A Multi-Field Coupled Numerical Simulation Method for Underground Coal Gasification

Abstract

This invention discloses a multi-field coupled numerical simulation method for underground coal gasification, relating to the field of energy development technology. The invention first constructs a multi-field coupled physicochemical control model, a porosity dynamic evolution model, a permeability dynamic evolution model, and a non-ideal gas thermodynamic model to realistically recreate the underground coal gasification process. Based on these models, a fully autonomous and controllable numerical simulation framework is built from the ground up. The finite volume method is used to spatially discretize the governing equations, and a fully implicit nonlinear solver is developed to perform fully coupled iterative solutions for all physical fields within the same time step. Using the structural parameters and internal medium parameters of the actual coal seam, the spatiotemporal evolution laws of the temperature field, pressure field, component concentration field, and porosity / permeability parameters during the underground coal gasification process are obtained using the constructed underground coal gasification numerical solver. By combining dynamic property evolution with accurate thermodynamic simulation, a precise simulation of the underground coal gasification process is achieved.

Description

Technical Field

[0001] This invention relates to the field of energy development technology, specifically to a multi-field coupled numerical simulation method for underground coal gasification. Background Technology

[0002] Currently, commonly used numerical simulation methods for underground coal gasification suffer from poor scalability and customization, loose multi-field coupling mechanisms, and poor simulation stability. These methods cannot incorporate the complex multi-step chemical reaction network and non-standard thermodynamic models unique to underground coal gasification, making it difficult to define the dynamic evolution of pores and permeability and accurately recreate the process. Furthermore, existing multi-field coupled numerical simulation methods often employ weak or sequential coupling strategies when dealing with the heat-fluidization multi-field problems in underground coal gasification. However, considering the extreme high temperatures, drastic phase transitions, and highly nonlinear reaction source-sink impacts inherent in actual underground coal gasification, the loose coupling methods used in current methods easily lead to simulation results that fail to accurately and stably reflect the transient strong feedback mechanisms between various physical quantities. Therefore, it is urgent to propose a multi-field coupled numerical simulation method for underground coal gasification to accurately reproduce the changes in multiple physical fields during the underground coal gasification process and achieve accurate reproduction of the underground coal gasification process. Summary of the Invention

[0003] This invention aims to solve the above-mentioned problems and proposes a multi-field coupled numerical simulation method for underground coal gasification. Based on a fully implicit and fully coupled framework, it combines dynamic property evolution with accurate thermodynamic simulation of the underground coal gasification process to obtain the multi-physics fields in the underground coal gasification process. This method overcomes the limitations of existing numerical simulation methods for underground coal gasification that use weak coupling or sequential coupling strategies when dealing with the multi-field problem of heat-fluidization-chemical transformation in the simulation process. It provides technical support for the scheme design and parameter optimization of underground coal gasification projects.

[0004] The present invention adopts the following technical solution:

[0005] A multi-field coupled numerical simulation method for underground coal gasification includes the following steps:

[0006] Step 1: Construct a multi-field coupled physicochemical control model to simulate the underground coal gasification process;

[0007] The physicochemical control model includes a multi-component mass conservation equation, an energy conservation equation, a momentum conservation equation, and a chemical reaction kinetics model.

[0008] Step 2: Establish dynamic evolution models for porosity and permeability;

[0009] Step 3: Construct a non-ideal gas thermodynamic model based on the PR-EOS equation of state to accurately simulate the real behavior of the gas phase in the coal seam;

[0010] Step 4: Based on the actual coal seam's structural parameters and internal medium parameters, and combined with the physicochemical control model, porosity dynamic evolution model, permeability dynamic evolution model, and non-ideal gas thermodynamic model constructed in Steps 1 to 3, establish a numerical simulation model for underground coal gasification. Based on the finite volume method, spatially discretize the numerical simulation model for underground coal gasification, divide the computational domain of the numerical simulation model for underground coal gasification into multiple non-overlapping control volumes, determine the control equation set and residual equation set of the numerical simulation model for underground coal gasification, and form a numerical solver for underground coal gasification.

[0011] Step 5: Using the numerical solver for underground coal gasification, solve the governing equations of the numerical simulation model of underground coal gasification to obtain multiple physical fields of the numerical simulation of underground coal gasification.

[0012] Preferably, in step 1, the multi-component mass conservation equation is:

[0013] ;

[0014] ;

[0015] ;

[0016] In the formula, For time steps; For fluid porosity; This refers to gas phase saturation. The molar density of the gas phase; For fluid component serial numbers; The component number is the solid phase component number. The chemical reaction number; For the first Mole fraction of each gas phase component; For gas injection wells and production wells; For gradient operators; This refers to the gas phase seepage velocity; For the first In the first chemical reaction Each gas phase component is used as the stoichiometric coefficient of the product; For the first In the first chemical reaction Each gaseous component serves as the stoichiometric coefficient of the reactants; For the first The reaction rate of a chemical reaction; The molar density of the liquid phase; Liquid phase saturation; The liquid phase percolation velocity; For liquid phase injection wells and production wells, it serves as a source and sink term; For the first In the first chemical reaction Each fluid component serves as the stoichiometric coefficient of the reactants; For the first In the first chemical reaction Each fluid component is used as the stoichiometric coefficient of the product; For the first The concentration of each solid component in the solid phase; For the first In the first chemical reaction Each solid component serves as the stoichiometric coefficient of the reactants; For the first In the first chemical reaction Each solid component is used as the stoichiometric coefficient of the product;

[0017] The energy conservation equation is as follows:

[0018] ;

[0019] In the formula, The coal seam medium is represented as follows: when the coal seam medium type is gas phase. When the coal seam medium type is liquid phase, it is represented as ; The internal energy of the coal seam medium; The molar density of the coal seam medium; The saturation level of the coal seam medium; Porosity; For the first The internal energy of each solid component; The molar density of the rock skeleton; The internal energy within the rock skeleton; The enthalpy of the coal seam medium; The seepage velocity of the coal seam medium; The overall thermal conductivity; The temperature of the coal seam; Source and sink terms for injection and production wells corresponding to the coal seam medium type; Injecting heat per unit volume per unit time into the heating well; For the first The enthalpy of a chemical reaction;

[0020] The momentum conservation equation is:

[0021] ;

[0022] In the formula, This refers to absolute penetration rate; The relative permeability of the coal seam medium; The viscosity of the coal seam medium; The flow potential of the coal seam medium;

[0023] The chemical reaction kinetic model is as follows:

[0024] ;

[0025] In the formula, For the first The rate constant of a chemical reaction , For the first The frequency factor of a chemical reaction It is a logarithmic function. For the first The activation energy of a chemical reaction. It is the universal gas constant; For the first The concentration of a fluid component in the fluid.

[0026] Preferably, in step 2, the porosity dynamic evolution model is:

[0027] ;

[0028] In the formula, For fluid porosity; Porosity; The component number is the solid phase component number. This represents the total number of solid components. For the first The concentration of each solid component in the solid phase; For the first Molar mass of each solid component; For the first Mass density of each solid component;

[0029] The dynamic evolution model of the penetration rate is as follows:

[0030] ;

[0031] In the formula, for The absolute permeability of the coal seam at any given moment; for The absolute permeability of the coal seam at any given moment; for Fluid porosity of the coal seam at any given time; for Fluid porosity of the coal seam at any given time; The Carmen-Kozeny coefficient.

[0032] Preferably, in step 3, the non-ideal gas thermodynamic model is:

[0033] ;

[0034] in,

[0035] ;

[0036] ;

[0037] In the formula, Coal seam pressure; It is the universal gas constant; The temperature of the coal seam; This represents the molar volume of the gas phase. The gravitational coefficient of the mixture; The repulsion coefficient of the mixture; , These are all fluid component serial numbers. ; This represents the total number of fluid components. For the first Mole fraction of each gas phase component; For the first Mole fraction of each fluid component; The coefficient represents the binary interaction coefficient. For the first The pure component repulsion coefficient of each gas phase component; coal seam temperature Time The gravitational coefficients of the pure components of each gas phase component; coal seam temperature Time The gravitational coefficient of each gas phase component in its pure form.

[0038] Preferably, in step 4, the computational domain of the numerical simulation model for underground coal gasification is discretized into multiple non-overlapping control volumes based on the finite volume method. The multi-component mass conservation equation and energy conservation equation are then integrated for each control volume to obtain:

[0039] ;

[0040] ;

[0041] ;

[0042] ;

[0043] In the formula, The first in the computational domain of the numerical simulation model for underground coal gasification The mesh volume of each mesh cell, in units of ; For the mesh volume;

[0044] Using the divergence theorem, the volume integral of the convection term in the discretized multi-component mass and energy conservation equations is transformed into the surface integral of the boundary flow. The center value of each grid cell represents the average value of all physical quantities within the entire grid cell. The boundary integral of the convection term in the transformed multi-component mass and energy conservation equations is then discretized into the sum of the boundary flow rates. The governing equations for the numerical simulation model of underground coal gasification are obtained as follows:

[0045] ;

[0046] ;

[0047] ;

[0048] In the formula, , All are the serial numbers of the grid cells; For time step The next time step; For the first The grid boundary of each grid cell; For grid boundaries; For time step; For the first Adjacent cells of a grid cell; For the first The grid cell and the first The average at the interface of each grid cell; This refers to the gas phase fluidity; For time step Time The grid cell and the first Transmission rate between individual grid cells; For time step Time Gas phase flow potential of each grid cell; For time step Time Gas phase flow potential of each grid cell; For time step Time Liquid phase flow potential of each grid cell; For time step Time Liquid phase flow potential of each grid cell;

[0049] The Newton-Raphson method was used to obtain the residual equations of the numerical simulation model of underground coal gasification. ,get:

[0050] ;

[0051] ;

[0052] ;

[0053] ;

[0054] In the formula, For time step Time Gas phase residual values ​​for each grid cell; For time step Time Liquid phase residual value of each grid cell; For time step Time Solid residual values ​​of each grid cell; For time step Time Energy residual value of each grid cell; For the first The grid cell and the first Contact area between grid cells; For the first The grid cell and the first The average overall thermal conductivity at the interface of each grid cell; For the first The distance from the center of each grid cell to the boundary; For the first The distance from the center of each grid cell to the boundary;

[0055] Based on the residual equations of the numerical simulation model of underground coal gasification, an iterative calculation formula is constructed, yielding:

[0056] ;

[0057] ;

[0058] in,

[0059] ;

[0060] In the formula, The main variable number is used as the main variable number. The main variables include fluid pressure, liquid saturation, fluid component mole fraction, coal seam temperature, and solid component concentration. This represents the number of iterations. Elements of a Jacobian matrix; For the first During the nth iteration calculation The change in each main variable; For time step Time In the grid cell, the th The residuals of each main variable; For time step Time The main variable vector is calculated in each iteration; , The first sequence During the nth iteration calculation One vector of main variables; For the first The change in the vector of each main variable; For the first One main variable vector.

[0061] Preferably, in step 5, the main variables are first selected to construct a set of unknown vectors. The main variables include fluid pressure, liquid saturation, fluid component mole fraction, coal seam temperature, and solid component concentration; based on the discretized residual equations... By numerical perturbation, the partial derivatives of each residual equation with respect to each principal variable are calculated, forming the Jacobian matrix. Solve the system of linear equations To obtain the change in the main variable At the current time step, each master variable is updated until the residual norm of each grid cell in the underground coal gasification numerical simulation model is lower than the preset tolerance. Then, the underground coal gasification numerical simulation calculation is stopped, and multiple physical fields of the underground coal gasification process are obtained, including pressure field, temperature field, gas phase component mole fraction field, fluid porosity field and permeability field.

[0062] The present invention has the following beneficial effects:

[0063] (1) This invention proposes a multi-field coupled numerical simulation method for underground coal gasification. It adopts the Newton-Raphson fully implicit solution algorithm built from the bottom layer to directly solve the fully coupled equations of all physical fields in the numerical simulation model of underground coal gasification. This fundamentally solves the computational divergence problem in the numerical simulation of underground coal gasification under extreme nonlinear conditions (such as high temperature and violent phase change), ensuring the convergence and stability of the numerical simulation of underground coal gasification and truly reflecting the transient strong feedback mechanism between multiple physical fields in underground coal seams. At the same time, by constructing an autonomous and controllable mechanism-level simulation framework, this invention allows researchers to freely add or delete fluid components, modify arbitrarily complex chemical reaction kinetic models, and customize pore-permeability evolution equations in the underlying code according to the actual physicochemical mechanism. This achieves in-depth customized simulation of the underground coal gasification process, providing unprecedented flexibility for mechanism research and engineering optimization.

[0064] (2) This invention proposes a multi-field coupled numerical simulation method for underground coal gasification. By introducing a porosity dynamic evolution model and a permeability dynamic evolution model into the numerical simulation process of underground coal gasification, the method accurately captures the phenomenon of a sharp increase in permeability caused by the consumption of solid coal in the coal seam, and accurately predicts the gas production and the formation of flow channels. At the same time, by using the PR-EOS equation of state to construct a non-ideal gas thermodynamic model, the method accurately calculates the thermophysical properties of multi-component mixed gas in the coal seam under high temperature and high pressure conditions, avoiding the calculation errors caused by the ideal gas assumption, making the simulation results more consistent with the real physical process, and providing a more reliable basis for the scheme design and parameter optimization of underground coal gasification projects. Attached Figure Description

[0065] Figure 1 This is a flowchart of a multi-field coupled numerical simulation method for underground coal gasification according to the present invention.

[0066] Figure 2 This is a schematic diagram illustrating the iterative calculation and solution process of the numerical simulation model for underground coal gasification of this invention.

[0067] Figure 3 The diagram shows the spatiotemporal distribution of coal. In the diagram, (a) is a schematic diagram of the initial coal distribution, (b) is a schematic diagram of the coal distribution 2 days after the underground gasification reaction, (c) is a schematic diagram of the coal distribution 10 days after the underground gasification reaction, and (d) is a schematic diagram of the coal distribution 50 days after the underground gasification reaction.

[0068] Figure 4 The diagram shows the spatiotemporal distribution of coal and coke. In the diagram, (a) is a schematic diagram of the initial coal and coke distribution, (b) is a schematic diagram of the coal and coke distribution 2 days after the underground gasification reaction, (c) is a schematic diagram of the coal and coke distribution 10 days after the underground gasification reaction, and (d) is a schematic diagram of the coal and coke distribution 50 days after the underground gasification reaction.

[0069] Figure 5 The figure shows the spatiotemporal distribution of the temperature field. In the figure, (a) is a schematic diagram of the initial temperature field, (b) is a schematic diagram of the temperature field 2 days after the underground gasification reaction, (c) is a schematic diagram of the temperature field 10 days after the underground gasification reaction, and (d) is a schematic diagram of the temperature field 50 days after the underground gasification reaction.

[0070] Figure 6 The figure shows the spatiotemporal distribution of CO2 components. In the figure, (a) is a schematic diagram of the initial CO2 component distribution, (b) is a schematic diagram of the CO2 component distribution after 2 days of underground gasification reaction, (c) is a schematic diagram of the CO2 component distribution after 10 days of underground gasification reaction, and (d) is a schematic diagram of the CO2 component distribution after 50 days of underground gasification reaction.

[0071] Figure 7 The figure shows the spatiotemporal distribution of fluid porosity. In the figure, (a) is a schematic diagram of the initial fluid porosity distribution, (b) is a schematic diagram of the fluid porosity distribution 2 days after the underground gasification reaction, (c) is a schematic diagram of the fluid porosity distribution 10 days after the underground gasification reaction, and (d) is a schematic diagram of the fluid porosity distribution 50 days after the underground gasification reaction. Detailed Implementation

[0072] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0073] Example 1

[0074] This invention proposes a multi-field coupled numerical simulation method for underground coal gasification, such as... Figure 1 As shown, it includes the following steps:

[0075] Step 1: Construct a multi-field coupled physicochemical control model, including multi-component mass conservation equations, energy conservation equations, momentum conservation equations, and chemical reaction kinetics models, to simulate the underground coal gasification process.

[0076] Specifically, considering the mass changes of various components in the coal seam under the influence of convection, diffusion, and chemical reactions during the underground coal gasification simulation, a multi-component mass conservation equation is established, yielding:

[0077] ;

[0078] ;

[0079] ;

[0080] In the formula, For time steps; For fluid porosity; This refers to gas phase saturation. The molar density of the gas phase, in units of ; For fluid component serial numbers; The component number is the solid phase component number. The chemical reaction number; For the first Mole fraction of each gas phase component; Source and sink terms for gas injection wells and production wells, in units of ; For gradient operators; The velocity is the vapor phase seepage velocity, in units of... ; For the first In the first chemical reaction Each gas phase component is used as the stoichiometric coefficient of the product; For the first In the first chemical reaction Each gaseous component serves as the stoichiometric coefficient of the reactants; For the first The reaction rate of a chemical reaction; The molar density of the liquid phase, in units of ; Liquid phase saturation; The liquid phase flow velocity is expressed in units of... ; Source and sink terms for liquid injection wells and production wells, in units of ; For the first In the first chemical reaction Each fluid component serves as the stoichiometric coefficient of the reactants; For the first In the first chemical reaction Each fluid component is used as the stoichiometric coefficient of the product; For the first The concentration of each solid component in the solid phase, in units of... ; For the first In the first chemical reaction Each solid component serves as the stoichiometric coefficient of the reactants; For the first In the first chemical reaction Each solid component serves as the stoichiometric coefficient of the product.

[0081] Considering heat conduction, convective heat transfer, and the heat effect of chemical reactions, an energy conservation equation is established, yielding:

[0082] ;

[0083] In the formula, The coal seam medium is represented as follows: when the coal seam medium type is gas phase. When the coal seam medium type is liquid phase, it is represented as ; The internal energy of the coal seam medium, in units of ; The molar density of the coal seam medium, in units of ; The saturation level of the coal seam medium; Porosity is the volume fraction of the fluid component and the solid organic matter. For the first The internal energy of each solid component, in units of ; The molar density of the rock skeleton, in units of ; Internal energy within the rock skeleton, measured in units of ; The enthalpy of the coal seam medium, in units of ; The seepage velocity of the coal seam medium, in units of ; The overall thermal conductivity is expressed in units of... ; Coal seam temperature, unit: ; Source and sink terms for injection and production wells corresponding to the coal seam medium type; The amount of heat injected per unit volume per unit time into the heating well, in units of ; For the first The enthalpy of a chemical reaction.

[0084] Based on Darcy's law to describe fluid flow in porous coal seams, a momentum conservation equation is established, yielding:

[0085] ;

[0086] In the formula, This is the absolute penetration rate, in units of ; The relative permeability of the coal seam medium; The viscosity of the coal seam medium, in units of... ; The fluid potential of the coal seam medium is expressed in units of 1000 kJ / m². .

[0087] Based on the Arrhenius equation describing the chemical reaction rates during underground coal gasification at each time step in the simulation, a chemical reaction kinetic model was established, yielding the following results:

[0088] ;

[0089] In the formula, For the first The rate constant of a chemical reaction , For the first The frequency factor of a chemical reaction, in units of _____. , It is a logarithmic function. For the first The activation energy of a chemical reaction, expressed in units of _____. , The universal gas constant has a value of [value missing]. ; For the first The concentration of a fluid component in the fluid, in units of .

[0090] Step 2: Establish dynamic evolution models for porosity and permeability.

[0091] Specifically, the porosity dynamic evolution model is as follows:

[0092] ;

[0093] In the formula, For fluid porosity; Porosity; The component number is the solid phase component number. This represents the total number of solid components. For the first The concentration of each solid component in the solid phase, in units of... ; For the first The molar mass of each solid component, in units of ; For the first The mass density of each gas phase component, in units of .

[0094] The dynamic evolution model of the penetration rate is as follows:

[0095] ;

[0096] In the formula, for Absolute permeability of coal seams at any given time, in units of ; for Absolute permeability of coal seams at any given time, in units of ; for Fluid porosity of the coal seam at any given time; for Fluid porosity of the coal seam at any given time; The Carmen-Kozeny coefficient.

[0097] Step 3: Construct a non-ideal gas thermodynamic model based on the PR-EOS equation of state to accurately simulate the real behavior of the gas phase in the coal seam.

[0098] Specifically, the non-ideal gas thermodynamic model is as follows:

[0099] ;

[0100] in,

[0101] ;

[0102] ;

[0103] In the formula, Coal seam pressure; It is the universal gas constant; The temperature of the coal seam; This represents the molar volume of the gas phase. The gravitational coefficient of the mixture; The repulsion coefficient of the mixture; , These are all fluid component serial numbers. ; This represents the total number of fluid components. For the first Mole fraction of each gas phase component; For the first Mole fraction of each fluid component; The coefficient represents the binary interaction coefficient. For the first The pure component repulsion coefficient of each gas phase component; coal seam temperature Time The gravitational coefficients of the pure components of each gas phase component; coal seam temperature Time The gravitational coefficient of each gas phase component in its pure form.

[0104] Step 4: Based on the actual structural parameters of the coal seam and the parameters of the medium inside the coal seam, and combined with the physicochemical control model, porosity dynamic evolution model, permeability dynamic evolution model and non-ideal gas thermodynamic model constructed in Steps 1 to 3, a numerical simulation model of underground coal gasification is established using the Fortran coding platform. The numerical simulation model of underground coal gasification is spatially discretized based on the finite volume method, and the computational domain of the numerical simulation model of underground coal gasification is divided into multiple non-overlapping control volumes. The control equation set and residual equation set of the numerical simulation model of underground coal gasification are determined, and a numerical solver for underground coal gasification is formed.

[0105] Specifically, based on the finite volume method, the computational domain of the numerical simulation model for underground coal gasification is discretized into multiple non-overlapping control volumes. The multi-component mass conservation equation and energy conservation equation are then integrated for each control volume, yielding:

[0106] ;

[0107] ;

[0108] ;

[0109] ;

[0110] In the formula, The first in the computational domain of the numerical simulation model for underground coal gasification The mesh volume of each mesh cell, in units of ; This represents the mesh volume.

[0111] Using the divergence theorem, the volume integral of the convection term in the discretized multi-component mass and energy conservation equations is transformed into the surface integral of the boundary flow, yielding:

[0112] ;

[0113] ;

[0114] ;

[0115] In the formula, It is the normal vector; For grid boundaries; For the first The grid boundary of each grid cell.

[0116] Using the center value of the grid cell to represent the average value of each physical quantity in the entire grid cell, the boundary integrals of the convection terms in the transformed multi-component mass conservation equation and energy conservation equation are discretized into the sum of each boundary flux, resulting in:

[0117] ;

[0118] ;

[0119] ;

[0120] In the formula, , All are the serial numbers of the grid cells; For time step The next time step; For the first The grid boundary of each grid cell; For time step; For the first Adjacent cells of a grid cell; For the first The grid cell and the first The average value at the interface of each grid cell is used. For the calculation of density, viscosity and relative permeability, the upstream windward weighted scheme is used, and the harmonic average value is used for permeability and thermal conductivity. Gas phase flow rate, unit: ; For time step Time The grid cell and the first Transmissivity between grid cells, in units of In this embodiment, the numerical simulation model of underground coal seam gasification is described in section [number missing]. The grid cell and the first The formula for calculating the conductivity between individual grid cells is: ,in, For the first One-sided conductivity of each grid cell For the first One-sided conductivity of each grid cell For the first The grid cell and the first Transmission rate between individual grid cells; For time step Time Gas phase flow potential of each grid cell; For time step Time Gas phase flow potential of each grid cell; For time step Time Liquid phase flow potential of each grid cell; For time step Time Liquid phase flow potential of each grid cell.

[0121] The Newton-Raphson method was used to obtain the residual equations of the numerical simulation model of underground coal gasification. ,get:

[0122] ;

[0123] ;

[0124] ;

[0125] ;

[0126] In the formula, For time step Time Gas phase residual values ​​for each grid cell; For time step Time Liquid phase residual value of each grid cell; For time step Time Solid residual values ​​of each grid cell; For time step Time Energy residual value of each grid cell; For the first The grid cell and the first Contact area between grid cells; For the first The grid cell and the first The average overall thermal conductivity at the interface of each grid cell; For the first The distance from the center of each grid cell to the boundary; For the first The distance from the center of each grid cell to the boundary.

[0127] Based on the residual equations of the numerical simulation model of underground coal gasification, an iterative calculation formula is constructed, yielding:

[0128] ;

[0129] ;

[0130] in,

[0131] ;

[0132] In the formula, The main variable number is used as the main variable number. The main variables include fluid pressure, liquid saturation, fluid component mole fraction, coal seam temperature, and solid component concentration. This represents the number of iterations. Elements of a Jacobian matrix; For the first During the nth iteration calculation The change in each main variable; For time step Time The main variable vector is calculated in each iteration; , The first sequence During the nth iteration calculation One vector of main variables; For the first The change in the vector of each main variable; For the first One main variable vector.

[0133] Step 5: Input the initial values ​​of fluid pressure, liquid phase saturation, mole fraction of each fluid component in the liquid phase, coal seam temperature, concentration of each solid component in the solid phase, and preset injection well production well parameters into the underground coal gasification numerical solver. Set the total simulation time, minimum time step, and initial time step of the underground coal gasification numerical solver. Use the underground coal gasification numerical solver to solve the governing equations of the underground coal gasification numerical simulation model, such as... Figure 2 As shown, the pressure field, temperature field, gas phase component mole fraction field, fluid porosity field, and permeability field in the numerical simulation model of underground coal seam gasification are obtained.

[0134] Specifically, first select the main variables to construct a set of unknown vectors. In this embodiment, the main variables include fluid pressure, liquid phase saturation, mole fraction of each fluid component in the liquid phase, coal seam temperature, and concentration of each solid component in the solid phase; based on the discretized residual equations... By numerical perturbation, the partial derivatives of each residual equation with respect to each principal variable are calculated, forming the Jacobian matrix. Solve the system of linear equations To obtain the change in the main variable At the current time step, update each master variable until the residual norm of each grid cell in the numerical simulation model of underground coal seam gasification is lower than the preset tolerance, then stop the numerical simulation calculation of underground coal seam gasification.

[0135] Example 2

[0136] This embodiment applies the multi-field coupled numerical simulation method for underground coal gasification described in Embodiment 1 to a coal seam to simulate the underground coal gasification process and conduct a numerical simulation study on the injection of pure oxygen into the coal seam. The specific process is as follows:

[0137] First, a numerical simulation model of the coal seam is established based on the structural and physical property parameters of the actual coal seam, and then the mesh is generated.

[0138] In this embodiment, the numerical simulation model for underground coal gasification has dimensions of 20m × 10m × 1m. Multiple 1m × 1m × 1m grid cells are uniformly divided within the model. An injection well is located in the middle of the left boundary of the model, and a flow rate of 3 × 10⁻⁶ m / s is introduced into the injection well. -5 Pure oxygen was injected, and the injection well was heated at a fixed temperature of 800K. A production well was set in the middle of the right boundary of the underground coal gasification numerical simulation model, and the pressure of the production well was set at 3.3 × 10⁻⁶. 6 All other boundaries of the numerical simulation model for underground coal gasification are set as non-flowing adiabatic boundaries.

[0139] Next, the material properties and reaction kinetic parameters of the numerical simulation model for underground coal gasification are set. These material properties include, but are not limited to, the density, specific heat capacity, and thermal conductivity of the coal body; the density, specific heat capacity, and thermal conductivity of water; gas phase viscosity model parameters; initial permeability; initial fluid porosity; and the pre-exponential factor and activation energy in the Arrhenius equation for each chemical reaction step. In this embodiment, the initial pressure, temperature, water saturation, gas saturation, initial porosity, fluid porosity, and permeability of the coal seam are set, as shown in Table 1.

[0140] Table 1 Material property parameters of coal seams

[0141] .

[0142] The numerical simulation model for underground coal gasification establishes a multiphase, multi-component system comprising gas, liquid, and solid phases. A multi-step chemical reaction kinetic model is introduced, encompassing coal pyrolysis, carbon combustion, carbon-water vapor gasification, carbon-carbon dioxide gasification, and water-gas shift reaction. In this embodiment, the gas phase includes... , , , , and The liquid phase includes liquid water, and the solid phase includes fixed carbon, char, and ash, involving multiple chemical reactions, including... , , , , , , , ,in, The frequency factor is 1.9 × 10⁻⁶. 16 / sky; The frequency factor is 1.8 × 10⁻⁶. 6 / day, activation energy is 100kJ / mol, reaction energy is -393kJ / mol; The frequency factor is 4.7 × 10⁻⁶. 8 / day, activation energy is 156kJ / mol, reaction energy is 131kJ / mol; The frequency factor is 6.4 × 10⁻⁶. 9 / day, activation energy is 249kJ / mol, reaction energy is 172kJ / mol; The frequency factor is 2.4 × 10⁻⁶. 3 / day, activation energy is 126kJ / mol, reaction energy is 41kJ / mol; The frequency factor is 2.7 × 10⁻⁶. 7 / day, activation energy is 30kJ / mol, reaction energy is 206kJ / mol.

[0143] Meanwhile, the thermodynamic calculations use the Peng-Robinson equation of state to describe the non-ideal behavior of multi-component mixed gases under high temperature and high pressure, and set the basic physical property data such as critical temperature, critical pressure and eccentricity factor of various gas components required for the Peng-Robinson equation of state.

[0144] In this embodiment, the control equations of the underground coal gasification numerical simulation model are solved using a numerical solver for underground coal gasification, with an initial time step of 1×10⁻⁶. -5 With a maximum time step of 1 day and an adaptive step size control strategy, the convergence tolerance for the nonlinear iterative solution is set to 3 × 10⁻⁶. -5 The maximum number of iterations was set to 10, and the type and preprocessing method of the numerical solver for underground coal gasification, as well as the frequency of result output and the total simulation duration, were set to 30 days.

[0145] Numerical simulation calculations are performed using a fully implicit and fully coupled solution process in the coal underground gasification numerical solver. At each time step, the residuals of each conservation equation are first calculated based on the current field variables. If the preset convergence tolerance is not reached, the Jacobian matrix is ​​assembled, and the linear system is solved to update each principal variable. This iteration is repeated until the preset convergence tolerance is reached. After the calculation converges, the porosity dynamic evolution model is called according to the consumption of solid components to update the fluid porosity of each grid cell in the coal underground gasification numerical simulation model. The permeability dynamic evolution model is used to update the permeability of each grid cell in the coal underground gasification numerical simulation model in real time. The updated porosity and permeability are then applied to the flow and heat transfer calculations in the next time step to continue iterative calculations until the preset total simulation time is reached.

[0146] After the simulation of the underground coal gasification process is completed, the spatiotemporal distribution data of the entire computational domain of the underground coal gasification numerical simulation model at each specified time will be output, including but not limited to key physical quantities such as pressure field, temperature field, mole fraction field of each component of the gas phase, fluid porosity field and permeability field. At the same time, the component flow rate and calorific value of the gas produced at the production well will be output as curves of change over time.

[0147] In this embodiment, the distribution of various physical properties of the underground coal gasification numerical simulation model after 50 days of underground coal gasification is obtained using a numerical solver for underground coal gasification. Figures 3-7 As shown, based on the distribution of various physical fields, the advancement of the reaction front, the evolution of syngas components, and the dynamic changes in the pore structure and transport capacity of the coal body during underground coal gasification can be intuitively displayed, thus providing technical support for the scheme design and parameter optimization of underground coal gasification projects.

[0148] Of course, the above description is not intended to limit the present invention, and the present invention is not limited to the examples given above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A multi-field coupled numerical simulation method for underground coal gasification, characterized in that, Includes the following steps: Step 1: Construct a multi-field coupled physicochemical control model to simulate the underground coal gasification process; The physicochemical control model includes a multi-component mass conservation equation, an energy conservation equation, a momentum conservation equation, and a chemical reaction kinetics model. Step 2: Establish dynamic evolution models for porosity and permeability; Step 3: Construct a non-ideal gas thermodynamic model based on the PR-EOS equation of state to accurately simulate the real behavior of the gas phase in the coal seam; Step 4: Based on the actual coal seam's structural parameters and internal medium parameters, and combined with the physicochemical control model, porosity dynamic evolution model, permeability dynamic evolution model, and non-ideal gas thermodynamic model constructed in Steps 1 to 3, establish a numerical simulation model for underground coal gasification. Based on the finite volume method, spatially discretize the numerical simulation model for underground coal gasification, divide the computational domain of the numerical simulation model for underground coal gasification into multiple non-overlapping control volumes, determine the control equation set and residual equation set of the numerical simulation model for underground coal gasification, and form a numerical solver for underground coal gasification. Step 5: Use the underground coal gasification numerical solver to solve the control equations of the underground coal gasification numerical simulation model to obtain multiple physical fields of the underground coal gasification numerical simulation. In step 2, the porosity dynamic evolution model is as follows: ； In the formula, For fluid porosity; Porosity; The component number is the solid phase component number. This represents the total number of solid components. For the first The concentration of each solid component in the solid phase; For the first Molar mass of each solid component; For the first Mass density of each solid component; The dynamic evolution model of penetration rate is as follows: ； In the formula, for The absolute permeability of the coal seam at any given moment; for The absolute permeability of the coal seam at any given moment; for Fluid porosity of the coal seam at any given time; for Fluid porosity of the coal seam at any given time; Carmen-Kozeny coefficients; In step 3, the non-ideal gas thermodynamic model is: ； in, ； ； In the formula, Coal seam pressure; It is the universal gas constant; The temperature of the coal seam; This represents the molar volume of the gas phase. The gravitational coefficient of the mixture; The repulsion coefficient of the mixture; , These are all fluid component serial numbers. ; This represents the total number of fluid components. For the first Mole fraction of each gas phase component; For the first Mole fraction of each fluid component; The coefficient represents the binary interaction coefficient. For the first The pure component repulsion coefficient of each gas phase component; coal seam temperature Time The gravitational coefficients of the pure components of each gas phase component; coal seam temperature Time The gravitational coefficient of each gas phase component in its pure form.

2. The multi-field coupled numerical simulation method for underground coal gasification according to claim 1, characterized in that, In step 1, the multi-component mass conservation equation is: ； ； ； In the formula, For time steps; For fluid porosity; This refers to gas phase saturation. The molar density of the gas phase; For fluid component serial numbers; The component number is the solid phase component number. The chemical reaction number; For the first Mole fraction of each gas phase component; For gas injection wells and production wells; For gradient operators; This refers to the gas phase seepage velocity; For the first In the first chemical reaction, the first Each gas phase component is used as the stoichiometric coefficient of the product; For the first In the first chemical reaction, the first Each gaseous component serves as the stoichiometric coefficient of the reactants; For the first The reaction rate of a chemical reaction; The molar density of the liquid phase; Liquid phase saturation; The liquid phase percolation velocity; For liquid phase injection wells and production wells, it serves as a source and sink term; For the first In the first chemical reaction Each fluid component serves as the stoichiometric coefficient of the reactants; For the first In the first chemical reaction, the first Each fluid component is used as the stoichiometric coefficient of the product; For the first The concentration of each solid component in the solid phase; For the first In the first chemical reaction, the first Each solid component serves as the stoichiometric coefficient of the reactants; For the first In the first chemical reaction, the first Each solid component is used as the stoichiometric coefficient of the product; The energy conservation equation is as follows: ； In the formula, The coal seam medium is represented as follows: when the coal seam medium type is gas phase. When the coal seam medium type is liquid phase, it is represented as ; The internal energy of the coal seam medium; The molar density of the coal seam medium; The saturation level of the coal seam medium; Porosity; For the first The internal energy of each solid component; The molar density of the rock skeleton; The internal energy within the rock skeleton; The enthalpy of the coal seam medium; The seepage velocity of the coal seam medium; The overall thermal conductivity; The temperature of the coal seam; Source and sink terms for injection and production wells corresponding to the coal seam medium type; Injecting heat per unit volume per unit time into the heating well; For the first The enthalpy of a chemical reaction; The momentum conservation equation is: ； In the formula, This refers to absolute penetration rate; The relative permeability of the coal seam medium; The viscosity of the coal seam medium; The flow potential of the coal seam medium; The chemical reaction kinetic model is as follows: ； In the formula, For the first The rate constant of a chemical reaction , For the first The frequency factor of a chemical reaction It is a logarithmic function. For the first The activation energy of a chemical reaction. It is the universal gas constant; For the first The concentration of a fluid component in the fluid.

3. The multi-field coupled numerical simulation method for underground coal gasification according to claim 2, characterized in that, In step 4, the computational domain of the numerical simulation model for underground coal gasification is discretized into multiple non-overlapping control volumes based on the finite volume method. The multi-component mass conservation equation and energy conservation equation are then integrated for each control volume to obtain: ； ； ； ； In the formula, The first in the computational domain of the numerical simulation model for underground coal gasification The mesh volume of each mesh cell, in units of ; For the mesh volume; Using the divergence theorem, the volume integral of the convection term in the discretized multi-component mass and energy conservation equations is transformed into the surface integral of the boundary flow. The center value of each grid cell represents the average value of all physical quantities within the entire grid cell. The boundary integral of the convection term in the transformed multi-component mass and energy conservation equations is then discretized into the sum of the boundary flow rates. The governing equations for the numerical simulation model of underground coal gasification are obtained as follows: ； ； ； In the formula, , All are the serial numbers of the grid cells; For time step The next step in time; For the first The grid boundary of each grid cell; For grid boundaries; For time step; For the first Adjacent cells of a grid cell; For the first The grid cell and the first The average at the interface of each grid cell; This refers to the gas phase fluidity; For time step Time The grid cell and the first Transmission rate between individual grid cells; For time step Time Gas phase flow potential of each grid cell; For time step Time Gas phase flow potential of each grid cell; For time step Time Liquid phase flow potential of each grid cell; For time step Time Liquid phase flow potential of each grid cell; The Newton-Raphson method was used to obtain the residual equations of the numerical simulation model of underground coal gasification. ,get: ； ； ； ； In the formula, For time step Time Gas phase residual values ​​of each grid cell; For time step Time Liquid phase residual value of each grid cell; For time step Time Solid residual values ​​of each grid cell; For time step Time Energy residual value of each grid cell; For the first The grid cell and the first Contact area between grid cells; For the first The grid cell and the first The average overall thermal conductivity at the interface of each grid cell; For the first The distance from the center of each grid cell to the boundary; For the first The distance from the center of each grid cell to the boundary; Based on the residual equations of the numerical simulation model of underground coal gasification, an iterative calculation formula is constructed, yielding: ； ； in, ； In the formula, The main variable number is used as the main variable number. The main variables include fluid pressure, liquid saturation, fluid component mole fraction, coal seam temperature, and solid component concentration. This represents the number of iterations. Elements of a Jacobian matrix; For the first During the nth iteration calculation The change in each main variable; For time step Time In the _ grid cell, the _ ... The residuals of each main variable; For time step Time The main variable vector is calculated in each iteration; , The first sequence During the nth iteration calculation One vector of main variables; For the first The change in the vector of each main variable; For the first One main variable vector.

4. The multi-field coupled numerical simulation method for underground coal gasification according to claim 1, characterized in that, In step 5, the main variables are first selected to construct a set of unknown vectors. The main variables include fluid pressure, liquid saturation, fluid component mole fraction, coal seam temperature, and solid component concentration; based on the discretized residual equations... By numerical perturbation, the partial derivatives of each residual equation with respect to each principal variable are calculated, forming the Jacobian matrix. Solve the system of linear equations To obtain the change in the main variable At the current time step, each master variable is updated until the residual norm of each grid cell in the underground coal gasification numerical simulation model is lower than the preset tolerance. Then, the underground coal gasification numerical simulation calculation is stopped, and multiple physical fields of the underground coal gasification process are obtained, including pressure field, temperature field, gas phase component mole fraction field, fluid porosity field and permeability field.

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