Advanced treatment method and system for fracturing flow-back fluid of oil and gas field

Abstract

The invention discloses a deep treatment method and system for fracturing flow-back fluid of an oil and gas field, and the method comprises the following steps: firstly, carrying out coagulation pretreatment on the fracturing flow-back fluid; adjusting the pH value of the fracturing flow-back fluid, wherein the pH value is less than 9; then, potassium ferrate or H2O2 and the potassium ferrate are adopted to conduct oxidation treatment on the fracturing flow-back fluid, and the molar mass ratio of the H2O2 to the potassium ferrate is 1: (1-3.5); treating the fracturing flow-back fluid subjected to oxidation treatment by adopting a sequencing batch bio-membrane reactor, and removing organic matters in the fracturing flow-back fluid; and finally, carrying out membrane filtration on the fracturing flow-back fluid by adopting a microfiltration membrane and a nanofiltration membrane in sequence. Based on an oxidation technology, a biological treatment technology and a membrane filtration technology in sequence, an efficient and economical fracturing flow-back fluid treatment scheme is constructed, and the oil content, the median particle size and the SS of treated effluent all meet the II-level water quality standard in Technical Requirements and Analysis Methods for Injection Water Quality Index of Clastic Rock Oil Reservoir (SY / T 5329-2022). Meanwhile, the first-grade standard of Integrated Wastewater Discharge Standard (GB 8978-1996) is met, meanwhile, the potential of treating and discharging the fracturing flowback fluid is achieved, and good practicability is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of oilfield sewage treatment, and in particular relates to a method and system for deep treatment of oil and gas field fracturing flowback fluid. Background Art

[0002] At present, my country's oil fields are mainly low-permeability and ultra-low-permeability reservoirs, which are difficult to exploit. Fracturing technology is a key way to develop low-permeability reservoirs and unconventional oil and gas. Its smooth implementation is inseparable from the support of a large amount of fracturing fluid. After the fracturing operation is completed, part of the fracturing fluid will flow back to the surface to form fracturing flowback fluid. Fracturing flowback fluid contains high mineralization, turbidity, salinity, complex pollutants and a variety of organic additives. Untreated fracturing flowback fluid will cause harm to the ecosystem and human health.

[0003] There are many methods for treating fracturing flowback fluid, which mainly include physical methods (gravity sedimentation and filtration), chemical methods (coagulation sedimentation), biological methods and membrane treatment methods. Considering the complex water quality of fracturing flowback fluid, it is unrealistic to expect to achieve harmless treatment through a certain technical unit. Through a combination of appropriate enhanced treatments, the fracturing flowback fluid can be well purified. The pretreatment process of fracturing flowback fluid does not rely solely on a single physical means such as coagulation and adsorption. The treatment technology of fracturing flowback fluid is usually a combination of multiple processes to meet the standards for reinjection. Its main pretreatment technology is also based on advanced oxidation treatment, supplemented by physical treatment, and through a series of enhanced treatment combination strategies, a good purification effect of fracturing flowback fluid is achieved.

[0004] Secondly, when biological treatment technology is used to treat fracturing flowback fluid, its effectiveness is hindered by high molecular weight and difficult-to-degrade organic matter (including phthalic acid, polycyclic aromatic hydrocarbons and petroleum hydrocarbons, etc.). Therefore, in order to improve the efficiency of biological treatment, it is necessary to introduce appropriate and effective oxidation technology. Therefore, suitable pre-oxidation technology is crucial to improve the treatment effect of fracturing flowback fluid. Finding green and efficient oxidation methods has become a research hotspot in the current oil and gas field wastewater field. Among them, enhanced ferrate Fe(VI) performs well in degrading pollutants. However, the degree of oxidation mineralization for some difficult-to-treat pollutants is not ideal. In addition, Fe(VI) and intermediate iron (Fe(IV) and Fe(V)) have limited degradation effects on organic pollutants that do not contain electron-rich groups. Secondly, the pH value of the aqueous solution has a significant effect on the stability and oxidation capacity of Fe(VI).

[0005] At present, the requirements for the quality of reinjection water for the exploitation of low-permeability oil reservoirs are getting higher and higher. In order to ensure that the reinjection process will not cause damage to the formation and meet the requirements of environmental protection and safe production, the oil industry has formulated relevant standards for reinjection water ("Technical Requirements and Analysis Methods for Water Quality Indicators in Clastic Reservoirs" (SY / T 5329-2022)). Some oil fields have also established local enterprise standards based on the nature of the formation, such as "Water Quality Indicators and Analysis Methods for Produced Water Treatment in Longdong Oilfield" (Q / SY CQ 08011-2019). These standards have put forward clear requirements for indicators such as suspended solids content (SS) and oil content. However, traditional treatment technologies are difficult to meet the requirements of reinjection. Therefore, a more efficient, environmentally friendly and economical combined deep treatment process for fracturing return fluid is urgently needed. Summary of the invention

[0006] The object of the present invention is to provide a method and system for deep treatment of oil and gas field fracturing flowback fluid, aiming to solve the above-mentioned problems based on oxidation technology, biological treatment technology and membrane filtration technology.

[0007] The present invention is mainly achieved through the following technical solutions:

[0008] A method for deep treatment of oil and gas field fracturing flowback fluid comprises the following steps:

[0009] Step S1: performing coagulation pretreatment on the fracturing flowback fluid;

[0010] Step S2: adjusting the pH value of the fracturing flowback fluid to a value less than 9; then, oxidizing the fracturing flowback fluid with potassium ferrate or H2O2 and potassium ferrate, wherein the molar mass ratio of H2O2 to potassium ferrate is 1:1-3.5;

[0011] Step S3: using a biofilm reactor to treat the fracturing flowback fluid after oxidation treatment to remove organic matter in the fracturing flowback fluid;

[0012] Step S4: using a microfiltration membrane and a nanofiltration membrane in sequence to perform membrane filtration on the fracturing flowback fluid.

[0013] In order to better implement the present invention, further, in step S2, the initial concentration of potassium ferrate is 2000-4000 mg / L. Preferably, the initial concentration of potassium ferrate is 3000 mg / L.

[0014] In order to better implement the present invention, further, in step S2, the pH value of the adjusted fracturing flowback fluid is 7-8, and the molar mass ratio of H2O2 and potassium ferrate is 1:2.

[0015] In order to better realize the present invention, further, in step S1, polyaluminium chloride PAC and polyacrylamide PAM are used to perform coagulation pretreatment on the fracturing flowback fluid; in the fracturing flowback fluid, the concentrations of polyaluminium chloride PAC and polyacrylamide PAM are 400mg / L-800mg / L and 10mg / L-40mg / L, respectively. Preferably, in the fracturing flowback fluid, the concentrations of polyaluminium chloride PAC and polyacrylamide PAM are 600mg / L and 20mg / L, respectively.

[0016] In order to better implement the present invention, further, in step S3, before treating the fracturing flowback fluid, first, the biofilm reactor is subjected to biofilm formation, and then, the biofilm reactor is subjected to salinity acclimation and pollutant acclimation in sequence.

[0017] In order to better realize the present invention, further, in the step S3, when treating the fracturing flowback fluid, the biofilm reactor sequentially operates in the anoxic stage, the aerobic stage, the anoxic stage and the aerobic stage, and the operating time ratio is 4:13:2.5:4.

[0018] In order to better implement the present invention, further, in the step S3, before filtering, the pH of the fracturing flowback fluid is adjusted to neutral; the inlet TOC:TN:TP of the biofilm reactor is 100:(1-1.5):(0.05-0.1).

[0019] A deep treatment system for oil and gas field fracturing flowback fluid is based on the above-mentioned deep treatment method for oil and gas field fracturing flowback fluid, and includes a flocculation pretreatment module, an oxidation treatment reaction tank, a biofilm filtration tank, a microfiltration membrane filtration tank and a nanofiltration membrane filtration tank which are arranged in sequence from front to back.

[0020] The beneficial effects of the present invention are as follows:

[0021] (1) The present invention aims to construct an efficient and economical fracturing flowback fluid treatment scheme by combining oxidation technology, biological treatment means and membrane filtration process, thereby achieving deep treatment of fracturing flowback fluid. The present invention uses hydrogen peroxide (H2O2) to strengthen potassium ferrate (Fe(VI)) oxidation treatment of fracturing flowback fluid, effectively improving the oxidation efficiency and biodegradability of fracturing flowback fluid; combining sequencing batch biofilm process (SBBR) to effectively remove organic matter in fracturing flowback fluid, and performing membrane filtration through microfiltration (MF) and nanofiltration (NF) to achieve deep treatment of fracturing flowback fluid. Among them, in the sequencing batch biofilm process, the present invention acclimates the salinity and pollutants of the activated sludge according to the water quality characteristics of the fracturing flowback fluid, so that its microorganisms can adapt to its living environment, and introduces the oxidized fracturing flowback fluid into the biological treatment system, effectively improving the efficiency of biological treatment, and has good practicality.

[0022] (2) The present invention uses 600 mg / L polyaluminium chloride (PAC) and 20 mg / L polyacrylamide (PAM) to pre-coagulate the fracturing flowback fluid. After coagulation, the removal rates of turbidity and suspended solids (SS) are 82.6% and 38.0% respectively. On this basis, Fe(VI) is used to oxidize the fracturing flowback fluid. When the Fe(VI) dosage is 3000 mg / L and the pH is 8.0, after 60 minutes of reaction, the chemical oxygen demand (COD), total organic carbon (TOC) and absorbance at 254 nm (UV 254 ) removal rates were 29.1%, 30.7% and 54.9% respectively. Furthermore, the addition of H2O2 in the ratio of 1:2 (Fe(VI):H2O2, n / n) enhanced the Fe(VI) oxidation, COD, TOC and UV 254 The removal rates of fracturing flowback fluid were increased to 39.3%, 46.0% and 66.6% respectively. The biodegradability of fracturing flowback fluid can be significantly improved after oxidation treatment. Using the effluent from the sewage treatment plant as the inoculation liquid, the ratio of biochemical oxygen demand to chemical oxygen demand (B / C) after Fe(VI)-H2O2 treatment was increased by 333.0% compared with the untreated fracturing flowback fluid; using the bacterial solution acclimated with fracturing flowback fluid as the inoculation liquid, the B / C ratio after Fe(VI)-H2O2 treatment reached 0.436.

[0023] (3) The fracturing flowback fluid after oxidation treatment was further treated by sequencing batch biofilm reactor (SBBR). After the Fe(VI) oxidation was enhanced, the removal capacity of organic matter by the SBBR reactor was improved. Among them, Fe(VI)-H2O2-SBBR had the best removal effect on organic matter. After the SBBR reactor was operated for 4 hours in anoxic stage, 13 hours in aerobic stage, 2.5 hours in anoxic stage, and 4 hours in aerobic stage, the COD, TOC and UV of the effluent of the Fe(VI)-H2O2-SBBR system were 254 The removal rates of all of them reached more than 90.0%. At the same time, the removal rate of TN in the effluent of this process can reach 75.0%, and the removal rate of TP can reach 60.0%. Analysis of the microbial community showed that the Fe(VI) or Fe(VI)-H2O2 oxidation process affected the diversity of the microbial community. Through linear discriminant analysis effect size (LEfSe) analysis, Fe(VI) or Fe(VI)-H2O2 oxidation can enhance the ability of microorganisms in the reactor to degrade aromatic organic matter, and the fracturing return fluid after enhanced oxidation is more conducive to microbial denitrification activities. Analysis of the relationship between environmental factors and microbial communities indicates that microorganisms can use organic matter in fracturing return fluid as a carbon source, and Fe(VI) oxidation-SBBR or Fe(VI)-H2O2 oxidation-SBBR processes can achieve effective treatment of fracturing return fluid.

[0024] (4) Microfiltration (MF) and nanofiltration (NF) were used to deeply treat the fracturing flowback fluid. Compared with the membrane flux of the water sample directly filtered after coagulation, the membrane flux after oxidation-SBBR treatment was significantly improved. The MF membrane flux increased from 8.0L·m-2·h-1 to more than 100.0L·m-2·h-1, and the NF membrane flux increased from 0.7L·m-2·h-1 to more than 25.0L·m-2·h-1. Combined with the characterization analysis of the membrane surface fouling layer, it was observed that the membrane surface of the oxidation-SBBR process treatment group had a more obvious infrared characteristic peak of the membrane material compared with the membrane surface of the coagulation treatment group after the membrane, indicating that oxidation-SBBR treatment can effectively improve membrane fouling. The TOC and COD removal rates of the effluent from ferrate oxidation-biofilm-membrane filtration reached more than 95.0%, and the salinity removal rate reached more than 65.0%. At the same time, it can remove highly toxic organic matter. Comparing the effluents of different oxidation methods, the Fe(VI)-H2O2-SBBR-MF-NF process has the best effluent quality. Its effluent oil content, median particle size and SS all meet the Grade II water quality standard in the Technical Requirements and Analysis Methods for Water Quality Indicators of Clastic Reservoirs (SY / T 5329-2022). At the same time, the TOC and COD of the effluent of this process are only 6.9 mg / L and 8.4 mg / L, which meet the Grade I standard of the Integrated Wastewater Discharge Standard (GB 8978-1996). Therefore, the Fe(VI)-H2O2-SBBR-MF-NF process has a good application prospect in treating the discharge of fracturing flowback fluid. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 (a) is the TOC curve of oil and gas field fracturing flowback fluid as the PAC concentration changes;

[0026] Figure 1 (b) is the turbidity change curve of oil and gas field fracturing flowback fluid with the change of PAM concentration;

[0027] Figure 2 (a) shows the variation curve of COD removal at different initial Fe(VI) concentrations;

[0028] Figure 2 (b) shows the variation curve of TOC removal at different initial Fe(VI) concentrations;

[0029] Figure 2 (c) shows the UV at different initial Fe(VI) concentrations. 254 The variation curve of removal;

[0030] Figure 3 (a) is the effect curve of different initial pH on COD degradation by Fe(VI);

[0031] Figure 3(b) is the effect curve of different initial pH on Fe(VI) degradation of TOC;

[0032] Figure 3 (c) shows the effect of different initial pH on the degradation of Fe(VI) by UV 254 The impact curve of

[0033] Figure 4 (a) is the effect curve of different H2O2 addition on Fe(VI) degradation of COD;

[0034] Figure 4 (b) is the effect curve of different H2O2 addition on Fe(VI) degradation of TOC;

[0035] Figure 4 (c) shows the degradation of Fe(VI) by UV at different H2O2 dosages. 254 The impact curve of

[0036] Figure 5 (a) is the EPR spectrum of active free radicals in different oxidation modes;

[0037] Figure 5 (b) is the EPR spectrum of OH radical quenching changes in different oxidation modes;

[0038] Figure 6 (a) is the curve showing the change of PMSO concentration, PMSO2 concentration and PMSO+PMSO2 generation in the Fe(VI) system;

[0039] Figure 6 (b) is the curve of the change of PMSO concentration, PMSO2 concentration and PMSO+PMSO2 generation in the Fe(VI)-H2O2 system;

[0040] Figure 7 (a) is the GC-MS spectrum of the product of the Coagulation system;

[0041] Figure 7 (b) is the GC-MS spectrum of the product of the Fe(VI) system;

[0042] Figure 7 (c) is the GC-MS spectrum of the product of the Fe(VI)-H2O2 system;

[0043] Figure 8 (a) is the effect curve of the influent of the Coagulation-SBBR system on the TOC removal of biological treatment;

[0044] Figure 8 (b) is the curve showing the effect of the influent of the Fe(VI)-SBBR system on the TOC removal of biological treatment;

[0045] Figure 8 (c) is the curve showing the effect of the influent of the Fe(VI)-H2O2-SBBR system on the TOC removal of biological treatment;

[0046] Fig. 9 (a) is the effect curve of the influent of the Coagulation-SBBR system on the COD removal of biological treatment;

[0047] Fig. 9 (b) is the curve showing the effect of the influent of the Fe(VI)-SBBR system on COD removal in biological treatment;

[0048] Fig. 9 (c) is the effect curve of the influent of Fe(VI)-H2O2-SBBR system on COD removal in biological treatment;

[0049] Fig.10 (a) shows the effect of influent on UV in the Coagulation-SBBR system. 254 Impact curve of removal;

[0050] Fig.10 (b) is the effect of the influent on the biological treatment UV of the Fe(VI)-SBBR system. 254 Impact curve of removal;

[0051] Fig.10 (c) is the effect of influent on biological treatment UV in Fe(VI)-H2O2-SBBR system. 254 Impact curve of removal;

[0052] Fig.11 (a) is the GC-MS spectrum of the effluent from the Coagulation-SBBR system;

[0053] Fig.11 (b) is the GC-MS spectrum of the effluent from the Fe(VI)-SBBR system;

[0054] Fig.11 (c) is the GC-MS spectrum of the effluent from the Fe(VI)-H2O2-SBBR system;

[0055] Fig.12 (a) is a schematic diagram of the relative abundance changes of organic matter in the effluent after biological treatment of Coagulation, Fe(VI) and Fe(VI)-H2O2 systems;

[0056] Fig.12 (b) is a schematic diagram of the number of corresponding organic matter types in the effluent organic matter after biological treatment of Coagulation, Fe(VI) and Fe(VI)-H2O2 systems;

[0057] Fig.13 (a) shows the change curves of the membrane flux of the microfiltration membrane in the Coagulation, Fe(VI) and Fe(VI)-H2O2 systems without biological treatment;

[0058] Fig.13 (b) shows the change curve of the membrane flux of the microfiltration membrane after biological treatment of Coagulation, Fe(VI) and Fe(VI)-H2O2 system;

[0059] Fig.14 (a) shows the infrared spectra of the fouling layer of the blank membrane, Coagulation, Fe(VI) and Fe(VI)-H2O2 system without biological treatment;

[0060] Fig.14 (b) is the infrared spectra of the fouling layer of the blank membrane, Coagulation, Fe(VI) and Fe(VI)-H2O2 system after biological treatment of the microfiltration membrane;

[0061] Fig.15 (a) shows the change curve of nanofiltration membrane flux of Coagulation-MF-NF, Fe(VI)-MF-NF and Fe(VI)-H2O2-MF-NF systems without biological treatment;

[0062] Fig.15 (b) is the change curve of nanofiltration membrane flux after biological treatment of Coagulation-MF-NF, Fe(VI)-MF-NF and Fe(VI)-H2O2-MF-NF systems;

[0063] Fig.16 (a) is the change curve of nanofiltration membrane flux of Coagulation-SBBR-NF, Fe(VI)-SBBR-NF and Fe(VI)-H2O2-SBBR-NF systems without microfiltration membrane filtration;

[0064] Fig.16 (b) is the change curve of nanofiltration membrane flux of Coagulation-SBBR-NF, Fe(VI)-SBBR-NF and Fe(VI)-H2O2-SBBR-NF systems after microfiltration membrane filtration;

[0065] Fig.17 (a) shows the infrared spectra of the fouling layer of the blank membrane, Coagulation, Fe(VI) and Fe(VI)-H2O2 system without biological treatment;

[0066] Fig.17(b) is the infrared spectra of the fouling layer of the blank membrane, Coagulation, Fe(VI) and Fe(VI)-H2O2 system after biological treatment;

[0067] Fig.18 (a) is the GC-MS spectrum of the product of the Coagulation-SBBR-MF system;

[0068] Fig.18 (b) is the GC-MS spectrum of the product of the Fe(VI)-SBBR-MF system;

[0069] Fig.18 (c) is the GC-MS spectrum of the product of the Fe(VI)-H2O2-SBBR-MF system;

[0070] Fig.19 (a) is the GC-MS spectrum of the product of the Coagulation-SBBR-MF-NF system;

[0071] Fig.19 (b) is the GC-MS spectrum of the product of the Fe(VI)-SBBR-MF-NF system;

[0072] Fig.19 (c) is the GC-MS spectrum of the product of the Fe(VI)-H2O2-SBBR-MF-NF system;

[0073] Fig. 20 (a) is a schematic diagram of the relative abundance changes of organic matter in the effluent after treatment by the CoagulationSBBR-MF-NF, Fe(VI)SBBR-MF-NF and Fe(VI)-H2O2SBBR-MF-NF systems;

[0074] Fig. 20 (b) is a schematic diagram of the number of corresponding organic matter types in the effluent organic matter after treatment by CoagulationSBBR-MF-NF, Fe(VI)SBBR-MF-NF and Fe(VI)-H2O2SBBR-MF-NF systems. DETAILED DESCRIPTION

[0075] Embodiment 1:

[0076] A method for deep treatment of oil and gas field fracturing flowback fluid, the fracturing flowback fluid used is from a well in Xinjiang oil and gas field, as shown in Table 1, the oil and gas field fracturing flowback fluid cannot meet the reinjection requirements without treatment, and will cause great harm to the environment if it is directly discharged. At the same time, its BOD5 value is extremely low (the inoculated liquid is the effluent from the sewage treatment plant), the B / C value is only 0.04, the biochemical performance is extremely poor, and the high salinity makes it difficult for microorganisms to survive, so it is not suitable for direct biochemical treatment.

[0077] Table 1

[0078]

[0079] The specific processing steps of the present invention are as follows:

[0080] Step 1: Pre-treat the fracturing flowback fluid with polyaluminium chloride (PAC) and polyacrylamide (PAM) to effectively remove suspended particles.

[0081] Add different concentrations of PAC to the fracturing flowback fluid, stir rapidly at 800r / min for 10 minutes, add 50mg / L PAM, stir slowly at 200r / min for 5 minutes, and then let it stand for 10 minutes. Figure 1 As shown in the figure, when the PAC content exceeds 600 mg / L, the growth of TOC removal rate gradually slows down and tends to be stable. Keep the PAC dosage at 600 mg / L, stir quickly at 800 r / min for 10 minutes, adjust the PAM dosage, stir slowly at 200 r / min for 5 minutes and then let it stand for 10 minutes. When the PAM concentration reaches 40 mg / L, the TOC removal rate drops sharply. This is because PAM is an organic polymer, and excessive concentration of PAM will contribute additional TOC. Therefore, considering all factors, the optimal PAM concentration should be controlled at 20 mg / L.

[0082] 600 mg / L polyaluminium chloride (PAC) and 20 mg / L polyacrylamide (PAM) were used for coagulation pretreatment of the fracturing flowback fluid. As shown in Table 2, the turbidity of the fracturing flowback fluid was greatly reduced, and SS was also significantly removed. The coagulation pretreatment can significantly remove suspended solids, but the effluent index still does not meet the reinjection requirements, and the fracturing flowback fluid needs to be oxidized.

[0083] Table 2

[0084]

[0085] Step 2: Take a certain volume of pre-treated fracturing flowback fluid, adjust the pH to the required value, and perform oxidation treatment.

[0086] (1)Fe(VI)

[0087] The prepared ferrate solution was added to the fracturing flowback fluid, and samples were taken after a certain period of time. Sodium thiosulfate (0.5 M) was added to quench the solution, and the COD value, TOC value and UV 254 .like Figure 2 As shown in the figure, after 60 min of oxidation treatment, as the initial concentration of Fe(VI) gradually increased from 1000 mg / L to 3000 mg / L, COD, TOC and UV 254The removal rate of COD, TOC and UV increased significantly. 254 The removal rate decreased instead. Therefore, when the initial concentration of Fe(VI) was 3000 mg / L, the oxidation effect was the best.

[0088] Five different initial pH values ​​were set at the optimal Fe(VI) concentration and the reaction time was 60 min. The effects of different initial pH values ​​on COD, TOC and UV of Fe(VI) oxidized fracturing flowback fluid were investigated. 254 removal effect. Figure 3 As shown, when the initial pH = 8.0, COD, TOC and UV 254 The removal effect is the best. When the pH is 2.0-4.0, it is usually in the form of H2FeO4. When the pH is 4-8, Fe(VI) is in the form of HFeO4 - The solution exists in the form of FeO4. 2- Form exists.

[0089] (2)Fe(VI)-H2O2

[0090] Different H2O2 reaction concentrations were set according to the optimal concentration of potassium ferrate in the Fe(VI) system (the ratio of potassium ferrate concentration to H2O2 concentration was 1:10, 3:10, 1:2, 7:10, 1:1 and 6:5). H2O2 was added to the fracturing flowback fluid to reach the specified concentration. After the solution was stirred evenly, the optimal concentration of ferrate solution was immediately added to the fracturing flowback fluid. After a certain period of time, samples were taken and sodium thiosulfate (0.5M) was added to quench. The COD value, TOC value and UV of the solution were measured. 254 .

[0091] like Figure 4 As shown in the figure, with the gradual increase of H2O2 dosage, the removal rate of organic matter first increased and then decreased. Compared with direct oxidation by Fe(VI), when the H2O2 dosage (Fe(VI):H2O2, n / n) was 1:2, the removal rate of COD in fracturing flowback fluid increased by 10.2%; the removal rate of TOC in fracturing flowback fluid increased by 15.3%; and the removal rate of UV in fracturing flowback fluid increased by 10.2%. 254The removal rate increased by 11.7%. The strengthening effect of H2O2 is to directly or indirectly accelerate the formation of Fe(IV) and Fe(V). Further, the reaction between Fe(V) or Fe(IV) and some organic matter can degrade the organic pollutants in the fracturing flowback fluid and generate Fe(II) and Fe(III). At the same time, the introduction of H2O2, the formation of Fe(V), Fe(IV), Fe(II) and Fe(III) in the system and the Fenton-like system produce ·OH, which achieves the purpose of degrading pollutants through the action of free radicals. With the increase of H2O2 addition, the degradation ability of pollutants in the fracturing flowback fluid gradually weakened, which may be due to the presence of excessive H2O2 in the solution, which becomes a scavenger of ·OH free radicals.

[0092] like Figure 5 As shown in the figure, the signal peak intensity of ·OH in the Fe(VI)-H2O2 system is more obvious. Adding H2O2 to Fe(VI) will improve the ability of Fe(VI) to degrade fracturing flowback fluid. Fe(VI) system: Adding tert-butyl alcohol TBA did not completely inhibit the degradation of pollutants, indicating that other active substances may exist in the reaction process. Fe(VI)-H2O2 system: After adding exogenous H2O2, the inhibitory effect of TBA is stronger, and a high-intensity ·OH signal peak is shown in EPR, which indicates that ·OH may be the main active substance in the system.

[0093] In order to further study the contribution of other active substances such as Fe(IV) / Fe(V), the amount of intermediate valence iron generated was indirectly measured by detecting the content of methyl phenyl sulfoxide PMSO and methyl phenyl sulfone PMSO2 during the reaction. Figure 6 As shown, the consumption of PMSO and the generation of PMSO2 indicate that Fe(IV) / Fe(V) indeed participates in the three reaction systems. The generation of Fe(IV) / Fe(V) is Fe(VI)>Fe(VI)-H2O2.

[0094] The test found that the B / C value of the untreated fracturing flowback fluid was only 0.040, and the B / C value after coagulation treatment was 0.058, but it was still difficult to treat biologically. After treatment with Fe(VI) and Fe(VI)-H2O2, its B / C ratio increased by 135.0% and 333.0%, respectively, indicating that Fe(VI) oxidation can be used as a pretreatment method for biological treatment of fracturing flowback fluid, making it have the potential for biological treatment. Among the oxidation methods, Fe(VI)-H2O2 has the most obvious ability to improve the biodegradability of fracturing flowback fluid.

[0095] However, it is generally believed that wastewater is considered biodegradable only when the B / C ratio is greater than 0.3. When the effluent from the sewage treatment plant is used as the inoculum, the B / C ratio is only 0.173 under the optimal oxidation conditions. Further use of the domesticated bacterial solution as the inoculum significantly improves BOD5. After treatment with Fe(VI) and Fe(VI)-H2O2, the B / C ratios reach 0.275 and 0.436, respectively. The fracturing flowback fluid after enhanced oxidation treatment is more suitable for biological treatment.

[0096] As shown in Table 3, the SS content and turbidity of the fracturing flowback fluid after coagulation treatment are still very high. After Fe(VI)-H2O2 enhanced oxidation, SS <20 mg / L, meeting the SS requirements of the Class III water quality indicators in the "Technical Requirements and Analysis Methods for Water Quality Indicators for Clastic Reservoirs" (SY / T 5329-2022). After oxidation treatment, the removal rate of oil content reached more than 90%, but only met the requirements of the oil content corresponding to Class V water quality in the standard. Although the SS can be significantly reduced after oxidation treatment, the median particle size of the suspended matter is still very high, which cannot meet the water injection requirements.

[0097] Table 3

[0098]

[0099] (Note: Except for pH, turbidity, B / C which are dimensionless and the median particle size is μm, other indicators are in mg / L: “-” represents a negative value; BOD5(1) and B / C(1) refer to the use of sewage treatment plant inoculum; BOD5(2) and B / C(2) refer to the use of domesticated effluent inoculum; the removal rate is calculated based on the coagulation effluent value.)

[0100] The strengthening effect of the Fe(VI)-H2O2 system is mainly due to the Fenton-like reaction between the intermediate valence iron of Fe(VI) and H2O2. Compared with the traditional Fenton system, the Fe(VI)-H2O2 system has milder pH conditions, and the pH range adapted to Fe(VI) is wider. In most cases, no additional adjustment of pH is required before the reaction. Secondly, the Fe(VI) system is less effective in removing chloride ions than the Fe(VI)-H2O2 system. The Fe(VI) system is mainly composed of Fe(IV) / Fe(V), which is not highly reactive with chloride ions. The Fe(VI)-H2O2 system mainly relies on ·OH oxidation.

[0101] like Figure 7As shown in the figure, compared with the coagulation effluent, the oxidation process of the Fe(VI) system has little effect on the types of organic matter, and mainly reduces the abundance of various organic matter. The Fe(VI)-H2O2 system has a more obvious removal effect on organic matter with a small relative molecular weight, especially the Fe(VI)-H2O2 system, in which the low molecular weight organic matter in the product is almost removed. At the same time, the relative abundance of organic matter in the product of the Fe(VI)-H2O2 system is lower than that of the Fe(VI) system, indicating that the removal effect of organic matter is better after strengthening ferrate.

[0102] Step 3: Use sequencing batch biofilm method (SBBR) to treat the fracturing return fluid.

[0103] The SBBR reactor experiment used fiber composite materials and simulated urban domestic sewage for 12 days of biofilm formation. After the SBBR reactor completed salinity and pollutant acclimation, it entered the formal biofilm treatment stage. The effects of fracturing flowback fluid before and after different oxidation methods on the microbial treatment effect were investigated through three treatment groups (Coagulation-SBBR, Fe(VI)-SBBR and Fe(VI)-H2O2-SBBR).

[0104] Twelve 1L beakers with a diameter of 11cm and a height of 16cm were selected as SBBR treatment reaction vessels. The fiber composite material after biofilm was transferred to the reactor. Each reactor was filled with a membrane sheet with a spacing of 30mm, totaling 3 sheets (upper, middle and lower). The pH of the fracturing return fluid with different oxidation treatments was adjusted to neutral with NaHCO3 and introduced into the reactor. The SBBR treatment process lasted for 21 days, with a cycle of 7 days. Three treatment groups were set up according to different oxidation methods, each with 3 parallel treatment groups. The A / O / A / O method was adopted, with a cycle of 24 hours, including anoxic (4 hours), aerobic (13 hours), anoxic (3 hours), and aerobic (4 hours). The water was changed after 2 days of circulation, and 90% of the water was discharged each time, with a gas-water ratio of about 10:1.

[0105] like Figure 8 As shown in the figure, the Fe(VI)-H2O2-SBBR effluent TOC removal rate was the best, reaching more than 95.0%, indicating that oxidation treatment helped improve effluent water quality. Secondly, the Fe(VI)-H2O2-SBBR effluent TOC dropped to 12.2 mg / L, indicating that the water quality after H2O2-enhanced Fe(VI) oxidation treatment was more easily utilized by microorganisms, and its better biodegradability also confirmed this point. Fig. 9As shown, Fe(VI)-H2O2-SBBR has the best effluent COD removal rate, which fluctuates in the range of 70.0% to 90.0% overall, and the effluent COD of Fe(VI)-H2O2-SBBR drops to 91.6 mg / L. Among the three treatment groups, Fe(VI)-H2O2-SBBR has the lowest effluent COD, which is similar to the effluent situation of TOC. This phenomenon may be attributed to two points: first, Fe(VI)-H2O2 has high oxidation efficiency and its influent COD is low, resulting in lower effluent COD; second, the system has high biodegradability and microorganisms have a stronger ability to degrade COD. Fig.10 As shown, Fe(VI)-H2O2-SBBR effluent UV 254 The removal rate showed a steady upward trend, indicating that the organic products of Fe(VI) after enhanced oxidation by H2O2 were more easily utilized by microorganisms. 254 The best removal rate reached more than 85.0%, indicating that microorganisms capable of degrading aromatic compounds may have evolved in the reactor during operation, thus 254 As shown in Table 4, after biological treatment, the organic matter (TOC, COD and UV 254 ) were significantly removed, and Fe(VI)-H2O2-SBBR had the best effect in treating organic matter, TOC, COD and UV 254 The removal rates of the three treatment groups were all greater than 90.0%. The removal effects of TP in the three treatment groups were similar, and the removal rate of TP in the effluent reached more than 60.0%.

[0106] Table 4

[0107]

[0108] At the same time, the oxidation-SBBR treatment process has a good removal effect on SS and oil content in fracturing flowback fluid. Although the SS and oil content of fracturing flowback fluid after oxidation-SBBR treatment can meet the Class II water quality indicators of "Technical Requirements and Analysis Methods for Water Quality Indicators of Clastic Reservoirs" (SY / T 5329-2022), oxidation-SBBR cannot effectively improve the median particle size of the effluent, and the median particle size of the effluent is still too large to meet the reinjection requirements. Secondly, the salinity of the effluent of each group increased after SBBR treatment. Therefore, after biological treatment, a suitable membrane treatment process is required to improve the effluent quality, on the one hand, to intercept suspended solids, colloids or organic matter with larger particle sizes; on the other hand, to remove salt ions in the fracturing flowback fluid.

[0109] In order to further reveal the treatment process of fracturing flowback fluid by SBBR, GC-MS analysis was performed on the treated water, such as Fig.11As shown in the figure, the abundance of organic matter in each system was significantly reduced after biological treatment, and the fracturing flowback fluid after oxidation treatment was more conducive to microbial degradation. Fig.12 As shown in the figure, the efficiency of biological treatment can be improved after enhanced Fe(VI) oxidation of fracturing flowback fluid. Compared with the Coagulation-SBBR system, the relative abundance of phenolic organic matter in the effluent of Fe(VI)-SBBR and Fe(VI)-H2O2-SBBR is lower, indicating that oxidation-SBBR has a better removal effect on most phenolic organic matter. Fe(VI)-H2O2-SBBR generates more alkane organic matter and completely degrades amine organic matter. Secondly, combined with the changes in the types of organic matter, the amount of effluent organic matter in the oxidized fracturing flowback fluid after SBBR treatment is further reduced, indicating that the biological treatment process can decompose some oxidation products into CO2 and H2O.

[0110] Step 4: After the effluent from the SBBR reactor is pre-treated, it is subjected to microfiltration membrane and nanofiltration membrane membrane tests in turn through a high-pressure flat membrane experimental equipment (TYLG-19).

[0111] (1) Effect of oxidation-SBBR treatment on mitigation of MF membrane fouling:

[0112] The effluent from the fracturing flowback fluid after coagulation (Coagulation), the fracturing flowback fluid treated by two oxidation methods (Fe(VI) and Fe(VI)-H2O2), and the fracturing flowback fluid after subsequent biological treatment were filtered by a high-pressure flat membrane device through a microfiltration membrane (MF). Fig.13 As shown in the figure, the membrane flux is improved after oxidation treatment. The oxidation process can convert some macromolecular organic matter into small molecular organic matter, thereby increasing the membrane flux and reducing MF membrane pollution. After biological treatment, the membrane flux is greatly improved, and it only takes 30 minutes to reach a stable flux. Biological treatment is the main process of degrading organic matter, and the oxidation treatment before biological treatment can indirectly increase the membrane flux. Fig.14 As shown in the figure, compared with the original membrane, the typical peak intensity of the membrane after Fe(VI)-MF and Fe(VI)-H2O2-MF was weakened. After biological treatment, it can be clearly seen that the typical peaks on the membranes of the three treatment groups are clearly evident. In addition, the typical peak signal is weaker after SBBR treatment, indicating that oxidation combined with biofilm treatment can effectively reduce the membrane fouling of MF membrane.

[0113] (2) Effect of oxidation-SBBR treatment on mitigation of NF membrane fouling

[0114] The filtrates from the MF were subjected to NF filtration to remove salt ions from the water samples, and the filtrates were filtered through a high-pressure flat membrane device using a nanofiltration membrane (NF). Fig.15As shown in the figure, after the untreated fracturing flowback fluid was filtered through the MF membrane, its effluent reached the stable membrane flux of the NF membrane after 60 min, among which the membrane flux of the two oxidation-MF-NF treatment groups was only 3.0 L·m -2 ·h -1 This is because the pore size of MF is large, which can only intercept some suspended matter and bacteria, and some organic matter cannot be intercepted. These organic matter will seriously affect the membrane efficiency of NF. Fe (VI) oxidation treatment can convert these high molecular organic matter into small molecular organic matter, thereby increasing membrane flux and reducing NF membrane pollution. However, after biological treatment-MF, the membrane flux has been greatly improved, and it only takes 30 minutes to reach a stable flux, and the membrane flux exceeds 25.0L·m -2 ·h -1 , indicating that SBBR biological treatment technology is the key to improving NF membrane flux.

[0115] like Fig.16 As shown in Figure 2, after MF filtration, the highest membrane flux in the oxidized-SBBR-NF treatment group increased by 6 L·m -2 ·h -1 This is because after biological treatment, there may still be residual microorganisms in the water sample after simple filtration. If it is not filtered by MF, the microorganisms in the solution will block the NF membrane pores and reduce the filtration efficiency of NF. Secondly, there are many small molecules in the water sample after oxidation-biological treatment, which will further intercept large molecules through the adsorption effect on the membrane surface and the interception effect of the internal network during the MF filtration process, thereby improving the filtration efficiency of NF. Therefore, MF is indispensable in the membrane filtration process.

[0116] like Fig.17 As shown in the figure, by comparison, it can be found that biological treatment has little effect on the NF membrane fouling layer. Combined with the membrane flux analysis, the fracturing return fluid treated by oxidation-SBBR may be mainly composed of small molecules, which will not quickly cause membrane plugging in the NF process. Fe(VI) oxidation and SBBR treatment play an important role in the entire process system. On the one hand, the oxidation process can improve the biodegradability of the fracturing return fluid and is conducive to biodegradation; on the other hand, there are some difficult-to-degrade organic matter in the fracturing return fluid, and biological treatment alone cannot achieve a better removal effect. These organic matter will seriously affect the membrane flux. Therefore, the combined effect of the two can improve the treatment effect of the fracturing return fluid and reduce the membrane pollution of the NF, extending the service life of the NF.

[0117] (3) Microfiltration (MF) effluent water quality analysis

[0118] As shown in Tables 5 and 6, MF has a certain interception effect on organic matter and nitrogen and phosphorus in fracturing flowback fluid, but the interception effect is poor. After the fracturing flowback fluid after oxidation treatment, the removal effect of organic matter is significantly improved after MF filtration, which is increased by about 20.0%. Among them, Fe(VI)-H2O2-MF has a certain interception effect on UV 254 The removal effect increased by about 40.0%. After biological treatment, the removal rates of organic matter and nitrogen and phosphorus were significantly improved, and the removal rates were all higher than 60.0%. Combined with oxidation treatment, the removal rate of organic matter can reach more than 90.0%. In addition, the chloride ion concentration, calcium and magnesium ion concentration and salinity of the MF filtered water (compared with the effluent after biological treatment) were slightly reduced. Due to the adsorption effect on the membrane surface and the interception effect of the internal network, some ions may also be intercepted or interact with the membrane material, thereby reducing the number of ions passing through the membrane. However, the MF membrane pores are large and cannot effectively remove organic matter and salt ions. The salt ion concentration of the effluent is still very high, which will still cause damage to the formation and equipment during the reinjection process. Therefore, NF is needed to further improve the effluent quality after MF.

[0119] Combination Fig.11 and Fig.18 It can be found that the types of organic matter after MF filtration are almost unchanged compared with those after biological treatment, but their relative abundance is significantly reduced, indicating that the MF process can remove some organic matter and improve the influent quality of the next membrane, thereby improving the efficiency of subsequent membrane processes and the service life of subsequent membranes.

[0120] Table 5

[0121]

[0122] Table 6

[0123]

[0124] (4) Analysis of water quality after nanofiltration (NF) filtration

[0125] As shown in Tables 7 and 8, the removal rate of organic matter in the fracturing flowback fluid after only NF treatment reached more than 90.0%, the organic matter removal rate reached more than 95.0% after oxidation treatment, and the organic matter removal rate reached more than 97.0% after oxidation-biological combined treatment, and the effluent COD can be lower than 20.0 mg / L. Oxidation-biological combined treatment can also improve the removal effect of TN in fracturing flowback fluid to more than 85.0%. After NF filtration, calcium and magnesium ions are less than 2.5 mg / L, and the removal rates of barium and strontium ions are as high as more than 95.0%. Based on multi-process combined treatment, the removal effect of chloride ions can be increased, and the removal rate of effluent salinity can reach 65%. In addition, it can be found that the removal rate of organic matter in the effluent of Coagulation-SBBR-MF-NF is not much different from that of the effluent of NF after oxidation-biological combined treatment, but the treatment process will cause greater membrane pollution, indicating that oxidation-biological treatment can reduce NF membrane pollution while ensuring the quality of effluent.

[0126] Table 7

[0127]

[0128] Table 8

[0129]

[0130] like Fig.19 As shown in the figure, the types and relative abundance of organic matter in the effluent after filtration by NF membrane are significantly reduced, indicating that NF can effectively remove organic matter. Fig. 20 As shown, it can be seen that the types of organic matter are reduced after NF membrane filtration, and the products are mainly phenols. In addition, comparing the amount of each type of organic matter in the product, Fe(VI)-H2O2-SBBR-MF-NF is the least. Combined with the water quality of its effluent, this process has the best treatment effect in the treatment of fracturing return fluid.

[0131] (6) Evaluation of effluent quality from membrane filtration process

[0132] As shown in Table 9, through the analysis of the water quality at each stage, after only enhanced oxidation treatment, the indicators of the effluent did not meet the Class I and Class II standards in the "Technical Requirements and Analysis Methods for Water Quality Indicators of Clastic Reservoirs" (SY / T 5329-2022), and could not be reinjected. After oxidation-SBBR treatment, SS and oil content reached the Class II reinjection water quality standard, indicating that biological treatment is a key step in improving the water quality of fracturing return fluid. Oxidation treatment improves the biodegradability of water quality, thereby further strengthening the biological treatment process. However, a higher concentration of salt ions is introduced during the biological treatment process. Although there is no requirement for salinity in the reinjection standard, high-salt fracturing return fluid will cause formation scaling, pipeline corrosion and other problems during the reinjection process. Secondly, the median particle size of the effluent after biological treatment is still high, and there are still problems such as difficulty in water injection. After combining MF and NF filtration, the effluent quality is significantly improved. On the one hand, most of the salt ions can be removed, and on the other hand, the SS and oil content can reach the Class I reinjection standard, and the median particle size can reach the Class II reinjection standard, which can realize the reinjection and reuse of oil and gas field fracturing return fluid.

[0133] As shown in Table 10, the effluent COD and TOC of the coagulation-biofilm-membrane filtration process and the oxidation-biofilm-membrane filtration process can meet the first-level indicators of the "Comprehensive Wastewater Discharge Standard" (GB 8978-1996), but after oxidation treatment, the effluent water quality is better than that of the coagulation-biofilm-membrane filtration process. In addition, in practical applications, combined with the membrane flux and considering the impact on membrane pollution, Fe(VI)-H2O2-SBBR-MF-NF has the best effluent effect while improving membrane pollution, indicating that the oil and gas field fracturing return fluid not only meets the requirements of reinjection water quality level II after being treated with H2O2-enhanced Fe(VI) oxidation-SBBR-MF-NF, but also has the potential to achieve the external discharge of fracturing return fluid treatment.

[0134] Table 9

[0135]

[0136] Table 10

[0137]

[0138] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for deep treatment of oil and gas field fracturing flowback fluid, characterized in that: The following steps are involved: Step S1: performing coagulation pretreatment on the fracturing flowback fluid; Step S2: adjusting the pH value of the fracturing flowback fluid to a value less than 9; then, oxidizing the fracturing flowback fluid with potassium ferrate or H2O2 and potassium ferrate, wherein the molar mass ratio of H2O2 to potassium ferrate is 1:1-3.5; Step S3: using a biofilm reactor to treat the fracturing flowback fluid after oxidation treatment to remove organic matter in the fracturing flowback fluid; Step S4: using a microfiltration membrane and a nanofiltration membrane in sequence to perform membrane filtration on the fracturing flowback fluid.

2. The deep treatment method of oil and gas field fracturing flowback fluid according to claim 1, characterized in that: In step S2, the initial concentration of potassium ferrate is 2000-4000 mg / L.

3. The deep treatment method of oil and gas field fracturing flowback fluid according to claim 2, characterized in that: The initial concentration of potassium ferrate is 3000 mg / L.

4. A method for deep treatment of oil and gas field fracturing flowback fluid according to any one of claims 1 to 3, characterized in that: In step S2, the pH value of the adjusted fracturing flowback fluid is 7-8, and the molar mass ratio of H2O2 to potassium ferrate is 1:

2.

5. The deep treatment method of oil and gas field fracturing flowback fluid according to claim 1, characterized in that: In the step S1, polyaluminium chloride (PAC) and polyacrylamide (PAM) are used to perform coagulation pretreatment on the fracturing flowback fluid; in the fracturing flowback fluid, the concentrations of polyaluminium chloride (PAC) and polyacrylamide (PAM) are 400 mg / L-800 mg / L and 10 mg / L-40 mg / L, respectively.

6. The deep treatment method of oil and gas field fracturing flowback fluid according to claim 1, characterized in that: In the step S3, before treating the fracturing flowback fluid, firstly, the biofilm reactor is subjected to biofilm formation, and then the biofilm reactor is subjected to salinity acclimation and pollutant acclimation in sequence.

7. The deep treatment method of oil and gas field fracturing flowback fluid according to claim 6, characterized in that: In the step S3, when treating the fracturing flowback fluid, the biofilm reactor sequentially operates in the anoxic stage, the aerobic stage, the anoxic stage and the aerobic stage, and the operating time ratio is 4:13:2.5:

4.

8. The method for deep treatment of oil and gas field fracturing flowback fluid according to claim 1, characterized in that: In the step S3, before filtering, the pH of the fracturing flowback fluid is adjusted to neutral; the influent TOC:TN:TP of the biofilm reactor is 100:(1-1.5):(0.05-0.1).

9. A deep treatment system for oil and gas field fracturing flowback fluid, based on the deep treatment method for oil and gas field fracturing flowback fluid according to any one of claims 1 to 8, characterized in that: The invention comprises a flocculation pretreatment module, an oxidation treatment reaction tank, a biofilm filtration tank, a microfiltration membrane filtration tank and a nanofiltration membrane filtration tank which are arranged in sequence from front to back.

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