An ester-based aerosol surfactant for reducing the CO 2 miscible pressure, its preparation method and application

Abstract

The present invention relates to the technical field of CO2 miscible flooding, in particular to an ester-based aerosoluble surfactant for reducing the CO2 miscibility pressure, its preparation method and application. The ester-based aerosoluble surfactant is prepared from polyglycerol fatty acid ester and acetic anhydride. The ester-based aerosoluble surfactant contains an unsaturated hydrocarbon chain with 8-18 carbon atoms. n is the degree of polymerization of the polyglycerol fatty acid ester, and n is any integer from 2 to 22. The ester-based aerosoluble surfactant has a solubility of 0.5 wt% - 1.5 wt% in supercritical carbon dioxide. The ester-based aerosoluble surfactant provided by the present invention does not require the addition of other auxiliaries during use, and can achieve good miscibility reduction effects for different crude oils by adjusting the number of carbon dioxide-philic groups and the length of the lipophilic unsaturated hydrocarbon chain.

Description

Technical Field

[0001] The present invention relates to the technical field of miscible flooding, and particularly to an ester-based gas-soluble surfactant for reducing the CO 2 miscible pressure, its preparation method and application. Background Art

[0002] Injecting carbon dioxide into oil reservoirs for oil displacement not only sequesters carbon dioxide but also can significantly improve the oil recovery rate, achieving a win-win situation of carbon dioxide emission reduction and efficient development of oil reservoirs, which has received extensive attention from relevant researchers. A series of carbon dioxide miscible flooding experiments have been carried out in the industry, and certain effects have been achieved. However, the miscible pressure between carbon dioxide and crude oil in most oil reservoirs is relatively high, making it difficult to achieve miscibility, which seriously affects the oil displacement effect in low-permeability oil reservoirs.

[0003] Currently, the methods for reducing the miscible pressure of carbon dioxide flooding mainly include injecting low-molecular-weight alkanes, low-carbon alcohols, enriched gas, etc. These additives have significant effects on reducing miscibility, but the required injection volume is relatively large, mostly above 0.2 PV, with high costs and poor economy.

[0004] Carbon dioxide is a non-polar molecule and has poor compatibility with polar macromolecules such as resins and asphaltenes in crude oil. To improve the miscibility between the two, a suitable surfactant can be added to reduce the interfacial tension between carbon dioxide and crude oil, thereby achieving the purpose of oil-gas miscibility. The surfactants for reducing interfacial tension are mainly divided into oil-soluble surfactants and carbon dioxide gas-soluble surfactants.

[0005] When oil-soluble surfactants are actually applied, they are usually injected in the form of slugs. Due to the heterogeneity of the formation, it is difficult for oil-soluble surfactants to contact crude oil, and they cannot effectively reduce the miscible pressure between carbon dioxide and crude oil. As a result, when oil-soluble surfactants are actually applied in the field, there are phenomena such as a large amount of chemical agent injection and low action efficiency, leading to high costs and poor effects.

[0006] Gas-soluble surfactants can have a high solubility in supercritical carbon dioxide, effectively reducing the interfacial tension between supercritical carbon dioxide and crude oil, thereby reducing the minimum miscible pressure between crude oil and carbon dioxide, enabling supercritical carbon dioxide to reach a miscible state with crude oil under formation temperature and pressure conditions, and achieving the purpose of improving the recovery rate of carbon dioxide flooding.

[0007] Aerosol surfactants are mainly divided into fluorocarbon-based, silicon-containing, and hydrocarbon-based types, etc. Fluorocarbon-based and silicon-containing surfactants have strong ability to reduce the interfacial tension. However, fluorine-containing surfactants are toxic, and silicon-containing surfactants are prone to hydrolysis in aqueous solutions. Both of them are expensive, which limits their applications. Existing hydrocarbon-based demulsifiers are further divided into those containing ethers, alcohols, esters, etc. Among ether surfactants, high-molecular polyethers containing more ether groups are widely used. However, as the relative molecular mass of the polyether increases, its solubility in carbon dioxide gradually decreases, which restricts its development. Alcohol surfactants generally have a general ability to reduce the interfacial tension and can only be used as co-surfactants. The ester groups in ester surfactants have good carbon dioxide affinity and good carbon dioxide solubility. The interaction strength between the ester group and carbon dioxide is higher than that of the carbonyl group and the ether group. However, existing ester surfactants cannot balance the carbon dioxide affinity and demulsification performance. Therefore, existing ester surfactants cannot achieve good effects in improving the recovery rate of carbon dioxide flooding.

[0008] Therefore, in order to improve the dissolution effect of aerosol surfactants in carbon dioxide and the demulsification effect on crude oil, it is urgent to provide a new type of ester-based aerosol surfactant. Summary of the Invention

[0009] The object of the present invention is to overcome the above-mentioned deficiencies of the prior art and provide an ester-based aerosol surfactant for reducing the CO 2 miscibility pressure, its preparation method and application. By controlling the number of carbon dioxide-philic groups, the dissolution effect in carbon dioxide is significantly improved, and by changing the length of the lipophilic tail chain, the miscibility pressure is effectively reduced.

[0010] To achieve the above technical effects, the present invention adopts the following technical solutions:

[0011] An ester-based aerosol surfactant for reducing the CO 2 miscibility pressure is prepared from polyglycerol fatty acid ester and acetic anhydride, and its structural formula is as follows:

[0012] Formula (I),

[0013] In Formula (I), R is an unsaturated hydrocarbon chain with 8-18 carbon atoms, n is the degree of polymerization, and n is any integer from 2 to 22.

[0014] The ester-based aerosol surfactant has a solubility of 0.5 wt%-1.5 wt% in supercritical carbon dioxide.

[0015] The ester-based aerosol surfactant provided by the present invention has an oleophilic unsaturated hydrocarbon chain of different lengths in the raw material polyglycerol fatty acid ester, such as polyglycerol stearate, polyglycerol octanoate, polyglycerol laurate or polyglycerol palmitate, so that the surfactant has an unsaturated hydrocarbon chain with 8-18 carbon atoms. According to the different degrees of polymerization n, the ester-based aerosol surfactant has n+2 carbon dioxide-philic ester groups, so that the surfactant has excellent carbon dioxide-philicity and can be dispersed in supercritical carbon dioxide; the oleophilic unsaturated hydrocarbon chains of different lengths contained in the ester-based aerosol surfactant make the surfactant have oleophilic characteristics, so that the surfactant can well reduce the miscibility pressure between carbon dioxide and crude oil.

[0016] At the same time, the action intensity of the ester group with CO 2 is higher than that of the carbonyl group and the ether group, and it has good application prospects in reducing the miscibility pressure. Selecting the ester group as the CO 2 group, the more ester groups represent the more polarizable CO 2 in the unit surfactant, and the more obvious the effect of reducing miscibility is. However, too many ester groups will lead to a decrease in the solubility of the surfactant in CO 2 . The ester-based aerosol surfactant provided in this application ensures that the surfactant has good solubility through the regulation of the number of ester groups.

[0017] Preferably, in formula (Ⅰ), R is an unsaturated hydrocarbon chain with 14-18 carbon atoms, n is the degree of polymerization, and n is 10.

[0018] The present invention also provides a preparation method of the above-mentioned ester-based aerosol surfactant, and the steps are as follows:

[0019] After heating and melting the polyglycerol fatty acid ester, it is put into a reaction vessel together with acetic anhydride and a catalyst, and reacted under water bath stirring. During the reaction, the water generated by the reaction is removed by vacuum pumping. The reaction ends when no condensate is generated, and an ester-based aerosol surfactant is obtained.

[0020] The preparation method provided in this application is simple and easy to operate. Only by removing water during the reaction can the yield be significantly improved, which is convenient for the wide application of ester-based aerosol surfactants.

[0021] Preferably, the molar ratio of acetic anhydride to the hydroxyl group in the polyglycerol fatty acid ester is 1:1. The number of hydroxyl groups in the polyglycerol fatty acid ester is the degree of polymerization n+1.

[0022] Preferably, the water bath temperature is 65°C-75°C, the stirring speed is 100 rpm-200 rpm, and the reaction duration is 10h-12h.

[0023] Preferably, concentrated sulfuric acid is used as the catalyst, and the catalyst is fed in at 0.1% of the total amount of substances, and the total amount of substances is the sum of the amount of acetic anhydride and the amount of polyglycerol fatty acid ester.

[0024] Preferably, the polyglycerol fatty acid ester is selected from any one of polyglycerol stearate, polyglycerol octanoate, polyglycerol laurate or polyglycerol palmitate.

[0025] The present invention also provides the application of the above ester-based aerosol surfactant or the ester-based aerosol surfactant prepared by the above preparation method in reducing the minimum miscibility pressure of carbon dioxide and crude oil in carbon dioxide flooding to improve the recovery rate.

[0026] Preferably, in the above application, the viscosity of the crude oil < 100 mPa·s.

[0027] Preferably, in the above application,

[0028] After the ester-based aerosol surfactant is fully mixed with liquid carbon dioxide on the ground, it is injected into the formation; or,

[0029] After the ester-based aerosol surfactant is fully mixed with liquid carbon dioxide on the ground, it is pressurized and heated to the supercritical state, and then injected into the formation.

[0030] Processing liquid carbon dioxide to the supercritical state on the ground can effectively avoid the problem that it is difficult to fully contact with crude oil when slug injecting oil-soluble surfactants or conventional surfactants.

[0031] Preferably, in the above application,

[0032] When the minimum miscibility pressure of carbon dioxide and the crude oil in the reservoir is less than or equal to 20 MPa, the addition amount of the ester-based aerosol surfactant is 0.3 wt%-0.9 wt% of the total mass of carbon dioxide;

[0033] When the minimum miscibility pressure of carbon dioxide and the crude oil in the reservoir is greater than 20 MPa, the addition amount of the ester-based aerosol surfactant is 0.9 wt%-1.5 wt% of the total mass of carbon dioxide.

[0034] Due to the increase in temperature, the solubility of the surfactant in CO 2 decreases. By adjusting the molecular structure of the surfactant, it is matched with different reservoir temperatures. When the reservoir temperature is less than or equal to 90 °C, when the length of the unsaturated hydrocarbon chain of the surfactant is 18 and the degree of polymerization n is 10, the effect of reducing the minimum miscibility pressure of crude oil and CO 2 is better; when the reservoir temperature is greater than 90 °C, when the length of the unsaturated hydrocarbon chain of the surfactant is 14-16 and the degree of polymerization n is 10, the effect of reducing the minimum miscibility pressure of crude oil and CO 2 is better.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] 1. When the ester-based aerosoluble surfactant provided by the present invention is used, no other additives are required, and by adjusting the number of carbon dioxide-philic groups and the length of the lipophilic unsaturated hydrocarbon chain, good demixing effects can be achieved for different crude oils;

[0037] 2. The preparation method of the ester-based aerosoluble surfactant provided by the present invention is simple and feasible. The esterification reaction is a reversible reaction, and by extracting the reaction-produced water during the preparation process, the product yield is effectively improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is the infrared spectrum of decaglycerol stearate used in Example 1;

[0039] Figure 2 is the infrared spectrum of the ester-based aerosoluble surfactant prepared in Example 1;

[0040] Figure 3 is the experimental flow chart of measuring the miscibility pressure by the interfacial tension disappearance method used in Experimental Example 1;

[0041] Figure 4 is the broken line graph of the chain length and solubility of the surfactant measured in Experimental Example 2;

[0042] Figure 5 is the broken line graph of the ester group number and solubility of the surfactant measured in Experimental Example 2;

[0043] Figure 6 is the experimental flow chart of measuring the oil recovery rate in Experimental Example 3;

[0044] Figure 7 is the broken line graph of the oil recovery rate measured for Crude Oil 1 in Experimental Example 3;

[0045] Figure 8 is the broken line graph of the oil recovery rate measured for Crude Oil 2 in Experimental Example 3;

[0046] Figure 9 is the broken line graph of the oil recovery rate measured for Crude Oil 3 in Experimental Example 3. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0047] The present invention will be further described below in conjunction with the embodiments and the drawings.

[0048] The raw materials and experimental equipment used in each of the examples and comparative examples are all commonly used commercially available raw materials or experimental equipment in the art, and their specific sources will not be elaborated here.

[0049] Example 1

[0050] A kind of reducing CO2 A miscible-pressure ester-based aerosol surfactant, the structural formula of which is as follows:

[0051] Formula (I),

[0052] In formula (I), R is an unsaturated hydrocarbon chain with 18 carbon atoms, n is the degree of polymerization, and n is 10.

[0053] The above-mentioned ester-based aerosol surfactant is prepared by the following method:

[0054] After heating 0.1 mol of decaglycerol stearate to 65 °C until it melts, 1.1 mol of acetic anhydride is added to the three-necked flask in small batches. The esterification reaction is an exothermic reaction, so avoid adding too much at once to prevent the solvent temperature from being too high. Under a 70 °C water bath and 200 rpm stirring, after all the acetic anhydride has been added to the three-necked flask, 0.0012 mol of concentrated sulfuric acid is added. During the 12-hour reaction, the water generated by the reaction is removed using a vacuum pump. When no more condensate is formed, the reaction is considered complete, and the ester-based aerosol surfactant is obtained.

[0055] As Figure 1 shown, 2916 cm -1 and 2847 cm -1 are the stretching vibration absorptions of C-H in methyl and methylene, 1465 cm -1 is the bending vibration absorption of C-H, and 1735 cm -1 is the characteristic absorption of the ester carbonyl. At the same time, 3362 cm -1 and 1070 cm -1 are respectively the stretching vibration absorption of O-H and the characteristic absorption peak of C-O-C in the polyglycerol molecule.

[0056] As Figure 2 shown, 2923 cm -1 and 2854 cm -1 are the stretching vibration absorptions of C-H in methyl and methylene, 1736 cm -1 is the characteristic absorption of the ester carbonyl, indicating the formation of an ester. At the same time, 1047 cm -1 are respectively the characteristic absorptions of C-O-C. 1372 cm -1 is related to the symmetric deformation vibration of -CH 3 (methyl). Figure 2 Compared with Figure 1 it is found that the O-H stretching vibration absorption peak at 3362 cm -1 disappears, and the characteristic absorption peak of the ester carbonyl at 1736 cm -1 is stronger, indicating that the hydroxyl group has been converted into an ester group.

[0057] Example 2

[0058] An ester-based aerosol surfactant for reducing the CO 2 miscibility pressure has the following structural formula:

[0059] Formula (I),

[0060] In Formula (I), R is an unsaturated hydrocarbon chain with 14 carbon atoms, n is the degree of polymerization, and n is 10.

[0061] The above ester-based aerosol surfactant is prepared by the following method:

[0062] After heating 0.1 mol of decaglycerol myristate to 65 °C to melt it, 1.1 mol of acetic anhydride is added to the three-necked flask in small batches. Under a water bath at 70 °C and stirring at 200 rpm, after all the acetic anhydride has been added to the three-necked flask, 0.0012 mol of concentrated sulfuric acid is added, and the reaction is carried out for 12 h. During the reaction process, the water generated by the reaction is pumped out using a vacuum pump. When no more condensate is formed, the reaction is considered to be over, and the ester-based aerosol surfactant is obtained.

[0063] Example 3

[0064] An ester-based aerosol surfactant for reducing the CO 2 miscibility pressure has the following structural formula:

[0065] Formula (I),

[0066] In Formula (I), R is an unsaturated hydrocarbon chain with 16 carbon atoms, n is the degree of polymerization, and n is 6.

[0067] The above ester-based aerosol surfactant is prepared by the following method:

[0068] After heating 0.1 mol of hexaglycerol palmitate to 65 °C to melt it, 0.7 mol of acetic anhydride is added to the three-necked flask in small batches. Under a water bath at 70 °C and stirring at 200 rpm, after all the acetic anhydride has been added to the three-necked flask, 0.0008 mol of concentrated sulfuric acid is added, and the reaction is carried out for 11 h. During the reaction process, the water generated by the reaction is pumped out using a vacuum pump. When no more condensate is formed, the reaction is considered to be over, and the ester-based aerosol surfactant is obtained.

[0069] Example 4

[0070] An ester-based aerosol surfactant for reducing the CO 2 miscibility pressure has the following structural formula:

[0071] Formula (I),

[0072] In Formula (I), R is an unsaturated hydrocarbon chain with 16 carbon atoms, n is the degree of polymerization, and n is 3.

[0073] The above ester-based aerosol surfactant is prepared by the following method:

[0074] After heating 0.1 mol of polyglycerol palmitate to 65 °C until it melts, 0.4 mol of acetic anhydride is added to the three-necked flask in small batches. Under a water bath at 70 °C and stirring at 200 rpm, after all the acetic anhydride has been added to the three-necked flask, 0.0005 mol of concentrated sulfuric acid is added, and the reaction is carried out for 10 h. During the reaction process, the water generated by the reaction is pumped out using a vacuum pump. When no more condensate is formed, it indicates that the reaction is complete, and the ester-based aerosol surfactant is obtained.

[0075] Comparative Example 1

[0076] The surfactant provided in this comparative example is Compound CAA8-X provided in the journal literature "Involvement and Miscibility Promotion Law of 'Oil-Carbon Dioxide Amphiphiles' Based on Multi-Ester Headgroups" (Acta Physico-Chimica Sinica. 2020, 36(10), 1907034).

[0077] Experimental Example 1

[0078] Miscibility reduction effect of ester-based aerosol surfactant on different crude oil viscosities

[0079] In this experimental example, Crude Oil 1 is the degassed crude oil from a certain block in the Daqing Oilfield. The viscosity of the crude oil under reservoir conditions is 3.6 mPa·s, the minimum miscibility pressure with carbon dioxide is 32 MPa, and the reservoir temperature is 95 °C; Crude Oil 2 is the degassed crude oil from a certain block in the Shengli Oilfield. The viscosity of the crude oil under reservoir conditions is 5.8 mPa·s, the minimum miscibility pressure with carbon dioxide is 19 MPa, and the reservoir temperature is 53 °C; Crude Oil 3 is the degassed crude oil from a certain block in the Jiangsu Oilfield. The viscosity of the crude oil under reservoir conditions is 1.24 mPa·s, the minimum miscibility pressure with carbon dioxide is 27 MPa, and the reservoir temperature is 117 °C.

[0080] Experimental procedure:

[0081] The experimental process of the high-temperature and high-pressure carbon dioxide interfacial properties is as Figure 3 shown. Clean the pipeline and the high-temperature and high-pressure reactor;

[0082] Draw deionized water into the syringe and calibrate the interfacial tensiometer at normal temperature and pressure;

[0083] Connect the experimental instruments, heat the high-temperature and high-pressure reactor to the corresponding temperature of each crude oil, add 1.5 wt% of the surfactant to the stirred intermediate container filled with carbon dioxide respectively, stir thoroughly at a speed of 200 rpm, and after stirring for 24 h, transfer the carbon dioxide gas to the high-temperature and high-pressure reactor.

[0084] After filling the syringe barrel with the sample crude oil, it is fixed on the syringe holder and installed in the high-temperature and high-pressure reactor;

[0085] When the temperature and pressure inside the entire reactor are in a stable state, control the drive motor to extrude a pendant droplet at the syringe needle tip. Enter the densities of carbon dioxide and the oil droplet at this temperature in the computer control panel, run the program, and the computer will automatically capture the droplet morphology for analysis. At the same time, a dynamic interfacial tension curve will be generated. The computer will also take images of the droplet morphology at different times according to the settings;

[0086] Change the pressure, re-extrude the droplet, repeat the above operations, and obtain the interfacial tension data at different pressures. Repeat each pressure point three times and take the average value of the final interfacial tension. According to the linear extrapolation method, obtain the pressure at which the interfacial tension is zero, which is the minimum miscibility pressure of the crude oil - carbon dioxide system.

[0087] Table 1 Minimum miscibility pressure after demixing of different crude oils

[0088]

[0089] In Table 1, except for the reservoir temperature, the units of all other data are MPa.

[0090] From the data in Table 1, it can be seen that for crude oil 1, the ester-based aerosol surfactant provided in Example 2 achieved the best demixing effect. This is because when the reservoir temperature is higher than 90 °C, the solubility of the surfactant provided in Example 1 in CO 2 decreases, resulting in a worse demixing effect. The number of C atoms in the unsaturated hydrocarbon chain of the surfactant provided in Example 2 is smaller than that of the surfactant provided in Example 1, increasing its solubility in CO 2 ;

[0091] For crude oil 2, the ester-based aerosol surfactant provided in Example 1 achieved the best demixing effect. This is because when the reservoir temperature is less than 90 °C, the solubility of the surfactant provided in Example 1 in CO 2 is better, and the number of C atoms in the unsaturated hydrocarbon chain is less different from the crude oil components, which can effectively reduce the interfacial tension between CO 2 and the crude oil;

[0092] For crude oil 3, the ester-based aerosol surfactant provided in Example 3 achieved the best demixing effect. This is because when the reservoir temperature is much higher than 90 °C, further shortening the length of the unsaturated hydrocarbon chain of the ester-based aerosol surfactant and increasing its solubility in CO 2 can effectively reduce the miscibility pressure.

[0093] Compared with Example 3, although the surfactant provided in Comparative Example 1 has the same number of ester groups and tail chain length, the demixing effect produced by Comparative Example 1 is relatively poor. This is because the surfactant used in Comparative Example 1 contains a sugar ring, which has a large steric hindrance, and the ester groups on the sugar ring cannot effectively contact with CO 2 effectively, resulting in a worse effect of reducing the miscibility pressure. However, the surfactant in Example 3 is in a long-chain shape, with a small steric hindrance, and the ester groups are more likely to come into full contact with carbon dioxide, enabling it to extend in carbon dioxide and better adsorb to the gas-liquid interface between carbon dioxide and crude oil, thereby reducing the miscibility pressure between carbon dioxide and crude oil.

[0094] Moreover, the raw material of the surfactant provided in Comparative Example 1 (sucrose palmitate) contains a sugar ring, which has a large steric hindrance, making it difficult to completely convert all hydroxyl groups into ester groups during the synthesis process. At the same time, the esterification reaction is exothermic, causing the temperature of the solution to rise. This substance is unstable, and sugar will turn into caramel at high temperatures, generating impurities, resulting in a low yield. And there are only 7 hydroxyl groups on the sugar ring of sucrose palmitate and carbon dioxide, and at most only 7 ester groups can be converted. It is very difficult to further increase the number of ester groups. However, the polyglycerol used in this application can easily increase the number of hydroxyl groups by changing the degree of polymerization and can be converted into more carbon dioxide-philic ester groups. Therefore, polyglycerol has a better effect of reducing the miscibility pressure of carbon dioxide.

[0095] The raw material of the surfactant provided in Comparative Example 1 (sucrose palmitate) has a high cost, but the raw material of the surfactant provided in the example, polyglycerol fatty acid ester, is inexpensive and has better prospects for industrial promotion.

[0096] Experimental Example 2

[0097] Experiment on the dissolution amount of ester-based aerosol surfactants in supercritical carbon dioxide

[0098] Experimental procedure:

[0099] Put a certain mass of ester-based aerosol surfactants with the same number of ester groups and different chain lengths or different numbers of ester groups and the same carbon chain length into a high-pressure visible autoclave. Control the mass of carbon dioxide added by the reading of the electronic scale (accuracy 0.01 g) under the carbon dioxide gas cylinder, and add a certain mass of carbon dioxide to it. Adjust the initial pressure to 25 MPa, turn on the thermostat and keep it at 65°C. The equilibrium time is 12 h. At this time, the ester-based aerosol surfactant and supercritical carbon dioxide in the high-pressure visible autoclave are in a single-phase state and are in equilibrium. Adjust the ISCO pump to a constant flow mode, and gradually reduce the pressure in the experimental system by discharging the water under the piston of the intermediate container at a constant speed. Record the resistance value of the photoresistor with a multimeter, and record the pressure in the experimental device corresponding to the resistance value of the photoresistor with a pressure sensor (accuracy 0.01 MPa).

[0100] Sort out the experimental data, draw the resistance-pressure curve graph, and obtain the cloud point pressure of the ester-based aerosol surfactant at this temperature at the inflection point of the curve. As Figure 4 and Figure 5 shown, through the solubility experiment, it is found that the shorter the carbon chain, the easier it is to dissolve in supercritical carbon dioxide; when the number of ester groups is 12, that is, when the degree of polymerization n is 10, the solubility of the ester-based aerosol surfactant in supercritical carbon dioxide is better.

[0101] Experimental Example 3

[0102] Enhanced oil recovery effect experiment

[0103] Experimental procedure:

[0104] To compare the influence of the injection concentration of the surfactant on the miscibility pressure, referring to the results of Experimental Example 1, the ester-based aerosol surfactant provided in Example 2 was selected for the experiment. As Figure 6 shown, using the method of one-dimensional core flooding experiment, the core was placed in a one-dimensional core holder. The core was first saturated with crude oil, and the ester-based aerosol surfactant provided in Example 2 with a concentration of 0.3wt%-1.5wt% was injected and mixed thoroughly with carbon dioxide to displace the oil, and the variation laws of the core pressure, gas-liquid flow rate, and crude oil recovery rate during the displacement process were recorded.

[0105] In this experimental example, Crude Oil 1 was the degassed crude oil from a certain block in the Daqing Oilfield, with a crude oil viscosity of 35 mPa·s under reservoir conditions and a minimum miscibility pressure with carbon dioxide of 32 MPa. Crude Oil 2 was the degassed crude oil from a certain block in the Shengli Oilfield, with a crude oil viscosity of 5.8 mPa·s under reservoir conditions and a minimum miscibility pressure with carbon dioxide of 19 MPa. Crude Oil 3 was the degassed crude oil from a certain block in the Jiangsu Oilfield, with a crude oil viscosity of 1.24 mPa·s under reservoir conditions and a minimum miscibility pressure with carbon dioxide of 27 MPa;

[0106] The influence of surfactants with different concentrations on the displacement effects of different crude oils was measured. As Figures 7 - 9 shown, it was found that when the minimum miscibility pressure of the crude oil in the reservoir is less than 20 MPa, the addition amount of the ester-based aerosol surfactant is 0.3-0.9wt%, and there will be a better miscibility reduction effect. When the minimum miscibility pressure of the crude oil in the reservoir is greater than 20 MPa, the addition amount of the ester-based aerosol surfactant is 0.9-1.5wt%, and there will be a better miscibility reduction effect.

Claims

1. An ester gas-soluble surfactant for reducing the miscible pressure of CO2 in carbon dioxide flooding to reduce the minimum miscible pressure of carbon dioxide and crude oil to improve the recovery rate, characterized in that: The ester gas-soluble surfactant for reducing the CO2 miscible pressure is prepared from polyglycerol fatty acid ester and acetic anhydride, and its structural formula is as follows: Formula (I), In formula (I), R is a saturated hydrocarbon chain having 14 to 18 carbon atoms, n is the degree of polymerization, and n is any integer from 2 to 22. The ester gas-soluble surfactant has a solubility of 0.5wt%-1.5wt% in supercritical carbon dioxide; The viscosity of crude oil is less than 100 mPa·s.

2. The use according to claim 1, characterized in that In formula (I), R is a saturated hydrocarbon chain having 14 to 18 carbon atoms, and n is the degree of polymerization, which is 10.

3. The use according to claim 1, characterized in that The preparation method of the ester aerosol surfactant comprises the following steps: After the polyglycerol fatty acid ester is heated and melted, it is put into a reaction container together with acetic anhydride and a catalyst, and reacted under stirring in a water bath, and vacuum is applied to remove the water generated by the reaction during the reaction, and the reaction is terminated until no condensate is generated, thereby obtaining an ester gas-soluble surfactant; The molar ratio of acetic anhydride to the hydroxyl group in the polyglycerol fatty acid ester is 1:1; The catalyst is added at 0.1% of the total amount of substance, which is the sum of the amount of acetic anhydride and the amount of polyglycerol fatty acid ester.

4. The use according to claim 3, characterized in that Concentrated sulfuric acid was selected as the catalyst.

5. The use according to claim 3, characterized in that The water bath temperature is 65°C-75°C, the stirring speed is 100 rpm-200 rpm, and the reaction time is 10h-12h.

6. The use according to claim 1, characterized in that The polyglyceryl fatty acid ester is selected from any one of polyglyceryl stearate, polyglyceryl caprylate, polyglyceryl laurate or polyglyceryl palmitate.

7. The use according to claim 1, characterized in that After the ester gas-soluble surfactant and liquid carbon dioxide are fully mixed on the ground, they are injected into the formation; or, The ester gas-soluble surfactant and liquid carbon dioxide are fully mixed on the ground, pressurized and heated to a supercritical state, and then injected into the formation.

8. The use according to claim 1, characterized in that When the minimum miscibility pressure of carbon dioxide and crude oil in the reservoir is less than or equal to 20 MPa, the amount of ester gas-soluble surfactant added is 0.3 wt%-0.8wt% of the total mass of carbon dioxide; When the minimum miscibility pressure of carbon dioxide and crude oil in the reservoir is greater than 20 MPa, the added amount of the ester gas-soluble surfactant is 0.8 wt%-1.5 wt% of the total mass of carbon dioxide.

9. The use according to claim 1, characterized in that When the reservoir temperature is less than or equal to 90°C, in formula (I), R is 18 and the degree of polymerization n is 10; When the reservoir temperature is greater than 90°C, in formula (I), R is 14-16 and the degree of polymerization n is 10.

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