A self-degradable double-crosslinked hydrogel temporary plugging agent and its preparation method and application

Abstract

The present invention discloses a self-degradable double-crosslinked hydrogel temporary plugging agent, a preparation method and application thereof, and belongs to the technical field of oil and gas extraction. The self-degradable double-crosslinked hydrogel temporary plugging agent of the present invention, based on the total mass of water, the raw materials for preparing the self-degradable double-crosslinked hydrogel temporary plugging agent include the following components in percentage by mass: 4%-8% acrylamide, 0.5%-0.8% initiator, 0.2%-0.6% polyethylene glycol-diacrylate and polydopamine dispersion; the polydopamine dispersion is obtained by dissolving dopamine hydrochloride in an alkaline aqueous solution and polymerizing it. The temporary plugging agent of the present invention solves the problems of uncontrollable gel breaking time, narrow adjustment window and low gel strength of the single ester-based crosslinking system. The strongly reducing free dopamine monomer not only inhibits the polymerization of acrylamide monomer and prolongs the gel time, but also shortens the length of the polyacrylamide molecular chain after gelation, and reduces the viscosity of the solution after the gel is spontaneously degraded.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas extraction, and in particular to a self-degradable double-crosslinked hydrogel temporary plugging agent and a preparation method and application thereof. Background Art

[0002] During oilfield development, temporary plugging technology, a key component of refracturing, workover operations, and drilling, directly impacts oil and gas production and development efficiency. Temporary plugging and diversion fracturing can create new fracture networks in the reservoir while reactivating existing fractures that haven't been fully depleted, thereby increasing the productivity of unconventional oil and gas reservoirs. Temporary plugging and workover technology can effectively address downhole faults, increase single-well production and recovery rates, and extend the well's production cycle. Temporary plugging drilling technology protects oil and gas reservoirs by sealing formation leakage pathways, reducing drilling fluid losses, and preventing drilling fluid from invading the producing strata.

[0003] In recent years, temporary plugging technologies in oil and gas fields have evolved from traditional mechanical plugging methods to chemical methods that are simple to operate, safer, and more efficient. A variety of functional chemical temporary plugging agents have been developed to meet diverse operational requirements. Among them, gel temporary plugging agents are materials that form a gel-like plugging layer under specific conditions, temporarily sealing fractures and pores in wellbores or formations. Depending on the method used to remove the temporary plugging, they are categorized as self-degrading or using a breaker. Self-degrading gels form a gel and degrade spontaneously after injection into the formation. Compared to traditional temporary plugging and degradation methods involving subsequent injection of degrading agents and premixed capsule breakers, they offer advantages such as no residue after degradation and a precisely controllable degradation time, attracting widespread attention. Xiao Zhang et al. developed a degradable gel particle temporary plugging agent suitable for high-temperature reservoirs (100-160°C) that completely degrades within 19-107 hours. However, the degradation performance of this system significantly decreases at low to medium temperatures (<80°C), and granular temporary plugging agents suffer from poor injectability, making their plugging effectiveness susceptible to reservoir fracture heterogeneity. Ying Wang's team developed a high-strength liquid temporary plugging agent using AM, AA (acrylic acid), and an unstable crosslinker, SEG (sulfonated epoxy resin), suitable for wellbore temporary plugging operations. This system exhibits good reservoir adaptability in the medium-low temperature range (60-80°C), with a gel-breaking time of 180-300 h. However, it suffers from issues such as high AM monomer concentration (>8 wt%), long self-degradation time of the SEG crosslinker, rapid gel formation rate, and high gel-breaking fluid viscosity (>50 mPa·s), resulting in poor injection and flowback performance. Furthermore, the introduction of AA in the system increases the loss modulus, which makes the gel susceptible to deformation during use, thus reducing its pressure-bearing capacity. Renjing Ji et al. developed a self-degradable temporary plugging valve for managed pressure drilling that degrades within 5-20 days at 90°C. While this system exhibits good injectability, the gel-breaking period is significantly prolonged at low temperatures (<80°C), and the controllable gel-breaking time is limited (only 15 days), resulting in a limited range of application scenarios. Comprehensive analysis shows that current research on temporary plugging agents in oil and gas fields primarily focuses on high-temperature reservoirs (>90°C), while research on specialized temporary plugging agents for medium- and low-temperature reservoirs (60-80°C) is relatively scarce. Existing medium- and low-temperature temporary plugging agents commonly suffer from technical bottlenecks such as rapid gelation rates (<30 minutes). Furthermore, the gel-breaking times of existing products vary widely (1-7 days or 7-15 days), making it difficult to universally apply specialized agents for different operational scenarios, such as fracturing temporary plugging agents and wellbore temporary plugging agents. Therefore, a new self-breaking gel temporary plugging agent suitable for medium- and low-temperature reservoirs (<90°C) is needed, characterized by a wide range of controllable gelation and gel-breaking times, ease of injection, and low gel-breaking viscosity. Summary of the Invention

[0004] To address the aforementioned issues in the prior art, the present invention provides a self-degradable double-crosslinked hydrogel temporary plugging agent, its preparation method, and its application. Using polydopamine (PDA) and polyethylene glycol diacrylate (PEGDA) as crosslinkers for polyacrylamide (PAM), the present invention designs a self-degradable double-crosslinked hydrogel (DSDG) temporary plugging agent suitable for medium- and low-temperature (60-100°C) reservoirs based on C=C double bond crosslinking and amino crosslinking.

[0005] The present invention first provides a self-degradable double-crosslinked hydrogel temporary plugging agent. The raw materials for preparing the self-degradable double-crosslinked hydrogel temporary plugging agent include the following components in percentage by weight, based on the total mass of water: 4%-8% acrylamide, 0.5%-0.8% initiator, 0.2%-0.6% polyethylene glycol-diacrylate and polydopamine dispersion;

[0006] The polydopamine dispersion is obtained by dissolving dopamine hydrochloride in an alkaline aqueous solution and polymerizing the solution.

[0007] In the above-mentioned self-degradable double-crosslinked hydrogel temporary plugging agent, the pH of the alkaline aqueous solution is 8-12, specifically 11;

[0008] The alkaline aqueous solution is sodium hydroxide solution and / or ammonia water;

[0009] The mass ratio of dopamine hydrochloride to acrylamide is 0.005:1-0.009:1.

[0010] In the above-mentioned self-degradable double-crosslinked hydrogel temporary plugging agent, the initiator is potassium persulfate and / or ammonium persulfate.

[0011] In the above-mentioned self-degradable double-crosslinked hydrogel temporary plugging agent, the self-degradable double-crosslinked hydrogel temporary plugging agent forms a gel at a pH value of 3-11 and a temperature of 60-100°C.

[0012] Specifically, the gelling temperature is 60-80°C or 80°C.

[0013] In the above-mentioned self-degradable double-crosslinked hydrogel temporary plugging agent, the raw materials for preparing the self-degradable double-crosslinked hydrogel temporary plugging agent include the following components in percentage by weight, calculated on the total mass of water: 4%-6% acrylamide, 0.5%-0.8% initiator, 0.3%-0.6% polyethylene glycol-diacrylate and polydopamine dispersion;

[0014] The mass ratio of the dopamine hydrochloride to acrylamide is 0.005:1-0.008:1.

[0015] In the above-mentioned self-degradable double-crosslinked hydrogel temporary plugging agent, the raw materials for preparing the self-degradable double-crosslinked hydrogel temporary plugging agent are the following components in percentage by weight, calculated on the total mass of water: acrylamide is 4%, 5%, or 6%; initiator is 0.5%, 0.6%, 0.7%, or 0.8%; polyethylene glycol-diacrylate is 0.3%, 0.4%, 0.5%, or 0.6%; polydopamine dispersion, wherein the mass ratio of dopamine hydrochloride to acrylamide is 0.005:1, 0.006:1, 0.007:1, or 0.008:1.

[0016] The self-degradable double-crosslinked hydrogel temporary plugging agent transforms from a liquid state to a solid-like gel within 30-180 minutes at 60-100°C.

[0017] The self-degradable double-crosslinked hydrogel temporary plugging agent spontaneously degrades from a quasi-solid gel to a low-viscosity liquid with a viscosity of less than 10 mPa·s within 1-20 days under the conditions of 60-100°C.

[0018] The present invention also provides a method for preparing the self-degradable double-crosslinked hydrogel temporary plugging agent, comprising the following steps: mixing the acrylamide, initiator, polyethylene glycol-diacrylate and polydopamine dispersion, and adjusting the pH value of the system to 3-11 to obtain the self-degradable double-crosslinked hydrogel temporary plugging agent.

[0019] In the above-mentioned preparation method, the preparation method of the polydopamine dispersion comprises the following steps: dissolving dopamine hydrochloride in an alkaline aqueous solution and polymerizing in an air atmosphere to obtain the polydopamine dispersion;

[0020] The pH value of the system was adjusted using hydrochloric acid solution.

[0021] In the above-mentioned preparation method, in the preparation method of the polydopamine dispersion, the polymerization time is 20-50 min; specifically, it can be 20 min, 25 min, 30 min, 40 min or 50 min;

[0022] The polymerization temperature is room temperature; the room temperature is well known to those skilled in the art and is generally 15-35°C.

[0023] Finally, the present invention provides an application of the self-degradable double-crosslinked hydrogel temporary plugging agent in improving the recovery rate of oil and gas reservoirs.

[0024] Specifically, the reservoir temperature of the oil and gas reservoir is 60-100°C; more specifically, 60-80°C.

[0025] The temporary plugging agent of this invention achieves high mechanical properties at low monomer concentrations. It utilizes the high-density catechol functional groups on the surface of PDA and an unstable crosslinker (PEGDA) to construct a dual-crosslinked structure. The controllable degradation of PDA under acidic and alkaline conditions disrupts the integrity of the gel network structure, thus resolving the issues of uncontrollable gel-breaking time, narrow adjustment window, and low gel strength associated with single ester-based crosslinking systems. The highly reducing free dopamine (DA) monomer not only inhibits the polymerization of acrylamide (AM) monomers, prolonging the gelation time, but also shortens the PAM molecular chain length after gelation and reduces the solution viscosity after spontaneous gel degradation.

[0026] The temporary plugging agent of this invention transforms from a liquid to a near-solid gel within 30-180 minutes at 60-100°C. Rheological results indicate that the low-viscosity phase accounts for 80% of the gel formation process, facilitating pumping of the temporary plugging agent. The unoxidized catechol groups, π-π stacking, and hydrogen bonds in the gel enhance its tensile strength, fracture toughness, and interfacial adhesion, ensuring effective plugging under complex working conditions.

[0027] The rapid degradation of PDA under acidic or alkaline conditions disrupts the integrity of the gel network, accelerating the gel-breaking process and achieving controllable gel-breaking time. Furthermore, free DA monomers inhibit AM polymerization, reducing the degree of polymerization of the PAM molecular chains and significantly lowering the viscosity of the degraded solution. At 60-100°C, the temporary plugging agent spontaneously degrades from a quasi-solid gel to a low-viscosity liquid with a viscosity of less than 10 mPa·s within 1-20 days. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Schematic diagram of different gel strengths.

[0029] Figure 2 Schematic diagram of the gel compressive strength testing device.

[0030] Figure 3 The gelling properties of DSDG prepared at different monomer concentrations.

[0031] Figure 4 The colloid formation properties of DSDG prepared at different initiator concentrations.

[0032] Figure 5 The gelling properties of DSDG prepared at different DA / AM mass ratios.

[0033] Figure 6 The gelling properties of DSDG prepared at different PEGDA concentrations.

[0034] Figure 7 The different pH values ​​of DSDG base solution affect the gelling properties of DSDG.

[0035] Figure 8 The gelling properties of DSDG prepared at different temperatures.

[0036] Figure 9 is the viscosity change of DSDG during the gelation process.

[0037] Figure 10 The effect of monomer concentration on the storage modulus of DSDG gel G' and loss modulus G'' impact.

[0038] Figure 11 The effect of DA / AM mass ratio on the storage modulus of DSDG gel G' and loss modulus G'' impact.

[0039] Figure 12 is the SEM image of the gel; Figure 12 (a) is PAM-PEGDA hydrogel; (b)-(d) are DSDG hydrogels, and the DA / AM mass ratios in (b)-(d) are 0.005, 0.007, and 0.009, respectively.

[0040] Figure 13 FTIR spectra of PAM-PEGDA gel, PDA and DSDG gel.

[0041] Figure 14 The self-degradation property of DSDG gel.

[0042] Figure 15 The pressure bearing performance of DSDG in the simulated wellbore.

[0043] Figure 16 The pressure bearing performance of DSDG in the matrix core. DETAILED DESCRIPTION

[0044] The present invention will be further described in detail below in conjunction with specific embodiments. The examples given are only for illustrating the present invention, not for limiting the scope of the present invention.

[0045] The experimental methods in the following examples are conventional methods unless otherwise specified.

[0046] The quantitative tests in the following examples were performed in triplicate unless otherwise specified, and the results were averaged.

[0047] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0048] Unless otherwise specified, all experiments in the following examples were carried out at 80°C.

[0049] The concentrations of monomers, initiators, and PEGDA described in the following examples all refer to the mass percentage of their weight relative to the weight of water in the system.

[0050] The test methods used in the following examples are as follows:

[0051] 1. Determination of gel formation time and degradation time

[0052] The gelation time and gel strength of DSDG were tested using the gel strength code method. This method involves inverting a glass centrifuge tube and observing the flow of the gel to quickly determine gel strength. This method allows for a wide range of gel strength classification. In this study, the glass centrifuge tube was slowly inverted 180° every 5 minutes, and the gel strength was recorded. Figure 1 The codes AI shown represent different gel strengths, wherein code I has the highest strength. The time required for the gel to increase from strength level A to strength level I is the gelation time.

[0053] In actual plugging operations in oil and gas fields, a reduction in strength grade to C is sufficient to achieve over 90% unblocking and resume operations. Therefore, gel degradation time is defined as the time required for the strength grade of the gel solution to drop to C after complete gelation. This degradation time is determined by continuously observing the gel flow in a blue-capped glass bottle.

[0054] 2. Rheological test

[0055] The rheological properties of DSDG were measured using a Haake rheometer (Thermo Scientific, Haake Mars iQ Air, Germany) with a P35 / Ti / SE rotor. The hydrogel sample was placed on the sample stage, and after setting the shear rate, the viscosity changes during the gelation and degradation of the DSDG were measured. Furthermore, the linear viscoelastic region of DSDG was measured at an oscillation frequency of 1.0 Hz and a strain range of 1.0-150%. After obtaining the corresponding stress test values, the storage modulus G' and loss modulus G'' of the gel were measured at frequencies of 0.01-10 Hz. The experimental temperature was 80°C.

[0056] 3. Scanning Electron Microscopy (SEM)

[0057] The DSDG base solution was heated at 80°C for 6 h to obtain DSDG. The DSDG gel sample was cut into small pieces, placed on a quartz plate, and freeze-dried in a freeze dryer. The polydopamine nanoparticles were washed three times in deionized water and ethanol to eliminate residual reactants and then dried in a 60°C oven. The treated gel and nanoparticle samples were observed using a HITACHI SU8010 field-emission scanning electron microscope.

[0058] 4. Fourier transform infrared spectroscopy (FTIR)

[0059] The FTIR spectra of pure PDA, pure PAM, and DSDG at room temperature were analyzed using an FTIR spectrometer (FTIR-850, ANGDONG, China) and the wavelength range of 4000–500 cm -1 The spectrum between.

[0060] 5. Gel temporary plugging performance test

[0061] Use Figure 2 The experimental setup shown here evaluates the pressure-bearing capacity of the DSDG. It consists of a nitrogen pressure supply system, a plunger pump, a constant-temperature oven, a pressure signal acquisition and processing system, a core holder, and a simulated wellbore. The wellbore model is made of N80 steel with an inner diameter of 120 mm. The inner surface of the wellbore is coated to prevent corrosion that could affect the gel's pressure-bearing performance. To test the pressure-bearing capacity of the gel temporary plugging agent within the wellbore, a gel solution is placed in the wellbore model and allowed to stand at 80°C for 6 hours. Nitrogen is then continuously injected into the wellbore bottom, and the injection pressure is monitored. When the injection pressure drops sharply, the pressure at the previous stage is recorded as the maximum bearing pressure of the DSDG.

[0062] Core flooding experiments were also conducted to investigate the gel's plugging ability in homogeneous cores. Core physical parameters are shown in Table 1. During the experiments, the confining pressure was 3 MPa higher than the injection pressure. Water was first injected until the injection pressure reached a stable value, followed by the injection of 2 PV of DSDG base fluid. The core holder was heated in an 80°C oven for 5 hours. After complete gel formation, water was injected again until the pressure reached another stable value.

[0063] After the core flooding experiment, the valves at the inlet and outlet of the core holder were closed, and the core holder was placed in an 80°C constant-temperature oven. After the gel inside the core was completely degraded, water flooding was performed until the pressure stabilized. The DSDG temporary plugging performance and the degree of damage to the core were evaluated by comparing the changes in water flooding pressure before and after temporary plugging.

[0064] Table 1 Physical parameters of artificial core

[0065]

[0066] Example 1

[0067] 1. Preparation of self-degradable double-crosslinked hydrogel (DSDG)

[0068] (1) Preparation of polydopamine (PDA)

[0069] Dopamine hydrochloride powder (DA) was added to a beaker containing a NaOH aqueous solution (pH = 11). After fully dissolved, the solution was stirred and self-polymerized at room temperature in an air atmosphere for 20 min to obtain a PDA dispersion.

[0070] (2) AM (acrylamide, monomer), KPS (potassium persulfate, initiator), PEGDA (polyethylene glycol diacrylate) and the above-mentioned PDA dispersion were stirred and mixed under nitrogen protection. After mixing for 10 minutes, dilute hydrochloric acid was added dropwise to adjust the solution to pH = 11 to obtain a self-degradable double-crosslinked hydrogel base solution (DSDG base solution).

[0071] Finally, pour approximately 15 mL of the DSDG base solution into a glass centrifuge tube (maximum capacity, 25 mL) and approximately 80 mL into a blue-capped glass bottle (maximum capacity, 100 mL). Seal with nitrogen. Store the container containing the DSDG base solution in a constant-temperature oven at 60-100°C and observe the gel formation and degradation behavior.

[0072] It should be noted that since DA can also be induced to undergo secondary polymerization and consume KPS, the preparation of DSDG hydrogel requires an excess of KPS (not less than 10% of the mass of the polymer monomer).

[0073] 2. Gelation Performance

[0074] 1. Effect of monomer concentration

[0075] The effect of monomer concentration in the range of 2.0-7.0 wt% on gelation time was investigated by gel strength code method at 80 ℃ and pH=11. The initiator concentration was 0.6 wt%, the PEGDA concentration was 0.4 wt%, and the DA / AM mass ratio was 5×10 -3 .

[0076] Figure 3The effect of monomer concentration on gelation time was demonstrated, with the gelation time of DSDG decreasing with increasing monomer concentration. Repeated experimental results at 80°C showed that when the monomer concentration was less than 2.0 wt%, DSDG failed to form a gel (Code A), while when the monomer concentration was between 2.0-3.0 wt%, DSDG only formed a weak gel (Code B or Code C). Therefore, when the DSDG monomer concentration was less than 3.0 wt%, its gel strength did not meet the requirements for temporary plugging in fracturing, drilling, or workover operations. When the monomer concentration was between 4.0-6.0 wt%, DSDG achieved Code I gel strength within 30-55 minutes, achieving good pumpability and effective temporary plugging. Furthermore, higher monomer concentrations increase the frequency of collisions between free radicals and monomer molecules, accelerating the growth rate of polymer chains. While this can form a denser three-dimensional network structure and increase gel strength, the rapid reaction rate results in a short gelation time, making injection impossible. When the monomer concentration is greater than 7 wt%, the gelation time of DSDG is shortened to <30 min, which may cause blockage in the near-wellbore area or the tubing string. In summary, the optimal monomer concentration of DSDG is 4.0-6.0 wt%.

[0077] 2. Influence of initiator concentration

[0078] Because DA monomers have strong reducing properties and inhibit free radical polymerization, DSDG gelation requires an excess of initiator. Free radicals generated by initiator decomposition can initiate monomer polymerization through a chain initiation-propagation-termination reaction to form segments. The initiator concentration in the formulation directly determines the free radical concentration and has a key influence on gelation kinetics. The effect of initiator concentrations ranging from 0.5 to 0.9 wt% on gelation time was systematically investigated at 80°C and pH 11, with fixed monomer concentrations of 5 wt%, PEGDA concentrations of 0.4 wt%, and a DA / AM mass ratio of 5×10 -3 The final gel strength of all samples reached Code I level.

[0079] Figure 4 The effect of initiator concentration on gelation time was demonstrated. The gelation time of DSDG decreased with increasing initiator concentration, but the downward trend slowed when the initiator concentration exceeded 0.6 wt%. This is because the free radical concentration reached saturation, the polymerization rate approached its limit, and further improvement was limited. When the initiator concentration fell below 0.5 wt% (10% of the monomer mass), the reducing nature of the DA monomer significantly decreased the initiator efficiency, preventing effective polymerization of the AM monomer and ultimately leading to gelation failure. Considering the requirements for gelation time and gel strength, the optimal initiator concentration for DSDG is 0.5-0.8 wt%. This condition strikes a balance between reaction rate and control requirements.

[0080] 3. Influence of DA / AM mass ratio

[0081] The crosslinker chemically crosslinks with the polymer chains in the gel to form covalent bonds or coordination bonds, connecting the linear polymer chains into a three-dimensional network structure. The dense catechol groups on the surface of PDA enable it to form a synergistic double crosslinked network with the unstable crosslinker PEGDA. -3 -9×10 -3 Effect of the gelation time on the monomer concentration, PEGDA concentration and initiator concentration were fixed at 5 wt%, 0.4 wt% and 0.6 wt%.

[0082] Figure 5 The effect of the DA / AM mass ratio on the gelation time was demonstrated. The gelation time of DSDG was prolonged with the increase of crosslinker concentration. This is because the free DA monomers that were not completely polymerized in the DSDG base liquid have strong reducing properties. Their catechol groups can quench free radicals and inhibit the free radical polymerization reaction of AM monomers. In addition, when the DA / AM mass ratio exceeds 10×10 -3 When the gelation time is too high, the excess free DA monomer in the gel solution will lead to unstable gelation process, and even some samples cannot form a complete three-dimensional network. Therefore, based on the balance between gelation time and final gel strength, the optimal DA / AM mass ratio of DSDG should be controlled at 5×10 -3 -9×10 -3 .

[0083] 4. Effect of cross-linker PEGDA concentration

[0084] PEGDA, as a crosslinker, forms a covalent crosslinking network with the amide groups on the polymer chain through the acrylate double bonds. The effect of PEGDA concentration (0.2-0.6 wt%) on the gelation behavior was systematically studied at 80°C and pH 11. The monomer concentration was fixed at 5 wt% and the DA / AM mass ratio was 8×10 -3 and initiator concentration 0.6 wt%.

[0085] The higher the concentration of the crosslinker PEGDA, the more active groups in the system can participate in the crosslinking reaction, which accelerates the crosslinking reaction and shortens the gelation time. Figure 6 The study demonstrated the effect of crosslinker PEGDA concentration on gelation time, showing that the gelation time of DSDG decreased with increasing crosslinker concentration. When the crosslinker PEGDA concentration was below 0.2 wt%, the excessive gelation time resulted in extensive degradation of PDA in an alkaline environment, significantly reducing the mechanical and adhesive properties of the gel. Therefore, considering both gelation time and gelation properties, the optimal concentration range of the crosslinker PEGDA should be controlled within the range of 0.3-0.6 wt%.

[0086] 5. Effect of pH

[0087] The effect of pH (3-11) on gelation time was investigated at 80 °C, with fixed monomer concentration of 5 wt%, PEGDA concentration of 0.4 wt%, and DA / AM mass ratio of 8×10 -3 and initiator concentration 0.6 wt%.

[0088] Under alkaline conditions, the amino group of acrylamide is deprotonated to form -NH2, which is beneficial to the polymerization reaction. However, as the alkalinity increases, the self-polymerization rate of PDA accelerates, consuming a large number of free radicals, which in turn inhibits the acrylamide polymerization initiated by free radicals. Figure 7 The effect of pH on gelation time is shown. When the pH range of DSDG base liquid is 7-11, its gelation time gradually increases with the increase of alkalinity. Under acidic conditions, the amino group (-NH2) of acrylamide will be protonated to form -NH3 + , inhibiting the free radical polymerization reaction. At the same time, PDA's oxidation rate is slow under acidic conditions, preserving its strong reducing properties. Therefore, when the pH of the DSDG base fluid is in the range of 3-7, its gelation time gradually increases with increasing acidity. Furthermore, subsequent degradation experiments have shown that pH also affects the degradation time of the gel. In summary, the pH of the DSDG base fluid is adjusted within the range of 3-11 based on actual on-site needs.

[0089] 6. Influence of temperature

[0090] Temperature has a direct impact on the rate at which the initiator decomposes into free radicals, which can initiate the polymerization of acrylamide and dopamine monomers. The gel-forming properties of DSDG were investigated at 60-100°C, with a fixed monomer concentration of 5 wt%, PEGDA concentration of 0.4 wt%, and DA / AM mass ratio of 8×10 -3 , pH = 11 and initiator concentration 0.6 wt%.

[0091] Since the initiator KPS cannot decompose when the temperature is lower than 60 ℃, DSDG cannot form gel. Figure 8 The effect of temperature on gelation time is demonstrated. When the gelation temperature is in the range of 60-100 ℃, the DSDG gelation time gradually increases with increasing temperature, and the gelation time meets the formation injection requirements.

[0092] Example 2, Rheological properties

[0093] (1) Preparation of DSDG base liquid: The preparation method is the same as that in Example 1, wherein the monomer concentration is 5 wt%, the PEGDA concentration is 0.4 wt%, and the DA / AM mass ratio is 8×10 -3, base liquid pH=11, initiator concentration is 0.6 wt%.

[0094] (2) Preparation of PAM-PEGDA gel solution: The preparation method is the same as that of DSDG base solution, except that PDA is not added. The monomer concentration is 5 wt%, the PEGDA concentration is 0.4 wt%, the base solution pH is 11, and the initiator concentration is 0.6 wt%.

[0095] The rheological properties of DSDG were measured using a Haake rheometer, including the viscosity change during the gelation process, the viscosity change during the degradation process, the storage modulus, and the viscosity of the DSDG. G' and loss modulus G'' .

[0096] First, the viscosity change of DSDG during the gelation process was investigated at 80 ℃. During the experiment, a sample hood (PEEK) was used to prevent the evaporation of DSDG water. Figure 9 The viscosity change of DSDG during the gelation process at 80 °C is shown. In the range of 0-4000 s, the viscosity of the DSDG base liquid does not change significantly. This is because the free radical polymerization reaction has just begun at this stage, the monomer content in the system is high, and the polymer molecular weight is small, so the viscosity of the system does not change significantly. In the range of 4000-4200 s, due to the growth of the molecular chain and the aggregation of some chain segments to form a more compact structure, the viscosity of the base liquid begins to rise slowly. In the range of 4200-5000 s, due to the continuous increase in the cross-linking points between the polymer chains, a three-dimensional network structure is formed, and the viscosity of the base liquid begins to increase sharply and eventually gels. The experimental results show that the low viscosity state of DSDG occupies more than 80% of the total gelation process, which is beneficial to the injection operation on site. In addition, after the final gelation, the viscosity of DSDG can reach 1.31×10 5 mPa·s, ensuring effective plugging in the formation / wellbore environment. However, the viscosity of the PAM-PEGDA gel solution without DA was observed to increase within just 2000 s, and the viscosity began to increase sharply after 2100 s. The duration of its low viscosity was only 50% of that of DSDG, and the final viscosity after gelation was only 70% of that of DSDG. This indicates that pure PAM gel without DA has poor injectability and is difficult to form an effective plug in the formation / wellbore.

[0097] Next, the gel's storage modulus (G') and loss modulus (G'') were measured at frequencies between 0.01 and 10 Hz to assess its viscoelastic properties. The effects of monomer concentration and the DA / AM mass ratio on the rheological properties of DSDG were also investigated. The base liquid was poured into a blue-capped glass bottle and placed in an 80°C constant-temperature oven under nitrogen. After the gel strength reached Code I, the temperature was maintained in the oven for another 5 hours to ensure stable sample performance. Figure 10 The study demonstrated the effect of monomer concentration on the gel's storage modulus, G', and loss modulus, G''. As the acrylamide (AM) monomer concentration increased, the DSDG's storage modulus, G', increased, while its loss modulus, G'', gradually climbed. This is because the increased monomer concentration in the gel increases the molecular weight of the polymer segments and the crosslinking density of the gel network, strengthening the interactions and constraints between the molecular chains. Furthermore, the increased crosslinking density between the molecular chains enables the network to better maintain its shape and structure when subjected to external forces, thereby increasing the gel's elastic modulus and loss modulus.

[0098] Figure 11 The effect of DA / AM mass ratio on the storage modulus G' and loss modulus G'' of the gel is shown. When the DA / AM mass ratio is 5×10 -3 -7×10 -3 As the concentration of polydopamine (PDA) in the DSDG base liquid increases, the storage modulus G' of DSDG gradually increases and the loss modulus G'' increases. This is because the catechol groups on the PDA chain segments can cross-link with the amino groups on the PAM chain segments to form a denser three-dimensional network structure. At the same time, the mutual stacking of PDA chains forms a large number of recoverable non-covalent bonds, such as π-π bond stacking and hydrogen bonds. The breakage of these non-covalent bonds can effectively dissipate energy and prevent the expansion of cracks during stretching, thereby enhancing the toughness of the gel. In addition, the DSDG gel network introduces a large number of covalent cross-links through the PDA chains, realizing load sharing with non-covalent bonds. The cross-linking points of covalent and non-covalent bonds can disperse stress through synergistic effects, thereby improving the overall elasticity of the gel. When the DA / AM mass ratio is greater than 7×10 -3 When the DA / AM mass ratio increases, the reducibility of the gel base fluid gradually increases, inhibiting the formation of the gel network structure and gradually reducing the crosslinking density. At the same time, the flexibility and reversibility of hydrogen bonds make the gel network more susceptible to elastic deformation when subjected to stress. The storage modulus G' of DSDG gradually decreases, and the loss modulus G'' gradually decreases.

[0099] Increasing the storage modulus of the gel can enhance the elasticity of the gel, making it less likely to deform under external forces, but it will reduce the flexibility of the gel. Too high a storage modulus will cause the gel to break easily under high pressure. The loss modulus can increase the flexibility and ductility of the gel and avoid breaking under high pressure conditions, but gels with too high a loss modulus are prone to deformation during use. The network structure of gels with low loss modulus maintains stability. This stability makes the gel network less likely to be structurally damaged when subjected to external forces, thereby maintaining its structural integrity. The easy breakage or deformation of the gel will have an adverse effect on its sealing effect. Figure 10 and Figure 11 DSDG, prepared using polydopamine (PDA) and polyethylene glycol diacrylate (PEGDA) as crosslinkers, exhibits high elastic modulus and low loss modulus. Furthermore, the gel contains a large number of recoverable non-covalent bonds, including π-π stacking and hydrogen bonds. The rupture of these non-covalent bonds effectively dissipates energy, preventing crack propagation during gel stretching. This makes the gel less susceptible to deformation and prevents rupture under high-pressure formation conditions.

[0100] Example 3: Microstructure of DSDG gel

[0101] (1) Preparation of DSDG

[0102] The preparation method of DSDG base liquid is the same as that of Example 1, wherein the monomer concentration is 5 wt%, the PEGDA concentration is 0.4 wt%, and the DA / AM mass ratio is 5×10 -3 -9×10 -3 , base liquid pH = 11, initiator concentration is 0.6 wt%. After the DSDG base liquid is prepared, it is placed in a constant temperature oven at 80 °C for 6 h to obtain DSDG.

[0103] (2) Preparation of PAM-PEGDA hydrogel

[0104] The PAM-PEGDA base liquid was prepared as follows: AM monomer, potassium persulfate, PEGDA, and deionized water were stirred under nitrogen for 10 minutes. Dilute hydrochloric acid was then added dropwise to adjust the pH of the solution to 11 to obtain the PAM-PEGDA base liquid. The base liquid was then placed in an 80°C oven for 6 hours to obtain the PAM-PEGDA hydrogel. The monomer concentration was 5 wt%, the PEGDA concentration was 0.4 wt%, and the initiator concentration was 0.06 wt%.

[0105] (3) Preparation of pure PDA nanoparticles

[0106] Dopamine hydrochloride was added to a sodium hydroxide aqueous solution with a pH of 11 and stirred for 20 min to obtain a PDA nanoparticle dispersion. The PDA nanoparticles were washed three times in deionized water and ethanol to eliminate the reactant residue in the sample and then dried in a constant temperature oven at 60°C.

[0107] The microstructure of the gel was observed using field emission scanning electron microscopy. Figure 12 The microscopic network structures of two different formula gels, PAM-PEGDA hydrogel and DSDG, are shown at 100 times magnification. Figure 12 As shown in (a), the network structure of the PAM-PEGDA hydrogel without PDA is loose and the cross-linking density is low. Figure 12 As shown in (b), (c) and (d), at the same magnification, the DA / AM mass ratio is 5×10 -3 -9×10 -3 The cross-linking density of DSDG network in the range is much greater than that of PAM-PEGDA hydrogel. -3 -7×10 -3 In the range of DA / AM mass ratio, the cross-linking density of the gel network is proportional to the DA / AM mass ratio. -3 -9×10 -3 Within this range, the cross-linking density of the gel network is inversely proportional to the DA / AM mass ratio.

[0108] The possible existence of double cross-linked structures between PDA, PEGDA and PAM was explored by analyzing the FTIR of PAM-PEGDA hydrogel, pure PDA nanoparticles and DSDG. The FTIR spectra of 4000-500 cm were recorded using an FTIR spectrometer (FTIR-850, ANGDONG, China). -1 Before measurement, pure PDA nanoparticle samples were washed three times in deionized water and ethanol to eliminate residual reactants and then dried in a 60°C oven. Hydrogel samples were dried in a freeze dryer at -50°C for one week.

[0109] Figure 13 The FTIR spectra of PAM-PEGDA hydrogel, pure PDA nanoparticles and DSDG were recorded. The PAM-PEGDA hydrogel had a wavelength of 3000-3500 cm -1 A band appears between the two bands, corresponding to the stretching vibration of NH at 2938 cm -1 The stretching vibration of CH is at 1607 cm -1 (NH deformation vibration of primary amine), 1460 cm -1 (CH2 in-plane vibration), 1356 cm -1(CH deformation vibration) and 1120 cm -1 (C-O-C stretching vibration) bands were also detected. -1 No characteristic absorption peak of C=C was found in the range, which indicates that the double bonds contained in PEGDA have been completely opened and participated in cross-linking. The disappearance of the C=C absorption peak and the appearance of the COC absorption peak prove the occurrence of the PAM-PEGDA cross-linking reaction. In the FTIR spectrum of PDA, the characteristic absorption band of the aromatic ring appears at 1500 cm -1 Near 3400 cm -1 The absorption band at is the result of the overlap between the absorption bands of hydroxyl groups (-OH) and water molecules adsorbed in the PDA polymer and the amine groups (-NH2) in PDA.

[0110] Compared with PAM-PEGDA hydrogel and PDA, the spectrum of DSDG hydrogel at 1320 cm -1 A new peak appeared at 1665 cm, which corresponds to the CN stretching vibration of phenylamine. The presence of this band indicates that there is an interaction between the -NH2 group of PAM and the catechol group of PDA. -1 The new peak at 1630-1650 cm corresponds to the stretching vibration of Schiff base C=N. The presence of this band indicates that there is a covalent cross-link between PAM and PDA. -1 No characteristic absorption peak of C=C was found in the above range, which proves the double cross-linked structure of DSDG.

[0111] Example 4: Self-degradability

[0112] This study used the unstable crosslinker PEGDA to form a stable double-crosslinked structure with PDA. The strong reducing property of the DA monomer inhibited the C=C double bond crosslinking on the PEGDA, reducing the number of crosslinking points. The crosslinking density was supplemented through the interaction between the catechol groups on the PDA segments and the amino groups on the PAM network. PDA is pH-sensitive and degrades under both acidic and alkaline conditions. Its rapid degradation compromised the integrity of the gel network structure, accelerating the gel breakage process and ultimately achieving controlled self-breakage of DSDG within 1-20 days. Temperature directly affects the rate at which the initiator decomposes into free radicals, which can initiate the polymerization of acrylamide and dopamine monomers.

[0113] The specific experimental methods are as follows:

[0114] Dopamine hydrochloride powder (DA) was added to a beaker containing a NaOH aqueous solution (pH 11) and allowed to autopolymerize under stirring for 20 minutes in an air atmosphere to form a PDA dispersion. AM, KPS, and PEGDA were then stirred and mixed with the PDA dispersion under nitrogen. After mixing for 10 minutes, dilute hydrochloric acid was added dropwise to adjust the pH of the solution to 3-11. Finally, approximately 80 mL of the prepared gel base solution was injected into a blue-capped glass bottle (maximum capacity 100 mL) and sealed with nitrogen. The container containing the gel base solution was stored in a constant temperature oven at 60-100°C and the gel formation and degradation behavior of the gel were observed.

[0115] Firstly, the gel strength code method was used to investigate the effects of DA / AM mass ratio, KPS concentration, monomer concentration, stirring time of PDA dispersion and PEGDA concentration on the degradation time and viscosity of the gel breaking solution at 80 ℃ and pH=11. Figure 14 Panel (a) shows the effect of the DA / AM mass ratio on the gel breaking time and gel breaking solution viscosity. As the DA / AM mass ratio increases, the gel degradation time first decreases and then increases, while the gel breaking solution viscosity decreases with the increasing DA / AM mass ratio. The experiments were conducted at fixed monomer concentrations of 5 wt%, PEGDA concentrations of 0.4 wt%, and initiator concentrations of 0.6 wt%. This is because as the DA / AM mass ratio increases, the number of PDA crosslinking sites within the gel network gradually increases. Furthermore, the enhanced reducibility of the gel solution further inhibits the PEGDA crosslinking reaction. Consequently, the proportion of PDA crosslinking sites in the resulting gel network increases, which, upon degradation, further damages the integrity of the gel network and ultimately shortens the degradation time of DSDG. However, increasing the DA / AM mass ratio also leads to the formation of more complex crosslinking structures (multiple crosslinks between DA, AM, and PEGDA). These structures increase the number of cleavage pathways required for degradation, making degradation more difficult and resulting in a longer gel degradation time. At the same time, as the DA / AM mass ratio increases, the polymer molecular weight decreases. Due to the hydrophilicity of PDA, it forms more hydrogen bonds with water molecules during the degradation process, making the polymer molecules more easily surrounded and dispersed by water molecules, thereby reducing the interactions between polymer molecules. This helps reduce the viscosity of the gel breaking solution. Increasing KPS concentration can also reduce the polymer molecular weight, and short polymer segments favor gel degradation. However, when the initiator concentration exceeds 0.6 wt%, the rate of reduction in DSDG degradation time gradually slows. This is because the polymer segment length approaches its minimum value after reaching a high initiator concentration. This is also confirmed by the change in the viscosity of the gel breaking solution in the figure.

[0116] Figure 14(b) shows the effect of initiator concentration on the gel breaking time and gel breaking solution viscosity. The degradation time of DSDG decreases with the increase of initiator concentration. In the experiment, the monomer concentration is 5 wt%, the PEGDA concentration is 0.4 wt%, and the DA / AM mass ratio is 5×10 -3 ; The viscosity of the gel-breaking liquid decreases with the increase of initiator concentration.

[0117] Figure 14 (c) shows the effect of monomer concentration on the gel-breaking time and gel-breaking solution viscosity. The degradation time of DSDG is prolonged with the increase of monomer concentration, and the gel-breaking solution viscosity is increased with the increase of initiator concentration. In the experiment, the PEGDA concentration is fixed at 0.4 wt% and the DA / AM mass ratio is 8×10 -3 , initiator concentration 0.6 wt%. Increasing the AM concentration increases the chain length, which is detrimental to gel degradation. It also forms a denser three-dimensional network structure, reducing the water content within the network and hindering the hydrolysis of ester groups and PDA, which in turn increases the gel viscosity after degradation.

[0118] Figure 14 (d) summarizes the effect of stirring time on gel degradation time and gel breaking solution viscosity when preparing PDA dispersion. In the experiment, the monomer concentration was fixed at 5 wt%, the PEGDA concentration was 0.4 wt%, and the DA / AM mass ratio was 8×10 -3 and an initiator concentration of 0.6 wt%. The PDA particle size is proportional to the length of the prepolymerization stirring time. As the stirring time increases, the PDA particle size gradually increases, the number of free DA monomers decreases, the reducing property gradually decreases, and its specific surface area also decreases. This leads to a gradual decrease in the number of PDA surface active sites and weakens the inhibitory effect on the PEGDA cross-linking reaction, resulting in a decrease in the proportion of PDA cross-linking points within the gel network, which is not conducive to the degradation of DSDG gel. However, the stirring process causes a large number of catechol groups to be oxidized, reducing the strength and adhesion of the gel. The low-strength gel network structure is loose, the stability is poor, and it is easily degraded. The viscosity of the gel breaker after degradation is also lower. Therefore, the viscosity of the gel breaker decreases with the increase of the stirring time when preparing the PDA dispersion.

[0119] Figure 14 (e) shows the effect of PEGDA concentration on the gel breaking time and gel breaking solution viscosity. With the increase of the concentration of PEGDA, another cross-linking agent in the DSDG formula, the gel degradation time first decreases and then increases, and the gel breaking solution viscosity first decreases and then increases. In the experiment, the monomer concentration was fixed at 5 wt% and the DA / AM mass ratio was 8×10 -3, initiator concentration 0.6 wt%. This is because the gel crosslinking time is too long at low PEGDA concentrations, and the PDA in the gel solution is almost completely degraded, losing its ability to affect the integrity of the gel network structure and inhibit monomer polymerization, resulting in a prolonged degradation time and a decrease in the viscosity of the gel-breaking solution. At the same time, an increase in PEGDA concentration will form more crosslinking points between polyacrylamide chains. Since PEGDA degrades longer than PDA, and the increase in PEGDA crosslinking points reduces the proportion of PDA crosslinking points in the gel network, the degradation time is prolonged and the viscosity of the gel-breaking solution increases.

[0120] The effects of the ambient temperature of DSDG gel and the pH value of DSDG base solution on the degradation time were also investigated. The monomer concentration was fixed at 5 wt%, PEGDA concentration was 0.4 wt%, and DA / AM mass ratio was 8×10 -3 , initiator concentration 0.6 wt%. Figure 14 Figure (f) shows the effect of temperature on the breaking time and viscosity of the breaking solution. Higher temperatures accelerate gel degradation. This is because higher ambient temperatures provide more energy, making it easier to reach the activation energy required for the degradation of ester bonds and PDA. Furthermore, higher temperatures promote rearrangement or repolymerization of degradation products and increase entanglement between degraded polymer segments, leading to an increase in gel viscosity after degradation.

[0121] Figure 14 (g) shows the effect of DSDG base solution pH on the breaking time and breaking solution viscosity. In the experiment, the monomer concentration was fixed at 5 wt%, the PEGDA concentration was 0.4 wt%, and the DA / AM mass ratio was 8×10 -3 , initiator concentration 0.6 wt%. Since the ester bond hydrolyzes faster under alkaline conditions than under acidic conditions, and PDA hydrolyzes faster under strongly alkaline conditions, the degradation time required for DSDG under alkaline conditions is shorter, and the degradation time decreases with increasing alkalinity. The gel-breaking time of DSDG increases with increasing acidity. This is because although PDA can accelerate degradation under acidic conditions, too low a pH value will cause the amino groups on the PDA surface to be protonated, making its structure more stable, thereby slowing down the degradation rate. Under neutral conditions, PDA is almost impossible to degrade, and the ester bond hydrolysis rate is much lower than that in acidic and alkaline environments. Therefore, the degradation time required for DSDG is the longest at pH = 7, and the gel-breaking solution has a low viscosity under different pH conditions.

[0122] Figure 14 (h) shows the classic formula: monomer concentration 5 wt%, PEGDA concentration 0.4 wt%, DA / AM mass ratio 8×10 -3The spontaneous degradation process of the gel prepared with 0.6 wt% initiator concentration at pH = 11 and 80 ℃ shows that the gel is gradually degraded rather than destroyed instantly. The effects of crosslinker PEGDA concentration and DA / AM mass ratio on gel breaking time were further tested, and the experimental results were used to draw a Figure 14 (i) and (j) in the figure demonstrate that DSDG can achieve controllable self-breakage within 1-20 days by adjusting the ratio of PEGDA and AM / DA in the formula.

[0123] Example 5: Sealing performance of gel

[0124] The preparation method of the DSDG base liquid in this embodiment is the same as that in Example 1.

[0125] Use Figure 2 The experimental setup shown here evaluates the pressure-bearing capacity of DSDG in wellbore and matrix core. The formulation was fixed at pH 11, monomer concentration 5 wt%, PEGDA concentration 0.4 wt%, and initiator concentration 0.6 wt%. The effect of the DA / AM mass ratio in the DSDG gel formulation on the pressure-bearing capacity was investigated using a wellbore model at 80°C. The experimental results are shown in Figure 2. Figure 15 The maximum pressure of DSDG has a peak value when the DA / AM mass ratio is lower than 7×10 -3 When the DA / AM mass ratio increases, the storage modulus of the gel gradually increases. The abundant catechol groups in PDA give the gel excellent adhesion properties on a variety of substrates and environments, and improve the cross-linking density of the gel network, so that the pressure bearing capacity of DSDG in the wellbore increases with the increase of the DA / AM mass ratio. -3 When the DA / AM mass ratio is greater than 7×10 -3 When the reducibility of the gel solution gradually increases, the formation of the gel network structure is inhibited, the crosslinking density gradually decreases, and finally the gel storage modulus gradually decreases. The gel with low storage modulus is easily deformed under the action of external force, resulting in liquid leakage along the wellbore wall and eventually plugging failure. Although when the DA / AM mass ratio is greater than 7×10 -3 The pressure bearing capacity of DSDG decreases with the increase of DA / AM mass ratio, but it is still strong when the DA / AM mass ratio is equal to 9×10 -3 Even with the addition of DA, the pressure still reached 0.45 MPa / m. This is higher than the control group without DA, which only had a pressure bearing capacity of 0.35 MPa / m. This is because the PAM-PEGDA hydrogel relies primarily on physical adsorption to bond with the substrate, a relatively weak binding force. Furthermore, the cross-linking density of the PAM-PEGDA gel is lower than that of a dual-cross-linked gel prepared from DA and PEGDA, resulting in a looser network structure and lower pressure bearing capacity.

[0126] Through homogeneous core displacement experiments, the changes in the gel system's plugging capacity caused by the DA / AM mass ratio and permeability in the absence of fractures were studied. The experiments were fixed at a monomer concentration of 5 wt%, a PEGDA concentration of 0.4 wt%, an initiator concentration of 0.6 wt%, a pH of 11, and a temperature of 80°C. As shown in Table 1, cores 1-3 were used to study the effect of permeability on the pressure bearing capacity of DSDG, and cores 4-7 were used to examine the effect of the DA / AM mass ratio on the pressure bearing capacity. The experimental results are shown in Table 1. Figure 16 shown. Figure 16 (a) shows a DA / AM mass ratio of 8×10 -3 The pressure bearing capacity of the gel at different permeabilities. During the first water flooding process, the core has a higher permeability. When the gel solution is injected into the homogeneous core, the injection pressure slowly increases as the viscosity of the gel solution increases during the gelation process. Among them, the injection pressure of the 5 mD homogeneous core is the highest, and the pressure reaches 0.61 MPa when the injection PV is 2 PV. After the injected hydrogel is gelled, the migration phenomenon in the homogeneous core after the gel breakthrough is observed during the second water flooding. After the first drop of liquid is discharged from the tail end of the core clamp, the gel sealing pressure continues to rise and stabilizes after continuing to inject water for a period of time. This is because DA-type gel has good elasticity and adhesion properties, and is not easy to break under high-pressure conditions. After breaking through under the action of pressure, it can still adapt to different channel shapes in the core pores through deformation and migration, and form a more stable sealing structure under the action of pore throats. Gel is easy to migrate in high-permeability cores, but its sealing performance is relatively weak. The pore structure of low-permeability cores is denser, and it is easier to form a stable sealing structure after gel injection. By Figure 16 As shown in (a), the gel has the best plugging performance in the 5 mD core, with a startup pressure of 119 MPa / m and a breakthrough pressure of 119 MPa / m. The most significant migration occurs in the 50 mD core, with a startup pressure of 34.6 MPa / m and a breakthrough pressure of 86.6 MPa / m.

[0127] Figure 16 (b) shows the DA / AM mass ratio in the formula (0×10 -3 , 5×10 -3 , 7×10 -3 , 9×10 -3 ) at the same permeability (10 mD) on the pressure bearing capacity. The pressure difference during the first water injection process ranged from 0.09 to 0.17 MPa. After injecting the gel solution into the homogeneous core, it was observed that the DA / AM mass ratio was 5×10 -3 -9×10 -3The injection pressure of the three groups of formulas rose at a slow rate, and the maximum pressure only reached 0.54 MPa after injecting 2 PV gel solution. However, the pressure of the control group formula without DA added reached as high as 1.52 MPa after injecting 2 PV. This shows that the introduction of DA improves the injection performance of the gel. This is because the strong reducing property of the DA monomer inhibits the formation of the gel, allowing the gel with added DA to remain in a low viscosity state for a longer time. After the injected hydrogel was gelled, it was observed during the secondary water flooding that the three groups of gels with the introduction of DA all migrated in the core after breaking through. In the control group experiment without DA added, the pressure reached a stable value very shortly after the breakthrough, and no obvious migration occurred. This is because the mechanical strength of the pure PAM gel is low, and it is easy to break under the action of fluid shear force, losing the ability to migrate further. Among them, the DA / AM mass ratio is 7×10 -3 The time migration phenomenon is most obvious, with a starting pressure of 110.2 MPa / m and a breakthrough pressure of 184.6 MPa / m. When the DA / AM mass ratio is lower than / higher than 7×10 -3 When the elastic modulus of DSDG is low, it is easier to deform under external force, and the plugging structure is more likely to be destroyed under the action of fluid pressure, resulting in plugging failure, as discussed previously. -3 , 9×10 -3 When DA was added, the starting pressures reached 79.6 MPa / m and 68.0 MPa / m, respectively, and the breakthrough pressures reached 154.8 MPa / m and 119.4 MPa / m, respectively. This was still higher than the control group without DA, which had a starting pressure of only 2.78 MPa / m and a breakthrough pressure of only 3.08 MPa / m. Therefore, the DSDG gel system shows great potential for plugging porous media, mainly core rocks.

[0128] The formation protection performance of temporary plugging agents is key to effectively removing the plug and restoring formation permeability after temporary plugging. Using artificial rock cores with permeabilities of 5-50 mD (Tables 1, 1-7), the formation protection performance of the different formulations listed in Table 2 at pH = 11 and 80°C was evaluated by comparing the water flooding pressure before injection and the water flooding pressure after gel breakdown. All formulations were fixed at a monomer concentration of 5 wt%, a PEGDA concentration of 0.4 wt%, and an initiator concentration of 0.6 wt%. The initial water flooding pressure curves before gel solution injection and after degradation are shown in Figure 2. Figure 16The water drive pressure before injection and after gel breaking are shown in Table 2. As shown in Table 2, the water drive pressure after gel breaking of the prepared DSDG in artificial cores of 5-50 mD did not exceed the water drive pressure before injection. This is partly because DSDG can completely break after injection into the core, resulting in a low core damage rate. On the other hand, the gel squeezes and releases the core pores during the plugging and unplugging process, causing some tiny pores to merge or expand, improving pore connectivity, thereby increasing the core permeability and ultimately reducing the water drive pressure after the core is unplugged. In contrast, the water drive pressure after gel breaking of the control group without DA was much greater than the water drive pressure before injection. The above results indicate that the matrix core can be completely displaced from the spontaneously degraded gel breaking fluid of DSDG, with little impact on the formation permeability and productivity, and excellent formation protection performance.

[0129] Table 2 Gel plugging performance in homogeneous core

[0130] .

Claims

1. A self-degradable double-crosslinked hydrogel temporary plugging agent, characterized by: The raw materials for preparing the self-degradable double-crosslinked hydrogel temporary plugging agent include the following components in percentage by weight based on the total mass of water: 4%-6% acrylamide, 0.5%-0.8% initiator, 0.2%-0.6% polyethylene glycol-diacrylate and polydopamine dispersion; The polydopamine dispersion is obtained by dissolving dopamine hydrochloride in an alkaline aqueous solution and polymerizing the solution; The mass ratio of dopamine hydrochloride to acrylamide is 0.005:1-0.009:1; The initiator is potassium persulfate and / or ammonium persulfate; The self-degradable double-crosslinked hydrogel temporary plugging agent forms gel at a pH value of 3-11 and a temperature of 60-100°C.

2. The self-degradable double-crosslinked hydrogel temporary plugging agent according to claim 1, characterized in that: The pH of the alkaline aqueous solution is 8-12; The alkaline aqueous solution is sodium hydroxide solution and / or ammonia water.

3. The self-degradable double-crosslinked hydrogel temporary plugging agent according to claim 1, characterized in that: The raw materials for preparing the self-degradable double-crosslinked hydrogel temporary plugging agent include the following components in percentage by weight based on the total mass of water: 4%-6% acrylamide, 0.5%-0.8% initiator, 0.3%-0.6% polyethylene glycol-diacrylate and polydopamine dispersion; The mass ratio of the dopamine hydrochloride to acrylamide is 0.005:1-0.008:

1.

4. A method for preparing the self-degradable double-crosslinked hydrogel temporary plugging agent according to any one of claims 1 to 3, comprising the steps of: mixing the acrylamide, an initiator, polyethylene glycol-diacrylate, and a polydopamine dispersion, and adjusting the pH value of the system to 3-11 to obtain the self-degradable double-crosslinked hydrogel temporary plugging agent.

5. The preparation method according to claim 4, characterized in that: The preparation method of the polydopamine dispersion comprises the following steps: dissolving dopamine hydrochloride in an alkaline aqueous solution and polymerizing in an air atmosphere to obtain the polydopamine dispersion; The pH value of the system was adjusted using hydrochloric acid solution.

6. The preparation method according to claim 5, characterized in that: In the preparation method of the polydopamine dispersion, the polymerization time is 20-50 minutes.

7. Use of the self-degradable double-crosslinked hydrogel temporary plugging agent according to any one of claims 1 to 3 in improving the recovery rate of oil and gas reservoirs.

8. The use according to claim 7, characterized in that: The reservoir temperature of the oil and gas reservoir is 60-100°C.

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