A compound surfactant for assisting co2 flooding and sequestration and its use

Abstract

The present disclosure relates to the technical field of oil exploitation, and particularly relates to a composite surfactant for assisting CO2 oil displacement and storage and application thereof. The composite surfactant comprises fluorocarbon surfactant, non-ionic co-surfactant and organic cosolvent in a mass ratio of 1:(6-12):(10-30). The present disclosure combines oil-gas amphiphilic fluorocarbon surfactant with non-ionic surfactant, and assists with organic cosolvent to form a composite surfactant. When the composite surfactant is used to assist CO2 oil displacement, the interfacial tension between CO2 and crude oil and the minimum miscibility pressure can be effectively reduced, and then the crude oil recovery rate of CO2 oil displacement and the CO2 storage rate are improved. The composite surfactant disclosed in the present disclosure can make the crude oil recovery rate increase by more than 9.56% and the storage efficiency increase by more than 11.50% when the composite surfactant is used to assist CO2 oil displacement, and the driving and storage are synergistically increased, which provides a new direction for realizing efficient CO2 displacement and storage in complex and dense reservoirs.

Description

Technical Field

[0001] This disclosure relates to the field of oil extraction technology, and in particular to a composite surfactant for assisting CO2 flooding and storage and its application. Background Technology

[0002] Tight reservoirs are a key alternative energy source for increasing oil and gas reserves and production nationwide, and their efficient development is crucial to energy security and low-carbon transformation. With the continuous depletion of conventional resources, tight oil accounts for more than 40% of unconventional oil and gas production. Conventional extraction methods such as depletion recovery and waterflooding have problems such as insufficient injection and extraction, making gas flooding (CO2, N2, CH4) the main extraction method for tight reservoirs.

[0003] In tight reservoirs, CO2 has become the preferred choice for developing low-permeability tight reservoirs due to its low miscibility pressure, strong extractability, high density in the supercritical state, and low interfacial tension with crude oil. However, the ability of a single oil displacement system to improve crude oil recovery is limited. Surfactant-assisted CO2 displacement can effectively reduce the interfacial tension and miscibility pressure between CO2 and crude oil. Tight reservoir development has evolved from a single issue of increasing oil and gas production to a multi-objective development strategy that combines oil displacement and burial.

[0004] Therefore, how to provide a surfactant displacement system that can effectively assist in CO2 displacement has become an urgent problem to be solved. Summary of the Invention

[0005] To address the aforementioned technical problems, this disclosure provides a composite surfactant for assisting CO2 oil displacement and storage, and its application. This disclosure, by compounding fluorocarbon surfactants, nonionic co-surfactants, and organic co-solvents to assist CO2 displacement, can effectively reduce the minimum miscibility pressure between CO2 and crude oil, improve crude oil recovery and CO2 storage efficiency, and has good application prospects.

[0006] In a first aspect, this disclosure provides a composite surfactant for assisting CO2 oil displacement and storage, including fluorocarbon surfactants, nonionic co-surfactants and organic co-solvents;

[0007] The mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent is 1:(6-12):(10-30).

[0008] Specifically, the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent 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, etc., but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0009] This disclosure employs an amphiphilic fluorocarbon surfactant combined with a nonionic surfactant and an organic co-solvent to reduce the CO2-crude oil interfacial tension and minimum miscibility pressure during CO2 displacement, thereby improving oil displacement efficiency. The fluorocarbon surfactant has a fluorocarbon chain with good affinity for CO2 at one end and a hydrocarbon chain with good affinity for crude oil at the other, effectively reducing the interfacial tension between crude oil and CO2, and consequently lowering the minimum miscibility pressure. The CO2-loving functional group exhibits good solubility in CO2 and can be stably dispersed within it. Simultaneously, the hydrocarbon chain can interact with polar components in crude oil, forming an oriented molecular layer at the oil-CO2 interface through adsorption. Furthermore, the fluorocarbon surfactant exhibits good stability under high temperature and high salinity conditions. The nonionic surfactant forms a flexible adsorption film at the CO2-water interface, preventing rapid aggregation of CO2 bubbles. The resulting micelles encapsulate and dissolve CO2, forming a "CO2-surfactant" composite micelle, prolonging the residence time of CO2 in the reservoir, and ensuring stable existence even in high-salinity reservoirs. Organic cosolvents can improve the solubility of CO2 in the aqueous phase and the dispersibility of surfactants in the CO2 phase, promoting the dissolution and diffusion of more CO2 into the oil phase. At the same time, they can alleviate the aggregation problem of fluorocarbon surfactants at the gas-liquid interface, enhance the extraction capacity of CO2 for crude oil, and achieve swelling and viscosity reduction of crude oil.

[0010] Furthermore, a suitable ratio of fluorocarbon surfactants to nonionic surfactants helps improve oil displacement and storage efficiency. If too much fluorocarbon surfactant is added to the composite surfactant, an excessively thick adsorption film will form at the oil-water interface, leading to an abnormal increase in interfacial tension and disrupting the system's ultra-low interfacial tension state. At the same time, excessive fluorocarbon molecules tend to aggregate at the interface to form a dense interfacial film, hindering the dissolution and diffusion of CO2 in the oil phase, reducing CO2 mass transfer efficiency, and negatively impacting the expansion of CO2 coverage and the stability of the storage process. If too little is added, ultra-low interfacial tension cannot be achieved, and the system is prone to surfactant precipitation and aggregation in high-salinity, high-temperature tight reservoir environments, leading to system failure and inability to adapt to complex formation conditions.

[0011] The following are preferred technical solutions of this disclosure, but are not intended to limit the technical solutions provided by this disclosure. The technical objectives and beneficial effects of this disclosure can be better achieved through the following technical solutions.

[0012] As a preferred technical solution of this disclosure, the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent is 1:(9-11):(14-16).

[0013] As a preferred technical solution of this disclosure, the fluorocarbon surfactant is selected from one or more of the structures shown in Formula I;

[0014] Formula I

[0015] Where R1 represents Any one of alkyl groups with 12-18 carbon atoms, where R2 represents any one of (CF2)3CF3, (CF2)5CF3 or (CF2)7CF3.

[0016] It should be noted that the hydrocarbon chains involved in the structure shown in Formula I are either straight chains or branched chains.

[0017] As a preferred technical solution of this disclosure, the nonionic co-surfactant includes fatty alcohol polyoxyethylene ether and / or fatty acid polyoxyethylene ester.

[0018] As a preferred technical solution of this disclosure, the fatty alcohol polyoxyethylene ether is selected from one or more having the structure shown in Formula II;

[0019] Formula II

[0020] Wherein, R3 represents any alkyl group with 12-13 carbon atoms, and n is any one of 7-15. It should be noted that alkyl groups with 12 carbon atoms have a straight-chain structure, and alkyl groups with 13 carbon atoms are isotridecylalkyl groups.

[0021] Preferably, the fatty acid polyoxyethylene ester is selected from one or more having the structure shown in Formula III;

[0022] Formula III

[0023] Wherein, R4 represents any alkyl group with 8-10 carbon atoms, and n2 is any alkyl group with 5-15 carbon atoms; it should be noted that the hydrocarbon chain can be a straight chain or a branched chain structure.

[0024] As a preferred technical solution of this disclosure, the organic co-solvent includes one or more of ethanol, methanol or n-butanol, preferably ethanol.

[0025] Secondly, this disclosure provides an application of a composite surfactant as described in the first aspect for assisting CO2 flooding and storage in the field of CO2 flooding and CO2 storage.

[0026] Thirdly, this disclosure provides a method for CO2 flooding, the method comprising: injecting a mixture of supercritical CO2 with a composite surfactant used to assist CO2 flooding and burying, as described in the first aspect, into the reservoir for oil displacement.

[0027] 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. Other unlisted values ​​within this range are also applicable.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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%, but is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0033] In CO2 flooding applications, controlling the appropriate amount of fluorocarbon surfactants can further improve oil displacement efficiency. Excessive fluorocarbon surfactant addition can lead to an abnormally high oil-water interfacial tension, disrupting the system's ultra-low interfacial tension state. Simultaneously, the resulting dense interfacial film reduces CO2 mass transfer efficiency. Excessive fluorocarbon surfactants also affect the system's HLB value, causing emulsion demulsification. The precipitated oil phase and surfactant aggregate and deposit at the rock pore throats, clogging the pores.

[0034] The technical solution provided in this disclosure has the following advantages compared with the prior art:

[0035] This disclosure combines an amphiphilic fluorocarbon surfactant with a nonionic surfactant and an organic co-solvent to form a composite surfactant. When used to assist CO2 flooding, it can effectively reduce the interfacial tension and minimum miscibility pressure between CO2 and crude oil, and reduce the flow resistance of CO2 in porous media, making it easier for gas to enter the micro-nano pores to displace crude oil. Using the composite surfactant provided in this disclosure to assist CO2 flooding can increase crude oil recovery by more than 9.56% and storage efficiency by more than 11.50%, achieving synergistic growth in both flooding and storage, and providing a new direction for achieving efficient CO2 displacement and storage in complex and tight reservoirs. Detailed Implementation

[0036] To better understand the above-mentioned objectives, features, and advantages of this disclosure, the solutions disclosed herein will be further described below. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0037] Numerous specific details are set forth in the following description in order to provide a full understanding of this disclosure, but this disclosure may also be implemented in other ways different from those described herein; obviously, the embodiments in the specification are only some, and not all, of the embodiments of this disclosure.

[0038] To better understand the above-mentioned objectives, features, and advantages of this disclosure, the solutions disclosed herein will be further described below. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other.

[0039] Numerous specific details are set forth in the following description in order to provide a full understanding of this disclosure, but this disclosure may also be implemented in other ways different from those described herein; obviously, the embodiments in the specification are only some, and not all, of the embodiments of this disclosure.

[0040] Example 1

[0041] This embodiment provides a compound system of a composite surfactant and supercritical CO2, wherein the composite surfactant comprises:

[0042] Fluorocarbon surfactants: ;

[0043] Nonionic cosurfactants: That is, AEO-7;

[0044] And, organic co-solvent: ethanol;

[0045] The mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent is 1:10:20.

[0046] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.3%, and 0.6% of the mass of the supercritical CO2, respectively.

[0047] Example 2

[0048] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that:

[0049] nonionic co-surfactants (AEO-7) replaced with (AEO-9).

[0050] Example 3

[0051] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that:

[0052] nonionic co-surfactants (AEO-7) replaced with (AEO-15).

[0053] Example 4

[0054] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that:

[0055] nonionic co-surfactants (AEO-7) replaced with (Isotridecyl alcohol polyoxyethylene ether EO-9).

[0056] Example 5

[0057] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that:

[0058] nonionic co-surfactants (AEO-7) replaced with .

[0059] Example 6

[0060] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that:

[0061] Fluorocarbon surfactants Replace with .

[0062] Example 7

[0063] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that:

[0064] Fluorocarbon surfactants Replace with .

[0065] Example 8

[0066] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that:

[0067] Fluorocarbon surfactants Replace with .

[0068] Example 9

[0069] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:6:20.

[0070] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.18%, and 0.6% of the mass of the supercritical CO2, respectively.

[0071] Example 10

[0072] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:12:20.

[0073] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.36%, and 0.6% of the mass of the supercritical CO2, respectively.

[0074] Example 11

[0075] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 2, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:6:20.

[0076] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.18%, and 0.6% of the mass of the supercritical CO2, respectively.

[0077] Example 12

[0078] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 2, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:12:20.

[0079] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.36%, and 0.6% of the mass of the supercritical CO2, respectively.

[0080] Example 13

[0081] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 3, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:6:20.

[0082] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.18%, and 0.6% of the mass of the supercritical CO2, respectively.

[0083] Example 14

[0084] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 3, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:12:20.

[0085] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.36%, and 0.6% of the mass of the supercritical CO2, respectively.

[0086] Example 15

[0087] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 4, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:6:20.

[0088] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.18%, and 0.6% of the mass of the supercritical CO2, respectively.

[0089] Example 16

[0090] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 4, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:12:20.

[0091] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.36%, and 0.6% of the mass of the supercritical CO2, respectively.

[0092] Example 17

[0093] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 5, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:6:20.

[0094] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.18%, and 0.6% of the mass of the supercritical CO2, respectively.

[0095] Example 18

[0096] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 5, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:12:20.

[0097] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.36%, and 0.6% of the mass of the supercritical CO2, respectively.

[0098] Example 19

[0099] This embodiment provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 2, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 3:30:40.

[0100] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.3%, and 0.4% of the mass of the supercritical CO2, respectively.

[0101] Example 20

[0102] The embodiment provides a compound system of composite surfactant and supercritical CO2, which is the same as the compound system in Example 2, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 3:30:80.

[0103] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.03%, 0.3%, and 0.8% of the mass of the supercritical CO2, respectively.

[0104] Comparative Example 1

[0105] This comparative example provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that the fluorocarbon surfactant is replaced with an equal mass of nonionic co-surfactant.

[0106] Comparative Example 2

[0107] This comparative example provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that: the fluorocarbon surfactant is used. Replace with (Sodium dodecylbenzenesulfonate).

[0108] Comparative Example 3

[0109] This comparative example provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is changed to 1:3:20.

[0110] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.0825%, 0.2475%, and 0.6% of the mass of the supercritical CO2, respectively.

[0111] Comparative Example 4

[0112] This comparative example provides a compound system of a composite surfactant and supercritical CO2, which is the same as the compound system in Example 1, except that the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant and the organic co-solvent is 1:20:20.

[0113] The amounts of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent added are 0.0157%, 0.3143%, and 0.6% of the mass of the supercritical CO2, respectively.

[0114] Oil displacement experiments were conducted using the compound systems obtained from the examples and comparative examples.

[0115] The experimental crude oil and core samples used are as follows:

[0116] Degassed crude oil from Block D: Formation (reservoir) temperature 95℃, formation (injection) pressure 20MPa; crude oil viscosity is 43.25mPa at 25℃ and 0.1MPa. s, density is 0.886 g / cm³ 3 .

[0117] The experimental core was from the tight sandstone reservoir in Block D. The core had a gas permeability of 0.176 mD and a gas porosity of 12.63%.

[0118] Test 1: Determination of minimum miscibility pressure using the interfacial tension disappearance method

[0119] Referring to the standard "SY / T 5370-2018", the interfacial tension between the pure CO2 system and the crude oil system was determined by the pendant drop method, and the interfacial tension between the compound system and the crude oil system described in Examples 1-20 and Comparative Examples 1-4 was determined. Then, the minimum miscibility pressure for CO2 displacement was calculated by the interfacial tension disappearance method.

[0120] The steps for determining interfacial tension using the pendant drop method include:

[0121] (1) Place pure CO2 or the compound system in an intermediate container and keep it at 95℃ and 6MPa initial pressure for 5h to form a stable state;

[0122] (2) Pure CO2 or a compound system is introduced into the reactor in stages by a gas booster pump, and the mixture is allowed to stand for 100 minutes. Then, the experimental crude oil is allowed to form droplets through the probe. The state of the droplets is observed, and the droplet morphology is captured by a microscopic imaging system. Based on the droplet morphology, the interfacial tension is calculated by substituting it into the Young-Laplace equation.

[0123] (3) Keep the temperature constant, gradually increase the pressure, repeat step (2), and then plot the interfacial tension as the ordinate and the pressure as the abscissa to fit the linear equation (R). 2 >0.98), the value when the interfacial tension is zero is calculated using the extrapolation method, which is the minimum miscibility pressure;

[0124] One value was measured at 1 MPa intervals, and the interfacial tension was measured at at least 8 pressures.

[0125] The test results for the minimum miscibility pressure are shown in Table 1.

[0126] Table 1

[0127]

[0128] As shown in Table 1, this disclosure combines an amphiphilic fluorocarbon surfactant with a nonionic surfactant and an organic co-solvent to form a composite surfactant. When used to assist CO2 flooding, it can effectively reduce the interfacial tension and minimum miscibility pressure between CO2 and crude oil.

[0129] Compared to Example 1, Comparative Example 1, which did not contain fluorocarbon surfactants, showed a smaller reduction in minimum miscibility pressure.

[0130] Compared with Example 1, Comparative Example 2 used a conventional surfactant instead of a fluorocarbon surfactant, but the effect was still not as good as the composite surfactant described in this disclosure.

[0131] Compared with Example 1, the amount of fluorocarbon surfactant added in Comparative Example 3 increased, while the amount of co-surfactant added decreased, resulting in the formation of a thicker fluorocarbon adsorption layer at the CO2-crude oil interface, which affected the diffusion of CO2 into the crude oil phase and increased the minimum miscibility pressure.

[0132] Compared with Example 1, the amount of fluorocarbon surfactant added in Comparative Example 4 was reduced, while the amount of co-surfactant added was increased, which affected the formation of the effective interface adsorption layer and resulted in a lower reduction in minimum miscibility.

[0133] Test 2: Determination of Oil Displacement and Storage Efficiency

[0134] Referring to the standard “SY / T 7454-2019”, indoor physical simulation experiments were conducted to carry out indoor oil displacement experiments. Under formation temperature and pressure, pure “CO2 flooding” and “composite surfactant-assisted CO2 flooding” experiments were carried out respectively, and the crude oil recovery rate and CO2 burial rate of pure “CO2 flooding” and “composite surfactant-assisted CO2 flooding” were calculated respectively.

[0135] Specifically, the oil displacement efficiency and burial efficiency of the compound systems described in Examples 1-6, 9-10 and Comparative Examples 1-4 were tested.

[0136] The specific steps are as follows:

[0137] (1) The experimental core was washed with oil and then dried at 120℃ for 24h. After drying, the experimental core was placed in the core holder and vacuumed for 12h. Then, the core was saturated with formation water of 12wt% manganese chloride 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.

[0138] (2) Heat the experimental crude oil to 95°C, and then inject it into the experimental core at a constant rate of 0.1 ml / min to replace the formation water. Stop the injection when no water is produced in the outlet section. After the crude oil is fully saturated, seal the experimental core back into the core holder.

[0139] (3) Inject the compound system or pure supercritical CO2 into the experimental core at a constant displacement rate of 0.1 ml / min, and control the confining pressure applied around the experimental core to always be higher than the displacement pressure by 3 MPa. Stop the injection when no more oil is produced; record the oil production and fluid output, and calculate the oil recovery rate and CO2 burial rate. The calculation formula for the oil recovery rate is shown in Equation (1), and the calculation formula for the CO2 burial rate is shown in Equations (2)-(6):

[0140] (1)

[0141] in, The crude oil recovery rate is %; The mass of saturated crude oil before displacement is expressed in g. The quality of crude oil produced after displacement is measured in g.

[0142] (2)

[0143] in, The amount of CO2 gas injected, in mol; The injection pressure of CO2, in Pa; Let m be the CO2 injection volume. 3 The CO2 injection temperature, in K; The ideal gas constant is 8.314 J / (mol). K).

[0144] (3)

[0145] in, The mass of injected CO2 is expressed in grams. The molar mass of CO2 is 44 g / mol.

[0146] (4)

[0147] in, The amount of CO2 gas produced, expressed in mol. The CO2 production pressure is 1.013 × 10⁻⁶. 5 Pa; m is the volume of CO2 produced. 3 The CO2 production temperature is 298.15 K; The ideal gas constant is 8.314 J / (mol). K).

[0148] (5)

[0149] in, The mass of CO2 extracted is expressed in grams. The molar mass of CO2 is 44 g / mol.

[0150] (6)

[0151] in, CO2 burial rate, % For the mass of injected CO2, The mass of CO2 produced.

[0152] The results of the crude oil recovery rate test are shown in Table 2, and the CO2 sequestration rate is shown in Table 3.

[0153] Table 2

[0154]

[0155] Table 3

[0156]

[0157] As shown in Tables 2 and 3, the use of the composite surfactant provided in this disclosure to assist CO2 flooding can effectively improve crude oil recovery and CO2 storage rate. Compared with pure CO2 flooding, crude oil recovery can be increased by more than 9.56%, and CO2 storage rate can be increased by more than 11.50%.

[0158] The above description is merely a specific embodiment of this disclosure, enabling those skilled in the art to understand or implement it. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this disclosure. Therefore, this disclosure is not to be limited to the embodiments described herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A composite surfactant for assisting CO2 oil displacement and storage, characterized in that, This includes fluorocarbon surfactants, nonionic cosurfactants, and organic cosolvents; The mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent is 1:(6-12):(10-30); The fluorocarbon surfactant is selected from one or more having the structure shown in Formula I; Equation I Where R1 represents —C6H 13 Any one of the alkyl groups with 12-18 carbon atoms, where R2 represents (CF2)3CF3; The nonionic cosurfactant includes fatty alcohol polyoxyethylene ether and / or fatty acid polyoxyethylene ester. The organic co-solvent includes one or more of ethanol, methanol, or n-butanol.

2. The composite surfactant for assisting CO2 oil displacement and storage according to claim 1, wherein the mass ratio of the fluorocarbon surfactant, the nonionic co-surfactant, and the organic co-solvent is 1:(9-11):(14-16).

3. The composite surfactant for assisting CO2 oil displacement and storage according to claim 1, characterized in that, The fatty alcohol polyoxyethylene ether is selected from one or more having the structure shown in Formula II; Formula II Wherein, R3 represents any one of alkyl groups with 12-13 carbon atoms, and n1 is any one of 7-15; And / or, the fatty acid polyoxyethylene ester is selected from one or more having the structure shown in Formula III; Formula III Wherein, R4 represents any alkyl group with 8-10 carbon atoms, and n2 is any alkyl group with 5-15 carbon atoms.

4. The application of a composite surfactant as described in any one of claims 1-3 in the field of CO2 flooding and CO2 storage.

5. A method for CO2-driven oil displacement, characterized in that, The method includes: injecting a mixture of the composite surfactant as described in any one of claims 1-3 and supercritical CO2 into the reservoir for oil displacement.

6. The method according to claim 5, characterized in that, The total salinity of the reservoir is 10. 4 -10 5 mg / L, at a temperature of 80-150℃; The crude oil in the reservoir has a viscosity of 1-50 mPa at 80-150℃. s; The gas permeability of the reservoir core is 0.01-0.05 mD.

7. The method according to claim 5, characterized in that, During the injection into the reservoir, the injection pressure is 10-20 MPa; And / or, the mass of the added fluorocarbon surfactant is 0.01-0.1% of the mass of the supercritical CO2.

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