Method for multi-scale characterization of wax-bearing pipeline crude oil crystallization nucleation characteristic influence

Abstract

The invention discloses a method for multi-scale characterization of wax-bearing pipeline crude oil crystallization nucleation characteristic influence, which comprises the following steps: obtaining characteristic component parameters of wax-bearing crude oil liquid alkane and wax crystals, and determining basic physical property parameters of wax-bearing crude oil molecule simulation; determining molecular models of different basic physical property parameters, performing coarse graining processing according to topological structures of different molecules, and calculating interaction parameters among particles; and verifying the interaction parameters among the particles, calculating density and solubility parameters, comparing the density and solubility parameters with experimental data, determining a simulation method and verifying the accuracy of force field parameters. Through the microcosmic action mechanism of molecules, the formation process of the three-dimensional network structure formed by paraffin crystals is accurately described under the mesoscale, and technical support is provided for improving the gelling characteristic and modification mode of wax-containing crude oil.

Description

Technical Field

[0001] The present invention relates to the technical field of structural and thermophysical property parameter analysis of waxy crude oil transported in micro-nano scale pipelines under different transport temperature and pressure conditions, and in particular to a method for characterizing the influence of multi-scale crystallization nucleation characteristics of waxy crude oil transported in pipelines. Background Art

[0002] At present, oil resources still occupy a dominant position in the energy structure, and the efficient development and utilization of oil and gas resources are crucial to national energy security. With the deepening of crude oil exploitation, crude oil flooding technology has gradually developed from water flooding to polymer flooding, the difficulty of crude oil exploitation has increased, and the crude oil recovery rate has decreased. In addition, 80% of the current crude oil produced is waxy crude oil. During the crude oil gathering and transportation process, when the temperature is lower than the wax precipitation point, the paraffin molecules in the crude oil will gradually precipitate and crystallize. The paraffin molecules form an insoluble network structure through microscopic interactions, and will further provide nucleation sites for the subsequent formation of paraffin crystal nuclei, gradually attached to the crude oil pipe wall, resulting in an increase in the freezing point of crude oil, an increase in viscosity, and a deterioration in the overall fluidity of crude oil, which brings huge challenges to the transportation and maintenance of oil pipelines.

[0003] According to the pipeline transportation standard SY / T 5536-2016, the transportation temperature of pipeline crude oil must be 3°C to 5°C higher than the freezing point of waxy crude oil. In order to ensure the fluidity of the transportation process, the main method used in the existing technology is to establish heating stations in sections, but this method consumes a lot of energy and has low efficiency.

[0004] With the development of computers and the advancement of characterization technology, some scholars have used experimental methods to analyze the phase change evolution process of crude oil near the wax precipitation point, and used scanning electron microscope (SEM) and thermogravimetric analyzer (FTR) and other experimental equipment to characterize and analyze the crystallization process of crude oil. However, the three-dimensional network structure of wax crystals is only analyzed from a qualitative perspective, and it is impossible to explore the mechanism of wax crystal nucleation from the microscopic interaction law of molecules (atoms). Therefore, the mechanism of paraffin crystallization nucleation in waxy crude oil is revealed from the micro-mesoscopic level, and the thermophysical properties and surface characteristics of the fluid-solid coupling interface are summarized, which proposes a new research idea for improving the rheological properties of waxy crude oil.

[0005] The current research methods of microscopic action mechanisms can be divided into the following three types based on scale: (1) Macroscale: numerical simulation (CFD) method. This method divides structured (unstructured) grids, solves fluid mechanics equations, and obtains numerical results of specific flow fields. (2) Mesoscale: dissipative particle dynamics (DPD), which optimizes the interaction forces between particles by coarsening molecular groups to achieve large-scale and high-precision simulation. (3) Microscale: molecular dynamics (MD), which describes the relevant physical properties of fluids by establishing atomic interaction forces. Among the above three methods, the macroscale CFD method can accurately simulate the temperature and mechanical properties of complex fluids, but cannot reveal the intermolecular action mechanism. Although the microscale MD method can accurately describe the microscopic action mechanism, facing the waxy petroleum hydrocarbon compound model with high molecular weight, this method does not have a certain degree of consistency and completeness for the overall description of the fluid, and the calculation efficiency is low. Therefore, the mesoscale DPD method is between the above two scales. It simplifies the topological structure of molecules in a certain way and can maintain the accuracy of intermolecular interaction forces. It can be used to explore the crystallization nucleation problem of waxy crude oil. Summary of the invention

[0006] In order to solve the above technical problems, the present invention proposes a method for multi-scale characterization of the influence of waxy pipeline crude oil on crystallization nucleation characteristics. Based on the dissipative particle dynamics method, a mesoscopic scale model of pipeline waxy crude oil is established. The wetting characteristics and thermal conductivity of waxy crude oil at interfaces with different roughness are analyzed by the dynamics open source software LAMMPS, and the factors affecting the crystallization nucleation of paraffin molecules are quantitatively analyzed.

[0007] On the one hand, to achieve the above-mentioned purpose, the present invention provides a method for characterizing the influence of multi-scale on the crystallization nucleation characteristics of waxy crude oil in pipeline, comprising:

[0008] Obtain the characteristic component parameters of liquid alkanes and wax crystals in waxy crude oil, and determine the basic physical property parameters of molecular simulation of waxy crude oil;

[0009] Determine the molecular models of different basic physical property parameters, perform coarse-graining processing according to the topological structures of different molecules, and calculate the interaction parameters between particles;

[0010] Based on the interaction parameters between the particles, a dynamic simulation is performed to calculate the change in the specific heat capacity of the simulation system. The influence of the specific heat capacity of the simulation system on the activation energy and the critical nucleation conditions in the wax crystal nucleation process at different times and under different wall conditions is determined through the change in the specific heat capacity. Through the basic concept of critical nucleation pressure, the influence of wettability and specific heat capacity on the critical nucleation pressure of wax crystals is obtained.

[0011] Preferably, the basic physical property parameters of the waxy crude oil molecular simulation include: transportation pressure, viscosity and density of pipeline waxy crude oil.

[0012] Preferably, the coarse graining process comprises:

[0013] The molecular structure was calculated using the microscopic molecular dynamics method, and the molecular cohesive energy was calculated using the Monte Carlo method to obtain the solubility parameters of the components at different temperatures. Finally, the interparticle Flory-Huggins constant was calculated based on the solubility parameters to obtain the interparticle interaction parameters.

[0014] Preferably, the method for calculating the molecular cohesive energy is:

[0015] E coh =-(E inter )= <E intra >- <E tatal >

[0016] In the formula, E inter is the total energy of all molecules, E total is the total energy of the system, E intra is the intramolecular energy, <> is the average value of the ensemble in which it is located, E coh is the molecular cohesive energy.

[0017] Preferably, the method for obtaining the solubility parameters of the components at different temperatures is:

[0018]

[0019] Where CED is the cohesive energy density, V i 0 represents the volume of molecule i, δ ij is the solubility parameter;

[0020] Among them, for any two substances mixed with each other, at temperature T, the solubility parameters calculated are δ i (T) and δ j (T), according to the solubility parameters, the volume ratio of the two substances is obtained. When , the energy change relationship expression before and after mixing is:

[0021]

[0022] in, represents the volume fraction of the mixed component, and v represents the partial molar volume of the mixed system.

[0023] Preferably, the interparticle Flory-Huggins constant is calculated based on the solubility parameter, comprising:

[0024] Calculate the Flory-Huggins constant from the mixing energy:

[0025]

[0026] In the formula, χ ij is the Flory-Huggins constant, k B is the Boltzmann constant, is the mixing energy, T is the temperature of the system, and R is the gas constant.

[0027] Preferably, calculating the interaction parameters between the particles comprises:

[0028] The interaction parameters between particles are calculated according to the Flory-Huggins constant:

[0029] a ij =a ii +b ij

[0030] In the formula, a ij is the interaction parameter between particles, a ii is the interaction parameter between the same particles, and the value of b is determined according to the dimensionless number density of the selected DPD.

[0031] Preferably, the method further comprises verifying the interaction parameters between the particles, calculating the density and solubility parameters, and comparing them with the experimental data, determining the simulation method and verifying the accuracy of the force field parameters.

[0032] On the other hand, to achieve the above-mentioned purpose, the present invention also provides a simulation system based on a method for characterizing the influence of multi-scale on the crystallization nucleation characteristics of waxy crude oil transported in pipelines, comprising:

[0033] Model building module: used to build a bulk model and a wall model of pipeline-transported waxy crude oil, wherein the wall model includes a rough wall model, a smooth wall model, a graphene wall model and a hydroxylated wall model;

[0034] Interaction parameter setting module: used to assign the obtained particle interaction parameters to the established model; among them, DPD interaction includes bond interaction and non-bonded interaction. Non-bonded interaction uses Lennard-Jones potential energy to describe the interaction force between fluid beads and solid wall. Bond interaction includes angular interaction and bond energy interaction.

[0035] Dynamic simulation module: used to perform dynamic simulation by setting temperature, pressure conditions, simulation system, simulation time step and total simulation time. The initial velocity of the simulation adopts Gaussian random distribution, and the linear momentum and angular momentum between particles are set to zero.

[0036] Preferably, the method for calculating the bonding interaction is:

[0037]

[0038] In the formula, E total is the bonding interaction, E bond is the bond energy interaction, E angle is the angular interaction, is the bond energy expansion coefficient, r ij is the actual distance between atom i and atom j, is the theoretical distance between atom i and atom j, is the bond angle bending force coefficient, θ ijk is the theoretical bond angle value, is the actual bond angle value.

[0039] Compared with the prior art, the present invention has the following advantages and technical effects:

[0040] (1) The present invention combines macroscopic experimental feature calibration and characterization, mesoscopic dissipative ion dynamics simulation and microscopic molecular dynamics methods to explore the interaction between the fluid-solid interface of complex crude oil systems through a cross-scale research method, providing a new theoretical basis for exploring the crystallization nucleation problem of waxy crude oil.

[0041] (2) Based on the microscopic action mechanism of molecules, the present invention combines the molecular dynamics method and the dissipative particle dynamics method to accurately describe the formation process of the three-dimensional network structure formed by paraffin crystals at the micro-mesoscopic scale, providing technical support for improving the gelling properties and modification methods of waxy crude oil.

[0042] (3) The present invention calculates the specific heat capacity of the waxy crude oil system from the perspective of thermodynamic analysis. The calculation result of the specific heat capacity can further reflect the change of the system energy, thereby qualitatively describing the influence of the wax deposition and crystallization process on the heat absorption and release process of the system under different pipe wall types, thereby providing new ideas for the internal dewaxing process and interface treatment method of the pipeline crude oil wall.

[0043] (4) The present invention combines the nucleation kinetics theory and provides the concept of critical nucleation pressure for the first time. By calculating the critical nucleation pressure at different temperatures, a critical value range is provided for the pipeline pressure during the transportation of waxy crude oil, providing parameter support for the safe transportation of crude oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0045] Figure 1 This is a flow chart of a method for characterizing the influence of multi-scale on the crystallization nucleation characteristics of waxy pipeline crude oil according to an embodiment of the present invention;

[0046] Figure 2 It is a schematic diagram of the force field parameter verification result of an embodiment of the present invention;

[0047] Figure 3 Schematic diagrams of four different wall models and a bulk model of pipeline-transported waxy crude oil according to an embodiment of the present invention;

[0048] Figure 4 Schematic diagram of the processing process of the power simulation module of an embodiment of the present invention; wherein (a) is a schematic diagram of the distribution of heat sources and heat sinks in the simulation system, (b) is a schematic diagram of the energy balance verification of the system, and (c) is a schematic diagram of the temperature distribution along the heat conduction direction;

[0049] Figure 5 A schematic diagram of the temperature gradient of a system according to an embodiment of the present invention;

[0050] Figure 6 Schematic diagram of thermal conductivity calculation results under four different wall types of embodiments of the present invention, wherein (a) is a smooth wall, (b) is a rough wall, (c) is a hydroxylated wall, and (d) is a graphene wall;

[0051] Figure 7 It is a bar graph of average thermal conductivity distribution of different wall surfaces according to an embodiment of the present invention. DETAILED DESCRIPTION

[0052] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0053] It should be noted that the steps shown in the flowcharts of the accompanying drawings can be executed in a computer system such as a set of computer executable instructions, and that, although a logical order is shown in the flowcharts, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0054] The present invention proposes a method for characterizing the influence of waxy crude oil crystallization nucleation characteristics at multiple scales, comprising:

[0055] Obtain the characteristic component parameters of liquid alkanes and wax crystals in waxy crude oil, and determine the basic physical property parameters of molecular simulation of waxy crude oil;

[0056] Determine the molecular models of different basic physical property parameters, perform coarse-graining processing according to the topological structures of different molecules, and calculate the interaction parameters between particles;

[0057] The interaction parameters between the particles are verified, the density and solubility parameters are calculated and compared with the experimental data, the simulation method is determined and the accuracy of the force field parameters is verified.

[0058] This example combines macroscopic experimental feature calibration and characterization, mesoscopic dissipative ion dynamics simulation and microscopic molecular dynamics methods, and explores the interaction between the fluid-solid interface of complex crude oil systems through a cross-scale research method, providing a new theoretical basis for exploring the crystallization nucleation problem of waxy crude oil.

[0059] Furthermore, the basic physical property parameters of the molecular simulation of waxy crude oil include: transportation pressure, viscosity and density of pipeline waxy crude oil.

[0060] Further, the coarse graining process is performed, including:

[0061] The molecular structure was calculated using the microscopic molecular dynamics method, and the molecular cohesive energy was calculated using the Monte Carlo method to obtain the solubility parameters of the components at different temperatures. Finally, the interparticle Flory-Huggins constant was calculated based on the solubility parameters to obtain the interparticle interaction parameters.

[0062] Specifically, the method for calculating the molecular cohesive energy is:

[0063] E coh =-(E inter )= <E intra >- <E tatal >

[0064] In the formula, E inter is the total energy of all molecules, E total is the total energy of the system, E intra is the intramolecular energy, <> is the average value of the ensemble in which it is located, E coh is the molecular cohesive energy.

[0065] The method to obtain the solubility parameters of components at different temperatures is:

[0066]

[0067] Where CED is the cohesive energy density, V i 0 represents the volume of molecule i, δ ij is the solubility parameter.

[0068] The interparticle Flory-Huggins constant is calculated based on the solubility parameters, including:

[0069] Calculate the Flory-Huggins constant from the mixing energy:

[0070]

[0071] In the formula, χ ij is the Flory-Huggins constant, are the volume constants of mixed gases A and B, respectively, and k B is the Boltzmann constant, is the mixing energy, T is the temperature of the system, and R is the gas constant;

[0072] Furthermore, the interaction parameters between particles are calculated, including:

[0073] The interaction parameters between particles are calculated according to the Flory-Huggins constant:

[0074] a ij =a ii +b ij

[0075] In the formula, a ij is the interaction parameter between particles, a ii is the interaction parameter between the same particles, and the value of b is determined according to the dimensionless number density of the selected DPD.

[0076] This embodiment also provides a simulation system based on a method for characterizing the influence of the crystallization nucleation characteristics of waxy crude oil in pipelines on a multi-scale basis, comprising:

[0077] Model building module: used to build a bulk model and a wall model of pipeline-transported waxy crude oil, wherein the wall model includes a rough wall model, a smooth wall model, a graphene wall model and a hydroxylated wall model;

[0078] Interaction parameter setting module: used to assign the obtained particle interaction parameters to the established model; among them, DPD interaction includes bond interaction and non-bonded interaction. Non-bonded interaction uses Lennard-Jones potential energy to describe the interaction force between fluid beads and solid wall. Bond interaction includes angular interaction and bond energy interaction.

[0079] Dynamic simulation module: used to perform dynamic simulation by setting temperature, pressure conditions, simulation system, simulation time step and total simulation time. The initial velocity of the simulation adopts Gaussian random distribution, and the linear momentum and angular momentum between particles are set to zero.

[0080] Furthermore, the method for calculating the bond interactions is:

[0081]

[0082] In the formula, E totalis the bonding interaction, E bond is the bond energy interaction, E angle is the angular interaction, is the bond energy expansion coefficient, r ij is the actual distance between atom i and atom j, is the theoretical distance between atom i and atom j, is the bond angle bending force coefficient, θ ijk is the theoretical bond angle value, is the actual bond angle value.

[0083] In order to more clearly express the technical solution of the present invention, the following specific embodiments are provided to introduce the solution:

[0084] A multi-scale method to characterize the influence of waxy pipeline crude oil crystallization nucleation characteristics, such as Figure 1 This method aims to solve the problem of waxy crude oil crystallizing during transportation, which causes blockage of crude oil transportation network.

[0085] Specifically include:

[0086] Step 1: Through experiments, the specific component information of the actual waxy crude oil in a certain oil field in Daqing, China was determined, and its main components were used as the waxy crude oil composition of the simulation system. Through the determination, the following components were selected as the simulated crude oil component information, as shown in Table 1:

[0087] Table 1

[0088]

[0089] Step 2: Coarse-graining of molecular models. The DPD simulation process is used to rearrange the topological structure of atoms. In this embodiment, the microscopic molecular dynamics method (MD) is used to accurately calculate the molecular structure, and the Monte Carlo method (MC) is used to calculate the molecular cohesive energy (E coh ) to accurately calculate the solubility parameters δ of the components at different temperatures ij (T), and finally based on δ ij (T) Calculate the interparticle Flory-Huggins (FH) constant χ ij , and obtain the interaction parameters between particles.

[0090] E coh =-(E inter )= <E intra >- <E tatal >

[0091] In the formula, E inter is the total energy of all molecules, E total is the total energy of the system, E intrais the intramolecular energy, <> is the average value of the ensemble in which it is located, E coh is the molecular cohesive energy.

[0092] Intramolecular cohesive energy E coh and enthalpy of vaporization ΔH vap Closely related, it represents the heat absorbed when a substance changes from liquid to gas, and cohesive energy is the part of energy in the heat composition.

[0093] E coh =ΔH vap -Δ(pV)=ΔH vap -RT

[0094] Where p is the system pressure, V is the system volume, R is the gas constant, and T is the system temperature.

[0095] The cohesive energy of a unit system is the cohesive energy density CED, which is expressed as:

[0096]

[0097] The unit of cohesive energy is the same as that of pressure, expressed as MPa (or kcal / cm 3 ), V i 0 represents the volume of molecule i

[0098] Solubility parameter δ ij The expression of (T) is:

[0099]

[0100] In this example, for two substances, the solubility parameters are calculated to be δ i (T) and δ j (T), according to the solubility parameter, we can get the volume ratio of the two substances Hildebrand and Scatchard proposed the expression of energy change before and after mixing:

[0101]

[0102] in, Indicates the volume fraction of mixed component i, in cm 3 / mol,v b It represents the partial molar volume (also called average molar volume) of a mixed system, in units of

[0103] The FH constant χ can be obtained based on the mixing energy ij :

[0104]

[0105] In the formula, χ ij is the Flory-Huggins constant, is the volume fraction of the mixed component, k B is the Boltzmann constant, is the mixing energy, T is the temperature of the system, and R is the gas constant;

[0106] The FH constant χ is obtained by iteration ij :

[0107]

[0108] Among them, k B represents the Boltzmann constant, and T represents the temperature of the system.

[0109] Calculate the interaction parameter a of the conservative force based on the FH parameter ij :

[0110] a ij =a ii +b ij

[0111] In the formula, a ij is the interaction parameter between particles, a ii is the interaction parameter between the same particles, and the value of b is determined according to the dimensionless number density of the selected DPD.

[0112] In addition, the molar volume calculation method of different molecules:

[0113]

[0114] Among them, M w It represents the molecular molar volume in g / mol, ρ r Indicates the density of the molecule in g / cm 3 .

[0115] In this embodiment, the interaction parameter a between the same particles is ii is 25, where the value of b is determined by the dimensionless number density ρ of the selected DPD. r Determine. r When =3 or 5, b is 3.5 or 1.45.

[0116] Step 3: Force field parameter verification. Set the simulation system temperature to 298.15K and the solubility δ ij The maximum relative error between the calculated result of (T) and the simulated result is 5.13%. This indicates that the solubility δ calculated by the MD methodij (T) is in good agreement with the experimental results. Figure 2 .

[0117] A simulation system based on a method for multi-scale characterization of the influence of waxy pipeline crude oil crystallization nucleation characteristics. In this embodiment, the simulation environment is based on the open source software (Large-Scale Atomic / Molecular Massively Parallel Simulator) LAMMPS. The visualization software is implemented by OVITO. The simulation results are obtained by Python programming analysis. The specific details of the simulation are as follows:

[0118] Model building module: used to build the bulk model and wall model of pipeline waxy crude oil, wherein the wall model includes a rough wall model, a smooth wall model, a graphene wall model and a hydroxylated wall model, such as Figure 3 shown.

[0119] Interaction parameter setting module: used to assign the obtained particle interaction parameters to the established model; among them, DPD interaction includes bond interaction and non-bonded interaction. Non-bonded interaction uses Lennard-Jones potential energy to describe the interaction force between fluid beads and solid wall. Bond interaction includes angular interaction and bond energy interaction.

[0120] Dynamic simulation module: set different temperatures and pressures; in this embodiment, the simulation system selects NVT, the simulation time step is 0.01 (DPD dimensionless parameter), the total simulation time is 10ns, the initial velocity of the simulation adopts Gaussian random distribution, and the linear momentum and angular momentum between particles are set to zero.

[0121] The calculation of specific heat is mainly based on the non-equilibrium molecular dynamics (NEMD) method, and its principle is mainly based on Fourier's theorem to calculate the thermal conductivity of a finite length sample. This embodiment mainly uses non-equilibrium heat flow to calculate thermal conductivity. This method can be applied to a non-periodic simulation system with a fixed boundary. By setting a cold source and a heat sink in the simulation system, the system is processed in blocks along the Z direction. After the system reaches stability, the average temperature is calculated by the average kinetic energy of the molecules to obtain the temperature gradient of the system. The thermal conductivity of a finite system is calculated according to the temperature gradient and Fourier's law.

[0122] Heat flux: Q ext =dE / dt

[0123] Among them, Q ext is the given heat flux, dE is the input energy, and dt is the time interval.

[0124] Finite volume thermal conductivity is calculated from Fourier's law:

[0125] Where S is the cross-sectional area, Q ext is the non-equilibrium heat flux, ▽T is the temperature gradient.

[0126] In this embodiment, when calculating the thermal conductivity of the waxy crude oil system, due to the non-periodic boundary conditions, the lower fixed wall model is selected as the heat source part, along the system heat transport direction (Z direction) ( Figure 4 (a) shows that the simulation time is 50ns, the simulation step is 2fs, and the average heat flux of the system is taken every 1ns. The calculation results are shown in Figure 4 As shown in (b), it can be seen that at this time, the sum of the heat flowing into the system and the heat flowing out is basically balanced with the heat provided by the outside and the total energy of the system, indicating that the system is in a steady state and energy is conserved. At this time, the average temperature of the simulated system along the transport direction is output as follows: Figure 4 As shown in (c), the temperature of the wax crystal adsorption layer on the tube wall was calculated respectively. The simulation average time was 1ns, 2ns, and 3ns respectively, and their mean was calculated. The slope was calculated to be the temperature gradient of the system at this time (such as Figure 5 ). From this, the thermal conductivity of the system can be calculated as Figure 6 (a)-(d) show the change of thermal conductivity of the system under different wall types. Figure 7 The average thermal conductivity distribution bar graphs of different wall surfaces were statistically calculated. It can be found that κ ss <κ HS <κ HS <κ Rs It was found that changing the wall roughness, coating with graphene and hydroxylating the wall all improved the thermal conductivity of the wall to a certain extent. The heat transport capacity of the wall also directly determines the temperature of the waxy crude oil, thus affecting the formation of paraffin crystals.

[0127] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A method for characterizing the influence of multi-scale on the crystallization nucleation characteristics of waxy crude oil in pipelines, characterized in that: include: Obtain the characteristic component parameters of liquid alkanes and wax crystals in waxy crude oil, and determine the basic physical property parameters of molecular simulation of waxy crude oil; Determine the molecular models of different basic physical property parameters, perform coarse-graining processing according to the topological structures of different molecules, and calculate the interaction parameters between particles; Based on the interaction parameters between the particles, a dynamic simulation is performed to calculate the change in the specific heat capacity of the simulation system. The influence of the specific heat capacity of the simulation system on the activation energy and the critical nucleation conditions in the wax crystal nucleation process at different times and under different wall conditions is determined through the change in the specific heat capacity. Through the basic concept of critical nucleation pressure, the influence of wettability and specific heat capacity on the critical nucleation pressure of wax crystals is obtained.

2. The method for multi-scale characterization of the influence of waxy pipeline crude oil crystallization nucleation characteristics according to claim 1, characterized in that: The basic physical property parameters of the waxy crude oil molecular simulation include: transportation pressure, viscosity and density of pipeline-transported waxy crude oil.

3. The method for multi-scale characterization of the influence of waxy pipeline crude oil crystallization nucleation characteristics according to claim 1, characterized in that: The coarse graining process comprises: The molecular structure was calculated using the microscopic molecular dynamics method, and the molecular cohesive energy was calculated using the Monte Carlo method to obtain the solubility parameters of the components at different temperatures. Finally, the interparticle Flory-Huggins constant was calculated based on the solubility parameters to obtain the interparticle interaction parameters.

4. The method for multi-scale characterization of the influence of waxy pipeline crude oil crystallization nucleation characteristics according to claim 3, characterized in that: The method for calculating the molecular cohesive energy is: AND coh =-(And int er )= <E int ra >- <E tatal > In the formula, E inter is the total energy of all molecules, E total is the total energy of the system, E intra is the intramolecular energy, <> is the average value of the ensemble in which it is located, E coh is the molecular cohesive energy.

5. The method for multi-scale characterization of the influence of waxy pipeline crude oil crystallization nucleation characteristics according to claim 4, characterized in that: The method for obtaining the solubility parameters of the components at different temperatures is: Where CED is the cohesive energy density, V i 0 represents the volume of molecule i, δ ij is the solubility parameter; Among them, for any two substances mixed with each other, at temperature T, the solubility parameters calculated are δ i (T) and δ j (T), according to the solubility parameters, the volume ratio of the two substances is obtained. When , the energy change relationship expression before and after mixing is: In the formula, represents the volume fraction of the mixed component, and v represents the partial molar volume of the mixed system.

6. The method for multi-scale characterization of the influence of waxy pipeline crude oil crystallization nucleation characteristics according to claim 5, characterized in that: The interparticle Flory-Huggins constant is calculated based on the solubility parameters, including: Calculate the Flory-Huggins constant from the mixing energy: In the formula, χ ij is the Flory-Huggins constant, k B is the Boltzmann constant, is the mixing energy, T is the temperature of the system, and R is the gas constant.

7. The method for multi-scale characterization of the influence of waxy pipeline crude oil crystallization nucleation characteristics according to claim 6, characterized in that: Calculating the interaction parameters between the particles, including: The interaction parameters between particles are calculated according to the Flory-Huggins constant: to ij =a ii +bχ ij In the formula, a ij is the interaction parameter between particles, a ii is the interaction parameter between the same particles, and the value of b is determined according to the dimensionless number density of the selected DPD.

8. The method for multi-scale characterization of the influence of waxy pipeline crude oil crystallization nucleation characteristics according to claim 1, characterized in that: The method also includes verifying the interaction parameters between the particles, calculating density and solubility parameters, and comparing them with experimental data to determine the simulation method and verify the accuracy of force field parameters.

9. A simulation system based on a multi-scale characterization method for the influence of waxy crude oil crystallization nucleation characteristics, characterized in that: include: Model building module: used to build a bulk model and a wall model of pipeline-transported waxy crude oil, wherein the wall model includes a rough wall model, a smooth wall model, a graphene wall model and a hydroxylated wall model; Interaction parameter setting module: used to assign the obtained particle interaction parameters to the established model; among them, DPD interaction includes bond interaction and non-bonded interaction. Non-bonded interaction uses Lennard-Jones potential energy to describe the interaction force between fluid beads and solid wall. Bond interaction includes angular interaction and bond energy interaction. Dynamic simulation module: used to perform dynamic simulation by setting temperature, pressure conditions, simulation system, simulation time step and total simulation time. The initial velocity of the simulation adopts Gaussian random distribution, and the linear momentum and angular momentum between particles are set to zero.

10. The simulation system according to claim 9, characterized in that: The method for calculating the bonding interactions is: In the formula, E total is the bonding interaction, E bond is the bond energy interaction, E angle is the angular interaction, is the bond energy expansion coefficient, r ij is the actual distance between atom i and atom j, is the theoretical distance between atom i and atom j, is the bond angle bending force coefficient, θ ijk is the theoretical bond angle value, is the actual bond angle value.

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