Composite surfactant for assisting CO2 oil displacement and burying and application thereof

Abstract

The invention relates to the technical field of oil exploitation, in particular to a composite surfactant for assisting CO2 oil displacement and burying and application of the composite surfactant. The composite surfactant comprises a fluorocarbon surfactant, a nonionic cosurfactant and an organic cosolvent in a mass ratio of 1: (6-12): (10-30). According to the invention, the oil-gas amphiphilic fluorocarbon surfactant is matched with the nonionic surfactant and is supplemented with the organic cosolvent to form the composite surfactant, and when the composite surfactant is used for assisting CO2 oil displacement, the interfacial tension and minimum miscible pressure between CO2 and crude oil can be effectively reduced, so that the crude oil recovery rate and CO2 storage rate of CO2 oil displacement are improved. When the composite surfactant is used for assisting CO2 oil displacement, the crude oil recovery rate can be increased by 9.56% or above, the burying efficiency can be increased by 11.50% or above, the synergistic growth of displacement and burying is realized, and a new direction is provided for realizing high-efficiency CO2 displacement and burying of a complex tight reservoir.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of oil production, in particular to a composite surfactant for assisting CO2 flooding and storage and application thereof. BACKGROUND

[0002] As a key replacement energy for increasing oil and gas reserves and production nationwide, the efficient development of tight reservoirs is related to energy security and low-carbon transformation. With the continuous decline of conventional resources, the proportion of tight oil in unconventional oil and gas production has broken through 40%. Conventional recovery methods such as depletion recovery and water flooding have problems such as injection and production, and gas flooding (CO2, N2, CH4) has become the main development method of tight reservoirs.

[0003] In tight reservoirs, CO2 is the first choice for the development of low-permeability tight reservoirs due to its low miscibility pressure, strong extraction capacity, supercritical state high density, and low interfacial tension between oil and oil. However, the single oil displacement system has limited ability to improve oil recovery, and the surfactant system assisted CO2 displacement can effectively reduce the interfacial tension and miscibility pressure between CO2 and oil. The development of tight reservoirs has been upgraded from single oil and gas production to driving and burying synergistic multi-target development.

[0004] Therefore, how to provide a surfactant displacement system that can effectively assist CO2 displacement has become a current problem to be solved. SUMMARY

[0005] To solve the above technical problems, the present disclosure provides a composite surfactant for assisting CO2 flooding and storage and application thereof. The present disclosure can effectively reduce the minimum miscibility pressure of CO2 and oil by compounding fluorocarbon surfactant, non-ionic co-surfactant and organic cosolvent to assist CO2 displacement, improve oil recovery and CO2 storage efficiency, and has good application prospect.

[0006] In a first aspect, the present disclosure provides a composite surfactant for assisting CO2 flooding and storage, comprising fluorocarbon surfactant, non-ionic co-surfactant and organic cosolvent. The mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:(6-12):(10-30).

[0007] Specifically, the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent can be 1:6:10, 1:7:12, 1:8:20, 1:9:20, 1:10:20, 1:11:20, 1:12:20 or 1:13:30, but is not limited to the listed values. Other values not listed in this range are also applicable.

[0008] The present disclosure adopts oil-gas amphiphilic fluorocarbon surfactant compounded with non-ionic surfactant, and is assisted by organic cosolvent, so as to reduce the CO2-oil interfacial tension and the minimum miscibility pressure in the process of CO2 displacement, and improve the oil displacement efficiency. Among them, the fluorocarbon surfactant has a fluorocarbon chain with good affinity with CO2 at one end and a carbon-hydrogen chain with good affinity with crude oil at the other end, which can effectively reduce the interfacial tension between crude oil and CO2, and the minimum miscibility pressure between the two is also reduced. The CO2-philic functional group has good solubility in CO2 and can be stably dispersed in CO2; at the same time, the carbon-hydrogen chain can interact with the polar components in the crude oil, form a directional molecular layer by adsorbing at the oil and CO2 interface, and the fluorocarbon surfactant has good stability in high temperature and high salt environment; the non-ionic surfactant can form a flexible adsorption film at the CO2-water interface, avoid the rapid coalescence of CO2 bubbles, form micelles to dissolve CO2, form "CO2-surfactant" composite micelles, prolong the residence time of CO2 in the reservoir, and at the same time, the micelles can stably exist in the high salinity reservoir. The organic cosolvent can improve the solubility of CO2 in the water phase and the dispersibility of the surfactant in the CO2 phase, promote more CO2 to dissolve and diffuse to the oil phase, at the same time, alleviate the problem of aggregation of fluorocarbon surfactant at the gas-liquid interface, enhance the extraction capacity of CO2 for crude oil, and realize the swelling and viscosity reduction of crude oil.

[0009] Further, the appropriate ratio of fluorocarbon surfactant and non-ionic surfactant helps to improve the oil displacement and storage efficiency. If the addition amount of fluorocarbon surfactant in the composite surfactant is too much, a too thick adsorption film will be formed at the oil-water interface, which will cause the interfacial tension to abnormally increase, destroy the ultra-low interfacial tension state of the system; at the same time, excessive fluorocarbon molecules are easy to aggregate at the interface to form a dense interface film, which hinders the dissolution and diffusion of CO2 in the oil phase, reduces the mass transfer efficiency of CO2, and is not conducive to the expansion of the swept area of CO2 and the stability of the storage process; too little will not be able to achieve ultra-low interfacial tension, and the system is easy to precipitate and aggregate in the high salinity and high temperature dense reservoir environment, resulting in system failure and inability to adapt to complex formation conditions.

[0010] The following is a preferred technical solution of the present disclosure, but not as a limitation of the technical solutions provided by the present disclosure. Through the following technical solutions, the technical purposes and beneficial effects of the present disclosure can be better achieved and realized.

[0011] As a preferred technical solution of the present disclosure, the mass ratio of the fluorocarbon surfactant, the non-ionic surfactant and the organic cosolvent is 1:(9-11):(14-16).

[0012] As a preferred technical solution of the present disclosure, the fluorocarbon surfactant is selected from one or more of the structures shown in formula I; Formula I wherein R1 represents any one of alkyl groups with 12-18 carbon atoms, and R2 represents any one of (CF2)3CF3, (CF2)5CF3 or (CF2)7CF3.

[0013] It should be noted that the carbon-hydrogen chain involved in the structure shown in Formula I is a straight chain or a branched chain structure.

[0014] As a preferred technical solution of the present disclosure, the non-ionic co-surfactant comprises a fatty alcohol polyoxyethylene ether and / or a fatty acid polyoxyethylene ester.

[0015] As a preferred technical solution of the present disclosure, the fatty alcohol polyoxyethylene ether is selected from one or more of the structures shown in Formula II; Formula II wherein R3 represents any one of alkyl groups with 12-13 carbon atoms, and n is any one of 7-15. It should be noted that the alkyl group with 12 carbon atoms is a straight chain structure, and the alkyl group with 13 carbon atoms is an isomeric tridecyl group.

[0016] Preferably, the fatty acid polyoxyethylene ester is selected from one or more of the structures shown in Formula III; Formula III wherein R4 represents any one of alkyl groups with 8-10 carbon atoms, and n2 is any one of 5-15; it should be noted that the carbon-hydrogen chain is a straight chain or a branched chain structure.

[0017] As a preferred technical solution of the present disclosure, the organic co-solvent comprises one or more of ethanol, methanol or n-butanol, and is preferably ethanol.

[0018] In a second aspect, the present disclosure provides a use of the composite surfactant for assisting CO2 flooding and sequestration in the field of CO2 flooding and CO2 sequestration, as described in the first aspect.

[0019] In a third aspect, the present disclosure provides a method for CO2 flooding, which comprises: injecting a homogeneous fluid formed by mixing the composite surfactant for assisting CO2 flooding and sequestration, as described in the first aspect, with supercritical CO2 into an oil reservoir to perform flooding.

[0020] Preferably, the temperature of the homogeneous fluid formed by mixing the composite surfactant with supercritical CO2 is -20℃ to 60℃, such as -20℃, 0℃, 20℃, 40℃ or 60℃, but is not limited to the listed values, and other unlisted values within this range are also applicable.

[0021] ​As a preferred technical solution of this disclosure, the total salinity of the reservoir is 10. 4 -10 5 mg / L, for example 10 4 mg / L, 2 10 4 mg / L, 4 10 4 mg / L, 6 10 4 mg / L, 8 10 4 mg / L or 10 5 mg / L, etc.; temperature is 80-150℃, such as 80℃, 100℃, 120℃ or 150℃, etc., but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0022] Preferably, the crude oil in the reservoir has a viscosity of 1-50 mPa at 80-150°C. s, for example 1 mPa s, 5mPa s, 10mPa s, 15mPa s, 20mPa s, 25mPa s, 30mPa s, 35mPa s, 40mPa s, 45mPa s or 50mPa s, etc., but not limited to the listed values; other unlisted values ​​within this range also apply.

[0023] Preferably, the gas permeability of the reservoir core is 0.01-0.05 mD, such as 0.01 mD, 0.02 mD, 0.03 mD, 0.04 mD or 0.05 mD, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0024] As a preferred technical solution of this disclosure, during the injection into the reservoir, the injection pressure is 10-20 MPa, such as 10 MPa, 12 MPa, 14 MPa, 16 MPa, 18 MPa or 20 MPa, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] And / or, the added mass of the fluorocarbon surfactant is 0.01-0.1% of the mass of the supercritical CO2, for example, 0.01%, 0.02%, 0.03%, 0.05%, 0.06%, 0.08% or 0.1%, etc., but not limited to the listed values, and other values not listed in the range are also applicable.

[0026] In the application of CO2 flooding, controlling the reasonable addition amount of fluorocarbon surfactant can further improve the oil displacement efficiency. If the addition amount of fluorocarbon surfactant is too much, it will cause the oil-water interfacial tension to abnormally increase, destroy the system ultra-low interfacial tension state, and at the same time, the dense interfacial film formed reduces the CO2 mass transfer efficiency, and the excess fluorocarbon surfactant affects the HLB value of the system, causing the emulsion to break, and the oil phase and surfactant precipitated will be deposited in the rock pore throat to block the pores.

[0027] The technical scheme provided by the embodiments of the present disclosure has the following advantages compared with the prior art: The present disclosure combines the oil-gas amphiphilic fluorocarbon surfactant with the non-ionic surfactant, and assists with the organic cosolvent to form a complex surfactant, which can effectively reduce the interfacial tension between CO2 and crude oil and the minimum miscibility pressure when used to assist CO2 flooding, and reduce the flow resistance of CO2 in the porous medium, so that the gas is more easily enters the micro-nano pores to displace the crude oil; using the complex surfactant provided by the present disclosure to assist CO2 flooding can increase the crude oil recovery by more than 9.56%, and the storage efficiency is increased by more than 11.50%, realizing the synergistic growth of displacement and storage, and providing a new direction for efficient displacement and storage of CO2 in complex and dense reservoirs. DETAILED DESCRIPTION

[0028] In order to more clearly understand the above-mentioned purposes, features and advantages of the present disclosure, the schemes of the present disclosure will be further described below. It should be noted that the embodiments of the present disclosure and the features in the embodiments can be combined with each other without conflict.

[0029] In the following description, many specific details are set forth in order to provide a thorough understanding of the present disclosure, but the present disclosure can also be implemented in other ways different from those described herein; obviously, the embodiments in the description are only a part of the embodiments of the present disclosure, not all the embodiments.

[0030] In order to more clearly understand the above-mentioned purposes, features and advantages of the present disclosure, the schemes of the present disclosure will be further described below. It should be noted that the embodiments of the present disclosure and the features in the embodiments can be combined with each other without conflict.

[0031] Many specific details are set forth in the following description in order to provide a thorough understanding of the present disclosure. However, the present disclosure can be practiced according to other embodiments that do not require some of the specific details described below. It is understood that the present disclosure is well suited to implement embodiments like those described below, but that the present disclosure is not limited to any single embodiment described. Embodiment 1 The present embodiment provides a complex surfactant and supercritical CO2 compound system, wherein the complex surfactant comprises: a fluorocarbon surfactant; ; a nonionic co-surfactant; i.e. AEO-7; and an organic cosolvent: ethanol; The mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic cosolvent is 1:10:20; The added amount of the fluorocarbon surfactant, the nonionic co-surfactant and the organic cosolvent is 0.03%, 0.3% and 0.6% of the mass of the supercritical CO2, respectively. Embodiment 2 The present embodiment provides a complex surfactant and supercritical CO2 compound system, which refers to the compound system in Embodiment 1, and the difference is only that: the nonionic co-surfactant (AEO-7) is replaced by (AEO-9).

[0032] Embodiment 3 The present embodiment provides a complex surfactant and supercritical CO2 compound system, which refers to the compound system in Embodiment 1, and the difference is only that: the nonionic co-surfactant (AEO-7) is replaced by (AEO-15).

[0033] Embodiment 4 The present embodiment provides a complex surfactant and supercritical CO2 compound system, which refers to the compound system in Embodiment 1, and the difference is only that: the nonionic co-surfactant (AEO-7) is replaced by (iso-tridecanol polyoxyethylene ether EO-9).

[0034] Embodiment 5 The present embodiment provides a complex surfactant and supercritical CO2 compound system, which refers to the compound system in Embodiment 1, and the difference is only that: the nonionic co-surfactant (AEO-7) is replaced by .

[0035] Embodiment 6 The embodiment provides a complexing system of composite surfactant and supercritical CO2, referring to the complexing system in the embodiment 1, the difference is only that: The fluorocarbon surfactant is replaced by .

[0036] Embodiment 7 The embodiment provides a complexing system of composite surfactant and supercritical CO2, referring to the complexing system in the embodiment 1, the difference is only that: The fluorocarbon surfactant is replaced by .

[0037] Embodiment 8 The embodiment provides a complexing system of composite surfactant and supercritical CO2, referring to the complexing system in the embodiment 1, the difference is only that: The fluorocarbon surfactant is replaced by .

[0038] Embodiment 9 The embodiment provides a complexing system of composite surfactant and supercritical CO2, referring to the complexing system in the embodiment 1, the difference is only that: the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:6:20; The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.18%, 0.6% of the mass of the supercritical CO2 respectively.

[0039] Embodiment 10 The embodiment provides a complexing system of composite surfactant and supercritical CO2, referring to the complexing system in the embodiment 1, the difference is only that: the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:12:20; The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.36%, 0.6% of the mass of the supercritical CO2 respectively.

[0040] Embodiment 11 The embodiment provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 2, and the difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:6:20. The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.18% and 0.6% of the mass of the supercritical CO2 respectively.

[0041] Embodiment 12 The embodiment provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 2, and the difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:6:20. The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.18% and 0.6% of the mass of the supercritical CO2 respectively.

[0042] Embodiment 13 The embodiment provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 3, and the difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:6:20. The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.18% and 0.6% of the mass of the supercritical CO2 respectively.

[0043] Embodiment 14 The embodiment provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 3, and the difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:6:20. The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.18% and 0.6% of the mass of the supercritical CO2 respectively.

[0044] Embodiment 15 The embodiment provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 4, and the difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:6:20. The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.18% and 0.6% of the mass of the supercritical CO2 respectively.

[0045] Example 16 This example provides a complex surfactant and supercritical CO2 complex system, referring to the complex system in Example 4, the only difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:12:20; The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.36% and 0.6% of the mass of the supercritical CO2 respectively.

[0046] Example 17 This example provides a complex surfactant and supercritical CO2 complex system, referring to the complex system in Example 5, the only difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:6:20; The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.18% and 0.6% of the mass of the supercritical CO2 respectively.

[0047] Example 18 This example provides a complex surfactant and supercritical CO2 complex system, referring to the complex system in Example 5, the only difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:12:20; The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.36% and 0.6% of the mass of the supercritical CO2 respectively.

[0048] Example 19 This example provides a complex surfactant and supercritical CO2 complex system, referring to the complex system in Example 2, the only difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 3:30:40; The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.3% and 0.4% of the mass of the supercritical CO2 respectively.

[0049] Example 20 The embodiment provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 2, and the difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 3:30:80. The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.03%, 0.3% and 0.8% of the mass of the supercritical CO2 respectively.

[0050] Comparative example 1 The comparative example provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 1, and the difference is that the fluorocarbon surfactant is replaced by the same mass of non-ionic co-surfactant.

[0051] Comparative example 2 The comparative example provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 1, and the difference is that the fluorocarbon surfactant is replaced by (sodium dodecyl benzene sulfonate).

[0052] Comparative example 3 The comparative example provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 1, and the difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is changed to 1:3:20. The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.0825%, 0.2475% and 0.6% of the mass of the supercritical CO2 respectively.

[0053] Comparative example 4 The comparative example provides a complex surfactant and supercritical CO2 compound system, referring to the compound system in the embodiment 1, and the difference is that the mass ratio of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 1:20:20. The adding amount of the fluorocarbon surfactant, the non-ionic co-surfactant and the organic cosolvent is 0.0157%, 0.3143% and 0.6% of the mass of the supercritical CO2 respectively.

[0054] The compound systems obtained in the embodiment and the comparative examples are used for oil displacement experiment tests.

[0055] In the embodiment, the experimental crude oil and the experimental core used are as follows: D block degassing crude oil: formation (reservoir) temperature 95℃, formation (injection) pressure 20 MPa; the viscosity of the crude oil at 25℃, 0.1 MPa is 43.25 mPa s, the density is 0.886 g / cm 3 .

[0056] The experimental core is a dense sandstone reservoir core in D block, and the gas measured permeability of the core is 0.176 mD, and the gas measured porosity is 12.63%.

[0057] Test 1: Determination of minimum miscibility pressure by interfacial tension disappearance method Referring to the standard "SY / T 5370-2018", the interfacial tension of the pure CO2 system and the crude oil system, the determination of the interfacial tension of the complex system described in examples 1-20 and comparative examples 1-4 and the crude oil system is determined, and then the minimum miscibility pressure of CO2 flooding is calculated by the interfacial tension disappearance method.

[0058] The steps of the pendant drop method for determining the interfacial tension include: (1) Put pure CO2 or complex system in the middle container, keep it at 95℃, 6 MPa initial pressure for 5h to form a stable state; (2) The pure CO2 or complex system is charged into the reaction kettle by the gas booster pump in stages, and then the experimental crude oil is formed into droplets by the probe, and the droplet state is observed, the droplet morphology is taken by the microscopic camera system, and the interfacial tension is calculated according to the Young-Laplace equation; (3) Keep the temperature unchanged, increase the pressure step by step, repeat step (2), then plot the interfacial tension as the ordinate and the pressure as the abscissa, fit the linear equation (R 2 > 0.98), and the value when the interfacial tension is zero is calculated by extrapolation, which is the minimum miscibility pressure; Among them, at least 8 values of interfacial tension under different pressures are determined every 1 MPa.

[0059] The test results of the minimum miscibility pressure are shown in Table 1.

[0060] Table 1

[0061] According to Table 1, the oil-gas amphiphilic fluorocarbon surfactant is matched with the non-ionic surfactant, and the organic cosolvent is used to form a complex surfactant, which can effectively reduce the interfacial tension between CO2 and crude oil and the minimum miscibility pressure when used to assist CO2 flooding.

[0062] Compared with example 1, comparative example 1 does not add fluorocarbon surfactant, and the reduction range of the minimum miscibility pressure is lower.

[0063] Compared with Example 1, the effect of Comparative Example 2, in which the fluorocarbon surfactant is replaced by a conventional surfactant, is still not as good as that of the composite surfactant described in the present disclosure.

[0064] Compared with Example 1, the amount of fluorocarbon surfactant is increased in Comparative Example 3, and the amount of co-surfactant is correspondingly reduced, resulting in the formation of a relatively thick fluorocarbon adsorption layer at the interface between CO2 and crude oil, affecting the diffusion of CO2 to the crude oil phase, and the minimum miscibility pressure is increased.

[0065] Compared with Example 1, the amount of fluorocarbon surfactant is reduced in Comparative Example 4, and the amount of co-surfactant is correspondingly increased, affecting the formation of the effective interfacial adsorption layer, and the minimum miscibility pressure is reduced at a lower rate.

[0066] Test two: determination of oil displacement efficiency and storage efficiency According to the standard "SY / T 7454-2019", indoor oil displacement experiments were carried out using indoor physical simulation experiments. Under the conditions of formation temperature and pressure, pure "CO2 flooding" and "composite surfactant assisted CO2 flooding" experiments were carried out respectively, and the oil recovery and CO2 storage rate of pure "CO2 flooding" and "composite surfactant assisted CO2 flooding" were calculated respectively.

[0067] Specifically, the oil displacement efficiency and storage efficiency of the complex system described in Examples 1-6, 9-10 and Comparative Examples 1-4 were tested.

[0068] The specific operation steps are as follows: (1) The experimental core was washed with oil and then dried at 120°C for 24h. After drying, the experimental core was placed in a core holder and vacuumed for 12h, and then saturated with 12wt% manganese chloride formation water at a displacement rate of 0.1ml / min. During the process, the confining pressure applied around the experimental core was always 3MPa higher than the displacement pressure; (2) The experimental crude oil was heated to 95°C, and then injected into the experimental core at a constant speed of 0.1ml / min to replace the formation water. When no water was produced at the outlet section, the injection was stopped. After the oil saturation was complete, the experimental core was sealed back into the core holder; (3) The complex system or pure supercritical CO2 was injected into the experimental core at a constant displacement rate of 0.1ml / min, and the confining pressure applied around the experimental core was always 3MPa higher than the displacement pressure. When no oil was produced, the injection was stopped. The oil production and fluid production were recorded, and the oil recovery and CO2 storage rate were calculated. The calculation formula of oil recovery is shown in formula (1), and the calculation formula of CO2 storage rate is shown in formula (2)-(6): (1) wherein, is the oil recovery, %; is the pre-displacement saturated oil mass, g; is the post-displacement oil production mass, g.

[0069] (2) wherein, is the amount of substance of CO2 gas injected, mol; is the injection pressure of CO2, Pa; is the injection volume of CO2, m 3 is the injection temperature of CO2, K; is the ideal gas constant, 8.314 J / (mol K).

[0070] (3) wherein, is the mass of CO2 injected, g; is the molar mass of CO2, 44 g / mol.

[0071] (4) wherein, is the amount of substance of produced CO2 gas, mol; is the production pressure of CO2, 1.013 x 10 5 Pa; is the production volume of CO2, m 3 is the production temperature of CO2, 298.15 K; is the ideal gas constant, 8.314 J / (mol K).

[0072] (5) wherein, is the mass of produced CO2, g; is the molar mass of CO2, 44 g / mol.

[0073] (6) wherein, is the CO2 storage rate, %; is the mass of CO2 injected, is the mass of produced CO2.

[0074] The results of the oil recovery test are shown in Table 2, and the CO2 storage rate is shown in Table 3.

[0075] Table 2

[0076] Table 3

[0077] According to Table 2 and Table 3, the composite surfactant assisted CO2 flooding provided by the present disclosure can effectively improve the oil recovery rate and the CO2 storage rate. Compared with pure CO2 flooding, the oil recovery rate can be increased by more than 9.56%, and the CO2 storage rate can be increased by more than 11.50%.

[0078] The above description is merely a specific implementation of the present disclosure, which enables those skilled in the art to understand or implement the present disclosure. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to these embodiments described herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A composite surfactant for assisting CO2 flooding and sequestration, characterized by, The fluorocarbon surfactant, the non-ionic co-surfactant and the organic co-solvent are in a mass ratio of 1: (6-12) : (10-30). The fluorocarbon surfactant, the non-ionic co-surfactant and the organic co-solvent are in a mass ratio of 1: (9-11) : (14-16).

2. The complex surfactant for assisting CO2 flooding and sequestration according to claim 1, wherein The fluorocarbon surfactant is selected from one or more of the structures shown in Formula I; 3. The complex surfactant for assisting CO2 flooding and sequestration according to claim 1, wherein The non-ionic co-surfactant includes fatty alcohol polyoxyethylene ether and / or fatty acid polyoxyethylene ester. Formula I wherein R1represents , 12-18 carbon atoms, R2represents any one of (CF2)3CF3, (CF2)5CF3or (CF2)7CF3.

4. The complex surfactant for assisting CO2 flooding and sequestration according to claim 1, wherein The fatty alcohol polyoxyethylene ether is selected from one or more of the structures shown in Formula II; 5. The complex surfactant for assisting CO2 flooding and sequestration according to claim 4, wherein wherein R3 represents any one of alkyl groups with 12-13 carbon atoms, and n is any one of 7-15; Formula II and / or, the fatty acid polyoxyethylene ester is selected from one or more of the structures shown in Formula III; wherein R4 represents any one of alkyl groups with 8-10 carbon atoms, and n2 is any one of 5-15. Formula III The organic co-solvent includes one or more of ethanol, methanol or n-butanol.

6. The complex surfactant for assisting CO2 flooding and sequestration according to claim 1, wherein 7. Use of the composite surfactant for assisting CO2 flooding and sequestration according to any one of claims 1-6 in the field of CO2 flooding and CO2 sequestration. The method comprises: mixing the composite surfactant according to any one of claims 1-6 with supercritical CO2, and then injecting the mixture into an oil reservoir for oil displacement.

8. A method of CO2 flooding, characterized by, The gas permeability of the core of the oil reservoir is 0.01-0.05 mD.

9. The method of claim 8, wherein, The total mineralization of the reservoir is 10 4 -10 5 mg / L at a temperature of 80-150°C; The crude oil of the reservoir has a viscosity of 1-50 mPa s at 80-150°C. During the injection into the oil reservoir, the injection pressure is 10-20 MPa.

10. The method of claim 8, wherein, and / or, the added mass of the fluorocarbon surfactant is 0.01-0.1% of the mass of the supercritical CO2. ​

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