Composite water clarifying agent for oilfield sewage treatment and preparation method thereof

Abstract

The application relates to the technical field of oilfield sewage treatment, and particularly discloses a composite water clarifying agent for oilfield sewage treatment and a preparation method thereof. The composite water clarifying agent for oilfield sewage treatment is prepared from the following raw materials: double-modified polyacrylamide, imidazoline quaternary ammonium salt graft copolymer, scale inhibitor, adsorbent, dodecyl dimethyl betaine, coagulant aid and deionized water; the double-modified polyacrylamide is obtained by double modification of 2,3-epoxypropyl trimethyl ammonium chloride and a hydrophobic monomer on polyacrylamide; and the imidazoline quaternary ammonium salt graft copolymer is a graft copolymer of imidazoline quaternary ammonium salt, acrylamide and acrylamide-2-methylpropane sulfonic acid. The composite water clarifying agent can be used for treating high-salt and high-oil sewage generated in the middle and late stages of oilfield exploitation, has the advantages of strong salt resistance, complete demulsification and good flocculation effect, can efficiently remove oil and suspended solids, ensures that the effluent meets the standards, and meets the complex sewage treatment requirements of oilfields.

Description

Technical Field

[0001] This application relates to the field of oilfield wastewater treatment technology, and more specifically, it relates to a composite water purification agent for oilfield wastewater treatment and its preparation method. Background Technology

[0002] Oilfield development is a core industry for ensuring energy supply. Throughout the drilling, oil extraction, and gathering and transportation processes, a large amount of oilfield wastewater is inevitably generated. As a "byproduct" of oil and gas extraction, this wastewater is massive in volume and complex in composition, representing typical high-difficulty industrial wastewater. Its treatment effectiveness directly impacts ecological safety and the sustainable development of the oilfield industry. Oilfield wastewater contains not only suspended oil and emulsified oil, but also high concentrations of inorganic salts, suspended solids such as clay and silt, recalcitrant organic matter, and residual chemical additives. Direct discharge or reuse without effective treatment can lead to soil salinization, water hypoxia, and may also clog formation pores and corrode treatment equipment, severely hindering the green development of oilfields. To achieve the harmless discharge and resource reuse of oilfield wastewater, water purification agents are widely used as core treatment agents. Through demulsification, flocculation, and adsorption, they effectively remove oil, suspended solids, and some organic matter from wastewater, reducing pollutant concentrations and meeting the requirements for oilfield reinjection water or compliant discharge. They are an indispensable key material in oilfield wastewater treatment systems.

[0003] Currently, various patents related to water-cleaning agents for oilfield wastewater treatment have been published in the industry, forming different types of technical solutions to adapt to various wastewater treatment needs. For example, patent application CN113121812A discloses a water-cleaning agent for treating oily wastewater in oilfields, its preparation method, and its application. This water-cleaning agent is prepared by a stepwise reaction of dimethylamine aqueous solution, dodecyl dimethyl tertiary amine, epichlorohydrin, and polyethylene polyamine. It has the characteristics of low viscosity and small dosage, and is suitable for treating oily wastewater in offshore oilfields. When treating oily wastewater with an oil content of 200-1500 mg / L, the oil removal rate can reach 86%-95%. For example, patent document CN114276549B discloses an anionic, highly surface-active organosilicon water purifier and its preparation method. The highly branched organosilicon polymer prepared by this method has a large number of -NH2 and / or -OH functional groups, good water solubility, high surface activity, and low flocculent production. It exhibits excellent water purification effects in polymer flooding oilfield wastewater treatment, effectively removing oil and suspended solids from the wastewater. In addition, there are various water purifiers designed for conventional operating conditions, focusing on optimizing different performance indicators such as demulsification efficiency and flocculation speed, providing diversified technical options for oilfield wastewater treatment.

[0004] However, existing water purification agents and conventional water purification agents in the industry are mostly suitable for conventional operating conditions. When treating wastewater generated in the mid-to-late stages of oilfield development, tight oil extraction, and during the reuse of pressure flowback fluid, this type of wastewater is characterized by high salt and high oil content. Conventional water purification agents are ineffective in treating this type of wastewater, specifically exhibiting incomplete demulsification, poor flocculation, insufficient removal of oil and suspended solids, and the treated effluent failing to meet discharge or reinjection water standards. This is mainly because existing water purification agents have a simple active component structure and lack salt-resistant modification design. High-salt environments disrupt their molecular charge balance and inhibit activity. Furthermore, their adsorption and chelation capabilities are insufficient, and active functional groups are easily coated by salt ions, failing to fully react with oil droplets and suspended solids, leading to reduced treatment efficiency and making it difficult to meet the increasingly complex wastewater treatment needs of oilfields. Therefore, there is an urgent need to develop a high-efficiency composite water purification agent specifically for oilfield wastewater with high salt and high oil content. Summary of the Invention

[0005] To enhance the purification effect of water-cleaning agents on oilfield wastewater with high salt and oil content, this application provides a composite water-cleaning agent for oilfield wastewater treatment and its preparation method.

[0006] This application provides a composite water cleaning agent for oilfield wastewater treatment, employing the following technical solution:

[0007] A composite water cleaning agent for oilfield wastewater treatment comprises the following raw materials in parts by weight:

[0008] 18-28 parts of double-modified polyacrylamide;

[0009] 10-16 parts of imidazoline quaternary ammonium salt graft copolymer;

[0010] 6-12 parts scale inhibitor;

[0011] 5-10 parts of adsorbent;

[0012] 1-3 parts of dodecyl dimethyl betaine;

[0013] 3-7 parts of coagulant aid;

[0014] 38-58 parts deionized water;

[0015] The dual-modified polyacrylamide is obtained by dual modification of polyacrylamide with 2,3-epoxypropyltrimethylammonium chloride and hydrophobic monomer;

[0016] The imidazoline quaternary ammonium salt graft copolymer is a graft copolymer of imidazoline quaternary ammonium salt with acrylamide and acrylamide-2-methylpropanesulfonic acid.

[0017] By adopting the above technical solution, 2,3-epoxypropyltrimethylammonium chloride is selected as the cationic etherifying agent. Compared with conventional cationic etherifying agents, it has higher reactivity and can fully combine with the polyacrylamide molecular chain, accurately controlling the cationicity at 35%-55%, and significantly enhancing the positive charge density of the polyacrylamide molecules. In high-salt wastewater, it can effectively resist the interference of a large number of salt ions (chloride ions, sodium ions, etc.), avoid the disruption of molecular charge balance, and solve the core pain point of inhibited activity of conventional water purification agents. At the same time, the positive charge can form strong electrostatic adsorption with the negatively charged emulsified oil droplets and suspended solids in high-salt wastewater, fundamentally improving the problems of poor flocculation effect and insufficient removal of suspended solids. Hexadecyl dimethyl allyl ammonium chloride is introduced as a hydrophobic monomer, and a polymerization reaction is initiated by an initiator to graft hydrophobic groups onto the polyacrylamide molecular chain, forming a hydrophobic associated structure. This design breaks through the limitations of conventional modification that only focuses on charge properties. Utilizing the hydrophobic effect of hydrophobic groups, the polyacrylamide molecular chains undergo hydrophobic association in a high-salt environment, forming a spatial network structure. On one hand, this network structure enables highly efficient bridging flocculation, firmly encapsulating fine oil droplets and suspended solids, solving the problems of conventional flocs being small, loose, and easily broken, significantly improving the suspended solids removal rate. On the other hand, the hydrophobic groups can interact hydrophobically with oil droplets, enhancing the adsorption capacity for oil droplets and further improving the oil removal effect, achieving a dual effect of "flocculation + adsorption." The dual-modified polyacrylamide in this application solves the "salt resistance activity" problem through cationization modification and the "flocculation strength + oil removal assistance" problem through hydrophobic association modification. The synergistic effect of these two methods allows the dual-modified polyacrylamide to maintain stable activity in high-salt, high-oil wastewater while achieving highly efficient flocculation and oil droplet adsorption, fundamentally improving the problems of poor flocculation and insufficient suspended solids removal.

[0018] Imidazoline quaternary ammonium salts are grafted onto the molecular chain of imidazoline quaternary ammonium salts via an initiator, using acrylamide and 2-acrylamide-2-methylpropanesulfonic acid to form multifunctional graft copolymers. The acrylamide group enhances the water solubility and bridging ability of the molecule, forming a synergistic flocculation effect with the doubly modified polyacrylamide. The 2-acrylamide-2-methylpropanesulfonic acid group possesses strong salt resistance and hydrophilicity, effectively improving the solubility of the graft copolymer in high-salt wastewater and avoiding the precipitation and decreased activity issues associated with conventional imidazoline quaternary ammonium salts in high-salt environments. Imidazoline quaternary ammonium salts inherently possess demulsifying properties. After graft copolymerization, the density of quaternary ammonium salt groups on the molecular chain increases, and the synergistic effect of multiple functional groups allows the molecules to quickly penetrate to the oil-water interface, reducing the interfacial tension. Compared to conventional imidazoline quaternary ammonium salts, the reduction in interfacial tension is increased by more than 30%, enabling rapid disruption of stable oil-water emulsion films in high-salt and high-oil wastewater (these emulsion films are extremely stable due to the high-salt environment, making them difficult for conventional demulsifiers to penetrate), achieving highly efficient demulsification and solving the core pain points of incomplete demulsification and insufficient oil removal by conventional water cleaning agents. The introduction of acrylamide-2-methylpropanesulfonic acid groups not only enhances the salt resistance of the graft copolymer, but also complexes some metal ions in high-salt wastewater, reducing the coating of salt ions on the active groups of imidazoline quaternary ammonium salt, and further enhancing its demulsification activity. At the same time, the molecular structure of the graft copolymer can intertwine with the network structure of the double-modified polyacrylamide, so that the oil droplets after demulsification are quickly captured by the flocculants, realizing the simultaneous occurrence of "demulsification + flocculation", which greatly improves the treatment efficiency of high-salt and high-oil wastewater.

[0019] The composite water purification agent provided in this technical solution can maintain a high efficiency and stable purification capacity under complex working conditions of high salt, high oil, and high impurities. It achieves simultaneous multi-site effects of demulsification, flocculation, adsorption, and resistance to salt interference, ultimately enabling the treated effluent to meet the requirements of oilfield reinjection water or discharge standards. This significantly improves the adaptability to working conditions of highly challenging wastewater such as oilfield late-stage development, tight oil extraction, and fracturing flowback fluid.

[0020] Optionally, the dual-modified polyacrylamide is prepared using the following method:

[0021] 2,3-epoxypropyltrimethylammonium chloride was added to polyacrylamide, and a cationization reaction was carried out at 55-75℃ and pH 7.0-8.0 for 3-4 hours. Then, a hydrophobic monomer and an initiator were added, and the reaction was carried out at 60-70℃ for 2-4 hours. After the reaction was completed, the polyacrylamide was obtained by precipitation, washing, vacuum drying and pulverization.

[0022] The above technical solution uses a two-step modification method for dual-modified polyacrylamide, which first completes the cationization modification to enhance the charge density, and then introduces hydrophobic monomers to form a hydrophobic association structure. In a high-salt system, the molecular chains are more likely to unfold and form a spatial network, effectively avoiding loose and small flocs, and further enhancing the ability to capture oil droplets and suspended matter.

[0023] Optionally, the amount of 2,3-epoxypropyltrimethylammonium chloride added is 15%-25% of the mass of polyacrylamide.

[0024] The aforementioned amount of 2,3-epoxypropyltrimethylammonium chloride enables polyacrylamide to obtain suitable cationic strength, ensuring charge stability in high-salt environments and achieving efficient adsorption of negatively charged colloids and oil droplets. This avoids the problems of weak salt resistance due to insufficient cationic content or decreased solubility due to excessive cationic content.

[0025] Optionally, the hydrophobic monomer is hexadecyl dimethyl allyl ammonium chloride, and the amount of the hydrophobic monomer added is 5%-10% of the mass of polyacrylamide.

[0026] Optionally, the initiator is a mixture of ammonium persulfate and sodium bisulfite in a mass ratio of 1:(0.5-1); the amount of the initiator added is 0.3%-0.8% of the total mass of the reaction system.

[0027] The above technical solution, employing an oxidation-reduction initiation system, allows polymerization to be completed under mild conditions. This facilitates the formation of dual-modified polyacrylamide with uniform structure and appropriate molecular weight, ensuring stable flocculation performance while improving raw material utilization and product reproducibility.

[0028] Optionally, the imidazoline quaternary ammonium salt graft copolymer is prepared by the following method:

[0029] First, mix imidazoline quaternary ammonium salt with acrylamide, add deionized water to dissolve, adjust the pH to 6.5-7.5 to obtain a mixed reaction solution, heat the mixed reaction solution to 70-80℃, add an initiator, react for 1-2 hours, then add 2-acrylamido-2-methylpropanesulfonic acid, continue the reaction for 2-3 hours, cool to room temperature, and obtain the imidazoline quaternary ammonium salt graft copolymer by vacuum distillation.

[0030] The above scheme prepares imidazoline quaternary ammonium salt graft copolymers through stepwise graft copolymerization. First, acrylamide is introduced to form a hydrophilic backbone, and then salt-resistant monomers are added, so that the product has good water solubility, interfacial activity and salt resistance, which significantly improves the demulsification effect and stability in high-salt wastewater.

[0031] Optionally, the mass ratio of the imidazoline quaternary ammonium salt, acrylamide, and deionized water is 1:(1.2-1.8):(8-15).

[0032] Optionally, the initiator is azobisisobutyramidine hydrochloride, and the amount of the initiator added is 0.5%-1.0% of the mass of the mixed reaction solution; the amount of 2-acrylamide-2-methylpropanesulfonic acid added is 70%-90% of the mass of imidazoline quaternary ammonium salt.

[0033] Optionally, the scale inhibitor is diethyl hydroxyphosphonate.

[0034] Using diethyl hydroxyphosphonate as a scale inhibitor can effectively complex calcium, magnesium and other polyvalent metal ions in high-salt wastewater, reduce the interference and coating of salts on active components, protect the demulsification and flocculation capabilities of the core components of the water cleaning agent, and improve long-term stability.

[0035] This application also provides a method for preparing a composite water purification agent for oilfield wastewater treatment, using the following technical solution:

[0036] A method for preparing a composite water-cleaning agent for oilfield wastewater treatment includes the following steps:

[0037] S1. Mix deionized water and double-modified polyacrylamide, heat to 45-55℃, and stir until completely dissolved to obtain the base solution;

[0038] S2. Add imidazoline quaternary ammonium salt graft copolymer to the base liquid, heat to 60-70℃, keep warm and stir until uniformly mixed, then add scale inhibitor, adsorbent, dodecyl dimethyl betaine and coagulant in sequence, continue to keep warm and stir for 30-60 minutes, cool to room temperature to obtain composite water cleaning agent.

[0039] The above preparation method uses stepwise dissolution, segmented temperature control, and sequential feeding to fully disperse and synergistically enhance the components, avoids clumping of polymer components or excessively high local concentrations, and ensures that the final product is uniform, stable, and reliable, making it suitable for industrial production and field applications.

[0040] In summary, this application has the following beneficial effects:

[0041] 1. The technical solution of this application effectively solves the core defect of conventional water purification agents' activity decay in high-salt environments, improving the stability of water purification agents in high-salt and high-oil wastewater. Addressing the problems of existing water purification agents having a single active component structure, lacking salt-resistant modification design, and being susceptible to charge balance disruption and activity inhibition due to salt ion interference, this application performs dual modification of polyacrylamide through cationization and hydrophobic association. A highly reactive cationic etherifying agent is selected to enhance the positive charge density of polyacrylamide molecules, resisting salt ion interference. Simultaneously, hydrophobic monomers are introduced to form a hydrophobic association structure, preventing molecular chain coiling. At the same time, salt-resistant monomers are incorporated into the imidazoline quaternary ammonium salt graft copolymer, improving its solubility in high-salt wastewater and preventing the active component from leaching out. The modification design of these two core components fundamentally solves the pain point of water purification agent activity decay in high-salt environments, ensuring stable activity under complex high-salt and high-oil conditions, providing a foundation for subsequent demulsification and flocculation.

[0042] 2. The composite water cleaning agent provided in this application can significantly improve the problems of incomplete demulsification and insufficient oil removal in high-salt and high-oil wastewater, thereby improving demulsification efficiency and oil removal effect. Addressing the shortcomings of conventional water cleaning agents, which struggle to disrupt stable oil-water emulsion films in high-salt environments and exhibit poor demulsification effects, the imidazoline quaternary ammonium salt graft copolymer in this application, through multi-functional group graft modification, significantly enhances interfacial activity, allowing it to quickly penetrate to the oil-water interface and disrupt the stable emulsion film, achieving highly efficient demulsification. Simultaneously, the graft copolymer and the double-modified polyacrylamide form a synergistic effect. The hydrophobic groups of the double-modified polyacrylamide interact hydrophobically with oil droplets, enhancing their adsorption capacity. Oil droplets after demulsification by the graft copolymer can be rapidly captured by the flocs formed by the polyacrylamide, achieving simultaneous "demulsification + flocculation," effectively solving the problems of insufficient oil removal and excessive oil content in the treated effluent, ensuring that the effluent meets discharge or reinjection water standards.

[0043] 3. The dual-modified polyacrylamide of this application achieves efficient electrostatic adsorption of negatively charged suspended solids and oil droplets through cationization modification, and then forms a spatial network structure through hydrophobic association to achieve efficient bridging flocculation, firmly encapsulating fine oil droplets and suspended solids, enhancing the strength and settling performance of the flocs, and preventing floc breakage and loss. Simultaneously, the auxiliary and core components work synergistically: the scale inhibitor reduces the coating of active functional groups by salt ions, the coagulant accelerates floc settling, and the adsorbent further adsorbs fine impurities, comprehensively improving the removal effect of suspended solids. The entire composite system achieves multi-site synergy of demulsification, flocculation, adsorption, and salt interference resistance, significantly improving the purification efficiency of high-salt and high-oil wastewater, and adapting to the complex wastewater treatment needs of oilfields. Detailed Implementation

[0044] The present application will be further described in detail below with reference to the embodiments.

[0045] Preparation example of dual-modified polyacrylamide

[0046] Preparation Example 1

[0047] The dual-modified polyacrylamide was prepared using the following method:

[0048] Weigh 100g of polyacrylamide, add 15g of 2,3-epoxypropyltrimethylammonium chloride, adjust the pH of the system to 7.0 with 5% sodium hydroxide solution, heat to 55℃, and stir at a constant temperature for 3h for cationization reaction; after the reaction, add 5g of hydrophobic monomer hexadecyldimethylallylammonium chloride, and then add an initiator composed of ammonium persulfate and sodium bisulfite in a mass ratio of 1:0.5, the amount of initiator added is 0.3% of the total mass of the reaction system, adjust the temperature to 60℃, and continue stirring for 2h; after the reaction, add acetone to precipitate, wash the precipitate 3 times with deionized water, dry it in a vacuum drying oven, and pulverize it to 100 mesh to obtain double-modified polyacrylamide.

[0049] Preparation Example 2

[0050] The dual-modified polyacrylamide was prepared using the following method:

[0051] Weigh 100g of polyacrylamide, add 20g of 2,3-epoxypropyltrimethylammonium chloride, adjust the pH of the system to 7.5 with 5% sodium hydroxide solution, heat to 65℃, and stir at a constant temperature for 3.5h for cationization reaction; after the reaction, add 7.5g of hydrophobic monomer hexadecyldimethylallylammonium chloride, and then add an initiator composed of ammonium persulfate and sodium bisulfite in a mass ratio of 1:0.75, the amount of initiator added being 0.55% of the total mass of the reaction system, adjust the temperature to 65℃, and continue stirring for 3h; after the reaction, add acetone for precipitation, wash the precipitate three times with deionized water, dry it in a vacuum drying oven, and pulverize it to 100 mesh to obtain double-modified polyacrylamide.

[0052] Preparation Example 3

[0053] The dual-modified polyacrylamide was prepared using the following method:

[0054] Weigh 100g of polyacrylamide, add 25g of 2,3-epoxypropyltrimethylammonium chloride, adjust the pH of the system to 8.0 with 5% sodium hydroxide solution, heat to 75℃, and stir at a constant temperature for 4h for cationization reaction; after the reaction, add 10g of hydrophobic monomer hexadecyldimethylallylammonium chloride, and then add an initiator composed of ammonium persulfate and sodium bisulfite in a mass ratio of 1:1, the amount of initiator added being 0.8% of the total mass of the reaction system, adjust the temperature to 70℃, and continue stirring for 4h; after the reaction, add acetone for precipitation, wash the precipitate three times with deionized water, dry it in a vacuum drying oven, and pulverize it to 100 mesh to obtain double-modified polyacrylamide.

[0055] Preparation Example 4

[0056] Cationic modified polyacrylamide was prepared by the following method:

[0057] Weigh 100g of polyacrylamide, add 20g of 2,3-epoxypropyltrimethylammonium chloride, adjust the pH of the system to 7.5 with 5% sodium hydroxide solution, add an appropriate amount of deionized water to dissolve, heat to 65℃, and stir at a constant temperature for 3.5h for cationization reaction. After the reaction is completed, add acetone to precipitate, wash the precipitate 3 times with deionized water, dry it in a vacuum drying oven, and pulverize it to 100 mesh to obtain cationized modified polyacrylamide.

[0058] Preparation example of imidazoline quaternary ammonium salt graft copolymer

[0059] Preparation Example 5

[0060] The imidazoline quaternary ammonium salt graft copolymer was prepared by the following method:

[0061] Weigh 10g of imidazoline quaternary ammonium salt and 12g of acrylamide, add 80g of deionized water, stir until completely dissolved, adjust the pH of the system to 6.5 with 5% sodium hydroxide solution to obtain a mixed reaction solution; heat the mixed reaction solution to 70℃, add 0.5g of azobisisobutyramidine hydrochloride, and stir at a constant temperature for 1h; after the reaction is completed, add 7g of 2-acrylamido-2-methylpropanesulfonic acid, and continue to stir at a constant temperature for 2h; cool to room temperature, and remove excess water by vacuum distillation to obtain the imidazoline quaternary ammonium salt graft copolymer.

[0062] Preparation Example 6

[0063] The imidazoline quaternary ammonium salt graft copolymer was prepared by the following method:

[0064] Weigh 10g of imidazoline quaternary ammonium salt and 15g of acrylamide, add 115g of deionized water, stir until completely dissolved, adjust the pH of the system to 7.0 with 5% sodium hydroxide solution to obtain a mixed reaction solution; heat the mixed reaction solution to 75℃, add 0.85g of azobisisobutyramidine hydrochloride, and stir at a constant temperature for 1.5h; after the reaction is completed, add 8g of 2-acrylamido-2-methylpropanesulfonic acid, and continue to stir at a constant temperature for 2.5h; cool to room temperature, and remove excess water by vacuum distillation to obtain the imidazoline quaternary ammonium salt graft copolymer.

[0065] Preparation Example 7

[0066] The imidazoline quaternary ammonium salt graft copolymer was prepared by the following method:

[0067] Weigh 10g of imidazoline quaternary ammonium salt and 18g of acrylamide, add 150g of deionized water, stir until completely dissolved, adjust the pH of the system to 7.5 with 5% sodium hydroxide solution to obtain a mixed reaction solution; heat the mixed reaction solution to 80℃, add 1.78g of azobisisobutyramidine hydrochloride, and stir at a constant temperature for 2h; after the reaction is completed, add 9g of 2-acrylamido-2-methylpropanesulfonic acid, and continue to stir at a constant temperature for 3h; cool to room temperature, and remove excess water by vacuum distillation to obtain the imidazoline quaternary ammonium salt graft copolymer.

[0068] Example

[0069] Example 1

[0070] A composite water cleaning agent for oilfield wastewater treatment, the raw material components and formulations of which are shown in Table 1, wherein the double-modified polyacrylamide is the double-modified polyacrylamide prepared in Preparation Example 1; the imidazoline quaternary ammonium salt graft copolymer is the imidazoline quaternary ammonium salt graft copolymer prepared in Preparation Example 5; the scale inhibitor is diethyl hydroxyphosphonate; the adsorbent is montmorillonite; and the coagulant aid is polyaluminum chloride with an alumina content of 30%.

[0071] A method for preparing a composite water-cleaning agent for oilfield wastewater treatment includes the following steps:

[0072] S1. Mix deionized water and double-modified polyacrylamide, heat to 45°C, and stir until completely dissolved to obtain the base solution;

[0073] S2. Add imidazoline quaternary ammonium salt graft copolymer to the base liquid, heat to 60℃, keep warm and stir until uniformly mixed, then add scale inhibitor, adsorbent, dodecyl dimethyl betaine and coagulant in sequence, continue to keep warm and stir for 30 minutes, cool to room temperature to obtain composite water purification agent.

[0074] Example 2

[0075] A composite water cleaning agent for oilfield wastewater treatment, the raw material components and formulations of which are shown in Table 1, wherein the double-modified polyacrylamide is the double-modified polyacrylamide prepared in Preparation Example 2; the imidazoline quaternary ammonium salt graft copolymer is the imidazoline quaternary ammonium salt graft copolymer prepared in Preparation Example 6; the scale inhibitor is diethyl hydroxyphosphonate; the adsorbent is montmorillonite; and the coagulant aid is polyaluminum chloride with an alumina content of 30%.

[0076] A method for preparing a composite water-cleaning agent for oilfield wastewater treatment includes the following steps:

[0077] S1. Mix deionized water and double-modified polyacrylamide, heat to 50°C, and stir until completely dissolved to obtain the base solution;

[0078] S2. Add imidazoline quaternary ammonium salt graft copolymer to the base liquid, heat to 65℃, keep warm and stir until uniformly mixed, then add scale inhibitor, adsorbent, dodecyl dimethyl betaine and coagulant in sequence, continue to keep warm and stir for 45 minutes, cool to room temperature to obtain composite water cleaning agent.

[0079] Example 3

[0080] A composite water cleaning agent for oilfield wastewater treatment, the raw material components and formulations of which are shown in Table 1, wherein the double-modified polyacrylamide is the double-modified polyacrylamide prepared in Preparation Example 3; the imidazoline quaternary ammonium salt graft copolymer is the imidazoline quaternary ammonium salt graft copolymer prepared in Preparation Example 7; the scale inhibitor is diethyl hydroxyphosphonate; the adsorbent is montmorillonite; and the coagulant aid is polyaluminum chloride with an alumina content of 30%.

[0081] A method for preparing a composite water-cleaning agent for oilfield wastewater treatment includes the following steps:

[0082] S1. Mix deionized water and double-modified polyacrylamide, heat to 55°C, and stir until completely dissolved to obtain the base solution;

[0083] S2. Add imidazoline quaternary ammonium salt graft copolymer to the base liquid, heat to 70℃, keep warm and stir until uniformly mixed, then add scale inhibitor, adsorbent, dodecyl dimethyl betaine and coagulant in sequence, continue to keep warm and stir for 60 min, cool to room temperature to obtain composite water cleaning agent.

[0084] Table 1. Raw material components and proportions (g) of the water-clearing agents in Examples 1-3

[0085]

[0086] Example 4

[0087] A composite water purification agent for oilfield wastewater treatment differs from Example 2 in that the dual-modified polyacrylamide used in this example is the dual-modified polyacrylamide prepared in Preparation Example 1.

[0088] Example 5

[0089] A composite water purification agent for oilfield wastewater treatment differs from Example 2 in that the imidazoline quaternary ammonium salt graft copolymer used in this example is the imidazoline quaternary ammonium salt graft copolymer prepared in Preparation Example 7.

[0090] Example 6

[0091] A composite water cleaning agent for oilfield wastewater treatment differs from Example 2 in that the scale inhibitor used in this example is aminotrimethylenephosphonic acid.

[0092] Example 7

[0093] A composite water purification agent for oilfield wastewater treatment differs from Example 2 in that the adsorbent used in this example is activated carbon with a specific surface area of ​​1200 m². 2 / g.

[0094] Comparative Example

[0095] Comparative Example 1

[0096] A water-cleaning agent was prepared according to Example 1 in the patent application document with publication number CN113121812A entitled "A water-cleaning agent for treating oily wastewater in oil fields and its preparation method and application".

[0097] Comparative Example 2

[0098] A composite water cleaning agent for oilfield wastewater treatment differs from Example 2 in that an equal amount of polyacrylamide is used instead of double-modified polyacrylamide, and an equal amount of imidazoline quaternary ammonium salt is used instead of imidazoline quaternary ammonium salt graft copolymer.

[0099] Comparative Example 3

[0100] A composite water purification agent for oilfield wastewater treatment differs from Example 2 in that an equal amount of cationic modified polyacrylamide prepared in Preparation Example 4 is used instead of double-modified polyacrylamide in this comparative example.

[0101] Comparative Example 4

[0102] A composite water purification agent for oilfield wastewater treatment differs from Example 2 in that an equal amount of imidazoline quaternary ammonium salt is used instead of the imidazoline quaternary ammonium salt graft copolymer in this comparative example.

[0103] Performance testing

[0104] I. Samples for testing

[0105] The water-clearing agents prepared in Examples 1-7 and Comparative Examples 1-4 were selected as test samples. Three copies of each sample were prepared in parallel, and the average value of the test results was taken to ensure the reliability of the data.

[0106] II. The wastewater comes from the mixed wastewater of oil production wastewater and pressure flowback fluid from the tight oil blocks in the middle and late stages of oilfield development. The initial oil content of the wastewater was 8250 mg / L; the salt content (mainly sodium chloride, calcium chloride and magnesium chloride) was 18.7%; the suspended solids content (mainly clay mud, crude oil emulsion residue and a small amount of drilling mud residue particles) was 823 mg / L; and the pH value was 7.5.

[0107] III. Testing Items

[0108] 1. Oil removal rate

[0109] Take 1000 mL of simulated high-salt, high-oil wastewater and place it in a beaker. Incubate the beaker in a constant-temperature water bath (25℃) for 10 min. Add 80 mg / L of the test sample (water purifier), stir (200 r / min) for 10 min, and allow to settle for 60 min. Take 50 mL of the supernatant and determine the oil content using GB / T16488-1996 "Determination of Petroleum and Animal / Vegetable Oils in Water - Infrared Spectrophotometry". Calculate the oil removal rate. The formula for calculating the oil removal rate is: Oil removal rate (%) = (Initial oil content - Oil content after treatment) / Initial oil content × 100%.

[0110] 2. Suspended solids removal rate

[0111] Under the same test conditions as test item 1, after settling for 60 minutes, take 50 mL of the supernatant and determine the suspended solids content in the supernatant using GB / T11901-1989 "Determination of Suspended Solids in Water - Gravimetric Method", and calculate the suspended solids removal rate. The formula for calculating the suspended solids removal rate is: Suspended solids removal rate (%) = (Initial suspended solids content - Treated suspended solids content) / Initial suspended solids content × 100%.

[0112] 3. Settling velocity of flocs

[0113] Under the same test conditions as test item 1, after adding the water-clearing agent and stirring, start timing immediately, observe and record the time required for the flocs to begin settling to the bottom of the beaker (settling height ≥ 90%), which is the floc settling speed (unit: min). The shorter the time, the higher the floc strength and the better the settling performance.

[0114] 4. Verification of effluent compliance

[0115] Under the same test conditions as test item 1, after settling for 60 minutes, the oil content and suspended solids content of the supernatant were measured. The effluent was judged to meet the standards by referring to the oilfield reinjection water index (oil content ≤50mg / L, suspended solids content ≤30mg / L) and the industrial wastewater discharge standard (oil content ≤10mg / L, suspended solids content ≤20mg / L).

[0116] IV. Test results are shown in Table 2

[0117] Table 2 Detection Results

[0118]

[0119] Looking at the changes in oil removal rate data, the oil removal rates of Examples 1-7 all remained above 97.8%, with Example 2 achieving the best rate of 99.1%. In contrast, the oil removal rates of Comparative Examples 1-4 were only 75.6%-88.9%, showing a significant difference. The core reason is that the imidazoline quaternary ammonium salt graft copolymer used in the examples of this application, by incorporating the salt-resistant monomer 2-acrylamide-2-methylpropanesulfonic acid, improves the solubility and interfacial activity in high-salt environments, enabling it to quickly penetrate and destroy the stable oil-water emulsion film in high-salt wastewater. In contrast, the existing water-cleaning agent in Comparative Example 1 did not undergo salt-resistant modification, Comparative Example 2 did not use modified core components, and Comparative Example 4 did not use graft copolymers, all of which failed to effectively demulsify, resulting in a low oil removal rate. This precisely corresponds to the technical problem in the background art of "incomplete demulsification and insufficient oil removal by conventional water-cleaning agents."

[0120] The data on suspended solids removal rate and floc settling velocity showed a clear synergistic trend. The suspended solids removal rate of Examples 1-7 was ≥97.1%, and the floc settling velocity was only 22-30 min. In contrast, Comparative Example 2 had the lowest suspended solids removal rate (73.2%) and the longest settling velocity (68 min), while Comparative Example 3 also achieved a settling velocity of 42 min. This is because the dual-modified polyacrylamide used in the Examples achieved electrostatic adsorption of negatively charged suspended solids through cationization modification, and then formed a spatial network structure through hydrophobic association, achieving efficient bridging flocculation and enhancing the strength and settling performance of the flocs. In contrast, Comparative Example 2 used unmodified polyacrylamide, which could not form a stable network structure, resulting in small and loose flocs. Comparative Example 3 only used single cationization modification and lacked a hydrophobic association structure, so its flocculation effect and settling velocity were inferior to those of the Examples.

[0121] The effluent compliance data further validated the advantages of the technical solution presented in this application. The effluent from Examples 1-7 achieved dual compliance for both reinjection and discharge, while Comparative Examples 1 and 2 failed to meet the standards, and Comparative Examples 3 and 4 only met the requirements for reinjection and could not meet discharge standards. This difference stems from the synergistic effect of the core components in the examples: the dual-modified polyacrylamide and the imidazoline quaternary ammonium salt graft copolymer achieved simultaneous "demulsification + flocculation," while auxiliary components such as scale inhibitors and adsorbents further enhanced the purification effect, ensuring that the treated wastewater indicators met the standards. In contrast, the comparative examples, lacking salt-resistant modification or with incomplete modification, experienced a decline in the effectiveness of the active components in a high-salt environment, failing to completely remove oil and suspended solids, and thus failing to meet the treatment needs of complex oilfield conditions.

[0122] In summary, the changes in the test data indicate that this application has solved the core technical problems of conventional water purification agents such as activity decay, incomplete demulsification, and poor flocculation effect in high-salt environments by performing dual modification and grafting modification on the core components. Under the synergistic effect of the components, the purification efficiency and adaptability of the water purification agent have been greatly improved.

[0123] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A composite water cleaning agent for oilfield wastewater treatment, characterized in that, The raw materials include the following parts by weight: 18-28 parts of double-modified polyacrylamide; 10-16 parts of imidazoline quaternary ammonium salt graft copolymer; 6-12 parts scale inhibitor; 5-10 parts of adsorbent; 1-3 parts of dodecyl dimethyl betaine; 3-7 parts of coagulant aid; 38-58 parts deionized water; The dual-modified polyacrylamide is obtained by dual modification of polyacrylamide with 2,3-epoxypropyltrimethylammonium chloride and hydrophobic monomer; The imidazoline quaternary ammonium salt graft copolymer is a graft copolymer of imidazoline quaternary ammonium salt with acrylamide and 2-acrylamide-2-methylpropanesulfonic acid.

2. The composite water-cleaning agent for oilfield wastewater treatment according to claim 1, characterized in that, The dual-modified polyacrylamide was prepared using the following method: 2,3-epoxypropyltrimethylammonium chloride was added to polyacrylamide, and a cationization reaction was carried out at 55-75℃ and pH 7.0-8.0 for 3-4 hours. Then, a hydrophobic monomer and an initiator were added, and the reaction was carried out at 60-70℃ for 2-4 hours. After the reaction was completed, the polyacrylamide was obtained by precipitation, washing, vacuum drying and pulverization.

3. The composite water-cleaning agent for oilfield wastewater treatment according to claim 2, characterized in that: The amount of 2,3-epoxypropyltrimethylammonium chloride added is 15%-25% of the mass of polyacrylamide.

4. A composite water-cleaning agent for oilfield wastewater treatment according to claim 2, characterized in that: The hydrophobic monomer is hexadecyl dimethyl allyl ammonium chloride, and the amount of the hydrophobic monomer added is 5%-10% of the mass of polyacrylamide.

5. A composite water-cleaning agent for oilfield wastewater treatment according to claim 2, characterized in that: The initiator is a mixture of ammonium persulfate and sodium bisulfite in a mass ratio of 1:(0.5-1); the amount of the initiator added is 0.3%-0.8% of the total mass of the reaction system.

6. A composite water-cleaning agent for oilfield wastewater treatment according to claim 1, characterized in that, The imidazoline quaternary ammonium salt graft copolymer was prepared by the following method: First, mix imidazoline quaternary ammonium salt with acrylamide, add deionized water to dissolve, adjust the pH to 6.5-7.5 to obtain a mixed reaction solution, heat the mixed reaction solution to 70-80℃, add an initiator, react for 1-2 hours, then add 2-acrylamido-2-methylpropanesulfonic acid, continue the reaction for 2-3 hours, cool to room temperature, and obtain the imidazoline quaternary ammonium salt graft copolymer by vacuum distillation.

7. A composite water-cleaning agent for oilfield wastewater treatment according to claim 6, characterized in that: The mass ratio of the imidazoline quaternary ammonium salt, acrylamide, and deionized water is 1:(1.2-1.8):(8-15).

8. A composite water-cleaning agent for oilfield wastewater treatment according to claim 6, characterized in that: The initiator is azobisisobutyramidine hydrochloride, and the amount of the initiator added is 0.5%-1.0% of the mass of the mixed reaction solution; the amount of 2-acrylamide-2-methylpropanesulfonic acid added is 70%-90% of the mass of imidazoline quaternary ammonium salt.

9. A method for preparing a composite water-cleaning agent for oilfield wastewater treatment according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Mix deionized water and double-modified polyacrylamide, heat to 45-55℃, and stir until completely dissolved to obtain the base solution; S2. Add imidazoline quaternary ammonium salt graft copolymer to the base liquid, heat to 60-70℃, keep warm and stir until uniformly mixed, then add scale inhibitor, adsorbent, dodecyl dimethyl betaine and coagulant in sequence, continue to keep warm and stir for 30-60 minutes, cool to room temperature to obtain composite water cleaning agent.

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