A method and system for treating oilfield fracturing flowback fluid

Abstract

This application belongs to the field of fracturing flowback fluid treatment technology, specifically relating to a method and system for treating oilfield fracturing flowback fluid. The method includes a pretreatment step and a wind-powered evaporation step. The pretreatment step reduces the suspended solids (SS), total hardness (TDS), COD, petroleum hydrocarbons, and viscosity of the flowback fluid, and adjusts the pH value of the flowback fluid to provide qualified feed water for the wind-powered evaporation step. The wind-powered evaporation step evaporates the feed water using a wind-powered carrier and removes volatile COD through gas-liquid mass transfer. The pretreatment step of this application effectively removes SS, petroleum hydrocarbons, TDS, and COD; the wind-powered evaporation step effectively removes TDS and volatile COD. Simultaneously, the wind-powered evaporation step of this application constructs a gas-liquid contact interface using a high specific surface area carrier, accelerating mass transfer with wind power as the core driving force, significantly reducing energy consumption and operating costs.

Description

Technical Field

[0001] This application belongs to the field of fracturing flowback fluid treatment technology, specifically relating to a method and system for treating oilfield fracturing flowback fluid. Background Technology

[0002] For the development of low-permeability oil and gas resources, fracturing has become a core technology for enhancing oil recovery. However, the flowback fluid generated by fracturing operations, characterized by "four highs and two complexities" (high suspended solids, high hardness, high salinity, high COD, complex composition, and complex water quality fluctuations), has become a key bottleneck restricting the green development of oilfields. Traditional treatment technologies generally face problems such as high energy consumption, high operating costs, and incomplete salt separation.

[0003] Taking Yulin Oilfield as an example, located in the northeastern part of the Ordos Basin, the oilfield has proven geological reserves of over 2 billion tons of oil. The main reservoirs are low-permeability to ultra-low-permeability rock formations, with porosity generally below 15% and permeability of only 0.1-10 mD. Data from 2023 shows that the oilfield's annual fracturing operations reached 12,000 well operations, with a single well fracturing fluid consumption of 1,000-5,000 m³, a flowback rate of 30%-60%, and an annual production of 1.8-3.6 million m³ of fracturing flowback fluid, showing a year-on-year increasing trend. The fracturing flowback fluid in this area primarily uses guar gum-based fracturing fluid as the matrix, forming a complex system after mixing with reservoir fluids. Monitoring data from five typical treatment stations, including Jingbian and Dingbian, showed that the backflow fluid contained SS concentrations of 256-1430 mg / L, COD of 1600-4100 mg / L, TDS of 20000-60000 mg / L, total hardness (as CaCO3) of 6000-22000 mg / L, and petroleum hydrocarbons of 0-1500 mg / L. Furthermore, the pollutant concentrations were significantly higher in the initial stage (1-3 days) of backflow than in the later stages. The area surrounding the Yulin Oilfield is an ecologically fragile zone on the edge of the Mu Us Desert. Untreated discharge of this wastewater would lead to soil salinization and groundwater pollution; direct reuse would exacerbate reservoir damage and pipeline blockage, making treatment an urgent necessity.

[0004] Current fracturing flowback fluid treatment technologies can be divided into three categories, but all have obvious limitations: physicochemical methods (coagulation, flotation, etc.) can only remove suspended solids and petroleum, with a TDS removal rate of less than 5%; biological treatment methods are inhibited by high salt, and the COD removal rate is less than 30% when TDS > 15000 mg / L; evaporation concentration methods (MVR, multi-effect evaporation) can achieve salt separation, but MVR unit energy consumption is 80-120 kWh / ton of water, treatment cost is 40-60 yuan / ton of water, and it is prone to scaling and clogging, with a continuous operation cycle of only 45 days. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art or related technologies, namely, the problems of low COD removal rate, high energy consumption, high operating cost and incomplete salt separation in the process of treating fracturing flowback fluid.

[0006] In view of this, the first aspect of the present invention provides a method for treating fracturing flowback fluid in oilfields. This method can achieve a SS removal rate of 99.7%, a petroleum hydrocarbon removal rate of 99.9%, a total hardness removal rate of 99.2%, a TDS removal rate of 95.0%, and a COD removal rate of 74.3%. At the same time, wind power carrier technology is introduced into the treatment of fracturing flowback fluid. By constructing a gas-liquid contact interface through a high specific surface area carrier, and using wind power as the core driving force to accelerate mass transfer, energy consumption and operating costs are significantly reduced.

[0007] A second aspect of the present invention provides a treatment system for oilfield fracturing flowback fluid, which is used to treat oilfield fracturing flowback fluid by the above-described treatment method.

[0008] Specifically, the following technical solutions are included: According to a first aspect of the embodiments of this application, a method for treating oilfield fracturing flowback fluid is provided. The method includes a pretreatment step and a wind-powered carrier evaporation step. The pretreatment step provides qualified feed water for the wind-powered carrier evaporation step by reducing the suspended solids (SS), total hardness, COD, petroleum hydrocarbons, and viscosity of the flowback fluid, and by adjusting the pH value of the flowback fluid. The wind-powered carrier evaporation step evaporates the feed water using a wind-powered carrier and removes volatile COD through gas-liquid mass transfer.

[0009] Furthermore, the pretreatment steps include: S1: Pre-sedimentation adjustment: Through stirring and stopping, large suspended particles in the liquid are precipitated and removed, so that SS ≤ 426 mg / L; S2: Colloidal breaking: Ammonium persulfate is added to the liquid to break the colloidal structure in the liquid and release the encapsulated suspended particles and grease, so that the liquid viscosity < 1.5 mPa·s and COD ≤ 2600 mg / L; S3: Hardness removal: Lime is added to the liquid to precipitate magnesium ions into Mg(OH)2, and soda ash is added to the liquid to precipitate calcium ions into CaCO3, so that the liquid hardness ≤ 200 mg / L; S4: High-density sedimentation: PAC and PAM are added to the liquid to coagulate and precipitate small suspended particles and colloidal substances in the liquid, so that SS ≤ 5 mg / L; S5: Air flotation: Dissolved air is introduced into the liquid, so that light and non-settling suspended particles and grease in the liquid adhere to microbubbles and float up quickly and are scraped off, so that the petroleum concentration ≤ 0.8 mg / L.

[0010] Preferably, in step S1, the stirring frequency is 90 r / min to 100 r / min; and the resting time is 4 h to 5 h.

[0011] Preferably, in step S2, the amount of ammonium persulfate added is 0.8 g / L to 0.9 g / L, the reaction temperature is 50°C to 60°C, and the reaction time is 2 h to 3 h.

[0012] Preferably, in step S3, the step of adding lime to the liquid makes the pH value of the liquid 10.5 to 11; the amount of soda ash added is 1.2 to 1.3 times the theoretical requirement.

[0013] Preferably, in step S4, the dosage of PAC is 100 mg / L to 110 mg / L, the dosage of PAM is 3 mg / L to 4 mg / L, and the precipitation time is 60 min to 70 min.

[0014] Preferably, in step S5, the pressure of the dissolved gas is 0.3 MPa to 0.4 MPa, and the water reflux ratio of the dissolved gas is 30% to 35%.

[0015] According to a second aspect of the embodiments of this application, a treatment system for oilfield fracturing flowback fluid is provided, for treating oilfield fracturing flowback fluid by the method described in any of the above technical solutions, including a pretreatment system and a wind-powered carrier evaporation unit connected in series; the pretreatment system includes a pre-settling adjustment unit, a degelatinizing unit, a hardening removal unit, a high-density sedimentation unit, and an air flotation unit connected in series; the wind-powered carrier evaporation unit includes multiple water distribution pipes and multiple evaporation curtains; the water distribution pipes are connected to a water storage tank for storing flowback fluid, and the multiple water distribution pipes are arranged parallel to each other on the same horizontal plane; a vertically suspended evaporation curtain is arranged parallel to each water distribution pipe at its bottom; multiple water spray holes are provided on the water distribution pipes, and the spray direction of the water spray holes is towards the surface of the evaporation curtain.

[0016] Preferably, the distance between two adjacent evaporation curtains is 50mm to 60mm.

[0017] Preferably, the evaporation curtain comprises a silicon-based evaporation fabric curtain.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: This application achieves removal rates of 99.7% for suspended solids (SS), 99.9% for petroleum hydrocarbons, and 99.2% for total hardness through a pretreatment step, effectively solving the problems of clogging and scaling in the evaporation system. It also degrades some organic polymers, resulting in a COD removal rate of 36.2%. In the wind-powered evaporation step, a TDS removal rate of 95.0% is achieved through wind-powered evaporation, while simultaneously removing 38.1% of volatile COD through gas-liquid mass transfer. Furthermore, the wind-powered evaporation step utilizes a high specific surface area carrier to construct a gas-liquid contact interface, accelerating mass transfer with wind power as the core driving force, significantly reducing energy consumption and operating costs. Attached Figure Description

[0019] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of this application. In the drawings: Figure 1 This is a flowchart illustrating an embodiment of this application; Figure 2 This is a schematic diagram of the processing system according to an embodiment of this application; Figure 3 This is a schematic diagram of the structure of the wind power carrier evaporation unit in an embodiment of this application; Figure 4 This is a comparison table of the embodiments of this application and the prior art.

[0020] The attached figures are labeled as follows: 1-Water distribution pipe; 2-Evaporation curtain. Detailed Implementation

[0021] To better understand the above technical solutions, the technical solutions of the embodiments of this application will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments of this application and the specific features in the embodiments are detailed descriptions of the technical solutions of the embodiments of this application, rather than limitations on the technical solutions of this application. In the absence of conflict, the embodiments of this application and the technical features in the embodiments can be combined with each other.

[0022] In view of this, according to a first aspect of the embodiments of this application, a method for treating oilfield fracturing flowback fluid is provided, such as... Figure 1 As shown, the method includes a pretreatment step and a wind power carrier evaporation step; the pretreatment step provides qualified feed water for the wind power carrier evaporation step by reducing the SS, total hardness, COD, petroleum hydrocarbons, and viscosity of the return liquid, and adjusting the pH value of the return liquid; the wind power carrier evaporation step evaporates the feed water through the wind power carrier and removes volatile COD through gas-liquid mass transfer.

[0023] Specifically, the core objective of the pretreatment step is to control SS (suspended solids) below 5 mg / L, reduce total hardness to below 200 mg / L, and achieve a viscosity ≤1.5 mPa·s, providing qualified influent for the wind-powered belt evaporation unit and preventing evaporation curtain blockage and scaling. The wind-powered belt evaporation step is the core unit of the treatment method in this application, offering advantages such as low carbon footprint, high efficiency, and pollution resistance. This wind-powered belt evaporation technology is based on Dalton's law of evaporation, constructing a gas-liquid contact interface through a high specific surface area carrier, using wind energy as the core driving force to accelerate mass transfer, i.e., maximizing the contact area between the liquid and air, allowing the liquid to evaporate quickly under the influence of wind energy. Simultaneously, a portion of the volatile COD is also removed in the wind-powered belt evaporation step, with a removal rate reaching 38.1%–38.3%. Furthermore, experiments show that the evaporation rate of the liquid is significantly positively correlated with temperature and significantly negatively correlated with relative humidity, with a clear interaction between the two. That is, under the same humidity conditions, when the temperature rises from 5℃ to 30℃, the evaporation rate increases by 317%. The increased temperature not only increases the kinetic energy of water molecules, accelerating their escape from the liquid film surface, but also increases the saturated vapor pressure difference of the air, providing a greater driving force for mass transfer. Under the same temperature conditions, when the relative humidity rises from 40% to 70%, the evaporation rate decreases by 38.5%, and when the relative humidity exceeds 80%, the air approaches saturation, the driving force for mass transfer weakens sharply, and the evaporation rate drops below 1 kg / (m²·h). Therefore, when nature is in a period of low temperature and high humidity, the evaporation rate can be increased to over 1.2 kg / (m²·h) by using solar energy as the primary energy source while simultaneously using solar-assisted heating (raising the water temperature by 5-8℃), thus ensuring the efficiency of the wind-driven evaporation step.

[0024] Furthermore, in one embodiment, such as Figure 1 As shown, the pretreatment steps include: S1: Pre-sedimentation adjustment: Through stirring and stopping, large suspended particles in the liquid are precipitated and removed, so that SS ≤ 426 mg / L; S2: Colloidal breaking: Ammonium persulfate is added to the liquid to break the colloidal structure in the liquid and release the encapsulated suspended particles and grease, so that the liquid viscosity < 1.5 mPa·s and COD ≤ 2600 mg / L; S3: Hardness removal: Lime is added to the liquid to precipitate magnesium ions into Mg(OH)2, and soda ash is added to the liquid to precipitate calcium ions into CaCO3, so that the liquid hardness ≤ 200 mg / L; S4: High-density sedimentation: PAC and PAM are added to the liquid to coagulate and precipitate small suspended particles and colloidal substances in the liquid, so that SS ≤ 5 mg / L; S5: Air flotation: Dissolved air is introduced into the liquid, so that light and non-settling suspended particles and grease in the liquid adhere to the microbubbles and float up quickly and are scraped off, so that the petroleum concentration ≤ 0.8 mg / L.

[0025] Preferably, in a specific embodiment, in step S1, the stirring frequency is 90 r / min to 100 r / min; and the resting time is 4 h to 5 h.

[0026] Specifically, taking a water sample with SS of 1430 mg / L as an example: when the retention time was extended from 2 hours to 4 hours, the SS removal rate jumped from 42.3% to 68.5%, and the effluent SS decreased from 826 mg / L to 446 mg / L. This is because large particulate suspended solids (such as proppant debris and rock particles) settled sufficiently. When the retention time exceeded 4 hours, the increase in removal rate tended to level off, with only a 0.3% difference between the removal rates of 6 hours and 8 hours, and the effluent SS stabilized at around 426 mg / L. Considering both treatment efficiency and equipment footprint, the optimal retention time for pre-sedimentation was determined to be 4 hours. This stage can remove approximately 70% of large particulate suspended solids, creating favorable conditions for subsequent S2 and S3 steps.

[0027] Preferably, in a specific embodiment, in step S2, the amount of ammonium persulfate added is 0.8 g / L to 0.9 g / L, the reaction temperature is 50°C to 60°C, and the reaction time is 2 h to 3 h.

[0028] Specifically, the unbroken guar gum polymers in the fracturing flowback fluid are the main cause of increased liquid viscosity, affecting the uniformity of water distribution and evaporation efficiency in the wind power carrier evaporation step. Therefore, it is necessary to reduce the viscosity to below 1.5 mPa·s through debonding treatment. Experiments show that the order of influence of various factors on viscosity removal rate during debonding is: debonding agent dosage > reaction temperature > reaction time. Therefore, the debonding agent dosage is the core influencing factor. When the debonding agent dosage is insufficient, the guar gum molecular chains are not fully broken, and the viscosity decreases slowly; while excessive debonding agent dosage will increase the COD content. Experiments show that when the debonding agent dosage is 1.0 g / L, the COD content increases by 5% compared to when the debonding agent dosage is 0.8 g / L, and it also results in agent waste. Considering both viscosity removal rate and economic efficiency, the optimal process parameters for gel breaking were determined to be: ammonium persulfate dosage of 0.8 g / L, reaction temperature of 50℃, and reaction time of 2 h. Under these conditions, the effluent viscosity was reduced to 1.2 mPa·s, the viscosity removal rate reached 83.3%, and the COD was controlled at around 2600 mg / L. This not only met the requirements of subsequent processes but also avoided the cost issues caused by excessive reagent dosage.

[0029] Preferably, in a specific embodiment, in step S3, the step of adding lime to the liquid makes the pH value of the liquid 10.5 to 11; the amount of soda ash added is 1.2 to 1.3 times the theoretical requirement.

[0030] Specifically, the high concentration of Ca²⁺ in the backflow solution + Mg² +This is a major cause of scaling in evaporation systems. A lime-soda composite descaling process is used, adjusting the pH value with lime to reduce Mg²⁺. + Mg(OH)₂ precipitate is formed, and then Ca²⁺ is reacted with soda ash. + CaCO3 precipitate is formed, thus removing hardness. This is because the solubility product of Mg(OH)2 (Ksp = 1.8 × 10⁻⁶) is high. - ¹¹) Lower, Mg² at pH ≥ 10.5 + Only with sufficient pH can CaCO3 precipitation be fully achieved. However, excessively high pH levels can cause the CaCO3 precipitation-dissolution equilibrium to shift in the opposite direction, which is detrimental to hardness removal. Taking a water sample with a total hardness of 18000 mg / L as an example, experiments showed that when the pH increased from 9.5 to 10.5, the total hardness removal rate increased from 72.3% to 93.8%, and the effluent hardness decreased from 5040 mg / L to 1116 mg / L. When the pH exceeded 10.5, the removal rate increased slowly, reaching 94.2% at pH=11.0, with an effluent hardness of 1044 mg / L. Therefore, the optimal pH value was determined to be 10.5. Under the condition of pH=10.5, experiments showed that when the dosage of soda ash increased from 1.0 times to 1.2 times the theoretical value, Ca²⁺… + The removal rate increased from 88.5% to 99.2%, and the effluent Ca²... + The concentration was reduced from 1480 mg / L to 120 mg / L. When the dosage of soda ash exceeded 1.2 times the theoretical value, the removal rate remained relatively stable. When the dosage reached 1.5 times the theoretical value, the removal rate only increased by 0.3%, but the reagent cost increased by 25%. Therefore, the optimal dosage of soda ash was determined to be 1.2 times the theoretical requirement. In summary, the total hardness of the effluent from step S3 was reduced to 180 mg / L, with a removal rate of 98.9%, fully meeting the requirements of the wind-powered belt-driven evaporation system for influent hardness (≤200 mg / L), effectively avoiding the problem of scaling on the evaporation curtain surface.

[0031] Preferably, in a specific embodiment, in step S4, the dosage of PAC is 100 mg / L to 110 mg / L, the dosage of PAM is 3 mg / L to 4 mg / L, and the precipitation time is 60 min to 70 min.

[0032] Specifically, after step S3, the return liquid still contains a small amount of fine precipitates and petroleum hydrocarbons, requiring further purification through steps S4 and S5. In step S4, the addition of a coagulant (PAC) and a flocculant (PAM) causes the tiny suspended particles and colloidal substances in the water to coagulate into large, dense flocs, which settle rapidly in the sedimentation tank, thereby further removing suspended solids (SS). Experiments show that with a PAC dosage of 100 mg / L, a PAM dosage of 3 mg / L, and a sedimentation time of 60 min, the SS removal rate reaches 99.2%, and the effluent SS drops to below 5 mg / L.

[0033] Preferably, in a specific embodiment, in step S5, the pressure of the dissolved gas is 0.3 MPa to 0.4 MPa, and the water reflux ratio of the dissolved gas is 30% to 35%.

[0034] Specifically, in step S5, dissolved air is introduced into the liquid to generate microbubbles. Lighter, less sediment-forming suspended solids and grease adhere to these microbubbles, quickly rising to the surface and being scraped off, thus reducing the concentration of petroleum hydrocarbons. Experiments show that when the dissolved air pressure is 0.3 MPa and the dissolved air-to-water reflux ratio is 30%, the petroleum hydrocarbon removal rate reaches 99.0%, and the effluent petroleum hydrocarbon concentration is ≤0.8 mg / L.

[0035] In summary, taking a water sample with SS of 1430 mg / L, total hardness of 18000 mg / L, COD of 4100 mg / L, and petroleum hydrocarbons of 1500 mg / L as an example, after the aforementioned pretreatment steps, the effluent water quality indicators are: pH 7.8, SS 3.2 mg / L, total hardness 180 mg / L, COD 2400 mg / L, petroleum hydrocarbons 0.6 mg / L, and viscosity 1.2 mPa·s. These fully meet the influent requirements of the wind-powered evaporation system, laying a solid foundation for subsequent enhanced evaporation treatment.

[0036] According to a second aspect of the embodiments of this application, a system for treating oilfield fracturing flowback fluid is provided, for treating oilfield fracturing flowback fluid by means of the method described in any of the above technical solutions, such as... Figure 2 As shown, it includes a pretreatment system and a wind-powered carrier evaporation unit connected in series; the pretreatment system includes a pre-settling and adjusting unit, a debinding unit, a hardening removal unit, a high-density sedimentation unit, and an air flotation unit connected in series; as shown... Figure 3 As shown, the wind-powered evaporation unit includes multiple water distribution pipes 1 and multiple evaporation curtains 2; the water distribution pipes 1 are connected to a water storage tank for storing return liquid, and the multiple water distribution pipes 1 are arranged in parallel on the same horizontal plane; the evaporation curtains 2 are arranged in parallel at the bottom of each water distribution pipe 1 and are suspended vertically; multiple water spray holes are opened on the water distribution pipes 1, and the spray direction of the water spray holes is towards the surface of the evaporation curtains 2.

[0037] Specifically, in the wind-powered evaporation unit, the spray holes uniformly spray liquid onto the evaporation curtain 2, and the large number of evaporation curtains 2 allows the liquid to contact the air to the maximum extent, thereby improving the evaporation efficiency.

[0038] Preferably, in a specific embodiment, the distance between two adjacent evaporation curtains 2 is 50mm to 60mm.

[0039] Specifically, the performance of the wind-powered evaporation unit directly determines the efficiency, energy consumption, and cost of the entire treatment system, while the distance between two adjacent evaporation curtains 2 determines the uniformity of airflow distribution and the gas-liquid contact time. Experiments show that when the spacing of the evaporation curtains 2 is too small (≤30mm), the resistance of the airflow through the evaporation curtains 2 increases, leading to uneven airflow distribution, forming dead zones in some areas, preventing timely renewal of the liquid film, and limiting the evaporation rate. When the spacing of the evaporation curtains 2 is too large (≥70mm), the gas-liquid contact time is shortened, water molecules fail to diffuse sufficiently into the airflow, and the mass transfer efficiency decreases. Therefore, the optimal distance between two adjacent evaporation curtains 2 is determined to be 50mm.

[0040] Preferably, in a specific embodiment, the evaporation curtain 2 includes a silicon-based evaporation fabric curtain.

[0041] Specifically, the evaporation curtain 2, as the core carrier for gas-liquid mass transfer, directly affects evaporation efficiency and system lifespan due to its hydrophilicity, corrosion resistance, specific surface area, and cost. The silicon-based evaporation curtain possesses the following comprehensive properties: First, excellent hydrophilicity, enabling rapid adsorption of pretreated effluent and the formation of a uniform liquid film, preventing localized dry areas or liquid accumulation; second, strong corrosion resistance, showing no corrosion or aging after immersion in high-salt wastewater with a TDS of 38000 mg / L for 30 days, with an estimated service life of up to 18 months, far exceeding other materials; third, a specific surface area of ​​850 m² / m³, providing ample interface for gas-liquid mass transfer; and fourth, reasonable cost, offering significant economic advantages.

[0042] In summary, such as Figure 4 As shown, the nursing method and system of this application exhibit significant comprehensive advantages over existing technologies: First, the treatment efficiency is close to that of MVR and multi-effect evaporation, with a TDS removal rate of 95.2%, and the effluent meets the requirements for discharge and reinjection; Second, it has outstanding advantages in energy consumption and cost, with unit energy consumption reduced by 78.6% compared to MVR, and treatment cost only 30.3% of MVR; Third, it has significant advantages in operational stability and land use, with a continuous operating cycle twice that of MVR, and a land area only 5% of that of natural evaporation. Among these, although natural evaporation has the lowest energy consumption, its efficiency is extremely low (evaporation rate is only 0.033 kg / (m²·h)) and it requires a large land area, which cannot meet the needs of large-scale oilfield treatment.

[0043] In the description of this specification, the terms "one embodiment," "some embodiments," "specific embodiment," etc., refer to a specific feature, structure, material, or characteristic described in connection with that embodiment or example, which is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0044] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. 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 treating oilfield fracturing flowback fluid, characterized in that, The method includes a pretreatment step and a wind energy carrier evaporation step; The pretreatment step provides qualified feed water for the wind power carrier evaporation step by reducing the SS, total hardness, COD, petroleum hydrocarbons, and viscosity of the backflow liquid, and adjusting the pH value of the backflow liquid. The wind power carrier evaporation step evaporates the incoming water using a wind power carrier and removes 38.1% to 38.3% of volatile COD through gas-liquid mass transfer. The preprocessing steps include: S1: Pre-sedimentation adjustment: Through stirring and stopping, large suspended solids in the liquid are precipitated and removed, so that SS≤426mg / L; S2: Colloidal Break: Ammonium persulfate is added to the liquid to break down the colloidal structure in the liquid and release the encapsulated suspended matter and oil, so that the liquid viscosity is <1.5mPa·s and COD≤2600mg / L; S3: Hardness Removal: Add lime to the liquid to cause magnesium ions in the liquid to form Mg(OH)2 precipitate, and add soda ash to the liquid to cause calcium ions in the liquid to form CaCO3 precipitate, so that the hardness of the liquid is ≤200mg / L. S4: High-density precipitation: PAC and PAM are added to the liquid to cause the tiny suspended particles and colloidal substances in the liquid to coagulate and precipitate, so that SS≤5mg / L; S5: Air flotation: Dissolved air is introduced into the liquid, causing light and non-settling suspended matter and oil in the liquid to adhere to the microbubbles, thus quickly floating to the surface and being scraped off, so that the concentration of petroleum is ≤0.8mg / L. The wind energy carrier evaporation step uses a silicon-based evaporation cloth as the evaporation carrier, which can quickly adsorb the pretreated water and form a uniform liquid film. The distance between adjacent silicon-based evaporation cloths is 50mm to 60mm.

2. The method for treating oilfield fracturing flowback fluid according to claim 1, characterized in that, In step S1, the stirring frequency is 90 r / min to 100 r / min; the resting time is 4 h to 5 h.

3. The method for treating oilfield fracturing flowback fluid according to claim 1, characterized in that, In step S2, the amount of ammonium persulfate added is 0.8 g / L to 0.9 g / L, the reaction temperature is 50℃ to 60℃, and the reaction time is 2 h to 3 h.

4. The method for treating oilfield fracturing flowback fluid according to claim 1, characterized in that, In step S3, The step of adding lime to the liquid makes the pH value of the liquid 10.5 to 11; The amount of soda ash added is 1.2 to 1.3 times the theoretical requirement.

5. The method for treating oilfield fracturing flowback fluid according to claim 1, characterized in that, In step S4, the dosage of PAC is 100 mg / L to 110 mg / L, the dosage of PAM is 3 mg / L to 4 mg / L, and the precipitation time is 60 min to 70 min.

6. The method for treating oilfield fracturing flowback fluid according to claim 1, characterized in that, In step S5, the pressure of the dissolved gas is 0.3 MPa to 0.4 MPa, and the water reflux ratio of the dissolved gas is 30% to 35%.

7. A system for treating oilfield fracturing flowback fluid, used to treat oilfield fracturing flowback fluid by the method described in any one of claims 1-6, characterized in that, Includes a connected pretreatment system and a wind-powered evaporation unit; The pretreatment system includes a pre-settling adjustment unit, a debinding unit, a hardening removal unit, a high-density sedimentation unit, and an air flotation unit connected in sequence. The wind-powered evaporation unit includes multiple water distribution pipes (1) and multiple evaporation curtains (2); The water distribution pipe (1) is connected to the water storage tank for storing the backflow liquid, and multiple water distribution pipes (1) are arranged in parallel on the same horizontal plane. Each of the water distribution pipes (1) is provided with a vertically suspended evaporation curtain (2) at its bottom. Multiple water spray holes are provided on the water distribution pipe (1), and the spray direction of the water spray holes is towards the surface of the evaporation curtain (2).

8. The oilfield fracturing flowback fluid treatment system according to claim 7, characterized in that, The distance between two adjacent evaporation curtains (2) is 50mm to 60mm.

9. The oilfield fracturing flowback fluid treatment system according to claim 7, characterized in that, The evaporation curtain (2) includes a silicon-based evaporation cloth curtain.

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