Compound and preparation method thereof, gemini surfactant and low-water cleaning fracturing fluid

Abstract

The invention discloses a compound and a preparation method thereof, a gemini surfactant and a low-water cleaning fracturing fluid, and belongs to the technical field of fracturing fluids. The structure of the compound is shown as a formula 1, and R is selected from the group consisting of; r1 is selected from the group consisting of R1 and R2

Description

Technical Field

[0001] This invention relates to the field of fracturing fluid technology, and in particular to a compound and its preparation method, a gemini surfactant, and a low-water clean fracturing fluid. Background Technology

[0002] my country possesses abundant unconventional natural gas resources (shale gas, coalbed methane, etc.), with recoverable resources ranking among the world's top. The efficient development of unconventional natural gas is crucial for optimizing my country's energy structure and serves as an important alternative resource for achieving a clean and low-carbon energy transition. However, unconventional natural gas reservoirs generally exhibit characteristics such as low porosity and low permeability, with limited natural fracture development. Therefore, hydraulic fracturing is essential to establish efficient seepage channels for commercial exploitation. Fracturing fluid, as the core working medium in fracturing operations, not only transmits pressure and creates fractures and carries proppant, but its flowback performance directly impacts the desorption and production release of unconventional natural gas.

[0003] While traditional water-based fracturing fluids are technically mature, they face significant challenges in unconventional reservoir stimulation: First, large-scale volumetric fracturing consumes enormous amounts of freshwater, placing immense pressure on water supply in water-scarce regions; second, the injection of large volumes of water can easily trigger reservoir water sensitivity and water-locking effects, and the retention of broken-gel residue significantly reduces fracture conductivity. To avoid water-phase damage, CO2 dry fracturing technology has emerged. This technology uses liquid or supercritical CO2 as the fracturing medium, effectively avoiding water-sensitive damage and achieving rapid flowback after fracturing. However, pure CO2 fluid has extremely low viscosity, resulting in severely insufficient proppant-carrying capacity, and its application requires specialized closed-loop equipment, leading to high costs and limiting its large-scale application. To balance low damage and high proppant carrying capacity, the concept of low-water fracturing fluid (or quasi-dry fracturing fluid) has been proposed. This involves mixing a small amount of water-based fluid with CO2 (typically with an aqueous phase of 15%-35%), aiming to combine the excellent proppant carrying capacity of water-based fluids with the low-damage properties of CO2, demonstrating significant potential in water conservation, environmental protection, and increased production. However, existing low-water fracturing fluid systems typically require the separate addition of CO2 thickeners and water-based thickeners, making the system complex, costly, and generally exhibiting poor temperature resistance, failing to meet the stimulation requirements of deep, high-temperature reservoirs. To address these needs, existing technologies mainly achieve synergistic thickening of the CO2 and aqueous phases through composite thickening systems. Patent CN 111363533 A uses two types of polymer-based thickeners—a water-based thickener and a CO2-phase thickener—to separately regulate the aqueous and CO2 phases, but this method is costly and the polymer-based thickeners do not completely break down the gel. Patent CN 114909117 A proposes a low-water CO2 fracturing fluid system composed of liquid CO2 mixed with water-based slickwater or guar gum fracturing fluid. This system mainly relies on polymer thickening in the aqueous phase to provide proppant carrying capacity. Although it reduces water consumption, it still cannot avoid the damage of polymer residues to low-permeability reservoirs. Patent CN 120555043A discloses a mixed-phase viscosity-enhancing fracturing fluid system, which consists of an aqueous phase fracturing fluid containing an acrylamide copolymer thickener and a CO2 phase fracturing fluid containing a dedicated CO2 thickener. This system also uses two different types of thickeners to thicken the two phases separately. However, due to the easy degradation of the aqueous phase polymer at high temperatures, the viscosity of the overall fracturing fluid system at 90°C is only 15 mPa·s under a water-to-carbon ratio of 40:60. In addition, although existing clean fracturing fluids use viscoelastic surfactants (VES) to construct worm-like micelles, they are mostly used in pure aqueous phase systems and are difficult to maintain stably in CO2 / water mixed phases.

[0004] Therefore, developing a new, efficient, and temperature-resistant low-water clean fracturing fluid system that can achieve simultaneous thickening of CO2 and water phases, and possesses excellent sand-carrying capacity, temperature resistance, and low-damage characteristics, is crucial for promoting the efficient and green development of unconventional oil and gas resources. Summary of the Invention

[0005] In view of this, the technical problem to be solved by the present invention is to provide a compound and its preparation method, a gemini surfactant, and a low-water clean fracturing fluid. The structure of the compound is shown in Formula 1. Its structure is novel and unique, and it can be used as a gemini surfactant. The low-water clean fracturing fluid prepared by the gemini surfactant has excellent temperature resistance and shear resistance, as well as good high-temperature proppant carrying capacity.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] This invention provides a compound with the structure shown in Formula 1:

[0008]

[0009] Formula 1;

[0010] Where R is selected from ;

[0011] R1 is selected from or ;

[0012] The n is selected from 5, the m is selected from 5-9, and the k is selected from 12-18.

[0013] In the structure of Formula 1 described in this invention, R represents adjacent, intermediate, or para substitution.

[0014] Preferably, in this invention, R is a meta-substitution, selected from... ;

[0015] Preferably, n is selected from 5, m is selected from 5 or 9, and k is selected from 12, 14 or 18.

[0016] The present invention also provides a method for preparing the above-mentioned compound, comprising the following steps:

[0017] (1) N-methyl-2,2'-diaminodiethylamine and nitrobenzaldehyde as shown in Formula 2 are mixed and heated to react to obtain intermediate A;

[0018] (2) Reduce intermediate A to obtain intermediate B;

[0019] (3) Intermediate B is reacted with formaldehyde and sodium borohydride to obtain intermediate C;

[0020] (4) Mix intermediate C with palladium on carbon and hydrazine hydrate, and heat to react to obtain intermediate D;

[0021] (5) The compound shown in Formula 1 was prepared by reacting intermediate D with the fatty acid shown in Formula 3;

[0022] Formula 2 Intermediate A

[0023] Intermediate B Intermediate C Intermediate D Formula 3.

[0024] The range of the R1 group in Formula 3 is the same as the range of the R1 group mentioned above, and will not be repeated here.

[0025] When n is selected from 5 and m is selected from 5, Formula 3 is oleic acid;

[0026] When n is selected from 5 and m is selected from 9, Equation 3 is erucic acid;

[0027] When k is selected from 12, Formula 3 is palmitic acid;

[0028] When k is selected from 14, Formula 3 is stearic acid;

[0029] When k is selected from 18, Equation 3 is behenic acid.

[0030] Preferably, the heating reaction temperature in step (1) of this invention is 50℃-70℃; more preferably 60℃.

[0031] The heating reaction in step (1) is carried out under an inert atmosphere.

[0032] The inert atmosphere is selected from atmospheres such as nitrogen or argon.

[0033] The solvents used in the heating reaction of step (1) include, but are not limited to, ethanol, isopropanol, methanol, etc.

[0034] Preferably, the reducing agent used in step (2) of this invention is selected from one or more of sodium borohydride, triethylsilane, and sodium triacetoxyborohydride; more preferably, sodium borohydride.

[0035] The reduction in step (2) is carried out under an inert atmosphere.

[0036] The inert atmosphere is selected from atmospheres such as nitrogen or argon.

[0037] The solvents used in the reduction reaction of step (2) include, but are not limited to, methanol, ethanol, isopropanol, etc.

[0038] After the reduction reaction is completed, post-processing steps such as vacuum distillation, extraction, and drying are also included.

[0039] The present invention does not have any particular limitation on the solvent used for extraction, and it can be any solvent known to those skilled in the art.

[0040] In some specific embodiments of the present invention, the solvent for extraction is selected from dichloromethane and water.

[0041] The present invention does not specifically limit the drying method, and can be any drying method known to those skilled in the art, such as vacuum drying, freeze drying, vacuum drying, etc.

[0042] After the reaction in step (3) is completed, post-processing such as filtration, vacuum distillation, extraction, and drying are carried out in sequence.

[0043] Before extraction, the filtrate after vacuum distillation needs to be adjusted to alkalinity.

[0044] The solvent used for extraction is selected from dichloromethane.

[0045] Preferably, the heating reaction temperature in step (4) of this invention is 50°C-70°C; more preferably 60°C.

[0046] Step (5) also contains the dehydrating agent EDC·HCl.

[0047] The solvent for the reaction in step (5) is selected from dichloromethane.

[0048] The synthetic route of the above preparation method is shown below:

[0049] .

[0050] The present invention also provides a gemini surfactant, comprising the above-described compound or the compound prepared by the above-described preparation method.

[0051] In some specific embodiments of the present invention, the above-mentioned compounds are used directly as gemini surfactants. Specifically, they can be represented as oleamide m-phenyltritertiary amine gemini surfactants, erucamide m-phenyltritertiary amine gemini surfactants, palmitamide m-phenyltritertiary amine gemini surfactants, stearamide m-phenyltritertiary amine gemini surfactants, or behenamide m-phenyltritertiary amine gemini surfactants, etc.

[0052] The present invention also provides a CO2-VES low-water clean fracturing fluid, which is composed of a CO2 phase and an aqueous phase;

[0053] The aqueous phase comprises the following components:

[0054] The aforementioned gemini surfactant: 1.2wt%-2.4wt%;

[0055] Aromatic anionic small molecules: 0.3wt%-0.6wt%; Stabilizer: 0.5wt%-1wt%;

[0056] Deionized water: Balance;

[0057] The mass ratio of the gemini surfactant to the aromatic anionic small molecule is 4:1.

[0058] Preferably, the volume percentage of the CO2 phase in this invention is 60%-90%.

[0059] Preferably, the above-mentioned compound is used directly as a gemini surfactant in CO2-VES low-water clean fracturing fluid. This significantly improves the performance of the CO2-VES low-water clean fracturing fluid, for the following reasons:

[0060] (i) The structure of the Gemini surfactant incorporates a rigid benzene ring structure, which improves its high-temperature stability and can effectively enhance the structural integrity of the entire micelle network at high temperatures, significantly improving the temperature resistance of the fracturing fluid system (up to 90℃-130℃).

[0061] (ii) The multi-site tertiary amine groups in the structure of the Gemini surfactant serve as CO2-responsive units, which are protonated under CO2 stimulation to form multiple cation centers. These centers then interact with specific counterions (aromatic anionic small molecules) through strong electrostatic interactions, π-π stacking, and hydrogen bonding to construct a more stable worm-like micelle network at high temperatures, thereby significantly improving the viscosity of the fracturing fluid.

[0062] (iii) The structure of the Gemini surfactant is a Gemini molecular configuration, which has high surface activity and can connect two hydrophobic chains through covalent bonds, thereby enhancing the orderly aggregation ability of molecules in solution and forming a supramolecular structure with higher entanglement and stronger mechanical strength, thus significantly improving the thickening efficiency and sand carrying capacity of fracturing fluid.

[0063] The CO2-VES low-water clean fracturing fluid of this invention exhibits strong acidity after thickening, which can moderately dissolve inorganic minerals (such as carbonate minerals) in the formation, clear microfractures in the reservoir, and enhance fracture conductivity. Furthermore, the fracturing fluid leaves no residue after gel breaking, is easily flowed back, and causes minimal damage to the reservoir.

[0064] This invention utilizes the CO2-responsive protonation properties of the aforementioned gemini surfactant to simultaneously achieve co-thickening of the CO2 and aqueous phases, significantly reducing water consumption (aqueous phase volume percentage ≤ 40%). This reduction in the aqueous phase and chemical content of the fracturing fluid significantly lowers costs. Furthermore, the self-assembly of its gemini structure promotes the formation of worm-like micelles, enhancing proppant carrying capacity. The principle behind the CO2-responsive protonation properties is as follows:

[0065] .

[0066] The range of values ​​for R is the same as described above, and will not be repeated here.

[0067] This invention optimizes the structure of the twin surfactant and combines it with a specific counterion, thereby achieving the goal of integrating CO2 responsiveness, high-efficiency thickening properties, excellent temperature resistance, and low-damage characteristics into a single surfactant system. This provides an efficient and environmentally friendly technical option for reservoir fracturing in deep, high-temperature, and water-scarce areas.

[0068] Preferably, the aromatic anionic small molecule is selected from one or more of sodium salicylate, sodium benzoate, sodium benzenesulfonate, sodium p-toluenesulfonate, sodium p-styrenesulfonate, and sodium p-hydroxybenzenesulfonate.

[0069] The content of Gemini surfactant in the CO2-VES low-water clean fracturing fluid of the present invention is preferably 1.6wt%-2.2wt%; more preferably 2wt%.

[0070] The content of the aromatic anionic small molecules is preferably 0.4wt%-0.6wt%; more preferably 0.5wt%.

[0071] In the above-mentioned CO2-VES low-water clean fracturing fluid, the stabilizer can not only act as a clay stabilizer to inhibit clay expansion, but also the ions ionized in water can compress the double layer formed by micelles, which is beneficial to enhancing the stability of micelles.

[0072] The stabilizer is preferably potassium chloride or ammonium chloride.

[0073] In this invention, the potassium chloride content in the fracturing fluid is preferably 0.8 wt%-1 wt%; more preferably 1 wt%. The volume percentage of the CO2 phase is preferably 65%-75%; more preferably 70%.

[0074] Compared with the prior art, the compound structure provided by the present invention is shown in Formula 1, wherein R is selected from... R1 is selected from or The n is selected from 5, m from 5-9, and k from 12-18. The compound has a novel and unique structure and can be used as a gemini surfactant to prepare low-water clean fracturing fluid. The gemini surfactant contains a rigid benzene ring and is CO2-responsive. Protonation of its polytert-amine groups can form multiple cations with a gemini structure. These multiple cations self-assemble with small aromatic anionic molecules to form a more stable worm-like micelle network, thereby achieving co-thickening of the CO2 and aqueous phases. This results in the low-water clean fracturing fluid exhibiting excellent temperature and shear resistance and good high-temperature proppant carrying capacity. Attached Figure Description

[0075] Figure 1 The 1H NMR spectrum of the erucamide m-phenylenediamine gemini surfactant synthesized in Example 1;

[0076] Figure 2 The temperature resistance and shear viscosity test curve of the CO2-VES low-water clean fracturing fluid prepared in Example 3;

[0077] Figure 3 The image shows the sand-carrying capacity of the CO2-VES low-water clean fracturing fluid prepared in Example 3 at high temperature, as observed through the viewing window. Detailed Implementation

[0078] To further illustrate the present invention, the following detailed description of the compounds and their preparation methods, the gemini surfactant, and the low-water clean fracturing fluid provided by the present invention is provided in conjunction with the embodiments.

[0079] The performance of the CO2-VES low-water clean fracturing fluid described below is mainly reflected by its viscosity; the higher the viscosity, the stronger its sand-carrying capacity. Viscosity is obtained through rheometer testing.

[0080] Example 1

[0081] Preparation of erucamide m-phenyltritertiary amine gemini surfactant

[0082] Step (1): Synthesis of bis(3-aminobenzyl)tetramethyltriamine (BABT)

[0083] (1) In an ethanol solution of 4.00 g N-methyl-2,2'-diaminodiethylamine (34.0 mmol), 10.3 g 3-nitrobenzaldehyde (68.2 mmol) was added under nitrogen protection. The mixture was heated at 60 °C for 12 h, cooled to room temperature, filtered, and dried to obtain 10.24 g of intermediate A.

[0084] (2) 8.00 g of intermediate A (20.8 mmol) was dissolved in methanol, and 3.14 g of sodium borohydride (83.4 mmol) was added under nitrogen protection. The mixture was stirred at room temperature for 12 hours. After the reaction was complete, methanol was removed under reduced pressure, and the residue was diluted with dichloromethane and deionized water. The organic layer was separated, dried over anhydrous sodium sulfate, and concentrated under reduced pressure to obtain a yellow viscous substance. This substance was then further dried under high vacuum to obtain 7.98 g of intermediate B.

[0085] (3) 9.00 g of intermediate B (22.4 mmol) was dissolved in a mixed solution of acetonitrile (240 mL) and acetic acid (60 mL), and formaldehyde solution (33.2 mL) was added. The mixture was stirred at room temperature for 2 hours. Subsequently, 4.98 g of sodium borohydride (134.0 mmol) was added in small portions at 0°C, and stirring was continued for 2 hours. Then, the mixture was brought to room temperature and stirred for another 12 hours. After the reaction was completed, the white precipitate was removed by filtration, and the filtrate was concentrated under reduced pressure. The filtrate was adjusted to alkaline, and the resulting yellow solution was extracted with dichloromethane. The organic phase was dried over anhydrous sodium sulfate and concentrated under reduced pressure to obtain 8.00 g of intermediate C.

[0086] (4) Under nitrogen protection, hydrazine hydrate (60.0 mL) was added to an ethanol mixture of Pd / C and 5.48 g of intermediate C (186.2 mmol). The temperature of the reaction mixture was then slowly raised to 60°C and stirred for 8 hours. After the reaction was completed, the mixture was cooled to room temperature. The mixture was then filtered, and the filtrate was concentrated under reduced pressure. The resulting colorless liquid was extracted with dichloromethane, and the organic phase was dried over anhydrous sodium sulfate and concentrated again to give 3.5 g of bis(3-aminobenzyl)tetramethyltriamine (BABT), which was a colorless liquid.

[0087] Step (II): Synthesis of erucamide-m-phenylenediamine gemini surfactant

[0088] 5.2 g of EDC·HCl (25 mmol) and 3.35 g of BABT (10 mmol) were added to a round-bottom flask containing 150 mL of dichloromethane, and the reaction mixture was placed at 0°C. Then, over 0.5 hours, 7.45 g of erucic acid (22 mmol) was slowly added dropwise to the mixture. The reaction mixture was then brought to room temperature and stirred for 18 hours. After the reaction was complete, the organic phase was washed three times with water, followed by removal of the organic solvent under reduced pressure. The crude product was purified by column chromatography to obtain a pale yellow solid product in 81% yield. Figure 1 The photon NMR spectrum of the erucamide m-phenylenediamine gemini surfactant synthesized in Example 1 is shown.

[0089] The synthetic route for the erucamide m-phenyltritertiary amine gemini surfactant is shown below:

[0090] .

[0091] Example 2

[0092] Preparation of oleamide m-phenyltritertiary amine gemini surfactant

[0093] Step (1): Synthesis of bis(3-aminobenzyl)tetramethyltriamine (BABT)

[0094] The steps are the same as in Example 1 (I).

[0095] Step (II): Synthesis of oleamide-m-phenylenediamine gemini surfactant

[0096] 5.2 g of EDC·HCl (25 mmol) and 3.35 g of BABT (10 mmol) were added to a round-bottom flask containing 150 mL of dichloromethane, and the reaction mixture was placed at 0°C. Then, over 0.5 hours, 6.21 g of oleic acid (22 mmol) was slowly added dropwise to the mixture. The reaction mixture was then brought to room temperature and stirred for 18 hours. After the reaction was complete, the organic phase was washed three times with water, followed by removal of the organic solvent under reduced pressure. The crude product was purified by column chromatography, and the final product was a viscous, pale yellow solid of oleic acid amide m-phenylenediamine gemini surfactant in 84% yield.

[0097] Example 3

[0098] Preparation of CO2-VES low-water clean fracturing fluid

[0099] Viscoelastic surfactants, abbreviated as VES, are used to formulate fracturing fluids that are easy to break down and leave no residue, hence the name VES clean fracturing fluid.

[0100] The following is a formulation for a temperature-resistant CO2-VES low-water clean fracturing fluid, prepared using the erucamide m-phenylenediamine gemini surfactant synthesized in Example 1:

[0101] (1) First, weigh 115.8g of deionized water into a 200mL beaker, add it to the rotor and stir and heat to 50℃. Then weigh 2.4g of erucamide m-phenyl tritertiary amine gemini surfactant synthesized in Example 1, 0.6g of sodium p-toluenesulfonate and 1.2g of potassium chloride, add them to the beaker in sequence and mix to dissolve. Continue stirring for 10 minutes to ensure complete dissolution. The aqueous phase of CO2-VES low-water clean fracturing fluid with a mass fraction of 2wt% is obtained, which is light yellow milky.

[0102] (2) Then, the high-temperature and high-pressure visualization reactor is heated to 50°C using a constant-temperature oil bath. The aqueous phase of the prepared fracturing fluid is then transferred into the reactor. After the reactor is sealed, the exhaust valve is opened to introduce CO2 to expel the air from the reactor. Then, the exhaust valve is closed, and liquid CO2 is continuously introduced using a booster pump until the pressure inside the reactor reaches the target pressure of 8 MPa. At this point, the liquid CO2 content in the reactor is 70%, the aqueous phase content is 30%, and the CO2 in the reactor is in a supercritical state. The stirrer is turned on and stirred for 10 minutes. The preparation of CO2-VES low-water clean fracturing fluid is now complete.

[0103] Performance testing

[0104] (a) Temperature resistance and shear resistance test

[0105] The high-temperature resistant CO2-VES low-water clean fracturing fluid prepared in Example 3 was subjected to a high-temperature shear resistance test to verify its performance. The steps are as follows:

[0106] The sealed container of the HAAKE MARS 60 rheometer was preheated to 50°C. The pipeline connecting the reactor and the sealed container of the rheometer was connected, and the connecting valve was opened. CO2-VES low-water clean fracturing fluid from the reactor was injected into the rheometer. During the process, the pressure in the sealed container of the rheometer was adjusted to 8 MPa using the hand-cranked valve of the reactor. The connecting valve was then closed. A high-temperature shear viscosity test was then performed using a PZDG38 dual-slit rotor. The test conditions were: the system temperature was raised from 50°C to a high temperature of 120°C, and then the shear viscosity test was performed at 120°C. The entire process lasted 7200 seconds, with the shear rate maintained at 170 seconds. -1 Constant and unchanging. Figure 2 The temperature resistance and shear viscosity test curve of the CO2-VES low-water clean fracturing fluid prepared in Example 3 is shown. Figure 2 The results show that the system can still maintain a viscosity of 39.2 mPa·s at 120℃ and 16.3 MPa, which meets the construction requirements (>25 mPa·s) and has excellent temperature resistance and shear resistance.

[0107] (II) High-Temperature Sand Carrying Performance Test

[0108] The high-temperature proppant carrying capacity of the temperature-resistant CO2-VES low-water clean fracturing fluid prepared in Example 3 was tested, and the steps are as follows:

[0109] The prepared fracturing fluid aqueous phase was transferred to a high-temperature, high-pressure, visually controlled reactor. Quartz sand proppant was then added at a volume ratio of 20%. After the reactor was sealed, the vent valve was opened to introduce CO2 to purge the air from the reactor. The vent valve was then closed, and CO2 was continuously introduced using a gas booster pump until the pressure inside the reactor reached the target pressure of 15 MPa. Simultaneously, the reactor was heated to the target temperature of 100°C using a constant-temperature oil bath. At this point, the CO2 inside the reactor was in a supercritical state. The stirrer was turned on for 10 minutes to achieve uniform distribution of the proppant in the fracturing fluid. After stirring was stopped, the static proppant suspension performance of the CO2-VES low-water clean fracturing fluid was investigated by monitoring the settling displacement and time of individual quartz sand particles. The results showed that 90% of the proppant had a settling time greater than 30 minutes, meeting the proppant carrying capacity standard (<5 cm / min). Figure 3 The image shows the sand-carrying capacity of the CO2-VES low-water clean fracturing fluid prepared in Example 3 at high temperature, as observed through the viewing window.

[0110] The above description of the embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A compound, characterized in that, Its structure is shown in Equation 1: Formula 1; Where R is selected from ; R1 is selected from The n is selected from 5, the m is selected from 5-9, and the k is selected from 12-18.

2. The compound according to claim 1, characterized in that, The R is a meta-substitution, selected from #imgpt4#; The n is selected from 5, the m is selected from 5 or 9, and the k is selected from 12, 14 or 18.

3. A method for preparing the compound according to claim 1 or 2, characterized in that, Includes the following steps: (1) N-methyl-2,2'-diaminodiethylamine and nitrobenzaldehyde as shown in Formula 2 are mixed and heated to react to obtain intermediate A; (2) Reduce intermediate A to obtain intermediate B; (3) Intermediate B is reacted with formaldehyde and sodium borohydride to obtain intermediate C; (4) Mix intermediate C with palladium on carbon and hydrazine hydrate, and heat to react to obtain intermediate D; (5) The compound shown in Formula 1 was prepared by reacting intermediate D with the fatty acid shown in Formula 3; 4. The preparation method according to claim 3, characterized in that, The heating temperature in step (1) is 50℃-70℃.

5. The preparation method according to claim 3, characterized in that, The reducing agent used in step (2) is selected from one or more of sodium borohydride, triethylsilane, and triacetoxysodium borohydride.

6. The preparation method according to claim 3, characterized in that, The heating temperature in step (4) is 50℃-70℃.

7. A gemini surfactant, characterized in that, This includes the compound described in claim 1 or 2, or the compound prepared by the preparation method described in any one of claims 3-6.

8. A CO2-VES low-water clean fracturing fluid, characterized in that, Composed of a CO2 phase and an aqueous phase; The aqueous phase comprises the following components: The gemini surfactant of claim 7: 1.2wt%-2.4wt%; Aromatic anionic small molecules: 0.3wt%-0.6wt%; Stabilizer: 0.5wt%-1wt%; Deionized water: Balance; The mass ratio of the gemini surfactant to the aromatic anionic small molecule is 4:

1.

9. The CO2-VES low-water clean fracturing fluid according to claim 8, characterized in that, The volume percentage of the CO2 phase is 60%-90%.

10. The CO2-VES low-water clean fracturing fluid according to claim 8 or 9, characterized in that, The aromatic anionic small molecule is selected from one or more of sodium salicylate, sodium benzoate, sodium benzenesulfonate, sodium p-toluenesulfonate, sodium p-styrenesulfonate, and sodium p-hydroxybenzenesulfonate.

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