In-situ iron removal and corrosion prevention and scale inhibition synergistic treatment process for high-salt and high-iron discharged and collected water

Abstract

The invention discloses an in-situ iron removal and corrosion prevention and scale inhibition cooperative treatment process for high-salt and high-iron discharged and produced water, and relates to the technical field of coal bed gas exploitation water treatment. The method comprises the following steps: directionally converting dissolved Fe < 2 + > and generating an iron-containing hydrolysate in situ through homogeneous regulation, pH regulation and aeration enhanced oxidation, completing coagulation and flocculation by taking the iron-containing hydrolysate as an inorganic coagulation core and cooperating with a polymeric flocculation aid, carrying out solid-liquid separation and fine filtration to obtain reinjection effluent, and meanwhile, adding a corrosion and scale inhibition agent to realize corrosion and scale inhibition cooperative control. According to the method, coagulant does not need to be additionally added, the agent consumption and the sludge yield can be greatly reduced, the total iron removal rate and the suspended matter removal rate both reach 99% or above, and effluent meets the stratum reinjection index. The process disclosed by the invention is good in salt-tolerant adaptability and stable in operation, can effectively solve the problems of difficulty in iron removal, prominent corrosion and scaling, easiness in blockage during reinjection and the like of the high-salt and high-iron water body, and is high in engineering applicability.

Description

Technical Field

[0001] This invention relates to the field of coalbed methane extraction water treatment technology, and in particular to a synergistic process for in-situ iron removal and corrosion and scale inhibition in high-salt and high-iron drainage water. Background Technology

[0002] In the large-scale development of coalbed methane resources, deep coalbed methane extraction commonly employs large-scale hydraulic fracturing technology. This results in high water consumption and flowback fluid production per well, with complex composition and significant water quality fluctuations in the flowback fluid. Improper handling can easily lead to water environment risks. In engineering applications, reinjection is the mainstream technical approach for the resource utilization and emission reduction of fracturing flowback fluid. The treated flowback fluid is sealed back into the formation via dedicated injection wells, with tight sandstone layers often used as safe reinjection formations. The long-term stable operation of the reinjection system is highly dependent on the control of reinjection water quality. Substandard water quality can easily cause particle blockage, chemical compatibility imbalance, and equipment corrosion damage in the reinjection pipeline network and reservoir section, ultimately leading to reduced injection capacity and formation damage.

[0003] Deep coalbed methane fracturing flowback fluids generally exhibit typical characteristics of high salinity, high iron content, high turbidity, and strong corrosivity, with high water mineralization (TDS reaching 2×10⁻⁶). 5 High salinity (mg / L), high total iron content (200–400 mg / L), high turbidity and suspended solids concentration, often weakly acidic pH, and highly corrosive. Increased ionic strength in high-salt environments alters ion activity and migration behavior; anions such as chloride ions can complex and stabilize Fe. 2+ This reduces dissolved oxygen mass transfer efficiency, making the iron oxidation and particle formation-removal process more complex. Simultaneously, high-salt, high-chlorine systems easily damage the passivation film on metal surfaces, leading to pitting and uniform corrosion, posing significant corrosion risks to surface facilities and reinjection tubing. Existing treatment technologies mostly rely on adding exogenous inorganic coagulants to remove iron and turbidity. However, in high-salt systems, these technologies generally suffer from problems such as large dosages, increased ion load in the water, and insufficient corrosion control, making it difficult to balance the long-term stability of reinjected water with engineering economics.

[0004] Therefore, there is an urgent need to develop an integrated treatment process that is suitable for the high-salt, high-iron, high-turbidity, and highly corrosive characteristics of water quality for the reinjection of fracturing flowback fluid in deep coalbed methane. Through optimization of key operating parameters and multi-unit coordinated control, the process can achieve a synergistic improvement in iron and turbidity removal, corrosion control, and reinjection adaptability, providing stable and reliable technical support for the safe and efficient reinjection of fracturing flowback fluid in the development of deep coalbed methane. Summary of the Invention

[0005] The purpose of this invention is to provide a synergistic process for in-situ iron removal and corrosion and scale inhibition in high-salt and high-iron drainage water, so as to solve the problems existing in the prior art.

[0006] To achieve the above objectives, the present invention provides the following solution: This invention provides a pretreatment method for coalbed methane drainage water reinjection, comprising the following steps: (1) After homogenization and adjustment, the pH of the raw water is adjusted to maintain the pH value of the water body in the range of 7.0 to 9.2; (2) Aeration and enhanced oxidation are carried out on the water obtained in step (1) to remove dissolved Fe from the water. 2+ Directed transformation, and in-situ generation of iron-containing hydrolysis products; (3) Add corrosion and scale inhibitors to the water after aeration and enhanced oxidation to achieve synergistic and stable regulation of corrosion and scaling; (4) Using the iron-containing hydrolysis products generated in situ as the coagulation core, a polymeric flocculant is added to complete the coagulation and flocculation reaction; (5) The water after coagulation and flocculation is subjected to solid-liquid separation and fine filtration in sequence to obtain treated effluent that meets the reinjection requirements.

[0007] Furthermore, the aeration-enhanced oxidation is performed using microporous aeration, with an aeration time of 20-80 min and an air-to-water ratio of 24:1-72:1.

[0008] Furthermore, the polymeric flocculant is anionic polyacrylamide (PAM), with a dosage of 1~6 mg / L.

[0009] Furthermore, coagulation and flocculation include rapid mixing for 0.5 to 5 minutes and slow flocculation for 5 to 40 minutes.

[0010] Furthermore, the dosage of the corrosion and scale inhibitor is 0.1~2.0 mL / L. More preferably, the dosage is 0.6 mL / L.

[0011] Furthermore, the solid-liquid separation is sedimentation, inclined tube separation, or air flotation; the fine filtration is sand filtration.

[0012] The present invention also provides a reinjection pretreatment system for implementing the above method, comprising, in sequence: a raw water inlet unit, a homogenization adjustment unit, a pH control unit, an aeration-enhanced oxidation unit, a synergistic stabilization control unit, an in-situ coagulation and flocculation unit, a solid-liquid separation unit, a fine filtration unit, and an effluent unit.

[0013] In the process of this invention, anionic polyacrylamide is used for coagulation and flocculation. The iron-containing hydrolysis products generated in situ serve as the inorganic coagulation core. Combined with anionic PAM, floc formation is achieved, sedimentation performance is optimized, suspended solids and total iron are efficiently removed, and the risk of fine particles entering the reinjection system is reduced.

[0014] When the oil content in the raw water is high, an oil removal / demulsification unit can be set up between the pH control unit and the aeration-enhanced oxidation unit. At the same time, cationic PAM is used as a demulsification and flocculant aid. Through charge neutralization and bridging, it promotes the destabilization of emulsified oil and the aggregation of oil droplets, so as to achieve floating or settling separation and eliminate the interference of oil relative to subsequent iron oxidation and floc structure.

[0015] Furthermore, the reinjection pretreatment system may also include an online monitoring unit and an automatic dosing control unit.

[0016] The purpose of this invention is to provide a pretreatment method and system for reinjection of water from high-salt, high-iron, and highly corrosive coalbed methane drainage. Through process chain integration and key unit functional coupling, a stable and engineerable treatment route is constructed. (1) By regulating the pH range and enhancing oxidation through aeration, the directional conversion of dissolved iron is achieved, and iron-containing hydrolysis products are generated in situ. (2) Using the iron-containing hydrolysis products generated in situ as the inorganic coagulation core, combined with a small amount of polymer flocculant, to achieve efficient solid-liquid separation; (3) Reduce the risk of system corrosion and scaling through synergistic regulation of corrosion inhibition and scale inhibition; (4) Combining solid-liquid separation with fine filtration reduces the entry of fine particles into the reinjection system and improves reinjection adaptability and operational stability.

[0017] In this invention, the raw water, after homogenization and adjustment, enters the pH control unit to maintain the pH in the neutral to slightly alkaline range; subsequently, it enters the aeration-enhanced oxidation unit to promote Fe... 2+ The process involves the conversion and in-situ generation of iron-containing hydrolysis products. In conditions requiring reduced corrosion risk, a synergistic stabilization and control unit is installed after aeration oxidation, and corrosion and scale inhibitors are added. The water then enters an in-situ coagulation and flocculation unit, utilizing the iron-containing hydrolysis products as the in-situ inorganic coagulation core. High-molecular-weight flocculants are added to achieve floc formation and growth. A solid-liquid separation unit removes the solid phase from the flocs, and the clarified water enters a sand filtration unit for further fine particle retention, yielding pre-treated effluent for reinjection. Furthermore, an oil removal / demulsification unit can be installed based on oil content characteristics; reinjection requirements can be met without additional disinfection, and online monitoring and automatic dosing can be configured for closed-loop operation.

[0018] This invention determines key operating parameter ranges through multi-factor synergistic regulation, establishing a stable matching relationship between pH control, aeration oxidation, and scale and corrosion inhibitor dosage, thereby reducing corrosion rates while ensuring particle removal efficiency. By controlling the dosage of polymeric flocculant and coagulation time, the floc structure and settling performance are optimized, reducing the risk of fine particles entering the reinjection system. The core technology of this invention lies in the synergistic optimization of operating parameters within a specific range.

[0019] The core control unit of this invention is pH range adjustment and aeration-enhanced oxidation. In single-well or low-fluctuation water quality conditions, raw water can directly enter the pH adjustment unit without the need for a separate homogenization unit. The homogenization unit serves only as an engineering buffer module for multi-well mixing or significant water flow fluctuations, its function being to reduce the impact of instantaneous water quality fluctuations on precise pH control and iron conversion efficiency. Therefore, the technical essence of this invention does not rely on a homogenization unit, but rather on pH regulation to construct a stable chemical environment for iron conversion.

[0020] When the oil content of the influent is high, the oil removal enhancement unit is preferably located after pH adjustment and before the aeration oxidation reaction. At this stage, the system pH is already in the neutral to slightly alkaline range, which is conducive to the destabilization of emulsified oil and the neutralization of interfacial charges. The oil removal enhancement unit uses cationic PAM as a demulsifier and flocculant aid, which promotes oil droplet aggregation and achieves floating or settling separation through charge neutralization and bridging, thereby reducing the interference of oil relative to the floc structure and settling behavior of subsequent iron oxidation products. When the oil content of the raw water is low, this unit can be omitted, and the aeration oxidation reaction stage can proceed directly.

[0021] The process chain of this invention does not include a separate disinfection unit. After pH adjustment, aeration-enhanced oxidation, in-situ coagulation and sedimentation, and fine filtration, suspended particles and their carrier microorganisms are effectively removed from the system. Simultaneously, the high-salt environment and oxidizing conditions inhibit microbial survival, and no total bacterial count was detected in the effluent. Therefore, no additional disinfection step is required to meet reinjection requirements.

[0022] The present invention discloses the following technical effects: This invention addresses the reinjection treatment needs of coalbed methane drainage water in high-salt, high-iron, and highly corrosive environments, achieving an integrated improvement in in-situ iron removal, corrosion and scale prevention, and water quality stabilization.

[0023] The process of this invention focuses on pH control and aeration-enhanced oxidation, using dissolved Fe... 2+ This invention utilizes targeted conversion and in-situ generation of iron hydrolysis products, which serve as the core of inorganic coagulation to achieve "iron-based iron control," eliminating the need for external coagulants and significantly reducing reagent costs and sludge production. Combined with a small amount of polymeric flocculant, it rapidly forms dense flocs. Through solid-liquid separation and fine filtration, the removal rates of total iron, suspended solids, and turbidity all exceed 99%, resulting in effluent particle size and turbidity levels far superior to reinjection standards. Furthermore, this invention effectively inhibits corrosion and scaling in high-salt, high-chlorine systems through synergistic regulation of corrosion and scale inhibition, keeping the corrosion rate within a safe range.

[0024] The present invention has stable overall process operation and strong engineering adaptability. It improves processing efficiency while reducing operating costs, and provides reliable technical support for the safe reinjection of coalbed methane drainage water. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the in-situ iron removal and corrosion and scale inhibition synergistic treatment process for high-salt and high-iron drainage water of the present invention.

[0027] Figure 2 The effects of different pH values ​​on effluent water quality and corrosion performance are shown in Figure 1. Figure 2 shows the effects of different pH values ​​on effluent turbidity and suspended solids (SS). Figure 3 shows the effects of different pH values ​​on effluent total iron content and total iron removal rate. Figure 4 shows the corrosion rate of N80 steel over time under different pH conditions.

[0028] Figure 3 The effect of different aeration air-to-water ratios on the corrosion rate of effluent.

[0029] Figure 4 To compare the effects of different dosing strategies on the removal of suspended solids and turbidity in effluent.

[0030] Figure 5 The effect of different PAM dosages on the removal rate of SS and turbidity in effluent.

[0031] Figure 6 The effect of different dosages of corrosion and scale inhibitors on the corrosion rate and corrosion inhibition rate of effluent.

[0032] Figure 7 This is a comparison of the raw water and effluent water quality indicators under optimal process parameters. Detailed Implementation

[0033] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0034] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0035] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0036] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0037] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0038] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0039] To determine the optimal operating range of the process of this invention, key parameters such as pH, aeration intensity, dosing method, flocculant dosage, and corrosion inhibitor dosage were first systematically optimized. The original water sample used had the following characteristics: pH 6.51, turbidity 1630 NTU, suspended solids (SS) 792 mg / L, petroleum hydrocarbons 1.82 mg / L, total iron 444 mg / L, median particle size (D50) 74.33 μm; K + 65.4 mg / L, Na + 1.44×10 4 mg / L, Ca 2+ 9.69×10 3 mg / L, Mg 2+ 1.02×10 3 mg / L, chloride 4.89×10 4 mg / L, CO3 2- (Calculated as CaCO3) 0 mg / L, HCO3 - (As CaCO3) 1.02×10 3 mg / L; sulfate concentration was below the detection limit of 8 mg / L, total hardness was 3.03 × 10 mg / L. 4 mg / L, anionic surfactant 0.097 mg / L, total bacterial count 2 CFU / mL, conductivity 166.1 ms / cm.

[0040] (1) pH parameter optimization: 5000 mL of raw water was collected and divided into 5 groups of 1000 mL each. Basic water quality indicators such as suspended solids (SS), turbidity, pH, ORP, and TDS were measured. The pH of each group was adjusted to 7.0, 7.5, 8.0, 8.5, and 9.0 using NaOH, respectively. Aeration was performed for 30 min at an air flow rate of 1600 mL / min and an air-to-water ratio of 48:1. Anionic PAM (2 mg / L) was added to each group at 120 rpm, followed by slow stirring at 70 rpm for 15 min. After settling for 30 min, the supernatant was collected and sand-filtered, and the effluent parameters were measured.

[0041] The results showed that the effluent SS and turbidity first decreased and then increased with increasing pH. The particle control effect was best at pH 8.0, with effluent SS at 2.95 mg / L and turbidity at 5.3 NTU. Within the experimental pH range, the total iron removal rate remained above 99%. At pH 8.5, the total iron concentration in the effluent was below 0.1 mg / L, and the removal rate was close to 100%.

[0042] Corrosion performance tests were conducted according to the standard "Determination of Corrosion Rate of Metallic Materials in Oil and Gas Field Media" (SY / T 0026.1-2024). The tests showed that the corrosion rate of the unadjusted pH group continuously increased over time, the corrosion rate of the pH 7.5 group remained at a high level, and the corrosion rate of the pH 8.5 group decreased significantly from day 3 onwards and stabilized at its lowest value.

[0043] Based on the overall requirements of the reinjection project, the pH control range is determined to be 7.5-8.5. When particle control is the main objective, 8.0 is preferred, and when long-term corrosion control is the main objective, 8.5 is preferred.

[0044] (2) Optimization of aeration intensity: Based on the above experiments, the aeration intensity was optimized. The pH of each group was uniformly adjusted to 7.5, the air flow rate was kept at 1600 mL / min, and aeration experiments were carried out with air-to-water ratios of 24:1, 48:1, 72:1, 96:1, and 120:1, respectively, while the other operating steps remained unchanged.

[0045] The results show that a moderate aeration intensity can reduce the risk of corrosion while ensuring sufficient oxidation of ferrous iron. Under the conditions of this experiment, the corrosion control effect is best with an air-to-water ratio of 36:1. Excessive aeration intensity will aggravate oxygen absorption corrosion. Therefore, the preferred range of air-to-water ratio is 24:1-72:1.

[0046] (3) Optimization of drug administration method: A comparative experiment on drug administration methods was conducted, with four control groups: a blank control group without any drugs, a group with only PAC 125 mg / L, a group with only PAM 2 mg / L, and a group with a combination of PAC 125 mg / L and PAM 2 mg / L. All other experimental conditions remained the same.

[0047] The results showed that the PAM group alone achieved the best particle removal effect, with effluent SS of about 3.5 mg / L and turbidity of about 6.7 NTU. The treatment effect was better than that of the PAC group alone and the PAC and PAM combined group. This indicates that the iron-containing hydrolysis products generated in situ after aeration-enhanced oxidation can be used as inorganic coagulation cores. With the addition of a small amount of polymeric flocculant, efficient coagulation and sedimentation can be achieved without the need for additional addition of inorganic coagulants such as PAC.

[0048] (4) Optimization of flocculant dosage The dosage of polymeric flocculant was optimized, and different anionic PAM dosage gradients were set to complete the entire process of rapid mixing, slow flocculation, sedimentation, and sand filtration.

[0049] The results showed that the removal rates of SS and turbidity in the effluent reached a clear inflection point at 2 mg / L. When the PAM dosage was below 2 mg / L, the removal effect increased significantly with the increase of dosage. However, when the dosage exceeded 2 mg / L, a colloidal protection phenomenon occurred, and the removal rate decreased. Therefore, the optimal dosage of PAM was determined to be 2 mg / L.

[0050] (5) Optimization of corrosion inhibitor dosage An optimization experiment for corrosion inhibitor dosage was conducted. 5000 mL of original water sample was divided into 5 groups of 1000 mL each. After determining basic water quality indicators, the pH was adjusted to 8.5 with NaOH. Aeration was performed under a fixed air-to-water ratio. Anionic PAM2 mg / L was added, followed by coagulation, sedimentation, and sand filtration. Corrosion inhibitors were added at concentrations of 0.2, 0.4, 0.6, 0.8, and 1.0 mL / L, respectively. Three parallel experiments were set up for each group, and the corrosion rate and other indicators of the effluent were measured. The results showed that the corrosion rate of N80 steel initially decreased and then increased with the amount of corrosion inhibitor added. The lowest corrosion rate (0.053 mm / a) was observed at a dosage of 0.6 mL / L. When the dosage was increased to 1.0 mL / L, the corrosion rate rebounded to 0.065 mm / a, indicating an optimal dosage threshold. Therefore, the preferred dosage of corrosion inhibitor is 0.6 mL / L.

[0051] The schematic diagram of the in-situ iron removal and corrosion and scale inhibition synergistic treatment process for high-salt and high-iron drainage water of this invention is shown below. Figure 1 As shown.

[0052] Example 1 This embodiment provides a pretreatment method for the reinjection of drainage water from high-salt, high-iron, and highly corrosive coalbed methane, using optimized process parameters to treat the raw water: The raw water quality of the high-salt, high-iron, and highly corrosive coalbed methane drainage water used was as follows: pH 6.51, turbidity 1630 NTU, suspended solids (SS) 792 mg / L, petroleum hydrocarbons 1.82 mg / L, total iron 444 mg / L, median particle size (D50) 74.33 μm; K + 65.4 mg / L, Na + 1.44×10 4 mg / L, Ca 2+ 9.69×10 3 mg / L, Mg 2+ 1.02×10 3 mg / L, chloride 4.89×10 4 mg / L, CO3 2- (Calculated as CaCO3) 0 mg / L, HCO3 - (As CaCO3) 1.02×10 3 mg / L; sulfate concentration was below the detection limit of 8 mg / L, total hardness was 3.03 × 10 mg / L. 4 mg / L, anionic surfactant 0.097 mg / L, total bacterial count 2 CFU / mL, conductivity 166.1 ms / cm.

[0053] The processing steps are as follows: (1) The raw water from the high-salt and high-iron coalbed methane drainage is introduced into the raw water intake unit and then into the homogenization and conditioning unit to complete the buffering and homogenization of water quality and quantity.

[0054] (2) After homogenization, the effluent enters the pH control unit, where NaOH solution is used to adjust the pH value of the water to 8.5, providing a stable and suitable environment for iron ion oxidation and coagulation processes.

[0055] (3) The effluent is introduced into the aeration-enhanced oxidation unit, where microporous aeration is used for enhanced oxidation. The aeration time is 30 minutes, and the air-to-water ratio is controlled at 48:1 to ensure that the dissolved Fe in the water is enhanced. 2+ It is fully oxidized and generates iron-containing hydrolysis products (such as Fe(OH)3 colloids) in situ.

[0056] (4) After oxidation, the effluent enters the synergistic stabilization control unit, and 0.6 mL / L of organic phosphoric acid and polycarboxylic acid compound (mass ratio 1:2) corrosion and scale inhibitor is added to the water body to achieve synergistic stabilization control of corrosion and scaling risks.

[0057] (5) After oxidation, the effluent enters the in-situ coagulation and flocculation unit, and 2 mg / L of anionic polyacrylamide (PAM) with a molecular weight of 10 million and a degree of hydrolysis of 26% is added. The flocculation is carried out at a slow speed of 70 rpm for 15 min to promote the full growth and aggregation of flocs.

[0058] (6) After coagulation, the water sample is passed into the solid-liquid separation unit and allowed to settle for 30 minutes to complete the solid-liquid separation. The supernatant is then passed into the fine filtration unit, finely filtered through the sand filtration unit, and collected through the effluent unit to obtain treated effluent that meets the requirements of formation reinjection.

[0059] After treatment by the process described in this embodiment, the quality of the effluent (such as...) Figure 7 The following parameters were observed: pH 7.08, turbidity 3.67 NTU, SS 1.11 mg / L, petroleum hydrocarbons 0.06 mg / L, total iron 0.16 mg / L, and D50 1.095 μm. + 81.6 mg / L, Na + 1.70×10 4 mg / L, Ca 2+ 9.43×10 3 mg / L, Mg 2+ 1.16×10 3 mg / L, chloride 4.94×10 4 mg / L, CO3 2- (Calculated as CaCO3) 0 mg / L, HCO3 - (Calculated as CaCO3) 184 mg / L; Total hardness 2.84 × 10⁻⁶ 4 mg / L. Sulfate concentration, anionic surfactant concentration, and total bacterial count were all undetectable. The effluent conductivity was 146.3 mS / cm. Figure 7 In this context, 8L and 0.05L refer to the limits of detection being 8 mg / L and 0.05 mg / L, respectively (the actual concentration in the water sample was lower than these limits of detection). Water quality tests were conducted according to the "Technical Requirements and Analysis Methods for Water Quality Indicators in Clastic Rock Reservoirs SY / T 5329-2022". The results showed that the turbidity removal rate was 99.77%, the SS removal rate was 99.85%, the petroleum removal rate was 96.70%, the total iron removal rate was 99.96%, and the effluent corrosion rate was 0.053 mm / a. All indicators met the formation reinjection standards.

[0060] Example 2 This embodiment provides a pretreatment method for the reinjection of high-salt, high-iron, and highly corrosive coalbed methane drainage water, configured with an online monitoring and automatic dosing control unit, to verify the stable operation effect of the process under continuous water intake and water quality fluctuation conditions.

[0061] The raw water quality used in this embodiment is the same as in Embodiment 1. The difference lies in the addition of an online monitoring unit and an automatic dosing control unit to the system. The online monitoring unit is installed at the outlets of the homogenization unit, pH control unit, aeration-enhanced oxidation unit, and effluent unit, respectively, to monitor operating parameters such as pH, ORP, dissolved oxygen, turbidity, flow rate, and conductivity in real time. The automatic dosing control unit is linked to the alkali dosing pump, corrosion and scale inhibitor dosing pump, and flocculant dosing pump, respectively, and automatically adjusts the dosage of each agent based on the online monitoring signals.

[0062] The processing steps are as follows: (1) The raw water from the drainage of high-salt and high-iron coalbed methane is introduced into the raw water inlet unit, and after being buffered and homogenized by the homogenization and adjustment unit, it enters the subsequent treatment system. (2) When the raw water enters the pH control unit, the online pH monitor provides real-time feedback on the changes in the acidity and alkalinity of the water. The automatic dosing control unit controls the NaOH metering pump to operate according to the set value, so that the pH of the effluent is stably maintained in the range of 7.5 to 8.5. (3) After pH adjustment, the water enters the aeration-enhanced oxidation unit. Online ORP and dissolved oxygen monitoring signals are used to determine the iron oxidation state and degree, and the operating status is corrected by feedback based on the front-end pH adjustment results, so that the dissolved Fe 2+ Continuous transformation and in-situ generation of ferric hydroxide colloid; (4) After aeration and oxidation, the water enters the coordinated stabilization control unit. The automatic dosing control unit adjusts the dosage of corrosion inhibitor and scale inhibitor according to the flow rate and online corrosion control target, so as to maintain it in the range of 0.4 to 0.8 mL / L. (5) The water then enters the flocculation unit. The online turbidity signal is used to help adjust the dosage of anionic PAM to maintain it in the range of 1 to 3 mg / L, so as to ensure the floc formation and sedimentation separation effect. (6) After coagulation, the water enters the sedimentation unit and the filtration unit in sequence to obtain treated effluent that meets the reinjection requirements.

[0063] The results show that, under conditions of continuous operation and fluctuations in raw water quality, after setting up an online monitoring and automatic dosing control unit, the system can achieve closed-loop control of pH adjustment, scale and corrosion inhibitor dosing, and flocculant dosing. The total iron, suspended solids, and turbidity in the effluent remain stable, and the corrosion rate is maintained within a safe range.

[0064] This embodiment demonstrates that the present invention can effectively reduce operational fluctuations caused by manual, experience-based dosing through online monitoring and automatic dosing control, thereby improving the continuous operational stability and engineering adaptability of the treatment system.

[0065] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A pretreatment method for coalbed methane drainage water reinjection, characterized in that, Includes the following steps: (1) After homogenization and adjustment, the pH of the raw water is adjusted to maintain the pH value of the water body in the range of 7.0 to 9.2; (2) Aeration and enhanced oxidation are carried out on the water obtained in step (1) to remove dissolved Fe from the water. 2+ Directed transformation, and in-situ generation of iron-containing hydrolysis products; (3) Add corrosion and scale inhibitors to the water after aeration and enhanced oxidation to achieve synergistic and stable regulation of corrosion and scaling; (4) Using the iron-containing hydrolysis products generated in situ as the coagulation core, a polymeric flocculant is added to complete the coagulation and flocculation reaction; (5) The water after coagulation and flocculation is subjected to solid-liquid separation and fine filtration in sequence to obtain treated effluent that meets the reinjection requirements.

2. The pretreatment method for coalbed methane drainage and reinjection water according to claim 1, characterized in that, The enhanced oxidation process is microporous aeration, with an aeration time of 20-80 min and an air-to-water ratio of 24:1-72:

1.

3. The pretreatment method for coalbed methane drainage and reinjection water according to claim 1, characterized in that, The polymeric flocculant is anionic polyacrylamide, and the dosage is 1~6 mg / L.

4. The pretreatment method for coalbed methane drainage water reinjection according to claim 1, characterized in that, The dosage of the corrosion and scale inhibitor is 0.1~2.0 mL / L.

5. The pretreatment method for coalbed methane drainage water reinjection according to claim 1, characterized in that, The solid-liquid separation is performed by sedimentation, inclined tube separation, or air flotation; the fine filtration is performed by sand filtration.

6. A reinjection pretreatment system for implementing the method according to any one of claims 1 to 5, characterized in that, It includes the following units connected in sequence: raw water inlet unit, homogenization adjustment unit, pH control unit, aeration enhanced oxidation unit, synergistic stabilization control unit, in-situ coagulation and flocculation unit, solid-liquid separation unit, fine filtration unit, and effluent unit.

7. The system according to claim 6, characterized in that, An oil removal / demulsification unit is installed between the pH control unit and the aeration-enhanced oxidation unit.

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