CN114085661B

CN114085661B - Gel particle emulsion liquid system and method for improving recovery ratio thereof

Info

Publication number: CN114085661B
Application number: CN202111306069.3A
Authority: CN
Inventors: 王沫然; 雷文海; 刘洋; 鲁旭康
Original assignee: Tsinghua University
Current assignee: Tsinghua University
Priority date: 2021-11-05
Filing date: 2021-11-05
Publication date: 2022-09-13
Anticipated expiration: 2041-11-05
Also published as: CN114085661A

Abstract

The application provides a gel particle emulsion system and a method for improving recovery ratio thereof, and relates to a displacement fluid, wherein the displacement fluid comprises gel particles, the gel particles are copolymers, and the copolymers comprise a first monomer and a second monomer; a part of the gel particles in the displacement fluid are wrapped by oil drops to form microspheres; the first monomer is a nonionic water-soluble monomer; the second monomer is a water-soluble anionic monomer; the displacement fluid also comprises gel particles wrapped by oil drops to form microspheres; the weight ratio of the microspheres to the gel particles is (1-10): 100. The method for improving the recovery ratio ensures good injectability of small-sized gel particles, and realizes a blocking-dispersing process to generate a pressure fluctuation effect through a nucleation blocking mechanism of spontaneously formed microspheres in a porous medium and the cooperation of discrete gel particles, so that the recovery ratio is greatly improved.

Description

Gel particle emulsion liquid system and method for improving recovery ratio thereof

Technical Field

The invention relates to the fields of new energy and high efficiency and energy conservation, in particular to a gel particle emulsion system and a method for improving recovery ratio thereof in the fields of exploration, development and utilization of petroleum and natural gas.

Background

The exploration and development of oil and gas resources are related to the development of energy safety and society in China, and a method for improving the recovery ratio is an important means for stably improving the oil and gas yield. But these present significant challenges for enhanced recovery due to the natural heterogeneous structures in the formation, such as dominant channels and the like. The gel particle suspension is a novel material capable of effectively improving the oil recovery ratio, and has been applied to most oil fields in China in recent years. The main action mechanism is that the water flow is guided to the low-permeability layer to play a role by blocking the high-permeability layer, so that the selection of proper particle size to block the high-permeability layer is particularly important. Too large a particle size can cause blockage of the percolation path resulting in a sharp decrease in permeability and affect the recovery efficiency, and too small a particle size can cause direct flow away and fail to function. Although the matching relationship between the size of the gel particles and the pore throat of the rock is widely researched, the optimal matching relationship obtained by the current research result is distributed in the range that the size of the microspheres is 0.1 to 4 times of the pore throat size, and the wide optimal matching relationship makes the gel particles difficult to be effectively applied. The current microgel particles are various but have poor effectiveness, and the flowing mechanism of the microgel particle suspension in an oil reservoir porous medium is not known, so that the design of the microgel particles and the development of related recovery efficiency improving methods are influenced.

In fact, not only are uniformly distributed gel particles in the gel particle suspension, but also an oil phase is often present in the gel particle suspension, and phenomena such as self-assembly, aggregation, dispersion and the like also occur among the particles, so that the gel particle suspension needs to be comprehensively analyzed to obtain the most favorable configuration scheme for improving the recovery ratio.

Disclosure of Invention

The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

The application provides a gel particle emulsion and a method for improving the recovery ratio thereof, wherein a system containing the gel particle emulsion is designed to form the optimal matching relation between a gel particle suspension system (namely a displacement fluid) and pores in a porous medium so as to realize the effect of flow field oscillation and achieve the aim of improving the recovery ratio.

The designed single gel particle has a size much smaller than that of the channel, but based on a gel particle stock solution prepared by a reverse phase emulsion method, a gel particle suspension (i.e. the displacement fluid) with an appropriate concentration of microspheres (oil droplet-encapsulated gel particles) can be formulated by adding an oily solvent, a water-soluble surfactant, an emulsifier, etc. The microspheres can aggregate the gel particles in the form of emulsion so as to realize the condition of being larger than the channel size, and the proper microspheres and the discrete gel particles realize the pressure fluctuation effect in the dominant channel region through the synergistic action, thereby realizing the effect of improving the recovery ratio in the heterogeneous oil reservoir. The method ensures good injectability of small-sized gel particles, and realizes a blocking-dispersing process to generate a pressure fluctuation effect through the cooperation of a nucleation blocking mechanism of spontaneously formed microspheres in a porous medium and discrete gel particles, thereby greatly improving the recovery ratio.

The application provides a displacement fluid, which comprises gel particles, wherein the gel particles are copolymers, and the copolymers comprise a first monomer and a second monomer; part or all of the gel particles in the displacement fluid are wrapped by oil drops to form microspheres;

the first monomer is a nonionic water-soluble monomer, optionally, the first monomer is selected from any one or more of acrylamide, methacrylamide, methyl methacrylate and dimethylaminoethyl methacrylate;

the second monomer is a water-soluble anionic monomer, optionally, the second monomer is selected from any one or more of acrylic acid, sodium acrylate, methacrylic acid, sodium methacrylate, 2-acrylamide-2-methylpropanesulfonic acid and sodium styrene sulfonate;

the weight ratio of the microspheres to the gel particles is (1-10): 100 (the "gel particles" comprise the gel particles in the microspheres)

In one embodiment provided herein, the concentration of the microspheres in the displacement fluid is from 0.0000001wt.% to 5wt.%, preferably the concentration of the microspheres in the displacement fluid is from 0.000005wt.% to 0.06 wt.%.

In one embodiment provided herein, the displacement fluid further comprises an emulsifier and a water-soluble surfactant;

in one embodiment provided herein, the concentration of the emulsifier in the displacement fluid is from 0.001wt.% to 10wt.%, preferably the concentration of the emulsifier in the displacement fluid is from 0.1wt.% to 10 wt.%;

in one embodiment provided herein, the concentration of the water-soluble surfactant in the displacement fluid is from 0.001wt.% to 3wt.%, preferably the concentration of the water-soluble surfactant in the displacement fluid is from 0.01wt.% to 1wt.%

In one embodiment provided herein, the gel particles have a size of 50nm to 50 μm;

in one embodiment provided herein, the average diameter of the gel particles is smaller than the average pore size of the target formation (the average characteristic pore diameter of the reservoir) to achieve good injectability of the displacement fluid, which can be guaranteed to be delivered deep into the formation for action.

In one embodiment provided herein, the microspheres are gel particles that are not completely encapsulated by oil droplets;

in one embodiment provided herein, the gel particles in the microspheres comprise from 5% to 50% by weight of the microspheres; optionally, the gel particles in the microspheres comprise 10% to 30% by weight of the microspheres.

In one embodiment provided herein, the water soluble surfactant is selected from any one or more of thiobetaine 12, sodium dodecylbenzenesulfonate, polyethylene glycol, alkyl glycoside, tween 80, tween 20 and polyethylene glycol octylphenyl ether;

in one embodiment provided herein, the emulsifier has a hydrophilic-lipophilic balance value of 3 to 10, preferably, the emulsifier is selected from any one or more of span-20, span 40, span 60, span 65, span 80, span 85, tween 20, tween 40, tween 60, tween 65, tween 80 and tween 85

In another aspect, the method for preparing the displacement fluid adopts a method that the oil phase is a continuous phase and the water phase is a dispersed phase, and the surfactant is used for dividing the monomer aqueous solution into a plurality of reaction micro-regions to prepare the displacement fluid stock solution. Gel particles in a micro-nano size range are synthesized by adopting a non-ionic water-soluble monomer/anionic monomer.

Optionally, the preparation method of the displacement fluid comprises:

(1) preparation of an aqueous monomer solution: mixing 10 to 40 parts by weight of the nonionic water-soluble monomer, 15 to 30 parts by weight of water and 5 to 20 parts by weight of the water-soluble anionic monomer until the monomers are completely dissolved, and then mixing the monomers with 1 to 10 parts by weight of a cross-linking agent until the monomers are completely dissolved;

(2) preparation of a first oily solvent containing a surfactant: mixing 5 to 20 parts by weight of a surfactant with 20 to 60 parts by weight of a first oily solvent until complete dissolution;

(3) inverse emulsion polymerization system: mixing and stirring the monomer aqueous solution prepared in the step (1) and the first oily solvent containing the surfactant prepared in the step (2);

(4) thermal polymerization: uniformly stirring 0.1 to 10 parts by weight of thermal initiator and the inverse emulsion polymerization system prepared in the step (3), removing oxygen in the mixed solution, slowly heating to 40 to 70 ℃ in a deoxygenation environment, and keeping for 4 to 12 hours to prepare a mixed solution, wherein solid matters in the mixed solution are the gel particles;

in one embodiment provided herein, the product of step (4) is always in the form of an opaque emulsion.

In one embodiment provided herein, in step (1), after the cross-linking agent is dissolved, the pH of the solution is adjusted to be neutral;

in one embodiment provided herein, the cross-linking agent is selected from a bifunctional or polyfunctional water-soluble cross-linking agent, preferably, the cross-linking agent is selected from any one or more of diallyl dimethyl ammonium chloride, polyethylene glycol diacrylate, pentaerythritol triacrylate, N-methylene bisacrylamide.

In one embodiment provided herein, in step (2), the first oily solvent is mineral spirit or an alkane; preferably, the solvent oil is selected from any one or more of kerosene, mineral oil, vegetable oil and diesel oil; preferably, the alkane is selected from any one or more of decane, octane and hexadecane;

in one embodiment provided herein, the surfactant is any one or more of span series surfactants, tween series surfactants, sodium dodecyl sulfate, and cetyltrimethylammonium bromide; preferably, the span series surfactant is selected from any one or two of span-60 and span-80; preferably, the tween series surfactant is selected from any one or two of tween-40 and tween-60;

in one embodiment provided herein, in the step (3), the aqueous monomer solution is slowly added to the first oily solvent containing the surfactant and stirred uniformly.

In one embodiment provided by the present application, in the step (4), an inert gas is introduced into the mixed solution for more than 1 hour;

the thermal initiator is selected from a single-component cracking oil-soluble thermal initiator, preferably, the thermal initiator is selected from any one or more of benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin dimethyl ether, dicumyl peroxide, benzoyl peroxide tert-butyl ester, azobisisobutyronitrile and azobisisobutyramidine hydrochloride;

in one embodiment provided herein, an inert atmosphere gas is used to exclude oxygen in the mixed liquid, optionally, the inert atmosphere gas is selected from any one or more of helium, neon, argon, nitrogen, and carbon dioxide.

In one embodiment provided herein, the preparation method of the displacement fluid further comprises mixing the mixed solution prepared in step (4), a second oily solvent, water for dilution, the emulsifier and the water-soluble surfactant; the kind of the second oily solvent is the same as that of the first oily solvent.

In one embodiment provided herein, the dilution water has a degree of mineralization of less than 5000 ppm;

in one embodiment provided herein, the volume ratio of the second oily solvent to the mixed solution prepared in step (4) is 1 (0.5 to 2), so as to obtain a mixture;

in one embodiment provided herein, the concentration of the mixture in the dilution water is 0.01vol.% to 3.0vol.%, preferably the concentration of the mixture in the dilution water is 0.5vol.% to 1.5 vol.%;

in one embodiment provided herein, the concentration of the water-soluble surfactant in the dilution water is from 0.01 to 1wt.%, preferably the concentration of the water-soluble surfactant in the dilution water is from 0.025wt.% to 0.075 wt.%;

in one embodiment provided herein, the amount of the emulsifier is 1wt.% to 10wt.% of the total weight of the mixed solution prepared in step (4), the second oily solvent, the water and the water-soluble surfactant.

In one embodiment provided herein, the preparation method of the displacement fluid may be: firstly, dissolving a water-soluble surfactant in dilution water, and then diluting a mixture obtained by mixing the second oily solvent and the mixed solution prepared in the step (4) into an aqueous solution containing the water-soluble surfactant to form a displacement fluid with a certain microsphere content, wherein an emulsifier is also added into the displacement fluid.

In yet another aspect, the present application provides the use of the displacement fluid described above;

in one embodiment provided herein, the application is injecting the displacement fluid into the formation through an injection well;

in one embodiment provided by the present application, the displacement fluid is injected first during displacement, and then the displacement fluid and water are alternately injected.

In a suspension of gel particles having a content of microspheres (i.e. the displacement fluid), a small number of microspheres are a plurality of gel particles in the form of an emulsion surrounded by oil droplets, and a large number of discrete gel particles are still present in the suspension. Reservoir conditions can be selected during use where the prevailing channel size is comparable to that of the microspheres, and the matrix pore size is larger than the rock pore structure of a single gel particle. The gel particles are blocked by a small amount of microspheres (the size of the microspheres is larger than the rock pore structure of a main flow dominant channel), so that the subsequent dispersed single gel particles are filtered and blocked at the position, and the pressure of a large particle retention area is increased; and further pressure rise causes the retention zone formed by the microspheres and the gel particles to be dispersed and the pressure to be reduced; pressure fluctuations are then created to encourage the production of residual oil from the trapped matrix region.

Additional features and advantages of the application will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the application. Other advantages of the present application may be realized and attained by the invention in its aspects as described in the specification.

Drawings

The accompanying drawings are included to provide an understanding of the present disclosure and are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and together with the examples serve to explain the principles of the disclosure and not to limit the disclosure.

FIG. 1 is a schematic diagram of a process for forming microspheres;

FIG. 2 shows gel particles in a gel particle stock solution and microspheres in a diluted displacement fluid photographed under a cryoelectron microscope;

FIG. 3 is a fluorescent microscope photograph of the formation of microspheres;

FIG. 4 is a fluorescence micrograph of a displacement fluid obtained by diluting a second stock solution in an aqueous solution containing different concentrations of a surfactant;

FIG. 5 is a graph of the particle size distribution and rheological properties of a displacement fluid resulting from mixing a second dope of different concentration with an aqueous solution of a surfactant of different concentration;

(ii) the (a) particle size distribution and (b) rheology profile of a displacement fluid obtained by mixing 0.3 vol.% of the second dope with 0.05 to 0.4 wt.% of an aqueous surfactant solution;

1.0 vol.% of a second stock solution 0.05 wt.% to 0.4 wt.% of an aqueous surfactant solution to obtain a displacement fluid having (c) a particle size distribution and (d) a rheology profile;

3.0vol.% of the second dope mixed with 0.05 to 0.4 wt.% of an aqueous surfactant solution to obtain a displacement fluid having (e) a particle size distribution and (f) a rheology profile;

FIG. 6 shows the oil-water two-phase distribution and recovery ratio of the heterogeneous porous medium displaced by the displacement fluid obtained by mixing the second stock solution with different concentrations and the surfactant aqueous solution with different concentrations;

FIG. 7 is a recovery ratio curve of a displacing fluid displacing a heterogeneous porous medium obtained by mixing a second stock solution with different concentrations and surfactant aqueous solutions with different concentrations and recovery ratios thereof in a breakthrough phase and a final phase;

fig. 8 is a graph showing pressure fluctuations and flow field oscillations caused by microsphere retention during displacement of a displacement fluid formed from 1.0 vol.% of the second stock solution and 0.05 wt.% of an aqueous surfactant solution. In the present example, the process control is a constant flow rate, because the subsequent gel particles are largely retained due to the retention of the microspheres to increase the pressure, and after the pressure is increased, the pressure is dispersed to form pressure fluctuation.

Detailed Description

In order to make the objects, technical solutions and advantages of the present application more apparent, embodiments of the present application are described in detail below. It should be noted that the embodiments and features of the embodiments in the present application may be arbitrarily combined with each other without conflict.

The first stock solution is a liquid (distributed in water) which only contains gel particles with good dispersibility (the gel particles in the stock solution are distributed in an oil phase), and the gel particle suspension (namely the displacement fluid) contains the gel particles and the microspheres at the same time and has good dispersibility.

Examples

The oil reservoir chip used in the embodiment is an oil reservoir chip which is heterogeneous and is designed according to the core structure of the Changqing oil field; the core sample of the Changqing oil field can also be directly used; the preparation method of the oil reservoir chip can refer to the preparation method of the oil reservoir chip in Chinese patent CN 110302853B.

In the embodiment of the application, the size of the mainstream dominant channel of the oil reservoir chip is 60-320 microns, the size of the microsphere is about 100 microns, and the average pore size (pore diameter) of the substrate (low-permeability porous medium substrate) of the oil reservoir chip is 63.5 microns.

The preparation of the displacement fluid comprises the following steps:

1) preparation of the first stock solution (gel particles):

this example a first stock solution (mixture containing gel particles) was prepared from the following materials: 10 parts by weight of a water-soluble anionic monomer, 20 parts by weight of a nonionic water-soluble monomer, 10 parts by weight of a surfactant, 30 parts by weight of a first oily solvent, 20 parts by weight of water, 9 parts by weight of a crosslinking agent, and 1 part by weight of a thermal initiator.

The nonionic water-soluble monomer is acrylamide, the water-soluble anionic monomer is selected from acrylic acid, the surfactant is a mixture of span-60 and tween-60 in a ratio of 1:1, the first oily solvent is n-decane, the crosslinking agent is a mixture of diallyl dimethyl ammonium chloride, polyethylene glycol diacrylate and pentaerythritol triacrylate in a ratio of 1:1:1, the thermal initiator is a mixture of benzoin, benzoin methyl ether and benzoin ethyl ether in a ratio of 1:1:1, and the inert gas is helium.

(1) Preparation of an aqueous monomer solution: dissolving the non-ionic water-soluble monomer into the deionized water, stirring to dissolve the non-ionic water-soluble monomer, adding the water-soluble anionic monomer, adding the cross-linking agent after the non-ionic water-soluble monomer is completely dissolved, fully stirring until the solution is clear and has no solid insoluble substances, and adjusting the pH value of the solution to 7.

(2) Preparation of a first oily solvent containing a surfactant: adding the surfactant into the first oily solvent, and fully stirring until the surfactant is dissolved uniformly.

(3) Inverse emulsion polymerization system: the aqueous solution of the water-soluble monomer is slowly added to the first oily solvent containing the surfactant, and sufficiently stirred.

(4) Thermal polymerization: adding the thermal initiator into an inverse emulsion polymerization system, uniformly stirring, introducing inert gas for 1.5 hours to replace oxygen of the reaction system, carrying out the reaction under a deoxidation condition, slowly heating to 50 ℃, and keeping for 8 hours, wherein the reaction system is always kept in an opaque emulsion state, and solid particles in the emulsion are gel particles.

The water-soluble surfactant used hereinafter is selected from thiobetaine 12; the emulsifier is formed by compounding an oil-soluble surfactant and a water-soluble surfactant, and the hydrophilic-lipophilic balance value of the surfactant is 7; the oil-soluble surfactant is selected from span-20; the water-soluble surfactant is selected from tween-20;

2) preparing a second stock solution:

the opaque emulsion prepared in (4) of the first stock solution (i.e., "step 1"), which contains gel particles, is mixed with a second oily solvent, sufficiently stirred, and sonicated in 80Hz ultrasound to form a second stock solution having a high oil content and good dispersibility. The volume of the second oily solvent added was 150% of the volume of the first stock solution. The second oily solvent is n-decane.

3) Preparation of gel particle suspension with microspheres (oil droplet encapsulated gel particles) (i.e. the displacement fluid): dissolving water-soluble surfactants with different concentrations in deionized water (namely dilution water), diluting a second stock solution in deionized water containing the water-soluble surfactants according to a certain concentration ratio to form a displacement fluid with a certain microsphere content, and simultaneously adding 5wt.% of emulsifier to improve the long-term stability of the microspheres.

In the process, microspheres are spontaneously generated, the schematic diagram of the formation process is shown in fig. 1, the in-situ cryoelectron microscope pictures of the gel particles and the microspheres before and after dilution are shown in fig. 2, and fig. 3 shows the spontaneous formation process of the microspheres, wherein the gel particles in the microspheres account for 5 to 50 percent of the weight of the microspheres, and the concentration of the microspheres in the displacement fluid is about 0.000005 to 0.06 percent by weight.

The concentrations of the water-soluble surfactant dissolved in deionized water (dilution water) were set at 0.05 wt.%, 0.1wt.%, and 0.4 wt.%;

the dilution concentration of the second stock solution in deionized water (dilution water) was set to three of 0.3 vol.%, 1.0 vol.%, and 3.0 vol.%;

it can be seen from fig. 4 that the content of microspheres produced by diluting the second stock solution with different dilution water (different concentrations of water-soluble surfactant) is different, i.e. the microspheres produced spontaneously by this dilution process are controllable.

The particle size distribution and rheological properties of microspheres obtained by mixing different concentrations of the second stock solution with different dilution water (different concentrations of water-soluble surfactant) are measured as shown in fig. 5.

A small amount of microspheres exist in the displacement fluid, the microspheres are a plurality of gel particles wrapped by oil drops in the form of emulsion, and a large amount of discrete gel particles still exist in the displacement fluid.

The average size of the gel particles dispersed in the displacement fluid was 5 microns. The gel particle size is smaller than the pore size (the average size is 63.5 microns), so that the displacement fluid can be well injected, and the displacement fluid can be conveyed to the deep part of the porous medium structure to play a role.

The oil reservoir chip is saturated with n-decane which is dyed by fluorescein (Nile red), then the displacement fluids with different formulas are injected into the chip at the speed of 1 mul/min, and the extraction of the oil phase is observed by a microscope to reflect the recovery efficiency. By comparing the recovery ratio of the resulting displacement fluids containing 0.3 vol.%, 1.0 vol.% and 3.0vol.% of the second stock solution diluted in dilution water containing 0.05 wt.% and 0.4 wt.% of surfactant, respectively, it can be seen that the best recovery ratio can be obtained for the displacement fluids obtained by diluting 1.0 vol.% of the second stock solution in dilution water containing 0.05 wt.% of surfactant.

FIG. 6 shows the oil-water two-phase distribution and recovery ratio of the heterogeneous porous medium displaced by the displacement fluid obtained by mixing the second stock solution with different concentrations and the dilution water containing different concentrations of the surfactant.

As can be seen from fig. 7 and 8, the recovery profile exhibits a strongly stepped character, which is precisely the effect of the pressure fluctuations generated by the appropriate displacement fluid.

In the embodiment of the application, the first stock solution and the second stock solution are both in the state of gel particles in the oil phase, because the polymer of the gel particles is water-soluble, a gel particle suspension (namely, the displacement fluid) with good dispersibility can be formed only by forming water drops in the oil phase, and because the gel particles need to be used in an aqueous phase environment after the preparation is completed, the gel particles can be diluted into water.

Although the embodiments disclosed in the present application are described above, the descriptions are only for the purpose of facilitating understanding of the present application, and are not intended to limit the present application. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the disclosure as defined by the appended claims.

Claims

1. A displacement fluid, wherein the displacement fluid comprises gel particles, wherein the gel particles are a copolymer comprising a first monomer and a second monomer; a part of the gel particles in the displacement fluid are wrapped by oil drops to form microspheres;

the first monomer is a nonionic water-soluble monomer, and is selected from any one or more of acrylamide, methacrylamide and dimethylaminoethyl methacrylate;

the second monomer is a water-soluble anionic monomer, and is selected from any one or more of acrylic acid, sodium acrylate, methacrylic acid, sodium methacrylate, 2-acrylamide-2-methylpropanesulfonic acid and sodium styrene sulfonate;

the weight ratio of the microspheres to the gel particles is (1 to 10): 100;

the concentration of the microspheres in the displacement fluid is 0.0000001wt.% to 5 wt.%;

the displacement fluid also comprises an emulsifier and a water-soluble surfactant;

the concentration of the emulsifier in the flooding fluid is 0.001wt.% to 10 wt.%;

the concentration of the water-soluble surfactant in the displacement fluid is 0.001wt.% to 3 wt.%;

the size of the gel particles is 50nm to 50 μm;

the average diameter of the gel particles is less than the average pore size of the target formation;

the microspheres are gel particles which are not completely wrapped by oil drops;

the gel particles in the microspheres constitute from 5% to 50% by weight of the microspheres;

the water-soluble surfactant is selected from any one or more of thiobetaine 12, sodium dodecyl benzene sulfonate, polyethylene glycol, alkyl glycoside, tween 80, tween 20 and polyethylene glycol octyl phenyl ether;

the hydrophilic-lipophilic balance value of the emulsifier is 3 to 10;

the preparation method of the displacement fluid comprises the following steps:

(3) inverse emulsion polymerization system: mixing and stirring the aqueous monomer solution prepared in the step (1) and the first oily solvent containing the surfactant prepared in the step (2);

(4) thermal polymerization: uniformly stirring 0.1 to 10 parts by weight of thermal initiator and the inverse emulsion polymerization system prepared in the step (3), removing oxygen in the mixed solution, slowly heating to 40 to 70 ℃ in a deoxygenation environment, and keeping for 4 to 12 hours, wherein solid matters in the mixed solution are the gel particles;

mixing the mixed solution prepared in the step (4), a second oily solvent, diluting water, the emulsifier and the water-soluble surfactant; the second oily solvent is the same kind as the first oily solvent.

2. The displacement fluid of claim 1, wherein the concentration of the microspheres in the displacement fluid is 0.000005wt.% to 0.06 wt.%.

3. The displacement fluid of claim 1, wherein the concentration of the emulsifier in the displacement fluid is 0.1wt.% to 10 wt.%.

4. The displacement fluid of claim 1, wherein the concentration of the water-soluble surfactant in the displacement fluid is 0.01wt.% to 1 wt.%.

5. The displacement fluid of claim 1, wherein the gel particles in the microspheres comprise 10% to 30% by weight of the microspheres.

6. The displacement fluid of claim 1, wherein the emulsifier is selected from any one or more of span-20, span 40, span 60, span 65, span 80 and span 85.

7. A method of preparing a displacement fluid according to any one of claims 1 to 6, wherein the method includes the steps of:

(4) thermal polymerization: and (4) uniformly stirring 0.1 to 10 parts by weight of thermal initiator and the inverse emulsion polymerization system prepared in the step (3), removing oxygen in the mixed solution, slowly heating to 40 to 70 ℃ in a deoxygenation environment, and keeping for 4 to 12 hours, wherein the solid matter in the mixed solution is the gel particles.

8. The method for preparing a displacement fluid according to claim 7, wherein in the step (1), after the crosslinking agent is dissolved, the pH of the solution is adjusted to be neutral.

9. The method of preparing a displacement fluid according to claim 8, wherein the cross-linking agent is selected from a di-or multi-functional water-soluble cross-linking agent.

10. The method of preparing a displacement fluid according to claim 7 or 8, wherein the cross-linking agent is selected from any one or more of diallyldimethylammonium chloride, polyethylene glycol diacrylate, pentaerythritol triacrylate, N-methylenebisacrylamide.

11. The method for preparing a displacement fluid according to claim 7, wherein in the step (2), the first oily solvent is solvent naphtha or alkane.

12. The method of preparing a displacement fluid according to claim 11, wherein the mineral spirits are selected from any one or more of kerosene, mineral oil, vegetable oil and diesel oil.

13. A method of producing a displacement fluid according to claim 11 or 12, wherein the alkane is selected from any one or more of decane, octane and hexadecane.

14. The method of preparing a displacement fluid according to claim 7, wherein the surfactant is any one or more of span series surfactants, tween series surfactants, sodium dodecyl sulfate and cetyltrimethylammonium bromide.

15. The method of preparing a displacement fluid according to claim 14 wherein the span-series surfactant is selected from any one or two of span-60 and span-80.

16. The method of preparing a displacement fluid according to claim 14 or 15 wherein the tween series surfactant is selected from any one or both of tween-40 and tween-60.

17. The preparation method of the displacement fluid according to claim 7, wherein in the step (3), the aqueous monomer solution is slowly added to the first oily solvent containing the surfactant and stirred uniformly.

18. The method for preparing a displacement fluid according to claim 7, wherein in the step (4), an inert gas is introduced into the mixed solution for 1 hour or more;

the thermal initiator is selected from one-component cracking oil-soluble thermal initiators.

19. A method of preparing a displacement fluid according to claim 18, wherein the thermal initiator is selected from any one or more of benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin dimethyl ether, dicumyl peroxide, benzoyl peroxide t-butyl, azobisisobutyronitrile and azobisisobutyramidine hydrochloride.

20. The method of preparing a displacement fluid according to claim 18 or 19, wherein an inert atmosphere gas is used to exclude oxygen in the mixed liquid.

21. A method of preparing a displacement fluid according to claim 20 wherein the inert atmosphere gas is selected from any one or more of helium, neon, argon, nitrogen and carbon dioxide.

22. The method of preparing a displacement fluid according to any one of claims 7, 8, 9, 11, 14, 17 and 18, wherein the mixed solution prepared in step (4), a second oily solvent, dilution water, the emulsifier and the water-soluble surfactant are mixed; the second oily solvent is the same kind as the first oily solvent.

23. A method of preparing a displacement fluid according to claim 22 wherein the dilution water has a degree of mineralization of less than 5000 ppm.

24. The method for preparing a displacement fluid according to claim 22, wherein the volume ratio of the second oily solvent to the mixed solution prepared in step (4) is 1 (0.5 to 2), so as to obtain a mixture.

25. A method of preparing a displacement fluid according to claim 22 wherein the concentration of the mixture in the dilution water is from 0.01 to 3.0 vol.%.

26. A method of preparing a displacement fluid according to claim 22 wherein the concentration of the mixture in the dilution water is from 0.5 to 1.5 vol.%.

27. A method of preparing a displacement fluid according to claim 22 wherein the concentration of the water soluble surfactant in the dilution water is from 0.01 to 1 wt.%.

28. A method of preparing a displacement fluid according to claim 22 wherein the concentration of the water-soluble surfactant in the dilution water is from 0.025 to 0.075 wt.%.

29. The method of preparing a displacement fluid according to claim 22, wherein the emulsifier is used in an amount of 1wt.% to 10wt.% based on the total weight of the mixed solution prepared in step (4), the second oily solvent, the diluting water and the water-soluble surfactant.

30. Use of a displacement fluid according to any one of claims 1 to 6 wherein the use is injection of the displacement fluid into a formation through an injection well.

31. Use according to claim 30, wherein the displacement is performed by injecting the displacement fluid first and then by alternating injection of displacement fluid and water.