A surfactant system, its preparation method, application and in-situ emulsification assisted CO2 flooding method

Abstract

The application discloses a surfactant system and a preparation method and application thereof and a method for in-situ emulsification assisted CO2 flooding. The system comprises water and 0.1-0.5wt% of surfactants in the water; wherein the surfactants are any ratio mixture of betaine surfactants, spreading surfactants and sulfonate surfactants, and 0.01-5wt% of additives are further included in the system. The surfactant system of the application starts from the interfacial film strength of a single surfactant, adjusts the stability of the emulsion by changing the compounding ratio of multiple surfactants, and the in-situ emulsification assisted CO2 flooding surfactant system with controllable CO2 strength developed by the application can realize emulsification plugging of CO2 and reduce the oil-water interfacial tension at the same time, and has a wide application prospect. The in-situ emulsification assisted CO2 flooding surfactant system provided by the application has excellent enhanced oil recovery effect for CO2 flooding reservoirs, and the in-situ emulsification assisted CO2 flooding recovery rate of core displacement can be increased by more than 10% compared with CO2-WAG flooding.

Description

Technical Field

[0001] This invention relates to a surfactant system, its preparation method, application, and an in-situ emulsification-assisted CO2 flooding method. Background Technology

[0002] CO2 is a relatively inexpensive gas that can interact with crude oil, thus benefiting crude oil development. CO2 dissolves in crude oil, causing it to expand in volume, decrease in viscosity, and reduce the oil-water interfacial tension, thereby improving its fluidity. When reservoir pressure is above 9.6 MPa and temperature is below 105℃, CO2 gas, through repeated contact with crude oil, continuously extracts light components, increasing the concentration of these light components and gradually achieving a near-miscible or miscible state. Furthermore, CO2 injection reduces air pollution and mitigates the greenhouse effect, contributing to environmental protection and representing an enhanced oil recovery technology for the rational use of resources. However, CO2 injection development is influenced by numerous factors, such as reservoir heterogeneity and unfavorable mobility ratios between CO2 and crude oil. These issues can lead to CO2 leakage, and once leakage channels form, the gas flows along these channels, making it difficult to reach the oil-rich matrix, resulting in ineffective gas circulation and severely impacting reservoir development. Therefore, effectively blocking gas channeling is the primary issue in gas injection development (Yuan Shiyi, Petroleum Exploration and Development, 2022, 49(04): 828-834. Peng Hao, China University of Petroleum (Beijing), 2018).

[0003] Currently, surfactants are commonly used adjuvants to enhance oil recovery through CO2 flooding. On one hand, surfactants can generate foam upon contact with CO2, effectively blocking high-permeability channels and expanding the swept volume. On the other hand, surfactants also reduce interfacial tension, emulsify and disperse crude oil, and improve wettability during the oil displacement process. Based on these needs, the performance of surfactant systems focuses on reducing interfacial tension, foaming and foam stability, emulsion stability, wettability, viscosity reduction, miscibility reduction, and solubilization. However, most studies often neglect the impact of CO2 dissolution in the oil-water phase and fail to consider the system composition and effectiveness analysis from the perspective of in-situ emulsification-assisted CO2 flooding.

[0004] Secondly, the surfactant system used to assist CO2 in generating foam to improve oil recovery is one that can adsorb at the gas-liquid interface to reduce surface tension, thereby generating stable foam during the flow of CO2 and solution in porous media, thus blocking gas channeling. Some articles also consider the foam formed by supercritical CO2 and water to be an "emulsion" (Song Xinmin, Petroleum Exploration and Development, 2023, 50(01):206-218). To achieve the above effects, the surface tension, foaming height, foam half-life, foam seepage process, and foam displacement effect of the surfactant are usually considered, and surfactants with strong surface tension reduction ability, high foaming height, long foam half-life, large foam seepage resistance, and good foam displacement effect are preferred. However, this system usually ignores the consideration of the properties of the oil-water emulsion. Summary of the Invention

[0005] In order to at least partially solve the above-mentioned technical problems existing in the prior art and further enrich the types of surfactant systems used for in-situ emulsification-assisted CO2 flooding, the present invention provides an in-situ emulsification-assisted CO2 flooding surfactant combination system with high stability and strong pore throat sealing ability, thereby improving the recovery rate of CO2 flooding.

[0006] As one aspect of the present invention, a surfactant combination system is provided, the combination system comprising water and a surfactant comprising 0.1 to 0.5 wt% of water; wherein the surfactant is a mixture of betaine-type surfactants, spreadable surfactants and sulfonate surfactants in any ratio.

[0007] In one or more possible embodiments, the mass ratio of the betaine-type surfactant, the extended surfactant, and the sulfonate surfactant is (0-10):(0-10):(0-10).

[0008] In one or more possible embodiments, the betaine-type surfactant is alkyl sulfobetaine;

[0009] The alkyl sulfonyl betaine is as shown in formula (I): In formula (I), R is selected from long-chain alkyl groups of C12 to C18.

[0010] In one or more possible embodiments, the extended surfactant is an alkyl alcohol polyoxypropylene polyoxyethylene sulfonate;

[0011] The alkyl alcohol polyoxypropylene polyoxyethylene sulfonate is as shown in formula (II):

[0012]

[0013] In formula (II): n is one of 2, 4, 6, 8 or 10, and R is selected from long-chain alkyl groups of C12 to C18.

[0014] In one or more possible embodiments, the sulfonate surfactant is a petroleum sulfonate;

[0015] The petroleum sulfonate is a mixture of the first-line, second-line, and third-line products in any ratio.

[0016] In one or more possible embodiments, the mixing ratio of the reduced first-line product, the reduced second-line product and the reduced third-line product is (0-5):(0-10):(0-10).

[0017] In one or more possible embodiments, the combined system further includes 0.01 to 5 wt% of additives.

[0018] In one or more possible embodiments, the additive is an alkaline substance or a neutral salt.

[0019] In one or more possible embodiments, when the additive is an alkaline substance, the mass fraction of the additive added to the combined system is 0.01 to 0.3%.

[0020] When the additive is a neutral salt, the mass fraction of the additive added to the combined system is 0.1% to 5%.

[0021] When the additive is a mixture of alkaline substances and neutral salts, the mass fraction of the additive added to the system is 0.01% to 5%.

[0022] In one or more possible embodiments, the alkaline substance is selected from one or more of sodium carbonate, sodium bicarbonate, or sodium hydroxide;

[0023] The neutral salt is selected from one or more of sodium chloride, potassium chloride, calcium chloride, magnesium chloride, or sodium sulfate.

[0024] As another aspect of the present invention, a method for preparing the above-mentioned surfactant combination system is provided, the method comprising: mixing the surfactant, the additive and the water to obtain an emulsion surfactant combination system.

[0025] As another aspect of the present invention, it relates to the application of the above-mentioned surfactant combination system in in-situ emulsification-assisted CO2 flooding.

[0026] As another aspect of the present invention, a method for in-situ emulsification-assisted CO2 displacement of oil is provided, wherein the method uses the above-mentioned surfactant combination system to perform in-situ emulsification-assisted CO2 displacement of oil.

[0027] In one or more possible embodiments, the reservoir conditions during the in-situ emulsification-assisted CO2 flooding are as follows:

[0028] The total salinity of water in the reservoir is 3000–80000 mg / L;

[0029] The reservoir temperature is 30–100℃;

[0030] The viscosity of the crude oil in the reservoir at the specified reservoir temperature is 1–20 cp;

[0031] The permeability of the reservoir is 0.01–50 mD.

[0032] The surfactant combination system provided by this invention, starting from the interfacial film strength of a single surfactant, regulates the stability of the emulsion by changing the compound ratio of multiple surfactants. The in-situ emulsification-assisted CO2 displacement surfactant combination system with controllable emulsification and CO2 blocking strength developed by this invention can reduce the oil-water interfacial tension while achieving emulsification and CO2 blocking, and has broad application prospects.

[0033] The in-situ emulsification-assisted CO2 flooding surfactant combination system provided by this invention has excellent enhanced oil recovery effect for CO2 flooding reservoirs. The in-situ emulsification-assisted CO2 flooding recovery rate of core displacement can be increased by more than 10% compared with CO2-WAG flooding (gas-water flooding).

[0034] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention are realized and obtained in accordance with the structures particularly pointed out in the description, claims and drawings. Attached Figure Description

[0035] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0036] Figure 1 This is a graph showing the variation trend of the interfacial expansion modulus of the surfactant system in an embodiment of the present invention with periodic oscillations.

[0037] Figure 2 This is a graph showing the trend of water separation rate of the surfactant system as a function of shaking time in an embodiment of the present invention.

[0038] Figure 3 This is a partial phenomenon diagram of a visualization microscopic oil displacement experiment in Embodiment 2 of the present invention; wherein, the blank part simulates the rock skeleton, the dark shaded part is the oil phase (internal phase), and the light shaded part is the water-in-oil emulsion droplets generated in situ by the external phase. Detailed Implementation

[0039] Exemplary embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0040] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0041] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0042] In the description of this invention, it should be noted that the terms "comprising", "including", "having", "containing", etc., are all open-ended terms, meaning that they include but are not limited to.

[0043] The betaine surfactant used in the examples is an industrial-grade product, with an effective content generally of 40%. This substance is selected from C12-C18 long-chain alkyl sulfonyl betaines, wherein the long-chain alkyl group can be dodecyl, hexadecyl, or octadecyl, etc.

[0044] The extended surfactant used in the examples is an industrial-grade product with an effective content of 50%. The substance is selected from C12 to C18 long-chain alkyl alcohol polyoxypropylene sulfonate, wherein the long-chain alkyl can be dodecyl, hexadecyl or octadecyl, etc., and the number of addition segments of the polyoxypropylene group can be any one of 2, 4, 6, 8 or 10.

[0045] The petroleum sulfonate surfactants used in the examples are industrial-grade products, with an effective content generally of 50%. This substance consists of first-line, second-line, and third-line surfactants, mixed in a ratio of (0–5):(0–10):(0–10), including but not limited to 1:4:7, 2:5:3, 2:3:6, etc.

[0046] The surfactant system prepared in the examples, when used for CO2 flooding, has the following reservoir conditions: total water salinity in the reservoir is 3000–80000 mg / L; reservoir temperature is 30–100°C; crude oil viscosity at the reservoir temperature is 1–20 cp; and reservoir permeability is 0.01–50 mD. In any embodiment, this includes, but is not limited to, a total water salinity of 20000 mg / L, a reservoir temperature of 70°C, crude oil viscosity of 3.1 cp at the reservoir temperature, and reservoir permeability of 50 mD for high-permeability and 5 mD for low-permeability.

[0047] The present invention will be further described below with reference to specific embodiments. The scope of protection of the present invention is not limited by the following embodiments. The main materials involved in the embodiments are all conventional commercially available products, and the water source used in the embodiments is injection water from the target oil field.

[0048] Example 1

[0049] The surfactant system of this embodiment includes 1.2% additive (sodium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0050] The surfactant used was betaine, and the water used in the experiment was water injected into a test area.

[0051] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0052] Example 2

[0053] The surfactant system of this embodiment includes 1.2% additive (potassium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0054] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0055] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0056] Example 3

[0057] The surfactant system of this embodiment includes 1.2% additive (sodium sulfate) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0058] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 2:6:2, and the water used in the experiment was the same as in Example 1.

[0059] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0060] Example 4

[0061] The surfactant system of this embodiment includes 1.2% additive (sodium chloride and sodium sulfate mixed in a mass ratio of 2:1) by mass percentage. The surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of the water.

[0062] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 2:2:6, and the water used in the experiment was the same as in Example 1.

[0063] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0064] Example 5

[0065] The surfactant system of this embodiment includes 1.2% additives (sodium chloride, calcium chloride and sodium sulfate mixed in a mass ratio of 1:1:3) by mass percentage. The surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of the water.

[0066] The surfactant used was a sulfonate surfactant, and the water used in the experiment was the same as in Example 1.

[0067] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0068] Example 6

[0069] The surfactant system of this embodiment includes 1.2% additive (sodium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.5 wt% of water.

[0070] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0071] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0072] Example 7

[0073] The surfactant system of this embodiment includes 1.2% additive (sodium hydroxide) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0074] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0075] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0076] Example 8

[0077] The surfactant system of this embodiment includes 2.4% additive (sodium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0078] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0079] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0080] Example 9

[0081] The surfactant system of this embodiment includes 1.2% additive (sodium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.1 wt% of water.

[0082] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0083] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0084] Example 10

[0085] The surfactant system of this embodiment includes 1.2% additive (sodium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0086] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 1:1:1, and the water used in the experiment was the same as in Example 1.

[0087] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0088] Example 11

[0089] The surfactant system of this embodiment includes 1.2% additive (sodium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0090] The surfactant used was an extended surfactant, and the water used in the experiment was the same as in Example 1.

[0091] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0092] Example 12

[0093] The surfactant system of this embodiment includes 1.2% additive (sodium hydroxide and sodium bicarbonate mixed in a mass ratio of 1:1) by mass percentage. The surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of the water.

[0094] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 2:6:2, and the water used in the experiment was the same as in Example 1.

[0095] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0096] Example 13

[0097] The surfactant system of this embodiment includes 0.01% additive (sodium hydroxide) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0098] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0099] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0100] Example 14

[0101] The surfactant system of this embodiment includes 0.15% additive (sodium hydroxide) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0102] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0103] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0104] Example 15

[0105] The surfactant system of this embodiment includes 0.3% additive (sodium hydroxide) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0106] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0107] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0108] Example 16

[0109] The surfactant system of this embodiment includes 0.1% additive (sodium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0110] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0111] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0112] Example 17

[0113] The surfactant system of this embodiment includes 0.25% additive (sodium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0114] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0115] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0116] Example 18

[0117] The surfactant system of this embodiment includes 5% additive (sodium chloride) by mass percentage, and the surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of water.

[0118] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0119] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0120] Example 19

[0121] The surfactant system of this embodiment includes 1.2% additive (sodium chloride and sodium hydroxide mixed in a mass ratio of 1:1) by mass percentage. The surfactant combination system of this embodiment also includes water and a surfactant accounting for 0.3 wt% of the water.

[0122] The surfactant was composed of betaine surfactant, extended surfactant and sulfonate surfactant in a mass ratio of 6:2:2, and the water used in the experiment was the same as in Example 1.

[0123] The surfactant system of this embodiment is prepared by the following steps: surfactants and additives are mixed in the above-mentioned mass percentages and injected into the water in the test area to obtain an emulsion surfactant system.

[0124] The surfactant systems prepared in Examples 1 to 18 above were subjected to the following performance tests.

[0125] Test 1 Interface Expansion Modulus Test

[0126] When an interface is subjected to periodic compression and expansion, the interfacial tension also changes periodically. The expansion modulus is defined as the ratio of the change in interfacial tension to the change in relative interfacial area, that is:

[0127]

[0128] In equation (III), ε is the expansion modulus, γ is the interfacial tension, and A is the interfacial area.

[0129] This experiment employed an interfacial expansion rheology and dynamic contact angle measuring instrument to conduct interfacial tension and expansion rheology experiments. Periodic oscillations were performed on suspended droplets of the surfactant systems prepared in Examples 1–19 under high CO2 pressure. Instantaneous changes in liquid surface area were captured by a camera, and dynamic interfacial tension and dynamic interfacial expansion properties were determined using droplet shape analysis methods. The interfacial expansion rheological properties were then measured. The results are shown in Table 1.

[0130] Test 2 Stability Test

[0131] For the surfactant systems prepared in Examples 1-19, the stability of the emulsions was determined using the bottle test method under reservoir conditions where CO2 reached dissolution equilibrium. The test temperature was set at 70°C. The experimental water was water injected from a test area, and the experimental oil was simulated oil prepared from dehydrated crude oil from a test area, with an oil-water volume ratio of 1:1. The emulsions were generated by shaking at a constant frequency 100 times, and the time required for 20% water separation was observed. The results are shown in Table 1 and... Figure 2 As shown.

[0132] Table 1 Performance Test Results

[0133]

[0134] Combination Figure 1 As shown in Table 2, the interfacial expansion modulus of the surfactant system prepared by this invention is generally high from low frequency to high frequency, indicating that the corresponding emulsion film generally possesses anti-instability properties within the low-frequency to high-frequency oscillation range. Specifically, Examples 2, 6, and 7 exhibit higher interfacial expansion moduli than other examples, demonstrating more significant anti-instability properties. Furthermore, considering Examples 12-19, the inventors hypothesize that by controlling the amount of surfactant added to 0.3-0.5% of the water mass, and further controlling the ratio of betaine surfactant, extended surfactant, and sulfonate surfactant to 3:1:1, and then adding 1.2% of additives, the surfactant system of this invention can achieve even better interfacial expansion modulus. Therefore, the surfactant system prepared by this invention possesses excellent interfacial expansion modulus, indicating that this series of systems can effectively resist liquid film disturbances and stabilize oil-water liquid films in the presence of CO2.

[0135] Combination Figure 2 As shown in the performance test data in Table 2, the surfactant system prepared by this invention has a relatively high overall emulsion water separation time of 20%, indicating that the emulsion has high stability. Among them, the emulsion stability of Example 2 is higher than that of other examples, with the highest emulsion stability. Based on the time consumed for 20% water separation of the surfactant system prepared by this invention, the inventors believe that this series of systems can effectively stabilize the oil-water film and emulsion in the presence of CO2, thereby achieving the expected effect of long-term sealing of pore throats and preventing CO2 gas migration.

[0136] The inventors conducted a visual oil displacement experiment on the surfactant systems prepared in Examples 1-19 above. Specific operations included:

[0137] Reference Figure 3 As shown, the experimental temperature was set at 70℃, the experimental water was water injected on-site in a certain test area, and the experimental oil was simulated oil prepared from dehydrated crude oil in a certain test area. Before the experiment, the simulated oil was injected into the heterogeneous glass model at a rate of 20 μL / min until the simulated oil filled the entire glass model and no air bubbles were present in the pore throat. During the experiment, different types of surfactant solutions and CO2 were injected into the glass model alternately at a constant injection rate of 0.1 μL / min. The oil displacement results in the glass model were recorded by microscope, the area ratio of clustered residual oil was measured, and the swept volume was calculated as 1 - the area ratio of clustered residual oil. The results are shown in Table 2, and the visualized microscopic oil displacement local phenomenon of Example 2 is also given. Figure 3 .

[0138] The inventors conducted actual oil displacement tests on the surfactant systems prepared in Examples 1-18 above. Specific operations included:

[0139] Simulating target reservoir conditions, oil displacement tests were conducted on the in-situ emulsified surfactant systems prepared in Examples 1-19 using saturated oil double-tube parallel core samples. Core gas permeability was 50 mD for high permeability and 5 mD for low permeability. The experimental temperature was 70℃, and the experimental water was field-injected water from a test area. The core outlet back pressure was 10 MPa. The injection system was the surfactant system prepared in Examples 1-18, with an injection rate of 0.1 ml / min. The injection method was water flooding until the water cut reached 98% + CO2 flooding until the gas-liquid ratio was >2000 + 0.1 PV gas-liquid (CO2 + surfactant system) alternating 4 times + subsequent CO2 flooding until the gas-liquid ratio was >2000. The results of oil displacement tests and improved oil recovery for different in-situ emulsified surfactant systems are shown in Table 2.

[0140] Table 2 shows the swept volume and enhanced oil recovery of the surfactant systems prepared in Examples 1-18.

[0141]

[0142]

[0143] Using 1.2% NaCl(aq) as the comparative example of this invention, when conducting oil displacement tests, Comparative Example 1, without emulsification, lacked a sufficient pressure gradient to allow CO2 to enter the low-permeability zone, and the measured crude oil recovery rate was 10.29%.

[0144] The data in Table 2 show that the swept volume of this series of systems is generally larger than that of the comparative example, indicating that it has the effect of expanding CO2 sweep volume. Correspondingly, the recovery rate of this series of systems is also generally greater than that of the comparative example, indicating that the surfactant system prepared by this invention can expand CO2 sweep volume and improve recovery rate.

[0145] Furthermore, in conjunction with Example 2 and Appendix Figure 3 The image shows a localized phenomenon of oil displacement under microscopic visualization. The white area represents the simulated rock skeleton. It is clearly visible that in the lower half of the image, there are in-situ generated oil-in-water emulsion droplets with the oil phase (black) as the inner phase and the brown area as the outer phase. These droplets remain in the displacement path, effectively hindering the flow of CO2 bubbles and other fluids, thereby expanding the swept volume.

[0146] In summary, the surfactant combination system provided by this invention, starting from the interfacial film strength of a single surfactant, regulates the stability of the emulsion by changing the compounding ratio of multiple surfactants. The in-situ emulsification-assisted CO2 flooding surfactant combination system developed by this invention, which has controllable emulsification and CO2 blocking strength, can reduce the oil-water interfacial tension while achieving CO2 emulsification and blocking. Furthermore, the in-situ emulsification-assisted CO2 flooding surfactant combination system provided by this invention has excellent enhanced oil recovery effects for CO2 flooding reservoirs. The in-situ emulsification-assisted CO2 flooding recovery rate of core displacement can be increased by more than 10% compared with CO2-WAG flooding.

[0147] Although the invention has been described in considerable detail and particularly with regard to several of the described embodiments, it is not intended to limit itself to any of these details or embodiments or any particular embodiment, thereby effectively covering the intended scope of the invention. Furthermore, the invention has been described above with respect to embodiments foreseeable by the inventors in order to provide a useful description, and non-substantial modifications to the invention that have not yet been foreseen may still represent equivalent modifications.

[0148] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0149] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A surfactant system, characterized in that, The system includes water and a surfactant comprising 0.1 to 0.5 wt% of the water; the surfactant is a mixture of betaine-type surfactant, extended surfactant, and sulfonate surfactant; the mass ratio of the betaine-type surfactant, the extended surfactant, and the sulfonate surfactant is 3:1:

1. The betaine-type surfactant is alkyl sulfobetaine; The alkyl sulfonyl betaine is as shown in formula (I): （Ⅰ）； In formula (I), R1 is selected from long-chain alkyl groups of C12 to C18; The extended surfactant is shown in formula (II): （Ⅱ）； In formula (II): n is one of 2, 4, 6, 8 or 10, and R2 is selected from long-chain alkyl groups of C12 to C18; The sulfonate surfactant is a petroleum sulfonate, which is a mixture of first-line, second-line, and third-line products in any ratio.

2. The surfactant system according to claim 1, characterized in that, The system also includes 0.01 to 5 wt% of additives.

3. The surfactant system according to claim 2, characterized in that, The additive is an alkaline substance and / or a neutral salt.

4. The surfactant system according to claim 3, characterized in that, When the additive is an alkaline substance, the mass fraction of the additive added to the system is 0.01~0.3%; When the additive is a neutral salt, the mass fraction of the additive added to the system is 0.1% to 5%. When the additive is a mixture of alkaline substances and neutral salts, the mass fraction of the additive added to the system is 0.01~5%.

5. The surfactant system according to claim 3 or 4, characterized in that, The alkaline substance is selected from one or more of sodium carbonate, sodium bicarbonate, or sodium hydroxide; The neutral salt is selected from one or more of sodium chloride, potassium chloride, calcium chloride, magnesium chloride, or sodium sulfate.

6. The application of the surfactant system as described in any one of claims 1-5 in in-situ emulsification-assisted CO2 flooding.

7. A method for in-situ emulsification-assisted CO2 displacement of oil, characterized in that, The method uses the surfactant system as described in any one of claims 1 to 5 to perform in-situ emulsification-assisted CO2 oil displacement.

8. The in-situ emulsification-assisted CO2 displacement method according to claim 7, characterized in that, During the in-situ emulsification-assisted CO2 flooding operation, the reservoir conditions are as follows: The total salinity of water in the reservoir is 3000~80000 mg / L; The reservoir temperature is 30~100℃; The viscosity of the crude oil in the reservoir at the specified reservoir temperature is 1~20 cp; The permeability of the reservoir is 0.01~50 mD.

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