Prediction method and application of diffusion behavior of hydrogen in pore scale of water-containing clay minerals

Abstract

The application of prediction method for diffusion behavior of underground hydrogen in pore scale of water-containing clay mineral belongs to the technical field of underground hydrogen energy storage and unconventional gas migration prediction, and the key point is to fill water molecules and salt ions into the pores according to the target water saturation and salinity; the initial model of the water-containing clay mineral pores is pre-equilibrated by the molecular dynamics method; hydrogen molecules are filled under the given temperature and pressure conditions by the grand canonical Monte Carlo method, and the hydrogen pressure is corrected by the SRK-EOS state equation; the hydrogen diffusion simulation is carried out by the molecular dynamics method with temperature and fugacity as input parameters, the mean square displacement of hydrogen in the pores is calculated, and the self-diffusion coefficient is obtained; the application can quantitatively characterize the diffusion rate change rule of hydrogen in the clay pores under different water film thickness, salinity and temperature and pressure conditions, and reveal the diffusion inflection point, diffusion inhibition stage and migration restriction mechanism caused by the water film or water bridge structure.

Description

Technical Field

[0001] This invention belongs to the field of underground hydrogen energy storage and unconventional gas reservoir behavior prediction technology. Specifically, it discloses a molecular-scale simulation method for simulating and predicting the diffusion behavior of hydrogen in the pores of hydrous clay minerals. This method is applicable to the study of hydrogen diffusion behavior in natural hydrogen reservoirs, underground hydrogen storage facilities, shale-clay capping systems, and geological bodies with clay minerals as the main porous medium. It provides a theoretical basis and microscopic mechanism support for the safety assessment of underground hydrogen storage and the prediction of hydrogen retention behavior. Background Technology

[0002] The migration and redistribution of hydrogen in underground formations are key control factors affecting hydrogen resource extraction efficiency, reservoir injectability, and hydrogen storage safety. During this process, hydrogen is influenced by numerous factors, including pore structure, water state, interfacial potential energy distribution, mineral surface charge, and dissolved ions, resulting in complex nonlinear response characteristics in its diffusion process. Underground reservoirs and caprocks are generally rich in clay minerals, whose layered structure, strong hydrophilicity, and surface charge characteristics determine their important micro-regulatory role in hydrogen migration. Especially in the water-bearing state, water films, water bridges, and interfacial water-bound structures easily form in clay pores. The presence of these water-mineral-gas multiphase interfaces significantly affects the hydrogen diffusion path and rate. Therefore, in-depth research on the diffusion behavior of hydrogen in the pores of water-bearing clay is of significant scientific importance for understanding underground hydrogen loss, predicting hydrogen migration range, and assessing reservoir sealing performance.

[0003] Existing research primarily relies on experimental diffusion tests, pore-scale theoretical models, or macroscopic transport equations (such as the Fick diffusion model) for analysis. However, these methods are usually based on idealized assumptions about pores, making it difficult to accurately reflect the water-gas-mineral interface effects in nanoscale pores, and also unable to effectively characterize microscopic mechanisms such as water film thickness variations, interfacial potential barriers, and diffusion channel contraction. Furthermore, actual underground conditions are characterized by large variations in water saturation, wide temperature and pressure ranges, and complex ionic compositions, resulting in significant nonlinear and phased changes in hydrogen diffusion behavior under different pore states. Existing studies have largely focused on dry or fully water-saturated systems, neglecting the gas-water coexistence state commonly found in underground strata. In this unsaturated pore environment, hydrogen diffusion is not only controlled by free gas diffusion but also influenced by interfacial trapping, localized retention, and pore water rearrangement, making its migration characteristics even more complex. Molecular simulation methods (including GCMC and MD) provide important technical means for studying the diffusion of various gases such as CO2 and shale gas in clay pores. However, a systematic model and simulation framework for the key issue of hydrogen diffusion behavior in water-bearing clay pores is currently lacking. Therefore, there is an urgent need for a methodological system that can construct realistic water-bearing clay pore structures at the nanoscale, simulate hydrogen diffusion trajectories, and quantify interface effects. This would reveal the microscopic control mechanisms of hydrogen migration underground, providing a scientific basis for underground hydrogen storage safety analysis, diffusion coefficient calibration, and long-term reservoir stability evaluation. Summary of the Invention

[0004] The purpose of this invention is to propose a method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals. By constructing a pore model of hydrous clay minerals and combining large canonical Monte Carlo and molecular dynamics simulation methods, the diffusion behavior of hydrogen under different water saturation, temperature, pressure and salinity conditions is quantified, revealing the interaction mechanism of the hydrogen-water-clay interface, thereby providing microscopic mechanism support for the safety assessment of underground hydrogen storage and the prediction of reservoir diffusion capacity.

[0005] The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to some embodiments of this application includes the following steps:

[0006] Based on the given surface hydroxyl distribution and force field charge distribution of clay mineral crystal structure parameters, and given the pore interlayer spacing, water molecules and salt ions are filled into the pores according to the target water saturation and salinity to obtain the initial pore model of hydrous clay minerals under different water saturation and salinity conditions.

[0007] The initial pore model of hydrous clay minerals was minimized and pre-equilibriumed using molecular dynamics methods to obtain a stable pore model of hydrous clay minerals.

[0008] Based on the pore stability model of hydrous clay minerals, hydrogen molecules were filled under given temperature and pressure conditions using the giant canonical Monte Carlo method, and the hydrogen pressure was corrected using the SRK-EOS equation of state to obtain the initial model for diffusion simulation.

[0009] Based on the initial diffusion simulation model, with temperature and fugacity as input parameters, hydrogen diffusion simulation was carried out using molecular dynamics methods to calculate the mean square displacement of hydrogen in the pores and obtain the self-diffusion coefficient.

[0010] The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to some embodiments of this application also includes the density distribution of the output hydrogen, and / or

[0011] The self-diffusion coefficient is used to characterize the diffusion behavior of hydrogen in the pores of water-bearing clay, depending on at least one of the following: water saturation, temperature, pressure, and salinity.

[0012] According to the method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals in some embodiments of this application, the clay minerals include kaolinite.

[0013] The unit cell parameters of the kaolinite model are a = 5.149 Å, b = 8.934 Å, c = 7.384 Å, α = 91.93°, β = 105.042°, and γ = 89.7910°; where a represents the unit vector length along the x-axis, b represents the unit vector length along the y-axis, and c represents the unit vector length along the z-axis. This represents the angle between the b-axis and the c-axis. This represents the angle between the a-axis and the c-axis. This represents the angle between the a-axis and the b-axis;

[0014] In MaterialsStudio software, the kaolinite model was hydrogenated, with three hydroxyl groups facing the aluminum oxide outer surface and the fourth hydroxyl group facing the siloxane surface, forming a surface hydroxyl distribution. Force field parameters and charge parameters were assigned, and nano-slit pores were constructed by cutting along the (001) crystal plane, where the (001) crystal plane is a set of planes parallel to the clay mineral structure layer plane. A vacuum layer was set and 8×4×2 cell expansion was performed to create two clay mineral substrates, with a given pore size of 50 Å, forming an upper and lower symmetrical wall model with x, y, and z direction dimensions of 41.192 Å, 35.736 Å, and 81.536 Å, respectively.

[0015] Based on the method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals in some embodiments of this application, the number of water molecules and salt ions is calculated based on the target water saturation and salinity and added to the pores to obtain an initial pore model of hydrous clay minerals under different water saturation and salinity conditions. The calculation formula is as follows:

[0016]

[0017] in, The average density of water molecules; This represents the molar mass of a water molecule. The pore surface area; The aperture size; is Avogadro's constant.

[0018] The salt ion number is obtained based on the following calculation: the volume of 1 mol of water is calculated to be 18 ml / mol, and the volume of 1 water molecule is 3 × 10⁻⁶. -26 L is determined by the number of water molecules and salinity to determine the number of NaCl molecules.

[0019] According to the method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals in some embodiments of this application, the potential energy function of the initial model of hydrous clay minerals consists of bond stretching energy, bond angle energy, van der Waals potential energy, and Coulomb potential energy, and the total potential energy U is shown in the following formula:

[0020]

[0021] In the formula, Represents total energy. Indicates the bonding energy. Indicates bond angle energy. It means that van der Waals can, Represents Coulomb potential energy;

[0022] Among them, bonds and energy It is expressed as follows:

[0023]

[0024] In the formula, The bond stretching force constant; This is the actual bond length; Standard bond length;

[0025] Among them, bond angle energy It is expressed as follows:

[0026]

[0027] In the formula, The constant of the key angle bending force; This refers to the actual bond angle; Standard bond angle;

[0028] Among them, van der Waals can It is expressed as follows:

[0029]

[0030] In the formula, and For different particles, For particles and particles Between potential well depths, For the same particles Between potential well depths, For the same particles Between potential well depths, For particles and particles Collision distance between them For the same particles Collision distance between them For the same particles The collision distance between particles was calculated using the Lorentz-Berthelot mixing rule, where the LJ parameters between different particles were... , , For particles and particles The distance between them;

[0031] Among them, Coulomb potential energy It is expressed as follows:

[0032]

[0033] In the formula, Where is the dielectric constant. and It is a particle and particles The charge, For particles and particles The distance between them.

[0034] According to the method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals in some embodiments of this application, an energy minimization and pre-equilibrium method is used to perform energy minimization and pre-equilibrium on the initial pore model of hydrous clay minerals to obtain a stable pore model of hydrous clay minerals, including:

[0035] Clay mineral force field setting Force field, water molecule model set The model, the hydrogen model is set to The unit point model, with the unit system set as follows: , Periodic boundary conditions are set in all three directions, and the atomic style is set to... The time step is set to 1.0. Runtime set to 1 The mineral substrate is set as a rigid body, and the bonds and bond angles of water molecules are set as rigid, while maintaining the number of particles in the system. ,volume and temperature Under certain conditions, in large-scale atomic and molecular parallel simulators A simulation was conducted to obtain a pore stability model for water-bearing clay minerals.

[0036] According to the method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals in some embodiments of this application, the giant canonical Monte Carlo method is used to fill hydrogen molecules under given temperature and pressure conditions, including setting the temperature to 300–420°C. Pressure 50–250 The time step is set to 1.0. The runtime is 1 Execute once every 100 steps The cycle consists of 100 attempts to insert or remove hydrogen molecules, and is based on... The criteria determine whether to accept or reject.

[0037] The SRK-EOS equation of state is corrected for hydrogen pressure, and the calculation is as follows:

[0038]

[0039]

[0040]

[0041]

[0042]

[0043]

[0044]

[0045] In the formula, For the degree of fugacity, For pressure, As the compression factor, The fugacity coefficient. The critical pressure. Thermodynamic temperature The critical temperature. The eccentricity factor for hydrogen;

[0046] Based on the initial diffusion simulation model, using temperature and fugacity as input parameters, hydrogen diffusion simulation was conducted using molecular dynamics methods. This included: at a given temperature and fugacity, performing simulations using only molecular dynamics methods to obtain the diffusion behavior and distribution characteristics of hydrogen in mineral pores, with a time step of 1.0. ,exist Running under the ensemble 4 and adopt The thermostat maintains a constant temperature, the first 2 Used for balance, the last 2 It is used to analyze trajectories.

[0047] According to the method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals in some embodiments of this application, the self-diffusion coefficient is expressed as follows:

[0048]

[0049] In the formula, For particles diffusion coefficient, For the relevant time, For particles The number of molecules, For particles No. molecule Location at any given moment For particles No. The initial position of each molecule;

[0050] in: This represents the mean square displacement of a hydrogen molecule.

[0051] According to the method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals in some embodiments of this application, the hydrogen self-diffusion coefficient is as follows under the external force caused by the concentration difference:

[0052]

[0053] In the formula, For particles diffusion coefficient, The gas constant is Thermodynamic temperature This refers to the external force applied to hydrogen molecules, with a numerical range of 0.2-0.45. Let t be the position of the hydrogen molecule at time t.

[0054] The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to some embodiments of this application is used to identify the nonlinear response of hydrogen diffusion caused by changes in water saturation, temperature, pressure and salinity.

[0055] Beneficial Effects: The proposed method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals can accurately reveal the migration mechanism of hydrogen molecules within the pores of hydrous clay based on hydrogen diffusion characteristics under different water saturation, temperature, pressure, and salinity conditions, and achieve refined prediction of the diffusion coefficient at the pore scale. Compared with existing studies that rely on macroscopic diffusion models or idealized pore assumptions, this invention constructs a molecular-scale simulation system that can realistically reflect the microenvironment of underground clay pores. It comprehensively considers the combined influence of water film structure, pore geometric constraints, and interfacial potential barriers on the hydrogen diffusion process, thereby more comprehensively capturing key microscopic phenomena such as interfacial trapping, diffusion inhibition zones, and diffusion path rearrangement during hydrogen migration. The method of this invention can identify the nonlinear stages and diffusion inflection points in the diffusion behavior, significantly improving the reliability of underground hydrogen diffusion prediction under complex water-gas coexistence conditions. Furthermore, this invention can be flexibly extended to different water saturation levels, pore structure types, and underground working conditions, and has wide applicability. It can provide important microscopic mechanism foundations and theoretical support for key engineering problems such as hydrogen loss assessment, effective diffusion coefficient calibration, and caprock sealing analysis during underground hydrogen storage. Attached Figure Description

[0056] Figure 1 This is a flowchart illustrating the method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to an embodiment of the present invention.

[0057] Figure 2 The following are simulation diagrams: (a) is a simulation diagram under dry conditions in an embodiment of the present invention; (b) is a simulation diagram under 0.3 water saturation conditions in an embodiment of the present invention; and (c) is a simulation diagram under 0.7 water saturation conditions in an embodiment of the present invention.

[0058] Figure 3 This is a schematic diagram of the hydrogen self-diffusion coefficient calculated under different water saturation levels in an embodiment of the present invention.

[0059] Figure 4 This is a schematic diagram of the hydrogen self-diffusion coefficient calculated under different salinities in an embodiment of the present invention.

[0060] Figure 5 This is a schematic diagram of the hydrogen self-diffusion coefficient calculated at different temperatures in an embodiment of the present invention.

[0061] Figure 6 This is a schematic diagram of the hydrogen self-diffusion coefficient calculated under different pressures in an embodiment of the present invention. Detailed Implementation

[0062] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0063] Example: This example provides a method for predicting the diffusion behavior of underground hydrogen at the pore scale of hydrous clay minerals, including the following steps:

[0064] S1. Constructing an initial pore model for hydrous clay minerals: Based on the given surface hydroxyl distribution and force field charge distribution of the clay mineral crystal structure parameters, and the given pore interlayer spacing, water molecules and salt ions are filled into the pores according to the target water saturation and salinity to obtain an initial pore model for hydrous clay minerals under different water saturation and salinity conditions. Specifically, step S1 includes the following steps:

[0065] S101. Kaolinite was selected as the representative mineral. The unit cell parameters were derived from an inorganic crystal structure database to ensure high accuracy in the model construction. The individual unit cell parameters are a = 5.149 Å, b = 8.934 Å, c = 7.384 Å, α = 91.93°, β = 105.042°, and γ = 89.7910°; where a represents the unit vector length along the x-axis, b represents the unit vector length along the y-axis, and c represents the unit vector length along the z-axis. This represents the angle between the b-axis and the c-axis. This represents the angle between the a-axis and the c-axis. This represents the angle between the a-axis and the b-axis.

[0066] S102. In Materials Studio software, the kaolinite model is hydrogenated, with three hydroxyl groups facing the aluminum oxide outer surface and the fourth hydroxyl group facing the siloxane surface to form a surface hydroxyl distribution. Force field parameters and charge parameters are assigned, and nano-slit pores are constructed by cutting along the (001) crystal plane. The (001) crystal plane is a set of planes parallel to the clay mineral structure layer plane. A vacuum layer is set and 8×4×2 cell expansion is performed. Then, two clay mineral substrates are created, with a given pore size of 50 Å, thus forming an upper and lower symmetrical wall model with x, y, and z direction dimensions of 41.192 Å, 35.736 Å, and 81.536 Å, respectively.

[0067] S103. Based on the water molecule density and target water saturation and salinity, calculate the number of water molecules and salt ions, and add them to the pores to obtain the initial pore model of water-bearing clay minerals under different water saturation and salinity conditions. The water molecule calculation formula is as follows:

[0068]

[0069] in, This is the average density of water molecules; detailed values ​​can be found in the NIST database. This represents the molar mass of a water molecule. The pore surface area; The aperture size; This is Avogadro's constant. For example, there are 744 water molecules at a water saturation level of 0.3 and 1730 water molecules at a water saturation level of 0.7.

[0070] The determination of the number of salt ions is based on the following assumptions: the volume of the solution is approximately the volume of water; the density of water is 1 g / cm³. 3 The molar mass of a water molecule is 18 g / mol; Avogadro's constant N A ≈6×10 23 / mol; the volume of 1 mol of water is calculated to be 18 ml / mol, and the volume of 1 water molecule is 3 × 10⁻⁶. -26 L is then used to determine the number of NaCl molecules based on the number of water molecules and the salinity. For example, at a water saturation of 0.3 and a salinity of 5M, the number of NaCl molecules is 67, and at a water saturation of 0.7 and a salinity of 5M, the number of NaCl molecules is 156.

[0071] S2. Obtaining a stable porosity model for hydrous clay minerals: Molecular dynamics methods were used to minimize and pre-equilibrium the initial porosity model of hydrous clay minerals under different water saturation and salinity conditions in step S1. This included setting the force field, unit system, boundary conditions, atomic pattern, time step, water-mineral treatment method, and NVT relaxation step to establish a stable porosity model for hydrous clay minerals. Specifically, the settings included: based on the initial porosity model of hydrous clay minerals obtained in step S1, the CLAYFF force field was selected for the clay mineral force field; the SPC / E model, compatible with the CLAYFF force field, was used for water molecules; Michels' unit point model was used for hydrogen; the unit system was set to real; periodic boundary conditions were set in all three directions; the atomic pattern was set to full; the time step was set to 1.0 fs; the running time was 1 ns; the mineral substrate was kept rigid; and the bonds and bond angles of water molecules were kept rigid. Simulations were conducted while keeping the number of particles N, volume V, and temperature T constant.

[0072] The potential energy function of the initial model of the pore size of hydrous clay minerals consists of bond stretching energy, bond angle energy, van der Waals potential energy, and Coulomb potential energy. The total potential energy U is shown in the following equation:

[0073]

[0074] Represents total energy. Indicates the bonding energy. Indicates bond angle energy. It means that van der Waals can, This represents the Coulomb potential energy.

[0075] Among them, bonds and energy It is expressed as follows:

[0076]

[0077] in, The bond stretching force constant is expressed in units of 1 / 3. ; For actual bond length, in units ; Standard bond length, unit .

[0078] Among them, bond angle energy It is expressed as follows:

[0079]

[0080] in, The key angle bending force constant, in units of ; Actual bond angle, unit ; Standard bond angle, unit .

[0081] Among them, van der Waals can It is expressed as follows:

[0082]

[0083] in, and For different particles, For particles and particles Between potential well depths, For the same particles Between potential well depths, For the same particles Between potential well depths, For particles and particles Collision distance between them For the same particles Collision distance between them For the same particles The collision distance between particles is calculated using the Lorentz-Berthelot mixing rule, which determines the LJ parameters between different particles. , , For particles and particles The distance between them.

[0084] Among them, Coulomb potential energy It is expressed as follows:

[0085]

[0086] in, Where is the dielectric constant. and It is a particle and particles The charge, For particles and particles The distance between them.

[0087] S3. Generate the initial model for hydrogen diffusion simulation. Based on the pore stability model of hydrous clay minerals from step S2, use the giant canonical Monte Carlo method to fill the model with hydrogen molecules under given temperature, pressure, water saturation, and salinity conditions. Correct for pressure using the SRK-EOS equation of state to obtain the initial diffusion simulation model. Specifically, the model includes setting up the SRK equation to correlate the compressibility factor and fugacity coefficient, calculated as follows:

[0088]

[0089]

[0090]

[0091]

[0092]

[0093]

[0094]

[0095] in, For the fugacity, For pressure, As the compression factor, The fugacity coefficient. The critical pressure. Thermodynamic temperature The critical temperature. The eccentricity factor of hydrogen is denoted as , where the eccentricity factor of hydrogen is... Critical pressure and critical temperature The values ​​are -0.216, 1.28 MPa, and 33.2 K, respectively.

[0096] The Grand Canonical Monte Carlo (GCMC) method is used to simulate the filling of hydrogen molecules to obtain an initial diffusion model. The temperature is set to 300–420 K, the pressure to 50–250 atm, the water saturation to 0.3, the salinity to 1 M, the time step to 1.0 fs, the run time to 1 ns, and a GCMC cycle is executed every 100 steps. Each cycle contains 100 attempts to insert or remove hydrogen molecules, and the acceptance or rejection is determined according to the Metropolis criterion.

[0097] S4. Conduct hydrogen diffusion simulation. Based on the initial diffusion simulation model from step S3, using temperature and fugacity as input parameters, calculate the mean square displacement (MSD) of hydrogen within the pores using molecular dynamics methods to obtain the self-diffusion coefficient. Further output the hydrogen density distribution and the relationship between the diffusion coefficient and water saturation, temperature, pressure, and salinity to characterize the diffusion behavior of hydrogen in the pores of water-bearing clay. Specifically, the settings are as follows: at a given temperature (e.g., 300 K) and fugacity (e.g., 200 atm), use only the MD method to simulate and study the diffusion behavior and distribution characteristics of hydrogen in mineral pores. The time step remains 1.0 fs, running for 4 ns under the NVT ensemble, and using a Nosé-Hoover thermostat for isothermal control. The first 2 ns are used for equilibration, and the last 2 ns are used for trajectory analysis. Specifically, the hydrogen self-diffusion coefficient is calculated using the following formula:

[0098]

[0099] in: For particles The diffusion coefficient; For the relevant time; For particles The number of molecules; For particles No. The position of each molecule, The mean square displacement of hydrogen molecules represents the average square of the deviation of hydrogen molecules from their initial positions during continuous movement. By tracking the positional changes of particles, the mean square displacement of particles can be obtained. Then, by linearly fitting the mean square displacement curve and processing the slope, the corresponding diffusion coefficient can be obtained. By outputting the mass density distribution of hydrogen along the pore height at a certain number of frames, and averaging multiple frames, the density distribution of hydrogen under a specific condition can be obtained. Then, simulations under multiple conditions are performed to obtain the relationship between density distribution and diffusion coefficient with water saturation, temperature, pressure, and salinity.

[0100] In the presence of external forces such as concentration differences, the hydrogen self-diffusion coefficient can be expressed as:

[0101]

[0102] in, For particles diffusion coefficient, The gas constant is Thermodynamic temperature This refers to the external force applied to hydrogen molecules, with a numerical range of 0.2-0.45. Let t represent the position of the hydrogen molecules at time t. The concentration difference is due to the solvent, such as pure water or salt water. Finally, the calculated self-diffusion coefficient under external force and the self-diffusion coefficient obtained by calculating the mean square displacement are not significantly different; therefore, only the self-diffusion coefficient obtained by the mean square displacement is included in the attached figure.

[0103] In this embodiment, Figure 3 This is a schematic diagram of the hydrogen self-diffusion coefficient calculated under different water saturation levels in an embodiment of the present invention; Figure 4 This is a schematic diagram of the hydrogen self-diffusion coefficient calculated at different salinities in an embodiment of the present invention; Figure 5 This is a schematic diagram of the hydrogen self-diffusion coefficient at different temperatures calculated in this embodiment of the invention; Figure 6 This is a schematic diagram of the hydrogen self-diffusion coefficient calculated under different pressures in an embodiment of the present invention; as shown. Figure 3-6 As shown, the self-diffusion coefficient of hydrogen under different water saturation levels exhibits a stepwise decreasing trend, indicating that hydrogen migration is hindered under water-containing conditions, which is unfavorable for hydrogen extraction and flow. The self-diffusion coefficient decreases with increasing salinity, and the effect is more pronounced under low salinity conditions, indicating that the presence of salt ions also weakens hydrogen flow. Under dry conditions, the self-diffusion coefficient shows an approximately exponential increasing or decreasing nonlinear trend with increasing temperature or pressure. The presence of water weakens this nonlinear change, and the effect increases with increasing water saturation, indicating that high temperature and low pressure are conducive to hydrogen migration, but the enhancing effect weakens with increasing water saturation. This effectively characterizes the hydrogen diffusion behavior.

[0104] This invention belongs to the field of underground hydrogen energy storage and unconventional gas migration prediction technology, proposing a method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals. Based on the actual pore structure of clay minerals, the characteristics of the water-gas coupling interface, and the microscopic interaction mechanism of hydrogen on the mineral surface, this method constructs a pore-scale hydrogen diffusion dynamics model. Utilizing a strategy combining statistical mechanics and molecular simulation theory, the initial filling configuration of hydrogen under different water saturation, temperature, pressure, and salinity conditions is generated using the grand canonical Monte Carlo method (GCMC). The mean square displacement of hydrogen in the pores is calculated using molecular dynamics (MD) methods to obtain the self-diffusion coefficient. This invention can quantitatively characterize the variation law of hydrogen diffusion rate in clay pores under different water film thicknesses, salinity, and temperature and pressure conditions, revealing diffusion inflection points, diffusion inhibition stages, and migration-restricted mechanisms caused by water film or water bridge structures.

[0105] The embodiments of the present invention are given for illustrative and descriptive purposes only, and are not intended to be exhaustive or to limit the invention to the forms disclosed. Many modifications and variations will be apparent to those skilled in the art. The embodiments were chosen and described in order to better illustrate the principles and practical application of the invention, and to enable those skilled in the art to understand the invention and to design various embodiments with various modifications suitable for a particular purpose.

Claims

1. A method for predicting the diffusion behavior of underground hydrogen at the pore scale in hydrous clay minerals, characterized in that, Includes the following steps: Based on the given surface hydroxyl distribution and force field charge distribution of clay mineral crystal structure parameters, and given the pore interlayer spacing, water molecules and salt ions are filled into the pores according to the target water saturation and salinity to obtain the initial pore model of hydrous clay minerals under different water saturation and salinity conditions. The initial pore model of hydrous clay minerals was minimized and pre-equilibriumed using molecular dynamics methods to obtain a stable pore model of hydrous clay minerals. Based on the pore stability model of hydrous clay minerals, hydrogen molecules were filled under given temperature and pressure conditions using the giant canonical Monte Carlo method, and the hydrogen pressure was corrected using the SRK-EOS equation of state to obtain the initial model for diffusion simulation. Based on the initial diffusion simulation model, using temperature and fugacity as input parameters, hydrogen diffusion simulation was carried out using molecular dynamics methods to calculate the mean square displacement of hydrogen in the pores and obtain the self-diffusion coefficient; Among them, the filling of hydrogen molecules under given temperature and pressure conditions using the giant canonical Monte Carlo method includes setting the temperature to 300–420°C. Pressure 50–250 The time step is set to 1.

0. The runtime is 1 Execute once every 100 steps The cycle consists of 100 attempts to insert or remove hydrogen molecules, and is based on... The criteria determine whether to accept or reject. The SRK-EOS equation of state is corrected for hydrogen pressure, and the calculation is as follows: In the formula, For the fugacity, For pressure, As the compression factor, The fugacity coefficient. The critical pressure. Thermodynamic temperature The critical temperature. The eccentricity factor for hydrogen; Based on the initial diffusion simulation model, using temperature and fugacity as input parameters, hydrogen diffusion simulation was conducted using molecular dynamics methods. This included: at a given temperature and fugacity, performing simulations using only molecular dynamics methods to obtain the diffusion behavior and distribution characteristics of hydrogen in mineral pores, with a time step of 1.

0. ,exist Running under the ensemble 4 and adopt The thermostat maintains a constant temperature, the first 2 Used for balance, the last 2 It is used to analyze trajectories.

2. The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to claim 1, characterized in that, It also includes the density distribution of the output hydrogen gas, and / or The self-diffusion coefficient is used to characterize the diffusion behavior of hydrogen in the pores of water-bearing clay, by varying the relationship between water saturation, temperature, pressure and salinity.

3. The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to claim 1, characterized in that, Clay minerals include kaolinite; The unit cell parameters of the kaolinite model are a = 5.149 Å, b = 8.934 Å, c = 7.384 Å, α = 91.93°, β = 105.042°, and γ = 89.7910°; where a represents the unit vector length along the x-axis, b represents the unit vector length along the y-axis, and c represents the unit vector length along the z-axis. This represents the angle between the b-axis and the c-axis. This represents the angle between the a-axis and the c-axis. This represents the angle between the a-axis and the b-axis; In Materials Studio software, the kaolinite model was hydrogenated, with three hydroxyl groups facing the aluminum oxide outer surface and the fourth hydroxyl group facing the siloxane surface, forming a surface hydroxyl distribution. Force field parameters and charge parameters were assigned, and nano-slit pores were constructed by cutting along the 001 crystal plane, where the 001 crystal plane is a set of planes parallel to the clay mineral structure layer plane. A vacuum layer was set and 8×4×2 cell expansion was performed to create two layers of clay mineral substrate. Given a pore size of 50 Å, a symmetrical upper and lower wall model with x, y, and z direction dimensions of 41.192 Å, 35.736 Å, and 81.536 Å, respectively, was formed.

4. The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to claim 3, characterized in that, The number of water molecules and salt ions were calculated based on the target water saturation and salinity, and then incorporated into the pores to obtain an initial pore model of hydrous clay minerals under different water saturation and salinity conditions. The calculation formula is as follows: in, The average density of water molecules; This represents the molar mass of a water molecule. The pore surface area; The aperture size; It is Avogadro's constant; The salt ion number is obtained based on the following calculation: the volume of 1 mol of water is calculated to be 18 ml / mol, and the volume of 1 water molecule is 3 × 10⁻⁶. -26 L is determined by the number of water molecules and salinity to determine the number of NaCl molecules.

5. The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to claim 1, characterized in that, The potential energy function of the initial model of hydrous clay minerals consists of bond stretching energy, bond angle energy, van der Waals potential energy, and Coulomb potential energy. The total potential energy U is shown in the following equation: In the formula, Represents total energy. Indicates the bonding energy. Indicates bond angle energy. It means that van der Waals can, Represents Coulomb potential energy; Among them, bonds and energy It is expressed as follows: In the formula, The bond stretching force constant; This is the actual bond length; Standard bond length; Among them, bond angle energy It is expressed as follows: In the formula, The constant of the key angle bending force; This refers to the actual bond angle; Standard bond angle; Among them, van der Waals can It is expressed as follows: In the formula, and For different particles, For particles and particles Between potential well depths, For the same particles Between potential well depths, For the same particles Between potential well depths, For particles and particles Collision distance between them For the same particles Collision distance between them For the same particles The collision distance between particles was calculated using the Lorentz-Berthelot mixing rule, where the LJ parameters between different particles were... , , For particles and particles The distance between them; Among them, Coulomb potential energy It is expressed as follows: In the formula, Where is the dielectric constant. and It is a particle and particles The charge, For particles and particles The distance between them.

6. The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to claim 5, characterized in that, An energy minimization and pre-equilibrium process was performed on the initial porosity model of hydrous clay minerals using molecular dynamics methods to obtain a stable porosity model for hydrous clay minerals, including: Clay mineral force field setting Force field, water molecule model set The model, the hydrogen model is set to The unit point model, with the unit system set as follows: , Periodic boundary conditions are set in all three directions, and the atomic style is set to... The time step is set to 1.

0. Runtime set to 1 The mineral substrate is set as a rigid body, and the bonds and bond angles of water molecules are set as rigid, while maintaining the number of particles in the system. ,volume and temperature Under certain conditions, in large-scale atomic and molecular parallel simulators A simulation was conducted to obtain a pore stability model for water-bearing clay minerals.

7. The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to claim 1, characterized in that, The self-diffusion coefficient is expressed as follows: In the formula, For particles diffusion coefficient, For the relevant time, For particles The number of molecules, For particles No. molecule Location at any given moment For particles No. The initial position of each molecule; in: This represents the mean square displacement of a hydrogen molecule.

8. The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals according to claim 1, characterized in that, Under the external force resulting from a concentration difference, the hydrogen self-diffusion coefficient is: In the formula, For particles diffusion coefficient, The gas constant is... Thermodynamic temperature This refers to the external force applied to hydrogen molecules, with a numerical range of 0.2-0.

45. Let t be the position of the hydrogen molecule at time t.

9. The method for predicting the diffusion behavior of underground hydrogen in the pore scale of hydrous clay minerals as described in claim 1, for the application of identifying the nonlinear response of hydrogen diffusion caused by changes in water saturation, temperature, pressure and salinity.

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