Method for predicting high-temperature permeability of coal underground gasification surrounding rock based on thermal stress model

Abstract

The invention relates to the technical field of permeability calculation, and discloses a method for predicting the high-temperature permeability of coal underground gasification surrounding rock based on a thermal stress model, which comprises the following steps: collecting samples, and preprocessing the samples to determine mineral components of the samples; carrying out heat treatment on the sample at different temperature levels, and then carrying out experimental testing to obtain parameters of the sample; simulating, coupling and calculating the strain energy of the sample by adopting a finite element method, and calculating the fracture amount by combining the strain energy and the geometrical shape of the fracture at high temperature; and calculating and predicting the high-temperature permeability by using the geometrical shape and the fracture amount of the fracture at high temperature. According to the method, finite element thermal stress simulation and fracture mechanics theories are combined, the stress causing fracture is accurately calculated, the fracture amount is calculated in combination with observation of the geometrical shape of the fracture after high temperature action under a microscopic image, and the fracture seepage rate is calculated based on the fracture amount and discontinuity of sudden change of the high-temperature seepage rate of the geometrical shape of the fracture after high temperature action. And the high-temperature permeability is calculated and predicted, and the method has very strong prediction performance.

Description

Technical Field

[0001] The present invention relates to the technical field of permeability calculation, and particularly to a method for predicting the high-temperature permeability of surrounding rock in underground coal gasification based on a thermal stress model. Background Art

[0002] Underground coal gasification (UCG) is a key technology for the clean utilization of coal under the goal of carbon emission reduction and may also be the only feasible method for large-scale hydrogen production. According to the current understanding, the underground coal gasification system consists of a coal seam designated for combustion and reaction and surrounding rock for enclosing syngas. The high temperature generated by the combustion of the coal seam during the gasification process causes the surrounding rock to heat up. Since the surrounding rock is mostly composed of mineral particles and has an expansion characteristic at high temperature, cracks are formed in the rock at high temperature, further leading to an increase in permeability and syngas leakage.

[0003] Due to the limitations of instrument materials, traditional experimental methods cannot directly measure the permeability at high temperature. Currently, the Darcy method can only measure the high-temperature permeability below 500°C. The indirect method based on image reconstruction of a three-dimensional model can overcome the above problems to a certain extent, but it also has certain defects. For example, the method of reconstructing a three-dimensional model based on CT images and then performing finite element method simulation through the Navier-Stokes equation has insufficient accuracy in measuring smaller pores and channels. Another example is using acoustic emission at high temperature to estimate the characteristics of cracks. However, this method is hindered by unclear physical meaning and the lack of a widely accepted mathematical model. Another example is using a high-temperature atomic force microscope (AFM) to evaluate the changes in pore structure at high temperature. Due to the extreme simplification of the grain shape, arrangement, and pore structure in its assumptions, it has limitations in practical applications.

[0004] Based on the defects of existing experimental methods, further consideration is given to establishing a model for predicting high-temperature permeability in the prior art. The method with an empirical formula as the core is relatively simple, but due to its lack of clear physical interpretation and its applicability only to specific lithologies in specific regions, its generalization is limited. The method with a theoretical model as the core has only two directions. One focuses on deformation and calculates the change in pore space through the deformation of the rock matrix or pores. The other considers energy and calculates the change in the number and morphology of cracks based on energy changes.

[0005] However, considering comprehensively, the method with a theoretical model as the core still has the following problems: The property changes of rocks at high temperature, including changes in permeability and mechanical properties, are discontinuous processes. Most rocks have a threshold temperature. Above this temperature, a large number of cracks will rapidly expand or form. This critical temperature is affected by factors such as the thermal expansion characteristics of minerals and chemical reactions. Existing models do not consider these discontinuous phenomena. Summary of the Invention

[0006] The object of the present invention is to provide a method for predicting the high-temperature permeability of surrounding rock in underground coal gasification based on a thermal stress model, so as to solve the technical problem that the high-temperature permeability of surrounding rock cannot be accurately predicted by direct or indirect methods in the prior art.

[0007] To solve the above technical problems, the present invention specifically provides the following technical solutions: A method for predicting the high-temperature permeability of surrounding rock in underground coal gasification based on a thermal stress model, characterized by comprising the following steps: Step 100: After collecting samples, perform pretreatment on the samples respectively to determine their mineral compositions, and perform heat treatment on the samples at different temperature levels and then conduct experimental tests to obtain the parameters of the samples; Step 200: Use the finite element method to simulate and couple the calculation of the strain energy of the samples, and calculate the fracture amount by combining the strain energy and the crack geometry at high temperature; Step 300: Calculate and predict the high-temperature permeability by using the crack geometry and fracture amount at high temperature.

[0008] Further, in Step 100, a closed furnace is used for heat treatment of the samples and carried out under a protective gas to simulate the oxygen-deficient environment of the formation; Among them, the heating rate is the same when heating at different temperature levels, and after sufficient heating, it is naturally cooled to room temperature.

[0009] Further, the experimental tests include: Mercury intrusion porosimetry experiment to determine the change in the porosity of the samples; X-CT and / or scanning electron microscope observation experiment to observe the cracks formed or expanded in the samples after heat treatment; Transient permeability experiment to measure the change in the permeability of the samples after heat treatment.

[0010] Further, in Step 200, the strain energy of the samples is obtained through a thermal stress model, and the thermal stress model includes a temperature field described by a heat conduction equation and a stress field described by a force balance equation, and the temperature field and the stress field are coupled through a thermal expansion equation to calculate the strain energy density.

[0011] Further, the method for establishing the temperature field of the samples is as follows: It is assumed that there is no pressure difference in the samples, and the thermal properties of the mineral particles in the samples are isotropic; By combining Fourier's law with the energy balance equation, the heat transfer equation is obtained: ; Wherein: , is the heat flux density ( ) is the thermal conductivity ( ); is the rock density ( ), is the specific heat capacity of the rock ( ), is the partial derivative symbol, is the temperature (K), is the time (s), is the divergence operator, H is the energy source ( ); The temperature field inside the sample is calculated using the steady - state heat transfer equation.

[0012] Furthermore, the pressure field of the sample is established as follows: It is assumed that the mineral particles in the sample are elastic and isotropic; The static equilibrium equation is obtained through Hooke's law and the momentum equation: ; where: ; ; In the formula, is the divergence operator, is the stress vector ( ), is the bulk stress ( ), is the shear modulus ( ), is the strain vector, v is the Poisson's ratio, dimensionless, is the trace of the strain tensor, is the Kronecker operator, is the Young's modulus ( ); The pressure field inside the sample is calculated using the static equilibrium equation.

[0013] Furthermore, the temperature field and the stress field are coupled through the thermal expansion equation to calculate the strain energy density, where the thermal expansion equation is: ; where: ; ; In the formula: is the rock volume ( ), which is a function of temperature, is the reference volume ( ), and are the temperature and reference temperature ([[]] ), is the coefficient of volumetric thermal expansion, is the volumetric strain, is the strain vector; Set the strain energy density as U, then: .

[0014] Furthermore, the specific representation of the crack geometry at high temperature is: Set the semi-major axis length of the crack cross-section in the sample and the ratio of the semi-minor axis to the semi-major axis length and the stress have the following relationship: ; ; ; wherein, is the initial semi-major axis length (m), is the ratio of the initial semi-minor axis to the semi-major axis length of the crack cross-section, is the Young's modulus (MPa) and is the Poisson's ratio, is the confining pressure (MPa); Under high temperature conditions, the thermal stress generated inside the sample is , and its expression is: ; wherein, is the thermal stress gradient ([[]] ), and , is the coefficient of thermal expansion , is the temperature (K), is the initial temperature (K); Apply the thermal stress to the sample, then the crack geometry at high temperature is expressed as: ; ; wherein, the semi-major axis length of the crack cross-section at high temperature and the ratio of the semi-minor axis to the semi-major axis length of the crack at high temperature .

[0015] Furthermore, the calculation method of the fracture amount is: Set that all the strain energy is converted into the surface energy required for crack formation or expansion, then: ; Among them, U is the strain energy density, is the number of newly generated cracks at high temperature, and S is the cross-sectional area of cracks at high temperature ( ), is the strain energy required for crack growth per unit area ( ), is the initial number of cracks, is the initial cross-sectional area of cracks ( ); The fracture amount at high temperature is the sum of the number of newly generated cracks and the initial number of cracks, .

[0016] Furthermore, the specific method for calculating and predicting the high-temperature permeability by using the crack geometry and fracture amount at high temperature is as follows: If the permeability of the sample is k, then: ; In the formula: is the proportion contributing to seepage in the cracks, is the total number of cracks, is the third-order origin moment of the ratio of the semi-minor axis to the semi-major axis of the crack cross-section, is the fifth-order origin moment of the length of the semi-major axis of the crack cross-section; Among them, the permeability is corrected by using the proportion contributing to seepage in the cracks; ; ; ; In the formula: is the geometric intermediate variable, is the connectivity probability, and as and increase, gradually approaches 1, is the length of the semi-major axis of the crack cross-section.

[0017] The present invention has the following beneficial effects compared with the prior art: The present invention combines finite element thermal stress simulation and fracture mechanics theory to accurately calculate the stress leading to fracture. By combining the observation of the geometric shape of cracks after high-temperature action under microscopic images, the fracture amount is calculated. Based on the fracture amount and the geometric shape of cracks after high-temperature action, the discontinuity of the sudden change in high-temperature seepage rate is presented, and the high-temperature permeability is calculated and predicted. This method has strong prediction performance, and the prediction R² of the permeability of most surrounding rocks after high-temperature action exceeds 0.85, and the root mean square error remains within an acceptable range. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only exemplary, and those of ordinary skill in the art can obtain other implementation drawings by extending based on the provided drawings without creative efforts.

[0019] Figure 1 It is a schematic flow chart provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0020] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0021] As Figure 1 shown, the present invention provides a method for predicting the high-temperature permeability of surrounding rocks in underground coal gasification based on a thermal stress model, including the following steps: Step 100: After collecting samples, preprocess the samples respectively to determine their mineral compositions, and conduct experimental tests on the samples after heat treatment at different temperature levels to obtain the parameters of the samples; When heat-treating the samples, a closed furnace is used and carried out under a protective gas to simulate the oxygen-deficient environment of the formation; Among them, the heating rate is the same when heating at different temperature levels, and after sufficient heating, it is naturally cooled to room temperature; Step 200: Use the finite element method to simulate and couple the calculation of the strain energy of the samples, and calculate the fracture amount by combining the strain energy and the crack geometry at high temperature; Step 300: Calculate and predict the high-temperature permeability by using the crack geometry and fracture amount at high temperature.

[0022] The following will be described in conjunction with specific embodiments.

[0023] Underground coal gasification is usually carried out in sedimentary strata, and sedimentary rocks with different grain sizes exhibit different characteristics at high temperatures. To enhance the broad representativeness of the research, sedimentary rocks with grain sizes ranging from coarse to fine were selected for testing. The samples included conglomeratic sandstone, sandstone, muddy siltstone, and mudstone from the Taiyuan Formation in the eastern margin of the Ordos Basin, Shaanxi Province, China.

[0024] After the samples were collected, X-ray diffraction (XRD) analysis was carried out to determine their mineral compositions. The results showed that quartz was the main component, all contained plagioclase, and contained a certain amount of clay minerals, mainly illite and kaolinite. The muddy siltstone contained a small amount of chlorite, and sandstone 1 and calcareous siltstone also contained a small amount of dolomite. Some samples contained trace amounts of anatase (Table 1).

[0025] The heat treatment was carried out using a closed tube furnace.

[0026] The samples were heated at eight different temperature levels: 25°C (room temperature), 200°C, 400°C, 600°C, 800°C, 1000°C, and 1200°C. The heating was carried out in an argon environment to simulate oxygen deficiency in the formation. The gas flow rate was 1 liter per minute, and the heating rate during the heating stage was set at 10°C per minute. The temperature was maintained for 2 hours to ensure that the samples were fully treated, and then they were naturally cooled to room temperature.

[0027] The permeability of the rock after heat treatment can be regarded as its high-temperature permeability.

[0028] Mercury intrusion porosimetry (MIP) experiments, X-CT, scanning electron microscopy observations, and transient permeability experiments were carried out on the treated samples.

[0029] Among them: Through mercury intrusion porosimetry experiments to determine the change in the porosity of the samples. The experimental results showed that the porosity of the heat-treated rock samples increased, and increased sharply between 400°C and 600°C. The porosity of the conglomeratic sandstone increased the most significantly, followed by sandstone 1, sandstone 2, and arsenic-bearing siltstone, while the change in mudstone was the smallest.

[0030] X-CT and scanning electron microscopy techniques were used to observe the fractures formed in the rock after heat treatment. For sandstone and muddy siltstone with relatively large grain sizes, a Bruker Skyscan-1272 CT scanner with a resolution of 1 micron was used. Due to the relatively fine grain size of the mudstone, X-CT could not effectively observe the microfractures, so a Hitachi SU8010 scanning electron microscope was used to examine it.

[0031] The results show that, compared with the samples at room temperature, the X-CT and SEM images of sandstone 2, siltstone and mudstone treated at 400 °C do not show significant differences, and relatively few fractures are observed. After treatment at 600 °C, a sudden increase in the number of fractures is observed, mainly concentrated around the grain boundaries. In the samples treated at 800 °C, the number of fractures further increases. This indicates that there is a threshold temperature between 400 °C and 600 °C, which leads to a sudden increase in fracture formation. The sudden increase in fractures results in a sudden rise in porosity measured by MIP. This change in fractures will cause a sudden change in the rock permeability at high temperatures.

[0032] The high-temperature rock permeability prediction method of the present invention is consistent with the sudden change of fractures. Therefore, the influence of the fracture phase is considered in the calculation. First, the strain energy in the rock is calculated, then the number of fractures is calculated using the strain energy and the high-temperature fracture geometry, and finally the high-temperature permeability is calculated based on the high-temperature fracture geometry and the number of fractures.

[0033] In step 200, the strain energy of the sample is obtained through a steady-state thermal stress model. The steady-state thermal stress model includes a temperature field described by a steady-state heat conduction equation and a stress field described by a force balance equation, and the temperature field and the stress field are coupled through a thermal expansion equation to calculate the strain energy density.

[0034] The temperature field of the sample is established as follows: It is assumed that there is no pressure difference in the sample and the fluid flow cannot be driven, so convective heat transfer can be ignored. And the thermal properties of the mineral particles in the sample are isotropic. By combining Fourier's law with the energy balance equation, the steady-state heat transfer equation is obtained: where , is the thermal conductivity ( ), is the rock density ( ), is the specific heat capacity of the rock ( ), is the partial derivative symbol, is the temperature (K), is the time (s), is the divergence operator, is the heat flux density ( ), H is the energy source ( ); The temperature field inside the sample is calculated using the steady-state heat transfer equation.

[0035] The pressure field of the sample is established as follows: It is assumed that the mineral particles in the sample are elastic and isotropic, and the heterogeneity caused by mineral lattice defects is ignored. The static equilibrium equation is obtained through Hooke's law and the momentum equation: ; where: ; ; In the formula, is the stress vector ( ), is the bulk stress ( ), is the shear modulus ( ), is the strain vector, v is the Poisson's ratio (dimensionless), is the trace of the strain tensor, is the Kronecker operator, is the Young's modulus ( ); The pressure field inside the sample is calculated using the static equilibrium equation.

[0036] The temperature field and the stress field are coupled through the thermal expansion equation to calculate the strain energy density. In the present invention, chemical reactions of rocks (such as clay minerals) at high temperatures are not considered. Therefore, the thermal expansion coefficient of clay minerals is set as a constant.

[0037] Among them, the thermal expansion equation is: ; where: ; ; In the formula: is the rock volume ( ), which is a function of temperature, is the reference volume ( ), and are the temperature and the reference temperature ( ), is the volume thermal expansion coefficient, is the volume strain, is the strain vector; In this embodiment, the volume thermal expansion coefficients of each mineral are shown in Table 2.

[0038] The ultimate goal of the finite element simulation is to obtain the tensile strain energy density. It is assumed that the strain energy density is U, then: 。

[0039] It should be noted that before performing finite element simulation, it is necessary to first construct a geometric model.

[0040] For coarse-grained rocks such as gravel-bearing sandstone and sandstone, optical microscope images can be used for geometric modeling; For fine-grained rocks such as argillaceous siltstone and mudstone, optical microscopes cannot effectively distinguish mineral grains, so backscattered scanning electron microscope (SEM) images are used for geometric modeling.

[0041] The rock sample is first processed to enhance image contrast and clarify the boundaries of mineral grains; then the mineral types are identified and the grain boundaries are outlined; then the mineral properties are assigned to each grain; finally, an adaptive mesh is generated in the constructed geometric model.

[0042] The finite element simulation is carried out based on the geometric model of this adaptive mesh and the above control equations.

[0043] In this embodiment, fixed constraints are applied to the outer boundary of the geometric model, but to mitigate the extreme stress accumulation caused by constrained strain, thermal expansion sub-nodes are introduced, and these thermal expansion sub-nodes calculate thermal strain by integrating the thermal expansion coefficient function. The interfaces between minerals are modeled as linearly elastic thin layers, and their mechanical parameters are measured experimentally.

[0044] On the other hand, the specific representation of the fracture geometry at high temperature is as follows: It is set that the semi-major axis length of the fracture cross-section in the sample and the ratio of the semi-minor axis to the semi-major axis length have the following relationship with the stress : ; ; ; where is the initial semi-major axis length (m), is the ratio of the initial semi-minor axis to the semi-major axis length of the fracture cross-section, is the Young's modulus (MPa) and is the Poisson's ratio, is the confining pressure (MPa); Under high temperature conditions, thermal stress will be generated inside the rock, and the thermal stress generated inside the sample is , and its expression is: ; where is the thermal stress gradient ( ), and , is the coefficient of thermal expansion , is the temperature (K), is the initial temperature (K); Applying thermal stress to the sample, assuming that the Young's modulus and Poisson's ratio of the rock remain unchanged at high temperature, the fracture geometry at high temperature is expressed as: ; ; where, the semi-major axis length of the fracture cross-section at high temperature and the ratio of the semi-minor axis to the semi-major axis length of the fracture at high temperature .

[0045] The cracking of rock at high temperature is a complex process involving various mechanisms such as intergranular cracking, intragranular cracking, and interlayer peeling of clay minerals. The formation and expansion of internal fractures in rock at high temperature are mainly attributed to the tensile stress caused by thermal expansion of minerals. When the tensile stress at any position in the rock exceeds its tensile strength, new fractures will be formed or existing fractures will be promoted to expand.

[0046] Therefore, the calculation method of the fracture amount is: Assuming that all strain energy is converted into the surface energy required for fracture formation or expansion, then: ; where, U is the strain energy density, is the number of newly generated fractures at high temperature, S is the cross-sectional area of the fractures at high temperature( ), which can be calculated by and , is the strain energy required for fracture expansion per unit area( ), is the initial number of fractures, is the initial cross-sectional area of the fractures( ); The surface strain energy of the tensile fracture can be calculated by the stress intensity factor as follows: ; The fracture amount at high temperature is the sum of the number of newly generated fractures and the initial number of fractures, .

[0047] The permeability of crystalline rock can be considered to depend entirely on fractures. If the permeability of the sample is k, then: ; In the formula: is the proportion contributing to seepage in fractures, is the total number of fractures, is the third-order origin moment of the ratio of the semi-minor axis to the semi-major axis of the fracture cross-section, is the fifth-order origin moment of the length of the semi-major axis of the fracture cross-section; Among them, only some fractures contribute to permeability. For example, dead-end fractures do not affect permeability. The proportion of fractures contributing to permeability is calculated based on seepage theory, and the permeability is corrected using the proportion contributing to seepage in fractures ; ; ; ; In the formula: is the geometric intermediate variable, is the connectivity probability, and as and increase, gradually approaches 1, is the length of the semi-major axis of the fracture cross-section.

[0048] Among the above parameters, the Young's modulus, Poisson's ratio, and coefficient of thermal expansion are obtained through experiments; the initial semi-major axis length, initial aspect ratio, and stress intensity factor are obtained through iterative fitting with experimental data at the initial temperature and 200°C. The calculation parameters in this embodiment are shown in Table 3. In this embodiment, the simulation prediction results are further verified.

[0049] The results show that it has high accuracy in predicting the permeability of rocks at high temperatures. For most rock types, the determination coefficient R² exceeds 0.85. The R² of sandstone 2 is relatively low, being 0.7677, because its permeability has an order-of-magnitude change at different temperatures, and this change is particularly significant in the high-value range. Since R² is mainly affected by high values, this amplifies the deviation of the model. However, the root mean square error (RMSE) of this sample is 4.1509 mD, still within the acceptable range for rock permeability research. The RMSE values of other samples also fall within the acceptable range corresponding to their particle size levels.

[0050] The above embodiments are only exemplary embodiments of the present application and are not used to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art can make various modifications or equivalent replacements within the essence and protection scope of the present application, and such modifications or equivalent replacements should also be regarded as falling within the protection scope of the present application.

Claims

1. A method for predicting the high-temperature permeability of surrounding rocks in underground coal gasification based on a thermal stress model, characterized in that, It includes the following steps: Step 100: After collecting the samples, preprocess the samples separately to determine their mineral compositions, and conduct experimental tests on the samples after heat treatment at different temperature levels to obtain the parameters of the samples; Step 200: Use the finite element method to simulate and couple the calculation of the strain energy density of the samples, and calculate the fracture amount by combining the strain energy density and the crack geometry at high temperature; Among them, the strain energy of the samples is obtained through a thermal stress model, the thermal stress model includes a temperature field described by a heat conduction equation and a stress field described by a force balance equation, and the temperature field and the stress field are coupled through a thermal expansion equation to calculate the strain energy density; Step 300: Calculate and predict the high-temperature permeability by using the crack geometry and fracture amount at high temperature.

2. The method for predicting the high-temperature permeability of surrounding rocks in underground coal gasification based on a thermal stress model according to claim 1, wherein In Step 100, a closed furnace is used for heat treatment of the samples and it is carried out under a protective gas to simulate the oxygen-deficient environment of the formation; Among them, the heating rate is the same when heating at different temperature levels, and after sufficient heating, it is naturally cooled to room temperature.

3. The method for predicting the high-temperature permeability of surrounding rock in underground coal gasification based on a thermal stress model according to claim 1, characterized in that, The experimental tests include: Mercury intrusion porosimetry experiment to determine the change in the porosity of the samples; X-CT and / or scanning electron microscope observation experiment to observe the cracks formed or expanded in the samples after heat treatment; Transient permeability experiment to measure the change in the permeability of the samples after heat treatment.

4. The method for predicting the high-temperature permeability of surrounding rocks in underground coal gasification based on a thermal stress model according to claim 1, wherein The temperature field of the samples is established as follows: It is set that there is no pressure difference in the samples, and the thermal properties of the mineral particles in the samples are isotropic; By combining Fourier's law and the energy balance equation, the heat transfer equation is obtained: ； Where: , is the heat flux density ( ), is the thermal conductivity ( ); is the rock density ( ), is the specific heat capacity of the rock ( ), is the symbol of partial derivative, is the temperature (K), is the time (s), is the divergence operator, H is the energy source ( ); Use the steady-state heat transfer equation to calculate the temperature field inside the samples.

5. The method for predicting the high-temperature permeability of surrounding rocks in underground coal gasification based on the thermal stress model according to claim 4, wherein, The pressure field of the samples is established as follows: It is set that the mineral particles in the samples are elastic and isotropic; Obtain the static equilibrium equation through Hooke's law and the momentum equation: ； Where: ； ； In the formula, is the divergence operator, is the stress vector ( ), is the volumetric stress ( ), is the shear modulus ( ), is the strain vector, v is the Poisson's ratio, dimensionless, is the trace of the strain tensor, is the Kronecker operator, is the Young's modulus ( ); Use the static equilibrium equation to calculate the pressure field inside the samples.

6. The method for predicting the high-temperature permeability of surrounding rocks in underground coal gasification based on a thermal stress model according to claim 5, characterized in that, The temperature field and the stress field are coupled through a thermal expansion equation to calculate the strain energy density, where the thermal expansion equation is: ； Where: ； ； Wherein: is the rock volume ( ), which is a function of temperature, is the reference volume ( ), and are the temperature and the reference temperature ( ), respectively, is the coefficient of volume thermal expansion, is the volume strain, is the strain vector; It is set that the strain energy density is U, then: 。 7. The method for predicting the high-temperature permeability of surrounding rock in underground coal gasification based on the thermal stress model according to claim 6, wherein The specific representation of the crack geometry at high temperature is as follows: Setting, the semi-major axis length of the crack cross-section in the sample and the ratio of the semi-minor axis to the semi-major axis length have the following relationship with the stress as follows: ； ； ； Among them, is the initial semi-major axis length (m), is the ratio of the initial semi-minor axis to the semi-major axis of the fracture cross-section, is the Young's modulus (MPa) and is the Poisson's ratio, is the confining pressure (MPa); Under high temperature conditions, the thermal stress generated inside the sample is , and its expression is: ； Among them, is the thermal stress gradient ( ), and , is the coefficient of thermal expansion , is the temperature (K), is the initial temperature (K); Apply thermal stress to the samples, then the crack geometry at high temperature is expressed as: ； ； Among them, the semi-major axis length of the crack cross-section at high temperature and the ratio of the semi-minor axis to the semi-major axis length of the crack at high temperature .

8. The method for predicting the high-temperature permeability of surrounding rocks in underground coal gasification based on a thermal stress model according to claim 7, characterized in that, The calculation method of the fracture amount is: It is set that all the strain energy is converted into the surface energy required for crack formation or expansion, then: ； Among them, U is the strain energy density, is the number of newly generated cracks at high temperature, and S is the cross-sectional area of cracks at high temperature ( ), is the strain energy required for crack propagation per unit area ( ), is the initial number of cracks, is the initial cross-sectional area of cracks ( ); Fracture amount at high temperature is the sum of the number of newly generated cracks and the number of initial cracks, .

9. The method for predicting the high-temperature permeability of surrounding rock in underground coal gasification based on the thermal stress model according to claim 8, wherein The specific method of calculating and predicting the high-temperature permeability by using the crack geometry and fracture amount at high temperature is: The permeability of the samples is k, then: ； In the formula: is the proportion contributing to seepage in the fracture, is the total number of fractures, is the third-order origin moment of the ratio of the semi-minor axis to the semi-major axis of the fracture cross-section, is the fifth-order origin moment of the length of the semi-major axis of the fracture cross-section; Among them, the contribution ratio of the fractures to seepage is used to correct the permeability. ； ； ； Wherein: is a geometric intermediate variable, is the connectivity probability, which gradually approaches 1 as and increase, and is the semi-major axis length of the fracture cross-section.

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