CN115326647B - Surfactant-CO2Method for researching crude oil miscible interface behavior - Google Patents

Abstract

The invention discloses a method for researching the mixed phase interface behavior of a surfactant-CO ₂ -crude oil, belonging to the technical field of CO ₂ oil displacement. The method for researching the mixed phase interface behavior of the surfactant-CO ₂ -crude oil comprises the following steps: providing a surfactant-CO ₂ -crude oil miscible system model, developing and optimizing a coarse-particle molecular dynamics force field, performing coarse-particle molecular dynamics simulation based on the coarse-particle molecular dynamics force field, and acquiring data every 1ps to calculate interface parameters of a simulation system. According to the invention, the influence rule of multi-component crude oil on the minimum miscible pressure of CO ₂ miscible flooding by the surfactant and the influence behavior of the CO ₂ -philic surfactant with different functional groups on the stability of an oil-gas interface are researched and analyzed in an organic matter-mineral slit of a shale reservoir, so that the influence rule of the surfactant on the oil-gas interface is revealed at a molecular level, and the optimal surfactant suitable for CO ₂ flooding of a shale oil reservoir is conveniently screened.

Description

A method for studying the interfacial behavior of surfactant-CO2-crude oil

Technical Field

The invention belongs to the technical field of _CO2 oil recovery, and in particular relates to a method for studying the behavior of a surfactant- _CO2 -crude oil miscible interface.

Background Art

Since shale reservoirs have low porosity, ultra-low permeability, and widespread nanopores, experimental research on shale oil is usually subject to great limitations, such as high temperature and high pressure conditions. In recent years, molecular dynamics simulation technology has shown great application potential in the quantitative characterization of fluid-solid adsorption and interfacial tension in nanomaterials, and has gradually gained favor in the oil field.

At present, the research on surfactants to reduce the minimum miscibility pressure of CO ₂ / crude oil is still in the exploratory stage. The existing molecular dynamics simulation of surfactants on interface behavior mainly focuses on the study of oil-water interface behavior. Most of them use dissipative particle dynamics methods to simulate the dynamics of a single crude oil component, and do not consider the influence of shale organic matter and minerals, resulting in simulation results that are not in line with reality and difficult to guide production practice. In addition, the existing laboratory simulation methods are time-consuming and costly to operate.

Summary of the invention

The purpose of the present invention is to provide a method for studying the interfacial behavior of surfactant- _CO2 -crude oil miscible phases. The interfacial behavior and influence law of _CO2 miscible phase flooding surfactants are studied based on coarse-grained force field molecular dynamics (CG-MD) simulation, so as to screen out the surfactant most suitable for improving the _CO2 oil recovery efficiency of shale oil reservoirs. This not only greatly reduces the experimental cost, but also solves the technical problem that the existing molecular dynamics simulation results do not conform to the actual production, thereby being able to guide practical applications at the theoretical level.

In order to achieve the above objectives, the embodiments of the present invention adopt the following technical solutions:

In an embodiment of the present invention, a method for studying the behavior of a surfactant-CO ₂ -crude oil miscible interface is provided, comprising the following steps:

S101: Provide a surfactant-CO ₂ -crude oil miscible system model;

S102: Develop and optimize coarse-grained molecular dynamics force fields, including:

Simulating the surfactant-CO ₂ -crude oil mixed phase system model into an all-atom system model, and performing an all-atom molecular dynamics simulation to obtain an initial all-atom system model;

Converting the initial model of the all-atom system into a coarse-grained molecular structure, and representing the coarse-grained molecular structure with predefined beads to obtain a coarse-grained system model;

In the coarse-grained system model, the bond lengths and bond angles of the coarse-grained bead structures are counted, and the potential energy parameters K _b , R ₀ , K ₀ and θ ₀ of the interaction between beads of the same coarse-grained size are calculated using formulas (1) and (2) respectively;

Wherein, in formula (1), U _b is the bond stretching potential energy, K _b is the factor, R is the bond length, and R ₀ is the bond length equilibrium value; in formula (2), U _θ is the bond angle bending potential energy, K ₀ is the factor, θ is the bond angle, and θ ₀ is the angle equilibrium value;

And, using formula (3) to fit the free energy curve of the interaction between beads of different coarse particle sizes, the Lennar-Jones potential energy parameters D ₀ and R ₀ were obtained;

Wherein, in formula (3), U _LJ9-6 is the van der Waals force, R is the bond length, R ₀ is the bond length equilibrium value, and D ₀ is the potential well depth;

The Lennar-Jones potential parameters D ₀ and R ₀ were fitted and optimized to make the macroscopic performance parameters of the coarse-grained system model consistent with the experimental data, and the coarse-grained molecular dynamics force field was obtained.

S103: Conduct molecular dynamics simulations, including:

Based on the coarse-grained molecular dynamics force field, the surfactant-CO ₂ -crude oil miscible system model was pre-simulated for 10-20 ps under the canonical ensemble to allow the simulation system to relax and balance. Then, the simulation was carried out for 2.0 ns under the canonical ensemble, data was collected every 1 ps, and the interface parameters of the simulation system were calculated.

In a preferred implementation of the present invention, the method for constructing the surfactant-CO ₂ -crude oil miscible system model includes:

Provide crude oil molecular model, surfactant molecular model and organic matter-mineral slit model;

CO ₂ , the crude oil molecular model and the surfactant molecular model are simultaneously filled into the organic matter-mineral slit model to obtain a surfactant-CO ₂ -crude oil mixed phase system model.

In a preferred implementation of the embodiment of the present invention, the method for constructing the surfactant-CO ₂ -crude oil miscible system model further includes:

The surfactant-CO ₂ -crude oil mixed phase system model was subjected to energy minimization processing to converge its energy to 1×10 ^-4 kcal/mol.

In a preferred implementation of the embodiment of the present invention, the filling numbers of the crude oil molecule model, the CO ₂ and the surfactant molecule model in the organic matter-mineral slit model are 800, 2000 and 10 respectively.

In a preferred implementation of the embodiment of the present invention, the method for constructing the crude oil molecular model includes:

After the mixed oil is formed according to the components and mass percentages shown in Table 1 below, the molecular model of crude oil is constructed using molecular simulation software;

Table 1 - Components and mass percentages of crude oil molecular models

In a preferred implementation of the embodiment of the present invention, the method for constructing the surfactant molecular model includes:

A variety of CO ₂ -philic surfactants with different functional groups were used as research objects, and the surfactant molecular models were constructed using molecular simulation software.

In a preferred implementation of the embodiment of the present invention, the method for constructing the organic matter-mineral slit model includes:

Selecting five-layer graphene as the organic wall of the narrow cracks in the shale reservoir, and selecting a hydroxylated quartz surface as the mineral wall of the narrow cracks in the shale reservoir;

In the molecular simulation software, the selected five-layer graphene and the quartz surface are simulated and combined to obtain an organic matter-mineral slit model.

In a preferred implementation of the embodiment of the present invention, in the molecular dynamics simulation, the Nosé-Hoover algorithm is used to control the temperature and pressure of the calculation system, and the temperature of the relaxation equilibrium is 344.3K.

In a preferred implementation of the embodiment of the present invention, in the molecular dynamics simulation, periodic boundary conditions are adopted, and the particle-particle-particle addition method is used to calculate the long-range electrostatic interaction, and the cut-off radius is 2 nm.

In a preferred implementation of the embodiment of the present invention, in the molecular dynamics simulation, the velocity of the system is initially distributed by the Maxwell-Boltzmann distribution and controlled by the Velocity-Verlet algorithm.

Compared with the prior art, the advantages or beneficial effects of the embodiments of the present invention include at least:

The research method of surfactant- _CO2 -crude oil miscible interface behavior provided in the embodiment of the present invention provides a surfactant- _CO2 -crude oil miscible model, develops and optimizes a coarse-grained molecular dynamics force field for molecular dynamics simulation, and can study the influence of multi-component crude oil on surfactants reducing the minimum miscible pressure of _CO2 miscible drive and the influence of _CO2- philic surfactants with different functional groups on the stability of the oil-gas interface, thereby revealing the influence of surfactants on the oil-gas interface at the molecular level, and facilitating the screening of the best surfactant suitable for shale oil reservoirs. More importantly, the molecular dynamics simulation method not only greatly reduces the cost, but also solves the problem that the existing molecular dynamics simulation results do not conform to the actual production, so that it can guide practical applications at the theoretical level.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments recorded in the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

FIG1 is a molecular structure of a surfactant used to construct a surfactant molecular model in an embodiment of the present invention;

FIG2 is an organic matter-mineral slit model constructed according to an embodiment of the present invention;

FIG3 is a surfactant-CO ₂ -crude oil mixed phase system model constructed in an embodiment of the present invention;

FIG4 is a curve showing the variation of CO ₂ -shale oil interfacial tension with pressure provided by an embodiment of the present invention;

FIG. 5 is a diagram showing the influence of different crude oil components on the effect of surfactants on reducing the minimum miscible pressure of CO ₂ provided in an embodiment of the present invention.

DETAILED DESCRIPTION

The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

The embodiment of the present invention provides a method for studying the behavior of a surfactant-CO2-crude oil miscible interface, comprising the following steps:

S101: In Material Studio molecular simulation software, a surfactant-CO ₂ -crude oil mixed phase system model is constructed, as shown in FIG3 ;

In the embodiment of the present invention, constructing the surfactant-CO ₂ -crude oil mixed phase system model preferably includes:

In Material Studio molecular simulation software, crude oil molecular models, surfactant molecular models and organic matter-mineral slit models were constructed respectively, and then crude oil molecular models, CO ₂ and surfactant molecular models were filled into the organic matter-mineral slit model to obtain a surfactant-CO ₂ -crude oil mixed phase system model, wherein the numbers of crude oil molecular models, CO ₂ and surfactant molecular models filled in the organic matter-mineral slit model were 2000, 800 and 10 respectively;

The energy of the surfactant-CO ₂ -crude oil miscible system model was minimized using the conjugate gradient algorithm, and its energy converged to 1×10 ^-4 kcal/mol.

In an embodiment of the present invention, constructing a crude oil molecular model preferably includes:

The mixed oil was formed according to the components and mass percentages shown in Table 1 below, and then the crude oil molecular model was constructed using Material Studio molecular simulation software;

Table 1 - Components and mass percentages of crude oil molecular models

In an embodiment of the present invention, constructing a surfactant molecular model preferably includes:

Four _CO2 -philic surfactants with different functional groups were used as research objects, and the surfactant molecular model was constructed using MaterialStudio molecular simulation software. Among them, the four preferred surfactants in this embodiment are AOT surfactants, three-chain anionic surfactant TC1 ₄ , low-foaming surfactant LS ₄₅ and peracetyl glucose dodecane.

S102: Develop and optimize the coarse-grained molecular dynamics force field preferably including:

In Material Studio molecular simulation software, the surfactant-CO ₂ -crude oil mixed phase system model was simulated into an all-atom system model, and an all-atom molecular dynamics simulation was performed to obtain an initial model of the all-atom system;

The initial model of the all-atom system is converted into a coarse-grained molecular structure, and the coarse-grained molecular structure is represented by predefined beads to obtain a coarse-grained system model, wherein the predefined bead types are shown in Table 2 below;

Table 2 - Predefined bead types

Bead Name Represents all-atom fragments Relative quality A1 CH ₃ -CH ₂ -CH ₂ - 43 A2 CH ₃ -CH ₂ 29 A3 -CH2 _- CH2 _{- CH2-} 42 A4 CO ₂ 44 A5 -COO- 44 A6 C-OAc 71 A7 -C(CH ₃ ) ₃ 57 A8 -C＝O-O-C- 56 A9 -C-SO ₃ 92

In the coarse-grained system model, the bond length and bond angle of the coarse-grained bead structure are statistically analyzed using the average and standard deviation methods, and the potential energy parameters K _b , R ₀ , K ₀ and θ ₀ of the interaction between beads of the same coarse-grained size are calculated using formulas (1) and (2), respectively;

Wherein, in formula (1), U _b is the bond stretching potential energy, K _b is the factor, R is the bond length, and R ₀ is the bond length equilibrium value; in formula (2), U _θ is the bond angle bending potential energy, K ₀ is the factor, θ is the bond angle, and θ ₀ is the angle equilibrium value;

And, using formula (3) to fit the free energy curve of the interaction between beads of different coarse particle sizes, the Lennar-Jones potential energy parameters D ₀ and R ₀ were obtained;

Wherein, in formula (3), U _LJ9-6 is the van der Waals force, R is the bond length, R ₀ is the bond length equilibrium value, and D ₀ is the potential well depth;

The Lennar-Jones potential parameters D ₀ and R ₀ are fitted and optimized to make the macroscopic performance parameters (density, interfacial tension, etc.) of the coarse-grained system model consistent with the experimental data and obtain the coarse-grained molecular dynamics force field.

S103: Conduct molecular dynamics simulations, including:

Based on the coarse-grained molecular dynamics force field, the surfactant-CO ₂ -crude oil miscible system model was pre-simulated for 10-20ps under the canonical ensemble using periodic boundary conditions, so that the temperature of the simulation system rose to a certain threshold, and the pre-simulation was continued for 2.5ns to relax the simulation system. Then, the simulation was carried out for 2.0ns, data was collected every 1ps, and the interface parameters of the simulation system were calculated. Among them, periodic boundary conditions were used in the x, y, and z directions.

In the embodiment of the present invention, the Nosé-Hoover algorithm is preferably used to control the temperature and pressure of the calculation system, and the temperature of the relaxation equilibrium is 344.3K.

In the embodiment of the present invention, periodic boundary conditions are adopted, preferably the particle-particle-particle addition method is used to calculate the long-range electrostatic interaction, and the cut-off radius is 2 nm. The calculation accuracy is 1.0×10 ^-4 .

In an embodiment of the present invention, the velocity of the system is initially distributed by the Maxwell-Boltzmann distribution and controlled by the Velocity-Verlet algorithm.

The technical solution of the present invention will be further described in detail below in conjunction with specific embodiments.

Example 1

This embodiment 1 provides a method for studying the behavior of a surfactant-CO ₂ -crude oil miscible interface, comprising the following steps:

Constructing a crude oil molecular model: The crude oil mixture was assembled according to the components and mass percentages shown in Table 1 below, and then a crude oil molecular model was constructed in Material Studio molecular simulation software.

Table 1 - Components and mass percentages of crude oil molecular models

Construction of surfactant molecular model: Four surfactants, including AOT surfactants, three-chain anionic surfactant TC1 ₄ , low-foaming surfactant LS ₄₅ and all-acetyl glucose dodecane, were taken as research objects, and the surfactant molecular model was constructed using MaterialStudio molecular simulation software, as shown in Figure 1.

Constructing an organic-mineral slit model: Five layers of graphene were selected as the organic wall of the slit in the shale reservoir and the hydroxylated quartz surface was selected as the mineral wall of the slit. In the Material Studio molecular simulation software, five layers of graphene and hydroxylated quartz were combined to obtain an organic-mineral slit model, as shown in Figure 2.

Constructing a surfactant-CO ₂ -crude oil miscible system model: In the Material studio molecular simulation software, 800 crude oil molecular models, 2000 CO ₂ and 10 surfactant molecular models were filled into the organic matter-mineral slit model to obtain a surfactant-CO ₂ -crude oil miscible model;

The surfactant-CO ₂ -crude oil miscible model was energy minimized using the conjugate gradient algorithm to converge the energy to 1×10 ^-4 kcal/mol, and the surfactant-CO ₂ -crude oil miscible system model was obtained, as shown in FIG3 .

S102: Develop and optimize a coarse-grained molecular dynamics force field, including:

(1) Quantitative characterization of bond interaction parameters

In Material studio molecular simulation software, the surfactant-CO ₂ -crude oil mixed phase system model is simulated into an all-atom system model, and an all-atom molecular dynamics simulation is performed to obtain an initial model of the all-atom system;

The initial model of the all-atom system is converted into a coarse-grained molecular structure, and the coarse-grained molecular structure is represented by predefined beads to obtain a coarse-grained system model, wherein the predefined bead types are shown in Table 2 below;

Table 2 - Predefined bead types

Bead Name Represents all-atom fragments Relative quality A1 CH ₃ -CH ₂ -CH ₂ - 43 A2 CH ₃ -CH ₂ 29 A3 -CH2 _- CH2 _{- CH2-} 42 A4 CO ₂ 44 A5 -COO- 44 A6 C-OAc 71 A7 -C(CH ₃ ) ₃ 57 A8 -C＝O-O-C- 56 A9 -C-SO ₃ 92

In the coarse-grained system model, the bond lengths and bond angles of the coarse-grained bead structures are counted, and the potential energy parameters K _b , R ₀ , K ₀ and θ ₀ of the interaction between beads of the same coarse-grained size are calculated using formulas (1) and (2) respectively;

Wherein, in formula (1), U _b is the bond stretching potential energy, K _b is the factor, R is the bond length, and R ₀ is the bond length equilibrium value; in formula (2), U _θ is the bond angle bending potential energy, K ₀ is the factor, θ is the bond angle, and θ ₀ is the angle equilibrium value;

(2) Quantitative characterization of non-bonded interaction parameters

Non-bonded interactions include van der Waals interactions and Coulomb electrostatic interactions. The Lennar-Jones potential function is used to describe the van der Waals interactions between different beads. The free energy curve of the interaction between beads of different coarse grains is fitted using formula (3) to obtain the Lennar-Jones potential parameters D ₀ and R ₀ ; then the Lennar-Jones potential parameters D ₀ and R ₀ are fitted and optimized so that the macroscopic property parameters such as density and interfacial tension of the simulated system are consistent with the experimental data;

Wherein, in formula (3), U _LJ9-6 is the van der Waals force, R is the bond length, R ₀ is the bond length equilibrium value, and D ₀ is the potential well depth;

S103: Conduct molecular dynamics simulations, including:

In Material studio molecular simulation software, based on the coarse-grained molecular dynamics force field, the canonical ensemble (NVT) is used, and the temperature and pressure of the calculation system are controlled by the Nosé-Hoover algorithm, and the temperature of the relaxation equilibrium is 323.15-348.15K; periodic boundary conditions are used, and the particle-particle-particle addition method is used to calculate the long-range electrostatic interaction, and the cut-off radius is 2nm; the velocity of the system is initially distributed by the Maxwell-Boltzmann distribution and controlled by the Velocity-Verlet algorithm. The surfactant-CO ₂ -crude oil mixed phase system model is simulated for 2.0ns, the simulation step is 1fs, and data is collected every 1ps to analyze the interface parameters of the simulation system.

1) Analysis of CO ₂ -Crude Oil Interfacial Tension

At a temperature of 344.3K, the interfacial tension of _CO2 -shale oil changes with pressure before and after adding surfactants, as shown in Figure 4. According to Figure 4, it can be seen that: under different pressures, there is a good linear relationship between the interfacial tension and pressure between _CO2 and crude oil. The pressure when the interfacial tension is zero is obtained by linearly extrapolating the curve, and this pressure is the minimum miscibility pressure. At a temperature of 344.3K, the minimum miscibility pressure is 10.4MPa before adding surfactants, and the minimum miscibility pressure is reduced to varying degrees after adding surfactants. Among them, the minimum miscibility pressure is 7.4MPa after adding all-acetyl glucose dodecane. Therefore, all-acetyl glucose dodecane can significantly reduce the minimum miscibility pressure of _CO2 /crude oil, and is the optimal surfactant.

2) Analysis of CO ₂ - crude oil interface stability after adding surfactant

Based on the equilibrium configuration of surfactant adsorption at the crude oil/ _CO2 interface, the embodiment of the present invention uses formula (4) to calculate the interface formation energy of the crude oil/ _CO2 system. The results are shown in Table 3 below.

Where IFE is the interfacial formation energy of the surfactant; E _total is the total energy of the system after the surfactant reaches equilibrium at the interface; E _Sur is the energy of one surfactant molecule; E _ref is the total energy of the corresponding system before surfactant adsorption; and n is the number of surfactants.

Table 3 - Interfacial formation energy of surfactants at the oil-gas interface (kJ·mol ^-1 )

According to Table 3, after adding four _CO2 -philic surfactants with different functional groups in this embodiment, the interface formation energy (IFE) of _CO2 /crude oil is negative, indicating that the adsorption of these four surfactants at the oil-gas interface enhances the thermodynamics of the interface and is conducive to the formation of _CO2 miscible phase. The smaller the value, the more conducive it is to the formation of a thermodynamically stable interface.

3) Effects of different crude oil components on reducing the minimum miscible pressure of CO ₂

With other conditions unchanged, the mass percentage of crude oil molecules was changed to construct three different cases, as shown in Table 4 below.

Table 4 - Different crude oil components

Case 1 Case 2 Case 3 CH ₄ 100 64 55 C ₂ H ₆ 88 60 50 _{C6H14 ​} 60 60 50 C ₉ H ₂₀ 60 88 76 _{C16H34 ​} 64 100 80 C ₉ H ₇ N 10 10 31 C ₆ H ₁₂ O ₂ 8 8 27 _CH4S 10 10 31

The simulation results are shown in Figure 5. According to Figure 5, it can be seen that when the proportion of alkanes decreases, _CO2 can be miscible with crude oil at a relatively low pressure. However, as the carbon number of alkanes increases, the minimum miscibility pressure of _CO2 and crude oil shows a very obvious increasing trend, especially when the molecular weight ratio is high.

According to the above description, this embodiment uses molecular dynamics simulation method to study the influence of _{CO2 -} philic surfactants containing different CO2-philic functional groups on reducing _CO2 miscible pressure, and introduces the interaction between shale wall and _CO2 /surfactant/crude oil system, revealing the flow law of molecular particles in shale nanopores under the action of molecules, molecules and walls, etc. at the molecular level, thereby greatly reducing the manpower, material and financial resources of the simulation experiment, and can provide guidance for practical applications at the theoretical level.

It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be considered exemplary and non-restrictive in all respects, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims be included in the present invention.

Claims (9)

1. A research method for surfactant-CO ₂ -crude oil miscible interface behavior is characterized by comprising the following steps:

S101: providing a surfactant-CO ₂ -crude oil miscible system model;

s102: the development and optimization of the coarse-grained molecular dynamics force field specifically comprises the following steps:

Simulating the surfactant-CO ₂ -crude oil miscible system model into a full-atom system model, and performing full-atom molecular dynamics simulation to obtain a full-atom system initial model;

Converting the initial model of the all-atomic system into a coarse-granularity molecular structure, and representing the coarse-granularity molecular structure by predefined beads to obtain a coarse-granularity system model;

Counting the bond length and bond angle of the coarse-grained bead structure in the coarse-grained system model, and respectively calculating potential energy parameters K _b、R₀、K₀ and theta ₀ of interaction between the same coarse-grained beads by utilizing formulas (1) and (2);

In the formula (1), U _b is bond expansion potential energy, K _b is a factor, R is bond length, and R ₀ is bond length balance value; in the formula (2), U _θ is the key angle bending potential energy, K ₀ is a factor, θ is the key angle, and θ ₀ is an angle balance value;

And fitting a free energy curve of interaction between the beads of different coarse sizes by using a formula (3) to obtain Lennar-Jones potential energy parameters D ₀ and R ₀;

In the formula (3), U _LJ9-6 is Van der Waals force, R is bond length, R ₀ is bond length balance value, and D ₀ is potential well depth;

Fitting and optimizing Lennar-Jones potential energy parameters D ₀ and R ₀ to ensure that the macroscopic performance parameters of the coarse-grained system model are consistent with experimental data, and obtaining a coarse-grained molecular dynamics force field;

s103: the molecular dynamics simulation is carried out, and the method specifically comprises the following steps:

Based on the coarse-particle molecular dynamics force field, pre-simulating the surfactant-CO ₂ -crude oil miscible system model for 10-20ps under a regular ensemble, so that the relaxation balance of a simulation system is realized, then performing simulation for 2.0ns under the regular ensemble, acquiring data every 1ps, and calculating interface parameters of the simulation system;

The construction method of the surfactant-CO ₂ -crude oil miscible system model comprises the following steps:

Providing a crude oil molecular model, a surfactant molecular model and an organic matter-mineral slit model;

And simultaneously filling the CO ₂, the crude oil molecular model and the surfactant molecular model in the organic matter-mineral slit model to obtain a surfactant-CO ₂ -crude oil miscible system model.

2. The method for studying surfactant-CO ₂ -crude oil miscible phase interface behavior according to claim 1, wherein the method for constructing surfactant-CO ₂ -crude oil miscible phase system model further comprises:

And (3) performing energy minimization treatment on the surfactant-CO ₂ -crude oil miscible system model to enable the energy to be converged to 1X 10 ^-4 kcal/mol.

3. The method for studying the mixed phase interface behavior of surfactant-CO ₂ -crude oil according to claim 2, wherein the filling numbers of the crude oil molecular model, the CO ₂ and the surfactant molecular model in the organic matter-mineral slit model are 800, 2000 and 10, respectively.

4. The method for studying the mixed phase interface behavior of a surfactant-CO ₂ -crude oil according to claim 1, wherein the method for constructing a crude oil molecular model comprises the following steps:

after the components and the mass percentages shown in the following table 1 are combined into mixed oil, a crude oil molecular model is constructed by utilizing molecular simulation software;

TABLE 1 composition and mass percent of crude oil molecular model

5. The method for studying the miscible interfacial behavior of surfactant-CO ₂ -crude oil according to claim 1, wherein the method for constructing the surfactant molecular model comprises the following steps:

And (3) taking a plurality of CO ₂ -philic surfactants with different functional groups as research objects, and constructing a surfactant molecular model by using molecular simulation software.

6. The method for studying surfactant-CO ₂ -crude oil miscible interface behavior according to claim 1, wherein the method for constructing the organic matter-mineral slit model comprises the following steps:

five-layer graphene is selected as an organic matter wall surface of a slit in a shale reservoir, and a quartz surface subjected to hydroxylation treatment is selected as a mineral wall surface of the slit in the shale reservoir;

And in molecular simulation software, performing simulation combination on the selected five-layer graphene and the quartz surface to obtain an organic matter-mineral slit model.

7. The method for studying the mixed phase interface behavior of surfactant-CO ₂ -crude oil according to claim 1, wherein in the molecular dynamics simulation, the temperature and pressure of the computing system are controlled by using a Nos-Hoover algorithm, and the temperature of the relaxation equilibrium is 344.3K.

8. The method for studying surfactant-CO ₂ -crude oil miscible interfacial behavior according to claim 7, wherein in the molecular dynamics simulation, a periodic boundary condition is adopted, and long-range force electrostatic interaction is calculated by using a particle-particle addition method, and the cut-off radius is 2nm.

9. The method of studying surfactant-CO ₂ -crude oil miscible interfacial behavior as claimed in claim 7, wherein in said molecular dynamics simulation, the Velocity of the system is initially distributed by maxwell-boltzmann distribution and is controlled by the Velocity-Verlet algorithm.