Ester gas-soluble surfactant for reducing CO2 miscible-phase pressure as well as preparation method and application of ester gas-soluble surfactant

Abstract

The invention relates to the technical field of CO2 miscible-phase oil displacement, in particular to an ester gas-soluble surfactant for reducing CO2 miscible-phase pressure and a preparation method and application thereof.The ester gas-soluble surfactant is prepared from polyglycerol fatty acid ester and acetic anhydride, the ester gas-soluble surfactant contains unsaturated hydrocarbon chains with the carbon atom number being 8-18, and the unsaturated hydrocarbon chains are connected with the polyglycerol fatty acid ester. N is the polymerization degree of the polyglycerol fatty acid ester, n is any integer in the range of 2-22, and the ester gas-soluble surfactant has the solubility of 0.5 wt%-1.5 wt% in supercritical carbon dioxide. When the ester gas-soluble surfactant provided by the invention is used, other auxiliaries do not need to be added, and a good mixing reduction effect can be achieved aiming at different crude oils by adjusting the number of carbon dioxide-philic groups and the length of oleophylic unsaturated hydrocarbon chains.

Description

Technical Field

[0001] The invention relates to the technical field of miscible phase oil displacement, in particular to an ester gas-soluble surfactant for reducing the CO2 miscible phase pressure, and a preparation method and application thereof. Background Art

[0002] Injecting carbon dioxide into oil reservoirs to drive oil not only seals carbon dioxide, but also greatly increases crude oil recovery, achieving a win-win situation of carbon dioxide emission reduction and efficient development of oil reservoirs, which has attracted wide attention from relevant researchers. The industry has conducted a series of carbon dioxide miscible oil recovery experiments, all of which have achieved certain results, but the miscible pressure of carbon dioxide and crude oil in most oil reservoirs is high, making it difficult to achieve miscibility, which has seriously affected the oil recovery effect of low permeability oil reservoirs.

[0003] At present, the main methods to reduce the miscibility pressure of carbon dioxide flooding include injecting low molecular weight alkanes, low carbon alcohols, enriched gas, etc. These additives have significant mixing reduction effects, but the required injection volume is large, mostly above 0.2 PV, which is costly and has poor economic efficiency.

[0004] Carbon dioxide is a non-polar molecule and has poor solubility with polar macromolecules such as colloids and asphaltene in crude oil. In order to improve the miscibility of the two, suitable surfactants can be added to reduce the interfacial tension between carbon dioxide and crude oil, thereby achieving the purpose of oil-gas miscibility. Surfactants that reduce interfacial tension are mainly divided into oil-soluble surfactants and carbon dioxide gas-soluble surfactants.

[0005] In actual application, oil-soluble surfactants are usually injected in the form of plugs. Due to the heterogeneity of the formation, it is difficult for oil-soluble surfactants to come into contact with crude oil and they cannot effectively reduce the miscibility pressure between carbon dioxide and crude oil. As a result, when oil-soluble surfactants are actually used in mines, there are large injection volumes and low efficiency, resulting in high costs and poor effects.

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

[0007] Gas-soluble surfactants are mainly divided into fluorocarbon, silicon-containing, hydrocarbon and the like. Fluorocarbon and silicon-containing surfactants have strong ability to reduce interfacial tension, but fluorochemical surfactants are toxic and silicon-containing surfactants are easily hydrolyzed in aqueous solution. Both are expensive, which limits their application; existing hydrocarbon demixing agents are divided into ether, alcohol, ester and the like. Among ether surfactants, high molecular weight polyethers contain more ether groups and are widely used. However, as the relative molecular mass of polyether increases, its solubility in carbon dioxide gradually decreases, which restricts its development. Alcohol surfactants have a general ability to reduce interfacial tension and can only be used as co-surfactants. The ester group in ester surfactants has good affinity for carbon dioxide and good solubility in carbon dioxide. The interaction strength between ester groups and carbon dioxide is higher than that between carbonyl groups and ether groups, but existing ester surfactants cannot take into account both affinity for carbon dioxide and demixing performance. Therefore, existing ester surfactants cannot achieve good results in improving the recovery rate of carbon dioxide flooding.

[0008] Therefore, in order to improve the solubility of gas-soluble surfactants in carbon dioxide and the downmixing effect of crude oil, it is urgent to provide a new type of ester gas-soluble surfactant. Summary of the invention

[0009] The purpose of the present invention is to overcome the deficiencies of the above-mentioned prior art and provide an ester gas-soluble surfactant for reducing the CO2 miscible pressure and its preparation method and application. By controlling the number of carbon dioxide-philic groups, the solubility effect in carbon dioxide is significantly improved, and by changing the length of the lipophilic tail chain, the miscible pressure is effectively reduced.

[0010] In order to achieve the above technical effects, the present invention adopts the following technical solutions: An ester gas-soluble surfactant for reducing the CO2 miscible phase 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 an unsaturated hydrocarbon chain having 8 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 0.5 wt%-1.5 wt% in supercritical carbon dioxide.

[0011] The ester gas-soluble surfactant provided by the present invention has lipophilic unsaturated hydrocarbon chains of different lengths, such as polyglycerol stearate, polyglycerol caprylate, polyglycerol laurate or polyglycerol palmitate, since the raw material polyglycerol fatty acid ester has unsaturated hydrocarbon chains of different lengths, such as polyglycerol stearate, polyglycerol caprylate, polyglycerol laurate or polyglycerol palmitate, so that the surfactant has an unsaturated hydrocarbon chain with a carbon number of 8-18. The ester gas-soluble surfactant has n+2 carbon dioxide-philic ester groups according to the different polymerization degrees n, so that the surfactant has excellent carbon dioxide-philicity and can be dispersed in supercritical carbon dioxide; the lipophilic unsaturated hydrocarbon chains of different lengths contained in the ester gas-soluble surfactant make the surfactant have both lipophilic characteristics, so that the surfactant can well reduce the miscible pressure of carbon dioxide and crude oil.

[0012] At the same time, the interaction intensity between ester groups and CO2 is higher than that between carbonyl groups and ether groups, and it has a good application prospect in reducing the miscibility pressure. By selecting ester groups as CO2-philic groups, the more ester groups there are, the more polarizable CO2 can be obtained per surfactant, and the more obvious the miscibility reduction effect is. However, too many ester groups will lead to a decrease in the solubility of the surfactant in CO2. The ester gas-soluble surfactant provided in the present application ensures that the surfactant has good solubility by regulating the number of ester groups.

[0013] Preferably, in formula (I), R is an unsaturated hydrocarbon chain having 14-18 carbon atoms, and n is the degree of polymerization, which is 10.

[0014] The present invention also provides a method for preparing the above-mentioned ester aerosol surfactant, the steps of which are as follows: 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. During the reaction, vacuum is applied to remove the water generated by the reaction. The reaction is terminated until no condensate is generated to obtain an ester gas-soluble surfactant.

[0015] The preparation method provided in the present application is simple and easy to implement. The yield can be significantly improved by only extracting water during the reaction process, which facilitates the widespread application of ester aerosol surfactants.

[0016] Preferably, the molar ratio of acetic anhydride to the hydroxyl groups 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.

[0017] Preferably, the water bath temperature is 65° C.-75° C., the stirring speed is 100 rpm-200 rpm, and the reaction time is 10 h-12 h.

[0018] Preferably, concentrated sulfuric acid is used as the catalyst, and the catalyst is added at 0.1% of the total amount of substance, where the total amount of substance is the sum of the amount of substance of acetic anhydride and the amount of substance of polyglycerol fatty acid ester.

[0019] Preferably, the polyglycerol fatty acid ester is selected from any one of polyglycerol stearate, polyglycerol caprylate, polyglycerol laurate or polyglycerol palmitate.

[0020] The present invention also provides the use of the above-mentioned ester gas-soluble surfactant or the ester gas-soluble surfactant prepared by the above-mentioned preparation method in reducing the minimum miscible pressure of carbon dioxide and crude oil in carbon dioxide flooding to improve the recovery rate.

[0021] Preferably, in the above application, the viscosity of crude oil is less than 100 mPa·s.

[0022] Preferably, in the above application, 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.

[0023] Treating liquid carbon dioxide to a supercritical state on the ground can effectively avoid the problem of oil-soluble surfactants or conventional surfactants having difficulty in fully contacting with crude oil when injecting plugs.

[0024] Preferably, in the above application, 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%-0.9wt% 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 amount of ester gas-soluble surfactant added is 0.9 wt%-1.5wt% of the total mass of carbon dioxide.

[0025] As the temperature rises, the solubility of surfactants in CO2 decreases. The surfactant molecular structure is adjusted to match different reservoir temperatures. When the reservoir temperature is less than or equal to 90°C, the unsaturated hydrocarbon chain length of the surfactant is 18, and the degree of polymerization n is 10, the effect of reducing the minimum miscible pressure of crude oil and CO2 is better; when the reservoir temperature is greater than 90°C, the unsaturated hydrocarbon chain length of the surfactant is 14-16, and the degree of polymerization n is 10, the effect of reducing the minimum miscible pressure of crude oil and CO2 is better.

[0026] Compared with the prior art, the present invention has the following beneficial effects: 1. The ester gas-soluble surfactant provided by the present invention does not require the addition of other additives when used, and can achieve good downmixing effects for different crude oils by adjusting the number of carbon dioxide-philic groups and the length of lipophilic unsaturated hydrocarbon chains; 2. The preparation method of the ester aerosol surfactant provided by the present invention is simple and easy, the esterification reaction is a reversible reaction, and the product yield is effectively improved by extracting the reaction water during the preparation process. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the infrared spectrum of ten polyglycerol stearate used in Example 1; Figure 2 is the infrared spectrum of the ester aerosol surfactant prepared in Example 1; Figure 3 is an experimental flow chart of the interfacial tension vanishing method used in Experimental Example 1 to measure the miscible phase pressure; Figure 4 is a line graph of the chain length and solubility of the surfactant measured in Experimental Example 2; Figure 5 is a line graph of the number of ester groups and the amount of dissolution of the surfactant measured in Experimental Example 2; Figure 6 is the experimental flow chart of the recovery factor measurement in Experimental Example 3; Figure 7 It is a line graph of the recovery factor measured for crude oil 1 in Experimental Example 3; Figure 8 It is a line graph of the recovery factor of crude oil 2 measured in Experimental Example 3; Fig. 9 It is a line graph of the recovery factor of crude oil 3 in Experimental Example 3. DETAILED DESCRIPTION

[0028] The present invention will be further described below in conjunction with embodiments and drawings.

[0029] The raw materials and experimental equipment used in each embodiment and comparative example are all commonly available commercially available raw materials or experimental equipment in the art, and their specific sources are not described here in detail.

[0030] Example 1 An ester gas-soluble surfactant for reducing the CO2 mixed phase pressure, the structural formula of which is as follows: Formula (I), In formula (I), R is an unsaturated hydrocarbon chain having 18 carbon atoms, and n is the degree of polymerization, which is 10.

[0031] The above-mentioned ester aerosol surfactant is prepared by the following method: After 0.1 mol of polyglycerol stearate was heated to 65°C and melted, 1.1 mol of acetic anhydride was added to a three-necked flask in small batches. The esterification reaction is an exothermic reaction, so avoid adding too much at one time to cause the solvent temperature to be too high. After all the acetic anhydride was added to the three-necked flask under 70°C water bath and stirring at 200rpm, 0.0012 mol of concentrated sulfuric acid was added and the reaction was continued for 12 hours. During the reaction, a vacuum pump was used to extract the water generated by the reaction. The reaction was completed when no condensate was generated, and an ester gas-soluble surfactant was obtained.

[0032] like Figure 1 As shown, 2916cm -1 and 2847cm -1 The stretching vibration absorption of CH in methyl and methylene is 1465cm -1 The bending vibration absorption of CH, 1735cm -1 This is the characteristic absorption of the ester carbonyl group. -1 and 1070cm -1 They are the OH stretching vibration absorption and COC characteristic absorption peaks in polyglycerol molecules respectively.

[0033] like Figure 2 As shown, 2923cm -1 and 2854cm -1 The stretching vibration absorption of CH in methyl and methylene is 1736 cm -1 This is the characteristic absorption of ester carbonyl, indicating that ester is generated. -1 They are COC characteristic absorption. 1372cm -1 It is related to the symmetric deformation vibration of -CH3 (methyl). Figure 2 and Figure 1 By comparison, 3362cm -1 The OH stretching vibration absorption peak disappears at 1736 cm -1 The characteristic absorption peak of the ester carbonyl group is stronger, indicating that the hydroxyl group has been converted into an ester group.

[0034] Example 2 An ester gas-soluble surfactant for reducing the CO2 mixed phase pressure, the structural formula of which is as follows: Formula (I), In formula (I), R is an unsaturated hydrocarbon chain having 14 carbon atoms, and n is the degree of polymerization, which is 10.

[0035] The above-mentioned ester aerosol surfactant is prepared by the following method: After 0.1 mol of decaglycerol myristic acid ester was heated to 65°C to melt, 1.1 mol of acetic anhydride was added to a three-necked flask in small batches. After all the acetic anhydride was added to the three-necked flask under 70°C water bath and stirring at 200 rpm, 0.0012 mol of concentrated sulfuric acid was added and the reaction was continued for 12 hours. During the reaction, a vacuum pump was used to extract the water generated by the reaction. The reaction was terminated when no condensate was generated, and an ester gas-soluble surfactant was obtained.

[0036] Example 3 An ester gas-soluble surfactant for reducing the CO2 mixed phase pressure, the structural formula of which is as follows: Formula (I), In formula (I), R is an unsaturated hydrocarbon chain having 16 carbon atoms, and n is the degree of polymerization, which is 6.

[0037] The above-mentioned ester aerosol surfactant is prepared by the following method: After 0.1 mol of hexapolyglycerol palmitate was heated to 65°C and melted, 0.7 mol of acetic anhydride was added to a three-necked flask in small batches. After all the acetic anhydride was added to the three-necked flask under 70°C water bath and stirring at 200 rpm, 0.0008 mol of concentrated sulfuric acid was added and the reaction was continued for 11 hours. During the reaction, a vacuum pump was used to extract the water generated by the reaction. The reaction was terminated when no condensate was generated, and an ester gas-soluble surfactant was obtained.

[0038] Example 4 An ester gas-soluble surfactant for reducing the CO2 mixed phase pressure, the structural formula of which is as follows: Formula (I), In formula (I), R is an unsaturated hydrocarbon chain having 16 carbon atoms, and n is the degree of polymerization, which is 3.

[0039] The above-mentioned ester aerosol surfactant is prepared by the following method: After 0.1 mol of tripolyglycerol palmitate was heated to 65°C and melted, 0.4 mol of acetic anhydride was added to a three-necked flask in small batches. After all the acetic anhydride was added to the three-necked flask under 70°C water bath and stirring at 200 rpm, 0.0005 mol of concentrated sulfuric acid was added and the reaction was continued for 10 hours. During the reaction, a vacuum pump was used to extract the water generated by the reaction. The reaction was completed when no condensate was generated, and an ester gas-soluble surfactant was obtained.

[0040] Comparative Example 1 The surfactant provided in this comparative example is the compound CAA8-X provided in the journal article "Oil-carbon dioxide amphiphilic molecules based on polyester head groups and their mixing aid rules" (Acta Physico-Chimica Sinica. 2020, 36(10), 1907034).

[0041] Experimental Example 1 Effect of ester gas-soluble surfactants on the mixing performance of crude oils with different viscosities In this experimental example, crude oil 1 is degassed crude oil from a block of Daqing Oilfield. The crude oil viscosity 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 degassed crude oil from a block of Shengli Oilfield. The crude oil viscosity 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 degassed crude oil from a block of Jiangsu Oilfield. The crude oil viscosity 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.

[0042] Experimental steps: The experimental process of high temperature and high pressure carbon dioxide interface characteristics is as follows Figure 3 As shown, clean pipelines and high temperature and high pressure reactors; Deionized water is sucked into the needle tube, and the interfacial tension meter is calibrated at normal temperature and pressure; Connect the experimental equipment, heat the high-temperature and high-pressure reactor to the corresponding temperature of each crude oil, add 1.5wt% of the surfactant into the stirring intermediate container filled with carbon dioxide, stir it thoroughly at 200rpm, and after stirring for 24h, transfer the carbon dioxide gas to the high-temperature and high-pressure reactor; After the syringe barrel is filled with sample crude oil, it is fixed on the syringe holder and installed in a high-temperature and high-pressure reactor; When the temperature and pressure in the entire reactor are in a stable state, control the drive motor to squeeze out a hanging droplet on the syringe needle, enter the density of carbon dioxide and oil droplets at this temperature in the computer control bar, run the program, and the computer will automatically capture the droplet morphology for analysis and generate a dynamic interfacial tension curve. The computer will also take images of the droplet morphology at different times according to the settings; Change the pressure, squeeze out the droplets again, repeat the above operation, and obtain the interfacial tension data under different pressures. Repeat each pressure point three times and take the average value of the final interfacial tension. According to the linear extrapolation method, the pressure at which the interfacial tension is zero is the minimum miscibility pressure of the crude oil-carbon dioxide system.

[0043] Table 1 Minimum miscibility pressure of different crude oils after downmixing

[0044] In Table 1, except for the reservoir temperature, the units of other data are MPa.

[0045] It can be seen from the data in Table 1 that for crude oil 1, the ester gas-soluble surfactant provided in Example 2 achieves the best mixing reduction effect. This is because when the reservoir temperature is higher than 90°C, the solubility of the surfactant provided in Example 1 in CO2 decreases, resulting in a poor mixing reduction effect. The surfactant provided in Example 2 has a smaller number of C atoms in the unsaturated hydrocarbon chain than the surfactant provided in Example 1, which increases its solubility in CO2. For crude oil 2, the ester gas-soluble surfactant provided in Example 1 achieves the best mixing reduction effect. This is because when the reservoir temperature is less than 90°C, the surfactant provided in Example 1 has good solubility in CO2, and the number of C atoms in the unsaturated hydrocarbon chain is slightly different from that of the crude oil component, which can effectively reduce the interfacial tension between CO2 and crude oil. For crude oil 3, the ester gas-soluble surfactant provided in Example 3 achieved the best mixing reduction effect. This is because when the reservoir temperature is much higher than 90°C, the length of the unsaturated hydrocarbon chain of the ester gas-soluble surfactant is further shortened, and its solubility in CO2 is increased, which can effectively reduce the miscibility pressure.

[0046] Compared with Example 3, although the surfactant provided in Comparative Example 1 has the same number of ester groups and tail chain length, the mixing reduction effect produced by Comparative Example 1 is poor. This is because the surfactant used in Comparative Example 1 contains a sugar ring, which has a large steric hindrance, and the ester group on the sugar ring cannot effectively contact CO2, making it less effective in reducing the miscible pressure. However, the surfactant in Example 3 is in the shape of a long chain, with small steric hindrance, and the ester group is more likely to fully contact with carbon dioxide, so that it can extend in carbon dioxide, better adsorb to the gas-liquid interface of carbon dioxide and crude oil, and reduce the miscible pressure of carbon dioxide and crude oil.

[0047] And the surfactant raw material (sucrose palmitate) provided in Comparative Example 1 contains sugar ring, steric hindrance is large, and it is difficult to convert all hydroxyl groups into ester groups during the synthesis process. At the same time, the esterification reaction is exothermic, so that the solution temperature is increased, and this material is unstable, and sugar will be converted into caramel at high temperature, generating impurities, so that the yield is relatively low. And there are only 7 hydroxyl groups on the sugar ring of sucrose palmitate with carbon dioxide, which can only be converted into 7 ester groups at most, and it is very difficult to further increase the number of ester groups. However, the polyglycerol used in this application is easy to increase the number of hydroxyl groups by changing the degree of polymerization, and can be converted into more ester groups of carbon dioxide affinity. Therefore, polyglycerol has a better effect in reducing the mixed phase pressure of carbon dioxide.

[0048] The surfactant raw material (sucrose palmitate) provided in Comparative Example 1 is relatively expensive, but the raw material polyglycerol fatty acid ester provided in the embodiment is cheap and has better prospects for industrial promotion.

[0049] Experimental Example 2 Experimental study on the solubility of ester gas-soluble surfactants in supercritical carbon dioxide Experimental steps: A certain mass of ester gas-soluble surfactants with the same number of ester groups and different chain lengths or different numbers of ester groups and the same number of carbon chains are placed in a high-pressure visual autoclave. The mass of carbon dioxide added is controlled by the reading of the electronic scale (accuracy 0.01 g) under the carbon dioxide cylinder. A certain mass of carbon dioxide is added. The initial pressure is adjusted to 25 MPa, and the thermostat switch is turned on and maintained at 65°C. The equilibrium time is 12 h. At this time, the ester gas-soluble surfactant and supercritical carbon dioxide in the high-pressure visual autoclave are in a single phase state, and the two are in equilibrium. Adjust the ISCO pump to the constant flow mode, discharge the water under the piston of the intermediate container at a constant speed, so that the pressure in the experimental system gradually decreases, record the resistance of the photoresistor through a multimeter, and record the pressure in the experimental device corresponding to the resistance of the photoresistor through a pressure sensor (accuracy 0.01 MPa).

[0050] Arrange the experimental data, draw the resistance-pressure curve, and obtain the cloud point pressure of the ester gas-soluble surfactant at this temperature at the inflection point of the curve. Figure 4 and Figure 5 As shown, through the solubility experiment, it was found that the shorter the carbon chain, the easier it is to dissolve in supercritical carbon dioxide; when the ester base number is 12, that is, when the degree of polymerization n is 10, the ester gas-soluble surfactant has a better solubility in supercritical carbon dioxide.

[0051] Experimental Example 3 Experiment on the effect of enhanced oil recovery Experimental steps: In order to compare the effect of surfactant injection concentration on the miscible pressure, referring to the results of Experimental Example 1, the ester gas-soluble surfactant provided in Example 2 was selected for the experiment. Figure 6 As shown, a one-dimensional core displacement experiment method is adopted, the core is placed in a one-dimensional core holder, the core is first saturated with crude oil, and 0.3wt%-1.5wt% of the ester gas-soluble surfactant provided in Example 2 is injected and fully mixed with carbon dioxide to drive oil, and the changes in core pressure, gas-liquid flow rate and crude oil recovery rate during the displacement process are recorded.

[0052] In this experimental example, crude oil 1 is degassed crude oil from a block of 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 is degassed crude oil from a block of 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 is degassed crude oil from a block of 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. The effects of different concentrations of surfactant on the displacement of different crude oils were determined. Figure 7-Figure 9 As shown in the figure, it is found that when the minimum miscibility pressure of crude oil in the reservoir is less than 20MPa, the addition amount of ester gas-soluble surfactant is 0.3-0.9wt%, which will have a better mixing reduction effect. When the minimum miscibility pressure of crude oil in the reservoir is greater than 20MPa, the addition amount of ester gas-soluble surfactant is 0.9-1.5wt%, which will have a better mixing reduction effect.

Claims

1. An ester gas-soluble surfactant for reducing the CO2 miscible pressure, characterized in that: It is made from polyglycerol fatty acid ester and acetic anhydride, and its structural formula is as follows: Formula (I), In formula (I), R is an unsaturated hydrocarbon chain having 8 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 0.5 wt%-1.5 wt% in supercritical carbon dioxide.

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

3. The method for preparing an ester aerosol surfactant according to any one of claims 1 or 2, characterized in that: Here are the 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 preparation method according to claim 3, characterized in that: Concentrated sulfuric acid was selected as the catalyst.

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

6. The preparation method according to claim 3, characterized in that: The polyglyceryl fatty acid ester is selected from any one of polyglyceryl stearate, polyglyceryl caprylate, polyglyceryl laurate or polyglyceryl palmitate.

7. 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 from any one of claims 1 or 2, or the ester gas-soluble surfactant for reducing the CO2 miscible pressure is prepared by the preparation method described in any one of claims 3 to 6, The viscosity of crude oil is less than 100 mPa·s.

8. The use according to claim 7, 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.

9. The use according to claim 7, 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.

10. The use according to claim 7, 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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