Method for pre-pumping coal face gas of directional inclined shaft staged fracturing coal seam

Abstract

The invention discloses a method for pre-pumping coal face gas of a directional inclined shaft staged fracturing coal seam, and belongs to the technical field of ground pre-pumping outburst elimination of gas in a coal mine area. A plurality of directional inclined shafts with second-opening well body structures are constructed on the ground with the plane distance of a mining coal seam drilling section being 60-80 m, second-opening well sections enter the coal seam from the position of a coal face air return roadway, and drilling is completed after the second-opening well sections are drilled to the position of a transportation roadway along the coal seam; a pre-punching wall protection casing pipe is tripped in, an annular channel between the casing pipe and the well wall is sealed through a plurality of external pipe packers, and a plurality of separated spaces are formed in the mining coal seam drilling section; small fracturing equipment is utilized on the ground to carry out coal seam mining small-scale segmented hydraulic fracturing in an oil pipe single-seal injection mode, drainage and mining equipment such as an electric submersible pump is put into a casing pipe after a well is dredged and washed, and drainage gas recovery and negative pressure extraction operation are carried out. According to the method, low-cost and rapid pre-drainage of coal seam gas of the coal face can be achieved, the gas outburst risk in the recovery process of the coal face is remarkably reduced, and the method has both environmental and social benefits.

Description

Technical Field

[0001] The present invention relates to a method for pre-extraction of gas from a coal mining face, in particular to a method for utilizing a gas pre-extraction directional inclined shaft to carry out segmented small-scale hydraulic fracturing transformation of coal seams, drainage gas extraction and negative pressure gas extraction, thereby realizing coal seam gas pre-extraction and outburst elimination and coal seam gas resource development and utilization in coal mining faces in coal mining areas, and belongs to the technical field of ground pre-extraction and outburst elimination of gas in coal mining areas. Background Art

[0002] In the process of coal mining, the extraction and management of coal seam gas is a crucial link, which is directly related to the safe production and mining efficiency of coal mines. With the continuous advancement of coal mining technology and the increasing requirements for safe production, how to pre-extract coal seam gas efficiently, safely and at low cost has become a key technical problem that needs to be solved in the underground mining of coal resources.

[0003] Traditional coal seam gas extraction methods, such as drilling extraction and vertical well surface extraction, can achieve gas extraction to a certain extent, but they often have disadvantages such as low efficiency, high cost, and reduced excavation and coal mining efficiency. As a result, traditional gas control methods are not only difficult to meet the needs of efficient and safe production in modern coal mines, but may also have an adverse impact on the sustainable mining of coal mines. Therefore, it is particularly important to explore a new, efficient, and low-cost coal seam gas extraction method.

[0004] In order to comprehensively promote coal seam gas control work within the scope of my country's coal mining areas, it is necessary to solve the problems of high cost, low efficiency and impact on coal mine production of traditional gas control methods, and actively explore new methods and technologies for ground pre-extraction of coal seam gas. New coal seam gas extraction methods need to be able to significantly improve gas extraction efficiency and significantly reduce gas extraction costs while ensuring safe production in coal mines. At the same time, the impact on the integrity of the coal seam roof and floor plates should be as small as possible, and avoid large-scale fracturing that causes adverse engineering geological problems in coal mines. This will not only improve the effect of coal mine gas control and the overall production efficiency of coal mines, but also maximize the exploitation and utilization of coal seam gas resources, generating comprehensive benefits in many aspects such as economy, environment and safety. Summary of the invention

[0005] Technical problem: The purpose of the present invention is to overcome the problems of large amount of drilling work for pre-extraction of coal seam gas in traditional underground along-layer and through-layer drilling in coal mining working face, low gas extraction efficiency, long extraction cycle and slow elimination of gas outburst effects of ground vertical wells, and provide a method of using gas pre-extraction directional inclined wells to carry out small-scale hydraulic fracturing transformation, drainage gas extraction and negative pressure gas extraction of coal seam segmentation, so as to realize pre-extraction and elimination of gas outbursts in coal seam mining working faces and development and utilization of coalbed methane resources, and improve the pre-extraction effect of coal seam gas.

[0006] Technical solution: In order to achieve the above-mentioned purpose, a method for pre-extracting gas from a coal mining face by staged fracturing of a coal seam by a directional inclined well of the present invention comprises the following steps:

[0007] (a) According to the planned mining area and the design drawings of the coal mining face, holes are opened at intervals on the ground outside the coal mining face near the return air tunnel, and multiple directional inclined wells for pre-extraction of gas from the coal mining face to the direction of the coal seam are deployed in a "straight" well arrangement mode. When the ground does not have the conditions for opening directional inclined wells, a "zigzag" well arrangement mode is adopted to deploy multiple directional inclined wells for pre-extraction of gas from the coal mining face to the direction of the coal seam, and the drilling trajectory is adjusted before the landing point so as not to change the drilling trajectory of the coal seam drilling section under the original "straight" well arrangement mode;

[0008] (b) The surface gas pre-extraction directional inclined well has a "two-opening" wellbore structure. On the ground, a vehicle-mounted drilling rig is used to perform "one-opening" drilling in an oblique hole-opening manner. The surface casing is put into the first-opening wellbore for cementing, forming a first-opening cement ring to provide wellbore conditions for the subsequent second-opening drilling. The "second-opening" directional drilling lands at the return air lane position of the coal mining working face and enters the mined coal seam. The drilling continues along the middle of the mined coal seam to the position of the transport lane of the coal mining working face and then is completed to form a second-opening wellbore;

[0009] (c) A production casing is run into the formed secondary wellbore, and a plurality of packers on the outside of the production casing are used to isolate the annular channel between the secondary wellbore and the production casing. The multiple packers on the outside of the production casing are used to realize ground pressurized sealing, and a plurality of separation spaces are formed in the drilling section of the mining coal seam. A pre-perforated screen pipe with a length of 2 to 3 m is provided on the production casing of each separation space, which serves as an annular channel between the secondary wellbore and the production casing and a channel for mining the coal seam during subsequent small-scale hydraulic fracturing for fracturing fluid and proppant to enter.

[0010] (d) connecting a small-scale hydraulic fracturing equipment on the ground, and using a fracturing injection pump to inject fracturing fluid and proppant in stages into the farthest end of the production casing, the farthest end of the annular channel between the second wellbore and the production casing, and the farthest end of the mined coal seam in a single oil pipe external packer upper sealing injection manner of the fracturing oil pipe, so that the proppant is accumulated and filled at the farthest end of the production casing and the farthest end of the annular channel between the second wellbore and the production casing, and vertically or horizontally extending fracturing cracks are generated in the mined coal seam, thereby improving the permeability of the mined coal seam around the wellbore of the ground gas pre-extraction directional inclined well;

[0011] (e) After the outer packer of the tubing is unsealed, the fracturing tubing is dragged upward to the position of the last fracturing section, and the fracturing fluid and proppant are injected into the production casing, the annular separation space between the last secondary wellbore and the production casing, and the mined coal seam in stages by using a fracturing injection pump in a manner of sealing and injecting a single outer packer of the fracturing tubing, so that the proppant is accumulated and filled in the annular channel between the production casing, the last secondary wellbore and the production casing, and vertically or horizontally extending fracturing cracks are generated in the mined coal seam, thereby improving the permeability of the mined coal seam around the wellbore of the surface gas pre-extraction directional inclined well;

[0012] (f) Repeat step (e) to unseal the outer packer of the tubing, drag the fracturing tubing upward to the position of the last fracturing section, and then seal and perform small-scale hydraulic fracturing operations until all the annular channels between the secondary wellbore and the production casing of the surface gas pre-extraction directional inclined well are filled with proppant accumulation, thereby realizing small-scale hydraulic sand fracturing transformation of the entire well section of the coal seam drilling section;

[0013] (g) After the completion of the segmented fracturing construction of the coal seam drilling section in the directional inclined well for gas pre-extraction from the ground, the overflow and well washing operations are carried out, and the drainage equipment in the well is placed into the production casing so that the bottom end of the wire-wound screen at the suction port of the submersible pump is located 2 to 3 meters above the top of the lowest pre-drilled screen. The ground is carried out with controlled drainage and gas production operations to gradually reduce the gas content and gas pressure of the mined coal seam. When the wellbore liquid level drops to the position of the wire-wound screen at the suction port of the submersible pump, the gas production pipeline is connected to the water ring vacuum pump and the motor to carry out negative pressure extraction of coal seam gas to further reduce the gas content and gas pressure of the mined coal seam.

[0014] In step (a), under the "straight line" or "broken line" well layout mode, the plane distance between the drilling sections of any two adjacent ground gas pre-extraction directional inclined wells for mining coal seams is controlled at 60 to 80 meters. When the second drilling reaches the landing point, the mining coal seam is entered, and then the mining coal seam is drilled steadily along the mining coal seam. The drilling rate of the mining coal seam is increased by logging while drilling and measurement while drilling, thereby improving the fracturing transformation and gas pre-extraction effect of the mining coal seam.

[0015] In step (b), the first wellbore is drilled obliquely to 10 to 15 meters below the bedrock, and after J55 steel grade surface casing is installed, G grade oil well cement is injected to form a first cementing cement ring; in the initial stage of drilling the second wellbore, the well inclination is slowly adjusted so that the second wellbore drill bit enters the mined coal seam tangentially at the return air lane position of the coal mining working face, and then steadily drills along the mined coal seam to the transportation lane position of the coal mining working face.

[0016] In step (c), the production casing is of N80 steel grade, with an inner diameter of ≥139 mm and a wall thickness of ≥9.7 mm; the multiple packers outside the production casing achieve a pressure resistance of ≥30 MPa after ground pressurization and sealing; the pre-perforated screen has a perforation density of 12 to 16 holes / m, a perforation diameter of 12 mm or more, a hole layout phase of 60°, and a positive spiral layout; small-scale hydraulic fracturing is a hydraulic fracturing transformation of coal reservoirs with low liquid volume, low displacement, and medium construction pressure. The fracturing fluid is an active water fracturing fluid, which is fully mixed with 98.8% clean water, 1.0% anti-swelling agent, 0.1% bactericide, and 0.1% drainage agent; the proppant is a mixture of 70-100 mesh fine-grained ceramic powder and 100-140 mesh ultra-fine-grained ceramic powder in a volume ratio of 1:1; when the mined coal seam (11) is shallowly buried and has a high original permeability, the proppant is a mixture of 70-100 mesh fine-grained quartz sand and 100-140 mesh ultra-fine-grained quartz sand in a volume ratio of 1:1.

[0017] In step (d), the fracturing oil pipe has an inner diameter of ≥50 mm, a wall thickness of ≥9 mm, and an ultimate pressure resistance of ≥50 MPa, and adopts ordinary oil pipes connected by threads, or adopts continuous oil pipes.

[0018] In step (e), the tubing external packer is a packer that can be used repeatedly or a packer that can be used once; the length of each fracturing section should be 60 to 70 m, and the fracturing tubing seat seal position is 2 to 4 m above the pre-perforated screen of the corresponding fracturing section, at the top plate position of hard siltstone and fine sandstone.

[0019] The staged injection refers to the injection in the early stage of fracturing in the manner of "low pressure, low speed, and low sand ratio", that is, at a wellhead injection pressure of 1-2 MPa, 0.3-0.5 m 3 / min injection displacement, 20% to 30% sand ratio, the mixture of fracturing fluid and proppant is injected into the annular channel and the toe of the production casing at a low speed and low pressure, or the mixture of fracturing fluid and proppant is injected into the separation space between the previous annular channel and the production casing at a low speed and low pressure, to achieve proppant accumulation and filling of the annular channel and the toe of the production casing, and then high-pressure, high-speed, low-sand ratio injection mining coal seam seam creation and fracturing fracture support work is carried out, with a wellhead injection pressure of 10 to 20 MPa and a 1 to 2m 3 / min injection displacement, 10% to 15% sand ratio, the mixture of fracturing fluid and proppant is injected into the annular channel and the toe of the production casing at high speed, so as to realize the proppant accumulation filling of the annular channel below the injection point and the toe of the production casing, or at a wellhead injection pressure of 10 to 20 MPa, 2 to 3 m 3With an injection displacement of 10% to 15% sand ratio, the mixture of fracturing fluid and proppant is injected into the annular channel and the lower part of the production casing at a high speed, so as to realize the proppant accumulation and filling of the annular channel and the production casing below the injection point, so as to achieve the purpose of reducing filtration loss and reducing the difficulty of coal seam gas extraction.

[0020] In step (f), the number of repeated small-scale hydraulic fracturing is determined by the inclination width of the working face, that is, the length of the drilling section for coal seam mining. Usually, the cumulative number of segmented fracturing sections is 6 to 7 sections / well. When the permeability of the coal reservoir is low or ≤-0.1mD, the number of fracturing sections is appropriately increased to ensure the effect of segmented fracturing transformation and gas pre-extraction in the directional inclined well mining coal seam drilling section for ground gas pre-extraction.

[0021] In step (g), the overflow release time is controlled at 3 to 5 days, and the well is washed by using a downhole gauge and a forward injection of formation water into the oil pipe to flush the sand to the bottom of the ground gas pre-extraction directional inclined well at one time; the well drainage equipment includes, in order from top to bottom, a drainage pipe, an electric submersible pump, a wire-wound screen pipe, a sand settling pipe, a downhole direct-reading thermometer and a downhole direct-reading pressure gauge.

[0022] In step (g), the drainage and gas production operation includes five stages: initial drainage, casing pressure control and gas production, flow pressure reduction and production increase, flow pressure control and stable production, and gas production attenuation. The duration of each stage is determined according to the stability of the liquid level drop, casing pressure, daily gas production, and daily water production during the drainage and pressure reduction process. The total duration of the drainage and gas production process is 120 to 280 days. When the drainage and gas production is completed, the wellbore liquid level gradually drops to below the position of the wire screen at the suction port of the electric submersible pump. When the water ring vacuum pump is used to extract gas under negative pressure, the negative pressure formed by the wellhead annulus of the surface gas pre-extraction directional inclined well is maintained at -0.06 to -0.04 MPa. When the pure amount of coal seam gas extracted under negative pressure is ≤60m 3 / d, or when the extracted gas concentration is lower than 8%, the pre-extraction work of coal seam gas is completed.

[0023] Beneficial effects: Due to the adoption of the above technical solution, the present invention overcomes the problems of large drilling workload and low gas extraction efficiency of traditional underground gas pre-extraction drilling in the layer and through the layer, long extraction cycle and slow elimination of gas outbursts in the ground vertical well gas extraction well, etc. Gas pre-extraction directional inclined wells are used to carry out small-scale hydraulic fracturing transformation, drainage gas extraction and negative pressure gas extraction in the mining coal seam, so as to realize the method of coal mining face mining coal seam gas pre-extraction elimination and coal seam gas resource development and utilization. First, multiple directional inclined wells for pre-extraction of coal seam gas are deployed in a "straight" or "zigzag" well arrangement mode. The second directional drilling is carried out to the return air tunnel of the coal mining face and lands in the mining coal seam. The drilling is continued along the middle of the mining coal seam to the transportation tunnel of the coal mining face and then completed. Secondly, multiple packers on the outside of the production casing are used to seal the annular channel between the second wellbore and the production casing, forming multiple separation spaces in the drilling section of the mining coal seam. The fracturing fluid and proppant are injected in stages by sealing the single outer packer of the fracturing tubing to improve the permeability of the mining coal seam around the wellbore. Thirdly, drainage and gas extraction and negative pressure gas extraction operations are carried out successively to continuously reduce the gas content and gas pressure of the mining coal seam and improve the pre-extraction and sudden elimination effect of the mining coal seam gas. The main advantages compared with the existing technology are:

[0024] 1) Adopting the "straight" or "broken line" directional inclined well layout method, the single well control area is reduced by increasing the number of gas pre-extraction wells, and the permeability transformation effect of the mining coal seam is significantly improved by carrying out staged fracturing in the drilling section of the mining coal seam, which greatly shortens the gas pre-extraction time of the coal mining working face;

[0025] 2) The drilling section of the coal seam is isolated by multiple production casing external packers, which realizes the segmented fracturing of the coal seam without cementing, which not only improves the safety and reliability of hydraulic fracturing construction, but also avoids the adverse effects of production casing cementing on the permeability of the coal seam, and does not affect the recovery work of the coal mining face;

[0026] 3) During the fracturing construction process, by controlling the fracturing scale and construction pressure, the risk of damage to the integrity of the coal seam roof is reduced, and the difficulty of roof management during the working face mining process is reduced;

[0027] 4) During the fracturing construction process, by controlling the amount of proppant added, the coal quality of the coal mined from the working face is prevented from being significantly reduced due to the mixing of proppant, thereby improving the economic benefits of coal mining enterprises. ⑤ The technical process is simple, the project implementation cost is low, and the economic, environmental and social benefits are good. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The present invention provides a plan projection diagram of the ground gas pre-extraction well layout and drilling trajectory of the method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam using a directional inclined well.

[0029] Figure 2 This is a cross-sectional diagram of the drilling trajectory and wellbore structure of a ground gas pre-extraction well for the method of pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by a directional inclined well according to the present invention.

[0030] Figure 3 This is a diagram of the first fracturing stage of the directional inclined well staged fracturing of a coal seam according to the present invention.

[0031] Figure 4 This is a diagram of the second fracturing stage fracturing method of the directional inclined well staged fracturing of the coal seam of the present invention.

[0032] Figure 5 This is a diagram of the method for pre-extracting gas after staged fracturing of a coal seam in a directional inclined well according to the present invention.

[0033] In the figure: 1-coal mining face; 2-return air lane; 3-transport lane; 4-cutting hole; 5-stop mining line; 6-airflow direction; 7-first opening hole; 8-second opening hole; 9-ground hole position; 10-ground; 11-mined coal seam; 12-landing point; 13-mined coal seam drilling section; 14-drilling trajectory; 15-surface casing; 16-first opening cement ring; 17-production casing; 18-1- The first casing external packer; 18-2-the second casing external packer; 18-3-the third casing external packer; 18-4-the fourth casing external packer; 18-5-the fifth casing external packer; 19-pre-perforated screen; 20-annular steel plate; 21-fracturing tubing; 22-tubing external packer; 23-proppant; 24-fracturing crack; 25-drainage pipe; 26-electric submersible pump; 27-wire screen. DETAILED DESCRIPTION

[0034] The present invention will be further described below with reference to the embodiments in the accompanying drawings:

[0035] like Figure 1 As shown, the method for pre-extracting gas from a coal mining face by staged fracturing of a coal seam by directional inclined well of the present invention comprises the following specific steps:

[0036] (a) According to the coal mine planning mining area and the design drawings of the coal mining face 1, a hole 9 is opened on the ground outside the coal mining face 1 near the return air tunnel 2, and multiple directional inclined shafts for pre-extraction of coal seam gas 11 are deployed in a "straight line" well arrangement mode. When the ground 10 does not have the conditions for opening directional inclined shafts, multiple directional inclined shafts for pre-extraction of coal seam gas 11 are deployed in a "broken line" well arrangement mode. By adjusting the drilling trajectory before the landing point 12, the original "straight line" well arrangement mode is not changed. The drilling trajectory 14 of the mining coal seam drilling section 13 under the "linear" or "broken line" well layout mode is as follows: under the "linear" or "broken line" well layout mode, the plane distance between the mining coal seam drilling section 13 of any two adjacent surface gas pre-extraction directional inclined wells is controlled at 60-80m, and the mining coal seam 11 is entered when the second drilling reaches the landing point 12, and then the mining coal seam 11 is drilled steadily along the mining coal seam 11, and the drilling rate of the mining coal seam 11 is improved by logging while drilling and measurement while drilling, thereby improving the fracturing transformation and gas pre-extraction effect of the mining coal seam 11.

[0037] (b) The surface gas pre-extraction directional inclined well is a "two-opening" wellbore structure. On the ground 10, a vehicle-mounted drilling rig is used to perform "one-opening" drilling in an oblique hole-opening manner. A surface casing 15 is lowered into the first-opening wellbore 7 for cementing, forming a first-opening cement ring 16 to provide wellbore conditions for subsequent second-opening drilling. The "two-opening" directional drilling reaches the return air lane 2 position of the coal mining working face 1, lands 12, enters the mining coal seam 11, and continues to drill along the middle of the mining coal seam 11 to the coal mining working face 1 for transportation. After the position of the lane 3 is reached, the drilling is completed to form a second wellbore 8; the first wellbore 7 is obliquely drilled to 10 to 15 meters below the bedrock and completed, and a J55 steel grade surface casing 15 is lowered and then G grade oil well cement is injected to form a first cementing cement ring 16; at the initial stage of drilling the second wellbore 5, the well inclination is slowly adjusted so that the second drill bit enters the mined coal seam 11 tangentially at the position of the return air lane 2 of the coal mining working face 1, and then the second wellbore 11 is steadily drilled along the mined coal seam 11 to the position of the transport lane 3 of the coal mining working face 1.

[0038] (c) A production casing 17 is lowered into the secondary wellbore 8, and a plurality of packers 18 outside the production casing 17 are used to isolate the annular channel between the secondary wellbore 8 and the production casing 17, so as to form a plurality of separation spaces in the coal seam drilling section 13. A pre-punched screen 19 with a length of 2 to 3 m is provided on the production casing 17 of each separation space, which serves as a channel for the fracturing fluid and the proppant 23 to enter the annular channel between the secondary wellbore 8 and the production casing 17 and the coal seam 11 during subsequent small-scale hydraulic fracturing. The production casing 17 is of N80 steel grade, with an inner diameter of ≥139 mm and a wall thickness of ≥9.7 mm. The plurality of packers 18 outside the production casing 17 are used to realize ground pressurized sealing, and the pressure resistance after sealing is ≥30 MPa. The pre-punched screen is punched. The hole density is 12-16 holes / m, the hole diameter is greater than or equal to 12 mm, the hole distribution phase is 60°, and the hole distribution method is a positive spiral type; small-scale fracturing is a hydraulic fracturing transformation of coal reservoirs with low liquid volume, low displacement, and medium construction pressure. The fracturing fluid used is an active water fracturing fluid, which is fully mixed with 98.8% clean water, 1.0% anti-swelling agent, 0.1% bactericide, and 0.1% drainage agent. The proppant 23 is a mixture of 70-100 mesh fine-grained ceramic powder and 100-140 mesh ultra-fine-grained ceramic powder in a volume ratio of 1:1; when the mined coal seam 11 is shallowly buried and has a high original permeability, a mixture of 70-100 mesh fine-grained quartz sand and 100-140 mesh ultra-fine quartz sand in a volume ratio of 1:1 can also be used as a proppant.

[0039] (d) connecting a small-scale hydraulic fracturing equipment to the surface, using a fracturing injection pump to inject fracturing fluid and proppant 23 in stages into the farthest end of the production casing 17, the farthest end of the annular channel between the second wellbore 8 and the production casing 17, and the farthest end of the mining coal seam 11 in a manner of sealing injection on a single oil pipe external packer 22 of the fracturing oil pipe 21, so that the proppant 23 is accumulated and filled at the farthest end of the production casing 17 and the farthest end of the annular channel between the second wellbore 8 and the production casing 17, and at the farthest end of the mining coal seam 11 to generate vertical or horizontal fracturing cracks 24, improve the permeability of the coal seam 11 mined around the surface gas pre-extraction directional inclined wellbore; the fracturing oil pipe 21 has an inner diameter of ≥50mm, a wall thickness of ≥9mm, and an ultimate pressure resistance of ≥50MPa, and can use ordinary oil pipes with threaded connections or continuous oil pipes; staged injection refers to the injection in the early stage of fracturing in a "low pressure, low speed, and low sand ratio" mode, that is, at a wellhead injection pressure of 1-2MPa, 0.3-0.5m 3 / min injection displacement, 20% to 30% sand ratio, the mixture of fracturing fluid and proppant 23 is injected into the annular channel and the toe end of the production casing at a low speed and low pressure to achieve the proppant 23 accumulation filling of the annular channel and the toe end of the production casing, and then the high-pressure, high-speed, low-sand ratio injection mining coal seam 11 seam creation and fracturing crack 24 support work is carried out, with a wellhead injection pressure of 10 to 20 MPa, 1 to 2m 3With an injection displacement of 10% to 15% sand ratio, a mixture of fracturing fluid and proppant 23 is injected into the annular channel and the toe end of the production casing at a high speed, so that the proppant 23 is accumulated and filled in the annular channel below the injection point and the toe end of the production casing, thereby achieving the purpose of reducing filtration loss and reducing the difficulty of gas extraction in coal seam 11.

[0040] (e) After the outer packer 22 of the tubing is unsealed, the fracturing tubing 21 is dragged upward to the position of the last fracturing stage, and the fracturing fluid and the proppant 23 are injected into the production casing 17, the annular separation space between the last second wellbore 8 and the production casing 17, and the mining coal seam 11 in stages by using the fracturing injection pump in a way that the outer packer 22 of the tubing is sealed and injected upward on the fracturing tubing 21, so that the proppant 23 is accumulated and filled in the annular channel between the production casing 17, the last second wellbore 8 and the production casing 17, and a vertically or horizontally extending fracturing fracture is generated in the mining coal seam 11. The seam 24 is used to improve the permeability of the coal seam 11 mined around the wellbore of the surface gas pre-extraction directional inclined well; the outer packer 22 of the tubing can be a packer that can be used repeatedly or a packer that can be used once; the length of each fracturing section should be 60 to 70 m, and the sealing position of the fracturing tubing 21 is 2 to 4 m above the pre-perforated screen pipe 19 of the corresponding fracturing section, and the top plate position of the siltstone and fine sandstone with hard lithology; the staged injection refers to the injection in the early stage of fracturing in the mode of "low pressure, low speed and low sand ratio", that is, at a wellhead injection pressure of 1 to 2 MPa, 0.3 to 0.5 m 3 / min injection displacement, 20% to 30% sand ratio, the mixture of fracturing fluid and proppant 23 is injected into the separation space between the upper annular channel and the production casing at low speed and low pressure to achieve the proppant 23 accumulation filling at the lower part of the annular channel and the production casing 17, and then the high-pressure, high-speed, low-sand ratio injection mining coal seam 11 seam creation and fracturing crack 24 support work is carried out, with a wellhead injection pressure of 10 to 20 MPa, 2 to 3m 3 With an injection displacement of 10% to 15% sand ratio, a mixture of fracturing fluid and proppant 23 is injected into the annular channel and the lower part of the production casing 17 at a high speed, so that the proppant 23 is piled up and filled in the annular channel below the injection point and the production casing 17, thereby achieving the purpose of reducing filtration loss and reducing the difficulty of gas extraction in the coal seam 11.

[0041] (f) Repeat the above steps of unsealing the outer packer 22 of the tubing, dragging the fracturing tubing 21 upward to the position of the last fracturing section, and then sealing and performing small-scale hydraulic fracturing operations until the annular channels between all the secondary wellbores 8 and the production casing 17 of the surface gas pre-extraction directional inclined well are filled with proppant 23, thereby realizing small-scale hydraulic sand fracturing transformation of the entire well section of the coal seam drilling section 13; the number of repeated fracturing is determined by the inclination width of the working face, that is, the length of the coal seam drilling section 13, and the cumulative number of fracturing sections is usually 6 to 7 sections / well. When the permeability of the coal reservoir is low or ≤-0.1mD, the number of fracturing sections can be appropriately increased to ensure the effect of staged fracturing transformation and gas pre-extraction of the surface gas pre-extraction directional inclined well coal seam drilling section (13).

[0042] (g) After the completion of the segmented fracturing construction of the 13th segment of the coal seam drilling section for ground gas pre-extraction directional inclined well mining, the overflow discharge and well washing operations are carried out, the well drainage equipment is lowered into the production casing 17, so that the bottom end of the wire screen 27 at the suction port of the electric submersible pump 26 is located 2 to 3 meters above the top of the lowest pre-perforated screen 19, and the ground is controlled to carry out drainage and gas production operations to gradually reduce the gas content and gas pressure of the pre-mined coal seam 9. When the wellbore liquid level drops to the position of the wire screen 27 at the suction port of the electric submersible pump 26, the gas production pipeline is connected to the water ring vacuum pump and the motor to carry out negative pressure gas extraction of the mined coal seam 11, so as to further reduce the gas content and gas pressure of the mined coal seam 11; the overflow discharge time should be controlled within 3 to 5 days, and the well washing adopts the method of lowering the well gauge and injecting formation water into the well by the oil pipe, and the sand is flushed once to the ground gas pre-extraction directional inclined well. The well bottom; the drainage equipment in the well includes drainage pipe 25, electric submersible pump 26, wire screen 27, sand settling pipe, downhole direct reading thermometer, downhole direct reading pressure gauge from top to bottom; the drainage gas production operation includes five stages: initial drainage, casing pressure control gas production, flow pressure reduction to increase production, flow pressure control to stabilize production, and gas production attenuation. The duration of each stage is determined according to the stability of the liquid level drop, casing pressure, daily gas production, and daily water production changes during the drainage and pressure reduction process. The total duration of the drainage gas production process is 120 to 280 days; at the end of drainage gas production, the wellbore liquid level gradually drops to below the position of the wire screen 27 at the suction port of the electric submersible pump 26; when the water ring vacuum pump is negatively pressurized to extract gas, the negative pressure formed by the wellhead annulus of the surface gas pre-extraction directional inclined well is maintained at -0.06 to -0.04 MPa; when the pure amount of negative pressure extraction of coal seam 11 gas is ≤60m 3 / d, or when the extracted gas concentration is lower than 8%, the pre-extraction work of coal seam 11 is completed.

Claims

1. A method for pre-extracting gas from a coal mining face by staged fracturing of a coal seam by a directional inclined well, characterized in that The following steps are involved: (a) According to the design drawings of the planned mining area and the coal mining face (1), holes (9) are opened at intervals on the ground outside the coal mining face (1) near the return air tunnel (2), and multiple gas pre-extraction directional inclined shafts are deployed in a "straight" well arrangement mode from the coal mining face (1) to the direction of the mined coal seam (11). When the ground (10) does not have the conditions for opening directional inclined shafts, multiple gas pre-extraction directional inclined shafts are deployed in a "zigzag" well arrangement mode from the coal mining face (1) to the direction of the mined coal seam (11), and the drilling trajectory is adjusted before the landing point (12) so that the drilling trajectory (14) of the mining coal seam drilling section (13) under the original "straight" well arrangement mode is not changed; (b) The surface gas pre-extraction directional inclined well is a "two-opening" wellbore structure. On the ground (10), a vehicle-mounted drilling rig is used to perform "one-opening" drilling in an oblique hole-opening manner. A surface casing (15) is lowered into the first-opening wellbore (7) for cementing, forming a first-opening cement ring (16) to provide wellbore conditions for subsequent second-opening drilling. The "two-opening" directional drilling is performed to the return air lane (2) position of the coal mining working face (1) and then lands (12) into the mining coal seam (11). The drilling is continued along the middle of the mining coal seam (11) to the position of the transport lane (3) of the coal mining working face (1) and then completed, forming a second-opening wellbore (8); (c) a production casing (17) is lowered into the formed secondary wellbore (8), and a plurality of packers (18) outside the production casing (17) are used to isolate the annular channel between the secondary wellbore (8) and the production casing (17), and a surface pressurized seal is achieved by using the plurality of packers (18) outside the production casing (17), so that a plurality of separation spaces are formed in the coal seam drilling section (13), and a pre-perforated screen (19) with a length of 2 to 3 m is provided on the production casing (17) of each separation space, which serves as a channel for fracturing fluid and proppant (23) to enter the annular channel between the secondary wellbore (8) and the production casing (17) and the coal seam (11) during subsequent small-scale hydraulic fracturing; (d) connecting a small-scale hydraulic fracturing equipment on the ground, and using a fracturing injection pump to inject fracturing fluid and proppant (23) in stages into the farthest end of the production casing (17), the farthest end of the annular channel between the second wellbore (8) and the production casing (17), and the farthest end of the mined coal seam (11) in a sealing injection manner through a single oil pipe external packer (22) of the fracturing oil pipe (21), so that the proppant (23) is accumulated and filled at the farthest end of the production casing (17) and the farthest end of the annular channel between the second wellbore (8) and the production casing (17), and a vertically or horizontally extending fracturing crack (24) is generated in the mined coal seam (11), thereby improving the permeability of the mined coal seam (11) around the wellbore of the ground gas pre-extraction directional inclined well; (e) after the outer packer (22) of the oil pipe is unsealed, the fracturing oil pipe (21) is dragged upward to the position of the last fracturing section, and the fracturing fluid and the proppant (23) are injected into the production casing (17), the annular separation space between the last second wellbore (8) and the production casing (17), and the mined coal seam (11) in stages by using a fracturing injection pump in a manner of sealing and injecting the fracturing oil pipe (21) with a single outer packer (22) of the oil pipe, so that the proppant (23) is accumulated and filled in the annular channel between the production casing (17), the last second wellbore (8) and the production casing (17), and a vertically or horizontally extending fracturing crack (24) is generated in the mined coal seam (11), thereby improving the permeability of the mined coal seam (11) around the wellbore of the surface gas pre-extraction directional inclined well; (f) Repeat step (e) to unseal the outer packer (22) of the tubing and drag the fracturing tubing (21) upward to the position of the last fracturing section, then seal it and perform small-scale hydraulic fracturing operations until all the annular channels between the secondary wellbore (8) and the production casing (17) of the surface gas pre-extraction directional inclined well are filled with proppant (23), thereby realizing small-scale hydraulic sand fracturing transformation of the entire well section of the coal seam drilling section (13); (g) After the completion of the staged fracturing construction of the coal seam drilling section (13) from the surface gas pre-extraction directional inclined well, the overflow discharge and well washing operations are carried out, and the drainage equipment in the well is placed into the production casing (17) so that the bottom end of the wire-wrap screen (27) at the suction port of the electric submersible pump (26) is located 2 to 3 meters above the top of the lowest pre-perforated screen (19). The ground is carried out with controlled drainage and gas production operations to gradually reduce the gas content and gas pressure of the mined coal seam (11). When the wellbore liquid level drops to the position of the wire-wrap screen (27) at the suction port of the electric submersible pump (26), the gas production pipeline is connected to the water ring vacuum pump and the motor to carry out negative pressure gas extraction of the mined coal seam (11) to further reduce the gas content and gas pressure of the mined coal seam (11).

2. The method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by directional inclined well according to claim 1, characterized in that: In step (a), under the "straight line" or "broken line" well arrangement mode, the plane distance between any two adjacent surface gas pre-extraction directional inclined wells for mining coal seams (13) is controlled to be 60 to 80 m, and the mining coal seams (11) are entered when the second drilling reaches the landing point (12), and then the mining coal seams (11) are drilled steadily along the mining coal seams (11), and the drilling rate of the mining coal seams (11) is increased by logging while drilling and measurement while drilling, thereby improving the fracturing transformation and gas pre-extraction effect of the mining coal seams (11).

3. The method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by directional inclined well according to claim 1, characterized in that: In step (b), the first wellbore (7) is drilled obliquely to 10 to 15 m below the bedrock, and then a J55 steel grade surface casing (15) is installed and then G grade oil well cement is injected to form a first well cementing cement ring (16); at the initial stage of drilling the second wellbore (5), the well inclination is slowly adjusted so that the second wellbore drill bit enters the mined coal seam (11) tangentially at the return air lane (2) position of the coal mining working face (1), and then the second wellbore drills steadily along the mined coal seam (11) to the transport lane (3) position of the coal mining working face (1).

4. The method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by directional inclined well according to claim 1, characterized in that: In step (c), the production casing (17) is of N80 steel grade, with an inner diameter of ≥139 mm and a wall thickness of ≥9.7 mm; the multiple packers (18) outside the production casing (17) achieve a pressure resistance of ≥30 MPa after ground pressure sealing; the pre-perforated screen (19) has a perforation density of 12 to 16 holes / m, a perforation diameter of 12 mm or more, a hole arrangement phase of 60°, and a positive spiral arrangement; small-scale hydraulic fracturing is a coal reservoir hydraulic fracturing with low liquid volume, low displacement, and medium construction pressure The fracturing fluid is an active water fracturing fluid, which is fully mixed with 98.8% clean water, 1.0% anti-swelling agent, 0.1% bactericide, and 0.1% drainage agent; the proppant (23) is a mixture of 70-100 mesh fine-grained ceramic powder and 100-140 mesh ultra-fine-grained ceramic powder in a volume ratio of 1:1; when the mined coal seam (11) is shallowly buried and has a high original permeability, the proppant (23) uses a mixture of 70-100 mesh fine-grained quartz sand and 100-140 mesh ultra-fine-grained quartz sand in a volume ratio of 1:

1.

5. The method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by directional inclined well according to claim 1, characterized in that: In step (d), the fracturing oil pipe (21) has an inner diameter of ≥50 mm, a wall thickness of ≥9 mm, and an ultimate pressure resistance of ≥50 MPa, and adopts an ordinary oil pipe connected by thread, or adopts a continuous oil pipe.

6. The method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by directional inclined well according to claim 1, characterized in that: In step (e), the outer packer (22) of the oil pipe is a packer that can be used repeatedly or a packer that can be used once; the length of each fracturing section should be 60 to 70 m, and the sealing position of the fracturing oil pipe (21) is 2 to 4 m above the pre-perforated screen (19) of the corresponding fracturing section, at the top plate position of hard siltstone or fine sandstone.

7. The method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by directional inclined well according to claim 1, characterized in that: The staged injection refers to the injection in the early stage of fracturing in the manner of "low pressure, low speed, and low sand ratio", that is, at a wellhead injection pressure of 1-2 MPa, 0.3-0.5 m 3 / min injection displacement, 20% to 30% sand ratio, a mixture of fracturing fluid and proppant (23) is injected into the annular channel and the toe end of the production casing at a low speed and low pressure, or a mixture of fracturing fluid and proppant (23) is injected into the separation space between the previous annular channel and the production casing at a low speed and low pressure, so as to realize the proppant (23) accumulation filling of the annular channel and the toe end of the production casing, and then high-pressure, high-speed, low-sand ratio injection is carried out to mine the coal seam (11) and support the fracture (24) at a wellhead injection pressure of 10 to 20 MPa and 1 to 2 m 3 / min injection displacement, 10% to 15% sand ratio, a mixture of fracturing fluid and proppant (23) is injected into the annular channel and the toe end of the production casing at a high speed, so that the proppant (23) is accumulated and filled in the annular channel below the injection point and the toe end of the production casing, or at a wellhead injection pressure of 10 to 20 MPa, 2 to 3 m 3 With an injection displacement of 10% to 15% sand ratio, a mixture of fracturing fluid and proppant (23) is injected into the annular channel and the lower part of the production casing (17) at a high speed, so that the proppant (23) is piled up and filled in the annular channel and the production casing (17) below the injection point, thereby achieving the purpose of reducing filtration loss and reducing the difficulty of gas extraction in the mining coal seam (11).

8. The method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by directional inclined well according to claim 1, characterized in that: In step (f), the number of repeated small-scale hydraulic fracturing is determined by the inclination width of the working face, that is, the length of the coal seam drilling section (13). Usually, the cumulative number of segmented fracturing sections is 6 to 7 sections / well. When the coal reservoir permeability is low or ≤-0.1mD, the number of fracturing sections is appropriately increased to ensure the effect of segmented fracturing transformation and gas pre-extraction in the directional inclined well mining coal seam drilling section (13) for ground gas pre-extraction.

9. The method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by directional inclined well according to claim 1, characterized in that: In step (g), the overflow discharge time is controlled within 3 to 5 days, and the well is washed by using a downhole gauge and a positive injection of formation water into the oil pipe to flush the sand to the bottom of the surface gas pre-extraction directional inclined well at one time; the well drainage equipment includes, from top to bottom, a drainage pipe (25), an electric submersible pump (26), a wire-wound screen pipe (27), a sand settling pipe, a downhole direct-reading thermometer and a downhole direct-reading pressure gauge.

10. The method for pre-extraction of gas from a coal mining face by staged fracturing of a coal seam by directional inclined well according to claim 1, characterized in that: In step (g), the drainage and gas production operation includes five stages: initial drainage, casing pressure control and gas production, flow pressure reduction and production increase, flow pressure control and stable production, and gas production attenuation. The duration of each stage is determined according to the stability of the liquid level drop, casing pressure, daily gas production, and daily water production changes during the drainage and pressure reduction process. The total duration of the drainage and gas production process is 120 to 280 days. At the end of the drainage and gas production, the wellbore liquid level gradually drops to below the position of the wire-wound screen pipe (27) at the suction port of the electric submersible pump (26). When the water ring vacuum pump is used to extract gas under negative pressure, the negative pressure formed by the wellhead annulus of the surface gas pre-extraction directional inclined well is maintained at -0.06 to -0.04 MPa. When the coal seam (11) is mined and the pure amount of gas extracted under negative pressure is ≤60m 3 / d, or when the extracted gas concentration is lower than 8%, the gas pre-extraction work of the mining coal seam (11) is completed.

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