A molecular dynamics simulation method for CO2-entrainer complex system huff and puff

Abstract

The application discloses a CO2-entrainer composite system stimulation molecular dynamics simulation method and belongs to the technical field of CO2 flooding oil. The CO2-entrainer composite system stimulation molecular dynamics simulation method comprises the following steps: providing a CO2-entrainer composite system model, developing and optimizing a coarse-grained molecular dynamics force field, performing molecular dynamics simulation based on the coarse-grained molecular dynamics force field, collecting component density distribution and mean square displacement data every 5 ps, and performing calculation and analysis. The CO2-entrainer composite system stimulation molecular dynamics simulation method provided by the application can not only simulate large-scale dynamic behavior, but also comprehensively characterize the interaction between molecules and the interaction between molecules and a wall in a shale oil reservoir. Meanwhile, the method can reveal the flow law of molecular particles in shale micro-nano channels under the action of the interaction between molecules, the interaction between molecules and a wall and the like at a molecular level, so that the shale oil reservoir production mechanism under the action of multiple mechanisms is revealed.

Description

Technical Field

[0001] This invention belongs to the field of CO2 flooding technology, and particularly relates to a molecular dynamics simulation method for the huff and puff of a CO2-entrainer composite system. Background Technology

[0002] Because shale reservoirs are characterized by low porosity, ultra-low permeability, well-developed nanopores, and complex wettability, experimental research on shale oil is limited by high temperature and high pressure conditions. It is difficult to reveal the molecular particle seepage law from a microscopic perspective, and the crude oil displacement effect can only be observed from a macroscopic perspective.

[0003] In recent years, molecular dynamics simulation technology has achieved significant research results in areas such as crude oil migration behavior, the reduction of CO2 / crude oil interfacial tension by surfactants, and surfactant aggregation and adsorption behavior. Among these, CO2 huff and puff not only enhances the recovery rate of shale oil reservoirs but also helps mitigate the greenhouse effect, making it a highly promising technology for improving shale oil recovery. However, despite its significant potential, CO2 huff and puff has not been widely applied in the field. The main reasons are: firstly, the mechanism and influencing factors of CO2 huff and puff for enhancing oil recovery in shale oil reservoirs are more complex than in conventional oil reservoirs, leading to an unclear understanding of the seepage mechanism of CO2 and crude oil in the micro- and nano-pores of shale; secondly, existing CO2 huff and puff technologies have limited effectiveness in improving shale oil recovery, failing to reach the economic minimum for large-scale field application. Therefore, it is necessary to clarify the mechanism of CO2 huff and puff in shale oil reservoirs and improve CO2 huff and puff technology to achieve more efficient development of shale oil reservoirs.

[0004] However, there is relatively little research on CO2 huff and puff for enhanced oil recovery in shale oil reservoirs. Moreover, most studies on the mechanisms of enhanced oil recovery are based on conventional oil reservoirs, and there is a lack of understanding of the synergistic mechanism of CO2 composite systems in shale oil reservoirs. The coarse-grained molecular dynamics force field of the CO2 composite system is not perfect enough to fully characterize the interaction between the CO2 composite system and shale organic matter / minerals-oil, which seriously restricts the exploration and research on the micro-permeability theory of CO2 composite system huff and puff in shale oil reservoirs. Summary of the Invention

[0005] The purpose of this invention is to provide a molecular dynamics simulation method for the huff and puff of a CO2-entrainer composite system. By developing a coarse-grained molecular dynamics force field for the "CO2-entrainer-crude oil-shale organic matter / mineral slit" system and conducting coarse-grained molecular dynamics simulations, it is possible not only to simulate large-scale dynamic behavior but also to more comprehensively characterize the interactions between molecules and between molecules and the walls in shale reservoirs. Simultaneously, by introducing a multi-mechanism synergistic mechanism between the CO2-entrainer composite system and shale organic matter, minerals, and crude oil, the flow law of molecular particles within the micro- and nano-pores of shale under the influence of intermolecular and molecular-wall interactions is revealed at the molecular level, thereby revealing the mobilization mechanism of shale reservoirs under multi-mechanism synergistic effects.

[0006] To achieve the above objectives, the technical solutions provided by the embodiments of the present invention are as follows:

[0007] In a first aspect, the present invention provides a molecular dynamics simulation method for the swallowing and expulsion of a CO2-entrainer composite system, comprising the following steps:

[0008] S101: Provides a model for a CO2-entrainer composite system;

[0009] S102: Develop and optimize coarse-grained molecular dynamics force fields, specifically including:

[0010] The CO2-entrainer composite system model was simulated into an all-atom system model, and all-atom molecular dynamics simulations were performed to obtain the initial model of the all-atom system.

[0011] The initial model of the all-atom system is transformed into a coarse-grained molecular structure, and the coarse-grained molecular structure is represented by predefined beads to obtain the coarse-grained system model;

[0012] In the coarse-grained system model, the bond lengths and bond angles of the coarse-grained bead structure are statistically analyzed, and the potential energy parameter K of the interaction between the same coarse-grained beads is calculated using formulas (1) and (2). b R0, K0, and θ0;

[0013] (1)

[0014] (2)

[0015] In formula (1), U b For bond stretching potential energy, K b R is the factor, R is the bond length, and R0 is the bond length equilibrium value; in formula (II), Let K0 be the bond angle bending potential energy, K0 be the factor, θ be the bond angle, and θ0 be the angle equilibrium value.

[0016] By fitting the free energy curves of the interaction between beads of different coarse grain size using formula (3), the Lennar-Jones potential parameters D0 and R0 are obtained;

[0017] (3)

[0018] In formula (3), U LJ9-6 R represents the van der Waals force, R represents the bond length, R0 represents the bond length balance value, and D0 represents the potential well depth.

[0019] Furthermore, the wetting angle parameter α between coarse-grained beads of different phases is fitted using formula (4);

[0020] (4)

[0021] In formula (4), α is the wetting angle, σ is the wetting angle, and σ is the wetting angle. sg For the solid-gas interface free energy, σ sl σ is the free energy at the solid-liquid interface. lg This is the free energy at the liquid-gas interface.

[0022] The Lennar-Jones potential parameters D0 and R0 were fitted and optimized to ensure that the various macroscopic properties of the coarse-grained system model were in good agreement with the experimental data, thus obtaining the coarse-grained molecular dynamics force field.

[0023] S103: Conduct molecular dynamics simulations, specifically including:

[0024] Based on the coarse-grained molecular dynamics force field, periodic boundary conditions are used to perform a pre-simulation of 10 ns under a canonical ensemble to allow the system to reach relaxation equilibrium. Then, the CO2-entrainer composite system model is simulated for 30 ns with a simulation step size of 1 fs. The required data are collected and calculated every 5 ps, including component density distribution, mean square displacement, etc.

[0025] In conjunction with the first aspect, in a preferred embodiment of the present invention, the method for constructing the CO2-entrainer composite system model includes:

[0026] Provides crude oil molecular models, entrainer molecular models, and organic matter-mineral slit models;

[0027] The organic matter-mineral slit model is filled with CO2, the crude oil molecular model, and the entrainer molecular model to obtain the CO2-entrainer composite system model.

[0028] In conjunction with the first aspect, in a preferred embodiment of the present invention, the method for constructing the CO2-entrainer composite system model further includes:

[0029] The conjugate gradient algorithm was used to minimize the energy of the CO2-entrainer composite system model, bringing the energy to 1×10⁻⁶. -4 kcal / mol.

[0030] In conjunction with the first aspect, in a preferred embodiment of the present invention, the crude oil molecule is n-decane.

[0031] In conjunction with the first aspect, in a preferred embodiment of the present invention, the entrainer is composed of peracetylglucosamine dodecane.

[0032] In conjunction with the first aspect, in a preferred embodiment of the present invention, the number of crude oil molecule models, CO2 and entrainer molecule models filled in the organic matter-mineral slit model are 500, 1000-2000 and 10-30, respectively.

[0033] In conjunction with the first aspect, in a preferred embodiment of the present invention, the number of entrainer molecule models filled is 20.

[0034] In conjunction with the first aspect, in a preferred embodiment of the present invention, the relaxation equilibrium temperature is 323.15-348.15 K during molecular dynamics simulation.

[0035] In conjunction with the first aspect, in a preferred embodiment of the present invention, the cutoff radius is 2 nm in molecular dynamics simulations.

[0036] Secondly, in this embodiment of the invention, the molecular dynamics simulation method of the CO2-entrainer composite system is provided for the application in exploring the synergistic mechanism of the CO2-entrainer composite system in improving the recovery rate of shale oil reservoirs.

[0037] Compared with the prior art, the advantages or beneficial effects of the embodiments of the present invention include at least the following:

[0038] This invention provides a molecular dynamics simulation method for the huff and puff of a CO2-entrainer composite system. By developing and optimizing the coarse-grained molecular dynamics force field of the "CO2-entrainer-crude oil-shale organic matter / mineral fracture" system, coarse-grained molecular dynamics simulations are conducted. Compared to relatively all-atom simulations, this method can not only simulate large-scale dynamic behavior but also more comprehensively characterize the interactions between molecules in shale reservoirs and between CO2 composite system molecules and shale organic matter / mineral walls. This allows for a theoretical exploration of the microscopic seepage of the CO2 composite system huff and puff in shale reservoirs. This study investigates and studies the synergistic effects of a multi-mechanism CO2-entrainer composite system with shale organic matter, minerals, and crude oil. At the molecular level, it reveals the interactions between molecules of the CO2-entrainer composite system and between the CO2-entrainer composite system molecules and the walls of shale organic matter / minerals. This further clarifies the multi-mechanism synergistic effect mechanism of the CO2-entrainer composite system and its mechanism for improving shale oil recovery, thus providing a theoretical basis for the efficient development of shale oil reservoirs. This research is of great significance for the industrial application of CO2 composite system huff and puff to enhance shale oil recovery. Attached Figure Description

[0039] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is a schematic diagram of the CO2-entrainer composite system model constructed according to an embodiment of the present invention;

[0041] Figure 2 These are number density curves of alkanes in mineral slits in composite systems under different calculation examples provided in the embodiments of the present invention;

[0042] Figure 3 This refers to the diffusion coefficient of alkanes in mineral slits in different composite systems provided in the embodiments of the present invention;

[0043] Figure 4 This is a number density distribution diagram of CO2 in the Z direction in mineral slits under different concentrations of entrainer provided in the embodiments of the present invention;

[0044] Figure 5 This is a number density distribution diagram of alkanes in the Z direction in mineral slits under different concentrations of entrainer provided in the embodiments of the present invention;

[0045] Figure 6 This is a number density distribution diagram of CO2 in the Z direction in a mineral fissure under the same entrainer concentration, temperature, and different pressure conditions provided in an embodiment of the present invention.

[0046] Figure 7 This is a number density distribution diagram of alkanes in the Z direction in a mineral slit under the same entrainer concentration, temperature, and different pressure conditions provided in an embodiment of the present invention.

[0047] Figure 8 This is a number density distribution diagram of CO2 in the Z direction in the organic matter-mineral slit under the same entrainer concentration, pressure, and different temperature conditions provided in the embodiments of the present invention.

[0048] Figure 9 This is a number density distribution diagram of alkanes in the Z direction in an organic matter-mineral slit under the same entrainer concentration, pressure, and different temperatures provided in an embodiment of the present invention.

[0049] Figure 10 This is a density distribution diagram of alkanes in organic matter and mineral crevices provided in the embodiments of the present invention. Detailed Implementation

[0050] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the embodiments described below are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0051] In this embodiment of the invention, a molecular dynamics simulation method for the swallowing and expulsion of a CO2-entrainer composite system is provided, comprising the following steps S101-S103.

[0052] S101 provides a CO2-entrainer complex system model: In MaterialStudio molecular simulation software, crude oil molecular models, entrainer molecular models, and organic matter-mineral slit models are constructed separately. Then, the organic matter-mineral slit model is sequentially filled with CO2, crude oil molecular models, and entrainer molecular models to obtain the CO2-entrainer complex system model. (See [link]). Figure 1 As shown.

[0053] S102 - Development and optimization of coarse-grained molecular dynamics force fields:

[0054] The CO2-entrainer composite system model was constructed into an all-atom system model in Materialstudio molecular simulation software, and all-atom molecular dynamics simulation was performed to obtain the initial model of the all-atom system.

[0055] The initial model of the all-atom system is transformed into a coarse-grained molecular structure, and the coarse-grained molecular structure is represented by predefined beads to obtain a coarse-grained system model. The types of the predefined beads are shown in Table 1 below.

[0056] Table 1 - Predefined Bead Types

[0057]

[0058] In the coarse-grained system model, the bond lengths and bond angles of the coarse-grained bead structure are statistically analyzed, and the potential energy parameter K of the interaction between the same coarse-grained beads is calculated using formulas (1) and (2). b R0, K0, and θ0;

[0059] (1)

[0060] (2)

[0061] In formula (1), U b For bond stretching potential energy, K b R is the factor, R is the bond length, and R0 is the bond length equilibrium value; in formula (II), Let K0 be the bond angle bending potential energy, K0 be the factor, θ be the bond angle, and θ0 be the angle equilibrium value.

[0062] The van der Waals forces between various coarse-grained beads were calculated using formula (3), and the free energy curves of the interaction between different beads were fitted to obtain the Lennar-Jones potential energy parameters D0 and R0.

[0063] (3)

[0064] In formula (3), U LJ9-6 R represents the van der Waals force, R represents the bond length, R0 represents the bond length balance value, and D0 represents the potential well depth.

[0065] Furthermore, the wetting angle parameter α between coarse-grained beads of different phases is fitted using formula (4);

[0066] (4)

[0067] In formula (4), α is the wetting angle, σ is the wetting angle, and σ is the wetting angle. sg For the solid-gas interface free energy, σ sl σ is the free energy at the solid-liquid interface. lg This is the free energy at the liquid-gas interface;

[0068] The Lennar-Jones potential parameters D0 and R0 were fitted and optimized to ensure that the various macroscopic properties of the coarse-grained system were in good agreement with the experimental data, and the coarse-grained molecular dynamics force field was obtained. The experimental data included the results of density test, solubility test, contact angle test, interfacial tension test and core swallowing test.

[0069] S103 - Conduct molecular dynamics simulations: In the Materialstudio molecular simulation software, based on the coarse-grained molecular dynamics force field, periodic boundary conditions are adopted, and a pre-simulation is performed for 10 ns under the canonical ensemble (NVT) to allow the system to reach relaxation equilibrium. Then, the CO2-entrainer composite system model is simulated for 30 ns with a simulation step size of 1 fs. Data is collected every 5 ps, including component density distribution, mean square displacement, and other relevant parameter data. The synergistic mechanism between the CO2 composite system and shale organic matter / mineral-oil is calculated, analyzed, and evolved.

[0070] It should be noted that the development of coarse-grained molecular dynamics force fields includes the quantitative characterization of bond interactions and non-bonded interactions. Among them, bond stretching potential energy and bond bending potential energy are bond interactions. After statistically analyzing the bond lengths and bond angles of the coarse-grained bead structure in the coarse-grained system model using the averaging and standard deviation methods, the potential energy parameters of the bead bond interactions are obtained by formulas (1) and (II), respectively. Van der Waals forces and wetting angles are non-bonded interactions. The van der Waals forces between each pair of various types of beads are calculated by formula (3). At the same time, the free energy of the solid-gas and liquid-gas interfaces remains constant in the simulation system. By adjusting the Lennar-Jones potential energy parameters D0 and R0 between the two types of beads, the free energy of the solid-liquid interface can be changed, thereby changing the wetting angle. In addition, the lipophilic and CO2-philic groups of the entrainer are not actual molecules, and their wetting angles on the solid surface cannot be examined. Therefore, the interaction parameters between the lipophilic and CO2-philic groups and the solid interface are approximately set to be the same as those between crude oil and CO2 and the solid surface.

[0071] In this embodiment of the invention, the preferred component of the crude oil molecular model is n-decane. However, those skilled in the art should understand that this embodiment of the invention does not impose any particular limitation on the specific components of the crude oil molecular model, as long as a crude oil molecular model that meets the chemical composition requirements is obtained.

[0072] In this embodiment of the invention, the component of the entrainer molecular model is preferably peracetylglucosamine dodecane. However, those skilled in the art should understand that this embodiment of the invention does not particularly limit the specific components of the entrainer, with the aim of obtaining a CO2-entrainer composite system that can effectively improve the recovery rate of shale oil reservoirs.

[0073] In this embodiment of the invention, the organic-mineral fracture model preferably uses five-layer graphene and surface-hydroxylated quartz as the organic matter and minerals of the fractures in the shale reservoir, respectively. The five-layer graphene and the surface-hydroxylated quartz are simulated and combined in molecular simulation software to obtain the organic-mineral fracture model. The surface-hydroxylated quartz is obtained by adding hydrogen atoms to the surface of quartz and performing hydroxylation treatment.

[0074] In this embodiment of the invention, the conjugate gradient algorithm is preferably used to minimize the energy of the CO2-entrainer composite system model, so that its energy converges to 1×10⁻⁶. -4 kcal / mol.

[0075] In this embodiment of the invention, the number of crude oil molecule models, CO2 and entrainer molecule models filled in the organic matter-mineral slit model are preferably 500, 1000-2000 and 10-30, respectively.

[0076] In this embodiment of the invention, the number of entrainer molecule models filled in the organic matter-mineral slit model is more preferably 20.

[0077] In this embodiment of the invention, molecular dynamics simulations are performed using Materialstudio molecular simulation software, and a canonical ensemble (NVT) is employed. The Nosé-Hoover algorithm is used to control the temperature and pressure of the computational system, with the relaxation equilibrium temperature preferably between 323.15 and 348.15 K. Periodic boundary conditions are used, and the long-range electrostatic interactions are calculated using the particle-particle-particle summation method, with a cutoff radius of 2 nm. The velocity of the system is initially assigned by the Maxwell-Boltzmann distribution and controlled by the Velocity-Verlet algorithm.

[0078] The molecular dynamics simulation method for the uptake and release of the CO2-entrainer composite system provided in this invention embodiment is applied to explore and study the synergistic mechanism of the CO2-entrainer composite system in improving the recovery rate of shale oil reservoirs.

[0079] The technical solution of the present invention will be further described in detail below with reference to specific embodiments.

[0080] Example 1

[0081] This embodiment 1 provides a molecular dynamics simulation method for the swallowing and expulsion of a CO2-entrainer composite system, including the following steps S101-S103.

[0082] S101 provides a CO2-entrainer complex system model: In MaterialStudio molecular simulation software, crude oil molecular models, entrainer molecular models, and organic matter-mineral slit models are constructed separately. Then, the organic matter-mineral slit model is sequentially filled with CO2, crude oil molecular models, and entrainer molecular models to obtain the CO2-entrainer complex system model. (See [link]). Figure 1 As shown.

[0083] S102 - Development and optimization of coarse-grained molecular dynamics force fields:

[0084] The CO2-entrainer composite system model was constructed into an all-atom system model in Materialstudio molecular simulation software, and all-atom molecular dynamics simulation was performed to obtain the initial model of the all-atom system.

[0085] The initial model of the all-atom system is transformed into a coarse-grained molecular structure, and the coarse-grained molecular structure is represented by predefined beads to obtain a coarse-grained system model. The types of the predefined beads are shown in Table 1 below.

[0086] Table 1 - Predefined Bead Types

[0087]

[0088] In the coarse-grained system model, the bond lengths and bond angles of the coarse-grained bead structure are statistically analyzed, and the potential energy parameter K of the interaction between the same coarse-grained beads is calculated using formulas (1) and (2). b R0, K0, and θ0;

[0089] (1)

[0090] (2)

[0091] In formula (1), U b For bond stretching potential energy, K b R is the factor, R is the bond length, and R0 is the bond length equilibrium value; in formula (II), Let K0 be the bond angle bending potential energy, K0 be the factor, θ be the bond angle, and θ0 be the angle equilibrium value.

[0092] The van der Waals forces between various coarse-grained beads were calculated using formula (3), and the free energy curves of the interaction between different beads were fitted to obtain the Lennar-Jones potential energy parameters D0 and R0.

[0093] (3)

[0094] In formula (3), U LJ9-6R represents the van der Waals force, R represents the bond length, R0 represents the bond length balance value, and D0 represents the potential well depth.

[0095] Furthermore, the wetting angle parameter α between coarse-grained beads of different phases is fitted using formula (4);

[0096] (4)

[0097] In formula (4), α is the wetting angle, σ is the wetting angle, and σ is the wetting angle. sg For the solid-gas interface free energy, σ sl σ is the free energy at the solid-liquid interface. lg This is the free energy at the liquid-gas interface;

[0098] The Lennar-Jones potential parameters D0 and R0 were fitted and optimized to ensure that the various macroscopic properties of the coarse-grained system were in good agreement with the experimental data, and the coarse-grained molecular dynamics force field was obtained. The experimental data included the results of density test, solubility test, contact angle test, interfacial tension test and core swallowing test.

[0099] S103 - Molecular Dynamics Simulation: In MaterialStudio molecular simulation software, based on the coarse-grained molecular dynamics force field, a canonical ensemble (NVT) was used. The Nosé-Hoover algorithm was employed to control the temperature and pressure of the computational system, with the relaxation equilibrium temperature ranging from 323.15 to 348.15 K. Periodic boundary conditions were used, and the particle-particle-particle summation method was employed to calculate long-range electrostatic interactions with a cutoff radius of 2 nm. The system velocity was initially assigned by a Maxwell-Boltzmann distribution and controlled by the Velocity-Verlet algorithm. The CO2-entrainer composite system model was simulated for 30 ns with a simulation step size of 1 fs. Data was collected every 5 ps, and parameters such as the component density distribution and mean square displacement of the simulated system were analyzed. The synergistic interaction mechanism between the CO2 composite system and shale organic matter / mineral-oil was calculated, analyzed, and evolved.

[0100] Based on the above molecular dynamics simulations, the research content that can be achieved in this embodiment is as follows:

[0101] 1) By using the curves showing the changes in the number of CO2 and crude oil molecules adsorbed on the organic matter wall of shale, the adsorption of CO2 and the stripping process of crude oil molecules from the wall can be quantitatively analyzed to evaluate the replacement efficiency of crude oil.

[0102] 2) By using the Einstein equation and combining it with the mean square displacement, the self-diffusion coefficient of crude oil is calculated, and its diffusion and transport process in micro- and nano-pores is quantitatively characterized.

[0103] 3) By simulating the density distribution of crude oil molecules adsorbed on the organic matter wall of shale, the oil recovery rate of crude oil in the channel after injecting CO2-entrainer composite system under different concentrations, temperatures and pressures of entrainer was evaluated, and the factors affecting the recovery rate were analyzed.

[0104] Figures 2 to 10 The research results are presented, specifically:

[0105] according to Figure 2 It can be seen that when only n-decane exists in the quartz channels, n-decane is adsorbed on the quartz wall, forming a distinct adsorption layer. This means that n-decane in the quartz channels is difficult to diffuse, which is not conducive to oil extraction. When CO2 molecules are added, the peak value of the adsorption layer decreases, and some n-decane is replaced by CO2 and migrates to the center of the quartz channels. When an entrainer is added, the peak value of the adsorption layer continues to decrease, the number density of n-decane replaced in the center of the quartz channels increases, and the replacement efficiency improves. This indicates that adding CO2 and entrainers can improve shale oil recovery.

[0106] according to Figure 3 It can be seen that adding a certain amount of entrainer to the CO2-alkane system is more conducive to the diffusion of n-decane in the quartz channels. Meanwhile, according to... Figures 4 to 5 It can be seen that after adding CO2 and the entrainer, CO2 is adsorbed on the quartz wall. At this time, most of the alkanes on the quartz wall are displaced to the center of the CO2-entrainer composite system, with a small portion remaining between the adsorption layers. Furthermore, the alkane displacement rate increases with the increase of the entrainer concentration. However, excessive entrainer concentration leads to increased system pressure, causing excessive crowding within the quartz pores and hindering the rapid diffusion of alkanes. Therefore, the optimal filler quantity for the entrainer is 20 cells.

[0107] according to Figures 6 to 7 It can be seen that as pressure increases, more alkanes are displaced by CO2 into the bulk phase, increasing the oil recovery rate within the pores. Meanwhile, according to... Figures 8 to 9 It can be seen that the higher the temperature of the simulation system, the less alkanes are displaced. Therefore, the optimal simulation temperature is 323.15-348.15 K.

[0108] according to Figure 10 It can be seen that in the organic matter-mineral slit, the organic matter wall has a stronger adsorption capacity for alkanes than the mineral wall.

[0109] Based on the above analysis, this embodiment is the first to develop a coarse-grained molecular dynamics force field for the "CO2-entrainer-crude oil-shale organic matter / mineral fracture" system, and conducts coarse-grained molecular dynamics simulations based on this force field. Compared with all-atom dynamics simulations, this embodiment can simulate large-scale dynamic behavior, and compared with mesoscopic simulations, this embodiment can reflect more information at the microscopic level. Compared with the coarse-grained force fields currently established for surfactant-crude oil-water-quartz systems and CO2-crude oil-surfactant systems, this embodiment can more comprehensively characterize the intermolecular and molecular-wall interactions in shale oil reservoirs, providing technical support for the study of the microscopic mechanism of CO2 composite systems in enhancing shale oil recovery. Meanwhile, current research on the mechanism of CO2 huff and puff for enhancing oil recovery mainly focuses on CO2 adsorption, displacement, and diffusion mechanisms, without addressing the synergistic effect of entrainers. This embodiment introduces for the first time the synergistic mechanism of a multi-mechanism CO2-entrainer composite system with shale organic matter, minerals, and crude oil, revealing the flow patterns of molecular particles within shale micro- and nano-pores under the influence of intermolecular and molecular-to-wall interactions at the molecular level. Compared with current single-mechanism studies, this embodiment reveals the dynamic mechanism of shale reservoirs under the synergistic effect of multiple mechanisms.

[0110] 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 invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method of molecular dynamics simulation of CO2-entrainer complex system huff and puff, characterized in that, The method comprises the following steps: S101-providing a CO2-entrainer complex system model; wherein, the construction method of the CO2-entrainer complex system model comprises: providing a crude oil molecule model, an entrainer molecule model and an organic matter-mineral slit model; filling CO2, the crude oil molecule model and the entrainer molecule model in the organic matter-mineral slit model to obtain the CO2-entrainer complex system model; S102-developing and optimizing a coarse-grained molecular dynamics force field: The CO2-entrainer complex system model is simulated into a full-atom system model, and full-atom molecular dynamics simulation is performed to obtain a full-atom system initial model; The full-atom system initial model is converted into a coarse-grained molecular structure, and the coarse-grained molecular structure is represented by a pre-defined bead to obtain a coarse-grained system model; The bond length and bond angle of the statistical coarse-grained bead structure in the coarse-grained system model are counted, and the potential energy parameters K of the interaction between the same coarse-grained beads are calculated by using formulas (1) and (2) b R0, K0 and θ0; （1） （2） wherein, in formula (1), U b is the bond-stretching potential, K b is a factor, R is the bond length, and R0 is the bond length equilibrium value; in formula (2), is the bond-angle bending potential, K0 is a factor, θ is the bond angle, and θ0 is the angle equilibrium value; Lennar-Jones potential energy parameters D0 and R0 are fitted by using formula (3) to fit the free energy curve of the interaction between different coarse-grained beads; (3) wherein, in formula (3), U LJ9-6 is the van der Waals force, R is the bond length, R0is the bond length equilibrium value, and D0is the potential well depth; And, the wetting angle parameter α between different phases of coarse-grained beads is fitted by using formula (4); （4） wherein in formula (4), a is a wetting angle, σ sg is a solid-gas interface free energy sl is a solid-liquid interface free energy lg is a liquid-gas interface free energy The Lennar-Jones potential energy parameters D0 and R0 are fitted and optimized so that various macroscopic properties of the coarse-grained system are consistent with experimental data, and a coarse-grained molecular dynamics force field is obtained; S103-conducting molecular dynamics simulation: Based on the coarse-grained molecular dynamics force field, periodic boundary conditions are adopted, and pre-simulation is performed for 10 ns under a regular ensemble to make the system reach relaxation equilibrium, and then simulation is performed for 30 ns on the CO2-entrainer complex system model, the simulation step is 1 fs, and the data of component density distribution and mean square displacement are collected every 5 ps and calculated and analyzed.

2. The method of molecular dynamics simulation of CO2-entrainer complex system huff and puff according to claim 1, wherein, The construction method of the CO2-entrainer complex system model further comprises: The CO2-solvent composite system model was subjected to energy minimization using the conjugate gradient algorithm to converge the energy to 1 x 10 -4 kcal / mol.

3. The method of molecular dynamics simulation of CO2-entrainer composite system huff and puff of claim 1, wherein, The components of the crude oil molecule are n-decane.

4. The method of molecular dynamics simulation of CO2-entrainer complex system huff and puff of claim 3, wherein, The components of the entrainer are all-acetyl glucose dodecane.

5. The method of molecular dynamics simulation of CO2-entrainer composite system huff and puff of claim 4, wherein, In the organic matter-mineral slit model, the filling numbers of the crude oil molecule model, CO2 and the entrainer molecule model are 500, 1000-2000 and 10-30 respectively.

6. The method of molecular dynamics simulation of CO2-entrainer composite system huff and puff of claim 5, wherein, The filling number of the entrainer molecule model is 20.

7. The method of molecular dynamics simulation of CO2-entrainer composite system huff and puff of claim 1, wherein, In the molecular dynamics simulation, the temperature of the relaxation equilibrium is 323.15-348.15 K.

8. The method of molecular dynamics simulation of CO2-entrainer composite system huff and puff of claim 7, wherein, In the molecular dynamics simulation, the cutoff radius is 2 nm.

9. Application of the molecular dynamics simulation method of the CO2-entrainer complex system in the CO2-entrainer complex system according to any one of claims 1-8 in exploring the synergistic mechanism of the CO2-entrainer complex system for improving the recovery ratio of shale oil reservoirs.

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