Advanced treatment system and method for fracturing flow-back fluid

Abstract

The invention belongs to the technical field of oil and gas field fracturing flow-back fluid treatment. The invention provides an advanced treatment system and method for fracturing flow-back fluid. The system comprises an electric flocculation device, an ultrasonic enhanced Fenton treatment device, a primary filtering device, a chelating nanofiltration device and a reverse osmosis purification device, the chelating nanofiltration device comprises a chelating tank, a nanofiltration device and a concentrated solution tank, the inlet end of the chelating tank is communicated with the outlet end of the primary filtering device, the outlet end of the chelating tank is communicated with the inlet end of the nanofiltration device, and two outlet ends of the nanofiltration device are respectively communicated with the inlet ends of the concentrated solution tank and the reverse osmosis purification device; the chelating tank is provided with a first feeding port for adding a chelating agent and a second feeding port for adding sodium hydroxide, the outlet end of the concentrated solution tank is communicated with the first feeding port, and the pH value of liquid in the chelating tank is 8.5-9.5 by adding sodium hydroxide. According to the invention, suspended matters and organic matters in the fracturing flow-back fluid can be more effectively removed, and the hardness and the mineralization degree are reduced.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field fracturing flowback fluid treatment technology, specifically relating to a deep treatment system and method for fracturing flowback fluid. Background Technology

[0002] Currently, fracturing technology is widely used in oil and gas field development to increase oil and gas production. As fracturing proceeds, a large amount of fracturing flowback fluid is often generated. This flowback fluid contains a large amount of suspended solids, organic matter, metal salt ions, and other chemical additives, and is characterized by high chemical oxygen demand (COD), high hardness, and high salinity.

[0003] Traditional treatment methods such as sedimentation, filtration, and oxidation are not only costly and inefficient, but also fail to meet the requirements for advanced treatment. Therefore, developing an efficient, economical, and environmentally friendly method for the advanced treatment of fracturing flowback fluid is of great significance. Summary of the Invention

[0004] To address the problems existing in the background technology, the present invention provides a deep treatment system and method for fracturing flowback fluid, which can more effectively remove suspended solids and organic matter from fracturing flowback fluid, reduce hardness and mineralization, and realize the reuse and compliant discharge of fracturing flowback fluid.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a deep treatment system for fracturing flowback fluid, comprising an electrocoagulation device, an ultrasonic-enhanced Fenton treatment device, a primary filtration device, a chelation nanofiltration device, and a reverse osmosis purification device connected sequentially along a pipeline. The chelation nanofiltration device includes a chelation tank, a nanofilter, and a concentrate tank. The inlet end of the chelation tank is connected to the outlet end of the primary filtration device, the outlet end of the chelation tank is connected to the inlet end of the nanofilter, and the two outlet ends of the nanofilter are respectively connected to the inlet end of the concentrate tank and the inlet end of the reverse osmosis purification device. The chelation tank is provided with a first feed port for adding chelating agent and a second feed port for adding sodium hydroxide. The outlet end of the concentrate tank is connected to the first feed port, and sodium hydroxide is added to the chelation tank through the second feed port to make the pH value of the liquid in the chelation tank 8.5-9.5.

[0006] Furthermore, the electrocoagulation device includes a flocculation tank with an anode and a cathode, and an external power source is connected between the anode and the cathode.

[0007] Furthermore, the distance between the anode and cathode is 0.5-3 cm, and the current density is 50-120 A / m. 2 .

[0008] Furthermore, the ultrasonic-enhanced Fenton treatment device includes a treatment tank, on which an ultrasonic generator, a stirrer, and a dosing device are provided, the dosing device being used to add Fenton agent to the treatment tank.

[0009] Furthermore, the primary filtration device includes any one or at least two of the following: a manganese sand filter, a diamond sand filter, and an activated carbon filter, connected in series.

[0010] Furthermore, the chelating agent is any one of ethylenediaminetetraacetic acid, aminotriacetic acid, and diethylenetriaminepentaacetic acid.

[0011] Furthermore, the reverse osmosis purification device is equipped with a reverse osmosis membrane, and the permeate flux of the reverse osmosis membrane is controlled at 5-10 L·m⁻¹. -2 ·h -1 .

[0012] Secondly, the present invention provides a method for deep treatment of fracturing flowback fluid, which employs the aforementioned deep treatment system for fracturing flowback fluid and includes the following steps: Step 1: The fracturing flowback fluid is fed into an electrocoagulation device for treatment. Metal cations are formed by sacrificial anodes, generating highly active flocculants in the solution, thereby removing total suspended solids and colloids from the fracturing flowback fluid. Anodic electrolytic oxidation can also remove some COD. Step 2: The fracturing flowback fluid treated in Step 1 is transported into an ultrasonic-enhanced Fenton treatment device for further processing; with the assistance of ultrasound, the Fenton oxidation reaction is enhanced, thereby efficiently degrading organic pollutants and reducing COD. Step 3: After adjusting the pH of the liquid obtained in Step 2 to neutral, it is then fed into the primary filtration device for further processing. Step 4: The liquid obtained after step 3 is fed into a chelation tank. Sodium hydroxide is added through the second feed port to adjust the pH to 8.5-9.5, and a chelating agent is added through the first feed port to form a complex. Then, the liquid is fed into a nanofiltration unit containing a nanofiltration membrane, resulting in a nanofiltration concentrate that did not pass through the membrane and a nanofiltration permeate that did. The nanofiltration concentrate enters a concentrate tank, where the chelating agent is regenerated using sodium hydroxide. The regenerated chelating agent is returned to the chelation tank for reuse. During the regeneration process, Mg(OH)₂ precipitate and Ca(OH)₂ are simultaneously generated. Then, CO₂ is used to precipitate Ca(OH)₂ to CaCO₃, achieving CaCO₃ precipitation. 2+ and Mg 2+ The removal; via chelating agents and Ca 2+ Mg 2+ The chelation process generates a complex, improving the nanofiltration membrane's ability to process Ca. 2+ Mg 2+The removal of high-valence salt ions effectively achieves efficient hardening and reduces mineralization, minimizing the risk of scaling in subsequent treatment system pipelines. The retained concentrate is fed into a nanofiltration concentrate tank for chelating agent regeneration and reuse. The synergistic effect of "chelation + nanofiltration" can reduce calcium... 2+ Mg 2+ The requirement for nanofiltration membrane pore size can be met by using nanofiltration membranes with larger pore sizes to achieve the desired Ca2+ filtration efficiency. 2+ Mg 2+ Excellent removal effect, while enabling nanofiltration membranes to achieve greater permeate flux at lower operating pressure; Step 5: The nanofiltration permeate obtained after step 4 is fed into a reverse osmosis purification unit for further processing. A deep desalination process using reverse osmosis membranes is employed to remove residual high-valence salt ions and potassium ions. + Na + The purified water produced is recycled for compounding fracturing fluid and discharged in compliance with standards. The retained concentrate can be used for subsequent salt extraction processes.

[0013] Furthermore, the purified water has total suspended solids ≤1 mg / L, COD ≤10 mg / L, mineralization ≤1000 mg / L, and Ca... 2+ ≤1mg / L, Mg 2+ ≤1mg / L.

[0014] This application has the following beneficial effects: 1. Based on the different pollutant components of fracturing flowback fluid and combined with the different pollutant removal patterns, this invention develops a deep treatment technology that combines efficient flocculation, advanced oxidation, and staged membrane separation desalination. The main directions are process optimization and skid-mounted process, which reduces the cost of fracturing flowback fluid treatment, enables the reuse of fracturing flowback fluid, provides technical support for the treatment of fracturing flowback fluid in oil and gas fields, and is of great significance for achieving energy conservation and emission reduction.

[0015] 2. In the chelation treatment stage of this invention, after adding sodium hydroxide, the pH rises to the alkaline range (pH 8.5-9.5), and EDTA is converted to γ-rays. 4- Morphology, chelation sites fully dissociate, and with Ca 2+ / Mg 2+ Formation of a more stable [Ca / Mg-EDTA] 2- Nanofiltration membranes, with their pore size sieving (1-10 nm) and charge repulsion (negatively charged membrane surface), have a higher retention rate for complexes, significantly reducing the concentration of free ions.

[0016] Sodium hydroxide enhances EDTA chelation and membrane separation efficiency through pH regulation, and works synergistically with nanofiltration membrane pretreatment to deeply remove Ca. 2+ / Mg 2+ Synergistically reduce the Ca content in the final purified water2+ and Mg 2+ content. Attached Figure Description

[0017] Figure 1 A schematic diagram of the deep treatment system for fracturing flowback fluid of the present invention; Figure 2 The Ca content of the purified water finally produced in Example 1 and Comparative Examples 1-3 of this invention. 2+ and Mg 2+ Content data comparison trend chart.

[0018] Explanation of reference numerals in the attached figures: 1. Electrocoagulation device; 101. Flocculation tank; 102. Anode; 103. Cathode; 104. External power supply; 2. Ultrasonic enhanced Fenton treatment device; 201. Treatment tank; 202. Ultrasonic generator; 203. Agitator; 204. Sludge tank; 3. Primary filtration device; 301. Emery filter; 302. Activated carbon filter; 4. Chelation nanofiltration device; 401. Chelation tank; 402. Nanofilter; 403. Concentrate tank; 5. Reverse osmosis unit. Detailed Implementation

[0019] The present application will be further described in detail below with reference to the embodiments.

[0020] Unless otherwise specified, the raw materials used in the embodiments and comparative examples of this application are all commercially available.

[0021] Example 1: As Figure 1 As shown, this embodiment provides a deep treatment system for fracturing flowback fluid, applicable to the treatment of fracturing flowback fluid and produced water generated during shale gas and coalbed methane extraction, and also suitable for the treatment of wastewater such as oilfield fracturing flowback fluid, produced water, and oilfield drilling waste fluid. The system includes an electrocoagulation device 1, an ultrasonic-enhanced Fenton treatment device 2, a primary filtration device 3, a chelation nanofiltration device 4, and a reverse osmosis purification device 5, connected sequentially along a pipeline.

[0022] The electrocoagulation device 1 includes a flocculation tank 101 with an anode 102 and a cathode 103, connected to an external power supply 104. The ultrasonic-enhanced Fenton treatment device 2 includes a treatment tank 201 with an ultrasonic generator 202, a stirrer 203, and a dosing device for adding Fenton reagent to the treatment tank 201. The treatment tank 201 is also connected to a sludge tank 204. The primary filtration device 3 includes a diamond filter 301 and an activated carbon filter 302. The chelation nanofiltration device 4 includes a chelation tank 401, a nanofilter 402, and a concentrate tank 403. The inlet of the chelation tank 401 is connected to the outlet of the primary filtration device 3, and the outlet of the chelation tank 401 is connected to the inlet of the nanofilter 402. The two outlets of the nanofilter 402 are respectively connected to the inlet of the concentrate tank 403 and the inlet of the reverse osmosis purification device 5. The chelation tank 401 is equipped with a first feed port for adding a chelating agent and a second feed port for adding sodium hydroxide. The outlet of the concentrate tank 403 is connected to the first feed port, and sodium hydroxide is added to the chelation tank 401 through the second feed port to maintain the pH value of the liquid in the chelation tank 401 at 8.5-9.5. The reverse osmosis purification device 5 is equipped with a reverse osmosis membrane.

[0023] This embodiment also provides a method for deep treatment of fracturing flowback fluid, which specifically includes the following steps: Step 1: The fracturing flowback fluid is transported into the electrocoagulation unit for processing.

[0024] Specifically, the total suspended solids = 1000 mg / L, mineralization = 50000 mg / L, and Ca... 2+ =5000 mg / L, Mg 2+ Fracturing flowback fluid with a concentration of 5000 mg / L and a COD of 2000 mg / L is pressurized by a centrifugal pump and then fed into the flocculation tank. Both the anode and cathode use aluminum electrodes, made of pure aluminum (Al ≥ 99.5%), with a thickness of approximately 4 mm. The effective area to throughput ratio is 1 m². 2 / 10m 3 •h. The electrode spacing is 1cm, and an external power supply provides current at a current density of 100A / m. 2 Metallic Al is produced through the dissolution of aluminum at the anode. 3+ The flocculating group combines with OH⁻ in water to form a hydroxyl complex. Under the adsorption and bridging effects of this highly active flocculating group, colloidal particles destabilize and aggregate into large flocs, thereby removing total suspended solids and colloids from the fracturing flowback fluid. Furthermore, the electrolytic oxidation at the anolyte removes some COD. After this step, the total suspended solids in the fracturing flowback fluid were 97 mg / L, and the COD was 1350 mg / L.

[0025] Step 2: The fracturing flowback fluid processed in Step 1 is transported into an ultrasonically enhanced Fenton treatment device for further processing.

[0026] Specifically, the liquid treated in step one is pressurized by a centrifugal pump and then fed into the treatment tank. The stirrer is turned on, and HCl is added via a dosing device to adjust the liquid acidity to pH=2. H₂O₂ and FeSO₄·7H₂O (molar ratio 1:3) are then added, with the H₂O₂ concentration at 600 mg / L and the FeSO₄·7H₂O concentration at 1800 mg / L. The Fe… 2+ The reaction with H₂O₂ produces ·OH, which oxidizes and degrades organic matter. An ultrasonic generator with a power of 1 kW and an ultrasonic frequency of 40 kHz was used. The localized high temperature and pressure generated by ultrasonic cavitation promoted more efficient decomposition of H₂O₂, increased the ·OH generation rate, enhanced the efficiency of Fenton oxidation in degrading organic matter, and reduced the COD of the fracturing flowback fluid. Furthermore, the cavitation effect promoted the Fe³⁺ / Fe²⁺ cycle through the high temperature and pressure environment, reduced Fe³⁺ accumulation, and maintained the continuous and efficient reaction. The COD of the liquid treated in this step was 83 mg / L.

[0027] Step 3: The liquid processed in Step 2 is then fed into the primary filtration device for further processing.

[0028] Specifically, after adjusting the pH to neutral, the water is filtered through a primary filtration device consisting of a diamond abrasive filter and an activated carbon filter to remove suspended solids and other impurities, thereby ensuring the stable operation of the subsequent membrane separation system.

[0029] Step 4: The liquid processed in Step 3 is transported into a chelation nanofiltration device for further processing.

[0030] Specifically, the liquid processed in step three is pressurized by a centrifugal pump and then fed into Ca. 2+ / Mg 2+ In the chelation tank, sodium hydroxide was added through the second feed port to adjust the pH to 9.0, and the chelating agent ethylenediaminetetraacetic acid (EDTA) was added through the first feed port. 2+ / Mg 2+ The molar ratio with EDTA is 1:1.05 (5% excess to ensure sufficient complexation), and the specific dosage is Ca... 2+ and Mg 2+ 105% of the total. Ca 2+ / Mg 2+ It forms a stable cyclic complex [Ca / Mg-EDTA] with the chelating agent ethylenediaminetetraacetic acid (EDTA). 2- Increased Ca 2+ / Mg 2+ The effective size is more conducive to the subsequent nanofiltration of Ca. 2+ / Mg2+ The nanofiltration unit is equipped with a nanofiltration membrane, which is a polyamide composite membrane with a molecular weight cutoff of 200-300 Da. The operating pressure is 0.8-1.2 MPa, and the membrane flux is 18 L / (m²·h). After treatment by the nanofiltration membrane, the nanofiltration concentrate that did not pass through the nanofiltration membrane and the nanofiltration permeate that passed through the nanofiltration membrane are obtained. The nanofiltration concentrate enters the concentrate tank and is regenerated with NaOH as a chelating agent. The regenerated chelating agent is then refluxed into the CaO / NaOH solution. 2+ / Mg 2+ The chelation tank is reused, and the regeneration process simultaneously produces Mg(OH)₂ precipitate and Ca(OH)₂. Then, CO₂ is used to precipitate Ca(OH)₂ into CaCO₃ precipitate, achieving Ca… 2+ and Mg 2+ The removal of suspended solids. The water quality indicators of the nanofiltration solution are: total suspended solids = 1 mg / L, COD = 26 mg / L, mineralization = 1830 mg / L, and Ca... 2+ =3.1 mg / L, Mg 2+ =2.8mg / L.

[0031] Step 5: The nanofiltration liquid obtained after step 4 is transported into the reverse osmosis purification device for further processing.

[0032] Specifically, the nanofiltration permeate is pressurized by a centrifugal pump and fed into a reverse osmosis purification unit, where a deep desalination process using reverse osmosis membranes is employed to further remove residual high-valence salt ions and potassium ions. + Na + The purified water, containing monovalent salt ions, is reused to formulate fracturing fluid and discharged in compliance with standards. The retained concentrate can be used for subsequent salt extraction processes (such as MVR or multi-effect evaporation). After this step, the final purified water has a total suspended solids concentration of 0.7 mg / L, a COD concentration of 8.6 mg / L, a mineralization concentration of 271 mg / L, and a calcium content of [missing information]. 2+ =0.5mg / L, Mg 2+ =0.4mg / L.

[0033] Example 2: The difference between this example and Example 1 is that in step four, the liquid treated in step three is pressurized by a centrifugal pump and then fed into Ca. 2+ / Mg 2+ In the chelation tank, sodium hydroxide is added through the second feed port to adjust the pH value to 8.5.

[0034] The final purified water has the following parameters: total suspended solids = 0.6 mg / L, COD = 9.0 mg / L, mineralization = 306 mg / L, and calcium content = 0.6 mg / L. 2+ =0.6mg / L, Mg 2+ =0.4mg / L.

[0035] Example 3: The difference between this example and Example 1 is that in step four, the liquid treated in step three is pressurized by a centrifugal pump and then fed into Ca. 2+ / Mg 2+ In the chelation tank, sodium hydroxide is added through the second feed port to adjust the pH value to 9.5.

[0036] The final purified water produced had a total suspended solids concentration of 0.9 mg / L, a COD concentration of 9.3 mg / L, a mineralization concentration of 294 mg / L, and a calcium content of [missing information]. 2+ =0.8mg / L, Mg 2+ =0.6mg / L.

[0037] Comparative Example 1: The difference between this comparative example and Example 1 is that in step four, sodium hydroxide is not added, and nanofiltration is not performed. That is, after chelation under neutral conditions, reverse osmosis membrane treatment is performed directly.

[0038] Specifically, a method for deep treatment of fracturing flowback fluid includes the following steps: Step 1: The fracturing flowback fluid is transported into the electrocoagulation unit for processing.

[0039] Specifically, the total suspended solids = 1000 mg / L, mineralization = 50000 mg / L, and Ca... 2+ =5000 mg / L, Mg 2+ Fracturing flowback fluid with a concentration of 5000 mg / L and a COD of 2000 mg / L was pressurized by a centrifugal pump and then fed into the flocculation tank. Both the anode and cathode used aluminum electrodes with a 1 cm electrode spacing, and an external power supply provided current at a current density of 100 A / m². 2 Metallic Al is produced through the dissolution of aluminum at the anode. 3+ The flocculating group combines with OH⁻ in water to form hydroxyl complexes. Under the adsorption and bridging effects of these highly active flocculating groups, colloidal particles destabilize and aggregate into large flocs, thereby removing total suspended solids and colloids from the fracturing flowback fluid. Furthermore, the electrolytic oxidation at the anolyte can remove some COD.

[0040] Step 2: The fracturing flowback fluid processed in Step 1 is transported into an ultrasonically enhanced Fenton treatment device for further processing.

[0041] Specifically, the liquid treated in step one is pressurized by a centrifugal pump and then fed into the treatment tank. The stirrer is turned on, and HCl is added through a dosing device to adjust the liquid acidity to pH=2. H₂O₂ and FeSO₄·7H₂O (molar ratio 1:3) are then added, and the solution is added through Fe... 2+The reaction with H₂O₂ produces ·OH, which oxidizes and degrades organic matter. An ultrasonic generator with a power of 1 kW is used; the localized high temperature and pressure generated by ultrasonic cavitation promotes more effective decomposition of H₂O₂, increases the ·OH generation rate, enhances the efficiency of Fenton oxidation in degrading organic matter, and reduces the COD of the fracturing flowback fluid. Furthermore, the cavitation effect promotes the Fe³⁺ / Fe²⁺ cycle through the high temperature and high pressure environment, reduces Fe³⁺ accumulation, and maintains the continuous and efficient progress of the reaction.

[0042] Step 3: The liquid processed in Step 2 is then fed into the primary filtration device for further processing.

[0043] Specifically, after adjusting the pH to neutral, the water is filtered through a primary filtration device consisting of a diamond abrasive filter and an activated carbon filter to remove suspended solids and other impurities, thereby ensuring the stable operation of the subsequent membrane separation system.

[0044] Step 4: The liquid processed in Step 3 is transported into the chelation device for further processing.

[0045] Specifically, the liquid processed in step three is pressurized by a centrifugal pump and then fed into Ca. 2+ / Mg 2+ In the chelation tank, chelating agent is added through the first feeding port.

[0046] Step 5: The liquid obtained after step 4 is directly fed into the reverse osmosis purification device for further processing.

[0047] Comparative Example 2: The difference between this comparative example and Example 1 is that sodium hydroxide is not added in step four. That is, after chelation under neutral conditions, nanofiltration membrane treatment is performed first, followed by reverse osmosis membrane treatment.

[0048] Comparative Example 3: The difference between this comparative example and Example 1 is that nanofiltration is not performed in step four. That is, after chelation under alkaline conditions with a pH of 9.0, reverse osmosis membrane treatment is performed directly.

[0049] Experimental Example: Test, statistically analyze, and record the COD and Ca of the purified water obtained from Examples 1-3 and Comparative Examples 1-3. 2+ Content and Mg 2+ Content data are shown in Table 1.

[0050] Table 1. Experimental Data Results Analysis: Analysis of Examples 1-3, combined with data from Table 1 and... Figure 2 It can be seen that the purified water produced in Examples 1-3 of this invention has a COD as low as below 9.3 mg / L and a Ca content as low as 1 mg / L. 2+ Content as low as below 0.8 mg / L, Mg2+ The content is as low as 0.6 mg / L or less.

[0051] Analyze Example 1 and Comparative Examples 1-3 and combine the data in Table 1 and Figure 2 Specifically, comparing Comparative Example 1 and Comparative Example 2, it can be seen that, compared to Comparative Example 1, Comparative Example 2, by adding nanofiltration membrane treatment before reverse osmosis membrane treatment, resulted in a higher Ca content in the final purified water. 2+ Content and Mg 2+ The content did not change significantly. This indicates that after chelation under neutral conditions, adding nanofiltration membrane treatment before reverse osmosis membrane treatment will affect the Ca content of the final purified water. 2+ Content and Mg 2+ The content is unaffected.

[0052] Specifically, comparing Comparative Example 1 and Comparative Example 3, it can be seen that, compared to Comparative Example 1, Comparative Example 3, by adding sodium hydroxide to adjust the pH to 9.0 and directly performing reverse osmosis membrane treatment, ultimately produces purified water with a lower Ca content. 2+ The content decreased from 5.9 mg / L (Comparative Example 1) to 1.8 mg / L (Comparative Example 3), Mg 2+ The concentration decreased from 5.1 mg / L (Comparative Example 1) to 1.4 mg / L (Comparative Example 3). This indicates that changing the chelation environment from neutral to alkaline (pH=9.0) can reduce the Ca concentration in the final purified water. 2+ Content and Mg 2+ content.

[0053] Comparing with Example 1, it can be seen that when the chelation environment is alkaline at pH 9.0, adding nanofiltration membrane treatment before reverse osmosis membrane treatment results in a lower Ca content in the final purified water. 2+ Content and Mg 2+ The content decreased significantly further.

[0054] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0055] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A deep treatment system for fracturing flowback fluid, characterized in that, It includes an electrocoagulation device (1), an ultrasonic enhanced Fenton treatment device (2), a primary filtration device (3), a chelation nanofiltration device (4), and a reverse osmosis purification device (5) connected in sequence through pipelines. The chelation nanofiltration device (4) includes a chelation tank (401), a nanofilter (402), and a concentrate tank (403). The inlet end of the chelation tank (401) is connected to the outlet end of the primary filtration device (3), the outlet end of the chelation tank (401) is connected to the inlet end of the nanofilter (402), and the two outlet ends of the nanofilter (402) are respectively connected to the inlet end of the concentrate tank (403) and the inlet end of the reverse osmosis purification device (5). The chelation tank (401) is provided with a first feed port for adding chelating agent and a second feed port for adding sodium hydroxide. The outlet end of the concentrate tank (403) is connected to the first feed port, and sodium hydroxide is added to the chelation tank (401) through the second feed port so that the pH value of the liquid in the chelation tank (401) is 8.5-9.

5.

2. The deep treatment system for fracturing flowback fluid according to claim 1, characterized in that, The electrocoagulation device (1) includes a flocculation tank (101) with an anode (102) and a cathode (103) thereon, and an external power supply (104) is connected between the anode (102) and the cathode (103).

3. The deep treatment system for fracturing flowback fluid according to claim 2, characterized in that, The distance between the anode (102) and the cathode (103) is 0.5-3cm, and the current density is 50-120A / m2.

4. The deep treatment system for fracturing flowback fluid according to claim 1, characterized in that, The ultrasonic-enhanced Fenton treatment device (2) includes a treatment tank (201) on which an ultrasonic generator (202), a stirrer (203) and a dosing device are provided. The dosing device is used to add Fenton agent to the treatment tank (201).

5. The deep treatment system for fracturing flowback fluid according to claim 1, characterized in that, The primary filtration device (3) includes any one or at least two of the following: a manganese sand filter, a diamond sand filter (301), and an activated carbon filter (302) connected in series.

6. The deep treatment system for fracturing flowback fluid according to claim 1, characterized in that, The chelating agent is any one of ethylenediaminetetraacetic acid, aminotriacetic acid, and diethylenetriaminepentaacetic acid.

7. The deep treatment system for fracturing flowback fluid according to claim 1, characterized in that, The reverse osmosis purification device (5) is equipped with a reverse osmosis membrane, and the permeate flux of the reverse osmosis membrane is controlled at 5-10 L·m. -2 ·h -1 .

8. A method for deep treatment of fracturing flowback fluid, characterized in that, The deep treatment system for fracturing flowback fluid as described in any one of claims 1-7 is used, and includes the following steps: Step 1: The fracturing flowback fluid is fed into the electrocoagulation unit (1) for treatment; Step 2: The fracturing flowback fluid after Step 1 is transported into the ultrasonic-enhanced Fenton treatment device (2) for further treatment; Step 3: After adjusting the pH of the liquid obtained in Step 2 to neutral, it is then fed into the primary filtration device (3) for further processing. Step 4: The liquid obtained after step 3 is transported into a chelation tank (401). Sodium hydroxide is added through the second feed port to adjust the pH value to 8.5-9.5, and a chelating agent is added through the first feed port to generate a complex. Then, it is transported into a nanofiltration unit (402). The nanofiltration unit (402) is equipped with a nanofiltration membrane to obtain nanofiltration concentrate that does not pass through the nanofiltration membrane and nanofiltration permeate that passes through the nanofiltration membrane. The nanofiltration concentrate enters a concentrate tank (403) and is regenerated using sodium hydroxide. The regenerated chelating agent is returned to the chelation tank (401) for reuse. During the regeneration process, Mg(OH)2 precipitate and Ca(OH)2 are generated simultaneously. Then, CO2 is used to generate CaCO3 precipitate from Ca(OH)2 to achieve CaCO3 precipitation. 2+ and Mg 2+ The removal; Step 5: The nanofiltration liquid obtained after step 4 is transported into the reverse osmosis purification device (5) for further processing. The purified water produced is reused to prepare fracturing fluid / discharged after meeting the standards.

9. The method for deep treatment of fracturing flowback fluid according to claim 8, characterized in that, The purified water has total suspended solids ≤1 mg / L, COD ≤10 mg / L, mineralization ≤1000 mg / L, and Ca2+ content ≤100 mg / L. 2+ ≤1mg / L, Mg 2+ ≤1mg / L.

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