Polyion liquid, preparation method thereof and application of polyion liquid in thickened oil recovery

Abstract

The invention provides a polyion liquid, a preparation method thereof and application of the polyion liquid in thickened oil recovery. The polyion liquid is formed by polymerizing polyvinyl imidazole cations and bis (trifluoromethanesulfonyl) imide anions through ionic bonds. The chemical structural general formula of the polyion liquid is shown in the specification. According to the polyion liquid disclosed by the invention, the viscosity reduction efficiency is remarkably improved through molecular structure regulation and combination with triple mechanisms of hydrogen bond competition, electrostatic shielding and steric hindrance, the physical adsorption limitation of a traditional surfactant is broken through, and the triple mechanisms of viscosity reduction-emulsification-wettability reversal are coordinated through molecular-level chemical action; meanwhile, the polyion liquid has high-temperature and high-salt stability, can adapt to a complex oil reservoir environment, integrates catalytic cracking and interfacial activity functions, improves the oil displacement efficiency, and promotes the upgrading of heavy oil recovery from low efficiency and high energy to precise environment protection.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of oilfield exploitation, and particularly relates to a polyionic liquid, a preparation method thereof and application of the polyionic liquid in heavy oil exploitation. BACKGROUND

[0002] Heavy oil contains a large amount of heavy oil and asphaltene, and the polar macromolecular structure of these components increases the viscosity of the heavy oil, which is difficult to flow naturally at the formation temperature, resulting in an increase in the difficulty of heavy oil exploitation. In order to improve the flowability of heavy oil, the traditional heat production technology is used, of which the most common method is steam stimulation. However, the above method has the disadvantages of high gas channeling rate and rapid increase in water content, resulting in a low recovery rate.

[0003] Ionic liquid is a kind of salt composed of anions and cations, and has a melting point lower than 100℃. It has unique physical and chemical properties such as low melting point, negligible vapor pressure and non-flammability, and is often used in engineering, synthesis, biomass conversion and other fields. In the development of heavy oil, ionic liquid can interact with the polar components (such as asphaltene, resin, etc.) in heavy oil, change its physical and chemical properties, and achieve the effect of low-temperature viscosity reduction. Ionic liquid is a green reagent for reducing the viscosity of heavy oil. It uses the synergistic effect of hydrogen bond competition, electrostatic shielding and steric hindrance to improve the recovery rate of heavy oil and reduce the cost, and breaks through the limitations of traditional heat production technology. It also has environmental friendliness.

[0004] At present, the methods for modifying and reducing the viscosity of ionic liquid heavy oil include ionic liquid microemulsion viscosity reducer, ionic liquid-surfactant oil displacement system and microbial-ionic liquid compound viscosity reducer. Chinese patent CN107286976B discloses an ionic liquid for demulsification of heavy crude oil. The ionic liquid is N-alkyl trimethyl ammonium bromide or N-alkyl trimethyl ammonium chloride. The use of the above ionic liquid can effectively improve the demulsification efficiency of heavy crude oil emulsion. The method of using microwave heating combined with ionic liquid demulsification can obtain high demulsification efficiency in a short time.

[0005] Although ionic liquid has multiple mechanisms for changing the viscosity of heavy oil and can improve the recovery rate of heavy oil to a certain extent, there are still the following problems:

[0006] (1) High cost: The synthesis cost of traditional ionic liquid is as high as $50 / kg, which is 3 times that of conventional surfactants, and the economic efficiency is poor, which restricts the commercial application of ionic liquid.

[0007] (2) Poor environmental adaptability: It is invalid under high temperature (>80℃) or high salt (salinity >20000mg / L) conditions, and the practical application is limited, especially in deep high-temperature reservoirs.

[0008] (3) Limited scope of action: The loss rate of the wellbore section is >60%, which makes it difficult to penetrate into the reservoir and achieve viscosity reduction in deep formations, affecting the overall use effect.

[0009] (5) Single function: Currently, viscosity reduction mainly relies on physical dispersion, lacks catalytic cracking function, cannot fundamentally change the chemical structure of heavy oil, and has limited viscosity reduction effect.

[0010] Therefore, there is an urgent need to develop a polyionic liquid with strong environmental adaptability, a wide range of effects, and a combination of multiple functions, and to apply it to heavy oil extraction to improve the extraction rate. Summary of the Invention

[0011] The purpose of the present invention is to provide a polyionic liquid and its preparation method and application in heavy oil production, so as to solve the problems of the above-mentioned ionic liquids used in heavy oil production, such as high cost, poor environmental adaptability, limited scope of action, and single function.

[0012] In order to achieve the above object, the present invention provides the following technical solutions:

[0013] The first aspect of the present invention provides a polyionic liquid, which is formed by ionic bonding of polyvinyl imidazolium cations and bistrifluoromethanesulfonimide anions.

[0014] The general chemical structure of polyionic liquids is:

[0015] .

[0016] A second aspect of the present invention provides a method for preparing a polyionic liquid, comprising the following steps:

[0017] S1: Quaternization reaction

[0018] 1-vinylimidazole is reacted with an alkylating agent to obtain 1-vinyl-3-methylimidazolium iodide;

[0019] S2: ion exchange reaction

[0020] 1-vinyl-3-methylimidazolium iodide is reacted with lithium bis(trifluoromethanesulfonyl)imide to obtain an intermediate monomer;

[0021] S3: Free Radical Polymerization

[0022] An initiator is added to the intermediate monomer to react and obtain a polyionic liquid.

[0023] Preferably, in step S1, the alkylating agent is methyl iodide, the molar ratio of 1-vinylimidazole to methyl iodide is 1:1.2-1.5, the reaction solvent is acetonitrile, and the amount of the reaction solvent added is 5-10 times the total mass of 1-vinylimidazole and methyl iodide.

[0024] Preferably, in step S1, the reaction temperature is 60-80° C. and the reaction time is 12-24 h.

[0025] Preferably, the specific reaction process of step S1 is:

[0026] 1-vinylimidazole and acetonitrile were added to a three-necked flask and deoxygenated by nitrogen for 10 minutes. Methyl iodide was then slowly added dropwise at a controlled rate to avoid violent exotherm. The temperature was then raised to 60-80°C and stirred until the solution changed from colorless to light yellow. The reaction was monitored by thin-layer chromatography (TLC, developing solvent: chloroform: methanol = 10:1 v / v) until the 1-vinylimidazole spot disappeared. Finally, post-treatment was performed, wherein the acetonitrile was removed by vacuum distillation to obtain a light yellow solid. The solid was washed with ethyl acetate 2-3 times (50 mL each time), filtered, and dried under vacuum (40°C, 12 hours) to obtain 1-vinyl-3-methylimidazolium iodide [VIm + ][I - ].

[0027] Preferably, in step S2, the molar ratio of 1-vinyl-3-methylimidazolium iodide to lithium bis(trifluoromethanesulfonyl)imide is 1:1-1.2, and the reaction solvent is water or acetone.

[0028] Preferably, in step S2, the reaction temperature is 20-25° C. and the reaction time is 24-48 hours.

[0029] Preferably, the specific reaction process of step S2 is:

[0030] Dissolve 1-vinyl-3-methylimidazolium iodide in deionized water (or acetone), add lithium bis(trifluoromethanesulfonyl)imide (LiNTf2) solid; stir at room temperature until the solution is clear (initially light yellow, colorless after the reaction); use nuclear magnetic resonance hydrogen spectrum ( 1 H NMR) to detect residual iodide ions: Take a small amount of the reaction solution and add AgNO3 solution. If there is no yellow precipitate (AgI), it indicates that the exchange is complete. Finally, the purification step is carried out: the reaction solution is decolorized by activated carbon (1-2g), filtered; the filtrate is passed through a cation exchange resin (Dowex50W-X8, H + Type) column to remove Li + The effluent was collected and concentrated under reduced pressure to 1 / 3 of the original volume, ether (200-300 mL) was added to precipitate, filtered and vacuum dried (50 ° C, 8 hours), and a white intermediate monomer [VIm + ][NTf2 - ], purity > 99%.

[0031] Preferably, in step S3, the initiator is azobisisobutyronitrile, the amount of initiator added is 0.5-2% of the mass of the intermediate monomer, the reaction solvent is N,N-dimethylformamide or dimethyl sulfoxide, and the amount of reaction solvent added is 50-100% of the mass of the intermediate monomer.

[0032] Preferably, in step S3, the reaction temperature is 55-65°C, and the time is 8-16h.

[0033] Preferably, the specific reaction process of step S3 is as follows:

[0034] The intermediate monomer and DMF are added to a polymerization bottle and ultrasonically dissolved; the initiator AIBN is added, nitrogen is introduced to remove oxygen for 15 minutes, and the polymerization bottle is sealed; then the system is stirred in an oil bath at 60°C, and the viscosity gradually increases until it becomes a light yellow gel; then methanol (50mL) is added to terminate the reaction, and stirring is performed for 10 minutes. Finally, the product is treated: the polymer is dissolved in tetrahydrofuran (THF) (50-100mL) and precipitated by n-hexane (200-300mL); the precipitate is collected by filtration, washed twice with n-hexane, and vacuum dried (60°C, 24 hours) to obtain a light yellow polyionic liquid solid, with a weight average molecular weight (M w )10 4 ~10 5 g / mol and a yield of 75%-80%.

[0035] Each synthesis step of the polyionic liquid in the present application has its corresponding role:

[0036] Step S1 is mainly a quaternization reaction of 1-vinylimidazole (an imidazole ring containing a double bond) and an alkylating agent (methyl iodide), which imparts a cationic charge to the ionic liquid and retains a polymerization site (vinyl group), and the chemical reaction equation is as follows:

[0037]

[0038] Step S2 is mainly an ion exchange reaction of 1-vinyl-3-methylimidazolium iodide [VIm + ][I - ] with lithium bistrifluoromethanesulfonimide, which introduces a target anion into the ionic liquid and optimizes the performance (such as stability and conductivity), and the chemical reaction equation is as follows:

[0039]

[0040] Step S3 is mainly a free radical polymerization reaction of [VIm ＋ ][NTf2 - ] under the action of an initiator, which forms a high molecular weight polymer and expands the application (such as electrolyte and catalyst carrier), and the chemical reaction equation is as follows:

[0041]

[0042] The third aspect of the present application provides a polyionic liquid and an application of the polyionic liquid in heavy oil exploitation.

[0043] Preferably, the polyionic liquid is applied to the specific process of heavy oil exploitation, which comprises the following steps:

[0044] (1) Reservoir adaptability evaluation and preliminary preparation

[0045] The core parameters in the reservoir evaluation process include reservoir temperature, crude oil composition, formation water salinity and rock wettability, and according to the evaluation results, the compatibility of the polyionic liquid with the target reservoir is confirmed;

[0046] (2) Preparation of polyionic liquid and debugging of injection equipment

[0047] The granular product of the polyionic liquid is added to water for mixing, and after complete dissolution, a uniform and transparent working solution is obtained, and the concentration of the working solution is configured according to the reservoir requirements;

[0048] The injection equipment is provided with a high-pressure pump, a corrosion-resistant pipeline and a pressure and flow detection instrument;

[0049] (3) Wellbore operation and in-situ implementation

[0050] The wellbore is pretreated, and then a segmented injection process is adopted to inject high-concentration ionic liquid, working solution and dispersion medium into the wellbore, and after the segmented injection is completed, the well is soaked;

[0051] (4) Production of crude oil

[0052] After the soaking is completed, the conventional oil displacement process is used to promote the flow of the viscosity-reduced crude oil to the bottom of the well, and then the heavy oil is lifted.

[0053] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0054] 1. The polyionic liquid prepared by the present application works synergistically through three molecular-level chemical action mechanisms of hydrogen bond competition and coordination, electrostatic shielding and charge neutralization, and steric hindrance effect, which can destroy the aggregation structure of asphaltene from multiple angles and maintain its dispersed state, thereby significantly reducing the viscosity of heavy oil and improving the flowability. Compared with traditional physical adsorption type surfactants, this chemical action dominated mechanism is more efficient and durable.

[0055] 2. The polyionic liquid prepared by the present application has good thermal stability and salt resistance, and can maintain stable performance under complex reservoir conditions such as high temperature and high salinity.

[0056] 3. The polyionic liquid prepared by the present invention not only has excellent viscosity reduction ability, but also integrates multiple functions such as catalytic cracking, interfacial activity regulation, and wettability inversion, realizing a new integrated heavy oil recovery model of "efficient displacement + green recovery" and improving the recovery rate of heavy oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 Schematic diagram of the aggregation state of asphaltene molecules;

[0058] Figure 2 Schematic diagram of the breaking state of hydrogen bonds between asphaltene molecules under the action of PILs molecules in an embodiment of the present invention;

[0059] Figure 3 This is a schematic diagram of the state of new hydrogen bond formation under the action of PILs molecules according to an embodiment of the present invention;

[0060] Figure 4 Schematic diagram of the double-layer structure of asphaltene particles without adding PILs molecules in an embodiment of the present invention;

[0061] Figure 5 Schematic diagram of the double-layer structure of asphaltene particles with the addition of PILs molecules according to an embodiment of the present invention;

[0062] Figure 6 This is a schematic diagram of the steric hindrance effect of adding PILs molecules according to an embodiment of the present invention;

[0063] Figure 7 FTIR spectrum of polyionic liquid. DETAILED DESCRIPTION

[0064] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0065] Example 1

[0066] This embodiment provides a method for preparing a polyionic liquid, comprising the following steps:

[0067] S1: Quaternization reaction

[0068] 1-vinylimidazole is reacted with an alkylating agent to obtain 1-vinyl-3-methylimidazolium iodide; the specific process is as follows:

[0069] Into a three-necked flask, 100 mmol of 1-vinylimidazole and 120 mL of acetonitrile were added, and the flask was purged with nitrogen for 10 minutes to remove oxygen; then 135 mmol of methyl iodide was slowly added dropwise, and the dropping speed was controlled to avoid violent heat release; then the temperature was raised to 70°C, and the reaction was stirred until the solution changed from colorless to light yellow, and the reaction time was 20 hours; the reaction was monitored by thin layer chromatography (TLC, developing agent: chloroform:methanol = 10:1 v / v) until the 1-vinylimidazole spot disappeared.

[0070] Finally, post-treatment was performed: acetonitrile was removed by distillation under reduced pressure to obtain a light yellow solid; the solid was washed with ethyl acetate for 3 times (50 mL each time), and then filtered and dried under vacuum (40°C, 12 hours) to obtain 1-vinyl-3-methylimidazolium iodide [VIm + ][I - ].

[0071] S2: Ion exchange reaction

[0072] 1-vinyl-3-methylimidazolium iodide was reacted with lithium bistrifluoromethanesulfonimide to obtain an intermediate monomer; the specific process was as follows:

[0073] 100 mmol of 1-vinyl-3-methylimidazolium iodide was dissolved in 120 mL of deionized water (or 100 mL of acetone), and then 110 mmol of lithium bistrifluoromethanesulfonimide (LiNTf2) solid was added; then the solution was stirred at room temperature until it was clear (initially light yellow, and colorless after the reaction was completed); the residual iodine ions were detected by nuclear magnetic resonance hydrogen spectrum (H NMR): a small amount of reaction solution was taken, and AgNO3 solution was added, and if there was no yellow precipitate (AgI), it indicated that the exchange was complete. 1

[0074] Finally, a purification step was performed: the reaction solution was decolorized by activated carbon (2 g) and filtered; the filtrate was passed through a cation exchange resin (Dowex 50W-X8, H + type) column to remove Li + , and the effluent was collected; acetonitrile was removed by concentration under reduced pressure to 1 / 3 of the original volume, ether (250 mL) was added for precipitation, and then filtered and dried under vacuum (50°C, 8 hours) to obtain a white intermediate monomer [VIm + ][NTf2 - ], with a purity of >99%.

[0075] S3: Free radical polymerization reaction

[0076] An initiator was added to the intermediate monomer to obtain a polyionic liquid. The specific reaction process was as follows:

[0077] ​Into a polymerization bottle, 50 g of intermediate monomer and 40 mL of DMF were added and dissolved by ultrasonic; 0.45 g of AIBN was added, and the polymerization bottle was sealed after oxygen was removed by nitrogen for 15 minutes; then the system was stirred in an oil bath at 60°C, and the viscosity gradually increased until it became a light yellow gel; the reaction was terminated by adding methanol (50 mL) and stirring for 10 minutes.

[0078] Finally, the product was treated: the polymer was dissolved in tetrahydrofuran (THF) (80 mL) and precipitated by n-hexane (250 mL); the precipitate was collected by filtration, washed twice with n-hexane, and dried in vacuum (60°C, 24 hours) to obtain a light yellow polyionic liquid solid with a weight average molecular weight (M w )10 4 ~10 5 g / mol and a yield of 78%.

[0079] The synthesized polyionic liquid is a polyvinylimidazole cation and a bistrifluoromethanesulfonimide anion polymerized by ionic bonds; the chemical structure of the polyionic liquid is as follows:

[0080]

[0081] The successful synthesis of the polyionic liquid was proved by infrared spectroscopy (FT-IR) (the test mode was ATR mode to collect signals):

[0082] Test objects: monomer [VIm + ][I - ] (S1 product), intermediate monomer [VIm + ][NTf2 - ] (S2 product), and polyionic liquid (S3 product);

[0083] As can be seen from Figure 7 : the monomer [VIm + ][I - ] has 1620 cm -1 (C=C) and 1630 cm -1 (imidazole ring), corresponding to the structure of the quaternary ammonium product; the intermediate monomer [VIm + ][NTf2 - ] has additional 1350 / 1250 cm -1 (NTf2 - ), proving the success of ion exchange; the 1620 cm -1 in the polyionic liquid disappears, and there is a wide peak of 2850~2950 cm -1 , proving the polymerization of the vinyl group and the formation of a polymer by the polymerization of the double bond.

[0084] The mechanism of the polyionic liquid (PILs molecule) prepared in this example and the thick oil is as follows:

[0085] (1) Hydrogen bond competition and coordination

[0086] Asphaltene molecules in heavy oil form stable aggregates through hydrogen bonds between -OH / -COOH groups (as shown in Figure 1 ). The cations in PILs molecules (such as imidazolium, guanidinium) form directional hydrogen bonds or coordination bonds with the polar groups of asphaltenes through strong electropositive centers (as shown in Figure 2 ), competitively replacing the weak hydrogen bonds between asphaltene molecules, and destroying their self-assembly network. PILs molecules form new hydrogen bonds between the hydrogen atoms in -OH / -COOH and the electronegative atoms (N / O) in PILs (as shown in Figure 3 ), which disperse asphaltene aggregates into small particles, reduce intermolecular internal friction, and significantly reduce the viscosity of heavy oil.

[0087] (2) Electrostatic shielding and charge neutralization

[0088] The surface of asphaltene particles is usually negatively charged, which gives them a certain repulsive force between them, but also allows them to interact with the surrounding substances, affecting the flowability of heavy oil. The cations on the PILs molecules can approach and adsorb on the surface of the asphaltene particles, neutralizing part of the negative charge. In the reservoir environment, the presence of heteroatoms (such as sulfur, nitrogen, oxygen, etc.) on the surface of asphaltene particles will cause ionization, resulting in a rich negative charge on the surface of the particles. These negative charges cause electrostatic repulsion between asphaltene particles, and at the same time attract cations in the surrounding medium, forming a double-layer structure. According to the double-layer theory, the double layer is composed of a compact layer and a diffuse layer. The cations in the compact layer are tightly bound to the surface of the asphaltene particles, while the cations in the diffuse layer are relatively loosely distributed around them. The presence of the double layer has an important influence on the aggregation state of asphaltene particles and the flowability of heavy oil, as it increases the effective distance between particles, hindering the free movement of particles, and thus increasing the viscosity of heavy oil.

[0089] When PILs molecules are added to the heavy oil system, their cations will migrate to and adsorb on the negatively charged surface of asphaltene particles. On the one hand, the cations in the PILs molecules neutralize part of the negative charge on the surface of the asphaltene particles, reducing the electrostatic repulsion between the particles. According to Coulomb's law, the force between charges is proportional to the product of the charge and inversely proportional to the square of the distance. The cations in the PILs molecules reduce the charge by neutralization, thereby weakening the electrostatic repulsion between particles. On the other hand, the adsorption of cations in PILs molecules will compress the thickness of the double layer. In a high-salinity reservoir environment, salt ions will compress the double layer, but the addition of PILs molecules can further optimize this effect. The reduction of double-layer thickness makes it easier for asphaltene particles to approach and disperse each other, reducing the intermolecular interaction resistance, and thus reducing the viscosity of heavy oil.

[0090] Without PILs (as Figure 4 ): presents two asphaltene particles with negative charge, double electric layer structure around, and obvious repulsion between particles.

[0091] With PILs (as Figure 5 ): shows that part of the negative charge is neutralized by PILs cations, the double electric layer thickness is reduced, the repulsion between particles is weakened, and the heavy oil fluidity is increased.

[0092] (3) Steric hindrance effect

[0093] When PILs molecules are adsorbed onto the surface of asphaltene particles, the molecular chains of PILs molecules will form a steric barrier around the particles, preventing asphaltene particles from aggregating together again, so that they remain in a dispersed state, thereby reducing the viscosity of heavy oil and enhancing the fluidity of heavy oil. When PILs molecules are adsorbed onto the surface of asphaltene particles, the side chains and substituents on the molecular chains of PILs molecules will form a steric hindrance layer around the particles. This steric hindrance layer acts as a physical barrier, preventing the approach and aggregation of other asphaltene particles. According to the theory of colloidal stability, the steric hindrance effect can increase the repulsive energy barrier between particles, making it difficult for particles to overcome this energy barrier and coalesce under the action of Brownian motion. Therefore, the steric hindrance effect of PILs molecules effectively maintains the dispersed state of asphaltene particles, reduces the intermolecular cohesion in heavy oil, and thus reduces the viscosity of heavy oil. The steric barrier formed by the molecular chains of PILs is represented by the purple curve (as shown in Figure 6 ).

[0094] Test Example 1

[0095] The polyionic liquid prepared in Example 1 was characterized and tested. The test results are as follows:

[0096] ① High ionic conductivity and interfacial activity

[0097] The polyionic liquid can reduce the oil-water interfacial tension (such as to 10 -2 mN / m level); promote heavy oil emulsification (oil droplet size <10 m), significantly reduce the viscosity of crude oil (reduction range up to 50%-80%); adsorb on the surface of reservoir pores, plug microcracks (reduce water channeling, improve sweep efficiency), while enhance the fluidity of crude oil (destroy the asphaltene network through ion-molecule interaction).

[0098] ② Thermal stability and salt tolerance

[0099] Polyionic liquid can resist high temperature of 80-150℃ (reservoir environment), anion structure does not decompose, keeps stable performance; does not precipitate to high salinity formation water (such as >20000mg / L NaCl), suitable for complex reservoir.

[0100] ③Synergistic effect of solubilization and viscosity reduction

[0101] Polyionic liquid can solubilize heavy components (asphaltene, resin) in heavy oil, reduce the cohesion of crude oil; adjust the charge distribution of formation through ion exchange, optimize the percolation channel, and improve the recovery rate (compared with conventional methods, the recovery rate is increased by 10%-20%).

[0102] Example 2

[0103] The specific process of polyionic liquid applied in heavy oil exploitation is as follows:

[0104] (1) Reservoir adaptability evaluation and preliminary preparation

[0105] Before applying polyvinylimidazole imidazolium bis-trifluoromethanesulfonimide salt (polyionic liquid), a systematic evaluation should be carried out for the target reservoir. This polyionic liquid is preferably suitable for heavy oil reservoirs with high viscosity and high asphaltene content, such as reservoirs with viscosity exceeding 5000 mPa·s, asphaltene content exceeding 10%, and reservoirs with poor conventional chemical flooding process effect under high temperature and high salinity formation. Its salt resistance characteristics can effectively adapt to complex water quality environment.

[0106] During the evaluation process, the core parameters include reservoir temperature (affecting the thermal stability of polyionic liquid), crude oil components (asphaltene and resin content determine the need for viscosity reduction), formation water salinity (determine whether to pre-treat to avoid ion reaction), and rock wettability.

[0107] Confirm the compatibility of polyionic liquid reagent and target reservoir to ensure the effectiveness of subsequent process. Polyionic liquid is preferably suitable for heavy oil reservoirs with high viscosity and high asphaltene content, including: conventional heavy oil reservoirs (viscosity >5000 mPa·s, asphaltene content >10%); complex reservoirs (high temperature, high salinity, oil-wet rock surface, and poor conventional chemical flooding effect). It can adapt to reservoir temperature of 60-150℃, formation water with salinity ≤200000mg / L, is suitable for high viscous oil reservoirs with asphaltene content ≥10% (especially >15%), and can reverse the oil-wet rock wettability to weak hydrophilic state with contact angle of 50°-70°.

[0108] (2) Ground preparation and injection equipment debugging

[0109] The polyionic liquid reagent is an industrialized synthesized granular product. It needs to be sealed in a moisture-proof container and stored in a cool and dry place to avoid moisture absorption and affect the performance. The polyionic liquid has high stability and salt resistance, and can maintain chemical activity in high salinity formations for a long time. It is especially suitable for medium-high temperature (60-150°C) reservoir environment. At the ground liquid preparation station, the polyionic liquid reagent is mixed with water or softened water according to the characteristics of the reservoir, and is fully stirred until completely dissolved to form a uniform transparent working solution. The imidazole ring and hydrophobic side chain in the molecular structure can quickly disperse in water to form a stable surface active system. The concentration is adjusted according to the principle of "reasonable concentration to meet the needs of the reservoir", for example, in high asphaltene content reservoirs, the concentration of ionic liquid needs to be increased to enhance the dispersion effect. The concentration of polyionic liquid working solution is positively correlated with the asphaltene content, and the specific corresponding relationship is: when the asphaltene content is ≤10%, the recommended working solution concentration is 1.5%-2.5%, which is suitable for medium-low asphaltene reservoirs, and a small amount of molecular action can disperse local aggregates; when the asphaltene content is 10%-15%, the concentration is adjusted to 2.5%-3.5% to enhance the effect of electrostatic shielding and steric hindrance to destroy the asphaltene network structure; if the asphaltene content is ≥15%, the concentration needs to be increased to 3.5%-4.0% to achieve the three mechanisms of "viscosity reduction-emulsification-wettability reversal" synergistically. In test example 2, the 3% concentration in the 15% asphaltene reservoir can reduce the oil-water interfacial tension to below 0.5 mN / m, and the recovery rate is increased to 92.1%. The corresponding relationship is determined based on the mechanism that the higher the asphaltene content, the denser the aggregate network, and higher concentration of polyionic liquid is needed to enhance the molecular level effect through hydrogen bond competition, electrostatic shielding and steric hindrance. At the same time, it needs to be combined with the parameters such as reservoir salinity and temperature to optimize through experiments.

[0110] In terms of injection equipment, high-pressure devices such as plunger pumps or screw pumps are equipped. Such pump types can withstand the high-pressure environment of thousands of meters of well depth and can handle the high viscosity characteristics of polyionic liquids. Corrosion-resistant stainless steel tubing is connected, which can resist chemical corrosion of ionic liquids and adapt to high temperature and high pressure conditions of the reservoir. Pressure and flow monitoring instruments are matched to real-time control injection parameters, and the pressure is controlled within a safe range below the formation fracture pressure. The low vapor pressure characteristics of ionic liquids can avoid gas locking caused by pressure fluctuations, ensuring the stability and safety of the injection process. In addition, the precise transmission design of the plunger pump can achieve accurate measurement of the polyionic liquid working solution, ensuring that the action range covers the target oil layer.

[0111] During the whole process, the high stability (resistant to temperature above 300°C) and fast dissolving characteristics of the ionic liquid make it compatible with conventional liquid preparation equipment without complex pretreatment in the ground preparation link. The combination of high-pressure injection system and corrosion-resistant pipe materials fully considers the long-term effectiveness of ionic liquids in the complex downhole environment, forming a full-chain technical guarantee from storage, preparation to injection.

[0112] (3) Wellbore operation and chemical action in the formation

[0113] Before formal injection, the wellbore needs to be pretreated to flush away impurities such as scale and sand through cleaning fluid, ensuring unobstructed passage and avoiding blockage of foreign matter affecting the injection efficiency of the reagent. After pretreatment, a segmented injection process is adopted: first, a small amount of high-concentration ionic liquid (3.5%-4.0%) is injected as a pre-pad to quickly act on the near-wellbore heavy oil viscosity with its strong dispersibility, remove local blockage and reduce the near-wellbore area heavy oil viscosity, achieving the purpose of breaking down local asphaltene aggregate blockage; then, the main working fluid (concentration of 2.5%-3.5%) is injected as the main pad to advance to the deep part of the reservoir in a constant pressure manner, expand the viscosity reduction range through hydrogen bond competition and electrostatic shielding mechanism, and form an oil-in-water emulsion to expand the viscosity reduction range; finally, water or weak gel is injected as a spacer pad to form a physical barrier to prevent the subsequent displacement fluid from directly mixing with the reagent, extending the action time of the ionic liquid in the formation. After segmented injection is completed, the well is immediately shut in for soakback, and the soakback duration is adjusted according to the reservoir temperature, as shown in Table 1.

[0114] Table 1 Correspondence between soakback duration and reservoir temperature

[0115]

[0116] During the soakback period, the chemical reaction between ionic liquid and crude oil, rock is accelerated by the formation temperature: on the one hand, ionic liquid molecules insert into asphaltene aggregates through π-π stacking and hydrogen bonding, disperse the network structure, and significantly reduce the viscosity of heavy oil, changing the flowability from "difficult to flow" to "pumpable"; on the other hand, ionic liquid is adsorbed at the oil-water interface, reducing the interfacial tension and promoting the formation of oil-in-water emulsion, reducing the flow resistance, and at the same time, adsorbing on the rock surface, changing the oil-wet wettability to neutral or weakly hydrophilic state, like a "cleaning agent" peeling off the retained oil adsorbed on the rock, ensuring that the effect spreads to the deep part of the reservoir.

[0117] (4) Connection of production process and dynamic production management

[0118] After the reagent injection is completed, the well needs to be closed for several hours to several days for soaking to allow the reagent to fully react with the crude oil and rock, and to ensure that the effect is spread to the deep part of the reservoir. Subsequently, a conventional oil displacement process such as water flooding, polymer flooding or gas flooding is used to push the crude oil with reduced viscosity to the bottom of the well by the displacement fluid pressure (for example, a polymer solution can expand the swept volume and improve the recovery rate). The lifting link is selected according to the viscosity of the crude oil after viscosity reduction: if the viscosity is reduced to below 1000 mPa·s, a conventional pumping unit (rod pump) is used; if it is still medium-high viscosity (higher than 1000 mPa·s), a large-displacement screw pump or electric submersible pump is used, and wellbore heating (such as electric heating rod) is used to assist the lifting of the flowability. The production process is monitored in real time to monitor the changes in liquid production, water cut and crude oil viscosity. If the viscosity is found to rise, a small amount of reagent can be supplemented to maintain the effect, and dynamic parameter optimization can be achieved.

[0119] (5) Summary

[0120] Through the chemical effects of viscosity reduction, emulsification and wetting reversal of ionic liquids, the bottleneck of high viscosity and strong rock adsorption in traditional exploitation is broken. Compared with thermal recovery technology, ionic liquid technology simplifies the implementation logic, does not require complex equipment such as steam boilers, is suitable for remote or low-temperature reservoirs, and greatly reduces the capital cost; at the same time, ionic liquids can play a long-term role in high-salt and high-temperature environments due to their high stability (temperature resistance > 300℃, salt resistance up to 200,000 mg / L), making up for the shortcomings of conventional surfactants that are prone to failure.

[0121] The overall application process of ionic liquids is centered on "precise adaptation to reservoirs - efficient injection reaction - cooperation with conventional oil recovery": the cation (such as polyvinylimidazolium) is adsorbed on the aromatic ring of asphaltene through π-π stacking, and the anion (bistrifluoromethylsulfonylimide salt) breaks up the aggregates by virtue of strong polarity, achieving interface control and dispersion at the molecular level, forming a complete technical chain of "injection viscosity reduction - improved flow - efficient recovery". This development scheme based on ionic liquids provides a low-cost (single-well cost reduced by 40% compared with thermal recovery) and highly adaptable solution for complex heavy oil reservoirs with asphaltene content > 15% and viscosity > 5000 mPa·s, combining technical innovation (such as a 30% increase in wetting reversal efficiency) with engineering feasibility.

[0122] Test Example 2

[0123] In order to prove the specific application results of polyionic liquids in heavy oil exploitation, this test example simulates the exploitation process for experiments, and compares the polyionic liquid prepared in Example 1 with a conventional surfactant solution as a comparative example.

[0124] The simulated exploitation process is as follows:

[0125] (1) Experimental preparation stage

[0126] ①Microfluidic chip fabrication (simulate formation pore structure)

[0127] Material selection: glass or PDMS (polydimethylsiloxane) chip, micron-scale pore network (pore size 10~100 μm, porosity 20%~25%, permeability 50~200 mD) is constructed by photolithography technology to simulate sandstone reservoir pore structure.

[0128] Surface modification:

[0129] Oily model: chip surface coated with octadecyltrichlorosilane (OTS), contact angle > 120°, simulating the oily rock surface of high asphaltene reservoir;

[0130] Neutral / hydrophilic model: retain glass surface hydroxyl (contact angle < 60°), or adjust wettability by plasma treatment, simulate conventional reservoir.

[0131] ②Solution preparation

[0132] Standard formation water:

[0133] High salinity: NaCl 200 g / L + CaCl2 20 g / L (simulate 200,000 mg / L salinity formation water);

[0134] Low salinity: NaCl 10 g / L (simulate injection water).

[0135] Simulated heavy oil:

[0136] Base oil: degassed crude oil or paraffin oil + 15% asphaltene (viscosity 5000 mPa·s, 50℃, simulating high asphaltene heavy oil).

[0137] Ionic liquid working solution:

[0138] Concentration 2%~4% ([PVIm][NTf2] particles dissolved in low salinity water, stirred to clarify, pH 6.5~7.5);

[0139] Control group: 0.3% conventional surfactant (SDBS) solution for displacement effect comparison.

[0140] ③Experimental equipment setup

[0141] Displacement system: microsyringe pump (precision ±0.1 μL / min), connected to chip inlet, control flow rate 0.01~0.5 mL / h;

[0142] Visual monitoring: inverted microscope (equipped with high-speed camera), real-time observation of fluid distribution in pores;

[0143] Temperature control: thermostat (temperature control accuracy ±0.5℃), simulate formation temperature (60~150℃).

[0144] (2) Experimental simulation steps

[0145] ① Step 1: Saturate with standard formation water (simulate the original formation water state)

[0146] Operation: Put the chip into the incubator and preheat to the target temperature (e.g. 90°C); inject high salinity formation water at a flow rate of 0.05 mL / h using a micro pump until no bubbles are discharged at the outlet end and the pressure is stable (about 2-4 hours); record the initial water phase saturation (Swir≈30%-40%) and confirm the pore water distribution through microscopic imaging.

[0147] Simulation purpose: Establish the initial state of the original formation water occupying the pores, simulate the pore water saturation during geological deposition.

[0148] ② Step 2: Saturate with simulated heavy oil (simulate the process of crude oil filling the formation)

[0149] Operation: Switch to simulated heavy oil and inject at a flow rate of 0.1 mL / h to displace the formation water until the water content at the outlet end is <5% (oil phase breakthrough); record the final oil phase saturation (Soi≈60%-70%) and observe the adsorption state of crude oil on the surface of the oil-wet chip (e.g. form a continuous oil film or cluster-like residual oil).

[0150] Simulation purpose: Reproduce the process of crude oil migration and displacement of formation water to form an experimental "oil-containing pore model".

[0151] ③ Step 3: Ionic liquid displacement experiment (simulate the process of chemical flooding in the formation)

[0152] Displacement sequence and parameters are shown in Table 2:

[0153] Table 2 Displacement sequence and parameters

[0154]

[0155] In the low salinity water flooding stage, the water phase flows along the pore center, and the residual oil in the oil-wet chip adheres to the pore wall in the form of "island", with a recovery of about 15%~20%, and the pressure curve shows an upward trend due to the large capillary resistance; while entering the ionic liquid flooding stage, the molecular viscosity reduction phenomenon occurs within 5~10 minutes after injection, the asphaltene aggregates are dispersed, the oil viscosity is visibly reduced, the flow pattern changes from "stagnation" to "continuous flow", at the same time, oil-in-water emulsion with droplet size <10 μm is formed in the pore, the flow resistance decreases significantly, the pressure drops by 30%~50%, and the oil film on the chip surface gradually peels off, the residual oil changes from "adsorbed state" to "discrete droplet state" and migrates with water flow, the contact angle decreases from 130° to 50°~70°, and the wettability reversal is realized. The recovery in this stage increases by 25%~35%, and the total recovery is increased by 40%~55% compared with water flooding, which is significantly better than conventional surfactant flooding (15%~20%).

[0156] ④Fourth step: soak well simulation (optional, to enhance the reaction effect)

[0157] Operation: After injecting ionic liquid, shut down the pump and soak the well for 1~2 hours (maintain constant temperature);

[0158] Reopen the pump and continue to displace at a flow rate of 0.1 mL / h, and observe the change in recovery after soaking (usually increased by 5%~8%).

[0159] Simulation purpose: to verify the long-term reaction effect of ionic liquid and asphaltene at formation temperature, and to simulate the field soaking process.

[0160] (3) Data collection and analysis

[0161] ① Real-time parameter recording

[0162] Pressure-time curve:

[0163] Water flooding stage: linear pressure rise, reflecting the dominance of capillary resistance;

[0164] Ionic liquid flooding stage: pressure drops sharply and then stabilizes, indicating a decrease in interfacial tension and improvement in wettability.

[0165] Recovery calculation:

[0166] Total recovery of ionic liquid group: 55%~65%;

[0167] Total recovery of conventional surfactant group: 35%~45%.

[0168] ② Microscopic visualization analysis

[0169] High-speed photography:

[0170] Record the emulsion formation process during ionic liquid displacement, the residual oil starting mode (such as "stretching fracture", "broken migration");

[0171] Compare the differences in the effects of ionic liquids in oleophilic / hydrophilic chips (wettability reversal is more significant in oleophilic chips).

[0172] Contact angle measurement:

[0173] Change of contact angle on chip surface before and after displacement, verify the efficiency of wettability reversal (oleophilic→weak hydrophilic).

[0174] ③ Fluid property test

[0175] Displacement fluid analysis:

[0176] Viscometer to measure the viscosity of displacement fluid (ionic liquid group viscosity reduced by 70%~80% compared with original oil);

[0177] Interfacial tension meter to measure oil-water interfacial tension (from 20 mN / m to below 1 mN / m).

[0178] (4) Experimental conclusion

[0179] The use of polyvinylimidazole bis-trifluoromethanesulfonimide salt ionic liquid to simulate heavy oil production experiments show that the ionic liquid can disperse asphaltene aggregates through π-π stacking and hydrogen bonding to achieve molecular level viscosity reduction (viscosity reduction of 70%~80%), form oil-in-water emulsion (droplet size <10 μm) to reduce interfacial tension to below 1 mN / m, and achieve wettability reversal by stripping the oil film on the rock surface, reducing the contact angle from 130° to 50°~70°. The triple mechanism makes the total recovery rate increase by 40%~55% compared with water flooding, which is significantly better than conventional surfactants (15%~20% increase), and performs better in high asphaltene (>10%), high salinity (>200,000 mg / L), and medium-high temperature (60~150°C) oleophilic reservoirs. Combined with pre-displacement low salinity water flooding, soaking (24~48 hours), and CO2 / polymer synergistic displacement, the effect can be further improved, with the economic and environmental benefits of efficient oil recovery and environmental recovery (recovery rate >80%), providing an innovative technical path for the development of complex heavy oil reservoirs.

[0180] (5) Control experiment (the experimental process takes 5000 ppm NaCl as an example)

[0181] ① Chip preparation: oleophilic modified microfluidic chip (contact angle >120°), saturated standard formation water→saturated simulated heavy oil (viscosity 5000 mPa·s, containing 15% asphaltene).

[0182] ② Low salinity water flooding: inject 5000 ppm NaCl to a water content of >90%, recovery rate about 64.5% (consistent with the control group and the experimental group).

[0183] ③Chemical displacement:

[0184] Control group: 0.2% BS-12, displacement for 40 minutes, total recovery rate 87.28%.

[0185] Experimental group: 3% polyvinylimidazolium bistrifluoromethyl sulfonimide ionic liquid, displacement for 40 minutes, total recovery rate 92.1%.

[0186] Table 3 Experimental results of BS-12 and polyionic liquid

[0187]

[0188] ④Conclusion: advantages of ionic liquid

[0189] By molecular viscosity reduction (viscosity reduction 80%), high efficiency emulsification (interfacial tension 0.5 mN / m), strong wetting reversal (contact angle reduction 60°), the recovery rate is significantly improved; high salt (200,000 mg / L), high temperature (150℃) stable, while the surfactant is easy to precipitate or thermal decomposition; small amount of reagent (2%~4%), recyclable (recovery rate >80%), cost reduction 20%~30%. Polyvinylimidazolium bistrifluoromethyl sulfonimide ionic liquid is better than traditional surfactant in heavy oil exploitation, especially suitable for complex reservoirs.

[0190] In summary, by regulating the molecular structure of PILs, combining the triple mechanism of hydrogen bond competition, electrostatic shielding and steric hindrance, the viscosity reduction efficiency is significantly improved; breaking the limitation of traditional surfactant physical adsorption, realizing the triple mechanism of “viscosity reduction-emulsification-wetting reversal” by molecular level chemical action; core-shell protection enhances high temperature and high salt stability, adapts to complex reservoir environment; integrates catalytic cracking and interfacial activity functions, improves oil displacement efficiency; creates “high efficiency displacement + green recovery” mode, promotes heavy oil exploitation from “low efficiency and high consumption” to “precise and environmental protection”.

[0191] The above is the preferred embodiment of the present application, it should be noted that for those skilled in the art, without departing from the principles of the present application, can make a number of improvements and refinements, these improvements and refinements should also be considered as the protection scope of the present application.

Claims

1. A polyionic liquid, characterized in that, The polyionic liquid is formed by polymerization of polyvinylimidazole cations and bistrifluoromethanesulfonimide anions through ionic bonds; The chemical structure of the polyionic liquid is as follows: 。 2. A method of preparing a polyionic liquid according to claim 1, characterized in that, The method comprises the following steps: S1: quaternary ammonium reaction 1-vinylimidazole is reacted with an alkylating agent to obtain 1-vinyl-3-methyl imidazolium iodide; S2: ion exchange reaction 1-vinyl-3-methyl imidazolium iodide is reacted with lithium bistrifluoromethanesulfonimide to obtain an intermediate monomer; S3: free radical polymerization reaction An initiator is added to the intermediate monomer to obtain the polyionic liquid.

3. The method of claim 2, wherein: In step S1, the alkylating agent is methyl iodide, the molar ratio of 1-vinylimidazole to methyl iodide is 1:1.2-1.5, the reaction solvent is acetonitrile, and the addition amount of the reaction solvent is 5-10 times the total mass of 1-vinylimidazole and methyl iodide.

4. The method of claim 2, wherein: In step S1, the reaction temperature is 60-80°C, and the reaction time is 12-24h.

5. The method of claim 2, wherein: In step S2, the molar ratio of 1-vinyl-3-methyl imidazolium iodide to lithium bistrifluoromethanesulfonimide is 1:1-1.2, and the reaction solvent is water or acetone.

6. The method of claim 2, wherein: In step S2, the reaction temperature is 20-25°C, and the reaction time is 24-48h.

7. The method of claim 2, wherein: In step S3, the initiator is azobisisobutyronitrile, the addition amount of the initiator is 0.5-2% of the mass of the intermediate monomer, the reaction solvent is N,N-dimethylformamide or dimethyl sulfoxide, and the addition amount of the reaction solvent is 50-100% of the mass of the intermediate monomer.

8. The method of claim 2, wherein: In step S3, the reaction temperature is 55-65°C, and the reaction time is 8-16h.

9. The polyionic liquid of claim 1 is used in heavy oil exploitation.

10. Use of the polyionic liquid according to claim 9 in heavy oil recovery, characterized in that, The specific process of the polyionic liquid in heavy oil exploitation is as follows: (1) Reservoir adaptability evaluation and preliminary preparation The core parameters in the reservoir evaluation process include reservoir temperature, crude oil composition, formation water salinity, and rock wettability. According to the evaluation results, the compatibility of the polyionic liquid with the target reservoir is confirmed; (2) Preparation of polyionic liquid and injection equipment debugging The granular product of the polyionic liquid is added to water and mixed, and after complete dissolution, a uniform and transparent working solution is obtained. The concentration of the working solution is configured according to the reservoir requirements; The injection equipment is equipped with a high-pressure pump, corrosion-resistant pipeline, and pressure and flow detection instruments; (3) Wellbore operation and in-situ implementation The wellbore is pretreated, and then the high-concentration ionic liquid, the working solution, and the dispersion medium are injected into the wellbore by using a segmented injection process. After the segmented injection is completed, the well is soaked; (4) Produced crude oil After the soaking is completed, the conventional oil displacement process is used to push the viscosity-reduced crude oil to flow to the bottom of the well, and then the heavy oil is lifted.

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