A thickener for oilfield fracturing fluid and its preparation method

Abstract

This invention relates to the field of oilfield chemical additives technology, and discloses an oilfield fracturing fluid thickener and its preparation method. The thickener is polymerized from the following monomers in parts by weight: 30-40 parts acrylamide, 10-20 parts 2-acrylamide-2-methylpropanesulfonic acid, 4-8 parts N-vinylpyrrolidone, 0.6-1 parts hydrophobic associating monomer, and 0.5-0.8 parts double-modified nano-silica. The process of this invention is mild and controllable, and the resulting thickener possesses excellent temperature and salt resistance, shear resistance, and rapid solubility. It has low levels of water-insoluble matter and gelling residue, resulting in minimal formation damage. It is suitable for fracturing operations in deep, high-temperature, and high-salinity oil reservoirs and is easily industrialized.

Description

Technical Field

[0001] This invention relates to the field of oilfield chemical additives technology, specifically to an oilfield fracturing fluid thickener and its preparation method. Background Technology

[0002] In the development of low-permeability and ultra-low-permeability oil and gas reservoirs, hydraulic fracturing is a core technology for increasing oil and gas well production and injection. The performance of fracturing fluid directly determines the success or failure of fracturing operations, and thickener is the core component of fracturing fluid. Its main functions are to increase viscosity, carry sand, and reduce filtration loss. At the same time, it must have good temperature and salt resistance, shear resistance, low damage, and easy flowback performance.

[0003] Currently, commonly used fracturing fluid thickeners in oilfields are mainly divided into two categories: natural plant gums and their modified products, and synthetic polymers. Natural plant gums, represented by guar gum and hydroxypropyl guar gum, have good thickening effects and are widely used. However, they suffer from drawbacks such as high water-insoluble content, excessive residue after breaking down, and significant damage to the formation and fracture conductivity. Furthermore, their temperature resistance limit is generally no more than 120℃, which cannot meet the fracturing requirements of deep, high-temperature oil and gas reservoirs. Synthetic polymers, represented by partially hydrolyzed polyacrylamide, have low residue content and moderate cost. However, they suffer from poor temperature and salt resistance. Under high temperature and high salinity conditions, the molecular chains are prone to hydrolysis, degradation, and coiling, leading to a significant decrease in system viscosity and loss of proppant carrying capacity. Simultaneously, their shear resistance is insufficient; under high-speed shear, the molecular chains break, resulting in irreversible viscosity loss.

[0004] In recent years, hydrophobic associating polymers have become a research hotspot for fracturing fluid thickeners. These polymers enhance thickening capacity and shear resistance by introducing small amounts of hydrophobic monomers into the molecular chain to form a hydrophobic associating network in aqueous solution. However, existing hydrophobic associating thickeners generally suffer from high hydrophobic monomer content, poor water solubility, long dissolution time, and difficulty in on-site preparation. Furthermore, their temperature and salt resistance still cannot meet the fracturing requirements of deep, high-temperature, and high-salt reservoirs above 160℃. Therefore, developing a fracturing fluid thickener that combines excellent temperature and salt resistance, shear resistance, low damage, and rapid dissolution has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide an oilfield fracturing fluid thickener and its preparation method, thereby solving the problems of poor temperature and salt resistance, insufficient shear resistance, high residue content, and significant formation damage of existing thickeners.

[0006] To achieve the above objectives, the present invention provides the following technical solution: an oilfield fracturing fluid thickener, polymerized from monomers comprising the following parts by weight: Acrylamide 30-40 parts; 10-20 parts of 2-acrylamide-2-methylpropanesulfonic acid; 4-8 parts of N-vinylpyrrolidone; 0.6-1 part of hydrophobic associating monomer 0.5-0.8 parts of double-modified nano-silica; Acrylamide, as the main monomer for polymerization, provides the basic molecular chain skeleton for the polymer, ensuring the thickener's basic thickening ability and water solubility; 2-acrylamide-2-methylpropanesulfonic acid contains a strong anionic sulfonic acid group in its molecular chain, which has strong hydration ability and large steric hindrance of the sulfonic acid group, making it less prone to hydrolysis and coiling under high temperature and high salinity conditions, thus significantly improving the polymer's temperature and salt resistance; N-vinylpyrrolidone contains a five-membered heterocyclic structure with high rigidity, which can introduce steric hindrance into the molecular chain, inhibiting the molecular chain breakage and degradation at high temperature, further improving the polymer's thermal stability, and at the same time improving the polymer's solubility. The dual-modified nano-silica consists of nano-silica particles with dual surface modifications of sulfonic acid groups and amino groups. On the one hand, nano-silica itself has extremely high rigidity and strength, and can be incorporated into polymer molecular chains through physical entanglement and chemical bonding to form an organic-inorganic interpenetrating network structure, which greatly improves the mechanical strength and thermal stability of polymer molecular chains and inhibits the breakage and degradation of molecular chains under high temperature and high shear conditions. On the other hand, the surface-modified sulfonic acid groups can further improve the hydration dispersibility and salt resistance of nanoparticles, and the amino groups can form hydrogen bonds with amide groups and sulfonic acid groups in polymer molecular chains to strengthen the stability of the network structure. At the same time, the amino groups can coordinate with the crosslinking agents in the fracturing fluid system to improve the crosslinking performance and further enhance the system's temperature resistance and shear resistance.

[0007] Furthermore, the method for preparing the hydrophobic associating monomer is as follows: S1. Polyvinyl alcohol is dissolved in dimethyl sulfoxide, then 4-formylbenzo15-crown-5 and p-toluenesulfonic acid catalyst are added, and the reaction is carried out at 75-80℃ for 8-10 h. After the reaction is completed, precipitation is carried out in anhydrous ethanol, and the precipitate is placed in a Soxhlet extractor. The precipitate is extracted with ethanol as the extractant for 10-12 hours. The precipitate is dried to obtain crown ether modified polyvinyl alcohol. S2. Add crown ether modified polyvinyl alcohol and undecenoic acid to N,N-dimethylformamide solvent, stir and mix, continue to add p-toluenesulfonic acid catalyst, react at 80-85℃ for 4-6h, after which vacuum distillation, wash and dry to obtain hydrophobic associating monomer; The hydrophobic associative monomer is a crown ether-functionalized hydrophobically modified polyvinyl alcohol (PVA), prepared through a two-step reaction: First, the crown ether group is grafted onto the PVA molecular chain by reacting the hydroxyl groups of PVA with the aldehyde group of 4-formylbenzo-15-crown-5; second, the remaining hydroxyl groups of PVA undergo esterification with the carboxyl group of undecenoic acid, introducing polymerizable carbon-carbon double bonds and a long-chain hydrophobic alkyl structure. On the one hand, the crown ether group possesses a unique cavity structure, enabling it to react with metal cations in aqueous solution (especially Na+). + K + Ca 2+ Mg 2+ The thickener undergoes specific complexation with scale-forming cations, shielding the charge shielding effect of metal cations on polymer molecular chains in high-mineralization environments, significantly improving the salt resistance of the thickener. Simultaneously, it forms scale ions, reducing the risk of formation scaling. Furthermore, the long-chain hydrophobic alkyl structure can undergo hydrophobic association in aqueous solutions, forming a reversible three-dimensional network structure, significantly enhancing the system's thickening ability and shear resistance. Under high-speed shearing, the hydrophobic association structure can undergo reversible dissociation and reconstruction, avoiding irreversible viscosity loss. Additionally, the polyvinyl alcohol backbone contains a large number of hydroxyl groups, which can significantly improve the water solubility of hydrophobic monomers, solving the problems of poor water solubility and long dissolution time of traditional hydrophobic associating monomers.

[0008] Further, in S1, the ratio of polyvinyl alcohol, dimethyl sulfoxide, 4-formylbenzo-15-crown-5, and p-toluenesulfonic acid catalyst is 0.4-0.5g: 80-85mL: 0.6-0.71g: 0.01-0.02g.

[0009] Further, in S2, the ratio of N,N-dimethylformamide, crown ether modified polyvinyl alcohol, undecenoic acid, and p-toluenesulfonic acid catalyst is 50-55 mL: 0.3-0.4 g: 0.1-0.15 g: 0.02-0.03 g.

[0010] Furthermore, it includes the following steps: Weigh 10-20 parts by weight of 2-acrylamido-2-methylpropanesulfonic acid, 4-8 parts by weight of N-vinylpyrrolidone, and 0.5-0.8 parts by weight of double-modified nano-silica and add them to deionized water. Adjust the pH of the system to 7-8 with a pH adjuster. Then add 30-40 parts by weight of acrylamide, 0.6-1 parts by weight of hydrophobic associating monomer, and 0.2-0.3 parts by weight of sodium dodecyl sulfate. After purging with nitrogen, add 0.06-0.08 parts by weight of the initiator ammonium persulfate-sodium bisulfite system (ammonium persulfate and sodium bisulfite mass ratio 2:1). Stir and react in a constant temperature water bath for 3-4 hours to obtain an oilfield fracturing fluid thickener.

[0011] Furthermore, the pH adjuster is a sodium hydroxide solution with a mass concentration of 8-10%.

[0012] Furthermore, the nitrogen purging time is 10-15 minutes.

[0013] Furthermore, the temperature of the stirring reaction in the constant temperature water bath is 40-45℃.

[0014] Compared with the prior art, the present invention has the following beneficial technical effects: The thickener of this invention effectively inhibits the hydrolysis, coiling and degradation of polymer molecular chains under high temperature and high salinity conditions by utilizing the strong hydration ability of sulfonic acid groups, the specific complexing effect of crown ether groups on metal cations, the rigid steric hindrance effect of five-membered heterocyclic structures, and the thermal stability enhancement effect of nanoparticles. It solves the problem of significant viscosity loss of traditional thickeners under extreme formation conditions and can meet the fracturing construction requirements of deep, high temperature and high salinity oil and gas reservoirs.

[0015] This invention introduces crown ether-functionalized hydrophobic associating monomers to construct a reversible hydrophobic associating three-dimensional network in aqueous solution. Combined with the mechanical enhancement effect of organic-inorganic interpenetrating networks, the polymer system can achieve reversible dissociation and reconstruction of its structure under high-speed shear, avoiding irreversible viscosity loss caused by molecular chain breakage. It can stably adapt to the high-speed shear conditions during fracturing operations and ensure the system's sand-carrying capacity.

[0016] This invention effectively solves the problems of poor water solubility, long dissolution time, and fisheye formation in traditional hydrophobic associating polymers by introducing a hydroxyl-rich polyvinyl alcohol backbone into the hydrophobic associating monomer and combining the synergistic effect of strongly hydrated sulfonic acid groups and hydrophilic heterocyclic structures. It significantly shortens the curing time for on-site preparation, eliminates the need for complex preparation equipment and processes, and is suitable for the rapid construction needs of oilfield sites.

[0017] The thickener system of this invention has an extremely low water-insoluble content and leaves little residue after degumming, which reduces the blockage and damage to the conductivity of formation pores and fractures; the degumming fluid has good fluidity and is easy to return to the source, meeting the environmental protection and construction requirements of green oilfield development. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] 4-Formylbenzo15-crown-5, CAS: 60835-73-6.

[0020] Polyvinyl alcohol (PVA) degree of polymerization (1750), degree of alcoholysis 99%, Tianjin Guangfu Fine Chemical Co., Ltd.

[0021] The preparation of double-modified nano-silica is based on the following method of synthesis of double-modified nano-SiO2: (1) Add 10g of nano-SiO2 solution and 40mL of anhydrous ethanol to a three-necked flask equipped with a condenser, a constant pressure dropping funnel and a magnetic rotor, and stir for 10min to make it fully and evenly mixed; then add 1mol / L hydrochloric acid dropwise through the dropping funnel until the pH value of the solution is 4, and then slowly add 2% KH590 modifier. Stir for 5min at room temperature to make it evenly mixed in the solution, and then heat to 80℃ and react for 8h to obtain mercapto-modified nano-SiO2; after the excess solvent is evaporated by rotary evaporation, mix SiO2 powder with 30% hydrogen peroxide at a mass ratio of 1:10, and stir and oxidize for 24h under light-proof and room temperature conditions to obtain sulfonate-modified nano-SiO2. (2) After washing the sulfonated nano-SiO2 with ethanol multiple times, the excess ethanol was removed by rotary evaporation to obtain powder. The powder was suspended in an ethanol-water solution (ethanol and water volume ratio of 4:1), the pH was adjusted to 10 with triethanolamine, and 2% KH550 modifier was slowly added dropwise. The reaction conditions and purification process in (1) were repeated to finally obtain nano-SiO2 particles with dual surface modification of sulfonation and amination (S-ANS).

[0022] Example 1

[0023] An oilfield fracturing fluid thickener, polymerized from monomers comprising the following parts by weight: 40 parts acrylamide; 20 parts of 2-acrylamide-2-methylpropanesulfonic acid; 8 parts of N-vinylpyrrolidone; 1 part of hydrophobic associating monomer 0.8 parts of double-modified nano-silica.

[0024] The method for preparing the hydrophobic associating monomer is as follows: S1. Dissolve 0.5 g of polyvinyl alcohol in 85 mL of dimethyl sulfoxide, then add 0.71 g of 4-formylbenzo15-crown-5 and 0.02 g of p-toluenesulfonic acid catalyst, react at 80 °C for 10 h, precipitate in anhydrous ethanol, place the precipitate in a Soxhlet extractor, extract the precipitate with ethanol as the extractant for 12 hours, and dry to obtain crown ether modified polyvinyl alcohol; S2. Add 0.4 g of crown ether modified polyvinyl alcohol and 0.15 g of undecenoic acid to 55 mL of N,N-dimethylformamide solvent, stir and mix, then add 0.03 g of p-toluenesulfonic acid catalyst, react at 85 °C for 6 h, after which distill under reduced pressure, wash and dry to obtain hydrophobic associating monomer.

[0025] The preparation method of oilfield fracturing fluid thickener includes the following steps: Weigh 20 parts by weight of 2-acrylamide-2-methylpropanesulfonic acid, 8 parts by weight of N-vinylpyrrolidone, and 0.8 parts by weight of double-modified nano-silica and add them to 160 parts by weight of deionized water. Adjust the pH of the system to 8 with a 10% sodium hydroxide solution. Then add 40 parts by weight of acrylamide, 1 part by weight of hydrophobic associating monomer, and 0.3 parts by weight of sodium dodecyl sulfate. After purging with nitrogen for 15 minutes, add 0.06-0.08 parts by weight of the initiator ammonium persulfate-sodium bisulfite system (ammonium persulfate to sodium bisulfite mass ratio 2:1). Stir and react in a constant temperature water bath at 45℃ for 4 hours to obtain an oilfield fracturing fluid thickener.

[0026] Example 2 An oilfield fracturing fluid thickener, polymerized from monomers comprising the following parts by weight: 30 parts acrylamide; 10 parts of 2-acrylamide-2-methylpropanesulfonic acid; 4 parts of N-vinylpyrrolidone; 0.6 parts of hydrophobic associating monomer 0.5 parts of double-modified nano-silica.

[0027] The method for preparing the hydrophobic associating monomer is as follows: S1. Dissolve 0.4 g of polyvinyl alcohol in 80 mL of dimethyl sulfoxide, then add 0.6 g of 4-formylbenzo15-crown-5 and 0.01 g of p-toluenesulfonic acid catalyst, react at 75 °C for 8 h, precipitate in anhydrous ethanol, place the precipitate in a Soxhlet extractor, extract the precipitate with ethanol as the extractant for 10 h, and dry to obtain crown ether modified polyvinyl alcohol; S2. Add 0.3 g of crown ether modified polyvinyl alcohol and 0.1 g of undecenoic acid to 50 mL of N,N-dimethylformamide solvent, stir and mix, then add 0.02 g of p-toluenesulfonic acid catalyst, react at 80 °C for 4 h, after which distill under reduced pressure, wash and dry to obtain hydrophobic associating monomer.

[0028] The preparation method of oilfield fracturing fluid thickener includes the following steps: Weigh 10 parts by weight of 2-acrylamide-2-methylpropanesulfonic acid, 4 parts by weight of N-vinylpyrrolidone, and 0.5 parts by weight of double-modified nano-silica and add them to 150 parts by weight of deionized water. Adjust the pH of the system to 7 with 8% sodium hydroxide solution. Then add 30 parts by weight of acrylamide, 0.6 parts by weight of hydrophobic associating monomer, and 0.2 parts by weight of sodium dodecyl sulfate. After purging with nitrogen for 10 minutes, add 0.06 parts by weight of the initiator ammonium persulfate-sodium bisulfite system (ammonium persulfate to sodium bisulfite mass ratio 2:1). Stir and react in a constant temperature water bath at 40℃ for 3 hours to obtain the oilfield fracturing fluid thickener.

[0029] Example 3 An oilfield fracturing fluid thickener, polymerized from monomers comprising the following parts by weight: 35 parts acrylamide; 15 parts of 2-acrylamide-2-methylpropanesulfonic acid; 6 parts of N-vinylpyrrolidone; 0.8 parts of hydrophobic associating monomer 0.65 parts of double-modified nano-silica.

[0030] The method for preparing the hydrophobic associating monomer is as follows: S1. Dissolve 0.45 g of polyvinyl alcohol in 82 mL of dimethyl sulfoxide, then add 0.65 g of 4-formylbenzo15-crown-5 and 0.015 g of p-toluenesulfonic acid catalyst, react at 78 °C for 9 h, precipitate in anhydrous ethanol, place the precipitate in a Soxhlet extractor, extract the precipitate with ethanol as the extractant for 11 h, and dry to obtain crown ether modified polyvinyl alcohol; S2. Add 0.35 g of crown ether modified polyvinyl alcohol and 0.12 g of undecenoic acid to 53 mL of N,N-dimethylformamide solvent, stir and mix, then add 0.025 g of p-toluenesulfonic acid catalyst, react at 82 °C for 5 h, after which distill under reduced pressure, wash and dry to obtain hydrophobic associating monomer.

[0031] The preparation method of oilfield fracturing fluid thickener includes the following steps: Weigh 15 parts by weight of 2-acrylamide-2-methylpropanesulfonic acid, 6 parts by weight of N-vinylpyrrolidone, and 0.7 parts by weight of double-modified nano-silica and add them to 155 parts by weight of deionized water. Adjust the pH of the system to 8 with a 9% sodium hydroxide solution. Then add 35 parts by weight of acrylamide, 0.8 parts by weight of hydrophobic associating monomer, and 0.25 parts by weight of sodium dodecyl sulfate. After purging with nitrogen for 12 minutes, add 0.07 parts by weight of the initiator ammonium persulfate-sodium bisulfite system (ammonium persulfate to sodium bisulfite mass ratio 2:1). Stir and react in a constant temperature water bath at 42℃ for 4 hours to obtain an oilfield fracturing fluid thickener.

[0032] Comparative Example 1 The only difference between this comparative example and Example 3 is that no hydrophobic associating monomer is added; the other components, amounts, and preparation methods are the same as in Example 3.

[0033] Comparative Example 2 The only difference between this comparative example and Example 3 is that double-modified nano-silica is not added; the other components, amounts, and preparation methods are the same as in Example 3.

[0034] Comparative Example 3 The only difference between this comparative example and Example 3 is that an equal part by weight of octadecylacrylamide is used to replace the hydrophobic associating monomer, while the other components, amounts, and preparation methods are the same as in Example 3.

[0035] The performance of the thickener was tested in accordance with SY / T5107-2016 "Performance Evaluation Method of Water-based Fracturing Fluid" and SY / T6376-2020 "General Technical Conditions for Fracturing Fluid". 1. Test of apparent viscosity and dissolution time of base liquid Test steps: (1) Prepare a 0.5% thickener aqueous solution: Measure 200mL of distilled water into a stirring cup, slowly add the accurately weighed thickener while stirring at 1500r / min, and use a stopwatch to record the time from the addition of the thickener until the powder is completely dissolved, there are no fish eyes, and the solution is homogeneous. This is the complete dissolution time.

[0036] (2) The base liquid was kept at a constant temperature of 25℃ in a water bath for 30 min, and the temperature was measured by a rotational rheometer at 170 s. -1 Apparent viscosity was tested at the shear rate, and the average value was taken from three parallel tests.

[0037] 2. Temperature resistance and shear strength test Instruments and reagents: HAAKEMARS60 high temperature and high pressure rheometer, electronic balance, constant temperature water bath, industrial grade organoboron crosslinking agent.

[0038] Test steps: (1) Prepare a thickener base liquid with a mass fraction of 0.5%, add an organic boron crosslinking agent at a mass of 0.3% of the base liquid, stir evenly, and then crosslink at a constant temperature of 25°C for 30 minutes to form a fracturing fluid gel.

[0039] (2) Load the gel into the rheometer test tube, and set the test temperature to 160℃ and the shear rate to 170s. -1 Continuous shearing for 120 min was performed, and the initial viscosity and the viscosity after 120 min of shearing were recorded. The viscosity retention rate was then calculated. Viscosity retention rate = (Viscosity after 120 min of shearing / Initial viscosity) × 100% 3. Salt resistance test Instruments and reagents: Same as 1.1, simulated formation water (total mineralization 25 × 10⁻⁶). 4 mg / L, of which Ca 2+ Mg 2+ Total content 8000 mg / L, simulating the characteristics of formation water in deep, high-salinity oil reservoirs.

[0040] Test steps: (1) Prepare a thickener aqueous solution with a mass fraction of 0.6% using simulated formation water, stir until completely dissolved, and keep it in a constant temperature water bath at 25℃ for 30 minutes.

[0041] (2) at 170s -1 The apparent viscosity was tested at the shear rate, and the viscosity retention rate in the brine was calculated using the viscosity of a base solution of the same concentration prepared with distilled water as a benchmark. Viscosity retention rate = (Simulated formation water-based fluid viscosity / Distilled water-based fluid viscosity) × 100% 4. Water-insoluble matter content test Instruments and reagents: G4 sand core crucible, vacuum filtration device, 105℃ vacuum drying oven, electronic balance, constant temperature water bath.

[0042] Test steps: (1) Dry the sand core crucible at 105℃ to constant weight and record the crucible mass m0.

[0043] (2) Accurately weigh 1.0000g of thickener sample, add 1000mL of distilled water, stir and dissolve in a 40℃ water bath for 2h to ensure complete dispersion.

[0044] (3) After the solution has reached constant weight, it is vacuum filtered in a sand core crucible. The filter residue is washed three times with hot distilled water. The crucible with the filter residue is dried at 105℃ to constant weight. The mass m1 is recorded, and the water-insoluble content is calculated: Water-insoluble matter content = (m1-m0) / sample mass × 100%, and the average value is taken from 3 parallel tests.

[0045] 5. Debonding performance and residue content test Instruments and reagents: Same as 4.1, high-temperature aging tank, constant temperature water bath, ammonium persulfate breaker, rotational rheometer.

[0046] Test steps: (1) Take the fracturing fluid gel for the temperature resistance and shear resistance test, add ammonium persulfate breaker at 0.05% of the gel mass, stir evenly and put it into a high temperature aging tank, and aging at 160℃ for 4 hours to complete the gel breaking.

[0047] (2) Cool the gelling liquid to 25°C and in 170s -1 The apparent viscosity of the ruptured liquid was tested at the shear rate.

[0048] (3) The debonded liquid is filtered through a constant-weight sand core crucible, and the filter residue is dried at 105°C to constant weight. The content of debonded residue is calculated in mg / L.

[0049] Table 1: Basic Performance Tests of Thickeners

[0050] Table 2: Test results of the temperature and shear resistance of the thickener (160℃, 170s) -1 )

[0051] Table 3: Salt resistance test results of thickener (25×10) 4 (mg / L mineralization)

[0052] Table 4: Thickener breaking performance test results (160℃, 4h breaking performance)

[0053] As shown in Tables 1-4, the only difference between Comparative Example 1 and Example 3 is the absence of the crown ether functionalized hydrophobic associating monomer of the present invention. All other components, dosages, and preparation methods are identical. Test results show that the apparent viscosity of the 0.5% concentration base liquid in this comparative example is only 71 mPa·s, indicating a decrease in thickening ability. At 160℃ and 170s... -1 Under continuous shearing conditions for 120 min, the viscosity retention rate was only 40.1%, and at 25×10⁻⁶... 4The viscosity retention rate in simulated formation water with a high salinity of mg / L was only 41.1%, and the core properties of temperature resistance, salt resistance, and shear resistance failed. Only the gel breaking performance was close to that of Example 3. The core reason for its performance degradation is that the missing hydrophobic associating monomer is the core of the system to build a reversible three-dimensional associative network and achieve efficient viscosity enhancement. At the same time, the crown ether group in the monomer can shield the charge interference of the high salinity environment by specifically complexing metal cations and inhibit the high-temperature hydrolysis of molecular chains through the rigid ring structure. Without this component, the polymer relies only on linear molecular entanglement for viscosity enhancement. Under high temperature and high salinity conditions, the molecular chains are very easy to curl and break, resulting in irreversible viscosity loss, which completely fails to meet the construction requirements of deep high temperature and high salinity reservoirs.

[0054] Comparative Example 2 The only difference between Comparative Example 2 and Example 3 is that no sulfonic acid and amino dual-modified nano-silica was added. The other components, dosages and preparation methods are completely the same. Test data show that the apparent viscosity of the 0.5% concentration base liquid in this comparative example is 84 mPa·s, and the basic viscosity-enhancing effect is significantly weakened. After high-temperature shearing at 160℃ for 120 min, the viscosity retention rate is only 44.2%, and the viscosity retention rate in high-mineralization formation water is only 43.7%. The high-temperature stability and salt resistance are severely deteriorated. Only the solubility and gel breaking performance are not significantly different from those of Example 3. The root cause of its performance degradation lies in the fact that the double-modified nano silica can form strong hydrogen bonds with polymer molecular chains through surface amino groups, constructing dense physical cross-linking points. At the same time, it forms an organic-inorganic interpenetrating network with its own high rigidity, inhibiting the slippage and breakage of molecular chains under high temperature and high shear. Its surface sulfonic acid groups can also enhance the hydration stability under high salt environment. Without this component, the density and mechanical strength of the polymer network will drop significantly, and the structure will be prone to collapse under high temperature and high salt environment, making it unsuitable for the extreme construction conditions of deep oil and gas reservoirs.

[0055] The only difference between Comparative Example 3 and Example 3 is that an equal weight of the traditional hydrophobic monomer octadecylacrylamide was used instead of the customized crown ether functionalized hydrophobic associating monomer of this invention. The other components, dosages, and preparation methods were completely identical. The test results showed that this comparative example not only had an apparent viscosity of only 95 mPa·s for the 0.5% concentration base liquid, significantly reducing the viscosity-enhancing efficiency, but also exhibited typical defects of traditional hydrophobic associating polymers, with a complete dissolution time as long as 18 min and a water-insoluble content of 0.12%. At the same time, its viscosity retention rate after high-temperature shearing at 160℃ was only 53.0%, and its viscosity retention rate in high-mineralization formation water was only 47.5%. Its temperature resistance, salt resistance, and shear resistance were far lower than those of Example 3, and the content of broken gel residue also increased to 41 mg / L, significantly increasing the risk of formation damage. The core reason for its overall performance degradation is that octadecylacrylamide is a strongly hydrophobic monomer with no hydrophilic modification structure, extremely poor water solubility, and can only form simple hydrophobic association structures. It is easily destroyed at high temperatures and cannot be reversibly reconstructed. At the same time, it lacks the cationic complexation effect of crown ether groups and cannot resist the charge shielding effect of high mineralization environments. Ultimately, it is far inferior to the functionalized hydrophobic association monomer of this invention in terms of solubility, temperature and salt resistance, and low damage.

[0056] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Unless otherwise specified, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0057] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A thickener for oilfield fracturing fluid, characterized in that, It is polymerized from monomers comprising the following parts by weight: Acrylamide 30-40 parts; 10-20 parts of 2-acrylamide-2-methylpropanesulfonic acid; 4-8 parts of N-vinylpyrrolidone; 0.6-1 part of hydrophobic associating monomer 0.5-0.8 parts of double-modified nano-silica; The dual-modified nano-silica is nano-silica particles with dual surface modification of sulfonic acid groups and amino groups; the hydrophobic associating monomer is hydrophobic modified polyvinyl alcohol functionalized with crown ether.

2. The oilfield fracturing fluid thickener according to claim 1, characterized in that, The method for preparing the hydrophobic associating monomer is as follows: S1. Polyvinyl alcohol is dissolved in dimethyl sulfoxide, then 4-formylbenzo15-crown-5 and p-toluenesulfonic acid catalyst are added, and the reaction is carried out at 75-80℃ for 8-10 h. After the reaction is completed, precipitation is carried out in anhydrous ethanol, and the precipitate is placed in a Soxhlet extractor. The precipitate is extracted with ethanol as the extractant for 10-12 hours. The precipitate is dried to obtain crown ether modified polyvinyl alcohol. S2. Add crown ether modified polyvinyl alcohol and undecenoic acid to N,N-dimethylformamide solvent, stir and mix, continue to add p-toluenesulfonic acid catalyst, react at 80-85℃ for 4-6 h, after which distill under reduced pressure, wash and dry to obtain hydrophobic associating monomer.

3. The oilfield fracturing fluid thickener according to claim 2, characterized in that, In step S1, the ratio of polyvinyl alcohol, dimethyl sulfoxide, 4-formylbenzo(15-crown-5), and p-toluenesulfonic acid catalyst is 0.4-0.5 g : 80-85 mL. 0.6-0.71g: 0.01-0.02g.

4. The oilfield fracturing fluid thickener according to claim 2, characterized in that, In S2, the ratio of N,N-dimethylformamide, crown ether modified polyvinyl alcohol, undecenoic acid, and p-toluenesulfonic acid catalyst is 50-55 mL: 0.3-0.4 g: 0.1-0.15 g: 0.02-0.03 g.

5. A method for preparing an oilfield fracturing fluid thickener as described in any one of claims 1-4, characterized in that, Includes the following steps: Weigh 10-20 parts by weight of 2-acrylamide-2-methylpropanesulfonic acid, 4-8 parts by weight of N-vinylpyrrolidone, and 0.5-0.8 parts by weight of double-modified nano-silica and add them to deionized water. Adjust the pH of the system to 7-8 with a pH adjuster. Then add 30-40 parts by weight of acrylamide, 0.6-1 parts by weight of hydrophobic associating monomer, and 0.2-0.3 parts by weight of sodium dodecyl sulfate. After purging with nitrogen, add 0.06-0.08 parts by weight of the initiator ammonium persulfate-sodium bisulfite system. Stir and react in a constant temperature water bath for 3-4 hours to obtain an oilfield fracturing fluid thickener.

6. The method for preparing the oilfield fracturing fluid thickener according to claim 5, characterized in that, The pH adjuster is a sodium hydroxide solution with a mass concentration of 8-10%; the mass ratio of ammonium persulfate to sodium bisulfite is 2:

1.

7. The method for preparing the oilfield fracturing fluid thickener according to claim 5, characterized in that, The nitrogen gas is introduced for 10-15 minutes.

8. The method for preparing the oilfield fracturing fluid thickener according to claim 5, characterized in that, The temperature of the stirring reaction in the constant temperature water bath is 40-45℃.

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