A polycarbonate surfactant for reducing the minimum miscibility pressure in CO2 flooding and its preparation method

Abstract

The application discloses a polycarbonate surfactant for reducing the minimum miscibility pressure of CO2 flooding and a preparation method thereof. Under the action of a catalyst, polycarbonate dihydric alcohol and fatty acid are reacted to obtain the polycarbonate surfactant for reducing the minimum miscibility pressure of CO2 flooding. The ester group and ether group in the polycarbonate dihydric alcohol serve as CO2-philic groups, and the hydrocarbon chain of the fatty acid serves as an oil-philic group. The surfactant preparation method is simple, the raw materials are cheap, widely sourced, green and low in cost; the obtained surfactant has good solubility in CO2, high CO2-philicity and can effectively reduce the minimum miscibility pressure of CO2 flooding. The surfactant can be prepared through only one step of reaction, and the preparation method and post-processing method are simple and easy to operate, and the surfactant has the advantages of environmental friendliness and low cost compared with carbon-fluorine surfactants and siloxane surfactants.

Description

Technical Field

[0001] This invention relates to the field of CO2 flooding technology, and in particular to a polycarbonate surfactant for reducing the minimum miscibility pressure in CO2 flooding and its preparation method. Background Technology

[0002] CO2 flooding, a key component of CCUS (Continuous Coal-Gas System) technology, injects CO2 into the oil reservoir. This significantly improves oil and gas field recovery while simultaneously sequestering CO2, achieving a win-win situation of efficient reservoir development and CO2 emission reduction. Based on the displacement mechanism, CO2 flooding can be categorized into immiscible, near-miscible, and miscible flooding. Theoretical research and field practice show that achieving near-miscible or miscible flooding significantly improves oil displacement efficiency. However, my country's low-permeability reservoirs are primarily continental sedimentary formations, characterized by high temperatures and high asphaltene content in crude oil. This results in high miscibility pressure between CO2 and crude oil, severely hindering the widespread application of CO2 flooding technology. Therefore, reducing the miscibility pressure between CO2 and crude oil and improving their mixing ability are crucial scientific challenges that urgently need to be addressed in CO2 flooding technology.

[0003] Currently, methods for reducing the minimum miscibility pressure (MMP) of CO2-crude oil are mainly divided into physical mixing aids and chemical mixing aids. The development of physical mixing aids is limited by the large injection volume and high cost of the slug injection. Chemical mixing aids utilize amphiphilic surfactants to directionally distribute at the oil-gas interface, reducing interfacial tension and thus promoting oil-gas miscibility. This method, with its advantages of small slug injection volume, low cost, and high mixing efficiency, has become the main research direction for reducing miscibility pressure. Hydrocarbon surfactants have the combined advantages of high surface activity, good environmental compatibility, and relatively low cost, making them a key research focus for amphiphilic surfactants. Therefore, developing a green, low-cost, and efficient amphiphilic hydrocarbon surfactant is a major task for reducing miscibility pressure and improving CO2 flooding recovery.

[0004] Currently available hydrocarbon surfactants are mostly oil-soluble and have poor solubility in CO2. Their limited CO2 affinity restricts their ability to reduce miscibility. Therefore, there is an urgent need to develop hydrocarbon surfactants that are highly soluble in CO2, have strong CO2 affinity, and are made from inexpensive, widely available, and environmentally friendly raw materials. Summary of the Invention

[0005] In order to overcome the above-mentioned problems in the prior art, the present invention proposes a polycarbonate surfactant for reducing the minimum miscibility pressure of CO2 drive and its preparation method.

[0006] The technical solution adopted by this invention to solve its technical problem is: a polycarbonate surfactant for reducing the minimum miscibility pressure of CO2 drive, which is prepared from polycarbonate diol and fatty acid, and its general structural formula is shown in Formula I: Formula I Wherein, R is a straight-chain alkyl group, R0 is a short-chain diol, x and y are the number of structural units, x ranges from 2 to 15, and y ranges from 2 to 10.

[0007] The polycarbonate surfactant described above for reducing the minimum miscibility pressure in CO2 flooding, wherein R is one of n-propyl, n-pentyl, n-heptyl, n-nonyl, undecyl, tridecyl, pentadecyl, and heptadecanyl.

[0008] The aforementioned polycarbonate surfactant for reducing the minimum miscibility pressure of CO2 drive is characterized in that RO is one of butanediol, pentanediol, and hexanediol.

[0009] The polycarbonate surfactant described above for reducing the minimum miscibility pressure in CO2 drive has a molar ratio of hydroxyl groups to fatty acids of 1:2 in the polycarbonate diol.

[0010] A method for preparing a polycarbonate surfactant that reduces the minimum miscibility pressure in CO2-driven processes, comprising the following steps: Step 1: Add the weighed polycarbonate diol and fatty acid to the drying reactor, and add the preset amount of catalyst. Shake the reactor to make the materials initially mixed. Step 2: Place the reactor equipped with a thermometer and stirrer in an oil bath, stir evenly, and then react under vacuum. Step 3: After the reaction is complete, wait for the reaction solution to cool naturally to 50-60°C, then add ethyl acetate to extract the product, so that the reaction product is fully dissolved in ethyl acetate. Step 4: Add 5% sodium bicarbonate solution to the organic phase, shake, let stand to separate the layers, and wash repeatedly until the organic phase is neutral; Step 5: Add sufficient color-changing silica gel to the organic phase obtained in step 4, dry it, filter the organic phase with qualitative filter paper, and collect the filtrate. Step 6: Rotary evaporate the collected filtrate, and vacuum dry the evaporated product to obtain a polycarbonate surfactant.

[0011] The method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 drive, wherein the molar ratio of hydroxyl groups to fatty acids in the polycarbonate diol is 1:1 to 1:2.

[0012] Preferably, the molar ratio of hydroxyl groups to fatty acids in the polycarbonate diol is 1:2. The method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding, wherein the fatty acid is one of butyric acid, hexanoic acid, octanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic acid, or octadecanoic acid.

[0013] In the above-mentioned method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure of CO2 drive, the catalyst in step 1 is one of p-toluenesulfonic acid, concentrated sulfuric acid, concentrated hydrochloric acid, and anhydrous sodium acetate.

[0014] In the above-mentioned method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure of CO2 drive, the mass of the catalyst is 0.5% to 2% of the total mass of the reactants.

[0015] Preferably, the mass of the catalyst is 1.5% of the total mass of the reactants.

[0016] In the above-mentioned method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure of CO2 drive, the oil bath temperature in step 2 is 100-130℃, the stirring speed is 400-500rpm, and the reaction time is 5-8h.

[0017] Preferably, the oil bath temperature is 100℃, the stirring speed is 500rpm, and the reaction time is 7h.

[0018] The beneficial effects of this invention are that it uses polycarbonate chains as carbonophilic groups, resulting in a high content of these groups, which enhances the surfactant's solubility in carbon dioxide and strengthens its ability to reduce miscibility. This invention uses polycarbonate diol copolymerized from propylene oxide and carbon dioxide as the starting material, offering advantages such as mature technology, low cost, and environmental friendliness. This invention requires only a one-step reaction, and the preparation and post-processing methods are simple and easy to operate. The surfactant in this invention is a hydrocarbon surfactant, which, compared to fluorocarbon and siloxane surfactants, has the advantages of being environmentally friendly and cost-effective. Attached Figure Description

[0019] Figure 1 This is the hydrogen nuclear magnetic resonance image of polycarbonate diol, the reactant in Example 3 of this invention.

[0020] Figure 2 The image shows the hydrogen nuclear magnetic resonance (NMR) spectrum of the polycarbonate diol butyrate surfactant prepared in Example 3 of this invention.

[0021] Figure 3 The effect of polycarbonate surfactants with different carbon chain lengths on the CO2-kerosene miscibility pressure. Detailed Implementation

[0022] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0023] Unless otherwise specified, the reagents, materials and equipment used in the examples are commercially available; the experimental methods described are conventional methods.

[0024] This invention discloses a polycarbonate surfactant for reducing the minimum miscibility pressure in CO2 flooding, which is prepared from polycarbonate diol and fatty acid, and its general structural formula is shown in Formula I: Formula I Wherein, R is a straight-chain alkyl group, R0 is a short-chain diol, x and y are the number of structural units, x ranges from 2 to 15, and y ranges from 2 to 10.

[0025] The preparation route of the polycarbonate surfactant of this invention is as follows: Example 1 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding includes the following steps: (1) Weigh 40.0456g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48mg KOH / g) and 3.6941g of n-butyric acid (analytical grade, purity ≥99%) according to the molar ratio of hydroxyl group of diol to n-butyric acid of 1:1 and add them to a dry 100ml three-necked flask. Then add 0.2187g of catalyst p-toluenesulfonic acid according to 0.5% of the total mass of reactants and gently shake the flask to mix the materials initially.

[0026] (2) Install the thermometer, mechanical stirrer and vacuum interface of the three-necked flask, and ensure that each interface is well sealed.

[0027] (3) Place the three-necked flask in an oil bath preheated to 100°C, turn on the mechanical stirrer, and set the stirring speed to 500 rpm.

[0028] (4) After the material is stirred evenly, turn on the vacuum pump, adjust the vacuum degree to 0.09MPa, and keep the vacuum system stable.

[0029] (5) Maintain the oil bath temperature, stirring rate and 0.09 MPa vacuum for 5 hours.

[0030] (6) Measure the acid value every 1 hour during the reaction and record the reaction temperature, vacuum degree and stirring rate. If the parameters fluctuate, adjust the equipment in time to maintain stability.

[0031] (7) After the reaction is complete, wait for the reaction solution to cool down to 50-60°C, add ethyl acetate to extract the product. The amount of ethyl acetate is 1.5-2 times the volume of the reaction solution. Stir for 5 minutes to fully dissolve the reaction product in ethyl acetate.

[0032] (8) Add 5% sodium bicarbonate solution to the organic phase in the separatory funnel, the amount of which is 1 / 2 of the volume of the organic phase. Gently shake the separatory funnel (avoid vigorous shaking to prevent emulsification) for 1 minute, and then let it stand to separate into layers. Repeat the washing 2 to 4 times until the organic phase is neutral.

[0033] (9) Transfer the washed organic phase to a dry beaker, add sufficient color-changing silica gel, dry for 2-4 hours, filter the organic phase with qualitative filter paper to remove the color-changing silica gel, and collect the filtrate in a round-bottom flask.

[0034] (10) Connect the round-bottom flask containing the filtrate to the rotary evaporator and set the vacuum distillation parameters: vacuum degree 0.08~0.09MPa, water bath temperature 80℃, and remove the ethyl acetate solvent by distillation.

[0035] (11) The product after rotary evaporation was transferred to a vacuum drying oven, and the vacuum degree was set to 0.09 MPa and the temperature to 60 °C. After drying for 12 h, polycarbonate surfactants were obtained.

[0036] The esterification rate was determined to be 46.37% by hydroxyl titration.

[0037] Example 2 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 39.9740 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 3.8043 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:1 to hydroxyl groups of the diol and n-butyric acid. 0.4394 g of p-toluenesulfonic acid catalyst is added at 1% of the total mass of the reactants. The reaction temperature is 110 °C, and the reaction time is 6 h.

[0038] The other steps and conditions are the same as in Example 1.

[0039] The esterification rate was determined to be 58.17% by hydroxyl titration.

[0040] Example 3 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 41.1502 g of commercially available polycarbonate diol (number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 3.7324 g of analytical grade n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of hydroxyl groups to n-butyric acid of 1:1. 0.6740 g of p-toluenesulfonic acid catalyst is added at 1.5% of the total mass of the reactants. The reaction temperature is 120 °C, and the reaction time is 7 h. The hydrogen nuclear magnetic resonance (HNMR) images of the reactant polycarbonate diol and the prepared polycarbonate diol butyrate surfactant are shown below. Figure 2 , Figure 3 As shown.

[0041] The other steps and conditions are the same as in Example 1.

[0042] The esterification rate was determined to be 61.83% by hydroxyl titration.

[0043] Example 4 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 40.0908 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 3.8080 g of n-butyric acid are added to a 100 ml three-necked flask at a molar ratio of 1:1 to hydroxyl groups of the diol and n-butyric acid. 0.8780 g of p-toluenesulfonic acid catalyst is added at 2% of the total mass of the reactants. The reaction temperature is 130 °C, and the reaction time is 8 h.

[0044] The other steps and conditions are the same as in Example 1.

[0045] The esterification rate was determined to be 56.10% by hydroxyl titration.

[0046] Example 5 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 39.9424 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 4.5391 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:1.25 to hydroxyl groups of the diol. 0.6723 g of p-toluenesulfonic acid catalyst is added at 1.5% of the total mass of the reactants. The reaction temperature is 100 °C, and the reaction time is 6 h.

[0047] The other steps and conditions are the same as in Example 1.

[0048] The esterification rate was determined to be 73.72% by hydroxyl titration.

[0049] Example 6 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 40.2116 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 4.777 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:1.25 to hydroxyl groups of the diol. 0.9080 g of p-toluenesulfonic acid catalyst is added at 2% of the total mass of the reactants. The reaction temperature is 110 °C, and the reaction time is 5 h.

[0050] The other steps and conditions are the same as in Example 1.

[0051] The esterification rate was determined to be 75.92% by hydroxyl titration.

[0052] Example 7 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. The method involves adding 40.0528 g of commercially available polycarbonate diol (number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 4.4897 g of n-butyric acid (analytical grade, purity ≥99%) to a 100 ml three-necked flask at a molar ratio of 1:1.25 to hydroxyl groups of the diol. Then, 0.2232 g of p-toluenesulfonic acid catalyst is added at 0.5% of the total mass of the reactants. The reaction temperature is 120 °C, and the reaction time is 8 h.

[0053] The other steps and conditions are the same as in Example 1.

[0054] The esterification rate was determined to be 56.65% by hydroxyl titration.

[0055] Example 8 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 39.9180 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 4.4695 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:1.25 to hydroxyl groups of the diol. 0.4445 g of p-toluenesulfonic acid catalyst is added at 1% of the total mass of the reactants. The reaction temperature is 130 °C, and the reaction time is 7 h.

[0056] The other steps and conditions are the same as in Example 1.

[0057] The esterification rate was determined to be 67.36% by hydroxyl titration.

[0058] Example 9 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 43.9406 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 5.9240 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:1.5 to hydroxyl groups of the diol. 0.9976 g of p-toluenesulfonic acid catalyst is added at 2% of the total mass of the reactants. The reaction temperature is 100 °C, and the reaction time is 7 h.

[0059] The other steps and conditions are the same as in Example 1.

[0060] The esterification rate was determined to be 87.20% by hydroxyl titration.

[0061] Example 10 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 42.0520 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 5.6599 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:1.5 between the diol hydroxyl group and n-butyric acid. 0.7167 g of p-toluenesulfonic acid catalyst is added at 1.5% of the total mass of the reactants. The reaction temperature is 110 °C, and the reaction time is 8 h.

[0062] The other steps and conditions are the same as in Example 1.

[0063] The esterification rate was determined to be 83.94% by hydroxyl titration.

[0064] Example 11 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 40.1802 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 5.4041 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:1.5 to hydroxyl groups of the diol. 0.4591 g of p-toluenesulfonic acid catalyst is added at 1% of the total mass of the reactants. The reaction temperature is 120 °C, and the reaction time is 5 h.

[0065] The other steps and conditions are the same as in Example 1.

[0066] The esterification rate was determined to be 83.04% by hydroxyl titration.

[0067] Example 12 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 34.9253 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 4.6929 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:1.5 to hydroxyl groups of the diol and n-butyric acid. 0.2001 g of p-toluenesulfonic acid catalyst is added at 0.5% of the total mass of the reactants. The reaction temperature is 130 °C, and the reaction time is 6 h.

[0068] The other steps and conditions are the same as in Example 1.

[0069] The esterification rate was determined to be 68.87% by hydroxyl titration.

[0070] Example 13 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 48.180 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 6.8586 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:2 to hydroxyl groups of the diol and n-butyric acid. 0.5600 g of p-toluenesulfonic acid catalyst is added at 1% of the total mass of the reactants. The reaction temperature is 100 °C, and the reaction time is 8 h.

[0071] The other steps and conditions are the same as in Example 1.

[0072] The esterification rate was determined to be 81.49% by hydroxyl titration.

[0073] Example 14 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 43.8642 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 6.2024 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:2 to hydroxyl groups of the diol and n-butyric acid. 0.2603 g of p-toluenesulfonic acid catalyst is added at 0.5% of the total mass of the reactants. The reaction temperature is 110 °C, and the reaction time is 7 h.

[0074] The other steps and conditions are the same as in Example 1.

[0075] The esterification rate was determined to be 85.04% by hydroxyl titration.

[0076] Example 15 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 36.1944 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 6.4724 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:2 to hydroxyl groups of the diol and n-butyric acid. 0.8454 g of p-toluenesulfonic acid catalyst is added at 2% of the total mass of the reactants. The reaction temperature is 120 °C, and the reaction time is 6 h.

[0077] The other steps and conditions are the same as in Example 1.

[0078] The esterification rate was determined to be 81.68% by hydroxyl titration.

[0079] Example 16 A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding is described in Example 1. 40.2929 g of polycarbonate diol (commercially available, number average molecular weight 2500, hydroxyl value 48 mg KOH / g) and 7.2373 g of n-butyric acid (analytical grade, purity ≥99%) are added to a 100 ml three-necked flask at a molar ratio of 1:2 to hydroxyl groups of the diol and n-butyric acid. 0.7132 g of p-toluenesulfonic acid catalyst is added at 1.5% of the total mass of the reactants. The reaction temperature is 130 °C, and the reaction time is 5 h.

[0080] The other steps and conditions are the same as in Example 1.

[0081] The esterification rate was determined to be 95.18% by hydroxyl titration.

[0082] Evaluation of ability to reduce miscibility pressure A high-temperature and high-pressure interfacial tension measuring device was used to measure the CO2-kerosene interfacial tension under different pressures after adding 1.5 wt% polycarbonate surfactant at 75℃. Linear regression fitting was performed on the series of interfacial tension data, and the minimum miscibility pressure between CO2 and kerosene when the interfacial tension is 0 was calculated by extrapolation.

[0083] like Figure 3As shown, the interfacial tension was measured using a high-temperature and high-pressure interfacial tension measuring device. Linear regression fitting was performed on the interfacial tension data, and extrapolation was used to calculate the MMP of CO2-kerosene to be 15.55 MPa. The MMP of the system with 1.5 wt% polycarbonate diol butyrate surfactant was 14.4 MPa, a reduction of 7.4%; the MMP of the system with 1.5 wt% polycarbonate diol hexanoate surfactant was 14.0 MPa, a reduction of 10%; and the MMP of the system with 1.5 wt% polycarbonate diol octanoate surfactant was... The MMP of the surfactant system was 13.6 MPa, a reduction of 12.5%; the MMP of the system with 1.5 wt% polycarbonate diol decanoate surfactant was 13.1 MPa, a reduction of 15.8%. Since the polycarbonate chain provides a large number of ester groups and ether bonds, the polycarbonate surfactant in this embodiment has better affinity for CO2. At the same time, as the lipophilic carbon chain increases, the product's ability to reduce mixing also increases. It can be seen that the polycarbonate surfactant prepared by this invention has great potential for reducing mixing.

[0084] The above embodiments are merely exemplary embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art can make various modifications or equivalent substitutions to the present invention within its scope and spirit, and such modifications or equivalent substitutions should also be considered to fall within the scope of protection of the present invention.

Claims

1. A polycarbonate surfactant for reducing the minimum miscibility pressure in CO2 flooding, characterized in that, It is prepared from polycarbonate diol and fatty acid, and its general structural formula is shown in Formula I: ； Formula I Wherein, R is a straight-chain alkyl group, R0 is a short-chain diol residue, x and y are the number of structural units, x ranges from 2 to 15, and y ranges from 2 to 10; R0 is one of butanediol residue, pentanediol residue, and hexanediol residue.

2. The polycarbonate surfactant for reducing the minimum miscibility pressure in CO2 flooding according to claim 1, characterized in that, R is one of n-propyl, n-pentyl, n-heptyl, n-nonyl, undecyl, tridecyl, pentadecyl, and heptadecanyl.

3. The polycarbonate surfactant for reducing the minimum miscibility pressure in CO2 flooding according to claim 1, characterized in that, The molar ratio of hydroxyl groups to fatty acids in the polycarbonate diol is 1:

2.

4. A method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding, characterized in that, The preparation of the polycarbonate surfactant according to any one of claims 1-2 comprises the following steps: Step 1: Add the weighed polycarbonate diol and fatty acid to the drying reactor, and add the preset amount of catalyst. Shake the reactor to make the materials initially mixed. Step 2: Place the reactor equipped with a thermometer and stirrer in an oil bath, stir evenly, and then react under vacuum. Step 3: After the reaction is complete, wait for the reaction solution to cool naturally to 50-60°C, then add ethyl acetate to extract the product, so that the reaction product is fully dissolved in ethyl acetate. Step 4: Add 5% sodium bicarbonate solution to the organic phase, shake, let stand to separate the layers, and wash repeatedly until the organic phase is neutral; Step 5: Add sufficient color-changing silica gel to the organic phase obtained in step 4, dry it, filter the organic phase with qualitative filter paper, and collect the filtrate. Step 6: Rotary evaporate the collected filtrate, and vacuum dry the evaporated product to obtain a polycarbonate surfactant.

5. The method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding according to claim 4, characterized in that, The molar ratio of hydroxyl groups to fatty acids in the polycarbonate diol is 1:1 to 1:

2.

6. The method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding according to claim 4, characterized in that, The fatty acid is one of butyric acid, hexanoic acid, octanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, hexadecanoic acid, or octadecanoic acid.

7. The method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding according to claim 4, characterized in that, The catalyst in step 1 is one of p-toluenesulfonic acid, concentrated sulfuric acid, concentrated hydrochloric acid, and anhydrous sodium acetate.

8. The method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding according to claim 4, characterized in that, The catalyst mass is 0.5% to 2% of the total mass of the reactants.

9. The method for preparing a polycarbonate surfactant to reduce the minimum miscibility pressure in CO2 flooding according to claim 4, characterized in that, In step 2, the oil bath temperature is 100-130℃, the stirring speed is 400-500 rpm, and the reaction time is 5-8 hours.

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