A pure water-hydrate system phase equilibrium calculation method based on molecular simulation and its application

Abstract

The present invention relates to the field of oil drilling and production technology, and specifically to a pure water-hydrate system phase equilibrium calculation method and application based on molecular simulation, including model establishment and accuracy verification, optimization of the hydrate system model, simulation and calculation of the hydrate isobaric decomposition process using the hydrate system model, calculation of the system thermodynamic quantities and AOP values ​​using the hydrate model to simulate the trajectory of the decomposition process, and prediction of the hydrate phase equilibrium temperature by thermodynamic quantities and AOP values. The present invention uses a molecular simulation method and is verified by experimental results to depict a pure water-hydrate phase equilibrium chart at 5-10MPa within an error range of 1.502%-1.792%, which can be used to predict the influence of wellbore working fluid (drilling fluid, cement slurry) filtrate and the like on hydrate stability and phase equilibrium during drilling and completion.

Description

Technical Field

[0001] The invention relates to the technical field of oil drilling and production, and in particular to a pure water-hydrate system phase equilibrium calculation method based on molecular simulation and its application. Background Art

[0002] Cementing technology is one of the key technologies for the long-term development of oil and gas resources and is of great significance to the long-term safe development of deepwater oil and gas resources.

[0003] Natural gas hydrates are easily formed in the low-temperature and high-pressure environment of deep-water shallow layers. The release of hydration heat during the setting period of cementing cement slurry and the influence of ions and admixture functional groups in the cement slurry filtrate on the hydrate phase equilibrium during cement slurry filtration will lead to hydrate decomposition. Therefore, the stability of hydrates is the main challenge for cementing deep-water shallow hydrate layers. When developing cement slurry systems for cementing deep-water shallow hydrate layers, it is necessary to fully consider the influence of cement slurry percolation on hydrate stability. Therefore, it is necessary to develop a molecular simulation prediction method for hydrate phase equilibrium under cement slurry percolation conditions to study the influence of cement slurry filtrate on the phase equilibrium of hydrate layers, so as to guide the research on reasonable cement slurry systems and cementing methods and solve the cementing problems of hydrate layers. Summary of the invention

[0004] Based on the above purpose, the present invention proposes a pure water-hydrate system phase equilibrium calculation method and application based on molecular simulation. First, a hydrate model is constructed, and then an organic aqueous solution containing cement slurry admixture functional groups is added above the hydrate model, and the hydrate phase equilibrium temperature under the action of the aqueous solution is simulated to study the influence of the cement slurry filtrate on the phase equilibrium of the hydrate layer, thereby guiding the study of a reasonable cement slurry system and cementing method to solve the cementing problem of the hydrate layer.

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

[0006] The present invention provides a pure water-hydrate system phase equilibrium calculation method based on molecular simulation. By changing the type and number of solutes in the liquid phase in the hydrate two-phase system model, the influence of cement slurry filtrate on the hydrate layer phase equilibrium can be simulated, and the hydrate phase equilibrium temperature under the action of the solution can be predicted.

[0007] A method for calculating the phase equilibrium temperature of a pure water-hydrate system based on molecular simulation comprises the following steps:

[0008] S1. Obtaining hydrate crystal information through X-ray crystal diffraction results, including the coordinates of oxygen atoms and carbon atoms in the hydrate crystal;

[0009] S2. Based on the hydrate crystal information obtained in S1, the Material Studio software was used to construct a hydrate 2×2×4 supercell molecular model, and Packmol was used to establish an aqueous solution model containing 200 water molecules;

[0010] S3, constructing a pure water-hydrate system model using the hydrate supercell molecular model and aqueous solution model established in S2, optimizing the geometric configuration of the pure water-hydrate system model, and performing energy minimization processing;

[0011] S4. Using the optimized pure water-hydrate system model in S3, LAMMPS software is used to calculate the molecular motion trajectory and thermodynamic quantities (pressure P, temperature T, potential energy PE) of the isobaric step-by-step heating decomposition process of hydrates at different pressures and temperatures;

[0012] S5, using the molecular motion trajectory calculated in S4 to calculate the water molecular order parameter AOP and the number of hydrogen bonds between water molecules in the pure water-hydrate system;

[0013] S6. Using the molecular motion trajectory of the pure water-hydrate system, potential energy PE, AOP, and the number of hydrogen bonds between water molecules calculated in S4 and S5, it is determined that the time when the methane hydrate is completely decomposed by continuing to heat up under the pressure, and the system temperature at the time is obtained by fitting the time average temperature curve with a fourth-order polynomial, i.e., the hydrate phase equilibrium temperature under the pressure;

[0014] S7. Compare the calculated hydrate phase equilibrium simulation temperature with the hydrate phase equilibrium experimental temperature to calculate the error.

[0015] In step S3, the construction of the pure water-hydrate system model includes the following steps:

[0016] S301. Use Material Studio software to export the pdb file of the hydrate supercell molecular model, use VMD to convert the xyz file and adjust the atomic numbers of water molecules so that the oxygen atoms and hydrogen atoms are arranged in the OHH order;

[0017] S302, re-importing the hydrate supercell molecular model adjusted in S301 into Material Studio software, combining it with the aqueous solution model, and establishing a two-phase stratified model pure water-hydrate system model;

[0018] S303. Convert the pure water-hydrate system model established in S302 into a data file supported by LAMMPS software, perform geometric optimization on the pure water-hydrate system model, and minimize the energy of the pure water-hydrate system model to ensure that subsequent calculation results are more accurate.

[0019] In step S4, the isobaric step-by-step temperature increase calculation of the pure water-hydrate system model includes the following steps:

[0020] S401, setting of force field parameters: water molecules use TIP4P potential energy model, bond length and bond angle are constrained by SHAKE algorithm, and methane molecules use OPLS-AA force field;

[0021] S402, setting of simulation parameters: temperature and pressure are constrained by Nosé-Hoover thermostat and barostat; periodic boundary conditions are used in the three directions of X, Y, and Z; the cutoff radius of short-range interaction (LJ action) is The long-range interaction force adopts the PPPM algorithm;

[0022] S403. The specific simulation process is as follows: set the initial velocity to conform to the Maxwell-Boltzman distribution at the initial temperature; use the NVT ensemble in the relaxation stage, and set the temperature to 10K lower than the hydrate phase equilibrium experimental temperature; use the NPT ensemble in the simulation stage, with a step length of 1fs for each heating process, a simulation time of 1ns, and a heating amplitude of 1K; output the system potential energy, system temperature, and system pressure through the thermo command, and output the trajectory file and equil file.

[0023] In step S5, the calculation of AOP and hydrogen bond number includes the following steps:

[0024] S501. Implement AOP calculation through Python programming. The AOP calculation method is as follows:

[0025]

[0026] In the formula, θ jik is the distance between oxygen atom and water molecule i The internal angle formed by any two oxygen atoms in water molecules within ;

[0027] S502, using the molecular motion trajectory calculated in step S4, using VMD to calculate the angle between water molecules, θ = 30°, The number of hydrogen bonds present.

[0028] In step S6, the calculation of the hydrate phase equilibrium temperature includes the following steps:

[0029] S601, simulation trajectory of hydrate decomposition process: the hydrate cage structure completely disappears, the system potential energy rises to a stable state, and the moment when the AOP calculated in step S5 reaches above 0.8 is the moment when the hydrate is completely decomposed;

[0030] S602. For the average temperature of the hydrate ensemble calculated in step S4, samples are taken every 250 steps forward based on 5000 steps, and a fluctuation curve of the system temperature averaged over time is drawn. A fourth-order polynomial fitting is performed to obtain the hydrate phase equilibrium temperature under the molecular simulation method.

[0031] The pure water-hydrate system phase equilibrium calculation method based on molecular simulation described in the present invention can be used to predict the hydrate phase equilibrium temperature under cement slurry percolation conditions, and also includes the following steps: S0, infrared testing of cement slurry admixtures to obtain characteristic conceptual energy group information of cement slurry admixtures, and constructing structural units characterizing admixture copolymers; and data writing of the obtained structural units under the OPLS-AA potential energy model.

[0032] Compared with the existing method, the beneficial effects of the present invention are:

[0033] 1. The pure water-hydrate phase equilibrium calculation method designed in the present invention takes into account the numerical error of the velocity, the heat exchange conditions are close to the experimental conditions, the calculated phase equilibrium temperature error is stable, and the pure water-hydrate phase equilibrium chart at 5-10MPa can be depicted within the error range of 1.502%-1.792%.

[0034] 2. The present invention realizes the decomposition of natural gas hydrates under different pressure, temperature, solute and other conditions by computer simulation; by adjusting the solute type, the properties of the hydrate decomposition process under the action of the wellbore working fluid (drilling fluid, cement slurry) filtrate during drilling and completion are simulated; by adjusting the temperature, the phase equilibrium conditions of hydrates are studied. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0036] Figure 1 is a flow chart of the prediction method of the present invention;

[0037] Figure 2 It is a model diagram of a hydrate single crystal in Example 1 and Example 2 of the present invention;

[0038] Figure 3 It is a hydrate supercell model diagram in Example 1 and Example 2 of the present invention;

[0039] Figure 4 is a pure water-hydrate model diagram in Example 1 of the present invention;

[0040] Figure 5 is a molecular model diagram of sodium dodecane sulfonate in Example 2 of the present invention;

[0041] Figure 6is a diagram of the aqueous solution-hydrate model in Example 2 of the present invention;

[0042] Figure 7 for Figure 2 Color map of

[0043] Figure 8 for Figure 3 Color map of

[0044] Fig. 9 for Figure 4 Color map of

[0045] Fig.10 for Figure 5 Color map of

[0046] Fig.11 for Figure 6 Color map of . DETAILED DESCRIPTION

[0047] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0048] like Figure 1-6 As shown, this embodiment proposes a hydrate phase equilibrium calculation prediction method, including:

[0049] The pre-processing stage includes setting the type and quantity of atoms, setting the type and quantity of bond angles, defining the atomic mass, defining the atomic position coordinates, and setting the atomic charge; obtaining hydrate crystal information through XRD diffraction experiments, using Material Studio software to construct a crystal model, setting the type and quantity of atoms, setting the type and quantity of bond angles, defining the atomic mass, defining the atomic position coordinates, and setting the atomic charge; using Packmol to construct a random water molecule solution; and using VMD to convert data formats.

[0050] The computational simulation stage includes the setting of boundary conditions, potential functions, ensembles, and constraint equations. Periodic boundary conditions are used in the X, Y, and Z directions. The TIP4P potential energy model is used for water molecules, and the OPLS-AA force field is used for organic molecules. The NVT ensemble is used in the relaxation stage, and the NPT ensemble is used in the simulation stage. The bond length and bond angle of water molecules are constrained using the SHAKE algorithm.

[0051] Post-processing stage, the post-processing stage includes the analysis of thermodynamic parameters such as potential energy and temperature, the calculation of AOP, and the calculation of the number of hydrogen bonds.

[0052] AOP calculation is implemented through Python programming. The AOP calculation method is as follows:

[0053]

[0054] In the formula, θ jik is the distance between oxygen atom and water molecule i The internal angle formed by any two oxygen atoms in water molecules within .

[0055] Using VMD, we calculated the angle between water molecules to be θ=30°. The number of hydrogen bonds present.

[0056] Example 1

[0057] like Figure 4 As shown, the hydrate phase equilibrium temperature is calculated by taking the pure water liquid phase of 200 water molecules and the SI-type hydrate 2×2×4 supercell molecular model as an example:

[0058] S1. Obtain hydrate crystal information through X-ray crystal diffraction results, including the coordinates of oxygen atoms and carbon atoms in the hydrate crystal; the space group is Pm3n, α=β=γ=90.00°, molecular formula (CH4)8(H2O) 46 ;

[0059] S2. Based on the hydrate crystal information obtained in S1, the Material Studio software was used to construct a hydrate 2×2×4 supercell molecular model, and Packmol was used to establish an aqueous solution model containing 200 water molecules;

[0060] S3, constructing a pure water-hydrate system model using the hydrate supercell molecular model and aqueous solution model established in S2, optimizing the geometric configuration of the pure water-hydrate system model, and performing energy minimization processing;

[0061] S4. Using the optimized pure water-hydrate system model in S3, LAMMPS software is used to calculate the molecular motion trajectory and thermodynamic quantities (pressure P, temperature T, potential energy PE) of the isobaric step-by-step heating decomposition process of hydrates at different pressures and temperatures;

[0062] S5, using the molecular motion trajectory calculated in S4 to calculate the water molecular order parameter AOP and the number of hydrogen bonds between water molecules in the pure water-hydrate system;

[0063] S6. Using the molecular motion trajectory of the pure water-hydrate system, potential energy PE, AOP, and the number of hydrogen bonds between water molecules calculated in S4 and S5, it is determined that the time when the methane hydrate is completely decomposed by continuing to heat up under the pressure, and the system temperature at the time is obtained by fitting the time average temperature curve with a fourth-order polynomial, i.e., the hydrate phase equilibrium temperature under the pressure;

[0064] S7. Compare the calculated hydrate phase equilibrium simulation temperature with the hydrate phase equilibrium experimental temperature to calculate the error.

[0065] In step S3, the construction of the pure water-hydrate system model includes the following steps:

[0066] S301. Use Material Studio software to export the pdb file of the hydrate supercell molecular model, use VMD to convert the xyz file and adjust the atomic numbers of water molecules so that the oxygen atoms and hydrogen atoms are arranged in the OHH order;

[0067] S302, re-importing the hydrate supercell molecular model adjusted in S301 into Material Studio software, combining it with the aqueous solution model, and establishing a two-phase stratified model pure water-hydrate system model;

[0068] S303. Convert the pure water-hydrate system model established in S302 into a data file supported by LAMMPS software, perform geometric optimization on the pure water-hydrate system model, and minimize the energy of the pure water-hydrate system model to ensure that subsequent calculation results are more accurate.

[0069] In step S4, the isobaric step-by-step temperature increase calculation of the pure water-hydrate system model includes the following steps:

[0070] S401, setting of force field parameters: water molecules use TIP4P potential energy model, bond length and bond angle are constrained by SHAKE algorithm, and methane molecules use OPLS-AA force field;

[0071] S402, setting of simulation parameters: temperature and pressure are constrained by Nosé-Hoover thermostat and barostat; periodic boundary conditions are used in the three directions of X, Y, and Z; the cutoff radius of short-range interaction (LJ action) is The long-range interaction force adopts the PPPM algorithm;

[0072] S403. The specific simulation process is as follows: set the initial velocity to conform to the Maxwell-Boltzman distribution at the initial temperature; use the NVT ensemble in the relaxation stage, and set the temperature to 10K lower than the hydrate phase equilibrium experimental temperature; use the NPT ensemble in the simulation stage, with a step length of 1fs for each heating process, a simulation time of 1ns, and a heating amplitude of 1K; output the system potential energy, system temperature, and system pressure through the thermo command, and output the trajectory file and equil file.

[0073] In step S5, the calculation of AOP and hydrogen bond number includes the following steps:

[0074] S501. Implement AOP calculation through Python programming. The AOP calculation method is as follows:

[0075]

[0076] In the formula, θ jik is the distance between oxygen atom and water molecule i The internal angle formed by any two oxygen atoms in water molecules within ;

[0077] S502, using the molecular motion trajectory calculated in step S4, using VMD to calculate the angle between water molecules, θ = 30°, The number of hydrogen bonds present.

[0078] In step S6, the calculation of the hydrate phase equilibrium temperature includes the following steps:

[0079] S601, simulation trajectory of hydrate decomposition process: the hydrate cage structure completely disappears, the system potential energy rises to a stable state, and the moment when the AOP calculated in step S5 reaches above 0.8 is the moment when the hydrate is completely decomposed;

[0080] S602. For the average temperature of the hydrate ensemble calculated in step S4, samples are taken every 250 steps forward based on 5000 steps, and a fluctuation curve of the system temperature averaged over time is drawn. A fourth-order polynomial fitting is performed to obtain the hydrate phase equilibrium temperature under the molecular simulation method.

[0081] Example 2

[0082] like Figure 6 As shown in the figure, the hydrate phase equilibrium temperature under cement slurry percolation conditions is calculated by taking the sodium dodecane sulfonate aqueous solution containing 200 water molecules and the SI-type hydrate 2×2×4 supercell molecular model as examples:

[0083] S1. Use Material Studio software to construct a molecular model of sodium dodecane sulfonate;

[0084] S2. Writing parameters of the obtained sodium dodecane sulfonate molecular model under the OPLS-AA potential energy model;

[0085] S3. Use Packmol to construct an aqueous solution model containing structural units and 200 water molecules;

[0086] S4. Obtaining hydrate crystal information through X-ray crystal diffraction results, including the coordinates of oxygen atoms and carbon atoms in the hydrate crystals;

[0087] S5. Based on the hydrate crystal information obtained in S4, the hydrate 2×2×4 supercell molecular model is constructed using Material Studio software;

[0088] S6. Using the hydrate supercell molecular model and the aqueous solution model established in S3 and S5 to construct an aqueous solution-hydrate system model, optimize the geometric configuration of the aqueous solution-hydrate system model, and perform energy minimization processing;

[0089] S7. Using the optimized aqueous solution-hydrate system model in S6, LAMMPS software is used to calculate the molecular motion trajectory and thermodynamic quantities (pressure P, temperature T, potential energy PE) of the isobaric step-by-step heating decomposition process of hydrates at different pressures and temperatures;

[0090] S8, using the molecular motion trajectory calculated in S7 to calculate the water molecular order parameter AOP and the number of hydrogen bonds between water molecules in the aqueous solution-hydrate system;

[0091] S9, using the molecular motion trajectory of the aqueous solution-hydrate system, potential energy PE, AOP, and the number of hydrogen bonds between water molecules calculated in S7 and S8, to determine the moment when the trace amount of methane hydrate under the pressure continues to rise in temperature and completely decomposes, and to perform a fourth-order polynomial fitting on the time-averaged temperature curve to obtain the system temperature at that moment, i.e., the predicted hydrate phase equilibrium temperature under the pressure;

[0092] In step S6, constructing the aqueous solution-hydrate system model includes the following steps:

[0093] S601. Use Material Studio software to export the pdb file of the hydrate supercell molecular model, use VMD to convert the xyz file and adjust the atomic numbers of water molecules so that the oxygen atoms and hydrogen atoms are arranged in the OHH order;

[0094] S602, re-importing the hydrate supercell molecular model adjusted in S601 into Material Studio software, combining it with the aqueous solution model, and establishing a two-phase stratified model aqueous solution-hydrate system model;

[0095] S603. Convert the aqueous solution-hydrate system model established in S602 into a data file supported by LAMMPS software, perform geometric optimization on the aqueous solution-hydrate system model, and minimize the energy of the aqueous solution-hydrate system model to ensure that subsequent calculation results are more accurate.

[0096] In step S7, the isobaric step-by-step temperature increase calculation of the aqueous solution-hydrate system model includes the following steps:

[0097] S701, setting of force field parameters: water molecules use TIP4P potential energy model, bond length and bond angle are constrained by SHAKE algorithm, and methane molecules use OPLS-AA force field;

[0098] S702, setting of simulation parameters: temperature and pressure are constrained by Nosé-Hoover thermostat and barostat; periodic boundary conditions are used in the three directions of X, Y, and Z; the cutoff radius of the short-range interaction (LJ action) is The long-range interaction force adopts the PPPM algorithm;

[0099] S703. The specific simulation process is as follows: set the initial velocity to conform to the Maxwell-Boltzman distribution at the initial temperature; use the NVT ensemble in the relaxation stage, and set the temperature to 10K lower than the pure water-hydrate phase equilibrium simulation temperature; use the NPT ensemble in the simulation stage, with a step size of 1fs for each heating process, a simulation time of 1ns, and a heating amplitude of 1K; output the system potential energy, system temperature, and system pressure through the thermo command, and output the trajectory file and equil file.

[0100] In step S8, the calculation of AOP and hydrogen bond number includes the following steps:

[0101] S801. Implement AOP calculation through Python programming. The AOP calculation method is as follows:

[0102]

[0103] In the formula, θ jik is the distance between oxygen atom and water molecule i The internal angle formed by any two oxygen atoms in water molecules within ;

[0104] S802, using the molecular motion trajectory calculated in step S7, using VMD to calculate the angle between water molecules, θ = 30°, The number of hydrogen bonds present.

[0105] In step S9, the calculation of the hydrate phase equilibrium temperature includes the following steps:

[0106] S901, simulation trajectory of hydrate decomposition process: the hydrate cage structure just completely disappears, the system potential energy rises to a stable state, and the moment when the AOP calculated in step S8 reaches above 0.8 is the moment when the hydrate just completely decomposes;

[0107] S902. For the average temperature of the hydrate ensemble calculated in step S7, samples are taken every 250 steps forward based on 5000 steps, and a fluctuation curve of the system temperature averaged over time is drawn. A fourth-order polynomial fitting is performed to obtain the hydrate phase equilibrium temperature for predicting cement slurry percolation conditions under the molecular simulation method.

Claims

1. A method for calculating the phase equilibrium of a pure water-hydrate system based on molecular simulation, comprising the following steps: S1. Obtaining hydrate crystal information through X-ray crystal diffraction results, including the coordinates of oxygen atoms and carbon atoms in the hydrate crystals; S2. Based on the hydrate crystal information obtained in S1, the Material Studio software was used to construct a hydrate 2×2×4 supercell molecular model, and Packmol was used to establish an aqueous solution model containing 200 water molecules; S3, constructing a pure water-hydrate system model using the hydrate supercell molecular model and aqueous solution model established in S2, optimizing the geometric configuration of the pure water-hydrate system model, and performing energy minimization processing; S4. Using the optimized pure water-hydrate system model in S3, LAMMPS software is used to calculate the molecular motion trajectory and thermodynamic quantities of the isobaric step-by-step heating decomposition process of hydrates at different pressures and temperatures; the thermodynamic quantities include pressure P and / or temperature T and / or potential energy PE; S5, using the molecular motion trajectory calculated in S4 to calculate the water molecular order parameter AOP and the number of hydrogen bonds between water molecules in the pure water-hydrate system; The calculation of AOP and hydrogen bond number includes the following steps: S501. AOP calculation is implemented through Python programming. The AOP calculation method is as follows: In the formula, θ jik is the distance between oxygen atom and water molecule i The internal angle formed by any two oxygen atoms in water molecules within ; S502, using the molecular motion trajectory calculated in step S4, using VMD to calculate the angle between water molecules, θ = 30°, The number of hydrogen bonds present; S6. Using the molecular motion trajectory of the pure water-hydrate system, potential energy PE, AOP, and the number of hydrogen bonds between water molecules calculated in S4 and S5, it is determined that the time when the methane hydrate is completely decomposed by continuing to heat up under the pressure, and the system temperature at the time is obtained by fitting the time average temperature curve with a fourth-order polynomial, i.e., the hydrate phase equilibrium temperature under the pressure; S7. Compare the calculated hydrate phase equilibrium simulation temperature with the hydrate phase equilibrium experimental temperature to calculate the error.

2. A method for calculating phase equilibrium of a pure water-hydrate system based on molecular simulation according to claim 1, characterized in that: In step S3, the construction of the pure water-hydrate system model includes the following steps: S301. Use Material Studio software to export the pdb file of the hydrate supercell molecular model, use VMD to convert the xyz file and adjust the atomic numbers of water molecules so that the oxygen atoms and hydrogen atoms are arranged in the OHH order; S302, re-importing the hydrate supercell molecular model adjusted in S301 into Material Studio software, combining it with the aqueous solution model, and establishing a two-phase stratified model pure water-hydrate system model; S303. Convert the pure water-hydrate system model established in S302 into a data file supported by LAMMPS software, perform geometric optimization on the pure water-hydrate system model, and minimize the energy of the pure water-hydrate system model to ensure that subsequent calculation results are more accurate.

3. The method for calculating phase equilibrium of a pure water-hydrate system based on molecular simulation according to claim 1, characterized in that: In step S4, the isobaric step-by-step temperature increase calculation of the pure water-hydrate system model includes the following steps: S401, setting of force field parameters: water molecules use TIP4P potential energy model, bond length and bond angle are constrained by SHAKE algorithm, and methane molecules use OPLS-AA force field; S402, setting of simulation parameters: temperature and pressure are constrained by Nosé-Hoover thermostat and barostat; periodic boundary conditions are used in the three directions of X, Y, and Z; the cutoff radius of short-range interaction is The long-range interaction force adopts the PPPM algorithm; S403. The specific simulation process is as follows: set the initial velocity to conform to the Maxwell-Boltzman distribution at the initial temperature; use the NVT ensemble in the relaxation stage, and set the temperature to 10K lower than the hydrate phase equilibrium experimental temperature; use the NPT ensemble in the simulation stage, with a step length of 1fs for each heating process, a simulation time of 1ns, and a heating amplitude of 1K; output the system potential energy, system temperature, and system pressure through the thermo command, and output the trajectory file and equil file.

4. The method for calculating phase equilibrium of a pure water-hydrate system based on molecular simulation according to claim 1, characterized in that: In step S6, the calculation of the hydrate phase equilibrium temperature includes the following steps: S601, simulation trajectory of hydrate decomposition process: the hydrate cage structure completely disappears, the system potential energy rises to a stable state, and the moment when the AOP calculated in step S5 reaches above 0.8 is the moment when the hydrate is completely decomposed; S602. For the average temperature of the hydrate ensemble calculated in step S4, samples are taken every 250 steps forward based on 5000 steps, and a fluctuation curve of the system temperature averaged over time is drawn. A fourth-order polynomial fitting is performed to obtain the hydrate phase equilibrium temperature under the molecular simulation method.

5. Application of a pure water-hydrate system phase equilibrium calculation method based on molecular simulation as described in any one of claims 1 to 4 in predicting the hydrate phase equilibrium temperature under cement slurry percolation conditions.

6. The application of a pure water-hydrate system phase equilibrium calculation method based on molecular simulation according to claim 5 in predicting the hydrate phase equilibrium temperature under cement slurry percolation conditions is characterized in that: The following steps are also included: S0. Conduct infrared testing on cement slurry admixtures to obtain characteristic concept energy group information of cement slurry admixtures and construct structural units to characterize admixture copolymers; The obtained structural units were subjected to data compilation under the OPLS-AA potential energy model.

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