Low-damage variable-viscosity fracturing fluid system, modified thickening agent and preparation method of low-damage variable-viscosity fracturing fluid system

Abstract

The invention belongs to the field of oilfield chemical auxiliaries, and discloses a low-damage variable-viscosity fracturing fluid system, a modified thickening agent and a preparation method of the low-damage variable-viscosity fracturing fluid system, the modified thickening agent and the preparation method of the low-damage variable-viscosity fracturing fluid system, and the preparation method of the modified thickening agent comprises the following steps: preparing a first polymer from first acrylamide and 2-acrylamido-2-methylpropanesulfonic acid; preparing a second polymer from second acrylamide and N, N '-dimethylacrylamide; preparing a third polymer from the third acrylamide and N-(2-(allylbromo) ethyl) oleamide; preparing a fourth polymer from the first polymer, the second polymer, and the third polymer; and preparing the modified thickening agent by using the fourth polymer. The fracturing fluid system prepared by the invention can realize real-time control of the viscosity of the fracturing fluid in different stages, and has the technical advantages of low damage, low friction resistance, good sand-carrying property, simple construction and the like.

Description

Technical Field

[0001] The invention belongs to the field of oilfield chemical additives, and in particular relates to a low-damage variable-viscosity fracturing fluid system, a modified thickener and a preparation method thereof. Background Art

[0002] With the continuous deepening of the development of unconventional oil and gas fields and the continuous development of fracturing technology, fracturing construction technology has continuously put forward new requirements for fluid performance. First, for the transformation of unconventional tight reservoirs, high-viscosity fracturing fluid is required to initially create long main fractures, and then inject low-viscosity fracturing fluid to create complex fractures, connecting more fine fractures to form multi-scale fractures consisting of main fractures, branch fractures and natural fractures; secondly, the sand carrying capacity of conventional slick water fracturing fluid is weak, and high and low viscosity fracturing fluids are usually injected alternately to improve the sand carrying capacity. According to the different degrees of natural fracture development in reservoirs and the high pumping pressure during construction, the fracturing fluid is required to have the ability to adjust the viscosity in real time to cope with the complex problems encountered during construction. Common methods for adjusting the viscosity of the fracturing fluid system include increasing the dosage and adding cross-linking agents, but for practical applications, increasing the dosage of fracturing fluid will significantly increase the cost, and a large amount of polymers may cause certain damage due to incomplete gel breaking after entering the formation. In recent years, it has been reported in the literature that when testing synthesized zwitterionic polymers, it was found that the drag reduction rate was inversely correlated with the solution viscosity. As the viscosity increased, the drag reduction rate decreased. Therefore, in practical applications, the correlation between viscosity and friction must be considered. The viscosity should not be increased too much, and the sand carrying performance should not be sacrificed.

[0003] Therefore, the market currently needs a low-damage, environmentally friendly, variable-viscosity fracturing fluid with fast dissolution rate, excellent drag reduction performance, and excellent sand carrying performance. Summary of the invention

[0004] In order to overcome the defects of the above-mentioned prior art, the purpose of the present invention is to provide a fracturing fluid system with real-time variable viscosity, which has the technical advantages of low damage, low friction, good sand carrying capacity, simple construction, etc. By controlling the concentration of the thickener during the construction process, real-time control of the viscosity of the fracturing fluid at different stages can be achieved, thereby solving the problems of poor sand carrying capacity of the fracturing fluid and excessive damage to the formation, and further increasing the output of unconventional oil and gas fields.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for preparing a modified thickener, comprising:

[0007] preparing a first polymer using a first acrylamide and 2-acrylamido-2-methylpropanesulfonic acid;

[0008] preparing a second polymer using a second acrylamide and N,N'-dimethylacrylamide;

[0009] preparing a third polymer using a third acrylamide and N-(2-(allyl bromideamino)ethyl)oleic acid amide;

[0010] preparing a fourth polymer using the first polymer, the second polymer and the third polymer;

[0011] A modified thickener is prepared using the fourth polymer.

[0012] In some embodiments of the present invention, the first polymer is prepared using a first acrylamide and 2-acrylamido-2-methylpropanesulfonic acid, comprising:

[0013] Mixing and stirring the first acrylamide, 2-acrylamido-2-methylpropanesulfonic acid and the first deionized water to obtain a first aqueous phase solution;

[0014] Adding the first kerosene and Span80 emulsifier to the first aqueous phase solution, stirring for 30 to 60 minutes under nitrogen atmosphere, then adding the first azobisisobutyronitrile initiator, and continuing to stir for 10 to 20 minutes to obtain a first mixed solution;

[0015] The first mixed solution is reacted in an oil bath at 30-60° C. for 4-8 hours to obtain a first polymer.

[0016] In some embodiments of the present invention, the mass ratio of the first acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, the first deionized water, the first kerosene, the Span80 emulsifier and the first azobisisobutyronitrile initiator is 20:10:60:60:4:1.

[0017] In some embodiments of the present invention, the second polymer is prepared using the second acrylamide and N,N'-dimethylacrylamide, comprising:

[0018] Mixing and stirring the second acrylamide, N,N'-dimethylacrylamide and the second deionized water to obtain a second aqueous phase solution;

[0019] Add the second kerosene and the first Tween80 emulsifier to the second aqueous phase solution, stir for 30 to 60 minutes under nitrogen atmosphere, then add the second azobisisobutyronitrile initiator, continue stirring for 10 to 20 minutes, and obtain a second mixed solution;

[0020] The second mixed solution is reacted in an oil bath at 30-60° C. for 3-6 hours to obtain a second polymer.

[0021] In some embodiments of the present invention, the mass ratio of the second acrylamide, N,N'-dimethylacrylamide, the second deionized water, the second kerosene, the first Tween80 emulsifier and the second azobisisobutyronitrile initiator is 15:15:60:60:4:1.

[0022] In some embodiments of the present invention, the third polymer is prepared using a third acrylamide and N-(2-(allyl bromide amino)ethyl)oleic acid amide, comprising:

[0023] Mixing and stirring a third acrylamide, N-(2-(allyl bromideamino)ethyl)oleic acid amide and a third deionized water to obtain a third aqueous phase solution;

[0024] Add the third kerosene and the second Tween80 emulsifier to the third aqueous phase solution, stir for 30 to 60 minutes under nitrogen atmosphere, then add the third azobisisobutyronitrile initiator, continue stirring for 10 to 20 minutes, and obtain a third mixed solution;

[0025] The third mixed solution is reacted in an oil bath at 30-60° C. for 3-6 hours to obtain a third polymer.

[0026] In some embodiments of the present invention, the mass ratio of the third acrylamide, N-(2-(allyl bromideamino)ethyl)oleic acid amide, the third deionized water, the third kerosene, the second Tween80 emulsifier and the third azobisisobutyronitrile initiator is 10:10:60:60:4:1.

[0027] In some embodiments of the present invention, preparing a fourth polymer using the first polymer, the second polymer and the third polymer comprises:

[0028] The first polymer, the second polymer and the third polymer are mixed in a mass ratio of 1:1:1, added into an oil bath at 30-60° C., stirred under a nitrogen atmosphere, and the fourth azobisisobutyronitrile is added and reacted for 4-6 hours to obtain a fourth polymer.

[0029] In some embodiments of the present invention, the modified thickener is prepared using the fourth polymer, comprising:

[0030] Add alkylphenol polyoxyethylene ether diverting agent to the fourth polymer and stir for 1 to 2 hours, and filter to obtain a modified thickener.

[0031] In a second aspect, the present invention discloses a modified thickener prepared by the above method.

[0032] In a third aspect, the present invention discloses a low-damage variable viscosity fracturing fluid system, comprising 0.8-1.5wt% of a modified thickener, 0.2-0.3wt% of a cross-linking agent, 0.1-0.3wt% of a cross-linking accelerator, 0.1-0.3wt% of a clay stabilizer, and 0.08-0.5wt% of a breaker;

[0033] The modified thickener is the main agent of the fracturing fluid system and is prepared by the above method;

[0034] The cross-linking agent includes one or more of boric acid, sodium borate, and sodium chromate;

[0035] The cross-linking accelerator includes one or more of aluminum citrate, zirconium oxychloride, and sodium bicarbonate;

[0036] Clay stabilizers include potassium chloride;

[0037] Breakers include ammonium persulfate.

[0038] Technical effects and advantages of the present invention:

[0039] 1. Compared with conventional fracturing fluid systems, the low-damage variable viscosity fracturing fluid system provided by the present invention has the ability to mix online, and does not require advance preparation at a preparation station, which can greatly reduce the preparation time and improve the efficiency of reservoir reconstruction construction.

[0040] 2. Compared with conventional fracturing fluid systems, the low-damage variable viscosity fracturing fluid system provided by the present invention has the ability of continuous viscosity change, and can achieve continuous viscosity change within 5-100 mPa·s by adjusting the amount of modified thickener. There is no need to prepare the fluid in advance at the fluid preparation station, which can greatly reduce the fluid preparation time and improve the efficiency of reservoir transformation construction.

[0041] 3. Compared with conventional fracturing fluid systems, the low-damage variable viscosity fracturing fluid system provided by the present invention has a thickener molecular weight of 6 to 8 million, which can achieve efficient gel breaking with low residue content under the action of a gel breaker.

[0042] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a flow chart of a method for preparing a modified thickener according to the present invention;

[0044] Figure 2 is a schematic diagram of the modified thickener of Example 1;

[0045] Figure 3 is a graph showing the relationship between the base fluid viscosity and concentration of the fracturing fluid system of Example 2;

[0046] Figure 4 This is a graph showing the temperature and shear resistance of the fracturing fluid system of Example 8. DETAILED DESCRIPTION

[0047] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0048] like Figure 1 As shown, the present invention provides a method for preparing a modified thickener, comprising:

[0049] preparing a first polymer using a first acrylamide and 2-acrylamido-2-methylpropanesulfonic acid;

[0050] preparing a second polymer using a second acrylamide and N,N'-dimethylacrylamide;

[0051] preparing a third polymer using a third acrylamide and N-(2-(allyl bromideamino)ethyl)oleic acid amide;

[0052] preparing a fourth polymer using the first polymer, the second polymer and the third polymer;

[0053] A modified thickener is prepared using the fourth polymer.

[0054] In some embodiments of the present invention, the first polymer is prepared using a first acrylamide and 2-acrylamido-2-methylpropanesulfonic acid, comprising:

[0055] Mixing and stirring the first acrylamide, 2-acrylamido-2-methylpropanesulfonic acid and the first deionized water to obtain a first aqueous phase solution;

[0056] Adding the first kerosene and Span80 emulsifier to the first aqueous phase solution, stirring for 30 to 60 minutes under nitrogen atmosphere, then adding the first azobisisobutyronitrile initiator, and continuing to stir for 10 to 20 minutes to obtain a first mixed solution;

[0057] The first mixed solution is reacted in an oil bath at 30-60° C. for 4-8 hours to obtain a first polymer.

[0058] In some embodiments of the present invention, the mass ratio of the first acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, the first deionized water, the first kerosene, the Span80 emulsifier and the first azobisisobutyronitrile initiator is 20:10:60:60:4:1.

[0059] In some embodiments of the present invention, the second polymer is prepared using the second acrylamide and N,N'-dimethylacrylamide, comprising:

[0060] Mixing and stirring the second acrylamide, N,N'-dimethylacrylamide and the second deionized water to obtain a second aqueous phase solution;

[0061] Add the second kerosene and the first Tween80 emulsifier to the second aqueous phase solution, stir for 30 to 60 minutes under nitrogen atmosphere, then add the second azobisisobutyronitrile initiator, continue stirring for 10 to 20 minutes, and obtain a second mixed solution;

[0062] The second mixed solution is reacted in an oil bath at 30-60° C. for 3-6 hours to obtain a second polymer.

[0063] In some embodiments of the present invention, the mass ratio of the second acrylamide, N,N'-dimethylacrylamide, the second deionized water, the second kerosene, the first Tween80 emulsifier and the second azobisisobutyronitrile initiator is 15:15:60:60:4:1.

[0064] In some embodiments of the present invention, the third polymer is prepared using a third acrylamide and N-(2-(allyl bromide amino)ethyl)oleic acid amide, comprising:

[0065] Mixing and stirring a third acrylamide, N-(2-(allyl bromideamino)ethyl)oleic acid amide and a third deionized water to obtain a third aqueous phase solution;

[0066] Add the third kerosene and the second Tween80 emulsifier to the third aqueous phase solution, stir for 30 to 60 minutes under nitrogen atmosphere, then add the third azobisisobutyronitrile initiator, continue stirring for 10 to 20 minutes, and obtain a third mixed solution;

[0067] The third mixed solution is reacted in an oil bath at 30-60° C. for 3-6 hours to obtain a third polymer.

[0068] In some embodiments of the present invention, the mass ratio of the third acrylamide, the N-(2-(allyl bromideamino)ethyl)oleic acid amide, the third deionized water, the third kerosene, the second Tween80 emulsifier and the third azobisisobutyronitrile initiator is 10:10:60:60:4:1.

[0069] In some embodiments of the present invention, preparing a fourth polymer using the first polymer, the second polymer and the third polymer comprises:

[0070] The first polymer, the second polymer and the third polymer are mixed in a mass ratio of 1:1:1, added into an oil bath at 30-60° C., stirred under a nitrogen atmosphere, and the fourth azobisisobutyronitrile is added and reacted for 4-6 hours to obtain a fourth polymer.

[0071] In some embodiments of the present invention, the modified thickener is prepared using the fourth polymer, comprising:

[0072] Add alkylphenol polyoxyethylene ether diverting agent to the fourth polymer and stir for 1 to 2 hours, and filter to obtain a modified thickener.

[0073] In a second aspect, the present invention discloses a modified thickener prepared by the above method.

[0074] In a third aspect, the present invention discloses a low-damage variable viscosity fracturing fluid system, comprising 0.8-1.5wt% of a modified thickener, 0.2-0.3wt% of a cross-linking agent, 0.1-0.3wt% of a cross-linking accelerator, 0.1-0.3wt% of a clay stabilizer, and 0.08-0.5wt% of a breaker;

[0075] Among them, the modified thickener is the main agent of the fracturing fluid system, which is prepared by the above method;

[0076] The cross-linking agent includes one or more of boric acid, sodium borate, and sodium chromate;

[0077] The cross-linking accelerator includes one or more of aluminum citrate, zirconium oxychloride, and sodium bicarbonate;

[0078] Clay stabilizers include potassium chloride;

[0079] Breakers include ammonium persulfate.

[0080] In order to better illustrate the present invention, the following examples are provided.

[0081] Example 1

[0082] The complete process of the preparation method of a modified thickener of the present invention is as follows, wherein the amount of each component is calculated by weight:

[0083] (1) adding 20 parts by weight of a first acrylamide and 10 parts by weight of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) into a three-necked flask filled with 60 parts by weight of deionized water to prepare a first aqueous phase solution, and stirring the solution with a stirrer to dissolve the monomers;

[0084] (2) During the stirring process, 60 parts by weight of the first kerosene and 4 parts by weight of the Span80 emulsifier were added, and the mixture was stirred for 30 to 60 minutes under a nitrogen atmosphere. Then, 1 part by weight of azobisisobutyronitrile initiator was added, and the mixture was stirred for 10 to 20 minutes to obtain a first mixed solution;

[0085] (3) transferring the first mixed solution obtained in step (2) into an oil bath preheated to 30 to 60° C. and reacting for 4 to 8 hours to obtain a first polymer;

[0086] (4) adding 15 parts by weight of a second acrylamide (AM) and 15 parts by weight of N,N'-dimethylacrylamide (DMAM) into a three-necked flask filled with 60 parts by weight of deionized water to prepare a second aqueous phase solution, and stirring the mixture with a stirrer to dissolve the monomers;

[0087] (5) During the stirring process, 60 parts by weight of the second kerosene and 4 parts by weight of the first Tween 80 emulsifier were added, and the mixture was stirred for 30 to 60 minutes under a nitrogen atmosphere. Then, 1 part by weight of azobisisobutyronitrile initiator was added, and the mixture was stirred for 10 to 20 minutes to obtain a second mixed solution.

[0088] (6) The solution obtained in step (5) is transferred into an oil bath preheated to 30-60°C and allowed to react for 3-6 hours.

[0089] The obtained product is polymer B;

[0090] (7) adding 10 parts by weight of a third acrylamide (AM) and 10 parts by weight of N-(2-(allylbromoamino)ethyl)oleic acid amide into a three-necked flask filled with 60 parts by weight of deionized water to prepare a third aqueous phase solution, and stirring the mixture with a stirrer to dissolve the monomers;

[0091] (8) During the stirring process, 60 parts by weight of the third kerosene and 4 parts by weight of the second Tween 80 emulsifier were added, and the mixture was stirred for 30 to 60 minutes under a nitrogen atmosphere. Then, 1 part by weight of azobisisobutyronitrile initiator was added, and stirring was continued for 10 to 20 minutes.

[0092] (9) The solution obtained in step (8) was transferred into an oil bath preheated to 30-60°C and allowed to react for 3-6 hours.

[0093] The obtained product is polymer C;

[0094] (10) The first polymer, the second polymer and the third polymer are mixed in a mass ratio of 1:1:1, added to an oil bath at 30 to 60° C., fully stirred under a nitrogen atmosphere, and 1 mass part of azobisisobutyronitrile is added, and the reaction is carried out for 4 to 6 hours to obtain a fourth polymer;

[0095] (11) Add 1 part by mass of alkylphenol polyoxyethylene ether diverting agent to the polymer D obtained in step (10), stir thoroughly for 1 to 2 hours, and filter the obtained product to obtain a modified thickener.

[0096] Figure 2 Schematic diagram of the modified thickener. The prepared modified thickener is a milky white polymer liquid with a molecular weight of 6 to 7 million and a viscosity of 300 to 350 mPa·s.

[0097] Example 2

[0098] A low-damage variable viscosity fracturing fluid system is configured and the base fluid properties of the system are investigated, specifically including the following processes:

[0099] Add 0.1wt%, 0.5wt%, 0.8wt%, 1.0wt%, 1.2wt% and 1.5wt% of modified thickener to water respectively under stirring. When adding the modified thickener, stir it fully and add it slowly to avoid the formation of fish eyes, so as to prepare a group of base liquids with different concentrations.

[0100] Use Haake rheometer at 25℃, 170s -1 The base fluid viscosity was measured.

[0101] The results are as follows Figure 3 As shown, the viscosity of the base fluid of the low-damage variable viscosity fracturing fluid system continuously changes between 5 and 100 mPa·s.

[0102] Example 3

[0103] A low-damage variable viscosity fracturing fluid system is configured and the system's gel breaking performance is investigated, specifically including the following processes:

[0104] Under stirring, add 0.1wt% clay stabilizer and 0.8wt% modified thickener to the water in turn. When adding the modified thickener, stir it thoroughly and add it slowly to avoid fish eyes. Add 0.12wt% gel breaker, then add 0.2wt% crosslinker and 0.2wt% crosslinking accelerator under stirring until a uniform gel is formed.

[0105] The fracturing fluid was added to a blue threaded bottle and placed in an oven at 90°C for 4 h. The viscosity of the breaker fluid was measured using a Hake rheometer. The residue content was measured according to step 7.10 of "SY / T 5107-2016 Performance Evaluation Method for Water-Based Fracturing Fluid".

[0106] The viscosity of the low-damage variable viscosity fracturing fluid system is 4.80mPa·s, and the residual content of the gel-breaking fluid is 24.44mg·L -1 , indicating that the gel breaking performance is good.

[0107] Example 4

[0108] A low-damage variable viscosity fracturing fluid system is configured and the system's gel breaking performance is investigated, specifically including the following processes:

[0109] Under stirring, add 0.1wt% clay stabilizer and 1wt% modified thickener to the water in turn. When adding the modified thickener, stir it thoroughly and add it slowly to avoid the formation of fish eyes. Add 0.1wt% gel breaker, then add 0.2wt% cross-linking agent and 0.2wt% cross-linking accelerator under stirring until a uniform gel is formed.

[0110] The fracturing fluid was added to a blue threaded bottle and placed in an oven at 120°C for 4 h. The viscosity of the breaker fluid was measured using a Hake rheometer. The residue content was measured according to step 7.10 of "SY / T 5107-2016 Performance Evaluation Method for Water-Based Fracturing Fluid".

[0111] The viscosity of the low-damage variable viscosity fracturing fluid system is 1.71mPa·s, and the residual content of the gel-breaking fluid is 41.67mg·L -1 , indicating that the gel breaking performance is good.

[0112] Example 5

[0113] A low-damage variable viscosity fracturing fluid system is configured and the system's gel breaking performance is investigated, specifically including the following processes:

[0114] Under stirring, add 0.1wt% clay stabilizer and 1.2wt% modified thickener to the water in turn. When adding the modified thickener, stir it thoroughly and add it slowly to avoid fish eyes. Add 0.08wt% gel breaker, then add 0.2wt% crosslinker and 0.25wt% crosslinking accelerator under stirring until a uniform gel is formed.

[0115] The fracturing fluid was added to the pressure vessel and placed in an oven at 160°C for 4 h. The viscosity of the breaker fluid was measured using a Hake rheometer. The residue content was measured according to step 7.10 of "SY / T 5107-2016 Performance Evaluation Method for Water-Based Fracturing Fluid".

[0116] The viscosity of the low-damage variable viscosity fracturing fluid system is 2.37mPa·s, and the residual content of the gel-breaking fluid is 73.56mg·L -1 , indicating that the gel breaking performance is good.

[0117] Example 6

[0118] A low-damage variable viscosity fracturing fluid system is configured and the system's gel breaking performance is investigated, specifically including the following processes:

[0119] Under stirring, add 0.1wt% clay stabilizer, 0.3wt% crosslinker, 0.3wt% high temperature stabilizer, and 1.2wt% modified thickener to the water in sequence. When adding the modified thickener, stir it thoroughly and add it slowly to avoid fish eyes. Add 0.2wt% broadband breaker, then add 0.2wt% crosslinker and 0.2wt% crosslinking accelerator under stirring until a uniform gel is formed.

[0120] The fracturing fluid was added to the pressure vessel and placed in an oven at 200°C for 4 hours. The viscosity of the breaker fluid was measured using a Hake rheometer. The residue content was measured according to step 7.10 of "SY / T 5107-2016 Performance Evaluation Method for Water-Based Fracturing Fluid".

[0121] The viscosity of the low-damage variable viscosity fracturing fluid system is 4.34mPa·s, and the residual content of the gel-breaking fluid is 81.0mg·L -1 , indicating that the gel breaking performance is good.

[0122] Example 7

[0123] A low-damage variable viscosity fracturing fluid system is configured and the system's gel breaking performance is investigated, specifically including the following processes:

[0124] Add 0.3wt% viscosity promoter, 0.12wt% high temperature stabilizer, and 1.5wt% modified thickener to the water in sequence while stirring. When adding the modified thickener, stir it thoroughly and add it slowly to avoid fish eyes. Add 0.5wt% broadband breaker and then 0.2wt% cross-linker while stirring until a uniform gel is formed.

[0125] The fracturing fluid was added to the pressure vessel and placed in an oven at 230°C for 4 h. The viscosity of the breaker fluid was measured using a Hake rheometer. The residue content was measured according to step 7.10 of "SY / T 5107-2016 Performance Evaluation Method for Water-Based Fracturing Fluid".

[0126] The viscosity of the low-damage variable viscosity fracturing fluid system is 3.25mPa·s, and the residual content of the gel-breaking fluid is 92.21mg·L -1 , indicating that the gel breaking performance is good.

[0127] Example 8

[0128] A low-damage variable viscosity fracturing fluid system is configured and the system's gel breaking performance is investigated, specifically including the following processes:

[0129] Under stirring, add 0.1wt% clay stabilizer, 0.3wt% cross-linker, 0.3wt% viscosity promoter, 0.3wt% high temperature stabilizer, and 1.2wt% modified thickener to the water in sequence. When adding the modified thickener, stir it thoroughly and add it slowly to avoid fish eyes. Add 0.2wt% broadband breaker and then 0.2wt% cross-linker under stirring until a uniform gel is formed.

[0130] Use Haake rheometer at 200℃, 170s -1 The fracturing fluid was sheared for 120 minutes to measure its temperature and shear resistance.

[0131] The results are as follows Figure 4 As shown in the figure, the final viscosity of the low-damage variable-viscosity fracturing fluid system is greater than 60 mPa·s, indicating that the fracturing fluid has strong resistance to temperature and shear.

[0132] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to make equivalent substitutions for some of the technical features therein. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a modified thickener, characterized in that: include: preparing a first polymer using a first acrylamide and 2-acrylamido-2-methylpropanesulfonic acid; preparing a second polymer using a second acrylamide and N,N'-dimethylacrylamide; preparing a third polymer using a third acrylamide and N-(2-(allyl bromideamino)ethyl)oleic acid amide; preparing a fourth polymer using the first polymer, the second polymer and the third polymer; The fourth polymer is used to prepare a modified thickener.

2. The method for preparing a modified thickener according to claim 1, characterized in that: The method of preparing the first polymer by using the first acrylamide and 2-acrylamide-2-methylpropanesulfonic acid comprises: Mixing and stirring the first acrylamide, the 2-acrylamido-2-methylpropanesulfonic acid and first deionized water to obtain a first aqueous phase solution; Adding the first kerosene and Span80 emulsifier to the first aqueous phase solution, stirring for 30 to 60 minutes under nitrogen atmosphere, then adding the first azobisisobutyronitrile initiator, and continuing to stir for 10 to 20 minutes to obtain a first mixed solution; The first mixed solution is reacted in an oil bath at 30-60° C. for 4-8 hours to obtain a first polymer.

3. The method for preparing a modified thickener according to claim 2, characterized in that: The mass ratio of the first acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, the first deionized water, the first kerosene, the Span80 emulsifier and the first azobisisobutyronitrile initiator is 20:10:60:60:4:

1.

4. The method for preparing a modified thickener according to claim 1, characterized in that: The method of preparing the second polymer by using the second acrylamide and N,N'-dimethylacrylamide comprises: Mixing and stirring the second acrylamide, the N,N'-dimethylacrylamide and the second deionized water to obtain a second aqueous phase solution; Adding the second kerosene and the first Tween 80 emulsifier to the second aqueous phase solution, stirring for 30 to 60 minutes under a nitrogen atmosphere, then adding the second azobisisobutyronitrile initiator, and continuing to stir for 10 to 20 minutes to obtain a second mixed solution; The second mixed solution is reacted in an oil bath at 30-60° C. for 3-6 hours to obtain a second polymer.

5. The method for preparing a modified thickener according to claim 4, characterized in that: The mass ratio of the second acrylamide, N,N'-dimethylacrylamide, the second deionized water, the second kerosene, the first Tween80 emulsifier and the second azobisisobutyronitrile initiator is 15:15:60:60:4:

1.

6. The method for preparing a modified thickener according to claim 1, characterized in that: The method of preparing the third polymer by using the third acrylamide and N-(2-(allyl bromide amino)ethyl)oleic acid amide comprises: The third acrylamide, the N-(2-(allyl bromideamino)ethyl)oleic acid amide and the third deionized water are mixed and stirred to obtain a third aqueous phase solution; Adding the third kerosene and the second Tween80 emulsifier to the third aqueous phase solution, stirring for 30 to 60 minutes under a nitrogen atmosphere, then adding the third azobisisobutyronitrile initiator, and continuing to stir for 10 to 20 minutes to obtain a third mixed solution; The third mixed solution is reacted in an oil bath at 30-60° C. for 3-6 hours to obtain a third polymer.

7. The method for preparing a modified thickener according to claim 6, characterized in that: The mass ratio of the third acrylamide, the N-(2-(allyl bromide amino)ethyl) oleic acid amide, the third deionized water, the third kerosene, the second Tween80 emulsifier and the third azobisisobutyronitrile initiator is 10:10:60:60:4:

1.

8. The method for preparing a modified thickener according to claim 1, characterized in that: The method of preparing a fourth polymer by using the first polymer, the second polymer and the third polymer comprises: The first polymer, the second polymer and the third polymer are mixed in a mass ratio of 1:1:1, added into an oil bath at 30-60° C., stirred under a nitrogen atmosphere, and a fourth azobisisobutyronitrile is added and reacted for 4-6 hours to obtain a fourth polymer.

9. The method for preparing a modified thickener according to claim 1, characterized in that: The method of preparing a modified thickener using the fourth polymer comprises: Add alkylphenol polyoxyethylene ether diverting agent to the fourth polymer and stir for 1 to 2 hours, and filter to obtain a modified thickener.

10. A modified thickener, characterized in that: The modified thickener is prepared by the method according to any one of claims 1 to 9.

11. A low-damage variable viscosity fracturing fluid system, characterized in that: The fracturing fluid system comprises 0.8-1.5wt% of a modified thickener, 0.2-0.3wt% of a cross-linking agent, 0.1-0.3wt% of a cross-linking accelerator, 0.1-0.3wt% of a clay stabilizer, and 0.08-0.5wt% of a breaker; The modified thickener is the main agent of the fracturing fluid system and is prepared by the method described in any one of claims 1 to 9; The cross-linking agent includes one or more of boric acid, sodium borate, and sodium chromate; The cross-linking accelerator includes one or more of aluminum citrate, zirconium oxychloride, and sodium bicarbonate; The clay stabilizer includes potassium chloride; The breaker includes ammonium persulfate.

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