NANO viscoelastic surfactant-based fracturing fluid and preparation method therefor

Abstract

A nano viscoelastic surfactant-based fracturing fluid and a preparation method therefor. Raw materials of the fracturing fluid comprise: a Gemini surfactant A, a nano core B, a dispersant, salt, and water. On the basis of the mass of the water being 100%, the amount of the Gemini surfactant A is 0.1%-5%, the amount of the nano core B is 0.01%-0.5%, the amount of the dispersant is 0.01%-5%, and the amount of the salt is 0.1%-5%. The Gemini surfactant A and the nano core B are linked by means of ionic bonds. The use of a nano viscoelastic surfactant having a Gemini structure for the preparation of a clean fracturing fluid can achieve the purpose of increasing the yield.

Description

Nano-viscoelastic surfactant fracturing fluid and preparation method thereof

[0001] Cross-reference information

[0002] This application claims priority to the Chinese patent application filed with the China Patent Office on December 28, 2023, with application number 202311831011.X and invention name “A nano-viscoelastic surfactant fracturing fluid and its preparation method”, the entire contents of which are incorporated by reference into this application. Technical Field

[0003] The invention belongs to the technical field of oil and gas field fracturing fluid, and particularly relates to a nano-viscoelastic surfactant fracturing fluid and a preparation method thereof. Background Art

[0004] With the continued development of oil and gas resources in low-permeability, tight reservoirs, hydraulic fracturing has become an effective means of increasing production from these reservoirs. Fracturing fluids are an indispensable component of fracturing and ultimately determine the success or failure of fracturing. Therefore, the development of advanced fracturing fluids that can meet complex and harsh reservoir conditions has been an active research topic, including guar gum fracturing fluids, hydrophobically modified polyacrylamide (HMPAM) fracturing fluids, and viscoelastic surfactant (VES) fracturing fluids. HMPAM and guar gum fracturing fluids are typically used with other essential additives such as crosslinkers, clay stabilizers, breakers, and biocides. Furthermore, due to the high molecular weight of both substances, complete gel breaking is difficult, which can result in insoluble residues that block pore throats and cause severe formation damage, thereby reducing fracturing efficiency. Furthermore, the presence of residual acrylamide monomer and degraded HMPAM are detrimental to the subsurface environment. Therefore, the development of stable, usable, and environmentally friendly alternative fracturing fluids is of both theoretical and practical significance.

[0005] VESs fracturing fluid is also known as clean fracturing fluid. As a thickener in fracturing fluid, VESs offers the following advantages over polymer fluids: fewer components, simple preparation, excellent proppant suspension, high drag reduction efficiency, and easy gel breaking by formation water and oil and gas. CN106543024A discloses a cationic gemini quaternary ammonium salt surfactant, which is applied in the fracturing fluid field. The resulting viscoelastic surfactant fracturing fluid exhibits excellent temperature resistance and stability. CN102181279A relates to a quaternary ammonium salt surfactant fracturing fluid, composed, by weight, of 0.1-7% of a quaternary ammonium salt surfactant with a diionic head group, a triionic head group, or a tetraionic head group, 0.1-2% of salt, and the balance of water. This surfactant is a multi-amphipatic surfactant that can meet the needs of high-temperature oil and gas reservoirs with reservoir temperatures exceeding 120°C. However, these two cationic quaternary ammonium salt VES fracturing fluids do not contain salt-tolerant groups and are not fortified with nanoparticles, resulting in limited viscosification and sand-carrying capacity. Nanoparticles are well known to optimize the viscoelastic properties 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.

[0006] However, how to better combine nanoparticles with surfactants to form a network structure, thereby simultaneously improving the salt resistance and heat resistance of VESs, remains a current technical challenge.

[0007] Summary of the Invention

[0008] To address the above technical problems, the present invention aims to provide a nano-viscoelastic surfactant fracturing fluid and a method for preparing the same. By combining nanocores with gemini surfactants, a nano-viscoelastic surfactant fracturing fluid with excellent viscosity-increasing effect and strong sand-carrying capacity can be prepared.

[0009] To achieve the above-mentioned object, the present invention provides a nano-viscoelastic surfactant fracturing fluid, wherein the raw materials of the fracturing fluid include: gemini surfactant A, nanocore B, dispersant, salt and water; based on the mass of water as 100%, the amount of the gemini surfactant A is 0.1%-5%, the amount of the nanocore B is 0.01%-0.5%, the amount of the dispersant is 0.01%-5%, and the amount of the salt is 0.1%-5%.

[0010] According to a specific embodiment of the present invention, preferably, the gemini surfactant A and the nanocore B are connected in an ion bond manner, and the structure after connection is shown in FIG9 .

[0011] According to a specific embodiment of the present invention, preferably, the gemini surfactant A has a structure shown in formula (I):

[0012] According to a specific embodiment of the present invention, preferably, the nano-core B is modified nano-silica, and the modified nano-silica has a structure shown in formula (II): wherein SiO2 on the left is a schematic diagram of silica particles.

[0013] 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, an alkylaryl group, an alkyl alkoxy group, and an unsaturated alkyl group having 12 to 30 carbon atoms, and R1 is a long-chain hydrophobic portion.

[0014] 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, eicosyl, docosyl, tetracosyl, hexacosyl, octacosyl, hexylphenyl, octylphenyl, nonylphenyl, dodecylphenyl, tetradecylphenyl, hexadecylphenyl, octadecylphenyl, dodecenyl, tetradecenyl, hexadecenyl, 9-octadecenyl, eicosyl, etc.

[0015] According to a specific embodiment of the present invention, preferably, in formula (I), R2 is selected from one or a combination of two or more of a straight-chain saturated alkyl group of 1-10 carbon atoms, a branched saturated alkyl group, an alkylaryl group, an alkyl alkoxy group, an alkyl ester group, and an unsaturated alkyl group.

[0016] In some specific implementation methods, preferably, R2 is selected from one or a combination of two or more 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.

[0017] According to a specific embodiment of the present invention, preferably, in formula (II), R is a saturated or unsaturated straight chain or branched group of 1 to 24 carbon atoms, preferably selected from one or a combination of two or more of saturated alkyl, alkylaryl, alkylalkoxy, alkylacyl, and unsaturated alkyl.

[0018] In some specific implementation methods, preferably, 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, phenbutyl, phenpentyl, phenyldodecyl, phenyltetradecyl, phenylhexadecyl, acetyl, propionyl, butyryl, dodecanoyl, hexadecanoyl, vinyl, propenyl, butenyl, dodecenyl, hexadecenyl, phenylphenyl, phenylpropenyl, phenylbutenyl, etc.

[0019] According to a specific embodiment of the present invention, preferably, the dispersant is an aqueous solution of an organic dispersant, including one or a combination of two or more of isopropyl alcohol, isobutyl alcohol, polyethylene glycol, sodium lauryl sulfate, hexadecyltrimethylammonium bromide, etc.

[0020] According to a specific embodiment of the present invention, preferably, the salt is a counterion salt, including one or a combination of two or more of potassium chloride, ammonium chloride, sodium chloride, sodium salicylate, sodium p-aminosalicylate, potassium salicylate, sodium benzenesulfonate, sodium p-aminobenzenesulfonate, potassium benzenesulfonate, ammonium benzenesulfonate, etc.

[0021] The present invention also provides a method for preparing the above-mentioned nano-viscoelastic surfactant fracturing fluid, wherein the preparation method comprises:

[0022] Modified nano-silica particles are added to water, and then a dispersant is added, and the mixture is stirred to obtain a modified nano-particle dispersion. The modified nano-particle dispersion is used as a base fluid, and gemini surfactant A and a counterion salt solution are added in sequence to obtain a nano-viscoelastic surfactant fracturing fluid, namely, a modified nano-particle enhanced clean fracturing fluid system.

[0023] In the above preparation method, preferably, the specific steps of the preparation method of the nano-viscoelastic surfactant fracturing fluid include:

[0024] Based on 100% water mass, 0.01-0.5% of modified nano-silica particles are added to water, followed by 0.01-5% of a dispersant. The mixture is magnetically stirred for 60 minutes until uniform. The solution is then placed in an ultrasonic cleaner and ultrasonically dispersed at 25-80°C (preferably 50°C) for 3 hours until the solution becomes clear and transparent, thereby obtaining a modified nano-particle dispersion. The modified nano-particle dispersion is then used as a base fluid, and 0.1-5% of a gemini surfactant A and 0.1-5% of a counterion salt are sequentially added to obtain a nano-viscoelastic surfactant fracturing fluid, i.e., a modified nano-particle enhanced clean fracturing fluid system.

[0025] According to a specific embodiment of the present invention, preferably, the preparation steps of the modified nano-silica particles include:

[0026] The method comprises mixing hydrophilic silica nanoparticles and a dispersing solvent at a mass ratio of 1-5:100-500 to obtain a nanosilica stock solution; mixing alkylsulfonic acid propylamino triethoxysilane with the nanosilica stock solution at a mass ratio of 1-5:100-200 to obtain a reaction solution of sulfonic acid-modified nanosilica; centrifuging the reaction solution to obtain a crude product of sulfonic acid-modified nanosilica; and vacuum drying and grinding the crude product to finally obtain sulfonic acid-modified modified silica nanoparticles.

[0027] In some specific embodiments, preferably, the dispersing solvent includes one or a combination of two or more of N,N-dimethylformamide, acetone, ethanol, propylene glycol, isopropyl alcohol, dichloromethane, etc.

[0028] According to a specific embodiment of the present invention, preferably, the preparation method of the alkylsulfonate propylamino triethoxysilane is: reacting 3-aminopropyltriethoxysilane and sodium chloroalkylsulfonate in a mass ratio of 2-10:1-5 in an aqueous solution of isopropanol, adjusting the pH to 8.5-9.5 (preferably 9) with NaOH, and heating at 50-90° C. for 5-10 hours to obtain alkylsulfonate propylamino triethoxysilane.

[0029] In some specific embodiments, preferably, the sodium chloroalkyl sulfonate includes one or a combination of two or more of sodium chloroethyl sulfonate, sodium chloropropyl sulfonate, sodium chlorododecyl sulfonate, sodium chlorohexadecyl sulfonate, sodium chlorostyrene sulfonate, sodium 4-chlorobenzene sulfonate, sodium methyl propylene sulfonate, etc.

[0030] In the above preparation method, preferably, the specific steps of the preparation method of the modified nano-silica particles include:

[0031] 3-Aminopropyltriethoxysilane and sodium chloroalkylsulfonate are reacted in an aqueous solution of isopropanol at a mass ratio of 2-10:1-5, the pH is adjusted to 8.5-9.5 (preferably 9) with NaOH, and heated at 50-90°C for 5-10 hours to obtain alkylsulfonate propylaminotriethoxysilane. Hydrophilic silica nanoparticles are added to a dispersing solvent (preferably N,N-dimethylformamide) and mixed at a mass ratio of 1-5:100-500. The mixture is stirred evenly at room temperature and ultrasonically dispersed at 25-80°C (preferably 50°C) until clear to obtain a nanosilica stock solution. Alkylsulfonate propylaminotriethoxysilane is mixed with the nanosilica stock solution at a mass ratio of 1-5:100-200 and magnetically stirred at room temperature for 12-36 hours (preferably 24 hours) to obtain a sulfonic acid-modified nanosilica reaction solution. The reaction solution was centrifuged at a speed of 1000-5000 r / min for 15 minutes, the supernatant liquid was removed and the centrifuged product was collected. The supernatant liquid was again centrifuged at a speed of 1000-5000 r / min, the supernatant liquid was discarded, and the centrifuged product was further collected. The centrifuged products collected twice were washed with ethanol multiple times to remove unreacted alkylsulfonic acid propylamino triethoxysilane to obtain a crude product of sulfonic acid-modified modified nano-silica. The crude product was vacuum-dried to remove the solvent. After drying, it was ground to finally obtain powdery sulfonic acid-modified modified silica nanoparticles.

[0032] According to a specific embodiment of the present invention, preferably, the Gemini surfactant A is a viscoelastic zwitterionic surfactant, and its preparation steps include:

[0033] (1) Take 4-8 g of alkylammonium and / or acrylamide, then take 15-25 g of sodium 3-chloro-2-hydroxypropanesulfonate, 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;

[0034] (2) Add 8-12 g of dihaloalkane and continue to reflux at 60-90° C. for 4-10 hours;

[0035] (3) Then add 20-35 g of alkyl acid chloride and / or chloroalkyl ester and 20-100 ml of triethylamine, and continue to reflux at 60-90 ° C for 6-12 hours;

[0036] (4) The reaction product is distilled under reduced pressure to remove the solvent, thereby obtaining Gemini surfactant A.

[0037] According to a specific embodiment of the present invention, preferably, in the step of preparing the gemini surfactant A, the solvent is one or a combination of two or more of isopropyl alcohol, 70% ethanol, dichloromethane, tetrahydrofuran, cyclohexane, ethyl acetate, etc.

[0038] According to a specific embodiment of the present invention, preferably, the dihaloalkane is a dibromoalkane and / or a dichloroalkane.

[0039] According to a specific embodiment of the present invention, preferably, the alkylammonium includes one or a combination of two or more of methylamine, ethylamine, propylamine, 2-methylpropylamine, butylamine, etc.

[0040] According to a specific embodiment of the present invention, preferably, the alkyl acid chloride includes one or a combination of two or more of dodecyl acid chloride, tetradecyl acid chloride, hexadecyl acid chloride, octadecyl acid chloride, etc.

[0041] According to a specific embodiment 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.

[0042] In the above preparation method, preferably, the specific steps of the preparation method of the gemini surfactant A include:

[0043] 4-8 g of alkylammonium and / or acrylamide and 15-25 g of sodium 3-chloro-2-hydroxypropanesulfonate are added to 50-250 ml of solvent, the pH is adjusted to 8.5-9.5 (preferably 9) with NaOH, and the mixture is refluxed at 60-90° C. for 4-6 hours. 8-12 g of a dihalogenated alkane is added, and the mixture is refluxed at 60-90° C. for a further 4-10 hours. 20-35 g of an alkyl chloride and / or a chloroalkyl ester is then added, along with 20-100 ml of triethylamine, and the mixture is refluxed at 60-90° C. for a further 6-12 hours. The reaction product is distilled under reduced pressure to remove the solvent, yielding a white paste product, namely, gemini surfactant A.

[0044] In the above preparation method, preferably, the preparation method of the nano-viscoelastic surfactant fracturing fluid comprises: first preparing sulfonic acid-modified nano-silica particles, secondly preparing gemini surfactant A, and finally preparing the nano-viscoelastic surfactant fracturing fluid. The specific process is shown in Figure 1.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] (1) The present invention introduces nanomaterials into clean fracturing fluid, further enhancing the high-temperature stability and viscosity-enhancing sand-carrying capacity of the system. The modified nanoparticles have a good dispersion effect in water, and the electrostatic attraction and additional hydrogen bonding brought by the surfactants enhance the interaction energy between molecules and improve the viscosity-enhancing sand-carrying effect. Nano-gemini surfactants form worm-like micelles with counterion salts, forming relatively stable and tight "nanoparticle-micelle" bridge points and double network structures, further enhancing the viscosity-enhancing sand-carrying effect of the system. Introducing special functional groups such as amino groups, hydroxyl groups and sulfonates into the structure of nano-viscoelastic surfactants can improve their salt resistance. The viscoelasticity of nano-VES fracturing fluid is due to the transient network formed by the self-assembly and entanglement process of surfactants triggered by counterions in water.

[0047] (2) The VES clean fracturing fluid containing modified nanoparticles prepared by the present invention is easy to break. When encountering formation water and oil phase, the hydrophilic head group and hydrophobic tail chain can solubilize it in oil and water, causing it to swell, prompting the micelle-nanoparticle cross-linked structure to transform into a loose non-sticky spherical structure, and the viscosity of the system is rapidly reduced, achieving complete breaking of the gel.

[0048] (3) The nano-viscoelastic surfactant of the present invention adopts a twin structure. The clean fracturing fluid prepared using it has the advantages of high viscosity, strong sand carrying capacity, no residue, low damage to the formation, and simple preparation. It 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

[0049] FIG1 is a schematic diagram of the synthesis method of nano-viscoelastic surfactant fracturing fluid.

[0050] FIG2 is a graph showing the particle size distribution of sulfonic acid-modified modified nano-silica particles in water.

[0051] FIG3 is a viscosity diagram of 1% Gemini surfactant 1 at 90° C. with different addition amounts of modified nano-SiO 2 .

[0052] FIG4 is a viscosity diagram of nano-VES fracturing fluid (0.1% modified nano-SiO2+1% Gemini surfactant 1) at 90° C. with different KCl addition amounts.

[0053] FIG5 is a graph showing the viscosity of nano-VES fracturing fluid (0.1% modified nano-SiO2+2% Gemini surfactant 1) at 90° C. with different KCl addition amounts.

[0054] FIG6 shows the test results of the surface tension of the nano-VES fracturing fluid (0.1% modified nano-SiO2+1% KCl) at different addition amounts of Gemini surfactant 1.

[0055] FIG7 shows the test results of the contact angle of nano-VES fracturing fluid (0.1% modified nano-SiO2+1% KCl) on the surface of dense sandstone at different addition amounts of Gemini surfactant 1.

[0056] FIG8 shows the viscosity of the nano-VES fracturing fluid prepared in Example 1 at 25° C. under different NaCl and CaCl 2 brine concentrations.

[0057] FIG9 is a schematic structural diagram of a nano-viscoelastic surfactant.

[0058] FIG10 is a schematic diagram of a micellar system of a nano-viscoelastic surfactant.

[0059] FIG11 is a H-NMR spectrum of Gemini surfactant 1.

[0060] FIG12 is the H-NMR spectrum of Gemini surfactant 2. DETAILED DESCRIPTION

[0061] In order to have a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention is now described in detail below, but it should not be understood as limiting the scope of implementation of the present invention.

[0062] Raw materials preparation:

[0063] (1) Preparation of sulfonic acid-modified nanosilica particles

[0064] To a 500 ml three-necked flask, 2.2 g of 3-aminopropyltriethoxysilane and 1.7 g of sodium chloroethylsulfonate were added in sequence in 200 ml of 70% aqueous isopropanol at room temperature. The pH was adjusted to 9 with NaOH, and the mixture was heated under reflux at 90°C for 6 hours. The solvent was removed by rotary evaporation to obtain ethylsulfonic acid propylaminotriethoxysilane.

[0065] 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 clear to obtain a nano-silica stock solution.

[0066] 2 g of ethylsulfonic acid propylamino triethoxysilane was added to the above nano-silica stock solution, and magnetic stirring was carried out at room temperature for about 24 hours to obtain a sulfonic acid-modified nano-silica reaction solution.

[0067] The reaction solution was centrifuged at 5000 r / min for 15 minutes, and the supernatant was removed and the centrifuged product was collected. The supernatant was centrifuged again at 5000 r / min, the supernatant was discarded, and the centrifuged product was further collected. The two collected centrifuged products were washed three times with ethanol to remove unreacted alkylsulfonic acid propylamino triethoxysilane, thereby obtaining a crude product of sulfonic acid-modified nano-silica.

[0068] The crude product is vacuum dried to remove the solvent; after drying, it is ground to finally obtain powdery sulfonic acid-modified modified nano-silica particles.

[0069] The particle size distribution of the prepared sulfonic acid-modified modified nano-silica particles in water is shown in FIG2 , and the peak value of the particle size distribution is 75 nm.

[0070] (2) Preparation of Gemini surfactant 1

[0071] 4.5g of ethylammonium and 19.7g of sodium 3-chloro-2-hydroxypropanesulfonate were added to 100ml of 70% ethanol, the pH was adjusted to 9 with NaOH, and the mixture was refluxed at 90°C for 6 hours. 11.1g of 1,4-dibromobutane was added, and the mixture was refluxed at 90°C for another 6 hours. The mixture was evaporated and dried using a rotary evaporator to obtain a concentrated intermediate product. This intermediate product was used as the reaction solution, and 22.0g of dodecyl acyl chloride and 100ml of triethylamine were added, and the mixture was refluxed at 90°C for another 8 hours. The reaction product was cooled and the solvent was removed by vacuum distillation to obtain a white paste having the structural formula shown in Formula (III), namely, Gemini surfactant 1.

[0072] The H NMR spectrum of Gemini surfactant 1 is shown in Figure 11: δ4.77 indicates the H of the hydroxyl group; δ4.2 indicates the H of the methine group connected to the hydroxyl group; δ3.67 indicates the H of the methylene group connected to the sulfonate; δ3.25 indicates the H of the methylene group connected to the N+; δ2.38 indicates the H of the methylene group closest to the carbonyl group on the long hydrophobic chain; δ1.56 indicates the H of the CH3 group on the ethyl group connected to the N+; δ1.26 indicates the H of the methylene group connecting the hydrophobic chain to the terminal methyl group; and δ0.88 indicates the H of the terminal methyl group of the hydrophobic chain. From this, it can be determined that the obtained product is the designed product.

[0073] (3) Preparation of Gemini surfactant 2

[0074] 8g of acrylamide and 22.2g of sodium 3-chloro-2-hydroxypropanesulfonate were added to 100ml of isopropanol solvent, the pH was adjusted to 9 with NaOH, and the mixture was refluxed at 90°C for 6 hours. 8.9g of 1,6-dichlorohexane was added, and the mixture was refluxed at 90°C for another 6 hours. The mixture was evaporated and dried using a rotary evaporator to obtain a concentrated intermediate product. This intermediate product was used as the reaction solution, and 30.9g of hexadecyl chloroformate and 100ml of triethylamine were added, and the mixture was refluxed at 90°C for another 8 hours. The reaction product was cooled and the solvent was removed by vacuum distillation to obtain a white paste product with the structural formula shown in Formula (IV), namely, Gemini surfactant 2.

[0075] The H-NMR spectrum of Gemini surfactant 2 is shown in Figure 12: δ7.40, 6.09, and 5.74 are H on the vinyl double bond; δ4.77 is the H of the hydroxyl group; δ4.2 is the H of the methine group connected to the hydroxyl group; δ3.67 is the H of the methylene group connected to the sulfonate; δ3.48 is the H of the N + The H on the connected methylene group; the H on the methylene group connecting the hydrophobic chain to the terminal methyl group at δ1.26; the H on the terminal methyl group of the hydrophobic chain at δ0.88. From this, it can be judged that the obtained product is the designed product.

[0076] The following is an investigation into the effects of the amount of raw materials added (based on the mass of water as 100%) and the concentration of nano-VES fracturing fluid on the performance of nano-VES fracturing fluid:

[0077] (1) The viscosity of 1% Gemini surfactant 1 at 90°C with different amounts of modified nano-SiO2 is shown in Figure 3. 500 mL of a 1% Gemini surfactant solution was prepared in distilled water at a speed of 200-800 r / min. After complete dissolution, different mass fractions (0.02%-0.5%) of modified nano-SiO2 were added. After stirring for 3-10 minutes until complete dissolution, the viscosity was measured at 90°C. The results show that modified nano-SiO2 enhances the thickening effect and heat resistance of the Gemini surfactant.

[0078] (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 under different KCl addition amounts are shown in Figures 4 and 5, respectively. 500 mL of a 1% Gemini surfactant solution was prepared in distilled water at a speed of 200-800 r / min. After complete dissolution, 0.1% modified nano-SiO2 was added and stirred for 3-10 minutes. After complete dissolution, different mass fractions of KCl were added and stirred for another 3-5 minutes. The viscosity was measured 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-increasing effect.

[0079] (3) The 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) is shown in Figure 6. The surface tension of the fracturing fluid was measured using a K100 surface tension meter. The process is 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, and then click the measurement button to automatically contact the platinum sheet and the fracturing fluid and measure. The results show that the critical micelle concentration CMC of the nano-VES fracturing fluid is 0.035%, and the surface tension at this concentration is 28.5mN / m, which has good surface activity. When the surfactant addition amount is 0.3%, the surface tension of the nano-VES fracturing fluid is 27.6mN / m. This result shows that it has a lower surface tension without adding a drainage agent, which is conducive to the return of the fracturing fluid.

[0080] (4) The 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 is shown in Figure 7. 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. The contact angle was calculated based on the first clear frame when the droplet landed on the rock surface. The results show that the contact angle increases with the increase of the surfactant concentration in the fracturing fluid. The 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.

[0081] Example 1

[0082] This embodiment provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0083] The prepared Gemini surfactant 1 was used to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process included adding 100 ml of water, 1 g of isopropyl alcohol, and 0.1 g of modified nano-silica particles into a 250 ml beaker at room temperature, magnetically stirring for 60 minutes, and then placing the solution in an ultrasonic cleaner and ultrasonically dispersing it at 50°C for 3 hours until the solution became clear and transparent, thereby obtaining a modified nanoparticle dispersion.

[0084] The modified nanoparticle dispersion was used as the base fluid, and 1 g of Gemini surfactant 1 was added to the base fluid at room temperature. After stirring for 10 minutes, 1 g of counterion salt KCl was added and stirred for 5 minutes to obtain a nano-viscoelastic surfactant fracturing fluid, i.e., the modified nanoparticle-enhanced clean fracturing fluid system 1.

[0085] The schematic structural diagram of the micelle system of the nano-viscoelastic surfactant fracturing fluid prepared in this embodiment is shown in Figure 10. Among them, (1) in Figure 10 is a nanoparticle-reinforced worm-like micelle system; (2) in Figure 10 is a double-layer coating structure of nanoparticles and surfactants; and (3) in Figure 10 is a nanoparticle-micelle bridge structure.

[0086] The following salt tolerance test was conducted on this example at different NaCl and CaCl2 dosages to explore the effect of brine concentration on the viscosity of the nano-viscoelastic surfactant fracturing fluid. The specific test results are as follows:

[0087] The viscosity of the nano-viscoelastic surfactant fracturing fluid prepared in Example 1 at 25°C under different NaCl and CaCl2 brine concentrations is shown in Figure 8. As can be seen from Figure 8, at 25°C, the initial viscosity of the nano-VES fracturing fluid prepared in Example 1 is 75mPa·s. When the NaCl brine concentration is 5%, the viscosity of the nano-VES fracturing fluid in the NaCl brine is 36mPa·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 the CaCl2 brine is 31.5mPa·s (the mass ratio of the nano-VES fracturing fluid to the brine is 1:100), indicating good salt tolerance.

[0088] Example 2

[0089] This embodiment provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0090] The prepared Gemini surfactant 1 was used to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process included adding 100 ml of water, 4 g of polyethylene glycol, and 0.2 g of modified nano-silica particles into a 250 ml beaker at room temperature, magnetically stirring for 60 minutes, and then placing the solution in an ultrasonic cleaner and ultrasonically dispersing it at 50°C for 3 hours until the solution became clear and transparent, thereby obtaining a modified nanoparticle dispersion.

[0091] The modified nanoparticle dispersion was used as the base fluid, and 2 g of the gemini surfactant 1 was added to the base fluid at room temperature. After stirring for 10 minutes, 2 g of the counterion 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.

[0092] Example 3

[0093] This embodiment provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0094] The prepared Gemini surfactant 2 was used to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process included: adding 100 ml of water, 2 g of hexadecyltrimethylammonium bromide and 0.5 g of modified nano-silica particles to a 250 ml beaker at room temperature, and magnetically stirring for 60 minutes. The solution was then placed in an ultrasonic cleaner and ultrasonically dispersed at 50°C for 3 hours until the solution became clear and transparent, thereby obtaining a modified nanoparticle dispersion.

[0095] The modified nanoparticle dispersion was used as the base fluid, and 4 g of the gemini surfactant 2 was sequentially added to the base fluid at room temperature. After stirring for 10 minutes, 2 g of the counterion salt KCl and 1 g of the counterion 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.

[0096] Example 4

[0097] This embodiment provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0098] The prepared Gemini surfactant 1 was used to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process included adding 100 ml of water, 5 g of isobutanol, and 0.4 g of modified nano-silica particles into a 250 ml beaker at room temperature, magnetically stirring for 60 minutes, and then placing the solution in an ultrasonic cleaner and ultrasonically dispersing it at 80°C for 3 hours until the solution became clear and transparent, thereby obtaining a modified nanoparticle dispersion.

[0099] The modified nanoparticle dispersion was used as the base fluid, and 5 g of the gemini surfactant 1 was added to the base fluid at room temperature. After stirring for 10 minutes, 5 g of sodium p-aminobenzenesulfonate was added and stirred for 5 minutes to obtain a nano-viscoelastic surfactant fracturing fluid, i.e., the modified nanoparticle-enhanced clean fracturing fluid system 4.

[0100] Example 5

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

[0102] The prepared Gemini surfactant 1 was used to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process included adding 100 ml of water, 0.01 g of sodium lauryl sulfate, and 0.01 g of modified nano-silica particles into a 250 ml beaker at room temperature, and magnetically stirring for 60 minutes. The solution was then placed in an ultrasonic cleaner and ultrasonically dispersed at 25°C for 3 hours until the solution became clear and transparent, thereby obtaining a modified nano-particle dispersion.

[0103] The modified nanoparticle dispersion was used as the base fluid, and 0.1 g of Gemini surfactant 1 was added to the base fluid in sequence at room temperature. After stirring for 10 minutes, 0.05 g of sodium chloride and 0.05 g of sodium p-aminosalicylate 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 5.

[0104] Example 6

[0105] This embodiment provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0106] The prepared Gemini surfactant 2 was used to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process included adding 100 ml of water, 0.05 g of isopropyl alcohol, and 0.05 g of modified nano-silica particles into a 250 ml beaker at room temperature, magnetically stirring for 60 minutes, and then placing the solution in an ultrasonic cleaner and ultrasonically dispersing it at 30°C for 3 hours until the solution became clear and transparent, thereby obtaining a modified nanoparticle dispersion.

[0107] The modified nanoparticle dispersion was used as the base fluid, and 0.2 g of Gemini surfactant 2 was added to the base fluid at room temperature. After stirring for 10 minutes, 0.2 g of potassium salicylate 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 6.

[0108] Example 7

[0109] This embodiment provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0110] The prepared Gemini surfactant 2 was used to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process included adding 100 ml of water, 0.1 g of isopropyl alcohol, and 0.1 g of modified nano-silica particles into a 250 ml beaker at room temperature, and magnetically stirring for 60 minutes. The solution was then placed in an ultrasonic cleaner and ultrasonically dispersed at 40°C for 3 hours until the solution became clear and transparent, thereby obtaining a modified nanoparticle dispersion.

[0111] The modified nanoparticle dispersion was used as the base fluid, and 0.5 g of Gemini surfactant 2 was added to the base fluid at room temperature. After stirring for 10 minutes, 0.5 g of ammonium benzenesulfonate 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 7.

[0112] Example 8

[0113] This embodiment provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0114] The prepared Gemini surfactant 2 was used to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process included adding 100 ml of water, 3 g of isopropyl alcohol, and 0.3 g of modified nano-silica particles into a 250 ml beaker at room temperature, and magnetically stirring for 60 minutes. The solution was then placed in an ultrasonic cleaner and ultrasonically dispersed at 60°C for 3 hours until the solution became clear and transparent, thereby obtaining a modified nanoparticle dispersion.

[0115] Using the modified nanoparticle dispersion as the base fluid, 3 g of gemini surfactant 2 was added to the base fluid at room temperature. After stirring for 10 minutes, 4 g of sodium benzenesulfonate 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 8.

[0116] Comparative Example 1

[0117] This comparative example provides a viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0118] At room temperature, 100 ml of water was added to a 250 ml beaker, and 4 g of Gemini surfactant 2 was added. After stirring for 10 minutes, 2 g of counterion salt KCl and 1 g of counterion salt sodium salicylate were added. The mixture was stirred for 5 minutes to obtain a fracturing fluid system 4.

[0119] Comparative Example 2

[0120] This comparative example provides a nano-viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0121] The prepared Gemini surfactant 2 was used to prepare a nano-viscoelastic surfactant fracturing fluid. The preparation process included adding 100 ml of water, 2 g of hexadecyltrimethylammonium bromide and 0.5 g of modified nano-silica particles into a 250 ml beaker at room temperature, and magnetically stirring for 60 minutes. The solution was then placed in an ultrasonic cleaner and ultrasonically dispersed at 50°C for 3 hours until the solution became clear and transparent, thereby obtaining a modified nano-particle dispersion.

[0122] The modified nanoparticle dispersion was used as a base fluid, 4 g of the gemini surfactant 2 was added to the base fluid at room temperature, and the mixture was stirred for 10 minutes to obtain a fracturing fluid system 5.

[0123] Comparative Example 3

[0124] This comparative example provides a viscoelastic surfactant fracturing fluid for unconventional oil and gas wells, and the preparation method thereof is as follows:

[0125] 100 ml of water was added to a 250 ml beaker at room temperature, and 4 g of Gemini surfactant 2 was added, followed by stirring for 10 minutes to obtain a fracturing fluid system 6.

[0126] The apparent viscosity 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 viscosity of the modified nanoparticle enhanced clean fracturing fluid systems 1-3 at 90°C and 170s -1 The viscosity of the fracturing fluid was tested by continuous shearing for 2 hours under the conditions of , and the viscosity retention rate η was calculated. The calculation formula is shown in formula (V); 70-140 mesh quartz sand was added to the graduated cylinder at 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; 1% kerosene was used as a breaker and heated at 90°C for 2 hours to test the gel breaking performance. The results are shown in Table 1.

[0127] Where η is the viscosity retention rate; μ0 is the apparent viscosity of the nano-VES fracturing fluid at 90 ° C; μ1 is the apparent viscosity of the nano-VES fracturing fluid at 90 ° C and 170s -1 Viscosity after continuous shearing for 2 hours under the conditions of .

[0128] Table 1. Test results of apparent viscosity, shear resistance, sand carrying capacity, and gel breaking performance of the nano-VES fracturing fluid system

[0129] The experimental results reported in Table 1 demonstrate that Examples 1-3 exhibit superior heat and shear resistance, as well as enhanced sand-carrying performance, compared to fracturing fluids without nanoparticle enhancement (Comparative Examples 1 and 3) or without the addition of counterion salts (Comparative Example 2). A 1% mass fraction of kerosene enables effective gel breaking in the modified nanoparticle-enhanced clean fracturing fluid prepared in this 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 by an ionic bond; The gemini surfactant A has the structure shown in formula (I): The nano-core B is modified nano-silica, and the modified nano-silica has the structure shown in formula (II): In formula (Ⅰ), R1 is selected from one or more combinations of saturated alkyl groups with 12 - 30 carbons, alkyl aryl groups, alkyl alkoxy groups, and unsaturated alkyl groups; R2 is selected from one or more combinations of straight-chain saturated alkyl groups, branched-chain saturated alkyl groups, alkyl aryl groups, alkyl alkoxy groups, alkyl ester groups, and unsaturated alkyl groups with 1 - 10 carbons; In formula (Ⅱ), R is selected from one or more combinations of saturated alkyl groups, alkyl aryl groups, alkyl alkoxy groups, alkyl acyl groups, and unsaturated alkyl groups with 1 - 24 carbons.

2. The fracturing fluid according to claim 1, wherein, The dispersant includes one or more combinations of isopropanol, isobutanol, 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.

5. The fracturing fluid according to claim 1, wherein, In the said formula (Ⅰ), 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.

6. The fracturing fluid according to claim 1, wherein, In the said formula (Ⅱ), 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, and styrylbutenyl.

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

8. The preparation method according to claim 7, wherein, The preparation steps of the modified nano-silica particles include: Mix hydrophilic silica nano-particles and a dispersion solvent at a mass ratio of 1-5:100-500 to obtain a nano-silica stock solution; mix 3-(alkylsulfonyl)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; 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.

9. The preparation method according to claim 8, wherein, The dispersion solvent includes one or a combination of two or more of N,N-dimethylformamide, acetone, ethanol, propylene glycol, isopropanol, and dichloromethane.

10. The preparation method according to claim 8, wherein, The preparation method of the 3-(alkylsulfonyl)propylamino triethoxysilane is: react 3-aminopropyltriethoxysilane and sodium chloroalkylsulfonate at a mass ratio of 2-10:1-5 in an aqueous solution of isopropanol. After the reaction, adjust the pH to 8.5-9.5 and heat at 50-90 °C for 5-10 hours to obtain 3-(alkylsulfonyl)propylamino triethoxysilane.

11. The preparation method according to claim 10, wherein, 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.

12. The preparation method according to claim 7, wherein, The preparation steps of the gemini surfactant A include: (1) Take 4-8 g of alkylammonium 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, and reflux at 60-90 °C for 4-8 hours; (2) Add 8-12 g of dihaloalkane and continue to reflux at 60-90 °C for 6-10 hours; (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; (4) Distill off the solvent from the reaction product under reduced pressure to obtain the gemini surfactant A.

13. The preparation method according to claim 12, wherein, The alkylammonium includes one or a combination of two or more of methylamine, ethylamine, propylamine, 2-methylpropylamine, and butylamine.

14. The preparation method according to claim 12, wherein, The alkyl acyl chloride includes one or a combination of two or more of dodecanoyl chloride, tetradecanoyl chloride, hexadecanoyl chloride, and octadecanoyl chloride.

15. The preparation method according to claim 12, wherein, 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.

16. The preparation method according to claim 12, 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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