Temporary plugging agent for acid fracturing, preparation method of temporary plugging agent and temporary plugging composition containing temporary plugging agent

Abstract

The invention relates to the technical field of fracturing, in particular to a temporary plugging agent for acid fracturing, a preparation method of the temporary plugging agent and a temporary plugging composition containing the temporary plugging agent, and the temporary plugging agent comprises the following components in percentage by weight: 0.1%-10% of a PGA-CS block copolymer, 5%-10% of acrylamide, 0.1%-2% of 2-acrylamido-2-methylpropanesulfonic acid and 0.1%-2% of methylene bisacrylamide. Wherein the PGA-CS block copolymer is prepared from the following components in percentage by weight: 0.1 to 10 percent of PEG (Polyethylene Glycol) chitosan and 0.1 to 10 percent of polyglycolide; the PEGylated chitosan is obtained by modifying chitosan by using polyethylene glycol; the temporary plugging composition comprises 1%-15% of a temporary plugging agent and 1%-10% of a carrying agent, and the carrying agent comprises 0.1%-10% of chitosan, 0.1%-5% of cellulose nanocrystals, 0.1%-5% of hydroxymethyl acrylamide and 0.1%-3% of imidazole. According to the temporary plugging agent, the preparation method thereof and the temporary plugging composition containing the temporary plugging agent, the plugging and unplugging effects can be effectively improved.

Description

Technical Field

[0001] The present invention relates to the technical field of fracturing, and particularly to a temporary plugging agent for acid fracturing, a preparation method thereof, and a temporary plugging composition containing the same. Background Art

[0002] Acid fracturing has always been the main measure for increasing production in carbonate reservoirs. The geological characteristics of carbonate reservoirs are complex, the fracture-cavity system is developed, and acid fluid filtration loss is serious, resulting in very low utilization rate of acid fluid during acid fracturing. Acid fracturing transformation must be based on communicating the fracture-cavity developed reservoir body to achieve the purpose of increasing production. Through on-site construction, it is found that the biggest problem in acid fracturing is that the length of the acid-etched fracture is too short to communicate the fracture-cavity system in the far-well zone, which is mainly affected by acid-rock reaction and acid fluid filtration loss. Adding a temporary plugging agent to the preflush fluid of acid fracturing is a common method for carbonate reservoir filtration loss. Commonly used acid fracturing temporary plugging agents mainly include powdered ceramics, degradable fibers, oil-soluble resins, etc.

[0003] In recent years, most of the temporary plugging agents used in acid fracturing are water-soluble temporary plugging agents, and their raw materials are mostly composite products of soluble (degradable) high molecular polymers. However, in high-temperature and high-acid oil and gas well environments, water-soluble temporary plugging agents are easily broken through by acid fluid, have a fast dissolution rate, greatly reduce the pressure-bearing strength, and have low plugging performance; during the plug removal process, soluble (degradable) high molecular polymers are prone to carbonization and are not easily unblocked. Therefore, the current temporary plugging agents cannot play a normal plugging and unplugging role in high-temperature and high-acid environments, the effect is unstable, which directly affects the effect of plugging measures and the output of oil and natural gas. Summary of the Invention

[0004] To solve the above technical problems, the present invention provides a temporary plugging agent for acid fracturing, a preparation method thereof, and a temporary plugging composition containing the same. The prepared temporary plugging agent has the characteristics of high pressure-bearing strength, controllable dissolution time, and strong acid resistance, can replace the existing acid fracturing temporary plugging agent products, significantly improve the construction effect of carbonate temporary plugging acid fracturing, reduce environmental pollution and personnel injuries; the prepared temporary plugging composition comprehensively utilizes the developed temporary plugging agent and CO2-responsive gel carrier, and has better plugging and unplugging effects.

[0005] The present invention is realized by adopting the following technical solutions: A temporary plugging agent for acid fracturing, calculated by weight percentage, includes 0.1% - 10% of PGA-CS block copolymer, 5% - 10% of acrylamide, 0.1% - 2% of 2-acrylamido-2-methylpropanesulfonic acid, and 0.1% - 2% of methylenebisacrylamide; wherein, the PGA-CS block copolymer includes 0.1% - 10% of PEGylated chitosan and 0.1% - 10% of polyglycolide; the PEGylated chitosan is obtained by modifying chitosan with polyethylene glycol.

[0006] It also includes α-ketoglutaric acid, and the α-ketoglutaric acid accounts for 0.3% of the weight of acrylamide.

[0007] The PEGylated chitosan is obtained by modifying chitosan with polyethylene glycol, and specifically includes the following steps: Step S 11 . Prepare methoxypolyethylene glycol acid and an aqueous chitosan acetate solution; Step S 12 . By weight percentage, add 0.1% - 5% of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, 0.1% - 5% of N-hydroxysuccinimide, and 1% - 15% of the aqueous chitosan acetate solution to the methoxypolyethylene glycol acid, react at room temperature and then dry and granulate to obtain PEGylated chitosan.

[0008] The preparation method of the PGA-CS block copolymer includes the following steps: Step S 21 . Add 0.1% - 10% of polyglycolide into water, keep the temperature below 30°C, let the polyglycolide particles swell sufficiently, and then raise the temperature to 60°C until the polyglycolide particles are completely dissolved; Step S 22 . Slowly add 0.1% - 10% of the PEGylated chitosan into an aqueous acetic acid solution with a concentration of 0.5% to obtain a viscous gel solution, and then add the aqueous polyglycolide solution in Step S 21 . Continue to stir for 2 h to obtain the PGA-CS block copolymer.

[0009] A preparation method of a temporary plugging agent for acid fracturing includes the following steps: Step S 41 . By weight percentage, add 5% - 10% of acrylamide, 0.1% - 2% of 2-acrylamido-2-methylpropanesulfonic acid, 0.1% - 2% of methylenebisacrylamide, and 0.1% - 10% of the PGA-CS block copolymer into water, stir to dissolve them, and then quickly add α-ketoglutaric acid accounting for 0.3% of the weight of acrylamide, stir evenly, and react for 2 h; Step S 42 . After the reaction product obtained in the above Step S 41 . is naturally dried, cut into pieces and dried to a constant weight to obtain a dry block, and the temporary plugging agent for acid fracturing is prepared.

[0010] A temporary plugging composition for acid fracturing, by weight percentage, includes 1% - 15% of a temporary plugging agent and 1% - 10% of a carrier; the carrier includes 0.1% - 10% of chitosan, 0.1% - 5% of cellulose nanocrystals, 0.1% - 5% of hydroxymethylacrylamide, and 0.1% - 3% of imidazole.

[0011] The cellulose nanocrystals are prepared from microcrystalline cellulose by an acid hydrolysis method.

[0012] The preparation method of the carrier comprises the following steps: Step S 31 . Add chitosan into an ethyl acetate solution with a concentration of 0.5% - 1%, and stir at room temperature until completely dissolved; Step S 32 . Uniformly disperse the cellulose nanocrystals in water, mix and stir evenly with the solution in Step S 31 to obtain a mixed solution; Step S 33 . Add hydroxymethylacrylamide into the mixed solution in Step S 32 . Stir until evenly mixed; evacuate and introduce nitrogen, raise the temperature to 60°C, inject an APS solution with a concentration of 5%, and react for 6 hours; Step S 34 . Add imidazole into the solution prepared in Step S 33 to obtain the carrier.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. In the present invention, first, polyethylene glycol (PEG) modification of chitosan (CS) is carried out, and the modified chitosan and polyglycolide (PGA) form a PGA-CS block copolymer. Then, acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, and methylenebisacrylamide are added into the block copolymer solution to generate a double-network polymer product, greatly increasing the gel strength, making it have stronger pressure-bearing capacity, plugging ability, erosion resistance ability, etc.

[0014] The acid-resistant temporary plugging agent of the present invention has the characteristics of high pressure-bearing strength, controllable dissolution time, strong acid resistance, simple processing technology, low cost, environmental friendliness, excellent regeneration ability, etc., can replace the existing acid fracturing temporary plugging agent products, significantly improve the construction effect of carbonate rock temporary plugging acid fracturing, and reduce environmental pollution and personnel injuries.

[0015] 2. In the present invention, the polyethylene glycolylation (PEGylation) of chitosan (CS) can fully maintain the biocompatibility of chitosan (CS), and realize its dissolution in aqueous solutions within a wider pH range through the selection of the PEG polymerization degree and the control of the grafting degree.

[0016] 3. In the present invention, the carrier is prepared by using chitosan, microcrystalline cellulose, hydroxymethylacrylamide, imidazole, etc., and a CO2-responsive sol can be obtained. When this carrier encounters CO2, it can cause the solution to gel, improving the temporary plugging effect. After the CO2 disappears, the gelation process is reversible, forming a low-viscosity sol, which is convenient for the degradation of the temporary plugging agent and the backflow of the residual acid. During the acid fracturing process, the acid reacts with the carbonate rock to generate CO2, which can make the CO2-responsive gel carrier play a corresponding role, making the carrier prepared in the present invention particularly suitable for acid fracturing construction.

[0017] 4. The temporary plugging composition for acid fracturing in the present invention includes a temporary plugging agent and a carrier. By comprehensively using the newly developed acid-resistant temporary plugging agent and the CO2-responsive gel carrier, the synergistic effect of each component is fully utilized to achieve self-adaptive plugging, with the effect of 1 + 1 > 2, which can improve the effect of the temporary plugging acid fracturing construction.

[0018] 5. In the present invention, PGA, PEG, etc. are all excellent biodegradable materials, which are green and environmentally friendly, and the degradation time is controllable.

[0019] 6. In the present invention, both the temporary plugging agent and the carrier use chitosan. Chitosan is widely distributed in nature and its reserves are only second to cellulose, being the second largest natural polymer. The annual biosynthesis amount of chitin is about 10 billion tons. It is a recyclable renewable resource, inexhaustible. These natural polymers are mainly distributed in coastal areas, countries such as India, Poland, Japan, the United States, Norway, and Australia. Chitosan has been commercially produced. Using it to produce the acid-resistant temporary plugging agent and the carrier can reduce the material cost.

[0020] 7. The preparation method in the present invention fully utilizes the advantages of high gel strength and good performance of block copolymers and interpenetrating network gels, and can be carried out at a relatively low temperature (not exceeding 80 °C), avoiding high-temperature preparation and facilitating production operations. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The present invention will be further described in detail below in conjunction with the specification drawings and specific embodiments, where: Figure 1 It is a schematic diagram of the dissolution rate of the temporary plugging agent in the present invention under the acidic condition of 20% mass fraction of HCL. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] Example 1 A temporary plugging agent for acid fracturing, by weight percentage, comprises 0.1% - 10% of PGA-CS block copolymer, 5% - 10% of acrylamide, 0.1% - 2% of 2-acrylamido-2-methylpropanesulfonic acid, and 0.1% - 2% of methylenebisacrylamide. Among them, the PGA-CS block copolymer comprises 0.1% - 10% of PEGylated chitosan and 0.1% - 10% of polyglycolide. The PEGylated chitosan is obtained by modifying chitosan with polyethylene glycol.

[0023] Specifically, the preparation method of the temporary plugging agent for acid fracturing comprises the following steps: Step 1. Prepare PEGylated chitosan. Among them, the PEGylated chitosan is obtained by modifying chitosan with polyethylene glycol, and specifically comprises the following steps: Step S 11 . Prepare methoxypolyethylene glycol acid and chitosan acetic acid aqueous solution.

[0024] Add 0.5% of KMnO4 into the polyethylene glycol methyl ether mPEG solution, heat up to 60°C and react for 2 h to generate methoxypolyethylene glycol acid mPEG-COOH.

[0025] Dissolve chitosan in an acetic acid solution with a concentration of 5% - 20%, and fully dissolve it to obtain a chitosan acetic acid aqueous solution.

[0026] Step S 12 . By weight percentage, add 0.1% - 5% of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, 0.1% - 5% of N-hydroxysuccinimide, and 1% - 15% of the chitosan acetic acid aqueous solution into the methoxypolyethylene glycol acid, react at room temperature for 3.0 h, and then dry and granulate to obtain PEGylated chitosan.

[0027] Step 2. Prepare PGA-CS block copolymer. The preparation method of the PGA-CS block copolymer comprises the following steps: Step S 21 . Add 0.1% - 10% of polyglycolide PGA into water, keep the temperature below 30°C, and place it for 1 h to fully swell the polyglycolide PGA particles, and then heat up to 60°C until the polyglycolide PGA particles are fully dissolved.

[0028] Step S 22 . Slowly add 0.1% - 10% of the PEGylated chitosan prepared in Step 1 into an acetic acid aqueous solution with a concentration of 0.5% to obtain a viscous gel solution, and then add the polyglycolide aqueous solution in Step S 21 . and continue stirring for 2 h to obtain the PGA-CS block copolymer.

[0029] Step 3. Add 5% - 10% acrylamide (AM), 0.1% - 2% 2 - acrylamido - 2 - methylpropanesulfonic acid (AMPS), 0.1% - 2% methylenebisacrylamide (MBAM), and 0.1% - 10% PGA - CS block copolymer into water, stir to dissolve them, then quickly add α - ketoglutaric acid accounting for 0.3% of the weight of acrylamide into the solution, stir evenly, and react for 2 h.

[0030] Step 4. Pour the reaction product obtained in Step 3 onto a container, spread it into a cake shape, after natural drying, cut it into blocks with a particle size diameter of 1 - 6 mm, dry it to a constant weight in an oven at 100 °C to obtain dry blocks, which are the acid - resistant temporary plugging agent for acid fracturing.

[0031] An acid fracturing temporary plugging composition, by weight percentage, includes 1% - 15% of the above - mentioned temporary plugging agent and 1% - 10% of a carrier. The carrier includes 0.1% - 10% chitosan, 0.1% - 5% cellulose nanocrystals, 0.1% - 5% hydroxymethylacrylamide, and 0.1% - 3% imidazole.

[0032] Among them, the preparation method of the acid fracturing temporary plugging composition is as follows: Step 1. Prepare the carrier. The preparation method of the carrier includes the following steps: Step S 31 . Add 0.1% - 10% chitosan by weight percentage into an ethyl acetate solution with a concentration of 0.5% - 1%, and stir at room temperature until completely dissolved.

[0033] Step S 32 . Prepare cellulose nanocrystals from microcrystalline cellulose by acid hydrolysis method. Uniformly disperse 0.1% - 5% cellulose nanocrystals in water, and mix and stir evenly with the solution in Step S 31 to obtain a mixed solution.

[0034] Step S 33 . Add 0.1% - 5% hydroxymethylacrylamide into the mixed solution in Step S 32 , stir until evenly mixed. Evacuate and introduce nitrogen, raise the temperature to 60 °C, inject an APS solution with a concentration of 5%, and react for 6 hours.

[0035] Step S 34 . Add 0.1% - 3% imidazole into the solution prepared in Step S 33 to obtain a CO2 - responsive sol, which is used as the carrier of the acid fracturing temporary plugging agent. This carrier can cause the solution to gel when encountering CO2, improving the temporary plugging effect. After the CO2 disappears, the gelation process is reversible, forming a low - viscosity sol, which is convenient for the degradation of the temporary plugging agent and the back - flow of spent acid.

[0036] Step 2. Add 1% - 15% of the temporary plugging agent and 1% - 10% of the carrier into water, and stir evenly to obtain the temporary plugging composition for acid fracturing.

[0037] Example 2 This example includes a temporary plugging agent for acid fracturing. By weight percentage, the temporary plugging agent for acid fracturing includes 10% of PGA-CS block copolymer, 5% of acrylamide, 0.5% of 2-acrylamido-2-methylpropanesulfonic acid, and 0.1% of methylenebisacrylamide. The PGA-CS block copolymer includes 10% of PEGylated chitosan and 5% of polyglycolide.

[0038] Among them, the preparation method of the PEGylated chitosan specifically includes the following steps: Step S 11 . Prepare methoxypolyethylene glycol acid and chitosan acetic acid aqueous solution.

[0039] Add 0.5% of KMnO4 into the methoxypolyethylene glycol mPEG solution, heat up to 60°C and react for 2 h to generate methoxypolyethylene glycol acid mPEG-COOH.

[0040] Dissolve chitosan in acetic acid solution with a concentration of 20%, and dissolve it fully to obtain chitosan acetic acid aqueous solution.

[0041] Step S 12 . By weight percentage, add 5% of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, 3% of N-hydroxysuccinimide, and 10% of chitosan acetic acid aqueous solution into methoxypolyethylene glycol acid, react at room temperature for 3.0 h and then dry and granulate to obtain PEGylated chitosan.

[0042] Using the PEGylated chitosan prepared in this example, combined with the preparation method of the PGA-CS block copolymer and the preparation method of the temporary plugging agent for acid fracturing in Example 1, the temporary plugging agent A for acid fracturing is prepared in this example.

[0043] Example 3 This example includes a temporary plugging agent for acid fracturing. By weight percentage, the temporary plugging agent for acid fracturing includes 5% of PGA-CS block copolymer, 5% of acrylamide, 0.5% of 2-acrylamido-2-methylpropanesulfonic acid, and 0.5% of methylenebisacrylamide. The PGA-CS block copolymer includes 3% of PEGylated chitosan and 0.5% of polyglycolide.

[0044] Among them, the preparation method of the PEGylated chitosan specifically includes the following steps: Step S 11 . Prepare methoxypolyethylene glycol acid and chitosan acetic acid aqueous solution.

[0045] Add 0.5% KMnO4 to the mPEG solution of polyethylene glycol methyl ether, heat it to 60 °C and react for 2 h to generate methoxypolyethylene glycol acid mPEG-COOH.

[0046] Dissolve chitosan in a 5% acetic acid solution and dissolve it fully to obtain an aqueous chitosan acetate solution.

[0047] Step S 12 . Add 0.1% 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, 5% N-hydroxysuccinimide and 1% aqueous chitosan acetate solution to methoxypolyethylene glycol acid by weight percentage, react at room temperature for 3.0 h and then dry and granulate to obtain PEGylated chitosan.

[0048] Using the PEGylated chitosan prepared in this example, combining with the preparation method of the PGA-CS block copolymer and the preparation method of the temporary plugging agent for acid fracturing in Example 1, the temporary plugging agent B for acid fracturing is prepared in this example.

[0049] Example 4 This example includes a carrier. By weight percentage, the carrier includes 0.5% chitosan, 0.5% cellulose nanocrystals, 0.5% hydroxymethylacrylamide and 0.5% imidazole. The preparation method of this carrier includes the following steps: Step S 31 . Add chitosan to a 1% ethyl acetate solution and stir at room temperature until completely dissolved; Step S 32 . Disperse cellulose nanocrystals evenly in water, mix and stir evenly with the solution in step S 31 to obtain a mixed solution; Step S 33 . Add hydroxymethylacrylamide to the mixed solution in step S 32 , stir until evenly mixed; evacuate and introduce nitrogen, raise the temperature to 60 °C, inject a 5% APS solution, and react for 6 hours; Step S 34 . Add imidazole to the solution prepared in step S 33 to obtain the carrier.

[0050] Through the above preparation method, the carrier is prepared in this example.

[0051] Example 5 This embodiment includes a temporary plugging composition for acid fracturing. By weight percentage, the temporary plugging composition for acid fracturing includes 3% of a temporary plugging agent and 5% of a carrier, with the balance being water. The temporary plugging agent is the acid fracturing temporary plugging agent B prepared in Example 3, and the carrier is the carrier prepared in Example 4.

[0052] A comparative test was conducted on a conventional water-soluble temporary plugging agent, the acid fracturing temporary plugging agent B prepared in Example 3, and the acid fracturing temporary plugging composition prepared in Example 5. The test results are as follows: For a fracture width of 3 mm, when using a conventional PVA+PLA water-soluble temporary plugging agent, the fracture mouth pressure continuously increased within 30 s; the mouth pressure reached the peak pressure of 8.0 MPa at 145 s. Thereafter, the mouth pressure rapidly fluctuated and decreased. The large-particle temporary plugging agent bridged and accumulated at the mouth to form a filter cake to plug the fracture. The thickness of the filter cake was 0.7 cm, and a small part entered the fracture to form a filtration loss shielding layer to jointly plug the fracture; the pressure-bearing capacity of this temporary plugging layer was 8.0 MPa.

[0053] For a fracture width of 3 mm, when using the acid fracturing temporary plugging agent B prepared in Example 3, the fracture mouth pressure continuously increased within 40 s; the mouth pressure reached the peak pressure of 11.3 MPa at 190 s. Thereafter, the mouth pressure rapidly fluctuated and decreased until it reached 0 MPa. The large-particle temporary plugging agent bridged and accumulated at the mouth to form a filter cake to plug the fracture. The thickness of the filter cake was 1.1 cm, and a small part entered the fracture to form a filtration loss shielding layer to jointly plug the fracture; the pressure-bearing capacity of this temporary plugging layer was 11.3 MPa.

[0054] For a fracture width of 3 mm, when using the acid fracturing temporary plugging composition prepared in Example 5, the fracture mouth pressure continuously increased within 35 s; the mouth pressure reached the peak pressure of 14.2 MPa at 155 s. Thereafter, the mouth pressure rapidly fluctuated and decreased until it reached 0 MPa. The large-particle temporary plugging agent bridged and accumulated at the mouth to form a filter cake to plug the fracture. The thickness of the filter cake was 1.3 cm, and a small part entered the fracture to form a filtration loss shielding layer to jointly plug the fracture; the pressure-bearing capacity of this temporary plugging layer was 14.2 MPa.

[0055] The above comparative test shows that compared with the conventional PVA+PLA water-soluble temporary plugging agent, the acid fracturing temporary plugging agent in this application has a better plugging effect, and the plugging effect is better when the acid fracturing temporary plugging agent is used in combination with the carrier.

[0056] The acid fracturing temporary plugging agent A prepared in Example 2 was subjected to an acid resistance test.

[0057] Experimental materials: The acid fracturing temporary plugging agent A prepared in Example 2 and hydrochloric acid.

[0058] Experimental equipment: Constant temperature water bath, electronic stirrer, electronic balance (minimum scale 0.001 g), measuring cylinder, beaker, rubber head dropper, glass rod.

[0059] Experimental steps: 1) Weigh 3 g of temporary plugging agent A for acid fracturing with an electronic balance; 2) Measure 200 ml of hydrochloric acid with a mass fraction of 20% using a measuring cylinder and place it in a 250 ml beaker; 3) At room temperature, add temporary plugging agent A for acid fracturing to the beaker and stir evenly; gradually heat up to the experimental temperature, observe the dissolution of the temporary plugging agent in the acid solution, pass the insoluble matter through a 60-mesh sieve, dry and weigh it, and calculate the dissolution ratio; 4) Repeat steps 1 - 3 to test the dissolution rate of the temporary plugging agent at different temperatures.

[0060] As can be seen from the attached Figure 1 specification, at 30°C, the temporary plugging agent A for acid fracturing hardly dissolves, and the particulate temporary plugging agent has the phenomenon of hydration and viscosity increase. During the construction process, attention should be paid to properly controlling the degree of particle hydration to prevent pump sticking and wellbore problems, which may affect the construction. The increase in temperature has a great impact on the dissolution of the temporary plugging agent A for acid fracturing. At 60°C, the initial degradation of the temporary plugging agent A for acid fracturing is relatively rapid, and it can be completely degraded in about 10 hours. The viscosity of the solution after degradation is equivalent to that of water, indicating that the temporary plugging agent degrades relatively completely and is easy to flow back after degradation, causing less damage to the reservoir.

[0061] In summary, after reading the present invention document, various other corresponding transformation schemes made by those of ordinary skill in the art without creative mental labor according to the technical solutions and technical concepts of the present invention all fall within the scope protected by the present invention.

Claims

1. A temporary plugging agent for acid fracturing, characterized in that: Comprising, by weight percentage, 0.1% - 10% of PGA-CS block copolymer, 5% - 10% of acrylamide, 0.1% - 2% of 2-acrylamido-2-methylpropanesulfonic acid, and 0.1% - 2% of methylenebisacrylamide; wherein, the PGA-CS block copolymer comprises 0.1% - 10% of PEGylated chitosan and 0.1% - 10% of polyglycolide; the PEGylated chitosan is obtained by modifying chitosan with polyethylene glycol.

2. The temporary plugging agent for acid fracturing according to claim 1, characterized in that: It further comprises α-ketoglutaric acid, and the α-ketoglutaric acid accounts for 0.3% of the weight of acrylamide.

3. The temporary plugging agent for acid fracturing according to claim 1, characterized in that: The PEGylated chitosan is obtained by modifying chitosan with polyethylene glycol, and specifically comprises the following steps: Step S 11 . Prepare methoxypolyethylene glycol acid and an aqueous chitosan acetate solution; Step S 12 . Add 0.1% - 5% of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide, 0.1% - 5% of N-hydroxysuccinimide, and 1% - 15% of chitosan acetic acid aqueous solution to methoxypolyethylene glycol acid by weight percentage. After reacting at room temperature, dry and granulate to obtain PEGylated chitosan.

4. The temporary plugging agent for acid fracturing according to claim 1, characterized in that: The preparation method of the PGA-CS block copolymer comprises the following steps: Step S 21 . Add 0.1% - 10% of polyglycolide into water, keep the temperature below 30°C, let the polyglycolide particles swell sufficiently, and then raise the temperature to 60°C until the polyglycolide particles are fully dissolved; Step S 22 . Slowly add 0.1% - 10% PEGylated chitosan into an aqueous acetic acid solution with a concentration of 0.5% to obtain a viscous gel solution, and then add the polyglycolide aqueous solution in Step S 21 . Continue stirring for 2 h to obtain a PGA-CS block copolymer.

5. A preparation method of the temporary plugging agent for acid fracturing according to any one of claims 1 to 4, characterized in that: Comprising the following steps: Step S 41 . Add 5% - 10% acrylamide, 0.1% - 2% 2 - acrylamido - 2 - methylpropanesulfonic acid, 0.1% - 2% methylenebisacrylamide, and 0.1% - 10% PGA - CS block copolymer to water by weight percentage, stir to dissolve it, then quickly add α - ketoglutaric acid accounting for 0.3% of the weight of acrylamide thereto, stir evenly, and react for 2 h; Step S 42 . After naturally drying the reaction product obtained from the above Step S 41 . The reaction product is chopped and dried to a constant weight to obtain a dry block, and a temporary plugging agent for acid fracturing is prepared.

6. A temporary plugging composition for acid fracturing, characterized in that: Comprising, by weight percentage, 1% - 15% of the temporary plugging agent according to any one of claims 1 - 4 and 1% - 10% of the carrier; the carrier comprises 0.1% - 10% of chitosan, 0.1% - 5% of cellulose nanocrystals, 0.1% - 5% of hydroxymethylacrylamide, and 0.1% - 3% of imidazole.

7. The temporary plugging composition for acid fracturing according to claim 6, characterized in that: The cellulose nanocrystals are prepared by an acid hydrolysis method from microcrystalline cellulose.

8. The temporary plugging composition for acid fracturing according to claim 6, characterized in that: The preparation method of the carrier comprises the following steps: Step S 31 . Add chitosan into an ethyl acetate solution with a concentration of 0.5% - 1%, and stir at room temperature until it is completely dissolved; Step S 32 . Uniformly disperse cellulose nanocrystals in water and mix and stir evenly with the solution in Step S 31 to obtain a mixed solution; Step S 33 . Add hydroxymethyl acrylamide to the mixed solution in Step S 32 , stir until evenly mixed; evacuate and introduce nitrogen, raise the temperature to 60 °C, inject an APS solution with a concentration of 5%, and react for 6 hours; Step S 34 . Add imidazole to the solution obtained in Step S 33 to obtain a carrier.

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