Fracturing flow-back fluid treatment system and treatment method

Abstract

The invention relates to a fracturing flow-back fluid treatment system and method, and the system comprises an adjusting precipitation unit, a dissolved air flotation unit, a direct filtration membrane filtration unit, a reverse osmosis membrane concentration unit, an MVR evaporative crystallization unit and an electrochemical oxidation unit which are sequentially connected along a treatment flow. The MVR evaporative crystallization unit comprises a steam-water separator, a vacuum device communicated with a steam outlet of the steam-water separator and a condensate pipeline communicated with a condensate outlet of the steam-water separator, a sodium sulfate adding device is arranged on the steam-water separator, and the vacuum device is used for controlling steam pressure in the steam-water separator. The water inlet end of the electrochemical oxidation unit is connected with a condensate pipeline of the MVR evaporative crystallization unit. The system has the beneficial effects that a plurality of treatment units are connected in sequence, so that suspended solids, colloids, oils, hard substances, silicon, high-concentration salt and refractory organic matters in the flowback fluid can be systematically removed, finally purified water which can meet the discharge or reuse standard is produced, and the targets of wastewater recycling and near-zero discharge are achieved.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field wastewater treatment technology, and in particular to a treatment system and method for fracturing flowback fluid. Background Technology

[0002] Fracturing technology is a key process in oil and gas extraction to increase well production. It involves injecting high-pressure fracturing fluid into the formation to create fractures, thereby increasing the channels for oil and gas flow. Water-based fracturing fluids are widely used due to their lower cost and controllable performance. They are typically based on clean water or thickened water, with the addition of thickeners (such as plant gums), crosslinking agents, breaker agents, bactericides, and other chemical additives. After fracturing operations, some of the fracturing fluid carries rock cuttings, crude oil, dissolved salts, and other substances back to the surface, forming fracturing flowback fluid. This fluid mainly originates from well washing wastewater, breaker flowback fluid, and unused fracturing base fluid, and its generation can reach 20% to 80% of the total injected fracturing fluid. It is characterized by intermittent discharge, complex composition, and high concentrations of contaminants.

[0003] The quality of fracturing flowback fluid varies significantly depending on the fracturing fluid formulation, flowback cycle, and formation conditions. It typically exhibits the following characteristics: high turbidity and complex composition, containing not only various chemical additives from the original fracturing fluid but also suspended solids, petroleum hydrocarbons, salts, and microorganisms from the formation; high viscosity and severe emulsification, forming a stable colloidal / emulsion system that makes solid-liquid separation difficult; high chemical oxygen demand (COD), mainly derived from recalcitrant thickeners (such as hydroxypropyl guanidine gum) and surfactants; high total dissolved solids (TDS) content, high hardness, prominent chloride ion concentration, and a certain amount of heavy metals and fluorides.

[0004] According to current industry standards, the main methods for disposing of fracturing flowback fluid include reuse, reinjection, and discharge. With increasingly stringent environmental requirements, reinjection is subject to stricter formation compatibility and environmental risk control restrictions; while reuse cannot completely absorb the generated flowback fluid, especially during non-fracturing operation periods, when large amounts of flowback fluid need to be discharged or used for other reuse methods. Currently, discharge or surface reuse (such as for farmland irrigation and urban miscellaneous uses) must meet the "Integrated Wastewater Discharge Standard" (GB 8978) or even stricter reuse water quality standards, which have high requirements for indicators such as COD, ammonia nitrogen, salinity, chloride ions, and hardness. Traditional oil-water separation-coagulation-filtration processes achieve a COD removal rate of less than 50%, which is far from meeting the treatment needs.

[0005] To achieve high emission standards or reuse, existing technologies typically employ multi-stage advanced treatment processes, including units such as air flotation, catalytic oxidation, softening sedimentation, biochemical treatment, membrane concentration, and evaporation crystallization. For example, some projects use a combined process of air flotation, advanced oxidation, multi-stage membrane concentration, MVR evaporation, and advanced oxidation. However, this type of process still faces the following prominent problems in practical applications:

[0006] (1) Weak process adaptability: The quality of fracturing flowback fluid is affected by multiple factors such as fracturing fluid formulation, formation lithology, and flowback stage, resulting in large differences between wells and drastic temporal fluctuations. Fixed process flow is difficult to dynamically respond to changes in water quality, which can easily lead to membrane system fouling, evaporation unit scaling, or biochemical system failure, resulting in poor operational stability;

[0007] (2) Stringent pretreatment requirements: Evaporation crystallization units (especially the mainstream MVR process in well sites) are limited by the compressor’s temperature rise capacity (usually only 20-24℃), requiring the influent hardness to be almost completely removed. Deep softening (such as high-density sedimentation, ion exchange) processes are lengthy and have strong chemical dependence. They are difficult to control when water quality fluctuates, and residual hardness can easily cause a sudden increase in energy consumption or even interruption of operation of the MVR system.

[0008] (3) High cost: Oil and gas field well sites generally lack public works conditions such as steam and circulating cooling water, which restricts the application of low-energy technologies such as multi-effect evaporation; MVR process has high power consumption, and the addition of reagent consumption of multi-stage membrane treatment, advanced oxidation and other units leads to high cost per ton of water treatment; in order to meet multiple objectives such as desalination, COD removal, hardening removal and oil removal, the process chain is too long, the equipment investment is large, the automation control requirements are high, and the operation and maintenance management in remote well site environments is significantly more difficult. Summary of the Invention

[0009] (a) Technical problems to be solved

[0010] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a fracturing flowback fluid treatment system and method, which solves the technical problems of poor adaptability of the treatment process to water quality fluctuations, stringent pretreatment requirements for hardening leading to lengthy processes and unstable operation, and high overall energy consumption and operating costs in the prior art.

[0011] (II) Technical Solution

[0012] To achieve the above objectives, the main technical solutions adopted by the present invention include:

[0013] In a first aspect, embodiments of the present invention provide a fracturing flowback fluid treatment system, comprising a conditioning sedimentation unit, a dissolved air flotation unit, a direct filtration membrane unit, a reverse osmosis membrane concentration unit, an MVR evaporation crystallization unit, and an electrochemical oxidation unit connected sequentially along the treatment process;

[0014] The MVR evaporation crystallization unit includes a steam-water separator, a vacuum device connected to the steam outlet of the steam-water separator, and a condensate pipeline connected to the condensate outlet of the steam-water separator. The steam-water separator is equipped with a sodium sulfate dosing device for adding sodium sulfate to the evaporating liquid inside the steam-water separator. The vacuum device is used to control the steam pressure inside the steam-water separator, and the condensate outlet is used to discharge condensate.

[0015] The water inlet of the electrochemical oxidation unit is connected to the condensate pipeline of the MVR evaporation and crystallization unit.

[0016] In a preferred embodiment of the present invention, the direct filtration membrane unit includes a primary dealkali reaction tank, a secondary dealkali reaction tank, and a direct filtration membrane assembly connected in sequence; the primary dealkali reaction tank is equipped with a first lime dosing device for adding lime to remove the bicarbonate alkalinity of the liquid to be treated in the primary dealkali reaction tank;

[0017] The secondary dealkali reaction tank is equipped with a second lime dosing device for continuing to add lime to generate magnesium hydroxide precipitate in the liquid to be treated in the secondary dealkali reaction tank and remove silicon; the filter membrane used in the direct filter membrane assembly is a hybrid membrane composed of inorganic materials and PTFE, and the filtration pore size of the hybrid membrane is 0.01-0.5µm.

[0018] In a preferred embodiment of the present invention, the primary dealkali reaction tank is further provided with a first online pH sensor, and the first lime dosing device is connected to the first online pH sensor for adding lime into the primary dealkali reaction tank according to the detection signal of the first online pH sensor in order to remove the bicarbonate alkalinity of the liquid to be treated.

[0019] The secondary dealkali reaction tank is also equipped with a second online pH sensor. The second lime dosing device is connected to the signal of the second online pH sensor and is used to continue adding lime into the secondary dealkali reaction tank according to the detection signal of the second online pH sensor, so that the liquid to be treated generates magnesium hydroxide precipitate and removes silicon.

[0020] In a preferred embodiment of the present invention, the secondary dealkali reaction tank is further provided with a silicon removal agent dosing device, and an online silicon content monitor is provided on the water inlet pipe of the secondary dealkali reaction tank. The silicon removal agent dosing device is connected to the online silicon content monitor.

[0021] When the online silicon content monitor detects that the silicon content in the influent is greater than 50 mg / L, the silicon removal agent dosing device is activated to add silicon removal agent; when the silicon content in the influent is detected to be less than 50 mg / L, the silicon removal agent dosing device is turned off.

[0022] In a preferred embodiment of the present invention, the steam-water separator is further provided with a calcium sulfate dosing device for adding calcium sulfate to the evaporating liquid in the steam-water separator.

[0023] In a preferred embodiment of the present invention, the reverse osmosis membrane concentration unit is provided with a concentrate circulation mechanism, which is provided with a variable frequency high-pressure pump, a TDS online monitoring instrument and a controller;

[0024] The TDS online monitoring instrument is installed on the concentrate pipeline of the reverse osmosis membrane concentration unit to detect the TDS concentration on the concentrate side in real time. The TDS online monitoring instrument is connected to the input terminal of the controller, and the output terminal of the controller is connected to the variable frequency high-pressure pump.

[0025] The controller can adjust the operating frequency of the variable frequency high-pressure pump according to the detection signal of the TDS online monitor, so as to control the TDS concentration on the concentrate side within the range of 75,000 mg / L to 100,000 mg / L.

[0026] The RO membrane module of the reverse osmosis membrane concentration unit is a spiral wound reverse osmosis membrane module with a pressure resistance of ≥90 bar.

[0027] In a preferred embodiment of the present invention, the dissolved air flotation unit includes a gel breaking reaction tank and a dissolved air flotation device connected in sequence;

[0028] The debonding reaction tank is equipped with a debonding agent dosing device for adding debonding agent to reduce the viscosity of the fracturing flowback fluid. The debonding agent dosing device can adjust the amount of debonding agent added according to the viscosity of the effluent and / or the concentration of dissolved organic matter.

[0029] The dissolved air flotation device includes a flotation tank, a dissolved air pump, and a dissolved air water tank. The inlet of the dissolved air pump is connected to the clear water zone of the flotation tank, the outlet of the dissolved air pump is connected to the inlet of the dissolved air water tank, and the outlet of the dissolved air water tank is connected to the inlet zone of the flotation tank through a release device to form a water return dissolved air structure.

[0030] The dissolved air flotation device has a reflux ratio of ≥50% and a dissolved air pressure of ≥0.5MPa.

[0031] In a preferred embodiment of the present invention, the electrochemical oxidation unit includes an electrochemical catalytic oxidation device for advanced oxidation treatment of the condensate from the MVR evaporation and crystallization unit to remove organic matter and ammonia nitrogen from the condensate.

[0032] As a preferred embodiment of the present invention, it further includes a sludge collection and discharge unit; the sludge collection and discharge unit is connected to the sludge discharge port of the regulating sedimentation unit, the dissolved air flotation unit and the direct filtration membrane unit, and is used to collect and discharge the sludge and waste generated by each unit.

[0033] Secondly, embodiments of the present invention provide a method for treating fracturing flowback fluid, employing the aforementioned treatment system, comprising the following steps:

[0034] S1. The fracturing flowback fluid is introduced into the regulating sedimentation unit for water quality and quantity homogenization and preliminary sedimentation treatment.

[0035] S2. The effluent from S1 is fed into the dissolved air flotation unit for debinding and flotation separation to remove suspended solids, colloids and some organic matter.

[0036] S3. The effluent from S2 is passed into the direct filtration membrane unit for dealkali removal, desilicon removal, and membrane filtration treatment.

[0037] S4. The effluent from S3 is passed into the reverse osmosis membrane concentration unit for concentration to obtain concentrated liquid and permeate.

[0038] S5. The concentrated solution obtained in S4 is passed into the MVR evaporation and crystallization unit. Sodium sulfate is added during the evaporation process, and the evaporation temperature is controlled not to exceed 60°C. Solid salt and condensate are obtained through evaporation and crystallization.

[0039] S6. The condensate produced by the MVR evaporation and crystallization unit in S5 is passed into the electrochemical oxidation unit for deep oxidation treatment to obtain product water.

[0040] In a preferred embodiment of the present invention, step S3 includes the following steps: 1) Passing the effluent from S2 into a primary dealkali reaction tank, adding lime and controlling the pH of the reaction system to 8.0-9.0 to remove bicarbonate alkalinity; 2) Passing the effluent into a secondary dealkali reaction tank, adding lime again and controlling the pH of the reaction system to 9.0-10.0, using the generated magnesium hydroxide precipitate for desiliconization, and selectively adding a desiliconizing agent according to the silica content of the influent to reduce the silica content of the effluent to below 50 mg / L.

[0041] In step S4, the TDS concentration of the concentrate is controlled within the range of 75,000 mg / L to 100,000 mg / L by controlling the concentrate circulation and variable frequency high-pressure pump of the reverse osmosis membrane concentration unit.

[0042] In step S5, the molar amount of sodium sulfate added is not less than the total molar amount of calcium and magnesium ions in the concentrate entering the MVR evaporation and crystallization unit.

[0043] (III) Beneficial Effects

[0044] The beneficial effects of this invention are as follows: The fracturing flowback fluid treatment system and method of this invention, by sequentially connecting multiple treatment units such as conditioning sedimentation, dissolved air flotation, direct filtration membrane filtration, reverse osmosis concentration, MVR evaporation crystallization, and electrochemical oxidation, form a pathway for the graded and targeted removal of various pollutants in the fracturing flowback fluid, ultimately forming a treatment system capable of achieving high-standard deep purification. It can systematically remove suspended solids, colloids, oils, hardness, silica, high-concentration salts, and recalcitrant organic matter from the flowback fluid, ultimately producing purified water that meets strict discharge or reuse standards, namely, TDS ≤ 1000 mg / L, COD ≤ 40 mg / L, BOD ≤ 10 mg / L, and ammonia nitrogen ≤ 5 mg / L, while simultaneously achieving the solidification and separation of inorganic salts, thus achieving the goal of wastewater resource utilization and near-zero discharge.

[0045] The MVR evaporation crystallization unit effectively solves the problems of high energy consumption, easy scaling, and unstable operation in the evaporation treatment of high-hardness wastewater in existing technologies. By adding sodium sulfate to the steam-water separator of the MVR evaporation crystallization unit, the precipitation sequence and crystallization morphology of salts in the concentrate are changed, converting calcium chloride and magnesium chloride, which easily lead to high boiling point rise and severe scaling, into calcium sulfate and magnesium sulfate, which are easier to precipitate. This can directly reduce the boiling point rise of the solution by 10℃-15℃, fundamentally alleviating the stringent requirement of near-zero hardness for the MVR process. At the same time, the system operating pressure is controlled at ≤20kPa (corresponding to a saturation temperature below 60℃) by a vacuum device, achieving low-temperature evaporation. The synergistic effect of low temperature conditions and sodium sulfate addition further suppresses the boiling point rise, reducing the demand on the temperature rise capacity of the steam compressor; on the other hand, it significantly reduces the transfer of volatile and semi-volatile organic compounds into the vapor phase, thereby reducing the treatment load of the evaporation condensate. In actual engineering, energy consumption per ton of water can be reduced by more than 40kW·h.

[0046] This invention employs a seed-based forced circulation evaporation technology. By adding calcium sulfate seed crystals to the system and maintaining the suspended solids (SS) content in the forced circulation liquid at over 10% during operation, sufficient crystal nuclei are ensured in the evaporation chamber. This allows sparingly soluble salts such as calcium sulfate and magnesium sulfate to complete their orderly crystal growth primarily within the steam-water separation chamber, effectively preventing scaling on critical components such as heat exchanger tube walls. The dosage of sodium sulfate does not require precise control; theoretically, the molar amount added equals the total molar amount of calcium and magnesium ions in the feed liquid. A slight excess is permissible in actual operation. Furthermore, the system can utilize surplus sodium sulfate from existing wastewater discharge processes as raw material, without increasing reagent costs. The crystallization product of this method is a common industrial salt, making disposal convenient.

[0047] The dissolved air flotation (DAF) unit achieves efficient demulsification and solid-liquid separation of high-viscosity, highly emulsified fracturing flowback fluid. Addressing the high viscosity and stable emulsion characteristics of fracturing flowback fluid, this unit incorporates a de-gelling reaction before DAF. By adding a de-gelling agent, the stability of the colloidal and emulsion system is effectively disrupted, reducing the effluent viscosity to below 2 mPa·s. With viscosity reduction as the goal, and by monitoring indicators such as dissolved organic matter in the effluent, excessive addition is avoided to prevent the over-decomposition of large organic molecules into smaller dissolved organic molecules. This control method ensures that most organic pollutants remain in suspended or colloidal states, allowing for efficient physical removal in subsequent DAF and filtration units (ensuring an overall organic matter removal rate of ≥50%), rather than being transformed into difficult-to-treat dissolved pollutants.

[0048] The dissolved air flotation (DAF) unit employs a high reflux ratio (≥50%) and high dissolved air pressure (≥0.5MPa), generating a large number of fine, stable bubbles. This significantly increases the collision probability and adhesion efficiency between the bubbles and oil droplets, colloids, and flocs in the water. The enhanced DAF system exhibits excellent capture and removal capabilities for oils released after floc breaking, the formed micro-flocs, and suspended solids, ensuring that the effluent oil content is reduced to below 50 mg / L and suspended solids to below 30 mg / L. This provides high-quality feed water for subsequent membrane treatment units, effectively reducing the risk of membrane fouling. Controlled chemical dosage minimizes the consumption of breaker agents and other chemicals while ensuring treatment effectiveness, avoiding increased operating costs and the risk of secondary pollution due to excessive chemical dosing.

[0049] This invention integrates chemical pretreatment and physical filtration technologies into a direct filtration membrane unit, achieving precise removal of specific contaminants from fracturing flowback fluid and significantly simplifying the process. This unit employs a two-stage dealkali removal reactor design in series, with each stage having a clear treatment objective and precise pH control. The first-stage dealkali removal (pH 8.0-9.0) removes bicarbonate alkalinity. Lime is added to convert calcium bicarbonate in the water into calcium carbonate precipitate. This prevents the thermal decomposition of bicarbonate into non-condensable CO2 during subsequent evaporation and crystallization, thus ensuring the heat transfer efficiency of the evaporation system and avoiding calcium carbonate scaling. This step does not aim to remove the permanent hardness of calcium and magnesium, greatly reducing lime consumption. The second-stage dealkali removal (pH 9.0-10.0) involves adding lime again to convert residual magnesium bicarbonate in the water into magnesium hydroxide precipitate, utilizing its strong adsorption and co-precipitation effect to remove silicates. This step also does not require complete removal of magnesium hardness, but rather focuses on generating sufficient magnesium hydroxide for silica removal. This staged design increases the proportion of active magnesium hydroxide in the second-stage tank, thereby enhancing the silica removal effect. The secondary dealkali removal reactor is also equipped with a silica removal agent dosing device, which is linked to the silica content of the influent (threshold set at 50 mg / L). When the silica content of the influent is not high, silica removal is achieved solely by lime; when the silica content exceeds the standard, auxiliary chemical dosing is automatically activated. This on-demand dosing mode ensures that the silica content of the effluent consistently meets the standard (<50 mg / L) while minimizing the cost of silica removal agents. A high-strength, fouling-resistant hybrid direct filter membrane (pore size 0.01-0.5 µm) composed of inorganic materials and PTFE is used, which can directly and efficiently filter wastewater containing fine precipitates after the aforementioned chemical conditioning. This integrates the multi-stage solid-liquid separation units required in traditional processes, such as sedimentation / clarification tanks, sand filtration, and ultrafiltration, into a single membrane filtration step, significantly simplifying the process flow and reducing the footprint. This membrane module features high flux, fouling resistance, and easy cleaning. It achieves excellent purification results without the need for PAM or other organic flocculants, producing effluent with an oil content below 2 mg / L and turbidity below 1 NTU. Furthermore, the total organic matter removal rate (relative to the raw water) can exceed 50%. This not only reduces operating costs but also avoids the membrane fouling risks associated with PAM addition, significantly extending membrane lifespan and improving the operational stability and reliability of the entire pretreatment unit.

[0050] The reverse osmosis (RO) membrane concentration unit abandons the traditional model that aims for a fixed recovery rate, instead focusing on controlling the salt concentration on the concentrate side. Specifically, by employing a concentrate recirculation process combined with the regulation of a variable frequency high-pressure pump, the TDS concentration of the concentrate is stably controlled within the range of 75,000 to 100,000 mg / L. Considering the operating limits of the membrane elements, high-pressure-resistant (e.g., 90 bar) spiral wound reverse osmosis (STRO) membrane elements are selected, achieving effective control of the high-salt concentrate while ensuring system safety. The concentrate after RO concentration enters a buffer tank and is then pumped to the subsequent evaporation and crystallization unit. This operating mode brings several significant advantages: First, it can flexibly adjust the circulation volume and pump frequency to cope with fluctuations in feed water quality, ensuring a stable and suitable feed concentration for the evaporation and crystallization unit; second, it effectively protects the membrane system, avoiding problems such as excessive concentration on the membrane surface, increased scaling risk, or excessive pressure differential caused by pursuing high recovery rates, thus improving the operational reliability and lifespan of the unit.

[0051] This invention places the electrochemical advanced oxidation unit at the very end of the entire treatment process, specifically for treating the condensate from the preceding MVR evaporation and crystallization unit. Since the condensate has already undergone multiple stages of treatment including conditioning sedimentation, dissolved air flotation, and evaporation crystallization, especially the phase change separation effect of evaporation crystallization which has solidified and removed most pollutants, the concentrations of organic matter and ammonia nitrogen in the condensate are extremely low, typically more than 90% lower than the raw water. This setup allows the high-energy-consuming unit to focus on deep oxidation of only a small amount of residual recalcitrant pollutants, resulting in a clear target and significantly reduced load. Furthermore, placing the high-energy-consuming advanced oxidation unit downstream avoids the enormous energy and reagent consumption associated with directly treating high-concentration, highly complex raw water, thereby significantly reducing the operating costs of the advanced oxidation process and making its application economically feasible. Simultaneously, it ensures that the final effluent's key indicators such as COD and ammonia nitrogen consistently meet the most stringent discharge or reuse standards, greatly improving the reliability and safety of the overall process effluent quality.

[0052] By setting up a sludge collection and discharge unit, and connecting it to the sludge discharge ports of the upstream solid waste generating units (regulating sedimentation, dissolved air flotation, and direct membrane filtration), the system achieves centralized collection and standardized discharge of various solid wastes such as sludge, scum, and chemical precipitates generated within the system. This design improves the system's material balance, ensuring the proper disposal of secondary pollutants (solid waste) generated during wastewater treatment. It demonstrates the integrity and environmental friendliness of the treatment system, preventing the accumulation of solid waste within the system from affecting operation or causing secondary pollution. In practical engineering, this unit can be subsequently connected to thickening and dewatering equipment to further reduce sludge volume.

[0053] In summary, the processing system and method provided by this invention, through innovative design and optimized integration of each unit, not only achieves high-standard purification of fracturing flowback fluid, but more importantly, it achieves significant synergistic improvements in processing efficiency, operating energy consumption, cost control, system stability, and adaptability. It provides an efficient, economical, and reliable solution to the current challenges in the field of deep treatment of fracturing flowback fluid, such as long process flow, high operating costs, and poor adaptability to water quality fluctuations. Attached Figure Description

[0054] Figure 1 This is a schematic diagram of the overall structure of a fracturing flowback fluid treatment system and method according to the present invention;

[0055] Figure 2 A process flow diagram of a fracturing flowback fluid treatment system and method for an invention.

[0056] [Explanation of Labels in the Attached Image]

[0057] 11. Settling tank; 12. Settling tank booster pump; 21. Debriding reaction tank; 22. First coagulation reaction tank; 23. Vertical flow flotation tank; 24. Dissolved air pump; 25. Dissolved air tank; 26. Flotation tank; 27. Flotation effluent pump; 31. Primary alkali removal reaction tank; 32. Secondary alkali removal reaction tank; 33. Second coagulation reaction tank; 34. Sludge balance tank; 35. Direct filter feed pump; 36. Direct filter membrane module; 37. Direct filter permeate tank; 38. 39. Direct filter backwash pump; 41. Direct filter permeate pump; 42. RO security filter; 43. RO high-pressure pump; 44. RO membrane module; 45. RO permeate tank; 46. RO permeate discharge pump; 47. RO concentrate tank; 58. RO concentrate pump; 59. MVR unit; 50. Mother liquor storage tank; 61. Mother liquor delivery pump; 62. Electrochemical catalytic oxidation unit; 63. Electrochemical circulating water tank; 64. Electrochemical circulating pump; 65. Condensate discharge pump. Detailed Implementation

[0058] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0059] To better understand the above technical solutions, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present 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 present invention can be understood more clearly and thoroughly, and that the scope of the present invention can be fully conveyed to those skilled in the art.

[0060] Example 1

[0061] like Figure 1 and Figure 2 As shown, this embodiment provides a fracturing flowback fluid treatment system. The fracturing flowback fluid treatment system includes a conditioning sedimentation unit, a dissolved gas flotation unit, a direct filtration membrane unit, a reverse osmosis membrane concentration unit, an MVR evaporation crystallization unit, and an electrochemical oxidation unit connected sequentially along the treatment process.

[0062] The regulating sedimentation unit, comprising a regulating sedimentation tank 11 and a sedimentation tank lift pump 12, is used for water quality and quantity balancing and preliminary solid-liquid separation. Sufficient hydraulic retention time (designed ≥3 hours) effectively buffers and balances drastic fluctuations in water quality and flow rate, providing stable influent conditions for subsequent treatment units. The regulating sedimentation tank 11 has a sedimentation function, effectively removing denser suspended solids and rock debris from the return wastewater. The sedimentation tank lift pump 12, located at the outlet of the regulating sedimentation tank 11, lifts the pre-sedimented wastewater to the subsequent dissolved air flotation unit.

[0063] Preferably, the inlet of the settling tank 11 is located at the top of the tank body, using a tangential vortex inlet method. This design simplifies the water distribution device, effectively reduces the direct impact of the incoming water on the water inside the tank, facilitates the settling of suspended solids, and reduces the risk of inlet blockage. To achieve efficient sedimentation and convenient sludge discharge, the bottom of the tank body is designed in a conical shape with a cone angle of not less than 55°, and is equipped with an automatic sludge discharge valve, which can realize timed or quantitative automatic gravity sludge discharge to prevent sludge accumulation.

[0064] Preferably, to enhance the sedimentation effect and adapt to continuous influent and intermittent effluent conditions, the regulating sedimentation tank is preferably designed with no fewer than two compartments. The system adopts a sequential batch operation logic, that is, one compartment is in the effluent effluent (and stops influent) state, using settling to complete deep sedimentation, while the other compartments simultaneously receive influent and buffer. Each compartment alternates this process, thereby achieving a near-static sedimentation effect in dynamic operation, significantly improving the removal efficiency of suspended solids.

[0065] Preferably, after the effluent from the settling tank 11 is lifted by the settling tank lift pump 12, a pipe mixer is installed on its outlet pipe for adding coagulant to destabilize the colloids and fine suspended solids in the wastewater, facilitating subsequent flocculation reactions.

[0066] The dissolved air flotation unit comprises a degumming reaction tank 21, a first coagulation reaction tank 22, and a dissolved air flotation device connected sequentially along the water flow direction. The degumming reaction tank 21 is equipped with a degumming agent dosing device for adding degumming agent to reduce the high viscosity and emulsification stability of the fracturing flowback fluid. This device is configured to adjust the amount of degumming agent added in real time based on the viscosity and dissolved organic matter concentration of the effluent from the degumming reaction tank 21, thereby achieving precise dosing and ensuring effective degumming while avoiding agent waste and abnormal increases in dissolved organic matter. The effluent from the degumming reaction tank 21 flows by gravity into the first coagulation reaction tank 22, where flocculants are added and coagulation occurs through stirring, forming easily separable micro-flocs. The effluent then flows by gravity into the dissolved air flotation device.

[0067] The dissolved air flotation (DAF) unit employs an effluent recirculation dissolved air process with a recirculation ratio of no less than 50% and a dissolved air pressure of no less than 0.5 MPa. This generates a large number of microbubbles, efficiently capturing and removing suspended solids, colloids, and some organic matter from the water. After treatment by this unit, the effluent viscosity is reduced to below 2 mPa·s, the oil content to below 50 mg / L, and the suspended solids content to below 30 mg / L.

[0068] It should be noted that suspended solids mainly include rock fragments, clay particles, and insoluble precipitates in the backflow fluid; colloids mainly include emulsified oil droplets, residues of polymeric thickeners, and micelles formed by surfactants; and some organic matter mainly refers to particulate organic matter that has been transformed into a non-dissolved state through the breaking of the colloid and can be separated by air flotation.

[0069] Specifically, the dissolved air flotation device includes a vertical flow flotation tank 23, a dissolved air pump 24, and a dissolved air water tank 25. The inlet of the dissolved air pump 24 is connected to the clear water zone of the vertical flow flotation tank 23 to extract a portion of the effluent. The outlet of the dissolved air pump 24 is connected to the dissolved air water tank 25, pressurizing water and sending it into the tank to fully contact with compressed air, forming dissolved air water. The outlet of the dissolved air water tank 25 is connected to the inlet zone of the vertical flow flotation tank 23 through a release device to form an effluent return dissolved air structure. The effluent from the dissolved air water tank 25 flows back to the reaction zone at the front end of the vertical flow flotation tank 23, where it is depressurized and releases a large number of microbubbles, thereby completing the flotation separation process.

[0070] The dissolved air flotation device also includes a flotation tank 26 and a flotation effluent pump 27. The effluent treated by the vertical flow flotation tank 23 flows by gravity into the flotation tank 26 for buffering and water quality equalization, and is then lifted by the flotation effluent pump 27 and transported to the downstream direct filtration membrane unit.

[0071] like Figure 1 As shown, the direct filtration membrane unit includes a primary dealkali reaction tank 31, a secondary dealkali reaction tank 32, a second coagulation reaction tank 33, a sludge balance tank 34, a direct filtration membrane module 36, and a direct filtration permeate tank 37, which are connected sequentially along the treatment process.

[0072] The primary dealkali removal reactor 31 is equipped with a first lime dosing device and a first online pH sensor. The first lime dosing device is connected to the first online pH sensor and is used to add lime into the primary dealkali removal reactor 31 according to the detection signal of the first online pH sensor to remove the bicarbonate alkalinity of the liquid to be treated, and to control the dosage to maintain the pH value of the reaction system between 8.0 and 9.0. Under these conditions, the following chemical reactions mainly occur:

[0073] Ca(HCO3)2+Ca(OH)2=2CaCO3↓+2H2O

[0074] The core purpose of this step is to preemptively eliminate bicarbonate ions that will decompose upon heating in the subsequent evaporation and crystallization unit and may cause scaling and non-condensable gas problems, rather than to perform deep softening.

[0075] The secondary alkali removal reactor 32 is equipped with a second lime dosing device and a second online pH sensor. The second lime dosing device is connected to the second online pH sensor and is used to continue adding lime into the secondary alkali removal reactor 32 according to the detection signal of the second online pH sensor, controlling the dosing amount to maintain the pH value of the reaction system between 9.0 and 10.0. Under these conditions, the following chemical reactions mainly occur:

[0076] Mg(HCO3)2+2Ca(OH)2=Mg(OH)2↓+2CaCO3↓+2H2O

[0077] By adding lime in stages, the in-situ generated magnesium hydroxide precipitate serves as a highly efficient adsorbent for magnesium-based silica removal, thereby reducing the silica content in the effluent to below 50 mg / L. Staged control increases the proportion of active magnesium hydroxide in the secondary dealkali removal reactor 32, enhancing the silica removal effect.

[0078] Preferably, the secondary dealkali removal reactor 32 is also equipped with a silicon removal agent dosing device. An online silicon content monitor is installed on the inlet pipe of the secondary dealkali removal reactor 32, and the silicon removal agent dosing device is connected to the online silicon content monitor. When the online silicon content monitor detects that the silicon content in the inlet water is greater than 50 mg / L, the silicon removal agent dosing device is activated to add silicon removal agent to assist in silicon removal; when the silicon content in the inlet water is detected to be less than 50 mg / L, the silicon removal agent dosing device is shut off, and silicon removal is carried out solely by magnesium hydroxide generated from lime, thereby optimizing reagent costs.

[0079] The effluent, after two-stage dealkali treatment, flows sequentially into the second coagulation reactor 33 and the sludge balance tank 34. In the second coagulation reactor 33, coagulant is added and stirred to further flocculate the formed precipitates and colloids; the sludge balance tank 34 buffers water volume fluctuations and promotes solid-liquid separation. Subsequently, the effluent is pumped by the direct filter feed pump 35 and sent to the direct filter membrane module 36 for filtration.

[0080] Specifically, the direct filtration membrane module 36 uses a hybrid membrane composed of inorganic materials and PTFE (polytetrafluoroethylene). Its filtration precision falls within the ultrafiltration category, with a pore size range of 0.01-0.5µm. This membrane features high strength, fouling resistance, and high flux, enabling efficient solid-liquid separation of pretreated wastewater. After treatment by this unit, the oil content of the effluent is reduced to below 2mg / L, the turbidity to below 1NTU, and the total removal rate of organic matter (calculated as COD) (compared to the raw water) can reach over 50%.

[0081] It should be noted that the residual impurities removed by the direct membrane filtration unit are hardness ions, silicates, and fine suspended matter. The residual hardness ions mainly include calcium and magnesium ions remaining after the two-stage dealkalization process; silicates refer to soluble silicates and colloidal silica; and fine suspended matter refers to fine particles and colloidal substances with a particle size between 0.01 and 0.5 µm.

[0082] Preferably, the direct filtration membrane unit further includes a direct filtration backwash pump 38. The inlet of the direct filtration backwash pump 38 is connected to the outlet of the direct filtration product water tank 37, and the outlet of the direct filtration backwash pump 38 is connected to the backwash inlet of the direct filtration membrane module 36.

[0083] Specifically, the direct filtration membrane module 36 adopts a fully automatic operation mode, including filtration cycle and backwash cycle. During the filtration cycle, the direct filtration feed water pump 35 sends wastewater into the direct filtration membrane module 36 for solid-liquid separation; when the set filtration time is reached or the transmembrane pressure difference reaches the set threshold, the system automatically switches to the backwash cycle. At this time, the direct filtration backwash pump 38 draws purified product water from the direct filtration product water tank 37, pressurizes it, and sends it back into the product water side of the direct filtration membrane module 36 to hydraulically flush and clean the contaminants attached to the membrane channels and membrane surface, thereby restoring membrane flux and extending membrane service life.

[0084] More preferably, the direct filter membrane filtration unit is also equipped with a compressed air system, which is connected to the air inlet of the direct filter membrane assembly 36 for periodic air wiping auxiliary cleaning, thereby further improving the backwashing effect.

[0085] To meet the feed water requirements of the subsequent membrane treatment unit, sulfuric acid is added to the pipeline after the permeate from the direct filtration membrane module 36 to adjust the pH of the effluent to the range of 7-8, and then it enters the direct filtration permeate tank 37 for temporary storage. Finally, the effluent from the direct filtration permeate tank 37 is transported to the subsequent reverse osmosis membrane concentration unit by the direct filtration permeate pump 39.

[0086] The reverse osmosis membrane concentration unit adopts an operating mode designed with controlling the salt concentration on the concentrate side as its core. Specifically, the unit includes a concentrate circulation mechanism, which comprises a variable frequency high-pressure pump, a TDS online monitoring instrument, and a controller. The TDS online monitoring instrument is located on the concentrate pipeline of the unit and is used to monitor the TDS concentration on the concentrate side in real time. The instrument is connected to the input of the controller, and the output of the controller is connected to the variable frequency high-pressure pump. The controller adjusts the operating frequency of the high-pressure pump based on the TDS signal from the online monitoring instrument to stably control the total dissolved solids (TDS) concentration of the concentrate within the target range of 75,000 mg / L to 100,000 mg / L, rather than pursuing a fixed recovery rate.

[0087] Specifically, such as Figure 1 As shown, the unit also includes an RO security filter 41, an RO high-pressure pump 42, an RO membrane module 43, an RO permeate tank 44, an RO permeate discharge pump 45, an RO concentrate tank 46, and an RO concentrate pump 47 arranged sequentially along the process flow. The treatment process is as follows: After the incoming water passes through the RO security filter 41 to remove residual particles, it is mixed with part of the concentrate from the RO membrane module 43 and then pressurized by the variable frequency RO high-pressure pump 42 and sent into the RO membrane module 43 for concentration and separation. The generated RO permeate enters the RO permeate tank 44 for temporary storage and is eventually discharged or reused by the RO permeate discharge pump 45; while most of the RO concentrate is recycled to maintain circulation, and a small portion enters the RO concentrate tank 46 for buffering, and is then stably transported to the downstream MVR evaporation and crystallization unit by the RO concentrate pump 47.

[0088] Preferably, the RO membrane module 43 is a spiral wound reverse osmosis (STRO) membrane element with a pressure resistance rating of not less than 90 bar. This selection provides a reliable equipment foundation for implementing the above-mentioned high-concentration concentration operation strategy, ensuring that the system can operate stably under high operating pressure, thereby achieving precise control of the concentrate TDS within the preset high concentration range.

[0089] The MVR evaporation and crystallization unit includes an MVR device 51, a mother liquor storage tank 52, and a mother liquor delivery pump 53. The MVR device 51 includes a steam-water separator, a vacuum device connected to the steam outlet of the steam-water separator, and a condensate pipeline connected to the condensate outlet of the steam-water separator. Specifically, the vacuum device is connected to the top steam outlet of the steam-water separator via a vacuum pipeline, which is equipped with a pressure regulating valve. The steam-water separator and the vacuum device work together to solve the problems of high energy consumption and easy scaling in the evaporation of high-salt, high-hardness wastewater.

[0090] A sodium sulfate dosing device is installed on the steam-water separator to add sodium sulfate (Na2SO4) to the evaporating raw liquid. This alters the composition and crystallization sequence of the salts in the solution, converting calcium chloride (CaCl2) and magnesium chloride (MgCl2), which cause high boiling point elevation and are prone to scaling, into more easily precipitated calcium sulfate (CaSO4) and magnesium sulfate (MgSO4) crystals. This lowers the boiling point elevation of the concentrate (by 10℃-15℃ in practical engineering), significantly reducing the temperature rise requirements of the steam compressor. This makes the MVR process feasible for treating high-hardness wastewater and effectively reduces operating energy consumption.

[0091] Specifically, after adding sodium sulfate, sulfate ions (SO4) 2- ) and calcium ions (Ca) in the concentrate 2+ ), magnesium ions (Mg 2+ The reaction occurs, producing calcium sulfate (CaSO4) and magnesium sulfate (MgSO4) crystals that precipitate out. Sodium ions (Na+) then react with these crystals. + Then, it reacts with the chloride ions (Cl) already present in the concentrate. - The sodium ions combine to form sodium chloride (NaCl), which remains in the solution. As evaporation proceeds, the sodium chloride concentration gradually increases and reaches saturation, eventually precipitating out as crystals or existing in a dissolved state in a small amount of the mother liquor. Sodium ions are ultimately discharged from the system as sodium chloride crystals or dissolved salts in the mother liquor. Because the solubility product of calcium sulfate and magnesium sulfate is much lower than that of sodium chloride, they preferentially reach saturation and crystallize out during evaporation, while sodium chloride crystallizes after further concentration. Furthermore, the molar amount of sodium sulfate added is not less than the total molar amount of calcium and magnesium ions in the concentrate entering the unit. A slight excess is allowed in actual operation, and the final crystallized product is a general industrial mixed salt, which is easy to dispose of.

[0092] Preferably, the sodium sulfate can be a byproduct generated during the zero-discharge treatment of other industrial wastewater, such as sodium sulfate byproducts from coal mine water desalination and sodium sulfate byproducts from the evaporation and crystallization of desulfurization wastewater from thermal power plants. These byproduct sodium sulfates are typically treated as solid waste, resulting in low costs. Furthermore, this invention does not require high purity sodium sulfate and can be used directly without additional purification, thereby significantly reducing reagent costs.

[0093] The vacuum device is used to control the steam pressure in the steam-water separator to not exceed 20 kPa (corresponding to a saturation temperature of approximately 60°C), achieving low-temperature evaporation. The low-temperature conditions bring dual benefits: firstly, they significantly reduce the transfer of volatile and semi-volatile organic compounds into the vapor phase, thereby significantly reducing the COD index of the evaporation condensate and alleviating the processing load and operating costs of subsequent advanced oxidation units; secondly, the low temperature itself also helps to lower the boiling point rise of the solution, synergistically with the effect of adding sodium sulfate, further optimizing the compressor's operating energy consumption.

[0094] Preferably, the MVR evaporation crystallization unit operates using a seed crystal method. Therefore, a calcium sulfate seed crystal addition device is installed on the steam-water separator to add calcium sulfate seed crystals to the evaporation stock solution during system startup or as needed. The purpose of adding seed crystals is to provide sufficient crystallization nuclei, guiding sparingly soluble salts such as calcium sulfate and magnesium sulfate to preferentially adhere to the seed crystal surface and grow, rather than forming hard scale on the surfaces of heat exchangers and other equipment. This ensures long-term, stable, and efficient system operation. Once the system stabilizes, it can maintain circulation using its own generated crystals, eliminating the need for continuous addition.

[0095] Specifically, the mother liquor generated by the MVR evaporation and crystallization unit is discharged into the mother liquor storage tank 52 through pipeline for buffering. The outlet of the mother liquor storage tank 52 is connected to the inlet of the mother liquor delivery pump 53. The mother liquor delivery pump 53 is used to discharge the buffered mother liquor on a timed or quantitative basis to the subsequent mother liquor drying treatment unit or for off-site disposal, so as to avoid the accumulation of salt and organic matter in the evaporation system and ensure the long-term stable operation of the evaporation and crystallization unit.

[0096] The steam generated by the MVR evaporation and crystallization unit is condensed to form condensate. To recover energy, the condensate first exchanges heat with the system's original feed water through a heat exchanger. The cooled condensate is then sent to the electrochemical oxidation unit for further treatment through the condensate pipeline.

[0097] Because the evaporation process may carry trace amounts of volatile and semi-volatile organic compounds, the COD and ammonia nitrogen levels in the condensate may still exceed stringent emission standards. Therefore, the system includes a dedicated electrochemical oxidation unit as a final guarantee for water quality. Specifically, the electrochemical oxidation unit includes an electrochemical catalytic oxidation device 61, which utilizes the generated highly reactive free radicals to perform advanced oxidative decomposition of residual recalcitrant organic matter and ammonia nitrogen in the condensate, thereby completely removing these pollutants.

[0098] Preferably, the electrochemical oxidation unit further includes an electrochemical circulating water tank 62, an electrochemical circulating pump 63, and a condensate discharge pump 64. Specifically, the condensate treated by the electrochemical catalytic oxidation device 61 enters the electrochemical circulating water tank 62 for buffering and homogenization. The outlet of the electrochemical circulating water tank 62 is connected to the inlet of the electrochemical circulating pump 63, and the outlet of the electrochemical circulating pump 63 is connected to the inlet of the electrochemical catalytic oxidation device 61, for continuously pumping the condensate into the electrochemical catalytic oxidation device 61 for advanced oxidation treatment. The treated effluent is returned to the electrochemical circulating water tank 62, forming a circulating treatment mode. The inlet of the condensate discharge pump 64 is connected to the outlet of the electrochemical circulating water tank 62, for discharging the treated condensate to the discharge port.

[0099] After treatment by this unit, the final produced water can stably meet the following key indicators: TDS≤1000mg / L, COD≤40mg / L, BOD≤10mg / L, and ammonia nitrogen (as N)≤5mg / L, satisfying the high-standard water quality requirements for discharge or reuse. It meets the Class I standard of the "Integrated Wastewater Discharge Standard" (GB 8978) and the relevant requirements of the "Water Quality Standard for Urban Reclaimed Wastewater for Miscellaneous Uses" (GB / T18920-2020), and can be directly discharged or reused in oilfield production, urban miscellaneous uses, and other fields.

[0100] Preferably, the treatment system further includes a sludge collection and discharge unit. The sludge collection and discharge unit is connected and communicates with the sludge discharge ports of the regulating sedimentation unit, the dissolved air flotation unit, and the direct membrane filtration unit, and is used to collect and discharge the sludge and waste generated by each unit.

[0101] Specifically, the sludge collection and discharge unit is connected to the sludge / slag discharge ports of the following devices: the regulating sedimentation tank 11 of the regulating sedimentation unit; the vertical flow flotation tank 23 of the dissolved air flotation unit; and the primary alkali removal reaction tank 31, the secondary alkali removal reaction tank 32, the secondary coagulation reaction tank 33, and the sludge balance tank 34 of the direct filtration membrane unit. Through the above connections, the sludge collection and discharge unit centrally collects and discharges the heavy sedimentation sludge, flotation scum, chemically softened precipitates, and pre-membrane buffer sludge generated during system operation, preventing the accumulation of solid waste within the system and ensuring the continuous and stable operation of each unit.

[0102] It should be noted that the de-gelling agents, coagulants, flocculants and desiliconizing agents added in this system can all be commercially available agents in the water treatment field. Their specific types and dosages can be determined through a limited number of routine tests based on the quality of the influent water, and are not the key limitations of this invention.

[0103] The method for treating fracturing flowback fluid using the above system is as follows:

[0104] (1) Adjustment and sedimentation treatment: The fracturing flowback fluid is introduced into the adjustment and sedimentation unit. The water quality and quantity of the incoming water are balanced and buffered by the batch operation of the adjustment sedimentation tank 11 (hydraulic residence time ≥ 3h), and the initial sedimentation and separation of heavy suspended solids are achieved by utilizing its conical structure and gravity sludge discharge function.

[0105] (2) De-gelling and flotation treatment: After the effluent from step (1) is lifted, coagulant is added first, and then it enters the de-gelling reaction tank 21 of the dissolved air flotation unit. De-gelling agent is added and stirred to react, reducing the liquid viscosity to below 2 mPa·s. Then the effluent enters the first coagulation reaction tank 22 to add flocculant to form micro flocs, and finally flows by gravity into the dissolved air flotation device to efficiently remove oil, colloids and suspended solids, so that the oil content of the effluent is <50mg / L and the suspended solids are <30mg / L.

[0106] (3) Staged dealkali and silica removal and direct membrane filtration: The effluent from step (2) is pumped sequentially into the primary dealkali reaction tank 31 and the secondary dealkali reaction tank 32 of the direct membrane filtration unit. Lime is added to the primary dealkali reaction tank 31 and the pH of the reaction system is controlled at 8.0-9.0 to remove bicarbonate alkalinity. Lime is added again to the secondary dealkali reaction tank 32 and the pH of the reaction system is controlled at 9.0-10.0 to generate magnesium hydroxide precipitate for adsorption and silica removal. Silica removal agent is selectively added according to the silica content of the influent (>50mg / L). Subsequently, after coagulation and sludge equilibration, the effluent is pumped into the direct membrane module 36 for filtration. The effluent oil content is <2mg / L, turbidity is <1NTU, and the total organic matter removal rate of the system reaches more than 50%. After filtration, acid is added to adjust the pH of the effluent to 7-8.

[0107] (4) Reverse osmosis membrane pre-concentration: The effluent from step (3) is pumped into the reverse osmosis membrane concentration unit. A concentrate circulation operation mode is adopted, and the TDS concentration on the concentration side is precisely controlled within the range of 75,000 mg / L to 100,000 mg / L by a variable frequency high-pressure pump. The generated RO permeate can be discharged or reused, while the concentrate (RO concentrate) is buffered and pumped to subsequent units.

[0108] (5) MVR Evaporation and Crystallization: The concentrate from step (4) is introduced into the MVR evaporation and crystallization unit. Sodium sulfate is added to the steam-water separator, with a molar addition not less than the total molar amount of calcium and magnesium ions in the feed liquid, so that calcium chloride and magnesium chloride are converted into calcium sulfate and magnesium sulfate crystals, thereby reducing the boiling point rise. At the same time, the system pressure is controlled at ≤20kPa (corresponding to an evaporation temperature of about 60℃) by a vacuum device for low-temperature evaporation. The system adopts a seed crystal method for forced circulation. Calcium sulfate seed crystals are added at startup, and the solid content of the circulating slurry is maintained at more than 10% to inhibit scaling and stabilize the crystallization process.

[0109] (6) Deep oxidation of condensate and final effluent: After heat recovery by exchanging heat between the evaporated condensate generated in step (5) and the raw water, it is introduced into the electrochemical oxidation unit. Electrochemical catalytic oxidation technology is used to deeply degrade the residual trace organic matter and ammonia nitrogen in the condensate, ensuring that the final effluent meets the following core indicators: TDS≤1000 mg / L, COD≤40 mg / L, BOD≤10 mg / L, ammonia nitrogen (as N)≤5 mg / L, so as to achieve standard discharge or resource reuse.

[0110] Application Example 1

[0111] The fracturing flowback fluid treatment system and method of Example 1 were used to treat the guar gum fracturing flowback fluid generated from an oil well in Jingbian to verify the technical effect of the present invention.

[0112] Test scale: In this application example, the treatment system is designed to have a processing capacity of 0.25 t / h (based on MVR evaporation rate) and to operate continuously and stably for 72 hours.

[0113] Water quality treated: The fracturing flowback fluid treated in this application example is guar gum fracturing flowback fluid from an oil well in Jingbian. This flowback fluid has typical characteristics of high viscosity, high COD, high TDS, and high hardness. For specific raw water quality indicators, please refer to Table 1.

[0114] Processing Results: After system processing in Example 1, the key indicators of the final product water quality are shown in Table 1. For ease of comparison, Table 1 also lists the test data of the raw water quality and the final product water quality.

[0115] Table 1

[0116]

[0117] Application Example 2

[0118] The fracturing flowback fluid treatment system and method of Example 1 were used to treat the high-elasticity fracturing flowback fluid generated from a gas well in Jingbian to verify the technical effect of the present invention.

[0119] Test scale: In this application example, the treatment system is designed to have a processing capacity of 0.25 t / h (based on MVR evaporation rate) and to operate continuously and stably for 72 hours.

[0120] Water quality treated: The fracturing flowback fluid treated in this application example is a high-elasticity fracturing flowback fluid from a gas well in Jingbian. For specific raw water quality indicators, please refer to Table 2.

[0121] Processing Results: After system processing in Example 1, the key indicators of the final product water quality are shown in Table 2. For ease of comparison, Table 2 also lists the test data of the raw water quality and the final product water quality.

[0122] Table 2

[0123]

[0124] Application Example 3

[0125] The fracturing flowback fluid treatment system and method of Example 1 were used to treat the slickwater fracturing flowback fluid generated in a gas well in Dingbian County to verify the technical effect of the present invention.

[0126] Test scale: In this application example, the treatment system is designed to have a processing capacity of 0.25 t / h (based on MVR evaporation rate) and to operate continuously and stably for 72 hours.

[0127] Water quality treated: The fracturing flowback fluid treated in this application example is the slickwater fracturing flowback fluid from a gas well in Dingbian. For specific raw water quality indicators, please refer to Table 2.

[0128] Processing Results: After system processing in Example 1, the key indicators of the final product water quality are shown in Table 3. For ease of comparison, Table 3 also lists the test data of the raw water quality and the final product water quality.

[0129] Table 3

[0130]

[0131] Based on the test data in Tables 1 to 3, the following analytical conclusions can be drawn:

[0132] The fracturing flowback fluid treatment system and method provided by this invention have demonstrated stable and efficient treatment performance in three typical application scenarios with significant differences in water quality: guar gum fracturing fluid (high COD, high TDS, high hardness) for Jingbian oil wells, high-elasticity fracturing fluid (medium TDS, low COD, high iron) for Jingbian gas wells, and slickwater fracturing fluid (extremely high TDS, extremely high hardness, extremely high barium) for Dingbian gas wells.

[0133] The final product water of Application Examples 1, 2 and 3 all had COD ≤35 mg / L, ammonia nitrogen ≤2.6 mg / L and TDS ≤286 mg / L, which are all better than the design indicators (COD ≤40 mg / L, ammonia nitrogen ≤5 mg / L, TDS ≤1000 mg / L), meeting the high standard requirements for discharge or reuse.

[0134] The total COD removal rate reaches 93.9%-99.6%; TDS removal rate ≥99.6%; calcium and magnesium hardness removal rate ≥99.99%; oil and suspended solids removal rate ≥99%; characteristic pollutants (barium, strontium, iron, manganese, fluorides, etc.) are removed simultaneously and efficiently.

[0135] Facing a TDS span of 2.78×10 4 -1.97×10 5 mg / L, COD range 427-7910 mg / L, hardness range 6.21×10 3 -3.62×10 4 Despite extreme water quality fluctuations of mg / L, the system remained scale-free, pollution-free, and without downtime throughout the entire process, with stable operating parameters for each unit. This demonstrates the strong adaptability, high stability, and economic advantages of this invention for complex and variable backflow solutions.

[0136] In summary, this invention solves the three major problems of poor process adaptability, stringent pretreatment requirements, and high operating costs of existing technologies, and provides a reliable technical solution for the deep treatment and resource reuse of fracturing flowback fluid in oil and gas fields.

[0137] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fracturing flowback fluid treatment system, characterized in that, It includes a conditioning and sedimentation unit, a dissolved air flotation unit, a direct filtration membrane unit, a reverse osmosis membrane concentration unit, an MVR evaporation and crystallization unit, and an electrochemical oxidation unit connected sequentially along the processing flow. The MVR evaporation crystallization unit includes a steam-water separator, a vacuum device connected to the steam outlet of the steam-water separator, and a condensate pipeline connected to the condensate outlet of the steam-water separator. The steam-water separator is equipped with a sodium sulfate dosing device for adding sodium sulfate to the evaporating liquid inside the steam-water separator. The vacuum device is used to control the steam pressure inside the steam-water separator, and the condensate outlet is used to discharge condensate. The water inlet of the electrochemical oxidation unit is connected to the condensate pipeline of the MVR evaporation and crystallization unit.

2. The fracturing flowback fluid treatment system as described in claim 1, characterized in that: The direct filtration membrane unit includes a primary dealkali reaction tank (31), a secondary dealkali reaction tank (32), and a direct filtration membrane assembly (36) connected in sequence. The primary dealkali reaction tank (31) is equipped with a first lime dosing device for adding lime to remove the bicarbonate alkalinity of the liquid to be treated in the primary dealkali reaction tank (31); The secondary dealkali reaction tank (32) is equipped with a second lime addition device, which is used to continue adding lime so that the liquid to be treated in the secondary dealkali reaction tank (32) generates magnesium hydroxide precipitate and removes silicon. The filter membrane used in the direct filtration membrane assembly (36) is a hybrid membrane composed of inorganic materials and PTFE, and the filtration pore size of the hybrid membrane is 0.01-0.5µm.

3. The fracturing flowback fluid treatment system as described in claim 2, characterized in that: The secondary dealkali reaction tank (32) is also equipped with a silicon removal agent dosing device. An online silicon content monitor is installed on the water inlet pipe of the secondary dealkali reaction tank (32). The silicon removal agent dosing device is connected to the online silicon content monitor. When the online silicon content monitor detects that the silicon content in the influent is greater than 50 mg / L, the silicon removal agent dosing device is activated to add silicon removal agent; when the silicon content in the influent is detected to be less than 50 mg / L, the silicon removal agent dosing device is turned off.

4. The fracturing flowback fluid treatment system as described in claim 1, characterized in that: The steam-water separator is also equipped with a calcium sulfate dosing device; Used to add calcium sulfate to the evaporating liquid in the steam-water separator.

5. The fracturing flowback fluid treatment system as described in claim 1, characterized in that: The reverse osmosis membrane concentration unit is equipped with a concentrate circulation mechanism, which includes a variable frequency high-pressure pump, a TDS online monitoring instrument, and a controller. The TDS online monitoring instrument is installed on the concentrate pipeline of the reverse osmosis membrane concentration unit to detect the TDS concentration on the concentrate side in real time. The TDS online monitoring instrument is connected to the input terminal of the controller, and the output terminal of the controller is connected to the variable frequency high-pressure pump. The controller can adjust the operating frequency of the variable frequency high-pressure pump according to the detection signal of the TDS online monitor, so as to control the TDS concentration on the concentrate side within the range of 75,000 mg / L to 100,000 mg / L. The RO membrane module (43) of the reverse osmosis membrane concentration unit is a spiral wound reverse osmosis membrane module with a pressure resistance of ≥90 bar.

6. The fracturing flowback fluid treatment system as described in claim 1, characterized in that: The dissolved air flotation unit includes a gel breaking reaction tank (21) and a dissolved air flotation device connected in sequence; The debonding reaction tank (21) is equipped with a debonding agent dosing device for adding debonding agent to reduce the viscosity of the fracturing flowback fluid. The debonding agent dosing device can adjust the amount of debonding agent added according to the viscosity of the effluent and / or the concentration of dissolved organic matter. The dissolved air flotation device includes a flotation tank, a dissolved air pump (24), and a dissolved air water tank (25). The inlet of the dissolved air pump (24) is connected to the clear water area of ​​the flotation tank, the outlet of the dissolved air pump (24) is connected to the inlet of the dissolved air water tank (25), and the outlet of the dissolved air water tank (25) is connected to the inlet area of ​​the flotation tank through a release device to form a water return dissolved air path. The dissolved air flotation device has a reflux ratio of ≥50% and a dissolved air pressure of ≥0.5MPa.

7. The fracturing flowback fluid treatment system as described in claim 1, characterized in that: The electrochemical oxidation unit includes an electrochemical catalytic oxidation device (61) for advanced oxidation treatment of the condensate from the MVR evaporation and crystallization unit to remove organic matter and ammonia nitrogen from the condensate.

8. The fracturing flowback fluid treatment system as described in claim 1, characterized in that: It also includes a sludge collection and discharge unit; The sludge collection and discharge unit is connected to the sludge discharge port of the regulating sedimentation unit, the dissolved air flotation unit, and the direct filtration membrane unit, and is used to collect and discharge the sludge and waste generated by each unit.

9. A method for treating fracturing flowback fluid, characterized in that, The processing system according to any one of claims 1-8 includes the following steps: S1. The fracturing flowback fluid is introduced into the regulating sedimentation unit for water quality and quantity homogenization and preliminary sedimentation treatment. S2. The effluent from S1 is fed into the dissolved air flotation unit for debinding and flotation separation to remove suspended solids, colloids and some organic matter. S3. The effluent from S2 is passed into the direct filtration membrane unit for dealkali removal, desilicon removal, and membrane filtration to remove residual impurities. S4. The effluent from S3 is passed into the reverse osmosis membrane concentration unit for concentration to obtain concentrated liquid and permeate. S5. The concentrated solution obtained in S4 is passed into the MVR evaporation and crystallization unit. Sodium sulfate is added during the evaporation process, and the evaporation temperature is controlled to be no higher than 60°C. Solid salt and condensate are obtained through evaporation and crystallization. S6. The condensate produced by the MVR evaporation and crystallization unit in S5 is passed into the electrochemical oxidation unit for deep oxidation treatment to obtain product water.

10. The method for treating fracturing flowback fluid as described in claim 9, characterized in that: In step S3, the dealkali and desiliconization treatment includes passing the effluent from S2 into a primary dealkali reaction tank, adding lime and controlling the pH of the reaction system to 8.0-9.0 to remove bicarbonate alkalinity; then passing the effluent into a secondary dealkali reaction tank, adding lime again and controlling the pH of the reaction system to 9.0-10.0, using the generated magnesium hydroxide precipitate to remove silicon, and selectively adding a desiliconizing agent according to the silicon content of the influent to reduce the silicon content of the effluent to below 50 mg / L; In step S4, the TDS concentration of the concentrate is controlled within the range of 75,000 mg / L to 100,000 mg / L by controlling the concentrate circulation and variable frequency high-pressure pump of the reverse osmosis membrane concentration unit. In step S5, the molar amount of sodium sulfate added is not less than the total molar amount of calcium and magnesium ions in the concentrate entering the MVR evaporation and crystallization unit.

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