Temperature-magnetic field dual-response shape memory particle with core-shell network structure as well as preparation method and application of temperature-magnetic field dual-response shape memory particle

Abstract

The invention provides a core-shell network structure temperature-magnetic field dual response shape memory particle and a preparation method and application thereof, and relates to the technical field of oil and gas field development. The memory particle comprises a magnetic shape memory core and a shape memory shell, the glass transition temperature Tg2 of the magnetic shape memory core is higher than the target oil reservoir temperature; the glass transition temperature of the shape memory shell is not higher than the target oil reservoir temperature, and the surface functionalized magnetic nanoparticles are connected with the polymer network of the magnetic shape memory core through covalent bonds. By constructing a core-shell hierarchical structure, a hierarchical response mechanism cooperatively triggered by the temperature and the magnetic field is achieved, the shell is recovered in advance at the oil reservoir temperature to promote deep migration of particles, the core triggers secondary recovery at a fixed point through the magnetothermal effect under the action of the alternating magnetic field, and accurate plugging of cracks is achieved; and the method is suitable for the development of low-permeability, ultra-low-permeability and unconventional fractured oil and gas reservoirs.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field development technology, and in particular to a core-shell network structure temperature-magnetic field dual-response shape memory particle, its preparation method and application. Background Technology

[0002] In the development of low-permeability, ultra-low-permeability, and unconventional oil and gas reservoirs, fracture systems, as important seepage channels, have a dual impact on crude oil production. On the one hand, fractures can provide efficient conductivity, helping to increase initial production; on the other hand, fractures can also easily become advantageous channels for edge and bottom water, injected water, or gas channeling, leading to premature water flooding, gas channeling, or ineffective circulation of displacement fluids in oil wells, severely restricting the improvement of ultimate recovery. Statistics show that the recovery rate of fractured reservoirs is generally below 25%, while in tight oil and shale oil reservoirs, the recovery rate is even less than 10%. Therefore, developing technologies that can effectively control fracture channeling and achieve precise deep sealing has become crucial for improving the development efficiency of such oil and gas resources.

[0003] Currently, commonly used fracture plugging materials mainly include fibers, rigid particles, expandable polymers, gels, and composite temporary plugging agents, with their action mechanism mostly following a "bridging-filling-cementing" model. However, these materials still have significant limitations in practical applications. First, they have poor compatibility with fracture scale, tending to accumulate prematurely in the near-wellbore zone to form a plug, making it difficult to migrate to deeper fractures. Second, the plugging behavior mainly relies on fluid carrying and passive accumulation, lacking active control over the plugging location and timing. Third, some materials have insufficient temperature resistance, shear resistance, and plugging strength, making it difficult to meet the long-term plugging requirements under high temperature, high salinity, and large pressure differential conditions.

[0004] Shape memory polymers, with their "deformation programming—stimulus-response recovery" characteristics, offer a new approach to active fracture control. Traditional temperature-responsive shape memory materials typically use formation temperature as the trigger condition, but their applications have significant limitations. First, traditional temperature-responsive shape memory materials are prone to premature recovery in high-temperature reservoirs, leading to particle accumulation near the wellbore and preventing penetration into deep fractures. Furthermore, inadequate temperature matching can result in incomplete recovery and low sealing strength. While magnetic field-triggered materials can achieve targeted heating and precise control, their triggering accuracy is severely affected if the material has already softened or partially recovered at reservoir temperatures. In addition, the dispersion stability of magnetic nanoparticles in the polymer matrix and their integration with the network structure directly influence the magnetocaloric conversion efficiency and the material's mechanical effects.

[0005] Therefore, there is an urgent need to develop a composite structure plugging material that can adapt to reservoir temperature conditions and has graded response capability to solve the above problems. Summary of the Invention

[0006] In view of this, the present invention provides a method for preparing core-shell network structured shape memory particles with dual temperature-magnetic field response. The present invention constructs core-shell structured particles with hierarchical response characteristics, forming a dual-response system triggered by synergistic temperature and magnetic field. The outer shell has a lower glass transition temperature (Tg1), allowing shape recovery to occur first at reservoir temperatures, thereby promoting particle migration into deeper fractures. The inner core has a higher glass transition temperature (Tg2) and is uniformly dispersed with surface-functionalized magnetic nanoparticles, enabling it to maintain shape stability at reservoir temperatures. Only when an alternating magnetic field is applied does the magnetocaloric effect of the magnetic nanoparticles raise the temperature of the inner core above its Tg2, triggering secondary shape recovery to precisely seal the fractures. This synergistic triggering mechanism is expected to overcome the shortcomings of existing materials, improve the controllability and reliability of deep fracture sealing, and has good application prospects in the efficient development of low-permeability, unconventional oil and gas reservoirs.

[0007] The first aspect of this invention is to provide a core-shell network structure temperature-magnetic field dual-response shape memory particle, the shape memory particle comprising a magnetic shape memory core and a shape memory shell; the magnetic shape memory core comprising a shape memory polymer matrix material and surface-functionalized magnetic nanoparticles dispersed therein, the glass transition temperature Tg2 of the magnetic shape memory core being higher than the target reservoir temperature, and the temperature difference between the two being 5℃-30℃; the shape memory shell continuously encapsulating the magnetic shape memory core, the glass transition temperature Tg1 of the shape memory shell being no higher than the target reservoir temperature, and the temperature difference between the two being 0℃-30℃; the surface-functionalized magnetic nanoparticles being connected to the polymer network of the magnetic shape memory core via covalent bonds.

[0008] A second aspect of the present invention is to provide a method for preparing the above-mentioned core-shell network structure temperature-magnetic field dual-response shape memory particles, comprising the following steps: S1. Selection of shape memory matrix material: Based on the target reservoir temperature, a shape memory matrix material is selected. The glass transition temperature (Tg2) of the core matrix material is higher than the target reservoir temperature; the glass transition temperature (Tg1) of the outer shell matrix material is not higher than the target reservoir temperature. The glass transition temperature of the polymer shell is lower than the reservoir temperature, which enables the shape memory shell to undergo preliminary shape memory recovery under reservoir temperature conditions, facilitating better particle penetration into the deep fracture. The glass transition temperature of the core is higher than the reservoir temperature, which ensures that the magnetic core maintains shape stability under reservoir temperature conditions. Shape memory recovery only occurs through point heating with a magnetic field, thus achieving precise sealing of the fracture. S2. Surface modification treatment of magnetic particles: Magnetic nanoparticles are dispersed in an organic solvent, and then their surfaces are modified with silane / titanium ester modifiers. By introducing active functional groups that can react with the core shape memory material on the surface of the magnetic particles, surface-functionalized magnetic nanoparticles are obtained. After surface functionalization, the magnetic particles are connected to the polymer network of the magnetic core through covalent bonds, thereby improving the dispersion stability of the magnetic nanoparticles in the magnetic core. S3, Preparation of high glass transition temperature magnetic cores: Magnetic nanoparticles with S2 surface functionalization were added to the core shape memory matrix material of S1. The magnetic particles were uniformly dispersed by mechanical stirring and / or ultrasonic dispersion. Then, a curing agent was added for cross-linking and curing. By increasing the amount of curing agent, a magnetic core shape memory material with high cross-linking density and glass transition temperature higher than the reservoir temperature was constructed. The cured magnetic core material was crushed and sieved to obtain magnetic core particles. S4. Construction of a low glass transition temperature shell: The magnetic core particles of S3 are added to the outer shell matrix material of S1 and stirred to disperse them evenly. Then, a curing agent is added for cross-linking and curing, forming a shape memory shell with low cross-linking density and glass transition temperature lower than the reservoir temperature on the outside of the magnetic core particles, thereby constructing a core-shell structure shape memory particle. In the core-shell structure particle, the shape memory shell continuously covers the magnetic shape memory core to form a complete core-shell network structure. S5, Shape Memory Programming of Core-Shell Structured Particles: The core-shell structure shape memory particles of S4 are heated to the programmed temperature window between Tg1 and Tg2 and an external force is applied to deform them. After cooling and shaping, core-shell network structure temperature-magnetic dual-response shape memory particles with temporary shapes are obtained.

[0009] Preferably, the temperature-magnetic field dual-response shape memory particles need to undergo graded response under reservoir temperature conditions, so the shape memory matrix materials of the particle core and shell can be the same or different.

[0010] Preferably, the temperature difference between the glass transition temperature of the core matrix material and the target reservoir temperature is 5℃-30℃, and the temperature difference between the glass transition temperature of the outer shell matrix material and the target reservoir temperature is 0℃-30℃.

[0011] Preferably, the shape memory matrix material is one or more of the shape memory polymers selected from polycaprolactone (PCL), polyurethane (PU), epoxy resin (EP), cross-linked polystyrene (CPS), and cross-linked polyethylene (XLPE).

[0012] Preferably, the curing agent is one or more of aliphatic amine curing agents, aromatic amine curing agents, and acid anhydride curing agents. More preferably, the aliphatic amine curing agent is one or more of ethylenediamine (EDA), polyetheramine (D230), diethylenetriamine (DETA), and triethylenetetramine (TETA); the aromatic amine curing agent is one or more of diaminodiphenyl sulfone (DDS), 4,4′-dichlorodiphenylmethane diamine (MOCA), 4,4′-diaminodiphenylmethane (MDA), and p-phenylenediamine; and the acid anhydride curing agent is one or more of methyltetrahydrophthalic anhydride (MeTHPA), hexahydrophthalic anhydride (HHPA), phthalic anhydride (PA), and tetrahydrophthalic anhydride (THPA).

[0013] Preferably, for epoxy resin (EP) systems, crosslinking density is controlled by aliphatic amines and acid anhydride curing agents to construct high Tg cores and low Tg shells respectively; for PCL or XLPE semi-crystalline shape memory polymers, Tg differentiation is achieved by controlling crystallinity and crosslinking degree, and its graded response mechanism is consistent with that of PU systems.

[0014] Preferably, the magnetic nanoparticles are ferrite-based magnetic materials, and more preferably, iron oxide magnetic nanoparticles with magnetocaloric effect.

[0015] Preferably, the modifier is one or more of 3-glycidyl etheroxypropyltrimethoxysilane (KH560), 3-methacryloyloxypropyltrimethoxysilane (KH570), and 3-aminopropyltriethoxysilane (APTES). More preferably, when the core matrix is ​​polyurethane or an isocyanate-containing system, the modifier is APTES, which introduces amino groups to react with NCO; when the core matrix is ​​an epoxy resin system, the modifier is KH560, whose epoxy groups can directly participate in crosslinking; when the core matrix is ​​a free radical polymerization or unsaturated system, the modifier is KH570.

[0016] Preferably, the content of magnetic nanoparticles in the magnetic core accounts for 5wt%-60wt% of the core mass fraction, so as to ensure that the magnetocaloric effect generated by the magnetic core under the action of an external magnetic field can make the core temperature exceed its own glass transition temperature.

[0017] Preferably, the temporary shape includes, but is not limited to, sheet-like, strip-like, or fibrous shapes that are easy to inject. The temporary shape (such as sheet-like or strip-like) is obtained by compressing and stretching a permanent shape (such as a sphere) at a specific temperature. The maximum axial dimension of the temporary shape is designed to be less than or equal to the characteristic dimension of the permanent shape. After trigger recovery, the particle undergoes deformation recovery, and its final characteristic dimension (such as the diameter of a sphere) can reach 2-5 times the maximum axial dimension of the temporary shape, thereby achieving effective sealing within the crack.

[0018] Preferably, the applied magnetic field is an alternating magnetic field, which generates a magnetocaloric effect by exciting the magnetic nanoparticles in the magnetic core, thereby achieving point heating of the magnetic core. The frequency of the alternating magnetic field is 50-1000 kHz, and the magnetic induction intensity is 10-800 Gauss.

[0019] A third aspect of this invention is to provide the application of the above-mentioned core-shell network structure temperature-magnetic field dual-response shape memory particles in oil and gas field production enhancement.

[0020] Preferably, the alternating magnetic field frequency of the temperature-magnetic field dual-response shape memory particle during use is 50-1000 kHz, and the magnetic induction intensity is 10-800 Gauss.

[0021] Preferably, the oil and gas well is a low-permeability oil and gas well, an ultra-low-permeability oil and gas well, or an unconventional fractured oil and gas well.

[0022] Compared with the prior art, the beneficial technical effects of the present invention are as follows: (1) This invention realizes graded intelligent response and precise plugging. By using the core-shell structure design, the particles are given graded response capability to external stimuli. The outer shell first undergoes shape recovery at downhole temperature, which effectively reduces the particle transmission resistance and promotes its migration to the deep part of the fracture. The core achieves fixed-point shape recovery under the trigger of an external alternating magnetic field, thereby realizing precise control of the plugging position and plugging strength, and avoiding the problem of ineffective plugging.

[0023] (2) This invention solves the limitations of single response materials. It effectively avoids the defect that single temperature response materials are prone to premature recovery in the wellbore or near-well zone, which can cause blockage. It also overcomes the disadvantage that single magnetic field response materials may weaken the triggering accuracy due to overall softening at reservoir temperature.

[0024] (3) This invention improves material stability and trigger reliability. By performing surface functionalization modification on magnetic nanoparticles, they are connected to the core polymer network in the form of covalent bonds, which significantly improves the dispersion stability and interfacial bonding force of magnetic nanoparticles in shape memory matrix, ensuring efficient and stable conversion and transmission of magnetocaloric effect.

[0025] (4) The present invention enhances the plugging adaptability and strength. The final recovery shape of the particle design (such as spherical) can play an effective role in wedging, recovery and filling in the crack, forming a high-strength plugging layer. At the same time, the core-shell integrated network structure also gives the material good overall mechanical properties.

[0026] (5) The process of this invention is controllable and widely applicable. By adjusting the matrix material, crosslinking density and magnetic particle content of the core and shell respectively, it can be flexibly adapted to reservoir conditions with different temperatures and different fracture scales, and has wide applicability and good engineering prospects. Attached Figure Description

[0027] The present invention will be further described below with reference to the accompanying drawings.

[0028] Figure 1 This is a schematic diagram of the core-shell network structure of the temperature-magnetic field dual-response shape memory particles of the present invention; Figure 2 This is a DSC test image of the temperature-magnetic field dual-response shape memory particle with core-shell network structure in Embodiment 1 of the present invention. Figure 3 This is a schematic diagram of the hierarchical response of shape memory particles in the temperature-magnetic field dual-response core-shell network structure of the present invention. Detailed Implementation

[0029] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0030] Unless otherwise specified, all experiments were repeated three times, and the results are expressed as averages.

[0031] Example 1: A method for preparing core-shell network structure temperature-magnetic field dual-response shape memory particles (target reservoir temperature 70℃), the steps of which are as follows: 1. Experimental materials: 1) The shape memory polymer matrix is ​​a diphenylmethane diisocyanate (MDI) type polyurethane prepolymer (NCO content is 3.0±0.2%, supplier: Xuzhou Peize New Material Chemical), which serves as the common matrix raw material for both the core and the shell (different crosslinking densities and glass transition temperatures (Tg) are achieved by adjusting the amount of curing agent). 2) Magnetic nanoparticles: Fe3O4 nanoparticles with an average particle size of 20 nm (purity >99%, supplier: Aladdin); 3) Curing agent: 4,4′-dichlorodiphenylmethane diamine (MOCA, industrial grade, supplier: Xuzhou Peize New Material Chemical). 4) Surface modifier: 3-aminopropyltriethoxysilane (APTES, purity ≥98%, supplier: McLean Reagents); 5) Solvent: Anhydrous ethanol (analytical grade).

[0032] 2. Surface amination modification of Fe3O4 nanoparticles: 1) 1.0 g Fe3O4 nanoparticles were dispersed in 200 mL of anhydrous ethanol and placed in an ultrasonic cleaner. The dispersion was ultrasonically carried out at room temperature for 3 h to obtain a uniform black suspension. 2) While maintaining sonication, dissolve 0.2 mL of APTES in 20 mL of anhydrous ethanol, and slowly add the above suspension dropwise over 30 min using a constant pressure dropping funnel; 3) Turn on the heating function of the ultrasonic cleaner, control the water bath temperature at 60±2℃, and continue the reaction for 18 hours; 4) After the reaction is complete, the product is separated by a permanent magnet and washed three times with anhydrous ethanol to remove the physically adsorbed silane. 5) The obtained solid was dried in a vacuum drying oven at 60℃ for 12 h to obtain APTES modified Fe3O4 powder (NH2-Fe3O4). After being lightly ground with an agate mortar, it was passed through a 400-mesh sieve for later use.

[0033] 3. Preparation of high Tg2 magnetic cores: 1) Formulation: 10.0 g polyurethane prepolymer, 15 phr (i.e. 1.5 g) of NH2-Fe3O4, and 10.8 phr (i.e. 1.08 g) of MOCA. 2) Mixing and dispersing: First, premix 1.5 g of NH2-Fe3O4 powder with about 1.0 g of prepolymer in a beaker using a spatula to form a paste; 3) Add the remaining 9.0 g of prepolymer and stir at 1800 rpm for 40 min using a high-speed mechanical stirrer; 4) The mixture was then placed in an ultrasonic cleaner cooled by an ice-water bath and ultrasonically dispersed for 60 min to obtain a uniform and fine magnetic prepolymer mixture. 5) After the above mixture has been allowed to stand to defoam, add 1.08 g MOCA and stir quickly by hand for about 90 seconds until it is evenly mixed. Then pour it into a polytetrafluoroethylene flat mold. 6) Curing process: Place the mold in a forced-air drying oven and cure it in a stepwise temperature increase program of 80℃ / 2 h + 100℃ / 2 h. This high-temperature post-curing is intended to promote the full reaction between isocyanate groups and amine groups in the system to form a polyurethane network structure with high cross-linking density. 7) Crushing and sieving: After the cured magnetic film is brittled by liquid nitrogen, it is initially crushed with a mortar and pestle, then finely ground, and graded by passing it through 200-mesh and 400-mesh standard sieves. Particles between 200-mesh and 400-mesh are collected as magnetic core particles and sealed for later use.

[0034] 4. Construction of a low Tg1 polymer shell: 1) Formulation: 10.0 g polyurethane prepolymer, the amount of magnetic core particles added is 50 phr (i.e. 5.0 g), and the amount of MOCA is reduced to 5.3 phr (i.e. 0.53 g) in order to construct a low crosslinking density network. 2) Mixing and coating: Add 5.0 g of magnetic core particles to 10.0 g of prepolymer and stir at 600 rpm for 25 min using a mechanical stirrer to fully wet and initially coat the core particles with the prepolymer; 3) Subsequently, ultrasonic-assisted dispersion was performed in an ice-water bath for 30 minutes to make the coating layer more uniform; 4) Curing: Add 0.53 g MOCA to the above mixture, stir quickly for about 2 minutes to mix evenly, and pour the mixture into the mold; 5) Curing process: To accommodate the formation of a low cross-linked network, a mild curing condition of 60℃ / 6 h was adopted. After curing, a blocky core-shell composite material was obtained. 6) Crushing and sieving: The solidified block material is crushed, ground, and sieved, and particles with a particle size range of 100-400μm are selected, which are the initially formed core-shell structure shape memory particles.

[0035] To further illustrate how the present invention achieves the glass transition temperature relationship defined in the claims by adjusting the material parameters of the core and shell under different reservoir temperature conditions, the MDI-type polyurethane prepolymer / MOCA curing system is used as an example to summarize the curing agent dosage, curing conditions and corresponding glass transition temperature range of the core and shell under different reservoir temperature conditions, as shown in Table 1 (it should be noted that the Fe3O4 content listed in Table 1 is expressed as phr, and its corresponding mass fraction can be converted according to the amount of prepolymer and curing agent).

[0036] Table 1. Parameter control results of core-shell structured shape memory particles 5. Shape memory programming of core-shell particles 1) Place the core-shell structured particles obtained in step 4 in a precision temperature-controlled oven and heat them to 65°C at a rate of 2°C / min. Hold the temperature at this temperature for 15 min to ensure that the outer shell polymer is fully softened and the chain segments are mobile, while the core remains rigid. 2) Quickly transfer the softened hot particles to a polytetrafluoroethylene mold preheated to the same temperature (65°C); apply axial pressure to the particles using a precision pressure device to control the axial compressive strain to 75%, so that the particles are temporarily deformed into a thin sheet with uniform thickness. The loading rate during the pressure application process is controlled at 1 mm / min, and the pressure is maintained after the target strain is reached. 3) Under the condition of maintaining constant pressure, start the program cooling and cool the particles and the mold together to 40°C (below the shell Tg1) at a rate of 5°C / min, and then cool to room temperature (below 25°C) at a rate of 10°C / min. This "pressure holding cooling" process is intended to freeze and fix the deformed state. 4) After cooling and shaping, release the pressure, demold and remove the particles to obtain a temporary sheet-like core-shell network structure temperature-magnetic dual-response shape memory particle. This programming process stores sufficient strain energy in the outer shell network while ensuring the stability of the core shape, laying the foundation for subsequent temperature and magnetic field triggered graded recovery.

[0037] The sheet-like particles obtained after shape memory programming in step 5 can achieve significant and controllable geometric expansion under triggered conditions. The specific expansion behavior was tested and is as follows: 1) Expansion range: During the process of a particle recovering from a temporary shape to a permanent shape, the expansion range of its linear size (such as diameter) is 200%-400% (that is, it expands to 3-5 times the size of the original temporary shape).

[0038] 2) Glass transition temperature test: DSC tests were performed on the final particles under nitrogen atmosphere, with a heating and cooling rate of 10℃ / min. The temperature was increased from room temperature to 150℃, then decreased from 150℃ to -30℃ to eliminate thermodynamic effects, and finally increased from -30℃ to 150℃. Two distinct glass transition regions were observed in the particles: one at 57.73℃ (outer shell Tg1) and the other at 76.51℃ (inner core Tg2), confirming the existence of a core-shell dual thermodynamic structure.

[0039] 3) Shape recovery rate test: The shape recovery rate of the particles was tested according to the conventional testing methods for shape memory materials. The results showed that under the combined triggering of reservoir temperature and alternating magnetic field, the shape recovery rate of the particles could reach 90%-98%.

[0040] 4) Shape fixation rate test: After programming and cooling, the shape fixation rate of the particles remained above 90%, indicating that their temporary shape had good stability during downhole transportation.

[0041] 5) Cyclic stability test: After 3-5 temperature-magnetic field cycle triggering, the glass transition temperature and shape recovery performance of the particles did not show significant (p>0.05) decay, the structure remained intact, and it has good cyclic stability.

[0042] Example 2 The difference from Example 1 is that, with low-temperature reservoir conditions (40°C) as the target, a core-shell network structure shape memory particles with a shell that responds first to temperature and a core that responds secondarily to magnetic field are constructed by selecting a semi-crystalline shape memory polymer system and controlling the degree of crosslinking.

[0043] The glass transition temperature (Tg2) of the magnetic core is 56.74℃; the glass transition temperature (Tg1) of the polymer shell is 27.42℃; the shape memory matrix material is a polycaprolactone (PCL) system (PCL, number average molecular weight Mn=45,000); the surface modifier of the magnetic particles is KH560; the curing agent is hexamethylene diisocyanate trimer (HDI trimer); the content of magnetic nanoparticles is 10 wt%; and the shape memory programming temperature window is 35℃.

[0044] Magnetic nanoparticles were surface-epoxidized using KH560 (denoted as Epoxy-Fe3O4 nanoparticles). The preparation steps for the high Tg2 magnetic core are as follows: 10.0 g of PCL was dissolved in 50 mL of DMF, 10 wt% of Epoxy-Fe3O4 nanoparticles (1.0 g) were added, and HDI trimer (the amount used was 1.5 times the theoretical molar amount of hydroxyl groups in PCL) was added. The mixture was cured at 80 °C for 12 h. The low Tg1 polymer shell was constructed by adding HDI trimer, the amount of which was only 30% of the theoretical molar amount of hydroxyl groups in PCL, and curing at 60 °C for 8 h.

[0045] DSC test results show that there are two clear glass transition regions in the particle system, corresponding to the outer shell Tg1 (27.42℃) and the core Tg2 (56.74℃), respectively. Under the action of an applied alternating magnetic field (conditions: 300 kHz, 200 Gauss), the magnetic core rapidly heats up through the magnetocaloric effect and exceeds Tg2, triggering secondary shape recovery. The shape recovery rate test results show that the shape recovery rate of the particles under the temperature-magnetic field synergistic triggering conditions is ≥90%.

[0046] Example 3 The difference from Example 1 is that, for high-temperature reservoir environments (target reservoir temperature of about 120°C), a high-temperature resistant epoxy resin system is selected as the shape memory matrix, and combined with aromatic amine curing agents and high-content magnetic nanoparticles, to construct core-shell network structure shape memory particles with distinct glass transition temperatures of the core and shell.

[0047] The target reservoir temperature is 120℃; the core glass transition temperature Tg2 is 147.21℃; and the outer shell glass transition temperature Tg1 is 108.57℃.

[0048] Shape memory matrix material: Both the core and the shell are made of bisphenol A type epoxy resin (EP) system; the surface modifier of magnetic particles is KH560 (denoted as Epoxy-Fe3O4); the curing agent is 4,4′-diaminodiphenyl sulfone (DDS); the amount of magnetic nanoparticles added is 40 wt% of the epoxy resin mass.

[0049] Preparation of high Tg2 magnetic core: Take 10.0 g epoxy resin, add 4.0 g EP-Fe3O4 powder, add DDS according to 105% of the theoretical amount of epoxy resin (3.3 g), and cure according to the step temperature program of 150℃ / 2 h + 180℃ / 2 h + 200℃ / 1 h. The Tg2 of the core is 145.37℃ according to DSC test.

[0050] Construction of a low Tg1 shell: Take 10.0 g of epoxy resin and add 6.0 g of magnetic core particles. The amount of DDS used is only 70% of the theoretical amount of epoxy resin (2.2 g). Curing is carried out according to a mild program of 120℃ / 2 h + 140℃ / 2 h. The Tg1 of the shell is 106.32℃ according to DSC test.

[0051] Shape memory programming of core-shell particles: The particles are kept at 125°C for 20 min to soften their outer shell, and then pressure is applied to compress and deform them into a thin sheet shape (axial strain of about 65%).

[0052] Performance verification: DSC testing clearly showed two glass transition stages; when the programmed particles were placed in a reservoir temperature environment of 120℃, their outer shell recovered first, and then an alternating magnetic field (300 kHz, 500 Gauss) was applied, the core temperature rose to above 150℃, triggering complete recovery, and its final shape recovery rate was ≥90%; after aging the particles at 150℃ for 100 h, their DSC characteristic peaks and shape recovery performance did not show significant attenuation.

[0053] Example 4: Influence of magnetic field parameters on particle properties By optimizing the frequency and intensity parameters of the alternating magnetic field, the magnetocaloric effect of the magnetic core is precisely controlled, so that it reaches and exceeds Tg2 within a set time, thereby achieving precise time and air control of shape memory recovery behavior.

[0054] The test sample was the core-shell structure shape memory particle (temporarily in sheet form) prepared in Example 1. The sample material and reservoir ambient temperature (70°C) were fixed. A binary experimental matrix was designed with magnetic field frequency (f) and magnetic induction intensity (B) as variables to evaluate the ability of the core to heat up from ambient temperature (70°C) to target temperature (80°C, which needs to exceed Tg2) within 2 minutes under different combinations of magnetic field parameters.

[0055] Table 2 Experimental matrix and key results for magnetic field parameter optimization Experimental results show that when the frequency or intensity is too low (100 kHz, 200 Gauss), the magnetocaloric effect is weak, and the core cannot reach the trigger temperature within the specified time. Considering the heating rate, energy consumption, and equipment requirements, f=300 kHz and B=300 Gauss is the optimal parameter combination. Under these conditions, the core can rapidly heat from 70℃ to over 80℃ within 48 seconds, with an average heating rate of 12.5℃ / min and a maximum equilibrium temperature of approximately 92.5℃. Experiments 2, 5, and 6 all triggered recovery within 2 minutes, demonstrating the feasibility of the parameter combination. Although experiment 4 had the fastest heating, the excessively high equilibrium temperature may be detrimental to the long-term stability of the material, and it also consumed a relatively high amount of energy.

[0056] In a 70℃ oil bath, the optimal magnetic field was applied to the programmed particles for 90 s. Infrared monitoring showed that the core temperature exceeded Tg2 at 50 s, and the particles then began to recover significantly. The shape recovery rate was measured to be 95%. The same batch of particle samples was subjected to the "heating-recovery-programming" cycle 5 times. Each cycle was triggered under the optimal magnetic field parameters. The results showed that the final shape recovery rate of the particles remained above 93%.

[0057] Example 5: Effect of Fe3O4 content on the magnetocaloric effect, thermodynamic properties, and final particle shape recovery of the magnetic core. With a fixed reservoir temperature (70℃) and outer shell composition, the mass fraction of NH2-Fe3O4 in the core was varied to design three groups of NH2-Fe3O4 content: control group A (lower limit group): NH2-Fe3O4 content was 5 wt%; experimental group B (middle value group): NH2-Fe3O4 content was 30 wt%; experimental group C (upper limit group): NH2-Fe3O4 content was 60 wt%.

[0058] Formula calculation (based on the total mass of the core composite material): Group A: 10.0 g PU prepolymer + 1.08 g MOCA + 0.58 g NH2-Fe3O4; Group B: 10.0 g PU prepolymer + 1.08 g MOCA + 4.75 g NH2-Fe3O4; Group C: 10.0 g PU prepolymer + 1.2 g MOCA + NH2-Fe3O4 16.80 g.

[0059] Characterization of the three groups of particles (alternating magnetic field 300 kHz, 300 Gauss) showed the following results: Group A (5 wt%): Measured Tg2 was 76.54℃, measured Tg1 was 57.32℃, and the time to heat to 80℃ was 247 s; Group B (30 wt%): Measured Tg2 was 78.65℃, measured Tg1 was 58.26℃, and the time to heat to 80℃ was 45 s; Group C (60 wt%): Measured Tg2 was 84.71℃, measured Tg1 was 58.89℃, and the time to heat to 80℃ was 28 s.

[0060] This embodiment demonstrates that shape memory particles with a core-shell structure can be prepared in polyurethane systems with a magnetic particle content ranging from 5wt% to 60wt%.

[0061] Comparative Example 1 The difference from Example 1 is that the Fe3O4 content is 0 wt%.

[0062] Test conditions: ambient temperature was 70°C, and an alternating magnetic field with the same parameters as in Example 5 (frequency 300 kHz, magnetic induction intensity 300 Gauss) was applied. The particles were initially in a programmed, sheet-like temporary shape.

[0063] Experimental results: When the ambient temperature rises to 70℃, the outer shell of the particle undergoes shape memory recovery, but the core remains stable in shape and does not undergo complete recovery; after applying an alternating magnetic field, the particle system does not show a significant temperature rise, the core temperature never exceeds its glass transition temperature Tg2, the particle does not undergo secondary shape memory recovery, and its geometric shape is basically the same as before the magnetic field was applied.

[0064] This comparative example shows that magnetic nanoparticles are key to achieving the "magnetic field-triggered secondary shape recovery" of this invention. Without magnetic particles, the material only has a single temperature response behavior and cannot achieve targeted and controllable sealing of cracks.

[0065] Comparative Example 2 The difference from Example 1 is that magnetic nanoparticles were introduced but the core-shell hierarchical structure was not constructed.

[0066] The magnetic nanoparticles account for 15 wt% of the total particle size; the particles do not distinguish between the core and the shell, do not construct a Tg hierarchical structure, and are a uniform cross-linked network; the overall glass transition temperature (Tg) of the particles is 65℃.

[0067] Preparation process: 15 phr (1.5 g) of NH2-Fe3O4 was dispersed in 10.0 g of PU prepolymer, and 8.8 phr (0.88 g) of MOCA was added. The mixture was then cured at 80℃ / 2 h + 100℃ / 2 h. The particles were compressed into sheets at 65℃.

[0068] Experimental results: When the ambient temperature rises to 70℃, the particles are close to or reach their Tg. The particles soften significantly or even partially recover their shape before entering the fracture or during migration. They are prone to premature recovery and accumulation in the near-wellbore area or at the fracture entrance. After the magnetic field is applied, the particles undergo overall, non-selective shape recovery. It is impossible to distinguish between the primary and secondary triggering processes, making it difficult to achieve precise control over the position and timing of shape recovery.

[0069] This comparative example demonstrates that simply introducing magnetic nanoparticles without constructing a core-shell hierarchical structure cannot achieve the hierarchical response behavior required by this invention. The core-shell structure is not merely a structural optimization method in this invention, but rather the key to achieving "dual temperature-magnetic field response, hierarchical triggering."

[0070] Comparative Example 3 The difference from Example 1 is that the magnetic nanoparticles were not surface functionalized and were not covalently linked with the core polymer network.

[0071] The magnetic nanoparticles are Fe3O4 nanoparticles with an average particle size of about 20 nm, and have not undergone any silane or other surface functionalization modification treatment. The unmodified Fe3O4 nanoparticles are directly added to the PU prepolymer and physically dispersed by mechanical stirring and ultrasonic dispersion without introducing surface modifiers.

[0072] Experimental results: During the preparation process, unmodified Fe3O4 nanoparticles are prone to agglomeration. During the curing process, the magnetic particles are unevenly distributed with obvious local enrichment. Under the action of alternating magnetic field, the overall temperature rise of the particles is significantly lower than that in Example 1, and the core temperature rise rate is slow, making it difficult to stabilize above Tg2 in a short time. During the magnetic field triggering stage, the secondary shape recovery of the particles is incomplete or uneven. After multiple temperature-magnetic field cycles, the magnetic particles further migrate and agglomerate, the shape recovery rate gradually decreases, and the performance deteriorates significantly.

[0073] This comparative example shows that surface functionalization modification of magnetic nanoparticles is the key to achieving stable and efficient magnetocaloric triggering in this invention. If only physical doping is performed without surface modification, the magnetocaloric effect and shape memory performance will be significantly weakened.

[0074] Comparative Example 4 The difference from Example 1 is that the glass transition temperature (Tg1) of the outer shell of the core-shell structure shape memory particle is higher than the reservoir temperature.

[0075] The target reservoir temperature is 70℃; the glass transition temperature Tg1 of the outer shell is 77.31℃ (higher than the reservoir temperature); the glass transition temperature Tg2 of the inner core is 93.54℃; although Tg2 is still higher than Tg1, the relative relationship between Tg1 and reservoir temperature is incorrect.

[0076] To obtain an outer shell Tg1 above 70°C, the amount of MOCA in the outer shell was significantly increased to 10.8 phr to construct a high cross-linking density outer shell network, raising its Tg1 above the reservoir temperature; the curing process used was the same high-temperature curing as the core (80°C / 2h + 100°C / 2h) to promote complete cross-linking; the shape memory programming temperature window was selected at 85°C for programming.

[0077] Experimental results: When the ambient temperature rises to 70℃, the outer shell Tg1 is higher than the reservoir temperature, so the outer shell is still in a glassy state. The overall rigidity of the particles is high, and almost no shape recovery occurs. The deformation and passage capacity of the particles in the fracture are significantly insufficient, and they are prone to getting stuck at the fracture entrance or near the well. Under the action of the magnetic field, when the core temperature exceeds Tg2, the core undergoes shape recovery. However, because the outer shell is still in a rigid state, the core recovery is severely constrained, the overall deformation of the particles is insufficient, and the degree of recovery is significantly limited.

[0078] This comparative example demonstrates that the relative relationship between the Tg gradient and reservoir temperature is one of the core aspects of this invention. If the Tg design is inappropriate, even with a core-shell structure, magnetic particles, and magnetic field triggering methods, the expected technical effect cannot be achieved.

[0079] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A core-shell network structure temperature-magnetic field dual-response shape memory particle, comprising a magnetic shape memory core and a shape memory shell, characterized in that, The magnetic shape memory core comprises a shape memory polymer matrix material and surface-functionalized magnetic nanoparticles dispersed therein. The glass transition temperature (Tg2) of the magnetic shape memory core is higher than the target reservoir temperature, and the temperature difference between the two is 5℃-30℃. The shape memory shell continuously encapsulates the magnetic shape memory core. The glass transition temperature (Tg1) of the shape memory shell is not higher than the target reservoir temperature, and the temperature difference between the two is 0℃-30℃. The surface-functionalized magnetic nanoparticles are connected to the polymer network of the magnetic shape memory core through covalent bonds.

2. The method for preparing the core-shell network structure temperature-magnetic field dual-response shape memory particles according to claim 1, characterized in that, Includes the following steps: S1. Selection of shape memory matrix material: Based on the target reservoir temperature, select the shape memory matrix material. The glass transition temperature Tg2 of the core matrix material is greater than the target reservoir temperature; the glass transition temperature Tg1 of the outer shell matrix material is not higher than the target reservoir temperature. S2. Surface modification treatment of magnetic particles: Magnetic nanoparticles are dispersed in an organic solvent, and then their surfaces are modified with silane / titanium ester modifiers to obtain surface-functionalized magnetic nanoparticles. S3, Preparation of high glass transition temperature magnetic core: Magnetic nanoparticles with S2 surface functionalization are added to the core shape memory matrix material of S1, uniformly dispersed, and then a curing agent is added for cross-linking and curing. The cured magnetic core material is then crushed and sieved to obtain magnetic core particles. S4. Construction of a low glass transition temperature shell: The magnetic core particles of S3 are added to the shell matrix material of S1 and dispersed evenly by stirring; then a curing agent is added for cross-linking and curing to construct core-shell structure shape memory particles. S5. Shape memory programming of core-shell structured particles: The core-shell structured shape memory particles of S4 are heated to the programming temperature window between Tg1 and Tg2 and an external force is applied to deform them. After cooling and shaping, core-shell network structure temperature-magnetic field dual-response shape memory particles with temporary shapes are obtained.

3. The preparation method according to claim 2, characterized in that, The temperature difference between the glass transition temperature of the core matrix material and the target reservoir temperature is 5℃-30℃, and the temperature difference between the glass transition temperature of the outer shell matrix material and the target reservoir temperature is 0℃-30℃.

4. The preparation method according to claim 2, characterized in that, The shape memory matrix material is one or more of the following: polycaprolactone, polyurethane, epoxy resin, cross-linked polystyrene, and cross-linked polyethylene shape memory polymer.

5. The preparation method according to claim 2, characterized in that, The curing agent is one or more of aliphatic amine curing agents, aromatic amine curing agents, and acid anhydride curing agents.

6. The preparation method according to claim 2, characterized in that, The magnetic nanoparticles are ferrite-based magnetic materials.

7. The preparation method according to claim 2, characterized in that, The modifier is one or more of 3-glycidyl etheroxypropyltrimethoxysilane, 3-methacryloyloxypropyltrimethoxysilane, and 3-aminopropyltriethoxysilane.

8. The preparation method according to claim 2, characterized in that, The magnetic nanoparticles in the magnetic core account for 5wt%-60wt% of the core mass fraction.

9. The preparation method according to claim 2, characterized in that, The applied magnetic field is an alternating magnetic field. The alternating magnetic field generates a magnetocaloric effect by exciting the magnetic nanoparticles in the magnetic core, thereby achieving point heating of the magnetic core. The frequency of the alternating magnetic field is 50-1000 kHz, and the magnetic induction intensity is 10-800 Gauss.

10. The application of the core-shell network structure temperature-magnetic field dual-response shape memory particles as described in claim 1 in oil and gas field production enhancement, wherein the oil and gas well is a low-permeability oil and gas well, an ultra-low-permeability oil and gas well, or an unconventional fractured oil and gas well.

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