Copolymers and methods for their preparation, polylactic acid fibers and methods for their preparation, copolymer fiber compositions, and fracturing fluids and applications

Abstract

The present application relates to the technical field of gas reservoir exploitation, and discloses a copolymer and a preparation method thereof, a polylactic acid fiber and a preparation method thereof, a copolymer fiber composition, a fracturing fluid and applications. The copolymer contains structural unit A shown in formula 1, structural unit B shown in formula 2, structural unit C shown in formula 3 and / or formula 4, and structural unit D shown in formula 5 and / or formula 6. The content of the structural unit A is 40-60 wt%, the content of the structural unit B is 20-30 wt%, the content of the structural unit C is 15-20 wt%, and the content of the structural unit D is 5-25 wt% based on the total weight of the copolymer. The copolymer has excellent salt resistance, high temperature resistance and stability. The fracturing fluid system containing the copolymer can be applied to high-temperature deep wells with a maximum temperature of 158 DEG C.

Description

Technical Field

[0001] This invention relates to the field of gas reservoir development technology, specifically to a copolymer and its preparation method, polylactic acid fiber and its preparation method, copolymer fiber composition and fracturing fluid and their applications. Background Technology

[0002] With the rapid increase in global demand for oil and natural gas, the focus is shifting from conventional reservoirs to the exploration and development of deep-well, high-temperature reservoirs. However, due to increasingly complex reservoir geological conditions, reservoir stimulation is becoming more challenging. Fracturing is a crucial technical measure for enhancing reservoir production, and fracturing fluid is an essential component of the fracturing process. Selecting a suitable, high-performance fracturing fluid is a key technical point for achieving successful reservoir fracturing stimulation.

[0003] Currently, fracturing fluids commonly used both domestically and internationally are divided into two categories: natural plant-based guar gum fracturing fluids and synthetic polymer fracturing fluid systems. Guar gum fracturing fluids have high viscosity and excellent proppant carrying capacity, but they also have high pipeline friction, high residue content, and significant reservoir damage. Furthermore, increasing the amount of guar gum to improve the temperature resistance of the fracturing fluid leads to a substantial increase in material costs. Slickwater systems in polymer fracturing fluids can create large-scale, complex fracture networks and connect natural formation fractures, making them the main fracturing fluid for volumetric fracturing in unconventional oil and gas reservoirs. However, conventional slickwater systems are not heat-resistant, have poor proppant carrying capacity, and cannot effectively carry proppant to fractures to form effective fracture channels. CN108841370A uses a reverse microemulsion method to prepare a polymer thickener and introduces high-strength polyvinyl acetal fibers to increase the proppant carrying capacity of the fracturing fluid. It can be used for fracturing and stimulation of reservoirs at medium and low temperatures (below 70 degrees Celsius). However, it still suffers from poor temperature resistance of slickwater fracturing fluid. Furthermore, it requires the use of designated equipment at the construction site to dissolve and disperse the fibers. Poor fiber dispersion performance severely reduces the efficiency of fiber fracturing fluid. Summary of the Invention

[0004] The purpose of this invention is to overcome the problem that existing fracturing fluids for deep well high-temperature reservoirs cannot simultaneously possess high-temperature resistance, easy proppant carrying capacity, low friction, easy flowback, and low damage performance. This invention provides a copolymer and its preparation method, polylactic acid fibers and their preparation method, copolymer fiber compositions, fracturing fluids, and their applications. This copolymer possesses salt- and high-temperature resistant functional groups and a rigid structure. Fracturing fluid systems containing this copolymer, in addition to being salt- and high-temperature resistant, also exhibit excellent properties such as easy proppant carrying capacity, high proppant carrying capacity, low friction, easy flowback, and low damage. Furthermore, the flowback fluid of this fracturing fluid is recyclable, conforming to the green and sustainable development strategy, and has low production costs.

[0005] To achieve the above objectives, a first aspect of the present invention provides a copolymer comprising structural unit A of Formula 1, structural unit B of Formula 2, structural unit C of Formula 3 and / or Formula 4, and structural unit D of Formula 5 and / or Formula 6.

[0006] Formula 1; Formula 2; Formula 3; Equation 4; Formula 5; Formula 6;

[0007] In Formula 1, R1 is hydrogen or methyl;

[0008] In Formula 2, R2 is hydrogen or methyl;

[0009] In Formula 3, R3 is hydrogen or methyl; R4 is In this context, R5 and R5' are each independently hydrogen or methyl;

[0010] R5 and R5' are not both hydrogen;

[0011] In Formula 4, R6 is hydrogen or methyl; R7 is methyl or benzyl; n is an integer from 1 to 4;

[0012] In Formula 5, R8 is hydrogen or methyl; R9 is , where R 14 C 10 -C 20 Straight-chain or branched alkyl groups;

[0013] In Equation 6, R 10 For hydrogen or methyl, R 11 for ;

[0014] Based on the total weight of the copolymer, the content of structural unit A is 40-60 wt%, the content of structural unit B is 20-30 wt%, the content of structural unit C is 15-20 wt%, and the content of structural unit D is 5-25 wt%.

[0015] A second aspect of the present invention provides a method for preparing a copolymer, wherein the method includes:

[0016] (1) A first mixture of monomer 1, monomer 2, monomer 3 and water is obtained to obtain an aqueous solution; wherein, monomer 1 has the structure shown in Formula I, monomer 2 has the structure shown in Formula II, and monomer 3 has the structure shown in Formula III and / or Formula IV;

[0017] (2) In the presence of a first solvent, monomer 4 and emulsifier are mixed for the second time to obtain an oil phase solution; wherein, monomer 4 has the structure shown in formula V and / or formula VI;

[0018] (3) In the presence of a protective atmosphere and an initiator, the aqueous solution and the oil solution are mixed and polymerized in a third stage to obtain the copolymer.

[0019] Formula I Formula II;

[0020] Formula III Formula IV;

[0021] Formula V Formula VI;

[0022] In Formula I, R1 is hydrogen or methyl;

[0023] In Formula II, R2 is hydrogen or methyl;

[0024] In formula III, R3 is hydrogen or methyl; R4 is In this context, R5 and R5' are each independently hydrogen or methyl;

[0025] R5 and R5' are not both hydrogen; in formula IV, R6 is hydrogen or methyl; R7 is methyl or benzyl; m is an integer from 1 to 4;

[0026] In formula V, R8 is hydrogen or methyl; R9 is , where R 14 C 10 -C 20 Straight-chain or branched alkyl groups;

[0027] In Equation VI, R 10 For hydrogen or methyl, R 11 for ;

[0028] Based on the total mass of monomers 1, 2, 3 and 4, the content of monomer 1 is 40-60 wt%, the content of monomer 2 is 20-30 wt%, the content of monomer 3 is 15-20 wt%, and the content of monomer 4 is 5-25 wt%.

[0029] A third aspect of the present invention provides a copolymer prepared by the above-described preparation method.

[0030] A fourth aspect of the present invention provides a polylactic acid fiber, wherein the polylactic acid fiber comprises polylactic acid nanospheres and polylactic acid microfibers;

[0031] The weight ratio of polylactic acid nanospheres to polylactic acid microfibers is 1:0.3-3.

[0032] The average particle size of the polylactic acid nanospheres is 122-143 nm.

[0033] The polylactic acid microfibers have an average length of 700-800 μm and an average diameter of 600-800 nm.

[0034] A fifth aspect of the present invention provides a method for preparing the above-mentioned polylactic acid fiber, wherein the method for preparing the polylactic acid nanospheres includes:

[0035] (1) Under ultrasonic conditions, polylactic acid is dissolved in organic solvent 1 to obtain organic phase 1;

[0036] (2) Add the organic phase 1 described in step (1) to water and mix to obtain the polylactic acid nanospheres;

[0037] The conditions for ultrasound in step (1) include: ultrasound time of 10-30 min, ultrasound power of 500-900 W, and temperature of 20-30℃.

[0038] A sixth aspect of the present invention provides a copolymer fiber composition, wherein the copolymer fiber composition comprises the above-described copolymer, the above-described polylactic acid fiber or polylactic acid fiber prepared by the above-described preparation method, a surfactant, and a polymerization inhibitor;

[0039] The copolymer fiber composition comprises:

[0040] The copolymer is 85-95 parts by weight;

[0041] The polylactic acid fiber is 0.5-1.5 parts by weight;

[0042] The surfactant is 5-10 parts by weight;

[0043] The polymerization inhibitor is used in amounts of 0.05-0.1 parts by weight.

[0044] A seventh aspect of the present invention provides a fracturing fluid, wherein the fracturing fluid comprises the above-mentioned copolymer fiber composition, a breaker and water;

[0045] The fracturing fluid includes:

[0046] The copolymer fiber composition is used in amounts of 0.5-1.5 parts by weight.

[0047] The amount of the de-adhesive is 0.1-0.5 parts by weight.

[0048] The eighth aspect of the present invention provides an application of the above-mentioned fracturing fluid in reservoir exploration and development.

[0049] The beneficial technical effects achieved by the present invention through the above technical solution are as follows:

[0050] (1) The copolymer obtained by the present invention has salt-resistant and high-temperature resistant functional groups and a rigid structure, which makes the copolymer have excellent salt resistance, high-temperature resistance and stability.

[0051] (2) The polylactic acid fiber provided by the present invention uses polylactic acid nanospheres and polylactic acid microfibers in a specific weight ratio. When applied to the fracturing fluid system, it can ensure the sand-carrying performance of the solution and the uniform dispersion of the solute.

[0052] (3) The fracturing fluid provided by the present invention is easy to flow back, which can reduce the damage of fracturing fluid to the reservoir, and can be quickly dissolved and thickened without the need for pre-mixing. It meets the technical requirements of large-volume and large-scale fracturing construction and can be applied to high-temperature deep wells with a maximum temperature of 158°C. Attached Figure Description

[0053] Figure 1 This is a diagram showing the temperature resistance and shear strength of the fracturing fluid used in Example 1. Detailed Implementation

[0054] 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.

[0055] A first aspect of the present invention provides a copolymer comprising structural unit A of Formula 1, structural unit B of Formula 2, structural unit C of Formula 3 and / or Formula 4, and structural unit D of Formula 5 and / or Formula 6.

[0056] Formula 1; Formula 2; Formula 3; Equation 4; Formula 5; Formula 6;

[0057] In Formula 1, R1 is hydrogen or methyl;

[0058] In Formula 2, R2 is hydrogen or methyl;

[0059] In Formula 3, R3 is hydrogen or methyl; R4 is In this context, R5 and R5' are each independently hydrogen or methyl;

[0060] R5 and R5' are not both hydrogen;

[0061] In Formula 4, R6 is hydrogen or methyl; R7 is methyl or benzyl; n is an integer from 1 to 4;

[0062] In Formula 5, R8 is hydrogen or methyl; R9 is , where R14 C 10 -C 20 Straight-chain or branched alkyl groups;

[0063] In Equation 6, R 10 For hydrogen or methyl, R 11 for ;

[0064] Based on the total weight of the copolymer, the content of structural unit A is 40-60 wt%, the content of structural unit B is 20-30 wt%, the content of structural unit C is 15-20 wt%, and the content of structural unit D is 5-25 wt%.

[0065] The content of each structural unit in the copolymer provided by this invention can be obtained by detection methods such as NMR and infrared spectroscopy, or determined by the monomers and the amount of monomers fed during preparation.

[0066] In this invention, structural units A and B are water-soluble and can improve the solubility and thickening properties of the copolymer. Structural unit C, which has salt-resistant and high-temperature-resistant functional groups, provides the copolymer with salt-resistant and high-temperature-resistant properties. Structural unit D, which has a rigid structure, brings structural stability to the copolymer. The specific content of structural units A, B, C, and D enables the copolymer to simultaneously possess salt-resistant, high-temperature-resistant properties and structural stability.

[0067] Furthermore, according to the present invention, based on the total weight of the copolymer, the content of structural unit A is 45-55 wt%, the content of structural unit B is 20-25 wt%, the content of structural unit C is 15-18 wt%, and the content of structural unit D is 5-17 wt%.

[0068] In this invention, the content of structural unit A, structural unit B, structural unit C and structural unit D in the copolymer is within the above range, which results in superior salt resistance, high temperature resistance and structural stability.

[0069] According to the present invention, in Formula 1, R1 is hydrogen.

[0070] According to the present invention, in Formula 2, R2 is hydrogen.

[0071] According to the present invention, in formula 3, R3 is hydrogen; R4 is... .

[0072] Furthermore, according to the present invention, R5 and R5' are methyl groups.

[0073] According to the present invention, in Formula 4, R6 is hydrogen or methyl; R7 is methyl or benzyl, and n is 2 or 3.

[0074] Furthermore, according to the present invention, R6 is methyl, R7 is methyl, and n is 2 or 3.

[0075] According to the present invention, in formula 5, R8 is hydrogen; R9 is... , where R 14 C 15 -C 20 Straight-chain or branched alkyl groups.

[0076] Furthermore, according to the present invention, R 14 for .

[0077] According to the present invention, in formula 6, R 10 For hydrogen, R 11 for .

[0078] In one specific embodiment of the present invention, preferably, the copolymer is selected from one of the following copolymer combinations:

[0079] Copolymer-1: Structural unit A is derived from acrylamide (structural formula as shown in Formula 1, R1 is hydrogen), structural unit B is derived from acrylic acid (structural formula as shown in Formula 2, R2 is hydrogen), and structural unit C is derived from N,N-dimethylacrylamide (structural formula as shown in Formula 3, R3 is hydrogen, R4 is...). (R5 and R5' are methyl groups), structural unit D comes from octadecyldimethylpropenylammonium chloride (structural formula shown in Formula 5, R8 is hydrogen, R9 is...). , where R 14 for );

[0080] Copolymer-2: Structural unit A is derived from acrylamide (structural formula as shown in Formula 1, R1 is hydrogen), structural unit B is derived from acrylic acid (structural formula as shown in Formula 2, R2 is hydrogen), structural unit C is derived from methacryloyloxyethyltrimethylammonium chloride (structural formula as shown in Formula 4, R6 is methyl, R7 is methyl, n is 2), and structural unit D is derived from sodium 4-acrylamide benzenesulfonate (structural formula as shown in Formula 6, R...). 10 For hydrogen, R 11 for );

[0081] According to the present invention, the weight-average molecular weight of the copolymer is 8 million g / mol to 10 million g / mol, preferably 8.5 million g / mol to 9 million g / mol.

[0082] A second aspect of the present invention provides a method for preparing a copolymer, wherein the method includes:

[0083] (1) A first mixture of monomer 1, monomer 2, monomer 3 and water is obtained to obtain an aqueous solution; wherein, monomer 1 has the structure shown in Formula I, monomer 2 has the structure shown in Formula II, and monomer 3 has the structure shown in Formula III and / or Formula IV;

[0084] (2) In the presence of a first solvent, monomer 4 and emulsifier are mixed for the second time to obtain an oil phase solution; wherein, monomer 4 has the structure shown in formula V and / or formula VI;

[0085] (3) In the presence of a protective atmosphere and an initiator, the aqueous solution and the oil solution are mixed and polymerized in a third stage to obtain the copolymer.

[0086] Formula I Formula II;

[0087] Formula III Formula IV;

[0088] Formula V Formula VI;

[0089] In Formula I, R1 is hydrogen or methyl;

[0090] In Formula II, R2 is hydrogen or methyl;

[0091] In formula III, R3 is hydrogen or methyl; R4 is In this context, R5 and R5' are each independently hydrogen or methyl;

[0092] R5 and R5' are not both hydrogen;

[0093] In Formula IV, R6 is hydrogen or methyl; R7 is methyl or benzyl; m is an integer from 1 to 4;

[0094] In formula V, R8 is hydrogen or methyl; R9 is , where R 14 C 10 -C 20 Straight-chain or branched alkyl groups;

[0095] In Equation VI, R 10 For hydrogen or methyl, R 11 for ;

[0096] Based on the total mass of monomers 1, 2, 3 and 4, the content of monomer 1 is 40-60 wt%, the content of monomer 2 is 20-30 wt%, the content of monomer 3 is 15-20 wt%, and the content of monomer 4 is 5-25 wt%.

[0097] Furthermore, according to the present invention, based on the total mass of monomer 1, monomer 2, monomer 3 and monomer 4, the content of monomer 1 is 45-55 wt%, the content of monomer 2 is 20-25 wt%, the content of monomer 3 is 15-18 wt%, and the content of monomer 4 is 5-17 wt%.

[0098] According to the present invention, in Formula I, R1 is hydrogen.

[0099] According to the present invention, in Formula II, R2 is hydrogen.

[0100] According to the present invention, in formula III, R3 is hydrogen; R4 is... R5 and R5' are methyl groups.

[0101] According to the present invention, in formula IV, R6 is hydrogen or methyl; R7 is methyl or benzyl, and n is 2 or 3.

[0102] According to the present invention, in formula V, R8 is hydrogen; R9 is... , where R 14 C 15 -C 20 Straight-chain or branched alkyl groups.

[0103] According to the present invention, in formula VI, R 10 For hydrogen, R 11 for .

[0104] In one specific embodiment of the present invention, monomer 1 is acrylamide (structural formula as shown in Formula I, R1 is hydrogen).

[0105] In one specific embodiment of the present invention, monomer 2 is acrylic acid (structural formula as shown in Formula II, R2 is hydrogen).

[0106] In one specific embodiment of the present invention, the monomer 3 is selected from N,N-dimethylacrylamide (structural formula shown in Formula III, where R3 is hydrogen and R4 is hydrogen). The following are at least one of the following: R5 and R5' are methyl; methacryloyloxyethyltrimethylammonium chloride (structural formula as shown in Formula IV, R6 is methyl, R7 is methyl, n is 2); methacryloyloxypropyltrimethylammonium chloride (structural formula as shown in Formula IV, R6 is methyl, R7 is methyl, n is 3); acryloyloxyethyltrimethylammonium chloride (structural formula as shown in Formula IV, R6 is hydrogen, R7 is methyl, n is 2); and 2-(acryloyloxy)-N-benzyl-N,N-dimethylethane-1-ammonium chloride (structural formula as shown in Formula IV, R6 is hydrogen, R7 is benzyl, n is 2).

[0107] In one specific embodiment of the present invention, the monomer 4 is selected from octadecyldimethylpropenylammonium chloride (structural formula shown in Formula V, R8 is hydrogen, R9 is...). , where R 14 for ) and / or sodium 4-acrylamide benzenesulfonate (structural formula as shown in Formula VI, R 10 For hydrogen, R 11 for ).

[0108] According to the present invention, the first mixing conditions in step (1) include: adjusting the pH to 6-7, the temperature to 20-30℃, and the time to 0.5-2h.

[0109] According to the present invention, the second mixing conditions in step (2) include: a stirring rate of 1000-1500 r / min, a stirring temperature of 20-30℃, and a stirring time of 10-30 min.

[0110] According to the present invention, the third mixing conditions in step (3) include: a stirring rate of 1000-1600 r / min, a stirring temperature of 20-30℃, and a stirring time of 5-30 min.

[0111] According to the present invention, the polymerization reaction conditions in step (3) include: a temperature of 36-45°C and a time of 2-3 hours.

[0112] According to the present invention, the polymerization in step (3) further includes adding a second solvent or initiator to make the polymerization reaction temperature 36-45°C.

[0113] According to the present invention, the concentration of monomer 1 in the aqueous solution is 30-40 wt%.

[0114] According to the present invention, there is no particular limitation on the concentration of monomer 4 in the oil phase solution, as long as the monomer 4 can be fully mixed with the emulsifier.

[0115] According to the present invention, based on the total mass of monomers, emulsifiers and initiators, the contents of monomer 1, monomer 2, monomer 3 and monomer 4 are 90-98.2 wt%, the contents of initiator are 0.2-1.2 wt%, and the contents of emulsifier are 1.6-9 wt%.

[0116] In this invention, the total mass of the monomers is the sum of the masses of monomer 1, monomer 2, monomer 3 and monomer 4.

[0117] In this invention, the content of the initiator is the total mass of the initiator in step (3).

[0118] According to the present invention, the initiator is at least one of azo initiators, peroxide initiators, and redox initiators, preferably at least one of azobisisobutyronitrile, benzoyl peroxide, potassium persulfate, tert-butyl hydroperoxide, sodium sulfite, and sodium bisulfite.

[0119] According to the present invention, the initiator is at least one selected from azobisisobutyronitrile, benzoyl peroxide, tert-butyl hydroperoxide-sodium bisulfite composite system and potassium persulfate-sodium bisulfite composite system.

[0120] In one specific embodiment of the present invention, the weight ratio of tert-butyl hydroperoxide to sodium bisulfite is 1:0.3-0.5.

[0121] In one specific embodiment of the present invention, the weight ratio of potassium persulfate to sodium sulfite is 1:1-2.

[0122] According to the present invention, the first solvent is at least one selected from kerosene, diesel oil, white oil, cyclohexane, benzene, toluene, and petroleum ether.

[0123] According to the present invention, the emulsifier is a nonionic surfactant, preferably at least one of sorbitan monostearate (Span 60), sorbitan oleate (Span 80), polyoxyethylene sorbitan monostearate (Tween 60), polysorbate 80 (Tween 80), and dodecylphenol polyoxyethylene ether (OP-10).

[0124] According to the present invention, the emulsifier is selected from one of the following: Span 60 / Span 80 compound emulsifier, Tween 60 / Tween 80 compound emulsifier, Span 80 / OP-10 compound emulsifier, or Tween 80 / OP-10 compound emulsifier.

[0125] In one specific embodiment of the present invention, the mass ratio of Span 60 to Span 80 is 1:1-2, the mass ratio of Tween 60 to Tween 80 is 1:0.6-1, the mass ratio of Span 80 to OP-10 is 1:1-2, and the mass ratio of Tween 80 to OP-10 is 1:1-2.

[0126] In this invention, a compound emulsifier is selected, which results in better emulsification and superior dispersion performance.

[0127] According to the present invention, the second solvent is water.

[0128] A third aspect of the present invention provides a copolymer prepared by the above-described preparation method.

[0129] In this invention, the copolymer prepared by the above method can be applied to reservoir fracturing fluid systems, which can improve the high temperature and salt resistance and shear resistance of the fracturing fluid system.

[0130] A fourth aspect of the present invention provides a polylactic acid fiber, wherein the polylactic acid fiber comprises polylactic acid nanospheres and polylactic acid microfibers;

[0131] The weight ratio of polylactic acid nanospheres to polylactic acid microfibers is 1:0.3-3.

[0132] The average particle size of the polylactic acid nanospheres is 122-143 nm.

[0133] The polylactic acid microfibers have an average length of 700-800 μm and an average diameter of 600-800 nm.

[0134] In this invention, the content of the levorotatory isomer of polylactic acid, which is used as the raw material for polylactic acid nanospheres and polylactic acid microfibers, is 90-99 wt%.

[0135] In this invention, polylactic acid fibers comprising polylactic acid nanospheres and polylactic acid microfibers in a specific weight ratio are applied to fracturing systems to ensure uniform dispersion of the solution and improve sand-carrying capacity.

[0136] A fifth aspect of the present invention provides a method for preparing the above-mentioned polylactic acid fiber, wherein the method for preparing the polylactic acid nanospheres includes:

[0137] (1) Under ultrasonic conditions, polylactic acid is dissolved in organic solvent 1 to obtain organic phase 1;

[0138] (2) Add the organic phase 1 described in step (1) to water and mix to obtain the polylactic acid nanospheres;

[0139] The conditions for ultrasound in step (1) include: ultrasound time of 10-30 min, ultrasound power of 500-900 W, and temperature of 20-30℃.

[0140] In this invention, polylactic acid nanospheres of different sizes are obtained by using specific ultrasonic and mixing conditions. Polylactic acid fibers composed of these nanospheres are then applied to fracturing fluid systems to achieve uniform dispersion of the fracturing fluid system.

[0141] According to the present invention, the mixing conditions in step (2) include: a stirring speed of 500-700 r / min and a stirring temperature of 20-30℃.

[0142] According to the present invention, the organic solvent 1 is a mixed solution of ethanol and acetone.

[0143] According to the present invention, the volume ratio of ethanol to acetone is 1:1-3.

[0144] According to the present invention, the content of the levorotatory isomer of the polylactic acid fiber is 90-99 wt%.

[0145] In this invention, polylactic acid can be prepared by the following method:

[0146] (1) Under anhydrous and oxygen-free conditions, lactic acid and stannous chloride are mixed and reacted at 120-150℃ for 5-10 h to obtain mixture 1;

[0147] (2) Dissolve the mixture in chloroform, add it to methanol, mix, and dry to obtain the polylactic acid.

[0148] According to the present invention, the weight ratio of stannous chloride to lactic acid is 1:3000-5000.

[0149] According to the present invention, in step (1), stannous chloride is prepared into a solution of 0.05-0.2 wt% and then mixed with lactic acid, wherein the solvent of the solution is chloroform.

[0150] According to the present invention, there is no particular limitation on the amount of chloroform used, as long as the mixture 1 is fully dissolved.

[0151] According to the present invention, in step (2), the mixing conditions include: until a large amount of white fibrous material is produced.

[0152] According to the present invention, the drying conditions include: a temperature of 80-100°C and a time of 5-10 hours, until the polylactic acid reaches a constant weight.

[0153] In this invention, polylactic acid microfibers can be prepared by the following method:

[0154] A 5-10 wt% polylactic acid dichloromethane solution is poured into a spinning tube. The spinning distance between the receiving screen and the spinneret is 15-20 cm. Spinning is carried out under the conditions of a spinning flow rate of 0.1-0.3 mL / h and a spinning voltage of 7-15 KV. The product is then dried at 20-30℃ to obtain polylactic acid microfibers.

[0155] A sixth aspect of the present invention provides a copolymer fiber composition, wherein the copolymer fiber composition comprises the above-described copolymer, the above-described polylactic acid fiber or polylactic acid fiber prepared by the above-described preparation method, a surfactant, and a polymerization inhibitor;

[0156] The copolymer fiber composition comprises:

[0157] The copolymer is 85-95 parts by weight;

[0158] The polylactic acid fiber is 0.5-1.5 parts by weight;

[0159] The surfactant is 5-15 parts by weight;

[0160] The polymerization inhibitor is used in amounts of 0.05-0.1 parts by weight.

[0161] In this invention, the copolymer fiber composition uses polylactic acid nanospheres and polylactic acid microfibers in a specific weight ratio. The polylactic acid nanospheres ensure the uniform dispersion of the solution, while the polylactic acid microfibers improve the sand-carrying capacity of the solution.

[0162] According to the present invention, the surfactant is selected from at least one of nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, and dodecylphenol polyoxyethylene ether.

[0163] According to the present invention, the polymerization inhibitor is at least one selected from polyvinyl alcohol, sodium polyacrylate and sodium alginate.

[0164] A seventh aspect of the present invention provides a fracturing fluid, wherein the fracturing fluid comprises the above-mentioned copolymer fiber composition, a breaker and water;

[0165] The fracturing fluid includes:

[0166] The copolymer fiber composition is used in amounts of 0.5-1.5 parts by weight.

[0167] The amount of the de-adhesive is 0.1-0.5 parts by weight.

[0168] According to the present invention, the de-gelling agent is at least one selected from potassium persulfate, sodium persulfate, and ammonium persulfate.

[0169] In this invention, compared to fracturing fluid systems composed of copolymers not described in this invention and high-strength polyvinyl acetal fibers, the fracturing fluid prepared in this invention can be applied to deep wells with a maximum temperature of 158°C, and can be rapidly dissolved and thickened without the need for pre-mixing, thereby meeting the technical requirements of large-volume and large-scale fracturing operations.

[0170] The eighth aspect of the present invention provides an application of the fracturing fluid described in the seventh aspect above in reservoir exploration and development.

[0171] The present invention will be described in detail below through embodiments, but the scope of protection of the present invention is not limited to the following description.

[0172] Unless otherwise specified in the following examples and comparative examples, all conditions were performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments used, unless otherwise specified, are all commercially available products.

[0173] The content of each structural unit in the copolymers obtained in the following examples and comparative examples was determined by the monomers used in preparation and the amount of monomers fed. In this invention, all monomers were completely reacted during preparation.

[0174] The weight-average molecular weight of the copolymer was determined by GPC.

[0175] The apparent viscosity of the fracturing fluid was tested according to "SY / T 6376-2008 General Technical Conditions for Fracturing Fluid". 350 mL was placed in a constant temperature water bath at 30℃ and kept at that temperature for 4 hours. The viscosity was measured using a Fann-35 rotational viscometer or a similar product at a rotation speed of 100 r / min (shear rate of 170 s⁻¹). -1 The apparent viscosity at that time.

[0176] The temperature resistance performance of the fracturing fluid was tested according to SY / T 5107-2005 "Performance Evaluation Method of Water-Based Fracturing Fluid". Specific conditions were as follows: the sample cup of the viscometer was filled with fracturing fluid, and the sample was heated. The heating rate was controlled at 3℃ / min ± 0.2℃ / min, starting the test at 30℃, while the rotor operated at a shear rate of 170 s⁻¹. -1 Rotation causes the fracturing fluid to undergo continuous shearing under heating conditions, resulting in an apparent viscosity reduction to 50 mPa. The temperature corresponding to time s represents the temperature resistance of the sample.

[0177] The surface tension of the fracturing fluid was tested according to SY / T 5370-1999 "Methods for Determining Surface and Interfacial Tension".

[0178] The drag reduction performance of the copolymer fiber composition was tested using an indoor pipe friction tester.

[0179] Preparation Example

[0180] Under anhydrous and oxygen-free conditions, 500 g of racemic lactide was mixed with 0.05 wt% stannous chloride and dehydrated under reduced pressure for 2 h. After sealing the ampoules with a high-temperature melt seal, the mixture underwent a dehydration condensation reaction at 130 °C for 5 h. The polylactic acid product was collected and dissolved in chloroform. The solution was poured into methanol and stirred until a large amount of white fibrous material was produced. The mixture was then dried under vacuum at 80 °C for 5 h to constant weight, yielding 400 g of polylactic acid. The content of the levorotatory isomer of polylactic acid was 95 wt%.

[0181] Preparation of copolymers

[0182] Example A-1

[0183] (1) 55 g of acrylamide (monomer 1, structural formula as shown in Formula I, R1 being hydrogen), 25 g of acrylic acid (monomer 2, structural formula as shown in Formula II, R2 being hydrogen), and 15 g of N,N-dimethylacrylamide (monomer 3, structural formula as shown in Formula III, R3 being hydrogen, R4 being hydrogen) were mixed. (R5 and R5' are methyl groups) and water were mixed at 25°C for 1 hour, and KOH was added to adjust the pH of the solution to 6 to obtain an aqueous solution with an acrylamide concentration of 30 wt%.

[0184] (2) Add 5 g of octadecyldimethylpropenylammonium chloride (monomer 4, structural formula as shown in formula V, R8 is hydrogen, R9 is...) , where R 14 for 5 g of emulsifier and white oil were stirred at 1000 r / min for 10 min at 25℃ until uniformly dispersed to obtain an oil phase solution. The emulsifier was a compound emulsifier system of Span 60 / Span 80, with a mass ratio of Span 60 to Span 80 of 1:2.

[0185] (3) Under a nitrogen atmosphere, at 25°C and 1000 r / min, the aqueous solution and the oil solution were mixed, and 0.2 g of initiator azobisisobutyronitrile was added dropwise. The mixture was polymerized at 40°C for 10 min to obtain copolymer A1.

[0186] Based on the total mass of monomers, emulsifiers and initiators, the total mass of monomers is 98.2 wt%, the content of initiators is 0.2 wt%, and the content of emulsifiers is 1.6 wt%.

[0187] The weight-average molecular weight is 9 × 10 6 Based on the total weight of the copolymer, the content of structural unit A is 55 wt%, the content of structural unit B is 25 wt%, the content of structural unit C is 15 wt%, and the content of structural unit D is 5 wt%.

[0188] Example A-2

[0189] The copolymer was prepared according to Example A-1, except that...

[0190] In step (1), monomer 3 is methacryloyloxyethyltrimethylammonium chloride (structural formula as shown in formula IV, R6 is methyl, R7 is methyl, n is 2).

[0191] In step (2), monomer 4 is sodium 4-acrylamide benzenesulfonate (structural formula as shown in formula VI, R 10 For hydrogen, R 11 for ).

[0192] Copolymer A2 was obtained.

[0193] Based on the total mass of monomers, emulsifiers and initiators, the total mass of monomers is 98.2 wt%, the content of initiators is 0.2 wt%, and the content of emulsifiers is 1.6 wt%.

[0194] The weight-average molecular weight is 8.9 × 10⁻⁶. 6 The acrylamide concentration is 35 wt%. Based on the total weight of the copolymer, the content of structural unit A is 55 wt%, the content of structural unit B is 25 wt%, the content of structural unit C is 15 wt%, and the content of structural unit D is 5 wt%.

[0195] Example A-3

[0196] The copolymer was prepared according to Example A-1, except that...

[0197] In step (1), 45g of acrylamide, 20g of acrylic acid, and monomer 3 is 18g of 2-acrylamido-2-methylpropanesulfonic acid (structural formula shown in Formula III, R3 is hydrogen, R4 is...). R5 is methyl, R5' is... , where R 12 for )

[0198] In step (2), 17g of octadecyldimethylpropenylammonium chloride and 10g of emulsifier are used.

[0199] In step (3), 1.1 g of initiator azobisisobutyronitrile.

[0200] Copolymer A3 was obtained. Based on the total mass of monomers, emulsifiers, and initiators, the total mass of monomers was 90 wt%, the content of initiators was 1 wt%, and the content of emulsifiers was 9 wt%.

[0201] The weight-average molecular weight is 8.8 × 10⁻⁶. 6 The concentration of acrylamide is 40 wt%. Based on the total weight of the copolymer, the content of structural unit A is 45 wt%, the content of structural unit B is 20 wt%, the content of structural unit C is 18 wt%, and the content of structural unit D is 17 wt%.

[0202] Example A-4

[0203] The copolymer was prepared according to Example A-1, except that the emulsifier in step (2) was a compound emulsifier system of Tween 60 / Tween 80, with a mass ratio of Tween 60 to Tween 80 of 1:0.6.

[0204] Copolymer A4 was obtained.

[0205] Based on the total mass of monomers, emulsifiers and initiators, the total amount of monomers is 98.2 wt%, the amount of initiators is 0.2 wt%, and the amount of emulsifiers is 1.6 wt%.

[0206] The weight-average molecular weight is 8.7 × 10⁻⁶. 6 The concentration of acrylamide is 30 wt%. Based on the total weight of the copolymer, the content of structural unit A is 55 wt%, the content of structural unit B is 25 wt%, the content of structural unit C is 15 wt%, and the content of structural unit D is 5 wt%.

[0207] Example A-5

[0208] The copolymer was prepared according to Example A-1, except that the emulsifier in step (2) was a compound emulsifier system of Span80 / OP-10, and the mass ratio of Span80 and OP-10 was 1:1.

[0209] Copolymer A5 was obtained.

[0210] Based on the total mass of monomers, emulsifiers and initiators, the total mass of monomers is 98.2 wt%, the content of initiators is 0.2 wt%, and the content of emulsifiers is 1.6 wt%.

[0211] The weight-average molecular weight is 8.7 × 10⁻⁶. 6 The acrylamide concentration is 30 wt%. Based on the total weight of the copolymer, the content of structural unit A is 55 wt%, the content of structural unit B is 25 wt%, the content of structural unit C is 15 wt%, and the content of structural unit D is 5 wt%.

[0212] Example A-6

[0213] The copolymer was prepared according to Example A-1, except that the emulsifier in step (2) was a compound emulsifier system of Tween80 / OP-10, and the mass ratio of Tween80 to OP-10 was 1:1.

[0214] Copolymer A6 was obtained.

[0215] Based on the total mass of monomers, emulsifiers and initiators, the total mass of monomers is 98.2 wt%, the content of initiators is 0.2 wt%, and the content of emulsifiers is 1.6 wt%.

[0216] The weight-average molecular weight is 8.5 × 10⁻⁶. 6The concentration of acrylamide is 30 wt%. Based on the total weight of the copolymer, the content of structural unit A is 55 wt%, the content of structural unit B is 25 wt%, the content of structural unit C is 15 wt%, and the content of structural unit D is 5 wt%.

[0217] Example A-7

[0218] The copolymer was prepared according to Example A-1, except that the initiator in step (3) was benzoyl peroxide.

[0219] Copolymer A7 was obtained.

[0220] Based on the total mass of monomers, emulsifiers and initiators, the total mass of monomers is 98.2 wt%, the content of initiators is 0.2 wt%, and the content of emulsifiers is 1.6 wt%.

[0221] The weight-average molecular weight is 8.6 × 10⁻⁶. 6 The acrylamide concentration is 30 wt%. Based on the total weight of the copolymer, the content of structural unit A is 55 wt%, the content of structural unit B is 25 wt%, the content of structural unit C is 15 wt%, and the content of structural unit D is 5 wt%.

[0222] Example A-8

[0223] The copolymer was prepared according to Example A-1, except that the initiator in step (3) was a tert-butyl hydroperoxide-sodium bisulfite composite system, and the weight ratio of tert-butyl hydroperoxide to sodium bisulfite was 1:0.5.

[0224] Copolymer A8 was obtained.

[0225] Based on the total mass of monomers, emulsifiers and initiators, the total mass of monomers is 98.2 wt%, the content of initiators is 0.2 wt%, and the content of emulsifiers is 1.6 wt%.

[0226] The weight-average molecular weight is 8.6 × 10⁻⁶. 6 The concentration of acrylamide is 30 wt%. Based on the total weight of the copolymer, the content of structural unit A is 55 wt%, the content of structural unit B is 25 wt%, the content of structural unit C is 15 wt%, and the content of structural unit D is 5 wt%.

[0227] Example A-9

[0228] The copolymer was prepared according to Example A-1, except that the initiator in step (3) was a potassium persulfate-sodium sulfite composite system, and the weight ratio of potassium persulfate to sodium sulfite was 1:2.

[0229] Copolymer A9 was obtained. Based on the total mass of monomers, emulsifiers, and initiators, the total mass of monomers was 98.2 wt%, the content of initiators was 0.2 wt%, and the content of emulsifiers was 1.6 wt%.

[0230] The weight-average molecular weight is 8.6 × 10⁻⁶. 6 The concentration of acrylamide is 30 wt%. Based on the total weight of the copolymer, the content of structural unit A is 55 wt%, the content of structural unit B is 25 wt%, the content of structural unit C is 15 wt%, and the content of structural unit D is 5 wt%.

[0231] Example A-10

[0232] The copolymer was prepared according to Example A-1, except that...

[0233] In step (1), there are 40g of acrylamide, 30g of acrylic acid, and 20g of N,N-dimethylacrylamide.

[0234] In step (2), 5g of octadecyldimethylpropenylammonium chloride is used.

[0235] Copolymer A10 was obtained. Based on the total mass of monomers, emulsifiers, and initiators, the total mass of monomers was 98.2 wt%, the content of initiators was 0.2 wt%, and the content of emulsifiers was 1.6 wt%.

[0236] The weight-average molecular weight is 8.5 × 10⁻⁶. 6 The concentration of acrylamide was 30 wt%. Based on the total weight of the copolymer, the content of structural unit A was 42.11 wt%, the content of structural unit B was 31.58 wt%, the content of structural unit C was 21.05 wt%, and the content of structural unit D was 5.26 wt%.

[0237] Comparative Example DA-1

[0238] The copolymer was prepared according to Example A-1, except that...

[0239] In step (1), there are 33g of acrylamide, 18g of acrylic acid, and 22g of N,N-dimethylacrylamide.

[0240] In step (2), 27g of octadecyldimethylpropenylammonium chloride is used.

[0241] Copolymer DA1 was obtained. Based on the total mass of monomers, emulsifiers, and initiators, the total mass of monomers was 98.2 wt%, the content of initiators was 0.2 wt%, and the content of emulsifiers was 1.6 wt%.

[0242] The weight-average molecular weight is 3.3 × 10⁻⁶. 5 The concentration of acrylamide is 30 wt%. Based on the total weight of the copolymer, the content of structural unit A is 33 wt%, the content of structural unit B is 18 wt%, the content of structural unit C is 22 wt%, and the content of structural unit D is 27 wt%.

[0243] Comparative Example DA-2

[0244] The copolymer was prepared according to Example A-1, except that...

[0245] In step (1), monomer 3 is 2-methyl-2-acrylamidopropanesulfonic acid.

[0246] In step (2), monomer 4 is N-vinylpyrrolidone.

[0247] Copolymer DA2 was obtained. Based on the total mass of monomers, emulsifiers, and initiators, the total mass of monomers was 98.2 wt%, the content of initiators was 0.2 wt%, and the content of emulsifiers was 1.6 wt%.

[0248] The weight-average molecular weight is 3.9 × 10⁻⁶. 5 The concentration of acrylamide is 30 wt%. Based on the total weight of the copolymer, the content of structural unit A is 55 wt%, the content of structural unit B is 25 wt%, the content of structural unit C is 15 wt%, and the content of structural unit D is 5 wt%.

[0249] Preparation of polylactic acid fibers

[0250] Example B-1

[0251] (1) Preparation of polylactic acid nanospheres: 20g of polylactic acid prepared in the preparation example was mixed with 400mL of ethanol and 600mL of acetone and sonicated at 25℃ and 500W for 30min to obtain a 2wt% polylactic acid solution. The solution was then added to distilled water and stirred at 500 r / min for 10min at 25℃ to obtain 17.5g of polylactic acid nanospheres. The average particle size of the polylactic acid nanospheres was 131nm.

[0252] (2) Preparation of polylactic acid microfibers: 200 mL of 10 wt% polylactic acid dichloromethane solution was poured into a spinning tube. The spinning distance between the receiving screen and the spinneret was 15 cm. Spinning was carried out under the conditions of a spinning flow rate of 0.1 mL / h and a spinning voltage of 12 KV. After drying, 19.2 g of polylactic acid microfibers were obtained. The average length of the polylactic acid microfibers was 743 μm and the average diameter was 758 nm.

[0253] 10g of polylactic acid nanospheres and 10g of polylactic acid microfibers were mixed in a weight ratio of 1:1 to obtain polylactic acid fiber B1.

[0254] Example B-2

[0255] Polylactic acid (PLA) fibers were prepared according to Example B-1, except that 9g of PLA nanospheres and 3g of PLA microfibers were mixed, with a weight ratio of PLA nanospheres to PLA microfibers of 3:1, to obtain PLA fiber B2.

[0256] Example B-3

[0257] Polylactic acid (PLA) fibers were prepared according to Example B-1, except that 3g of PLA nanospheres and 9g of PLA microfibers were mixed, with a weight ratio of PLA nanospheres to PLA microfibers of 1:3, to obtain PLA fiber B3.

[0258] Example B-4

[0259] Polylactic acid fibers were prepared according to Example B-1, except that in the preparation of polylactic acid nanospheres (1), the fibers were ultrasonicated at 30°C and 900W for 10 min to obtain polylactic acid fibers B4.

[0260] Example B-5

[0261] Polylactic acid fibers were prepared according to Example B-1, except that in the preparation of polylactic acid nanospheres (1), the mixture was stirred at 700 r / min for 30 min at 30 °C to obtain polylactic acid fiber B5.

[0262] Comparative Example DB-1

[0263] Polylactic acid (PLA) fibers were prepared according to Example B-1, except that 3g of PLA nanospheres and 12g of PLA microfibers were mixed, with a weight ratio of PLA nanospheres to PLA microfibers of 1:4, to obtain PLA fiber DB1.

[0264] Comparative Example DB-2

[0265] Polylactic acid (PLA) fibers were prepared according to Example B-1, except that 12g of PLA nanospheres and 3g of PLA microfibers were mixed, with a weight ratio of PLA nanospheres to PLA microfibers of 4:1, to obtain PLA fiber DB2.

[0266] Comparative Example DB-3

[0267] Polylactic acid fibers were prepared according to Example B-1, except that in the preparation of polylactic acid nanospheres (1), the fibers were ultrasonicated at 25°C and 1000W for 30 minutes to obtain polylactic acid fibers DB3.

[0268] Preparation of copolymer fiber compositions

[0269] The formulations of copolymer fiber compositions C1-C10 and DC1-DC7 are shown in Table 1.

[0270] Table 1

[0271]

[0272] The drag reduction rates of copolymer fiber compositions C1-C10 and DC1-DC7 with different contents (0.05 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%) were studied in water and 10 wt% standard brine at 30℃ and a tubing diameter of 10 mm. The standard brine was prepared according to SY / T 5107-2005 "Performance Evaluation Method of Water-Based Fracturing Fluids", specifically, the composition of the standard brine was: 2.0 wt% KCl + 5.5 wt% NaCl + 0.45 wt% MgCl2 + 0.55 wt% CaCl2. The results are shown in Table 2.

[0273] Table 2

[0274]

[0275] As can be seen from the results in Table 2, the indoor drag reduction rate of the copolymer fiber compositions of Examples 1-10 of the present invention is maintained above 70%, which shows excellent drag reduction.

[0276] Application examples

[0277] Application Example 1

[0278] 1.5 parts by weight of copolymer fiber composition C1, 0.1 parts by weight of potassium persulfate and 100 parts by weight of water are mixed to obtain fracturing fluid E1.

[0279] Application Example 2

[0280] Mix 1 part by weight of copolymer fiber composition C2, 0.3 parts by weight of sodium persulfate and 100 parts by weight of water to obtain fracturing fluid E2.

[0281] Application Example 3

[0282] 0.5 parts by weight of copolymer fiber composition C2, 0.1 parts by weight of ammonium persulfate and 100 parts by weight of water are mixed to obtain fracturing fluid E3.

[0283] Application Example 4-10

[0284] The fracturing fluid was prepared according to Application Example 1, except that the copolymer fiber compositions were C4-C10, resulting in fracturing fluids E4-E10.

[0285] Application Comparative Examples 1-7

[0286] The fracturing fluid was prepared according to Application Example 1, except that the copolymer fiber compositions were DC1-DC7, resulting in fracturing fluids DE1-DE7.

[0287] Application Comparative Example 8

[0288] Two parts by weight of copolymer fiber composition C1, 0.6 parts by weight of potassium persulfate, and water were mixed to obtain fracturing fluid DE8.

[0289] Test case

[0290] The performance parameters of fracturing fluids E1-E10 and DE1-DE8 are shown in Table 3. The appearance of the fluids was observed after standing at 25°C for 10 minutes.

[0291] Table 3

[0292]

[0293] As can be seen from the results in Table 3, the fracturing fluid prepared using the copolymers provided in Examples 1-10 of the present invention has excellent high temperature resistance, easy sand carrying capacity and low friction performance.

[0294] Figure 1 The figure shows the temperature resistance and shear strength of the fracturing fluid prepared in Example 1. As can be seen from the figure, the fracturing fluid provided by the present invention can still maintain a high apparent viscosity at 158°C and has excellent high temperature resistance and shear strength.

[0295] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combining the various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A copolymer, characterized in that, The copolymer contains structural unit A as shown in Formula 1, structural unit B as shown in Formula 2, structural unit C as shown in Formula 3 and / or Formula 4, and structural unit D as shown in Formula 5 and / or Formula 6. Formula 1; Formula 2; Formula 3; Equation 4; Formula 5; Formula 6; In Formula 1, R1 is hydrogen or methyl; In Formula 2, R2 is hydrogen or methyl; In Formula 3, R3 is hydrogen or methyl; R4 is In this context, R5 and R5' are each independently hydrogen or methyl; R5 and R5' are not both hydrogen; In Formula 4, R6 is hydrogen or methyl; R7 is methyl or benzyl; n is an integer from 1 to 4; In Formula 5, R8 is hydrogen or methyl; R9 is , where R 14 C 10 -C 20 Straight-chain or branched alkyl groups; In Equation 6, R 10 For hydrogen or methyl, R 11 for ; Based on the total weight of the copolymer, the content of structural unit A is 40-60 wt%, the content of structural unit B is 20-30 wt%, the content of structural unit C is 15-20 wt%, and the content of structural unit D is 5-25 wt%.

2. The copolymer according to claim 1, wherein, Based on the total weight of the copolymer, the content of structural unit A is 45-55 wt%, the content of structural unit B is 20-25 wt%, the content of structural unit C is 15-18 wt%, and the content of structural unit D is 5-17 wt%.

3. The copolymer according to claim 2, wherein, In Equation 1, R1 is hydrogen.

4. The copolymer according to claim 2, wherein, In Equation 2, R2 is hydrogen.

5. The copolymer according to claim 2, wherein, In Equation 3, R3 is hydrogen; R4 is... R5 and R5' are methyl groups.

6. The copolymer according to claim 2, wherein, In Formula 4, R6 is hydrogen or methyl; R7 is methyl or benzyl, and n is 2 or 3.

7. The copolymer according to claim 6, wherein, R6 is methyl, R7 is methyl, and n is 2 or 3.

8. The copolymer according to claim 2, wherein, In Equation 5, R8 is hydrogen; R9 is... , where R 14 C 15 -C 20 Straight-chain or branched alkyl groups.

9. The copolymer according to claim 8, wherein, R 14 for .

10. The copolymer according to claim 2, wherein, In Equation 6, R 10 It is hydrogen.

11. The copolymer according to any one of claims 1-10, wherein, The copolymer has a weight-average molecular weight of 8 million g / mol to 10 million g / mol.

12. The copolymer according to claim 11, wherein, The copolymer has a weight-average molecular weight of 8.5 million g / mol to 9 million g / mol.

13. A method for preparing a copolymer, characterized in that, The preparation method includes: (1) A first mixture of monomer 1, monomer 2, monomer 3 and water is obtained to obtain an aqueous solution; wherein, monomer 1 has the structure shown in Formula I, monomer 2 has the structure shown in Formula II, and monomer 3 has the structure shown in Formula III and / or Formula IV; (2) In the presence of a first solvent, monomer 4 and emulsifier are mixed for the second time to obtain an oil phase solution; wherein, monomer 4 has the structure shown in formula V and / or formula VI; (3) In the presence of a protective atmosphere and an initiator, the aqueous solution and the oil solution are mixed and polymerized in a third stage to obtain the copolymer. Formula I Formula II; Formula III Formula IV; Formula V Formula VI; In Formula I, R1 is hydrogen or methyl; In Formula II, R2 is hydrogen or methyl; In formula III, R3 is hydrogen or methyl; R4 is In this context, R5 and R5' are each independently hydrogen or methyl; R5 and R5' are not both hydrogen; In Formula IV, R6 is hydrogen or methyl; R7 is methyl or benzyl; m is an integer from 1 to 4; In formula V, R8 is hydrogen or methyl; R9 is , where R 14 C 10 -C 20 Straight-chain or branched alkyl groups; In Equation VI, R 10 For hydrogen or methyl, R 11 for ; Based on the total mass of monomers 1, 2, 3 and 4, the content of monomer 1 is 40-60 wt%, the content of monomer 2 is 20-30 wt%, the content of monomer 3 is 15-20 wt%, and the content of monomer 4 is 5-25 wt%.

14. The preparation method according to claim 13, wherein, The first mixing conditions described in step (1) include: adjusting the pH to 6-7, the temperature to 20-30℃, and the time to 0.5-2h.

15. The preparation method according to claim 14, wherein, The second mixing conditions described in step (2) include: a stirring rate of 1000-1500 r / min, a stirring temperature of 20-30℃, and a stirring time of 10-30 min.

16. The preparation method according to claim 14, wherein, The third mixing conditions described in step (3) include: a stirring rate of 1000-1600 r / min, a stirring temperature of 20-30℃, and a stirring time of 5-30 min.

17. The preparation method according to claim 14, wherein, The polymerization reaction conditions described in step (3) include: a temperature of 36-45℃ and a time of 2-3h.

18. The preparation method according to claim 14, wherein, The polymerization described in step (3) further includes adding a second solvent or initiator to make the polymerization reaction temperature 36-45℃.

19. The preparation method according to claim 14, wherein, The concentration of monomer 1 in the aqueous solution is 30-40 wt%.

20. The preparation method according to claim 14, wherein, Based on the total mass of monomers, emulsifiers and initiators, the contents of monomer 1, monomer 2, monomer 3 and monomer 4 are 90-98.2 wt%, the contents of initiator are 0.2-1.2 wt%, and the contents of emulsifier are 1.6-9 wt%.

21. The preparation method according to claim 14, wherein, The initiator is selected from at least one of azo initiators, peroxide initiators, and redox initiators.

22. The preparation method according to claim 21, wherein, The initiator is selected from at least one of azobisisobutyronitrile, benzoyl peroxide, potassium persulfate, tert-butyl hydroperoxide, sodium sulfite, and sodium bisulfite.

23. The preparation method according to claim 14, wherein, The first solvent is selected from at least one of kerosene, diesel oil, white oil, cyclohexane, benzene, toluene, and petroleum ether.

24. The preparation method according to claim 14, wherein, The emulsifier is a nonionic surfactant.

25. The preparation method according to claim 24, wherein, The emulsifier is selected from at least one of sorbitan monostearate, sorbitan oleate, polyoxyethylene sorbitan monostearate, polysorbate 80, and dodecylphenol polyoxyethylene ether.

26. The preparation method according to claim 18, wherein, The second solvent is water.

27. A copolymer prepared by any one of claims 13-26.

28. A copolymer fiber composition, characterized in that, The copolymer fiber composition comprises the copolymer as described in any one of claims 1-12 and 27, polylactic acid fiber, surfactant, and polymerization inhibitor; The copolymer fiber composition comprises: The copolymer is 85-95 parts by weight; The polylactic acid fiber is 0.5-1.5 parts by weight; The surfactant is 5-10 parts by weight; The polymerization inhibitor is used in amounts of 0.05-0.1 parts by weight.

29. The copolymer fiber composition according to claim 28, wherein, The polylactic acid fiber includes polylactic acid nanospheres and polylactic acid microfibers; The weight ratio of the polylactic acid nanospheres to the polylactic acid microfibers is 1:0.3-3. The average particle size of the polylactic acid nanospheres is 122-143 nm. The polylactic acid microfibers have an average length of 700-800 μm and an average diameter of 600-800 nm.

30. The copolymer fiber composition according to claim 29, wherein, The preparation method of the polylactic acid nanospheres includes: (1) Under ultrasonic conditions, polylactic acid is dissolved in organic solvent 1 to obtain organic phase 1; (2) Add the organic phase 1 described in step (1) to water and mix to obtain the polylactic acid nanospheres; The conditions for ultrasound in step (1) include: ultrasound time of 10-30 min, ultrasound power of 500-900 W, and temperature of 20-30℃.

31. The copolymer fiber composition according to claim 30, wherein, The mixing conditions in step (2) include: a stirring speed of 500-700 r / min, a stirring temperature of 20-30℃, and a mixing time of 10-30 min.

32. The copolymer fiber composition according to claim 30, wherein, The organic solvent 1 is a mixed solution of ethanol and acetone.

33. The copolymer fiber composition according to claim 30, wherein, The content of the levorotatory isomer of polylactic acid is 90-99 wt%.

34. The copolymer fiber composition according to claim 29, wherein, The preparation method of the polylactic acid microfiber includes: pouring a 5-10 wt% polylactic acid dichloromethane solution into a spinning tube, spinning at a spinning distance of 15-20 cm between the receiving screen and the spinneret, spinning at a spinning flow rate of 0.1-0.3 mL / h and a spinning voltage of 7-15 KV, and drying at 20-30℃ to obtain polylactic acid microfiber.

35. The copolymer fiber composition according to any one of claims 28-34, wherein, The surfactant is selected from at least one of nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, and dodecylphenol polyoxyethylene ether.

36. The copolymer fiber composition according to any one of claims 28-34, wherein, The polymerization inhibitor is selected from at least one of polyvinyl alcohol, sodium polyacrylate, and sodium alginate.

37. A fracturing fluid, characterized in that, The fracturing fluid comprises the copolymer fiber composition according to any one of claims 28-36, a breaker, and water; The fracturing fluid includes: The copolymer fiber composition is used in amounts of 0.5-1.5 parts by weight. The amount of the de-adhesive is 0.1-0.5 parts by weight.

38. The application of the fracturing fluid of claim 37 in reservoir exploration and development.

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