A method for calculating gas-to-gas diffusion concentration distribution considering the influence of gravity

Abstract

This invention discloses a method for calculating gas-gas diffusion concentration distribution considering the influence of gravity, relating to the field of oil and gas field development. This method introduces a gravity drift velocity term generated by density difference into the traditional diffusion equation, realizing a unified characterization of molecular diffusion and gravity differentiation. It constructs a dynamic mass transfer model applicable to multi-temperature, multi-pressure, and multi-component conditions, which is used to calculate the concentration distribution and corresponding dynamic diffusion coefficient of injected gas and formation gas during the vertical diffusion process. This provides a more consistent calculation basis for predicting the migration front and distribution pattern of injected gas, optimizing gas injection parameters, and assessing gas channeling risk. It has applicable value for improving the recovery rate and the operational safety of gas storage facilities.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field development, and in particular to a method for calculating gas-gas diffusion concentration distribution considering the influence of gravity. This method is especially suitable for gas diffusion simulation in gas-driven gas reservoir development. Background Technology

[0002] In the fields of oil and gas field development and underground gas storage, gas injection displacement and gas peak shaving operations rely heavily on the accurate characterization of mass transfer processes between different gases. In actual reservoirs, gas-gas systems often exhibit multi-factor coupled mass migration under high temperature and high pressure conditions. In addition to molecular diffusion and dispersion, gravitational differentiation caused by density differences and natural convection can change the vertical composition and concentration distribution, thereby affecting the migration front, fingering morphology, and overall development effect of the injected gas.

[0003] Existing research can be broadly categorized into experimental and theoretical approaches. On the experimental side, some researchers, focusing on CO2 and natural gas systems, employ stratified sampling and time-series monitoring to measure component concentrations at different depths and times. They then use analytical formulas based on Fick's law or convection-dispersion frameworks for fitting and regression analysis, obtaining characterizing parameters such as molecular diffusion coefficients and equivalent diffusion / dispersion coefficients (a method for determining the gravitational diffusion coefficient of carbon dioxide in natural gas, CN119164838B). Some works further introduce dimensionless gravity parameters to summarize observed phenomena. Other researchers have determined the longitudinal dispersion coefficients of supercritical CO2 and CH4 in porous media through core displacement experiments, establishing a one-dimensional convection-dispersion model. These methods are engineering-feasible and can reflect macroscopic migration characteristics to some extent when density differences exist; however, they often incorporate the "gravity-induced drift / convection" effect into the equivalent parameters, limiting the physical interpretability of the parameters. (CO2 sequestration for enhanced gas recovery: New measurements of supercritical CO2–CH4 dispersion in porousmedia and a review of recent research.[J]. International Journal ofGreenhouse Gas Control, 2012, Vol.9: 457-468.).

[0004] In terms of theoretical model research, the mainstream approach is to establish a multi-component diffusion coefficient matrix based on Fick's law or the Maxwell-Stefan equation, discuss the influence of thermodynamic non-ideals and molecular interactions on equivalent diffusion, and give an approximate relationship with the Fick coefficient under certain conditions (Calculation of multi-component gas-gas diffusion coefficient [J]. Natural Gas Industry, 2015, (8): 39-43.). Some scholars have established a diffusion coefficient calculation model based on Fick's second law for dry gas and condensate gas systems under supercritical conditions. However, compared with gas-oil diffusion, modeling work that directly addresses gas-gas systems and introduces gravity effects under supercritical conditions is still relatively limited (A method for calculating the dynamic diffusion coefficient of dry gas-condensate gas under supercritical conditions, CN115081217A). Existing models mostly focus on the expression of "concentration gradient-driving flux", and lack explicit inclusion and unified description of the drift term caused by density difference-gravity, the triggering conditions of natural convection under stable / unstable density stratification, and the coupling mechanism of gravity and dispersion at the level of the governing equation.

[0005] In summary, while existing methods can reflect gravity-related phenomena experimentally, they generally lack the ability to explicitly incorporate gravity terms into the governing equations at the theoretical level. Furthermore, modeling methods primarily based on molecular diffusion struggle to characterize the gravity-driven features of gas-gas systems. Based on the theoretical simulation requirements for engineering applications, it is necessary to propose a gas-gas diffusion modeling and parameterization approach that can simultaneously consider gravitational differentiation and dynamic diffusion processes, thereby supporting the design and optimization of gas injection schemes. Summary of the Invention

[0006] The purpose of this invention is to propose a method for calculating gas-gas diffusion concentration distribution that considers the influence of gravity. This method introduces a gravity drift velocity term generated by density difference into the traditional diffusion equation, achieving a unified characterization of molecular diffusion and gravitational differentiation. It constructs a dynamic mass transfer model applicable to various temperature, pressure, and component conditions, used to calculate the concentration distribution and corresponding dynamic diffusion coefficients of injected gas and formation gas during vertical diffusion. This provides a computational basis for frontal location prediction, injection parameter setting, and gas channeling risk assessment, and supports decision-making for enhanced oil recovery and gas storage facility operation.

[0007] To achieve the above technical objectives, the present invention adopts the following technical solution.

[0008] A method for calculating gas-to-gas diffusion concentration distribution considering the influence of gravity includes the following steps:

[0009] Step S1: Now consider a certain volume PVT cylinder containing two gases with known initial composition in a non-equilibrium state. The origin coordinates are located at the gas-gas interface, where the interface is in equilibrium. The upward direction is the positive z-axis, and the downward direction is the negative z-axis.

[0010] Step S2: Based on S1, considering the effects of molecular diffusion and gravity, calculate the total diffusion flux of the upper and lower gas sections. J The expression is as follows:

[0011] (1)

[0012] In the formula: J For the total diffusion flux, ; The flux caused by molecular diffusion, ; For the flux caused by gravity, .

[0013] Step S3: Based on Fick's second law, the basic form of diffusion flux is:

[0014] (2)

[0015] In the formula: D The diffusion coefficient is... ; molar concentration, z is the position coordinate. .

[0016] Step S4: Gas molecules in the system are affected by gravity. The mass transfer flux caused by gravity is:

[0017] (3)

[0018] In the formula: The drift velocity caused by gravity, .

[0019] Step S5: Based on Stokes' law, the expression for the velocity of a gas caused by gravity is:

[0020] (4)

[0021] In the formula: D m is the diffusion coefficient. 2 / s; The acceleration due to gravity is m / s². 2 ; The density difference between the two gases, in kg / m³ 3 ; The average molar mass of the gas is expressed in kg / mol. The average density of the gas is kg / m³. 3 ; R The universal gas constant is 8.314 kg·m. 2 / (s 2 (·mol·K);T Let K be the temperature.

[0022] Step S6: During the diffusion of the two gases, when gravity is involved, the gas-to-gas mass transfer equation is:

[0023] (5)

[0024] In the formula: t For diffusion time, .

[0025] Step S7: Combining the physical simulation process in Step S1 and the mass transfer equation in Step S5, the upper gas diffusion model takes the following form:

[0026] (6)

[0027] In the formula: the subscript 1 represents the upper part of the air; The initial molar concentration of the upper gas. ; This represents the molar concentration of the upper gas phase component at equilibrium at the gas-gas interface. ; This refers to the height at which the upper gas enters. m .

[0028] Step S8: Homogeneous boundary conditions. The upper gas diffusion model in step S7 can be transformed into the following form:

[0029] (7)

[0030] In the formula: As an additional attenuation term, satisfying ; For intermediate functions, satisfying ; For intermediate functions, satisfying .

[0031] Step S9: Using the mathematical physics method of separation of variables, the upper gas model is solved, and the expression for the upper gas concentration function is obtained as follows:

[0032] (8)

[0033] in:

[0034] In the formula: The characteristic coefficients of the upper gas concentration function are given.

[0035] Similarly, the expression for the lower gas concentration function can be obtained as follows:

[0036] (9)

[0037] in:

[0038] In the formula: the subscript 2 represents the lower part of the gas.

[0039] Step S10: Fit the experimental data using the least squares method to determine the diffusion coefficient considering the effect of gravity. D :

[0040] (10)

[0041] In the formula: The values ​​are the gas molar concentrations calculated by the model. ; The values ​​are the gas molar concentrations measured in the experiment. .

[0042] Furthermore, step S3 is as follows:

[0043] Step S31: In Fick's second law, the fundamental driving force of diffusion is the concentration gradient; the diffusion behavior of a gas is mainly driven by the concentration gradient. C-controlled diffusion flux expression is:

[0044] (3.1)

[0045] Step S32: Since diffusion only occurs in the z-direction, the concentration gradient... The expression is as follows:

[0046] (3.2)

[0047] Step S33: Substituting equation (3.2) into equation (3.1) yields:

[0048] (2)

[0049] Furthermore, step S5 is as follows:

[0050] Step S51: and These are the densities of the upper and lower gas layers, respectively. The density difference between the two gases is expressed as follows:

[0051] (5.1)

[0052] Step S52: y 1 and y 2 represents the mass fractions of the upper and lower gas components, respectively. The average density of the gas mixture is obtained by weighting the densities of the two gases together.

[0053] (5.2)

[0054] Step S53: The average molar mass of the gas mixture is obtained by weighting the molar masses of the two gases together.

[0055] (5.3)

[0056] Step S54: Based on Stokes' law, The expression is:

[0057] (5.4)

[0058] Step S55: Substituting equations (5.1), (5.2), and (5.3) into equation (5.4) yields:

[0059] (4)

[0060] Furthermore, step S6 is as follows:

[0061] Step S61: Total flux during the diffusion process J It is the sum of the diffusion flux driven by the concentration gradient and the drift flux caused by gravity. Substituting equations (2) and (3) into equation (1) yields:

[0062] (6.1)

[0063] Step S62: Substituting the total flux into the continuity equation, we obtain the following equation:

[0064] (6.2)

[0065] Step S63: During the diffusion of the two gases, the gas-to-gas mass transfer equation under the influence of gravity is:

[0066] (6.3)

[0067] Step S64: Assume Independent of position z, the final form of the mass transfer equation is:

[0068] (5)

[0069] Furthermore, step S7 is as follows:

[0070] Step S71: Based on the gas-to-gas diffusion physical simulation process described in step S1, the initial conditions for the upper gas are as follows:

[0071] (7.1)

[0072] Step S72: The upper boundary of diffusion is a closed, flux-free boundary, and the upper boundary condition is:

[0073] (7.2)

[0074] Step S73: During the gas-to-gas diffusion process, the pressure of the entire system remains constant, the interface is in equilibrium, and the lower boundary condition is:

[0075] (7.3)

[0076] Step S74: Based on the initial conditions and boundary conditions, combined with equation (5), the upper gas diffusion model takes the following form:

[0077] (6)

[0078] Similarly, the lower gas diffusion model takes the following form:

[0079] (7.4)

[0080] Furthermore, step S8 is as follows:

[0081] Step S81: Let After homogenizing the boundary conditions, equation (5) can be transformed into:

[0082] (8.1)

[0083] Step S82: Define We can obtain:

[0084] (7)

[0085] Similarly, the lower gas diffusion model can be transformed into:

[0086] (8.2)

[0087] Furthermore, step S9 is as follows:

[0088] Step S91: Solve the diffusion model using the method of separation of variables. Let:

[0089] (9.1)

[0090] In the formula: X for z Functions in space; T for t A function of time.

[0091] Step S92: Substitute equation (9.1) into equation (9) and divide both sides simultaneously. ,

[0092] (9.2)

[0093] In the formula: It is a parameter introduced to eliminate the convection term.

[0094] Step S93: The general solution for the time part is:

[0095] (9.3)

[0096] In the formula: These are coefficients to be determined.

[0097] Step S94: The spatial partial differential equation is:

[0098] (9.4)

[0099] Step S95: The general solution for the spatial part is:

[0100] (9.5)

[0101] In the formula: These are coefficients to be determined; These are coefficients to be determined.

[0102] Step S96: Therefore, the solution to equation (7) is:

[0103] (9.6)

[0104] Step S97: Combining the initial conditions, boundary conditions, and equation (9.6), the expression for the upper gas concentration function can be obtained as follows:

[0105] (8)

[0106] in:

[0107] (9)

[0108] Similarly, the expression for the lower gas concentration function can be obtained as follows:

[0109] (10)

[0110] in:

[0111] (11)

[0112] Furthermore, step S10 is as follows:

[0113] Step S101: Obtain the concentration distribution of the upper and lower gas through a gas-to-gas diffusion experiment;

[0114] Step S102: Provide initial values ​​for the diffusion coefficients of the upper and lower gases. D g1 and D g2 ;

[0115] Step S103: Calculate the concentration distribution of the upper and lower gases according to equations (8) and (10), and determine whether the difference between the model-calculated value and the experimentally measured value is less than 10 using the least squares method. -5 If the difference is less than 10 -5 Then the corresponding diffusion coefficient will be output. D ;

[0116] (12)

[0117] Step S104: If the difference between the model calculated value and the experimental measured value is greater than 10... -5 Then return to step S102, readjust the diffusion coefficient value, and perform subsequent calculations until the difference is less than 10. -5 until.

[0118] Compared with existing technologies, this invention, based on the natural convection mechanism caused by density differences, explicitly introduces a gravitational drift velocity term into the traditional diffusion equation, forming a gas-gas diffusion coupling model that simultaneously characterizes molecular diffusion and gravitational differentiation. Under multiple temperature, pressure, and component conditions, this model can calculate the vertical concentration distribution and the dynamic diffusion coefficient as a function of temperature and pressure, overcoming the shortcomings of models based on Fick's law that do not explicitly consider the influence of gravity, and more fully reflecting the vertical differentiation and convection behavior caused by density differences. Therefore, it can provide a more consistent calculation basis for predicting the migration front and distribution pattern of injected gas, optimizing injection parameters, and assessing gas channeling risk, and has applicable value for improving oil recovery and the operational safety of gas storage facilities. Attached Figure Description

[0119] Figure 1 This is a schematic diagram of the physical model;

[0120] Figure 2 The graph shows the distribution of CH4 mole fraction as a function of location at 10 MPa.

[0121] Figure 3 This is a graph showing the distribution of CO2 mole fraction as a function of location at 10 MPa.

[0122] Figure 4 Calculate the flowchart for the program. Detailed Implementation

[0123] The present invention will be further described below with reference to the accompanying drawings and examples to enable those skilled in the art to understand the invention. However, it should be understood that the present invention is not limited to the specific embodiments described herein. For those skilled in the art, any variations that fall within the spirit and scope of the invention as defined and determined by the appended claims are all within the scope of protection.

[0124] This embodiment of a method for calculating gas-to-gas diffusion concentration distribution considering the influence of gravity includes the following steps:

[0125] S1. Consider a fixed-volume PVT cylinder containing two gases, CO2 and CH4, in a non-equilibrium state with known initial compositions. A diffusion experiment is conducted on these two gases under isothermal conditions, with CO2 in the upper part of the PVT cylinder and natural gas in the lower part. The specific assumptions of the model are as follows: no chemical reaction occurs at the gas-gas interface; the mass flux of the components relative to the interface is continuous; the origin is located at the gas-gas interface, which is in equilibrium, with the positive z-axis pointing upwards and the negative z-axis pointing downwards. (See [reference]). Figure 1 The diagram shows a physical model.

[0126] Table 1 Basic Parameter Table

[0127]

[0128] S2 is based on the physical process of S1, considering the effects of molecular diffusion and gravity. The total diffusion flux of the upper and lower gas components at this point is... J for:

[0129] (1)

[0130] S31. In Fick's second law, the fundamental driving force of diffusion is the concentration gradient; the diffusion behavior of a gas is mainly driven by the concentration gradient. C is controlled, and the diffusion flux is:

[0131] (3.1)

[0132] S32. Since diffusion only occurs in the z-direction, the concentration gradient... as follows:

[0133] (3.2)

[0134] S3. Regarding molecular diffusion, based on Fick's second law, the concentration gradient is the fundamental driving force of the diffusion process. Under the action of this driving force, the diffusion flux is:

[0135] (2)

[0136] S4. Gas molecules in the system are affected by gravity. Gravity causes denser gas molecules to sink, while less dense gas molecules rise relatively, greatly affecting mass transfer. This mass transfer flux caused by gravity is:

[0137] (3)

[0138] S51. and These are the densities of the upper and lower gas layers, respectively. This represents the density difference between the two gases.

[0139] (5.1)

[0140] S52. y 1 and y 2 represents the mass fractions of the upper and lower gas components, respectively. The average density of the gas mixture is calculated by weighting the densities of the two gases.

[0141] (5.2)

[0142] S53. The average molar mass of the gas mixture is calculated by weighting the molar masses of the two gases.

[0143] (5.3)

[0144] S54. Based on Stokes' law, the upper air... for:

[0145] (5.4)

[0146] lower part of the air for:

[0147] (5.5)

[0148] S61. Total flux during diffusion process J It is the sum of the diffusion flux driven by the concentration gradient and the drift flux caused by gravity. Substituting equations (2) and (3) into equation (1) yields:

[0149] (6.1)

[0150] S62. Substituting the total flux into the continuity equation, we obtain the following equation:

[0151] (6.2)

[0152] S63. In the diffusion process of two gases, the gas-to-gas mass transfer equation under the influence of gravity is:

[0153] (6.3)

[0154] S6. In the diffusion process of two gases, when gravity is involved, the gas-to-gas mass transfer equation is:

[0155] (5)

[0156] S71. Based on the gas-to-gas diffusion physical simulation process described in S1, for the upper gas, the initial conditions are:

[0157] (7.1)

[0158] In the formula: Represents the initial molar concentration of the upper gas. .

[0159] S72. The upper boundary of diffusion is a closed, flux-free boundary, and the upper boundary condition is:

[0160] (7.2)

[0161] S73. During gas-to-gas diffusion, the pressure of the entire system remains constant, the interface is in equilibrium, and the lower boundary condition is:

[0162] (7.3)

[0163] S7. Combining the physical simulation process of S1 and the mass transfer equation of S5, the upper gas diffusion model takes the following form:

[0164] (6)

[0165] Similarly, the lower gas diffusion model takes the following form:

[0166] (7)

[0167] S8. Order With homogeneous boundary conditions, the upper gas diffusion model in S7 can be transformed into the following form:

[0168] (8)

[0169] in:

[0170] .

[0171] Similarly, the lower gas diffusion model can be transformed into:

[0172] (9)

[0173] S91. The diffusion model is solved using the method of separation of variables. Let:

[0174] (9.1)

[0175] S92. Substitute equation (9.1) into equation (9) and divide both sides simultaneously. ,

[0176] (9.2)

[0177] S93. The general solution for the time part is:

[0178] (9.3)

[0179] S94. The partial differential equation in space is:

[0180] (9.4)

[0181] S95. The general solution for the spatial part is:

[0182] (9.5)

[0183] S96. Therefore, the solution to equation (8) is

[0184] (9.6)

[0185] S9 uses the method of separation of variables from mathematical physics to solve the upper gas model, and the upper gas concentration function is obtained as follows, with z=L g1 For example, with a time interval of 0.0175m and t=3600s:

[0186] (11)

[0187] (10)

[0188] Similarly, the concentration distribution function of the lower gas is calculated:

[0189] (13)

[0190] (12)

[0191] The concentration distributions of CH4 and CO2 at different times and locations were calculated using the upper and lower gas concentration distribution functions, as shown below. Figure 2 and Figure 3 As shown:

[0192] S101. Obtain the concentration distribution of the upper and lower gas through gas-to-gas diffusion experiments.

[0193] S102. The initial values ​​of the diffusion coefficients for the upper and lower gas sections are given as 0.51 × 10⁻⁶. -8 m 2 / s and 0.38×10 - 8 m 2 / s.

[0194] S103. Calculate the concentration distribution of the upper and lower parts of the gas according to equations (11) and (12), and determine whether the difference between the calculated value and the measured value is less than 10 using the least squares method. -5 If the difference is less than 10 -5 Then the corresponding diffusion coefficient will be output. D .

[0195] (14)

[0196] S104. If the difference between the model calculated value and the experimental measured value is greater than 10... -5 Then return to step S102, readjust the diffusion coefficient value, and perform subsequent calculations until the difference is less than 10. -5 See details up to date. Figure 4 .

[0197] Table 2. Diffusion coefficient data of CH4 and CO2 at 10 MPa

[0198] .

Claims

1. A method for calculating concentration distribution of gas-gas diffusion considering the effect of gravity, characterized in that, Comprising the following steps: Step S1: now consider a certain volume PVT cylinder, containing two kinds of gas in a non-equilibrium state with known initial composition, the origin coordinate is located at the gas-gas interface, the interface is in equilibrium state, the upward is the positive direction of z axis, and the downward is the negative direction of z axis; Step S2: Based on S1, considering molecular diffusion and gravity effect, total flux of up and down gas diffusion J The expression is as follows: (1) wherein: J is the total flux of diffusion, ; is the flux induced by molecular diffusion, ; is the flux induced by gravity, ; Step S3: based on Fick's second law, the basic form of diffusion flux is: (2) wherein: D D is the diffusion coefficient, ; C is the molar concentration, ; z is the position coordinate, ; Step S4: the gas molecules in the system will be affected by gravity, and the mass transfer flux caused by gravity is: (3) In the formulae: is the drift velocity caused by gravity, ; Step S5: based on Stokes law, the velocity expression of the gas caused by gravity is: (4) wherein: D D is the diffusion coefficient, m 2 / s; g is the gravitational acceleration, m / s 2 ; is the difference in density between the two gases, kg / m 3 ; is the average molar mass of the gas, kg / mol; is the average density of the gas, kg / m 3 ; R R is the universal gas constant, 8.314 kg-m 2 / (s 2 ·mol·K); T T is the temperature, K; Step S6: in the process of diffusion of the two gases, when there is gravity, the mass transfer equation of gas-gas is: (5) In the formulae: t is the diffusion time, ; Step S7: combining the physical simulation process of step S1 and the mass transfer equation of step S5, the upper gas diffusion model is as follows: (6) where subscript 1 represents the upper gas; is the initial molar concentration of the upper gas, ; is the molar concentration of the upper gas phase component at equilibrium at the gas-gas interface, ; is the height of the upper gas transition, m ; Step S8: homogeneous boundary condition, the upper gas diffusion model in step S7 is transformed into the following form: (7) wherein: is an additional decay term satisfying ; is an intermediate function satisfying ; is an intermediate function satisfying ; Step S9: the upper gas concentration function expression is obtained by solving the upper gas model by using mathematical physics method of separation of variables: (8) wherein: wherein: is the upper gas concentration function characteristic coefficient; Similarly, the lower gas concentration function expression is: (9) wherein: wherein: subscript 2 represents lower air; Step S10: fitting the experimental data by least square method to determine the diffusion coefficient considering the effect of gravity D : (10) wherein: is the value of the molar concentration of the gas calculated from the model, ; is the value of the molar concentration of the gas measured experimentally, .

2. The method for calculating the concentration distribution of gas-gas diffusion considering the effect of gravity according to claim 1, characterized in that, The process of step S3 is as follows: Step S31: In Fick's second law, the basic driving force of the diffusion process is the concentration gradient, and the diffusion behavior of the gas is mainly determined by the concentration gradient C control, the diffusion flux expression is: (3.1) Step S32: Since diffusion occurs only in the z direction, the concentration gradient The expression is as follows: (3.2) Step S33: substitute formula (3.2) into formula (3.1) to obtain: (2)。 3. The method for calculating the concentration distribution of gas-gas diffusion considering the effect of gravity according to claim 1, characterized in that, The process of step S5 is as follows: Step S51: and respectively the densities of the upper and lower gases, the difference between the densities of the two gases, in the form: (5.1) Step S52: y 1 and y 2 are the mass fractions of the upper and lower gases, respectively, is the average density of the mixed gas, which is a weighted average of the densities of the two gases: (5.2) Step S53: For the average molar mass of the mixture gas, the molar masses of both gases are weighted: (5.3) Step S54: Based on the Stokes law, The expression is: (5.4) Step S55: substitute formula (5.1), formula (5.2) and formula (5.3) into formula (5.4) to obtain: (4)。 4. The method for calculating the concentration distribution of gas-gas diffusion considering the effect of gravity according to claim 1, characterized in that, The process of step S6 is as follows: Step S61: Total flux during diffusion J is the sum of the diffusion flux driven by the concentration gradient and the drift flux caused by the action of gravity, substituting equation (2) and equation (3) into equation (1) gives: (6.1) Step S62: substitute the total flux into the continuity equation to obtain the following formula: (6.2) Step S63: in the process of diffusion of the two gases, when there is gravity, the mass transfer equation of gas-gas is: (6.3) Step S64: Assuming The final form of the mass transfer equation is obtained regardless of the position z as follows: （5）。 5. The method for calculating the concentration distribution of gas-gas diffusion considering the effect of gravity according to claim 1, characterized in that, The process of step S7 is as follows: Step S71: according to the gas-gas diffusion physical simulation process described in step S1, for the upper gas, the initial condition is: (7.1) Step S72: the upper boundary of diffusion is a closed no-flux boundary, and the upper boundary condition is: (7.2) Step S73: in the process of gas-gas diffusion, the pressure of the whole system is constant, the interface is in equilibrium state, and the lower boundary condition is: (7.3) Step S74: according to the initial condition and the boundary condition, combining formula (5), the upper gas diffusion model is as follows: (6) Similarly, the lower gas diffusion model is as follows: (7.4)。 6. The method for calculating the concentration distribution of gas-gas diffusion considering the effect of gravity according to claim 1, characterized in that, The process of step S8 is as follows: Step S81: Let After the homogeneous boundary condition, the formula (5) is converted into: (8.1) Step S82: defining , to obtain: (7) Similarly, the lower gas diffusion model is transformed into: (8.2)。 7. The method for calculating the concentration distribution of gas-gas diffusion considering the effect of gravity according to claim 1, characterized in that, The process of step S9 is as follows: Step S91: the diffusion model is solved by using the separation of variables method, and it is assumed that: (9.1) wherein: X is z a function of space; T is t a function of time; Step S92: Substitute formula (9.1) into formula (9) while dividing both sides , (9.2) wherein: is a parameter introduced to eliminate the convection term; Step S93: the time part of the general solution is: (9.3) In the formulae: are undetermined coefficients; Step S94: the spatial part differential equation is: (9.4) Step S95: the spatial part general solution is: (9.5) wherein: are undetermined coefficients; are undetermined coefficients; Step S96: therefore, the solution of formula (7) is: (9.6) Step S97: combining the initial condition and the boundary condition and formula (9.6), the upper gas concentration function expression is obtained as follows: (8) Wherein: (9) Similarly, the lower gas concentration function expression is: (10) Wherein: (11)。 8. The method for calculating the concentration distribution of gas-gas diffusion considering the effect of gravity according to claim 1, wherein, The process of step S10 is as follows: Step S101: the concentration distribution of the upper gas and the lower gas is obtained through the gas-gas diffusion experiment; Step S102: Given initial values of the upper and lower gas diffusion coefficients D g1 and D g2 ; Step S103: Calculate the concentration distribution of the upper and lower parts of the gas according to formula (8) and formula (10), and determine whether the difference between the model calculation value and the experimental measurement value is less than 10 by the least square method -5 , if the difference is less than 10 -5 , output the corresponding diffusion coefficient D ; (12) Step S104: If the difference between the model calculated value and the experimentally measured value is greater than 10 -5 , return to step S102, re-adjust the diffusion coefficient value, and then perform subsequent calculations until the difference is less than 10 -5 .

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