A crosslinking agent for degradable hydrogels and degradable polyacrylamide hydrogels based on the crosslinking agent, and preparation method and use thereof

Abstract

The application discloses a crosslinking agent for degradable hydrogel and a preparation method of degradable polyacrylamide hydrogel based on the crosslinking agent; the crosslinking agent for degradable hydrogel is composed of hydantoin epoxy resin, N, N-dimethyl benzylamine and acrylamide; the degradable polyacrylamide hydrogel prepared by using the crosslinking agent comprises acrylamide, 1.8-4.6 parts of water, potassium persulfate and the crosslinking agent; the crosslinking agent for degradable hydrogel can realize the preparation of the degradable hydrogel by chemical crosslinking method, and by adjusting the crosslinking agent, the degradable hydrogel prepared based on the crosslinking agent is applicable in the temperature environment range of normal temperature to 150 DEG C, the degradable hydrogel has selectivity to gel breaking liquid, and can realize gel breaking only by adding alkaline gel breaking liquid, and the gel breaking time is adjustable in the range of 90 min to 72 h; in addition, the crosslinking agent and the preparation method of the hydrogel are simple, the conditions are easy to control, and have good market application and promotion prospect.

Description

Technical Field

[0001] This invention relates to the field of biodegradable hydrogel technology, and particularly to a crosslinking agent for biodegradable hydrogels, a biodegradable polyacrylamide hydrogel based on the crosslinking agent, its preparation method, and its uses. Background Technology

[0002] When performing well repair and fracturing operations on geothermal wells, it is often necessary to seal the formation. The traditional method is to use mechanical tools such as bridge plug packers. However, these mechanical tools are prone to getting stuck in the wellbore, making it impossible to remove the tools normally. A major overhaul operation is required to remove the tools.

[0003] To address the aforementioned issues, researchers have developed a hydrogel material that can degrade within the wellbore for sealing. This biodegradable hydrogel material is primarily composed of natural polymers such as guar gum and guar gum, or synthetic water-soluble polymers such as polyacrylamide. Solutions prepared from these water-soluble polymers can polymerize and cross-link at wellbore temperatures to form a hydrogel. After use, the hydrogel can be broken down by biological enzymes or peroxides such as hydrogen peroxide, potassium persulfate, and ammonium persulfate. The peroxides decompose within the wellbore to generate free radicals. These active free radicals can break the polymer chains, and the acidic substances produced during decomposition also contribute to this chain breakage, allowing the polymer gel to revert to a flowable solution and flow out of the wellbore. However, existing technologies have the following limitations: 1) Biological enzyme-based degreasing agents are only effective for hydrogels made from natural polymers such as guar gum, and their application temperature is relatively low. For hydrogels made of polyacrylamide-based synthetic polymers, decomposition is difficult; 2) When using peroxides such as potassium persulfate and ammonium persulfate as decomposition agents, to ensure the decomposition effect, the decomposition agent needs to be encapsulated and pre-embedded inside the plug. However, due to the half-life of peroxides, free radicals are quickly released inside the plug, thereby reducing the strength of the plug; therefore, if a decomposition solution made of peroxides is pumped into the wellbore after the plug has formed, the decomposition reaction will only occur in the area where the decomposition solution contacts the plug, and a large amount of the decomposition solution will decompose under high temperature without contacting the plug, thus being wasted. In addition, some peroxides, including hydrogen peroxide, may release oxygen under high temperature. If there are associated gases such as hydrogen sulfide or natural gas in the wellbore, there is a risk of explosion. If a decomposition agent is added to the outside of the gel after it has formed, it will be difficult to decompose the gel. Summary of the Invention

[0004] The purpose of this invention is to provide a crosslinking agent that can be used to prepare biodegradable hydrogels.

[0005] Another object of the present invention is to provide a method for preparing the crosslinking agent for the above-mentioned biodegradable hydrogel.

[0006] Another objective of this invention is to provide a biodegradable polyacrylamide hydrogel suitable for plugging in wells at normal, medium, and high temperatures and capable of degrading and unblocking under the action of specific alkaline solutions.

[0007] Another object of the present invention is to provide a method for preparing the above-mentioned biodegradable polyacrylamide hydrogel.

[0008] Another object of the present invention is to provide a suitable use of the above-mentioned biodegradable polyacrylamide hydrogel as a sealing agent.

[0009] Therefore, the technical solution of the present invention is as follows:

[0010] A crosslinking agent for preparing biodegradable hydrogels is composed of 100 parts by weight of hydantoin epoxy resin, 0.75 to 1.7 parts by weight of N,N-dimethylbenzylamine and 6 to 12 parts by weight of acrylamide.

[0011] A method for preparing a crosslinking agent for biodegradable hydrogels includes the following steps: adding hydantoin epoxy resin, N,N-dimethylbenzylamine and acrylamide to a reaction flask, turning on the stirrer and heating to 60-120°C, reacting under stirring conditions for 17-75 min, and then cooling to room temperature to obtain the crosslinking agent for biodegradable hydrogels.

[0012] A biodegradable polyacrylamide hydrogel prepared using the above-mentioned crosslinking agent comprises 0.2 to 0.5 parts by weight of acrylamide, 1.8 to 4.6 parts by weight of water, 0.002 to 0.005 parts by weight of potassium persulfate and 0.014 to 0.04 parts by weight of crosslinking agent.

[0013] A method for preparing the above-mentioned biodegradable polyacrylamide hydrogel includes the following steps: adding acrylamide, water, potassium persulfate and crosslinking agent into a reaction flask, stirring at room temperature for at least 5 minutes to obtain a homogeneous reaction system; then, placing the reaction flask in a high-pressure reactor, and then raising the temperature of the reaction system to 70℃~150℃ and keeping it at that temperature for 1~5 hours to obtain the biodegradable hydrogel.

[0014] The use of a biodegradable polyacrylamide hydrogel prepared with the above-mentioned crosslinking agent as a downhole plugging agent.

[0015] Compared with existing technologies, the crosslinking agent used in this biodegradable hydrogel enables the preparation of biodegradable hydrogels via chemical crosslinking. Furthermore, by adjusting the crosslinking agent, the biodegradable hydrogel prepared based on it can be used in a temperature range of room temperature to 150°C. Moreover, this biodegradable hydrogel is selective for the breaking solution, and can only be broken down when an alkaline breaking solution is added, with the breaking time adjustable from 90 min to 72 h. In addition, the crosslinking agent and hydrogel preparation method of this application are simple and the conditions are easy to control, which has good market application and promotion prospects. Attached Figure Description

[0016] Figure 1 This is a flowchart illustrating the preparation process of the biodegradable polyacrylamide hydrogel of the present invention.

[0017] Figure 2(a) shows the state of the polyacrylamide hydrogel prepared in Example 1 of the present invention before degradation;

[0018] Figure 2(b) shows the state of the polyacrylamide hydrogel prepared in Example 1 of the present invention after 60 min of degradation;

[0019] Figure 2(c) shows the state of the polyacrylamide hydrogel prepared in Example 1 of the present invention after 90 min of degradation;

[0020] Figure 2(d) shows the state of the polyacrylamide hydrogel prepared in Example 1 of the present invention after 120 min of degradation;

[0021] Figure 3(a) shows the state of the polyacrylamide hydrogel prepared in Example 2 of the present invention before degradation;

[0022] Figure 3(b) shows the state of the polyacrylamide hydrogel prepared in Example 2 of the present invention after 60 min of degradation;

[0023] Figure 3(c) shows the state of the polyacrylamide hydrogel prepared in Example 2 of the present invention after 90 min of degradation;

[0024] Figure 3(d) shows the state of the polyacrylamide hydrogel prepared in Example 2 of the present invention after 120 min of degradation;

[0025] Figure 3(e) shows the state of the polyacrylamide hydrogel prepared in Example 2 of the present invention after 180 min of degradation;

[0026] Figure 4(a) shows the state of the polyacrylamide hydrogel prepared in Example 3 of the present invention before degradation in a 20 wt.% NaOH aqueous solution;

[0027] Figure 4(b) shows the state of the polyacrylamide hydrogel prepared in Example 3 of the present invention after degradation in 20 wt.% NaOH aqueous solution for 60 min;

[0028] Figure 4(c) shows the state of the polyacrylamide hydrogel prepared in Example 3 of the present invention after degradation in 20 wt.% NaOH aqueous solution for 90 min;

[0029] Figure 5(a) shows the state of the polyacrylamide hydrogel prepared in Example 3 of the present invention before degradation in a 20 wt.% ammonium persulfate aqueous solution;

[0030] Figure 5(b) shows the state of the polyacrylamide hydrogel prepared in Example 3 of the present invention after 60 min of degradation;

[0031] Figure 5(c) shows the state of the polyacrylamide hydrogel prepared in Example 3 of the present invention after 90 min of degradation;

[0032] Figure 5(d) shows the state of the polyacrylamide hydrogel prepared in Example 3 of the present invention after 120 min of degradation;

[0033] Figure 6(a) shows the state of the polyacrylamide hydrogel prepared in Example 4 of the present invention before degradation;

[0034] Figure 6(b) shows the state of the polyacrylamide hydrogel prepared in Example 4 of the present invention after 60 min of hydrolysis;

[0035] Figure 6(c) shows the state of the polyacrylamide hydrogel prepared in Example 4 of the present invention after 180 min of degradation;

[0036] Figure 6(d) shows the state of the polyacrylamide hydrogel prepared in Example 4 of the present invention after 300 min of degradation;

[0037] Figure 7(a) shows the state of the polyacrylamide hydrogel prepared in Example 5 of the present invention before degradation;

[0038] Figure 7(b) shows the state of the polyacrylamide hydrogel prepared in Example 5 of the present invention after 120 min of degradation;

[0039] Figure 8(a) shows the state of the polyacrylamide hydrogel prepared in Example 6 of the present invention before degradation in a 20 wt.% NaOH aqueous solution;

[0040] Figure 8(b) shows the state of the polyacrylamide hydrogel prepared in Example 6 of the present invention after degradation in 20 wt.% NaOH aqueous solution for 60 min;

[0041] Figure 8(c) shows the state of the polyacrylamide hydrogel prepared in Example 6 of the present invention after degradation in 20 wt.% NaOH aqueous solution for 120 min;

[0042] Figure 8(d) shows the state of the polyacrylamide hydrogel prepared in Example 6 of the present invention after degradation in 20 wt.% NaOH aqueous solution for 180 min;

[0043] Figure 9(a) shows the state of the polyacrylamide hydrogel prepared in Example 6 of the present invention before degradation in NaCl aqueous solution;

[0044] Figure 9(b) shows the state of the polyacrylamide hydrogel prepared in Example 6 of the present invention after degradation in NaCl aqueous solution for 60 min;

[0045] Figure 9(c) shows the state of the polyacrylamide hydrogel prepared in Example 6 of the present invention after degradation in NaCl aqueous solution for 120 min;

[0046] Figure 9(d) shows the state of the polyacrylamide hydrogel prepared in Example 6 of the present invention after degradation in 20 wt.% NaCl aqueous solution for 550 min;

[0047] Figure 10(a) is a state diagram of the polyacrylamide hydrogel prepared in Example 8 of the present invention after 0 h of degradation;

[0048] Figure 10(b) shows the state of the polyacrylamide hydrogel prepared in Example 8 of the present invention after 24 hours of degradation;

[0049] Figure 10(c) is a state diagram of the polyacrylamide hydrogel prepared in Example 8 of the present invention after 48 hours of degradation;

[0050] Figure 10(d) shows the state of the polyacrylamide hydrogel prepared in Example 8 of the present invention after 72 hours of degradation;

[0051] Figure 11(a) shows the state of the polyacrylamide hydrogel prepared in Example 8 of the present invention before degradation in a 20 wt.% NaOH aqueous solution;

[0052] Figure 11(b) shows the state of the polyacrylamide hydrogel prepared in Example 8 of the present invention after degradation in 20 wt.% NaOH aqueous solution for 240 min;

[0053] Figure 12(a) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 of the present invention before degradation in a 20 wt.% NaOH aqueous solution;

[0054] Figure 12(b) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 of the present invention after degradation in 20 wt.% NaOH aqueous solution for 60 min;

[0055] Figure 12(c) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 of the present invention after degradation in 20 wt.% NaOH aqueous solution for 120 min;

[0056] Figure 12(d) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 of the present invention after degradation in 20 wt.% NaOH aqueous solution for 240 min;

[0057] Figure 13(a) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 of the present invention before degradation in a 20 wt.% ammonium persulfate aqueous solution;

[0058] Figure 13(b) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 of the present invention after degradation in 20 wt.% ammonium persulfate aqueous solution for 60 min;

[0059] Figure 13(c) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 of the present invention after degradation in 20 wt.% ammonium persulfate aqueous solution for 120 min;

[0060] Figure 13(d) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 of the present invention after degradation in 20 wt.% ammonium persulfate aqueous solution for 180 min. Detailed Implementation

[0061] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the following embodiments are by no means intended to limit the present invention.

[0062] Example 1

[0063] (I) The specific preparation steps for the crosslinking agent used in the preparation of biodegradable hydrogels are as follows:

[0064] 2g of hydantoin epoxy resin, 0.034g of N,N-dimethylbenzylamine, and 0.12g of acrylamide were added to a reaction flask. The mixture was stirred and heated to 60°C. After reacting for 75 minutes under stirring, the stirring was stopped, and the mixture was allowed to cool to room temperature to obtain a crosslinking agent for biodegradable hydrogels. The hydantoin epoxy resin is 5,5-dimethyl-1,3-di(epoxyethylenemethyl)imidazolidine-2,4-dione, specifically the hydantoin epoxy resin (epoxy value 0.7-0.74) provided by Shenzhen Huite Chemical Co., Ltd. The same applies to the following examples and comparative examples.

[0065] (II) Based on the above-mentioned crosslinking agent for biodegradable hydrogels, polyacrylamide hydrogels are prepared:

[0066] 0.4 g acrylamide, 4.6 g water, 0.004 g potassium persulfate, and 0.035 g of the prepared crosslinking agent were added to a reaction flask and stirred at room temperature for 5 min to obtain a homogeneous reaction system. Then, the reaction flask was placed in a water bath and the temperature of the reaction system was raised to 75 °C and kept at that temperature for 2 h to obtain a biodegradable hydrogel. During the reaction, it was observed that at 75 °C, when the reaction time was about 20 min, the mixture in the reaction flask began to coagulate and form a cylindrical hydrogel.

[0067] To facilitate observation of the degradation process during subsequent degradation experiments, a cylindrical glass sampling bottle with a cap and an inner diameter of 20 mm was used for the reaction flask. Initially, the volume of the hydrogel in the reaction flask was about 1 / 2 of the bottle's volume.

[0068] The polyacrylamide hydrogel prepared in (II) above was subjected to degradation experiments: 5g of 20wt.% NaOH aqueous solution was added to the reaction flask, the cap was tightened and the flask was placed in a 90℃ water bath for 120min to allow the hydrogel to fully undergo degradation reaction; during the entire degradation process, the reaction flask was removed from the water bath at 60min, 90min and 120min respectively, the reaction flask was inverted for about 5-15s, and after the liquid in the system in the reaction flask stopped flowing, a photo was taken to clearly observe the changes that occurred in the hydrogel during the degradation process; then the reaction flask was quickly uprighted and placed back into the water bath to continue the degradation.

[0069] Figure 2(a) shows the state of the polyacrylamide hydrogel prepared in Example 1 before degradation. As can be seen from the figure, the polyacrylamide hydrogel at this moment is a complete cylindrical hydrogel. Figure 2(b) shows the state of the polyacrylamide hydrogel prepared in Example 1 after 60 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel at this moment is significantly smaller, and the volume reduction exceeds 1 / 2 of the initial volume. Figure 2(c) shows the state of the polyacrylamide hydrogel prepared in Example 1 after 90 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel at this moment further decreases, and the liquid level of the NaOH aqueous solution continues to rise. Figure 2(d) shows the state of the polyacrylamide hydrogel prepared in Example 1 after 120 min of degradation. As can be seen from the figure, no polyacrylamide hydrogel can be observed in the bottle at this moment, and the liquid level of the NaOH aqueous solution continues to rise.

[0070] In summary, it can be seen from the entire degradation process that the polyacrylamide hydrogel prepared in Example 1 gradually degrades under the action of 90℃ and 20wt.% NaOH aqueous solution. When the degradation time reaches 120min, the polyacrylamide hydrogel completely recovers its fluidity, that is, the degradation is complete.

[0071] Example 2

[0072] (I) The specific preparation steps for the crosslinking agent used in the preparation of biodegradable hydrogels are as follows:

[0073] Add 2g of hydantoin epoxy resin, 0.02g of N,N-dimethylbenzylamine and 0.16g of acrylamide to a reaction flask, turn on the stirrer and heat to 100℃. After reacting for 17 minutes under stirring, stop stirring and let it cool to room temperature to obtain a crosslinking agent for biodegradable hydrogels.

[0074] (II) Based on the above-mentioned crosslinking agent for biodegradable hydrogels, polyacrylamide hydrogels are prepared:

[0075] 0.4 g acrylamide, 4.6 g water, 0.004 g potassium persulfate, and 0.035 g of the prepared crosslinking agent were added to a reaction flask and stirred at room temperature for 5 min to obtain a homogeneous reaction system. Then, the reaction flask was placed in a water bath, and the temperature of the reaction system was raised to 90 °C and kept at that temperature for 2 h to obtain a biodegradable hydrogel. During the reaction, it was observed that at 90 °C, when the reaction time was about 10 min, the mixture in the reaction flask had begun to coagulate and form a cylindrical hydrogel.

[0076] To facilitate observation of the degradation process during subsequent degradation experiments, a cylindrical glass sampling bottle with a cap and an inner diameter of 20 mm was used for the reaction flask. Initially, the volume of the hydrogel in the reaction flask was about 1 / 2 of the bottle's volume.

[0077] The polyacrylamide hydrogel prepared in (II) above was subjected to degradation experiments: 5g of 20wt.% NaOH aqueous solution was added to the reaction flask, the cap was tightened and the flask was placed in a 90℃ water bath for 180min to allow the hydrogel to fully undergo degradation reaction; during the entire degradation process, the reaction flask was removed from the water bath at 60min, 90min, 120min and 180min respectively, the reaction flask was inverted for about 5-15s, and after the liquid in the system in the reaction flask stopped flowing, a photograph was taken to clearly observe the changes that occurred in the hydrogel during the degradation process; then the reaction flask was quickly uprighted and placed back into the water bath to continue the degradation.

[0078] Figure 3(a) shows the state of the polyacrylamide hydrogel prepared in Example 2 before degradation. As can be seen from the figure, the polyacrylamide hydrogel at this moment is a complete cylindrical hydrogel. Figure 3(b) shows the state of the polyacrylamide hydrogel prepared in Example 2 after 60 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel has decreased significantly at this moment. Figure 3(c) shows the state of the polyacrylamide hydrogel prepared in Example 2 after 90 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel has decreased further at this moment, and the liquid level of the NaOH aqueous solution has continued to rise. Figure 3(d) shows the state of the polyacrylamide hydrogel prepared in Example 2 after 120 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel has decreased further at this moment, and the liquid level of the NaOH aqueous solution has continued to rise. Figure 3(e) shows the state of the polyacrylamide hydrogel prepared in Example 2 after 180 min of degradation. As can be seen from the figure, no polyacrylamide hydrogel can be observed in the bottle at this moment, and the liquid level of the NaOH aqueous solution continues to rise.

[0079] In summary, it can be seen from the entire degradation process that the polyacrylamide hydrogel prepared in Example 2 gradually degrades under the action of 90℃ and 20wt.% NaOH aqueous solution. When the degradation time reaches 180min, the polyacrylamide hydrogel completely recovers its fluidity, that is, the degradation is complete.

[0080] Example 3

[0081] (I) The specific preparation steps for the crosslinking agent used in the preparation of biodegradable hydrogels are as follows:

[0082] Add 2g of hydantoin epoxy resin, 0.015g of N,N-dimethylbenzylamine and 0.2g of acrylamide to a reaction flask, turn on the stirrer and heat to 120°C. React for 21 minutes under stirring, then stop stirring and let cool to room temperature to obtain a crosslinking agent for biodegradable hydrogels.

[0083] (II) Based on the above-mentioned crosslinking agent for biodegradable hydrogels, polyacrylamide hydrogels are prepared:

[0084] 0.4 g acrylamide, 4.6 g water, 0.004 g potassium persulfate and 0.025 g crosslinking agent were added to a reaction flask and stirred at room temperature for 5 min to prepare a homogeneous reaction system. Then, the reaction flask was placed in a water bath and the temperature of the reaction system was raised to 70 °C and kept at this temperature for 2 h to obtain a biodegradable hydrogel. During the reaction, it was observed that at 70 °C, when the reaction time was about 20 min, the mixture in the reaction flask began to coagulate and form a cylindrical hydrogel.

[0085] To facilitate observation of the degradation process during subsequent degradation experiments, a cylindrical glass sampling bottle with a cap and an inner diameter of 20 mm was used for the reaction flask. Initially, the volume of the hydrogel in the reaction flask was about 1 / 2 of the bottle's volume.

[0086] The polyacrylamide hydrogel prepared in (II) above was subjected to degradation experiments: 5g of 20wt.% NaOH aqueous solution was added to the reaction flask, the cap was tightened and the flask was placed in a 90℃ water bath for 90min to allow the hydrogel to fully undergo degradation reaction; during the entire degradation process, the reaction flask was removed from the water bath at 60min and 90min respectively, the reaction flask was inverted for about 5-15s, and after the liquid in the system in the reaction flask stopped flowing, a photograph was taken to clearly observe the changes that occurred in the hydrogel during the degradation process; then the reaction flask was quickly uprighted and placed back into the water bath to continue the degradation.

[0087] Figure 4(a) shows the state of the polyacrylamide hydrogel prepared in Example 3 before degradation. As can be seen from the figure, the polyacrylamide hydrogel at this moment is a complete cylindrical hydrogel. Figure 4(b) shows the state of the polyacrylamide hydrogel prepared in Example 3 after 60 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel at this moment is significantly smaller. Figure 4(c) shows the state of the polyacrylamide hydrogel prepared in Example 3 after 90 min of degradation. As can be seen from the figure, no polyacrylamide hydrogel can be observed in the bottle at this moment, and the liquid level of the NaOH aqueous solution has risen.

[0088] In summary, it can be seen from the entire degradation process that the polyacrylamide hydrogel prepared in Example 3 gradually degrades under the action of 90℃ and 20wt.% NaOH aqueous solution. When the degradation time reaches 90min, the polyacrylamide hydrogel completely recovers its fluidity, that is, the degradation is complete.

[0089] Similarly, the polyacrylamide hydrogel of Example 3 was prepared again, and 5g of 20wt.% ammonium persulfate aqueous solution was used as a breaker instead of 5g of 20wt.% NaOH aqueous solution and added to the reaction flask. The cap was tightened and the flask was placed in a 90℃ water bath for 120min to allow the hydrogel to fully degrade. During the degradation process, the reaction flask was removed from the water bath at 60min, 90min, and 120min, and the flask was inverted for about 5-15s. After the liquid in the system in the reaction flask stopped flowing, the flask was photographed to clearly observe the changes that occurred in the hydrogel during the degradation process. Then, the reaction flask was quickly placed upright and put back into the water bath to continue the degradation.

[0090] Figure 5(a) shows the state of the polyacrylamide hydrogel prepared in Example 3 before degradation. As can be seen from the figure, the polyacrylamide hydrogel at this point is a complete cylindrical hydrogel. Figure 5(b) shows the state of the polyacrylamide hydrogel prepared in Example 3 after 60 minutes of degradation. As can be seen from the figure, at this point, the volume of the polyacrylamide hydrogel did not decrease significantly, but the layer in contact with the ammonium persulfate aqueous solution turned brown. Figure 5(c) shows the state of the polyacrylamide hydrogel prepared in Example 3 after 90 minutes of degradation. As can be seen from the figure, the polyacrylamide hydrogel at this point... The gel state is similar to that at 60 min of degradation. Figure 5(d) shows the state of the polyacrylamide hydrogel prepared in Example 3 after 120 min of degradation. As can be seen from the figure, at this point, after 2 hours of degradation, the ammonium persulfate in the system should be basically decomposed and free radicals should be released. However, the volume of the polyacrylamide hydrogel has not changed much. The entire degradation solution is acidic, with pH=1. It can be seen that the polyacrylamide hydrogel prepared in Example 3 is difficult to degrade under the action of ammonium persulfate aqueous solution, and the hydrogel cannot regain its fluidity. That is, the degradation of the polyacrylamide hydrogel is selective for the degrading agent.

[0091] Example 4

[0092] Based on the crosslinking agent for the biodegradable hydrogel prepared in Example 3, a polyacrylamide hydrogel was prepared:

[0093] 0.5g acrylamide, 4.5g water, 0.005g potassium persulfate and 0.04g crosslinking agent were added to a reaction flask and stirred at room temperature for 5 min to prepare a homogeneous reaction system. Then, the reaction flask was placed in a high-pressure reactor and the temperature of the reaction system was raised to 120℃ and kept at that temperature for 1 h to obtain a biodegradable hydrogel.

[0094] To facilitate observation of the degradation process during subsequent degradation experiments, a cylindrical glass sampling bottle with a cap and an inner diameter of 20 mm was used for the reaction flask. Initially, the volume of the hydrogel in the reaction flask was about 1 / 2 of the bottle's volume.

[0095] The polyacrylamide hydrogel prepared above was subjected to degradation experiments: 5g of 20wt.% NaOH aqueous solution was added to the reaction flask, the cap was tightened and the flask was placed in a high-pressure reactor, and the temperature was raised to 120℃ to allow the hydrogel to fully undergo degradation reaction; during the entire degradation process, the reactor was cooled and the reaction flask was removed from the reactor at 60min, 180min and 300min respectively. The reaction flask was inverted for about 5-15s, and after the liquid in the system in the reaction flask stopped flowing, a photograph was taken to clearly observe the changes that occurred in the hydrogel during the degradation process; then the reaction flask was quickly uprighted and placed back into the reactor, and the temperature was raised to 120℃ to continue degradation.

[0096] Figure 6(a) shows the state of the polyacrylamide hydrogel prepared in Example 4 before degradation. As can be seen from the figure, the polyacrylamide hydrogel at this moment is a complete cylindrical hydrogel. Figure 6(b) shows the state of the polyacrylamide hydrogel prepared in Example 4 after 60 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel has decreased by about 1 / 3 at this moment. Figure 6(c) shows the state of the polyacrylamide hydrogel prepared in Example 4 after 180 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel has decreased significantly further at this moment, and the remaining hydrogel volume is about 1 / 3 of the volume of the hydrogel before degradation. The liquid level of the NaOH aqueous solution has risen. Figure 6(d) shows the state of the polyacrylamide hydrogel prepared in Example 4 after 300 min of degradation. As can be seen from the figure, no polyacrylamide hydrogel can be observed in the bottle at this moment, and the liquid level of the NaOH aqueous solution continues to rise.

[0097] In summary, it can be seen from the entire degradation process that the polyacrylamide hydrogel prepared in Example 4 gradually degrades under the action of 120℃ and 20wt.% NaOH aqueous solution. When the degradation time reaches 300min, the polyacrylamide hydrogel completely recovers its fluidity, that is, the degradation is complete.

[0098] Example 5

[0099] Based on the crosslinking agent for the biodegradable hydrogel prepared in Example 3, a polyacrylamide hydrogel was prepared:

[0100] 0.5g acrylamide, 4.5g water, 0.005g potassium persulfate and 0.04g crosslinking agent were added to a reaction flask and stirred at room temperature for 5 min to prepare a homogeneous reaction system. Then, the reaction flask was placed in a high-pressure reactor and the temperature of the reaction system was raised to 150℃ and kept at that temperature for 1 h to obtain a biodegradable hydrogel.

[0101] To facilitate observation of the degradation process during subsequent degradation experiments, a cylindrical glass sampling bottle with a cap and an inner diameter of 20 mm was used for the reaction flask. Initially, the volume of the hydrogel in the reaction flask was about 1 / 2 of the bottle's volume.

[0102] The polyacrylamide hydrogel prepared above was subjected to degradation experiments: 5g of 20wt.% NaOH aqueous solution was added to the reaction flask, the cap was tightened and the flask was placed in a 150℃ water bath for 120min to allow the hydrogel to fully undergo degradation reaction; after 120min, the reaction vessel was removed and cooled. After the reaction vessel cooled, the reaction flask was removed from the reaction vessel and inverted for about 5-15s. After the liquid in the system in the reaction flask stopped flowing, a photograph was taken to clearly observe the changes that occurred in the hydrogel during the degradation process.

[0103] Figure 7(a) shows the state of the polyacrylamide hydrogel prepared in Example 5 before degradation. As can be seen from the figure, the polyacrylamide hydrogel at this moment is a complete cylindrical hydrogel. Figure 7(b) shows the state of the polyacrylamide hydrogel prepared in Example 5 after 120 min of degradation. As can be seen from the figure, the polyacrylamide hydrogel can no longer be observed in the bottle at this moment.

[0104] In summary, it can be seen from the entire degradation process that the polyacrylamide hydrogel prepared in Example 5 completely regained its fluidity when the degradation time reached 120 min under the action of 150℃ and 20wt.% NaOH aqueous solution, that is, the degradation was complete.

[0105] Example 6

[0106] Based on the crosslinking agent for the biodegradable hydrogel prepared in Example 3, a polyacrylamide hydrogel was prepared:

[0107] 0.2g acrylamide, 1.8g water, 0.002g potassium persulfate and 0.014g crosslinking agent were added to a reaction flask and stirred at room temperature for 5 min to prepare a homogeneous reaction system. Then, the reaction flask was placed in a water bath and the temperature of the reaction system was raised to 70℃ and kept at that temperature for 5 h to obtain a biodegradable hydrogel. During the reaction, it was observed that at 70℃, when the reaction time was about 20 min, the mixture in the reaction flask began to coagulate and form a cylindrical hydrogel.

[0108] To facilitate observation of the degradation process during subsequent degradation experiments, a cylindrical glass sampling bottle with a cap and an inner diameter of 20 mm was used for the reaction flask. Initially, the volume of the hydrogel in the reaction flask was about 1 / 5 of the bottle's volume.

[0109] Degradation experiments were conducted on the polyacrylamide hydrogel prepared above. 2g of 20wt.% NaOH aqueous solution was added to the reaction flask, the cap was tightened, and the flask was placed in a 70℃ water bath for 180min to allow the hydrogel to fully degrade. During the degradation process, the reaction flask was removed from the water bath at 60min, 120min, and 180min, and inverted for approximately 5–15s. After the liquid in the system stopped flowing, photographs were taken to clearly observe the changes in the hydrogel during degradation. The reaction flask was then quickly placed upright back into the water bath to continue the degradation process.

[0110] Figure 8(a) shows the state of the polyacrylamide hydrogel prepared in Example 5 before degradation. As can be seen from the figure, the polyacrylamide hydrogel at this moment is a complete cylindrical hydrogel. Figure 8(b) shows the state of the polyacrylamide hydrogel prepared in Example 5 after 60 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel has decreased at this moment. Figure 8(c) shows the state of the polyacrylamide hydrogel prepared in Example 5 after 120 min of degradation. As can be seen from the figure, the volume of the polyacrylamide hydrogel has decreased significantly further at this moment, and the liquid level of the NaOH aqueous solution has risen. Figure 8(d) shows the state of the polyacrylamide hydrogel prepared in Example 5 after 180 min of degradation. As can be seen from the figure, the polyacrylamide hydrogel can no longer be observed in the bottle at this moment, and the liquid level of the NaOH aqueous solution continues to rise.

[0111] In summary, the degradation process shows that the polyacrylamide hydrogel prepared in Example 6 gradually degrades under the action of 70℃ and 20wt.% NaOH aqueous solution. When the degradation time reaches 180 min, the polyacrylamide hydrogel completely recovers its fluidity, indicating complete degradation. Furthermore, the polyacrylamide hydrogel of Example 6 was prepared repeatedly, and 2g of 20wt.% NaCl aqueous solution was added to the reaction flask as a breaker. The flask was then capped and placed in a 70℃ water bath for 550 min to allow for complete degradation. During the degradation process, the reaction flask was removed from the water bath at 60 min, 120 min, and 550 min, and inverted for approximately 5–15 s. After the liquid in the system stopped flowing, photographs were taken to clearly observe the changes in the hydrogel during degradation. The reaction flask was then quickly uprighted and returned to the water bath to continue degradation.

[0112] Figure 9(a) shows the state of the polyacrylamide hydrogel prepared in Example 6 before degradation. At this moment, the polyacrylamide hydrogel is a complete cylindrical hydrogel. Figure 9(b) shows the state of the polyacrylamide hydrogel prepared in Example 6 after 60 min of degradation. At this moment, the volume of the polyacrylamide hydrogel has not changed. Figure 9(c) shows the state of the polyacrylamide hydrogel prepared in Example 6 after 120 min of degradation. The state of the polyacrylamide hydrogel at this moment is similar to that at 60 min of degradation. Figure 9(d) shows the state of the polyacrylamide hydrogel prepared in Example 6 after 550 min of degradation. At this moment, the volume of the polyacrylamide hydrogel has increased slightly. It can be seen that the polyacrylamide hydrogel prepared in Example 6 is difficult to degrade under the action of NaCl aqueous solution, indicating that the polyacrylamide hydrogel of this application has specificity in the selection of degrading agent. Similarly, the polyacrylamide hydrogel of Example 6 was prepared again. A small piece of hydrogel was taken and weighed, and its weight was 69 mg. The 69 mg hydrogel was placed in a reaction flask, and 9 mL of water was added to the reaction flask. While stirring, the temperature of the reaction system was raised to 90°C, and the reaction was continued at 90°C for 1 hour. After 1 hour, it was observed that the hydrogel did not dissolve, but only swelled. The swollen hydrogel was taken out and weighed, and its weight was 317 mg, which proves that the crosslinking agent prepared in this application can indeed produce a chemical crosslinking reaction on polyacrylamide hydrogels.

[0113] Example 7

[0114] Based on the crosslinking agent for the biodegradable hydrogel prepared in Example 3, a polyacrylamide hydrogel was prepared:

[0115] 0.16g acrylamide, 1.84g water, 0.0016g potassium persulfate and 0.014g crosslinking agent were added to a reaction flask and stirred at room temperature for 5 min to prepare a homogeneous reaction system. Then, the reaction flask was placed in a water bath and the temperature of the reaction system was raised to 70℃ and kept at that temperature for 5 h to obtain a biodegradable hydrogel. During the reaction, it was observed that at 70℃, when the reaction time was about 20 min, the mixture in the reaction flask began to coagulate and form a cylindrical hydrogel.

[0116] To facilitate observation of the degradation process during subsequent degradation experiments, a cylindrical glass sampling bottle with a cap and an inner diameter of 20 mm was used for the reaction flask. Initially, the volume of the hydrogel in the reaction flask was about 1 / 5 of the bottle's volume.

[0117] Degradation experiments were conducted on the polyacrylamide hydrogel prepared above. 2g of 20wt.% NaOH aqueous solution was added to the reaction flask, the cap was tightened, and the flask was placed at room temperature to allow the hydrogel to undergo a full degradation reaction. During the entire degradation process, the reaction flask was inverted for about 5-15 seconds at 24h, 48h, and 72h, respectively. After the liquid in the system in the reaction flask stopped flowing, photographs were taken to clearly observe the changes that occurred in the hydrogel during the degradation process. Afterward, the reaction flask was quickly uprighted and the degradation continued.

[0118] Figure 10(a) shows the state of the polyacrylamide hydrogel prepared in Example 7 at 0h of degradation. At this moment, the polyacrylamide hydrogel is a complete cylindrical hydrogel. Figure 10(b) shows the state of the polyacrylamide hydrogel prepared in Example 7 at 24h of degradation. At this moment, the volume of the polyacrylamide hydrogel is small. Figure 10(c) shows the state of the polyacrylamide hydrogel prepared in Example 7 at 48h of degradation. As can be seen from the figure, at this moment, the volume of the polyacrylamide hydrogel has further decreased significantly, and the liquid level of the NaOH aqueous solution has risen. Figure 10(d) shows the state of the polyacrylamide hydrogel prepared in Example 5 at 72h of degradation. As can be seen from the figure, at this moment, no polyacrylamide hydrogel can be observed in the bottle, and the liquid level of the NaOH aqueous solution continues to rise.

[0119] In summary, it can be seen from the entire degradation process that the polyacrylamide hydrogel prepared in Example 7 can also gradually degrade under the action of room temperature and 20 wt.% NaOH aqueous solution. When the degradation time reaches 72 h, the polyacrylamide hydrogel completely recovers its fluidity, that is, the degradation is complete.

[0120] Example 8

[0121] (I) The specific preparation steps for the crosslinking agent used in the preparation of biodegradable hydrogels are as follows:

[0122] Add 2g of hydantoin epoxy resin, 0.02g of N,N-dimethylbenzylamine and 0.24g of acrylamide to a reaction flask, turn on the stir and heat to 120°C. React for 21 minutes under stirring, then stop stirring and let cool to room temperature to obtain a crosslinking agent for biodegradable hydrogels.

[0123] (II) Based on the above-mentioned crosslinking agent for biodegradable hydrogels, polyacrylamide hydrogels are prepared:

[0124] 0.4g acrylamide, 4.6g water, 0.004g potassium persulfate, and 0.035g crosslinking agent were added to a reaction flask and stirred at room temperature for 20 minutes. It was found that only part of the crosslinking agent dissolved. Then, the reaction flask was placed in a high-pressure reactor, and the temperature of the reaction system was raised to 70°C and kept at that temperature for 5 hours to obtain a biodegradable hydrogel. During the reaction, it was observed that at 70°C, when the reaction time was about 20 minutes, the mixture in the reaction flask began to coagulate and form a cylindrical hydrogel.

[0125] To facilitate observation of the degradation process during subsequent degradation experiments, a cylindrical glass sampling bottle with a cap and an inner diameter of 20 mm was used for the reaction flask. Initially, the volume of the hydrogel in the reaction flask was about 1 / 2 of the bottle's volume.

[0126] The polyacrylamide hydrogel prepared above was subjected to degradation experiments: 5g of 20wt.% NaOH aqueous solution was added to the reaction flask, the cap was tightened and the flask was placed in the reaction vessel, and the temperature of the reaction system was raised to 120℃ to allow the hydrogel to fully undergo degradation reaction; after 240min, the reaction vessel was removed and cooled. After the reaction vessel cooled, the reaction flask was removed from the reaction vessel, and the reaction flask was inverted for about 5-15s. After the liquid in the system in the reaction flask stopped flowing, a photograph was taken to clearly observe the changes that occurred in the hydrogel during the degradation process.

[0127] Figure 11(a) shows the state of the polyacrylamide hydrogel prepared in Example 8 before degradation. At this moment, the polyacrylamide hydrogel is a complete cylindrical hydrogel. Figure 11(b) shows the state of the polyacrylamide hydrogel prepared in Example 8 after 4 hours of degradation. As can be seen from the figure, at this moment, no polyacrylamide hydrogel can be observed in the bottle. The experimental results show that after 4 hours of degradation, the hydrogel in the reaction bottle in Example 8 has been completely degraded and has regained its fluidity.

[0128] Comparative Example 1

[0129] (I) A new crosslinking agent is prepared by replacing the hyaluronic acid epoxy resin in the crosslinking agent of the biodegradable hydrogel with bisphenol A type epoxy resin. The preparation steps are as follows:

[0130] 2.75g of bisphenol A type epoxy resin, 0.02g of N,N-dimethylbenzylamine and 0.16g of acrylamide were added to a reaction flask, the stirring was turned on and the temperature was raised to 100℃. After reacting for 17 minutes under stirring, the stirring was stopped and the mixture was cooled to room temperature to obtain the crosslinking agent. Among them, the bisphenol A type epoxy resin used was bisphenol A diglycidyl ether, which had an epoxy value of 0.51.

[0131] (II) Based on the above-mentioned crosslinking agent for biodegradable hydrogels, polyacrylamide hydrogels are prepared:

[0132] 0.4g acrylamide, 4.6g water, 0.004g potassium persulfate and 0.035g crosslinking agent were added to a reaction flask. After stirring at room temperature for 50 minutes, the mixture still could not dissolve in water to form a homogeneous solution. After heating the reaction system to 70°C and reacting for 5 hours, a viscous solution was obtained, and a hydrogel could not be obtained.

[0133] Based on the reaction phenomena in step (II), 0.0035 g of the crosslinking agent and 4.6 g of dimethyl sulfoxide prepared in step (I) were added to the reaction flask and stirred at room temperature for 20 min to obtain a homogeneous solution. It can be seen that the crosslinking agent is insoluble in water but soluble in the oil-soluble solvent dimethyl sulfoxide. That is, the crosslinking agent prepared by replacing the hydantoin epoxy resin with bisphenol A type epoxy resin cannot be dissolved in the aqueous system. Therefore, in step (II), the crosslinking agent cannot be dissolved in the aqueous system to react with acrylamide. Acrylamide can only self-polymerize to form a linear long-chain polyacrylamide polymer under the action of potassium persulfate, which is the viscous solution obtained after the reaction.

[0134] Comparative Example 2

[0135] Polyacrylamide hydrogels were prepared by replacing the crosslinking agent for biodegradable hydrogels in this application with the commonly used crosslinking agent N,N'-methylenebisacrylamide: 0.4 g acrylamide, 4.6 g water, 0.004 g potassium persulfate and 0.0045 g N,N-methylenebisacrylamide were added to a reaction flask and stirred at room temperature for 5 min to obtain a homogeneous reaction system; then, the reaction flask was placed in a high-pressure reactor and the temperature of the reaction system was raised to 90 °C and kept at that temperature for 2 h to obtain the hydrogel.

[0136] Degradation experiments were conducted on the polyacrylamide hydrogel prepared above. 5g of 20wt.% NaOH aqueous solution was added to the reaction flask, the cap was tightened, and the flask was placed in a 90℃ water bath for 240min to allow the hydrogel to fully degrade. During the entire degradation process, the reaction flask was removed from the water bath at 60min, 120min, and 240min, and inverted for approximately 5–15s. After the liquid in the system in the reaction flask stopped flowing, photographs were taken to clearly observe the changes that occurred in the hydrogel during the degradation process. The reaction flask was then quickly placed upright back into the water bath to continue the degradation process.

[0137] Figures 12(a), 12(b), 12(c), and 12(d) show the states of the polyacrylamide hydrogel prepared in Comparative Example 2 before degradation, and at 60 min, 120 min, and 240 min after degradation, respectively. As can be seen from the above four figures, the polyacrylamide hydrogel in the reaction flask remained as a cylindrical hydrogel throughout the entire degradation experiment, and no volume change occurred during the degradation process. That is, the polyacrylamide hydrogel prepared using N'N-methylenebisacrylamide as a crosslinking agent cannot be degraded by NaOH aqueous solution.

[0138] In addition, the polyacrylamide hydrogel of Comparative Example 2 was prepared repeatedly, and 5g of 20wt.% ammonium persulfate aqueous solution was added to the reaction flask as a degrading agent. The flask was then tightened and placed in a 90℃ water bath for 90min to allow the hydrogel to fully degrade. During the degradation process, the reaction flask was removed from the water bath at 60min, 120min, and 180min, and inverted for about 5-15s. After the liquid in the system in the reaction flask stopped flowing, photographs were taken to clearly observe the changes that occurred in the hydrogel during the degradation process. Afterward, the reaction flask was quickly placed upright and returned to the water bath to continue the degradation.

[0139] Figure 13(a) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 before degradation. At this moment, the polyacrylamide hydrogel is a complete cylindrical hydrogel. Figure 13(b) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 after 60 min of degradation. At this moment, the volume of the polyacrylamide hydrogel did not decrease significantly, but the surface in contact with the ammonium persulfate aqueous solution turned brown. Figure 13(c) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 after 120 min of degradation. The state of the polyacrylamide hydrogel at this moment is similar to that at 60 min of degradation. Figure 13(d) shows the state of the polyacrylamide hydrogel prepared in Comparative Example 2 after 180 min of degradation. At this moment, the state of the polyacrylamide hydrogel did not change further. It can be seen that the polyacrylamide hydrogel prepared in Comparative Example 2 is difficult to degrade under the action of ammonium persulfate aqueous solution, which proves the special application of the crosslinking agent prepared in this application in preparing biodegradable polyacrylamide hydrogels.

[0140] Comparative Example 3

[0141] 2g of hydantoin epoxy resin, 0.02g of N,N'-dimethylbenzylamine, and 0.2g of acrylamide were added to a reaction flask and stirred at room temperature for 5 minutes to obtain a mixed liquid. 0.4g of acrylamide, 0.46g of water, 0.004g of potassium persulfate, and 0.035g of the mixed liquid were added to a new reaction flask and stirred at room temperature for 2 minutes. The reaction flask was then placed in a high-pressure reactor, and the reaction system was heated to 70°C and reacted for 5 hours. The reaction was then stopped. At this point, it was observed that no hydrogel had formed in the reaction system of the reaction flask. Therefore, the raw material mixture of the crosslinking agent in this application cannot crosslink acrylamide to form a polyacrylamide hydrogel.

[0142] In summary, the crosslinking agent disclosed in this application is a special crosslinking agent capable of preparing biodegradable polyacrylamide hydrogels. Furthermore, the biodegradable polyacrylamide hydrogels prepared using the crosslinking agent of this application exhibit selective degradation characteristics, meaning they can only be degraded in alkaline solutions (such as 20 wt.% sodium hydroxide aqueous solution). Moreover, the biodegradable polyacrylamide hydrogels have a wide applicable temperature range, suitable for downhole temperatures from room temperature to 150°C, and the degradation time is adjustable within the range of 90 min to 72 h.

Claims

1. A crosslinking agent for preparing biodegradable hydrogels, characterized in that, It is composed of 100 parts by weight of hydantoin epoxy resin, 0.75 to 1.7 parts by weight of N,N-dimethylbenzylamine and 6 to 12 parts by weight of acrylamide; The preparation steps are as follows: Add hydantoin epoxy resin, N,N-dimethylbenzylamine and acrylamide to a reaction flask, turn on the stirrer and heat to 60~120℃, react for 17~75 min under stirring, and then cool to room temperature to obtain a crosslinking agent for biodegradable hydrogels.

2. A biodegradable polyacrylamide hydrogel prepared using the crosslinking agent according to claim 1, characterized in that, It includes 0.2 to 0.5 parts by weight of acrylamide, 1.8 to 4.6 parts by weight of water, 0.002 to 0.005 parts by weight of potassium persulfate and 0.014 to 0.04 parts by weight of crosslinking agent, which can achieve debonding when alkaline debonding solution is added.

3. A method for preparing the biodegradable polyacrylamide hydrogel according to claim 2, characterized in that, The steps are as follows: Add acrylamide, water, potassium persulfate and crosslinking agent to the reaction flask, stir at room temperature for at least 5 minutes, then raise the temperature of the reaction system to 70℃~150℃ and keep it at that temperature for 1~5 hours to obtain a biodegradable hydrogel.

4. The use of a biodegradable polyacrylamide hydrogel prepared with the crosslinking agent as described in claim 2 as a downhole plugging agent.

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