Water-soluble temporary plugging agent for oil well fracturing and preparation method thereof

Abstract

The invention relates to the technical field of temporary plugging agents, and discloses a water-soluble temporary plugging agent for oil well fracturing and a preparation method thereof. According to the prepared water-soluble temporary plugging agent, sodium lignin sulfonate / graphene oxide serves as a core, then the surface of the core layer is coated with a polyacrylic acid-polyacrylamide copolymer to form a first shell layer, and then the surface of the first shell layer is coated with acrylate modified chitosan to form a second shell layer; the temporary plugging agent is applied to oil well fracturing, not only has excellent plugging performance, dissolving performance and anti-shearing performance, but also can improve the transportation capacity of the temporary plugging agent in the oil well fracturing process.

Description

Technical Field

[0001] The present invention relates to the technical field of temporary plugging agents, and in particular to a water-soluble temporary plugging agent for oil well fracturing and a preparation method thereof. Background Art

[0002] Hydraulic fracturing is one of the core technologies for increasing oil and gas field production. It forms a fracture network by injecting high-pressure fluid into the formation and improving the permeability of the reservoir. In multi-layer or heterogeneous reservoirs, temporary plugging technology is needed to achieve directional expansion of fractures or repeated fracturing to achieve a balanced transformation effect. Temporary plugging agents are key materials that need to achieve precise control of temporary plugging and unplugging under specific conditions. Traditional temporary plugging agents (such as inert particles, fibers or chemically cross-linked gels) have the following limitations: (1) Insoluble residues: quartz sand, polymer gels, etc. can easily cause reservoir damage and reduce conductivity; (2) Incomplete unplugging: backflow or acid unplugging is required, which increases operation cost and complexity.

[0003] Water-soluble temporary plugging agents have emerged as the times require. Their core advantages lie in their self-degradation properties, controllability, and environmental friendliness. These advantages make water-soluble temporary plugging agents consistent with the trend of green oil and gas field development. However, the most commonly used acrylamide polymer water-based temporary plugging agents currently have some problems during use: (1) They absorb water and expand too quickly during use, resulting in complete expansion before reaching the target position, which leads to a low plugging rate; (2) They lack sufficient stability and strength, making it easy for the structure to be damaged and the membrane to disintegrate. Therefore, in response to the problems existing in existing water-soluble temporary plugging agents, researchers need to develop a water-soluble temporary plugging agent with excellent strength, water solubility, and high plugging rate to meet practical application needs. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention provides a water-soluble temporary plugging agent for oil well fracturing and a preparation method thereof.

[0005] The purpose of the present invention can be achieved through the following technical solutions: A method for preparing a water-soluble temporary plugging agent for oil well fracturing comprises the following steps: Step S1: 10 g of acrylic acid, 3-3.2 g of sodium hydroxide, and 50 mL of water were mixed and stirred uniformly, and reacted in an ice-water bath until the neutralization degree of the acrylic acid aqueous solution reached 60%-70%. 9-9.5 g of acrylamide, 0.18-0.21 g of aluminum nitrate, and 0.15-0.17 g of potassium persulfate were then added and stirred uniformly. 0.5-1 g of sodium lignin sulfonate / graphene oxide (LSC / GO) was then added and ultrasonically stirred for 20 minutes. The mixture was stirred and reacted at 60-80° C. for 5-7 hours, filtered, and dried to obtain LSC / GO@PAA-PAM particles (LSC / GO particles coated with polyacrylic acid-polyacrylamide copolymer). Step S2: 10 g of LSC / GO@PAA-PAM particles were stirred evenly in 200 mL of ethanol and heated to 80° C. 0.1-0.5 g of acrylate-modified chitosan was added and stirred for 30 min. 0.015-0.02 g of potassium persulfate and 0.02-0.03 g of aluminum nitrate were then added and stirred at the same temperature for 24 h. The mixture was filtered, washed, and dried to obtain a water-soluble temporary plugging agent for oil well fracturing.

[0006] Furthermore, the sodium lignin sulfonate / graphene oxide is prepared by the following steps: Step A1: Sodium lignin sulfonate (LSC), sodium hydroxide, and water are mixed, N-methyl-4-nitrobenzylamine is added, and the temperature is raised to 85° C., followed by addition of a 36 wt % formaldehyde aqueous solution. The mixture is condensed and refluxed for 5 h, and a 1 mol / L hydrochloric acid solution is added dropwise until no precipitate is formed. The mixture is filtered, washed, dried, ground, filtered, washed, and dried to obtain nitrated sodium lignin sulfonate. Furthermore, in step A1, the ratio of sodium lignin sulfonate, sodium hydroxide, water, N-methyl-4-nitrobenzylamine and formaldehyde solution is 10 g: 0.5-0.6 g: 50 mL: 2.8-3.5 g: 4-5 mL; Step A2, mixing nitrated sodium lignin sulfonate, reduced iron powder, methanol and water, stirring evenly, heating to 80°C, adding ammonium chloride, reflux stirring for 5-7h, filtering, washing and drying to obtain aminated sodium lignin sulfonate; Furthermore, in step A2, the ratio of nitrated sodium lignin sulfonate, reduced iron powder, methanol, water and ammonium chloride is 5g:1.5-3.5g:20mL:10mL:1-2g; Step A3, ultrasonically treat 5 g of graphene oxide in 200 mL of water for 1-2 hours, collect the upper dispersion, filter, and dry to obtain pretreated graphene oxide; the pretreated graphene oxide is evenly dispersed in a mixture of ethanol and water (the volume ratio of ethanol to water is 1:3), add 30 wt% potassium hydroxide aqueous solution to adjust the pH of the system to 8.5, add aminated sodium lignin sulfonate, ultrasonicate in a water bath for 30 minutes, react under nitrogen at 75-85°C for 5-7 hours, then add allylamine and maintain the temperature for 6 hours, filter, wash, and dry to obtain sodium lignin sulfonate / graphene oxide (LSC / GO); Furthermore, in step A3, the ratio of pre-treated graphene oxide, ethanol, water, aminated sodium lignin sulfonate and allylamine is 1 g:20 mL:60 mL:1-2 g:0.3-0.6 g.

[0007] Furthermore, the acrylate-modified chitosan is prepared by the following steps: Step B1, 3-chloro-1-propanol, dichloromethane and triethylamine were mixed, stirred in an ice-water bath for 30 minutes, acryloyl chloride dichloromethane solution was added, stirred for 1-2 hours, and then heated to 30-40°C for 12 hours, washed, rotary evaporated, purified by column chromatography, and rotary evaporated again to obtain an acrylate derivative; Furthermore, in step B1, the usage ratio of 3-chloro-1-propanol, dichloromethane, triethylamine and acryloyl chloride dichloromethane solution is 0.01-0.03 mol:50 mL:1-3 g:20 mL; Furthermore, the acryloyl chloride dichloromethane solution in step B1 is prepared by mixing acryloyl chloride and dichloromethane in a ratio of 0.01-0.03 mol:20 mL; Step B2, 5g of chitosan was swelled in 30mL of 3wt% glacial acetic acid for 30min, then washed with anhydrous ethanol until neutral, dried, and the pretreated chitosan was collected; the pretreated chitosan was mixed with 1-allyl-3-methylimidazolium chloride ionic liquid, stirred in an 80°C oil bath for 1.5-2.5h, then 1,2-epoxydodecane was added, the temperature was maintained and stirred for 8-10h, and an ethanol / acetone mixture (the volume ratio of ethanol to acetone was 1:1) was added and stirred for 10min, filtered, washed, and dried to obtain hydrophobically modified chitosan; Furthermore, in step B2, the amount ratio of pre-treated chitosan, 1-allyl-3-methylimidazolium chloride ionic liquid, 1,2-epoxydodecane and ethanol / acetone mixture is 1 g:10-20 g:2-3 g:100 mL; Step B3, mixing hydrophobically modified chitosan, sodium hydroxide and tetrahydrofuran, introducing nitrogen, and heating with stirring to reflux, then adding an acrylate derivative, condensing and refluxing under heating and stirring for 14-18 hours, washing with ether, washing with water, and drying to obtain acrylate-modified chitosan; Furthermore, in step B3, the ratio of the amount of hydrophobically modified chitosan, sodium hydroxide, tetrahydrofuran and acrylate derivative is 2g:2.5-3.5g:50mL:0.5-1.5g.

[0008] The invention also discloses a water-soluble temporary plugging agent for oil well fracturing prepared by the preparation method.

[0009] Beneficial effects of the present invention: The water-soluble temporary plugging agent prepared by the present invention uses sodium lignin sulfonate / graphene oxide as a core, and then coats the surface of the core layer with a polyacrylic acid-polyacrylamide copolymer to form a first shell layer, and then coats the surface of the first shell layer with acrylate-modified chitosan to form a second shell layer. The temporary plugging agent is used in oil well fracturing and not only has excellent plugging performance, solubility performance, and shear resistance, but also can improve the transportation capacity of the temporary plugging agent during the oil well fracturing process.

[0010] The sodium lignin sulfonate / graphene oxide in the core layer of the water-soluble temporary plugging agent of the present invention is coated with the polyacrylic acid-polyacrylamide copolymer in the first shell layer through chemical bonding. The graphene oxide contained therein synergistically acts with the benzene ring structure in the sodium lignin sulfonate to form a stable structure, which is beneficial to improving the strength of the temporary plugging agent and maintaining good shear strength before being transported to the target location. After water molecules enter the interior of the temporary plugging agent, the sodium lignin sulfonate can absorb water and expand, synergistically acting with the polyacrylic acid-polyacrylamide copolymer in the shell structure to increase the expansion multiple of the temporary plugging agent and the sealing effect. The acrylate-modified chitosan in the second shell is based on small molecule chitosan, and a small amount of hydrophobic alkyl long chain and acrylate structure are introduced on its surface by chemical grafting. Among them, the acrylate structure can be cross-linked with the first shell to achieve coating, and the hydrophobic long chain can use its hydrophobic effect to delay the water absorption and expansion of the temporary plugging agent in the initial stage. When the hydrophobic layer is hydrolyzed, water molecules enter the interior through the pores on the surface of the temporary plugging agent and use the internal hydrophilic groups to absorb water and expand. By introducing the hydrophobic structure on the surface of the temporary plugging agent, the migration ability of the temporary plugging agent in the formation is improved. DETAILED DESCRIPTION

[0011] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0012] Example 1: Sodium lignin sulfonate / graphene oxide was prepared by the following steps: Step A1, 10g of sodium lignin sulfonate, 0.5g of sodium hydroxide and 50mL of water were mixed, 2.8g of N-methyl-4-nitrobenzylamine was added, and the temperature was raised to 85°C, 4mL of 36wt% formaldehyde aqueous solution was added, and the reaction was condensed and refluxed for 5h, 1mol / L hydrochloric acid solution was added dropwise until no precipitate was precipitated, filtered, washed, dried, ground, filtered, washed, and dried to obtain nitrated sodium lignin sulfonate; Step A2, 5 g of nitrated sodium lignin sulfonate, 1.5 g of reduced iron powder, 20 mL of methanol and 10 mL of water were mixed and stirred evenly, and the mixture was heated to 80° C., 1 g of ammonium chloride was added, and the mixture was refluxed and stirred for 5 h, filtered, washed, and dried to obtain aminated sodium lignin sulfonate; Step A3, ultrasonically treat 5 g of graphene oxide in 200 mL of water for 1 hour, collect the upper dispersion, filter, and dry to obtain pretreated graphene oxide; evenly disperse 1 g of pretreated graphene oxide in a mixture of 20 mL of ethanol and 60 mL of water (the volume ratio of ethanol to water is 1:3), add 30 wt% potassium hydroxide aqueous solution to adjust the pH of the system to 8.5, add 1 g of aminated sodium lignin sulfonate, ultrasonicate in a water bath for 30 minutes, react under nitrogen at 75°C for 5 hours, then add 0.3 g of allylamine and maintain the temperature for 6 hours, filter, wash, and dry to obtain sodium lignin sulfonate / graphene oxide.

[0013] Acrylate-modified chitosan is prepared by the following steps: Step B1, 0.01 mol 3-chloro-1-propanol, 50 mL dichloromethane and 1 g triethylamine were mixed, stirred in an ice-water bath for 30 min, 20 mL acryloyl chloride dichloromethane solution was added and stirred for 1 h, then the temperature was raised to 30° C. and reacted for 12 h, washed, rotary evaporated, purified by column chromatography, and rotary evaporated again to obtain an acrylate derivative. The acryloyl chloride dichloromethane solution was prepared by mixing and stirring acryloyl chloride and dichloromethane in a ratio of 0.01 mol: 20 mL; Step B2, 5g of chitosan was swelled in 30mL of 3wt% glacial acetic acid for 30min, then washed with anhydrous ethanol until neutral, dried, and the pretreated chitosan was collected; 1g of pretreated chitosan was mixed with 10g of 1-allyl-3-methylimidazolium chloride ionic liquid, stirred in an 80°C oil bath for 1.5h, and then 2g of 1,2-epoxydodecane was added. The temperature was maintained and stirred for 8h. 100mL of ethanol / acetone mixture (the volume ratio of ethanol to acetone was 1:1) was added and stirred for 10min. The mixture was filtered, washed, and dried to obtain hydrophobically modified chitosan; Step B3: 2 g of hydrophobically modified chitosan, 2.5 g of sodium hydroxide and 50 mL of tetrahydrofuran were mixed, nitrogen was introduced, and the mixture was heated with stirring to reflux. Then, 0.5 g of an acrylate derivative was added, and the mixture was condensed and refluxed under heating and stirring for 14 h. The mixture was washed with ether, washed with water, and dried to obtain acrylate-modified chitosan.

[0014] Example 2: Sodium lignin sulfonate / graphene oxide was prepared by the following steps: Step A1, 10g of sodium lignin sulfonate, 0.55g of sodium hydroxide and 50mL of water were mixed, 3.1g of N-methyl-4-nitrobenzylamine was added, and the temperature was raised to 85°C, 4.5mL of 36wt% formaldehyde aqueous solution was added, and the reaction was refluxed under condensation for 5h, 1mol / L hydrochloric acid solution was added dropwise until no precipitate was precipitated, filtered, washed, dried, ground, filtered, washed, and dried to obtain nitrated sodium lignin sulfonate; Step A2, 5 g of nitrated sodium lignin sulfonate, 2.5 g of reduced iron powder, 20 mL of methanol and 10 mL of water were mixed and stirred evenly, and the mixture was heated to 80° C., 1.5 g of ammonium chloride was added, and the mixture was refluxed and stirred for 6 h, filtered, washed, and dried to obtain aminated sodium lignin sulfonate; Step A3, ultrasonically treat 5 g of graphene oxide in 200 mL of water for 1.5 h, collect the upper dispersion, filter, and dry to obtain pretreated graphene oxide; 1 g of pretreated graphene oxide is evenly dispersed in a mixture of 20 mL of ethanol and 60 mL of water (the volume ratio of ethanol to water is 1:3), add 30 wt% potassium hydroxide aqueous solution to adjust the pH of the system to 8.5, add 1.5 g of aminated sodium lignin sulfonate, ultrasonicate in a water bath for 30 min, react under nitrogen at 80 ° C for 6 h, then add 0.45 g of allylamine and maintain the temperature for 6 h, filter, wash, and dry to obtain sodium lignin sulfonate / graphene oxide.

[0015] Acrylate-modified chitosan is prepared by the following steps: Step B1, 0.02 mol 3-chloro-1-propanol, 50 mL dichloromethane and 2 g triethylamine were mixed, stirred in an ice-water bath for 30 min, 20 mL acryloyl chloride dichloromethane solution was added, stirred for 1.5 h, then heated to 35 ° C. for 12 h, washed, rotary evaporated, purified by column chromatography, and rotary evaporated again to obtain an acrylate derivative. The acryloyl chloride dichloromethane solution was prepared by mixing and stirring acryloyl chloride and dichloromethane in a ratio of 0.02 mol: 20 mL; Step B2, 5g of chitosan was swelled in 30mL of 3wt% glacial acetic acid for 30min, then washed with anhydrous ethanol until neutral, dried, and the pretreated chitosan was collected; 1g of pretreated chitosan was mixed with 15g of 1-allyl-3-methylimidazolium chloride ionic liquid, stirred in an 80°C oil bath for 2h, and then 2.5g of 1,2-epoxydodecane was added. The temperature was maintained and stirred for 9h. 100mL of ethanol / acetone mixture (the volume ratio of ethanol to acetone was 1:1) was added and stirred for 10min. The mixture was filtered, washed, and dried to obtain hydrophobically modified chitosan; Step B3: 2 g of hydrophobically modified chitosan, 3 g of sodium hydroxide and 50 mL of tetrahydrofuran were mixed, nitrogen was introduced, and the mixture was heated with stirring to reflux. Then, 1 g of an acrylate derivative was added, and the mixture was condensed and refluxed under heating and stirring for 16 h. The mixture was washed with ether, washed with water, and dried to obtain acrylate-modified chitosan.

[0016] Example 3: Sodium lignin sulfonate / graphene oxide was prepared by the following steps: Step A1, 10g of sodium lignin sulfonate, 0.6g of sodium hydroxide and 50mL of water were mixed, 3.5g of N-methyl-4-nitrobenzylamine was added, and the temperature was raised to 85°C, 5mL of 36wt% formaldehyde aqueous solution was added, and the reaction was refluxed under condensation for 5h, 1mol / L hydrochloric acid solution was added dropwise until no precipitate was precipitated, filtered, washed, dried, ground, filtered, washed, and dried to obtain nitrated sodium lignin sulfonate; Step A2, 5 g of nitrated sodium lignin sulfonate, 3.5 g of reduced iron powder, 20 mL of methanol and 10 mL of water were mixed and stirred evenly, and the mixture was heated to 80° C., 2 g of ammonium chloride was added, and the mixture was refluxed and stirred for 7 h, filtered, washed and dried to obtain aminated sodium lignin sulfonate; Step A3, ultrasonically treat 5 g of graphene oxide in 200 mL of water for 2 h, collect the upper dispersion, filter, and dry to obtain pretreated graphene oxide; 1 g of pretreated graphene oxide is evenly dispersed in a mixture of 20 mL of ethanol and 60 mL of water (the volume ratio of ethanol to water is 1:3), add 30 wt% potassium hydroxide aqueous solution to adjust the pH of the system to 8.5, add 2 g of aminated sodium lignin sulfonate, ultrasonicate in a water bath for 30 min, react under nitrogen at 85° C. for 7 h, then add 0.6 g of allylamine and maintain the temperature for 6 h, filter, wash, and dry to obtain sodium lignin sulfonate / graphene oxide.

[0017] Acrylate-modified chitosan is prepared by the following steps: Step B1, 0.03 mol 3-chloro-1-propanol, 50 mL dichloromethane and 3 g triethylamine were mixed, stirred in an ice-water bath for 30 min, 20 mL acryloyl chloride dichloromethane solution was added and stirred for 2 h, then the temperature was raised to 40 ° C. and reacted for 12 h, washed, rotary evaporated, purified by column chromatography, and rotary evaporated again to obtain an acrylate derivative. The acryloyl chloride dichloromethane solution was prepared by mixing and stirring acryloyl chloride and dichloromethane in a ratio of 0.03 mol: 20 mL; Step B2, 5g of chitosan was swelled in 30mL of 3wt% glacial acetic acid for 30min, then washed with anhydrous ethanol until neutral, dried, and the pretreated chitosan was collected; 1g of pretreated chitosan was mixed with 20g of 1-allyl-3-methylimidazolium chloride ionic liquid, stirred in an 80°C oil bath for 2.5h, and then 3g of 1,2-epoxydodecane was added. The temperature was maintained and stirred for 10h. 100mL of ethanol / acetone mixture (the volume ratio of ethanol to acetone was 1:1) was added and stirred for 10min. The mixture was filtered, washed, and dried to obtain hydrophobically modified chitosan; Step B3: 2 g of hydrophobically modified chitosan, 3.5 g of sodium hydroxide and 50 mL of tetrahydrofuran were mixed, nitrogen was introduced, and the mixture was heated with stirring to reflux. Then, 1.5 g of an acrylate derivative was added, and the mixture was condensed and refluxed under heating and stirring for 18 h. The mixture was washed with ether, washed with water, and dried to obtain acrylate-modified chitosan.

[0018] Example 4: A method for preparing a water-soluble temporary plugging agent for oil well fracturing comprises the following steps: Step S1, 10g of acrylic acid, 3g of sodium hydroxide and 50mL of water were mixed and stirred uniformly, and reacted in an ice-water bath until the neutralization degree of the acrylic acid aqueous solution reached 60%, then 9g of acrylamide, 0.18g of aluminum nitrate and 0.15g of potassium persulfate were added and stirred uniformly, and then 0.5g of sodium lignin sulfonate / graphene oxide prepared in Example 1 was added and ultrasonically mixed and stirred for 20min, and then stirred and reacted at 60°C for 5h, filtered and dried to obtain LSC / GO@PAA-PAM particles; Step S2: 10 g of LSC / GO@PAA-PAM particles were stirred uniformly in 200 mL of ethanol and heated to 80° C., 0.1 g of acrylate-modified chitosan prepared in Example 1 was added and stirred for 30 min, and then 0.015 g of potassium persulfate and 0.02 g of aluminum nitrate were added. The temperature was maintained and stirred for 24 h. The mixture was filtered, washed, and dried to obtain a water-soluble temporary plugging agent for oil well fracturing.

[0019] Example 5: A method for preparing a water-soluble temporary plugging agent for oil well fracturing comprises the following steps: Step S1: 10 g of acrylic acid, 3.1 g of sodium hydroxide, and 50 mL of water were mixed and stirred uniformly, and reacted in an ice-water bath until the neutralization degree of the acrylic acid aqueous solution reached 65%. 9.3 g of acrylamide, 0.2 g of aluminum nitrate, and 0.16 g of potassium persulfate were then added and stirred uniformly. 0.75 g of sodium lignin sulfonate / graphene oxide prepared in Example 2 was then added and ultrasonically mixed and stirred for 20 min. The mixture was stirred and reacted at 70° C. for 6 h, filtered, and dried to obtain LSC / GO@PAA-PAM particles. Step S2: 10 g of LSC / GO@PAA-PAM particles were stirred uniformly in 200 mL of ethanol and heated to 80° C., 0.3 g of acrylate-modified chitosan prepared in Example 2 was added and stirred for 30 min, and then 0.017 g of potassium persulfate and 0.025 g of aluminum nitrate were added. The temperature was maintained and stirred for 24 h. The mixture was filtered, washed, and dried to obtain a water-soluble temporary plugging agent for oil well fracturing.

[0020] Example 6: A method for preparing a water-soluble temporary plugging agent for oil well fracturing comprises the following steps: Step S1: 10 g of acrylic acid, 3.2 g of sodium hydroxide, and 50 mL of water were mixed and stirred uniformly, and reacted in an ice-water bath until the neutralization degree of the acrylic acid aqueous solution reached 70%. 9.5 g of acrylamide, 0.21 g of aluminum nitrate, and 0.17 g of potassium persulfate were then added and stirred uniformly. 1 g of sodium lignin sulfonate / graphene oxide prepared in Example 3 was then added and ultrasonically mixed and stirred for 20 min. The mixture was stirred and reacted at 80° C. for 7 h, filtered, and dried to obtain LSC / GO@PAA-PAM particles. Step S2: 10 g of LSC / GO@PAA-PAM particles were stirred uniformly in 200 mL of ethanol and heated to 80° C., 0.5 g of acrylate-modified chitosan prepared in Example 3 was added and stirred for 30 min, and then 0.02 g of potassium persulfate and 0.03 g of aluminum nitrate were added. The temperature was maintained and stirred for 24 h. The mixture was filtered, washed, and dried to obtain a water-soluble temporary plugging agent for oil well fracturing.

[0021] Comparative Example 1: This comparative example is a water-soluble temporary plugging agent. The difference from Example 6 is that graphene oxide is used instead of the sodium lignin sulfonate / graphene oxide prepared in Example 3, and the rest are the same.

[0022] Comparative Example 2: This comparative example is a water-soluble temporary plugging agent. The difference from Example 6 is that sodium lignin sulfonate is used instead of the sodium lignin sulfonate / graphene oxide prepared in Example 3, and the rest are the same.

[0023] Comparative Example 3: This comparative example is a water-soluble temporary plugging agent, which is the LSC / GO@PAA-PAM particles prepared in Example 6, and the rest are the same.

[0024] The water-soluble temporary plugging agents prepared in Examples 4-6 and Comparative Examples 1-3 were subjected to performance tests: Solubility test: The dissolution time of the test samples in 80°C simulated formation water (total salinity of 7000 mg / L) and fracturing fluid containing 0.5 wt% guar gum was measured. The test results are shown in Table 1. Shear strength test: The material was fully expanded in simulated formation water (total mineralization of 7000 mg / L) at 20°C for 350 minutes. After the expansion ratio remained essentially unchanged, the shear strength was measured using a volume expansion particle strength shear instrument. The test results are shown in Table 1. Plugging rate performance test: Tested according to the plugging rate test method in patent number CN115197364A; Expansion performance test: The sample was added to simulated formation water (total salinity of 7000 mg / L) and allowed to fully absorb water and swell at 20°C. The sample was taken out regularly and wiped dry with filter paper. The expansion multiple of the temporary plugging agent was measured at room temperature. The test results are shown in Table 2.

[0025] As can be seen from Table 1, the water-soluble temporary plugging agent prepared in the present invention has good solubility, shear strength and high plugging rate.

[0026]

[0027] As can be seen from Table 2, after the expansion performance test, the expansion multiple of the water-soluble temporary plugging agent prepared by the present invention is small in the early stage, and gradually increases with time, indicating that the hydrophobic component on the surface of the temporary plugging agent can delay the entry of water molecules into the interior in the early stage, thereby improving its migration ability in the formation.

[0028] The above content is merely an example and explanation of the concept of the present invention. Those skilled in the art may make various modifications or additions to the described specific embodiments or replace them in a similar manner. As long as they do not deviate from the scope defined by the concept of the invention, they should all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a water-soluble temporary plugging agent for oil well fracturing, characterized in that: The method comprises the following steps: Step S1: 10 g of acrylic acid, 3-3.2 g of sodium hydroxide, and 50 mL of water were mixed and stirred uniformly, and reacted in an ice-water bath until the neutralization degree of the acrylic acid aqueous solution reached 60%-70%. 9-9.5 g of acrylamide, 0.18-0.21 g of aluminum nitrate, and 0.15-0.17 g of potassium persulfate were then added and stirred uniformly. 0.5-1 g of sodium lignin sulfonate / graphene oxide was then added and ultrasonically stirred for 20 minutes. The mixture was stirred and reacted at 60-80° C. for 5-7 hours, filtered, and dried to obtain LSC / GO@PAA-PAM particles. Step S2: 10 g of LSC / GO@PAA-PAM particles were stirred uniformly in 200 mL of ethanol and heated to 80° C., 0.1-0.5 g of acrylate-modified chitosan was added and stirred for 30 min, and then 0.015-0.02 g of potassium persulfate and 0.02-0.03 g of aluminum nitrate were added. The temperature was maintained and stirred for 24 h. The mixture was filtered, washed, and dried to obtain a water-soluble temporary plugging agent for oil well fracturing. The sodium lignin sulfonate / graphene oxide is prepared by reacting pre-treated graphene oxide with aminated sodium lignin sulfonate and allylamine. The aminated sodium lignin sulfonate is prepared by reducing nitrated sodium lignin sulfonate with reduced iron powder and ammonium chloride. The nitrated sodium lignin sulfonate is prepared by reacting sodium lignin sulfonate with formaldehyde and N-methyl-4-nitrobenzylamine. The acrylate-modified chitosan is prepared by reacting hydrophobically modified chitosan and an acrylate derivative, wherein the hydrophobically modified chitosan is prepared by reacting pretreated chitosan and 1,2-epoxydodecane, and the acrylate derivative is prepared by reacting 3-chloro-1-propanol and acryloyl chloride.

2. The method for preparing a water-soluble temporary plugging agent for oil well fracturing according to claim 1, characterized in that: The sodium lignin sulfonate / graphene oxide is prepared by the following steps: Step A1: After mixing sodium lignin sulfonate, sodium hydroxide and water, add N-methyl-4-nitrobenzylamine, and heat to 85° C., then add 36 wt% formaldehyde aqueous solution, condense and reflux for 5 hours, add 1 mol / L hydrochloric acid solution dropwise until no precipitate is formed, filter, wash, dry, grind, filter with suction, wash, and dry to obtain nitrated sodium lignin sulfonate; Step A2, mixing nitrated sodium lignin sulfonate, reduced iron powder, methanol and water, stirring evenly, heating to 80°C, adding ammonium chloride, reflux stirring for 5-7h, filtering, washing and drying to obtain aminated sodium lignin sulfonate; Step A3, ultrasonically treat 5 g of graphene oxide in 200 mL of water for 1-2 h, collect the upper dispersion, filter, and dry to obtain pretreated graphene oxide; the pretreated graphene oxide is evenly dispersed in a mixture of ethanol and water, 30 wt% potassium hydroxide aqueous solution is added to adjust the pH of the system to 8.5, add aminated sodium lignin sulfonate, ultrasonicate in a water bath for 30 min, react under nitrogen at 75-85 ° C for 5-7 h, then add allylamine and maintain the temperature for 6 h, filter, wash, and dry to obtain sodium lignin sulfonate / graphene oxide.

3. The method for preparing a water-soluble temporary plugging agent for oil well fracturing according to claim 2, characterized in that: In step A1, the usage ratio of sodium lignin sulfonate, sodium hydroxide, water, N-methyl-4-nitrobenzylamine and formaldehyde aqueous solution is 10 g:0.5-0.6 g:50 mL:2.8-3.5 g:4-5 mL.

4. The method for preparing a water-soluble temporary plugging agent for oil well fracturing according to claim 2, characterized in that: In step A2, the usage ratio of nitrated sodium lignin sulfonate, reduced iron powder, methanol, water and ammonium chloride is 5g:1.5-3.5g:20mL:10mL:1-2g.

5. The method for preparing a water-soluble temporary plugging agent for oil well fracturing according to claim 2, characterized in that: In step A3, the ratio of pre-treated graphene oxide, ethanol, water, aminated sodium lignin sulfonate and allylamine is 1 g: 20 mL: 60 mL: 1-2 g: 0.3-0.6 g.

6. The method for preparing a water-soluble temporary plugging agent for oil well fracturing according to claim 1, characterized in that: The acrylate-modified chitosan is prepared by the following steps: Step B1, 3-chloro-1-propanol, dichloromethane and triethylamine were mixed, stirred in an ice-water bath for 30 minutes, acryloyl chloride dichloromethane solution was added, stirred for 1-2 hours, and then heated to 30-40°C for 12 hours, washed, rotary evaporated, purified by column chromatography, and rotary evaporated again to obtain an acrylate derivative; Step B2, 5g of chitosan was swelled in 30mL of 3wt% glacial acetic acid for 30min, then washed with anhydrous ethanol until neutral, dried, and the pretreated chitosan was collected; the pretreated chitosan was mixed with 1-allyl-3-methylimidazolium chloride ionic liquid, stirred in an 80°C oil bath for 1.5-2.5h, and then 1,2-epoxydodecane was added. The temperature was maintained and stirred for 8-10h. After adding an ethanol / acetone mixture and stirring for 10min, the mixture was filtered, washed, and dried to obtain hydrophobically modified chitosan; Step B3: Mix hydrophobically modified chitosan, sodium hydroxide and tetrahydrofuran, introduce nitrogen, heat and stir until reflux, add acrylate derivative, condense and reflux under heating and stirring for 14-18 hours, wash with ether, wash with water, and dry to obtain acrylate-modified chitosan.

7. The method for preparing a water-soluble temporary plugging agent for oil well fracturing according to claim 6, characterized in that: In step B1, the amount ratio of 3-chloro-1-propanol, dichloromethane, triethylamine and acryloyl chloride dichloromethane solution is 0.01-0.03 mol:50 mL:1-3 g:20 mL, and the acryloyl chloride dichloromethane solution is prepared by mixing acryloyl chloride and dichloromethane in an amount ratio of 0.01-0.03 mol:20 mL.

8. The method for preparing a water-soluble temporary plugging agent for oil well fracturing according to claim 6, characterized in that: In step B2, the amount ratio of pre-treated chitosan, 1-allyl-3-methylimidazolium chloride ionic liquid, 1,2-epoxydodecane and ethanol / acetone mixture is 1g:10-20g:2-3g:100mL.

9. The method for preparing a water-soluble temporary plugging agent for oil well fracturing according to claim 6, characterized in that: In step B3, the ratio of the amount of hydrophobically modified chitosan, sodium hydroxide, tetrahydrofuran and acrylate derivative is 2 g:2.5-3.5 g:50 mL:0.5-1.5 g.

10. A water-soluble temporary plugging agent for oil well fracturing, characterized in that: The water-soluble temporary plugging agent for oil well fracturing is prepared according to the method according to any one of claims 1 to 9.

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