Alcohol-based fracturing fluid containing modified nanoparticles and preparation method thereof

Abstract

The invention relates to the technical field of oil exploitation, and relates to an alcohol-based fracturing fluid containing modified nanoparticles and a preparation method thereof, the alcohol-based fracturing fluid comprises the following components: an alcohol solvent, modified nanoparticles, an anti-swelling agent, a thickening agent, a polyamide-amine dendritic polymer, a pH regulator, an optional suspending agent and water; the modified nano-particles are prepared by modifying nano-particles with an alkenyl silane coupling agent, then copolymerizing the modified nano-particles with alkenyl-containing imidazolium salt, and grafting polyimidazolium salt on the surfaces of the nano-particles in situ. The preparation method comprises the following steps: modifying nanoparticles by using an alkenyl silane coupling agent, copolymerizing with alkenyl-containing imidazolium salt to form polyimidazolium salt, and grafting the polyimidazolium salt on the surfaces of the nanoparticles in situ to form modified nanoparticles; the anti-swelling property, the cross-linking property and the sand-carrying capacity of the alcohol-based fracturing fluid are remarkably improved through the combined action of the modified nanoparticles and the polyamide-amine dendritic polymer, and the gel forming and hanging time is 60-90 s, so that the alcohol-based fracturing fluid is particularly suitable for medium-high-temperature deep wells.

Description

Technical Field

[0001] The present invention belongs to the technical field of petroleum extraction, and in particular relates to an alcohol-based fracturing fluid containing modified nanoparticles and a preparation method thereof. Background Art

[0002] In the shale oil extraction process, fracturing technology is one of the key means of increasing shale oil recovery. Fracturing fluid, at the heart of this technology, directly impacts fracturing effectiveness and shale oil extraction efficiency. Fracturing fluid is a fluid mixture pumped into underground shale reservoirs at extremely high pressures. Its primary function is to create and maintain flow pathways (fracture networks) within the dense shale formations, thereby releasing previously unmovable shale oil (and natural gas). Its core function is to use high pressure to create fractures in the shale and deliver proppant to maintain them open, thereby increasing the effective permeability of the reservoir and unlocking the oil and gas resources trapped in the shale pores, allowing them to flow continuously and economically to production wells.

[0003] Traditional water-based fracturing fluids present numerous challenges in practical applications. For example, clay minerals in shale reservoirs easily swell and disperse upon contact with water, resulting in reduced reservoir permeability. This is because water-based fracturing fluids are water-sensitive and unable to inhibit clay swelling. Alcohol-based fracturing fluids have attracted considerable attention due to their superior ability to inhibit viscosity swelling compared to water-based fluids. However, due to the difficulty of thickeners dissolving in pure alcohol systems and economic considerations, alcohol-based fracturing fluids cannot currently be used in 100% alcohol systems. Therefore, while the anti-swelling properties of existing alcohol-based fracturing fluids are superior to those of water-based fluids, further improvement is needed. In addition, the cross-linking and sand-carrying properties of alcohol-based fracturing fluids need to be improved. The fracturing fluid needs to maintain a high-viscosity gel state (forming a three-dimensional network through a cross-linking agent) to ensure that the proppant (quartz sand / ceramsite) is suspended and transported deep into the fracture. In particular, medium- and high-temperature deep wells (above 120°C) generally require a gelling time of 60 to 90 seconds. Otherwise, the fracturing fluid cannot form a gel with sufficient strength at the appropriate time, and it cannot ensure that the proppant is stably suspended in the fracturing fluid for a sufficient time, thereby failing to effectively form propped fractures, thereby affecting the conductivity of shale oil.

[0004] CN111574989B discloses a polyhydroxy thickener, a high-temperature resistant alcohol-based fracturing fluid system, and applications thereof. The polyhydroxy thickener is formed by copolymerizing three monomers: acrylamide, N-vinyl pyrrolidone, and N-acrylamidopropyl-N,N,N-dimethylhydroxyethyl ammonium chloride in an aqueous solution. The mass ratio of the three monomers, based on the total weight of the monomers, is 40-85% acrylamide, 5-35% N-vinyl pyrrolidone, and 3-25% N-acrylamidopropyl-N,N,N-dimethylhydroxyethyl ammonium chloride. The raw materials of the high-temperature resistant alcohol-based fracturing fluid system include the polyhydroxy thickener, a composite crosslinker (the composite crosslinker is obtained by the contact reaction of borax, zirconium oxychloride, glycerol, triethanolamine, lactic acid, sodium gluconate, and water), a regulator, a high-flash-point alcohol, and water. The alcohol-based fracturing fluid system of this application, through the introduction of a polyhydroxy thickener, has high heat and shear resistance, with a temperature resistance exceeding 160°C. The use of high-flash-point alcohols makes the fracturing system safer and allows the alcohol / water ratio to be increased to over 60%. This system offers numerous advantages, including high flowback rates, minimal formation damage, safety, and environmental protection. However, the gelling time of this fracturing fluid remains relatively long, meaning its sand-carrying capacity needs to be further improved.

[0005] Therefore, there is a need for a fracturing fluid that can effectively inhibit clay expansion and has good cross-linking and sand-carrying properties. Summary of the Invention

[0006] The present invention provides an alcohol-based fracturing fluid containing modified nanoparticles and a preparation method thereof, so as to solve the problems of clay swelling, insufficient sand carrying capacity and poor cross-linking performance existing in traditional fracturing fluids in shale oil production.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present invention provides an alcohol-based fracturing fluid containing modified nanoparticles, comprising the following components: an alcohol solvent, modified nanoparticles, an anti-swelling agent, a thickener, a polyamide-amine dendrimer, a pH adjuster, an optional suspending agent, and water; the modified nanoparticles are prepared by modifying the nanoparticles with an alkenyl silane coupling agent, then copolymerizing with an alkenyl-containing imidazole salt, and in-situ grafting the polyimidazole salt onto the surface of the nanoparticles.

[0008] Furthermore, the mass ratio of the nanoparticles, the alkenyl silane coupling agent, and the alkenyl-containing imidazole salt is 100:(6-10):(8-15), preferably 100:(6-10):(10-15).

[0009] Furthermore, the alkenyl silane coupling agent is at least one of vinyl triethoxysilane, vinyl trimethoxysilane, γ-methacryloxypropyl trimethoxysilane, and γ-methacryloxypropyl triethoxysilane; the alkenyl-containing imidazole salt is at least one of 1-vinyl-3-butyl imidazole bromide, 1-vinyl-3-ethyl imidazole bromide, 1-vinyl-3-hexyl imidazole chloride, 1-vinyl-3-hydroxyethyl imidazole tetrafluoroborate, and 1-allyl-3-butyl-imidazole bromide.

[0010] Preferably, the alkenyl silane coupling agent is γ-methacryloxypropyltrimethoxysilane or γ-methacryloxypropyltriethoxysilane, and the alkenyl-containing imidazole salt is 1-vinyl-3-hydroxyethyl imidazole tetrafluoroborate.

[0011] Furthermore, the nanoparticles are at least one of nano-titanium dioxide, nano-silicon dioxide, and nano-zinc oxide, and the particle size D50 is 60-100 nm.

[0012] Furthermore, the preparation method of the modified nanoparticles comprises the following steps: (L1) adding an alkenylsilane coupling agent to water for hydrolysis reaction to obtain a hydrolyzate; then placing the nanoparticles in the hydrolyzate at 50-70° C. with stirring for reaction, and purifying to obtain modified nanoparticles; (L2) The modified nanoparticles, the alkenyl-containing imidazole salt, and the initiator are added to water, and then copolymerized at 40-60°C. The modified nanoparticles are obtained after purification.

[0013] Furthermore, in step (L1), the ratio of the alkenylsilane coupling agent to water is 6-10 g:1000-1500 mL. The hydrolysis reaction conditions are: hydrolysis at 40-50°C for 30-50 minutes at a pH of 4-5; the stirring reaction time is 2-4 hours; and the purification treatment comprises: filtering the solid after the reaction, washing the solid with pure water until the filtrate is neutral, and drying at 60-80°C for 8-24 hours. The hydrolysis of the alkenylsilane coupling agent produces Si-OH groups, which react with -OH groups on the surface of the nanoparticles to form chemical bonds with the alkenylsilane coupling agent. This modification not only enhances the hydrophobicity of the nanoparticles, facilitating their dispersion in the system, but also facilitates the uniformity of the subsequent polymerization reaction on the nanoparticle surface.

[0014] Furthermore, in step (L2), the amount of the initiator used is 0.4-0.7 wt% based on the mass ratio of the nanoparticles, and the initiator is at least one of hydrogen peroxide, potassium persulfate, and ammonium persulfate. The copolymerization reaction lasts for 2-5 hours, preferably 3-4 hours. The alkenyl groups on the modified nanoparticles copolymerize with the alkenyl-containing imidazole salt in the presence of the initiator to form polyimidazolates that are in situ grafted onto the surface of the nanoparticles, thereby obtaining modified nanoparticles.

[0015] The polyamidoamine dendrimer (PAMAM) has a second-generation or third-generation structure, selected from G2-PAMAM or G3-PAMAM. PAMAM is a dendrimer whose structure radiates outward from a central core, resembling a number. It is divided into different generations, or generations. Higher generations have more complex branching structures. Each generation of the dendrimer possesses functional groups such as amine and amide groups, and higher generations increase the number of functional groups. In the present invention, PAMAM is used as a crosslinker. Compared to traditional organoboron and organozirconium crosslinkers, PAMAM offers the advantages of high crosslinking efficiency and network stability, forming a more homogeneous, high-density three-dimensional network. Furthermore, the amide bonds (-CO-NH-) of PAMAM are stable at high temperatures. After shear failure, the network can be rapidly reconstructed through hydrogen bonding and electrostatic interactions between the amine groups and the polymer, resulting in heat and shear resistance. Furthermore, due to its hierarchical structure and rich functional groups, PAMAM can be used as a crosslinker in a lower dosage than traditional crosslinkers.

[0016] Nanoparticles, due to their unique nano-effect, are commonly used in fracturing fluids to improve rheology and reduce fluid loss. This invention utilizes nanoparticles modified with silane coupling agents to initiate in situ copolymerization, forming polyimidazolate polymer chains grafted onto the nanoparticle surface, thus forming modified nanoparticles. The inventors have discovered that the combined action of modified nanoparticles and PAMAM significantly enhances the anti-swelling properties and crosslinking (sand-carrying capacity) of alcohol-based fracturing fluids. This is likely due to, firstly, the increased hydrophobicity of the modified nanoparticles, which forms an effective hydrophobic protective film on the clay surface, preventing direct contact between water and the clay and thus inhibiting clay swelling. Secondly, the polyimidazolate grafted onto the surface of the modified nanoparticles is a polyionic liquid (polycation), which efficiently binds to the negative charges on the shale rock surface, further significantly enhancing the anti-swelling effect. Thirdly, the PAMAM in the present invention has a highly branched structure and a large number of terminal active functional groups (-NH2), which can form hydrogen bonds or covalent crosslinks with the hydroxyl or amide groups of thickeners (such as hydroxyethyl cellulose and polyacrylamide). Specifically, the unique structure of PAMAM allows it to form a high density of crosslinking points with the thickener, thereby building a dense, stable three-dimensional network structure, thereby improving the sand-carrying capacity of the fracturing fluid. The polyimidazole salt polymer chains grafted onto the surface of the modified nanoparticles can entangle with PAMAM to form physical crosslinking points, further strengthening the three-dimensional network structure and thus the sand-carrying capacity of the fracturing fluid. Furthermore, the polyimidazole salt grafted onto the surface of the modified nanoparticles has a large heterocyclic structure, which provides a steric hindrance effect, thereby improving the dispersion of the nanoparticles in the fracturing fluid and enhancing the stability of the fracturing fluid.

[0017] Furthermore, the alcohol solvent is selected from at least one of isopropyl alcohol, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, glycerol, 1,2-butylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, n-pentanol, and n-hexanol. As the base solvent for fracturing fluids, alcohol solvents have low surface tension and good wettability, effectively reducing damage to shale reservoirs while providing a dispersion medium for other components.

[0018] Furthermore, the anti-swelling agent is a polyquaternium salt, such as polyquaternium-2 or polyquaternium-42. Polyquaternium salts are cationic polymers that neutralize the surface charge of the clay by exchanging with cations on the clay surface, effectively inhibiting the expansion and dispersion of the clay, thereby improving the anti-swelling rate of shale.

[0019] Furthermore, the thickener is selected from at least one of hydroxyethyl cellulose and nonionic polyacrylamide, preferably nonionic polyacrylamide. The hydroxyethyl cellulose has a weight-average molecular weight of 1.2 million to 1.5 million Da, and the nonionic polyacrylamide has a weight-average molecular weight of 5 million to 10 million Da. The thickener can increase the viscosity of the fracturing fluid, improving its sand-carrying capacity and suspension properties.

[0020] Furthermore, the pH adjuster is an aqueous solution of hydrochloric acid, acetic acid, formic acid, sodium bicarbonate, and potassium bicarbonate, and the pH adjuster adjusts the pH of the system to 7-9.

[0021] Furthermore, the suspending agent is selected from at least one of attapulgite and bentonite. The suspending agent can improve the suspension stability of the proppant (quartz sand or ceramsite) in the fracturing fluid.

[0022] Furthermore, based on 100% of the total weight of the alcohol-based fracturing fluid, the alcohol-based fracturing fluid containing modified nanoparticles includes the following components: Alcohol solvent 55~70wt%, Modified nanoparticles 2~5wt%, Anti-swelling agent 3~8wt%, Thickener 2~6wt%, Polyamide-amine dendrimer 0.05~0.15wt%, pH regulator 0.05~0.2wt%, Suspending agent 0~1wt%, Water 15~35wt%.

[0023] In a second aspect, the present invention provides a method for preparing the alcohol-based fracturing fluid containing the modified nanoparticles, comprising the following steps: (S1) adding the modified nanoparticles into an alcohol solvent and stirring at high speed to uniformly disperse the modified nanoparticles to obtain a suspension; (S2) adding an anti-swelling agent and a thickening agent to water and stirring until they are uniformly dissolved to form a viscous liquid; (S3) adding the viscous liquid of step (S2) to the suspension of step (S1), then adding a suspending agent and stirring evenly, and finally adding a pH adjuster to adjust the pH of the system to 7-9; (S4) Before fracturing, polyamide-amine dendrimer is added to form an alcohol-based fracturing fluid.

[0024] Furthermore, the high-speed stirring in step (S1) is 800-1200 r / min for 60-90 min; and the stirring in step (S2) is 300-500 r / min for 40-70 min.

[0025] After the construction is completed, a gel breaker is added to the alcohol-based fracturing fluid to make it flow back; the gel breaker is at least one of potassium persulfate, ammonium persulfate, and hydrogen peroxide, preferably ammonium persulfate.

[0026] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention modifies nanoparticles with an alkenyl silane coupling agent, which is then copolymerized with an alkenyl-containing imidazole salt to form a polyimidazolate that is in situ grafted onto the surface of the nanoparticles, thus forming modified nanoparticles. The modified nanoparticles and polyamide-amine dendrimer (PAMAM) work together to significantly enhance the anti-swelling and cross-linking properties (sand carrying capacity) of the alcohol-based fracturing fluid. The gel formation and hanging time is 60 to 90 seconds, making the alcohol-based fracturing fluid particularly suitable for medium- and high-temperature deep wells.

[0027] 2. The alcohol-based fracturing fluid of the present invention causes little damage to the reservoir and is safe and environmentally friendly. DETAILED DESCRIPTION

[0028] The present application will be further described in detail below with reference to the examples, but the examples are only used to explain the relevant invention rather than to limit the invention.

[0029] The experimental methods described in the following examples are conventional methods unless otherwise specified; the reagents and materials are commercially available unless otherwise specified.

[0030] The particle size D50 of nano-silica is 72 nm.

[0031] G2-PAMAM and G3-PAMAM were purchased from Chengdu Kele Biotechnology Co., Ltd.

[0032] Polyquaternium-2 and Polyquaternium-4 were purchased from Zhongshan Dixin Chemical Co., Ltd.

[0033] The weight average molecular weight of nonionic polyacrylamide is about 8 million Da~9 million Da.

[0034] The weight average molecular weight of hydroxyethyl cellulose is approximately 1.2 million Da to 1.3 million Da.

[0035] The organic zirconium crosslinker SL-EXP4206 was selected from Qingdao Lide Oilfield Services Co., Ltd.

[0036] Preparation of modified nano-silicon particles.

[0037] Preparation Example 1 (L1) 8 g of γ-methacryloyloxypropyltriethoxysilane was added to 1200 mL of water, the pH was adjusted to 4.5, and then hydrolyzed at 45°C for 40 min to obtain a hydrolyzate; 100 g of nano-silica was then placed in the hydrolyzate and stirred at 60°C for 2.5 h. After the reaction, a solid was obtained by filtration, and the solid was washed with pure water until the filtrate was neutral, and then dried at 70°C for 24 h to obtain modified nano-silica; (L2) All the modified nano-silica prepared in step (S1), 10 g of 1-vinyl-3-hydroxyethylimidazolium tetrafluoroborate, and 0.6 g of ammonium persulfate were added to water, and then reacted at 55°C. After the reaction was completed, a solid was obtained by filtration. The solid was washed with pure water until the filtrate was neutral, and then dried at 70°C for 24 hours to obtain modified nanoparticles-1.

[0038] Preparation Example 2 The rest is the same as Preparation Example 1, except that: in step (L2), the amount of 1-vinyl-3-hydroxyethyl imidazole tetrafluoroborate used is 8 g, and modified nanoparticles-2 are finally obtained.

[0039] Preparation Example 3 The rest is the same as Preparation Example 1, except that: in step (L2), the amount of 1-vinyl-3-hydroxyethylimidazolium tetrafluoroborate used is 12 g, and modified nanoparticles-3 are finally obtained.

[0040] Preparation Example 4 The rest is the same as Preparation Example 1, except that the amount of 1-vinyl-3-hydroxyethylimidazolium tetrafluoroborate used in step (L2) is 15 g, and modified nanoparticles-4 are finally obtained.

[0041] Preparation Example 5 The rest is the same as Preparation Example 1, except that: in step (L1), vinyltriethoxysilane is used in equal moles to replace γ-methacryloyloxypropyltriethoxysilane, and in step (L2), 1-vinyl-3-butylimidazole bromide is used in equal moles to replace 1-vinyl-3-hydroxyethylimidazole tetrafluoroborate, and finally modified nanoparticles-5 are obtained. Example 1

[0042] An alcohol-based fracturing fluid containing nano-modified particles, comprising the following components: 1,2-propylene glycol 62wt%, Modified nanoparticles-1 3wt%, Polyquaternium-2 5wt%, Nonionic polyacrylamide 4wt%, G3-PAMAM 0.1wt%, pH regulator sodium bicarbonate 0.1wt%, Bentonite 0.2wt%, Water 25.6wt%.

[0043] The above components are prepared into alcohol-based fracturing fluid according to the following steps: (S1) adding the modified nanoparticles-1 to 1,2-propylene glycol and stirring at 1000 rpm for 70 minutes to uniformly disperse the modified nanoparticles-1 to obtain a suspension; (S2) adding polyquaternium-2 and nonionic polyacrylamide to water and stirring at 400 rpm for 50 min until the mixture is uniformly dissolved to form a viscous liquid; (S3) adding the viscous liquid of step (S2) to the suspension of step (S1), then adding bentonite and stirring evenly, and finally adding sodium bicarbonate as a pH adjuster to adjust the pH of the system to 8.0; (S4) Before fracturing, G3-PAMAM is added to form an alcohol-based fracturing fluid. Example 2

[0044] The rest is the same as Example 1, except that the modified nanoparticles-2 prepared in Preparation Example 2 are used instead of the modified nanoparticles-1. Example 3

[0045] The rest is the same as Example 1, except that the modified nanoparticles-3 prepared in Preparation Example 3 are used instead of the modified nanoparticles-1. Example 4

[0046] The rest is the same as Example 1, except that the modified nanoparticles-4 prepared in Preparation Example 4 are used instead of the modified nanoparticles-1. Example 5

[0047] The rest is the same as Example 1, except that the modified nanoparticles-5 prepared in Preparation Example 5 are used instead of the modified nanoparticles-1. Example 6

[0048] The rest of the examples are the same as in Example 1, except that the proportions of the components of the alcohol-based fracturing fluid are different, specifically: 1,2-propylene glycol 70wt%, Modified nanoparticles-1 5wt%, Polyquaternium-2 3wt%, Nonionic polyacrylamide 6wt%, G3-PAMAM 0.05wt%, pH regulator sodium bicarbonate 0.1wt%, Bentonite 0.2wt%, Water 15.65wt%. Example 7

[0049] The rest of the examples are the same as in Example 1, except that the components and proportions of the alcohol-based fracturing fluid are different, specifically: 1,2-Butanediol 55wt%, Modified nanoparticles-1 2wt%, Polyquaternium-4 8wt%, Hydroxyethyl cellulose 2wt%, G2-PAMAM 0.15wt%, pH regulator sodium bicarbonate 0.1wt%, Water 32.75wt%.

[0050] The above components are prepared into alcohol-based fracturing fluid according to the following steps: (S1) adding the modified nanoparticles-1 to 1,2-butanediol and stirring at 1000 r / min for 70 min to uniformly disperse the modified nanoparticles-1 to obtain a suspension; (S2) adding polyquaternium-4 and hydroxyethyl cellulose to water and stirring at 400 rpm for 50 min until the mixture is uniformly dissolved to form a viscous liquid; (S3) adding the viscous liquid of step (S2) to the suspension of step (S1), and then adding a pH adjuster, sodium bicarbonate, to adjust the pH of the system to 7.5; (S4) Before fracturing, G2-PAMAM is added to form an alcohol-based fracturing fluid.

[0051] Comparative Example 1 The rest is the same as Example 1, except that the modified nanoparticles-1 are replaced by nano-silica of equal mass, that is, the nano-silica is not modified.

[0052] Comparative Example 2 The rest is the same as Example 1, except that an equal mass of organic zirconium cross-linking agent SL-EXP4206 is used instead of G3-PAMAM.

[0053] Testing and Analysis The alcohol-based fracturing fluids prepared in Examples 1-7 and Comparative Examples 1-2 were subjected to the following tests: Anti-swelling properties test: Standard core powder (quartz sand: kaolin: montmorillonite = 8:1:1, passed through a 200-mesh sieve) was dried in a 105°C oven for 24 hours. The core powder was then pressed (20 MPa for 10 minutes) into several core slices with a diameter of 2.5 cm and a thickness of 0.5 cm. The core slices were placed in the alcohol-based fracturing fluids prepared in Examples 6-12 and Comparative Examples 1-2, respectively, and immersed at 120°C and 1.5 MPa for 12 hours. After immersion, the core slices were removed, the surface residual fluid was washed, and the core slices were dried. The difference in thickness before and after immersion was measured, and the linear expansion ratio was calculated using the following formula. The linear expansion ratio is used to evaluate the anti-swelling properties of the fracturing fluid; the lower the linear expansion ratio, the better the anti-swelling properties of the fracturing fluid.

[0054] Linear expansion rate (%) =

[0055] Where H1 is the thickness of the core slice after impregnation, and H0 is the initial thickness of the core slice.

[0056] Gel-forming time test: Using an RH-30 rheometer, according to SY / T 6376-2008, the gel-forming time was measured at 120°C, with the viscosity reaching 200 mPa·s as the endpoint. The results are shown in Table 1. The gel was then broken by adding 0.05 wt% ammonium persulfate at 120°C for 2 hours. The viscosity of the gel after breaking was measured, as shown in Table 1.

[0057] Table 1 Performance test of alcohol-based fracturing fluid Alcohol-based fracturing fluid Linear expansion rate of core slice (%) Glue hanging time (s) Viscosity of gel breaking liquid (mPa·s) Example 1 13.5 78 3.5 Example 2 14.9 85 3.1 Example 3 12.8 72 3.6 Example 4 12.3 69 3.8 Example 5 13.7 82 4.2 Example 6 14.3 74 3.8 Example 7 14.4 82 4.0 Comparative Example 1 28.6 89 3.3 Comparative Example 2 18.2 150 4.5 As can be seen from Table 1, the linear expansion rates of the core slices all do not exceed 15%, indicating that the alcohol-based fracturing fluid produced by the present invention has excellent anti-swelling properties and can effectively inhibit clay expansion. Furthermore, the gelation and suspension time of the alcohol-based fracturing fluid of the present invention is between 60 and 90 seconds, indicating good crosslinking properties and thus good sand-carrying capacity. Furthermore, after gelation, conventional breakers can break the gel, and the viscosity after breaking does not exceed 5 mPa·s, indicating easy flowback and minimal damage to the reservoir. In contrast, the nanoparticles in Comparative Example 1 are unmodified, and the corresponding alcohol-based fracturing fluid has poor anti-swelling properties, with a linear expansion rate of nearly 30% for the core slices. The conventional crosslinker used in Comparative Example 2 results in a relatively long gelation and suspension time for the alcohol-based fracturing fluid, resulting in a correspondingly low sand-carrying capacity.

Claims

1. An alcohol-based fracturing fluid comprising modified nanoparticles, characterized in that: The invention comprises the following components: an alcohol solvent, modified nanoparticles, an anti-swelling agent, a thickener, a polyamide-amine dendrimer, a pH regulator, an optional suspending agent, and water; the modified nanoparticles are prepared by modifying the nanoparticles with an alkenyl silane coupling agent, copolymerizing the nanoparticles with an alkenyl-containing imidazole salt, and in-situ grafting the polyimidazolate salt on the surface of the nanoparticles.

2. The alcohol-based fracturing fluid containing modified nanoparticles according to claim 1, characterized in that: The mass ratio of the nanoparticles, the alkenyl silane coupling agent, and the alkenyl-containing imidazole salt is 100:(6-10):(8-15).

3. The alcohol-based fracturing fluid containing modified nanoparticles according to claim 1, characterized in that: The alkenyl silane coupling agent is at least one of vinyl triethoxysilane and γ-methacryloxypropyl triethoxysilane; the alkenyl-containing imidazole salt is at least one of 1-vinyl-3-butyl imidazole bromide, 1-vinyl-3-ethyl imidazole bromide, 1-vinyl-3-hexyl imidazole chloride, 1-vinyl-3-hydroxyethyl imidazole tetrafluoroborate, and 1-allyl-3-butyl imidazole bromide; and the nanoparticles are nano-silicon dioxide with a particle size D50 of 60 to 100 nm.

4. The alcohol-based fracturing fluid containing modified nanoparticles according to claim 1, characterized in that: The alkenyl silane coupling agent is γ-methacryloxypropyltriethoxysilane; and the alkenyl-containing imidazole salt is 1-vinyl-3-hydroxyethyl imidazole tetrafluoroborate.

5. The alcohol-based fracturing fluid containing modified nanoparticles according to claim 1, characterized in that: The preparation method of the modified nanoparticles comprises the following steps: (L1) adding an alkenylsilane coupling agent to water for hydrolysis reaction to obtain a hydrolyzate; then placing the nanoparticles in the hydrolyzate at 50-70° C. with stirring for reaction, and purifying to obtain modified nanoparticles; (L2) The modified nanoparticles, the alkenyl-containing imidazole salt, and the initiator are added to water, and then copolymerized at 40-60°C. The modified nanoparticles are obtained after purification.

6. The alcohol-based fracturing fluid containing modified nanoparticles according to claim 5, characterized in that: In step (L1), the ratio of the alkenyl silane coupling agent to water is 6-10 g:1000-1500 mL, the conditions of the hydrolysis reaction are: hydrolysis at 40-50° C. for 30-50 min when the pH is 4-5; the stirring reaction time is 2-4 h; the purification treatment is: after the reaction is completed, filtration is performed to obtain a solid, the solid is washed with pure water until the filtrate is neutral, and then dried at 60-80° C. for 8-24 h; and / or in step (L2), the amount of the initiator is 0.4-0.7 wt% of the mass ratio of the nanoparticles, and the initiator is at least one of hydrogen peroxide, potassium persulfate, and ammonium persulfate; and the copolymerization reaction time is 2-5 h.

7. The alcohol-based fracturing fluid containing modified nanoparticles according to claim 1, characterized in that: The polyamidoamine dendrimer has a second-generation structure or a third-generation structure, that is, is selected from G2-PAMAM or G3-PAMAM.

8. The alcohol-based fracturing fluid containing modified nanoparticles according to claim 1, characterized in that: The alcohol solvent is selected from at least one of isopropyl alcohol, ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, glycerol, 1,2-butylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, n-pentanol, and n-hexanol; and / or the anti-swelling agent is a polyquaternary ammonium salt; and / or the thickening agent is selected from at least one of hydroxyethyl cellulose and non-ionic polyacrylamide, the weight-average molecular weight of hydroxyethyl cellulose is 1.2 million Da to 1.5 million Da, and the weight-average molecular weight of non-ionic polyacrylamide is 5 million Da to 10 million Da; and / or the pH adjuster is an aqueous solution of hydrochloric acid, acetic acid, formic acid, sodium bicarbonate, and potassium bicarbonate, and the pH adjuster adjusts the pH of the system to 7 to 9; and / or the suspending agent is selected from at least one of attapulgite and bentonite.

9. The alcohol-based fracturing fluid containing modified nanoparticles according to claim 1, characterized in that: Based on the total weight of the alcohol-based fracturing fluid as 100%, the alcohol-based fracturing fluid containing modified nanoparticles includes the following components: Alcohol solvent 55~70wt%, Modified nanoparticles 2~5wt%, Anti-swelling agent 3~8wt%, Thickener 2~6wt%, Polyamide-amine dendrimer 0.05~0.15wt%, pH regulator 0.05~0.2wt%, Suspending agent 0~1wt%, Water 15~35wt%.

10. The method for preparing an alcohol-based fracturing fluid containing modified nanoparticles according to any one of claims 1 to 9, characterized in that: The following steps are involved: (S1) adding the modified nanoparticles into an alcohol solvent and stirring at high speed to uniformly disperse the modified nanoparticles to obtain a suspension; (S2) adding an anti-swelling agent and a thickening agent to water and stirring until they are uniformly dissolved to form a viscous liquid; (S3) adding the viscous liquid of step (S2) to the suspension of step (S1), then adding a suspending agent and stirring evenly, and finally adding a pH adjuster to adjust the pH of the system to 7-9; (S4) Before fracturing, polyamide-amine dendrimer is added to form an alcohol-based fracturing fluid.