A polyacrylamide thickener, its preparation method and application

Abstract

This invention belongs to the field of oilfield chemical additives technology, and discloses a polyacrylamide thickener, its preparation method, and its application. This thickener is synthesized by polymerizing acrylamide, acrylic acid, a monomer containing sulfonic acid groups, and a long-chain cyclodextrin functional monomer. Its core lies in the introduction of long-chain cyclodextrin monomers and salt-resistant monomers containing sulfonic acid groups into the polymer backbone through molecular design: the long carbon chains can form a hydrophobic network to maintain the flexibility of the molecular chain; the sulfonic acid monomers can enhance salt resistance through the charge barrier effect; and the cyclodextrin cavity structure can stabilize the hydrophobic network, inhibiting the destruction of the associated structure by salt ions, ultimately achieving a significant improvement in the salt resistance and shear resistance of the thickener. This thickener maintains a high apparent viscosity even at 150℃ and a salinity of 50 g / L, making it suitable for fracturing operations in complex oil and gas reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of oilfield chemical additives technology, and relates to a polyacrylamide thickener, its preparation method and application. Background Technology

[0002] With the continuous growth of my country's energy demand and the increasing depletion of conventional oil and gas resources, oil and gas exploration and development are constantly advancing into deep and ultra-deep reservoirs, which have become an important backup for ensuring national energy security. However, deep and ultra-deep oil and gas reservoirs generally have characteristics such as deep burial, high temperature, high pressure, extremely low natural permeability, and complex pore structure, resulting in poor fluid flow capacity, making it difficult to achieve economical and efficient development using traditional extraction technologies. Hydraulic fracturing technology, as a core means of reservoir modification and artificially constructing high-conductivity channels, has become a key technology for improving the recovery rate of low-permeability, tight, and unconventional oil and gas reservoirs, and is widely used globally. Fracturing fluid, as the "blood" of hydraulic fracturing operations, directly determines the initiation and propagation morphology of fractures, the effective delivery and placement of proppant, and thus affects the post-fracturing production enhancement effect.

[0003] In recent years, polyacrylamide (PAM) and its derivatives have been widely used as the main thickeners in water-based fracturing fluids due to their advantages such as long molecular chains, good solubility, high thickening efficiency, and excellent drag-reducing performance, achieving good application results in conventional reservoirs. Researchers have also modified PAM through copolymerization, crosslinking, or the introduction of functional monomers to improve its overall performance. However, facing the high temperatures (>120℃) commonly found in deep and ultra-deep reservoirs, existing PAM-based thickeners have revealed serious technical bottlenecks: on the one hand, the molecular chains are prone to thermal oxidative degradation under high-temperature conditions, leading to main chain breakage and a decrease in molecular weight; on the other hand, polyvalent cations (such as Ca2+) in high-salinity formation water... 2+ Mg 2+ Salt-induced shrinkage occurs when the negative charge on the polymer chain is shielded, causing the molecular chain to change from an extended state to a coiled conformation, significantly reducing the solution viscosity. Furthermore, during high-speed downhole pumping, intense shearing further exacerbates the mechanical degradation of the polymer chain, resulting in irreversible viscosity loss. The synergistic effect of these multiple degradation mechanisms leads to low viscosity retention, insufficient proppant carrying capacity, and short effective action time in traditional PAM fracturing fluids used in deep applications, making it difficult to form fractures of sufficient length and conductivity, and thus failing to meet the operational requirements under complex conditions. Therefore, developing a novel polyacrylamide-based thickener with high temperature resistance, strong salt resistance, and excellent shear stability has become a key technical challenge and research hotspot in the field of fracturing materials. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a polyacrylamide thickener, its preparation method, and its application, thereby solving the technical problem that polyacrylamide-based fracturing fluid thickeners in the prior art are prone to thermal degradation, salt-induced shrinkage, and mechanical fracture under high temperature, high salinity, and strong shear conditions in deep and ultra-deep oil and gas reservoirs, resulting in a sharp drop in viscosity and insufficient proppant carrying capacity, making it difficult to meet the requirements of efficient fracturing operations.

[0005] This invention is achieved through the following technical solution: A method for preparing a polyacrylamide thickener includes the following steps: S1: Dissolve the polymerizable monomers in water, including acrylamide, acrylic acid, monomers containing sulfonic acid groups, and cyclodextrin functional monomers. Then adjust the pH of the polymerization system to between 7.0 and 7.5 in an ice-water bath to obtain a polymerization solution. S2: The temperature of the polymerization solution is lowered to 0-5℃, nitrogen is introduced to remove oxygen, and then an oxidant and a reducing agent are added to carry out the polymerization reaction to obtain a gel-like polymer block. S3: The polymer block is granulated, crushed and sieved to obtain a polymer thickener.

[0006] Preferably, the mass concentration of the polymeric monomer in the polymerization solution is 30% to 35%.

[0007] Preferably, the molar ratio of acrylamide, acrylic acid, monomers containing sulfonic acid groups, and cyclodextrin functional monomers is 1:(0.10-0.50):(0.01-0.05):(0.005-0.01).

[0008] Preferably, the monomer containing the sulfonic acid group is one of 2-acrylamido-2-methylpropanesulfonic acid, sodium vinyl sulfonate, and sodium methacrylate-2-ethanesulfonate.

[0009] Preferably, the nitrogen deoxygenation time is 30–40 min.

[0010] Preferably, the polymerization reaction time is 4 to 5 hours.

[0011] Preferably, the concentration of the oxidant and reducing agent after being added to the polymerization solution is 40–100 mg / L.

[0012] Preferably, the mass ratio of oxidant to reducing agent is 1:1.5.

[0013] A polyacrylamide thickener is prepared by the method described above.

[0014] The above-mentioned polyacrylamide thickener is used in thickening fracturing fluid.

[0015] Compared with the prior art, the present invention has the following beneficial technical effects: This invention discloses a method for preparing a polyacrylamide thickener. This method, through unique molecular design and polymerization process, constructs a dual synergistic system of "charge barrier and hydrophobic network," effectively solving the problem of performance degradation of polyacrylamide in deep / ultra-deep environments in existing technologies. First, by introducing long-chain cyclodextrin functional monomers, the long carbon chains of their side chains form a hydrophobic association network in aqueous solution, physically expanding the molecular chains and effectively resisting mechanical breakage and molecular chain shrinkage caused by high temperature and high shear, maintaining the flexibility of the molecules. Second, the introduced monomers containing sulfonic acid groups utilize the strong electronegativity of the sulfonic acid groups to form a "charge barrier," significantly enhancing the polymer's salt resistance in high-mineralization environments and preventing the absorption of salt ions (such as Ca2+). 2+ Mg 2+ This shields against "salt-induced precipitation" or "salt-induced shrinkage" caused by electrical charges. Finally, the unique cavity structure of cyclodextrin provides steric protection for the hydrophobic network, further inhibiting the damage of salt ions to the associated structure. This synergistic effect allows the prepared thickener to maintain a high apparent viscosity even under high temperature (150℃), high salinity, and strong shear conditions, thus meeting the stringent requirements for proppant carrying and rheological properties in deep oil and gas reservoir fracturing operations.

[0016] Furthermore, in the polymerization solution, the mass concentration of the polymerizing monomer is 30% to 35%, a concentration range that ensures the polymerization reaction system has a suitable viscosity. Too low a concentration will result in insufficient molecular weight of the product and poor thickening effect; too high a concentration will make heat dissipation difficult, easily leading to explosive polymerization or an excessively wide molecular weight distribution. This range helps to obtain polymer blocks with uniform molecular weight distribution, moderate gel strength, and good solubility.

[0017] Furthermore, the molar ratio of acrylamide, acrylic acid, sulfonic acid-containing monomer, and cyclodextrin functional monomer is 1:(0.10-0.50):(0.01-0.05):(0.005-0.01), which precisely controls the ratio of hydrophilic to hydrophobic groups. Acrylic acid regulates hydrophilicity; sulfonic acid-containing monomer (0.01-0.05%) ensures salt resistance; and cyclodextrin monomer (0.005-0.01%) provides hydrophobic association sites. This specific ratio range ensures that while introducing sufficient salt resistance and hydrophobic groups, excessive functional monomers do not lead to polymerization difficulties or reduced water solubility, achieving an optimal balance between temperature resistance, salt resistance, and solubility.

[0018] Furthermore, the monomer containing the sulfonic acid group is one of 2-acrylamido-2-methylpropanesulfonic acid, sodium vinyl sulfonate, and sodium methacrylate-2-ethanesulfonate. All three specified monomers (AMPS, sodium vinyl sulfonate, and sodium methacrylate-2-ethanesulfonate) contain a sulfonic acid group (-SO3H). Compared to traditional carboxylic acid groups, sulfonic acid groups exhibit higher ionization under high salinity and high temperature conditions, stronger resistance to high-valence metal ions, effectively prevent salt-induced precipitation, and significantly improve the viscosity retention rate of the thickener in deep, high-salinity formation water.

[0019] Furthermore, the nitrogen purging time for deoxygenation is 30–40 minutes, as oxygen is an inhibitor of free radical polymerization. Under low-temperature polymerization conditions of 0–5°C, thorough nitrogen purging for 30–40 minutes can completely eliminate dissolved oxygen in the system, ensuring that the redox initiation system can effectively initiate the polymerization reaction, preventing prolonged induction period or incomplete polymerization due to residual oxygen, thereby ensuring the molecular weight and batch stability of the product.

[0020] Furthermore, the polymerization reaction time is 4–5 hours, a range based on the kinetics of the low-temperature (0–5°C) initiation system. A reaction time of 4–5 hours is sufficient to ensure near-complete monomer conversion, resulting in a high molecular weight polymer. Too short a time leads to high monomer residue and low molecular weight; too long a time reduces production efficiency and may increase chain transfer reactions, affecting product performance.

[0021] Furthermore, after the oxidant and reducing agent are added to the polymerization solution, the concentration is 40–100 mg / L. The concentration of the redox initiator directly determines the rate of free radical generation. This concentration range (40–100 mg / L) is appropriate, ensuring that the reaction is initiated at low temperatures while avoiding excessive free radicals and excessively fast polymerization rates due to excessive concentration, which could lead to explosive polymerization or excessively low molecular weight (increased chain transfer). This results in the preparation of a high-viscosity, highly elastic gel polymer.

[0022] Furthermore, the mass ratio of oxidant to reducing agent is 1:1.5, which usually means that the reducing agent is in excess. This ratio helps to ensure that the oxidant reacts completely, reduces the subsequent oxidative degradation of the polymer backbone by residual oxidizing substances, and at the same time ensures the efficient operation of the initiation system, improving the conversion rate of the polymerization reaction and the stability of the product. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is the reaction equation for the synthesis of cyclodextrin functional monomers in this invention; Figure 2 The image shows the temperature and shear resistance test results of the thickener prepared in Example 5 of this invention at 120°C. Figure 3 The graph shows the temperature and shear resistance test results of the thickener prepared in Example 5 of this invention at 150°C. Detailed Implementation

[0025] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0026] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0027] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0028] In this article, unless otherwise specified, “contains,” “includes,” “containing,” “has,” or similar terms cover the meanings of “composed of” and “mainly composed of,” for example, “A contains a” covers the meanings of “A contains a and others” and “A contains only a.”

[0029] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0030] This invention provides a method for preparing a polyacrylamide thickener, comprising the following steps: S1: Dissolve the polymerizable monomers in water, including acrylamide, acrylic acid, monomers containing sulfonic acid groups, and cyclodextrin functional monomers. Then adjust the pH of the polymerization system to between 7.0 and 7.5 in an ice-water bath to obtain a polymerization solution. In the polymerization solution, the mass concentration of the polymeric monomer is 30% to 35%. The pH of the polymerization system was adjusted to 7.0–7.5 using one of sodium hydroxide, sodium bicarbonate, and potassium carbonate. The monomer containing the sulfonic acid group is 2. Acrylamide group 2 One of methylpropanesulfonic acid, sodium vinyl sulfonate, and sodium 2-ethanesulfonate methacrylate.

[0031] The cyclodextrin functional monomer is prepared by reacting β-cyclodextrin with undecenoyl chloride, and its direct equation is shown below. Figure 1 The specific preparation process is as follows: β-cyclodextrin is dissolved in a mixed solvent of N,N-dimethylformamide (DMF) and water, triethylamine is added as a catalyst and stirred evenly, then undecenoyl chloride is slowly added dropwise, and the reaction is carried out at room temperature for 8 hours to obtain undecenoyl-β-cyclodextrin functional monomer. The molar ratio of reactants β-cyclodextrin:undecenoyl chloride:triethylamine = 1:1:0.5; Total solvent usage: Based on the mass of β-cyclodextrin, 1g of β-cyclodextrin corresponds to 10g of mixed solvent; The mass ratio of the mixed solvents is N,N-dimethylformamide (DMF):water = 1:1.

[0032] S2: The temperature of the polymerization solution is lowered to 0-5℃, nitrogen is introduced to remove oxygen, and then an oxidant and a reducing agent are added to carry out the polymerization reaction to obtain a gel-like polymer block. The nitrogen deoxygenation time is 30–40 min; the polymerization reaction time is 4–5 h. The oxidant is one of ammonium persulfate, potassium persulfate, and sodium persulfate; The reducing agent is one of sodium bisulfite, ascorbic acid, and sodium formaldehyde sulfoxylate. After the oxidant and reducing agent are added to the polymerization solution, the concentration is 40-100 mg / L; the oxidant and reducing agent act as initiators, that is, the concentration of the initiator is 40-100 mg / L. The mass ratio of oxidant to reducing agent is 1:1.5; S3: The polymer block is granulated, crushed and sieved to obtain a polymer thickener.

[0033] In addition, the present invention also discloses a polyacrylamide thickener prepared by the above scheme; the structural formula of the polyacrylamide thickener is shown below: ; Among them, taking the total number of moles of each structural unit as 100%, x is 60%~85%, y is 10%~30%, z is 3%~15%, and p is 0.5%~5%; Meanwhile, the present invention also discloses the application of the above-mentioned polyacrylamide thickener in thickening fracturing fluid.

[0034] This invention introduces cyclodextrin functional monomers with long carbon chains and sulfonic acid functional monomers into the main chain of polyacrylamide, forming a dual synergistic system of "charge barrier and hydrophobic network". The long carbon chain hydrophobic network keeps the molecular chain extended, while the charge barrier enhances salt resistance. At the same time, the cavity structure of cyclodextrin can stabilize the hydrophobic network, prevent salt ions from destroying association, and increase the viscosity of polyacrylamide in brine. This gives the product better salt resistance and shear strength, effectively improving the viscosity stability and shear resistance of fracturing fluid in high-salt environments, meeting the fracturing requirements of deep and ultra-deep oil and gas reservoirs.

[0035] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0036] The following examples use instruments and equipment conventional in the art. Experimental methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. All raw materials used in the following examples are conventional commercially available products with specifications conventional in the art. In this specification and the following examples, unless otherwise specified, "%" refers to weight percentage, "parts" refers to parts by weight, and "ratio" refers to weight proportion.

[0037] Example 1 A salt-resistant polyacrylamide thickener with hydrophobic cyclodextrin side chains is prepared by the following steps: Step 1: According to the monomers acrylamide, acrylic acid, 2 Acrylamide group 2 The molar ratio of methylpropanesulfonic acid and cyclodextrin functional monomers is 1:0.2:0.03:0.008. They are added to deionized water to prepare an aqueous solution with a total monomer mass concentration of 32%. The solution is stirred until it is completely dissolved to obtain a polymerization solution. The pH of the polymerization system is adjusted to 7.2 with sodium bicarbonate solution in an ice-water bath to obtain the polymerization solution. Step 2: Place the polymerization solution in a refrigerator to cool it down to below 3°C, then put it into a thermos flask and purge it with nitrogen for 40 minutes to remove oxygen. Add ammonium persulfate and sodium bisulfite in a mass ratio of 1:1.5 to form an initiator to initiate the polymerization reaction. The concentration of the initiator is 75 mg / L. Stop purging with nitrogen, keep the solution at a constant temperature and heat for 5 hours to obtain a gel-like and elastic polymer product. Step 3: Take out the above gel-like polymer product and cut it into small pieces. Dry it in a 70°C oven and then pulverize it with a pulverizer to obtain a white powdery polymer thickener.

[0038] Example 2 A salt-resistant polyacrylamide thickener with hydrophobic cyclodextrin side chains is prepared by the following steps: Step 1: Add the monomers acrylamide, acrylic acid, sodium methacrylate-2-ethanesulfonate, and cyclodextrin functional monomers in a molar ratio of 1:0.15:0.02:0.006 to deionized water to prepare an aqueous solution with a total monomer mass concentration of 35%. Stir until the solution is completely dissolved to obtain a polymerization solution. Adjust the pH of the polymerization system to 7.0 with sodium hydroxide solution in an ice-water bath to obtain the polymerization solution. Step 2: Place the polymerization solution in a refrigerator to cool it to below 3°C, then put it into a thermos flask and purge it with nitrogen for 30 minutes to remove oxygen. Add sodium persulfate and ascorbic acid in a mass ratio of 1:1.5 to form an initiator to initiate the polymerization reaction. The concentration of the initiator is 62.5 mg / L. Stop purging with nitrogen, keep the solution at the thermos flask, and react for 4.5 hours to obtain a gel-like and elastic polymer product. Step 3: Take out the above gel-like polymer product and cut it into small pieces. Dry it in a 70°C oven and then pulverize it with a pulverizer to obtain a white powdery polymer thickener.

[0039] Example 3 A salt-resistant polyacrylamide thickener with hydrophobic cyclodextrin side chains is prepared by the following steps: Step 1: Add the monomers acrylamide, acrylic acid, sodium methacrylate-2-ethanesulfonate, and cyclodextrin functional monomers in a molar ratio of 1:0.3:0.05:0.01 to deionized water to prepare an aqueous solution with a total monomer mass concentration of 32%. Stir until the solution is completely dissolved to obtain a polymerization solution. Adjust the pH of the polymerization system to 7.5 with potassium carbonate solution in an ice-water bath to obtain the polymerization solution. Step 2: Place the polymerization solution in a refrigerator to cool it to 5°C, then put it into a thermos flask and purge it with nitrogen for 35 minutes to remove oxygen. Add potassium persulfate and sodium formaldehyde sulfoxylate in a mass ratio of 1:1.5 to form an initiator to initiate the polymerization reaction. The concentration of the initiator is 80 mg / L. Stop purging with nitrogen, keep the solution at a constant temperature and heat it for 5 hours to obtain a gel-like and elastic polymer product. Step 3: Take out the above gel-like polymer product and cut it into small pieces. Dry it in a 70°C oven and then pulverize it with a pulverizer to obtain a white powdery polymer thickener.

[0040] Example 4 A salt-resistant polyacrylamide thickener with hydrophobic cyclodextrin side chains is prepared by the following steps: Step 1: According to the monomers acrylamide, acrylic acid, 2 Acrylamide group 2 The molar ratio of methylpropanesulfonic acid and cyclodextrin functional monomers is 1:0.1:0.01:0.005. They are added to deionized water to prepare an aqueous solution with a total monomer mass concentration of 35%. The solution is stirred until it is completely dissolved to obtain a polymerization solution. The pH of the polymerization system is adjusted to 7.0 with sodium hydroxide solution in an ice-water bath to obtain the polymerization solution. Step 2: Place the polymerization solution in a refrigerator to cool it to 0°C, then put it into a thermos flask and purge it with nitrogen for 30 minutes to remove oxygen. Add ammonium persulfate and ascorbic acid in a mass ratio of 1:1.5 to form an initiator to initiate the polymerization reaction. The concentration of the initiator is 100 mg / L. Stop purging with nitrogen, keep the solution at a constant temperature and heat it for 4 hours to obtain a gel-like and elastic polymer product. Step 3: Take out the above gel-like polymer product and cut it into small pieces. Dry it in a 70°C oven and then pulverize it with a pulverizer to obtain a white powdery polymer thickener.

[0041] Example 5 A salt-resistant polyacrylamide thickener with hydrophobic cyclodextrin side chains is prepared by the following steps: Step 1: According to the monomers acrylamide, acrylic acid, 2 Acrylamide group 2 The molar ratio of methylpropanesulfonic acid and cyclodextrin functional monomers is 1:0.2:0.04:0.005. They are added to deionized water to prepare an aqueous solution with a total monomer mass concentration of 32%. The solution is stirred until it is completely dissolved to obtain a polymerization solution. The pH of the polymerization system is adjusted to 7.0 with sodium hydroxide solution in an ice-water bath to obtain the polymerization solution. Step 2: Place the polymerization solution in a refrigerator to cool it to 5°C, then put it into a thermos flask and purge it with nitrogen for 30 minutes to remove oxygen. Add potassium persulfate and sodium bisulfite in a mass ratio of 1:1.5 to form an initiator to initiate the polymerization reaction. The concentration of the initiator is 75 mg / L. Stop purging with nitrogen, keep the solution at a constant temperature and heat it for 4 hours to obtain a gel-like and elastic polymer product. Step 3: Take out the above gel-like polymer product and cut it into small pieces. Dry it in a 70°C oven and then pulverize it with a pulverizer to obtain a white powdery polymer thickener.

[0042] Example 6 A salt-resistant polyacrylamide thickener with hydrophobic cyclodextrin side chains is prepared by the following steps: Step 1: Add the monomers acrylamide, acrylic acid, sodium methacrylate-2-ethanesulfonate, and cyclodextrin functional monomers in a molar ratio of 1:0.5:0.04:0.009 to deionized water to prepare an aqueous solution with a total monomer mass concentration of 35%. Stir until the solution is completely dissolved to obtain a polymerization solution. Adjust the pH of the polymerization system to 7.3 with sodium hydroxide solution in an ice-water bath to obtain the polymerization solution. Step 2: Place the polymerization solution in a refrigerator to cool it down to below 3°C, then put it into a thermos flask and purge it with nitrogen for 40 minutes to remove oxygen. Add sodium persulfate and sodium formaldehyde sulfoxylate in a mass ratio of 1:1.5 to form an initiator to initiate the polymerization reaction. The concentration of the initiator is 50 mg / L. Stop purging with nitrogen, keep the solution at a constant temperature and heat for 5 hours to obtain a gel-like and elastic polymer product. Step 3: Take out the above gel-like polymer product and cut it into small pieces. Dry it in a 70°C oven and then pulverize it with a pulverizer to obtain a white powdery polymer thickener.

[0043] The performance of the products obtained in the embodiments of the present invention was tested through the following tests: (1) Temperature resistance test Example 1: Testing with a rheometer 6. Apparent viscosity of the prepared thickener at different temperatures. The mass fraction of the thickener was 1%, and the shear rate was 170 s⁻¹. 1 The test results are shown in Table 1.

[0044] Table 1 Example 1 6. Apparent viscosity of the prepared thickener at different temperatures

[0045] (2) Salt resistance test Add 55g NaCl, 20g KCl, 5.5g CaCl2, and 4.5g MaCl2 to 1L of deionized water, then dilute to prepare brine with different mineralization levels. (Example 1) The prepared thickener was dissolved in salt water to prepare a polymer solution with a mass fraction of 1%. The apparent viscosity was tested using a rheometer at 30°C. The test results are shown in Table 2.

[0046] Table 2 Example 1 6. Salt resistance test results of the prepared thickener

[0047] Comparison of Example 1 with Tables 1 and 2 As can be seen from 6, Example 5 still has a high apparent viscosity under high temperature and high salinity conditions, and has good temperature and salt resistance. Therefore, Example 5 is the best example.

[0048] (3) Temperature and shear resistance test The temperature and shear resistance of the thickener prepared in Example 5 were tested using a HAAKE Mars 40 rheometer, and an aqueous solution with a thickener powder concentration of 1% was prepared.

[0049] Shear resistance was tested at 120℃ with a shear rate of 170 s. 1 The temperature was raised to 120℃ and sheared for 2 hours. The results were as follows: Figure 2 As shown, the apparent viscosity can reach 124.52 mPa·s after shearing for 2 hours at 120℃.

[0050] Shear resistance was tested at 150℃ with a shear rate of 170 s. 1 The temperature was raised to 150℃ and sheared for 2 hours. The results were as follows: Figure 3 As shown, the apparent viscosity can reach 110.24 mPa·s after shearing for 2 hours at 150℃.

[0051] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a polyacrylamide thickener, characterized in that, Includes the following steps: S1: Dissolve the polymerizable monomers in water, including acrylamide, acrylic acid, monomers containing sulfonic acid groups, and cyclodextrin functional monomers. Then adjust the pH of the polymerization system to between 7.0 and 7.5 in an ice-water bath to obtain a polymerization solution. S2: The temperature of the polymerization solution is lowered to 0-5℃, nitrogen is introduced to remove oxygen, and then an oxidant and a reducing agent are added to carry out the polymerization reaction to obtain a gel-like polymer block. S3: The polymer block is granulated, crushed and sieved to obtain a polymer thickener.

2. The method for preparing a polyacrylamide thickener according to claim 1, characterized in that, The polymer solution contains 30% to 35% by mass.

3. The method for preparing a polyacrylamide thickener according to claim 1, characterized in that, The molar ratio of acrylamide, acrylic acid, monomers containing sulfonic acid groups, and cyclodextrin functional monomers is 1:(0.10-0.50):(0.01-0.05):(0.005-0.01).

4. The method for preparing a polyacrylamide thickener according to claim 1, characterized in that, The monomer containing the sulfonic acid group is one of 2-acrylamido-2-methylpropanesulfonic acid, sodium vinyl sulfonate, or sodium methacrylate-2-ethanesulfonate.

5. The method for preparing a polyacrylamide thickener according to claim 1, characterized in that, The nitrogen deoxygenation time is 30-40 minutes.

6. The method for preparing a polyacrylamide thickener according to claim 1, characterized in that, The polymerization reaction takes 4 to 5 hours.

7. The method for preparing a polyacrylamide thickener according to claim 1, characterized in that, After the oxidizing agent and reducing agent are added to the polymerization solution, the concentration is 40-100 mg / L.

8. The method for preparing a polyacrylamide thickener according to claim 1, characterized in that, The mass ratio of oxidant to reducing agent is 1:1.

5.

9. A polyacrylamide thickener, characterized in that, It is prepared by the method described in any one of claims 1 to 8.

10. The application of the polyacrylamide thickener according to claim 9 in thickening fracturing fluid.

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