Simulation analysis method for heat transfer behavior of underground coal gasification under condition of multiple thin coal seams

Abstract

The present application relates to the field of coal underground gasification analysis, and relates to a simulation analysis method for heat transfer behavior of coal underground gasification under the condition of multiple thin coal seams. The present application divides the grid of each geological layer by the thickness of each geological layer in the target gasification coal seam and its overburden, initializes each parameter of each unit, analyzes the exothermic competition weight based on the endothermic and exothermic reaction rate of the target gasification coal seam, and further determines the net reaction heat source term, updates the temperature of the grid unit based on the net reaction heat source term of each unit, identifies the endothermic and exothermic alternating zone and locates the heat spot center in combination with the exothermic competition weight; the trajectory curvature and heat spot moving speed are obtained based on the associated heat spot center of adjacent time points, and the gasification stability is evaluated; the connected desorption domain is divided based on the overburden temperature, and the desorption window is divided according to the average temperature of the connected desorption domain at each time point, the heat action intensity index and effective desorption volume of each desorption window are calculated, and the basis for analyzing the methane thermal desorption is provided.

Description

Technical Field

[0001] This invention relates to the field of underground coal gasification analysis, and specifically to a method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams. Background Technology

[0002] Underground coal gasification, which converts solid coal in situ into combustible syngas, is a crucial technological pathway for the clean development of difficult-to-mine coal seams. For thin coal seams, characterized by small individual layer thickness and frequent interlayering of gangue, the heat transfer path from the target coal seam to the overlying layers during gasification is complex, directly impacting syngas production, gasification stability, and methane desorption from adjacent coal seams. Therefore, a simulation analysis method is urgently needed to precisely describe the heat transfer behavior during gasification under thin coal seam conditions and to provide quantitative data for gasification stability assessment and joint mining projects.

[0003] Existing technologies, such as Chinese Patent Publication No. CN116244926A, disclose a numerical simulation method for underground coal gasification processes that considers heat source movement and excavation disturbance. This method determines the mechanical and thermodynamic parameters of rock strata through indoor experiments, establishes theoretical models of temperature, stress, and damage fields in the underground coal seam gasification process, determines geometric models based on engineering geological conditions, and establishes numerical models of heat source movement boundary conditions and combustion zone formation processes in the coal gasification process, thereby solving for the temperature, stress, and damage fields.

[0004] Chinese Patent Publication No. CN121031456A discloses a numerical simulation method and system for underground coal gasification processes. It constructs a three-zone conceptual model of gasification chamber, chamber wall, and raw coal; establishes a kinetic model coupling five types of chemical reactions, and correlates porosity changes and chamber wall temperature gradient distribution; and realizes dynamic evolution prediction of semi-teardrop-shaped gasification chambers through a numerical process of cyclically updating porosity, marking the chamber, calculating reactions, and outputting syngas.

[0005] However, existing technologies have the following problems: 1. Existing technologies, by simulating the impact of the overall movement of the heat source on the stress field and damage field of the surrounding rock, cannot distinguish the spatial competition between combustion heat release and gasification heat absorption, resulting in the lack of hot spot center location and making it difficult to establish a quantitative correlation between hot spot movement characteristics and gasification stability at the level of heat transfer behavior analysis.

[0006] 2. Existing technologies describe the thermal field by pre-defining uniform temperature distribution within the cavity and non-uniform temperature distribution on the cavity wall, thus enabling dynamic evolution prediction of the gasification cavity morphology. However, they cannot reflect the influence of the interlayered structure of multiple thin coal seams on the heat transfer path and neglect to quantitatively evaluate the transfer effect of gasification heat to the overlying thin coal seam, resulting in the inability to accurately determine the extraction timing and area for joint mining projects. Summary of the Invention

[0007] This invention aims to address the shortcomings of existing technologies by providing a method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams. By calculating the exothermic competition weight to dynamically allocate the reactive heat source, locating the heat absorption and exothermic alternation zone and tracking the hot spot trajectory, and simultaneously identifying the desorption temperature zone of the overlying coal seam, this method provides a basis for assessing gasification stability and co-extraction of coalbed methane.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: The present invention provides a method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams, including: dividing each geological layer into grids based on the thickness of the target gasification coal seam and its overlying geological layers, and initializing the temperature of each unit.

[0009] Based on the analysis of the exothermic and endothermic reaction rates of the target gasified coal seam, the exothermic competition weight is analyzed. According to the exothermic competition weight of each unit and the coal volume ratio, the net reaction heat source term is analyzed. Based on the net reaction heat source term of each unit, the temperature of all grid units at the next time point is calculated. The next time point is used as the new current time point, and the process is repeated to obtain the temperature of all grid units at each time point.

[0010] Based on the exothermic competition weight, the hot spot center on the heat absorption and exothermic alternation zone and its adjacent exothermic side is located. The hot spot migration trajectory is constructed based on the hot spot center at each historical time point. The trajectory curvature and hot spot movement speed of the hot spot migration trajectory are obtained. The gasification stability is evaluated based on the trajectory curvature and hot spot movement speed.

[0011] Based on the current overlying layer temperature, the connected desorption domains are divided. According to the average temperature of the connected desorption domains at each time point, the desorption windows of the entire gasification process are divided, and the thermal intensity index and effective desorption volume of each desorption window are calculated.

[0012] Compared with the prior art, the present invention has the following beneficial effects: (1) Based on the thickness of the target gasified coal seam and the geological layers in the overlying layer, the present invention divides the geological layers into grids, initializes the temperature of each unit, realizes the precise matching of grid size and geological layer thickness, avoids the thick grid from obscuring the heat transfer details of the thin coal seam, and improves the accuracy of temperature field simulation.

[0013] (2) Based on the analysis of the exothermic reaction rate and endothermic reaction rate of the target gasification coal seam, the present invention analyzes the exothermic competition weight based on the exothermic competition weight of each unit and the coal volume ratio, and calculates the temperature of all grid units at the next time point based on the net reaction heat source of each unit, quantifies the endothermic and exothermic competition relationship under multiple thin coal seam conditions, and realizes the precise spatiotemporal allocation of reaction heat.

[0014] (3) Based on the heat competition weight, the present invention locates the hot spot center on the heat absorption and heat release alternation zone and its adjacent heat release side, constructs the hot spot migration trajectory based on the hot spot center at each historical time point, obtains the trajectory curvature and hot spot movement speed of the hot spot migration trajectory, evaluates the gasification stability based on the trajectory curvature and hot spot movement speed, accurately identifies the heat absorption and heat release alternation zone, and the hot spot positioning is more in line with the reaction mechanism and heat transfer law, realizes the visualization of hot spot migration, and improves the continuous reliability of gasification of multiple thin coal seams.

[0015] (4) Based on the current overlying layer temperature, the present invention divides the connected desorption domain, and divides the desorption window of the entire gasification process according to the average temperature of the connected desorption domain at each time point. The thermal intensity index and effective desorption volume of each desorption window are calculated to achieve quantitative desorption effect, provide support for guiding the extraction area, and help improve the gas recovery rate and resource utilization rate. Attached Figure Description

[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the method steps of the present invention;

[0018] Figure 2 This is a schematic diagram illustrating the specific steps of the hot spot center localization method in this invention;

[0019] Figure 3 This is a schematic diagram of the steps in the desorption window partitioning method of the present invention. Detailed Implementation

[0020] Various exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that, unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of the invention. Furthermore, it should be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale.

[0021] The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. Techniques, methods, and apparatus known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and apparatus should be considered part of the specification.

[0022] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values.

[0023] Please see Figure 1 As shown, the present invention provides a method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams, including: S1, dividing each geological layer into grids based on the thickness of the target gasification coal seam and its overlying geological layers, and initializing the temperature of each unit.

[0024] Considering the spatial characteristics of multiple thin coal seams—numerous layers and thin individual layers—using a uniform mesh size would result in an overly thick mesh that obscures the internal temperature gradient and heat transfer details of the thin coal seam, leading to simulation distortion. Furthermore, the thicknesses of different geological layers vary significantly, necessitating adaptive determination of mesh edge length and the number of layers based on their minimum thickness to ensure precise subdivision of each layer without crossing layers. Therefore, it is necessary to construct mesh cells of matching thickness, using the thinnest coal seam and the thinnest rock strata as benchmarks, to achieve refined heat transfer simulation.

[0025] Based on this, the specific steps for gridding various geological strata include: S11, obtaining the average thickness of each coal seam and each interstitial rock layer in the target gasification coal seam and its overlying strata; determining the grid side length of the basic coal seam based on the coal seam with the smallest average thickness; and determining the grid side length of the basic rock layer based on the interstitial rock layer with the smallest average thickness. In this embodiment, to ensure that each geological stratum reflects a temperature gradient in the vertical direction, the grid side lengths of the basic coal seam and the basic rock layer are set according to the coal seam with the smallest average thickness and the interstitial rock layer with the smallest average thickness. Specifically, 1 / n of the average thickness of the coal seam with the smallest average thickness is used as the grid side length of the basic coal seam, where 3 is used in this embodiment. Implementers can also determine other values ​​according to requirements, but it must not be less than 2.

[0026] The overlying strata include all coal seams above the target gasification coal seam and the interlayers between them.

[0027] S12. Using the side length of the basic coal seam grid as the horizontal length and width, each coal seam is uniformly divided into multiple grid columns on the horizontal plane. The ratio of the average thickness of each grid column to the side length of the basic coal seam grid is rounded down to obtain the pre-divided grid layer number of each grid column.

[0028] S13. The ratio of the average thickness of each grid column to the number of pre-divided grid layers is recorded as the actual grid height. The actual grid height is used as the vertical dimension of the grid to construct the grid unit corresponding to each grid column. Ensure that each coal seam is covered by the grid unit, and that each grid unit does not cross different geological layers.

[0029] S14. Similarly, construct the grid units for each interstitial rock layer according to the grid construction method of each coal seam. Specifically, firstly, divide the interstitial rock layer laterally with the side length of the basic rock layer grid as the width to obtain each grid column. Then, round down the ratio of the average thickness of each grid column to the side length of the basic rock layer grid to obtain the number of pre-divided grid layers for each grid column.

[0030] Finally, the ratio of the average thickness of each grid column to the number of pre-divided grid layers is recorded as the actual grid height. The actual grid height is used as the vertical dimension of the grid to construct the grid cell corresponding to each grid column.

[0031] Furthermore, in a specific embodiment of the present invention, the specific steps for initializing the temperature of each unit are as follows: obtain the surface temperature and geothermal gradient of the target area, and calculate its initial temperature in combination with the burial depth of each grid unit.

[0032] This invention divides the geological layers into grids based on the thickness of the target gasified coal seam and its overlying geological layers, initializes the temperature of each unit, and achieves precise matching between the grid size and the thickness of the geological layers. This avoids the thick grid from obscuring the heat transfer details of the thin coal seam and improves the accuracy of temperature field simulation.

[0033] S2. Analyze the exothermic competition weights, determine the net reaction heat source term based on the exothermic competition weights, and update the temperature of all grid cells.

[0034] Considering that during underground coal gasification, the exothermic combustion reaction and the endothermic gasification reaction occur simultaneously and compete with each other in the same space, a single reaction rate cannot reflect the true heat production characteristics of the region. Furthermore, coal does not fill the entire volume within a grid cell; therefore, the proportion of coal volume is necessary to accurately calculate the net heat source per unit volume.

[0035] Furthermore, considering that recursively updating the internal heat source temperature field based on the net reactive heat source term can accurately reflect the heat transfer law of heat absorption and release coupling within multiple thin coal seams, it is necessary to first calculate the heat release competition weight, then combine it with the coal volume ratio to obtain the net reactive heat source term, and then proceed with the temperature field iteration.

[0036] Based on this, the specific implementation steps of S2 include: S21, analyzing the exothermic reaction rate and endothermic reaction rate of the target gasification coal seam to determine the exothermic competition weight. The specific implementation steps include: S211, extracting the mole fractions of oxygen and water vapor and the gas pressure from the parameters of each grid cell, solving for the combustion exothermic reaction rate constant based on the current temperature of each grid cell, the preset combustion reaction activation energy, and the pre-exponential factor, and recording the product of the combustion exothermic reaction rate constant with the oxygen mole fraction and the gas pressure as the exothermic reaction rate. The gas pressure, oxygen mole fraction, and water vapor mole fraction corresponding to each grid cell at each time point are input as a known data sequence changing over time and are not involved in the iterative calculation of this method. These can be pre-calculated using Eclipse, a gas reservoir simulation software well-known in the field.

[0037] The formula for calculating the rate constant of the exothermic combustion reaction is as follows: .

[0038] in Represents the activation energy of the reaction. Represents the pre-exponential factor. Represents the rate constant of the exothermic combustion reaction. The current temperature. The value represents the molar gas constant, taken as 8.314 J / (mol·K). This formula is the exponential form of the Arrhenius equation, which is existing technology and will not be described in detail here. In a specific embodiment of this invention, the activation energy and pre-exponential factor are obtained by conducting experiments using a thermogravimetric analyzer under conditions of combustion reaction at a heating rate of 10 °C / min and an atmosphere of air, and vaporization reaction of 30% water vapor + 70% nitrogen. The activation energy and pre-exponential factor are obtained by kinetic fitting of the experimental data using the Coats-Redfern integral method. This fitting process is existing technology and will not be described in detail here.

[0039] S212. Based on the current temperature of each grid cell, the preset activation energy of the gasification reaction, and the pre-exponential factor, solve for the gasification endothermic reaction rate constant. The product of the gasification endothermic reaction rate constant, the water vapor mole fraction, and the gas pressure is denoted as the endothermic reaction rate. The calculation formula for the gasification endothermic reaction rate constant is consistent with the calculation method for the combustion exothermic reaction rate constant.

[0040] S213. Calculate the sum of the exothermic reaction rate and the endothermic reaction rate and record it as the total rate. Select the cell grids with a non-zero total rate and record them as reaction regions. Record the ratio of the exothermic reaction rate of each reaction region to the total rate as the exothermic competition weight.

[0041] S22. Analyze the net reaction heat source item based on the heat release competition weight and coal volume ratio of each unit. The specific implementation steps include: S221. Obtain the heat absorption of a unit mass of coal for complete gasification and the heat release of a unit mass of coal for complete combustion in the target gasification coal seam, and record them as unit heat absorption and unit heat release, respectively. Weight the unit heat absorption and unit heat release according to the heat release competition weight and calculate the difference to obtain the unit reaction heat of each reaction zone.

[0042] The specific formula for calculating the unit heat of reaction is as follows: .

[0043] in The unit of reaction heat, Represents the exothermic competitive weight. , These represent the unit of heat released and the unit of heat absorbed, respectively. Since the gasification reaction is an endothermic process, its contribution to the net reaction heat source term is negative, so it is calculated by subtraction.

[0044] Furthermore, the unit heat absorption and unit heat release can be obtained by performing thermal analysis experiments on coal samples from the target gasification coal seam using differential scanning calorimetry (DSC) to obtain the heat of reaction for complete combustion and complete gasification. The differential scanning calorimetry method is existing technology and will not be described in detail here.

[0045] S222. The ratio of the actual coal-bearing volume to the grid cell volume in each reaction zone is denoted as the coal volume ratio. The product of the unit heat of reaction in each reaction zone with the target gasification coal seam density, the coal volume ratio, and the total rate is denoted as the net heat source term. The net heat source term represents the net heat released per unit time and unit volume by the combined effects of combustion and gasification, i.e., the thermal power per unit volume, with units of W / m³. 3 .

[0046] S23. Calculate the temperature of all grid cells at the next time point based on the net reaction heat source term of each cell.

[0047] By taking the next time point as the new current time point and iterating in a loop, the temperature of all grid cells at each time point is obtained.

[0048] Specifically, after obtaining the net reaction heat source term for each grid cell, the finite volume method is used to numerically solve the unsteady-state heat conduction equation to determine the temperature distribution of each grid cell at the next time point, thus recursively obtaining the temperature of all grid cells at each time point. The specific steps are as follows: The time domain is discretized into a series of time points. For each grid cell, an unsteady-state heat conduction equation considering the reaction heat source is established. The temperature at the next time point is calculated at each time point, and its expression is as follows: .

[0049] For adjacent time intervals, Let i be the temperature at the next time point after the i-th time point. Let be the temperature at the i-th time point. Let be the thermal conductivity at the i-th time point, where the thermal conductivity can be obtained from the pre-calibrated thermal conductivity of each geological layer at each temperature based on the current temperature. Let i be the net reaction heat source term at time point i. The density of the medium within the grid cell, Specific heat capacity is an inherent parameter of each medium, and the corresponding specific heat capacity can be obtained from the underground gasification heat transfer behavior simulation and analysis database based on the medium within each grid cell. For the heat conduction term, in this embodiment, the finite volume method is used to discretize the heat conduction term, and the volume component of the heat conduction term of each grid can be converted into the sum of the heat flux through its six faces. The specific calculation formula is as follows: .

[0050] in The temperature at the center of the current grid cell. The temperature is the center of the adjacent grid cell adjacent to surface f. This is the distance between the center of the current grid cell and the centers of adjacent grid cells on face f. Let f be the area of ​​surface f. The equivalent thermal conductivity on surface f is the harmonic average of the thermal conductivity of two adjacent units.

[0051] It is necessary to calculate whether the interval time satisfies the requirement. If the conditions are not met, then the data collection interval for each time point will be set to [value]. .in It is the largest thermal diffusivity in the entire simulation region. ; This represents the minimum grid size.

[0052] This invention analyzes the exothermic and endothermic reaction rates of the target gasified coal seam to determine the exothermic competition weights. Based on the exothermic competition weights of each unit and the coal volume ratio, it analyzes the net reaction heat source terms. Based on the net reaction heat source terms of each unit, it calculates the temperature of all grid units at the next time point, quantifies the endothermic and exothermic competition relationship under multiple thin coal seam conditions, and achieves precise spatiotemporal allocation of reaction heat.

[0053] S3. Locate the hot spot center on the alternating heat absorption and release zone and its adjacent heat release side, and evaluate the gasification stability based on the trajectory curvature and hot spot movement speed of the hot spot center.

[0054] Considering that the hot spots on the adjacent sides of the heat absorption and release alternation zone, where the heat absorption and release competition is most intense, can better reflect the characteristics of the gasification process, and that abrupt changes in hot spot trajectory and velocity are direct indicators of gasification instability, gasification stability can be quantitatively assessed by using both trajectory curvature and movement speed as indicators.

[0055] Furthermore, considering that the reaction competition region cannot be identified by relying solely on the temperature field, the heat absorption and exothermic alternation region and hot spot can be located based on the exothermic competition weight.

[0056] Based on this, the specific implementation steps of S3 include: S31, locating the heat absorption / exhaustion alternation zone and the hot spot center on its adjacent heat-exhausting side based on the heat release competition weight. For example... Figure 2 As shown, the specific implementation steps include: S311, obtaining the theoretical range of the exothermic competition weight, taking the sum of the minimum value of the theoretical range of the exothermic competition weight and the preset boundary correction coefficient as the lower boundary of the alternating threshold, and taking the difference between the maximum value of the theoretical range of the exothermic competition weight and the preset boundary correction coefficient as the upper boundary of the alternating threshold.

[0057] Specifically, the theoretical range of the exothermic competition weight is [0, 1], where an exothermic competition weight of 0 represents pure endothermic dominance, and an exothermic competition weight of 1 represents pure exothermic dominance. The range between 0 and 1 indicates the existence of competition. In numerical simulation, a very small non-zero reaction rate may be generated due to rounding errors. If no boundary correction coefficient is set, these noise units will be misjudged as alternating regions. In this embodiment, the preset boundary correction coefficient is 0.2, so the range of the alternation threshold is [0.2, 0.8]. Implementers can also set other values ​​according to the situation, but they must be less than 0.5.

[0058] S312. Each grid cell whose heat exothermic competition weight is less than the upper boundary of the alternation threshold and greater than the lower boundary of the alternation threshold is denoted as a heat absorption / exothermic alternation cell.

[0059] S313. Mark the connectivity of adjacent heat absorption and release alternating units, and denote each connected region as a heat absorption and release alternating region.

[0060] S314. Obtain the heat release competition weights of all grid cells adjacent to the heat absorption and release alternation zone, and select grid cells whose heat release competition weights are greater than or equal to the upper boundary of the alternation threshold, and record them as adjacent heat release sides.

[0061] S315. Based on the temperatures of each grid cell on adjacent heat-exerting sides, select the geometric center of the grid cell with the highest temperature as the hot spot center. If there are no adjacent heat-exerting sides, then the grid cell with the highest heat-exerting competition weight in the heat absorption and release alternation zone is selected as the hot spot center.

[0062] S32. Construct hot spot migration trajectories based on hot spot centers at historical time points, obtain the trajectory curvature and hot spot movement speed, and evaluate vaporization stability based on trajectory curvature and hot spot movement speed. The calculation method for hot spot movement speed is as follows: obtain the Euclidean distance between the position coordinates of the hot spot center at each time point and its adjacent previous time point, and use the ratio of this distance to the adjacent time interval as the hot spot movement speed. The specific implementation steps are as follows: S321. Connect the hot spot centers at historical time points sequentially in chronological order to construct hot spot migration trajectories, obtain the triangle formed by the last three hot spot centers of the hot spot migration trajectory, obtain the radius of its corresponding smallest circumcircle, and record the reciprocal of the radius as the trajectory curvature. The acquisition frequency at each time point is 1 second, but the implementer can set other values ​​according to the actual situation. It should be noted that when the number of hot spot centers is less than 3, the trajectory curvature and hot spot movement speed are not calculated, and stability analysis is not performed.

[0063] S322. Obtain the trajectory curvature corresponding to each historical time point, combine it with the current trajectory curvature to form a curvature sequence, calculate the rate of change between adjacent trajectory curvatures, obtain the rate of change of curvature at each time point, and calculate the average and standard deviation of the historical rate of change of curvature.

[0064] S323. Set the curvature mutation threshold based on the mean and standard deviation. If the current rate of change of curvature is greater than the curvature mutation threshold, then the current point is recorded as the trajectory curvature mutation point.

[0065] S324. Similarly, determine whether the current point is a hotspot movement speed abruptly, using the method for judging abrupt changes in trajectory curvature. Specifically, based on the hotspot center at each time point and its adjacent previous time point, the ratio of this ratio to the unit time interval is recorded as the hotspot movement speed; obtain the hotspot movement speed corresponding to each historical time point, combine it with the current trajectory curvature to form a movement speed sequence, calculate the rate of change between adjacent movement speeds, obtain the rate of change of movement speed at each time point, and calculate the average and standard deviation of the historical rate of change of movement speed.

[0066] The threshold for sudden change in hot spot movement speed is set based on the average value and standard deviation. If the current rate of change in movement speed is greater than the threshold for sudden change in hot spot movement speed, then the current point is recorded as the point of sudden change in hot spot movement speed.

[0067] S325. If the current point is a sudden change in trajectory curvature or a sudden change in hot spot movement speed, the gasification stability is determined to be unstable; otherwise, the gasification stability is determined to be stable.

[0068] This invention locates the hot spot center on the heat absorption and release alternation zone and its adjacent heat release side based on the heat release competition weight. It constructs the hot spot migration trajectory based on the hot spot center at each historical time point, obtains the trajectory curvature and hot spot movement speed, evaluates gasification stability based on the trajectory curvature and hot spot movement speed, accurately identifies the heat absorption and release alternation zone, and makes the hot spot location more consistent with the reaction mechanism and heat transfer law, realizes the visualization of hot spot migration, and improves the continuous reliability of gasification in multiple thin coal seams.

[0069] S4. Divide the connected desorption domain, divide the desorption window for the entire gasification process, and calculate the thermal intensity index and effective desorption volume of each desorption window.

[0070] Considering that methane desorption from the overlying thin coal seam requires reaching a critical temperature, and that only connected high-temperature regions can form effective desorption zones, scattered single points lack engineering desorption value. Furthermore, the desorption process exhibits a three-stage pattern over time: heating up – a high-temperature plateau – cooling down. Desorption windows can be scientifically divided based on the temperature sequence of connected desorption domains. In addition, the intensity of thermal action and the effective desorption volume are core indicators for evaluating the efficiency of gasification heat utilization, directly guiding the timing and location of coalbed methane co-production.

[0071] Based on this, the specific implementation steps of S4 include: S41, dividing the connected desorption domain based on the current overburden temperature. The specific implementation steps are as follows: S411, obtaining the critical methane desorption temperature of each coal seam in the overburden, and marking each grid cell in each coal seam whose current temperature reaches the corresponding critical methane desorption temperature as a desorption initiation unit. The critical methane desorption temperature of each coal seam in the overburden is extracted from the underground gasification heat transfer behavior simulation analysis database. Specifically, the critical temperature at which methane begins to desorb for various types of coal can be experimentally calibrated and stored in the underground gasification heat transfer behavior simulation analysis database.

[0072] S412. Mark adjacent desorption start-up units within the same coal seam as connected components, remove connected components that contain only a single grid, and record the remaining connected components as connected desorption components.

[0073] S42. Divide the entire gasification process into desorption windows based on the average temperature of the connected desorption domain at each time point. For example... Figure 3 As shown, the specific implementation steps are as follows: S421, obtain the average temperature of each unit grid in all connected desorption domains of each coal seam at each time point in the entire gasification process, and form a temperature sequence of the connected desorption domains of the corresponding coal seam.

[0074] S422. Obtain the local maxima in the temperature sequence of the connected desorption domains of each coal seam, and use the time window formed by the first and last local maxima as the initial high-temperature plateau window for desorption. It should be explained that a local maximum is defined as a point whose temperature is greater than the temperatures of the preceding and following n consecutive points. In this embodiment, n is 3, but the implementer can set other specific values ​​according to actual conditions. It should also be noted that when the first and last local maximums occur at the same time point, the time between the first occurrence of the connected desorption domain and the last time point where the desorption domain exists is recorded as the initial high-temperature plateau window for desorption.

[0075] S423. Based on the local maxima within the initial desorption high-temperature platform window of each coal seam, divide the coal seam into candidate windows. Analyze the final desorption high-temperature platform window based on the time length and temperature standard deviation of each candidate window.

[0076] It should be explained that the analysis method of the final desorption high temperature plateau window includes: W1, obtaining initial candidate windows of various lengths within the initial desorption high temperature plateau window, with the starting point as the local maximum point and the subsequent local maximum points as the endpoints, and sliding the initial candidate windows of various lengths on the time axis at unit time intervals to obtain each candidate window.

[0077] W2. Obtain the standard deviation of the temperature at each time point within each candidate window, calculate the ratio of the time length to the standard deviation of each candidate window, and record the candidate window corresponding to the maximum value as the final desorption high-temperature platform window.

[0078] S424. The time point from the first appearance of the interconnected desorption domain in each coal seam to the starting point of the final desorption high temperature plateau window is recorded as the desorption heating window, and the time point from the final desorption high temperature plateau window to the last time point where the desorption domain exists is recorded as the desorption cooling window.

[0079] S43. Calculate the thermal intensity index and effective desorption volume for each desorption window. The specific implementation steps are as follows: S431. Multiply the temperature sequence of the connected desorption domain of each coal seam with the time interval respectively, and sum the results according to each desorption window to obtain the thermal intensity index of the corresponding desorption window.

[0080] S432. The effective desorption volume of the corresponding desorption window is obtained by summing the product of the volume of the connected desorption domain of each coal seam at each time point within each desorption window and the time.

[0081] The invention divides the connected desorption domain based on the current overlying layer temperature, and divides the desorption window of the entire gasification process according to the average temperature of the connected desorption domain at each time point. It calculates the thermal intensity index and effective desorption volume of each desorption window to achieve quantitative desorption effect, providing support for guiding the extraction area and helping to improve gas recovery rate and resource utilization rate.

[0082] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented using software, the above embodiments can be implemented, in whole or in part, in the form of a computer program product.

[0083] Those skilled in the art will recognize that the modules and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0084] In addition, the functional modules in the various embodiments of this application can be integrated into one processing module, or each module can exist physically separately, or two or more modules can be integrated into one module.

[0085] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

[0086] Finally, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams, characterized in that, include: Based on the thickness of the target gasification coal seam and the geological layers in its overlying layers, the geological layers are divided into grids, and the temperature of each grid cell is initialized. The exothermic reaction rate and endothermic reaction rate of the target gasification coal seam are analyzed to determine the exothermic competition weight. The net reaction heat source term is analyzed based on the exothermic competition weight and coal volume ratio of each unit. The temperature of all grid units at the next time point is calculated based on the net reaction heat source term of each unit. The next time point is used as the new current time point, and the temperature of all grid units at each time point is obtained by iteratively advancing. Based on the exothermic competition weight, the hot spot center on the heat absorption and exothermic alternation zone and its adjacent exothermic side is located. The hot spot migration trajectory is constructed based on the hot spot center at each historical time point. The trajectory curvature and hot spot movement speed of the hot spot migration trajectory are obtained. The gasification stability is evaluated based on the trajectory curvature and hot spot movement speed. Based on the current overlying layer temperature, the connected desorption domain is divided. According to the average temperature of the connected desorption domain at each time point, the desorption window of the entire gasification process is divided, and the thermal intensity index and effective desorption volume of each desorption window are calculated. The method for calculating the exothermic competition weight includes: The mole fractions of oxygen and water vapor and the pressure are extracted from the parameters of each grid cell. The combustion exothermic reaction rate constant is solved based on the current temperature of each grid cell, the preset combustion reaction activation energy and the pre-exponential factor. The product of the combustion exothermic reaction rate constant and the mole fraction of oxygen and the pressure is recorded as the exothermic reaction rate. The gasification endothermic reaction rate constant is solved based on the current temperature of each grid cell, the preset gasification reaction activation energy and the pre-exponential factor. The product of the gasification endothermic reaction rate constant, the water vapor mole fraction and the gas pressure is denoted as the endothermic reaction rate. The sum of the exothermic reaction rate and the endothermic reaction rate is calculated and recorded as the total rate. Each cell grid with a non-zero total rate is selected as a reaction region. The ratio of the exothermic reaction rate of each reaction region to the total rate is recorded as the exothermic competition weight. The analytical methods for the net reaction calorific value include: The heat absorbed by a unit mass of coal for complete gasification and the heat released by a unit mass of coal for complete combustion are recorded as the heat absorbed and the heat released, respectively. The heat absorbed and the heat released are weighted according to the heat release competition weight and the difference is used to obtain the heat of reaction of each reaction zone. The ratio of the actual coal-bearing volume to the grid cell volume in each reaction zone is recorded as the coal volume ratio. The product of the unit heat of reaction in each reaction zone with the target gasification coal seam density, the coal volume ratio, and the total rate is recorded as the net heat source term.

2. The method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams as described in claim 1, characterized in that, The specific steps for dividing the geological layers into grids include: Obtain the average thickness of each coal seam and each interstitial rock layer in the target gasification coal seam and its overlying strata. Determine the grid edge length of the basic coal seam based on the coal seam with the smallest average thickness, and determine the grid edge length of the basic rock layer based on the interstitial rock layer with the smallest average thickness. Using the side length of the basic coal seam grid as the horizontal length and width, each coal seam is uniformly divided into multiple grid columns on the horizontal plane. The ratio of the average thickness of each grid column to the side length of the basic coal seam grid is rounded down to obtain the pre-divided grid layer number of each grid column. The ratio of the average thickness of each grid column to the number of pre-divided grid layers is recorded as the actual grid height. The actual grid height is used as the vertical dimension of the grid to construct the grid cell corresponding to each grid column. Similarly, grid units for each inter-layer rock stratum are constructed using the same grid construction method as for each coal seam.

3. The method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams as described in claim 1, characterized in that, The method for locating the hot spot center includes: Obtain the theoretical range of the exothermic competition weight, and use the sum of the minimum value of the theoretical range of the exothermic competition weight and the preset boundary correction coefficient as the lower boundary of the alternating threshold, and use the difference between the maximum value of the theoretical range of the exothermic competition weight and the preset boundary correction coefficient as the upper boundary of the alternating threshold. Each grid cell whose heat exothermic competition weight is less than the upper boundary of the alternation threshold and greater than the lower boundary of the alternation threshold is denoted as a heat absorption / exothermic alternation cell. Adjacent heat absorption and release alternating units are marked with connectivity, and each connected region is recorded as a heat absorption and release alternating region; Obtain the heat release competition weights of all grid cells adjacent to the heat absorption and release alternation zone, and select grid cells whose heat release competition weights are greater than or equal to the upper boundary of the alternation threshold, and record them as adjacent heat release sides. Based on the temperature of each grid cell on the adjacent heat-exothermic side, the geometric center of the grid cell with the highest temperature is selected as the hot spot center.

4. The method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams as described in claim 1, characterized in that, The method for assessing gasification stability includes: Connect the hot spot centers at each historical time point in chronological order to construct the hot spot migration trajectory. Obtain the triangle formed by the last three hot spot centers of the hot spot migration trajectory, obtain the radius of the corresponding smallest circumcircle, and denote the reciprocal of the radius as the trajectory curvature. Obtain the trajectory curvature corresponding to each historical time point, combine it with the current trajectory curvature to form a curvature sequence, calculate the rate of change between adjacent trajectory curvatures, obtain the rate of change of curvature at each time point, and calculate the average and standard deviation of the historical rate of change of curvature. The curvature mutation threshold is set based on the mean and standard deviation. If the current rate of change of curvature is greater than the curvature mutation threshold, the current point is recorded as the trajectory curvature mutation point. Similarly, the method for determining abrupt changes in trajectory curvature should be used to determine whether the current point is a point of abrupt change in hot spot movement speed; If the current point is a sudden change in trajectory curvature or a sudden change in hot spot movement speed, the gasification stability is determined to be unstable; otherwise, the gasification stability is determined to be stable.

5. The method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams as described in claim 1, characterized in that, The method for dividing the connected desorption domain includes: Obtain the critical temperature of methane desorption in each coal seam in the overlying layer, and mark each grid cell in each coal seam whose current temperature reaches the corresponding critical temperature of methane desorption as a desorption start-up unit; Adjacent desorption initiation units within the same coal seam are marked as connected components. Components containing only a single grid are removed, and the remaining connected components are denoted as connected desorption components.

6. The method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams as described in claim 1, characterized in that, The method for dividing the desorption window includes: The average temperature of each cell grid in all connected desorption domains of each coal seam at each time point in the entire gasification process is obtained, and these are used to form the temperature sequence of the connected desorption domains of the corresponding coal seam. Local maxima are obtained in the temperature sequence of the interconnected desorption domain of each coal seam, and the time window composed of the first and last local maxima is used as the initial high-temperature plateau window for desorption. Based on the local maxima within the initial desorption high-temperature plateau window of each coal seam, each candidate window is obtained. The final desorption high-temperature plateau window is then analyzed based on the time length and temperature standard deviation of each candidate window. The time point from the first appearance of the interconnected desorption domain in each coal seam to the starting point of the final desorption high-temperature plateau window is recorded as the desorption heating window, and the time point from the final desorption high-temperature plateau window to the last time point where the desorption domain exists is recorded as the desorption cooling window.

7. The method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams as described in claim 6, characterized in that, The analysis method for the final desorption high-temperature plateau window includes: Within the initial desorption high-temperature platform window, initial candidate windows of various lengths are set, starting from the initial local maximum point and ending at each subsequent local maximum point. Each candidate window is obtained by sliding the initial candidate windows of various lengths on the time axis at unit time intervals. Obtain the standard deviation of temperature at each time point within each candidate window, calculate the ratio of time length to standard deviation for each candidate window, and record the candidate window corresponding to the maximum value as the final desorption high-temperature platform window.

8. The method for simulating and analyzing the heat transfer behavior of underground coal gasification under multiple thin coal seams as described in claim 7, characterized in that, The calculation methods for the thermal intensity index and effective desorption volume of each desorption window include: The temperature sequence of the connected desorption domain of each coal seam is multiplied by the time interval, and the results are accumulated according to each desorption window to obtain the thermal intensity index of the corresponding desorption window. The effective desorption volume of the corresponding desorption window is obtained by summing the products of the connected desorption domain volume of each coal seam at each time point within each desorption window and time.

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