Coal underground gasification combustion space shape measuring method based on radioactive isotope

Abstract

The invention relates to the technical field of coal underground gasification, and discloses a radioactive isotope-based coal underground gasification combustion space shape measurement method, which specifically comprises the following steps of: selecting and injecting radioactive isotopes: selecting the radioactive isotopes with moderate half-life period and injecting the radioactive isotopes into a combustion space; radiation detectors are arranged and data are collected, wherein the detectors are arranged on the basis of the periphery of a combustion space area where radioactive isotopes are injected, and radiation intensity data are collected; data processing: performing background radiation deduction based on the acquired radiation intensity data, and selecting a proper threshold value for a calculation result by adopting a maximum entropy threshold value method; outputting a result, dividing the boundary of the combustion air area based on the calculated model parameter m and the determined threshold value, and outputting a three-dimensional contour surface diagram and contour surface diagrams in all directions of the combustion air area; the measurement method provided by the invention is high in industrial practicability, high in measurement precision and relatively simple and convenient to operate, and provides important technical support for large-scale application of a coal underground gasification technology.

Description

Technical Field

[0001] The present invention relates to the technical field of underground coal gasification, and in particular to a method for measuring the shape of a combustion zone of underground coal gasification based on radioactive isotopes. Background Art

[0002] Underground coal gasification (ISC) is a process that converts coal directly into product gas through combustion and gasification reactions in the presence of an oxidant. This product gas, commonly known as syngas, can then be used as a feedstock for a variety of applications, including fuel production, chemical production, and power generation. This underground coal gasification technology is applicable to most coal deposits. Given the increasingly stringent environmental regulations in the mining industry and the associated labor and infrastructure costs, this technology is undoubtedly attractive. Surface wells are drilled directly into the coal seam, providing an efficient path for oxidant injection and product gas production.

[0003] A pair of wells connected or extended horizontally underground form a substantially horizontal wellbore (also referred to as a coal seam wellbore or connecting well). This wellbore facilitates oxidant injection, inter-combustion zone growth, and product gas transport. One well used for oxidant injection is called an "injection well," while the other well used for product gas production is called a "production well." Both directional horizontal and vertical wells can serve as injection and production wells. Underground coal gasification (ISC) may also require one or more vertical wells (e.g., functional wells and auxiliary wells) between the injection and production wells. When a coal seam contains injection wells, production wells, and a horizontal wellbore connecting them, this structure is called an underground coal gasification (ISC) unit or well pair. An ISC unit consists of a combustion zone, a gasification zone, and a pyrolysis zone. The combustion zone is located near the oxidant injection point in the coal seam; the gasification zone radially surrounds or lies downstream of the combustion zone, where coal is gasified and partially oxidized to produce product gas; and the pyrolysis zone is located downstream of the gasification zone, where pyrolysis of the coal typically occurs. The high-temperature product gas flows downstream from the gasification zone and is eventually transported to the surface from the wellhead of the production well. As the coal burns or gasifies, the ISC combustion zone in the coal seam grows and expands.

[0004] Patent publication CN102465692B (Method for Real-Time Determination of the Shape of the Gap Zone during Underground Coal Gasification) discloses real-time measurement of temperature data from various temperature measurement points within the gap zone during underground coal gasification; obtaining temperature distribution maps for each coal seam plane based on the coordinate data and temperature data of each temperature measurement point in different coal seam planes; obtaining contour maps of the gap zone for each coal seam plane based on the temperature distribution maps for each coal seam plane; and obtaining a three-dimensional image of the gap zone based on the contour maps of the gap zone for each coal seam plane. This technology requires the deployment of numerous temperature measurement points within the gap zone, which is difficult to implement in industrial underground coal gasification equipment because it is difficult to deploy so many temperature measurement points hundreds of meters underground. This limited deployment of these points is limited to laboratory underground coal gasification equipment, limiting the method's applicability.

[0005] Underground coal gasification (UCG) is a technology that converts underground coal into combustible gas. During the gasification process, the shape and extent of the combustion zone directly impacts gasification efficiency and safety. Therefore, an efficient and real-time method for measuring this zone is needed. Summary of the Invention

[0006] The purpose of the present invention is to provide a radioisotope-based method for measuring the shape of the burnout zone in underground coal gasification, which can efficiently and in real time measure and calculate the geometric shape of the burnout zone, and provide technical support for the monitoring, optimization and safety management of the gasification process.

[0007] The present invention provides a method for measuring the shape of underground coal gasification combustion zone based on radioactive isotopes

[0008] The following steps are involved:

[0009] S1: Selection and injection of radioisotopes: radioisotopes with moderate half-life are selected and injected into the combustion zone;

[0010] S2: Radiation detector placement and data acquisition: Detectors are placed around the combustion zone where radioactive isotopes are injected and radiation intensity data are collected;

[0011] S3: Data processing, background radiation subtraction is performed based on the collected radiation intensity data, and then the model parameter m is calculated using the grid regularization inversion method, and the maximum entropy threshold method is used to select an appropriate threshold for the calculation result;

[0012] S4: Result output: based on the calculated model parameter m and the determined threshold, the fuel-empty zone boundary is divided, and the three-dimensional isosurface map of the fuel-empty zone and the isosurface maps in each direction are output.

[0013] Furthermore, the radioactive isotope in step S1 is radon-222, which has a half-life of 3.8 days; the injection amount is calculated based on the estimated volume of the combustion zone, typically 1-10 microcuries of radioactive isotope per cubic meter of combustion zone; the injection methods include continuous injection and pulse injection, with the continuous injection flow rate being 0.5-2 liters / minute, and the pulse injection lasting 5-15 minutes each time and performed every 2-4 hours; the injection pressure is controlled within the range of 0.2-0.5 MPa.

[0014] Furthermore, in step S2, the detector arrangement area covers the projection range of the underground fuel void zone and extends appropriately outside the boundary, with an arrangement density of at least one detector per 10 square meters; the vertical arrangement of the detectors extends from the surface at least downward to above the top of the fuel void zone, and the detector spacing in the vertical direction is 1-3 meters; the detectors on the surface form a grid distribution with a grid spacing of 2-5 meters; the detectors in the borehole are evenly distributed along the borehole axis with a spacing of 1-2 meters.

[0015] Furthermore, in step S2, a CZT or HPGe radiation detector is used to measure the gamma rays emitted by the radioactive isotope; the sampling frequency is determined according to the development speed of the combustion-air zone, the sampling interval is 5-15 minutes, the rapid development stage is shortened to 1-5 minutes, and the stable stage is extended to 30-60 minutes; the data collection duration is 12-72 hours of continuous monitoring.

[0016] Furthermore, background radiation subtraction in step S3 is achieved by performing background radiation measurements before radioisotope injection for at least 24 hours, recording the background radiation levels at each detector location under natural conditions; performing isotope injection after background radiation measurements are completed; and calculating the net radiation intensity after subtracting the background radiation.

[0017] Furthermore, the grid regularized inversion method in step S3 is implemented by the following steps:

[0018] Construct the objective function:

[0019]

[0020] Among them, m is the model parameter vector to be solved, d is the radiation intensity data vector measured by the detector, G is the forward operator matrix, λ is the regularization parameter, L is the regularization matrix, ||.|| 2 represents the Euclidean norm;

[0021] Find the gradient of the objective function and set it to zero;

[0022] Solve for the model parameters:

[0023] m=(G T G+λL T L) -1G T d;

[0024] Where m is the model parameter vector, G is the forward operator matrix, G T is the transposed matrix of G, λ is the regularization parameter, L is the regularization matrix, L T is the transposed matrix of L, d is the radiation intensity data vector measured by the detector;

[0025]

[0026] r ij is the distance from the ith detector to the jth grid point, and ε is a small constant to avoid division by zero.

[0027] Furthermore, the maximum entropy threshold method in step S3 is implemented by the following steps:

[0028] Normalize the model parameter m:

[0029] M i '=(M i -M min ) / (M max -M min );

[0030] Among them, M i ' is the normalized array M i The value at a certain position i, M i is the value of the array M at a certain position i before normalization, M max and M min They are the maximum and minimum values in the array m before normalization, and finally the array M after array normalization is obtained. i ;

[0031] The normalized array M i Continuous values are discretized into a certain number of value intervals;

[0032] The probability p(i) of each numerical interval appearing in the statistical array m';

[0033] Traverse all possible thresholds t and calculate background entropy and foreground entropy;

[0034] Select the t value that maximizes the total entropy as the optimal threshold:

[0035] H(t)=H b (t)+H f (t);

[0036] Among them, H b (t) represents the background entropy, H f (t) represents the foreground entropy.

[0037] Furthermore, the result output in step S4 includes: marking the position points greater than the threshold as inside the fuel-empty zone, and marking the position points less than the threshold as outside the fuel-empty zone; outputting the three-dimensional shape diagram and XY plane diagram, YZ plane diagram, and XZ plane diagram of the fuel-empty zone; and automatically generating a fuel-empty zone volume calculation report, including the total volume, the maximum size in each direction, and the change from the last measurement result.

[0038] The beneficial effects of the present invention are as follows: the present invention adopts radioactive isotope injection combined with the arrangement of radiation detectors around the combustion void zone, thereby avoiding the difficulty of arranging sensors inside the combustion void zone and being more suitable for industrial applications; by adding radioactive isotopes to the gasification agent and using external detectors to monitor the radiation intensity distribution, non-invasive measurement of the combustion void zone shape is achieved, greatly improving the safety and feasibility of the measurement; by adopting the grid regularization inversion method and the maximum entropy threshold method for data processing, the three-dimensional shape of the combustion void zone is reconstructed with high precision, providing more accurate combustion void zone geometric information, and providing technical support for the monitoring, optimization and safety management of the gasification process. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is a flow chart of the method steps of the present invention;

[0040] Figure 2 It is an isosurface diagram of the combustion-air zone in the XY direction of the present invention;

[0041] Figure 3 It is an isosurface diagram of the combustion-air zone in the YZ direction of the present invention;

[0042] Figure 4 It is an isosurface diagram of the combustion-air zone in the XZ direction of the present invention;

[0043] Figure 5 It is a three-dimensional isosurface diagram of the combustion-air zone of the present invention. DETAILED DESCRIPTION

[0044] The subject matter described herein will now be discussed with reference to example embodiments. It should be understood that these embodiments are discussed solely to enable those skilled in the art to better understand and implement the subject matter described herein, and that the functions and arrangements of the elements discussed may be varied without departing from the scope of this specification. Various examples may omit, substitute, or add various processes or components as needed. Furthermore, features described in some examples may be combined in other examples.

[0045] At least one embodiment of the present invention discloses a method for measuring the shape of the underground coal gasification combustion zone based on radioactive isotopes, such as Figure 1 As shown, the following steps are included:

[0046] Step S1, selection and injection of radioactive isotopes, selecting radioactive isotopes with a moderate half-life and injecting them into the combustion zone;

[0047] Step S11: Isotope selection. A radioactive isotope with a moderate half-life and suitable radiation type and energy for detection is selected, such as radon-222 (Rn-222). Its half-life of 3.8 days makes it suitable for short-term monitoring, and its radiation energy is easily detectable. Furthermore, the selected isotope must have a low environmental impact and meet safety standards for the use and discharge of radioactive materials.

[0048] Step S12, isotope injection. The radioactive isotope is injected into the oxygen / air pipeline and injected into the combustion zone together with oxygen or air. The specific injection amount is calculated based on the estimated volume of the combustion zone. Usually, 1-10 microcuries (μCi) of radioactive isotope are injected per cubic meter of combustion zone. If the radioactive isotope is radon, it is about 6.5 to 65 picograms per cubic meter (1 picogram = 10-12 grams). The injection method adopts two methods: continuous injection and pulse injection. Continuous injection is suitable for situations where long-term monitoring is required, and pulse injection is suitable for situations where periodic monitoring is required. For continuous injection, a flow controller should be used to ensure a constant injection rate, usually 0.5-2 liters / minute; for pulse injection, each injection lasts for 5-15 minutes, and injection is performed every 2-4 hours. The injection pressure should match the gasification agent injection pressure, and is generally controlled within the range of 0.2-0.5MPa.

[0049] During implementation, the decay effect of radioactive isotopes should be considered, especially when choosing isotopes with short half-lives. For example, for radon-222 (half-life 3.8 days), its activity will drop to 25% of its initial value after 7.6 days. Therefore, the initial injection amount should be adjusted or a regular supplementary injection plan should be designed based on the monitoring duration and the half-life of the isotope. The decay formula can be used to calculate the activity at different time points:

[0050] A=A0e ^ (βt);

[0051] Where A0 is the initial activity, β is the decay constant (β = ln(2) / T, where T is the half-life), and t is the time. After the isotope is injected, it is necessary to wait 1-2 hours for it to fully diffuse in the combustion-air region and form a relatively uniform concentration distribution before measurement.

[0052] Step S2, arranging radiation detectors and collecting data, arranging detectors around the combustion zone where the radioactive isotope is injected and collecting radiation intensity data;

[0053] Step S21: Detector selection. Select a highly sensitive radiation detector, using a CZT (cadmium zinc telluride) or HPGe (high-purity germanium) detector to measure and detect gamma rays emitted by radioactive isotopes in the borehole and on the surface. Ensure that the detector's range and resolution meet measurement requirements.

[0054] Step S22, detector layout. The detector layout area should cover the projection range of the underground void zone and expand appropriately outside the boundary, with a layout density of at least 1 detector per 10 square meters. The vertical layout of the detector should take into account the depth and shape characteristics of the void zone, extending from the surface at least downward to above the top of the void zone, and the detector spacing in the vertical direction should be 1-3 meters to ensure that the vertical changes of the void zone can be captured. The detectors on the surface should be distributed in a grid shape with a grid spacing of 2-5 meters; the detectors in the borehole should be evenly distributed along the borehole axis with a spacing of 1-2 meters. Use a fixed bracket to fix the detector in the predetermined position to ensure its stability. All detectors should be accurately positioned and their three-dimensional coordinate information should be recorded.

[0055] Step S23, data collection. Start the detector and collect radiation intensity data in real time. Determine the sampling frequency according to the development speed of the fuel-air zone. Under normal circumstances, the sampling interval is 5-15 minutes. It can be shortened to 1-5 minutes in the rapid development stage and extended to 30-60 minutes in the stable stage. The duration of data collection should be determined according to the development of the fuel-air zone. Generally, it is 12-72 hours of continuous monitoring. The sampling frequency can be adjusted as needed during this period. Use the data acquisition system to record the radiation intensity value of each detector, and synchronously record the time and detector position information. During the collection process, regularly check the operating status of the detector to ensure data accuracy and continuity.

[0056] The data acquisition system consists of a sensor interface unit, a data transmission network, and a central processing unit. Each detector connects to a field data collector via a dedicated interface. The collector transmits data in real time to a central data server via a wired or wireless network. The transmission protocol uses industry-standard Modbus RTU or TCP / IP to ensure reliable and real-time data transmission. The central data server is equipped with a high-performance computer running dedicated data processing software to pre-process, store, and analyze the received data. 256-bit encryption is used throughout the data transmission process to ensure data security. The system features breakpoint resume and data backup functions to prevent data loss due to network interruptions or equipment failures.

[0057] S3: Data processing, background radiation subtraction is performed based on the collected radiation intensity data, and then the model parameter m is calculated using the grid regularization inversion method, and the maximum entropy threshold method is used to select an appropriate threshold for the calculation result;

[0058] Step S31, background radiation deduction. Before radioactive isotope injection, background radiation measurement should be carried out first. The measurement should last for at least 24 hours, and the background radiation level of each detector position in the natural environment should be recorded. After the background radiation measurement is completed, isotope injection is carried out according to step S12, and then formal monitoring begins. Assuming that the radiation intensity measured by a detector in the absence of radioactive isotopes is Ibackground, the radiation intensity measured by the detector at the same position in the presence of radioactive isotopes is Itotal, then the net radiation intensity after deducting the background radiation is: Inet = Itotal - Ibackground;

[0059] Where Itotal is the radiation intensity measured in the presence of radioactive isotopes, and Ibackground is the radiation intensity measured in the absence of radioactive isotopes. This method eliminates interference from factors such as the geological environment and cosmic rays on the measurement results.

[0060] Step S32, data processing: The data is processed using a grid regularization inversion method, and the threshold is selected using the maximum entropy threshold method.

[0061] Regularized inversion is a mathematical method used to solve inverse problems. It is widely used in fields such as geophysical exploration, medical imaging, signal processing, and machine learning. Its core idea is to transform an ill-posed problem into a well-posed problem by introducing a regularization term, thereby obtaining a stable and reasonable solution. The direct and inverse problems are explained as follows:

[0062] Forward Problem: Given a known model parameter vector m, calculate the observed data d using physical laws (such as the wave equation or diffusion equation).

[0063] d = G(m), where G is a forward operator (such as a finite element or finite difference model), d is the radiation intensity vector measured by the detector, and m is the model parameter vector (such as the distribution of radioactive isotopes in the combustion and empty regions).

[0064] Inverse Problem: Given the observed data d, find the model parameter m.

[0065] m=G -1 (d), where G -1 It represents the inverse operation of G, which usually cannot be calculated directly and needs to be solved through optimization methods.

[0066] The method we need to use, the regularized inversion method, is an inverse problem. The inverse problem refers to the process of inferring the model parameter m (the shape of the combustion-cavity region) from the radiation intensity data d detected by the detector. The inverse problem is usually ill-posed, as shown by:

[0067] The solution is not unique: multiple different m may correspond to the same d;

[0068] Unstable solution: A small perturbation of the data d causes a dramatic change in m;

[0069] The solution does not exist: the data d may not be exactly generated by any m.

[0070] The role of regularization:

[0071] Properties of the constraint solutions (e.g., smoothness, sparsity);

[0072] Suppress the influence of noise and improve the stability of the solution;

[0073] Ensure the uniqueness and rationality of the solution.

[0074] Regularized inversion is solved by minimizing the objective function:

[0075]

[0076] in:

[0077] ||G(m)-d|| 2 Measures the difference between model predictions and observed data;

[0078] R(m) is the regularization term that constrains the properties of m;

[0079] λ is the regularization parameter, balancing data fitting and model constraints;

[0080] is the objective function, which represents the total error to be optimized.

[0081] The selection of the regularization parameter λ is very critical, as it controls the trade-off between the data fitting term and the regularization term. If the λ value is too large, it will lead to oversmoothing and loss of detail information; if the λ value is too small, the influence of noise cannot be effectively suppressed. Common methods for selecting λ are:

[0082] L-curve method: plot the log-log curve of the data fitting error and the regularization term, and select the lambda value at the inflection point of the curve;

[0083] Generalized cross-validation method: select the best lambda value by minimizing the prediction error;

[0084] Empirical selection method: selection based on prior knowledge and experimental results, generally within 10 -5 to 10 -1 within the range.

[0085] This method uses the empirical selection method combined with the L curve method for verification to ultimately determine the λ value.

[0086] The calculation steps of regularized inversion are as follows:

[0087] Construct the objective function:

[0088]

[0089] Where m is the model parameter vector to be solved, d is the radiation intensity data vector measured by the detector, G is the forward operator matrix, λ is the regularization parameter, L is the regularization matrix, ||.|| 2 represents the Euclidean norm;

[0090] Find the gradient of the objective function and set it to zero:

[0091]

[0092] in Represents the gradient operator, which calculates the derivative of the objective function with respect to the model parameter m, is the objective function, G T is the transposed matrix of G, λ is the regularization parameter, L is the regularization matrix, L T is the transposed matrix of L, and d is the radiation intensity data vector measured by the detector.

[0093] Solve for the model parameters:

[0094] m=(G T G+λL T L) -1 G T d; (1)

[0095] In the formula is the objective function, which is to make the model prediction G(m) as close as possible to the detected data d and prevent the solution from being unstable. (G T G+λL T L) -1 Represents a matrix inversion operation.

[0096] G is a forward operator that describes how the model parameters m generate the measurement data d.

[0097]

[0098] r ijis the distance from the i-th detector to the j-th grid point, ε is a small constant to avoid division by zero, and k is a parameter related to the radiation intensity, which is related to the activity and attenuation characteristics of the radioactive isotope. The source of this formula is based on the attenuation formula of the point source radiation intensity. That is, the radiation intensity of a radiation point source at a certain location is inversely proportional to the square of the distance r from that location to the radiation point source.

[0099] First, a large space is selected as the area where the fuel-air zone is located. The selected area should include the entire actual fuel-air zone. The area is divided into grids. Assume that the number of grids in this area is U and the number of detectors is N. Then G is an NxU matrix, G T is the transposed matrix of matrix G.

[0100] L is the regularization matrix, L T It is the transposed matrix of matrix L. The regularization matrix of this model selects the identity matrix I. L is an identity matrix of size UxU. A sparse identity matrix can be used to store only non-zero elements. It is suitable for processing large-scale matrices and saving computer memory and computing time.

[0101] λ is the regularization parameter that controls the weight of the regularization term.

[0102] d is the known radiation intensity detected by the detector, and d is an Nx1 matrix.

[0103] m is the required model parameter. The value of each position in the array represents the probability that the position is within the gas-gap region. Larger values increase the probability of being within the gas-gap region, while smaller values decrease the probability. A threshold is required to determine the shape of the gas-gap region. Coordinates in m above this threshold are within the gas-gap region. Coordinates below this threshold are likely to be noise or non-gas-gap regions and are not displayed on the graph.

[0104] The threshold is selected using the Maximum Entropy Thresholding method. The Maximum Entropy Thresholding method is an image segmentation method based on information entropy theory. It is used to automatically determine the optimal segmentation threshold for an image. Here, we can apply it to the model parameter m. The core idea of the Maximum Entropy Thresholding method is to select a threshold that maximizes the sum of the entropies of the two regions (foreground and background) of the segmented array. Entropy is an indicator of system uncertainty in information theory; greater entropy indicates greater information content. The algorithm steps are as follows:

[0105] After the above calculation, we can get the array m, which is a series of numbers. First, we need to normalize the data:

[0106] M i '=(M i -M min ) / (Mmax -M min );

[0107] Among them, M i ' is the value of the normalized array m' at a certain position i, M i is the value of the array m at a certain position i before normalization, M max and M min They are the maximum and minimum values in the array m before normalization, and finally the array m' after array normalization is obtained.

[0108] Discretize the continuous values in the array m' into a certain number of value intervals;

[0109] The probability p(i) of each numerical interval appearing in the statistical array m';

[0110] Traverse all possible thresholds t and divide the array m' into two parts: background (0 to t) and foreground (t+1 to L-1), where L is the maximum numerical interval index after discretization. Calculate the entropy of the two parts:

[0111] Background entropy: H b (t)=-Σ[p(i) / P(t)*ln(p(i) / P(t))], i=0 to t;

[0112] Foreground entropy: H f (t)=-Σ[p(i) / (1-P(t))*ln(p(i) / (1-P(t))], i=t+1 to L-1;

[0113] Where P(t) = Σp(i), i = 0 to t; Σ represents the summation symbol, ln represents the natural logarithm, p(i) represents the probability of the i-th numerical interval, P(t) represents the total probability of the background area, H b (t) represents the background entropy, H f (t) represents the foreground entropy, t represents the current threshold, L represents the maximum numerical interval index after discretization, i represents the index of the numerical interval, and * represents a multiplication operation.

[0114] Find the optimal threshold t: select the total entropy H(t) = H b (t)+H f (t) The maximum t value is taken as the optimal threshold. All calculations are completed by Matlab programming.

[0115] S4: Output the results: divide the fuel-empty zone boundary based on the calculated model parameter m and the determined threshold value, and output the three-dimensional shape diagram of the fuel-empty zone and the projection diagrams in various directions;

[0116] Points with values greater than the threshold are marked as inside the void zone, while points with values less than the threshold are marked as outside. The system outputs a 3D shape diagram of the void zone and projections in different directions, including XY, YZ, and XZ planes, to visually demonstrate the void zone's geometry. After outputting the results, the system automatically generates a void zone volume calculation report, including the total volume, maximum dimensions in each direction, and the change from the previous measurement, to help engineers assess the void zone's development trend and rate.

[0117] Once the void zone shape is measured, the system can repeat this process hourly, daily, or weekly as needed, creating a dynamic database of the void zone shape over time, providing a basis for long-term monitoring and prediction. By comparing the void zone shape at different time points, the void zone expansion rate and direction can be calculated, providing a scientific basis for gasification process control.

[0118] Specific implementation case: The shape of the fuel void area is calculated based on the known detector radiation intensity. In order to verify the correctness of this method, the known fuel void area shape is used and it is assumed that the shape of the fuel void area is unknown in the calculation. Instead, the shape of the fuel void area is calculated based on the radiation intensity value of the detector to see whether the calculated fuel void area shape is the same or similar to the actual shape.

[0119] Assume that an underground void is an ellipsoid with a long radius of 8 meters in the X direction, a short radius of 4 meters in the Y direction, and a vertical radius of 4 meters in the Z direction. The coordinates of the ellipsoid's center are (0, 0, -6). Based on this ellipsoidal void, assume in this embodiment that the radioactive isotope concentration is uniform throughout the void, meaning that the radiation intensity is consistent. However, in practice, inconsistent radioactive isotope radiation intensity across the void can be measured. Based on the known detector layout, the radiation intensity from the void measured by each detector can be calculated. Furthermore, assume that the shape of the void is completely unknown, knowing only that it is within a 20x20x15 meter cube (this cube needs to encompass the actual void; care should be taken to keep it as small as possible while still encompassing the actual void to ensure calculation accuracy). The shape of the void is now calculated based on the two known quantities: the radiation intensity measured by the detectors and the fact that the void is within the 20x20x15 meter cube.

[0120] The coordinates of the center point of this 20x20x15 cube are (0,0,-7.5). The x, y, and z axes of this cube are divided into 30 grid points, so there are a total of 27,000 grids. 10 groups of detectors are evenly arranged in the x-axis direction (-10~10), with 10 detectors in each group. 10 groups of detectors are evenly arranged in the y-axis direction (-10~10), with 10 detectors in each group. Detectors are arranged in the z-axis direction using drilled holes and above the ground. From z=-2 meters to z=-12 meters, 10 groups of detectors are evenly arranged, with 10 detectors in each group, for a total of 1,000 detectors.

[0121] The detector number uses a three-dimensional coordinate system. For example, detector (3, 4, -5) means that the detector is located at x = 3, y = 4, z = -5.

[0122] The detectors should be arranged as many as possible. The more and denser the arrangement, the higher the calculation accuracy. Matlab programming is used for calculation. The radiation intensity measured by each detector can be calculated based on the position of the combustion-air zone and the position of the detector. The array d can be obtained. The calculation formula is as follows:

[0123]

[0124] Where d i represents the radiation intensity measured by the i-th detector, k = 1000, k is a parameter related to the radiation intensity of the combustion-air region, and here k is taken as 1000;

[0125] ε=10 -10 , ε takes a very small value to avoid division by 0.

[0126] Where U' is the number of grids in the U grids that are included in the combustion zone, that is, the contribution of the radioactive isotopes in the combustion zone to the radiation intensity of the detector. The grids in the non-combustion zone have no contribution to the radiation intensity measured by the detector. ij is the distance between the i-th detector and the j-th grid.

[0127] In this way, we get the radiation intensity value d of 1000 detectors. At this time, we assume that we do not know the shape of the gas-gap area, but only know that the gas-gap area is within 27000 grids in the area of 20x20x15 meters we set. According to formula (1), we can find the model parameter G:

[0128]

[0129] Where k = 1000, ε = 10 -10 , ε takes a very small value to avoid division by 0. ijis the distance between the i-th detector and the j-th grid. G is a matrix of size 1000x27000, 1000 is the number of detectors, 27000 is the number of the entire cubic grid, and λ in formula (1) is 10 -3 , L is the unit matrix with a size of 27000x27000. L can use a sparse matrix to store only non-zero elements, which is suitable for processing large-scale matrices and saving computer memory and computing time.

[0130] This calculates m. m is the required model parameter. The value of each position in the array represents the probability that the three-dimensional space is within the burn-out zone. Larger values indicate a higher probability of being within the burn-out zone, while smaller values indicate a lower probability of being within the burn-out zone. The normalized array m' and the optimal threshold are then calculated using the maximum entropy thresholding method described above. All calculations are performed using Matlab programming.

[0131] This paper uses the maximum entropy threshold method mentioned above to calculate the optimal threshold value of 0.5195, then according to the threshold value of array m', we can get the following: Figure 2-5 As shown in the diagram of the shape of the lower combustion void area, the coordinates of the values in the m array greater than 0.5195 are shown in the figure above. The values less than 0.5195 may be noise or non-combustion void areas and are not shown in the figure.

[0132] From the above figure we can see that Figure 2 The combustion and air zone in the XY direction is very close to the actual ellipsoid. Figure 3 The combustion and air zone in the YZ direction is also close to the actual shape. Figure 4 The XZ direction diagram of the combustion void zone may be slightly different from the actual ellipsoid. This is because the process characteristics of underground coal gasification determine that the detector cannot be placed in the combustion void zone, but can only be placed above the combustion void zone, which leads to some distortion.

[0133] The above describes an embodiment of the present invention, but this embodiment is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Ordinary technicians in this field can also make more forms of equivalent embodiments based on the inspiration of this embodiment, all of which are protected by this embodiment.

Claims

1. A method for measuring the shape of the burnout zone of underground coal gasification based on radioactive isotopes, characterized in that: The following steps are involved: S1: Selection and injection of radioisotopes: radioisotopes with moderate half-life are selected and injected into the combustion zone; S2: Radiation detector placement and data acquisition: Detectors are placed around the combustion zone where radioactive isotopes are injected and radiation intensity data are collected; S3: Data processing, background radiation subtraction is performed based on the collected radiation intensity data, and then the model parameter m is calculated using the grid regularization inversion method, and the maximum entropy threshold method is used to select an appropriate threshold for the calculation result; S4: Result output: based on the calculated model parameter m and the determined threshold, the fuel-empty zone boundary is divided, and the three-dimensional isosurface map of the fuel-empty zone and the isosurface maps in each direction are output.

2. The radioisotope-based method for measuring the shape of underground coal gasification burnout area according to claim 1, characterized in that: The radioactive isotope in step S1 is radon-222, which has a half-life of 3.8 days. The injection amount is calculated based on the estimated volume of the combustion zone, typically injecting 1-10 microcuries of radioactive isotope per cubic meter of combustion zone. The injection methods include continuous injection and pulse injection. The flow rate of continuous injection is 0.5-2 liters / minute, and the pulse injection lasts for 5-15 minutes each time and is performed every 2-4 hours; the injection pressure is controlled in the range of 0.2-0.5MPa.

3. The radioisotope-based method for measuring the shape of underground coal gasification burnout area according to claim 1, characterized in that: In step S2, the detector layout area covers the projection range of the underground fuel void zone and extends appropriately outside the boundary, with a layout density of at least one detector per 10 square meters; the vertical layout of the detectors extends from the surface at least downward to above the top of the fuel void zone, and the detector spacing in the vertical direction is 1-3 meters; the detectors on the surface form a grid distribution with a grid spacing of 2-5 meters; the detectors in the borehole are evenly distributed along the borehole axis with a spacing of 1-2 meters.

4. The radioisotope-based method for measuring the shape of underground coal gasification burnout zone according to claim 1, characterized in that: In step S2, a CZT or HPGe radiation detector is used to measure the gamma rays emitted by the radioactive isotope. The sampling frequency is determined according to the development speed of the combustion zone, and the sampling interval is 5-15 minutes, which is shortened to 1-5 minutes in the rapid development stage and extended to 30-60 minutes in the stable stage. The data collection duration is 12-72 hours of continuous monitoring.

5. The radioisotope-based method for measuring the shape of underground coal gasification burnout area according to claim 1, characterized in that: The background radiation subtraction in step S3 is achieved by: performing background radiation measurement before radioisotope injection, the measurement lasting at least 24 hours, and recording the background radiation level at each detector position in the natural environment; performing isotope injection after the background radiation measurement is completed; and the net radiation intensity after deducting the background radiation is: Inet = Itotal - Ibackground; Where Itotal is the radiation intensity measured under the condition of radioactive isotopes, and Ibackground is the radiation intensity measured under the condition of no radioactive isotopes.

6. The radioisotope-based method for measuring the shape of underground coal gasification burnout zone according to claim 1, characterized in that: The grid regularized inversion method in step S3 is implemented by the following steps: Construct the objective function: Where m is the model parameter vector to be solved, d is the radiation intensity data vector measured by the detector, G is the forward operator matrix, λ is the regularization parameter, L is the regularization matrix, ||·|| 2 represents the Euclidean norm; Find the gradient of the objective function and set it to zero; Solve for the model parameters: m=(G T G+λL T L) -1 G T d; Where m is the model parameter vector, G is the forward operator matrix, G T is the transposed matrix of G, λ is the regularization parameter, L is the regularization matrix, L T is the transposed matrix of L, d is the radiation intensity data vector measured by the detector; r ij is the distance from the i-th detector to the j-th grid point, ε is a small constant to avoid division by zero, and k is a parameter related to the radiation intensity.

7. The radioisotope-based method for measuring the shape of underground coal gasification burnout area according to claim 1, characterized in that: The maximum entropy threshold method in step S3 is implemented by the following steps: Normalize the model parameter m: M i ′=(M i -M min ) / (M max -M min ); Among them, M i ' is the value of the normalized array m' at a certain position i, M i is the value of the array m at a certain position i before normalization, M max and M min are respectively the maximum and minimum values in the array m before normalization, and finally the array m' after array normalization is obtained; The normalized array M i Continuous values are discretized into a certain number of value intervals; The probability p(i) of each numerical interval appearing in the statistical array m'; Traverse all possible thresholds t and calculate background entropy and foreground entropy; Select the t value that maximizes the total entropy as the optimal threshold: H(t)=H b (t)+H f (t); Among them, H b (t) represents the background entropy, H f (t) represents the foreground entropy.

8. The radioisotope-based method for measuring the shape of underground coal gasification burnout area according to claim 1, characterized in that: The result output in step S4 includes: marking the position points greater than the threshold as inside the gas-gap area, and marking the position points less than the threshold as outside the gas-gap area; outputting the three-dimensional shape diagram and XY plane diagram, YZ plane diagram, and XZ plane diagram of the gas-gap area; and automatically generating a gas-gap area volume calculation report, including the total volume, the maximum size in each direction, and the change from the last measurement result.

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