A gradient pore self-assembled drag reducer for nano-fracturing fluid and its preparation method

Abstract

This invention discloses a gradient pore self-assembled drag-reducing agent for nano-fracturing fluids and its preparation method. The drag-reducing agent comprises nanoparticles with a three-layer gradient self-assembled structure, the particle size of which is no greater than 50 nm. The three-layer gradient self-assembled structure comprises, from the inside out, a hydrophobic core layer, a middle viscoelastic layer, and an outer hydrophilic layer, and is composed of the following by mass percentage: hydrophobic core layer: 25–35% nonionic surfactant, 15–20% organic solvent; middle viscoelastic layer: 30–40% polymer; outer hydrophilic layer: 15–20% zwitterionic surfactant. This invention, through its three-layer gradient self-assembled structure design, integrates multiple functions such as high-efficiency drag reduction, strong proppant carrying capacity, reservoir modification, deep penetration, and temperature and salt resistance. It comprehensively solves the technical bottlenecks of existing technologies, such as low drag reduction efficiency, high reservoir damage, single function, and poor environmental adaptability, providing a novel fracturing fluid additive with superior performance for the efficient development of unconventional oil and gas resources.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field development technology, specifically to a gradient pore self-assembly drag-reducing agent for nano-fracturing fluid and its preparation method. Background Technology

[0002] Hydraulic fracturing is one of the key technologies for exploiting unconventional oil and gas resources, and its effectiveness largely depends on the performance of the fracturing fluid. Drag reducers, as the core additives in fracturing fluids, primarily function to reduce the frictional resistance of the fluid flowing through pipes and wellbores, and effectively carry proppant into formation fractures.

[0003] Currently, drag-reducing agents commonly used in oilfields mainly fall into two categories:

[0004] The first category is high molecular weight polymer drag reducers, such as polyacrylamide (PAM) and its derivatives (partially hydrolyzed polyacrylamide, HPAM). These drag reducers mainly reduce turbulent friction through the viscoelasticity of their long molecular chains. However, these drag reducers have obvious limitations: (1) Their molecular chain size is usually large (generally greater than 100 nm), while the pore throat size of tight reservoirs (such as shale and tight sandstone) is generally in the range of 100-200 nm. Large molecular chains are difficult to efficiently enter such small pore throats, which not only leads to a significant reduction in drag reduction efficiency at deep reservoirs, but also poses a risk of pore blockage and reservoir damage; (2) Their temperature and salt resistance is poor. In high-temperature and high-mineralization formation environments, the molecular chains are prone to curling, degradation, or cross-linking with high-valence ions, resulting in a sharp loss of viscoelasticity and affecting the fracturing fluid's fracture-creating and sand-carrying performance.

[0005] The second category is surfactant-based drag reducers, which provide drag reduction by forming micelle structures. Although their molecular size is small, these drag reducers also have inherent problems: (1) the micelle structures they form have poor stability under high temperature and high salt conditions and are easily destroyed, leading to the failure of drag reduction performance; (2) some surfactants may adsorb or react with formation fluids or minerals, posing a potential risk of reservoir damage; (3) in order to achieve nanoscale dispersion and stability, their preparation process is usually more complex, costly, and has insufficient long-term storage stability.

[0006] Furthermore, both polymer-based and surfactant-based drag reducers tend to have relatively singular functions, making it difficult to simultaneously achieve multiple objectives such as efficient drag reduction, precise sand carrying, and reservoir modification (e.g., wettability reversal). In existing technologies, nano-sizing of drag reducers to adapt to dense porous structures is a research trend; however, how to construct structurally stable, functionally integrated nano-drag reducers that can adapt to harsh operating conditions remains a pressing technical challenge in this field.

[0007] Therefore, there is an urgent need in the field for a novel drag-reducing agent that combines the advantages of small size, temperature and salt resistance, structural stability, and multifunctional synergy to overcome the aforementioned shortcomings of the prior art.

[0008] In view of this, the present invention is hereby proposed. Summary of the Invention

[0009] To address the aforementioned problems, this invention proposes a gradient pore self-assembled drag-reducing agent for nano-fracturing fluids and its preparation method. Specifically, the following technical solution is adopted:

[0010] A gradient pore self-assembled drag reducer for nano-fracturing fluid includes nanoparticles with a three-layer gradient self-assembled structure, wherein the particle size of the nanoparticles is no greater than 50 nm.

[0011] The three-layer gradient self-assembled structure comprises, from the inside out, a hydrophobic core layer, a middle viscoelastic layer, and an outer hydrophilic layer, and consists of, by mass percentage:

[0012] Core hydrophobic layer: 25–35% nonionic surfactant, 15–20% organic solvent;

[0013] Intermediate viscoelastic layer: 30–40% polymer, the polymer comprising partially hydrolyzed polyacrylamide and polyvinylpyrrolidone, comprising by weight percentage: 20–25% partially hydrolyzed polyacrylamide and 10–15% polyvinylpyrrolidone;

[0014] Outer hydrophilic layer: 15–20% zwitterionic surfactant.

[0015] As an optional embodiment of the present invention, the nonionic surfactant comprises alkylphenol polyoxyethylene ether and nonylphenol polyoxyethylene ether, and is composed of: 15-20% alkylphenol polyoxyethylene ether and 10-15% nonylphenol polyoxyethylene ether by mass percentage;

[0016] The organic solvent is an aromatic solvent.

[0017] As an optional embodiment of the present invention, the alkylphenol polyoxyethylene ether is dodecylphenol polyoxyethylene ether OP-10, and the organic solvent is xylene.

[0018] As an optional embodiment of the present invention, the zwitterionic surfactant of the outer hydrophilic layer is a betaine surfactant.

[0019] This invention also provides a method for preparing a gradient pore self-assembled drag-reducing agent for nano-fracturing fluids, comprising the following steps:

[0020] Step S1: Add nonionic surfactant and organic solvent to the reaction vessel, and synthesize the core hydrophobic layer under heating and stirring conditions;

[0021] Step S2: Add the polymer to the system of step S1, increase the reaction temperature and stirring speed, and synthesize the intermediate viscoelastic layer;

[0022] Step S3: Add the zwitterionic surfactant to the system of step S2, reduce the reaction temperature and stirring speed, and self-assemble to synthesize the outer hydrophilic layer;

[0023] Step S4: Homogenize the mixture obtained in step S3 so that the particle size of the nanoparticles is no greater than 50 nm.

[0024] Step S5: Spray dry the homogenized liquid from step S4 to obtain a solid powder;

[0025] Step S6: Mix the dried powder with the silane coupling agent and perform activation treatment.

[0026] As an optional embodiment of the present invention, the nonionic surfactant in step S1 includes alkylphenol polyoxyethylene ether and nonylphenol polyoxyethylene ether, and step S1 includes:

[0027] Step S101: Add alkylphenol polyoxyethylene ether, nonylphenol polyoxyethylene ether and organic solvent to the reaction vessel and heat to 48-57℃;

[0028] Step S102: Stir at a constant speed of 600-800 rpm for 30-60 minutes, then sonicate to eliminate bubbles until the interfacial tension is ≤3 mN / m, thus synthesizing the hydrophobic core layer.

[0029] As an optional embodiment of the present invention, the polymer in step S2 includes partially hydrolyzed polyacrylamide and polyvinylpyrrolidone, and step S2 includes:

[0030] Step S201: After partially hydrolyzing polyacrylamide and polyvinylpyrrolidone are premixed, they are slowly added to the reaction vessel;

[0031] Step S202: Gradually raise the temperature of the reactor to 68-77℃, with the heating rate controlled at 1-2℃ / min;

[0032] Step S203: After heating to the target temperature in step S202, stir at a speed of 800-1000 rpm for 1-2 hours to form a coated intermediate viscoelastic layer.

[0033] As an optional embodiment of the present invention, step S3 includes:

[0034] Step S301: Add the zwitterionic surfactant dropwise and gradually lower the temperature of the reactor to 55-60℃;

[0035] Step S302: Gently stir at 400-500 rpm for 2-3 hours to orient the hydrophilic groups and self-assemble to synthesize the outer hydrophilic layer.

[0036] As an optional embodiment of the present invention, step S4 includes:

[0037] The mixture obtained in step S3 is transferred to a high-pressure homogenizer for homogenization. The homogenizer is homogenized at 50 MPa 2-5 times to break up large particles, and homogenized at 100 MPa 2-5 times to make the particle size of the nanoparticles no larger than 50 nm.

[0038] Particle size is monitored in real time during homogenization. When the particle size is >50 nm, homogenization is continued until the particle size is ≤50 nm.

[0039] Step S5 includes: feeding the homogenized liquid from step S4 into a spray dryer for spray drying and curing. The feed rate of the spray dryer is 20-30 L / h, the atomization pressure is 0.2-1 MPa, the inlet temperature is 110-150℃ for rapid evaporation of moisture, and the outlet temperature is 55-65℃. The dried and cured drag-reducing agent has a moisture content of ≤5% and the powder of the drag-reducing agent has flowability.

[0040] In step S6, the mass ratio of the dried powder to the silane coupling agent is 100:1-200:1.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] The gradient pore self-assembled drag reducer for nano-fracturing fluids provided by this invention, through its unique three-layer gradient self-assembled structure and synergistic design at the nanoscale, has the following technical effects:

[0043] 1. Excellent reservoir permeability and low damage characteristics.

[0044] Because the drag-reducing agent consists of nanoparticles with an overall particle size ≤50 nm, its size is much smaller than the pore throat diameter of 100-200 nm in tight reservoirs (such as shale and tight sandstone). This allows the drag-reducing agent to efficiently enter and penetrate the reservoir's micropore network, achieving complete coverage of the fracturing fluid at depth. Unlike conventional macromolecular polymers (>100 nm) which are prone to bridging and clogging at the pore throat, this invention fundamentally reduces the risk of reservoir damage caused by physical blockage, thus helping to protect and maintain the reservoir's original permeability.

[0045] 2. Multifunctional integrated synergy of "drag reduction-sand carrying-modification".

[0046] This invention achieves multifunctional integration of a single dosage form through a precise three-layer structure design:

[0047] The hydrophobic core layer and nonionic surfactant provide the initial hydrophobic core for the entire structure and work synergistically with the organic solvent to ensure the initial stability of the structure in the oil phase environment, laying the foundation for the assembly of subsequent layers.

[0048] Intermediate viscoelastic layer and polymer: Provides the core viscoelasticity required by fracturing fluid. Through the extension and interaction of molecular chains, it effectively reduces the turbulent frictional resistance of fluid flow (i.e. drag reduction) and endows fracturing fluid with excellent proppant carrying capacity, ensuring effective suspension and delivery of proppant.

[0049] The outer hydrophilic layer and zwitterionic surfactants can achieve directional alignment on the rock surface, transforming it from oleophilic to hydrophilic. This wettability reversal function reduces the seepage resistance of fracturing fluid in the reservoir and facilitates post-fracturing flowback, reducing water-locking damage. Together with the intermediate viscoelastic layer, it improves the overall proppant carrying and flowback efficiency.

[0050] 3. Significantly enhanced temperature and salt resistance and long-term dispersion stability.

[0051] The three-layer gradient self-assembly structure described in this invention forms a stable "core-shell" nanocomposite. The hydrophobic core layer and the hydrophilic outer layer together constitute a protective barrier, effectively mitigating the attack and degradation of the polymer molecular chains in the intermediate viscoelastic layer by high temperature and high salt ions. Simultaneously, the component range defined in this invention (such as the content of zwitterionic surfactants) ensures that the nanoparticles maintain excellent dispersion stability in high-temperature, high-salt fracturing fluid systems, preventing aggregation, precipitation, or performance failure, thus guaranteeing long-term effectiveness and reliability under harsh operating conditions.

[0052] 4. Improved construction adaptability and efficiency.

[0053] The drag-reducing agent described in this invention, with its nanoscale structure and stable powder morphology (ensuring this through the preparation method), exhibits rapid dissolution and uniform dispersion in field applications, greatly facilitating on-site fluid preparation and application. Its multifunctional integrated characteristics simplify fracturing fluid formulations, achieving comprehensive performance optimization without the need for additional additives, thereby improving application efficiency and reducing operating costs.

[0054] In summary, this invention, through a three-layer gradient self-assembly structure design of "core hydrophobic layer - intermediate viscoelastic layer - outer hydrophilic layer" and strict control of the proportions of each component and the nanoscale of the final product, successfully integrates multiple functions such as efficient drag reduction, strong sand carrying capacity, reservoir modification, deep penetration, and temperature and salt resistance into a single product. The functions synergistically enhance each other, comprehensively solving the technical bottlenecks of existing technologies such as low drag reduction efficiency, high reservoir damage, single function, and poor environmental adaptability. This provides a novel fracturing fluid additive with superior performance for the efficient development of unconventional oil and gas resources.

[0055] The preparation method provided by this invention, through its unique, step-by-step design and precise control of process parameters, offers a reliable and industrially scalable production path for achieving the core structure and performance of the gradient pore self-assembled drag-reducing agent. The specific technical effects of this method are reflected in:

[0056] Precise and orderly self-assembly of gradient structures was achieved.

[0057] Step S1 (Core Formation): By synthesizing a hydrophobic core layer under heating and stirring conditions, sufficient self-organization conditions are provided for the nonionic surfactant and organic solvent, forming a stable hydrophobic core, which lays the foundation for subsequent multilayer coating.

[0058] Step S2 (Viscoelastic Layer Coating): By increasing the reaction temperature and stirring speed, the expansion and migration of polymer molecular chains are promoted, enabling them to uniformly and densely coat the hydrophobic core, thereby constructing a key viscoelastic layer that provides drag reduction and sand carrying capacity.

[0059] Step S3 (Self-assembly of the outer hydrophilic layer): By lowering the reaction temperature and stirring speed, a mild reaction environment is provided for the zwitterionic surfactant, enabling it to align itself dropwise and orderly outside the viscoelastic layer through intermolecular forces, ultimately forming a stable outer hydrophilic layer and achieving the function of reversing wettability on the rock surface. These three interconnected steps ensure the precise construction and functional integration of the "hydrophobic-viscoelastic-hydrophilic" three-layer gradient structure.

[0060] This ensures the uniformity and stability of the product at the nanoscale.

[0061] Step S4 (High-Pressure Homogenization): This step is crucial for controlling the particle size of the final product. High-intensity homogenizing shear force effectively breaks down and reconstructs the micelles or aggregates formed earlier, ensuring that the final nanoparticle size is strictly controlled within ≤50 nm. This meets the size requirements for entering dense pore throats and improves batch consistency.

[0062] A high-performance final dosage form that is easy to store and transport was obtained.

[0063] Step S5 (Spray Drying): This step efficiently transforms the uniform and stable nanoscale liquid dispersion into a free-flowing solid powder. This formulation greatly facilitates long-term storage and long-distance transportation of the product, avoiding problems such as sedimentation, flocculation, or freeze-thaw instability that may occur in liquid systems. At the same time, the powder form also facilitates rapid on-site dissolution and use.

[0064] It significantly improves the product's performance in the application environment.

[0065] Step S6 (Activation Treatment): The surface of the nanoparticles was activated and modified by introducing a silane coupling agent through dry mixing. This step significantly enhances the immediate dispersibility and long-term dispersion stability of the nanoparticles in fracturing fluid-based fluids (especially high-salinity water), prevents the powder from agglomerating upon contact with water, and ensures the full performance and durability of the product under harsh oilfield conditions.

[0066] In summary, the preparation method provided by this invention is a systematic chemical assembly and physical processing procedure with stringent process conditions. Through a series of synergistic processes including stepwise self-assembly, forced nano-sizing, solidification, and surface activation, this method successfully integrates multiple functional components into a structurally well-defined and performance-stable nano-drag reducer. Furthermore, its excellent theoretical properties (such as deep penetration, efficient drag reduction, and reservoir modification) are reliably reproduced and guaranteed in actual industrial products and applications. Attached Figure Description

[0067] Figure 1 A flowchart of a method for preparing a gradient pore self-assembled drag-reducing agent for nano-fracturing fluid according to an embodiment of the present invention. Detailed Implementation

[0068] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0069] Therefore, the following detailed description of embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely illustrates some embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0070] It should be noted that, unless otherwise specified, the embodiments and features and technical solutions in the present invention can be combined with each other.

[0071] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0072] In the description of this invention, it should be noted that the terms "upper," "lower," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. These terms are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. In addition, the terms "first," "second," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0073] This embodiment provides a gradient pore self-assembled drag-reducing agent for nano-fracturing fluid, comprising nanoparticles with a three-layer gradient self-assembled structure, wherein the particle size of the nanoparticles is no greater than 50 nm.

[0074] The three-layer gradient self-assembled structure comprises, from the inside out, a hydrophobic core layer, a middle viscoelastic layer, and an outer hydrophilic layer, and consists of, by mass percentage:

[0075] Core hydrophobic layer: 25–35% nonionic surfactant, 15–20% organic solvent;

[0076] Intermediate viscoelastic layer: 30–40% polymer;

[0077] Outer hydrophilic layer: 15–20% zwitterionic surfactant.

[0078] The gradient pore self-assembled drag reducer for nano-fracturing fluids provided by this invention, through its unique three-layer gradient self-assembled structure and synergistic design at the nanoscale, has the following technical effects:

[0079] 1. Excellent reservoir permeability and low damage characteristics.

[0080] Because the drag-reducing agent consists of nanoparticles with an overall particle size ≤50 nm, its size is much smaller than the pore throat diameter of 100-200 nm in tight reservoirs (such as shale and tight sandstone). This allows the drag-reducing agent to efficiently enter and penetrate the reservoir's micropore network, achieving complete coverage of the fracturing fluid at depth. Unlike conventional macromolecular polymers (>100 nm) which are prone to bridging and clogging at the pore throat, this invention fundamentally reduces the risk of reservoir damage caused by physical blockage, thus helping to protect and maintain the reservoir's original permeability.

[0081] 2. Multifunctional integrated synergy of "drag reduction-sand carrying-modification".

[0082] This invention achieves multifunctional integration of a single dosage form through a precise three-layer structure design:

[0083] The hydrophobic core layer and nonionic surfactant provide the initial hydrophobic core for the entire structure and work synergistically with the organic solvent to ensure the initial stability of the structure in the oil phase environment, laying the foundation for the assembly of subsequent layers.

[0084] Intermediate viscoelastic layer and polymer: Provides the core viscoelasticity required by fracturing fluid. Through the extension and interaction of molecular chains, it effectively reduces the turbulent frictional resistance of fluid flow (i.e. drag reduction) and endows fracturing fluid with excellent proppant carrying capacity, ensuring effective suspension and delivery of proppant.

[0085] The outer hydrophilic layer and zwitterionic surfactants can achieve directional alignment on the rock surface, transforming it from oleophilic to hydrophilic. This wettability reversal function reduces the seepage resistance of fracturing fluid in the reservoir and facilitates post-fracturing flowback, reducing water-locking damage. Together with the intermediate viscoelastic layer, it improves the overall proppant carrying and flowback efficiency.

[0086] 3. Significantly enhanced temperature and salt resistance and long-term dispersion stability.

[0087] The three-layer gradient self-assembly structure described in this invention forms a stable "core-shell" nanocomposite. The hydrophobic core layer and the hydrophilic outer layer together constitute a protective barrier, effectively mitigating the attack and degradation of the polymer molecular chains in the intermediate viscoelastic layer by high temperature and high salt ions. Simultaneously, the component range defined in this invention (such as the content of zwitterionic surfactants) ensures that the nanoparticles maintain excellent dispersion stability in high-temperature, high-salt fracturing fluid systems, preventing aggregation, precipitation, or performance failure, thus guaranteeing long-term effectiveness and reliability under harsh operating conditions.

[0088] 4. Improved construction adaptability and efficiency.

[0089] The drag-reducing agent described in this invention, with its nanoscale structure and stable powder morphology (ensuring this through the preparation method), exhibits rapid dissolution and uniform dispersion in field applications, greatly facilitating on-site fluid preparation and application. Its multifunctional integrated characteristics simplify fracturing fluid formulations, achieving comprehensive performance optimization without the need for additional additives, thereby improving application efficiency and reducing operating costs.

[0090] In summary, this invention, through a three-layer gradient self-assembly structure design of "core hydrophobic layer - intermediate viscoelastic layer - outer hydrophilic layer" and strict control of the proportions of each component and the nanoscale of the final product, successfully integrates multiple functions such as efficient drag reduction, strong sand carrying capacity, reservoir modification, deep penetration, and temperature and salt resistance into a single product. The functions synergistically enhance each other, comprehensively solving the technical bottlenecks of existing technologies such as low drag reduction efficiency, high reservoir damage, single function, and poor environmental adaptability. This provides a novel fracturing fluid additive with superior performance for the efficient development of unconventional oil and gas resources.

[0091] As an optional implementation of this embodiment, in a gradient pore self-assembly drag-reducing agent for nano-fracturing fluid in this embodiment, the nonionic surfactant includes alkylphenol polyoxyethylene ether and nonylphenol polyoxyethylene ether, and is composed of: 15-20% alkylphenol polyoxyethylene ether and 10-15% nonylphenol polyoxyethylene ether by mass percentage; the organic solvent is an aromatic solvent.

[0092] Specifically, in a gradient pore self-assembly drag-reducing agent for nano-fracturing fluid in this embodiment, the alkylphenol polyoxyethylene ether is dodecylphenol polyoxyethylene ether OP-10, and the organic solvent is xylene.

[0093] As an optional implementation of this embodiment, in a gradient pore self-assembled drag-reducing agent for nano-fracturing fluid in this embodiment, the polymer of the intermediate viscoelastic layer includes partially hydrolyzed polyacrylamide and polyvinylpyrrolidone, and is composed of: 20-25% partially hydrolyzed polyacrylamide and 10-15% polyvinylpyrrolidone by mass percentage.

[0094] As an optional implementation of this embodiment, in a gradient pore self-assembly drag-reducing agent for nano-fracturing fluid in this embodiment, the zwitterionic surfactant of the outer hydrophilic layer is a betaine surfactant.

[0095] like Figure 1 As shown, this embodiment also provides a method for preparing a gradient pore self-assembled drag-reducing agent for nano-fracturing fluid, including the following steps:

[0096] Step S1: Add nonionic surfactant and organic solvent to the reaction vessel, and synthesize the core hydrophobic layer under heating and stirring conditions;

[0097] Step S2: Add the polymer to the system of step S1, increase the reaction temperature and stirring speed, and synthesize the intermediate viscoelastic layer;

[0098] Step S3: Add the zwitterionic surfactant to the system of step S2, reduce the reaction temperature and stirring speed, and self-assemble to synthesize the outer hydrophilic layer;

[0099] Step S4: Homogenize the mixture obtained in step S3 so that the particle size of the nanoparticles is no greater than 50 nm.

[0100] Step S5: Spray dry the homogenized liquid from step S4 to obtain a solid powder;

[0101] Step S6: Mix the dried powder with the silane coupling agent and perform activation treatment.

[0102] The preparation method provided in this embodiment, through its unique, step-by-step design and precise control of process parameters, offers a reliable and industrially scalable production path for achieving the core structure and performance of the gradient pore self-assembled drag-reducing agent. The specific technical effects of this method are reflected in:

[0103] Precise and orderly self-assembly of gradient structures was achieved.

[0104] Step S1 (Core Formation): By synthesizing a hydrophobic core layer under heating and stirring conditions, sufficient self-organization conditions are provided for the nonionic surfactant and organic solvent, forming a stable hydrophobic core, which lays the foundation for subsequent multilayer coating.

[0105] Step S2 (Viscoelastic Layer Coating): By increasing the reaction temperature and stirring speed, the expansion and migration of polymer molecular chains are promoted, enabling them to uniformly and densely coat the hydrophobic core, thereby constructing a key viscoelastic layer that provides drag reduction and sand carrying capacity.

[0106] Step S3 (Self-assembly of the outer hydrophilic layer): By lowering the reaction temperature and stirring speed, a mild reaction environment is provided for the zwitterionic surfactant, enabling it to align itself dropwise and orderly outside the viscoelastic layer through intermolecular forces, ultimately forming a stable outer hydrophilic layer and achieving the function of reversing wettability on the rock surface. These three interconnected steps ensure the precise construction and functional integration of the "hydrophobic-viscoelastic-hydrophilic" three-layer gradient structure.

[0107] This ensures the uniformity and stability of the product at the nanoscale.

[0108] Step S4 (High-Pressure Homogenization): This step is crucial for controlling the particle size of the final product. High-intensity homogenizing shear force effectively breaks down and reconstructs the micelles or aggregates formed earlier, ensuring that the final nanoparticle size is strictly controlled within ≤50 nm. This meets the size requirements for entering dense pore throats and improves batch consistency.

[0109] A high-performance final dosage form that is easy to store and transport was obtained.

[0110] Step S5 (Spray Drying): This step efficiently transforms the uniform and stable nanoscale liquid dispersion into a free-flowing solid powder. This formulation greatly facilitates long-term storage and long-distance transportation of the product, avoiding problems such as sedimentation, flocculation, or freeze-thaw instability that may occur in liquid systems. At the same time, the powder form also facilitates rapid on-site dissolution and use.

[0111] It significantly improves the product's performance in the application environment.

[0112] Step S6 (Activation Treatment): The surface of the nanoparticles was activated and modified by introducing a silane coupling agent through dry mixing. This step significantly enhances the immediate dispersibility and long-term dispersion stability of the nanoparticles in fracturing fluid-based fluids (especially high-salinity water), prevents the powder from agglomerating upon contact with water, and ensures the full performance and durability of the product under harsh oilfield conditions.

[0113] In summary, the preparation method provided in this embodiment is a systematic chemical assembly and physical processing procedure with stringent process conditions. Through a series of synergistic processes including stepwise self-assembly, forced nano-sizing, solidification, and surface activation, this method successfully integrates multiple functional components into a structurally well-defined and performance-stable nano-drag reducer. Furthermore, its excellent theoretical properties (such as deep penetration, efficient drag reduction, and reservoir modification) are reliably reproduced and guaranteed in actual industrial products and applications.

[0114] Optionally, in the preparation method of this embodiment, the nonionic surfactant in step S1 includes alkylphenol polyoxyethylene ether and nonylphenol polyoxyethylene ether, and step S1 includes:

[0115] Step S101: Add alkylphenol polyoxyethylene ether, nonylphenol polyoxyethylene ether and organic solvent to the reaction vessel and heat to 48-57℃;

[0116] Step S102: Stir at a constant speed of 600-800 rpm for 30-60 minutes, then sonicate to eliminate bubbles until the interfacial tension is ≤3 mN / m, thus synthesizing the hydrophobic core layer.

[0117] Optionally, in the preparation method of this embodiment, the polymer in step S2 includes partially hydrolyzed polyacrylamide and polyvinylpyrrolidone, and step S2 includes:

[0118] Step S201: After partially hydrolyzing polyacrylamide and polyvinylpyrrolidone are premixed, they are slowly added to the reaction vessel;

[0119] Step S202: Gradually raise the temperature of the reactor to 68-77℃, with the heating rate controlled at 1-2℃ / min;

[0120] Step S203: After heating to the target temperature in step S202, stir at a speed of 800-1000 rpm for 1-2 hours to form a coated intermediate viscoelastic layer.

[0121] Optionally, in the preparation method of this embodiment, step S3 includes:

[0122] Step S301: Add the zwitterionic surfactant dropwise and gradually lower the temperature of the reactor to 55-60℃;

[0123] Step S302: Gently stir at 400-500 rpm for 2-3 hours to orient the hydrophilic groups and self-assemble to synthesize the outer hydrophilic layer.

[0124] Optionally, in the preparation method of this embodiment, step S4 includes:

[0125] The mixture obtained in step S3 is transferred to a high-pressure homogenizer for homogenization. The homogenizer is homogenized at 50 MPa 2-5 times to break up large particles, and homogenized at 100 MPa 2-5 times to make the particle size of the nanoparticles no larger than 50 nm.

[0126] The particle size is monitored in real time during the homogenization process. When the particle size is >50 nm, the homogenization process continues until the particle size is ≤50 nm.

[0127] Step S5 includes: feeding the homogenized liquid from step S4 into a spray dryer for spray drying and curing. The feed rate of the spray dryer is 20-30 L / h, the atomization pressure is 0.2-1 MPa, the inlet temperature is 110-150℃ for rapid evaporation of moisture, and the outlet temperature is 55-65℃. The dried and cured drag-reducing agent has a moisture content of ≤5% and the powder of the drag-reducing agent has flowability.

[0128] In step S6, the mass ratio of the dried powder to the silane coupling agent is 100:1-200:1. Example 1

[0129] A gradient pore self-assembled drag-reducing agent for nano-fracturing fluids has a three-layer gradient self-assembled structure, comprising a hydrophobic core layer, a middle viscoelastic layer, and an outer hydrophilic layer; it is composed of the following components in mass percentage:

[0130] The core hydrophobic layer comprises 15% alkylphenol polyoxyethylene ether, 10% nonylphenol polyoxyethylene ether, and 20% organic solvent;

[0131] In this embodiment, the alkylphenol polyoxyethylene ether used is dodecylphenol polyoxyethylene ether OP-10, the nonylphenol polyoxyethylene ether used is NP-9, and the organic solvent used is xylene.

[0132] The intermediate viscoelastic layer comprises 25% partially hydrolyzed polyacrylamide (HPAM, molecular weight 8 million) and 10% polyvinylpyrrolidone (PVP).

[0133] The outer hydrophilic layer is composed of 20% zwitterionic surfactant. In this embodiment, the zwitterionic surfactant is cocamidopropyl hydroxysulfonate betaine.

[0134] The drag-reducing agent has a particle size of ≤50 nm, ensuring that it enters the pore throat of the tight reservoir within the range of 100-200 nm and covers the area affected by the fracturing fluid.

[0135] A method for preparing a gradient pore self-assembled drag-reducing agent for nano-fracturing fluid includes the following steps:

[0136] Step S1: Synthesize the hydrophobic core layer; add OP-10, NP-9 and xylene to the reactor and heat to 50±2℃; stir at 600 rpm for 30 minutes, then sonicate to eliminate bubbles. The specific conditions for sonication are 40 kHz for 10 minutes.

[0137] Take a sample to test the interfacial tension. If the interfacial tension is ≤3 mN / m, proceed to the next step; otherwise, continue stirring until the standard is met.

[0138] Step S2: Slowly add premixed HPAM and PVP for directional coating of the viscoelastic layer;

[0139] HPAM and PVP were premixed and slowly added to the system in step 1, and then the temperature of the reactor was raised to 70±2℃.

[0140] The mixture was stirred for 1 hour using a high-speed shear emulsifier (800 rpm), and the viscosity change was monitored during the process. The target viscosity was 50 mPa•s.

[0141] In addition, the heating rate needs to be controlled at 2℃ / min to prevent polymer degradation.

[0142] Step S3: Self-assembly of the outer hydrophilic layer; add betaine surfactant dropwise and cool to 60°C; gently stir at 400 rpm for 2 hours to orient the hydrophilic groups;

[0143] The contact angle test ensures that the rock surface changes from oleophilic to hydrophilic (contact angle <90°).

[0144] Step S4: High-pressure homogenization and nano-sizing; the mixture is transferred to a high-pressure homogenizer for homogenization.

[0145] Among them, the homogenization is carried out three times at 50 MPa to crush large particles; and the homogenization is carried out twice at 100 MPa to make the particle size ≤50 nm.

[0146] In addition, the homogenization process is monitored in real time, and a laser particle size analyzer is used to detect the particle size in each cycle. When the particle size is >50 nm, the number of homogenization cycles is increased.

[0147] Step S5: Spray drying and curing;

[0148] The homogenized mixture is fed into a spray dryer for spray drying and curing. The feed rate of the spray dryer is 20 L / h, the atomization pressure is 0.3 MPa, the inlet temperature is 120℃ for rapid evaporation of moisture, and the outlet temperature is 60℃ to prevent the heat-sensitive material from deteriorating.

[0149] The drag-reducing agent, after drying and curing, has a moisture content of ≤5%, exhibits powder fluidity (i.e., an angle of repose of <30°), and is easy to dissolve quickly on-site.

[0150] Step S6: Activation pretreatment;

[0151] The dried powder was dry-mixed with silane coupling agent (KH-550) at a mass ratio of 100:1 to enhance the dispersion stability of nanoparticles in fracturing fluid. Example 2

[0152] A gradient pore self-assembled drag-reducing agent for nano-fracturing fluids has a three-layer gradient self-assembled structure, comprising a hydrophobic core layer, a middle viscoelastic layer, and an outer hydrophilic layer; it is composed of the following components in mass percentage:

[0153] The core hydrophobic layer comprises 20% alkylphenol polyoxyethylene ether, 15% nonylphenol polyoxyethylene ether, and 15% organic solvent;

[0154] In this embodiment, the alkylphenol polyoxyethylene ether used is dodecylphenol polyoxyethylene ether OP-10, the nonylphenol polyoxyethylene ether used is NP-9, and the organic solvent used is xylene.

[0155] The intermediate viscoelastic layer comprises 20% partially hydrolyzed polyacrylamide and 15% polyvinylpyrrolidone;

[0156] The outer hydrophilic layer is composed of 15% zwitterionic surfactant. In this embodiment, the zwitterionic surfactant is cocamidopropyl hydroxysulfonate.

[0157] A method for preparing a gradient pore self-assembled drag-reducing agent for nano-fracturing fluid includes the following steps:

[0158] Step S1: Precisely synthesize the hydrophobic core layer; add OP-10, NP-9 and xylene to the reactor and heat to 55±2℃; stir at 800 rpm for 60 minutes, then sonicate to eliminate bubbles. The specific conditions for sonication are 50 kHz for 15 minutes.

[0159] Take a sample to test the interfacial tension. If the interfacial tension is ≤3 mN / m, proceed to the next step; otherwise, continue stirring until the standard is met.

[0160] Step S2: Slowly add premixed HPAM and PVP for directional coating of the viscoelastic layer;

[0161] HPAM and PVP were premixed and slowly added to the system in step 1, and then the temperature of the reactor was raised to 75±2℃.

[0162] The mixture was stirred for 1 hour using a high-speed shear emulsifier (1000 rpm), and the viscosity change was monitored during the process. The target viscosity was 50 mPa•s.

[0163] In addition, the heating rate needs to be controlled at 1℃ / min to prevent polymer degradation.

[0164] Step S3: Self-assembly of the outer hydrophilic layer; add betaine surfactant dropwise and cool to 55°C; gently stir at 500 rpm for 3 hours to orient the hydrophilic groups;

[0165] The contact angle test ensures that the rock surface changes from oleophilic to hydrophilic (contact angle <90°).

[0166] Step S4: High-pressure homogenization and nano-sizing; the mixture is transferred to a high-pressure homogenizer for homogenization.

[0167] Among them, the homogenization is carried out three times at 50 MPa to crush large particles; and the homogenization is carried out twice at 100 MPa to make the particle size ≤50 nm.

[0168] In addition, the homogenization process is monitored in real time, and a laser particle size analyzer is used to detect the particle size in each cycle. When the particle size is >50 nm, the number of homogenization cycles is increased.

[0169] Step S5: Spray drying and curing;

[0170] The homogenized mixture is fed into a spray dryer for spray drying and curing. The feed rate of the spray dryer is 20 L / h, the atomization pressure is 0.3 MPa, the inlet temperature is 140℃ for rapid evaporation of moisture, and the outlet temperature is 55℃ to prevent the heat-sensitive material from deteriorating.

[0171] The drag-reducing agent, after drying and curing, has a moisture content of ≤5%, exhibits powder fluidity (i.e., an angle of repose of <30°), and is easy to dissolve quickly on-site.

[0172] Step S6: Activation pretreatment;

[0173] The dried powder was dry-mixed with silane coupling agent (KH-550) at a mass ratio of 100:1 to enhance the dispersion stability of nanoparticles in fracturing fluid. Example 3

[0174] A gradient pore self-assembled drag-reducing agent for nano-fracturing fluids has a three-layer gradient self-assembled structure, comprising a hydrophobic core layer, a middle viscoelastic layer, and an outer hydrophilic layer; it is composed of the following components in mass percentage:

[0175] The core hydrophobic layer comprises 15% alkylphenol polyoxyethylene ether, 15% nonylphenol polyoxyethylene ether, and 20% organic solvent;

[0176] In this embodiment, the alkylphenol polyoxyethylene ether used is dodecylphenol polyoxyethylene ether OP-10, the nonylphenol polyoxyethylene ether used is NP-9, and the organic solvent used is xylene.

[0177] The intermediate viscoelastic layer comprises 23% partially hydrolyzed polyacrylamide and 12% polyvinylpyrrolidone.

[0178] The outer hydrophilic layer is composed of 15% zwitterionic surfactant. In this embodiment, the zwitterionic surfactant is cocamidopropyl hydroxysulfonate.

[0179] When using the drag-reducing agent of the present invention in the field, it is necessary to add the drag-reducing agent of the present invention to the slickwater-based fluid at a ratio of 0.1-1.5% and stir for 10 minutes to dissolve it; at the same time, add 0.1-0.5% of nano-pressure drive agent to form an integrated fracturing fluid. Example 4

[0180] This embodiment presents a gradient pore self-assembled drag-reducing agent for nano-fracturing fluids, which has a three-layer gradient self-assembled structure, including a hydrophobic core layer, an intermediate viscoelastic layer, and an outer hydrophilic layer; it is composed of the following components in mass percentage:

[0181] Core hydrophobic layer: 15% dodecylphenol polyoxyethylene ether (OP-10), 10% nonylphenol polyoxyethylene ether (NP-9), 20% xylene;

[0182] Intermediate viscoelastic layer: 18% partially hydrolyzed polyacrylamide (HPAM, molecular weight 8 million), 12% polyvinylpyrrolidone (PVP);

[0183] Outer hydrophilic layer: 20% cocamidopropyl hydroxysulfonate betaine.

[0184] This embodiment describes a method for preparing a gradient pore self-assembled drag-reducing agent for nano-fracturing fluids, comprising:

[0185] OP-10, NP-9 and xylene were added to a reaction vessel, heated to 48°C, stirred at 600 rpm for 60 minutes, and then sonicated at 40 kHz for 15 minutes, resulting in an interfacial tension of 2.5 mN / m.

[0186] Slowly add the premixed HPAM and PVP, heat to 68°C at 1°C / min, and stir at 800 rpm for 2 hours;

[0187] Add the betaine surfactant dropwise, cool to 55°C, and stir at 400 rpm for 3 hours;

[0188] Homogenized twice at 50 MPa and five times at 100 MPa, the final particle size was 49 nm.

[0189] Spray drying: feed rate 20 L / h, inlet temperature 110℃, outlet temperature 55℃;

[0190] Mix with silane coupling agent KH-550 at a mass ratio of 200:1. Example 5

[0191] This embodiment presents a gradient pore self-assembled drag-reducing agent for nano-fracturing fluids, which has a three-layer gradient self-assembled structure, including a hydrophobic core layer, an intermediate viscoelastic layer, and an outer hydrophilic layer; it is composed of the following components in mass percentage:

[0192] Core hydrophobic layer: OP-10 18%, NP-9 12%, xylene 18%;

[0193] Intermediate viscoelastic layer: HPAM 28%, PVP 12%;

[0194] Outer hydrophilic layer: 15% betaine surfactant.

[0195] This embodiment describes a method for preparing a gradient pore self-assembled drag-reducing agent for nano-fracturing fluids, comprising:

[0196] Core synthesis: 57℃, 800 rpm stirring for 30 minutes, 50 kHz sonication for 10 minutes, interfacial tension 2.2 mN / m;

[0197] Viscoelastic layer coating: Heat to 77℃ at 2℃ / min, stir at 1000 rpm for 1 hour, and the viscosity reaches 65 mPa·s;

[0198] Outer layer self-assembly: 60℃, 500 rpm stirring for 2 hours;

[0199] Homogenization: Homogenize 5 times at 50 MPa and 2 times at 100 MPa, with a final particle size of 43 nm;

[0200] Spray drying: feed rate 30 L / h, inlet temperature 150℃, outlet temperature 65℃;

[0201] Activation treatment: Silane coupling agent to powder mass ratio 100:1. Example 6

[0202] This embodiment presents a gradient pore self-assembly drag-reducing agent for nano-fracturing fluid, with the same composition and ratio as in Embodiment 1.

[0203] Special parameters for preparation method:

[0204] Step S1: Lower limit temperature 48℃, lower limit speed 600 rpm, lower limit time 30 minutes;

[0205] Step S2: Lower limit heating rate of 1℃ / min, lower limit reaction temperature of 68℃;

[0206] Step S3: Lower limit temperature 55℃, lower limit speed 400 rpm;

[0207] Step S4: Homogenize twice at 50 MPa (lower limit), and twice at 100 MPa (lower limit).

[0208] Step S5: Feed rate 20 L / h (lower limit), inlet temperature 110℃ (lower limit);

[0209] Step S6: Silane coupling agent ratio 200:1 (upper limit);

[0210] Result: The final product has a particle size of 47 nm, and all performance indicators meet the standards.

[0211] Test case

[0212] The present invention verifies the test data according to the embodiments. During the construction process, in the pre-liquid stage, a 0.1% concentration low-viscosity liquid is used to create joints (viscosity 5 mPa•s); in the sand-carrying liquid stage, a 1.5% concentration high-viscosity liquid is used to transport sand (viscosity > 50 mPa•s); in the displacement liquid stage, a 0.3% concentration of gel breaking agent is used to assist drainage.

[0213] The comparative experimental data analysis is as follows:

[0214] 1. Comparative test of drag reduction performance.

[0215]

[0216] Comparative Example 1: Conventional HPAM drag reducer (molecular weight 8 million).

[0217] Comparative Example 2: Common surfactant drag reducer (hexadecyltrimethylammonium bromide).

[0218] 2. Reservoir adaptability test.

[0219]

[0220] 3. Temperature and salt resistance test.

[0221] High-temperature aging test (90℃, 24 hours):

[0222]

[0223] 4. Dispersion stability test.

[0224] Observe after standing at room temperature for 30 days:

[0225]

[0226] 5. Sand-carrying performance test.

[0227]

[0228] The above embodiments and systematic effect verification data of the present invention fully demonstrate that the present invention has the following outstanding advantages:

[0229] Significant performance advantages: Compared with two typical comparative examples, the product of this invention shows significant advantages in drag reduction rate, temperature and salt resistance, and reservoir adaptability, especially in terms of performance stability under high temperature and high salt conditions.

[0230] Multifunctional integration effect: A single product simultaneously achieves multiple functions such as efficient drag reduction, strong sand carrying, reservoir modification and deep permeability, which solves the technical bottleneck of single function in the existing technology.

[0231] Promising industrialization prospects: All embodiments were prepared using conventional equipment, with highly controllable process parameters and stable product quality, demonstrating promising prospects for industrial application and promotion.

[0232] The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described herein. Although the present invention has been described in detail with reference to the above embodiments, the present invention is not limited to the specific embodiments described above. Therefore, any modifications or equivalent substitutions to the present invention, as well as all technical solutions and improvements that do not depart from the spirit and scope of the invention, are covered within the scope of the claims of the present invention.

Claims

1. A gradient-pore self-assembled drag reducer for nano-fracturing fluid, characterized in that, The nanoparticles comprise a three-layer gradient self-assembly structure, and the particle size of the nanoparticles is not greater than 50 nm. The three-layer gradient self-assembly structure comprises, from inside to outside, a core hydrophobic layer, an intermediate viscoelastic layer, and an outer hydrophilic layer, and the mass percentage composition comprises: The core hydrophobic layer: non-ionic surfactant 25-35%, organic solvent 15-20%; The intermediate viscoelastic layer: polymer 30-40%, the polymer comprising partially hydrolyzed polyacrylamide and polyvinylpyrrolidone, and the mass percentage composition comprising: partially hydrolyzed polyacrylamide 20-25%, polyvinylpyrrolidone 10-15%; The outer hydrophilic layer: zwitterionic surfactant 15-20%.

2. The gradient-pore self-assembled drag reducer for nano-fracturing fluid according to claim 1, characterized in that, The non-ionic surfactant comprises alkylphenol polyoxyethylene ether and nonylphenol polyoxyethylene ether, and the mass percentage composition comprises: alkylphenol polyoxyethylene ether 15-20%, nonylphenol polyoxyethylene ether 10-15%; The organic solvent is an aromatic hydrocarbon solvent. 3.The gradient-pore self-assembled drag reducer for nano-fracturing fluid according to claim 2, characterized in that, The alkylphenol polyoxyethylene ether is dodecylphenol polyoxyethylene ether OP-10, and the organic solvent is xylene. 4.The gradient-pore self-assembled drag reducer for nano-fracturing fluid according to claim 1, characterized in that, The zwitterionic surfactant of the outer hydrophilic layer is a betaine surfactant.

5. A method for preparing the gradient-pore self-assembled drag reducer for nano-fracturing fluid according to any one of claims 1-4, characterized in that, The method comprises the following steps: Step S1, adding non-ionic surfactant and organic solvent into a reaction kettle to synthesize the core hydrophobic layer under heating and stirring; Step S2, adding polymer into the system of step S1 to synthesize the intermediate viscoelastic layer by increasing the reaction temperature and stirring speed; Step S3, adding zwitterionic surfactant into the system of step S2 to synthesize the outer hydrophilic layer by reducing the reaction temperature and stirring speed; Step S4, performing homogenization treatment on the mixed liquid obtained in step S3 to make the particle size of the nanoparticles not greater than 50 nm; Step S5, performing spray drying on the liquid after homogenization in step S4 to obtain solid powder; Step S6, mixing the dried powder with silane coupling agent and performing activation treatment.

6. The preparation method according to claim 5, characterized in that, The non-ionic surfactant in step S1 comprises alkylphenol polyoxyethylene ether and nonylphenol polyoxyethylene ether, and step S1 comprises: Step S101, adding alkylphenol polyoxyethylene ether, nonylphenol polyoxyethylene ether, and organic solvent into a reaction kettle, and heating to 48-57℃; Step S102, stirring at a speed of 600-800 rpm for 30-60 minutes, and then performing ultrasonic treatment to eliminate bubbles until the interfacial tension is ≤3 mN / m, to synthesize the core hydrophobic layer.

7. The production method according to claim 6, wherein The polymer in step S2 comprises partially hydrolyzed polyacrylamide and polyvinylpyrrolidone, and step S2 comprises: Step S201, pre-mixing partially hydrolyzed polyacrylamide and polyvinylpyrrolidone, and then slowly adding into a reaction kettle; Step S202, gradually increasing the temperature of the reaction kettle to 68-77℃ at a rate of 1-2℃ / min; Step S203, after the temperature in step S202 is increased to the target temperature, stirring at a speed of 800-1000 rpm for 1-2 hours to form the coated intermediate viscoelastic layer.

8. The production method according to claim 7, characterized by, Step S3 comprises: Step S301, the amphiphilic ionic surfactant is added drop by drop, and the temperature of the reaction kettle is gradually reduced to 55-60℃; Step S302, the hydrophilic group is arranged and assembled to synthesize the outer hydrophilic layer by stirring at a speed of 400-500 rpm for 2-3 hours.

9. The preparation method according to claim 5, characterized in that, The step S4 comprises: The mixed solution obtained in the step S3 is transferred to a high-pressure homogenizer for homogenization treatment, wherein the mixed solution is homogenized at 50 MPa for 2-5 times to break large particles, and the mixed solution is homogenized at 100 MPa for 2-5 times to make the particle size of the nanoparticles not greater than 50 nm; The particle size is monitored in real time during the homogenization process, and the homogenization treatment is continued until the particle size is less than or equal to 50 nm when the particle size is greater than 50 nm; The step S5 comprises: the liquid after the homogenization treatment in the step S4 is input into a spray dryer for spray drying and solidification, the feeding rate of the spray dryer is 20-30 L / h, the atomization pressure is 0.2-1 MPa, the inlet temperature is 110-150℃, and the outlet temperature is 55-65℃, so as to evaporate water quickly, and the water content of the drag reducer after the drying and solidification is less than or equal to 5%, and the powder of the drag reducer has flowability; The mass ratio of the drying powder to the silane coupling agent in the step S6 is 100:1-200:1.

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