Extraction method of coalbed methane by vertical displacement of coalbed methane through dual horizontal well circulation CO2 injection in complex coal-bearing formations

Abstract

The application discloses a kind of complex coal measures strata double horizontal well cyclic injection CO2 vertical displacement coalbed gas mining method, belong to coalbed gas development technical field.Deep coal measures strata is selected as purpose layer, and two single branch horizontal wells are drilled in the same well group.One horizontal section is located at the top of purpose layer for coalbed gas extraction and fracturing;Another horizontal section is located at the bottom of purpose layer for injecting liquid CO2.Before extraction, by continuously injecting liquid CO2 into the bottom of purpose layer, the displacement and displacement effect of CO2 on CH4 are used to improve the recovery of deep coalbed gas;During extraction, part of CO2 is extracted with CH4, and the extracted CO2 is liquefied and recycled into the bottom of purpose layer through CO2 separation device;At the end of extraction, most of CH4 has been extracted, and the horizontal well is sealed to prevent CO2 from leaking, achieving the purpose of CO2 storage.The method is simple and easy to implement, and can be used for complex structure of purpose layer.

Description

Technical Field

[0001] This invention relates to a method for extracting coalbed methane, and more particularly to a method for extracting coalbed methane by vertically driving it through CO2 injection in dual horizontal wells in complex coalbed strata, belonging to the field of coalbed methane surface extraction technology. Background Technology

[0002] Deep coalbed methane is difficult to extract due to its complex environment, but deep coalbed methane reservoirs can also store large amounts of CO2. Current technologies, exemplified by the Ordos and Qinshui basins, commonly employ "U"-shaped wells or a single injection-multiple extraction approach in CO2-driven coalbed methane projects. This means CO2 injection and extraction occur at the same horizontal level, and the target layer is generally the coal seam, without involving other gas-bearing reservoirs within the coalbed methane strata, such as shale gas or sandstone gas. The results of these projects have been largely unsatisfactory. Relying solely on injection pressure to drive CO2 replacement and CH4 displacement has limited effectiveness, resulting in only a small increase in coalbed methane recovery, low CO2 storage, and significant CO2 venting into the air.

[0003] A method for in-situ layered combustion and explosion fracturing of methane in a vertical well of a coal-bearing gas reservoir, disclosed in publication number CN115653561A, involves the following steps: constructing a vertical well from the surface into the target reservoir, passing through all target reservoirs; constructing branch wells from the vertical wellbore into specific layers; sealing the branch wells with packers while simultaneously monitoring the methane pressure within them in real time; when the methane pressure in the branch wells reaches a specified pressure, introducing an oxidizer and filling the vertical well with water; then igniting the methane-oxidizer mixture to create a fracture network around the branch wells; performing combustion and explosion fracturing layer by layer within the target reservoir from bottom to top until the entire target area is fractured, creating a large-scale artificial gas reservoir; and finally, draining the water from the vertical wells and beginning methane gas extraction. However, this method requires multiple extraction pipes at different depths to achieve multi-layer extraction, making the construction complex and impractical. Summary of the Invention

[0004] Technical Problem: The purpose of this invention is to overcome the problems of insignificant effects of traditional horizontal CO2-driven coalbed methane extraction, small improvement in coalbed methane resource recovery rate, large amount of CO2 venting, and poor economic benefits of coalbed methane development. This invention provides a method for vertically driving coalbed methane extraction using dual horizontal wells with circulating CO2 injection in complex coal-bearing formations.

[0005] Technical Solution: To achieve the above objectives, the present invention provides a method for extracting coalbed methane from complex coal-bearing strata using a dual-horizontal-well circulating CO2 injection system, comprising the following steps:

[0006] a. Select a deep and complex coal-bearing stratum as the target layer, construct a vertical shaft on the surface in the target layer's occurrence area, construct an inclined section at the top of the target layer, and construct a horizontal section along the top of the target layer to form a horizontal well with a three-section shaft structure.

[0007] b. Cementing is completed on the extraction horizontal well. Segmented perforation and fracturing are performed below the extraction horizontal well to form a target layer fracture network from top to inward within the target layer. The fracturing depth cannot penetrate the entire target layer. After fracturing, extraction pipelines are installed in the extraction horizontal well. The extraction pipelines are equipped with drainage pumps to perform pre-extraction and depressurization extraction of coal-bearing gas in the target layer.

[0008] c. Construct a vertical shaft 50-100 meters away from the wellhead of the extraction horizontal well until the bottom of the target layer. Then, construct a horizontal section along the bottom of the target layer through the directional section to form a CO2 injection horizontal well with a three-section well structure. After cementing, install a CO2 injection horizontal well section screen pipe in the horizontal section. Then, install a CO2 injection pipeline into the CO2 injection horizontal well until the end of the horizontal section. Install a packer between the directional section and the horizontal section.

[0009] d. Install a liquid CO2 gas source at the wellhead of the CO2 injection horizontal well and connect it to the CO2 injection pipeline;

[0010] e. After the gas production of the extraction horizontal well reaches its peak, liquid CO2 is continuously injected into the CO2 injection horizontal well. The liquid CO2 is discharged from the end of the CO2 injection pipeline and is blocked by the packer. Under the action of the screen pipe in the CO2 injection horizontal well section, some of the CO2 is adsorbed and stored in the surrounding rock layers and part of the target layer. Some of the CO2 is displaced and replaced by CH4 in the target layer with the assistance of the laminar fracture network. The liquid CO2 diffuses from the high pressure at the bottom of the target layer to the low pressure at the top. In the gas-liquid two-phase flow, the buoyancy of the gas moving upward drives CH4 into the extraction horizontal well.

[0011] f. Repeat step e. The liquid CO2 injected into the horizontal well is vaporized in the target layer and moves upward through the pore fracture channel until it enters the extraction well. At this time, the CO2 is separated from gases such as CH4 by the CO2 separator on the surface. After being liquefied by the CO2 liquefaction pump, it is injected into the CO2 storage tank to realize the cyclic injection of CO2.

[0012] g. Finally, when the CH4 content extracted from the extraction horizontal well is lower than the threshold, the extraction horizontal well is closed, while the CO2 injection horizontal well can continue to inject until the bottom pressure reaches the in-situ stress, thereby sealing CO2 in the target layer.

[0013] Furthermore, the target layer is a deep and complex coal-bearing stratum, a composite layer of multiple coal-bearing gas reservoirs, including coal seams, sandstone layers, and shale layers. The target layer does not contain a dense sealing layer that isolates CO2. The thickness of the target layer is about 40 meters, which means that the vertical distance between the two horizontal well sections is controlled to be about 40 meters, and there is no dense sealing layer in the target layer.

[0014] Furthermore, in the three-section wellbore structure for coalbed methane extraction horizontal wells, the vertical section is first constructed to the upper loose strata of the target layer, and the surface casing is cemented during vertical drilling; then, directional drilling is carried out to the top of the target layer, and technical casing is cemented; after drilling to the top of the target layer, the horizontal section is drilled for 800-1000m in the roof, and after completion, the production casing is cemented in the horizontal section.

[0015] Furthermore, the horizontal well for coalbed methane extraction adopts segmented hydraulic directional perforation to carry out horizontal section fracturing. Perforation fracturing is carried out in the direction from the top plate to the bottom plate of the target layer. The spacing between each perforation segment is set at 25m, and the length of a single hydraulic perforation segment does not exceed 2 / 3 of the thickness of the target layer, so as not to affect the stability of the CO2 injection horizontal well.

[0016] Furthermore, in the three-section wellbore structure, the CO2 injection horizontal well is first constructed by drilling the vertical section to the loose formation above the target layer, and then cementing the surface casing during vertical drilling; the directional drilling is then carried out to the bottom plate of the target layer, and the technical casing is cemented; the bottom plate of the target layer is drilled horizontally for 800-1000m, and the horizontal section of the CO2 injection horizontal well is kept parallel to the horizontal section of the production horizontal well. After drilling is completed, the production casing is cemented, and the CO2 injection horizontal well section screen pipe is installed in the horizontal section for completion.

[0017] Furthermore, after draining and depressurizing the horizontal well for a period of time, the wellhead pressure was stabilized at 12 MPa. When the gas production began to decline, liquid CO2 was injected into the CO2 injection horizontal well. The initial CO2 injection rate was 80 t / d. When the wellhead pressure of the CO2 injection horizontal well was lower than 20 MPa, the wellhead pressure of the CO2 injection horizontal well was gradually increased by 2 t / d. Afterward, the CO2 injection rate was continuously adjusted to keep the wellhead pressure of the CO2 injection horizontal well constant at 20 MPa, so as to ensure that there is a pressure difference between the top and bottom of the target layer, so that CO2 can better displace CH4 in the vertical direction and enter the horizontal well.

[0018] Furthermore, the CO2 gas source includes an interconnected CO2 storage tank and a plunger pump. The CO2 storage tank is equipped with a liquid CO2 storage tank temperature and pressure gauge. The inlet of the CO2 storage tank is equipped with a liquid CO2 main gate valve, which connects to the liquid CO2 tank truck via a liquid CO2 tank truck gate valve. The output of the CO2 storage tank is connected to the plunger pump via a liquid CO2 transport pipeline. The plunger pump is equipped with an electric motor. The liquid CO2 transport pipeline is equipped with a first liquid CO2 delivery gate valve. The plunger pump is connected to the end of the liquid CO2 injection pipeline. The end of the liquid CO2 injection pipeline is equipped with a liquid CO2 flow meter, a second liquid CO2 delivery gate valve, and a liquid CO2 injection pressure gauge in sequence.

[0019] Furthermore, the coalbed methane extraction pipeline is connected to an oil pumping unit at its surface end. The oil pumping unit is connected to a CO2 separator via a gas transport pipeline. The gas transport pipeline is equipped with a main gas production valve. The CO2 separator is equipped with a CO2 output pipeline and a CH4 output pipeline. The CO2 output pipeline is connected to a CO2 liquefaction pump and is equipped with a CO2 separation gate valve. The CH4 output pipeline is equipped with a CH4 separation gate valve. The CO2 liquefaction pump output port is connected to a main gate valve for storing liquid CO2 via a pipeline. The pipeline is also equipped with a CO2 liquefaction pump gate valve.

[0020] Furthermore, when the CH4 content in the gas produced by the extraction horizontal well is less than 10%, the extraction horizontal well is shut off, while the CO2 injection horizontal well can continue to operate, in order to achieve the purpose of sealing CO2 in complex coal-bearing formations.

[0021] Beneficial Effects: By adopting the above-mentioned technical solution, this invention overcomes the problems of insignificant effects, small increases in coalbed methane recovery, large amounts of CO2 venting, and poor economic benefits of traditional horizontal CO2-driven coalbed methane treatment. First, the target layer can include various reservoirs such as coal seams, shale layers, and sandstone layers, adapting to complex formations and offering a wider range of storage compared to the current single-coal-seam displacement method. Second, under the combined effects of CO2 displacement and replacement of CH4, diffusion from the high pressure floor to the low pressure roof, and buoyancy of gas moving upwards in the gas-liquid two-phase flow, the driving force for coalbed methane is greatly increased, significantly improving the coalbed methane recovery rate. Third, the extracted CO2 is separated, liquefied, and reinjected into the ground using a surface CO2 separator, greatly reducing CO2 venting. Finally, a large amount of CO2 was adsorbed in the deep coal-bearing strata after the coal-bearing gas was extracted, realizing the sequestration of CO2 in the deep coal-bearing strata and significantly improving the economic, environmental and social benefits of coal-bearing gas extraction. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the method for extracting coalbed methane using a dual-horizontal-well circulating CO2 injection system in complex coal-bearing formations according to the present invention.

[0023] In the diagram: 1-Target layer; 2-Horizontal well surface casing; 3-Horizontal well technical casing; 4-Horizontal well production casing; 5-Horizontal well drilling trajectory; 6-CO2 injection horizontal well; 7-Production horizontal well; 8-CO2 injection horizontal well section screen pipe; 9-Laminated fracture network; 10-Liquid CO2 injection pipeline; 11-Coalbed methane extraction pipeline; 12-Drainage pump; 13-Packer; 14-Liquid CO2 tanker truck; 15-Liquid CO2 tanker truck gate valve; 16-Liquid CO2 storage main gate valve; 17-Liquid CO2 storage... Tank; 18-Liquid CO2 storage tank temperature and pressure gauge; 19-First gate valve for liquid CO2 delivery; 20-Plunger pump; 21-Electric motor; 22-Liquid CO2 flow meter; 23-Second gate valve for liquid CO2 delivery; 24-Liquid CO2 injection pressure gauge; 25-Oil pumping unit; 26-Main gas production valve; 27-CO2 separator; 28-CO2 separation gate valve; 29-CH4 separation gate valve; 30-CO2 liquefaction pump; 31-CO2 liquefaction pump gate valve; 32-Liquid CO2 transport pipeline; 33-Gas transport pipeline. Detailed Implementation

[0024] The present invention will be further described below with reference to the embodiments shown in the accompanying drawings:

[0025] like Figure 1 As shown, the specific steps of the present invention for the extraction of coalbed methane from complex coal-bearing strata using a dual-horizontal-well circulating CO2 injection method are as follows:

[0026] (a) On the ground (34) of the target layer (1) occurrence area, construct a production horizontal well (7) with a three-section wellbore structure along the drilling trajectory (5), with the horizontal section located at the top of the target layer. The first section is drilled to the upper loose formation, and the surface casing (2) is cemented; the second section is drilled directionally to the top of the target layer, and the technical casing (3) is cemented; the third section is drilled to the top of the target layer, drilling 800-1000m in the top plate, and after completion, the production casing (4) is cemented in the horizontal section.

[0027] (b) After cementing the horizontal well (7), hydraulic perforation is performed in segments along the target layer direction of the horizontal section. The spacing between each perforation segment is set at 25m, and the length of a single hydraulic perforation segment does not exceed 2 / 3 of the target layer thickness to avoid affecting the stability of the CO2 injection horizontal well (6). Finally, a target layer fracture network (9) is formed. After fracturing, the extraction pipeline (11) and the drainage pump (12) are installed to carry out the early-stage pumping and depressurization extraction of coal-bearing gas.

[0028] (c) Along the drilling trajectory (5) 50-100 meters from the aforementioned extraction horizontal well (7), construct a CO2 injection horizontal well (6) with a three-section wellbore structure, with the horizontal section located at the bottom of the target formation. The first section is drilled to the upper loose formation, and the surface casing (2) is cemented. The second section is drilled directionally to the bottom of the target formation, and the technical casing (3) is cemented. The third section is drilled to the bottom of the target formation, and 800-1000m is drilled at the bottom. The direction of the horizontal section of the CO2 injection horizontal well (6) is kept parallel to the horizontal section of the extraction horizontal well (7). After drilling is completed, the production casing (4) is cemented, and the screen pipe (8) is installed in the horizontal section for completion. After cementing is completed, a packer (13) is installed at the end of the directional section, and the CO2 injection pipeline (10) is extended to the end of the CO2 injection horizontal well (6).

[0029] (d) A CO2 circulating injection device, including a CO2 separator (27), a CO2 liquefaction pump (30), a CO2 storage tank (17), a plunger pump (20), and an electric motor (21), is arranged on the ground.

[0030] (e) After draining and depressurizing the horizontal well (7) for a period of time, the wellhead pressure was stabilized at 12 MPa. When the gas production began to decline, liquid CO2 was injected into the CO2 injection horizontal well (6). Through the vertical displacement and replacement of CH4 by CO2, the diffusion effect from the high pressure at the bottom to the low pressure at the top, and the buoyancy effect of the gas moving upward in the gas-liquid two-phase flow, CH4 was driven into the horizontal well (7). The initial CO2 injection rate was 80 t / d. When the wellhead pressure of the CO2 injection horizontal well (6) was lower than 20 MPa, the wellhead pressure of the CO2 injection horizontal well (6) was gradually increased by 2 t / d. Afterwards, the CO2 injection rate was continuously adjusted to keep the wellhead pressure of the CO2 injection horizontal well (6) constant at 20 MPa, so as to ensure that there is a certain pressure difference between the top and bottom of the target layer, so that CO2 can better displace CH4 in the vertical direction and enter the horizontal well (7).

[0031] (f) In the middle and late stages of drainage, the liquid CO2 injected into the CO2 injection well (6) will be vaporized and enter the extraction well (7) through channels such as pores and fractures. At this time, the CO2 is separated from gases such as CH4 by the CO2 separator (27) on the ground. After being liquefied by the CO2 liquefaction pump (30), it is injected into the CO2 storage tank (17) to realize the cyclic injection of CO2.

[0032] (g) When the CH4 content in the gas produced by the extraction horizontal well (7) is less than 10%, the extraction horizontal well (7) shall be shut in. The CO2 injection horizontal well (6) may continue to operate in order to achieve the purpose of sealing CO2 in the deep coal-bearing strata.

[0033] The CO2 gas source includes an interconnected CO2 storage tank 17 and a plunger pump 20. The CO2 storage tank 17 is equipped with a liquid CO2 storage tank temperature and pressure gauge 18. The inlet of the CO2 storage tank 17 is equipped with a liquid CO2 main gate valve 16, which is connected to a liquid CO2 tank truck 14 via a liquid CO2 tank truck gate valve 15. The output of the CO2 storage tank 17 is connected to the plunger pump 20 via a liquid CO2 transport pipeline 32. The plunger pump 20 is connected to an electric motor 21. The liquid CO2 transport pipeline 32 is equipped with a first liquid CO2 gate valve 19. The plunger pump 20 is connected to the end of a liquid CO2 injection pipeline 10. The end of the liquid CO2 injection pipeline is equipped with a liquid CO2 flow meter 22, a second liquid CO2 gate valve 23, and a liquid CO2 pressure gauge 24 in sequence.

[0034] The end of the coalbed methane extraction pipeline 11 located on the ground is connected to an oil pumping unit 25. The oil pumping unit 25 is connected to a CO2 separator 27 via a gas transport pipeline 33. A main gas production valve 26 is installed on the gas transport pipeline 33. The CO2 separator 27 is equipped with a CO2 output pipeline and a CH4 output pipeline. The CO2 output pipeline is connected to a CO2 liquefaction pump 30 and is equipped with a CO2 separation gate valve 28. A CH4 separation gate valve 29 is installed on the CH4 output pipeline. The output port of the CO2 liquefaction pump 30 is connected to a liquid CO2 storage main gate valve 16 via a pipeline. A CO2 liquefaction pump gate valve 31 is installed on the pipeline.

Claims

1. A method for extracting coalbed methane from complex coal-bearing formations using a dual-horizontal-well circulating CO2 injection system, characterized in that... The steps are as follows: a. Select a deep and complex coal-bearing stratum as the target layer (1), construct a vertical shaft on the ground (34) in the area where the target layer (1) is located, construct an inclined section at the top of the target layer (1), construct a horizontal section along the top of the target layer (1), and form a three-section well structure extraction horizontal well (7). b. After cementing the extraction horizontal well (7), perform segmented perforation and fracturing below the extraction horizontal well (7) to form a target layer fracture network (9) from top to inward within the target layer (1). The fracturing depth cannot penetrate the entire target layer (1). After fracturing, run an extraction pipeline (11) into the extraction horizontal well (7). The extraction pipeline (11) is equipped with a drainage pump (12) to perform pre-extraction and depressurization extraction of coal-bearing gas in the target layer (1). The end of the extraction pipeline (11) located on the ground is connected to a pumping unit (25). A CO2 separator (27) is connected via a gas transport pipeline (33). A gas production main valve (26) is provided on the gas transport pipeline (33). The CO2 separator (27) is provided with a CO2 output pipeline and a CH4 output pipeline. The CO2 output pipeline is connected to the CO2 liquefaction pump (30) and is provided with a CO2 separation gate valve (28). A CH4 separation gate valve (29) is provided on the CH4 output pipeline. The output port of the CO2 liquefaction pump (30) is connected to the liquid CO2 main gate valve (16) via a pipeline. A CO2 liquefaction pump gate valve (31) is provided on the pipeline. c. Vertically construct a shaft 50-100 meters away from the wellhead of the extraction horizontal well (7) until the bottom depth of the target layer (1). Then, construct a horizontal section along the bottom of the target layer (1) through the directional section to form a CO2 injection horizontal well (6) with a three-section well body structure. After cementing, install a CO2 injection horizontal well section screen pipe (8) in the horizontal section. Then, install a CO2 injection pipeline (10) into the CO2 injection horizontal well (6) until the end of the horizontal section. Install a packer (13) between the directional section and the horizontal section. d. Install a liquid CO2 gas source at the wellhead of the CO2 injection horizontal well (6) and connect it to the CO2 injection pipeline (10); e. After the gas production of the extraction horizontal well (7) reaches its peak, liquid CO2 is continuously injected into the CO2 injection horizontal well (6). The liquid CO2 is discharged from the end of the CO2 injection pipe (10) and blocked by the packer (13). Under the action of the screen pipe (8) in the CO2 injection horizontal well section, it diffuses. Some of the CO2 is adsorbed and stored in the surrounding rock layers and part of the target layer (1). Some of the CO2 is displaced and replaced by CH4 in the target layer (1) with the assistance of the laminar fracture network (9). The liquid CO2 diffuses from the high pressure at the bottom to the low pressure at the top of the target layer (1), and under the buoyancy of the gas moving upward in the gas-liquid two-phase flow, it drives CH4 into the extraction horizontal well (7). f. Repeat step e. The liquid CO2 injected into the CO2 injection horizontal well (6) is vaporized in the target layer (1) and moves upward through the pore fracture channel until it enters the extraction horizontal well (7). At this time, the CO2 is separated from CH4 and other gases by the CO2 separator (27) on the ground. After being liquefied by the CO2 liquefaction pump (30), it is injected into the CO2 storage tank (17) to realize the cyclic injection of CO2. g. Finally, when the CH4 content extracted by the extraction horizontal well (7) is lower than the threshold, the extraction horizontal well (7) is closed, and the CO2 injection horizontal well (6) can continue to inject until the bottom pressure reaches the ground stress, thereby sealing CO2 in the target layer (1).

2. The method for extracting coalbed methane from complex coal-bearing strata using dual horizontal wells with CO2 circulation injection for vertical displacement, as described in claim 1, is characterized in that: The target layer (1) is a deep and complex coal-bearing stratum, which is a multi-type coal-bearing gas reservoir composite layer, including coal seam, sandstone layer and shale layer. The target layer (1) does not contain a dense sealing layer that isolates CO2. The thickness of the target layer is 40 meters, that is, the vertical distance between the two horizontal well sections is controlled to be 40 meters, and there is no dense sealing layer in the target layer. For a three-section coal-bearing gas extraction horizontal well (7), first, a vertical section is constructed to the upper loose stratum of the target layer (1). During vertical drilling, a surface casing (2) is installed and cemented. Then, directional drilling is performed to the top of the target layer (1), and a technical casing (3) is installed and cemented. After drilling to the top of the target layer, the horizontal section is drilled for 800-1000m along the top of the target layer (1) in the roof. After drilling is completed, a production casing (4) is installed and cemented in the horizontal section. The horizontal well (7) for coal gas extraction adopts segmented hydraulic directional perforation to carry out horizontal section fracturing. Perforation fracturing is carried out in the direction from the top plate to the bottom plate of the target layer (1). The spacing between each perforation segment is set to 25m. The length of a single hydraulic perforation segment does not exceed 2 / 3 of the thickness of the target layer (1) in order to avoid affecting the stability of the CO2 injection horizontal well (6). In a CO2 injection horizontal well with a three-section wellbore structure (6), the vertical section is first drilled to the loose formation above the target layer (1), and the surface casing (2) is installed and cemented during vertical drilling; the directional drilling is then carried out to the bottom plate of the target layer (1), and the technical casing (3) is installed and cemented. Drill into the bottom plate of the target layer, and drill horizontally along the bottom plate for 800-1000m. The direction of the horizontal section of the CO2 injection horizontal well (6) is parallel to the horizontal section of the extraction horizontal well (7). After drilling is completed, run the production casing (4) for cementing, and run the CO2 injection horizontal well section screen pipe in the horizontal section to complete the well. After draining and depressurizing the horizontal well (7) for a period of time, the wellhead pressure was stabilized at 12MPa. When the gas production began to decline, liquid CO2 was injected into the CO2 injection horizontal well (6). The initial CO2 injection rate was 80t / d. When the wellhead pressure of the CO2 injection horizontal well (6) was lower than 20MPa, the wellhead pressure of the CO2 injection horizontal well (6) was gradually increased by 2t / d. After that, the CO2 injection rate was continuously adjusted to keep the wellhead pressure of the CO2 injection horizontal well (6) constant at 20MPa, so as to ensure that there is a pressure difference between the top and bottom of the target layer, so that CO2 can better drive CH4 into the pumping horizontal well (7) in the vertical direction. When the CH4 content in the gas produced by the extraction horizontal well (7) is less than 10%, the extraction horizontal well (7) is shut off, while the CO2 injection horizontal well (6) can continue to operate, so as to achieve the purpose of sealing CO2 in complex coal-bearing strata.

3. The method for extracting coalbed methane from complex coal-bearing strata using dual horizontal wells with CO2 circulation injection for vertical displacement, as described in claim 1, is characterized in that: The CO2 gas source includes a CO2 storage tank (17) and a plunger pump (20) connected to each other. The CO2 storage tank (17) is equipped with a liquid CO2 storage tank temperature and pressure gauge (18). The inlet of the CO2 storage tank (17) is equipped with a liquid CO2 main gate valve (16). The liquid CO2 main gate valve (16) is connected to the liquid CO2 tank truck (14) through the liquid CO2 tank truck gate valve (15). The output of the CO2 storage tank (17) is connected to the plunger pump (20) through the liquid CO2 transport pipeline (32). The plunger pump (20) is connected to an electric motor (21). The liquid CO2 transport pipeline (32) is equipped with a liquid CO2 first gate valve (19). The plunger pump (20) is connected to the end of the liquid CO2 injection pipeline (10). The end of the liquid CO2 injection pipeline is equipped with a liquid CO2 flow meter (22), a liquid CO2 second gate valve (23), and a liquid CO2 pressure gauge (24).

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