Superhydrophobic nanoparticles, nanofluids, preparation, gas film drag reduction method and application

Abstract

The present invention belongs to the technical field of functional nanomaterials and oil and gas development, and discloses superhydrophobic nanoparticles, nanofluids, preparation methods, gas film drag reduction methods and applications. The preparation method of the superhydrophobic nanoparticles includes: under vacuum conditions, drying nano-SiO2 to remove adsorbed moisture to obtain product I; contacting product I, heptadecafluorodecyltrimethoxysilane and acetic acid in an acetone solution for oil bath heating and rotary evaporation concentration treatment to obtain product II; redissolving product II in xylene, and repeatedly purifying it by Soxhlet extraction to remove unreacted monomers, centrifuging the purified product II, and obtaining superhydrophobic nanoparticles through vacuum drying and grinding treatment. The water droplet contact angle of the superhydrophobic nanoparticles of the present invention reaches 165° in the air; the prepared superhydrophobic nanofluid has excellent effects in reducing pressure and increasing injection in low-permeability oil reservoirs, and the drag reduction rate is more than 1.35 times that of conventional nanomaterials.

Description

Technical Field

[0001] The present invention belongs to the technical field of functional nanomaterials and oil and gas development, and particularly relates to a superhydrophobic nanoparticle, nanofluid, preparation, gas film drag reduction method and application thereof. Background Art

[0002] With the continuous increase in oilfield exploration and development, the existing conventional oil and gas resources are decreasing year by year, the mining difficulty is increasing, and unconventional oil and gas resources are becoming more and more important. Due to insufficient natural energy, it is often necessary to inject water to supplement the formation energy in the later stage of oilfield development. However, unconventional reservoirs have the characteristics of low porosity and low permeability, resulting in a large injection pressure of water, and even water cannot be injected into the formation. Surface modification and drag reduction of reservoir rocks are considered to be an effective way to solve the problem of too high injection pressure in low-permeability reservoirs.

[0003] At present, this technology mainly includes surfactant modification and nanomaterial modification. The related technologies of surfactant modification are reported as follows: Chinese Patent CN111088008B reports a surfactant composition for reducing pressure and increasing injection in low-permeability reservoirs, mainly through the combination of non-ionic surfactants and cationic surfactants. Chinese Patent CN114644914A is prepared by sulfonating lactone-type sophorolipids with a sulfonating reagent, and is used as a pressure-reducing and injection-increasing agent in low-permeability oilfields. The related technologies of nanomaterial modification are reported as follows: Chinese Patent CN106479469B uses alkyl and alkyl acid dual groups to modify nano-silica particles, and mixes them with an aqueous NaOH solution to prepare an oilfield water injection agent for increasing injection. Chinese Patent CN110194949A reports a pressure-reducing and injection-increasing chemical agent, using the hydrophobic group provided by methacrylic acid-(3-alkoxysilyl)-alkyl ester and the phosphate group provided by mercaptoalkylcarbethyl phosphate as the modification groups covalently connected to the silica particles in the dual-grafted nano-silica. However, both of these methods have certain deficiencies. Briefly speaking, the effective period of surfactants is short and they are sensitive to the environment. Nanomaterials are prone to aggregation and have poor stability under formation conditions of high temperature and high salinity. Therefore, there is an urgent need for a new method for super drag reduction in water injection development of low-permeability reservoirs to achieve efficient pressure reduction and injection increase.

[0004] In recent years, the research and methods of bionics have been increasingly applied to various fields. By drawing on the characteristics of biological systems such as structure, principle, and function, new solutions are provided to effectively solve scientific and technological problems. In terms of reducing reservoir injection resistance, inspired by the "lotus effect", a bionic drag reduction can be achieved by using a superhydrophobic interface to anchor a nano gas film. Some studies have shown that the lotus leaf surface has a superhydrophobic micro / nano structure. Relevant researchers have proposed that when a superhydrophobic surface comes into contact with water, a nano gas layer that can be adsorbed and fixed on the surface is formed, thus forming a multiphase interface. The presence of the gaseous film can reduce the huge resistance caused by direct liquid-solid contact. The research and application of this technology in the oil field have also gradually developed in recent years. The currently recognized mechanism is that hydrophobic nanoparticles can form a stable hydrophobic surface film, but the formation principle and drag reduction mechanism of the hydrophobic film lack strong experimental evidence. At the same time, the dispersion stability of hydrophobic nanomaterials remains an urgent problem to be solved. Therefore, there is an urgent need to find a new drag reduction method with a long effective period, good stability, and a wide range of application scenarios in bionic drag reduction to efficiently achieve pressure reduction and injection increase.

[0005] Through the above analysis, the problems and defects existing in the prior art are as follows:

[0006] (1) The existing surface modification drag reduction technology has poor compatibility with the reservoir environment. The hydrophobic degree of the existing hydrophobic materials used in oil fields is limited, and the dispersion stability is poor.

[0007] (2) The existing superhydrophobic nanofluid has a large resistance rate and a slow flow rate;

[0008] (3) When the existing technology is applied to drag reduction of reservoir cores, the folds and grooves in the fluid flow channels in the cores cannot be effectively filled, and the surface roughness of the cores is high, resulting in a large resistance caused by direct liquid-solid contact. Summary of the Invention

[0009] To overcome the problems existing in the related technologies, the disclosed embodiments of the present invention provide a superhydrophobic nanoparticle, nanofluid, preparation, gas film drag reduction method, and application.

[0010] In particular, it relates to a new bionic "lotus leaf" superhydrophobic interface gas film drag reduction method applicable to water injection development in low-permeability oil reservoirs based on a fluorinated long-chain novel superhydrophobic nanoparticle composite surfactant to construct a nanofluid.

[0011] The technical solution is as follows: A preparation method of a superhydrophobic nanoparticle, comprising the following steps:

[0012] S101, under vacuum conditions, nano-SiO 2 is dried to remove adsorbed moisture to obtain product I;

[0013] S102. Under nitrogen protection, contact the product I, perfluorodecyltrimethoxysilane, and acetic acid in an acetone solution, and perform oil bath heating and rotary evaporation concentration treatment to obtain product II;

[0014] S103. Redissolve product II in xylene, and repeatedly purify it by Soxhlet extraction to remove unreacted monomers. Centrifuge the purified product II, and obtain superhydrophobic nanoparticles through vacuum drying and grinding treatment.

[0015] In one embodiment, in step S101, the original particle size of the nano-SiO 2 is 10 - 20 nm, and the vacuum drying temperature is 100 - 120 °C;

[0016] In step S102, contact the product I, perfluorodecyltrimethoxysilane, and acetic acid in an acetone solution, and perform oil bath heating and rotary evaporation concentration treatment to obtain product II; wherein, relative to 20 - 50 mL of the acetone solution, the mass ratio of product I to perfluorodecyltrimethoxysilane is (140 - 80):(8 - 1), and acetic acid adjusts the solution pH to 3 - 5;

[0017] The oil bath heating temperature is 45 - 70 °C, and the time is 2 - 4 h; the rotary evaporation concentration temperature is 50 - 80 °C;

[0018] Perfluorodecyltrimethoxysilane hydrolyzes in the solution to generate silanol, and reacts with the surface hydroxyl groups of the purified nano-SiO 2 to graft the fluorinated long chain onto the surface of the nano-SiO 2 surface;

[0019] In step S103, the rotation speed during the centrifugation separation process is 5000 - 10000 rpm, and the centrifugation time is 8 - 5 min; the vacuum drying temperature is 100 - 120 °C.

[0020] Another object of the present invention is to provide superhydrophobic nanoparticles containing long fluorinated chains and low surface energy prepared according to the above-mentioned preparation method. The superhydrophobic nanoparticles include FAS@SiO 2 .

[0021] Another object of the present invention is to provide a preparation method of a superhydrophobic functional nanofluid using the above-mentioned superhydrophobic nanoparticles, including the following steps:

[0022] S201. Mix the previously prepared superhydrophobic nanoparticles with an ethanol solution, and add an appropriate amount of water to obtain solution I;

[0023] S202, dissolving sodium α-olefin sulfonate and lauryl amide propyl betaine in water and mixing them, and adding the obtained mixed solution to the solution I to obtain a solution II;

[0024] S203, under a temperature rising condition, ultrasonically dispersing the solution II to obtain a super-hydrophobic functional nanofluid.

[0025] In one embodiment, in step S201, the mass ratio of superhydrophobic nanoparticles to water is (0.01-0.15):100;

[0026] In step S202, the mass ratio of sodium α-olefin sulfonate and lauryl amide propyl betaine is (1-2): (1-2) together forming a composite surfactant; the mass concentration of the composite surfactant / water solution is 0.1-0.5%;

[0027] In step S203, the ultrasonic dispersion power is 500W to 800W, the ultrasonic time is 12h to 24h, and the temperature is 50 to 80°C;

[0028] In step S202, the composite surfactant disperses the super-hydrophobic nanoparticles into the aqueous solution, and the hydrophobic tails of the composite surfactant molecules are adsorbed on the surface of the super-hydrophobic nanoparticles, exposing the hydrophilic groups to the outside, thereby reducing the interfacial energy of the system and making the super-hydrophobic nanoparticles evenly dispersed in the aqueous phase.

[0029] Another object of the present invention is to provide a super-hydrophobic functional nanofluid prepared by using the preparation method of the super-hydrophobic functional nanofluid.

[0030] Another object of the present invention is to provide a new method for super-hydrophobic interface air film drag reduction suitable for water injection development of low-permeability oil reservoirs based on the idea of ​​lotus leaf bionics, the method comprising: using nano-silicon dioxide as a raw material, adopting fluorine-containing long-chain coupling agent for grafting modification to prepare super-hydrophobic nanoparticles, and constructing a stable super-hydrophobic functional nanofluid through surfactant dispersion; super-hydrophobic nanoparticles form a large number of micro / nano rough structures on the surface of the core through self-assembly, fill the wrinkles and grooves in the fluid flow channel in the core, and reduce the surface roughness of the core; at the same time, the super-hydrophobic interface constructed based on the bionics idea captures microbubbles in the fluid, and embeds a nano-gas film at the solid-liquid contact surface; utilizing the barrier effect of the air film to convert the liquid-solid interface into a liquid-gas-solid three-phase interface, thereby reducing the resistance generated by direct liquid-solid contact.

[0031] Another object of the present invention is to provide an experimental device for completing the flow field change in the microchannel before and after the adsorption of the super-hydrophobic nanoparticles through a microchannel visual flow field experiment.

[0032] Another object of the present invention is to provide an experimental device for evaluating the drag reduction rate of the superhydrophobic functional nanofluid by means of core pressure change through a core flow experiment.

[0033] Another object of the present invention is to provide an application of the superhydrophobic functional nanofluid in pressure reduction and injection increase for water injection development in low-permeability oilfields.

[0034] Combining all the above technical solutions, the advantages and positive effects of the present invention are as follows:

[0035] First, aiming at the technical problems existing in the above-mentioned prior art and the difficulty of solving this problem, closely combining the technical solution to be protected by the present invention and the results and data in the R & D process, etc., analyze in detail and profoundly how the technical solution of the present invention solves the technical problems, and some creative technical effects brought after solving the problems, which are specifically described as follows:

[0036] The preparation method provided by the present invention includes: (1) Under vacuum conditions, nano-SiO 2 is dried to remove adsorbed moisture to obtain Product I; (2) Under nitrogen protection conditions, Product I, heptadecafluorodecyltrimethoxysilane, and acetic acid are contacted in an acetone solution for oil bath heating and rotary evaporation concentration treatment to obtain Product II; (3) Product II is redissolved in xylene, and the Soxhlet extraction method is used for repeated purification to remove unreacted monomers. The purified Product II is centrifuged and separated, and superhydrophobic nanoparticles (SHNPs) containing fluorinated long chains and low surface energy are obtained through vacuum drying and grinding treatment; (4) By compounding surfactants (sodium α-olefin sulfonate and lauramidopropyl betaine), a superhydrophobic nanofluid with long-term stability (>48 d) is formed. The water droplet contact angle of the superhydrophobic nanoparticles (SHNPs) of the present invention reaches 165° in the air; the prepared superhydrophobic nanofluid has excellent effects in pressure reduction and injection increase in low-permeability reservoirs, and the drag reduction rate reaches more than 1.35 times that of conventional nanomaterials (NPs), and the highest drag reduction rate reaches 25.7%; compared with injecting pure water, after injecting the SHNPs nanofluid, the central flow velocity in the subsequent water flooding process increases by 98.27% (the flow velocity decreases by 53.45% after injecting NPs).

[0037] Second, regarding the technical solution as a whole or from the perspective of the product, the technical effects and advantages of the technical solution to be protected by the present invention are specifically described as follows:

[0038] (1) The superhydrophobic nanoparticles (SHNPs) synthesized in the present invention select nano-SiO 2, Using heptadecafluorodecyltrimethoxysilane and acetic acid as reaction raw materials, a superhydrophobic nanoparticle (SHNPs) containing a fluorinated long chain and low surface energy was synthesized through a coupling reaction, and the water droplet contact angle in air reached 165° ± 2.4°.

[0039] (2) The superhydrophobic nanofluid prepared in the present invention is formed by compounding superhydrophobic nanoparticles (SHNPs) with a composite surfactant (sodium α-olefin sulfonate AOS and lauramidopropyl betaine LAB) to form a long-term stable superhydrophobic nanofluid. The particle size is stable at 28 nm within 1 month, and the zeta potential remains above -30 mV.

[0040] (3) The superhydrophobic nanofluid prepared in the present invention has excellent effects in reducing core resistance and flow resistance. Under the same conditions, the drag reduction rate reaches more than 1.35 times that of conventional nanomaterials (NPs), and the highest drag reduction rate reaches 25.7%; compared with injecting pure water, after injecting SHNPs nanofluid, the central flow velocity in the subsequent water flooding process increased by 98.27% (the flow velocity decreased by 53.45% after injecting NPs).

[0041] (4) The method for preparing the superhydrophobic nanofluid of the present invention is simple and the raw materials are inexpensive, and it is applicable to nano-SiO 2 particles of various actual application sizes, and has great application potential in aspects such as reducing core matrix resistance, reducing water injection pressure, and increasing water injection volume.

[0042] (5) When the superhydrophobic nanofluid prepared in the present invention is applied to reducing the resistance of reservoir cores, the carried superhydrophobic nanoparticles can self-assemble on the core surface to form a large number of micro / nano rough structures, filling the folds and gullies in the fluid flow channels in the core, effectively reducing the surface roughness of the core. At the same time, the superhydrophobic interface constructed based on the bionic idea can successfully capture microbubbles in the fluid, thereby embedding a nano gas film at the solid-liquid contact surface. Using the barrier effect of the gas film, the liquid-solid interface is transformed into a liquid-gas-solid three-phase interface, greatly reducing the large resistance generated by the direct contact between the liquid and the solid.

[0043] Third, as the creativity of the present invention, it is also reflected in the following important aspects:

[0044] (1) In recent years, nanomaterials have been more and more widely used in the oil and gas field development site. Especially for low porosity and low permeability oil and gas fields, different technological means of water injection development are currently adopted in oil fields in China. Due to their small size effect, nanomaterials have been widely used in large-scale production applications such as displacement agents, imbibition agents, and enhancers. The superhydrophobic nanofluid preparation method provided by the technical solution of the present invention is simple and has remarkable effects, and can complete pilot tests and final production through patent technology licensing, providing technical reserves for the long-term supply of oil field development.

[0045] (2) The principle explanation part of the technical solution of the present invention reveals the drag reduction mechanism of superhydrophobic materials. The superhydrophobic interface can successfully capture microbubbles in the fluid, thereby embedding a nano gas film at the solid-liquid contact surface. Utilizing the barrier effect of the gas film, the liquid-solid interface is transformed into a liquid-gas-solid triple interface, greatly reducing the large resistance generated by the direct contact between the liquid and the solid. And based on the bubble probe technology of the atomic force microscope, mechanical evidence for the formation of the nano gas film is given. It successfully provides a complete and accurate theoretical basis for the drag reduction technology of superhydrophobic nanomaterials. Description of the Drawings

[0046] The drawings here are incorporated into the specification and form a part of this specification, showing embodiments consistent with the present disclosure, and are used together with the specification to explain the principles of the present disclosure;

[0047] Figure 1 It is a flowchart of the preparation method of superhydrophobic nanoparticles (SHNPs) provided by an embodiment of the present invention;

[0048] Figure 2 It is a flowchart of the preparation method of superhydrophobic functional nanofluids provided by an embodiment of the present invention;

[0049] Figure 3a It is a schematic diagram of the synthesis route of hydrophobic nanoparticles (FAS@SiO 2 ) provided by an embodiment of the present invention;

[0050] Figure 3b It is a schematic diagram of the synthesis route of hydrophobic nanoparticles (DMDCS@SiO 2 ) provided by an embodiment of the present invention;

[0051] Figure 3c It is a schematic diagram of the synthesis route of hydrophobic nanoparticles (D4@SiO 2 ) provided by an embodiment of the present invention;

[0052] Figure 4a It is the FT-IR spectrogram of SiO 2 , FAS@SiO 2 , D4@SiO 2 and DMDCS@SiO 2 provided by an embodiment of the present invention;

[0053] Figure 4b It is the XPS energy spectrum of SiO 2 , FAS@SiO 2 , D4@SiO 2 and DMDCS@SiO 2 provided by an embodiment of the present invention;

[0054] Figure 4care the high-resolution C1s spectra of various samples provided by the embodiments of the present invention;

[0055] Figure 5a is SiO modified with different hydrophobic groups provided by the embodiments of the present invention 2 In the wettability of, the water contact angle of the bare glass surface is about 92°, and after the surface is covered with D4@SiO 2 and DMDCS@SiO 2 the contact angle of water in air increases to 145° and 136° respectively; schematic diagram

[0056] Figure 5b is SiO modified with different hydrophobic groups provided by the embodiments of the present invention 2 In the wettability of, the contact angle remains unchanged during the entire pressing process. Even when the water droplet is peeled off from the wafer with the SHNP coating, the water droplet will not deform due to adhesion; figure

[0057] Figure 6 is the particle size distribution result graph provided by the embodiments of the present invention;

[0058] Figure 7a is the TEM result in the transmission electron microscope of nanoparticles provided by the embodiments of the present invention, showing the particle size of nano-SiO 2 before FAS modification; effect diagram

[0059] Figure 7b is the TEM result in the transmission electron microscope of nanoparticles provided by the embodiments of the present invention, showing the particle size of nano-SiO 2 after FAS modification; effect diagram

[0060] Figure 8a is the particle size change graph of SHNP functional nanofluid after long-term aging provided by the embodiments of the present invention;

[0061] Figure 8b is the potential change graph of SHNP functional nanofluid after long-term aging provided by the embodiments of the present invention;

[0062] Figure 9a is the injection pressure change graph of NP nanofluid in a core with a permeability of 5 mD provided by the embodiments of the present invention;

[0063] Figure 9b is the injection pressure change graph of NP nanofluid in a core with a permeability of 1 mD provided by the embodiments of the present invention;

[0064] Figure 9c is the injection pressure change of SHNP nanofluid in a core with a permeability of 5 mD provided by the embodiments of the present invention

[0065] Figure 9dIt is the injection pressure change diagram of the SHNP nanofluid provided by the embodiment of the present invention in a core with a permeability of 1 mD;

[0066] Figure 9e It is the injection pressure change diagram of the pure surfactant solution provided by the embodiment of the present invention in a 5 mD core;

[0067] Figure 9f It is the water contact angle in the air on the core surface and the bubble contact angle in water before (left) and after (right) the injection of the SHNP nanofluid provided by the embodiment of the present invention;

[0068] Figure 10a It is the flow field distribution diagram of the subsequent water flooding in the simulation channel after injecting deionized water provided by the embodiment of the present invention;

[0069] Figure 10b It is the flow field distribution diagram of the subsequent water flooding in the simulation channel after injecting the NP nanofluid provided by the embodiment of the present invention;

[0070] Figure 10c It is the flow field distribution diagram of the subsequent water flooding in the simulation channel after injecting the SHNP nanofluid provided by the embodiment of the present invention, respectively, which is the flow field distribution diagram of the subsequent water flooding in the simulation channel;

[0071] Figure 10d It is the 2D velocity field diagram after injecting deionized water provided by the embodiment of the present invention;

[0072] Figure 10e It is the 2D velocity field diagram after injecting the NP nanofluid provided by the embodiment of the present invention;

[0073] Figure 10f It is the 2D velocity field diagram after injecting the SHNP nanofluid provided by the embodiment of the present invention;

[0074] Figure 10g It is the 3D velocity field diagram after injecting deionized water provided by the embodiment of the present invention;

[0075] Figure 10h It is the 3D velocity field diagram after injecting the NP nanofluid provided by the embodiment of the present invention;

[0076] Figure 10i It is the 3D velocity field diagram after injecting the SHNP nanofluid provided by the embodiment of the present invention;

[0077] Figure 11a It is the SiO without surfactant provided by the embodiment of the present invention 2 Mechanism diagram of nanofluid adsorption and assembly on the wall surface;

[0078] Figure 11b It is the SiO without surfactant provided by the embodiment of the present invention 2AFM images of nanofluids adsorbed and assembled on the wall;

[0079] Figure 11c is the SiO without surfactant provided by the embodiment of the present invention 2 Longitudinal sectional view of nanofluids adsorbed and assembled on the wall;

[0080] Figure 11d is the mechanism diagram of NP nanofluids adsorbed and assembled on the wall provided by the embodiment of the present invention;

[0081] Figure 11e is the AFM image of NP nanofluids adsorbed and assembled on the wall provided by the embodiment of the present invention;

[0082] Figure 11f is the longitudinal sectional view of NP nanofluids adsorbed and assembled on the wall provided by the embodiment of the present invention;

[0083] Figure 11g is the mechanism diagram of SHNP nanofluids adsorbed and assembled on the wall provided by the embodiment of the present invention;

[0084] Figure 11h is the AFM image of SHNP nanofluids adsorbed and assembled on the wall provided by the embodiment of the present invention;

[0085] Figure 11i is the longitudinal sectional view of SHNP nanofluids adsorbed and assembled on the wall provided by the embodiment of the present invention;

[0086] Figure 12 is the mechanism diagram of drag reduction of SHNPs superhydrophobic interface nano gas film in the embodiment of the present invention;

[0087] Figure 13 is the schematic diagram of the core flow experiment device in the embodiment of the present invention;

[0088] Figure 14 is the schematic diagram of the microchannel visible flow field experiment device in the embodiment of the present invention;

[0089] In the figure: 1 - ISCO pump, 2 - nanofluid intermediate container, 3 - six-way valve, 4 - pressure gauge, 5 - core holder, 6 - confining pressure pump, 7 - collecting graduated cylinder, 8 - display screen, 9 - flow controller, 10 - micro particle image velocimetry (μ-PIV), 11 - microchannel, 12 - micro-injection pump. Detailed implementation manners

[0090] To make the above objects, features, and advantages of the present invention more apparent and understandable, the following detailed description of the specific embodiments of the present invention will be provided in conjunction with the accompanying drawings. Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0091] I. Explanation of the embodiment:

[0092] The embodiment of the present invention proposes a new method for gas film drag reduction with a superhydrophobic interface suitable for water injection development in low-permeability oil reservoirs based on the bionic thinking of "lotus leaf". The main process is as follows: Using nano-silica as the raw material, superhydrophobic nanoparticles (SHNPs) are prepared by graft modification with a fluorinated long-chain coupling agent, and a stable superhydrophobic functional nanofluid is constructed by surfactant dispersion. The superhydrophobic nanoparticles can self-assemble on the core surface to form a large number of micro / nano rough structures, filling the folds and grooves in the fluid flow channels inside the core, effectively reducing the surface roughness of the core. At the same time, the superhydrophobic interface constructed based on bionics can successfully capture microbubbles in the fluid, thereby embedding a nano gas film at the solid-liquid contact surface. Using the barrier effect of the gas film, the liquid-solid interface is transformed into a liquid-gas-solid triple interface, greatly reducing the large resistance generated by the direct contact between the liquid and the solid. The modified superhydrophobic functional nanofluid exhibits good performance in reducing the injection pressure of the oil reservoir and the flow resistance.

[0093] In the following examples: The surface functional group composition and elemental distribution of particles in each modification stage were characterized by Fourier transform infrared spectroscopy (Bruker, VERTEX 80v) and X-ray photoelectron spectroscopy (Thermo scientific, Escalab 250Xi); the wettability was measured by a dynamic contact angle meter (POWEREACH, JC2000D); the dispersion process of nanoparticles and surfactants was carried out using an ultrasonic disperser (Newzhi Biotechnology, JY92-IIN); the particle size and zeta potential of nanofluids were measured by a dynamic light scattering instrument (Brookhaven, BI-200SM). The core flooding experimental device was self-assembled in the laboratory. The permeability and porosity of the cores used in the experiment were measured by a gas permeability meter (Beijing Yinen Petroleum Technology, ULP-613) and a helium porosity meter (Beijing Yinen Petroleum Technology, PMI-100), respectively; the surface morphology and elemental distribution before and after core flooding were measured by a field emission scanning electron microscope (FEI QUANTAFEG250), and the three-dimensional surface morphology and roughness of the core were measured by a laser confocal microscope (KEYENCE, VK-X250); the visualization flow field experimental device was self-assembled in the laboratory; the adsorption and assembly morphology of nanoparticles and surfactants on the wall were measured by an atomic force microscope (Bruker, MultiMode 8); the non-tip probe used in the measurement of the bubble-wall interaction force was a special customized product of the laboratory.

[0094] Raw material nano-SiO 2 (20 nm) was purchased from Bohuasi Nanotechnology Co., Ltd. SHNPs were prepared by grafting modification with silane coupling agents. The silane coupling agents used included trimethoxy(1H,1H,2H,2H-heptadecafluorodecyl)silane (abbreviation: FAS), dimethyldichlorosilane (abbreviation: DMDCS), and octamethylcyclotetrasiloxane (abbreviation: D4), which were purchased from Aladdin Reagent Co., Ltd. Surfactants such as sodium α-olefin sulfonate (abbreviation: AOS, purchased from Shanghai Guoyao), Triton X-100 (TX100, purchased from Shanghai Guoyao), lauramidopropyl betaine (abbreviation: LAB, purchased from Lusen Chemical Industry), and lauramidopropyl hydroxysulfobetaine (abbreviation: LHSB, purchased from Lusen Chemical Industry) were used to disperse SHNPs. Dodecanethiol for hydrophobic modification of the AFM bubble probe was purchased from Aladdin Reagent Co., Ltd. Deionized water was prepared by a laboratory ultrapure water machine (ULUPURE, UPT-II). NaCl was purchased from Aladdin Reagent for the preparation of mineralized water. All reagents were freshly prepared and used without further purification.

[0095] Example 1

[0096] As Figure 1 shown, an embodiment of the present invention provides a method for preparing superhydrophobic nanoparticles (SHNPs), including:

[0097] S101, under vacuum conditions, nano - SiO 2 is dried to remove adsorbed moisture to obtain Product I;

[0098] S102, under nitrogen protection conditions, the Product I, heptadecafluorodecyltrimethoxysilane, and acetic acid are contacted in an acetone solution for oil bath heating and rotary evaporation concentration treatment to obtain Product II;

[0099] S103, the Product II is redissolved in xylene, and is repeatedly purified by Soxhlet extraction to remove unreacted monomers. The purified Product II is centrifuged and separated, and superhydrophobic nanoparticles (SHNPs) are obtained through vacuum drying and grinding treatment.

[0100] The embodiment of the present invention also provides superhydrophobic nanoparticles (SHNPs) containing long fluorine chains and low surface energy prepared by the described preparation method.

[0101] In the embodiment of the present invention, heptadecafluorodecyltrimethoxysilane hydrolyzes in the solution to generate silanol. Such silanols have a large number of highly active hydroxyl groups, which can react with the surface hydroxyl groups of the purified nano - SiO 2 to graft fluorine - containing long chains onto the surface of nano - SiO 2 surface. The preparation of superhydrophobic surfaces mainly relies on constructing micro - nano rough structures and grafting low - surface - energy functional groups on the surface.

[0102] In the embodiment of the present invention, further, in step S101, the original particle size of the nano - SiO 2 is 10 - 20 nm, preferably 20 nm, the vacuum drying temperature is 100 - 120 °C, preferably 120 °C. The vacuum degree is usually - 0.1 MPa.

[0103] In the embodiment of the present invention, further, in step S102, the Product I, heptadecafluorodecyltrimethoxysilane, and acetic acid are contacted in an acetone solution for oil bath heating and rotary evaporation concentration treatment to obtain Product II; wherein, relative to 20 - 50 mL of the acetone solution, the mass ratio of the Product I to heptadecafluorodecyltrimethoxysilane is (140 - 80):(8 - 1), and acetic acid adjusts the solution pH to 3 - 5, preferably (120 - 100):(4 - 2), and the pH is 3 - 4.

[0104] In the embodiment of the present invention, further, in step S102, the oil bath heating temperature is 45 - 70 °C, preferably 50 - 60 °C, and the time is 2 - 4 h; the rotary evaporation concentration temperature is 50 - 80 °C, preferably 60 - 70 °C.

[0105] In an embodiment of the present invention, further, in step S103, the repeated purification by Soxhlet extraction is carried out until the xylene solution is clear during the condensation reflux process; the rotation speed during the centrifugal separation process is 5000 - 10000 rpm, preferably 6000 - 8000 rpm, and the centrifugation time is 8 - 5 min; the vacuum drying temperature is 100 - 120 °C, preferably 120 °C.

[0106] As a preferred embodiment of the present invention, a method for preparing superhydrophobic nanoparticles (SHNPs) includes:

[0107] (1′) Under vacuum conditions, 100 g of nano - SiO 2 (with a diameter of 20 nm) is dried to remove adsorbed moisture to obtain product I;

[0108] (2′) Under nitrogen protection, product I, 1,1,2,2,3,3,4,4,5,5,6,6,7,7,7 - heptadecafluorodecyltrimethoxysilane, and acetic acid are contacted in an acetone solution for oil bath heating and rotary evaporation concentration treatment to obtain product II; wherein, relative to 20 mL of the acetone solution, the mass ratio of product I to 1,1,2,2,3,3,4,4,5,5,6,6,7,7,7 - heptadecafluorodecyltrimethoxysilane is (120 - 100):(4 - 2), and the pH is adjusted to 3 - 4 using an acetic acid solution;

[0109] (3′) 50 g of product II is redissolved in 500 mL of xylene, and repeatedly purified by Soxhlet extraction until the xylene solution is clear during the condensation reflux process to remove unreacted monomers. The purified product II is centrifuged at 6000 - 8000 rpm for 8 - 5 min, and after vacuum drying at 120 °C, superhydrophobic nanoparticles (SHNPs) are obtained through grinding treatment.

[0110] Example 2

[0111] As Figure 2 shown, the embodiment of the present invention also provides a method for preparing a superhydrophobic functional nanofluid, including:

[0112] S201, mixing the superhydrophobic nanoparticles (SHNPs) prepared in the aforementioned Example 1 with a little ethanol solution, and adding an appropriate amount of water to obtain solution I;

[0113] S202, dissolving sodium α - olefin sulfonate (AOS) and lauramidopropyl betaine (LAB) in water and mixing them, and adding the obtained mixture to solution I to obtain solution II;

[0114] S203, under heating conditions, ultrasonically dispersing solution II to obtain a superhydrophobic functional nanofluid.

[0115] In the above preparation method, it can be understood that in step S201, the mixing is to mix nanoparticles into water with the assistance of ethanol; in step S202, the mixing is to first dissolve sodium alpha-olefin sulfonate and lauramidopropyl betaine in water to obtain a surfactant solution. Finally, the nanoparticle solution and the surfactant solution are mixed to obtain a functional nanofluid.

[0116] An embodiment of the present invention provides a superhydrophobic functional nanofluid prepared by the above preparation method.

[0117] The mechanism of action of the superhydrophobic nanofluid is as follows:

[0118] Through a certain shut-in time, SHNPs are adsorbed on the core surface and a large number of hydrophobic rough structures are formed. On the one hand, the micro / nano rough structure can effectively reduce the surface roughness of the core and inhibit wall turbulence; on the other hand, the superhydrophobic interface is successfully constructed and forms a gaseous film on the core surface through the adsorption of SHNPs. Through the barrier effect of the gas film, the liquid-solid interface is transformed into a liquid-gas-solid interface, reducing the large resistance generated by direct liquid-solid contact, thereby improving the water injection effect. The superhydrophobic nanofluid can be applied in the water injection development of low-permeability oilfields.

[0119] In an embodiment of the present invention, in order to successfully inject superhydrophobic nanoparticles (SHNPs) into the formation, a surfactant is needed to disperse SHNPs into an aqueous solution. The hydrophobic tail end of the surfactant molecule can adsorb on the nanoparticle surface, exposing the hydrophilic group outside, reducing the interfacial energy of the system, and thus dispersing the intrinsically hydrophobic nanoparticles into the aqueous phase.

[0120] In an embodiment of the present invention, further, in step S201, the mass ratio of superhydrophobic nanoparticles (SHNPs) to water is (0.01 - 0.15):100, preferably (0.05 - 0.10):100; the ethanol solution is used as a dispersion aid, and the addition amount only needs to submerge the superhydrophobic nanoparticles.

[0121] In an embodiment of the present invention, further, in step S202, the mass ratio of sodium alpha-olefin sulfonate (AOS) to lauramidopropyl betaine (LAB) is (1 - 2):(1 - 2), preferably 1:1; the mass concentration of the composite surfactant / aqueous solution is 0.1 - 0.5%, preferably 0.1 - 0.2%.

[0122] In an embodiment of the present invention, further, in step S203, the ultrasonic dispersion power is 500W - 800W, the ultrasonic time is 12h - 24h, and the temperature is 50 - 80°C, preferably 500W, the ultrasonic time is 12h, and the temperature is 50 - 60°C.

[0123] In an embodiment of the present invention, as a preferred mode of the embodiment of the present invention, the preparation method of the superhydrophobic nanofluid includes:

[0124] (a') Using an ethanol solution as a dispersion aid, the addition amount only needs to submerge the superhydrophobic nanoparticles. The mass ratio of the superhydrophobic nanoparticles (SHNPs) to water in the mixed solution is (0.05 - 0.10):100 to obtain Solution I;

[0125] (b') Dissolve a composite surfactant with a mass concentration of 0.1 - 0.2% (where the mass ratio of sodium α-olefin sulfonate (AOS) to lauramidopropyl betaine (LAB) is 1:1) in water for a second contact, and add the obtained mixed solution to Solution I to obtain Solution II;

[0126] (c') Under the condition of 50 - 60 °C, ultrasonically disperse Solution II at a power of 500 W for 12 h to obtain the superhydrophobic functional nanofluid.

[0127] Example 3

[0128] The embodiment of the present invention aims to illustrate the hydrophobic nanoparticles prepared by the preparation method of the present invention.

[0129] According to Figure 3a the synthesis route of the hydrophobic nanoparticles (FAS@SiO 2 ):

[0130] Place nano-SiO 2 in a vacuum oven at 120 °C for 3 - 4 h to remove adsorbed water. Add SiO 2 (5 g), trimethoxy(1H,1H,2H,2H-heptadecafluorodecyl)silane (FAS, 0.15 g) and acetic acid solution (pH = 4) to 20 mL of acetone solution, place it in an oil bath at 50 °C, and stir and react under nitrogen protection for 4 - 6 h. After cooling, centrifuge at 8000 rpm for 5 min to obtain FAS@SiO 2 nanoparticles. Use the Soxhlet extraction method to remove unreacted monomers with xylene as the solvent. Place the purified FAS@SiO 2 in a vacuum oven at 120 °C, dry for 5 - 6 h and then grind to obtain hydrophobic nanoparticles, labeled as Z1.

[0131] Example 4

[0132] The embodiment of the present invention aims to illustrate the hydrophobic nanoparticles prepared by the preparation method of the present invention.

[0133] According to Figure 3b the synthesis route of the hydrophobic nanoparticles (DMDCS@SiO 2 ):

[0134] Put nano-SiO 2 in a vacuum oven at 120 °C and dry for 3 - 4 h to remove adsorbed water. Add SiO 2 (5 g), dimethyldichlorosilane (DMDCS, 0.15 g) and acetic acid solution (pH = 4) to 20 mL of acetone solution, place it in an oil bath at 50 °C, and stir and react for 4 - 6 h under nitrogen protection. After cooling, centrifuge at 8000 rpm for 5 min to obtain DMDCS@SiO 2 nano-particles. Use Soxhlet extraction method to remove unreacted monomers with xylene as the solvent. Put the purified DMDCS@SiO 2 in a vacuum oven at 120 °C, dry for 5 - 6 h and then grind to obtain hydrophobic nano-particles, labeled as Z2.

[0135] Example 5

[0136] This example of the present invention aims to illustrate the hydrophobic nano-particles prepared by using the preparation method of the present invention.

[0137] According to Figure 3c the synthesis route of the shown hydrophobic nano-particles (D4@SiO 2 ):

[0138] Put nano-SiO 2 in a vacuum oven at 120 °C and dry for 3 - 4 h to remove adsorbed water. Add SiO 2 (5 g), octamethylcyclotetrasiloxane (D4, 0.15 g) and acetic acid solution (pH = 4) to 20 mL of acetone solution, place it in an oil bath at 50 °C, and stir and react for 4 - 6 h under nitrogen protection. After cooling, centrifuge at 8000 rpm for 5 min to obtain D4S@SiO 2 nano-particles. Use Soxhlet extraction method to remove unreacted monomers with xylene as the solvent. Put the purified D4@SiO 2 in a vacuum oven at 120 °C, dry for 5 - 6 h and then grind to obtain hydrophobic nano-particles, labeled as Z3.

[0139] In this example of the present invention, in the above Examples 3, 4, and 5, as Figure 4a shown, the FT-IR spectra of SiO 2 , FAS@SiO 2 , D4@SiO 2 and DMDCS@SiO 2 are presented. The absorption peaks at 1118 cm -1 , 801 cm -1 and 482 cm -1 are for SiO 2Characteristic peaks of Si-O-Si stretching and bending vibrations. 3460 cm -1 and 1638 cm -1 The absorption bands at are the symmetric stretching and bending vibration absorption peaks of -OH in bound water and adsorbed water, respectively. In the SiO 2 spectrum, the absorption band at 965 cm -1 is attributed to the bending of Si-OH. In FAS@SiO 2 , D4@SiO 2 and DMDCS@SiO 2 spectra, the disappearance of the Si-OH bending phenomenon indicates that the hydrolyzed silanols have undergone a condensation reaction with the surface hydroxyl groups. In the spectra of D4@SiO 2 and DMDCS@SiO 2 , an absorption band belonging to -CH 3 / -CH 2 appears (2850 cm -1 ~2960 cm -1 ). These results indicate that D4 and DMDCS have been successfully grafted onto the SiO 2 surface. The characteristic absorption peak of the C-F bond in the spectrum of FAS@SiO 2 is not obvious (~1210 cm -1 ), which may be due to the overlap of a broad characteristic peak at 1118 cm -1 with the C-F bond.

[0140] Figure 4b shows the XPS spectra of SiO 2 , FAS@SiO 2 , D4@SiO 2 and DMDCS@SiO 2 . The main peaks observed at 292.16 eV, 533.14 eV, 689.26 eV and 103.97 eV correspond to C1s, O1s, F1s and Si2p, respectively. The surface element compositions of the modified SiO 2 are shown in Table 1. The F content of FAS@SiO 2 reaches 24.13%, and the C / F atomic ratio is 0.47, which is consistent with the theoretical ratio in the FAS reagent. Compared with the 32.01% C content in D4@SiO 2 , the C content in DMDCS@SiO 2 is only 5.73%. This is mainly because the D4 molecule contains 4 times more methyl groups than the DMDCS molecule. The high-resolution C1s spectra of each sample are as shown in Figure 4c . For FAS@SiO 2, the C1s spectrum decomposes into three peaks at 285.8 eV, 292.2 eV, and 294.1 eV, which are attributed to C-C, -CF 3 and -CF 2 . Both D4@SiO2 and DMDCS@SiO 2 show only one peak (284.8 eV, -CH 3 ). Thus, the FT-IR and XPS results indicate that various hydrophobic groups have been successfully grafted onto the SiO 2 surface.

[0141] Table 1

[0142]

[0143]

[0144] In the embodiments of the present invention, as Figure 5a shown, for the wettability of SiO 2 modified with different hydrophobic groups, the water contact angle of the bare glass surface is about 92°. After the surface is covered with D4@SiO 2 and DMDCS@SiO 2 , the contact angle of water in air increases to 145° and 136° respectively. The results show that the hydrophobicity after D4 modification is slightly higher than that after DMDCS modification, which is consistent with the XPS results. This is mainly because the content of hydrophobic methyl groups on the surface of D4@SiO 2 is greater than that on the surface of DMDCS@SiO 2 .

[0145] In the embodiments of the present invention, FAS@SiO2 has superhydrophobicity of 165° because fluorine-containing materials generally have lower surface energy than carbon-containing materials. Therefore, FAS@SiO 2 is determined as the optimal SHNPs. The superhydrophobicity of SHNPs is further demonstrated by the adhesion experiment, as Figure 5b shown. For the wettability of SiO 2 modified with different hydrophobic groups, the contact angle remains unchanged during the entire pressing process. Even when the water droplet is peeled off from the wafer with the SHNP coating, the water droplet will not deform due to adhesion.

[0146] Example 6

[0147] The embodiments of the present invention are intended to illustrate the hydrophobic nanoparticles prepared by using the preparation method of the present invention.

[0148] Hydrophobic nanoparticles (FAS@SiO 2 ) are prepared according to the same method as in Example 3, with the difference that:

[0149] Relative to 10 g of SiO 2 , the amount of trimethoxy(1H,1H,2H,2H-heptadecafluorodecyl)silane used is 0.2 g. The finally prepared FAS@SiO 2 is labeled as Z4.

[0150] Example 7

[0151] This example of the present invention is to illustrate the hydrophobic nanoparticles prepared by using the preparation method of the present invention.

[0152] Hydrophobic nanoparticles (FAS@SiO 2 ) are prepared according to the same method as in Example 3, the difference is that:

[0153] Relative to 10 g of SiO 2 , the amount of trimethoxy(1H,1H,2H,2H-heptadecafluorodecyl)silane used is 0.4 g. The finally prepared FAS@SiO 2 is labeled as Z4.

[0154] Example 8

[0155] This example of the present invention is to illustrate the hydrophobic nanoparticles prepared by using the preparation method of the present invention.

[0156] Hydrophobic nanoparticles (FAS@SiO 2 ) are prepared according to the same method as in Example 3, the difference is that:

[0157] Relative to 10 g of SiO 2 , the amount of trimethoxy(1H,1H,2H,2H-heptadecafluorodecyl)silane used is 0.1 g. The finally prepared FAS@SiO 2 is labeled as Z6.

[0158] Example 9

[0159] This example of the present invention is to illustrate the hydrophobic nanoparticles prepared by using the preparation method of the present invention.

[0160] Hydrophobic nanoparticles (FAS@SiO 2 ) are prepared according to the same method as in Example 3, the difference is that:

[0161] The pH of the reaction system is adjusted to 3 by dropping acetic acid solution. The finally prepared FAS@SiO 2 is labeled as Z7.

[0162] Example 10

[0163] This example is to illustrate the superhydrophobic nanofluid prepared by using the method of the present invention, including the following steps:

[0164] (1) Using an ethanol solution as a dispersion aid, the addition amount only needs to submerge the superhydrophobic nanoparticles. The mass ratio of the superhydrophobic nanoparticles (FAS@SiO 2 ) to water in the mixed solution is 0.10:100 to obtain Solution I;

[0165] (2) Dissolve a composite surfactant with a mass concentration of 0.1% (where the mass ratio of sodium α-olefin sulfonate (AOS) to lauramidopropyl betaine (LAB) is 1:1) in water for a second contact, and add the obtained mixed solution to Solution I to obtain Solution II;

[0166] (3) Under the condition of 50 - 60 °C, ultrasonically disperse Solution II at a power of 500 W for 12 h to obtain a superhydrophobic functional nanofluid, labeled as S1.

[0167] In the dispersion of SHNPs by different surfactants, after adding 0.1% of the composite surfactant (where the mass ratio of sodium α-olefin sulfonate (AOS) to lauramidopropyl betaine (LAB) is 1:1), the SHNP nanofluid is clear and transparent. In addition to providing hydrophilic head groups, the anionic surfactant AOS can also effectively improve the electrostatic repulsion between SHNPs and enhance the stability of the SHNP functional nanofluid. The particle size distribution results ( Figure 6 ) show that SiO without adding surfactant 2 is easily agglomerated in water, and the particle size is about 110 nm. The addition of the surfactant significantly improves the stability of the nanofluid. The median particle sizes of the SiO2 + composite surfactant nanofluid and the SHNPs + composite surfactant nanofluid are 21 nm and 28 nm respectively.

[0168] Figure 7a For the TEM result of the nanoparticles, it shows the particle size effect diagram of nano-SiO 2 before FAS modification;

[0169] Figure 7b For the TEM result of the nanoparticles, it shows the particle size effect diagram of nano-SiO 2 after FAS modification; among them Figure 7a and Figure 7b both show no obvious change, which is consistent with the DLS result. The final optimal nanofluid composition is SHNPs and AOS / LAB.

[0170] Figure 8a It is the particle size change diagram of the SHNP functional nanofluid after long-term aging, Figure 8bIt is the potential change diagram of SHNP functional nanofluid after long-term aging. The results show that the SHNP functional nanofluid can remain stable within 48 days, with the median particle size maintained at about 30 nm and the Zeta potential value maintained at -32 mV.

[0171] Figure 9a and Figure 9b are the injection pressure changes of NP nanofluid in cores with permeabilities of 5 mD and 1 mD respectively; Figure 9c and Figure 9d are the injection pressure changes of SHNP nanofluid in cores with permeabilities of 5 mD and 1 mD respectively; Figure 9e is the injection pressure change of pure surfactant solution in a 5 mD core; Figure 9f are the water contact angle in air and the bubble contact angle (f) in water on the core surface before (left) and after (right) injecting SHNP nanofluid. To verify the drag reduction effect of the prepared SHNP nanofluid in the core, a core displacement experiment with different permeabilities was carried out using a nanofluid containing hydrophilic silica (NPs) as the control group. As Figure 9a shown, for the core with a permeability of 5 mD, the initial water flooding pressure was 0.74 MPa. After injecting NP nanofluid, the subsequent water injection pressure dropped to 0.59 MPa, and the drag reduction rate was 20.27%. Under the same conditions, before and after injecting SHNP nanofluid, the water flooding pressure dropped from 0.70 MPa to 0.52 MPa, and the drag reduction rate reached 25.71% ( Figure 9c ). Compared with the hydrophilic NP system, the drag reduction ability of SHNP nanofluid was increased by about 1.26 times. Under the condition of lower permeability (1 mD), the drag reduction advantage of SHNP nanofluid was more obvious, reaching 1.34 times that of NPs nanofluid. The drag reduction rates of SHNP and NP nanofluids were 19.9% ( Figure 9d ) and 14.9% ( Figure 9b ) respectively. To distinguish the contributions of surfactants and nanomaterials to core drag reduction, a core drag reduction experiment with only surfactant injection was carried out. The results are as Figure 9e shown, and the pressure reduction rate was only 1.43%, indicating that pure surfactant did not show obvious improvement in drag reduction. In Figure 9f , the original core surface showed strong hydrophilicity in air. After the displacement ended, due to the adsorption of SHNPs, the surface became strongly hydrophobic. The contact angle of bubbles on the original core surface was about 147°. After SHNPs adsorption, the bubbles spread on the core surface, and the contact angle was almost 0°, indicating that the bubbles existed in the form of a gaseous film on the SHNPs-adsorbed core surface.

[0172] Figure 10a 、 Figure 10b 、 Figure 10c are the flow field distributions in the simulation channel during subsequent water flooding respectively, Figure 10a, Figure 10b , Figure 10c correspond to after injecting deionized water, after injecting NP nanofluid, and after injecting SHNP nanofluid, respectively; Figure 10d and Figure 10g are the 2D / 3D velocity fields after injecting deionized water; Figure 10e and Figure 10h are the 2D / 3D velocity fields after injecting NP nanofluid; Figure 10f and Figure 10i are the 2D / 3D velocity fields after injecting SHNP nanofluid. In the control experiment without adding nanomaterials, the water flows uniformly along the channel, and the central flow velocity is 0.464 cm / s. The adsorption interface of hydrophilic NPs generates an obvious "viscous" phenomenon (as shown in Figure 10e ), the central flow velocity of water is 0.216 cm / s, 53.45% lower than that of pure water, and there is no obvious regular liquid-solid boundary.

[0173] In the embodiment of the present invention, Figure 10f after injecting SHNP nanofluid, the central flow velocity in the subsequent water flooding process is significantly increased to 0.920 cm / s, 98.27% higher than that of pure water, and 3.26 times that of NP nanofluid. Through the three-dimensional velocity field distribution ( Figure 10g - Figure 10h ), the comparison results of the central velocity can be obtained more clearly. The substantial increase in the flow velocity indicates that SHNPs can effectively reduce the fluid flow resistance and increase the injection volume. In addition, Figure 10f obvious liquid-solid stratification is observed, which may be due to the generation of the "gas film" slip phenomenon.

[0174] In the embodiment of the present invention, the mechanism diagrams, AFM images, and longitudinal profiles of different nanofluids adsorbed and assembled on the wall include: Figure 11a is the mechanism diagram of the adsorption and assembly of SiO 2 nanofluid without surfactant on the wall, Figure 11b is the AFM image of the adsorption and assembly of SiO 2 nanofluid without surfactant on the wall, Figure 11c is the longitudinal profile of the adsorption and assembly of SiO 2 nanofluid without surfactant on the wall;

[0175] Figure 11d is the mechanism diagram of the adsorption and assembly of NP nanofluid on the wall, Figure 11e is the AFM image of the adsorption and assembly of NP nanofluid on the wall, Figure 11f is the longitudinal profile of the adsorption and assembly of NP nanofluid on the wall;

[0176] Figure 11g is the mechanism diagram of the adsorption and assembly of SHNP nanofluid on the wall, Figure 11hAFM images of the adsorption and assembly of SHNP nanofluids on the wall surface Figure 11h Longitudinal sectional view of the adsorption and assembly of SHNP nanofluids on the wall surface;

[0177] As Figure 11a - Figure 11c shown, pure SiO 2 aggregates easily in water and settles on the surface, forming aggregates with a size of about 90 nm. NP nanofluids and SHNP nanofluids containing surfactants have good dispersion stability and can be evenly adsorbed on the solid surface, thus assembling to form a large number of uniform micro / nano structures. Under the same conditions, hydrophilic NP particles are more likely to be adsorbed on the hydrophilic solid surface. The adsorption thickness of NPs is about 70 nm, and the root mean square roughness (RMS) is 20.8 nm( Figure 11e , Figure 11f ), while the adsorption thickness of SHNPs is only 40 nm and the RMS is 13.9 nm( Figure 11h , Figure 11i ). This indicates that the micro / nano rough structures formed by NP nanofluids are richer than those of SHNP nanofluids. However, the results of core flooding experiments and visualization flow field experiments both show that SHNP has better drag reduction and injection enhancement effects, further proving that the assembled structure of superhydrophobic nanoparticles has special properties of gas film drag reduction in addition to reducing roughness.

[0178] Figure 12 is the mechanism of gas film drag reduction of the superhydrophobic interface of SHNPs in the water injection process in the embodiments of the present invention. The surface of the core is rough, and a large amount of water injection energy is lost on the pore wall, resulting in a high water injection pressure( Figure 12 Figure a in

[0179] By dispersing with surfactants, SHNPs can be successfully injected into the formation( Figure 12 Figure b in

[0180] Example 11

[0181] This example is to illustrate the superhydrophobic nanofluid prepared by the method of the present invention.

[0182] (1) Use an ethanol solution as a dispersion aid, and the addition amount only needs to submerge the superhydrophobic nanoparticles. The mass ratio of the superhydrophobic nanoparticles (FAS@SiO 2 ) to water in the mixed solution is 0.20:100 to obtain Solution I;

[0183] (2) Dissolve a composite surfactant with a mass concentration of 0.1% (where the mass ratio of sodium α-olefin sulfonate (AOS) to lauramidopropyl betaine (LAB) is 1:1) in water for the second contact, and add the obtained mixed solution to the above Solution I to obtain Solution II;

[0184] (3) Under the condition of 50 - 60 °C, ultrasonically disperse Solution II at a power of 500 W for 12 h to obtain a superhydrophobic functional nanofluid, labeled as S2.

[0185] Example 12

[0186] This example is to illustrate the superhydrophobic nanofluid prepared by using the method of the present invention.

[0187] (1) Use an ethanol solution as a dispersion aid, and the addition amount only needs to submerge the superhydrophobic nanoparticles. The mass ratio of the superhydrophobic nanoparticles (FAS@SiO 2 ) to water in the mixed solution is 0.05:100 to obtain Solution I;

[0188] (2) Dissolve a composite surfactant with a mass concentration of 0.1% (where the mass ratio of sodium α-olefin sulfonate (AOS) to lauramidopropyl betaine (LAB) is 1:1) in water for the second contact, and add the obtained mixed solution to the above Solution I to obtain Solution II;

[0189] (3) Under the condition of 50 - 60 °C, ultrasonically disperse Solution II at a power of 500 W for 12 h to obtain a superhydrophobic functional nanofluid, labeled as S3.

[0190] Example 13

[0191] This example is to illustrate the superhydrophobic nanofluid prepared by using the method of the present invention.

[0192] (1) Use an ethanol solution as a dispersion aid, and the addition amount only needs to submerge the superhydrophobic nanoparticles. The mass ratio of the superhydrophobic nanoparticles (FAS@SiO 2 ) to water in the mixed solution is 0.01:100 to obtain Solution I;

[0193] (2) Dissolve the composite surfactant with a mass concentration of 0.1% (where the mass ratio of sodium α-olefin sulfonate (AOS) to lauramidopropyl betaine (LAB) is 1:1) in water for the second contact, and add the obtained mixture to the solution I to obtain solution II;

[0194] (3) Under the condition of 50 - 60 °C, ultrasonically disperse solution II at a power of 500 W for 12 h to obtain a superhydrophobic functional nanofluid, labeled as S4.

[0195] Comparative Example 1

[0196] Use unmodified hydrophilic nano-SiO 2 as the displacement fluid for the core displacement experiment, and calculate the drag reduction rate, labeled as D1.

[0197] Comparative Example 2

[0198] Use the hydrophobic nanoparticles (DMDCS@SiO 2 ) prepared in Example 4 as the displacement fluid for the core displacement experiment, and calculate the drag reduction rate, labeled as D2.

[0199] Comparative Example 3

[0200] Use the hydrophobic nanoparticles (D4@SiO 2 ) prepared in Example 5 as the displacement fluid for the core displacement experiment, and calculate the drag reduction rate, labeled as D3.

[0201] Test Example

[0202] Take the core drag reduction rates of the nanofluids prepared in Examples 10 - 13 and Comparative Examples 1 - 3 as technical indicators for testing, and the results are shown in Table 2.

[0203] Table 2

[0204]

[0205] It can be seen from the results of Examples 10 - 13, Comparative Examples 1 - 3, and Table 2 that the superhydrophobic nanofluid prepared by the present invention exhibits good performance in reducing the core water injection pressure and reducing the core friction rate. The water droplet contact angle of the superhydrophobic nanoparticles in the air reaches 165°, and the drag reduction rate for a 5 mD low-permeability core is as high as 25.71%. It has great application potential in "reducing pressure and increasing injection" for water injection development in low-permeability oilfields.

[0206] In the above embodiments, the descriptions of each embodiment have their own emphases. For the parts not detailed or recorded in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0207] In an embodiment of the present invention, an evaluation of the application of the aforementioned superhydrophobic functional nanofluid in reducing the injection pressure in oil fields is also provided. The evaluation of the application of the superhydrophobic functional nanofluid in reducing the injection pressure in oil fields mainly evaluates the drag reduction rate of the superhydrophobic functional nanofluid by means of a core flow experiment with the aid of the core pressure change; and completes the change of the flow field in the microchannel before and after the adsorption of the superhydrophobic nanoparticles through a microchannel visible flow field experiment.

[0208] In an embodiment of the present invention, the evaluation of the application of the provided superhydrophobic functional nanofluid in reducing the injection pressure in oil fields mainly evaluates the drag reduction rate of the superhydrophobic functional nanofluid by means of a core flow experiment with the aid of the core pressure change; and completes the change of the fluid flow field in the microchannel before and after the adsorption of the superhydrophobic nanoparticles through a microchannel visible flow field experiment. The experiments are as follows:

[0209] Experiment

[0210] Evaluate the drag reduction rate of the superhydrophobic functional nanofluid by means of a core flow experiment with the aid of the core pressure change. The experimental device is as Figure 13 shown, including: an ISCO pump 1, a nanofluid intermediate container 2, a six-way valve 3, a pressure gauge 4, a core holder 5, an confining pressure pump 6, and a collection graduated cylinder 7.

[0211] The specific method is as follows:

[0212] 1) Place the water-saturated core in the core holder 5, adjust the six-way valve 3 to connect the outlet of the ISCO pump 1 to the inlet of the core holder 5. After the oven temperature stabilizes at 60 °C, increase the confining pressure to 3 MPa and keep the confining pressure 2 - 3 MPa higher than the injection pressure. Then turn on the constant flow mode of the ISCO pump 1 and inject water into the core at a flow rate of 0.5 mL·min -1 . Record the injection pressure at the inlet of the core holder 5 through the pressure acquisition system, and the stabilized pressure is denoted as P1;

[0213] 2) After adding 0.05 wt% SHNP nanofluid to the nanofluid intermediate container 2, adjust the six-way valve 3 to connect the outlet of the nanofluid intermediate container 2 filled with the nanofluid to the core inlet. Inject the SHNP nanofluid into the core at a flow rate of 0.2 mL·min -1 , and stop when the injection volume reaches 0.5 PV. Close the inlet and outlet of the core holder 5 and shut in the well for 8 h (to provide wall adsorption time for the nanoparticles);

[0214] 3) Finally, inject the core again at a constant flow rate of 0.5 mL·min -1 for subsequent water flooding, record the inlet pressure of the core holder 5 until the pressure stabilizes, and denote it as P2.

[0215] The pressure gauge 4 is connected to the six-way valve 3. The core holder 5 is connected to the confining pressure pump 6 through a valve, and the outlet of the core holder 5 is connected to the collection graduated cylinder 7.

[0216] 4) Calculate the drag reduction rate using formula (1), and formula (1) is as follows:

[0217]

[0218] According to the present invention, the change in the flow field in the microchannel before and after the adsorption of superhydrophobic nanoparticles is completed through a microchannel visual flow field experiment. The experimental device is as Figure 14 shown, including: a display screen 8, a flow controller 9, a micro-particle image velocimetry (μ-PIV) 10, a microchannel 11, and a micro-injection pump 12.

[0219] The specific method is as follows:

[0220] a) Place the microchannel 11 model in a 30°C vacuum oven for drying;

[0221] b) Slowly inject deionized water (control group), hydrophilic nanofluid, and SHNP nanofluid into the microchannel 11 model at a speed of 0.01 mL·min -1 for 8 hours continuously;

[0222] c) Drop 0.1 wt% of monodisperse fluorescent solution (mainly composed of polyethylene microspheres with a particle size of 0.2 μm) into 50 mL of water and ultrasonically disperse it at 40°C for 10 minutes. Load the dispersed liquid into the Hamilton micro-injection pump 12, and then inject it into the microchannel 11 at a constant flow rate of 0.05 mL·min -1 . The flow field distribution in the microchannel before and after the injection of the nanofluid can be obtained through the micro-particle image velocimetry (μ-PIV) 10. Adjust the flow field observation field of view range to 400 μm × 400 μm through the flow controller 9, and finally all visual data is transmitted to the display screen 8.

[0223] The above is only a relatively preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. A preparation method of superhydrophobic functional nanofluid, characterized in that, This method is prepared using superhydrophobic nanoparticles containing long fluorinated chains and low surface energy. The superhydrophobic nanoparticles include FAS@SiO 2 , and the preparation method of the superhydrophobic functional nanofluid specifically includes the following steps: S201, mix the prepared superhydrophobic nanoparticles with an ethanol solution, and add water to obtain Solution I; S202, dissolve sodium α-olefin sulfonate and lauramidopropyl betaine in water and mix them, and add the obtained mixture to Solution I to obtain Solution II; S203, under the condition of temperature increase, ultrasonically disperse Solution II to obtain superhydrophobic functional nanofluid; In step S201, the mass ratio of superhydrophobic nanoparticles to water is (0.01~0.15):100; In step S202, the mass ratio of sodium α-olefin sulfonate to lauramidopropyl betaine is (1~2):(1~2), sodium α-olefin sulfonate and lauramidopropyl betaine form a composite surfactant, and the mass concentration of the composite surfactant / aqueous solution is 0.1~0.5%; In step S203, the ultrasonic dispersion power is 500 W~800 W, the ultrasonic time is 12h~24h, and the temperature of the temperature increase condition is 50~80°C; The preparation method of the hydrophobic nanoparticles includes the following steps: S101, under vacuum conditions, remove the adsorbed moisture of nano-SiO 2 to obtain Product I; S102, under the condition of nitrogen protection, perform oil bath heating and rotary evaporation concentration treatment on Product I, perfluorodecyltrimethoxysilane, and acetic acid in an acetone solution to obtain Product II; S103, dissolve Product II in a xylene solution, and repeatedly purify it by Soxhlet extraction to remove unreacted monomers. Centrifuge the purified Product II, and obtain superhydrophobic nanoparticles through vacuum drying and grinding treatment; In step S101, the nano-SiO 2 has an original particle size of 10 to 20 nm; In step S102, the acetone solution is 20~50 mL, the mass ratio of Product I to perfluorodecyltrimethoxysilane is (140~80):(8~1), and acetic acid adjusts the pH of the solution to 3~5; the oil bath heating temperature is 45~70°C, and the heating time is 2~4h; the rotary evaporation concentration temperature is 50~80°C; In step S103, the rotation speed of the centrifugal separation is 5000~10000 rpm, and the centrifugal time is 8~5 min; the vacuum drying temperature is 100~120°C.

2. A superhydrophobic functional nanofluid prepared by using the preparation method of the superhydrophobic functional nanofluid described in claim 1.

3. A new method for superhydrophobic interfacial gas film drag reduction applicable to water injection development in low-permeability oil reservoirs based on the bionic thinking of lotus leaves, characterized in that, It is implemented by using the superhydrophobic functional nanofluid described in claim 2; this new method for gas film drag reduction includes: using nano-silica as a raw material, grafting and modifying it with a fluorinated long-chain coupling agent to prepare superhydrophobic nanoparticles, and constructing a stable superhydrophobic functional nanofluid through surfactant dispersion; the superhydrophobic nanoparticles self-assemble on the core surface to form a large number of micro / nano rough structures, filling the folds and grooves in the fluid flow channels in the core and reducing the surface roughness of the core; at the same time, the superhydrophobic interface constructed based on bionic thinking captures microbubbles in the fluid, and embeds a nano gas film at the solid-liquid contact surface; by utilizing the barrier effect of the gas film, the liquid-solid interface is transformed into a liquid-gas-solid three-phase interface, reducing the resistance generated by the direct contact between the liquid and the solid.

4. Application of the superhydrophobic functional nanofluid as described in Claim 2 in reducing pressure and increasing injection during the water injection development of low-permeability oilfields.

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