A method for preparing a surfactant for use in oil fields

Abstract

The application belongs to the technical field of oil field chemicals, and particularly discloses a preparation method of a surfactant for oil fields, wherein the raw material of the surfactant is easy to obtain and the cost is controllable, the high activity of the gemini and the high salt resistance of the betaine are realized at the molecular level through the introduction of the carboxyl betaine group of the gemini skeleton, the emulsification adaptability to heavy oil is enhanced through the design of the asymmetric hydrophobic chain, and the chemical flooding development suitable for the difficult-to-produce reserves such as high-temperature high-salt, low-permeability and heavy oil is realized.

Description

Technical Field

[0001] This invention belongs to the field of oilfield chemical technology, and specifically relates to a method for preparing a surfactant for oilfield use. Background Technology

[0002] With the continued growth of global energy demand and the increasing depletion of conventional oil and gas resources, enhanced oil recovery (EOR) has become a core issue in the petroleum industry. Chemical flooding, as one of the most promising methods in tertiary oil recovery, relies on surfactants to reduce oil-water interfacial tension and alter rock wettability, thereby effectively displacing residual oil that is difficult to recover using conventional waterflooding. However, as oilfield development enters its later stages, more and more reservoirs exhibit harsh geological characteristics such as high temperature (>120°C), high salinity (>100,000 mg / L), and high calcium and magnesium ion content (>5000 mg / L), placing extremely stringent performance requirements on surfactants used for oil displacement.

[0003] Currently, the most commonly used oil displacement surfactants in oilfields both domestically and internationally include anionic surfactants such as petroleum sulfonates and heavy alkylbenzene sulfonates, as well as betaine-type zwitterionic surfactants. Petroleum sulfonates and heavy alkylbenzene sulfonates have achieved large-scale application in oilfields such as Daqing and Shengli due to their wide availability of raw materials and relatively low cost. However, these anionic surfactants have significant technical shortcomings: the sulfonic acid groups in their molecular structure are susceptible to damage from high-valence metal ions (Ca). 2+ Mg 2+ Anionic surfactants are extremely sensitive and readily form insoluble precipitates with divalent ions in highly salinized formation water, leading to a loss of surface activity. Simultaneously, under high-temperature conditions, their molecular chains are prone to thermal degradation, resulting in a sharp decline in interfacial activity, making it difficult to meet the requirements of reservoirs with temperatures exceeding 100°C. Furthermore, anionic surfactants exhibit weak electrostatic repulsion with formation rocks (usually negatively charged), leading to significant adsorption losses and resulting in low effective utilization of chemical agents and persistently high oil displacement costs.

[0004] Betaine-type zwitterionic surfactants, due to the presence of both positive and negative charged groups within their molecules, are insensitive to electrolytes and maintain good interfacial activity even in high-salt environments, making them a research hotspot in recent years. CN102757775A discloses an oil displacement composition comprising a compound of a betaine-type surfactant and a nonionic surfactant, which can reduce the oil-water interfacial tension to 10 under weakly alkaline conditions. -2The concentration is on the order of mN / m. However, the betaine surfactant used in this scheme is still a traditional single-chain, single-headed structure with a high critical micelle concentration (cmc) (typically in the range of 1-10 mM). This means that a higher concentration is required to achieve the desired oil displacement effect, which increases the cost of the reagent. At the same time, the interfacial coverage density of single-chain betaine molecules is limited, making them unsuitable for low-permeability and ultra-low-permeability reservoirs.

[0005] In terms of molecular structure design, Gemini surfactants connect two traditional surfactant monomers through spacer chains, exhibiting lower CMC values ​​and higher interfacial activity than single-chain surfactants, theoretically significantly reducing dosage. CN115093351A discloses a modified betaine-type Gemini surfactant, whose molecule has two positively charged centers and four anionic groups, exhibiting overall negative charge, which can reduce formation adsorption to some extent. However, the patent uses bi-terminal epoxy glycidyl ether as the linking group, introducing polyether segments, which makes the molecule prone to ether bond breakage at high temperatures, limiting its temperature resistance (<120°C); furthermore, its betaine group is sulfobetaine, the synthesis process involves multiple steps, resulting in a low overall yield, posing challenges for industrial production.

[0006] To address the development needs of low-permeability reservoirs, CN117757023A discloses a high-salt, low-permeability reservoir displacement agent containing a betaine-type surfactant. The surfactant is formed by polymerizing perfluorononenoxybenzenesulfonyl diethylenetriamine with lysine betaine. Although the perfluoroalkyl group imparts excellent temperature and salt resistance, the fluorinated raw materials are expensive and pose potential risks of bioaccumulation and environmental persistence, which does not conform to the current development direction of green chemistry.

[0007] In summary, existing surfactant technologies for oilfields suffer from the following shortcomings: single-chain structures have limited interfacial activity, require large quantities, and are costly; twin structures have insufficient temperature resistance or complex synthesis processes; and fluorinated structures pose significant environmental risks and are excessively expensive. This invention aims to develop a surfactant for oilfields that is easy to synthesize, cost-controllable, possesses ultra-low interfacial tension and low adsorption loss, and is suitable for challenging extraction scenarios such as high temperature, high salinity, and low permeability. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention employs a two-step method to synthesize a surfactant with readily available raw materials and controllable costs. This surfactant achieves molecular-level synergy between the high activity of the twin framework and the high salt resistance of betaine by introducing carboxyl betaine groups into the twin framework. At the same time, the asymmetric hydrophobic chain design enhances its emulsification adaptability to heavy oil, making it suitable for chemical flooding development of difficult-to-extract reserves such as high-temperature, high-salt, low-permeability, and heavy oil.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a method for preparing a surfactant for oilfield use, specifically comprising the following steps: S1. Mix alkyl dimethyl tertiary amine and anhydrous ethanol; dissolve dihaloalkane in anhydrous ethanol to form a linker, and add the linker dropwise; after the addition is complete, heat to reflux reaction; take samples every 4 hours to monitor the reaction progress; after the reaction is completed, rotary evaporate, distill under reduced pressure to obtain intermediate, analyze the intermediate by chromatography, and take the target intermediate as product 1. S2. The following operations are all carried out under an inert atmosphere: Dissolve product 1 in deionized water, add halocarboxylate in batches under cooling conditions, stirring until completely dissolved after each batch before adding the next batch; adjust the pH to 9.0-10.0; heat and keep stirring; take samples every 2 hours to monitor the reaction progress; after the reaction is completed, adjust the pH to 2.0-3.0, extract, combine the organic phases, wash the organic phases, dry, distill under reduced pressure, recrystallize, and vacuum dry to obtain the surfactant for oilfield use.

[0010] The method for preparing surfactants for this oil field uses readily available raw materials, all of which are industrial-grade chemicals; the reaction conditions are mild under normal pressure, requiring no expensive catalysts, making it suitable for industrial production.

[0011] In some embodiments, the molecular structural formula of the oilfield surfactant obtained by the preparation method is as follows: , Where: R1 and R2 are C8~C 18 Straight-chain or branched alkyl groups; n is an integer from 2 to 5.

[0012] The surfactant used in this oilfield employs a structure in which two quaternary ammonium head groups are rigidly linked by a short alkyl chain, causing the molecule to arrange itself in a U-shaped or bridge-shaped conformation at the oil-water interface, resulting in a higher interfacial coverage density compared to single-chain structures. The introduction of betaine groups endows the molecule with zwitterionic characteristics, solving the problem of interfacial activity loss caused by charge shielding in traditional anionic surfactants under high-salt conditions. Simultaneously, the critical micelle concentration (CMC) is lower than that of conventional petroleum sulfonates, allowing for reduced dosage and lower costs in practical applications.

[0013] In some embodiments, R1 and R2 are each selected from straight-chain or branched alkyl groups with a carbon chain length of C8 to C18.

[0014] The asymmetric selection of R1 and R2 structures improves the emulsification and viscosity reduction effect on heavy oil (viscosity > 5000 mPa·s) compared to the symmetric structure, and the resulting emulsion has moderate stability, ensuring both oil carrying capacity and facilitating subsequent demulsification. Furthermore, the asymmetric structure reduces the close packing of molecules on the rock surface, resulting in lower adsorption losses compared to the symmetric structure.

[0015] In some embodiments, R1 and R2 are each selected from straight-chain alkyl groups with a carbon chain length of C12, C16, or C18.

[0016] In some implementations, n=3.

[0017] The length of the spacer chain affects the conformational freedom and head-to-group spacing of the twin molecules. When n=3, the spacing between the two quaternary ammonium head-to-groups matches the thickness of the electric double layer at the oil-water interface (and the charge distribution on the rock surface). Too short a spacing may lead to excessive electrostatic repulsion of the head-to-groups, resulting in a loose interfacial arrangement; too long a spacing will increase molecular flexibility and decrease the interfacial packing density.

[0018] In some embodiments, in step S1, the molar ratio of the alkyl dimethyl tertiary amine to the dihaloalkane is 1:(0.5-0.6).

[0019] In some embodiments, in step S1, the dripping rate of the binder is 2 to 5 mL / min.

[0020] In some embodiments, in step S1, the sampling and monitoring are performed using TLC, and the TLC developing solvent is a chloroform-methanol solution with a volume ratio of (7-10):1.

[0021] In some embodiments, in step S2, the molar ratio of the alkyl dimethyl tertiary amine to the halocarboxylic acid salt is 1:(1.05 to 1.15).

[0022] In some embodiments, in step S1, the dihaloalkane is a straight-chain bromoalkane with a carbon chain length of C2 to C5.

[0023] Compared with the prior art, the present invention has the following beneficial effects: 1. The surfactant provided by this invention has a critical micelle concentration lower than that of conventional single-chain surfactants. It can spontaneously assemble into micelles at extremely low concentrations to exert interfacial activity, directly reducing the reagent cost of chemical flooding in practical oilfield applications. Furthermore, even if the concentration decreases due to dilution during formation migration, the molecules can still maintain the micelle structure and interfacial activity, thereby extending the effective range of chemical flooding and improving the sweep efficiency. Simultaneously, this surfactant can maintain ultra-low interfacial tension between crude oil and formation water, improving the effective stripping and displacement of residual oil droplets.

[0024] 2. The surfactant provided by this invention has a high CN bond energy in its twin skeleton and the spacer chain is a saturated alkyl chain, without functional groups such as ether bonds and ester bonds that are easily broken in high-temperature aqueous solutions; the zwitterionic structure of the betaine group maintains charge balance at high temperatures and is not easily thermally degraded or hydrolyzed, making it suitable for a large number of high-temperature deep oil reservoirs in western my country and the South China Sea, thus expanding the application boundaries of chemical flooding technology.

[0025] 3. The zwitterionic properties of the betaine group in the surfactant provided by this invention enable the positive and negative charges within the molecule to shield each other, making it electrically neutral externally and unaffected by the electrolyte shielding effect, thus exhibiting excellent salt resistance. At the same time, the carboxyl group forms a soluble chelate with high-valence metal ions instead of precipitation, fundamentally eliminating the deactivation problem caused by divalent ions. This makes it suitable for high-salinity oil reservoirs in basins such as Tarim and Junggar in my country, as well as seawater displacement scenarios in offshore oil fields.

[0026] 4. The surfactant molecules provided by this invention are electrically neutral as a whole. The combination of asymmetric hydrophobic chain structure disrupts the orderly and compact packing of molecules on the rock surface, weakens the electrostatic interaction with the rock surface, reduces rock adsorption loss, and allows more surfactant to participate in the oil displacement process, thereby improving the effective utilization rate of the agent. Detailed Implementation

[0027] The present invention will be described below with reference to specific implementation schemes. It should be noted that the following embodiments and comparative examples are examples of the present invention and are used only to illustrate the invention, not to limit it. Other combinations and various modifications within the scope of the present invention can be made without departing from its spirit or scope. It is worth noting that, unless otherwise specified, the raw materials used in the following preparation examples and embodiments are all from any commercially available manufacturer.

[0028] Example 1 A method for preparing an oilfield surfactant specifically includes the following steps: S1. Mix 0.1 mol dodecyl dimethyl tertiary amine, 0.1 mol hexadecyl dimethyl tertiary amine, and 150 mL anhydrous ethanol at 60°C and 300 rpm. Dissolve 0.11 mol 1,3-dibromopropane in 50 mL anhydrous ethanol to form a linker, and add the linker dropwise at a rate of 3 mL / min. After the addition is complete, reflux the mixture at 78°C. Take samples every 4 hours for TLC monitoring (using a chloroform-methanol solution with a volume ratio of 9:1) until the starting material spot disappears. After the reaction is complete, remove the ethanol by rotary evaporation, and distill under reduced pressure to obtain three intermediates. Analyze the intermediates by chromatography, and take the asymmetric intermediate as product 1. S2. The following operations were all carried out under nitrogen protection: Product 1 was dissolved in 100 mL of deionized water, and sodium chloroacetate (0.22 mol total) was added in three equal batches at 5°C. After each batch was added, the mixture was stirred until completely dissolved before adding the next batch. The pH was adjusted to 9.5 with 1 M NaOH solution. The temperature was raised to 80°C and stirred. Samples were taken every 2 hours for monitoring with high performance liquid chromatography until the content of the raw material was ≤0.1%, indicating the end of the reaction. After the reaction, the pH was adjusted to 3.0 with 6 M HCl, and the mixture was extracted with ethyl acetate. The organic phases were combined, washed with saturated NaCl solution until neutral, dried over anhydrous sodium sulfate, and distilled under reduced pressure. The mixture was recrystallized three times with an ethanol-water solution (9:1 v / v). The product was dried under vacuum at 50°C to obtain the surfactant for oilfield use. The structural formula is as follows: .

[0029] The NMR and infrared data of the oilfield surfactant obtained in this embodiment are as follows: 1 H NMR (D2O, 400MHz): δ 3.2-3.5 ppm (N + -CH2), δ 3.6-3.8 ppm (N + -CH2-COO - ); 13 C NMR (D2O, 100MHz): δ 50-55 ppm (quaternary ammonium carbon), δ 170 ppm (carboxyl carbon); FT-IR: 1640 cm -1 (COO - ), 1470 cm -1 (N + -CH3).

[0030] Example 2 A method for preparing an oilfield surfactant specifically includes the following steps: S1. Mix 0.1 mol dodecyl dimethyl tertiary amine, 0.1 mol octadecyl dimethyl tertiary amine, and 150 mL anhydrous ethanol at 60°C and 300 rpm. Dissolve 0.11 mol 1,3-dibromopropane in 100 mL anhydrous ethanol to form a linker, and add the linker dropwise at a rate of 5 mL / min. After the addition is complete, reflux the mixture at 78°C. Take samples every 4 hours for TLC monitoring (using a 7:1 volume ratio chloroform-methanol solution as the developing solvent) until the starting material spot disappears. After the reaction is complete, remove the ethanol by rotary evaporation, and distill under reduced pressure to obtain three intermediates. Analyze the intermediates by chromatography, and take the asymmetric intermediate as product 1. S2. The following operations were all carried out under nitrogen protection: Product 1 was dissolved in 100 mL of deionized water, and sodium chloroacetate (0.23 mol total) was added in three equal batches at 5°C. After each batch was added, the mixture was stirred until completely dissolved before adding the next batch. The pH was adjusted to 10.0 with 1 M NaOH solution. The temperature was raised to 80°C and stirred. The reaction progress was monitored by high performance liquid chromatography every 2 hours. After the reaction was completed, the pH was adjusted to 3.0 with 6 M HCl, extracted with ethyl acetate, and the organic phases were combined. The organic phases were washed with saturated NaCl solution until neutral, dried with anhydrous sodium sulfate, distilled under reduced pressure, and recrystallized three times with an ethanol-water solution at a volume ratio of 9:1. The product was dried under vacuum at 50°C to obtain the surfactant for oilfield use.

[0031] Example 3 A method for preparing an oilfield surfactant, specifically comprising the following steps, wherein the raw materials are in parts by weight: S1. Mix 0.1 mol octadecyl dimethyl tertiary amine, 0.1 mol hexadecyl dimethyl tertiary amine, and 150 mL anhydrous ethanol at 60°C and 300 rpm. Dissolve 0.12 mol 1,3-dibromopropane in 50 mL anhydrous ethanol to form a linker, and add the linker dropwise at a rate of 2 mL / min. After the addition is complete, reflux the mixture at 78°C. Take samples every 4 hours for TLC monitoring (using a 10:1 volume ratio chloroform-methanol solution as the developing solvent) until the starting material spot disappears. After the reaction is complete, remove the ethanol by rotary evaporation, and distill under reduced pressure to obtain three intermediates. Analyze the intermediates by chromatography, and take the asymmetric intermediate as product 1. S2. The following operations were all carried out under nitrogen protection: Product 1 was dissolved in 100 mL of deionized water, and sodium chloroacetate was added in three equal batches at 5°C, with each batch stirred until completely dissolved before adding the next batch; the pH was adjusted to 9.0 with 1 M NaOH solution; the temperature was raised to 80°C and stirred; samples were taken every 2 hours to monitor the reaction progress using high performance liquid chromatography; after the reaction was completed, the pH was adjusted to 3.0 with 6 M HCl, extracted with ethyl acetate, the organic phases were combined, washed with saturated NaCl solution until neutral, dried with anhydrous sodium sulfate, distilled under reduced pressure, and recrystallized three times with an ethanol-water solution with a volume ratio of 9:1; dried under vacuum at 50°C to obtain the surfactant for oilfield use.

[0032] Example 4 This embodiment provides a method for preparing a surfactant for oil fields. The specific implementation method is the same as in Embodiment 1, except that: dodecyl dimethyl tertiary amine is used instead of hexadecyl dimethyl tertiary amine in the same molar amount.

[0033] Example 5 This embodiment provides a method for preparing a surfactant for oilfield use. The specific implementation method is the same as in Embodiment 1, except that 1,3-dibromopropane is replaced with an equal molar amount of 1,6-dibromohexane.

[0034] Comparative Example 1 A method for preparing an oilfield surfactant specifically includes the following steps: 0.2 mol of dodecyl dimethyl tertiary amine was dissolved in 150 mL of anhydrous ethanol, and the mixture was heated to 60°C and stirred at 300 rpm. 0.21 mol of sodium chloroacetate was dissolved in 50 mL of deionized water and slowly added dropwise to the reaction system. After the addition was complete, the pH was adjusted to 9.5 with 1M NaOH, and the mixture was refluxed at 80°C for 12 h. After the reaction was completed, the pH was adjusted to 3.0 with 6M HCl, extracted with ethyl acetate, and the organic phases were combined. The mixture was washed with saturated NaCl solution until neutral, dried over anhydrous sodium sulfate, distilled under reduced pressure, and recrystallized three times with an ethanol-water solution. The product was then dried under vacuum at 50°C to obtain the surfactant for oilfield use.

[0035] Comparative Example 2 A method for preparing an oilfield surfactant specifically includes the following steps: S1. The operation is the same as in Example 1; S2. Dissolve product 1 in 100 mL of deionized water, heat to 60°C and stir for 2 h for hydrolysis. After the reaction is complete, extract with ethyl acetate, combine the organic phases, wash with saturated NaCl solution until neutral, dry with anhydrous sodium sulfate, distill under reduced pressure, recrystallize three times with ethanol-water solution, and dry under vacuum at 50°C to obtain the oilfield surfactant.

[0036] Performance testing The oilfield surfactants provided in the examples and comparative examples were tested as follows, and the results are shown in Table 1: 1. Determination of critical micelle concentration and surface tension: Prepare aqueous solutions of surfactants with different concentration gradients. Under constant temperature of 25°C, measure the surface tension of each concentration solution using the pendant method. Plot the surface tension against the logarithm of concentration. The concentration corresponding to the inflection point of the curve is the critical micelle concentration cmc, and the surface tension at this point is denoted as γcmc.

[0037] 2. Oil-water interfacial tension (IFT) determination: Simulated formation water preparation: total salinity 100,000 mg / L (NaCl 95,000 mg / L, CaCl2 3,000 mg / L, MgCl2 2,000 mg / L); each surfactant was prepared into a 0.3 wt% simulated formation water solution; using a rotating drop interfacial tensiometer at 60°C and 5000 rpm, the IFT was measured with dehydrated crude oil (viscosity 45 mPa·s, density 0.85 g / cm³). 3 The interfacial tension is recorded; the equilibrium interfacial tension value is recorded.

[0038] 3. Temperature resistance test: Prepare a 0.3 wt% simulated formation aqueous solution for each surfactant (same as test 2); seal the solution in an ampoule and place it in a 150°C constant temperature oven; take samples every 7 days to measure IFT, for a period of 28 days; at the same time, observe the changes in the appearance of the solution (transparency, precipitation, color).

[0039] 4. Rock Adsorption Loss Test: Quartz sandstone with a permeability of approximately 200 mD was crushed and sieved to obtain 40-60 mesh particles. Each surfactant was prepared into a 0.3 wt% simulated formation aqueous solution and mixed with quartz sandstone powder at a liquid-to-solid ratio of 10:1. The mixture was shaken in a 60°C constant temperature shaker for 24 hours and then centrifuged. The surfactant concentration in the supernatant was determined by two-phase titration, and the static adsorption capacity was calculated.

[0040] 5. Heavy oil emulsification viscosity reduction test: Take a heavy oil sample (viscosity 8500 mPa·s at 50°C, content of gum + asphaltene 42wt%); prepare a 0.5 wt% hot aqueous solution of each surfactant at 60°C; mix the heavy oil and surfactant solution at an oil-to-water ratio of 7:3 (mass ratio); use a high-speed shear emulsifier to shear at 8000 rpm for 5 min; measure the viscosity of the emulsified system at 50°C and calculate the viscosity reduction rate.

[0041] Table 1 Performance Test Results

[0042] As shown in Table 1, Examples 1 (C12 / C16 asymmetric), 2 (C12 / C18 asymmetric), and 3 (C16 / C18 asymmetric) exhibited excellent overall performance in all test items. Among the three, Example 2, with its longer chain side being C18, showed stronger hydrophobic interactions, resulting in a slightly lower CMC and slightly better interfacial tension, but the cost of C18 raw materials was slightly higher. Example 3, with both sides consisting of relatively long chains, had the lowest CMC, but its interfacial tension was slightly higher than that of Examples 1 and 2. This may be because the longer hydrophobic chains caused the molecules to pack too tightly at the interface, which in turn limited the extension of the hydration layer of the betaine groups.

[0043] Compared to Example 1, Example 4 has a symmetrical C12 structure on both sides, lacking the interfacial adaptability of asymmetric design. This leads to increased CMC, interfacial tension, and adsorption capacity, resulting in poorer stability at 72 hours and a decrease in the viscosity reduction rate of heavy oil. In Example 5, increasing the spacer chain from n=3 to n=6 increases the distance between the two quaternary ammonium head groups, exceeding the optimal matching range (0.5-1.0 nm) for the thickness of the double electric layer at the oil-water interface. This leads to a decrease in molecular packing density at the interface and reduced interfacial activity. Furthermore, the longer spacer chain may interact hydrophobically with the rock surface, which could actually increase the adsorption capacity.

[0044] Compared to Example 1, Comparative Example 1's single-chain structure lacks the synergistic effect of the bi-headed groups in a twin backbone, resulting in a loose molecular arrangement at the interface, low interface coverage density, high CMC, and weak interfacial activity. Furthermore, the single-headed structure limits the adsorption sites on the rock surface, but electrostatic interactions are not effectively shielded, leading to higher adsorption than twin betaine. Simultaneously, without the constraint of rigid spacer chains, the molecular conformation is highly free, making it prone to desorption from the interface at high temperatures and exhibiting poor thermal stability. It also lacks sufficient solubilization and emulsification capabilities for high molecular weight components in heavy oil, resulting in poor emulsion stability and limited viscosity reduction. Comparative Example 2 lacks the zwitterionic group of betaine, resulting in an overall positively charged molecule that exhibits strong electrostatic attraction to the negatively charged sandstone surface, leading to a dramatic increase in adsorption. While the shielding effect of the positive charge is weak in high-salt environments, high-valence anions may cause molecular aggregation and precipitation, resulting in turbidity. Moreover, the thermal stability of the quaternary ammonium salt head group is inferior to that of the betaine structure, leading to molecular degradation at high temperatures.

[0045] The embodiments and comparative examples described above do not limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a surfactant for oilfield use, characterized in that, Specifically, it includes the following steps: S1. Mix alkyl dimethyl tertiary amine and anhydrous ethanol; dissolve dihaloalkane in anhydrous ethanol to form a linker, and add the linker dropwise; after the addition is complete, heat to reflux reaction; take samples every 4 hours to monitor the reaction progress; after the reaction is completed, rotary evaporate, distill under reduced pressure to obtain intermediate, analyze the intermediate by chromatography, and take the target intermediate as product 1. S2. The following operations are all carried out under an inert atmosphere: Dissolve product 1 in deionized water, add halocarboxylate in batches under cooling conditions, stirring until completely dissolved after each batch before adding the next batch; adjust the pH to 9.0-10.0; heat and keep stirring; take samples every 2 hours to monitor the reaction progress; after the reaction is completed, adjust the pH to 2.0-3.0, extract, combine the organic phases, wash the organic phases, dry, distill under reduced pressure, recrystallize, and vacuum dry to obtain the surfactant for oilfield use.

2. The method for preparing an oilfield surfactant according to claim 1, characterized in that, The molecular structural formula of the oilfield surfactant obtained by the preparation method is as follows: ， Where: R1 and R2 are C8~C 18 Straight-chain or branched alkyl groups; n is an integer from 2 to 5.

3. The method for preparing an oilfield surfactant according to claim 2, characterized in that, R1 and R2 are each selected from straight-chain or branched alkyl groups with a carbon chain length of C8 to C18.

4. The method for preparing an oilfield surfactant according to claim 3, characterized in that, R1 and R2 are each selected from straight-chain alkyl groups with a carbon chain length of C12, C16, or C18.

5. The method for preparing an oilfield surfactant according to claim 2, characterized in that, The n=3.

6. The method for preparing an oilfield surfactant according to claim 1, characterized in that, In step S1, the molar ratio of the alkyl dimethyl tertiary amine to the dihaloalkane is 1:(0.5-0.6).

7. The method for preparing an oilfield surfactant according to claim 1, characterized in that, In step S1, the dripping rate of the connecting agent is 2-5 mL / min.

8. The method for preparing an oilfield surfactant according to claim 1, characterized in that, In step S1, the sampling and monitoring are performed using TLC, and the TLC developing solvent is a chloroform-methanol solution with a volume ratio of (7-10):

1.

9. The method for preparing an oilfield surfactant according to claim 1, characterized in that, In step S2, the molar ratio of the alkyl dimethyl tertiary amine to the halocarboxylic acid salt is 1:(1.05-1.15).

10. The method for preparing an oilfield surfactant according to claim 1, characterized in that, In step S1, the dihaloalkane is a straight-chain bromoalkane with a carbon chain length of C2 to C5.

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