A permanent plugging structure and method for a coal mine overlapping area natural gas disposal well

Abstract

The application discloses a kind of permanent plugging structure of coal mine overlapping area natural gas disposal well, including primary barrier, secondary barrier and tertiary barrier, the outside of production casing is sequentially arranged fresh water layer, coal seam, cover layer and production layer, the fresh water layer, coal seam, cover layer and production layer are sequentially connected, one side of the secondary barrier is connected with coal seam and fresh water layer, the other side of the secondary barrier is connected with production casing;One side of the tertiary barrier is connected with production layer and cover layer, the other side of the tertiary barrier is connected with production casing.The permanent plugging method includes the following steps: determining the thickness of fresh water layer, coal seam, cover layer and production layer;According to the thickness of fresh water layer, coal seam, cover layer, production layer, corresponding primary barrier is set;By different temperature, pressure, position parameters, determine the structure parameters of secondary barrier and tertiary barrier;According to the parameter results determined, from bottom to top, set the structure of secondary barrier and tertiary barrier, complete permanent plugging.

Description

Technical Field

[0001] This invention relates to well repair engineering technology for natural gas wells in overlapping areas, specifically a permanent sealing structure and method for abandoned natural gas wells in overlapping coal mine areas. Background Technology

[0002] Currently, natural gas wells in coal mining overlap areas are mainly distributed in basin regions. The basin's structure slopes from west to east, and vertically, it consists primarily of overlapping freshwater, coal, and gas layers. Coal mining is mainly conducted through open-pit or shallow surface mining, resulting in relatively little overlap between coal and natural gas mining rights.

[0003] However, in recent years, with the development of coal mining technology, the exploitable depth of coal mines has increased significantly, and coal mining areas have expanded westward, creating a large overlap of mining rights with oil and gas field development areas. Mutual avoidance between coal mining and gas field development has become inevitable. For natural gas wells in coal-mining overlap areas, modern perforation and fracturing technologies are required for efficient development. Therefore, before coal mining, the cement sheath of the oil layer in natural gas wells already contains cracks and defects caused by perforation, fracturing, and development, which to some extent damages the integrity of the cement sheath and wellbore in the coal-mining overlap area.

[0004] Furthermore, after a natural gas well is abandoned in an overlapping coal mining area, coal mining will damage the wellbore cement sheath and casing within the coal seam, leading to the complete destruction of the wellbore integrity. Therefore, permanent sealing technology for abandoned natural gas wells in overlapping coal mining areas has become an important guarantee for safe coal mining. However, existing standards and processes for permanent sealing of natural gas wells do not consider the destruction of the underground tubing integrity after sealing, and thus cannot meet the requirements for permanent sealing of abandoned natural gas wells in overlapping coal mining areas.

[0005] Therefore, there is an urgent need to study a new, economical, safe, environmentally friendly, and applicable method and technology for the permanent sealing of abandoned natural gas wells in overlapping coal mine areas.

[0006] Existing technologies and methods are mainly aimed at sealing gas storage facilities, primarily using cement plugs inside pipes to seal unusable abandoned wells in gas storage facilities.

[0007] Specifically, a predetermined amount of cement slurry is injected into the abandoned well to be sealed. After the cement slurry solidifies in the well, it forms a cement plug, thereby sealing the abandoned well.

[0008] Based on the characteristics of coal mine overlapping areas, analysis revealed at least the following problems with the relevant technologies: 1) The damage to the cement sheath and casing and the integrity of the wellbore within the coal seam section caused by coal mining was not considered; 2) During coal mining, toxic and harmful gases such as methane, CO2, and H2S from the producing seam enter the coal seam, causing accidents such as natural disasters, explosions, and poisoning; 3) During coal mining, toxic and harmful gases from the coal seam enter the freshwater layer and the air, causing environmental pollution; 4) The impact of perforation, pressure, and internal defects and cracks in the cement sheath caused during the development process on subsequent sealing was not considered.

[0009] Therefore, it is still necessary to address the problem of the combined effects of perforation fracturing development and coal mining damaging the cement sheath and wellbore integrity of natural gas wells in overlapping coal mining areas. Summary of the Invention

[0010] To address the problem in existing technologies where the combined effects of perforation fracturing and coal mining damage the cement sheath and wellbore integrity of natural gas wells in overlapping coal mining areas, this invention provides a permanent sealing structure and method for abandoned natural gas wells in overlapping coal mining areas. This not only achieves permanent sealing of abandoned natural gas wells in overlapping coal mining areas but also allows for safe coal mining and environmental protection. It simultaneously solves the technical challenges of permanent sealing of abandoned natural gas wells in overlapping coal mining areas, safe coal mining operations, and the pollution of freshwater layers and air. This structure and method are low-cost, simple to operate, and readily adaptable to real-world working conditions in various wellbore environments.

[0011] This invention is achieved through the following technical solution:

[0012] A permanent sealing structure for abandoned natural gas wells in overlapping coal mine areas includes a primary barrier, a secondary barrier, and a tertiary barrier. A freshwater layer, a coal seam, a caprock, and a producing layer are sequentially arranged axially on the outer side of the production casing. These layers are connected sequentially. One side of the secondary barrier is connected to the coal seam and the freshwater layer, and the other side is connected to the production casing. One side of the tertiary barrier is connected to the producing layer and the caprock, and the other side is connected to the production casing.

[0013] Furthermore, the primary barrier includes a standard casing, an easy-to-drill alloy casing, and a high-strength casing, which are connected in sequence. The standard casing is installed on the inner wall of the freshwater layer and the coal seam, while the caprock and the producing layer are installed on the inner wall of the standard casing, the easy-to-drill alloy casing, and the high-strength casing.

[0014] Furthermore, the secondary barrier includes a surface cement ring, a surface sleeve, and a quick-setting cement ring; the inner wall of the freshwater layer is provided with a surface sleeve, a high-strength sleeve is provided between the surface sleeve and the ordinary sleeve, and the quick-setting cement ring is provided on the outside of the connection end between the ordinary sleeve and the freshwater layer.

[0015] Furthermore, the secondary barrier includes a first cement plug and a first bridge plug, with the first bridge plug disposed below the first cement plug. The first cement plug and the first bridge plug are coaxially arranged, and the outer diameter of the first cement plug is in contact with the inner wall surface of the ordinary sleeve.

[0016] Furthermore, the third-level barrier includes ultrafine cement slurry and dense perforations. The dense perforations are arranged perpendicularly to the first-level barrier. The ultrafine cement slurry is disposed within the dense perforations and enters the dense perforations through fracturing perforations on the high-strength casing.

[0017] Furthermore, the three-stage barrier includes an oil layer cement ring, a second bridge plug, and a second cement plug. The second cement plug is disposed inside the easy-drill alloy casing, and the outer wall surface of the second cement plug is in contact with the inner wall surface of the easy-drill alloy casing. The lower end surface of the second cement plug is connected to the upper end surface of the second bridge plug.

[0018] Furthermore, a quick-setting cement ring is provided between the surface sleeve and the ordinary sleeve, and the quick-setting cement ring is located on the side closer to the freshwater layer. The quick-setting cement ring and the annular space are of equal diameter.

[0019] A method for permanently sealing natural gas abandoned wells in overlapping coal mine areas, as described above, includes the following steps:

[0020] S1: Determine the thickness settings for freshwater layer, coal seam, caprock, and producing seam;

[0021] S2: First-level barriers are set up according to the thickness of freshwater layer, coal seam, caprock, and producing seam;

[0022] S3: Determine the structural parameters of the secondary and tertiary barriers by using different temperature, pressure, and location parameters;

[0023] S4: Based on the determined parameter results, set up the structures of the secondary and tertiary barriers from bottom to top to complete the permanent sealing.

[0024] Furthermore, based on the parameters of the oil layer cement sheath and fracturing perforations near the producing layer, the parameters of the ultrafine cement slurry in this region are determined; based on the parameters of the oil layer cement sheath and dense perforations near the caprock, the parameters of the ultrafine cement slurry in this region are also determined.

[0025] Furthermore, based on the temperature and pressure of the producing seam, coal seam, and surrounding area, as well as the inner diameter of the primary barrier and the friction coefficient of the cement plug, the length of the cement plug and the quick-setting cement ring, and the amount of mud used are determined.

[0026] Compared with the prior art, the present invention has the following beneficial technical effects:

[0027] This invention provides a method for permanently sealing natural gas abandoned wells in overlapping coal mine areas. By considering the temperature, fluid, and pressure characteristics of the producing seam and the coal seam, a permanent sealing structure for natural gas abandoned wells in overlapping coal mine areas is proposed.

[0028] The permanent sealing structure for natural gas abandoned wells in overlapping coal mine areas provided by this invention mainly employs a three-stage barrier consisting of pressurized injection of ultrafine cement slurry into densely packed perforated holes, pressurized injection of ultrafine cement slurry into fracturing perforations, and a second cement plug to block and seal the producing layer and coal seam. A secondary barrier consisting of annular cement rings and a first bridge plug is used to block and seal the freshwater layer and coal seam, thereby achieving permanent sealing of natural gas abandoned wells in overlapping coal mine areas and providing important protection for subsequent safe coal mining. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the pre-sealing structure of a permanent sealing structure for a natural gas abandoned well in an overlapping area of ​​a coal mine, provided by an embodiment of the present invention.

[0031] Figure 2 This is a schematic diagram of a permanent sealing structure for a natural gas abandoned well in an overlapping area of ​​a coal mine, provided as an embodiment of the present invention.

[0032] Figure 3 This is a schematic diagram of the cross-section of a densely perforated section after the permanent sealing structure of a natural gas abandoned well in an overlapping area of ​​a coal mine is provided in an embodiment of the present invention.

[0033] Figure 4 This is a schematic diagram of the cross-section of the fracturing perforation section after the permanent sealing structure of a natural gas abandoned well in an overlapping area of ​​a coal mine, provided as an embodiment of the present invention.

[0034] In the diagram: 1. Surface casing; 2. Annulus; 3. Oil layer cement sheath; 4. Freshwater layer; 5. Surface cement sheath; 6. Coal seam; 7. Caprock; 8. Production layer; 9. Fracturing perforation; 10. Ordinary casing; 11. Easy-drill alloy casing; 12. High-strength casing; 13. Quick-setting cement sheath; 14. First cement plug; 15. First bridge plug; 16. Second cement plug; 17. Second bridge plug; 18. Dense perforation; 19. Ultrafine cement slurry; 20. Crack. Detailed Implementation

[0035] In the following description, only certain exemplary embodiments are briefly described. As those skilled in the art will recognize, the described embodiments can be modified in various ways without departing from the spirit or scope of the invention. Therefore, the drawings and description are considered to be exemplary in nature and not restrictive.

[0036] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0037] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0038] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a communication connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0039] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature being directly above or diagonally above the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0040] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0041] Example 1:

[0042] As attached Figure 1 As shown, the present invention proposes a permanent sealing structure for abandoned natural gas wells in overlapping coal mine areas. The sealing structure mainly includes: surface casing 1, annulus 2, oil layer cement sheath 3, freshwater layer 4, surface cement sheath 5, coal seam 6, caprock 7, producing layer 8, fracturing perforation 9, ordinary casing 10, easy-drill alloy casing 11, high-strength casing 12, quick-setting cement sheath 13, first cement plug 14, first bridge plug 15, second cement plug 16, second bridge plug 17, dense perforation 18, ultrafine cement slurry 19, and crack 20.

[0043] From top to bottom, the formation consists of a freshwater layer 4, a coal seam 6, a caprock 7, and a producing layer 8. The freshwater layer 4 is located at the top, the coal seam 6 is located between the caprock 7 and the freshwater layer 4, and the producing layer 8 is located at the bottom. The ordinary casing 10, the easy-drill alloy casing 11, and the high-strength casing 12 constitute the production casing. It is mainly used as a primary barrier in the production process to prevent fluid leakage between the producing layer, the coal seam, and the freshwater layer.

[0044] The sealing structure mainly includes a secondary barrier located above coal seam 6 and a tertiary barrier located above producing seam 8.

[0045] The secondary barrier consists of a surface cement ring 5, a surface sleeve 1, a quick-setting cement ring 13, a first cement plug 14, and a first bridge plug 15. The secondary barrier is connected to the ordinary sleeve 10. The secondary barrier is mainly used to prevent toxic and harmful gases in the coal seam 6 from entering the freshwater layer 4 and the air, thus preventing environmental pollution. At the same time, it prevents water in the freshwater layer 4 from entering the coal seam 6, thus preventing flooding. The quick-setting cement ring 13 between the surface sleeve 1 and the ordinary sleeve 10 is used to prevent fluid crosstalk in the annulus 2, and the first cement plug 14 is used to prevent fluid crosstalk within the ordinary sleeve 10.

[0046] The three-stage barrier consists of ultrafine cement slurry 19 solidified inside the fracturing perforation 9, dense perforations 18, and ultrafine cement slurry 19 placed inside the dense perforations 18. The three-stage barrier also includes an oil layer cement ring 3, a second bridge plug 17, and a second cement plug 16. The three-stage barrier is connected to an easy-drill alloy casing 11 and a high-strength casing 12. It is mainly used to block formation water, CO2, H2S and other toxic and harmful gases from the producing layer 8 from entering the coal seam 6.

[0047] The ultrafine cement slurry 19 injected into the perforation hole 9 is used to repair the cracks 20 generated inside the cement sheath 3 of the oil layer near the producing layer 8 during the first cementing, perforation, fracturing and production process, and to prevent formation water, CO2 and H2S from entering the coal seam 6 of the producing layer 8.

[0048] The ultrafine cement slurry 19 inside the dense perforations 18 is used to repair the cracks 20 generated inside the cement sheath 3 of the oil layer near the caprock 7 during the first cementing and production process, and to block formation water, CO2, and H2S from entering the coal seam 6 from the producing layer 8.

[0049] To make the objectives, technical solutions, and advantages of this invention clearer,

[0050] The following is in conjunction with the appendix Figure 1 Appendix Figure 2 Appendix Figure 3 and attached Figure 4 A method for permanently sealing natural gas abandoned wells in overlapping coal mine areas is described, and its implementation method is as follows:

[0051] Step 1: Analyze on-site geological data to determine the depth and thickness of the freshwater layer 4, coal seam 6, caprock 7, and producing layer 8 in the overlapping area of ​​the coal mine;

[0052] Step 2: Design the production casing of the natural gas well in the coal mine overlapping area from top to bottom as ordinary casing 10, easy-drill alloy casing 11, and high-strength casing 12. Ordinary casing 10 is located in freshwater layer 4 and coal seam 6, easy-drill alloy casing is located in cap layer 7, and high-strength casing 12 is located in production layer 8.

[0053] Step 3: Based on the logging parameters of the cement sheath 3 of the oil layer near the production layer 8 of the abandoned natural gas well, the density, diameter and depth of the fracturing perforation 9, determine the amount of ultrafine cement slurry 19 required to seal the fracturing perforation 9 and repair the internal crack 20 of the cement sheath 3 of the oil layer near the production layer 8, Q1.

[0054] Step 4: Use a perforation gun to perform dense perforation 18 on the easy-drill alloy casing 11 near the cap layer 7. Based on the logging parameters of the cement sheath 3 of the oil layer near the cap layer 7 of the abandoned natural gas well, the density, diameter and depth of the dense perforation 18 holes, determine the amount of ultrafine cement slurry 19 Q2 required to seal the dense perforation 18 holes and repair the internal cracks 20 of the cement sheath 3 of the oil layer near the cap layer 7.

[0055] Step 5: Based on the temperature and pressure of the producing layer 8 of the abandoned natural gas well, the inner diameter of the easy-drill alloy casing 11 and the friction coefficient between it and the second cement plug 16, determine the length of the second cement plug 16 and the required amount of G-grade cement slurry Q3.

[0056] Step 6: Based on the temperature and pressure of the coal seam 6 in the abandoned natural gas well, the inner diameter of the ordinary casing 10 and the friction coefficient between it and the first cement plug 14, determine the length of the first cement plug 14 and the required amount of G-grade cement slurry Q4.

[0057] Step 7: Determine the length of the quick-setting cement ring 13 and the formula and dosage of quick-setting cement slurry Q5 based on the temperature, pressure, annulus 2 gap, and friction coefficient between the quick-setting cement ring 13 and the ordinary casing 10 in the coal seam 6 of the abandoned natural gas well.

[0058] Step 8: Under an injection pressure of 10-15 MPa, inject Q1 ultrafine cement slurry 19 into the perforation hole 9. After solidification, repair and seal the cracks 20 inside the perforation hole 9 and the cement sheath 3 of the oil layer near the production layer 8, and prevent formation water, CO2, and H2S in the production layer 8 from entering the coal seam 6 through the cement sheath 3 of the oil layer near the production layer 8 and the production casing.

[0059] Step 9: Under an injection pressure of 5-10 MPa, inject Q2 ultrafine cement slurry 19 into the dense perforation 18 holes to achieve repair and sealing of the cracks 20 inside the dense perforation 18 holes and the cement sheath 3 of the oil layer near the caprock 7, thereby preventing formation water, CO2, and H2S from the producing layer 8 from entering the coal seam 6 through the cement sheath 3 of the oil layer near the caprock 7.

[0060] Step 10: Insert the second bridge plug 17 into the easy-drill alloy casing 11 above the dense perforation 18, and inject G-grade cement slurry Q3 into the easy-drill alloy casing 11 through the oil pipe. After solidification, a second cement plug 16 is formed, which further blocks the formation water, CO2, and H2S of the producing layer 8 from entering the coal seam 6 through the production casing.

[0061] Step 11: Insert the first bridge plug 15 into the ordinary casing 10, and inject G-grade cement slurry Q4 directly into the ordinary casing 10. After solidification, the first cement plug 14 is formed, which prevents toxic and harmful gases in the coal seam 6 from entering the freshwater layer 4 and the air through the ordinary casing 10 and causing environmental pollution. At the same time, it prevents water in the freshwater layer 4 from entering the coal seam 6 through the ordinary casing 10 and causing flooding.

[0062] Step 12: Inject quick-setting cement slurry Q5 into the annulus 2 gap between the surface casing 1 and the ordinary casing 10. After solidification, a quick-setting cement ring 13 is formed, which prevents toxic and harmful gases in the coal seam 6 from entering the freshwater layer 4 and the air through the annulus 2 and causing environmental pollution. At the same time, it prevents water in the freshwater layer 4 from entering the coal seam 6 through the annulus 2 and causing flooding.

[0063] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description, and thus all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0064] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can be appropriately combined to form other embodiments that can be understood by those skilled in the art. The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A permanent sealing structure for natural gas abandoned wells in overlapping coal mine areas, characterized in that, The system includes a primary barrier, a secondary barrier, and a tertiary barrier. A freshwater layer (4), a coal seam (6), a caprock (7), and a producing layer (8) are sequentially arranged on the outer axial side of the production casing. The freshwater layer (4), coal seam (6), caprock (7), and producing layer (8) are connected in sequence. One side of the secondary barrier is connected to the coal seam (6) and the freshwater layer (4), and the other side of the secondary barrier is connected to the production casing. One side of the tertiary barrier is connected to the producing layer (8) and the caprock (7), and the other side of the tertiary barrier is connected to the production casing. The primary barrier includes a common casing (10), an easy-drill alloy casing (11), and a high-strength casing (12), which are connected in sequence. The common casing (10) is installed on the inner wall of the freshwater layer (4) and the coal seam (6), and the common casing (10), the easy-drill alloy casing (11), and the high-strength casing (12) are installed on the inner wall of the caprock (7) and the producing layer (8). The secondary barrier includes a surface cement ring (5), a surface sleeve (1), and a quick-setting cement ring (13); the inner wall of the freshwater layer (4) is provided with a surface sleeve (1), a high-strength sleeve (12) is provided between the surface sleeve (1) and the ordinary sleeve (10), and the quick-setting cement ring (13) is provided on the outside of the connection end between the ordinary sleeve (10) and the freshwater layer (4); The secondary barrier includes a first cement plug (14) and a first bridge plug (15). The first bridge plug (15) is disposed below the first cement plug (14). The first cement plug (14) and the first bridge plug (15) are coaxially arranged. The outer diameter of the first cement plug (14) is in contact with the inner wall surface of the ordinary sleeve (10). The third-level barrier includes ultrafine cement slurry (19) and dense perforations (18). The dense perforations (18) and the first-level barrier are arranged vertically. The ultrafine cement slurry (19) is placed inside the dense perforations (18). The ultrafine cement slurry (19) enters the dense perforations (18) through the fracturing perforations (9) on the high-strength casing (12). The three-level barrier includes an oil layer cement ring (3), a second bridge plug (17), and a second cement plug (16). The second cement plug (16) is disposed inside the easy-drill alloy casing (11). The outer wall surface of the second cement plug (16) is in contact with the inner wall surface of the easy-drill alloy casing (11). The lower end surface of the second cement plug (16) is connected to the upper end surface of the second bridge plug (17). The ordinary casing (10), the easy-drill alloy casing (11), and the high-strength casing (12) constitute the production casing.

2. The permanent sealing structure for natural gas abandonment wells in overlapping coal mine areas according to claim 1, characterized in that, A quick-setting cement ring (13) is provided between the surface sleeve (1) and the ordinary sleeve (10). The quick-setting cement ring (13) is located on the side close to the freshwater layer (4). The quick-setting cement ring (13) and the annulus (2) are of the same diameter.

3. A method for permanently sealing natural gas abandonment wells in overlapping coal mine areas based on the permanent sealing structure of natural gas abandonment wells in overlapping coal mine areas as described in claim 1 or 2, characterized in that, The permanent sealing method includes the following steps: S1: Determine the thickness settings of the freshwater layer (4), coal seam (6), caprock (7) and producing seam (8); S2: Based on the thickness of the freshwater layer (4), coal seam (6), caprock (7), and producing seam (8), a primary barrier is set up accordingly; S3: Determine the structural parameters of the secondary and tertiary barriers by using different temperature, pressure, and location parameters; S4: Based on the determined parameter results, set up the structures of the secondary and tertiary barriers from bottom to top to complete the permanent sealing.

4. The method for permanently sealing natural gas abandoned wells in overlapping coal mine areas according to claim 3, characterized in that, Based on the parameters of the oil layer cement sheath (3) and fracturing perforations (9) near the production layer (8), the parameters of the ultrafine cement slurry (19) near the production layer (8) are determined; based on the parameters of the oil layer cement sheath (3) and dense perforations (18) near the caprock (7), the parameters of the ultrafine cement slurry (19) near the caprock (7) are determined.

5. A method for permanently sealing natural gas abandoned wells in overlapping coal mine areas according to claim 3, characterized in that, Based on the temperature and pressure of the producing seam (8), the coal seam (6), the surrounding temperature and pressure, the inner diameter of the primary barrier, and the friction coefficient of the cement plug, determine the length of the cement plug and the quick-setting cement ring (13) and the amount of mud used.

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