Viscoelastic surfactant oil displacement agent for binary combination flooding and preparation method thereof

Abstract

The application discloses a surfactant oil displacement agent for viscoelastic binary composite flooding and a preparation method thereof, and relates to the technical field of oil exploitation. The oil displacement agent is a polymer and surfactant binary composite system, which is compounded by a water phase and an oil phase in a specific proportion. The water phase accounts for 5-45% and the oil phase accounts for 55-95% to expand the range. The water phase contains alcohol and water, and the oil phase contains a surfactant, a self-made thickening agent and white oil. Through reasonable compounding of components and optimization of the preparation process, the application solves the technical problems of poor temperature resistance and salt resistance, weak environmental protection and high cost of the existing viscoelastic surfactant, has excellent viscoelasticity, oil washing efficiency and environmental compatibility, can significantly reduce the oil / water interfacial tension, promote the desorption and dispersion of crude oil, improve the recovery rate, and is suitable for high-temperature and high-salt oil reservoir exploitation.

Description

Technical Field

[0001] This invention relates to the field of petroleum extraction technology, specifically to a viscoelastic binary composite flooding surfactant and its preparation method. Background Technology

[0002] In oil extraction, viscoelastic surfactants, which form unique worm-like micelles, impart excellent viscoelasticity to aqueous solutions, making them key additives for enhancing oil recovery. Currently, most viscoelastic surfactants on the market are cationic (mainly including long-chain alkyl quaternary ammonium salts and long-chain alkyl halogenated pyridines) and gemini. Cationic surfactants readily form worm-like micelles in water, while gemini surfactants exhibit high low-temperature solubility and low biotoxicity, demonstrating higher temperature resistance and viscoelasticity compared to traditional quaternary ammonium salts.

[0003] However, existing viscoelastic surfactants still have many technical drawbacks in practical applications: First, adsorption retention and formation damage are prominent issues. Formation rock surfaces are negatively charged, and high-concentration cationic viscoelastic surfactants are easily adsorbed and retained in the formation, causing wetting reversal, increasing capillary resistance in oil and gas flow, and affecting oil recovery. Second, their temperature resistance is limited. Most viscoelastic surfactant fracturing fluids can only withstand temperatures around 70℃. At high temperatures, filtration is severe and viscosity decreases significantly, failing to meet the needs of high-temperature reservoir development. Third, their environmental friendliness is insufficient. The rupture fluid from the backflow contains a large amount of non-degradable cationic surfactants, easily causing environmental pollution. Fourth, their cost is high. Viscoelastic surfactants require large quantities and have strong filtration properties, making them more expensive than conventional guar gum fracturing fluids, limiting their large-scale application. Furthermore, conventional viscoelastic surfactants also have limited salt and hard water resistance. Their micelle structure relies on ionic groups for stability; high concentrations of salt can disrupt charge stability, leading to micelle disintegration and a sharp drop in solution viscosity, further affecting oil displacement.

[0004] In view of the above-mentioned defects in the existing technology, the purpose of this invention is to provide a viscoelastic binary composite flooding surfactant oil displacement agent and a method for preparing the oil displacement agent, which solves the technical problems of poor temperature and salt resistance, weak environmental protection, high cost and easy formation damage of existing viscoelastic surfactants. Through the synergistic effect of polymer and surfactant, the viscoelasticity, oil washing efficiency and environmental compatibility of the oil displacement agent are improved, so as to meet the exploitation needs under different reservoir conditions. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a viscoelastic binary composite surfactant for oil displacement and its preparation method, solving the problems mentioned in the background section. To achieve the above objectives, this invention is implemented through the following technical solution: A viscoelastic binary composite surfactant for oil displacement, wherein the oil displacement agent is a binary composite system of polymer and surfactant, and the components are in the following mass percentages: aqueous phase 5%-45%, oil phase 55%-95%, and the sum of the mass percentages of the components is 100%. The aqueous phase comprises highly polar organic small molecules and water, wherein the highly polar organic small molecules account for 5%-15% of the total mass of the binary composite system, and water accounts for 30%-40% of the total mass of the binary composite system; The oil phase comprises surfactants, thickeners, and white oil, wherein the surfactants account for 15%-60% of the total mass of the binary composite system, the thickeners account for 20%-40% of the total mass of the binary composite system, and the white oil accounts for 5%-35% of the total mass of the binary composite system. The surfactant is one or more of the following: sodium dodecyl sulfonate, α-olefin sulfonate, secondary alkyl sulfonate, sodium dodecylbenzene sulfonate, petroleum sulfonate, naphthalene sulfonate formaldehyde condensate, succinate sulfonate, lignin sulfonate, alkylphenol polyoxyethylene ether sulfonate, Span 80 (sorbitan monooleate), Span 60, Tween 85, Tween 80, fatty alcohol polyoxyethylene ether, oleamide propyl betaine, diethanolamine oleate, lauramidopropyl betaine, and diethanolamine lauryl acid; it can significantly reduce the oil / water interfacial tension. The highly polar organic small molecules are one or more of the following: ethylene glycol, 1,2-propanediol, n-octanol, isopropanol, methanol, 1,2-butanediol, 1,3-butanediol, xylitol, sorbitol, maltitol, diethylene glycol, triethylene glycol, threitol, formic acid, lactic acid, methylamine, ethylamine, dimethylamine, glycine, and alanine. The thickener is a self-made polymer that enhances the viscoelasticity of the system; the white oil serves as an oil phase carrier and optimizes the dispersibility of the system.

[0006] The preparation method of the above-mentioned viscoelastic binary composite flooding surfactant oil displacement agent includes the following steps: Step 1: Prepare the thickener; Step 2: Preparation of aqueous complex system: Mix alcohol and water in a certain proportion and stir evenly to obtain aqueous complex system; Step 3: Preparation of oil phase complex system: Mix the thickener, white oil and surfactant prepared in step 1 in proportion and stir evenly to obtain the oil phase complex system; Step 4: Preparation of oil displacement agent: Mix the aqueous phase compound system and the oil phase compound system, stir evenly, and obtain a viscoelastic binary composite surfactant oil displacement agent.

[0007] The specific steps for preparing the thickener are as follows: S1 Preparation of aqueous phase: Dissolve polymer monomers and catalyst in water, adjust pH to neutral with ammonia, and control system temperature to be less than 26℃; S2 Preparation of oil phase: Mix white oil, surfactant and initiator in proportion and stir at 300 r / min for 30 min; S3 Polymerization reaction: Pour the oil phase prepared in step S2 into the aqueous phase prepared in step S1, and perform high-speed shearing until the emulsion viscosity reaches a certain value. Then, introduce nitrogen gas and initiate the polymerization reaction at a certain temperature. During the reaction, control the temperature to gradually increase to avoid explosive polymerization and obtain an oil-soluble thickener. S4. Add a certain mass of phase inversion agent to the oil-soluble thickener in S3 and stir at a constant speed for 30 minutes to obtain the final water-soluble reverse emulsion polymerization thickener.

[0008] Furthermore, in the above-mentioned method for preparing the water-soluble reverse emulsion polymerization thickener, the polymer monomer is at least one of acrylamide, acrylic acid, styrene, N-hydroxymethylacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, sodium methacrylate sulfonate, sodium styrene sulfonate, sodium acrylate sulfonate, methacryloyloxyethyltrimethylammonium chloride, dimethyl diallyl ammonium chloride, octadecylmethylacrylamide, and dodecyl dimethyl allyl ammonium chloride, and the polymer monomer accounts for 3.0%-35% of the total raw materials; Furthermore, in the above-mentioned method for preparing the water-soluble reverse emulsion polymerization thickener, the mass of acrylamide accounts for 12.0%-22.0% of the total mass of the raw materials; Furthermore, in the above-mentioned method for preparing the water-soluble reverse emulsion polymerization thickener, the mass of acrylic acid accounts for 12.0%-22.0% of the total mass of the raw materials; Furthermore, in the above-mentioned method for preparing the water-soluble reverse emulsion polymerization thickener, the catalyst is at least one selected from 1,4-butanediol, 1,6-hexanediol, bis-hydroxyethyl (or hydroxypropyl) bisphenol A, ethylene glycol, glycerol, propylene glycol, 1,4-bis(hydroxyethyl) hydroquinone, hydrogenated bisphenol A, 1,4-cyclohexanediol, 1,4-bis(hydroxyethyl)piperazine, pentaerythritol, trimethylolpropane, tetramethylethylenediamine, diethylamine, diethylene glycol, dimethylaminopropionitrile, triethylene glycol, neopentyl glycol, hydrogenated bisphenol A, 1,4-cyclohexanediol, sorbitol, diethylaminoanhydrous ethanol, N,N'-methylenebisacrylamide, and sodium formate, and its mass accounts for 0.005%-1% of the total mass of the raw materials; Furthermore, in the above-mentioned method for preparing the water-soluble reverse emulsion polymerization thickener, the initiator is at least one selected from sodium bisulfite, sodium thiosulfate, dicyclohexyl peroxide, ammonium persulfate, azobisisobutyronitrile, azobisisobutyramidoline hydrochloride, azobisisobutyramidine hydrochloride, potassium persulfate, azodicyanovalerate, azobisisopropylimidazoline, and sodium persulfate, and its mass accounts for 0.001-0.1% of the total mass of the raw materials; Furthermore, in the above-mentioned method for preparing the water-soluble reverse emulsion polymerization thickener, the surfactant is at least one of the following: dodecylaminopropionic acid, octadecylamidopropylamine oxide, dodecyl dimethyl sulfopropyl betaine, dodecyl ethoxy sulfobenzene, tetradecylamidopropyl hydroxypropyl sulfobenzene, octadecyl dihydroxyethylamine oxide, sodium dodecyl sulfate, dodecyl dimethyl hydroxypropyl sulfobenzene, hexadecyl dimethyl hydroxypropyl phosphate betaine, dodecyl alcohol polyoxyethylene ether, 2-acrylamido-2-dimethylpropanesulfonic acid, decyl dimethyl hydroxypropyl sulfobenzene, sodium dodecyl sulfonate, Span 80 (sorbitan monooleate), Span 60, Tween 85, Tween 80, fatty alcohol polyoxyethylene ether, hexadecyl trimethylammonium bromide, oleamidopropyl betaine, diethanolamine oleate, laurylamidopropyl betaine, and diethanolamine laurate, wherein the surfactant accounts for 0.5-3.0% of the total mass of the raw materials. Furthermore, in the above-mentioned method for preparing the water-soluble reverse emulsion polymerization thickener, the phase inversion agent is at least one of methanol, ethanol, isopropanol, n-propanol, ethylene glycol, propylene glycol, ethylene glycol monoethyl ether, acetic acid, propionic acid, Tween 20, Tween 40, Tween 80, PEG-400, PEG-600, glycerol, cocamidopropylamine oxide, coconut oil monoethanolamide, octylphenol polyoxyethylene ether, and castor oil polyoxyethylene ether, and its mass accounts for 2.0%-5% of the total mass of the raw materials; Furthermore, in step S3, the viscosity of the emulsion after high-speed shearing must be stabilized within the range of 800 mPa·s-1000 mPa·s before nitrogen gas can be introduced to initiate polymerization, in order to ensure the stability of the polymerization reaction and the performance of the thickener.

[0009] The advantages of this application are: 1. Excellent temperature and salt resistance, suitable for the needs of high-temperature and high-salinity oil reservoir extraction. In oilfield development, high temperature and high salinity are the core challenges affecting the stability of displacement agents. Existing single surfactant displacement systems, due to their molecular structure characteristics, are prone to increased molecular chain thermal motion and decreased structural stability when exposed to temperatures above 70°C. Simultaneously, in high salinity environments, salt ions interact with surfactant molecules, disrupting their original micelle structure and causing a sharp decrease in system viscosity. This makes them unable to meet the stability requirements during displacement, thus limiting their application in high-temperature, high-salinity reservoirs. This invention constructs a binary composite system of a thickener and a surfactant, leveraging their synergistic effect to fundamentally overcome the shortcomings of single surfactants in terms of temperature and salt resistance. The thickener, with its unique molecular structure, can form stable interactions with surfactant molecules, providing strong support for the micelle structure and significantly improving its stability in complex environments. Even in high-temperature and high-salt environments exceeding 70°C, this composite system maintains good viscoelasticity, effectively preventing sudden viscosity drops and ensuring sufficient momentum for the oil displacement agent to propagate uniformly and stably within the formation. This makes it suitable for high-temperature and high-salt oil reservoir extraction operations, breaking through the limitations of existing technologies on reservoir environments.

[0010] 2. It is highly environmentally friendly and meets the environmental protection requirements of green mining. With increasing environmental awareness, the environmental impact of oil displacement agents during oilfield extraction is receiving more and more attention. Some existing oil displacement agents use chemical components that are difficult to degrade in the natural environment, and the resulting backflow fluid contains a large number of harmful substances. This not only pollutes the soil, water bodies, and other natural environments, but also requires significant investment in subsequent treatment, which is inconsistent with the current trend of green mining development.

[0011] This invention fully considers environmental friendliness in its component selection. The thickener used is a polyacrylamide polymer, which has good biodegradability and can be gradually decomposed through hydrolysis by microorganisms in the natural environment. The acrylic acid and ammonia nitrogen produced during the decomposition process are both harmless substances and will not cause secondary pollution to the environment. Furthermore, this invention is compounded with an environmentally friendly surfactant. Through the synergistic effect of the two, not only is the oil displacement performance ensured, but the overall environmental harm of the oil displacement agent is further reduced. The pollutant content of the backflow fluid generated during use is significantly reduced, and environmentally friendly discharge or reinjection standards can be met without complex deep treatment, fully complying with the current environmental protection requirements for oilfield development and providing a feasible technical solution for green oilfield development. 3. Minimal formation damage, effectively ensuring oil and gas recovery rate. Formations are crucial carriers of oil and gas storage and seepage, and the degree of damage caused by oil displacement agents directly impacts oil and gas recovery rates. Some existing oil displacement agents utilize cationic surfactant systems. Since formation rock surfaces typically carry a negative charge, cationic surfactant molecules readily undergo electrostatic adsorption, leading to significant surfactant retention on the formation rock surface. This adsorption and retention not only wastes the agent but also triggers a formation wettability reversal, transforming the rock surface from a water-wetness favorable for crude oil desorption to an oil-wetness unfavorable for extraction. Simultaneously, it increases capillary resistance during oil and gas seepage, clogs formation pore channels, and causes irreversible damage to the formation, ultimately resulting in a decrease in oil and gas recovery rates. This invention innovatively employs a non-cationic surfactant compound system, fundamentally avoiding the electrostatic adsorption problem between cationic surfactants and formation rock surfaces, significantly reducing the amount of surfactant adsorbed and retained on the formation rock surface. This design not only reduces reagent consumption but also effectively inhibits wetting reversal, maintaining the original water-wetting characteristics of the formation rock. Simultaneously, it reduces capillary resistance during oil and gas seepage, ensuring unobstructed formation pore channels and achieving effective formation protection. By protecting the original seepage capacity of the formation, it provides a guarantee for efficient oil and gas extraction, helping to maintain a long-term stable recovery rate.

[0012] 4. High oil washing efficiency, significantly improving oil production efficiency. In the middle and late stages of oilfield development, a large amount of crude oil remains in the formation pores. Due to the constraint of the oil-water interface tension, this residual oil is tightly adsorbed on the rock surface or retained in tiny cracks. Conventional oil displacement methods are unable to effectively displace it, resulting in low oil production efficiency and low utilization rate of oil and gas resources. The surfactant component in this invention possesses excellent interfacial activity, enabling it to rapidly act on the oil-water interface, significantly reducing the oil-water interfacial tension and weakening the adhesion between crude oil and the rock surface, thus creating favorable conditions for crude oil desorption from the rock surface. Simultaneously, the surfactant promotes the formation of a stable dispersion system after desorption, preventing the crude oil from re-aggregating and adsorbing. More importantly, this invention constructs an oil displacement system with good viscoelasticity through the combined action of a thickener and a surfactant. This system generates sufficient driving force during flow, not only propelling the dispersed crude oil towards the production well but also penetrating deep into tiny formation fissures, effectively displacing residual oil that was previously trapped in these fissures through a "dragging" effect. Through this series of actions, the utilization rate of residual formation oil is significantly improved, thereby significantly enhancing overall oil production efficiency.

[0013] 5. Costs are controllable, and it has the potential for large-scale promotion and application. The cost of oil displacement agents is a significant factor limiting their large-scale industrial application. To ensure effectiveness, some existing high-performance oil displacement agents often require large quantities of expensive surfactants, resulting in high overall costs. Simultaneously, some systems exhibit poor filtration properties, leading to significant agent loss through formation pores during injection, further increasing material losses during extraction and raising actual extraction costs, thus limiting their application in large-scale oilfield development. This invention, through precise optimization of the proportions of each component, significantly reduces the amount of expensive surfactants used while ensuring the expected oil displacement performance, directly lowering the raw material cost of the oil displacement agent. Furthermore, the unique structure of the composite system improves the system's filtration performance, reducing ineffective losses of the agent during injection, further lowering material consumption costs during extraction. Through multi-dimensional cost control, the oil displacement agent of this invention achieves excellent performance while maintaining a reasonable cost advantage. Simultaneously, this system requires no large-scale modification to existing oilfield injection equipment during use, is easy to operate, and has strong adaptability, laying a solid foundation for its large-scale application in oilfields of different sizes. Detailed Implementation

[0014] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0015] Example 1: I. Oil displacement agent components Aqueous phase: 20%, of which highly polar organic small molecule ethylene glycol accounts for 6% of the total mass of the binary composite system and water accounts for 14% of the total mass of the binary composite system; Oil phase: 80%, of which the surfactant sodium dodecylbenzenesulfonate and petroleum sulfonate are compounded in a 1:1 ratio, accounting for 35% of the total mass of the binary composite system, the self-made thickener accounts for 37% of the total mass of the binary composite system, and the white oil accounts for 8% of the total mass of the binary composite system. II. Preparation Method Step 1: Prepare homemade thickener S1 Preparation of the aqueous phase: Dissolve 18% acrylamide, 15% acrylic acid, 0.05% tetramethylethylenediamine catalyst, and 0.03% N,N'-methylenebisacrylamide in water, adjust the pH to 7.0 with ammonia, and control the system temperature at 24℃; S2 Preparation of the oil phase: Mix 60% white oil, 1.2% Tween 80 and 0.8% Span 80, 0.04% ammonium persulfate initiator and 0.02% sodium bisulfite in proportion, and stir at 300 r / min for 30 min to ensure uniform mixing; S3 Polymerization Reaction: The oil phase prepared in S2 is slowly poured into the aqueous phase prepared in S1, and sheared using a high-speed shear machine at a speed of 3000 r / min until the emulsion viscosity stabilizes at 900 mPa·s. High-purity nitrogen is introduced into the system for 30 min to remove oxygen, and the polymerization reaction is initiated at 18.5℃. During the reaction, the temperature is controlled by a water bath, and the temperature is gradually increased to 45℃ at a rate of 5℃ per hour to avoid local burst polymerization. After 6 hours of reaction, the oil-soluble thickener is obtained. S4 Phase Inversion Treatment: Add 5% op-10 phase inversion agent to the oil-soluble thickener obtained in S3, and stir at a constant speed of 200 r / min for 30 min to obtain the final water-soluble reverse emulsion polymerization thickener for later use. Step 2: Preparation of aqueous complex system Add 6% ethylene glycol and 14% water to a stirred tank in a specific ratio, and stir at 250 r / min for 20 min to ensure complete mixing without stratification, thus obtaining an aqueous complex system. Step 3: Preparation of oil-phase complex system Add 37% of the self-made thickener prepared in step one, 8% of white oil, 35% of sodium dodecylbenzenesulfonate and petroleum sulfonate compound product to a mixing tank in proportion, stir at 300 r / min for 40 min, and control the temperature at 30℃ during the process to form a uniform and stable oil phase system, thus obtaining the oil phase compound system.

[0016] Step 4: Preparation of oil displacement agent The 20% aqueous phase compound system was slowly added to the 80% oil phase compound system while stirring at a speed of 280 r / min for 30 min. The temperature of the mixed system was controlled at 25℃, and a homogeneous, precipitation-free viscoelastic binary composite surfactant for oil displacement was finally obtained.

[0017] Example 2: I. Oil displacement agent components Aqueous phase: 15%, of which highly polar organic small molecule 1,2-propanediol accounts for 5% of the total mass of the binary composite system and water accounts for 10% of the total mass of the binary composite system; Oil phase: 85%, of which surfactants oleamide propyl betaine 20%, sodium dodecyl sulfonate 10%, and diethanolamine oleate 10% account for the total mass of the binary composite system, self-made thickener accounts for 40% of the total mass of the binary composite system, and white oil accounts for 5% of the total mass of the binary composite system. II. Preparation Method Step 1: Prepare homemade thickener S1 Preparation of the aqueous phase: Dissolve 15% of the polymer monomer acrylamide, 12% of 2-acrylamide-2-methylpropanesulfonic acid, 0.08% of the catalyst ethylene glycol, and 0.02% of sodium formate in water, adjust the pH value to 6.8 with ammonia water, and control the system temperature at 25℃; S2 Preparation of the oil phase: Mix 65% white oil, surfactant (Tween 80 and Span 60 in a 2:1 ratio, accounting for 1.5% of the total mass of the thickener raw materials), 0.03% azobisisobutyronitrile initiator, and 0.02% potassium persulfate, all accounting for the total mass of the thickener raw materials, and stir at 300 r / min for 30 min. S3 Polymerization reaction: The oil phase is poured into the aqueous phase and sheared at a high speed of 3200 r / min until the emulsion viscosity stabilizes at 800 mPa.s. Nitrogen gas is introduced for 40 min to remove oxygen, and polymerization is initiated at 18℃. During the reaction, the temperature is increased to 42℃ at a rate of 4℃ per hour. After 7 h of reaction, the oil-soluble thickener is obtained. S4 Phase Inversion Treatment: Add Tween 20, a phase inversion agent, to the oil-soluble thickener, accounting for 6% of the total mass of the thickener. Stir at 180 r / min for 30 min to obtain a water-soluble reverse emulsion polymerization thickener. Step 2: Preparation of aqueous complex system Add 5% 1,2-propanediol and 10% water to a mixing tank and stir at 220 rpm for 25 minutes to ensure uniform mixing and obtain an aqueous complex system. Step 3: Preparation of oil-phase complex system 40% of the self-made thickener, 5% of white oil, 20% of oleamide propyl betaine, 10% of sodium dodecyl sulfonate, and 10% of diethanolamine oleate were added to a mixing vessel in proportion, stirred at 300 r / min for 45 min, and the temperature was controlled at 28℃ to obtain a homogeneous oil phase compound system. Step 4: Preparation of oil displacement agent Add 15% aqueous phase compound system to 85% oil phase compound system, stir at 260 r / min for 35 min, and control the temperature at 24℃ to obtain viscoelastic binary composite surfactant oil displacement agent.

[0018] Example 3: I. Oil displacement agent components Aqueous phase: 25%, of which highly polar organic small molecule n-octanol accounts for 10% of the total mass of the binary composite system and water accounts for 15% of the total mass of the binary composite system; Oil phase: 75%, of which surfactant Tween 80 10% and diethanolamine laurate 10% account for 20% of the total mass of the binary composite system, the self-made thickener accounts for 30% of the total mass of the binary composite system, and white oil accounts for 25% of the total mass of the binary composite system. II. Preparation Method Step 1: Prepare homemade thickener S1 Preparation of the aqueous phase: Mix 15% acrylamide, 5% sodium styrene sulfonate, 15% acrylic acid, 0.06% trimethylolpropane catalyst, and 0.04% diethylamine with ammonia to adjust the pH to 7.2 (neutral), and control the system temperature at 23℃. S2 Preparation of the oil phase: Mix 70% white oil, surfactant (Tween 85 and Span 60 in a 3:1 ratio, accounting for 2.5% of the total mass of the thickener raw materials), 0.05% azobisisobutylamidine hydrochloride initiator, and 0.01% sodium persulfate, all accounting for the total mass of the thickener raw materials, and stir at 300 r / min for 30 min. S3 Polymerization reaction: The oil phase is poured into the aqueous phase and subjected to high-speed shearing at 2800 r / min until the emulsion viscosity stabilizes at 1000 mPa.s. Nitrogen gas is purged for 35 min to remove oxygen, and polymerization is initiated at 19°C. The temperature is increased to 48°C at a rate of 6°C per hour, and the reaction is carried out for 5 h to obtain the oil-soluble thickener. S4 Phase inversion treatment: Add 4% castor oil polyoxyethylene ether as a phase inversion agent, stir at 220 r / min for 30 min to obtain a water-soluble reverse emulsion polymerization thickener. Step 2: Preparation of aqueous complex system Add 10% n-octanol and 15% water to a mixing tank and stir at 240 rpm for 22 min. After mixing evenly, an aqueous complex system is obtained. Step 3: Preparation of oil-phase complex system Add 30% of the self-made thickener, 25% of the white oil, 10% of Tween 80, and 10% of diethanolamine laurate to a mixing vessel in a certain proportion, stir at 300 r / min for 50 min, and control the temperature at 32℃ to obtain an oil phase complex system.

[0019] Step 4: Preparation of oil displacement agent Add 25% of the aqueous phase compound system to 75% of the oil phase compound system, stir at 270 r / min for 32 min, and control the temperature at 26℃ to obtain a viscoelastic binary composite surfactant for oil displacement.

[0020] Comparative Example 1: The method for preparing the oil displacement agent is the same as in Example 1. The only change is that no additional surfactant is added to the oil phase in the compounding step; only thickener and white oil are added.

[0021] Comparative Example 2: The method for preparing the oil displacement agent is the same as in Example 1. The only change is that the self-made thickener is not added to the oil phase in the compounding step; only surfactants and white oil are added.

[0022] Comparative Example 3: The method for preparing the oil displacement agent is the same as in Example 1. The only change is that the high-speed shearing step is removed in the preparation of the thickener. After mixing and stirring, the polymer reaction is carried out directly, and the finished product is used as a self-made thickener for compounding.

[0023] 1. Product appearance and stability Take 50 mL of the final product sample into a 100 mL colorimetric tube, keep it at 25 °C for 30 min, and visually observe the product condition.

[0024] Table 1: Table 1 shows the appearance results of the viscoelastic binary composite oil displacement agent. 2. Temperature and salt resistance A simulated formation water with a salinity of 50,000 mg / L (mainly NaCl, containing Ca²⁺ and Mg²⁺) was prepared. A 1 wt% viscoelastic binary composite oil displacement agent aqueous solution was prepared using the simulated formation water. The state and viscosity were observed after being placed in an oven at 80℃ for 24 hours. The test results are shown in Table 2. The dispersion state was uniform dispersion and stratification. Uniform dispersion indicates that the oil displacement agent can withstand this temperature and salinity. Stratification or a significant decrease in viscosity indicates that the viscoelastic binary composite oil displacement agent with surfactant cannot be used at high temperatures and high salinity, and its performance is at risk.

[0025] Table 2: 3. Oil washing efficiency Simulated formation sand and target block crude oil (crude oil from Changqing Oilfield Plant 9) were mixed at a mass ratio of 4:1 and placed in an oven for constant aging at reservoir temperature for 7 days, stirring once a day to ensure uniform mixing. The mixture was sealed during aging to prevent crude oil volatilization. Approximately 5g (accurate to 0.001g) of the aged sand was weighed into an Erlenmeyer flask to obtain m1. Add 50.0 g of a 0.4 wt% viscoelastic binary composite oil displacement agent mixture (prepared with water injected into the target block) to a conical flask, mix thoroughly, and let stand at reservoir temperature for 48 h. Use clean cotton yarn to remove the crude oil floating in the sample solution and the crude oil adhering to the bottle wall after standing, and pour out the sample solution. Then place the conical flask in a 100℃ oven to dry to constant weight to obtain m2. Use petroleum ether with a boiling range of 90℃-120℃ to elute the sample dried to constant weight until the petroleum ether is colorless. Place the conical flask from which crude oil has been eluted in a 120℃ oven to dry to constant weight and weigh to obtain m3. Calculate the oil-washing capacity using the following formula: In the formula: σ — Washing power, % m1 — Total mass of the conical flask and formation sand before oil washing, in grams m2 — Mass of the conical flask and formation sand after oil washing, in grams m3 — Mass of the conical flask and the formation sand after washing, in grams Table 3: 4. Interfacial tension The test should be conducted using a TX-500C or an instrument capable of achieving equivalent results. The instrument must be calibrated before testing. The test temperature is the bottom temperature (60℃), the rotation speed is 5000 r / min, the light phase is on-site crude oil (from Changqing Oilfield Plant 9), and the high-density phase is a 0.4 wt% viscoelastic binary composite oil displacement agent aqueous solution (prepared with injection water from the target block). Fill the measuring tube with the 0.4 wt% viscoelastic binary composite oil displacement agent aqueous solution using a syringe, then inject approximately 0.5 μL of on-site crude oil into the middle of the measuring tube using a micro-syringe. A small amount of water from the oil displacement agent aqueous solution will be expelled. Refill the measuring tube with the oil displacement agent aqueous solution until the liquid level is above the tube opening, and plug the tube (there should be no air bubbles inside the tube). Clean the outer wall of the measuring tube with lens paper and insert the measuring tube into the rotating shaft of the instrument, tightening the pressure cap (holding the shaft end prevents rotation). After starting the measurement, adjust the instrument balance to ensure the crude oil remains in the middle of the measuring tube, and measure the interfacial tension values ​​at 1 min and 5 min.

[0026] Table 4: Experimental conclusion: Surfactants are key to reducing oil / water interfacial tension and improving oil washing efficiency: Comparative Example 1 (without surfactant) only reduced the interfacial tension to the order of 10⁻¹mN / m, with an oil washing efficiency of less than 60%, while the Example achieved an interfacial tension of 10⁻¹mN / m due to the action of surfactants. 4 With a concentration of mN / m and an oil washing efficiency exceeding 80%, the interfacial modification and oil washing effects of surfactants were verified.

[0027] The self-made thickener is the key to ensuring the stability and temperature and salt resistance of the system: Comparative Example 2 (without self-made thickener) had a viscosity retention rate of only 44.4% at 90°C and high salt environment and showed stratification, while the Example had a viscosity retention rate of over 94% and uniform dispersion due to the synergistic effect of the thickener and surfactant, proving that the thickener can enhance the stability of the micelle structure.

[0028] High-speed shearing is crucial to the performance of thickeners: In Comparative Example 3 (without high-speed shearing), the thickener polymerization was uneven, resulting in agglomeration and precipitation in the system, which significantly reduced the temperature and salt resistance and oil washing efficiency. However, after high-speed shearing, the performance of the thickener in the Example was optimized, and the overall performance of the system was excellent.

[0029] The binary composite system (surfactant + self-made thickener) and key processes in Examples 1-3 effectively solve the performance defects of single components or simplified processes, and have excellent stability, temperature and salt resistance, oil washing efficiency and interfacial activity, which meet the needs of high temperature and high salt reservoir exploitation.

[0030] In summary, the product of this invention has been verified through performance testing and comparative experiments: in terms of appearance stability, no stratification or precipitation occurred after 30 minutes at a constant temperature of 25℃; the core indicators such as temperature and salt resistance, oil washing efficiency, and interfacial activity far exceed those of the control system without surfactants, thickeners, or high-speed shearing processes. It can effectively solve problems such as uneven water drive and reduced recovery rate in ultra-low permeability reservoirs, providing an oil displacement technology that is efficient, environmentally friendly, and economical for the oil extraction field, with broad application prospects.

[0031] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A viscoelastic binary composite surfactant for oil displacement, characterized in that, The oil displacement agent is a binary composite system of polymer and surfactant. The components, by mass percentage, are: aqueous phase 5%-45%, oil phase 55%-95%, and the sum of the mass percentages of all components is 100%. The aqueous phase comprises highly polar organic small molecules and water, wherein the highly polar organic small molecules account for 5%-15% of the total mass of the binary composite system, and water accounts for 30%-40% of the total mass of the binary composite system; The oil phase comprises surfactants, thickeners, and white oil, wherein the surfactants account for 15%-60% of the total mass of the binary composite system, the thickeners account for 20%-40% of the total mass of the binary composite system, and the white oil accounts for 5%-35% of the total mass of the binary composite system. The highly polar organic small molecules are one or more of the following: ethylene glycol, 1,2-propanediol, n-octanol, isopropanol, methanol, 1,2-butanediol, 1,3-butanediol, xylitol, sorbitol, maltitol, diethylene glycol, triethylene glycol, threitol, formic acid, lactic acid, methylamine, ethylamine, dimethylamine, glycine, and alanine. The surfactant is two or more of the following: sodium dodecyl sulfonate, α-olefin sulfonate, secondary alkyl sulfonate, sodium dodecylbenzene sulfonate, petroleum sulfonate, naphthalene sulfonate formaldehyde condensate, succinate sulfonate, lignin sulfonate, alkylphenol polyoxyethylene ether sulfonate, Span 80 (sorbitan monooleate), Span 60, Tween 85, Tween 80, fatty alcohol polyoxyethylene ether, hexadecyltrimethylammonium bromide, oleamidopropyl betaine, diethanolamine oleate, lauroamidopropyl betaine, and diethanolamine lauryl acid.

2. The viscoelastic binary composite surfactant for oil displacement according to claim 1, characterized in that: The thickener is a self-made thickener.

3. The preparation method of the viscoelastic binary composite flooding surfactant oil displacement agent according to claim 2, characterized in that: Includes the following steps: Step 1: Prepare the thickener; Step 2: Preparation of aqueous complex system: Mix alcohol and water in a certain proportion and stir evenly to obtain aqueous complex system; Step 3: Preparation of oil phase complex system: Mix the thickener, No. 10 white oil and surfactant prepared in step 1 in proportion and stir evenly to obtain the oil phase complex system; Step 4: Preparation of oil displacement agent: Mix the aqueous phase compound system and the oil phase compound system, stir evenly, and obtain a viscoelastic binary composite surfactant oil displacement agent.

4. The method for preparing the viscoelastic binary composite flooding surfactant oil displacement agent according to claim 3, characterized in that: The specific steps in Step One are as follows: S1, Preparation of the aqueous phase: Dissolve the polymer monomers and catalyst in water, adjust the pH to neutral with ammonia, and control the system temperature to below 26℃. S2, Preparation of the oil phase: Mix white oil, surfactant, and initiator in a certain proportion, and stir at 300 r / min for 30 min. S3, Polymerization reaction: The oil phase prepared in step S2 is poured into the aqueous phase prepared in step S1, and subjected to high-speed shearing until the emulsion viscosity reaches a certain value. Nitrogen gas is then introduced to initiate the polymerization reaction at a certain temperature. During the reaction, the temperature is gradually increased to avoid explosive polymerization, resulting in an oil-soluble thickener. S4. Add a certain mass of phase inversion agent to the oil-soluble thickener in S3 and stir at a constant speed for 30 minutes to obtain the final water-soluble reverse emulsion polymerization thickener.

5. The preparation method of the viscoelastic binary composite flooding surfactant oil displacement agent according to claim 4, characterized in that: The polymer monomer is at least one of acrylamide, acrylic acid, styrene, N-hydroxymethylacrylamide, N-isopropylacrylamide, N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, sodium methpropylene sulfonate, sodium styrene sulfonate, sodium propylene sulfonate, methacryloyloxyethyltrimethylammonium chloride, dimethyldiallylammonium chloride, octadecylmethylacrylamide, and dodecyldimethylallylammonium chloride, and the polymer monomer accounts for 3.0%-35% of the total raw material.

6. The method for preparing the viscoelastic binary composite flooding surfactant oil displacement agent according to claim 4, characterized in that: The catalyst is at least one of the following: 1,4-butanediol, 1,6-hexanediol, bis-hydroxyethyl (or hydroxypropyl) bisphenol A, ethylene glycol, glycerol, propylene glycol, 1,4-bis(hydroxyethyl)hydroquinone, hydrogenated bisphenol A, 1,4-cyclohexanediol, 1,4-bis(hydroxyethyl)piperazine, pentaerythritol, trimethylolpropane, tetramethylethylenediamine, diethylamine, diethylene glycol, dimethylaminopropionitrile, triethylene glycol, neopentyl glycol, hydrogenated bisphenol A, 1,4-cyclohexanediol, sorbitol, diethylaminoanhydrous ethanol, N,N'-methylenebisacrylamide, and sodium formate, and its mass accounts for 0.005%-1% of the total mass of the raw materials.

7. The method for preparing the viscoelastic binary composite flooding surfactant oil displacement agent according to claim 4, characterized in that: The initiator is at least one selected from sodium bisulfite, sodium thiosulfate, dicyclohexyl peroxide, ammonium persulfate, azobisisobutyronitrile, azobisisobutyramidoline hydrochloride, azobisisobutyramidine hydrochloride, potassium persulfate, azobiscyanopentanoic acid, azobisisopropylimidazoline, and sodium persulfate, and its mass accounts for 0.001-0.1% of the total mass of the raw materials.

8. The method for preparing the viscoelastic binary composite flooding surfactant oil displacement agent according to claim 4, characterized in that: The surfactant is at least one of the following: dodecylaminopropionic acid, octadecylamidopropylamine oxide, dodecyl dimethyl sulfopropyl betaine, hexadecyl trimethylammonium chloride, dodecyl ethoxy sulfobetaine, tetradecylamidopropyl hydroxypropyl sulfobetaine, octadecyl dihydroxyethylamine oxide, sodium dodecyl sulfate, dodecyl dimethyl hydroxypropyl sulfobetaine, hexadecyl dimethyl hydroxypropyl phosphate betaine, dodecyl alcohol polyoxyethylene ether, 2-acrylamido-2-dimethylpropanesulfonic acid, decyl dimethyl hydroxypropyl sulfobetaine, sodium dodecyl sulfonate, Span 80 (sorbitan monooleate), Span 60, Tween 85, Tween 80, fatty alcohol polyoxyethylene ether, hexadecyl trimethylammonium bromide, oleamidopropyl betaine, diethanolamine oleate, lauramide propyl betaine, and diethanolamine laurate, and its mass accounts for 0.5-3.0% of the total mass of the raw materials.

9. The method for preparing the viscoelastic binary composite flooding surfactant oil displacement agent according to claim 4, characterized in that: The phase inversion agent is at least one of methanol, ethanol, isopropanol, n-propanol, ethylene glycol, propylene glycol, ethylene glycol monoethyl ether, acetic acid, propionic acid, Tween 20, Tween 40, Tween 80, PEG-400, PEG-600, glycerol, cocamidopropylamine oxide, coconut oil monoethanolamide, octylphenol polyoxyethylene ether, and castor oil polyoxyethylene ether, and its mass accounts for 2.0%-5% of the total mass of the raw materials.

10. The method for preparing the viscoelastic binary composite flooding surfactant oil displacement agent according to claim 4, characterized in that: In step S3, the viscosity of the emulsion after high-speed shearing must be stabilized within the range of 800 mPa·s-1000 mPa·s before nitrogen gas can be introduced to initiate polymerization.

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