Preparation method and application of pva reinforced chitosan-based semi-interpenetrating network hydrogel

Abstract

The application discloses a preparation method and application of PVA reinforced chitosan-based semi-interpenetrating network hydrogel and belongs to the technical field of gel temporary plugging agents. The preparation method is as follows: polyvinyl alcohol aqueous solution and acid chitosan aqueous solution are mixed, a crosslinking agent and a pH regulator are added for pre-crosslinking to obtain nascent gel; the nascent gel is soaked in a reinforcing medium and then subjected to crosslinking reaction to obtain PVA reinforced chitosan-based semi-interpenetrating network hydrogel with a compact crosslinking network. The hydrogel is suitable for well leakage plugging, water plugging of oil and gas wells, temporary plugging and diversion fracturing and the like, and has the application advantages that the gel type temporary plugging agent is in a low-viscosity liquid state in an initial state, is convenient to inject, can be automatically degraded and broken, does not need to add a breaker, is a low-viscosity liquid after being broken, is easy to flow back, has very small damage to a well, has the characteristics of reservoir protection, environmental friendliness and the like.

Description

Technical Field

[0001] This invention relates to the field of gel temporary plugging agents, and more specifically, to a method for preparing and applying a PVA-reinforced chitosan-based semi-interpenetrating network hydrogel. Background Technology

[0002] Temporary plugging agents are functional materials that can temporarily form a sealing barrier in the formation. They are injected into the wellbore in combination with water-soluble polymers, relying on pressure differential to rapidly construct a dense temporary plugging layer, thereby effectively controlling reservoir fluid production or working fluid loss. This temporary plugging layer can be released automatically or through external intervention within a preset time, restoring the original permeability of the formation. Existing temporary plugging systems mainly include granular, fibrous, and gel types; however, they all have certain application limitations. For example, if the particle size of granular temporary plugging agents is not properly matched with the pore throat, it can easily cause mis-plugging or be difficult to degrade, leading to reservoir damage. Fiber materials generally face problems of insufficient plugging strength and poor degradation performance. As for gel-type temporary plugging agents, the controllability of their gelation and unplugging processes is low, and the construction process is relatively complex.

[0003] Current research on gels largely focuses on improving the mechanical strength of materials, with insufficient attention paid to the systematic regulation of their degradation behavior. Existing unblocking methods mainly rely on physical methods such as mechanical drilling or chemical treatment with strong oxidants, but these methods have significant limitations in application, especially for gel systems with high strength suitable for pressurized operations, where their cross-linked structures are often difficult to break down using conventional methods. In common low-temperature wellbore environments (60-80℃), the decomposition efficiency of oxidants is low, further increasing the difficulty of gel breaking. Therefore, exploring gel self-degradation mechanisms based on the controllable breaking of chemical bonds has become an important approach to improving the unblocking properties of temporary plugging agents. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing and applying a PVA-reinforced chitosan-based semi-interpenetrating network hydrogel. The gel temporary plugging agent degrades on its own within a certain period of time, and after degradation, it becomes a low-viscosity liquid that is discharged automatically.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for preparing a PVA-reinforced chitosan-based semi-interpenetrating network hydrogel includes the following steps:

[0007] (a) Mix polyvinyl alcohol aqueous solution and acidic chitosan aqueous solution, add crosslinking agent and pH adjuster for pre-crosslinking to obtain nascent gel;

[0008] (b) The nascent gel was immersed in a reinforcing medium and then subjected to a cross-linking reaction to obtain a PVA-reinforced chitosan-based semi-interpenetrating network hydrogel with a dense cross-linked network.

[0009] The present invention is further configured such that the mass ratio of polyvinyl alcohol to chitosan and crosslinking agent is (1-15):(1-3):(1-4), and polyvinyl alcohol accounts for 1-2% of the total mass of polyvinyl alcohol aqueous solution and acidic chitosan aqueous solution.

[0010] The present invention is further configured such that the crosslinking agent is at least one of citric acid, propylene oxide, and epichlorohydrin.

[0011] The present invention is further configured such that the pH adjuster is used to adjust the pH value of the system to 6-7, and the pH adjuster is at least one of sodium bicarbonate and sodium acetate.

[0012] The present invention is further configured such that the pre-crosslinking reaction temperature is 40-100℃ and the reaction time is 10-180min.

[0013] The present invention is further configured such that the reinforcing medium is an aqueous solution of at least one of sodium citrate, sodium sulfate, disodium hydrogen phosphate, and borax, with a concentration of 0.1 mol / L to saturation concentration.

[0014] The treatment temperature for nascent gels in the reinforcing medium is 5℃, and the treatment time is 12-24h.

[0015] The present invention is further configured such that the soaking temperature is 5°C and the soaking time is 12-24h.

[0016] The present invention is further configured such that the crosslinking reaction temperature in step (b) is 40-100℃ and the reaction time is 120min.

[0017] The application of a PVA-reinforced chitosan-based semi-interpenetrating network hydrogel as a temporary plugging agent in well leakage sealing, water shut-off in oil and gas wells, temporary plugging and directional fracturing, and pressurized well workover operations in water injection wells.

[0018] In summary, the present invention has the following beneficial effects:

[0019] (1) The PVA-reinforced chitosan-based semi-interpenetrating network hydrogel of the present invention is formed by heating and cross-linking a liquid low-viscosity gel precursor liquid (30-90 mPa·s) at a certain temperature (40-100℃) for a certain time. The formed hydrogel can be automatically broken down by heating at a certain temperature (40-100℃) for a certain period of time and then become a low-viscosity liquid (viscosity between 60-80 mPa·s). Therefore, this principle of "reversible phase transformation from liquid to gel to liquid" can be used to apply the hydrogel to well leakage sealing and oil and gas... This technology is applicable to temporary formation sealing in well water shut-off, temporary plugging and fracturing, and pressurized well workover operations in water injection wells under medium and high temperature environments. The process involves injecting a low-viscosity liquid gel precursor followed by cross-linking and plugging. The injection operation is convenient, and the gel system degrades after temporary plugging without the need for breaker agents. The degraded low-viscosity liquid is easily flushed back from the wellbore and causes minimal damage to the producing formation, significantly improving formation recovery capabilities after the operation and reducing the risk of contamination to the wellbore and reservoir. This technology is of great value in promoting the development and large-scale application of this type of workover technology.

[0020] (2) By adjusting the crosslinking agent, pH value and content of the system, the degradation time of the hydrogel temporary plugging agent can be controlled to 10-30 days, which can meet the requirements of the degradation time of the temporary plugging agent under different construction time. Attached Figure Description

[0021] Figure 1 Here is a scanning electron microscope image of the CSPE-1 hydrogel from Example 1;

[0022] Figure 2 This is a scanning electron microscope image of the CSPE-2 hydrogel from Example 2;

[0023] Figure 3 Here is a scanning electron microscope image of the CSPE-3 hydrogel from Example 3;

[0024] Figure 4 Here is a scanning electron microscope image of the CSPE-4 hydrogel from Example 4;

[0025] Figure 5 Here is a scanning electron microscope image of the CSPE-5 hydrogel from Example 5;

[0026] Figure 6 The infrared spectra of CSPE-1 hydrogel (Example 1) and CSPE-2 hydrogel (Example 2) are shown. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0028] The hydrogel plugging agent described in this invention is prepared through a synergistic process involving chemical crosslinking and physical interpenetration. Its core lies in using a small-molecule crosslinking agent to link biopolymers and synthetic polymers, forming a stable three-dimensional network that encapsulates water molecules. The hydrogel constructed through the synergistic combination of chemical crosslinking and physical interpenetration integrates the stability of covalent bonds with the dynamism of non-covalent bonds, exhibiting superior mechanical properties, environmental responsiveness, controllable degradation, and application reliability compared to traditional purely chemically crosslinked hydrogels. It is particularly suitable for applications such as high-performance plugging, controllable remediation, and complex reservoir conditions. The specific preparation method is as follows:

[0029] (a) Mix polyvinyl alcohol aqueous solution and acidic chitosan aqueous solution, add crosslinking agent and pH adjuster, and pre-crosslink at 40-100℃ for 10-180 min to obtain primary gel. The mass ratio of polyvinyl alcohol to chitosan and crosslinking agent is (1-15):(1-3):(1-4), with polyvinyl alcohol accounting for 1-2% of the total mass of the polyvinyl alcohol aqueous solution and acidic chitosan aqueous solution; the crosslinking agent is at least one of citric acid, propylene oxide, and epichlorohydrin; the pH adjuster is used to adjust the pH of the system to 6-7, and the pH adjuster is at least one of sodium bicarbonate and sodium acetate.

[0030] (b) The nascent gel is immersed in a reinforcing medium at 5°C for 12-24 hours, followed by a crosslinking reaction at 40-100°C for 120 minutes to obtain a PVA-reinforced chitosan-based semi-interpenetrating network hydrogel with a dense crosslinked network. The reinforcing medium is an aqueous solution of at least one of sodium citrate, sodium sulfate, disodium hydrogen phosphate, and borax, with a concentration ranging from 0.1 mol / L to saturation.

[0031] Example 1

[0032] (1) Weigh 5g of PVA and add it to 95g of deionized water. Stir and dissolve at 95℃ for 3h to obtain a 5wt% PVA solution.

[0033] (2) Weigh 3g of chitosan and add it to a 3wt% acetic acid solution. Stir and dissolve at room temperature for 4h to obtain a 3wt% CS solution.

[0034] (3) Mix PVA solution and CS solution at a mass ratio of 3:1, add 1% citric acid of the total mass of PVA solution and CS solution, add sodium acetate to adjust the pH of the system to 7, stir at room temperature for 2 hours; inject the mixed solution into a polytetrafluoroethylene mold and heat it in an 80℃ oil bath for 3 hours.

[0035] (4) Take out the gel after molding and soak it in 1 mol / L sodium citrate solution for 24 hours at a soaking temperature of 5℃ to obtain a hydrogel mixture solution.

[0036] (5) The hydrogel mixture was injected into a colorimetric tube and heated in an oil bath at 80°C. The phenomenon was observed every 0.5 hours, and the final gelation time was recorded as 2 hours. The final product was named CSPE-1 hydrogel. The CSPE-1 hydrogel was placed in an oil bath at 80°C, and it was found that it took 20 days to completely turn into a liquid.

[0037] Example 2

[0038] (1) Weigh 10g of PVA and add it to 90g of deionized water. Stir and dissolve at 95℃ for 3h to obtain a 10wt% PVA solution.

[0039] (2) Weigh 1g of chitosan and add it to a 1wt% acetic acid solution. Stir and dissolve at room temperature for 4h to obtain a 1wt% CS solution.

[0040] (3) Mix PVA solution and CS solution at a mass ratio of 1:1, add propylene oxide accounting for 1% of the total mass of PVA solution and CS solution, stir at room temperature for 2 hours; inject the mixed solution into a polytetrafluoroethylene mold, and heat it in an 80℃ oil bath for 3 hours.

[0041] (4) Take out the gel after molding and soak it in 0.5 mol / L sodium citrate solution for 24 hours at a soaking temperature of 5℃ to obtain a hydrogel mixture solution.

[0042] (5) Inject the hydrogel mixture into a colorimetric tube, place it in an oil bath and heat it at 80°C. Observe the phenomenon every 0.5 hours and record the final gelation time as 1.5 hours. The final product is called CSPE-2 hydrogel.

[0043] When the CSPE-2 hydrogel was placed in an 80°C oil bath, it was found that it took 25 days to completely turn into a liquid.

[0044] Example 3

[0045] (1) Weigh 8g of PVA and add it to 92g of deionized water. Stir at 95℃ for 3h to dissolve and obtain 8wt% PVA solution.

[0046] (2) Weigh 2g of chitosan and add it to a 2wt% acetic acid solution. Stir and dissolve at room temperature for 4h to obtain a 2wt% CS solution.

[0047] (3) Mix PVA solution and CS solution at a mass ratio of 2:1, add citric acid accounting for 1% of the total mass of PVA solution and CS solution, stir at room temperature for 2 hours; pour the mixed solution into a polytetrafluoroethylene mold and heat it in an 80℃ oil bath for 3 hours.

[0048] (4) Take out the gel after molding and soak it in 0.5 mol / L sodium citrate solution for 24 hours at a soaking temperature of 5℃ to obtain a hydrogel mixture solution.

[0049] (5) Inject the hydrogel mixture into a colorimetric tube and heat it in an oil bath at 80°C. Observe the phenomenon every 0.5 hours and record the final gelation time as 3 hours. The final product is called CSPE-3 hydrogel.

[0050] When the CSPE-3 hydrogel was placed in an 80°C oil bath, it was found that it took 16 days to completely turn into a liquid.

[0051] Example 4

[0052] (1) Weigh 6g of PVA and add it to 94g of deionized water. Stir at 95℃ for 3h to dissolve and obtain a 6wt% PVA solution.

[0053] (2) Weigh 1.6g of chitosan and add it to a 1.2wt% acetic acid solution. Stir and dissolve at room temperature for 4h to obtain a 1.6wt% CS solution.

[0054] (3) Mix PVA solution and CS solution at a mass ratio of 4:1, add citric acid accounting for 1% of the total mass of PVA solution and CS solution, and stir at room temperature for 2 hours.

[0055] (4) Inject the mixed solution into a polytetrafluoroethylene mold and heat it in an oil bath at 80°C for 3 hours; take out the gel after molding and soak it in a 2 mol / L sodium citrate solution for 24 hours at a soaking temperature of 5°C to obtain a hydrogel mixed solution.

[0056] (5) Inject the hydrogel mixture into a colorimetric tube and heat it in an oil bath at 80°C. Observe the phenomenon every 0.5 hours and record the final gelation time as 2.5 hours. The final product is called CSPE-4 hydrogel.

[0057] When the CSPE-4 hydrogel was placed in an 80°C oil bath, it was found that it took 28 days to completely turn into a liquid.

[0058] Example 5

[0059] (1) Weigh 3g of PVA and add it to 97g of deionized water. Stir at 95℃ for 3h to dissolve and obtain a 3wt% PVA solution.

[0060] (2) Weigh 2.4g of chitosan and add it to a 3.6wt% acetic acid solution. Stir and dissolve at room temperature for 4h to obtain a 2.4wt% CS solution.

[0061] (3) Mix PVA solution and CS solution at a mass ratio of 5:1, add citric acid accounting for 1% of the total mass of PVA solution and CS solution, stir at room temperature for 2 hours; pour the mixed solution into a polytetrafluoroethylene mold and heat it in an 80℃ oil bath for 3 hours.

[0062] (4) Take out the gel after molding and soak it in 0.8 mol / L sodium citrate solution for 24 hours at a soaking temperature of 5℃ to obtain a hydrogel mixture solution.

[0063] (5) Inject the hydrogel mixture into a colorimetric tube and heat it in an oil bath at 80°C. Observe the phenomenon every 0.5 hours and record the final gelation time as 2 hours. The final product is called CSPE-5 hydrogel.

[0064] When the CSPE-5 hydrogel was placed in an 80°C oil bath, it was found that it took 32 days to completely turn into a liquid.

[0065] The raw materials used in the above embodiments are:

[0066] Polyvinyl alcohol (PVA) is polyvinyl alcohol 088-20 from Shenzhen Baishun Chemical Co., Ltd., with a degree of polymerization of 1750±50.

[0067] Chitosan (CS) was purchased from Qingdao Yunzhou Biotechnology Co., Ltd. It is an industrial-grade chitosan with a specific viscosity and a degree of deacetylation >95%.

[0068] Scanning electron microscope images of the hydrogels prepared in Examples 1-5 are shown below. Figures 1-5 As shown; the infrared spectra of the hydrogels prepared in Examples 1 and 2 are shown in the figure. Figure 6 As shown.

[0069] The rheological properties of the hydrogels prepared in Examples 1-5 were tested using a HAAKEMARS 60 rheometer. -1 The viscosity changes of the CSPE gel suspension and the broken gel were measured at a fixed shear rate. The test results are shown in Table 1.

[0070] Table 1

[0071]

[0072] Table 1 shows that the viscosity of the precursor solution and the viscosity of the solution after gel breaking in each embodiment are both low. Both are flowable, low-viscosity liquids, without lumps, easy to flow back, and do not damage the formation. With the increase of PVA content in the system, the gel strength can reach the H level, indicating that the addition of PVA enhances the gel to a certain extent.

[0073] The data from the above embodiments show that the hydrogel of the present invention, as a temporary plugging agent, can meet the pressure requirements of the gel temporary plugging agent during the well workover process of water injection wells. It is suitable for processes such as well leakage sealing, water shut-off in oil and gas wells, and temporary plugging and diversion fracturing. It can automatically degrade after a certain period of time, and the gel formation and breaking time of the gel temporary plugging agent system are controllable. It can adapt to a temperature range of 40-80℃, greatly simplifying the construction process, and has the characteristics of reservoir protection and environmental friendliness.

[0074] The hydrogel plugging agents prepared in each embodiment exhibited moderate strength. This invention uses the visual code table of gel strength (GSC) proposed by Sydansk (1988) to evaluate gel strength (see Table 2). Gel strength is classified into AJ grades; higher strength indicates higher pressure resistance. Given the current trend of oilfield development towards deep and ultra-deep wells, higher pressure resistance has broader applicability in oilfield development. The method of this invention can successfully prepare high-strength hydrogels, with the highest strength grade reaching Level I: rigid gel.

[0075] Table 2

[0076]

[0077] A core with a physical model size of 2.5 cm in diameter, 10 cm in length, and a permeability of 36 mD (fluid conductivity of the core pores) was used to test the hydrogel plugging performance. Test procedure: After measuring the core pore volume and pre-plugging permeability, a discharge rate of 200 mL / h (3.3 mL / min) was selected, and the hydrogel mixture obtained in step (4) of the example was squeezed into the core in the same direction. The inlet and outlet valves of the core were closed, and the core was allowed to solidify at the temperature and time specified in the technical requirements of the plugging agent product (80℃, 3h).

[0078] The sealing performance test results of the hydrogel in Example 1 are as follows: the breakthrough pressure is 1.63 MPa, and the calculated breakthrough pressure gradient is 16.3 MPa / m (core length is 10 cm).

[0079] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. The application of a PVA-reinforced chitosan-based semi-interpenetrating network hydrogel as a temporary plugging agent, characterized in that, Includes the following steps: (a) Mix polyvinyl alcohol aqueous solution and acidic chitosan aqueous solution, add crosslinking agent and pH adjuster for pre-crosslinking to obtain primary gel; The mass ratio of polyvinyl alcohol to chitosan and crosslinking agent is (1-15):(1-3):(1-4), and polyvinyl alcohol accounts for 1-2% of the total mass of polyvinyl alcohol aqueous solution and acidic chitosan aqueous solution; the crosslinking agent is at least one of citric acid, propylene oxide, and epichlorohydrin; the pre-crosslinking reaction temperature is 40-80℃; (b) The nascent gel is immersed in a reinforcing medium and then subjected to a cross-linking reaction to obtain a PVA-reinforced chitosan-based semi-interpenetrating network hydrogel with a dense cross-linked network; the reinforcing medium is an aqueous solution of sodium citrate with a concentration of 0.1 mol / L to saturation concentration; After soaking treatment, a hydrogel mixture solution is obtained, which is a liquid low-viscosity gel precursor with a viscosity of 30-90 mPa·s; the crosslinking reaction temperature is 40-100℃; The PVA-reinforced chitosan-based semi-interpenetrating network hydrogel is used as a temporary plugging agent in well leakage sealing, water shut-off in oil and gas wells, temporary plugging and fracturing, and pressurized well workover operations in water injection wells. During the process, a liquid low-viscosity gel precursor is injected and then cross-linked for plugging. The formed hydrogel is then automatically broken down after being heated at 40-100℃, and then becomes a low-viscosity liquid with a viscosity between 60-80 mPa·s.

2. The application of the PVA-reinforced chitosan-based semi-interpenetrating network hydrogel according to claim 1 as a temporary plugging agent, characterized in that, The pH adjuster is used to adjust the pH value of the system to 6-7, and the pH adjuster is at least one of sodium bicarbonate and sodium acetate.

3. The application of the PVA-reinforced chitosan-based semi-interpenetrating network hydrogel according to claim 1 as a temporary plugging agent, characterized in that, The soaking temperature is 5℃ and the soaking time is 12-24h.

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