System and method for cyclic injection of coal-bed methane into coal seams using staged heating of small quantities of coal-bed methane

Abstract

This application provides a system and method for extracting coalbed methane (CBM) through cascade heating and circulating injection into coal seams, belonging to the field of CBM extraction. It solves the problems of low energy efficiency, difficulty in separating injected CO2, and water lock caused by injected steam in traditional methods. The system includes a CBM circulating injection and extraction system deployed in the target coal seam, a cascade circulating heating system, and an intelligent control system. The cascade circulating heating system includes surface preheating equipment, downhole enhanced heating equipment, and gas circulation energy enhancement equipment. The surface preheating equipment is used to initially heat the CBM before injecting it into the CBM injection wellbore, and then the downhole enhanced heating equipment performs secondary heating on the CBM. The gas circulation energy enhancement equipment is used to back-transport a portion of the CBM output from the extraction well to the cascade circulating heating system. This application achieves safe and efficient CBM extraction by constructing a three-dimensional well network, a surface and downhole cascade circulating heating system, and gas circulation energy enhancement.

Description

Technical Field

[0001] This application relates to the field of coalbed methane extraction technology, and in particular to a system and method for extracting coalbed methane by cascade heating and circulating injection of small amounts of coalbed methane into the coal seam. Background Technology

[0002] Coalbed methane (CBM), as a clean and efficient unconventional natural gas resource, plays a crucial role in low-carbon development and improving the safety of coal mine production through large-scale exploitation and utilization. Currently, the mainstream technologies for CBM extraction include drainage depressurization, hydraulic fracturing, gas displacement, and thermal injection. However, all of these technologies face technical bottlenecks under complex coal seam geological conditions. Traditional drainage depressurization relies on continuous drainage to reduce reservoir pressure. However, due to the potential for low permeability and heterogeneous regions in CBM reservoirs, gas desorption and migration become uneven, easily forming "dead zones" in the outer areas, affecting CBM recovery rates. While hydraulic fracturing can create a fracture network in the coal seam through high-pressure water injection, increasing permeability, it is prone to water-locking, which blocks coal seam pores and affects CBM desorption. Furthermore, while traditional CO2 displacement technology can improve recovery rates by adsorbing and replacing CBM, competitive adsorption can easily alter the reservoir structure. Additionally, using CO2 as a displacement gas requires off-site transportation, resulting in high costs, and it is difficult to separate CO2 from CH4. Although water vapor thermal displacement can heat underground coal seams and reduce the adsorption capacity of CH4 on the coal seam surface, the water vapor will remain in the coal seam fractures after condensation, blocking the coalbed methane seepage channels, exacerbating the water-locking effect, and long-term high temperature may damage the coal seam reservoir structure.

[0003] CH4, as the main component of coalbed methane, has a narrow explosion limit range (5%-15%), a lower density than air, and diffuses more easily after leakage. Compared to other hydrocarbon gases, it is less prone to explosion, exhibiting outstanding safety. Furthermore, CH4 can be directly obtained during the drilling process, effectively avoiding the additional costs associated with long-distance transportation. In addition, CH4 has a specific heat capacity 2.5 times that of CO2, meaning that the same volume of CH4 can carry more energy to the coal seam, thus more effectively improving coal seam desorption efficiency. However, existing heat injection technologies rely on external heat sources to heat the coal seam, resulting in low energy utilization. Summary of the Invention

[0004] To overcome the shortcomings of existing technologies, this application proposes a system and method for extracting coalbed methane by cascade heating and circulating injection into coal seams. By constructing a three-dimensional well network, a surface and underground cascade circulating heating system, and gas circulation energy enhancement, the system solves the problems of low energy efficiency of traditional electric heating, difficulty in separating injected CO2, and water lock in injected steam, thus achieving safe and efficient extraction of coalbed methane.

[0005] The technical solution adopted in this application is: a system for extracting coalbed methane by cascade heating and circulating injection of small amounts of coalbed methane into the coal seam, including a coalbed methane circulating injection and extraction system deployed in the target coal seam, a cascade circulating heating system and an intelligent control system. The coalbed methane circulating injection and extraction system includes a composite well network composed of coalbed methane injection wells, horizontal wells, coalbed methane extraction wells and circulating pipelines, and horizontal well screens are installed in the horizontal sections of the horizontal wells. The coalbed methane extraction wells are connected to the horizontal wells through a fracture network.

[0006] The cascaded circulating heating system includes a surface preheating device, an underground enhanced heating device, and a gas circulation energy enhancement device connected by circulating pipelines. The surface preheating device is used to initially heat the coalbed methane and inject it into the coalbed methane injection well. Then, the coalbed methane is heated a second time by the underground enhanced heating device. The gas circulation energy enhancement device is used to back-transport a portion of the coalbed methane output from the coalbed methane extraction well to the cascaded circulating heating system.

[0007] The intelligent control system is used to intelligently control and regulate the electrical equipment in the coalbed methane circulating injection and production system and the cascade circulating heating system.

[0008] Furthermore, the ground preheating equipment includes a pressurization device and a ground heater deployed on the ground. One end of the pressurization device is connected to the wellhead of the coalbed methane injection well through a circulation pipeline, and the other end of the pressurization device is connected to one end of the ground heater through a circulation pipeline. The ground heater is powered by a green electricity device.

[0009] Furthermore, the downhole enhanced heating equipment includes a downhole heater deployed in the wellbore of the coalbed methane injection well, wherein the downhole heater is located in the wellbore of the coalbed methane injection well where the target coal seam is located.

[0010] Furthermore, the gas circulation energy enhancement equipment includes a circulation pump, a waste heat recovery device, a drying device, and an extraction pump deployed on the ground. The other end of the ground heater is connected to the waste heat gas outlet of the waste heat recovery device through a circulation pipe. The other end of the ground heater is also connected to one end of the circulation pump through a circulation pipe. The other end of the circulation pump is connected to the inlet of the waste heat recovery device through a circulation pipe.

[0011] The outlet of the waste heat recovery device is connected to the inlet of the drying device through a pipeline, and the outlet of the drying device is connected to the coalbed methane storage equipment through a conveying pipeline; one end of the extraction pump is connected to the wellhead of the coalbed methane extraction well, and the other end of the extraction pump is connected to the circulation pipeline between the circulation pump and the waste heat recovery device.

[0012] Furthermore, the intelligent control system includes control devices deployed on the ground, various sensors arranged at different locations in the coalbed methane circulation injection and production system and the cascade circulation heating system, as well as valves with different functions installed at the wellheads of the coalbed methane injection wells and coalbed methane extraction wells. The control devices regulate the ground heaters and downhole heaters by monitoring the data from each sensor in real time.

[0013] The sensors include: temperature sensors installed in the outlet pipe section of the surface heater and inside the shell of the downhole heater; pressure sensors installed at the connecting flange of the circulation pipeline and the coalbed methane injection well; oxygen sensors installed on the circulation pipeline; CH4 concentration detectors installed between the outlet of the extraction pump and the inlet pipe of the circulation pump; and flow meters installed in the pipeline of the extraction pump.

[0014] The valves include: explosion-proof valves and high-pressure needle valves installed at the wellhead of coalbed methane injection wells; pneumatic anti-clogging ball valves installed at the wellhead of coalbed methane extraction wells; proportional control valves installed on the reverse transmission branch of the circulating pump; one-way shut-off valves installed at the inlet of the waste heat recovery device; and Hastelloy valves installed on the inlet pipeline of the drying device. The first port of the proportional control valve is connected to the circulating pump, the second port of the proportional control valve is connected to the one-way shut-off valve, and the third port of the proportional control valve is connected to the extraction pump.

[0015] Furthermore, the coalbed methane injection wellbore is equipped with a double-layer nitrogen protection chamber for protecting the downhole heater. The double-layer nitrogen protection chamber includes an inner spiral guide chamber and an outer annular gas storage chamber. The double-layer nitrogen protection chamber forms a closed loop with the surface inerting system.

[0016] Furthermore, the opening degree of the proportional control valve and the detection value of the flow meter form a feedback control loop. Based on the real-time monitoring data of the flow meter, the opening degree of the proportional control valve is dynamically adjusted to maintain the circulating coalbed methane at 10%-15% of the total coalbed methane volume. Moreover, the return flow rate of the circulating pump and the coalbed methane flow rate extracted by the extraction pump form a closed-loop interlock control.

[0017] A method for extracting coalbed methane by cascade heating and circulating injection of small amounts of coalbed methane, based on the aforementioned system for extracting coalbed methane by cascade heating and circulating injection of small amounts of coalbed methane, includes the following steps:

[0018] a. Deploy a coalbed methane recirculation injection and production system in the target coal seam section;

[0019] b. After implementing horizontal well screens on the target coal seam section of the horizontal well, a radial fracture network is formed through segmented hydraulic fracturing;

[0020] c. Vacuum the wellbore of the target coal seam section using a vacuum pump, then inject high-purity nitrogen into the wellbore. Repeat the vacuuming and nitrogen injection cycle multiple times until the residual oxygen content in the wellbore is less than 1%.

[0021] d. Utilize green electricity devices to power the primary and secondary heating processes, drive the surface heater to initially heat the coalbed methane to 100-150℃, and inject it into the target coal seam along the coalbed methane injection well; then, use the downhole heater installed in the wellbore of the target coal seam section to perform secondary heating on the coalbed methane, so that the final temperature of the coalbed methane reaches 150-250℃.

[0022] e. Use extraction pumps to extract desorbed coalbed methane, and circulate 10%-15% of the total volume of coalbed methane through circulation pumps. After being pressurized by a booster device, the coalbed methane is returned to the coalbed methane circulation injection and extraction system.

[0023] f. The remaining coalbed methane is cooled to 50-80℃ by a waste heat recovery device. The heat energy recovered during the cooling process is used to preheat the inlet gas. The coalbed methane is dehydrated by a drying device and then enters the conveying pipeline.

[0024] g. The oxygen concentration is monitored in real time by the oxygen sensor installed on the circulation pipeline. When the concentration exceeds the standard, the oxygen sensor 28 transmits the signal to the intelligent control system. The intelligent control system immediately starts the nitrogen inerting system 31. The explosion-proof valve 27 automatically opens to release pressure when the system pressure exceeds its set safety value.

[0025] Furthermore, the initial injection of coalbed methane uses a small amount of coalbed methane generated during the drilling process for cyclic injection. As the coalbed methane desorption process continues, the total amount of free coalbed methane in the target coal seam gradually increases. The subsequent total amount of cyclic injection is dynamically adjusted based on the real-time monitoring value of the free gas reserves.

[0026] Furthermore, during the extraction process, the CH4 concentration of the extracted coalbed methane is monitored in real time using a CH4 concentration detector. When the CH4 concentration is detected to be lower than a set threshold, a side-drilling replacement well mechanism is initiated. This mechanism involves directionally drilling a new branch well within a 100m radius around the original well and connecting it to the original fracture network via directional fracturing.

[0027] The advantages of this application over the prior art are as follows:

[0028] 1. By using cascade heating at the surface and underground, combined with staged fracturing and replacement well mechanisms to expand the thermal field coverage, the desorption rate of coalbed methane has been significantly improved.

[0029] 2. Renewable energy sources are used to replace fossil fuels, and the efficient heat exchange mechanism of waste heat recovery devices reduces energy consumption;

[0030] 3. By vacuuming and nitrogen injection circulation, the oxygen concentration in the well is controlled to within 1%, and explosion-proof valves are installed at the wellheads of coalbed methane injection wells used as heating wells; the well network pipelines are sealed with a combination of metal corrugated sealing pipes and graphite gaskets, which improves the safety of the well.

[0031] 4. Based on real-time monitoring and dynamic control technology, a small amount of coalbed methane is circulated and injected into the target coal seam. With the concentration adaptive switching mechanism, the returned coalbed methane accounts for 10%-15% of the total volume, ensuring that the production system can produce continuously. Attached Figure Description

[0032] The following description, in conjunction with the accompanying drawings, further illustrates this application:

[0033] Figure 1 A schematic diagram of a system structure for the staged heating and small-scale circulating injection of coalbed methane into coal seams for coalbed methane extraction, provided in an embodiment of this application;

[0034] Figure 2 A schematic diagram of coalbed methane circulation during coalbed methane extraction using complementary surface and underground heating, provided as an embodiment of this application.

[0035] In the diagram: 1-Control device; 2-Pressure sensor; 3-Pressure booster; 4-Temperature sensor; 5-Surface heater; 6-Circulation pump; 7-One-way shut-off valve; 8-Waste heat recovery device; 9-Drying device; 10-Hastelloy valve; 11-Horizontal well; 12-Transportation pipeline; 13-High-pressure needle valve; 14-Coalbed methane injection well; 15-Downhole heater; 16-Packer I; 17-Fractured network; 18-Horizontal well screen; 19-Filter device; 20-Packer II; 21-Coalbed methane extraction well; 22-Pneumatic anti-clogging ball valve; 23-Extraction pump; 24-CH4 concentration detector; 25-Green electricity device; 26-Circulation pipeline; 27-Explosion-proof valve; 28-Oxygen sensor; 29-Proportional regulating valve; 30-Flow meter; 31-Nitrogen inerting system; Double-dotted arrows indicate the direction of surface coalbed methane circulation; Dashed arrows indicate the direction of downhole coalbed methane flow. Detailed Implementation

[0036] like Figure 1 and Figure 2 As shown, this application provides a system for the cascade heating and small-scale circulating injection of coalbed methane into a coal seam for coalbed methane extraction. The system includes a coalbed methane circulating injection and extraction system deployed in the target coal seam, a cascade circulating heating system, and an intelligent control system. The coalbed methane circulating injection and extraction system comprises a composite well network consisting of a coalbed methane injection well 14, a horizontal well 11, a coalbed methane extraction well 21, and a circulating pipeline 26. The well network and pipelines are sealed using a combination of corrugated metal sealing pipes and graphite gaskets. The coalbed methane injection well 14 vertically penetrates the top and bottom plates of the coal seam, extending to below the bottom plate, and is connected to the horizontal well 11 via a sloping section. The horizontal section extends 100-300m and is equipped with a horizontal well screen pipe 18 as a coal dust prevention screen pipe. The coalbed methane extraction well 21 forms a connected network with the horizontal well 11 through a fracture network 17. The fracture network 17 is formed by segmented hydraulic fracturing to create a 30-50m radial fracture network.

[0037] The cascaded circulating heating system includes a surface preheating device, an underground enhanced heating device, and a gas circulation energy enhancement device connected by a circulation pipe 26. The surface preheating device includes a pressurization device 3 and a surface heater 5 deployed on the surface. The underground enhanced heating device includes an underground heater 15 arranged in the wellbore of the coalbed methane extraction well 21. The gas circulation energy enhancement device includes an extraction pump 23 deployed on the surface, consisting of a circulation pump 6, a waste heat recovery device 8, and a drying device 9. One end of the pressurization device 3 is connected to the wellhead of the coalbed methane injection well 14 through the circulation pipe 26, and the other end of the pressurization device 3 is connected to one end of the surface heater 5 through the circulation pipe 26. The ground heater 5 is connected to the waste heat gas outlet of the waste heat recovery device 8 via a circulation pipe 26. The other end of the ground heater 5 is also connected to one end of the circulation pump 6 via a circulation pipe 26. The other end of the circulation pump 6 is connected to the inlet of the waste heat recovery device 8 via a circulation pipe 26. The outlet of the waste heat recovery device 8 is connected to the inlet of the drying device 9 via a pipeline. The outlet of the drying device 9 is connected to the coalbed methane storage equipment via a conveying pipe 12. One end of the extraction pump 23 is connected to the wellhead of the coalbed methane extraction well 21. The other end of the extraction pump 23 is connected to the circulation pipe 26 between the circulation pump 6 and the waste heat recovery device 8.

[0038] The downhole heater 15 is specifically deployed in the wellbore of the coalbed methane injection well 14 located in the target coal seam section, and is arranged in a closed structure. The surface heater 5 is driven by the green power device 25 to achieve the initial heating of the coalbed methane. After the coalbed methane enters the coalbed methane injection well 14, it is then heated a second time by the downhole heater 15.

[0039] The green electricity device 25 uses wind and solar power as the main energy sources and is equipped with an energy storage unit as a backup energy supplement device. When the wind and solar power generation is lower than the heating demand, the energy storage unit supplements the energy to achieve green electricity heating throughout the entire process.

[0040] The intelligent control system includes a control device 1 deployed on the ground, various sensors arranged at different locations in the coalbed methane circulation injection and production system and the cascade circulation heating system, and valves with different functions installed at the wellheads of the coalbed methane injection well 14 and the coalbed methane extraction well 21. The control device 1 regulates equipment such as the surface heater 5 and the downhole heater 15 by monitoring the data from each sensor in real time. Specifically, temperature sensors 4 are installed in the outlet pipe section of the surface heater 5 and inside the shell of the downhole heater 15, respectively. Pressure sensors 2 are installed at the connecting flange of the circulation pipeline 26 and the coalbed methane injection well 14, and oxygen sensors 28 are installed on the circulation pipeline 26 to monitor the oxygen concentration in the circulation pipeline 26 in real time. A CH4 concentration detector 24 is installed between the outlet of extraction pump 23 and the inlet pipe of circulation pump 6. A pneumatic anti-blocking ball valve 22 is installed at the wellhead of coalbed methane extraction well 21. A flow meter 30 is installed in the pipeline of extraction pump 23. A proportional regulating valve 29 is installed on the reverse transmission branch of circulation pump 6. A one-way shut-off valve 7 is installed at the inlet of waste heat recovery device 8. A Hastelloy valve 10 is installed on the inlet pipe of drying device 9.

[0041] A flow meter 30 installed in the pipeline of extraction pump 23 is used to detect the coalbed methane flow rate in real time. The opening degree of the proportional regulating valve 29 installed on the return branch of circulating pump 6 and the detection value of flow meter 30 form a feedback regulation loop. Based on the real-time monitoring data of flow meter 30, the opening degree of proportional regulating valve 29 is dynamically adjusted to maintain the circulating coalbed methane at 10%-15% of the total coalbed methane volume. The return flow rate of circulating pump 6 and the coalbed methane extraction flow rate of extraction pump 23 form a closed-loop interlocked control.

[0042] The wellhead of coalbed methane injection well 14 is equipped with an explosion-proof valve 27 and a high-pressure needle-type regulating valve 13. The explosion-proof valve 27 is an intelligent safety pressure relief device that is resistant to high pressure, has a fast response, and automatically resets. It includes a main valve body structure, a shell, a sealing device, and a pressure relief channel. The main valve body structure is made of reinforced silicon carbide composite material. The shell adopts an inner layer-pressure-bearing layer-outer protective layer design. The inner layer is high-purity silicon carbide to resist the erosion of high-pressure airflow and gas corrosion. The pressure-bearing layer is made of fiber-reinforced composite material to provide the main structural strength. The outer protective layer is an impact-resistant coating. Flange interfaces are provided at both ends of the shell for seamless connection with the upstream pipeline of the wellhead pipe. The sealing device includes... It consists of a shape memory alloy drive ring, a high-strength conical valve core, and a super-elastic sealing gasket; the pressure relief channel is a Laval nozzle-type variable cross-section flow channel, the front half of the pressure relief channel narrows towards the middle to a narrow throat, and then expands outward from the narrow throat. The inner wall of the pressure relief channel is diamond polished and coated with a diamond-like carbon coating to minimize flow resistance and prevent particle adhesion and corrosion; the explosion-proof valve 27 is linked with the pressure sensor 2. When the pressure sensor 2 detects that the wellhead pressure fluctuation exceeds the set threshold ±15%, the control unit sends a trigger signal to the shape memory alloy drive ring of the explosion-proof valve 27 within 10 milliseconds to start the pressure relief procedure.

[0043] At the interface between the directional section of the coalbed methane injection well 14 and the screen pipe 18 of the horizontal well, a packer I 16 with a temperature resistance of not less than 300℃ is installed; at the end of the coalbed methane extraction well 21, a filter device 19 is installed, which includes an outer filter screen and an inner adsorption layer; in the wellbore of the coalbed methane extraction well 21, expansion packers II 20 are installed in sections.

[0044] The target coal seam in this application is a medium-deep coal seam with a burial depth of 800-1500m, the top and bottom plates of the coal seam are low thermal conductivity shale layers, and the coal seam thickness is not less than 10m; the distance between the coalbed methane injection well 14 and the coalbed methane extraction well 21 is 100-200m, the coalbed methane injection well 14 adopts a Φ250mm casing, and the coalbed methane extraction well 21 adopts a screen pipe structure.

[0045] The surface heater 5 is a spiral baffled heater, exchanging heat between shell-side air and tube-side gas. The downhole heater 15 is an armored resistance type, with its heating unit composed of a spiral nickel-chromium alloy resistance wire wrapped with a 0.5mm ceramic fiber insulation layer. A K-type thermocouple is deployed at the center of the resistance wire to monitor the heating temperature in real time. The coalbed methane extraction well 21 has a double-layer nitrogen protection chamber inside the wellbore to protect the downhole heater 15. The inner layer is a spiral flow guide chamber to ensure N2 covers the surface of the resistance wire and prevents CH4 from contacting the high-temperature components. The outer layer is an annular gas storage chamber to store and buffer nitrogen, maintaining a positive pressure environment within the protection chamber. The downhole heater 15 is connected to the surface control device 1. The double-layer nitrogen protection chamber of the downhole heater 15 forms a closed loop with the nitrogen inerting system 21, ensuring N2 purity ≥ 99.99%. The oxygen concentration in the wellbore is monitored in real time by an oxygen sensor 28; if the concentration exceeds the standard, a shutdown is triggered and emergency inerting is initiated.

[0046] CH4 concentration detector 24 monitors the CH4 concentration of the extracted coalbed methane in real time. When the CH4 concentration is detected to be lower than the set threshold, the side-drilling replacement well mechanism is activated, that is, a new branch well is directionally drilled within a 100m range around the original well and connected to the original fracture network 17 through directional fracturing.

[0047] The waste heat recovery device 8 adopts a counter-flow heat pipe structure with an inclination angle of 30-45°. The heat pipe is made of corrosion-resistant stainless steel coiled tubes. The internal cavity of the heat pipe is divided into a high-temperature gas phase channel and a low-temperature liquid phase channel. The high-temperature gas phase channel is connected to the circulation pipe 26 (i.e., the exhaust pipe), and the pipe wall is inlaid with fins to enhance the heat exchange effect. The low-temperature liquid phase channel forms a working fluid circulation path that flows in the opposite direction to the high-temperature gas phase channel through a flow guide baffle. The working fluid of the heat pipe is a mixture of water and ethanol to recover the waste heat of the exhaust gas and preheat the coalbed methane. A one-way shut-off valve 7 with a temperature resistance of 300℃ is installed at the inlet end of the waste heat recovery device 8, and the outlet is connected to the inlet section of the ground heater 5 through a flange to control the preheated gas temperature at 50-80℃.

[0048] Based on the above system, this application also proposes a method for extracting coalbed methane by cascade heating and circulating injection of a small amount of coalbed methane into the coal seam. Using the small amount of coalbed methane produced from the production well as the heat transfer fluid, a cascade circulating heating system of "surface preheating + downhole enhanced heating + gas circulation energy enhancement" is constructed. Green energy devices 25, such as wind, solar, and natural gas, can drive a surface heater 5 to initially heat the coalbed methane to 100-150℃ before injecting it into the wellbore. Secondly, a downhole heater 15 deployed in the wellbore further heats the coalbed methane, raising its temperature to 150-250℃, thus promoting the energy-enhancing desorption of a large amount of adsorbed coalbed methane. An intelligent control system is used to pressurize 10%-15% of the total produced coalbed methane via a circulating pump 6 and return it to the cascade circulating heating system. Simultaneously, the remaining 85%-90% of the coalbed methane is dried and directly enters the delivery pipeline 12. Throughout the extraction process, real-time monitoring of system pressure and gas concentration ensures safe and efficient extraction of coalbed methane.

[0049] The method specifically includes the following steps:

[0050] a. Deploy a coalbed methane recirculation injection and production system in the target coal seam section;

[0051] b. After completing the horizontal well screen 18 in the target coal seam section of horizontal well 11, a radial fracture network 17 of 30-50m is formed by segmented hydraulic fracturing.

[0052] c. Vacuuming is performed on injection well 14 and extraction well 21 of the target coal seam section using a vacuum pump. Then, high-purity nitrogen is injected into the wellbore. The vacuuming and nitrogen injection cycle is repeated 3 times to ensure that the residual oxygen content in the wellbore is less than 1%.

[0053] d. The ground heater 5 is driven by the green electricity device 25 to heat the coalbed methane to 100-150℃ for the first time, and then injected into the coal seam along the coalbed methane injection well 14; then the coalbed methane is heated a second time by the downhole heater 15 installed in the wellbore of the target coal seam section, so that the final temperature of the coalbed methane reaches 150-250℃.

[0054] e. The desorbed coalbed methane is extracted by extraction pump 23, and 10%-15% of the total volume of coalbed methane is produced by circulation pump 6. After being pressurized by pressurization device 3, it is returned to the coalbed methane circulation injection and extraction system.

[0055] f. The remaining coalbed methane is cooled to 50-80℃ by the waste heat recovery device 8. The heat energy recovered during the cooling process is used to preheat the inlet gas. After being dehydrated by the drying device 9, the coalbed methane enters the conveying pipeline 12.

[0056] g. The oxygen concentration is monitored in real time by an oxygen sensor 28 installed on the circulation pipeline 26. When the concentration exceeds the standard, the oxygen sensor 28 transmits a signal to the intelligent control system, which immediately activates the nitrogen inerting system 31 for inerting. The explosion-proof valve 27 automatically activates when the system pressure exceeds its set safety value for emergency pressure relief.

[0057] The initial injection of coalbed methane in this application uses a small amount of coalbed methane generated during the drilling process for cyclic injection. As the coalbed methane desorption process continues, the total amount of free coalbed methane in the target coal seam gradually increases. The total amount of subsequent cyclic injection is dynamically adjusted based on the real-time monitoring value of the free gas reserves.

[0058] The present application will be further described below through different embodiments. Example 1

[0059] The coalbed methane reservoir is buried at a depth of 1200m, with a coal seam thickness of 15m and low thermal conductivity roof and floor layers. The following process will be implemented:

[0060] A vertical coalbed methane injection well 14 is drilled, using a casing with a diameter of 250 mm, penetrating the coal seam, extending to the bottom below the coal seam, and connected to a horizontal well 11 via a directional drilling section at the top. The horizontal section extends 200 m and is equipped with a horizontal well screen pipe 18. The coalbed methane extraction well 21 is completed using a laser-cut slotted screen pipe, with a well spacing of 150 m. A pneumatic anti-clogging ball valve 22 is installed at the wellhead, and it is connected to the horizontal well 11 through a fracture network 17. A circulation pipeline 26 connects the coalbed methane injection well 14, the coalbed methane extraction well 21, and the surface equipment.

[0061] After three cycles of vacuuming and nitrogen injection in the wellbore of the target coal seam section (including coalbed methane injection well 14 and coalbed methane extraction well 21), the oxygen content in the wellbore of coalbed methane injection well 14 and coalbed methane extraction well 21 was measured to be 0.8%. An oxygen sensor 28 was installed along the circulation pipeline 26. When the oxygen concentration exceeded the standard, the signal was transmitted to the intelligent control system, which immediately activated the nitrogen inerting system 31. At the same time, the explosion-proof valve 27 automatically opened to release pressure when the system pressure exceeded its set safety value. A photovoltaic-powered spiral baffle heater (surface heater 5) first heated the coalbed methane to 140°C, and then used an underground armored resistance heater (underground heater 15) to heat the coalbed methane a second time to 190°C. The heat was evenly covered by a double-layer nitrogen protection chamber (inner spiral guide, outer annular gas storage), and the staged heated coalbed methane was injected into the coal seam along the coalbed methane injection well 14, promoting the desorption of adsorbed coalbed methane.

[0062] The return flow rate of the circulating pump 6 is linked with the flow rate of the extraction pump 23 in real time. The proportion of return gas is controlled to 12% by the flow meter 30 and the proportional regulating valve 29. The remaining coalbed methane recovers waste heat through a countercurrent heat pipe inclined at 40° to preheat the inlet gas to 70°. The dried coalbed methane is then stored in a storage tank.

[0063] Based on the underground heat diffusion and coalbed methane production efficiency, the heating power of the surface heater 5 and the downhole heater 15 is adjusted in real time. When the CH4 concentration detector 24 located at the outlet of the extraction pump 23 detects that the methane concentration is below 85%, the sidetracking replacement well mechanism is activated. A new branch well is directionally drilled within a range of 50m from the original well and connected to the original fracture network 17 through directional fracturing. Example 2

[0064] The coalbed methane reservoir is buried at a depth of 950m, with a coal seam thickness of 18m. The roof and floor are medium-deep shale coal seams, and there are local low-pressure areas. The following process will be implemented:

[0065] The distance between coalbed methane injection well 14 and coalbed methane extraction well 21 is 100m. The inclined section of coalbed methane injection well 14 is equipped with a packer I16 with a temperature resistance of 300℃. The screen pipe 18 of the horizontal well extends 250m. The horizontal section is subjected to segmented fracturing to form a 30m fracture network 17.

[0066] By performing three vacuuming and nitrogen injection cycles on the wellbore of the target coal seam section (including coalbed methane injection well 14 and coalbed methane extraction well 21), the oxygen content inside the wellbore of coalbed methane injection well 14 and coalbed methane extraction well 21 is reduced to below 0.5%. The coalbed methane is initially heated to 110°C using a green electric device 25 to drive the surface heater 5. The downhole heater 15 has a built-in K-type thermocouple for real-time temperature measurement, and the coalbed methane is then heated a second time to 180°C using the green electric device 25. The downhole heater 15 employs a double-layer nitrogen protection chamber to isolate the resistance wire from oxygen. A temperature sensor 4 monitors the outlet temperature of the surface heater 5, and the heating power is dynamically adjusted by a control device 1.

[0067] The return flow rate of the circulating pump 6 is linked with the extraction flow rate of the extraction pump 23 in real time. By dynamically adjusting the speed, the proportion of the return gas volume to the total gas production is stabilized at 10%. The separated concentrated coalbed methane is pressurized by the pressurizing device 3 and then returned to the inlet of the coalbed methane circulation injection and extraction system to mix with the supplemented coalbed methane. The mixed gas is initially heated by the ground heater 5, and the remaining coalbed methane enters the countercurrent heat pipe heat exchanger arranged at an inclination of 45° to recover waste heat and preheat the inlet gas to 60°. Finally, the dried coalbed methane enters the transmission pipeline.

[0068] Based on the underground heat diffusion and coalbed methane production efficiency, the heating power of the surface heater 5 and the downhole heater 15 is adjusted in real time. A CH4 concentration detector 24 is deployed at the outlet of the extraction pump 23. When the concentration drops to 85%, the sidetracking replacement well mechanism is triggered, and a new branch well is directionally drilled at a position 60m away from the original well. It is then connected to the original fracture network 17 through directional fracturing. Example 3

[0069] The coalbed methane reservoir is buried at a depth of 1300m, with a coal seam thickness of 20m and a low-permeability sandstone roof. The following process will be implemented:

[0070] The coalbed methane injection well 14 has a vertical depth of 1300m, and the horizontal well 11 extends in two branches, each branch being 600m long. The coalbed methane extraction well 21 is equipped with a double-layer filter device 19 at its end, which includes an outer screen and a 50mm thick inner activated carbon fiber adsorption layer.

[0071] By performing three vacuuming and nitrogen injection cycles on the wellbore of the target coal seam section (including coalbed methane injection well 14 and coalbed methane extraction well 21), the oxygen content in the wellbore of coalbed methane injection well 14 and coalbed methane extraction well 21 is reduced to less than 1%. The surface heater 5 and the downhole heater 15 are powered by the green electricity device 25, which drives the surface heater 5 to heat the circulating gas to 150°C; the downhole heater 15 heats the coalbed methane a second time to 200°C, and preheats the inlet gas to 80°C through the waste heat recovery device 8; 99.99% pure N2 circulates in the double-layer nitrogen protection chamber of the downhole heater 15 and is connected to the nitrogen inerting system 31.

[0072] The return flow rate of circulating pump 6 accounts for 15%, and the proportional deviation is controlled to ±1.5% by proportional regulating valve 29; when the CH4 concentration in the extracted gas is below 85% for 10 minutes, the PLC triggers the replacement well sidetracking, and ceramic aggregate is used to support the fracture to connect the original well network; the new branch is connected to the original well through fracture network 17.

[0073] This application utilizes extracted coalbed methane as a heat transfer fluid injected into the target coal seam. Through gradient heating, it preferentially injects the gas into the "core zone" with high permeability and desorption potential, while thermal diffusion drives heat migration to the low-permeability "edge zone," achieving tiered desorption in heterogeneous reservoirs. Simultaneously, gaseous circulation avoids liquid phase intrusion and water-locking effects. Furthermore, the weak polarity of CH4 molecules inhibits competitive adsorption with CO2, resulting in minimal changes in reservoir framework stress. Most importantly, the method of injecting a small amount of high-temperature CH4 to develop coalbed methane eliminates the gas separation process, significantly improving efficiency and reducing costs. This technology recovers residual heat through an internal circulation system to reduce heat loss, and combined with the wide explosion limit range of CH4, it significantly improves the safety of coalbed methane extraction, achieving a desorption rate of over 85%, providing a new pathway for the safe and efficient development of coalbed methane.

[0074] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A system for extracting coalbed methane by cascade heating and circulating injection of small amounts of coalbed methane into the coal seam, characterized in that: The system includes a coalbed methane circulation injection and production system, a cascade circulation heating system and an intelligent control system deployed in the target coal seam. The coalbed methane circulation injection and production system includes a composite well network consisting of coalbed methane injection wells (14), horizontal wells (11), coalbed methane extraction wells (21) and circulation pipelines (26). Horizontal well screens (18) are installed in the horizontal section of the horizontal wells (11). The coalbed methane extraction wells (21) form a network with the horizontal wells (11) through a fracture network (17). The cascaded circulation heating system includes a ground preheating device, an underground enhanced heating device and a gas circulation energy enhancement device connected by a circulation pipeline (26). The ground preheating device is used to inject the coalbed methane into the coalbed methane injection well (14) after initial heating. Then, the coalbed methane is heated a second time by the underground enhanced heating device. The gas circulation energy enhancement device is used to back-transport part of the coalbed methane output from the coalbed methane extraction well (21) to the cascaded circulation heating system. The ground preheating equipment includes a pressurizing device (3) and a ground heater (5) deployed on the ground. One end of the pressurizing device (3) is connected to the wellhead of the coalbed methane injection well (14) through a circulation pipe (26), and the other end of the pressurizing device (3) is connected to one end of the ground heater (5) through a circulation pipe (26). The gas circulation energy enhancement equipment includes a circulation pump (6), a waste heat recovery device (8), a drying device (9), and an extraction pump (23) deployed on the ground. The other end of the ground heater (5) is connected to the waste heat gas outlet of the waste heat recovery device (8) through a circulation pipe (26). The other end of the ground heater (5) is also connected to one end of the circulation pump (6) through a circulation pipe (26). The other end of the circulation pump (6) is connected to the inlet of the waste heat recovery device (8) through a circulation pipe (26). The outlet of the waste heat recovery device (8) is connected to the inlet of the drying device (9) through a pipeline, and the outlet of the drying device (9) is connected to the coalbed methane storage equipment through the conveying pipeline (12); one end of the extraction pump (23) is connected to the wellhead of the coalbed methane extraction well (21), and the other end of the extraction pump (23) is connected to the circulation pipeline (26) between the circulation pump (6) and the waste heat recovery device (8); The intelligent control system is used to intelligently control and regulate the electrical equipment in the coalbed methane circulating injection and production system and the cascade circulating heating system.

2. The system for staged heating and circulating injection of small amounts of coalbed methane into coal seams for coalbed methane extraction according to claim 1, characterized in that: The ground heater (5) is powered by a green electricity device (25).

3. The system for staged heating and circulating injection of small amounts of coalbed methane into coal seams for coalbed methane extraction according to claim 2, characterized in that: The downhole enhanced heating equipment includes a downhole heater (15) deployed in the wellbore of the coalbed methane injection well (14), the downhole heater (15) being located in the wellbore of the coalbed methane injection well (14) where the target coal seam is located.

4. The system for staged heating and circulating injection of small amounts of coalbed methane into coal seams for coalbed methane extraction according to claim 3, characterized in that: The intelligent control system includes a control device (1) deployed on the ground, various sensors arranged at different locations in the coalbed methane circulation injection and production system and the cascade circulation heating system, and valves with different functions installed at the wellheads of the coalbed methane injection well (14) and the coalbed methane extraction well (21). The control device (1) controls the ground heater (5) and the downhole heater (15) by monitoring the data of each sensor in real time. Each sensor includes: a temperature sensor (4) installed in the outlet pipe section of the surface heater (5) and inside the shell of the downhole heater (15); a pressure sensor (2) installed at the connecting flange of the circulation pipe (26) and the coalbed methane injection well (14); an oxygen sensor (28) installed on the circulation pipe (26); a CH4 concentration detector (24) installed between the outlet of the extraction pump (23) and the inlet pipe of the circulation pump (6); and a flow meter (30) installed in the pipeline of the extraction pump (23). The valves include: an explosion-proof valve (27) and a high-pressure needle valve (13) installed at the wellhead of the coalbed methane injection well (14); a pneumatic anti-blocking ball valve (22) installed at the wellhead of the coalbed methane extraction well (21); a proportional regulating valve (29) installed on the reverse transmission branch of the circulating pump (6); a one-way shut-off valve (7) installed at the inlet of the waste heat recovery device (8); and a Hastelloy valve (10) installed on the inlet pipe of the drying device (9). The first port of the proportional regulating valve (29) is connected to the circulating pump (6), the second port of the proportional regulating valve (29) is connected to the one-way shut-off valve (7), and the third port of the proportional regulating valve (29) is connected to the extraction pump (23).

5. A system for staged heating and circulating injection of small amounts of coalbed methane into coal seams for coalbed methane extraction according to claim 4, characterized in that: The coalbed methane injection well (14) is equipped with a double-layer nitrogen protection chamber for protecting the downhole heater (15). The double-layer nitrogen protection chamber includes an inner spiral guide chamber and an outer annular gas storage chamber. The double-layer nitrogen protection chamber forms a closed loop with the surface inerting system.

6. The system for staged heating and circulating injection of small amounts of coalbed methane into coal seams for coalbed methane extraction according to claim 4, characterized in that: The opening degree of the proportional control valve (29) and the detection value of the flow meter (30) constitute a feedback control loop. Based on the real-time monitoring data of the flow meter (30), the opening degree of the proportional control valve (29) is dynamically adjusted so that the circulating coalbed methane is maintained at 10%-15% of the total coalbed methane volume. The return flow rate of the circulating pump (6) and the coalbed methane flow rate extracted by the extraction pump (23) form a closed-loop interlock control.

7. A method for extracting coalbed methane by staged heating and circulating injection of small amounts of coalbed methane into the coal seam, characterized in that: The system for extracting coalbed methane based on the cascade heating and small-scale coalbed methane circulation injection system as described in claim 5 includes the following steps: a. Deploy a coalbed methane recirculation injection and production system in the target coal seam section; b. After implementing the horizontal well screen (18) on the target coal seam section of the horizontal well (11), a radial fracture network (17) is formed by segmented hydraulic fracturing. c. Vacuum the wellbore of the target coal seam section using a vacuum pump, then inject high-purity nitrogen into the wellbore. Repeat the vacuuming and high-purity nitrogen injection steps until the residual oxygen content in the wellbore is less than 1%. d. Using the green power device (25) to provide energy for the primary and secondary heating, drive the ground heater (5) to heat the coalbed methane to 100-150℃ and inject it into the target coal seam along the coalbed methane injection well (14); then, the coalbed methane is heated a second time by the downhole heater (15) installed in the wellbore of the target coal seam section, so that the final temperature of the coalbed methane reaches 150-250℃. e. Use extraction pump (23) to extract desorbed coalbed methane, and circulate it through circulation pump (6) to produce 10%-15% of the total volume of coalbed methane. After being pressurized by pressurizing device (3), it is returned to the coalbed methane circulation injection and extraction system. f. The remaining coalbed methane is cooled to 50-80℃ by the waste heat recovery device (8). The heat energy recovered during the cooling process is used to preheat the inlet gas. The coalbed methane is dehydrated by the drying device (9) and then enters the conveying pipeline (12). g. The oxygen concentration is monitored in real time by the oxygen sensor (28) installed on the circulation pipeline (26). When the concentration exceeds the standard, the oxygen sensor (28) transmits the signal to the intelligent control system. The intelligent control system immediately starts the nitrogen inerting system (31). The explosion-proof valve (27) automatically opens to release pressure when the system pressure exceeds its set safety value.

8. The method for extracting coalbed methane by staged heating and circulating injection of a small amount of coalbed methane into the coal seam according to claim 7, characterized in that: The initial injection of coalbed methane uses a small amount of coalbed methane generated during the drilling process for cyclic injection. As the coalbed methane desorption process continues, the total amount of free coalbed methane in the target coal seam gradually increases. The subsequent total amount of cyclic injection is dynamically adjusted based on the real-time monitoring value of the free gas reserves.

9. The method for extracting coalbed methane by staged heating and circulating injection of a small amount of coalbed methane into the coal seam according to claim 7, characterized in that: During the mining process, the CH4 concentration of the extracted coalbed methane is monitored in real time by a CH4 concentration detector (24). When the CH4 concentration is detected to be lower than the set threshold, the side-drilling replacement well mechanism is activated. The side-drilling replacement well mechanism is to directionally drill a new branch well within 100m around the original well and connect it to the original fracture network (17) through directional fracturing.

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