A water treatment method for re-injection of low-permeability oilfields after treatment of oilfield wastewater containing polymer

Abstract

The application discloses a water treatment method for re-injection of low-permeability oilfields after treatment of poly-containing oilfield sewage. The method comprises the following steps: on the basis of adjusting water quantity and homogenizing water quality, an oxidizing agent is added into the poly-containing oilfield sewage to perform a pre-oxidation reaction to obtain pre-oxidized water; a modified flocculating agent and a coagulant aid are added to perform a sedimentation reaction to obtain water from which oil and suspensions are removed; the water is introduced into a gas floatation device, a demulsifying agent is added to obtain gas floatation effluent; inorganic high-molecular flocculating agents and cationic polyacrylamide are added, and magnetic media induced by a permanent magnet are simultaneously introduced to perform a sedimentation to obtain magnetically coagulated water; then, multi-stage filtration is performed to obtain multi-stage filtered water; and then, ultrafiltration is performed to reach the re-injection standard of offshore oilfields, so that the poly-containing oilfield sewage is used to drive oil in low-permeability oilfields. The application realizes deep removal of SS and oil and improves the resource utilization rate of the poly-containing oilfield sewage.

Description

Technical Field

[0001] This invention belongs to the field of polymer-containing water treatment technology in low-permeability oilfields, and relates to a water treatment method for reinjecting treated wastewater from polymer-containing oilfields into low-permeability oilfields. Background Technology

[0002] Offshore oilfield development involves drilling fluid drainage and fracturing flowback fluid. Some of these drainages contain polymers with a concentration reaching 1500 mg / L. With the continuous development of new wells, these drainages mix with production water and enter the oilfield wastewater treatment system. The resulting mixed water can contain up to 140 mg / L of polymers. Offshore low-permeability oilfields have strict requirements for water injection indicators, requiring oil content not to exceed 6 mg / L, suspended solids not to exceed 2 mg / L, and a median particle size not to exceed 1.5 μm. Suspended solids readily combine with polymers, increasing the viscosity of production water with increasing polymer content. This increased viscosity increases the stability of colloidal particles in the water, interfering with flocculation and sedimentation. Simultaneously, polymers may adsorb onto the surface of oil droplets, causing them to disperse as tiny particles in the water through electrostatic repulsion, making oil-water separation difficult and increasing the complexity of water treatment.

[0003] The drawbacks of conventional treatment include: 1) Polymers are difficult to degrade, and oily wastewater has high viscosity, which interferes with the flocculation and sedimentation effect; 2) Emulsified oil is difficult to separate from water; 3) The pretreatment is substandard, making it difficult to meet the influent water quality requirements of fine filtration devices, and the ultrafiltration membrane system is easily blocked or even broken; 4) The concentration and particle size of suspended solids in the effluent are difficult to control, which can easily block the pores of low-permeability reservoirs. Summary of the Invention

[0004] To address the problems of high viscosity, complex composition, difficulty in oil-water separation, and stable colloids in the current wastewater from polymer-containing oilfields, which makes it difficult to meet standards and reinject into low-permeability oilfields, the present invention aims to provide a water treatment method for treating wastewater from polymer-containing oilfields and then reinjecting it into low-permeability oilfields.

[0005] This invention aims to address the problems of polymer residues, difficulty in removing suspended solids, and unstable oil-water emulsions in polymer-containing oilfield wastewater, which can easily clog reservoir pores and reduce oil displacement efficiency during reinjection into low-permeability oilfields. The technology employs a treatment process involving "pre-oxidation - oil removal tank - pressurized dissolved air flotation - magnetic coagulation - walnut shell filter - multi-media filter - security filter - ultrafiltration membrane filtration" to achieve deep removal of suspended solids (SS), oil, and polymers, thereby improving the resource utilization rate of polymer-containing oilfield wastewater. The treated water will have an oil content ≤6 mg / L, SS ≤2 mg / L, and median particle size ≤1.5 μm.

[0006] The present invention provides a water treatment method for reinjecting polymer-containing oilfield wastewater into a low-permeability oilfield after treatment, comprising the following steps: 1) On the basis of adjusting the water volume and homogenizing the water quality, an oxidant is added to the polymer-containing oilfield wastewater to carry out a pre-oxidation reaction to obtain pre-oxidized water; 2) Add modified flocculant and coagulant aid to the pre-oxidized water and carry out sedimentation reaction to obtain water with oil and suspension removed; 3) The de-oiled and desuspended water is introduced into the air flotation device, and a demulsifier is added to reduce the oil content and suspended solids content in the water to obtain air flotation effluent; 4) Add inorganic polymer flocculant and cationic polyacrylamide to the air-flotated water, and simultaneously introduce a magnetic medium induced by a permanent magnet to form high-density magnetic flocs, which are then settled to obtain water treated by magnetic coagulation. 5) The water treated by magnetic coagulation is subjected to multi-stage filtration to obtain multi-stage filtered water; The multi-stage filtration process is as follows: oil content in the water is removed by a walnut shell filter, and then suspended solids are further removed by a multi-media filter; 6) The water after multi-stage filtration is filtered through ultrafiltration to meet the reinjection standards of offshore oil fields, so as to reinject oil into low-permeability oil fields.

[0007] In the above method, step 1) further includes the step of separating the polymer-containing oilfield wastewater through a three-phase separator before adding the oxidant, and then the water separated by the three-phase separator enters the regulating buffer tank. In step 1), the oxidant is selected from at least one of Fenton's reagent, Fenton-like reagent, persulfate, and ozone; specifically, it can be hydrogen peroxide (H2O2) + ferrous sulfate (FeSO4), or hydrogen peroxide (H2O2) + Fe... 3+ / Cu 2+ Na2S2O8, (NH4)2S2O8, or ozone.

[0008] In this invention, in step 1), the pre-oxidation reaction destroys the long-chain structure of the polymer and decomposes some organic matter, thereby degrading the macromolecular polymer into small molecules and reducing the viscosity of the wastewater.

[0009] In the above method, in step 2), the sedimentation reaction time can be 6 to 9 hours, specifically 6 hours, to remove polymers, suspended solids and floating oil.

[0010] In the above method, the sedimentation reaction in step 2) is carried out in an oil removal tank, which is a natural oil removal tank and / or a flocculation oil removal tank.

[0011] In the above method, in step 2), the modified flocculant is polyaluminum ferric silicate; The coagulant is polyacrylamide.

[0012] In the above method, the air flotation device is selected from pressurized dissolved air flotation and / or vortex air flotation device to further reduce the oil content and suspended solids content.

[0013] In this invention, the dissolved air pressure used in the air flotation device can specifically be 0.4 MPa.

[0014] In the above method, in step 4), the inorganic polymeric flocculant includes polyaluminum chloride; The magnetic medium includes magnetite powder, the composition of which is Fe3O4.

[0015] In the above method, in step 4), the settling time can be 15-30 minutes to remove suspended solids from the water.

[0016] In step 5) of this invention, the walnut shell filter: utilizes the adsorption properties of walnut shells to deeply remove oil, controlling the oil content to ≤8mg / L; The multi-media filter uses quartz sand, corundum, and anthracite as filter media to intercept suspended solids with a particle size ≥5μm.

[0017] In the above method, in step 6), the ultrafiltration is performed using at least one of a metal membrane filtration device, an organic membrane filtration device, and a ceramic membrane filtration device.

[0018] In this invention, a security filter is connected after the multi-media filter: a 5μm filter element is used to protect the subsequent ceramic membrane filter.

[0019] In this invention, the water treatment system used in the method of treating and reinjecting polymer-containing oilfield wastewater into low-permeability oilfields specifically consists of the following components connected in sequence: a pre-oxidation settling unit, an oil removal tank, a pressurized dissolved air flotation device, a magnetic coagulation reaction device, a multi-stage filtration device, and an ultrafiltration membrane filter. The multi-stage filtration device includes, in sequence, a walnut shell filter, a multi-media filter, and a security filter.

[0020] The present invention has the following beneficial effects: (1) Synergistic degradation of pre-oxidation + modified flocculation: The polymer molecular chains are broken by oxidizing the Fenton reagent and other oxidants, and the polymer-suspension flocculation is achieved by combining the new modified flocculant and coagulant aid (modified aluminum salt flocculant + polyacrylamide); (2) Enhanced sedimentation of magnetic coagulation: magnetic flocs are induced to form by permanent magnets. During the movement of the magnetic flocs, they actively collide with fine particles and stably adsorb the fine particles on their surface. The rate is increased to 3 times that of conventional coagulation through "heterogeneous nucleation flocculation + dynamic adsorption capture".

[0021] (3) Gradient interception of multi-stage filtration: Gradient filtration of walnut shell (oil removal) - multi-media (suspended solids removal) - security filtration - ultrafiltration membrane (fine filtration) to achieve precise interception of pollutants of different particle sizes. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the water treatment method for reinjecting polymer-containing oilfield wastewater into low-permeability oilfields according to the present invention. Detailed Implementation

[0023] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0024] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0025] This application provides a water treatment method for reinjecting polymer-containing oilfield wastewater into a low-permeability oilfield, as follows: Figure 1 The process shown includes the following steps: (1) Pre-oxidation and settling unit at the front end: On the basis of adjusting the water volume and homogenizing the water quality, and on the premise of ensuring the oxidation time of the incoming water in the tank, add oxidant to start the pre-oxidation reaction, so that the macromolecular polymer is degraded into small molecules and the viscosity of polymer-containing oilfield wastewater is reduced. (2) The pre-oxidized water enters the oil removal tank. Modified flocculant and coagulant aid are added to the oil removal tank to ensure that the oily wastewater settles in the oil removal tank for 6-8 hours. The floating oil is removed through the oil collection system, and some polymers and SS in the water are removed at the same time. (3) The water that has been de-oiled and desuspended in the oil removal tank is further introduced to the pressurized dissolved air flotation device. A demulsifier is added to the device, and the microbubbles released by the dissolved air flotation adsorb the fine oil droplets and fine suspended matter, further reducing the oil content and SS content in the water. (4) The air flotation effluent enters the magnetic coagulation reactor, where polyaluminum chloride and cationic polyacrylamide are added, and at the same time, a magnetic medium induced by a permanent magnet is introduced to form high-density magnetic flocs. After settling for 15-30 minutes, SS in the water is further removed. (5) Multi-stage filtration: Walnut shell filter: Utilizes the adsorption properties of walnut shells to deeply remove oil, controlling oil content to ≤8mg / L; Multi-media filter: uses quartz sand, corundum, and anthracite as filter media to intercept suspended solids with a particle size ≥5μm; Security filter: Uses a 5μm filter element to protect the subsequent ceramic membrane filter; (6) Ultrafiltration membrane filter (such as ceramic membrane filter) A ceramic membrane with a pore size of 0.1 μm is used to finely filter the wastewater, ensuring that the median particle size of the effluent is ≤1.5 μm, which meets the reinjection standard of low-permeability oilfields.

[0026] The method of this application is further described with reference to the accompanying drawings. The drawings show a schematic diagram of the flow direction of oily wastewater.

[0027] Furthermore, the oxidant can be at least one of Fenton's reagent, Fenton-like reagent, ozone, and persulfate. In this embodiment, Fenton's reagent is used for oxidation. The dosage of Fenton's reagent needs to be optimized in conjunction with the wastewater quality, and the pH is adjusted to 2-4. The reaction time between Fenton's reagent and oilfield wastewater is 60-90 minutes. After oxidation, the wastewater is led to a neutralization and conditioning tank to restore the pH to 6-9. Specifically, it can be hydrogen peroxide (H2O2) + ferrous sulfate (FeSO4) or hydrogen peroxide (H2O2) + Fe 3+ / Cu 2+ Na2S2O8, (NH4)2S2O8, or ozone.

[0028] Furthermore, through the pre-oxidation unit reaction, polymers in polymer-containing oilfield wastewater can be degraded into smaller molecules, reducing the viscosity of the polymer-containing wastewater and minimizing its impact on subsequent oil and suspension removal. Further, the oilfield wastewater from the pre-oxidation unit enters the oil removal tank. For oilfield wastewater containing polymers, the retention time in the oil removal tank is considered to be 6-9 hours. The vertical oil removal tank is equipped with a water distribution system, a water collection system, an oil collection system, a sludge discharge system, and a central cylinder, etc. Annular oil collection tanks, radial oil collection tanks, and floating oil collection devices can be used. Sludge discharge is achieved using hydrostatic pressure, hydraulic discharge, or negative pressure suction. Modified flocculants and appropriate amounts of coagulants can be added to the oil removal tank to specifically break down the stable O / W emulsion system containing polymers, disrupting the oil-solid-water encapsulation structure, causing oil droplets to quickly aggregate and float to the surface. Simultaneously, Fenton iron sludge and polymer degradation products are adsorbed, forming large-particle flocs, accelerating the sedimentation of suspended solids (SS) to the bottom of the tank, and then discharged to the sludge thickening unit through the sludge discharge system.

[0029] Further, the oily wastewater, after being degreased in the oil removal tank, enters the dissolved air flotation (DAF) unit. The DAF unit can be either a vortex DAF unit or a pressurized dissolved air flotation unit, with pressurized DAF unit being preferred. The hydraulic retention time is 10-20 minutes, the reflux dissolved air ratio is 15-30%, and the operating pressure of the dissolved air tank is 0.4-0.5 MPa. A demulsifier and an appropriate amount of PAM can be added to the DAF unit to remove incompletely demulsified emulsified oil from the oil removal tank, while simultaneously promoting the formation of flocs from iron sludge and other materials. The flocculated wastewater is then discharged to the sludge thickening unit via the sludge discharge system.

[0030] Further, the oilfield wastewater from the air flotation unit enters the magnetic coagulation unit, where flocculants and coagulants are added to enhance the air flotation effect. In one embodiment, the flocculant can be selected from a combination of iron salts, aluminum salts, and polyacrylamide, preferably a combination of polyaluminum chloride and polyacrylamide. The dosage of polyaluminum chloride is 50~300 mg / L, preferably 100~200 mg / L; the dosage of polyacrylamide is 2~10 mg / L, preferably 2~5 mg / L; the reaction time of magnetic coagulation is 15~30 min, removing suspended solids, undegraded small molecule organic matter, and colloids from the water. A reflux system is set up and magnetic powder is loaded. The particle size and specific gravity of the magnetic flocs are larger than those of conventional flocs. The sludge produced by the magnetic coagulation unit is discharged into the sludge thickening unit.

[0031] Furthermore, after a large amount of suspended solids (SS) is removed by the magnetic coagulation unit, the wastewater enters the walnut shell filter and the multi-media filter, and then undergoes fine filtration treatment through the ultrafiltration membrane unit to meet the reinjection standards of low-permeability offshore oilfields before reinjection.

[0032] In one embodiment, the fine filtration device can be a ceramic membrane, an organic membrane, a metal membrane, etc., with a ceramic membrane being preferred. The operating pressure is 0.1~1.0 MPa, preferably 0.4~0.5 MPa; the membrane flux is 150~500 L / m³. 2 Preferred concentration: 150~200 L / m 2 A security filter is installed before the membrane to reduce the load on the fine filtration unit. Large polymer molecules deposited on the membrane surface can easily form concentration polarization, resulting in membrane fouling. Small organic molecules and dissolved organic matter can easily be adsorbed and retained in the membrane pores, thus clogging the membrane pores. Backwashing is performed periodically according to the transmembrane pressure difference, using a combination of hydraulic backwashing and alkaline washing.

[0033] Example 1: 1. Basic parameters Processing capacity: 450 m³ / h (continuous operation); Influent water quality: polymer 140mg / L, oil 458mg / L, suspended solids (SS) 484mg / L, mineralization 22000mg / L, water temperature 40~45℃; Target indicators: A2-level reinjection standard for low-permeability offshore oilfields (oil content ≤ 6 mg / L, SS ≤ 2 mg / L, median particle size ≤ 1.5 μm).

[0034] 2. Processing procedure ① Pre-oxidation unit: Two units, each with a volume of 500m³, employing a closed reaction tank. These tanks combine multiple functions, including water volume regulation, water quality homogenization, and pre-oxidation. Equipped with mechanical seal stirring, sulfuric acid is first added to control the pH to 3.5-3.8, followed by the addition of 1mL / L 30% H₂O and 600mg / L ferrous sulfate. The oxidation reaction lasts for 1 hour, resulting in polymer degradation to 30mg / L and initial flocculation of some suspended solids. The oxidized wastewater then enters a neutralization tank, where alkaline solution is added. The pH is adjusted to 7-8 by controlling the dosage using an online pH meter.

[0035] ② Oil removal tanks: There are two tanks in total, each with a volume of 1500m³. Modified aluminum salt flocculant 100mg / L + PAM 2mg / L are added. The settling time is 6h, the oil collection rate is 80%, and the effluent contains 246mg / L of oil, 235mg / L of SS, and 22mg / L of polymer.

[0036] ③ Pressurized dissolved air flotation: dissolved air pressure 0.4MPa, reflux ratio of refluxed dissolved air water 20%, demulsifier 100mg / L added, residence time 25min, microbubbles adsorb oil droplets and fine flocs, effluent oil content 25mg / L, SS 150mg / L.

[0037] ④ Magnetic coagulation unit: Add 100mg / L of PAC, stir rapidly at 180rpm / 3min to quickly mix the magnetic powder, PAC and wastewater to form tiny flocs, stir slowly at 70rpm / 15min, add 1.5mg / L of cationic PAM to promote the formation of dense magnetic-floc composites of magnetic powder-flocs-pollutants, settle for 15min, the magnetic floc recovery rate is 95%, the effluent SS is 30mg / L and the polymer is 15mg / L.

[0038] ⑤ Multi-stage filtration: Walnut shell filter (filtration speed 12m / h), backwashing intensity 18L / (m²·s); Multi-media filter (filtration speed 8m / h); Security filter (5μm filter element); The effluent contained 8 mg / L of oil and 5 mg / L of suspended solids (SS).

[0039] ⑥ Ceramic membrane filtration: 0.1μm pore size ceramic membrane, transmembrane pressure difference 0.2MPa, filtration flux 150L / (m²·h), periodically cleaned according to transmembrane pressure difference, final effluent contains 4mg / L oil, 1.5mg / L SS, and median particle size 1.32μm.

[0040] Example 2: 1. Basic parameters Processing capacity: 450 m³ / h.

[0041] Influent water quality: polymer 125mg / L, oil 350mg / L, SS 202mg / L, mineralization 21000mg / L, water temperature 40~45℃.

[0042] 2. Processing procedure ① Pre-oxidation unit: Add 0.8 mL / L of 30% H2O and 500 mg / L of ferrous sulfate, control the pH at 3.8~4.0, and oxidize for 1 hour until the polymer degradation reaches 28 mg / L. The oxidized wastewater enters the neutralization tank, where alkaline solution is added. The pH is adjusted to 7~8 by controlling the dosage using an online pH meter.

[0043] ② Oil removal tanks: There are two tanks in total, each with a volume of 1500m³. Modified flocculant 100mg / L + PAM 2.0mg / L are added. The settling time is 6h. The effluent contains 138mg / L of oil, 120mg / L of SS, and 12mg / L of polymer.

[0044] ③ Pressurized dissolved air flotation: dissolved air pressure 0.38MPa, reflux ratio of refluxed dissolved air water 20%, demulsifier 70mg / L added, residence time 25min, effluent oil content 22mg / L, SS 68mg / L.

[0045] ④ Magnetic coagulation unit: Add PAC 50mg / L, stir quickly at 180rpm / 2min, stir slowly at 65rpm / 12min, add cationic PAM 1.2mg / L, settle for 15min, magnetic floc recovery rate 95%, effluent SS 18mg / L, polymer 11mg / L.

[0046] ⑤ Multi-stage filtration + ceramic membrane: walnut shell filter (filtration speed 12m / h), multi-media filter (filtration speed 8m / h), security filter, ceramic membrane, final effluent contains 3mg / L oil, 1.2mg / L SS, and median particle size of 1.14μm.

[0047] Example 3: 1. Basic parameters Processing capacity: 450 m³ / h. Influent water quality: polymer 45mg / L, oil 286mg / L, SS 180mg / L, mineralization 23000mg / L, water temperature 40~45℃.

[0048] 2. Processing procedure ① Pre-oxidation unit: Add 0.4 mL / L of 30% H2O and 200 mg / L of ferrous sulfate, control the pH at 4.0~4.2, and oxidize for 1 hour until the polymer degradation reaches 10 mg / L. The oxidized wastewater enters the neutralization tank, where alkaline solution is added. The pH is adjusted to 7~8 by controlling the dosage using an online pH meter.

[0049] ② Oil removal tanks: There are two tanks in total, each with a volume of 1500m³. Modified flocculant 50mg / L + PAM 1.8mg / L are added. The settling time is 8h. The effluent contains 114mg / L of oil, 93mg / L of SS, and 8mg / L of polymer.

[0050] ③ Pressurized dissolved air flotation: dissolved air pressure 0.35MPa, reflux ratio of refluxed dissolved air water 20%, demulsifier 60mg / L added, residence time 25min, effluent oil content 19mg / L, SS 42mg / L.

[0051] ④ Magnetic coagulation unit: Add PAC 30mg / L, stir quickly at 160rpm / 2min, stir slowly at 60rpm / 10min, add cationic PAM 1mg / L, settle for 15min, magnetic floc recovery rate 95%, effluent SS 12mg / L, polymer 6mg / L.

[0052] ⑤ Multi-stage filtration + ceramic membrane: walnut shell filter (filtration speed 12m / h), multi-media filter (filtration speed 8m / h), ceramic membrane (flux 150L / (m²·h)), final effluent contains 2mg / L oil, 0.8mg / L SS, and median particle size 1.16μm.

[0053] Comparative Example 1: As a comparative example of Embodiment 1 of the present invention, Comparative Example 1 skips the pre-oxidation unit, and the raw water directly enters the oil removal tank unit. The remaining treatment units, equipment specifications, reagent dosage, operating parameters, etc. are consistent with Embodiment 1 of the present invention. Specifically, no oxidizing agents such as H2O2 and ferrous sulfate are added to the raw water, and no polymer oxidation degradation is performed. The raw water directly enters the oil removal tank. Since the polymer in the raw water is not degraded, the stability of emulsified oil and suspended solids is relatively strong. The effluent contains 375 mg / L of oil, 382 mg / L of SS, and 114 mg / L of polymer. The pressurized dissolved air flotation is affected by the high concentration of polymer and emulsified oil, and the microbubble adsorption effect is greatly reduced. The effluent contains 176 mg / L of oil and 257 mg / L of SS. The effluent from the magnetic coagulation unit contains 143 mg / L of oil and 108 mg / L of SS. After passing through the walnut shell filter, multi-media filter, and security filter, the filtration unit has a high risk of clogging. The effluent contains 31 mg / L of oil, 27 mg / L of SS, and 91 mg / L of polymer. Considering the excessively high suspended solids (SS), oil content, and polymer content in the effluent after multi-stage filtration, the effluent was not directed to the ceramic membrane filtration unit to prevent fouling and damage to the membrane.

[0054] Comparative Example 2: As a comparative example of Embodiment 1 of the present invention, Comparative Example 2 bypasses the magnetic coagulation unit. The remaining treatment units, equipment specifications, reagent dosages, and operating parameters are consistent with Embodiment 1. The pressurized dissolved air flotation effluent bypasses the magnetic coagulation unit and is directly led to the multi-stage filtration unit. The pressurized dissolved air flotation effluent contains 25 mg / L of oil and 150 mg / L of suspended solids (SS). After multi-stage filtration, the effluent from the multi-media filter contains 9 mg / L of oil and 21 mg / L of SS. After interception by the security filter and filtration by the ceramic membrane, the final effluent contains 4 mg / L of oil and 1.8 mg / L of SS. Although the final effluent meets the requirements, the high SS content easily leads to a rapid increase in transmembrane pressure differential, a significant increase in the frequency of backwashing and chemical cleaning, rapid membrane flux decay, and irreversible fouling, among other problems.

[0055] In the method of this invention, the process steps, reagent types, equipment parameters, etc., can be replaced or adjusted according to the actual water quality conditions, treatment requirements, and on-site working conditions. Modifications, equivalent substitutions, and improvements made within the scope of the concept of this invention are included within the protection scope of the claims of this invention.

Claims

1. A water treatment method for reinjecting polymer-containing oilfield wastewater into a low-permeability oilfield after treatment, comprising the following steps: 1) On the basis of adjusting the water volume and homogenizing the water quality, adding an oxidant to the polymer-containing oilfield wastewater to carry out a pre-oxidation reaction to obtain pre-oxidized water; 2) Add modified flocculant and coagulant aid to the pre-oxidized water and carry out sedimentation reaction to obtain water with oil and suspension removed; 3) The de-oiled and desuspended water is introduced into the air flotation device, and a demulsifier is added to reduce the oil content and suspended solids content in the water to obtain air flotation effluent; 4) Add inorganic polymer flocculant and cationic polyacrylamide to the air-flotated water, and simultaneously introduce a magnetic medium induced by a permanent magnet to form high-density magnetic flocs, which are then settled to obtain water treated by magnetic coagulation. 5) The water treated by magnetic coagulation is subjected to multi-stage filtration to obtain multi-stage filtered water; The multi-stage filtration process is as follows: oil content in the water is removed by a walnut shell filter, and then suspended solids are further removed by a multi-media filter; 6) The water after multi-stage filtration is filtered through ultrafiltration to meet the reinjection standards of offshore oil fields, so as to reinject oil into low-permeability oil fields.

2. The method according to claim 1, characterized in that, Step 1) includes the step of separating the polymer-containing oilfield wastewater through a three-phase separator before adding the oxidant, and then having the separated water enter a regulating buffer tank. In step 1), the oxidant is selected from at least one of Fenton reagent, Fenton-like reagent, persulfate and ozone.

3. The method according to claim 1 or 2, characterized in that, In step 2), the sedimentation reaction takes 6 to 9 hours.

4. The method according to any one of claims 1-3, characterized in that, The sedimentation reaction described in step 2) is carried out in an oil removal tank, which is a natural oil removal tank and / or a flocculation oil removal tank.

5. The method according to any one of claims 1-4, characterized in that, In step 2), the modified flocculant is polyaluminum ferric silicate; The coagulant is polyacrylamide.

6. The method according to any one of claims 1-5, characterized in that, The air flotation device is selected from pressurized dissolved air flotation and / or vortex air flotation devices.

7. The method according to any one of claims 1-6, characterized in that, In step 4), the inorganic polymeric flocculant includes polyaluminum chloride; The magnetic medium includes magnetite powder.

8. The method according to any one of claims 1-7, characterized in that, In step 4), the settling time is 15-30 minutes to remove suspended solids (SS) from the water.

9. The method according to any one of claims 1-8, characterized in that, In step 6), the ultrafiltration is performed using at least one of a metal membrane filtration device, an organic membrane filtration device, and a ceramic membrane filtration device.

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