A Degradable Fracturing Fluid Composite Thickening Agent and Its Preparation Method

Abstract

The present invention discloses a degradable fracturing fluid composite thickener and a preparation method thereof. By mixing acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide, a modified additive, modified montmorillonite and deionized water, and polymerizing under the action of an initiator, the modified additive contains cyclodextrin molecules. Compared with traditional guar gum, cyclodextrin has better high-temperature resistance and good shear resistance, enabling the thickener to exhibit a stable chemical structure, good rheological properties and sand-carrying capacity during high-temperature use. Thus, it can greatly reduce the dissolution of the salt layer by the fracturing fluid, and at the same time, there is no residue after gel breaking, which can effectively reduce the damage to the reservoir. The polyvinyl alcohol structure contained can increase the salt resistance of the thickener, enabling the thickener to be applicable to complex formation environments. At the same time, the organic part in the thickener can be degraded, thereby reducing the possibility of pollution to soil and water bodies.

Description

Technical Field

[0001] The present invention relates to the technical field of preparation of fracturing fluid thickeners, and particularly relates to a degradable fracturing fluid composite thickener and a preparation method thereof. Background Art

[0002] Fracturing fluid is mainly a liquid used to ensure the operation intensity during oil and gas exploitation. This liquid can conduct the ground pressure to form fractures in the formation; carry proppants and make the proppants spread in the fractures; reduce the loss of external liquid in the formation. The viscosity of natural water-based fracturing fluid is relatively low. During use, in order to expand the application market of water-based fracturing fluid, enhance the viscosity of the water-based fracturing fluid system, reduce the filtration loss performance of the fracturing fluid to achieve a better sand-carrying effect, the modification of thickeners has become the main research direction of domestic and foreign scholars. Traditional thickeners include three types: natural plant gum derivatives, cellulose derivatives, and synthetic polymers. The use of a single thickener will cause problems such as high-temperature degradation, residue formation, and salt sensitivity, resulting in the fact that some fracturing fluids cannot be widely used or cause harm to the environment. Therefore, there is an urgent need for a thickener with good thickening effect, high temperature resistance, salt tolerance, and low residue generation. Summary of the Invention

[0003] The purpose of the present invention is to provide a degradable fracturing fluid composite thickener and a preparation method thereof, which solve the problem of poor salt and high temperature resistance of fracturing fluid at the present stage.

[0004] The purpose of the present invention can be achieved by the following technical solutions:

[0005] A preparation method of a degradable fracturing fluid composite thickener specifically includes the following steps:

[0006] Step A1: Mix sodium montmorillonite and deionized water evenly. Under the conditions of a rotation speed of 800 - 1000 r / min and a temperature of 80 - 85 °C, stir for 30 - 40 min, then add cetyltrimethylammonium bromide, and continue to stir for 2 - 3 h. Filter to remove the filtrate and dry to obtain modified montmorillonite.

[0007] Step A2: Mix acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide, modified additive, modified montmorillonite, and deionized water evenly, introduce nitrogen to expel air. Under the conditions of a rotation speed of 120 - 150 r / min, a temperature of 60 - 70 °C, and a pH value of 7, stir and add an initiator, and carry out the reaction for 5 - 7 h, and dry to obtain the degradable fracturing fluid composite thickener.

[0008] Further, the dosage ratio of the sodium montmorillonite, deionized water, and cetyltrimethylammonium bromide in Step A1 is 10 g:200 mL:6.5 g.

[0009] Further, the mass ratio of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and the modified additive described in step A2 is 74:16:8:2:5. The dosage of the modified montmorillonite is 1% of the sum of the masses of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and the modified additive. The dosage of the initiator is 1‰ of the sum of the masses of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and the modified additive. The initiator is prepared by mixing ammonium persulfate and sodium bisulfite in a mass ratio of 2:1.

[0010] Further, the modified additive is prepared by the following steps:

[0011] Step B1: Mix β-cyclodextrin and pyridine, and under the conditions of a rotation speed of 200 - 300 r / min, a temperature of 0 - 5 °C, and a pH value of 13 - 14, stir and add 4-toluenesulfonyl chloride, and react for 25 - 30 h to obtain pretreated cyclodextrin. Mix the pretreated cyclodextrin and ammonia water, and under the conditions of a rotation speed of 300 - 500 r / min and a temperature of 50 - 60 °C, react for 15 - 20 h to obtain aminoated cyclodextrin;

[0012] Step B2: Mix polyvinyl alcohol, boron trifluoride diethyl ether and deionized water evenly, and under the conditions of a rotation speed of 200 - 300 r / min and a temperature of 60 - 65 °C, stir and reflux and add epichlorohydrin, and react for 2 - 3 h. Then add sodium hydroxide solution, raise the temperature to 75 - 80 °C, and continue to react for 3 - 5 h to obtain functionalized polyvinyl alcohol. Mix the aminoated cyclodextrin, functionalized polyvinyl alcohol and deionized water, and under the conditions of a rotation speed of 150 - 200 r / min, a temperature of 60 - 70 °C, and a pH value of 11 - 12, react for 4 - 6 h to obtain a precursor;

[0013] Step B3: Mix the precursor, lauric acid, dicyclohexylcarbodiimide and deionized water evenly, and under the conditions of a rotation speed of 150 - 200 r / min and a temperature of 95 - 100 °C, react for 2 - 3 h to obtain a modified precursor. Mix the modified precursor, acrylic acid, p-toluenesulfonic acid and deionized water evenly, and under the conditions of a rotation speed of 150 - 200 r / min and a temperature of 95 - 100 °C, react for 6 - 8 h to obtain the modified additive.

[0014] Further, the dosage ratio of β-cyclodextrin, pyridine and 4-toluenesulfonyl chloride described in step B1 is 10 mmol:60 mL:73 mmol. The dosage ratio of the pretreated cyclodextrin and ammonia water is 1 g:25 mL, and the mass fraction of ammonia water is 25%.

[0015] Further, the dosage ratio of polyvinyl alcohol, boron trifluoride diethyl etherate, deionized water, epichlorohydrin, and sodium hydroxide solution described in step B2 is 40 g: 0.45 g: 200 mL: 0.1 mol: 10 mL. The model of polyvinyl alcohol is 0388, the degree of polymerization is 300, the molecular weight is 10,600, and the mass ratio of amino cyclodextrin to functionalized polyvinyl alcohol is 3:5.

[0016] Further, the molar ratio of amino group, lauric acid, and dicyclohexylcarbodiimide on the precursor described in step B3 is 1:1:1.2. The dosage of the modified precursor and acrylic acid is in a mass ratio of 8:13, and the dosage of p-toluenesulfonic acid is 2% of the mass of acrylic acid.

[0017] Beneficial effects of the present invention: A degradable fracturing fluid composite thickener disclosed by the present invention uses sodium montmorillonite as a raw material, which is treated with cetyltrimethylammonium bromide for organic intercalation modification to obtain modified montmorillonite. Acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide, a modified additive, modified montmorillonite, and deionized water are mixed and polymerized under the action of an initiator to form a polyacrylamide structure on the surface of the modified montmorillonite, thereby obtaining the degradable fracturing fluid composite thickener.

[0018] The modified additive uses β-cyclodextrin as a raw material and is treated with 4-toluenesulfonyl chloride, so that the C6 hydroxyl group on β-cyclodextrin reacts with the acyl chloride on 4-toluenesulfonyl chloride to obtain pretreated cyclodextrin. The pretreated cyclodextrin is treated with ammonia water to undergo a nucleophilic substitution reaction to obtain amino cyclodextrin. Polyvinyl alcohol and epichlorohydrin are reacted so that some of the hydroxyl groups on polyvinyl alcohol react with the epoxy groups on epichlorohydrin, and then a new epoxy group is formed by ring closure under the action of sodium hydroxide solution to obtain functionalized polyvinyl alcohol. The functionalized polyvinyl alcohol and amino cyclodextrin are reacted so that the epoxy group on the functionalized polyvinyl alcohol reacts with the amino group on the amino cyclodextrin to obtain a precursor. The precursor and lauric acid are reacted so that the unreacted amino group in the precursor and the carboxyl group on lauric acid undergo a dehydration reaction to obtain a modified precursor. The modified precursor and acrylic acid are reacted so that the hydroxyl group on the modified precursor and the carboxyl group on acrylic acid are esterified to obtain the modified additive.

[0019] The modified additive contains cyclodextrin molecules. Compared with traditional guar gum, cyclodextrin has better high-temperature resistance and shear resistance, enabling the thickener to exhibit a stable chemical structure, good rheological properties, and sand-carrying capacity during high-temperature use. As a result, it can greatly reduce the dissolution of the salt layer by the fracturing fluid, and there is no residue after gel breaking, causing little damage to the formation, effectively reducing the damage to the reservoir. The polyvinyl alcohol structure it contains can increase the salt resistance of the thickener, making the thickener applicable to complex formation environments. At the same time, the organic part in the thickener can be degraded, thereby reducing the possibility of pollution to the soil and water bodies. The addition of modified montmorillonite further enhances the sand-carrying capacity of the thickener. Detailed implementation mode

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present invention.

[0021] Example 1, a preparation method of a degradable fracturing fluid composite thickener, specifically includes the following steps:

[0022] Step A1: Mix sodium-based montmorillonite and deionized water evenly. Under the conditions of a rotation speed of 800 r / min and a temperature of 80 °C, stir for 30 min, then add cetyltrimethylammonium bromide, continue to stir for 2 h, filter to remove the filtrate and dry to obtain modified montmorillonite.

[0023] Step A2: Mix acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide, modified additive, modified montmorillonite, and deionized water evenly, introduce nitrogen to expel air, and under the conditions of a rotation speed of 120 r / min, a temperature of 60 °C, and a pH value of 7, stir and add an initiator, and react for 5 h, then dry to obtain the degradable fracturing fluid composite thickener.

[0024] The dosage ratio of the sodium-based montmorillonite, deionized water, and cetyltrimethylammonium bromide in Step A1 is 10 g:200 mL:6.5 g.

[0025] The mass ratio of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and the modified additive described in step A2 is 74:16:8:2:5. The dosage of the modified montmorillonite is 1% of the sum of the masses of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and the modified additive. The dosage of the initiator is 1‰ of the sum of the masses of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and the modified additive. The initiator is prepared by mixing ammonium persulfate and sodium bisulfite in a mass ratio of 2:1.

[0026] The described modified additive is prepared by the following steps:

[0027] Step B1: Mix β-cyclodextrin and pyridine, and under the conditions of a rotation speed of 200 r / min, a temperature of 0 °C, and a pH value of 13, stir and add 4-toluenesulfonyl chloride, and react for 25 h to obtain pretreated cyclodextrin. Mix the pretreated cyclodextrin and ammonia water, and under the conditions of a rotation speed of 300 r / min and a temperature of 50 °C, react for 15 h to obtain aminoated cyclodextrin;

[0028] Step B2: Mix polyvinyl alcohol, boron trifluoride diethyl ether and deionized water evenly, and under the conditions of a rotation speed of 300 r / min and a temperature of 65 °C, stir and reflux and add epichlorohydrin. After reacting for 3 h, add sodium hydroxide solution, raise the temperature to 80 °C, and continue to react for 5 h to obtain functionalized polyvinyl alcohol. Mix aminoated cyclodextrin, functionalized polyvinyl alcohol and deionized water, and under the conditions of a rotation speed of 200 r / min, a temperature of 70 °C, and a pH value of 12, react for 6 h to obtain a precursor;

[0029] Step B3: Mix the precursor, lauric acid, dicyclohexylcarbodiimide and deionized water evenly, and under the conditions of a rotation speed of 200 r / min and a temperature of 100 °C, react for 3 h to obtain a modified precursor. Mix the modified precursor, acrylic acid, p-toluenesulfonic acid and deionized water evenly, and under the conditions of a rotation speed of 200 r / min and a temperature of 100 °C, react for 8 h to obtain the modified additive.

[0030] The dosage ratio of β-cyclodextrin, pyridine and 4-toluenesulfonyl chloride described in step B1 is 10 mmol:60 mL:73 mmol. The dosage ratio of pretreated cyclodextrin and ammonia water is 1 g:25 mL, and the mass fraction of ammonia water is 25%.

[0031] The dosage ratio of polyvinyl alcohol, boron trifluoride diethyl etherate, deionized water, epichlorohydrin and sodium hydroxide solution described in step B2 is 40 g: 0.45 g: 200 mL: 0.1 mol: 10 mL. The model of polyvinyl alcohol is 0388, the degree of polymerization is 300, and the molecular weight is 10,600. The mass ratio of amino cyclodextrin to functionalized polyvinyl alcohol is 3:5.

[0032] The molar ratio of amino group, lauric acid and dicyclohexylcarbodiimide on the precursor described in step B3 is 1:1:1.2. The dosage ratio of the modified precursor to acrylic acid is 8:13 by mass, and the dosage of p-toluenesulfonic acid is 2% of the mass of acrylic acid.

[0033] Example 2, a preparation method of a degradable fracturing fluid composite thickening agent, specifically includes the following steps:

[0034] Step A1: Mix sodium montmorillonite and deionized water evenly. Under the conditions of a rotation speed of 800 r / min and a temperature of 85 °C, stir for 35 min, then add cetyltrimethylammonium bromide, and continue to stir for 2 h. Filter to remove the filtrate and dry it to obtain modified montmorillonite.

[0035] Step A2: Mix acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide, modified additive, modified montmorillonite and deionized water evenly, introduce nitrogen to discharge air. Under the conditions of a rotation speed of 150 r / min, a temperature of 65 °C and a pH value of 7, stir and add an initiator, and carry out the reaction for 6 h, then dry to obtain the degradable fracturing fluid composite thickening agent.

[0036] The dosage ratio of sodium montmorillonite, deionized water and cetyltrimethylammonium bromide described in step A1 is 10 g: 200 mL: 6.5 g.

[0037] The mass ratio of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and modified additive described in step A2 is 74:16:8:2:5. The dosage of modified montmorillonite is 1% of the sum of the masses of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and modified additive. The dosage of the initiator is 1‰ of the sum of the masses of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and modified additive. The initiator is prepared by mixing ammonium persulfate and sodium bisulfite in a mass ratio of 2:1.

[0038] The described modified additive is prepared by the following steps:

[0039] Step B1: Mix β-cyclodextrin and pyridine, stir and add 4-toluenesulfonyl chloride under the conditions of a rotation speed of 200 r / min, a temperature of 3 °C, and a pH value of 13, and react for 30 h to obtain pretreated cyclodextrin. Mix the pretreated cyclodextrin and ammonia water, and react for 18 h under the conditions of a rotation speed of 300 r / min and a temperature of 55 °C to obtain amino-functionalized cyclodextrin;

[0040] Step B2: Mix polyvinyl alcohol, boron trifluoride diethyl ether, and deionized water evenly, stir and reflux under the conditions of a rotation speed of 300 r / min and a temperature of 60 °C, add epichlorohydrin, react for 3 h, add sodium hydroxide solution, raise the temperature to 75 °C, and continue to react for 4 h to obtain functionalized polyvinyl alcohol. Mix amino-functionalized cyclodextrin, functionalized polyvinyl alcohol, and deionized water, and react for 5 h under the conditions of a rotation speed of 150 r / min, a temperature of 65 °C, and a pH value of 11 to obtain a precursor;

[0041] Step B3: Mix the precursor, lauric acid, dicyclohexylcarbodiimide, and deionized water evenly, and react for 2 h under the conditions of a rotation speed of 150 r / min and a temperature of 100 °C to obtain a modified precursor. Mix the modified precursor, acrylic acid, p-toluenesulfonic acid, and deionized water evenly, and react for 7 h under the conditions of a rotation speed of 200 r / min and a temperature of 95 °C to obtain a modified additive.

[0042] The dosage ratio of β-cyclodextrin, pyridine, and 4-toluenesulfonyl chloride described in Step B1 is 10 mmol: 60 mL: 73 mmol, the dosage ratio of pretreated cyclodextrin and ammonia water is 1 g: 25 mL, and the mass fraction of ammonia water is 25%.

[0043] The dosage ratio of polyvinyl alcohol, boron trifluoride diethyl ether, deionized water, epichlorohydrin, and sodium hydroxide solution described in Step B2 is 40 g: 0.45 g: 200 mL: 0.1 mol: 10 mL. The model of polyvinyl alcohol is 0388, the degree of polymerization is 300, and the molecular weight is 10,600. The mass ratio of amino-functionalized cyclodextrin and functionalized polyvinyl alcohol is 3: 5.

[0044] The molar ratio of amino group, lauric acid, and dicyclohexylcarbodiimide on the precursor described in Step B3 is 1: 1: 1.2. The dosage ratio of the modified precursor and acrylic acid is 8: 13 in mass, and the dosage of p-toluenesulfonic acid is 2% of the mass of acrylic acid.

[0045] Example 3, A preparation method of a degradable fracturing fluid composite thickener, specifically includes the following steps:

[0046] Step A1: Mix sodium montmorillonite and deionized water evenly. Under the conditions of a rotation speed of 1000 r / min and a temperature of 85 °C, stir for 40 min, then add cetyltrimethylammonium bromide, and continue stirring for 3 h. Filter to remove the filtrate and dry it to obtain modified montmorillonite.

[0047] Step A2: Mix acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide, modified additive, modified montmorillonite and deionized water evenly. Pass nitrogen to expel air. Under the conditions of a rotation speed of 150 r / min, a temperature of 70 °C, and a pH value of 7, stir and add an initiator, and carry out the reaction for 7 h, then dry to obtain a degradable fracturing fluid composite thickening agent.

[0048] The dosage ratio of the sodium montmorillonite, deionized water and cetyltrimethylammonium bromide described in Step A1 is 10 g: 200 mL: 6.5 g.

[0049] The mass ratio of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and modified additive described in Step A2 is 74:16:8:2:5. The dosage of modified montmorillonite is 1% of the sum of the masses of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and modified additive. The dosage of the initiator is 1‰ of the sum of the masses of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and modified additive. The initiator is prepared by mixing ammonium persulfate and sodium bisulfite in a mass ratio of 2:1.

[0050] The described modified additive is prepared by the following steps:

[0051] Step B1: Mix β-cyclodextrin and pyridine. Under the conditions of a rotation speed of 300 r / min, a temperature of 5 °C, and a pH value of 14, stir and add 4-toluenesulfonyl chloride, and carry out the reaction for 30 h to obtain pretreated cyclodextrin. Mix the pretreated cyclodextrin and ammonia water, and under the conditions of a rotation speed of 500 r / min and a temperature of 60 °C, carry out the reaction for 20 h to obtain amino-functionalized cyclodextrin.

[0052] Step B2: Mix polyvinyl alcohol, boron trifluoride diethyl ether and deionized water evenly. Under the conditions of a rotation speed of 300 r / min and a temperature of 65 °C, stir and reflux and add epichlorohydrin, and carry out the reaction for 3 h, then add a sodium hydroxide solution, raise the temperature to 80 °C, and continue the reaction for 5 h to obtain functionalized polyvinyl alcohol. Mix the amino-functionalized cyclodextrin, functionalized polyvinyl alcohol and deionized water, and under the conditions of a rotation speed of 200 r / min, a temperature of 70 °C, and a pH value of 12, carry out the reaction for 6 h to obtain a precursor.

[0053] Step B3: Mix the precursor, lauric acid, dicyclohexylcarbodiimide, and deionized water evenly. React for 3 h under the conditions of a rotation speed of 200 r / min and a temperature of 100 °C to obtain a modified precursor. Then mix the modified precursor, acrylic acid, p-toluenesulfonic acid, and deionized water evenly and react for 8 h under the conditions of a rotation speed of 200 r / min and a temperature of 100 °C to obtain a modified additive.

[0054] The dosage ratio of β-cyclodextrin, pyridine, and 4-toluenesulfonyl chloride described in Step B1 is 10 mmol: 60 mL: 73 mmol. The dosage ratio of the pretreated cyclodextrin and ammonia water is 1 g: 25 mL, and the mass fraction of ammonia water is 25%.

[0055] The dosage ratio of polyvinyl alcohol, boron trifluoride diethyl etherate, deionized water, epichlorohydrin, and sodium hydroxide solution described in Step B2 is 40 g: 0.45 g: 200 mL: 0.1 mol: 10 mL. The model of polyvinyl alcohol is 0388, the degree of polymerization is 300, and the molecular weight is 10,600. The mass ratio of the amino-functionalized cyclodextrin to the functionalized polyvinyl alcohol is 3:5.

[0056] The molar ratio of the amino group, lauric acid, and dicyclohexylcarbodiimide on the precursor described in Step B3 is 1:1:1.2. The mass ratio of the modified precursor to acrylic acid is 8:13, and the dosage of p-toluenesulfonic acid is 2% of the mass of acrylic acid.

[0057] Comparative Example 1: Compared with Example 1, montmorillonite is used instead of modified montmorillonite in this comparative example, and the remaining steps are the same.

[0058] Comparative Example 2: Compared with Example 1, ethylenediamine is used instead of amino-functionalized cyclodextrin in this comparative example, and the remaining steps are the same.

[0059] Comparative Example 3: Compared with Example 1, acetic acid is used instead of lauric acid in this comparative example, and the remaining steps are the same.

[0060] Comparative Example 4: Compared with Example 1, the amino-functionalized cyclodextrin, acrylic acid, dicyclohexylcarbodiimide, and deionized water are mixed evenly and reacted for 8 h under the conditions of a rotation speed of 200 r / min and a temperature of 30 °C. The obtained product is used to replace the modified additive, and the remaining steps are the same.

[0061] The thickeners prepared in Examples 1-3 and Comparative Examples 1-4 are blended in the following mass percentages: 0.6% thickener, 0.25% potassium chloride, 0.1% tetradecyltrimethylammonium bromide, 0.1% ammonium persulfate, and 0.2% OP-10, with the balance being water. Adjust the pH value to 7 to obtain a fracturing base fluid at a fixed shear rate of 170 s -1, under the condition that the temperature is 20 °C, use a six-speed rotational viscometer to detect the apparent viscosity. After heating to 120 °C and shearing for 1.5 h, detect the apparent viscosity. Add sodium chloride to the fracturing base fluid to form a solution with a sodium ion concentration of 30,000 mg / L, and detect the apparent viscosity. The detection results are shown in Table 1 below.

[0062] Table 1

[0063]

[0064] It can be seen from Table 1 that this application has good salt resistance and high temperature resistance effects.

[0065] The above content is only an example and illustration of the concept of the present invention. Those skilled in the art of this technology can make various modifications or supplements to the described specific embodiments or use similar methods to replace them. As long as they do not deviate from the concept of the invention or exceed the scope defined by this claim book, they should all belong to the protection scope of the present invention.

Claims

1. A method for preparing a degradable composite thickener for fracturing fluid, characterized in that: The specific steps include: Step A1: After mixing sodium montmorillonite and deionized water, cetyltrimethylammonium bromide is added, stirring is continued, the filtrate is filtered to remove the filtrate and dried to obtain modified montmorillonite; Step A2: uniformly mixing acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide, a modified additive, modified montmorillonite and deionized water, introducing nitrogen to exhaust air, stirring and adding an initiator, reacting, and drying to obtain a degradable fracturing fluid composite thickener; The modified additive is prepared by the following steps: Step B1: β-cyclodextrin and pyridine are mixed and stirred, and 4-toluenesulfonyl chloride is added to react to obtain pretreated cyclodextrin, and the pretreated cyclodextrin is mixed and reacted with ammonia water to obtain amino cyclodextrin; Step B2: polyvinyl alcohol, boron trifluoride etherate and deionized water are mixed, stirred and refluxed, and epichlorohydrin is added. After the reaction, sodium hydroxide solution is added, and the temperature is increased to continue the reaction to obtain functionalized polyvinyl alcohol. Aminated cyclodextrin, functionalized polyvinyl alcohol and deionized water are mixed and reacted to obtain a precursor; Step B3: The precursor, lauric acid, dicyclohexylcarbodiimide and deionized water are mixed and reacted to obtain a modified precursor, and the modified precursor, acrylic acid, p-toluenesulfonic acid and deionized water are mixed and reacted to obtain a modified additive.

2. The method for preparing a degradable composite thickener for fracturing fluid according to claim 1, characterized in that: The amount ratio of sodium montmorillonite, deionized water and hexadecyltrimethylammonium bromide described in step A1 is 10g:200mL:6.5g.

3. The method for preparing a degradable composite thickener for fracturing fluid according to claim 1, characterized in that: The mass ratio of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and the modifying additive described in step A2 is 74:16:8:2:5, and the amount of modified montmorillonite is 1% of the sum of the mass of acrylamide, acrylic acid, 2-methyl-2-acrylamidopropanesulfonic acid, N,N-dimethylacrylamide and the modifying additive.

4. The method for preparing a degradable composite thickener for fracturing fluid according to claim 1, characterized in that: The dosage ratio of β-cyclodextrin, pyridine and 4-toluenesulfonyl chloride in step B1 is 10mmol:60mL:73mmol, and the dosage ratio of pretreated cyclodextrin and ammonia water is 1g:25mL.

5. The method for preparing a degradable composite thickener for fracturing fluid according to claim 1, characterized in that: The amount ratio of polyvinyl alcohol, boron trifluoride etherate, deionized water, epichlorohydrin and sodium hydroxide solution in step B2 is 40g:0.45g:200mL:0.1mol:10mL, and the mass ratio of amino cyclodextrin to functionalized polyvinyl alcohol is 3:

5.

6. The method for preparing a degradable composite thickener for fracturing fluid according to claim 1, characterized in that: The molar ratio of amino group, lauric acid and dicyclohexylcarbodiimide on the precursor in step B3 is 1:1:1.2, and the mass ratio of the modified precursor to acrylic acid is 8:

13.

7. A degradable composite thickener for fracturing fluid, characterized in that: Prepared according to any one of claims 1 to 6.

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