Environmental response surfactant, coalbed methane clean fracturing fluid and method and application of environmental response surfactant and coalbed methane clean fracturing fluid

Abstract

The invention discloses an environmental response surfactant, a coal bed gas clean fracturing fluid and a method and application thereof, and belongs to the technical field of oil and gas field exploration and development and applied chemistry. The environmental response surfactant is a block copolymer composed of a hydrophilic thermosensitive chain segment and a hydrophobic association chain segment, and the structural general formula of the block copolymer is P (NIPMAM-co-AMPS)-b-PSMA, wherein the hydrophilic thermosensitive chain segment is a random copolymer of N-isopropyl methacrylamide and 2-acrylamido-2-methylpropanesulfonic acid, and the hydrophilic thermosensitive chain segment is a hydrophilic thermosensitive chain segment. The hydrophobic association chain segment is polyoctadecyl methacrylate. The invention also provides a clean fracturing fluid for coal bed gas, which comprises the following components in percentage by weight: 0.4-2.0% of environment response surfactant, 0.5-2.0% of clay anti-swelling agent, 0.01-0.5% of pH regulator and the balance of water. The technical problems that an existing clean fracturing fluid is poor in temperature resistance and salt resistance and cannot be recycled, and an existing heat-sensitive surfactant cannot meet the thickening and salt-resistant requirements of the fracturing fluid are solved.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field exploration, development and applied chemistry technology, specifically relating to an environmentally responsive surfactant, a coalbed methane clean fracturing fluid, and their methods and applications. Background Technology

[0002] With the increasing depletion of conventional oil and gas resources globally, coalbed methane (CBM), as a clean and efficient unconventional energy source, holds significant strategic importance for optimizing the energy structure, alleviating the supply-demand imbalance of natural gas, and improving the safety and production environment of coal mines. However, coal reservoirs generally exhibit characteristics of "three lows and one softness" (low permeability, low reservoir pressure, low gas saturation, and soft coal and rock texture). Hydraulic fracturing technology is currently the most effective means of enhancing CBM well production. Its core principle is to use high-pressure pump sets to inject fracturing fluid carrying proppant into the formation, fracturing the coal and rock to create artificial fractures. The proppant maintains the conductivity of the fractures, thereby constructing efficient gas flow channels. As the "blood of fracturing operations," the performance of the fracturing fluid directly determines the fracture creation effect and the final gas production.

[0003] Existing coalbed methane fracturing fluids have significant technical limitations. While active water fracturing fluids are low-cost and have low friction, their extremely low viscosity (<5 mPa·s) and poor sand-carrying capacity easily lead to sand blockage accidents, and their large filtration loss causes clay swelling damage to water-sensitive coal and rock. Guar gum fracturing fluids, although possessing excellent thickening and sand-carrying properties, leave residues after breaking down, trapped in the fine cleavage and pores of the coal and rock, causing severe "matrix damage," with a permeability damage rate as high as 30%–50%. Viscoelastic surfactant (VES) fracturing fluids, while not causing residue damage, face three major bottlenecks: poor temperature resistance (viscosity drops sharply above 80℃), insufficient salt resistance (prone to phase separation and precipitation in high-salt environments), and non-recyclability and high cost (costing 2–3 times that of guar gum, and difficult to separate and recover after flowback).

[0004] To address the challenges of chemical recycling, the academic community has proposed the concept of "environmentally responsive" materials. Manfredini et al. (ACS Appl. Polym. Mater. 2022, 4, 270-279) synthesized a thermosensitive block copolymer surfactant using RAFT polymerization. This system is stable at low temperatures and exhibits phase separation and reversibility at high temperatures (>64°C). However, the direct application of this technology to coalbed methane fracturing fluid faces four major obstacles: First, the chemical structure has poor hydrolysis resistance; the ester groups of methacrylate monomers are easily hydrolyzed under high-temperature and non-neutral pH conditions at the bottom of the well. Second, it lacks salt resistance; the hydrophilic segment of the PEG group encounters a strong salting-out effect in saline water. Third, it has weak thickening ability; the hydrophobic monomer BMA side chain contains only 4 carbon atoms, which cannot provide sufficient hydrophobic association energy to form a high-viscosity network structure. Fourth, the phase transition temperature windows are mismatched; the Tcp of the P(PEGMA-co-BMA) system reported by Manfredini et al. is about 64℃, while the LCST of a single PNIPMAM homopolymer is about 44℃, neither of which can meet the requirements of coalbed methane wells with well temperatures of 70-90℃, and phase separation will occur before reaching the target layer.

[0005] In summary, existing technologies cannot simultaneously meet the four core requirements of high temperature resistance (90℃), high salt resistance (50,000 mg / L), high viscosity sand carrying capacity, and thermally recyclable properties. Therefore, the development of environmentally responsive surfactants based on novel monomer combinations and molecular structure designs has become an urgent need in the field of clean coalbed methane production enhancement. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the present invention aims to provide an environmentally responsive surfactant, a coalbed methane clean fracturing fluid, and the method and application thereof, in order to solve the technical problems of poor temperature and salt resistance and non-recyclability of existing clean fracturing fluids, as well as the inability of existing heat-sensitive surfactants to meet the requirements of thickening and salt resistance of fracturing fluids.

[0007] To achieve the above objectives, the present invention employs the following technical solution: This invention provides an environmentally responsive surfactant, which is a block copolymer composed of hydrophilic thermosensitive segments and hydrophobic associating segments, and the general structural formula of the block copolymer is P(NIPMAM-co-AMPS)-b-PSMA. The hydrophilic thermosensitive segment is a random copolymer of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid; the hydrophobic associative segment is polyoctadecyl methacrylate. The molar ratio of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid is 90:10 to 70:30, and the number-average molecular weight of the hydrophilic thermosensitive chain segment is 20,000-50,000 g / mol. The degree of polymerization of the octadecyl methacrylate unit in the hydrophobic associative chain segment is 5-20.

[0008] This invention also provides a method for preparing an environmentally responsive surfactant, comprising the following steps: S1, N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid are dissolved in the reaction medium, RAFT reagent and the first initiator are added, nitrogen gas is introduced to remove oxygen and the reaction is stirred to obtain a reaction solution, and then precipitation, centrifugation and drying are carried out in sequence to obtain a macromolecular chain transfer agent. S2, a macromolecular chain transfer agent is dissolved in water as a macromolecular emulsifier, octadecyl methacrylate is added to form a pre-emulsion, a second initiator is added, and after deoxygenation, a polymerization reaction is carried out to obtain a block copolymer; S3, an excess of a second initiator is added to the block copolymer to carry out an addition-fracture reaction to remove end groups, thus obtaining an environmentally responsive surfactant.

[0009] In one embodiment, in S1, the reaction medium is deionized water or a mixed solvent of water and ethanol, wherein the volume ratio of water to ethanol in the mixed solvent is 2:1 to 4:1; the RAFT reagent is 4-cyano-4-(dodecylthioalkylthiocarbonyl)valeric acid or 4-cyano-4-(phenylcarbonthioylthio)valeric acid; and the first initiator is 4,4'-azobis(4-cyanovaleric acid).

[0010] In one embodiment, in S1, the molar ratio of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid is 90:10 to 70:30; the mass-to-volume ratio of the total mass of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid monomers to the volume of the reaction medium is 0.3-0.5 g: 1 mL; the molar ratio of the total molar amount of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid monomers to the molar amount of the RAFT reagent is 200:1 to 400:1; and the molar ratio of the RAFT reagent to the first initiator is 3:1 to 5:1.

[0011] In one embodiment, in S2, the mass-to-volume ratio of the macromolecular chain transfer agent to water is 2.0-5.0 g: 100 mL; the molar ratio of the macromolecular chain transfer agent to octadecyl methacrylate is 1:5 to 1:20; and the molar ratio of the macromolecular chain transfer agent to the second initiator is 3:1 to 10:1.

[0012] In one embodiment, in S2 and S3, the second initiator is 2,2'-azobis(2-methylpropanediamine) dihydrochloride or azobisisobutyronitrile; in S3, the molar ratio of the excess second initiator to the residual RAFT end groups in the block copolymer is 2:1 to 5:1.

[0013] In one embodiment, in S1, the temperature of the stirring reaction is 60-70°C and the time is 12-24 hours; in S2, the temperature of the polymerization reaction is 65-75°C and the time is 24 hours; in S3, the temperature of the addition-fracture reaction is 80°C and the time is 4-12 hours.

[0014] The present invention also provides a clean fracturing fluid for coalbed methane, comprising, by weight percentage, 0.4%-2.0% of an environmentally responsive surfactant, 0.5%-2.0% of a clay anti-swelling agent, 0.01%-0.5% of a pH adjuster, and the balance being water; The clay anti-swelling agent is one or more of potassium chloride, ammonium chloride, tetramethylammonium chloride, and polyquaternary ammonium salt; the pH adjuster is one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, hydrochloric acid, and citric acid.

[0015] This invention also provides a method for using coalbed methane clean fracturing fluid, comprising the following steps: An environmentally responsive surfactant was mixed with clay anti-swelling agent, pH adjuster and water to prepare a clean fracturing fluid for coalbed methane fracturing operations. The polymer was completely dissolved at fracturing operation temperatures below 90°C. After the fracturing operation was completed, the flowback fluid was collected. The flowback fluid is heated to a temperature higher than the critical phase transition temperature of the clean fracturing fluid for coalbed methane. The polymer undergoes a conformational change and aggregates and settles to obtain an intermediate product. After separating the supernatant of the intermediate product, the bottom polymer paste is collected, cooled to room temperature, and then the polymer is redissolved and recycled for the preparation of clean fracturing fluid for coalbed methane.

[0016] This invention also provides the application of environmentally responsive surfactants and coalbed methane cleaning fracturing fluid in the hydraulic fracturing process of coalbed methane wells.

[0017] Compared with the prior art, the present invention has the following beneficial effects: This invention provides an environmentally responsive surfactant and a clean fracturing fluid for coalbed methane (CBM) fracturing. It constructs a temperature- and salt-resistant molecular framework using amide and sulfonic acid monomers to enhance the polymer's chemical stability in high-temperature and high-salt environments. It achieves strong thickening by introducing long-chain hydrophobic monomers (C18 and above) to enhance hydrophobic association, forming a high-viscosity supramolecular network structure at low concentrations to meet fracturing proppant carrying requirements. It precisely controls the response temperature by adjusting the hydrophilic monomer ratio, regulating the critical phase transition temperature (LCST) to the 90-100℃ range, ensuring a dissolved and thickened state in downhole environments below 90℃, and achieving solid-liquid separation when heated to above 100℃ on the surface. It establishes a green recycling process, achieving controllable molecular weight based on RAFT polymerization, and eliminating sulfur chemical residues through end-group treatment, enabling efficient recovery (>90%) and recycling of surfactants in fracturing flowback fluids. Specifically, this invention breaks through the temperature resistance limits of traditional clean fracturing fluids. Traditional VES fracturing fluids exhibit an exponential decrease in viscosity at temperatures exceeding 80°C. Existing thermosensitive polymers have relatively low phase transition temperatures (approximately 64°C), which cannot meet the development requirements of medium-deep coalbed methane. This invention significantly improves the temperature resistance of the system by employing NIPMAM with an amide bond structure and the strong electrolyte AMPS, combined with a C18 long-chain hydrophobic monomer. Experiments show that this fracturing fluid can maintain a viscosity >60 mPa·s (170 s⁻¹) even at 90°C. -1The effective viscosity of this invention meets the fracturing requirements of coalbed methane wells with temperatures below 90℃, solving the problem of insufficient performance of existing clean fracturing fluids in this temperature range. This invention achieves excellent salt resistance and stability. Traditional VES fracturing fluids suffer from double-layer compression in high-salt environments, leading to phase separation or precipitation. Existing heat-sensitive polymers (such as PEG-based systems) experience strong salting-out effects in saline environments, and may precipitate at room temperature in coalbed methane produced water (typically containing 2%~5% NaCl / KCl), making them unpumpable downhole. This invention, through the introduction of sulfonic acid groups via AMPS units, not only avoids precipitation in 50,000 mg / L high-salinity brine but also maintains a viscosity retention rate of >70%. This invention establishes an environmentally friendly recycling model. By precisely controlling the critical phase transition temperature to the 90-100℃ range, this invention achieves an ideal response window for "downhole viscosity enhancement - surface recovery": maintaining a dissolved and viscous state in the downhole environment (60-90℃), and achieving solid-liquid separation at the surface by heating to approximately 100℃. The surfactant recovery rate is over 90%, and after three cycles of "heating-recovery-redissolving," the viscosity retention rate at 90℃ remains above 90%, significantly reducing chemical costs compared to single-use VES fracturing fluid. This invention overcomes the weakness of existing technologies in thickening ability while avoiding residue damage. Existing thermosensitive polymers (such as the BMA-based system reported by Manfredini et al.) have hydrophobic side chains containing only C4, which cannot provide sufficient hydrophobic association energy to form a high-viscosity physical cross-linked network. When used as a fracturing fluid thickener, their viscosity is extremely low, making them unable to carry proppant. This invention significantly enhances hydrophobic association by introducing C18 long-chain hydrophobic monomers (SMA), forming a long-range entangled three-dimensional network structure at a concentration of 1.0%, achieving a viscosity of 62 mPa·s at 90°C, achieving the sand-carrying effect of conventional guar gum (0.4%-0.5%). Further research shows that at a concentration of 0.6%, the viscosity at 90°C can still reach above 35 mPa·s, meeting the basic sand-carrying requirements for low-sand-ratio fracturing operations. Through precise control of molecular weight and hydrophilic / hydrophobic ratio via RAFT polymerization, the system exhibits excellent shear dilution characteristics (facilitating pumping) and shear recovery ability (facilitating sand laying within fractures). Compared to guar gum fracturing fluids, this invention is a purely synthetic polymer system, free of water-insoluble residues from natural plant gums, avoiding severe matrix damage caused by residue retention after gum breakdown (guar gum penetration damage rate is as high as 30%~50%), and significantly improving the desorption and seepage efficiency of coalbed methane. Furthermore, the process provided by this invention offers strong process controllability and stable product quality. This invention employs RAFT (Reactive Radical Induction) polymerization technology to achieve precise control of molecular weight distribution (PDI < 1.5), ensuring the sensitivity and reproducibility of phase transition temperature. Compared to the poor batch stability of polymer products prepared by traditional radical polymerization, the preparation method of this invention offers controllable process parameters, good product performance consistency, and is suitable for industrial-scale production and field applications. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the synthetic route for the environmentally responsive surfactant P(NIPMAM-co-AMPS)-b-PSMA of the present invention; Figure 2 A comparison of the viscosity-temperature curves of different surfactants of the present invention at high temperatures; Figure 3 A comparison of the viscosity retention performance of surfactants in brine with different mineralization levels according to the present invention; Figure 4 This invention provides a comparison of the thermally induced phase separation and recycling performance of the surfactants. Figure 5 This invention provides a comparison of the effects of different monomer ratios on surfactant performance. Figure 6 A comparison of the proppant-carrying capacity of different fracturing fluids; Figure 7 Comparison of reservoir damage caused by different fracturing fluids; Figure 8 The infrared spectrum of P(NIPMAM-co-AMPS)-b-PSMA prepared in Example 1 of this invention is shown. Detailed Implementation

[0019] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0020] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0021] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0022] In this article, unless otherwise specified, “contains,” “includes,” “containing,” “has,” or similar terms cover the meanings of “composed of” and “mainly composed of,” for example, “A contains a” covers the meanings of “A contains a and others” and “A contains only a.”

[0023] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0024] This invention provides an environmentally responsive surfactant, a clean fracturing fluid for coalbed methane, and methods and applications thereof, belonging to the field of oil and gas field exploration and development and applied chemistry technology. Specifically, it relates to a functional chemical additive for enhancing the production of unconventional natural gas (especially coalbed methane, CBM) reservoirs. More specifically, this invention relates to a block copolymer surfactant prepared based on reversible addition-fracture chain transfer (RAFT) polymerization technology, possessing high temperature resistance (90°C), high salt resistance, and thermally induced phase separation and recyclability, and its application in the formulation of recyclable clean fracturing fluids.

[0025] On the one hand, an environmentally responsive surfactant for clean fracturing fluid of coalbed methane is provided. The surfactant is a block copolymer with the general structural formula P(NIPMAM-co-AMPS)-b-PSMA, which is composed of hydrophilic thermosensitive segments and hydrophobic associative segments.

[0026] The hydrophilic thermosensitive segment is a random copolymer of N-isopropylmethacrylamide (NIPMAM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS), and the hydrophobic associative segment is polyoctadecyl methacrylate (PSMA).

[0027] Preferably, the molar ratio of NIPMAM to AMPS is 90:10 to 70:30. Preferably, the number-average molecular weight of the hydrophilic thermosensitive segment is 20,000-50,000 g / mol. Preferably, the degree of polymerization of the SMA unit in the hydrophobic associating segment is 5-20.

[0028] One method provides a method for preparing an environmentally responsive surfactant for clean fracturing fluid in coalbed methane, comprising the following steps: Step 1: Synthesis of hydrophilic thermosensitive macromolecular chain transfer agent P(NIPMAM-co-AMPS)-CTA.

[0029] N-Isopropylmethacrylamide (NIPMAM) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) were dissolved in a reaction medium at a molar ratio of 90:10 to 70:30. A RAFT reagent and a first initiator were added, and the mixture was purged with nitrogen to remove oxygen. The reaction was then stirred at 60-70°C for 12-24 hours. The reaction solution was precipitated, centrifuged, and dried to obtain a macromolecular chain transfer agent. The reaction medium was deionized water or a water / ethanol mixture, with a water-to-ethanol volume ratio of 2:1 to 4:1. The RAFT reagent was 4-cyano-4-(dodecylthioalkylthiocarbonyl)pentanoic acid (CDTPA) or 4-cyano-4-(phenylcarbonylthio)pentanoic acid (CPA). The mass-to-volume ratio of the total monomer mass of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid to the volume of the reaction medium was 0.3-0.5 g:1 mL. The ratio of the total molar amount of the monomer to the molar amount of the RAFT reagent is 200:1 to 400:1, and the molar ratio of the RAFT reagent to the first initiator is 3:1 to 5:1. The first initiator is 4,4'-azobis(4-cyanopentanoic acid) (ACVA).

[0030] The role of NIPMAM is to provide thermoresponsiveness; its amide bond structure is stable and not easily hydrolyzed at high temperatures, and it has a higher critical phase transition temperature (approximately 44°C) compared to NIPAM. The role of AMPS is to introduce strong anionic sulfonic acid groups, providing strong hydration ability and electrostatic repulsion, preventing the polymer from curling and precipitating in high-salt environments, and precisely raising the critical phase transition temperature of the copolymer to the 90-100°C range by adjusting its content.

[0031] Step 2: Synthesis of block copolymer P(NIPMAM-co-AMPS)-b-PSMA.

[0032] The macromolecular chain transfer agent obtained in step one is dissolved in water as a macromolecular emulsifier. Octadecyl methacrylate (SMA) is added, and the mixture is dispersed by ultrasonication or high-speed shearing to form a pre-emulsion. A second initiator is added, and after deoxygenation, the mixture is reacted at 65-75°C for 24 hours to obtain a block copolymer. The molar ratio of the macromolecular chain transfer agent to SMA is 1:5 to 1:20. The second initiator is 2,2'-azobis(2-methylpropanediamine) dihydrochloride (V-50) or azobisisobutyronitrile (AIBN). The mass-to-volume ratio of the macromolecular chain transfer agent to water is 2.0-5.0 g:100 mL; the molar ratio of the macromolecular chain transfer agent to the second initiator is 3:1 to 10:1.

[0033] The role of the SMA is to provide C18 long carbon chains, which form physical cross-linking points through strong hydrophobic association. When the polymer concentration exceeds the critical overlap concentration, different polymer chains are connected to form a three-dimensional network structure, giving the fluid high viscosity and elasticity as a suspension support.

[0034] Step 3: End-base removal treatment.

[0035] After polymerization in step two, an excess of the second initiator is added to the emulsion, the temperature is raised to 80°C, and the reaction is carried out for 4-12 hours. The terminal sulfoxocarbonyl sulfide groups are replaced or oxidized and removed through an addition-fracture reaction. The purpose of this end-group removal treatment is to eliminate the sulfoxocarbonyl sulfide groups at the ends of the RAFT polymerization product, remove color and odor, and improve the chemical stability of the product. The molar ratio of the excess second initiator to the residual RAFT end groups in the block copolymer is 2:1 to 5:1.

[0036] like Figure 1 The figure shows the entire process of first synthesizing hydrophilic Macro-CTA with trithioester end groups, second inserting SMA monomers for chain extension, and third treating with AIBN to remove end groups and convert it into inert end groups.

[0037] On the other hand, a clean fracturing fluid for coalbed methane containing the aforementioned environmentally responsive surfactant is also provided, with the following weight percentage composition: 0.4%-2.0% environmentally responsive surfactant, 0.5%-2.0% clay anti-swelling agent, 0.01%-0.5% pH adjuster, and water as the balance. The clay anti-swelling agent is one or more of potassium chloride, ammonium chloride, tetramethylammonium chloride, and polyquaternary ammonium salt; the pH adjuster is one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, hydrochloric acid, and citric acid.

[0038] Furthermore, a method for using a coalbed methane clean fracturing fluid containing the aforementioned environmentally responsive surfactant is also provided, comprising the following steps: (1) The environmentally responsive surfactant is mixed with clay anti-swelling agent, pH adjuster and water to prepare fracturing fluid. At fracturing construction temperature below 90℃, the polymer is completely dissolved, the hydrophobic segment aggregates to form physical cross-linking points, and the system exhibits high viscoelasticity. (2) After the fracturing operation is completed, the flowback fluid is heated to a temperature higher than the critical phase transition temperature (e.g., heated to 100-105℃), and the polymer undergoes a conformational change and aggregates and settles. (3) After separating the supernatant, collect the bottom polymer paste, cool it to room temperature, and then the polymer is redissolved and recycled for the preparation of fracturing fluid.

[0039] Preferably, the fracturing operation temperature is 60-90℃, which is below the critical phase transition temperature; and the heating temperature is 100-110℃, which is above the critical phase transition temperature.

[0040] On the other hand, the application of the above-mentioned environmentally responsive surfactant and the above-mentioned coalbed methane cleaning fracturing fluid in the hydraulic fracturing process of coalbed methane wells is also provided.

[0041] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0042] The following examples use instruments and equipment conventional in the art. Experimental methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. All raw materials used in the following examples are conventional commercially available products with specifications conventional in the art. In this specification and the following examples, unless otherwise specified, "%" refers to weight percentage, "parts" refers to parts by weight, and "ratio" refers to weight proportion.

[0043] The specific information regarding the reagents used in the following examples and comparative examples is as follows: N-Isopropylmethacrylamide (NIPMAM, molecular weight 157.21 g / mol), 2-acrylamido-2-methylpropanesulfonic acid (AMPS, molecular weight 207.25 g / mol), and octadecyl methacrylate (SMA, molecular weight 338.57 g / mol) were all purchased from Sigma-Aldrich. Before use, the polymerization inhibitors were removed by passing the solution through an alkaline alumina column. The chain transfer agent 4-cyano-4-(dodecylthioalkylthiocarbonyl)valerate (CDTPA, molecular weight 403.67 g / mol), the initiator 4,4'-azobis(4-cyanovaleric acid) (ACVA, molecular weight 280.28 g / mol), and azobisisobutyronitrile (AIBN, molecular weight 164.21 g / mol) were all analytical grade. Inorganic salts such as potassium chloride and sodium chloride were commercially available analytical grade. The water used was deionized water.

[0044] Example 1 Preparation of environmentally responsive surfactant P(NIPMAM-co-AMPS)-b-PSMA (Best Example) Step 1: Synthesis of macromolecular chain transfer agent (Macro-CTA): In a 250 mL round-bottom flask equipped with a magnetic stirrer, NIPMAM (12.58 g, 80 mmol) and AMPS (4.15 g, 20 mmol) (monomer molar ratio 80:20, total 100 mmol) were added and dissolved in 40 mL of a mixed solvent of deionized water and ethanol (volume ratio 3:1). RAFT reagent CDTPA (0.161 g, 0.4 mmol) and initiator ACVA (0.028 g, 0.1 mmol) were added, bringing the monomer / RAFT molar ratio to 250:1 and the RAFT / initiator molar ratio to 4:1. The flask was sealed, and high-purity nitrogen was purged for 30 minutes to remove oxygen. The reaction was stirred in an oil bath at 70 °C for 20 hours. After the reaction was complete, the reaction solution was added dropwise to excess cold diethyl ether to precipitate the product. The product was collected by centrifugation and dried under vacuum to obtain approximately 12 g of a hydrophilic macromolecular chain transfer agent, P(NIPMAM-co-AMPS)-CTA, a pale yellow powder. GPC testing (DMF as the mobile phase, polystyrene standard for calibration) showed a number-average molecular weight (Mn) of approximately 32,500 g / mol and a PDI of 1.15.

[0045] Step 2, Synthesis of block copolymers: Dissolve the above-mentioned Macro-CTA (6.50 g, 0.20 mmol, calculated based on Mn=32,500) in 200 mL of deionized water and adjust the pH to 7 to serve as a macromolecular emulsifier. Mix and dissolve the hydrophobic monomer SMA (0.68 g, 2.0 mmol) with the initiator AIBN (6.6 mg, 0.04 mmol) and add this mixture to the above-mentioned macromolecular emulsifier aqueous solution (Macro-CTA to SMA molar ratio 1:10, RAFT end group to initiator molar ratio 5:1). Disperse the mixture for 30 minutes in an ultrasonic cell disruptor (300W power, pulse mode: 2s on, 3s off) under ice bath conditions to form a stable oil-in-water pre-emulsion. Transfer the emulsion to a reaction flask, purge with nitrogen for 30 minutes to remove oxygen, and then place it in a 70°C oil bath with stirring for 24 hours.

[0046] Determination of degree of polymerization of the hydrophobic segment: use 1 The final product was characterized by ¹H-NMR (solvent: DMSO-d6 / D₂O mixed solvent, temperature 25℃). The actual degree of polymerization of the hydrophobic segment was calculated by comparing the integral area ratio of the terminal methyl peak (-CH₃, 3H) of the SMA unit at δ=0.85 ppm with that of the methine peak (-CH(CH₃)₂, 1H) of the NIPMAM unit at δ=3.85 ppm.

[0047] Result: Theoretical degree of polymerization (based on feed ratio): 10 1The actual degree of polymerization determined by H-NMR is 8-11 (slight differences between batches), and the conversion rate is approximately 80-100%.

[0048] Conclusion: The measured degree of polymerization in the hydrophobic segment agrees well with the theoretical design value, proving the controllability of SMA monomers in emulsion RAFT polymerization.

[0049] Step 3, end-base removal: After the reaction was complete, without separation and purification, excess initiator AIBN (0.10 g, 0.61 mmol, approximately three times the equivalent of the residual RAFT end groups) was added directly to the emulsion. The temperature was raised to 80°C and the reaction continued for 6 hours. The free radicals generated by the decomposition of the initiator underwent an addition-cleavage reaction with the trithioester groups at the chain ends, removing the sulfur carbonyl end groups. The reaction solution was dialyzed in deionized water for 3 days (with water changed every 8 hours) through a dialysis bag (molecular weight cutoff 3500 Da). After freeze-drying, approximately 6.5 g of the white powdery final product P(NIPMAM-co-AMPS)-b-PSMA was obtained.

[0050] Characterization of end-group removal effect: The polymer solutions before and after end-group removal were characterized using a UV-Vis spectrophotometer. Test conditions: 0.5 wt% aqueous solution, scanning wavelength 200-500 nm. Results: Before end-group removal: A distinct trithioester characteristic absorption peak (attributed to the C=S π (ground state) → π (excited state) transition) was observed at λ=309 nm, and the solution was pale yellow. After end-group removal: The absorption peak at 309 nm completely disappeared, and the solution was colorless and transparent. Conclusion: UV-Vis spectroscopy confirms that after AIBN treatment, the sulfur carbonyl sulfide groups at the polymer ends were effectively removed, improving the chemical stability of the product and eliminating odor.

[0051] Polymer structure characterization: The structure of the final product P(NIPMAM-co-AMPS)-b-PSMA was characterized using Fourier transform infrared spectroscopy (FT-IR, Nicolet iS50). Test conditions: KBr pellet method, scanning range 4000-400 cm⁻¹. -1 4 cm resolution -1 The number of scans was 32. The infrared spectrum is as follows: Figure 8 As shown, the characteristic absorption peaks are assigned as follows:

[0052] Note: br = broad peak, ν = stretching vibration, δ = bending vibration, ρ = rocking vibration, as = asymmetric, s = symmetric Conclusion: The characteristic amide absorptions of the NIPMAM unit (3295, 1642, 1538 cm⁻¹) were simultaneously detected in the FT-IR spectrum. -1), characteristic absorptions of sulfonic acid groups in AMPS units (1178, 1038 cm⁻¹) -1 ) and the ester carbonyl group of the PSMA unit (1728 cm) -1 ) and long-chain methylene absorption (2918, 2850, 718 cm⁻¹) -1 This confirms the successful synthesis of the target block copolymer P(NIPMAM-co-AMPS)-b-PSMA.

[0053] Example 2: Preparation of surfactants with different monomer ratios The operation steps are the same as in Example 1, except that the molar ratio of NIPMAM to AMPS in step (1) is adjusted to 90:10, and the specific amount of feed is: NIPMAM (14.15 g, 90 mmol) and AMPS (2.07 g, 10 mmol).

[0054] Characterization results: GPC test: number average molecular weight Mn is approximately 31,200 g / mol, and PDI is 1.18.

[0055] Critical phase transition temperature (LCST): The cloud point temperature of a 1.0 wt% aqueous solution is 82 °C, determined by turbidity method.

[0056] Temperature resistance: 1.0 wt% aqueous solution at 80℃ for 170 s -1 The viscosity is 58 mPa·s.

[0057] Scope of application: Suitable for shallow coalbed methane wells with well temperatures ≤80℃.

[0058] Example 3: Preparation of surfactants with different monomer ratios The operation steps are the same as in Example 1, except that the molar ratio of NIPMAM to AMPS in step (1) is adjusted to 70:30, and the specific amounts of NIPMAM (11.00 g, 70 mmol) and AMPS (6.22 g, 30 mmol) are:

[0059] Characterization results: GPC test: number average molecular weight Mn is approximately 34,800 g / mol, and PDI is 1.22.

[0060] Critical phase transition temperature (LCST): 105 °C for a 1.0 wt% aqueous solution, determined by turbidity method.

[0061] Temperature resistance: A 1.0 wt% aqueous solution at 90℃ for 170 s -1 The viscosity at 100℃ is 70 mPa·s; the viscosity at 100℃ is 55 mPa·s.

[0062] Salt resistance: In a 50,000 mg / L NaCl solution, the viscosity at 90°C is 56 mPa·s, and the retention rate is 80%.

[0063] Scope of application: Suitable for medium-deep coalbed methane wells with well temperatures of 85-100℃, but the recovery heating temperature needs to be increased to above 110°C.

[0064] Example 4 Low molecular weight end (monomer / RAFT = 200:1) The operation steps are the same as in Example 1, except that in step one, the amount of RAFT reagent CDTPA is adjusted to 0.202 g (0.5 mmol), so that the ratio of the total molar amount of monomer to the molar amount of RAFT reagent is 200:1.

[0065] Characterization results: GPC tests showed that the number-average molecular weight Mn was approximately 22,000 g / mol and the PDI was 1.12.

[0066] Performance testing: 1.0 wt% aqueous solution at 90℃ for 170 s -1 The viscosity is 55 mPa·s and the critical phase transition temperature is 93℃.

[0067] Conclusion: The product prepared under low molecular weight end conditions still has good thickening properties and thermal response characteristics, meeting the requirements for fracturing fluid use.

[0068] Example 5 High molecular weight endpoint (monomer / RAFT = 400:1) The operation steps are the same as in Example 1, except that in step one, the amount of RAFT reagent CDTPA is adjusted to 0.101 g (0.25 mmol), so that the ratio of the total molar amount of monomer to the molar amount of RAFT reagent is 400:1.

[0069] Characterization results: GPC tests showed that the number-average molecular weight Mn was approximately 48,500 g / mol and the PDI was 1.28.

[0070] Performance testing: The viscosity of a 1.0 wt% aqueous solution at 90℃ and 170 s⁻¹ is 68 mPa·s, and the critical phase transition temperature is 94℃.

[0071] Conclusion: Under high molecular weight endpoint conditions, the viscosity of the product increases slightly, but the PDI remains within a controllable range and meets the requirements for fracturing fluid use.

[0072] Example 6a RAFT / Initiator Ratio Endpoint (3:1) The operation steps are the same as in Example 1, except that in step one, the amount of initiator ACVA is adjusted to 0.037 g (0.133 mmol), so that the molar ratio of RAFT reagent to initiator is 3:1.

[0073] Characterization results: Mn is approximately 30,800 g / mol, PDI is 1.20, and viscosity at 90℃ is 60 mPa·s.

[0074] Conclusion: A RAFT / initiator ratio of 3:1 yields products with controllable molecular weight and stable performance.

[0075] Example 6b RAFT / Initiator Ratio Endpoint (5:1) The operation steps are the same as in Example 1, except that in step one, the amount of initiator ACVA is adjusted to 0.022 g (0.08 mmol), so that the molar ratio of RAFT reagent to initiator is 5:1.

[0076] Characterization results: Mn is approximately 33,200 g / mol, PDI is 1.13, and viscosity at 90℃ is 63 mPa·s.

[0077] Conclusion: A RAFT / initiator ratio of 5:1 yields a product with controllable molecular weight and stable performance.

[0078] Example 7 Low hydrophobicity endpoint (Macro-CTA / SMA=1:5) The operation steps are the same as in Example 1, except that in step two, the amount of SMA is adjusted to 0.34 g (1.0 mmol), so that the molar ratio of macromolecular chain transfer agent to SMA is 1:5.

[0079] Characterization results: 1H-NMR analysis showed that the actual degree of polymerization of the hydrophobic segment was 4-5.

[0080] Performance testing: The viscosity of a 1.0 wt% aqueous solution at 90℃ and 170 s⁻¹ is 42 mPa·s, and the critical phase transition temperature is 96℃.

[0081] Conclusion: The viscosity decreased under low hydrophobicity conditions, but still met the basic sand carrying requirements (>30 mPa·s).

[0082] Example 8 High hydrophobicity endpoint (Macro-CTA / SMA=1:20) The operation steps are the same as in Example 1, except that in step two, the amount of SMA is adjusted to 1.36 g (4.0 mmol), so that the molar ratio of macromolecular chain transfer agent to SMA is 1:20.

[0083] Characterization results: 1H-NMR analysis showed that the actual degree of polymerization of the hydrophobic segment was 16-18.

[0084] Performance testing: The viscosity of a 1.0 wt% aqueous solution is 75 mPa·s at 90℃ and 170 s⁻¹, and the critical phase transition temperature is 91℃.

[0085] Conclusion: Under high hydrophobicity conditions, hydrophobic association is enhanced, and viscosity is significantly increased, but the phase transition temperature is slightly reduced, which is still within an acceptable range.

[0086] Example 9 Endpoint verification of reaction conditions Example 9a (S1 Low Temperature Short Time: 60℃ / 12h): The operation steps are the same as in Example 1, except that the reaction temperature in step one is 60℃ and the reaction time is 12 hours. Characterization results: Mn is approximately 28,500 g / mol, PDI is 1.18, and the conversion rate is approximately 85%. The viscosity at 90℃ is 58 mPa·s.

[0087] Example 9b (S2 Low Temperature: 65℃): The operation steps are the same as in Example 1, except that the reaction temperature in step two is 65℃. Characterization results: The degree of polymerization of the hydrophobic segment is 8-10, and the viscosity at 90℃ is 60 mPa·s.

[0088] Example 9c (S2 high temperature: 75℃): The operation steps are the same as in Example 1, except that the reaction temperature in step two is 75℃. Characterization results: The degree of polymerization of the hydrophobic segment is 9-11, and the viscosity at 90℃ is 64 mPa·s.

[0089] Example 9d (S3 short time: 4h): The operation steps are the same as in Example 1, except that the reaction time for the end-group removal in step three is 4 hours. Characterization results: UV-Vis shows that the intensity of the 309 nm absorption peak is reduced by about 90%, and the product is pale yellow.

[0090] Example 9e (S3 long time: 12h): The operation steps are the same as in Example 1, except that the reaction time for the end-group removal in step three is 12 hours. Characterization results: UV-Vis showed that the 309 nm absorption peak completely disappeared, and the product was colorless.

[0091] Conclusion: Within the temperature and time ranges specified in the claims, products with satisfactory performance can be obtained.

[0092] Comparative Example 1: The polymer P(NIPMAM)-b-PSMA was synthesized without AMPS. The procedure was the same as in Example 1, but AMPS was not added in step (1), and only NIPMAM monomer (15.72 g, 100 mmol) was used. Due to the lack of the strong hydrophilic contribution of AMPS, and the introduction of the hydrophobic PSMA segment further reducing the overall hydrophilicity, the measured LCST of this block copolymer was approximately 58-62 °C, and phase separation occurred above this temperature. This was used to verify the key contribution of AMPS to improving the phase transition temperature and salt tolerance.

[0093] Comparative Example 2: Short-chain hydrophobic polymers P(NIPMAM-co-AMPS)-b-PBMA was synthesized. The procedure was the same as in Example 1, but in step (2), butyl methacrylate (BMA, 0.284 g, 2.0 mmol, molecular weight 142.20 g / mol) was used instead of octadecyl methacrylate (SMA). This was used to verify the contribution of long-chain SMA to thickening properties.

[0094] Comparative Example 3: Commercially available VES fracturing fluid A commercially available conventional erucamide propyl betaine type viscoelastic surfactant (30% effective content) was selected as a control and prepared according to the same effective concentration.

[0095] Application Example: Fracturing Fluid Performance Evaluation and Recycling Test This embodiment provides a clean fracturing fluid for coalbed methane. The preparation process is as follows: Taking the preparation of 100 g of fracturing fluid as an example, weigh 1.0 g of the polymer / surfactant prepared in Examples 1-3 and Comparative Examples 1-3, add 97.0 g of deionized water, stir and dissolve at room temperature for 2 hours to prepare a 1.0 wt% base solution. Add 1.0 g (1.0 wt%) of potassium chloride as an anti-swelling agent, and adjust the pH to 7.0 ± 0.2 with 0.1 mol / L sodium hydroxide solution or 0.1 mol / L hydrochloric acid solution. The amount of pH adjuster used is approximately 0.5-2.0 mL. After thorough mixing, the solution is ready for testing.

[0096] A clean fracturing fluid for coalbed methane containing the aforementioned environmentally responsive surfactant has the following weight percentage composition: 0.4%-2.0% environmentally responsive surfactant, 0.5%-2.0% clay swelling inhibitor, 0.01%-0.5% pH adjuster, and water as the balance. The clay swelling inhibitor is one or more of potassium chloride, ammonium chloride, tetramethylammonium chloride, and polyquaternary ammonium salt; the pH adjuster is one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, hydrochloric acid, and citric acid.

[0097] Temperature resistance test (corresponding) Figure 2 ) A rheometer (Haake Mars 60) was used with a coaxial cylindrical rotor (CC27) and a fixed shear rate of 170 s⁻¹. -1 The apparent viscosity of each fracturing fluid was continuously measured at different temperatures, with the temperature increased from 25℃ to 110℃ at a heating rate of 3℃ / min.

[0098] Results Analysis: The temperature resistance performance of Example 1 and the Comparative Example were compared. Test conditions: 1.0 wt% aqueous solution, shear rate 170 s⁻¹. -1 The heating rate was 3℃ / min. Example 1 maintained a viscosity above 60 mPa·s at 90℃, superior to the control sample. Example 1 exhibited a viscosity of 62 mPa·s at 90℃, demonstrating good temperature resistance. Control Example 1 (without AMPS) had a LCST of approximately 60℃, at which point phase separation occurred, making it impossible to obtain effective viscosity data at higher temperatures. Control Example 2 (short-chain BMA) had a viscosity of only 8 mPa·s at 90℃, indicating insufficient thickening ability and failing to meet sand-carrying requirements. Control Example 3 (commercially available VES) had a viscosity of 45 mPa·s at 90℃, lower than Example 1.

[0099] Salt resistance test (corresponding) Figure 3 ) The viscosities of Example 1 and Comparative Example 2 were measured at different mineralization levels (NaCl concentration 0-60,000 mg / L) under constant temperature conditions of 90°C. Note: Comparative Example 1, due to its LCST of approximately 60°C, had already undergone phase separation under the 90°C test conditions, making effective salt tolerance testing impossible.

[0100] Results Analysis: In a 50,000 mg / L NaCl solution, Example 1 maintained a viscosity of 45 mPa·s, with a retention rate of approximately 72.6% (>70% technical specification). In contrast, Example 2 (short-chain BMA) showed almost no thickening effect in brine due to its extremely low base viscosity (only 8 mPa·s). The excellent salt resistance of Example 1 demonstrates that the introduction of AMPS sulfonic acid groups effectively resists salting out using the polyelectrolyte effect.

[0101] At 90℃, the shear rate is 170 s. -1 Under test conditions of 1.0 wt% surfactant concentration, see [reference needed]. Figure 3 The viscosity changes of Example 1 and Comparative Example 2 under different NaCl concentrations are shown.

[0102] Recycling performance test (corresponding) Figure 4 ) Simulates the process of "downhole application - surface recovery - reuse": Initial state: 100 mL of the fracturing fluid prepared in Example 1 was taken and the initial viscosity was measured to be 62 mPa·s at 90°C.

[0103] Recovery procedure: Place the fracturing fluid in a 105°C water bath (above its LCST) for 30 minutes. Observe that the polymer gradually aggregates into a white paste and settles to the bottom, while the supernatant becomes clear. Pour off the supernatant to separate it, collect the paste at the bottom, and weigh it.

[0104] Re-preparation: Cool the collected paste to room temperature, add deionized water to the original volume according to the amount of loss, and stir again for 2 hours to completely dissolve the polymer.

[0105] Cyclic testing: The regenerated fracturing fluid was heated back to 90℃ and its viscosity was measured. The above recovery-re-preparation-testing process was repeated 3 times. The results showed that after 3 cycles, the viscosity retention rate was 90.3%, and the average recovery rate was >90%, proving that the system has excellent recycling performance.

[0106] See Figure 4 This demonstrates the viscosity retention of the fracturing fluid at 90°C after three cycles of recycling in Example 1.

[0107] See Figure 5 The effects of different monomer ratios on surfactant performance show that increasing AMPS content significantly improves LCST and salt tolerance, but excessively high content leads to excessively high recycling temperatures and increased energy consumption. The 80:20 ratio in Example 1 achieves the best balance between temperature tolerance, salt tolerance, and recyclability, and is the recommended formulation.

[0108] Sand-carrying performance test (corresponding) Figure 6 ) Test method: Referring to SY / T 6376-2008 "General Technical Conditions for Fracturing Fluids", the static suspended proppant method was used to evaluate the proppant carrying capacity of the fracturing fluid. Ceramsite proppant (20 / 40 mesh, apparent density 1.6 g / cm³) was added. 3 Add the proppant to the fracturing fluid at a ratio of 20% (by volume), stir well, and then place it in a constant temperature environment of 90°C. Record the time required for the proppant to completely settle to the bottom of the container (sand suspension time).

[0109] See Figure 6 Results analysis: Example 1 showed a sand suspension time of 42 minutes at a concentration of 1.0%, which is comparable to that of 0.45% guar fracturing fluid, meeting the sand carrying requirements of conventional fracturing operations (usually requiring a sand suspension time of >30 min).

[0110] Example 1, with a sand suspension time of 18 minutes at a concentration of 0.6%, meets the requirements for fracturing operations with low sand ratios or short fractures. Comparative Example 2, due to its excessively low viscosity, has a sand suspension time of only 3 minutes, making it unsuitable for fracturing sand carrying.

[0111] Reservoir damage assessment (corresponding) Figure 7 ) Test Method: Referring to SY / T 5107-2016 "Performance Evaluation Method of Water-Based Fracturing Fluid", core flow experiments were used to evaluate the degree of damage of fracturing fluid to the coal matrix. Core samples from the No. 3 coal seam in the Qinshui Basin (gas permeability approximately 0.5 mD) were selected, and the original permeability K0 and the permeability K1 after fracturing fluid contamination were measured sequentially. The permeability damage rate η was calculated as η = (K0 - K1) / K0 × 100%.

[0112] See results Figure 7 The penetration damage rate of Example 1 was only 9.6%, far lower than the 39.2% of guar fracturing fluid, proving that the pure synthetic polymer system of the present invention has no residue retention problem. Compared with commercially available VES (damage rate 16.0%), the damage rate of the present invention is lower, which may be related to the thermally induced phase separation characteristics of the polymer—the polymer can more easily desorb from the pores of coal and rock during the flowback process with temperature changes.

[0113] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of this invention.

Claims

1. An environmentally responsive surfactant, characterized in that, The environmentally responsive surfactant is a block copolymer composed of hydrophilic thermosensitive segments and hydrophobic associative segments, and the general structural formula of the block copolymer is P(NIPMAM-co-AMPS)-b-PSMA. The hydrophilic thermosensitive segment is a random copolymer of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid; the hydrophobic associative segment is polyoctadecyl methacrylate. The molar ratio of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid is 90:10 to 70:30, and the number-average molecular weight of the hydrophilic thermosensitive chain segment is 20,000-50,000 g / mol. The degree of polymerization of the octadecyl methacrylate unit in the hydrophobic associative chain segment is 5-20.

2. A method for preparing an environmentally responsive surfactant as described in claim 1, characterized in that, Includes the following steps: S1, N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid are dissolved in the reaction medium, RAFT reagent and the first initiator are added, nitrogen gas is introduced to remove oxygen and the reaction is stirred to obtain a reaction solution, and then precipitation, centrifugation and drying are carried out in sequence to obtain a macromolecular chain transfer agent. S2, a macromolecular chain transfer agent is dissolved in water as a macromolecular emulsifier, octadecyl methacrylate is added to form a pre-emulsion, a second initiator is added, and after deoxygenation, a polymerization reaction is carried out to obtain a block copolymer; S3, an excess of a second initiator is added to the block copolymer to carry out an addition-fracture reaction to remove end groups, thus obtaining an environmentally responsive surfactant.

3. The method for preparing an environmentally responsive surfactant according to claim 2, characterized in that, In S1, the reaction medium is deionized water or a mixed solvent of water and ethanol, wherein the volume ratio of water to ethanol in the mixed solvent is 2:1 to 4:1; the RAFT reagent is 4-cyano-4-(dodecylthioalkylthiocarbonyl)valeric acid or 4-cyano-4-(phenylcarbonylthio)valeric acid; and the first initiator is 4,4'-azobis(4-cyanovaleric acid).

4. The method for preparing an environmentally responsive surfactant according to claim 2, characterized in that, In S1, the molar ratio of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid is 90:10 to 70:30; the mass-to-volume ratio of the total mass of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid monomers to the volume of the reaction medium is 0.3-0.5 g:1 mL; the molar ratio of the total molar amount of N-isopropylmethacrylamide and 2-acrylamido-2-methylpropanesulfonic acid monomers to the molar amount of RAFT reagent is 200:1 to 400:1; and the molar ratio of the RAFT reagent to the first initiator is 3:1 to 5:

1.

5. The method for preparing an environmentally responsive surfactant according to claim 2, characterized in that, In S2, the mass-to-volume ratio of the macromolecular chain transfer agent to water is 2.0-5.0 g: 100 mL; the molar ratio of the macromolecular chain transfer agent to octadecyl methacrylate is 1:5 to 1:20; and the molar ratio of the macromolecular chain transfer agent to the second initiator is 3:1 to 10:

1.

6. The method for preparing an environmentally responsive surfactant according to claim 2, characterized in that, In S2 and S3, the second initiator is 2,2'-azobis(2-methylpropanediamine) dihydrochloride or azobisisobutyronitrile; in S3, the molar ratio of the excess second initiator to the residual RAFT end groups in the block copolymer is 2:1 to 5:

1.

7. The method for preparing an environmentally responsive surfactant according to claim 2, characterized in that, In S1, the temperature of the stirring reaction is 60-70℃ and the time is 12-24 hours; in S2, the temperature of the polymerization reaction is 65-75℃ and the time is 24 hours; in S3, the temperature of the addition-fracture reaction is 80℃ and the time is 4-12 hours.

8. A clean fracturing fluid for coalbed methane, characterized in that, By weight percentage, it includes 0.4%-2.0% of the environmentally responsive surfactant as described in claim 1, 0.5%-2.0% of the clay anti-swelling agent, 0.01%-0.5% of the pH adjuster, and the balance being water; The clay anti-swelling agent is one or more of potassium chloride, ammonium chloride, tetramethylammonium chloride, and polyquaternary ammonium salt; the pH adjuster is one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, hydrochloric acid, and citric acid.

9. A method of using the coalbed methane clean fracturing fluid as described in claim 8, characterized in that, Includes the following steps: An environmentally responsive surfactant was mixed with clay anti-swelling agent, pH adjuster and water to prepare a clean fracturing fluid for coalbed methane fracturing operations. The polymer was completely dissolved at fracturing operation temperatures below 90°C. After the fracturing operation was completed, the flowback fluid was collected. The flowback fluid is heated to a temperature higher than the critical phase transition temperature of the clean fracturing fluid for coalbed methane. The polymer undergoes a conformational change and aggregates and settles to obtain an intermediate product. After separating the supernatant of the intermediate product, the bottom polymer paste is collected, cooled to room temperature, and then the polymer is redissolved and recycled for the preparation of clean fracturing fluid for coalbed methane.

10. The application of the environmentally responsive surfactant as described in claim 1 and the coalbed methane clean fracturing fluid as described in claim 8 in the hydraulic fracturing process of a coalbed methane well.

Images