Electrochemical pretreatment process for shale gas fracturing flow-back fluid

Abstract

The invention relates to the field of wastewater treatment, and discloses an electrochemical pretreatment process for shale gas fracturing flow-back fluid. Comprising the following steps: step 1, electrochemical pretreatment: introducing the shale gas fracturing flow-back fluid into an electrocatalytic oxidation pond, and reacting under an electrifying condition to obtain a pretreatment fluid; step 2, plate microporous filtration: filtering the pretreatment liquid to obtain filtrate; 3, reverse osmosis treatment: carrying out reverse osmosis treatment on the filtrate by adopting a disc type reverse osmosis membrane to obtain fresh water 1; and step 4, electrodialysis: carrying out electrodialysis concentration on the fresh water 1 to obtain outlet produced water, concentrated water 1 with the TDS content of more than or equal to 20% and concentrated water 2 with the TDS content of less than or equal to 4%, controlling the conductivity of the outlet produced water to be less than or equal to 0.10 [mu] s / cm, returning the concentrated water 2 to the step 1, and repeating the treatment of the steps 1-4. According to the electrochemical pretreatment process for the shale gas fracturing flow-back fluid, the removal rate of calcium ions and magnesium ions can be increased, and meanwhile the treatment cost is reduced.

Description

Technical Field

[0001] The present invention relates to the field of wastewater treatment, and particularly to an electrochemical pretreatment process for shale gas fracturing flowback fluid. Background Art

[0002] Shale gas refers to unconventional natural gas stored in reservoir rock series mainly composed of organic-rich shale. Shale gas is usually extracted by fracturing technology. Therefore, flowback fluid will be generated during the extraction process. A large amount of metal ions, organic compounds and ammonia nitrogen elements are contained in this flowback fluid. Direct discharge will pollute the surrounding water sources, so it needs to be treated before discharge.

[0003] Regarding the treatment of the flowback fluid generated during the shale gas extraction process, traditional treatment methods, as shown in a pretreatment device for shale gas flowback fluid disclosed in the application number CN202123382352.2 applied by our company, add chemical agents such as PAC and PAM through a dosing system for precipitation. However, it is found in actual application that when the metal ions in the flowback fluid include calcium ions and magnesium ions, there is a problem of incomplete precipitation during the precipitation with agents, resulting in relatively low removal rates of calcium and magnesium ions. And the cost of the agents is relatively high. In the case of a large amount of flowback fluid, using a large amount of agents will undoubtedly generate huge treatment costs. Summary of the Invention

[0004] The present invention aims to provide an electrochemical pretreatment process for shale gas fracturing flowback fluid to improve the removal rates of calcium ions and magnesium ions and reduce the treatment cost at the same time.

[0005] To achieve the above object, the present invention adopts the following technical scheme: An electrochemical pretreatment process for shale gas fracturing flowback fluid, comprising the following steps:

[0006] Step 1: Electrochemical pretreatment. Introduce the shale gas fracturing flowback fluid into an electrocatalytic oxidation cell. Under the condition of energization, a pretreatment liquid is obtained by reaction, and the effluent turbidity of the pretreatment liquid should be ≤20.

[0007] Step 2: Flat plate microfiltration. Filter the pretreatment liquid to obtain a filtrate.

[0008] Step 3: Reverse osmosis treatment. Perform reverse osmosis treatment on the filtrate using a disc type reverse osmosis membrane to obtain fresh water 1.

[0009] Step 4: Electrodialysis. Concentrate the fresh water 1 by electrodialysis to obtain outlet product water, concentrated water 1 with a TDS content greater than or equal to 20%, and concentrated water 2 with a TDS content less than or equal to 4%. Control the conductivity of the outlet product water ≤0.10 μs / cm, and return the concentrated water 2 to Step 1 to repeat the treatment of Steps 1-4.

[0010] The beneficial effects of this solution are:

[0011] 1. This solution removes organic matter in wastewater through electrochemical pretreatment. Under the condition of power-on, electric energy acts as a catalyst, and hydrogen peroxide, oxygen, and ozone are used as oxidants for oxidation reactions, thereby reducing the hardness of shale gas fracturing flowback fluid. During the oxidation reaction process, hydroxyl radical intermediates (·HO) are generated in the oxidation system, and the hydroxyl radical intermediates are used as the main oxidants to react with organic matter. At the same time, organic radicals or organic peroxide radicals can be generated during the reaction and continue to react, enabling more efficient and thorough decomposition of organic matter.

[0012] 2. After the microfiltration in Step 2, the penetrability of the microfiltration membrane is used to isolate some impurities, and then colloidal silicon and suspended solids can be further removed.

[0013] Furthermore, sludge is obtained simultaneously in Step 1, and Step 5 also includes: sludge treatment, dehydrating the sludge until the moisture content of the sludge is less than or equal to 60%, and treating the sewage obtained by dehydration through Steps 1 to 4.

[0014] The beneficial effects of this solution are as follows: This solution can separate the sewage in the sludge, further reduce the volume and weight of the sludge and lower the water content of the sludge, facilitating the unified collection of the sludge and transporting the sludge to another treatment process for treatment. The separated sewage can be treated together with the shale gas fracturing flowback fluid to be treated by this method without additional treatment steps, and the operation is simple and convenient.

[0015] Furthermore, backwash water is obtained simultaneously in Step 2. In Step 5, the backwash water is mixed with the sludge obtained in Step 1, and then sewage dehydration treatment is carried out.

[0016] The beneficial effects of this solution are as follows: The ultrafiltration backwash water aims to remove the inorganic substances attached to the microfiltration membrane. The microfiltration membrane is rinsed by a backwash pump and a blower. The wastewater after rinsing contains aggregates of inorganic substances and various fine impurities. By mixing with the pretreated sludge, it is uniformly sent to a sludge dehydrator for treatment.

[0017] Furthermore, the water with a TDS content less than or equal to 250 mg / L obtained in Step 3 is marked as passing the detection.

[0018] The beneficial effects of this solution are as follows: The water marked as passing the detection can be directly discharged or recycled for reuse.

[0019] Furthermore, the concentrated water 1 is collected in Step 4.

[0020] The beneficial effects of this solution are as follows: After collecting the concentrated water 1 in this solution, it is convenient to uniformly transport it to another treatment process for centralized treatment. Description of the Drawings

[0021] Figure 1 Process flow chart of Embodiment 1 of the present invention;

[0022] Figure 2 Process flow chart of Comparative Example 2 of the present invention;

[0023] Figure 3 Process flow chart of Comparative Example 3 of the present invention. Detailed implementation manners

[0024] The following is a further detailed description through specific implementation manners:

[0025] Embodiment 1

[0026] Electrochemical pretreatment process for shale gas fracturing flowback fluid, combined Figure 1 , including the following steps:

[0027] Step 1: Electrochemical pretreatment. Introduce the shale gas fracturing flowback fluid into the electrocatalytic oxidation cell. Under the condition of energization, a pretreatment liquid is obtained through reaction, and the effluent turbidity of the pretreatment liquid should be ≤20;

[0028] Step 2: Flat microporous filtration. Filter the pretreatment liquid to obtain a filtrate;

[0029] Step 3: Reverse osmosis treatment. Use a disc-type reverse osmosis membrane to perform reverse osmosis treatment on the filtrate. The water with a TDS content less than or equal to 250 mg / L obtained is marked as qualified for detection, so as to facilitate the unified discharge or recycling of the qualified fresh water; the water with a TDS content greater than 250 mg / L is marked as fresh water 1;

[0030] Step 4: Electrodialysis. Concentrate the fresh water 1 through electrodialysis to obtain concentrated water 1 with a TDS content greater than or equal to 20% and concentrated water 2 with a TDS content less than or equal to 4%; collect the concentrated water 1 for unified treatment, and return the concentrated water 2 to Step 1 to repeat the treatment of Steps 1-4;

[0031] Step 5: Sludge treatment. After using for a certain period of time, perform backwashing on the flat microporous filtration structure in Step 2 to obtain backwash water. Specifically, the time interval between two backwashes is determined according to the amount of sludge remaining in the flat microporous filtration structure, and backwashing is performed before the flat microporous filtration structure is blocked by sludge; mix the backwash water with the sludge obtained in Step 1, dehydrate the sludge until the moisture content of the sludge is less than or equal to 60%, and perform the treatment of Steps 1-4 on the sewage obtained by dehydration.

[0032] The present invention uses the treatment method of a pretreatment device for shale gas flowback fluid with the application number CN202123382352.2 applied by our company as Comparative Example 1. The qualified fresh water obtained in Step 3 of the above embodiment is marked as produced water; in Comparative Example 1, the gas field water transported to the storage tank is marked as produced water.

[0033] Comparative Example 2 and Comparative Example 3 are also disclosed:

[0034] In Comparative Example 2, as Figure 2 shown, after the sewage is precipitated, the sewage is filtered in the same filtration method as in Step 2 of this embodiment to obtain a filtrate, and then the filtrate is subjected to osmosis treatment by DTRO reverse osmosis technology, and the obtained fresh water is recorded as product water;

[0035] In Comparative Example 3, as Figure 3 shown, the DTRO reverse osmosis technology in Comparative Example 2 is replaced with BWRO reverse osmosis technology, and the obtained fresh water is also recorded as product water.

[0036] The product water obtained in Example 1 of this embodiment and Comparative Examples 1 to 3 is divided by the untreated sewage, and the result is recorded as the total product water yield. At the same time, the desalination rate of the product water is measured compared with the sewage before treatment, and the treatment cost of treating 1 ton of sewage with the same degree of pollution is counted. The results are shown in the following table:

[0037]

[0038] According to the above table, it is found that the treatment cost of treating sewage by using Example 1 of this embodiment is lower, but the product water is more, and the desalination effect is not significantly reduced. Therefore, the pretreatment process of Example 1 takes into account the treatment cost, treatment efficiency and treatment quality at the same time.

[0039] The above are only the embodiments of the present invention. Specific technical solutions and / or common knowledge such as characteristics well known in the art are not described in detail herein. It should be noted that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can be made, and these should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners described in the specification can be used to explain the content of the claims.

Claims

1. The electrochemical pretreatment process of shale gas fracturing flowback fluid is characterized by: The following steps are involved: Step 1: Electrochemical pretreatment: introduce shale gas fracturing flowback fluid into an electrocatalytic oxidation tank, react under power-on conditions to obtain a pretreated fluid, and control the effluent turbidity of the pretreated fluid to be ≤20; Step 2: Filter the pretreated liquid by flat plate microfiltration to obtain a filtrate; Step 3: Reverse osmosis treatment, using a disc-type reverse osmosis membrane to perform reverse osmosis treatment on the filtrate to obtain fresh water 1; Step 4: electrodialysis. Concentrate the fresh water 1 by electrodialysis to obtain outlet water, concentrated water 1 with a TDS content greater than or equal to 20%, and concentrated water 2 with a TDS content less than or equal to 4%. Control the conductivity of the outlet water to be ≤0.10μs / cm, return the concentrated water 2 to step 1, and repeat the processing of steps 1 to 4.

2. The electrochemical pretreatment process for shale gas fracturing flowback fluid according to claim 1, characterized in that: Step 2 also obtains sludge, and also includes step 5: sludge treatment, dehydrating the sludge until the water content of the sludge is less than or equal to 60%, and treating the wastewater obtained by dehydration in steps 1 to 4.

3. The electrochemical pretreatment process for shale gas fracturing flowback fluid according to claim 2, characterized in that: After a certain period of use, the flat microporous filtration structure in step 2 is backwashed to obtain backwash water. In step 5, the backwash water is mixed with the sludge obtained in step 1, and then the sewage is dehydrated.

4. The electrochemical pretreatment process for shale gas fracturing flowback fluid according to claim 1, characterized in that: Step 3: Mark the water with a TDS content of less than or equal to 250 mg / L as meeting the test standard.

5. The electrochemical pretreatment process for shale gas fracturing flowback fluid according to claim 1, characterized in that: Step 4 collects concentrated water 1.

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