A twin-tail type betaine surfactant, preparation method thereof and fracturing fluid

Abstract

The present invention relates to the technical field of preparing surfactant-based clean fracturing fluids, and particularly to a gemini betaine surfactant, a preparation method thereof, and a fracturing fluid. By successively carrying out alkylation reaction and quaternization reaction on N,N-dimethyl-1,3-propanediamine and halogenated hydrocarbons, the surfactant prepared has two carbon chains with double gemini tails, both of which are C 16 ~C 22 , wherein at least one of the two long chains contains a double bond. The double bond on the double gemini tail can easily react with a sulfonating group under formation conditions to self-break gel without the addition of an external gel breaker, effectively solving the problem of gel breaking of clean fracturing fluids. The preparation method is simple, the reaction conditions are mild, and it is easy to industrialize. Moreover, the prepared surfactant has higher temperature resistance and viscosity, good sand-carrying capacity, and can self-break gel, solving the problems of poor temperature resistance, low viscosity, poor sand-carrying capacity, and difficult gel breaking of viscoelastic surfactant fracturing fluids in the prior art.

Description

Technical Field

[0001] The present invention relates to the technical field of preparing surfactant-based clean fracturing fluids, and particularly to a gemini betaine surfactant, a preparation method thereof, and a fracturing fluid. Background Art

[0002] Petroleum is known as the "industrial blood". In recent years, 70% of the newly discovered oil and gas reserves are low-permeability reservoirs. However, due to the deep reservoir and complex formation conditions, the average recovery rate is only 23.3% at most, which severely restricts the sustainable exploitation of oil and gas. The hydraulic fracturing technology can efficiently transform the geological reservoirs of special oil and gas reservoirs, thereby increasing the production of oil and gas wells and the injection of water wells. The liquid used in the hydraulic fracturing process is a fracturing fluid. A fracturing fluid is a liquid with a certain viscosity injected into the formation through a high-pressure pump during the fracturing operation. After being injected into the formation, it can cause fractures in the formation and introduce proppants, thereby achieving the purpose of increasing oil and gas production. In addition, the fracturing fluid also needs to meet conditions such as "good performance, low damage, and low cost". Currently, hydraulic fracturing fluids mainly include polymer fracturing fluids and clean fracturing fluids, also called viscoelastic surfactant fracturing fluids. Polymer fracturing fluids need to add crosslinking agents and breaker agents during use, and their breaker residues are not easily returned, causing great damage to the formation. In addition, their temperature resistance and salt resistance are poor, and they are prone to shear thinning, resulting in unsatisfactory application performance. Compared with polymer fracturing fluids, viscoelastic surfactant fracturing fluids have the advantages of low damage, shear resistance, and excellent sand-carrying ability, and have a broader application prospect. However, the viscoelastic surfactant fracturing fluids developed so far still have problems such as poor temperature resistance, low viscosity, poor sand-carrying ability, and difficult gel breaking, which greatly limit their popularization and use. Summary of the Invention

[0003] Aiming at the problems of poor temperature resistance, low viscosity, poor sand-carrying ability, and difficult gel breaking of viscoelastic surfactant fracturing fluids in the prior art, the present invention provides a gemini betaine surfactant, a preparation method thereof, and a fracturing fluid.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] The present invention provides a gemini betaine surfactant, and the surfactant has the following structure:

[0006]

[0007] wherein, R1 and R2 are saturated or unsaturated carbon chains with a carbon chain length in the range of C 16 -C 22 range, and at least one of R1 and R2 contains an unsaturated bond.

[0008] The present invention also provides a preparation method of the above gemini betaine surfactant, including the following steps:

[0009] Dissolve N,N-dimethyl-1,3-propanediamine and a halogenated hydrocarbon in a solvent for an alkylation reaction. After the reaction is completed, an intermediate product, a double long-chain tertiary amine, is formed.

[0010] Add a sodium chloroacetate solution to the intermediate product, the double long-chain tertiary amine, for a quaternization reaction. After the reaction, remove the solvent to obtain the final product, a twin-tailed betaine surfactant.

[0011] Preferably, the solvent is a mixed solution of absolute ethanol and cesium carbonate. Among them, the molar ratio of absolute ethanol to the halogenated hydrocarbon is 2:1, and the molar ratio of cesium carbonate to N,N-dimethyl-1,3-propanediamine is 2:1.

[0012] Preferably, the molar ratio of N,N-dimethyl-1,3-propanediamine to the halogenated hydrocarbon is (1 - 1.5):(2 - 2.5).

[0013] Preferably, the reaction temperature of the alkylation reaction is 60°C to 75°C, and the reaction time is 10 to 13 h.

[0014] Preferably, the molar ratio of sodium chloroacetate to the intermediate product is (1 - 1.5):(1 - 1.5).

[0015] Preferably, the temperature of the quaternization reaction is 60°C to 75°C, and the reaction time is 2 to 3 h with stirring.

[0016] Preferably, the halogenated hydrocarbon is a mixture of X1-R1 and X2-R2, where X1 and X2 are both halogens, and R1 and R2 are C 16 -C 22 , and at least one of R1 or R2 contains a double bond.

[0017] The present invention provides a fracturing fluid, which includes the above-mentioned twin-tailed betaine surfactant.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention provides a twin-tailed betaine surfactant. Both carbon chains of the double twin-tails of this surfactant are C 16 ~C 22 , and the critical micelle concentration (cmc) is lower, and the regularity is good, making the surfactant have good associativity, and thus making it have better temperature resistance and shear resistance. After testing, its temperature resistance performance reaches 90 - 100°C, which is much higher than the current products (60 - 80°C). The above characteristics can endow the surfactant with good sand-carrying ability under high-salt, high-temperature and high-shear conditions. At the same time, the double bonds on the double twin-tails can easily react with sulfonating groups under formation conditions to break the gel by themselves, without the need for an external gel breaker, effectively solving the problem of gel breaking of clean fracturing fluids, being convenient to use and easy to promote.

[0020] The present invention also provides a preparation method of a twin-tailed betaine surfactant. By sequentially performing alkylation reaction and quaternization reaction on N,N-dimethyl-1,3-propanediamine and halogenated hydrocarbon, the twin-tailed betaine surfactant can be obtained. The preparation method is simple, the reaction conditions are mild, it is easy to industrialize, and the prepared surfactant has higher temperature resistance and viscosity, good sand-carrying property, and can break gel by itself, which is an advanced technology in the field of fracturing fluid preparation technology.

[0021] The present invention also provides a fracturing fluid prepared by using the twin-tailed betaine surfactant. When in use, a small amount of the above surfactant is taken and dissolved in brine, and stirred evenly. It avoids the problems of large dust in dry powder feeding, difficult to mix evenly, easy to generate powder particles and local gel caking in the on-site preparation of the current fracturing fluid, which affect the fracturing performance. Moreover, it can be efficiently dissolved in various solutions, has good compatibility and wide application range. After testing, when the shear rate is 170s -1 and the temperature is 120 °C, its viscosity is still 89 mPa·s, meeting the construction requirements. Description of the Drawings

[0022] Figure 1 is a flow chart of a preparation method of a twin-tailed betaine surfactant of the present invention.

[0023] Figure 2 is a test result diagram of the 1H NMR spectrum of the surfactant in the embodiment of the present invention.

[0024] Figure 3 is a curve graph of the shear recovery test result of the fracturing fluid prepared in the embodiment of the present invention. Detailed Embodiments

[0025] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Generally, the components of the embodiments of the present invention described and illustrated herein can be arranged and designed in various different configurations.

[0026] Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed present invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0027] It should be noted that like reference numerals and letters refer to like items in the following figures. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0028] In the description of the embodiments of the present invention, it should be noted that if terms such as "upper", "lower", "horizontal", "inner", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the figures, or the orientation or positional relationship in which the invention product is usually placed during use, it is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be construed as a limitation on the present invention. In addition, terms such as "first", "second", etc. are only used for distinguishing descriptions and cannot be understood as indicating or implying relative importance.

[0029] In addition, if the term "horizontal" appears, it does not mean that the component is required to be absolutely horizontal, but it can be slightly inclined. For example, "horizontal" only means that its direction is more horizontal relative to "vertical", and does not mean that the structure must be completely horizontal, but it can be slightly inclined.

[0030] In the description of the embodiments of the present invention, it should also be noted that unless otherwise clearly specified and limited, if terms such as "set", "installed", "connected", "connected" are understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0031] The following further describes the present invention in detail with specific embodiments, which is an explanation rather than a limitation of the present invention.

[0032] The present invention discloses a twin-tailed betaine surfactant, and the surfactant has the following structure:

[0033]

[0034] Among them, R1 and R2 are saturated carbon chains or unsaturated carbon chains with a carbon chain length in the range of C 16 -C 22 , and R1 and R2 contain at least one unsaturated bond.

[0035] See Figure 1 , the present invention also provides a preparation method of the above twin-tailed betaine surfactant, including the following steps:

[0036] S1: Dissolve N,N-dimethyl-1,3-propanediamine and halogenated hydrocarbon in a solvent for alkylation reaction. After the reaction is completed, the intermediate product, double long-chain tertiary amine, is formed. The specific operation is as follows: Add N,N-dimethyl-1,3-propanediamine and halogenated hydrocarbon to a mixed solution containing absolute ethanol and cesium carbonate according to a molar ratio of (1 - 1.5):(2 - 2.5). Place it in a reaction vessel equipped with a condenser and a stirrer for alkylation reaction. The reaction conditions are 60°C - 75°C, and the stirring time is 10 - 13 h to obtain the intermediate product, double long-chain tertiary amine. Among them, the molar ratio of absolute ethanol to halogenated hydrocarbon is 2:1, and the molar ratio of cesium carbonate to N,N-dimethyl-1,3-propanediamine is 2:1. The halogenated hydrocarbons are X1-R1 and X2-R2, where X1 and X2 are both halogens, and R1 and R2 are C 16 -C 22 , where at least one of R1 or R2 contains a double bond.

[0037] S2: Add sodium chloroacetate solution to the intermediate product, double long-chain tertiary amine, for quaternization reaction. After the reaction, remove the solvent to obtain the final product, twin-tailed betaine surfactant. The specific operation is as follows: Dissolve sodium chloroacetate in water to prepare a sodium chloroacetate solution. Drop the prepared sodium chloroacetate solution into the intermediate product, double long-chain tertiary amine, for quaternization reaction. The reaction conditions are 60°C - 75°C, and the stirring reaction time is 2 - 3 h. The molar ratio of sodium chloroacetate to the intermediate product, double long-chain tertiary amine, is (1 - 1.5):(1 - 1.5). The concentration of the sodium chloroacetate solution is 35% - 45%.

[0038] The reaction principle is as follows:

[0039] Alkylation:

[0040]

[0041] Quaternization:

[0042]

[0043] That is, using ethanol as the solvent and cesium carbonate to provide an alkaline environment. Alkylate N,N-dimethyl-1,3-propanediamine and halogenated hydrocarbon to form double long-chain tertiary amine, and prepare twin-tailed betaine surfactant by quaternizing this tertiary amine with sodium chloroacetate.

[0044] The present invention also provides a fracturing fluid prepared by using the above-mentioned twin-tailed betaine surfactant. The fracturing fluid comprises a twin-tailed betaine surfactant and brine. Among them, the mass ratio of the twin-tailed betaine surfactant to brine is (6-8):(94-92). The brine is standard brine, and the brine components are: 5.5% of NaCl, 2% of KCl, 0.45% of MgCl, and 0.55% of CaCl, with the balance being water. When formulating, only need to dissolve the twin-tailed betaine surfactant in brine and stir for 25-40 min to obtain the fracturing fluid.

[0045] Example 1

[0046] Add 102 g of N,N-dimethyl-1,3-propanediamine, 305 g of hexadecyl bromide, and 305 g of hexadecene bromide to a dry reactor equipped with a condenser and a stirrer. Add 1075 g of Cs2CO3 to provide an alkaline environment and dissolve it in ethanol. Stir and react at 65 °C for 10 h. After cooling, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer, and a dropping funnel. Add 116 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 65 °C for 2 h. Remove the solvent by rotary evaporation to obtain the final product. After cooling, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 6 g of the twin-tailed betaine surfactant in 94 g of brine and stir for 30 min to obtain the fracturing fluid.

[0047] Example 2

[0048] Add 153 g of N,N-dimethyl-1,3-propanediamine, 457 g of hexadecyl bromide, 457 g of hexadecene bromide, and 1075 g of Cs2CO3 to ethanol. Stir and react at 75 °C for 13 h. After cooling, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer, and a dropping funnel. Add 116 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 65 °C for 2 h. Remove the solvent by rotary evaporation to obtain the final product. After cooling, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 6 g of this twin-tailed betaine surfactant in 94 g of brine and stir for 30 min to obtain the fracturing fluid.

[0049] Example 3

[0050] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 320 g of hexadecyl bromide, 320 g of hexadecenyl bromide, and 1075 g of Cs2CO3 in ethanol. Stir and react at 60 °C for 12 h. After it cools down, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer, and a dropping funnel. Add 116 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 75 °C for 2 h. Remove the solvent by rotary evaporation to obtain the final product. After it cools down, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 6 g of this twin-tailed betaine surfactant in 94 g of brine and stir for 30 min to prepare a fracturing fluid.

[0051] Example 4

[0052] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 305 g of hexadecyl bromide, 305 g of hexadecenyl bromide, and 1075 g of Cs2CO3 in ethanol. Stir and react at 60 °C for 10 h. After it cools down, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer, and a dropping funnel. Add 130 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 60 °C for 2 h. Remove the solvent by rotary evaporation to obtain the final product. After it cools down, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 6 g of this twin-tailed betaine surfactant in 94 g of brine and stir for 30 min to prepare a fracturing fluid.

[0053] Example 5

[0054] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 305 g of hexadecyl bromide, 305 g of hexadecenyl bromide, and 1075 g of Cs2CO3 in ethanol. Stir and react at 65 °C for 10 h. After it cools down, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer, and a dropping funnel. Add 116 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 65 °C for 3 h. Remove the solvent by rotary evaporation to obtain the final product. After it cools down, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 7 g of this twin-tailed betaine surfactant in 93 g of brine and stir for 40 min to prepare a fracturing fluid.

[0055] Example 6

[0056] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 305 g of hexadecyl bromide, 305 g of hexadecenyl bromide, and 1000 g of Cs2CO3 in ethanol. Stir and react at 65 °C for 10 h. After cooling, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer, and a dropping funnel. Add 150 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 65 °C for 3 h. Remove the solvent by rotary evaporation to obtain the final product. After cooling, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 6 g of this twin-tailed betaine surfactant in 94 g of brine and stir for 30 min to prepare a fracturing fluid.

[0057] Example 7

[0058] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 389 g of docosyl bromide, 389 g of docosenyl bromide, and 1000 g of Cs2CO3 in ethanol. Stir and react at 75 °C for 11 h. After cooling, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer, and a dropping funnel. Add 150 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 75 °C for 2 h. Remove the solvent by rotary evaporation to obtain the final product. After cooling, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 7 g of this twin-tailed betaine surfactant in 93 g of brine and stir for 30 min to prepare a fracturing fluid.

[0059] Example 8

[0060] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 317 g of 1-chloroeicosane, 316.5 g of 1-chloro-19-eicosenes, and 1075 g of Cs2CO3 in ethanol. Stir and react at 70 °C for 12 h. After cooling, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer, and a dropping funnel. Add 150 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 70 °C for 3 h. Remove the solvent by rotary evaporation to obtain the final product. After cooling, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 8 g of this twin-tailed betaine surfactant in 94 g of brine and stir for 35 min to prepare a fracturing fluid.

[0061] Example 9

[0062] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 289 g of chlorooctadecane, 288.5 g of chlorooctadecene and 1075 g of Cs2CO3 in ethanol. Stir and react at 65 °C for 13 h. After cooling, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer and a dropping funnel. Add 150 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 65 °C for 3 h. Remove the solvent by rotary evaporation to obtain the final product. After cooling, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 8 g of this twin-tailed betaine surfactant in 94 g of brine and stir for 35 min to prepare a fracturing fluid.

[0063] Example 10

[0064] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 578 g of chlorooctadecene and 1075 g of Cs2CO3 in ethanol. Stir and react at 65 °C for 13 h. After cooling, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer and a dropping funnel. Add 150 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 65 °C for 3 h. Remove the solvent by rotary evaporation to obtain the final product. After cooling, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 8 g of this twin-tailed betaine surfactant in 94 g of brine and stir for 35 min to prepare a fracturing fluid.

[0065] Example 11

[0066] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 289 g of chlorooctadecane, 288.5 g of chlorooctadecatriene and 1075 g of Cs2CO3 in ethanol. Stir and react at 65 °C for 13 h. After cooling, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer and a dropping funnel. Add 150 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 65 °C for 3 h. Remove the solvent by rotary evaporation to obtain the final product. After cooling, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 8 g of this twin-tailed betaine surfactant in 94 g of brine and stir for 35 min to prepare a fracturing fluid.

[0067] Example 12

[0068] Dissolve 102 g of N,N-dimethyl-1,3-propanediamine, 289 g of chlorinated octadecadiene, 288.5 g of chlorinated octadecatetraene and 1075 g of Cs2CO3 in ethanol. Stir and react at 65 °C for 13 h. After cooling, remove the solvent from the solution by rotary evaporation to obtain the intermediate product, double long-chain tertiary amine. Add the double long-chain tertiary amine to a dry reactor equipped with a condenser, a stirrer and a dropping funnel. Add 150 g of sodium chloroacetate solution to the dropping funnel. Stir and react at 65 °C for 3 h. Remove the solvent by rotary evaporation to obtain the final product. After cooling, remove the solvent by rotary evaporation to obtain the final product, twin-tailed betaine surfactant. Dissolve 8 g of this twin-tailed betaine surfactant in 94 g of brine and stir for 35 min to prepare a fracturing fluid.

[0069] See Figure 2 , perform 1H NMR test on the twin-tailed betaine surfactant prepared in Example 6 above. 1H NMR (300 MHz, DMSO): δ 5.43 (m, H), 4.18 (s, H), 3.30 (s, H), 3.22 (t, H), 3.01 (t, H), 2.46 (t, H), 2.16 (m, H), 1.83 (m, H), 1.36 (m, H), 1.29 (m, H), 1.26 (m, H), 0.88 (m, H) ppm. From the NMR data, it can be seen that the surfactant with the target structure was successfully prepared in the present invention.

[0070] In order to characterize the application performance of the fracturing fluid of the twin-tailed betaine surfactant, the following tests are carried out on it:

[0071] First of all, viscosity is an important parameter to evaluate the performance of fracturing fluid. Due to the complex formation environment during the actual construction process, the fracturing fluid is required to have a certain temperature resistance. Under the condition of a shear rate of 170 s -1 , test the change of viscosity of the fracturing fluid of Example 1 with temperature. The test results are shown in the following table:

[0072]

[0073] It can be seen that at 120 °C, the viscosity of the surfactant fracturing fluid is 89 mPa·s, which meets the construction requirements of the fracturing fluid, indicating that the fracturing fluid prepared in the present invention has better high-temperature resistance.

[0074] During the fracturing construction, the fracturing fluid is in a high-shear state when passing through the wellbore, and is in a low-shear state when entering the formation fracture. This requires the fracturing fluid to have good shear recovery. See Figure 3 , for the test results of the shear viscosity recovery of the fracturing fluid prepared in Example 1, it can be seen that the fracturing fluid has very good shear recovery.

[0075] The gel-breaking performance of the fracturing fluid was tested. NaHSO3 was added to the fracturing fluids prepared in Examples 1-3 to simulate the formation environment. After the gel turned into an aqueous solution, the gel-breaking was completed. Optionally, the concentration of the fracturing fluid was 5-10 wt%; the addition amount of NaHSO3 accounted for 2-5 wt% of the mass of the fracturing fluid. Optionally, the simulated formation temperature was 80 °C and the pH value was 5. The test results are shown in the following table:

[0076] Example Viscosity before gel breaking Viscosity after gel breaking Gel breaking time 1 150 0.81 19 min 2 169 0.78 20 min 3 144 3.55 20 min

[0077] It can be seen that the prepared fracturing fluid can achieve self-gel-breaking within 20 minutes without adding a gel breaker, and the gel-breaking of this fracturing fluid is complete after the fracturing operation, causing little damage to the formation.

[0078] Viscosity reduction test of the fracturing fluid on heavy oil: Test conditions: At 50 °C, using an RV30 viscometer, 170 s -1 , and the shear time was 30 minutes. The crude oil viscosity was 1832 mPa·S. The viscosity reduction test results of the fracturing fluid of Example 3 on heavy oil are shown in the following table:

[0079] Viscosity mPa.S Degree of viscosity reduction Crude oil: fracturing fluid = 8:1 1680 8.3％

[0080] Filtration loss performance test of the fracturing fluid: The filtration loss performance of the fracturing fluid of Example 5 was tested, and no filter cake was formed and there was no wall-building performance. When the formation permeability was lower than 5 millidarcies, it was difficult for this viscoelastic liquid to enter the pore throats.

[0081] Determination of the residue content of the fracturing fluid: Take 50 ml of the gel-breaking liquid after the gel-breaking of the fracturing fluid of Example 4 with water, which was a transparent and slightly milky white liquid. After centrifugation at 3000 r / min for 30 minutes, the liquid did not separate and no precipitate residue appeared, showing very good stability.

[0082] Proppant-carrying performance test of the fracturing fluid: The proppant-carrying performance test results showed that the natural settlement speed of the proppant of the fracturing fluid was 0.08-0.18 mmS -1 to meet the proppant suspension requirements of the fracturing fluid. The proppant suspension performance of fracturing fluids with different concentrations under different salinities is shown in the following table:

[0083]

[0084] As can be seen from the above table, the proppant suspension ability of the fracturing fluid increases with the increase of the surfactant concentration and decreases with the increase of the salinity. The fracturing fluid formed with a surfactant concentration of 3% can meet the proppant suspension requirements of the fracturing fluid when the salinity is higher than 1000 mg / L -1 and lower than 8000 mg / L -1 .

[0085] In summary, the twin-tail type betaine surfactant fracturing fluid of the present invention has unique rheological properties, high liquid working efficiency, ideal fracture creating ability, simple on-site preparation, wide applicable temperature range, complete gel breaking and little formation damage, etc. It is an ideal low-damage clean surfactant. Moreover, the prepared fracturing fluid can be efficiently dissolved in various solutions, has good compatibility and a wide application range.

[0086] The above are only the preferred embodiments of the present invention and are not used to limit the technical solutions of the present invention in any way. Those skilled in the art should understand that, without departing from the spirit and principle of the present invention, the technical solutions can be subject to several simple modifications and substitutions, and these modifications and substitutions also fall within the protection scope covered by the claims.

Claims

1. A twin-tailed betaine surfactant, characterized in that, The surfactant has the following structure: ， Among them, R1 and R2 are saturated or unsaturated carbon chains with a carbon chain length in the range of C 16 -C 22 range, R 1、 R2 contains at least one unsaturated bond, and R1 is C 16 H 33 、C 17 H 35 、C 18 H 37 、C 19 H 39 、C 20 H 41 、C 21 H 43 or C 22 H 45 ; R2 is C 16 H 31 、C 17 H 33 、C 18 H 35 、C 19 H 37 、C 20 H 39 、C 21 H 41 or C 22 H 43 .

2. The preparation method of the twin-tailed betaine surfactant according to claim 1, characterized in that, The method includes the following steps: Dissolve N,N-dimethyl-1,3-propanediamine and halogenated hydrocarbon in a solvent for alkylation reaction. After the reaction is completed, an intermediate product, double long-chain tertiary amine, is formed. Add sodium chloroacetate solution to the intermediate product, double long-chain tertiary amine, for quaternization reaction. After the reaction, remove the solvent to obtain the final product, twin-tailed betaine surfactant.

3. The preparation method of the twin-tailed betaine surfactant according to claim 2, characterized in that, The solvent is a mixed solution of absolute ethanol and cesium carbonate, wherein the molar ratio of absolute ethanol to halogenated hydrocarbon is 2:1, and the molar ratio of cesium carbonate to N,N-dimethyl-1,3-propanediamine is 2:

1.

4. The preparation method of the twin-tailed betaine surfactant according to claim 2, characterized in that, The molar ratio of N,N-dimethyl-1,3-propanediamine to halogenated hydrocarbon is (1 - 1.5):(2 - 2.5).

5. The preparation method of the twin-tailed betaine surfactant according to claim 2, characterized in that, The reaction temperature of the alkylation reaction is 60°C to 75°C, and the reaction time is 10 to 13 h.

6. The preparation method of the twin-tailed betaine surfactant according to claim 2, characterized in that, The molar ratio of sodium chloroacetate to the intermediate product is (1 - 1.5):(1 - 1.5).

7. The preparation method of the twin-tailed betaine surfactant according to claim 2, characterized in that, The temperature of the quaternization reaction is 60°C to 75°C, and the reaction time is 2 to 3 h with stirring.

8. The preparation method of the twin-tailed betaine surfactant according to any one of claims 2-7, characterized in that, The halogenated hydrocarbon is a mixture of X1-R1 and X2-R2, where X1 and X2 are both halogens, and R1 and R2 are C 16 -C 22 , where at least one of R1 or R2 contains a double bond.

9. A fracturing fluid, characterized in that, It includes the twin-tailed betaine surfactant described in claim 1.

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