Closed-loop extraction and isotope tracing method for hot flue gas displacement of coal bed methane

Abstract

The application discloses a closed-loop extraction and isotope tracing method for hot flue gas displacement of coal bed methane, which comprises the following steps: first, using CH4 combustion and explosion fracturing technology to form a crack network in the in-situ coal seam by synergistic combustion and explosion reaction impact fracturing; then, injecting hot flue gas into the combustion and explosion fractured coal seam; the hot flue gas can promote the desorption of CH4 gas in the coal body due to its high temperature; the CO2, SO2 and NO2 in the hot flue gas can replace a large amount of CH4 gas adsorbed in the in-situ coal seam due to the competitive adsorption advantage; the extracted CH4 and hot flue gas can be continuously used in a closed loop; the hot flue gas storage is realized, and the coal bed methane extraction efficiency is improved; in addition, the combustion and explosion gas and the hot flue gas are marked by specific isotopes, and the migration characteristics and interaction of the combustion and explosion gas, the hot flue gas and the extracted CH4 gas in the target deep coal seam can be analyzed and reversed; therefore, the injection parameters of the combustion and explosion gas and the hot flue gas can be dynamically regulated and controlled, and the hot flue gas storage and CH4 extraction efficiency of the deep in-situ coal seam are maximized.

Description

Technical Field

[0001] This invention relates to a method for hot flue gas storage and CH4 extraction, specifically a closed-loop extraction and isotope tracing method for hot flue gas displacing coal seam methane, belonging to the field of carbon fixation and CH4 extraction technology. Background Technology

[0002] Hydraulic fracturing is currently the most widely used reservoir fracturing and permeability enhancement method for CO2 sequestration and CH4 extraction. However, hydraulic fracturing generally suffers from problems such as high water consumption and limited fracture propagation, resulting in a limited scope for hot flue gas sequestration and CH4 extraction. Currently, hydraulic fracturing technology cannot achieve closed-loop operation in hot flue gas sequestration and CH4 displacement of coal seams. That is, after the injected H2O and fracturing fluid complete fracturing and permeability enhancement in the coal seam, they need to be extracted before subsequent hot flue gas sequestration and CH4 extraction can be carried out, making a closed-loop reuse process impossible. In contrast, combustion-explosion fracturing technology utilizes in-situ desorption of CH4 from the coal seam, and the high-temperature, high-pressure gas impact generated by the synergistic combustion of O2 and CH4. Fracturing deep, unminable coal seams efficiently creates a three-dimensional fracture network, greatly increasing the amount of hot flue gas stored and CH4 extracted from these seams. However, no research has yet combined combustion-explosion fracturing with hot flue gas storage to displace CH4 extraction from the coal seam. The "combustion-explosion fracturing-hot flue gas storage-CH4 extraction" process involves the interaction of multiple gases such as CO2, CH4, and N2. However, most existing studies can only analyze the components of the extracted mixed gas, lacking a clear understanding of the specific sources of these components. Furthermore, there is a lack of dynamic tracking of the migration characteristics of combustion gases, hot flue gas, and CH4 within the coal seam throughout the entire process, making it impossible to determine the migration patterns of various gases within deep, unminable coal seams.

[0003] Therefore, to address the shortcomings of current methods for thermal flue gas sequestration and CH4 extraction in deep, unminable coal seams, a new approach is proposed. This method combines combustion-explosion fracturing and thermal flue gas sequestration technologies to simultaneously achieve efficient closed-loop extraction of CH4 from deep, unminable coal seams. It dynamically tracks the migration characteristics of combustion gases, thermal flue gas, and extracted CH4 gas throughout the entire process of "combustion-explosion fracturing - thermal flue gas sequestration - CH4 extraction," and optimizes sequestration and extraction parameters based on these migration characteristics. This approach reduces the cost of deep, unminable coalbed methane extraction and maximizes the efficiency of thermal flue gas sequestration and CH4 extraction in deep, unminable coal seams, providing a new research approach to support carbon neutrality and the development of the coalbed methane industry. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides a closed-loop extraction and isotope tracing method for displacing coalbed methane with hot flue gas. Based on the combination of combustion-explosion fracturing technology and hot flue gas storage technology, it simultaneously achieves efficient closed-loop extraction of CH4 from deep unminable coalbed methane. It dynamically tracks the migration characteristics of combustion-explosion gas, hot flue gas, and extracted CH4 gas throughout the entire process of "combustion-explosion fracturing - hot flue gas storage - CH4 extraction", and optimizes the storage and extraction parameters based on the migration characteristics. This reduces the cost of deep unminable coalbed methane extraction and maximizes the efficiency of hot flue gas storage and CH4 extraction in deep unminable coalbed methane.

[0005] To achieve the above objectives, the technical solution adopted by this invention is: a closed-loop extraction and isotope tracing method for displacing coal seam methane with hot flue gas, the specific steps of which are as follows:

[0006] A. Drilling construction: First, determine the location of the unminable coal seam, then construct vertical shafts and extraction wells from the ground through the rock strata to the unminable coal seam. After the vertical shaft is formed, use a directional drilling rig to drill horizontal wells from the deepest part of the vertical shaft along the direction of the unminable coal seam. Drilling work is stopped after completion.

[0007] B. Deployment of Hot Flue Gas Injection and Coalbed Methane Extraction System: Injection plugs and extraction plugs are installed in the vertical shaft and extraction well, respectively. The injection plug is located at the interface between the rock strata and the coal seam, creating a combustion-explosion sealed space between the vertical shaft below the injection plug and the entire horizontal shaft. The extraction plug is located at the wellhead of the extraction well, sealing it. The injection plug is equipped with a combustion-explosion gas inlet pipe and a hot flue gas inlet pipe. The two ends of the combustion-explosion gas inlet pipe are located on either side of the injection plug, within the combustion-explosion sealed space. One end of the device is equipped with a solenoid valve, and the other end is connected to one end of the flammable gas injection pipe extending into the vertical shaft. The two ends of the hot flue gas inlet pipe are located on either side of the gas injection plug, with the end inside the flammable confined space equipped with a solenoid valve and the other end connected to the hot flue gas injection pipe extending into the vertical shaft. The gas injection plug in the flammable confined space is equipped with a multi-parameter monitoring sensor and an ignition head. Both the multi-parameter monitoring sensor and the ignition head are connected to the ground control center via a multi-parameter monitoring data transmission line. Flammable gas is sequentially arranged on the ground. Gas storage tank, explosive gas injection pump, hot flue gas injection pump, hot flue gas storage tank, hot flue gas emission device for coal-fired power plants, power generation device for coal-fired power plants, gas separation device, gas filtration device, heat exchanger assembly, and extraction pump; 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 gas extraction pipe passes through the extraction plug and extends into the extraction well, and the other end of the gas extraction pipe is connected to the inlet of the extraction pump; the extraction pump is connected to the heat exchanger assembly via pipelines. The gas filtration device and the gas separation device are connected. The CH4 gas passing through the gas separation device is connected to the inlet of the coal-fired power plant's power generation unit and the combustion and explosion gas storage tank through the CH4 delivery pipeline. The hot flue gas entering the gas separation device and the hot flue gas entering 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 delivery 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.

[0008] C. Explosive fracturing of unmineable coal seams: Before fracturing, use... 2 H and 18 O stable isotope labeling of the explosive gas in the storage tank, wherein the explosive gas is used 2 H-labeled CH4 gas, 18 O is marked as O2 gas; used 13 C and 15 The hot flue gas in the N stable isotope labeled hot flue gas storage tank (i.e., 13 CO2, 15(N2); When fracturing begins, the combustion gas injection pump is started and the solenoid valve of the combustion gas inlet pipe is opened, allowing the combustion gas labeled with a specific isotope to be continuously injected into the combustion-explosion confined space through the combustion gas injection pipe and the inlet pipe. Multi-parameter monitoring sensors are used to measure the pressure of the combustion gas, CH4 gas concentration, and temperature in the combustion-explosion confined space in real time. When the parameters in the combustion-explosion confined space reach the combustion-explosion condition threshold, the combustion gas injection pump is stopped and the solenoid valve of the combustion gas inlet pipe is closed. Then, the igniter is activated, causing the injected combustion gas in the combustion-explosion confined space to undergo an in-situ combustion-explosion reaction, producing a large amount of... 18 The O-labeled CO2 gas, the transient shock wave generated by the combustion and explosion reaction, and the high temperature and high pressure gas act on the surrounding coal body to produce a large number of cracks. The fractured rock mass can desorb a large amount of CH4 gas, which continues to react with O2 gas, thus forming a pulse fracturing effect in the coal seam.

[0009] D. Hot flue gas injection, storage, and CH4 gas desorption: The hot flue gas injection pump and the solenoid valve of the hot flue gas inlet pipe are opened. Hot flue gas labeled with a specific isotope is injected from the hot flue gas storage tank into the combustion-explosion sealed space through the hot flue gas injection pipe and inlet pipe. The high temperature of the hot flue gas itself promotes the desorption of CH4 gas adsorbed in the unminable coal seam. Simultaneously, the CH4 gas in the hot flue gas... 13 C-labeled CO2, SO2, and NO2, due to their competitive adsorption advantage, cause unminable coal seams to adsorb CO2 gas and precipitate CH4 gas through displacement, thus achieving a large-scale production of CH4 gas from unminable coal seams. This not only seals up hot flue gas but also improves the efficiency of CH4 mining in coal seams.

[0010] E. CH4 Gas Extraction and Hot Flue Gas Recycling: After a period of time, the extraction pump is started to extract the mixed gas from the extraction well through the gas extraction pipe. The mixed gas then passes through a heat exchanger group, where its temperature is lowered to room temperature. The extracted mixed gas first passes through a gas filtration device to remove other gas impurities. The treated mixed gas then enters a gas separation device, which separates the treated mixed gas into the main components of hot flue gas and CH4 gas. A portion of the separated gas... CH4 gas is injected into the coal-fired power plant's generating unit through a CH4 gas delivery pipeline to mix with coal for combustion and power generation, improving efficiency. The remaining separated CH4 gas is injected into a combustion and explosion gas storage tank to mix with an oxidizer to generate combustion and explosion gas for subsequent combustion and explosion fracturing. The main components of the separated hot flue gas and the hot flue gas from the hot flue gas emission device are injected into the hot flue gas storage tank through a hot flue gas delivery pipeline to mix. Finally, the hot flue gas is injected into the fracturing coal seam through a hot flue gas injection pump to continuously store the hot flue gas and displace the remaining CH4 gas in the coal seam.

[0011] F. Isotope Tracing Monitoring, CH4 Gas Recirculation Extraction, and Hot Flue Gas Recirculation and Seizure: During the hot flue gas seizure and CH4 gas extraction from the displaced coal seam, isotope analysis of the separated gas is performed at regular intervals to determine the CO2 content. 18 CO2 labeled with O 13 C-labeled CO2, N2 15 N, N2, CH4 and 2 The relative contents of each H-labeled CH4 are used to determine the source and evolution of the extracted mixed gas. The migration of combustion and explosion gases, hot flue gas, and extracted CH4 gas in the unmineable coal seam is acquired in real time. The interaction between combustion and explosion gases, hot flue gas, and coal seam CH4 is inverted to provide data support for subsequent fracturing parameter determination. When the relative content of coal seam CH4 gas in the mixed gas decreases to below the critical extraction CH4 concentration, steps B to E are repeated to continuously perform multiple combustion and explosion fracturing, hot flue gas sealing, and CH4 extraction processes in a closed loop until the extraction operation is completed. The combustion and explosion gas injection pipe, hot flue gas injection pipe, and gas extraction pipe are removed from the vertical shaft and extraction well, respectively, and the vertical shaft and extraction well are sealed to seal the large amount of hot flue gas adsorbed in the unmineable coal seam.

[0012] Furthermore, the multi-parameter monitoring sensor includes a pressure sensor, a gas concentration sensor, and a temperature sensor, wherein the pressure sensor monitors the pressure of the flammable gas in the flammable and explosive confined space in real time; the gas concentration sensor is used to monitor the concentration of CH4 gas in the flammable and explosive confined space in real time; and the temperature sensor is used to monitor the temperature in the flammable and explosive confined space in real time.

[0013] Furthermore, a casing is installed inside the vertical shaft, and the casing is sealed to the inner wall of the vertical shaft. A screen pipe is installed inside the horizontal shaft, and multiple mesh holes are opened on the screen pipe. The ground distance between the vertical shaft and the extraction well does not exceed 2000m.

[0014] Furthermore, the main components of the hot flue gas include CO2 gas, SO2 gas, NO2 gas, N2 gas, and H2O(g).

[0015] Furthermore, the explosion condition thresholds are: the explosion gas pressure threshold is 10 MPa, and the explosion CH4 concentration threshold is 5%~20%.

[0016] Furthermore, the critical extraction CH4 concentration is 20%.

[0017] Compared with existing technologies, this invention combines combustion-explosive fracturing of unmineable coal seams and displacement of CH4 gas from coal seams with hot flue gas, while also sealing CO2 gas in the hot flue gas. This approach offers the following advantages:

[0018] (1) The methane combustion and explosion fracturing technology of the present invention uses the high temperature and high pressure gas generated by the in-situ synergistic combustion and explosion reaction of O2 and CH4 gas in deep unminable coal seams to impact and fracture the coal seam to form a large number of initial fractures. At the same time, the CO2 gas generated by the high temperature and high pressure explosion of the combustion and explosion reaction continuously weds into the fractures, causing the fractures to extend and expand further. In addition, the fractured coal body desorbs a large amount of CH4 gas in situ, which continues to react with O2 gas to form a pulse fracturing effect in the coal seam, efficiently creating a complex fracture network, and providing an efficient transport channel for the storage of hot flue gas and CH4 extraction in deep unminable coal seams.

[0019] (2) The hot flue gas used in this invention is a high-temperature mixed gas composed of N2, CO2, H2O(g), O2, SO2, and NO2, with a temperature ranging from 50 to 1500℃. After the hot flue gas is injected into the coal seam, its high temperature enhances 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, promoting CH4 desorption while weakening the adsorption and expansion effect of CO2, thereby improving the injectability of CO2 and enhancing reservoir permeability; CO2, SO2, and NO2 in the hot flue gas, as strongly adsorbent gases, have a competitive adsorption advantage after being injected into the coal seam, promoting the large-scale production of CH4 gas in difficult-to-mine and unmineable coal seams, thereby improving the efficiency of coal seam methane mining while achieving hot flue gas sequestration; and simultaneously improving the efficiency of coal seam methane mining while achieving the large-scale sequestration of greenhouse gases and toxic and harmful gases in deep coal seams. The volume of H2O(g) in the flue gas eliminates the need for desulfurization and denitrification in coal-fired power plants, saving related costs. In addition, H2O(g) in the flue gas acts as a good heat transfer medium, which can greatly reduce heat loss during the transportation of the flue gas to the coal seam, so that the 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 heat energy in the flue gas through its high specific heat capacity during transportation, and reduce the heat loss of the 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 a heat insulation layer, further reducing the heat conduction and radiation heat loss of the flue gas.

[0020] (3) This invention uses a combustion and explosive gas labeled with a specific isotope ( 2 H(CH4) 18 O(O2)) and hot flue gas ( 13 C(CO2), 15 N(N2) is injected into deep, unminable coal seams for "combustion-explosive fracturing - hot flue gas sealing - methane extraction". Isotope analysis (CO2, C2, C4) is then performed on the extracted gases. 18 O2, 13 CO2, N2, 15 N2, CH4 and C 2H4 can be used to invert the migration characteristics and interactions of combustion and explosion gases, hot flue gas, and extracted CH4 gas in the target deep coal seam in real time during the entire process of "combustion and explosion fracturing - hot flue gas storage - methane extraction". Therefore, the injection parameters of combustion and explosion gases and hot flue gas can be dynamically adjusted to achieve stable hot flue gas storage and CH4 extraction process, and provide data support for subsequent fracturing parameter determination.

[0021] (4) In this invention, a portion of the CH4 gas extracted from the coal seam is injected into a coal-fired power plant to mix and burn with the coal, converting it into electrical energy. The remaining extracted CH4 gas can be mixed with an oxidizer as a combustion and explosion gas, and then injected back into the fracturing coal seam for multiple CH4 combustion and explosion fracturing operations. In addition, the main components of the hot flue gas extracted from the coal seam (CO2, SO2, NO2, N2, H2O(g)) can be continuously reinjected into the fracturing coal seam after mixing with the hot flue gas generated by the coal-fired power plant, which promotes further desorption and displacement of CH4 in the fracturing coal seam, so as to continuously store the hot flue gas and displace the remaining CH4 gas in the coal seam. This makes the entire "combustion and explosion fracturing - hot flue gas storage - CH4 gas extraction" process form a closed loop, maximizing the amount of hot flue gas stored and CH4 extracted in the deep unminable coal seam, and realizing the efficient closed-loop utilization of extracted CH4 gas resources. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall layout of the present invention;

[0023] Figure 2 yes Figure 1 Axial sectional view of the gas injection plug.

[0024] In the diagram: 1- Explosive gas storage tank; 2- Explosive gas injection pump; 3- Hot flue gas injection pump; 4- Hot flue gas storage tank; 5- Hot flue gas emission device; 6- Coal-fired power plant generator; 7- Gas separation device; 8- Gas filtration device; 9- Heat exchanger group; 10- Extraction pump; 11- Injection plug; 12- Extraction plug; 13- Ignition head; 14- Multi-parameter monitoring sensor; 15- Explosive fracturing fracture; 16- Explosive gas injection pipe; 17- Hot flue gas injection pipe; 18- Gas extraction pipe; 19- Hot flue gas delivery pipeline; 20- CH4 delivery pipeline; 21- Multi-parameter monitoring data transmission line; 22- Ground control center; 23- Vertical shaft; 24- Extraction well; 25- Horizontal well; 26- Rock strata; 27- Coal seam; 29- Connector; 29- Hot flue gas inlet pipe; 30- Solenoid valve; 31- Explosive gas inlet pipe. Detailed Implementation

[0025] The present invention will be further described below.

[0026] like Figure 1 As shown, the specific steps of this invention are as follows:

[0027] A. Drilling Construction: First, determine the location of the unminable coal seam. Then, construct vertical shaft 23 and extraction shaft 24 from the ground through the rock stratum 26 towards the unminable coal seam. After the vertical shaft 23 is formed, use a directional drilling rig to drill a horizontal shaft 25 from the deepest point of the vertical shaft 23 along the direction of the unminable coal seam. After completion, stop drilling work. A casing is installed in the vertical shaft 23, and the casing is sealed to the inner wall of the vertical shaft. A screen pipe is installed in the horizontal shaft 25, and multiple mesh openings are made on the screen pipe. The ground distance between the vertical shaft 23 and the extraction shaft 24 does not exceed 2000m.

[0028] B. Deployment of hot flue gas injection and coalbed methane extraction system: Gas injection plugger 11 and extraction plugger 12 are respectively installed in vertical shaft 23 and extraction shaft 24. Gas injection plugger 11 is located at the interface between rock stratum 26 and coal seam 27, creating a combustion-explosion sealed space between vertical shaft 23 below gas injection plugger 11 and the entire horizontal shaft 25. Extraction plugger 12 is located at the wellhead of extraction shaft 24, sealing it. Figure 2As shown, the gas injection sealing device 11 is equipped with a combustion and explosion gas inlet pipe 31 and a hot flue gas inlet pipe 29. The two ends of the combustion and explosion gas inlet pipe 31 are located on both sides of the gas injection sealing device 11, with one end within the combustion and explosion sealed space equipped with a solenoid valve 30, and the other end connected to one end of the combustion and explosion gas injection pipe 16 extending into the vertical shaft 23. The two ends of the hot flue gas inlet pipe 29 are located on both sides of the gas injection sealing device 11, with one end within the combustion and explosion sealed space equipped with a solenoid valve 23, and the other end connected to one end of the hot flue gas injection pipe 17 extending into the vertical shaft 23. The gas injection plug 11 inside is equipped with a multi-parameter monitoring sensor 14 and an ignition head 13; both the multi-parameter monitoring sensor 14 and the ignition head 13 are connected to the ground control center 22 via a multi-parameter monitoring data transmission line 21; the multi-parameter monitoring sensor 14 includes a pressure sensor, a gas concentration sensor and a temperature sensor, wherein the pressure sensor monitors the pressure of the flammable and explosive gas in the flammable and explosive confined space in real time; the gas concentration sensor is used to monitor the concentration of CH4 gas in the flammable and explosive confined space in real time; and the temperature sensor is used to monitor the temperature in the flammable and explosive confined space in real time. On the ground, the following components are arranged in sequence: a flammable gas storage tank 1, a flammable gas injection pump 2, a hot flue gas injection pump 3, a hot flue gas storage tank 4, a hot flue gas emission device for a coal-fired power plant 5, a power generation device for a coal-fired power plant 6, a gas separation device 7, a gas filtration device 8, a heat exchanger group 9, and an extraction pump 10. The flammable gas storage tank 1 is connected to the inlet of the flammable gas injection pump 2, and the outlet of the flammable gas injection pump 2 is connected to the other end of the flammable gas injection pipe 16. One end of the gas extraction pipe 14 passes through the extraction plug 12 and extends into the extraction well 24; the other end of the gas extraction pipe 14 is connected to the air inlet of the extraction pump 10. The extraction pump 10 is connected to... The system is connected in sequence to heat exchanger group 9, gas filter device 8 and gas separator device 7 via pipelines. The CH4 gas passing through gas separator device 7 is connected to the inlet of coal-fired power plant generator 6 and combustion and explosion gas storage tank 1 via CH4 delivery pipeline 20. The hot flue gas entering gas separator device 7 and the hot flue gas of coal-fired power plant tail gas treatment system are both connected to the inlet of hot flue gas storage tank 4 via hot flue gas delivery pipeline 19. The outlet of hot flue gas storage tank 4 is connected to the inlet of hot flue gas injection pump 3, and the outlet of hot flue gas injection pump 3 is connected to the other end of hot flue gas injection pipe 17, thus completing the system layout.

[0029] C. Explosive fracturing of unmineable coal seams: Before fracturing, use... 2 H and 18 O stable isotope labeling of the explosive gas in the storage tank, wherein the explosive gas is used 2 H-labeled CH4 gas, 18 O is marked as O2 gas; used 13 C and 15The hot flue gas in the hot flue gas storage tank is labeled with N stable isotopes. The main components of the hot flue gas include CO2, SO2, NO2, N2, and H2O(g), wherein... 13 C is used to label CO2 gas. 15 N-labeled N2 gas; at the start of fracturing, the combustion gas injection pump 2 is started and the solenoid valve 30 of the combustion gas inlet pipe 31 is opened, allowing the specific isotope-labeled combustion gas to be continuously injected into the combustion-explosion confined space through the combustion gas injection pipe 16 and the inlet pipe. The pressure of the combustion gas, the concentration of CH4 gas, and the temperature in the combustion-explosion confined space are measured in real time by the multi-parameter monitoring sensor 14. When the parameters in the combustion-explosion confined space reach the combustion-explosion condition threshold, specifically including: the combustion gas pressure threshold of 10 MPa and the combustion CH4 concentration threshold of 5%~20%, the combustion gas injection pump 2 is stopped and the solenoid valve 23 of the combustion gas inlet pipe 31 is closed. Then, the igniter 13 is started, causing the combustion gas injected into the combustion-explosion confined space to undergo an in-situ combustion-explosion reaction, producing a large amount of 18 The O-labeled CO2 gas, the transient shock wave generated by the combustion and explosion reaction, and the high temperature and high pressure gas act on the surrounding coal body to produce a large number of cracks. The fractured rock mass can desorb a large amount of CH4 gas, which continues to react with O2 gas, thus forming a pulse fracturing effect in the coal seam.

[0030] D. Hot flue gas injection, sealing, and CH4 gas desorption: The hot flue gas injection pump 3 is turned on, and the solenoid valve 30 of the hot flue gas inlet pipe 29 is opened. Hot flue gas labeled with a specific isotope is injected from the hot flue gas storage tank 4 into the combustion-explosion sealed space through the hot flue gas injection pipe 29 and the inlet pipe. The high temperature of the hot flue gas itself promotes the desorption of CH4 gas adsorbed in the unminable coal seam. Simultaneously, the hot flue gas... 13 C-labeled CO2, SO2, and NO2, due to their competitive adsorption advantage, cause unminable coal seams to adsorb hot flue gas and precipitate CH4 gas through displacement, thus achieving a large-scale production of CH4 gas from unminable coal seams. This not only seals up the hot flue gas but also improves the CH4 mining efficiency of the coal seam.

[0031] E. CH4 gas extraction and its hot flue gas recycling: After a period of time, the extraction pump 10 is started to extract the mixed gas from the extraction well 24 through the gas extraction pipe 18. Then the mixed gas passes through the heat exchanger group 9, and the temperature of the mixed gas is reduced to room temperature after heat exchange. The mixed gas mainly consists of: flue gas main component gases with isotope labels (i.e., 13 CO2, SO2, NO2 15 The flue gas is composed of N2, CH4, and other gaseous impurities. The main components of the flue gas originate from C2, CH4, and some combustion reaction products generated during the combustion-explosion fracturing process. 18 O2 gas, the original CO2 gas present in the coal seam, and the residual flue gas components during the hot flue gas sealing and displacement process.13 CO2 gas 15 N2 gas; CH4 gas originates from the non-ignition gas component C during the combustion-explosive fracturing process. 2 The extracted mixed gas consists of H4 gas and CH4 gas originally present in the coal seam. First, it passes through a gas filtration device 8 to remove other gaseous impurities. The treated mixed gas then enters a gas separation device 7, which separates the treated mixed gas into the main components of hot flue gas and CH4 gas. A portion of the separated CH4 gas is injected into the coal-fired power plant's generator unit 6 via a CH4 gas delivery pipeline to mix with coal for combustion and power generation, improving efficiency. The remaining separated CH4 gas is injected into the combustion and explosion gas storage tank 1 to mix with an oxidizer (O2 gas) to generate combustion and explosion gas for subsequent combustion and explosion fracturing. The separated main components of the hot flue gas and the hot flue gas from the hot flue gas emission device 5 are both injected into the hot flue gas storage tank 4 via the hot flue gas delivery pipeline 19 and mixed. Finally, the mixture is injected into the fracturing coal seam via the hot flue gas injection pump 3 to continuously store the hot flue gas and displace the remaining CH4 gas in the coal seam.

[0032] F. Isotope Tracing Monitoring, CH4 Gas Recirculation Extraction, and Hot Flue Gas Recirculation Sequestration: During the hot flue gas sequestration and CH4 gas extraction from the displaced coal seam, isotope analysis of the separated gas is performed at regular intervals to determine the CO2 and C content. 18 O2, 13 CO2, N2, 15 N2, CH4 and C 2 The relative content of each H4 is determined to identify the source and evolution of the extracted mixed gas. The migration of combustion and explosion gases, hot flue gas, and extracted CH4 gas in the unminable coal seam is acquired in real time, and the interaction between combustion and explosion gases, hot flue gas, and coal seam CH4 is inverted. When the relative content of coal seam CH4 gas in the mixed gas decreases to below the critical extraction CH4 concentration of 20%, steps B to E are repeated to continuously perform multiple combustion and explosion fracturing, hot flue gas sealing, and CH4 extraction processes in a closed loop until the extraction operation is completed. The combustion and explosion gas injection pipe 16, hot flue gas injection pipe 17, and gas extraction pipe 18 are taken out from the vertical shaft 23 and extraction shaft 24, respectively, and the vertical shaft 23 and extraction shaft 24 are sealed to seal the large amount of hot flue gas adsorbed in the unminable coal seam.

[0033] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A closed-loop extraction and isotope tracing method for displacing coal seam methane with hot flue gas, characterized in that, The specific steps are as follows: A. Drilling construction: First, determine the location of the unminable coal seam, then construct vertical shafts and extraction wells from the ground through the rock strata to the unminable coal seam. After the vertical shaft is formed, use a directional drilling rig to drill horizontal wells from the deepest part of the vertical shaft along the direction of the unminable coal seam. Drilling work is stopped after completion. B. Deployment of Hot Flue Gas Injection and Coalbed Methane Extraction System: Injection plugs and extraction plugs are installed in the vertical shaft and extraction well, respectively. The injection plug is located at the interface between the rock strata and the coal seam, creating a combustion-explosion sealed space between the vertical shaft below the injection plug and the entire horizontal shaft. The extraction plug is located at the wellhead of the extraction well, sealing it. The injection plug is equipped with a combustion-explosion gas inlet pipe and a hot flue gas inlet pipe. The two ends of the combustion-explosion gas inlet pipe are located on either side of the injection plug, within the combustion-explosion sealed space. One end of the device is equipped with a solenoid valve, and the other end is connected to one end of the flammable gas injection pipe extending into the vertical shaft. The two ends of the hot flue gas inlet pipe are located on either side of the gas injection plug, with the end inside the flammable confined space equipped with a solenoid valve and the other end connected to the hot flue gas injection pipe extending into the vertical shaft. The gas injection plug in the flammable confined space is equipped with a multi-parameter monitoring sensor and an ignition head. Both the multi-parameter monitoring sensor and the ignition head are connected to the ground control center via a multi-parameter monitoring data transmission line. Flammable gas is sequentially arranged on the ground. Gas storage tank, explosive gas injection pump, hot flue gas injection pump, hot flue gas storage tank, hot flue gas emission device for coal-fired power plants, power generation device for coal-fired power plants, gas separation device, gas filtration device, heat exchanger assembly, and extraction pump; 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 gas extraction pipe passes through the extraction plug and extends into the extraction well, and the other end of the gas extraction pipe is connected to the inlet of the extraction pump; the extraction pump is connected to the heat exchanger assembly via pipelines. The gas filtration device and the gas separation device are connected. The CH4 gas passing through the gas separation device is connected to the inlet of the coal-fired power plant's power generation unit and the combustion and explosion gas storage tank through the CH4 delivery pipeline. The hot flue gas entering the gas separation device and the hot flue gas entering 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 delivery 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. Explosive fracturing of unmineable coal seams: Before fracturing, use... 2 H and 18 O stable isotope labeling of the explosive gas in the storage tank, wherein the explosive gas is used 2 H-labeled CH4 gas, 18 O is marked as O2 gas; used 13 C and 15 Nitrogen stable isotope labeling of hot flue gas in a hot flue gas storage tank, wherein... 13 C is used to label CO2 gas. 15 N-labeled N2 gas; at the start of fracturing, the combustion gas injection pump is activated and the solenoid valve of the combustion gas inlet pipe is opened, allowing the specific isotope-labeled combustion gas to be continuously injected into the combustion-explosion confined space through the combustion gas injection pipe and the combustion gas inlet pipe. Multi-parameter monitoring sensors are used to measure the combustion gas pressure, CH4 gas concentration, and temperature in the combustion-explosion confined space in real time. When the parameters in the combustion-explosion confined space reach the combustion-explosion condition threshold, the combustion gas injection pump is stopped and the solenoid valve of the combustion gas inlet pipe is closed. Then, the igniter is activated, causing the injected combustion gas in the combustion-explosion confined space to undergo an in-situ combustion-explosion reaction, producing a large amount of... 18 The CO2 gas labeled with O, the transient shock wave generated by the combustion and explosion reaction and the high temperature and high pressure gas act on the surrounding coal body to produce a large number of cracks, and then 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. D. Hot flue gas injection, storage, and CH4 gas desorption: The hot flue gas injection pump and the solenoid valve of the hot flue gas inlet pipe are opened. Hot flue gas labeled with a specific isotope is injected from the hot flue gas storage tank into the combustion-explosion confined space through the hot flue gas injection pipe and the hot flue gas inlet pipe. The high temperature of the hot flue gas itself promotes the desorption of CH4 gas adsorbed in the unminable coal seam. Simultaneously, the CH4 gas in the hot flue gas... 13 C-labeled CO2, SO2 and NO2, due to their competitive adsorption advantage, cause unminable coal seams to adsorb hot flue gas and precipitate CH4 gas through displacement, thus achieving a large output of CH4 gas from unminable coal seams, which improves the CH4 mining efficiency of coal seams while sealing hot flue gas. E. 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 well through the gas extraction pipe. The mixed gas then passes through a heat exchanger group, where its temperature is lowered to room temperature. The mixed gas first passes through a gas filtration device to remove other gas impurities. The treated mixed gas then enters a gas separation device, which separates the treated extracted mixed gas into the main components of hot flue gas and CH4 gas. A portion of the separated CH4 gas... CH4 gas is injected into the coal-fired power plant's generating unit through a CH4 gas delivery pipeline to mix with coal for combustion and power generation, improving efficiency. The remaining separated CH4 gas is injected into a combustion and explosion gas storage tank to mix with an accelerant to generate combustion and explosion gas for subsequent combustion and explosion fracturing. The main components of the separated hot flue gas and the hot flue gas from the hot flue gas emission device are both injected into the hot flue gas storage tank through a hot flue gas delivery pipeline to mix. Finally, the hot flue gas is injected into the fracturing coal seam through a hot flue gas injection pump to continuously store the hot flue gas and displace the remaining CH4 gas in the coal seam. F. Isotope Tracing Monitoring, CH4 Gas Recirculation Extraction, and Hot Flue Gas Recirculation and Seizure: During the hot flue gas seizure and CH4 gas extraction from the displaced coal seam, isotope analysis of the separated gas is performed at regular intervals to determine the CO2 content. 18 CO2 labeled with O 13 C-labeled CO2, N2 15 N-labeled N2, CH4, and 2 The relative contents of each H-labeled CH4 are used to determine the source and evolution of the extracted mixed gas. The migration of combustion and explosion gases, hot flue gas, and extracted CH4 gas in the unminable coal seam is acquired in real time, and the interaction between combustion and explosion gases, hot flue gas, and coal seam CH4 is inverted. When the relative content of coal seam CH4 gas in the mixed gas decreases to below the critical extraction CH4 concentration, steps B to E are repeated to continuously perform multiple combustion and explosion fracturing, hot flue gas sealing, and CH4 extraction processes in a closed loop until the extraction operation is completed. The combustion and explosion gas injection pipe, hot flue gas injection pipe, and gas extraction pipe are removed from the vertical shaft and extraction well, respectively, and the vertical shaft and extraction well are sealed to seal the large amount of hot flue gas adsorbed in the unminable coal seam.

2. The closed-loop extraction and isotope tracing method for coal-bed methane by hot flue gas according to claim 1, characterized in that, The multi-parameter monitoring sensor includes a pressure sensor, a gas concentration sensor, and a temperature sensor. The pressure sensor monitors the pressure of the flammable gas in the flammable and explosive confined space in real time; the gas concentration sensor monitors the concentration of CH4 gas in the flammable and explosive confined space in real time; and the temperature sensor monitors the temperature in the flammable and explosive confined space in real time.

3. The closed-loop extraction and isotope tracing method for displacing coal seam methane with hot flue gas according to claim 1, characterized in that, The vertical shaft is equipped with a casing, which is sealed to the inner wall of the vertical shaft. The horizontal shaft is equipped with a screen pipe with multiple mesh openings. The ground distance between the vertical shaft and the extraction well does not exceed 2000m.

4. The closed-loop extraction and isotope tracing method of coal-bed methane by hot flue gas according to claim 1, characterized in that, The main components of the hot flue gas include CO2, SO2, NO2, N2, and water vapor.

5. The closed-loop extraction and isotope tracing method for displacing coal seam methane with hot flue gas 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%~20%.

6. The closed-loop extraction and isotope tracing method for displacing coal seam methane with hot flue gas according to claim 1, characterized in that, The critical extraction CH4 concentration is 20%.

Images