Degradable temporary plugging fiber for fracturing

Abstract

The present application relates to the technical field of temporary plugging fiber, and particularly relates to a degradable temporary plugging fiber for fracturing. The fiber has a three-layer composite structure: the inner core layer is composed of PLGA-ONB-polypeptide block copolymer with a photosensitive protecting group; the middle layer is a PVA matrix embedding pH-sensitive microcapsules resistant to high-temperature protease; and the outer layer is a PCL-lipoic acid dynamic covalent cross-linking network. The degradation process is programmable controlled by external instructions. First, the outer network provides high-strength temporary plugging and self-adaptive softening at reservoir temperature; second, the injection of alkaline activating liquid releases protease from the microcapsules, realizing preliminary weakening; finally, specific wavelength light is applied to remove the shielding of the inner core enzyme cutting site, triggering the protease to catalyze the disintegration of the inner core, realizing rapid and complete degradation. The present application solves the problem that the degradation time of traditional materials is uncontrollable and high strength and rapid plug removal cannot be considered, and has good application prospect.

Description

Technical Field

[0001] This invention relates to the field of temporary plugging fiber technology, and more specifically to a biodegradable temporary plugging fiber for fracturing. Background Technology

[0002] In the development of unconventional oil and gas resources (such as shale gas and tight oil and gas), hydraulic fracturing is a core technology for improving reservoir production. To increase the productivity of a single well, temporary plugging and diversion technology is often used. This involves injecting a temporary plugging agent into the wellbore after the initial fracturing to temporarily seal the existing fractures, forcing subsequent fracturing fluids to divert and open up a new fracture network, thereby more fully transforming the reservoir.

[0003] Temporary plugging fibers are an important type of temporary plugging agent, widely used due to their excellent bridging and filling capabilities. Ideally, temporary plugging fibers should maintain sufficient mechanical strength during construction to effectively seal fractures and degrade rapidly and completely after a predetermined time to avoid secondary damage to the reservoir.

[0004] Currently, the following types of temporary plugging fibers are mainly found in domestic and international research and applications: Conventional biodegradable fibers are mainly made from polylactic acid (PLA), polyglycolic acid (PGA), or their copolymers (PLGA). Their degradation depends on the hydrolysis of the polymer, and the degradation rate is passively controlled by environmental factors such as downhole temperature and pH. The degradation time window is wide and unpredictable, making it impossible to accurately match different reservoir conditions and operational timing requirements.

[0005] Composite / modified biodegradable fibers: Degradation rates are adjusted through blending, surface coating, or the addition of plasticizers. For example, fillers such as calcium carbonate are mixed into PLA to accelerate hydrolysis. However, such modifications are essentially physical blending or simple modification, and the degradation behavior is still dominated by the environment. They often come at the cost of initial strength, making it difficult to achieve a dynamic balance between high-strength temporary sealing and rapid, complete degradation.

[0006] Exploration of responsive materials: Some studies have attempted to apply pH-sensitive or temperature-sensitive materials to oilfield chemistry. For example, cross-linked polymers are used to swell or disintegrate at specific pH levels. However, directly applying such materials to fracturing plugging faces significant challenges: First, their response thresholds (such as pH abrupt change points) are difficult to match with the actual conditions of fracturing engineering; second, single response mechanisms (such as pH alone) have poor reliability in complex and variable downhole environments, and are prone to unexpected degradation or plugging failure; third, there is a lack of programmed control over the degradation process, making it impossible to achieve a smooth transition from plugging to unplugging.

[0007] In summary, existing temporary plugging fiber technologies suffer from core drawbacks such as uncontrollable degradation, passive performance dependence on the environment, and inability to be actively programmed. Their technological development is largely limited to material substitution or physical blending, failing to construct, at the molecular design level, a smart temporary plugging material capable of actively responding to multiple engineering signals and executing strength maintenance, weakening, and degradation according to a pre-set program. Summary of the Invention

[0008] The present invention provides a biodegradable temporary plugging fiber for fracturing to solve the problems mentioned in the background art.

[0009] To achieve the above objectives, in one aspect, the present invention provides a biodegradable temporary plugging fiber for fracturing, the fiber having a three-layer composite structure, consisting of, from the inside out, an inner core layer (final disintegration layer), a middle layer (signal response and function release layer), and an outer layer (strength and adaptive layer): The inner core layer is composed of a block copolymer formed by chemically linking polylactic-glycolic acid copolymer (PLGA) and a custom polypeptide. The custom polypeptide chain contains an amino acid sequence (e.g., a Leu-Ala repeat unit) that can be specifically recognized and cleaved by a specific protease (e.g., subtilisin). To enable instruction triggering, the side chains of key amino acids (e.g., glutamic acid) in this sequence are modified with photosensitive protecting groups (e.g., o-nitrobenzyl ester) to shield the cleavage sites before light exposure.

[0010] The intermediate layer consists of a polyvinyl alcohol (PVA) matrix and pH-sensitive microcapsules uniformly embedded therein. The core material of the microcapsules is a heat-resistant protease (such as a Bacillus subtilis protease variant with a temperature resistance of >80℃), and the wall material is a material that can dissolve or rupture at a specific pH value (such as an alkaline environment), such as an alkali-sensitive modified calcium alginate / polylysine composite membrane or Eudragit S100 enteric material.

[0011] The outer layer is composed of a polymer network formed by dynamic covalent chemical crosslinking of polycaprolactone (PCL) and lipoic acid (LA). The dynamic disulfide bonds formed by the ring-opening polymerization of lipoic acid endow the polymer network with reversible heat exchange properties, allowing its mechanical properties to be adaptively adjusted with temperature.

[0012] Based on the percentage of total solid mass of the fiber, the preferred proportions for each layer are: outer layer 50%-70%, middle layer 20%-35%, and inner core layer 10%-25%.

[0013] A second aspect of the present invention provides a method for preparing the above-mentioned temporary plugging fiber, wherein the fiber is prepared by a process comprising the following steps, such that the above components form the three-layer composite structure: S1. Preparation of inner core spinning solution: Dissolve PLGA-ONB-peptide block copolymer in hexafluoroisopropanol (HFIP) to form a homogeneous solution.

[0014] S2. Preparation of intermediate layer spinning solution: PVA is dissolved in hot water, and after cooling, pH-sensitive microcapsules loaded with protease are uniformly dispersed in the PVA solution.

[0015] S3. Preparation of outer spinning solution: PCL and thioctic acid are dissolved in chloroform in proportion, a catalyst is added, and partial prepolymerization is carried out to form a prepolymer solution with a dynamic disulfide bond network.

[0016] S4. Coaxial electrospinning: Using a three-channel coaxial spinning needle, the solutions prepared by S1, S2 and S3 are used as the inner, middle and outer layer fluids, respectively. Spinning is carried out under specific voltage, flow rate and receiving distance, and the fiber membrane is collected.

[0017] S5. Post-crosslinking and curing: The collected fiber membrane is heat-treated to fully crosslink and cure the dynamic disulfide bonds of the outer PCL-lipoic acid network, while the middle PVA layer is partially crystallized and cured, and the microcapsules are stably embedded.

[0018] The working principle of this invention is reflected in its step-by-step triggering and programmed degradation mechanism, where each layer of the structure functions in a preset order: Phase 1: High-intensity temporary plugging and adaptive adjustment (0-24 hours). After the fiber is injected into the fracture, the outer PCL-lipoic acid dynamic network provides initial high mechanical strength (fiber monofilament strength > 20MPa) to ensure effective bridging and temporary plugging. Downhole temperature (60-120℃) triggers the dynamic exchange of lipoic acid disulfide bonds, allowing the material to undergo topological restructuring under external force. Macroscopically, this manifests as gradual softening of the fiber and a decrease in modulus, but without overall degradation. During this phase, the intermediate layer microcapsules and the inner core polypeptide chains are in an inactive state.

[0019] The second stage: chemical signal activation and initial weakening (actively triggered by operation). When the unblocking procedure needs to be initiated, a small amount of weakly alkaline activation solution (such as a buffer solution with pH=10.0) is injected into the wellbore. After the liquid arrives, the pH-sensitive microcapsule wall material in the intermediate PVA matrix dissolves in the alkaline environment, releasing the core material protease. At the same time, trace amounts of reducing agents (such as cysteine) in the activation solution permeate to the outer layer, selectively partially reducing dynamic disulfide bonds, causing the overall strength and modulus of the fiber to decrease significantly within a controllable range (such as a decrease of more than 60%), achieving the initial release of temporary plugging pressure. In this stage, because the enzyme cleavage sites of the inner core polypeptide chain are shielded by photosensitive protective groups, the released protease cannot play a role, and the degradation process is effectively blocked.

[0020] The third stage: physical command triggering and final rapid degradation (actively triggered by the operation). After the fiber has weakened, a specific wavelength of ultraviolet light (e.g., 365nm) is emitted into the temporarily blocked area through a downhole light source. The ultraviolet light is absorbed and photolyzed by the photosensitive protective groups (ONB) on the inner core polypeptide chain, thereby exposing complete protease-specific recognition and cleavage sites. At this time, the protease, which is already in a standby state inside the fiber, rapidly acts on these sites, catalyzing the breakage of the polypeptide chain. The breakage of the polypeptide chain, which is the chemical bond connecting the PLGA chain segments, leads to the collapse of the entire inner core polymer network. The rapid decomposition of the inner core immediately causes the fiber to lose its structural integrity. The remaining PCL and PVA fragments are rapidly hydrolyzed into small molecules (such as adipic acid, glycolic acid, acetic acid, etc.) under the high temperature and high pressure environment downhole, and are carried out by the flowback fluid, achieving complete unblocking without any solid residue.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: In this biodegradable plugging fiber for fracturing, the weakening and eventual degradation of the fiber are triggered stepwise by two independent external commands (injection of chemical activation fluid and application of specific light). The degradation time window can be precisely controlled by the operator, changing the unpredictable degradation situation of traditional materials. The outer dynamic network enables it to undergo controllable topological rearrangement at high downhole temperatures, achieving a gradual decrease in strength (adaptive softening), avoiding the construction risks caused by sudden material failure, while its performance is less affected by environmental fluctuations.

[0022] Secondly, the initial high strength ensures reliable temporary plugging; the photo-triggered enzymatic hydrolysis mechanism ensures the targeted disintegration of the inner core network, thereby achieving rapid and thorough degradation, resolving the contradiction between high strength and rapid degradation that is difficult to achieve with traditional materials. The final degradation products are water-soluble small molecules with no solid residue, causing minimal damage to reservoir permeability.

[0023] This invention integrates dynamic chemistry, bio-enzyme engineering, and photochemistry technologies. The raw materials and preparation processes involved (such as electrospinning) are mature and have the potential for industrial production and field application, providing a new direction for the development of intelligent fracturing chemical materials. Attached Figure Description

[0024] Figure 1 This is an overall flowchart of Embodiment 1 of the present invention. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0026] To enable those skilled in the art to implement this invention, the preparation method of the key customized raw materials is first disclosed.

[0027] Firstly: Synthesis of PLGA-ONB-peptide block copolymer Step 1: Using solid-phase synthesis, a Leu-Ala polypeptide chain with 30 repeating units was synthesized, wherein the glutamate side chain at a specific position was protected in the form of o-nitrobenzyl ester (ONB) to obtain a protective polypeptide (H2N-Pep(ONB)-COOH).

[0028] Step 2: Dissolve PLGA (molecular weight 50,000, LA:GA=75:25) with a carboxyl group at one end in anhydrous dichloromethane, add excess N,N'-diisopropylcarbodiimide and N-hydroxysuccinimide, and activate at 0-5℃ for 2 hours to obtain PLGA-NHS active ester solution.

[0029] Step 3: Dissolve the protective polypeptide obtained in Step 1 in dimethylformamide, add it dropwise to the activation solution in Step 2, and add triethanolamine as a catalyst. React at room temperature for 24 hours.

[0030] Step 4: After the reaction is complete, the solution is added dropwise to excess ice-cold ether to precipitate, filtered, washed several times with ether, and dried under vacuum to obtain a white solid product, PLGA-ONB-peptide block copolymer.

[0031] Secondly: Preparation of pH-sensitive microcapsules of thermoresistant Bacillus subtilis protease variant Step 1, Enzyme source: Use a commercially available thermostable Bacillus subtilis protease variant (e.g., Savinase® series from Novozymes, or obtained through site-directed mutagenesis), with an enzyme activity ≥10000 U / g.

[0032] Step 2, Microencapsulation: Using the sharp-pore coagulation bath method, 100 mg of the above-mentioned protease powder was dispersed in 10 mL of 2% (w / v) sodium alginate (screened for high G segment and partially esterified to impart alkali sensitivity) aqueous solution.

[0033] Step 3: Using a syringe pump, the above mixture is dripped into a coagulation bath containing 1.5% (w / v) calcium chloride and 0.5% (w / v) polylysine aqueous solution through a 200 μm diameter needle.

[0034] Step 4: After the addition is complete, continue stirring and solidifying for 30 minutes. Filter and collect the microspheres, wash three times with deionized water, and freeze-dry to obtain pH-sensitive microcapsules loaded with protease, with an average particle size of approximately 5 μm.

[0035] Example 1: This embodiment of the invention provides a biodegradable temporary plugging fiber for fracturing, the preparation method of which is as follows: S1. Preparation of inner core spinning solution: Weigh 10 grams of PLGA-ONB-peptide block copolymer, dissolve it in 90 grams of hexafluoroisopropanol (HFIP), and stir until completely clear.

[0036] S2. Preparation of intermediate layer spinning solution: Weigh 20g of polyvinyl alcohol (PVA1788, degree of alcoholysis 87-89%) and dissolve it in 80g of hot water at 80℃. Cool to room temperature and add 5g of calcium alginate / polylysine microcapsules loaded with thermostable Bacillus subtilis protease variant (average particle size 5μm, enzyme activity ≥10000U / g). Stir at low speed to disperse evenly.

[0037] S3. Preparation of outer spinning solution: Weigh 60g of polycaprolactone (PCL, molecular weight 80,000) and 15g of thioctic acid (LA) and dissolve them in 125g of chloroform. Add 0.3g of dibutyltin dilaurate as a catalyst and react at 50℃ for 2 hours to obtain a partially cross-linked prepolymer solution.

[0038] S4. Coaxial electrospinning: The three solutions mentioned above are loaded into three injection pumps respectively, using a three-channel coaxial needle (inner / middle / outer layer diameters are 0.4 / 0.7 / 1.2 mm respectively). Process parameters are set as follows: inner, middle, and outer layer flow rates are 0.5, 1.0, and 1.5 mL / h respectively; spinning voltage is 18 kV; receiving distance is 15 cm; ambient temperature is 25℃±3℃, humidity is <40%, and the fiber membrane is collected on a roller receiver.

[0039] S5. Post-treatment: The collected fiber membrane is placed in a vacuum oven at 60°C for 12 hours for heat treatment. This process allows the outer PCL-LA dynamic network to fully cross-link and solidify, while the middle PVA layer partially crystallizes, and the microcapsules are firmly embedded.

[0040] Calculations show that in the fibers obtained in this embodiment, the outer solid layer accounts for approximately 62%, the middle layer approximately 28%, and the inner core layer approximately 10%. The average fiber diameter is approximately 25 μm.

[0041] Example 2: Adjusting the mechanical properties of the outer layer This embodiment aims to demonstrate that by reducing the density of the outer dynamic covalent crosslinked network, fibers with lower initial modulus and faster softening at downhole temperatures can be obtained. The preparation method is as follows: S1. Preparation of inner core spinning solution: exactly the same as in Example 1.

[0042] S2. Preparation of intermediate layer spinning solution: exactly the same as in Example 1.

[0043] S3. Preparation of outer spinning solution (key difference): Weigh 60g of polycaprolactone (PCL, molecular weight 80,000) and 5g of thioctic acid (LA) and dissolve them in 125g of chloroform. Add 0.15g of dibutyltin dilaurate as a catalyst and react at 45°C for 1.5 hours. This formula and reaction conditions are designed to form a dynamic network with fewer crosslinking points.

[0044] S4. Coaxial electrospinning: The process parameters are the same as in Example 1.

[0045] S5. Post-treatment: The collected fiber membrane is placed in a vacuum oven at 55°C for 10 hours to heat-treat it so that the outer layer reaches the fully cross-linked state under this formula. The average diameter of the resulting fiber is about 24μm.

[0046] Example 3: Adjusting the pH value of the intermediate layer response This embodiment aims to demonstrate that the pH threshold for chemical activation can be adjusted from near-neutral to alkaline by changing the solubility properties of the microcapsule wall material. The preparation method is as follows: S1. Preparation of inner core spinning solution: exactly the same as in Example 1.

[0047] S2. Preparation of intermediate spinning solution (key difference): Weigh 20g of polyvinyl alcohol (PVA1788) and dissolve it in 80g of hot water at 80℃. Cool to room temperature and add 5g of Eudragit S100 enteric microcapsules loaded with the same thermostable Bacillus subtilis protease variant (average particle size 5μm; this acrylic resin material is insoluble at pH < 7.0 and soluble at pH > 7.0).

[0048] S3. Preparation of outer spinning solution: exactly the same as in Example 1.

[0049] S4. Coaxial electrospinning: The process parameters are the same as in Example 1.

[0050] S5. Post-processing: exactly the same as in Example 1.

[0051] Example 4: Adjusting the core optical response wavelength This embodiment aims to demonstrate that by altering the molecular structure of the photosensitive protective group, the wavelength of light that ultimately triggers degradation can be shifted from the ultraviolet region to the visible light region, thereby improving the safety of downhole operations. The preparation method is as follows: S1. Preparation of inner core spinning solution (key difference): Weigh 10 grams of PLGA-ONB-peptide block copolymer and dissolve it in 90 grams of hexafluoroisopropanol (HFIP), and stir until completely clear; the glutamic acid side chain of the polypeptide chain in this copolymer is protected by 4,5-dimethoxy-2-nitrobenzyl (DMNB), and its photolysis wavelength is about 400-420nm (visible blue-violet light).

[0052] S2. Preparation of intermediate layer spinning solution: exactly the same as in Example 1.

[0053] S3. Preparation of outer spinning solution: exactly the same as in Example 1.

[0054] S4. Coaxial electrospinning: The process parameters are the same as in Example 1.

[0055] S5. Post-processing: exactly the same as in Example 1.

[0056] Comparative Example 1: Traditional temperature / hydrolysis degradation fibers This comparative example represents a traditional temporarily plugging fiber that is currently widely used and whose degradation depends entirely on ambient temperature and the material's own hydrolysis rate. Its preparation method is as follows: S1. Preparation of spinning solution: Weigh 15 grams of polylactic acid (PLA, average molecular weight 100,000) and dissolve it in 85 grams of a mixed solvent composed of dichloromethane (DCM) and N,N-dimethylformamide (DMF) in a volume ratio of 7:3. Stir until completely dissolved.

[0057] S2. Electrospinning: Single-axis spinning is adopted, and the process parameters are set as follows: spinning solution flow rate is 1.0 mL / h; spinning voltage is 15 kV; receiving distance is 12 cm; ambient temperature is 25℃.

[0058] S3. Post-treatment: The collected fiber membrane was placed in a ventilated area at room temperature to evaporate the solvent for 12 hours. The resulting PLA fibers were of a single homogeneous structure with an average diameter of approximately 20 μm.

[0059] Comparative Example 2: Single pH-sensitive degradable fiber This comparative example represents a comparative technique that relies solely on a single chemical signal (pH) for triggering and where the enzyme preparation is not protected. The preparation method is as follows: S1. Preparation of spinning solution: Weigh 8 g of PLGA (75:25) and 2 g of poly(1,4-cyclohexanediethanol-co-orthoester) (PCDE) and dissolve them in 90 g of tetrahydrofuran (THF). After stirring and dissolving, add 0.5 g of Bacillus subtilis protease powder (unmicroencapsulated, enzyme activity ≥10000U / g) and sonicate for 30 minutes.

[0060] S2. Electrospinning: Single-axis spinning is adopted, and the process parameters are set as follows: spinning solution flow rate is 1.2 mL / h; spinning voltage is 20 kV; receiving distance is 10 cm; ambient temperature is 25℃.

[0061] S3. Post-treatment: The collected fiber membrane was placed in a vacuum oven at 40°C and dried for 6 hours to remove residual solvent. The resulting fiber was a monolayer structure in which the enzyme was directly dispersed in a pH-sensitive polymer matrix.

[0062] Comparative Example 3: Programmed fibers without a dynamic outer layer This comparative example is used to verify the importance of the outer dynamic cross-linking network in the adaptive softening stage, and its preparation method is as follows: S1. Preparation of inner core spinning solution: exactly the same as in Example 1.

[0063] S2. Preparation of intermediate layer spinning solution: exactly the same as in Example 1.

[0064] S3. Preparation of outer spinning solution (key difference): Weigh 60 grams of pure PCL (molecular weight 80,000) and dissolve it in 140 grams of chloroform, stirring until completely dissolved. No lipoic acid (LA) or catalyst is added.

[0065] S4. Coaxial electrospinning: The process parameters are the same as in Example 1.

[0066] S5. Post-treatment: The collected fiber membrane is placed in a vacuum oven at 50°C for 8 hours for heat treatment. This is only used to remove solvent and stabilize the PVA layer. The outer layer of PCL does not undergo cross-linking. The resulting fiber has a three-layer structure, but the outer layer is a thermoplastic linear polymer.

[0067] Comparative Example 4: Programmed fibers without photosensitivity protection This comparative example is used to verify the key role of the core photosensitive protective group in triggering the final degradation step. Its preparation method is as follows: S1. Preparation of inner core spinning solution (key difference): Weigh 10 grams of PLGA-ONB-peptide block copolymer (PLGA molecular weight 50,000, polypeptide chain consists of 30 Leu-Ala repeating units, key glutamic acid side chain is unprotected, i.e., in free carboxylic acid form) and dissolve it in 90 grams of hexafluoroisopropanol (HFIP), stirring until completely clear.

[0068] S2. Preparation of intermediate layer spinning solution: exactly the same as in Example 1.

[0069] S3. Preparation of outer spinning solution: exactly the same as in Example 1.

[0070] S4. Coaxial electrospinning: The process parameters are the same as in Example 1.

[0071] S5. Post-processing: exactly the same as in Example 1.

[0072] Experimental Example 1: Temporary Plugging Strength Maintenance and Adaptive Softening Performance Test Methods: The fibers prepared in Examples 1 and 2 and Comparative Examples 1 and 3 were compressed into temporary plugging agent balls of the same specifications (3 mm in diameter). These balls were placed in a high-temperature, high-pressure reactor to simulate reservoir conditions (temperature 90°C, pressure 30 MPa, formation water salinity). Samples were taken out at 0 h, 2 h, 6 h, 12 h, and 24 h, respectively, and their single-ball compressive strength at room temperature was tested. The test results are shown in Table 1.

[0073] Table 1: Temporary plugging strength retention and adaptive softening performance (Indicators: single-sphere compressive strength, MPa) As shown in Table 1, the fiber balls in Example 1 have high initial strength (30 MPa), and their strength gradually decreases to about 40% of the initial value within 12-24 hours, demonstrating the "adaptive softening" of the dynamic network. The fiber balls in Example 2 have moderate initial strength (e.g., 20 MPa) and a faster softening rate.

[0074] Comparative Example 1 (pure PLA) fiber balls had high initial strength, but their strength decreased very slowly or showed no significant change during the testing period, lacking a controllable weakening process. Comparative Example 3 (without a dynamic outer layer) fiber balls may have high initial strength, but their strength decreased rapidly and disorderly at high temperatures due to PCL softening, potentially leading to premature failure.

[0075] Experimental Example 2: Validation of Stepwise Triggered Degradation Procedure Method: Fiber samples from Examples 1, 3, 4 and Comparative Examples 2, 4 were placed in simulated formation water at 90°C.

[0076] Phase 1 (0-4h): No trigger signal is applied.

[0077] Second stage (4h): Inject alkaline buffer solution (pH=10.0, containing trace amounts of cysteine).

[0078] The third stage (6 hours): The sample was irradiated with ultraviolet light at a specific wavelength (365 nm for Examples 1 and 3, and 405 nm for Example 4) for 10 minutes. The mass retention rate of the fiber sample and the concentration of small molecule products in the solution were monitored throughout the process. The test results are shown in Table 2.

[0079] Table 2: Validation of the step-triggered degradation procedure (Indicator: mass retention rate, %) As shown in Table 2, the fiber in Example 1 showed no mass loss in the first stage; in the second stage (after chemical triggering), the mass decreased slightly (approximately 10-20%), corresponding to initial weakening of the outer layer; in the third stage (after photo-triggered), the mass rapidly decreased by more than 80% within 1 hour. The fiber in Example 3 only exhibited a similar response in the second stage after injection of an activation solution with pH > 7.0, demonstrating that the pH triggering threshold is adjustable. The fiber in Example 4 only underwent rapid degradation after irradiation with 405 nm light, demonstrating that the photo-triggered wavelength is adjustable.

[0080] Comparative Example 2 (single pH sensitive) fiber began to degrade rapidly in the second stage (pH triggering), failing to realize light as an independent and safe final command. Comparative Example 4 (no photosensitivity protection) fiber began to degrade rapidly without light after the second stage (chemical triggering enzyme release), losing the key step of programmed control.

[0081] Experimental Example 3: Adaptability Test to Complex Reservoir Environment Methods: The fibers of Example 1 and Comparative Examples 1 and 2 were placed in different simulated environments: (A) high temperature (120°C), (B) low temperature (60°C), and (C) high-mineralization (200,000 ppm) formation water. After 48 hours without applying a trigger signal, the fiber morphology was observed and the strength retention rate was tested. Then, a trigger signal was applied to the fiber of Example 1 according to a prescribed procedure, and its degradation behavior was observed to be affected. The test results are shown in Table 3.

[0082] Table 3: Adaptability to complex reservoir environments (Indicators: Intensity retention rate before 48 hours without triggering, % / Whether degradation can proceed according to the program after triggering) As shown in Table 3, under the three adverse conditions A, B, and C, the degradation behavior of Comparative Example 1 (temperature-dependent) fluctuates drastically (too fast at high temperatures and too slow at low temperatures). Comparative Example 2 (pH-dependent), under high mineralization, suffers from pH-responsive microcapsule failure or enzyme inactivation due to the influence of salt ions.

[0083] In Example 1, the fibers maintained their basic structural integrity in the above-mentioned environment before being triggered, with strength changes mainly controlled by their own dynamic network and minimally affected by environmental interference. Even after a standard trigger signal was applied, they continued to degrade effectively according to the prescribed program.

[0084] Experimental Example 4: Core Flow Experiment to Evaluate Unblocking Effect and Reservoir Damage Methods: Artificial fracture core modules were used. First, a fiber-based temporary plugging agent was injected to form an effective seal, and the plugging pressure was recorded. Then, the following procedures were performed: (1) For the core injected with fibers from Example 1, a standard step-by-step triggering procedure was executed (first injecting alkaline solution, then irradiating with ultraviolet light). (2) For the core injected with fibers from Comparative Example 1, the process was allowed to proceed naturally based solely on formation conditions. (3) For the core injected with fibers from Comparative Example 2, the process was triggered solely by injecting alkaline solution. The temporary plugging pressure drop curves, final unblocking time, and the recovery rate of core permeability after backflow were monitored and compared among the three groups of experiments. The test results are shown in Table 4.

[0085] Table 4: Unblocking effect of core flow experiment (Indicators: unblocking time and permeability recovery rate) As shown in Table 4, Example 1 can achieve rapid and complete unblocking under command control from high pressure temporary blockage, with controllable unblocking time (e.g., within 2 hours), and the core permeability recovery rate is expected to be >95%.

[0086] Comparative Example 1 shows that the unblocking time is uncontrollable and long (may take several days to several weeks), and incomplete degradation may lead to a low permeability recovery rate (e.g., <80%).

[0087] Comparative Example 3 may unblock faster, but the process is violent and difficult to control, and there may be enzyme or polymer residues, posing a potential risk to the recovery of permeability.

[0088] Therefore, it can be seen that the procedural unblocking method of the present invention can achieve efficient and thorough unblocking to the greatest extent and protect reservoir permeability to the greatest extent.

[0089] Example 5: Preliminary assessment of safety and biocompatibility Methods: The final solution of the fiber from Example 1 after the complete triggered degradation process was collected and subjected to: (1) cytotoxicity test (MTT method, using mouse fibroblast L929 cells); (2) chemical oxygen demand (COD) determination; (3) analysis of major degradation product components (liquid chromatography-mass spectrometry, LC-MS). The test results are shown in Table 5.

[0090] Table 5: Safety and Biocompatibility Assessment As shown in Table 5, the final degradation products of the materials of this invention have low cytotoxicity, are easy to handle, have clearly defined components, and are environmentally friendly, fully meeting the requirements of green and sustainable development of current oilfield chemicals.

[0091] In summary, as shown in Tables 1 and 2, the product of this invention (Example 1) achieves precise and segmented control from high-intensity temporary blockage to adaptive softening and then to command-triggered rapid disintegration. Its intensity evolution is gradual and controllable (retaining 40% of the initial value after 24 hours), and the degradation process strictly depends on external commands (chemical and photo-triggered), which is in stark contrast to the passive, singular, or uncontrollable degradation of Comparative Examples 1, 2, and 4.

[0092] As shown in Table 3, under extreme simulated reservoir conditions such as high temperature, low temperature, and high mineralization, the core performance (strength retention and programmed degradation capability) of the product of this invention is less affected by the environment and exhibits high stability. In contrast, the performance of Comparative Examples 1 and 2 is heavily dependent on environmental conditions, showing risks such as excessively rapid degradation, failure, or incomplete degradation. This demonstrates the advanced nature and reliability of the design concept of this invention, which is driven by instructions rather than by the environment.

[0093] Core flow experiments (Table 4) demonstrate that the product of this invention can achieve rapid (<2 hours) and thorough unblocking, and brings extremely high permeability recovery rate (>95%), far exceeding Comparative Example 1 (recovery rate <80%) which relies on natural degradation and Comparative Example 2 which has residual risk, directly solving the core problem of reservoir damage in fracturing and temporary plugging operations.

[0094] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A biodegradable temporary plugging fiber for fracturing, characterized in that, The fiber has a three-layer composite structure, consisting of an inner core layer, a middle layer, and an outer layer from the inside out. The inner core layer is composed of a block copolymer formed by chemically linking polylactic acid-glycolic acid copolymer and a customized polypeptide. The customized polypeptide chain contains an amino acid sequence that can be specifically recognized and cleaved by proteases, and the side chains of key amino acids in the amino acid sequence are modified by photosensitive protecting groups. The intermediate layer is composed of a polyvinyl alcohol matrix and pH-sensitive microcapsules uniformly embedded therein, and the core material of the microcapsules is a heat-resistant protease. The outer layer is composed of a polymer network formed by dynamic covalent chemical crosslinking of polycaprolactone and thioctic acid.

2. The biodegradable plugging fiber for fracturing according to claim 1, characterized in that, In the inner core layer, the photosensitive protecting group is o-nitrobenzyl ester or 4,5-dimethoxy-2-nitrobenzyl; the amino acid sequence in the customized polypeptide chain is a Leu-Ala repeating unit.

3. The biodegradable temporary plugging fiber for fracturing according to claim 1, characterized in that, In the intermediate layer, the wall material of the pH-sensitive microcapsule is an alkali-sensitive modified calcium alginate / polylysine composite membrane or Eudragit S100 enteric material.

4. The biodegradable plugging fiber for fracturing according to claim 1, characterized in that, In the intermediate layer, the thermoresistant protease is a variant of Bacillus subtilis protease that is resistant to temperatures greater than 80°C.

5. The biodegradable temporary plugging fiber for fracturing according to claim 1, characterized in that, In the outer layer, the dynamic disulfide bonds formed by the ring-opening polymerization of thioctic acid endow the polymer network with thermally reversible exchange properties.

6. The biodegradable plugging fiber for fracturing according to claim 1, characterized in that, Based on the percentage of total solid mass of the fiber, the outer layer accounts for 50%-70%, the middle layer accounts for 20%-35%, and the inner core layer accounts for 10%-25%.

7. The biodegradable plugging fiber for fracturing according to any one of claims 1-6, characterized in that, The fiber is prepared by coaxial electrospinning, so that the inner core layer, the middle layer and the outer layer form a coaxial composite structure.

8. The biodegradable plugging fiber for fracturing according to claim 7, characterized in that, The coaxial electrospinning process employs a three-channel coaxial spinning needle to simultaneously spin the inner core spinning solution, the intermediate layer spinning solution, and the outer layer spinning solution. The inner core spinning solution contains a block copolymer of polylactic acid-glycolic acid copolymer and a customized polypeptide. The intermediate layer spinning solution contains polyvinyl alcohol and pH-sensitive microcapsules dispersed therein. The outer layer spinning solution contains a prepolymer solution of polycaprolactone and thioctic acid.

9. The biodegradable temporary plugging fiber for fracturing according to claim 1, characterized in that, The strength of the fiber monofilament is greater than 200 MPa.

10. The biodegradable plugging fiber for fracturing according to claim 1, characterized in that, The degradation process of the fiber includes: In the first stage, the outer network provides initial strength and undergoes adaptive softening at reservoir temperature; In the second stage, the release of protease from the intermediate layer microcapsules is triggered by the injection of alkaline activation solution, which further weakens the outer layer network. In the third stage, the photosensitive protective groups of the inner core layer are removed by applying light of a specific wavelength, exposing the enzyme cleavage sites, and the released protease catalyzes the disintegration of the inner core layer.

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