A composite nonionic surfactant for reducing the minimum miscible pressure of crude oil and CO2 and its application

Abstract

The present invention relates to the field of oil production technology, specifically a compound nonionic surfactant for reducing the minimum miscible pressure of crude oil and CO2 and its application. The compound nonionic surfactant is prepared by mixing equal masses of Span 20 and 1712 fluorosurfactant SMF. Equal masses of Span 20 and 1712 fluorosurfactant SMF are stirred evenly, sealed and kept warm at 60°C for 3 hours to fully mix them to obtain the compound nonionic surfactant. The compound nonionic surfactant provided by the present invention can effectively reduce the interfacial tension between crude oil and CO2, effectively enhance the interaction energy between CO2 and crude oil, increase the solubility of CO2 in crude oil, thereby reducing the minimum miscible pressure of crude oil CO2 and crude oil. Wherein, hydrocarbon surfactants can promote the solubility of hydrocarbon surfactants in crude oil and CO2 under the synergistic effect of fluorosurfactants.

Description

Technical Field

[0001] The invention relates to the technical field of oil extraction, in particular to a composite nonionic surfactant for reducing the minimum miscible pressure of crude oil and CO2 and application thereof. Background Art

[0002] Since the Industrial Revolution, with the rapid development of the global economy and the surge in population, CO2 emissions have increased year by year. Before the Industrial Revolution, the average concentration of CO2 in the atmosphere was about 278 mg / L; in 2021, the average concentration of CO2 in the atmosphere rose to 414.3 mg / L, 149% of the pre-industrial level; according to research by NOAA (National Oceanic and Atmospheric Administration), CO2 can remain in the atmosphere for up to 200 years. Even if CO2 emissions into the atmosphere are stopped, the greenhouse effect produced by previously emitted CO2 will continue to exist for many generations. Therefore, large-scale capture of CO2 and efficient resource utilization technology have become a hot topic in the study of reducing the concentration of CO2 in the atmosphere.

[0003] At present, CO2 resource utilization technology is mainly focused on CO2 oil and gas production technology. According to the IEA (International Energy Agency), the world's oil resources suitable for CO2 enhanced oil recovery technology are about 3×10 8 ~6×10 8bbl, equivalent to 14% to 28% of the world's total recoverable oil resources. Many foreign oil fields have successively carried out CO2 flooding, and the results of field tests have shown that CO2 enhanced crude oil recovery technology is conducive to reservoir development and carbon sequestration. Among them, the effect of CO2 miscible drive on improving crude oil recovery is significantly higher than that of CO2 immiscible drive. As of 2020, there are about 142 active CO2 gas drive enhanced oil recovery projects in the United States, with a total of 23 operators involved. The annual CO2 gas drive enhanced oil recovery is close to 100 million barrels (about 13.43 million tons), mainly contributed by the following four regions: Permian production area (contribution of about 68%), southeastern Gulf of Mexico production area (contribution of about 14%), central production area (contribution of about 4%), and states along the Rocky Mountains (contribution of about 14%). At the same time, the gas sources of CO2 gas drive enhanced oil recovery projects are becoming more and more diversified. The progress of CO2 capture technology has made it possible to obtain CO2 gas sources from industrial stations (natural gas processing stations, fertilizer plants, ethanol processing plants, hydrogen fuel preparation plants, etc.) at a lower cost. At present, the annual CO2 injection volume in the United States is about 29.89 million tons, of which about 24.29 million tons come from natural CO2 gas reservoirs and about 5.61 million tons come from industrially captured CO2. The statistics show that the economic benefits are the main determining factor for CO2 flooding reservoirs implemented abroad. In order to achieve a higher oil recovery effect, CO2 enhanced oil recovery technology is mainly based on miscible flooding. For reservoirs that cannot be developed by CO2 miscible flooding, either other oil recovery methods should be considered for development, or certain measures or technical means should be taken to promote CO2 immiscible flooding to achieve miscible flooding or near-miscible flooding, so as to maximize the overall technical and economic effects of CO2 flooding.

[0004] With the deeper exploitation of oil reservoirs, a large number of oil reservoirs have entered the stage of tertiary oil recovery. In the process of CO2 flooding for some deep buried oil reservoirs and heavy crude oil, it was found that the minimum miscible pressure (MMP) of CO2 flooding in these oil reservoirs was high and higher than the formation fracture pressure, so that these oil reservoirs can only be flooded with CO2 immiscible phase. If the minimum miscible pressure of these oil reservoirs can be effectively reduced so as to carry out CO2 miscible flooding, the efficiency of CO2 flooding will be significantly improved, bringing huge economic benefits. Since the dissolution of CO2 in crude oil and the volume expansion of crude oil play an important role in the CO2 flooding process, if the dissolution of CO2 in crude oil and the volume expansion of crude oil can be increased under the same conditions, the miscibility of CO2 and crude oil will be further promoted and the minimum miscible pressure of CO2 and crude oil will be reduced.

[0005] Song Xinmin et al. pointed out in their study "Progress and Prospect of Carbon Dioxide Capture, Oil Displacement and Storage Technology in China's Petroleum" that although it is generally believed that C2-C6 is the key component that promotes miscibility in crude oil, their research innovatively proposed that C6-C 15It is also a key component that affects the miscibility of crude oil and CO2, which explains why the eastern oil reservoirs with low C2-C6 components can still achieve miscibility with CO2 under reservoir pressure. However, the miscibility reducer is greatly affected by the components of crude oil. Generally, a surfactant can only achieve a good effect of reducing the miscibility pressure in specific oil reservoirs. Therefore, for C6-C 15 Reservoir crude oil with high content urgently needs to develop a good performance reducer.

[0006] At present, the main methods for reducing the miscible pressure of CO2 flooding include adding co-solvents, supercritical CO2 microemulsion and CO2-philic non-ionic surfactants, each of which has its own advantages and disadvantages.

[0007] The method of adding co-solvents can achieve the purpose of reducing the miscible pressure of CO2 flooding, but it is difficult to carry out such applications in oil fields that lack hydrocarbon gases, which is not conducive to its popularization and use. In addition, the hydrocarbon gas injected into the reservoir may separate from the CO2 during the displacement process, thus failing to achieve the purpose of miscible oil recovery.

[0008] Supercritical CO2 microemulsion can effectively reduce the minimum miscibility pressure of crude oil-CO2, but the miscibility of surfactants and supercritical CO2 in supercritical CO2 microemulsion is poor, which limits its dissolution of macromolecules such as colloid asphaltene in crude oil.

[0009] Only when hydrocarbon non-ionic surfactants reach a certain solubility in CO2 and crude oil can they effectively reduce the minimum miscible pressure of CO2 flooding. In the study of "Study on Regulators for Reducing Minimum Miscibility Pressure of CO2 Flooding", Wu Sha et al. mixed 0.3% benzene and 0.2% ethylene glycol butyl ether, and the reduction rate was 17.86%; in the study of "Technical Research on Improving CO2 Oil Flooding Effect in Yushulin Oilfield", Liu Yingjie et al. found that the reduction rate of isobutyl citrate reached 22.29%; in the study of "Selection of Non-ionic Low Molecular Surfactants and Research on Oil Flooding Efficiency", the surfactant CAE synthesized by Guo Ping et al. can reduce the miscibility pressure by 22.3%; in the study of "Reducing the Miscibility Pressure in GasInjection Oil Recovery Processes", Rommerskirchen found that the reduction rate of miscibility pressure of five surfactants ME-1, ME-2, ME-4, ME-5 and ME-6 was 10%-18%; in the study of "Application and Mixing Mechanism of a New CO2 / Crude Oil Flooding Aid CAA8-X", Ma Cheng et al. found that CAA8-X can reduce the miscibility pressure by 20.5%.

[0010] The reduction range of the mixing agent in the existing research is mostly below 23%, so it is urgent to study a new type of mixing agent that can not only reduce C6-C 15Reservoirs with high component content and a mixing reducer with a high degree of mixing reduction.

[0011] At the same time, the downhole temperature varies according to the depth and location of the reservoir. When the reservoir temperature changes, how to ensure the demixing effect must also be taken into consideration. Summary of the invention

[0012] The purpose of the present invention is to solve the deficiencies of the above-mentioned prior art and provide a composite non-ionic surfactant for reducing the minimum miscibility pressure of crude oil and CO2 and its application, which is conducive to further promoting the development of CO2 oil recovery technology.

[0013] In order to achieve the above technical effects, the present invention adopts the following technical solutions:

[0014] A compound nonionic surfactant for reducing the minimum miscible pressure of crude oil and CO2 is prepared by mixing equal masses of Span 20 and 1712 fluorosurfactant SMF. The equal masses of Span 20 and 1712 fluorosurfactant SMF are stirred evenly, sealed and kept warm at 60°C for 3 hours to fully mix them, thereby obtaining the compound nonionic surfactant.

[0015] The compound nonionic surfactant in this study can effectively enhance the interaction energy between CO2 and crude oil and increase the solubility of CO2 in crude oil, thereby effectively improving the degree to which the compound nonionic surfactant reduces the miscibility pressure.

[0016] The present invention also provides the use of the above-mentioned composite nonionic surfactant for reducing the minimum miscible pressure of crude oil and CO2, comprising the following steps:

[0017] (1) Injection: First, the compound nonionic surfactant and supercritical CO2 (under the conditions of 31°C and 7.3MPa) are co-injected into the reservoir through an injection well, and the mass concentration of the compound nonionic surfactant is adjusted according to different reservoir temperatures;

[0018] (2) Well soaking: After the injection is completed, the well is soaked so that the compound non-ionic surfactant can diffuse into the reservoir to a greater depth with the supercritical CO2, thereby increasing the fluidity of the crude oil.

[0019] (3) Displacement: After the well is shut down, CO2 gas is injected to start oil displacement;

[0020] (4) When the daily oil production drops to 1 t / d, the current round of production is terminated and steps (1) to (3) are repeated until the cumulative oil production reaches the expected value and oil production is stopped.

[0021] Preferably, the mass of the compounded nonionic surfactant accounts for 0.5%-1.3% of the mass of the crude oil in the reservoir.

[0022] Preferably, when the temperature t of the reservoir is between 60°C≤t<75°C, a compound nonionic surfactant accounting for 0.5wt% of the mass of the crude oil in the reservoir is selected for injection; when the temperature of the reservoir is between 75°C≤t<90°C, a compound nonionic surfactant accounting for 0.8wt% of the mass of the crude oil in the reservoir is selected for injection; when the temperature of the reservoir is between 90°C≤t<105°C, a compound nonionic surfactant accounting for 1.0wt% of the mass of the crude oil in the reservoir is selected for injection; when the temperature of the reservoir is between 105°C≤t≤120°C, a compound nonionic surfactant accounting for 1.3wt% of the mass of the crude oil in the reservoir is selected for injection.

[0023] Preferably, in step (1), the volume ratio of the compounded nonionic surfactant to the supercritical CO2 is 1:(100-120); further preferably, the volume ratio of the compounded nonionic surfactant to the supercritical CO2 is 1:100.

[0024] Preferably, in step (2), the soaking time is 30 days.

[0025] Preferably, in step (3), a near-miscible drive is used. Experimental verification shows that the oil displacement efficiency of near-miscible drive is close to that of miscible drive, and near-miscible drive has lower injection pressure, less cost and higher economic benefits than miscible drive.

[0026] Preferably, in step (3), the injection pressure of CO2 is selected to be (0.85 to 0.9) MMP.

[0027] Preferably, the reservoir type is C6-C 15 The oil reservoir has a content of 50%-90%; further preferably, the reservoir type is C6-C 15 Oil reservoirs with a content of 70%-90%.

[0028] Beneficial effects of the present invention:

[0029] (1) The composite nonionic surfactant for reducing the minimum miscible pressure between crude oil and CO2 provided by the present invention can effectively reduce the interfacial tension between crude oil and CO2, effectively enhance the interaction energy between CO2 and crude oil, and increase the solubility of CO2 in crude oil, thereby reducing the minimum miscible pressure between crude oil, CO2 and crude oil. Among them, the hydrocarbon surfactant can promote the solubility of the hydrocarbon surfactant in crude oil and CO2 under the synergistic effect of the fluorosurfactant.

[0030] (2) The present invention is directed to crude oil components C6-C 15 88.78% of the oil reservoirs can effectively reduce their minimum miscibility pressure for CO2 flooding, and the reduction range reaches 25.35%.

[0031] (3) The compound nonionic surfactant provided by the present invention has a large temperature applicability range and can be used in different concentrations within the range of 60 to 120°C, achieving a good demixing effect at different reservoir temperatures.

[0032] (4) The near-miscible drive used in the present invention during injection can achieve a recovery factor that is almost the same as that of the miscible drive. At the same time, the near-miscible drive can reduce the injection pressure of MMP by (10% to 15%) on the basis of reducing the miscible pressure by 25.35%. The cost required for the near-miscible drive is less than that of the miscible drive, and the economic benefit is higher. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a bar graph showing the degree to which different surfactants reduce the miscible pressure in Experimental Example 2;

[0034] Figure 2 This is a bar graph showing the degree of reduction of miscible pressure by different proportions of Span20 and 1712 fluorosurfactant SMF in the compound nonionic surfactant of Experimental Example 3;

[0035] Figure 3 This is a bar graph showing the degree to which the miscible pressure is reduced by compounding nonionic surfactants at different concentrations in Experimental Example 4;

[0036] Figure 4 This is a bar chart of crude oil recovery in Experimental Example 6 without adding a composite nonionic surfactant;

[0037] Figure 5 This is a bar graph of crude oil recovery factor when 1.0 wt% of a composite nonionic surfactant is added to Experimental Example 6;

[0038] Figure 6 This is a visualization diagram of oil-gas miscibility without adding surfactant in simulation example 1;

[0039] Figure 7 This is a visualization diagram of the oil-gas miscibility when 1.0 wt% of a compound nonionic surfactant is added to Simulation Example 1. DETAILED DESCRIPTION

[0040] The compound nonionic surfactant for reducing the minimum miscibility pressure of crude oil and CO2 is prepared by the following method: equal masses of Span 20 and 1712 fluorosurfactant SMF are added into a container and stirred evenly, and the mixture is sealed and placed in a 60°C oven for 3 hours to mix and then dissolved in water to obtain the compound nonionic surfactant (the compound nonionic surfactant is referred to as SF for short).

[0041] The specific implementation plan includes the following steps:

[0042] (1) Injection: Inject the demixing agent SF and supercritical CO2 in a volume ratio of 1:(100-120) into the well.

[0043] (2) Well soaking: After the injection is completed, the well is soaked for 30 days to allow SF to diffuse into the reservoir to a greater depth with the supercritical CO2, thereby increasing the fluidity of the crude oil.

[0044] (3) After the well is shut down, CO2 gas is injected to start oil recovery. The injection pressure of CO2 is selected to be (0.85-0.9) MMP, and a near-miscible drive method is adopted.

[0045] (4) When the daily oil production drops to 1 t / d, the current round of production is terminated and steps (1) to (3) are repeated until the cumulative oil production reaches the expected value.

[0046] When the temperature t of the reservoir is between 60°C≤t<75°C, a compound nonionic surfactant accounting for 0.5wt% of the mass of the crude oil in the reservoir is selected for injection; when the temperature of the reservoir is between 75°C≤t<90°C, a compound nonionic surfactant accounting for 0.8wt% of the mass of the crude oil in the reservoir is selected for injection; when the temperature of the reservoir is between 90°C≤t<105°C, a compound nonionic surfactant accounting for 1.0wt% of the mass of the crude oil in the reservoir is selected for injection; when the temperature of the reservoir is between 105°C≤t≤120°C, a compound nonionic surfactant accounting for 1.3wt% of the mass of the crude oil in the reservoir is selected for injection.

[0047] Experimental Example 1

[0048] Experimental oil carbon number distribution determination

[0049] Source of experimental oil: crude oil from a low-permeability reservoir.

[0050] Table 1

[0051] Crude oil components Mole fraction / % <![CDATA[C2-C6]]> 0.0882 <![CDATA[C6-C 15 ]]> 88.7812 <![CDATA[C 15+ ]]> 11.1306 total 100

[0052] Table 1 is the C2-C 15 The crude oil used this time is C6-C 15 The crude oil component content is as high as 88.7812%. The following experiments are all carried out based on this experimental oil.

[0053] Experimental Example 2

[0054] Screening of nonionic decomposition agents by using the interfacial tension vanishing method

[0055] 1. Purpose of the experiment

[0056] The IFT (equilibrium interfacial tension) changes with pressure after eight surfactants were added to crude oil were tested. The MMP of crude oil after adding different surfactants was measured using the interfacial tension disappearance method, and the downmixing rate was calculated.

[0057] 2. Experimental conditions

[0058] (1) Experimental temperature: 90°C;

[0059] (2) Experimental gas: carbon dioxide, purity 99.99%, produced by Qingdao Xinkeyuan Technology Co., Ltd.

[0060] (3) Experimental nonionic surfactants: 1712 Fluorosurfactant SMF is a nonionic fluorinated surfactant with high surface activity, which can effectively reduce the IFT of oil and gas. It was purchased from Fujian Sanmei New Materials Technology Co., Ltd. FCO-80 is a nonionic fluorinated surfactant that is easily soluble in organic solvents such as ketones and halogenated hydrocarbons. It has extremely high surface activity, thermal stability, chemical stability and good compatibility. Surfactant FCO-80 can reduce the IFT of gas and liquid, enhance the dispersibility, leveling and wettability of oil-based systems; therefore, they are also widely used in cleaning, coating and other industries. It was purchased from Guangzhou Jieluhua Biotechnology Co., Ltd., China. Span20 / 60 / 80 is usually used as a stationary liquid, emulsifier and wetting agent for gas chromatography, and was purchased from Sinopharm Co., Ltd.; APEO (alkylphenol polyoxyethylene ether), Linyi Yiqun Chemical, content 99%; AEO-9 (fatty alcohol polyoxyethylene ether) was purchased from Wuxi Sandray Technology Development Co., Ltd., with an active matter content of ≥99%, and is a colorless and transparent liquid; SPO-15 (fatty alcohol polyoxypropylene ether), purchased from Hai'an Petrochemical Plant, Jiangsu Province.

[0061] (4) Experimental oil: Experimental Example 1 used crude oil;

[0062] (5) Experimental instrument: Tracker-H interfacial tension meter, manufactured by TECLIS, France, with a maximum working pressure of 20 MPa and a maximum working temperature of 200 °C.

[0063] 3. Experimental steps: 1) Clean the pipeline and high-temperature and high-pressure reactor. 2) Inhale deionized water into the needle tube and calibrate the interfacial tension meter at room temperature and pressure. 3) Connect the experimental instrument, heat the high-temperature and high-pressure reactor to the experimental temperature (90°C), inject CO2 gas to the experimental pressure, and check the airtightness of the device. 4) Add surfactant to the crude oil, stir it thoroughly, seal it, and place it in an oven for 12 hours to fully dissolve it. 5) After the syringe barrel is filled with crude oil sample, fix it on the syringe holder and install it in the high-temperature and high-pressure reactor. Use carbon dioxide gas to evacuate the air in the reactor, and adjust the height of the instrument base so that the needle appears in the visual window. 6) Set the temperature in the reactor to the experimental temperature, use a carbon dioxide intermediate container to pressurize the reactor, and pay attention to adding carbon dioxide into the reactor at a slow speed during the pressurization process. 7) When the temperature and pressure in the entire reactor are in a stable state, control the drive motor to press out a suspended 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. 8) Change the pressure, re-extrude the droplet, repeat the above operation, and obtain the interfacial tension data at different pressure points. 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 obtained, which is the minimum miscibility pressure of the crude oil and CO2 system.

[0064] 4. Experimental Results and Analysis

[0065] Each nonionic surfactant was added to crude oil at a concentration of 1 wt % and the crude oil downmixing rate was measured (see Figure 1 ). From the results, it can be concluded that SMF and Span20 have the best downmixing effect and can reach 15.42% and 15.76% downmixing rates. Therefore, 1712 fluorosurfactant SMF and Span20 were finally selected for subsequent compounding.

[0066] Experimental Example 3

[0067] Compounding of Span20 and 1712 fluorosurfactant SMF in different proportions

[0068] 1. Experimental purpose: To test Span20 and 1712 fluorosurfactant SMF with a total mass concentration of 1wt% (the proportion of compounded non-ionic surfactant to the total mass of crude oil, the same below), the IFT (equilibrium interfacial tension) after adding different mass ratios of crude oil changes with pressure, and the MMP after adding different proportions of surfactants to crude oil is measured by the interfacial tension disappearance method, and the mixing reduction rate is calculated.

[0069] 2. Experimental conditions

[0070] (1) Experimental temperature: 90°C;

[0071] (2) Experimental gas: carbon dioxide, purity 99.99%, produced by Qingdao Xinkeyuan Technology Co., Ltd.

[0072] (3) Nonionic surfactants used in the experiment: Span 20, 1712 fluorosurfactant SMF;

[0073] (4) Experimental oil: Experimental Example 1 used crude oil;

[0074] (5) Experimental instrument: Tracker-H interfacial tension meter, manufactured by TECLIS, France, with a maximum working pressure of 20 MPa and a maximum working temperature of 200 °C.

[0075] 3. Experimental steps:

[0076] The steps are the same as those in Experimental Example 1.

[0077] 4. Experimental Results and Analysis

[0078] like Figure 2 As shown in the figure, by compounding with seven mass ratios of 2:8, 3:7, 4:6, 1:1, 6:4, 7:3, and 8:2, it was finally determined that when the total mass concentration of Span20 and 1712 fluorosurfactant SMF was 1wt% and the mass ratio of Span20 to SMF was 1:1, the downmixing rate was 25.35%, and the downmixing effect was the best. Moreover, the downmixing rate that can be achieved by the compounded SF is much higher than the downmixing rate that can be achieved when Span20 or SMF is used alone, which shows that the compounding of the two achieves a synergistic effect and promotes SF to further improve the downmixing rate.

[0079] Experimental Example 4

[0080] Compounding of Span20 and 1712 Fluorosurfactant SMF at different concentrations

[0081] 1. Experimental purpose: To test the IFT (equilibrium interfacial tension) of Span20 and 1712 fluorosurfactants SMF with equal mass ratio after adding them into crude oil at different mass concentrations, and to measure the MMP of crude oil after adding different proportions of surfactants by interfacial tension vanishing method, and calculate the downmixing rate.

[0082] 2. Experimental conditions

[0083] (1) Experimental temperature: 90°C;

[0084] (2) Experimental gas: carbon dioxide, purity 99.99%, produced by Qingdao Xinkeyuan Technology Co., Ltd.

[0085] (3) Experimental nonionic surfactants: Span20, 1712 fluorosurfactant SMF;

[0086] (4) Experimental oil: Experimental Example 1 used crude oil;

[0087] (5) Experimental instrument: Tracker-H interfacial tension meter, manufactured by TECLIS, France, with a maximum working pressure of 20 MPa and a maximum working temperature of 200 °C.

[0088] 3. Experimental steps:

[0089] The steps are the same as those in Experimental Example 1.

[0090] 4. Experimental Results and Analysis

[0091] like Figure 3 As shown, by mixing the compound nonionic surfactants with six concentration ranges of 0.25%, 0.5%, 0.8%, 1.0%, 1.3% and 1.5% with crude oil, the final test was carried out. When equal masses of Span20 and 1712 fluorosurfactant SMF were compounded into SF, when the experimental conditions were limited to 90°C, when the compound nonionic surfactant with a mass concentration of 1.0wt% was added to the crude oil, the downmixing effect was the best, and the downmixing rate was 25.35%.

[0092] Experimental Example 5

[0093] Study on the temperature adaptability of 0.5wt%, 0.8wt%, 1.0wt% and 1.3wt% SF composite surfactants by using the interfacial tension vanishing method

[0094] 1. Experimental purpose: To test the degree to which SF reduces miscible pressure at different temperatures and concentrations.

[0095] 2. Experimental conditions

[0096] (1) Experimental temperature: 60℃, 75℃, 90℃, 105℃, 120℃;

[0097] (2) Experimental gas: carbon dioxide, purity 99.99%, produced by Qingdao Xinkeyuan Technology Co., Ltd.

[0098] (3) Experimental nonionic surfactants: 0.5 wt% SF, 0.8 wt% SF, 1.0 wt% SF, and 1.3 wt% SF;

[0099] (4) Experimental oil: Experimental Example 1 used crude oil;

[0100] (5) Experimental instrument: Tracker-H interfacial tension meter, manufactured by TECLIS, France, with a maximum working pressure of 20 MPa and a maximum working temperature of 200 °C.

[0101] 3. Experimental steps:

[0102] After adding SF of different concentrations into crude oil and stirring thoroughly, the crude oil is placed in a constant temperature oven for a period of time to ensure that the surfactant is evenly dispersed in the crude oil. After that, the crude oil samples are tested for IFT changes with pressure at different IFT temperatures.

[0103] 4. Experimental Results and Analysis

[0104] At higher temperatures, the energy of oil and gas molecules is higher, and the forces between molecules are more intense, so more surfactant molecules are needed to achieve a better mixing effect. However, when the temperature is low, the forces between oil and gas molecules are weakened, and excessive surfactant molecules will combine with CO2 molecules, reducing the dissolution of CO2 in crude oil. Table 2 shows the surfactant concentration with the best effect of reducing the miscible pressure at each temperature after adding different concentrations of surfactants. By comparing the MMP values ​​at different temperatures, from the final results, when the temperature t is between 60℃≤t<75℃, the SF with a mass concentration of 0.5% has a better mixing effect; when the temperature t is between 75℃≤t<90℃, the SF with a mass concentration of 0.8% has a better mixing effect; when the temperature t is between 90℃≤t<105℃, the SF with a mass concentration of 1.0% has a better mixing effect; when the temperature t is between 105℃≤t≤120℃, the SF with a mass concentration of 1.3% has a better mixing effect. Therefore, the compound surfactant SF has a larger temperature adaptability range, and the compound surfactant SF can be used from 60 to 120°C, and can at least ensure a 24.31% reduction rate.

[0105] Table 2

[0106] Temperature / ℃ Concentration / wt% Downmix rate / % 60 0.5 24.31 75 0.8 24.94 90 1.0 25.35 105 1.3 25.03 120 1.3 24.93

[0107] Experimental Example 6

[0108] The recovery factor changes at different pressures after adding 1wt% surfactant SF to crude oil were determined by one-dimensional core flooding experiment.

[0109] 1. Experimental purpose: To test the change of oil recovery from near-miscible to miscible pressure after adding surfactant SF, so as to verify the increase of oil recovery by surfactant SF in actual production process.

[0110] 2. Experimental conditions

[0111] (1) Experimental temperature: 90°C;

[0112] (2) Experimental gas: carbon dioxide, purity 99.99%, produced by Qingdao Xinkeyuan Technology Co., Ltd.

[0113] (3) Experimental nonionic surfactant: 1.0 wt% SF;

[0114] (4) Experimental oil: Experimental Example 1 used crude oil;

[0115] (5) Experimental core: natural core, water permeability 30-40 mD;

[0116] (6) Experimental instruments: core holder, back pressure valve, plunger pump, pressure collector, intermediate container;

[0117] (7) Displacement pressure point: The displacement experiment was designed with two control experiments, one with and one without the addition of surfactant SF to crude oil. Five displacement pressure points were set for each experiment. At the same time, three immiscible pressure points, one near-miscible pressure point, and one miscible pressure point were selected for each experiment.

[0118] The MMP value of crude oil and CO2 without the addition of surfactant SF is 23.90MPa, so the miscible pressure point needs to be greater than 23.90MPa, and is selected as 24.51MPa based on actual tests; the near-miscible pressure point is 0.9 times the MMP, and is selected as 21.53MPa based on actual tests; and the three immiscible pressure points can be far less than the miscible pressure, which are 16MPa, 10.06MPa and 6.02MPa respectively.

[0119] Similarly, after adding surfactant SF to the crude oil, the MMP of crude oil and CO2 was reduced to 17.84MPa. The same oil production method was used to select the miscible drive pressure point as 18.84MPa, the near-miscible drive pressure point as 16.06MPa, the immiscible pressure points as 12.02MPa, 6.02MPa and 4.00MPa.

[0120] 3. Experimental steps:

[0121] (1) Evacuate the core and saturate it with water to measure the pore volume and water permeability;

[0122] (2) Carry out oil-water displacement at the experimental temperature to determine the oil saturation and irreducible water saturation of the core;

[0123] (3) Inject CO2 for gas drive. Stop injection after accumulating 2.0 PV of CO2 gas. Close the valves at both ends of the injection and production, and then start depressurization production after the well is shut down for half an hour.

[0124] 4. Experimental Results and Analysis

[0125] Before adding surfactant, the recovery rates of near-miscible and miscible flooding were 78.06% and 81.38%, respectively (see Figure 4 ), after adding surfactant SF, CO2 recovery increased to 88.8% and 89.97% (see Figure 5). The recovery factors of near-miscible and miscible drives are similar, and the crude oil recovery factor is greatly improved after adding surfactants. Based on the above experimental research results, it is believed that near-miscible drive can also obtain an oil recovery factor close to that of miscible drive, while reducing the cost of CO2 injection equipment. In relevant literature research, some studies set the pressure range of near-miscible drive at (0.8-1.0) MMP, but there is no fixed definition standard. This test selected 0.9 MMP as the near-miscible drive pressure point, and the obtained crude oil recovery factor is very close to the miscible drive crude oil recovery factor. Therefore, according to the experimental results, it is predicted that the final near-miscible drive pressure range should be greater than (0.9-1.0) MMP, so it is finally determined that the near-miscible drive pressure range is between (0.85-1.0) MMP. Therefore, after adding the mixing reducer, the use of near-miscible drive can not only ensure a higher recovery factor, but also reduce the displacement pressure of (10% to 15%) MMP. It can greatly increase the production of crude oil and reduce the investment in ground gas injection equipment.

[0126] Simulation Example 1

[0127] The microscopic mechanism of reducing the miscibility pressure of the surfactant SF after adding 1wt% SF to crude oil was tested by molecular simulation software. 1. Simulation purpose: To visually see the miscibility of the added surfactant at the molecular scale through simulation.

[0128] 2. Simulation conditions

[0129] (1) Simulation temperature: 90°C;

[0130] (3) Molecular formula of nonionic surfactant used in the experiment: Span20:C 18 H 34 O6; 1712 Fluorosurfactant SMF: C 25 H 37 9F 15 ;

[0131] 3. Simulation steps:

[0132] (1) First, use the Visualizer Tools module to construct the oil phase molecules, CO2 molecules, and surfactant molecules

[0133] (2) The AC module was then used to build a box consisting of an oil phase box, a CO2 box, and a box for molecular dynamics simulation. The model consists of an oil phase and a gas phase, where the size of the oil phase and the gas phase are both 5×5×5nm. 3 The size of the model used for molecular dynamics simulation is 5×5×10 nm. 3 .

[0134] (3) The actual crude oil C3 to C9 are simplified to C9 and C9 in the oil phase model. 14to C 35 In the oil phase model, it is simplified to C 22 , then after correction, C9, C 10 , C 11 , C 12 , C 13 , C 22 The mass concentrations of Span 20 and SMF were set to 3.8wt%, 14.7wt%, 24.7wt%, 28.1wt%, 22.8wt% and 5.9wt%, respectively, where the amount of surfactant was added according to the total mass concentration of about 1wt%, and when SF was added, it was added according to the mass concentration ratio of Span 20 and SMF of 1:1.

[0135] (4) The model uses a constant temperature and pressure ensemble (NPT). The number of particles, temperature and pressure in the ensemble remain constant. The temperature of the system is controlled by the Nosé-Hoover temperature control method, and the pressure is controlled by the Berendsen method.

[0136] 4. Simulation Results and Analysis

[0137] In the visualization results of the simulation, the red represents CO2 molecules, the gray represents oil phase molecules, and at the end of the simulation, no surfactant molecules were added to the simulation box, and there was still an obvious phase interface (see Figure 6 After adding SF, the oil and gas phases in the simulation box are highly miscible (see Figure 7 ).

[0138] By analyzing the oil-gas interaction energy after adding surfactant molecules, it can be learned that the average interaction energy of the oil-gas system without adding surfactant is -1002.29kCal / mol, and the average interaction energy of the system after adding Span 20, 1712 fluorosurfactants SMF and SF is -1128.75kCal / mol, -1252.68kCal / mol, and -1519.17kCal / mol respectively. When one of the compound surfactants is added alone, the interaction energy between oil and gas is less than the oil-gas mixing energy after adding the compound surfactant, so the compound surfactant can more effectively promote the interaction between oil and gas molecules and promote oil-gas miscibility.

[0139] The coordination number of surfactants and CO2 was then studied. It was found that after adding Span20, 1712 fluorosurfactants SMF and SF, the coordination numbers of the system were 4.95, 5.37 and 8.43, respectively. The more electronegative fluorine atoms in the 1712 fluorosurfactant SMF can have specific interactions with the electron-deficient carbon atoms in CO2, increasing the interaction force between the surfactant and CO2, thereby increasing the solubility.

Claims

1. A composite nonionic surfactant for reducing the minimum miscible pressure of crude oil and CO2, characterized in that: The compound nonionic surfactant is prepared by mixing equal masses of Span 20 and 1712 fluorosurfactant SMF. Equal masses of Span 20 and 1712 fluorosurfactant SMF are stirred evenly, sealed and kept warm at 60°C for 3 hours to fully mix them, thereby obtaining the compound nonionic surfactant.

2. An application of the compound nonionic surfactant as claimed in claim 1, characterized in that: The steps include: (1) Injection: First, the compound nonionic surfactant and supercritical CO2 are injected into the reservoir through the injection well, and the mass concentration of the compound nonionic surfactant is adjusted according to different reservoir temperatures; (2) Well soaking: Well soaking after injection is completed; (3) Displacement: After the well is shut down, CO2 gas is injected to start oil displacement; (4) When the daily oil production drops to 1 t / d, the current round of production is terminated and steps (1) to (3) are repeated until the cumulative oil production reaches the expected value and oil production is stopped.

3. The use according to claim 2, characterized in that The mass of the compound nonionic surfactant accounts for 0.5%-1.3% of the mass of crude oil in the reservoir.

4. The use according to claim 2, characterized in that When the temperature t of the reservoir is between 60°C≤t<75°C, a compound nonionic surfactant accounting for 0.5wt% of the mass of the crude oil in the reservoir is selected for injection; when the temperature of the reservoir is between 75°C≤t<90°C, a compound nonionic surfactant accounting for 0.8wt% of the mass of the crude oil in the reservoir is selected for injection; when the temperature of the reservoir is between 90°C≤t<105°C, a compound nonionic surfactant accounting for 1.0wt% of the mass of the crude oil in the reservoir is selected for injection; when the temperature of the reservoir is between 105°C≤t≤120°C, a compound nonionic surfactant accounting for 1.3wt% of the mass of the crude oil in the reservoir is selected for injection.

5. The use according to claim 2, characterized in that In the step (1), the volume ratio of the compounded nonionic surfactant to the supercritical CO2 is 1:(100-120).

6. The use according to claim 5, characterized in that In the step (1), the volume ratio of the compounded nonionic surfactant to the supercritical CO2 is 1:

100.

7. The use according to claim 2, characterized in that In the step (2), the soaking time is 30 days.

8. The use according to claim 2, characterized in that In the step (3), a near-miscible phase drive is adopted.

9. The use according to claim 8, characterized in that In the step (3), the injection pressure of CO2 is selected to be (0.85 to 0.9) MMP.

10. The use according to any one of claims 2 to 9, characterized in that: Reservoir type is C6-C 15 Oil reservoirs with a content of 50%-90%.

11. The use according to claim 10, characterized in that Reservoir type is C6-C 15 Oil reservoirs with a content of 70%-90%.

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