Method and system for closed circulation heat removal of depleted deep shale reservoir

Abstract

The invention discloses a closed circulation heat extraction method and system for an exhausted deep shale reservoir, and belongs to the technical field of energy exploitation and utilization. The method comprises the following steps: (1) selecting a target well group; (2) transforming a target well group; the fracturing section sleeve of the target well group is sealed; the toe ends of the horizontal sections of the adjacent production wells of the target well group are communicated through the new well drilling section, and a closed circulation loop is established; (3) a fluid circulation test is carried out, and it is ensured that fluid leakage does not occur; after the test is completed, a heat-carrying working medium is injected into the closed circulation loop, and closed circulation heat removal is carried out. According to the invention, the existing waste-stopped cluster fracturing production well is transformed to carry out closed circulating heat extraction, so that the geothermal energy can be fully developed. And in addition, an ultra-long heating path is provided for the heat-carrying working medium through the long horizontal section, and the closed circulation heat removal effect can be effectively improved. In addition, the geothermal development cost can be effectively reduced, and good application prospects are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of energy exploitation and utilization, and in particular to a closed-cycle heat extraction method and system for depleted deep shale reservoirs based on side-drilling transformation 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 circulation 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 for heat extraction rate in ground application scenarios, the existing closed heat extraction systems such as coaxial heat exchange and U-well heat exchange systems have limited length of heat exchange sections in the heat storage, which makes it difficult to operate at high circulation rates (such as water circulation rates > 1000m 3 / day) to maintain high wellhead production temperature and heat recovery rate, which limits its large-scale promotion and application.

[0003] In addition, as deep shale oil and gas reservoirs are gradually depleted, the established shale gas production wells are often directly abandoned and permanently sealed, resulting in a waste of resources. Moreover, due to the high temperature of deep shale oil and gas reservoirs, the rich geothermal energy they contain cannot be well 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] The casing of the fracturing section of the target well group is sealed; the toes of the horizontal sections of the adjacent production wells of the target well group are connected 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 > 100°C and a distance between the hot end and the development well platform < 10 km; and three parallel horizontal production wells with production casing inner diameters of more than 118.62 mm 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 condition is checked by using downhole imaging or acoustic wave monitoring; expansion casing is lowered into the horizontal sections of the three production wells of the target well group, and high thermal conductivity cement is injected, whose thermal conductivity is above 1.0 W / m / K, 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 wells, the window target positions and side drilling trajectories of the toes of the horizontal sections of the production wells on both sides of the target well group are determined to ensure effective connection with the toes of the horizontal sections of the middle production wells through the newly drilled section; the milling tool is lowered to perform casing window openings at the window target positions at the toes of the horizontal sections 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 of the horizontal section of the middle production well; the casing is lowered into the newly drilled section of the side drilling and high thermal conductivity cement is injected for completion, 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 inclination 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 inclination 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, a heat-carrying medium is injected into production well two located in the middle of the target well group, flows to the toe of production well two through the vertical well section, vertical inclination section and horizontal section of production well two, and then is diverted to the toes of production wells one and three on both sides through the side-drilling horizontal section, and then flows through the horizontal section, vertical inclination section and vertical well section of production wells one and three on both sides to complete a closed circulation; after the above-mentioned closed circulation process, the heat-carrying medium is heated by the heat storage and then produced from the wellheads of production wells one and three on both sides.

[0018] The present invention also provides a closed-cycle heat extraction system for a depleted deep shale reservoir, which comprises a production well 1, a production well 2 and a production well 3, wherein the production well 2 is located between the production well 1 and the production well 3; the production well 1, the production well 2 and the production well 3 all comprise a vertical well section, a vertical deflection section and a horizontal section connected in sequence; the horizontal section of the production well 1 and the horizontal section of the production well 3 are both connected to a side-drilling deflection section at the toe end, and the side-drilling deflection section connected to the production well 1 and the production well 3 both extend toward the direction of the production well 2;

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

[0020] Preferably, a plugging area 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 section lengths of the production well 1, production well 2 and production well 3 are set between 1000 and 3000 m, and the spacing between the horizontal sections of the production well 1 and production well 2, and the spacing between the horizontal sections of the production well 2 and production well 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 wells one and three.

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

[0024] The present invention creatively selects existing abandoned cluster fracturing production wells and purposefully transforms them to perform closed-loop heat extraction, thereby fully developing geothermal energy. The present invention provides an ultra-long heating path for the heat-carrying medium through a long horizontal section, which can effectively improve the closed-loop heat extraction effect. The present invention can not only effectively extend the life cycle of shale gas wells, extend the development life of the reservoir, and maximize the potential of the reservoir, but also transforms existing abandoned wells, which can effectively reduce the cost of geothermal development and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the closed cycle heat extraction method for depleted deep shale reservoir 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 It is a top view schematic diagram of the closed cycle heat extraction system for depleted deep shale reservoir of the present invention.

[0028] In the figure: 1. A vertical section of a production well; 2. A vertical deflection section of a production well; 3. A horizontal section of a production well; 4. A vertical section of a production well; 5. A vertical deflection section of a production well; 6. A horizontal section of a production well; 7. A vertical section of a production well; 8. A vertical deflection section of a production well; 9. A horizontal section of a production well; 10. A side-drilling deflection section at the toe end of a production well; 11. A horizontal section connecting the toe ends of production wells one and two; 12. A side-drilling deflection section at the toe end of a production well; 13. A horizontal section connecting the toe ends of production wells two and three 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-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 / day) to maintain high wellhead production temperature and heat extraction rate, which limits its large-scale promotion and application. In addition, the development of traditional heat storage, such as hydrothermal geothermal and hot dry rock, has high well construction costs. Deep shale oil and gas reservoirs have high temperatures and rich geothermal resources. Based on this, the present invention creatively transforms cluster production wells and proposes 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 consideration is given to selecting development well platforms that meet the requirements of depleted shale reservoir temperature > 100°C and a distance between the hot end and the development well platform < 10km. In the same development well platform, three relatively parallel production wells with production casing inner diameters of more than 118.62mm and no casing deformation are selected as the target well group to be transformed.

[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 fracturing section casing of the target well group is sealed; the toes of the horizontal sections of adjacent production wells of 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, pull out the original oil pipe in the production well, and use downhole imaging or acoustic monitoring to check the wellbore condition. Figure 2 , Figure 3 As shown, expansion casings are lowered into the horizontal sections of the three production wells of the target well group, namely, expansion casing 19 lowered into the horizontal section of production well 1, expansion casing 21 lowered into the horizontal section of production well 2, and expansion casing 23 lowered into the horizontal section of production well 3, and high thermal conductivity cement 15 is injected, whose thermal conductivity is above 1.0W / m / K, to seal the already fractured casings of the horizontal sections of the three production wells, namely, the original fractured casing 18 of the horizontal section of production well 1, the original fractured casing 20 of the horizontal section of production well 2, and the original fractured casing 22 of the horizontal section of production well 3, and to improve the strength of the well wall. Combined with the wellbore trajectory of the original production well, the window opening target position and side drilling trajectory near 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 middle production well through the newly drilled well section. The milling tool is lowered to perform casing window opening at the window opening target position at the toe end of the horizontal section of the production wells on both sides. Using flexible drilling tools and small-sized drill bits (size 3 1 / 2" to 4 1 / 2"), side-drill along the designed well trajectory to the designed target position near the toe of the horizontal section of the intermediate production well. The drilling trajectory is monitored in real time by the measurement while drilling system to ensure that the wellbore trajectory meets the design requirements. Casing is lowered into the new drilling section of the side-drilling and high thermal conductivity cement 15 is injected for completion. The thermal conductivity of the cement is above 1.0W / m / K. The connection between the side-drilling section casing and the expansion casing of the production well is sealed with a metal sealing ring 25 to establish 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, and production well 2 is located between production well 1 and production well 3. The three production wells all include a straight 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 to the side drilling inclination section at the toe end, which are the side drilling inclination section 10 at the toe end of production well 1 and the side drilling inclination section 12 at the toe end of production well 3, respectively. Then, through the side drilling horizontal section, the horizontal section 11 connected to the toe end of production well 1 and production well 2, the horizontal section 13 connected to the toe end of production well 2 and production well 3 are connected to the toe end of the horizontal section 6 of production well 2 through a three-way pipe 14, and the connection is sealed by a metal sealing ring 25. More specifically, taking the connection between production well 1 and production well 2 as an example, one end of the side drilling and deflection section 10 at the toe end of production well 1 is inserted into the expansion casing 19 lowered into the horizontal section of production well 1, and the metal sealing ring 25 is used to seal the connection, and the other end of the side drilling and deflection section 10 at the toe end of production well 1 is connected to one end of the horizontal section 11 connecting the toe ends of production well 1 and production well 2, and the other end of the horizontal section 11 connecting the toe ends of production well 1 and production well 2 is inserted into one of the ports of the three-way pipe 14, and the metal sealing ring 25 is used to seal the connection; the second port of the three-way pipe 14 is inserted into the expansion casing 21 lowered into the horizontal section of production well 2, and the metal sealing ring 25 is used to seal the connection. As above, the third port of the three-way pipe 14 is inserted into the horizontal section 13 connecting the toe ends of production well 2 and production well 3, and the metal sealing ring 25 is used to seal the connection. The connecting horizontal section 13 at the toe end of production well 2 and production well 3 is connected to the expansion casing 23 lowered into the horizontal section of production well 3 through the side drilling and deflection section 12 at the toe end of 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 to carry out closed circulation heat extraction.

[0041] Specifically, 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 4, the vertical deflection section 5 and the horizontal section 6 of the production well 2, and then flows to the toe of the horizontal section of the production well 1 and the production well 3 on both sides through the side drilling horizontal section, that is, the horizontal section 11 connected to the toe of the production well 1 and the production well 2 and the horizontal section 13 connected to the toe of the production well 1 and the production well 3, and then flows through the horizontal section, vertical deflection section and vertical well section of the production well 1 and the production well 3 on both sides to complete the closed cycle. After the above closed cycle process, the heat-carrying medium is heated by the heat storage and then produced from the wellheads of the production well 1 and the production well 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 m, and the distance between two adjacent production wells is between 300 and 900 m. 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 opened at the toe end of the production well for side drilling to establish the connection relationship 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 end side drilling section to construct a closed cycle heat exchange system with an ultra-long circulation loop. In the closed cycle heat extraction process, the circulating working fluid is injected from the middle production well, and after reaching its toe end, it is diverted to the production wells on both sides, and the flow rate is further reduced to improve the heat exchange effect. This closed cycle heat extraction method can efficiently extract the associated geothermal energy of 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, and production well 2 is located between production well 1 and production well 3. Production well 1 includes a production well straight section 1, a production well 1 vertical deflection section 2 and a production well 1 horizontal section 3 connected in sequence, production well 2 includes a production well 2 straight section 4, a production well 2 vertical deflection section 5 and a production well 2 horizontal section 6 connected in sequence, and production well 3 includes a production well 3 straight section 7, a production well 3 vertical deflection section 8 and a production well 3 horizontal section 9 connected in sequence. The production well 1 horizontal section 3 and the production well 3 horizontal section 9 are both connected to the side drilling and deflection sections at the toe end, namely, the production well 1 toe side drilling and deflection section 10 and the production well 3 toe side drilling and deflection section 12. The production well 1 toe side drilling and deflection section 10 and the production well 3 toe side drilling and deflection section 12 both extend toward the production well 2. The toe end of the horizontal section 6 of the production well 2 is connected to one of the ports of the three-way pipe 14, and the other two ports of the three-way pipe 14 are respectively connected to the side drilling and deflection sections of the production well 1 and the production well 3 through two side drilling horizontal sections, and the connection points are sealed with metal sealing rings. The two side drilling horizontal sections are the horizontal section 11 connected to the toe ends of the production well 1 and the production well 2, and the horizontal section 13 connected to the toe ends of the production well 2 and the production well 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 fractures 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 Well 1, Production Well 2 and Production Well 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 continuously.

[0047] The working process of the closed-cycle heat extraction system for depleted deep shale reservoirs based on cluster production well side-drilling transformation is as follows: 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 end of the horizontal section of the production well 2 through the vertical well section 4 of the production well 2, the vertical deflection section 5 of the production well 2, and the horizontal section 6 of the production well 2, and then flows to the toe ends of the horizontal sections of the production well 1 and the production well 3 on both sides through the side-drilling horizontal section, that is, the horizontal section 11 connected to the toe ends of the production well 1 and the production well 2 and the horizontal section 13 connected to the toe ends of the production well 2 and the production well 3, and then flows through the horizontal section 3 of the production well 1, the vertical deflection section 2 of the production well 1, and the vertical well section 1 of the production well 1, as well as the horizontal section 9 of the production well 3, the vertical deflection section 8 of the production well 3, and the vertical well section 7 of the production well 3 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 well 1 and the production well 3 on both sides.

[0048] The closed-loop heat extraction system proposed in the present invention is based on the existing shale reservoir abandoned cluster production wells. By side-drilling and transformation at the toe of the production well, the three production wells are connected to form a closed loop, which expands the control volume of the wellbore for the heat storage and effectively increases the heat exchange time of the heat-carrying medium in the wellbore. In the closed-loop heat extraction process, the heat-carrying medium is injected from the middle well and extracted from the wells on both sides. The heat exchange medium is divided after reaching the toe of the horizontal section of the middle production well. The heat-carrying medium in the heat exchange wells on both sides has a smaller flow rate, and the heat exchange is more sufficient, the wellhead extraction temperature and heat extraction power are improved, and the total flow rate of the heat-carrying medium produced on the ground is not affected. At the same time, the thermal insulation coating is evenly sprayed on the inner wall of the upper casing of the extraction section or the vertical well section of the production wells on both sides, which can further avoid the heat loss of the heat-carrying medium. The system can effectively extend the life cycle of shale gas wells, extend the development life of the reservoir, and maximize the potential of the reservoir. At the same time, transformation based on the existing abandoned wells can effectively reduce the cost of well construction for geothermal development.

[0049] The parts not mentioned in the above methods can be realized 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 protection scope of the present invention.

Claims

1. A closed cycle heat extraction method for depleted deep shale reservoir, characterized in that The following steps are involved: (1) Select the target well group; (2) Carry out transformation on the target well group; The casing of the fracturing section of the target well group is sealed; the toes of the horizontal sections of the adjacent production wells of the target well group are connected 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 fluid into the closed circulation loop to carry out closed circulation heat extraction.

2. A closed cycle heat extraction method for depleted deep shale reservoir according to claim 1, characterized in that: In step (1): a development well platform is selected that meets the requirement of depleted shale reservoir temperature > 100°C and the distance between the hot end and the development well platform is < 10 km; and three parallel horizontal production wells with production casing inner diameters of more than 118.62 mm and no deformation of the production casing are selected from the same development well platform as the target well group for transformation.

3. A closed cycle heat extraction method for depleted deep shale reservoir according to claim 2, characterized in that: The length of the horizontal section 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.

4. A closed cycle heat extraction method for depleted deep shale reservoir according to claim 1, characterized in that: In step (2): for the selected target well group, the original oil pipe in the production well is pulled out, and the wellbore condition is checked by using downhole imaging or acoustic wave monitoring; expansion casing is lowered into the horizontal sections of the three production wells in the target well group, and high thermal conductivity cement is injected, whose thermal conductivity is above 1.0 W / m / K, to seal the fracturing casing of the horizontal sections of the three production wells and improve the strength of the well wall.

5. A closed cycle heat extraction method for depleted deep shale reservoir according to claim 4, characterized in that: Combined with the wellbore trajectory of the original production wells, determine the window target position and sidetracking trajectory at the toe of the horizontal section of the production wells on both sides of the target well group to ensure effective connection with the toe of the horizontal section of the middle production well through the newly drilled section; lower the milling tool to perform casing window opening at the window target position at the toe of the horizontal section of the production wells on both sides; use flexible drilling tools and drill bits to sidetrack along the sidetracking trajectory to the toe of the horizontal section of the middle production well; lower the casing into the newly drilled section and inject high thermal conductivity cement for completion, with the thermal conductivity of the cement being above 1.0W / m / K.

6. A closed cycle heat extraction method for depleted deep shale reservoir according to claim 5, characterized in that: 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.

7. A closed cycle heat extraction method for depleted deep shale reservoir according to claim 2, 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 inclination section and a horizontal section connected in sequence; the horizontal section of production well 1 and the horizontal section of production well 3 are connected to the side drilling inclination 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.

8. A closed cycle heat extraction method for depleted deep shale reservoir according to claim 7, characterized in that: 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.

9. A closed-cycle heat extraction system for depleted deep shale reservoirs transformed by any of the methods of claims 1 to 8, characterized in that: It includes production well 1, production well 2 and production well 3, and production well 2 is 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 inclination 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 inclination section at the toe end, and the side drilling inclination section connected to production well 1 and production well 3 both extends toward the direction of production well 2; The toe end of the horizontal section of the production well 2 is connected to one of the ports of the tee pipe, and the other two ports of the tee pipe are connected to the side drilling and deflection sections of the production wells 1 and 3 through two side drilling horizontal sections respectively.

10. A closed-cycle heat extraction system for depleted deep shale reservoirs according to claim 9, characterized in that: A plugging area is set in the horizontal sections of production wells 1, 2 and 3; The horizontal section lengths of the production well 1, production well 2 and production well 3 are set between 1000 and 3000 m, and the spacing between the horizontal sections of the production well 1 and production well 2, and the spacing between the horizontal sections of the production well 2 and production well 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 wells one and three.

Images