A friction reducer for slickwater used in shale gas reservoir reconstruction and a preparation method and application thereof

Abstract

The application belongs to the field of oil and gas exploitation, and relates to a slick water resistance reducer for shale gas reservoir reconstruction as well as a preparation method and application thereof. The resistance reducer comprises structural units shown in formula (1), formula (2), formula (3) and formula (4). In formula (1), R1 is H or C1-C10 alkyl. In formula (2), R2 is H or C1-C10 alkyl, and R3 is C1-C24 alkyl. In formula (4), R4 is H or an alkali metal, and p is an integer of 1-8. The slick water resistance reducer provided by the application has good stability, low reservoir damage and good resistance reduction effect.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas extraction, specifically relating to a slickwater drag-reducing agent for shale gas reservoir stimulation, its preparation method, and its application. Background Technology

[0002] Shale gas is an unconventional petroleum resource. Its reservoirs are characterized by low porosity and low permeability, making extraction difficult. Artificial stimulation methods such as horizontal well drilling and fracturing are necessary for economical and effective development. Currently, slickwater fracturing is the primary extraction method. The slickwater fracturing fluid system, composed of water and a drag-reducing agent, is a commonly used fracturing fluid system for hydraulic fracturing of shale oil and gas reservoirs. The drag-reducing agent is the core of the slickwater fracturing fluid system, used to reduce friction between the fracturing fluid and the tubing string, increase pump flow rate, and compensate for problems such as low viscosity and poor proppant carrying capacity of the fracturing fluid system.

[0003] Acrylamide-based polymers, as an economical and effective drag-reducing agent for slickwater, have been widely researched and applied in recent years. Drag-reducing agents used in the field are generally available in two formulations: polyacrylamide powder and emulsion. Powder products are low-cost and easy to transport, but generally have a slow dissolution rate, are prone to forming "fisheye" patterns, and are difficult to meet the requirements of continuous online mixing, making them unsuitable for large-scale, on-site mixing processes. Emulsion products are widely used due to their fast dissolution rate, ease of on-site mixing, and low reservoir damage; however, currently used emulsion products still suffer from poor stability, long dissolution time, low viscosity, and poor salt and shear resistance, making them difficult to meet the requirements of on-site construction.

[0004] Therefore, there is an urgent need to develop a drag-reducing agent that is easy to prepare, suitable for shale gas fracturing, has good stability, fast dissolution rate, and excellent salt and shear resistance. Summary of the Invention

[0005] The purpose of this invention is to address the deficiencies and shortcomings of existing technologies by providing a slickwater drag-reducing agent for shale gas reservoir stimulation and its preparation method. Specific functional groups are introduced into the macromolecular chain of polyacrylamide, thereby further improving the polymer's solubility, viscosity, salt resistance, and shear strength. The slickwater drag-reducing agent prepared using this invention has the advantages of rapid dissolution, salt resistance, high drag reduction rate, and stable storage.

[0006] To achieve the above objectives, a first aspect of the present invention provides a slickwater drag-reducing agent for shale gas reservoir stimulation, the drag-reducing agent comprising the structural units shown in formula (1), formula (2), formula (3) and formula (4);

[0007]

[0008] In formula (1), R1 is H or a C1-C10 alkyl group, preferably H or CH3;

[0009] In formula (2), R2 is H or a C1-C10 alkyl group, preferably H or CH3, and R3 is a C1-C24 alkyl group, preferably a C5-C18 alkyl group;

[0010] In formula (4), R4 is H or an alkali metal, preferably Na and / or K, and p is an integer from 1 to 8.

[0011] A second aspect of the present invention provides a method for preparing a slickwater drag-reducing agent for shale gas reservoir stimulation, comprising the following steps:

[0012] The monomers shown in formulas (5), (6), (7), and (8) are mixed, an initiator is added to carry out the reaction, and a phase inversion agent is added after the reaction is completed to obtain the drag-reducing agent.

[0013]

[0014] In formula (5), R1′ is H or a C1-C10 alkyl group, preferably H or CH3;

[0015] In formula (6), R2′ is H or a C1-C10 alkyl group, preferably H or CH3, and R3′ is a C1-C24 alkyl group, preferably a C5-C18 alkyl group;

[0016] In formula (8), R4′ is H or an alkali metal, preferably Na and / or K, and p is an integer from 1 to 8.

[0017] A third aspect of the present invention provides a slickwater drag reducer for shale gas reservoir stimulation prepared by the aforementioned preparation method.

[0018] The fourth aspect of the present invention provides the application of the slickwater drag reducer for shale gas reservoir stimulation in shale gas reservoir stimulation, preferably for the preparation of shale gas fracturing fluid.

[0019] The beneficial effects of this invention are:

[0020] The drag-reducing agent for slickwater provided by this invention is an emulsion type, exhibiting good stability, a medium molecular weight (4-9 million), and low reservoir damage. It demonstrates excellent drag-reducing effect, achieving good results even with low dosage; a 0.25% by mass aqueous solution can achieve a drag reduction rate of approximately 75%. It dissolves quickly, in 30-50 seconds, without forming "fisheye" patterns, making it suitable for continuous on-site mixing or online mixing processes. This meets the needs of preparing large volumes of slickwater and significantly improves construction efficiency. Furthermore, the drag-reducing agent of this invention incorporates sulfonic acid groups as salt-resistant units in its molecular chain, enhancing its salt resistance and maintaining good drag-reducing performance even in a 30,000 mg / L salt solution.

[0021] The acrylamide emulsion copolymer provided by this invention, as a drag reducer for slickwater fracturing fluids, can achieve a high and stable drag reduction rate. The specific application method is well-known to those skilled in the art. Preferably, the aqueous solution concentration of the acrylamide copolymer used as a drag reducer in slickwater fracturing fluids can be 0.05-0.5% by weight.

[0022] Common additives known to those skilled in the art for use in shale gas fracturing fluids, such as corrosion inhibitors, acidifiers, and clay stabilizers, can also be added to the drag reducer containing the acrylamide copolymer, resulting in good compatibility.

[0023] Other features and advantages of the present invention will be described in detail in the following detailed description section. Detailed Implementation

[0024] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0025] To achieve the above objectives, a first aspect of the present invention provides a slickwater drag-reducing agent for shale gas reservoir stimulation, the drag-reducing agent comprising the structural units shown in formula (1), formula (2), formula (3) and formula (4);

[0026]

[0027] In formula (1), R1 is H or a C1-C10 alkyl group, preferably H or CH3;

[0028] In formula (2), R2 is H or a C1-C10 alkyl group, preferably H or CH3, and R3 is a C1-C24 alkyl group, preferably a C5-C18 alkyl group;

[0029] In formula (4), R4 is H or an alkali metal, preferably Na and / or K, and p is an integer from 1 to 8.

[0030] According to the present invention, preferably, based on the total weight of all structural units in the drag-reducing agent, the content of the structural unit shown in formula (1) is 30-70 wt%, preferably 40-60 wt%, the content of the structural unit shown in formula (2) is 0.5-5 wt%, preferably 1-3 wt%, the content of the structural unit shown in formula (3) is 25-60 wt%, preferably 35-50 wt%, and the content of the structural unit shown in formula (4) is 1-10 wt%, preferably 3-8 wt%.

[0031] According to the present invention, preferably, the drag-reducing agent has a viscosity-average molecular weight of 4 million to 9 million, a bulk viscosity of 200 to 1200 mPa·s, a dissolution time of less than 50 s, and a drag reduction rate of ≥60%.

[0032] A second aspect of the present invention provides a method for preparing a slickwater drag-reducing agent for shale gas reservoir stimulation, comprising the following steps:

[0033] The monomers shown in formulas (5), (6), (7), and (8) are mixed, an initiator is added to carry out the reaction, and a phase inversion agent is added after the reaction is completed to obtain the drag-reducing agent.

[0034]

[0035] In formula (5), R1′ is H or a C1-C10 alkyl group, preferably H or CH3;

[0036] In formula (6), R2′ is H or a C1-C10 alkyl group, preferably H or CH3, and R3′ is a C1-C24 alkyl group, preferably a C5-C18 alkyl group;

[0037] In formula (8), R4′ is H or an alkali metal, preferably Na and / or K, and p is an integer from 1 to 8.

[0038] The essential difference between this invention and the prior art lies in:

[0039] This invention increases the solubility, thickening, and salt shear resistance of the prepared acrylamide emulsion polymer by introducing the aforementioned three specific monomers into the macromolecular chain structure of polyacrylamide, combined with other features. By introducing a small amount of long-chain alkyl acrylamide with hydrophobic association function, the structural stability of the molecular chain is enhanced through hydrophobic interactions, increasing the polymer's viscosity and salt shear resistance. Introducing sulfonic acid groups improves the polymer's salt resistance, and introducing thiourea groups increases the polymer's water solubility and dissolution rate.

[0040] According to the present invention, preferably, based on the total weight of the monomers shown in formulas (5), (6), (7), and (8), the amount of monomer shown in formula (5) is 30-70 wt%, preferably 40-60 wt%, the amount of monomer shown in formula (6) is 0.5-5 wt%, preferably 1-3 wt%, the amount of monomer shown in formula (7) is 25-60 wt%, preferably 35-50 wt%, and the amount of monomer shown in formula (8) is 1-10 wt%, preferably 3-8 wt%.

[0041] According to the present invention, preferably, the preparation method is carried out by emulsion polymerization, and includes the following steps:

[0042] Step 1: Dissolve the monomers shown in formulas (5), (7), and (8) in an aqueous solvent, optionally add a complexing agent, and add a pH adjuster to adjust the pH of the system to 6-8 to obtain an aqueous solution.

[0043] Step 2: Dissolve the monomer and emulsifier shown in formula (6) in an organic solvent and stir until homogeneous to obtain an oil phase solution;

[0044] Step 3: Mix the obtained aqueous solution and oil solution and pre-emulsify them to obtain a pre-emulsion system;

[0045] Step 4: Under inert gas protection, an initiator is added to carry out the reaction, and finally the temperature is raised to remove the residual monomer;

[0046] Step 5: Add a phase inversion agent to the emulsion system after the above reaction is completed to obtain the drag-reducing agent.

[0047] This invention uses emulsion polymerization, which involves no toxic or harmful solvents, generates no waste, is environmentally friendly and energy-efficient, and has a safe, gentle and easy-to-operate production process.

[0048] According to the present invention, preferably, in step 1, the aqueous solvent is deionized water; and the pH adjuster is sodium hydroxide and / or 2-acrylamido-2-methylpropanesulfonic acid.

[0049] In this invention, the complexing agent is a compound capable of forming complex ions with metal ions, which can reduce the influence of impurities in the system. Preferably, it is disodium ethylenediaminetetraacetate (EDTA-2Na). In this invention, the complexing agent is introduced into the polymerization system in the form of an aqueous solution. Preferably, the complexing agent is an aqueous solution of EDTA-2Na with a mass concentration of 1% by weight.

[0050] According to the present invention, preferably, based on the total weight of the aqueous solution, the amount of monomers shown in formulas (5), (7), and (8) added is 40-60 wt%, preferably 45-55 wt%, and the amount of complexing agent is 0-0.01 wt%, preferably 0.001-0.005 wt%.

[0051] According to the present invention, preferably, in step 2, the organic solvent is a non-polar or low-polar organic solvent that is immiscible with water and is used in the art for emulsion polymerization, preferably at least one of cyclohexane, n-heptane, isoalkanes, isoparaffins, and white oil.

[0052] The emulsifier is any nonionic emulsifier used in emulsion polymerization in the art, preferably selected from at least one of polyoxyethylene sorbitan fatty acid ester, nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, benzylphenol oxypolyoxyethylene ether, phenethylphenol polyoxyethylene ether, isomeric undecyl alcohol polyoxyethylene ether, isomeric tridecyl alcohol polyoxyethylene ether, nonylphenol polyoxyethylene ether, phenethylphenol polyoxypropylene polyoxyethylene ether, and benzyl dimethylphenol polyoxyethylene ether. It is preferably a composite emulsifier, more preferably a composite emulsifier of sorbitan fatty acid ester and polyoxyethylene sorbitan fatty acid ester, wherein the mass ratio of sorbitan fatty acid ester to polyoxyethylene sorbitan fatty acid ester is 3:1-5:1.

[0053] Based on the total weight of the oil phase solution, the amount of monomer shown in formula (6) added is 0.5-5 wt%, preferably 1-3 wt%, and the amount of emulsifier added is 2-8 wt%, preferably 4-6 wt%.

[0054] The pre-emulsification conditions include: a rotation speed of 8000-12000 r / min and a time of 2-10 min.

[0055] According to the present invention, preferably, in step 3, the amount of the aqueous phase solution is 50-80 wt%, preferably 55-75 wt%, and the amount of the oil phase solution is 20-50 wt%, preferably 25-45 wt%, based on the total weight of the pre-emulsion system.

[0056] According to the present invention, preferably, in step 4, the initiator can be a conventional redox system initiator in the prior art, preferably a persulfate oxidant and a sulfite reductant. Specifically, the persulfate oxidant can be, for example, potassium persulfate, ammonium persulfate, etc. The sulfite reductant can be, for example, potassium bisulfite, sodium bisulfite, etc.

[0057] In this invention, based on the total weight of the monomers shown in (5), (6), (7), and (8), the amount of the persulfate oxidant is 0.01-0.05 wt%; and the amount of the sulfite reducing agent is 0.005-0.025 wt%.

[0058] In this invention, preferably, the persulfate oxidant is a potassium persulfate aqueous solution and / or an ammonium persulfate aqueous solution with a mass concentration of 2% by weight; the sulfite reducing agent is a potassium bisulfite aqueous solution and / or a sodium bisulfite aqueous solution with a mass concentration of 1% by weight.

[0059] According to the present invention, the conditions for the emulsion polymerization reaction include: the emulsion polymerization reaction is carried out under an inert atmosphere, wherein the inert atmosphere can be provided by an inert gas commonly used in the prior art, such as nitrogen.

[0060] In this invention, the reaction conditions include: a temperature of 10-40℃ and a time of 120-300 min; the conditions for heating to remove residual monomers include: a temperature of 30-50℃ and a time of 60-180 min.

[0061] According to the present invention, after the polymerization reaction, the polymerization product is contacted with a phase inversion agent so that the product obtained from the polymerization reaction can be rapidly dissolved in water. The phase inversion agent can be any of the phase inversion agents used in the art for emulsion polymerization, ensuring that the polymerization product can be rapidly dissolved in water under emulsion stability conditions in drag-reducing agent applications. For example, it can be at least one of polyoxyethylene sorbitan fatty acid ester, nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, benzylphenol oxyethylene ether, phenethylphenol polyoxyethylene ether, isomeric undecyl alcohol polyoxyethylene ether, isomeric tridecyl alcohol polyoxyethylene ether, nonylphenol polyoxyethylene ether, phenethylphenol polyoxypropylene polyoxyethylene ether, and benzyl dimethylphenol polyoxyethylene ether.

[0062] In this invention, based on the total weight of the emulsion system after the reaction is completed, the amount of the phase inversion agent is 1-5 wt%, preferably 2-3 wt%.

[0063] A third aspect of the present invention provides a slickwater drag reducer for shale gas reservoir stimulation prepared by the aforementioned preparation method.

[0064] The fourth aspect of the present invention provides the application of the slickwater drag reducer for shale gas reservoir stimulation in shale gas reservoir stimulation, preferably for the preparation of shale gas fracturing fluid.

[0065] According to the present invention, preferably, the concentration of the slickwater drag reducer for shale gas reservoir stimulation is 0.05-0.5 wt%, based on the total weight of the fracturing fluid used for shale gas.

[0066] The present invention will be further described below with reference to the embodiments, but the scope of the present invention is not limited to these embodiments.

[0067] Unless otherwise specified in the examples, the conditions were performed under standard conditions or as recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified were either readily available or prepared using methods disclosed in the prior art. Unless otherwise stated, all reagents used were commercially available chemically pure reagents, and identical products originated from the same source.

[0068] The monomer N used in the embodiments is the monomer shown in formula (8), where p = 1 or 8.

[0069] Example 1

[0070] Operating steps:

[0071] Step 1: Add 84g of deionized water to a beaker, and under stirring, add 225g of 50wt% sodium 2-acrylamido-2-methylpropanesulfonate solution, 112g of acrylamide, 10g of monomer N (p=1, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.), and 1.5g of 1wt% EDTA-2Na aqueous solution. After complete dissolution, add sodium hydroxide to adjust the pH to 7.0.

[0072] Step 2: Add 180g white oil, 5g octadecylacrylamide, 26g sorbitan fatty acid ester (Span-80), and 7g polyoxyethylene sorbitan fatty acid ester (Tween-80) to the reaction vessel.

[0073] Step 3: Under stirring conditions, add the aqueous solution from step 1 to the oil phase from step 2, and emulsify at high speed for 30 minutes to obtain a stable pre-emulsion.

[0074] Step 4: Transfer the pre-emulsion into the reactor, start stirring, control the temperature at 10°C, and purge with nitrogen for 30 minutes. Then, add 4g of 2% potassium persulfate aqueous solution dropwise. The addition is completed in 1 hour. Stop temperature control and continue to add 4g of 1% sodium bisulfite aqueous solution dropwise. The addition is completed in 1.5 hours. Raise the temperature of the polymerization system and then lower it to room temperature before discharging.

[0075] Step 5: Add 17g of phase inversion agent OP-10 to the emulsion polymer prepared in step 4 above, and stir for 30 minutes to carry out phase inversion.

[0076] Example 2

[0077] The procedure was carried out according to Example 1, except that the amount of monomer used in the aqueous phase was different. Specifically, in step 1, 145g of deionized water, 150g of 50wt% sodium 2-acrylamido-2-methylpropanesulfonate solution, and 150g of acrylamide were added, and the rest was the same as in Example 1.

[0078] Example 3

[0079] The procedure was carried out according to the method of Example 1, except that the amount of monomer used in the aqueous phase was different. Specifically, in step 1, 95g of deionized water, 270g of 50wt% sodium 2-acrylamido-2-methylpropanesulfonate solution, and 85g of acrylamide were added, and the rest was the same as in Example 1.

[0080] Example 4

[0081] The procedure was carried out according to Example 1, except that the amount of monomer N in the aqueous phase was reduced. Specifically, in step 1, 84g of deionized water, 225g of 50wt% sodium 2-acrylamido-2-methylpropanesulfonate solution, 115g of acrylamide, and 3g of monomer N (p=1, source same as in Example 1) were added, and the rest was the same as in Example 1.

[0082] Example 5

[0083] The procedure was carried out according to Example 1, except that the amount of monomer N in the aqueous phase was increased. Specifically, 20g of monomer N (p=1, source same as in Example 1) was added in step 1, and everything else was the same as in Example 1.

[0084] Example 6

[0085] The method of Example 1 was followed, except that the p value in the monomer N structure was different. Specifically, in step 1, the same amount of monomer N (p=8, purchased from Beijing Innocare Technology Co., Ltd.) was used instead of monomer N (p=1, sourced from the same source as in Example 1), and everything else was the same as in Example 1.

[0086] Example 7

[0087] The procedure was carried out according to Example 1, except that 2g of monomer octadecylacrylamide was added in step 2.

[0088] Example 8

[0089] The procedure was carried out according to Example 1, except that 11g of monomer octadecylacrylamide was added in step 2.

[0090] Example 9

[0091] The method of Example 1 was followed, except that the water-oil ratio was changed and the amount of white oil was reduced. Specifically, in step 2, 130g of white oil, 5g of octadecylacrylamide, 24g of sorbitan fatty acid ester (Span-80), and 6.5g of polyoxyethylene sorbitan fatty acid ester (Tween-80) were added.

[0092] Example 10

[0093] The method of Example 1 was followed, except that the water-oil ratio was changed and the amount of white oil was increased. Specifically, in step 2, 250g of white oil, 5g of octadecylacrylamide, 29g of sorbitan fatty acid ester (Span-80), and 7.8g of polyoxyethylene sorbitan fatty acid ester (Tween-80) were added.

[0094] Example 11

[0095] The only difference from Example 1 is that in step 2, the amount of polyoxyethylene sorbitan fatty acid ester (Tween-80) added is 26g, and the amount of sorbitan fatty acid ester (Span-80) added is 9g.

[0096] Example 12

[0097] The only difference from Example 1 is that in step 2, the amount of sorbitan fatty acid ester (Span-80) added is 26g, and the amount of polyoxyethylene sorbitan fatty acid ester (Tween-80) added is 4.3g.

[0098] Comparative Example 1

[0099] The method of Example 1 was followed, except that the composition of the monomer mixture was different. Specifically, in step 1, the monomer N (p=1) was not added, while the rest was the same as in Example 1.

[0100] Comparative Example 2

[0101] The procedure was carried out according to the method of Example 1, except that the composition of the monomer mixture was different. Specifically, in step 2, the monomer octadecylacrylamide was not added, while the rest was the same as in Example 1.

[0102] Comparative Example 3

[0103] Polyacrylamide emulsion drag reducer (provided by Shengli Chemical Co., Ltd. of Shengli Oilfield, viscosity-average molecular weight 10 million).

[0104] Test Example 1

[0105] This test is used to test the performance of the emulsion copolymer provided by the present invention, and the test is conducted in accordance with the petroleum industry standard Q / SHCG0160-2021.

[0106] Table 1. Basic properties of emulsion copolymers

[0107]

[0108]

[0109] Test Example 2

[0110] This test was used to evaluate the performance of the slickwater drag reducer solution formulated with the emulsion copolymer provided by this invention. The testing was conducted according to petroleum industry standard Q / SH CG0160-2021.

[0111] At a temperature of 25°C, the solutions of Examples 1-12 and Comparative Examples 1-3 were dissolved in deionized water and saline solution with a mineralization of 30000 mg / L, respectively, to prepare solutions with a concentration of 0.25%. The drag reduction rate was tested using a pipeline friction meter (test conditions: pipeline test section inner diameter 15.8 mm, pipeline test section length 4 m, and flow rate 30 L / min). The results are shown in Table 2.

[0112] Table 2 Solubility and drag reduction properties of emulsion copolymers

[0113]

[0114]

[0115] The test results show that the fast-dissolving, salt-resistant emulsion polymer prepared by this invention has the characteristics of good stability, rapid salt resistance and dissolution, and high drag reduction rate. The emulsion polymer has a moderate bulk viscosity and can remain stable without stratification for 30 days. Compared with Comparative Examples 1 and 3, the dissolution time of Examples 1-10 is greatly shortened, dissolving within 30-50 seconds, meeting the requirements for continuous on-site or online mixing. The salt resistance is good; the drag reduction rate of Examples 1-9 is significantly higher than that of Comparative Examples 2-3. The drag reduction rate of a 0.25% drag-reducing agent solution in 30000 mg / L salt water can reach up to 73.9%, which is significantly better than that of Comparative Examples 2-3.

[0116] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

[0117] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

Claims

1. A slickwater drag-reducing agent for shale gas reservoir stimulation, characterized in that, The drag-reducing agent includes the structural units shown in formula (1), formula (2), formula (3) and formula (4); In formula (1), R1 is H or a C1-C10 alkyl group, preferably H or a C1-C3 alkyl group, and more preferably H or CH3; In formula (2), R2 is H or a C1-C10 alkyl group, preferably H or a C1-C3 alkyl group, more preferably H or CH3, and R3 is a C1-C24 alkyl group, preferably a C5-C18 alkyl group; In formula (4), R4 is H or an alkali metal, preferably Na and / or K, and p is an integer from 1 to 8.

2. The slickwater drag-reducing agent for shale gas reservoir stimulation according to claim 1, wherein, Based on the total weight of all structural units in the drag-reducing agent, the content of the structural unit shown in formula (1) is 30-70 wt%, preferably 40-60 wt%, the content of the structural unit shown in formula (2) is 0.5-5 wt%, preferably 1-3 wt%, the content of the structural unit shown in formula (3) is 25-60 wt%, preferably 35-50 wt%, and the content of the structural unit shown in formula (4) is 1-10 wt%, preferably 3-8 wt%.

3. The slickwater drag-reducing agent for shale gas reservoir stimulation according to claim 1, wherein, The drag-reducing agent has a viscosity-average molecular weight of 4-9 million, a bulk viscosity of 200-1200 mPa·s, a dissolution time of less than 50 s, and a drag reduction rate of ≥60%.

4. A method for preparing a slickwater drag-reducing agent for shale gas reservoir stimulation, characterized in that, Includes the following steps: The monomers shown in formulas (5), (6), (7), and (8) are mixed, an initiator is added to carry out the reaction, and a phase inversion agent is added after the reaction is completed to obtain the drag-reducing agent. In formula (5), R1′ is H or a C1-C10 alkyl group, preferably H or CH3; In formula (6), R2′ is H or a C1-C10 alkyl group, preferably H or CH3, and R3′ is a C1-C24 alkyl group, preferably a C5-C18 alkyl group; In formula (8), R4′ is H or an alkali metal, preferably Na and / or K, and p is an integer from 1 to 8.

5. The preparation method according to claim 4, wherein, Based on the total weight of the monomers shown in Equations (5), (7), (6), and (8), the amount of monomer shown in Equation (5) is 30-70 wt%, preferably 40-60 wt%, the amount of monomer shown in Equation (6) is 0.5-5 wt%, preferably 1-3 wt%, the amount of monomer shown in Equation (7) is 25-60 wt%, preferably 35-50 wt%, and the amount of monomer shown in Equation (8) is 1-10 wt%, preferably 3-8 wt%.

6. The preparation method according to claim 4 or 5, wherein, The preparation method employs emulsion polymerization and includes the following steps: Step 1: Dissolve the monomers shown in formulas (5), (7), and (8) in an aqueous solvent, optionally add a complexing agent, and add a pH adjuster to adjust the pH of the system to 6-8 to obtain an aqueous solution; Step 2: Dissolve the monomer and emulsifier shown in formula (6) in an organic solvent and stir until homogeneous to obtain an oil phase solution; Step 3: Mix the obtained aqueous solution and oil solution and pre-emulsify them to obtain a pre-emulsion system; Step 4: Under inert gas protection, an initiator is added to carry out the reaction, and finally the temperature is raised to remove the residual monomer; Step 5: Add a phase inversion agent to the emulsion system after the above reaction is completed to obtain the drag-reducing agent.

7. The preparation method according to claim 6, wherein, In step 1, the aqueous solvent is deionized water; The pH adjuster is sodium hydroxide and / or 2-acrylamido-2-methylpropanesulfonic acid; The complexing agent is a compound capable of forming a complex ion with metal ions, preferably disodium ethylenediaminetetraacetate; Based on the total weight of the aqueous solution, the amount of monomers added according to formulas (5), (7), and (8) is 40-60 wt%, preferably 45-55 wt%, and the amount of complexing agent is 0-0.01 wt%, preferably 0.001-0.005 wt%.

8. The preparation method according to claim 6, wherein, In step 2, the organic solvent is selected from at least one of cyclohexane, n-heptane, isoalkanes, isoparaffins, and white oil; The emulsifier is selected from at least one of polyoxyethylene sorbitan fatty acid ester, nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, benzylphenol oxypolyoxyethylene ether, phenethylphenol polyoxyethylene ether, isomeric undecyl alcohol polyoxyethylene ether, isomeric tridecyl alcohol polyoxyethylene ether, nonylphenol polyoxyethylene ether, phenethylphenol polyoxypropylene polyoxyethylene ether, and benzyl dimethylphenol polyoxyethylene ether, preferably a composite emulsifier, more preferably a composite emulsifier of sorbitan fatty acid ester and polyoxyethylene sorbitan fatty acid ester, wherein the mass ratio of sorbitan fatty acid ester to polyoxyethylene sorbitan fatty acid ester is 3:1-5:1; Based on the total weight of the oil phase solution, the amount of monomer shown in formula (6) added is 0.5-5 wt%, preferably 1-3 wt%, and the amount of emulsifier added is 2-8 wt%, preferably 4-6 wt%. The pre-emulsification conditions include: a rotation speed of 8000-12000 r / min and a time of 2-10 min.

9. The preparation method according to claim 6, wherein, In step 3, based on the total weight of the pre-emulsion system, the amount of the aqueous phase solution is 50-80 wt%, preferably 55-75 wt%, and the amount of the oil phase solution is 20-50 wt%, preferably 25-45 wt%.

10. The preparation method according to claim 6, wherein, In step 4, the initiator is a persulfate oxidant and / or a sulfite reducing agent, the persulfate oxidant is potassium persulfate and / or ammonium persulfate, and the sulfite reducing agent is potassium bisulfite and / or sodium bisulfite. Based on the total weight of the monomers shown in (5), (6), (7), and (8), the amount of the persulfate oxidant is 0.01-0.05 wt%; the amount of the sulfite reducing agent is 0.005-0.025 wt%. The inert gas is nitrogen; The reaction conditions include: temperature of 10-40℃ and time of 120-300 min; The conditions for removing residual monomers by heating include: a temperature of 30-50℃ and a time of 60-180 min.

11. The preparation method according to claim 6, wherein, In step 5, the phase inversion agent is at least one of the following: polyoxyethylene sorbitan fatty acid ester, nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, benzylphenol oxypolyoxyethylene ether, phenethylphenol polyoxyethylene ether, isomeric undecyl alcohol polyoxyethylene ether, isomeric tridecyl alcohol polyoxyethylene ether, nonylphenol polyoxyethylene ether, phenethylphenol polyoxypropylene polyoxyethylene ether, and benzyl dimethylphenol polyoxyethylene ether. Based on the total weight of the emulsion system after the reaction is completed, the amount of the phase inversion agent is 1-5 wt%, preferably 2-3 wt%.

12. A slickwater drag reducer for shale gas reservoir stimulation prepared by the preparation method according to any one of claims 4-11.

13. The application of the slickwater drag reducer for shale gas reservoir stimulation according to any one of claims 1-3 and 12 in shale gas reservoir stimulation, preferably for the preparation of shale gas fracturing fluid.

14. The application according to claim 13, wherein, Based on the total weight of the shale gas fracturing fluid, the concentration of the slickwater drag reducer used for shale gas reservoir stimulation is 0.05-0.5 wt%.

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