A closed-cycle heat extraction method and system for depleted deep shale reservoirs

Abstract

The present invention discloses a closed-cycle heat extraction method and system for depleted deep shale reservoirs, belonging to the field of energy extraction and utilization technology. The method comprises the following steps: (1) selecting a target well group; (2) performing a modification on the target well group; sealing the casing of the fracturing section of the target well group; connecting the toes of the horizontal sections of adjacent production wells of the target well group through a newly drilled section to establish a closed-cycle loop; (3) conducting a fluid circulation test to ensure that no fluid leakage occurs; after the test is completed, injecting a heat-carrying medium into the closed-cycle loop to perform closed-cycle heat extraction. The present invention can fully develop geothermal energy by modifying existing abandoned cluster fracturing production wells to perform closed-cycle heat extraction. Moreover, the present invention can effectively improve the closed-cycle heat extraction effect by providing an ultra-long heating path for the heat-carrying medium through a long horizontal section. The present invention can also effectively reduce the cost of geothermal development and has good application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of energy mining and utilization, and in particular to a closed-cycle heat extraction method and system for depleted deep shale reservoirs based on sidetracking modification of cluster production wells. Background Art

[0002] Geothermal energy is a clean and renewable energy source with abundant reserves, wide distribution, stability and reliability. Its efficient development and utilization are of great significance to the adjustment of the national energy structure. Geothermal extraction usually adopts an open heat extraction system or a closed heat extraction system. In a closed heat extraction system, the heat-carrying medium is not in direct contact with the heat storage rock, which can significantly reduce the risk of pipeline corrosion and scaling. However, the closed heat extraction system mainly extracts heat through heat conduction, and its heat extraction rate is limited by the length of the heat exchange well section. Taking into account the high requirements of ground application scenarios for heat extraction rate, the existing closed heat extraction systems such as coaxial heat exchange and U-well heat exchange systems have limited length of heat exchange section in the heat storage, which is difficult to operate at high circulation rates (such as water circulation rate >1000m 3 / day) to maintain high wellhead production temperature and heat recovery rate, which limits its large-scale promotion and application.

[0003] Furthermore, as deep shale oil and gas reservoirs gradually deplete, established shale gas wells are often abandoned and permanently sealed, resulting in a waste of resources. Furthermore, due to the high temperatures of deep shale oil and gas reservoirs, the rich geothermal energy they contain cannot be fully utilized. Summary of the Invention

[0004] Based on the above technical problems, the present invention proposes a closed-cycle heat extraction method and system for depleted deep shale reservoirs.

[0005] The technical solution adopted by the present invention is:

[0006] A closed-cycle heat extraction method for a depleted deep shale reservoir comprises the following steps:

[0007] (1) Select the target well group;

[0008] (2) Carry out transformation on the target well group;

[0009] Seal the casing of the fracturing section of the target well group; connect the toes of the horizontal sections of adjacent production wells in the target well group through the newly drilled section to establish a closed circulation loop;

[0010] (3) Conduct fluid circulation test to ensure that there is no fluid leakage; after the test is completed, inject heat-carrying fluid into the closed circulation loop to carry out closed circulation heat extraction.

[0011] Preferably, in step (1): a development well platform is selected that meets the requirement of a depleted shale reservoir temperature greater than 100°C and a distance between the hot end and the development well platform less than 10 km; and three parallel horizontal production wells with production casing inner diameters of 118.62 mm or more and no deformation of the production casing are selected from the same development well platform as the target well group for transformation.

[0012] Preferably, the length of the horizontal section of the selected production well is between 1000 and 3000 m, and the distance between the horizontal sections of two adjacent production wells is between 300 and 900 m.

[0013] Preferably, in step (2): for the selected target well group, the original oil pipe in the production well is pulled out, and the wellbore working condition is checked by using downhole imaging or acoustic monitoring; expansion casing is lowered into the horizontal sections of the three production wells in the target well group, and high thermal conductivity cement with a thermal conductivity of more than 1.0 W / m / K is injected to seal the fracturing casing of the horizontal sections of the three production wells and improve the strength of the well wall.

[0014] Preferably, in combination with the wellbore trajectory of the original production well, the window target position and side drilling trajectory of the toe end of the horizontal section of the production wells on both sides of the target well group are determined to ensure effective connection with the toe end of the horizontal section of the intermediate production well through the newly drilled section; the milling tool is lowered to perform casing windowing at the window target position at the toe end of the horizontal section of the production wells on both sides; the flexible drilling tool and the drill bit are used to side drill along the side drilling trajectory toward the toe end of the horizontal section of the intermediate production well; the casing is lowered into the newly drilled section and high thermal conductivity cement is injected to complete the well, and the thermal conductivity of the cement is above 1.0 W / m / K.

[0015] Preferably, the size of the drill bit is 3 1 / 2" to 4 1 / 2"; during the sidetracking process, the drilling trajectory is monitored in real time by a measurement while drilling system to ensure that the wellbore trajectory meets the design requirements; the connection between the sidetracking section casing and the production well expansion casing is sealed with a metal sealing ring to establish a closed circulation loop.

[0016] Preferably, in step (2): the three production wells are production well 1, production well 2 and production well 3, and production well 2 is located between production well 1 and production well 3; the three production wells all include a vertical well section, a vertical deflection section and a horizontal section connected in sequence; the horizontal section of production well 1 and the horizontal section of production well 3 are both connected to the side drilling deflection section at the toe end, and then connected to the toe end of the horizontal section of production well 2 through the side drilling horizontal section through a three-way pipe.

[0017] Preferably, the heat-carrying medium is injected into the production well 2 located in the middle of the target well group, and flows to the toe of the production well 2 through the vertical well section, vertical deflection section and horizontal section of the production well 2, and then is diverted to the toes of the production wells 1 and 3 on both sides through the side drilling horizontal section, and then flows through the horizontal section, vertical deflection section and vertical well section of the production wells 1 and 3 on both sides to complete the closed circulation; after the above closed circulation process, the heat-carrying medium is heated by the heat storage and then produced from the wellheads of the production wells 1 and 3 on both sides.

[0018] The present invention also provides a closed-cycle heat extraction system for a depleted deep shale reservoir, comprising a first production well, a second production well, and a third production well, wherein the second production well is located between the first and third production wells; the first production well, the second production well, and the third production well each comprise a vertical well section, a vertical deflection section, and a horizontal section connected in sequence; the horizontal section of the first production well and the horizontal section of the third production well are both connected to a sidetracking section at their toes, and the sidetracking sections connected to the first and third production wells each extend toward the second production well;

[0019] The toe end of the horizontal section of production well 2 is connected to one end of the tee pipe, and the other two ends of the tee pipe are connected to the side drilling and deflection sections of production well 1 and production well 3 through two side drilling horizontal sections.

[0020] Preferably, a plugging zone is provided in the horizontal sections of production well 1, production well 2 and production well 3 to plug the casing of the fractured section;

[0021] The horizontal lengths of the production wells 1, 2 and 3 are set between 1000 and 3000 m, and the spacing between the horizontal sections of the production wells 1 and 2, as well as the spacing between the horizontal sections of the production wells 2 and 3, are both set between 300 and 900 m.

[0022] The thermal insulation coating is evenly sprayed on the inner wall of the upper casing of the vertical well section of production well 1 and production well 3.

[0023] The beneficial technical effects of the present invention are:

[0024] This invention creatively selects and purposefully transforms existing abandoned cluster fracturing production wells to enable closed-loop heat extraction, thereby fully developing geothermal energy. By providing an ultra-long heating path for the heat-carrying medium through a long horizontal section, this invention effectively enhances the closed-loop heat extraction efficiency. This invention not only effectively extends the lifecycle of shale gas wells, prolongs the development life of reservoirs, and maximizes the reservoir's potential, but also transforms existing abandoned wells, effectively reducing geothermal development costs and offering promising application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a schematic flow chart of the closed-cycle heat extraction method for depleted deep shale reservoirs of the present invention;

[0026] Figure 2 This is a schematic diagram of the structural principle of the closed-cycle heat extraction system for depleted deep shale reservoirs of the present invention;

[0027] Figure 3 This is a top view schematic diagram of the closed-cycle heat extraction system for depleted deep shale reservoirs of the present invention.

[0028] In the figure: 1. A vertical section of a production well; 2. A vertical deflection section of production well 1; 3. A horizontal section of production well 1; 4. A vertical section of production well 2; 5. A vertical deflection section of production well 2; 6. A horizontal section of production well 2; 7. A vertical section of production well 3; 8. A vertical deflection section of production well 3; 9. A horizontal section of production well 3; 10. A sidetracking section at the toe of production well 1; 11. A horizontal section connecting the toes of production wells 1 and 2; 12. A sidetracking section at the toe of production well 3; 13. A horizontal section connecting the toes of production wells 2 and 3 Section; 14. Tee pipe; 15. High thermal conductivity cement; 16. Overlying strata; 17. Shale oil and gas reservoir; 18. Existing fracturing casing in the first horizontal section of the production well; 19. Expansion casing lowered into the first horizontal section of the production well; 20. Existing fracturing casing in the second horizontal section of the production well; 21. Expansion casing lowered into the second horizontal section of the production well; 22. Existing fracturing casing in the third horizontal section of the production well; 23. Expansion casing lowered into the third horizontal section of the production well; 24. Thermal insulation coating for the vertical well section; 25. Metal sealing ring. DETAILED DESCRIPTION

[0029] The existing coaxial heat exchange and U-shaped well heat exchange systems have limited heat exchange section length in the heat storage, making it difficult to operate at high circulation rates (>1000m 3 Maintaining high wellhead temperatures and heat recovery rates at low temperatures (per day) limits its large-scale application. Furthermore, the development of traditional heat reservoirs, such as hydrothermal geothermal and hot dry rock, is associated with high well construction costs. Deep shale oil and gas reservoirs possess relatively high temperatures and are rich in geothermal resources. Based on this, the present invention creatively transforms cluster production wells to propose a closed-loop heat extraction method and circulating heat extraction system for depleted deep shale reservoirs.

[0030] The following is a detailed description with reference to the accompanying drawings and specific embodiments.

[0031] Example 1

[0032] like Figure 1 As shown, a closed-cycle heat extraction method for a depleted deep shale reservoir comprises the following steps:

[0033] (1) Select the target well group;

[0034] Specifically, comprehensive considerations were used to select development well platforms that meet the requirements for depleted shale reservoir temperatures greater than 100°C and a distance between the hot end and the development well platform less than 10 km. Within the same development well platform, three relatively parallel production wells with production casing inner diameters of 118.62 mm or greater and no casing deformation were selected as target well groups for stimulation.

[0035] The length of the horizontal section of the production wells in the target well group to be transformed is between 1000 and 3000 meters, and the distance between the horizontal sections of two adjacent production wells is between 300 and 900 meters.

[0036] (2) Carry out transformation on the target well group;

[0037] The casing of the fracturing section of the target well group is sealed; the toes of the horizontal sections of adjacent production wells in the target well group are connected through the newly drilled section to establish a closed circulation loop.

[0038] Specifically, for the selected target well group, the original oil pipes in the production wells are pulled out, and the wellbore conditions are checked using downhole imaging or acoustic monitoring. Figure 2 、 Figure 3 As shown, expansion casings are run into the horizontal sections of the three production wells in the target well group. These expansion casings are 19 in the first horizontal section, 21 in the second horizontal section, and 23 in the third horizontal section. High-thermal-conductivity cement 15 with a thermal conductivity of at least 1.0 W / m / K is then injected to seal the existing fractured casings in the three horizontal sections of the production wells (18, 20, and 22, respectively) and to strengthen the wellbore wall. Based on the wellbore trajectory of the existing production wells, target window locations and sidetracking trajectories are determined near the toes of the horizontal sections of the production wells on both sides of the target well group to ensure effective connectivity with the toes of the intermediate production wells through the newly drilled section. Milling tools are then run to perform casing windowing at the target window locations at the toes of the horizontal sections of the production wells on both sides. Using flexible drilling tools and a small drill bit (sizes 3 1 / 2" to 4 1 / 2"), sidetracking was performed along the planned well trajectory to a target location near the toe of the horizontal section of the intermediate production well. The drilling trajectory was monitored in real time using a measurement-while-drilling system to ensure that the wellbore trajectory met the planned requirements. Casing was run into the newly drilled section and completed with high-thermal-conductivity cement 15, with a thermal conductivity of at least 1.0 W / m / K. A metal seal 25 was used to seal the connection between the sidetracked section casing and the production well expansion casing, establishing a closed circulation loop.

[0039] like Figure 2 、 Figure 3As shown, the three production wells are Production Well 1, Production Well 2, and Production Well 3, with Production Well 2 located between Production Well 1 and Production Well 3. Each of the three production wells includes a vertical well section, a vertical inclination section, and a horizontal section connected in sequence. The horizontal section 3 of Production Well 1 and the horizontal section 9 of Production Well 3 are connected at their toes to side-drilled inclination sections, namely, the side-drilled inclination section 10 at the toe of Production Well 1 and the side-drilled inclination section 12 at the toe of Production Well 3. These sections are then connected to the toe of the horizontal section 6 of Production Well 2 through a tee pipe 14, with metal sealing rings 25 used to seal the connection. More specifically, taking the connection between production wells 1 and 2 as an example, one end of the sidetracking section 10 at the toe of production well 1 is inserted into the expansion casing 19 lowered into the horizontal section of production well 1, and the connection is sealed with a metal seal 25. The other end of the sidetracking section 10 at the toe of production well 1 is connected to one end of the horizontal section 11 connecting the toes of production wells 1 and 2. The other end of the horizontal section 11 connecting the toes of production wells 1 and 2 is inserted into one end of the tee pipe 14, and the connection is sealed with a metal seal 25. The second end of the tee pipe 14 is inserted into the expansion casing 21 lowered into the horizontal section of production well 2, and the connection is sealed with a metal seal 25. Similarly, the third end of the tee pipe 14 is inserted into the horizontal section 13 connecting the toes of production wells 2 and 3, and the connection is sealed with a metal seal 25. The connecting horizontal section 13 at the toe end of the production well 2 and the production well 3 is connected to the expansion casing 23 lowered into the horizontal section of the production well 3 through the side-drilling and deflecting section 12 at the toe end of the production well 3.

[0040] (3) Conduct fluid circulation test to ensure that there is no fluid leakage. After the test is completed, inject heat-carrying fluid into the closed circulation loop and carry out closed circulation heat extraction.

[0041] Specifically, a heat-carrying medium is injected into Production Well 2, located in the middle of the target well group. It flows through the vertical section 4, the vertical deflection section 5, and the horizontal section 6 of Production Well 2 to the toe of Production Well 2. It then flows through the sidetracked horizontal sections, namely the horizontal section 11 connecting the toes of Production Wells 1 and 2, and the horizontal section 13 connecting the toes of Production Wells 2 and 3, to the toes of the horizontal sections of Production Wells 1 and 3 on either side. It then flows through the horizontal sections, vertical deflection sections, and vertical sections of Production Wells 1 and 3 on either side, completing a closed cycle. After this closed cycle, the heat-carrying medium is heated by the heat storage and then produced from the wellheads of Production Wells 1 and 3 on both sides.

[0042] In summary, the present invention is based on a cluster production well platform for shale oil and gas reservoirs, and selects three abandoned production wells as the target well group for transformation. The horizontal section length of the selected abandoned production wells is between 1000 and 3000 meters, and the distance between two adjacent production wells is between 300 and 900 meters. High thermal conductivity cement is injected into the target well group to seal the artificial cracks in the fracturing section of the production well to improve the strength of the well wall. A window is drilled sideways at the toe of the production well to establish a connection between the three production wells. Heat exchange pipelines are lowered into the horizontal sections of the three production wells, and metal sealing rings are used to seal the inclination section and the toe side drilling section to construct a closed-loop heat exchange system with an ultra-long circulation loop. During the closed-loop heat extraction process, the circulating working fluid is injected from the middle production well and diverted to the production wells on both sides after reaching its toe. The flow rate is further reduced, thereby improving the heat exchange effect. This closed-loop heat extraction method can efficiently extract geothermal energy associated with deep depleted shale oil and gas reservoirs.

[0043] Example 2

[0044] like Figure 2 、 Figure 3 As shown, a closed-cycle heat extraction system for a depleted deep shale reservoir includes production well 1, production well 2, and production well 3, with production well 2 located between production wells 1 and 3. Production well 1 includes a sequentially connected production well vertical section 1, production well 1 vertical deflection section 2, and production well 1 horizontal section 3. Production well 2 includes a sequentially connected production well vertical section 4, production well 2 vertical deflection section 5, and production well 2 horizontal section 6. Production well 3 includes a sequentially connected production well vertical section 7, production well 3 vertical deflection section 8, and production well 3 horizontal section 9. Both production well 1 horizontal section 3 and production well 3 horizontal section 9 are connected at their toes to sidetracking sections: production well 1 toe sidetracking section 10 and production well 3 toe sidetracking section 12, respectively. Both production well 1 toe sidetracking section 10 and production well 3 toe sidetracking section 12 extend toward production well 2. The toe of horizontal section 6 of Production Well 2 is connected to one end of a tee pipe 14. The other two ends of tee pipe 14 are connected to the sidetracking sections of Production Wells 1 and 3 through two sidetracking horizontal sections, respectively. Metal sealing rings are used at these connections. These two sidetracking horizontal sections are horizontal section 11 connecting the toes of Production Wells 1 and 2, and horizontal section 13 connecting the toes of Production Wells 2 and 3.

[0045] Example 3

[0046] The basic structure is the same as that of Example 1, except that: a plugging area is provided in the horizontal section of production well 1, production well 2 and production well 3. The artificial cracks in the fracturing section of the production well can be plugged by the plugging area to improve the strength of the well wall. That is to say, the fracturing section casing is plugged by inserting an expansion casing into the horizontal section, and sealed and cemented with high thermal conductivity cement. The horizontal section lengths of the above-mentioned production wells 1, 2 and 3 are set between 1000 and 3000 m, and the spacing between the horizontal sections of production well 1 and production well 2, as well as the spacing between the horizontal sections of production well 2 and production well 3 are both set between 300 and 900 m. The thermal insulation coating is also evenly sprayed on the inner wall of the upper casing of the vertical well section of production well 1 and production well 3, such as Figure 2 and Figure 3 The casing here is the part left after the original oil pipe is pulled out from the vertical section of the production well and can be used again.

[0047] The closed-circulation heat extraction system for depleted deep shale reservoirs based on cluster production well sidetracking works as follows: Heat-carrying fluid is injected into production well 2, located in the middle of the target well group. It flows through production well 2's vertical section 4, production well 2's vertical deflection section 5, and production well 2's horizontal section 6 to the toe of production well 2's horizontal section. It then flows through the sidetracked horizontal sections, namely, the horizontal section 11 connecting the toes of production wells 1 and 2, and the horizontal section 13 connecting the toes of production wells 2 and 3, to the toes of the horizontal sections of production wells 1 and 3 on either side. It then flows through production well 1's horizontal section 3, production well 1's vertical deflection section 2, production well 1's vertical section 1, production well 3's horizontal section 9, production well 3's vertical deflection section 8, and production well 3's vertical section 7, completing the closed circulation. After this closed circulation process, the heat-carrying fluid is heated by the heat reservoir and then extracted from the wellheads of production wells 1 and 3 on both sides.

[0048] The closed-loop heat extraction system proposed in this invention utilizes existing abandoned cluster production wells in shale reservoirs. By side-drilling the production well toes, the three production wells are connected, forming a closed loop. This expands the wellbore's controlled heat storage volume and effectively increases the heat exchange time of the heat-carrying fluid within the wellbore. During the closed-loop heat extraction process, the heat-carrying fluid is injected into the center well and extracted from the two side wells. The heat exchange fluid is split upon reaching the toe of the horizontal section of the center production well. The heat-carrying fluid in the two side wells maintains a lower flow rate, enabling more efficient heat exchange, improving wellhead extraction temperature and heat extraction efficiency, while maintaining the total surface heat-carrying fluid flow rate. Furthermore, a uniformly sprayed insulating coating is applied to the inner wall of the upper casing of the extraction section, or vertical well section, of the two production wells to further minimize heat loss from the heat-carrying fluid. This system can effectively extend the lifecycle of shale gas wells, extend the reservoir's development lifespan, and maximize the reservoir's potential. Furthermore, by retrofitting existing abandoned wells, the system can significantly reduce well construction costs for geothermal development.

[0049] Parts not described in the above methods can be achieved by adopting or drawing on existing technologies.

[0050] Finally, it should be noted that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any improvements, equivalent substitutions, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A closed cycle heat extraction method for depleted deep shale reservoirs, characterized in that The following steps are involved: (1) Select the target well group; (2) Carry out transformation on the target well group; Seal the casing of the fracturing section of the target well group; connect the toes of the horizontal sections of adjacent production wells in the target well group through the newly drilled section to establish a closed circulation loop; (3) Conduct fluid circulation test to ensure that there is no fluid leakage; after the test is completed, inject heat-carrying medium into the closed circulation loop and carry out closed circulation heat extraction; In step (1): a development well platform is selected that meets the requirement of a depleted shale reservoir temperature greater than 100°C and a distance between the hot end and the development well platform less than 10 km; three parallel horizontal production wells with production casing inner diameters of 118.62 mm or more and no deformation of the production casing are selected from the same development well platform as the target well group for transformation; The horizontal section length of the selected production wells is between 1000 and 3000 m, and the distance between the horizontal sections of two adjacent production wells is between 300 and 900 m; In step (2): for the selected target well group, pull out the original oil pipe in the production well, and use downhole imaging or acoustic monitoring to check the wellbore working condition; Expandable casings were run into the horizontal sections of the three production wells in the target well group, and high-thermal-conductivity cement with a thermal conductivity of more than 1.0 W / m / K was injected to seal the fracturing casings in the horizontal sections of the three production wells and improve the strength of the wellbore wall. Based on the wellbore trajectory of the original production wells, the window target positions and sidetracking trajectories at the toes of the horizontal sections of the production wells on both sides of the target well group are determined to ensure effective connectivity with the toes of the horizontal sections of the intermediate production wells through the newly drilled section. Milling tools are lowered to perform casing windowing at the window target positions at the toes of the horizontal sections of the production wells on both sides. Flexible drilling tools and drill bits are used to sidetrack along the sidetracking trajectory toward the toes of the horizontal sections of the intermediate production well. Casing is lowered into the newly drilled section and high-thermal-conductivity cement is injected for completion. The thermal conductivity of the cement is above 1.0 W / m / K.

2. The closed-cycle heat extraction method for depleted deep shale reservoirs according to claim 1, characterized in that: The drill bit size ranges from 3 1 / 2" to 4 1 / 2". During the sidetracking process, the drilling trajectory is monitored in real time through a measurement-while-drilling system to ensure that the wellbore trajectory meets design requirements. The connection between the sidetracking section casing and the production well expansion casing is sealed with a metal sealing ring to establish a closed circulation loop.

3. The closed-cycle heat extraction method for depleted deep shale reservoir according to claim 1, characterized in that: In step (2): the three production wells are production well 1, production well 2 and production well 3, and production well 2 is located between production well 1 and production well 3; the three production wells all include a vertical well section, a vertical deflection section and a horizontal section connected in sequence; the horizontal section of production well 1 and the horizontal section of production well 3 are both connected to the side drilling deflection section at the toe end, and then connected to the toe end of the horizontal section of production well 2 through the side drilling horizontal section through a three-way pipe.

4. The closed-cycle heat extraction method for depleted deep shale reservoirs according to claim 3, characterized in that: The heat-carrying medium is injected into the production well 2 located in the middle of the target well group, flows to the toe of the production well 2 through the vertical well section, vertical deflection section and horizontal section of the production well 2, and then is diverted to the toes of the production wells 1 and 3 on both sides through the side drilling horizontal section, and then flows through the horizontal section, vertical deflection section and vertical well section of the production wells 1 and 3 on both sides to complete the closed circulation; after the above closed circulation process, the heat-carrying medium is heated by the heat storage and then produced from the wellheads of the production wells 1 and 3 on both sides.

5. A closed-cycle heat extraction system for depleted deep shale reservoirs obtained by the method of any one of claims 1 to 4, characterized in that: The system includes production well 1, production well 2, and production well 3, with production well 2 located between production well 1 and production well 3. Production well 1, production well 2, and production well 3 all include a vertical well section, a vertical deflection section, and a horizontal section connected in sequence. The horizontal section of production well 1 and the horizontal section of production well 3 are both connected to a sidetracking section at their toes, and the sidetracking section connecting production well 1 and production well 3 extends toward production well 2. The toe end of the horizontal section of production well 2 is connected to one end of the tee pipe, and the other two ends of the tee pipe are connected to the side drilling and deflection sections of production well 1 and production well 3 through two side drilling horizontal sections.

6. The closed-cycle heat extraction system for depleted deep shale reservoirs according to claim 5, characterized in that: A plugging zone is set up in the horizontal sections of production wells 1, 2 and 3; The horizontal lengths of the production wells 1, 2 and 3 are set between 1000 and 3000 m, and the spacing between the horizontal sections of the production wells 1 and 2, as well as the spacing between the horizontal sections of the production wells 2 and 3, are both set between 300 and 900 m. The thermal insulation coating is evenly sprayed on the inner wall of the upper casing of the vertical well section of production well 1 and production well 3.

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