A method for inverse determination of thermal conductivity of coal-rock matrix

Abstract

The invention discloses a method for inversion determination of thermal conductivity of coal rock matrix. The method comprises the following steps: firstly, after CT scanning of coal rock sample, specific software is used to extract coal skeleton structure and fracture structure thereof for three-dimensional reconstruction, a CT grid model is constructed and imported into COMSOL software for simulation; a transient simulation experiment of thermal conductivity of coal rock sample is conducted; then a golden section search calculation model is constructed, and a conjugate heat transfer model is established based on the solid and fluid heat transfer modules of COMSOL software; the simulation boundary condition setting of the conjugate heat transfer model is consistent with the experimental condition, and then the experimental data and the conjugate heat transfer model are combined, and a specific algorithm is used to re-assign the golden section search calculation model, and after the specific condition is met, the thermal conductivity k of the sample matrix is ​​finally calculated. ture The present invention can obtain the thermal conductivity of the solid matrix itself in the coal rock mass, thereby applying the thermal conductivity to digital core modeling, providing data support for the subsequent research and development of abandoned mine geothermal mining technology.

Description

Technical Field

[0001] The invention relates to an inversion determination method for thermal conductivity of a coal rock matrix, and belongs to the technical field of digital core thermal physical property testing. Background Art

[0002] Abandoned mines refer to mines that have been abandoned because coal resources have been mined or production has stopped for other reasons. However, abandoned mines still contain a large amount of geothermal resources. Abandoned mine geothermal resources refer to the geothermal energy underground that can be scientifically and reasonably developed from abandoned mines under current technical, economic and geological environmental conditions. Geothermal energy is also a renewable clean energy.

[0003] The thermal conductivity of coal rock is an important indicator for evaluating the efficiency of geothermal extraction from rocks. The thermal conductivity of coal rock is divided into thermal conductivity and effective thermal conductivity. At present, the traditional method to determine the effective thermal conductivity of coal rock mass is mainly based on experimental measurement based on steady-state method or non-steady-state method. Since coal is a porous medium with a fracture system, the solid phase is also called solid matrix, and the remaining space is fracture. Digital core technology can accurately characterize the pore and fracture structure of coal mass, reconstruct the CT image of the internal structure of the core and convert it into a high-precision grid structure that can be recognized by the current common simulation calculation software. However, when conducting relevant heat transfer modeling, the thermal conductivity of the coal rock material matrix is ​​required (that is, the thermal conductivity of the coal solid matrix itself, excluding cracks), and the coal rock measured in existing experiments is the effective thermal conductivity, which already includes cracks during measurement. Since the underground coal rock has obvious crack development after the impact of mining, digital core modeling will also simulate the development of cracks in the coal rock after mining. This will result in a large error between the initial measured effective thermal conductivity and the actual thermal conductivity after the development of coal rock cracks after mining, and ultimately the initial measured effective thermal conductivity cannot be applied to digital core modeling.

[0004] Therefore, how to obtain the thermal conductivity of the solid matrix itself in the coal rock mass, and then apply the thermal conductivity to digital core modeling to provide data support for the subsequent research and development of abandoned mine geothermal mining technology, is the research direction required by the present invention. Summary of the invention

[0005] In view of the problems existing in the above-mentioned prior art, the present invention provides a method for inversion determination of thermal conductivity of coal rock matrix, which can obtain the thermal conductivity of the solid matrix itself in the coal rock mass, thereby applying the thermal conductivity to digital core modeling, and providing data support for the subsequent research and development of abandoned mine geothermal mining technology.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: a method for inversion determination of thermal conductivity of coal rock matrix, the specific steps are:

[0007] Step 1: first collect coal and rock mass in the abandoned mine, and process the coal and rock mass into coal and rock samples;

[0008] Step 2: Perform CT scanning on the coal rock sample prepared in step 1, and use AVIZO reconstruction software to comprehensively apply interactive threshold segmentation and interactive top-hat method to the CT slice image, extract its coal skeleton structure and fracture structure for three-dimensional reconstruction, and output it as a surface mesh file; import the surface mesh file into the mesh repair software for geometric repair, remove self-intersections and some small features, and then build a CT mesh model;

[0009] Step 3: Place the coal rock sample prepared in step 1 on the holder, and make one end face of the coal rock sample fit closely with the constant temperature heat source to form a heat source face, and the end face opposite to the heat source face is provided with a temperature monitoring point to form a monitoring face for collecting temperature data on the monitoring face, and wrap the rest of the surface of the coal rock sample with insulation material; then start the constant temperature heat source, and at the same time, the temperature monitoring point collects the temperature data of the monitoring face every 1 minute and feeds it back to the computer for storage; in order to prevent excessive heat from damaging the structure of the coal rock body, that is, generating new cracks, the heating power setting of this constant temperature heat source needs to make the temperature difference of each position of the coal rock sample not exceed 50°C;

[0010] Step 4: Use the CT mesh model constructed in step 2 to import the cracks and matrix inside the coal rock sample into the COMSOL software for simulation, build a golden section search calculation model through a computer, and establish a conjugate heat transfer model based on the solid and fluid heat transfer modules of the COMSOL software; the simulation boundary conditions of the conjugate heat transfer model are set in accordance with the experimental conditions of step 3, and then use the COMSOL software solver to calculate the transient three-dimensional temperature field results of the coal rock material when the matrix thermal conductivity is k1 and k2, and extract the temperature T(k,t n ), where t n is the experimental recording time, n is the experimental time step;

[0011] Step 5: According to the golden section search calculation model and conjugate heat transfer model established in step 4, multiple sets of different temperature data are obtained, and the optimal solution inversion is performed on the multiple sets of temperature data. Specifically, the multiple sets of temperature data T(k,t n ) and the experimentally measured T(t n ) into the objective function S calculation model, and calculate the value S of the objective function calculation model corresponding to k1 and k2 in each set of temperature data respectively n (k n1 ) and S n (k n2 );

[0012] Step 6: For each group S obtained in step 5 n (kn1 ) and S n (k n2 ) to make a judgment, and according to the judgment conditions, k in the golden section search calculation model is low and k high Reassignment;

[0013] Step 7. Repeat steps 5 and 6 until all |S n (k1)-S n (k2)|<1×10 -6 When , stop the calculation, and then calculate k according to the following formula ture is the thermal conductivity of the sample matrix;

[0014]

[0015] Furthermore, the golden section search calculation model in step 4 is specifically:

[0016]

[0017]

[0018] In the formula, k low k is the lower limit of thermal conductivity of coal rock without cracks, high is the upper limit of thermal conductivity of coal rock without cracks, is the golden ratio,

[0019] Furthermore, in step 4, the conjugate heat transfer model is established based on the solid and fluid heat transfer module of COMSOL software as follows:

[0020] The transient temperature equation derived from the energy conservation equation is:

[0021]

[0022] Where, d z is the model thickness, ρ is the density of the material; C p is the constant pressure heat capacity of the matrix; u is the velocity vector; is the Laplace operator; T is the temperature; q is the conduction heat flux vector; Q is the heat source term; q0 is the inward heat flux;

[0023] in, Where k is the thermal conductivity; F is the external force acting on the fluid;

[0024] The creeping flow equation and continuity equation derived from the Navier-Stoks steady-state equation are as follows:

[0025]

[0026]

[0027] The energy conservation equation for the heat transfer module is:

[0028]

[0029]

[0030] Where q is the conduction heat flux, C p is the heat capacity of a solid at constant pressure.

[0031] Furthermore, the calculation model of the objective function S in step 5 is specifically:

[0032] S n =[T(t n )-T(k,t n )] 2 .

[0033] Furthermore, the judgment condition in step 6 is specifically: when S n (k n1 )>S n (k n2 ), let k low =k n1 ; When S n (k n1 ) n (k n2 ), let k high =k n2 .

[0034] Compared with the prior art, the present invention has the following advantages:

[0035] 1. The method of the present invention combines the digital core simulation research method and the transient simulation experiment with the golden section optimization algorithm, and can calculate the thermal conductivity of the crack-free matrix of the coal rock mass. In this way, the thermal conductivity can be applied in the digital core simulation, effectively improving the accuracy of the digital core heat transfer simulation.

[0036] 2. The measurement process of the present invention is implemented based on digital core simulation technology, which simplifies the complex boundary conditions and testing methods required for the experiment, reduces the difficulty of thermal conductivity testing and improves experimental efficiency.

[0037] 3. The test results of the present invention are controllable in sampling time step and sampling quantity when calculating the thermal conductivity of unfractured coal rock mass. The inversion algorithm using multiple optimization algorithms can increase the accuracy of calculating the thermal conductivity of unfractured matrix of coal rock mass. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 ​It is a schematic diagram of the overall process of the present invention;

[0039] Figure 2 It is a schematic diagram of the experimental system in the present invention.

[0040] In the figure: 1-constant temperature heating power supply, 2-heating plate, 3-insulating material, 4-coal and rock sample, 5-temperature measuring piece, 6-data transmission line, 7-computer. DETAILED DESCRIPTION

[0041] The present invention will be further described below.

[0042] There is underground geothermal energy that can be developed inside an abandoned mine. In order to use digital core technology to determine the geothermal mining technology for the abandoned mine, the thermal conductivity of the coal rock matrix is ​​measured by the present invention, such as Figure 1 As shown, the specific steps are:

[0043] Step 1: first collect coal and rock mass in an abandoned mine, and process the coal and rock mass into coal and rock samples, wherein the size of the coal and rock samples is a cube of 50mm*50mm*50mm;

[0044] Step 2: Perform CT scanning on the coal rock sample prepared in step 1, and use AVIZO reconstruction software to comprehensively apply interactive threshold segmentation and interactive top-hat method to the CT slice image, extract its coal skeleton structure and fracture structure for three-dimensional reconstruction, and output it as a surface mesh file; import the surface mesh file into the mesh repair software for geometric repair, remove self-intersections and some small features, and then build a CT mesh model;

[0045] Step 3: Figure 2 As shown, the coal rock sample prepared in step 1 is placed on the holder, and one end face of the coal rock sample is tightly fitted with the heating plate 2 to form a heat source surface. The heating plate is powered by a constant temperature heating power supply 1 to form a uniform constant temperature heat source. A temperature monitoring point is provided on the end face opposite to the heat source surface to form a monitoring surface. A temperature measuring piece 5 is attached to the temperature monitoring point to collect temperature data on the monitoring surface and feed it back to the computer 7 through a data transmission line 6, and the remaining surfaces of the coal rock sample are wrapped with insulation material 3; then the constant temperature heat source is started, and at the same time, the temperature monitoring point collects the temperature data of the monitoring surface every 1 minute and feeds it back to the computer 7 for storage; in order to prevent excessive heat from damaging the structure of the coal rock body, that is, generating new cracks, the heating power of this constant temperature heat source is set to 5w, so that the temperature difference at each position of the coal rock sample does not exceed 50°C;

[0046] Step 4: Using the CT mesh model constructed in step 2, the cracks and matrix inside the coal and rock samples are imported into COMSOL software for simulation;

[0047] The golden section search calculation model is constructed by computer, specifically:

[0048]

[0049]

[0050] In the formula, k low k is the lower limit of thermal conductivity of coal rock without cracks, high is the upper limit of thermal conductivity of coal rock without cracks, is the golden ratio,

[0051] A conjugate heat transfer model is established based on the solid and fluid heat transfer modules of COMSOL software, specifically:

[0052] The transient temperature equation derived from the energy conservation equation is:

[0053]

[0054] Where, d z is the model thickness, ρ is the density of the material; C p is the constant pressure heat capacity of the matrix; u is the velocity vector; ▽ is the Laplace operator; T is the temperature; q is the conduction heat flux vector; Q is the heat source term; q0 is the inward heat flux;

[0055] in, Where k is the thermal conductivity; F is the external force acting on the fluid;

[0056] The creeping flow equation and continuity equation derived from the Navier-Stoks steady-state equation are as follows:

[0057]

[0058]

[0059] The energy conservation equation for the heat transfer module is:

[0060]

[0061]

[0062] Where q is the conduction heat flux, C p is the heat capacity of the solid at constant pressure;

[0063] The simulation boundary conditions of the conjugate heat transfer model are consistent with the experimental conditions in step 3, specifically: the ambient temperature is set to the experimental temperature of 25°C, the heat source is set to 5W, and the four walls are set to an adiabatic state; then the COMSOL software solver is used to calculate the transient three-dimensional temperature field results of the coal rock material when the matrix thermal conductivity is k1 and k2, and the temperature T(k,t n ), where t n is the experimental recording time, n is the experimental time step, set n = 5;

[0064] Step 5: According to the golden section search calculation model and conjugate heat transfer model established in step 4, multiple sets of different temperature data are obtained, and the optimal solution inversion is performed on the multiple sets of temperature data. Specifically, the multiple sets of temperature data T(k,t n ) and the experimentally measured T(t n ) into the objective function S calculation model, specifically: S n =[T(t n )-T(k,t n )] 2 ; Calculate the value S of the objective function calculation model corresponding to k1 and k2 in each set of temperature data respectively n (k n1 ) and S n (k n2 );

[0065] Step 6: For each group S obtained in step 5 n (k n1 ) and S n (k n2 ) to make a judgment, and according to the judgment conditions, k in the golden section search calculation model is low and k high Re-assignment, the judgment condition is as follows: when S n (k n1 )>S n (k n2 ), let k low =k n1 ; When S n (k n1 ) n (k n2 ), let k high =k n2 .

[0066] Step 7. Repeat steps 5 and 6 until all |S n (k1)-S n (k2)|<1×10 -6 When , stop the calculation, and calculate k according to the following formula ture is the thermal conductivity of the sample matrix;​

[0067]

[0068] Finally, the thermal conductivity of the abandoned mine sample matrix was calculated to be k ture =0.748W / (m·K).

[0069] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for inverse determination of thermal conductivity of coal rock matrix, characterized in that: The specific steps are: Step 1: first collect coal and rock mass in the abandoned mine, and process the coal and rock mass into coal and rock samples; Step 2: Perform CT scanning on the coal rock sample prepared in step 1, and use AVIZO reconstruction software to comprehensively apply interactive threshold segmentation and interactive top-hat method to the CT slice image, extract its coal skeleton structure and fracture structure for three-dimensional reconstruction, and output it as a surface mesh file; Import the surface mesh file into the mesh repair software for geometric repair, remove self-intersections and some small features, and then build the CT mesh model; Step 3: Place the coal rock sample prepared in step 1 on the holder, and make one end face of the coal rock sample fit closely with the constant temperature heat source to form a heat source surface, and set a temperature monitoring point on the end face opposite to the heat source surface to form a monitoring surface for collecting temperature data on the monitoring surface, and wrap the rest of the surface of the coal rock sample with insulation material; then start the constant temperature heat source, and at the same time, the temperature monitoring point collects the temperature data of the monitoring surface every 1 minute and feeds it back to the computer for storage; Step 4: Use the CT grid model constructed in step 2 to import the cracks and matrix inside the coal and rock samples into COMSOL software for simulation, build a golden section search calculation model through a computer, and establish a conjugate heat transfer model based on the solid and fluid heat transfer modules of COMSOL software; The simulation boundary conditions of the conjugate heat transfer model are set in accordance with the experimental conditions in step 3. Then, the transient three-dimensional temperature field results of the coal-rock material matrix thermal conductivity are calculated using the solver of COMSOL software when the matrix thermal conductivity is k1 and k2, and the temperature T(k,t n ), where t n is the experimental recording time, n is the experimental time step; Step 5: According to the golden section search calculation model and conjugate heat transfer model established in step 4, multiple sets of different temperature data are obtained, and the optimal solution inversion is performed on the multiple sets of temperature data. Specifically, the multiple sets of temperature data T(k,t n ) and the experimentally measured T(t n ) into the objective function S calculation model, and calculate the value S of the objective function calculation model corresponding to k1 and k2 in each set of temperature data respectively n (k n1 ) and S n (k n2 ); Step 6: For each group S obtained in step 5 n (k n1 ) and S n (k n2 ) to make a judgment, and according to the judgment conditions, k in the golden section search calculation model is low and k high Reassignment; Step 7. Repeat steps 5 and 6 until all |S n (k1)-S n (k2)|<1×10 -6 When , stop the calculation, and then calculate k according to the following formula ture is the thermal conductivity of the sample matrix; 2. The method for inverse determination of thermal conductivity of coal rock matrix according to claim 1, characterized in that: The golden section search calculation model in step 4 is specifically as follows: In the formula, k low k is the lower limit of thermal conductivity of coal rock without cracks, high is the upper limit of thermal conductivity of coal rock without cracks, is the golden ratio, 3. The method for inverse determination of thermal conductivity of coal rock matrix according to claim 1, characterized in that: In the fourth step, the conjugate heat transfer model is established based on the solid and fluid heat transfer modules of COMSOL software as follows: The transient temperature equation derived from the energy conservation equation is: Where, d z is the model thickness, ρ is the density of the material; C p is the constant pressure heat capacity of the matrix; u is the velocity vector; ▽ is the Laplace operator; T is the temperature; q is the conduction heat flux vector; Q is the heat source term; q0 is the inward heat flux; Where q = -d z k▽T, where k is the thermal conductivity; F is the external force acting on the fluid; The creeping flow equation and continuity equation derived from the Navier-Stoks steady-state equation are as follows: 0=-▽p+▽·(μ(▽u+(▽u) T ))+F ρ▽·u=0 The energy conservation equation for the heat transfer module is: ρC p u·▽T+▽·q=Q q=-λ▽T Where q is the conduction heat flux, C p is the heat capacity of a solid at constant pressure.

4. The method for inverse determination of thermal conductivity of coal rock matrix according to claim 1, characterized in that: The calculation model of the objective function S in step 5 is specifically: S n =[T(t n )-T(k,t n )] 2 。 5. The method for inverse determination of thermal conductivity of coal rock matrix according to claim 1, characterized in that: The judgment condition in step 6 is specifically: when S n (k n1 )>S n (k n2 ), let k low =k n1 ; When S n (k n1 ) n (k n2 ), let k high =k n2 .​

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