Acetylated surfactant for reducing minimum miscible pressure of CO2 miscible flooding as well as preparation method and application of acetylated surfactant

Abstract

The invention provides an acetylated surfactant capable of reducing the minimum miscible pressure of CO2 miscible displacement as well as a preparation method and application of the acetylated surfactant, and belongs to the technical field of CO2 oil displacement. The preparation method of the acetylated surfactant comprises the following steps: reacting a gluconic acid aqueous solution with n-caprylic alcohol under the action of a catalyst A to obtain n-octyl gluconate; under the action of a catalyst B, n-octyl gluconate and acetic anhydride react to obtain the acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding. The preparation method of the surfactant is simple, and the raw materials are cheap, wide in source, green and low in cost; the obtained surfactant can interact with CO2, has high CO2 affinity, and can effectively reduce the minimum miscible pressure of CO2 miscible flooding.

Description

Technical Field

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

[0002] CO2 flooding technology is divided into miscible flooding, near-miscible flooding, and immiscible flooding. Among them, miscible flooding has a high oil washing efficiency because it can achieve miscibility with crude oil and completely eliminate interfacial tension. Minimum miscibility pressure (MMP) is an important parameter for evaluating whether miscible flooding can be formed. Crude oil produced from reservoirs with poor continental deposition and accumulation conditions has the characteristics of high density, high viscosity, and high heavy components. This type of crude oil often has a high miscibility pressure. In the actual production process, it shows that the miscibility pressure is greater than the formation fracture pressure. Therefore, to achieve miscible flooding, the reservoir environment must meet the miscibility conditions, which requires reducing the minimum miscibility pressure.

[0003] At present, there are three main methods commonly used to reduce the miscible pressure, namely, the miscible solvent method, the supercritical microemulsion method, and the surfactant method. Among them, the miscible solvent method is limited by defects such as the source of gas and separation from crude oil under formation conditions; the supercritical microemulsion method is limited by the limited pressure reduction and the instability caused by the presence of water; the surfactant method is a CO2-philic-lipophilic surfactant developed by being inspired by hydrophilic-lipophilic surfactants. It can act on the CO2-oil interface and reduce MMP by eliminating interfacial tension. This method was first discovered by T. Hoefling in 1991 that perfluorinated alkyl ethers have good solubility in supercritical CO2, and thus proposed the design concept of CO2-philic surfactants.

[0004] CO2-philic surfactants are mainly divided into three categories: fluorine-containing, siloxane, and hydrocarbon. Among them, fluorine-containing surfactants show excellent performance in the field of CO2-philic. F atoms can produce specific interaction forces with CO2, so fluorine-containing surfactants can be well dissolved in CO2; but fluorine-containing surfactants have the problems of being environmentally unfriendly and high cost. Siloxane surfactants have high surface activity, and the main chain of siloxane has high chain flexibility, which can be tightly arranged on the interface, thereby greatly reducing the oil-CO2 interfacial tension and thus reducing the minimum miscible pressure; but due to the large amount of surfactants used on the oil field, the cost of siloxanes is high, and siloxane surfactants are easily hydrolyzed in water, resulting in a waste of surfactants. Therefore, many scientific researchers are currently looking for hydrocarbon surfactants that can dissolve well in CO2, are green, and are cheap.

[0005] Hydrocarbon surfactants are green and low-cost. If a CO2-philic hydrocarbon surfactant with good CO2 solubility can be developed, the minimum miscibility pressure of CO2 miscible flooding can be greatly reduced, which will be of practical significance for the promotion of CO2 miscible flooding. At present, hydrocarbon surfactants with good CO2 solubility are mainly non-ionic surfactants, which mainly contain CO2-philic groups such as carbonyl and ether bonds. Studies have found that oxygen-containing groups can produce strong Lewis acid-base reactions with CO2, thereby enhancing its solubility in CO2 and reducing the minimum miscibility pressure. Guo Ping (Petroleum Drilling Technology, 2012, 34(2): 81-84.) et al. invented an oil-soluble surfactant CAE, which can reduce the minimum miscibility pressure by 22.3%. Liao (Acta Physico-Chimica Sinica, 2019, 36(10): 1907034.) et al. studied a hexadecyl acetyl octaester, which can reduce the miscibility pressure by 16.04%. Currently available hydrocarbon surfactants are limited by their weak affinity for CO2, which limits their ability to reduce the miscible pressure. Therefore, there is an urgent need to develop a hydrocarbon surfactant that has strong affinity for CO2, can effectively reduce the miscible pressure, and is green, low-cost, and has a wide range of raw material sources. Summary of the invention

[0006] In view of the shortcomings of the prior art, especially the poor solubility of the existing hydrocarbon surfactants in CO2, which cannot achieve the effect of reducing MMP, the present invention provides an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding and its preparation method and application. The surfactant preparation method of the present invention is simple, the raw materials are cheap, the source is wide, it is green and low in cost; the obtained surfactant can interact with CO2, has a high affinity for CO2, and can effectively reduce the minimum miscible pressure of CO2 miscible flooding.

[0007] The technical solution of the present invention is as follows: An acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding has a structure shown in the following formula I:

[0008] Ⅰ.

[0009] The method for preparing the acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding comprises the steps of: (1) Under the action of catalyst A, aqueous gluconic acid solution and n-octanol react to obtain n-octyl gluconic acid ester II;

[0010] II; (2) Under the action of catalyst B, n-octyl gluconate and acetic anhydride react to obtain an acetylated surfactant that reduces the minimum miscible pressure of CO2 miscible flooding.

[0011] Preferably, according to the present invention, in step (1), the mass concentration of the gluconic acid aqueous solution is 45-55%; the molar ratio of gluconic acid to n-octanol is 1-2.2:1, preferably 1:1.

[0012] Preferably, in step (1), catalyst A is selected from concentrated sulfuric acid with a mass concentration of 95-98%, concentrated phosphoric acid with a mass concentration greater than or equal to 85%, potassium hydroxide or phosphotungstic acid; and the molar amount of catalyst A is 0.5-3% of the molar amount of gluconic acid, preferably 3%.

[0013] Preferably, the phosphotungstic acid is SiO2-loaded phosphotungstic acid, and the specific preparation steps are as follows: dissolve phosphotungstic acid (HPW) in an ethanol aqueous solution, add the oxide carrier SiO2, stir at room temperature for 20-30 hours, remove the solvent by rotary evaporation, and calcine at 200-400°C in an air atmosphere for 2-4 hours; wherein the volume ratio of ethanol to water is 1:1, the mass ratio of phosphotungstic acid to the volume ratio of ethanol aqueous solution is 0.005-0.02g / mL, and the mass ratio of phosphotungstic acid (HPW) to the oxide carrier SiO2 is 1:0.5-2.

[0014] Preferably, according to the present invention, in step (1), when catalyst A is liquid, n-octanol and catalyst A are added to the gluconic acid aqueous solution by dropwise addition, respectively, or, the gluconic acid aqueous solution and catalyst A are added to n-octanol by dropwise addition, respectively; when catalyst A is solid, catalyst A is fully dispersed in the gluconic acid aqueous solution, and then n-octanol is added dropwise, or, catalyst A is fully dispersed in n-octanol, and then the gluconic acid aqueous solution is added dropwise.

[0015] Preferably, in step (1), the reaction temperature is 100-120° C., the reaction time is 3-5 h, and the reaction is carried out under stirring; preferably, the reaction temperature is 110-115° C., and the reaction time is 3-4 h.

[0016] Preferably, in step (1), the post-treatment method of the reaction solution obtained by the reaction comprises the steps of: washing the reaction solution with water, sodium carbonate aqueous solution and water in sequence, drying the organic phase with magnesium sulfate and distilling at normal pressure to obtain n-octyl gluconate II.

[0017] According to the preferred embodiment of the present invention, in step (1), the preparation method of gluconic acid n-octyl ester II comprises the following steps: under stirring conditions, n-octanol and catalyst A are added dropwise to a gluconic acid aqueous solution, stirred for reaction, and then washed, dried, and distilled at normal pressure to obtain gluconic acid n-octyl ester II; Alternatively, under stirring conditions, the aqueous solution of gluconic acid and the catalyst A are added dropwise to n-octanol simultaneously, stirred for reaction, and then washed, dried, and distilled under normal pressure to obtain n-octyl gluconic acid ester II; Alternatively, the catalyst A is fully dispersed in an aqueous solution of gluconic acid, n-octanol is added dropwise under stirring, reacted by stirring, and then washed, dried, and distilled under normal pressure to obtain n-octyl gluconic acid ester II; Alternatively, the catalyst A is fully dispersed in n-octanol, and an aqueous solution of gluconic acid is added dropwise under stirring, and the mixture is reacted by stirring, and then washed, dried, and distilled under normal pressure to obtain n-octyl gluconic acid ester II.

[0018] Preferably according to the present invention, in step (2), the molar ratio of acetic anhydride to n-octyl gluconate is 5-10:1, preferably 5-6:1.

[0019] Preferably, in step (2), the catalyst B is selected from one of iodine, concentrated sulfuric acid with a mass concentration of 95-98%, thionyl chloride, p-toluenesulfonic acid or cupric perchlorate; and the molar amount of the catalyst B is 3-5% of the molar amount of n-octyl gluconate, preferably 3%.

[0020] Preferably, according to the present invention, in step (2), the reaction temperature is 0-8°C, the reaction time is 1-3h, and the reaction is carried out under stirring conditions; preferably, the reaction temperature is 0-5°C, and the reaction time is 1-1.5h.

[0021] Preferably, in step (2) of the present invention, the post-treatment method of the reaction solution obtained by the reaction comprises the steps of: adding ice water to the reaction solution, extracting with chloroform, taking the organic phase, adding triethylamine to the organic phase, stirring sufficiently, and then washing with a saturated sodium thiosulfate aqueous solution, drying the organic phase with anhydrous sodium sulfate, and distilling to obtain an acetylated surfactant that reduces the minimum miscible pressure of CO2 miscible drive.

[0022] Preferably, in step (2) of the present invention, the method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding comprises the following steps: uniformly mixing acetic anhydride and n-octyl gluconate, cooling to 0-8°C, adding catalyst B, reacting by stirring, then adding ice water, extracting, washing, drying and distilling the organic phase to obtain the acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding.

[0023] The above-mentioned acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding is used in CO2 oil recovery.

[0024] Preferably, according to the present invention, the acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding is used in CO2 miscible flooding.

[0025] The preparation route of the acetylated surfactant of the present invention is as follows:

[0026] The technical features and beneficial effects of the present invention are as follows: 1. The surfactant of the present invention introduces a large number of CO2-philic groups (acetoxy groups), which improves the solubility of hydrocarbon surfactants in CO2 and has the advantage of low usage concentration.

[0027] 2. The surfactant of the present invention has both CO2-philic groups (acetoxy groups) and lipophilic groups (alkyl chains), which can act on the interface between CO2 and oil, reduce the CO2-oil interfacial tension, and then reduce the minimum miscible pressure, promote miscibility, and improve the miscible drive recovery rate.

[0028] 3. The surfactant of the present invention is a small molecule hydrocarbon surfactant with excellent CO2 solubility performance. Unlike large molecule CO2-philic surfactants, it does not need to be used with a co-solvent when used.

[0029] 4. The surfactant of the present invention is a hydrocarbon surfactant, which has the advantages of being green and low cost compared to fluorocarbon and siloxane aerosol surfactants with good solubility.

[0030] 5. The present invention uses gluconic acid as a starting material, which is green, cheap and widely available. After a series of reactions, an acetylated surfactant is prepared. The preparation method is simple, green and low-cost.

[0031] 6. The purpose of acetylation after obtaining n-octyl gluconate in the present invention is to provide more oxygen-containing groups, because the oxygen-containing groups can provide more and stronger Lewis acid-base interactions with C in CO2. At the same time, the ester group is in a loose and stretched state, which can increase the chance of contact with CO2, thereby improving the affinity with CO2, enhancing the solubility of CO2, and carrying more surfactants into the formation. If acetylation is not performed, the strong hydrogen bonding between hydroxyl groups will lead to strong interaction between surfactant molecules, thereby reducing the interaction between surfactant and CO2 and reducing the solubility. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is the infrared spectrum of acetylated n-octyl gluconate prepared in Example 1.

[0033] Figure 2 This is the hydrogen nuclear magnetic resonance image of acetylated n-octyl gluconate prepared in Example 1.

[0034] Figure 3 This is an experimental diagram of the solubility of acetylated gluconic acid n-octyl ester prepared in Example 1 with CO2.

[0035] Figure 4 This is a graph showing the relationship between light power and pressure in the CO2 solubility experiment of acetylated n-octyl gluconate prepared in Example 1.

[0036] Figure 5 These are the downmixing experimental diagrams of acetylated n-octyl gluconate / CO2 / diesel (lower) and CO2 / diesel (upper) prepared in Example 1. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with specific embodiments.

[0038] Unless otherwise specified, the reagents, materials and equipment used in the examples can be obtained from commercial sources; the experimental methods described are conventional methods unless otherwise specified.

[0039] Example 1 A method for preparing an acetylated surfactant for reducing the minimum miscible phase pressure of CO2 miscible phase flooding comprises the following steps: (1) Add 31.62 mL (0.1 mol) of 50% gluconic acid aqueous solution to a 500 mL three-necked flask equipped with a reflux device, start stirring and heat up, and when the temperature reaches 110°C, use a constant pressure funnel to drop 15.74 mL (0.1 mol) of n-octanol solution (one drop every 4-6 seconds), and at the same time, slowly drop 0.16 mL (0.003 mol) of 98% concentrated sulfuric acid from the other end of the three-necked flask, one drop every 3-5 seconds. After the n-octanol solution is added, keep the temperature in the three-necked flask at 110°C and stir for 3 hours to obtain a brown oily product. Transfer the oily product to a separatory funnel, wash with water, 10 wt% sodium carbonate aqueous solution, and water respectively, dry the organic phase over magnesium sulfate, and then distill at atmospheric pressure to obtain n-octyl gluconate in a collection flask. The single-step yield is 83.92%.

[0040] (2) Place 9.6 mL (0.1 mol) of acetic anhydride in a 500 mL single-necked flask, then add 6.41 mL (0.02 mol) of n-octyl gluconate and mix well. Place the single-necked flask in an ice bath to ensure that the temperature is between 0 and 5 °C, slowly add 0.03 mL (0.0006 mol) of 98% concentrated sulfuric acid, and stir for 1 hour. After the reaction is completed, pour the reactant into 100 mL of ice water at a temperature of 0 to 5 °C, continue stirring, and add chloroform to extract. After the extraction is completed, use a separatory funnel to separate the organic phase, add triethylamine to the organic phase, stir well to remove acetic anhydride, then add saturated sodium thiosulfate aqueous solution to wash, dry the organic phase over anhydrous sodium sulfate, and then distill to remove chloroform to obtain the final product acetylated surfactant with a single-step yield of 84.4%.

[0041] The infrared spectrum and H spectrum of the product synthesized in Example 1 are as follows: Figure 1 , Figure 2 shown. Figure 1 The characteristic absorption peak of -CH2- bond appears at 2950 cm -1 ; -CH3 stretching vibration occurs at 2993 cm -1 The C=O (ester) group stretching vibration absorption peak appears at 1731 cm -1 The asymmetric stretching vibration peak of -COC- appears at 1269cm -1 and 1241cm -1 The symmetrical stretching vibration peak of -COC- appears at 1193 cm -1 This indicates that the present invention successfully synthesized the acetylated surfactant (acetylated gluconic acid n-octyl ester).

[0042] Example 2 A method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding is as described in Example 1, except that in step (1), a slightly excess amount of 50% gluconic acid aqueous solution (35 mL, 0.22 mol) is used.

[0043] The other steps and conditions are the same as in Example 1.

[0044] The single-step yield of n-octyl gluconate in step (1) is 79.93%; the single-step yield of the product acetylated surfactant is 81.12%.

[0045] Example 3 A method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding is as described in Example 1, except that in step (1), the dropwise addition order in the esterification reaction is changed. The specific steps are as follows: Add 15.74mL (0.1mol) of n-octanol solution to a 500mL three-necked flask equipped with a reflux device, start stirring and heating, and when the temperature reaches 110°C, use a constant pressure funnel to drop 31.62mL (0.1mol) of 50% gluconic acid aqueous solution (one drop every 4-6s), and slowly drop 0.16mL (0.003mol) of 98% concentrated sulfuric acid from the other end of the three-necked flask (one drop every 3-5s). After the addition of the gluconic acid aqueous solution is completed, keep the temperature in the three-necked flask at 110°C and stir for 3h to obtain a brown oily product. Transfer the oily product to a separatory funnel, wash with water, 10wt% sodium carbonate aqueous solution, and water respectively, dry the organic phase with magnesium sulfate, and then distill at normal pressure to obtain n-octyl gluconic acid in a collection flask.

[0046] The other steps and conditions are the same as in Example 1.

[0047] The single-step yield of n-octyl gluconate in step (1) is 82.3%; the single-step yield of the product acetylated surfactant is 83.25%. Example 4 A method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding is as described in Example 1, except that in step (1), the reaction temperature of the esterification reaction is changed to 115°C.

[0048] The other steps and conditions are the same as in Example 1.

[0049] The single-step yield of n-octyl gluconate in step (1) is 77.37%; the single-step yield of the product acetylated surfactant is 78.25%.

[0050] Example 5 A method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding is as described in Example 1, except that in step (1), the esterification reaction time is changed to 4 hours.

[0051] The other steps and conditions are the same as in Example 1.

[0052] The single-step yield of n-octyl gluconate in step (1) is 79.71%; the single-step yield of the product acetylated surfactant is 85%.

[0053] Example 6 A method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding is as described in Example 1, except that in step (2), the amount of acetic anhydride is changed to 0.12 mol, and the molar ratio of acetic anhydride to n-octyl gluconate is 6:1.

[0054] The other steps and conditions are the same as in Example 1.

[0055] The single-step yield of the obtained acetylated surfactant was 82.5%.

[0056] Example 7 A method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding is as described in Example 1, except that in step (2), the acetylation reaction time is changed to 1.5 h.

[0057] The other steps and conditions are the same as in Example 1.

[0058] The single-step yield of the obtained acetylated surfactant was 83.56%.

[0059] Example 8 A method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding is as described in Example 1, except that in step (2), the catalyst in the acetylation reaction is changed to 0.15 g (0.0006 mol) of elemental iodine.

[0060] The other steps and conditions are the same as in Example 1.

[0061] The single-step yield of the obtained acetylated surfactant was 80.52%.

[0062] Example 9 A method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding, as described in Example 1, except that: in step (1), the catalyst used is 2.88 g SiO2-supported phosphotungstic acid catalyst (50 PW / SiO2, wherein the phosphotungstic acid content is 50 wt %), wherein the molar amount of phosphotungstic acid is 0.5% of the molar amount of gluconic acid; The specific preparation steps of SiO2-supported phosphotungstic acid catalyst are as follows: dissolve phosphotungstic acid (HPW) in ethanol aqueous solution (the volume ratio of ethanol to water is 1:1), and the volume ratio of phosphotungstic acid to ethanol aqueous solution is 0.01 g / mL; add oxide carrier SiO2, and the mass ratio of phosphotungstic acid (HPW) to oxide carrier SiO2 is 1:1; stir at room temperature for 24 hours, remove the solvent by rotary evaporation, and calcine at 300°C in air atmosphere for 3 hours, and dry for use.

[0063] In a 500mL three-necked flask equipped with a reflux device, add 31.62mL (0.1mol) of 50% gluconic acid aqueous solution, add 2.88g SiO2-loaded phosphotungstic acid catalyst, start stirring and heating, and when the temperature reaches 110℃, use a constant pressure funnel to drop 15.74mL (0.1mol) of n-octanol solution (one drop every 4~6s). After the n-octanol solution is added, keep the temperature in the three-necked flask at 110℃ and stir the reaction for 3h to obtain a brown oily product. Filter the oily product to remove the catalyst, then transfer it to a separatory funnel, wash it with water, 10wt% sodium carbonate aqueous solution, and water respectively, dry the organic phase with magnesium sulfate, and then distill it at normal pressure to obtain n-octyl gluconate in a collection bottle.

[0064] The other steps and conditions are the same as in Example 1.

[0065] The single-step yield of n-octyl gluconate in step (1) is 80.13%; the single-step yield of the product acetylated surfactant is 82.36%; Comparative Example 1 The product used in Comparative Example 1 is the commercially available oil-soluble surfactant AEO-3.

[0066] Comparative Example 2 The product used in Comparative Example 2 is n-octyl gluconate synthesized in step (1) of Example 1.

[0067] Test example: 1. Solubility performance test The surfactant obtained in Example 1 was placed in a high-pressure visual kettle, and a certain amount of CO2 (the mass of the surfactant was 0.2wt.% of the mass of CO2) was added into the visual kettle using an ISCO pump. The experiment was carried out at a temperature of 45°C. When the pressure reached 20MPa, the visual kettle became clear. The pressure in the visual kettle was gradually reduced to observe whether turbidity appeared in the visual kettle. The pressure when turbidity appeared was the cloud point pressure. The CO2 solubility of the product was judged by the cloud point pressure. The smaller the cloud point pressure, the better the CO2 solubility.

[0068] Figure 3 The figure shows the dissolution in the visible kettle under different pressures. When the pressure is 20MPa, the visible kettle is a clear and transparent phase, indicating that acetylated octyl gluconate can be well dissolved in CO2. When the pressure reaches 15MPa, the visible kettle is still a clear and transparent phase. When the pressure reaches 10.85MPa, the visible kettle suddenly becomes turbid, indicating that under this pressure, acetylated octyl gluconate gradually precipitates from CO2. Therefore, 10.85MPa is the cloud point pressure under the corresponding experimental conditions.

[0069] The error of observing the turbidity point by naked eyes is too large. Therefore, the turbidity point is determined by measuring the optical power. The result is as follows: Figure 4 shown. Figure 4 This is the relationship between pressure and optical power recorded during the experiment. It can be found that as the pressure decreases, the optical power will have an inflection point. By taking the first-order derivative of the optical power-pressure curve, the turbidity point can be obtained more accurately. Figure 4 The inflection point appears at 10.63 MPa, which indicates that at pressures higher than 10.63 MPa, acetylated n-octyl gluconate can be dissolved in CO2 and has good solubility. Compared with the CO2 miscible pressure of 30 MPa, it can be used to reduce the minimum miscible pressure of CO2 miscible flooding.

[0070] 2. Evaluation of MMP Reduction Capacity Acetylated n-octyl gluconate can be well dissolved in CO2 and has good CO2 solubility, so it can be used to reduce the minimum miscibility pressure of CO2 miscible drive. 60 mL of diesel and the surfactant obtained in Example 1 were placed in a visual kettle, and CO2 (the mass of the surfactant was 0.2 wt.% of the mass of CO2) was injected into the visual kettle using an ISCO pump until a clear and transparent phase was reached in the visual kettle, and then the pressure in the visual kettle was gradually reduced to observe the corresponding pressure point when the cloud point appeared, i.e., the minimum miscibility pressure, and to compare the cloud point pressure, i.e., the minimum miscibility pressure, when no surfactant sample was added, to judge the mixing reduction ability of acetylated n-octyl gluconate.

[0071] Figure 5 The turbidity of the visual kettle can be seen. It can be seen that in the system without acetylated gluconic acid n-octyl ester, the diesel + CO2 system becomes turbid at 19.2MPa, indicating that the minimum miscible pressure of the system is 19.2MPa. The system with 0.2wt.% acetylated gluconic acid n-octyl ester becomes turbid at 14MPa, and the minimum miscible pressure decreases by 27.08% compared with the system without acetylated gluconic acid n-octyl ester. Compared with the commercially available product (Comparative Example 1), since this product provides more CO2-philic groups (oxygen-containing ester groups), it is more fluffy in spatial structure than ether bonds, and can provide more opportunities for contact with CO2, so it has stronger CO2-philicity than ether bonds; while Comparative Example 2 is unacetylated gluconic acid n-octyl ester, because unacetylated gluconic acid n-octyl ester contains more hydroxyl groups, although it also contains oxygen groups, there is a strong hydrogen bond between hydroxyl groups, resulting in poor CO2 solubility, which is also verified by the experimental results. Acetylated octyl gluconate is a green hydrocarbon mixing reducer with good mixing performance.

[0072] The miscibility and mixing performance of the products obtained in the examples and comparative examples are shown in Table 1 below.

[0073] Table 1 Evaluation results of miscibility and demixing ability

[0074] The cloud point pressure in Table 1 refers to the cloud point pressure tested in the solubility performance experiment, and the minimum miscible pressure refers to the cloud point pressure tested in the MMP reduction ability evaluation experiment.

Claims

1. An acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding, characterized in that: It has the structure shown in the following formula I: Ⅰ。 2. The method for preparing the acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding according to claim 1, comprising the steps of: (1) Under the action of catalyst A, aqueous gluconic acid solution and n-octanol react to obtain n-octyl gluconic acid ester II; Catalyst A is selected from one of concentrated sulfuric acid with a mass concentration of 95-98%, concentrated phosphoric acid with a mass concentration greater than or equal to 85%, potassium hydroxide or phosphotungstic acid; Ⅱ； (2) Under the action of catalyst B, n-octyl gluconate and acetic anhydride react to obtain an acetylated surfactant that reduces the minimum miscible pressure of CO2 miscible flooding; Catalyst B is selected from one of iodine, concentrated sulfuric acid with a mass concentration of 95-98%, thionyl chloride, p-toluenesulfonic acid or copper perchlorate.

3. The method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding according to claim 2, characterized in that: In step (1), the mass concentration of the gluconic acid aqueous solution is 45-55%; and the molar ratio of gluconic acid to n-octanol is 1-2.2:

1.

4. The method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding according to claim 2, characterized in that: In step (1), one or more of the following conditions are included: i. The molar amount of catalyst A is 0.5-3% of the molar amount of gluconic acid; ii. When catalyst A is liquid, n-octanol and catalyst A are added dropwise to the gluconic acid aqueous solution separately and simultaneously, or, the gluconic acid aqueous solution and catalyst A are added dropwise to n-octanol separately and simultaneously; when catalyst A is solid, catalyst A is fully dispersed in the gluconic acid aqueous solution, and then n-octanol is added dropwise, or, catalyst A is fully dispersed in n-octanol, and then the gluconic acid aqueous solution is added dropwise.

5. The method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding according to claim 2, characterized in that: In step (1), the reaction temperature is 100-120° C., the reaction time is 3-5 h, and the reaction is carried out under stirring conditions.

6. The method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding according to claim 2, characterized in that: In step (2), one or more of the following conditions are included: i. The molar ratio of acetic anhydride to n-octyl gluconate is 5-10:1; ii. The molar amount of catalyst B is 3-5% of the molar amount of n-octyl gluconate.

7. The method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding according to claim 2, characterized in that: In step (2), the reaction temperature is 0-8°C, the reaction time is 1-3h, and the reaction is carried out under stirring conditions.

8. The method for preparing an acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding according to claim 2, characterized in that: In step (2), the preparation method of the acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding comprises the following steps: uniformly mixing acetic anhydride and n-octyl gluconate, cooling to 0-8°C, adding catalyst B, reacting by stirring, then adding ice water, extracting, washing, drying and distilling the organic phase to obtain the acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding.

9. Use of the acetylated surfactant for reducing the minimum miscible pressure of CO2 miscible flooding as claimed in claim 1 in CO2 oil recovery.

10. The use according to claim 9, characterized in that: The acetylated surfactant for reducing the minimum miscible phase pressure of CO2 miscible phase flooding is used in CO2 miscible phase flooding.

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