A method for preventing sand and coal dust in underground coalbed methane wells with combined drainage

Abstract

The present invention discloses a method for underground sand and coal dust prevention in a combined-layer coalbed methane well. Two upper and lower drainage devices are provided in the combined-layer coalbed methane well. First, a combined-layer development coalbed methane well with a two-wellbore structure is constructed on the ground, and the lower structural coal and the upper native structural coal are subjected to fracturing transformation in sequence. A packer is used to suspend a lower drainage pipe string at the top plate of the structural coal producing layer, so that the return fluid, formation water, and coalbed methane produced by the structural coal producing layer can enter the interior of the lower drainage pipe string through the wire-wound screen pipe. At the same time, the proppant and coal dust produced in large quantities by the structural coal producing layer are blocked from entering the drainage pipe string, and the proppant and coal dust are caused to sink to the bottom of the artificial well under the action of gravity. The lower drainage pipe string is used to prevent sand and coal dust, reduce the risk of pump sticking, and ensure the continuity of the drainage process of the combined-layer development coalbed methane well. It is particularly suitable for coalbed methane drainage under conditions where the coal body structure of the main coalbed methane producing layer in the middle and lower coal measures is broken and a large amount of sand and dust is expelled during combined-layer drainage.

Description

Technical Field

[0001] The present invention relates to a method for underground sand and coal dust prevention, and is particularly suitable for underground sand and coal dust prevention in coalbed methane wells with combined drainage production, where the main producing layer of coalbed methane development is located in the lower part of the coal system, the coal body structure of the main producing layer is incomplete, and a large amount of sand and coal dust are expelled during the combined drainage production process. Background Art

[0002] my country is rich in coalbed methane resources. It is estimated that the geological resources of coalbed methane at a depth of less than 2000m are 36.8×10 12 m 3 , which is comparable to conventional natural gas resources. Large-scale development of coalbed methane can alleviate my country's shortage of conventional oil and gas energy, reduce the probability of coal mine gas accidents, and reduce greenhouse gas emissions from coal mine production, generating significant economic, environmental, safety, and social benefits.

[0003] Due to the low abundance of CBM resources within a single coal seam, multi-layer CBM development is necessary in the Permian coal measure's multi-layer development areas to improve CBM well production and the economic benefits of CBM development. Currently, the specific process for CBM multi-layer development and drainage in multi-layer coal measure areas involves drilling vertical or directional CBM wells through multiple coal seams on the surface. After running production casing and cementing, the well is perforated and completed in the development coal seam section. Multiple coal seams are then fractured using either single-layer partial pressure or multi-layer fracturing. Finally, CBM resources are developed in multiple layers through drainage and pressure reduction. However, under multi-layer CBM development conditions, the majority of thick, gas-rich, and structurally fractured tectonic coal seams experience severe sand and powder expulsion during drainage. This often leads to pump sticking and buried pumps in CBM wells, impacting the continuity of the CBM multi-layer drainage process and causing irreversible damage to the permeability of the coal reservoir.

[0004] In order to further improve the gas production effect of combined-layer coalbed methane wells and ensure that coalbed methane wells have a long-term, stable and continuous drainage process, it is necessary to solve the problems of underground sand and coal seam prevention by optimizing the underground drainage pipe structure and drainage methods of coalbed methane wells.

[0005] Existing combined-layer coalbed methane wells all use a single drainage string during the drainage process. During the drainage process, a large amount of coal dust and proppant generated by the structural coal seam mixes with the flowback liquid containing solid particles from the primary structural coal, and then continuously enters the tubular pump barrel during the long-term drainage process, frequently causing pump jams and pump burial accidents. In order to improve the problem of frequent pump jams and pump burials during the drainage process of coalbed methane wells, wire-wound screens were previously used to filter coal dust under single-row production string conditions. Although this method has a certain effect in preventing sand and coal dust, it causes coal dust and fine-grained proppant to frequently clog the wire-wound screen and the suction port when the wellbore liquid is sucked into the tubular pump, and causes the lifting pump efficiency of the tubular pump to drop rapidly and significantly. Therefore, it is difficult to ensure long-term and stable drainage of coalbed methane wells, and it is impossible to solve the current problems of underground sand and coal dust prevention in combined-layer coalbed methane wells. Summary of the Invention

[0006] Technical problem: The purpose of the present invention is to overcome the shortcomings of the existing technology and provide a method for underground sand and coal powder prevention in coalbed methane wells, which can greatly reduce the coal powder and proppant returned from the structural coal seam and enter the pump barrel of the tubular pump, and the underground sand and coal powder prevention of coalbed methane wells.

[0007] Technical solution: The present invention relates to a method for underground sand and coal dust prevention in a combined-layer coalbed methane well. An upper and a lower set of drainage devices are arranged in the combined-layer coalbed methane well. The upper drainage device includes an upper drainage pipe string, a tubular pump and a screen pipe arranged in sequence in the upper drainage pipe string, and an upper wire plug arranged at the bottom of the upper drainage pipe string; the lower drainage device includes a lower drainage pipe string, a packer arranged at the top of the lower drainage pipe string, a wire screen pipe wound on the lower drainage pipe string, and a lower wire plug arranged at the bottom of the lower drainage pipe string, for forming a The flowback fluid, formation water, and coalbed methane produced by the coal-forming seam pass through and enter the lower drainage pipe string, and continuously move toward the wellhead during the combined layer drainage process. At the same time, the proppant and coal powder produced in large quantities by the structural coal seam are prevented from entering the drainage pipe string, so that the blocked proppant and coal powder continue to sink to the bottom of the artificial well under the action of gravity; the flowback fluid and formation water from which most of the solid particles have been removed enter the upper drainage pipe string through the screen pipe and are lifted to the surface by a tubular pump. The specific steps are as follows:

[0008] (a) A vertical coalbed methane well with a two-wellbore structure is constructed on the surface of a coal-bearing multi-coal seam development area. The first wellbore of the coalbed methane vertical well is drilled to 15-20 meters below the interface between the loose layer and the bedrock, and then the surface casing is run and cemented. The second wellbore of the coalbed methane vertical well is drilled to about 80 meters below the bottom of the structural coal seam and completed. After the production casing is run and cemented in the second wellbore section of the coalbed methane vertical well to form an artificial well bottom, perforations are sequentially perforated at the locations of the production casing in the structural coal seam and the primary structural coal seam, and the structural coal seam and the primary structural coal seam are subjected to separate-layer hydraulic fracturing.

[0009] (b) Using downhole oil pipes, transport the lower drainage device to the vicinity of the structural coal seam, and make the packer at the top of the lower drainage pipe string 2-3 meters above the top surface of the structural coal seam for pressure sealing;

[0010] (c) After the packer is pressurized and sealed, the downhole operating oil pipe is pulled out, and then the upper drainage device is lowered into the production casing of the coalbed methane vertical well, and the distance between the upper wire plug at the bottom of the upper drainage pipe string and the packer at the top of the lower drainage pipe string is 3-5m, so as to ensure that the coal production layer (5) is deeply depressurized during the coalbed methane well combined drainage process, thereby fully releasing the gas production potential;

[0011] (d) Install drainage auxiliary equipment according to existing technology and carry out combined layer drainage according to the established drainage operation system.

[0012] The artificial well bottom formed after the production casing is cemented is not less than 60m away from the surface of the structural coal producing seam.

[0013] The perforation at the location of the production casing in the structural coal seam is limited between the top and bottom surfaces of the structural coal seam, and the whole coal seam section perforation or the partial coal seam section perforation is adopted according to the thickness of the coal seam.

[0014] The perforation at the production casing located at the primary structure coal seam is limited to between the top and bottom surfaces of the primary structure coal seam. When the thickness of the primary structure coal seam is less than 1.5m, the perforation is expanded 0.5m above the top surface of the primary structure coal seam.

[0015] The packer is a hydraulic expansion type external oil pipe packer, which is sealed by ground pressure and plays the role of suspending the drainage pipe string during the drainage process.

[0016] The inner diameter of the wire-wound screen tube is not less than 75 mm, the length is not less than 2 times the thickness of the structural coal seam, the screen hole diameter is 4 mm, the screen hole density is 60-90 holes / m, the wire-wound material is copper, and the diameter is 1.5 mm.

[0017] The vertical distance between the lower wire plug and the bottom of the artificial well is not less than 45m.

[0018] The inner diameter of the sieve tube is not less than 75 mm, the length is not less than 8 m, the sieve hole diameter is 2-3 mm, and the sieve hole density is 120-150 holes / m.

[0019] The vertical distance between the lower end surface of the screen pipe and the top surface of the structural coal seam is less than 8m.

[0020] Beneficial effects: Due to the adoption of the above technical solution, the present invention overcomes the problems of poor sand and coal dust prevention of the underground production string using a single production string in the combined production mode, and the frequent pump jams and buried pumps during the production of coalbed methane wells and the frequent blockage of the suction port during the production process. First, a semi-enclosed space is formed by using the packer and the lower production string to allow the return fluid, formation water and coalbed methane produced by the coal production layer to pass through and enter the lower production string, and the blocked proppant and coal dust are continuously sunk to the bottom of the artificial well under the action of gravity, realizing the separation of gas, liquid and solid particles; secondly, the upper production string is used to lift the return fluid and formation water from which most of the solid particles have been removed to the ground, which not only improves the production pump efficiency, but also reduces the risk of pump jams and buried pumps. It is particularly suitable for the conditions where the coal structure of the main coalbed methane producing seams in the middle and lower coal measures is broken and a large amount of sand and coal powder is spitted out during the combined layer drainage process. The wire-wound screen pipe on the lower drainage pipe string is used to prevent proppant and coal powder from entering the working space of the upper drainage pipe string, reducing the risk of tubular pump sticking and ensuring the continuity of the drainage process of the combined layer drainage coalbed methane well. The main advantages compared with existing technologies are:

[0021] ①Using upper and lower production strings, efficient separation of gas, liquid and solid phases is achieved underground, significantly reducing the risk of pump sticking;

[0022] ②Do not increase energy consumption and capital investment in the drainage process;

[0023] ③Low risk in underground operations and high success rate of one-time operations;

[0024] ④ The technology and process are simple, the project implementation cost is low, and the economic, environmental and social benefits are good. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the method for underground sand and coal dust prevention in a combined-layer drainage coalbed methane well of the present invention.

[0026] In the figure: 1-ground; 2-surface casing; 3-production casing; 4-primary structural coal seam; 5-tectonic coal seam; 6-cementing cement sheath; 7-upper drainage pipe string; 8-tubing pump; 9-screen pipe; 10-upper wire plug; 11-packer; 12-lower drainage pipe string; 13-wire-wound screen pipe; 14-lower wire plug; 15-artificial well bottom. DETAILED DESCRIPTION

[0027] The present invention will be further described below with reference to the embodiments in the accompanying drawings:

[0028] The invention provides a method for underground sand and coal dust prevention in a combined layer coalbed methane well: an upper and a lower set of drainage devices are set in the combined layer coalbed methane well, the upper drainage device includes an upper drainage pipe string 7, a tubular pump 8, a screen pipe 9 and an upper wire plug 10 arranged at the bottom of the upper drainage pipe string 7 in sequence; the lower drainage device includes a lower drainage pipe string 12, a packer 11 arranged at the top of the lower drainage pipe string 12, a wire screen pipe 13 wound on the lower drainage pipe string 12 and a lower wire plug 14 arranged at the bottom of the lower drainage pipe string 12, which is used to allow the return fluid, formation water and coalbed methane produced by the coal production layer 5 to pass through and enter the interior of the lower drainage pipe string 12, and continuously flow to the bottom of the lower drainage pipe string 12 during the combined layer drainage process. The wellhead moves in the direction of the wellhead, and at the same time prevents the proppant and coal powder produced in large quantities by the structural coal seam 5 from entering the drainage pipe string 12, so that the blocked proppant and coal powder continue to sink to the artificial well bottom 15 under the action of gravity; the return fluid and formation water from which most of the solid particles have been removed enter the upper drainage pipe string 7 through the screen pipe 9, and are lifted to the ground by the tubing pump 8; the inner diameter of the wire-wound screen pipe 13 is not less than 75mm, the length is not less than 2 times the thickness of the structural coal seam 5, the sieve hole diameter is 4mm, the sieve hole density is 60-90 holes / m, the wire is made of copper and has a diameter of 1.5mm; the vertical distance between the lower wire plug 14 and the artificial well bottom 15 is not less than 45m. The inner diameter of the screen tube 9 is not less than 75mm, the length is not less than 8m, the sieve hole diameter is 2-3mm, and the sieve hole density is 120-150 holes / m; the vertical distance between the lower end surface of the screen tube 9 and the top surface of the structural coal seam 5 is less than 8m.

[0029] The specific steps are as follows:

[0030] (a) A coalbed methane vertical well with a two-wellbore structure is constructed on the surface 1 of a coal-bearing multi-coal seam development area. The first wellbore of the coalbed methane vertical well is drilled to 15-20 m below the interface between the loose layer and the bedrock, and then the surface casing 2 is run in and cemented. The second wellbore of the coalbed methane vertical well is drilled to about 80 m below the bottom of the structural coal production layer 5 and cemented. In the second wellbore of the coalbed methane vertical well, a production casing 3 is run in and cemented to form an artificial well bottom 15. Perforations are sequentially perforated in the production casing 3 at positions located at the structural coal production layer 5 and the primary structural coal production layer 4, and the structural coal production layer 5 and the primary structural coal production layer 4 are subjected to stratified hydraulic pressure. Fracture transformation; the artificial well bottom 15 formed after the production casing 3 is cemented is not less than 60m away from the surface of the structural coal production seam 5; the perforation at the position of the production casing 3 located in the structural coal production seam (5) is limited to between the top and bottom surfaces of the structural coal production seam 5, and the whole coal seam section perforation or the coal seam section partial perforation is adopted according to the thickness of the coal seam; the perforation at the position of the production casing 3 located in the primary structure coal production seam 4 is limited to between the top and bottom surfaces of the primary structure coal production seam 4, and when the thickness of the primary structure coal production seam 4 is less than 1.5m, the perforation is expanded 0.5m above the top surface of the primary structure coal production seam 4.

[0031] (b) The lower drainage device is transported to the vicinity of the structural coal production layer 5 by using the downhole operation oil pipe, and the packer 11 on the top of the lower drainage pipe string 12 is positioned 2-3m above the top surface of the structural coal production layer 5 for pressure sealing; the packer 11 adopts a hydraulic expansion type oil pipe external packer, which is sealed by pressure sealing on the ground and serves to suspend the lower drainage pipe string 12 during the drainage process.

[0032] (c) After the packer 11 is pressurized and sealed, the downhole operating oil pipe is pulled out, and then the upper drainage device is lowered into the production casing 3 of the coalbed methane vertical well, and the distance between the upper wire plug 10 at the bottom of the upper drainage pipe string 7 and the packer 11 at the top of the lower drainage pipe string 12 is 3-5m, so as to ensure that the coal production layer 5 is deeply depressurized during the coalbed methane well combined layer drainage process, thereby fully releasing the gas production potential;

[0033] (d) Install drainage auxiliary equipment according to existing technology and carry out combined layer drainage according to the established drainage operation system.

[0034] Since the tectonic coal seam is located below one or more primary structural coal seams and has the characteristics of large coal seam thickness, good gas content and high resource abundance, it is comprehensively evaluated as a water production contributing layer and a gas production contributing layer for the combined development coalbed methane well. The wire-wound screen pipe on the lower drainage and production string is used to block the proppant and coal powder from entering the working space of the upper drainage and production string, reducing the risk of pump sticking and pump burial, and ensuring the continuity of the drainage process of the combined development coalbed methane well.

[0035] Example:

[0036] (a) constructing a coalbed methane vertical well with a two-wellbore structure on the ground 1 in a coal-bearing multi-coal seam development area, the coalbed methane vertical well has a first-well drill bit diameter of φ311.1 mm, and the well is drilled to 20 m below the bottom of the Quaternary system, and then a J55 steel grade surface casing 2 with an outer diameter of φ244.5 mm is run into the well for cementing, the coalbed methane vertical well has a second-well drill bit diameter of φ215.9 mm, and the well is drilled to 80 m below the bottom of the structural coal production layer 5, and after cementing, a production casing 3 with an outer diameter of φ177.8 mm is run into the second-well section of the coalbed methane vertical well, and then perforations are sequentially made at the production casing 3 positions of the structural coal production layer 5 and the primary structural coal production layer 4, and the structural coal production layer 5 and the primary structural coal production layer 4 are subjected to stratified hydraulic fracturing transformation; the second-well section The artificial well bottom 15 formed after cementing the well section production casing 3 is no less than 60m from the surface of the structural coal seam 5; the structural coal seam 5 is located below one or more primary structural coal seams 4 and has the characteristics of large coal seam thickness, good gas content, and high resource abundance. Comprehensively evaluated, it is a water-producing and gas-producing contributing layer for the combined development of coalbed methane wells; the perforation position of the production casing 3 of the structural coal seam 5 section is limited to between the top and bottom surfaces of the structural coal seam 5, and perforation of the entire coal seam section or partial perforation of the coal seam section is adopted; the perforation position of the casing 3 of the primary structural coal seam 4 section is limited to between the top and bottom surfaces of the primary structural coal seam 4, and when the thickness of the primary structural coal seam 4 is less than 1.5m, the perforation is expanded 0.5m above the top surface of the primary structural coal seam 4;

[0037] (b) Using downhole oil pipes, the lower drainage pipe string 12 with the packer 11 on the top is transported to the vicinity of the structural coal production layer 5, and the packer 11 on the top of the lower drainage pipe string 12 is located 2m above the top surface of the structural coal production layer 5; the lower drainage pipe string 12 includes the packer 11, the wire screen 13, and the lower wire plug 14, which mainly allows the return fluid, formation water, and coalbed methane produced by the structural coal production layer 5 to pass through and enter the lower drainage pipe string 12, and continuously move toward the wellhead during the combined layer drainage process, while preventing the proppant and coal powder produced in large quantities from the structural coal production layer 5 from entering. The packer 11 is a hydraulically expandable external packer for the oil pipe, which is sealed by ground pressure and serves to suspend the lower production pipe 12 during the production process. The wire-wound screen 13 has an inner diameter of not less than 75 mm and a length not less than twice the thickness of the tectonic coal seam 5. The screen hole diameter is 4 mm and the screen hole density is 60-90 holes / m. The wire is made of copper and has a diameter of 1.5 mm. The vertical distance between the lower wire plug 14 and the bottom of the artificial well is not less than 45 m.

[0038] (c) After the packer 11 is pressurized and sealed, the downhole oil pipe is raised, and then the upper drainage pipe string 7 with an upper screw plug 10 at the bottom is lowered into the coalbed methane vertical well production casing 3, and the upper screw plug 10 at the bottom of the upper drainage pipe string 7 is 3-5m away from the top packer 11 of the lower drainage pipe string 12, so as to ensure that the structural coal production layer 5 can be deeply depressurized during the coalbed methane well combined layer drainage process, thereby fully releasing the gas production potential; the upper drainage pipe string 7 includes a tubular pump 8, a screen pipe 9, and an upper screw plug 10, and its main function is to lift the return fluid and formation water from which most of the solid particles have been removed to the ground; the inner diameter of the screen pipe 9 is not less than 75mm, the length is not less than 8m, the sieve hole diameter is 2-3mm, and the sieve hole density is 120-150 holes / m; the vertical distance between the lower end face of the screen pipe 9 and the top surface of the structural coal production layer 5 is less than 8m;

[0039] (d) After installing the sucker rod, start the walking beam pumping unit and carry out combined layer production according to the established production operation system.

Claims

1. A method for underground sand and coal dust prevention in a combined-layer coalbed methane well, characterized by: Two sets of drainage devices, upper and lower, are provided in a combined-layer drainage coalbed methane well. The upper drainage device in the two sets of drainage devices comprises an upper drainage pipe string (7), a tubular pump (8) and a screen pipe (9) sequentially arranged in the upper drainage pipe string (7), and an upper wire plug (10) arranged at the bottom of the upper drainage pipe string (7); the lower drainage device in the two sets of drainage devices comprises a lower drainage pipe string (12), a packer (11) arranged at the top of the lower drainage pipe string (12), a wire screen pipe (13) wound on the lower drainage pipe string (12), and a lower wire plug (14) arranged at the bottom of the lower drainage pipe string (12), for constructing The flowback fluid, formation water and coalbed methane produced by the coal producing layer (5) pass through and enter the interior of the lower drainage pipe string (12), and continuously move toward the wellhead during the combined layer drainage process, while preventing the proppant and coal powder produced in large quantities by the structural coal producing layer (5) from entering the interior of the lower drainage pipe string (12), so that the blocked proppant and coal powder continuously sink to the bottom of the artificial well (15) under the action of gravity; the flowback fluid and formation water from which most of the solid particles have been removed pass through the screen pipe (9) into the interior of the upper drainage pipe string (7), and are lifted to the ground (1) by the tubular pump (8); the specific steps are as follows: (a) A coalbed methane vertical well with a two-wellbore structure is constructed on the surface (1) of a coal-bearing multi-coal seam development area. The first wellbore of the coalbed methane vertical well is drilled to 15-20 m below the interface between the loose layer and the bedrock, and then the surface casing (2) is lowered for cementing. The second wellbore of the coalbed methane vertical well is drilled to about 80 m below the bottom of the structural coal production layer (5). After cementing, a production casing (3) is lowered into the second well section of the coalbed methane vertical well to form an artificial well bottom (15). Perforations are sequentially perforated at the positions of the production casing (3) at the structural coal production layer (5) and the primary structural coal production layer (4), and the structural coal production layer (5) and the primary structural coal production layer (4) are subjected to layered hydraulic fracturing transformation; (b) using downhole oil pipes to transport the lower drainage device to the vicinity of the structural coal producing layer (5), and to make the packer (11) at the top of the lower drainage pipe string (12) be located 2-3 meters above the top surface of the structural coal producing layer (5) for pressure sealing; (c) After the packer (11) is pressurized and sealed, the downhole operation oil pipe is raised, and then the upper drainage device is lowered into the production casing (3) of the coalbed methane vertical well, and the distance between the upper wire plug (10) at the bottom of the upper drainage pipe string (7) and the top packer (11) of the lower drainage pipe string (12) is 3-5m, so as to ensure that the depth of the coal production layer (5) is reduced during the coalbed methane well combined drainage process, thereby fully releasing the gas production potential; (d) Install drainage auxiliary equipment according to existing technology and carry out combined layer drainage according to the established drainage operation system.

2. The method for underground sand and coal dust prevention in a combined-layer coalbed methane well according to claim 1, characterized in that: The artificial well bottom (15) formed after the production casing (3) is cemented is not less than 60m away from the surface of the structural coal production layer (5).

3. The method for underground sand and coal dust prevention in a combined-layer coalbed methane well according to claim 1 or 2, characterized in that: The perforation at the location of the production casing (3) in the structural coal producing seam (5) is limited between the top and bottom surfaces of the structural coal producing seam (5), and the whole coal seam section perforation or the coal seam section partial perforation is adopted according to the thickness of the coal seam.

4. The method for underground sand and coal dust prevention in a combined-layer coalbed methane well according to claim 1 or 2, characterized in that: The perforation at the position where the production casing (3) is located in the primary structure coal production seam (4) is limited to between the top surface and the bottom surface of the primary structure coal production seam (4), and when the thickness of the primary structure coal production seam (4) is less than 1.5m, the perforation is expanded 0.5m above the top surface of the primary structure coal production seam (4).

5. The method for underground sand and coal dust prevention in a combined-layer coalbed methane well according to claim 1, characterized in that: The packer (11) is a hydraulic expansion type external packer of the oil pipe, which is sealed by ground pressure and plays the role of suspending the lower drainage pipe string (12) during the drainage process.

6. The method for underground sand and coal dust prevention in a combined-layer coalbed methane well according to claim 1, characterized in that: The inner diameter of the wire-wound screen tube (13) is not less than 75 mm, the length is not less than 2 times the thickness of the structural coal producing layer (5), the screen hole diameter is 4 mm, the screen hole density is 60-90 holes / m, the wire-wound material is copper, and the diameter is 1.5 mm.

7. The method for underground sand and coal dust prevention in a combined-layer coalbed methane well according to claim 1, characterized in that: The vertical distance between the lower wire plug (14) and the bottom of the artificial well (15) is not less than 45m.

8. The method for underground sand and coal dust prevention in a combined-layer coalbed methane well according to claim 1, characterized in that: The inner diameter of the sieve tube (9) is not less than 75 mm, the length is not less than 8 m, the sieve hole diameter is 2-3 mm, and the sieve hole density is 120-150 holes / m.

9. The method for underground sand and coal dust prevention in a combined-layer coalbed methane well according to claim 1 or 8, characterized in that: The vertical distance between the lower end surface of the screen tube (9) and the top surface of the structural coal producing layer (5) is less than 8m.

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