Crack selective plugging method based on temperature-magnetic field dual response shape memory particles

Abstract

The invention provides a crack selective plugging method based on temperature-magnetic field dual response shape memory particles, and relates to the technical field of oil and gas field development. Temperature-magnetic field dual-response shape memory particles with core-shell structures are dispersed in carrier liquid to be injected into a target oil layer, and a particle shell network is converted into a high-elastic state under the action of the in-situ temperature of an oil reservoir, so that adaptive entering and pre-filling of cracks with different sizes are realized; then, an alternating magnetic field is only applied to the target well section, the inner core network is activated and subjected to shape memory recovery through the magnetothermal effect generated by magnetic components in the inner core network, the particles are driven to be subjected to geometric dimension expansion in the crack space, and the plugging structure with the mechanical build-in characteristic is formed. And after the alternating magnetic field is removed, the shape memory recovery structure is fixed, deep, accurate and stable plugging of the crack channeling channel is achieved, and treatment of low-permeability, ultra-low-permeability and unconventional oil and gas reservoir crack channeling channels is achieved.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field development technology, and in particular to a method for selectively sealing fractures based on temperature-magnetic field dual-response shape memory particles. Background Technology

[0002] As conventional oil and gas resources gradually deplete, global oil and gas exploration and development is accelerating its shift towards unconventional reservoirs such as low-permeability, ultra-low-permeability, tight oil and gas, and shale oil and gas. These reservoirs generally feature extremely low matrix permeability, strong heterogeneity, and well-developed natural fractures. During development, they easily form high-permeability channels dominated by fractures. Injected media or edge water rapidly flows along fractures during displacement, leading to severe fingering at the displacement front, limited swept volume, early water breakthrough in the wellbore, rapid increase in water cut, and consistently low reservoir recovery. Fracture channeling not only weakens the efficiency of injected energy utilization but also induces severe inter-well interference and water channeling risks, making it one of the major challenges restricting the efficient development of low-permeability and unconventional oil and gas reservoirs.

[0003] To address the treatment of high-permeability channels in fractures, reservoir engineering has developed a series of profile control, water shut-off, and fracture sealing technologies. These mainly include: chemical gel sealing systems represented by polymer gels; particle sealing systems primarily composed of inorganic or organic solid particles; and composite sealing systems combining gel and particle synergies. Chemical gel systems typically achieve selective sealing of high-permeability channels by forming a spatial network structure through in-situ cross-linking within the formation. However, in reservoirs with complex fracture geometry, strong shear forces, and sensitive lithology, they are prone to problems such as severe filtration loss, difficulty in deep placement, and insufficient stability of the sealing body, making precise control of the sealing location and strength difficult. Conventional particle sealing agents rely on particle bridging and accumulation to form sealing bands, and their sealing capacity is highly correlated with particle size and fracture width. However, most particle materials have limited deformation capacity, making it difficult to adapt to complex multi-level fracture width variations, and they suffer from shortcomings such as difficulty in entering deep flow channels and poor erosion resistance. Although composite plugging systems take into account both strength and filling properties to a certain extent, most of them are still "passive adaptation" materials, lacking the ability to actively control the plugging location and process, and are difficult to meet the needs of unconventional oil and gas reservoirs for deep and refined control.

[0004] In recent years, various smart responsive materials have received widespread attention in the field of oil and gas engineering plugging and control, including shape memory polymers (SMPs), magnetic nanocomposites, and multi-stimuli responsive materials such as those responsive to light, electricity, and pH. Current research focuses on thermally responsive SMP plugging agents and temporary plugging and redirecting agents, preliminarily verifying their feasibility and plugging enhancement effects in drilling and completion plugging, fracturing, and other engineering scenarios. Furthermore, related studies have shown that magnetically responsive materials can be used for magnetic field guidance of plugging agents, directional control of oil displacement systems, and remote activation of profile control agents, improving the precision of construction. However, existing smart responsive plugging materials mostly revolve around a "single stimulus response mechanism," with shape memory materials relying solely on formation temperature, making it difficult to control the timing and location of deformation triggering. Magnetic particle systems relying solely on magnetocaloric effects often exhibit mechanisms of thermal expansion or volume expansion, making it difficult to form high-strength, stable interlocking plugging structures.

[0005] Therefore, existing fracture sealing technologies still lack a method that can achieve "first migration, then sealing" deep within fractures, and actively control the sealing location and process in stages. In particular, there is a lack of a sealing technology that can synergistically utilize the reservoir temperature field and an external magnetic field to maintain good adaptability of the material during the migration phase, and then activate it at the target location to form a high-strength interlocking structure. This would address the problems of poor sealing selectivity, unstable sealing, and difficulty in deep application in unconventional oil and gas reservoir fracture channeling control. Summary of the Invention

[0006] In view of this, the present invention provides a method for selective fracture plugging based on temperature-magnetic field dual-response shape memory particles. This method utilizes temperature-magnetic field dual-response shape memory particles with a core-shell network structure to achieve deep, precise, and stable plugging of fracture channeling under the synergistic effect of reservoir temperature and an applied alternating magnetic field. By controlling the plugging process in stages, the shape memory particles maintain good fracture entry capability during injection and migration, and are then activated at the target fracture location and undergo deformation recovery. This overcomes the problems of easy well access plugging, uncontrolled plugging location, and insufficient plugging strength in existing fracture plugging technologies, and is suitable for the treatment of fracture channeling in low-permeability, ultra-low-permeability, and unconventional oil and gas reservoirs.

[0007] The first aspect of this invention is to provide a method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles, comprising the following steps: S1. Preparation and injection of sealing fluid: Temperature-magnetic field dual-response shape memory particles are used as the sealing medium. The shape memory particles are dispersed in the carrier fluid to form a sealing working fluid. The sealing working fluid is then injected into the target oil layer, allowing the shape memory particles to migrate along the fracture flow channel with the carrier fluid to the deeper part of the formation. The shape memory particles are core-shell structured particles, including a shape memory outer shell network and a shape memory inner shell network. The glass transition temperature of the outer shell network is lower than the target reservoir temperature, and the glass transition temperature of the inner shell network is higher than the target reservoir temperature. Magnetic particles are dispersed in the inner shell network. The shape memory particles are in a temporary shape state after being programmed with shape memory before injection. The temporary shape is sheet-like, rod-like, or fibrous to reduce flow resistance when entering the crack. S2. Initial Temperature Response and Deep Pre-filling Stage: Under the influence of in-situ reservoir temperature, the outer shell network of the shape memory particles is in a highly elastic state, thereby improving the deformation capacity and crack adaptability of the particles. This is beneficial for the particles to enter cracks of different scales and achieve deep migration and pre-filling within the crack space, while the core network remains inactive during this stage. S3, Alternating Magnetic Field Fixed-Point Activation Stage: After pre-filling is completed, an alternating magnetic field is applied only to the target well section to induce a magnetocaloric effect in the magnetic particles in the core network and raise the temperature of the core network above its glass transition temperature, thereby triggering the shape memory particles to undergo deformation recovery. S4, Shape Memory Recovery Blocking Stage: During the deformation recovery process, the shape memory particles undergo geometric expansion within the crack space and form mechanical embedding structures such as wedge-type and lock-type with the crack wall, thereby sealing the crack flow channel. S5, Stabilization stage of the sealing structure: After the alternating magnetic field is removed, the shape memory particles regain their original shape and the resulting sealing structure remains stable under geological conditions, achieving deep, precise, and long-term stable sealing of fracture flow channels.

[0008] Preferably, the magnetic particles in the core network are one or both of nano-Fe3O4 and nano-γ-Fe2O3, the mass fraction of the magnetic particles in the shape memory particles is 5-60 wt%, and the average particle size of the shape memory particles is 50-500 μm.

[0009] Preferably, the carrier liquid is one or a combination of several of the following: water, formation water, prepared brine, polymer solution, etc., and the mass concentration of the shape memory particles in the carrier liquid is 0.1-10 wt%.

[0010] Preferably, the frequency of the alternating magnetic field is 50-800 kHz and the magnetic induction intensity is 10-500 Gauss.

[0011] Preferably, the alternating magnetic field is generated in the target well section by a downhole alternating magnetic field generator. The device is run into the target well section via a cable or coiled tubing and positioned therein to achieve local activation of only the target well section. The downhole alternating magnetic field generator includes: a downhole induction coil (sole coil or saddle coil), a non-magnetic heat-resistant shell, a centerer, and a transmission cable or coiled tubing cable connected to a surface power supply / controller. The effective length of the coil is 0.5-10 m.

[0012] After the aforementioned device is lowered into the wellbore, precise alignment is achieved using logging depth / casing coupling positioning or perforation layer comparison. Packers (or bridge plug / packer combinations) can be installed above and below the target layer to form localized isolation sections, thereby limiting the activation range to the target well section. Under conditions where the formation conductivity and wellbore structure meet requirements, the alternating magnetic field effectively activates fracture channels within a certain radial distance outward from the wellbore.

[0013] Preferably, the fixed-point activation involves applying an alternating magnetic field to a preset deep target area, causing the particles to recover their shape and form a sealing layer within that target area.

[0014] Preferably, during the initial temperature response stage, the glass transition temperature Tg(a) of the outer shell network is not higher than the target reservoir temperature, and the difference between the two temperatures is 0℃-30℃.

[0015] Preferably, the glass transition temperature Tg(b) of the core network is higher than the glass transition temperature Tg(a) of the outer shell network, and the difference between the core network and the target reservoir temperature is 5℃-30℃.

[0016] Preferably, the target reservoir is a fractured area in a low-permeability reservoir, an ultra-low-permeability reservoir, a tight reservoir, or a shale reservoir, and the target of the sealing is the fracture channel in the wellbore / deep fracture or the fracture channel between wells.

[0017] Preferably, the method for preparing the temperature-magnetic field dual-response shape memory particles includes the following steps: (1) Selection of shape memory matrix material: Based on the target reservoir temperature, shape memory matrix materials are selected. The glass transition temperature (Tg2) of the core matrix material is 5℃-30℃ 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, and the temperature difference between the two is 0℃-30℃. (2) Surface modification treatment of magnetic particles: Magnetic nanoparticles were dispersed in an organic solvent and ultrasonically dispersed. Then, their surfaces were 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 were obtained after magnetic separation, washing, and drying. (3) Fabrication of magnetic cores with high glass transition temperature: The surface-functionalized magnetic nanoparticles in (2) are added to the core shape memory matrix material in (1). The magnetic particles are uniformly dispersed by mechanical stirring and / or ultrasonic dispersion. Then, a curing agent is 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 is constructed. The cured magnetic core material is crushed and sieved to obtain magnetic core particles. (4) Construction of a shell with a low glass transition temperature: The magnetic core particles in (3) 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, 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. (5) Shape memory programming of core-shell structured particles: The core-shell structure shape memory particle in (4) is heated to the programming temperature window between Tg1 and Tg2 and an external force is applied to deform it. It is held at the programming temperature for 5-15 min, and then cooled at a rate of 1-10℃ / min to at least 10℃ below the shell network Tg1 or below room temperature while maintaining the external force. The external force is then released to obtain a temperature-magnetic field dual-response shape memory particle with a temporary shape.

[0018] Preferably, the shape memory matrix material is a polyurethane system, formed by stepwise polymerization of isocyanate, polyol and chain extender; the isocyanate is diphenylmethane diisocyanate, the polyol is polyether polyol, and the chain extender is 4,4′-dichlorodiphenylmethane diamine.

[0019] Preferably, the magnetic nanoparticles are one or both of nano-Fe3O4 and nano-γ-Fe2O3, with an average particle size of 10-50 nm; the modifier is 3-aminopropyltriethoxysilane; the surface modification reaction is carried out at 50-70°C for 12-24 h, and the surface-functionalized magnetic nanoparticles are connected to the polymer network of the magnetic core through covalent bonds.

[0020] Preferably, the method for preparing the core shape memory matrix material is as follows: The isocyanate and polyol are reacted at a molar ratio of NCO:OH = 2.5:1 - 3.5:1 at 70 - 80 °C for 2 - 3 h to form a prepolymer with terminal -NCO groups; magnetic nanoparticles with surface functionalization are added to the prepolymer at a dosage of 10 - 30 wt% of the core mass, and through high-speed mechanical stirring and ultrasonic treatment, a uniform magnetic prepolymer dispersion is formed; a chain extender is added to the dispersion, and the molar ratio of amino group (-NH2) to isocyanate group (-NCO) ([NH2] / [NCO]) is controlled to be between 1.05 and 1.25; after preliminary curing at 80 °C for 2 h, post-curing is carried out at 100 °C for 2 - 3 h; the cured bulk material is embrittled by liquid nitrogen, ground, and classified and sieved to obtain magnetic core particles with a particle size range of 30 - 80 μm.

[0021] Preferably, the preparation method of the outer shell matrix material is as follows: A prepolymer is synthesized by using isocyanate and polyol at a molar ratio of NCO:OH = 2.0:1 - 2.2:1; magnetic core particles with a dosage of 30 - 70 wt% of the outer shell prepolymer mass are dispersed in the outer shell prepolymer, and uniform coating is achieved through stirring; a chain extender is added, and the [NH2] / [NCO] molar ratio is controlled to be between 0.6 and 0.9; curing is carried out at 60 °C for 6 - 8 h; after curing, a core-shell composite material is obtained, which is broken, ground, and multi-stage sieved to obtain shape memory particles with a particle size of 50 - 500 μm.

[0022] Preferably, the shape memory programming steps are as follows: The particles are heated to the programming temperature Tp, where Tg(a) < Tp < Tg(b), so that the outer shell softens while the core remains rigid; an external force is applied to the particles in a displacement / strain control mode, and the loading strain rate is 0.01 - 0.2 s -1 , for spherical particles, the compressive stress is controlled to be between 0.5 and 3 MPa, so that the particles reach the target programming strain ε of 50% - 80%, and the load is maintained at the programming temperature Tp for 5 - 15 min; under the condition of not releasing the target strain ε, the temperature is cooled at a rate of 1 - 10 °C / min to be at least 10 °C below the Tg(a) of the outer shell network or below room temperature, and then the external force is removed to fix the temporary shape.

[0023] Preferably, the content of magnetic nanoparticles in the magnetic core accounts for 5 wt% - 60 wt% of the core mass fraction.

[0024] Preferably, the temporary shape is sheet-like, rod-like, or fibrous. The temporary shape (such as sheet-like, rod-like, or fibrous) is obtained by applying external force to deform and cool particles with permanent shapes such as spherical or polyhedral forms within a programmed temperature window above the glass transition temperature (Tg1) of the outer shell and below the glass transition temperature (Tg2) of the inner shell, followed by fixation. During this process, the maximum axial dimension of the particles is compressed to 1 / 5 to 1 / 2 of the characteristic dimension of the permanent shape. When the particles are triggered to recover, their size expands to the original permanent shape, and the characteristic dimension (such as the diameter of a sphere) can reach 2-5 times the maximum axial dimension of the temporary shape, thereby generating significant expansion and wedging within the crack, forming an effective seal.

[0025] Compared with the prior art, the beneficial technical effects of the present invention are as follows: This invention uses a phased response method of "first transport, then activation" to enable shape memory particles to deform and recover at a designated location deep within the fracture, avoiding premature sealing in the near-wellbore area and increasing the sealing depth by more than 2 times.

[0026] The particles of this invention are in a temporary form such as sheet-like, rod-like or fibrous before injection, and maintain good deformation ability under the action of reservoir temperature, which is conducive to entering the fracture channel of different scales and complex shapes. Their migration depth can reach the middle section of the fracture and the deep wide fracture area, far exceeding that of single response particles.

[0027] The sealing process of this invention relies on the deformation recovery of shape memory particles, rather than simple thermal expansion or passive accumulation, so that the particles form a wedge-like, lock-like mechanical embedding structure in the crack, which has higher shear resistance and erosion resistance, and significantly improves the sealing pressure difference.

[0028] In this invention, the temperature response is used only to improve particle transport performance, and the magnetic field response is used only for point-triggered plugging. The two response mechanisms are independent of each other, making this method applicable to reservoir environments with different temperature conditions and different fracture sizes.

[0029] The method of this invention maintains a plugging rate of over 97.4% after 30 days of accelerated aging and scouring tests, with a plugging pressure differential attenuation rate of only 20.8%, demonstrating excellent long-lasting plugging effect. Attached Figure Description

[0030] The present invention will be further described below with reference to the accompanying drawings.

[0031] Figure 1 This is a DSC test image of the temperature-magnetic field dual-response shape memory particle of Embodiment 1 of the present invention; Figure 2 This is a schematic diagram of the temperature-magnetic field dual-response shape memory particles used to seal cracks in the sealing method of the present invention; Figure 3This is a comparison diagram of the sealing cracks using dual-response shape memory particles and single-response particles in the sealing method of this invention. Detailed Implementation

[0032] 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.

[0033] Unless otherwise specified, all experiments were repeated three times. Results are expressed as mean ± standard deviation, and P < 0.05 indicates a significant difference.

[0034] Example 1: Preparation and Blocking Effect Verification of Dual-Response Shape Memory Particles I. Experimental Reagents Isocyanate: Diphenylmethane diisocyanate (MDI, PM-200, supplier: Chengxin Chemicals), providing rigid segments and reaction sites.

[0035] Polyol: Polyether polyol (DL-2000D, molecular weight 2000 g / mol, supplier: Chengxin Chemical), provides flexible segments, the proportion of which determines the phase transition temperature of the soft segments in the material.

[0036] Chain extender and curing agent: 4,4′-dichlorodiphenylmethane diamine (MOCA, industrial grade, supplier: Chengxin Chemicals), as a hard segment chain extender and crosslinking point.

[0037] Magnetic particles: Fe3O4 nanoparticles with an average particle size of 20 nm (purity >99%, supplier: McLean).

[0038] Surface modifier: 3-aminopropyltriethoxysilane (APTES, purity ≥98%, supplier: McLean).

[0039] II. Preparation of Temperature-Magnetic Field Dual-Response Shape Memory Particles The response temperature window of the particles is designed based on the target reservoir temperature T=70℃.

[0040] (1) Surface amination modification of Fe3O4 nanoparticles Fe3O4 nanoparticles were dispersed in anhydrous ethanol and sonicated. Under stirring and heating (60℃), an ethanol solution of APTES was slowly added dropwise and the reaction was carried out for 18 h. After the reaction was completed, the nanoparticles were magnetically separated, washed with ethanol and dried under vacuum to obtain functionalized Fe3O4 (Fe3O4-NH2) with amino groups (-NH2) on the surface. These amino groups react with the -NCO groups in the subsequent polyurethane prepolymer to form chemical bonds.

[0041] (2) Preparation of high Tg(b) magnetic cores: Under dry nitrogen protection, MDI and polyether polyol were reacted at 75°C for 3 hours at a molar ratio of NCO:OH = 3:1; Fe3O4-NH2 powder (added at 15 wt% of the core mass) was premixed with 1.5 g of prepolymer and subjected to high-speed mechanical stirring and ultrasonic treatment to form a uniform magnetic prepolymer dispersion; a metered amount of chain extender MOCA was added to the dispersion. The amount of MOCA added was in excess relative to the -NCO groups in the prepolymer, and the molar ratio of amine groups (-NH2) to isocyanate groups (-NCO) ([NH2] / [NCO]) was controlled at 1.20. The mixture was poured into a mold and initially cured at 80℃ for 2 h, followed by curing at 100℃ for 3 h to complete the full crosslinking reaction. The cured block material was subjected to liquid nitrogen brittle fracture, grinding, and grading sieving to obtain magnetic core particles with a particle size range of 30-80 μm. The DSC characterization results are as follows: Figure 1 As shown.

[0042] (3) Coating with a low Tg(a) polymer shell: MDI and polyether polyol were used to synthesize a shell prepolymer at an NCO:OH molar ratio of 2.1:1. The magnetic core particles (30 wt% of the shell prepolymer mass) were dispersed in the shell prepolymer, and uniform coating was achieved through gentle stirring. MOCA was added for curing. To impart a low Tg(a) to the shell, the amount of MOCA added was insufficient, and the [NH2] / [NCO] molar ratio was controlled at 0.7. The low amount of curing agent resulted in fewer crosslinking points, forming a loose network structure, thus significantly reducing the glass transition temperature. Curing was carried out at 60℃ for 8 h; the mild curing conditions avoided excessive crosslinking. After curing, a core-shell composite material was obtained. After crushing, grinding, and multi-stage sieving, the final shape memory particles with a particle size of 150-250 μm were obtained. DSC characterization results are shown below. Figure 1 As shown.

[0043] (4) Particle shape memory programming: The original (memory) shape is spherical, and the temporary (programmed) shape is sheet-like (elongated forms such as rods, fibers, or sheets). The particles are heated to the programmed temperature Tp = 65℃ (e.g., 65-70℃) to soften the outer shell (highly elastic state) while maintaining the rigidity of the core. The particles are programmed to deform using a displacement / strain control method, with a strain rate of 0.1 s⁻¹. -1 The compressive stress is controlled at 2MPa. After the particles reach the target programmed strain ε60%, they are held at the programmed temperature Tp for 15 minutes to complete the stable pre-storage of strain energy. Cooling is carried out while maintaining the target strain ε, at a cooling rate of 5℃ / min, until the temperature is 10℃ lower than the shell network Tg(a). Then the external force is released to fix the temporary shape, while the external force remains unchanged during the cooling process.

[0044] The mechanical work applied during programming is converted into strain energy, which is mainly stored in the deformation network of the shape memory material and released under triggering conditions to drive the particle to undergo shape memory recovery. The magnitude of the pre-stored strain energy is directly controlled by the programmed strain ε; the larger ε is, the more pre-stored energy is, and the stronger the subsequent recovery driving force. The final recovery expansion range of the particle is mainly determined by the programmed strain ε, and the two are positively correlated. To ensure that the expansion required for sealing is achieved, ε is precisely controlled within a predetermined range to ensure that the particle can achieve the designed size expansion after triggering.

[0045] III. Verification of Blocking Effectiveness (1) Experimental design 1) Experimental model Core: Artificial sandstone core (permeability 100 mD), dimensions 30 cm × 5 cm × 5 cm.

[0046] Crack: A single parallel crack was formed by inserting a shim along the core axis, with a crack width of 0.8 mm and a crack length of 25 cm.

[0047] 2) Experimental Grouping Experimental group (this invention): Injected temperature-magnetic field dual-response core-shell shape memory particles prepared according to this embodiment.

[0048] Control group (single temperature response): injected with traditional shape memory particles with only temperature response function (overall Tg=60℃, no magnetic components, particle size 150-250 μm).

[0049] The conventional shape memory particles were prepared using a one-step method, employing the same polyurethane prepolymer as the present invention, but without the addition of any magnetic nanoparticles. The method is as follows: 0.86 g of curing agent MOCA was added (controlling the -NH2 / -NCO molar ratio to 0.9) to form a uniform cross-linked network after curing, and the overall glass transition temperature (Tg) was adjusted to 60℃ (70℃ lower than the target reservoir temperature); the cured bulk material was crushed and sieved to obtain spherical particles with a particle size of 150-250 μm; finally, the spherical particles were compressed and deformed at 65℃ and cooled to fix them, resulting in control particles with a temporary shape (such as flakes). These particles softened as a whole at reservoir temperatures and underwent overall shape recovery, lacking core-shell structure, magnetic components, and staged response characteristics.

[0050] 3) Experimental conditions Simulated formation temperature: constant at 70℃.

[0051] Displacement / carrying solution: 3wt% NaCl solution.

[0052] Magnetic field parameters (used only during the activation phase of the experimental group): frequency 300 kHz, magnetic induction intensity 300 Gauss.

[0053] (2) Blocking operations and data recording In both experiments, a plugging fluid containing 5 wt% particles was injected at a flow rate of 1.0 mL / min until particles were observed at the outlet. The initial fracture conductivity was recorded. Injection was then stopped, and the model was allowed to stand at 70°C for 1 h.

[0054] Control group: Traditional temperature-responsive particles soften as a whole and recover their shape, forming a seal in the crack.

[0055] Experimental group: In this embodiment, only the outer shell of the particles softened, and the particles as a whole softened and moved in accordance with the cracks, but there was no significant recovery, and the particles were able to move further into the cracks.

[0056] An alternating magnetic field was applied to the middle part of the experimental model (15 cm from the inlet) for 30 min to trigger core recovery. After standing for 2 h, a displacement test was performed at a flow rate of 0.5 mL / min. After stabilization, the driving pressure difference (ΔP) before and after plugging was recorded, and the effective permeability (K) and plugging rate (η) of the fracture were calculated. The experimental results are shown in Table 1.

[0057] Table 1 Comparison before and after crack sealing (3) Results Analysis The final sealing pressure differential (1.2 MPa) of the method of this invention (experimental group) was three times that of the control group (0.4 MPa), and the sealing rate (99.5%) was also significantly higher than that of the control group (92%). Core analysis revealed that the particles in the control group, due to overall recovery at 70℃, mostly accumulated and sealed near the fracture entrance (within 5 cm), resulting in "shallow sealing but not deep sealing." In contrast, the particles in the experimental group only softened and migrated during the temperature stage, ultimately forming a sealing layer in the deep region (10-20 cm) targeted by the magnetic field, effectively solving the problem that a single temperature-responsive material cannot penetrate and seal the deep parts of the fracture.

[0058] Based on the mechanism of this invention and experimental verification, the term "deep" is defined as follows: Mechanism definition: "Deep" refers to the fracture region where shape memory particles mainly undergo outer shell softening and migration under the influence of in-situ reservoir temperatures, without triggering the core shape memory recovery that would allow them to reach and distribute. This region must be located outside the significant influence range of wellbore stress disturbances and temperature field changes.

[0059] Geographic definition of operations: In field applications, "deep" typically refers to a fracture network area at a radial distance of more than 5 times the wellbore diameter (5D) from the wellbore. For a typical wellbore diameter (e.g., 0.2m), this means fracture space at a distance of more than 1 meter.

[0060] Experimental Verification and Quantification Standards: As shown in Example 1, in simulated fractures, the single temperature-responsive particles of the control group only blocked the fracture at the inlet end (0-5 cm, i.e., the near-wellbore region), while the particles of this invention, under magnetic field-activated localization, formed a high-strength blockage at 10-20 cm. The ratio of the effective blocking position front (10 cm) to the near-wellbore blocking region front (5 cm) of the particles of this invention reached 2:1. Therefore, "deep" is quantified as: the distance from the fracture inlet to the main formation location of the blocking structure is at least twice the depth of the near-wellbore accumulation blocking region of the single temperature-responsive particles under the control conditions. This ratio (≥2) comprehensively reflects the advantages of this invention in promoting particle migration into the fracture interior and delaying activation.

[0061] On-site judgment criteria: In actual construction, the sealing effect is indirectly confirmed to occur deep in the fracture by changes in the construction pressure curve, tracer monitoring, or subsequent production dynamics (such as the increase in the detection radius shown by pressure recovery test, and the significant delay in the time of agent exposure in the corresponding water well).

[0062] The above definition of "deep" is used to clarify the technical effects and implementation mechanism of the present invention, and should not be construed as a limitation on the scope of protection of the claims.

[0063] Example 2: Effect of different magnetic particle contents on magnetocaloric response behavior The difference from Example 1 is that the magnetic particle content is 5wt%, 30wt%, and 60wt%, respectively.

[0064] Shape memory particles with different magnetic particle contents were placed in an alternating magnetic field apparatus for magnetocaloric response testing. Alternating magnetic field frequency: 300 kHz; magnetic induction intensity: 300 Gauss; initial ambient temperature: 70℃; magnetic field duration: 30 min. Sample temperature changes were recorded in real time using thermocouples or infrared thermometry.

[0065] The test results are shown in Table 2.

[0066] Table 2. Magnetothermic effect data with different magnetic particle contents As shown in Table 2, the magnetic particle content is positively correlated with the magnetocaloric effect. Increasing the content from 5 wt% to 60 wt% significantly improves the initial heating rate and drastically shortens the time required to reach the core activation temperature. At a content of 5 wt%, although the heating rate is slower, under continuous magnetic field, it can still stably exceed the core Tg(b) (reaching 92℃), indicating its ability to activate the core shape memory recovery and achieve the sealing function. At a content of 60 wt%, it exhibits an extremely fast heating rate and the highest final temperature, proving the effectiveness of magnetocaloric heating at high contents.

[0067] It can be seen that the magnetic particle content mainly affects the magnetothermal heating rate and efficiency. However, within the range of magnetic particle content defined in this invention, the core network is effectively activated without changing the core technical effect of "temperature-assisted transport - magnetic field point activation - shape memory recovery blocking". This verifies the rationality of setting the range of magnetic particle content.

[0068] Example 3: Influence of shape memory particles of different sizes on crack entry and sealing behavior The difference from Example 1 is that by controlling the crushing, grinding and grading sieving processes, three groups of particle samples with different particle sizes were obtained: fine particles (50-100 μm), medium particles (150-250 μm), and coarse particles (400-500 μm).

[0069] Experimental model: A crack model of an artificial sandstone plate (30 cm × 5 cm) was constructed to form a crack with continuously varying width: the crack width at the inlet end (0-10 cm) is 100 μm, the crack width in the middle section (10-20 cm) expands to 300 μm, and the crack width in the deep part (20-30 cm) is 800 μm.

[0070] At 70°C, three groups of particles of plugging fluid (5wt% concentration) were injected at a flow rate of 1.0 mL / min. An alternating magnetic field (300 kHz, 300 Gauss) was locally applied for 30 minutes in the deep, wide-slit section (20-25 cm) of the model. After the magnetic field was removed, a displacement test was performed (0.5 mL / min), and the stable driving pressure difference before and after plugging was recorded, and the plugging rate was calculated.

[0071] The experimental results are shown in Table 3.

[0072] Table 3 Comparison of crack adaptability and sealing performance of particles with different sizes After activation by the magnetic field, shape memory particles of all sizes undergo geometric expansion within the crack space, forming a wedge-like, lock-like mechanical embedding structure with the crack wall, significantly blocking the crack flow channels. Experiments show that particles of different sizes can achieve effective blocking after magnetic field activation. The blocking location and form vary with particle size and crack size, but the basic blocking mechanism under the dual temperature-magnetic field response remains unchanged.

[0073] Meanwhile, particles with a diameter in the range of 50-500 μm are suitable for sealing cracks of different scales, from micro-cracks to large cracks. The specific size can be selected or compounded according to the scale distribution of the target crack, and particles with appropriate diameters can be selected within this range to optimize the whole process of "entry-migration-sealing".

[0074] The above results demonstrate that particle size mainly affects its crack entry capability and the scale of the sealing structure. Within the range of 50-500 μm, particles can complete the entire process of "migration-activation-shape memory recovery-crack sealing" based on the aforementioned dual response mechanism, thus verifying the rationality of the particle size range setting.

[0075] Example 4: Effect of different alternating magnetic field frequencies on the magnetocaloric activation behavior of shape memory particles The difference from Example 1 is that the test was conducted using a fixed magnetic induction intensity (300 Gauss) and varying frequencies (50 kHz, 300 kHz, 800 kHz).

[0076] Using the same variable-width artificial crack model as in Example 3, the standard plugging procedure was repeated in the crack model. Then, the magnetic fields of the three frequencies mentioned above were used for point activation (30 minutes), followed by displacement tests to evaluate the plugging strength.

[0077] The experimental results are shown in Table 4.

[0078] Table 4. Magnetothermic effect and sealing performance data at different magnetic field frequencies Experimental results show that shape memory particles exhibit observed magnetocaloric heating under different alternating magnetic field frequencies, and the heating rate varies with frequency. Even at the lowest frequency of 50 kHz, the magnetic particles still produce a significant magnetocaloric effect, with their final temperature (98℃) significantly higher than the core activation temperature Tg(b) (79.5℃), effectively driving the particles to complete shape recovery and achieving efficient deep crack sealing (sealing rate of 98.5%). Under 800 kHz conditions, the particle heating rate is further increased, completing the magnetocaloric activation of the core network in a shorter time.

[0079] The above phenomena demonstrate that within the 50-800 kHz alternating magnetic field frequency range, shape memory particles can achieve magnetocaloric heating under appropriate magnetic field conditions (by adjusting the magnetic field duration and magnetic induction intensity), triggering the shape memory recovery process of the core network. The alternating magnetic field frequency primarily affects the magnetocaloric heating rate and the activation time, without altering the particle's magnetocaloric activation and shape memory recovery. Within this frequency range, effective activation of the core network can be achieved by adjusting the magnetic field parameters.

[0080] Example 5: Effect of different magnetic induction intensities on the magnetocaloric activation behavior of shape memory particles The difference from Example 1 is that the test was conducted using a fixed frequency (300 kHz) and varying magnetic induction intensity (10 Gauss, 100 Gauss, 300 Gauss, 500 Gauss).

[0081] The particle samples were placed in an adiabatic environment, and alternating magnetic fields of varying magnetic induction intensities were applied. The sample temperature was continuously monitored, and the shortest time required for it to reach and stabilize above the preset core activation temperature Tg(b) = 79.5℃ was recorded. Point activation was performed on a standard deep wide-crack segment of the crack model, followed by displacement tests to evaluate the sealing strength.

[0082] The experimental results are shown in Table 5.

[0083] Table 5. Magnetothermic effect and sealing performance data under different magnetic induction intensities Experimental results show that shape memory particles exhibit magnetocaloric heating under different magnetic induction intensities, but the heating rate and activation efficiency differ significantly. Even at the lowest intensity of 10 Gauss, the particle temperature eventually stabilizes at 82℃, clearly exceeding the glass transition temperature Tg(b) of the core (79.5℃). Although the time required to heat to the activation temperature under this condition is relatively long (180 s), by correspondingly extending the magnetic field application time (360 s in this experiment), the particles still successfully complete shape memory recovery and form effective deep sealing (sealing rate of 98.0%), which fully demonstrates the feasibility of 10 Gauss as the lower limit of intensity. Under the condition of 500 Gauss, the particle heating rate is further improved, and the magnetocaloric activation of the core network is completed in a shorter time. Therefore, within the magnetic induction intensity range of 10-500 Gauss, by reasonably matching the magnetic field application time, magnetic field frequency, and magnetic particle content, shape memory particles can all generate a magnetocaloric effect sufficient to trigger the shape memory recovery of the core network.

[0084] Example 6: Influence of the glass transition temperature difference between the outer shell network and the reservoir temperature on particle migration and pre-packing behavior. The difference from Example 1 is that the amount of chain extender MOCA used in the outer shell synthesis was adjusted (controlling the NH2 / NCO molar ratio) to prepare three groups of shape memory particles with different outer shell Tg(a).

[0085] Group A: NH2 / NCO molar ratio = 0.6, MOCA dosage: 18.3 g; Tg(a) = 40℃, temperature difference ΔT1 = Tres - Tg(a) = 30℃ Group B: NH2 / NCO molar ratio = 0.75, MOCA dosage: 22.8 g; Tg(a) = 60℃, temperature difference ΔT2 = 10℃ Group C: NH2 / NCO molar ratio = 0.9, MOCA dosage: 27.4 g; Tg(a) = 70℃, temperature difference ΔT3 = 0℃ A long-scale (50 cm) homogeneous narrow fracture model (fracture width 200 μm) was used. A plugging fluid containing 5 wt% particles was injected at a constant flow rate (1.0 mL / min) at 70 °C. Injection pressure-time curves were recorded, and the migration front position of the bulk particles was measured. After particle injection and deep migration, an alternating magnetic field (300 kHz, 300 Gauss) was applied to the deep, wide fracture section (800 μm) to activate the particles, and the final plugging strength and plugging rate were evaluated.

[0086] The experimental results are shown in Table 6.

[0087] Table 6. Data on particle transport and plugging performance under different Tg(a) temperature differences When the glass transition temperature (Tg(a)) of the outer shell network is 30°C lower than the reservoir temperature, the particle shell is extremely soft in the reservoir, exhibiting excellent deep migration capability. When Tg(a) equals the reservoir temperature, the shell is at the glass transition point at initial injection. Although it exhibits some initial resistance, it still undergoes significant softening and deformation under the continuous influence of formation temperature and fluid carrying, achieving effective deep migration (migration distance greater than 20 cm). Generally, the larger the temperature difference (Tres-Tg(a)), the higher the initial softness of the particles in the reservoir and the stronger their migration capability; the smaller the temperature difference, the relatively greater the initial migration resistance, but the actual engineering needs can still be met by optimizing the injection process (such as appropriately increasing the injection pressure or using pre-fluid). Under both of the above temperature difference conditions, the particles can achieve reliable sealing at depth through magnetic field triggering. The difference between the glass transition temperature Tg(a) of the outer shell network and the reservoir temperature mainly affects the flexibility of the particles during the migration and pre-filling stages and their adaptability to fractures. Within the range of 0-30℃, shape memory particles can maintain a highly elastic state under the influence of reservoir temperature, meeting the functional requirements of entering fractures and completing pre-filling.

[0088] Example 7: Effect of different injection flow rates on the position of the particle migration front The difference from Example 1 is that a transparent viewing plate crack model with a length (L) of 100 cm × width (W) of 5 cm × internal slit height (H) of 0.5 cm is used, the slit width (w) is fixed at 0.5 mm, and 5 different constant injection flow rates are set: 0.2 mL / min, 0.5 mL / min, 1.0 mL / min, 2.0 mL / min, and 5.0 mL / min.

[0089] The prepared sealing fluid was injected into the model inlet at a set constant flow rate. The time it took for the particle migration front to reach each scale position was visually observed and recorded through the observation window. Injection was stopped after 1 hour (or when the particle front reached the outlet). After standing for 10 minutes, the entire model was photographed to record the macroscopic distribution of particles within the fracture. The fracture model was divided into 10 equal-length sections (10 cm each), and the particles in each section were flushed out, dried, weighed, and the axial mass fraction of particles in the fracture was calculated.

[0090] The experimental results are shown in Table 7.

[0091] Table 7. Particle transport and distribution characteristics at different injection velocities. Experimental results show a positive correlation between injection flow rate and particle migration front depth. When the flow rate increases from 0.2 mL / min to 1.0 mL / min, the particle migration depth increases significantly while maintaining good uniformity of distribution. However, when the flow rate is too high (≥2.0 mL / min), although the migration speed increases, it triggers a significant viscous fingering effect, causing the carrier liquid to form dominant channels in the cracks, resulting in severely uneven particle distribution. Simultaneously, a large number of particles are flushed out of the cracks, causing a significant decrease in retention rate, thereby reducing material utilization efficiency and deep sealing performance. 0.5–1.0 mL / min represents a balance window for achieving effective deep particle migration and good uniformity of distribution. This ensures that particles achieve deep migration while maintaining high uniformity of distribution and retention rate, creating ideal pre-filling conditions for subsequent magnetic field-guided activation.

[0092] Example 8: Long-term stability verification experiment of the sealing structure The difference from Example 1 is that, after particle injection and magnetic field activation were completed in a standard crack model (crack width 0.8 mm), the following four experimental groups were set up for long-term stability testing: Group A (Control): The sealing structure was left to stand at 70°C without flushing.

[0093] Group B (gentle flushing): 3wt% NaCl solution was continuously injected at 0.1 mL / min at 70℃.

[0094] Group C (Enhanced rinsing): At 70℃, high and low flow rates were applied periodically: first rinsed at 0.5 mL / min for 2 h, then rinsed at 2.0 mL / min for 0.5 h, and then returned to 0.1 mL / min.

[0095] Group D (Temperature Cycling): Based on Group B, perform daily temperature cycling: 70℃ (20 h) → 90℃ (4 h) → 70℃.

[0096] All experiments lasted for 30 days (720 h). Inlet pressure, outlet flow rate, and presence of particle carryover were monitored daily. A standard displacement test (0.5 mL / min) was performed every 7 days to measure the plugging pressure differential and plugging rate.

[0097] The experimental results are shown in Table 8.

[0098] Table 8. Long-term stability test results of the sealing structure

[0099] Note: Closure pressure differential decay rate = (initial pressure differential - 30-day pressure differential) / initial pressure differential × 100% All groups of plugging structures maintained a plugging rate of over 97.4% during the 30-day experimental period. Even under the most stringent Group C (enhanced scouring) conditions, the plugging rate still reached 97.4%, indicating that the plugging structure formed by the present invention has excellent long-term stability.

[0100] In the experiment, the slight attenuation of the sealing effect mainly stemmed from the following two aspects: First, the initial adaptation period, during which the sealing structure and the fracture wall further integrated and tended towards a final stable state under stress and temperature within the first 7 days; second, slight surface erosion, where long-term erosion caused the breakage of a very small number of polymer chain segments on the particle surface, but did not destroy the overall "wedge-lock" mechanical embedding structure. The main reasons for long-term stability are: In terms of structural stability, the "wedge-lock" structure formed by shape memory recovery has three-dimensional mechanical interlocking characteristics, which has stronger structural integrity compared with traditional particle stacking or gel sealing; In terms of material stability, shape memory polyurethane materials exhibit excellent resistance to hydrolysis and aging within the reservoir temperature range (usually below 120℃), and the core is in a glassy state (Tg(b) higher than the reservoir temperature), thus ensuring the long-term "freezing" of the recovered morphology; In terms of interface stability, the normal compressive force generated between the particles and the fracture wall during the shape memory recovery process provides continuous and sufficient contact stress, significantly enhancing the interfacial bonding strength.

[0101] Comparative Example 1: Study on the blocking behavior of a single magnetic response shape memory particle The difference from Example 1 is that a single-structure shape memory particle with only magnetic response function is prepared.

[0102] Polyurethane prepolymers were synthesized according to an NCO:OH molar ratio of 2.8:1. 15 wt% Fe3O4-NH2 was uniformly dispersed in the prepolymer. MOCA was added, and the NH2 / NCO molar ratio was controlled at 1.15 to achieve an overall Tg of 80℃. After curing, crushing, and sieving, spherical particles with a particle size of 150-250 μm were obtained. The spherical particles were compressed into rod shapes at 90℃ (>Tg) and then cooled and fixed, with the programmed strain controlled at 60%.

[0103] The same single parallel crack model (crack width 0.8 mm, crack length 25 cm) and experimental procedure as in Example 1 were used.

[0104] The experimental results are shown in Table 9.

[0105] Table 9 Comparison of plugging performance between single magnetic response particles and particles of the present invention

[0106] The results of this comparative example, from the opposite perspective, confirm the necessity of the core-shell structure and the dual temperature-magnetic field response mechanism of this invention. First, because the overall glass transition temperature of the particles (80°C) is higher than the reservoir temperature (70°C), they remain rigid throughout the injection and migration stages, lacking temperature-assisted pre-deformation capabilities. This makes it difficult for the particles to adapt to fracture morphology, resulting in high injection pressure and difficulty in deep migration, with most particles prematurely accumulating in the near-wellbore zone. Second, under the action of a magnetic field, individual magnetically responsive particles mainly undergo overall thermal softening through heat generation from magnetic components. Their shape recovery is a passive, uniform thermal response behavior that relies on pre-stored strain energy. The heat dispersion leads to weak recovery driving force and low efficiency, ultimately resulting in a limited strength of the sealing structure (sealing pressure difference of only 0.25 MPa). In contrast, the particles of this invention selectively heat the core under a magnetic field, while the outer shell remains in a highly elastic state because its glass transition temperature Tg(a) is lower than the reservoir temperature. The shape memory recovery energy of the core acts as a "rigid core" to powerfully drive the entire particle to undergo anisotropic expansion, forming a high-strength mechanical interlocking structure (the sealing pressure difference reaches 1.20 MPa). Thirdly, the activation of a single magnetic response particle depends entirely on the external magnetic field, which cannot achieve precise time-temperature control of "first migration, then fixed-point activation"—applying the magnetic field too early will lead to near-wellbore blockage, and if activation is delayed after migration, it is difficult to achieve deep sealing due to its poor deep migration ability.

[0107] This invention achieves precise phased control of "temperature-assisted transport (stage 1)" and "magnetic field fixed-point activation and blocking (stage 2)" through the decoupling design of temperature and magnetic field.

[0108] Comparative Example 2: Effects of low magnetic particle content (2 wt%) on the magnetocaloric response and sealing performance of shape memory particles. The difference from Example 1 is that the content of magnetic particles (Fe3O4) is reduced to 2wt%, the outer shell Tg(a) = 60℃ and the inner core Tg(b) = 80℃ remain unchanged, the particle size is 150-250 μm, and the programmed strain is controlled at 60%.

[0109] The experimental results are shown in Table 10.

[0110] Table 10 Comparison of magnetocaloric effect and plugging performance under different magnetic particle contents

[0111] The content of magnetic particles is the material basis for the magnetocaloric effect. Results showed that when the content was as low as 2 wt%, under typical magnetic field conditions (300 kHz, 300 Gauss, 30 min), the heat generated by the particles was weak, only raising the temperature by 2-5 °C. The highest temperature (75 °C) was significantly lower than the activation threshold Tg(b) (80 °C) of the core network, preventing the triggering of the core's shape memory function and the inability of the particles to recover their deformation. Due to the lack of an active shape memory recovery mechanism, the particles could only function as ordinary rigid particles, achieving plugging through simple bridging and stacking. This passive plugging method was inefficient, weak in strength, and unstable, resulting in an extremely low plugging rate (25%) and negligible plugging pressure difference (0.08 MPa).

[0112] The contrast between this comparative example (2 wt%) and Example 2 (5 wt%) demonstrates that 5 wt% is near the critical content required to achieve effective magnetocaloric activation under the typical magnetic field conditions described in this invention. Below this content, shape memory recovery triggering cannot be ensured within a reasonable engineering time window.

[0113] Comparative Example 3: Study on the transport and blocking behavior of shape memory particles with excessively high shell Tg The difference from Example 1 is that the shell Tg(a) is 75°C, the NH2 / NCO molar ratio of the shell prepolymer to MOCA is 1.10, it is cured at 80°C for 6 h, the magnetic particle content is 15 wt%, the particle size is 150-250 μm, and it is programmed at 85°C with a programming strain of 60%.

[0114] The experimental results are shown in Table 11.

[0115] Table 11. Particle behavior and properties when the outer shell Tg(a) is higher than the reservoir temperature.

[0116] The results of this comparative example directly demonstrate that if the glass transition temperature Tg(a) of the outer shell network is higher than the reservoir temperature, the "temperature-magnetic field dual response staged triggering" mechanism will fail, making it difficult to achieve the expected technical effect.

[0117] First, the "temperature-responsive migration stage" function is lost. This invention utilizes reservoir temperature to soften the outer shell, thereby endowing particles with the ability to deform and migrate deep. When Tg(a) is higher than the reservoir temperature, the outer shell remains rigid in a glassy state within the reservoir, and the particles behave like ordinary rigid particles, losing the crucial ability to "adaptively deform and enter fractures," leading to a surge in injection pressure and near-wellbore blockage. Second, deep pre-filling cannot be achieved. Because particles have difficulty entering and migrating to the depths of fractures, the "migration first, activation later" strategy loses its premise, resulting in a large accumulation of particles in the near-wellbore area. Even if a magnetic field is applied subsequently, it can only produce limited sealing effects in non-target areas. Third, the "magnetic field-responsive sealing stage" effect is limited. Even if the core is activated by a magnetic field, the driving force generated by its shape recovery needs to be transmitted through the outer shell and act on the fracture wall. The rigid outer shell not only cannot coordinate deformation but also restricts the core recovery, significantly weakening the overall expansion force and wedging effect, resulting in low sealing strength.

[0118] Comparative Example 4: Comparison of sealing performance of conventional chemical gel sealing systems in cracks The difference from Example 1 is that a partially hydrolyzed polyacrylamide (HPAM)-aluminum citrate delayed crosslinking gel system was selected as the comparison object.

[0119] Formulation: Polymer HPAM (molecular weight ~15 million, degree of hydrolysis 25%), concentration 3000 mg / L; crosslinking agent aluminum citrate, concentration 300 mg / L; stabilizer thiourea, concentration 100 mg / L; solvent is 3wt% NaCl solution.

[0120] At 70°C, the gel working solution was injected at a flow rate of 1.0 mL / min until liquid appeared at the outlet. Injection was stopped, and the entire model was placed in a 70°C incubator for 48 h to allow for complete gelation of the delayed crosslinking system. After curing, a displacement test was performed at a flow rate of 0.5 mL / min, and the breakthrough pressure and final stable displacement pressure were recorded to calculate the plugging rate.

[0121] The experimental results are shown in Table 12.

[0122] Table 12 Comparison of the plugging performance of conventional gel systems and the dual-response particles of this invention

[0123] The comparative results demonstrate the fundamental differences and inherent limitations between conventional chemical gel plugging systems and the present invention. First, there is a significant difference in plugging strength and stability: gel plugging relies on the physical filling of a polymer network, and its strength is limited by polymer concentration and cross-linking degree. Furthermore, its viscoelastic nature makes it prone to creep and structural damage under sustained shear. Experiments show that its stable plugging pressure differential (0.50 MPa) is far lower than that of the present invention (1.20 MPa), and it fails rapidly under accelerated scouring conditions. In contrast, the "wedge-lock" structure formed by the present invention has a mechanically interlocked rigid / semi-rigid connection characteristic, exhibiting higher compressive and shear resistance, and a stable and durable plugging effect. Second, the plugging depth and location are "passively uncontrolled": the gel working fluid enters and fills all the pores it occupies like a fluid, forming a gel as a whole. It cannot distinguish between deep flow channels that need to be plugged and seepage channels that need to be preserved, resulting in a lack of selectivity in plugging, easily leading to over-plugging and damage to effective oil and gas flow paths. This invention utilizes the movement of solid particles and the precise intervention of an external magnetic field to achieve selective deep sealing that can "hit the target" and minimize the impact on non-target areas.

[0124] Furthermore, the two mechanisms of action differ fundamentally: gel plugging is a form of "passive adaptation and filling," its effectiveness largely dependent on formation conditions (such as pore throat size, ionic strength, and temperature), and the gelation process is complex and difficult to precisely control. In contrast, this invention employs "active deformation and locking," where particle behavior is actively controlled in stages and predictively through a preset glass transition temperature (Tg) and an external magnetic field. It is less affected by formation fluid chemistry, significantly improving reliability. Moreover, regarding engineering adaptability: gel systems are sensitive to formation water salinity and pH, facing stability challenges in high-temperature, high-salinity reservoirs, and are subject to shear degradation risks during injection. The particles used in this invention are physical entities with stable chemical properties; their temperature and salt resistance depends on the selected polymer material (such as polyurethane), thus offering broader applicability.

[0125] Comparative Example 5: Verification of the control over crack sealing location and sealing strength by ordinary thermal expansion microspheres The difference from Example 1 is that commercially available thermally expandable polymer microspheres for oil well plugging are used, with a core of low-boiling-point hydrocarbon foaming agent and a shell of thermoplastic polymer.

[0126] Key parameters: Particle size 50-150 μm (dry state); initial expansion temperature 70-75℃; maximum expansion ratio ≥3 times. The thermally expanded microspheres are dispersed at a mass concentration of 5 wt% in a 3 wt% NaCl solution to form the sealing solution.

[0127] The same single parallel fracture model as in Example 1 (fracture width 0.8 mm, fracture length 25 cm) and experimental temperature (70 °C) were adopted. The working fluid containing 5 wt% thermally expandable microspheres was injected at a flow rate of 1.0 mL / min. Injection was stopped, and the model was allowed to stand at 70 °C for 2 h. Displacement tests were carried out at a flow rate of 0.5 mL / min to evaluate the plugging effect.

[0128] The experimental results are shown in Table 13.

[0129] Table 13 Comparison of plugging performances between ordinary thermally expandable microspheres and the dual-responsive particles of the present invention

[0130] This comparative example reveals the fundamental differences in the action mechanism between ordinary thermally expandable microspheres as a temperature-responsive material and the present invention, as well as the resulting engineering limitations. First, in terms of the response logic, the expansion of thermally expandable microspheres is an inherent passive physical property. Once the ambient temperature exceeds its initial expansion temperature, the expansion process occurs automatically and irreversibly, and it is impossible to achieve the spatio-temporal control of "first migrating to the deep part and then triggering at the target point". Therefore, plugging necessarily occurs prematurely at the nearest end where the temperature reaches (such as the near-wellbore zone). In contrast, the shape memory recovery of the present invention is a programmable active behavior that requires specific conditions (temperature higher than Tg(b)) to trigger. At reservoir temperature (Tg(a) < temperature < Tg(b)), the particles remain in a "standby" state, and only when an external magnetic field intervenes precisely can the recovery be initiated at the specified position, thus achieving complete control over the triggering timing and position. Second, in terms of the deformation mode, thermally expandable microspheres exhibit isotropic volume expansion, mainly generating uniform extrusion pressure, which is limited and prone to relaxation due to material creep. The shape memory recovery of the particles of the present invention is the active recovery from a temporary shape (such as a rod shape) to a memory shape (a spherical shape), involving complex geometric reconstruction and anisotropic dimensional changes, and can generate a strong directional wedging force, making it easier to form a mechanical self-locking structure. Third, in terms of the structural performance, the expanded microspheres are essentially soft and compressible elastomers, and the formed plugging body is similar to a "foam plastic wall", with limited pressure-bearing capacity and poor long-term stability. After the particles of the present invention recover, their cores are in a glassy state, providing rigid support points for the plugging structure, and through interlocking with the fracture wall surface and other particles, a high-strength stable structure similar to a "crushed stone arch bridge" is constructed.

[0131] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention patent. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention patent shall be subject to the appended claims.

Claims

1. A method for selectively sealing cracks based on temperature-magnetic field dual-response shape memory particles, characterized in that, Includes the following steps: Temperature-magnetic field dual-response shape memory particles are used as the sealing medium. The shape memory particles are dispersed in the carrier fluid to form a sealing working fluid. The sealing working fluid is then injected into the target oil layer, allowing the shape memory particles to migrate along the fracture flow channel with the carrier fluid to the deep formation for pre-filling. After pre-filling is completed, an alternating magnetic field is applied only to the target well section to induce a magnetocaloric effect in the magnetic particles in the core network and raise the temperature of the core network above its glass transition temperature, triggering the shape memory particles to deform and recover, thereby sealing the fracture channel. The shape memory particles are core-shell structured particles, including a shape memory outer shell network and a shape memory inner core network. The glass transition temperature of the outer shell network is lower than the target reservoir temperature, and the glass transition temperature of the inner core network is higher than the target reservoir temperature. Magnetic particles are dispersed in the inner core network. The shape memory particles are in a temporary shape state after being programmed with shape memory before injection, and the temporary shape is sheet-like, rod-like, or fibrous.

2. The method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles according to claim 1, characterized in that, The magnetic particles in the core network are one or both of nano Fe3O4 and nano γ-Fe2O3, and the mass fraction of the magnetic particles in the shape memory particles is 5-60 wt%, and the average particle size of the shape memory particles is 50-500 μm.

3. The method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles according to claim 1, characterized in that, The carrier liquid is one or a combination of several of the following: water, formation water, prepared brine, polymer solution, etc., and the mass concentration of the shape memory particles in the carrier liquid is 0.1-10 wt%.

4. The method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles according to claim 1, characterized in that, The frequency of the alternating magnetic field is 50-800 kHz, and the magnetic induction intensity is 10-500 Gauss.

5. The method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles according to claim 1, characterized in that, The alternating magnetic field is generated in the target well section by a downhole alternating magnetic field generator. The downhole alternating magnetic field generator includes: a downhole induction coil (sole coil or saddle coil), a non-magnetic heat-resistant shell, a centerer, and a transmission cable or continuous tubing cable connected to the surface power supply / controller. The effective length of the coil is 0.5-10 m.

6. The method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles according to claim 1, characterized in that, The glass transition temperature of the temperature-magnetic field dual-response shape memory particle outer shell network is not higher than the target reservoir temperature, and the difference between the two temperatures is 0℃-30℃; the glass transition temperature of the inner core network is higher than that of the outer shell network, and the difference between the two temperatures is 5℃-30℃.

7. The method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles according to claim 1, characterized in that, The target reservoir is a fractured area in a low-permeability reservoir, ultra-low-permeability reservoir, tight reservoir, or shale reservoir. The target for sealing is the fracture channel in the wellbore / deep fracture or the cross-flow channel of the fracture between wells.

8. The method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles according to claim 1, characterized in that, The method for preparing the temperature-magnetic field dual-response shape memory particles includes the following steps: Based on the target reservoir temperature, shape memory matrix materials are selected. The glass transition temperature (Tg2) of the core matrix material is 5℃-30℃ 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, and the temperature difference between the two is 0℃-30℃. Magnetic nanoparticles were dispersed in an organic solvent and ultrasonically dispersed. Then, their surfaces were 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 were obtained after magnetic separation, washing, and drying. Surface-functionalized magnetic nanoparticles are added to the core shape memory matrix material, uniformly dispersed, and solidified. The solidified magnetic core material is then crushed and sieved to obtain magnetic core particles. Magnetic core particles are added to the outer shell matrix material, uniformly dispersed, and solidified to obtain core-shell structure shape memory particles; The core-shell structure shape memory particle is heated to the programmed temperature window between Tg1 and Tg2 and an external force is applied to deform it. The load is maintained and the particle is cooled to at least 10°C below the outer shell network Tg1 or below room temperature. Then the external force is removed to obtain a temperature-magnetic dual-response shape memory particle with a temporary shape.

9. A method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles according to claim 8, characterized in that, The magnetic nanoparticles in the magnetic core account for 5wt%-60wt% of the core mass fraction.

10. A method for selective crack sealing based on temperature-magnetic field dual-response shape memory particles according to claim 8, characterized in that, The diameter of the fully recovered temperature-magnetic dual-response shape memory particle is 2-5 times the maximum axial dimension of the temporary shape.

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