Polymer thickening agent, preparation method thereof and prepared fracturing fluid

Abstract

The invention provides a polymer thickening agent, a preparation method thereof and fracturing fluid prepared from the polymer thickening agent, and belongs to the technical field of fracturing fluid. The invention relates to a water-soluble polymer, which is prepared from a polymer monomer, acrylamide, methacryloyloxyethyl trimethyl ammonium chloride and 1-vinyl-2-pyrrolidone according to a mass ratio of (111-218): (21-36): (16-23): (41-52). The polymer thickening agent prepared by the invention has better tackifying performance, shearing resistance, salt resistance, excellent temperature resistance, good sand-carrying performance and better acid resistance, reduces the acid rock reaction rate, improves the penetration depth of acid etching cracks, is non-toxic, safe and environment-friendly, is safer in gel breaking and lower in damage, and has a wide application prospect.

Description

Technical Field

[0001] This invention relates to the field of fracturing fluid technology, specifically to a polymer thickener, its preparation method, and the fracturing fluid obtained therefrom. Background Technology

[0002] Currently, the thickeners used in fracturing fluids both domestically and internationally are mainly plant gums and their derivatives, viscoelastic surfactants (VES), and synthetic polymers. Plant gums and their derivatives have good thickening effects, but they contain a large amount of water-insoluble plant fibers, and the large amount of residue after breaking the gum can clog the fracturing fractures, directly affecting the fracturing effect. Chemically modified plant gums have improved performance, but cost control is difficult given the unstable price of plant gums. VES fracturing fluid systems do not contain water-insoluble substances, break the gum easily without residue, and have the advantages of simple system preparation, no other chemical additives, and simple construction. However, this system requires a large amount of viscoelastic surfactant, resulting in high cost, which limits its application in oilfields. Synthetic polymers do not contain water-insoluble substances, have good viscoelasticity and thixotropy, and require low dosage, showing great potential for development.

[0003] The literature (Du Daijun, Development and Performance Evaluation of Supramolecular Self-Assembled Fracturing Fluid Thickener [D], Southwest Petroleum University, 2017) introduced functional monomers vinyl-modified β-cyclodextrin and N-phenylethyl-methacrylamide into polyacrylamide to prepare a supramolecular self-assembled fracturing fluid thickener. The aim was to achieve a large fracturing fluid system with stable viscosity of 124.90 mPa·s and 60.54 mPa·s after shearing for 120 min at temperatures of 90℃ and 118℃ and a shear rate of 170 s⁻¹. This indicates that the currently synthesized polymer exhibits good water solubility, thickening properties, viscoelasticity, and shear dilution properties, but its temperature resistance and shear resistance need further improvement.

[0004] US Patent 5,551,516 discloses a clean fracturing fluid based on small-molecule surfactants. Its viscosity is adjusted by regulating the type of self-assembly structure of the small-molecule surfactant, without using any polymers, successfully solving the problem of residue in formations from plant-based fracturing fluids. However, because its main active ingredient, geraniol dihydroxyethyl ammonium chloride, requires selective catalytic hydrogenation under high temperature and high pressure, the process is not only complex and dangerous but also expensive, costing more than twenty times that of conventional plant-based fracturing fluids, making large-scale application difficult.

[0005] The small-molecule surfactant fracturing fluid proposed by Chinese patents CN1439691A and CN1621484B, which mainly consists of long-chain alkyl trimethyl quaternary ammonium salt and oleic acid, has a significantly lower cost than foreign clean fracturing fluids, about three times the cost of plant-based rubber fracturing fluids. However, because the surfactants used have shorter hydrophobic chains and weaker intermolecular hydrophobic interactions, they cannot form stable worm-like aggregates in high-temperature environments and are only suitable for hydraulic fracturing operations in low-temperature formations with temperatures below 80°C. Summary of the Invention

[0006] The purpose of this invention is to provide a polymer thickener, its preparation method, and the fracturing fluid obtained therefrom. This fluid has good thickening properties, excellent shear resistance, salt resistance, and temperature resistance, good sand carrying capacity, good acid resistance, reduces the acid-rock reaction rate, increases the penetration depth of acid-etched fractures, is non-toxic, safe and environmentally friendly, and is safer to break down with less damage, thus having broad application prospects.

[0007] The technical solution of this invention is implemented as follows:

[0008] This invention provides a polymer thickener having the structure shown in Formula I:

[0009]

[0010] Where a:b:c:d = 10-15:30-50:15-20:20-25.

[0011] As a further improvement of the present invention, it is prepared from polymer monomers, acrylamide, methacryloyloxyethyltrimethylammonium chloride, and 1-vinyl-2-pyrrolidone in a mass ratio of 111-218:21-36:16-23:41-52, and the structural formula of the polymer monomers is shown in Formula II:

[0012]

[0013] R1 and R2 are C6-C12 alkyl chains.

[0014] As a further improvement of the present invention, the preparation method of the polymer monomer is as follows:

[0015] S1. Tetrahydrophthalic anhydride, ethanol, and concentrated sulfuric acid were heated to reflux, stirred, and the pH of the solution was adjusted to obtain intermediate 1, with the following structure:

[0016] S2. Intermediate 1 and alkylamine were heated and stirred to react, yielding intermediate 2, with the following structure:

[0017] S3. Intermediate 2 and sodium 3-chloro-2-hydroxypropanesulfonate were heated and stirred to prepare intermediate 3, with the following structure:

[0018] S4. Intermediate 3 is reacted with a haloalkane to prepare intermediate 4, with the following structure:

[0019] S5. React intermediate 4 with butyryl lactone to obtain the product.

[0020] As a further improvement of the present invention, in step S1, the mass ratio of tetrahydrophthalic anhydride, ethanol, and concentrated sulfuric acid is 20-30:50-70:2-3, the mixture is heated to reflux, and stirred for 3-5 hours, with the pH of the solution adjusted to 7.5-8.5; in step S2, the molar ratio of intermediate 1 to alkylamine is 1:2.2-2.5, the temperature of the heating and stirring reaction is 120-130°C, and the time is 28-35 hours, wherein the alkylamine is selected from at least one of n-hexylamine, n-heptylamine, n-nonylamine, n-octylamine, n-decylamine, n-undecylamine, and n-dodecylamine.

[0021] As a further improvement of the present invention, in step S3, the molar ratio of intermediate 2 and sodium 3-chloro-2-hydroxypropanesulfonate is 1-1.1:2, and the heating and stirring reaction temperature is 60-70°C for 4-6 hours; in step S4, the molar ratio of intermediate 3 and haloalkane is 1-1.1:2, the reaction temperature is 50-60°C for 3-5 hours, and a base is added, wherein the base is selected from at least one of triethylamine, NaOH, and KOH. The haloalkane is selected from at least one of 1-chlorohexane, 1-chloroheptane, 1-chlorooctane, 1-chlorononane, 1-chlorodecane, 1-chloroundecane, 1-chlorododecane, 1-bromohexane, 1-bromoheptane, 1-bromooctane, 1-bromononane, 1-bromodecane, 1-bromoundecane, and 1-bromododecane; the molar ratio of intermediate 4 and butyrylamide in step S5 is 1:2.2-2.5, the reaction temperature is 45-55℃, and the reaction time is 20-24h.

[0022] As a further improvement of the present invention, the method for preparing the polymer monomer specifically includes the following steps:

[0023] S1. Mix 20-30 parts by weight of tetrahydrophthalic anhydride, 50-70 parts by weight of ethanol, and 2-3 parts by weight of concentrated sulfuric acid, heat under reflux, stir and react for 3-5 hours, remove ethanol under reduced pressure, add toluene, wash with water, adjust the pH of the solution to 7.5-8.5, wash with water, dry, remove toluene under reduced pressure, and obtain intermediate 1.

[0024] S2. Add 0.1 molar equivalent of intermediate 1 and 0.22-0.25 molar equivalent of alkylamine to xylene, heat to 120-130℃, stir and react for 28-35 h, filter, wash and dry to obtain intermediate 2;

[0025] S3. Dissolve 0.2 molar equivalents of sodium 3-chloro-2-hydroxypropanesulfonate in ethylene glycol, add 0.1-0.11 molar equivalents of intermediate 2, heat to 60-70℃, adjust the pH of the mixed solution to 7.5-8.5 with 40% NaOH aqueous solution, stir the reaction for 4-6 hours, filter, wash, and dry to obtain intermediate 3;

[0026] S4. Add 0.1-0.11 molar equivalent of intermediate 3, 0.2 molar equivalent of haloalkane, and 0.5 molar equivalent of base to acetonitrile, heat to 50-60℃, stir and react for 3-5 hours, remove solvent under reduced pressure, wash, filter, and dry to obtain intermediate 4.

[0027] S5. Dissolve 0.1 molar equivalent intermediate 4 and 0.22-0.25 molar equivalent butyryl lactone in acetone, heat to 45-55℃, stir and react for 20-24 hours, filter, wash, and dry to obtain the product.

[0028] As a further improvement of the present invention, a raw material co-solvent and an initiation system are added during the preparation of the polymer thickener; the mass of the co-solvent is 0.25-0.35 wt% of the total mass of the monomers, and the mass of the initiation system is 0.02-0.05 wt% of the total mass of the monomers; the co-solvent is acetamide; the initiation system is a mixture of an initiator and sodium sulfite in a mass ratio of 0.9-1.2:0.7, and the initiator is selected from at least one of sodium persulfate, ammonium persulfate, and potassium persulfate.

[0029] This invention further protects a method for preparing the above-mentioned polymer thickener, comprising the following steps:

[0030] (1) Add polymer monomers, acrylamide, methacryloyloxyethyltrimethylammonium chloride, 1-vinyl-2-pyrrolidone, and cosolvent to water to prepare an aqueous solution with a total monomer mass concentration of 32-37 wt%.

[0031] (2) Under the protection of inert gas, add the initiation system and water to prepare an aqueous solution with a total monomer mass concentration of 27-30 wt%. Stir the reaction at 40-50℃ for 2-4 h. Filter, wash, dry and crush the product to obtain the polymer thickener.

[0032] The present invention further protects a fracturing fluid containing the above-mentioned polymer thickener.

[0033] As a further improvement of the present invention, it is prepared from the following raw materials in parts by weight: 0.2-2 parts polymer thickener, 0.2-0.4 parts crosslinking agent, 0.1-0.5 parts drainage aid, and 90-95 parts water.

[0034] Preferably, the discharge aid is selected from at least one of methanol, ethanol, isopropanol, and cyclohexanol.

[0035] Preferably, the crosslinking agent is selected from at least one of sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, sodium tetradecyl sulfate, sodium hexadecyl sulfate, sodium octadecylbenzenesulfonate, hexadecyltrimethylammonium bromide, hexadecyldimethylbenzylammonium bromide, hexadecyltrimethylammonium chloride, dodecyltrimethylammonium chloride, tetradecyldimethylbenzylammonium chloride, and octadecyltrimethylammonium bromide.

[0036] The crosslinking agent is a mixture of sodium dodecyl sulfate and dodecyltrimethylammonium chloride in a mass ratio of 4-6:1-2.

[0037] The present invention has the following beneficial effects:

[0038] This invention prepares a polymer thickener by designing and synthesizing a polymer monomer with a rigid ring structure, polysulfonic acid groups, and quaternary ammonium salt structures, and is linked to a hydrophobic long alkyl chain. The rigid structure significantly improves the polymer's heat resistance and shear resistance, the sulfonic acid group structure improves the polymer's temperature and salt resistance, and the quaternary ammonium salt structure improves the polymer's acid solubility and temperature resistance. The hydrophobic long alkyl chain also enhances the polymer's salt thickening effect and improves the polymer's temperature and salt resistance. By introducing the polymer monomer of this invention, the thickener's shear resistance, salt resistance, and temperature resistance are greatly improved, while also exhibiting good sand-carrying capacity. In addition, the rigid five-membered ring monomer 1-vinyl-2-pyrrolidone monomer is added during the preparation of the thickener, which can also synergistically improve the polymer's shear resistance and heat resistance, while also providing high-temperature retardation. Methacryloxyethyltrimethylammonium chloride is also added, which improves the thickener's acid solubility, provides good thickening effect, and also exhibits good acid resistance.

[0039] This invention incorporates a crosslinking agent composed of a combination of anionic and cationic surfactants, which can form physical crosslinks and significantly increase the viscosity of the solution. Compared with chemical crosslinking, the physical crosslinking method is simpler and more thorough, resulting in less residue and less damage.

[0040] The polymer thickener prepared by this invention has good thickening properties, excellent shear resistance, salt resistance, and temperature resistance, good sand carrying capacity, good acid resistance, reduces the acid-rock reaction rate, increases the penetration depth of acid-etched cracks, is non-toxic, safe and environmentally friendly, makes debonding safer and less harmful, and has broad application prospects. Detailed Implementation

[0041] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and 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.

[0042] Preparation Example 1: Preparation of Polymer Monomers

[0043] Synthesis route:

[0044]

[0045] Specifically, the following steps are included:

[0046] S1. Mix 20g tetrahydrophthalic anhydride, 50g ethanol, and 2g concentrated sulfuric acid, heat to reflux, stir and react for 3h, remove ethanol under reduced pressure, add 50mL toluene, wash with water, adjust the pH of the solution to 7.5, wash with water, dry, remove toluene under reduced pressure, and obtain intermediate 1.

[0047] S2. Add 0.1 mol of intermediate 1 and 0.22 mol of n-hexylamine to xylene, heat to 120°C, stir and react for 28 h, filter, wash and dry to obtain intermediate 2;

[0048] S3. Dissolve 0.2 mol of sodium 3-chloro-2-hydroxypropanesulfonate in 200 mL of ethylene glycol, add 0.1 mol of intermediate 2, heat to 60 °C, adjust the pH of the mixed solution to 7.5 with 40% NaOH aqueous solution, stir for 4 h, filter, wash, and dry to obtain intermediate 3;

[0049] S4. Add 0.1 mol of intermediate 3, 0.2 mol of 1-chlorohexane and 0.5 mol of NaOH to 200 mL of acetonitrile, heat to 50 °C, stir and react for 3 h, remove the solvent under reduced pressure, wash, filter and dry to obtain intermediate 4;

[0050] S5. Dissolve 0.1 mol of intermediate 4 and 0.22 mol of butyryl lactone in 200 mL of acetone, heat to 45 °C, stir for 20 h, filter, wash, and dry to obtain the product with a total yield of 61.2%.

[0051] Preparation Example 2: Preparation of Polymer Monomers

[0052] Specifically, the following steps are included:

[0053] S1. Mix 30g tetrahydrophthalic anhydride, 70g ethanol, and 3g concentrated sulfuric acid, heat to reflux, stir and react for 5h, remove ethanol under reduced pressure, add 70mL toluene, wash with water, adjust the pH of the solution to 8.5, wash with water, dry, remove toluene under reduced pressure, and obtain intermediate 1.

[0054] S2. Add 0.1 mol of intermediate 1 and 0.25 mol of n-dodecylamine to xylene, heat to 130℃, stir and react for 35 h, filter, wash and dry to obtain intermediate 2;

[0055] S3. Dissolve 0.2 mol of sodium 3-chloro-2-hydroxypropanesulfonate in 200 mL of ethylene glycol, add 0.11 mol of intermediate 2, heat to 70 °C, adjust the pH of the mixed solution to 8.5 with 40% NaOH aqueous solution, stir for 6 h, filter, wash, and dry to obtain intermediate 3;

[0056] S4. Add 0.11 mol of intermediate 3, 0.2 mol of 1-chlorododecane and 0.5 mol of KOH to 200 mL of acetonitrile, heat to 60 °C, stir for 5 h, remove solvent under reduced pressure, wash, filter and dry to obtain intermediate 4;

[0057] S5. Dissolve 0.1 mol of intermediate 4 and 0.25 mol of butyryl lactone in 200 mL of acetone, heat to 55 °C, stir for 24 h, filter, wash, and dry to obtain the product with a total yield of 64.7%.

[0058] Preparation Example 3: Preparation of Polymer Monomers

[0059] Specifically, the following steps are included:

[0060] S1. Mix 25g tetrahydrophthalic anhydride, 60g ethanol, and 2.5g concentrated sulfuric acid, heat to reflux, stir and react for 4h, remove ethanol under reduced pressure, add 60mL toluene, wash with water, adjust the pH of the solution to 8, wash with water, dry, remove toluene under reduced pressure, and obtain intermediate 1.

[0061] S2. Add 0.1 mol of intermediate 1 and 0.235 mol of n-nonylamine to xylene, heat to 125℃, stir and react for 32 h, filter, wash and dry to obtain intermediate 2;

[0062] S3. Dissolve 0.2 mol of sodium 3-chloro-2-hydroxypropanesulfonate in 200 mL of ethylene glycol, add 0.105 mol of intermediate 2, heat to 65 °C, adjust the pH of the mixed solution to 8 with 40% NaOH aqueous solution, stir for 5 h, filter, wash, and dry to obtain intermediate 3;

[0063] S4. Add 0.105 mol of intermediate 3, 0.2 mol of 1-bromoheptane, and 0.5 mol of NaOH to 200 mL of acetonitrile, heat to 55 °C, stir and react for 4 h, remove the solvent under reduced pressure, wash, filter, and dry to obtain intermediate 4.

[0064] S5. Dissolve 0.1 mol of intermediate 4 and 0.235 mol of butyryl lactone in 200 mL of acetone, heat to 50 °C, stir for 22 h, filter, wash, and dry to obtain the product with a total yield of 63.4%.

[0065] Example 1

[0066] This embodiment provides a method for preparing a polymer thickener, including the following steps:

[0067] (1) 111 parts by weight of the polymer monomer prepared in Preparation Example 1, 21 parts by weight of acrylamide, 16 parts by weight of methacryloyloxyethyltrimethylammonium chloride, 41 parts by weight of 1-vinyl-2-pyrrolidone and acetamide were added to water to prepare an aqueous solution with a total monomer mass concentration of 32 wt%.

[0068] (2) Under nitrogen protection, an initiation system and water were added to prepare an aqueous solution with a total monomer mass concentration of 27wt%. The mixture was stirred and reacted at 40°C for 2 hours. The product was filtered, washed, dried, and pulverized to obtain a polymer thickener.

[0069] The mass of the acetamide is 0.25 wt% of the total mass of the monomers;

[0070] The initiation system comprises 0.02 wt% of the total monomer mass; the initiation system is a mixture of sodium persulfate and sodium sulfite in a mass ratio of 0.9:0.7.

[0071] The infrared spectrum analysis is as follows: 3407 cm⁻¹ and 3152 cm⁻¹ are the antisymmetric stretching vibration absorption peaks and symmetric stretching vibration absorption peaks of the NH bond in the amide group, respectively; 2932 cm⁻¹ and 2825 cm⁻¹ are the stretching vibration absorption peaks of the CH bond in the methyl and methylene groups; 1682 cm⁻¹ is the stretching vibration absorption peak of the C=O bond; 1471 cm⁻¹ is the bending vibration absorption peak of the CH bond in the methyl and methylene groups; 1234 cm⁻¹ and 1141 cm⁻¹ are the stretching vibration absorption peaks of the CN bond in 1-vinyl-2-pyrrolidone; 925-1120 cm⁻¹ are the COC stretching vibration peaks; 952 cm⁻¹ is the characteristic absorption peak of the quaternary ammonium salt group; 947 cm⁻¹, 612 cm⁻¹ and 529 cm⁻¹ are the stretching vibration peaks of the sulfonate group; and 721 cm⁻¹ is the in-plane rocking vibration absorption peak of -(CH₂)n-.

[0072] Example 2

[0073] This embodiment provides a method for preparing a polymer thickener, including the following steps:

[0074] (1) 218 ​​parts by weight of the polymer monomer prepared in Preparation Example 2, 36 parts by weight of acrylamide, 23 parts by weight of methacryloyloxyethyltrimethylammonium chloride, 52 parts by weight of 1-vinyl-2-pyrrolidone and acetamide were added to water to prepare an aqueous solution with a total monomer mass concentration of 37 wt%.

[0075] (2) Under nitrogen protection, an initiation system and water were added to prepare an aqueous solution with a total monomer mass concentration of 30 wt%. The mixture was stirred at 50 °C for 4 h. The product was filtered, washed, dried, and pulverized to obtain a polymer thickener.

[0076] The mass of the acetamide is 0.35 wt% of the total mass of the monomers;

[0077] The initiation system comprises 0.05 wt% of the total monomer mass; the initiation system is a mixture of ammonium persulfate and sodium sulfite in a mass ratio of 1.2:0.7.

[0078] Example 3

[0079] This embodiment provides a method for preparing a polymer thickener, including the following steps:

[0080] (1) 177 parts by weight of the polymer monomer prepared in Preparation Example 3, 32 parts by weight of acrylamide, 20 parts by weight of methacryloyloxyethyltrimethylammonium chloride, 47 parts by weight of 1-vinyl-2-pyrrolidone and acetamide were added to water to prepare an aqueous solution with a total monomer mass concentration of 35 wt%.

[0081] (2) Under nitrogen protection, an initiation system and water were added to prepare an aqueous solution with a total monomer mass concentration of 28 wt%. The mixture was stirred and reacted at 45°C for 3 h. The product was filtered, washed, dried, and pulverized to obtain a polymer thickener.

[0082] The mass of the acetamide is 0.3 wt% of the total mass of the monomers;

[0083] The initiation system comprises 0.03 wt% of the total monomer mass; the initiation system is a mixture of potassium persulfate and sodium sulfite in a mass ratio of 1:0.7.

[0084] Comparative Example 1

[0085] The difference compared to Example 3 is that no polymer monomer was added.

[0086] Comparative Example 2

[0087] The difference from Example 3 is that methacryloyloxyethyltrimethylammonium chloride was not added.

[0088] Comparative Example 3

[0089] The difference from Example 3 is that 1-vinyl-2-pyrrolidone was not added.

[0090] Test Example 1

[0091] The polymer thickeners prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests. The results are shown in Tables 1-3.

[0092] Acid solubility test: Add 0.5 wt% of thickener to 20 wt% hydrochloric acid and stir continuously. Measure the apparent viscosity every 10 minutes. After the viscosity change stabilizes, measure the apparent viscosity every 20 minutes. When the viscosity tends to stabilize, measure the acid solubility time of the thickener.

[0093] Thickening test: The thickener was mixed with 20wt% hydrochloric acid to prepare a thickening acid with a concentration of 0.5wt%, and the apparent viscosity was measured at 30℃ and 170s-1.

[0094] Acid resistance test: The thickener was prepared by adding 0.5 wt% of hydrochloric acid to form thickened acid, and the apparent viscosity was measured at 30℃ and 170 s⁻¹.

[0095] Table 1

[0096]

[0097] As can be seen from the table above, the polymer thickeners prepared in Examples 1-3 of this invention have good acid solubility and acid resistance, and excellent thickening effect.

[0098] Temperature and shear resistance test: The thickener was prepared into a thickened acid by adding 0.5 wt% of thickener and 20 wt% of hydrochloric acid. The rheological properties of the thickened acid at 150 and 220℃ were measured using a rheometer. The heating rate was 0.05℃ / s. After the temperature reached the test temperature, it was continuously sheared at a rate of 170 s⁻¹ for 60 min and the apparent viscosity was measured.

[0099] Salt tolerance test: NaCl and CaCl2 were mixed in a 1:1 ratio (mass ratio) to form a composite salt, which was then dissolved in 20wt% hydrochloric acid to a concentration of 10wt%. This solution was used as the base liquid and mixed with 0.5wt% thickener to form a thickened acid. The apparent viscosity was measured at 30℃ and 170s⁻¹.

[0100] Table 2

[0101]

[0102] As can be seen from the table above, the polymer thickeners prepared in Examples 1-3 of this invention have good salt resistance, temperature resistance, and shear resistance.

[0103] Slow-release test: Thickener was added at 0.5 wt% and mixed with 20 wt% hydrochloric acid to prepare thickened acid. Cores of the same size were drilled from carbonate rock slabs and placed into the prepared thickened acid and blank hydrochloric acid respectively. The reaction was carried out at 90, 150 and 220 ℃ using an acid-rock reaction rotating rock disk apparatus for 10 min. The static reaction rate of the acid was determined by the weight loss method, and the acid-rock reaction rate was calculated.

[0104] Table 3

[0105]

[0106] As can be seen from the table above, the polymer thickeners prepared in Examples 1-3 of this invention have good retardation properties and can meet the requirements of deep well acidizing operations.

[0107] Example 4

[0108] This embodiment provides a fracturing fluid, prepared by mixing 1 part by weight of the polymer thickener obtained in Example 1, 0.2 parts by weight of the crosslinking agent, 0.1 parts by weight of cyclohexanol, and 90 parts by weight of water evenly. The crosslinking agent is a mixture of sodium dodecyl sulfate and dodecyltrimethylammonium chloride in a mass ratio of 4:1.

[0109] Example 5

[0110] This embodiment provides a fracturing fluid, prepared by mixing 2 parts by weight of the polymer thickener obtained in Example 2, 0.4 parts by weight of the crosslinking agent, 0.5 parts by weight of ethanol, and 95 parts by weight of water evenly. The crosslinking agent is a mixture of sodium dodecyl sulfate and dodecyltrimethylammonium chloride in a mass ratio of 6:2.

[0111] Example 6

[0112] This embodiment provides a fracturing fluid, prepared by mixing 1.4 parts by weight of the polymer thickener, 0.3 parts by weight of the crosslinking agent, 0.3 parts by weight of isopropanol, and 92 parts by weight of water obtained in Example 3 until homogeneous. The crosslinking agent is a mixture of sodium dodecyl sulfate and dodecyltrimethylammonium chloride in a mass ratio of 5:1.5.

[0113] Comparative Examples 4-6

[0114] The difference from Example 6 is that the polymer thickener was prepared from Comparative Examples 1-3.

[0115] Test Example 2

[0116] Static suspension tests were used to determine the settling velocity of proppant particles in the fracturing fluids prepared in Examples 4-6 and Comparative Examples 4-6. Fracturing fluid was poured into a 100ml graduated cylinder, and the liquid level height h was measured with a ruler. Propped proppant particles were gently placed onto the liquid surface, and the time t for the first particle to reach the bottom of the graduated cylinder was recorded at different temperatures. The proppant ratio was 20%. The results are shown in Table 4.

[0117] The method for calculating the settling velocity is: settling velocity v = liquid level height h / settling time t.

[0118] Table 4

[0119]

[0120] As can be seen from the table above, the fracturing fluids prepared in Examples 4-6 of this invention have good sand-carrying capacity.

[0121] Test Example 3

[0122] The fracturing fluids prepared in Examples 4-6 and Comparative Examples 4-6 were tested to assess their damage to the core samples. The results are shown in Table 5.

[0123] Table 5

[0124]

[0125] As can be seen from the table above, the fracturing fluids prepared in Examples 4-6 of this invention have a low damage rate to core permeability, and after the fracturing fluid breaks down, the damage to the reservoir is relatively small, thus meeting the low damage requirement.

[0126] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A polymer thickener, characterized in that, It has the structure shown in Equation I: Where a:b:c:d = 10-15:30-50:15-20:20-25.

2. The polymer thickener according to claim 1, characterized in that, It is prepared from polymer monomers, acrylamide, methacryloyloxyethyltrimethylammonium chloride, and 1-vinyl-2-pyrrolidone in a mass ratio of 111-218:21-36:16-23:41-52, and the structural formula of the polymer monomers is shown in Formula II: R1 and R2 are C6-C12 alkyl chains.

3. The polymer thickener according to claim 2, characterized in that, The preparation method of the polymer monomer is as follows: S1. Tetrahydrophthalic anhydride, ethanol, and concentrated sulfuric acid were heated to reflux, stirred, and the pH of the solution was adjusted to obtain intermediate 1, with the following structure: S2. Intermediate 1 and alkylamine were heated and stirred to react, yielding intermediate 2, with the following structure: S3. Intermediate 2 and sodium 3-chloro-2-hydroxypropanesulfonate were heated and stirred to prepare intermediate 3, with the following structure: S4. Intermediate 3 is reacted with a haloalkane to prepare intermediate 4, with the following structure: S5. React intermediate 4 with butyryl lactone to obtain the product.

4. The polymer thickener according to claim 3, characterized in that, In step S1, the mass ratio of tetrahydrophthalic anhydride, ethanol, and concentrated sulfuric acid is 20-30:50-70:2-3. The mixture is heated to reflux and stirred for 3-5 hours. The pH of the solution is adjusted to 7.5-8.

5. In step S2, the molar ratio of intermediate 1 to alkylamine is 1:2.2-2.

5. The heating and stirring reaction is carried out at a temperature of 120-130°C for 28-35 hours. The alkylamine is selected from at least one of n-hexylamine, n-heptylamine, n-nonylamine, n-octylamine, n-decylamine, n-undecanamine, and n-dodecylamine.

5. The polymer thickener according to claim 3, characterized in that, In step S3, the molar ratio of intermediate 2 and sodium 3-chloro-2-hydroxypropanesulfonate is 1-1.1:2, and the heating and stirring reaction is carried out at a temperature of 60-70°C for 4-6 hours. In step S4, the molar ratio of intermediate 3 and haloalkane is 1-1.1:2, the reaction is carried out at a temperature of 50-60°C for 3-5 hours, and a base is added. The base is selected from at least one of triethylamine, NaOH, and KOH. The mixture is selected from at least one of 1-chlorohexane, 1-chloroheptane, 1-chlorooctane, 1-chlorononane, 1-chlorodecane, 1-chloroundecane, 1-chlorododecane, 1-bromohexane, 1-bromoheptane, 1-bromooctane, 1-bromononane, 1-bromodecane, 1-bromoundecane, and 1-bromododecane; the molar ratio of intermediate 4 and butyrylolactone in step S5 is 1:2.2-2.5, the reaction temperature is 45-55℃, and the reaction time is 20-24h.

6. The polymer thickener according to claim 3, characterized in that, The preparation method of the polymer monomer specifically includes the following steps: S1. Mix 20-30 parts by weight of tetrahydrophthalic anhydride, 50-70 parts by weight of ethanol, and 2-3 parts by weight of concentrated sulfuric acid, heat under reflux, stir and react for 3-5 hours, remove ethanol under reduced pressure, add toluene, wash with water, adjust the pH of the solution to 7.5-8.5, wash with water, dry, remove toluene under reduced pressure, and obtain intermediate 1. S2. Add 0.1 molar equivalent of intermediate 1 and 0.22-0.25 molar equivalent of alkylamine to xylene, heat to 120-130℃, stir and react for 28-35 h, filter, wash and dry to obtain intermediate 2; S3. Dissolve 0.2 molar equivalents of sodium 3-chloro-2-hydroxypropanesulfonate in ethylene glycol, add 0.1-0.11 molar equivalents of intermediate 2, heat to 60-70℃, adjust the pH of the mixed solution to 7.5-8.5 with 40% NaOH aqueous solution, stir the reaction for 4-6 hours, filter, wash, and dry to obtain intermediate 3; S4. Add 0.1-0.11 molar equivalent of intermediate 3, 0.2 molar equivalent of haloalkane, and 0.5 molar equivalent of base to acetonitrile, heat to 50-60℃, stir and react for 3-5 hours, remove solvent under reduced pressure, wash, filter, and dry to obtain intermediate 4. S5. Dissolve 0.1 molar equivalent intermediate 4 and 0.22-0.25 molar equivalent butyryl lactone in acetone, heat to 45-55℃, stir and react for 20-24 hours, filter, wash, and dry to obtain the product.

7. The polymer thickener according to claim 1, characterized in that, The preparation process of the polymer thickener also includes the addition of a raw material co-solvent and an initiation system; the mass of the co-solvent is 0.25-0.35 wt% of the total mass of the monomers, and the mass of the initiation system is 0.02-0.05 wt% of the total mass of the monomers; the co-solvent is acetamide; the initiation system is a mixture of an initiator and sodium sulfite in a mass ratio of 0.9-1.2:0.7, and the initiator is selected from at least one of sodium persulfate, ammonium persulfate, and potassium persulfate.

8. A method for preparing a polymer thickener as described in any one of claims 1-7, characterized in that, Includes the following steps: (1) Add polymer monomers, acrylamide, methacryloyloxyethyltrimethylammonium chloride, 1-vinyl-2-pyrrolidone, and cosolvent to water to prepare an aqueous solution with a total monomer mass concentration of 32-37 wt%. (2) Under the protection of inert gas, add the initiation system and water to prepare an aqueous solution with a total monomer mass concentration of 27-30 wt%. Stir the reaction at 40-50℃ for 2-4 h. Filter, wash, dry and crush the product to obtain the polymer thickener.

9. A fracturing fluid, characterized in that, Contains the polymer thickener according to any one of claims 1-7.

10. The fracturing fluid according to claim 9, characterized in that, It is prepared from the following raw materials in parts by weight: 0.2-2 parts polymer thickener, 0.2-0.4 parts crosslinking agent, 0.1-0.5 parts drainage aid, and 90-95 parts water.

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