Molecular simulation method for quantifying lag degree of shale gas adsorption-desorption process

Abstract

The invention discloses a molecular simulation method for quantifying the lag degree of a shale gas adsorption-desorption process, and belongs to the technical field of shale gas exploration and development. The method comprises the following steps: S1, constructing a kerogen pore model; s2, acquiring initial structure parameters of the matrix pore model; s3, calibrating a conversion relation between the fugacity and the pressure; s4, shale gas adsorption process simulation is carried out; s5, shale gas desorption process simulation is carried out; and S6, quantitatively analyzing the lag degree of the adsorption-desorption process. According to the method, construction of the real shale kerogen pore structure is achieved, the provided GCMC + MD simulation method is combined, the adsorption-desorption process of shale gas in the shale kerogen pore structure is reduced on the molecular scale, meanwhile, the deformation characteristics of kerogen are considered, and the method has the advantages of being high in practicability and high in practicability. Therefore, micromechanism interpretation and quantitative characterization of the adsorption-desorption hysteresis phenomenon are achieved, and theoretical guidance is provided for shale gas productivity prediction and selection of yield increasing measures.

Description

Technical Field

[0001] The invention belongs to the technical field of shale gas exploration and development, and in particular relates to a molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process. Background Art

[0002] Against the backdrop of growing global energy demand and depletion of traditional fossil energy, the development and utilization of unconventional natural gas resources has attracted widespread attention from the international community. As a key component of unconventional natural gas resources, the effective development of shale gas has great strategic significance for optimizing energy structure and ensuring energy security. Shale gas is mainly distributed in shale reservoirs rich in organic matter. Due to its extremely low porosity and permeability and other geological characteristics, the relevant extraction technology is more complex and challenging than conventional natural gas.

[0003] The development of shale gas involves many technical fields such as geological exploration, reservoir transformation and production-increasing measures. Among them, molecular simulation technology plays a vital role in the development of shale gas. This technology enables researchers to deeply understand the adsorption and desorption behavior of shale gas in kerogen pores at the microscopic level and evaluate the specific impact of these behaviors on gas production capacity. However, the adsorption and desorption behavior of shale gas is affected by many factors, including kerogen type, pore structure, pressure and temperature. In actual shale gas development, the desorption process often shows obvious hysteresis, that is, the irreversibility of the adsorption and desorption process, which has a far-reaching impact on the production capacity evaluation of shale gas and the formulation of production-increasing strategies. Summary of the invention

[0004] The purpose of the present invention is to propose a molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process. This method can reproduce the adsorption and desorption behavior of shale gas in the pore structure of shale kerogen at the molecular level, and fully consider the deformation effect of kerogen, thereby revealing the microscopic coupling mechanism of adsorption-desorption hysteresis degree and pore deformation.

[0005] The technical solution adopted by the present invention is:

[0006] A molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process comprises the following steps:

[0007] S1. Construct kerogen matrix pore model;

[0008] S2. Obtaining initial structural parameters of the kerogen matrix pore model;

[0009] S3. Calibrate the conversion relationship between fugacity and pressure;

[0010] S4. Conduct simulation of shale gas adsorption process;

[0011] The kerogen matrix pore model constructed by S1 is used, with temperature and fugacity as input parameters. The grand canonical Monte Carlo method is first used to simulate the gas molecule adsorption process of the kerogen matrix pore model under a certain pressure condition to obtain the adsorption configuration under the pressure condition; then a molecular dynamics simulation is performed to obtain the equilibrium configuration after deformation; then, the adsorbed gas molecules in the pores are deleted to obtain the structural parameters of the kerogen matrix pore model under the pressure condition, and it is used as the initial system configuration of the next pressure point; the above simulation process is repeated, and the simulation pressure is gradually increased to obtain the isothermal adsorption curve and the structural parameters of the kerogen matrix pore model under different pressures during the adsorption process;

[0012] S5. Carry out simulation of shale gas desorption process;

[0013] The pore equilibrium state at the maximum pressure point simulated by the S4 shale gas adsorption process is used as the initial configuration for shale gas desorption process simulation; the simulation pressure is gradually reduced, and the simulation process in S4 is used to obtain the adsorption amount under different pressure conditions and the equilibrium configuration after the kerogen matrix deformation; for each pressure point, the structural parameters of the kerogen matrix pore model under the pressure condition are obtained and used as the initial system configuration for the next pressure point; through the above process, the isothermal desorption curve and the structural parameters of the kerogen matrix pore model under different pressures during the desorption process are obtained;

[0014] S6. Quantitative analysis of the hysteresis degree of the adsorption-desorption process;

[0015] The hysteresis degree of shale gas adsorption-desorption process was quantified based on the isothermal adsorption curve obtained by S4 simulation and the isothermal desorption curve obtained by S5 simulation.

[0016] The beneficial technical effects of the present invention are as follows:

[0017] The present invention constructs a kerogen matrix pore model, obtains its initial structural parameters, and establishes the corresponding relationship between gas fugacity and pressure, and then uses the grand canonical Monte Carlo-molecular dynamics (GCMC+MD) simulation method to simulate the adsorption and desorption process of shale gas. This method can reproduce the adsorption and desorption behavior of shale gas in the pore structure of shale kerogen at the molecular level, and fully consider the deformation effect of kerogen, so as to quantify the hysteresis degree of shale gas adsorption-desorption process, and reveal the microscopic coupling mechanism of adsorption-desorption hysteresis degree and pore deformation. This provides important theoretical guidance and scientific basis for in-depth understanding of the adsorption behavior and desorption hysteresis law of supercritical gas in shale organic pores, and then for shale gas production capacity prediction and optimization of production increase measures. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1A flow chart of a molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process according to the present invention;

[0019] Figure 2 It is a schematic diagram of shale gas desorption hysteresis curve;

[0020] Figure 3 Schematic diagram of the initial kerogen matrix equilibrium configuration;

[0021] Figure 4 is the initial kerogen matrix pore size distribution;

[0022] Figure 5 This is the fugacity-pressure relationship diagram of methane at 353.15K;

[0023] Figure 6 Isothermal adsorption / desorption curve of methane in kerogen at 353.15K;

[0024] Figure 7 Schematic diagram of the change of kerogen volume strain with pressure;

[0025] Figure 8 Schematic diagram of the change of kerogen pore volume and porosity with pressure. DETAILED DESCRIPTION

[0026] The present invention provides a molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process in nanopores of shale organic matter, and the method comprises the following steps: S1. constructing a kerogen matrix pore model; S2. obtaining the initial structural parameters (pore volume and porosity) of the kerogen matrix pore model; S3. calibrating the fugacity-pressure conversion relationship; S4. carrying out shale gas adsorption process simulation; S5. carrying out shale gas desorption process simulation; S6. quantitative analysis of the hysteresis degree of the adsorption-desorption process. The present invention realizes the construction of a real shale kerogen pore structure and combines the proposed GCMC+MD simulation method to restore the adsorption-desorption process of shale gas in the shale kerogen pore structure at the molecular scale, while considering the deformation characteristics of kerogen, thereby realizing the microscopic mechanism explanation and quantitative characterization of the adsorption-desorption hysteresis phenomenon. The method and idea provided by the present invention are conducive to understanding the adsorption behavior and desorption hysteresis mechanism of supercritical gas in shale organic matter pores, and further provide theoretical guidance for shale gas production capacity prediction and the selection of production increase measures.

[0027] The present invention will be further described below in conjunction with the accompanying drawings and specific implementations.

[0028] like Figure 1 As shown, a molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process includes the following steps:

[0029] S1. Construct kerogen matrix pore model;

[0030] The present invention adopts the kerogen molecular model proposed by Ungerer et al. and performs a series of geometric optimization and annealing treatments on it to ensure that the stable configuration with the lowest energy is obtained. Subsequently, according to the predetermined density, an appropriate amount of kerogen molecules are randomly arranged in the initial configuration of the simulation box; under the NVT ensemble conditions, the kerogen molecules are fully stretched through a high-temperature relaxation process to simulate their natural state; finally, the final equilibrium state of the kerogen matrix is ​​obtained by performing NPT ensemble simulation under the conditions of real formation temperature and pressure.

[0031] S2. Obtaining initial structural parameters of the kerogen matrix pore model;

[0032] Using the Atom Volumes & Surfaces module in the Materials Studio simulation software, the spherical probe method was first used to accurately calculate the pore volume and surface area of ​​the matrix model based on the dynamic diameter of helium molecules (0.26nm). On this basis, a series of decreasing free pore volume data were obtained by gradually increasing the diameter of the virtual probe molecules (0-0.5nm). By differentially calculating the free pore volumes corresponding to probe molecules of different sizes, the pore size distribution curve of the organic pore model was obtained, thereby providing initial pore structure information for subsequent molecular simulation studies of adsorption and desorption processes.

[0033] S3. Calibrate the conversion relationship between fugacity and pressure;

[0034] In a blank simulation box without kerogen matrix, the GCMC method was used to simulate the adsorption behavior of methane under different fugacity conditions. The corresponding system density was obtained by analyzing the number of adsorbed molecules, and compared with the pressure data at the same density in the NIST database, thus establishing the conversion relationship between gas fugacity and pressure at a specific temperature, providing the necessary input parameters for subsequent molecular simulation studies of adsorption and desorption processes under specific pressure conditions.

[0035] S4. Conduct simulation of shale gas adsorption process;

[0036] Using the final equilibrium configuration obtained in step S1, with temperature and fugacity as input parameters, the GCMC method is first used to simulate the adsorption process of the kerogen pore model under a relatively low pressure condition to obtain the adsorption configuration under the pressure. On this basis, the kerogen pore model is simulated by MD, firstly by 100ps kinetic relaxation under the NVT ensemble; then by 4000ps NPT ensemble simulation under zero effective stress to fully reflect the deformation characteristics of the kerogen pore model due to gas adsorption under specific pressure conditions; finally, 2000ps kinetic relaxation is performed under the NVT ensemble to obtain the adsorption equilibrium configuration after deformation. Subsequently, the adsorbed gas molecules in the pores are deleted, and the method described in step S2 is used to obtain the kerogen matrix pore structure parameters under the pressure condition, and it is used as the initial system configuration of the next pressure point. Through the above GCMC+MD simulation process, the simulation pressure is gradually increased to obtain the isothermal adsorption curve under specific temperature conditions and the pore structure parameters under different pressures during the adsorption process.

[0037] S5. Carry out simulation of shale gas desorption process;

[0038] The pore equilibrium state at the maximum pressure point of the adsorption process in step S4 is used as the initial configuration for simulating the desorption process. The simulation pressure is gradually reduced, and the GCMC+MD method described in step S4 is used to obtain the adsorption amount under different pressure conditions and the equilibrium configuration after the kerogen matrix is ​​deformed. For each pressure point, the method described in step S2 is used to obtain the kerogen matrix pore structure parameters under this condition, and it is used as the initial system configuration for the next pressure point. Through the above process, the isothermal desorption curve under specific temperature conditions and the pore structure parameters under different pressures during the desorption process are obtained.

[0039] S6. Quantitative analysis of the hysteresis degree of the adsorption-desorption process;

[0040] According to the isothermal adsorption and desorption curves simulated in steps S4 and S5, the hysteresis degree of shale gas adsorption-desorption process is quantified based on the desorption hysteresis index (HI). At the same time, combined with the pore structure parameters at different pressure points during the adsorption-desorption process, the kerogen volume strain characteristics are analyzed to reveal the microscopic coupling mechanism between kerogen matrix deformation and shale gas desorption hysteresis.

[0041] The hysteresis coefficient can be calculated using the following formula:

[0042]

[0043] Where: A hys is the area of ​​the hysteresis loop ( Figure 2 middle red shaded area), A des is the area corresponding to the desorption curve ( Figure 2 Medium red + black shadow part), A ads is the area corresponding to the adsorption curve ( Figure 2 Middle black shadow part); p max is the maximum adsorption pressure; V i des 、V i ads The pressure is p i The adsorption amount corresponding to the desorption process and adsorption process.

[0044] The volumetric strain is calculated using the following formula:

[0045]

[0046] Where: Vε is the volume strain (dimensionless), V and V 0 are the total volumes of the kerogen model after adsorption deformation and the initial volume, respectively.

[0047] In the above step S1, the process of constructing the kerogen matrix pore model may generally include the following steps:

[0048] S11. Import kerogen molecular units and perform geometry optimization and annealing simulation processes to obtain the lowest energy stable structure.

[0049] S12. Determine the size (X, Y, Z) of the periodic unit cell box, and randomly place a certain number of kerogen molecules into the unit cell box at a predetermined density.

[0050] S13. Perform a series of geometric structure optimizations on the unit cell box to reduce the system energy: first, perform high-temperature relaxation under the NVT ensemble; second, reduce the temperature at a certain gradient under the NPT ensemble to perform molecular dynamics simulation; finally, perform full relaxation under certain reservoir temperature and pressure conditions to obtain the final stable configuration.

[0051] The GCMC+MD method used in the above steps S4 and S5 is further explained as follows: within the pore structure of kerogen, this method uses the Grand Canonical Monte Carlo (GCMC) technique to simulate the insertion, removal and migration of shale gas molecules based on the Metropolis sampling algorithm; it uses the molecular dynamics (MD) technique to simulate the dynamic diffusion of shale gas molecules inside the pores based on Newton's laws of mechanics, and at the same time reveals the deformation of the kerogen skeleton under specific temperature and pressure conditions, including the generation of new pores, the expansion of existing pores, and the collapse of some original pores. The above phenomenon is the result of the combined effect of the expansion effect caused by gas adsorption and the compression effect caused by external pressure, and will change with the change of simulation conditions. In the simulation system, the pore free volume is the key target area that determines the gas adsorption capacity, and it is also the core of revealing the microscopic coupling mechanism of adsorption-desorption hysteresis and pore deformation. Since each execution of GCMC+MD operation will cause changes in the pore volume of the kerogen matrix, in order to ensure the continuity of the adsorption and desorption processes, the equilibrium configuration of each pressure point is used as the initial state of the simulation of the next pressure point, thereby ensuring the accuracy and reliability of the simulation process.

[0052] This paper combines the cutting-edge technologies of Monte Carlo and molecular dynamics simulation to reveal the physical adsorption and desorption process of shale gas in real kerogen pore structures under different temperature and pressure conditions. This method can not only successfully reproduce the adsorption-desorption hysteresis phenomenon of supercritical shale gas observed in the experiment, but also deeply analyze the microscopic mechanism and influencing factors behind this phenomenon, thereby providing important theoretical guidance and scientific basis for shale gas production capacity prediction and optimization of production increase measures.

[0053] The present invention is further illustrated by taking the adsorption and desorption process of methane in the pores of shale type IIIA kerogen matrix as an example.

[0054] The specific steps include:

[0055] S1. Construct kerogen matrix pore model;

[0056] The present invention is based on the type IIIA kerogen molecular model (C 233 H 204 N 4 O 27 ), the energy was reduced by geometry optimization and 10 annealing cycles were performed, with the temperature gradually increasing from 300K to 800K and then decreasing to 300K to obtain the lowest energy molecular configuration. A blank simulation box was set up and the 310 optimized kerogen molecules are randomly placed at a preset density. At a temperature of 800K, the simulation system first undergoes a high-temperature relaxation of 1000ps under the NVT ensemble to ensure that the kerogen molecules are fully stretched and evenly fill the entire simulation box. Then, under a pressure of 30MPa, the simulation temperature is gradually lowered, and NPT ensemble simulations are performed under temperature conditions of 800K, 600K, 400K and 300K, respectively, with a simulation time of 500ps at each temperature. During the simulation process, the energy and density changes of the system are continuously monitored to ensure that the simulation time is sufficient and the results converge. Finally, the above kerogen matrix model is further relaxed for 2000ps at 353.15K and 30MPa to obtain a stable structure reflecting the actual formation temperature and pressure conditions, such as Figure 3 shown.

[0057] The specific simulation process for constructing the shale kerogen matrix model is shown in Table 1.

[0058] Table 1

[0059] step Ensemble Temperature (K) Pressure(MPa) Simulation time (ps) 1 NVT 800 30 1000 2 NPT 800 30 500 3 NPT 600 30 500 4 NPT 400 30 500 5 NPT 300 30 500 6 NPT 353.15 30 2000

[0060] S2. Obtaining initial structural parameters of the matrix pore model;

[0061] Using the Atom Volumes & Surfaces module in the Materials Studio software, the initial free pore volume and porosity of the kerogen matrix model constructed in step S1 are calculated using a spherical probe with a helium molecular dynamics diameter (0.26 nm) as the standard. On this basis, a series of gradually decreasing free pore volumes are obtained by gradually increasing the diameter of the virtual probe molecule (0-0.5 nm), and the pore size distribution of the initial kerogen model is calculated by differentiating the free pore volumes under probe molecules of different diameters, as shown in Figure 1. Figure 4 shown.

[0062] Table 2 shows the structural parameters of the initial pore model.

[0063] Table 2

[0064]

[0065] S3. Calibrated fugacity and pressure conversion relationship:

[0066] At a temperature of 353.15K, the GCMC method was used to simulate the methane adsorption of a blank simulation box without kerogen matrix. The fugacity input in the simulation is a value calculated based on the Peng-Robinson equation of state. Based on the number of methane molecules obtained from the adsorption simulation and the volume of the blank simulation box, the average bulk density of methane in the box under specific fugacity conditions can be calculated. Furthermore, the calculated density is compared with the density-pressure relationship curve retrieved from the NIST database at the same temperature, thereby establishing the conversion relationship between methane fugacity and pressure at this temperature, such as Figure 5 shown.

[0067] S4. Conduct simulation of shale gas adsorption process;

[0068] Using the final equilibrium configuration obtained in step S1, with temperature and fugacity as input parameters, the GCMC method was first used to simulate the adsorption process of the kerogen pore model under low pressure conditions (1 MPa) to obtain the methane adsorption under this condition. The number of equilibrium steps and production steps were both set to 2×10 7 . On this basis, an MD simulation was performed on the adsorption configuration. First, a 100ps kinetic relaxation was performed under the NVT ensemble; then a 4000ps NPT ensemble simulation was performed under zero effective stress to allow the kerogen pores to fully deform under the adsorption conditions; finally, the deformed kerogen matrix was kinetically balanced under the NVT ensemble, with a simulation time of 2000ps. Subsequently, the adsorbed gas molecules in the pores were deleted, and the method described in step S2 was used to obtain the pore structure parameters of the kerogen matrix under the pressure conditions, and used as the initial system configuration for the next pressure point. Through the above-mentioned GCMC+MD simulation process, the simulation pressure was gradually increased to 30MPa, and the isothermal adsorption curve under the temperature conditions was obtained (such as Figure 6 as shown) and pore structure parameters under different pressure conditions.

[0069] S5. Carry out simulation of shale gas desorption process;

[0070] The pore equilibrium state at the maximum pressure point (30MPa) of the adsorption process in step S4 is used as the initial configuration for simulating the desorption process. The simulation pressure is gradually reduced. For each pressure point, the GCMC simulation is first used to obtain the adsorption configuration under this condition; then, the system undergoes 100ps of NVT ensemble relaxation to balance the configuration; next, 4000ps is simulated under the NPT ensemble, and the kerogen undergoes expansion and compression until it reaches an equilibrium state; finally, the system is further balanced by a 2000ps NVT ensemble, and the adsorbed gas molecules are deleted to obtain the final pore configuration. For each pressure point, the method described in step S2 is used to obtain the kerogen matrix pore structure parameters under this condition, and it is used as the initial system configuration for the next pressure point. Through the above process, the isothermal desorption curve under this temperature condition is obtained (such as Figure 6 as shown) and pore structure parameters under different pressure conditions.

[0071] S6. Quantitative analysis of the hysteresis degree of the adsorption-desorption process;

[0072] According to the isothermal adsorption curve and desorption curve obtained from the simulation process of steps S4 and S5, based on formulas (1) and (2), the desorption hysteresis coefficient HI = 0.129 can be calculated, thereby realizing the quantification of the hysteresis degree of the adsorption-desorption process of shale gas (methane) in type IIIA kerogen pores under the condition of 353.15K in the pressure range of 0-30MPa. Figure 7 The volume strain of type IIIA kerogen during the adsorption-desorption process is shown. Figure 8 is the change of pore volume and porosity during adsorption and desorption, and the change law is similar to that of volume strain. It can be seen that the formation of the hysteresis loop in this application example is mainly due to the irreversible deformation of the pore structure of kerogen after methane adsorption, which leads to different degrees of increase in the free pore volume during the desorption process compared with the adsorption process, so that the adsorption amount during the desorption process is greater than that during the adsorption process under the same pressure. Using similar methods and ideas, the degree of adsorption-desorption hysteresis of shale gas under different simulation conditions can be quantified, and the influence of different factors (such as temperature, maximum adsorption pressure, kerogen type, kerogen water content, etc.) can be explored.

Claims

1. A molecular simulation method for quantifying the hysteresis of shale gas adsorption-desorption process, characterized in that The following steps are involved: S1. Construct kerogen matrix pore model; S2. Obtaining initial structural parameters of the kerogen matrix pore model; S3. Calibrate the conversion relationship between fugacity and pressure; S4. Conduct simulation of shale gas adsorption process; The kerogen matrix pore model constructed by S1 was used. With temperature and fugacity as input parameters, the Grand Canonical Monte Carlo method was first used to simulate the gas molecule adsorption process of the kerogen matrix pore model under a certain pressure condition to obtain the adsorption configuration under the pressure condition. Then, molecular dynamics simulation was performed to obtain the equilibrium configuration after deformation. Subsequently, the adsorbed gas molecules in the pores are deleted, and the structural parameters of the kerogen matrix pore model under the pressure condition are obtained, and used as the initial system configuration of the next pressure point; the above simulation process is repeated, and the simulation pressure is gradually increased to obtain the isothermal adsorption curve and the structural parameters of the kerogen matrix pore model under different pressures during the adsorption process; S5. Carry out simulation of shale gas desorption process; The pore equilibrium state at the maximum pressure point simulated by the S4 shale gas adsorption process is used as the initial configuration for the shale gas desorption process simulation; the simulation pressure is gradually reduced, and the simulation process in S4 is used to obtain the adsorption amount under different pressure conditions and the equilibrium configuration after the kerogen matrix deformation; for each pressure point, the structural parameters of the kerogen matrix pore model under the pressure condition are obtained and used as the initial system configuration for the next pressure point; Through the above process, the isothermal desorption curve and the structural parameters of the kerogen matrix pore model at different pressures during the desorption process are obtained; S6. Quantitative analysis of the hysteresis degree of the adsorption-desorption process; The hysteresis degree of shale gas adsorption-desorption process was quantified based on the isothermal adsorption curve obtained by S4 simulation and the isothermal desorption curve obtained by S5 simulation.

2. A molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process according to claim 1, characterized in that: S1 includes the following steps: S11. Import kerogen molecules, and perform geometry optimization and annealing on them to obtain the stable configuration with the lowest energy; S12. Randomly place the kerogen molecules with stable configuration obtained in S11 into the simulation box according to a predetermined density; S13. The simulation box is processed to reduce the system energy: first, high-temperature relaxation is performed under the NVT ensemble; secondly, the NPT ensemble simulation is performed to obtain the final equilibrium state of the kerogen matrix.

3. A molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process according to claim 1, characterized in that: S2 includes the following steps: The spherical probe method was used to calculate the free pore volume and surface area of ​​the kerogen matrix pore model based on the dynamic diameter of helium molecules. A series of decreasing free pore volume data were obtained by gradually increasing the diameter of the virtual probe molecules. The pore size distribution curve of the kerogen matrix pore model was obtained by differentially calculating the free pore volumes corresponding to probe molecules of different sizes.

4. A molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process according to claim 1, characterized in that: S3 includes the following steps: In a blank simulation box without kerogen matrix, the Grand Canonical Monte Carlo method was used to simulate the adsorption behavior of methane under different fugacity conditions. The corresponding system density was obtained by analyzing the number of adsorbed molecules, and compared with the pressure data at the same density in the NIST database to establish the conversion relationship between gas fugacity and pressure at a specific temperature.

5. The molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process according to claim 1, characterized in that: The molecular dynamics simulation process in S4 includes the following steps: First, kinetic relaxation is performed under the NVT ensemble. Then, NPT ensemble simulation is performed under zero effective stress to fully reflect the deformation characteristics of the kerogen matrix pore model caused by gas adsorption under specific pressure conditions. Finally, kinetic relaxation is performed again under the NVT ensemble to obtain the adsorption equilibrium configuration after deformation.

6. A molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process according to claim 1, characterized in that: In S6: quantifying the hysteresis degree of shale gas adsorption-desorption process based on the desorption hysteresis coefficient; A hys =A des -A ads (2) Where: HI is the desorption hysteresis coefficient, A hys is the area of ​​the hysteresis loop, A des is the area corresponding to the desorption curve, A ads is the area corresponding to the adsorption curve.

7. A molecular simulation method for quantifying the hysteresis degree of shale gas adsorption-desorption process according to claim 1, characterized in that: The following steps are also included: The degree of volumetric strain is calculated using the following formula: Where: Vε is the volume strain, V and V0 are the total volumes of the kerogen matrix pore model after adsorption deformation and the initial pore volume, respectively.

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