Dry powder friction reducer for salt-tolerant fracturing fluid and preparation method thereof

Abstract

The application discloses a dry powder drag reducer for salt-resistant fracturing fluid and a preparation method thereof, and belongs to the technical field of organic polymer compounds. The dry powder drag reducer comprises the following raw materials: phenolic acid compounds, hydroxyethyl methacrylate, dicyclohexyl carbodiimide, 4-dimethylamino pyridine, tetrahydrofuran, sodium bisulfite, polyformaldehyde, sodium acrylate, acrylamide, an initiator and water. In the preparation, the phenolic acid compounds are first esterified with the hydroxyethyl methacrylate to introduce polymerizable double bonds, then sulfomethylization is performed to connect sulfonic acid groups on benzene rings to obtain a multifunctional modifier, and finally, the multifunctional modifier is copolymerized with the sodium acrylate and the acrylamide to obtain the dry powder drag reducer. Compared with the prior art, the phenolic acid compounds derived from natural polyphenols are used as functional raw materials in the application, and the source of the raw materials is wide. The obtained product has excellent drag reduction performance and sand carrying capacity in high salinity brine, and the problem that the salt resistance and sand carrying capacity of the existing dry powder drag reducer are difficult to be considered simultaneously is solved.

Description

Technical Field

[0001] This invention relates to the field of organic polymer compound technology, and in particular to a dry powder drag reducer for salt-resistant fracturing fluid and its preparation method. Background Technology

[0002] As shale oil and gas resource development expands into deeper, high-salinity formation water areas, slickwater fracturing technology places higher demands on the salt resistance of drag-reducing agents. Dry powder drag-reducing agents are widely used due to their high effective content and convenient storage and transportation. However, conventional polyacrylamide-based dry powder drag-reducing agents are prone to molecular chain shrinkage in high-salinity brine due to the charge shielding effect of metal cations, leading to a significant decrease in drag reduction efficiency. Furthermore, these agents lack crosslinkable groups, resulting in insufficient proppant carrying capacity, making it difficult to meet the dual requirements of drag reduction and proppant carrying in fracturing operations.

[0003] The patent with publication number CN120923681A discloses a dry powder drag reducer for fracturing fluid. It is modified by copolymerization by preparing a polymerizable modifier containing aromatic rings and sulfonic acid ions, and introduces catechol structure into the molecular chain to form a five-membered ring borate ester with boron-based crosslinking agent. This solves the crosslinking compatibility problem when the dry powder drag reducer is used with guar gum drag reducer. However, the preparation of the modifier depends on dopamine as the starting material, and the synthesis route is relatively complicated.

[0004] The patent with publication number CN120737281A discloses a dry powder drag reducer that introduces a double-tailed hydrophobic monomer and modified nanoparticles. It improves salt and temperature resistance through hydrophobic association and nano-reinforcement mechanism. However, its components contain bromine-containing intermediates and nanomaterials, making the system relatively complex. Furthermore, it does not involve a scheme to construct a boron coordination crosslinking network using natural polyphenol structures.

[0005] In existing technologies, it is difficult to simultaneously achieve drag reduction performance and sand-carrying capacity in high-salinity brine using dry powder drag-reducing agents. Furthermore, routes using synthetic functional monomers suffer from limitations such as limited raw material sources or complex processes. Therefore, there is an urgent need to develop a dry powder drag-reducing agent that has widely available raw materials, a simple preparation process, and exhibits excellent drag reduction performance and sand-carrying capacity in high-salinity brine. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention aims to provide a dry powder drag reducer for salt-resistant fracturing fluids and its preparation method.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A dry powder drag-reducing agent for salt-resistant fracturing fluid comprises the following raw materials in parts by weight: 2-8 parts phenolic acid compounds, 1-5 parts hydroxyethyl methacrylate, 3-8 parts dicyclohexylcarbodiimide, 0.1-0.5 parts 4-dimethylaminopyridine, 100-150 parts tetrahydrofuran, 2-7 parts sodium bisulfite, 1-5 parts paraformaldehyde, 15-30 parts sodium acrylate, 100-130 parts acrylamide, 0.05-0.3 parts initiator, and 380-620 parts water.

[0008] The phenolic acid compound is at least one of gallic acid, 3,4-dihydroxybenzoic acid, 2,3-dihydroxybenzoic acid, 3,5-dihydroxybenzoic acid, 3,4-dihydroxycinnamic acid, and 2,3,4-trihydroxybenzoic acid.

[0009] The initiator is a mixture of potassium persulfate and sodium bisulfite in a mass ratio of 6-8:8-10.

[0010] Preferably, the phenolic acid compound is composed of gallic acid and 3,4-dihydroxycinnamic acid in a mass ratio of 0.5-2:0.5-2.

[0011] The preparation method of the dry powder drag-reducing agent for salt-resistant fracturing fluid is as follows: Step 1: Phenolic acid compounds, hydroxyethyl methacrylate, dicyclohexylcarbodiimide, 4-dimethylaminopyridine and tetrahydrofuran are mixed and stirred at 50-60℃ for 10-30 hours under nitrogen atmosphere. After the reaction is completed, the precipitate is removed by filtration. The filtrate is concentrated by vacuum distillation, redissolved in ethyl acetate, washed successively with saturated brine, dried with anhydrous sodium sulfate, and the solvent is recovered by vacuum distillation to obtain the pretreated product. Step 2: Add the pretreated material prepared in Step 1 to water, add sodium bisulfite and paraformaldehyde, adjust the pH of the system to 8.5-9.5, stir the reaction, adjust the pH to neutral after the reaction is completed, concentrate by vacuum distillation, and freeze dry to obtain the multifunctional modifier. Step 3: Mix the multifunctional modifier prepared in Step 2, sodium acrylate, acrylamide, and water in a reaction vessel, stir until completely dissolved, adjust the pH of the system to 7-8, add the initiator to the system, let the reaction stand to obtain the polymerization product, then granulate the polymerization product by extrusion, vacuum dry it, and then crush and sieve it to obtain the dry powder drag reducer.

[0012] Preferably, the preparation method of the dry powder drag-reducing agent for salt-resistant fracturing fluid is as follows: Step 1: Gallic acid, hydroxyethyl methacrylate, dicyclohexylcarbodiimide, 4-dimethylaminopyridine, and tetrahydrofuran are mixed and stirred at 50-60°C for 8-16 hours under nitrogen atmosphere. Then, 3,4-dihydroxycinnamic acid, hydroxyethyl methacrylate, dicyclohexylcarbodiimide, 4-dimethylaminopyridine, and anhydrous tetrahydrofuran are added to the reaction system, and the mixture is stirred at 50-60°C for 4-8 hours under nitrogen atmosphere. After the reaction is completed, the precipitate is removed by filtration. The filtrate is concentrated by vacuum distillation, redissolved in ethyl acetate, washed successively with saturated brine, dried over anhydrous sodium sulfate, and the solvent is recovered by vacuum distillation to obtain the pretreated product. Step 2: Add the pretreated material prepared in Step 1 to water, add sodium bisulfite and paraformaldehyde, adjust the pH of the system to 8.5-9.5, stir the reaction, adjust the pH to neutral after the reaction is completed, concentrate by vacuum distillation, and freeze dry to obtain the multifunctional modifier. Step 3: Mix the multifunctional modifier prepared in Step 2, sodium acrylate, acrylamide, and water in a reaction vessel, stir until completely dissolved, adjust the pH of the system to 7-8, add the initiator to the system, let the reaction stand to obtain the polymerization product, then granulate the polymerization product by extrusion, vacuum dry it, and then crush and sieve it to obtain the dry powder drag reducer.

[0013] Each of the vacuum distillation concentrations is independently concentrated to 1 / 3 to 1 / 10 of its original volume.

[0014] In step 2, the stirring reaction is carried out at 60-80°C for 5-20 hours.

[0015] In step 2, the pH of the system is adjusted to 8.5-9.5 using a 0.1-1 mol / L sodium hydroxide aqueous solution; the pH is adjusted to neutral using a 0.5-2 mol / L hydrochloric acid solution.

[0016] In step 3, the pH of the system is adjusted to 7-8 using 0.5-2 mol / L hydrochloric acid.

[0017] In step 3, the extrusion granulation is to produce particles with a particle size of 1-5 mm.

[0018] In step 3, the vacuum drying is performed at 50-70°C until the moisture content is no higher than 10%.

[0019] In step 3, the sieving is done through a 20-100 mesh sieve.

[0020] Existing dry powder drag-reducing agents suffer from significant drag reduction due to the charge-shielding effect of metal cations in highly saline solutions, leading to polymer chain shrinkage. Furthermore, conventional polyacrylamide drag-reducing agents lack crosslinkable groups, resulting in insufficient sand-carrying capacity and crosslinking compatibility issues when mixed with guar gum drag-reducing agents. To address these problems, this invention introduces phenolic acid compounds containing polyphenolic hydroxyl structures to copolymerize and modify polyacrylamide. Sulfonation introduces sulfonic acid groups onto the molecular chain through sulfonation, providing electrostatic repulsion to counteract the charge-shielding effect. Simultaneously, the catechol or pyrogallol structure forms a crosslinking network with the borax crosslinking agent, enabling the dry powder drag-reducing agent to possess both salt-resistant drag reduction and sand-carrying capacity.

[0021] In single phenolic acid systems, gallic acid, due to its pyrogallol structure, can incorporate more sulfonic acid groups and provide multiple sets of ortho-crosslinking sites after sulfonation, resulting in superior overall performance. To further enhance performance, this invention discovered that combining gallic acid with 3,4-dihydroxycinnamic acid can produce a synergistic effect: the gallic acid derivative provides a high density of sulfonic acid groups and multiple crosslinking sites, while the carboxyl group of 3,4-dihydroxycinnamic acid is connected to the benzene ring via a carbon-carbon double bond. After esterification, the benzene ring is connected to the polymer backbone by unsaturated spacer arms, causing the benzene ring to extend outward in space. Its steric hindrance effect can further inhibit the coiling of the molecular chain in brine and enhance the stability of the crosslinking network. The two complement each other, resulting in a compound with superior performance compared to single phenolic acids.

[0022] Based on the compound formulation, this invention further discovered that the esterification kinetics of the two phenolic acids differ. During one-step mixed esterification, they competitively react with hydroxyethyl methacrylate, easily leading to uneven esterification. The actual ratio of the two functional derivatives deviates from the feed ratio, affecting the full realization of the synergistic effect. Therefore, this invention adopts a stepwise esterification strategy: first, gallic acid is added for pre-esterification, and then 3,4-dihydroxycinnamic acid is added for further reaction. This makes the esterification degree of both more consistent, the derivative ratio closer to the expected value, and the synergistic effect more fully realized, thereby obtaining a dry powder drag reducer with further improved performance.

[0023] Compared with the prior art, the present invention has the following beneficial technical effects: 1) This invention modifies the phenolic acid compound containing polyphenolic hydroxyl groups by sulfonation copolymerization, simultaneously attaching sulfonic acid groups and catechol or pyrogallol structures to the molecular chain, so that the dry powder drag reducer has good drag reduction performance in high salinity brine, and can be cross-linked with borax to form a network, significantly improving sand carrying capacity and high temperature viscosity retention rate.

[0024] 2) This invention uses gallic acid and 3,4-dihydroxycinnamic acid in combination. The former provides high-density sulfonic acid groups and multiple cross-linking sites, while the latter, through the steric hindrance effect generated by the connection of benzene rings with unsaturated spacer arms, further inhibits the shrinkage of molecular chains in brine and enhances the stability of the cross-linking network. The two complement each other and produce synergistic effects, with overall performance superior to single phenolic acids.

[0025] 3) This invention adopts a stepwise esterification process to address the differences in esterification activity of the compound components. Gallic acid and 3,4-dihydroxycinnamic acid are added sequentially for esterification reaction, so that the degree of esterification of the two tends to be consistent, the proportion of the resulting derivatives is closer to the expectation, the synergistic effect is more fully exerted, and the drag reduction rate and viscosity retention rate are further improved. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0027] The specific details of the raw materials used in the embodiments of the present invention are as follows: Hydroxyethyl methacrylate: CAS No. 868-77-9, colorless transparent liquid, purity ≥99%, commercially available.

[0028] Dicyclohexylcarbodiimide: CAS No. 538-75-0, purity ≥99.0%, commercially available.

[0029] 4-Dimethylaminopyridine: CAS No. 1122-58-3, purity ≥98%, commercially available.

[0030] Paraformaldehyde: CAS No. 30525-89-4, white powder, average degree of polymerization 10-12, commercially available, purity 95%.

[0031] Sodium acrylate: CAS No. 7446-81-3, white powder, purity ≥98%, commercially available.

[0032] Acrylamide: CAS No. 79-06-1, white crystalline solid, purity ≥99%, commercially available.

[0033] Gallic acid: CAS No. 149-91-7, purity ≥98%, commercially available.

[0034] 3,4-Dihydroxybenzoic acid: CAS No. 99-50-3, purity ≥97%, commercially available.

[0035] 2,3-Dihydroxybenzoic acid: CAS No. 303-38-8, purity ≥97%, commercially available.

[0036] 3,5-Dihydroxybenzoic acid: CAS No. 99-10-5, purity ≥98%, commercially available.

[0037] 3,4-Dihydroxycinnamic acid: CAS No. 331-39-5, purity ≥98%, commercially available.

[0038] 2,3,4-Trihydroxybenzoic acid: CAS No. 610-02-6, purity ≥97%, commercially available.

[0039] In the embodiments and comparative examples of this invention, all raw materials are commercially available products.

[0040] Example 1 The preparation method of a dry powder drag-reducing agent for salt-resistant fracturing fluid is as follows, by weight: Step 1: Mix 5 parts of phenolic acid compound, 3.5 parts of hydroxyethyl methacrylate, 5.5 parts of dicyclohexylcarbodiimide, 0.3 parts of 4-dimethylaminopyridine and 120 parts of tetrahydrofuran. Stir and react at 55°C for 18 hours under nitrogen atmosphere. After the reaction is complete, filter to remove the precipitate. Concentrate the filtrate to 1 / 5 of the original volume by vacuum distillation. Redissolve it in ethyl acetate, wash with saturated brine, dry with anhydrous sodium sulfate, and then recover the solvent by vacuum distillation to obtain the pretreated product. Step 2: Add the pretreated material prepared in Step 1 to 100 parts of water, add 4.5 parts of sodium bisulfite and 3 parts of paraformaldehyde, adjust the pH of the system to 9.0 with 0.5 mol / L sodium hydroxide aqueous solution, stir the reaction at 75℃ for 8 hours to carry out sulfonylation, after the reaction is completed, adjust the pH to neutral with 1 mol / L hydrochloric acid, concentrate to 1 / 5 of the original volume by vacuum distillation, and freeze dry to obtain the multifunctional modifier; Step 3: Mix the multifunctional modifier prepared in Step 2, 22 parts sodium acrylate, 110 parts acrylamide, and 400 parts water in a reaction vessel and stir until completely dissolved. Then, adjust the pH of the system to 7.5 with 1 mol / L hydrochloric acid, add 0.1 parts initiator to the system, and let it stand for 3 hours to obtain the polymerization product. Then, granulate the polymerization product by extrusion to form particles with a particle size of 3 mm, vacuum dry at 60°C until the water content is no more than 10%, and then crush and pass through a 50-mesh sieve to obtain the dry powder drag reducer.

[0041] The initiator is a mixture of potassium persulfate and sodium bisulfite in a mass ratio of 7:9.

[0042] The phenolic acid compound is gallic acid.

[0043] Example 2 The preparation method of a dry powder drag reducer for salt-resistant fracturing fluid is basically the same as that in Example 1, except that the phenolic acid compound is 3,4-dihydroxybenzoic acid.

[0044] Example 3 The preparation method of a dry powder drag reducer for salt-resistant fracturing fluid is basically the same as that in Example 1, except that the phenolic acid compound is 2,3-dihydroxybenzoic acid.

[0045] Example 4 The preparation method of a dry powder drag reducer for salt-resistant fracturing fluid is basically the same as that in Example 1, except that the phenolic acid compound is 3,5-dihydroxybenzoic acid.

[0046] Example 5 The preparation method of a dry powder drag reducer for salt-resistant fracturing fluid is basically the same as that in Example 1, except that the phenolic acid compound is 3,4-dihydroxycinnamic acid.

[0047] Example 6 The preparation method of a dry powder drag reducer for salt-resistant fracturing fluid is basically the same as that in Example 1, except that the phenolic acid compound is 2,3,4-trihydroxybenzoic acid.

[0048] Example 7 The preparation method of a dry powder drag reducer for salt-resistant fracturing fluid is basically the same as that in Example 1, except that the phenolic acid compound is composed of gallic acid and 3,4-dihydroxycinnamic acid in a mass ratio of 1:1.

[0049] Example 8 The preparation method of a dry powder drag reducer for salt-resistant fracturing fluid is basically the same as that in Example 1, except that the phenolic acid compound is composed of 3,4-dihydroxybenzoic acid and 2,3-dihydroxybenzoic acid in a mass ratio of 1:1.

[0050] Example 9 The preparation method of a dry powder drag-reducing agent for salt-resistant fracturing fluid is as follows, by weight: Step 1: Mix 2.5 parts gallic acid, 1.75 parts hydroxyethyl methacrylate, 2.75 parts dicyclohexylcarbodiimide, 0.15 parts 4-dimethylaminopyridine, and 60 parts tetrahydrofuran. Stir and react at 55°C for 12 hours under nitrogen atmosphere. Then add 2.5 parts 3,4-dihydroxycinnamic acid, 1.75 parts hydroxyethyl methacrylate, 2.75 parts dicyclohexylcarbodiimide, 0.15 parts 4-dimethylaminopyridine, and 60 parts anhydrous tetrahydrofuran to the reaction system. Continue stirring and reacting at 55°C for 6 hours under nitrogen atmosphere. After the reaction is complete, filter to remove the precipitate. Concentrate the filtrate to 1 / 5 of its original volume by vacuum distillation. Redissolve it in ethyl acetate, wash with saturated brine, dry with anhydrous sodium sulfate, and then recover the solvent by vacuum distillation to obtain the pretreated product. Step 2: Add the pretreated material prepared in Step 1 to 100 parts of water, add 4.5 parts of sodium bisulfite and 3 parts of paraformaldehyde, adjust the pH of the system to 9.0 with 0.5 mol / L sodium hydroxide aqueous solution, stir the reaction at 75℃ for 8 hours to carry out sulfonylation, after the reaction is completed, adjust the pH to neutral with 1 mol / L hydrochloric acid, concentrate to 1 / 5 of the original volume by vacuum distillation, and freeze dry to obtain the multifunctional modifier; Step 3: Mix the multifunctional modifier prepared in Step 2, 22 parts sodium acrylate, 110 parts acrylamide, and 400 parts water in a reaction vessel and stir until completely dissolved. Then, adjust the pH of the system to 7.5 with 1 mol / L hydrochloric acid, add 0.1 parts initiator to the system, and let it stand for 3 hours to obtain the polymerization product. Then, granulate the polymerization product by extrusion to form particles with a particle size of 3 mm, vacuum dry at 60°C until the water content is no more than 10%, and then crush and pass through a 50-mesh sieve to obtain the dry powder drag reducer.

[0051] The initiator is a mixture of potassium persulfate and sodium bisulfite in a mass ratio of 7:9.

[0052] Comparative Example 1 The preparation method of a dry powder drag reducer for salt-resistant fracturing fluid is basically the same as that in Example 1, except that the phenolic acid compound is not added.

[0053] Test Example 1 Drag reduction rate test: A fracturing fluid tubing friction testing device was used, with a test string having an inner diameter of 10 mm and a length of 3 m. The brine used in the test was simulated high-salinity formation water from an oilfield (total salinity not less than 20,000 mg / L, of which Ca...). 2+ +Mg 2+ (Content not less than 2000 mg / L). Salt solutions containing 0.08 wt% of the dry powder drag-reducing agent prepared in the embodiments and comparative examples of this invention were prepared as drag-reducing hydraulic fracturing fluids, with the same salt solution without drag-reducing agent as a blank reference. At room temperature (25±2℃), the drag-reducing hydraulic fracturing fluid was pumped through a test string at a constant flow rate of 10 L / min, and the pressure difference data across the string was collected. The drag reduction rate was calculated using the following formula: DR = (ΔP0 - ΔP) / ΔP0 × 100% In the formula: DR is the drag reduction rate (%); ΔP0 is the pressure difference (kPa) when blank brine passes through the tubing; ΔP is the pressure difference (kPa) when drag-reduced hydraulic fracturing fluid passes through the tubing. Each test group was repeated 3 times and the average value was taken.

[0054] The test results are shown in Table 1.

[0055] Example 1 74.3 Example 2 70.5 Example 3 69.2 Example 4 68.1 Example 5 71.6 Example 6 72.8 Example 7 78.5 Example 8 70.1 Example 9 80.2 Comparative Example 1 51.8 Test Example 2 High-temperature shear viscosity retention test: 0.2 wt% of the dry powder drag-reducing agent prepared in the embodiments and comparative examples of this invention, 0.3 wt% of guar gum powder, and 0.1 wt% of borax crosslinking agent were added to water, and stirred until homogeneous to prepare a crosslinked fracturing fluid with a pH of 10. A high-temperature and high-pressure rheometer equipped with a concentric cylinder measurement system was used. Test conditions: shear rate set to 170 s. -1 The fracturing fluid was continuously sheared in a 90℃ water bath for 120 min, and the retention viscosity at the shear endpoint was recorded. Each test was repeated 3 times, and the viscosity (mPa·s) of each fracturing fluid at 90℃ was recorded. The viscosity retention rate was calculated using the following formula: η=(μ / μ0)×100%, where: η is the viscosity retention rate (%); μ is the viscosity of the fracturing fluid at 90℃ for 170 min. -1 The final viscosity (mPa·s) of the fracturing fluid after shearing for 120 min under the specified conditions; μ0 is the viscosity of the fracturing fluid at 90℃ for 170 s. -1 The viscosity (mPa·s) at the initial shear time under the given conditions was calculated. The average value was taken, and the relevant test data are summarized in Table 2.

[0056] Table 2 Example 1 62.4 Example 2 58.1 Example 3 57.3 Example 4 50.6 Example 5 59.2 Example 6 61.7 Example 7 68.3 Example 8 58.0 Example 9 73.1 Comparative Example 1 18.2 Comparative Example 1 of this invention did not add any phenolic acid compounds, and the resulting polymer molecular chain had neither sulfonic acid groups nor crosslinked ortho-phenolic hydroxyl groups. In highly saline solutions, the charge shielding effect of metal cations causes the molecular chain to curl up, significantly reducing drag reduction capability; simultaneously, due to the lack of boron-coordinated crosslinking sites, viscosity is maintained only by guar gum and borax under high temperature and high shear conditions, resulting in low viscosity retention. Examples 1 to 6 introduced phenolic acid compounds with different phenolic hydroxyl structures, respectively. By incorporating sulfonic acid groups through sulfonation, their electrostatic repulsion can counteract the charge shielding effect, allowing the molecular chain to maintain a more extended conformation in the saline solution, thereby achieving better drag reduction performance; at the same time, the catechol or pyrogallol structures on the molecular chain can form a crosslinking network with borax, improving viscosity retention. Among them, Example 1 used gallic acid, whose pyrogallol structure can introduce more sulfonic acid groups and provide more than one set of ortho-crosslinking sites, resulting in a relatively higher crosslinking density. Therefore, it exhibits better drag reduction and viscosity retention effects in the single phenolic acid series.

[0057] Example 7, which combines gallic acid and 3,4-dihydroxycinnamic acid, shows superior drag reduction and viscosity retention compared to examples using either component alone. This may be due to the complementary functions of the two phenolic acid derivatives. Gallic acid, containing a pyrogallol structure, can incorporate numerous sulfonic acid groups after sulfonation, providing strong electrostatic repulsion to counteract the charge shielding effect of metal cations in brine. Simultaneously, its multiple ortho- and ortho-hydroxyl groups can form a high density of boron-coordinated crosslinking sites. In 3,4-dihydroxycinnamic acid, the carboxyl group is linked to a benzene ring via a vinyl group. After esterification and integration into the main chain, the benzene ring is connected to the main chain by unsaturated spacer arms, extending the benzene ring outwards. This steric hindrance further inhibits chain curling in brine, and the structure enhances network stability within the crosslinking network. The electrostatic repulsion of the sulfonic acid groups and the steric hindrance generated by the outwardly extending benzene ring form a composite salt-resistant mechanism, while the high-density crosslinking sites and stable network framework form a composite reinforcement mechanism, resulting in superior overall performance of the combined compound compared to a single phenolic acid. Example 8 uses a combination of two isomers, 3,4-dihydroxybenzoic acid and 2,3-dihydroxybenzoic acid. Since the two are similar in structure and have similar functions, it is difficult to form the above-mentioned complementary effect. Therefore, the performance of the combination is comparable to that of using either one alone.

[0058] Example 9, based on the compounding in Example 7, adopted a stepwise esterification strategy to address the kinetic differences between the two phenolic acids in the esterification reaction. Gallic acid has a carboxyl group directly attached to the benzene ring, while 3,4-dihydroxycinnamic acid has a carboxyl group attached to the benzene ring via a double bond. Due to their different structures, they competitively react with hydroxyethyl methacrylate during one-step mixed esterification, easily leading to uneven esterification and a final derivative ratio deviating from the feed ratio. Example 9 pre-esterified the derivatives with gallic acid before adding 3,4-dihydroxycinnamic acid for further reaction, resulting in a more consistent esterification degree. The ratio of the two derivatives in the resulting multifunctional modifier was closer to the expected ratio, allowing for a more fully realized synergistic effect. Consequently, the drag reduction rate and viscosity retention rate were further improved compared to Example 7.

Claims

1. A method for preparing a dry powder drag-reducing agent for salt-resistant fracturing fluid, characterized in that, The raw materials include the following parts by weight: 2-8 parts phenolic acid compounds, 1-5 parts hydroxyethyl methacrylate, 3-8 parts dicyclohexylcarbodiimide, 0.1-0.5 parts 4-dimethylaminopyridine, 100-150 parts tetrahydrofuran, 2-7 parts sodium bisulfite, 1-5 parts paraformaldehyde, 15-30 parts sodium acrylate, 100-130 parts acrylamide, 0.05-0.3 parts initiator, and 380-620 parts water; The phenolic acid compound is composed of gallic acid and 3,4-dihydroxycinnamic acid in a mass ratio of 0.5-2:0.5-2; The initiator is a mixture of potassium persulfate and sodium bisulfite in a mass ratio of 6-8:8-10. The preparation method of the dry powder drag-reducing agent for salt-resistant fracturing fluid is as follows: Step 1: Phenolic acid compounds, hydroxyethyl methacrylate, dicyclohexylcarbodiimide, 4-dimethylaminopyridine and tetrahydrofuran are mixed and stirred at 50-60℃ for 10-30 hours under nitrogen atmosphere. After the reaction is completed, the precipitate is removed by filtration. The filtrate is concentrated by vacuum distillation, redissolved in ethyl acetate, washed successively with saturated brine, dried with anhydrous sodium sulfate, and the solvent is recovered by vacuum distillation to obtain the pretreated product. Step 2: Add the pretreated material prepared in Step 1 to water, add sodium bisulfite and paraformaldehyde, adjust the pH of the system to 8.5-9.5, stir the reaction, adjust the pH to neutral after the reaction is completed, concentrate by vacuum distillation, and freeze dry to obtain the multifunctional modifier. Step 3: Mix the multifunctional modifier prepared in Step 2, sodium acrylate, acrylamide, and water in a reaction vessel, stir until completely dissolved, adjust the pH of the system to 7-8, add the initiator to the system, let the reaction stand to obtain the polymerization product, then granulate the polymerization product by extrusion, vacuum dry it, and then crush and sieve it to obtain the dry powder drag reducer.

2. The method as described in claim 1, characterized in that, Step 1 can also be performed using the following methods: Step 1: Gallic acid, hydroxyethyl methacrylate, dicyclohexylcarbodiimide, 4-dimethylaminopyridine, and tetrahydrofuran are mixed and stirred at 50-60°C for 8-16 hours under nitrogen atmosphere. Then, 3,4-dihydroxycinnamic acid, hydroxyethyl methacrylate, dicyclohexylcarbodiimide, 4-dimethylaminopyridine, and anhydrous tetrahydrofuran are added to the reaction system, and the mixture is stirred at 50-60°C for another 4-8 hours under nitrogen atmosphere. After the reaction is complete, the precipitate is removed by filtration. The filtrate is concentrated by vacuum distillation, redissolved in ethyl acetate, washed successively with saturated brine, dried over anhydrous sodium sulfate, and the solvent is recovered by vacuum distillation to obtain the pretreated product.

3. The method as described in claim 1 or 2, characterized in that, Each of the vacuum distillation concentrations is independently concentrated to 1 / 3 to 1 / 10 of its original volume.

4. The method as described in claim 1, characterized in that, In step 2, the stirring reaction is carried out at 60-80℃ for 5-20 hours; the pH of the system is adjusted to 8.5-9.5 using a 0.1-1 mol / L sodium hydroxide aqueous solution; and the pH is adjusted to neutral using 0.5-2 mol / L hydrochloric acid.

5. The method as described in claim 1, characterized in that, In step 3, the pH of the system is adjusted to 7-8 using 0.5-2 mol / L hydrochloric acid.

6. The method as described in claim 1, characterized in that, In step 3, the extrusion granulation is to produce particles with a particle size of 1-5 mm; the vacuum drying is to vacuum dry at 50-70℃ until the moisture content is not higher than 10%; and the sieving is to pass through a 20-100 mesh sieve.