1. An in situ-generated mid-phase microemulsion wash oil system, comprising, in mass fractions: 0.5 to 6.5 percent of surfactant, 1.5 to 3.5 percent of cosurfactant, 2.5 to 4.5 percent of salt and the balance of water; the surfactant is formed by mixing a nonionic surfactant and an anionic surfactant according to a mass ratio of 1: 2-5: 1.

2. The in situ-forming mid-phase microemulsion washing oil system of claim 1, wherein the non-ionic surfactant is selected from one of polyoxyethylene fatty acid ester, alkylphenol ethoxylate and tween-20.

3. The in situ-forming mid-phase microemulsion oil system of claim 1 wherein the anionic surfactant is selected from the group consisting of sodium fatty alcohol-polyoxyethylene ether sulfate, sodium alpha-olefin sulfonate and sodium fatty acid methyl ester sulfonate.

4. The in situ-forming mid-phase microemulsion system of claim 1 wherein the co-surfactant is selected from any of n-hexanol, n-butanol or n-pentanol.

5. The in situ-forming, mid-phase microemulsion system of claim 1 wherein the salt is potassium chloride or sodium chloride.

6. A process for the preparation of an in situ formed mid-phase microemulsion system according to any one of claims 1 to 5 comprising the steps of: adding salt into water to prepare a salt solution; and sequentially adding the nonionic surfactant, the anionic surfactant and the cosurfactant into the salt solution, and stirring to obtain the in-situ generated middle-phase micro-emulsion oil-washing system.

Background

Along with the increasing of oil and gas development strength, the oil reservoir resource reserves are reduced continuously, and the oil field production-maintaining difficulty is increased gradually. The low-permeability oil reservoir has become a key object for increasing oil and potential of each oil field due to the characteristics of wide distribution, rich reserves and the like. The WCXX oil field ZJ1-3U/L oil is located in the west of the south China sea, and is transferred to waterflood development due to poor reservoir physical properties and insufficient natural energy. The oil has high mud content (clay content 16.3-24.1%, wherein content of illite-montmorillonite mixed layer exceeds 50%), low permeability (logging permeability is 16.3-24.1 × 10)^-3μm²Diameter of throat 8.0 × 10^-6～2.8×10^-5m) and the likeIn the process of operation and well flushing, reservoir fluid returns to a shaft, and in the process of secondary injection, as part of crude oil returns to a near-well reservoir, the oil-water interface tension of the reinjection produced water and the crude oil is large, the injection pressure is high, and the difficulty of water injection is increased.

Microemulsion flooding is an advanced method in enhanced oil recovery, and can improve the crude oil recovery ratio to 80-90%. The microemulsion is a transparent stable system for increasing and dissolving oil and water, the radius of the liquid drop is between 1 and 100nm, the microemulsion is easier to enter a target reservoir tiny pore throat compared with an emulsion (the diameter is micron), and the oil washing efficiency is higher. The microemulsion is divided into three types, namely a lower phase microemulsion (Winsor I type), a middle phase microemulsion (Winsor III type) and an upper phase microemulsion (Winsor II), wherein the middle phase microemulsion can form a miscible phase with reservoir residual oil to realize high-efficiency displacement, and the oil washing efficiency is much higher compared with that of other two types of microemulsions. The existing in-situ generated middle-phase microemulsion is developed aiming at microemulsion flooding, the time for forming the stable middle-phase microemulsion is generally 7 days, and the effect is poor when the in-situ generated middle-phase microemulsion is applied to reinjection priming after well flushing in water well operation.

Disclosure of Invention

Aiming at the problem that the start injection pressure is increased due to the fact that reservoir fluid returns to a shaft in the process of low-permeability reservoir operation and well flushing, the invention provides an in-situ generated middle-phase microemulsion flushing oil system which can rapidly form stable anti-swelling middle-phase microemulsion with crude oil in situ and can rapidly form middle-phase microemulsion with high solubilizing capability.

The invention provides an in-situ generated middle-phase microemulsion washing oil system, which comprises the following components in percentage by mass: 0.5 to 6.5 percent of surfactant, 1.5 to 3.5 percent of cosurfactant, 2.5 to 4.5 percent of salt and the balance of water; the surfactant is formed by mixing a nonionic surfactant and an anionic surfactant according to a mass ratio of 1: 2-5: 1.

Further, the nonionic surfactant is selected from one of polyoxyethylene fatty acid ester, alkylphenol polyoxyethylene or tween-20.

Further, the anionic surfactant is any one of fatty alcohol-polyoxyethylene ether sodium sulfate, alpha-alkenyl sodium sulfonate or fatty acid methyl ester sodium sulfonate; the nonionic surfactant and the anionic surfactant contain ethoxy, the ethoxy has good solubilizing power, and the middle-phase microemulsion can be dissolved into more oil due to the strong solubilizing power, so that the oil washing efficiency is improved.

Furthermore, the cosurfactant is any one of n-hexanol, n-butanol or n-pentanol, and the n-hexanol, the n-butanol or the n-pentanol can be dissolved in the surfactant at a dissolving speed of less than 10 min.

Further, the salt is potassium chloride or sodium chloride.

The invention also provides a preparation method of the in-situ generated middle-phase microemulsion washing oil system, which comprises the following steps: adding salt into water to prepare a salt solution; and (3) sequentially adding the nonionic surfactant, the anionic surfactant and the cosurfactant into the salt solution at room temperature, and stirring for 3 minutes to obtain the in-situ generated middle-phase microemulsion oil washing system.

Further, the concentration of the salt solution differs from the formation water salinity by no more than ± 10%.

Further, the maximum mass fraction of surfactant and co-surfactant is the apex of the intersection of the line forming the maximum co-surfactant loading fit for the middle phase microemulsion and the dilution line.

The action mechanism of the middle-phase microemulsion oil washing system provided by the invention for displacing oil sand is as follows: the microemulsion is a system in a stable state of thermodynamics, and the interfacial tension between the middle-phase microemulsion and the surplus water phase and oil phase can reach 10^-2～10^{- 3}mN/m, the net curvature of the oil-water interface is zero. The middle-phase microemulsion is a bicontinuous microemulsion and is in a state between oil-in-water type microemulsion and water-in-oil type microemulsion, the middle-phase microemulsion can form a mixed phase with reservoir residual oil to realize efficient displacement of the oil washing, the particle size of the liquid drop is 1-100 nm, the liquid drop is easier to enter the fine pore throat of a target reservoir compared with the emulsion, and the oil washing efficiency is higher.

The technical scheme provided by the invention has the beneficial effects that: after being injected into a reservoir, the middle-phase microemulsion oil washing system provided by the invention can enter a small pore throat of a target reservoir to form a miscible phase with residual oil of the reservoir, so that high-efficiency displacement is realized, the oil washing efficiency can reach more than 98%, and the oil washing time is 1-2 h.

Detailed Description

To make the objects, technical solutions and advantages of the present invention more apparent, the following embodiments of the present invention

The embodiment of the invention provides an in-situ generated middle-phase microemulsion washing oil system which comprises the following components in percentage by mass: 0.5 to 6.5 percent of surfactant, 1.5 to 3.5 percent of cosurfactant, 2.5 to 4.5 percent of salt and the balance of water, wherein the sum of the mass fractions of the components is 100 percent; the surfactant is formed by mixing a nonionic surfactant and an anionic surfactant according to a mass ratio of 1: 2-5: 1; the nonionic surfactant is selected from one of polyoxyethylene fatty acid ester, alkylphenol polyoxyethylene or tween-20; the anionic surfactant is any one of fatty alcohol-polyoxyethylene ether sodium sulfate, alpha-alkenyl sodium sulfonate or fatty acid methyl ester sodium sulfonate; the cosurfactant is any one of n-hexanol, n-butanol or n-pentanol; the salt is potassium chloride or sodium chloride.

The preparation method of the in-situ generated middle-phase microemulsion washing oil system comprises the following steps: adding salt into water to prepare a salt solution; and (3) adding the nonionic surfactant, the anionic surfactant and the cosurfactant into the salt solution in sequence at room temperature, and stirring to obtain the in-situ generated middle-phase microemulsion washing oil system.

The process of utilizing the in-situ generated middle-phase microemulsion washing oil system to wash oil comprises the following steps: mixing the in-situ generated middle-phase microemulsion oil washing system with oil sand according to the mass ratio of 1: 1-1: 8, standing for 30 min-2 h at the constant temperature of 25 ℃, pouring supernatant, and repeatedly washing with stratum water for 3 times to finish the oil washing process.

The in-situ formed middle-phase microemulsion oil system and the preparation method thereof provided by the invention are explained in detail by combining the examples.

Example 1:

adding 3g of potassium chloride into 92.5g of distilled water to prepare a potassium chloride solution; and (3) sequentially adding 1g of polyoxyethylene fatty acid ester (OEO), 1g of fatty alcohol-polyoxyethylene ether sodium sulfate (AES) and 2.5g of n-hexanol into a potassium chloride solution at 25 ℃, and stirring for 3 minutes to obtain the in-situ generated middle-phase micro-emulsion oil-washing system.

Mixing the in-situ generated middle-phase micro-emulsion oil washing system prepared in example 1 with oil sand with the oil content of 25% according to the mass ratio of 4:6 at 25 ℃, standing for 1h at constant temperature, pouring supernatant, and repeatedly washing with stratum water for 3 times, wherein the oil washing efficiency reaches 98.6%.

Example 2:

adding 3.5g of potassium chloride into 90g of distilled water to obtain a potassium chloride solution; and (2) sequentially adding 2.5g of alkylphenol polyoxyethylene OP-10, 1g of fatty alcohol polyoxyethylene ether sodium sulfate and 3g of n-butyl alcohol into a potassium chloride solution at 25 ℃, and stirring for 3 minutes to obtain the in-situ generated middle-phase microemulsion washing oil system.

Mixing the in-situ generated middle-phase micro-emulsion oil washing system prepared in the example 2 with oil sand with the oil content of 25% according to the mass ratio of 3:7 at 25 ℃, standing for 1h at constant temperature, pouring supernatant liquid, and repeatedly washing with stratum water for 3 times, wherein the oil washing efficiency reaches 98.8%.

Example 3:

adding 4g of sodium chloride into 89g of distilled water to obtain a sodium chloride solution; and (3) sequentially adding 3g of polyoxyethylene fatty acid ester, 2g of sodium fatty acid methyl ester sulfonate and 2g of n-amyl alcohol into the potassium chloride solution at 25 ℃, and stirring for 3 minutes to obtain the in-situ generated middle-phase microemulsion oil-washing system.

Mixing the in-situ generated middle-phase micro-emulsion oil washing system prepared in example 3 with oil sand with the oil content of 15% according to the mass ratio of 5:5 at 25 ℃, standing for 1h at constant temperature, pouring supernatant, and repeatedly washing with stratum water for 3 times, wherein the oil washing efficiency reaches 99.2%.

Example 4:

adding 3.5g of potassium chloride into 89g of distilled water to obtain a potassium chloride solution; and (3) sequentially adding 1.5g of tween-20, 3g of alpha-sodium alkenyl sulfonate and 3g of n-hexanol into the potassium chloride solution at 25 ℃, and stirring for 3 minutes to obtain the in-situ generated middle-phase microemulsion oil-washing system.

Mixing the in-situ generated middle-phase micro-emulsion oil washing system prepared in example 4 with oil sand with the oil content of 15% according to the mass ratio of 2:8 at 25 ℃, standing for 1h at constant temperature, pouring supernatant, and repeatedly washing with stratum water for 3 times, wherein the oil washing efficiency reaches 98.9%.

Example 5:

adding 4g of potassium chloride into 90.5g of distilled water to obtain a potassium chloride solution; and (3) sequentially adding 2g of polyoxyethylene fatty acid ester, 1g of fatty alcohol-polyoxyethylene ether sodium sulfate and 2.5g of n-hexanol into a potassium chloride solution at 25 ℃, and stirring for 3 minutes to obtain the in-situ generated middle-phase microemulsion washing oil system.

Mixing the in-situ generated middle-phase micro-emulsion oil washing system prepared in example 5 with oil sand with the oil content of 15% according to the mass ratio of 1:5 at 25 ℃, standing for 1h at constant temperature, pouring supernatant, and repeatedly washing with stratum water for 3 times, wherein the oil washing efficiency reaches 98.4%.

Example 6:

adding 3.5g of potassium chloride into 89.5g of distilled water to obtain a potassium chloride solution; and (3) sequentially adding 3g of polyoxyethylene fatty acid ester, 1g of fatty alcohol-polyoxyethylene ether sodium sulfate and 3g of n-hexanol into a potassium chloride solution at 25 ℃, and stirring for 3 minutes to obtain the in-situ generated middle-phase microemulsion oil-washing system.

Mixing the in-situ generated middle-phase micro-emulsion oil washing system prepared in example 6 with oil sand with the oil content of 15% according to the mass ratio of 1:6 at 25 ℃, standing for 1h at constant temperature, pouring supernatant, and repeatedly washing with stratum water for 3 times, wherein the oil washing efficiency reaches 98.6%.

Example 7:

adding 4.5g of sodium chloride into 85.5g of distilled water to obtain a sodium chloride solution; and (3) sequentially adding 5g of polyoxyethylene fatty acid ester, 1.5g of fatty alcohol-polyoxyethylene ether sodium sulfate and 3.5g of n-hexanol into the potassium chloride solution at 25 ℃, and stirring for 3 minutes to obtain the in-situ generated middle-phase microemulsion washing oil system.

The in-situ generated middle-phase microemulsion washing oil system prepared in example 7 is mixed with oil sand with the oil content of 15% according to the mass ratio of 1:8 at 25 ℃, the mixture is kept stand for 1 hour at a constant temperature, supernatant liquid is poured, and the mixture is repeatedly washed by using stratum water for 3 times, wherein the oil washing efficiency reaches 98.2%.

The features of the embodiments and embodiments described herein above may be combined with each other without conflict.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.