A method for hot flue gas displacement combustion and explosion fracturing coal seam methane extraction and closed loop carbon sequestration

Abstract

This invention discloses a method for methane extraction and closed-loop carbon fixation in coal seams using hot flue gas-driven combustion-explosion fracturing. First, each horizontal branch well is divided into sections, and perforations are constructed in each section. Then, CH4 is used in each section. 4 Combustion-explosive fracturing technology utilizes in-situ synergistic combustion-explosion reactions to generate gas impact perforation fractures, allowing them to further develop and expand into a fracture network. Then, hot flue gas is sequentially injected into each horizontal branch well. Due to its high temperature, the hot flue gas promotes CH4 (chemical oxygen demand) in the coal seam. 4 Gas desorption, and CO in hot flue gas 2 SO 2 NO 2 Due to its competitive adsorption advantage, it can displace a large amount of CH4 adsorbed in extra-thick coal seams. 4 Gas, then hot flue gas and CH 4 The gas continues to be utilized in a closed loop, improving the efficiency of coal seam methane extraction while simultaneously achieving hot flue gas sequestration. Through this continuous closed-loop utilization process, the efficiency of methane combustion and fracturing, hot flue gas sequestration, and CH4 extraction in various areas of extra-thick coal seams is maximized. 4 Extraction efficiency.

Description

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

[0001] This invention relates to a method for hot flue gas storage and CH4 extraction, specifically a method for extracting methane from coal seams by hot flue gas displacement combustion and detonation fracturing and for closed-loop carbon fixation, belonging to the technical field of carbon fixation and CH4 extraction. 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 range for thermal gas sequestration and CH4 extraction. Currently, hydraulic fracturing technology cannot achieve closed-loop construction in thermal gas sequestration and CH4 extraction from 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 thermal gas sequestration and CH4 extraction can be carried out, making a closed-loop reuse process impossible. Combustion-explosive fracturing technology utilizes in-situ desorption of CH4 from the coal seam. The high-temperature, high-pressure gas generated by the synergistic combustion of O2 and CH4 impacts and fracturing deep, unminable coal seams, thereby efficiently creating a three-dimensional fracture network and greatly increasing the thermal gas sequestration and CH4 extraction rates in deep, unminable coal seams. However, there is currently no research combining combustion-explosive fracturing with thermal gas sequestration and CH4 extraction from unminable coal seams.

[0003] Therefore, to address the shortcomings of current methods for hot flue gas sequestration and CH4 extraction in deep, extra-thick coal seams, a new approach is proposed. This approach combines combustion-explosion fracturing and hot flue gas sequestration technologies to achieve efficient closed-loop extraction of CH4 in vast, unminable deep coal seams. This achieves efficient closed-loop utilization of combustion-explosion fracturing, hot flue gas sequestration, and extracted CH4 resources throughout the entire process of "combustion-explosion fracturing - hot flue gas sequestration - methane extraction," thereby reducing the cost of deep, unminable coalbed methane extraction, maximizing the utilization of resources in horizontally vast, difficult-to-mine or unminable coal seams, and improving the efficiency of hot flue gas sequestration and CH4 extraction in deep, extra-thick, unminable coal seams. This provides 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 method for methane extraction and closed-loop carbon fixation in coal seams using hot flue gas displacement combustion-explosion fracturing. By combining combustion-explosion fracturing technology and hot flue gas storage technology, this method simultaneously achieves efficient closed-loop extraction of CH4 from extra-thick coal seams, thereby reducing the cost of deep, unminable coalbed methane extraction, maximizing the utilization of deep, difficult-to-mine or unminable coal seam resources, and improving the efficiency of hot flue gas storage and CH4 extraction in deep, unminable, extra-thick coal seams.

[0005] To achieve the above objectives, the technical solution adopted by this invention is: a method for methane extraction and closed-loop carbon fixation in coal seams driven by hot flue gas displacement combustion and fracturing, the specific steps of which are as follows:

[0006] A. Drilling construction: First, determine the location of the extra-thick, difficult-to-mine, and unmineable coal seams. Then, construct vertical shafts and extraction wells from the ground through the rock strata into the coal seam. After the vertical shafts are formed, use a directional drilling rig to drill a horizontal branch well from the vertical shaft at different depths in the coal seam, forming multiple horizontal branch wells at different depths. Drilling work is stopped after completion.

[0007] B. Deployment of the hot flue gas injection and coalbed methane extraction system: An extraction plug is installed at the wellhead of the extraction well to seal it; on the surface, a combustible gas storage tank, a combustible gas injection pump, a hot flue gas injection pump, a hot flue gas storage tank, a coal-fired power plant hot flue gas emission device, a coal-fired power plant power generation device, a gas separation device, a gas filtration device, a heat exchanger assembly, and an extraction pump are installed sequentially; the combustible gas storage tank is connected to the inlet of the combustible gas injection pump, and the outlet of the combustible gas injection pump is connected to one end of the combustible gas injection pipe; one end of the extraction pipe passes through the extraction plug and extends into the extraction well, and the other end of the extraction pipe is connected to... The inlet of the extraction pump is connected, and the extraction pump is connected in sequence to the heat exchanger group, gas filter and gas separator through pipelines. The CH4 gas passing through the gas separator is connected to the inlet of the coal-fired power plant generator and the combustion and explosion gas storage tank through the CH4 delivery pipeline. The hot flue gas entering the gas separator and the hot flue gas of the coal-fired power plant 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 one end of the hot flue gas injection pipe, completing the system layout.

[0008] C. Deploying a multi-stage combustion-explosive gas injection system and section combustion-explosive fracturing in horizontal branch wells: First, select a horizontal branch well, then divide it into multiple sections at equal intervals. Insert a perforating gun from the vertical shaft into the horizontal branch well, and sequentially create multiple perforated fractures perpendicular to the horizontal branch well in each section using the perforating gun. After completion, retrieve the perforating gun. Next, deploy a multi-stage combustion-explosive gas injection system at the deepest section of the horizontal branch well. This system includes a gas injection plug, multi-parameter monitoring sensors, an ignition head, and an inlet pipe. The gas injection plug is fixed at the connection between the deepest section and its adjacent sections, ensuring that the deepest section... The section forms a closed space, with the two ends of the air inlet pipe located on either side of the gas injection plugger. The end inside the closed space is equipped with a solenoid valve. The gas injection plugger inside the closed 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. The other end of the combustion gas injection pipe is extended through a vertical shaft into the horizontal branch well and connected to the air inlet pipe outside the closed space, completing the multi-stage combustion gas injection system setup for this section. When fracturing begins, the combustion gas injection pump is started first, and the solenoid valve is opened to initiate combustion. Gas is injected into a sealed space through the combustion gas injection pipe and the inlet pipe. Multi-parameter monitoring sensors continuously measure the pressure of the combustion gas and the concentration of CH4 gas within the sealed space. When the parameters within the sealed space reach the combustion and explosion threshold, the combustion gas injection pump is stopped and the solenoid valve is closed. Then, the igniter is activated, causing the injected combustion gas in the sealed space to undergo an in-situ combustion and explosion reaction. The transient shock wave and high-temperature, high-pressure gas generated by the combustion and explosion reaction act on the surrounding coal seam, causing the perforated fractures in the current section to expand and develop under impact. This further fractures the deeper parts of the coal seam, desorbing a large amount of CH4 gas, which then continues to react with O2 gas. A reaction occurs, creating a pulse fracturing effect in the coal seam. The process continues until the combustion reaction in that section ends. The multi-stage combustion injection system for that section is retained, and its connection to the other end of the combustion gas injection pipe is disconnected. Another multi-stage combustion injection system is then installed in the next section along the horizontal branch drilling, and the section combustion fracturing process is repeated. This process is repeated multiple times to achieve sequential combustion fracturing of each section until the fracturing process of all sections of the horizontal branch drilling is completed. After completion, only the multi-stage combustion injection system at the connection between the horizontal branch drilling and the vertical shaft is retained, and the multi-stage combustion injection systems in the remaining sections are removed.

[0009] D. Overall combustion and explosive fracturing of the coal seam: Select another horizontal branch well and repeat the fracturing process in step C. Repeat this process multiple times until combustion and explosive fracturing is completed for each horizontal branch well, thus achieving overall combustion and explosive fracturing of the coal seam.

[0010] E. Hot flue gas injection and storage, and CH4 gas desorption: First, select a horizontal branch well and connect the other end of the hot flue gas injection pipe to the intake pipe of the multi-stage combustion and explosion gas injection system of that horizontal branch well through a vertical shaft; turn on the hot flue gas injection pump and open the solenoid valve of the intake pipe to transport the hot flue gas in the hot flue gas storage tank to the horizontal branch well through the hot flue gas injection pipe. The high temperature of the hot flue gas itself can promote the desorption of CH4 gas adsorbed by the coal seam around each horizontal branch well. At the same time, CO2, SO2 and NO2 gases in the hot flue gas, due to their competitive adsorption advantage, cause the coal seam around each horizontal branch well to adsorb the hot flue gas and precipitate CH4 gas through displacement. This improves the CH4 mining efficiency of the coal seam while storing the hot flue gas. During the injection process, multi-parameter monitoring sensors measure in real time the pressure, CH4 concentration, and temperature of the main components of the hot flue gas and the CH4 mixture in the horizontal branch well. The injection parameters of the hot flue gas injected into the horizontal branch well are dynamically adjusted to ensure continuous and efficient injection of hot flue gas. This continues until a sudden change occurs in the monitored hot flue gas and CH4 concentration and pressure values, indicating that the injection volume of hot flue gas has reached the injection limit of the coal seam surrounding the horizontal branch well. The hot flue gas injection pump is then shut down, and the other end of the hot flue gas injection pipe is disconnected. Then, another horizontal branch well is selected, and the other end of the hot flue gas injection pipe is connected to it. The hot flue gas injection process is repeated in this way until the hot flue gas injection process of all horizontal branch wells is completed.

[0011] F. CH4 Gas Extraction and Hot Flue Gas Sequestration: After a waiting period, the extraction pump is started to extract the mixed gas from the extraction well through the gas extraction pipe. The source of this mixed gas includes some CO2 gas generated during the combustion-explosion fracturing process, residual CO2, SO2, NO2, and N2 from the hot flue gas sequestration and displacement process, and the original CO2, SO2, and N2 present in the coal seam. The CH4 in the extracted mixed gas originates from the CH4 components that did not undergo combustion-explosion during the combustion-explosion fracturing process, as well as the CH4 gas displaced by the hot flue gas in the coal seam. The mixed gas is then passed through a heat exchanger group, where its temperature is lowered to ambient temperature. The extracted mixed gas is first filtered by a gas filtration device to remove the extracted mixed gas. Other gaseous impurities in the gas are treated to meet standards before the extracted mixed gas enters a gas separation device. The gas separation device separates the treated extracted mixed gas into the main component gas of hot flue gas and CH4 gas. A portion of the separated CH4 gas is injected into the power generation unit of a coal-fired power plant through the CH4 gas delivery pipeline to mix with coal for combustion and power generation to improve efficiency. The remaining separated CH4 gas is injected into the combustion and explosion gas storage tank to mix with the combustion aid (i.e., O2 gas) to generate combustion and explosion gas for subsequent combustion and explosion fracturing. The separated main component gas of 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 the hot flue gas delivery pipeline for mixing and subsequent hot flue gas injection.

[0012] G. When the relative content of CH4 gas in the coal seam in the mixed gas decreases to below the critical extraction CH4 concentration, repeat steps B to F to continuously perform multiple combustion and fracturing, hot flue gas sealing and CH4 extraction processes in a closed loop until the hot flue gas sealing amount of each horizontal branch well reaches the set value or above, and the hot flue gas sealing and CH4 extraction operations are completed.

[0013] Furthermore, the multi-parameter monitoring sensor includes a pressure sensor, a gas concentration sensor, and a temperature sensor. The pressure sensor monitors the pressure changes of the combustion and explosive gases and hot flue gas in the confined space in real time; the gas concentration sensor monitors the concentration changes of the main component gases and CH4 gas in the flue gas in the confined space in real time; and the temperature sensor monitors the temperature changes in the confined space in real time.

[0014] Furthermore, a casing is installed inside the vertical shaft, and the casing is sealed to the inner wall of the vertical shaft. Each horizontal branch well is equipped with a combination of screen pipe and casing, wherein the casing is installed at the connection between the horizontal branch well and the vertical shaft, and the remaining part is equipped with screen pipe.

[0015] Furthermore, the injected combustion gas is a mixture of CH4 and O2; the main components of the hot flue gas include CO2, SO2, NO2, N2 and H2O(g).

[0016] Furthermore, the injection parameters include gas injection pressure and gas injection flow rate.

[0017] Furthermore, the explosion condition thresholds are: the explosion gas pressure threshold is 10 MPa, and the explosion CH4 concentration threshold is 5%~20%.

[0018] Furthermore, the critical extraction CH4 concentration is 10-20%.

[0019] Compared with existing technologies, this invention employs a combination of combustion-explosive fracturing of extra-thick coal seams and hot flue gas displacement of CH4 gas in the coal seam, along with the sealing of CO2, SO2, and NO2 gases in the hot flue gas, which has the following advantages:

[0020] (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 extra-thick coal seams to impact and fracturing each horizontal branch well to form a large number of initial fractures. At the same time, the CO2 gas generated by the high temperature and high pressure explosion generated by the combustion and explosion reaction continues to wed 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 migration channel for the storage of hot flue gas and CH4 extraction in extra-thick coal seams.

[0021] (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 extra-thick coal seams, thus improving the efficiency of coal seam methane extraction while achieving hot flue gas sequestration; and simultaneously improving the efficiency of coal seam methane extraction while achieving the large-scale sequestration of greenhouse gases and toxic and harmful gases in deep coal seams. The presence of H2O(g) in the hot flue gas eliminates the need for desulfurization and denitrification in coal-fired power plants, saving related costs. In addition, H2O(g) in the hot flue gas acts as a good heat transfer medium, which can greatly reduce heat loss during the transportation of 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 heat energy in the hot flue gas through its high specific heat capacity during transportation, 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 a heat insulation layer, further reducing the heat conduction and radiation heat loss of the hot flue gas.

[0022] (3) 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 (i.e., O2 gas) 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

[0023] Figure 1 is a schematic diagram of the overall layout of the present invention.

[0024] Figure 2 is an axial sectional view of the multi-stage combustion and explosion gas injection system in Figure 1.

[0025] In the diagram: 1-Rock strata; 2-Coal seam; 3-Vertical shaft; 4-Drainage well; 5-Horizontal branch well; 6-Drainage plugging device; 7-Explosive gas storage tank; 8-Explosive gas injection pump; 9-Hot flue gas injection pump; 10-Hot flue gas storage tank; 11-Hot flue gas emission device; 12-Coal-fired power plant generator; 13-Gas separation device; 14-Gas filtration device; 15-Heat exchanger group; 16-Drainage pump; 17-Explosive gas injection pipe; 18-CH4 gas delivery pipeline; 19-Hot flue gas delivery pipeline; 20-Hot flue gas injection pipe; 21-Casing; 22-Screen pipe; 23-Perforation fracture; 24-Injection plugging device; 25-Multi-parameter monitoring sensor; 26-Ignition head; 27-Inlet pipe; 28-Solenoid valve; 29-Multi-parameter monitoring data transmission line; 30-Ground control center. Detailed Implementation

[0026] The present invention will be further described below.

[0027] As shown in Figure 1, the specific steps of this invention are as follows:

[0028] A. Drilling Construction: First, determine the location of the extra-thick, difficult-to-mine, and unmineable coal seams. Then, construct vertical shafts 3 and extraction shafts 4 from the surface through rock strata 1 to coal seam 2. After the vertical shafts 3 are formed, use a directional drilling rig to drill a horizontal branch well 5 at different depths in coal seam 2 from the vertical shafts 3, forming multiple horizontal branch wells 5 at different depths. Casing 21 is installed inside the vertical shafts 3, and the casing 21 is sealed to the inner wall of the vertical shafts 3. Screen pipes 22 and casing 21 are arranged in each horizontal branch well 5. The casing 21 is installed at the connection between the horizontal branch well 5 and the vertical shaft 3, and the remaining part is installed with screen pipes 22. After completion, drilling work is stopped.

[0029] B. Deployment of Hot Flue Gas Injection and Coalbed Methane Extraction System: An extraction plug 6 is installed at the wellhead of extraction well 4 to seal it. On the surface, the following components are sequentially installed: a combustible gas storage tank 7, a combustible gas injection pump 8, a hot flue gas injection pump 9, a hot flue gas storage tank 10, a coal-fired power plant hot flue gas emission device 11, a coal-fired power plant power generation device 12, a gas separation device 13, a gas filtration device 14, a heat exchanger assembly 15, and an extraction pump 16. The combustible gas storage tank 7 is connected to the inlet of the combustible gas injection pump 8, and the outlet of the combustible gas injection pump 8 is connected to one end of the combustible gas injection pipe 17. One end of the extraction pipe passes through the extraction plug 6 and extends into the extraction well 4; the other end of the extraction pipe is connected to the extraction pipe... The inlet of the extraction pump 16 is connected to the heat exchanger group 15, the gas filter device 14 and the gas separator device 13 in sequence through pipelines. The CH4 gas passing through the gas separator device 13 is connected to the inlet of the coal-fired power plant generator 12 and the combustion and explosion gas storage tank 8 through the CH4 delivery pipeline 18. The hot flue gas entering the gas separator device 13 and the hot flue gas of the coal-fired power plant tail gas treatment system are both connected to the inlet of the hot flue gas storage tank 10 through the hot flue gas delivery pipeline 19. The outlet of the hot flue gas storage tank 10 is connected to the inlet of the hot flue gas injection pump 9, and the outlet of the hot flue gas injection pump 9 is connected to one end of the hot flue gas injection pipe 20, thus completing the system layout.

[0030] C. Deploying a multi-stage combustion-explosive gas injection system and section combustion-explosive fracturing of horizontal branch wells: First, select a horizontal branch well 5, and then divide it into multiple sections at equal intervals. Insert a perforating gun from the vertical shaft 3 into the horizontal branch well 5, and use the perforating gun to sequentially form multiple perforated fractures 23 perpendicular to the horizontal branch well 5 in each section. After completion, retrieve the perforating gun. Next, deploy a multi-stage combustion-explosive gas injection system at the deepest section of the horizontal branch well 5, as shown in Figure 2. This system includes a gas injection plug 24, a multi-parameter monitoring sensor 25, an ignition head 26, and an air inlet pipe 27. The gas injection plug 24 is fixed at the connection between the deepest section and its adjacent sections, creating a sealed space in the deepest section. The two ends of the air inlet pipe 27 are respectively located at... On both sides of the gas injection plug 24, the end inside the sealed space is equipped with a solenoid valve 28; a multi-parameter monitoring sensor 25 and an ignition head 26 are installed on the gas injection plug 24 inside the sealed space; both the multi-parameter monitoring sensor 25 and the ignition head 26 are connected to the ground control center 30 through a multi-parameter monitoring data transmission line 29; the multi-parameter monitoring sensor 25 includes a pressure sensor, a gas concentration sensor and a temperature sensor, wherein the pressure sensor monitors the pressure changes of the combustion and explosive gases and hot flue gas in the sealed space in real time; the gas concentration sensor is used to monitor the concentration changes of the main component gases of the flue gas and CH4 gas in the sealed space in real time; and the temperature sensor is used to monitor the temperature changes in the sealed space in real time.The other end of the combustion gas injection pipe is extended through a vertical shaft into the horizontal branch well and connected to the intake pipe outside the confined space, completing the multi-stage combustion gas injection system layout for this section. When fracturing begins, the combustion gas injection pump 8 is started and the solenoid valve 28 is opened, allowing the combustion gas to be injected into the confined space through the combustion gas injection pipe 17 and the intake pipe 27. The injected combustion gas is a mixture of CH4 and O2. The pressure of the combustion gas and the concentration of CH4 in the confined space are measured in real time by a multi-parameter monitoring sensor 25. When the parameters in the confined space reach the combustion and explosion thresholds, i.e., the combustion gas pressure reaches 10 MPa and the CH4 concentration reaches 15%, the combustion gas injection pump 8 is stopped and the solenoid valve 28 is closed. Then, the igniter 26 is started, causing the injected combustion gas in the confined space to undergo an in-situ combustion and explosion reaction. The transient shock wave and high-temperature, high-pressure gas generated will act on the perforation fracture 23 in the current section, causing the perforation fracture 23 to extend, develop, and expand further, thereby fracturing the deep coal seam and desorbing a large amount of CH4 gas, which will continue to react with O2 gas to form a pulse fracturing effect in the coal seam. Until the combustion and explosion reaction in this section ends, the multi-stage combustion and explosion gas injection system in this section is retained and disconnected from the other end of the combustion and explosion gas injection pipe 17. Another multi-stage combustion and explosion gas injection system is then deployed in the next section along the horizontal branch well 5, and the section combustion and explosion fracturing process of this step is repeated. This process is repeated many times to achieve sequential combustion and explosion fracturing of each section until the fracturing process of all sections of the horizontal branch well 5 is completed. After completion, only the multi-stage combustion and explosion gas injection system at the connection between the horizontal branch well 5 and the vertical shaft 3 is retained, and the multi-stage combustion and explosion gas injection systems of the other sections are removed.

[0031] D. Overall combustion and explosion fracturing of the coal seam: Select another horizontal branch well 5 and repeat the fracturing process in step C. Repeat this process multiple times until combustion and explosion fracturing is completed in each horizontal branch well 5, thus realizing the overall combustion and explosion fracturing process of the coal seam.

[0032] E. Hot flue gas injection, storage, and CH4 gas desorption: First, select a horizontal branch well 5, and connect the other end of the hot flue gas injection pipe 20 to the inlet pipe 27 of the multi-stage combustion and explosion gas injection system of the horizontal branch well 5 through the vertical shaft 3; turn on the hot flue gas injection pump 9 and open the solenoid valve 28 of the inlet pipe 27 to transport the hot flue gas in the hot flue gas storage tank 10 to the horizontal branch well 5 through the hot flue gas injection pipe 20. The main components of the hot flue gas include CO2 gas, SO2 gas, NO2 gas, N2 gas, and H2O(g). The high temperature of the hot flue gas promotes the desorption of CH4 gas adsorbed by the coal seam around each horizontal branch well 5. Simultaneously, the competitive adsorption of CO2, SO2, and NO2 gases in the hot flue gas leads to the adsorption of hot flue gas and the release of CH4 gas by the coal seam around each horizontal branch well 5 through displacement, thus sealing the hot flue gas and improving the CH4 mining efficiency of the coal seam. During the injection process, multi-parameter monitoring sensors 25 measure in real time the pressure, CH4 concentration, and temperature of the main components of the hot flue gas and the CH4 mixture within the horizontal branch well 5, and dynamically adjust the injection of the hot flue gas into the horizontal branch well. The hot flue gas injection parameters of well 5 (including gas injection pressure and gas injection flow rate) are used to ensure that the hot flue gas can be injected continuously and efficiently until the monitored hot flue gas and CH4 concentration and pressure values ​​change abruptly, indicating that the hot flue gas injection volume has reached the injection limit of the coal seam around the horizontal branch well 5. Then, the hot flue gas injection pump 9 is shut down and the other end of the hot flue gas injection pipe 20 is disconnected. Then, another horizontal branch well 5 is selected, and the other end of the hot flue gas injection pipe 20 is connected to it. The hot flue gas injection process of this step is repeated. This process is repeated many times until the hot flue gas injection process of all horizontal branch wells 5 is completed.

[0033] F. CH4 Gas Extraction and Hot Flue Gas Sequestration: After a period of waiting, the extraction pump 16 is started to extract the mixed gas from the extraction well 4 through the gas extraction pipe. The source of this mixed gas includes some CO2 gas generated during the combustion-explosion fracturing process, residual CO2, SO2, NO2, and N2 from the hot flue gas sequestration and displacement process, and the original CO2, SO2, and N2 present in the coal seam. The CH4 in the extracted mixed gas originates from the CH4 component that did not undergo combustion-explosion during the combustion-explosion fracturing process and the CH4 gas displaced by the hot flue gas in the coal seam. The mixed gas is then passed through heat exchanger group 15, where the temperature of the mixed gas is reduced to ambient temperature after heat exchange. The extracted mixed gas first passes through gas filtration device 14 to remove other components from the extracted mixed gas. After processing to meet standards, the extracted mixed gas, free of impurities, enters the gas separation device 13. The gas separation device 13 separates the extracted mixed gas into the main component gas of hot flue gas and CH4 gas. A portion of the separated CH4 gas is injected into the coal-fired power plant generator 12 through the CH4 gas delivery pipeline 18 to mix with coal for combustion and power generation, thereby improving efficiency. The remaining separated CH4 gas is injected into the combustion and explosion gas storage tank 7 to mix with the combustion aid (i.e., O2 gas) to generate combustion and explosion gas for subsequent combustion and explosion fracturing. The separated main component gas of hot flue gas and the hot flue gas from the hot flue gas emission device 11 are both injected into the hot flue gas storage tank 10 through the hot flue gas delivery pipeline 19 for mixing and subsequent injection of hot flue gas.

[0034] G. When the relative content of CH4 gas in the coal seam in the mixed gas is reduced to below 10% of the critical extraction CH4 concentration, repeat steps B to F to continuously perform multiple combustion and fracturing, hot flue gas sealing and CH4 extraction processes in a closed loop until the hot flue gas sealing amount of each horizontal branch well 5 reaches the set value or above, and the hot flue gas sealing and CH4 extraction operations are completed.

[0035] 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 method for methane extraction and closed-loop carbon fixation in coal seams subjected to thermal flue gas displacement combustion-explosion fracturing, characterized in that, The specific steps are as follows: A. Drilling construction: First, determine the location of the extra-thick, difficult-to-mine, and unmineable coal seams. Then, drill vertical shafts and extraction wells from the surface through the rock strata into the coal seam. After the vertical shafts are formed, use a directional drilling rig to drill a horizontal branch well at different depths in the coal seam, forming multiple horizontal branch wells at different depths. Drilling work is stopped after completion. B. Deployment of hot flue gas injection and coalbed methane extraction systems: Install extraction plugs at the wellheads of the extraction wells to seal them. On the surface, sequentially install combustion and explosion gas storage tanks, combustion and explosion gas injection pumps, hot flue gas injection pumps, hot flue gas storage tanks, and coal-fired power plants. The system includes a flue gas emission device, a coal-fired power plant generator, a gas separation device, a gas filtration device, a heat exchanger assembly, and an extraction pump. The inlet of the explosive gas storage tank is connected to the explosive gas injection pump, and the outlet of the explosive gas injection pump is connected to one end of the explosive gas injection pipe. One end of the extraction pipe passes through an extraction plug and extends into the extraction well; the other end of the extraction pipe is connected to the inlet of the extraction pump. The extraction pump is connected sequentially to the heat exchanger assembly, the gas filtration device, and the gas separation device via pipelines. CH4 gas passing through the gas separation device is connected to the inlets of the coal-fired power plant generator and the explosive gas storage tank via CH4 gas delivery pipelines. The hot flue gas from the separation unit and the hot flue gas from the coal-fired power plant's hot flue gas emission unit are both connected to the inlet of the hot flue gas storage tank via hot flue gas delivery pipelines; 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 one end of the hot flue gas injection pipe, completing the system layout; C. Deploying a multi-stage combustion and explosion gas injection system and section combustion and explosion fracturing of horizontal branch wells: First, select a horizontal branch well, then divide it into multiple sections at equal intervals. Extend the perforating gun from the vertical shaft into the horizontal branch well, and use the perforating gun to sequentially form multiple perforated fractures perpendicular to the horizontal branch well in each section. After completion, the perforating gun is retrieved; then, a multi-stage combustion and explosion gas injection system is deployed in the deepest section of the horizontal branch drilling, which includes a gas injection plug, a multi-parameter monitoring sensor, an ignition head, and an air inlet pipe. The gas injection plug is fixed at the connection between the deepest section and its adjacent sections, forming a sealed space in the deepest section. The two ends of the air inlet pipe are located on both sides of the gas injection plug, with a solenoid valve installed at the end inside the sealed space; the gas injection plug inside the sealed 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 through a multi-parameter monitoring data transmission line.The other end of the flammable gas injection pipe is extended through a vertical shaft into the horizontal branch well and connected to the intake pipe located outside the confined space, completing the multi-stage flammable gas injection system setup for this section. At the start of fracturing, the flammable gas injection pump is first started and the solenoid valve is opened, allowing flammable gas to be injected into the confined space through the flammable gas injection pipe and the intake pipe. Multi-parameter monitoring sensors are used to measure the flammable gas pressure and CH4 gas concentration within the confined space in real time. When all parameters within the confined space reach the flammability threshold, the flammable gas injection pump is stopped and the solenoid valve is closed. Then, the ignition head is activated, injecting flammable gas into the confined space. The introduced combustible gas undergoes an in-situ combustion and explosion reaction. The transient shock wave and high-temperature, high-pressure gas generated by the combustion and explosion reaction act on the surrounding coal seam, causing the perforated fractures in the current section to expand and develop under impact. This leads to the desorption of a large amount of CH4 gas from the deeper parts of the coal seam, which continues to react with O2 gas, thus creating a pulse fracturing effect in the coal seam. This process continues until the combustion and explosion reaction in this section ends. The multi-stage combustion and explosion gas injection system for this section is retained, and the connection to the other end of the combustion and explosion gas injection pipe is disconnected. Another multi-stage combustion and explosion gas injection system is then installed in the next section along this horizontal branch, and the section combustion and explosion fracturing process of this step is repeated. This process is repeated multiple times to achieve... The process involves sequentially performing combustion-explosive fracturing on each section until all sections of the horizontal branch well are fracturing. After completion, only the multi-stage combustion-explosive gas injection system at the connection between the horizontal branch well and the vertical shaft is retained, while the multi-stage combustion-explosive gas injection systems in the remaining sections are dismantled. D. Overall combustion-explosive fracturing of the coal seam: Select another horizontal branch well and repeat the fracturing process in step C. This process is repeated multiple times until combustion-explosive fracturing is completed on all horizontal branch wells, thus achieving overall combustion-explosive fracturing of the coal seam. E. Hot flue gas injection and storage, and CH4 gas desorption: First, select one horizontal branch well and inject hot flue gas into the other... The end is connected to the intake pipe of the multi-stage combustion and explosion gas injection system of the horizontal branch well through the vertical shaft; the hot flue gas injection pump is turned on and the solenoid valve of the intake pipe is opened, and the hot flue gas in the hot flue gas storage tank is transported to the horizontal branch well through the hot flue gas injection pipe. The high temperature of the hot flue gas itself can promote the desorption of CH4 gas adsorbed by the coal seam around each horizontal branch well. At the same time, CO2 gas, SO2 gas and NO2 gas in the hot flue gas have competitive adsorption advantages, and through the displacement effect, the coal seam around each horizontal branch well adsorbs the hot flue gas and precipitates CH4 gas, thereby improving the CH4 mining efficiency of the coal seam while sealing the hot flue gas.During the injection process, multi-parameter monitoring sensors continuously measure the pressure, CH4 concentration, and temperature of the main components of the hot flue gas and the CH4 mixture within the horizontal branch well. The injection parameters are dynamically adjusted to ensure continuous and efficient injection of the hot flue gas. This continues until a sudden change occurs in the monitored hot flue gas and CH4 concentration and pressure values, indicating that the injection volume has reached the injection limit of the coal seam surrounding the horizontal branch well. The hot flue gas injection pump is then shut off, and the other end of the hot flue gas injection pipe is disconnected. Next, another horizontal branch well is selected, and the other end of the hot flue gas injection pipe is connected to it. This process is repeated multiple times until the hot flue gas injection process is completed for all horizontal branch wells. F. CH4 Gas Extraction and Hot Flue Gas Storage: After a period of time, the extraction pump is started, and the mixed gas is extracted from the extraction well through the extraction pipe. The mixed gas is then passed through a heat exchanger group, where the temperature is lowered to room temperature after heat exchange. The mixed gas is first filtered by a gas filtration device to remove the mixed gas. Other gaseous impurities in the gas mixture are removed, and the treated gas enters a gas separation device. The gas separation device separates the treated 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 through a CH4 gas 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 separated main components of the hot flue gas and the hot flue gas from the coal-fired power plant's flue gas emission device are both injected into the hot flue gas storage tank through a hot flue gas pipeline for subsequent hot flue gas injection. G. When the relative content of coal seam CH4 gas in the mixed gas decreases below the critical extraction CH4 concentration, steps B to F are repeated, continuously performing multiple combustion and explosion fracturing, hot flue gas sealing, and CH4 extraction processes in a closed loop until the hot flue gas sealing volume of each horizontal branch well reaches or exceeds the set value, completing the hot flue gas sealing and CH4 extraction operation.

2. The method for methane extraction and closed-loop carbon fixation in coal seams driven by hot flue gas displacement combustion-explosion fracturing 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 changes of the combustible gas and hot flue gas in the confined space in real time. The gas concentration sensor monitors the concentration changes of the main components of the hot flue gas and CH4 gas in the confined space in real time. The temperature sensor monitors the temperature changes in the confined space in real time.

3. The method for methane extraction and closed-loop carbon fixation in coal seams subjected to hot flue gas displacement combustion-explosion fracturing 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. Each horizontal branch well is equipped with a combination of screen pipe and casing, with the casing installed at the connection between the horizontal branch well and the vertical shaft, and the remaining part equipped with screen pipe.

4. The method for methane extraction and closed-loop carbon fixation in coal seams subjected to hot flue gas displacement combustion-explosion fracturing according to claim 1, characterized in that, The injected combustion gas is a mixture of CH4 and O2; the main components of the hot flue gas include CO2, SO2, NO2, N2 and water vapor.

5. The method for methane extraction and closed-loop carbon fixation in coal seams driven by hot flue gas displacement combustion-explosion fracturing according to claim 1, characterized in that, The injection parameters include gas injection pressure and gas injection flow rate.

6. The method for methane extraction and closed-loop carbon fixation in coal seams driven by hot flue gas displacement combustion-explosion fracturing 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%.

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

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