A surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs and its preparation and application

Abstract

The present invention provides a surfactant nanocarrier oil-displacing agent suitable for low-permeability / tight oil reservoirs and its preparation and application, wherein the oil-displacing agent is composed of the following components by mass fraction: 0-2wt% anionic surfactant or anionic nonionic surfactant, 0.4-8wt% nonionic surfactant, 0.5-10wt% oil phase, and the rest is deionized water. The oil-displacing agent of the present invention wraps the surfactant inside the nanocarrier, and as the temperature rises during the stratum migration process of the low-permeability / tight oil reservoir, the nanocarrier is transformed from a solid into a liquid, gradually releasing the surfactant and the oil phase therein. Thus, while reducing the adsorption loss during the surfactant flooding process, diluting the residual oil adsorbed on the surface of the formation rock, the oil-water interfacial tension can be significantly reduced and the residual oil can be stripped off, thereby improving the oil displacement efficiency; and at the same time, the effect of pressure reduction and injection is shown.

Description

Technical Field

[0001] The invention relates to the technical field of petroleum extraction, and in particular to a surfactant nano-carrier oil displacement agent suitable for low-permeability / tight oil reservoirs and the preparation and application thereof. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention, and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] With the decrease of recoverable reserves in medium and high permeability reservoirs and the steady growth of global oil demand, the development of low permeability reservoirs has attracted more and more attention. Low permeability reservoir resources are very rich in the world and widely distributed. Basically, all oil-producing countries have such reservoirs. With the advancement of exploration and development, the proportion of low permeability / tight oil reservoirs is increasing, but low permeability / tight oil reservoirs have the characteristics of low permeability, poor reservoir properties, narrow pore throats, and strong Jamin effect, which lead to problems such as high injection pressure and low recovery rate in conventional water drive. Therefore, it is imperative to seek technologies suitable for low permeability / tight oil reservoirs to efficiently improve the recovery rate.

[0004] Currently, surfactant flooding has been proven to be an effective way to improve the development of low-permeability / tight oil reservoirs. The surfactant system can reduce the oil-water interfacial tension and injection pressure, and can emulsify, disperse and strip crude oil, thereby increasing the recovery rate.

[0005] However, the problems existing in the application of surfactant flooding in low permeability / tight oil reservoirs have gradually become prominent. There is a strong interaction between surfactant molecules and formation rocks, resulting in serious adsorption losses, which affects the migration distance and effect of surfactants in the formation, resulting in a decrease in the effect of improving oil recovery, and bringing about a relatively serious high cost problem. Therefore, reducing adsorption losses is particularly important for the effect of surfactant flooding to improve oil recovery. Summary of the invention

[0006] In order to solve the deficiencies of the prior art, the present invention provides a surfactant nanocarrier oil displacement agent and its preparation and application, specifically a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs and its preparation and application.

[0007] In order to achieve the above object, the technical solution of the present invention is:

[0008] In a first aspect of the present invention, a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs is provided, wherein the oil displacement agent is composed of the following components by mass fraction: 0-2 wt% of anionic surfactant or anionic-nonionic surfactant, 0.4-8 wt% of nonionic surfactant, 0.5-10 wt% of oil phase, and the remainder is deionized water;

[0009] Wherein, the ionic surfactant or anionic-nonionic surfactant and nonionic surfactant together account for 0.5-10wt% of the total mass fraction of the oil displacement agent;

[0010] The oil phase is blended wax, which is composed of wax and additives;

[0011] The wax is beeswax, paraffin, microcrystalline wax, Fischer-Tropsch wax or polyethylene wax;

[0012] The additives are paraffin, limonene, C4-C 16 Any one or a mixture of two straight-chain or branched alkanes within the range, the mass ratio of the additive to the wax is 0:10 to 4:1, and the amount of the additive is not 0;

[0013] The nonionic surfactant is fatty alcohol polyoxyethylene ether or alkylphenol polyoxyethylene ether with a low cloud point.

[0014] The second aspect of the present invention provides a method for preparing a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs as described in the first aspect, comprising:

[0015] A certain amount of additive is mixed with wax to form blended wax, wherein the mass ratio of the additive to the wax is 0:10 to 4:1, and the amount of the additive is not 0;

[0016] At a first temperature, dissolving a nonionic surfactant in a melted blended wax to obtain a wax / surfactant mixture, adding deionized water to the wax / surfactant mixture, and fully stirring (once) to obtain a white emulsion;

[0017] The white emulsion is lowered to a second temperature, and when the white emulsion becomes translucent, an appropriate amount of anionic surfactant or anionic-nonionic surfactant is added, and further stirred (secondary stirring) to obtain a transparent light blue nanoemulsion;

[0018] The transparent light blue nanoemulsion is rapidly cooled in an ice water bath to obtain a nano-carrier oil displacement agent.

[0019] The third aspect of the present invention is the use of the surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs described in the first aspect or the surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs prepared by the preparation method described in the second aspect in oil production.

[0020] The beneficial effects of the present invention are:

[0021] (1) The nanocarrier oil-displacing agent of the present invention adopts a low-energy emulsification method, has a simple preparation process and low cost, and the prepared nanocarrier oil-displacing agent has a small size (1 to 300 nm), and can migrate in the pore throats of low-permeability / tight oil reservoir formations by virtue of the "small size" effect, effectively expanding the swept volume of the micro-nano pore throat matrix, and greatly reducing the water injection start-up pressure gradient;

[0022] (2) The nanocarrier oil displacement agent of the present invention has high interfacial activity (depending on the different encapsulated surfactants, the oil-water interfacial tension can be reduced to 10 -2 ~10 -3 mN·m -1 ) and wetting reversal ability, and also has strong stability (stable for more than 30 days under reservoir conditions);

[0023] (3) The nanocarrier oil displacement agent of the present invention is divided into two forms, nanoparticles (below the melting point) and nanoemulsions (nanoemulsions), according to the different melting points of the nanocarrier oil phase. The nanocarrier is dissolved in crude oil to release surfactants, and the latter releases faster than the former. Therefore, by adjusting the melting point of the oil phase, a nanocarrier oil displacement agent suitable for different target oil layer temperatures and with adjustable release rate can be obtained;

[0024] (4) The nanocarrier oil displacement agent of the present invention, because the surfactant molecules are encapsulated inside and on the interface of the nanocarrier, reduces the interaction between the surfactant molecules and the formation rocks during the formation migration process, thereby effectively reducing the adsorption loss of the surfactant during the oil displacement process of low permeability / tight oil reservoirs;

[0025] (5) The nanocarrier oil displacement agent described in the present invention can effectively strip away the residual oil on the surface of the formation rock by reducing the adsorption loss of surfactant during the oil displacement process of low permeability / tight oil reservoirs, using the oil phase to dilute the residual oil adsorbed on the surface of the formation rock, and reducing the oil-water interfacial tension, thereby significantly improving the oil displacement efficiency of low permeability / tight oil reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0027] Figure 1 The melting point of beeswax in the present invention changes with the proportion of additives;

[0028] Figure 2 It is the relationship among water content, recovery rate, pressure and injection pore volume in the present invention. DETAILED DESCRIPTION

[0029] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0030] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0031] The purpose of the present invention is to provide a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs and its preparation and application, wherein the oil displacement agent comprises the following components: anionic surfactant or anionic-nonionic surfactant, nonionic surfactant, oil phase, and deionized water. The oil displacement agent of the present invention wraps the surfactant inside the nanocarrier, and as the temperature rises during the migration of the formation, the nanocarrier changes from solid to liquid, and after contacting with crude oil, it dissolves in the crude oil and gradually releases the surfactant and oil phase therein. Thus, while reducing the adsorption loss during the surfactant oil displacement process, the residual oil adsorbed on the surface of the formation rock is diluted, the oil-water interfacial tension can be significantly reduced, the residual oil can be stripped off, and the oil displacement efficiency can be improved; at the same time, the effect of reducing pressure and increasing injection is exhibited.

[0032] In a typical embodiment of the present invention, a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs is provided, wherein the oil displacement agent is composed of the following components by mass fraction: 0-2 wt% of anionic surfactant or anionic-nonionic surfactant, 0.4-8 wt% of nonionic surfactant, 0.5-10 wt% of oil phase, and the rest is deionized water;

[0033] Wherein, the ionic surfactant or anionic-nonionic surfactant and nonionic surfactant together account for 0.5-10wt% of the total mass fraction of the oil displacement agent;

[0034] The oil phase is blended wax, which is composed of wax and additives;

[0035] The wax is beeswax, paraffin, microcrystalline wax, Fischer-Tropsch wax or polyethylene wax;

[0036] The additives are paraffin, limonene, C4-C 16 Any one or a mixture of two straight-chain or branched alkanes within the range, the mass ratio of the additive to the wax is 0:10 to 4:1, and the amount of the additive is not 0;

[0037] The nonionic surfactant is fatty alcohol polyoxyethylene ether or alkylphenol polyoxyethylene ether with a low cloud point.

[0038] In some examples of this embodiment, the additive is a mixture of any two of paraffin, limonene, and dodecane;

[0039] Preferably, when the additive is paraffin, the mass ratio of paraffin to beeswax is 2 to 5, more preferably 4;

[0040] Preferably, when the additive is limonene, the mass ratio of limonene to beeswax is 1 to 2, more preferably 1.5;

[0041] Preferably, when the additive is dodecane, the mass ratio of dodecane to beeswax is 1 to 2, more preferably 1.5.

[0042] In some examples of this embodiment, the anionic surfactant is dodecylbenzene sulfonate, and the anionic-nonionic surfactant is fatty alcohol polyoxyethylene ether carboxylate;

[0043] Preferably, the amount of anionic surfactant or anionic-nonionic surfactant is 0.1wt%;

[0044] Preferably, the amount of the nonionic surfactant is 0.4 wt%.

[0045] In some examples of this embodiment, the cloud point is lower than 60°C;

[0046] Furthermore, the average particle size of the oil-displacing agent emulsion is 1 to 300 nm.

[0047] In another typical embodiment of the present invention, a method for preparing the above-mentioned surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs is provided, comprising:

[0048] First, a certain amount of additives and beeswax are mixed to form blended wax, wherein the mass ratio of the additives to the blended wax is 0 to 4, and the amount of the additives is not 0;

[0049] Then, the nanocarrier is prepared by a low-energy emulsification method. At a first temperature, a nonionic surfactant is dissolved in a melted blending wax to obtain a wax / surfactant mixture, and deionized water is added to the wax / surfactant mixture, and a white emulsion is fully stirred (stirred once);

[0050] The white emulsion is cooled to a second temperature by transferring the emulsion to a water bath at the second temperature. As the surfactant molecules migrate from the oil phase to the oil-water interface, the emulsification efficiency increases, the droplet size decreases, and the solution becomes translucent. When the white emulsion becomes translucent, an appropriate amount of anionic surfactant or anionic-nonionic surfactant is added, and the mixture is stirred fully (secondary stirring) to obtain a transparent light blue nanoemulsion.

[0051] Finally, the transparent light blue nanoemulsion was rapidly cooled in an ice-water bath (0°C) to obtain a nanocarrier oil displacement agent suitable for low-permeability / tight oil reservoirs.

[0052] In some examples of this embodiment, the first temperature is 2-5°C higher than the melting temperature of the blending wax and the cloud point of the nonionic surfactant, and the cloud point is less than 60°C.

[0053] In some examples of this embodiment, the second temperature is 2-5°C lower than the cloud point of the nonionic surfactant, and the cloud point is less than 60°C.

[0054] In some examples of this embodiment, the temperature of the deionized water is the same as the first temperature.

[0055] In some examples of this embodiment, the sufficient stirring and the continued sufficient stirring are both performed using a mechanical stirrer;

[0056] Further, the mixture was stirred at 200 r / min for 10 min.

[0057] Further, continue to stir fully (secondary stirring) at 400 r / min for 10 min.

[0058] In another typical embodiment of the present invention, the use of the above-mentioned surfactant nanocarrier oil-displacing agent suitable for low permeability / tight oil reservoirs or the surfactant nanocarrier oil-displacing agent suitable for low permeability / tight oil reservoirs prepared by the above-mentioned preparation method in oil production is provided.

[0059] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with specific embodiments.

[0060] Example 1

[0061] Preparation of the blended wax:

[0062] According to the different melting points of the nanocarrier oil phase, the nanocarrier is dissolved in crude oil to release surfactants in two forms: nanoparticles (below the melting point) and nanoemulsions (nanoemulsions), and the latter releases faster than the former. In order to better use the nanocarrier oil displacement agent in the target formation (the melting point is controlled within 2 to 5 °C below the formation temperature, taking the formation temperature of 45 °C as an example), it is necessary to regulate the melting point of the wax through physical modification to obtain a blended wax. Three additives (paraffin, limonene, and dodecane) are selected and mixed with beeswax in different mass proportions. Taking the melting point as an indicator, the blended wax formula that meets the melting point requirements (42 to 47 °C) is preferred. Using the observation method, the blended wax with different additive mass proportions is added to the test tube, placed in a 62 °C (2 °C higher than the melting point of beeswax) water bath, and the water bath temperature is gradually reduced (1 °C / 10min). After stabilization, the state of the system is observed. When the system solidifies, the corresponding temperature is recorded as the melting point of the blended wax.

[0063] It was observed that the three additives could form a uniform and transparent blended wax after being mixed with beeswax, and no stratification occurred after standing, indicating that the three additives had a good solubility effect in beeswax. Figure 1 The melting point of beeswax changes with the proportion of additives. As the proportion of additives increases, the melting point of beeswax gradually decreases. Among them, the addition of limonene has the most significant effect on the reduction of the melting point of beeswax. When the mass ratio of additives to beeswax is 4, limonene can reduce the melting point of wax to 33°C; n-dodecane is second, and paraffin has the worst effect on reducing the melting point of beeswax, which is 47°C. Combined with the formation temperature of the target reservoir (45°C), the melting point of the blended wax should be near the formation temperature (43-47°C). The melting points of the three systems of blended wax 1 (paraffin to beeswax mass ratio of 4), blended wax 2 (dodecane to beeswax mass ratio of 1.5), and blended wax 3 (limonene to beeswax mass ratio of 1.5) meet the requirements.

[0064] The viscosity of the blending wax at 45°C was measured by a Physica MCR301 rheometer (Anton Paar, Austria) to further screen the best blending wax formula. At 45°C, the viscosities of the three systems of blending wax 1, blending wax 2, and blending wax 3 were 121 mPa·s, 3.81 mPa·s, and 2.31 mPa·s, respectively. Blending wax 3 had the lowest viscosity at the corresponding temperature, and its lower melting point (42.2°C) would also ensure that it was liquid at formation temperature (45°C). Blending wax 3 (the mass ratio of limonene to beeswax was 1.5) was determined as the best blending wax formula.

[0065] Example 2

[0066] Preparation of the nano-carrier phase oil displacement agent:

[0067] Add 1wt% blended wax 3 (the mass ratio of limonene to beeswax is 1.5), 0.4wt% nonionic surfactant (fatty alcohol polyoxyethylene ether) AEO7 and appropriate amount of deionized water to a 60°C beaker and stir thoroughly (200r / min, 10min) to obtain a white emulsion; transfer the emulsion to a 45°C water bath, wait until the solution becomes translucent, add 0.1wt% anionic surfactant dodecylbenzene sulfonate, continue to stir thoroughly (400r / min, 10min) to obtain a light blue transparent nanoemulsion. The nanoemulsion is rapidly cooled in an ice water bath (0°C) to obtain a nanocarrier oil-displacing agent 1. The nanocarrier oil-displacing agent 1 was subjected to a performance test (formation temperature 45°C, mineralization: 9243mg / L), and the results are shown in Table 1:

[0068] Table 1 Performance test results of sample 1

[0069] project index Average particle size, nm 56 Zeta potential, mV -50.53 Package rate, % 88.9 <![CDATA[Surface activity, mN·m -1 > <![CDATA[8.7×10 -2 ]]> Change wettability Wetting reversal possible Adsorption is less than that of surfactant solution, % 94.1

[0070] Example 3

[0071] Preparation of the nano-carrier phase oil displacement agent:

[0072] Add 1.5wt% blended wax 3 (the mass ratio of limonene to beeswax is 1.5), 0.4wt% non-ionic surfactant AEO7 and appropriate amount of deionized water to a 60℃ beaker and stir thoroughly (200r / min, 10min) to obtain a white emulsion; transfer the emulsion to a 45℃ water bath, wait until the solution becomes translucent, add 0.1wt% anionic-nonionic surfactant fatty alcohol polyoxyethylene ether carboxylate (SS231), continue to stir thoroughly (400r / min, 10min) to obtain a light blue transparent nanoemulsion. The nanoemulsion is rapidly cooled in an ice water bath (0℃) to obtain a nanocarrier oil-displacing agent 2. The nanocarrier oil-displacing agent 2 is subjected to performance testing (formation temperature 45℃, mineralization: 9243mg / L), and the results are shown in Table 2:

[0073] Table 2 Performance test results of sample 2

[0074] project index Average particle size, nm 92 Zeta potential, mV -48.13 Package rate, % 90.9 <![CDATA[Surface activity, mN·m -1 > <![CDATA[6.8×10 -2 ]]> Change wettability Wetting reversal possible Adsorption is less than that of surfactant solution, % 93.6

[0075] Example 4

[0076] Preparation of nano-carrier phase oil displacement agent:

[0077] Add 1.0wt% blended wax 3 (the mass ratio of limonene to beeswax is 1.5), 0.4wt% non-ionic surfactant AEO7 and appropriate amount of deionized water to a 60℃ beaker and stir thoroughly (200r / min, 10min) to obtain a white emulsion; transfer the emulsion to a 45℃ water bath, wait until the solution becomes translucent, add 0.1wt% anionic-nonionic surfactant fatty alcohol polyoxyethylene ether carboxylate (SS231), continue to stir thoroughly (400r / min, 10min) to obtain a light blue transparent nanoemulsion. The nanoemulsion is rapidly cooled in an ice water bath (0℃) to obtain a nanocarrier oil-displacing agent 3. The nanocarrier oil-displacing agent 3 is subjected to performance testing (formation temperature 45℃, mineralization: 9243mg / L), and the results are shown in Table 3:

[0078] Table 3 Performance test results of nanocarrier oil displacement agent 3

[0079] project index Average particle size, nm 45 Zeta potential, mV -49.20 Package rate, % 89.1 <![CDATA[Surface activity, mN·m -1 > <![CDATA[7.2×10 -2 ]]> Change wettability Wetting reversal possible Adsorption is less than that of surfactant solution, % 91.7

[0080] The nanocarrier oil-displacing agent 3 was used for the evaluation of oil-displacing performance (formation temperature 45° C., salinity: 9243 mg / L).

[0081] (1) Oil washing experiment

[0082] In order to study the oil washing effect of nanocarrier oil displacement agent 3, the weight loss method was used to test the oil washing efficiency of nanocarrier oil displacement agent 3 on oil sands with different oil contents at 45°C. The simulated oil and 80-mesh quartz sand were prepared into oil sands with an oil content of 5-25wt% in different proportions, and aged for 4h at formation temperature (45°C); 5g of oil sands were weighed and placed in a weighed test tube (mass w1); 20mL of nanocarrier oil displacement agent 3 was added, stirred gently, and the temperature was kept constant for 24h; the separated oil and liquid were removed, and the total mass of the test tube and oil sand after oil washing was measured w2 after drying and weighing.

[0083] The oil washing efficiency φ is calculated according to formula (2):

[0084]

[0085] In the formula: φ is the oil washing efficiency of the oil displacement agent, %; w1 is the mass of the test tube, g; w2 is the total mass of the test tube and oil sand after oil washing, g; x is the oil content of the oil sand, %.

[0086] Table 3 shows the oil washing efficiency of nanocarrier oil displacing agent 3 under different oil contents of oil sands. At 45°C, the oil washing efficiency of nanocarrier oil displacing agent 3 for oil sands with different oil contents increases with the increase of oil content, and can reach up to 92.8%. After adding nanocarrier oil displacing agent 3 to the oil sands, the nanocarrier gradually dissolves in the simulated oil on the oil sands to release surfactants, significantly reducing the interfacial tension, thereby reducing the adhesion work on the surface of the oil sands. Therefore, the adsorbed simulated oil is separated from the sand, thereby improving the oil washing efficiency. It can be concluded that nanocarrier oil displacing agent 3 has good oil washing efficiency while having anti-adsorption effect, so it can be used for tertiary oil recovery.

[0087] Table 4 The oil washing efficiency of nanocarrier oil displacement agent 3 on oil sands with different oil contents at 45°C

[0088] Oil content / wt% 5 10 15 20 25 Oil washing efficiency / % 86.3 90.3 90.6 92.3 92.8

[0089] (2) Low permeability core physical flooding experiment

[0090] The physical displacement device was used to evaluate the oil displacement performance of the nanocarrier oil displacement agent 3. The physical displacement experimental steps are as follows:

[0091] 1) Take the artificial core, measure the dry weight, and use a constant flow pump to pump at a constant speed (0.1, 0.3, 0.5 mL min -1 ) Displace formation water into the core, record pressure changes, calculate water permeability; measure the wet weight of the core, and calculate the pore volume;

[0092] 2) At a constant speed (0.1 mL min -1 ) Displace crude oil into the core until no water is produced at the core production end, and record the volume of water in the produced fluid. The volume of water is the volume of saturated oil input, and the oil saturation is calculated;

[0093] 3) At a constant speed (0.1 mL min -1 ) displaces formation water (mineralization 9243 mg·L) into the core -1 ), record the data every half hour, record the volume of oil and water in the produced fluid, and drive to the water content of the produced fluid of 98%;

[0094] 4) At a constant speed (0.1 mL min -1 ) injecting nano-carrier oil displacement agent 3 into the core, and the injection plug volume is 0.5 PV;

[0095] 5) Carry out subsequent water flooding on the core until the water content reaches 98%, and calculate the recovery factor.

[0096] The core with gas permeability of 30mD, water permeability of 3.4mD, porosity of 12.2% and saturation of 65% was selected. Figure 2Figure 3 is the relationship between recovery factor, water content, pressure and injected pore volume. After injecting 2.6 PV of injected water, the water content remained above 98% and the water drive recovery factor was 46.4%. After injecting 0.5 PV of nanocarrier oil displacement agent 3, the recovery factor gradually increased and the final total recovery factor was 61.4%. After injecting the nanocarrier plug, the recovery factor increased by about 15.0% compared with water drive.

[0097] Analyzing the pressure changes in the displacement experiment, the injection pressure first increases and then decreases during the pre-water drive, and then gradually stabilizes. This is because the formation oil production requires a starting pressure. With the continuous injection of liquid, the internal pressure gradually increases until the starting pressure is reached, and the production end begins to discharge liquid, accompanied by a pressure drop. As the liquid discharge rate from the production end stabilizes, the pressure will reach a position lower than the starting pressure and remain stable. After the oil is gradually produced, when the water drive recovery rate reaches 98%, it begins to switch to nanocarrier displacement. When it reaches a stable state, the pressure will also drop. This is because the injection of the system reduces the oil-water interfacial tension and reduces the capillary resistance, making it easier for the system to flow in the pores, starting the remaining oil that is difficult to produce, thereby improving the recovery rate. In addition, the surfactant released by the nanocarrier can emulsify and disperse the larger oil droplets in the pores into small oil droplets, no longer blocking the pores, and effectively reducing the water injection pressure. The equilibrium pressure of the post-water drive is about 30% lower than that of the pre-water drive. The nanocarrier shows the effect of reducing pressure and increasing injection during the oil displacement process.

[0098] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs, characterized in that: The oil displacement agent is composed of the following components by mass fraction: 0.5-10wt% of oil phase, 0-2wt% of anionic surfactant or anionic-nonionic surfactant, 0.4-8wt% of nonionic surfactant, and the rest is deionized water; Among them, the anionic surfactant or anionic-nonionic surfactant and nonionic surfactant together account for 0.5-10wt% of the total mass fraction of the oil displacement agent; The oil phase is blended wax, which is composed of wax and additives; The wax is beeswax; The additive is limonene, and the mass ratio of limonene to beeswax is 1-2; The nonionic surfactant is fatty alcohol polyoxyethylene ether or alkylphenol polyoxyethylene ether; The average particle size of the oil displacement agent emulsion is 1 to 300 nm; The anionic surfactant is dodecylbenzene sulfonate, and the anionic-nonionic surfactant is fatty alcohol polyoxyethylene ether carboxylate; The preparation method of the surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs is as follows: taking a certain amount of additives and mixing with beeswax to form a blended wax, wherein the mass ratio of the additives to the beeswax is 1-2; At a first temperature, dissolving a nonionic surfactant in a melted blended wax to obtain a wax / surfactant mixture, adding deionized water to the wax / surfactant mixture, and fully stirring a white emulsion; The white emulsion is lowered to a second temperature, and when the white emulsion becomes translucent, an appropriate amount of anionic surfactant or anionic-nonionic surfactant is added, and the mixture is stirred fully to obtain a transparent light blue nanoemulsion; The transparent light blue nanoemulsion was rapidly cooled in an ice water bath to obtain a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs; The first temperature is 2-5°C higher than the melting temperature of the blending wax and the cloud point of the nonionic surfactant, and the cloud point is less than 60°C; The second temperature is 2-5°C lower than the cloud point of the nonionic surfactant, and the cloud point is less than 60°C.

2. The surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs according to claim 1, characterized in that: The additive is limonene, and the mass ratio of limonene to beeswax is 1.

5.

3. The surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs according to claim 1, characterized in that: The amount of anionic surfactant or anionic-nonionic surfactant used is 0.1wt%.

4. The surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs according to claim 1, characterized in that: The dosage of the nonionic surfactant is 0.4 wt %.

5. A method for preparing a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs according to any one of claims 1 to 4, characterized in that: include: A certain amount of additives and beeswax are mixed to form blended wax, wherein the mass ratio of the additives to the beeswax is 1 to 2; At a first temperature, dissolving a nonionic surfactant in a melted blended wax to obtain a wax / surfactant mixture, adding deionized water to the wax / surfactant mixture, and fully stirring a white emulsion; The white emulsion is lowered to a second temperature, and when the white emulsion becomes translucent, an appropriate amount of anionic surfactant or anionic-nonionic surfactant is added, and the mixture is stirred fully to obtain a transparent light blue nanoemulsion; The transparent light blue nanoemulsion is rapidly cooled in an ice water bath to obtain a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs.

6. The method for preparing a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs according to claim 5, characterized in that: The temperature of the deionized water is the same as the first temperature.

7. The method for preparing a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs according to claim 5, characterized in that: The sufficient stirring and continued sufficient stirring are both performed using a mechanical stirrer.

8. The method for preparing a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs according to claim 5, characterized in that: The stirring speed is 200r / min and the stirring time is 10min.

9. The method for preparing a surfactant nanocarrier oil displacement agent suitable for low permeability / tight oil reservoirs according to claim 5, characterized in that: Continue to stir fully (secondary stirring) at 400r / min for 10min.

10. Use of a surfactant nanocarrier oil-displacing agent suitable for low-permeability / tight oil reservoirs as claimed in any one of claims 1 to 4 or a surfactant nanocarrier oil-displacing agent suitable for low-permeability / tight oil reservoirs prepared by the preparation method as claimed in any one of claims 5 to 9 in oil production.

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