Shale gas fracturing flowback fluid up-to-standard external discharge pretreatment method and system

Abstract

The application discloses a shale gas fracturing flow-back fluid standard discharge pretreatment method and system, relates to the technical field of waste liquid treatment, and can optimize the existing shale gas flow-back fluid treatment process, reduce the process flow unit, and reduce the reagent dosage. The method comprises the following steps: S1, high ferrate and solid catalyst are added to the fracturing flow-back fluid for oxidation-coagulation treatment, then a flocculating agent is added, and first supernatant is obtained; S2, the pH of the first supernatant is adjusted to be alkaline by adding sodium hydroxide, and persulfate and sodium carbonate are added to perform oxidation-softening treatment, and second supernatant is obtained; and S3, the second supernatant is subjected to ultrafiltration treatment, and third supernatant is obtained. The application can effectively remove turbidity, organic matter content, scale-forming ion concentration and bacteria in the flow-back fluid, improve the subsequent membrane water quality, reduce the membrane pollution risk in the membrane system desalination process, prolong the service life of the membrane and the evaporation desalination system, and ensure that the fracturing flow-back fluid treatment meets the standard discharge.

Description

Technical Field

[0001] This invention relates to the field of waste liquid treatment technology, specifically to a method and system for pre-treatment of shale gas fracturing flowback fluid to meet discharge standards. Background Technology

[0002] With my country's ever-growing demand for natural gas, shale gas, as a relatively clean and efficient unconventional energy source, has attracted widespread attention. The fracturing flowback fluid generated after fracturing operations has a complex composition, containing high concentrations of polymers, microorganisms, organic matter, surfactants, suspended solids, and soluble inorganic salts. It is characterized by high suspension, high salinity, poor biodegradability, and difficulty in biodegradation. Improper disposal and direct discharge of fracturing flowback fluid could pollute surface water sources, causing environmental accidents and posing a serious threat to the environment and human health. Previously, the main disposal methods included reinjection, reuse, and external discharge. Due to the potential risks of reinjection and the time and site limitations of reuse, compliant external discharge has become the primary focus.

[0003] The treatment process for fracturing flowback fluid mainly includes pretreatment, desalination, and evaporation. The desalination system is the primary process; however, the high organic content can cause equipment blockage and waste salt generation, while suspended solids and scaling ions can lead to scale buildup in the evaporator, reducing equipment lifespan and posing safety hazards. Therefore, finding an economical, scientific, reasonable, and efficient pretreatment technology is crucial.

[0004] In the process of realizing this invention, the inventors discovered that existing shale gas fracturing flowback fluid pretreatment generally consists of a coagulation and flocculation unit, an oxidation unit, a softening unit, and a sterilization unit, which respectively remove suspended solids, organic matter, scale ions, and bacteria. The active agents and reaction tanks of each unit must be considered separately. For example, the pretreatment part in patent application number 201611175024.6 includes a pre-sedimentation tank unit, a coagulation and sedimentation unit, a sand filtration unit, and an UF system. The coagulation and sedimentation unit alone requires the addition of agents such as aluminum sulfate, polyaluminum chloride, ferric chloride, ferrous sulfate, and magnesium carbonate. Moreover, the treated water sample still contains a large amount of organic matter, color, hardness, etc. Although patent application number 201810812821.3 can obtain a relatively high quality of pretreated water, the process is cumbersome, including first adding coagulants, flocculants, and descaling agents, then adding sterilizing agents, and then including sand filtration, activated carbon filtration, and ultrafiltration treatment. In general, the current process is relatively complex, requires the addition of many types of reagents in large doses, involves many unit processes, has long residence time, and occupies a large area, making it unsuitable for shale gas development operations in mountainous environments. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a pretreatment method and system to optimize the existing fracturing flowback fluid treatment process, reduce the number of process units, and reduce the amount of reagents added.

[0006] This invention is achieved through the following technical solution:

[0007] On the one hand, the present invention provides a method for pretreatment of shale gas fracturing flowback fluid to meet discharge standards, including the following steps:

[0008] S1. Obtaining primary supernatant: Ferrate and solid catalyst are added to the fracturing flowback fluid to fully mix the ferrate and solid catalyst with the fracturing flowback fluid for oxidation-coagulation treatment. Then, flocculant is added to cause the flocs generated in S1 to aggregate into clumps. After settling by gravity, the flocs generated during the settling process are removed to obtain primary supernatant.

[0009] S2. Obtaining the secondary supernatant: The pH of the primary supernatant is adjusted to alkaline by adding sodium hydroxide, then persulfate is added and stirred until fully mixed to remove barium ions while oxidizing. Then sodium carbonate is added as a softening agent and stirred until fully mixed to carry out oxidation-softening treatment. After standing and settling, the secondary supernatant is obtained.

[0010] S3. Obtaining the tertiary supernatant: The secondary supernatant is subjected to ultrafiltration to remove macromolecular substances in the solution and obtain the tertiary supernatant.

[0011] Optionally, in step S1, the ferrate is at least one of potassium ferrate and sodium ferrate, preferably potassium ferrate, and the amount of ferrate added is 400-1000 mg / L.

[0012] Optionally, in step S1, the solid catalyst is a FeO(OH)-montmorillonite composite material, the amount of the solid catalyst is 1-3 g / L, and the FeO(OH)-montmorillonite composite material can be reused more than 3 times by magnetic separation technology.

[0013] Among them, the solid-phase catalyst α-FeO(OH)-montmorillonite can form a Fenton-like oxidation system with hydrogen peroxide generated in situ from ferrate, producing highly active hydroxyl radicals that greatly improve the removal efficiency of recalcitrant organic matter.

[0014] Further optionally, the preparation method of the FeO(OH)-montmorillonite composite material includes:

[0015] By weight, 1-50 parts of montmorillonite were added to 100-1000 parts of ultrapure water and stirred at 300-1000 rpm for 18-24 hours. The montmorillonite was uniformly suspended in the ultrapure water to obtain liquid 1.

[0016] Sodium dodecyl sulfate was added to liquid 1, with the weight of sodium dodecyl sulfate being 0.1%-0.3% of montmorillonite. The mixture was stirred at a stirring speed of 300-1000 rpm for 20-40 minutes to obtain liquid 2.

[0017] FeSO4·7H2O was added to liquid 2, Fe 2+ The amount of substance and the weight ratio of montmorillonite were 8-16 mmol / g. The mixture was stirred at 300-1000 rpm for 5-15 min to obtain liquid 3.

[0018] Sodium acetate was added to liquid 3, with the weight of sodium acetate being 80%-120% of montmorillonite. The mixture was stirred at a stirring speed of 300-1000 rpm for 4-6 hours to obtain a brown precipitate.

[0019] The brown precipitate was repeatedly washed with anhydrous ethanol and ultrapure water, then placed in an oven at 65-75℃ for 18-30 hours. After drying, it was ground into powder to obtain ginger-yellow FeO(OH)-montmorillonite material.

[0020] Optionally, in step S1, the pH of the fracturing flowback fluid during the oxidation-coagulation treatment is 7-8, and the stirring speed during the oxidation-coagulation treatment is 300-350 rpm.

[0021] Optionally, in step S1, the flocculant is anionic polyacrylamide, and the dosage of the flocculant is 1-4 mg / L.

[0022] Optionally, in step S2, the alkaline pH value is 10-11.

[0023] Optionally, in step S2, the persulfate is at least one of sodium persulfate and potassium persulfate, preferably sodium persulfate, and the amount of persulfate added is 500-1000 mg / L;

[0024] The dosage of sodium carbonate is 400-600 mg / L.

[0025] Optionally, in step S1, the oxidation-coagulation treatment time is 20-40 min, and the settling time is 15-40 min;

[0026] In step S2, the oxidation-softening treatment takes 35-45 minutes, and the settling time is 25-40 minutes.

[0027] On the other hand, the present invention provides a shale gas fracturing flowback fluid pretreatment system for compliant discharge, applied to any of the above-described shale gas fracturing flowback fluid compliant discharge pretreatment methods, the system comprising:

[0028] Storage tank, the storage tank being used to store fracturing flowback fluid;

[0029] The first pretreatment unit is used to receive fracturing flowback fluid and perform step S1, and output primary supernatant.

[0030] The second pretreatment unit is used to receive the primary supernatant and perform step S2, and output the secondary supernatant.

[0031] A tubular ultrafiltration unit is used to receive the secondary supernatant and perform step S3, and output the tertiary supernatant;

[0032] The storage tank, the first pretreatment unit, the second pretreatment unit, and the tubular ultrafiltration unit are connected in sequence via pipelines.

[0033] The present invention has the following advantages and beneficial effects:

[0034] This invention provides a pretreatment method for compliant discharge of shale gas fracturing flowback fluid. In the pretreatment of the fracturing flowback fluid, ferrate, solid-phase catalyst, persulfate, and sodium carbonate are added sequentially. Ferrate is a strong oxidant that can oxidize contaminants and destroy bacterial structures. Simultaneously, after oxidation, ferrate forms ferric hydroxide (Fe(OH)3), which can simultaneously coagulate and flocculate to remove suspended solids (SS). Meanwhile, persulfate can generate free radicals under alkaline conditions to oxidize remaining organic matter, while simultaneously reducing it to sulfate ions (SO42-). 2- Adding sodium carbonate as a softening agent can create a softening method similar to Na2SO4-NaOH-Na2CO3, which can simultaneously and efficiently remove Ca. 2+ Mg 2+ Ba 2+ Ions are effectively removed from the flowback fluid, thereby reducing turbidity, organic matter content, scale-forming ion concentration, and bacteria, improving the quality of subsequent membrane feed water, reducing the risk of membrane fouling during desalination, extending the service life of the membrane and evaporative desalination system, and ensuring that the fracturing flowback fluid meets discharge standards. Furthermore, compared to existing technologies, this system achieves multiple functions such as oxidation, coagulation, softening, and sterilization of the fracturing flowback fluid with fewer types of reagents, reducing the corresponding processes and the total dosage of reagents required. It reduces pretreatment costs while ensuring the effectiveness of pretreatment. Based on this, the shale gas fracturing flowback fluid discharge pretreatment system provided by this invention achieves multiple functions such as oxidation, coagulation, softening, and sterilization through two process units: a first pretreatment unit and a second pretreatment unit, thereby reducing reagent dosage, lowering treatment costs, and streamlining the treatment process. Attached Figure Description

[0035] The accompanying drawings, which are included to provide a further understanding of embodiments of the invention and form part of this application, do not constitute a limitation thereof. In the drawings:

[0036] Figure 1 This is a flowchart illustrating the steps of one embodiment of the present invention;

[0037] Figure 2 This is a schematic diagram of the pretreatment process flow in one embodiment of the present invention;

[0038] Figure 3 This is a schematic diagram of the entire process flow up to and including discharge in one embodiment of the present invention. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings. The illustrative embodiments and descriptions of the present invention are only used to explain the present invention and are not intended to limit the present invention.

[0040] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that these specific details are not necessary to practice the invention. In other embodiments, well-known structures, materials, or methods have not been specifically described in order to avoid obscuring the invention.

[0041] Throughout this specification, references to "an embodiment," "an example," or "an example" mean that a particular feature, structure, or characteristic described in connection with that embodiment or example is included in at least one embodiment of the invention. Therefore, the phrases "an embodiment," "an example," "an example," or "an example" appearing in various places throughout the specification do not necessarily refer to the same embodiment or example. Furthermore, specific features, structures, or characteristics can be combined in one or more embodiments or examples in any suitable combination and / or sub-combination. Moreover, those skilled in the art will understand that the illustrations provided herein are for illustrative purposes and are not necessarily drawn to scale. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0042] It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. In this invention, unless otherwise stated, the terms "first," "second," etc., are used to distinguish similar objects, rather than to describe a specific order or sequence.

[0043] the term:

[0044] Coagulants: Coagulants are substances that can cause suspended solids, colloidal particles and other impurities in water to form flocculent precipitates.

[0045] Flocculants: Any substance used to precipitate solutes, colloids, or suspended particles in an aqueous solution into flocculent precipitates is called a flocculant.

[0046] Coagulation: When suspended particles in water are small enough, the energy of their Brownian motion is sufficient to counteract gravity, preventing sedimentation. Such suspensions can remain stable for extended periods. Furthermore, the surfaces of suspended particles are often charged (usually negatively charged), and the repulsive force between particles of the same charge makes them less likely to coalesce and grow larger, thus increasing the stability of the suspension. The coagulation process involves adding a positively charged coagulant to neutralize the negative charge on the particle surfaces, causing the particles to "destabilize." As a result, the particles combine and grow larger through collisions, surface adsorption, and van der Waals forces, facilitating separation from the water.

[0047] Flocculation: Flocculation is the process by which polymer chains bridge between suspended particles. "Bridging" means that different segments of the polymer molecule adsorb onto different particles, promoting particle aggregation.

[0048] Oxidation: Generally speaking, oxidation is the loss of electrons, and reduction is the gain of electrons. The reaction in which organic matter introduces oxygen or removes hydrogen is called oxidation; the introduction of hydrogen or the loss of oxygen is called reduction. A slow reaction of a substance with oxygen results in a slow increase in temperature and no light emission.

[0049] Oxidation is called slow oxidation, such as metal corrosion and biological respiration. Vigorous oxidation involving light and heat is called combustion.

[0050] Oxidizing agent: A substance that gains electrons in a redox reaction. Oxidizing agents have oxidizing properties; when they gain electrons, their oxidation state decreases, leading to a reduction reaction and the production of reduction products.

[0051] Example 1:

[0052] 1. Shale gas fracturing flowback fluid treatment project. The shale gas fracturing flowback fluid, after testing, has the following water quality characteristics:

[0053] Table 1. Raw water quality data of a certain fracturing flowback fluid

[0054] index 1# Turbidity (NTU) 163 pH 7.28 K + (mg / L) 610.9 <![CDATA[Na + (mg / L)]]> 11250 <![CDATA[Ca 2+ (mg / L)]]> 286 <![CDATA[Mg 2+ (mg / L)]]> 57.75 <![CDATA[Ba 2+ (mg / L)]]> 186 <![CDATA[Sr 2+ (mg / L)]]> 94.3 Total iron (mg / L) ND Total silicon (mg / L) 47.8 <![CDATA[Br - (mg / L)]]> 102 <![CDATA[HCO3 - (mg / L)]]> 51 <![CDATA[Cl - (mg / L)]]> 13180.31 <![CDATA[SO4 2- (mg / L)]]> 1.68 TDS (mg / L) 25867.74 SS (mg / L) 74 COD (mg / L) 357 TOC (mg / L) 23.24 Petroleum products (mg / L) 4.89 Sulfate-reducing bacteria (CFU / ml) 1100000 Iron bacteria (cities / ml) 70 Saprophytic bacteria (cFU / ml) 70

[0055] Among them, such as Figures 1 to 2 As shown, the pretreatment method for shale gas fracturing flowback fluid includes the following steps:

[0056] S1. Potassium ferrate and FeO(OH)-montmorillonite composite material are added to the fracturing flowback fluid for oxidation and coagulation, i.e., oxidation-coagulation treatment. The preparation of the FeO(OH)-montmorillonite composite material is as follows: 200g of montmorillonite is added to 2000g of ultrapure water and stirred at 300rpm for 20h, resulting in a uniform suspension of montmorillonite in the ultrapure water, yielding liquid 1-1; 0.2g of sodium dodecyl sulfate is added to liquid 1-1 and stirred at 300rpm for approximately 25min, yielding liquid 1-2; 556g of FeSO4·7H2O is added to liquid 1-2, Fe... 2+ The amount of substance and the weight ratio of sodium acetate to montmorillonite were 10 mmol / g. The mixture was stirred at 300 rpm for 10 min to obtain liquid 1-3. 200 g of sodium acetate was added to liquid 1-3 and stirred at 300 rpm for 4 h to obtain a brown precipitate. The brown precipitate was washed three times with anhydrous ethanol and ultrapure water and then placed in a 70 ℃ oven for 24 h. After drying, it was ground into powder to obtain ginger-yellow FeO(OH)-montmorillonite material.

[0057] The dosage of potassium ferrate was 400 mg / L, and the dosage of FeO(OH)-montmorillonite composite material was 1 g / L. The mixture was reused more than three times via magnetic separation. The pH during the oxidation-coagulation treatment was 8, the stirring speed was 300 rpm, and the stirring time during the potassium ferrate oxidation treatment was 20 min (including the addition time).

[0058] Anionic polyacrylamide, a flocculant with a viscosity-average molecular weight of 12 million, was added to the fracturing flowback fluid treated with potassium ferrate and allowed to settle. The dosage of the anionic polyacrylamide was 1 mg / L, the pH during flocculation was 8, and the settling time was 30 min (including the addition time, the time for separating the flocs and the supernatant).

[0059] After separating the flocs and the supernatant, a primary supernatant is obtained. The water quality of the primary supernatant is shown in Table 2.

[0060] Table 2. Water quality data of primary supernatant

[0061]

[0062] S2. Add sodium hydroxide to the primary supernatant to adjust the pH to 11 for 10 minutes.

[0063] Adding 1000 mg / L of sodium persulfate oxidizes and softens the Ba while removing it. 2+ Let it stand and settle. Then add another softening agent, sodium carbonate, 500 mg / L. The pH of the oxidation-softening treatment is 11, and the stirring time for oxidation-softening is 40 min.

[0064] After settling for 30 minutes, the secondary supernatant was obtained. The water quality of the secondary supernatant at this time is shown in Table 3.

[0065] Table 3. Water quality data of secondary supernatant

[0066]

[0067] S3. The secondary supernatant is then subjected to ultrafiltration through a tubular micro-ultrafiltration membrane for solid-liquid separation for 10 minutes to remove macromolecular substances from the solution and obtain the tertiary supernatant.

[0068] Based on the steps described in this embodiment, the time required by different preprocessing systems is compared in Table 4.

[0069] Table 4 Comparison of time for different pretreatment systems

[0070]

[0071] The water quality after different pretreatment systems is shown in Table 5.

[0072] Table 5 Water quality after treatment by the three systems

[0073]

[0074] Ultimately, as Figure 3 As shown, the third-stage supernatant can be used as water to enter the reverse osmosis system for filtration, reducing the SS of the fracturing flowback fluid to 3 mg / L, the turbidity to below 1 NTU, and the TOC to 4 mg / L. Then, it enters the desalination system for concentration before being discharged.

[0075] As can be seen from Tables 4 and 5, after the shale gas fracturing flowback fluid is pretreated by the method described in this patent, it can not only effectively shorten the process flow and residence time, but also significantly reduce the turbidity, bacterial count, chemical oxygen demand, and total organic carbon content in the reverse osmosis feed water quality indicators. This can reduce reverse osmosis membrane fouling, extend membrane life, and increase economic benefits.

[0076] Example 2:

[0077] The shale gas fracturing flowback fluid used was the shale gas fracturing flowback fluid in Example 1, and the water quality is shown in Table 1.

[0078] Among them, such as Figures 1 to 2 As shown, the pretreatment method for shale gas fracturing flowback fluid includes the following steps:

[0079] S1. Add sodium ferrate and FeO(OH)-montmorillonite composite material to the fracturing flowback fluid for oxidation reaction and coagulation, i.e., oxidation-coagulation treatment. The preparation of FeO(OH)-montmorillonite composite material is as follows: 400g of montmorillonite was added to 2000g of ultrapure water and stirred at 500rpm for 24h, so that the montmorillonite was uniformly suspended in the ultrapure water, resulting in liquid 2-1; 0.8g of sodium dodecyl sulfate was added to liquid 2-1 and stirred at 500rpm for about 30min, resulting in liquid 2-2; 1556.8g of FeSO4·7H2O was added to liquid 2-2, Fe... 2+ The amount of substance and the weight ratio of sodium acetate to montmorillonite were 14 mmol / g. The mixture was stirred at 500 rpm for 10 min to obtain liquid 2-3. 320 g of sodium acetate was added to liquid 2-3 and stirred at 500 rpm for 5 h to obtain a brown precipitate. The brown precipitate was washed three times with anhydrous ethanol and ultrapure water and then placed in a 75 ℃ oven for 24 h. After drying, it was ground into powder to obtain ginger-yellow FeO(OH)-montmorillonite material.

[0080] The dosage of sodium ferrate was 600 mg / L, and the dosage of FeO(OH)-montmorillonite composite material was 2 g / L. The mixture was reused more than three times via magnetic separation. The pH during the oxidation-coagulation treatment was 8, the stirring speed was 350 rpm, and the stirring time during the potassium ferrate oxidation treatment was 30 min (including the addition time).

[0081] Anionic polyacrylamide, a flocculant with a viscosity-average molecular weight of 15 million, was added to the fracturing flowback fluid treated with potassium ferrate and allowed to settle. The dosage of the anionic polyacrylamide was 2.5 mg / L, the pH during flocculation was 8, and the settling time was 40 min (including the addition time, the time for separating the flocs and the supernatant).

[0082] After separating the flocs and the supernatant, a primary supernatant is obtained. The water quality of the primary supernatant is shown in Table 6.

[0083] Table 6. Water quality data of primary supernatant

[0084]

[0085] S2. Add sodium hydroxide to the primary supernatant to adjust the pH to 10.5 for 10 minutes.

[0086] Adding 800 mg / L of potassium persulfate oxidizes and softens the Ba while removing it. 2+ Let it stand and settle. Then add another softening agent, sodium carbonate, 400 mg / L. The pH of the oxidation-softening treatment is 10.5, and the stirring time for oxidation-softening is 40 min.

[0087] After settling for 30 minutes, the secondary supernatant was obtained. The water quality of the secondary supernatant at this time is shown in Table 7.

[0088] Table 7. Water quality data of secondary supernatant

[0089]

[0090] S3. The secondary supernatant is then subjected to ultrafiltration through a tubular micro-ultrafiltration membrane for solid-liquid separation for 10 minutes to remove macromolecular substances from the solution and obtain the tertiary supernatant.

[0091] Based on the steps described in this embodiment, the time required for different preprocessing systems is compared in Table 8.

[0092] Table 8 Comparison of time for different pretreatment systems

[0093]

[0094] The water quality after different pretreatment systems is shown in Table 9.

[0095] Table 9 Water quality after treatment by the three systems

[0096]

[0097] Ultimately, as Figure 3 As shown, the third-stage supernatant can be used as water to enter the reverse osmosis system for filtration, reducing the SS of the fracturing flowback fluid to 2 mg / L, the turbidity to below 1 NTU, and the TOC to 3 mg / L. Then, it enters the desalination system for concentration before being discharged.

[0098] As can be seen from Tables 8 and 9, after the shale gas fracturing flowback fluid is pretreated by the method described in this patent, it can not only effectively shorten the process flow and residence time, but also significantly reduce the turbidity, bacterial count, chemical oxygen demand, and total organic carbon content in the reverse osmosis feed water quality indicators. This can reduce reverse osmosis membrane fouling, extend membrane life, and increase economic benefits.

[0099] Example 3:

[0100] 2. Shale gas fracturing flowback fluid treatment project. The shale gas fracturing flowback fluid, after testing, has the following water quality characteristics:

[0101] Table 10 Original water quality data of a certain fracturing flowback fluid

[0102] index 2# Turbidity (NTU) 532 pH 7.01 <![CDATA[K + (mg / L)]]> 1098 <![CDATA[Na + (mg / L)]]> 22512 <![CDATA[Ca 2+ (mg / L)]]> 436 <![CDATA[Mg 2+ (mg / L)]]> 87 <![CDATA[Ba 2+ (mg / L)]]> 89 <![CDATA[Sr 2+ (mg / L)]]> 41 Total iron (mg / L) 31 Total silicon (mg / L) 37.7 <![CDATA[Br - (mg / L)]]> 156 <![CDATA[HCO3 - (mg / L)]]> 88 <![CDATA[Cl - (mg / L)]]> 26360 <![CDATA[SO4 2- (mg / L)]]> 2.55 TDS (mg / L) 50938.25 SS (mg / L) 587 COD (mg / L) 441 TOC (mg / L) 33.21 Petroleum products (mg / L) 5.01 Sulfate-reducing bacteria (CFU / ml) 1100000 Iron bacteria (cities / ml) 25000 Saprophytic bacteria (cFU / ml) 25000

[0103] Among them, such as Figures 1 to 2 As shown, the pretreatment method for shale gas fracturing flowback fluid includes the following steps:

[0104] S1. Potassium ferrate and sodium ferrate, along with FeO(OH)-montmorillonite composite material, are added to the fracturing flowback fluid for oxidation and coagulation, i.e., oxidation-coagulation treatment. The preparation of the FeO(OH)-montmorillonite composite material is as follows: 700g of montmorillonite is added to 2000g of ultrapure water and stirred at 350rpm for 24h, resulting in a uniform suspension of montmorillonite in the ultrapure water, yielding liquid 3-1; 1.75g ​​of sodium dodecyl sulfate is added to liquid 3-1 and stirred at 350rpm for approximately 35min, yielding liquid 3-2; 3113.6g of FeSO4·7H2O is added to liquid 3-2, Fe... 2+ The amount of substance was 16 mmol / g of sodium acetate at a mass ratio of 16 mmol / g to montmorillonite. The mixture was stirred at 350 rpm for 15 min to obtain liquid 3-3. 560 g of sodium acetate was added to liquid 3-3 and stirred at 350 rpm for 5 h to obtain a brown precipitate. The brown precipitate was washed three times with anhydrous ethanol and ultrapure water and then placed in a 65 ℃ oven for 30 h. After drying, it was ground into powder to obtain ginger-yellow FeO(OH)-montmorillonite material.

[0105] The dosage of potassium ferrate was 400 mg / L, the dosage of sodium ferrate was 400 mg / L, and the dosage of FeO(OH)-montmorillonite composite material was 2 g / L. The mixture was reused more than three times via magnetic separation. The pH during the oxidation-coagulation treatment was 7.5, the stirring speed was 350 rpm, and the stirring time during the potassium ferrate oxidation treatment was 30 min (including the addition time).

[0106] Anionic polyacrylamide, a flocculant with a viscosity-average molecular weight of 8 million, was added to the fracturing flowback fluid after ferrate treatment, and the mixture was allowed to settle. The dosage of the anionic polyacrylamide was 4 mg / L, the pH during flocculation was 7.5, and the settling time was 40 min (including the addition time, the time for separating the flocs and the supernatant).

[0107] After separating the flocs and the supernatant, a primary supernatant is obtained. The water quality of the primary supernatant is shown in Table 11.

[0108] Table 11 Water quality data of primary supernatant

[0109]

[0110] S2. Add sodium hydroxide to the primary supernatant to adjust the pH to 10.5 for 10 minutes.

[0111] Adding sodium persulfate at 800 mg / L oxidizes and softens to remove Ba. 2+Let it stand and settle. Then add another softening agent, sodium carbonate, 700 mg / L. The pH of the oxidation-softening treatment is 10.5, and the stirring time for oxidation-softening is 40 min.

[0112] After settling for 30 minutes, the secondary supernatant was obtained. The water quality of the secondary supernatant at this time is shown in Table 12.

[0113] Table 12 Water quality data of secondary supernatant

[0114]

[0115] S3. The secondary supernatant is then subjected to ultrafiltration through a tubular micro-ultrafiltration membrane to separate solids and liquids and remove macromolecules from the solution. The process takes 10 minutes to obtain the tertiary supernatant.

[0116] Based on the steps described above in this embodiment, the time required for different preprocessing systems is compared in Table 13.

[0117] Table 13 Comparison of time for different pretreatment systems

[0118]

[0119]

[0120] The water quality after different pretreatment systems is shown in Table 14.

[0121] Table 14 Water quality after treatment by the three systems

[0122]

[0123] Ultimately, as Figure 3 As shown, the third-stage supernatant can be used as water to enter the reverse osmosis system for filtration, reducing the SS of the fracturing flowback fluid to 3 mg / L, the turbidity to below 1 NTU, and the TOC to 2 mg / L. Then, it enters the desalination system for concentration before being discharged.

[0124] As can be seen from Tables 13 and 14, after the shale gas fracturing flowback fluid is pretreated by the method described in this patent, it can not only effectively shorten the process flow and residence time, but also significantly reduce the turbidity, bacterial count, chemical oxygen demand, and total organic carbon content in the reverse osmosis feed water quality indicators. This can reduce reverse osmosis membrane fouling, extend membrane life, and increase economic benefits.

[0125] Example 4:

[0126] The shale gas fracturing flowback fluid used was the shale gas fracturing flowback fluid in Example 3, and the water quality is shown in Table 10.

[0127] Among them, such as Figures 1 to 2As shown, the pretreatment method for shale gas fracturing flowback fluid includes the following steps:

[0128] S1. Potassium ferrate and sodium ferrate, along with FeO(OH)-montmorillonite composite material, are added to the fracturing flowback fluid for oxidation and coagulation, i.e., oxidation-coagulation treatment. The preparation of the FeO(OH)-montmorillonite composite material is as follows: 100g of montmorillonite is added to 2000g of ultrapure water and stirred at 800rpm for 18h, resulting in a uniform suspension of montmorillonite in the ultrapure water, yielding liquid 4-1; 0.3g of sodium dodecyl sulfate is added to liquid 4-1 and stirred at 300rpm for 20min, yielding liquid 4-2; 222.4g of FeSO4·7H2O is added to liquid 4-2, Fe... 2+ The amount of substance was 8 mmol / g of sodium acetate at a mass ratio of 8 mmol / g to montmorillonite. The mixture was stirred at 500 rpm for 12 min to obtain liquid 4-3. 90 g of sodium acetate was added to liquid 4-3 and stirred at 500 rpm for 5 h to obtain a brown precipitate. The brown precipitate was washed three times with anhydrous ethanol and ultrapure water and then placed in a 75 ℃ oven for 24 h. After drying, it was ground into powder to obtain ginger-yellow FeO(OH)-montmorillonite material.

[0129] The dosage of potassium ferrate was 500 mg / L, the dosage of sodium ferrate was 500 mg / L, and the dosage of FeO(OH)-montmorillonite composite material was 2.5 g / L. The mixture was reused more than three times via magnetic separation. The pH during the oxidation-coagulation treatment was 8, the stirring speed was 320 rpm, and the stirring time during the ferrate oxidation treatment was 25 min (including the addition time).

[0130] Anionic polyacrylamide, a flocculant with a viscosity-average molecular weight of 6 million, was added to the fracturing flowback fluid after ferrate treatment, and the mixture was allowed to settle. The dosage of the anionic polyacrylamide was 2 mg / L, the pH during flocculation was 8, and the settling time was 40 min (including the addition time, the time for separating the flocs and the supernatant).

[0131] After separating the flocs and the supernatant, a primary supernatant is obtained. The water quality of the primary supernatant is shown in Table 15.

[0132] Table 15 Water quality data of primary supernatant

[0133]

[0134] S2. Add sodium hydroxide to the primary supernatant to adjust the pH to 10.5 for 10 minutes.

[0135] Add 500 mg / L sodium persulfate and 400 mg / L potassium persulfate to oxidize and soften the Ba while removing it.2+ The mixture was allowed to settle. Then, 400 mg / L sodium carbonate and 200 mg / L potassium carbonate were added as softening agents. The pH for the oxidation-softening treatment was 10.5, and the stirring time was 40 min.

[0136] After settling for 30 minutes, the secondary supernatant was obtained. The water quality of the secondary supernatant at this time is shown in Table 16.

[0137] Table 16 Water Quality Data of Secondary Supernatant

[0138]

[0139] S3. The secondary supernatant is then subjected to ultrafiltration through a tubular micro-ultrafiltration membrane for solid-liquid separation for 10 minutes to remove macromolecular substances from the solution and obtain the tertiary supernatant.

[0140] Based on the steps described in this embodiment, the time required for different preprocessing systems is compared in Table 17.

[0141] Table 17 Comparison of time for different pretreatment systems

[0142]

[0143] The water quality after different pretreatment systems is shown in Table 18.

[0144] Table 18 Water quality after treatment by the three systems

[0145]

[0146]

[0147] Ultimately, as Figure 3 As shown, the third-stage supernatant can be used as water to enter the reverse osmosis membrane system for filtration, reducing the SS of the fracturing flowback fluid to 1 mg / L, the turbidity to below 1 NTU, and the TOC to 2 mg / L. Then, it enters the desalination system for concentration before being discharged.

[0148] As can be seen from Tables 17 and 18, after the shale gas fracturing flowback fluid is pretreated by the method described in this patent, it can not only effectively shorten the process flow and residence time, but also significantly reduce the turbidity, bacterial count, chemical oxygen demand, and total organic carbon content in the reverse osmosis feed water quality indicators. This can reduce reverse osmosis membrane fouling, extend membrane life, and increase economic benefits.

[0149] It should be noted that, in Examples 1-4, the pretreatment method 1 specifically includes: an air flotation unit, a chemical softening unit, a coagulation and sedimentation unit, and an activated carbon particle adsorption unit; the pretreatment method 2 specifically includes: a coagulation and sedimentation unit, a two-stage Fenton oxidation unit, a softening and clarification unit, an electrolytic oxidation unit, and a multi-media filtration unit. Both are commonly used discharge pretreatment systems in the field, and therefore will not be described in detail here.

[0150] Example 5:

[0151] This invention also provides a shale gas fracturing flowback fluid compliant discharge system, which can be applied to the shale gas fracturing flowback fluid compliant discharge pretreatment method mentioned in any of the above embodiments. The system includes:

[0152] Storage tank, the storage tank being used to store fracturing flowback fluid;

[0153] The first pretreatment unit is used to receive fracturing flowback fluid and perform step S1, and output primary supernatant.

[0154] The second pretreatment unit is used to receive the primary supernatant and perform step S2, and output the secondary supernatant.

[0155] A tubular ultrafiltration unit is used to receive the secondary supernatant and perform step S3, and output the tertiary supernatant;

[0156] The storage tank, the first pretreatment unit, the second pretreatment unit, and the tubular ultrafiltration unit are connected in sequence via pipelines.

[0157] During the pretreatment of fracturing flowback fluid, the fracturing flowback fluid in the storage tank is fed into the first pretreatment unit via a booster pump for step S1, thereby outputting primary supernatant. The second pretreatment unit receives the primary supernatant, performs step S2, and outputs secondary supernatant. The tubular ultrafiltration unit receives the secondary supernatant, performs step S3, and outputs tertiary supernatant, thus completing the pretreatment of the fracturing flowback fluid. At this point, the tertiary supernatant can be used as water to enter the reverse osmosis membrane system for filtration, and then enters the desalination system for concentration before being discharged.

[0158] It is understood that the specific steps in steps S1 and S2 are limited to the addition of the reagent, the stirring and mixing between the solution and the reagent, and the acquisition of the supernatant after standing. These are all conventional technical means for those skilled in the art, and the specific structures of the first pretreatment unit and the second pretreatment unit will not be described in detail here.

[0159] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for pre-treating shale gas fracturing flowback fluid to meet discharge standards, characterized in that, Including the following steps: S1. Obtaining the primary supernatant: Ferrate and solid-phase catalyst are added to the fracturing flowback fluid to fully mix them, and oxidation-coagulation treatment is performed. Then, flocculant is added, and after settling, the primary supernatant is obtained. The pH of the fracturing flowback fluid during the oxidation-coagulation treatment is 7-8. The solid-phase catalyst is a FeO(OH)-montmorillonite composite material, and the preparation method of the FeO(OH)-montmorillonite composite material includes: By weight, 1-50 parts of montmorillonite were added to 100-1000 parts of ultrapure water and stirred at 300-1000 rpm for 18-24 h. The montmorillonite was uniformly suspended in the ultrapure water to obtain liquid 1. Sodium dodecyl sulfate was added to liquid 1, with the weight of sodium dodecyl sulfate being 0.1%-0.3% of montmorillonite. The mixture was stirred at a stirring speed of 300-1000 rpm for 20-40 minutes to obtain liquid 2. FeSO4•7H2O was added to liquid 2, Fe 2+ The amount of substance and the weight ratio of montmorillonite were 8-16 mmol / g. The mixture was stirred at 300-1000 rpm for 5-15 min to obtain liquid 3. Sodium acetate was added to liquid 3, with the weight of sodium acetate being 80%-120% of montmorillonite. The mixture was stirred at a stirring speed of 300-1000 rpm for 4-6 hours to obtain a brown precipitate. The brown precipitate was repeatedly washed with anhydrous ethanol and ultrapure water and then placed in an oven at 65-75℃ for 18-30 hours. After drying, it was ground into powder to obtain ginger-yellow FeO(OH)-montmorillonite material. S2. Obtaining secondary supernatant: The pH of the primary supernatant is adjusted to alkaline by adding sodium hydroxide, and then persulfate and sodium carbonate are added sequentially and stirred until fully mixed to carry out oxidation-softening treatment. After standing and settling, the secondary supernatant is obtained. The alkaline pH value is 10-11. S3. Obtain the tertiary supernatant: Perform ultrafiltration on the secondary supernatant to remove macromolecular substances in the solution and obtain the tertiary supernatant.

2. The method for pretreatment of shale gas fracturing flowback fluid to meet discharge standards according to claim 1, characterized in that, In step S1, the ferrate is at least one of potassium ferrate and sodium ferrate, and the amount of ferrate added is 400-1000 mg / L.

3. The method for pretreatment of shale gas fracturing flowback fluid to meet discharge standards according to claim 1, characterized in that, In step S1, the amount of solid catalyst added is 1-3 g / L.

4. A method for pretreatment of shale gas fracturing flowback fluid to meet discharge standards according to any one of claims 1-3, characterized in that, In step S1, the stirring speed during the oxidation-coagulation treatment is 300-350 rpm.

5. The method for pretreatment of shale gas fracturing flowback fluid to meet discharge standards according to claim 1, characterized in that, In step S1, the flocculant is anionic polyacrylamide, and the dosage of the flocculant is 1-4 mg / L.

6. The method for pretreatment of shale gas fracturing flowback fluid to meet discharge standards according to claim 1, characterized in that, In step S2, the persulfate is at least one of sodium persulfate and potassium persulfate, and the amount of persulfate added is 500-1000 mg / L; The dosage of sodium carbonate is 400-600 mg / L.

7. The method for pretreatment of shale gas fracturing flowback fluid to meet discharge standards according to claim 1, characterized in that, In step S1, the oxidation-coagulation treatment time is 20-40 min, and the settling time is 15-40 min; In step S2, the oxidation-softening treatment takes 35-45 minutes, and the settling time is 25-40 minutes.

8. A shale gas fracturing flowback fluid pretreatment system for compliant discharge, characterized in that, The system applied to the shale gas fracturing flowback fluid pretreatment method according to any one of claims 1-7, the system comprising: Storage tank, the storage tank being used to store fracturing flowback fluid; The first pretreatment unit is used to receive fracturing flowback fluid and perform step S1, and output primary supernatant. The second pretreatment unit is used to receive the primary supernatant and perform step S2, and output the secondary supernatant. A tubular ultrafiltration unit is used to receive the secondary supernatant and perform step S3, and output the tertiary supernatant; The storage tank, the first pretreatment unit, the second pretreatment unit, and the tubular ultrafiltration unit are connected in sequence via pipelines.

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