Coal bed gas multi-branch horizontal well ground coal extraction rapid pressure relief extraction system and method

Abstract

The invention relates to the technical field of coal and coal bed gas co-mining, in particular to a coal bed gas multi-branch horizontal well ground coal extraction rapid pressure relief extraction system and method.The method comprises the steps that a ground horizontal well is arranged, specifically, a first drilling well is drilled to the position 60 m below a stable bed rock, and a second drilling well is drilled to the position within the range of 1-2 m from the end of a deflecting section to a coal bed top plate / coal bed bottom plate; the horizontal main branch section and the coal seam are drilled in parallel, windowing drilling of each branch section is completed in a backward mode from the far-end horizontal main branch section deviating from the first drilling well in sequence, the branch sections are provided with diameter-expanding coal digging caves, and a drilling track penetrating through the coal seam between a coal seam top plate and a coal seam bottom plate is formed; and drainage and mining equipment is installed for negative-pressure extraction, and rapid gas seeing and continuous high yield of the coal-bed gas well are achieved. The mode that the horizontal main branch section is drilled in the stable coal seam top plate or coal seam bottom plate is adopted, compared with coal seam drilling, the stable and hard coal seam top plate or coal seam bottom plate is easier, the well wall is more stable, collapse is not prone to occurring in the extraction process, and the method is suitable for long-term production.

Description

Technical Field

[0001] The present invention relates to the technical field of co - mining of coal and coalbed methane, and particularly to a surface coal - mining rapid pressure - relief extraction system and method for multi - branched horizontal wells of coalbed methane. Background Art

[0002] Since 2010, the coal mine gas extraction technology in China has gradually developed into a comprehensive treatment stage mainly based on regional extraction. The "three - zone linkage vertical and horizontal extraction mode in coal mines in Shanxi key coal mining areas" has gradually developed into a "four - zone linkage (planning area, preparation area, production area (mining area), goaf)" co - mining mode of coal and coalbed methane in the whole time period, whole coal seam, and whole region (CN115341877A). In the southern high - rank coal in the Qinshui Basin of Shanxi coal mining area, commercial exploitation has been realized. The preparation area adopts the combined extraction method of underground and surface, attempting to improve the coalbed methane extraction efficiency through the linkage extraction of underground and surface. For the broken - soft and low - permeability tectonic coal seams, the surface coalbed methane wells in the planning area are used for advanced pre - extraction. A large number of extraction projects show that the ground fracturing transformation effect of broken - soft coal seams is poor, it is difficult to form an effective seepage network and high - speed seepage channels in the coal seams, the extraction effect is extremely poor, the single - well output is low, and it is difficult to recover the cost. Through a large number of stimulation tests, the effect is very little. The high - efficiency 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] CN101936155A, a distribution structure of the horizontal section of a multi - branched horizontal well for coalbed methane, proposes a wellbore structure of a multi - branched horizontal well. By increasing the number of main wellbores of the multi - branched horizontal well and locating the branch wellbores outside the outermost two 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. The above - mentioned structure is aimed at high - rank coal with relatively hard coal quality, and to a certain extent, it improves the coalbed methane extraction efficiency. However, during the drilling process in broken - soft coal seams, problems such as wellbore collapse, caving, and other problems are likely to occur, resulting in the failure of well completion. At the same time, although multi - branched 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 - branched horizontal wells are open - hole completed wells and cannot be fractured for stimulation.

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

[0005] CN102979498B A coalbed methane multi-branch horizontal well system proposes a drilling method for multi-branch horizontal wells in the roof or floor of a coal seam. The L-shaped horizontal well pattern is adopted. 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. This technical method only solves the problem of the gas production channel. The problems of coal seam pollution, wellbore collapse and high well completion difficulty generated during the drilling process of the branches and vein branches remain unsolved.

[0006] CN214576923U Coalbed methane horizontal well system proposes an L-shaped double-layer parallel single-branch horizontal well system; CN113605874B A method for extracting coalbed methane from the double-layer horizontal wells in the roof and floor of broken soft coal seams 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 technologies are the same as those of the method for extracting coalbed methane from the segmented fracturing horizontal wells in the roof or floor of broken soft and low-permeability coal seams proposed in CN112983385B, but only technical optimization has been carried out on this basis, and the gas production and drainage channels have been separated. The above three patent technical principles are only technically optimized on the basis of constructing the gas production channel in the coal seam roof. The coal seam is enhanced and transformed through directional fracturing. As mentioned above, a large number of engineering practices have shown that enhancing and transforming broken soft coal seams through various hydraulic fracturing is a failure. The fractures generated by the fracturing will quickly close with the drainage and pressure reduction of the coalbed methane, and it is difficult to form a lasting fracture network for the diffusion and migration of coalbed methane.

[0007] CN108798516B and CN108843241B propose a method for in-situ coalbed methane horizontal well cave pressure relief mining of tectonic coal. This technology adopts the U-shaped well pattern, reams the horizontal section of several hundred meters or even more than a thousand meters, and the broken coal powder is discharged to the ground through the butt vertical well using a special lifting system. The biggest problem with the above technical solution is still that during the reaming process of the horizontal section, the problems of wellbore collapse and high well completion difficulty cannot be solved. Even after the well is completed, large-area wellbore collapse will still occur during the drainage process, resulting in well completion failure and loss of the gas production channel and inability to produce gas.

[0008] CN112593912B A method for dynamic diameter expansion, pressure relief and permeability enhancement extraction of coalbed methane in horizontal wells. A single-branch horizontal well is adopted, and the horizontal section is arranged in the coal seam. Mechanical diameter expansion and high-pressure air-powered coal cutting for pressure relief are carried out on the ground. Coal cutting 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 ground coalbed methane well. This patent has optimized the technology on the basis of the overall coal cutting and pressure relief in the horizontal section of CN108798516B. A fiberglass-reinforced plastic casing is run into the horizontal section, and then the fiberglass-reinforced plastic casing in the horizontal section is milled. Then, mechanical diameter expansion and dynamic diameter expansion coal cutting and cavity formation are carried out, and the cavity is supported with a filling material rich in holes and fissures. This scheme requires a large amount of time and cost. First, the downhole operation cost of coiled tubing is expensive. Second, during the cavity formation process of this technical scheme, 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 sealing. 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 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] In view of the international problem of coalbed methane extraction in soft and broken coal seams, as well as the deficiencies and defects of the existing technology, based on the current situation that the downhole coal cutting and pressure relief extraction effect is good, but the project volume is large, the cost is high, the construction period is long, and there are potential safety hazards, the present invention proposes a method for rapid pressure relief extraction of coalbed methane by ground coal cutting in multi-branch horizontal wells in soft and broken coal seams to achieve the efficient and large-scale exploitation of coalbed methane in soft and broken coal seams.

[0010] The technical solution of the present invention is as follows: (I) Ground coal cutting rapid pressure relief extraction system for multi-branch horizontal wells of coalbed methane The ground coal cutting rapid pressure relief extraction system for multi-branch horizontal wells of coalbed methane is characterized in that a ground horizontal well is arranged along the strike of the mining face in the coal mine preparation area; wherein, the ground horizontal well includes a first-opening drilled well, a second-opening drilled well, a third-opening horizontal main branch section and branch sections; the horizontal main branch section is located at the coal seam floor / coal seam roof, and drills into the opposite coal seam roof / coal seam floor through the branch sections. The branch sections are provided with diameter expansion coal cutting and cavity formation, and the maximum cavity formation diameter of the diameter expansion coal cutting and cavity formation is 3 times the original diameter of the branch section.

[0011] The mining direction of the planned mining face in the coal mine preparation area is downward dip. When the coal seam dip angle > 15°, the well type is a horizontal main branch section with multiple branch sections of L-shaped coal seam roof; when the coal seam dip angle < 15°, the well type is a horizontal main branch section with multiple branch sections of L-shaped coal seam floor.

[0012] The mining direction of the coal face planned in the coal mine preparation area is upward. It also includes a row of production wells, which are connected to the horizontal main branch section located at the coal seam floor. The well type is a multi-branch horizontal well with a U-shaped horizontal main branch section at the coal seam floor.

[0013] When the number of the horizontal main branch sections = 1, the well type is a multi-branch horizontal well with an L-shaped single horizontal main branch section at the coal seam roof or a multi-branch horizontal well with an L-shaped single horizontal main branch section at the coal seam floor; when the number of the horizontal main branch sections > 1, the well type is a multi-branch horizontal well with an L-shaped multi-horizontal main branch section at the coal seam roof or a multi-branch horizontal well with an L-shaped multi-horizontal main branch section at the coal seam floor.

[0014] When the number of the horizontal main branch sections = 1, the well type is a multi-branch horizontal well with a U-shaped single horizontal main branch section at the coal seam floor, and the number of the production wells = 1; when the number of the horizontal main branch sections > 1, the well type is a multi-branch horizontal well with a U-shaped multi-horizontal main branch section at the coal seam floor, the number of the production wells is equal to the number of the horizontal main branch sections, and the production wells correspond to the horizontal main branch sections one by one.

[0015] The reaming rates of the reaming and coal mining for creating cavities are 150%, 200%, and 300% in sequence.

[0016] The distance between the horizontal main branch section and the coal seam is 1 - 2 m, and the end of the branch section extends into the coal seam roof / coal seam floor and the distance from the coal seam is 1 - 2 m.

[0017] The number of the branch sections provided on the horizontal main branch section > 1, the distance between two adjacent branch sections is 8 - 10 m, and the branch sections are inclined upward with a well deviation angle of 92° - 95°; the drilling of the branch sections starts from the far end away from the first drilled well and retreats to drill to the first drilled well; after the drilling of each branch section is completed, a packer is provided in the horizontal main branch section, and the distance between the packer and the next branch section < the distance from the current branch section.

[0018] (2) A method for rapid pressure relief and extraction of coalbed methane by ground coal mining in multi-branch horizontal wells, using the rapid pressure relief and extraction system for coalbed methane by ground coal mining in multi-branch horizontal wells as described above. The method is as follows: Set up the ground horizontal well: The first drilled well is drilled to 60 m below the stable bedrock, and the second drilled well is drilled to within the range of 1 - 2 m from the coal seam roof / coal seam floor at the end of the build section; the horizontal main branch section is drilled parallel to the coal seam, and starting from the far-end horizontal main branch section away from the first drilled well in sequence, the window drilling of each branch section is completed in a retrogressive manner. The branch sections are provided with reaming and coal mining for creating cavities to form a drilling trajectory that penetrates the coal seam between the coal seam roof and the coal seam floor; Install the production equipment for negative pressure extraction to achieve rapid gas breakthrough and continuous high production of the coalbed methane well.

[0019] More preferably, when arranging the ground horizontal well, If the mining direction of the coal mining face planned in the coal mine preparation area is downward dip, when the coal seam dip angle > 15°, the well type is a multi-branch horizontal well with an L-shaped horizontal main branch section on the coal seam roof, and the horizontal main branch section is located on the coal seam roof; when the coal seam dip angle < 15°, the well type is a multi-branch horizontal well with an L-shaped horizontal main branch section on the coal seam floor, and the horizontal main branch section is located on the coal seam floor; a jet pump is installed at the wellhead of the surface horizontal well for drainage, and a water ring vacuum pump is installed in the oil-casing annulus for negative pressure air extraction; in the initial stage of drainage and production, drainage is carried out through the jet pump; subsequently, drainage and negative pressure air extraction are carried out simultaneously. If the mining direction of the coal mining face planned in the coal mine preparation area is upward dip, it also includes a production vertical well, and the production vertical well is connected to the horizontal main branch section located on the coal seam floor. The well type is a multi-branch horizontal well with a U-shaped horizontal main branch section on the coal seam floor, and the horizontal main branch section is located on the coal seam floor; a screw pump is installed in the production vertical well for drainage, and a negative pressure extraction device is installed at the wellhead of the surface horizontal well for gas production, so as to realize the rapid gas appearance and continuous high production of the coalbed methane well.

[0020] The technical effects of the present invention are as follows: ① By adopting the method of drilling the horizontal main branch section in the stable coal seam roof or coal seam floor, it is easier to drill in the stable and hard coal seam roof or coal seam floor than in the coal seam. The wellbore is more stable and is not prone to collapse during the extraction process, which is suitable for long-term production. ② After each part of the horizontal main branch section and the branch section is drilled, well flushing and packer sealing are carried out to avoid the continuous immersion and scouring of the drilling fluid during the construction process of the subsequent branch section, resulting in wellbore pollution and collapse. ③ By enlarging the diameter and excavating coal to create cavities, the exposed area of the coal seam is increased, promoting the release of coal seam stress and forming a three-dimensional network of fractures, greatly improving the permeability, and the effect is better than that of coal seam fracturing. ④ The present invention has relatively few processes, the construction process is simple and easy to implement, the cost is low, and the feasibility is good. ⑤ It can be put into operation for drainage and production after rapid pressure release. For conventional fractured wells, in order to avoid the closure of the fracture, it is necessary to carry out slow pressure release for 15 days or even several months before putting into operation. The present invention can release pressure quickly or without pressure release. After putting into operation, it can quickly drain and produce gas to achieve rapid gas appearance and gas production peak, improving economic benefits. ⑥ The large-diameter three-opening wellbore structure increases the seepage channel while having fast drilling speed and relatively short construction period. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a flow chart of the present invention.

[0022] Figure 2 It is a schematic diagram of a multi-branch horizontal well with a single L-shaped horizontal main branch section on the coal seam roof.

[0023] Figure 3 It is a schematic diagram of a multi-branch horizontal well with a double L-shaped horizontal main branch section on the coal seam roof.

[0024] Figure 4 Schematic diagram of a multi-branch horizontal well with a single horizontal main branch section of L-shaped coal seam floor

[0025] Figure 5 Schematic diagram of a multi-branch horizontal well with a double horizontal main branch section of L-shaped coal seam floor

[0026] Figure 6 Schematic diagram of a multi-branch horizontal well with a single horizontal main branch section of U-shaped coal seam floor

[0027] Figure 7 Schematic diagram of a multi-branch horizontal well with a double horizontal main branch section of U-shaped coal seam floor

[0028] Figure 8 Logging interpretation result diagram of No. 15 coal seam in Block A

[0029] Reference numerals: 1, coal seam roof; 2, coal seam; 3, coal seam floor; 4, first-opening well; 5, second-opening well; 6, horizontal main branch section; 7, branch section; 8, hole enlargement for coal extraction by coal cutting; 9, packer; 10, production and drainage vertical well Specific implementation method

[0030] Specific experimental case 1 In a certain basin in the eastern part, the average thickness of Coal Seam 2 of No. 15 is about 5.03 m. The dip angle of Coal Seam 2 is 8° - 20°. There is one parting, and the thickness of the parting is 0.34 m mudstone. The false roof is sandy mudstone with an average thickness of 0.95 m. The immediate roof above is fine-grained sandstone with an average thickness of 7.1 m. The false floor is sandy mudstone with an average thickness of 0.72 m. The immediate floor below is fine-grained sandstone with an average thickness of 3.6 m (see Figure 8 ), refer to Figure 2 、 Figure 4 、 Figure 6 . The 7.1 m fine sandstone of the immediate roof is equivalent to the coal seam roof 1 in the figure, the 3.6 m fine-grained sandstone of the immediate floor is equivalent to the coal seam floor 3 in the figure, and Coal Seam 2 of No. 15 is equivalent to Coal Seam 2 in the figure

[0031] In the case where the mining direction of the planned mining face in the coal mine preparation area is down-dip, at this time, the coal mining method is downhill coal mining method. When the dip angle of Coal Seam 2 > 15°, the implementation steps are as follows

[0032] First step, to facilitate the drainage and gas production of the branch section 7 and provide a gas production channel for the horizontal main branch section 6, the well type is a multi-branch horizontal well with a single horizontal main branch section of L-shaped coal seam roof ( Figure 2 ); adopt a large-diameter three-opening wellbore structure

[0033] Step 2, Well Location: The wellhead is deployed on the coal pillar outside the stop line of the planned coal mining face in the coal mine preparation area. According to the collected lithological and mechanical parameters of the coal seam roof 1 and the coal seam floor 3, the distance between the horizontal main branch section 6 and the coal seam 2 is controlled within the range of 1 - 2 m. The actual longitudinal (up and down) trajectory of the horizontal main branch section 6 has an error of no more than 1 m from the design, and the lateral (left and right) error should not exceed 1 m. The trajectory of the horizontal main branch section 6 extends in the direction opposite to the coal mining direction of the coal mining face.

[0034] Step 3, Complete the first-stage drilling 4, the second-stage drilling 5, the third-stage horizontal main branch section 6 and the branch section 7. The aperture of the first-stage drilling 4 is Φ444.5 mm, reaching 60 m below the stable bedrock, and a Φ339.7 mm surface casing is run in and cemented back to the wellhead. The aperture of the second-stage drilling 5 is Φ311.15 mm, reaching the end of the build section, and a Φ244.5 mm technical casing is run in and cemented back to the wellhead. The aperture of the third-stage drilling is Φ215.9 mm to drill to the design target point with open hole completion without cementing. Combining with seismic fine interpretation and near-bit trajectory control, the second-stage drilling 5 drills to the end of the build section located within the coal seam roof 1 and is 1 - 2 m away from the coal seam 2, and the third-stage horizontal main branch section 6 is parallel to the coal seam 2 and drilled within the coal seam roof 1.

[0035] Step 4, Complete the staged mechanical reaming and coal extraction cavity formation 8 for the farthest branch section 7; After the horizontal main branch section 6 is drilled to the target depth, pull out the drill pipe and replace it with a specially designed mechanical reaming tool. The drill string design is: kelly + drill pipe + drill collar + drill tool combination, and run it to the designed position of the farthest branch section 7. For easy drainage, drill the branch section 7 at a well inclination angle of 92° - 95°, starting from the coal seam roof 1 and drilling into the coal seam floor 3 to a depth of 1 - 2 m to form a drilling trajectory penetrating the coal seam 2; Among them, to reduce the disturbance of the surrounding soil or rock layer caused by single reaming and reduce the risk of hole wall collapse, the reaming and coal extraction cavity formation 8 is carried out in a three-stage reaming method, and the reaming rates of each stage are 150%, 200%, and 300% in sequence. The mechanical reaming tool adopts a three-stage reaming tool, and the drill tool design scheme is designed with reference to the reaming tool and drilling method of CN108071344B; the drilling rig provides power for the drill tool. The aperture of the first-stage bit of the drill tool is a 215.9 mm bit. The first and second stages of reaming are completed using a plunger bit, and the third stage of reaming is completed using a blade. The reaming activation adopts a hydraulic control method; Carry out three-stage reciprocating reaming and coal extraction cavity formation 8 within the coal seam 2. The broken coal powder is transported to the ground with the drilling fluid and flows into the sedimentation tank for separation of water and coal powder. Among them, the drilling fluid for the branch section 7 is clear water drilling fluid; the aperture after the reaming and coal extraction cavity formation 8 is 3 times the original aperture of the branch section 7.

[0036] Step 5, Wash the well of the farthest branch section 7 with clear water.

[0037] Step 6: After completing the hole enlargement and coal extraction for the outermost branch section 7 to create the cavity 8, lower a drillable or dissolvable open-hole packer 9 into the horizontal main branch section 6 to seal the hole enlargement and coal extraction cavity 8 in the outermost branch section 7. The position of the packer 9 should be close to the window opening of the next branch section 7. The purpose is to minimize the contamination of the completed hole enlargement and coal extraction cavity 8 and the drilling passage when drilling the next branch section 7.

[0038] Step 7: Drill the next branch section 7 in a reverse manner to complete the hole enlargement and coal extraction for all branch sections 7 in sequence. The distance between adjacent branch sections 7 is designed to be 8 - 10 m.

[0039] Step 8: Drill and mill / dissolve all the drillable or dissolvable open-hole packers 9 that have been lowered.

[0040] Step 9: Install the production and drainage equipment. Compared with the conventional fracturing for production increase, after completing the cavity creation and well completion for the hole enlargement and coal extraction, the pressure can be quickly released and then the production and drainage can be put into operation. For conventional fractured wells, to avoid the closure of the fracture, the pressure needs to be slowly released before putting into operation. This technology can effectively shorten the pressure release time and achieve rapid gas breakthrough and gas production peak. Install a jet pump at the wellhead of the surface horizontal well for drainage and install a liquid ring vacuum pump in the tubing-casing annulus for negative pressure gas extraction. In the initial stage of production and drainage, there is a large amount of well flushing fluid in the wellbore, which is drained by the jet pump. Subsequently, drainage and negative pressure gas extraction are carried out simultaneously to achieve rapid gas breakthrough and continuous high production of the coal seam 2 gas well.

[0041] To increase the coal extraction volume and gas production, the horizontal well with an L-shaped single horizontal main branch section and multiple branch sections in the coal seam roof ( Figure 2 ) is optimized to be a horizontal well with an L-shaped double horizontal main branch section and multiple branch sections in the coal seam roof (see Figure 3 ).

[0042] Specific experimental case 2 In the case where the mining direction of the planned mining face in the coal mine preparation area is downward, the coal mining method is downhill coal mining. When the dip angle of the coal seam 2 < 15°, the implementation steps are as follows.

[0043] Step 1: To facilitate drainage and gas production in the branch section 7 and provide a gas production channel in the horizontal main branch section 6, use a horizontal well with an L-shaped single horizontal main branch section and multiple branch sections in the coal seam floor (see Figure 4 ).

[0044] Step 2 is the same as in Specific experimental case 1.

[0045] Step 3: Complete the first-stage drilling 4, second-stage drilling 5, third-stage horizontal main branch section 6 and branch section 7. The aperture of the first-stage drilling 4 is Φ444.5 mm, reaching 60 m below the stable bedrock, and casing pipe with an outer diameter of Φ339.7 mm is run in and cemented in place with the cement returning to the wellhead. The aperture of the second-stage drilling 5 is Φ311.15 mm, reaching the end of the build section, and casing pipe with an outer diameter of Φ244.5 mm is run in and cemented in place with the cement returning to the wellhead. The aperture of the third-stage drilling is Φ215.9 mm and the well is completed with open hole drilling to the designed target point without cementing. Combining with the fine seismic interpretation and near-bit trajectory control, the second-stage drilling 5 reaches the end of the build section and is located within the coal seam floor 3 and is 1 - 2 m away from the coal seam 2. The main branch section of the third-stage horizontal section is located within the coal seam floor 3 and is drilled parallel to the coal seam 2.

[0046] Step 4: Complete the staged mechanical reaming and coal cavity formation 8 for the outermost branch section 7; for drainage convenience, the well deviation angle of the branch section 7 is 92° - 95° and slopes upward, starting from the coal seam floor 3 and drilling into the coal seam roof 1 to a depth of 1 - 2 m, forming a drilling trajectory that penetrates the coal seam 2.

[0047] Steps 5 to 9 are the same as in the specific experimental case 1.

[0048] To increase the coal extraction volume and gas production volume of the horizontal well, the L-shaped single horizontal main branch multi-branch horizontal well in the coal seam floor ( Figure 4 ) is optimized to be the L-shaped double horizontal main branch multi-branch horizontal well in the coal seam floor ( Figure 5 ).

[0049] Specific experimental case 3 In the case where the mining direction of the planned coal mining face in the coal mine preparation area is upward, the coal mining method is the uphill coal mining method at this time, as shown in Figure 6 ; the implementation steps are as follows.

[0050] Step 1: Determine the well type as the U-shaped single horizontal main branch multi-branch horizontal well in the coal seam floor and adopt a large-caliber three-stage wellbore structure.

[0051] Step 2 is the same as in the specific experimental case 2.

[0052] Step 3: Complete the drilling of the drainage vertical well 10, adopting a three-stage wellbore structure. The aperture of the first-stage drilling 4 is Φ444.5 mm, reaching 60 m below the stable bedrock, and casing pipe with an outer diameter of Φ339.7 mm is run in and cemented in place with the cement returning to the wellhead. The aperture of the second-stage drilling 5 is Φ311.15 mm to the coal seam floor 3, and casing pipe with an outer diameter of Φ244.5 mm is run in and cemented in place with the cement returning to the wellhead. The aperture of the third stage is Φ215.9 mm and the well is completed with open hole drilling to 40 m below the coal seam floor 3. A cavity is formed at the docking position about 1 - 2 m below the coal seam floor 3 to facilitate the docking with the horizontal main branch section 6.

[0053] Step 4: Complete the first-stage drilling of the horizontal well 4 and the second-stage drilling of the horizontal well 5, and connect the horizontal main branch section 6 with the cavity-making section; the aperture of the first-stage drilling 4 is Φ444.5 mm, reaching 60 m below the stable bedrock, and casing pipe with an outer diameter of Φ339.7 mm is run in, and the cement for well cementing returns to the wellhead; the aperture of the second-stage drilling 5 is Φ311.15 mm, reaching the end of the build section, and casing pipe with an outer diameter of Φ244.5 mm is run in, and the cement for well cementing returns to the wellhead; the aperture of the third-stage drilling is Φ215.9 mm, and the horizontal main branch section 6 is connected with the cavity-making section of the drainage vertical well 10 without well cementing. Combining fine seismic interpretation and near-bit trajectory control, the second-stage drilling 5 reaches the end of the build section and is located within the coal seam floor 3 and is 1 - 2 m away from the coal seam 2, and the main branch section of the third-stage horizontal section is drilled parallel to the coal seam 2 within the coal seam floor 3.

[0054] Step 5: First, use the open-hole packer 9 to plug the cavity-making section of the drainage vertical well 10, and complete the staged mechanical reaming and coal-removing cavity-making 8 of the outermost branch section 7; for the convenience of drainage, the well deviation angle of the branch section 7 is 92° - 95° and slopes upward. Drill into the coal seam roof 1 from the coal seam floor 3 to a depth of 1 - 2 m to form a drilling trajectory that penetrates the coal seam 2.

[0055] Steps 6 to 9 are the same as Steps 5 to 8 of the specific experimental case 1.

[0056] Step 10: Install a progressing cavity pump in the drainage vertical well 10 for drainage, and install a negative pressure extraction device at the wellhead of the surface horizontal well for gas production to achieve rapid gas appearance and continuous high production of the gas well in the coal seam 2.

[0057] To increase the coal-removing amount and gas production volume, the U-shaped single horizontal main branch multi-branch horizontal well in the coal seam floor ( Figure 6 ) is optimized to be the U-shaped double horizontal main branch multi-branch horizontal well in the coal seam floor ( Figure 7 ). The multiple horizontal main branch sections 6 are correspondingly connected with multiple drainage vertical wells 10. After the reaming and coal-removing cavity-making 8 of all the branch sections 7 of the first horizontal main branch section 6 are completed, they are plugged. The packer 9 is preferably a dissolvable open-hole packer 9. Then, the second horizontal main branch section 6 is connected with the second drainage vertical well 10, and then the reaming and coal-removing cavity-making 8 of the branch section 7 of the second horizontal main branch section 6 are carried out.

Claims

1. A rapid pressure relief and extraction system for surface coal mining of multi-branch horizontal wells in coalbed methane, characterized in that, In the coal mine preparation area, a surface horizontal well is arranged along the strike of the coal mining face; among them, the surface horizontal well includes a first-opening drill well (4), a second-opening drill well (5), a third-opening horizontal main branch section (6) and a branch section (7); the horizontal main branch section (6) is located at the coal seam floor (3) / coal seam roof (1), and drills into the opposite coal seam roof (1) / coal seam floor (3) through the branch section (7). The branch section (7) is provided with an enlarged-diameter coal-cutting cavity (8), and the maximum cavity diameter of the enlarged-diameter coal-cutting cavity (8) is 3 times the diameter of the original branch section (7).

2. The surface coal mining and rapid pressure relief drainage system for multi-branch horizontal wells in coalbed methane according to claim 1, wherein The coal mining direction of the planned coal mining face in the coal mine preparation area is downward dip. When the dip angle of the coal seam (2) > 15°, the well type is a horizontal well with a multi-branch section of an L-shaped horizontal main branch section in the coal seam roof; when the dip angle of the coal seam (2) < 15°, the well type is a horizontal well with a multi-branch section of an L-shaped horizontal main branch section in the coal seam floor.

3. The coalbed methane multi-branch horizontal well surface coal mining rapid pressure relief extraction system according to claim 1, characterized in that, The coal mining direction of the planned coal mining face in the coal mine preparation area is upward dip. It also includes a row of production vertical wells (10). The production vertical wells (10) are communicated with the horizontal main branch section (6) located at the coal seam floor (3), and the well type is a horizontal well with a multi-branch section of a U-shaped horizontal main branch section in the coal seam floor.

4. The coalbed methane multi-branch horizontal well surface coal mining rapid pressure relief drainage system according to claim 2, wherein, When the number of the horizontal main branch sections (6) = 1, the well type is a horizontal well with a multi-branch section of a single L-shaped horizontal main branch section in the coal seam roof or a horizontal well with a multi-branch section of a single L-shaped horizontal main branch section in the coal seam floor; when the number of the horizontal main branch sections (6) > 1, the well type is a horizontal well with a multi-branch section of a multi-L-shaped horizontal main branch section in the coal seam roof or a horizontal well with a multi-branch section of a multi-L-shaped horizontal main branch section in the coal seam floor.

5. The coalbed methane multi-branch horizontal well surface coal mining rapid pressure relief extraction system according to claim 3, characterized in that, When the number of the horizontal main branch sections (6) = 1, the well type is a horizontal well with a multi-branch section of a single U-shaped horizontal main branch section in the coal seam floor, and the number of the production vertical wells (10) = 1; when the number of the horizontal main branch sections (6) > 1, the well type is a horizontal well with a multi-branch section of a multi-U-shaped horizontal main branch section in the coal seam floor, and the number of the production vertical wells (10) is equal to the number of the horizontal main branch sections (6), and the production vertical wells (10) and the horizontal main branch sections (6) are in one-to-one correspondence.

6. The coalbed methane multi-branch horizontal well surface coal mining rapid pressure relief extraction system according to claim 1, wherein The diameter expansion rates of the enlarged-diameter coal-cutting cavity (8) are 150%, 200% and 300% in sequence.

7. The coalbed methane multi-branch horizontal well surface coal mining rapid pressure relief extraction system according to claim 1, characterized in that, The distance between the horizontal main branch section (6) and the coal seam (2) is 1 - 2 m, and the end of the branch section (7) penetrates into the coal seam roof (1) / coal seam floor (3) and the distance from the coal seam (2) is 1 - 2 m.

8. The coalbed methane multi-branch horizontal well surface coal mining rapid pressure relief extraction system according to claim 1, wherein The number of the branch sections (7) provided on the horizontal main branch section (6) > 1. The distance between two adjacent branch sections (7) is 8 - 10 m, and the branch section (7) is inclined upward, and the well deviation angle is 92° - 95°; the drilling of the branch section (7) starts from the distal end away from the first-opening drill well (4) and retreats to drill to the first-opening drill well (4); after the drilling of each branch section (7) is completed, a packer is provided in the horizontal main branch section (6), and the distance between the packer and the next branch section (7) < the distance from the current branch section (7).

9. A method for rapid pressure relief extraction of coalbed methane from multi-branch horizontal wells by surface coal extraction, characterized in that, When using the surface coal-cutting rapid pressure relief and extraction system for multi-branch horizontal wells of coalbed methane as described in claim 1 above, the method is as follows: Set up a surface horizontal well: In the first drilling stage (4), drill to 60 m below the stable bedrock. In the second drilling stage (5), drill to a range of 1 - 2 m from the coal seam roof (1) / coal seam floor (3) at the end of the build section. The horizontal main branch section (6) drills parallel to the coal seam (2). Starting from the distal horizontal main branch section (6) away from the first drilling well (4), perform the window - opening drilling of each branch section (7) in a backward manner. The branch section (7) is provided with an enlarged - diameter coal - mining cavity (8) to form a drilling trajectory that penetrates the coal seam (2) between the coal seam roof (1) and the coal seam floor (3). Install drainage and extraction equipment for negative - pressure extraction to achieve rapid gas appearance and continuous high - yield production of the coal seam (2) gas well.

10. A method for rapid pressure relief and extraction of coalbed methane from multi-branch horizontal wells by surface coal mining according to claim 9, characterized in that, When arranging the surface horizontal well, If the mining direction of the planned mining face in the coal mine preparation area is downward dip and the dip angle of the coal seam (2) > 15°, the well type is a horizontal well with an L - shaped horizontal main branch section and multiple branch sections on the coal seam roof. The horizontal main branch section (6) is located on the coal seam roof (1). When the dip angle of the coal seam (2) < 15°, the well type is a horizontal well with an L - shaped horizontal main branch section and multiple branch sections on the coal seam floor. The horizontal main branch section (6) is located on the coal seam floor (3). Install a jet pump at the wellhead of the surface horizontal well for drainage and install a liquid - ring vacuum pump in the oil - casing annulus for negative - pressure gas extraction. In the initial stage of drainage and extraction, drain water through the jet pump. Subsequently, drain water and perform negative - pressure gas extraction simultaneously. If the mining direction of the planned mining face in the coal mine preparation area is upward dip, it also includes a drainage vertical well (10). The drainage vertical well (10) is connected to the horizontal main branch section (6) located on the coal seam floor (3). The well type is a horizontal well with a U - shaped horizontal main branch section and multiple branch sections on the coal seam floor. The horizontal main branch section (6) is located on the coal seam floor (3). Install a screw pump in the drainage vertical well (10) for drainage and install negative - pressure extraction equipment at the wellhead of the surface horizontal well for gas production to achieve rapid gas appearance and continuous high - yield production of the coal seam (2) gas well.

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