Modified nanofluid oil-displacing agent and preparation method thereof

Abstract

The invention relates to the technical field of petroleum development, and provides a modified nanofluid oil-displacing agent and a preparation method thereof, and the modified nanofluid oil-displacing agent comprises the following components: cyclodextrin modified loaded magnetic nano Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets and a surfactant. The composite material is prepared from the following components in parts by mass: 9 to 12 parts of cyclodextrin modified loaded magnetic nano Fe3O4 multi-walled carbon nanotubes; the coating is prepared from the following components in parts by weight: 21-36 parts of carboxylated cellulose nanofibers, 3-6 parts of molybdenum disulfide nanosheets, 24-30 parts of a surfactant and 25-40 parts of a diluent. Two kinds of cyclodextrin modified loaded magnetic nano Fe3O4 multi-walled carbon nanotubes with different characteristics and a one-dimensional structure, carboxylated cellulose nanofibers, magnetic nano Fe3O4 with a 0-dimensional structure and molybdenum disulfide nanosheets with a two-dimensional structure are compounded to form a composite material with good dispersity, good fluid fluidity, reduced viscosity of thickened oil, low possibility of pore blockage, high viscosity, high viscosity, high viscosity, high viscosity, high viscosity, high viscosity, high viscosity and the like. The modified nanofluid oil-displacing agent is good in oil-displacing effect, and the recovery efficiency is effectively improved.

Description

Technical Field

[0001] This application relates to the technical field of oil development, and particularly relates to a modified nanofluid oil displacement agent and a preparation method thereof. Background Art

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

[0003] Low-permeability oil reservoirs have the characteristics of low permeability, small pore throat radius, large seepage resistance, small porosity (below 40%, etc.), serious throat blockage, and heavy oil. Especially for heavy oil in low-permeability oil reservoirs, it has high viscosity, etc. The interfacial tension between heavy oil and water is high, and the viscosity of crude oil is large, making it easier to be trapped in pores and difficult to flow out. Therefore, there are technical problems of "difficult to inject and difficult to produce" in the field.

[0004] A nanofluid oil displacement agent is a nanofluid that is prepared by adding solid nanoparticles to a liquid solvent and obtaining a homogeneous suspension system under mechanical agitation. As a new type of oil displacement agent, the nanofluid oil displacement agent has great advantages compared with traditional chemical oil displacement agents, such as a high specific surface area, excellent biocompatibility, high oil recovery rate, etc., and has received increasing attention. Classifying the nanoparticles in the nanofluid oil displacement agent according to dimensions: 0D (nanospheres, nanoparticles), 1D (nanowires, nanotubes), 2D (nanosheets), etc.

[0005] Currently, most of the nanomaterials used to improve oil recovery are 0D spherical nanoparticles, which can reduce the interfacial tension and can adsorb on the solid surface to change the wettability, thereby improving the oil recovery rate. However, because of its easy adsorption, large loss, and the spherical shape resulting in insufficient interfacial contact, low efficiency, particle aggregation, flocculation precipitation, and small particle size being prone to clogging fine pores. For example, under harsh conditions (such as high-salt conditions), the electrostatic repulsion between nanoparticles will be weakened, easily leading to particle aggregation, flocculation precipitation, resulting in difficult injection or serious adsorption phenomena. In addition, 0D spherical nanoparticles cannot effectively reduce the viscosity of heavy oil, resulting in difficulty in further reducing the interfacial tension between oil and water, affecting the oil recovery rate.

[0006] Among 1D nanomaterials, there are studies showing that hydrophobic multi-walled carbon nanotubes, single-walled carbon nanotubes, activated carbon nanoparticles, etc. have oil displacement effects and have a certain ability to change wettability and reduce interfacial tension. However, the strong hydrophobicity and easy aggregation of carbon nanotubes, as well as low stability, etc. affect their oil displacement effects.

[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 has more sufficient contact with oil and water during the oil displacement process and has better interfacial effects, etc. However, there are still problems such as the huge surface free energy of the nanosheets, obvious agglomeration and sedimentation trends, easy blockage of fine pore throats when injected into porous media, causing semi-permanent or permanent damage to the reservoir, and it is difficult to achieve both good dispersion stability and interfacial adsorption.

[0008] In summary, for the nanofluid oil displacement agent, there are problems such as aggregation and flocculation precipitation of nanoparticles, poor overall stability, limited abilities to reduce the oil-water interfacial tension, improve the oil-rock separation ability, and improve the fluidity of the fluid, pore blockage, large adsorption loss at the rock interface, and short-lived effects. How to obtain a nanofluid oil displacement agent that can effectively reduce the viscosity of heavy oil, has good fluid dispersion, good stability, is not easily blocked, and has small adsorption loss is an important technical improvement direction for the effective utilization of low-permeability oil reservoirs. Summary of the Invention

[0009] Overcoming the deficiencies of the prior art, this application provides a modified nanofluid oil displacement agent and a preparation method thereof, which have the advantages of good dispersion, good fluidity, reducing the viscosity of heavy oil, not easily blocking pores, small adsorption loss, good oil displacement effect, and improving oil recovery rate.

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

[0011] In the first aspect, an example of this application provides a modified nanofluid oil displacement agent, which includes the following components: cyclodextrin-modified magnetic nano-Fe 3 O 4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactant and diluent;

[0012] The mass fractions of each component include: cyclodextrin-modified magnetic nano-Fe 3 O 4 multi-walled carbon nanotubes are 9 - 12 parts; carboxylated cellulose nanofibers are 21 - 36 parts, molybdenum disulfide nanosheets are 3 - 6 parts, surfactant is 24 - 30 parts, and diluent is 25 - 40 parts.

[0013] Optionally, in the cyclodextrin-modified magnetic nano-Fe 3 O 4 multi-walled carbon nanotubes: multi-walled carbon nanotubes with an outer diameter of 15 - 40 nm and a length of 400 nm - 3 μm are used as raw materials for preparation, and the magnetic nano-Fe 3 O 4 loading amount is 15 - 30%, and cyclodextrin and silane coupling agent are in the following mass ratio, cyclodextrin: silane coupling agent: loaded magnetic nano-Fe 3 O 4The mass ratio of the carbon nanotubes is 1:(0.3 - 0.6):(1 - 1.6).

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

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

[0016] Optionally, the surfactant includes an anionic surfactant and / or a non-ionic surfactant.

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

[0018] Optionally, the cyclodextrin is 2-hydroxypropyl-β-cyclodextrin or methyl-β-cyclodextrin; the silane coupling agent is kh570.

[0019] In a second aspect, the present application example provides a preparation method of a modified nanofluid oil displacement agent for preparing the modified nanofluid oil displacement agent as described above.

[0020] The method includes: modifying and loading magnetic nano-Fe 3 O 4 Multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactant and diluent are mixed according to the mass parts of each component, stirred, and ultrasonically dispersed to obtain a modified nanofluid oil displacement agent.

[0021] Optionally, the preparation of the cyclodextrin-modified and loaded magnetic nano-Fe 3 O 4 Multi-walled carbon nanotubes is as follows:

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

[0023] S2. Ferric chloride and ferrous sulfate are made into a mixed aqueous solution, the oxidized multi-walled carbon nanotubes obtained in step S1 are added, the pH of the mixed solution is adjusted to 8 - 9.5, and after suction filtration, carbon nanotube powder loaded with magnetic nano-Fe 3 O 4 is obtained. The loading amount of magnetic nano-Fe 3 O 4 is 15 - 30% by mass percentage based on the total mass of the carbon nanotube powder loaded with magnetite.

[0024] S3. Mix cyclodextrin, silane coupling agent, and aqueous ethanol solution to prepare a cyclodextrin dispersion solution. Add 0.2 - 0.3 g of the carbon nanotube powder loaded with magnetic nano-Fe 3 O 4 prepared in step S2 to 60 - 80 ml of the cyclodextrin dispersion solution, ultrasonically disperse for 5 - 15 min, perform magnetic stirring treatment, then centrifuge, repeatedly rinse with distilled water, and vacuum dry to obtain cyclodextrin-modified magnetic nano-Fe 3 O 4 multi-walled carbon nanotubes; wherein, the volume ratio of ethanol in the aqueous ethanol 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-Fe 3 O 4 is 1:(0.3 - 0.6):(1 - 1.6).

[0026] Beneficial effects include:

[0027] A modified nanofluid oil displacement agent provided by the present invention has a magnetically drivable property due to the 0D magnetic nano-Fe stably loaded by the multi-walled carbon nanotubes with strong reactivity of the one-dimensional structure. Utilize the one-dimensional structure characteristics to form a 3D stable cross-network resistant to high temperature, pull the oil droplets into filaments, improve the fluid flow ability, and promote the viscosity reduction of heavy oil. Through cyclodextrin modification, it makes up for the adverse effects such as the increase in density, the increase in the attraction between nanoparticles, and the certain aggregation tendency of the multi-walled carbon nanotubes itself. Obtain cyclodextrin-modified magnetic nano-Fe 3 O 4 multi-walled carbon nanotubes have the effects of good dispersibility, not easy to agglomerate, certain magnetorheological properties, high hydrophilicity, improving rock wettability, reducing the viscosity of heavy oil, reducing the oil-water interfacial tension, etc., and also help to reduce the loss of 0D magnetic nano-Fe 3 O 4 nanoparticles adsorbed on the rock interface, which can greatly improve the performance of the modified nanofluid oil displacement agent system and increase the oil recovery rate after use. 3 O 4 By introducing carboxylated cellulose nanofibers with a one-dimensional structure of high aspect ratio, good dispersibility, and thixotropy into the modified nanofluid oil displacement agent system, it is beneficial to combine with the cyclodextrin-modified magnetic nano-Fe with a small aspect ratio 3 O 4 whose aspect ratio is small

[0028] 3 O 4 ​Multi-walled carbon nanotubes form a combination of thick and thin particle sizes, weaving a dense three-dimensional cross-network, improving the barrier to the mutual migration of different substances such as oil and water, enhancing the stable stripping ability of oil, improving the interfacial wettability, having better dispersion performance, and reducing the load of magnetic nano-Fe 3 O 4 The sedimentation risk of multi-walled carbon nanotubes is reduced, improving the static stability of the system. At the same time, due to its thixotropy, the modified nanofluid flooding agent system can be optionally combined with magnetic modification, etc. when injecting and reducing the need to block nano-pores. The double-promoted rheological characteristics are combined, with better variable fluidity dynamically, greatly enhancing the performance of the modified nanofluid flooding agent system and increasing the oil recovery rate after use.

[0029] By introducing molybdenum disulfide nanosheets with a two-dimensional structure, under the interaction of one-dimensional structures such as multi-walled carbon nanotubes and carboxylated cellulose nanofibers in highly dispersed cyclodextrin-modified magnetic nano-Fe 3 O 4 Without modification, it also has better dispersion, is not easy to settle, reducing the difficulty of applying two-dimensional structures; the two-dimensional structure with a small thickness and short sheet diameter is arranged in an oriented manner, is not easy to flip compared with nanoparticles, is conducive to spreading out and overlapping adsorption on the interface to improve the barrier ability of the three-dimensional cross-network and the stability of the three-dimensional cross-network. It blocks the contact between oil droplets and between oil droplets and rocks in more dimensions, is conducive to guiding the system to form a wedge-shaped structural molecular film to increase the structural partial pressure, is conducive to the propulsion and separation of the fluid, and promotes the peeling of crude oil from the rock surface.

[0030] In summary, through the combination of cyclodextrin-modified magnetic nano-Fe 3 O 4 with one-dimensional structures of multi-walled carbon nanotubes and carboxylated cellulose nanofibers, and magnetic nano-Fe 3 O 4 with a zero-dimensional structure, and molybdenum disulfide nanosheets with a two-dimensional structure, a modified nanofluid flooding agent with good dispersion, good fluid fluidity, reduced viscosity of heavy oil, not easy to block pores, and good oil displacement effect is formed, effectively increasing the oil recovery rate. A preparation method of a modified nanofluid flooding agent does not require complex modification of multiple nanostructures, the preparation process is simple, and it is relatively easier to stably prepare the corresponding preparation. Specific embodiments

[0031] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the protection scope of the present invention.

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

[0033] Those skilled in the art can understand that, unless specifically stated, the singular forms "a", "an", "the" and "said" used herein may also include the plural forms. It should be further understood that the term "comprising" used in the specification of the present invention means the presence of the stated features, integers, steps, operations, elements and / or components, but does not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or their groups. The phrase "and / or" used herein includes any and all units and all combinations of one or more of the associated listed items.

[0034] Regarding the problems of nanoparticle aggregation, flocculation precipitation, poor overall stability, limited ability to reduce the oil-water interfacial tension, improve the oil-rock separation ability, fluid mobility, and pore blockage in nanofluid oil displacement agents, an embodiment of the present invention provides a modified nanofluid oil displacement agent and a preparation method.

[0035] Exemplarily, a modified nanofluid oil displacement agent is provided, which includes the following components: cyclodextrin-modified magnetic nano-Fe 3 O 4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactants, and diluents.

[0036] The mass fractions of each component include: cyclodextrin-modified magnetic nano-Fe 3 O 4 The multi-walled carbon nanotubes are 9 - 12 parts (which can be 9, 10, 11, 12, etc.); the carboxylated cellulose nanofibers are 21 - 36 parts (which can be 21, 22, 24, 26, 28, 31, 33, 34, 35, 36, etc.), the molybdenum disulfide nanosheets are 3 - 6 parts (which can be 3, 4, 5, 6, etc.), the surfactants are 24 - 30 parts (which can be 24, 26, 28, 29, 30, etc.), and the diluents are 25 - 40 parts (which can be 25, 26, 28, 29, 32, 24, 34, 35, 37, 38, 40, etc.).

[0037] Cyclodextrin-modified magnetic nano-Fe 3 O 4Multi-walled carbon nanotubes have the effects of good dispersibility, not easy to agglomerate, certain magnetorheological properties, high hydrophilicity, improving rock wettability, reducing the viscosity of heavy oil, and reducing the oil-water interfacial tension, etc. They can greatly improve the performance of the modified nanofluid flooding agent system and increase the oil recovery rate after use.

[0038] Compared with single-walled carbon nanotubes, which have obvious deficiencies such as stable chemical properties but strong hydrophobicity and easy aggregation, multi-walled carbon nanotubes (MWCNT) mainly consist of coaxial single-layer nanotube groups with nanoscale interlayer spacing arranged along the fiber axis. They have a certain stiffness, are not easy to bend and wind, have a small aspect ratio (short length and large outer diameter carbon nanotubes), the surface of multi-walled carbon nanotubes is active (strong reactivity), good thermal conductivity, and strong heat resistance. In the emulsion formed by the modified nanofluid flooding agent system, it is easy to form a three-dimensional stable cross-network with high temperature resistance, which can improve the emulsion stability. The high activity is easy to be compounded with surfactants, modifiers, etc. to form ideal properties, and improve the bonding stability of the loaded magnetic nano-Fe 3 O 4 At the same time, in the oil / solid / nanofluid three-phase contact area, it is easier to form a stable thin film. At the same time, its viscosity-reducing effect promotes the reduction of the adhesion work for the oil droplets to peel off from the pore rock wall surface, promotes the occurrence of imbibition. Its one-dimensional rigid structure characteristics are also conducive to pulling the oil droplets into filaments and improving the pulling orientation effect, improving the fluid flow ability, and increasing the oil recovery rate.

[0039] Preferably, the multi-walled carbon nanotubes are short-length and large-outer-diameter carbon nanotubes, with an outer diameter of 15 - 40 nm and a length of 400 nm - 3 μm, preferably 500 nm - 800 nm; it can also be an outer diameter of 20 - 30 nm and a length of 500 nm - 800 nm, such as TNSM5 (Chengdu Organic Chemistry Co., Ltd., Chinese Academy of Sciences); the multi-walled carbon nanotubes are short-length and large-outer-diameter carbon nanotubes. The large outer diameter is beneficial to serving as the framework for forming a three-dimensional stable cross-network with high temperature resistance. When the outer diameter is less than this range and the three-dimensional cross-network is dense, the risk of agglomeration between them increases; when the outer diameter is greater than this range, the particle size is too large, which is not conducive to the solution entering the nano-scale fine throats and micro-throats to improve wettability, and it is easy to cause local blockage of the rock micro-pores, and the contact with different phases at the oil / solid / nanofluid three-phase interface is reduced, which is not conducive to reducing the interfacial tension and changing the rock wettability.

[0040] By loading magnetic nano-Fe 3 O 4 onto multi-walled carbon nanotubes, the dot-like magnetic nano-Fe 3 O 4 particles are stabilized, and at the same time, it can promote its ability to change the fluidity under the action of magnetic and external forces. Loading magnetic nano-Fe 3 O 4, due to the adverse effects such as increased density, increased attraction between nanoparticles, and the inherent aggregation tendency of multi-walled carbon nanotubes themselves, through the modification with a small amount of cyclodextrin, the influence of the aforementioned adverse factors is compensated, and the hydrophilicity of the nanoparticles is improved and they have better dispersibility, which can promote the change of rock wettability and improve oil recovery.

[0041] Cellulose nanofibers are a kind of nanomaterial with relatively wide sources and low preparation costs, and their use can reduce the cost of nanomaterials. Carboxylated cellulose nanofibers are prepared by introducing carboxyl functional groups on the surface of cellulose nanofibers through a chemical modification method. 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, and they are easily soluble in water and have high dispersibility. Carboxylated cellulose nanofibers have the characteristics of a high aspect ratio, with a fiber diameter of 2 - 10 nm and a length of 400 nm - 3 μm (optionally: carboxylated cellulose nanofibers produced by Guilin Qihong Technology Co., Ltd., with a diameter of 3 - 5 nm and a fiber length of 500 - 1000 nm), and they have thixotropy (that is, in a low mass fraction solution (below 0.3%), without shear force, they can be in a stable suspension state, and when the shear action such as agitation is small, they can become a flowing solution). By introducing carboxylated cellulose nanofibers with a high aspect ratio, good dispersibility, and thixotropy into the modified nanofluid oil displacement agent system, it is beneficial to form a combination of thick and thin particle sizes with multi-walled carbon nanotubes with a small aspect ratio to form a dense three-dimensional cross-network, so that a stable thin film is more easily formed in the oil / solid / nanofluid three-phase contact area, improving the barrier to the mutual migration of different substances such as oil and water, and improving oil stable stripping, interfacial wettability, etc.; it has better dispersion performance, supports the dispersion effect, and can reduce the sedimentation risk of multi-walled carbon nanotubes loaded with magnetic nano-Fe 3 O 4 Multi-walled carbon nanotubes, forming a combination of thick and thin particle sizes, weaving a dense three-dimensional cross-network, so that a stable thin film is more easily formed in the oil / solid / nanofluid three-phase contact area, improving the barrier to the mutual migration of different substances such as oil and water, and improving oil stable stripping, interfacial wettability, etc.; it has better dispersion performance, supports the dispersion effect, and can reduce the sedimentation risk of multi-walled carbon nanotubes loaded with magnetic nano-Fe 3 O 4 Multi-walled carbon nanotubes, improve the stability of the system; due to the one-dimensional structure characteristics, it is also beneficial to pull oil droplets into filaments and improve the flow ability of the fluid; it has a viscosity reduction effect and is beneficial to reducing the viscosity of heavy oil; because it has thixotropy, when the oil displacement agent system is injected, under the action of a high shear rate, the oil displacement agent has high fluidity, which is convenient for the injection of the fluid and increases the penetration area; when it is intertwined with the modified multi-walled carbon nanotubes to form a three-dimensional network, under the conditions of high shear forces such as agitation and increased shear forces such as magnetic force, it can promote the decomposition and deformation of the three-dimensional network formed with the modified multi-walled carbon nanotubes as the backbone, which is beneficial to the controllable rheological change of the system. In some cases, when the nanofluid is blocked or difficult to enter nano-scale fine throats and micro-throats, it can promote the modified nanofluid oil displacement agent to have a certain rheology and improve the oil displacement performance. The use ratio of carboxylated cellulose nanofibers is greater than that of cyclodextrin-modified magnetic nano-Fe 3 O 4Multi-walled carbon nanotubes are beneficial for a denser three-dimensional cross-network, which can reduce the risk of carbon nanotube deposition, improve stability, and also reduce costs. However, if the usage ratio is too high, due to its high hydrophilicity, there is a certain possibility of aggregation, and hydrophobic modification is required. If the usage ratio is too low, its corresponding advantages cannot be fully utilized.

[0042] Most two-dimensional sheet-like nano-oil displacement agents are currently in the research and development stage. Molybdenum disulfide nanosheets are a nano-component in a mainstream two-dimensional sheet-like nano-oil displacement agent, which have certain effects in reducing interfacial tension and improving reservoir wettability. They also have characteristics such as self-adsorption at the oil-water interface that ordinary nanomaterials do not have. However, there are currently problems such as 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-layer molybdenum disulfide nanosheets with two-dimensional structures into the modified nanofluid oil displacement agent system, in the highly dispersible cyclodextrin-modified magnetic nano-Fe 3 O 4 Under the interaction of one-dimensional structures such as multi-walled carbon nanotubes and carboxylated cellulose nanofibers, without modification, they also have better dispersibility and are not easy to settle. Two-dimensional structures with small thickness and short sheet diameter have an oriented arrangement, are not easy to flip compared with nanoparticles, are conducive to spreading and overlapping adsorption on the interface, can improve the molecular diffusion barrier between the inner and outer phases, that is, improve the three-dimensional cross-network barrier ability and stability. The cooperation between its two-dimensional sheet structure and one-dimensional structure enriches the dispersion and migration behaviors, which is conducive to improving the fluidity. Two-dimensional structures with small thickness and sheet diameter block the contact between oil droplets and between oil droplets and rocks in more dimensions, form a wedge-shaped molecular film to increase the structural partial pressure, are conducive to the propulsion and separation of the fluid, and promote the peeling of crude oil from the rock surface. And the highly dispersible cyclodextrin-modified magnetic nano-Fe 3 O 4 One-dimensional structures such as multi-walled carbon nanotubes and carboxylated cellulose nanofibers cooperate with it to form a barrier after peeling, thereby improving the water wettability of the rock surface, enhancing the peeling effect, maintaining interface stability, and at the same time the fluid has good injection and flow performance, and they cooperate with each other to improve the oil recovery effect.

[0043] The molybdenum disulfide nanosheets can be selected as small-diameter thin-layer molybdenum disulfide nanosheets, with a sheet diameter of 10 - 700 nm and a thickness of less than 8 nm. Preferably, the sheet 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 sheet diameter are conducive to improving the rheology of the fluid and avoiding unexpected blockage of nano-scale fine throats and micro throats.

[0044] The viscosity of heavy oil is reduced by surfactants, the dispersibility is improved, and the oil recovery rate is increased. The surfactants include anionic biosurfactants and / or non-ionic surfactants.

[0045] Optionally, an anionic surfactant is used as the surfactant. The anionic surfactant is selected from the anionic biosurfactant rhamnolipid, which has good chemical and biological properties, is amphiphilic with oil and water, can reduce the surface tension of water, can be used as a wetting agent, promotes the decrease of the oil-water interfacial tension, has biological activity, can activate the indigenous microorganisms in the formation, plays a role in synergistic oil recovery, and has good biodegradability. When used in combination with carboxylated cellulose nanofibers and flaky nanosheets with biodegradability, it promotes the formation of a mixed adsorption layer between the oil-water interfaces, resulting in a decrease in the oil-water interfacial tension. Due to its biodegradability, it can greatly reduce the impact of the oil displacement agent on the environment.

[0046] Optionally, a nonionic surfactant is used as the surfactant. The nonionic surfactant can be polyoxyethylene sorbitan monolaurate, which has both hydrophilic and lipophilic parts on the molecule, can balance hydrophilicity and lipophilicity, has good dispersibility for carboxylated cellulose nanofibers, etc., and at the same time has good ability to promote the decrease of the oil-water interfacial tension.

[0047] Optionally, an anionic biosurfactant and a nonionic surfactant are used as the surfactant. The ratio of the anionic biosurfactant to the nonionic surfactant is 2 - 3:1. By compounding the anionic biosurfactant and the nonionic surfactant, the characteristics of different surfactants can be fully utilized, which is beneficial to the formation of a mixed adsorption layer between the oil-water interfaces, resulting in a decrease in the oil-water interfacial tension, can reduce the impact of the oil displacement agent on the environment, and improve the oil recovery rate. Using a higher proportion of rhamnolipid, due to its biodegradability, it can greatly reduce the impact of the oil displacement agent on the environment.

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

[0049] For the above-mentioned modified nanofluid oil displacement agent, a preparation method of the modified nanofluid oil displacement agent is provided, and the method includes:

[0050] Preparation of cyclodextrin-modified magnetic nano Fe 3 O 4 Preparation of multi-walled carbon nanotubes:

[0051] S1. The multi-walled carbon nanotubes are treated with a strong acid mixture of concentrated sulfuric acid and concentrated nitric acid with a volume ratio of 3:1 at 30 - 50 °C (preferably 40 °C) and stirred and refluxed for 1 - 4 hours, washed with deionized water until neutral, then filtered and dried to obtain oxidized multi-walled carbon nanotubes. The oxidized multi-walled carbon nanotubes have oxygen-containing active groups such as hydroxyl (-OH) and carboxyl (-COOH), thereby improving the hydrophilicity and dispersibility of the multi-walled carbon nanotubes, which is beneficial for loading magnetic nano Fe 3 O 4。

[0052] S2. Prepare an aqueous mixed solution of ferric chloride and ferrous sulfate, add the oxidized multi-walled carbon nanotubes obtained in step S1, adjust the pH of the mixed solution to 8-9.5, and obtain carbon nanotube powder loaded with magnetic nano-Fe 3 O 4 by suction filtration. The loading amount of magnetic nano-Fe 3 O 4 is 15-30% (the loading amount is calculated as the mass percentage of magnetite in the total mass of the carbon nanotube powder loaded with magnetite). The amounts of ferric chloride and ferrous sulfate can be adjusted according to the loading amount, and no specific limitation is made here. By loading magnetic nano-Fe 3 O 4 onto the multi-walled carbon nanotubes, the multi-walled carbon nanotubes can be driven by magnetic force. When using the modified nanofluid oil displacement agent, for example, when serious problems such as rock formation pore blockage occur, and when it is necessary to enhance the fluidity, etc., under the action of magnetic force, it can promote the displacement of the cyclodextrin-modified multi-walled carbon nanotubes loaded with magnetic nano-Fe 3 O 4 to change the stability of the nanofluid oil displacement agent at the oil-water interface structure, and enhance its variable fluidity. It is also beneficial to the disintegration of structures such as nanoparticle blockage of pores and entry into structures such as nano-micro throats. The stable binding of magnetic nano-Fe 3 O 4 on the outer wall of the carbon nanotubes increases the contact between the carbon nanotubes and the oil, and is not prone to agglomeration and aggregation, which is beneficial to improving the performance of the dot-like nanoparticles.

[0053] When the loading amount of magnetic nano-Fe 3 O 4 is greater than the above range, the specific gravity of magnetic nano-Fe 3 O 4 is relatively large, which may exacerbate the deposition risk of multi-walled carbon nanotubes and is not conducive to the stability of the modified nanofluid oil displacement agent system; when it is less than the above range, the proportion of magnetic nano-Fe 3 O 4 is too low, resulting in an insignificant effect of magnetic force driving on the multi-walled carbon nanotubes. When using the modified nanofluid oil displacement agent, when serious problems such as rock formation pore blockage occur, and when it is necessary to enhance the fluidity, etc., it is impossible to reduce problems such as pore blockage and improve the rheology under the action of magnetic force.

[0054] S3. Mix cyclodextrin, silane coupling agent, and ethanol aqueous solution to prepare a cyclodextrin dispersion solution, and use 0.2-0.3 g of the product prepared in step S2 loaded with magnetic nano-Fe 3 O 4The carbon nanotube powder is added to 60 - 80 ml of cyclodextrin dispersion solution, ultrasonically dispersed for 5 - 15 min, magnetically stirred, centrifuged, repeatedly rinsed with distilled water, and vacuum dried to obtain cyclodextrin - modified magnetic nano - Fe 3 O 4 multi - walled carbon nanotubes. Among them, the volume ratio of ethanol in the ethanol - aqueous solution is 20 - 40%.

[0055] Cyclodextrin and silane coupling agent are added according to the following mass ratio: cyclodextrin: silane coupling agent: magnetic nano - Fe loaded on multi - walled carbon nanotubes 3 O 4 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 loaded with magnetic nano - Fe 3 O 4 , due to the adverse effects such as increased density, increased attraction between nanoparticles, and the inherent aggregation tendency of multi - walled carbon nanotubes themselves, cyclodextrin is an inexpensive cyclic supramolecular host compound with a large number of hydrophilic hydroxyl groups on the outside and a hydrophobic cavity formed by hydrogen bonding inside. Its hydrophobic interior forms a pocket - like structure with a unique molecular structure, which can form inclusion compounds with multi - walled carbon nanotubes, and then can better disperse multi - walled carbon nanotubes into the solution. Thus, through a small amount of cyclodextrin modification, the influence of the foregoing adverse factors is compensated, and the hydrophilicity of nanoparticles and their better dispersibility are improved, which can promote the change of rock wettability and improve oil recovery.

[0057] Cyclodextrin is preferably 2 - hydroxypropyl - β - cyclodextrin, methyl - β - cyclodextrin, etc., to improve the dispersibility of magnetic nano - Fe 3 O 4 loaded multi - walled carbon nanotubes in water, and then improve the use stability of the modified nanofluid flooding agent. If the usage ratio of cyclodextrin is greater than the above range, the cyclodextrin is excessive, resulting in an increase in the viscosity of the solution system and affecting the interaction between the nano - components and the oil phase; if it is less than the above range, it is not conducive to improving the dispersibility.

[0058] The silane coupling agent can be selected as kh570; the coupling agent kh570 is a kind of substance with two different functional groups. One part of the functional groups in their molecules can react with organic molecules, and the other part can react with the surface of inorganic substances, thereby improving the bonding between cyclodextrin and magnetic nano - Fe 3 O 4 loaded multi - walled carbon nanotubes containing oxygen - containing active groups such as hydroxyl (-OH) and carboxyl (-COOH). If the proportion of the silane coupling agent is less than the above range, the bonding between cyclodextrin and multi - walled carbon nanotubes loaded with magnetic nano - Fe 3O 4 The reactivity decreases, which is not conducive to the formation of a stable complex between the two. If it is greater than the above ratio, it will cause unnecessary waste, and the excessive use of silane coupling agent will also lead to unexpected changes in the solution system, affecting its use effect.

[0059] Cyclodextrin-modified supported magnetic nano-Fe 3 O 4 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, and oil displacement is carried out by injecting it into the oil reservoir to improve the recovery rate.

[0060] The characteristics and performance of the present application are further described in detail below in conjunction with the embodiments:

[0061] Example 1

[0062] A modified nanofluid oil displacement agent, the mass fractions of each component are: cyclodextrin-modified supported magnetic nano-Fe 3 O 4 10 parts of multi-walled carbon nanotubes; 27 parts of carboxylated cellulose nanofibers, 5 parts of molybdenum disulfide nanosheets, 26 parts of surfactants, and 38 parts of diluents.

[0063] Cyclodextrin-modified supported magnetic nano-Fe 3 O 4 The preparation method of multi-walled carbon nanotubes is as follows:

[0064] S1. The multi-walled carbon nanotubes are treated with a strong acid mixture of concentrated sulfuric acid and concentrated nitric acid with a volume ratio of 3:1, stirred and refluxed at 40 °C for 2 hours, washed with deionized water to neutrality, filtered, and dried to obtain oxidized multi-walled carbon nanotubes. The multi-walled carbon nanotubes are short-length and 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 made into a mixed aqueous solution, and the oxidized multi-walled carbon nanotubes obtained in step S1 are added, and the pH of the mixed solution is adjusted to 9. After suction filtration, the carbon nanotube powder loaded with magnetic nano-Fe 3 O 4 is obtained, and the loading amount of magnetic nano-Fe 3 O 4 is 28%.

[0066] S3. Mix cyclodextrin, silane coupling agent, and an ethanol aqueous solution to prepare a cyclodextrin dispersion solution. Add 0.25 g of the carbon nanotube powder loaded with magnetic nano-Fe 3 O 4 prepared in step S2 to 70 ml of the cyclodextrin dispersion solution, ultrasonically disperse for 10 min, perform magnetic stirring treatment, then centrifuge, repeatedly rinse with distilled water, and vacuum dry to obtain cyclodextrin-modified magnetic nano-Fe 3 O 4 multi-walled carbon nanotubes. Among them, the volume ratio of ethanol in the ethanol aqueous solution is 30%.

[0067] Cyclodextrin and silane coupling agent are added according to the following mass ratio: the mass ratio of cyclodextrin: silane coupling agent: carbon nanotubes loaded with magnetic nano-Fe 3 O 4 is 1:0.4:1.5. The cyclodextrin is methyl-β-cyclodextrin, and the silane coupling agent is kh570.

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

[0069] Molybdenum disulfide nanosheets, with a sheet 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 an anionic biosurfactant (rhamnolipid): a nonionic surfactant (polyoxyethylene sorbitan monolaurate) in a ratio of 2:1. The diluent is a mixed solution of water and ethanol, and in terms of volume ratio, water: ethanol is 7:1.

[0071] Mix the cyclodextrin-modified magnetic nano-Fe 3 O 4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactant, and diluent according to the mass parts of each component, stir, and ultrasonically disperse to obtain a modified nanofluid oil displacement agent. Add the modified nanofluid oil displacement agent to water at a mass percentage of 3% to prepare a modified nanofluid oil displacement agent system.

[0072] Performance test:

[0073] Wettability: The oil production effects of cores with different wettabilities vary greatly. The more water-wet the core is, the greater the driving force for displacing crude oil, which promotes the imbibition displacement of oil and water between the matrix and fractures. The more crude oil is displaced, the higher the recovery factor. Wettability is characterized by the contact angle between water and oil. The contact angle of water droplets or oil droplets on the surface of rock slices is measured by taking high-resolution images of core slices with a high-speed camera. The contact angle of oil droplets on the rock surface is measured. The test oil droplet used is: crude oil density 0.91 tons per cubic meter, viscosity (at room temperature): 1100 mPa·s. Cores are drilled from a certain heavy oil well (porosity 27%) to make rock slices. Through the contact angle measurement experiment, the contact angle between the oil droplet and the rock slice is measured to be 24°, which is an oil-wet rock slice. The rock slice is soaked in a modified nanofluid flooding agent system prepared by adding water for 24 hours. The water droplet contact angle is 36°, and the oil droplet contact angle is 146°. This shows that the modified nanofluid flooding agent can significantly change the wettability of the oil-wet rock, making it have strong hydrophilicity and oleophobicity, which is beneficial to the discharge of crude oil and the improvement of recovery factor. For highly water-wet cores, it can also reduce the fluid flow resistance.

[0074] Interfacial tension test: Measured by a fully automatic rotating drop ultra-low interfacial tension meter TX-500C at a temperature of 80°C and a rotation speed of 5000 revolutions per minute. Using the modified nanofluid flooding agent system as the aqueous phase and the test oil droplet as the oil phase, the interfacial tension is 4.2×10 - 3 mN / m. The ultra-low interfacial tension has strong interfacial activity, which reduces the interfacial tension, capillary force, and improves the oil displacement efficiency.

[0075] Dispersion stability test: Take 100 ml of solution in a beaker and observe and record the time when aggregation and flocculation occur in the modified nanofluid flooding agent system every 1 hour to test the dispersion stability. The flocculation time is 48 hours; to test the dispersion stability of the system under the influence of magnetic force, a magnetic field is applied outside the beaker with a magnetic field intensity of 1000 Gs. The flocculation time under the applied magnetic field is 44 hours. The dispersion is good, and the modified nanofluid flooding agent system can be affected by the magnetic field.

[0076] Recovery factor test: An oil displacement experiment is carried out using a core displacement device to simulate reservoir conditions. Multiple artificial heterogeneous cores are taken, and the basic parameters of the cores are measured and recorded. The experimental temperature is 80°C. The core is water-flooded at a rate of 0.01 mL / min until the water cut mass fraction of the outlet liquid exceeds 98%. The water-flood recovery factor is recorded as 18.4%; after water flooding, the prepared modified nanofluid flooding agent system is injected without applying an external magnetic field and with an external magnetic field (magnetic field intensity 1000 Gs) respectively. Finally, the total recovery factors of the cores are recorded as 65% and 67% respectively. It has a significant effect on improving the recovery factor. Under the action of the magnetic field, it is more conducive to enhancing the oil displacement driving force.

[0077] Example 2

[0078] It is basically the same as the composition and preparation method of the modified nanofluid oil displacement agent in Example 1, and the main difference is that: for multi-walled carbon nanotubes, the outer diameter is 10 - 16 nm and the length is 700 nm - 1000 nm.

[0079] Performance test:

[0080] Wettability: The water droplet contact angle is 37°, and the oil droplet contact angle is 143°. Compared with Example 1, it can be seen that the water droplet contact angle becomes slightly larger, the oil droplet contact angle becomes slightly smaller, the hydrophilic wettability becomes slightly worse, and the interfacial tension is 5.1×10 -3 mN / m, the flow resistance becomes slightly larger, the flocculation time appears at 44 h, and the flocculation time appears at 40 h with an external magnetic field, and the dispersibility becomes significantly worse; without an external magnetic field and with an external magnetic field, the total recovery rates of the core are 63% and 64% respectively, and the recovery rate decreases. The possible reason is that the outer diameter of the multi-walled carbon nanotubes becomes smaller, the aggregation risk increases, and it affects the improvement of the recovery rate.

[0081] Example 3

[0082] It is basically the same as the composition and preparation method of the modified nanofluid oil displacement agent in Example 1, and the main difference is that: for multi-walled carbon nanotubes, the outer diameter is 40 - 50 nm and the length is 700 nm - 1000 nm.

[0083] Performance test:

[0084] Wettability: The water droplet contact angle is 39°, and the oil droplet contact angle is 140°. Compared with Example 1, it can be seen that the water droplet contact angle becomes slightly larger, the oil droplet contact angle becomes slightly smaller, the hydrophilic wettability becomes slightly worse, and the interfacial tension is 6.4×10 -3 mN / m, the flow resistance becomes slightly larger, the flocculation time appears at 43 h, and the flocculation time appears at 40 h with an external magnetic field, and the dispersibility becomes significantly worse; without an external magnetic field and with an external magnetic field, the total recovery rates of the core are 61% and 62% respectively, and the recovery rate decreases. The possible reason is that the outer diameter of the multi-walled carbon nanotubes becomes larger, which is not conducive to reducing the interfacial tension and affects the improvement of the recovery rate.

[0085] Example 4

[0086] It is basically the same as the composition and preparation method of the modified nanofluid oil displacement agent in Example 1, and the main difference is that: the loading amount of magnetic nano-Fe 3 O 4 is 10%.

[0087] Performance test:

[0088] Wettability: The water droplet contact angle is 36°, and the oil droplet contact angle is 145°. Compared with Example 1, it can be seen that the changes in the water droplet contact angle and the oil droplet contact angle are small, the change in hydrophilic wettability is small, and the interfacial tension is 4.4×10 -3mN / m, with a relatively small change. The flocculation time is 48 h, and the flocculation time is also 48 h under an external magnetic field. The change in dispersibility under the magnetic field condition is not obvious. With or without an external magnetic field, the total recovery rate of the core is 65.2% and 65.2% respectively, and the change in recovery rate is not obvious, but there is no obvious magnetic field response.

[0089] Example 5

[0090] It is basically the same as the composition and preparation method of the modified nanofluid flooding agent in Example 1. The main difference is that the magnetic nano-Fe 3 O 4 loading is 40%.

[0091] Performance test:

[0092] Wettability: The water droplet contact angle is 40°, and the oil droplet contact angle is 140°. Compared with Example 1, it can be seen that the water droplet contact angle becomes larger, the oil droplet contact angle becomes smaller, the hydrophilic wettability becomes worse, and the interfacial tension is 5.7×10 -3 mN / m, which becomes larger, the flow resistance becomes larger, the flocculation time is 38 h, the flocculation time under an external magnetic field is 32 h, and the dispersibility becomes worse. With or without an external magnetic field, the total recovery rate of the core is 61% and 63% respectively, the recovery rate decreases, and the external magnetic field has a more obvious effect on improving its recovery rate, but the recovery rate is still lower than that in Example 1.

[0093] Example 6

[0094] It is basically the same as the composition and preparation method of the modified nanofluid flooding agent in Example 1. The main difference is that the molybdenum disulfide nanosheets have a sheet diameter of 600 - 700 nm and a thickness of 15 nm.

[0095] Performance test:

[0096] Wettability: The water droplet contact angle is 36°, and the oil droplet contact angle is 145°. Compared with Example 1, it can be seen that the change in hydrophilic wettability is relatively small, the interfacial tension is 4.7×10 -3 mN / m, which becomes larger, the flow resistance becomes larger, the flocculation time is 47 h, the flocculation time under an external magnetic field is 43 h, and the dispersibility becomes worse. With or without an external magnetic field, the total recovery rate of the core is 64% and 66% respectively, and the recovery rate decreases.

[0097] Example 7

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

[0099] Performance test:

[0100] Wettability: The contact angle of water droplets is 38°, and the contact angle of oil droplets is 144°. Compared with Example 1, it can be seen that the hydrophilic wettability deteriorates slightly. The interfacial tension is 4.8×10 -3 mN / m, the flow resistance increases slightly, the flocculation time is 48 h, and the flocculation time is 45 h with an external magnetic field. The dispersibility difference is not significant. With or without an external magnetic field, the total recovery rates of the core are 63% and 65% respectively, and the recovery rate decreases.

[0101] Example 8

[0102] It is basically the same as the composition and preparation method of the modified nanofluid oil displacement agent in Example 1. The main difference is that the surfactant uses the non-ionic surfactant polyoxyethylene sorbitan monolaurate.

[0103] Performance test:

[0104] Wettability: The contact angle of water droplets is 39°, and the contact angle of oil droplets is 147°. Compared with Example 1, it can be seen that the hydrophilic wettability deteriorates slightly. The interfacial tension is 5.0×10 -3 mN / m, the flow resistance increases slightly, the flocculation time is 48 h, and the flocculation time is 46 h with an external magnetic field. The dispersibility difference is not significant. With or without an external magnetic field, the total recovery rates of the core are 61% and 64% respectively, and the recovery rate decreases.

[0105] Example 9

[0106] It is basically the same as the composition and preparation method of the modified nanofluid oil displacement agent in Example 1. The main difference is that the mass ratio of cyclodextrin: silane coupling agent: carbon nanotubes loaded with magnetic nano-Fe 3 O 4 is 1:0.4:1.4.

[0107] Performance test:

[0108] Wettability: The contact angle of water droplets is 34°, and the contact angle of oil droplets is 147°. Compared with Example 1, it can be seen that by increasing the use of cyclodextrin, the hydrophilic wettability is better. The interfacial tension is 3.8×10 -3 mN / m, the flow resistance decreases, the flocculation time is 49 h, and the flocculation time is 46 h with an external magnetic field. The dispersibility is better. With or without an external magnetic field, the total recovery rates of the core are 66% and 68% respectively, and the recovery rate increases.

[0109] Comparative Example 1:

[0110] It is basically the same as the composition and preparation method of the modified nanofluid oil displacement agent in Example 1. The main difference is that the multi-walled carbon nanotubes loaded with magnetic nano-Fe 3 O 4 are not modified with cyclodextrin.

[0111] Performance test:

[0112] Wettability: The water droplet contact angle is 46°, and the oil droplet contact angle is 134°. Compared with Example 1, it can be seen that the water droplet contact angle becomes larger, the oil droplet contact angle becomes smaller, the hydrophilic wettability becomes worse, and the interfacial tension is 2.1×10 -2 mN / m, which becomes larger, the flow resistance becomes larger, the flocculation time appears at 36 h, and the flocculation time appears at 27 h with an external magnetic field. The dispersibility becomes worse, especially the influence of the magnetic field increases; with or without an external magnetic field, the total recovery rate of the core is 55% and 53% respectively. The recovery rate decreases, and when cyclodextrin modification is not carried out, applying an external magnetic field does not help to increase the recovery rate, and there is even a possibility of decrease. The possible reason is that although the external magnetic field has a certain effect on changing the rheology, the dispersion stability of the system becomes worse. As a comprehensive result, it has no improvement effect on increasing the recovery rate.

[0113] Comparative Example 2:

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

[0115] Performance test:

[0116] Wettability: The water droplet contact angle is 63°, and the oil droplet contact angle is 115°. Compared with Example 1, it can be seen that the water droplet contact angle increases significantly, the oil droplet contact angle decreases significantly, the hydrophilic wettability deteriorates significantly, and the interfacial tension is 7.5×10 -2 mN / m. Without adding carboxylated cellulose nanofibers, it becomes relatively larger, the flow resistance increases significantly, the flocculation time appears at 40 h, and the flocculation time appears at 33 h with an external magnetic field. The dispersibility deteriorates to some extent; with or without an external magnetic field, the total recovery rate of the core is 42% and 44% respectively. The recovery rate decreases. Without adding carboxylated cellulose nanofibers, it is not conducive to improving the recovery rate.

[0117] Comparative Example 3:

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

[0119] Performance test:

[0120] Wettability: The water droplet contact angle is 42°, and the oil droplet contact angle is 131°. Compared with Example 1, it can be seen that the water droplet contact angle becomes larger, the oil droplet contact angle becomes smaller, the hydrophilic wettability decreases to some extent, and the interfacial tension is 1.1×10 -2mN / m, the flow resistance increases; the flocculation time is 43 h, and the flocculation time with an external magnetic field is 40 h, and the dispersibility becomes slightly worse; with or without an external magnetic field, the total recovery rate of the core is 57% and 59% respectively, and the recovery rate decreases.

[0121] Comparative Example 4:

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

[0123] Performance test:

[0124] Wettability: The water droplet contact angle is 65°, and the oil droplet contact angle is 109°. Comparing with Example 1, it can be seen that the water droplet contact angle increases significantly, the oil droplet contact angle decreases significantly, the hydrophilic wettability becomes significantly worse, and the interfacial tension is 9.7×10 -2 mN / m, relatively increases, the flow resistance increases, the flocculation time is 39 h, and the flocculation time with an external magnetic field is 31 h, and the dispersibility becomes slightly worse; with or without an external magnetic field, the total recovery rate of the core is 40% and 43% respectively, and the recovery rate decreases.

[0125] Comparative Example 5:

[0126] It is basically the same as the composition and preparation method of the modified nanofluid oil displacement agent in Example 1, and the main difference is that: the cyclodextrin-modified magnetic nano-Fe 3 O 4 Multi-walled carbon nanotubes are replaced by single-walled carbon nanotubes.

[0127] Performance test:

[0128] Wettability: The water droplet contact angle is 43°, and the oil droplet contact angle is 127°. Comparing with Example 1, it can be seen that the water droplet contact angle increases, the oil droplet contact angle decreases, the hydrophilic wettability becomes worse, and the interfacial tension is 1.0×10 -2 mN / m, increases, the flow resistance increases, the flocculation time is 35 h, and the dispersibility becomes worse; the total recovery rate of the core without an external magnetic field is 53%, and the recovery rate decreases.

[0129] Comparative Example 6

[0130] It is basically the same as the composition and preparation method of the modified nanofluid oil displacement agent in Example 1, and the main difference is that: the cyclodextrin-modified magnetic nano-Fe 3 O 4 Multi-walled carbon nanotubes are 3 parts; carboxylated cellulose nanofibers are 50 parts, molybdenum disulfide nanosheets are 3 parts, surfactant is 24 parts, and diluent is 38 parts.

[0131] Performance test:

[0132] Wettability: the contact angle of water droplets is 35°, and the contact angle of oil droplets is 147°. Compared with Example 1, it can be seen that the hydrophilic wettability is better, and the interfacial tension is 8.4×10 -2 mN / m. Without adding carboxylated cellulose nanofibers, it becomes relatively larger, the flow resistance increases significantly, the flocculation time is 33 h, and the flocculation time is 32 h when an external magnetic field is applied. The dispersibility becomes worse; whether an external magnetic field is applied or not, the total oil recovery rate of the core is 47% and 47% respectively. When the carboxylated cellulose nanofibers are too high, the dispersibility becomes worse, which is not conducive to the improvement of the oil recovery rate.

[0133] In summary, through the cyclodextrin-modified magnetic nano-Fe with one-dimensional structures of two different characteristics 3 O 4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, and 0-dimensional magnetic nano-Fe 3 O 4 , and the compounding of two-dimensional molybdenum disulfide nanosheets, a modified nanofluid oil displacement agent with good dispersibility, good fluid fluidity, reduced viscosity of heavy oil, not easy to block pores, and good oil displacement effect is formed, effectively improving the oil recovery rate. A preparation method of a modified nanofluid oil displacement agent does not require complex modification of a variety of nanostructures, the preparation process is simple, and it is relatively easier to stably prepare the corresponding preparation.

[0134] The preferred embodiments of the present invention have been described in detail above, which are only used to illustrate the technical solutions of the present invention and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A modified nanofluid oil displacing agent, characterized in that: The invention comprises the following components: cyclodextrin-modified magnetic nano-Fe3O4 multi-walled carbon nanotubes, carboxylated cellulose nanofibers, molybdenum disulfide nanosheets, surfactants and diluents; The mass proportions of the components include: 9-12 parts of cyclodextrin-modified loaded magnetic nano Fe3O4 multi-walled carbon nanotubes; 21-36 parts of carboxylated cellulose nanofibers, 3-6 parts of molybdenum disulfide nanosheets, 24-30 parts of surfactants, and 25-40 parts of diluents.

2. The modified nanofluid oil-displacing agent according to claim 1, characterized in that: Cyclodextrin modified multi-walled carbon nanotubes loaded with magnetic nano-Fe3O4: multi-walled carbon nanotubes with an outer diameter of 15-40nm and a length of 400nm-3μm are used as raw materials for preparation, the magnetic nano-Fe3O4 loading amount is 15-30%, and the cyclodextrin and silane coupling agent are 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).

3. The modified nanofluid oil-displacing agent according to claim 1, characterized in that: The fiber diameter of carboxylated cellulose nanofibers is 2-10 nm and the length is 400 nm-3 μm.

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

5. The modified nanofluid oil-displacing agent according to claim 1, characterized in that: The surfactant includes anionic surfactants and / or nonionic surfactants.

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

1.

7. The modified nanofluid oil-displacing agent according to claim 2, characterized in that: The cyclodextrin is 2-hydroxypropyl β-cyclodextrin or methyl β-cyclodextrin; and the silane coupling agent is kh570.

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

9. A method for preparing a modified nanofluid oil-displacing agent according to claim 8, characterized in that: Preparation of the cyclodextrin-modified multi-walled carbon nanotubes loaded with magnetic nano-Fe3O4: S1, treating the multi-walled carbon nanotubes with a concentrated sulfuric acid mixture of concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1, stirring and refluxing at 30-50° C. for 1-4 hours, washing with deionized water until neutral, filtering, and drying to obtain oxidized multi-walled carbon nanotubes; S2, preparing a mixed aqueous solution of ferric chloride and ferrous sulfate, adding the oxidized multi-walled carbon nanotubes obtained in step S1, adjusting the pH of the mixed solution to 8-9.5, and filtering to obtain a carbon nanotube powder loaded with magnetic nano-Fe3O4, wherein the loading amount of the magnetic nano-Fe3O4 is 15-30% by mass percentage of ferroferric oxide to the total mass of the carbon nanotube powder loaded with ferroferric oxide; S3, cyclodextrin, silane coupling agent, ethanol aqueous solution are mixed to prepare cyclodextrin dispersion solution, 0.2-0.3g of carbon nanotube powder loaded with magnetic nano-Fe3O4 prepared in step S2 is added to 60-80ml of cyclodextrin dispersion solution, ultrasonically dispersed for 5-15min, centrifuged after magnetic stirring, repeatedly rinsed with distilled water, and vacuum dried to obtain cyclodextrin-modified multi-walled carbon nanotubes loaded with magnetic nano-Fe3O4; wherein the volume ratio of ethanol in the ethanol aqueous solution is 20-40%.

10. The method for preparing a modified nanofluid oil-displacing agent according to claim 9, 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).