A method and system for treating fracturing flowback fluid

Abstract

This invention belongs to the field of fracturing flowback fluid treatment technology, and discloses a method and system for treating fracturing flowback fluid. The system includes a fracturing flowback fluid inlet pipeline, a pretreatment tank, a micron-sized aerated water pump device, an air storage tank, an air flotation cyclone separator, a metal membrane electrocoagulation device, a vacuum storage tank, and a two-stage DTL membrane treatment device. This invention achieves the treatment and reuse of fracturing flowback fluid through a combined process of pretreatment + air flotation cyclone treatment + metal membrane electrocoagulation treatment + DTL membrane treatment.

Description

Technical Field

[0001] This invention belongs to the field of fracturing flowback fluid treatment technology, and more specifically, relates to a fracturing flowback fluid treatment method and system. Background Technology

[0002] Most oil fields have been in operation for many years. Due to factors such as declining well productivity and permeability, oil production is showing a downward trend. To increase oil production, more and more oil fields are adopting fracturing technology. Currently, the most widely used fracturing technology is hydraulic fracturing. During production, the generation of fracturing flowback fluid is unavoidable. Hydraulic fracturing fluid is a mixture of water, additives, and sand, therefore it is characterized by high oil content, high suspended solids (SS), high salt content, and high COD. The treatment and reuse of fracturing flowback fluid significantly impacts oil field production.

[0003] The most commonly used processes for treating fracturing flowback fluid are combinations of flocculation sedimentation, flotation, biological treatment, advanced oxidation, and membrane processes. Flocculation sedimentation primarily involves adding agents such as PAC and PAM to remove some oil, suspended solids (SS), and COD through bridging and adsorption. Flocculation sedimentation is often used as a pretreatment unit. Flotation equipment utilizes a large number of air bubbles to adsorb and capture fine particulate matter and oil pollutants during their rise, achieving solid-liquid separation and removing SS, oil, and COD from the water. Biological treatment tanks primarily use activated sludge to remove easily treatable organic matter and reduce COD. Advanced oxidation mainly uses ozone catalytic oxidation for deep COD removal. Ozone introduced into the water is catalyzed by a catalyst to decompose and generate hydroxyl radicals. Hydroxyl radicals are highly oxidizing substances that can break down large organic molecules into smaller ones and even oxidize organic matter into carbon dioxide and water. Ozone catalytic oxidation can remove most pollutants from the water, thus meeting the standards for flowback fluid discharge and reuse. Membrane technology primarily utilizes membrane separation to remove pollutants from water. To meet reuse standards, a dual-membrane method (ultrafiltration + reverse osmosis) is typically employed. However, to achieve reuse of the wastewater, reverse osmosis is often used as a final, more advanced treatment step. The abundant additives, sand, and oils present in the wastewater can cause significant fouling of the RO membrane, drastically impacting its flux, permeability, and even lifespan. Furthermore, silica compounds have a significant impact on the membrane, and reverse osmosis imposes strict limits on silica content. To prevent silica scale formation, the silica content in the concentrate is generally required to be less than 200 mg / L.

[0004] Moreover, the fracturing flowback fluid treatment process currently used in engineering mainly includes a multi-stage pretreatment + deep treatment combination process. This process has high energy consumption, large mud production, large footprint, serious membrane fouling, and many problems in actual operation.

[0005] Therefore, in summary, there is an urgent need to propose a new method and system for treating fracturing flowback fluid. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method and system for treating fracturing flowback fluid. This invention achieves the treatment and reuse of fracturing flowback fluid through a combined process of pretreatment, air flotation cyclone treatment, metal membrane electrocoagulation treatment, and DTL membrane treatment.

[0007] To achieve the above objectives, the present invention provides a fracturing flowback fluid treatment system, the system comprising a fracturing flowback fluid inlet pipeline, a pretreatment tank, a micron-sized aeration pump device, an air storage tank, an air flotation cyclone separator, a metal membrane electrocoagulation device, a vacuum water storage tank, and a two-stage DTL membrane treatment device.

[0008] The micron-sized aerated water pump device includes a centrifugal pump and a gas-liquid mixing nozzle;

[0009] The gas storage tank has a first exhaust pipe and a second exhaust pipe connected to its outlet.

[0010] The fracturing flowback fluid inlet pipeline is connected to the upper inlet of the pretreatment tank; the silica removal agent pipeline is connected to the top inlet of the pretreatment tank; the lower outlet of the pretreatment tank is connected to the inlet of the centrifugal pump; the outlet of the centrifugal pump is connected to the inlet of the gas-liquid mixing nozzle; the first exhaust pipeline is connected to the air inlet of the gas-liquid mixing nozzle; the outlet of the gas-liquid mixing nozzle is connected to the tangential inlet of the air flotation cyclone separator; and the lower outlet of the air flotation cyclone separator is connected to the upper inlet of the metal membrane electrocoagulation device.

[0011] The metal membrane electrocoagulation device is equipped with a titanium metal mesh, a metal membrane water collection pipe, and a pulse aeration device. The titanium metal mesh serves as the cathode of the metal membrane electrocoagulation device and is used to filter and separate the mud-water mixture within the device. The metal membrane water collection pipe allows the filtered reaction liquid to enter the vacuum storage tank. The anode of the metal membrane electrocoagulation device is an aluminum electrode. The pulse aeration device is located at the bottom of the metal membrane electrocoagulation device and is connected to the second exhaust pipe.

[0012] The vacuum water storage tank is connected to a vacuum pump and a residual chlorine removal agent dosing pipeline; the lower outlet of the vacuum water storage tank is connected to the inlet of the two-stage DTL membrane treatment device; the compliant recycled water outlet of the two-stage DTL membrane treatment device is connected to the outside.

[0013] According to the present invention, preferably, a stirring device is provided in the pretreatment tank.

[0014] According to the present invention, preferably, the silicon removal agent pipeline includes a sodium aluminate agent dosing pipeline and a PAC agent dosing pipeline.

[0015] According to the present invention, preferably, the air flotation cyclone is a single-stage air flotation cyclone.

[0016] According to the present invention, preferably, the pore size of the titanium metal mesh is 0.3-0.8 μm.

[0017] According to the present invention, preferably, the titanium metal mesh is further provided with a backflush pipe; the outlet of the gas storage tank is further connected to a third exhaust pipe; the third exhaust pipe is connected to the backflush pipe.

[0018] According to the present invention, preferably, there are multiple vacuum water storage tanks; each vacuum water storage tank is equipped with a stirring device and an ORP probe.

[0019] In this invention, as a preferred embodiment, there are two vacuum water storage tanks, one in operation and one on standby.

[0020] According to the present invention, preferably, the pretreatment tank is provided with a first bottom sludge discharge port;

[0021] The air flotation cyclone separator is equipped with a top oil discharge port and a bottom slag discharge port;

[0022] The metal membrane electrocoagulation device is provided with a second bottom sludge discharge port;

[0023] The first bottom sludge discharge port, the top oil discharge port, the bottom slag discharge port, and the second bottom sludge discharge port are all connected to the sludge tank.

[0024] According to the present invention, preferably, the lower outlet of the vacuum water storage tank is connected to the inlet of the first-stage DTL membrane treatment device in the two-stage DTL membrane treatment device;

[0025] The first-stage DTL membrane treatment equipment is equipped with a primary product water outlet and a primary concentrate discharge outlet, and the primary product water outlet is connected to the inlet of the second-stage DTL membrane treatment equipment.

[0026] The second-stage DTL membrane treatment equipment is equipped with a secondary product water outlet and a secondary concentrate discharge outlet. The secondary product water outlet is the compliant reuse product water outlet of the two-stage DTL membrane treatment device.

[0027] The primary and secondary concentrated wastewater discharge outlets are connected to the sludge tank.

[0028] Another aspect of the present invention provides a method for treating fracturing flowback fluid, the method employing the above-described system and comprising the following steps:

[0029] S1: The fracturing flowback fluid and desiliconizing agent are sent into the pretreatment tank, and after reaction, sludge sediment and pretreatment clear liquid are obtained;

[0030] S2: The pretreated clear liquid is pressurized by the centrifugal pump and sent into the gas-liquid mixing nozzle to mix with the gas sent into the gas-liquid mixing nozzle from the first exhaust pipe to obtain a water-gas-solid three-phase mixture;

[0031] S3: The water-gas-solid three-phase mixture is tangentially fed into the air flotation hydrocyclone, and after cyclone separation, bubble-oil droplet adhering body, mud-sand mixture and air flotation hydrocyclone clear liquid are obtained;

[0032] S4: The clarified liquid from the air flotation cyclone separator is fed into the metal membrane electrocoagulation device, where it is electrolyzed under pulse aeration conditions and undergoes flocculation and adsorption to obtain adsorbed precipitate and reaction liquid; the reaction liquid is then filtered into a vacuum water storage tank through a titanium metal mesh membrane under the negative pressure drive of the vacuum pump.

[0033] S5: In the vacuum water storage tank, the reaction solution is mixed with the residual chlorine removal agent and reacted to obtain a reduction reaction solution; the reduction reaction solution is sent to the two-stage DTL membrane treatment device to obtain compliant recycled permeate and secondary concentrate.

[0034] According to the present invention, preferably, in step S1:

[0035] The silicon removal agent includes sodium aluminate and PAC agent. The mass concentration of sodium aluminate agent in the pretreatment tank is (0.8-1.2):(0.8-1.2) of silicon in the fracturing flowback fluid in the pretreatment tank. The mass concentration of PAC agent in the pretreatment tank is 30-50 mg / L.

[0036] The hydraulic retention time in the pretreatment tank is 15-30 minutes.

[0037] The method also includes discharging the sludge into a sludge pond.

[0038] According to the present invention, preferably, in step S2: in the gas-liquid mixing nozzle, the pretreated clear liquid is mixed with the gas fed into the gas-liquid mixing nozzle from the first exhaust pipe to produce bubbles with a diameter of 100-200 μm, thereby obtaining a water-gas-solid three-phase mixture.

[0039] In this invention, a large amount of pretreated clarified liquid directly enters the centrifugal pump, and after being pressurized by the pump, it enters the gas-liquid mixing nozzle at a high flow rate. Due to the high flow rate, a high vacuum region (approximately -0.08 MPa) is formed inside the nozzle. The pretreated clarified liquid in the gas-liquid mixing nozzle mixes thoroughly with the gas in the intake gas storage tank, producing microbubbles with a diameter of 100-200 μm, generating a water-gas-solid three-phase mixture. The water-gas-solid three-phase mixture enters the air flotation hydrocyclone tangentially at a high flow rate. During the cyclone separation process, the bubbles collide and adsorb with oil droplets in the water to form bubble-oil droplet adhesies. Based on the density difference, under centrifugal force, the bubble-oil droplet adhesies are discharged from the top oil outlet of the air flotation hydrocyclone into the sludge tank. Sand particles in the water and a large amount of sludge form a high-density sludge-mud mixture. Under the action of centrifugal force, the sludge-mud mixture flows on the inner wall of the air flotation hydrocyclone and is finally discharged from the bottom of the air flotation hydrocyclone into the sludge tank. The clarified liquid from the air flotation hydrocyclone, after separation, is discharged from the lower outlet of the air flotation hydrocyclone. This invention removes a large amount of silica scum, sand, and oil from the fracturing flowback fluid after treatment in the pretreatment tank and air flotation hydrocyclone.

[0040] According to the present invention, preferably, in step S3: the method further includes discharging the bubble-oil droplet adhering body into the sludge tank from the top oil outlet, and discharging the sludge mixture into the sludge tank from the bottom sludge outlet.

[0041] According to the present invention, preferably, in step S4:

[0042] The pulse aeration conditions include: aeration for 10 seconds followed by a 50-second pause.

[0043] The current density for electrolysis is determined based on the water quality of the fracturing flowback fluid, and is preferably 5-50 mA / m³. 2 ;

[0044] The negative pressure driven by the negative pressure is from -50 kPa to -25 kPa;

[0045] The method also includes discharging the adsorbed sediment into a sludge tank;

[0046] In this invention, the clarified liquid from the air flotation cyclone separator enters the metal membrane electrocoagulation device:

[0047] The metal membrane electrocoagulation device uses aluminum plates as the anode for electrolysis. The aluminum anode produces aluminum ions, which undergo a series of hydrolysis and polymerization reactions to form polynuclear hydroxyl complexes. These complexes can react with residual silicon, fluorine, calcium, and magnesium ions in the clarified liquid from the air flotation cyclone separator to form adsorption precipitates, thus achieving deep removal. Simultaneously, the polynuclear hydroxyl complexes can also adsorb large molecular organic matter in the water, thereby achieving partial COD removal.

[0048] The cathode of the metal membrane electrocoagulation device is a titanium metal mesh with a pore size of 0.5 micrometers. The titanium metal mesh serves as both the cathode and the membrane for filtration separation. The mud-water mixture in the metal membrane electrocoagulation device is filtered by external pressure filtration in the form of submerged ultrafiltration. Under the negative pressure drive (-50 kPa) of the vacuum storage tank, the titanium metal mesh performs filtration separation on the mud-water mixture in the metal membrane electrocoagulation device. The separated reaction liquid is then pumped into the vacuum storage tank.

[0049] The bottom of the metal membrane electrocoagulation device is equipped with a pulse aeration device, with a pulse duration of 10 seconds followed by a 50-second pause. Under the action of pulse aeration, the mass transfer effect within the metal membrane electrocoagulation device is ensured. At the same time, pulse aeration generates high-velocity large bubbles that scour the surface of the titanium metal mesh membrane, maintaining the flux of the titanium metal mesh membrane.

[0050] As a preferred embodiment, the method further includes backflushing the titanium metal mesh using a backflushing pipeline; the backflushing pressure is 0.8-1.2 bar, and the time is 2.5-3.5 min.

[0051] According to the present invention, preferably, the metal membrane electrocoagulation device is operated in a sequential batch manner, and the steps included in the operation cycle are: simultaneous electrolysis and filtration separation, stopping electrolysis and filtration separation, backflushing the titanium metal mesh membrane - stopping backflushing, settling and sedimentation, and sludge discharge.

[0052] According to the present invention, preferably, the electrolysis time is 40-50 minutes.

[0053] According to the present invention, preferably, the backflushing treatment time is 2.5-3.5 min.

[0054] According to the present invention, preferably, the settling time is 8-12 minutes.

[0055] According to the present invention, preferably, the sludge discharge time is 1.5-2.5 min.

[0056] According to the present invention, preferably, in step S5:

[0057] During operation, the vacuum water storage tank is operated by a vacuum pump, and the vacuum degree inside the vacuum water storage tank is -55kPa to -45kPa.

[0058] The residual chlorine removal agent is sodium bisulfite;

[0059] In this invention, during the electrolysis process, chloride ions in the water inevitably undergo anodic oxidation to produce residual chlorine and other substances. Residual chlorine has strong oxidizing properties and can damage the subsequent DTL membrane. Based on the value of the ORP probe in the vacuum water storage tank, sodium bisulfite is added to the vacuum water storage tank. After stirring, the added reagent mixes and reacts with the residual chlorine in the water to reduce the residual chlorine in the water.

[0060] The dosage of the residual chlorine removal agent is determined based on the ORP value measured by the ORP probe in the vacuum water storage tank; the ORP value in the vacuum water storage tank is controlled between 100-150 mV.

[0061] The reduction reaction solution is fed into the first-stage DTL membrane treatment unit of the two-stage DTL membrane treatment device to obtain primary permeate and primary concentrate; the primary permeate is fed into the second-stage DTL membrane treatment unit to obtain the qualified reuse permeate and secondary concentrate; the primary concentrate and secondary concentrate are discharged into the sludge tank.

[0062] The recovery rate of the first-stage DTL membrane treatment equipment is controlled at 75-85%;

[0063] The recovery rate of the second-stage DTL membrane treatment device is controlled at 90-95%. In this invention, the total recovery rate of the two-stage DTL membrane treatment devices is controlled at 75%.

[0064] The beneficial effects of the technical solution of the present invention are as follows:

[0065] This invention utilizes a pretreatment tank as the primary silica removal unit for pretreating the influent before membrane treatment. In the pretreatment tank, silica removal is mainly achieved by adding sodium aluminate. The addition of flocculant PAC further removes significant amounts of silica and suspended solids (SS) from the water. Simultaneously, flocculation and sedimentation remove some of the oil from the fracturing flowback fluid, reducing its viscosity. This invention eliminates the need for a sludge-water separation zone within the pretreatment tank; the pretreated clarified liquid, generated from uniform chemical mixing, directly enters an air flotation cyclone separator for sludge-water separation. Compared to conventional coagulation and sedimentation tanks, this reduces residence time, decreases tank size, saves land, and lowers construction costs.

[0066] The pretreated clarified liquid of this invention is aerated by a micron-sized aeration pump and then processed by an air flotation hydrocyclone. Within the flow field of the air flotation, oil droplets collide and adhere to air bubbles, forming a bubble-oil droplet adhering body. Influenced by centrifugation and air flotation, this results in efficient separation of oil, water, and sludge. After treatment by the air flotation hydrocyclone, most of the oil and suspended solids (SS) in the water are removed. This invention uses a micron-sized aeration pump to aerate and fuse the pretreated clarified liquid. The air bubbles mixed in the pretreated clarified liquid are at the micron level, with an average diameter of approximately 100-200 μm and an upward velocity of approximately 150 s / m. The smaller the bubble diameter, the longer the contact time, approximately 3 minutes, which is similar to the hydraulic residence time of the air flotation hydrocyclone, ensuring the effectiveness of centrifugal oil-water separation. The characteristics of the air flotation hydrocyclone are utilized to achieve efficient separation of water, oil, and sludge.

[0067] The metal membrane electrocoagulation device of this invention uses a titanium metal mesh as the cathode, integrating electrocoagulation and membrane separation. It ensures effective electrocoagulation removal while achieving in-situ solid-liquid separation, reducing process units and saving space. This invention selects a titanium metal mesh with a 0.5-micron pore size, which can trap suspended solids generated during electrolysis, ensuring that the SDI and other indicators meet the requirements before membrane entry. Using aluminum as the anode, the aluminum salts generated during electrolysis undergo hydrolysis and polymerization to form polynuclear hydroxyl complexes, which can enhance the removal of calcium, magnesium, silicon, and fluorine ions in water. Under the influence of the electric field, the titanium metal mesh acts as the cathode, reducing the impact of negatively charged groups clogging the membrane.

[0068] During electrolysis, a side reaction of chloride ion oxidation also occurs at the anode, generating chlorine gas. Some of this chlorine gas dissolves in water, producing residual chlorine. Residual chlorine has a certain oxidizing property and can damage the membrane. Sodium bisulfite is added to the vacuum storage tank to remove residual chlorine from the water, thus preventing damage to the membrane.

[0069] This invention addresses substances that cause severe membrane fouling by incorporating a series of pretreatment processes to ensure the efficient operation of a two-stage DTL membrane treatment unit. After treatment by a series of process units including a pretreatment tank, an air flotation cyclone separator, and a metal membrane electrocoagulation device, the return liquid enters the two-stage DTL membrane treatment unit. The DTL membrane has high flux, and its unique mesh design avoids concentration polarization within the channel, while also balancing permeate quality and energy consumption.

[0070] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0071] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of the invention.

[0072] Figure 1A schematic diagram of a fracturing flowback fluid treatment system provided in Embodiment 1 of the present invention is shown.

[0073] The annotations in the attached figures are explained as follows:

[0074] 1. Fracturing flowback fluid inlet pipeline, 2. Pretreatment tank, 3. Centrifugal pump, 4. Gas-liquid mixing nozzle, 5. Gas storage tank, 6. Air flotation cyclone, 7. Metal membrane electrocoagulation device, 8. Vacuum water storage tank, 9. Two-stage DTL membrane treatment device, 10. Sludge tank, 11. Vacuum pump, 12. Standard-compliant reusable water, 13. Exhaust, 14. First exhaust pipeline, 15. Second exhaust pipeline, 16. Third exhaust pipeline. Detailed Implementation

[0075] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0076] In the following embodiments:

[0077] The two-stage DTL membrane treatment unit was purchased from Luxin Tiandiren Environmental Technology (Anhui) Group Co., Ltd., featuring a special concentrate grid: open N-type flow channel; effective membrane area 29.5m². 2 Pressure rating: 75 bar / 90 bar.

[0078] Example 1

[0079] This embodiment provides a fracturing flowback fluid treatment system, such as... Figure 1 As shown, the system includes a fracturing flowback fluid inlet pipeline 1, a pretreatment tank 2, a micron-sized aeration pump device, an air storage tank 5, an air flotation cyclone separator 6, a metal membrane electrocoagulation device 7, a vacuum water storage tank 8, and a two-stage DTL membrane treatment device 9.

[0080] The pretreatment tank 1 is equipped with a stirring device;

[0081] The micron-sized aerated water pump device includes a centrifugal pump 3 and a gas-liquid mixing nozzle 4;

[0082] The outlet of the gas storage tank 5 is connected to a first exhaust pipe 14, a second exhaust pipe 15, and a third exhaust pipe 16.

[0083] The air flotation cyclone 6 is a single-stage air flotation cyclone;

[0084] The fracturing flowback fluid inlet pipe 1 is connected to the upper inlet of the pretreatment tank 2; the silica removal agent pipe (not shown) is connected to the top inlet of the pretreatment tank 2, and the silica removal agent pipe includes a sodium aluminate agent dosing pipe and a PAC agent dosing pipe; the lower outlet of the pretreatment tank 2 is connected to the inlet of the centrifugal pump 3; the outlet of the centrifugal pump 3 is connected to the inlet of the gas-liquid mixing nozzle 4, and the first exhaust pipe 14 is connected to the air inlet of the gas-liquid mixing nozzle 4; the outlet of the gas-liquid mixing nozzle 4 is connected to the tangential inlet of the air flotation cyclone separator 6; the lower outlet of the air flotation cyclone separator 6 is connected to the upper inlet of the metal membrane electrocoagulation device 7.

[0085] The metal membrane electrocoagulation device 7 is equipped with a titanium metal mesh, a metal membrane water collection pipe, and a pulse aeration device. The titanium metal mesh has a pore size of 0.5 μm and serves as the cathode of the metal membrane electrocoagulation device 7 and is used to filter and separate the mud-water mixture within the device. The titanium metal mesh is also equipped with a backflush pipe, and the third exhaust pipe 16 is connected to the backflush pipe. The metal membrane water collection pipe allows the reaction liquid obtained from the filtration separation to enter the vacuum water storage tank. The anode of the metal membrane electrocoagulation device is an aluminum electrode. The pulse aeration device is located at the bottom of the device and is connected to the second exhaust pipe 15.

[0086] Two vacuum water storage tanks 8 are provided, one in operation and one on standby. Each vacuum water storage tank 8 is connected to a vacuum pump 11 and a residual chlorine removal agent dosing pipeline. Each vacuum water storage tank 8 is equipped with a stirring device and an ORP probe. The lower outlet of each vacuum water storage tank 8 is connected to the inlet of the first-stage DTL membrane treatment device in the two-stage DTL membrane treatment unit 9. The first-stage DTL membrane treatment device is equipped with a primary product water outlet and a primary concentrate discharge outlet. The primary product water outlet is connected to the inlet of the second-stage DTL membrane treatment device. The second-stage DTL membrane treatment device is equipped with a secondary product water outlet and a secondary concentrate discharge outlet. The secondary product water outlet is the compliant reuse product water outlet of the two-stage DTL membrane treatment unit.

[0087] The pretreatment tank 2 is provided with a first bottom sludge discharge port; the air flotation cyclone 6 is provided with a top oil discharge port and a bottom slag discharge port; the metal membrane electrocoagulation device 7 is provided with a second bottom sludge discharge port; the first bottom sludge discharge port, the top oil discharge port, the bottom slag discharge port, the second bottom sludge discharge port, the primary concentrate discharge port and the secondary concentrate discharge port are all connected to the sludge tank 10.

[0088] This embodiment also provides a method for treating fracturing flowback fluid, wherein the fracturing flowback fluid is fracturing flowback fluid from an oilfield in Heilongjiang Province, and the water quality before and after treatment in this embodiment is shown in Table 1.

[0089] Table 1

[0090]

[0091] The method employs the system described above and includes the following steps:

[0092] S1: The fracturing flowback fluid and desiliconizing agent are fed into the pretreatment tank 2 and rapidly mixed by the stirring equipment in the pretreatment tank 2. After the reaction, sludge precipitate (a small amount of large precipitate) and pretreatment clear liquid are obtained; the sludge precipitate is periodically discharged into the sludge tank 10.

[0093] The mass concentration ratio of sodium aluminate in pretreatment tank 2 to silicon in fracturing flowback fluid in pretreatment tank 2 is 1:1; the mass concentration of PAC in pretreatment tank 2 is 30-50 mg / L.

[0094] The hydraulic retention time of the pretreatment tank 2 is 15-30 minutes;

[0095] S2: The pretreated clear liquid is pressurized by the centrifugal pump 3 and sent into the gas-liquid mixing nozzle 4, where it is mixed with the gas sent into the gas-liquid mixing nozzle 4 by the first exhaust pipe 14, producing bubbles with a diameter of 100μm, and obtaining a water-gas-solid three-phase mixture.

[0096] S3: The water-gas-solid three-phase mixture is tangentially fed into the air flotation hydrocyclone 6, and after cyclone separation, bubble-oil droplet adhering body, mud-sand mixture and air flotation hydrocyclone clear liquid are obtained;

[0097] The bubble-oil droplet adhering body is discharged into the sludge tank 10 through the top oil outlet, and the mud-sand mixture is discharged into the sludge tank 10 through the bottom sludge outlet;

[0098] S4: The clarified liquid from the air flotation cyclone separator is fed into the metal membrane electrocoagulation device 7. The metal membrane electrocoagulation device 7 operates in a batch mode, and the steps included in the operating cycle are: electrolysis (current density of 20mA / m³) is carried out simultaneously under pulse aeration conditions (aeration for 10s followed by a 50s stop aeration). 2 The process involves electrolysis (45 min) followed by filtration and separation, stopping electrolysis and filtration, backflushing the titanium metal mesh at 1 bar pressure for 3 min, stopping backflushing, allowing it to settle for 10 min, discharging sludge for 2 min, and completing one cycle of 1 h to obtain adsorption precipitate and reaction solution.

[0099] The reaction liquid is drawn into the vacuum storage tank 8 under the negative pressure drive (-50 kPa) of the vacuum pump 11; the adsorbed precipitate is discharged into the sludge tank 10.

[0100] S5: In the vacuum storage tank 8 (vacuum degree -50kpa), the reaction solution is mixed with sodium bisulfite and reacted to obtain a reduction reaction solution; the reduction reaction solution is sent to the first stage DTL membrane treatment equipment of the two-stage DTL membrane treatment device 9 to obtain primary permeate and primary concentrate; the primary permeate is sent to the second stage DTL membrane treatment equipment to obtain the qualified reuse permeate and secondary concentrate; the primary concentrate and secondary concentrate are discharged into the sludge tank.

[0101] The dosage of sodium bisulfite is determined based on the ORP value measured by the ORP probe in the vacuum water storage tank 8; the ORP value in the vacuum water storage tank 8 is controlled between 100-150 mV.

[0102] The recovery rate of the first-stage DTL membrane treatment equipment is controlled at 80%; the recovery rate of the second-stage DTL membrane treatment equipment is controlled at 95%, and the total recovery rate of the two-stage DTL membrane treatment devices 9 is controlled at 75%.

[0103] The fracturing flowback fluid (i.e., compliant recycled production water) processed by the system and method of this invention can meet the "General Technical Conditions for Fracturing Fluids" SY / T, and can realize the reuse of fracturing fluid.

[0104] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for treating fracturing flowback fluid, characterized in that, The system used in the method includes a fracturing flowback fluid inlet pipeline, a pretreatment tank, a micron-sized aerated water pump device, an air storage tank, an air flotation cyclone separator, a metal membrane electrocoagulation device, a vacuum water storage tank, and a two-stage DTL membrane treatment device. The micron-sized aerated water pump device includes a centrifugal pump and a gas-liquid mixing nozzle; The gas storage tank has a first exhaust pipe and a second exhaust pipe connected to its outlet. The fracturing flowback fluid inlet pipeline is connected to the upper inlet of the pretreatment tank; the silica removal agent pipeline is connected to the top inlet of the pretreatment tank; the lower outlet of the pretreatment tank is connected to the inlet of the centrifugal pump; the outlet of the centrifugal pump is connected to the inlet of the gas-liquid mixing nozzle; the first exhaust pipeline is connected to the air inlet of the gas-liquid mixing nozzle; the outlet of the gas-liquid mixing nozzle is connected to the tangential inlet of the air flotation cyclone separator; and the lower outlet of the air flotation cyclone separator is connected to the upper inlet of the metal membrane electrocoagulation device. The metal membrane electrocoagulation device is equipped with a titanium metal mesh, a metal membrane water collection pipe, and a pulse aeration device. The titanium metal mesh serves as the cathode of the metal membrane electrocoagulation device and is used to filter and separate the mud-water mixture within the device. The metal membrane water collection pipe allows the filtered reaction liquid to enter the vacuum storage tank. The anode of the metal membrane electrocoagulation device is an aluminum electrode. The pulse aeration device is located at the bottom of the metal membrane electrocoagulation device and is connected to the second exhaust pipe. The pore size of the titanium metal mesh is 0.3-0.8 μm; The vacuum water storage tank is connected to a vacuum pump and a residual chlorine removal agent dosing pipeline; the lower outlet of the vacuum water storage tank is connected to the inlet of the two-stage DTL membrane treatment device; the compliant recycled water outlet of the two-stage DTL membrane treatment device is connected to the outside. The two-stage DTL membrane treatment unit was purchased from Luxin Tiandiren Environmental Technology (Anhui) Group Co., Ltd., featuring a special concentrate grid: open N-type flow channel; effective membrane area 29.5m². 2 Pressure rating: 75 bar / 90 bar; The method includes the following steps: S1: The fracturing flowback fluid and desiliconizing agent are sent into the pretreatment tank, and after reaction, sludge sediment and pretreatment clear liquid are obtained; S2: The pretreated clear liquid is pressurized by the centrifugal pump and sent into the gas-liquid mixing nozzle to mix with the gas sent into the gas-liquid mixing nozzle from the first exhaust pipe to obtain a water-gas-solid three-phase mixture; S3: The water-gas-solid three-phase mixture is tangentially fed into the air flotation hydrocyclone, and after cyclone separation, bubble-oil droplet adhering body, mud-sand mixture and air flotation hydrocyclone clear liquid are obtained; S4: The clarified liquid from the air flotation cyclone separator is fed into the metal membrane electrocoagulation device, where it is electrolyzed under pulse aeration conditions and undergoes flocculation and adsorption to obtain adsorbed precipitate and reaction liquid; the reaction liquid is then filtered into a vacuum water storage tank through a titanium metal mesh membrane under the negative pressure drive of the vacuum pump. The metal membrane electrocoagulation device operates in a sequential batch mode, and the steps included in the operating cycle are: simultaneous electrolysis and filtration separation, stopping electrolysis and filtration separation, backflushing the titanium metal mesh membrane - stopping backflushing, settling and sedimentation, and sludge discharge. S5: In the vacuum water storage tank, the reaction solution is mixed with the residual chlorine removal agent and reacted to obtain a reduction reaction solution; the reduction reaction solution is sent to the two-stage DTL membrane treatment device to obtain compliant recycled permeate and secondary concentrate.

2. The fracturing flowback fluid treatment method according to claim 1, wherein, The pretreatment tank is equipped with a stirring device; The silicon removal agent pipeline includes a sodium aluminate agent dosing pipeline and a PAC agent dosing pipeline; The air flotation cyclone is a single-stage air flotation cyclone; The titanium metal mesh is also equipped with a backflush pipe; the outlet of the gas storage tank is also connected to a third exhaust pipe; the third exhaust pipe is connected to the backflush pipe. The vacuum water storage tanks are multiple; each vacuum water storage tank is equipped with a stirring device and an ORP probe; The pretreatment tank is provided with a first bottom sludge discharge port; The air flotation cyclone separator is equipped with a top oil discharge port and a bottom slag discharge port; The metal membrane electrocoagulation device is provided with a second bottom sludge discharge port; The first bottom sludge discharge port, the top oil discharge port, the bottom slag discharge port, and the second bottom sludge discharge port are all connected to the sludge tank.

3. The fracturing flowback fluid treatment method according to claim 1, wherein, The lower outlet of the vacuum water storage tank is connected to the inlet of the first-stage DTL membrane treatment device in the two-stage DTL membrane treatment device. The first-stage DTL membrane treatment equipment is equipped with a primary product water outlet and a primary concentrate discharge outlet, and the primary product water outlet is connected to the inlet of the second-stage DTL membrane treatment equipment. The second-stage DTL membrane treatment equipment is equipped with a secondary product water outlet and a secondary concentrate discharge outlet. The secondary product water outlet is the compliant reuse product water outlet of the two-stage DTL membrane treatment device. The primary and secondary concentrated wastewater discharge outlets are connected to the sludge tank.

4. The fracturing flowback fluid treatment method according to claim 1, wherein, In step S1: The silicon removal agent includes sodium aluminate and PAC agent. The mass concentration of sodium aluminate agent in the pretreatment tank is (0.8-1.2):(0.8-1.2) of silicon in the fracturing flowback fluid in the pretreatment tank. The mass concentration of PAC agent in the pretreatment tank is 30-50 mg / L. The hydraulic retention time in the pretreatment tank is 15-30 minutes. The method also includes discharging the sludge into a sludge pond.

5. The fracturing flowback fluid treatment method according to claim 1, wherein, In step S2: inside the gas-liquid mixing nozzle, the pretreated clear liquid is mixed with the gas fed into the gas-liquid mixing nozzle from the first exhaust pipe to produce bubbles with a diameter of 100-200μm, thereby obtaining a water-gas-solid three-phase mixture.

6. The fracturing flowback fluid treatment method according to claim 1, wherein, In step S3: the method further includes discharging the bubble-oil droplet adhering body into the sludge tank through the top oil outlet, and discharging the sludge mixture into the sludge tank through the bottom sludge outlet.

7. The fracturing flowback fluid treatment method according to claim 1, wherein, In step S4: The pulse aeration conditions include: aeration for 10 seconds followed by a 50-second pause. The current density of the electrolysis is 5-50 mA / m 2 ; The negative pressure driven by the negative pressure is from -50 kPa to -25 kPa; The method also includes discharging the adsorbed sediment into a sludge tank; The method further includes backflushing the titanium metal mesh using a backflushing pipeline; the backflushing pressure is 0.8-1.2 bar and the time is 2.5-3.5 min.

8. The fracturing flowback fluid treatment method according to claim 7, wherein, The electrolysis time is 40-50 minutes; The backflushing process takes 2.5-3.5 minutes. The settling time is 8-12 minutes; The sludge removal time is 1.5-2.5 minutes.

9. The fracturing flowback fluid treatment method according to claim 1, wherein, In step S5: The vacuum level inside the vacuum water storage tank is -55 kPa to -45 kPa; The residual chlorine removal agent is sodium bisulfite; The dosage of the residual chlorine removal agent is determined based on the ORP value measured by the ORP probe in the vacuum water storage tank; the ORP value in the vacuum water storage tank is controlled between 100-150 mV. The reduction reaction solution is fed into the first-stage DTL membrane treatment unit of the two-stage DTL membrane treatment device to obtain primary permeate and primary concentrate; the primary permeate is fed into the second-stage DTL membrane treatment unit to obtain the qualified reuse permeate and secondary concentrate; the primary concentrate and secondary concentrate are discharged into the sludge tank. The recovery rate of the first-stage DTL membrane treatment equipment is controlled at 75-85%; The recovery rate of the second-stage DTL membrane treatment equipment is controlled at 90-95%.

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