Cationic copolymer, nano-microsphere as well as preparation method and application of nano-microsphere

Abstract

The invention relates to the technical field of CO2 oil displacement, in particular to a cationic copolymer, a nano-microsphere, a preparation method of the nano-microsphere and application of the nano-microsphere. According to the cationic copolymer, supercritical CO2 and water can be self-emulsified under the oil field stratum condition to form stable emulsion; the cationic copolymer has a positively charged hydrophilic chain link structure, a CO2-philic chain link structure and a propyl alcohol type lipophilic chain link structure at the same time, and has various affinity characteristics of hydrophilicity, lipophilicity and CO2 affinity in a supercritical CO2 fluid. Therefore, the cationic copolymer provided by the invention can form a CO2 / water emulsion when encountering formation water in a relatively severe complex reservoir environment, and the emulsion is subjected to a mixed-phase process when encountering oil to form a quasi-mixed state. The quasi-mixed state can solve the key problem of stratum water influence in the aperture system, the effect of replacing crude oil with CO2 in the matrix is further enhanced, and the crude oil recovery rate is increased.

Description

Technical Field

[0001] The present invention relates to the technical field of CO2 enhanced oil recovery, and specifically to a cationic copolymer, nano microspheres, and their preparation methods and applications. Background Art

[0002] In order to achieve the goals of "carbon peak and carbon neutrality" (abbreviated as "dual carbon"), a series of carbon capture, utilization, and storage (Carbon Capture, Utilization and Storage, CCUS) measures being taken by various industries are considered to be one of the effective ways to reduce carbon emissions. As one of the CCUS technologies in the oil and gas development field, the carbon dioxide enhanced oil recovery (CO2-EOR) technology has developed rapidly due to its advantages such as good injectability, high oil displacement efficiency, and CO2 storage rate. The CO2-EOR technology is divided into CO2 immiscible flooding and CO2 miscible flooding according to the degree of miscibility between the displacing agent (injected gas) and the crude oil. Due to the large gas-liquid density difference and capillary force, the sweep efficiency of immiscible flooding is low, and the oil displacement effect is poor. Miscible flooding means that the displacing agent (injected gas) diffuses, mass transfers, and dissolves with the crude oil, eliminating the surface, so that the displacing agent and the crude oil form a single miscible state. At this time, the capillary force trapping effect of the crude oil disappears, and the oil displacement efficiency is higher than that of immiscible flooding, which is the current main development direction. Research and practice have shown that injecting compressed liquid CO2 fluid into the formation and controlling CO2 to exist in a supercritical state under reservoir temperature and pressure conditions to form a miscible oil displacement method has the best oil displacement efficiency and storage efficiency. Promoting the large-scale application and development of CO2 flooding has great strategic and economic significance.

[0003] In the 1980s, with the exploitation of natural CO2 gas fields in the United States and the laying of gas transmission pipelines, injection CO2 development tests were successively carried out in multiple oil fields such as Paradis and Shoemaker, and CO2 flooding gradually became the main means of improving oil recovery in North America. The foreign field test CO2 flooding technology still mainly uses miscible flooding, accounting for more than 90%; the continental sedimentary reservoir environment in China is more complex and more heterogeneous, and most oil reservoirs have higher temperatures. The crude oil composition is mainly heavy components, and the content of C2-C 15 component content is significantly low, the crude oil viscosity is large, and the supercritical CO2 front is extremely easy to break through the underground fluid to form channeling, increasing the difficulty of miscibility between supercritical CO2 and residual oil. Moreover, as the proportion of residual oil decreases, the proportion of formation water in the pore-fracture system increases significantly. The supercritical CO2 / crude oil miscible process is easily affected by underground fluids such as formation water, and it is difficult to achieve an ideal single miscible state. It has a greater impact on the displacement efficiency of supercritical CO2 flooding.

[0004] The research on supercritical CO2 flooding mainly focuses on two aspects: regulating the mobility of CO2 to improve the sweep efficiency and enhancing the miscibility efficiency of CO2 and crude oil. Urgent research is needed to eliminate the important influence of the objectively existing formation free water on the two processes of supercritical CO2 migration and miscible flooding. The basic research on supercritical CO2-water emulsion in the field of using CO2 to displace and exploit natural gas hydrates provides a new idea for solving this problem. The CO2-water emulsification process can transform the free formation water into the dispersed phase of the emulsion, maximizing the reduction of its negative impact on the supercritical CO2 / crude oil miscible process.

[0005] However, the application of CO2 emulsion in oil and gas exploitation is still in its infancy. The basic research on the application of CO2-water emulsion in the field of oil and gas exploitation mainly focuses on using CO2 to displace and exploit natural gas hydrates. ConocoPhillips completed the world's first field test project on the replacement potential of CO2 / CH4 in the West Sak well area of the North Slope Oilfield in Alaska in 2012. To enhance the CO2 / CH4 replacement efficiency, in 2008, Zhou et al. tried to prepare gaseous CO2-water microfoams and supercritical CO2-water emulsions with different water-carbon ratios and found that the replacement rate of hydrates by CO2 emulsion was higher than that of pure supercritical CO2. In 2014, Yuan et al. formed a CO2-water emulsion with sodium dodecyl sulfate and polyoxyethylene sorbitan monooleate as emulsifiers and found that its stability was mainly affected by temperature, pressure, and salinity. In 2021, Yan Yu et al. screened out octylphenol-10 (OP-10) with better stability performance for CO2 emulsion than the previous Tween 80 and sorbitan monopalmitate emulsifier systems by trying the emulsion stability under different initial components and reaction conditions. Although it has been developed for more than a decade, the application of CO2 emulsion in oil and gas exploitation is still in its infancy. Due to the limitations in the field of natural gas application, supercritical CO2 emulsion must be prepared before injection. And restricted by the temperature and pressure gradients of terrestrial oilfield reservoirs, this method cannot be directly applied to the field of supercritical CO2 flooding. There is no report on the research of the emulsification method to enhance mass transfer in improving the miscibility effect of CO2 / crude oil. The research on the formation and enhanced mass transfer mechanism of CO2 emulsion in the continental sedimentary reservoir environment needs to be carried out urgently. Summary of the Invention

[0006] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art and provide a cationic copolymer, nano microspheres, and their preparation methods and applications. Among them, the cationic copolymer can enable supercritical CO2 to achieve good emulsification with water in the oilfield formation. The nano microspheres prepared from the cationic copolymer and anionic polymer have a slow-release effect and can be injected into the oilfield formation together with supercritical CO2 to achieve in-situ synergistic regulation of supercritical CO2 emulsion.

[0007] To achieve the above technical effects, the present invention adopts the following technical solutions: A cationic copolymer is prepared by grafting and growing monomer A on a long chain formed by the growth of monomer B. The average molecular weight of the cationic copolymer is 1000 Da - 25000 Da. The molar ratio of monomer A to monomer B is (3 - 8):(1.5 - 2.5). Among them, monomer A is selected from at least one of dimethyldiallylammonium chloride (DMDAAC), dimethylaminoethyl methacrylate (DMAEMA), vinylpyrrolidone (NVP), 1H,1H,2H,2H - perfluorodecyl acrylate (FDA), perfluoroacrylate (TPA), hexafluorobutyl methacrylate (HFBMA), or cationic acrylamide (pAM), and monomer B is propylene oxide.

[0008] When preparing the cationic copolymer, an alkali metal hydroxide is needed to achieve the ring - opening polymerization reaction of monomer B, and a chain transfer agent and an esterification reaction catalyst are added to achieve the grafting and growth polymerization reaction of monomer A on the main chain formed by monomer B. Among them, the chain transfer agent includes 4 - cyano - 4 - thiopropanethiol pentanoic acid (CTPPA), the alkali metal hydroxide is any one of potassium hydroxide (KOH) and sodium hydroxide (NaOH), and the esterification reaction catalyst is N - (3 - dimethylaminopropyl) - N'-ethylcarbodiimide hydrochloride (EDAC).

[0009] In the cationic copolymer, monomer A is a CO2 - philic monomer and a cationic monomer, and monomer B is an oil - philic monomer.

[0010] The cationic copolymer provided by the present invention can make supercritical CO2 and water form a stable emulsion by self - emulsification under oil - field formation conditions. The cationic copolymer simultaneously has a positively charged hydrophilic chain segment structure, a CO2 - philic chain segment structure, and a propanol - type oil - philic chain segment structure, and has multiple affinity characteristics of hydrophilicity, oil - philicity, and CO2 - philicity in supercritical CO2 fluid. Therefore, the cationic copolymer provided by the present invention can form a CO2 / water emulsion when encountering formation water in a relatively harsh reservoir complex environment, and the emulsion undergoes a miscible process when encountering oil to form a pseudo - miscible state. This pseudo - miscible state can solve the key problem of the influence of formation water in the pore - fracture system, further enhance the effect of CO2 displacing crude oil in the matrix, and improve the crude oil recovery rate.

[0011] Preferably, in the cationic copolymer, the molar ratio of monomer A to monomer B is (3.5 - 6):(2 - 2.5).

[0012] Preferably, monomer A is selected from dimethyldiallylammonium chloride (DMDAAC), dimethylaminoethyl methacrylate (DMAEMA), or vinylpyrrolidone (NVP). Further preferably, monomer A is selected from dimethyldiallylammonium chloride (DMDAAC) or dimethylaminoethyl methacrylate (DMAEMA).

[0013] Preferably, the preparation method of the cationic copolymer is as follows: Step 1: Dissolve the initiator in the first organic reagent at room temperature to obtain an initiator solution; Step 2: Dissolve the alkali metal hydroxide and monomer B in the second organic reagent and carry out ring-opening polymerization to form an alkoxide anion, which generates a long-chain polymer as the mother chain through continuous ring-opening reactions with monomer B during the growth stage; Step 3: Dissolve monomer A, the chain transfer agent, and the esterification reaction catalyst in the second organic reagent, add the mixed organic reagent solution prepared in Step 1 and Step 2, mix well, remove oxygen by passing nitrogen, and then heat and stir for copolymerization reaction to obtain the cationic copolymer.

[0014] Preferably, the alkali metal hydroxide is selected from KOH or NaOH; the initiator is selected from at least one of ammonium persulfate (APS), 4,4'-azobis(4-cyanovaleric acid) (ACPA), and azobisisobutyronitrile (AIBN); the first organic reagent is selected from tetrahydrofuran (THF); the second organic reagent is selected from any one of dichloromethane (DCM), cyclohexane, heptane (HPT), octane, or kerosene; the reaction temperature in Step 2 is 45°C - 65°C; the reaction temperature in Step 3 is 45 - 80°C; the mass ratio of the initiator to monomer B is (0.02 - 0.12):1.

[0015] The present invention also provides a kind of nano-microspheres, which are prepared by reacting an anionic polymer and at least one of the above cationic copolymers, and the molar ratio of the anionic polymer to the cationic copolymer is (1.28 - 0.69):1; the reaction includes electrostatic adsorption and crosslinking; the anionic polymer is selected from at least one of sodium polyvinyl sulfate (Na-PVS), polyacrylic acid (PAA), polyanionic cellulose PAC, polyanionic cellulose DRISPAC, or carboxymethyl cellulose (CMC); in the selection of the anionic polymer, the effects that each anionic polymer can achieve are basically similar; the average particle size of the nano-microspheres is 150nm - 600nm.

[0016] Preferably, the anionic polymer is selected from Na-PVS.

[0017] The preparation method of the above nano-microspheres is as follows: Put the cationic copolymer and the anionic polymer into solvents respectively to obtain a cationic dispersion and an anionic dispersion, drop the cationic dispersion into the anionic dispersion, and obtain the nano-microspheres after concentration and washing after the reaction.

[0018] Preferably, deionized water is selected as the solvent in the cationic dispersion; deionized water is selected as the solvent in the anionic dispersion; further preferably, the mass concentration of the cationic copolymer in the cationic dispersion is 0.01 wt% - 1.10 wt%, and the mass concentration of the anionic polymer in the anionic dispersion is 0.01 wt% - 1.50 wt%.

[0019] Preferably, the dropping rate is 60 - 90 drops / min.

[0020] The method for preparing the nano - microspheres further includes putting the nano - microspheres into a stabilizer solution to obtain a nano - microsphere dispersion with a mass concentration of 0.01 wt% - 1.50 wt% for standby; further preferably, a Poloxmar type (Pluronic F - 127) non - ionic surfactant is selected as the stabilizer; more preferably, the concentration of the stabilizer in the stabilizer solution is 0.02 wt% - 0.2 wt%, and deionized water is selected as the solvent in the stabilizer solution.

[0021] The storage time is prolonged by adding a stabilizer to the nano - microsphere dispersion.

[0022] The present invention also provides the application of the above - mentioned nano - microspheres or the nano - microspheres prepared by the above - mentioned preparation method in enhancing oil recovery in supercritical CO2 flooding. The nano - microspheres are put into supercritical CO2 and then injected into the oilfield reservoir together, and there are channeling channels in the oilfield reservoir.

[0023] Preferably, the mass ratio of the nano - microspheres to supercritical CO2 is (0.05 - 4.5):100; further preferably, the input method of the nano - microspheres is direct input and / or input in the form of a nano - microsphere dispersion.

[0024] After the nano - microspheres provided by the present invention enter the reservoir with supercritical CO2, they migrate underground with supercritical CO2 and then enter the channeling channels. The free water existing in the channeling channels contacts the nano - microspheres, changing their surface electrical properties and pH values, causing the nano - microspheres to swell, dissociate and release the cationic copolymer, and de - protonating the charged functional groups of the cationic copolymer, realizing the function of "targeted release" of the cationic copolymer in response to the environment during its migration with CO2; the entire process of swelling, dissociating and releasing the cationic copolymer takes several hours to dozens of hours, and this process is determined by the composition, particle size of the nano - microspheres and the chemical properties of the free water in the channeling channels; the released cationic copolymer can stimulate the in - situ self - emulsification of supercritical CO2 and formation water to form a CO2 - water emulsion to achieve in - situ thickening of CO2, and the emulsion can also promote the CO2 / crude oil miscibility efficiency after encountering oil, realizing the in - situ synergistic regulation of self - emulsification thickening / directional mass transfer miscibility in supercritical CO2 flooding.

[0025] The nano - microspheres provided by the present invention are nano - scale microsphere materials with a uniform structure or core - shell structure formed by the electrostatic adsorption of cationic copolymers and anionic polymers. Their structural characteristics are regulated by the chain lengths, charge densities, and polarities of the cationic copolymers and anionic polymers. The nano - microspheres have a particle size of 150nm - 600nm and can have good dispersibility and reservoir migration ability when applied in unconventional low - permeability reservoirs such as tight sandstone and shale.

[0026] After the nano - microspheres are injected into the oilfield reservoir, when the nano - microspheres encounter water, the C - N bond of the quaternary ammonium group on the cationic copolymer is deprotonated, reducing the positive charge density and total amount of the cationic copolymer. As a result, it undergoes electrostatic dissociation with the anionic polymer. The dissociated cationic copolymer forms a CO2 emulsion with supercritical CO2 and water. This process is called "in - situ release of CO2 - philic cationic copolymer emulsifier upon encountering water". At the same time, the anionic polymer selected in the present invention, in addition to being able to form nano - microspheres with the cationic copolymer, should also have functions such as drag reduction, permeability enhancement, or slow - release scale inhibition after release, rather than becoming a residual blocking substance in the pore - fracture system that requires additional degradation agents for treatment.

[0027] When preparing nano - microspheres by the cross - linking reaction of cationic copolymers and anionic copolymers, although the cross - linking agent used is for cross - linking other functional groups on the cation, the selected cross - linking agent is a complexing type (Cr cross - linking agent), and the formed bond is also a reversible complexing bond, which can partially extend the release time of the cationic polymer.

[0028] Advantages of the present invention: 1. The cationic copolymer provided by the present invention can enable supercritical CO2 and water to form a stable emulsion by self - emulsification under oilfield formation conditions. The cationic copolymer simultaneously has a positively charged hydrophilic chain segment structure, a CO2 - philic chain segment structure, and a propanol - type lipophilic chain segment structure, and has multiple affinity characteristics of hydrophilicity, lipophilicity, and CO2 - philicity in supercritical CO2 fluid. Therefore, the cationic copolymer provided by the present invention can form a CO2 / water emulsion upon encountering formation water in a relatively harsh reservoir complex environment. The emulsion undergoes a miscible process upon encountering oil to form a pseudo - miscible state. This pseudo - miscible state can solve the key problem of the influence of formation water in the pore - fracture system, further enhance the effect of CO2 displacing crude oil in the matrix, and improve the crude oil recovery rate.

[0029] 2. The nano - microspheres provided by the present invention can not only release the cationic copolymer in - situ upon encountering water in the oilfield reservoir, enabling supercritical CO2 to form an emulsion in - situ in the reservoir, but also the dissociated anionic polymer should have functions such as drag reduction, permeability enhancement, or slow - release scale inhibition, rather than becoming a residual blocking substance in the pore - fracture system that requires additional degradation agents for treatment. Description of the Drawings

[0030] Figure 1 It is the FTIR spectrum of the cationic copolymer prepared in Example 1 of the present invention; Figure 2 1H-NMR spectrum of the nano-microspheres prepared in Example 1 of the present invention; Figure 3 1H-NMR spectrum of the nano-microspheres prepared in Example 2 of the present invention; Figure 4 FTIR spectrum of the cationic copolymer prepared in Example 9 of the present invention; Figure 5 Scanning electron microscope photograph of the nano-microspheres prepared in Example 9 of the present invention; Figure 6 Schematic diagram of the test device used in Experimental Example 1 of the present invention. Detailed implementation manners

[0031] The present invention will be further described below in conjunction with examples and drawings.

[0032] The raw materials and equipment used in each example, comparative example and experimental example are all common commercially available raw materials and equipment in the art, and their specific sources will not be elaborated here.

[0033] Example 1 A cationic copolymer is prepared by grafting and growing the polymerization of DMDAAC and DMAEMA monomers on the main chain formed by the ring-opening polymerization of propylene oxide. The molar ratio of DMDAAC, propylene oxide and DMAEMA is 4.6:1.9:3.3, and the average molecular weight of the cationic copolymer is 18000 Da - 22000 Da; A kind of nano-microspheres prepared by electrostatic adsorption of Na-PVS and the above cationic copolymer.

[0034] The preparation methods of the above cationic copolymer and nano-microspheres are as follows: S1. Dissolve 0.02 g of AIBN initiator in 5 mL of THF at room temperature to obtain an initiator solution; S2. Dissolve 0.1 g of potassium hydroxide in 5 mL of DCM, then add 1.12 g of propylene oxide and heat and stir to cause ring-opening polymerization to form an alkoxide anion, which generates a long-chain polymer as the main chain through continuous ring-opening reactions with monomer molecules during the growth stage; S3. Dissolve 7.5 g of DMDAAC, 5.2 g of DMAEMA, 0.12 g of CTPPA and 0.05 g of EDAC in 5 mL of DCM, pour the mixed solution prepared in S1 and S2 into it and mix well. After deoxygenating by passing nitrogen, carry out a copolymerization reaction by stirring at 60 °C to obtain a cationic copolymer.

[0035] During the reaction, the DCM solvent evaporated upon heating, and finally the remaining solvent was THF. The solution of the reaction product was diluted with THF to a concentration of 0.3% to obtain a cationic copolymer solution for standby. After pressing the cationic copolymer with KBr, its functional groups were analyzed using an infrared spectrometer (FT-IR). The obtained infrared spectrum is as shown in Figure 1 shown. As can be seen from Figure 1 , the peaks at 2890 cm -1 (-C-H), 2830 cm -1 (-CH2-OH), and 1762 cm -1 (-CHOH) are characteristic vibration peaks of the polypropylene glycol chain segment and the poly(dimethylaminoethyl methacrylate) (PDMAEMA) chain segment; meanwhile, the vibration peak at 1050 -1 - 1150 cm -1 also proves the existence of the ether group (C-O-C) on the polypropylene glycol chain segment; the vibration peaks at 1150 - 1350 cm -1 and 1500 - 1700 cm -1 prove the existence of the quaternary ammonium functional group on the poly(dimethyldiallylammonium chloride) (PDMDAAC). From the infrared analysis results, it can be seen that the cationic copolymer was successfully prepared in this example; the reaction product was dried to form a white powder solid, and a 0.2% sample was prepared with a deuterium oxide solution for hydrogen nuclear magnetic resonance scanning (H-NMR). The results are as shown in Figure 2 shown.

[0036] S4. Prepare a 0.075% Na-PVS solution with deionized water for standby. Take 15 mL of the Na-PVS solution and stir it at a speed of 800 rpm at room temperature. Take 4 mL of the cationic copolymer solution prepared in step S3 and add it dropwise at a speed of 60 drops / min. After an electro-adsorption reaction under stirring, a nano-microsphere dispersion is obtained. After concentrating and washing the nano-microspheres by centrifugation, a 0.035 wt% low-concentration nano-microsphere dispersion is prepared. The solvent in the dispersion is deionized water. Add the non-ionic stabilizer Pluronic F-127 to the dispersion and make its final concentration 0.02 wt%, then it can be stored for a long time, and the storage time exceeds 6 months.

[0037] The nano-microspheres prepared in this example have a particle size of 200 nm - 400 nm. Example 2 A cationic copolymer is prepared by copolymerizing DMDAAC, NVP, propylene oxide, and FDA. It is prepared by grafting and growing the polymerization of DMDAAC, FDA, and DMAEMA on the main chain formed by the ring-opening polymerization of propylene oxide. The molar ratio of DMDAAC, FDA, propylene oxide, and DMAEMA is 2.2:0.11:1.9:3.3, and the average molecular weight of the cationic copolymer is 12000 - 13500 Da; A kind of nano - microspheres prepared by electrostatic adsorption of Na - PVS and the above cationic copolymer.

[0038] In this embodiment, in the preparation methods of the cationic copolymer and the nano - microspheres, the difference from Example 1 is that In step S3, 3.6 g of DMDAAC and 0.9 g of FDA are used instead of 7.5 g of DMDAAC in Example 1; In step S4, 12 mL of a Na - PVS solution with a concentration of 0.09% is taken for the reaction to prepare nano - microspheres, and the concentration of the low - concentration dispersion of the nano - microspheres is 0.04 wt%; Other conditions are the same as those in Example 1. The solution of the reaction product is diluted to a concentration of 0.3% with THF to obtain a cationic copolymer solution for standby. After pressing the cationic copolymer into a KBr tablet, its functional groups are analyzed using an infrared spectrometer, and the obtained infrared spectrum is as Figure 1 shown. The reaction product is dried to form a white powder solid, and a 0.2% sample is prepared with a deuterium oxide solution for hydrogen nuclear magnetic resonance scanning (H - NMR), and the results are as Figure 3 shown.

[0039] Example 3 The first kind of cationic copolymer is prepared by the S1 - S3 steps of Example 1 from DMDAAC, propylene oxide, and DMAEMA. The molar ratio of DMDAAC, propylene oxide, and DMAEMA is 4.6:1.9:3.3, and the average molecular weight of the cationic copolymer is 18000 Da - 22000 Da; The second kind of cationic copolymer is prepared by the S1 - S3 steps of Example 2 from DMDAAC, FDA, propylene oxide, and DMAEMA. The molar ratio of DMDAAC, NVP, propylene oxide, and FDA is 2.2:0.11:1.9:3.3, and the average molecular weight of the cationic copolymer is 12000 - 13500 Da.

[0040] A kind of nano - microspheres prepared by electrostatic adsorption of Na - PVS and the above two kinds of cationic copolymers.

[0041] In this embodiment, in the preparation method of the nano - microspheres, the difference from step S3 in Example 2 is that 2 mL of the first kind of cationic copolymer solution and 2.5 mL of the second kind of cationic copolymer solution are taken instead of 4 mL of the cationic copolymer solution in Example 2, and the concentration of the low - concentration dispersion of the nano - microspheres is 0.05 wt%; Other conditions are the same as those in Example 2.

[0042] Example 4 A kind of nano - microspheres prepared by cross - linking a cationic copolymer made from Na - PVS and the one prepared in Example 1, the preparation method is as follows: S1. Prepare a 0.075% Na - PVS solution for standby. Take 15 mL of the Na - PVS solution and stir it at a speed of 800 rpm at room temperature. Take 4 mL of the cationic copolymer solution prepared in step S2 in Example 1 (the molar ratio of DMDAAC, propylene oxide and DMAEMA is 4.6:1.9:3.3) and add it dropwise at a speed of 80 drops / min, then stir well. S2. After the suspension system is stable, add 0.5 mL of a 5% KCr(SO4)2 cross - linker dilution solution, and continue to stir for 3 hours to prepare the final nano - microsphere suspension. Concentrate and wash the nano - microspheres by centrifugation to prepare a 0.1 wt% low - concentration dispersion of nano - microspheres. Add the non - ionic stabilizer Pluronic F - 127 to make its concentration 0.02 wt% for long - term storage.

[0043] Example 5 This example provides a cationic copolymer and nano - microspheres prepared therefrom. The difference in the preparation method from Example 1 is that 4.7 g of TPA is used instead of 5.2 g of DMAEMA, and the molar ratio of the reaction monomers DMDAAC, propylene oxide and TPA is 4.6:1.9:0.88, and other conditions are the same as those in Example 1.

[0044] Example 6 This example provides a cationic copolymer and nano - microspheres prepared therefrom. The difference in the preparation method from Example 1 is that 4.3 g of HFBMA is used instead of 5.2 g of DMAEMA, and the molar ratio of DMDAAC, propylene oxide and HFBMA is 4.6:1.9:1.42, and other conditions are the same as those in Example 1.

[0045] Example 7 This example provides a cationic copolymer and nano - microspheres prepared therefrom. The difference in the preparation method from Example 1 is that in step S3, DMDAAC is not added, and the molar ratios of other monomers remain unchanged.

[0046] Example 8 This example provides a cationic copolymer and nano - microspheres prepared therefrom. The difference in the preparation method from Example 1 is that in step S3, DMAEMA is not added.

[0047] Example 9 A cationic copolymer is prepared by copolymerizing DMDAAC, propylene oxide, and DMAEMA. The molar ratio of DMDAAC, propylene oxide, and DMAEMA is 4.6:1.9:3.3, and the average molecular weight of the cationic copolymer is 18,000 Da - 22,000 Da; A kind of nano - microspheres prepared by the electrostatic adsorption of Na - PVS and the above - mentioned cationic copolymer.

[0048] The preparation methods of the above - mentioned cationic copolymer and nano - microspheres are as follows: S1. Dissolve 0.02 g of AIBN initiator in 5 mL of THF at room temperature to obtain an initiator solution; S2. Dissolve 7.5 g of DMDAAC, 1.12 g of propylene oxide, and 5.2 g of DMAEMA in 10 mL of DCM. Add the initiator solution prepared in S1 and mix well. After purging with nitrogen to remove oxygen, carry out a stirring copolymerization reaction at 60 °C to obtain a cationic copolymer.

[0049] During the reaction process, the DCM solvent evaporates due to heat, and the final remaining solvent is THF. Dilute the solution of the reaction product with THF to a concentration of 0.3% to obtain a cationic copolymer solution for standby. After pressing the cationic copolymer with KBr tablets, use an infrared spectrometer to analyze its functional groups. The obtained infrared spectrum is as Figure 4 shown. From Figure 4 it can be seen that the peaks at 2890 cm -1 (-C - H), 2830 cm -1 (-CH2 - OH), and 1762 cm -1 (-CHOH) are the characteristic vibration peaks of the polypropylene glycol chain segment and the poly(dimethylaminoethyl methacrylate) (PDMAEMA) chain segment; at the same time, the vibration peaks at 1050 -1 -1150 cm -1 also prove the existence of the ether group (C - O - C) on the polypropylene glycol chain segment; the vibration peaks at 1500 -1 -1700 cm -1 prove the existence of the quaternary ammonium functional group on the poly(dimethyldiallylammonium chloride) (PDMDAAC). From the infrared analysis results, it can be known that the cationic copolymer is successfully prepared in this example; S3. Prepare a 0.075% Na-PVS solution with deionized water for later use. Take 15 mL of the Na-PVS solution and stir it at 800 rpm at room temperature. Take 4 mL of the cationic copolymer solution prepared in step S2 and add it dropwise at a rate of 60 drops / min. After an electroadsorption reaction under stirring, a nano-microsphere dispersion is obtained. After concentrating and washing the nano-microspheres by centrifugation, a 0.035 wt% low-concentration nano-microsphere dispersion is prepared. The solvent in the dispersion is deionized water. Add the nonionic stabilizer Pluronic F-127 to the dispersion and make its final concentration 0.02 wt%, then it can be stored for a long time, and the storage time exceeds 6 months.

[0050] As Figure 5 shown, the nano-microspheres prepared in this example have a particle size of 200 nm - 400 nm. Example 10 A cationic copolymer is prepared by copolymerizing DMDAAC, NVP, propylene oxide, and FDA. The molar ratio of DMDAAC, NVP, propylene oxide, and FDA is 12.3:19.3:4.5:1.2, and the average molecular weight of the cationic copolymer is 11000 Da - 14500 Da. A kind of nano-microspheres prepared by electroadsorption of Na-PVS and the above cationic copolymer.

[0051] In this example, in the preparation methods of the cationic copolymer and the nano-microspheres, the differences from Example 9 are as follows. In step S2, use 2.5 g of DMDAAC and 5 g of NVP to replace 7.5 g of DMDAAC in Example 1, and use 5 g of FDA to replace 5.2 g of DMAEMA in Example 1. In step S3, take 12 mL of a 0.09% Na-PVS solution for reaction to prepare nano-microspheres, and the concentration of the low-concentration nano-microsphere dispersion is 0.04 wt%. Other conditions are the same as those in Example 9.

[0052] Example 11 The first cationic copolymer is prepared by copolymerizing DMDAAC, propylene oxide, and DMAEMA. The molar ratio of DMDAAC, propylene oxide, and DMAEMA is 4.6:1.9:3.3, and the average molecular weight of the cationic copolymer is 18000 Da - 22000 Da. Its preparation method is the same as steps S1 and S2 of Example 9. The second cationic copolymer is prepared by copolymerizing DMDAAC, NVP, propylene oxide and FDA. The molar ratio of DMDAAC, NVP, propylene oxide and FDA is 12.3:19.3:4.5:1.2. The average molecular weight of the cationic copolymer is 11,000 Da - 14,500 Da. Its preparation method is the same as steps S1 and S2 of Example 9.

[0053] A kind of nano - microspheres prepared by electrostatic adsorption of Na - PVS and the above two cationic copolymers.

[0054] In this example, in the preparation method of the nano - microspheres, the difference from step S3 in Example 10 is that Take 2 mL of the first cationic copolymer solution and 2.5 mL of the second cationic copolymer solution to replace the 4 mL cationic copolymer solution in Example 10. The concentration of the low - concentration dispersion of the nano - microspheres is 0.05 wt%. Other conditions are the same as those in Example 10.

[0055] Example 12 A kind of nano - microspheres prepared by cross - linking Na - PVS and the cationic copolymer prepared in Example 9. The preparation method is as follows: S1. Prepare a 0.075% Na - PVS solution for standby. Take 15 mL of the Na - PVS solution and stir it at a speed of 800 rpm at room temperature. Take 4 mL of the cationic copolymer solution prepared in step S2 of Example 1 (the molar ratio of DMDAAC, propylene oxide and DMAEMA is 4.6:1.9:3.3) and add it dropwise at a speed of 80 drops / min, and then stir well. S2. After the suspension system is stable, add 0.5 mL of a 5% KCr(SO4)2 cross - linker dilution solution, and continue to stir for 3 hours to prepare the final nano - microsphere suspension. Concentrate and wash the nano - microspheres by centrifugation to prepare a 0.1 wt% low - concentration dispersion of the nano - microspheres, and add the non - ionic stabilizer Pluronic F - 127 to make its concentration 0.02 wt% for long - term preservation.

[0056] Example 13 This example provides a cationic copolymer and the nano - microspheres prepared therefrom. The difference in the preparation method from Example 9 is that 4.7 g of TPA is used instead of 5.2 g of DMAEMA. The molar ratio of the reaction monomers DMDAAC, propylene oxide and TPA is 4.6:1.9:0.88, and other conditions are the same as those in Example 9.

[0057] Example 14 This example provides a cationic copolymer and the nanospheres prepared therefrom. The difference in the preparation method from Example 9 is that 5.7 g of FDA is used instead of 5.2 g of DMAEMA, and the molar ratio of DMDAAC, propylene oxide, and FDA is 4.6:1.9:1.36, and other conditions are the same as those in Example 9.

[0058] Example 15 This example provides a cationic copolymer and the nanospheres prepared therefrom. The difference in the preparation method from Example 9 is that DMDAAC is not added in step S2, and the molar ratios of other monomers remain unchanged.

[0059] Example 16 This example provides a cationic copolymer and the nanospheres prepared therefrom. The difference in the preparation method from Example 9 is that DMAEMA is not added in step S2.

[0060] Comparative Example 1 The difference between this comparative example and Example 1 is that the content of propylene oxide monomer in step S2 is changed, so that the molar ratio of monomers DMDAAC, propylene oxide, and DMAEMA is 4.6:3.1:3.3, and the average molecular weight of the cationic copolymer is 16000 Da - 22000 Da.

[0061] Comparative Example 2 A nanoparticle emulsifier with aminopropyltriethoxysilane (APTES) is used to improve the stability of the CO2 foam dispersion system. The preparation process is from a published paper (Ning Chuang, Zhang Liwei, Tong Changbing, Jing Zhengzheng. Modification of nano-SiO2 particles to improve the stability of CO2 foam. Contemporary Chemical Industry, 2024, 53(07): 1550 - 1554.). The preparation method of the above nanoparticle emulsifier with aminopropyltriethoxysilane (APTES) is as follows: S1. Add 2.5 g of APTES to the suspension of 2.5 g of silica powder in 150 mL of toluene. Then stir the suspension at room temperature for 12 h; after that, separate the solid by centrifugation (3000 r / min, centrifugation time 10 min), and then wash it twice with ethanol (50 mL); finally, put it into an oven (1 h) and dry it at 70 °C to obtain surface-modified silica nanoparticles called modified silica (MS) nanoparticles.

[0062] S2. Before the experiment, nanomaterials (silica, modified silica or ZnO) with a pre-specified mass and SDS were added to a specific amount of distilled water, and then sonicated with an ultrasonic probe for 30 min to obtain a homogeneous nanofluid; the stability of the nanofluid was studied using the dynamic light scattering (DLS) method and the HORIBA Zetasizer SZ-100 ζ potential analyzer.

[0063] Comparative Example 3 A bifunctional modified CO2-responsive particulate emulsifier is used in the supercritical CO2 emulsification process. The preparation process is from a published paper (Li Xixi, Han Xia, Liu Honglai. Preparation and Emulsifying Properties of CO2-Responsive Particulate Emulsifiers. China Adhesives, 2019, 28(06): 1-4+17.).

[0064] The preparation method of the above bifunctional modified CO2-responsive particulate emulsifier is as follows: S1. Take 1.0 g of fumed silica particles and place them in a flask. After drying in a vacuum drying oven at 100 °C for 12 h, add 20 mL of anhydrous toluene pre-dried with 4A molecular sieve, and sonicate for 30 min to uniformly disperse the particles in toluene; S2. Add 1.0 g of tetramethylguanidinopropyltrimethoxysilane and 1.0 g of cetyltrimethoxysilane to the toluene dispersion of silica simultaneously, and react for 12 h under an oil bath at 110 °C in a nitrogen atmosphere to obtain a bifunctional modified CO2-responsive particulate emulsifier.

[0065] Experimental Example 1 Use Figure 6 The shown test device was used to dry the nanospheres prepared in Examples 1-16 and Comparative Examples 1-3, and then perform cationic polymer loading rate and encapsulation rate test experiments, and measure the particle size of the nanosphere samples using a Malvern Zetasizer Nano LAB dynamic light scattering instrument. The results are shown in Table 1.

[0066] Table 1 Measured data

[0067] In the table, the calculation methods of the cationic copolymer loading rate and the cationic copolymer encapsulation rate are as follows: ; 。

[0068] As can be seen from the data in Table 1, the copolymer embedding rates of Examples 1-7 are all over 35%. In Comparative Example 1, due to the presence of a long polyoxypropylene bond in the copolymer long chain and the low water solubility of the polymer, the embedding rate is only 25.7%. The water solubility of FDA added in Example 6 and Example 14 is relatively low, and it is difficult to closely entangle with the anionic polymer during the preparation of microspheres. Therefore, the loading rates are relatively low (37.3%, 26.4%), the embedding rates are also relatively low (35%, 31.4%), and the particle sizes of the formed microspheres are relatively small, being 256.4 nm and 210.2 mm. Except for Examples 6 and 14, the dispersion particle sizes of other examples in 0.02 wt% Pluronic F-127 solution are in the range of 298.9-476.8 nm.

[0069] Experimental Example 2 Test the particle sizes of the nano-microspheres for deep in-situ self-emulsifying profile control in supercritical CO2 flooding after being dispersed in deionized water, 1% NaCl, 0.02% Pluronic F-127, and 0.02% Pluronic F-127 + 1% NaCl solution for 6 hours to reach equilibrium. The nano-microspheres provided in Preparation Examples 1-6 were respectively placed in different solutions and waited for 6 hours to swell and reach equilibrium, and the hydrodynamic diameters of the samples were measured using a Malvern dynamic light scattering instrument (Zetasizer Lab). The obtained experimental data are shown in Table 2.

[0070] Table 2 Particle Sizes of Nano-Microspheres

[0071] In Table 2, the unit of each data is nm.

[0072] As can be seen from the data in Table 2, the particle sizes of Examples 1-8 and Comparative Example 1 have obvious changes after standing for 6 hours in different solution environments. The particle sizes of all samples become larger in 1% NaCl solution, mainly because the nano-microspheres undergo a swelling phenomenon. On the contrary, when 0.02% Pluronic F-127 is added, the particle sizes of the nano-microspheres are more stable, and the particle sizes do not change within 6 hours regardless of the presence of NaCl; when the NaCl concentration is 5%, 0.02% Pluronic F-127 still has the effect of stabilizing the nano-microspheres. Table 2 shows the particle sizes of Examples 1-8 and Comparative Example 1 in four different dispersion environments. Therefore, during the industrial implementation of the product, more preferably, when the salinity is in the range of 0-2%, the dosage of Pluronic F-127 is 0.02 wt%.

[0073] Experimental Example 3 Test the particle sizes of the nano-microspheres for in-situ deep self-emulsifying profile control in supercritical CO2 flooding after 6 hours of equilibrium dispersion in deionized water, 1% NaCl, 0.02% Pluronic F-127, and 0.02% Pluronic F-127 + 1% NaCl solution respectively. Place the nano-microspheres provided in Examples 9-16 into different solutions and wait for 6 hours to allow them to swell and reach equilibrium. Use a Malvern dynamic light scattering instrument (Zetasizer Lab) to measure the hydrodynamic diameter of the samples. The experimental data obtained are shown in Table 3.

[0074] Table 3 Particle Sizes of Nano-Microspheres

[0075] In Table 3, the unit of each data is nm.

[0076] From the data in Table 3, it can be seen that there are obvious changes in the particle sizes of Examples 9-16 after standing for 6 hours in different solution environments. The particle sizes of all samples increase in 1% NaCl solution, mainly because the nano-microspheres undergo a swelling phenomenon. On the contrary, when 0.02% Pluronic F-127 is added, the particle sizes of the nano-microspheres are more stable, and the particle sizes do not change within 6 hours regardless of the presence of NaCl. However, when the NaCl concentration is 5%, 0.02% Pluronic F-127 cannot stabilize the nano-microspheres, and its concentration needs to be increased to 0.05%. Table 3 shows the particle sizes of Examples 9-16 in four different dispersion environments. Therefore, during the industrial implementation of the product, according to the formation water salinity, Pluronic F-127 with a concentration of 0.02 wt% to 0.05 wt% can be used to regulate the swelling of nano-microspheres and the time for releasing cationic copolymers. More preferably, when the salinity is in the range of 0 - 0.02%, the dosage of Pluronic F-127 is 0.02 wt%, and when the salinity is in the range of 0.02 - 0.05%, the dosage of the stabilizer is 0.05 wt%.

[0077] Example 4 Test the particle sizes of the nano-microspheres for in-situ deep self-emulsifying profile control in supercritical CO2 flooding. Take the nano-microspheres obtained in Examples 1-16 and Comparative Example 1 and disperse them in a 0.02% Pluronic F-127 + 1% NaCl solution (concentration 15 mg / ml). Pass CO2 (20 ml / min) into the dispersion, and measure the changes in the particle sizes of the nano-microspheres at different temperatures after passing CO2.

[0078] Table 4 Data of Particle Size Changes

[0079] In Table 4, the unit of temperature is °C, the unit of particle size is mm, and the unit of Zeta is mV.

[0080] As shown in Table 4, Examples 1-6 show that as the CO2 introduction time increases, the stability of the nanospheres is inversely correlated with temperature. When the test temperature is 65°C and 90°C, the particle size of the nanospheres increases significantly after 6 hours of CO2 introduction, showing a swelling phenomenon. At 90°C, the particle size of some nanospheres could not be measured continuously after 6 hours of CO2 introduction. Therefore, according to the actual situation requirements, the temperature, pH sensitivity characteristics of the nanospheres and the CO2 introduction time can be used to design the controlled release time of the nanospheres. The particle size change of Example 7 was small after 18 hours of CO2 introduction at 90°C, and the particles remained intact, but it could not meet the ability to control the release of the cationic copolymer. In Comparative Example 1, it could not be stably dispersed, and the particle size could not be measured after 6 hours of CO2 introduction (the particles degraded). It can be seen from Table 4 that when the temperature is 50°C, Examples 1-3, Example 6 and Example 8 have a better temperature response time range; when the temperature is 65°C, Examples 4 and 5 have a better temperature response time range; when the temperature is 90°C, Example 7 has a better temperature response time range.

[0081] At the same time, Examples 9-14 show that as the CO2 introduction time increases, the stability of the nanospheres is inversely correlated with temperature. When the test temperature is 65°C and 90°C, the particle size of the nanospheres increases significantly after 6 hours of CO2 introduction, showing a swelling phenomenon. At 90°C, the particle size of some nanospheres could not be measured continuously after 6 hours of CO2 introduction. Therefore, according to the actual situation requirements, the temperature, pH sensitivity characteristics of the nanospheres and the CO2 introduction time can be used to design the controlled release time of the nanospheres. The particle size change of Example 15 was small after 18 hours of CO2 introduction at 90°C, and the particles remained intact, but it could not meet the ability to control the release of the cationic copolymer. It can be seen from Table 4 that when the temperature is 50°C, Examples 9-11, Example 14 and Example 16 have a better temperature response time range; when the temperature is 65°C, Examples 12 and 13 have a better temperature response time range; when the temperature is 90°C, Example 15 has a better temperature response time range.

[0082] Experimental Example 5 Using Figure 6 the device shown in the figure to measure the influence of the cationic copolymer and nanospheres prepared in Example 1 on the emulsification effect of supercritical CO2-deionized water at 65°C and 90°C under a pressure of 12 ± 1.5 MPa. A certain proportion of cationic polymer or nanospheres and a certain amount of H2O were placed in a high-temperature and high-pressure observation kettle. After introducing a certain proportion of supercritical CO2, the emulsification of CO2-deionized water was observed after stirring for 5 minutes and 120 minutes respectively.

[0083] Table 5 Emulsification situation

[0084] In Table 5, the unit of the injection volume is mL, the unit of the concentration is wt%, and the unit of the temperature is °C.

[0085] As shown in Table 5, after stirring for 5 minutes, the cationic copolymer can cause the self-emulsification of CO2-deionized water to form a stable emulsion at both low and high concentrations, and the emulsification effect is better at high concentrations (1.233% and 1.399%). The influence of temperature on the emulsification effect of the cationic polymer is not obvious. Since the nano-microspheres do not release free cationic polymers, they do not have strong emulsifying ability. When the stirring reaction lasts for 120 minutes, the emulsification reaction of Samples 5-7 appears, and Sample 8 is further completely emulsified, proving that the nano-microspheres have released cationic polymers between 5 minutes and 120 minutes.

[0086] Experimental Example 6 Adopt Figure 6 The device shown in the figure is used to measure the influence of the cationic copolymer and nano-microspheres prepared in Example 9 on the emulsification effect of supercritical CO2-deionized water at 65 °C and 90 °C under a pressure of 11 ± 0.7 MPa. A certain proportion of the cationic polymer or nano-microspheres and a certain amount of H2O are placed in a high-temperature and high-pressure observation kettle. After introducing a certain proportion of supercritical CO2, the emulsification situation of CO2-deionized water is observed respectively after stirring for 5 minutes and stirring for 120 minutes.

[0087] Table 6 Emulsification Situation

[0088] In Table 6, the unit of the injection volume is mL, the unit of the concentration is wt%, and the unit of the temperature is °C.

[0089] As shown in Table 6, after stirring for 5 minutes, the cationic copolymer can cause the self-emulsification of CO2-deionized water to form a stable emulsion at both low and high concentrations, and the emulsification effect is better at high concentrations (1.23% and 1.4%). The influence of temperature on the emulsification effect of the cationic polymer is not obvious. Since the nano-microspheres do not release free cationic polymers, they do not have strong emulsifying ability. When the stirring reaction lasts for 120 minutes, the emulsification reaction of Samples 5-7 appears, and Sample 8 is further completely emulsified, proving that the nano-microspheres have released cationic polymers between 5 minutes and 120 minutes.

[0090] Experimental Example 7 A supercritical CO2 flooding deep in-situ self-emulsifying nano-microsphere prepared by using Examples 1-16 and Comparative Examples 1-3 is used for a supercritical CO2 flooding oil simulation experiment on a tight reservoir, and the oil recovery rate is calculated by collecting the produced fluid. The specific experimental process is as follows: The experimental temperature was 70°C. The core used in the experiment was a cylindrical tight core from the Chang 8 reservoir in Hongde, Huanjiang, Ordos Basin (with a cross-sectional diameter of 3.5 cm and a length of 15 cm), and its permeability was 21 mD ± 0.1 mD. The core was saturated with oil. The crude oil used in the experiment was the produced crude oil from the Seventh Oil Production Plant of PetroChina Changqing Oilfield Company (obtained from the three-phase separation at the Huan Silian water injection station), and the backpressure was 6.1 MPa at a temperature of 70°C.

[0091] First, the core was subjected to supercritical CO2 flooding until the pressures at both the inlet and outlet ends were stable (about 2.5 PV) and no more crude oil flowed out; then, 1 PV of the nano-microspheres (0.2% wt) prepared in Examples 1-16 and Comparative Example 1 was injected, and the model was sealed at both ends and left static for 24 hours; finally, supercritical CO2 was injected again until no more crude oil flowed out.

[0092] Table 7 Crude oil recovery

[0093] As shown in Table 7, the recovery rate of the pre-injected supercritical CO2 flooding was 53.1 - 62.3%. This fluctuation was due to the inconsistent results of saturated crude oil and primary displacement at the beginning of each experiment, and this fluctuation was within the normal experimental range. When the CO2 bifunctional polyelectrolyte nanoparticles prepared in Examples 1-16 were injected with supercritical CO2, a small amount of crude oil was produced (0 - 3.41%). This was mainly because after entering the core, the nano-microspheres adsorbed on the rock surface, improving the flow ability of supercritical CO2. When the core was sealed for 24 hours, some of the CO2 bifunctional polyelectrolyte nanoparticles underwent shell-core separation changes, further improving the oil displacement ability. The recovery rate in this process for Examples 1-16 could reach 1.98 - 8.42%. For Comparative Example 2 and Comparative Example 3, since they both belonged to the surface modification of SiO2 nanoparticles, their particles were not elastic and had a large particle size, so it was difficult to enter the core pores. Only 0.23 PV and 0.11 PV were injected respectively, and then the injection could not continue due to the too rapid increase in pressure. From the change in the crude oil recovery rate of the two-stage supercritical CO2 flooding, it can be seen that the supercritical CO2 flooding deep in-situ self-emulsifying nano-microspheres proposed in the present invention have good effects in improving CO2 flooding in tight oil reservoirs.

[0094] The above are only specific embodiments of the present invention, and the scope of the invention cannot be limited by them. Therefore, the replacement of equivalent components or the equivalent changes and modifications made according to the scope of protection of the present invention should still fall within the scope covered by this patent. In addition, the technical features in the present invention can be freely combined with each other between technical features, between technical features and technical inventions, and between technical inventions.

Claims

1. A cationic copolymer is prepared by grafting and growing polymerization of monomer A on a long chain formed by the growth of monomer B. The average molecular weight of the cationic copolymer is 1000 Da - 25000 Da. The molar ratio of monomer A to monomer B is (3 - 8):(1.5 - 2.5). Among them, Monomer A is selected from at least one of dimethyldiallylammonium chloride, dimethylaminoethyl methacrylate, vinylpyrrolidone, 1H,1H,2H,2H - perfluorodecyl acrylate, perfluoroacrylate, hexafluorobutyl methacrylate, or cationic acrylamide, and monomer B is propylene oxide.

2. The cationic copolymer according to claim 1, characterized in that, The molar ratio of monomer A to monomer B is (3.5 - 6):(2 - 2.5).

3. The cationic copolymer according to claim 1, wherein Monomer A is selected from dimethyldiallylammonium chloride, dimethylaminoethyl methacrylate, or vinylpyrrolidone; further preferably, monomer A is selected from dimethyldiallylammonium chloride or dimethylaminoethyl methacrylate.

4. The cationic copolymer according to any one of claims 1-3, characterized in that, The preparation method of the cationic copolymer is as follows: Step 1: Dissolve the initiator in the first organic reagent at room temperature to obtain an initiator solution. Step 2: Dissolve the alkali metal hydroxide and monomer B in the second organic reagent and carry out ring - opening polymerization to form an alkoxide anion, which generates a long - chain polymer through continuous ring - opening reactions with monomer B during the growth stage as the main chain. Step 3: Dissolve monomer A, the chain transfer agent, and the esterification reaction catalyst in the second organic reagent, add the organic reagent mixture prepared in Step 1 and Step 2, mix well, remove oxygen by passing nitrogen, and then heat and stir for copolymerization reaction to obtain the cationic copolymer.

5. The cationic copolymer according to claim 4, characterized in that, The initiator is selected from at least one of ammonium persulfate, 4,4 - azobis(4 - cyanovaleric acid), and azobisisobutyronitrile. The first organic reagent is selected from any one of dichloromethane (DCM), cyclohexane, heptane (HPT), octane, or kerosene. The second organic reagent is tetrahydrofuran (THF). The reaction temperature in Step 1 is 45°C - 65°C. The reaction temperature in Step 3 is 45 - 80°C. The mass ratio of the initiator to monomer B is (0.02 - 0.12):

1.

6. A nano - microsphere is prepared by reacting an anionic polymer and at least one cationic copolymer provided in Claims 1 - 5. The molar ratio of the anionic polymer to the cationic copolymer is (1.28 - 0.69):

1. The reaction includes electrostatic adsorption and cross - linking. The anionic polymer is selected from at least one of sodium polyvinyl sulfate, polyacrylic acid, polyanionic cellulose, or carboxymethyl cellulose. The average particle size of the nano - microsphere is 150 nm - 600 nm.

7. The preparation method of the nano - microsphere according to Claim 6, wherein the cationic copolymer and the anionic polymer are respectively added into a solvent to obtain a cationic dispersion and an anionic dispersion, the cationic dispersion is added dropwise to the anionic dispersion, and after the reaction, the product is concentrated and washed to obtain the nano - microsphere.

8. The preparation method according to claim 7, characterized in that, Deionized water is selected as the solvent in the cationic dispersion; deionized water is selected as the solvent in the anionic dispersion; further preferably, the mass concentration of the cationic copolymer in the cationic dispersion is 0.01 wt% - 1.10 wt%, and the mass concentration of the anionic polymer in the anionic dispersion is 0.01 wt% - 1.50 wt%; preferably, the dropping rate is 60 - 90 drops / min.

9. The preparation method according to claim 7, characterized in that, The preparation method further includes putting the nano - microspheres into a stabilizer solution to obtain a nano - microsphere dispersion with a mass concentration of 0.01 wt% - 1.50 wt% for standby; further preferably, a Poloxmar - type non - ionic surfactant is selected as the stabilizer; more preferably, the concentration of the stabilizer in the stabilizer solution is 0.02 wt% - 0.2 wt%, and deionized water is selected as the solvent in the stabilizer solution.

10. Application of nano - microspheres in enhancing oil recovery in supercritical CO2 flooding The nano - microspheres are provided by any one of claims 6 or 7, or the nano - microspheres are prepared by the preparation method described in any one of claims 8 or 9. In the application, the nano - microspheres are put into supercritical CO2 and injected into the oilfield reservoir together. There are channeling channels in the oilfield reservoir; preferably, the mass ratio of the nano - microspheres to supercritical CO2 is (0.05 - 4.5):100; further preferably, the input method of the nano - microspheres is direct input and / or input in the form of a nano - microsphere dispersion.

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