A numerical simulation method for diffusion of buried natural gas pipeline leakage to the ground environment

Abstract

The application discloses a numerical simulation method for diffusion of buried natural gas pipeline leakage to the ground environment, comprising the following steps: step 1, determining the leakage working condition and designing the physical model during natural gas leakage; step 2, establishing the geometric model of the diffusion area of the buried natural gas pipeline leakage and dividing the grid; step 3, establishing the influence relationship of the soil moisture content on the diffusion of natural gas in the soil; step 4, establishing the buried natural gas pipeline leakage model and solving; step 5, analyzing the diffusion characteristics of natural gas under different working conditions; the application considers the relationship between the soil moisture content and the soil viscosity resistance coefficient, the soil viscosity resistance coefficient influences the diffusion of natural gas in the soil, the soil moisture content is directly related to the diffusion of natural gas in the soil through the relationship, and the influence of the change of the soil moisture content on the diffusion law of natural gas in the soil is obtained. The change of the soil moisture content factor makes the simulation result more in line with the actual situation.

Description

Technical Field

[0001] This invention relates to the field of oil and gas safety technology, specifically a numerical simulation method for the diffusion of buried natural gas pipeline leaks to the surface environment. Background Technology

[0002] With rapid urbanization, buried natural gas pipelines, as core infrastructure for urban energy supply, have formed a comprehensive underground network system. Under high-pressure transmission conditions, this network is susceptible to leaks due to factors such as geological subsidence, corrosion, or human-caused damage. Buried gas pipelines, due to their harsh and concealed environments, are prone to leaks that are difficult to detect, and the probability of explosions is even higher. Some developed countries have begun research on transporting hydrogen through gas pipelines; however, hydrogen has a wider explosive limit range and greater destructive potential in the event of a leak. Leaked natural gas diffuses through the soil to the surface, and when the methane concentration reaches its explosive limit, it creates a significant risk of deflagration. Preventing any gas pipeline accidents and avoiding the resulting casualties, property damage, and gas supply interruptions, ensuring the stable and safe operation of the gas pipeline network, is a goal pursued by all gas companies and even nations. Therefore, real-time monitoring of natural gas pipeline leaks and analysis of methane diffusion patterns in the soil and surface environment are crucial for establishing an accident early warning system, especially important for the safe operation and maintenance of high-pressure natural gas pipelines in densely populated areas. Therefore, establishing an accurate leak diffusion prediction model can provide a scientific basis for risk management in the safe operation of urban gas systems.

[0003] Currently, buried natural gas transmission technology is basically mature. However, when buried natural gas pipelines leak at different locations or under different weather conditions, these factors affect soil conditions and the surface environment, thus altering the diffusion of leaked natural gas in the soil. Therefore, it is necessary to analyze different soil conditions. Changes in soil moisture content affect the diffusion of natural gas in the soil during simulation. In simulating natural gas diffusion in soil, porous media fluids are used to simulate the soil region. Changes in the viscous drag coefficient of the porous media fluid correspond to changes in soil moisture content. Therefore, by establishing the relationship between the soil viscous drag coefficient and moisture content, the viscous drag coefficient can be changed during the simulation to simulate soils with different moisture contents. Simultaneously, changes in the air domain, such as the presence of strong winds, can be achieved by setting the air domain velocity inlet to alter the air domain environment and simulate different weather conditions.

[0004] Patent CN 107590336 B describes a numerical simulation method for the impact of gas pipeline leakage on the internal flow field. This invention uses Fluent software to analyze the changes in the internal flow field of the pipeline before and after a gas pipeline leak, obtaining the changes in the internal flow field of different types of pipelines under different influencing factors. However, this method only obtains the flow changes inside the gas pipeline and does not analyze the area outside the leak point; it simply sets up an outlet to influence the transformation of the internal flow field.

[0005] Patent CN 118569138 A discloses an atmospheric diffusion simulation method for pipeline flange leaks based on Fluent. This invention simulates leaks occurring at the connection flanges of natural gas pipelines, defining the leak points on both sides of the pipeline-flange connection. This invention improves upon the orifice model, using a virtual width to characterize the leak at the flange connection, thus obtaining a leak pattern consistent with reality. However, the external basin is defined only as the ordinary atmospheric environment, without discussing the situation when the pipeline is in soil.

[0006] Patent CN 117933134 A discloses a CFD-based simulation method for the leakage characteristics of high-pressure hydrogen-blended natural gas pipelines. This method divides the leakage of high-pressure hydrogen-blended natural gas pipelines into two processes: a near-field jet stage and a far-field diffusion stage. These two processes are modeled and simulated separately, with the output of the near-field jet model used as the input of the far-field diffusion model to obtain the complete diffusion process of the high-pressure hydrogen-blended natural gas pipeline leakage. However, errors exist in the data transfer process between the two models, which may cause the simulation process to deviate from reality, reducing the accuracy of the simulation results.

[0007] A summary and analysis of the above studies revealed limited research on buried natural gas pipeline leaks. Among these studies, changes in soil moisture content significantly impact natural gas diffusion, directly affecting soil physical properties. This study provides a research method for natural gas leaks in different soil types. The leak area is divided into two parts and incorporated into a single model. Key areas within each region are further subdivided. To ensure the accuracy of the numerical simulation at the leak point during the leak process, the mesh at the leak point is refined. Furthermore, to simulate the leak point's appearance during pipeline operation, the leak point is initially set as a wall surface. The pipeline is first filled with methane, and then the leak point is set as an internal surface, thus reflecting the actual leak process. External factors are then considered, including soil analysis. A model of soil coefficient changes at different moisture contents is constructed, and the results are input into the leak model to reflect real soil conditions. This invention proposes a numerical simulation method for the diffusion of buried natural gas pipeline leaks to the surface environment. The leakage area of ​​the buried natural gas pipeline is divided into soil diffusion and air diffusion. The pipeline inlet interface is set as the natural gas delivery inlet, and part of the boundary of the air domain is set as the pressure outlet. The soil domain is set with parameters such as viscous drag coefficient and inertial drag coefficient, which can achieve an accurate description of the entire leakage field. Summary of the Invention

[0008] The purpose of this invention is to provide a numerical simulation method for the diffusion of buried natural gas pipeline leaks to the surface environment, so as to solve the problems mentioned in the background art.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment, comprising the following steps: Step 1: Determine the leakage conditions and design a physical model for natural gas leakage. Divide the leakage diffusion area of ​​the buried natural gas pipeline into three regions: pipeline region, soil region, and air region. Clarify the diffusion process of natural gas leakage in the soil region and air region, and determine the leakage conditions parameters, including the natural gas pipeline operating pressure, leak size, atmospheric wind speed, soil type, and soil moisture content. Step 2: Establish a geometric model of the buried natural gas pipeline leakage diffusion area and divide it into grids. The geometric model covers the natural gas pipeline, soil and ambient atmosphere. A structured grid division method is used to locally refine the grid in the area near the leakage hole and to verify the grid independence. Step 3: Establish the influence of soil moisture content on the diffusion of natural gas in the soil. Treat the soil as a porous medium, characterize the soil properties through the viscosity resistance coefficient and the inertial resistance coefficient, use the Van Genuchten-Mualem model to describe the relationship between the soil viscosity resistance coefficient and the moisture content, and derive the correlation between soil moisture content and natural gas diffusion. Step 4: Establish and solve the buried natural gas pipeline leakage model. The governing equations of the model include the mass conservation equation, momentum conservation equation, energy conservation equation and component transport equation. The k-ε turbulence model is selected, and the boundary conditions of the pipeline, soil domain and air domain are set. The pressure-velocity coupled solution is performed using a double-precision pressure solver and the SIMPLE algorithm. Step 5: Analyze the diffusion characteristics of natural gas under different operating conditions, calculate the mass fraction of natural gas in soil and air, and determine the dangerous explosion time at different locations after a buried natural gas pipeline leak.

[0010] Preferably, the leakage condition parameters in step 1 are determined as follows: through on-site investigation and engineering data statistics, a typical pipeline operating pressure range of 0.1-10MPa, a leak size of 1-100mm, an atmospheric wind speed of 0-10m / s, soil types including sand, loam, and clay, and a soil moisture content range of 5%-40% are selected.

[0011] Preferably, the process of establishing the geometric model in step 2 is as follows: a three-dimensional geometric model is constructed based on the actual pipeline burial depth, soil layer thickness, and atmospheric environment range. The pipeline burial depth is set to 0.5-3m, the soil domain size is 10-50m extending along the pipeline axis, with a width and height of 5-20m, and the air domain height is 5-30m. The mesh is divided using a hexahedral structure mesh, with the mesh size near the leak hole being 0.01-0.1m and the mesh size in the area away from the leak hole being 0.1-1m.

[0012] Preferably, in step 3: Van Genuchten-Mualem permeability model calculation formula: , ,

[0013] In the formula: Saturated permeability, in units of ; Residual volumetric moisture content, in units of ; This represents the saturated volumetric water content, in units of... ; For Van Genuchten shape parameters; For auxiliary parameters; Effective saturation; This refers to the volumetric water content. Formula for calculating viscous drag coefficient:

[0014] In the formula: The dynamic viscosity of water, in units of , This is the viscous resistance coefficient.

[0015] Preferably, the momentum conservation equation in step 3 takes into account the resistance effect of the porous soil medium, and the source term of the momentum equation... The calculation formula is:

[0016] In the formula: This represents the gas diffusion velocity, measured in m / s. This is the inertial drag coefficient, in units of... ; This refers to the density of a gas, expressed in kg / m³. 3 ; These are parameters related to penetration rate; The momentum conservation equation affected by porosity is:

[0017] In the formula: Porosity; Time, in seconds; Absolute pressure, unit: Pa; This is the second-order stress tensor, with units of Pa. This represents the component of gravitational acceleration, in units of... ; , These are spatial coordinate components.

[0018] Preferably, the boundary conditions in step 4 are set as follows: the natural gas pipeline inlet adopts a pressure inlet boundary, and the inlet pressure is consistent with the pipeline operating pressure; the pipeline outlet adopts a pressure outlet boundary, and the outlet pressure is atmospheric pressure; the soil domain's surrounding boundaries adopt pressure outlet boundaries; velocity inlet boundaries are set on both sides of the air domain to simulate atmospheric wind speed, and the top and corresponding side boundaries of the air domain adopt pressure outlet boundaries; the contact boundaries between the natural gas pipeline leak and the soil and air domains are set as internal surfaces and share a mesh.

[0019] Preferably, the solution parameters in step 4 are set as follows: gravitational acceleration is set to 9.8 m / s². 2 The time step is 10. -3 -10 -1The time step is 1000-100000; the maximum number of iterations is 10-50; in the component transport equation, the natural gas component is mainly methane, and the initial mole fraction is set to 100%.

[0020] Preferably, the method for determining the dangerous explosion time in step 5 is as follows: based on the explosion limit of natural gas, with a volume fraction of 5%-15%, by monitoring the change of methane mass fraction at different locations in the soil and air domains over time, the time when the methane mass fraction at a certain location first reaches the lower or upper limit of the explosion limit is the dangerous explosion time at that location.

[0021] Compared with the prior art, the beneficial effects of the present invention are: (1) The present invention proposes a numerical simulation method for the diffusion of buried natural gas pipeline leakage on the surface environment. This method considers the relationship between soil moisture content and soil viscosity resistance coefficient. The soil viscosity resistance coefficient affects the diffusion of natural gas in the soil. By directly establishing a link between soil moisture content and the diffusion of natural gas in the soil, the influence of soil moisture content changes on the diffusion law of natural gas in the soil is obtained. Incorporating the change of soil moisture content makes the simulation results more consistent with reality.

[0022] (2) The present invention proposes a numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment. It integrates three regions in the leakage diffusion process into a geometric model, which reduces the error of data transmission when simulating separately and improves the accuracy of natural gas diffusion data in the simulation results. Attached Figure Description

[0023] Figure 1 A flowchart illustrating a numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment, provided in an embodiment of the present invention; Figure 2 This is a diagram showing the leakage area distribution in the geometric model domain of the buried natural gas pipeline in this invention example; Figure 3 This is a mesh partitioning diagram of a buried natural gas pipeline leakage model in an example of the present invention; Figure 4 This is a graph showing the relationship between the soil cohesive resistance coefficient and water content in an example of the present invention. Figure 5 This is a graph showing the methane mass fraction in the soil and air domains of a leak model of a buried natural gas pipeline in this invention, as a function of time. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] Please see Figure 1 This invention provides a numerical simulation method for the diffusion of buried natural gas pipeline leaks to the surface environment, comprising the following steps: Step 1: Determine the leakage conditions and design a physical model for natural gas leakage. Divide the leakage diffusion area of ​​the buried natural gas pipeline into three regions: the pipeline domain, the soil domain, and the air domain. Clarify the diffusion process of natural gas leakage in the soil and air domains, and determine the leakage condition parameters. These parameters include the natural gas pipeline operating pressure, leak size, atmospheric wind speed, soil type, and soil moisture content. The leakage condition parameters are determined by: through field surveys and engineering data statistics, selecting typical pipeline operating pressure ranges of 0.1-10 MPa, leak size ranges of 1-100 mm, atmospheric wind speeds of 0-10 m / s, soil types including sand, loam, and clay, and soil moisture content ranges of 5%-40%. Step 2: Establish a geometric model of the buried natural gas pipeline leakage diffusion area and mesh it. The geometric model covers the natural gas pipeline, soil, and ambient atmosphere. A structured meshing method is used to refine the mesh in the area near the leak hole, and mesh independence verification is performed. The geometric model is established as follows: a three-dimensional geometric model is constructed based on the actual pipeline burial depth, soil layer thickness, and atmospheric environment range. The pipeline burial depth is set to 0.5-3m, the soil domain size is 10-50m extending along the pipeline axis, with a width and height of 5-20m, and the air domain height is 5-30m. A hexahedral structure mesh is used for meshing, with a mesh size of 0.01-0.1m near the leak hole and a mesh size of 0.1-1m in areas far from the leak hole. Step 3: Establish the influence of soil moisture content on natural gas diffusion in the soil. Treating the soil as a porous medium, soil properties are characterized by the viscous drag coefficient and inertial drag coefficient. The Van Genuchten-Mualem model is used to describe the relationship between the soil viscous drag coefficient and moisture content, deriving the correlation between soil moisture content and natural gas diffusion. Van Genuchten-Mualem permeability model calculation formula: , ,

[0026] In the formula: Saturated permeability, in units of ; Residual volumetric moisture content, in units of ; This represents the saturated volumetric water content, in units of... ; For Van Genuchten shape parameters; For auxiliary parameters; Effective saturation; This refers to the volumetric water content. Formula for calculating viscous drag coefficient:

[0027] In the formula: The dynamic viscosity of water, in units of , This is the viscous resistance coefficient.

[0028] The momentum conservation equation takes into account the resistance effect of porous soil media; the source term of the momentum equation... The calculation formula is:

[0029] In the formula: This represents the gas diffusion velocity, measured in m / s. This is the inertial drag coefficient, in units of... ; This refers to the density of a gas, expressed in kg / m³. 3 ; These are parameters related to penetration rate; The momentum conservation equation affected by porosity is:

[0030] In the formula: Porosity; Time, in seconds; Absolute pressure, unit: Pa; This is the second-order stress tensor, with units of Pa. This represents the component of gravitational acceleration, in units of... ; , These are spatial coordinate components; Step 4: Establish and solve the buried natural gas pipeline leakage model. The governing equations of the model include the mass conservation equation, momentum conservation equation, energy conservation equation, and component transport equation. A k-ε turbulence model is selected, and boundary conditions are set for the pipeline, soil domain, and air domain. A double-precision pressure solver and the SIMPLE algorithm are used for pressure-velocity coupled solution. The boundary conditions are set as follows: the natural gas pipeline inlet uses a pressure inlet boundary, with the inlet pressure consistent with the pipeline operating pressure; the pipeline outlet uses a pressure outlet boundary, with the outlet pressure being atmospheric pressure; the soil domain's four boundaries use pressure outlet boundaries; velocity inlet boundaries are set on both sides of the air domain to simulate atmospheric wind speed, and the top and corresponding side boundaries of the air domain use pressure outlet boundaries; the contact boundaries between the natural gas pipeline leak and the soil and air domains are set as internal surfaces and share the mesh. The solution parameters are set as follows: gravitational acceleration is set to 9.8 m / s². 2 The time step is 10. -3 -10 -1 The time step is 1000-100000; the maximum number of iterations is 10-50; in the component transport equation, the natural gas component is mainly methane, and the initial mole fraction is set to 100%. Step 5: Analyze the diffusion characteristics of natural gas under different operating conditions, calculate the mass fraction of natural gas in the soil and air, and determine the dangerous explosion time at different locations after a buried natural gas pipeline leak. The method for determining the dangerous explosion time is as follows: based on the explosion limit of natural gas, with a volume fraction of 5%-15%, monitor the change of methane mass fraction at different locations in the soil and air domains over time. When the methane mass fraction at a certain location first reaches the lower or upper limit of the explosion limit, the corresponding time is the dangerous explosion time at that location.

[0031] In this example, the buried natural gas pipeline has a burial depth of 0.8m, an operating temperature of 300K, an operating pressure of 0.6MPa, and a leakage orifice diameter of 20mm. Based on the method of this invention, the leakage characteristics of this buried pipeline are analyzed to determine the possible time for an explosion to occur at different locations after a leak. The implementation steps are as follows:

[0032] Step 1: The leakage process of a buried natural gas pipeline at a depth of 0.8m is selected as the research object. The leakage diffusion area of ​​the buried natural gas pipeline is divided into the pipeline zone, the soil zone, and the air zone, as follows: Figure 2 As shown, the leakage of buried natural gas pipelines is divided into soil diffusion and air diffusion; the pipeline operating pressure is determined to be 0.6 MPa, the leakage orifice diameter is 20 mm, the wind speed in the atmospheric environment is 0 m / s, and the soil type is set as loam.

[0033] Step 2: Use a Python program to construct a geometric model of the buried natural gas pipeline leak and generate a mesh, as shown below. Figure 3As shown, the leakage model consists of a natural gas pipeline, soil, and ambient atmosphere, with an overall computational domain size of 1m (length) × 1m (width) × 2m (height). A structured meshing method is used to mesh the leakage model. Since the velocity and pressure gradients at the natural gas pipeline leak are relatively large, the mesh near the leak is locally refined to reduce computation time and ensure the accuracy of the numerical simulation. The computational domain is discretized using hexahedral elements, and the mesh independence of the buried natural gas pipeline leakage model is verified. Step 3: Establish the influence of soil moisture content on the diffusion of natural gas in the soil; in the leakage model, porous media are used to simulate soil, and the relationship between the soil's viscosity resistance coefficient and moisture content is described by the Van Genuchten-Mualem model; calculate the relationship between the soil viscosity resistance coefficient and moisture content, and the calculation results are as follows. Figure 4 As shown; at this time, the viscosity resistance coefficient of the loam is 2.45×10¹¹, and the calculated water content in the soil is 16.11%; Step 4: Establishment and solution of buried natural gas pipelines; governing equations include mass, momentum, and energy conservation equations, and component transport equations are used to calculate the mole fraction of each component in the natural gas; turbulence model is selected. Model: The natural gas pipeline inlet boundary condition uses a pressure inlet, and 100% methane is selected as the natural gas. The natural gas pipeline outlet boundary condition uses a pressure outlet, the soil domain's four-sided boundary condition uses a pressure outlet, and the two side boundary conditions in the air domain use a velocity inlet with a velocity magnitude set to 0 m / s. The corresponding two side boundaries and the upper boundary in the air domain use a pressure outlet. The contact boundary between the natural gas pipeline leak point boundary and the soil and air domains uses an internal surface, and the contact surface is shared to ensure the correctness of numerical transmission when natural gas passes through the internal surface. Specific numerical boundary conditions are shown in Table 1; the gravitational acceleration is set to 9.8. A double-precision pressure solver was used, and the SIMPLE algorithm was employed for pressure-velocity coupling. The time step was set to 10⁻² s, the number of time steps was 18000, the maximum number of iterations for the time step was 20, and the total duration of the simulated leakage was calculated to be 180 s. Table 1 Boundary Condition Definitions Boundary position Boundary type Pipe inlet Pressureinlet Pipeline outlet Pressureoutlet Pipe leak Interior Soil-Surface Environment Contact Surface Interior pipe wall wall Surface environment pressure outlet Pressureoutlet Soil perimeter Pressureoutlet Bottom soil boundary wall Air zone wind speed inlet Velocityinlet Air pressure outlet Pressureoutlet Step 5: Analyze the diffusion characteristics of buried natural gas pipeline leaks on the surface environment. The methane mass fraction in the soil and air domains changes over time, as shown in the graphs. Figure 5As shown, 10 seconds after the leak, the natural gas first diffused and accumulated in the soil, with a methane mass fraction of 3.13%. When methane entered the air through the soil, the methane mass fraction in the soil decreased to 2.5%, and subsequently increased over time. After 50 seconds, the methane mass fraction in the air reached 5.2%, hitting the explosion limit. 180 seconds after the leak, the air was filled with methane. At this point, the methane mass fraction in the soil medium at 0.5m below the ground was 20.5%, and at 0.4m below the ground was 20.6%.

[0034] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A numerical simulation method for the diffusion of buried natural gas pipeline leaks to the surface environment, characterized in that: Includes the following steps: Step 1: Determine the leakage conditions and design a physical model for natural gas leakage. Divide the leakage diffusion area of ​​the buried natural gas pipeline into three regions: pipeline region, soil region, and air region. Clarify the diffusion process of natural gas leakage in the soil region and air region, and determine the leakage conditions parameters, including the natural gas pipeline operating pressure, leak size, atmospheric wind speed, soil type, and soil moisture content. Step 2: Establish a geometric model of the buried natural gas pipeline leakage diffusion area and divide it into grids. The geometric model covers the natural gas pipeline, soil and ambient atmosphere. A structured grid division method is used to locally refine the grid in the area near the leakage hole and to verify the grid independence. Step 3: Establish the influence of soil moisture content on the diffusion of natural gas in the soil. Treat the soil as a porous medium, characterize the soil properties through the viscosity resistance coefficient and the inertial resistance coefficient, use the Van Genuchten-Mualem model to describe the relationship between the soil viscosity resistance coefficient and the moisture content, and derive the correlation between soil moisture content and natural gas diffusion. Step 4: Establish and solve the buried natural gas pipeline leakage model. The governing equations of the model include the mass conservation equation, momentum conservation equation, energy conservation equation and component transport equation. The k-ε turbulence model is selected, and the boundary conditions of the pipeline, soil domain and air domain are set. The pressure-velocity coupled solution is performed using a double-precision pressure solver and the SIMPLE algorithm. Step 5: Analyze the diffusion characteristics of natural gas under different operating conditions, calculate the mass fraction of natural gas in soil and air, and determine the dangerous explosion time at different locations after a buried natural gas pipeline leak.

2. The numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment according to claim 1, characterized in that: The method for determining the leakage condition parameters in step 1 is as follows: through on-site investigation and engineering data statistics, a typical pipeline operating pressure range of 0.1-10MPa, a leak size of 1-100mm, an atmospheric wind speed of 0-10m / s, soil types including sand, loam, and clay, and a soil moisture content range of 5%-40% are selected.

3. The numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment according to claim 1, characterized in that: The process of establishing the geometric model in step 2 is as follows: a three-dimensional geometric model is constructed based on the actual pipeline burial depth, soil layer thickness, and atmospheric environment range. The pipeline burial depth is set to 0.5-3m, the soil domain size is 10-50m extending along the pipeline axis, with a width and height of 5-20m, and the air domain height is 5-30m. The mesh is divided using a hexahedral structure mesh, with the mesh size near the leak hole being 0.01-0.1m and the mesh size in areas far from the leak hole being 0.1-1m.

4. The numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment according to claim 1, characterized in that: In step 3: Van Genuchten-Mualem permeability model calculation formula: ； In the formula: Saturated permeability, in units of ; Residual volumetric moisture content, in units of ; This represents the saturated volumetric water content, in units of... ; For Van Genuchten shape parameters; For auxiliary parameters; Effective saturation; This refers to the volumetric water content. Formula for calculating viscous drag coefficient: In the formula: The dynamic viscosity of water, in units of , This is the viscous resistance coefficient.

5. The numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment according to claim 1, characterized in that: The momentum conservation equation in step 3 takes into account the resistance effect of porous soil media, and the source term of the momentum equation... The calculation formula is: In the formula: This represents the gas diffusion velocity, measured in m / s. This is the inertial drag coefficient, in units of... ; This refers to the density of a gas, expressed in kg / m³. 3 ; These are parameters related to penetration rate; The momentum conservation equation affected by porosity is: In the formula: Porosity; Time, in seconds; Absolute pressure, unit: Pa; This is the second-order stress tensor, with units of Pa. This represents the component of gravitational acceleration, in units of... ; , These are spatial coordinate components.

6. The numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment according to claim 1, characterized in that: The boundary conditions in step 4 are set as follows: the natural gas pipeline inlet adopts a pressure inlet boundary, and the inlet pressure is consistent with the pipeline operating pressure; the pipeline outlet adopts a pressure outlet boundary, and the outlet pressure is atmospheric pressure; the soil domain's four sides adopt a pressure outlet boundary; velocity inlet boundaries are set on both sides of the air domain to simulate atmospheric wind speed, and the top of the air domain and its corresponding two sides adopt pressure outlet boundaries; the contact boundary between the natural gas pipeline leak and the soil and air domains is set as an internal surface and shares a mesh.

7. The numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment according to claim 1, characterized in that: The solution parameters in step 4 are set as follows: gravitational acceleration is set to 9.8 m / s². 2 The time step is 10. -3 -10 -1 The time step is 1000-100000; the maximum number of iterations is 10-50; in the component transport equation, the natural gas component is mainly methane, and the initial mole fraction is set to 100%.

8. The numerical simulation method for the diffusion of buried natural gas pipeline leakage to the surface environment according to claim 1, characterized in that: The method for determining the dangerous explosion time in step 5 is as follows: based on the explosion limit of natural gas, with a volume fraction of 5%-15%, by monitoring the change of methane mass fraction at different locations in the soil and air domains over time, the time when the methane mass fraction at a certain location first reaches the lower or upper limit of the explosion limit is the dangerous explosion time at that location.

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