Coal underground gasification selected area evaluation method

Abstract

The invention discloses a coal underground gasification area selection evaluation method, which comprises the following steps of: collecting geological parameters, hydrological parameters, coal quality parameters and environmental parameters of candidate areas; the parameters comprise coal seam thickness, buried depth, coal seam gas pressure, underground water level, aquifer distribution, surrounding rock strength, permeability coefficient, coal industrial analysis index, coal reaction activity index, underground water pollution risk factor and ground surface settlement risk factor; four dimensions including a geological stability index, a hydrological safety index, a coal reactivity index and an environmental risk index are included according to index types, a hierarchical structure of evaluation indexes is constructed by adopting an analytic hierarchy process, and the weight of each evaluation parameter in an index system is calculated through a pairwise comparison method and consistency check. Therefore, the problems that the evaluation factor is single, the result is subjective, and multi-factor coupling is not considered are solved, and the scientificity, objectivity and engineering practicability of coal underground gasification selected area analysis are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of coal mining, and in particular to an underground coal gasification zone selection evaluation method. Background Art

[0002] Underground coal gasification (UCG) is a technology that converts unmined underground coal seams into combustible gas in situ. It offers advantages such as low mining costs, high resource utilization, and minimal surface damage. Existing UCG technology typically analyzes basic geological parameters, such as coal seam thickness, burial depth, and gas reservoir pressure, as the primary basis for site selection evaluation. These parameters are easily accessible, simplifying the evaluation process and making them suitable for early feasibility studies. Consequently, they are widely adopted in current engineering practice. However, as underground coal gasification projects continue to advance, technicians have gradually discovered significant shortcomings in these traditional zone selection and evaluation methods. The primary issue lies in their single evaluation factor and their lack of comprehensive analysis of the geological environment in which the coal seam resides. In practice, the hydrogeological conditions of the coal seam, the stability of the surrounding rock structure, the coupling relationship between the permeability of the coal seam and the surrounding rock, and the impact of coal quality on gasification reaction efficiency all play a key role in the feasibility and safety of zone selection. Traditional methods fail to fully account for the interactions of these multiple factors, which can easily lead to misjudgments in zone selection, which in turn can cause problems such as gas leakage, groundwater contamination, or low gasification efficiency, severely restricting the stable operation and sustainable development of underground coal gasification technology. Summary of the Invention

[0003] The present invention aims to solve one of the technical problems in the related art at least to a certain extent.

[0004] To this end, this paper proposes a method for evaluating underground coal gasification (UCG) site selection. By incorporating the analytic hierarchy process (AHP) and a fuzzy comprehensive evaluation model, a multi-factor weighting system and a suitability scoring mechanism are established, improving the scientific and practical nature of UCG site selection. Finally, regional suitability levels are assigned based on the comprehensive scores, significantly enhancing the accuracy and environmental safety of UCG site selection.

[0005] To achieve the above object, the present invention proposes a method for evaluating underground coal gasification area selection, comprising the following steps: S1. Data collection: Collect geological parameters, hydrological parameters, coal quality parameters, and environmental parameters of the candidate area, including coal seam thickness, burial depth, coal seam gas pressure, groundwater level, aquifer distribution, surrounding rock strength, permeability, coal industry analysis indicators, coal reactivity indicators, groundwater pollution risk factors, and surface subsidence risk factors; S2. Parameter processing: normalize the various parameters and classify them into four dimensions according to the indicator type: geological stability index, hydrological safety index, coal reactivity index, and environmental risk index; S3. Weight setting: A hierarchical structure of evaluation indicators is constructed using the analytic hierarchy process, and the weight of each evaluation parameter in the indicator system is calculated through paired comparison and consistency test; S4. Model calculation: Based on the normalized parameters and the indicator weights, a fuzzy comprehensive evaluation method is used to construct a fuzzy judgment matrix, fuzzify the matrix using a membership function, and perform a fuzzy comprehensive operation to calculate a comprehensive evaluation score for the candidate area. S5. Result determination: Compare the comprehensive score with the preset suitability threshold and classify the candidate areas into three categories: suitable, basically suitable, and unsuitable, to guide the site selection decision for underground coal gasification.

[0006] The present invention provides an underground coal gasification zone selection evaluation method, which solves the problems existing in the background technology such as single evaluation factors, subjective results, and failure to consider multi-factor coupling by constructing a systematic parameter system, integrating a weight model and a fuzzy logic algorithm, thereby significantly improving the scientificity, objectivity and engineering practicality of underground coal gasification zone selection analysis.

[0007] Specifically, the coal quality parameters in the data collection step include the volatile matter content, fixed carbon content, ash content and calorific value of the coal, which serve as a basis for evaluating the reactivity of the coal and are used for calculating the coal quality reactivity index.

[0008] Specifically, the hydrological parameters include the thickness of the aquifer, the groundwater flow direction and the thickness of the aquiclude between the coal seams, which are used to determine the risk level of groundwater pollution.

[0009] Specifically, the surface subsidence risk factors include coal seam burial depth, goaf predicted volume and regional geological structure stability index, and the subsidence trend is assessed based on the geotechnical stability model.

[0010] Specifically, the surrounding rock stability parameters are obtained by conducting uniaxial compression tests, triaxial shear tests and stability numerical simulations, and are used to establish a geological stability index system and participate in fuzzy comprehensive calculations.

[0011] Specifically, the permeability parameters are obtained through water injection tests, coal rock porosity tests, and seepage simulation methods, and are used to describe the distribution characteristics of gas permeable channels.

[0012] Specifically, the fuzzy comprehensive evaluation method uses a weighted average fuzzy operator to perform weighted synthesis on different evaluation indicators, and the obtained results reflect the overall suitability of each candidate area.

[0013] Specifically, in the suitability determination step, the comprehensive score is divided into a suitable zone if it is greater than 0.7, a basically suitable zone if it is between 0.5 and 0.7, and an unsuitable zone if it is less than 0.5.

[0014] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which: Figure 1 This is a schematic diagram of the underground coal gasification zone selection and evaluation method of the present invention. DETAILED DESCRIPTION

[0016] The embodiments of the present invention are described in detail below. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention and are not to be construed as limiting the present invention. On the contrary, the embodiments of the present invention include all variations, modifications, and equivalents that fall within the spirit and scope of the appended claims.

[0017] The following describes an underground coal gasification zone selection and evaluation method according to an embodiment of the present invention with reference to the accompanying drawings.

[0018] like Figure 1 As shown, the underground coal gasification zone selection evaluation method according to the embodiment of the present invention may include the following steps: S1. Data collection: Collect geological parameters, hydrological parameters, coal quality parameters and environmental parameters of the candidate area. The parameters include coal seam thickness, burial depth, coal seam gas pressure, groundwater level, aquifer distribution, surrounding rock strength, permeability coefficient, coal industry analysis indicators, coal reactivity indicators, groundwater pollution risk factors and surface subsidence risk factors.

[0019] It should be noted that the data collection steps described in this embodiment obtain parameter data through field drilling, geophysical exploration, remote sensing analysis and integration of historical geological data. Coal industry analysis indicators include ash, volatile matter, moisture and fixed carbon; coal reactivity indicators are determined by laboratory thermogravimetric analysis, reactivity index testing, etc.; groundwater pollution risk factors are derived from the water-isolating conditions between the aquifer and the coal seam, groundwater flow direction and water chemical sensitivity assessment; comprehensive collection of the above information can provide reliable basic data support for subsequent multi-dimensional comprehensive evaluation.

[0020] S2. Parameter processing: normalize various parameters and classify them into four dimensions according to the indicator type: geological stability index, hydrological safety index, coal quality reactivity index and environmental risk index.

[0021] It should be noted that the parameter processing steps described in this embodiment include dimensional unification and linear or nonlinear normalization of the original data, and determining the parameter classification based on principal component analysis (PCA) or expert experience; for example, the surrounding rock strength, permeability and coal seam burial depth are classified into the geological stability dimension, volatile matter and fixed carbon are classified into the coal quality reactivity dimension, and the aquifer thickness and impermeable layer thickness are classified into the hydrological safety dimension, thereby providing a scientific parameter hierarchical structure for the construction of a comprehensive evaluation model.

[0022] S3. Weight setting: A hierarchical analysis method is used to construct the hierarchical structure of evaluation indicators, and the weight of each evaluation parameter in the indicator system is calculated through paired comparison method and consistency test.

[0023] It should be noted that the weight setting step described in this embodiment includes constructing a three-layer structure of target layer-criteria layer-indicator layer. Technical personnel formulate a pairwise comparison matrix based on expert research, historical engineering experience and field survey results, and use the consistency ratio (CR) to check whether the matrix consistency is less than 0.1 to ensure the logical consistency and credibility of the weight distribution. At the same time, the integrated expert voting average method can be used to optimize the weight error.

[0024] S4. Model calculation, based on the normalized parameters and indicator weights, adopts the fuzzy comprehensive evaluation method to construct the fuzzy judgment matrix, performs fuzzification processing through the membership function and performs fuzzy comprehensive operation to calculate the comprehensive evaluation score of the candidate area.

[0025] It should be noted that the model calculation steps described in this embodiment use a weighted average fuzzy operator (such as a multiplication type or a weighted average type), and construct a membership function according to the evaluation level of each indicator, such as a triangular or trapezoidal membership function, to map the normalized quantitative data into fuzzy linguistic variables (such as "high", "medium", and "low"); the fuzzy matrix outputs a unified evaluation value after weighted operation, thereby realizing effective integration of multiple uncertain factors.

[0026] S5. Result determination: Compare the comprehensive score with the preset suitability threshold and classify the candidate areas into three categories: suitable, basically suitable, and unsuitable, to guide the site selection decision for underground coal gasification.

[0027] It should be noted that in the result determination step described in this embodiment, the suitability level is determined based on the regression analysis of historical underground gasification project results. For example, a comprehensive score greater than 0.7 is defined as a suitable area, 0.5 to 0.7 as a basically suitable area, and less than 0.5 as an unsuitable area. In addition, a spatial distribution map can be generated in conjunction with a GIS system, and the area can be graphically output to assist site selectors in making intuitive judgments, thereby achieving effective transformation from numerical results to engineering practice.

[0028] Specifically, through the integration of field drilling, remote sensing, and historical data, comprehensive parameter information covering key factors such as geology, hydrology, coal quality, and environmental impacts was collected to ensure the integrity and representativeness of the data sources. Subsequently, normalization techniques were used to unify parameter dimensions. Principal component analysis and expert knowledge were combined to scientifically classify each indicator into four dimensions: geological stability, hydrological safety, coal quality reactivity, and environmental risk, thus constructing a clear indicator structure. To further reflect the importance of each indicator in the evaluation process, the method used the Analytic Hierarchy Process (AHP) to establish a three-tiered evaluation system. Pairwise comparisons and consistency tests were used to calculate indicator weights, enhancing the scientific and verifiable nature of the weightings. A fuzzy comprehensive evaluation method was employed in the model calculation phase. By fuzzifying parameters and constructing a fuzzy judgment matrix, uncertain factors were transformed into an operational membership model. Finally, a weighted fuzzy synthesis was used to determine the comprehensive scores of candidate areas. Finally, based on the set suitability thresholds, the areas were classified into three categories: suitable, moderately suitable, and unsuitable. Spatial heat maps were generated using GIS technology to assist engineers in intuitively selecting areas. This technical solution overcomes the defects of existing technologies that rely only on single geological parameters such as coal seam thickness or burial depth, the evaluation results are subjective and arbitrary, and the coupled environmental risks are not considered. It realizes the integrated processing of multi-source information, quantitative judgment of multiple indicators and engineering visualization output of the final results, significantly improving the scientificity, accuracy and safety of underground coal gasification site selection.

[0029] In one embodiment of the present invention, Figure 1 As shown, the coal quality parameters in the data collection step include the volatile matter content, fixed carbon content, ash content and calorific value of the coal, which serve as the basis for evaluating the reactivity of the coal and are used for calculating the coal quality reactivity index.

[0030] It should be noted that the coal quality parameters described in this embodiment can be obtained through standard industrial analysis methods, among which the volatile matter content and fixed carbon content reflect the gasification ability of coal, the ash content indirectly affects the amount of ash generated and the risk of reactor blockage, and the calorific value reflects the gasification gas production potential; these indicators are weighted and summarized to form the coal quality reaction activity grade, which is used to construct the membership function of the coal quality factor in the fuzzy comprehensive evaluation.

[0031] In one embodiment of the present invention, Figure 1As shown, hydrological parameters include aquifer thickness, groundwater flow direction and aquiclude thickness between coal seams, which are used to determine the risk level of groundwater pollution.

[0032] It should be noted that the hydrological parameter data described in this embodiment are obtained through hydrogeological drilling, water pressure testing and groundwater dynamics modeling, wherein the groundwater flow direction is determined by tracer testing in monitoring wells, and the thickness of the aquiclude is determined by logging curves and lithological analysis; by establishing a risk level matrix, the region can be divided into high, medium and low pollution risk levels, which are input into the comprehensive evaluation model as key factors in the environmental risk dimension.

[0033] In one embodiment of the present invention, Figure 1 As shown in the figure, the surface subsidence risk factors include coal seam burial depth, goaf predicted volume and regional geological structure stability index, and the subsidence trend is assessed based on the geotechnical stability model.

[0034] It should be noted that the surface subsidence risk analysis described in this embodiment uses three-dimensional numerical simulation (such as FLAC3D, Plaxis, etc.) to predict the deformation of the goaf, the coal seam burial depth and thickness are used to estimate the structural changes of the overburden layer, and the structural stability is characterized by the fault density and structural activity assessment index; the prediction results are converted into quantitative subsidence indicators and participate in the weighted calculation of the environmental dimension in the suitability scoring system.

[0035] In one embodiment of the present invention, Figure 1 As shown in Figure 2, the surrounding rock stability parameters are obtained by conducting uniaxial compression tests, triaxial shear tests and stability numerical simulations, which are used to establish a geological stability index system and participate in fuzzy comprehensive calculations.

[0036] It should be noted that the surrounding rock stability assessment described in this embodiment combines laboratory mechanical parameter testing with engineering geological model simulation. The rock samples collected are from the coal seam surrounding rock distribution zone. The test data include peak strength, elastic modulus and Poisson's ratio. The surrounding rock stress changes and failure modes during mining are simulated in the model. The calculated stability level serves as an important input for the "high-medium-low" judgment basis in the fuzzy matrix.

[0037] In one embodiment of the present invention, Figure 1 As shown in Figure 2, the permeability parameters are obtained through water injection tests, coal rock porosity tests and seepage simulation methods, and are used to describe the distribution characteristics of gas permeable channels.

[0038] It should be noted that the permeability evaluation described in this embodiment uses surface water injection and core permeability meter testing, combined with gas flow field simulation (such as COMSOL or ANSYS Fluent) to predict the channel path, determine the gas channel network that may be formed in the coal seam and surrounding rock during the gasification process, and evaluate whether they have good gas flow and combustion front advancement conditions. This is an important auxiliary factor in evaluating gasification efficiency.

[0039] In one embodiment of the present invention, Figure 1 As shown in the figure, the fuzzy comprehensive evaluation method uses a weighted average fuzzy operator to perform weighted synthesis of different evaluation indicators, and the obtained results reflect the overall suitability of each candidate area.

[0040] It should be noted that the fuzzy operator described in this embodiment preferably adopts the weighted average model (WAO), which multiplies and accumulates the output value of the membership function of each dimensional indicator with its corresponding weight, and finally obtains a comprehensive membership value between 0 and 1; this value can be used for subsequent classification judgment to ensure that the comprehensive result has good stability, interpretability and engineering adaptability.

[0041] In one embodiment of the present invention, Figure 1 As shown in the figure, in the suitability determination step, the comprehensive score is divided into the suitable zone if it is greater than 0.7, the basically suitable zone if it is between 0.5 and 0.7, and the unsuitable zone if it is less than 0.5.

[0042] It should be noted that the suitability grade classification standard described in this embodiment is determined based on the scoring regression analysis of multiple typical underground gasification projects at home and abroad. The set score boundaries can be flexibly adjusted to adapt to the coal seam conditions and policy requirements of different regions; the evaluation results can be linked with the geographic information system (GIS) to output the suitability layer, realize the image visualization of the spatial site selection results, and provide decision-making support for project deployment and environmental approval. Example

[0043] This example selects geological stability as the primary controlling factor and uses a typical western coalfield block as a case study to illustrate the application of the present invention's area selection and evaluation method. Western coalfields typically have relatively simple and stable geological structures, deep groundwater, and a relatively arid environment, which are conducive to the implementation of underground gasification projects. During the underground gasification process, the geological body in which the coal seam is located serves as the carrier of the gasifier, and the geological endowment of the coal seam directly constrains the feasibility of the project. Therefore, the purpose of this example is to determine suitable underground gasification areas through a systematic fuzzy comprehensive evaluation, under the premise of favorable geological conditions, to ensure the safe and efficient operation of the gasification process.

[0044] Materials and Methods According to the method steps of claim 1, this embodiment includes five main steps: data collection, normalization processing, weight setting, fuzzy evaluation calculation and judgment, which are as follows: Data Collection: Basic data related to underground gasification suitability, including geological, hydrological, and coal seam parameters, will be collected. For a representative western coalfield block, the focus will be on obtaining geological stability indicators such as coal seam thickness, coal seam depth, fault development, and roof and floor rock strength. Hydrogeological parameters (groundwater level depth, aquiclude thickness) will also be collected, along with information on coal resource availability and quality. It is assumed that the coal seam in this block is approximately 10 meters thick and 500 meters deep, with a simple regional structure and no major faults. The groundwater level is approximately 150 meters deep, and the overlying aquiclude is a clay layer at least 30 meters thick. These data provide the foundation for subsequent evaluation.

[0045] Normalization: All collected raw data is normalized to make indicators of different dimensions comparable. Based on the indicator's attributes, appropriate methods are used to map the indicator values ​​to dimensionless values ​​between 0 and 1. For positive indicators such as coal seam thickness, aquiclude thickness, and groundwater depth, where "larger values ​​are more favorable," larger original values ​​result in higher normalized scores. For negative indicators such as fault density, where "larger values ​​are less favorable," larger original values ​​result in lower normalized scores. Through normalization, indicators of different units and dimensions are converted to comparable scales, paving the way for subsequent fuzzy evaluation calculations.

[0046] Weight setting: The weight of each indicator is determined based on expert experience scoring or the analytic hierarchy process (AHP). In view of the fact that geological stability is taken as the main controlling factor in this embodiment, a higher weight is given to geological indicators in the weight distribution. For example, compared with environmental and economic indicators, geological factors such as the degree of fault development, coal seam burial depth, and surrounding rock stability have the highest weight ratio to reflect their decisive influence on underground gasification site selection decisions. In addition, environmental risks are considered as secondary factors due to the relatively favorable hydrological conditions in this block (deep groundwater burial depth and thick aquiclude), and are therefore given a relatively low weight. Such weight setting makes the evaluation results mainly reflect the pros and cons of geological conditions, which is in line with the original intention of "geology-dominated" in this example.

[0047] Fuzzy evaluation calculation: A fuzzy comprehensive evaluation model is established and calculated. First, a membership function is set for each evaluation indicator to characterize the degree of influence of different indicator values ​​on the suitability level of the selected area (such as "highly suitable", "generally suitable", "unsuitable", etc.). Substitute the normalized indicator values ​​into their respective membership functions to obtain the membership of the indicator for each evaluation level. Then, the membership of all indicators is combined to form an evaluation matrix, and fuzzy operations (such as weighted summation or fuzzy synthesis) are performed according to the weight vector determined above to calculate the fuzzy membership results of the block relative to each comprehensive evaluation level. Due to the heavy weight of geological indicators and the superior geological conditions of the block, the results of the fuzzy comprehensive calculation show that the block has the highest membership to the "suitable" category, which is significantly higher than the membership of other levels.

[0048] Judgment: According to the results of the fuzzy comprehensive evaluation, the suitability level of the block is determined. The maximum membership principle or the threshold discrimination method can be used: that is, if the membership of the evaluation result at a certain level (such as "highly suitable") is the highest and exceeds the preset threshold, the block is determined to belong to this level. In combination with this embodiment, the maximum membership of the evaluation result corresponds to the "highly suitable" level, so the western coalfield block is determined to be the preferred area for underground gasification. If the evaluation results show that a block has only a moderate degree of suitability, it must be considered conditionally feasible, and it is recommended to conduct further demonstration or take reinforcement measures before implementation. In this embodiment, after the above steps, it is finally confirmed that the selected western coalfield block has good conditions for conducting coal underground gasification experiments: the geological structure is stable, the resource endowment is good, the environmental risks are controllable, and it is suitable as an underground gasification demonstration area.

[0049] Results and Discussion The evaluation results of this example indicate that the suitability level for underground gasification in this typical western coalfield block is assessed as "highly suitable," with a high overall score. The results indicate that due to the block's stable geological structure, moderate and thick coal seams, favorable geological conditions, and minimal impact from environmental factors (such as groundwater), the block achieved significant advantages in the fuzzy comprehensive evaluation. This result is reasonable and consistent with practical understanding: stable geological conditions mean lower geological risks, which facilitates the construction and stable operation of gasifiers. For example, the scarcity of faults and hard surrounding rock conditions reduce the risk of collapse and gas leakage during the gasification process, while the deep groundwater level and thick aquiclude ensure that environmental safety is not a constraint. Therefore, the combination of various indicators in this block meets the requirements for high suitability, and the evaluation conclusions are credible and have engineering guidance significance.

[0050] Through the application of this method, it can be seen that compared with the existing technology, the present invention can overcome the defects of the lack of systematicity and the majority of qualitative judgments in the previous area selection evaluation. Traditional underground gasification site selection often lacks a unified evaluation system, especially in the quantitative assessment of geological risks, which was an important reason why underground gasification projects have been difficult to succeed for a long time. This embodiment uses fuzzy comprehensive evaluation to quantify key factors such as geological stability and assign high weights to ensure that the impact of geological conditions on site selection results is fully reflected, avoiding the previous situation where site selection failed due to ignoring geological factors. In addition, this method combines expert experience with quantitative calculations, improves the scientific nature and objectivity of decision-making, and makes the evaluation results more reliable. In summary, Example 1 proves that the method of the present invention can reasonably select suitable blocks while emphasizing the main geological control conditions, which not only ensures the safety of underground gasification project site selection, but also provides a clear basis for project implementation. Example

[0051] This embodiment takes environmental risk as the main limiting factor and selects complex hydrogeological conditions blocks with high groundwater levels and thin impermeable layers for evaluation and analysis. Among the environmental impacts of underground coal gasification, groundwater pollution is considered to be one of the most serious potential risks. For areas with high aquifers and weak impermeable barriers, environmental safety must be given priority to prevent the gasification process from polluting groundwater. This embodiment aims to identify the restrictive effects of environmental conditions on the selected areas through evaluation methods, ensuring that under complex hydrological conditions, only areas with controllable environmental risks will be considered suitable for underground gasification, avoiding potential environmental accidents at the source.

[0052] Materials and Methods This embodiment also follows the five steps of claim 1: data collection, normalization processing, weight setting, fuzzy evaluation calculation and judgment. The specific implementation process is as follows: Data collection: Collect various data related to the complex hydrogeological block and the underground gasification area. Due to the complex groundwater conditions in this block, first conduct a detailed investigation of environmental risk-related indicators, including the depth of the groundwater level, the thickness of the aquifer and aquiclude, the characteristics of groundwater flow, and the water quality. At the same time, obtain the necessary geological condition data (such as coal seam thickness, coal seam burial depth, fracture and fissure development, and surrounding rock water-proof performance) as well as coal resources and gasification process parameters. Assume that the coal seam in the block selected in this example is about 300 meters deep and 8 meters thick; but the aquifer above it is only 50 meters deep, the aquiclude is less than 10 meters thick, and there are water-conducting fractures in some areas. The above data reflect that the environmental conditions in this block pose a severe challenge to underground gasification.

[0053] Normalization: Normalize the collected raw data to convert different indicators into dimensionless, comparable values. In situations with high groundwater levels and weak aquicludes, special attention should be paid to the normalization method for environmental risk indicators. For indicators such as groundwater depth, where "the larger the value, the more favorable (the larger the safety margin)," an appropriate normalization function is used to make the normalized score closer to 1 as the original value increases. The same applies to the thickness of the aquiclude: the thicker the thickness, the better the hydrological barrier, and the higher the normalized score. Conversely, for indicators such as fracture density or pollutant concentration, where "the larger the value, the more unfavorable," the larger the original value, the lower the normalized score. In this way, all environmental and geological indicators are converted to a unified scale of 0 to 1, providing a quantitative basis for comprehensive evaluation.

[0054] Weight setting: Experts in the fields of environment, geology, etc. will judge and score the importance of each indicator based on the actual situation of the block to determine the final weight distribution. In this embodiment, since environmental safety is the primary limiting factor in site selection, the highest weight ratio is given to environmental risk-related indicators. For example, indicators such as groundwater depth and aquiclude thickness occupy a major share in the weight distribution, while geological indicators (such as coal seam thickness and fault development degree) and economic indicators are assigned relatively low weights to highlight the dominant role of environmental factors in district selection decisions. Such weight setting ensures that in the comprehensive evaluation, if the environmental conditions in a certain area are not good, it will be difficult to obtain a high score even if the resources and geological conditions are good, thereby achieving the effect of strict screening.

[0055] Fuzzy evaluation calculation: According to the established weights and normalized index values, a fuzzy comprehensive evaluation calculation is performed on the block. First, the corresponding fuzzy membership function is set according to the influence of each indicator on the suitability of the selected area, which is used to describe the degree to which the index value corresponds to the evaluation level such as "suitable" or "unsuitable". The normalized value of each indicator is substituted into its membership function to obtain the membership matrix for different evaluation levels. Subsequently, the weight vector is applied to perform fuzzy operations on the matrix, and the overall evaluation result of the block is obtained by comprehensively considering the influence of each indicator. In this embodiment, since the weight of the environmental risk indicator is dominant, if the environmental conditions of the block are poor (such as a very thin aquiclude resulting in a high risk of pollution), the membership corresponding to the "unsuitable" level in the fuzzy calculation result will increase significantly, exceeding the membership of the "suitable" level. Through fuzzy comprehensive calculation, the influence of multiple factors is integrated into a comprehensive evaluation vector, which provides a basis for the final judgment.

[0056] Determination: Determine whether the block is suitable for underground gasification development based on the fuzzy comprehensive evaluation results. Usually, the maximum membership principle or the preset threshold method can be used for judgment: if the evaluation result has the highest membership for the "suitable" or "relatively suitable" level and exceeds the set threshold, the block is considered to have development potential; conversely, if the membership for the "unsuitable" level is the highest, then the block is determined to be unsuitable as an underground gasification site at present. According to the assumed data of this embodiment, due to the shallow groundwater level and extremely thin aquiclude in this block, the environmental safety score is very low, resulting in the highest comprehensive membership for the "unsuitable" level. Based on this result, it can be determined that the environmental risk of this block is unacceptable. Unless major engineering measures are taken to reduce the risk, it is not recommended to use it as an implementation target for underground gasification. In practical applications, the evaluation conclusions can also be used to guide the next step of work, such as proposing risk mitigation plans or selecting alternative sites for blocks with slightly worse environmental conditions but resource potential.

[0057] Results and Discussion After the above evaluation process, the results of this example indicate that this complex hydrological block, characterized by a high groundwater level and a weak aquitard, was judged "unsuitable" for underground coal gasification. Disadvantages in environmental factors significantly reduced the comprehensive evaluation score: The two key limiting indicators, shallow groundwater depth and a thin aquitard, both scored very low, directly lowering the overall suitability rating, ultimately placing the block in the "unsuitable" category in the fuzzy comprehensive evaluation. This result is realistic: although the block's resource conditions, such as coal seam thickness and depth, are assumed to be acceptable, the excessive risk of groundwater leakage and contamination is sufficient to negate its development prospects. If underground gasification were to proceed without appropriate measures, it would likely result in serious consequences such as groundwater contamination. Therefore, the evaluation model made a conservative assessment, reflecting strict environmental safety considerations. This is consistent with the principle of "safety first, environmental protection first" in underground coal gasification projects, demonstrating the rationality and reliability of the evaluation results.

[0058] This example shows that the method of the present invention effectively makes up for the shortcomings of traditional site selection evaluation in terms of environmental risk considerations. In the past, some underground gasification site selections placed too much emphasis on coal resources and technical and economic factors, and did not pay enough attention to environmental safety issues. There were cases where the weight of environmental risks such as groundwater pollution was ranked secondary. In contrast, the present method quantifies environmental factors and assigns high weights through fuzzy comprehensive evaluation, strictly reflecting the importance of environmental safety in the evaluation. In this way, the defect of underestimation of environmental indicators in the past is overcome, ensuring that only candidate blocks with controllable environmental risks will be recommended for underground gasification projects, fundamentally improving the safety and reliability of site selection decisions. At the same time, the method establishes a multi-indicator comprehensive evaluation mechanism, implements the "environment first" site selection concept, and echoes the requirements of foreign countries for strict site selection standards for underground gasification. By actively identifying and avoiding environmental geological hazards during the site selection stage, the method of the present invention helps to avoid environmental pollution problems that may be caused by underground gasification from the source, and provides scientific guidance for the safe implementation of underground coal gasification under complex conditions.

[0059] In summary, the underground coal gasification area selection and evaluation method of the embodiment of the present invention firstly utilizes a combination of field drilling, remote sensing and historical data to comprehensively collect key parameters such as coal seam thickness, burial depth, gas pressure, coal quality, hydrogeology, permeability, surrounding rock strength and environmental risk in the evaluation area, thereby providing complete data support for subsequent evaluation; secondly, the collected data is normalized, and through dimensional unification and parameter classification, a standardized indicator system of four dimensions, namely geological stability, hydrological safety, coal quality reactivity and environmental risk, is formed, thereby effectively solving the problem of parameter heterogeneity; and then, a hierarchical analysis method is used to construct a multi-level indicator system. Structure, and combined with paired comparison matrix and consistency test to set the weight of each evaluation factor, so that the weight distribution has logical consistency and expert experience basis; in the model calculation stage, the fuzzy comprehensive evaluation method is introduced, and the uncertainty and subjectivity of each factor are converted into an operational comprehensive score through fuzzy processing and weighted operation to ensure that the evaluation output has credibility and stability; finally, by comparing the comprehensive score with the empirical threshold and combining it with the GIS system to output the heat map, a complete closed loop from quantitative analysis to spatial selection visualization is achieved, and finally the candidate areas are scientifically divided into three categories: suitable, basically suitable and unsuitable. The present invention solves the problems of single evaluation factors, subjective results and failure to consider multi-factor coupling in the background technology by constructing a systematic parameter system, integrating weight models and fuzzy logic algorithms, and significantly improves the scientificity, objectivity and engineering practicality of underground coal gasification selection analysis.

[0060] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and deform the above embodiments within the scope of the present invention.

Claims

1. A method for evaluating underground coal gasification area selection, characterized in that: The following steps are involved: S1. Data collection: Collect geological parameters, hydrological parameters, coal quality parameters, and environmental parameters of the candidate area, including coal seam thickness, burial depth, coal seam gas pressure, groundwater level, aquifer distribution, surrounding rock strength, permeability, coal industry analysis indicators, coal reactivity indicators, groundwater pollution risk factors, and surface subsidence risk factors; S2. Parameter processing: normalize the various parameters and classify them into four dimensions according to the indicator type: geological stability index, hydrological safety index, coal quality reactivity index, and environmental risk index; S3. Weight setting: A hierarchical structure of evaluation indicators is constructed using the analytic hierarchy process, and the weight of each evaluation parameter in the indicator system is calculated through paired comparison and consistency test; S4. Model calculation: Based on the normalized parameters and the indicator weights, a fuzzy comprehensive evaluation method is used to construct a fuzzy judgment matrix, fuzzify the matrix using a membership function, and perform a fuzzy comprehensive operation to calculate a comprehensive evaluation score for the candidate area. S5. Result determination: Compare the comprehensive score with the preset suitability threshold and classify the candidate areas into three categories: suitable, basically suitable, and unsuitable, to guide the site selection decision for underground coal gasification.

2. The underground coal gasification area selection evaluation method according to claim 1, characterized in that: The coal quality parameters in the data collection step include the volatile matter content, fixed carbon content, ash content and calorific value of the coal, which serve as a basis for evaluating the reactivity of the coal and are used for calculating the coal quality reactivity index.

3. The underground coal gasification area selection evaluation method according to claim 1, characterized in that: The hydrological parameters include the thickness of the aquifer, the direction of groundwater flow and the thickness of the aquiclude between the coal seams, and are used to determine the risk level of groundwater pollution.

4. The method for evaluating underground coal gasification area selection according to claim 1, wherein: The surface subsidence risk factors include coal seam burial depth, goaf predicted volume and regional geological structure stability index, and the subsidence trend is assessed based on the geotechnical stability model.

5. The underground coal gasification area selection and evaluation method according to claim 1, characterized in that: The surrounding rock stability parameters are obtained by conducting uniaxial compression tests, triaxial shear tests and stability numerical simulations, and are used to establish a geological stability index system and participate in fuzzy comprehensive calculations.

6. The method for evaluating underground coal gasification area selection according to claim 1, characterized in that: The permeability parameters are obtained through water injection tests, coal rock porosity tests and seepage simulation methods, and are used to describe the distribution characteristics of gas permeable channels.

7. The method for evaluating underground coal gasification area selection according to claim 1, characterized in that: The fuzzy comprehensive evaluation method adopts a weighted average fuzzy operator to perform weighted synthesis on different evaluation indicators, and the obtained result reflects the overall suitability of each candidate area.

8. The method for evaluating underground coal gasification area selection according to claim 1, characterized in that: In the suitability determination step, the comprehensive score is divided into a suitable zone if it is greater than 0.7, a basically suitable zone if it is between 0.5 and 0.7, and an unsuitable zone if it is less than 0.5.

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