Preparation method and application of nano composite salt-resistant and temperature-resistant drag reducer

Abstract

The invention provides a preparation method of a nano composite salt-resistant and temperature-resistant drag reducer. The method comprises the following steps: preparing a hydrolyzed KH550 solution, a modified SiO2 dispersion liquid, a cellulose nanofiber dispersion liquid and a KH-550 modified CNF-SiO2 composite suspension liquid; the preparation method comprises the following steps: adding methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid into deionized water to obtain a monomer premixed solution, then adding the KH-550 modified CNF-SiO2 composite suspension, then adding ammonium persulfate and sodium hydrogen sulfite, and finally preparing the nano composite salt-resistant temperature-resistant drag reducer. The nano-composite salt-resistant and temperature-resistant drag reducer has excellent properties of easy dissolution, salt resistance, shear resistance and high temperature resistance, can be used for a high-salt (greater than or equal to 50000ppm) slick water system prepared from fracturing flow-back fluid to reduce the dependence on fresh water resources, and can also be used for hydraulic fracturing of shale oil gas and compact oil gas to reduce the pressure of the fracturing flow-back fluid. And the requirements of high-temperature (greater than or equal to 150 DEG C) extreme working conditions of ultra-deep well fracturing can be met

Description

Technical Field

[0001] This invention belongs to the technical field of drag-reducing agents for oilfield fracturing, specifically relating to a preparation method and application of a nanocomposite salt-resistant and temperature-resistant drag-reducing agent. Background Technology

[0002] In oilfield development, fracturing technology is one of the key methods to improve oil and gas well production. In fracturing fluid systems, drag-reducing agents can significantly reduce the flow resistance of fluids in pipelines and formation pores, improve fracturing fluid injection efficiency, reduce equipment energy consumption, and thus lower production costs, playing a crucial role in improving oil and gas recovery. Currently, commonly available fracturing drag-reducing agents mainly include: (1) Polyacrylamide drag reducers: These are synthetic polymers based on acrylamide. Their molecular chains reduce fluid friction resistance through turbulence suppression and viscoelastic effects, thereby achieving drag reduction. Polyacrylamide drag reducers have good water solubility and drag reduction performance, and are most widely used in fracturing fluid systems with low mineralization. However, conventional products have problems such as poor high-temperature resistance and molecular chain shear sensitivity, which need to be improved through structural optimization (such as comb-like or hydrophobic association).

[0003] (2) Natural polymer drag reducers: These are mainly based on bio-based materials such as xanthan gum and polysaccharide guanidine. They rely on the thickening effect of natural polymer chains to reduce friction and achieve drag reduction. These drag reducers are widely available, low in cost, and have good biodegradability, resulting in less environmental pollution compared to synthetic materials. However, they have a high residue content after gel breaking, which can easily clog the pores of low-permeability reservoirs. They also have poor temperature and salt resistance, and are prone to degradation and instability in high-temperature and high-mineralization environments, affecting the drag reduction effect.

[0004] (3) Surfactant-based drag reducers: These reduce drag by decreasing the surface and interfacial tension of the fluid through micelles, thereby reducing turbulent friction. Surfactant-based drag reducers have drag reduction potential at low concentrations and can take effect rapidly under turbulent conditions. Novel stimulus-responsive surfactants (such as betaine derivatives) can maintain their performance in highly saline solutions. However, the micelle structure of these drag reducers is easily destroyed under high-speed shear, resulting in a sharp drop in drag reduction effect. Some surfactants are highly toxic and require the formulation of multiple additives, posing certain environmental risks with long-term use.

[0005] In summary, polyacrylamide-based agents exhibit superior performance and are the mainstream choice for both conventional and unconventional reservoir fracturing. With the continued advancement of sustainable water resource development, utilizing fracturing flowback fluid for fluid preparation is becoming an inevitable trend in current oil and gas field fracturing operations. However, conventional polyacrylamide-based drag reducers still have several shortcomings when dealing with high-salinity flowback fracturing systems: (1) Insufficient salt resistance Highly mineralized flowback solutions contain large amounts of salt ions (such as Na⁺, Ca⁺, and Ca²⁺). 2 ⁺、Mg 2 These ions (⁺) compress polymer molecular chains through the polyelectrolyte effect, causing the molecular chains to shrink, aggregate, and even precipitate, thereby reducing the viscosity and drag-reducing performance of the drag-reducing agent. Traditional polyacrylamide drag-reducing agents experience a significant decrease in drag reduction rate under high-mineralization environments, failing to meet the demands of actual production.

[0006] (2) Poor solubility and shear resistance During fracturing, fracturing fluid needs to be injected into the formation via a high-pressure pump, during which it is subjected to intense shearing. Traditional dry powder drag reducers dissolve slowly in flowback fluid (requiring 10-15 minutes), and the high-speed pumping shearing can easily cause molecular chain breakage, resulting in a decrease in drag reduction rate.

[0007] (3) Poor temperature resistance To address the aforementioned core issues, existing research focuses on improvements through molecular structure modification and product morphology optimization. For example, functional monomers (such as monomers containing sulfonic acid groups or hydrophobic groups) are incorporated to integrate rigid nanostructure units. Through the synergistic effect between functional monomers and rigid nanostructure units, salt resistance, temperature resistance, and shear resistance are optimized. Furthermore, by combining nanostructure units and modifying interfaces, a balance between hydrophobic and hydrophilic groups in the copolymer is achieved, forming a unique network structure that improves the viscosity stability of the copolymer, enhances shear resistance, and shortens dissolution time. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a preparation method and application of a nanocomposite salt-resistant and temperature-resistant drag-reducing agent to address the shortcomings of the prior art. This nanocomposite salt-resistant and temperature-resistant drag-reducing agent has excellent properties such as easy solubility, salt resistance, shear resistance and high temperature resistance. It can not only be used in the slickwater system for fracturing flowback fluid preparation to reduce reliance on freshwater resources, but also in hydraulic fracturing in shale gas and oil and gas fields. It is suitable for extreme fracturing conditions in oil and gas field deep wells with temperatures ≥150℃ and salt concentrations ≥50000ppm.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for preparing a nanocomposite salt-resistant and temperature-resistant drag-reducing agent, the method being as follows: S1. At room temperature, anhydrous ethanol, water and glacial acetic acid are mixed and magnetically stirred for 5 min to 10 min. Then, silane coupling agent KH550 is added and magnetically stirred for 30 min to 40 min to obtain a hydrolyzed KH550 solution. S2. At room temperature, SiO2 nanopowder is dispersed in anhydrous ethanol and ultrasonically treated for 30 min to 40 min. Then, it is added dropwise to the hydrolyzed KH550 solution obtained in S1. The reaction is then stirred for 6 h to 7 h in a water bath at 70 °C to obtain the reaction product. S3. After centrifuging the reaction product obtained in S2, wash the precipitate twice with anhydrous ethanol and then dry it under vacuum to obtain modified SiO2. S4. Disperse the modified SiO2 obtained in S3 in deionized water and sonicate for 15 min to 20 min to obtain a modified SiO2 dispersion. S5. Disperse cellulose nanofibers (CNF) in deionized water and sonicate for 30 min to 40 min to obtain cellulose nanofiber dispersion; S6. After mixing the modified SiO2 dispersion obtained in S4 and the cellulose nanofiber dispersion obtained in S5, ultrasonic treatment is carried out for 1h~2h to obtain KH-550 modified CNF-SiO2 composite mother liquor. S7. Dilute the CNF-SiO2 composite mother liquor obtained in S6 with deionized water to obtain KH-550 modified CNF-SiO2 composite suspension. By modifying CNF-SiO2 with silane coupling agent KH-550, a chemically bonded composite is achieved, forming a "fiber-particle" dual-scale rigid network. The synergistic effect of SiO2 nanoparticles and CNF long fiber skeleton can effectively improve the salt resistance and shear resistance of drag-reducing agent. The covalent bonding interface formed by the condensation of silanol and CNF-SiO2 can further improve the high temperature stability of drag-reducing agent. S8. While stirring, add methyl methacrylate (MMA), acrylamide (AM), acrylic acid (AA) and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) sequentially to deionized water. After dissolution, a monomer premix is ​​obtained. S9. Add the KH-550 modified CNF-SiO2 composite suspension obtained in S7 to the monomer premix obtained in S8. Adjust the pH of the system to 6.5-7.0 with a NaOH aqueous solution with a mass concentration of 0.1 mol / L. Stir magnetically for 30-40 minutes at room temperature to obtain a mixed solution. S10. Add ammonium persulfate and sodium bisulfite to the mixture obtained in S9, and then react for 2-6 hours under N2 atmosphere and at a temperature of 30℃-50℃. After washing the reaction product three times with anhydrous ethanol and drying it under vacuum, a nano-composite salt-resistant and temperature-resistant drag-reducing agent is obtained.

[0010] Preferably, the ratio of anhydrous ethanol, water, glacial acetic acid, and silane coupling agent KH550 in S1 is 10 mL: 10 mL: 0.1 mL: (0.05~0.25) g; The ratio of SiO2 nanopowder, anhydrous ethanol, and hydrolyzed KH550 solution in S2 is 1g:20mL:20mL; The ratio of modified SiO2 to deionized water in S4 is 1g:20mL; the ratio of cellulose nanofibers to deionized water in S5 is 1g:20mL.

[0011] Preferably, the conditions for ultrasonic treatment in S2, S4, and S6 are: frequency of 40kHz and power of 200W; the conditions for centrifugation in S3 are: rotation speed of 12000rpm and time of 10min; and the conditions for vacuum drying in S3 and S10 are: temperature of 60℃ and time of 12h.

[0012] Preferably, in S6, the mass ratio of cellulose nanofibers in the cellulose nanofiber dispersion to modified SiO2 in the modified SiO2 dispersion is 1:(0.5-2); in S7, the mass fraction of KH-550 modified CNF-SiO2 composite mother liquor in the KH-550 modified CNF-SiO2 composite suspension is 1%-1.5%.

[0013] Preferably, in S8, the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid accounts for 20wt% to 25wt% of the monomer premix; the mass of 2-acrylamide-2-methylpropanesulfonic acid accounts for 10wt% to 15wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid; the mass of methyl methacrylate accounts for 1wt% to 2wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid; and the mass ratio of acrylamide to acrylic acid is (7-8):(2-3).

[0014] Preferably, the mass of the KH-550 modified CNF-SiO2 composite suspension in S9 accounts for 0.5wt% to 2wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix.

[0015] Preferably, the mass ratio of ammonium persulfate (APS) to sodium bisulfite (SHS) in S10 is 1:(1-2); the total mass of ammonium persulfate and sodium bisulfite in S10 accounts for 0.1wt% to 0.2wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix.

[0016] Preferably, the average particle size of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent in S10 is 80 nm to 140 nm.

[0017] This invention also provides the application of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent prepared by the above preparation method, wherein the nanocomposite salt-resistant and temperature-resistant drag-reducing agent is used for oilfield fracturing.

[0018] Preferably, the nanocomposite salt-resistant and temperature-resistant drag-reducing agent is used in fracturing conditions in oil and gas field deep wells with temperatures ≥150℃ and salt concentrations ≥50000ppm.

[0019] Compared with the prior art, the present invention has the following advantages: 1. The nanocomposite salt-resistant and temperature-resistant drag-reducing agent prepared by this invention exhibits excellent salt resistance. Conventional PAM suffers from molecular chain shrinkage and a sharp decrease in viscoelasticity in high-mineralization environments due to charge shielding effects. This invention enhances salt resistance by introducing a sulfonic acid monomer (2-acrylamide-2-methylpropanesulfonic acid), a synergistic hydrophobic associating monomer (methyl methacrylate), and an integrated rigid structural unit (CNF-SiO2).

[0020] 2. The present invention modifies CNF-SiO2 with KH-550 to improve its hydrophilicity, thereby shortening the dissolution time of the drag reducer to ≤3 minutes (conventional products require 5–10 minutes); CNF-SiO2 enhances interfacial stability through KH-550 modification, CNF long fibers disperse stress and inhibit chain breakage, and SiO2 nanoparticles reduce inter-chain friction, reduce flow energy consumption, and synergistically improve shear resistance while enhancing solubility.

[0021] 3. The thermally stable framework constructed by cellulose nanofibers (CNF) and SiO2 in this invention can inhibit the high-temperature shrinkage of the polymer backbone and improve the high-temperature tolerance of drag reducers. High-temperature aging test experiments show that the tolerance temperature can reach 180°C, which can meet the requirements of high temperature (≥150°C) and high salinity (>50,000 ppm) reservoir conditions in oil and gas fields.

[0022] The present invention will be further described in detail below with reference to the embodiments. Detailed Implementation

[0023] Example 1 The preparation method of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent in this embodiment is as follows: S1. At room temperature, mix 10 mL of anhydrous ethanol, 10 mL of water and 0.1 mL of glacial acetic acid, stir magnetically for 5 min, then add 0.15 g of silane coupling agent KH550, stir magnetically for 30 min, and obtain the hydrolyzed KH550 solution. S2. At room temperature, 1g of SiO2 nanoparticles were dispersed in 20mL of anhydrous ethanol and sonicated for 30min at a frequency of 40kHz and a power of 200W. Then, the mixture was added dropwise to 20mL of the hydrolyzed KH550 solution obtained in S1. The mixture was then stirred in a water bath at 70℃ for 6h to obtain the reaction product. S3. After centrifuging the reaction product obtained in S2 at 12000 rpm for 10 min, the precipitate was washed twice with anhydrous ethanol and then vacuum dried at 60℃ for 12 h to obtain modified SiO2. S4. The modified SiO2 obtained in S3 is dispersed in deionized water and ultrasonically treated for 15 minutes at a frequency of 40kHz and a power of 200W to obtain a modified SiO2 dispersion; the ratio of modified SiO2 to deionized water is 1g:20mL. S5. Disperse cellulose nanofibers (CNF) in deionized water and sonicate for 30 min to obtain a cellulose nanofiber dispersion; the ratio of cellulose nanofibers to deionized water is 1 g: 20 mL. Cellulose nanofibers (CNF) were purchased commercially from Shenzhen Seruna Technology Co., Ltd. S6. The modified SiO2 dispersion obtained in S4 and the cellulose nanofiber dispersion obtained in S5 are mixed and ultrasonically treated for 1 hour at a frequency of 40kHz and a power of 200W to obtain KH-550 modified CNF-SiO2 composite mother liquor; the mass ratio of cellulose nanofibers in the cellulose nanofiber dispersion to modified SiO2 in the modified SiO2 dispersion is 1:1. S7. Dilute the CNF-SiO2 composite mother liquor obtained in S6 with deionized water to obtain a KH-550 modified CNF-SiO2 composite suspension; the mass fraction of the KH-550 modified CNF-SiO2 composite mother liquor in the KH-550 modified CNF-SiO2 composite suspension is 1.5%; S8. While stirring, add methyl methacrylate (MMA), acrylamide (AM), acrylic acid (AA) and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) sequentially to deionized water. After dissolution, a monomer premix is ​​obtained. The total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid accounts for 20 wt% of the monomer premix. The 2-acrylamide-2-methylpropanesulfonic acid comprises 15 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The methyl methacrylate comprises 1.5 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The mass ratio of acrylamide to acrylic acid is 7:3; S9. Add the KH-550 modified CNF-SiO2 composite suspension obtained in S7 to the monomer premix obtained in S8. Adjust the pH of the system to 7.0 with a NaOH aqueous solution with a mass concentration of 0.1 mol / L. Stir magnetically for 30 min at room temperature to obtain a mixed solution. The mass of the KH-550 modified CNF-SiO2 composite suspension accounts for 1.5 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix. S10. Add ammonium persulfate (APS) and sodium bisulfite (SHS) to the mixture obtained in S9. Then react for 5 h at 40 °C under N2 atmosphere. Wash the reaction product three times with anhydrous ethanol and then vacuum dry it for 12 h at 60 °C to obtain a nanocomposite salt-resistant and temperature-resistant drag-reducing agent with an average particle size of 100 nm. The mass ratio of ammonium persulfate to sodium bisulfite is 1:1; The total mass of ammonium persulfate and sodium bisulfite accounts for 0.1 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix.

[0024] Example 2 The preparation method of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent in this embodiment is as follows: S1. At room temperature, mix 10 mL of anhydrous ethanol, 10 mL of water and 0.1 mL of glacial acetic acid, stir magnetically for 10 min, then add 0.05 g of silane coupling agent KH550, stir magnetically for 40 min, and obtain the hydrolyzed KH550 solution. S2. At room temperature, 1g of SiO2 nanopowder was dispersed in 20mL of anhydrous ethanol and ultrasonically treated for 40min at a frequency of 40kHz and a power of 200W. Then, it was added dropwise to 20mL of the hydrolyzed KH550 solution obtained in S1. The reaction was then stirred for 7h in a water bath at a temperature of 70℃ to obtain the reaction product. S3. After centrifuging the reaction product obtained in S2 at 12000 rpm for 10 min, the precipitate was washed twice with anhydrous ethanol and then vacuum dried at 60℃ for 12 h to obtain modified SiO2. S4. Disperse the modified SiO2 obtained in S3 in deionized water and sonicate it for 20 min at a frequency of 40 kHz and a power of 200 W to obtain a modified SiO2 dispersion; the ratio of modified SiO2 to deionized water is 1 g: 20 mL. S5. Disperse cellulose nanofibers (CNF) in deionized water and sonicate for 40 min to obtain a cellulose nanofiber dispersion; the ratio of cellulose nanofibers to deionized water is 1 g: 20 mL. Cellulose nanofibers (CNF) were purchased commercially from Shenzhen Seruna Technology Co., Ltd. S6. The modified SiO2 dispersion obtained in S4 and the cellulose nanofiber dispersion obtained in S5 are mixed and ultrasonically treated for 2 hours at a frequency of 40kHz and a power of 200W to obtain KH-550 modified CNF-SiO2 composite mother liquor; the mass ratio of cellulose nanofibers in the cellulose nanofiber dispersion to modified SiO2 in the modified SiO2 dispersion is 1:0.5. S7. Dilute the CNF-SiO2 composite mother liquor obtained in S6 with deionized water to obtain a KH-550 modified CNF-SiO2 composite suspension; the mass fraction of the KH-550 modified CNF-SiO2 composite mother liquor in the KH-550 modified CNF-SiO2 composite suspension is 1%; S8. While stirring, add methyl methacrylate (MMA), acrylamide (AM), acrylic acid (AA) and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) sequentially to deionized water. After dissolution, a monomer premix is ​​obtained. The total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid accounts for 20 wt% of the monomer premix. The 2-acrylamide-2-methylpropanesulfonic acid comprises 15 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The methyl methacrylate comprises 1 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The mass ratio of acrylamide to acrylic acid is 7:3; S9. Add the KH-550 modified CNF-SiO2 composite suspension obtained in S7 to the monomer premix obtained in S8. Adjust the pH of the system to 6.5 with a NaOH aqueous solution with a mass concentration of 0.1 mol / L. Stir magnetically for 40 min at room temperature to obtain a mixed solution. The mass of the KH-550 modified CNF-SiO2 composite suspension accounts for 0.5 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix. S10. Add ammonium persulfate (APS) and sodium bisulfite (SHS) to the mixture obtained in S9, and then react for 6 hours under N2 atmosphere and temperature of 30°C. Wash the obtained reaction product three times with anhydrous ethanol, and then vacuum dry it for 12 hours at temperature of 60°C to obtain a nanocomposite salt-resistant and temperature-resistant drag-reducing agent with an average particle size of 80nm. The mass ratio of ammonium persulfate to sodium bisulfite is 1:1.2; The total mass of ammonium persulfate and sodium bisulfite accounts for 0.2 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix.

[0025] Example 3 The preparation method of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent in this embodiment is as follows: S1. At room temperature, mix 10 mL of anhydrous ethanol, 10 mL of water and 0.1 mL of glacial acetic acid, stir magnetically for 8 min, then add 0.25 g of silane coupling agent KH550, stir magnetically for 35 min to obtain the hydrolyzed KH550 solution. S2. At room temperature, 1g of SiO2 nanoparticles were dispersed in 20mL of anhydrous ethanol and sonicated for 35min at a frequency of 40kHz and a power of 200W. Then, the mixture was added dropwise to 20mL of the hydrolyzed KH550 solution obtained in S1. The mixture was then stirred in a water bath at 70℃ for 6.5h to obtain the reaction product. S3. After centrifuging the reaction product obtained in S2 at 12000 rpm for 10 min, the precipitate was washed twice with anhydrous ethanol and then vacuum dried at 60℃ for 12 h to obtain modified SiO2. S4. The modified SiO2 obtained in S3 is dispersed in deionized water and ultrasonically treated for 18 minutes at a frequency of 40kHz and a power of 200W to obtain a modified SiO2 dispersion; the ratio of modified SiO2 to deionized water is 1g:20mL. S5. Disperse cellulose nanofibers (CNF) in deionized water and sonicate for 35 min to obtain a cellulose nanofiber dispersion; the ratio of cellulose nanofibers to deionized water is 1 g: 20 mL. Cellulose nanofibers (CNF) were purchased commercially from Shenzhen Seruna Technology Co., Ltd. S6. The modified SiO2 dispersion obtained in S4 and the cellulose nanofiber dispersion obtained in S5 are mixed and ultrasonically treated for 1.5 h at a frequency of 40 kHz and a power of 200 W to obtain KH-550 modified CNF-SiO2 composite mother liquor; the mass ratio of cellulose nanofibers in the cellulose nanofiber dispersion to modified SiO2 in the modified SiO2 dispersion is 1:2. S7. Dilute the CNF-SiO2 composite mother liquor obtained in S6 with deionized water to obtain a KH-550 modified CNF-SiO2 composite suspension; the mass fraction of the KH-550 modified CNF-SiO2 composite mother liquor in the KH-550 modified CNF-SiO2 composite suspension is 1.5%; S8. While stirring, add methyl methacrylate (MMA), acrylamide (AM), acrylic acid (AA) and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) sequentially to deionized water. After dissolution, a monomer premix is ​​obtained. The total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid accounts for 25 wt% of the monomer premix. The mass of the 2-acrylamide-2-methylpropanesulfonic acid accounts for 20 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The methyl methacrylate comprises 2 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The mass ratio of acrylamide to acrylic acid is 8:2; S9. Add the KH-550 modified CNF-SiO2 composite suspension obtained in S7 to the monomer premix obtained in S8. Adjust the pH of the system to 7.0 with a NaOH aqueous solution with a mass concentration of 0.1 mol / L. Stir magnetically for 35 min at room temperature to obtain a mixed solution. The mass of the KH-550 modified CNF-SiO2 composite suspension accounts for 2 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix. S10. Add ammonium persulfate (APS) and sodium bisulfite (SHS) to the mixture obtained in S9. Then react for 2 hours at 50°C under N2 atmosphere. Wash the obtained reaction product three times with anhydrous ethanol and then vacuum dry it for 12 hours at 60°C to obtain a nanocomposite salt-resistant and temperature-resistant drag-reducing agent with an average particle size of 140 nm. The mass ratio of ammonium persulfate to sodium bisulfite is 1:2; The total mass of ammonium persulfate and sodium bisulfite accounts for 0.15 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix.

[0026] Comparative Example 1 The preparation method of the salt-resistant drag-reducing agent in this comparative example is as follows: S1. While stirring, add methyl methacrylate (MMA), acrylamide (AM), acrylic acid (AA) and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) sequentially to deionized water. After dissolution, a monomer premix is ​​obtained. The total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid accounts for 20 wt% of the monomer premix. The 2-acrylamide-2-methylpropanesulfonic acid comprises 15 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The methyl methacrylate comprises 1.5 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The mass ratio of acrylamide to acrylic acid is 7:3; S2. Add ammonium persulfate (APS) and sodium bisulfite (SHS) to the monomer premix obtained in S1, and then react for 5 h at 40 °C under N2 atmosphere. Wash the obtained reaction product three times with anhydrous ethanol and then vacuum dry it for 12 h at 60 °C to obtain a salt-resistant drag-reducing agent without nano-components. The mass ratio of ammonium persulfate to sodium bisulfite is 1:1; The total mass of ammonium persulfate and sodium bisulfite accounts for 0.1 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix.

[0027] Comparative Example 2 The preparation method of the unmodified KH550 nanocomposite salt-resistant and temperature-resistant drag-reducing agent in this comparative example is as follows: S1. Disperse cellulose nanofibers (CNF) in deionized water and sonicate for 30 min to obtain a cellulose nanofiber dispersion; the ratio of cellulose nanofibers to deionized water is 1 g: 20 mL. Cellulose nanofibers (CNF) were purchased commercially from Shenzhen Seruna Technology Co., Ltd. S2. After mixing SiO2 nanopowder and the cellulose nanofiber dispersion obtained in S1, the mixture is ultrasonically treated for 1 hour at a frequency of 40kHz and a power of 200W to obtain CNF-SiO2 composite mother liquor; the mass ratio of cellulose nanofibers to SiO2 nanopowder in the cellulose nanofiber dispersion is 1:1. S3. Dilute the CNF-SiO2 composite mother liquor obtained in S2 with deionized water to obtain a CNF-SiO2 composite suspension; the mass fraction of CNF-SiO2 composite mother liquor in the CNF-SiO2 composite suspension is 1.5%; the CNF-SiO2 composite suspension has not been modified by KH-550. S4. While stirring, add methyl methacrylate (MMA), acrylamide (AM), acrylic acid (AA) and 2-acrylamide-2-methylpropanesulfonic acid (AMPS) sequentially to deionized water. After dissolution, a monomer premix is ​​obtained. The total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid accounts for 20 wt% of the monomer premix. The 2-acrylamide-2-methylpropanesulfonic acid comprises 15 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The methyl methacrylate comprises 1.5 wt% of the total mass of the methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid. The mass ratio of acrylamide to acrylic acid is 7:3; S5. Add the CNF-SiO2 composite suspension obtained in S3 to the monomer premix obtained in S4. Adjust the pH of the system to 7.0 with a NaOH aqueous solution with a mass concentration of 0.1 mol / L. Stir magnetically for 30 min at room temperature to obtain a mixed solution. The CNF-SiO2 composite suspension accounts for 1.5 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix. S6. Add ammonium persulfate (APS) and sodium bisulfite (SHS) to the mixture obtained in S5, and then react for 5 h at 40 °C under N2 atmosphere. Wash the reaction product three times with anhydrous ethanol and then vacuum dry it for 12 h at 60 °C to obtain a nanocomposite salt-resistant and temperature-resistant drag-reducing agent with an average particle size of 220 nm without KH550 modification. The mass ratio of ammonium persulfate to sodium bisulfite is 1:1; The total mass of ammonium persulfate and sodium bisulfite accounts for 0.1 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix.

[0028] Performance testing includes: (1) Salt resistance The drag-reducing agent products obtained in Examples 1-3 and Comparative Examples 1-2 were dissolved in 20,000 ppm and 50,000 ppm NaCl aqueous solutions, respectively, to prepare solutions with a concentration of 0.15 wt%. These solutions were kept at a constant temperature of 25 ± 0.5 ℃ for 30 min, with the shear rate fixed at 170 s⁻¹. -1 The apparent viscosity was measured using a rotational viscometer, and the results are shown in Table 1.

[0029] Table 1. Apparent viscosity of drag-reducing agent at different brine concentrations The nanocomposite salt-resistant and temperature-resistant drag-reducing agents prepared in Examples 1-3 had viscosities of 65.4 mPa·s, 52.3 mPa·s, and 71.2 mPa·s in clean water, respectively. With increasing salt ion concentration, the charge shielding effect intensifies, molecular chain coiling worsens, and the apparent viscosity decreases with increasing brine concentration. The apparent viscosities of Examples 1 and 3 in 50,000 ppm brine were 43.1 mPa·s and 47.5 mPa·s, respectively. In Example 2, when the contents of AMPS, MMA, and KH550-modified CNF-SiO2 decreased, the synergistic mechanism of the three weakened simultaneously, resulting in a lower viscosity than Examples 1 and 3, with an apparent viscosity of 33.9 mPa·s in 50,000 ppm brine. In Comparative Example 1, due to the lack of a rigid CNF-SiO2 nanoframework, the apparent viscosity in pure water was 38.4 mPa·s, while the viscosity in 50,000 ppm salt water was only 15.6 mPa·s. Compared with Example 1, Comparative Example 2 showed a rapid decrease in apparent viscosity in 20,000 ppm and 50,000 ppm salt water, indicating that the covalently bonded nanonetwork formed by the condensation of KH550 and CNF-SiO2 has a significant effect on anti-salt stability.

[0030] In summary, the nanocomposite salt-resistant and temperature-resistant drag-reducing agents prepared in Examples 1-3 exhibit excellent salt resistance in 50,000 ppm brine. AMPS (2-acrylamide-2-methylpropanesulfonic acid) and KH-550-modified CNF-SiO2 are the key factors for the salt resistance performance of the drag-reducing agents.

[0031] (2) Temperature resistance The drag-reducing agent products obtained in Examples 1-3 and Comparative Examples 1-2 were dissolved in water and NaCl aqueous solutions of 20,000 ppm and 50,000 ppm, respectively, to prepare solutions with a concentration of 0.15 wt%. These solutions were then placed in a 20 MPa high-temperature reactor and aged at 180°C for 2 hours. The viscosity was measured using a rotational viscometer at 170 s⁻¹. -1 Viscosity retention rate at shear rate is shown in Table 2.

[0032] Table 2. Aging viscosity retention rate of drag-reducing agents at different brine concentrations Comparing Examples 1 and 2, when the total monomer mass fraction is maintained at 20 wt% (i.e., the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid in step S8 accounts for 20 wt% of the monomer premix), the CNF-SiO2 skeleton in the polymer provides support and inhibition, resulting in a viscosity retention rate ≥70% after aging at 180°C for 2 hours. Furthermore, the thermal stability of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent in Example 1 is superior to that in Example 2, achieving a viscosity retention rate of 65% in 500,000 ppm brine. When the total monomer mass fraction is increased to 25 wt% (Example 3, where the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid in step S8 accounts for 25 wt% of the monomer premix), the molecular weight increases and hydrophobic association is enhanced, further improving thermal stability, with a viscosity retention rate of 75% in 500,000 ppm brine (Example 3). In Comparative Example 1, due to the lack of a stable CNF-SiO2 framework, the polymer molecular chains broke after aging at 180℃, resulting in a viscosity retention rate of only 38% in clean water. Comparative Example 2 exhibited slightly higher thermal stability than Comparative Example 1, but due to the aggregation of the unmodified CNF-SiO2 composite copolymer at high temperatures, which triggered localized hot spots, the drag-reducing agent prepared in Comparative Example 2 had a viscosity retention rate of 55% in clean water and only 40% in 50,000 ppm brine. Therefore, it can be confirmed that the nanocomposite salt-resistant and temperature-resistant drag-reducing agents prepared in Examples 1-3 can meet the fracturing conditions of deep wells ≥150℃. KH550-modified CNF-SiO2 is the core element for temperature resistance; the absence or lack of modification of the nano-rigid components will lead to high-temperature failure.

[0033] (3) Solubility The drag-reducing agent products obtained in Examples 1-3 and Comparative Examples 1-2 were dissolved in 20,000 ppm and 50,000 ppm NaCl aqueous solutions to prepare solutions with a concentration of 0.15 wt%. The water bath temperature was controlled at 25°C, the stirring speed at 400 rpm, and the shear rate was fixed at 170 s⁻. 1 The rotational viscometer was started to record the time Tᵩ when the viscosity reached 90% of the stable value. The results are shown in Table 3.

[0034] Table 3 Dissolution time of drag-reducing agent at different saline concentrations Comparing Examples 1 and 2, when the amount of AMPS added was reduced from 15% to 10% (i.e., in Example 1, the mass of 2-acrylamide-2-methylpropanesulfonic acid accounted for 15 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid; in Example 2, the mass of 2-acrylamide-2-methylpropanesulfonic acid accounted for 10 wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid), the reduction in the salt-resistant component led to a decrease in the degree of ionization, resulting in poorer particle dispersibility in the initial stage of dissolution and a prolonged dissolution time. In a brine environment, the electrostatic repulsion was further weakened due to the compression of the electric double layer by salt ions, accompanied by a tendency for particle aggregation, further prolonging the dissolution time. In Example 3, the increased total amount of monomers (i.e., the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid in Example 3 accounted for 25 wt% of the monomer premix, while in Examples 1-2 the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid accounted for 20 wt% of the monomer premix) increased the molecular weight of the polymer, requiring a longer time to eliminate molecular chain entanglement. The high-salt environment further inhibited chain extension, with a Tᵩ of 167 s at 50,000 ppm. The dissolution times Tᵩ measured in water and salt water for Comparative Examples 1 and 2 were significantly higher than those in Examples 1-3. In Comparative Example 1, due to the lack of rigid CNF-SiO2 support, the molecular chains excessively coiled, forming a dense structure. Furthermore, the CNF-SiO2 nanophase can reduce polymer molecular aggregation and promote dispersion through the interaction between surface hydroxyl groups and polymer chains. In Comparative Example 1, AMPS, which provides strong hydration, lacks the dispersion effect of the CNF-SiO2 nanophase, resulting in excessively high local charge density. Electrostatic repulsion between molecular chains leads to entanglement, causing delayed dissolution. In a brine environment, the compressed electric double layer intensifies molecular chain coiling, forming hydrogen bonds between chains and hindering water molecule diffusion. At 50,000 ppm, Tᵩ reaches 204s, exceeding the allowable limit of 180s for fracturing operations. The drag reducer in Comparative Example 2, without KH550 modification, easily forms aggregates due to hydroxyl association, blocking the polymer surface. In a brine environment, salt ions penetrate the gaps in the aggregates, and after forced dispersion by mechanical stirring, hard aggregates can form, hindering water molecule pathways and leading to dissolution failure.

[0035] (4) Shear resistance The examples and comparative examples were dissolved in NaCl aqueous solution in 50,000 ppm brine to prepare a 0.15 wt% solution. The shear rate was adjusted at 25℃ and 150℃ for 30 min respectively using a high temperature and high pressure ring rheometer to simulate the whole pumping time of fracturing fluid and measure the final viscosity. The viscosity recovery rate was calculated, and the results are shown in Table 4.

[0036] Table 4 Viscosity recovery rate of drag-reducing agents at different shear rates At room temperature of 25°C, the drag-reducing agents of Examples 1 and 3 were tested for 5000 s⁻ 1 Viscosity recovery rate at shear rate > 80% in all cases, Example 2 at 3000 s⁻ 1 Under shear rates, the viscosity recovery rate can be >80%. Comparative Example 1, lacking nanostructure support, experiences molecular chain breakage under high shear forces, failing to reconstruct the network, resulting in a viscosity recovery rate of only 58%. The drag-reducing agent in Comparative Example 2 achieves a viscosity recovery rate of >80% at 5000 s⁻¹. 1 Although the viscosity recovery rate can be maintained at 70% under shear rate, unmodified CNF-SiO2 is prone to agglomeration, creating stress concentration points and easily detaching, causing frictional loss. In a high-temperature environment of 150℃, the drag-reducing agents of Examples 1-3 show a viscosity recovery rate of 3000 s⁻ 1 The viscosity recovery rate at shear rate is >70%, with Example 3 showing the best viscosity recovery rate of 89%. Examples 1, 2, and Comparative Example 2 follow in sequence, with Comparative Example 1 showing only 40%. This demonstrates that appropriately increasing the total monomer content can improve shear resistance. KH550-modified CNF-SiO2 is the main factor ensuring the drag-reducing agent's temperature and shear resistance. If KH-570 modification is omitted, the drag-reducing agent's viscosity recovery rate at 150℃ for 3000 s⁻¹ is significantly lower. 1 Shear resistance decreases by more than 30% at higher shear rates.

[0037] (5) Drag reduction According to the SY / T 6376-2008 standard, a pipeline flow test device was used to simulate the flow conditions in actual formations. The drag reduction performance of Examples 1-3 and Comparative Examples 1-2 in clean water, 20000 ppm, 50000 ppm and 80000 ppm brine was tested. The results are shown in Table 5.

[0038] Table 5. Evaluation of drag reduction performance of drag-reducing agents Increased salt concentration is accompanied by increased ionic strength, enhancing the real charge shielding effect, causing polymer molecular chains to coil up, reducing the size of turbulent eddies, and resulting in a general decreasing trend in drag reduction with increasing salt concentration. The drag reduction agents in Examples 1-3 all exhibited drag reduction rates >70% in clean water, meeting the technical specifications for polyacrylamide drag reduction agents stipulated in NB / T14003.2-2016, and can be used for hydraulic fracturing in shale oil and gas and tight oil and gas. Examples 1-3 also showed drag reduction rates >50% in 80,000 ppm brine, suitable for high-salt fracturing flowback fluid formulations. In contrast to Comparative Examples 1-2, the absence or lack of CNF-SiO2 led to a drastic decrease in drag reduction rate, indicating that KH550-modified CNF-SiO2 is a core element for high-temperature drag reduction.

[0039] The nanocomposite salt-resistant and temperature-resistant drag-reducing agent prepared by this invention has excellent properties such as easy solubility, salt resistance, shear resistance and high temperature resistance. It can not only be used in the preparation of high-salt (≥50000ppm) slickwater systems for fracturing flowback fluid to reduce dependence on freshwater resources, but also in shale oil and gas and tight oil and gas hydraulic fracturing, which can meet the high-temperature extreme working conditions (≥150℃) requirements of ultra-deep well fracturing.

[0040] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention in any way. Any simple modifications, alterations, and equivalent changes made to the above embodiments based on the inventive essence shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a nanocomposite salt-resistant and temperature-resistant drag-reducing agent, characterized in that, The method is as follows: S1. At room temperature, anhydrous ethanol, water and glacial acetic acid are mixed and magnetically stirred for 5 min to 10 min. Then, silane coupling agent KH550 is added and magnetically stirred for 30 min to 40 min to obtain a hydrolyzed KH550 solution. S2. At room temperature, SiO2 nanopowder is dispersed in anhydrous ethanol and ultrasonically treated for 30 min to 40 min. Then, it is added dropwise to the hydrolyzed KH550 solution obtained in S1. The reaction is then stirred for 6 h to 7 h in a water bath at 70 °C to obtain the reaction product. S3. After centrifuging the reaction product obtained in S2, wash the precipitate twice with anhydrous ethanol and then dry it under vacuum to obtain modified SiO2. S4. Disperse the modified SiO2 obtained in S3 in deionized water and sonicate for 15 min to 20 min to obtain a modified SiO2 dispersion. S5. Disperse cellulose nanofibers in deionized water and sonicate for 30 min to 40 min to obtain cellulose nanofiber dispersion. S6. After mixing the modified SiO2 dispersion obtained in S4 and the cellulose nanofiber dispersion obtained in S5, ultrasonic treatment is carried out for 1h~2h to obtain KH-550 modified CNF-SiO2 composite mother liquor. S7. Dilute the CNF-SiO2 composite mother liquor obtained in S6 with deionized water to obtain KH-550 modified CNF-SiO2 composite suspension. S8. While stirring, add methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid to deionized water in sequence. After dissolution, a monomer premix is ​​obtained. S9. Add the KH-550 modified CNF-SiO2 composite suspension obtained in S7 to the monomer premix obtained in S8. Adjust the pH of the system to 6.5-7.0 with a NaOH aqueous solution with a mass concentration of 0.1 mol / L. Stir magnetically for 30-40 minutes at room temperature to obtain a mixed solution. S10. Add ammonium persulfate and sodium bisulfite to the mixture obtained in S9, and then react for 2-6 hours under N2 atmosphere and at a temperature of 30℃-50℃. After washing the reaction product three times with anhydrous ethanol and drying it under vacuum, a nano-composite salt-resistant and temperature-resistant drag-reducing agent is obtained.

2. The preparation method of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent according to claim 1, characterized in that, The ratio of anhydrous ethanol, water, glacial acetic acid, and silane coupling agent KH550 in S1 is 10 mL: 10 mL: 0.1 mL: (0.05~0.25) g; The ratio of SiO2 nanopowder, anhydrous ethanol, and hydrolyzed KH550 solution in S2 is 1g:20mL:20mL; The ratio of modified SiO2 to deionized water in S4 is 1g:20mL; the ratio of cellulose nanofibers to deionized water in S5 is 1g:20mL.

3. The preparation method of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent according to claim 1, characterized in that, The conditions for ultrasonic treatment in S2, S4, and S6 are: frequency of 40kHz and power of 200W; the conditions for centrifugation in S3 are: rotation speed of 12000rpm and time of 10min; the conditions for vacuum drying in S3 and S10 are: temperature of 60℃ and time of 12h.

4. The preparation method of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent according to claim 1, characterized in that, The mass ratio of cellulose nanofibers in the cellulose nanofiber dispersion to modified SiO2 in the modified SiO2 dispersion in S6 is 1:(0.5~2); the mass fraction of KH-550 modified CNF-SiO2 composite mother liquor in the KH-550 modified CNF-SiO2 composite suspension in S7 is 1%~1.5%.

5. The preparation method of a nanocomposite salt-resistant and temperature-resistant drag-reducing agent according to claim 1, characterized in that, In S8, the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid accounts for 20wt% to 25wt% of the monomer premix; the mass of 2-acrylamide-2-methylpropanesulfonic acid accounts for 10wt% to 15wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid; the mass of methyl methacrylate accounts for 1wt% to 2wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid, and 2-acrylamide-2-methylpropanesulfonic acid; and the mass ratio of acrylamide to acrylic acid is (7-8):(2-3).

6. The preparation method of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent according to claim 1, characterized in that, The mass of the KH-550 modified CNF-SiO2 composite suspension in S9 accounts for 0.5wt% to 2wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix.

7. The preparation method of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent according to claim 1, characterized in that, The mass ratio of ammonium persulfate and sodium bisulfite in S10 is 1:(1-2); the total mass of ammonium persulfate and sodium bisulfite in S10 accounts for 0.1wt% to 0.2wt% of the total mass of methyl methacrylate, acrylamide, acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid in the monomer premix.

8. The preparation method of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent according to claim 1, characterized in that, The average particle size of the nanocomposite salt-resistant and temperature-resistant drag-reducing agent described in S10 is 80 nm to 140 nm.

9. An application of a nanocomposite salt-resistant and temperature-resistant drag-reducing agent prepared by the preparation method according to any one of claims 1-8, characterized in that, The nanocomposite salt-resistant and temperature-resistant drag-reducing agent is used for oilfield fracturing.

10. The application according to claim 9, characterized in that, The nanocomposite salt-resistant and temperature-resistant drag-reducing agent is used in oilfield deep wells with temperatures ≥150℃ and salt concentrations ≥50000ppm for fracturing operations.

Images