A salt-tolerant nanoparticle-stabilized foam flooding system and its preparation method

Abstract

The present invention discloses a salt-tolerant nano-particle stabilized foam flooding system, which is composed of the following raw materials in parts by weight: 0.3 - 0.5 parts of an asymmetric hydrophilic group anionic gemini surfactant, 0.05 - 0.2 parts of an inorganic potassium salt, 0.05 - 0.15 parts of a foam stabilizer, and 100 parts of water; meanwhile, the present invention also discloses a preparation method of the flooding system. The flooding system provided by the present invention can be applicable to the reservoir formation water environment with a calcium ion content as high as 20000 mg / L, shows certain viscoelasticity, is beneficial to improving the foam stability; has a low interfacial tension and has a good oil washing effect; has a high foam half-life and a high foam comprehensive index, belongs to a strong foam system, and has excellent foaming performance and foam stabilizing performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of enhancing oil recovery in oilfield development, and particularly relates to a salt-tolerant nanoparticle-stabilized foam flooding system and a preparation method thereof. Background Art

[0002] Foam flooding is a tertiary oil recovery technology that can effectively achieve continuous development. It has the advantages of low implementation cost, simple process, high safety, and combines the advantages of gas flooding and surfactant flooding. During the foam flooding process, gas dissolves in the crude oil, causing the crude oil volume to expand, increasing the elastic energy, and easily forming immiscible flooding. At the same time, a foam displacement fluid of gas-liquid two phases is formed on the reservoir, increasing the flow resistance and swept volume, improving the oil displacement effect. The surfactant has the functions of reducing the oil-water interfacial tension and emulsifying and washing the oil, thereby increasing the recovery rate. So far, foam flooding field tests have been carried out in many oilfields and certain results have been achieved. Foam flooding can increase the recovery rate by more than 5% compared with water flooding.

[0003] At present, the traditional foam systems with good foaming and foam-stabilizing properties are mainly fluorocarbon-based and betaine-based, which have problems such as poor environmental protection, low interfacial activity, poor stability against high calcium ions (calcium ion concentration less than 5000 mg / L), and poor economy. They cannot meet the formation water environment with high calcium ions (calcium ion concentration up to 20000 mg / L), and the effect of reducing the oil-water interfacial tension needs to be further improved, seriously restricting the application and popularization of the foam flooding technology for enhancing oil recovery. The main technical problems of the existing foam systems are as follows:

[0004] (1) Under the conditions of high salinity (especially divalent ions such as high calcium and magnesium), the foam stability is poor, affecting the foam flooding effect. It is necessary to develop new foaming agents and foam stabilizers with excellent performance suitable for high salinity and high calcium ion reservoir environments to solve the problems of foam generation, stability, and seepage under complex reservoir conditions;

[0005] (2) At present, the interfacial tension between the foaming agent and the crude oil is relatively high, resulting in poor oil washing effect. It is necessary to further reduce the oil-water interfacial tension and strengthen the oil washing efficiency to further improve the foam flooding recovery rate. Summary of the Invention

[0006] Aiming at the defects of the existing technology, the present invention provides a salt-tolerant nanoparticle-stabilized foam flooding system and a preparation method thereof. The flooding system has certain viscoelasticity under high salinity, can reduce the interfacial tension with the crude oil, and has excellent foaming performance and foam-stabilizing performance.

[0007] A salt-tolerant nano-particle stabilized foam flooding system is composed of the following raw materials in parts by weight: 0.3 - 0.5 part of an asymmetric hydrophilic group anionic gemini surfactant, 0.05 - 0.2 part of an inorganic potassium salt, 0.05 - 0.15 part of a foam stabilizer, and 100 parts of water; wherein, the molecular structural formula of the asymmetric hydrophilic group anionic gemini surfactant is as follows:

[0008] n = 2 - 4; m = 13 - 17, both are integers.

[0009] Preferably, the asymmetric hydrophilic group anionic gemini surfactant is prepared by the following method:

[0010] (1) Add the linker into a flask. According to the molar ratio of linker, sodium chloroacetate, and 2-chloroethyl sulfonate of 1:(1 - 1.2):(1 - 1.2), dissolve sodium chloroacetate and 2-chloroethyl sulfonate in water and then dropwise add them into the linker. React at room temperature for 24 h, then raise the temperature to 85 - 95 °C and reflux for 13 - 18 h. During the reflux reaction, maintain the pH value of the system at 9 - 10. After the reaction, remove the solvent by reduced pressure distillation, then wash with absolute ethanol, filter by suction, and dry to obtain intermediate A; wherein, the linker is ethylenediamine, 1,3-propanediamine, or 1,4-butanediamine, and the corresponding n is 2, 3, or 4 respectively;

[0011] (2) Dissolve intermediate A in distilled water, adjust the pH to 7 with formic acid, then dropwise add epichlorohydrin according to the molar ratio of intermediate A to epichlorohydrin of 1:(2 - 2.4). React at a constant temperature of 30 °C for 4 - 6 h, and remove the solvent by rotary evaporation to obtain a light brownish-yellow gel-like product, denoted as intermediate B;

[0012] (3) Dissolve the intermediate B in distilled water and raise the temperature to 85 - 95 °C. According to the molar ratio of intermediate B to amine of 1:2, add amine to it, maintain the pH of the reaction system at 9 - 10 and react at a constant temperature for 12 - 16 h. Then add absolute ethanol to it, cool naturally and filter by suction. Recrystallize the solid product 2 - 3 times with a mixed solution of chloroform and methanol, and finally dry the product to obtain the asymmetric hydrophilic group anionic gemini surfactant; the amine is tetradecylamine, pentadecylamine, hexadecylamine, heptadecylamine, or octadecylamine, and the corresponding m is 13, 14, 15, 16, or 17 respectively.

[0013] Preferably, in the mixed solution of chloroform and methanol in step (3), the volume ratio of chloroform to methanol is 9:1.

[0014] Preferably, the foam stabilizer is modified SiO2 nanoparticles, and the modified SiO2 nanoparticles are prepared by the following method:

[0015] (a) Add nano-silica into the mixed solution of ethanol and water, stir for 1 - 1.5 h, and then ultrasonically disperse for 30 - 40 min;

[0016] (b) Add silane coupling agent into deionized water, adjust the pH to 3 - 4 with acetic acid, and let it stand for hydrolysis for 1 - 1.5 h;

[0017] (c) Mix the nano-silica solution obtained in step (a) with the silane coupling agent solution obtained in step (b), adjust the pH to 5 - 7, stir and react at 75 - 85 °C for 3.5 - 4.5 h, filter by suction, wash, and dry to obtain modified SiO₂ nanoparticles.

[0018] Preferably, in step (c), the mass ratio of nano-silica to silane coupling agent is 1:(0.05 - 0.1).

[0019] Preferably, the silane coupling agent is KH550, KH560, KH570, KH792 or DL6021.

[0020] Preferably, in the mixed solution of ethanol and water, the volume ratio of ethanol to water is 1:1.

[0021] Preferably, the drying condition is drying to constant weight at 80 - 85 °C.

[0022] Preferably, the inorganic potassium salt is any one of potassium chloride, potassium sulfate, potassium carbonate, and potassium bicarbonate.

[0023] The preparation method of the salt-tolerant nanoparticle-stabilized foam flooding system: Dissolve the foam stabilizer in water, ultrasonically disperse for 30 - 40 min, and then sequentially add the asymmetric hydrophilic group anionic gemini surfactant and the inorganic potassium salt under stirring, and continue to stir for 3 - 8 min to obtain the flooding system.

[0024] In the asymmetric hydrophilic group anionic gemini surfactant solution, when an inorganic potassium salt is introduced, although the monovalent potassium ion directly contacts and reacts with the negatively charged hydrophilic group of the surfactant, due to its small charge amount, it has a weak shielding effect on the electrostatic repulsion between the hydrophilic groups, and the arrangement of the surfactant molecules is less affected by it. Adding a small amount of potassium ions will compress and arrange the diffuse double layer more closely, reduce the charge repulsion between the hydrophilic groups, reduce the dissociation and recombination between molecules, make the surfactant easier to aggregate into larger micelles, and thus increase the solution viscosity, which helps to improve the stability of the foam.

[0025] The modified SiO2 nanoparticles play a role in stabilizing the foam by means of monolayer, bilayer bridging or multi-layer bridging reticulation connection within the thin film, slowing down the drainage rate of the liquid film, reducing the disproportionation rate of the foam, and delaying the foam rupture rate, thereby enhancing the salt tolerance and calcium resistance of the foam system.

[0026] Advantages of the present invention:

[0027] (1) The oil displacement system provided by the present invention is a salt-tolerant low interfacial tension foaming system with an asymmetric hydrophilic group anionic gemini surfactant as the main agent and an inorganic potassium salt as the auxiliary agent, and a modified SiO2 nanoparticle is introduced as a foam stabilizer to further improve the salt tolerance and stability of the foam system;

[0028] (2) The oil displacement system can be applied to the reservoir formation water environment with a calcium ion content of up to 20,000 mg / L, showing certain viscoelasticity, which is beneficial to the improvement of foam stability; it has a low interfacial tension and good oil washing effect; it has a high foam half-life and a high foam comprehensive index, belonging to a strong foam system, and has excellent foaming performance and foam stabilizing performance. Description of the drawings

[0029] Figure 1 FT-IR spectrum of the asymmetric hydrophilic group anionic gemini surfactant;

[0030] Figure 2 of the asymmetric hydrophilic group anionic gemini surfactant 1 1H NMR spectrum. Detailed implementation manners

[0031] In actual use, there is no requirement for the water used to prepare the oil displacement system, and it can meet the requirements of the present invention. For preparing the oil displacement system, the requirements for pure water or tap water are low, and the effect of the prepared oil displacement system is better; for water with a certain salinity, the requirements are strict, and the effect of the prepared oil displacement system is worse than that prepared with pure water. Therefore, in the embodiments of the present invention, injected water or formation water with a certain salinity is used for preparation.

[0032] In the embodiments of the present invention, the water is prepared formation water or injected water. The salinity of the injected water is 6788 mg / L, and the calcium ion concentration is 326 mg / L; the salinity of the formation water is 118212 mg / L, and the calcium ion concentration is 20439 mg / L.

[0033] Example 1

[0034] 1. A salt-tolerant nanoparticle-stabilized foam oil displacement system is composed of the following raw materials in parts by weight: 0.4 part of an asymmetric hydrophilic group anionic gemini surfactant, 0.1 part of potassium chloride, 0.1 part of a foam stabilizer, and 100 parts of water;

[0035] Among them, the foam stabilizer is modified SiO2 nanoparticles, and the modified SiO2 nanoparticles are prepared by the following method:

[0036] (a) Add 1 g of nano-silica into a mixed solution with a volume ratio of ethanol to water of 1:1, stir for 1 h, and then ultrasonically disperse for 30 min;

[0037] (b) Add 0.1 g of silane coupling agent KH570 into deionized water, adjust the pH to 3 - 4 with acetic acid, and let it stand for hydrolysis for 1 h;

[0038] (c) Mix the nano-silica solution obtained in step (a) with the silane coupling agent solution obtained in step (b), adjust the pH to 5 - 6, stir and react in a water bath at 80 °C for 4 h, filter by suction, wash, and dry at 80 °C to constant weight to obtain modified SiO2 nanoparticles;

[0039] The molecular structural formula of the asymmetric hydrophilic group anionic gemini surfactant is shown as follows:

[0040]

[0041] The asymmetric hydrophilic group anionic gemini surfactant is prepared by the following method:

[0042] (1) Add the linker 1,3-propanediamine into a flask. According to the molar ratio of linker, sodium chloroacetate, and 2-chloroethyl sulfonate of 1:1:1, dissolve sodium chloroacetate and 2-chloroethyl sulfonate in water and then dropwise add them into the linker. React at room temperature for 24 h, then raise the temperature to 90 °C and reflux for 16 h. During the reflux reaction, maintain the pH value of the system at 9 - 10. After the reaction, remove the solvent water by vacuum distillation, then wash with absolute ethanol, filter by suction, and dry at 80 °C to constant weight to obtain intermediate A;

[0043]

[0044] (2) Dissolve intermediate A in distilled water, adjust the pH to 7 with formic acid, and then dropwise add epichlorohydrin according to the molar ratio of intermediate A to epichlorohydrin of 1∶2. React at a constant temperature of 30 °C for 5 h, and remove the solvent water by rotary evaporation to obtain a light brownish-yellow gel-like product, denoted as intermediate B;

[0045]

[0046] (3) Dissolve the intermediate B in distilled water and heat it to 90 °C. According to the molar ratio of intermediate B to hexadecylamine being 1:2, add hexadecylamine to it, maintain the pH of the reaction system at 9 - 10 for constant-temperature reaction for 14 h, then add absolute ethanol to it. After natural cooling, filter by suction. Recrystallize the solid product 3 times with a mixed solution of chloroform and methanol with a volume ratio of 9:1, and finally dry it to constant weight at 80 °C to obtain the asymmetric hydrophilic group anionic gemini surfactant;

[0047]

[0048] Use Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance hydrogen spectrum ( 1 1H NMR) to characterize the structure of the asymmetric hydrophilic group anionic gemini surfactant. Its FT-IR spectrum and 1 1H NMR spectrum are shown in Figure 1 and Figure 2 respectively;

[0049] From Figure 1 the FT-IR spectrum of the product, it can be seen that the absorption peak at a wave number of 3353.77 cm -1 is attributed to the stretching vibration of -OH; the absorption peak at 1466.01 cm -1 is attributed to the bending vibration of C—N; the absorption peak at 1570.20 cm -1 is attributed to the in-plane bending stretching vibration of -CH-; the absorption peak at 2955.30 cm -1 is attributed to the out-of-plane bending vibration of -NH-; the absorption peaks at 1177.41 and 1049.05 cm -1 are attributed to the asymmetric and symmetric stretching vibrations of -SO3; the absorption peak at 2918.25 cm -1 is attributed to the stretching vibrations of methyl and methylene; the absorption peak at 719.71 cm -1 is attributed to the long-chain methylene chain in the molecule. The FT-IR analysis results show that -OH, N—C bond, N—H bond, C—H bond, -SO3, -CH3 and -CH- exist in the synthesized product, which is consistent with the functional groups in the molecular structure of the target product;

[0050] Because the molecular structure of the asymmetric hydrophilic group anionic gemini surfactant is basically symmetric, except for the different hydrophilic groups, only half of its structure is analyzed. From Figure 2 the 1 1H NMR spectrum, it can be seen that δ: 0.89~0.92 (d, 3H, -CH3), 1.21~1.42 [d, 28H, H3C—(CH2) 14-], 1.51 - 1.67 (d, 2H, NCH2—CH2—CH2N), 2.43 - 2.64 (m, 2H, -CH2—SO3), 2.82 (m, 4H, -CH2—N—CH2-), 2.94 (t, 4H, -CH2—NH—CH2-), 3.05 - 3.22 (t, 2H, -CH2—N-), 4.23 - 4.31 (m, 1H, -CH—OH); The attribution of the hydrogen atoms of the main functional groups in the product is consistent with the molecular structure of the target product.

[0051] 2. Preparation method of the salt - tolerant nanoparticle - stabilized foam flooding system: Dissolve the foam stabilizer in water, ultrasonically disperse for 30 min, then successively add the asymmetric hydrophilic - group anionic gemini surfactant and inorganic potassium salt under stirring, and continue stirring for 5 min to obtain the flooding system.

[0052] Example 2

[0053] A salt - tolerant nanoparticle - stabilized foam flooding system is composed of the following raw materials in parts by weight: 0.5 part of the asymmetric hydrophilic - group anionic gemini surfactant described in Example 1, 0.2 part of potassium carbonate, 0.15 part of foam stabilizer, and 100 parts of water; others are the same as in Example 1.

[0054] Example 3

[0055] A salt - tolerant nanoparticle - stabilized foam flooding system is composed of the following raw materials in parts by weight: 0.3 part of the asymmetric hydrophilic - group anionic gemini surfactant described in Example 1, 0.05 part of potassium sulfate, 0.05 part of foam stabilizer, and 100 parts of water; others are the same as in Example 1.

[0056] Example 4

[0057] The molecular structural formula of the asymmetric hydrophilic - group anionic gemini surfactant is as follows:

[0058]

[0059] It is prepared by the following preparation method:

[0060] (1) Add ethylenediamine as the linker into the flask. According to the molar ratio of linker, sodium chloroacetate, and 2 - chloroethyl sulfonate of 1:1.2∶1.2, dissolve sodium chloroacetate and 2 - chloroethyl sulfonate in water and then drop - add them into the linker. React at room temperature for 24 h, then raise the temperature to 85 °C and reflux for 18 h. During the reflux reaction, maintain the pH value of the system at 9 - 10. After the reaction, remove the solvent water by vacuum distillation, then wash with absolute ethanol, filter by suction, and dry at 85 °C to constant weight to obtain intermediate A;

[0061] (2) Dissolve intermediate A in distilled water, adjust the pH to 7 with formic acid, and then dropwise add epichlorohydrin thereto according to the molar ratio of intermediate A to epichlorohydrin of 1:2.4. React at a constant temperature of 30 °C for 6 h, and remove the solvent water by rotary evaporation to obtain a light brownish-yellow gel-like product, denoted as intermediate B;

[0062] (3) Dissolve the intermediate B in distilled water and heat it to 85 °C. Add tetradecylamine thereto according to the molar ratio of intermediate B to tetradecylamine of 1:2, and keep the pH of the reaction system at 9-10 for constant-temperature reaction for 16 h. Then add anhydrous ethanol thereto, cool naturally and filter by suction. Recrystallize the solid product twice with a mixed solution of chloroform and methanol with a volume ratio of 9:1, and finally dry it to constant weight at 85 °C to obtain the asymmetric hydrophilic group anionic gemini surfactant;

[0063] Others are the same as in Example 1.

[0064] Example 5

[0065] The molecular structural formula of the asymmetric hydrophilic group anionic gemini surfactant is as follows:

[0066]

[0067] It is prepared by the following preparation method:

[0068] (1) Add the linker 1,4-butanediamine to a flask. Dissolve sodium chloroacetate and 2-chloroethylsulfonate in water and dropwise add them to the linker according to the molar ratio of linker, sodium chloroacetate, and 2-chloroethylsulfonate of 1:1:1.2. React at room temperature for 24 h, then heat to 95 °C and reflux for 13 h. Keep the pH value of the system at 9-10 during the reflux reaction. After the reaction, remove the solvent water by vacuum distillation, then wash with anhydrous ethanol, filter by suction, and dry to constant weight at 85 °C to obtain intermediate A;

[0069] (2) Dissolve intermediate A in distilled water, adjust the pH to 7 with formic acid, and then dropwise add epichlorohydrin thereto according to the molar ratio of intermediate A to epichlorohydrin of 1:2.4. React at a constant temperature of 30 °C for 6 h, and remove the solvent water by rotary evaporation to obtain a light brownish-yellow gel-like product, denoted as intermediate B;

[0070] (3) Dissolve the intermediate B in distilled water and heat it to 95 °C. Add octadecylamine thereto according to the molar ratio of intermediate B to octadecylamine of 1:2, and keep the pH of the reaction system at 9-10 for constant-temperature reaction for 12 h. Then add anhydrous ethanol thereto, cool naturally and filter by suction. Recrystallize the solid product twice with a mixed solution of chloroform and methanol with a volume ratio of 9:1, and finally dry it to constant weight at 85 °C to obtain the asymmetric hydrophilic group anionic gemini surfactant;

[0071] The others are the same as in Example 1.

[0072] Example 6

[0073] The foam stabilizer is modified SiO2 nanoparticles, and the modified SiO2 nanoparticles are prepared by the following method:

[0074] (a) Add 1 g of nano-silica into a mixed solution of ethanol and water with a volume ratio of 1:1, stir for 1.5 h, and then ultrasonically disperse for 40 min;

[0075] (b) Add 0.05 g of silane coupling agent KH570 into deionized water, adjust the pH to 3 - 4 with acetic acid, and let it stand for hydrolysis for 1.5 h;

[0076] (c) Mix the nano-silica solution obtained in step (a) with the silane coupling agent solution obtained in step (b), adjust the pH to 6 - 7, stir and react in a water bath at 85 °C for 3.5 h, filter by suction, wash, and dry at 85 °C to constant weight to obtain modified SiO2 nanoparticles;

[0077] The others are the same as in Example 1.

[0078] Performance detection

[0079] I. Atmospheric pressure foam performance test

[0080] (1) Take 250 mL of the above-mentioned oil displacement system, turn on the Ross foam meter, set the experimental temperature to 50 °C, and then take 50 mL of the oil displacement system into the constant-temperature graduated cylinder of the Ross foam meter;

[0081] (2) Put the remaining 200 mL of the oil displacement system into a constant-temperature water bath and heat it to 50 °C;

[0082] (3) Add all the heated 200 mL of the foam oil displacement system into the separatory funnel of the Ross foam meter;

[0083] (4) Open the valve of the separatory funnel to the maximum, let the foam liquid flow out freely and impact the foam liquid in the constant-temperature graduated cylinder of the Ross foam meter to form foam;

[0084] (5) When all the fluid in the separatory funnel has flowed out, immediately read and record the volume of the foam (foaming) in the constant-temperature graduated cylinder, and at the same time time with a stopwatch. When the foam volume decays to half, read and record the time at this time, and this time is the half-life of the foam; each group of foam fluids is measured three times, and the average value of the foaming volume and the half-life is taken. The measurement results are shown in Table 1.

[0085] Table 1 Foam performance of the salt-tolerant nanoparticle-stabilized foam oil displacement system (50 °C, atmospheric pressure)

[0086]

[0087] It can be seen from this that the oil displacement system prepared with injected water in Example 1 has excellent foam performance after being tested by a Ross-Miles foam meter. At the simulated reservoir temperature (50 °C), this foam system has excellent foaming performance and foam stability performance, with a half-life higher than 60 min (the half-life of conventional foaming agents and foam systems is about 10 - 40 min). Its foam comprehensive index reaches 14377.5 mL·min, the viscosity of the foam liquid is 3.42 mPa·s, and the oil-water interfacial tension drops to 0.012 mN·m -1 。

[0088] The foam flooding system prepared with formation water in Example 1 also has good foam performance after being tested by a Ross-Miles foam meter. At the simulated reservoir temperature (50 °C), this foam system has good foaming performance and foam stability performance, with a half-life higher than 60 min (the half-life of conventional foaming agents and foam systems is about 10 - 40 min). Its foam comprehensive index reaches 9562.5 mL·min, the viscosity of the foam liquid is 2.83 mPa·s, and the oil-water interfacial tension drops to 0.025 mN·m -1 。

[0089] In other examples, the foam systems prepared with injected water and formation water all have good foaming performance and foam stability performance, with a certain viscosity and low interfacial tension (10 -2 order of magnitude).

[0090] II. Foam performance test under simulated reservoir temperature and pressure

[0091] ① Clean the visual foam reactor in the high-temperature and high-pressure foam performance evaluation device, and open the valve at the lower part of the reactor to drain the cleaning waste liquid;

[0092] ② Close the valve at the lower part of the reactor, open the gas injection system in the device, introduce a certain amount of nitrogen into the reactor, and raise the pressure in the reactor to the reservoir pressure of 9 MPa. Turn on the heating system of the device to raise the temperature in the reactor to the reservoir temperature of 60 °C;

[0093] ③ Continue to add 250 mL of the oil displacement system to the high-temperature and high-pressure visual foam reactor;

[0094] ④ Start the stirrer in the visual reactor, stir at a speed of 3000 r / min for 5 min, stop stirring, observe and record the initial volume of the foam and the volume after standing for 12 h;

[0095] Table 2 Foam performance of the salt-tolerant nanoparticle-stabilized foam flooding system (60 °C, 9 MPa)

[0096]

[0097] As can be seen from Table 2, the oil displacement system prepared with injection water provided in Example 1 has an initial foam volume of 700 mL. After standing for 12 h, the foam volume only decreases by 100 mL. This shows that the oil displacement system prepared with injection water provided in Example 1 has very excellent foaming and foam-stabilizing properties under the reservoir temperature and pressure conditions (60 °C, 9 MPa).

[0098] The foam flooding system prepared with formation water in Example 1 has an initial foam volume of 650 mL. After standing for 12 hours, the foam volume only decreases by 260 mL. This shows that the oil displacement system prepared with formation water provided in Example 1 has good foaming and foam-stabilizing properties under the reservoir temperature and pressure conditions (60 °C, 9 MPa).

[0099] As can be seen from Table 1 and Table 2, the defoaming time of the foam is greatly extended under high-pressure conditions. After standing for 12 h, the foam volume only decreases by 100 mL. If the half-life (the time required for the foam volume to decrease by half) is to be measured, a longer time is required. To reduce the experimental time consumption, the stability of the foam system is reflected by measuring the foam volume and the reduction amount after standing for 12 h. The higher the initial foam volume, the better the foaming performance. The smaller the reduction amount of the foam volume after standing for 12 h, the more stable the foam.

Claims

1. A salt-tolerant nano-particle stabilized foam flooding system, characterized in that: It consists of the following raw materials in parts by weight: 0.3 - 0.5 part of an asymmetric hydrophilic group anionic gemini surfactant, 0.05 - 0.2 part of an inorganic potassium salt, 0.05 - 0.15 part of a foam stabilizer, and 100 parts of water; wherein, the molecular structural formula of the asymmetric hydrophilic group anionic gemini surfactant is as follows: n = 2 - 4; m = 13 - 17, both are integers.

2. The salt-tolerant nanoparticle-stabilized foam flooding system according to claim 1, wherein: The asymmetric hydrophilic group anionic gemini surfactant is prepared by the following method: (1) Add the linker into a flask. According to the molar ratio of linker, sodium chloroacetate, and 2-chloroethyl sulfonate of 1:(1 - 1.2):(1 - 1.2), dissolve sodium chloroacetate and 2-chloroethyl sulfonate in water and then dropwise add them into the linker. React at room temperature for 24 h, then raise the temperature to 85 - 95 °C and reflux for 13 - 18 h. During the reflux reaction, maintain the pH value of the system at 9 - 10. After the reaction, remove the solvent water by vacuum distillation, then wash with absolute ethanol, filter by suction, and dry to obtain intermediate A; wherein, the linker is ethylenediamine, 1,3-propanediamine, or 1,4-butanediamine; (2) Dissolve intermediate A in distilled water, adjust the pH to 7 with formic acid, then dropwise add epichlorohydrin according to the molar ratio of intermediate A to epichlorohydrin of 1:(2 - 2.4), and react at a constant temperature of 30 °C for 4 - 6 h. Rotate evaporate to remove the solvent water to obtain a light brown yellow gel-like product, denoted as intermediate B; (3) Dissolve the intermediate B in distilled water and raise the temperature to 85 - 95 °C. According to the molar ratio of intermediate B to amine of 1:2, add amine thereto, maintain the pH of the reaction system at 9 - 10 and react at a constant temperature for 12 - 16 h, then add absolute ethanol thereto. After natural cooling, filter by suction. Recrystallize the solid product 2 - 3 times with a mixed solution of chloroform and methanol, and finally dry the product to obtain the asymmetric hydrophilic group anionic gemini surfactant; the amine is tetradecylamine, pentadecylamine, hexadecylamine, heptadecylamine, or octadecylamine.

3. The salt-tolerant nanoparticle-stabilized foam flooding system according to claim 2, wherein: In the mixed solution of chloroform and methanol in step (3), the volume ratio of chloroform to methanol is 9:

1.

4. The salt-tolerant nanoparticle-stabilized foam flooding system according to claim 1, wherein: The foam stabilizer is a modified SiO₂ nanoparticle, and the modified SiO₂ nanoparticle is prepared by the following method: (a) Add nano-silica into a mixed solution of ethanol and water, stir for 1 - 1.5 h, and then ultrasonically disperse for 30 - 40 min; (b) Add a silane coupling agent into deionized water, adjust the pH to 3 - 4 with acetic acid, and let it stand for hydrolysis for 1 - 1.5 h; (c) Mix the nano-silica solution obtained in step (a) with the silane coupling agent solution obtained in step (b), adjust the pH to 5 - 7, stir and react at 75 - 85 °C for 3.5 - 4.5 h, filter by suction, wash, and dry to obtain the modified SiO₂ nanoparticle.

5. The salt-tolerant nanoparticle-stabilized foam flooding system according to claim 4, characterized in that: In step (c), the mass ratio of nano-silica to the silane coupling agent is 1:(0.05 - 0.1).

6. The salt-tolerant nanoparticle-stabilized foam flooding system according to claim 5, wherein: The silane coupling agent is KH550, KH560, KH570, KH792, or DL6021.

7. The salt-tolerant nanoparticle-stabilized foam flooding system according to claim 5, wherein: In the mixed solution of ethanol and water, the volume ratio of ethanol to water is 1:

1.

8. The salt-tolerant nanoparticle-stabilized foam flooding system according to claim 2 or 4, characterized in that: The drying conditions are drying to constant weight at 80 - 85 °C.

9. The salt-tolerant nanoparticle-stabilized foam flooding system according to claim 1, wherein: The inorganic potassium salt is any one of potassium chloride, potassium sulfate, potassium carbonate, and potassium bicarbonate.

10. The preparation method of the salt-tolerant nanoparticle-stabilized foam flooding system according to claim 1, characterized in that: The preparation method is to dissolve the foam stabilizer in water, ultrasonically disperse for 30 - 40 min, and then sequentially add the asymmetrical hydrophilic group anionic gemini surfactant and the inorganic potassium salt under stirring, and continue stirring for 3 - 8 min to obtain the oil displacement system.

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