Middle-phase microemulsion, and preparation method therefor and use thereof

Abstract

The present invention relates to the technical field of tertiary oil recovery in oil fields. Disclosed are a middle-phase microemulsion, and a preparation method therefor and the use thereof. The middle-phase microemulsion contains 0.4-1.6 wt% of a main surfactant, 0.1-0.5 wt% of a co-surfactant, 7-14 wt% of an inorganic salt, 45-55 wt% of an oil phase and 35-45 wt% of water, wherein the main surfactant is selected from at least one of monoalkyl benzene sulfonate and dialkyl benzene sulfonate which have a C11-C22 alkyl chain, and the co-surfactant is sodium lauryl polyoxypropylene sulfate. When used for oil displacement, the middle-phase microemulsion of the present invention can greatly reduce the interfacial tension, and improve the recovery rate of crude oil; moreover, the middle-phase microemulsion does not require the addition of other auxiliaries such as an alkali, such that the concentration of the reagent used is low and the economic benefits are high. The middle-phase microemulsion can be used after chemical flooding to further increase the recovery rate.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The application claims the benefit of the Chinese patent application No. “202211329487.9”, filed on October 27, 2022, the content of which is specifically and entirely incorporated herein by reference. TECHNICAL FIELD

[0002] The present disclosure relates to the technical field of tertiary oil recovery in oil fields, in particular to a middle-phase microemulsion, a preparation method therefor and use thereof. BACKGROUND ART

[0003] Crude oil extraction can be divided into three stages according to the classification of oil recovery stages and technical means: in the initial stage of oil field development, the natural flow exploitation is carried out by means of the oil reservoir energy, the recovery rate is only 15%-20%, the stage is called primary oil recovery; to supplement the insufficient stratum energy, the oil reservoir energy is replenished by artificial water injection or gas injection to recover oil, the recovery rate can reach 25%-40%, the stage is called secondary oil recovery; for the sake of extracting most of the residual crude oil, the residual oil is continuously recovered based on secondary oil recovery by using new technologies such as physical, chemical and biological technologies, and such recovery means is collectively called tertiary oil recovery. The modes for performing tertiary oil recovery mainly comprise chemical flooding, gas flooding, thermal flooding, and the like.

[0004] In the chemical oil flooding, the micro-emulsion oil displacement achieves breakthrough results in tertiary oil recovery of low penneability reservoirs, and the recovery rate of the crude oil is greatly improved. Microemulsion is a transparent or translucent dispersion system having thermodynamic stability, isotropy, and low viscosity spontaneously formed with oils and water by the action of surfactants and co-surfactants under certain conditions. Two or more kinds of immiscible liquid in the microemulsion are blended and emulsified to form a liquid drop system having a diameter within the range of 5-100nm, the main principle is that during the oil exploitation process, the oil displacement is performed by initially adding a surfactant and a portion of macromolecular compound and then injecting water. In an oil well, an aqueous surfactant solution and an original solution form a bicontinuous phase microemulsion. The microemulsion coexists with superfluous water and excessive oil, thereby greatly reducing the interfacial tension of the crude oil and water.

[0005] The microemulsion is divided into two types, namely multiphase microemulsion (Winsor I type microemulsion, Winsor II type microemulsion, and Winsor III type microemulsion) and single-phase microemulsion (Winsor IV type microemulsion), according to the phase number of the microemulsion, wherein the Winsor I microemulsion is formed by the coexistence of excessive oil components and O / W type microemulsion, the Winsor I type microemulsion is also called lower phase microemulsion, wherein a surfactant is mainly dissolved in a microemulsion phase at a lower part of the system, and upper part oil components also contain surfactant monomers with a low concentration. Winsor II type microemulsion is formed by the coexistence of W / O type microemulsion and excessive water components, the Winsor II type microemulsion is also called upper phase microemulsion, wherein the surfactant is mainly dissolved at the upper part of the system, and the lower part water components also contain the surfactant with a low concentration. Winsor III type microemulsion is formed by the coexistence of a microemulsion with excessive water and oil components, it is a middlephase microemulsion. The Winsor III type microemulsion is an intermediate structure in a continuous conversion path of Winsor I type microemulsion and Winsor II type microemulsion, the system contains two interfaces and a total of three phases, is composed of a bicontinuous phase rich in a surfactant, an oil phase containing a small amount of the surfactant at the upper part of the system, and a water phase containing a small amount of the surfactant at the lower part of the system, a middle-phase of the Winsor III type microemulsion is actually a bicontinuous microemulsion.

[0006] Generally, a high surfactant concentration (> 1%) and various auxiliaries are required to construct a middle-phase microemulsion, alkaline substances shall be added into the formulation to construct the middle-phase microemulsion. The formation of such a middle-phase microemulsion requires a high cost, and the addition of an alkali is prone to cause problems such as corrosion or scaling of a pipe.

[0007] Therefore, it is very necessary to develop a middle-phase microemulsion that can solve the above technical problems, a preparation process therefor, and use thereof. SUMMARY

[0008] The present disclosure aims to overcome the problems of the middle-phase microemulsion in the prior art and provides a middle-phase microemulsion, a preparation method therefor, and use thereof. The middle-phase microemulsion is used for oil displacement, can greatly reduce interfacial tension, and improve the recovery rate of the crude oil, the middle-phase microemulsion of the present disclosure does not require the addition of an alkali, only needs a small amount of auxiliaries, has low concentration of a main agent in use, high economic benefits, can greatly improve the recovery rate, and can be used after chemical flooding (e.g., polymer flooding) to further increase the recovery rate.

[0009] To fulfill the above purpose, the first aspect of the present disclosure provides a middle-phase microemulsion comprising 0.4-1.6 wt% of a main surfactant, 0.1-0.5 wt% of a co-surfactant, 7-14 wt% of an inorganic salt, 45-55 wt% of an oil phase and 35-45 wt% of water, wherein the main surfactant is at least one selected from monoalkyl benzene sulfonate and dialkyl benzene sulfonate which have a C11-C22 alkyl chain, and the co-surfactant is sodium lauryl polyoxypropylene sulfate.

[0010] Preferably, the main surfactant is one or at least a mixture of two or more selected from monoalkyl benzene sulfonate and dialkyl benzene sulfonate having a C14 alkyl chain, a Ci6 alkyl chain, and a Cis alkyl chain.

[0011] Preferably, a structural formula of the co-surfactant is represented by formula (1) as follows: CH3 CH^CH^oCHjTo-C-C^SOW |0012] formula (I)

[0013] wherein n is an integer from 3 to 6.

[0014] Preferably, the inorganic salt is sodium chloride and / or potassium chloride.

[0015] Preferably, the oil phase is a straight-chain alkane or a mixture of n-alkane and isoalkane.

[0016] Further preferably, the straight-chain alkane is n-tetradecane.

[0017] Further preferably, the mixture of n-alkane and isoalkane is white oil.

[0018] Preferably, the water is deionized water.

[0019] Preferably, the middle-phase microemulsion is a transparent or translucent liquid.

[0020] The second aspect of the present disclosure provides a method for preparing the above-mentioned middle-phase microemulsion comprising: mixing a main surfactant, a co-surfactant, an inorganic salt, an oil phase, and water.

[0021] Preferably, the mixing process comprises: stirring and mixing a main surfactant, a co-surfactant, an inorganic salt, an oil phase, and water, and then subjecting the mixture to standing still.

[0022] The third aspect of the present disclosure provides a use of the above-mentioned middle-phase microemulsion as an oil-displacing agent.

[0023] When used for oil displacement, the middle-phase microemulsion of the present disclosure can greatly reduce the interfacial tension, and improve the recovery rate of crude oil; moreover, the middle-phase microemulsion does not require the addition of other auxiliaries such as an alkali, such that the concentration of the reagent used is low and the economic benefits are high. The middle-phase microemulsion can be used after chemical flooding to further increase the recovery rate.

[0024] The middle-phase microemulsion of the present disclosure has a simple preparation process, it can be prepared by directly and uniformly mixing the components, and is beneficial to large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 illustrates a small-angle X-ray diagram of the middle-phase microemulsion prepared in Example 1;

[0026] FIG. 2 illustrates a cryo-scanning electron microscope image of the middle-phase microemulsion prepared in Example 1;

[0027] FIG. 3 illustrates a small-angle X-ray diagram of the middle-phase microemulsion prepared in Example 2;

[0028] FIG. 4 illustrates a cryo-scanning electron microscope image of the middle-phase microemulsion prepared in Example 2. DESCRIPTION OF THE PREFERRED EMBODIMENT

[0029] The preferred embodiments of the present disclosure will be described in detail below with reference to the drawings. It should be understood that the preferred embodiments described herein merely serve to illustrate and explain the present disclosure, instead of imposing limitations thereto.

[0030] The terminals and any value of the ranges disclosed herein are not limited to the precise ranges or values, such ranges or values shall be comprehended as comprising the values adjacent to the ranges or values. As for numerical ranges, the endpoint values of the various ranges, the endpoint values and the individual point values of the various ranges, and the individual point values may be combined with one another to produce one or more new numerical ranges, which should be deemed to have been specifically disclosed herein.

[0031] The middle-phase microemulsion of the present disclosure contains a main surfactant, a co-surfactant, an inorganic salt, an oil phase, and water, and does not contain alkali and other auxiliaries.

[0032] In the middle-phase microemulsion of the present disclosure, the main surfactant is at least one selected from monoalkyl benzene sulfonate and di alkyl benzene sulfonate which have a Ci 1-622 alkyl chain. Specifically, the main surfactant is one or at least a mixture of two or more selected from monoalkyl benzene sulfonate and dialkyl benzene sulfonate having a C14 alkyl chain, a Ci6 alkyl chain, and a Cis alkyl chain.

[0033] In the middle-phase microemulsion of the present disclosure, the middle-phase microemulsion contains 0.4-1.6 wt% of a main surfactant, 0.1-0.5 wt% of a co-surfactant, 7-14 wt% of an inorganic salt, 45-55 wt% of an oil phase and 35-45 wt% of water, the total amount of the middlephase microemulsion is 100wt%. In a case of preferably, the middle-phase microemulsion contains 0.4-1.2 wt% of a main surfactant, 0.1-0.3 wt% of a co-surfactant, 8-12 wt% of an inorganic salt, 48-52 wt% of an oil phase and 38-42 wt% of water.

[0034] In the middle-phase microemulsion of the present disclosure, the co-surfoctant is sodium lauryl polyoxypropylene sulfate. In a case of preferably, a structural formula of the co-surfactant is represented by formula (I) as follows: h2 H3 CH3(CH2)mCH2“f“O“~C -“C-^-SO^Na*

[0035] ° formula (I)

[0036] wherein n is an integer from 3 to 6, the specific examples of n may be 3, 4, 5, or 6.

[0037] In the middle-phase microemulsion of the present disclosure, the inorganic salt may be sodium chloride and / or potassium chloride, preferably sodium chloride.

[0038] In the middle-phase microemulsion of the present disclosure, the oil phase may be a straight-chain alkane or a mixture of n-alkane and isoalkane. The straight-chain alkane is preferably n-tetradecane. The mixture of n-alkane and isoalkane is preferably white oil.

[0039] In the middle-phase microemulsion of the present disclosure, the water is preferably deionized water.

[0040] In the present disclosure, the middle-phase microemulsion is a transparent or translucent liquid.

[0041] The preparation method of the middle-phase microemulsion may comprise: mixing a main surfactant, a co-surfactant, an inorganic salt, an oil phase, and water.

[0042] In a case of preferably, the mixing process comprises: stirring and mixing a main surfactant, a co-surfactant, an inorganic salt, an oil phase, and water, and then subjecting the mixture to standing still. Further preferably, the mixing process comprises: dissolving a main surfactant, a co-surfactant, and an inorganic salt in water to obtain an aqueous phase, stirring and mixing the aqueous phase with the oil phase, and then subjecting the mixture to standing still. The stirring can be a mechanical stirring, the stirring rate may be within the range of 50-200 rpm, and the stirring time is within the range of 5-30 minutes.

[0043] The present disclosure also provides a use of the middle-phase microemulsion as an oil-displacing agent. The middle-phase microemulsion, when used as an oil-displacing agent for oil displacement, can greatly reduce interfacial tension and improve the recovery rate of crude oil and can be used after chemical flooding (e.g., polymer flooding) to further increase the recovery rate.

[0044] The middle-phase microemulsion, a preparation method therefor, and use thereof according to the present disclosure are further illustrated with reference to the following examples. The examples are implemented on the premise of the technical scheme of the present disclosure, and provide the detailed embodiments and specific operation procedure, but the protection scopes of the present disclosure are not limited to the following examples.

[0045] Uniess otherwise specified in the present disclosure, the experimental methods in the following examples are conventional in the art. The experimental materials used in the following examples are commercially available unless otherwise specified.

[0046] Example 1

[0047] 0 ,4wt% of heavy alkylbenzene sulfonate (mainly a monoalkyl benzene sulfonate having a Ci6 alkyl chain), 0.2wt% of sodium lauryl polyoxypropylene sulfonate (a compound represented by formula (I), wherein n was 3), 10wt% of sodium chloride, 50wt% of n-tetradecane, and 39.4wt% of deionized water were weighed. The weighed surfactant and sodium chloride were dissolved in deionized water to obtain a water phase; the weighed oil phase and the prepared water phase were uniformly mixed, and the mixture was added into a beaker and stirred at the rotation speed of 100 rpm at the normal temperature for 10 minutes, the mixture was then subjected to standing still to prepare a middle-phase microemulsion Al.

[0048] Wherein the heavy alkylbenzene sulfonate was supplied by PetroChina Xinjiang Oilfield Branch, sodium lauryl polyoxypropylene sulfate was purchased from Sasol Corporation (Sandton, South Africa), n-tetradecane (99%) was commercially available from Adamas-Beta (Shanghai) Limited Company, and NaCl (AR) was purchased from Kelong Reagent Company (Chengdu). Deionized water (with a resistivity of 18.25 MQcm) was voluntarily prepared by a laboratory with an ultrapure water purification device CDUPT-III (manufactured by Chengdu Ultrapure Technology Co., Ltd., China).

[0049] The prepared middle-phase microemulsion Al was encapsulated in a quartz capillary tube, which was then placed in SAXSpace (Anton Paar, Austria, Cu-Ka, X = 0.154 nm) for subjecting to the Small Angle X-ray Scattering (SAXS) measurement, the relationship between scattering intensity and scattering vector was obtained, a “Teubner Strey” model was used for fitting an SAXS curve, and the correlation length and the bicontinuous structural domain periodicity d were obtained, the relevant result is shown in FIG. 1.

[0050] As shown in FIG. 1, the relationship between scattering intensity and scattering vector of the component complied with the Teubner Strey (T-S) model, the middle-phase was verified to be a middle-phase microemulsion with a bicontinuous structure.

[0051] The prepared middle-phase microemulsion Al was subjected to a Cryo-SEM test using FEI-Helios G5 cryomicroscope (manufactured by FEI, USA). Before sample preparation, the middle-phase microemulsion was stabilized at 40°C for 30min, and then dripped into a copper bracket with conductive adhesive and mounted on a freezing bracket. The sample was then quenched in liquid nitrogen (-196°C) for 10 s. The sample was subsequently bent with two tweezers, it was fractured and a new cross section was exposed. The frozen sample was then transferred to a sample chamber, sublimed at -90°C for lOmin, and subjected to a gold sputtering at the current of 10mA, the sample was transferred to an observation chamber at -140°C, and the images were captured using secondary or backscattered electrons (2 keV, 60 pA) under a pressure from vacuum to 1 x 10'5Pa, the relevant results were shown in FIG. 2.

[0052] As illustrated by FIG. 2, a typical bicontinuous structure appeared in the electron microscope results, which demonstrated that the middle phase was exactly the middle-phase microemulsion.

[0053] Example 2

[0054] 0.8wt% of heavy alkylbenzene sulfonate (mainly a monoalkyl benzene sulfonate having a Ci6 alkyl chain), 0.2wt% of sodium lauryl polyoxypropylene sulfonate (a compound represented by formula (I), wherein n was 3), 10wt% of sodium chloride, 50% of white oil, and 39% of deionized water were weighed. The weighed surfactant and sodium chloride were dissolved in deionized water to obtain a water phase; the weighed oil phase and the prepared water phase were uniformly mixed, and the mixture was added into a beaker and stirred at the rotation speed of 100 rpm at the normal temperature for 10 minutes, the mixture was then subjected to standing still to prepare a middle-phase microemulsion A2.

[0055] Wherein the heavy alkylbenzene sulfonate was supplied by PetroChina Xinjiang Oilfield Branch, sodium lauryl polyoxypropylene sulfate was purchased from Sasol Corporation (Sandton, South Africa), white oil (15 #) was commercially available from Lingzhong Lubricant Co., Ltd. (Chengdu), and NaCl (AR) was purchased from Kelong Reagent Company (Chengdu). Deionized water (with a resistivity of 18.25 MG cm) was voluntarily prepared by a laboratory with an ultrapure water purification device CDUPT-III (manufactured by Chengdu Ultrapure Technology Co., Ltd., China).

[0056] The prepared middle-phase microemulsion A2 was encapsulated in a quartz capillary tube, which was then placed in SAXSpace (Anton Paar, Austria, Cu-Ka, X = 0.154 nm) for subjecting to the SAXS measurement, the relationship between scattering intensity and scattering vector was obtained, a “Teubner Strey” model was used for fitting an SAXS curve, and the correlation length £ and the bicontinuous structural domain periodicity d were obtained, the relevant result is shown in FIG. 3.

[0057] As shown in FIG. 3, the relationship between scattering intensity and scattering vector of the component complied with the Teubner □ Strey (T-S) model, the middle phase was verified to be a middle-phase microemulsion with a bicontinuous structure.

[0058] The prepared middle-phase microemulsion A2 was subjected to a Cryo-SEM test using the FEI-Helios G5 cryomicroscope (manufactured by FEI, USA). Before sample preparation, the middle-phase microemulsion was stabilized at 40°C for 30min, and then dripped into a copper bracket with conductive adhesive and mounted on a freezing bracket. The sample was then quenched in liquid nitrogen (-196°C) for 10 s. The sample was subsequently bent with two tweezers, it was fractured and a new crosssection was exposed. The frozen sample was then transferred to a sample chamber, sublimed at -90°C for lOmin, and subjected to a gold sputtering at the current of 10mA, the sample was transferred to an observation chamber at -140°C, and the images were captured using secondary or backscattered electrons (2 keV, 60 pA) under a pressure from vacuum to 1 x 10'5Pa, the relevant results were shown in FIG. 4.

[0059] As illustrated by FIG. 4, a typical bicontinuous structure appeared in the electron microscope results, which demonstrated that the middle phase was exactly the middle-phase microemulsion.

[0060] Example 3

[0061] I .2wt% of heavy alkylbenzene sulfonate (mainly a monoalkyl benzene sulfonate having a Ci6 alkyl chain), 0.4wt% of sodium lauryl polyoxypropylene sulfonate (a compound represented by formula (I), wherein n was 6), 10wt% of sodium chloride, 52wt% of n-tetradecane, and 36.4wt% of deionized water were weighed. The weighed surfactant and sodium chloride were dissolved in deionized water to obtain a water phase; the weighed oil phase and the prepared water phase were uniformly mixed, and the mixture was added into a beaker and stirred at the rotation speed of 100 rpm at the normal temperature for 10 minutes, the mixture was then subjected to standing still to prepare a middle-phase microemulsion A3.

[0062] Example 4

[0063] 1 ,6wt% of heavy alkylbenzene sulfonate (mainly a monoalkyl benzene sulfonate having a Ci6 alkyl chain), 0.4wt% of sodium lauryl polyoxypropylene sulfonate (a compound represented by formula (I), wherein n was 4), 10wt% of sodium chloride, 50wt% of white oil, and 38wt% of deionized water were weighed. The weighed surfactant and sodium chloride were dissolved in deionized water to obtain a water phase; the weighed oil phase and the prepared water phase were uniformly mixed, and the mixture was added into a beaker and stirred at the rotation speed of 100 rpm at the normal temperature for 10 minutes, the mixture was then subjected to standing still to prepare a middle-phase microemulsion A4.

[0064] Example 5

[0065] 0.8wt% of heavy alkylbenzene sulfonate (mainly a monoalkyl benzene sulfonate having a Cis alkyl chain), 0.2wt% of sodium lauryl polyoxypropylene sulfonate (a compound represented by formula (I), wherein n was 5), 12wt% of potassium chloride, 50wt% of white oil, and 37wt% of deionized water were weighed. The weighed surfactant and potassium chloride were dissolved in deionized water to obtain a water phase; the weighed oil phase and the prepared water phase were uniformly mixed, the mixture was added into a beaker, and stirred at the rotation speed of 100 rpm at the normal temperature for 10 minutes, the mixture was then subjected to standing still to prepare a middle-phase microemulsion A5.

[0066] Comparative Example 1

[0067] The middle-phase microemulsion was prepared according to the method in Example 1, except that the heavy alkylbenzene sulfonate was replaced with the same weight of cycloalkylaryl sulfonate to prepare a middle-phase microemulsion DI.

[0068] Comparative Example 2

[0069] The middle-phase microemulsion was prepared according to the method in Example 1, except that the sodium lauryl polyoxypropylene sulfate was replaced with the same weight of sodium dodecylbenzene sulfonate to prepare a middle-phase microemulsion D2.

[0070] Test Example

[0071] The surfactant systems screened by the aforementioned Examples and Comparative Examples were subjected to an oil displacement test, the specific test process was as follows:

[0072] The tests were performed using natural cores taken from the target formation: firstly, a rock core was cleaned and dried, and the basic physical property parameters of the rock core were measured; the rock core was saturated with the simulated brine by using a core vacuumizing saturation device, the rock core shall be soaked in the simulated brine after taking out for later use. The rock core displacement test was carried out at the specific reservoir temperature of 40°C, the test was divided into three processes namely saturated oil, water flooding, surfactant flooding and subsequent water flooding, the specific steps were as follows:

[0073] 1) Saturated oil: the rock core was placed in a holder, and displaced in a pressure pipeline by using crude oil until the outlet did not discharge water anymore but stably exuded oil, the displacement water volume % was collected and recorded to obtain a rock core saturated with oil; after completion of saturation, the rock core was placed at a high temperature and kept for 1-2 days, the purpose was to uniformly distribute the crude oil in the rock core and simulate the crude oil distribution of a real oil reservoir as much as possible;

[0074] 2) Water flooding: the simulated saline was injected into the rock core at a flow rate of 1 mL-mnr1 until the water content of the output liquid was more than 98%, the volume Kw of the displaced oil was recorded, and the water flooding recovery rate was calculated according to the following formula:

[0075] Ew = -^xl00% (2)

[0076] 3) Surfactant flooding and subsequent water flooding: after 1 PV of a surfactant solution was quantitatively injected into the rock core at a flow rate of 1 mLmin’1, the simulated saline was further injected into the rock core at a flow rate of 1 mL inin-1, the subsequent water flooding was terminated when the water content of the output liquid reached 98% or more again. The accumulated total volume V of the displaced oil in the water flooding process, the surfactant flooding process, and the subsequent water flooding process was recorded, and the total recovery rate E of the crude oil and the recovery rate improved by the surfactant flooding were calculated according to the following formulas:

[0077] E= — x 100% (3)

[0078] ES = E-EW (4)

[0079] The core parameters and test results were shown in Table 1 below, wherein 1 denoted the rock core length, h denoted the rock core diameter, PV denoted the pore volume, denoted the porosity, Ke denoted permeability, Ew denoted the water flooding recovery rate, and Es denoted the surfactant flooding recovery rate.

[0080] Table 1 Number I (cm) h (cm) PP(mL) 0 (%) Ke (mD) Ew (%) Es (%) Example 1 29.4 3.8 32.8 9.8 154.0 46.14 10.12 Example 2 29.5 3.8 33.1 9.9 153.7 45.27 12.44 Example 3 29.5 3.8 32.3 9.7 148.7 46.77 17.57 Example 4 29.4 3.8 33.0 9.9 153.5 40.50 17.71 Example 5 29.4 3.8 33.0 9.9 154.5 44.50 15.88 Comparative Example 1 29.3 3.8 33.4 10 148.3 43.16 6.98 Comparative Example 2 29.5 3.8 32.9 9.8 150.1 47.47 7.61

[0081] The results in Table 1 show that the surfactant oil displacement system in the present disclosure can greatly improve the recovery rate after water flooding, and the amplitude of recovery rate improvement is increased along with the increased concentration of the main surfactant. Compared with the formula in the present disclosure, after the main surfactant and auxiliaries are changed, the amplitude of recovery rate improvement is obviously reduced, it demonstrates superiority of the system in the present disclosure.

[0082] The above content describes in detail the preferred embodiments of the present disclosure, but the present disclosure is not limited thereto. A variety of simple modifications can be made in regard to the technical solutions of the present disclosure within the scope of the technical concept of the present disclosure, including a combination of individual technical features in any other suitable manner, such simple modifications and combinations thereof shall also be regarded as the content disclosed by the present disclosure, each of them falls into the protection scope of the present disclosure.

Claims

1. A middle-phase microemulsion comprising 0.4-1.6 wt% of a main surfactant, 0.1-0.5 wt% of a co-surfactant, 7-14 wt% of an inorganic salt, 45-55 wt% of an oil phase and 35-45 wt% of water, wherein the main surfactant is at least one selected from monoalkyl benzene sulfonate and dialkyl benzene sulfonate which have a C11-C22 alkyl chain, and the cosurfactant is sodium lauryl polyoxypropylene sulfate.

2. The middle-phase microemulsion according to claim 1, wherein the main surfactant is one or at least a mixture of two or more selected from monoalkyl benzene sulfonate and dialkyl benzene sulfonate having a C14 alkyl chain, a Ci6 alkyl chain, and a Cis alkyl chain.

3. The middle-phase microemulsion according to claim 1, wherein a structural formula of the co-surfactant is represented by formula (I) as follows:CH3 CH3(CH2)10CH24"O”?^ formula (I) wherein n is an integer from 3 to 6.

4. The middle-phase microemulsion according to claim 1, wherein theinorganic salt is sodium chloride and / or potassium chloride.

5. The middle-phase microemulsion according to claim 1, wherein the oil phase is a straight-chain alkane or a mixture of n-alkane and isoalkane.

6. The middle-phase microemulsion according to claim 5, wherein thestraight-chain alkane is n-tetradecane.

7. The middle-phase microemulsion according to claim 5, wherein themixture of n-alkane and isoalkane is white oil.

8. The middle-phase microemulsion according to claim 1, wherein thewater is deionized water.

9. The middle-phase microemulsion according to any of claims 1-8, wherein the middle-phase microemulsion is a transparent or translucent liquid.

10. A method for preparing the middle-phase microemulsion according to any one of claims 1-9 comprising: mixing main surfactant, co-surfactant, inorganic salt, oil phase, and water.

11. The method according to claim 10, wherein the mixing process comprises: stirring and mixing a main surfactant, a co-surfactant, an inorganic salt, an oil phase, and water, and then subjecting the mixture to standing still.

12. A use of the middle-phase microemulsion according to any one of claims 1-9 as an oil-displacing agent.

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