High-temperature-resistant self-degradation type composite acid fracturing fluid system and preparation method thereof

Abstract

The invention discloses a high-temperature-resistant self-degradation type composite acid fracturing fluid system and a preparation method thereof, and relates to the technical field of acid fracturing fluids, the high-temperature-resistant self-degradation type composite acid fracturing fluid system comprises a base fluid, a thickening agent, a complexing agent, a cleanup additive, a corrosion inhibitor and a solvent; the base liquid is citric acid-tartaric acid-hydrochloric acid composite acid liquid, the thickening agent is AM / DMC / AA / AMPS / NVP quinary copolymer, the complexing agent is dihydroxyethyl glycine, the cleanup additive is a fluorocarbon surfactant, the corrosion inhibitor is an imidazoline and pyridine compound corrosion inhibitor, and the solvent is deionized water. The high-temperature-resistant self-degradable composite acid fracturing fluid system has the advantages of high-temperature retardance, self-degradation property, environmental protection property, salt resistance, shear resistance and temperature resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of acid fracturing fluids, and particularly relates to a high-temperature resistant self-degrading composite acid fracturing fluid system and a preparation method thereof. Background Art

[0002] The acid fracturing technology is an effective means to improve the permeability through the chemical dissolution of the acid solution on the fracture wall surface of the reservoir. However, there are still the following technical bottlenecks in practical applications: For example, the acid retardation is insufficient: the reaction rate of the conventional hydrochloric acid system with carbonate rocks is too fast, resulting in a short action distance of the acid solution and making it difficult to achieve deep reservoir transformation. The temperature resistance of the thickener is poor: the polyacrylamide-based thickeners widely used in the prior art are prone to molecular chain breakage and a sharp decrease in viscosity in high-temperature environments, unable to meet the construction requirements of deep well high-temperature reservoirs. The environmental compatibility is low: the cationic monomers in traditional thickeners have poor biodegradability, and the residues are likely to cause formation pore blockage and environmental pollution, not meeting the current environmental protection requirements of green development. The flowback efficiency is low: the existing systems rely on breaker agents to degrade the thickener, but incomplete breaking of the gel will lead to the retention of residual gel in the formation, thereby reducing the flowback rate and possibly causing secondary pollution problems of the flowback fluid. Summary of the Invention

[0003] In order to solve the problems of the prior art, the present invention provides a high-temperature resistant self-degrading composite acid fracturing fluid system and a preparation method thereof.

[0004] On the one hand, a high-temperature resistant self-degrading composite acid fracturing fluid system is provided, including: a base fluid, a thickener, a complexing agent, a drainage aid, an inhibitor, and a solvent; The base fluid is a citric acid-tartaric acid-hydrochloric acid composite acid solution, the thickener is a five-component copolymer of AM / DMC / AA / AMPS / NVP, the complexing agent is dihydroxyethylglycine, the drainage aid is a fluorocarbon surfactant, the inhibitor is a compound inhibitor of imidazoline and pyridine, and the solvent is deionized water.

[0005] Further, by mass percentage, the thickener is 0.8 - 1.2%, the complexing agent is 1 - 2%, the drainage aid is 0.1 - 0.3%, the inhibitor is 0.5 - 1%, hydrochloric acid in the base fluid is 15 - 20%, citric acid in the base fluid is 10 - 15%, tartaric acid in the base fluid is 5 - 10%, and the balance is the solvent.

[0006] Further, in the base fluid, the concentration of hydrochloric acid is 15 - 20%, the concentration of citric acid is 5 - 8%, and the concentration of tartaric acid is 3 - 5%.

[0007] Further, the molar ratio of AM, DMC, AA, AMPS, and NVP in the thickening agent is 85:3:10:1:1.

[0008] Further, the mass ratio of imidazoline type to pyridine type in the corrosion inhibitor is 2:1.

[0009] On the other hand, a method for preparing the high-temperature resistant self-degrading composite acid fracturing fluid system is provided, including the following steps: Prepare the AM / DMC / AA / AMPS / NVP five-component copolymer; Dissolve dihydroxyethylglycine, tartaric acid, citric acid, and hydrochloric acid in deionized water to obtain a high-temperature resistant acid base fluid; Mix the high-temperature resistant acid base fluid with deionized water, and then sequentially add the AM / DMC / AA / AMPS / NVP five-component copolymer, fluorocarbon surfactant, and the compound corrosion inhibitor of imidazoline type and pyridine type to obtain the high-temperature resistant self-degrading composite acid fracturing fluid system.

[0010] Further, the preparation of the AM / DMC / AA / AMPS / NVP five-component copolymer specifically includes: Dissolve AM, DMC, AA, AMPS, and NVP in deionized water according to the molar ratio of 85:3:10:1:1, adjust the pH to 7.0, purge with nitrogen to remove oxygen for half an hour, dropwise add a 0.08 wt% ammonium persulfate-sodium bisulfite initiator at 45°C, react for 12 h, granulate, and dry in vacuum at 60°C to obtain the AM / DMC / AA / AMPS / NVP five-component copolymer.

[0011] The beneficial effects of the technical solution provided by the present invention are as follows: The high-temperature resistant self-degrading composite acid fracturing fluid system in the present invention has: 1. High-temperature retarding property: The multi-acid synergistic complexing retarding mechanism and the compound corrosion inhibitor technology take into account both the corrosion inhibition efficiency and the system stability; 2. Self-degrading property: The rigid chain segments of NVP and AMPS in the copolymer control the degradation time, and no breaker is required; 3. Environmental friendliness: DMC replaces the traditional toxic cationic monomer, improving the biodegradation rate; 4. Salt and shear resistance: In the case of high salt, the sulfonic acid group of AMPS inhibits the compression of the electric double layer by salt ions through electrostatic repulsion, making its salt tolerance performance excellent; 5. Temperature resistance: At high temperatures, the rigid molecular chain structure inhibits degradation, making its viscosity decay smoothly. Description of the Drawings

[0012] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0013] Figure 1 It is the infrared spectrogram of a five - component copolymer of AM / DMC / AA / AMPS / NVP provided by the present invention; Figure 2 It is the molecular structural formula diagram of a five - component copolymer of AM / DMC / AA / AMPS / NVP provided by the present invention; Figure 3 It is the viscosity comparison diagram of a high - temperature resistant self - degradable composite acid fracturing fluid system provided by the present invention and a traditional polyacrylamide fracturing fluid system at different temperatures; Figure 4 It is the test result diagram of the viscosity stability of a high - temperature resistant self - degradable composite acid fracturing fluid system provided by the present invention and a traditional polyacrylamide fracturing fluid system in a high salinity environment; Figure 5 It is the sand suspension performance comparison diagram of a high - temperature resistant self - degradable composite acid fracturing fluid system provided by the present invention and a traditional polyacrylamide fracturing fluid system; Figure 6 It is the gel - breaking performance comparison diagram of a high - temperature resistant self - degradable composite acid fracturing fluid system provided by the present invention and a traditional polyacrylamide fracturing fluid system. Detailed implementation manners

[0014] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present application.

[0015] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings.

[0016] Example 1

[0017] A high - temperature resistant self - degradable composite acid fracturing fluid system includes the following components: base fluid, complexing agent, corrosion inhibitor, thickening agent, flow - back aid, and solvent.

[0018] Among them, the base fluid is a citric acid - tartaric acid - hydrochloric acid composite acid solution, the thickening agent is a five - component copolymer of AM / DMC / AA / AMPS / NVP, the complexing agent is dihydroxyethylglycine, the flow - back aid is a fluorocarbon - based surfactant, the corrosion inhibitor is a compound corrosion inhibitor of imidazoline and pyridine, and the solvent is deionized water.

[0019] In this high-temperature resistant self-degrading composite acid fracturing fluid system, by mass percentage, the AM / DMC / AA / AMPS / NVP five-component copolymer is 1%, dihydroxyethylglycine is 1.5%, fluorocarbon surfactant is 0.2%, the compound corrosion inhibitor of imidazoline and pyridine is 0.8%, hydrochloric acid in the base fluid is 18%, citric acid is 12%, tartaric acid is 8%, and the balance is deionized water; among them, the molar ratio of AM, DMC, AA, AMPS, and NVP in the AM / DMC / AA / AMPS / NVP five-component copolymer is 85:3:10:1:1, and the mass ratio of imidazoline and pyridine in the compound corrosion inhibitor of imidazoline and pyridine is 2:1.

[0020] Among them, the fluorocarbon surfactant is perfluorooctane sulfonate (PFOS): DuPont Zonyl® FSA (CAS: 29457-72-5), the imidazoline corrosion inhibitor is perfluorooctyl imidazoline quaternary ammonium salt, CAS: N / A, trade name: FluoroImid-12, and the pyridine corrosion inhibitor is cetylpyridinium bromide (CPB) (CAS: 140-72-7), trade name: Pyridinium-16.

[0021] The specific preparation method is as follows: Step (101): Synthesis of thickening agent: Add 85 parts of AM, 1 part of AMPS, 1 part of NVP, 3 parts of DMC, 10 parts of AA, and 298 parts of deionized water to the reaction kettle, adjust the pH to 7.0, pass nitrogen for half an hour to remove oxygen, then heat up to 45°C, dropwise add 0.5 part of initiator ammonium persulfate-sodium bisulfite, react for 12 h, granulate, and dry in vacuum at 60°C to obtain a powder with a molecular weight of 10 million - 12 million, which is the target thickening agent.

[0022] After preparing the thickening agent, take 1.2 g and dissolve it in 200 g of experimental water. After fully dissolving, use a viscometer to measure its viscosity, and its apparent viscosity is 85 mPa·s, with a good thickening effect.

[0023] Step (102): Preparation of high-temperature resistant acid base fluid. Dissolve dihydroxyethylglycine, tartaric acid with a concentration of 4%, citric acid with a concentration of 6.5%, and hydrochloric acid with a concentration of 18% in deionized water according to the above ratio to obtain a high-temperature resistant acid base fluid.

[0024] Step (103): Mix the high-temperature resistant acid base fluid with deionized water, and successively add the AM / DMC / AA / AMPS / NVP five-component copolymer, fluorocarbon surfactant, and the compound corrosion inhibitor of imidazoline and pyridine in the above ratio, and let it stand for 2 h. Finally, obtain a high-temperature resistant self-degrading composite acid fracturing fluid system.

[0025] Example 2 A high-temperature resistant self-degrading composite acid fracturing fluid system. By mass percentage, the AM / DMC / AA / AMPS / NVP five-component copolymer is 0.8%, dihydroxyethylglycine is 1%, fluorocarbon surfactant is 0.1%, the compound corrosion inhibitor of imidazoline and pyridine is 0.5%, hydrochloric acid in the base fluid is 15%, citric acid is 10%, tartaric acid is 5%, and the balance is deionized water; among them, the molar ratio of AM, DMC, AA, AMPS, and NVP in the AM / DMC / AA / AMPS / NVP five-component copolymer is 85:3:10:1:1, and the mass ratio of imidazoline and pyridine in the compound corrosion inhibitor of imidazoline and pyridine is 2:1.

[0026] Among them, the fluorocarbon surfactant is perfluorooctane sulfonate (PFOS): DuPont Zonyl® FSA (CAS: 29457-72-5), the imidazoline corrosion inhibitor is perfluorooctyl imidazoline quaternary ammonium salt, CAS: N / A, trade name: FluoroImid-12, and the pyridine corrosion inhibitor is cetylpyridinium bromide (CPB) (CAS: 140-72-7), trade name: Pyridinium-16.

[0027] The specific preparation method is as follows: Step (201): The same as in Example 1.

[0028] Step (202): Preparation of the high-temperature resistant acid base fluid. Dissolve dihydroxyethylglycine, tartaric acid with a concentration of 3%, citric acid with a concentration of 5%, and hydrochloric acid with a concentration of 15% in deionized water according to the above ratios to obtain the high-temperature resistant acid base fluid.

[0029] Step (203): Mix the high-temperature resistant acid base fluid with deionized water, and successively add the AM / DMC / AA / AMPS / NVP five-component copolymer, fluorocarbon surfactant, and the compound corrosion inhibitor of imidazoline and pyridine in the above ratios, and let it stand for 1 h. Finally, obtain the high-temperature resistant self-degrading composite acid fracturing fluid system.

[0030] Example 3 A high-temperature resistant self-degrading composite acid fracturing fluid system. By mass percentage, the AM / DMC / AA / AMPS / NVP five-component copolymer is 1.2%, dihydroxyethylglycine is 2%, fluorocarbon surfactant is 0.3%, the compound corrosion inhibitor of imidazoline and pyridine is 1%, hydrochloric acid in the base fluid is 20%, citric acid is 15%, tartaric acid is 10%, and the balance is deionized water; among them, the molar ratio of AM, DMC, AA, AMPS, and NVP in the AM / DMC / AA / AMPS / NVP five-component copolymer is 85:3:10:1:1, and the mass ratio of imidazoline and pyridine in the compound corrosion inhibitor of imidazoline and pyridine is 2:1.

[0031] Among them, the fluorocarbon surfactant is perfluorooctane sulfonate (PFOS): DuPont Zonyl® FSA (CAS: 29457-72-5), the imidazoline corrosion inhibitor is perfluorooctyl imidazoline quaternary ammonium salt, CAS: N / A, trade name: FluoroImid-12, and the pyridine corrosion inhibitor is cetylpyridinium bromide (CPB) (CAS: 140-72-7), trade name: Pyridinium-16.

[0032] The specific preparation method is as follows: Step (301): The same as in Example 1.

[0033] Step (302): Prepare the high-temperature resistant acid-base liquid base. Dissolve dihydroxyethylglycine, tartaric acid with a concentration of 5%, citric acid with a concentration of 8%, and hydrochloric acid with a concentration of 20% in deionized water according to the above ratio to obtain the high-temperature resistant acid-base liquid base.

[0034] Step (303): Mix the high-temperature resistant acid-base liquid base with deionized water, and sequentially add the AM / DMC / AA / AMPS / NVP five-component copolymer, fluorocarbon surfactant, and the compound corrosion inhibitor of imidazoline and pyridine in the above ratio, and let it stand for 3 h. Finally, obtain the high-temperature resistant self-degrading composite acid fracturing fluid system.

[0035] It should be noted that in the AM / DMC / AA / AMPS / NVP five-component copolymer thickener, acrylamide (AM) is the main chain skeleton of the polymer molecule, methylacryloyloxyethyl trimethyl ammonium chloride (DMC) is the cationic modification of the polymer molecule to enhance the adsorption property, acrylic acid (AA) increases the solubility of the polymer, 2-acrylamido-2-methylpropanesulfonic acid (AMPS) sulfonic acid group improves the salt tolerance, and N-vinylpyrrolidone (NVP) pyrrolidone ring improves the chain rigidity and increases the temperature resistance.

[0036] Figure 2 is the molecular structural formula of the prepared AM / DMC / AA / AMPS / NVP five-component copolymer, Figure 1 is the infrared spectrum of the AM / DMC / AA / AMPS / NVP five-component copolymer. From Figure 1 it can be seen that the infrared spectrum verifies the successful synthesis of the copolymer. Specifically: at 3450 cm -1 is the N-H stretching vibration peak (AM characteristic absorption); at 1668 cm -1 is the C=O stretching vibration peak (AA and AMPS characteristic absorption); at 1180 cm -1 is the S=O symmetric stretching vibration peak (AMPS sulfonic acid group characteristic absorption); at 1050 cm -1 is the C-O-C stretching vibration peak (DMC cationic group characteristic absorption).

[0037] In addition, different comparative examples were set by adjusting the molar ratios of AM, DMC, AA, AMPS, and NVP in the thickener, and a traditional polyacrylamide thickener (molecular weight 10 million) was used as a comparative example. The performance of the thickener in Example 1 was compared. Thickener solutions with a concentration of 0.6% were prepared with deionized water, and the viscosity retention rate of each thickener at 120 °C was measured to characterize its temperature resistance, and the biodegradation rate of each thickener was measured. The specific results are shown in Table 1.

[0038] Table 1 Group Thickener type Molecular weight (×10,000) Dissolution time (min) Viscosity retention rate at 120°C (%) Biodegradation rate (30 days, %) Example 1 The molar ratio of AM, DMC, AA, AMPS, and NVP is 85:3:10:1:1 1100 10 87.2 92.5 Comparative Example 1 The molar ratio of AM, DMC, AA, AMPS, and NVP is 85:4:14:1:1 1250 15 84.4 71.4 Comparative Example 2 The molar ratio of AM, DMC, AA, AMPS, and NVP is 85:5:13:1:1 980 18 82.1 43.5 Comparative Example 3 The molar ratio of AM, DMC, AA, AMPS, and NVP is 80:3:15:1:1 890 15 76.8 85.2 Comparative Example 4 The molar ratio of AM, DMC, AA, AMPS, and NVP is 80:1:17:1:1 750 15 52.3 89.7 Comparative Example 5 Polyacrylamide thickener 1000 38 28.9 39.7 From the data in Table 1, it can be seen that the influence of the molecular weight on the thickener: too high molecular weight (>12 million) leads to difficult dissolution, and too low molecular weight (<8 million) results in a decrease in temperature resistance; the relationship with environmental protection: when the DMC content > 5%, the biodegradation rate decreases significantly. Therefore, the molar ratio of AM, DMC, AA, AMPS, and NVP in the AM / DMC / AA / AMPS / NVP five-component copolymer is 85:3:10:1:1, showing the best comprehensive performance; and the performance of the thickener in Example 1 is better than that of the traditional polyacrylamide thickener.

[0039] It should also be noted that different comparative examples were set by adjusting the mass percentages of dihydroxyethylglycine, tartaric acid, citric acid, and hydrochloric acid in the high-temperature resistant acid-base fluid, and the high-temperature resistant acid-base fluid in step (102) of Example 1, step (202) of Example 2, and step (302) of Example 3 was used for Fe 3+ complexation efficiency comparison.

[0040] The experimental implementation steps are as follows: 1. Preparation of Fe³⁺ solution: Weigh 4.84 g of FeCl 3 ·6H 2 O and dissolve it in 1 L of deionized water to prepare a standard solution with a Fe³⁺ concentration of 1000 mg / L. 2. Preparation of acid solution samples: Prepare the high-temperature resistant acid-base fluid according to the mass percentages in Table 2. For example, the high-temperature resistant acid-base fluid in Example 1 is: hydrochloric acid (20%), citric acid (12%), tartaric acid (8%), dihydroxyethylglycine (1.5%). 3. Mixing and reaction: Take 50 mL of the high-temperature resistant acid-base fluid and 50 mL of the Fe³⁺ solution (1000 mg / L), mix them, place them in a constant temperature water bath, and react at 90 °C for 2 hours. After the reaction, transfer the mixed solution to a centrifuge tube and centrifuge at 4000 rpm for 15 minutes to separate the precipitate and the supernatant. 4. Determination of Fe³⁺ concentration: Filter the supernatant (0.45 μm filter membrane) and measure the free Fe³⁺ concentration using ICP-OES. The specific results are shown in Table 2.

[0041] Table 2 Group Hydrochloric acid (%) Citric acid (%) Tartaric acid (%) Bis(2-hydroxyethyl)glycine (%) Acid solution viscosity (mPa·s) Fe³⁺ complexation efficiency (%) Example 1 18 12 8 1.5 85 98.2 Example 2 15 10 5 1.0 82 95.7 Example 3 20 15 10 2.0 88 97.5 Comparative Example 1 20 0 20 1.5 72 63.4 (precipitation formed) Comparative Example 2 25 12 8 0 68 81.3 (complexation failure) Comparative Example 3 20 8 8 1.5 78 89.6 Among them, the hydrochloric acid concentration and tartaric acid concentration in Comparative Example 1 in Table 2 are the same as the hydrochloric acid concentration and tartaric acid concentration in Example 1, respectively, and the hydrochloric acid concentration, tartaric acid concentration and citric acid concentration in Comparative Example 2 and Comparative Example 3 are the same as the hydrochloric acid concentration, tartaric acid concentration and citric acid concentration in Example 1, respectively.

[0042] As can be seen from Table 2, the absence of citric acid (Comparative Example 1) leads to Fe 3+ precipitation; the absence of the complexing agent (Comparative Example 2) significantly reduces the complexing efficiency. Therefore, it can be seen from Table 2 that the synergistic complexation of citric acid - tartaric acid - hydrochloric acid in the present invention inhibits hydrolysis precipitation and maintains the acidification effect.

[0043] It should also be noted that taking the high-temperature self-degradable composite acid fracturing fluid system in Example 1 as an example, different comparative examples are set by adjusting the type and concentration of the corrosion inhibitor, and the other components are the same as those in Example 1. The corrosion rate and stability of each fracturing fluid system are detected. The test method: N80 steel sheets are immersed in the acid solution with each fracturing fluid system at 90 °C for 4 h, and the corrosion rate is measured according to SY / T 5405-2019. The specific results are shown in Table 3.

[0044] Table 3 Group Corrosion inhibitor type Total concentration of corrosion inhibitor (%) Corrosion rate (g / m²·h) Stability (precipitation after 72h) Example 1 Imidazoline:pyridine = 2:1 0.8 0.75 None Comparative Example 1 Imidazoline:pyridine = 1:1 1.0 0.82 Trace flocculation Comparative Example 2 Imidazoline:pyridine = 3:1 1.2 1.05 Precipitation formed Comparative Example 3 Single pyridine type 0.8 1.32 None Comparative Example 4 Commercially available corrosion inhibitor 1.5 0.68 Severe stratification As can be seen from Table 3, the corrosion inhibitor type (imidazoline:pyridine = 2:1) in Example 1 takes into account both the corrosion inhibition efficiency and the system stability.

[0045] Among them, the single pyridine type in Comparative Example 3 is (cetylpyridinium bromide (CPB) (CAS: 140-72-7), trade name: Pyridinium-16), and the commercially available corrosion inhibitor is BASF Keromix series: Keromix 600.

[0046] It should also be noted that the AM / DMC / AA / AMPS / NVP five-component copolymer in Example 1 is replaced with a commercially available polyacrylamide to form a fracturing fluid system. The other components are the same as those in Example 1, and the salt tolerance performance tests of the high-temperature self-degradable composite acid fracturing fluid system in Example 1 under different salinities are carried out. The test steps: prepare brine solutions containing 5×10 4 mg / L and 10×10 4 mg / L; add the solution of each fracturing fluid system at a concentration of 0.6% and measure the viscosity. The specific results are shown in Table 4.

[0047] Table 4 Group Total dissolved solids (mg / L) <![CDATA[NaCl:CaCl 2 (by mass)]]> Initial viscosity (mPa·s) Viscosity after shearing at 120°C (mPa·s) Precipitation of complexing agent Example 1 <![CDATA[5×10 4 > 3:1 83 56 None Example 1 <![CDATA[10×10 4 > 3:1 80 49 None Comparative Example 1 <![CDATA[5×10 4 > 3:1 52 18 A large amount of precipitation Comparative Example 1 <![CDATA[10×10 4 > 3:1 48 15 A large amount of precipitation In addition, Figure 4It is the graph showing the viscosity stability test results of the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1 and the traditional polyacrylamide fracturing fluid system (viscosity after shearing at 120°C). Refer to Figure 4 and Table 4. When the salinity is 5×10 4 mg / L, the initial viscosity of the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1 is 83 mPa·s, and the viscosity after shearing is 56 mPa·s. When the salinity is 10×10 4 mg / L, the initial viscosity of the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1 is 80 mPa·s, and the viscosity after shearing is 49 mPa·s. This is because the sulfonic acid groups of AMPS inhibit the compression of the electrical double layer by salt ions through electrostatic repulsion, resulting in excellent salt tolerance of the system. The sulfonic acid groups of AMPS can support the stable operation of the system at a salinity of 10×10 4 mg / L; while in Comparative Example 1, the salt tolerance of polyacrylamide is too poor, resulting in the failure of the complexing agent to bind with Ca 2+ , and the traditional polyacrylamide acid fracturing system lacks salt-resistant groups, and its viscosity is significantly affected by the salting-out effect.

[0048] It should also be noted that the AM / DMC / AA / AMPS / NVP five-component copolymer in Example 1 is replaced with commercially available polyacrylamide to form a fracturing fluid system, and the other components are the same as those in Example 1. The viscosity comparison at different temperatures is carried out with the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1. The fracturing fluid system solution is prepared at a concentration of 0.6%, and stirred with a Waring blender until completely dissolved; after standing for 4 h, the viscosity values at temperatures (30°C, 60°C, 90°C, 120°C, 150°C) and a shear rate of 170 s⁻¹ are measured using a Fann-35 six-speed rotary viscometer; for the specific results, refer to Figure 3 , and it can be seen from Figure 3 that the initial viscosity of the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1 is 86 mPa·s at 30°C and remains 52 mPa·s at 150°C, indicating that the viscosity attenuation is gentle, and the rigid molecular chain structure inhibits degradation at high temperatures. The initial viscosity of the traditional polyacrylamide fracturing fluid system is 62 mPa·s at 30°C and only 15 mPa·s remains at 150°C. The molecular chain breaks significantly at high temperatures, and the viscosity drops sharply.

[0049] Secondly, the suspension sand performance of the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1 is compared with that of the traditional polyacrylamide fracturing fluid system (quartz sand with a diameter of 0.42 - 0.84 mm (20 - 40 mesh) is selected as the proppant, and it is prepared according to the mass ratio of the fracturing fluid system to the proppant of 7:3. The settlement time of the sand and gravel is recorded, and the settlement rate of the proppant is calculated), and for the specific results, refer to Figure 5 , and from Figure 5It can be seen that initially the height of the proppant is 18 cm for all cases. Finally, for the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1, the corresponding proppant height is 10 cm, while for the traditional polyacrylamide fracturing fluid system, the corresponding proppant height is 1 cm. This is because the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1 forms a three-dimensional network structure, which can effectively suspend the proppant. The entanglement of thickener molecular chains and hydrogen bonds can significantly enhance the sand suspension ability compared with the traditional polyacrylamide fracturing fluid system.

[0050] In addition, the gel-breaking performance of the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1 was compared with that of the traditional polyacrylamide fracturing fluid system (0.15% ammonium persulfate was added to the fracturing fluid system solution, and the gel-breaking experiment was carried out at 90 °C). The specific results are shown in Figure 6 , from Figure 6 it can be seen that the introduction of the NVP segment in the high-temperature resistant self-degradable composite acid fracturing fluid system in Example 1 controls the degradation rate, realizes controllable self-degradation, and the degradation time reaches the industrial standard within two hours.

[0051] It should be noted that through the above experimental data, the significant advantages of the high-temperature resistant self-degradable composite acid fracturing fluid system in the present invention in terms of temperature resistance, salt resistance, sand suspension performance and self-degradation ability can be seen.

[0052] The serial numbers of the above embodiments of the present invention are only for description and do not represent the advantages or disadvantages of the embodiments.

[0053] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A high temperature resistant self-degradable composite acid fracturing fluid system, characterized in that: include: Base fluid, thickener, complexing agent, drainage aid, corrosion inhibitor and solvent; The base liquid is a citric acid-tartaric acid-hydrochloric acid composite acid liquid, the thickener is a five-component copolymer of AM / DMC / AA / AMPS / NVP, the complexing agent is dihydroxyethylglycine, the drainage aid is a fluorocarbon surfactant, the corrosion inhibitor is an imidazoline and pyridine composite corrosion inhibitor, and the solvent is deionized water.

2. The high temperature resistant self-degradable composite acid fracturing fluid system according to claim 1, characterized in that: In terms of mass percentage, the thickener is 0.8-1.2%, the complexing agent is 1-2%, the drainage agent is 0.1-0.3%, the corrosion inhibitor is 0.5-1%, the hydrochloric acid in the base liquid is 15-20%, the citric acid in the base liquid is 10-15%, the tartaric acid in the base liquid is 5-10%, and the balance is the solvent.

3. The high temperature resistant self-degradable composite acid fracturing fluid system according to claim 2, characterized in that: In the base liquid, the concentration of the hydrochloric acid is 15-20%, the concentration of the citric acid is 5-8%, and the concentration of the tartaric acid is 3-5%.

4. The high temperature resistant self-degradable composite acid fracturing fluid system according to claim 3, characterized in that: The molar ratio of AM, DMC, AA, AMPS and NVP in the thickener is 85:3:10:1:

1.

5. The high temperature resistant self-degradable composite acid fracturing fluid system according to claim 4, characterized in that: The mass ratio of imidazolines to pyridines in the corrosion inhibitor is 2:

1.

6. A method for preparing the high temperature resistant self-degradable composite acid fracturing fluid system according to any one of claims 1 to 5, characterized in that: The steps include: Preparation of AM / DMC / AA / AMPS / NVP pentacrylamide; Dissolve dihydroxyethylglycine, tartaric acid, citric acid and hydrochloric acid in deionized water to obtain a high temperature resistant acid base liquid; The high temperature resistant acid base fluid is mixed with deionized water, and then the AM / DMC / AA / AMPS / NVP quintuplex, fluorocarbon surfactant, and imidazoline and pyridine compound corrosion inhibitors are added in sequence to obtain the high temperature resistant self-degradable composite acid fracturing fluid system.

7. The method for preparing a high temperature resistant self-degradable composite acid fracturing fluid system according to claim 6, characterized in that: The preparation of the AM / DMC / AA / AMPS / NVP five-element copolymer specifically comprises: AM, DMC, AA, AMPS and NVP were dissolved in deionized water at a molar ratio of 85:3:10:1:1, the pH was adjusted to 7.0, nitrogen was passed to deoxygenate for half an hour, ammonium persulfate-sodium bisulfite initiator was added dropwise at 45° C., the reaction was carried out for 12 hours, granulation was performed, and vacuum drying was performed at 60° C. to obtain the AM / DMC / AA / AMPS / NVP pentacrylic copolymer.

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