High-salt-tolerance structured fracturing fluid and preparation method thereof

Abstract

The invention discloses a high-salt-tolerance structured fracturing fluid and a preparation method thereof, and the fracturing fluid is prepared by taking a flowback fluid as a base fluid and adding a high-molecular polymer and a macromolecular surfactant. Wherein the high-molecular polymer is prepared by polymerizing five monomers, namely acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, a hydrophobic monomer A, sodium p-styrenesulfonate and N-vinyl pyrrolidone; the hydrophobic monomer A is long-chain alkyl quaternary ammonium salt. The macromolecular surfactant is prepared by polymerizing five monomers, namely acrylamide, butyl acrylate, acrylic acid, a hydrophobic monomer B and sodium p-styrenesulfonate; the hydrophobic monomer B is selected from at least one of 2-(acrylamide) sodium dodecyl sulfonate, 2-(acrylamide) sodium tetradecanesulfonate and 2-(acrylamide) sodium hexadecanesulfonate. The fracturing fluid prepared by adopting the flowback fluid or saline water with the total mineralization degree of 85,000 as the base fluid still keeps good viscosity, and good salt resistance and sand-carrying performance are guaranteed.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of oil field fracturing fluid, in particular to a high-salt-tolerant structured fracturing fluid and a preparation method thereof. BACKGROUND

[0002] With the large-scale development of unconventional oil and gas resources (such as shale gas and tight oil), hydraulic fracturing technology has become a key means to improve reservoir productivity. However, a large amount of high-salinity flowback fluid generated during fracturing operation not only contains high concentrations of inorganic salt ions such as Na + , K + , Ca 2+ , Mg 2+ , but also may contain heavy metals and organic additives, which poses a serious threat to the environment if directly discharged or treated. Therefore, how to realize the resource utilization of flowback fluid (such as directly used for preparing fracturing fluid) has become an important research direction for current oilfield environmental protection and cost reduction and efficiency improvement.

[0003] Currently, conventional polymer fracturing fluids (such as guanidine gum and polyacrylamide) have a significant performance decline in a high-salinity environment. This is mainly because high concentrations of salt ions compress the double electric layer of polymer molecular chains, resulting in a sharp decrease in viscosity, for example, hydroxypropyl guanidine gum is prone to crosslinking failure in the presence of Ca 2+ . In addition, the salting-out effect destroys the polymer network structure, and the sand-carrying capacity is greatly reduced. Moreover, the traditional crosslinking agent (such as organic boron and zirconium) is affected by other high-valence metal ions and impurities in the flowback fluid, the crosslinking effect is reduced, and the gel breaking is difficult, which aggravates the reservoir damage (core damage rate > 30%). In addition, the metal ions in the flowback fluid will complex with the carboxylate in the conventional fracturing polyacrylamide (partially hydrolyzed polyacrylamide), resulting in a decrease in the dissolution rate of the polymer and a low viscosity release rate, which ultimately affects the sand-carrying performance. Clean fracturing fluids (such as VES viscoelastic surfactant fracturing fluid and low molecular weight polymer fracturing fluid) have certain salt tolerance and low damage, low filtration loss, and convenient operation, but their application in a high-salinity environment still faces challenges. High salinity (especially Ca 2+ , Mg 2+ ) can destroy the micellar structure, resulting in a decrease in sand-carrying performance.

[0004] Therefore, it is urgent to develop a method for preparing fracturing fluid directly using high-salinity flowback fluid as base fluid, which can ensure that the fracturing fluid still maintains excellent sand-carrying performance and low environmental damage in an extremely high-salinity environment to meet the demand for green exploitation. SUMMARY

[0005] In view of the problems of poor salt tolerance and sand-carrying performance in the existing technology of preparing fracturing fluid using high-salinity flowback fluid as base fluid, the present application provides a high-salt-tolerant structured fracturing fluid and a preparation method thereof.

[0006] The high-salt-tolerant structured fracturing fluid provided by the application is prepared by adding a high-molecular polymer and a macromolecular surfactant into flowback fluid as a base fluid.

[0007] The high-molecular polymer is prepared by free radical polymerization of acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, a hydrophobic monomer A, sodium styrene sulfonate and N-vinylpyrrolidone.

[0008] The hydrophobic monomer A is a long-chain alkyl quaternary ammonium salt. Preferably, the hydrophobic monomer A is at least one selected from tetradecyldimethylallyl ammonium chloride, hexadecyldimethylallyl ammonium chloride and octadecyldimethylallyl ammonium chloride.

[0009] The preparation method of the high-molecular polymer is as follows:

[0010] S1: acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, sodium styrene sulfonate, N-vinylpyrrolidone and the hydrophobic monomer A are added into water and stirred and dissolved, and the pH of the solution is adjusted to 7.0-7.5 by sodium hydroxide. The mass ratio of acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, sodium styrene sulfonate, N-vinylpyrrolidone, the hydrophobic monomer A and water is (80-90):(5-10):(1-5):(1-2):(0.5-1):(300-350).

[0011] S2: azobisimidozolinium hydrochloride is added into the solution of step S1, and in order to improve the molecular weight of the polymerization product, low-temperature initiation is adopted, the solution system is cooled to 0℃, then transferred to a sealed and heat-insulated reaction container, deoxygenated by nitrogen, and then ammonium persulfate and cerium nitrate ammonium are added, stirred and deoxygenated by nitrogen for 10 min, then sodium bisulfite is added, stirred for several minutes, then stopped stirring, and deoxygenated by nitrogen until the solution becomes sticky, then continue to seal and heat-insulate and react for 5-7 h to obtain a rubber block.

[0012] S3: the rubber block is crushed, dried and ground to obtain a white powder of the high-molecular polymer.

[0013] The macromolecular surfactant is prepared by free radical polymerization of acrylamide, butyl acrylate, acrylic acid, a hydrophobic monomer B and sodium styrene sulfonate. The hydrophobic monomer B is at least one selected from 2-(acrylamido) dodecane sodium sulfonate, 2-(acrylamido) tetradecane sodium sulfonate and 2-(acrylamido) hexadecane sodium sulfonate.

[0014] The preparation method of the macromolecular surfactant is as follows:

[0015] S1: Add acrylamide, acrylic acid, sodium styrene sulfonate, butyl acrylate and hydrophobic monomer B to water and stir to dissolve. Adjust the pH of the solution to 7.0-7.5 with sodium hydroxide. The mass ratio of acrylamide, acrylic acid, sodium styrene sulfonate, butyl acrylate, hydrophobic monomer B and water is (20-30):(10-20):(10-15):(10-20):(5-10):(750-800).

[0016] S2: Add azobisisobutyrazoline hydrochloride to the solution in step S1, stir for several minutes and then stop stirring. Then place the reaction vessel in a constant temperature oven at 50-60℃ and let it stand for 7-9 hours to obtain a macromolecular surfactant solution.

[0017] Preferably, the fracturing fluid is prepared by mixing a high molecular weight polymer, a macromolecular surfactant, and flowback fluid in a mass ratio of (0.3-0.5):(0.5-1):100.

[0018] Compared with the prior art, the advantages of the present invention are:

[0019] (1) The introduction of 2-acrylamido-2-methylpropanesulfonic acid and sodium styrene sulfonate into the polymer of the present invention can effectively resist the compression effect of salt ions on the double layer and avoid a sudden drop in viscosity; a low temperature (0°C) initiation system is used in the preparation of the polymer to increase the molecular weight of the polymer; the polymer molecule does not contain salt-sensitive groups, ensuring good solubility in high-mineralized salt water and preventing salting out; the hydrophobic monomer A (long-chain alkyl quaternary ammonium salt) forms a dynamic physical cross-linking network through hydrophobic association, which can maintain molecular chain extension under high salt conditions and has good salt resistance.

[0020] (2) In this invention, sodium styrene sulfonate and butyl acrylate in the macromolecular surfactant synergistically enhance micellar stability, while the long-chain hydrophobic group of hydrophobic monomer B can interact with the hydrophobic chain in the polymer to enhance the polymer's performance. Moreover, by selecting different types of hydrophobic monomers for the polymer and the macromolecular surfactant, the interaction between the two hydrophobic monomers can be strengthened through counterionic interaction, resulting in a greater improvement in fracturing fluid performance.

[0021] (3) This invention employs a mixed micelle system formed by a polymer and a macromolecular surfactant. The long-chain backbone of the polymer provides structural viscosity, while the macromolecular surfactant forms a micelle network with the polymer through hydrophobic and electrostatic interactions. Furthermore, in high-mineralization environments, the micelles form more compactly, resulting in a more stable structure and better sand-carrying capacity and salt resistance.

[0022] (4) The polymer of the present invention does not contain carboxylic acid groups and will not complex with metal ions in the flowback fluid; if the polymer contains carboxylic acid groups, its complexation with metal ions will lead to a significant decrease in polymer solubility and a low viscosity release rate, ultimately resulting in a significant decrease in the performance of fracturing fluid.

[0023] (5) This invention directly uses high-mineralization flowback fluid as the base fluid to formulate fracturing fluid, which significantly reduces wastewater treatment costs and freshwater consumption. The fracturing fluid system contains no heavy metal crosslinking agents (such as organic boron / zirconium), avoiding reservoir damage caused by precipitation in traditional fracturing fluids, and has good environmental benefits.

[0024] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0025] Figure 1 This is a scanning electron microscope image of the freeze-dried fracturing fluid from Example 1.

[0026] Figure 2 The rheological diagram is a viscosity test result of the fracturing fluid in Example 1.

[0027] Figure 3 The rheological diagram is for the viscosity test of the fracturing fluid in Comparative Example 2.

[0028] Figure 4 The results are the fracturing fluid viscosity test results of Examples 1-5 and Comparative Examples 1-6.

[0029] Figure 5 The figures show the proppant carrying capacity test results of fracturing fluids under ambient temperature conditions for Examples 1-5 and Comparative Examples 1-6.

[0030] Figure 6 The figures show the proppant carrying capacity test results of the fracturing fluids in Examples 1-5 and Comparative Examples 1-6 under high temperature (90°C) conditions. Detailed Implementation

[0031] The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0032] In the following examples, all quantities of raw materials are by weight.

[0033] Example 1

[0034] A method for preparing a highly salt-tolerant structured fracturing fluid includes the following steps:

[0035] (1) 90 parts acrylamide, 10 parts 2-acrylamido-2-methylpropanesulfonic acid, 5 parts sodium styrene sulfonate, 1 part N-vinylpyrrolidone, 0.2 parts tetradecyl dimethyl allyl ammonium chloride, 0.3 parts hexadecyl dimethyl allyl ammonium chloride, and 300 parts water were added to a beaker and stirred to dissolve. The pH of the solution was adjusted to 7.2 using a 20% sodium hydroxide aqueous solution as a pH adjuster. Then, 0.05 parts azobisisobutyrazoline hydrochloride was added as an initiator. To increase the molecular weight of the product, low-temperature initiation was used. The solution was then cooled to 0℃. The mixture was then transferred to a thermos flask. N2 was bubbled through the flask for 30 minutes to remove oxygen from the reaction system. Then, 0.01 parts of ammonium persulfate and 0.01 parts of cerium ammonium nitrate were added and stirred until homogeneous. Nitrogen gas was bubbled through the flask for another 10 minutes. Then, 0.01 parts of sodium bisulfite were added and stirred until homogeneous. The stirring was stopped, and the mixture was allowed to stand until the solution became viscous. Nitrogen gas (N2) was then stopped, and the mixture was allowed to stand in adiabatic conditions for another 6 hours to obtain a gel block. The gel block was then crushed into particles with a diameter of 4-6 mm and dried at 80°C. After drying, the particles were further ground to obtain a white solid powder, which is the polymer.

[0036] (2) Add 20 parts acrylamide, 10 parts acrylic acid, 15 parts sodium styrene sulfonate, 20 parts butyl acrylate, 4 parts sodium 2-(acrylamido)tetradecanesulfonate, 6 parts sodium 2-(acrylamido)hexadecanesulfonate and 750 parts water to a beaker and stir to dissolve. Then use 20% sodium hydroxide aqueous solution as a pH adjuster to adjust the pH of the system to 7.0. Add 0.5 parts azobisisobutyrazoline hydrochloride and then place the beaker in a 50℃ constant temperature oven and let it stand for 8 hours to obtain a macromolecular surfactant solution.

[0037] (3) Take 5 parts of polymer powder, 5 parts of macromolecular surfactant solution and 1000 parts of simulated mineralized water with a total mineralization of 85,000 and add them into the stirring tank. Mix them evenly at a stirring speed of 500 rpm to obtain fracturing fluid.

[0038] The simulated mineralized water was prepared from NaCl, CaCl2 and MgCl2·6H2O. The composition ratio of the mineralized water is shown in Table 1.

[0039] Table 1. Dosage of each component in 1L of mineralized water

[0040] Salinity (mg / L) NaCl (g) KCl (g) CaCl2(g) MgCl2-6H2O (g) FeCl3(g) 85000 55 20 5.5 9.6 0.58

[0041] Example 2

[0042] Based on Example 1, in step (1), "0.3 parts hexadecyl dimethyl allyl ammonium chloride" is replaced with an equal amount of tetradecyl dimethyl allyl ammonium chloride, that is, only one hydrophobic monomer A and 0.5 parts tetradecyl dimethyl allyl ammonium chloride are used. Other steps remain unchanged, and fracturing fluid is prepared.

[0043] Example 3

[0044] Based on Example 1, in step (1), "0.3 parts hexadecyl dimethyl allyl ammonium chloride" was replaced with an equal amount of octadecyl dimethyl allyl ammonium chloride. The other steps remained unchanged, and the fracturing fluid was prepared.

[0045] Example 4

[0046] Based on Example 1, the amounts of the six monomers in step (1) were adjusted to: 94 parts acrylamide, 10 parts 2-acrylamido-2-methylpropanesulfonic acid, 1 part sodium styrene sulfonate, 1 part N-vinylpyrrolidone, 0.2 parts tetradecyl dimethyl allyl ammonium chloride, and 0.3 parts hexadecyl dimethyl allyl ammonium chloride. Other steps remained unchanged, and the fracturing fluid was prepared.

[0047] Example 5

[0048] Based on Example 1, in step (2), "4 parts of sodium 2-(acrylamido)tetradecanesulfonate" is replaced with an equal amount of sodium 2-(acrylamido)hexadecanesulfonate, that is, only one hydrophobic monomer B and 10 parts of sodium 2-(acrylamido)hexadecanesulfonate are used. The other steps remain unchanged, and the fracturing fluid is prepared.

[0049] Comparative Example 1

[0050] Based on Example 1, in step (1), "0.2 parts tetradecyl dimethyl allyl ammonium chloride and 0.3 parts hexadecyl dimethyl allyl ammonium chloride" are removed, meaning that hydrophobic monomer A is not used in the preparation of the polymer. The polymer is synthesized from four monomers: acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, sodium styrene sulfonate, and N-vinylpyrrolidone. All other steps remain unchanged, and the fracturing fluid is prepared.

[0051] Comparative Example 2

[0052] Based on Example 1, in step (2), "4 parts of sodium 2-(acrylamido)tetradecanesulfonate and 6 parts of sodium 2-(acrylamido)hexadecanesulfonate" are removed, meaning that hydrophobic monomer B is not used in the preparation of the macromolecular surfactant. The macromolecular surfactant is polymerized from four monomers: acrylamide, acrylic acid, sodium styrene sulfonate, and butyl acrylate ketone. All other steps remain unchanged, and the fracturing fluid is prepared.

[0053] Comparative Example 3

[0054] Based on Example 1, step (2) is deleted. In step (3), 5 parts of the polymer prepared in step (1) and 1000 parts of simulated mineralized water with a total mineralization of 85,000 are directly added to the stirred tank and mixed evenly at a stirring speed of 500 rpm to obtain fracturing fluid.

[0055] Comparative Example 4

[0056] Based on Example 1, step (2) is deleted. In step (3), 5 parts of the polymer prepared in step (1), 0.5 parts of sodium hexadecyl sulfonate and 1000 parts of simulated mineralized water with a total mineralization of 85,000 are directly added to the stirred tank and mixed evenly at a stirring speed of 500 rpm to obtain fracturing fluid.

[0057] Comparative Example 5

[0058] Based on Example 5, in step (2), "15 parts of sodium styrene sulfonate" was removed, and a macromolecular surfactant polymerized from acrylamide, acrylic acid, butyl acrylate, and sodium 2-(acrylamido)hexadecanesulfonate was prepared. The other steps remained unchanged, and a fracturing fluid was prepared.

[0059] Comparative Example 6

[0060] Based on Example 1, step (1) is deleted, and step (3) specifically involves: mixing 5 parts of the macromolecular surfactant prepared in step (2) and 5 parts of partially hydrolyzed polyacrylamide (molecular weight 2×10⁻⁶). 7 Add 1000 parts of simulated mineralized water (g / mol, degree of hydrolysis of 20%) and 85,000 parts of total mineralization to a stirred tank and mix at 500 rpm for 5 min to obtain fracturing fluid.

[0061] The performance tests of the fracturing fluids prepared in Examples 1-5 and Comparative Examples 1-6 are as follows:

[0062] (1) Scanning electron microscopy (SEM) test: The fracturing fluid from Example 1 was freeze-dried under a vacuum of 0.1 Pa for 72 hours to completely remove water. The dried sample was then spread on conductive adhesive, sputtered with gold, fixed to the stage, and its surface morphology was analyzed and observed using a Hitachi SU-8100 field emission scanning electron microscope. The scanning voltage was 3 kV. The test results are shown in […]. Figure 1 As can be seen, a tight three-dimensional network structure is formed between the polymer chains and the surfactant chains. This three-dimensional network structure increases the hydrodynamic volume and thus increases the viscosity of the fracturing fluid system. This demonstrates that the interaction between the polymer and the surfactant can improve the temperature resistance, salt resistance, and proppant carrying capacity of the fracturing fluid system.

[0063] (2) Rheological property testing: The rheological properties of the fracturing fluids obtained in Example 1 and Comparative Example 2 were tested using a HAAKE MARS rheometer from Hacker GmbH, Germany. The shear rate was 170 s. -1 The temperature ranged from room temperature to 90℃, and the viscosity change of the fracturing fluid was observed. The test results are shown below. Figure 2 and Figure 3 It can be seen that... Figure 2 The fracturing fluid in Example 1, as shown, has a viscosity of approximately 80 mPa·s at 90°C. Figure 3 The fracturing fluid in Comparative Example 2 shown has a viscosity of only 15 mPa·s. In Comparative Document 2, the viscosity of the fracturing fluid decreased sharply due to the absence of hydrophobic monomer B in the macromolecular surfactant, thus demonstrating the importance of hydrophobic monomer B in the fracturing fluid of this invention. In Example 1, the presence of hydrophobic monomer B resulted in a stable three-dimensional network structure formed by the polymer and macromolecular surfactant in the fracturing fluid at high temperatures, exhibiting good temperature and shear resistance. This also proves that the fracturing fluid of this invention requires the synergistic use of hydrophobic monomers A and B; if only hydrophobic monomer A is used, the temperature and shear resistance of the resulting fracturing fluid decreases significantly.

[0064] (3) Apparent viscosity test: The apparent viscosity of the fracturing fluids obtained in Examples 1-5 and Comparative Examples 1-6 was tested. An HTD-6ST digital display six-speed rotary viscometer from Qingdao Hengtaida Electromechanical Equipment Co., Ltd. was used. The test temperature was 25℃, and the rotation speed was 100 r / min. The test results are as follows: Figure 4 As shown, the fracturing fluid systems of Examples 1-5 of this invention exhibit good salt resistance and high viscosity in brine. This indicates that when both the polymer and the macromolecular surfactant contain hydrophobic monomers, the synergistic effect of the two hydrophobic monomers increases the viscosity of the fracturing fluid system. The stronger the hydrophobicity of the monomers, the stronger the effect and the higher the viscosity. Moreover, the selection of different types of hydrophobic monomers A and B allows for stronger interaction between the two hydrophobic monomers through counterionic interactions, resulting in a greater improvement in fracturing fluid performance. The fracturing fluid prepared using the polymer alone has a lower viscosity. When the polymer reacts with the small-molecule surfactant sodium hexadecyl sulfonate, the viscosity of the fracturing fluid increases, but it is still significantly lower than the viscosity of the fracturing fluid prepared with the synergistic effect of the polymer and the macromolecular surfactant. Furthermore, the benzene ring structure in the macromolecular surfactant can promote micelle formation, and the viscosity decreases significantly after removing the benzene ring structure. The low viscosity of the fracturing fluid in Comparative Example 6 indicates that the partial hydrolysis of polyacrylamide in simulated brine containing iron ions has a slow dissolution rate and a low viscosity release rate. This also demonstrates that partially hydrolyzed polyacrylamide that has not undergone hydrophobic modification cannot synergize with macromolecular surfactants.

[0065] (4) Sand-carrying performance test: Take 100g of fracturing fluid from Examples 1-5 and Comparative Examples 1-6 into a beaker, then add 32g of 40 / 70 mesh quartz sand, stir at 500rpm for 1min, pour the mixture into a 100ml graduated cylinder and observe the accumulation height of the quartz sand in the graduated cylinder after 15min. The results are as follows. Figure 5 As shown, the fracturing fluids of Examples 1-5 of this invention exhibit good salt resistance, with almost no sedimentation of the quartz sand after 15 minutes. In contrast, Comparative Examples 1-6 show poor sand-carrying capacity, with almost complete sedimentation of the quartz sand after 15 minutes. This further demonstrates that the association structure formed between the polymer and the large-molecule surfactant of this invention increases the "viscoelasticity" of the fracturing fluid, resulting in better and more stable sand-carrying performance. It also further verifies that long-chain hydrophobic monomers and benzene ring structures are more conducive to forming a compact micelle structure. Although the polymer alone can also interact with small-molecule surfactants, the interaction is relatively weak, and the resulting structure cannot effectively carry sand. Conventional partially hydrolyzed polyacrylamide is affected by Ca... 2+ Mg 2+ Fe 3+ The viscosity decreases significantly due to the influence of Fe. 3+ Due to the influence of [the substance], the solubility decreases significantly, and the sand-carrying capacity also decreases accordingly.

[0066] Take 100g of fracturing fluid from Examples 1-5 and Comparative Examples 1-6 respectively and place them in beakers. Then add 32g of 40 / 70 mesh quartz sand. Shear the mixture in a 90℃ water bath at 500rpm for 5 minutes. Then pour the mixture into a 100ml graduated cylinder and place the graduated cylinder in a 90℃ water bath. Observe the accumulation height of the quartz sand in the graduated cylinder after 5 minutes. The results are as follows. Figure 6 As shown in the figure, after shearing at 90℃ for 5 minutes, the fracturing fluids of Examples 1, 3, and 5 still exhibit good proppant carrying capacity. The proppant carrying capacity of the fracturing fluids of Examples 2 and 4 decreases slightly. The fracturing fluids of Comparative Examples 1-6 fail to carry proppant. This indicates that the fracturing fluid of the present invention has a stable structure and good temperature and shear resistance. Introducing rigid monomers and more hydrophobic monomers into the polymer can effectively improve the temperature and shear resistance of the system. Fracturing fluids prepared using polymers alone show a significant decrease in proppant carrying capacity at high temperatures due to increased molecular motion, weakened intermolecular interactions, and decreased viscosity. Furthermore, the structure formed by the polymer and the small-molecule surfactant sodium hexadecyl sulfonate is also destroyed at high temperatures, leading to a significant decrease in proppant carrying capacity. This also indicates that even under high-temperature stirring, the proppant carrying capacity is significantly reduced due to Fe... 3+ Due to the influence of conventional partially hydrolyzed polyacrylamide, it cannot be fully dissolved, resulting in a low viscosity release rate. Furthermore, the high temperature and Ca... 2+ Mg 2+ Due to the influence of [the factors], the sand-carrying capacity further decreased.

[0067] In summary, this invention, through the combined use of high-molecular polymers and macromolecular surfactants, produces a fracturing fluid formulated with mineralized water of a total salinity of 85,000 as the base fluid that still possesses the advantages of high salt tolerance and high proppant carrying capacity. This demonstrates that this invention can directly utilize high-salinity flowback fluid as the base fluid to formulate fracturing fluid, significantly reducing wastewater treatment costs and freshwater consumption. Furthermore, the fracturing fluid does not require the use of heavy metal crosslinking agents (such as organoboron / zirconium), avoiding reservoir damage caused by precipitation in traditional fracturing fluids, thus exhibiting significant environmental benefits.

[0068] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing a highly salt-tolerant structured fracturing fluid, characterized in that, It is formulated with backflow liquid as the base liquid, and high molecular weight polymers and macromolecular surfactants are added; The polymer is prepared by free radical polymerization of five monomers in aqueous solution: acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, hydrophobic monomer A, sodium styrene sulfonate, and N-vinylpyrrolidone; the hydrophobic monomer A is a long-chain alkyl quaternary ammonium salt. The macromolecular surfactant is prepared by free radical polymerization of five monomers: acrylamide, butyl acrylate, acrylic acid, hydrophobic monomer B, and sodium styrene sulfonate in aqueous solution; the hydrophobic monomer B is selected from at least one of sodium 2-(acrylamido)dodecanesulfonate, sodium 2-(acrylamido)tetradecanesulfonate, and sodium 2-(acrylamido)hexadecanesulfonate.

2. The method for preparing the high salt-tolerant structured fracturing fluid as described in claim 1, characterized in that, The mass ratio of the polymer, the surfactant, and the backflow solution is (0.3-0.5):(0.5-1):

100.

3. The method for preparing the high salt-tolerant structured fracturing fluid as described in claim 1, characterized in that, The preparation method of the polymer is as follows: S1: Add acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, sodium styrene sulfonate, N-vinylpyrrolidone and hydrophobic monomer A to water and stir to dissolve. Adjust the pH of the solution to 7.0-7.

5. S2: Add azobisisobutyrazoline hydrochloride to the solution in step S1. After cooling the solution system to 0℃, transfer it to a sealed and heat-insulated reaction container. After purging with nitrogen to remove oxygen, add ammonium persulfate and cerium ammonium nitrate. Stir and continue to purge with nitrogen for 10 minutes. Then add sodium bisulfite. Stir for several minutes and then stop stirring. Let the reaction stand until the solution becomes viscous. Stop purging with nitrogen and continue to let the reaction stand in a sealed and heat-insulated container for 5-7 hours to obtain a gel block. S3: Crush, dry and grind the glue block to obtain a white powdery polymer.

4. The method for preparing the high salt-tolerant structured fracturing fluid as described in claim 3, characterized in that, In step S1, the mass ratio of acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, sodium styrene sulfonate, N-vinylpyrrolidone, hydrophobic monomer A, and water is (80-90):(5-10):(1-5):(1-2):(0.5-1):(300-350).

5. The method for preparing the high salt-tolerant structured fracturing fluid as described in claim 3, characterized in that, The hydrophobic monomer A is selected from at least one of tetradecyl dimethyl allyl ammonium chloride, hexadecyl dimethyl allyl ammonium chloride, and octadecyl dimethyl allyl ammonium chloride.

6. The method for preparing the high salt-tolerant structured fracturing fluid as described in claim 3, characterized in that, In step S1, the pH value is adjusted using an aqueous sodium hydroxide solution.

7. The method for preparing the high salt-tolerant structured fracturing fluid as described in claim 1, characterized in that, The preparation method of the macromolecular surfactant is as follows: S1: Add acrylamide, acrylic acid, sodium styrene sulfonate, butyl acrylate and hydrophobic monomer B to water and stir to dissolve. Adjust the pH of the solution to 7.0-7.

5. S2: Add azobisisobutyrazoline hydrochloride to the solution in step S1, stir for several minutes and then stop stirring. Then place the reaction vessel in a constant temperature oven at 50-60℃ and let it stand for 7-9 hours to obtain a macromolecular surfactant solution.

8. The method for preparing the high salt-tolerant structured fracturing fluid as described in claim 7, characterized in that, In step S1, the mass ratio of acrylamide, acrylic acid, sodium styrene sulfonate, butyl acrylate, hydrophobic monomer B, and water is (20-30):(10-20):(10-15):(10-20):(5-10):(750-800).

9. The method for preparing the high salt-tolerant structured fracturing fluid as described in claim 7, characterized in that, In step S1, the pH value is adjusted using an aqueous sodium hydroxide solution.

10. A highly salt-tolerant structured fracturing fluid, characterized in that, It is prepared by the method described in any one of claims 1-9.

Images