Modified nanofluid oil displacement agent and preparation method thereof

Abstract

The application relates to the technical field of oil development, and provides a modified nanofluid oil displacement agent and a preparation method, which comprise the following components: cyclodextrin modified magnetic nanometer Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets and a surfactant. The mass fractions of the components include the following: 9-12 parts of cyclodextrin modified magnetic nanometer Fe3O4 multi-walled carbon nanotubes; 21-36 parts of carboxylated cellulose nanofibers; 3-6 parts of molybdenum disulfide nanosheets; 24-30 parts of the surfactant; and 25-40 parts of a diluent. The modified nanofluid oil displacement agent is prepared by compounding the cyclodextrin modified magnetic nanometer Fe3O4 multi-walled carbon nanotubes with one-dimensional structure, the carboxylated cellulose nanofibers with one-dimensional structure, the magnetic nanometer Fe3O4 with zero-dimensional structure and the molybdenum disulfide nanosheets with two-dimensional structure, and has the advantages of good dispersity, good fluid flowability, reduced thick oil viscosity, non-pore blockage and good oil displacement effect, so that the recovery efficiency is effectively improved.

Description

Technical Field

[0001] This application relates to the field of petroleum development technology, specifically to a modified nanofluid oil displacement agent and its preparation method. Background Technology

[0002] With the continuous development of technology and the deepening of oil and gas exploration, unconventional oil reservoirs such as low-permeability reservoirs are oil and gas resources with great potential development value.

[0003] Low-permeability oil reservoirs are characterized by low permeability, small pore throat radius, high seepage resistance, low porosity (below 40%), severe throat blockage, and heavy oil. In particular, for heavy oil in low-permeability reservoirs, the high viscosity and high interfacial tension between heavy oil and water make it easier to be trapped in the pores and difficult to flow out. Therefore, there is a technical problem of "cannot be injected and cannot be extracted" in the field.

[0004] Nanoparticle-based oil displacement agents are nanofluids prepared by adding solid nanoparticles to a liquid solvent and then mechanically agitating them to form a homogeneous suspension system. As a novel type of oil displacement agent, nanoparticle-based oil displacement agents offer significant advantages over traditional chemical oil displacement agents, such as high specific surface area, excellent biocompatibility, and high recovery rate, and are attracting increasing attention. Nanoparticles in nanoparticle-based oil displacement agents can be classified according to their dimensionality as follows: 0-dimensional (nanospheres, nanoparticles), 1-dimensional (nanowires, nanotubes), and 2-dimensional (nanofashelves).

[0005] Currently, most nanomaterials used to enhance oil recovery are zero-dimensional spherical nanoparticles. These can reduce interfacial tension and adsorb onto solid surfaces to change wettability, thereby improving oil recovery. However, due to their easy adsorption, large losses, and insufficient interfacial contact caused by their spherical shape, they are inefficient, prone to particle aggregation, flocculation and sedimentation, and their small particle size makes them easy to clog pores. For example, under harsh conditions (such as high salinity), the electrostatic repulsion between nanoparticles is weakened, which can easily lead to particle aggregation, flocculation and sedimentation, making injection difficult or causing severe adsorption. In addition, zero-dimensional spherical nanoparticles cannot effectively reduce the viscosity of heavy oil, making it difficult to further reduce the oil-water interfacial tension and affecting the oil recovery rate.

[0006] Among one-dimensional nanomaterials, studies have shown that hydrophobic multi-walled carbon nanotubes, single-walled carbon nanotubes, and activated carbon nanoparticles have oil displacement effects and can modify wettability and reduce interfacial tension to some extent. However, the strong hydrophobicity, easy aggregation, and low stability of carbon nanotubes affect their oil displacement effect.

[0007] Two-dimensional sheet-like nanomaterials have a larger specific surface area and more active sites than zero-dimensional and one-dimensional nanomaterials. The sheet-like structure allows for more thorough contact between oil and water during oil displacement, resulting in better interfacial interactions. However, problems remain, such as the high surface free energy of nanosheets, a significant tendency to aggregate and settle, and the potential for clogging of fine pores when injected into porous media, causing semi-permanent or permanent damage to the reservoir. Furthermore, achieving both dispersion stability and interfacial adsorption simultaneously is challenging.

[0008] In summary, nanofluid flooding agents suffer from nanoparticle aggregation, flocculation, and sedimentation, resulting in poor overall stability. Their abilities to reduce oil-water interfacial tension, improve oil-rock separation, and enhance fluid flow are limited. They also clog pores, suffer significant adsorption losses at the rock interface, and their effects are not long-lasting. Therefore, developing nanofluid flooding agents that can effectively reduce heavy oil viscosity, exhibit good fluid dispersibility, stability, and are less prone to clogging and adsorption losses is a crucial technological improvement direction for the effective utilization of low-permeability oil reservoirs. Summary of the Invention

[0009] To overcome the shortcomings of existing technologies, this application provides a modified nanofluid oil displacement agent and its preparation method, which has advantages such as good dispersibility, good fluid flowability, reduced viscosity of heavy oil, less pore clogging, low adsorption loss, good oil displacement effect, and improved oil recovery rate.

[0010] The embodiments of this application are implemented as follows:

[0011] In a first aspect, this application provides a modified nanofluid oil displacement agent comprising the following components: cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactants, and diluents.

[0012] The mass fractions of each component include: 9-12 parts of cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes; 21-36 parts of carboxylated cellulose nanofibers; 3-6 parts of molybdenum disulfide nanosheets; 24-30 parts of surfactant; and 25-40 parts of diluent.

[0013] Optionally, the cyclodextrin-modified loaded magnetic Fe3O4 multi-walled carbon nanotubes are prepared using multi-walled carbon nanotubes with an outer diameter of 15-40 nm and a length of 400 nm-3 μm as raw materials, with a magnetic Fe3O4 loading of 15-30%, and the cyclodextrin and silane coupling agent are prepared in the following mass ratio: cyclodextrin:silane coupling agent:carbon nanotubes loaded with magnetic Fe3O4 in a mass ratio of 1:(0.3-0.6):(1-1.6).

[0014] Optionally, the carboxylated cellulose nanofibers have a diameter of 2-10 nm and a length of 400 nm-3 μm.

[0015] Optionally, the molybdenum disulfide nanosheets are small-diameter, thin-layer molybdenum disulfide nanosheets with a diameter of 10-700 nm and a thickness of less than 8 nm.

[0016] Optionally, the surfactant includes anionic surfactants and / or nonionic surfactants.

[0017] Optionally, the diluent is a mixture of water and ethanol, wherein the water:ethanol ratio is (7-8):1 by volume.

[0018] Optionally, the cyclodextrin is 2-hydroxypropylβ-cyclodextrin or methylβ-cyclodextrin; the silane coupling agent is KH570.

[0019] In a second aspect, this application provides a method for preparing a modified nanofluid oil displacement agent, wherein the modified nanofluid oil displacement agent as described above is prepared.

[0020] The method includes: mixing cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactants and diluents according to the mass fractions of each component, stirring, and ultrasonically dispersing to obtain a modified nanofluid oil displacement agent.

[0021] Optionally, the preparation of the cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes:

[0022] S1. Multi-walled carbon nanotubes are subjected to strong acid treatment with a mixture of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1. The mixture is stirred and refluxed at 30-50℃ for 1-4 hours. After washing with deionized water until neutral, the mixture is filtered and dried to obtain oxidized multi-walled carbon nanotubes.

[0023] S2. Prepare a mixed aqueous solution of ferric chloride and ferrous sulfate, add the oxide multi-walled carbon nanotubes obtained in step S1, adjust the pH of the mixed solution to 8-9.5, filter and obtain carbon nanotube powder loaded with magnetic nano Fe3O4. The magnetic nano Fe3O4 loading is 15-30% of the total mass of the carbon nanotube powder loaded with Fe3O4, based on the mass percentage of Fe3O4 in the total mass of the Fe3O4 loaded carbon nanotube powder.

[0024] S3. Prepare a cyclodextrin dispersion by mixing cyclodextrin, silane coupling agent, and ethanol aqueous solution. Add 0.2-0.3g of carbon nanotube powder loaded with magnetic nano-Fe3O4 prepared in step S2 to 60-80ml of cyclodextrin dispersion solution, sonicate for 5-15min, magnetically stir, centrifuge, rinse repeatedly with distilled water, and vacuum dry to obtain cyclodextrin-modified magnetic nano-Fe3O4 multi-walled carbon nanotubes. The volume ratio of ethanol in the ethanol aqueous solution is 20-40%.

[0025] Optionally, in step S3, cyclodextrin and silane coupling agent are added in the following mass ratio: the mass ratio of cyclodextrin:silane coupling agent:carbon nanotubes loaded with magnetic nano Fe3O4 is 1:(0.3-0.6):(1-1.6).

[0026] Beneficial effects include:

[0027] This invention provides a modified nanofluid oil displacement agent. It utilizes one-dimensional, highly reactive multi-walled carbon nanotubes to stabilize and load zero-dimensional magnetic Fe3O4 nanoparticles, giving them magnetically driven properties. The one-dimensional structure forms a high-temperature resistant, stable three-dimensional cross-network, which pulls oil droplets into filaments, improving fluid flowability and promoting viscosity reduction in heavy oil. Cyclodextrin modification compensates for the adverse effects of increased density, increased interparticle attraction, and the inherent aggregation tendency of multi-walled carbon nanotubes loaded with magnetic Fe3O4. The cyclodextrin-modified multi-walled carbon nanotubes loaded with magnetic Fe3O4 exhibit good dispersibility, are not prone to aggregation, possess certain magnetorheological properties, and have high hydrophilicity, improving rock wettability, reducing heavy oil viscosity, and lowering oil-water interfacial tension. It also helps reduce the loss of zero-dimensional magnetic Fe3O4 nanoparticles adsorbed at the rock interface, significantly improving the performance of the modified nanofluid oil displacement agent system and increasing the recovery rate after use.

[0028] By introducing carboxylated cellulose nanofibers with high aspect ratio, good dispersibility, and thixotropic one-dimensional structure into the modified nanofluid oil displacement agent system, it is beneficial to form a coarse-fine particle size combination with cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes with small aspect ratio, creating a dense three-dimensional cross network. This improves the barrier against the migration of different substances such as oil and water, enhances oil stability and stripping ability, improves interfacial wettability, and provides better dispersion performance. It can reduce the sedimentation risk of the magnetic Fe3O4 multi-walled carbon nanotubes and improve the static stability of the system. At the same time, due to its thixotropic nature, the modified nanofluid oil displacement agent system can be selectively combined with magnetic properties to reduce the blockage of nanopores during injection, thus promoting rheological properties and exhibiting better dynamic variable flowability. This greatly enhances the performance of the modified nanofluid oil displacement agent system and improves the recovery rate after use.

[0029] By introducing two-dimensional molybdenum disulfide nanosheets, and through the interaction of highly dispersible cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes and carboxylated cellulose nanofibers with one-dimensional structures, superior dispersibility and reduced sedimentation are achieved without modification, simplifying the application of two-dimensional structures. The thin, short-diameter two-dimensional structures, with their oriented arrangement, are less prone to flipping compared to nanoparticles, facilitating their spread and overlapping adsorption at interfaces, thus enhancing the barrier capacity and stability of the three-dimensional cross-network. This multi-dimensional barrier prevents contact between oil droplets and between oil droplets and rocks, guiding the formation of wedge-shaped molecular films to increase structural partial pressure, promoting fluid propulsion and separation, and facilitating the stripping of crude oil from the rock surface.

[0030] In summary, by combining one-dimensional cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes and carboxylated cellulose nanofibers with different properties, one-dimensional magnetic Fe3O4 nanotubes, and two-dimensional molybdenum disulfide nanosheets, a modified nanofluid oil displacement agent with good dispersibility, good fluid flowability, reduced heavy oil viscosity, minimal pore clogging, and good oil displacement effect is formed, effectively improving oil recovery. A method for preparing this modified nanofluid oil displacement agent does not require complex modification of multiple nanostructures; the preparation process is simple and relatively easier to achieve stable preparation of the corresponding formulation. Detailed Implementation

[0031] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It should also be understood that terms such as those defined in general dictionaries should be understood to have the same meaning as in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless specifically defined as herein.

[0033] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The term “and / or” as used herein includes all or any units and all combinations of one or more associated listed items.

[0034] To address the problems of nanofluid flooding agents, such as nanoparticle aggregation, flocculation and precipitation, poor overall stability, limited ability to reduce oil-water interfacial tension, improve oil-rock separation capacity, limited fluid flowability, and pore blockage, this invention provides a modified nanofluid flooding agent and its preparation method.

[0035] For example, a modified nanofluid oil displacement agent is provided, comprising the following components: cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactants, and diluents.

[0036] The mass fractions of each component include: 9-12 parts of cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes (optional, 9, 10, 11, 12, etc.); 21-36 parts of carboxylated cellulose nanofibers (optional, 21, 22, 24, 26, 28, 31, 33, 34, 35, 36, etc.); 3-6 parts of molybdenum disulfide nanosheets (optional, 3, 4, 5, 6, etc.); 24-30 parts of surfactant (optional, 24, 26, 28, 29, 30, etc.); and 25-40 parts of diluent (optional, 25, 26, 28, 29, 32, 24, 34, 35, 37, 38, 40, etc.).

[0037] Cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes exhibit good dispersibility, are not prone to aggregation, possess certain magnetorheological properties, and have high hydrophilicity. They can improve rock wettability, reduce heavy oil viscosity, and reduce oil-water interfacial tension, thus greatly improving the performance of modified nanofluid oil displacement agent systems and increasing oil recovery rates after use.

[0038] Compared to single-arm carbon nanotubes, which are chemically stable but have significant drawbacks such as strong hydrophobicity and easy aggregation, multi-arm carbon nanotubes (MWCNTs) are mainly composed of coaxial monolayer nanotube groups with nanoscale interlayer spacing arranged along the fiber axis. They possess a certain rigidity, are not easily bent or coiled, have a small aspect ratio (short length and large outer diameter carbon nanotubes), and their multi-walled carbon nanotubes have a more active surface (high reactivity), good thermal conductivity, and strong heat resistance. In modified nanofluid oil displacement agent systems, they easily form high-temperature resistant, stable, three-dimensional cross-networks, which can improve emulsion stability. Their high activity makes them easy to combine with surfactants, modifiers, etc., to form ideal properties, improve the bonding stability of loaded magnetic nano-Fe3O4, and more easily form stable films in the oil / solid / nanofluid three-phase contact area. At the same time, their viscosity-reducing effect reduces the adhesion work of oil droplets peeling off from the porous rock wall, promoting the occurrence of percolation. Their one-dimensional rigid structure also helps to pull oil droplets into filaments and improve the pulling orientation effect, thereby improving the fluid flow capacity and increasing the recovery rate.

[0039] Preferably, the multi-walled carbon nanotubes are short-length, large-diameter carbon nanotubes with an outer diameter of 15-40 nm and a length of 400 nm-3 μm, preferably 500 nm-800 nm; alternatively, they can have an outer diameter of 20-30 nm and a length of 500 nm-800 nm, TNSM5 (Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences). The large outer diameter of the multi-walled carbon nanotubes is beneficial for forming a framework for a high-temperature resistant, stable 3D cross-linked network. If the outer diameter is smaller than this range, the risk of aggregation between the particles increases when the 3D cross-linked network is dense. If the outer diameter is larger than this range, the particle size is too large, which is not conducive to the solution entering the nanoscale fine throats and micro-throats to improve wettability, and is prone to causing local blockage of the micropores of the rock. In addition, the contact between different phases in the oil / solid / nanofluid three-phase interface is reduced, which is not conducive to reducing interfacial tension and changing the wettability of the rock.

[0040] By loading magnetic Fe3O4 nanoparticles onto multi-walled carbon nanotubes, the dot-like magnetic Fe3O4 nanoparticles were stabilized, and their ability to alter fluid flow under magnetic force was enhanced. However, loading magnetic Fe3O4 nanoparticles onto multi-walled carbon nanotubes presents several drawbacks, including increased density, stronger interparticle attraction, and a tendency for aggregation inherent in the multi-walled carbon nanotubes themselves. Modification with a small amount of cyclodextrin mitigated these negative effects, improved the hydrophilicity and dispersibility of the nanoparticles, and thus promoted changes in rock wettability, ultimately increasing oil recovery.

[0041] Cellulose nanofibers are a relatively widely available and low-cost nanomaterial, whose use can reduce the cost of nanomaterials. Carboxylated cellulose nanofibers are prepared by introducing carboxyl functional groups onto the surface of cellulose nanofibers through chemical modification. This modification not only changes the surface properties of the fibers but also enhances their compatibility and reactivity with other substances. Due to the introduction of carboxyl groups, the hydrophilicity of cellulose nanofibers is significantly improved, making them readily soluble in water and highly dispersible. Carboxylated cellulose nanofibers are characterized by a high aspect ratio, with fiber diameters ranging from 2-10 nm and lengths from 400 nm to 3 μm (optional: carboxylated cellulose nanofibers produced by Guilin Qihong Technology Co., Ltd., with diameters of 3-5 nm and fiber lengths of 500-1000 nm). They also exhibit thixotropy (i.e., in low-concentration solutions (below 0.3%), they can remain in a stable suspension state without shear force, and can become a free-flowing solution under low shear forces such as agitation). By introducing carboxylated cellulose nanofibers with high aspect ratio, good dispersibility, and thixotropic properties into the modified nanofluid oil displacement agent system, it is beneficial to form a coarse-to-fine particle size distribution and a dense three-dimensional cross-network with cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes with a small aspect ratio. This facilitates the formation of a stable film in the oil / solid / nanofluid three-phase contact area, improves the barrier against the migration of different substances such as oil and water, and enhances oil stability, exfoliation, and interfacial wettability. It also exhibits better dispersibility, supporting the dispersion effect and reducing the risk of sedimentation of the magnetic Fe3O4 multi-walled carbon nanotubes, thus improving system stability. Furthermore, its one-dimensional structure facilitates the stretching of oil droplets into filaments, improving flow... The fluidity of the liquid has a viscosity-reducing effect, which is beneficial for reducing the viscosity of heavy oil. Due to its thixotropic properties, when injected into the oil displacement agent system, under the action of high shear rate, the oil displacement agent has high fluidity, which facilitates fluid injection and increases the penetration area. When intertwined with modified multi-walled carbon nanotubes to form a three-dimensional network, under conditions of high shear force such as stirring and increased shear force such as magnetic force, it can promote the decomposition and deformation of the three-dimensional network formed by the modified multi-walled carbon nanotubes as the skeleton, which is beneficial for the controllable rheological changes of the system. Under certain circumstances, if the nanofluid blocks or has difficulty entering the nanoscale throats and micro-throats, it can promote the modified nanofluid oil displacement agent to have certain rheological properties and improve the oil displacement performance. The proportion of carboxylated cellulose nanofibers used is greater than that of cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes, which is beneficial for a denser 3D cross network, which can reduce the risk of carbon nanotube deposition, improve stability, and also reduce costs. If the proportion used is too high, due to its high hydrophilicity, there is a certain possibility of aggregation, which requires hydrophobic modification. If the proportion used is too low, its corresponding advantages cannot be fully utilized.

[0042] Two-dimensional sheet-like nano-displacement agents are mostly under research. Molybdenum disulfide nanosheets are a mainstream nano-component in two-dimensional sheet-like nano-displacement agents. They have certain effects on reducing interfacial tension and improving reservoir wettability, and also possess properties not found in ordinary nanomaterials, such as self-adsorption at the oil-water interface. However, they currently suffer from poor dispersibility, overly complex processes such as Janus amphiphilic modification, and limited ability to reduce interfacial tension. By introducing a small amount of small-diameter, thin-layered molybdenum disulfide nanosheets with a two-dimensional structure into the modified nanofluid displacement agent system, and through the interaction of highly dispersible cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes and carboxylated cellulose nanofibers, they exhibit superior dispersibility and are less prone to sedimentation without modification. The thin, short-diameter, and oriented two-dimensional structure is less prone to flipping compared to nanoparticles, which facilitates flat spreading and overlapping adsorption at the interface, improving the barrier to molecular diffusion between the internal and external phases, i.e., enhancing the 3D cross-network barrier ability and improving stability. Its two-dimensional sheet structure, combined with the one-dimensional structure, enriches dispersion and migration behavior, which is beneficial to improving fluid flowability. The thin, small-diameter two-dimensional structure blocks the contact between oil droplets and between oil droplets and rocks in more dimensions, forming a wedge-shaped molecular film to increase structural partial pressure, which is beneficial to the propulsion and separation of fluids and promotes the stripping of crude oil from the rock surface. Meanwhile, the highly dispersible cyclodextrin-modified loaded magnetic Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers and other one-dimensional structures work together to form a barrier after stripping, thereby improving the wettability of the rock surface, improving the stripping effect, maintaining interface stability, and the fluid has good injection and flow properties. The combination of these elements improves the recovery rate.

[0043] Molybdenum disulfide nanosheets can be selected as small-diameter, thin-layer molybdenum disulfide nanosheets, with a diameter of 10-700 nm and a thickness of less than 8 nm. Preferably, the diameter is 20-500 nm and the thickness is 1-5 nm (Xianfeng Nano, XF137, small-diameter, thin-layer molybdenum disulfide nanosheets). The thin layer and small diameter are beneficial to improving the rheological properties of the fluid and avoiding unintended blockage by nanoscale throats and micro-throats.

[0044] Surfactants can reduce the viscosity of heavy oil, improve its dispersibility, and thus increase its recovery rate. Surfactants include anionic biosurfactants and / or nonionic surfactants.

[0045] Optionally, anionic surfactants are used, selected from anionic biosurfactants such as rhamnolipids. These surfactants possess both good chemical and biological properties, exhibiting amphiphilicity to both oil and water. They can reduce water surface tension, act as wetting agents, promote a decrease in oil-water interfacial tension, and possess biological activity. They can activate native formation microorganisms, playing a synergistic role in oil recovery. Furthermore, they have good biodegradability. When used in conjunction with biodegradable carboxylated cellulose nanofibers and sheet-like nanosheets, they promote the formation of a mixed adsorption layer between the oil and water interfaces, thereby reducing the oil-water interfacial tension. Due to their biodegradability, they can significantly reduce the environmental impact of the oil displacement agent.

[0046] Optionally, a nonionic surfactant is used, which may be polyoxyethylene sorbitan monolaurate. The nonionic surfactant has both hydrophilic and lipophilic parts in its molecule, which can take into account both hydrophilic and lipophilic properties. It has good dispersibility for carboxylated cellulose nanofibers and has a good ability to promote the reduction of oil-water interfacial tension.

[0047] Optionally, anionic biosurfactants and nonionic surfactants can be used as surfactants, with an anionic biosurfactant to nonionic surfactant ratio of 2-3:1. By combining anionic biosurfactants and nonionic surfactants, the characteristics of different surfactants can be fully utilized, which is conducive to the formation of a mixed adsorption layer between the oil and water interface, thereby reducing the interfacial tension between oil and water, reducing the environmental impact of the oil displacement agent, and improving the oil recovery rate. Using a higher proportion of rhamnolipids, due to their biodegradability, can greatly reduce the environmental impact of the oil displacement agent.

[0048] The diluent is a mixture of water and ethanol to promote mixing of the components; wherein, by volume ratio, water:ethanol is (7-8):1.

[0049] Regarding the above-mentioned modified nanofluid oil displacement agent, a method for preparing the modified nanofluid oil displacement agent is provided, the method comprising:

[0050] Preparation of cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes:

[0051] S1. Multi-walled carbon nanotubes are subjected to strong acid treatment with a mixture of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1. The mixture is stirred and refluxed at 30-50℃ (preferably 40℃) for 1-4 hours. After washing with deionized water until neutral, the mixture is filtered and dried to obtain oxidized multi-walled carbon nanotubes. Oxidized multi-walled carbon nanotubes have oxygen-containing active groups such as hydroxyl (-OH) and carboxyl (-COOH), which improves the hydrophilicity and dispersibility of multi-walled carbon nanotubes and facilitates the loading of magnetic nano-Fe3O4.

[0052] S2. Prepare a mixed aqueous solution of ferric chloride and ferrous sulfate, add the oxide multi-walled carbon nanotubes obtained in step S1, adjust the pH of the mixture to 8-9.5, and filter to obtain carbon nanotube powder loaded with magnetic Fe3O4 nanoparticles. The loading of magnetic Fe3O4 nanoparticles is 15-30% (the loading is the mass percentage of Fe3O4 in the total mass of the Fe3O4-loaded carbon nanotube powder). The amounts of ferric chloride and ferrous sulfate can be adjusted according to the loading amount and are not specifically limited here. By loading magnetic Fe3O4 nanoparticles onto multi-walled carbon nanotubes, the multi-walled carbon nanotubes can be driven by magnetic force. When using modified nanofluid oil displacement agents, such as when there are problems such as severe blockage of rock pores, or when it is necessary to enhance fluid fluidity, the one-dimensional cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes can be displaced under the action of magnetic force, changing the stability of the nanofluid oil displacement agent at the oil-water interface and enhancing its variable fluidity. It also facilitates the disintegration of nanostructures such as blockage pores and the entry into nanostructures. The stable bonding of magnetic Fe3O4 nanoparticles to the outer wall of carbon nanotubes increases the contact between carbon nanotubes and oil, and makes them less prone to agglomeration, which is beneficial to improving the performance of dot-shaped nanoparticles.

[0053] If the loading of magnetic nano-Fe3O4 exceeds the above range, the relatively high specific gravity of magnetic nano-Fe3O4 may exacerbate the deposition risk of multi-walled carbon nanotubes, which is detrimental to the stability of the modified nanofluid oil displacement agent system. If the loading is less than the above range, the proportion of magnetic nano-Fe3O4 is too low, resulting in an insignificant magnetic driving effect on multi-walled carbon nanotubes. When using modified nanofluid oil displacement agents, problems such as severe pore blockage in rock formations and the need to enhance fluid flowability cannot be addressed by reducing pore blockage and improving rheological properties under magnetic force.

[0054] S3. Prepare a cyclodextrin dispersion by mixing cyclodextrin, a silane coupling agent, and an ethanol-water solution. Add 0.2-0.3 g of the carbon nanotube powder loaded with magnetic Fe3O4 nanoparticles prepared in step S2 to 60-80 ml of the cyclodextrin dispersion solution, ultrasonically disperse for 5-15 min, magnetically stir, centrifuge, repeatedly wash with distilled water, and vacuum dry to obtain cyclodextrin-modified magnetic Fe3O4 nanoparticle multi-walled carbon nanotubes. The volume ratio of ethanol in the ethanol-water solution is 20-40%.

[0055] The cyclodextrin and silane coupling agent are added in the following mass ratio: the mass ratio of cyclodextrin:silane coupling agent:carbon nanotubes loaded with magnetic nano-Fe3O4 is 1:(0.3-0.6):(1-1.6), and can be selected as 1:0.4:1.5, 1:0.3:1, 1:0.5:1.3, 1:0.5:1.5, 1:0.5:1.1, 1:0.5:1.6, etc.

[0056] Multi-walled carbon nanotubes (MWCNTs) loaded with magnetic Fe3O4 nanoparticles suffer from several adverse effects, including increased density, enhanced interparticle attraction, and the inherent aggregation tendency of MMCNTs. Cyclodextrin, an inexpensive cyclic supramolecular host compound, possesses numerous hydrophilic hydroxyl groups on the outside and hydrophobic cavities formed internally through hydrogen bonding. This hydrophobic interior creates a pocket-like structure with a unique molecular structure, allowing it to form inclusion complexes with MMCNTs. This enables better dispersion of the MMCNTs in solution. Therefore, modification with a small amount of cyclodextrin mitigates the aforementioned adverse effects, improves the hydrophilicity and dispersibility of the nanoparticles, and can promote changes in rock wettability, thereby increasing oil recovery.

[0057] Cyclodextrin is preferably 2-hydroxypropyl β-cyclodextrin and methyl β-cyclodextrin, etc., to improve the dispersibility of carbon nanotubes loaded with magnetic Fe3O4 nanoparticles in water, thereby improving the stability of the modified nanofluid oil displacement agent. If the proportion of cyclodextrin used is greater than the above range, excessive cyclodextrin will lead to an increase in the viscosity of the solution system and will affect the interaction between the nano-components and the oil phase; if it is less than the above range, it will not be conducive to improving dispersibility.

[0058] The silane coupling agent can be KH570. KH570 is a substance with two functional groups of different properties. Some of these functional groups can react with organic molecules, while others can react with inorganic surfaces, thereby improving the adhesion between cyclodextrin and carbon nanotubes loaded with magnetic Fe3O4 containing oxygen-containing active groups such as hydroxyl (-OH) and carboxyl (-COOH). If the proportion of silane coupling agent is less than the above range, the reactivity between cyclodextrin and multi-walled carbon nanotubes loaded with magnetic Fe3O4 decreases, which is not conducive to the formation of a stable complex. If the proportion is greater than the above range, it will cause unnecessary waste, and excessive use of silane coupling agent will also lead to unpredictable changes in the solution system, affecting its effectiveness.

[0059] Cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactants, and diluents are mixed according to the mass fractions of each component, stirred, and ultrasonically dispersed to obtain a modified nanofluid oil displacement agent. The modified nanofluid oil displacement agent is added to water (which can be oilfield water) at a mass percentage of 2-6% to prepare a modified nanofluid oil displacement agent system, which is then injected into the oil reservoir to displace oil and improve the recovery rate.

[0060] The features and performance of this application will be further described in detail below with reference to embodiments:

[0061] Example 1

[0062] A modified nanofluid oil displacement agent, wherein the mass parts of each component are as follows: 10 parts of cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes; 27 parts of carboxylated cellulose nanofibers; 5 parts of molybdenum disulfide nanosheets; 26 parts of surfactant; and 38 parts of diluent.

[0063] The preparation method of cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes is as follows:

[0064] S1. Multi-walled carbon nanotubes were subjected to strong acid treatment with a mixture of concentrated sulfuric acid and concentrated nitric acid (volume ratio 3:1), stirred and refluxed at 40°C for 2 hours, washed with deionized water until neutral, filtered, and dried to obtain oxidized multi-walled carbon nanotubes. The multi-walled carbon nanotubes were short-length, large-diameter carbon nanotubes, with an outer diameter of 20-30 nm and a length of 500 nm-800 μm, TNSM5 (Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences).

[0065] S2. Ferric chloride and ferrous sulfate are mixed into an aqueous solution, and the oxidized multi-walled carbon nanotubes obtained in step S1 are added. The pH of the mixture is adjusted to 9. After filtration, carbon nanotube powder loaded with magnetic nano Fe3O4 is obtained, with a magnetic nano Fe3O4 loading of 28%.

[0066] S3. Prepare a cyclodextrin dispersion by mixing cyclodextrin, a silane coupling agent, and an ethanol-water solution. Add 0.25g of the carbon nanotube powder loaded with magnetic Fe3O4 nanoparticles prepared in step S2 to 70ml of the cyclodextrin dispersion solution, ultrasonically disperse for 10min, magnetically stir, centrifuge, repeatedly wash with distilled water, and vacuum dry to obtain cyclodextrin-modified magnetic Fe3O4 nanoparticle multi-walled carbon nanotubes. The volume ratio of ethanol in the ethanol-water solution is 30%.

[0067] Cyclodextrin and silane coupling agent were added in the following mass ratio: cyclodextrin:silane coupling agent:carbon nanotubes loaded with magnetic Fe3O4 nanoparticles in a mass ratio of 1:0.4:1.5. The cyclodextrin was methylβ-cyclodextrin, and the silane coupling agent was KH570.

[0068] Carboxylated cellulose nanofibers, with a diameter of 3-5 nm and a fiber length of 500-1000 nm, are produced by Guilin Qihong Technology Co., Ltd.

[0069] Molybdenum disulfide nanosheets, with a diameter of 20-500 nm and a thickness of 1-5 nm (Xianfeng Nano, XF137, small diameter thin-layer molybdenum disulfide nanosheets).

[0070] The surfactant is selected from anionic biosurfactant (rhamnolipid) and nonionic surfactant (polyoxyethylene sorbitan monolaurate) in a ratio of 2:1. The diluent is a mixed solution of water and ethanol, with a volume ratio of water to ethanol of 7:1.

[0071] Cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactants, and diluents were mixed according to the mass fractions of each component, stirred, and ultrasonically dispersed to obtain a modified nanofluid oil displacement agent. The modified nanofluid oil displacement agent was then added to water at a mass percentage of 3% to prepare a modified nanofluid oil displacement agent system.

[0072] Performance testing:

[0073] Wettability: Core samples with different wettability exhibit significantly different recovery effects. The more hydrophilic the core, the greater the driving force for displacing crude oil, promoting the permeation and replacement of oil and water between the matrix and fractures, resulting in a greater amount of displaced crude oil and a higher recovery rate. Wettability is characterized by the contact angle between water and oil. High-resolution images of core slices taken with a high-speed camera are used to measure the contact angle of water or oil droplets on the rock slice surface. The oil droplets used in the test were selected with a crude oil density of 0.91 tons per cubic meter and a viscosity (at room temperature) of 1100 mPa·s. Core samples (porosity 27%) from a heavy oil well were drilled to prepare rock slices. The contact angle measurement experiment showed that the contact angle between the oil droplet and the rock slice was 24°, indicating an oleophilic rock slice. Rock slices were immersed in a modified nanofluid oil displacement agent system containing water for 24 hours. The water droplet contact angle was 36° and the oil droplet contact angle was 146°. This indicates that the modified nanofluid oil displacement agent can significantly change the wettability of oleophilic rocks, making them more hydrophilic and oleophobic. This is beneficial for crude oil discharge and improves oil recovery. Highly hydrophilic cores can also reduce fluid flow resistance.

[0074] Interfacial tension testing: Measured using a fully automated rotating drop ultra-low interfacial tension meter TX-500C at 80℃ and 5000 rpm. The modified nanofluid oil displacement agent system was used as the aqueous phase, and the oil droplets used in the test were used as the oil phase. The interfacial tension was 4.2 × 10⁻⁶. - 3 With an interfacial tension of mN / m, it exhibits strong interfacial activity, reduces interfacial tension, decreases capillary force, and improves oil displacement efficiency.

[0075] Dispersion stability test: 100 ml of solution was taken from a beaker, and the time for aggregation and flocculation of the modified nanofluid oil displacement agent system was observed and recorded every 1 hour to test the dispersion stability. The flocculation time was 48 hours. To test the dispersion stability of the system under the influence of magnetic force, an external magnetic field with a strength of 1000 Gs was applied to the beaker. The flocculation time under the applied magnetic field was 44 hours. The dispersibility is good, and the modified nanofluid oil displacement agent system can be affected by a magnetic field.

[0076] Oil recovery test: An oil displacement experiment was conducted using a core displacement device to simulate reservoir conditions. Multiple artificial heterogeneous core samples were taken, and their basic parameters were measured and recorded. The experimental temperature was 80℃. Water was used to displace the core samples at a rate of 0.01 mL / min until the water content of the outlet liquid exceeded 98%. The water displacement recovery rate was recorded as 18.4%. After water displacement, the prepared modified nanofluid oil displacement agent system was injected with and without an external magnetic field (magnetic field strength 1000 Gs). The total oil recovery rates of the core samples were recorded as 65% and 67%, respectively. This significantly improves oil recovery, and the magnetic field further enhances the oil displacement dynamics.

[0077] Example 2

[0078] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being: multi-walled carbon nanotubes with an outer diameter of 10-16 nm and a length of 700 nm-1000 nm.

[0079] Performance testing:

[0080] Wettability: The water droplet contact angle is 37°, and the oil droplet contact angle is 143°. Compared with Example 1, the water droplet contact angle is slightly larger, the oil droplet contact angle is slightly smaller, the hydrophilic wettability is slightly worse, and the interfacial tension is 5.1 × 10⁻⁶. -3 With mN / m, the flow resistance increased slightly, and the flocculation time was 44h. With the external magnetic field, the flocculation time was 40h, and the dispersibility deteriorated significantly. Without the external magnetic field and with the external magnetic field, the total recovery rate of the core was 63% and 64%, respectively. The recovery rate decreased, possibly because the outer diameter of the multi-walled carbon nanotubes decreased, increasing the risk of aggregation and affecting the recovery rate.

[0081] Example 3

[0082] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being: multi-walled carbon nanotubes with an outer diameter of 40-50 nm and a length of 700 nm-1000 nm.

[0083] Performance testing:

[0084] Wettability: The water droplet contact angle is 39°, and the oil droplet contact angle is 140°. Compared with Example 1, the water droplet contact angle is slightly larger, the oil droplet contact angle is slightly smaller, the hydrophilic wettability is slightly worse, and the interfacial tension is 6.4 × 10⁻⁶. -3 With mN / m, the flow resistance increased slightly, and the flocculation time was 43h. With the external magnetic field, the flocculation time was 40h, and the dispersibility deteriorated significantly. Without the external magnetic field and with the external magnetic field, the total recovery rate of the core was 61% and 62%, respectively. The recovery rate decreased, possibly because the outer diameter of the multi-walled carbon nanotubes increased, which is not conducive to reducing interfacial tension and affecting the recovery rate.

[0085] Example 4

[0086] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that the loading of magnetic nano Fe3O4 is 10%.

[0087] Performance testing:

[0088] Wettability: The water droplet contact angle is 36°, and the oil droplet contact angle is 145°. Compared with Example 1, the changes in the water droplet contact angle and oil droplet contact angle are small, indicating a small change in hydrophilic wettability. The interfacial tension is 4.4 × 10⁻⁶. -3 The mN / m value showed little change, with flocculation occurring at a time of 48 hours. Flocculation also occurred at a time of 48 hours under the applied magnetic field. The dispersibility did not change significantly under the magnetic field conditions. The total recovery rate of the core was 65.2% under both the absence and application of the external magnetic field. The recovery rate did not change significantly, but there was no obvious magnetic field response.

[0089] Example 5

[0090] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that the loading of magnetic nano Fe3O4 is 40%.

[0091] Performance testing:

[0092] Wettability: The water droplet contact angle is 40°, and the oil droplet contact angle is 140°. Compared with Example 1, the water droplet contact angle is larger, and the oil droplet contact angle is smaller, indicating poorer hydrophilic wettability. The interfacial tension is 5.7 × 10⁻⁶. -3 As the mN / m increases, the flow resistance increases, and the flocculation time is 38 hours. With the external magnetic field applied, the flocculation time is 32 hours, and the dispersibility becomes worse. Without the external magnetic field and with the external magnetic field applied, the total recovery rate of the core is 61% and 63%, respectively, indicating a decrease in recovery rate. The external magnetic field has a more significant effect on improving the recovery rate, but it is still lower than that of Example 1.

[0093] Example 6

[0094] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being: molybdenum disulfide nanosheets with a diameter of 600-700 nm and a thickness of 15 nm.

[0095] Performance testing:

[0096] Wettability: Water droplet contact angle is 36°, oil droplet contact angle is 145°. Compared with Example 1, the change in hydrophilic wettability is small, and the interfacial tension is 4.7 × 10⁻⁶. -3As the mN / m increases, the flow resistance increases, and the flocculation time is 47 hours. With the external magnetic field applied, the flocculation time is 43 hours, and the dispersibility becomes worse. Without the external magnetic field and with the external magnetic field applied, the total recovery rate of the core is 64% and 66%, respectively, indicating a decrease in recovery rate.

[0097] Example 7

[0098] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that only the anionic surfactant rhamnolipid is used as the surfactant.

[0099] Performance testing:

[0100] Wettability: Water droplet contact angle is 38°, oil droplet contact angle is 144°. Compared with Example 1, the hydrophilic wettability is slightly worse, and the interfacial tension is 4.8 × 10⁻⁶. -3 With mN / m, the flow resistance increased slightly, and the flocculation time was 48h. With the external magnetic field, the flocculation time was 45h, and the dispersibility was not significantly different. Without the external magnetic field and with the external magnetic field, the total recovery rate of the core was 63% and 65%, respectively, and the recovery rate decreased.

[0101] Example 8

[0102] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that the surfactant used is the nonionic surfactant polyoxyethylene dehydrated sorbitan monolaurate.

[0103] Performance testing:

[0104] Wettability: Water droplet contact angle is 39°, oil droplet contact angle is 147°. Compared with Example 1, the hydrophilic wettability is slightly worse, and the interfacial tension is 5.0 × 10⁻⁶. -3 With mN / m, the flow resistance increased slightly, and the flocculation time was 48h. With the external magnetic field, the flocculation time was 46h, and the difference in dispersibility was not significant. Without the external magnetic field and with the external magnetic field, the total recovery rate of the core was 61% and 64%, respectively, and the recovery rate decreased.

[0105] Example 9

[0106] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that the mass ratio of cyclodextrin: silane coupling agent: carbon nanotubes loaded with magnetic nano Fe3O4 is 1:0.4:1.4.

[0107] Performance testing:

[0108] Wettability: Water droplet contact angle is 34°, and oil droplet contact angle is 147°. Compared with Example 1, it can be seen that improving the use of cyclodextrin results in better hydrophilic wettability, with an interfacial tension of 3.8 × 10⁻⁶. -3With mN / m, the flow resistance decreased, and flocculation occurred in 49 hours. With the external magnetic field applied, flocculation occurred in 46 hours, indicating good dispersion. Without the external magnetic field and with the external magnetic field applied, the total recovery rate of the core was 66% and 68%, respectively, showing an improvement in recovery rate.

[0109] Comparative Example 1:

[0110] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that the magnetic nano Fe3O4 multi-walled carbon nanotubes loaded with magnetic nanoparticles were not modified with cyclodextrin.

[0111] Performance testing:

[0112] Wettability: The water droplet contact angle is 46°, and the oil droplet contact angle is 134°. Compared with Example 1, the water droplet contact angle is larger, and the oil droplet contact angle is smaller, indicating poorer hydrophilic wettability. The interfacial tension is 2.1 × 10⁻⁶. -2 As the mN / m ratio increases, the flow resistance increases, and the flocculation time becomes 36 hours. With an external magnetic field, the flocculation time becomes 27 hours, and the dispersibility deteriorates, especially due to the increased influence of the magnetic field. The total recovery rate of the core is 55% with and without an external magnetic field, respectively, indicating a decrease in recovery rate. Without cyclodextrin modification, the external magnetic field does not help improve the recovery rate and may even reduce it. The possible reason is that although the external magnetic field has a certain effect on changing the rheological properties, it worsens the dispersion stability of the system. In summary, it does not improve the recovery rate.

[0113] Comparative Example 2:

[0114] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that carboxylated cellulose nanofibers were not added, and other components were added in proportion.

[0115] Performance testing:

[0116] Wettability: The water droplet contact angle is 63°, and the oil droplet contact angle is 115°. Compared with Example 1, the water droplet contact angle is significantly larger, and the oil droplet contact angle is significantly smaller, indicating a significant decrease in hydrophilic wettability. The interfacial tension is 7.5 × 10⁻⁶. -2 The mN / m ratio was relatively larger without the addition of carboxylated cellulose nanofibers, resulting in significantly increased flow resistance and a flocculation time of 40 hours. With the external magnetic field applied, the flocculation time was 33 hours, indicating a decrease in dispersibility. The total recovery rate of the core was 42% without the external magnetic field and 44% with the external magnetic field applied, indicating a decrease in recovery rate. The absence of carboxylated cellulose nanofibers was not conducive to improving the recovery rate.

[0117] Comparative Example 3:

[0118] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that molybdenum disulfide nanosheets were not added, and other components were added in proportion.

[0119] Performance testing:

[0120] Wettability: The water droplet contact angle is 42°, and the oil droplet contact angle is 131°. Compared with Example 1, the water droplet contact angle is larger and the oil droplet contact angle is smaller, indicating a decrease in hydrophilic wettability. The interfacial tension is 1.1 × 10⁻⁶. -2 mN / m, the flow resistance increases; flocculation time is 43h, and flocculation time is 40h with external magnetic field applied, the dispersibility is worse; without external magnetic field and with external magnetic field applied, the total recovery rate of core is 57% and 59% respectively, the recovery rate decreases.

[0121] Comparative Example 4:

[0122] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that: molybdenum disulfide nanosheets and carboxylated cellulose nanofibers were not added, and other components were added in proportion.

[0123] Performance testing:

[0124] Wettability: The water droplet contact angle is 65°, and the oil droplet contact angle is 109°. Compared with Example 1, the water droplet contact angle is significantly larger, and the oil droplet contact angle is significantly smaller, indicating a significant decrease in hydrophilic wettability. The interfacial tension is 9.7 × 10⁻⁶. -2 The relative increase in mN / m leads to increased flow resistance and a flocculation time of 39 hours. With an external magnetic field, the flocculation time is 31 hours, and the dispersibility deteriorates. Without and with an external magnetic field, the total recovery rate of the core is 40% and 43%, respectively, indicating a decrease in recovery rate.

[0125] Comparative Example 5:

[0126] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, the main difference being that the cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes are replaced with single-arm carbon nanotubes.

[0127] Performance testing:

[0128] Wettability: The water droplet contact angle is 43°, and the oil droplet contact angle is 127°. Compared with Example 1, the water droplet contact angle is larger, and the oil droplet contact angle is smaller, indicating poorer hydrophilic wettability. The interfacial tension is 1.0 × 10⁻⁶. -2 As the mN / m ratio increases, the flow resistance increases, flocculation occurs at a time of 35 hours, and the dispersibility deteriorates. The total recovery rate of the core without an external magnetic field is 53%, indicating a decrease in recovery rate.

[0129] Comparative Example 6

[0130] The composition and preparation method of the modified nanofluid oil displacement agent in Example 1 are basically the same, with the main difference being: 3 parts of cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes; 50 parts of carboxylated cellulose nanofibers; 3 parts of molybdenum disulfide nanosheets; 24 parts of surfactant; and 38 parts of diluent.

[0131] Performance testing:

[0132] Wettability: Water droplet contact angle is 35°, and oil droplet contact angle is 147°. Compared with Example 1, the hydrophilic wettability is better, and the interfacial tension is 8.4 × 10⁻⁶. -2 The mN / m ratio was relatively large without the addition of carboxylated cellulose nanofibers, resulting in a significant increase in flow resistance and a flocculation time of 33 hours. With the external magnetic field applied, the flocculation time was 32 hours, indicating poor dispersibility. The total recovery rate of the core was 47% with and without the external magnetic field applied. The excessively high dispersibility of carboxylated cellulose nanofibers was detrimental to improving the recovery rate.

[0133] In summary, by combining one-dimensional cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes and carboxylated cellulose nanofibers with different properties, one-dimensional magnetic Fe3O4 nanotubes, and two-dimensional molybdenum disulfide nanosheets, a modified nanofluid oil displacement agent with good dispersibility, good fluid flowability, reduced heavy oil viscosity, minimal pore clogging, and good oil displacement effect is formed, effectively improving oil recovery. A method for preparing this modified nanofluid oil displacement agent does not require complex modification of multiple nanostructures; the preparation process is simple and relatively easier to achieve stable preparation of the corresponding formulation.

[0134] The preferred embodiments of the present invention have been described in detail above, and are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A modified nanofluid oil displacement agent, characterized in that: The components include: cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactants, and diluents. The mass fractions of each component include: 9-12 parts of cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes; 21-36 parts of carboxylated cellulose nanofibers; 3-6 parts of molybdenum disulfide nanosheets; 24-30 parts of surfactant; and 25-40 parts of diluent. The cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes are prepared using multi-walled carbon nanotubes with an outer diameter of 15-40 nm and a length of 400 nm-3 μm as raw materials. The magnetic Fe3O4 loading is 15-30%. The cyclodextrin and silane coupling agent are used in the following mass ratio: cyclodextrin:silane coupling agent:carbon nanotubes loaded with magnetic Fe3O4 at a mass ratio of 1:(0.3-0.6):(1-1.6). Preparation of the cyclodextrin-modified supported magnetic Fe3O4 multi-walled carbon nanotubes: S1. Multi-walled carbon nanotubes are subjected to strong acid treatment with a mixture of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:

1. The mixture is stirred and refluxed at 30-50℃ for 1-4 hours. After washing with deionized water until neutral, the mixture is filtered and dried to obtain oxidized multi-walled carbon nanotubes. S2. Prepare a mixed aqueous solution of ferric chloride and ferrous sulfate, add the oxide multi-walled carbon nanotubes obtained in step S1, adjust the pH of the mixed solution to 8-9.5, filter and obtain carbon nanotube powder loaded with magnetic nano Fe3O4. The magnetic nano Fe3O4 loading is 15-30% of the total mass of the carbon nanotube powder loaded with Fe3O4, based on the mass percentage of Fe3O4 in the total mass of the Fe3O4 loaded carbon nanotube powder. S3. Prepare a cyclodextrin dispersion solution by mixing cyclodextrin, silane coupling agent, and ethanol aqueous solution. Add 0.2-0.3g of carbon nanotube powder loaded with magnetic nano Fe3O4 prepared in step S2 to 60-80ml of cyclodextrin dispersion solution, sonicate for 5-15min, magnetically stir, centrifuge, rinse repeatedly with distilled water, and vacuum dry to obtain cyclodextrin-modified magnetic nano Fe3O4 multi-walled carbon nanotubes. The volume ratio of ethanol in the ethanol aqueous solution is 20-40%.

2. The modified nanofluid oil displacement agent according to claim 1, characterized in that, Carboxylated cellulose nanofibers have a diameter of 2-10 nm and a length of 400 nm-3 μm.

3. The modified nanofluid oil displacement agent according to claim 1, characterized in that, Molybdenum disulfide nanosheets are small-diameter, thin-layered molybdenum disulfide nanosheets with a diameter of 10-700 nm and a thickness of less than 8 nm.

4. The modified nanofluid oil displacement agent according to claim 1, characterized in that, Surfactants include anionic surfactants and / or nonionic surfactants.

5. The modified nanofluid oil displacement agent according to claim 1, characterized in that, The diluent is a mixture of water and ethanol, wherein the volume ratio of water to ethanol is (7-8):

1.

6. The modified nanofluid oil displacement agent according to claim 1, characterized in that, The cyclodextrin is 2-hydroxypropylβ-cyclodextrin or methylβ-cyclodextrin; the silane coupling agent is KH570.

7. A method for preparing a modified nanofluid oil displacement agent, comprising preparing the modified nanofluid oil displacement agent as described in any one of claims 1-6, characterized in that, The method includes: mixing cyclodextrin-modified magnetic Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactants and diluents according to the mass fractions of each component, stirring, and ultrasonically dispersing to obtain a modified nanofluid oil displacement agent.

8. The preparation method of the modified nanofluid oil displacement agent according to claim 7, characterized in that, In step S3, cyclodextrin and silane coupling agent are added in the following mass ratio: the mass ratio of cyclodextrin:silane coupling agent:carbon nanotubes loaded with magnetic nano Fe3O4 is 1:(0.3-0.6):(1-1.6).