An amphiphilic carbon dot, a clean fracturing fluid and its application

Abstract

This invention discloses an amphiphilic carbon dot, a clean fracturing fluid, and their applications. The amphiphilic carbon dot is prepared by a solvothermal reaction of an organic acid and a hydrophobic aliphatic compound. By changing the type of hydrophobic aliphatic compound, the amphiphilic carbon dot provided by this invention can be easily prepared with different structures and properties. The clean fracturing fluid prepared using this amphiphilic carbon dot not only has high-temperature resistance, making it suitable for reservoirs with higher temperatures, but also exhibits high-salt resistance, demonstrating excellent fracturing performance in hydraulic fracturing operations in low-permeability and high-salinity reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of hydraulic fracturing technology, specifically relating to an amphiphilic carbon dot, a clean fracturing fluid, and their applications. Background Technology

[0002] Low-permeability oil and gas reservoirs in my country represent an important unconventional oil and gas resource, rich in reserves but challenging to develop. Hydraulic fracturing, a key technology for developing low-permeability oil and gas reservoirs, is a production enhancement and injection measure that utilizes hydraulic action to create fractures in the formation, improving gas and oil flow. Fracturing fluid plays a crucial role in oil and gas well fracturing operations; its main function is to increase recoverable oil and gas reserves by generating and maintaining fractures. Therefore, the performance of fracturing fluid has a decisive impact on fracturing effectiveness and the economic benefits of reservoir development. Currently, guar gum polymer fracturing fluid is widely used due to its good sand-carrying capacity and economic benefits. However, it generates a large amount of guar gum residue after fracturing operations, causing significant formation damage. Furthermore, conventional fracturing fluids have poor salt tolerance, require large amounts of freshwater for preparation, and involve complex post-fracturing flowback and treatment processes. These problems limit its wider application.

[0003] Against this backdrop, viscoelastic surfactant (VES) fracturing fluid has rapidly developed as a novel fracturing fluid system in the oil and gas field development field. The viscosity of VES fracturing fluid is mainly generated by the worm-like micelles formed by the surfactant in aqueous solution. Compared with traditional guar gum polymer fracturing fluids, VES fracturing fluid has significant advantages in proppant carrying capacity, fracturing effect, and environmental friendliness. For example, it can completely break down hydrocarbons without leaving residue, resulting in less formation damage. The main component of VES fracturing fluid is small-molecule surfactant, so it is easy to flow back after breaking down, hence it is called a "clean fracturing fluid." However, VES fracturing fluid still faces certain challenges in terms of high-temperature resistance and cost.

[0004] Therefore, how to prepare a clean fracturing fluid that is resistant to high temperatures and salt has become an urgent problem to be solved. Summary of the Invention

[0005] To address the aforementioned shortcomings, the present invention aims to provide an amphiphilic carbon dot, a clean fracturing fluid, and its application, thereby solving the technical problems of poor high-temperature resistance and salt resistance of traditional clean fracturing fluids.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] In a first aspect, the present invention provides an amphiphilic carbon dot, which is prepared by a solvothermal reaction of an organic acid and a hydrophobic aliphatic compound.

[0008] Preferably, the organic acid includes one or more of citric acid, tartaric acid, malic acid, salicylic acid, and hydroxybenzoic acid; the hydrophobic aliphatic compound includes one or more of fatty acids, fatty alcohols, and alkylamines.

[0009] Preferably, the fatty acids include one or more of saturated fatty acids, unsaturated fatty acids, and aromatic fatty acids; the fatty alcohols include one or more of n-octanol, n-dodecanol, n-tetradecanool, and n-hexadecanool; and the alkylamines include one or more of n-octylamine, dodecylamine, and tetradecylamine.

[0010] Preferably, the mass ratio of organic acid to hydrophobic aliphatic compound is (1:1) to (1:5).

[0011] Preferably, the solvent used in the solvothermal reaction includes one or more of water, ethanol, DMF, DMSO, CCl4, and PEG.

[0012] Preferably, the reaction temperature of the solvothermal reaction is 140~190℃, and the reaction time is 8~16 h.

[0013] In a second aspect, the present invention provides a clean fracturing fluid comprising a cationic surfactant, mineralized water, and amphiphilic carbon dots as described in the first aspect; wherein the mineralization degree of the mineralized water is 50,000~250,000 mg / L.

[0014] Preferably, the total mass of the cationic surfactant and the amphiphilic carbon dots is 1.0% to 4.0% of the total mass of the clean fracturing fluid; the mass ratio of the cationic surfactant to the amphiphilic carbon dots is (6 to 9): (1 to 4).

[0015] Preferably, the cationic surfactant is one or more of dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, and octadecyltrimethylammonium bromide; the salt in the mineralized water includes inorganic salts.

[0016] Thirdly, the present invention provides the application of the clean fracturing fluid as described in the second aspect in hydraulic fracturing of low-permeability reservoirs and / or high-salinity reservoirs.

[0017] The beneficial effects of this invention are:

[0018] The amphiphilic carbon dots provided by this invention are obtained by a one-step solvothermal method using organic acids and hydrophobic aliphatic compounds. By changing the type of hydrophobic aliphatic compound, hydrophobic groups with different properties and carbon chain lengths can be introduced, thus enabling the simple preparation of amphiphilic carbon dots with different structures and properties.

[0019] The amphiphilic carbon dots proposed in this invention are nanoscale in size. When used as a key component of clean fracturing fluid, their nanoscale effect can effectively penetrate into the tight pore throats and microfractures of low-permeability reservoirs, reduce fluid flow resistance, extend the fracturing range, and thus significantly improve the fracturing operation effect.

[0020] The amphiphilic carbon dots proposed in this invention can significantly improve the viscosity of cationic surfactants in highly salinized water. By screening the optimal ratio of the two, the amount of clean fracturing fluid can be greatly reduced, thus lowering production costs.

[0021] This invention utilizes amphiphilic carbon dots to prepare a clean fracturing fluid that not only has high temperature resistance, making it suitable for reservoirs with higher temperatures, but also has high salt resistance, avoiding dependence on fresh water. It can be prepared using seawater or fracturing flowback fluid, realizing the recycling of water resources, meeting the requirements of green environmental protection and sustainable development, and providing an environmentally friendly and efficient solution for the application of clean fracturing fluid. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly described below.

[0023] Figure 1 The curve showing the effect of the mass ratio of CTAB to CDs-2 on the viscosity of clean fracturing fluid;

[0024] Figure 2 The curve showing the effect of the total amount of CTAB and CDs-2 on the viscosity of clean fracturing fluid;

[0025] Figure 3 The curves showing the effect of mineralized water ion type and concentration on the viscosity of clean fracturing fluid;

[0026] Figure 4 Viscosity-temperature curves of the clean fracturing fluids in Examples 18 and 19 are shown. Detailed Implementation

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

[0028] Carbon dots, as a novel nanomaterial, exhibit good adaptability to low-permeability reservoirs, mainly in the following aspects:

[0029] 1) The nano-size effect of carbon dots enables them to effectively penetrate the pore-throat structure of low-permeability reservoirs, serving as an additive in fracturing fluids to improve fracture propagation capacity, especially in microfractures and capillaries, thereby enhancing reservoir permeability. Simultaneously, their high permeability effectively reduces the amount of carbon dots required, lowering the overall cost of fracturing fluid usage. 2) The high interfacial activity of carbon dots significantly reduces the interfacial tension between fracturing fluid and crude oil, altering rock wettability and facilitating effective penetration of fracturing fluid into the reservoir environment, extending the fracturing distance. Their excellent surface activity also aids in the flowback of fracturing fluid after operation. 3) Due to their fluorescent properties, carbon dots can be used as tracers to monitor the flow path and distribution of fracturing fluid in the reservoir, helping to assess fracturing effectiveness and adjust fracturing strategies promptly. 4) Carbon dots have a wide range of raw material sources, and their preparation methods are simple and diverse, offering good cost-effectiveness. 5) Carbon dots exhibit good biocompatibility and environmental degradability, thus having minimal environmental impact during the fracturing process. The inventors realized that amphiphilic carbon dots have hydrophilic and oleophilic properties. If used as nanoscale surfactants, they will have good compatibility with low-permeability reservoirs. Therefore, using amphiphilic carbon dots to replace the traditional VES system is expected to reduce the cost of clean fracturing fluid and improve fracturing effect, which has good application prospects for promoting the efficient development of low-permeability reservoirs.

[0030] In view of this, in a first aspect, embodiments of this application provide an amphiphilic carbon dot, which is prepared by a solvothermal reaction of an organic acid and a hydrophobic aliphatic compound.

[0031] In some preferred embodiments, the organic acid includes one or more of citric acid, tartaric acid, malic acid, salicylic acid, and hydroxybenzoic acid; the hydrophobic aliphatic compound includes one or more of fatty acids, fatty alcohols, and alkylamines. The fatty acids include, but are not limited to, one or more of saturated fatty acids, unsaturated fatty acids, and aromatic fatty acids; for example, saturated fatty acids may be one or more of lauric acid, palmitic acid, and stearic acid; unsaturated fatty acids may be one or more of oleic acid, linoleic acid, and ricinoleic acid; and aromatic fatty acids may be one or more of benzoic acid and p-methylbenzoic acid. The fatty alcohols include, but are not limited to, one or more of n-octanol, n-dodecanol, n-tetradecylol, and n-hexadecylol; and the alkylamines include, but are not limited to, one or more of n-octylamine, dodecylamine, and tetradecylamine.

[0032] In some preferred embodiments, a method for preparing amphiphilic carbon dots specifically includes the following steps:

[0033] An organic acid, a hydrophobic aliphatic compound, and a solvent are mixed and then subjected to a solvothermal reaction at 140–190 °C for 8–16 h. After post-treatment, amphiphilic carbon dots are obtained. The mass ratio of the organic acid to the hydrophobic aliphatic compound is (1:1) to (1:5). The solvent includes, but is not limited to, one or more of water, ethanol, DMF, DMSO, CCl4, and PEG; the mass ratio of the organic acid to the solvent is (1:10) to (1:100).

[0034] In some preferred embodiments, the post-processing steps after the reaction can be as follows: after the reaction is completed, the reaction mixture is cooled, then centrifuged and filtered, and the solvent is evaporated to obtain a crude product; the crude product is dissolved in water and then dialyzed (using a dialysis bag with a capacity of 500-2000 Da) and lyophilized to obtain amphiphilic carbon dots.

[0035] In a second aspect, embodiments of this application provide a clean fracturing fluid, which includes a cationic surfactant, mineralized water, and amphiphilic carbon dots as described in the first aspect; wherein the mineralization degree of the mineralized water is 50,000~250,000 mg / L; the total mass of the cationic surfactant and the amphiphilic carbon dots is 1.0%~4.0% of the total mass of the clean fracturing fluid; and the mass ratio of the cationic surfactant to the amphiphilic carbon dots is (6~9):(1~4).

[0036] In some preferred embodiments, the cationic surfactant is one or more of dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, and octadecyltrimethylammonium bromide. The mineralized water is prepared by dissolving inorganic salts in water, including but not limited to one or more of sodium chloride, calcium chloride, and potassium chloride.

[0037] Thirdly, embodiments of this application provide the application of the clean fracturing fluid as described in the second aspect in hydraulic fracturing of low-permeability reservoirs and / or high-salinity reservoirs.

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be described in more detail below with reference to the accompanying drawings.

[0039] Example 1

[0040] An amphiphilic carbon dot was prepared as follows: 1.0 g of salicylic acid and 1.5 g of dodecanol were weighed and placed in a 50 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene. 40 mL of anhydrous ethanol was added, and the mixture was dissolved and thoroughly mixed. The sealed reactor was placed in an oil bath and heated at 160 °C for 8 h. After the high-pressure reactor cooled to room temperature, the reaction solution was transferred to a centrifuge tube and centrifuged at 5000 rpm to remove insoluble matter. The solvent was then concentrated and evaporated by rotary evaporation to obtain a crude product. The crude product was redispersed in water and dialyzed for 48 h using a dialysis bag with a molecular weight cutoff of 500 Da. After dialysis, the amphiphilic carbon dot, denoted as CDs-1, was obtained by freeze-drying.

[0041] Example 2

[0042] An amphiphilic carbon dot was prepared as follows: 1.0 g of citric acid, 1.5 g of n-octylamine, and 1.0 g of tetradecyl alcohol were weighed and placed in a 50 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene. 10 mL of deionized water and 30 mL of ethanol were added, and the mixture was dissolved and thoroughly mixed. The sealed reactor was placed in an oil bath and heated at 180 °C for 12 h. After the high-pressure reactor cooled to room temperature, the reaction solution was transferred to a centrifuge tube and centrifuged at 5000 rpm to remove insoluble matter. The solvent was then concentrated and evaporated by rotary evaporation to obtain a crude product. The crude product was redispersed in water and dialyzed for 48 h using a dialysis bag with a molecular weight cutoff of 500 Da. After dialysis, the amphiphilic carbon dot, denoted as CDs-2, was obtained by freeze-drying.

[0043] Example 3

[0044] An amphiphilic carbon dot was prepared as follows: 1.5 g of malic acid, 3.5 g of dodecylamine, and 0.2 g of stearic acid were weighed and placed in a 50 mL stainless steel high-pressure reactor lined with polytetrafluoroethylene. 20 mL of DMF and 20 mL of CCl4 were added, dissolved, and mixed thoroughly. The sealed reactor was placed in an oil bath and heated at 180 °C for 14 h. After the high-pressure reactor cooled to room temperature, the reaction solution was removed into a centrifuge tube, centrifuged at 5000 rpm to remove insoluble matter, and then concentrated by rotary evaporation to obtain a crude product. The crude product was redispersed in water and dialyzed for 48 h using a dialysis bag with a molecular weight cutoff of 500 Da. After dialysis, the amphiphilic carbon dot, denoted as CDs-3, was obtained by freeze-drying.

[0045] Example 4

[0046] A clean fracturing fluid is prepared as follows: 200g of NaCl is weighed into a beaker and dissolved in 500mL of distilled water, then diluted to a 1000mL volumetric flask to obtain a mineralized aqueous solution; 1.2g of the amphiphilic carbon point CDs-1 prepared in Example 1 and 3.6g of dodecyltrimethylammonium bromide are weighed into a 250mL beaker, and then 150mL of mineralized water is added. The mixture is stirred at 700rpm for 60min to obtain the clean fracturing fluid.

[0047] Example 5

[0048] A clean fracturing fluid is prepared as follows: 200g NaCl and 10g CaCl2 are weighed into a beaker and dissolved in 500mL distilled water. The solution is then diluted to a 1000mL volumetric flask to obtain a mineralized aqueous solution. 0.6g of CDs-2 prepared in Example 2 and 2.4g CTAB are weighed into a 250mL beaker, and 150mL of mineralized water is added. The mixture is stirred at 700rpm for 60min to obtain the clean fracturing fluid.

[0049] Example 6

[0050] A clean fracturing fluid is prepared as follows: 200g NaCl and 10g CaCl2 are weighed into a beaker, dissolved in 500mL distilled water, and the solution is diluted to a 1000mL volumetric flask to obtain a mineralized aqueous solution; 0.6g of CDs-3 prepared in Example 3 and 2.4g CTAB are weighed into a 250mL beaker, and 150mL of mineralized water is added. The mixture is stirred at 700rpm for 60min to obtain the clean fracturing fluid.

[0051] Example 7

[0052] A clean fracturing fluid is prepared as follows: 200g NaCl and 10g CaCl2 are weighed into a beaker, dissolved in 500mL distilled water, and the solution is diluted to a 1000mL volumetric flask to obtain a mineralized aqueous solution; 2.0g of the amphiphilic carbon dots CDs-3 prepared in Example 3 and 2.0g CTAB are weighed into a 250mL beaker, and 150mL of mineralized water is added. The mixture is stirred at 700rpm for 60min to obtain the clean fracturing fluid.

[0053] Example 8

[0054] A clean fracturing fluid is prepared in a manner that is basically the same as that in Example 5, except that CDs-2 is 0.3 g and CTAB is 2.7 g, i.e., the mass ratio of CTAB to CDs-2 is 9:1.

[0055] Example 9

[0056] A clean fracturing fluid is prepared in a manner that is basically the same as that in Example 5, except that CDs-2 is 0.9 g and CTAB is 2.1 g, i.e., the mass ratio of CTAB to CDs-2 is 7:3.

[0057] Example 10

[0058] A clean fracturing fluid is prepared in a manner that is basically the same as that in Example 5, except that CDs-2 is 1.2 g and CTAB is 1.8 g, i.e., the mass ratio of CTAB to CDs-2 is 6:4.

[0059] Example 11

[0060] A clean fracturing fluid is prepared in a manner that is essentially the same as that in Example 5, except that the total mass of CTAB and CDs-2 is 1.0% of the total mass of the fracturing fluid.

[0061] Example 12

[0062] A clean fracturing fluid is prepared in a manner that is essentially the same as that in Example 5, except that the total mass of CTAB and CDs-2 is 1.5% of the total mass of the fracturing fluid.

[0063] Example 13

[0064] A clean fracturing fluid is prepared in a manner essentially the same as in Example 5, except that the total mass of CTAB and CDs-2 is 3.0% of the total mass of the fracturing fluid.

[0065] Example 14

[0066] A clean fracturing fluid is prepared in a manner that is essentially the same as that in Example 5, except that the total mass of CTAB and CDs-2 is 4.0% of the total mass of the fracturing fluid.

[0067] Example 15

[0068] A clean fracturing fluid, prepared in essentially the same way as in Example 5, except that the mineralized water is 5 × 10⁻⁶. 4 mg / L, 10×10 4 mg / L, 15×10 4 mg / L, 20×10 4 A mg / L NaCl solution.

[0069] Example 16

[0070] A clean fracturing fluid, prepared in essentially the same way as in Example 5, except that the mineralized water is 5 × 10⁻⁶. 4 mg / L, 10×10 4 mg / L, 15×104 mg / L, 20×10 4 mg / L KCl solution.

[0071] Example 17

[0072] A clean fracturing fluid, prepared in essentially the same way as in Example 5, except that the mineralized water is 5 × 10⁻⁶. 4 mg / L, 10×10 4 mg / L, 15×10 4 mg / L, 20×10 4 mg / L CaCl2 solution.

[0073] Example 18

[0074] A clean fracturing fluid is prepared in a manner essentially the same as in Example 5, except that the mineralizing water is a 200,000 mg / L NaCl solution.

[0075] Example 19

[0076] A clean fracturing fluid is prepared in a manner that is essentially the same as that in Example 5, except that the mineralized water is 200,000 mg / L NaCl and the total mass of CTAB and CDs-2 is 4.0% of the total mass of the fracturing fluid.

[0077] Comparative Example 1

[0078] The preparation method is basically the same as that in Example 5, except that 0.6g of CDs-2 is replaced with CTAB to obtain a clean fracturing fluid without amphiphilic carbon dots.

[0079] Comparative Example 2

[0080] A clean fracturing fluid is prepared in a manner that is basically the same as that in Example 5, except that CDs-2 is 1.5 g and CTAB is 1.5 g, that is, the mass ratio of CTAB to CDs-2 is 5:5.

[0081] Comparative Example 3

[0082] A clean fracturing fluid is prepared in a manner that is basically the same as that in Example 5, except that CDs-2 is 2.1 g and CTAB is 0.9 g, i.e., the mass ratio of CTAB to CDs-2 is 3:7.

[0083] Comparative Example 4

[0084] A clean fracturing fluid is prepared in a manner that is basically the same as that in Example 5, except that CDs-2 is 2.7 g and CTAB is 0.3 g, i.e., the mass ratio of CTAB to CDs-2 is 1:9.

[0085] Comparative Example 5

[0086] A clean fracturing fluid is prepared in a manner that is basically the same as that in Example 5, except that CDs-2 is 3.0 g and CTAB is 0 g, i.e., the mass ratio of CTAB to CDs-2 is 0:10.

[0087] Comparative Example 6

[0088] The preparation method is basically the same as that in Example 5, except that the mineralized water is replaced with tap water to obtain a clean fracturing fluid that does not contain high-mineralization salt ions.

[0089] Comparative Example 7

[0090] The preparation method is basically the same as that in Example 5, except that the mineralized water is replaced with distilled water to obtain a clean fracturing fluid that does not contain metal salt ions.

[0091] Performance testing

[0092] The clean fracturing fluids prepared in each example and comparative example were measured using an eight-speed rotational viscometer. Test conditions: 25℃, 170s. -1 .

[0093] Figure 1 The curves showing the effect of the mass ratio of CTAB to CDs-2 on viscosity are shown for Examples 5, 8-10, and Comparative Examples 1-5. The total amount of both was 2%, and the mineralized water composition included 200,000 mg / L NaCl and 10,000 mg / L CaCl2. It can be seen that when the ratio of CTAB to CDs-2 was adjusted from 10:0 to 6:4, the viscosity of the fracturing fluid first increased and then decreased with increasing CDs-2 content. However, the viscosity of the compound system was always greater than that of pure CTAB, indicating that CDs had a viscosity-enhancing effect on CTAB. When the proportion of CDs-2 exceeded 50%, it had an adverse effect on viscosity. Therefore, the optimal ratio of the two was 8:2, and Example 5 showed the best effect.

[0094] Figure 2 The graph shows the effect of the total amount of CTAB and CDs-2 mixture on viscosity in Examples 5 and 11-14. The CTAB:CDs-2 ratio was 8:2 (mass ratio), and the mineralized water composition included 200,000 mg / L NaCl and 10,000 mg / L CaCl2. As shown in the graph, the viscosity of the clean fracturing fluid significantly increases with increasing total amount; a 4% dosage is sufficient to achieve 300 mPa·s. Compared to the 6% dosage required for traditional clean fracturing fluids, this reduces the dosage and lowers production costs to some extent.

[0095] Figure 3Examples 15-17 show the effect of mineralized water ion type and concentration on the viscosity of clean fracturing fluid. The CTAB:CDs-2 ratio was 8:2 (mass ratio), with a total dosage of 2%. The results show that the viscosity-enhancing effect is not significant when the mineralization is below 50,000 mg / L. Viscosity increases with increasing mineralization; comparatively, the mineralized water achieves optimal viscosity in a 200,000 mg / L NaCl solution.

[0096] Figure 4 The viscosity-temperature curves of the clean fracturing fluids in Examples 18 and 19 are shown. The ratio of CTAB to CDs-2 is 8:2 (mass ratio), and the mineralized water composition is 200,000 mg / L NaCl. The results show that the viscosity of the fracturing fluid first increases and then decreases with increasing temperature, reaching its maximum viscosity at 40℃. The industry standard SY / T 6376-2008, "General Technical Conditions for Fracturing Fluids," requires that the apparent viscosity of such fracturing fluids, representing their "temperature resistance and shear resistance," be ≥20 mPa·s. It can be seen that the fracturing fluid of this invention, at a dosage of 2%, can withstand a maximum temperature of 75℃, while at a dosage of 4%, it can reach 92℃, thus solving the problem of traditional clean fracturing fluids' inability to withstand high temperatures.

[0097] Compared with Example 5, the viscosity of Comparative Examples 6 and 7 is 0, indicating that the metal salt ions in the clean fracturing fluid system play an important role in its viscosity. This also shows that the fracturing fluid provided by the present invention has good salt resistance and can be used for the development of low-permeability and high-salinity reservoirs.

[0098] It should be noted that all the above embodiments belong to the same inventive concept, and the descriptions of each embodiment have different focuses. Where the description in a particular embodiment is not detailed, please refer to the description in other embodiments.

[0099] The embodiments described above are merely illustrative of implementation methods of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A clean fracturing fluid, characterized by, The cationic surfactant, mineralized water, and amphiphilic carbon dots are included; wherein the mineralized water has a salinity of 50,000-250,000 mg / L; The amphiphilic carbon dots are prepared by a solvothermal reaction of an organic acid and a hydrophobic aliphatic compound; The organic acid includes one or more of citric acid, tartaric acid, malic acid, salicylic acid, and hydroxybenzoic acid; and the hydrophobic aliphatic compound includes one or more of a fatty acid, a fatty alcohol, and an alkyl amine. The fatty acid includes one or more of a saturated fatty acid, an unsaturated fatty acid, and an aromatic fatty acid; the fatty alcohol includes one or more of n-octanol, n-dodecanol, n-tetradecanol, and n-hexadecanol; and the alkyl amine includes one or more of n-octylamine, dodecylamine, and tetradecylamine. The total mass of the cationic surfactant and the amphiphilic carbon dots is 1.0%-4.0% of the total mass of the clean fracturing fluid. The mass ratio of the cationic surfactant to the amphiphilic carbon dots is (6-9):(1-4).

2. The clean fracturing fluid of claim 1, wherein, The mass ratio of the organic acid to the hydrophobic aliphatic compound is (1:1)-(1:5).

3. The clean fracturing fluid of claim 1, wherein, The solvent used in the solvothermal reaction includes one or more of water, ethanol, DMF, DMSO, CCl4, and PEG.

4. The clean fracturing fluid of claim 1, wherein, The reaction temperature of the solvothermal reaction is 140-190℃, and the reaction time is 8-16 h.

5. The clean fracturing fluid of claim 1, wherein, The cationic surfactant is one or more of dodecyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, hexadecyltrimethylammonium bromide, and octadecyltrimethylammonium bromide; and the mineralized water contains inorganic salts.

6. Use of the clean fracturing fluid according to any one of claims 1-5 in hydraulic fracturing of a low-permeability oil reservoir and / or a high-salinity oil reservoir.

Images