Crushed soft coal seam top and bottom plate double-layer horizontal well ground coal extraction pressure relief extraction system and method

Abstract

The broken soft coal seam top and bottom plate double-layer horizontal well ground coal extraction pressure relief extraction system comprises a top plate horizontal well and a bottom plate horizontal well, and the top plate horizontal well comprises a top plate horizontal well first opening, a top plate horizontal well second opening and a top plate horizontal well third opening horizontal section main branch which are sequentially connected; the top plate horizontal well branches are obliquely arranged below the three-opening horizontal section main branch of the top plate horizontal well, and caves are formed in the top plate horizontal well branches; the bottom plate horizontal well comprises a first bottom plate horizontal well opening, a second bottom plate horizontal well opening, a third bottom plate horizontal well opening horizontal section main branch and a plurality of bottom plate horizontal well branches obliquely arranged above the third bottom plate horizontal well opening horizontal section, wherein the first bottom plate horizontal well opening, the second bottom plate horizontal well opening and the third bottom plate horizontal well opening horizontal section main branch are sequentially connected. And each top plate horizontal well branch is communicated with one bottom plate horizontal well branch through one cave. The device can quickly release pressure or does not need to release pressure, can quickly discharge and extract after being put into operation, and realizes quick gas seeing and gas production peak.

Description

Technical Field

[0001] The invention belongs to the technical field of co - mining of coal and coalbed methane, and particularly relates to a rapid pressure - relief extraction system and method for ground coal extraction of a double - layer horizontal well in the roof and floor of a broken - soft coal seam. Background Technique

[0002] The pore structure of broken - soft and low - permeability coal seams is relatively complex, with small pore sizes and poor connectivity, which increases the difficulty of desorbing coalbed methane molecules from the adsorbed state. The permeability of the coal seam is extremely low, which means that the migration channels of coalbed methane in the coal seam are very limited. After coalbed methane desorbs from the pores of the coal matrix, it needs to migrate to the wellbore through channels such as fractures. However, the degree of fracture development in broken - soft coal seams is often low, and due to coal body fragmentation, fractures are easily blocked by coal chips and the like. For conventional coalbed methane extraction, hydraulic fracturing is a commonly used stimulation measure. However, in broken - soft and low - permeability coal seams, due to coal body fragmentation, fracturing fluid is easily lost along the fracture surface, and it is difficult to form effective support fractures in the coal seam. Moreover, the stress distribution in broken - soft coal seams is complex, and the extension direction and shape of fractures are difficult to control during the fracturing process. As a result, the stimulation measures cannot achieve the expected effects of improving coal seam permeability and promoting coalbed methane migration. The efficient extraction technology and equipment for coalbed methane in broken - soft coal seams have become one of the main technical bottlenecks restricting the rapid development of China's coalbed methane industry.

[0003] Chinese Patent CN101936155A discloses a distribution structure of the horizontal section of a coalbed methane multi - branch horizontal well, and proposes a wellbore structure of a coalbed methane multi - branch horizontal well. By increasing the number of main wellbores of the multi - branch horizontal well and locating the branch wellbores outside the two outermost main wellbores, the purpose of reducing the drainage pressure of the main wellbores and increasing the footage utilization rate of the main wellbores is achieved. However, the above - mentioned structure is for high - rank coal with relatively hard coal quality, which improves the coalbed methane extraction efficiency to a certain extent, but in the process of drilling in broken - soft coal seams, problems such as wellbore collapse, caving, and other problems are likely to occur, resulting in well failure. At the same time, although multi - branch horizontal wells can increase the gas - drainage area of the reservoir, during the process of drilling branches, other already - formed branches are polluted by the drilling fluid entering, resulting in a decrease in permeability and affecting production capacity. And most multi - branch horizontal wells are open - hole completed wells and cannot be subjected to fracturing stimulation.

[0004] Chinese Patent CN201865653U discloses a coalbed methane horizontal well system, and proposes a method of using the vertical well section of a multi - branch horizontal well as a cooperating well for another multi - branch horizontal well. Compared with the prior art 1, it only plays a role in cost reduction and efficiency improvement, and does not fundamentally solve the problems of coal seam pollution, wellbore collapse, and high well - completion difficulty generated during the drilling process of multi - branch horizontal wells.

[0005] Chinese Patent CN102979498A discloses a coalbed methane multi-branch horizontal well system and proposes a drilling method for multi-branch horizontal wells in the roof or floor of a coal seam. The main branch drills in the roof or floor of the coal seam to provide a stable drainage channel. The branches control the drainage area, and the vein branches dredge the coal seam. Through the circulation of 2 production drainage vertical wells and the vertical wells, a stable well flushing channel is established to remove the residual mud and coal ash in the main wellbore. During the subsequent construction of the branches and vein branches, the main branch, as well as the completed branches and vein branches, are soaked for a long time, resulting in pollution and collapse. For the branches and vein branches, there are blind ends and they are not open channels. During the well flushing process, the mud and coal ash in the vein branches and branches cannot be effectively circulated for well flushing.

[0006] Chinese Patent CN214576923U discloses a coalbed methane horizontal well system and proposes an L-shaped double-layer parallel single-branch horizontal well system; CN113605874A discloses a method for extracting coalbed methane from a double-layer horizontal well in the roof and floor of a soft broken coal seam and proposes a U-shaped double-layer parallel single-branch horizontal well system. The principle of both is to establish virtual gas production and drainage channels, conduct directional fracturing on the coal seam in the horizontal section of the coal seam roof to construct a high-speed gas production channel, and conduct drainage and pressure relief on the coal seam floor to construct a drainage channel. The technical principles of these two extraction techniques are the same as those of the method for extracting coalbed methane from a segmented fractured horizontal well in the roof or floor of a soft broken and low-permeability coal seam proposed in CN112983385A. Only on this basis, technical optimization is carried out, and the gas production and drainage channels are separated. The above three technical solutions only optimize the technology on the basis of constructing the gas production channel in the coal seam roof. The coal seam is enhanced and transformed by directional fracturing. As mentioned above, a large number of engineering practices have shown that the effect of enhancing and transforming soft broken coal seams through various hydraulic fracturing is poor. The fractures generated by the fracturing will quickly close with the drainage and pressure reduction of coalbed methane, and it is difficult to form a lasting fracture network for the diffusion and migration of coalbed methane.

[0007] Chinese Patents CN108798516A and CN108843241A propose a method for in-situ coalbed methane horizontal well cave pressure relief mining of tectonic coal. This technology uses a U-shaped well to ream and extract coal in the horizontal section of several hundred meters or even more than a thousand meters. The broken coal powder is removed to the ground through the butt joint vertical well using a special lifting system. The biggest problem is that the gas production channel is located in the coal seam. After the well is completed, during the drainage process, the soft coal seam will still show the phenomenon of wellbore collapse, losing the gas production channel and being unable to produce gas, and unable to continue production subsequently.

[0008] Chinese Patent CN112593912A discloses a method for dynamic diameter expansion, pressure relief, and permeability enhancement of coalbed methane in horizontal wells. A single-branch horizontal well is used, and the horizontal section is arranged in the coal seam. Mechanical diameter expansion and high-pressure air-powered coal removal for pressure relief are carried out on the ground. Coal removal and cavity formation are carried out on the ground at each section of the horizontal section to release the coal seam stress and release the production capacity of the surface coalbed methane well. This patent optimizes the technology on the basis of the overall coal removal and pressure relief in the horizontal section of CN108798516A. A fiberglass casing is lowered into the horizontal section, and then the fiberglass casing in the horizontal section is milled. Then, mechanical diameter expansion and dynamic diameter expansion for cavity formation are carried out, and the cavity is supported with a filling material rich in holes and fractures. However, it takes a lot of time and cost. First, the downhole operation cost of coiled tubing is expensive. Second, during the cavity formation process of this technical solution, it experiences ① casing section milling and window opening, ② coiled tubing well flushing, ③ mechanical diameter expansion, ④ secondary coiled tubing well flushing, ⑤ high-pressure air-powered diameter expansion, ⑥ filling and supporting of cavity formation materials, and ⑦ inter-hole isolation. The process is very cumbersome, and it is necessary to frequently replace downhole tools and working media, which requires a long construction time and increases the downhole construction difficulty. Third, the porous material for filling can only be transported through drill pipes or tubing. There is no effective method provided for how to transport the porous material to the cavity formation space within the limited range of the main branch and achieve effective support, and it is also difficult to achieve in actual operation. Summary of the Invention

[0009] Aiming at the defects of the prior art, the present invention provides a double-layer horizontal well ground coal removal pressure relief extraction system and method for broken and soft coal seams to achieve the efficient and large-scale exploitation of coalbed methane in broken and soft coal seams.

[0010] The double-layer horizontal well ground coal removal pressure relief extraction system for broken and soft coal seams includes a roof horizontal well and a floor horizontal well. The roof horizontal well includes a roof horizontal well first open section, a roof horizontal well second open section, and a roof horizontal well third open section main branch connected in sequence, and a number of roof horizontal well branches inclined below the roof horizontal well third open section main branch. Cavities are provided on the roof horizontal well branches. The floor horizontal well includes a floor horizontal well first open section, a floor horizontal well second open section, and a floor horizontal well third open section main branch connected in sequence, and a number of floor horizontal well branches inclined above the floor horizontal well third open section. Each roof horizontal well branch is connected to a floor horizontal well branch through a cavity. The roof horizontal well third open section main branch and the floor horizontal well third open section main branch are respectively located in the coal seam roof and the coal seam floor, and the cavities are arranged in the coal seam. The number of roof horizontal well branches, floor horizontal well branches, and cavities corresponds. The roof horizontal well third open section main branch and the floor horizontal well third open section main branch connected through the cavity are arranged in parallel and in the same vertical plane.

[0011] Preferably, packers are provided on the main branch of the third open horizontal section of the roof horizontal well, in front of the window opening position of the roof horizontal well branch, at the end of the window opening position of the roof horizontal well branch close to the wellhead on the main branch of the third open horizontal section of the roof horizontal well, and on the main branch of the third open horizontal section of the floor horizontal well, in front of the window opening position of the floor horizontal well branch.

[0012] In the present invention, the first open is surface drilling; the second open is the technical casing section, from the bottom of the surface casing to the end of the build-up section.

[0013] Preferably, the distances between the main branch of the third open horizontal section of the roof horizontal well and the top of the coal seam, and between the main branch of the third open horizontal section of the floor horizontal well and the bottom of the coal seam are 2 - 3 m respectively.

[0014] Preferably, the maximum diameter of the cave is 3 times the diameter of the roof horizontal well branch.

[0015] Preferably, the well inclination angle of the roof horizontal well branch is 30° smaller than that of the main branch of the third open horizontal section of the roof horizontal well, which is convenient for drainage.

[0016] Preferably, the distance between two adjacent roof horizontal well branches is 8 - 10 m.

[0017] Preferably, the packer is a drillable or dissolvable open-hole packer.

[0018] Preferably, there are multiple main branches of the third open horizontal section of the roof horizontal well and the main branches of the third open horizontal section of the floor horizontal well respectively, and the numbers are the same; all the main branches of the third open horizontal section of the roof horizontal well are connected to the second open of the roof horizontal well, and all the main branches of the third open horizontal section of the floor horizontal well are connected to the second open of the floor horizontal well.

[0019] The ground coal extraction pressure relief and drainage method for the double-layer horizontal well in the soft broken coal seam roof and floor is carried out by using the above system, and includes the following steps: (1) Collect the geological data of the target area and the relevant parameters of adjacent exploration wells or coalbed methane wells, determine the well location deployment of the double-layer horizontal well, determine the distances between the main branches of the third open horizontal section of the roof horizontal well and the floor horizontal well and the coal seam, and the drilling target horizons; (2) Drill the first open and the second open of the roof horizontal well, and perform cementing; (3) On the coal seam roof, along the down-dip direction of the coal seam, complete the drilling of the main branch of the third open horizontal section of the roof horizontal well; (4) Wash the well of the main branch of the third open horizontal section of the roof horizontal well; (5) On the main branch of the third open horizontal section of the roof horizontal well, drill and excavate coal to form caves for the roof horizontal well branch from far to near, and lower the corresponding packer during the drilling process; (6) Drill the first open and the second open of the floor horizontal well, and perform cementing; (7) At the coal seam floor, along the up-dip direction of the coal seam, complete the drilling of the main branch of the horizontal section of the third spud of the floor horizontal well. (8) Drill the branches of the floor horizontal well in sequence from far to near in the main branch of the horizontal section of the third spud of the floor horizontal well, ensuring that the main branch of the horizontal section of the third spud of the floor horizontal well connected by the cave is parallel to the main branch of the horizontal section of the third spud of the roof horizontal well and remains in the same vertical plane; during the drilling process, lower the corresponding packer. (9) Drill or dissolve the lowered packer. (10) Inject dilute acid into the roof horizontal well through its wellhead and let it stand for 24 h. (11) Drain the floor horizontal well. After the drainage is completed, install a negative pressure extraction device at the wellhead of the roof horizontal well for negative pressure extraction; if water is produced from the coal seam during the extraction process, continue to drain the floor horizontal well at the same time, and the drainage of the floor horizontal well does not affect the negative pressure extraction of the roof horizontal well.

[0020] Preferably, the drilling of the branches of the roof horizontal well and the coal mining and cavity formation from far to near in step (5) specifically include the following steps: (51) Lower a packer in front of the window opening position of the outermost and farthest branch of the roof horizontal well in the main branch of the horizontal section of the third spud of the roof horizontal well for plugging. (52) Pull out the drill pipe, lower it to the target position of the outermost and farthest cavity in the main branch of the horizontal section of the third spud of the roof horizontal well, drill the outermost and farthest branch of the roof horizontal well, start drilling from the coal seam roof to the coal seam, ream the hole to form a cavity, and the pulverized coal after being broken is transported to the ground along with the drilling fluid. (53) Wash the well with clear water. (54) Adopt a backward method, repeat steps (51)-(53), and sequentially carry out the drilling of the next branch of the roof horizontal well and the coal mining and cavity formation from far to near. (55) After completing the drilling of all branches of the roof horizontal well and the coal mining and cavity formation, lower a packer at the end of the window opening position of the last branch of the roof horizontal well for plugging.

[0021] Preferably, the drilling of the branches of the floor horizontal well in sequence from far to near in step (8) is specifically as follows: Start drilling from the outermost and farthest branch of the floor horizontal well, make it communicate with the corresponding branch of the roof horizontal well through the corresponding cave. Before drilling, lower a packer in front of the window opening position of the corresponding branch of the floor horizontal well for plugging. After each branch of the floor horizontal well is drilled, wash the well, and repeat this step to sequentially complete the drilling of all branches of the floor horizontal well.

[0022] Preferably, in step (52), the hole reaming and cavity formation adopt three-stage hole reaming for cavity formation, and the three-stage hole reaming rates are 150%, 200%, and 300% respectively.

[0023] Preferably, the distance between the main branch of the horizontal section in the third opening of the roof horizontal well and the top of the coal seam, and the main branch of the horizontal section in the third opening of the floor horizontal well and the bottom of the coal seam is 2 - 3 m; the first opening of the roof horizontal well and the first opening of the floor horizontal well are both drilled to a depth of 60 m below the bedrock and then completed.

[0024] Preferably, when there are more carbonate minerals in the coalbed methane, the dilute acid is dilute hydrochloric acid with a concentration of 10 wt% - 15 wt%; when there are more clay and silicate minerals in the coal seam, the dilute acid is a mixture of dilute hydrochloric acid with a concentration of 5 wt% - 12 wt% and hydrofluoric acid with a concentration of 1 wt% - 3 wt%.

[0025] In the present invention, "from far to near" refers to from far to near relative to the wellhead.

[0026] Advantages of the present invention: 1) By adopting the method of drilling the main branch of the horizontal section in the third opening in the stable roof or floor of the coal seam, it is easier to drill in the stable and hard roof and floor compared with drilling in the coal seam, the wellbore is more stable, and it is not easy to collapse during the extraction process, which is suitable for long-term production; 2) Before and after the branch is drilled, the well is washed and the packer is sealed to avoid the continuous immersion and scouring of the drilling fluid during the subsequent branch construction process, resulting in wellbore pollution and collapse; 3) The roof horizontal well and the floor horizontal well have established an effective channel through the cave, and there is no dead end. Under the action of gravity, the roof horizontal well injects and the floor horizontal well drains water, and the permeability can be continuously improved by flushing. The coal fall and collapse can also be carried out through the liquid circulation of the double-layer horizontal well; 4) By creating a cave to expand the exposed area of the coal seam, promoting the release of coal seam stress and forming a three-dimensional network of fractures, the permeability is greatly improved, and the effect is better than that of coal seam fracturing; 5) By injecting dilute acid for soaking, the carbonate and silicate minerals in the coal and parting can be dissolved to enhance the permeability; 6) It can be quickly depressurized and then put into operation for drainage and extraction. For conventional fractured wells, in order to avoid the closure of the fracture, it is necessary to slowly depressurize for 15 days or even several months before putting into operation. The present invention can quickly depressurize or does not need to depressurize, and can quickly drain and extract after being put into operation to achieve quick gas production and peak gas production. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the roof horizontal well and the cave; Figure 2 It is a schematic diagram of the wellbore of the ground coal mining and pressure relief extraction well for the double-layer horizontal well of the roof and floor; Figure 3 It is a logging interpretation result diagram of the 15# coal seam in Block A; Figure 4 It is a schematic diagram of the wellbore of the ground coal mining and pressure relief extraction well for the double-layer multi-main-branch horizontal well of the roof and floor; Among them, the roof of Coal Seam 1, Coal Seam 2, the floor of Coal Seam 3, the first opening of the horizontal well in the roof 4, the first opening of the horizontal well in the floor 4-1, the second opening of the horizontal well in the roof 5, the second opening of the horizontal well in the floor 5-1, the main branch of the horizontal section of the third opening of the horizontal well in the roof 6, the main branch of the horizontal section of the third opening of the horizontal well in the floor 6-1, the branch of the horizontal well in the roof 7, the branch of the horizontal well in the floor 7-1, the cave 8, and the packer 9. Detailed implementation method

[0028] Embodiment 1 The double-layer horizontal well ground coal mining pressure relief and drainage system for broken soft coal seam roof and floor includes a horizontal well in the roof and a horizontal well in the floor. The horizontal well in the roof includes the first opening of the horizontal well in the roof 1, the second opening of the horizontal well in the roof 5, and the main branch of the horizontal section of the third opening of the horizontal well in the roof 6, which are connected in sequence, and several branches of the horizontal well in the roof 7 that are inclined and arranged below the main branch of the horizontal section of the third opening of the horizontal well in the roof 6. A cave 8 is arranged on the branch of the horizontal well in the roof 7; the horizontal well in the floor includes the first opening of the horizontal well in the floor 4-1, the second opening of the horizontal well in the floor 5-1, the main branch of the horizontal section of the third opening of the horizontal well in the floor 6-1, which are connected in sequence, and several branches of the horizontal well in the floor 7-1 that are inclined and arranged above the horizontal section of the third opening of the horizontal well in the floor; each branch of the horizontal well in the roof 7 is connected to a branch of the horizontal well in the floor 7-1 through a cave 8; the main branch of the horizontal section of the third opening of the horizontal well in the roof 6 and the main branch of the horizontal section of the third opening of the horizontal well in the floor 6-1 are respectively located in the roof 1 of the coal seam and the floor 3 of the coal seam, and the cave 8 is arranged in the coal seam 2; the number of branches of the horizontal well in the roof 7, branches of the horizontal well in the floor 7-1, and caves 8 corresponds; the main branch of the horizontal section of the third opening of the horizontal well in the roof 6 and the main branch of the horizontal section of the third opening of the horizontal well in the floor 6-1 that are connected through the cave 8 are arranged in parallel and in the same vertical plane.

[0029] By connecting the horizontal well in the roof and the horizontal well in the floor through the cave 8, under the action of gravity, the horizontal well in the roof injects and the horizontal well in the floor drains liquid, and continuous flushing can be carried out to improve permeability. The coal seam caving and collapse can also be carried out through the liquid circulation of the double-layer horizontal well and carried out, and it can be put into operation without pressure release or rapid pressure release, and rapid gas production can be achieved.

[0030] Embodiment 2 1. On the basis of Embodiment 1, packers 9 are arranged on the main branch of the horizontal section of the third opening of the horizontal well in the roof 6, in front of the window opening position of the branch of the horizontal well in the roof 7, at the end of the window opening position of the branch of the horizontal well in the roof 7 near the wellhead, and on the main branch of the horizontal section of the third opening of the horizontal well in the floor 6-1, in front of the window opening position of the branch of the horizontal well in the floor 7-1.

[0031] The distances between the main branch of the horizontal section of the third opening of the horizontal well in the roof 6 and the main branch of the horizontal section of the third opening of the horizontal well in the floor 6-1 and the top and bottom of the coal seam 2 are 2-3m respectively.

[0032] The maximum diameter of the cave 8 is three times the diameter of the roof horizontal well branch 7.

[0033] The well deviation angle of the roof horizontal well branch 7 is 30° smaller than that of the main branch 6 of the third open horizontal section of the roof horizontal well, which is convenient for drainage.

[0034] The distance between two adjacent roof horizontal well branches 7 is 8 - 10 m.

[0035] The packer 9 is a drillable or dissolvable open-hole packer.

[0036] 2. The ground coal extraction pressure relief drainage method for the double-layer horizontal well in the top and bottom plates of the broken soft coal seam is carried out by using this system, including the following steps: (1) Collect the geological data of the target area and the relevant parameters of adjacent exploration wells or coalbed methane wells, determine the well position deployment of the double-layer horizontal well, and determine the distances of the main branch 6 of the third open horizontal section of the roof horizontal well and the main branch 6-1 of the third open horizontal section of the floor horizontal well from the coal seam and the drilling target horizon according to the collected lithological and mechanical parameters of the top and bottom plates of the coal seam; among them, the distances between the main branch 6 of the third open horizontal section of the roof horizontal well and the main branch 6-1 of the third open horizontal section of the floor horizontal well and the top and bottom of the coal seam 2 are 2 - 3 m respectively, and the longitudinal (up and down) actual trajectory of the main branch of the roof horizontal well and the main branch of the floor horizontal well has an error not exceeding 2 m from the design, and the lateral error should not exceed 1 m; (2) The roof horizontal well adopts a large-diameter three-opening wellbore structure. The first open hole 1 of the roof horizontal well (surface drilling) is drilled to a depth of 60 m below the bedrock and then completed, and the surface casing is run in and cemented; the second open hole 5 of the roof horizontal well (technical casing section) is drilled from the bottom of the surface casing to the end of the build-up (when it ends, the distance from the top of the coal seam 2 is 2 - 3 m), and the technical casing is run in and cemented; (3) In order to facilitate drainage and pressure reduction, in the coal seam roof 1, along the down-dip direction of the coal seam 2, the main branch 6 of the third open horizontal section of the roof horizontal well is drilled, parallel to the coal seam 2; (4) The main branch 6 of the third open horizontal section of the roof horizontal well is washed. (5) In the main branch 6 of the third open horizontal section of the roof horizontal well, the roof horizontal well branch 7 is drilled and coal extraction and cavity formation are carried out from far to near: (51) A packer 9 is run in and blocked in front of the window opening position of the most distal roof horizontal well branch 7 in the main branch 6 of the third open horizontal section of the roof horizontal well. The purpose is to avoid contaminating the main branch of the roof horizontal well when communicating with the floor horizontal well; (52) Lift the drill pipe and lower it to the target position of the cave 8 at the farthest end in the main branch 6 of the horizontal section of the third section of the roof horizontal well. Drill the farthest roof horizontal well branch 7, starting from the coal seam roof 1 and drilling to the coal seam 2. Ream and create a cavity. The pulverized coal after being broken is transported to the ground with the drilling fluid (using water) and flows into the sedimentation tank for separation of the drilling fluid and the pulverized coal. Among them, reaming and creating a cavity adopt three-stage reaming. The three-stage reaming rates are 150%, 200%, and 300% respectively. Existing reaming tools and reaming methods can be used. For example, referring to Patent CN108071344B, three-stage reaming can be carried out. For the first and second stages of reaming, a plunger bit is used, and for the third stage of reaming, a blade wing is used. Reciprocating progressive reaming and cavity creation are carried out in the coal seam 2 to reduce the disturbance of the surrounding soil or rock layer caused by single-stage reaming and reduce the risk of hole wall collapse. (53) Wash the well with clean water; (54) In a backward manner, repeat steps (51)-(53) to drill and excavate coal and create cavities for the next roof horizontal well branch 7 in sequence from far to near. (55) After completing the drilling and excavation of coal and creating cavities for all roof horizontal well branches 7, lower a packer 9 at the end of the window opening position of the last roof horizontal well branch 7 for plugging to prevent the vertical section and the build-up section of the floor horizontal well from communicating with those of the roof horizontal well. (6) The floor horizontal well adopts a large-diameter three-section wellbore structure. For the first section 4-1 of the floor horizontal well (surface drilling), drill to 60 m below the bedrock and complete the drilling, then lower the surface casing and cement it. For the second section 5-1 of the floor horizontal well (technical casing section), drill from the bottom of the surface casing to the end of the build-up (when ending, the distance from the bottom of the coal seam 2 is 2-3 m), then lower the technical casing and cement it. (7) At the coal seam floor 3, along the up-dip direction of the coal seam 2, complete the drilling of the main branch 6-1 of the horizontal section of the third section of the floor horizontal well, parallel to the coal seam 2. (8) During the drilling of the main branch 6-1 of the horizontal section of the third section of the floor horizontal well, drill the floor horizontal well branches 7-1 in sequence from far to near: Start drilling from the farthest floor horizontal well branch 7-1 to make it communicate with the roof horizontal well branch 7 through the corresponding cave 8. Before drilling, lower a packer 9 for plugging in front of the window opening position of the corresponding floor horizontal well branch 7-1. After each floor horizontal well branch 7-1 completes drilling, wash the well. Repeat this step to complete the drilling of all floor horizontal well branches 7-1 in sequence. Ensure that the main branch 6-1 of the horizontal section of the third section of the floor horizontal well connected through the cave 8 is parallel to the main branch 6 of the horizontal section of the third section of the roof horizontal well and remains in the same vertical plane. (9) Remove the packer 9: Drill and mill or dissolve all the lowered packers 9; (10) injecting dilute acid into the horizontal well through the wellhead of the roof, and letting it stand for more than 24 hours to dissolve the soluble minerals therein and increase the permeability; when there are more carbonate minerals in the coalbed methane, the dilute acid is a dilute hydrochloric acid with a concentration of 10wt%~15wt%; when there are more clay and silicate minerals in the coal seam, the dilute acid is a mixture of 5wt%~12wt% dilute hydrochloric acid and 1wt%~3wt% hydrofluoric acid; (11) In the early stage of drainage, a large amount of well washing fluid (the well washing fluid in the present invention is water) remains in the wellbore. A screw pump or a rodless pump is used to quickly drain the bottom plate horizontal well. After the drainage is completed, a water ring vacuum pump is installed at the wellhead of the roof plate horizontal well to carry out negative pressure extraction, thereby achieving rapid gas production and sustained high production of the coalbed methane well. If water production occurs in the coal seam during the extraction process, the bottom plate horizontal well is continuously drained without affecting the negative pressure gas production of the roof plate horizontal well.

[0037] Example 3 Block A is located in the eastern part of the basin. The No. 15 coal seam has a dip angle of 8°-20° and a thickness of about 5.03m. The No. 15 coal seam contains a layer of intercalated gangue, which is 0.34m thick mudstone. The pseudo-top is 0.95m thick sandy mudstone, the upper direct top is 7.1m thick fine sandstone, the pseudo-bottom is 0.72m thick sandy mudstone, and the lower direct bottom is 3.6m thick fine sandstone. Figure 3 ; Combination Figure 1 , Figure 2 , Figure 3 The present invention is further described as follows: The method for surface coal excavation and pressure relief extraction of double-layer horizontal wells in the top and bottom plates of broken soft coal seams is carried out using the system described in the above-mentioned embodiment 2, wherein the maximum diameter of the cave 8 is 0.65m, and the method comprises the following steps: (1) Collect geological data of the above-mentioned target area and relevant parameters of nearby exploration wells or coalbed methane wells to determine the well location deployment of the double-layer horizontal wells. According to the collected lithology and mechanical parameters of the coal seam roof and floor, determine the distance between the main branch 6 of the three-opening horizontal section of the roof horizontal well and the main branch 6-1 of the three-opening horizontal section of the floor horizontal well and the coal seam and the drilling target layer; wherein, the main branch 6 of the three-opening horizontal section of the roof horizontal well and the main branch 6-1 of the three-opening horizontal section of the floor horizontal well are 2-3m away from the top and bottom of the coal seam 2 respectively, and the actual longitudinal (up and down) trajectory of the main branch of the roof horizontal well and the main branch of the floor horizontal well shall not exceed 2m from the design, and the lateral error shall not exceed 1m; the upper direct roof is 7.1m of fine sandstone equivalent to the coal seam roof 1, the lower direct bottom is 3.6m of fine-grained sandstone equivalent to the coal seam floor 3, and the 15# coal seam is equivalent to the coal seam 2; (2) The roof horizontal well adopts a large-diameter three-opening wellbore structure and selects a ZJ40 drilling rig. For the first opening 1 of the roof horizontal well, the hole diameter of the first opening 1 of the roof horizontal well is Φ444.5 mm, reaching 60 m below the stable bedrock, and a Φ339.7 mm surface casing is run in and cemented to return to the wellhead; for the second opening 5 of the roof horizontal well, the hole diameter is Φ311.15 mm, and a Φ244.5 mm technical casing is run in and cemented to the ground. (3) To facilitate drainage and pressure reduction, at the coal seam roof 1, along the down-dip direction of the coal seam 2, a Φ215.9 mm bit is used to complete the drilling of the main branch 6 of the horizontal section of the third opening of the roof horizontal well. The distance between the horizontal section of the third opening of the roof horizontal well and the top of the coal seam is maintained within 2 - 3 m and is parallel to the coal seam 2. (4) Use clear water to wash the main branch 6 of the horizontal section of the third opening of the roof horizontal well. (5) In the main branch 6 of the horizontal section of the third opening of the roof horizontal well, drill and excavate coal to form cavities for the roof horizontal well branch 7 from far to near: (51) Run in a packer 9 in front of the window opening position of the farthest roof horizontal well branch 7 inside the main branch 6 of the horizontal section of the third opening of the roof horizontal well for plugging, aiming to avoid contaminating the main branch of the roof horizontal well when communicating with the floor horizontal well. (52) Pull out the drill pipe and run in the target interval of the farthest cavity 8 in the horizontal section of the third opening of the roof horizontal well, drill the farthest roof horizontal well branch 7, start drilling from the coal seam roof 1 to the coal seam 2, ream and excavate the cavity, and the pulverized coal after being broken is transported to the ground with the drilling fluid (using water) and flows into the sedimentation tank for separation of the drilling fluid and the pulverized coal; among them, the reaming tool and the reaming method refer to Patent CN108071344B for three-stage reaming, and the three-stage reaming rates are 150%, 200%, and 300% respectively. (53) Wash the well with clear water. (54) Adopt a backward method to repeat steps (51)-(53) to drill and excavate coal to form cavities for the next roof horizontal well branch 7 from far to near in sequence. (55) After completing the drilling and excavating coal to form cavities for all roof horizontal well branches 7, run in a packer 9 at the end of the window opening position of the last roof horizontal well branch 7 for plugging to avoid the floor horizontal well communicating with the vertical section and the deflecting section of the roof horizontal well. (6) The floor horizontal well adopts a large-diameter three-opening wellbore structure. For the first opening 4 - 1 of the floor horizontal well (surface drilling), drill to a depth of 60 m below the bedrock to complete the drilling, run in the surface casing and cement it; for the second opening 5 - 1 of the floor horizontal well (technical casing section), drill from the bottom of the surface casing to the end of deflection (when ending, the distance from the bottom of the coal seam 2 is 2 - 3 m), run in the technical casing and cement it; among them, the drilling parameters are the same as those in step (2). (7) At the coal seam floor 3, along the upward dip direction of the coal seam 2, drill the main branch 6-1 of the horizontal section of the third section of the floor horizontal well. The parameters are the same as those in step (3). The distance between the main branch 6-1 of the horizontal section of the third section of the floor horizontal well and the bottom of the coal seam is kept within 2-3 m, parallel to the coal seam 2. The connection line between the wellhead of the floor horizontal well and the wellhead of the roof horizontal well is the dip direction of the coal seam 2; (8) Drill the branches 7-1 of the floor horizontal well in sequence from far to near along the main branch of the floor horizontal well: The trajectory of each branch is precisely controlled by near-bit steering technology. Start drilling from the farthest floor horizontal well branch 7-1, and make it communicate with the corresponding roof horizontal well branch 7 through the corresponding cave 8. Before drilling, lower a packer 9 at the window-opening position of the corresponding floor horizontal well branch 7-1 for plugging. The purpose is to avoid the pollution of the completed floor horizontal well branch 7-1 and the wellbore of the main branch of the floor horizontal well during the construction of the subsequent floor horizontal well branch 7-1. After each floor horizontal well branch 7-1 is drilled, wash the well. Repeat this step to complete the drilling of all floor horizontal well branches 7-1 in sequence; ensure that the main branch 6-1 of the horizontal section of the third section of the floor horizontal well connected through the cave 8 is parallel to the main branch 6 of the horizontal section of the third section of the roof horizontal well and is in the same vertical plane; (9) Remove the packer 9: Drill and mill or dissolve all the open-hole packers 9 that have been lowered; (10) Inject dilute acid into the roof horizontal well through its wellhead, let it stand for 24 h, dissolve the soluble minerals in it, and increase the permeability; because this coal seam contains more clay and silicate minerals, the dilute acid is a mixture of 5 wt% - 12 wt% dilute hydrochloric acid and 1 wt% - 3 wt% hydrofluoric acid; (11) In the initial stage of drainage, there is a large amount of well-washing fluid water in the wellbore. Use a screw pump to quickly drain the floor horizontal well. After the drainage is completed, install a water-ring vacuum pump at the wellhead of the roof horizontal well for vacuum drainage to achieve rapid gas production and continuous high production of the coalbed methane well; if water production occurs during the drainage process, continue to drain the floor horizontal well, and the drainage does not affect the vacuum drainage.

[0038] Example 4 On the basis of the above Example 2, in order to increase the coal extraction volume and gas production volume, set the main branches 6 of the horizontal section of the third section of the roof horizontal well and the main branches 6-1 of the horizontal section of the third section of the floor horizontal well to multiple, specifically as follows: The main branches 6 of the horizontal section of the third section of the roof horizontal well and the main branches 6-1 of the horizontal section of the third section of the floor horizontal well are respectively set to multiple, and the numbers are the same; all the main branches 6 of the horizontal section of the third section of the roof horizontal well are connected to the second section 5 of the roof horizontal well, and all the main branches 6-1 of the horizontal section of the third section of the floor horizontal well are connected to the second section 5-1 of the floor horizontal well.

[0039] More preferably, there are 2 main horizontal wells on the top plate and the bottom plate respectively, specifically as shown in Figure 4 .

Claims

1. Double-layer horizontal well surface coal extraction pressure relief drainage system for top and bottom plates of soft broken coal seams, characterized in that: It includes a top - plate horizontal well and a bottom - plate horizontal well. The top - plate horizontal well includes a first - stage top - plate horizontal well (4), a second - stage top - plate horizontal well (5), and a main branch of the third - stage horizontal section of the top - plate horizontal well (6) connected in sequence, as well as a number of top - plate horizontal well branches (7) inclined and arranged below the main branch of the third - stage horizontal section of the top - plate horizontal well (6). A cave (8) is provided on the top - plate horizontal well branch (7). The bottom - plate horizontal well includes a first - stage bottom - plate horizontal well (4 - 1), a second - stage bottom - plate horizontal well (5 - 1), a main branch of the third - stage horizontal section of the bottom - plate horizontal well (6 - 1), and a number of bottom - plate horizontal well branches (7 - 1) inclined and arranged above the third - stage horizontal section of the bottom - plate horizontal well. Each top - plate horizontal well branch (7) is connected to a bottom - plate horizontal well branch (7 - 1) through a cave (8). The main branch of the third - stage horizontal section of the top - plate horizontal well (6) and the main branch of the third - stage horizontal section of the bottom - plate horizontal well (6 - 1) are respectively located in the coal - seam roof (1) and the coal - seam floor (3), and the cave (8) is arranged in the coal seam (2). The numbers of the top - plate horizontal well branches (7), the bottom - plate horizontal well branches (7 - 1), and the caves (8) correspond to each other. The main branch of the third - stage horizontal section of the top - plate horizontal well (6) and the main branch of the third - stage horizontal section of the bottom - plate horizontal well (6 - 1) connected through the cave (8) are parallel and in the same vertical plane.

2. The double-layer horizontal well ground coal mining pressure relief extraction system for the top and bottom plates of soft broken coal seams according to claim 1, wherein: Packers (9) are provided on the main branch of the third - stage horizontal section of the top - plate horizontal well (6) before the window - opening position of the top - plate horizontal well branch (7), at the end of the window - opening position of the top - plate horizontal well branch (7) close to the wellhead on the main branch of the third - stage horizontal section of the top - plate horizontal well (6), and before the window - opening position of the bottom - plate horizontal well branch (7 - 1) on the main branch of the third - stage horizontal section of the bottom - plate horizontal well (6 - 1).

3. The double-layer horizontal well surface coal extraction pressure relief and drainage system for the roof and floor of soft broken coal seams according to claim 2, wherein: The distances between the main branch of the third - stage horizontal section of the top - plate horizontal well (6) and the top of the coal seam (2), and between the main branch of the third - stage horizontal section of the bottom - plate horizontal well (6 - 1) and the bottom of the coal seam (2) are 2 - 3 m respectively.

4. The double-layer horizontal well ground coal mining pressure relief extraction system for the roof and floor of soft broken coal seams according to claim 2, characterized in that: The maximum diameter of the cave (8) is 3 times the diameter of the top - plate horizontal well branch (7).

5. The double-layer horizontal well surface coal extraction pressure relief and drainage system for the roof and floor of broken soft coal seams according to claim 2, characterized in that: The well - deviation angle of the top - plate horizontal well branch (7) is 30° smaller than the well - deviation angle of the main branch of the third - stage horizontal section of the top - plate horizontal well (6).

6. The double-layer horizontal well ground coal mining pressure relief and drainage system for the roof and floor of soft broken coal seams according to claim 2, characterized in that: The distance between two adjacent top - plate horizontal well branches (7) is 8 - 10 m.

7. The double - layer horizontal well ground coal extraction and pressure relief drainage system for the top and bottom plates of broken soft coal seams according to claim 6, wherein: There are multiple main branches of the third - stage horizontal section of the top - plate horizontal well (6) and the main branches of the third - stage horizontal section of the bottom - plate horizontal well (6 - 1), and the numbers are the same. All the main branches of the third - stage horizontal section of the top - plate horizontal well (6) are connected to the second - stage top - plate horizontal well (5), and all the main branches of the third - stage horizontal section of the bottom - plate horizontal well (6 - 1) are connected to the second - stage bottom - plate horizontal well (5 - 1).

8. Ground coal extraction and pressure relief drainage method for double-layer horizontal wells in the roof and floor of soft broken coal seams, characterized in that: Using the system described in any one of claims 2 - 7, it includes the following steps: (1) Collect geological data of the target area and relevant parameters of adjacent exploration wells or coal - bed methane wells, determine the well - position deployment of the double - layer horizontal well, determine the distances between the main branch of the third - stage horizontal section of the top - plate horizontal well (6), the main branch of the third - stage horizontal section of the bottom - plate horizontal well (6 - 1) and the coal seam (2), and the drilling target horizon; (2) Drill the first - stage top - plate horizontal well (4) and the second - stage top - plate horizontal well (5), and perform cementing; (3) At the coal seam roof (1), along the down-dip direction of the coal seam (2), drill the main branch (6) of the horizontal section of the third section of the roof horizontal well; (4) Wash the main branch (6) of the horizontal section of the third section of the roof horizontal well; (5) At the main branch (6) of the horizontal section of the third section of the roof horizontal well, drill and excavate coal to form cavities for the roof horizontal well branches (7) from far to near. During the drilling process, lower the corresponding packer (9); (6) Drill the first section (4-1) and the second section (5-1) of the floor horizontal well, and perform cementing; (7) At the coal seam floor (3), along the up-dip direction of the coal seam (2), drill the main branch (6-1) of the horizontal section of the third section of the floor horizontal well; (8) Drill the floor horizontal well branches (7-1) from far to near in sequence at the main branch (6-1) of the horizontal section of the third section of the floor horizontal well, ensuring that the main branch (6-1) of the horizontal section of the third section of the floor horizontal well connected to the main branch (6) of the horizontal section of the third section of the roof horizontal well through the cave (8) is parallel and remains in the same vertical plane; among them, lower the corresponding packer (9) during the drilling process; (9) Drill or dissolve the lowered packer (9); (10) Inject dilute acid into the roof horizontal well through its wellhead and let it stand for 24 h; (11) Drain the floor horizontal well. After the drainage is completed, install negative pressure extraction equipment at the wellhead of the roof horizontal well for negative pressure extraction; if water is produced from the coal seam during the extraction process, continue to drain the floor horizontal well at the same time.

9. The ground coal extraction pressure relief and drainage method for double - layer horizontal wells in the roof and floor of soft - broken coal seams according to claim 8, characterized in that: The specific steps of drilling and excavating coal to form cavities for the roof horizontal well branches (7) from far to near in step (5) are as follows: (51) Lower a packer (9) in the main branch (6) of the horizontal section of the third section of the roof horizontal well to block the position in front of the window-opening position of the outermost roof horizontal well branch (7); (52) Lift the drill pipe, lower it to the target position of the outermost cave (8) in the main branch (6) of the horizontal section of the third section of the roof horizontal well, drill the outermost roof horizontal well branch (7), start drilling from the coal seam roof (1) to the coal seam (2), ream the hole to form a cavity, and the pulverized coal after being broken is transported to the ground with the drilling fluid; (53) Wash the well with clean water; (54) In a backward manner, repeat steps (51)-(53) to drill and excavate coal to form cavities for the next roof horizontal well branch (7) from far to near in sequence; (55) After completing the drilling and excavating coal to form cavities for all roof horizontal well branches (7), lower a packer (9) at the end of the window-opening position of the last roof horizontal well branch (7) to block it; The specific process of drilling the floor horizontal well branches (7-1) from far to near in step (8) is as follows: Start drilling from the outermost floor horizontal well branch (7-1) to make it communicate with the roof horizontal well branch (7) through the corresponding cave (8). Before drilling, lower a packer (9) in front of the window-opening position of the corresponding floor horizontal well branch (7-1) to block it. After each floor horizontal well branch (7-1) is drilled, wash the well, and repeat this step to complete the drilling of all floor horizontal well branches (7-1) in sequence.

10. The ground coal extraction pressure relief and drainage method for double-layer horizontal wells in the roof and floor of soft broken coal seams according to claim 9, characterized in that: The hole enlargement and cavity formation adopt three - stage hole diameter expansion for cavity formation, and the three - stage hole diameter expansion rates are 150%, 200%, and 300% respectively; the first - stage drilling (4) of the horizontal well in the roof and the first - stage drilling (4 - 1) of the horizontal well in the floor are both drilled to a depth of 60 m below the bedrock and then completed.

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