Fracturing fluid thickening agent, its preparation method and application

Abstract

The present disclosure belongs to the technical field of oil and gas resource development, and provides a fracturing fluid thickening agent and a preparation method and application thereof. The thickening agent comprises a non-ionic acrylamide monomer, a vinyl imidazole sulfonic acid inner salt monomer and a cationic acrylate functional monomer. The preparation method comprises the following steps: mixing the monomers to obtain a mixed solution; adding an initiator and a co-initiator to the mixed solution and mixing to obtain a gel product; and drying, crushing and sieving the gel product to obtain the thickening agent. The application of the fracturing fluid thickening agent in any one of unconventional oil and gas development and offshore oil and gas field seawater-based fracturing operation is also disclosed. The fracturing fluid thickening agent has excellent salt resistance, high-temperature resistance and shear resistance, has a high viscosity retention rate under high-temperature and high-salinity conditions, can be directly used for fracturing fluid preparation with formation flowback water or seawater, has a small dosage, has a good viscosity stabilization effect, and is suitable for unconventional oil and gas and seawater oil and gas field fracturing operation requirements.

Description

Technical Field

[0001] This disclosure relates to the field of oil and gas resource development technology, and in particular to a fracturing fluid thickener, its preparation method, and its application. Background Technology

[0002] As my country's oil and gas exploration and development continues to advance into deep, ultra-deep, and unconventional reservoirs such as shale oil, these reservoirs generally exhibit extreme operating conditions characterized by high temperatures, high salinity, and high calcium and magnesium divalent ions. This places extremely high demands on the comprehensive performance of water-based fracturing fluid thickeners in terms of temperature resistance, salt resistance, and shear resistance. Currently, commonly used fracturing fluid thickeners in the field are mainly acrylamide-based linear synthetic polymers, natural plant gums, and conventional hydrophobic associative polymers. However, these products all have significant technical shortcomings in extreme formation environments, making it difficult to meet the needs of efficient fracturing operations in deep unconventional oil and gas formations.

[0003] Conventional partially hydrolyzed polyacrylamide and other linear acrylamide polymers primarily rely on the electrostatic repulsion between anionic groups on their molecular chains to fully extend the chains in solution and generate viscosity. In high-salinity formation water, backflow, and seawater conditions, a large number of salt ions shield the surface charge of the polymer molecular chains, disrupting the electrostatic repulsion. This causes the molecular chains to rapidly retract from an extended state into a clustered mass, resulting in a sharp decrease in the apparent viscosity of the fracturing fluid and a loss of its sand-carrying and fracture-creating capacity. Simultaneously, when the formation temperature exceeds 90°C, the polymer backbone is prone to thermal oxidative cracking, and the amide groups in the side chains undergo deep hydrolysis. This not only causes a significant decrease in molecular weight but also generates a large number of carboxyl groups, further exacerbating the polymer's sensitivity to salt ions. Under high-temperature, long-term shear coupling, traditional linear polymers are highly susceptible to molecular chain degradation and conformational collapse, leading to rapid viscoelastic failure of the fracturing fluid and increasing the risk of engineering accidents such as sand-carrying failure and sand blockage.

[0004] Natural plant-based thickeners have inherent drawbacks such as high water-insoluble content, large amount of residue, severe reservoir damage, and limited temperature and salt resistance. In addition, their resource supply is limited and their cost is relatively high, making them unsuitable for large-scale fracturing operations in deep, high-temperature, and high-salinity reservoirs.

[0005] To address the insufficient temperature and salt resistance of linear polymers, the industry has developed conventional hydrophobic associative polymers. These polymers introduce hydrophobic side chains to form an intermolecular physical association network in aqueous solutions, thereby enhancing the viscoelasticity and static viscosity recovery of fracturing fluids through hydrophobic association. However, these polymers still face significant bottlenecks: First, hydrophobic association is a weak physical cross-linking process. Under high-speed shear conditions in the field, shear forces easily break the intermolecular hydrophobic association bonds, causing a sharp drop in viscosity, which cannot be effectively recovered in dynamic shear environments. Second, hydrophobic association is highly sensitive to temperature. Under high-temperature conditions, the movement of molecular chain segments intensifies, significantly weakening the hydrophobic association effect and severely reducing thickening and viscosity stabilization performance. Third, conventional hydrophobic associative polymers have weak resistance to divalent calcium and magnesium ions, resulting in significant performance degradation in high-salinity brine systems, making them unsuitable for the field requirements of direct preparation of fracturing fluids from high-salinity formation water and seawater.

[0006] In summary, current fracturing fluid thickeners generally suffer from three major drawbacks: susceptibility to high temperatures and high salinity, and poor shear resistance. They cannot be directly formulated using formation backflow water or seawater, rely heavily on freshwater resources, and incur high wastewater treatment costs, thus hindering the efficient and economical development of deep, unconventional, and offshore oil and gas resources. Therefore, developing a novel fracturing fluid thickener that combines excellent temperature resistance, salt tolerance, and shear resistance, and can be directly formulated into high-salinity water bodies, is a pressing technical challenge in the oil and gas fracturing field. Summary of the Invention

[0007] This disclosure provides a fracturing fluid thickener, its preparation method, and its application, in order to at least solve the above-mentioned technical problems existing in the prior art.

[0008] According to a first aspect of this disclosure, a fracturing fluid thickener is provided, comprising, by molar fraction, 65% to 80% of nonionic acrylamide monomers, 15% to 34% of vinylimidazolium sulfonate monomers, and 1% to 5% of cationic acrylate functional monomers.

[0009] Specifically, this disclosure provides a fracturing fluid thickener, which is designed as a ternary copolymer of nonionic acrylamide monomers, vinyl imidazole sulfonic acid inner salt monomers, and cationic acrylate functional monomers in a limited molar ratio. Through the synergistic effect of specific monomer structures and the optimization of the ratio, it solves the technical defects of traditional fracturing fluid thickeners at the molecular structure level, such as poor high temperature resistance, weak salt resistance, and rapid long-term shear viscosity decay: (1) It imparts an anti-electrolyte effect to the polymer. This disclosure introduces a vinyl imidazole sulfonic acid inner salt structure, which enables the polymer to have unique conformational recovery characteristics in a high-saltification brine environment. When subjected to high shear force, the molecular chain only undergoes temporary orientation and physical stretching, exhibiting shear thinning characteristics. After the shear force is removed, the extended conformation is its thermodynamically stable state in brine. The molecular chain can instantly rebound and recover its extended shape like a nano-spring by relying on the internal electrical balance of the single chain, without relying on complex intermolecular recombination, which greatly improves the system's shear resistance and viscosity self-recovery ability, and overcomes the disadvantage of conventional hydrophobic associating polymers being difficult to re-adhere after shearing. (2) It introduces an imidazole rigid ring structure to enhance the stability of the molecular chain. The imidazole rigid ring is inserted into the polymer molecular backbone, which significantly increases the resistance to thermal motion of the molecular chain, and transforms the polymer molecule from the traditional flexible coil structure to a rigid rod-shaped comb structure. This effectively inhibits the molecular chain curling, thermal degradation and chain segment movement disorder under high temperature conditions, and significantly improves the polymer's thermal stability and long-term shear resistance. It can still maintain a high effective viscosity under high temperature, high salinity and long-term shear conditions. (3) The precise molar ratio of ternary monomers is used for synergistic effect. The molar fraction limits the proportion of each monomer: nonionic acrylamide monomers 65%~80%, vinyl imidazole sulfonic acid inner salt monomers 15%~34%, and cationic acrylate functional monomers 1%~5%. This ratio range takes into account the basic viscosity-enhancing ability of the polymer, the proportion of inner salt structure and the cationic functional regulation. The three work together to avoid the problems of molecular curling under salt of traditional linear polymers and high temperature association failure of ordinary hydrophobic associating polymers. It can be adapted to direct preparation of high salinity formation water and return water. It still meets the viscosity requirements of fracturing fluid for sand carrying construction under high temperature and high salinity conditions.

[0010] In one embodiment, the nonionic acrylamide monomer is selected from acrylamide.

[0011] Specifically, nonionic acrylamide monomers serve as the main monomers and backbone monomers, forming the basis of the copolymer polymer backbone and providing the basic molecular weight and long molecular chain of the polymer; they have strong hydrophilic amide groups, which endow the polymer with excellent water solubility, ensuring that the thickener can dissolve quickly in water and spread and thicken; the good flexibility of the molecular chain provides the basic viscosity and gelling ability of the overall polymer, and is the matrix backbone for thickening fracturing fluid.

[0012] In one embodiment, the vinylimidazolium sulfonate monomer is selected from 1-propylsulfonic acid-3-vinylimidazolium sulfonate.

[0013] Specifically, the vinylimidazolium sulfonic acid inner salt monomer contains an imidazolium rigid ring, which can increase the rigidity and steric hindrance of the molecular chain, inhibit the shrinkage and thermal degradation of the molecular chain at high temperatures, and significantly improve temperature resistance and shear resistance. The sulfonic acid imidazolium inner salt structure has an anti-polyelectrolyte effect, and high-mineralization salt ions do not shield the charge, but instead promote the stretching of the molecular chain, achieving salt resistance and salt thickening, and is not afraid of calcium ions and magnesium ions. The zwitterionic structure of the inner salt can optimize the molecular chain conformation, and can quickly rebound and restore viscosity after shearing, improving the shear recovery of fracturing fluid.

[0014] In one embodiment, the cationic acrylate functional monomer is selected from dimethylaminoethyl methacrylate (DMEAEMA).

[0015] Specifically, the cationic acrylate functional monomers introduce cationic groups, which form intermolecular electrostatic interactions with anionic groups in the system, appropriately constructing a weak cross-linked network of molecular chains, further improving the viscoelasticity and sand-carrying capacity of the solution; improving the adsorption compatibility of the polymer on the surface of formation rocks, while also helping to enhance the molecular chain association stability of the polymer in high-salt environments, compensating for the sensitivity defects of single acrylamide salts; adjusting the microstructure of the copolymer, synergistically with imidazole inner salt monomers, further enhancing the overall high-temperature stability and shear resistance of the system.

[0016] According to a second aspect of this disclosure, a method for preparing the above-mentioned fracturing fluid thickener is provided, comprising the following steps: S1: Mix nonionic acrylamide monomers, vinyl imidazole sulfonate monomers, and cationic acrylate functional monomers to obtain a mixed solution; S2: Add an initiator and a co-initiator to the mixed solution, and react to obtain a gel product; S3: The gel product is dried, pulverized, and sieved to obtain the fracturing fluid thickener.

[0017] In one embodiment, the mass concentration of the mixed solution in step S1 is 24% to 26%.

[0018] In a preferred embodiment, the mass concentration of the mixed solution in step S1 is 25%.

[0019] In one embodiment, step S2 first deoxygenates the mixed solution, including the following steps: introducing an inert gas into the mixed solution for 25-35 minutes to deoxygenate.

[0020] Specifically, the inert gas is selected from at least one of nitrogen and argon.

[0021] In one embodiment, the initiator in step S2 is selected from at least one of ammonium persulfate, potassium persulfate, sodium persulfate, and hydrogen peroxide.

[0022] In one embodiment, the co-initiator in step S2 is selected from at least one of sodium bisulfite, sodium sulfite, ammonium bisulfite, and sodium thiosulfate.

[0023] In one embodiment, the total amount of the initiator and the co-initiator in step S2 is 0.1% to 0.3% of the total mass of the monomer, and the mass ratio of the initiator to the co-initiator is 1:1 to 1.5.

[0024] In one embodiment, the mixing speed in step S2 is 150~250 r / min, the reaction temperature is 40~60℃, and the time is 8~12 h.

[0025] In one embodiment, the drying temperature in step S3 is 35~45°C.

[0026] According to a third aspect of this disclosure, the application of the aforementioned fracturing fluid thickener is provided in any one of unconventional oil and gas development and seawater-based fracturing operations in offshore oil and gas fields; the unconventional oil and gas development includes any one of shale gas reservoir fracturing and shale oil reservoir fracturing.

[0027] Specifically, this disclosure allows for the direct use of high-mineralization formation produced water or backflow water to prepare fracturing fluid. Leveraging its unique anti-polyelectrolyte effect, it solves the problem of the sudden viscosity drop of traditional thickeners in brine environments, significantly reducing freshwater transportation costs and wastewater treatment pressure, and substantially improving the construction efficiency and environmental protection level of shale formations.

[0028] This disclosure also enables immediate preparation and use in seawater environments. Its superior resistance to monovalent and divalent ions (such as calcium and magnesium ions) allows the fracturing fluid to be directly prepared on offshore platforms without complex softening treatment, simplifying the construction process and offering extremely high economic benefits and operational convenience.

[0029] According to one possible implementation of this disclosure, at least the following beneficial effects are achieved: Compared with existing traditional fracturing fluid thickeners, this disclosure has excellent salt resistance, shear resistance and high temperature stability, effectively solving the three major technical problems of conventional thickeners being afraid of salt, heat and shear. Under extreme working conditions, only a low dosage is needed to maintain high viscosity and stable viscosity, and the comprehensive application advantages are significant.

[0030] First, it possesses excellent salt thickening properties. This disclosure introduces an imidazole sulfonic acid inner salt structure, which, relying on the anti-polyelectrolyte effect, can effectively dissolve the internal mutual attraction between inner salt molecules as the salt concentration of the system increases. This causes the polymer molecular chains to transform from a clustered and coiled state to a stretched and extended state, overcoming the defects of traditional polymers that suffer from charge shielding and a sharp drop in viscosity when exposed to salt. It is suitable for use in preparing solutions for highly mineralized water bodies.

[0031] Second, the shear resistance is significantly improved. The viscosity of the polymer disclosed in this invention no longer relies on the fragile hydrophobic association between molecules, but on the electrostatic spreading effect generated by the sulfonic acid groups, which allows the polymer molecular chains to fully extend and stabilize in aqueous solution. Under high-speed shear conditions, it is not prone to structural damage and precipitous viscosity decline. After the shearing effect is eliminated, the molecular conformation can be quickly restored, and it can still maintain a stable basic viscosity under long-term dynamic shear.

[0032] Third, it exhibits outstanding high-temperature thermal stability. The introduction of a rigid imidazole ring structure into the molecular chain significantly enhances the rigidity and steric hindrance of the polymer backbone, effectively suppressing polymer backbone coiling, thermal decomposition, and excessive hydrolysis under high-temperature conditions. This prevents molecular chain degradation and collapse, ensuring that the polymer maintains stable molecular conformation and viscosity properties under high-temperature conditions.

[0033] In summary, the thickener disclosed herein exhibits excellent comprehensive performance in terms of salt resistance, temperature resistance, and shear resistance. It can be used directly to prepare fracturing fluids from high-salinity formation produced water, backflow water, and seawater, reducing reliance on freshwater and wastewater treatment costs, and simplifying on-site construction processes. It has good economic value and application prospects in unconventional oil and gas and offshore oil and gas field fracturing operations.

[0034] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of this disclosure, nor is it intended to limit the scope of this disclosure. Other features of this disclosure will become readily apparent from the following description. Detailed Implementation

[0035] To make the objectives, features, and advantages of this disclosure more apparent and understandable, the technical solutions in the embodiments of this disclosure will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. Based on the embodiments in this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.

[0036] Example 1 This embodiment prepares a fracturing fluid thickener, as detailed below: (1) Take 80% acrylamide, 17% 1-propylsulfonic acid-3-vinylimidazolium salt and 3% dimethylaminoethyl methacrylate (DMEAEMA) by molar fraction to prepare a mixed solution with a mass concentration of 25%.

[0037] (2) Pour the mixed solution into a 500 mL glass reactor and place it in a 40 °C water bath. Pass N2 through to remove oxygen for 30 min. Then add ammonium persulfate and sodium bisulfite in a mass ratio of 1:1.2, with a total amount of 0.2% of the total monomer mass. Stir at a speed of 200 r / min until the solution shows signs of climbing the rod. Stop stirring and react for 10 h to obtain a gel-like product.

[0038] (3) Take out the gel-like product, cut it into pieces with scissors, put it in a 40°C oven to dry, then crush the block sample with a pulverizer, sieve the sample with a sieve, and collect the white powdery polymer sample, which is the fracturing fluid thickener.

[0039] Example 2 This embodiment prepares a fracturing fluid thickener, as detailed below: (1) Take 80% acrylamide, 19% 1-propylsulfonic acid-3-vinylimidazolium inner salt and 1% DMEAEMA by molar fraction to prepare a mixed solution with a mass concentration of 25%.

[0040] (2) Pour the mixed solution into a 500 mL glass reactor and place it in a 40 °C water bath. Pass N2 through to remove oxygen for 30 min. Then add ammonium persulfate and sodium bisulfite in a mass ratio of 1:1.2, with a total amount of 0.2% of the total monomer mass. Stir at a speed of 200 r / min until the solution shows signs of climbing the rod. Stop stirring and react for 10 h to obtain a gel-like product.

[0041] (3) Take out the gel-like product, cut it into pieces with scissors, put it in a 40°C oven to dry, then crush the block sample with a pulverizer, sieve the sample with a sieve, and collect the white powdery polymer sample, which is the fracturing fluid thickener.

[0042] Example 3 This embodiment prepares a fracturing fluid thickener, as detailed below: (1) Take 80% acrylamide, 15% 1-propylsulfonic acid-3-vinylimidazolium inner salt and 5% DMEAEMA by molar fraction to prepare a mixed solution with a mass concentration of 25%.

[0043] (2) Pour the mixed solution into a 500 mL glass reactor and place it in a 40 °C water bath. Pass N2 through to remove oxygen for 30 min. Then add ammonium persulfate and sodium bisulfite in a mass ratio of 1:1.2, with a total amount of 0.2% of the total monomer mass. Stir at a speed of 200 r / min until the solution shows signs of climbing the rod. Stop stirring and react for 10 h to obtain a gel-like product.

[0044] (3) Take out the gel-like product, cut it into pieces with scissors, put it in a 40°C oven to dry, then crush the block sample with a pulverizer, sieve the sample with a sieve, and collect the white powdery polymer sample, which is the fracturing fluid thickener.

[0045] Example 4 This embodiment prepares a fracturing fluid thickener, as detailed below: (1) Take 65% acrylamide, 34% 1-propylsulfonic acid-3-vinylimidazolium inner salt and 1% DMEAEMA by molar fraction to prepare a mixed solution with a mass concentration of 25%.

[0046] (2) Pour the mixed solution into a 500 mL glass reactor and place it in a 40 °C water bath. Pass N2 through to remove oxygen for 30 min. Then add ammonium persulfate and sodium bisulfite in a mass ratio of 1:1.2, with a total amount of 0.2% of the total monomer mass. Stir at a speed of 200 r / min until the solution shows signs of climbing the rod. Stop stirring and react for 10 h to obtain a gel-like product.

[0047] (3) Take out the gel-like product, cut it into pieces with scissors, put it in a 40°C oven to dry, then crush the block sample with a pulverizer, sieve the sample with a sieve, and collect the white powdery polymer sample, which is the fracturing fluid thickener.

[0048] Example 5 This embodiment prepares a fracturing fluid thickener, as detailed below: (1) Take 65% acrylamide, 30% 1-propylsulfonic acid-3-vinylimidazolium inner salt and 5% DMEAEMA by molar fraction to prepare a mixed solution with a mass concentration of 25%.

[0049] (2) Pour the mixed solution into a 500 mL glass reactor and place it in a 40 °C water bath. Pass N2 through to remove oxygen for 30 min. Then add ammonium persulfate and sodium bisulfite in a mass ratio of 1:1.2, with a total amount of 0.2% of the total monomer mass. Stir at a speed of 200 r / min until the solution shows signs of climbing the rod. Stop stirring and react for 10 h to obtain a gel-like product.

[0050] (3) Take out the gel-like product, cut it into pieces with scissors, put it in a 40°C oven to dry, then crush the block sample with a pulverizer, sieve the sample with a sieve, and collect the white powdery polymer sample, which is the fracturing fluid thickener.

[0051] Example 6 This embodiment prepares a fracturing fluid thickener, as detailed below: (1) Take 65% acrylamide, 32% 1-propylsulfonic acid-3-vinylimidazolium inner salt and 3% DMEAEMA by molar fraction to prepare a mixed solution with a mass concentration of 25%.

[0052] (2) Pour the mixed solution into a 500 mL glass reactor and place it in a 40 °C water bath. Pass N2 through to remove oxygen for 30 min. Then add ammonium persulfate and sodium bisulfite in a mass ratio of 1:1.2, with a total amount of 0.2% of the total monomer mass. Stir at a speed of 200 r / min until the solution shows signs of climbing the rod. Stop stirring and react for 10 h to obtain a gel-like product.

[0053] (3) Take out the gel-like product, cut it into pieces with scissors, put it in a 40°C oven to dry, then crush the block sample with a pulverizer, sieve the sample with a sieve, and collect the white powdery polymer sample, which is the fracturing fluid thickener.

[0054] Comparative Example 1 This comparative example provides a commercially available partially hydrolyzed polyacrylamide (HPAM) polymer with a molecular weight of 12 million and a degree of hydrolysis of 22%.

[0055] Comparative Example 2 This comparative example prepared a fracturing fluid thickener, which differs from Example 1 in that it did not contain the 1-propylsulfonic acid-3-vinylimidazolium inner salt binary polymer. Details are as follows: (1) Take 97% acrylamide and 3% DMEAEMA by molar fraction to prepare a mixed solution with a mass concentration of 25%.

[0056] (2) Pour the mixed solution into a 500 mL glass reactor and place it in a 40 °C water bath. Pass N2 through to remove oxygen for 30 min. Then add ammonium persulfate and sodium bisulfite in a mass ratio of 1:1.2, with a total amount of 0.2% of the total monomer mass. Stir at a speed of 200 r / min until the solution shows signs of climbing the rod. Stop stirring and react for 10 h to obtain a gel-like product.

[0057] (3) Take out the gel-like product, cut it into pieces with scissors, put it in a 40°C oven to dry, then crush the block sample with a pulverizer, sieve the sample with a sieve, and collect the white powdery polymer sample, which is the fracturing fluid thickener.

[0058] Comparative Example 3 This comparative example provides a fracturing fluid thickener. Unlike Example 1, the fracturing fluid thickener in this comparative example is a conventional hydrophobic associating polymer. Details are as follows: (1) Take 75% acrylamide, 22% acrylic acid and 3% octadecyl methacrylate by molar fraction to prepare a mixed solution with a mass concentration of 25%.

[0059] (2) Pour the mixed solution into a 500 mL glass reactor and place it in a 40 °C water bath. Pass N2 through to remove oxygen for 30 min. Then add ammonium persulfate and sodium bisulfite in a mass ratio of 1:1.2, with a total amount of 0.2% of the total monomer mass. Stir at a speed of 200 r / min until the solution shows signs of climbing the rod. Stop stirring and react for 10 h to obtain a gel-like product.

[0060] (3) Take out the gel-like product, cut it into pieces with scissors, put it in a 40°C oven to dry, then crush the block sample with a pulverizer, sieve the sample with a sieve, and collect the white powdery polymer sample, which is the fracturing fluid thickener.

[0061] Test case The base viscosity and shear resistance of the fracturing fluid thickeners in Examples 1-6 and Comparative Examples 1-3 were tested.

[0062] The fracturing fluid thickeners from Examples 1-6 and Comparative Examples 1-3 were all directly prepared into 0.5% solutions by mass using simulated formation water with a salinity of 100,000-220,000. These solutions were then tested at 90°C and a shear rate of 170 s⁻¹. -1 Its apparent viscosity and shear rate were measured at 170 s⁻¹. -1 The viscosity after shearing for 2 hours is shown in Table 1.

[0063] Table 1

[0064] Table 1 shows that in Examples 1-6, with the increase of the amount of 1-propylsulfonic acid-3-vinylimidazolium inner salt monomer, the viscosity reduction after high-temperature shear of the polymer gradually decreased, and the shear stability was significantly improved. Examples 4-6 (30%-34%) with higher inner salt content showed a viscosity reduction of only 22-43 mPa·s, with high viscosity retention after shear. In contrast, Examples 1-3 (15%-19%) with lower inner salt content showed a larger viscosity reduction, and the viscosity decay under long-term high-temperature shear was more pronounced. This indicates that introducing a high content of imidazolium sulfonic acid inner salt structure can significantly improve the high-temperature shear resistance of the polymer by relying on the electrostatic interaction of the sulfonic acid groups and the rigid structure of the imidazolium ring.

[0065] Examples 1-3, with an acrylamide content of 80%, generally had higher initial apparent viscosity. Among them, Example 3 had the lowest imidazole internal salt content and the highest acrylamide content, with an initial viscosity of 195 mPa·s. Examples 4-6, with acrylamide reduced to 65% and imidazole internal salt increased to 30%-34%, had slightly lower initial viscosity, but better high-temperature stability and shear resistance. The optimal ratio can be flexibly selected according to the on-site construction requirements for initial viscosity and long-term stability.

[0066] Comparative Example 1: The commercially available HPAM polymer exhibits extremely low initial and post-shear viscosity. Under high temperature and high salt conditions, its molecular chains easily coil and undergo severe thermal degradation, essentially losing its thickening and sand-carrying capacity. Comparative Example 2: Although it does not introduce imidazole sulfonic acid inner salt monomers and has a higher initial viscosity, its viscosity drops significantly after high-temperature shear, resulting in poor viscosity stability. Comparative Example 3: Conventional hydrophobic associative polymers rely on intermolecular hydrophobic association to form a network. Under high temperature and strong shear, these hydrophobic association bonds are easily broken, leading to a large viscosity decrease. After 2 hours of shear, the viscosity is only 24 mPa·s. In contrast, the apparent viscosity of each embodiment in this disclosure after 2 hours of shear is not less than 78 mPa·s, far exceeding the minimum standard of 50 mPa·s required by the "General Technical Conditions for Fracturing Fluids SY / T6376-2008". It demonstrates outstanding sand-carrying capacity under high temperature, high salt, and long-term shear conditions, overcoming the defects of traditional thickeners that are sensitive to heat, salt, and shear.

[0067] It should be understood that the various forms of processes shown above can be used to rearrange, add, or delete steps. For example, the steps described in this disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution of this disclosure can be achieved, and this is not limited herein.

[0068] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this disclosure, "a plurality of" means two or more, unless otherwise explicitly specified.

[0069] The above description is merely a specific embodiment of this disclosure, but the scope of protection of this disclosure is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this disclosure should be included within the scope of protection of this disclosure. Therefore, the scope of protection of this disclosure should be determined by the scope of the claims.

Claims

1. A fracturing fluid thickener, characterized in that, According to a molar fraction, the fracturing fluid thickener comprises 65% to 80% nonionic acrylamide monomers, 15% to 34% vinylimidazolium sulfonate monomers, and 1% to 5% cationic acrylate functional monomers.

2. The fracturing fluid thickener according to claim 1, characterized in that, The nonionic acrylamide monomer is selected from acrylamide.

3. The fracturing fluid thickener according to claim 1, characterized in that, The vinylimidazolium sulfonic acid inner salt monomer is selected from 1-propylsulfonic acid-3-vinylimidazolium inner salt.

4. The fracturing fluid thickener according to claim 1, characterized in that, The cationic acrylate functional monomer is selected from dimethylaminoethyl methacrylate.

5. A method for preparing the fracturing fluid thickener according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1: Mix nonionic acrylamide monomers, vinyl imidazole sulfonate monomers, and cationic acrylate functional monomers to obtain a mixed solution; S2: Add an initiator and a co-initiator to the mixed solution, and react to obtain a gel product; S3: The gel product is dried, pulverized, and sieved to obtain the fracturing fluid thickener.

6. The preparation method according to claim 5, characterized in that, The mass concentration of the mixed solution in step S1 is 24%~26%.

7. The preparation method according to claim 5, characterized in that, In step S2, the initiator is selected from at least one of ammonium persulfate, potassium persulfate, sodium persulfate, and hydrogen peroxide; The co-initiator is selected from at least one of sodium bisulfite, sodium sulfite, ammonium bisulfite, and sodium thiosulfate; The total amount of the initiator and the co-initiator is 0.1% to 0.3% of the total mass of the monomer, and the mass ratio of the initiator to the co-initiator is 1:1 to 1.

5.

8. The preparation method according to claim 5, characterized in that, In step S2, the mixing speed is 150~250 r / min, the reaction temperature is 40~60℃, and the time is 8~12 h.

9. The preparation method according to claim 5, characterized in that, The drying temperature in step S3 is 35~45℃.

10. The application of the fracturing fluid thickener according to any one of claims 1 to 4 in any one of unconventional oil and gas development and seawater-based fracturing operations in offshore oil and gas fields, characterized in that, The unconventional oil and gas development includes either shale gas reservoir fracturing or shale oil reservoir fracturing.

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