THMC multi-field coupling simulation method and system for process of displacing CH4 by coal seam CO2

Abstract

The invention is applicable to the technical field of coal bed gas development and geological sequestration, and provides a THMC multi-field coupling simulation method and system for a process of displacing CH4 by coal bed CO2, and the method comprises the following steps: collecting multi-source parameter information of a target coal bed; according to the multi-source parameter information, constructing a coal seam structure model comprising a matrix and a crack system; constructing a control equation system based on the coal seam structure model, and introducing a multi-field coupling mechanism to obtain a fully-coupled mathematical model; performing numerical solution on the fully-coupled mathematical model to obtain a spatio-temporal evolution result of the key field variables; and according to the spatio-temporal evolution result of the key field variable, evaluating the index of the target coal seam. According to the invention, a heat-force-gas-chemical multi-physical field full-coupling simulation system is constructed, so that the CH4 recovery rate and the CO2 storage stability can be accurately predicted. The method is suitable for gas injection-storage coupling optimization of a multi-coal-rank and low-permeability coal seam, and has high precision, high adaptability and remarkable engineering guidance value.

Description

Technical Field

[0001] This invention belongs to the field of coalbed methane development and geological storage technology, and particularly relates to a THMC multi-field coupling simulation method and system for the process of CO2 displacement of CH4 in coal seams. Background Technology

[0002] As an unconventional natural gas resource, coalbed methane (CBM) holds significant strategic importance in the synergistic utilization of low-carbon energy transition and carbon sequestration. Traditional CBM extraction primarily relies on reducing formation pressure to drive CH4 desorption and dispersion. However, in deep, high-rank, and low-permeability coal seams, the small porosity, low permeability, and high adsorption energy limit gas transport and generally result in low recovery rates, making it difficult to meet both economic and carbon emission reduction requirements.

[0003] To improve enhanced coal recovery efficiency while meeting carbon sequestration requirements, the CO2-ECBM (CO2-driven CH4 displacement) development model has become a hot research and engineering application area. This technology utilizes the stronger adsorption affinity of CO2 molecules compared to CH4, allowing them to competitively adsorb and occupy active sites on the coal surface. This promotes the desorption of methane from the coal matrix and its migration to the fracture system, achieving simultaneous CH4 recovery and CO2 sequestration. Furthermore, some CO2 can be adsorbed in the coal micropores or react with minerals to form stable carbonates, enabling long-term geological sequestration.

[0004] However, coal seams are multi-scale media with an organic-inorganic porous composite structure, undergoing various processes during CO2 injection, including heat conduction, mechanical deformation, gas adsorption-desorption, and mineral-fluid reactions. These processes interact and are nonlinearly coupled, exhibiting complex coupling effects of adsorption-induced strain, pore reconstruction, and permeability evolution. Existing studies mostly employ single-field or weakly coupled models, which struggle to characterize the stress-seepage-thermal field feedback caused by competitive adsorption and cannot reflect the multi-field evolution characteristics of the coal body under dynamic injection and production conditions in real time.

[0005] In addition, traditional numerical simulations often use fixed boundaries and static parameter settings, which cannot achieve rolling correction of the model as temperature, stress and gas concentration change, resulting in large deviations in the results and making it difficult to meet the dynamic solution requirements of deep CO2-CH4 interaction processes.

[0006] To address the aforementioned issues, there is an urgent need to establish a fully coupled method that can connect adsorption-strain-percolation multi-field feedback with a multi-platform rolling adaptive solution mechanism. This method should not only reveal the pore structure reconstruction and permeability evolution driven by competitive adsorption, but also achieve real-time linkage updates of temperature, stress, and gas composition parameters during the calculation process. Summary of the Invention

[0007] The purpose of this invention is to provide a THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams. It aims to systematically reveal the coupling law of coal body structure evolution, permeability change and storage stability during CO2 injection through multi-physics field collaborative simulation, so as to achieve high-precision dynamic prediction of the enhanced mining efficiency and storage safety of deep low-permeability coal seams. This solves the problems of weak multi-field coupling, lack of consideration of adsorption strain, model response lag and non-real-time boundary update in existing simulations of the CO2 displacement of CH4 process in coal seams.

[0008] This invention is implemented as follows: a THMC multi-field coupled simulation method for the CO2 displacement of CH4 process in coal seams, comprising the following steps: Collect multi-source parameter information of the target coal seam; the multi-source parameter information includes mechanical parameters, thermal property parameters, adsorption characteristic parameters, coal petrology information, and fracture distribution information; Based on multi-source parameter information, a coal seam structure model including matrix and fracture systems is constructed. Based on the coal seam structure model, a governing equation system is constructed, and a multi-field coupling mechanism is introduced to obtain a fully coupled mathematical model. The governing equation system includes one or more of the following: heat conduction equation, elasticity equation, multi-component seepage equation, competitive adsorption model, and chemical reaction-property evolution model. The multi-field coupling mechanism includes one or more coupling paths of the following: heat-stress, stress-permeability, permeability-chemistry, and heat-chemistry-permeability. Numerical solutions were obtained for the fully coupled mathematical model to yield the spatiotemporal evolution results of the key field variables; Based on the spatiotemporal evolution of key field variables, the indicators of the target coal seam are evaluated; the indicators include one or more of CH4 recovery rate, CO2 storage stability, fracture propagation degree and thermal stress sensitivity.

[0009] Furthermore, the heat conduction equation is as follows: ; in, r 1 represents the density of the rock mass; c Specific heat capacity; T For temperature; t For time; l 1 represents thermal conductivity; Q This is an internal heat source item.

[0010] Furthermore, the elasticity equation is as follows: ; in, s For stress tensor; f It is the volumetric force density; e ij For strain tensor; l2 , m Let be the Lamé constant, where m It is the shear modulus. l 2 Related to Young's modulus and Poisson's ratio; d ij It is the Kronecker delta function; e kk For volumetric strain.

[0011] Furthermore, the multi-component seepage equation is as follows: ; in, ϕ Porosity; r 2 represents the fluid density; v q represents Darcy velocity; q represents source and sink terms; t represents time.

[0012] Furthermore, the competitive adsorption model is as follows: ; in, q i Let i be the amount of coal adsorbed per unit mass of gas i; V L ᵢ Let be the Langmuir capacity constant of gas i; bᵢ Let be the Langmuir constant for gas i; pᵢ Let i be the partial pressure of the gas; j For all types of adsorbed gases; β The adsorption strain sensitivity coefficient, The effective stress is used to reflect the inhibitory effect of stress on adsorption capacity.

[0013] Furthermore, the chemical reaction-property evolution model is as follows: ; in, D The diffusion coefficient is denoted as . D 0 represents the initial diffusion coefficient; E α R is the diffusion activation energy; R is the gas constant; T is the temperature.

[0014] Furthermore, the stress-permeability coupling formula is as follows: ; in, K This represents the current penetration rate. K 0 initial permeability; γ is the stress sensitivity coefficient; Effective stress; Adsorption-induced volumetric strain; orThe adsorption coupling coefficient is denoted as . The formula for the coupling of heat and stress is as follows: ; in, s th Thermal stress; E Young's modulus; α Δ is the coefficient of thermal expansion; T For temperature change; The coupling formula for heat, chemistry, and permeability is as follows: ; in, D The diffusion coefficient is denoted as . D 0 represents the initial diffusion coefficient; E α R is the diffusion activation energy; R is the gas constant; T is the temperature.

[0015] Furthermore, the steps of numerically solving the fully coupled mathematical model to obtain the spatiotemporal evolution results of the key field variables specifically include: The fully coupled mathematical model is discretized using the finite element method or the finite difference method. Local mesh refinement is set in combination with the crack guiding structure. An implicit-explicit joint time step control method is selected. Multiphysics joint solution is performed using at least one platform among TOUGH2, FLAC3D and COMSOL. The spatiotemporal evolution results of key field variables are output.

[0016] Furthermore, the steps for evaluating the indicators of the target coal seam based on the spatiotemporal evolution results of key field variables specifically include: Based on the spatiotemporal evolution results of key field variables, obtain one or more evolution diagrams of temperature field, stress field, seepage field and gas concentration field; The indicators of the target coal seam are evaluated based on one or more evolution diagrams of the temperature field, stress field, seepage field, and gas concentration field.

[0017] Another objective of this invention is to provide a THMC multi-field coupled simulation system for the CO2 displacement of CH4 process in coal seams, used to implement the above-mentioned THMC multi-field coupled simulation method, comprising: The multi-source parameter information acquisition module is used to acquire multi-source parameter information of the target coal seam; the multi-source parameter information includes mechanical parameters, thermal property parameters, adsorption characteristic parameters, coal petrology information, and fracture distribution information; The coal seam structure model building module is used to build a coal seam structure model including the matrix and fracture system based on multi-source parameter information. The coupling mechanism setting module is used to construct a system of governing equations based on a coal seam structure model and introduce a multi-field coupling mechanism to obtain a fully coupled mathematical model. The system of governing equations includes one or more of the following: heat conduction equation, elasticity equation, multi-component seepage equation, competitive adsorption model, and chemical reaction-property evolution model. The multi-field coupling mechanism includes one or more coupling paths of the following: heat-stress, stress-permeability, permeability-chemistry, and heat-chemistry-permeability. The rolling solver module is used to numerically solve the fully coupled mathematical model and obtain the spatiotemporal evolution results of key field variables; The evolution results analysis module is used to evaluate the indicators of the target coal seam based on the spatiotemporal evolution results of key field variables.

[0018] This invention can achieve a detailed simulation of the entire process of CO2 displacement and simultaneous sequestration of CH4 in deep coal seams using a multiphysics field. Compared with existing technologies, this invention has the following significant advantages and innovations: 1. Mechanism innovation: An adsorption-strain-seepage feedback mechanism is introduced to establish a quantitative relationship between coal pore structure reconstruction and permeability evolution driven by competitive adsorption, breaking through the limitation of traditional CO2-ECBM simulation that only considers pressure and seepage effects.

[0019] 2. Model dynamism: The FLAC3D and COMSOL joint rolling solution system realizes real-time linkage of multi-field data and adaptive boundary adjustment, which overcomes the error accumulation problem caused by fixed parameters in static models and improves the accuracy and stability of the simulation of CO2–CH4 coupling process in deep coal seams.

[0020] 3. Strong engineering adaptability: It is suitable for the combined design of enhanced mining and storage in deep (>800m), high-rank, and low-permeability coal seams, and can provide decision support for CO2 injection and production well network layout, storage capacity assessment and injection and production control strategies.

[0021] 4. Results visualization and intelligentization: The model can output three-dimensional field distribution maps, time-series response curves and sequestration safety criteria. It can be linked with monitoring data to achieve real-time comparison and optimization, providing intelligent tools for green mining and carbon sequestration of coalbed methane. Attached Figure Description

[0022] Figure 1 This is a schematic flowchart of the THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams provided in an embodiment of the present invention.

[0023] Figure 2 This is a schematic diagram of a typical coal seam structure model.

[0024] Figure 3 A diagram illustrating the interaction mechanism between multiple fields (thermal, mechanical, percolation, and chemical) provided for embodiments of the present invention.

[0025] Figure 4 The evolution curve of storage efficiency with gas injection time shows three stages: initial rapid growth, stable injection, and plateau stabilization. Figure 5 The diagram shows the evolution of the coal seam temperature field, illustrating the spatial distribution and expansion trend of heat conduction during CO2 injection. Figure 6 This is a porosity variation diagram, which uses color layers to show the evolution characteristics of the pore structure of the coal seam during the gas injection-reaction-expansion process.

[0026] Figure 7 The simulation results are shown in the example of the Qinshui Basin.

[0027] Figure 8 This is a schematic diagram of the inter-well coordinated injection-production-storage path. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0029] This invention proposes a thermo-mechanical-gas-chemical (THMC) multi-field fully coupled simulation method for the CO2 displacement of CH4 process in coal seams. This method integrates heat conduction, mechanical response, multi-component seepage, and adsorption-reaction kinetics, introducing an adsorption-strain-seepage feedback mechanism to simulate the dynamic evolution of the coal seam pore and fracture system after CO2 injection. Simultaneously, a multi-field rolling adaptive solver system based on the FLAC3D and COMSOL joint interface is constructed to achieve real-time coupled calculation of temperature, stress, gas pressure, and chemical reaction parameters. This method overcomes the limitation of traditional UCG gasification models, which are only applicable to high-temperature combustion scenarios, and can accurately predict the competitive adsorption and storage behavior of CO2–CH4 under medium- and low-temperature, non-combustion conditions, providing theoretical and engineering support for enhanced coal seam mining and carbon sequestration optimization.

[0030] like Figure 1 As shown, in one embodiment of the present invention, a THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams is provided, comprising the following steps: S100: Collect multi-source parameter information of the target coal seam; S200. Based on multi-source parameter information, construct a coal seam structure model that includes the matrix and fracture system; S300. Based on the coal seam structure model, a system of governing equations is constructed, and a multi-field coupling mechanism is introduced to obtain a fully coupled mathematical model. S400. Numerical solution of the fully coupled mathematical model to obtain the spatiotemporal evolution results of key field variables; S500: Evaluate the indicators of the target coal seam based on the spatiotemporal evolution results of key field variables.

[0031] In this embodiment of the invention, a multi-field response model based on the adsorption-strain-permeation feedback mechanism is constructed to quantitatively describe the pore reconstruction and dynamic evolution of permeability caused by competitive adsorption. In addition, by developing the FLAC3D and COMSOL rolling adaptive solution system, real-time linkage of thermo-mechanical-gas-chemical multi-field parameters and automatic correction of boundary conditions are realized.

[0032] Specifically, in step S100, the multi-source parameter information includes: 1. Mechanical parameters: Young's modulus, Poisson's ratio, compressive strength, and initial ground stress distribution, used to establish a mechanical response model; 2. Thermal properties: thermal conductivity, specific heat capacity, and geothermal gradient, used for calculating the heat conduction field; 3. Adsorption characteristic parameters: The Langmuir isotherm parameters (V) of CO2 and CH4 were obtained experimentally. L (and b value), construct an adsorption-desorption model; 4. Coal petrology and mineralogy information: including the main components (quartz, clay, carbonates, etc.) and their reactivity, for use in subsequent chemical reaction modeling; 5. Crack distribution information: Crack density, opening, direction and other parameters are obtained from CT scans, geological outcrops or manual tests to provide boundary conditions for crack modeling.

[0033] Based on the aforementioned multi-source parameter information, combined with well logging profiles and regional geological maps, two-dimensional or three-dimensional coal seam structure models are constructed using modeling software (such as COMSOL, TOUGH Builder, or FLAC3D Preprocessor). These models distinguish between the matrix and fracture systems, achieving a precise representation of the multi-scale structure of the coal seam. Specifically, for example... Figure 2 As shown, this figure displays the three-dimensional spatial structure of the coal seam in the form of geological modeling software output, including the differences in the matrix and fracture system. The model clearly shows the distribution of the main coal seam and interbedded gangue layers, the fracture orientation, density, and aperture, and overlays the spatial arrangement of injection and production wells. Different colors or transparency are used to distinguish fracture conductivity in the figure, and wellbore parameters (such as depth, completion section location, etc.) are also included to guide the reasonable layout of injection and production wells and the initial settings of the coupled simulation.

[0034] In step S200, the governing equation system includes the heat conduction equation, the elasticity equation, the multi-component percolation equation, the competitive adsorption model, and the chemical reaction-property evolution model (adsorption-desorption-reaction kinetic equation). An adsorption-strain-percolation feedback mechanism is introduced into the model. The Langmuir competitive adsorption model is used to describe the volumetric strain and permeability changes caused by the CO2-CH4 interaction. The Arrhenius dependence of the gas diffusion coefficient on temperature is established to reflect the temperature-chemical coupling effect.

[0035] The heat conduction equation (Fourier's Law) is as follows: ; in, r 1 represents the density of the rock mass (kg / m³) 3 ); c Specific heat capacity (J / (kg·K)); T Temperature (K); t Time (s); l 1 represents thermal conductivity (W / (m·K)); Q Internal heat source term (W / m) 3 For example, heat release during the gas injection process.

[0036] The equations of elasticity (linear elasticity equilibrium equations) are as follows: ; in, s For stress tensor (Pa); f Volumetric force density (N / m³) 3 ), such as gravity; e ij For strain tensor; l 2 , m Let be the Lamé constant, where m It is the shear modulus. l 2 Related to Young's modulus and Poisson's ratio; d ij For the Kronecker delta function (identity matrix); e kk This represents volumetric strain (strain trace).

[0037] Furthermore, the multi-component seepage equation (Darcy–Fick model) is as follows: ; in, ϕ Porosity (dimensionless); r 2 represents the fluid density (kg / m³) 3 ); vDarcy velocity (m / s); q represents the source and sink terms (kg / (m²)). 3 ·s), such as gas injection or production; t is time (s).

[0038] The competitive adsorption model (stress-corrected Langmuir equation) is as follows: ; in, q i The adsorption capacity of gas i per unit mass of coal (m³) 3 / t); V L ᵢ Let m be the Langmuir capacity constant of gas i. 3 / t); bᵢ Let i be the Langmuir constant (MPa). -1 ); pᵢ Let i be the partial pressure of the gas (MPa); j For all types of adsorbed gases (e.g., CO2 and CH4); β The adsorption strain sensitivity coefficient is 1 / Pa. The effective stress is used to reflect the inhibitory effect of stress on adsorption capacity.

[0039] The chemical reaction-property evolution model (Arrhenius equation) is as follows: ; in, D The diffusion coefficient (m) 2 / s); D 0 is the initial diffusion coefficient (m 2 / s); E α R is the diffusion activation energy (J / mol); R is the gas constant (J / (mol·K)); T is the temperature (K), ensuring the dynamic response relationship of each physical field.

[0040] In addition, such as Figure 3 As shown, when constructing the thermo-mechanical-permeability-chemical (THMC) multiphysics simulation model of the CO2 displacement of CH4 process in coal seams, the system introduces a multi-field coupling mechanism, including four typical coupling paths: thermo-stress, stress-permeability, permeability-chemistry, and thermo-chemical-permeability, in order to comprehensively describe the response process of coal under complex thermo-fluidization conditions. Figure 3In this diagram, arrows and nodes are used to construct a causal path diagram, representing the interaction paths and feedback relationships between different physical fields. For example, the thermal field (temperature increase) can induce thermal expansion, thereby changing the stress field (thermo-mechanical coupling); stress changes affect crack opening and permeability, thus affecting the fluid seepage path (mechanical-permeability coupling); CO2 carried by the fluid reacts chemically with coal minerals, affecting pore structure and adsorption capacity (permeability-chemical coupling); and the heat released or absorbed by the chemical reaction is fed back to the thermal field (thermo-chemical coupling). This diagram clearly depicts the closed-loop coupling relationship and main control path between the four fields of THMC, providing a basis for mechanism analysis and modeling parameter setting.

[0041] Specifically, to realize the dynamic interaction between various physical fields, the model embeds the following three types of core equations: The stress-permeability coupling formula (stress-controlled permeability model) is as follows: ; in, K Current penetration rate (m 2 ); K 0 Initial penetration rate (m 2 ); γ is the stress sensitivity coefficient (1 / Pa); Effective stress (Pa); The volumetric strain (Pa) induced by adsorption; or The adsorption coupling coefficient is 1 / Pa. The coupling formula for heat and stress (expression of thermal stress) is as follows: ; in, s th Thermal stress (Pa); E Young's modulus (Pa); α Δ is the coefficient of thermal expansion (1 / K); T Temperature change (K); The coupling formula of heat-chemistry-permeability (temperature-dependent diffusion coefficient expression) is as follows: ; in, D The diffusion coefficient (m) 2 / s); D 0 is the initial diffusion coefficient (m 2 / s); E α R is the diffusion activation energy (J / mol); R is the gas constant (J / (mol·K)); T is the temperature (K).

[0042] The above-mentioned multi-field coupling paths are explained as follows: 1. Thermal-stress coupling path: to more realistically simulate the coal body response process; 2. Stress-permeability coupling path: Effective stress ( The change affects the degree of fracture opening, thereby controlling the dynamic change of permeability (K) and regulating the gas flow channel; 3. Permeability-chemical coupling pathway: Gas concentration distribution affects adsorption-desorption rate; mineral reaction products (such as carbonate precipitation) can further alter pore structure and flow channels; 4. Thermal-chemical-permeability coupling path: Temperature changes regulate reaction and diffusion rates, which in turn feed back into the permeation field and adsorption behavior, enabling dynamic simulation of the entire process.

[0043] In this embodiment of the invention, the pore reconstruction effect caused by adsorption-induced strain is incorporated into the seepage control equation for the first time. By introducing an adsorption-strain-seepage feedback mechanism, the dynamic feedback between competitive adsorption, stress, and pore structure is revealed, and the process adaptive response of gas migration behavior is realized.

[0044] In a preferred embodiment of the present invention, the step of numerically solving the fully coupled mathematical model to obtain the spatiotemporal evolution results of key field variables specifically includes: The fully coupled mathematical model is discretized using the finite element method (FEM) or the finite difference method (FDM). Local mesh refinement is set in conjunction with the crack flow guiding structure. An implicit-explicit joint time step control method is selected. Multiphysics joint solution is performed using at least one platform among TOUGH2, FLAC3D and COMSOL. The spatiotemporal evolution results of key field variables are output.

[0045] In practical applications, a joint FLAC3D and COMSOL solution system can be used to achieve rolling adaptive updates of multi-field data and linked correction of boundary conditions. The FLAC3D and COMSOL joint solution system includes a data interface module, a boundary monitoring module, and a real-time feedback module. Temperature, stress, and seepage parameters are updated synchronously through the FLAC3D and COMSOL interface to achieve dynamic equilibrium for numerical convergence. FLAC3D is responsible for the real-time updates of the mechanical and seepage fields, while COMSOL handles the calculations for heat conduction and chemical reactions. The two are implemented through the data interface module. (1) Parameter linkage: real-time bidirectional transmission of temperature, pore pressure, stress and gas concentration; (2) Boundary rolling: Automatically corrects boundary conditions and field variables over time steps; (3) Convergence monitoring: A dynamic time step and adaptive iteration strategy are adopted to ensure computational stability and accuracy.

[0046] In a preferred embodiment of the present invention, the step of evaluating the indicators of the target coal seam based on the spatiotemporal evolution results of key field variables specifically includes: Based on the spatiotemporal evolution results of key field variables, obtain one or more evolution diagrams of temperature field, stress field, seepage field and gas concentration field; The indicators of the target coal seam are evaluated based on one or more evolution diagrams of the temperature field, stress field, seepage field, and gas concentration field. Among these indicators are CH4 recovery rate, CO2 storage stability, fracture propagation degree, and thermal stress sensitivity.

[0047] The model supports automatic mesh refinement and local solution optimization for detailed calculations in fracture zones and high-gradient regions. The output spatiotemporal evolution results include temperature field, stress field, seepage field, and concentration distributions of components such as CO2 / CH4 (gas concentration field), generating evolution diagrams such as storage efficiency time-series curves, CO2 / CH4 concentration isosurface maps, fracture opening rate variation diagrams, and storage stability index curves. These are compared and verified with measured data to generate an evaluation report for engineering decision-making. Inversion verification is performed using field or experimental data to achieve rolling model correction.

[0048] The method provided in this embodiment of the invention is adaptable to the development and design of coal seam gas injection in different regions, coal grades and stress backgrounds, and has good versatility and expansion potential.

[0049] In another embodiment of the present invention, a THMC multi-field coupling simulation system for the CO2 displacement of CH4 process in coal seams is also provided, for implementing the above-mentioned THMC multi-field coupling simulation method, comprising: The multi-source parameter information acquisition module is used to acquire multi-source parameter information of the target coal seam; the multi-source parameter information includes mechanical parameters, thermal property parameters, adsorption characteristic parameters, coal petrology information, and fracture distribution information; The coal seam structure model building module is used to build a coal seam structure model including the matrix and fracture system based on multi-source parameter information. The coupling mechanism setting module is used to construct a system of governing equations based on a coal seam structure model and introduce a multi-field coupling mechanism to obtain a fully coupled mathematical model. The system of governing equations includes one or more of the following: heat conduction equation, elasticity equation, multi-component seepage equation, competitive adsorption model, and chemical reaction-property evolution model. The multi-field coupling mechanism includes one or more coupling paths of the following: heat-stress, stress-permeability, permeability-chemistry, and heat-chemistry-permeability. The rolling solver module is used to numerically solve the fully coupled mathematical model and obtain the spatiotemporal evolution results of key field variables; The evolution results analysis module is used to evaluate the indicators of the target coal seam based on the spatiotemporal evolution results of key field variables.

[0050] The system provided in this invention can run in a combined FLAC3D and COMSOL environment, supporting custom interfaces and secondary development. The rolling solver module features inter-field adaptive iteration, automatically adjusting the calculation step size and boundary constraints in response to changes in temperature, stress, and adsorption pressure. This system is suitable for deep, low-permeability, high-rank coal seams with permeability less than 1 mD, and the coal rank range covers high-volatile coal to anthracite.

[0051] It should be noted that each of the above modules can be implemented as a computer program, which can run on a computer device. The computer device's memory can store the computer program that makes up each module, enabling the processor to execute each step of the above method.

[0052] The following examples illustrate the application of the THMC multi-field coupling simulation method and system for the CO2 displacement of CH4 process in coal seams provided by the present invention in actual coal seam research.

[0053] Example 1: A deep coal seam on the northwestern edge of the Junggar Basin in Xinjiang was used as the research object. The coal seam was buried at a depth of about 1150 meters, and the coal rank was lean to poor coal with relatively closed fractures. The original gas content was about 14.7 m³. 3 / t, CO2 adsorption capacity is significantly higher than CH4. The selected area is a typical low-permeability, high-stress coal seam system, which meets the combined requirements of CO2 sequestration and enhanced coal production.

[0054] I. Model input parameters include: Thermophysical properties: thermal conductivity 0.19 W / (m·K), specific heat capacity 870 J / (kg·K); Mechanical parameters: Young's modulus 3.6 GPa, Poisson's ratio 0.21, initial ground stress is vertical stress > horizontal principal stress; Adsorption characteristic parameters: Langmuir capacity is V L(CO2) =28.4m 3 / t, V L(CH4) =16.1m 3 / t, constant b CO2 =0.37MPa -1 ; The simulation uses the COMSOL multiphysics platform to construct a 3D coal seam structure and well network layout, and sets the single-well gas injection pressure to 10MPa, with continuous gas injection for 120 days.

[0055] Among them, the numerical simulation results of the coal seam CO2 injection-storage process in the Junggar Basin implementation example are as follows: Figure 4 , Figure 5 , Figure 6 As shown, the three figures together reflect the impact of the thermo-mechanical-chemical response mechanism of the Junggar coal seam on the storage efficiency, providing a model basis for the spatiotemporal optimization and control of the storage process.

[0056] II. Simulation results show that: 1. During CO2 injection, the coal seam temperature rises by about 5-8℃, causing increased local expansion stress and permeability by about 4.8 times; 2. CH4 recovery rate increased by 17.6%, and the storage stability index (based on gas residual diffusion gradient and porosity variation) score reached Grade A; 3. The mineral-CO2 reaction is mainly concentrated in the calcite-rich area, forming secondary pores and enhancing the adsorption and storage capacity; 4. Compared with the comparative simulation of the uncoupled thermodynamic process, the coupled model predicts an increase in recovery efficiency of 13.2% and a reduction in storage simulation bias of 21%.

[0057] This implementation example demonstrates that the method provided by the present invention still possesses excellent prediction and storage optimization capabilities under complex geological and high-stress environments, providing effective technical support for the deployment of CO2-ECBM projects in deep coal seams in western China.

[0058] Example 2: A medium-deep coal seam in the southern Qinshui Basin of Shanxi Province was used as the research object. The coal seam was buried at a depth of about 720 meters, and the coal rank was gas coal to lean coal. The fractures were relatively well developed, and the original gas content was 18.3 m³. 3 The area possesses a good natural drainage network. It is a typical medium-permeability, stress-sensitive coal seam system, suitable for research on CO2-CH4 competitive adsorption-enhanced extraction and long-term storage.

[0059] I. Model input parameters include: Thermophysical properties: thermal conductivity 0.27 W / (m·K), specific heat capacity 930 J / (kg·K); Mechanical parameters: Young's modulus 4.2 GPa, Poisson's ratio 0.24, initial principal stresses exhibit a normal stress gradient distribution (σv>σH>σh); Adsorption characteristic parameters: Langmuir capacity is V L(CO2) =30.5m 3 / t, V L(CH4) =19.2m 3 / t, constant b CO2 =0.41MPa -1 ; The simulation platform uses the FLAC3D–TOUGH2 joint calculation module to construct a 1.5km×1.5km three-dimensional coal seam model and sets up a five-point injection-production well network of "central gas injection – four-sided gas production". The gas injection pressure is set to 8MPa and the cumulative injection cycle is 180 days.

[0060] Among them, the simulation results of the Qinshui Basin implementation example are as follows: Figure 7As shown in the figure, the multiphysics response of the coal seam during CO2 injection is illustrated: 1. The CO2 seepage path is depicted through color gradients to represent the evolution of the CO2 saturation isosurface; 2. The CH4 concentration field is displayed as isochoric lines showing the concentration changes before and after extraction; 3. The fracture response is shown through vector arrows or permeability change layers, illustrating the evolution of fracture opening direction and scale. This figure integrates three-dimensional spatial field and temporal dynamic characteristics, intuitively reflecting the comprehensive effect of gas injection on the coal seam structure and component migration.

[0061] II. Simulation results show: 1. The temperature rise in the gas injection area is 3-6℃, and the coal volume expansion rate is approximately 1.2%; 2. The permeability increased to a maximum of 3.9 times the original value as the coal body expanded, and the CH4 extraction rate reached 23.1%; 3. There is a significant competitive adsorption zone between CO2 and CH4, and the interface is controlled by the initial gas saturation and the conductivity of local cracks. 4. The overall storage stability index score is A−, and the residual diffusion flux remains low, indicating good long-term storage stability. 5. The multi-field linkage of heat, force and gas significantly improves the accuracy of sequestration prediction. Compared with single-physics field simulation, the effective CO2 sequestration capacity increases by about 19.5%.

[0062] This embodiment illustrates that the method provided by the present invention is applicable to coal seam systems with medium burial depth and well-developed fractures, and can effectively improve CBM mining efficiency and enhance the long-term stability of sealing behavior.

[0063] Example 3: Taking a shallow coal seam on the eastern edge of the Ordos Basin in Inner Mongolia as the research object, the coal seam is buried at a depth of 460 meters, is a long-flame coal, and has well-developed fractures. However, due to the loose and easily deformable nature of the coal, the original permeability is low. This area is a typical shallow-buried, weakly stressed, high-mineral-content coal seam, possessing the advantages of economic injection-production and mineral preservation coupling. The inter-well coordinated injection-production-preservation path is as follows: Figure 8 As shown in the figure, a typical CO2 injection well and CH4 production well layout is illustrated. Red arrows indicate the path of CO2 migration from the injection well through the fracture system into the coal seam and its gradual expansion. The blue shaded area represents the evolution of the CH4 production space as the injection process progresses. The fracture network is represented by lines, reflecting the coal seam's conductivity and channel structure. Stress response curves are included at the bottom, showing the coal seam expansion, fracture opening, and permeability enhancement processes caused by gas injection. The dip zone at the top reflects the coal seam's bedding tendency. The overall graphic integrates structure, injection-production relationship, and mechanical response, revealing the spatial evolution characteristics of CO2 displacing CH4 and the path of enhanced sequestration in a multi-well system.

[0064] I. Model input parameters include: Thermophysical properties: thermal conductivity 0.22 W / (m·K), specific heat capacity 910 J / (kg·K); Mechanical parameters: Young's modulus 2.9 GPa, Poisson's ratio 0.19, principal stress is horizontal principal stress > vertical stress, strong sensitivity to crack closure; Adsorption characteristic parameters: Langmuir capacity V L(CO2) =25.7m 3 / t, V L(CH4) =14.6m 3 / t, constant b CO2 =0.34MPa -1 ; A self-built THMC coupled numerical platform was used to simulate a typical "injection-production-mineral reaction" linkage process. The gas injection pressure was set to 7 MPa and the simulation period was 90 days.

[0065] II. The simulation results indicate that: 1. Due to the lower temperature in the shallow layer, heat conduction is mainly concentrated in the area 5-10m around the injection well, with a temperature rise of about 2-3℃; 2. Stress changes trigger slight closure of the cracks, leading to an initial decrease in permeability. However, the subsequent mineral-CO2 reaction generates carbonate cement, which strengthens the support of the crack walls, allowing the permeability to gradually recover to 1.3 times its initial value. 3. There is a clear "mineral dissolution-reprecipitation-pore conditioning" process, which has a positive effect on storage stability; 4. CH4 recovery rate increased by 12.4%, CO2 residual diffusion and re-adsorption coefficients decreased significantly, and storage stability index was rated B+; This embodiment demonstrates that the method provided by the present invention is also applicable to low-stress, high-mineral coal seam environments, and is particularly suitable for developing a combined "mineral reaction + physical adsorption" sequestration scheme to enhance the technological added value of shallow CBM engineering.

[0066] In summary, this invention proposes a THMC multi-field coupled simulation method applicable to the CO2 displacement of CH4 process in coal seams. For the first time, it integrates heat conduction, mechanical response, multiphase flow, and adsorption-chemical reaction systems into a unified modeling framework, comprehensively reflecting the co-evolution mechanism of various physical fields in coal seams during gas injection development and carbon sequestration. This method achieves high-precision prediction through numerical simulation and can adapt to coalbed methane development and sequestration evaluation under different coal ranks, burial depths, and tectonic stress conditions, demonstrating good scalability and engineering applicability. This invention can significantly improve CH4 recovery efficiency and CO2 sequestration stability, providing strong support for unconventional natural gas development and carbon emission reduction technologies. This method is applicable to gas injection-storage coupling optimization in multi-rank, low-permeability coal seams, exhibiting high accuracy, strong adaptability, and significant engineering guidance value.

[0067] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this patent should be determined by the appended claims.

Claims

1. A THMC multi-field coupled simulation method for the CO2 displacement of CH4 process in coal seams, characterized in that, Includes the following steps: Collect multi-source parameter information of the target coal seam; the multi-source parameter information includes mechanical parameters, thermal property parameters, adsorption characteristic parameters, coal petrology information, and fracture distribution information; Based on multi-source parameter information, a coal seam structure model including matrix and fracture systems is constructed. Based on the coal seam structure model, a governing equation system is constructed, and a multi-field coupling mechanism is introduced to obtain a fully coupled mathematical model. The governing equation system includes one or more of the following: heat conduction equation, elasticity equation, multi-component seepage equation, competitive adsorption model, and chemical reaction-property evolution model. The multi-field coupling mechanism includes one or more coupling paths of the following: heat-stress, stress-permeability, permeability-chemistry, and heat-chemistry-permeability. Numerical solutions were obtained for the fully coupled mathematical model to yield the spatiotemporal evolution results of the key field variables; Based on the spatiotemporal evolution of key field variables, the indicators of the target coal seam are evaluated; the indicators include one or more of CH4 recovery rate, CO2 storage stability, fracture propagation degree and thermal stress sensitivity.

2. The THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams according to claim 1, characterized in that, The heat conduction equation is as follows: ； in, ρ 1 represents the density of the rock mass; c Specific heat capacity; T For temperature; t For time; λ 1 represents thermal conductivity; Q This is an internal heat source item.

3. The THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams according to claim 1, characterized in that, The elasticity equation is as follows: ； in, σ For stress tensor; f It is the volumetric force density; ε ij For strain tensor; λ 2 , μ Let be the Lamé constant, where μ It is the shear modulus. λ 2 Related to Young's modulus and Poisson's ratio; δ ij It is the Kronecker delta function; ε kk For volumetric strain.

4. The THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams according to claim 1, characterized in that, The multi-component seepage equation is as follows: ； in, ϕ Porosity; ρ 2 represents the fluid density; v q represents Darcy velocity; q represents source and sink terms; t represents time.

5. The THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams according to claim 1, characterized in that, The competitive adsorption model is as follows: ； in, q i Let i be the amount of coal adsorbed per unit mass of gas i; V L ᵢ Let be the Langmuir capacity constant of gas i; bᵢ Let be the Langmuir constant for gas i; pᵢ Let i be the partial pressure of the gas; j For all types of adsorbed gases; β The adsorption strain sensitivity coefficient, The effective stress is used to reflect the inhibitory effect of stress on adsorption capacity.

6. The THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams according to claim 1, characterized in that, The chemical reaction-property evolution model is as follows: ； in, D The diffusion coefficient is denoted as . D 0 represents the initial diffusion coefficient; E α R is the diffusion activation energy; R is the gas constant; T is the temperature.

7. The THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams according to claim 1, characterized in that, The stress-permeability coupling formula is as follows: ； in, K This represents the current penetration rate. K 0 initial permeability; γ is the stress sensitivity coefficient; Effective stress; Adsorption-induced volumetric strain; η The adsorption coupling coefficient is denoted as . The formula for the coupling of heat and stress is as follows: ； in, σ th Thermal stress; E Young's modulus; α Δ is the coefficient of thermal expansion; T For temperature change; The coupling formula for heat, chemistry, and permeability is as follows: ； in, D The diffusion coefficient is denoted as . D 0 represents the initial diffusion coefficient; E α R is the diffusion activation energy; R is the gas constant; T is the temperature.

8. The THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams according to claim 1, characterized in that, The steps for numerically solving the fully coupled mathematical model to obtain the spatiotemporal evolution results of key field variables specifically include: The fully coupled mathematical model is discretized using the finite element method or the finite difference method. Local mesh refinement is set in combination with the crack guiding structure. An implicit-explicit joint time step control method is selected. Multiphysics joint solution is performed using at least one platform among TOUGH2, FLAC3D and COMSOL. The spatiotemporal evolution results of key field variables are output.

9. The THMC multi-field coupling simulation method for the CO2 displacement of CH4 process in coal seams according to claim 1, characterized in that, The steps for evaluating the indicators of the target coal seam based on the spatiotemporal evolution results of key field variables specifically include: Based on the spatiotemporal evolution results of key field variables, obtain one or more evolution diagrams of temperature field, stress field, seepage field and gas concentration field; The indicators of the target coal seam are evaluated based on one or more evolution diagrams of the temperature field, stress field, seepage field, and gas concentration field.

10. A THMC multi-field coupled simulation system for the CO2 displacement of CH4 process in coal seams, used to implement the THMC multi-field coupled simulation method according to any one of claims 1-9, characterized in that, include: The multi-source parameter information acquisition module is used to acquire multi-source parameter information of the target coal seam; The multi-source parameter information includes mechanical parameters, thermophysical parameters, adsorption characteristic parameters, coal petrology information, and fracture distribution information; The coal seam structure model building module is used to build a coal seam structure model including the matrix and fracture system based on multi-source parameter information. The coupling mechanism setting module is used to construct a system of governing equations based on a coal seam structure model and introduce a multi-field coupling mechanism to obtain a fully coupled mathematical model. The system of governing equations includes one or more of the following: heat conduction equation, elasticity equation, multi-component seepage equation, competitive adsorption model, and chemical reaction-property evolution model. The multi-field coupling mechanism includes one or more coupling paths of the following: heat-stress, stress-permeability, permeability-chemistry, and heat-chemistry-permeability. The rolling solver module is used to numerically solve the fully coupled mathematical model and obtain the spatiotemporal evolution results of key field variables; The evolution results analysis module is used to evaluate the indicators of the target coal seam based on the spatiotemporal evolution results of key field variables.

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