Method for methane extraction and closed-loop carbon fixation in stacked coal seams using hot flue gas displacement

Abstract

A method for methane extraction and closed-loop carbon fixation in stacked coal seams using hot flue gas displacement. First, a CH4 combustion explosion fracturing technique is used to perform in-situ coordinated combustion explosion reactions in coal seam sections to generate gases to impact and fracture the coal seam sections, so as to form a fracture network; then, hot flue gases injected into the coal seam sections can promote desorption of CH4 gases from coal due to a relatively high temperature of the hot flue gases, and due to competitive adsorption advantages, CO2, SO2 and NO2 in the hot flue gases can displace a large amount of CH4 gases adsorbed in stacked coal seams for closed-loop utilization, such that the hot flue gases are stored, and the efficiency of methane extraction in the coal seams is also improved. According to the current changes in physical property parameters of different coal seam sections, an established multi-source monitoring data inversion and intelligent regulation system can design optimal combustion explosion gas injection parameters and optimal hot flue gas injection parameters which meet the coal seams, so as to perform layered, dynamic and precise regulation on gas injection parameters of the coal seam sections, thereby maximally improving the methane combustion explosion fracturing, hot flue gas storage and CH4 extraction efficiency of the coal seam sections of the stacked coal seams.

Description

A method for methane extraction and closed-loop carbon fixation in superimposed coal seams by hot flue gas displacement Technical Field

[0001] The present invention relates to a hot flue gas storage and CH4 extraction method, specifically a hot flue gas displacement superimposed coal seam CH4 extraction and multi-directional closed-loop carbon fixation method, belonging to the technical field of carbon fixation and CH4 extraction. Background Art

[0002] Based on the national energy structure of "rich in coal, poor in oil and little gas", coal accounts for more than 50% of my country's energy consumption. Among them, coal-fired power plants are the main form of coal consumption. The annual average CO2 emissions from coal-fired power plants account for about 30% to 50% of the hot flue gas. In order to reduce the emission of CO2 into the atmosphere, CO2 storage is generally considered to be the most effective potential solution to achieve carbon neutrality. my country's deep unminable coal seams have a huge CH4 adsorption volume and have huge prospects for CH4 resource utilization. In addition, these coal seams are buried deep underground and are generally difficult to directly mine, which can provide better geological conditions for the storage of hot flue gas. Some scholars predict that the CO2 storage capacity of unminable coal seams 1500 to 2000 meters underground is about 55.8 billion tons. However, deep unminable coal seams generally have the characteristics of high ground stress, low porosity and low permeability, which increases the difficulty of hot flue gas injection and CH4 extraction. It is necessary to use fracturing and permeability-enhancing measures to promote the development of coal seam pore and fracture structure to provide efficient migration channels for the storage of hot flue gas and CH4 extraction.

[0003] Hydraulic fracturing is currently the most widely used reservoir fracturing and permeability enhancement method for CO2 storage and CH4 extraction. However, hydraulic fracturing generally has problems such as high water consumption and single fracture extension, which leads to a limited range of hot flue gas storage and CH4 extraction. At present, hydraulic fracturing technology cannot achieve closed-loop construction in hot flue gas storage and displacement of coal seam CH4. That is, after the injected H2O and fracturing fluid complete the fracturing and permeability enhancement in the coal seam, they need to be extracted from the coal seam for subsequent hot flue gas storage and CH4 extraction, and neither can achieve a closed-loop reuse process. The explosive fracturing technology uses the in-situ desorption of CH4 from the coal seam, and the high-temperature, high-pressure gas generated by the synergistic explosion of O2 and CH4 is used to impact and fracture deep, unmineable coal seams, thereby efficiently creating a three-dimensional fracture network. The thermal gas storage and CH4 extraction capacity of deep unmineable coal seams can be greatly improved. However, no research has yet combined explosive fracturing with thermal gas storage and displacement for CH4 extraction in superimposed coal seams. In addition, the properties of the various coal seam sections in the superimposed coal seams vary greatly. The traditional multi-layer unified injection technology uses a unified injection parameter injection between horizontal wells, which will lead to problems such as low injection efficiency of a single well. During the entire explosive fracturing-hot gas storage-CH4 extraction process, if the changes in the physical parameters of each coal seam section cannot be well monitored, then the explosive fracturing, hot gas storage and CH4 extraction efficiency of each coal seam section will be low. In addition, the excessive development of cracks in the coal seam section that is not monitored may cause the caprock to rupture, resulting in environmental safety issues such as hot gas leakage.

[0004] Therefore, in response to the current shortcomings in hot flue gas storage and CH4 extraction in deep superimposed coal seams, a new method is provided. Based on the combination of explosive fracturing technology and hot flue gas storage technology, efficient closed-loop extraction of CH4 is achieved in each coal seam section of the superimposed coal seam, thereby achieving efficient closed-loop utilization of explosive gas, hot flue gas and extracted CH4 gas resources in the whole process of "explosive fracturing-hot flue gas storage-methane extraction", thereby reducing the cost of deep unminable coalbed methane mining, maximizing the utilization of deep difficult-to-mine or unminable superimposed coal seam resources, and improving the efficiency of hot flue gas storage and CH4 mining in deep unminable and extra-thick coal seams, providing a new research idea for helping carbon neutrality and the development of the coalbed methane industry.

[0005] Summary of the Invention

[0006] In response to the problems existing in the above-mentioned prior art, the present invention provides a method for methane extraction and closed-loop carbon fixation in superimposed coal seams by replacing hot flue gas. Based on the combination of explosive fracturing technology and hot flue gas storage technology, efficient closed-loop extraction of CH4 from superimposed coal seams is achieved simultaneously, thereby reducing the cost of deep unminable coalbed methane mining, maximizing the utilization of deep difficult-to-mine or unminable superimposed coal seam resources, and improving the efficiency of hot flue gas storage and CH4 mining in deep unminable and extra-thick coal seams.

[0007] To achieve the above objectives, the present invention adopts a technical solution: a method for methane extraction and closed-loop carbon sequestration in superimposed coal seams by hot flue gas displacement, which specifically comprises the following steps:

[0008] A. Drilling Construction: First, determine the location of each coal seam segment and its rock cap in the superimposed coal seams. Then, construct vertical shafts and extraction-monitoring combined wells from the surface through each rock cap to the deepest coal seam segment. After the vertical shafts are constructed, use a directional drill to drill a horizontal well from the vertical shaft along the direction of each coal seam segment. Drilling stops after completion.

[0009] B. Laying out hot flue gas injection and coalbed methane extraction system: a gas injection plug is set in each horizontal drilling well of the vertical shaft, so that a closed space is formed inside each horizontal drilling well, and an extraction plug is installed at the wellhead of the extraction-monitoring well to seal the extraction-monitoring well; a layered injection device is installed at the connection between each horizontal drilling well and the vertical shaft, one end of the hot flue gas injection pipe and one end of the explosion gas injection pipe are extended into the vertical shaft, and each layered injection device is connected in series in sequence; an explosion gas inlet pipe is set on each gas injection plug and hot flue gas inlet pipe; the two ends of the explosive gas inlet pipe are respectively on both sides of the gas injection plug, of which one end in the closed space is equipped with a solenoid valve, and the other end is connected to the stratified injection device closest to it; the two ends of the hot flue gas inlet pipe are respectively on both sides of the gas injection plug, of which one end in the closed space is equipped with a solenoid valve, and the other end is connected to the stratified injection device closest to it; the gas injection plug in the closed space is equipped with a multi-parameter monitoring sensor and an ignition head; the multi-parameter monitoring sensor and the ignition head are connected to the ground through a multi-parameter monitoring data transmission line The control center is connected; on the ground, there are explosive gas storage tanks, explosive gas injection pumps, hot flue gas injection pumps, hot flue gas storage tanks, hot flue gas emission devices of coal-fired power plants, power generation devices of coal-fired power plants, gas separation devices, gas filtration devices, heat exchanger groups and extraction pumps in sequence; the explosive gas storage tanks are connected to the inlet of the explosive gas injection pump, and the outlet of the explosive gas injection pump is connected to the other end of the explosive gas injection pipe; one end of the extraction pipe passes through the extraction plug and extends into the extraction-monitoring combined well, and the other end of the extraction pipe is connected to the air inlet of the extraction pump, and the extraction pump is connected through The pipelines are sequentially connected to the heat exchanger group, the gas filtration device, and the gas separation device. The CH4 gas passing through the gas separation device is connected to the coal-fired power plant's power generation device and the air inlet of the explosion gas storage tank through the CH4 transmission pipeline. The hot flue gas entering the gas separation device and the hot flue gas from the coal-fired power plant's tail gas treatment system are both connected to the inlet of the hot flue gas storage tank through the hot flue gas transmission pipeline. The outlet of the hot flue gas storage tank is connected to the inlet of the hot flue gas injection pump, and the outlet of the hot flue gas injection pump is connected to the other end of the hot flue gas injection pipe, completing the system layout.

[0010] C. Deployment of the Monitoring System: Multiple integrated multi-parameter monitoring devices are sent into the combined extraction and monitoring wells. Multiple integrated multi-parameter monitoring devices are deployed at equal intervals in each coal seam segment along the wellbore of the combined extraction and monitoring wells. Each integrated multi-parameter monitoring device in each coal seam segment is connected to the ground control center via a multi-parameter monitoring data transmission line to obtain real-time information on fracture development and temperature changes within each coal seam segment, completing the deployment of the monitoring system.

[0011] D. Establish a multi-source monitoring data inversion and intelligent control system: First, use various integrated multi-parameter monitoring devices to monitor the geological conditions of different coal seam sections, and image the temperature, microseismic, acoustic wave and resistivity data of each coal seam section at different depths in the initial state; then, drill coal samples at all integrated multi-parameter monitoring device layout points in each coal seam section, and conduct laboratory measurements on samples at different depths in each coal seam section; combine the on-site multi-source monitoring data imaging results with laboratory measurement results, use deep learning algorithms to train the acquired data, and establish a multi-source monitoring data inversion and intelligent control system; this system can monitor the changes in physical parameters of different depths of the superimposed coal seam (temperature, ground stress state, mechanical strength, porosity, permeability, fluid density, viscosity and saturation parameters) in real time, and can provide the optimal combustion gas injection parameters and optimal hot flue gas injection parameters that meet the needs of each coal seam section based on the changes in the physical parameters of each coal seam section;

[0012] E. Perform explosive fracturing on the superimposed coal seams: When starting fracturing, first start the explosive gas injection pump and open the solenoid valve of the explosive gas inlet pipe of each coal seam section, so that the explosive gas is transported to each layered injection device through the explosive gas injection pipe. The layered injection device corresponding to each coal seam section controls the amount of explosive gas entering the closed space of each coal seam section according to the optimal explosive gas injection parameters determined in step D, and measures the explosive gas pressure and CH4 gas concentration in the closed space in real time through their respective multi-parameter monitoring sensors. When the parameters in the closed space reach the explosive condition threshold, stop the explosive gas injection pump and close the solenoid valve of the explosive gas inlet pipe, and then start the ignition head to cause the explosive gas injected into the closed space to undergo an in-situ explosive reaction. The transient shock wave and high-temperature, high-pressure gas generated by the explosion reaction act on the surrounding coal body to produce a large number of cracks, and the fractured rock mass can desorb a large amount of CH4 gas, which continues to react with O2 gas to form a pulse fracturing effect in the coal seam. After this round of explosion reaction is completed, the multi-source monitoring data inversion and intelligent control system adjusts the explosion gas injection parameters of different coal seam sections according to the different explosion fracturing conditions of each coal seam section, restarts the explosion gas injection pump, repeats the explosion fracturing steps multiple times, and performs multiple explosion fracturing processes around the horizontal wells of each coal seam section. During this period, the explosion fracturing conditions of each coal seam section are continuously monitored by an integrated multi-parameter monitoring device. When it is detected that the cracks in each horizontal well have extended to the connection between the coal seam and the caprock, the explosion fracturing process is completed.

[0013] F. Injecting hot flue gas for storage and displacing CH4 gas desorption: Turn on the hot flue gas injection pump and the solenoid valve of the hot flue gas inlet pipe to transport the hot flue gas in the hot flue gas storage tank to each layered injection device through the hot flue gas injection pipe. The layered injection device corresponding to each coal seam section controls the amount of hot flue gas entering the closed space of each coal seam section according to the optimal hot flue gas injection parameters determined in step D. The higher temperature of the hot flue gas itself can promote the desorption of CH4 gas adsorbed in each coal seam section. At the same time, due to the competitive adsorption advantage of CO2 gas, SO2 gas and NO2 gas in the hot flue gas, each coal seam section adsorbs CO2 gas and precipitates CH4 gas through replacement, thereby achieving large-scale CH4 gas production in each coal seam section, sealing the hot flue gas while improving the CH4 mining efficiency of the coal seam;

[0014] G. CH4 gas extraction and hot flue gas storage: After a period of time, the extraction pump is started to extract the mixed gas from the extraction-monitoring well through the gas extraction pipe, and then the mixed gas passes through the heat exchanger group. After heat exchange, the temperature of the mixed gas is reduced to room temperature; the mixed gas first passes through the gas filter device to remove other gas impurities in the extracted mixed gas, the sources of which include part of the CO2 gas generated during the explosive fracturing process, the residual CO2, SO2, NO2 and N2 during the hot flue gas storage and displacement process, and the original CO2, SO2, N2 in the coal seam. ; The CH4 in the extracted mixed gas comes from the CH4 gas component that did not explode during the explosive fracturing process and the CH4 gas displaced by the hot flue gas in the coal seam; the extracted mixed gas that has been treated to meet the standards enters the gas separation device, and the gas separation device separates the extracted mixed gas that has been treated to meet the standards into the main component gas of hot flue gas and CH4 gas; a part of the separated CH4 gas is injected into the power generation device of the coal-fired power plant through the CH4 gas transmission pipeline and mixed with coal to generate electricity to improve efficiency, and the remaining separated CH4 gas is injected into the explosive gas storage tank Mixed with combustion-supporting agent to generate combustion gas for subsequent combustion-explosion fracturing; the separated main component gas of hot flue gas and the hot flue gas of hot flue gas discharge device are injected into the hot flue gas storage tank through the hot flue gas transmission pipeline and mixed, and finally enter the fractured coal seam through the hot flue gas injection pump, so as to continuously seal the hot flue gas and displace the remaining CH4 gas in the coal seam; during the hot flue gas injection and sealing period, the temperature, acoustic wave, microseismic parameters and resistivity data of different coal seam sections at different depths are collected in real time through the integrated multi-parameter monitoring device; according to the multi-source data inversion and The intelligent control system performs real-time inversion of the hot flue gas injection conditions in different sections of the superimposed coal seam and adjusts the optimal hot flue gas injection parameters for different sections. At this time, the ground control center uses multi-parameter monitoring sensors to monitor the pressure, CH4 concentration, and temperature of the main components of the hot flue gas (i.e., CO2, SO2, NO2, N2) and the CH4 mixed gas in each confined space in real time. Based on the real-time monitoring data, the hot flue gas injection parameters in each horizontal well are dynamically adjusted to the optimal hot flue gas injection parameters to ensure continuous and efficient hot flue gas injection in each fractured section of the superimposed coal seam.

[0015] H. When the relative content of coal seam CH4 gas in the mixed gas drops below the critical extraction CH4 concentration, repeat steps B to G, and continue the closed-loop process of multiple explosive fracturing, hot flue gas sealing, and CH4 extraction until the hot flue gas sealing volume of each coal seam section of the superimposed coal seam reaches above the set value, completing the hot flue gas sealing and CH4 extraction operations.

[0016] Furthermore, the multi-parameter monitoring sensor includes an air pressure sensor, a gas concentration sensor and a temperature sensor, wherein the air pressure sensor monitors the pressure of the explosion gas in the explosion-proof confined space in real time; the gas concentration sensor is used to monitor the concentration of CH4 gas in the explosion-proof confined space in real time; and the temperature sensor is used to monitor the temperature in the explosion-proof confined space in real time.

[0017] Furthermore, casing is arranged in the vertical shaft, and the casing is sealed against the inner wall of the vertical shaft. A screen and casing combination is arranged in each horizontal drilling well, wherein the casing is arranged at the connection between the horizontal drilling well and the vertical shaft, and the screen is arranged in the remaining part.

[0018] Furthermore, the main component gases of the hot flue gas include CO2 gas, SO2 gas, NO2 gas, N2 gas and H2O(g).

[0019] Furthermore, the integrated multi-parameter monitoring device includes: a temperature sensor, an electromagnetic wave probe, a microseismic monitoring probe and an acoustic wave sensor; the temperature sensor can monitor the temperature changes of different coal seam sections at different depths in real time, and invert the temperature field evolution of the explosion gas, hot flue gas and coal seam gas migration in the whole process of explosion fracturing-hot flue gas displacement-CH4 extraction-hot flue gas storage according to the temperature anomaly; the electromagnetic wave probe can monitor the resistivity changes of coal seam sections at different depths in the whole process of explosion fracturing-hot flue gas displacement-CH4 extraction-hot flue gas storage in real time, reflecting the changes in the lithologic parameters of coal porosity and permeability in the whole process; the microseismic monitoring probe can monitor the explosion fracturing-hot flue gas displacement-CH4 extraction-hot flue gas storage in real time The intensity and frequency changes of acoustic emission events during the fracturing process can be used to obtain the stress changes of reservoirs at different depths in different coal seam sections, and the effect and range of explosive fracturing can be inverted in real time; the acoustic wave sensor can monitor the acoustic wave velocity, attenuation and reflection characteristics of different coal seam sections at different depths in real time. Since the propagation characteristics of acoustic waves passing through different gas media (CH4, CO2, SO2, NO2, N2) in the reservoir are different, the relative content, distribution range and distribution characteristics of the main components of hot flue gas (i.e. CO2, SO2, NO2, N2) and CH4 gas in the target deep superimposed coal seams during the entire process of explosive fracturing-hot flue gas displacement-CH4 extraction-hot flue gas storage can be obtained in real time.

[0020] Furthermore, the explosion condition thresholds are: the explosion gas pressure threshold is 10 MPa, and the explosion CH4 concentration threshold is 5% to 20%.

[0021] Furthermore, the critical extraction CH4 concentration is 10-20%.

[0022] Compared with the existing technology, the present invention combines the use of explosive fracturing of superimposed coal seams with the displacement of CH4 gas from coal seams by hot flue gas, and the sealing of CO2 gas in the hot flue gas, which has the following advantages:

[0023] (1) The methane explosion fracturing technology of the present invention uses the high-temperature, high-pressure gas generated by the in-situ synergistic explosion reaction of O2 and CH4 gases in the superimposed coal seams to impact and fracture each coal seam segment to form a large number of initial cracks. At the same time, the CO2 gas generated by the high-temperature and high-pressure explosion generated by the explosion reaction continues to wedge into the cracks, causing the cracks to further extend and expand. In addition, the fractured coal body will desorb a large amount of CH4 gas in situ, which continues to react with the O2 gas to form a pulse fracturing effect in the coal seam, efficiently creating a complex fracture network, and providing an efficient migration channel for the storage of hot flue gas in the superimposed coal seams and the extraction of CH4.

[0024] (2) The hot flue gas used in the present invention is a high-temperature mixed gas composed of N2, CO2, H2O(g), O2, SO2, and NO2, and the temperature of the hot flue gas can be between 50-1500°C. After the hot flue gas is injected into the coal seam, the high temperature of the hot flue gas can enhance the desorption of adsorbed CH4 and the diffusion of free CH4; the N2 in the hot flue gas can reduce the partial pressure and concentration of CH4, promote the desorption of CH4, and weaken the adsorption and expansion effect of CO2, thereby improving the injectability of CO2 and enhancing the permeability of the reservoir; CO2, SO2 and NO2 in the hot flue gas, as strong adsorption gases, have competitive adsorption advantages after being injected into the coal seam, promoting the large-scale production of CH4 gas in the superimposed coal seam, and improving the efficiency of coal seam methane extraction while realizing the sealing of hot flue gas; while improving the efficiency of coal seam methane extraction, a large amount of greenhouse gases and toxic and harmful gases in deep coal seams are sealed. The H2O(g) in the hot flue gas acts as a good heat carrier, which can greatly reduce the heat loss during the transportation of the hot flue gas to the coal seam, so that the hot flue gas reaching the target coal seam still has a high-temperature effect on the coal seam: compared with other components of the flue gas, H2O(g) has a higher specific heat capacity and latent heat of vaporization. Therefore, it can retain the thermal energy in the hot flue gas through its high specific heat capacity during the transportation process, and reduce the loss of heat from the hot flue gas to the surrounding environment through its high vaporization potential. In addition, the water vapor condensed on the inner wall of the pipe will form an insulating layer, further reducing the heat conduction and radiation heat loss of the hot flue gas.

[0025] (3) The present invention adopts the extraction-monitoring combined well technology. By deploying multiple integrated multi-parameter monitoring devices at corresponding positions of different coal seam sections in the extraction-monitoring combined well, it can simultaneously realize the functions of coalbed methane extraction and distributed dynamic monitoring of changes in coal seam physical parameters, thereby improving the utilization efficiency of a single well. In addition, the extraction-monitoring combined well combines temperature, microseismic, acoustic and electromagnetic wave monitoring technologies to realize the monitoring of the entire process of "combustion and fracturing-hot flue gas storage-methane extraction". In conjunction with the deep learning algorithm, a multi-source monitoring data inversion and intelligent control system is established to design the optimal combustion and explosion gas injection parameters and the optimal hot flue gas injection parameters that meet the needs of each coal seam according to the changes in the physical parameters of the current different coal seam sections, thereby performing layered dynamic and precise control of the gas injection parameters of each coal seam section, thereby maximizing the methane combustion and explosion fracturing, hot flue gas storage and CH4 extraction efficiency of each coal seam section of the superimposed coal seam.

[0026] (4) In the present invention, a portion of the CH4 gas extracted from the coal seam is injected into a coal-fired power plant for mixed combustion with coal to convert it into electrical energy. The remaining extracted CH4 gas can be mixed with a combustion-supporting agent as a combustion gas and injected into the fractured coal seam again to perform multiple CH4 combustion-explosion fracturing. In addition, the main components of the hot flue gas (CO2, SO2, NO2, N2) extracted from the coal seam can be mixed with the hot flue gas generated by the coal-fired power plant and continuously injected into the fractured coal seam, thereby promoting further desorption and displacement of CH4 in the fractured coal seam, so as to continuously seal the hot flue gas and displace the remaining CH4 gas in the coal seam. The entire "combustion-explosion fracturing-hot flue gas sealing-CH4 gas extraction" work process forms a closed loop, maximizing the storage capacity of hot flue gas and the CH4 extraction capacity in deep unmineable coal seams, and realizing efficient closed-loop utilization of extracted CH4 gas resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG1 is a schematic diagram of the overall layout of the present invention;

[0028] FIG2 is an axial cross-sectional view of the gas injection plugger in FIG1 ;

[0029] FIG3 is a cross-sectional view of the integrated multi-parameter monitoring device in FIG1 .

[0030] In the figure: 1-1-first cap rock; 1-2-second cap rock; 1-3-third cap rock; 2-1-first coal seam section; 2-2-second coal seam section; 2-3-third coal seam section; 3-1-first horizontal drilling well; 3-2-second horizontal drilling well; 3-3-third horizontal drilling well; 4-vertical shaft; 5-extraction-monitoring combined well; 6-explosion gas storage tank; 7-explosion gas injection pump; 8-hot flue gas injection pump; 9-hot flue gas storage tank; 10-hot flue gas discharge device; 11-coal-fired power plant power generation device; 12-gas separation device; 13-gas filtration device; 14-heat exchanger group; 15 - extraction pump; 16- explosion gas injection pipe; 17- hot flue gas injection pipe; 18- casing; 19- screen pipe; 20- gas injection plug; 21- layered injection device; 22- explosion gas inlet pipe; 23- solenoid valve; 24- multi-parameter monitoring sensor; 25- ignition head; 26- multi-parameter monitoring data transmission line; 27- ground control center; 28- integrated multi-parameter monitoring device; 29- extraction plug; 30- CH4 transmission pipeline; 31- hot flue gas transmission pipeline; 32- temperature sensor; 33- electromagnetic wave probe; 34- microseismic monitoring probe; 35- acoustic wave sensor. DETAILED DESCRIPTION

[0031] The present invention will be further described below.

[0032] As shown in Figure 1, the specific steps of the present invention are:

[0033] A. Drilling construction: first determine the positions of the three coal seam sections and their rock cap layers in the superimposed coal seams, which are, from top to bottom, the first cap layer 1-1, the first coal seam section 2-1, the second cap layer 1-2, the second coal seam section 2-2, the third cap layer 1-3, and the third coal seam section 2-3. Then, construct a vertical shaft 4 and a combined extraction-monitoring well 5 from the ground through the three cap layers to the third coal seam section 2-3 at the deepest part. After the vertical shaft 4 is formed, a directional drilling rig is used to drill a horizontal well from the vertical shaft 4 along the direction of each coal seam section, namely the first horizontal well 3-1, the second horizontal well 3-2, and the third horizontal well 3-2. After completion, the drilling work is stopped. Casing 18 is laid in the vertical shaft 4, and the casing 18 is sealed against the inner wall of the vertical shaft. A screen 19 and a combination of casing 18 are laid in each horizontal well, wherein the casing 18 is laid at the connection between the horizontal well and the vertical shaft, and the screen 19 is laid in the rest.

[0034] B. Laying out hot flue gas injection and coalbed methane extraction system: a gas injection plug 20 is set in each of the three horizontal wells of the vertical shaft 4, so that a closed space is formed inside the three horizontal wells, and an extraction plug 29 is installed at the wellhead of the extraction-monitoring combined well 5 to seal the extraction-monitoring combined well 5; a layered injection device 21 is installed at the connection between each horizontal well and the vertical shaft 4, one end of the hot flue gas injection pipe 17 and one end of the explosion gas injection pipe 16 are both extended into the vertical shaft 4, and the layered injection devices 21 are connected in series in sequence; each gas injection plug 20 is provided with an explosion gas inlet pipe 22 and a hot flue gas inlet pipe; the two ends of the explosion gas inlet pipe 22 are respectively on both sides of the gas injection plug 20, wherein the end in the closed space is equipped with a solenoid valve 23, and the other end is connected to the layered injection device closest to it. injection device 21; the two ends of the hot flue gas inlet pipe are respectively located on both sides of the gas injection plug 20, wherein one end in the enclosed space is equipped with a solenoid valve 23, and the other end is connected to the layered injection device 21 closest to it; as shown in Figure 2, a multi-parameter monitoring sensor 24 and an ignition head 25 are installed on the gas injection plug 20 in the enclosed space; the multi-parameter monitoring sensor 24 and the ignition head 25 are both connected to the ground control center 27 through a multi-parameter monitoring data transmission line 26; the multi-parameter monitoring sensor 24 includes an air pressure sensor, a gas concentration sensor and a temperature sensor, wherein the air pressure sensor monitors the combustion and explosion gas pressure in the enclosed space in real time; the gas concentration sensor is used to monitor the concentration of CH4 gas in the enclosed space in real time; the temperature sensor is used to monitor the temperature in the enclosed space in real time. On the ground, there are arranged in sequence a combustion gas storage tank 6, a combustion gas injection pump 7, a hot flue gas injection pump 8, a hot flue gas storage tank 9, a hot flue gas exhaust device 10 of a coal-fired power plant, a power generation device 11 of a coal-fired power plant, a gas separation device 12, a gas filter device 13, a heat exchanger group 14 and an extraction pump 15; the combustion gas storage tank 6 is connected to the inlet of the combustion gas injection pump 7, and the outlet of the combustion gas injection pump 7 is connected to the other end of the combustion gas injection pipe 16; one end of the extraction pipe passes through the extraction plug 29 and extends into the extraction-monitoring combined well 5, and the other end of the extraction pipe is connected to the air inlet of the extraction pump 15, and the extraction pump 15 is connected through the pipe The pipeline is connected to the heat exchanger group 14, the gas filter device 13 and the gas separation device 12 in sequence. The CH4 gas passing through the gas separation device 12 is connected to the coal-fired power plant power generation device 11 and the air inlet of the explosion gas storage tank 6 through the CH4 transmission pipeline 30. The hot flue gas entering the gas separation device 12 and the hot flue gas from the coal-fired power plant tail gas treatment system are both connected to the inlet of the hot flue gas storage tank 9 through the hot flue gas transmission pipeline 31; the outlet of the hot flue gas storage tank 9 is connected to the inlet of the hot flue gas injection pump 8, and the outlet of the hot flue gas injection pump 8 is connected to the other end of the hot flue gas injection pipe 17, completing the system layout work;

[0035] C. Deployment of monitoring system: multiple integrated multi-parameter monitoring devices 28 are sent into the extraction-monitoring well 5, and multiple integrated multi-parameter monitoring devices 28 are respectively deployed at equal intervals at the positions of the three coal seam sections on the wellbore of the extraction-monitoring well 5; the integrated multi-parameter monitoring devices 28 of the three coal seam sections are connected to the ground control center 27 through the multi-parameter monitoring data transmission line 26, which is used to obtain the crack development and temperature changes in the three coal seam sections in real time, and complete the deployment of the monitoring system; as shown in Figure 3, the integrated multi-parameter monitoring device 28 includes: a temperature sensor 32, an electromagnetic wave probe 33, a microseismic monitoring probe 34 and an acoustic wave sensor 35; the temperature sensor 32 can monitor the temperature changes of different coal seam sections at different depths in real time, and invert the temperature field evolution of the explosion gas, hot flue gas and coal seam gas migration in the whole process of explosive fracturing-hot flue gas displacement-CH4 extraction-hot flue gas sealing according to the temperature anomaly; the electromagnetic wave probe 33 can monitor in real time the resistivity changes of coal seam sections at different depths during the entire process of explosive fracturing-hot flue gas displacement-CH4 extraction-hot flue gas storage, reflecting the changes in the lithologic parameters of coal porosity and permeability during the entire process; the microseismic monitoring probe 34 can monitor in real time the intensity and frequency changes of acoustic emission events during the explosive fracturing process, thereby obtaining the stress changes of the reservoirs at different depths in the three coal seam sections, and inverting the effect and range of the explosive fracturing in real time; the acoustic wave sensor 35 can monitor in real time the acoustic wave velocity, attenuation and reflection characteristics of the three coal seam sections at different depths. Since the propagation characteristics of acoustic waves in the reservoir through different gas media (CH4, CO2, SO2, NO2, N2) are different, the relative content, distribution range and distribution characteristics of the main components of hot flue gas (i.e., CO2, SO2, NO2, N2) and CH4 gas in the target deep superimposed coal seams during the entire process of explosive fracturing-hot flue gas displacement-CH4 extraction-hot flue gas storage can be obtained in real time.

[0036] D. Establish a multi-source monitoring data inversion and intelligent control system: First, use each integrated multi-parameter monitoring device 28 to monitor the geological conditions of the three coal seam sections, and image the temperature, microseismic, acoustic wave and resistivity data of the three coal seam sections at different depths in the initial state; then, drill coal samples at all the points where the integrated multi-parameter monitoring devices 28 are arranged in the three coal seam sections, and conduct laboratory tests on the samples at different depths of the three coal seam sections; combine the on-site multi-source monitoring data imaging results with the laboratory measurement results, use the existing deep learning algorithm to train the acquired data, and then establish a multi-source monitoring data inversion and intelligent control system; this system can monitor the changes in physical parameters of the overlapping coal seams at different depths (temperature, ground stress state, mechanical strength, porosity, permeability, fluid density, viscosity and saturation parameters, etc.) in real time, and can provide the optimal combustion gas injection parameters and optimal hot flue gas injection parameters that meet the needs of each coal seam section based on the changes in the physical parameters of each coal seam section;

[0037] E. Perform explosive fracturing on the superimposed coal seams: When starting fracturing, first start the explosive gas injection pump 7 and open the solenoid valve 23 of the explosive gas inlet pipe 22 of each coal seam section, so that the explosive gas is transported to each layered injection device 21 through the explosive gas injection pipe 16. The layered injection device 21 corresponding to each coal seam section controls the amount of explosive gas entering the enclosed space of each coal seam section according to the optimal explosive gas injection parameters determined in step D, and measures the explosive gas pressure and CH4 gas concentration in the enclosed space in real time through their respective multi-parameter monitoring sensors 24. When the parameters in the enclosed space reach the explosive condition threshold, that is, the explosive gas pressure threshold is 10Mpa, and the explosive CH4 concentration threshold is 5% to 20%. At this time, the explosive gas injection pump 7 is stopped and the solenoid valve 23 of the explosive gas inlet pipe 22 is closed, and then the ignition head is started, so that the explosive gas injected in the confined space undergoes an in-situ explosive reaction, and the transient shock wave and high-temperature and high-pressure gas generated by the explosive reaction act on the surrounding coal body to produce a large number of cracks, and then the broken rock mass can desorb a large amount of CH4 gas, which continues to react with O2 gas to form a pulse fracturing effect in the coal seam; after this round of explosive reaction is completed, the multi-source monitoring data inversion and intelligent control system adjusts the explosive gas injection parameters of different coal seam sections according to the different explosive fracturing conditions of the three coal seam sections, restarts the explosive gas injection pump 7, repeats the explosive fracturing steps multiple times, and performs multiple explosive fracturing processes around the horizontal wells of the three coal seam sections; during this period, the explosive fracturing conditions of each coal seam section are continuously monitored by the integrated multi-parameter monitoring device 28, and when it is monitored that the cracks in each horizontal well extend to the connection between the coal seam and the caprock, the explosive fracturing process is completed;

[0038] F. Injecting hot flue gas for storage and displacing CH4 gas desorption: Turn on the hot flue gas injection pump 8 and the solenoid valve of the hot flue gas inlet pipe to transport the hot flue gas in the hot flue gas storage tank 9 to each layered injection device 21 through the hot flue gas injection pipe 17. The main components of the hot flue gas include CO2 gas, SO2 gas, NO2 gas, N2 gas, and H2O (g). The layered injection device 21 corresponding to each coal seam segment controls the amount of hot flue gas entering the enclosed space of each coal seam segment according to the optimal hot flue gas injection parameters determined in step D. The higher temperature of the hot flue gas itself can promote the desorption of CH4 gas adsorbed in the three coal seam segments. At the same time, due to the competitive adsorption advantage of CO2 in the hot flue gas, the three coal seam segments adsorb CO2 gas, SO2 gas, and NO2 gas through replacement and precipitate CH4 gas, thereby achieving large-scale CH4 gas production in the three coal seam segments, sealing the hot flue gas while improving the coal seam CH4 mining efficiency.

[0039] G. CH4 gas extraction and hot flue gas storage: After a period of time, the extraction pump 15 is started to extract the mixed gas from the extraction-monitoring well 5 through the gas extraction pipe. The mixed gas is then passed through the heat exchanger group 14. After heat exchange, the temperature of the mixed gas is reduced to room temperature. The extracted mixed gas is first filtered to remove other gas impurities in the extracted mixed gas. The sources of the impurities include part of the CO2 gas generated during the explosive fracturing process, the residual CO2, SO2, NO2 and N2 during the hot flue gas storage and displacement process, and the original CO2, SO2 and N2 in the coal seam. The CH4 in the extracted mixed gas comes from the CH4 gas component that did not explode during the explosive fracturing process and the CH4 gas displaced by the hot flue gas in the coal seam; the mixed gas that has been treated to meet the standards enters the gas separation device 12, and the gas separation device 12 separates the extracted mixed gas that has been treated to meet the standards into the main component gas of the hot flue gas and CH4 gas; a part of the separated CH4 gas is injected into the coal-fired power plant power generation device 11 through the CH4 gas transmission pipeline and mixed with coal to generate electricity to improve efficiency, and the remaining separated CH4 gas is injected into the explosive gas storage tank 6 to mix with the combustion aid (i.e. O2 gas) is mixed to generate explosive gas for subsequent explosive fracturing; the separated main component gas of the hot flue gas and the hot flue gas of the hot flue gas discharge device 10 are injected into the hot flue gas storage tank 9 through the hot flue gas transmission pipeline 31 for mixing, and finally enter the fracturing coal seam through the hot flue gas injection pump 8, so as to continuously seal the hot flue gas and displace the remaining CH4 gas in the coal seam; during the hot flue gas injection and sealing period, the temperature, acoustic wave, microseismic parameters and resistivity data of different coal seam sections at different depths are collected in real time by the integrated multi-parameter monitoring device 28; according to the multi-parameter monitoring data of the ground control center 27 The source data inversion and intelligent control system inverts the hot flue gas injection conditions of different coal seam sections of the superimposed coal seam in real time and adjusts the optimal hot flue gas injection parameters for different coal seam sections. At this time, the ground control center 27 monitors the pressure, CH4 concentration, and temperature of the main component gases (i.e., CO2, SO2, NO2, N2) and CH4 mixed gas in each confined space in real time through the multi-parameter monitoring sensor 24, and dynamically adjusts the hot flue gas injection parameters in each horizontal drilling well to the optimal hot flue gas injection parameters based on the real-time monitoring data to ensure continuous and efficient hot flue gas injection in the three coal seam sections of the superimposed coal seam.

[0040] H. When the relative content of coal seam CH4 gas in the mixed gas drops below the critical extraction CH4 concentration of 15%, repeat steps B to G, and continue the closed-loop process of multiple explosive fracturing, hot flue gas sealing and CH4 extraction until the hot flue gas sealing volume of the three coal seam sections of the superimposed coal seam reaches above the set value, completing the hot flue gas sealing and CH4 extraction operations.

[0041] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for extracting methane from superimposed coal seams and closed-loop carbon fixation by hot flue gas displacement, characterized in that: The specific steps are: A. Well drilling construction: First determine the location of each coal seam section and its rock cap in the superimposed coal seam, then construct vertical shafts and extraction-monitoring combined wells from the ground through each rock cap to the deepest coal seam section. After the vertical shaft is formed, use a directional drill to drill a horizontal well from the vertical shaft along the direction of each coal seam section, and stop drilling after completion; B. Laying out hot flue gas injection and coalbed methane extraction system: a gas injection plug is set in each horizontal well of the vertical shaft, so that a closed space is formed inside each horizontal well, and an extraction plug is installed at the wellhead of the extraction-monitoring combined well to seal the extraction-monitoring combined well; a layered injection device is installed at the connection between each horizontal well and the vertical shaft, one end of the hot flue gas injection pipe and one end of the explosion gas injection pipe are extended into the vertical shaft, and each layered injection device is connected in series in sequence; an explosion gas inlet pipe is set on each gas injection plug and hot flue gas inlet pipe; the two ends of the explosive gas inlet pipe are respectively located on both sides of the gas injection plug, one end in the closed space is equipped with a solenoid valve, and the other end is connected to the stratified injection device closest to it; the two ends of the hot flue gas inlet pipe are respectively located on both sides of the gas injection plug, one end in the closed space is equipped with a solenoid valve, and the other end is connected to the stratified injection device closest to it; a multi-parameter monitoring sensor and an ignition head are installed on the gas injection plug in the closed space; the multi-parameter monitoring sensor and the ignition head are connected to the ground through a multi-parameter monitoring data transmission line The control center is connected; on the ground, there are explosive gas storage tanks, explosive gas injection pumps, hot flue gas injection pumps, hot flue gas storage tanks, hot flue gas emission devices of coal-fired power plants, power generation devices of coal-fired power plants, gas separation devices, gas filtering devices, heat exchanger groups and extraction pumps in sequence; the explosive gas storage tank is connected to the inlet of the explosive gas injection pump, and the outlet of the explosive gas injection pump is connected to the other end of the explosive gas injection pipe; one end of the extraction pipe passes through the extraction plugger and extends into the extraction-monitoring combined well, and the other end of the extraction pipe is connected to the air inlet of the extraction pump, and the extraction pump is connected through The pipeline is connected to the heat exchanger group, the gas filter device and the gas separation device in sequence. The CH4 gas passing through the gas separation device is connected to the gas inlet of the coal-fired power plant power generation device and the explosion gas storage tank through the CH4 transmission pipeline. The hot flue gas entering the gas separation device and the hot flue gas of the coal-fired power plant tail gas treatment system are connected to the inlet of the hot flue gas storage tank through the hot flue gas transmission pipeline; the outlet of the hot flue gas storage tank is connected to the inlet of the hot flue gas injection pump, and the outlet of the hot flue gas injection pump is connected to the other end of the hot flue gas injection pipe, thus completing the system layout; C. Deployment of monitoring system: multiple integrated multi-parameter monitoring devices are sent into the extraction-monitoring well, and multiple integrated multi-parameter monitoring devices are deployed at equal intervals at the positions of each coal seam section on the wellbore of the extraction-monitoring well; the integrated multi-parameter monitoring devices of each coal seam section are connected to the ground control center through a multi-parameter monitoring data transmission line to obtain the fracture development and temperature changes in each coal seam section in real time, and complete the deployment of the monitoring system; D. Establish a multi-source monitoring data inversion and intelligent control system: First, use various integrated multi-parameter monitoring devices to monitor the geological conditions of different coal seam sections, and image the temperature, microseismic, acoustic wave and resistivity data of different depths of each coal seam section in the initial state; then drill coal samples at all integrated multi-parameter monitoring device layout points in each coal seam section, and conduct laboratory measurements on samples at different depths of each coal seam section; combine the on-site multi-source monitoring data imaging results with the laboratory measurement results, use deep learning algorithms to train the acquired data, and establish a multi-source monitoring data inversion and intelligent control system; The system can monitor the changes in physical parameters of superimposed coal seams at different depths in real time, and can provide the best combustion and explosion gas injection parameters and the best hot flue gas injection parameters that meet the needs of each coal seam section according to the changes in physical parameters of each coal seam section. E. Perform explosive fracturing on the superimposed coal seams: When starting fracturing, first start the explosive gas injection pump and open the solenoid valve of the explosive gas inlet pipe of each coal seam section, so that the explosive gas is transported to each layered injection device through the explosive gas injection pipe. The layered injection device corresponding to each coal seam section controls the amount of explosive gas entering the closed space of each coal seam section according to the optimal explosive gas injection parameters determined in step D, and measures the explosive gas pressure and CH4 gas concentration in the closed space in real time through their respective multi-parameter monitoring sensors. When the parameters in the closed space reach the explosive condition threshold, the explosive gas injection pump is stopped and the electromagnetic valve of the explosive gas inlet pipe is closed. The valve is then turned on, and the ignition head is started, so that the explosive gas injected into the confined space undergoes an in-situ explosive reaction. The transient shock wave and high-temperature and high-pressure gas generated by the explosive reaction act on the surrounding coal body to produce a large number of cracks, and then the broken rock mass can desorb a large amount of CH4 gas, which continues to react with O2 gas to form a pulse fracturing effect in the coal seam. After this round of explosive reaction is completed, the multi-source monitoring data inversion and intelligent control system adjusts the explosive gas injection parameters of different coal seam sections according to the different explosive fracturing conditions of each coal seam section, restarts the explosive gas injection pump, repeats the explosive fracturing steps for multiple times, and performs multiple explosive fracturing processes around the horizontal drilling of each coal seam section. During this period, the explosive fracturing conditions of each coal seam section are continuously monitored through an integrated multi-parameter monitoring device. When the cracks in each horizontal well are monitored to extend to the connection between the coal seam and the caprock, the explosive fracturing process is completed; F. Injection of hot flue gas for storage and displacement of CH4 gas desorption: Open the hot flue gas injection pump and the solenoid valve of the hot flue gas inlet pipe, and transport the hot flue gas in the hot flue gas storage tank to each layered injection device through the hot flue gas injection pipe. The layered injection device corresponding to each coal seam section controls the amount of hot flue gas entering the closed space of each coal seam section according to the optimal hot flue gas injection parameters determined in step D. The higher temperature of the hot flue gas itself can promote the desorption of CH4 gas adsorbed in each coal seam section. At the same time, due to the competitive adsorption advantage, CO2, SO2 and NO2 in the hot flue gas make each coal seam section adsorb CO2 gas and precipitate CH4 gas through replacement, thereby achieving a large amount of CH4 gas output in each coal seam section, sealing the hot flue gas while improving the CH4 mining efficiency of the coal seam; G. CH4 gas extraction and hot flue gas sealing: After a period of time, the extraction pump is started to extract the mixed gas from the extraction-monitoring well through the gas extraction pipe, and then the mixed gas passes through the heat exchanger group. After heat exchange, the mixed gas The temperature of the mixed gas is reduced to room temperature; the mixed gas first passes through a gas filtering device to remove other gas impurities in the mixed gas, and the mixed gas that meets the treatment standard enters a gas separation device, and the gas separation device separates the mixed gas that meets the treatment standard into hot flue gas main component gas and CH4 gas; a part of the separated CH4 gas is injected into the power generation device of the coal-fired power plant through the CH4 gas transmission pipeline to be mixed with coal for combustion to generate electricity and improve efficiency, and the remaining separated CH4 gas is injected into the explosion gas storage tank and mixed with the combustion aid to generate explosion gas for subsequent explosion fracturing; the separated hot flue gas main component gas and the hot flue gas from the hot flue gas discharge device are injected into the hot flue gas storage tank through the hot flue gas transmission pipeline to mix, and finally enter the fracturing coal seam through the hot flue gas injection pump for continuous fracturing. The hot flue gas is sealed and the remaining CH4 gas in the coal seam is displaced. During the hot flue gas injection and sealing period, the temperature, acoustic wave, microseismic parameters and resistivity data of different coal seam sections at different depths are collected in real time through the integrated multi-parameter monitoring device. The hot flue gas injection conditions of different coal seam sections of the superimposed coal seam are inverted in real time according to the multi-source data inversion and intelligent control system of the ground control center, and the optimal hot flue gas injection parameters of different coal seam sections are adjusted. At this time, the ground control center monitors the pressure, CH4 concentration and temperature of the hot flue gas and CH4 mixed gas in each confined space in real time through the multi-parameter monitoring sensor, and dynamically adjusts the hot flue gas injection parameters in each horizontal drilling to the optimal hot flue gas injection parameters according to the real-time monitoring data, so as to ensure the continuous and efficient injection of hot flue gas in each fractured coal seam section of the superimposed coal seam. H. When the relative content of coal seam CH4 gas in the mixed gas drops below the critical extraction CH4 concentration, repeat steps B to G, and continue the closed-loop process of multiple explosion fracturing, hot flue gas sealing and CH4 extraction until the hot flue gas sealing volume of each coal seam section of the superimposed coal seam reaches above the set value, completing the hot flue gas sealing and CH4 extraction operations.

2. The method for methane extraction and closed-loop carbon fixation in superimposed coal seams by hot flue gas displacement according to claim 1, characterized in that: The multi-parameter monitoring sensor includes an air pressure sensor, a gas concentration sensor and a temperature sensor, wherein the air pressure sensor monitors the pressure of the explosion gas in the explosion-proof enclosed space in real time; the gas concentration sensor is used to monitor the concentration of CH4 gas in the explosion-proof enclosed space in real time; and the temperature sensor is used to monitor the temperature in the explosion-proof enclosed space in real time.

3. The method for methane extraction and closed-loop carbon fixation in superimposed coal seams by hot flue gas displacement according to claim 1, characterized in that: Casing is arranged in the vertical shaft, and the casing is sealed with the inner wall of the vertical shaft. Screen pipes and casing combinations are arranged in each horizontal drilling well, wherein the casing is arranged at the connection between the horizontal drilling well and the vertical shaft, and the screen pipes are arranged in the remaining parts.

4. The method for methane extraction and closed-loop carbon fixation in superimposed coal seams by hot flue gas displacement according to claim 1, characterized in that: The main component gases of the hot flue gas include CO2 gas, SO2 gas, NO2 gas, N2 gas and H2O(g).

5. The method for methane extraction and closed-loop carbon fixation in superimposed coal seams by hot flue gas displacement according to claim 1, characterized in that: The integrated multi-parameter monitoring device includes: a temperature sensor, an electromagnetic wave probe, a microseismic monitoring probe and an acoustic wave sensor; the temperature sensor can monitor the temperature changes of different coal seam sections at different depths in real time, and invert the temperature field evolution of the explosion gas, hot flue gas and coal seam gas migration in the whole process of explosive fracturing-hot flue gas displacement-CH4 extraction-hot flue gas sealing according to the temperature anomaly; the electromagnetic wave probe can monitor the resistivity changes of coal seam sections at different depths in the whole process of explosive fracturing-hot flue gas displacement-CH4 extraction-hot flue gas sealing in real time, reflecting the changes in lithological parameters of coal body porosity and permeability in the whole process; the microseismic monitoring probe can monitor the intensity and frequency changes of acoustic emission events in the process of explosive fracturing in real time, so as to obtain the stress changes of reservoirs at different depths in different coal seam sections, and invert the effect and range of explosive fracturing in real time; the acoustic wave sensor can monitor the acoustic wave velocity, attenuation and reflection characteristics of different coal seam sections at different depths in real time.

6. The method for methane extraction and closed-loop carbon fixation in superimposed coal seams by hot flue gas displacement according to claim 1, characterized in that: The explosion condition thresholds are: the explosion gas pressure threshold is 10 MPa, and the explosion CH4 concentration threshold is 5% to 20%.

7. The method for methane extraction and closed-loop carbon fixation in superimposed coal seams by hot flue gas displacement according to claim 1, characterized in that: The critical extraction CH4 concentration is 10-20%.

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