A gel-breaking flocculating material for fracturing flowback fluid treatment and a preparation method and application thereof

Abstract

This invention belongs to the field of oilfield wastewater treatment and polymer composite materials. It discloses a depolymerizing flocculant for treating fracturing flowback fluid, its preparation method, and its application. The depolymerizing flocculant is prepared by aqueous solution free radical copolymerization using a main chain backbone, crosslinking monomers, dual-cationic synergistic monomer pairs, and hydrophobic associating functional monomers as reactants, under a pre-set crosslinking agent. The polymer linear chains of the depolymerizing flocculant have a moderately crosslinked structure introduced between them. Specifically, the moderately crosslinked structure is a partially bridging structure dominated by linear segments and supplemented by local network nodes, or a lightly crosslinked structure. This invention uses a main chain backbone, crosslinking monomers, dual-cationic synergistic monomer pairs, and hydrophobic associating functional monomers as reactants, and is prepared by aqueous solution free radical copolymerization with the introduction of a moderately crosslinked structure. This achieves efficient and stable removal of COD, oil content, and suspended solids from fracturing flowback fluid, with high treatment stability and low risk of secondary pollution.

Description

Technical Field

[0001] This invention belongs to the technical field of oilfield wastewater treatment and polymer composite materials, and specifically relates to a depolymerizing flocculant for fracturing flowback fluid treatment, its preparation method, and its application. Background Technology

[0002] Fracturing flowback fluid is a complex industrial wastewater produced during hydraulic fracturing operations in unconventional oil and gas extraction processes such as shale gas and tight oil. It is characterized by "three highs and one strong": high salinity (total dissolved solids, typically ≥80 g / L), high oil content (including free oil, dispersed oil, and stable emulsified oil), and high chemical oxygen demand (COD). Furthermore, the colloidal and emulsified oil droplet system in fracturing flowback fluid is extremely stable and also contains residual fracturing fluid additives, formation particles, and various dissolved inorganic salts. Therefore, if fracturing flowback fluid is discharged or reinjected without effective treatment, it will not only damage surface water, soil, and the ecological environment, but the high concentration of salts and organic impurities can also cause formation blockage, thus affecting oil and gas recovery rates and posing a dual challenge to both environmental protection and production efficiency in oil and gas extraction.

[0003] Currently, the main technologies for treating fracturing flowback fluid include coagulation sedimentation, air flotation, advanced oxidation (such as Fenton and ozone), membrane separation (such as ultrafiltration and reverse osmosis), and combined processes. Among these, coagulation sedimentation is the most widely used due to its low infrastructure investment, simple process, large treatment throughput, and ease of management. Its treatment effect mainly depends on the selection of coagulants and flocculants and their compatibility with the wastewater system.

[0004] Traditional water treatment coagulants, such as inorganic salt coagulants like polyaluminum chloride (PAC) and polyferric sulfate (PFS), and organic polymeric flocculants like polyacrylamide (PAM) and its cationic derivatives (CPAM), have shown good performance in treating ordinary municipal sewage or low-salinity industrial wastewater. However, when faced with highly mineralized fracturing flowback fluid, traditional water treatment coagulants generally suffer from severe charge shielding, molecular chain collapse, low demulsification efficiency, loose flocs and slow settling, large dosage and unstable effects, thus failing to meet the requirements for efficient treatment of fracturing flowback fluid.

[0005] Specifically, high concentrations of salt ions, especially polyvalent cations such as Ca2+, are beneficial. 2+ Mg 2+ It will produce a strong charge shielding effect and salting-out effect in aqueous solution; for cationic polymeric flocculants, such as CPAM, the positively charged groups carried on their molecular chains are blocked by high concentrations of counterions (such as Cl-). -The neutralization effect of electrostatic charge on polymer chains significantly weakens the electrostatic repulsion between them, causing the molecular chains to change from an extended state to a coiled and collapsed conformation. This conformational change reduces the effective size of the polymer molecules, drastically decreases the charge neutralization capacity, and shortens the adsorption bridging range. This manifests as: low destabilization and demulsification efficiency for stable emulsified oil droplets and colloidal particles; the resulting flocs (lumps) are small, loose, and fluffy, containing a large amount of water, with poor mechanical strength, and easily broken under slight hydraulic shear; the flocs settle slowly, resulting in poor mud-water separation, large fluctuations in effluent turbidity, oil content, and COD, and unstable water quality. Furthermore, traditional water treatment coagulants mainly rely on charge neutralization and bridging, and have limited ability to specifically capture nano- and micron-sized emulsified oil droplets stabilized by surfactants. Often, extremely large doses are required to achieve a certain treatment effect, significantly increasing reagent costs and potentially leading to excessive levels of residual aluminum or iron ions in the water or the introduction of new organic pollutants, creating secondary environmental risks. Summary of the Invention

[0006] To address the technical problems existing in the prior art, this invention provides a degelatinizing flocculant material for fracturing flowback fluid treatment, its preparation method, and its application, in order to solve the technical problems of traditional water treatment coagulants, such as severe charge shielding, molecular chain collapse, low demulsification efficiency, loose flocs and slow settling, large dosage, and unstable effects.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This invention provides a depolymerizing flocculant for treating fracturing flowback fluid. The depolymerizing flocculant is prepared by using a main chain backbone and crosslinking monomers, dual cationic synergistic monomer pairs and hydrophobic associating functional monomers as reactive monomers, and by aqueous free radical copolymerization under a preset crosslinking agent. The polymer linear chains of the depolymer flocculant are introduced with a moderate cross-linking structure; The moderately cross-linked structure is a partially bridging structure or a lightly cross-linked structure with linear segments as the main component and local network nodes as auxiliary components.

[0008] Furthermore, the main chain backbone and bridging monomer are acrylamide; The dual-cationic co-monomer pair includes dimethyl diallyl ammonium chloride and methacryloyloxyethyltrimethylammonium chloride; The hydrophobic associative functional monomer is a hydrophobic alkyl acrylate monomer; wherein the alkyl chain length of the hydrophobic alkyl acrylate monomer is 9-12.

[0009] Furthermore, the hydrophobic alkyl acrylate monomer is one or more selected from butyl acrylate, octyl acrylate, and dodecyl acrylate.

[0010] Furthermore, the molar ratio of acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, and hydrophobic alkyl acrylate monomer is (30-50):(25-40):(10-25):(1-5).

[0011] Furthermore, the preset crosslinking agent is N,N′-methylenebisacrylamide.

[0012] Furthermore, the amount of N,N′-methylenebisacrylamide added is 0.1wt%-3.0wt% of the total mass of the reactants.

[0013] Furthermore, the number-average molecular weight of the depolymerizing flocculant is controlled at 2.5 × 10⁻⁶. 6 -7.1×10 6 Da.

[0014] Furthermore, the cationicity of the depolymerizing flocculant is 38% to 52%.

[0015] This invention also provides a method for preparing a degelatinizing flocculant for fracturing flowback fluid treatment, comprising: The main chain backbone, bridging monomers, dual-cationic synergistic monomer pairs, and hydrophobic associative functional monomers are added to water, dissolved, and mixed evenly to obtain a monomer mixed aqueous solution. The pH of the monomer mixture aqueous solution was adjusted to 4.5-5.5, and then deoxygenated to obtain a deoxygenated monomer mixture aqueous solution. Under a nitrogen-protected atmosphere, the deoxygenated monomer mixed aqueous solution was heated to the preset polymerization temperature; then, a chain transfer agent, an initiator, and a preset crosslinking agent were added sequentially to carry out the polymerization reaction and obtain the reaction product. The reaction products are cooled, post-treated, and molded to obtain a depolymerized flocculant material.

[0016] The present invention also provides an application of a de-gelling flocculant for fracturing flowback fluid treatment, wherein the de-gelling flocculant is used in the de-gelling treatment process of fracturing flowback fluid.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: The degelatinizing flocculant provided by this invention for treating fracturing flowback fluid is prepared by aqueous free radical copolymerization of a main chain backbone with crosslinking monomers, dual-cationic synergistic monomer pairs, and hydrophobic associative functional monomers, and introducing a moderately crosslinked structure. It achieves highly efficient and stable removal of COD, oil content, and suspended solids from fracturing flowback fluid, exhibiting high treatment stability and low risk of secondary pollution. It is particularly suitable for treating high-salt, high-oil, and high-COD fracturing flowback fluids with TDS ≥ 80 g / L, requiring no large-scale equipment modification and facilitating rapid on-site application in oilfields. The flocs formed by the degelatinizing flocculant after treatment have high density, high mechanical strength, and strong shear resistance, thus significantly reducing [the impact of the flocculant on the flowback fluid]. It improves sedimentation velocity and mud-water separation effect, and significantly reduces reagent dosage. Among them, by constructing a moderately cross-linked structure with linear segments as the main component and local network nodes as auxiliary components, it effectively overcomes the problem of molecular chain coiling and collapse caused by charge shielding effect in high mineralization environment, and maintains the extended conformation and effective radius of polymer chain in solution. By introducing dual-cationic synergistic monomers, it significantly enhances the charge neutralization stability of the material in high salt system and avoids charge failure caused by counterion surrounding. In addition, by introducing hydrophobic associative functional monomers to establish a dual demulsification mechanism of charge neutralization and hydrophobic adsorption, it significantly enhances the exclusive capture ability of stable emulsified oil droplets and colloidal particles.

[0018] Furthermore, acrylamide, as a nonionic water-soluble monomer, can form a flexible long-chain backbone, providing a structural basis for adsorption bridging and net-like sweeping, thus solving the problem of insufficient bridging ability of traditional flocculants. Dimethyl diallyl ammonium chloride can provide a high-density quaternary ammonium salt positive charge, achieving strong charge neutralization. Methacryloxyethyltrimethylammonium chloride and dimethyl diallyl ammonium chloride can form a synergistic effect of steric hindrance and charge distribution, so as to counteract charge shielding and inhibit excessive coiling of molecular chains in high-salt environments, maintain charge neutralization stability, and overcome the defects of conventional cationic flocculants, such as strong salt sensitivity and easy charge shielding. Hydrophobic alkyl acrylate monomers with alkyl chain lengths of 9-12 have hydrophobic side chain groups that can target and capture emulsified oil droplets through hydrophobic association. The hydrophobic association is not affected by ionic strength, making up for the shortcomings of traditional agents in capturing stable emulsified oil droplets. It can be seen that the coupling of dual cationic synergistic monomer pairs and hydrophobic associative functional monomers can fundamentally overcome the limitations of high-salt environments on flocculation performance.

[0019] Furthermore, controlling the molar ratio of acrylamide to 30-50 ensures the molecular chain length and basic water solubility, meeting the bridging requirements; controlling the ratio of dimethyl diallyl ammonium chloride to methacryloyloxyethyl trimethyl ammonium chloride to (25-40): (10-25) ensures sufficient cation density, achieving efficient charge neutralization and destabilization, while avoiding excessive ratio leading to poor solubility and rigid molecular chains; controlling the ratio of hydrophobic associating functional monomers to 1-5 can form effective hydrophobic association, enhance demulsification and oil removal, and avoid damaging the polymer's water solubility due to excessive hydrophobic groups, thus synergistically achieving a balance between high salt resistance, charge neutralization, bridging, and hydrophobic adsorption.

[0020] Furthermore, N,N′-methylenebisacrylamide is a water-soluble bifunctional crosslinking agent with good compatibility with the reactant monomers and matching copolymerization reactivity. It can precisely form chemical crosslinking points between the linear long chains of polymers. Compared with other conventional crosslinking agents, its crosslinking strength is moderate and will not cause excessive crosslinking of polymers to form insoluble gels. It can stably construct a slightly crosslinked structure with linear as the main component and local network structure, which enhances the toughness and density of flocs and solves the defects of traditional flocs that form loose flocs, are easy to break, and have slow sedimentation.

[0021] Furthermore, by controlling the addition amount of N,N′-methylenebisacrylamide to 0.1wt%-3.0wt% of the total mass of the reactants, the crosslinking density can be precisely controlled, ensuring that the flocs are dense, shear-resistant, and settle quickly, without affecting the solubility and chain extension of the polymer in high-salt wastewater. This optimizes the floc performance from a structural perspective and effectively solves the bottlenecks of poor floc strength and low separation efficiency in traditional flocs.

[0022] Furthermore, the number-average molecular weight of the depolymerizing flocculant was controlled at 2.5 × 10⁻⁶. 6 -7.1×10 6 Da can balance bridging ability and solubility, enabling it to maintain effective chain length in high-salt environments, achieving efficient bridging and aggregation of micro-flocs to form large and dense flocs, thus solving the problems of bridging failure and small flocs caused by unreasonable molecular weight of traditional flocculants.

[0023] Furthermore, by controlling the cationicity of the depolymerizing flocculant to 38%-52%, it can provide sufficient and stable positive charge, efficiently neutralize and destabilize the charge, and at the same time maintain the flexibility and water solubility of the molecular chain, enabling it to resist charge shielding in a high-salt environment, ensuring stable flocculation effect, and solving the problems of poor treatment effect and large dosage caused by the cationicity imbalance of conventional cationic flocculants.

[0024] The preparation method and application of the degelatinizing flocculant for fracturing flowback fluid treatment provided by this invention possess all the advantages of the aforementioned degelatinizing flocculant for fracturing flowback fluid treatment. Attached Figure Description

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

[0026] Figure 1 A process flow diagram of the de-gelling treatment process of the de-gelling flocculant provided in this application for use in fracturing flowback fluid; Figure 2 These are micrographs showing the floc morphology formed when the degelatinized flocculants prepared in Examples 1-5 were used to treat experimental raw water. Detailed Implementation

[0027] To make the technical problems, technical solutions, and beneficial effects solved by this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0028] This invention provides a depolymerizing flocculant for fracturing flowback fluid treatment. The depolymerizing flocculant (hereinafter referred to as SRB coagulant) is prepared using a main-chain backbone with crosslinking monomers, dual-cationic synergistic monomer pairs, and hydrophobic associating functional monomers as reactive monomers, and via aqueous solution free radical copolymerization under a pre-defined crosslinking agent. The number-average molecular weight of the depolymerizing flocculant is controlled at 2.5 × 10⁻⁶. 6 -7.1×10 6 Da, the cationicity of the depolymerizing flocculant is 38%-52%.

[0029] The main chain backbone and bridging monomer are acrylamide (AM). As a non-ionic water-soluble monomer, acrylamide forms a flexible polyacrylamide long-chain backbone after polymerization, which can provide basic solubility and long-chain properties for the entire molecule. It is the structural basis for realizing adsorption bridging and net trapping sweeping effects.

[0030] The dual-cationic synergistic monomer pair, comprising dimethyl diallyl ammonium chloride (DMDAAC) and methacryloyloxyethyltrimethylammonium chloride (DMC), effectively enhances the charge stability of the material. Dimethyl diallyl ammonium chloride provides a high-density quaternary ammonium salt-type permanent positive charge, unaffected by solution pH, and is primarily responsible for strongly neutralizing negatively charged emulsified oil droplets and colloidal particles, destabilizing them. Methacryloxyethyltrimethylammonium chloride also provides a quaternary ammonium salt-type positive charge, but the spatial arrangement of ester bonds and cationic groups in its molecular structure differs from that of dimethyl diallyl ammonium chloride. The introduced methacryloyloxyethyltrimethylammonium chloride can produce a synergistic effect of steric hindrance and charge distribution with dimethyl diallyl ammonium chloride, helping to partially offset excessive chain coiling caused by charge shielding in high-salt ion environments, maintaining the charge effectiveness and a certain degree of chain rigidity in local polymer chain regions, thereby improving charge neutralization stability under high salinity.

[0031] The hydrophobic associating functional monomer is a hydrophobic alkyl acrylate monomer; wherein the alkyl chain length of the hydrophobic alkyl acrylate monomer is 9-12; preferably, the hydrophobic alkyl acrylate monomer is one or more of butyl acrylate, octyl acrylate, and dodecyl acrylate; it should be noted that using a hydrophobic alkyl acrylate monomer with an alkyl chain length of 9-12, that is, an alkyl acrylate monomer with an alkyl chain length of C4 butyl to C12 dodecyl, as the hydrophobic associating functional monomer, allows its hydrophobic alkyl chain to be introduced into the polymer side chain during polymerization; wherein, the hydrophobic alkyl chain, as a hydrophobic group, tends to aggregate with each other in a high aqueous environment, generating a hydrophobic association effect; the hydrophobic association effect is not affected by the ionic strength of the solution, and can directly and strongly capture hydrophobic emulsified oil droplets and organic matter in wastewater, achieving the adhesion and aggregation of oil droplets through hydrophobic-hydrophobic interactions, which complements and strengthens the charge neutralization effect of cationic groups.

[0032] The preset crosslinking agent is N,N′-methylenebisacrylamide (MBA). N,N′-methylenebisacrylamide is a water-soluble bifunctional crosslinking agent with good compatibility with the reactants and matching copolymerization reactivity. It can precisely form chemical crosslinking points between the linear long chains of the polymer. Compared with other conventional crosslinking agents, its crosslinking strength is moderate and will not cause the polymer to be over-crosslinked and form an insoluble gel. It can stably construct a slightly crosslinked structure with linear as the main component and local network structure, which enhances the toughness and density of the flocs and solves the defects of traditional flocs that form loose flocs, are easy to break, and have slow sedimentation. Preferably, the amount of N,N′-methylenebisacrylamide added is 0.1wt%-3.0wt% of the total mass of the reactants.

[0033] In the above embodiments, the depolymerizing and flocculating material is prepared by free radical copolymerization of aqueous solution with main chain backbone, crosslinking monomers, dual cationic synergistic monomer pairs and hydrophobic associating functional monomers as reactive monomers and the introduction of appropriate crosslinking structure. The material can still efficiently depolymerize, destabilize, flocculate and remove contaminants in high mineralization systems. The dosage is significantly lower than that of traditional agents. The treatment effect is stable and there is no risk of secondary pollution. It is suitable for the on-site treatment needs of oilfield fracturing flowback fluid.

[0034] In this invention, the introduced dual-cationic synergistic monomer pair can counteract charge shielding, maintain the effectiveness of local charge and chain rigidity of molecular chains in high-salt environments, prevent polymer chain segment collapse due to salt ion interference, and ensure stable charge neutralization capability. By introducing hydrophobic associative functional monomers, the hydrophobic associative side chains, based on hydrophobic interactions unaffected by salinity, can accurately capture stable emulsified oil droplets, making up for the shortcomings of traditional agents in capturing nano / micro-sized oil droplets. In addition, by introducing appropriate cross-linking structures between the linear long chains of polymers, the overall toughness and density of flocs can be strengthened, the shear resistance of flocs can be improved, and the sedimentation rate can be accelerated.

[0035] In this invention, the molar ratio of acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, and hydrophobic alkyl acrylate monomer is (30-50):(25-40):(10-25):(1-5), ensuring that the depolymerizing flocculant simultaneously possesses sufficient molecular weight, suitable cationic density, effective hydrophobic modification, and good water solubility. Specifically, controlling the molar ratio of acrylamide to 30-50 ensures the molecular chain length and basic water solubility, meeting the bridging requirements. The goal is to control the ratio of dimethyl diallyl ammonium chloride to methacryloyloxyethyl trimethyl ammonium chloride at (25-40): (10-25) to ensure sufficient cation density, achieve efficient charge neutralization and destabilization, and avoid excessive ratio leading to poor solubility and rigid molecular chains. The goal is to control the ratio of hydrophobic associative functional monomers at 1-5, which can form effective hydrophobic association, enhance demulsification and oil removal, and avoid damaging the polymer's water solubility due to excessive hydrophobic groups, thus achieving a balance between high salt resistance, charge neutralization, bridging, and hydrophobic adsorption.

[0036] In this invention, a moderately cross-linked structure is introduced between the linear long chains of the polymer in the depolymerizing flocculant. Specifically, this moderately cross-linked structure is a partially bridging structure dominated by linear segments and supplemented by local network nodes, or a lightly cross-linked structure. It should be explained in detail that, in order to enhance the mechanical strength and density of the formed flocs, a low dose of N,N′-methylenebisacrylamide is introduced into the polymerization reaction system as a cross-linking agent. The amount of N,N′-methylenebisacrylamide is controlled to be 0.1wt%-3.0wt% of the total mass of the reactants. By controlling the timing and amount of N,N′-methylenebisacrylamide addition, moderate chemical cross-linking points are formed between the linear long chains of the polymer, constructing a partially bridging or lightly cross-linked structure dominated by linear segments and supplemented by local network nodes. This improves the integrity and toughness of the flocculation network, making the formed flocs denser and stronger, enhancing shear resistance, accelerating sedimentation, and reducing the likelihood of breakage and redispersion.

[0037] In this invention, the number-average molecular weight (Mn) of the depolymerizing flocculant is controlled at 2.5 × 10⁻⁶ through polymerization process control. 6 -7.1×10 6 Within the range of Da, it can balance bridging ability and solubility, maintaining effective chain length in high-salt environments, achieving efficient bridging and aggregation of micro-flocs, forming large and dense flocs, and solving the problems of bridging failure and small flocs caused by unreasonable molecular weight of traditional flocculants; when the molecular weight is below 2.5×10 6 At a molecular chain length of 7.1 × 10⁻⁶, the molecular chains are too short, resulting in insufficient adsorption bridging and netting / sweeping capabilities, making it difficult to form large-sized flocs; while at lengths higher than 7.1 × 10⁻⁶, the molecular chains are too short. 6 At high salt levels, polymers are difficult to dissolve and their molecular chains are severely entangled, making it impossible for them to fully extend in high-salt systems and reducing bridging efficiency. Controlling the cationicity of the depolymerizing flocculant (i.e., the molar percentage of quaternary ammonium salt cationic units on the polymer chain to the total monomer units) to 35%-55% ensures sufficient charge neutralization capacity while avoiding the decrease in polymer solubility in water or excessively rigid molecular chains due to excessive cationic units.

[0038] Preparation method: This invention also provides a method for preparing a degelatinizing flocculant for fracturing flowback fluid treatment, comprising the following steps: Step 1: Add the main chain backbone, bridging monomers, dual-cationic co-monomer pairs, and hydrophobic associating functional monomers to water, dissolve and mix evenly to obtain a monomer mixed aqueous solution.

[0039] Specifically, acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, and hydrophobic alkyl acrylate monomers are precisely weighed according to a pre-designed molar ratio; wherein acrylamide and dimethyl diallyl ammonium chloride are usually in aqueous solution form, and methacryloyloxyethyl trimethyl ammonium chloride is usually in aqueous solution form; the weighed acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, and hydrophobic alkyl acrylate monomers are added together to a reaction vessel containing deionized water, and stirred at room temperature until all reactants are completely dissolved and mixed evenly to form a clear or microemulsified monomer mixed aqueous solution, thus obtaining a monomer mixed aqueous solution; preferably, the total concentration of reactants in the monomer mixed aqueous solution is 20wt%-40wt%.

[0040] Step 2: Adjust the pH of the monomer mixed aqueous solution to 4.5-5.5, and then deoxygenate to obtain a deoxygenated monomer mixed aqueous solution.

[0041] It should be noted that acrylamide is more stable under slightly acidic conditions, which also facilitates the initiation of subsequent redox reactions. Therefore, the pH of the monomer mixture aqueous solution is adjusted to a weakly acidic range of 4.5-5.5 using a pre-set concentration of dilute acid or dilute alkali. Examples of dilute acids include dilute sulfuric acid or dilute nitric acid, and dilute alkalis include dilute sodium hydroxide. Subsequently, nitrogen or an inert gas such as argon is continuously introduced into the reaction system for 15-30 minutes to fully remove dissolved oxygen and obtain a deoxygenated monomer mixture aqueous solution. Since oxygen is an inhibitor of free radical polymerization, the deoxygenation step is crucial for obtaining high molecular weight products.

[0042] Step 3: Under a nitrogen-protected atmosphere, heat the deoxygenated monomer mixed aqueous solution to the preset polymerization temperature; then, add the chain transfer agent, initiator and preset crosslinking agent in sequence to carry out the polymerization reaction and obtain the reaction product.

[0043] Specifically, under a nitrogen-protected atmosphere, the deoxygenated monomer mixture aqueous solution is heated to 50-60℃. After the temperature stabilizes, the chain transfer agent and initiator are added sequentially, and the polymerization reaction is carried out at this temperature, with the total reaction time controlled at 2-5 hours. The chain transfer agent is mercaptoacetic acid or mercaptopropionic acid, used to adjust the molecular weight and redox properties. The initiator is a mixture of ammonium persulfate and sodium bisulfite, which can efficiently generate free radicals at relatively low temperatures. When the polymerization reaction reaches the middle and late stages, such as 1.5-3 hours after the start of the polymerization reaction, and the viscosity of the reaction system increases significantly, a pre-dissolved crosslinking agent solution is slowly added to the reaction system, and the reaction continues until the predetermined total reaction time. The crosslinking agent is N,N′-methylenebisacrylamide (MBA). The stepwise addition of the chain transfer agent, initiator, and crosslinking agent helps to control the crosslinking density, form an ideal local network structure, and introduce a moderate crosslinking structure between the linear long chains of the polymer in the depolymerizing flocculant.

[0044] Step 4: Cool the reaction products, perform post-processing and product molding to obtain the depolymerized flocculant material.

[0045] Specifically, after the polymerization reaction is completed, the heat source is removed, and the reaction product is allowed to cool naturally to room temperature. At this point, a transparent or translucent elastic gel polymer is obtained. The elastic gel polymer is then mechanically broken into gel blocks of a predetermined size using a shearing machine. The gel blocks of the predetermined size are then added to deionized water and allowed to swell and dissolve under moderate stirring to form a uniform viscous solution. Next, the uniform viscous solution is filtered through a filter screen, such as a 100-mesh screen, to remove any undissolved gel particles or insoluble matter. Finally, water is added or evaporated, and the filtrate is adjusted to a uniform viscous mother liquor product with a solid content of 8wt%-12wt%, which is used as the depolymerizing and flocculant material. Alternatively, if a powder is required, the above-mentioned uniform viscous mother liquor product can be pretreated and then sent to a spray drying tower for spray drying, or placed in a vacuum drying oven and dried at a suitable temperature, and ground to obtain a free-flowing powder product, which is used as the depolymerizing and flocculant material.

[0046] In this invention, adjusting the pH of the monomer mixed aqueous solution to 4.5-5.5 stabilizes the performance of acrylamide, ensures redox initiation efficiency, avoids monomer hydrolysis side reactions, and improves product purity. Deoxygenation treatment eliminates free radical inhibition factors, ensuring complete polymerization and controllable molecular weight. Stepwise addition under nitrogen protection allows for precise control of molecular weight and cross-linking structure. Initiating polymerization before adding the cross-linking agent avoids uneven molecular chain distribution caused by early cross-linking, resulting in an ideal, mildly cross-linked structure. The preparation process of this invention is mild and highly controllable, stably producing high-salt-resistant, high-performance depolymerizing flocculants, solving the problems of difficult molecular structure control and large product performance fluctuations in traditional preparation processes, and is suitable for industrial production and field applications.

[0047] It should be noted that, for ease of storage, transportation and use, the SRB coagulant can be finally processed into two formulations: one is a homogeneous viscous mother liquor product with a solid content of 8 wt%-12 wt%, which is used as a mother liquor; the other is a water-soluble white or light yellow powder made by spray drying or vacuum drying process, which is used as a powder.

[0048] Application process: The degelatinizing and flocculant material for fracturing flowback fluid treatment provided by this invention can be used in the degelatinizing process of fracturing flowback fluid; as shown in the attached figure. Figure 1 The specific application process is as follows: Step 100: Water Pretreatment. First, the raw water from the fracturing flowback is coarsely filtered through a screen or simple coarse filter to remove any larger particles and floating matter. Then, the pH of the raw water is adjusted to a weakly acidic range of 6.0-6.5 using acids such as hydrochloric acid or sulfuric acid, resulting in pH-adjusted wastewater. Adjusting the pH of the raw water to 6.0-6.5 is beneficial for the subsequent hydrolysis of inorganic coagulants and for SRB coagulants to achieve optimal charge neutralization and flocculation efficiency.

[0049] Step 200: Inorganic Coagulation Aid Enhancement and Destabilization. An inorganic polymeric coagulant is added as a coagulant aid to the pH-adjusted wastewater; preferably, polyferric sulfate (PFS) or polyaluminum chloride (PAC) is used as the inorganic polymeric coagulant, with the dosage based on the effective component Fe. 3+ Or Al 3+ The concentration is 200-300 mg / L. After addition, start the agitator for rapid mixing, with the stirring speed controlled at 300-600 rpm for 1-3 minutes to obtain rapidly mixed water. The purpose of this stage is to utilize the polynuclear hydroxy complexes generated by the rapid hydrolysis of inorganic polymer coagulants to perform preliminary charge neutralization, double-layer compression, and adsorption destabilization on most colloidal particles and some emulsified oil in the wastewater, forming a large number of micro-flocs, thus creating favorable conditions for subsequent organic flocculation.

[0050] Step 300: Organic Flocculation and Bridged Network Capture. The SRB coagulant is added to the water after rapid stirring. If mother liquor is used, the dosage, based on the volume of the mother liquor, is equivalent to 30-50 mg / L of the effective ingredient. After addition, the stirring speed is significantly reduced to a slow stirring state of 50-100 rpm and maintained for 8-12 minutes. During this slow stirring stage, the long-chain molecules of the SRB coagulant are fully extended, and the cationic groups on its chains continue to neutralize the incompletely destabilized fine particles and oil droplets. Simultaneously, the hydrophobic groups strongly adsorb oil droplets. More importantly, its long-chain molecules bridge between the micro-flocs and, through a local network structure, capture and sweep them, connecting, encapsulating, and agglomerating countless micro-flocs into visible, coarse, dense, and high-strength flocs, i.e., alum flocs.

[0051] Step 400: Sedimentation and Effluent Discharge. After slow stirring, stop stirring completely to allow the wastewater to enter the settling stage, with a settling time of 20-40 minutes. Under gravity, the large, dense flocs quickly settle to the bottom of the container, forming a concentrated sludge layer, while the upper liquid becomes clear. After settling, solid-liquid separation can be achieved by decanting, overflow, or using a sludge pump. The separated bottom sludge can be further dewatered, such as by centrifugation or filter press. If the supernatant meets the standards for reuse or discharge, it can be directly introduced into subsequent processes. If higher water quality is required, it can be introduced into advanced treatment units such as filtration, advanced oxidation, or membrane treatment.

[0052] The following specific embodiments further explain the depolymerizing flocculant material for fracturing flowback fluid treatment provided by the present invention: Example 1 This embodiment 1 provides a depolymerizing flocculant for fracturing flowback fluid treatment. The depolymerizing flocculant is prepared by aqueous free radical copolymerization using acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride and hydrophobic monomer butyl acrylate as reactants and N,N'-methylenebisacrylamide as a pre-set crosslinking agent.

[0053] In this Example 1, the depolymerizing flocculant has the characteristics of moderate cationicity and hydrophobic modification ratio; the preparation process is as follows: Step 1: In the reactor, add 35.5g of acrylamide (corresponding to 0.5mol), 66.7g of dimethyl diallyl ammonium chloride (concentration of 60%, corresponding to 0.25mol), 53.3g of methacryloyloxyethyl trimethyl ammonium chloride (concentration of 75%, corresponding to 0.15mol), and 6.5g of butyl acrylate (corresponding to 0.05mol). Add deionized water to the reactor to make the total mass concentration of the reactants 30wt%, and obtain a monomer mixed aqueous solution. The molar ratio of acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, and hydrophobic monomer butyl acrylate is 35:25:15:5.

[0054] Step 2: Adjust the pH of the monomer mixed aqueous solution to 5.0 using dilute sulfuric acid; then, purge nitrogen gas for 25 minutes to remove oxygen from the monomer mixed aqueous solution, obtaining a deoxygenated monomer mixed aqueous solution.

[0055] Step 3: Under a nitrogen-protected atmosphere, the deoxygenated monomer mixed aqueous solution is heated to 55°C. After the temperature stabilizes, 0.15g of mercaptoacetic acid (0.1wt% of the total monomer mass) is added sequentially as a chain transfer agent, and 0.16g of ammonium persulfate and 0.08g of sodium bisulfite are added as initiators. After reacting at a constant temperature for 3 hours, 0.75g of pre-dissolved N,N'-methylenebisacrylamide (0.5wt% of the total monomer mass) is added, and the reaction is continued for 1 hour to obtain the reaction product.

[0056] Step 4: After the polymerization reaction is completed and cooled, a transparent or translucent elastic gel polymer is obtained. This transparent or translucent elastic gel polymer is sheared, dissolved, and filtered to prepare a homogeneous viscous mother liquor product with a solid content of 10 wt%, labeled as product S1. Characterization shows that the number average molecular weight Mn of product S1 is 3.8 × 10⁻⁶. 6 Da has a cationicity of 40%.

[0057] Simulation experiment: The depolymerizing flocculant material (i.e., product S1) prepared in Example 1 was used to treat experimental raw water A to verify the effect of the depolymerizing flocculant material prepared in Example 1. The experimental raw water A was used to simulate fracturing flowback fluid. The experimental raw water A had the following properties: TDS=85g / L, COD=4500mg / L, oil content=650mg / L, SS=1200mg / L, and pH=8.2.

[0058] Processing technology: Take 1L of experimental raw water A and adjust the pH to 6.2 with dilute hydrochloric acid; use Fe 3+The following steps were taken: 250 mg / L of polyferric sulfate was added and stirred rapidly at 500 rpm for 2 minutes; then, product S1 was added to make the effective ingredient dosage 35 mg / L; the speed was reduced to 80 rpm and stirred slowly for 10 minutes; after stirring was stopped, the mixture was allowed to stand and settle for 30 minutes.

[0059] Processing effect description: After treatment with product S1, the experimental raw water A showed a COD removal rate of 71%, an oil removal rate of 87%, an SS removal rate of 90%, and a floc settling velocity of 4.8 m / h; the flocs were dense and the settling interface was clear.

[0060] Example 2 This embodiment 2 provides a depolymerizing flocculant for fracturing flowback fluid treatment. The depolymerizing flocculant is prepared by aqueous free radical copolymerization using acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride and octyl acrylate (OA) as reactive monomers and N,N'-methylenebisacrylamide as a preset crosslinking agent.

[0061] In this Example 2, the depolymerizing flocculant has high cationicity and molecular weight; the preparation process is as follows: Step 1: In the reactor, add 42.5g of acrylamide (corresponding to 0.6mol), 83.3g of dimethyl diallyl ammonium chloride (concentration of 60%, corresponding to 0.3125mol), 44.4g of methacryloyloxyethyl trimethyl ammonium chloride (concentration of 75%, corresponding to 0.125mol), and 11.0g of octyl acrylate (corresponding to 0.065mol). Add deionized water to the reactor to make the total mass concentration of the reactants 30wt%, and obtain a monomer mixed aqueous solution. The molar ratio of acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, and octyl acrylate is 40:31.25:12.5:6.25.

[0062] Step 2: Adjust the pH of the monomer mixed aqueous solution to 5.1 using dilute sulfuric acid; then, purge nitrogen gas for 30 minutes to remove oxygen from the monomer mixed aqueous solution, and obtain a deoxygenated monomer mixed aqueous solution.

[0063] Step 3: Under a nitrogen-protected atmosphere, the deoxygenated monomer aqueous solution is heated to 60°C. After the temperature stabilizes, 0.3 g of mercaptopropionic acid (0.2 wt% of the total monomer mass) is added sequentially as a chain transfer agent, and 0.16 g of ammonium persulfate and 0.08 g of sodium bisulfite are added as initiators. After reacting at a constant temperature for 3.5 h, 1.5 g of pre-dissolved N,N'-methylenebisacrylamide (1.0 wt% of the total monomer mass) is added, and the reaction is continued for 1.25 h to obtain the reaction product.

[0064] Step 4: After the polymerization reaction is completed and cooled, a transparent or translucent elastic gel polymer is obtained. This transparent or translucent elastic gel polymer is sheared, dissolved, and filtered to prepare a homogeneous viscous mother liquor product with a solid content of 10 wt%, labeled as product S2. Characterization shows that product S2 has a number-average molecular weight Mn of 5.5 × 10⁻⁶. 6 Da has a cationicity of 48%.

[0065] Simulation experiment: The depolymerizing flocculant material (i.e., product S2) prepared in Example 2 was used to treat the experimental raw water A to verify the effect of the depolymerizing flocculant material prepared in Example 2.

[0066] Processing technology: Take 1L of experimental raw water A and adjust the pH to 6.2 with dilute hydrochloric acid; use Fe 3+ The following steps were taken: 260 mg / L of polyferric sulfate was added and stirred rapidly at 500 rpm for 2 minutes; then, product S2 was added to make the effective ingredient dosage 40 mg / L; the speed was reduced to 80 rpm and stirred slowly for 10 minutes; after stopping stirring, the mixture was allowed to settle for 30 minutes.

[0067] Processing effect description: After treatment with product S2, the COD removal rate of raw water A was 79%, the oil removal rate was 92%, the SS removal rate was 93%, and the floc settling velocity was 5.3 m / h.

[0068] Example 3 This embodiment 3 provides a depolymerizing flocculant for fracturing flowback fluid treatment. The depolymerizing flocculant is prepared by aqueous free radical copolymerization using acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride and dodecyl acrylate (LA) as reactive monomers and N,N'-methylenebisacrylamide as a pre-set crosslinking agent.

[0069] In this embodiment 3, the depolymerizing flocculant has a moderate cationicity and a high proportion of hydrophobic modification; the preparation process is as follows: Step 1: In the reactor, add 49.0 g of acrylamide (corresponding to 0.69 mol), 93.3 g of dimethyl diallyl ammonium chloride (concentration of 60%, corresponding to 0.35 mol), 35.5 g of methacryloyloxyethyl trimethyl ammonium chloride (concentration of 75%, corresponding to 0.10 mol), and 14.0 g of dodecyl acrylate (corresponding to 0.0585 mol). Add deionized water to the reactor to make the total mass concentration of the reactants 31 wt%, and obtain a monomer mixed aqueous solution. The molar ratio of acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, and dodecyl acrylate is 46.9:33.4:9.6:5.6.

[0070] Step 2: Adjust the pH of the monomer mixed aqueous solution to 4.9 using dilute sulfuric acid; then, purge nitrogen gas for 25 minutes to remove oxygen from the monomer mixed aqueous solution, and obtain a deoxygenated monomer mixed aqueous solution.

[0071] Step 3: Under a nitrogen-protected atmosphere, the deoxygenated monomer aqueous solution was heated to 65°C. After the temperature stabilized, 0.15 g of mercaptoacetic acid (0.1 wt% of the total monomer mass) was added sequentially as a chain transfer agent, and 0.16 g of ammonium persulfate and 0.08 g of sodium bisulfite were added as initiators. After reacting at a constant temperature for 3 h, 0.25 g of pre-dissolved N,N'-methylenebisacrylamide (1.7 wt% of the total monomer mass) was added, and the reaction was continued for 1.75 h to obtain the reaction product.

[0072] Step 4: After the polymerization reaction is completed and cooled, a transparent or translucent elastic gel polymer is obtained. This transparent or translucent elastic gel polymer is sheared, dissolved, and filtered to prepare a homogeneous viscous mother liquor product with a solid content of 11 wt%, labeled as product S3. Characterization shows that product S3 has a number-average molecular weight Mn of 7.1 × 10⁻⁶. 6 Da has a cationicity of 52%.

[0073] Simulation experiment: The depolymerizing flocculant material (i.e., product S1) prepared in Example 3 was used to treat the experimental raw water A to verify the effect of the depolymerizing flocculant material prepared in Example 3; wherein, the simulated fracturing flowback fluid raw water A had TDS=85g / L, COD=4500mg / L, oil content=650mg / L, SS=1200mg / L, and pH=8.2.

[0074] Processing technology: Take 1 L of simulated fracturing flowback fluid raw water A, and adjust the pH to 6.2 with dilute hydrochloric acid; use Al 3+The following steps were taken: 250 mg / L of polyaluminum chloride was added and stirred rapidly at 500 rpm for 2 min; then, product S3 was added to bring the effective ingredient dosage to 45 mg / L; the stirring speed was reduced to 80 rpm and stirred slowly for 10 min; after stirring was stopped, the mixture was allowed to settle for 30 min.

[0075] Treatment effect description: After treatment with product S3, the raw water A has a COD removal rate of 85%, an oil removal rate of 95%, an SS removal rate of 94%, and a floc settling velocity of 5.0 m / h; the flocs are particularly dense.

[0076] Example 4 Example 4 provides a depolymerizing flocculant for fracturing flowback fluid treatment. To verify the material's adaptability under extremely high salt conditions with TDS far exceeding 80 g / L, acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, and hexyl acrylate (HA) were used as reactants. The depolymerizing flocculant was prepared by aqueous free radical copolymerization with N,N'-methylenebisacrylamide as a pre-set crosslinking agent.

[0077] In this embodiment 4, the preparation process of the depolymerizing flocculant is as follows: Step 1: In the reactor, add 40.0g of acrylamide, 81.0g of dimethyl diallyl ammonium chloride (concentration of 60%), 47.0g of methacryloyloxyethyl trimethyl ammonium chloride (concentration of 75%) and 9.0g of ethyl acrylate. Add deionized water to the reactor to make the total mass concentration of the reactants 30wt%, and obtain a monomer mixed aqueous solution.

[0078] Step 2: Adjust the pH of the monomer mixed aqueous solution to 5.1 using dilute sulfuric acid; then, purge nitrogen gas for 30 minutes to remove oxygen from the monomer mixed aqueous solution, and obtain a deoxygenated monomer mixed aqueous solution.

[0079] Step 3: Under a nitrogen-protected atmosphere, the deoxygenated monomer aqueous solution is heated to 60°C. After the temperature stabilizes, 0.3 g of mercaptopropionic acid (0.2 wt% of the total monomer mass) is added sequentially as a chain transfer agent, and 0.16 g of ammonium persulfate and 0.08 g of sodium bisulfite are added as initiators. After reacting at a constant temperature for 3.5 h, pre-dissolved N,N'-methylenebisacrylamide (1.25 wt% of the total monomer mass) is added, and the reaction is continued for another 1.25 h to obtain the reaction product.

[0080] Step 4: After the polymerization reaction is completed and cooled, a transparent or translucent elastic gel polymer is obtained. This transparent or translucent elastic gel polymer is sheared, dissolved, and filtered to prepare a homogeneous viscous mother liquor product with a solid content of 10 wt%, labeled as product S4. Characterization shows that the number average molecular weight Mn of product S4 is 4.65 × 10⁻⁶. 6 Da has a cationicity of 46%.

[0081] Simulation experiment: The depolymerizing flocculant material (i.e., product S4) prepared in Example 4 was used to treat experimental raw water B to verify the effect of the depolymerizing flocculant material prepared in Example 4. The experimental raw water B was used to simulate raw water with higher salinity. The experimental raw water B had TDS=120g / L, COD=5800mg / L, oil content=850mg / L, SS=1500mg / L, and pH=7.8.

[0082] Processing technology: Take 1L of experimental raw water B and adjust the pH to 6.1 with dilute hydrochloric acid; use Fe 3+ The following steps were taken: 280 mg / L of polyferric sulfate was added and stirred rapidly at 500 rpm for 2 min; then, product S4 was added to bring the effective ingredient dosage to 46 mg / L; the stirring speed was reduced to 80 rpm and stirred slowly for 12 min; after stirring was stopped, the mixture was allowed to settle for 40 min.

[0083] Processing effect description: After treatment with product S4, the COD removal rate of raw water B was 66%, the oil removal rate was 85%, the SS removal rate was 88%, and the floc settling velocity was 4.1 m / h. This proves that product S4 has excellent treatment effect under extremely high salinity, demonstrating its excellent high-salinity resistance.

[0084] Example 5 This embodiment 5 provides a depolymerizing flocculant for fracturing flowback fluid treatment. The depolymerizing flocculant is prepared by aqueous free radical copolymerization using acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride and butyl acrylate (BA) as reactive monomers and N,N'-methylenebisacrylamide as a preset crosslinking agent.

[0085] In this embodiment 5, the depolymerizing flocculant has the characteristics of moderate cationicity and slightly low molecular weight, aiming to achieve rapid charge neutralization and flocculation; the preparation process is as follows: Step 1: In the reactor, add 30.0g of acrylamide (corresponding to 0.422mol), 58.0g of dimethyl diallyl ammonium chloride (concentration of 60%, corresponding to 0.2175mol), 40.0g of methacryloyloxyethyl trimethyl ammonium chloride (concentration of 75%, corresponding to 0.1125mol), and 5.0g of butyl acrylate (corresponding to 0.039mol). Add deionized water to the reactor to make the total mass concentration of the reactants 30wt%, and obtain a monomer mixed aqueous solution.

[0086] Step 2: Adjust the pH of the monomer mixed aqueous solution to 5.0 using dilute sulfuric acid; then, purge nitrogen gas for 25 minutes to remove oxygen from the monomer mixed aqueous solution, obtaining a deoxygenated monomer mixed aqueous solution.

[0087] Step 3: Under a nitrogen-protected atmosphere, the deoxygenated monomer mixed aqueous solution is heated to 55°C. After the temperature stabilizes, 0.15 g of mercaptoacetic acid (0.1 wt% of the total monomer mass) is added sequentially as a chain transfer agent, and 0.16 g of ammonium persulfate and 0.08 g of sodium bisulfite are added as initiators. After reacting at a constant temperature for 3 h, pre-dissolved N,N'-methylenebisacrylamide (0.25 wt% of the total monomer mass) is added, and the reaction is continued for 1 h to obtain the reaction product.

[0088] Step 4: After the polymerization reaction is completed and cooled, a transparent or translucent elastic gel polymer is obtained. This transparent or translucent elastic gel polymer is sheared, dissolved, and filtered to form a homogeneous viscous mother liquor product with a solid content of 10 wt%, labeled as product S5. Characterization shows that product S5 has a number-average molecular weight Mn of 2.5 × 10⁻⁶. 6 Da has a cationicity of 38%.

[0089] Simulation experiment: The depolymerizing flocculant material (i.e., product S5) prepared in Example 5 was used to treat the experimental raw water C to verify the effect of the depolymerizing flocculant material prepared in Example 5. The experimental raw water C was used to simulate raw water with high suspended solids, slightly low salinity but still in the high salt range, with TDS=95g / L, COD=3200mg / L, oil content=500mg / L, SS=1500mg / L, and pH=8.5.

[0090] Processing technology: Take 1L of the experimental raw water C and adjust the pH to 6.35 with dilute hydrochloric acid; use Al 3+ The following steps were taken: 220 mg / L of polyaluminum chloride was added and stirred rapidly at 500 rpm for 2 min; then, product S5 was added to bring the effective ingredient dosage to 31 mg / L; the stirring speed was reduced to 80 rpm and stirred slowly for 10 min; after stirring was stopped, the mixture was allowed to settle for 30 min.

[0091] Processing effect description: After treatment with product S5, the COD removal rate of raw water C was 78%, the oil removal rate was 89%, the SS removal rate was 94%, and the floc settling velocity was 5.6 m / h. During the treatment process, product S5 demonstrated excellent removal ability for suspended solids and rapid settling characteristics.

[0092] Comparative Example We selected commercially available polyacrylamide (CPAM) with a nominal cationicity of 60% for parallel comparison.

[0093] Treatment experiment: The same experimental raw water A and basic process (PFS 250 mg / L, pH 6.2) as in Example 1 were used; to achieve visible flocculation, the effective component of CPAM needs to be added at 60 mg / L.

[0094] Experimental results: In the experimental raw water A treated with polyacrylamide, the COD removal rate was only 45%, the oil removal rate was 58%, the SS removal rate was 70%, the settling velocity was 2.1 m / h, and the flocs were small and loose.

[0095] As attached Figure 2 As shown, attached Figure 2 The document presents microscopic photographs of the floc morphology formed by the de-gelling flocculants prepared in Examples 1-5 when treating experimental raw water, and also presents microscopic photographs of the floc morphology formed by the polyacrylamide in the comparative example when treating experimental raw water; Table 1 presents the comparison results of the indicators of the de-gelling flocculants prepared in Examples 1-5 and the polyacrylamide in the comparative example after treating experimental raw water.

[0096] Comparison of indicators of the deflocculating flocculants in Examples 1-5 and the polyacrylamide used in the comparative examples after treating the raw water.

[0097] From the appendix Figure 2 As can be seen from Table 1 above, the debinding and flocculant materials prepared in Examples 1-5, by introducing dual-cationic synergistic monomer pairs, hydrophobic associating monomers, and moderately cross-linked structures into the molecular chain, can still efficiently debind, destabilize, flocculate, and remove contaminants in high-mineralization systems, with stable treatment effects and no risk of secondary pollution; among them, the number-average molecular weight of the debinding and flocculant materials can reach 2.5 × 10⁻⁶. 6 -7.1×10 6The cationicity of the Da component can reach 38%-52%; the dosage is 31-46 mg / L, which is significantly lower than that of traditional agents; in terms of application effect, the COD removal rate is 66%-85%, the oil removal rate is 85%-95%, the SS removal rate is 88%-94%, and the settling velocity is 4.1-5.6 m / h, which is significantly better than the performance of traditional commercially available CPAM under high salinity conditions; the preparation method of the described depolymerizing flocculant is reliable and the application process is highly adaptable, providing an efficient, stable and economical technical solution for solving the problem of treating high-mineralization fracturing flowback fluid.

[0098] The depolymerizing flocculant of this invention, starting from molecular structure design, uses the main chain backbone and crosslinking monomers, dual-cationic synergistic monomer pairs and hydrophobic associative functional monomers as reactive monomers. It is prepared by free radical copolymerization in aqueous solution and the introduction of a moderate crosslinking structure. It can effectively overcome the defects of conventional inorganic and organic polymer coagulants and flocculants in the treatment of high-salinity fracturing flowback fluids, such as severe charge shielding, molecular chain collapse, low demulsification efficiency, loose flocs and slow sedimentation, large dosage and unstable effect. It is particularly suitable for the treatment of fracturing flowback fluids with high salt TDS≥80g / L, high oil and high COD. Through its unique molecular structure design, this material can maintain excellent performance in high-salt environments.

[0099] In this invention, the depolymerizing flocculant maintains good molecular chain extensibility under high salt ion strength conditions, possesses efficient and stable charge neutralization capabilities, and also exhibits strong hydrophobic adsorption of oil droplets. This meets the treatment requirements for special water qualities such as high-salt fracturing flowback fluid, and has significant practical significance and application value for promoting the green and environmentally friendly development of unconventional oil and gas extraction and the recycling of water resources. The preparation process is based on aqueous solution free radical copolymerization technology, with a clear process route, mild and controllable reaction conditions, good repeatability, and easy industrial-scale production. The application process is simple and can be seamlessly integrated with existing coagulation sedimentation, flotation, and other water treatment facilities without large-scale equipment modifications, facilitating rapid promotion and application in oilfields.

[0100] In this invention, the molecular design concept of synergistic steric hindrance and charge distribution between dual-cationic monomer pairs, combined with the hydrophobic side chain association of hydrophobic associating functional monomers, fundamentally overcomes the shielding effect of high salinity environments on the charge neutralization capacity of polymers. The dual-cationic structure enhances charge stability, while the hydrophobic interaction is a physical force whose strength is basically unaffected by the concentration of inorganic salt ions in the water. Therefore, SRB coagulant can still maintain efficient demulsification, destabilization, and flocculation capabilities in harsh water environments with high mineralization (TDS) ≥ 80 g / L or even higher, resulting in stable and reliable treatment effects.

[0101] In this invention, thanks to the slight cross-linking structure introduced during the polymerization process, the internal network of the resulting flocs is more robust, exhibiting a large, dense, and uniform floc morphology with clear boundaries and low bound water content. The flocs have high mechanical strength and are not easily broken during subsequent transportation or light stirring. Their settling speed is significantly faster than that of flocs formed by traditional agents, with measured settling speeds reaching over 4 m / h, or even exceeding 5 m / h, greatly shortening the solid-liquid separation time and improving the efficiency of the treatment facility.

[0102] Addressing the core contamination issues of COD, grease, and suspended solids in fracturing flowback fluids, SRB coagulant achieves highly efficient removal of various contaminants, especially difficult-to-treat emulsified oils, through a multi-synergistic mechanism of charge neutralization, hydrophobic adsorption, and bridging trapping. Under optimized application conditions, for typical high-salt fracturing flowback fluids, COD removal rates can stably reach 60%-90%, oil removal rates can reach 80%-97%, and suspended solids (SS) removal rates can reach 85%-95%, resulting in significant improvement in effluent quality.

[0103] Due to its targeted and efficient nature, SRB coagulant can achieve the same or better treatment results in practical applications with a dosage that is 30%-50% lower than that of traditional cationic PAM (CPAM) agents. This significant reduction in agent cost, coupled with the potential reduction in sludge volume (i.e., denser flocs with lower moisture content), makes the overall water treatment process more cost-effective. SRB coagulant is available in both liquid and solid formulations to meet the needs of different on-site storage and dosing conditions. The accompanying application process is simple with clearly defined parameters, requiring no special equipment or complex modifications to existing coagulation sedimentation tanks or flotation machines. It is very easy to rapidly promote and apply in various oilfield produced water treatment stations and fracturing flowback fluid centralized treatment plants, demonstrating broad industrialization prospects.

[0104] The above embodiments are merely one of the implementation methods for achieving the technical solution of the present invention. The scope of protection claimed by the present invention is not limited to this embodiment, but also includes any variations, substitutions and other implementation methods that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention.

Claims

1. A degelatinizing flocculant for treating fracturing flowback fluid, characterized in that, Using the main chain backbone and crosslinking monomers, dual cationic synergistic monomer pairs and hydrophobic associative functional monomers as reactive monomers, and under the condition of a preset crosslinking agent, an aqueous free radical copolymerization method was adopted to prepare a depolymerizing flocculant. The polymer linear chains of the depolymer flocculant are introduced with a moderate cross-linking structure; The moderately cross-linked structure is a partially bridging structure or a lightly cross-linked structure with linear segments as the main component and local network nodes as auxiliary components.

2. The degelatinizing flocculant for fracturing flowback fluid treatment according to claim 1, characterized in that, The main chain backbone and bridging monomer are acrylamide; The dual-cationic co-monomer pair includes dimethyl diallyl ammonium chloride and methacryloyloxyethyltrimethylammonium chloride; The hydrophobic associative functional monomer is a hydrophobic alkyl acrylate monomer; wherein the alkyl chain length of the hydrophobic alkyl acrylate monomer is 9-12.

3. The degelatinizing flocculant for treating fracturing flowback fluid according to claim 2, characterized in that, The hydrophobic alkyl acrylate monomer is one or more of butyl acrylate, octyl acrylate and dodecyl acrylate.

4. The degelatinizing flocculant for fracturing flowback fluid treatment according to claim 2, characterized in that, The molar ratio of acrylamide, dimethyl diallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride and hydrophobic alkyl acrylate monomer is (30-50):(25-40):(10-25):(1-5).

5. The degelatinizing flocculant for treating fracturing flowback fluid according to claim 1, characterized in that, The preset crosslinking agent is N,N′-methylenebisacrylamide.

6. The degelatinizing flocculant for fracturing flowback fluid treatment according to claim 5, characterized in that, The amount of N,N′-methylenebisacrylamide added is 0.1wt%-3.0wt% of the total mass of the reactants.

7. The degelatinizing flocculant for treating fracturing flowback fluid according to claim 1, characterized in that, The number-average molecular weight of the depolymerizing flocculant is controlled at 2.5 × 10⁻⁶. 6 -7.1×10 6 Da.

8. The degelatinizing flocculant for treating fracturing flowback fluid according to claim 1, characterized in that, The cationicity of the depolymerizing flocculant is 38%-52%.

9. A method for preparing a degelatinizing flocculant for fracturing flowback fluid treatment as described in any one of claims 1-8, characterized in that, include: The main chain backbone, bridging monomers, dual-cationic synergistic monomer pairs, and hydrophobic associative functional monomers are added to water, dissolved, and mixed evenly to obtain a monomer mixed aqueous solution. The pH of the monomer mixture aqueous solution was adjusted to 4.5-5.5, and then deoxygenated to obtain a deoxygenated monomer mixture aqueous solution. Under a nitrogen-protected atmosphere, the deoxygenated monomer mixed aqueous solution was heated to the preset polymerization temperature; then, a chain transfer agent, an initiator, and a preset crosslinking agent were added sequentially to carry out the polymerization reaction and obtain the reaction product. The reaction products are cooled, post-treated, and molded to obtain a depolymerized flocculant material.

10. The application of the degelatinizing flocculant for fracturing flowback fluid treatment as described in any one of claims 1-8, characterized in that, The de-gelling and flocculant material is used in the de-gelling process of fracturing flowback fluid.

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