Degradable liquid rubber plug and method for plugging shaft by using same

Abstract

The invention discloses a degradable liquid rubber plug and a method for plugging a shaft by using the same, and relates to the technical field of petroleum engineering well plugging. The invention provides a degradable liquid rubber plug and a shaft plugging method. The rubber plug is prepared from 10 wt%-20 wt% of polylactic acid particles, 3 wt%-6 wt% of sodium alginate, 1 wt%-3 wt% of nano silicon dioxide, 0.2 wt%-0.5 wt% of ammonium persulfate and the balance water. In a shaft environment at 90-130 DEG C, the initial viscosity of the rubber plug system is 80 + / -5 mPa.s, the rubber plug system is thickened to 5000 mPa.s within 20 min to form gel, and the pressure bearing capacity is larger than or equal to 20 MPa; and when meeting an acidic solution with the pH value of 1-3, the coating can be completely degraded within 60-120 minutes without solid phase residues. The temporary plugging device is suitable for temporary plugging of the well depth within 3000m, and drilling and milling operation is not needed for unplugging.

Description

Technical Field

[0001] This invention belongs to the field of well sealing technology in petroleum engineering, specifically relating to a biodegradable liquid plug and a method for sealing wellbores using it. Background Technology

[0002] In oil and gas well drilling and completion operations, temporary wellbore plugging is a crucial step in ensuring operational safety. Current technologies primarily rely on cement-based plugging agents or mechanical packers, but these have significant drawbacks: cement slurry requires 8-12 hours to set, making subsequent plugging operations prone to damaging the producing formation; and in complex wellbores with a dogleg greater than 15° / 30m, the failure rate of mechanical packers can reach as high as 30%. During staged fracturing of shale gas horizontal wells, existing tools achieve a setting success rate of less than 70% in the three-dimensional wellbore trajectory, resulting in a significant risk of stuck pipe during the pull-out process. Statistics show that in a certain domestic shale gas block, non-productive time due to plugging failure accounts for 18% of the operational cycle, highlighting the urgent need to overcome the dual bottlenecks of timeliness and wellbore adaptability.

[0003] While chemical liquid plugs can circumvent the limitations of mechanical tools, existing systems suffer from uncontrollable degradation and reservoir damage. Chromium-crosslinked gels degrade in 2-8 days at 90°C well temperatures, leaving a residual solid content of up to 18wt%, clogging reservoir pores and throats and causing a 40% decrease in permeability. Meanwhile, polylactic acid (PLA)-based materials dissolve in over 48 hours at temperatures below 80°C, and their pressure resistance decreases to <10MPa at temperatures above 120°C. Even more serious is the problem of starch-based plugs reacting with highly salinized formation water (>10×10⁻⁶ m³ / s). 4 When the concentration of sulfur dioxide (mg / L) is reduced by 50%, the sealing depth is limited to within 1500m, which cannot meet the high temperature and high pressure requirements of deep wells.

[0004] With increasingly stringent environmental regulations and the deepening development of unconventional oil and gas, the industry has raised three core requirements for plugging technology: rapid response (achieving a pressure bearing capacity of ≥15MPa within <2 hours), precise and controllable degradation (complete dissolution within 2-3 hours), and zero solid residue (permeability recovery rate ≥98%). Existing systems struggle to achieve all three; for example, organic crosslinking agents pose a risk of heavy metal contamination, and biopolymers lack sufficient thermal stability. Especially in large-scale staged fracturing of shale gas horizontal wells, there is an urgent need to develop a green plugging solution that can adapt to well depths up to 3000m, well temperatures of 130℃, and is 100% flowback-capable, in order to reduce operating costs and protect the reservoir. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a green plugging solution that can adapt to well depths of up to 3000m, well temperatures of 130℃ and 100% returnable.

[0006] The technical solution adopted by this invention to solve its technical problem is as follows: A biodegradable liquid rubber stopper, characterized in that: the components include 10-20wt% polylactic acid particles, 3-6wt% sodium alginate, 1-3wt% nano-silica, 0.2-0.5wt% initiator, and the balance being water.

[0007] Preferably, the polylactic acid has a relative molecular weight of 15,000-25,000 and a particle size D50 of 80±10μm.

[0008] Preferably, the viscosity of the sodium alginate is 40-60 mPa·s (1% aqueous solution, 25°C).

[0009] Preferably, the initiator is selected from ammonium persulfate or potassium persulfate.

[0010] A biodegradable liquid plug and a method for sealing wellbores using it, characterized by the following steps: S1. With a depth of 0.5-1.0m 3 Pumping rubber plug fluid into the target section of the wellbore at a displacement of / min; S2. A gel-like sealing layer forms 20-40 minutes after well shut-in; S3. Pressurize the oil pipe to 15-25MPa to verify its sealing performance.

[0011] Preferably, the pH of the wellbore fluid is adjusted to 8-9 before injection in S1.

[0012] Preferably, pulse injection is used in S1, 0.2m 3 / min low speed range and 0.8m 3 The high-speed section is alternating at a time ratio of 3:1.

[0013] Preferably, the annular pressure is maintained at 3-5 MPa during the well shut-in period in S2.

[0014] Preferably, the unsealing operation involves injecting an acidic solution with a pH of 1-3 into the wellbore, with the injection volume being 1.5 times the volume of the sealed section.

[0015] Preferably, the acidic solution is a citric acid solution or an acetic acid solution.

[0016] The beneficial effects of this invention are as follows: (1) The present invention can form a plug quickly, and the viscosity can be increased from 80 mPa·s to ≥5000 mPa·s in 20 minutes, which is 10 times more efficient than cement in setting time; (2) The high-pressure seal of this invention can withstand a pressure of 20MPa at 90℃ (corresponding to a well depth of 1500m), and the seal retention rate is 100% when the pressure difference fluctuates by ±5MPa; (3) The present invention is controllable and completely dissolved in 60 min at 90℃ / pH=2, and the dissolution time at 130℃ is ≤120 min; (4) This invention is zero-pollution; the degradation liquid is filtered through a 0.22μm filter membrane with no solid residue, and the core permeability recovery value is ≥98%. (5) The raw materials of this invention are renewable, with PLA and sodium alginate accounting for more than 60% of the total mass and 95% of the bio-based content. Detailed Implementation

[0017] The present invention will be further described below with reference to the embodiments.

[0018] Unless otherwise specified, the raw materials or chemical reagents used in the embodiments and comparative examples of this invention were obtained through conventional commercial channels. The experimental equipment used in the embodiments and comparative examples of this invention included a 50L high-temperature and high-pressure reactor, an online pH monitor, a Φ139.7mm casing × 30m simulated wellbore, a ±0.2MPa precision pressurization pump set, a core flow meter, and a 0.22μm microporous filter membrane.

[0019] Example 1 of a biodegradable liquid rubber stopper raw material Example 1 of the biodegradable liquid rubber stopper raw material includes: 15g PLA particles (Mw=20000), 4.5g sodium alginate (viscosity 50mPa·s), 2g nano SiO2, 0.3g ammonium persulfate, and 78.2g water.

[0020] Example 1: A biodegradable liquid plug and a method for sealing wellbores using it According to the components of Example 1 of a biodegradable liquid rubber stopper raw material, 15g of polylactic acid particles (Mw=20000), 4.5g of sodium alginate (viscosity 50mPa·s), 2g of nano-silica, 0.3g of ammonium persulfate, and 78.2g of water were first mixed and stirred for 30min, and the initial viscosity was measured. Then, 300ml of the mixture was injected into a 50L high-temperature and high-pressure reactor and thickened at 90℃ and normal pressure for 20min to form a gel. Then, 500ml of citric acid solution with pH=2 was injected into the reactor, and the gel was completely dissolved after reacting at 90℃ for 60min. Finally, the solution was tested using a 0.22μm filter membrane. To verify the high-temperature adaptability, the above steps were repeated at 130℃ to test the dissolution time.

[0021] To evaluate the effectiveness of Example 1 of the present invention—a biodegradable liquid plug and a method for sealing wellbores using the same—in terms of dissolution rate, residue control, and high-temperature adaptability, the following standard test methods were used for comprehensive evaluation: Initial viscosity: The viscosity of the mixture was measured using a Brookfield DV2T rotor viscometer (RV4 rotor, 60 rpm, 25℃) in accordance with SY / T 5108-2016 "Test Procedures for Water-Based Drilling Fluids". Dissolution time determination: According to ASTM D5155-19, the dissolution endpoint is defined as when the viscosity of the system drops to ±5% of the initial viscosity and no gel fragments are visually observed. Quantitative analysis of residue: Referring to SY / T 6335-2013, the solution was vacuum filtered through a 0.22μm polytetrafluoroethylene (Millipore) filter membrane, dried at 105℃, and weighed to constant weight; High-temperature verification: During the retest in a 130℃ autoclave (PARR 4575), a real-time pH / conductivity probe (Mettler Toledo InPro4260) was used to monitor the ion release rate and verify the degradation reaction kinetics.

[0022] Testing revealed that, after treatment with a biodegradable liquid plug and its application in wellbore sealing, the initial viscosity of the application embodiment 1 of this invention stabilized at 82 mPa·s. This viscosity control stems from the synergistic effect of polylactic acid (PLA) particles (D50 = 80 μm) and sodium alginate: PLA provides a rigid framework to delay sedimentation, while sodium alginate thickens the plug through hydrogen bonding; simultaneously, nano-silica adsorbs free water molecules, forming steric hindrance. This characteristic ensures that the plug fluid thins under shear during wellbore pumping (pumping viscosity < 30 mPa·s) and immediately recovers its structural viscosity upon reaching the target layer, preventing sealing failure due to liquid leakage or premature thickening, and laying a rheological foundation for subsequent rapid gelation.

[0023] Testing showed that, after treatment with a biodegradable liquid plug and its application in sealing wellbores, the first application example of this invention completely dissolved within 60 minutes at 90°C / pH=2. This precise degradation time was achieved by a free radical reaction initiated by ammonium persulfate: the acidic environment triggered the reaction of persulfate ions (S₂O₈). 2- ) decomposes to produce reactive free radicals (SO4) - The attack on polylactic acid bonds causes molecular chain breakage; simultaneously, citric acid chelates calcium ions, accelerating the disintegration of the sodium alginate network. The temperature-acidity dual control mechanism exponentially increases the hydrolysis rate (dissolution at 70℃ takes 145 minutes), which is 24 times more efficient than traditional chromium rubber stoppers, ensuring that the sealing layer intelligently deseals within the target time window.

[0024] Testing revealed that, after treatment with a biodegradable liquid plug and its application in Example 1 of this invention for wellbore sealing, the residue on the 0.22μm filter membrane was 0mg. This zero-residue characteristic stems from a molecular-level degradation design: polylactic acid (Mw=20000) is acid-hydrolyzed to generate lactic acid monomers (molecular weight 90), sodium alginate is degraded into mannouronic acid (molecular weight 198), and nano-silica is dispersed in a colloidal state (particle size <50nm). All dissolved products are soluble in the aqueous phase and have a particle size <0.1μm, completely avoiding the technical defects of traditional plugging agents that clog reservoir pore throats (average pore size 0.5-2μm) due to residual crosslinking agents (such as chromium ions) or undegraded polymers (>1μm).

[0025] Testing showed that, after treatment with a biodegradable liquid plug and its application in sealing wellbores, Application Example 1 of this invention completed degradation in 108 minutes at 130°C. The high-temperature stability stems from the "thermal shield effect" of nano-silica: its surface silanol groups form coordination bonds with polylactic acid carbonyl groups, inhibiting the high-temperature pyrolysis of the polymer backbone; simultaneously, the activation energy for ammonium persulfate decomposition decreases, accelerating free radical yield. Increased temperature raises the degradation reaction rate constant k, matching the aging requirements for high-temperature deep wells (>3000m).

[0026] Example 2 of a biodegradable liquid rubber stopper raw material Example 2 of the biodegradable liquid stopper raw material includes: the same stopper liquid as in Example 1.

[0027] Example 2 of a biodegradable liquid plug and a method for sealing wellbores using it According to the components and weight proportions of a biodegradable liquid rubber stopper raw material in Example 2, it was first poured into a simulated wellbore with a diameter of Φ139.7mm × 30m at a depth of 0.8m. 3 / min displacement pumped into 3m 3 Example 1: The prepared rubber plug fluid (corresponding to a 30m plugging section volume) was then shut in for 40 minutes to thicken the rubber plug (maintaining annular pressure of 4MPa during this period). The viscosity was monitored in real time and reached 5125mPa·s to form a gel plugging layer. Subsequently, the pressure was stepped up at the tubing end using a ±0.2MPa precision pressurization pump set. When the pressure reached 20MPa, it was stabilized for 30 minutes, and the pressure drop was 0MPa. The pressure was continuously increased to 25MPa to verify the structural integrity (no rupture or leakage). Finally, a 15-20MPa pressure cycle test was performed 100 times (pressure increase / decrease rate 1MPa / min).

[0028] To evaluate the effectiveness of the biodegradable liquid plug and the method for sealing wellbores according to Embodiment 2 of the present invention in terms of high-pressure sealing performance, ultimate pressure bearing capacity, and dynamic cycle stability, the following standard test methods were used for comprehensive evaluation: 20MPa Stabilized Pressure Test: Section 5.3 of API 10B-2 "Evaluation Specification for Downhole Plugging Materials" Static Pressure Test, the pass criterion is pressure drop ≤0.3MPa / 30min.

[0029] 25MPa ultimate pressure test: ISO 10426-6 "Destructive testing methods for packers in the oil and gas industry", failure criteria: pressure drop > 5% or acoustic emission energy > 100dB.

[0030] 100-cycle pressure cycle sealing test: API 19B ​​"Completion Packer Dynamic Sealing Evaluation Procedure", passing the standard: cumulative leakage <5ml and no interface debonding.

[0031] Testing showed that, after treatment with a biodegradable liquid plug and its application in sealing wellbores, the pressure drop from 20 MPa to 0 MPa after 30 minutes of stabilization was achieved in Application Example 2 of this invention. Polylactic acid particles (D50=80μm) form a rigid supporting framework in the cross-linked network, achieving a compressive modulus of 280 MPa; ② Sodium alginate and nano-silica are reacted through Ca... 2+ The bridging effect creates nanoscale pores, resulting in a gel compressibility of <0.5%; ③ The annular pressure of 4MPa inhibits the propagation of microcracks, and the Poisson's ratio of the sealing layer is stabilized at 0.38±0.02 (compared to 0.45±0.05 for traditional cement plugs), achieving zero leakage.

[0032] Testing revealed that, after treatment with a biodegradable liquid plug and its application in sealing wellbores, the structure of the plug remained intact even after pressurization to 25 MPa in Application Example 2 of this invention. The increased compressive strength is attributed to a stress dissipation mechanism: nano-silica adsorbs PLA molecular chains to form "nanohexes," dissipating energy through elastic deformation in the low-pressure zone of 15-20 MPa; in the high-pressure zone of 20-25 MPa, it triggers plastic flow of PLA particles, increasing the energy absorption density per unit volume. The phase transformation process raises the critical stress intensity factor (KIC) for crack propagation to 2.8 MPa·m. 1 / 2 It is 2.3 times that of traditional chrome rubber stoppers.

[0033] Testing revealed that, after treatment with a biodegradable liquid plug and its application in Example 2 of this invention for wellbore sealing, the seal retention rate remained at 100% after 100 cycles of 15-20 MPa pressure cycling. The fatigue resistance stems from dynamic bonding: ① Sodium alginate β-mannuronic acid segments rebuild the hydrogen bond network during pressure relief; ② Residual ammonium persulfate free radicals (concentration 1.3 μmol / g) continuously induce PLA chain breakage and rearrangement, resulting in a cyclic loss rate of storage modulus G' < 2%.

[0034] Example 3 of a biodegradable liquid rubber stopper raw material The biodegradable liquid stopper raw material of Example 3 includes: artificial sandstone core (Φ25×100mm, gas permeability 100.3mD) and degradation liquid of Example 1.

[0035] Example 3: A biodegradable liquid plug and a method for sealing wellbores using it According to the components and weight parts of Example 3 of a biodegradable liquid rubber stopper raw material, firstly, an artificial sandstone core (Φ25×100mm, porosity 19.2%) with a gas permeability of 100.3mD was saturated with simulated formation water (mineralization 8.7×10⁻⁶). 4 mg / L, Ca 2+An initial permeability baseline was established using a core flow apparatus at 90℃ with a concentration of 4200 mg / L. Subsequently, 50 PV of rubber stopper degradation solution (prepared by completely dissolving 300 ml of rubber stopper from Example 1 at 90℃ / pH=2 and pre-filtering with 0.22 μm) was injected at a constant flow rate of 0.2 ml / min. After 12 hours of continuous injection, the experiment was stopped, and the core was dried in an oven at 85℃ for 48 hours. The final core permeability was measured to be 98.7 mD, and the permeability recovery rate was calculated as (98.7 / 100.3) × 100% = 98.4%. Simultaneously, the effluent was collected and filtered through a 0.22 μm microporous membrane, and the solid residue was measured to be 0.003 g / L (below the equipment detection limit of 0.01 g / L).

[0036] To evaluate the effectiveness of Example 3 of the present invention—a biodegradable liquid plug and a method for sealing wellbores using the same—in terms of reservoir permeability compatibility and control of solid residue in the degradation fluid, the following standard test methods were used for comprehensive evaluation: Core permeability recovery rate test: GB / T 29172-2012 "Core Analysis Methods" Section 7.4 Formation damage assessment, qualification criterion: η ≥ 90% (industry first-class standard); Quantitative analysis of solid residue: GB / T 13217.7-2004 "Determination of Filtration Residue of Liquid Ink Filter Membrane", detection limit: 0.01 g / L.

[0037] Testing showed that, after treatment with a biodegradable liquid plug and its application in Example 3 of this invention, the core permeability recovery rate reached 98.4%. Polylactic acid hydrolyzes in an acidic environment into water-soluble lactic acid monomers (molecular weight 90 Da, particle size <0.1 nm), and sodium alginate depolymerizes into oligosaccharides (molecular weight <500 Da), with sizes much smaller than the average pore throat size of the core (0.52 μm). The dissolved products do not undergo physical adsorption when passing through the rock pores, and the nano-silica, due to the protonation of its surface hydroxyl groups (-OH), carries a positive charge (ζ potential +32 mV at pH=2), generating electrostatic repulsion with the negatively charged sandstone surface (ζ potential -15 mV), completely avoiding damage caused by clay expansion or particle migration.

[0038] Testing revealed that, after treatment with a biodegradable liquid rubber stopper and its application in sealing wellbores, the residue on the 0.22μm filter membrane in Application Example 3 of this invention was only 0.003g / L. This near-zero residue characteristic stems from a triple purification mechanism: ① Ammonium persulfate free radicals (·OH) completely sever the PLA backbone, leaving no macromolecular fragments >1000Da remaining; ② The citric acid chelation system integrates Ca... 2+ / Mg 2+Ions (complexation constant 9.1) prevent the regeneration of calcium alginate microgels; ③ Nano-silica exists as monomeric silicic acid (H4SiO4) in an acidic environment, with a concentration (23 mg / L) lower than the solubility threshold (120 mg / L), ensuring that the degradation solution is a true solution rather than a sol. The residue is only the background noise of the instrument and is considered as zero solid phase in actual use scenarios.

[0039] Comparative Example 1: A Traditional Liquid Rubber Stopper Raw Material The conventional liquid rubber stopper raw material, Comparative Example 1, includes: 0.6g of hydroxypropyl guanidine gum, 0.4g of chromium acetate, and 99g of water.

[0040] Comparative Example 1: A conventional liquid rubber plug and a method for sealing wellbores using it According to the components and weight parts of a traditional liquid plugging material, 0.6g of hydroxypropyl guar gum and 0.4g of chromium acetate were first dissolved in 99g of water and mechanically stirred for 30min to obtain a control plugging agent with an initial viscosity of 105mPa·s. 300ml of the solution was injected into a 50L high-temperature and high-pressure autoclave and thickened at 90℃ for 35min until the viscosity increased to 4800mPa·s to form a gel. Subsequently, a pressure of 20MPa was applied to the gel through a simulated wellbore pressurization system, and after stabilizing the pressure for 30min, the pressure drop was recorded as 2.1MPa. Then, 500ml of citric acid solution with pH=2 was injected and treated at 90℃ for 120min. The gel dissolution rate was measured to be 43% (mass loss method). Finally, the solution was filtered using a 0.22μm filter membrane, dried, and weighed to determine the solid residue amount as 1.6g / L. The permeability recovery rate of the core sample treated under the same conditions was measured to be 76.2%.

[0041] To evaluate the effectiveness of a conventional liquid rubber plug and its method for sealing wellbores in Comparative Example 1 in terms of acid responsiveness, solid phase contamination risk, and reservoir damage, the following standard test methods were used for comprehensive assessment: Gel solubility test: Section 4.2, mass loss method, of SY / T 5796-2018 "Methods for Determination of Solubility of Oilfield Chemical Agents"; Quantitative analysis of solid residue and permeability recovery rate test: Same as in Example 3.

[0042] Testing revealed that, after treatment with a traditional liquid rubber plug and its application in wellbore sealing, Comparative Example 1 showed a solubility of only 43% after 120 minutes of acid treatment at 90°C / pH=2. The fundamental reason lies in the irreversible cross-linking of chromium: chromium acetate (Cr... 3+The hydroxypropyl guar gum forms an octahedral coordination structure with the carboxyl groups. In an acidic environment, only some ionic bonds break, and the covalent cross-linked network maintains the integrity of the framework. The residual cross-linked gel has a high swelling ratio, and the actual effective degradation material is less than 40%. The residue contains a Cr(OH)3 precipitate phase (2θ=19.1°) and carbonized guar gum (C / O ratio > 5:1), causing the chemical dissolution to kinetically stall.

[0043] Testing revealed that after treatment with a traditional liquid rubber stopper and its application in sealing the wellbore in Comparative Example 1, the residue on the 0.22μm filter membrane reached 1.6g / L. The main pollutants were three insoluble substances: ①Cr 3+ Hydrolysis at pH > 2 produces Cr(OH)3 colloid; ② Unhydrolyzed polyguanidine gum coking products; ③ CrO4 generated by chromium acetate oxidation. 2- With Ca 2+ It combines to form CaCrO4 precipitate. The high density of the residue causes irreversible bridging at the pore throat.

[0044] Testing revealed that, after treatment with a traditional liquid rubber plug and its application in sealing the wellbore, the core permeability recovery rate of Comparative Example 1 was only 76.2%. Cr(OH)3 flocs blocked the sandstone pore throats, reducing the effective flow cross-section; residual chromium ions caused montmorillonite lattice expansion; and coking guar gum adsorbed acidic components of crude oil, forming sludge.

[0045] In summary, after Comparative Example 1 was treated with a traditional liquid rubber plug and the method of sealing the wellbore using it, only 43% effective dissolution rate was achieved in 120 minutes in an acidic environment of 90℃ / pH=2. The solid residue measured on a 0.22μm filter membrane was as high as 1.6g / L, and the permeability recovery rate was only 76.2%. In contrast, after being treated with a biodegradable liquid rubber plug, Example 1 of the present invention achieved 100% complete dissolution in 60 minutes under the same conditions (the ammonium persulfate free radical chain reaction completely cut off the PLA ester bond), with 0mg of residue, a core permeability recovery rate of 98.4% (the molecular weight of the dissolved product is <500Da, which is much smaller than the median radius of the core pore throat of 0.52μm), and the gelation time was shortened to 20 minutes (75% faster than the comparative example), and the pressure drop at 20MPa was zero (the cross-linked network compression rate is <0.5%). This invention achieves a technological leap over traditional chromium rubber stoppers in four dimensions: controllable degradation (dissolution rate +3.3 times), environmental friendliness (residue pollution reduced by 533 times), reservoir protection (permeability recovery gap of 22.2%), and operational efficiency (gelation efficiency increased by 75%).

Claims

1. A biodegradable liquid stopper, characterized in that: The components include 10-20 wt% polylactic acid particles, 3-6 wt% sodium alginate, 1-3 wt% nano silica, 0.2-0.5 wt% initiator, and the balance being water.

2. The biodegradable liquid stopper according to claim 1, characterized in that: The polylactic acid has a relative molecular weight of 15,000-25,000 and a particle size D50 of 80±10μm.

3. The biodegradable liquid stopper according to claim 1, characterized in that: The viscosity of the sodium alginate is 40-60 mPa·s (1% aqueous solution, 25℃).

4. The biodegradable liquid stopper according to claim 1, characterized in that: The initiator is selected from ammonium persulfate or potassium persulfate.

5. A biodegradable liquid plug and a method for sealing wellbores using it, characterized in that: Includes the following steps: S1. With a depth of 0.5-1.0m 3 Pumping rubber plug fluid into the target section of the wellbore at a displacement of / min; S2. A gel-like sealing layer forms 20-40 minutes after well shut-in; S3. Pressurize the oil pipe to 15-25MPa to verify its sealing performance.

6. The biodegradable liquid plug and the method for sealing wellbores using it according to claim 5, characterized in that: Before injection in S1, the pH of the wellbore fluid is adjusted to 8-9.

7. The biodegradable liquid plug and the method for sealing wellbores using it according to claim 5, characterized in that: In S1, pulse injection is used, 0.2m 3 / min low speed range and 0.8m 3 The high-speed section is alternating at a time ratio of 3:

1.

8. The biodegradable liquid plug and the method for sealing wellbores using it according to claim 5, characterized in that: During the well-closing period in S2, the annular pressure is maintained at 3-5 MPa.

9. The biodegradable liquid plug and the method for sealing wellbores using it according to claim 5, characterized in that: For the unsealing operation, inject an acidic solution with pH=1-3 into the wellbore, with the injection volume being 1.5 times the volume of the sealed section.

10. The biodegradable liquid plug and the method for sealing wellbores using it according to claim 9, characterized in that: The acidic solution is a citric acid solution or an acetic acid solution.