Nano viscoelastic surfactant fracturing fluid and preparation method thereof

Abstract

The invention provides a nano viscoelastic surfactant fracturing fluid and a preparation method thereof. The fracturing fluid is prepared from the following raw materials: a gemini surfactant A, a nano core B, a dispersing agent, salt and water, the dosage of the gemini surfactant A is 0.1%-5%, the dosage of the nano core B is 0.01%-0.5%, the dosage of the dispersing agent is 0.01%-5%, and the dosage of the salt is 0.1%-5% based on the mass of the water being 100%; wherein the gemini surfactant A and the nano core B are connected in an ionic bond manner. The clean fracturing fluid prepared by adopting the nano viscoelastic surfactant with the gemini structure has the advantages of high viscosity, strong sand-carrying capacity, no residue, low damage to strata, simplicity in preparation and the like, and reservoir fracturing transformation of low-permeability oil and gas reservoirs can be effectively carried out to achieve the purpose of yield increase.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil and gas field fracturing fluids, and particularly relates to a nano viscoelastic surfactant fracturing fluid and a preparation method thereof. Background Art

[0002] With the further development of oil and gas resources in low-permeability tight reservoirs, hydraulic fracturing has become an effective stimulation means for increasing production in such reservoirs. Fracturing fluid is an indispensable component of fracturing and ultimately determines the success or failure of fracturing. Therefore, developing advanced fracturing fluids to meet complex and harsh reservoir conditions has always been an active research topic, including guar gum fracturing fluid, hydrophobically modified polyacrylamide (HMPAM) fracturing fluid, and viscoelastic surfactant (VESs) fracturing fluid. HMPAM and guar gum fracturing fluids are usually used together with other basic additives such as crosslinking agents, clay stabilizers, breaker agents, and bactericides. In addition, due to the high molecular weights of these two substances, it is difficult to completely break them down, which may lead to the blockage of pore throats by insoluble residues, causing serious formation damage, thereby reducing the efficiency of fracturing operations. At the same time, the presence of residual acrylamide monomers and degraded HMPAM is unfavorable to the underground environment. Therefore, developing stable, utilizable, and environmentally friendly alternative fracturing fluids has both theoretical and practical significance.

[0003] VESs fracturing fluid is also known as clean fracturing fluid. As a thickening agent in fracturing fluid, VESs has the following advantages compared with polymer fluids: less components, simple preparation, good proppant suspension ability, high drag reduction efficiency, and easy to be broken by formation water and oil and gas. CN106543024A discloses a cationic gemini quaternary ammonium salt surfactant, which is applied to the field of fracturing fluid, and the prepared viscoelastic surfactant fracturing fluid has excellent temperature resistance performance and stability. CN102181279A relates to a quaternary ammonium salt type surfactant fracturing fluid, which is composed of the following components by weight percentage: quaternary ammonium salt type surfactant with double ionic head groups, three ionic head groups, or four ionic head groups is 0.1 - 7%, salt is 0.1 - 2%, and water is the balance. This surfactant belongs to a surfactant with a multi-amphiphilic structure and can meet the needs of high-temperature oil and gas reservoirs with reservoir temperatures higher than 120°C. However, these two cationic quaternary ammonium salt VES fracturing fluids do not contain salt-tolerant groups and are not strengthened by nanoparticles, and their viscosity-increasing and sand-carrying abilities are limited. As is well known, nanoparticles can optimize the viscoelasticity of many fluid formulations. Researchers have reported that nanoparticles can improve the performance and thermal stability of VESs-based fluids by increasing the viscosity, rheology, and microstructure of micellar solutions.

[0004] However, how to better combine nanoparticles with surfactants to form a network structure, thereby simultaneously improving the salt tolerance and heat resistance of VESs remains a current technical challenge. Summary of the Invention

[0005] To solve the above technical problems, the object of the present invention is to provide a nano viscoelastic surfactant fracturing fluid and a preparation method thereof. By combining a nano core with a gemini surfactant, a nano viscoelastic surfactant fracturing fluid with good thickening effect and strong sand-carrying capacity can be prepared.

[0006] To achieve the above object, the present invention provides a nano viscoelastic surfactant fracturing fluid, wherein the raw material composition of the fracturing fluid includes: gemini surfactant A, nano core B, dispersant, salt and water; based on the mass of water being 100%, the dosage of the gemini surfactant A is 0.1% - 5%, the dosage of the nano core B is 0.01% - 0.5%, the dosage of the dispersant is 0.01% - 5%, and the dosage of the salt is 0.1% - 5%.

[0007] According to a specific embodiment of the present invention, preferably, the gemini surfactant A and the nano core B are connected in an ionic bond manner, and the structure after connection is as Figure 9 shown.

[0008] According to a specific embodiment of the present invention, preferably, the gemini surfactant A has the structure shown in formula (Ⅰ):

[0009]

[0010] According to a specific embodiment of the present invention, preferably, the nano core B is modified nano silica, and the modified nano silica has the structure shown in formula (Ⅱ): where the SiO2 on the left is a schematic diagram of the silica particle.

[0011]

[0012] According to a specific embodiment of the present invention, preferably, in formula (Ⅰ), R1 is selected from one or a combination of two or more of a saturated alkyl group with 12 - 30 carbons, an alkyl aryl group, an alkyl alkoxy group, and an unsaturated alkyl group, and R1 is a long-chain hydrophobic part.

[0013] In some specific implementation methods, preferably, R1 is selected from one or a combination of two or more of dodecyl, tetradecyl, hexadecyl, octadecyl, eicosyl, docosyl, tetracosyl, hexacosyl, octacosyl, dodecyloxy, tetradecyloxy, hexadecyloxy, octadecyloxy, eicosyloxy, docosyloxy, tetracosyloxy, hexacosyloxy, octacosyloxy, hexylphenyl, octylphenyl, nonylphenyl, dodecylphenyl, tetradecylphenyl, hexadecylphenyl, octadecylphenyl, dodecenyl, tetradecenyl, hexadecenyl, 9-octadecenyl, eicosenyl, etc.

[0014] According to a specific embodiment of the present invention, preferably, in formula (I), R2 is selected from one or more combinations of a straight-chain saturated alkyl group having 1 to 10 carbon atoms, a branched-chain saturated alkyl group, an alkyl aryl group, an alkyl alkoxy group, an alkyl ester group, and an unsaturated alkyl group.

[0015] In some specific implementation methods, preferably, R2 is selected from one or more combinations of methyl, ethyl, propyl, butyl, pentyl, hexyl, 2-methylpropyl, 2-methylbutyl, 2-methylpentyl, ethoxy, propoxy, butoxy, pentyloxy, benzyl, phenethyl, acetoxy, propionyloxy, butyryloxy, acryloyl, acryloyloxyethyl, etc.

[0016] According to a specific embodiment of the present invention, preferably, in formula (II), R is a saturated or unsaturated straight-chain or branched-chain group having 1 to 24 carbon atoms, preferably selected from one or more combinations of a saturated alkyl group, an alkyl aryl group, an alkyl alkoxy group, an alkyl acyl group, and an unsaturated alkyl group.

[0017] In some specific implementation methods, preferably, R is selected from one or more combinations of methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, eicosyl, docosyl, tetracosyl, ethoxy, propoxy, butoxy, pentyloxy, dodecyloxy, tetradecyloxy, hexadecyloxy, phenyl, benzyl, phenethyl, phenylbutyl, phenylpentyl, phenyldodecyl, phenyltetradecyl, phenylhexadecyl, acetyl, propionyl, butyryl, dodecanoyl, hexadecanoyl, vinyl, propenyl, butenyl, dodecenyl, hexadecenyl, styryl, cinnamyl, phenylbutenyl, etc.

[0018] According to a specific embodiment of the present invention, preferably, the dispersant is an aqueous solution of an organic dispersant, including one or more combinations of isopropyl alcohol, isobutyl alcohol, polyethylene glycol, sodium dodecyl sulfate, cetyltrimethylammonium bromide, etc.

[0019] According to a specific embodiment of the present invention, preferably, the salt is an anti-ion salt, including one or more combinations of potassium chloride, ammonium chloride, sodium chloride, sodium salicylate, sodium para-aminosalicylate, potassium salicylate, sodium benzenesulfonate, sodium para-aminobenzenesulfonate, potassium benzenesulfonate, ammonium benzenesulfonate, etc.

[0020] The present invention also provides a preparation method of the above-mentioned nano viscoelastic surfactant fracturing fluid, wherein the preparation method includes:

[0021] Add the modified nano-silica particles into water, then add a dispersant, and stir and mix to obtain a dispersion of modified nano-particles; using the dispersion of modified nano-particles as the base fluid, sequentially add gemini surfactant A and an anti-ion salt solution to obtain a nano viscoelastic surfactant fracturing fluid, that is, a modified nano-particle enhanced clean fracturing fluid system.

[0022] In the above preparation method, preferably, the specific steps of the preparation method of the nano viscoelastic surfactant fracturing fluid include:

[0023] Based on the mass of water being 100%, add 0.01 - 0.5% of modified nano-silica particles into water, add 0.01% - 5% of a dispersant, magnetically stir for 60 minutes until uniform, then put the solution into an ultrasonic cleaner, and ultrasonically disperse at 25 - 80 °C (preferably 50 °C) for 3 hours until the solution is clear and transparent to obtain a dispersion of modified nano-particles. Using the dispersion of modified nano-particles as the base fluid, sequentially add 0.1% - 5% of gemini surfactant A and 0.1% - 5% of an anti-ion salt to obtain a nano viscoelastic surfactant fracturing fluid, that is, a modified nano-particle enhanced clean fracturing fluid system.

[0024] According to the specific embodiments of the present invention, preferably, the preparation steps of the modified nano-silica particles include:

[0025] Mix hydrophilic silica nano-particles and a dispersion solvent in a mass ratio of 1 - 5:100 - 500 to obtain a nano-silica stock solution; mix 3-aminopropyltriethoxysilane and the nano-silica stock solution in a mass ratio of 1 - 5:100 - 200 to obtain a reaction solution for sulfonic acid-modified nano-silica; perform centrifugal separation on the reaction solution to obtain a crude product of sulfonic acid-modified nano-silica; perform vacuum drying and grinding on the crude product to finally obtain sulfonic acid-modified nano-silica particles.

[0026] In some specific embodiments, preferably, the dispersion solvent includes one or a combination of two or more of N,N-dimethylformamide, acetone, ethanol, propylene glycol, isopropanol, dichloromethane, etc.

[0027] According to the specific embodiments of the present invention, preferably, the preparation method of 3-aminopropyltriethoxysilane is: react 3-aminopropyltriethoxysilane and sodium chloroalkylsulfonate in an aqueous solution of isopropanol in a mass ratio of 2 - 10:1 - 5, adjust the pH to 8.5 - 9.5 (preferably 9) with NaOH, and heat at 50 - 90 °C for 5 - 10 hours to obtain 3-aminopropyltriethoxysilane.

[0028] In some specific embodiments, preferably, the sodium chloroalkyl sulfonate includes one or a combination of two or more of sodium 2-chloroethyl sulfonate, sodium 3-chloropropyl sulfonate, sodium dodecyl chloro sulfonate, sodium hexadecyl chloro sulfonate, sodium 4-chlorostyrene sulfonate, 4-chlorobenzenesulfonic acid sodium salt, sodium methallyl sulfonate, etc.

[0029] In the above preparation method, preferably, the specific steps of the preparation method of the modified nano-silica particles include:

[0030] React 3-aminopropyltriethoxysilane and sodium chloroalkyl sulfonate in an aqueous solution of isopropanol at a mass ratio of 2-10:1-5, adjust the pH to 8.5-9.5 (preferably 9) with NaOH, heat at 50-90 °C for 5-10 hours to obtain alkylsulfonylpropylaminotriethoxysilane. Add hydrophilic silica nanoparticles to a dispersion solvent (preferably N,N-dimethylformamide), mix at a mass ratio of 1-5:100-500, stir evenly at room temperature, and ultrasonically disperse until clear at 25-80 °C (preferably 50 °C) to obtain a nano-silica stock solution. Mix alkylsulfonylpropylaminotriethoxysilane with the nano-silica stock solution at a mass ratio of 1-5:100-200, magnetically stir at room temperature for 12-36 hours (preferably 24 hours) to obtain a sulfonic acid-modified nano-silica reaction solution. Centrifuge the reaction solution at a speed of 1000-5000 r / min for 15 minutes, take out the upper layer liquid and collect the centrifuged product, and centrifuge the upper layer liquid again at a speed of 1000-5000 r / min, pour off the upper layer liquid, and further collect the centrifuged product. Wash the centrifuged product collected twice with ethanol multiple times to remove unreacted alkylsulfonylpropylaminotriethoxysilane to obtain a crude product of sulfonic acid-modified nano-silica. Vacuum dry the crude product to remove the solvent. After drying is completed, perform grinding treatment to finally obtain powdered sulfonic acid-modified nano-silica particles.

[0031] According to the specific embodiments of the present invention, preferably, the gemini surfactant A is a viscoelastic zwitterionic surfactant, and its preparation steps include:

[0032] (1) Take 4-8 g of alkyl ammonium and / or acrylamide, and then take 15-25 g of 3-chloro-2-hydroxypropanesulfonic acid sodium salt, add 50-250 ml of solvent, adjust the pH to 8.5-9.5 (preferably 9), and reflux at 60-90 °C for 4-6 hours;

[0033] (2) Add 8-12 g of dihaloalkane and continue to reflux at 60-90 °C for 4-10 hours;

[0034] (3) Then add 20 - 35 g of alkyl acyl chloride and / or chloroalkyl ester, and add 20 - 100 ml of triethylamine, and continue to reflux at 60 - 90 °C for 6 - 12 hours;

[0035] (4) Distill off the solvent from the reaction product under reduced pressure to obtain gemini surfactant A.

[0036] According to the specific embodiments of the present invention, preferably, in the step of preparing gemini surfactant A, the solvent is one or a combination of two or more of isopropanol, 70% ethanol, dichloromethane, tetrahydrofuran, cyclohexane, ethyl acetate, etc.

[0037] According to the specific embodiments of the present invention, preferably, the dihaloalkane is dibromoalkane and / or dichloroalkane.

[0038] According to the specific embodiments of the present invention, preferably, the alkylammonium includes one or a combination of two or more of methylamine, ethylamine, propylamine, 2 - methylpropylamine, butylamine, etc.

[0039] According to the specific embodiments of the present invention, preferably, the alkyl acyl chloride includes one or a combination of two or more of dodecyl acyl chloride, tetradecyl acyl chloride, hexadecyl acyl chloride, octadecyl acyl chloride, etc.

[0040] According to the specific embodiments of the present invention, preferably, the chloroalkyl ester includes one or a combination of two or more of dodecyl chloroformate, dodecyl chloroacetate, tetradecyl chloroformate, tetradecyl chloroacetate, hexadecyl chloroformate, hexadecyl chloroacetate, octadecyl chloroformate, octadecyl chloroacetate, etc.

[0041] In the above preparation method, preferably, the specific steps of the preparation method of gemini surfactant A include:

[0042] Take 4 - 8 g of alkylammonium and / or acrylamide, then take 15 - 25 g of 3 - chloro - 2 - hydroxypropanesulfonate, add 50 - 250 ml of solvent, adjust the pH to 8.5 - 9.5 (preferably 9) with NaOH, reflux at 60 - 90 °C for 4 - 6 hours; add 8 - 12 g of dihaloalkane, continue to reflux at 60 - 90 °C for 4 - 10 hours; then add 20 - 35 g of alkyl acyl chloride and / or chloroalkyl ester, and add 20 - 100 ml of triethylamine, continue to reflux at 60 - 90 °C for 6 - 12 hours; distill off the solvent from the reaction product under reduced pressure to obtain a white paste product, namely gemini surfactant A.

[0043] In the above preparation method, preferably, the preparation method of the nano - viscoelastic surfactant fracturing fluid includes: first prepare sulfonic acid - modified nano - silica particles, secondly prepare gemini surfactant A, and finally prepare the nano - viscoelastic surfactant fracturing fluid. The specific process is asFigure 1 as shown

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] (1) The present invention introduces nanomaterials into the clean fracturing fluid, further enhancing the high-temperature stability and viscosity-increasing sand-carrying capacity of the system. The modified nanoparticles have good dispersion in water. Through electrostatic attraction and the additional hydrogen bonding effect brought by the surfactant between the nanoparticles and the gemini surfactant, the intermolecular interaction energy is enhanced, improving the viscosity-increasing sand-carrying effect. The gemini surfactant and the counterion salt form worm-like micelles, forming relatively stable and tight "nanoparticle-micelle" bridging points and a double network structure, further enhancing the viscosity-increasing sand-carrying effect of the system. Introducing special functional groups such as amino groups, hydroxyl groups, and sulfonates into the structure of the nano viscoelastic surfactant can improve its salt tolerance. The viscoelasticity of the nano VES fracturing fluid is due to the transient network formed by the self-assembly and entanglement process of the surfactant induced by counterions in water.

[0046] (2) The VES clean fracturing fluid containing modified nanoparticles prepared by the present invention is easy to break gel. When it encounters formation water and oil phase, the hydrophilic head group and hydrophobic tail chain can solubilize it in oil and water, causing swelling, promoting the transformation of the micelle-nanoparticle cross-linked structure into a non-viscous spherical structure with a loose structure, and the viscosity of the system rapidly decreases, achieving complete gel breaking.

[0047] (3) The nano viscoelastic surfactant of the present invention adopts a gemini structure. The clean fracturing fluid prepared by using it has the advantages of high viscosity, strong sand-carrying ability, no residue, low formation damage, and simple preparation, and can effectively carry out reservoir fracturing transformation of low-permeability oil and gas reservoirs to achieve the purpose of increasing production. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 Schematic diagram of the synthesis method of the nano viscoelastic surfactant fracturing fluid.

[0049] Figure 2 Particle size distribution diagram of sulfonic acid-modified nano-silica particles in water.

[0050] Figure 3 Viscosity diagram of 1% gemini surfactant 1 at 90 °C with different dosages of modified nano-SiO2.

[0051] Figure 4 Viscosity diagram of nano VES fracturing fluid (0.1% modified nano-SiO2 + 1% gemini surfactant 1) at 90 °C with different dosages of KCl.

[0052] Figure 5Viscosity diagram of nano-VES fracturing fluid (0.1% modified nano-SiO2 + 2% gemini surfactant 1) at 90°C with different KCl dosages.

[0053] Figure 6 Test results of the solution surface tension of nano-VES fracturing fluid (0.1% modified nano-SiO2 + 1% KCl) with different dosages of gemini surfactant 1.

[0054] Figure 7 Test results of the contact angle of nano-VES fracturing fluid (0.1% modified nano-SiO2 + 1% KCl) on the tight sandstone surface with different dosages of gemini surfactant 1.

[0055] Figure 8 Viscosity of the nano-VES fracturing fluid prepared in Example 1 at 25°C with different NaCl and CaCl2 brine concentrations.

[0056] Figure 9 Schematic diagram of the structure of nano-viscoelastic surfactant.

[0057] Figure 10 Schematic diagram of the micelle system of nano-viscoelastic surfactant.

[0058] Figure 11 1H NMR spectrum of gemini surfactant 1.

[0059] Figure 12 1H NMR spectrum of gemini surfactant 2. Detailed implementation manners

[0060] For a clearer understanding of the technical features, objectives, and beneficial effects of the present invention, the technical solutions of the present invention are described in detail below, but it should not be construed as a limitation on the implementable scope of the present invention.

[0061] Raw material preparation:

[0062] (1) Preparation of sulfonic acid-modified nano-silica particles

[0063] At room temperature, 2.2 g of 3-aminopropyltriethoxysilane and 1.7 g of sodium 2-chloroethylsulfonate were successively added to a 500 ml three-necked flask and reacted in 200 ml of 70% isopropyl alcohol aqueous solution. The pH was adjusted to 9 with NaOH, and the mixture was heated under reflux at 90°C for 6 hours. After removing the solvent by rotary evaporation, ethylsulfonylpropylaminotriethoxysilane was obtained.

[0064] 5 g of hydrophilic silica nanoparticles were added to 100 ml of N,N-dimethylformamide, stirred at room temperature for 2 hours, and ultrasonically dispersed at 50°C until clarified to obtain a nano-silica stock solution.

[0065] Add 2 g of ethylsulfonylpropylaminotriethoxysilane to the above-mentioned nano-silica stock solution, and magnetically stir for about 24 hours at room temperature to obtain a sulfonic acid-modified nano-silica reaction solution.

[0066] After centrifuging the above reaction solution at a high speed of 5000 r / min for 15 minutes, take out the upper layer liquid and collect the centrifuged product, and centrifuge the upper layer liquid again at a speed of 5000 r / min, pour off the upper layer liquid, and further collect the centrifuged product. Wash the centrifuged product collected twice with ethanol 3 times to remove the unreacted alkylsulfonylpropylaminotriethoxysilane to obtain a crude product of sulfonic acid-modified nano-silica.

[0067] Vacuum dry the above crude product to remove the solvent; after drying is completed, perform grinding treatment to finally obtain powdered sulfonic acid-modified nano-silica particles.

[0068] The particle size distribution diagram of the prepared sulfonic acid-modified nano-silica particles in water is as Figure 2 shown, and the peak value of its particle size distribution is 75 nm.

[0069] (2) Preparation of Gemini surfactant 1

[0070] Take 4.5 g of ethylamine and 19.7 g of 3-chloro-2-hydroxypropanesulfonate, add 100 ml of 70% ethanol solvent, adjust the pH to 9 with NaOH, and reflux at 90 °C for 6 hours. Add 11.1 g of 1,4-dibromobutane, continue refluxing at 90 °C for 6 hours, and evaporate and dry the mixed solution using a rotary evaporator to obtain a concentrated intermediate product. Take this intermediate product as the reaction solution, add 22.0 g of dodecanoyl chloride and 100 ml of triethylamine, and continue refluxing at 90 °C for 8 hours. After cooling the reaction product, perform vacuum distillation to remove the solvent to obtain a white paste product with the structural formula shown in formula (Ⅲ), that is, Gemini surfactant 1.

[0071]

[0072] The 1H NMR spectrum of Gemini surfactant 1 is as Figure 11 shown: The H of the hydroxyl group is at δ 4.77; the H of the methylene group connected to the hydroxyl group is at δ 4.2; the H of the methylene group connected to the sulfonate is at δ 3.67; the H of the methylene group connected to N+ is at δ 3.25; the H of the methylene group closest to the carbonyl group on the hydrophobic long chain is at δ 2.38; the H of the CH3 on the ethyl group connected to N+ is at δ 1.56; the H of the methylene group where the hydrophobic chain is connected to the terminal methyl group is at δ 1.26; the H of the terminal methyl group on the hydrophobic chain is at δ 0.88. From this, it can be judged that the obtained product is the designed product.

[0073] (3) Preparation of Gemini surfactant 2

[0074] Take 8 g of acrylamide and 22.2 g of 3-chloro-2-hydroxypropanesulfonic acid sodium salt, add 100 ml of isopropanol solvent, adjust the pH to 9 with NaOH, and reflux at 90 °C for 6 hours. Add 8.9 g of 1,6-dichlorohexane, continue to reflux at 90 °C for 6 hours, evaporate and dry the mixture using a rotary evaporator to obtain the concentrated intermediate product. Take this intermediate product as the reaction solution, add 30.9 g of cetyl chloroformate and 100 ml of triethylamine, and continue to reflux at 90 °C for 8 hours. After cooling the reaction product, carry out vacuum distillation to remove the solvent to obtain a white paste product with the structural formula shown in Formula (IV), namely gemini surfactant 2.

[0075]

[0076] The 1H NMR spectrum of gemini surfactant 2 is as Figure 12 shown: The H on the vinyl double bond is at δ 7.40, 6.09, 5.74; the H of the hydroxyl group is at δ 4.77; the H of the methylene group connected to the hydroxyl group is at δ 4.2; the H of the methylene group connected to the sulfonate is at δ 3.67; the H of the methylene group connected to N + is at δ 3.48; the H of the methylene group where the hydrophobic chain is connected to the terminal methyl is at δ 1.26; the H of the terminal methyl of the hydrophobic chain is at δ 0.88. Thus, it can be judged that the obtained product is the designed product.

[0077] Next, explore the influence of the addition amount of raw materials (calculated based on the mass of water being 100%) and the concentration of the nano-VES fracturing fluid on the performance of the nano-VES fracturing fluid:

[0078] (1) The viscosities of 1% gemini surfactant 1 at 90 °C with different addition amounts of modified nano-SiO2 are as Figure 3 shown. Prepare a 500 mL gemini surfactant solution with a concentration of 1% at a rotation speed of 200 - 800 r / min using distilled water. After complete dissolution, add modified nano-SiO2 with different mass fractions (0.02% - 0.5%), stir for 3 - 10 minutes until completely dissolved, and then measure the viscosity at 90 °C. The results show that the modified nano-SiO2 enhances the thickening effect and heat resistance of the gemini surfactant.

[0079] (2) The viscosities of (0.1% modified nano-SiO2 + 1% gemini surfactant 1) VES fracturing fluid and (0.1% modified nano-SiO2 + 2% gemini surfactant 1) VES fracturing fluid at 90 °C with different addition amounts of KCl are respectively as Figure 4 、 Figure 5As shown. Prepare 500mL of 1% Gemini surfactant solution with distilled water at a speed of 200-800r / min, add 0.1% modified nano-SiO2 after complete dissolution, stir for 3-10 minutes, add different mass fractions of KCl, continue stirring for 3-5 minutes, and measure the viscosity at 90°C. The results show that the fracturing fluid formed by the nano viscoelastic surfactant and the counterion salt KCl has a good viscosity-enhancing effect.

[0080] (3) Effect of the amount of Gemini surfactant 1 on the surface tension of the prepared nano-VES fracturing fluid (0.1% modified nano-SiO2 + 1% KCl) Figure 6 As shown. The surface tension of the fracturing fluid was measured using a K100 surface tension meter, and the process was as follows: slowly raise the container containing the fracturing fluid to be tested until the bottom edge of the platinum sheet is close to the liquid surface, then click to measure, so that the platinum sheet and the fracturing fluid are automatically in contact and measured. The results showed that the critical micelle concentration CMC of the nano-VES fracturing fluid was 0.035%, and the surface tension at this concentration was 28.5 mN / m, which has good surface activity. When the surfactant addition was 0.3%, the surface tension of the nano-VES fracturing fluid was 27.6 mN / m. This result shows that it has a lower surface tension without adding a drainage agent, which is beneficial to the return of the fracturing fluid.

[0081] (4) Effect of the amount of Gemini surfactant 1 on the contact angle of the prepared nano-VES fracturing fluid (0.1% modified nano-SiO2 + 1% KCl) on the surface of dense sandstone Figure 7 As shown. The DSA-100 contact angle meter was used to record and analyze the video of the fracturing fluid dropping on the horizontal dense sandstone rock surface, and the contact angle was calculated based on the first clear frame when the droplet dropped on the rock surface. The results show that the contact angle increases with the increase of surfactant concentration in the fracturing fluid. Nano-VES fracturing fluid can significantly increase the water-phase non-wettability of the rock surface, thereby further reducing the capillary pressure of the dense sandstone reservoir and reducing the bound water on the dense sandstone surface.

[0082] Example 1

[0083] This embodiment provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0084] The prepared Gemini surfactant 1 was used to prepare a nano viscoelastic surfactant fracturing fluid, and the preparation process included: adding 100 ml of water, 1 g of isopropanol and 0.1 g of modified nano silica particles into a 250 ml beaker at room temperature, stirring magnetically for 60 minutes, and then placing the solution into an ultrasonic cleaner and ultrasonically dispersing it at 50° C. for 3 hours until the solution was clear and transparent, thereby obtaining a modified nano particle dispersion.

[0085] Using the modified nanoparticle dispersion as the base fluid, 1 g of gemini surfactant 1 was added to the base fluid in sequence at room temperature. After stirring for 10 minutes, 1 g of anti-ion salt KCl was added, and stirring was continued for 5 minutes to obtain a nano-viscoelastic surfactant fracturing fluid, namely modified nanoparticle enhanced clean fracturing fluid system 1.

[0086] The structural schematic diagram of the micelle system of the nano-viscoelastic surfactant fracturing fluid prepared in this example is as Figure 10 shown. Among them, Figure 10 (1) in is a nanoparticle enhanced worm-like micelle system; Figure 10 (2) in is a bilayer coating structure of nanoparticles and surfactants; Figure 10 (3) in is a nanoparticle-micelle bridging structure.

[0087] Next, the salt tolerance of this example was tested under different addition amounts of NaCl and CaCl2 to explore the influence of brine concentration on the viscosity of the nano-viscoelastic surfactant fracturing fluid. The specific test results are as follows:

[0088] The viscosities of the nano-viscoelastic surfactant fracturing fluid prepared in Example 1 at 25 °C under different NaCl and CaCl2 brine concentrations are as Figure 8 shown. It can be seen from Figure 8 that at 25 °C, the initial viscosity of the nano-VES fracturing fluid prepared in Example 1 is 75 mPa·s. When the NaCl brine concentration is 5%, the viscosity of the nano-VES fracturing fluid in this NaCl brine is 36 mPa·s (the mass ratio of the nano-VES fracturing fluid to the brine is 1:100); when the CaCl2 brine concentration is 0.3%, the viscosity of the nano-VES fracturing fluid in this CaCl2 brine is 31.5 mPa·s (the mass ratio of the nano-VES fracturing fluid to the brine is 1:100), indicating good salt tolerance.

[0089] Example 2

[0090] This example provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and its preparation method is as follows:

[0091] Take the prepared gemini surfactant 1 to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process includes: adding 100 ml of water and then 4 g of polyethylene glycol and 0.2 g of modified nano-silica particles to a 250 ml beaker at room temperature, magnetically stirring for 60 minutes, and then putting the solution into an ultrasonic cleaner for ultrasonic dispersion at 50 °C for 3 hours until the solution is clear and transparent to obtain a modified nanoparticle dispersion.

[0092] Using the modified nanoparticle dispersion as the base fluid, 2 g of gemini surfactant 1 was added to the base fluid successively at room temperature. After stirring for 10 minutes, 2 g of anti-ion salt NH4Cl was added, and stirring was continued for 5 minutes to obtain a nano-viscoelastic surfactant fracturing fluid, namely the modified nanoparticle enhanced clean fracturing fluid system 2.

[0093] Example 3

[0094] This example provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and its preparation method is as follows:

[0095] Take the prepared gemini surfactant 2 to prepare the nano-viscoelastic surfactant fracturing fluid. The preparation process includes: adding 100 ml of water to a 250 ml beaker at room temperature, then adding 2 g of cetyltrimethylammonium bromide and 0.5 g of modified nano-silica particles, and magnetically stirring for 60 minutes. Then the solution was placed in an ultrasonic cleaner and ultrasonically dispersed at 50 °C for 3 hours until the solution was clear and transparent to obtain the modified nanoparticle dispersion.

[0096] Using the modified nanoparticle dispersion as the base fluid, 4 g of gemini surfactant 2 was added to the base fluid successively at room temperature. After stirring for 10 minutes, 2 g of anti-ion salt KCl and 1 g of anti-ion salt sodium salicylate were added, and stirring was continued for 5 minutes to obtain a nano-viscoelastic surfactant fracturing fluid, namely the modified nanoparticle enhanced clean fracturing fluid system 3.

[0097] Comparative Example 1

[0098] This comparative example provides a viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and its preparation method is as follows:

[0099] Add 100 ml of water to a 250 ml beaker at room temperature, add 4 g of gemini surfactant 2, stir for 10 minutes, then add 2 g of anti-ion salt KCl and 1 g of anti-ion salt sodium salicylate, and continue stirring for 5 minutes to obtain the fracturing fluid system 4.

[0100] Comparative Example 2

[0101] This comparative example provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and its preparation method is as follows:

[0102] Take the prepared gemini surfactant 2 to prepare the nano-viscoelastic surfactant fracturing fluid. The preparation process includes: adding 100 ml of water to a 250 ml beaker at room temperature, then adding 2 g of cetyltrimethylammonium bromide and 0.5 g of modified nano-silica particles, and magnetically stirring for 60 minutes. Then the solution was placed in an ultrasonic cleaner and ultrasonically dispersed at 50 °C for 3 hours until the solution was clear and transparent to obtain the modified nanoparticle dispersion.

[0103] Using the modified nanoparticle dispersion as the base fluid, 4 g of gemini surfactant 2 was added to the base fluid at room temperature and stirred for 10 minutes to obtain the fracturing fluid system 5.

[0104] Comparative Example 3

[0105] This comparative example provides a viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and its preparation method is as follows:

[0106] At room temperature, 100 ml of water was added to a 250 ml beaker, 4 g of gemini surfactant 2 was added, and stirred for 10 minutes to obtain the fracturing fluid system 6.

[0107] Next, the apparent viscosities of the modified nanoparticle enhanced clean fracturing fluid systems 1 - 3 in Examples 1 - 3 and the fracturing fluid systems 4 - 6 in Comparative Examples 1 - 3 at 90 °C, and the viscosities after continuous shearing for 2 h under the conditions of 90 °C and 170 s -1 were tested, and the viscosity retention rate η was calculated. Its calculation formula is shown in Equation (Ⅴ); and quartz sand with a particle size of 70 - 140 mesh was added to the graduated cylinder according to 25% of the total mass, and the proppant settling time was tested at 90 °C to further reflect the sand - carrying capacity of the fracturing fluid. The longer the proppant settling time, the stronger the sand - carrying capacity; the gel - breaking performance was tested by heating with 1% kerosene as the gel - breaker at 90 °C for 2 hours. The results are shown in Table 1.

[0108]

[0109] In the formula, η is the viscosity retention rate; μ0 is the apparent viscosity of the nano - VES fracturing fluid at 90 °C; μ1 is the viscosity of the nano - VES fracturing fluid after continuous shearing for 2 h under the conditions of 90 °C and 170 s -1 and.

[0110] Table 1. Test results of apparent viscosity, shear resistance, sand - carrying performance, and gel - breaking performance of nano - VES fracturing fluid systems

[0111]

[0112] The experimental results recorded in Table 1 show that: compared with the fracturing fluid systems without nano - enhancement (Comparative Examples 1, 3) or without adding counter - ion salts (Comparative Example 2), the fracturing fluid systems in Examples 1 - 3 have better temperature and shear resistance and viscosity - increasing and sand - carrying performance. 1% mass fraction of kerosene can effectively break the gel of the modified nanoparticle enhanced clean fracturing fluid system prepared in the present invention.

Claims

1. A nano viscoelastic surfactant fracturing fluid, wherein, The raw material composition of the fracturing fluid includes: gemini surfactant A, nano-core B, dispersant, salt and water; Based on the mass of water being 100%, the dosage of the gemini surfactant A is 0.1%-5%, the dosage of the nano-core B is 0.01%-0.5%, the dosage of the dispersant is 0.01%-5%, and the dosage of the salt is 0.1%-5%; Among them, the gemini surfactant A and the nano-core B are connected in an ionic bond manner; The gemini surfactant A has the structure shown in formula (Ⅰ): The nano-core B is modified nano-silica, and this modified nano-silica has the structure shown in formula (Ⅱ): In formula (Ⅰ), R1 is selected from one or more combinations of saturated alkyls with 12-30 carbons, alkyl aryls, alkyl alkoxys, and unsaturated alkyls; R2 is selected from one or more combinations of straight-chain saturated alkyls, branched-chain saturated alkyls, alkyl aryls, alkyl alkoxys, alkyl ester groups, and unsaturated alkyls with 1-10 carbons; In formula (Ⅱ), R is selected from one or more combinations of saturated alkyls, alkyl aryls, alkyl alkoxys, alkyl acyls, and unsaturated alkyls with 1-24 carbons.

2. The fracturing fluid according to claim 1, wherein The dispersant includes one or more combinations of isopropyl alcohol, isobutyl alcohol, polyethylene glycol, sodium dodecyl sulfate, and cetyltrimethylammonium bromide.

3. The fracturing fluid according to claim 1, wherein, The salt is an anti-ion salt, including one or more combinations of potassium chloride, ammonium chloride, sodium chloride, sodium salicylate, sodium para-aminosalicylate, potassium salicylate, sodium benzenesulfonate, sodium para-aminobenzenesulfonate, potassium benzenesulfonate, and ammonium benzenesulfonate.

4. The fracturing fluid according to claim 1, wherein, In the said formula (Ⅰ), R1 is selected from one or more combinations of dodecyl, tetradecyl, hexadecyl, octadecyl, eicosyl, docosyl, tetracosyl, hexacosyl, octacosyl, dodecyloxy, tetradecyloxy, hexadecyloxy, octadecyloxy, eicosyloxy, docosyloxy, tetracosyloxy, hexacosyloxy, octacosyloxy, hexylphenyl, octylphenyl, nonylphenyl, dodecylphenyl, tetradecylphenyl, hexadecylphenyl, octadecylphenyl, dodecenyl, tetradecenyl, hexadecenyl, 9-octadecenyl, and eicosenyl; Preferably, R2 is selected from one or more combinations of methyl, ethyl, propyl, butyl, pentyl, hexyl, 2-methylpropyl, 2-methylbutyl, 2-methylpentyl, ethoxy, propoxy, butoxy, pentyloxy, benzyl, phenethyl, acetoxy, propionyloxy, butyryloxy, acryloyl, and acryloyloxyethyl.

5. The fracturing fluid according to claim 1, wherein, In the formula (II), R is selected from one or a combination of two or more of methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, eicosyl, docosyl, tetracosyl, ethoxy, propoxy, butoxy, pentyloxy, dodecyloxy, tetradecyloxy, hexadecyloxy, phenyl, benzyl, phenethyl, phenylbutyl, phenylpentyl, phenyldodecyl, phenyltetradecyl, phenylhexadecyl, acetyl, propionyl, butyryl, dodecanoyl, hexadecanoyl, vinyl, propenyl, butenyl, dodecenyl, hexadecenyl, styryl, cinnamyl, phenylbutenyl.

6. The preparation method of the nano viscoelastic surfactant fracturing fluid according to any one of claims 1-5, wherein, The preparation method includes: Adding modified nano-silica particles into water, then adding a dispersant, and stirring and mixing to obtain a modified nano-particle dispersion; using the modified nano-particle dispersion as a base liquid, and sequentially adding gemini surfactant A and an anti-ion salt solution to obtain a nano-viscoelastic surfactant fracturing fluid.

7. The preparation method according to claim 6, wherein, The preparation steps of the modified nano-silica particles include: Mixing hydrophilic nano-silica particles and a dispersion solvent at a mass ratio of 1-5:100-500 to obtain a nano-silica stock solution; mixing alkylsulfonic acid propylamino triethoxysilane and the nano-silica stock solution at a mass ratio of 1-5:100-200 to obtain a reaction solution for sulfonic acid-modified nano-silica; performing centrifugal separation on the reaction solution to obtain a crude product of sulfonic acid-modified nano-silica; performing vacuum drying and grinding on the crude product to finally obtain sulfonic acid-modified nano-silica particles; Preferably, the dispersion solvent includes one or a combination of two or more of N,N-dimethylformamide, acetone, ethanol, propylene glycol, isopropanol, and dichloromethane.

8. The preparation method according to claim 7, wherein, The preparation method of the alkylsulfonic acid propylamino triethoxysilane is: reacting 3-aminopropyltriethoxysilane and sodium chloroalkylsulfonate at a mass ratio of 2-10:1-5 in an aqueous solution of isopropanol, after the reaction ends, adjusting the pH to 8.5-9.5, and heating at 50-90 °C for 5-10 hours to obtain alkylsulfonic acid propylamino triethoxysilane; Preferably, the sodium chloroalkylsulfonate includes one or a combination of two or more of sodium chloroethylsulfonate, sodium chloropropylsulfonate, sodium chlorododecylsulfonate, sodium chlorohexadecylsulfonate, sodium chlorostyrenesulfonate, 4-chlorobenzenesulfonate, and sodium methallylsulfonate.

9. The preparation method according to claim 6, wherein, The preparation steps of the gemini surfactant A include: (1) Taking 4-8 g of alkylammonium and / or acrylamide, and then taking 15-25 g of 3-chloro-2-hydroxypropanesulfonate, adding 50-250 ml of a solvent, adjusting the pH to 8.5-9.5, and refluxing at 60-90 °C for 4-8 hours; (2) Adding 8-12 g of a dihaloalkane, and continuing to reflux at 60-90 °C for 6-10 hours; (3) Then adding 20-35 g of an alkyl acyl chloride and / or a chloroalkyl ester, and adding 20-100 ml of triethylamine, and continuing to reflux at 60-90 °C for 6-12 hours; (4) Distilling off the solvent from the reaction product under reduced pressure to obtain the gemini surfactant A.

10. The preparation method according to claim 9, wherein, The alkylammonium includes one or a combination of two or more of methylamine, ethylamine, propylamine, 2-methylpropylamine, and butylamine; Preferably, the alkyl acyl chloride includes one or a combination of two or more of dodecyl acyl chloride, tetradecyl acyl chloride, hexadecyl acyl chloride, and octadecyl acyl chloride; Preferably, the chloroalkyl ester includes one or a combination of two or more of dodecyl chloroformate, dodecyl chloroacetate, tetradecyl chloroformate, tetradecyl chloroacetate, hexadecyl chloroformate, hexadecyl chloroacetate, octadecyl chloroformate, and octadecyl chloroacetate.

11. The preparation method according to claim 9, wherein, The solvent is one or a combination of two or more of isopropanol, 70% ethanol, dichloromethane, tetrahydrofuran, cyclohexane, and ethyl acetate.

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