CN112414890B

CN112414890B - Molecular simulation method for kerogen expansion behavior of shale oil reservoir

Info

Publication number: CN112414890B
Application number: CN202011164897.3A
Authority: CN
Inventors: 黄亮; 周文; 王璐; 徐浩; 周秋媚; 邹杰
Original assignee: Chengdu Univeristy of Technology
Current assignee: Chengdu Univeristy of Technology
Priority date: 2020-10-27
Filing date: 2020-10-27
Publication date: 2021-07-27
Anticipated expiration: 2040-10-27
Also published as: CN112414890A

Abstract

The invention provides a molecular simulation method for kerogen expansion behavior of a shale oil reservoir, which comprises the following steps: s1, initializing a simulation device; s2, coagulating by using a simulation device; s3, shaping the simulation device; s4, simulating the expansion of kerogen; and S5, analyzing a coupling mechanism of liquid molecule occurrence and kerogen expansion. According to the invention, by constructing a kerogen expansion microscopic simulation device and combining the proposed geometric restriction insertion-molecular dynamics coupling simulation method, the physical process that the kerogen expands in the excess liquid hydrocarbon can be reduced at the maximum molecular scale, the kerogen expansion behavior obtained through experimental tests can be reproduced, and the microscopic mechanism of kerogen expansion can be explained. The technical scheme provided by the invention is beneficial to the research of the expansion behavior of kerogen in different liquid hydrocarbon components, the occurrence form of hydrocarbons and the coupling mechanism of kerogen expansion, so that the understanding of the chemical separation phenomenon in the kerogen is deepened, and a theoretical basis is laid for the accurate evaluation of shale oil reservoir reserves.

Description

Molecular simulation method for kerogen expansion behavior of shale oil reservoir

Technical Field

The invention belongs to the technical field of petroleum exploration, and particularly relates to a molecular simulation method for kerogen expansion behavior of a shale oil reservoir.

Background

Various hydrocarbon components in shale oil reservoirs are mainly generated by shale kerogen decomposition. Kerogen is an amorphous organic matter, and the macromolecular structure of the kerogen is composed of aromatic rings, aliphatic units and heteroatom functional groups. The thermal maturation evolution of kerogen includes both bond-forming reactions and bond-breaking reactions. The bond forming reaction is beneficial to improving the crosslinking degree of the kerogen structural network, and the bond breaking reaction enables organic carbon on the kerogen skeleton to be converted into various hydrocarbon components. Common hydrocarbon components include normal alkanes, cycloalkanes, and aromatics, among others. Due to the abundant nanopores and the huge specific surface area, kerogen can store a large amount of hydrocarbon components generated by decomposition. These hydrocarbon components are mainly present in kerogen in the form of an adsorbed phase, a dissolved phase and a free phase. The kerogen has a flexible pore structure, and the interaction of various hydrocarbon components with the kerogen can cause the kerogen to expand to different degrees. The presence of hydrocarbon components and the expansion of kerogen are in dynamic equilibrium. The expansion behavior of kerogen in different hydrocarbon components is determined, the coupling mechanism of the hydrocarbon occurrence form and the kerogen expansion is found out, the understanding of the chemical separation phenomenon in the kerogen can be deepened, and a theoretical basis can be laid for the accurate evaluation of the shale oil reservoir reserves.

At present, scholars at home and abroad adopt a test tube expansion device to carry out a large amount of experimental tests aiming at the expansion behaviors of kerogen in various liquid solvents. The kerogen test tube expansion experiment principle is simple, the operation is convenient, the test cost is low, and the method is an expansion test method which is generally adopted at present. The basic operation flow of the method is as follows: firstly, weighing 0.2g of kerogen powder sample, and putting a glass round tube with the outer diameter of 3mm and uniform thickness; then, tapping the kerogen sample for multiple times to realize the close accumulation among the particles; centrifuging the sample tube for 5min twice at the rotating speed of 2500 rpm, and recording the initial height of the dried sample when the height of the sample is unchanged within 5 hours; then slowly adding liquid hydrocarbon components into the glass round tube, adding the reagent and simultaneously carrying out beating, ultrasonic and low-speed centrifugal operation on the sample, so as to extrude air in the sample and realize the full mixing of the kerogen sample and the reagent, and in addition, in order to ensure that the sample fully expands in the reagent, adding excess reagent, wherein the liquid level at the top end of the reagent should exceed the end face of the kerogen by 3-4 cm; finally, the tube containing the sample and reagents was placed vertically for 24 hours, the tube was centrifuged at 5000 rpm for 5 minutes, the centrifugation step was repeated until the height of the sample in the tube did not change, the final sample height was recorded, and the rate of volume expansion of the kerogen was calculated in combination with the initial height of the kerogen. In reported studies of the kerogen swelling experiments, the swelling tests were generally carried out on the basis of a plurality of different types of kerogen samples and a plurality of different types of liquid reagents, the differential swelling behaviour of a single type of kerogen in liquid hydrocarbon components of the same type but of different sizes not being clearly understood. Recently, the system tests the swelling behavior of Kimmeridge kerogen in n-alkanes, cycloalkanes and aromatics with different sizes respectively, and perfects the kerogen swelling theory. The experimental test is the most direct method for researching the expansion behavior of the kerogen, and the molecular simulation method is a powerful means for researching the micro mechanism of the expansion of the kerogen. Currently, there are few studies on the molecular scale that study the swelling behavior of kerogen. Some researchers have tried molecular simulation studies on the swelling behavior of kerogen and coal in solvents, respectively, but the swelling was insufficient because the kerogen and coal were not brought into contact with the excessive solvent in the simulation.

At present, few molecular simulation researches aiming at the expansion behavior of organic matters in a liquid solvent are carried out, and Pathak et al adopt a Molecular Dynamics (MD) method to research the expansion behavior of kerogen in a series of liquid fluid molecules, and the basic scheme is as follows: and randomly putting the kerogen molecules and the fluid molecules into a large cubic box, wherein the mass of the fluid molecules is about 20% of that of the kerogen molecules, performing structural relaxation on the mixture of the kerogen and the fluid molecules by adopting a strict MD annealing process, finally obtaining the balanced kerogen volume, and calculating the expansion rate of the kerogen in the fluid molecules by combining the kerogen volume without the fluid molecules. Similarly, Niekerk et al also used MD to study the swelling behavior of coal molecules in liquid solvents, the basic scheme is: mixing coal molecules and 500 solvent molecules together, putting the mixture into a large box, then adopting a series of MD steps to relax the structure of the mixture, and finally calculating the expansion rate of the coal molecules based on the balanced structure volume. These two solutions are similar, the first one is to mix a fixed mass fraction of liquid solvent molecules in kerogen molecules, and the second one is to mix a fixed number of solvent molecules in coal molecules. The main drawback of these two solutions is that the expansion of the kerogen or coal is not sufficient and does not reflect the true expansion rate, because the interaction between kerogen or coal and the excess liquid molecules cannot be simulated.

Currently, a part of scholars study the expansion behavior of kerogen absorbing gas micromolecules by adopting a giant canonical Monte Carlo and a coupling method of molecular dynamics (GCMC-MD). The method adopts GCMC algorithm to realize the insertion and deletion of gas molecules in kerogen, and adopts MD algorithm to realize the expansion of kerogen and the movement of gas molecules. Specifically, the method is to perform GCMC and MD simulation at intervals, namely, fixing the pore structure for a certain number of steps of GCMC simulation, stopping the GCMC simulation for a certain number of steps of MD simulation, stopping the MD simulation for GCMC simulation, and repeating the steps. This method cannot be directly applied to the swelling study of kerogen in liquid solvents because the GCMC-MD method controls the loading of molecules by chemical potential or pressure, whereas in the kerogen tube swelling experiment, the test pressure is 1atm, and too low a pressure cannot achieve the loading of liquid molecules. In addition, besides low algorithm efficiency and high calculation cost, the GCMC-MD method can have the technical problems of low sampling probability, parallel calculation error, incorrect track generation and the like when being applied to liquid macromolecules.

Disclosure of Invention

The invention aims to provide a molecular simulation method for kerogen expansion behavior of a shale oil reservoir. The method can reduce the physical process of expansion of the kerogen in the liquid solvent in the test tube expansion experiment, duplicate the kerogen expansion behavior observed in the experiment and explain the microscopic mechanism of the coupling of the kerogen expansion and the liquid molecular occurrence mode. In particular, the set of methods can simultaneously achieve the following functions: (1) the full expansion of the kerogen is realized through the interaction of the reduced kerogen and the excessive liquid molecules; (2) effectively distinguishing the occurrence forms of liquid molecules in the expanded kerogen; (3) the process of equilibration of the swollen kerogen with excess hydrocarbon components is significantly accelerated.

A molecular simulation method for kerogen expansion behavior of a shale oil reservoir comprises the following steps:

s1. initialization of simulation device

Determining an initial simulated cassette size for the device, wherein the x and y dimensions are set to final target values and the z dimension is set to be sufficiently large; putting a certain amount of kerogen molecules and virtual particles with different sizes into an initial simulation box; the two end faces of the simulation box in the z direction were bonded to the piston and the quartz layer, respectively, to form an initialized simulation apparatus.

S2. coagulation of simulation device

And applying a certain compression acting force on a piston of the simulation device along the z direction, simulating the pressure in the NPT ensemble, and developing the structural relaxation of the kerogen matrix based on a strict annealing simulation flow to ensure that the kerogen matrix is condensed. The dimensions of the simulation setup in the x and y directions remain fixed and the dimension in the z direction gradually decreases with the simulation and reaches equilibrium.

S3, analog device sizing

And (3) aiming at the simulation device which achieves the condensation in the step S2, stretching the piston for a certain distance along the z direction to form an excess fluid area, deleting virtual particles in the system to form irregular large pores inside kerogen, and then performing structural relaxation on the simulation system under the NVT (noise vibration and harshness) ensemble to eliminate stress concentration. And then, selecting a certain number of carbon skeleton atoms around the irregular macropores in the kerogen for fixation, wherein the positions of the fixed atoms are always kept unchanged in the subsequent simulation process, so as to prevent the macropores from collapsing in the expansion simulation process. The position and concentration of the fixed atoms are optimized and adjusted, so that the fixed atoms can effectively preserve macropores and have small influence on the expansion rate of kerogen. The constructed microcosmic simulation device is formed by sequentially combining a quartz layer, a kerogen substrate, an excess fluid area and a piston plate, and can maximally reduce the physical process of the kerogen test tube expansion experiment, wherein the quartz layer represents the bottom of a test tube in the experiment test and is kept fixed in the simulation process; kerogen matrix corresponds to a tightly packed powder sample in the experiment; the surplus fluid area can realize the contact between kerogen and surplus liquid molecules, so that the full expansion of the kerogen is realized; the piston sheet can reflect the external environment pressure in experimental tests by adding external force in the expansion process.

S4. kerogen expansion simulation

The microscopic kerogen expansion simulation device constructed based on the step S3 adopts the proposed geometric constraint insertion-molecular dynamics coupling method to carry out the expansion simulation of the kerogen, and an external acting force corresponding to the experimental environment pressure is applied to the piston in the simulation process.

The geometric constraint insertion method judges whether the fluid molecules can be successfully inserted according to unoccupied pore spaces, when the residual space in the pores can accommodate more fluid molecules, new fluid molecules are inserted, otherwise, the pore spaces are considered to be saturated with the fluid molecules. The method does not consider the interaction between fluid molecules and between the fluid molecules and pores when the fluid molecules are inserted, so that various technical problems when the GCMC method is applied to liquid macromolecules can be avoided, and the efficiency of kerogen saturated fluid molecules can be effectively improved.

The kerogen expansion simulation method comprises the following specific steps:

(1) recording the initial height of the kerogen prior to expansion, and saturating fluid molecules in the excess fluid zone and kerogen matrix and macropores in a kerogen expansion microscopic simulation device using geometrically constrained fluid insertion techniques;

(2) then pausing the fluid insertion process, and realizing the equilibrium relaxation of kerogen and fluid molecules by adopting a molecular dynamics method;

(3) then suspending the molecular dynamics process, and saturating the surplus fluid area and the kerogen macropores by using a fluid insertion technique again;

repeating steps (2) and (3) until a new fluid molecule is not successfully inserted, indicating that the kerogen swelling reaches equilibrium, recording the kerogen matrix height at that time, and calculating the kerogen swelling rate by combining the kerogen initial height.

S5, analyzing coupling mechanism of liquid molecule occurrence and kerogen expansion

And (4) combining the initial fluid distribution in the microscopic simulation device and the fluid molecular distribution after expansion balance, quantitatively distinguishing different occurrence forms of the liquid molecules, and analyzing the coupling relation between the kerogen expansion and the occurrence forms of the fluid molecules by combining the kerogen expansion rate in the step S4.

Step S5, specifically, determining the bulk phase weight of the fluid molecules in the kerogen based on the initial fluid molecule distribution in the kerogen, and defining the physical boundary of the dissolved phase weight statistics by the bulk phase weight boundary; determining the total fluid molecular weight in kerogen based on the fluid molecular distribution after expansion equilibrium, and defining another physical boundary of dissolved phasor statistics by the matrix front edge after kerogen expansion; subtracting the volume phase amount from the total fluid molecular weight to obtain the absorption amount of fluid molecules in the kerogen, including the absorption amount of the macroporous surface and the dissolution amount in the kerogen matrix; determining the dissolved phase amount of fluid molecules in the kerogen from the physical boundary of the defined dissolved phase amount statistics; the amount of the dissolved phase is subtracted from the amount of the fluid molecules adsorbed, and the amount of the fluid molecules adsorbed in the kerogen is obtained.

The invention provides a molecular simulation method for kerogen expansion behavior of a shale oil reservoir, which has the advantages that the physical process of the expansion of kerogen in excess liquid hydrocarbon can be reduced in a molecular scale maximization manner by constructing a kerogen expansion microscopic simulation device and combining the proposed geometric constraint insertion-molecular dynamics coupling simulation method, the kerogen expansion behavior obtained by experimental tests can be reproduced, and the microscopic mechanism of kerogen expansion can be explained. The technical scheme provided by the invention is beneficial to the research of the expansion behavior of kerogen in different liquid hydrocarbon components, the occurrence form of hydrocarbons and the coupling mechanism of kerogen expansion, so that the understanding of the chemical separation phenomenon in the kerogen is deepened, and a theoretical basis is laid for the accurate evaluation of shale oil reservoir reserves.

Drawings

FIG. 1 is a flow chart of a molecular simulation study of the kerogen swelling behavior of an example shale reservoir;

FIG. 2 shows the distribution of atoms fixed around the irregular macropores of the kerogen of the example;

FIG. 3 is a graph showing the effect of example fixed atoms on the pore structure of the kerogen matrix;

FIG. 4 is a microscopic simulation of the expansion of an example kerogen;

FIG. 5A is a graph of the expansion rate of kerogen in various liquid n-alkanes according to the examples;

FIG. 5B is the overrun of the example kerogen in different liquid cycloalkanes;

FIG. 5C is a graph of the expansion rate of the kerogen of the examples in various liquid aromatic hydrocarbons;

FIG. 6 is the initial distribution of example n-pentane in kerogen and the distribution after equilibrium of the expansion;

FIG. 7 is a graph of the relative amounts of various forms of occurrence of the example liquid hydrocarbons;

FIG. 8A is a graph of the coupling of the expansion rate of kerogen to the sorption of liquid hydrocarbon for the examples;

FIG. 8B is a graph showing the coupling of the expansion rate of kerogen to the amount of liquid hydrocarbon adsorbed in the examples;

FIG. 8C is a graph showing the coupling of the swelling ratio of kerogen to the amount of dissolved liquid hydrocarbon in the examples.

Detailed Description

The invention is further described below with reference to the figures and examples.

This example illustrates the swelling behavior of kerogen in various liquid n-alkanes, cycloalkanes, and aromatics, and describes a specific embodiment of the present invention according to the scheme shown in FIG. 1.

S1, initializing a simulation device. Constructing an initial cuboid simulation box, wherein the sizes of the box in the x direction, the y direction and the z direction are respectively

First, 15 kerogen type IA molecules (C) were randomly dosed into a simulated box₂₅₁H₃₈₅O₁₃N₇S₃) Then, 3 pieces of LJ with a diameter of 3 LJ are randomly put into the box

And 4 LJ diameters of

The virtual particle of (1); combining the upper end face of the simulated box in the z direction with a graphite sheet layer to form the boxThe lower end face of the seed in the z direction is combined with a quartz layer to form an initialized simulation system.

And S2, coagulating by using a simulation device. Taking the graphite sheet layer in the initial system as a piston, converting the pressure in the NPT ensemble into a compressive acting force applied to the piston along the z direction, and performing structural relaxation by referring to the annealing simulation step in the table 1.

TABLE 1 shale matrix annealing simulation procedure

Step (ii) of	Heald	Temperature (K)	Pressure (atm)	Analog time (ps)
					1	NPT	298.15	10	1000
2	NPT	500	10	1000
					3	NPT	700	10	1000
4	NPT	500	10	1000
					5	NPT	298.15	10	1000
6	NPT	298.15	100	1000

And S3, the simulation device is shaped. Aiming at the simulation device achieving the condensation in the step S2, the graphite sheet layer is stretched to 3nm along the z direction to form an excess fluid area, the virtual particles in the kerogen matrix are deleted to form an irregular large pore, and then the simulation system is subjected to 1000ps of structure relaxation in NVT (noise, vibration and harshness) ensemble under 298.15K, so that stress concentration in the original virtual particles and the kerogen matrix near the original graphite sheet layer is eliminated. And then, selecting a certain number of carbon skeleton atoms around the irregular macropores in the kerogen for fixation, wherein the positions of the fixed atoms are always kept unchanged in the subsequent simulation process, so as to prevent the macropores from collapsing in the expansion simulation process.

Figure 2 shows the fixed atoms selected around the irregular macropores of kerogen, with a concentration of only 0.15% of the total kerogen atoms.

FIG. 3 compares the pore structure in the kerogen matrix with fixed and non-fixed atoms, and shows that the fixed atoms are effective in preserving the macroporosity inside the kerogen. The results of the quantitative analysis show that 18.9% reduction in the porosity of the kerogen matrix occurs when the simulation is performed with the non-fixed fraction of atoms. The results of the analysis of the amount of expansion of kerogen showed that the simulation results for the fixed portion of atoms (1.078. + -. 0.008) varied only slightly compared to the simulation results for the non-fixed portion of atoms (1.083. + -. 0.008). Thus, the fixed atom distribution and concentration in FIG. 2 is effective in preserving large pores with less effect on the kerogen expansion ratio.

Fig. 4 is a microscopic simulation of the expansion of kerogen upon completion of the construction, consisting of a quartz layer, kerogen matrix, excess fluid zone, and graphite sheet layer in sequence. The quartz substrate in the apparatus remained stationary during the simulation, similar to the bottom of a glass test tube in the experimental test; the agglomerated kerogen matrix can simulate tightly packed kerogen powder in an experiment; contacting kerogen with excess liquid molecules in the experiment by saturating the fluid in the excess fluid zone; by applying an external force to the graphite sheet during the simulation, the external ambient pressure during the experimental test can be simulated. The microcosmic simulation device can maximally reduce the physical process of the test tube expansion experiment of the dry roots.

And S4, simulating the expansion of the kerogen. The microscopic simulation device of expansion of kerogen (fig. 4) constructed based on the step S3, developed the simulation of expansion of kerogen using the proposed geometric-constrained intercalation-molecular dynamics coupling method, in which an external force corresponding to 1atm was applied to the graphite sheet during the simulation.

The kerogen expansion simulation comprises the following specific processes: (1) recording the initial height of the kerogen prior to expansion, and saturating fluid molecules in the excess fluid zone and kerogen matrix and macropores in a kerogen expansion microscopic simulation device using geometrically constrained fluid insertion techniques; (2) then pausing the fluid insertion process, adopting a molecular dynamics method to perform 1000ps simulation in an NVT ensemble of 298.15K, and realizing the equilibrium relaxation of kerogen and fluid molecules; (3) then suspending the molecular dynamics process, and saturating the surplus fluid area and the kerogen macropores by using a fluid insertion technique again; repeating steps (2) and (3) until a new fluid molecule is not successfully inserted, indicating that the kerogen swelling reaches equilibrium, recording the kerogen matrix height at that time, and calculating the kerogen swelling rate by combining the kerogen initial height.

Fig. 5A, fig. 5B, and fig. 5C show the swelling ratios of the kerogen obtained through simulation in different liquid n-alkanes, cycloalkanes, and aromatics, and the simulation has a substantially consistent variation trend with the experimental test results, which indicates that the constructed microscopic simulation device and the proposed swelling simulation method can effectively replicate the swelling behavior of the kerogen obtained through experimental observation.

And S5, analyzing a coupling mechanism of liquid molecule occurrence and kerogen expansion. FIG. 6 shows the initial distribution of n-pentane (n-C5) in the kerogen expansion simulation device and the distribution after expansion equilibrium. During the initial saturation process, the n-pentane molecules fill the large pores inside the kerogen and the excess fluid zone, and there is no distribution of n-pentane molecules in the kerogen dense matrix. Since the fluid-pore interaction is not considered during the insertion of the n-pentane molecules, the n-pentane molecules in the large pores inside the kerogen exist in the form of a free phase, where the effect of the fluid-fluid contact is neglected. From the right boundary of the n-pentane free phase molecules in the macropores, the region I of the free phase and the adsorption phase and the left boundary of the n-pentane molecular dissolved phase can be defined. After the kerogen is expanded to reach the equilibrium, the kerogen matrix front edge is pushed to the right for a certain distance, the right boundary of the n-pentane molecular dissolved phase can be defined by the balanced front edge, and then the region II of the n-pentane molecular dissolved phase is determined. Counting the molecules in region I and region II, the total n-pentane molecular weight can be obtained. The adsorption amount of the n-pentane molecules in the kerogen can be obtained by subtracting the volume phase amount of the total n-pentane molecular weight. Counting the molecules in the region II, the dissolved phase amount of the n-pentane molecules can be obtained. The adsorption phase amount of the n-pentane molecules can be obtained by subtracting the dissolved phase amount from the adsorption amount of the n-pentane molecules.

FIG. 7 is a graph showing the relative amounts of various occurrences of each hydrocarbon component, showing that the relative amount of the dissolved phase of the hydrocarbon component is about 20% and contributes significantly to the liquid hydrocarbon reserves; as the liquid hydrocarbon molecule size increases, the contribution of the free phase decreases and the contribution of the adsorbed phase increases. In combination with the kerogen expansion ratio and the content of the occurrence form of the liquid hydrocarbon component in fig. 5, the coupling relationship between the kerogen expansion and the occurrence form of the liquid hydrocarbon component can be analyzed.

FIGS. 8A, 8B, and 8C show the relationship between the expansion ratio of kerogen and the amount of liquid hydrocarbon adsorbed, and dissolved phases. The results show that as the molecular size of the liquid hydrocarbon is reduced, the expansion rate of kerogen is increased, and the absorption quantity, the adsorption quantity and the dissolution quantity of the liquid hydrocarbon are increased; the amount of dissolved liquid hydrocarbon is more affected by the molecular shape of the liquid hydrocarbon than the amount of adsorbed liquid hydrocarbon, and more naphthenic and aromatic hydrocarbons need to be dissolved in kerogen than n-alkane in order to achieve the same kerogen expansion ratio.

Claims

1. A molecular simulation method for kerogen expansion behavior of a shale oil reservoir is characterized by comprising the following steps:

s1. initialization of simulation device

Determining an initial simulated cassette size for the device, wherein the x and y dimensions are set to final target values and the z dimension is set to be sufficiently large; putting a certain amount of kerogen molecules and virtual particles with different sizes into an initial simulation box; combining two end faces of the simulation box in the z direction with the piston and the quartz layer respectively to form an initialized simulation device;

s2. coagulation of simulation device

Applying a certain compression acting force on a piston of the simulation device along the z direction, simulating the pressure in an NPT ensemble, and developing the structural relaxation of the kerogen matrix based on a strict annealing simulation process to enable the kerogen matrix to be condensed; the sizes of the simulation device in the x direction and the y direction are kept fixed, and the size in the z direction is gradually reduced along with the simulation and reaches balance;

s3, analog device sizing

Aiming at the simulation device which achieves the coagulation in the step S2, the piston is stretched for a certain distance along the z direction to form an excess fluid area, the virtual particles in the system are deleted to form irregular large pores inside kerogen, and then the simulation system is subjected to structural relaxation under the NVT ensemble to eliminate stress concentration; then, selecting a certain number of carbon skeleton atoms around the irregular macropores in the kerogen for fixation, wherein the positions of the fixed atoms are kept unchanged all the time in the subsequent simulation process, so as to prevent the macropores from collapsing in the expansion simulation process; the position and the concentration of the fixed atoms are optimally adjusted, so that the fixed atoms can effectively preserve macropores and have small influence on the expansion rate of kerogen; the constructed microcosmic simulation device is formed by sequentially combining a quartz layer, a kerogen substrate, an excess fluid area and a piston plate, and can maximally reduce the physical process of the kerogen test tube expansion experiment, wherein the quartz layer represents the bottom of a test tube in the experiment test and is kept fixed in the simulation process; kerogen matrix corresponds to a tightly packed powder sample in the experiment; the surplus fluid area realizes the contact of kerogen and surplus liquid molecules, and further realizes the full expansion of the kerogen; external acting force is added in the piston sheet in the expansion process to reflect the external environment pressure in the experimental test;

s4. kerogen expansion simulation

Based on the kerogen expansion microscopic simulation device constructed in the step S3, expansion simulation of the kerogen is carried out by adopting a proposed geometric constraint insertion-molecular dynamics coupling method, and an external acting force corresponding to the experimental environment pressure is applied to the piston in the simulation process;

2. The method of claim 1, wherein in step S4, the geometric constraint insertion method determines whether the fluid molecules can be inserted successfully according to the unoccupied pore space, and inserts new fluid molecules when the remaining space in the pore space can accommodate more fluid molecules, otherwise, the pore space is considered to be saturated with fluid molecules.

3. The molecular simulation method for kerogen swelling behavior of shale oil reservoirs as claimed in claim 1 or 2, wherein in step S4, the specific process of kerogen swelling simulation is as follows:

4. The molecular simulation method for the kerogen swelling behavior of the shale oil reservoir as claimed in claim 1, wherein in step S5, the specific process is as follows: determining the bulk phase weight of the fluid molecules in the kerogen based on the initial fluid molecule distribution in the kerogen, and defining a physical boundary of dissolved phase weight statistics by the bulk phase weight boundary; determining the total fluid molecular weight in kerogen based on the fluid molecular distribution after expansion equilibrium, and defining another physical boundary of dissolved phasor statistics by the matrix front edge after kerogen expansion; subtracting the volume phase amount from the total fluid molecular weight to obtain the absorption amount of fluid molecules in the kerogen, including the absorption amount of the macroporous surface and the dissolution amount in the kerogen matrix; determining the dissolved phase amount of fluid molecules in the kerogen from the physical boundary of the defined dissolved phase amount statistics; the amount of the dissolved phase is subtracted from the amount of the fluid molecules adsorbed, and the amount of the fluid molecules adsorbed in the kerogen is obtained.