A method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation

Abstract

The present invention discloses a method for calculating the melting point of crude oil dewax based on molecular dynamics simulation, and belongs to the technical field of oil and gas field development. The method comprises the following steps: S1: obtaining a target crude oil sample, separating and obtaining crude oil dewax, and measuring the real component information of the crude oil dewax; S2: establishing a molecular model of each single-component normal alkane in the crude oil dewax, and mixing the molecular models of each single-component normal alkane to obtain a molecular model of crude oil dewax of a real component; S3: establishing a molecular force field of each single-component normal alkane, and optimizing the dihedral angle torsion term potential energy parameters and atomic charges in the molecular force field to obtain optimized molecular force field parameters; S4: performing molecular dynamics simulation at different temperatures based on the molecular model of crude oil dewax of a real component and combining the optimized molecular force field parameters; S5: determining the melting point of crude oil dewax based on the simulation results. The present invention can accurately calculate the melting point of crude oil dewax using molecular dynamics simulation.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas field development, and in particular to a method for calculating the melting point of crude oil dewaxing wax based on molecular dynamics simulation. Background Art

[0002] During the extraction or transportation of crude oil, due to changes in temperature and pressure conditions, the ability of light hydrocarbons in crude oil to dissolve heavy hydrocarbon components decreases, and the wax molecules that were originally evenly dispersed in the oil phase agglomerate with each other, eventually precipitating wax crystals. The precipitated wax crystals continue to deposit on the wall, reducing the effective flow area and even causing blockage of the production and transportation system, ultimately causing huge production and economic losses. At present, the melting point of paraffin is mainly determined by the endothermic peak in the heating stage of the DSC curve, providing the required temperature for on-site thermal dewaxing and unblocking of pipelines, so as to formulate dewaxing and unblocking plans.

[0003] At present, when molecular dynamics simulates paraffin-related systems, researchers consider the problem of computing power, and most molecular models use single-component or two-component straight-chain alkanes (such as n-tetracosane, n-hexacosane, n-octacosane and n-trioctacosane, etc.) to represent wax. In terms of simulating force field models, when the existing force field simulates the paraffin melting process system, the molecular structure does not change significantly or significantly overestimates the melting temperature of paraffin molecules. It is necessary to optimize the model (molecular model and force field model) so that it can more realistically reflect the melting process of paraffin in crude oil and accurately simulate the melting temperature of paraffin. Summary of the invention

[0004] In view of the above problems, the present invention aims to provide a method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation.

[0005] The technical solution of the present invention is as follows:

[0006] A method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation, comprising the following steps:

[0007] S1: obtaining a target crude oil sample, separating crude oil dewax from the target crude oil sample, and measuring real component information of the crude oil dewax;

[0008] S2: establishing a molecular model of each single-component normal alkane in the crude oil dewax according to the real component information, and mixing the molecular models of each single-component normal alkane according to the real component information to obtain a real component crude oil dewax molecular model;

[0009] S3: establishing a molecular force field for each single-component normal alkane, and optimizing the potential energy parameters of the dihedral angle torsion term and the atomic charge in the molecular force field to obtain optimized molecular force field parameters;

[0010] S4: performing molecular dynamics simulation at different temperatures according to the wax molecular model of the real component crude oil and the optimized molecular force field parameters;

[0011] S5: Determine the melting point of the wax removed from the crude oil according to the results of molecular dynamics simulation at different temperatures.

[0012] Preferably, in step S1, the real component information is obtained by chromatograph testing.

[0013] Preferably, step S2 specifically includes the following sub-steps:

[0014] S21: Determine the components contained in the wax removed from the crude oil and the mass composition of each component according to the real component information;

[0015] S22: Calculate the molar composition of each component according to the mass composition of each component;

[0016] S23: establishing a molecular model in which the total number of normal alkanes in each component is greater than 1000 according to the molar composition of each component;

[0017] S24: The molecular models of the single-component normal alkanes are mixed according to the molar composition ratio of each component to obtain the real component crude oil dewaxing molecular model.

[0018] Preferably, step S23 further includes the step of establishing a normal alkane crystal model of one of the components, and when mixing is performed in step S24, the normal alkane crystal model is mixed with the molecular models of the remaining single-component normal alkanes.

[0019] Preferably, step S3 specifically includes the following sub-steps:

[0020] S31: Establish the OPLS-AA force field of each single-component n-alkanes, and unify and merge the OPLS-AA force field parameters of each single-component n-alkanes;

[0021] S32: selecting one of the n-alkane containing at least four methylene groups, and performing a flexible scanning on the dihedral angle of the n-alkane using the quantum chemical M06-2X / def2-TZVP method to obtain a potential energy curve of the dihedral angle;

[0022] S33: Remove the 1-4 non-bonded interactions in the dihedral angle potential energy and calculate the potential energy curves of the CT-CT-CT-CT, HC-CT-CT-HC and CT-CT-CT-HC dihedral angle torsion terms in n-alkanes respectively;

[0023] S34: The potential energy curves of the dihedral angle torsion terms are fitted by the following formula to obtain the dihedral angle parameters of each type of dihedral angle in n-alkanes:

[0024] (1)

[0025] Where: is the potential energy generated by the torsion of the intramolecular dihedral angle; is a constant; is the dihedral angle;

[0026] S35: determining a potential energy calculation formula for dihedral angle torsion according to the dihedral angle parameter, and calculating the potential energy generated by intramolecular dihedral angle torsion of other normal alkanes according to the potential energy calculation formula for dihedral angle torsion;

[0027] S36: Keep the net charge of all single-component n-alkane molecules to zero, and adjust the charge of hydrogen atoms in the force field of some single-component n-alkane molecules until the simulated melting point of the single-component n-alkane molecules is consistent with the experimental melting point;

[0028] S37: According to the potential energy generated by the intramolecular dihedral angle torsion obtained in step S35 and the hydrogen atom charge obtained in step S36, the OPLS-AA force field parameters are updated to obtain optimized P-OPLS force field parameters.

[0029] Preferably, in step S32, when performing flexible scanning, scanning is performed every 2°, and 180 steps are scanned.

[0030] Preferably, in step S33, the atomic charge of hydrogen atoms in the calculation process is 0.06e, the charge of methylene carbon atoms is -0.12e, and the charge of methyl carbon atoms is -0.18e.

[0031] Preferably, in step S4, when performing molecular dynamics simulation, the conjugate gradient method and the steepest descent method are used together to minimize energy, the NPT ensemble is used, the LINCS algorithm is adopted to constrain the bonds connected to the hydrogen atoms, the simulation step size is 2fs, the electrostatic interaction is calculated using the PME algorithm, the van der Waals interaction uses the cut-off algorithm, the cutoff distances of the van der Waals interaction and the electrostatic interaction are both set to 1.4nm, the temperature is controlled by the V-rescale method, the pressure is controlled by the C-rescale method, and the simulation pressure remains unchanged during the simulation process.

[0032] Preferably, when the real component crude oil wax molecular model contains a crystal model, during molecular dynamics simulation, the crystal model is frozen in the first stage and the simulation calculation is performed for 10 ns, and in the second stage, the freezing is cancelled and the simulation calculation is performed for 150 ns.

[0033] Preferably, in step S5, the melting point of the wax removed from the crude oil is determined based on the density difference between the wax liquid phase and the wax solid phase combined with the molecular model structure.

[0034] The beneficial effects of the present invention are:

[0035] The present invention establishes a molecular model of crude oil wax removal based on the real component information of crude oil wax removal, and optimizes the force field parameters so that the present invention can accurately calculate the melting point of crude oil wax removal. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0037] Figure 1 It is a schematic flow chart of the method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation of the present invention;

[0038] Figure 2 This is the atomic number diagram of n-decane molecule;

[0039] Figure 3 is a relative potential energy curve of dihedral angles of four methylene carbon atoms C11-C14-C17-C20 of n-decane in a specific implementation;

[0040] Figure 4 is a calculation curve and a fitting curve of the relative potential energy of the CT-CT-CT-CT dihedral angle torsion term of n-decane in a specific implementation;

[0041] Figure 5 A calculation curve and a fitting curve of the relative potential energy of the dihedral angle torsion term of n-decane HC-CT-CT-HC in a specific implementation;

[0042] Figure 6 The density variation curve of the wax removal system of crude oil from Well A-1 and Well A-2 in a specific implementation is as a function of temperature;

[0043] Figure 7 It is a schematic diagram of molecular dynamics simulation results of wax removal from crude oil of Well A-1 under different temperature conditions in a specific implementation;

[0044] Figure 8 A schematic diagram of molecular dynamics simulation results of wax removal from crude oil of Well A-2 under different temperature conditions in a specific implementation;

[0045] Fig. 9 is a DSC curve diagram of wax removal from crude oil of Well A-1 in a specific implementation;

[0046] Fig.10 The DSC curve of wax removal from crude oil of Well A-2 in a specific implementation is shown. DETAILED DESCRIPTION

[0047] The present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, in the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those generally understood by those of ordinary skill in the art to which this application belongs. The words "including" or "comprising" and the like used in the disclosure of the present invention mean that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0048] like Figure 1 As shown, the present invention provides a method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation, comprising the following steps:

[0049] S1: Obtain a target crude oil sample, separate crude oil dewax from the target crude oil sample, and measure the real component information of the crude oil dewax.

[0050] In a specific embodiment, when separating the crude oil to remove wax, the separation is performed with reference to SY / T7550-2012 "Determination of wax, colloid and asphaltene content in crude oil", and the true component information is obtained by chromatograph testing.

[0051] S2: establishing molecular models of each single component normal alkane in the crude oil dewax according to the real component information, and mixing the molecular models of each single component normal alkane according to the real component information to obtain a real component crude oil dewax molecular model.

[0052] In a specific embodiment, step S2 specifically includes the following sub-steps:

[0053] S21: Determine the components contained in the wax removed from the crude oil and the mass composition of each component according to the real component information;

[0054] S22: Calculate the molar composition of each component according to the mass composition of each component;

[0055] S23: establishing a molecular model in which the total number of normal alkanes in each component is greater than 1000 according to the molar composition of each component;

[0056] S24: The molecular models of the single-component normal alkanes are mixed according to the molar composition ratio of each component to obtain the real component crude oil dewaxing molecular model.

[0057] In a specific embodiment, step S23 further includes the step of establishing a normal alkane crystal model of one of the components, and when mixing is performed in step S24, the normal alkane crystal model is mixed with the molecular models of the remaining single-component normal alkanes.

[0058] In the above embodiment, by establishing the normal alkane crystal model, the wax crystals composed of normal alkane molecules in the wax molecule model of the real component crude oil can be removed, thereby avoiding the influence of supercooling on the simulation results and greatly saving the amount of calculation.

[0059] S3: Establishing the molecular force field of each single-component normal alkane, and optimizing the potential energy parameters of the dihedral angle torsion term and the atomic charge in the molecular force field, to obtain the optimized molecular force field parameters.

[0060] In a specific embodiment, step S3 specifically includes the following sub-steps:

[0061] S31: Establish the OPLS-AA force field of each single-component n-alkanes, and unify and merge the OPLS-AA force field parameters of each single-component n-alkanes;

[0062] S32: selecting one of the n-alkane containing at least four methylene groups, and performing a flexible scanning on the dihedral angle of the n-alkane using the quantum chemical M06-2X / def2-TZVP method to obtain a potential energy curve of the dihedral angle;

[0063] S33: Remove the 1-4 non-bonded interactions in the dihedral angle potential energy and calculate the potential energy curves of the CT-CT-CT-CT, HC-CT-CT-HC and CT-CT-CT-HC dihedral angle torsion terms in n-alkanes respectively;

[0064] S34: The potential energy curves of the dihedral angle torsion terms are fitted by the following formula to obtain the dihedral angle parameters of each type of dihedral angle in n-alkanes:

[0065] (1)

[0066] Where: is the potential energy generated by the torsion of the intramolecular dihedral angle; is a constant; is the dihedral angle;

[0067] S35: determining a potential energy calculation formula for dihedral angle torsion according to the dihedral angle parameter, and calculating the potential energy generated by intramolecular dihedral angle torsion of other normal alkanes according to the potential energy calculation formula for dihedral angle torsion;

[0068] S36: Keep the net charge of all single-component n-alkane molecules to zero, and adjust the charge of hydrogen atoms in the force field of some single-component n-alkane molecules until the simulated melting point of the single-component n-alkane molecules is consistent with the experimental melting point;

[0069] S37: According to the potential energy generated by the intramolecular dihedral angle torsion obtained in step S35 and the hydrogen atom charge obtained in step S36, the OPLS-AA force field parameters are updated to obtain optimized P-OPLS force field parameters.

[0070] In the above embodiment, by optimizing the parameters of the dihedral angle torsion term of the carbon chain in the original OPLS-AA force field, simulating the melting point of each single-component normal alkane, and optimizing the charge parameters of each single-component normal alkane, the optimized P-OPLS force field parameters obtained enable the present invention to more accurately calculate the melting point of wax removed from crude oil.

[0071] In a specific embodiment, in step S32, when performing flexible scanning, scanning is performed every 2°, and 180 steps are scanned; in step S33, the atomic charge of hydrogen atoms in the calculation process is 0.06e, the charge of methylene carbon atoms is -0.12e, and the charge of methyl carbon atoms is -0.18e.

[0072] S4: According to the real component crude oil wax removal molecular model, combined with the optimized molecular force field parameters, molecular dynamics simulations are performed at different temperatures.

[0073] In a specific embodiment, when performing molecular dynamics simulation, the conjugate gradient method and the steepest descent method are used together to minimize energy, the NPT ensemble is used, the LINCS algorithm is used to constrain the bonds connected to the hydrogen atoms, the simulation step is 2fs, the electrostatic interaction is calculated using the PME algorithm, the van der Waals interaction uses the cut-off algorithm, the cutoff distances of the van der Waals interaction and the electrostatic interaction are both set to 1.4nm, the temperature is controlled by the V-rescale method, the pressure is controlled by the C-rescale method, and the simulation pressure remains unchanged during the simulation process.

[0074] In a specific embodiment, when the real component crude oil wax molecular model contains a crystal model, during molecular dynamics simulation, the crystal model is frozen in the first stage and the simulation calculation is performed for 10ns. In the second stage, the freeze is canceled and the simulation calculation is performed for 150ns.

[0075] S5: Determine the melting point of the wax removed from the crude oil according to the results of molecular dynamics simulation at different temperatures.

[0076] In a specific embodiment, the melting point of the wax removed from the crude oil is determined based on the density difference between the wax liquid phase and the wax solid phase combined with the molecular model structure.

[0077] In a specific embodiment, taking the crude oil of a certain reservoir A-1 well and A-2 well as an example, the method for calculating the melting point of crude oil wax removal based on molecular dynamics simulation of the present invention is used to calculate the melting point of the crude oil wax removal, which specifically includes the following steps:

[0078] (1) obtaining a target crude oil sample, separating crude oil dewax from the target crude oil sample, and measuring true component information of the crude oil dewax;

[0079] In this embodiment, the crude oil wax is separated from the crude oil of Well A-1 and Well A-2 with reference to SY / T7550-2012 "Determination of wax colloid asphaltene content in crude oil", and then the real component information is obtained by chromatography testing.

[0080] (2) Establishing a molecular model of wax removal from crude oil with real components;

[0081] A molecular model of the wax solution of the crude oil of Well A-1 and Well A-2 with a total number of not less than 1000 real components is established. Specifically, the number of each single component normal alkane molecule in the wax solution of the crude oil of Well A-1 and Well A-2 is calculated based on the real component information obtained in step (1). The results are shown in Table 1:

[0082] Table 1 Number of single-component n-alkane molecules in the wax removed from crude oils from Well A-1 and Well A-2

[0083]

[0084] The packmol software was used to combine and model the n-alkane molecules of each component. The box size was 20nm×20nm×20nm, and the real component crude oil wax molecular models of well A-1 and well A-2 were established respectively.

[0085] (3) Establish the molecular force field of each single-component n-alkanes and optimize it to obtain the optimized molecular force field parameters;

[0086] The OPLS-AA force field of each single-component n-alkanes is established, and the OPLS-AA force field parameters of each single-component n-alkanes are unified and merged;

[0087] Select the four methylene carbon atoms C11-C14-C17-C20 of n-decane, such as Figure 2 As shown, the dihedral angle is flexibly scanned using the quantum chemical M06-2X / def2-TZVP method, scanning once every 2° for 180 steps, and the potential energy curve of the dihedral angle is obtained. The results are shown in Figure 3 As shown;

[0088] After removing the influence of 1-4 non-bonded interactions in the dihedral angle potential energy, the potential energy curves of the dihedral angle torsion terms of CT-CT-CT-CT and HC-CT-CT-HC were calculated respectively, and fitted with the function shown in formula (1). The results are shown in Figure 4 and Figure 5As shown, the CT-CT-CT-HC and HC-CT-CT-HC dihedral angle torsion terms use the same parameters, and the dihedral angle parameters of each bond are shown in Table 2:

[0089] Table 2 Parameters of the Ryckaert−Bellemans potential function of the P-OPLS force field

[0090]

[0091] Determining a potential energy calculation formula for dihedral angle torsion according to the dihedral angle parameters, and calculating the potential energy generated by intramolecular dihedral angle torsion of other normal alkanes according to the potential energy calculation formula for dihedral angle torsion;

[0092] The net charge of all single-component n-alkane molecules is kept at zero, and the charge size of hydrogen atoms in the force field of some single-component n-alkane molecules is adjusted until the simulated melting point of the single-component n-alkane molecules is consistent with the experimental melting point (i.e., the single-component n-alkane molecules in Table 3 are selected, and the experimental melting point value is selected as the approximate target value. Through multiple experimental simulations, the charge parameters of the single-component saturated straight-chain alkanes are adjusted so that the simulation results are gradually consistent with the target values). The results are shown in Table 3:

[0093] Table 3 Hydrogen atom charge optimization

[0094]

[0095] According to the hydrogen atom charge adjustment results in Table 3, the atomic charges of single-component n-alkanes are determined, as shown in Table 4:

[0096] Table 4 Atomic charges of normal alkanes

[0097]

[0098] According to the intramolecular dihedral angle torsion potential energy parameters and the atomic charges obtained above, the OPLS-AA force field parameters are updated to obtain the optimized P-OPLS force field parameters.

[0099] (4) performing molecular dynamics simulation at different temperatures, and determining the melting point of the wax removed from the crude oil according to the simulation results;

[0100] The real component crude oil dewaxing molecular model established in step (2) was used as the initial model. The optimized P-OPLS force field parameters obtained in step (3) were applied using gromacs software. Multiple temperature points were set between 40 and 70 °C by the dichotomy method to perform molecular dynamics simulations at different temperatures.

[0101] When performing molecular dynamics simulation, the conjugate gradient method and the steepest descent method are used to minimize energy. The NPT ensemble is used, and the LINCS algorithm is used to constrain the bonds connected to hydrogen atoms. The simulation step is 2fs. To ensure that the simulation system reaches equilibrium, the first stage is simulated and calculated at a lower temperature for a period of time to obtain a model of wax crystals and liquid mixtures in the crude oil of A-1 and A-2 wells. In the second stage, the simulation time is 100ns. The PME algorithm is used for electrostatic interaction calculation, and the cut-off algorithm is used for van der Waals interaction. The cut-off distances of van der Waals interaction and electrostatic interaction are both set to 1.4nm. The V-rescale method is used for temperature control, and the C-rescale method is used for pressure control. The simulation pressure remains unchanged during the simulation process.

[0102] The final conformations and motion trajectories generated under various temperature conditions were viewed using VMD software, the mass density of the final simulation results at different temperatures was calculated, and the melting point of the wax removed from crude oil was determined based on the density difference between the wax liquid and solid phases combined with the molecular model structure.

[0103] In this embodiment, the density of well A-1 and well A-2 changes with temperature as shown in the following figure: Figure 6 As shown. Figure 6 It can be seen that the melting point of the wax removed from the crude oil of well A-1 is 57.35℃, and the melting point of the wax removed from the crude oil of well A-2 is 51.45℃. The final conformation and motion trajectory at this temperature are shown in Figure 7 and Figure 8 shown.

[0104] DSC experiments were conducted on the wax removal of crude oil from Well A-1 and Well A-2. The obtained DSC curves of wax removal of crude oil are shown in the following table: Fig. 9 and Fig.10 As shown. Fig. 9 and Fig.10 It can be seen that the melting point of the wax removed from the crude oil of well A-1 is 55.5°C, and the melting point of the wax removed from the crude oil of well A-2 is 50.1°C. In addition, the molecular dynamics simulation was performed using the unoptimized original OPLS-AA force field, and the simulation results are shown in Table 5:

[0105] Table 5 Comparison of molecular dynamics simulation results before and after force field optimization

[0106]

[0107] It can be seen from Table 5 that the present invention can accurately calculate the melting point of wax removed from crude oil.

[0108] In summary, the present invention only needs to test the component data of crude oil wax removal, and can use it to build a model and calculate the melting point of the crude oil wax removal. When using the present invention, it is also possible to simulate and calculate the melting point of crude oil wax removal under higher pressure conditions, providing theoretical guidance for crude oil wax deposition research. Compared with the prior art, the present invention has significant progress.

[0109] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention without departing from the content of the technical solution of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation, characterized in that: The following steps are involved: S1: obtaining a target crude oil sample, separating crude oil dewax from the target crude oil sample, and measuring real component information of the crude oil dewax; S2: establishing a molecular model of each single-component normal alkane in the crude oil dewax according to the real component information, and mixing the molecular models of each single-component normal alkane according to the real component information to obtain a real component crude oil dewax molecular model; S3: Establishing the molecular force field of each single-component normal alkane, and optimizing the potential energy parameters of the dihedral angle torsion term and the atomic charge in the molecular force field to obtain the optimized molecular force field parameters; specifically comprising the following sub-steps: S31: Establish the OPLS-AA force field of each single-component n-alkanes, and unify and merge the OPLS-AA force field parameters of each single-component n-alkanes; S32: selecting one of the n-alkane containing at least four methylene groups, and performing a flexible scanning on the dihedral angle of the n-alkane using the quantum chemical M06-2X / def2-TZVP method to obtain a potential energy curve of the dihedral angle; S33: Remove the 1-4 non-bonded interactions in the dihedral angle potential energy and calculate the potential energy curves of the CT-CT-CT-CT, HC-CT-CT-HC and CT-CT-CT-HC dihedral angle torsion terms in n-alkanes respectively; S34: The potential energy curves of the dihedral angle torsion terms are fitted by the following formula to obtain the dihedral angle parameters of each type of dihedral angle in n-alkanes: （1） Where: is the potential energy generated by the torsion of the intramolecular dihedral angle; is a constant; is the dihedral angle; S35: determining a potential energy calculation formula for dihedral angle torsion according to the dihedral angle parameter, and calculating the potential energy generated by intramolecular dihedral angle torsion of other normal alkanes according to the potential energy calculation formula for dihedral angle torsion; S36: Keep the net charge of all single-component n-alkane molecules to zero, and adjust the charge of hydrogen atoms in the force field of some single-component n-alkane molecules until the simulated melting point of the single-component n-alkane molecules is consistent with the experimental melting point; S37: updating the OPLS-AA force field parameters according to the potential energy generated by the intramolecular dihedral angle torsion obtained in step S35 and the hydrogen atom charge obtained in step S36 to obtain the optimized P-OPLS force field parameters; S4: performing molecular dynamics simulation at different temperatures according to the wax molecular model of the real component crude oil and the optimized molecular force field parameters; S5: Determine the melting point of the wax removed from the crude oil according to the results of molecular dynamics simulation at different temperatures.

2. The method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation according to claim 1, characterized in that: In step S1, the real component information is obtained through chromatograph testing.

3. The method for calculating the melting point of crude oil based on molecular dynamics simulation according to claim 1, characterized in that: Step S2 specifically includes the following sub-steps: S21: Determine the components contained in the wax removed from the crude oil and the mass composition of each component according to the real component information; S22: Calculate the molar composition of each component according to the mass composition of each component; S23: establishing a molecular model in which the total number of normal alkanes in each component is greater than 1000 according to the molar composition of each component; S24: The molecular models of the single-component normal alkanes are mixed according to the molar composition ratio of each component to obtain the real component crude oil dewaxing molecular model.

4. The method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation according to claim 3, characterized in that: Step S23 also includes the step of establishing a normal alkane crystal model of one of the components. When mixing is performed in step S24, the normal alkane crystal model is mixed with the molecular models of the remaining single-component normal alkanes.

5. The method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation according to claim 1, characterized in that: In step S32, when performing flexible scanning, scanning is performed every 2°, and 180 steps are scanned.

6. The method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation according to claim 1, characterized in that: In step S33, during the calculation process, the atomic charge of hydrogen atoms is 0.06e, the charge of methylene carbon atoms is -0.12e, and the charge of methyl carbon atoms is -0.18e.

7. The method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation according to claim 1, characterized in that: In step S4, when performing molecular dynamics simulation, the conjugate gradient method and the steepest descent method are used together to minimize energy, the NPT ensemble is used, the LINCS algorithm is used to constrain the bonds connected to the hydrogen atoms, the simulation step size is 2 fs, the PME algorithm is used to calculate the electrostatic interaction, the cut-off algorithm is used for the van der Waals interaction, the cutoff distances for the van der Waals interaction and the electrostatic interaction are both set to 1.4 nm, the V-rescale method is used for temperature control, the C-rescale method is used for pressure control, and the simulation pressure remains unchanged during the simulation process.

8. The method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation according to claim 7, characterized in that: When the molecular model of the wax removed from the real component crude oil contains a crystal model, during the molecular dynamics simulation, the crystal model is frozen in the first stage and the simulation calculation is performed for 10 ns. In the second stage, the freezing is cancelled and the simulation calculation is performed for 150 ns.

9. The method for calculating the melting point of wax removed from crude oil based on molecular dynamics simulation according to claim 1, characterized in that: In step S5, the melting point of the wax removed from the crude oil is determined based on the density difference between the wax liquid phase and the wax solid phase combined with the molecular model structure.

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