S-dpg shielded temperature-resistant and salt-tolerant degradable preformed particle gel temporary plugging agent and preparation method and application thereof

Abstract

This invention discloses an S-DPG shield-type temperature- and salt-resistant biodegradable pre-formed granular gel temporary plugging agent, its preparation method, and its application, belonging to the technical field of temporary plugging and diversion for acid fracturing and control of fractured formation leakage in oil and gas fields. The temporary plugging agent is composed of acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, N-hydroxyethylacrylamide, polyethylene glycol diacrylate, 2-amino-2-methyl-1-propanol, persulfate initiator, and water, and is prepared through aqueous solution free radical polymerization, drying, pulverization, and sieving. Under high temperature and high salt conditions, the temporary plugging agent can absorb water and expand to form a fracture plug. Under the action of acid, hot water, or oxidants, it can gradually degrade and restore core permeability, thus meeting the requirements of stable plugging during the plugging stage and low-damage recovery during the degradation and flowback stage. This material is suitable for temporary plugging and diversion and fracture leakage control in deep, high-temperature, and high-salt fractured oil and gas reservoirs, and has good prospects for field application.

Description

Technical Field

[0001] This invention relates to the fields of oil and gas field development, acid fracturing temporary plugging and diversion, and fractured formation leakage control, specifically to an S-DPG shield-type temperature- and salt-resistant biodegradable pre-formed granular gel temporary plugging agent suitable for deep, high-temperature, and high-salt formations, its preparation method, and its application. Background Technology

[0002] As oil and gas exploration progresses to deeper and ultra-deeper reservoirs, these reservoirs generally exhibit characteristics such as high temperature, high pressure, high salinity, and well-developed fractures. During acidizing, acid fracturing, and hydraulic fracturing operations, the working fluid tends to preferentially enter high-permeability channels, resulting in insufficient stimulation of low-permeability areas. To improve reservoir stimulation, temporary plugging agents are often used to seal dominant channels and achieve pressure diversion. Existing sealing materials include rigid particles, fibers, soluble resins, viscoelastic surfactants, in-situ gels, and pre-formed particle gels. Among them, rigid particles have poor adaptability, fibers degrade incompletely, and in-situ gels are affected by temperature and mineralization. However, pre-formed particle gels can cross-link on the ground and selectively seal through water absorption and expansion, showing promising application prospects. However, conventional polyacrylamide pre-formed granular gels still face risks of strength decay, dehydration shrinkage, incomplete degradation, and reservoir damage under high temperature and high salinity conditions. In acidizing operations, the plugging agent needs to maintain strength during the plugging stage and also needs to undergo controllable degradation during the degradation stage to restore conductivity. Therefore, developing a pre-formulated particulate gel temporary plugging agent that combines temperature and salt resistance, particle size distribution for plugging, controllable degradation, and low-damage recovery capabilities is of great engineering significance. Summary of the Invention

[0003] The purpose of this invention is to solve the problems of insufficient sealing strength, poor salt resistance, incomplete degradation, and low permeability recovery rate after degradation and backflow of existing pre-formed granular gel temporary plugging agents in deep, high-temperature, and high-salt formations. This invention proposes an S-DPG shield-type temperature- and salt-resistant degradable pre-formed granular gel temporary plugging agent, its preparation method, and its application. This temporary plugging agent can form a high-strength temporary plug during the plugging stage and can gradually degrade into low-viscosity degradation products under the action of acid, hot water, or persulfate, thus achieving both fracture plugging, pressure diversion, and low-damage relief.

[0004] The technical solution of the present invention is as follows: An S-DPG shield-type temperature- and salt-resistant biodegradable pre-formed granular gel temporary sealing agent, comprising, by total mass of gel solution, 3.0%–6.0% acrylamide, 0.5%–2.0% 2-acrylamide-2-methylpropanesulfonic acid or its salt, 0.5%–3.0% N-hydroxyethylacrylamide, 0.4%–1.2% polyethylene glycol diacrylate 200, 0.05%–0.40% 2-amino-2-methyl-1-propanol, 0.02%–0.10% persulfate, with the balance being water; preferably, acrylamide is 4.0%, 2-acrylamide-2-methylpropanesulfonic acid or its salt is 1.0%, N-hydroxyethylacrylamide is 1.5%, polyethylene glycol diacrylate 200 is 0.8%, 2-amino-2-methyl-1-propanol is 0.2%, persulfate is 0.05%, with the balance being water.

[0005] In this invention, acrylamide is used as the main chain monomer, 2-acrylamido-2-methylpropanesulfonic acid or its salt is used as the temperature and salt resistant monomer, N-hydroxyethylacrylamide is used as the hydrogen bond reinforcing monomer, polyethylene glycol diacrylate 200 is used as the ester-containing degradable crosslinking agent, 2-amino-2-methyl-1-propanol is used as the pH adjuster and degradation promoter, and persulfate is used as the initiator. The invention constructs a controllable degradable gel network through the AM / AMPS temperature and salt resistant main chain, N-hydroxyethylacrylamide hydrogen bond reinforcement, and PEGDA200 breakable crosslinking points.

[0006] A method for preparing the above-mentioned S-DPG temporary plugging agent includes the following steps: adding acrylamide, 2-acrylamido-2-methylpropanesulfonic acid or its salt and N-hydroxyethylacrylamide to water and stirring to dissolve them to form a first mixture; adding polyethylene glycol diacrylate 200 and 2-amino-2-methyl-1-propanol to the first mixture and stirring to form a second mixture; adjusting the pH of the second mixture to 7.5-8.5, adding persulfate under nitrogen purging and deoxygenation conditions, and polymerizing at 60-70°C for 2.0-3.0 h to obtain a bulk gel; cutting, washing, drying, pulverizing and sieving the bulk gel to obtain a pre-prepared particulate gel temporary plugging agent.

[0007] The pre-made granular gel temporary sealing agent can be sieved into at least two types of particles from 20-40 mesh, 40-80 mesh, and 80-160 mesh according to the target crack width and used in a graded manner, so that the large-diameter particles form a bridging skeleton, and the medium and small-diameter particles fill the skeleton pores and form a tight seal after absorbing water and expanding.

[0008] This invention also provides the application of the above-mentioned S-DPG temporary plugging agent in acidizing temporary plugging and redirection, acid fracturing temporary plugging and redirection, hydraulic fracturing temporary plugging and redirection, repeated hydraulic fracturing and redirection, profile control and water control, and fracture leakage control in deep or ultra-deep high-temperature and high-salinity fractured oil and gas reservoirs. In application, the pre-prepared granular gel can be formulated into a 1.0 wt% to 5.0 wt% granular dispersion and injected into the target fracture or high-permeability dominant channel. After construction, its degradation and backflow can be triggered by 5 wt% to 15 wt% hydrochloric acid, 0.3 wt% to 1.0 wt% persulfate solution, or hot water.

[0009] The beneficial effects of this invention include: (1) The sulfonic acid groups of AMPS improve the hydration stability of the polymer chain in a high-salt environment, and N-hydroxyethylacrylamide provides hydrogen bond enhancement sites, so that the gel can maintain a relatively complete network structure in a high-temperature and high-salt environment. (2) Dry gel particles can absorb water and swell, forming particle bridging, fine particle filling and compaction deformation structures in cracks, thereby increasing the flow resistance of dominant channels; (3) The ester bonds in PEGDA200 can be broken under acid, hot water or oxidant conditions, causing the three-dimensional network to gradually disintegrate and form low-viscosity degradation products, thereby improving the degradation rate and permeability recovery rate and reducing the risk of long-term reservoir damage. (4) The preparation process is aqueous solution free radical polymerization, the raw materials are readily available, and it is convenient to expand production, storage, transportation and on-site compatibility. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the S-DPG preparation process; Figure 2 This is a schematic diagram of the S-DPG structure and degradation process. Detailed Implementation

[0011] The present invention will be further described below with reference to the embodiments. The following embodiments are used to explain the present invention and not to limit the scope of protection of the present invention; unless otherwise specified, the percentages in the embodiments are all mass percentages.

[0012] The S-DPG particle dispersion of the present invention can be graded using at least two types of particles selected from 20-40 mesh, 40-80 mesh, and 80-160 mesh according to the crack width. Preferably, a combination of large particle bridging, medium particle filling, and small particle densification is adopted. For cracks of 0.5-2.0 mm, the ratio of particles of different sizes can be adjusted according to the crack width, target sealing pressure, and expected backflow requirements.

[0013] Example 1: Weigh 4.0 g acrylamide, 1.0 g AMPS and 1.5 g N-hydroxyethylacrylamide, add to 92.45 g deionized water, and stir at room temperature until completely dissolved; add 0.8 g PEGDA200 and 0.2 g 2-amino-2-methyl-1-propanol, and continue stirring for 20-30 min to make the system homogeneous; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, stir quickly and evenly, pour into a sealed mold, and react at 65℃ for 2.5 h to obtain bulk gel; cut the bulk gel, wash quickly with deionized water 2-3 times, vacuum dry at 50℃ to constant weight, pulverize and sieve into 20-40 mesh, 40-80 mesh and 80-160 mesh particles.

[0014] Example 2: Weigh 4.0 g acrylamide, 0.5 g AMPS and 1.5 g N-hydroxyethylacrylamide, add to 92.95 g deionized water, and stir at room temperature until completely dissolved; add 0.8 g PEGDA200 and 0.2 g 2-amino-2-methyl-1-propanol, and continue stirring for 20-30 min to make the system homogeneous; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, stir quickly and evenly, pour into a sealed mold, and react at 65℃ for 2.5 h to obtain bulk gel; cut, wash, vacuum dry to constant weight, pulverize and sieve to obtain S-DPG particles with low AMPS content.

[0015] Example 3: Weigh 4.0 g acrylamide, 1.5 g AMPS and 1.5 g N-hydroxyethylacrylamide, add to 91.95 g deionized water, and stir at room temperature until completely dissolved; add 0.8 g PEGDA200 and 0.2 g 2-amino-2-methyl-1-propanol, and continue stirring for 20-30 min to make the system homogeneous; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, and react at 65℃ for 2.5 h; after the reaction is completed, cut, wash, vacuum dry, pulverize and sieve the obtained bulk gel to obtain S-DPG particles with high AMPS content.

[0016] Example 4: Weigh 4.0 g acrylamide, 1.0 g AMPS and 1.5 g N-hydroxyethylacrylamide, add to 92.85 g deionized water, and stir at room temperature until completely dissolved; add 0.4 g PEGDA200 and 0.2 g 2-amino-2-methyl-1-propanol, and continue stirring for 20-30 min; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, and react at 65℃ for 2.5 h; after the reaction is complete, cut, wash, vacuum dry, pulverize and sieve the obtained bulk gel to obtain S-DPG particles with low crosslinking density.

[0017] Example 5: Weigh 4.0 g acrylamide, 1.0 g AMPS and 1.5 g N-hydroxyethylacrylamide, add to 92.05 g deionized water, and stir at room temperature until completely dissolved; add 1.2 g PEGDA200 and 0.2 g 2-amino-2-methyl-1-propanol, and continue stirring for 20-30 min; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, and react at 65℃ for 2.5 h; after the reaction is complete, cut, wash, vacuum dry, pulverize and sieve the obtained bulk gel to obtain S-DPG particles with high crosslinking density.

[0018] Example 6: Weigh 4.0 g acrylamide, 1.0 g AMPS and 2.0 g N-hydroxyethylacrylamide, add to 91.95 g deionized water, and stir at room temperature until completely dissolved; add 0.8 g PEGDA200 and 0.2 g 2-amino-2-methyl-1-propanol, and continue stirring for 20-30 min; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, and react at 65℃ for 2.5 h; after the reaction is complete, cut, wash, vacuum dry, pulverize and sieve the obtained bulk gel to obtain S-DPG particles with high hydrogen bond monomer content.

[0019] Comparative Example 1: Weigh 4.0 g acrylamide and 1.0 g AMPS, add them to 93.95 g deionized water, and stir at room temperature until completely dissolved; add 0.8 g PEGDA200 and 0.2 g 2-amino-2-methyl-1-propanol, and continue stirring for 20-30 min; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, and react at 65℃ for 2.5 h; after the reaction is completed, cut, wash, vacuum dry, pulverize and sieve to obtain control particles that do not contain N-hydroxyethylacrylamide.

[0020] Comparative Example 2: Weigh 4.0 g acrylamide and 1.5 g N-hydroxyethyl acrylamide, add them to 93.45 g deionized water, and stir at room temperature until completely dissolved; add 0.8 g PEGDA200 and 0.2 g 2-amino-2-methyl-1-propanol, and continue stirring for 20-30 min; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, and react at 65℃ for 2.5 h; after the reaction is completed, cut, wash, vacuum dry, pulverize and sieve to obtain AMPS-free control particles.

[0021] Comparative Example 3: Weigh 4.0 g acrylamide, 1.0 g AMPS and 1.5 g N-hydroxyethylacrylamide, add to 93.20 g deionized water, and stir at room temperature until completely dissolved; add 0.05 g N,N′-methylenebisacrylamide and 0.2 g 2-amino-2-methyl-1-propanol, and continue stirring for 20-30 min; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, and react at 65℃ for 2.5 h; after the reaction is completed, cut, wash, vacuum dry, pulverize and sieve to obtain the control particles with MBA replacing PEGDA200.

[0022] Comparative Example 4: Weigh 4.0 g acrylamide, 1.0 g AMPS and 1.5 g N-hydroxyethyl acrylamide, add to 92.65 g deionized water, and stir at room temperature until completely dissolved; add 0.8 g PEGDA200, and continue stirring for 20-30 min; adjust the pH of the system to 7.5-8.5, and purge with nitrogen for 30 min to remove oxygen; add 0.05 g ammonium persulfate, and react at 65℃ for 2.5 h; after the reaction is completed, cut, wash, vacuum dry, pulverize and sieve to obtain control particles without degradation promoters.

[0023] To verify the temperature and salt resistance, plugging performance, and degradation recovery ability of the S-DPG temporary plugging agent of the present invention, the expansion performance, high temperature and high salt aging stability, plugging strength, degradation performance, and core permeability recovery of the example and comparative samples were evaluated by experiments.

[0024] 1. Expansion, aging stability and sealing strength testing To evaluate the water absorption and swelling capacity, network stability, and fracture plugging performance of S-DPG particles under high temperature and high salinity conditions, 0.5 g of dried particles were placed in 50 mL of simulated formation water. The simulated formation water had a salinity of 80,000–150,000 mg / L and contained NaCl and CaCl2. The particles were allowed to swell at 90–140°C, and were weighed at preset time intervals. The swelling ratio was calculated using the following formula: ; In the formula, S is the expansion ratio (g / g), m0 is the mass of dried particles (g), and m t The value is the mass of the particles after water absorption, in grams.

[0025] After reaching equilibrium expansion, the particles were placed in simulated formation water and aged for 30 days under high salinity conditions at 140℃. The change in storage modulus G′ was measured using a rheometer to evaluate the structural stability of the gel network in a high-temperature and high-salt environment. The storage modulus retention rate was calculated using the following formula: ; In the formula, G0′ is the energy storage modulus before aging, in Pa, G t ′ represents the energy storage modulus after aging, in Pa.

[0026] Particle dispersions were injected using fractured core models or visualized fractured plates, and breakthrough or plugging pressures were recorded to evaluate the bridging, filling, and compaction capabilities of samples within fractures.

[0027] Table 1. Test results of expansion, aging stability and sealing strength of each sample ; As shown in Table 1, Example 1 achieved a good balance between expansion ratio, aging stability, and crack bearing capacity; Example 3, due to its higher AMPS content, exhibited improved storage modulus retention after high-temperature and high-salt aging; Example 5, with its higher PEGDA200 content, had the highest bearing capacity but a decreased expansion ratio; Example 4, due to its lower crosslinking density, had a higher expansion ratio but a relatively lower bearing capacity; Comparative Examples 1 and 2 both had low expansion ratios and bearing capacities, indicating that N-hydroxyethyl acrylamide and AMPS have a positive effect on gel network stability and crack sealing strength; Although Comparative Example 3 had high storage modulus retention and bearing capacity, it used non-degradable MBA crosslinking, and therefore its low-damage relief advantage cannot be solely demonstrated.

[0028] 2. Degradation, plugging rate, and permeability recovery tests After reaching equilibrium expansion, the gel particles were placed in 10 wt% hydrochloric acid, 0.5 wt% persulfate solution, and hot water at 90–140°C, respectively, for degradation experiments under constant temperature conditions. Samples were taken at preset time intervals, filtered, washed with deionized water, and vacuum dried at 50°C to constant weight. The mass of residual solids was measured, and the degradation rate was calculated using the following formula: ; In the formula, D is the degradation rate, %; m d0 The dry weight of the sample before degradation is in g, m d The dry weight of the residual solids after degradation is in g.

[0029] The plugging and permeability recovery performance of the particulate dispersion was evaluated using an artificial fracture core model. The permeability of the experimental cores ranged from 1000 to 3000 mD, and the fracture width ranged from 0.5 to 2.0 mm. The particulate dispersion was injected at a constant flow rate, and the permeability changes before and after plugging and after degradation and backflow were measured after the system stabilized. The plugging rate E and the permeability recovery rate R were calculated according to the following formulas: ; In the formula, k0 is the initial permeability before plugging, k1 is the permeability after plugging, and k2 is the permeability after degradation and backflow.

[0030] Table 2. Degradation, plugging rate, and permeability recovery test results for each sample. ; As shown in Table 2, Example 1 exhibits a good overall balance between plugging rate, degradation rate, and permeability recovery rate; Example 4 has a high degradation rate and permeability recovery rate, but as shown in Table 1, its pressure resistance is relatively low; Example 5 has a high plugging rate, but the degradation rate and permeability recovery rate are reduced, indicating that excessive crosslinking density will affect the decomposition effect; Comparative Example 3 has a high plugging rate, but the degradation rate and permeability recovery rate are significantly reduced, indicating that the cleavable ester bond of PEGDA200 is the key structure for achieving low-damage decomposition; Comparative Example 4 shows a decrease in degradation rate and permeability recovery rate after the addition of 2-amino-2-methyl-1-propanol, indicating that this component has a promoting effect on acid / thermal triggered degradation.

[0031] As can be seen from Tables 1 and 2, the present invention, through the synergistic effect of AMPS temperature and salt resistant monomers, N-hydroxyethyl acrylamide hydrogen bond enhancement, PEGDA200 fracture crosslinking and degradation promoter, enables the pre-prepared particulate gel to have high sealing strength in the sealing stage, and can effectively degrade and restore core permeability after the action of acid, hot water or persulfate. Example 1 shows a good comprehensive balance between sealing ability in the sealing stage and low damage relief in the degradation stage.

[0032] In summary, the S-DPG shield-type temperature- and salt-resistant biodegradable pre-formed granular gel temporary plugging agent of this invention still exhibits high expansion stability, plugging pressure resistance, and fracture plugging effect under high temperature and high salt conditions. Simultaneously, the synergistic effect of the fracture-resistant cross-linked structure of PEGDA200 and the degradation promoter enables it to gradually degrade and restore core permeability under the action of acid, hot water, or oxidants, thus balancing the need for effective plugging during the plugging stage with the requirement for low-damage removal during the degradation stage. This material is suitable for temporary plugging and diversion in deep fractured oil and gas reservoirs and for fracture leakage control, showing promising prospects for field applications.

Claims

1. A S-DPG shielded temperature-resistant and salt-tolerant degradable preformed particle gel temporary plugging agent, characterized in that, The temporary sealing agent is a pre-formed granular gel formed by free radical polymerization of aqueous solution, drying, pulverizing, and sieving. Based on the total mass of the gel solution, it comprises the following components: 3.5%–5.0% acrylamide, 0.8%–1.5% 2-acrylamido-2-methylpropanesulfonic acid or its salt, 1.2%–2.0% N-hydroxyethylacrylamide, 0.6%–1.0% polyethylene glycol diacrylate 200, 0.10%–0.30% 2-amino-2-methyl-1-propanol, 0.03%–0.08% ammonium persulfate, with the balance being water. The mass ratio of N-hydroxyethylacrylamide to polyethylene glycol diacrylate 200 is 1.5–3.0, and the mass ratio of 2-amino-2-methyl-1-propanol to polyethylene glycol diacrylate 200 is 0.15–0.

35. The temporary sealing agent does not contain chromium crosslinking agents, phenolic crosslinking agents, or N,N′-methylenebisacrylamide crosslinking agents.

2. The temporary occlusion agent of claim 1, wherein, The polyethylene glycol diacrylate 200 is an ester-linking crosslinking agent, and the 2-amino-2-methyl-1-propanol is used to adjust the pH of the polymerization system and promote the hydrolysis of the ester-linking crosslinked structure.

3. Temporary occlusion agent according to claim 1 or 2, characterized in that, The temporary plugging agent retains a storage modulus of no less than 75% after aging in simulated formation water at 140℃ and a mineralization of 150,000 mg / L for 30 days; and a degradation rate of no less than 80% after being treated with 10 wt% hydrochloric acid at 90℃ for 12 hours, with an apparent viscosity of no more than 15 mPa·s.

4. The temporary sealing agent according to claim 1, characterized in that, The pre-formed granular gel is sieved to form 20-40 mesh, 40-80 mesh and 80-160 mesh particles; for cracks of 0.5-2.0 mm, at least two particle sizes are mixed and injected, so that the large-diameter particles form a bridging skeleton, and the medium and small-diameter particles enter the skeleton pores and expand after absorbing water to form a compacted sealing body.

5. A method for preparing the temporary plugging agent according to any one of claims 1 to 4, characterized in that, The process includes the following steps: acrylamide, 2-acrylamide-2-methylpropanesulfonic acid or its salt, and N-hydroxyethylacrylamide are added to water and stirred to dissolve, forming a first mixture; polyethylene glycol diacrylate 200 and 2-amino-2-methyl-1-propanol are added to the first mixture and stirred to obtain a second mixture; nitrogen gas is passed through the second mixture to remove oxygen, and ammonium persulfate is added, and polymerization is carried out at 60-70°C for 2.0-3.0 h to obtain a bulk gel; the bulk gel is cut, washed, vacuum dried at 40-60°C to constant weight, pulverized, and sieved to obtain the S-DPG shield-type temperature- and salt-resistant degradable pre-formed granular gel temporary sealing agent.

6. The preparation method according to claim 5, characterized in that, The pH of the second mixture is adjusted to 7.5-8.5 before adding ammonium persulfate, and nitrogen is passed through for 20-40 minutes to remove oxygen. The bulk gel is washed 2-3 times, vacuum dried at 45-55°C, and then pulverized and sieved into 20-40 mesh, 40-80 mesh, and 80-160 mesh particles, respectively.

7. The application of the temporary plugging agent according to any one of claims 1 to 4 in acidizing, acid fracturing, hydraulic fracturing, temporary plugging and diversion, or fracture leakage control in deep or ultra-deep high-temperature and high-salinity fractured oil and gas reservoirs, characterized in that, The temporary sealing agent is injected into the target crack or high-permeability dominant channel in the form of a 1.0 wt% to 5.0 wt% particulate dispersion. During the construction period, it forms a temporary sealing body through particle bridging, fine particle filling, water absorption expansion and compaction deformation, causing the subsequent working fluid to be redirected.

8. The application according to claim 7, characterized in that, The application environment temperature is 90–160℃, and the mineralization is 50,000–250,000 mg / L. After the temporary plugging and diversion are completed, the degradation of the temporary plugging agent is triggered by 5 wt%–15 wt% hydrochloric acid, 0.3 wt%–1.0 wt% persulfate solution or hot water. After degradation and backflow, the core permeability recovery rate is not less than 80%.

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