternary copolymer in the injected water is 500-3000 mg / L.

10. The application of the polymer flooding agent according to claim 9 in low-permeability reservoirs, high-salinity reservoirs or heterogeneous reservoirs.

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