A method for rapidly and efficiently extracting gas in a soft coal seam coal mine

Abstract

This invention discloses a rapid and efficient method for gas extraction in soft, fractured coal seams, belonging to the field of coalbed methane development and coal mine gas control technology. The method involves determining the geological characteristics of the coal mining planning area, deploying a combined treatment system of surface U-shaped wells and underground bottom-extraction rock-drainage boreholes, and adopting a well-factory model to achieve large-scale regional treatment. It utilizes combined surface and underground microseismic monitoring and underground transient electromagnetic methods to detect the fracturing range and fracturing fluid distribution characteristics, optimizing the layout of the cross-layer boreholes. The cross-layer boreholes are divided into drainage holes and gas extraction holes, and a coordinated extraction system is established based on different drainage stages. This invention solves the problems of low efficiency, long cycle, and blind spots in gas extraction from soft, fractured coal seams, significantly shortening treatment time and reducing costs, thus providing a guarantee for safe and efficient coal mine production.

Description

Technical Field

[0001] This invention belongs to the field of coalbed methane development and coal mine gas control technology, specifically relating to a rapid and efficient method for gas extraction in soft coal seams. Background Technology

[0002] Gas control is a prerequisite for safe and efficient coal mine production. In my country, the low mechanical strength of coal and rock, poor permeability, high gas pressure, and the difficulty of borehole collapse when drilling along the seam have resulted in poor gas extraction, which has always been a technical bottleneck restricting gas control in such coal mines.

[0003] Currently, my country's gas control mainly employs surface coalbed methane development, underground gas extraction, and combined surface and underground extraction methods. Compared to drilling within the coal seam for gas control in native coal seams, surface coalbed methane development in fractured and soft coal seams faces challenges such as borehole collapse, difficulty in propagating hydraulic fracturing fractures within the coal seam, dust production during extraction, and pump blockage. Underground gas extraction along the coal seam faces problems such as stuck drill bits, borehole collapse, inability to drill long drills, low borehole formation rate, and low extraction efficiency. To address the technical challenges of gas control in fractured and soft coal seams, surface coalbed methane development generally employs drilling along the coal seam roof, such as L-shaped or U-shaped roof wells; underground gas control typically utilizes drilling through the roof and floor rock tunnels for extraction. However, roof L-shaped and roof U-shaped wells typically employ large-scale hydraulic fracturing, resulting in large drainage volumes and long timeframes for gas breakthrough and achieving stable production. Furthermore, the production cycle for coalbed methane wells is usually 10-15 years or more. Additionally, due to the use of staged fracturing for production enhancement, areas not reached by fracturing exhibit drainage blind spots and dead zones, making it impossible to achieve full-area drainage compliance. Roof and floor drilling for drainage in coal mines suffers from small drainage radii and coverage areas, leading to dense drilling operations, large workloads, high labor intensity for workers, and short-lasting pre-drainage effects, resulting in poor coalbed methane drainage.

[0004] With the increasing mechanization, intelligence, and automation of coal mining, high-extraction, long fully mechanized mining faces are becoming more common. While accelerating coal mining, this also demands that gas control be completed in a shorter time and in a more efficient manner to meet the needs of coal production capacity succession. In particular, gas control in high-gas-content, soft, and fractured coal seams presents significant challenges, requiring more suitable and efficient extraction methods. Summary of the Invention

[0005] This invention provides a rapid and efficient method for gas extraction in soft coal seams. It employs a combined surface and underground extraction approach, using a U-shaped well on the surface and bottom-drainage rock roadway through-layer drilling underground to improve the gas extraction effect in soft coal seams.

[0006] To solve the above problems, the technical solution provided by the present invention is as follows:

[0007] This invention provides a method for rapid and efficient gas extraction in soft coal seams, comprising the following steps:

[0008] Step 1: Determine the geological characteristics of the coal mining planning area;

[0009] Step 2: Design a combined system of surface U-shaped wells and underground bottom drainage roadways for the coal mining planning area. Multiple surface U-shaped wells are deployed simultaneously. The horizontal section of a single U-shaped well is located near the central axis of the working face in the coal mining planning area. The bottom drainage roadway is located in the bottom rock strata of the coal seam between the working faces of the two coal mining planning areas along the trajectory of the horizontal wellhead and is connected by a through roadway.

[0010] Step 3: Develop a gas control and extraction plan for the U-shaped wells on the ground;

[0011] Step 4: The ground microseismic monitoring network for each fracturing section is arranged according to a specific grid structure with the ground projection of the horizontal section of the H well of the U-shaped well as the center. The underground microseismic monitoring network for each fracturing section is arranged in the bottom rock extraction tunnels on both sides to monitor and interpret fracturing microseismic events in real time to determine the crack propagation law and spatial distribution pattern.

[0012] Step 5: Deploy transient electromagnetic detection points in the bottom pumping tunnel corresponding to each fracturing section, use downhole transient electromagnetic method to detect the fracturing fluid expansion range, compare the low resistivity anomaly characteristics and change features before and after fracturing, and determine the formation water-rich anomaly area.

[0013] Step 6: Based on the microseismic monitoring of fracture development and the transient electromagnetic detection of formation water abundance, formulate a plan for the layout of cross-layer drilling holes in the bottom dredging tunnel.

[0014] Step 7: Combine the layout of the cross-layer drilling in the bottom rock extraction tunnel in the mine, and formulate the extraction system by adjusting the relationship between the bottom flow pressure and the original formation pressure, desorption pressure and casing pressure, so as to extract coal seam gas quickly and efficiently.

[0015] Preferably, in step 2, the U-shaped well location design avoids areas with faults and collapse columns. The horizontal section of the H-shaped well intersects the direction of the main fracture at a certain angle. The V-shaped well is located in the downdip direction of the coal seam. The horizontal section of the H-shaped well is 0-2m away from the top of the coal seam, and the length of the horizontal section is 800-1200m, located near the central axis of the planned coal mining face. The bottom drainage roadway is 15-25m away from the bottom of the coal seam, depending on the lithology and structure of the strata. The bottom drainage roadway is connected by a through roadway, and the through roadway is equipped with bottom through-layer boreholes to extract gas from the coal seam near the vertical shaft end of the H-shaped well.

[0016] Preferably, in step 3, well H adopts a three-section wellbore structure. Well H uses a cable pump bridge plug + perforation combined operation process to carry out segmented perforation and segmented fracturing construction on the horizontal section. The perforation azimuth is determined according to the specific wellbore trajectory. The segmented fracturing parameters and pumping construction procedures are determined according to specific geological conditions. Well V adopts a two-section or three-section wellbore structure according to the formation conditions. Well V is used as a drainage well and uses an electric submersible screw pump for drainage.

[0017] Preferably, in step 3, during the segmented perforation and fracturing construction of the horizontal section of well H, each segment is divided into sections of 70-80m in length, and the specific length is adjusted according to the coal or rock conditions of the wellbore trajectory and the fault development. The perforation location avoids the casing coupling location, and different perforation schemes are adopted according to the actual wellbore trajectory. The fracturing parameters and pumping construction procedures for each segment are determined according to the specific geological conditions.

[0018] Preferably, in step 4, the ground microseismic monitoring network for each fracturing section is divided into a 6×7 grid structure centered on the ground projection of the horizontal section of the U-shaped well H well, with a station spacing of 80m, a coverage range of 400m along the trajectory of the horizontal section, and a coverage range of 480m along the radial direction of the horizontal section; the underground microseismic monitoring network for each fracturing section is equipped with 9 microseismic geophones in each bottom dredging tunnel, arranged at 20m intervals, with the horizontal projection of the fracturing section located in the middle of the detection point, and the rolling mobile geophones are used to carry out microseismic monitoring of the next fracturing section after each fracturing operation is completed.

[0019] Preferably, in step 5, during the downhole transient electromagnetic method detection, based on the detection distance of the transient electromagnetic instrument and the number of horizontal segmented fracturing sections, two transient electromagnetic detection points are arranged in the bottom rock extraction tunnel corresponding to each fracturing section location, with the same spacing between adjacent fracturing section detection points; each detection point is designed with five detection directions, namely an elevation angle of 90° (vertically upward), an elevation angle of 60°, an elevation angle of 30°, an elevation angle of 15°, and directly in front of the tunnel wall (horizontal direction).

[0020] Preferably, in step 6, the drainage holes of the bottom-extraction rock tunnel cross-layer drilling are initially used for drainage and pressure reduction, and later connected to negative pressure pipelines for air extraction; the air extraction holes of the bottom-extraction rock tunnel cross-layer drilling are mainly used for air extraction, and also serve a partial drainage function when there is water in the initial coal seam; one drainage hole is arranged in the bottom-extraction rock tunnel corresponding to each fracturing section, and the drainage hole is arranged in the bottom-extraction rock tunnel at a low altitude and near the farthest water-rich area within the influence range of each fracturing fracture; after drilling, the bottom-extraction rock tunnel cross-layer drilling is subjected to high-pressure flushing, and the addition of sodium lignosulfonate as a coal powder dispersant is determined according to the coal slag situation and the flushing and slag removal capacity of the clean water medium; the drainage holes must be flushed under high pressure, and the air extraction holes are flushed under high pressure depending on the drilling slag and water situation; after the bottom-extraction rock tunnel cross-layer drilling is completed, PVC screen pipes are used as drainage and air extraction pipelines and the wellhead is sealed; the drainage holes corresponding to each fracturing section are constructed first, after the fracturing operation and venting are completed and the formation pressure is stable, and the drainage holes corresponding to the first fracturing section are constructed first.

[0021] Preferably, in step 7, the specific sampling procedures for different stages of the sampling process are as follows:

[0022] Drainage stage: When pumping begins, if the bottom hole pressure is greater than the original formation pressure, adjust the surface electric submersible screw pump speed to near the maximum and open the drainage valves to the maximum. When the bottom hole pressure is lower than the original formation pressure but greater than the desorption pressure, reduce the pumping rate and reduce the number of drainage valves in the well. Control the bottom hole pressure drop rate to 0.03-0.09 MPa / d and ensure that the daily pressure drop fluctuation does not exceed 20% of the previous day's pressure drop.

[0023] Production ramp-up phase: When the bottom hole pressure is less than the desorption pressure, further reduce the pump discharge rate and close the drainage holes at the high-altitude fracturing sections in the horizontal formation. Control the bottom hole pressure drop rate at 0.01-0.03 MPa / d. As coalbed methane desorbs and casing pressure increases, open the surface pipeline valves to start gas production and gradually increase the gas production valves to ramp up production. Control the casing pressure at 0.1 MPa-0.5 MPa, maintain a slow decrease in bottom hole pressure, and control large fluctuations in casing pressure to avoid gas leakage from the discharge pump.

[0024] Stable production phase: After the gas production stabilizes, the drainage hole is gradually closed according to the water production, and the pumping rate is further reduced. The pressure drop rate at the bottom of the well is controlled at 0-0.01MPa / d to maintain the balance between the bottom of the well and the casing pressure, ensuring that the dynamic fluid level is 10 meters higher than the pumping pump. The downhole gas extraction hole is opened, and the negative pressure is 5-20KPa. The negative pressure gradually increases with the extraction time, and the gas extraction hole valve is adjusted according to the gas concentration and gas purity.

[0025] Attenuation stage: Maintain a stable relationship between bottom hole flowing pressure and casing pressure by adjusting pump speed and gas production valves, so that gas production gradually decreases; increase downhole extraction intensity, fully open the gas extraction port and drainage port valves to the maximum, increase the extraction negative pressure to 20-40 kPa, and adjust the gas extraction port valves according to gas concentration and gas purity.

[0026] Compared with existing technologies, this invention provides a rapid and efficient gas extraction method for soft coal seams, which has the following advantages: The gas control method involves deploying multiple surface U-shaped wells and underground bottom-extraction rock-drainage perforations throughout the coal mining planning area, forming a well factory and regional gas control effect. This reduces the number of surface well sites and bottom-extraction rock-drainage wells. Simultaneously, multi-well regional extraction is more likely to cause inter-well interference and facilitate gas desorption and production. The combined extraction method of surface V-wells and underground perforations solves the problems of long gas exposure time and long extraction cycles. The perforation layout scheme in the bottom-extraction rock-drainage utilizes microseismic monitoring of fracture development and transient electromagnetic detection of formation water abundance to effectively address gas control in dead zones and areas not affected by fracturing, achieving full-area coverage. It also solves the problems of blind drilling, dense drilling, and large engineering workload associated with perforation. Furthermore, the establishment of extraction systems for different stages of gas extraction in combined surface and underground gas control enables rapid and efficient extraction of coal seam gas. This invention can significantly shorten the gas control time at coal mining faces, reduce the cost of gas control per ton of coal, and improve extraction efficiency. It can meet the requirements of intelligent fully mechanized mining technology and provide strong protection for the safe production of coal mines. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic diagram of the surface U-shaped well and the underground bottom rock extraction tunnel cross-layer drilling extraction mode provided in the embodiments of the present invention.

[0029] Figure 2 This is a schematic diagram of the direction arrangement for the transient electromagnetic method for advance detection provided in an embodiment of the present invention.

[0030] Figure 3 A flowchart of a method for efficient gas extraction in soft coal seams provided in an embodiment of the present invention.

[0031] Figure 4 This is a schematic diagram of the location of the U-shaped well on the ground and the deployment of the bottom rock extraction tunnel provided in an embodiment of the present invention.

[0032] Figure 5 This is a schematic diagram of the U-shaped well shaft structure provided in an embodiment of the present invention.

[0033] Figure 6 This is a schematic diagram of the main fracture projection for hydraulic fracturing microseismic monitoring of a horizontal section of a U-shaped well, provided in an embodiment of the present invention.

[0034] Figure 7This is a schematic diagram of the reservoir stimulation volume for microseismic monitoring in a horizontal section of a U-shaped well, provided in an embodiment of the present invention.

[0035] Figure 8 The image shows a sector view of the apparent resistivity profile at a certain measuring point before U-shaped hydraulic fracturing (elevation angle 15°) provided in an embodiment of the present invention.

[0036] Figure 9 This is a sector-shaped profile (elevation angle 15°) of apparent resistivity detection at a certain measuring point after U-shaped hydraulic fracturing, provided in an embodiment of the present invention.

[0037] Figure 10 This is a schematic diagram of the arrangement of drainage holes and air extraction holes in a U-shaped bottom rock extraction tunnel provided in an embodiment of the present invention. Detailed Implementation

[0038] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application. The terms "upper," "lower," "front," "rear," "left," and "right," etc., used when describing the installation position or direction of the structure or components in this embodiment are based on the orientation shown in the accompanying drawings. They are merely for convenience of description, used to distinguish the relative positions of various components or directions, and do not represent the orientation of the system or functional components in this embodiment during use.

[0039] This invention provides a method for rapid and efficient gas extraction in soft coal seams, comprising the following steps:

[0040] Step 1: Determine the geological characteristics of the coal mining planning area, such as strata, structure, coal seam fracture development, and geostress. Using geophysical exploration and geological drilling combined with mine geological surveys, clarify the stress distribution, principal stress orientation, fault and collapse column development, coal seam dip angle, coal body structure, and coal seam fracture system development characteristics of the coal mining planning area.

[0041] Step 2: Design a combined system of surface U-shaped wells and underground bottom drainage roadways with cross-layer drilling in the coal mining planning area. The surface U-shaped well group adopts multiple U-shaped wells deployed simultaneously to form a well factory and regional gas control effect. The horizontal section of a single U-shaped well is arranged near the central axis of the working face in the coal mining planning area. The bottom drainage roadway is arranged in the bottom rock strata of the coal seam between the two planned coal mining working faces along the direction of the horizontal wellbore trajectory and is connected by a through roadway.

[0042] Step 3: Develop a gas control and extraction plan for the surface U-shaped wells. The surface U-shaped wells include well H and well V. Well H adopts a three-section wellbore structure. Well H uses a cable pump bridge plug + perforation combined operation process to carry out segmented perforation and segmented fracturing construction on the horizontal section. The perforation orientation is determined according to the specific wellbore trajectory. The segmented fracturing parameters and pumping construction procedures are determined according to specific geological conditions. Well V adopts a two-section or three-section wellbore structure according to the formation conditions. Well V is used as a drainage well and uses an electric submersible screw pump for drainage.

[0043] Step 4: Use a combined surface and downhole microseismic monitoring method to characterize the hydraulic fracturing stimulation range. The surface microseismic monitoring network for each fracturing section is arranged according to a specific grid structure with the surface projection of the horizontal section of the U-shaped well and H-well as the center. The downhole microseismic monitoring network for each fracturing section is arranged in the bottom rock extraction tunnels on both sides to monitor and interpret fracturing microseismic events in real time to determine the fracture propagation law and spatial distribution morphology.

[0044] Step 5: Use downhole transient electromagnetic method to detect the expansion range of fracturing fluid in the formation after fracturing. Set up transient electromagnetic detection points in the bottom pumping tunnel corresponding to each fracturing section. Design multiple detection directions for each detection point and conduct detection before and after fracturing. Compare and analyze the low-resistivity anomaly characteristics and change features to determine the spatial distribution of the water-rich anomaly zone, thereby judging the expansion range of fracturing fluid and identifying the water-rich anomaly area in the formation.

[0045] Step 6: Based on the microseismic monitoring of fracture development and the transient electromagnetic detection of formation water abundance, formulate a borehole layout plan for the bottom venting tunnel. In this embodiment, the bottom venting tunnel boreholes are divided into drainage holes and air extraction holes. Drainage holes are arranged near the farthest water-rich area within the influence range of each fracturing fracture and in the bottom venting tunnel at a low altitude. Air extraction holes are arranged in areas not affected by the fracturing fracture. The borehole depth is 5-10m above the coal seam roof, and the hole bottom spacing is 10-20m, arranged in a fan shape.

[0046] Step 7: Combining the layout of the cross-layer drilling in the underground bottom drainage tunnel, and by adjusting the relationship between the bottom-hole flowing pressure and the original formation pressure, desorption pressure, and casing pressure, a drainage system is formulated to rapidly and efficiently extract coal seam gas. This embodiment formulates a drainage system for different drainage stages of joint surface and underground management. The system adjusts the parameters of the surface electric submersible screw pump rate, underground drainage and extraction port valves, and drainage negative pressure in the drainage stage, production increase stage, production stabilization stage, and attenuation stage to achieve rapid and efficient extraction of coal seam gas.

[0047] Specifically, in step 2, the U-shaped well location design avoids areas with faults and collapse columns. The horizontal section of the H-shaped well intersects the direction of the main fracture at a certain angle. The V-shaped well is located in the downdip direction of the coal seam. The horizontal section of the H-shaped well is 0-2m away from the top of the coal seam, and the length of the horizontal section is 800-1200m and located near the central axis of the planned coal mining face. The bottom drainage roadway is 15-25m away from the bottom of the coal seam, depending on the lithology and structure of the strata. The bottom drainage roadway is connected by a through roadway. The through roadway is equipped with bottom through-layer boreholes to extract gas from the coal seam near the vertical end of the H-shaped well.

[0048] refer to Figure 1 In this embodiment, gas control is carried out using surface U-shaped wells and underground bottom-extraction roadways with cross-layer drilling. Based on the geological development characteristics of the coal mining planning area, the U-shaped wells and bottom-extraction roadways are deployed in combination at suitable locations. The surface U-shaped well group adopts a multi-well, integrated deployment design, forming a well factory and a regional gas control effect. This reduces surface drilling space and costs, while minimizing the impact on adjacent wells during hydraulic fracturing operations. Multi-well regional drainage is more likely to create inter-well interference, which is beneficial for gas desorption and production. The U-shaped well locations are designed to avoid areas with faults, collapse columns, and other structurally developed areas. The horizontal section of the U-shaped well H intersects the main fracture direction at a certain angle, and the U-shaped well V is located in the downdip direction of the coal seam, facilitating subsequent drainage and gas production. The horizontal section of the U-shaped well H is 0-2m from the coal seam roof, and the horizontal section of the U-shaped well is located near the central axis of the planned coal mining face.

[0049] Bottom-extraction tunnels R1 and R2 are arranged along the horizontal wellbore trajectory in the floor strata of the coal seam between two planned coal mining faces. Each bottom-extraction tunnel can provide floor-penetrating gas drainage holes (drainage holes, air extraction holes) to two adjacent coal mining faces, reducing the number of bottom-extraction tunnels required. Bottom-extraction tunnels R1 and R2 are connected by bottom-extraction tunnel R3, which contains floor-penetrating boreholes for extracting gas from the coal seam near the vertical shaft end of the U-shaped H-shaped well. Drilling sites Z1i, Z2i, and Z3i are respectively set in bottom-extraction tunnels R1, R2, and R3 to accommodate the cross-penetrating boreholes. The distance between the bottom-extraction tunnel and the coal seam floor is h2, which is 15-25m, depending on the lithology and geological structure.

[0050] In step 3, during the segmented perforation and fracturing of the horizontal section of the U-shaped H well, each segment is 70-80m long, adjusted according to the coal or rock conditions and fault development of the wellbore trajectory. Each segment has 3 perforation clusters, each cluster having 1.5m of perforation, totaling 4.5m of perforation per segment, with a perforation density of 15 perforations / m. Perforation locations avoid casing couplings, and different perforation schemes are adopted based on the actual wellbore trajectory: vertical downward perforation is used when the horizontal wellbore trajectory is more than 2m away from the coal seam in the roof strata; oblique downward and vertical downward perforation are used when the distance is 0-2m from the coal seam; spiral perforation is used within the coal seam; fracturing employs a high-volume, high-sand-ratio injection method, with a construction volume of 12-24m³. 3 The fracturing fluid is prepared at a flow rate of 10%-20% per minute, with a sand ratio of 10%-20%. The fluid used is activated water, mixed with clean water, 1.0% KCl, and 0.05% bactericide. A drag-reducing agent can be added as needed based on the fracturing pressure. The single-stage fluid volume is 1200-2000 m³ / min. 3 The proppant is made of 20 / 40 mesh or 16 / 20 mesh quartz sand, with a single section containing 100-200 m³ of quartz sand. 3 .

[0051] In step 4, the surface microseismic monitoring network for each fracturing section is divided into a 6×7 grid structure centered on the surface projection of the horizontal section of the U-shaped H-well. The station spacing is 80m, the coverage range along the horizontal section trajectory is 400m, and the radial coverage range along the horizontal section is 480m. Downhole, the microseismic monitoring network for each fracturing section deploys 9 microseismic geophones at 20m intervals in each bottom venting tunnel, with the horizontal projection of the fracturing section located in the center of the monitoring points. After each fracturing section is completed, a rolling mobile geophone is used to monitor the microseismic events of the next fracturing section. This embodiment employs a combined surface and downhole microseismic monitoring method to monitor microseismic events generated during hydraulic fracturing in real time, locate and interpret the fracturing microseismic events, infer the fracture length, height, and azimuth, analyze the fracture propagation pattern and spatial distribution morphology, and characterize the effective influence range of hydraulic fracturing.

[0052] In step 5, during the underground transient electromagnetic method detection, based on the detection distance of the transient electromagnetic instrument and the number of horizontal fracturing sections, two transient electromagnetic detection points are arranged in the bottom dredging tunnel corresponding to each fracturing section location, with the same spacing between adjacent fracturing section detection points. Each detection point is designed with five detection directions: 90° elevation (vertically upward), 60° elevation, 30° elevation, 15° elevation, and directly in front of the tunnel wall (horizontal direction). A mine-grade transient electromagnetic instrument and a high-power transmitter are used, with a detection depth of approximately 100m. An overlapping loop combination device is adopted, with a 1.5m side length square frame for transmitting and receiving, 4 turns of transmitting coil, and 40 turns of receiving coil. At least 30 superpositions are performed at each measurement point to improve the signal-to-noise ratio.

[0053] refer to Figure 2In this embodiment, based on the transient electromagnetic instrument's detection distance and the number of horizontal fracturing segments, two transient electromagnetic detection points are arranged in the bottom dredging tunnel corresponding to each fracturing segment location. The spacing between transient electromagnetic detection points in adjacent fracturing segments is the same. Each detection point is designed with five detection directions: 90° elevation (vertically upward), 60° elevation, 30° elevation, 15° elevation, and directly in front of the tunnel wall (horizontal direction). Detection is performed once before and once after fracturing operations. The low-resistivity anomaly characteristics and changes in the detection range before and after hydraulic fracturing are compared and analyzed to determine the spatial distribution of the water-rich anomaly zone after fracturing, thereby judging the extent of fracturing fluid propagation.

[0054] In step 6, the drainage holes of the bottom-extraction rock tunnel cross-layer boreholes are initially used for drainage and pressure reduction, and later connected to negative pressure pipelines for air extraction; the air extraction holes of the bottom-extraction rock tunnel cross-layer boreholes are mainly used for air extraction, and also serve a partial drainage function when there is water in the initial coal seam; one drainage hole is arranged in the bottom-extraction rock tunnel corresponding to each fracturing section, and the drainage hole is located in the bottom-extraction rock tunnel at a low altitude and near the farthest water-rich area within the influence range of each fracturing fracture; after drilling, the cross-layer boreholes of the bottom-extraction rock tunnels are subjected to high-pressure flushing, and the addition of sodium lignosulfonate as a coal powder dispersant is determined based on the coal slag discharge and the flushing and slag removal capacity of the clean water medium. Drainage holes must be flushed, and air extraction holes are subject to high-pressure flushing based on the slag and water discharge conditions during drilling; after flushing, PVC screen pipes are installed as drainage and air extraction pipelines and the holes are sealed; the drainage holes corresponding to each fracturing section are constructed first, after the fracturing operation and venting are completed and the formation pressure is stable, and the drainage holes corresponding to the first fracturing section are constructed first.

[0055] This embodiment uses the results of microseismic monitoring and transient electromagnetic detection to determine the layout of drainage holes and air extraction holes in the bottom venting tunnel, including the drilling site spacing, number, length, and orientation of the holes. One drainage hole is placed in the bottom venting tunnel corresponding to each fracturing section. These drainage holes are located in the lower elevation of the bottom venting tunnel and are positioned near the furthest water-rich area within the influence range of each fracturing fracture. Air extraction holes are placed in areas unaffected by the fracturing fracture. The final depth of both drainage and air extraction holes is 5-10m above the coal seam roof, with a spacing of 10-20m between the bottoms of each hole, arranged in a fan shape.

[0056] After drilling, the bottom-extraction rock tunnel cross-layer boreholes require high-pressure flushing to remove the soft coal fragments inside and around the borehole. The addition of a coal powder dispersant (sodium lignosulfonate) is determined based on the amount of coal slag produced and the flushing capacity of the water medium. Drainage holes must be flushed, while the need for high-pressure flushing of air extraction holes depends on the amount of slag and water produced during drilling. After flushing, PVC screen pipes are installed as drainage and air extraction pipelines, and the boreholes are sealed to ensure a tight seal.

[0057] To further shorten the overall drainage time, the drainage holes corresponding to each fracturing section are constructed before the air extraction holes after the fracturing operation is completed and the formation pressure is stable, and the drainage holes corresponding to the first fracturing section are constructed first.

[0058] A combined extraction method using surface electric submersible screw pumps and underground bottom-drainage rock extraction tunnels with cross-layer boreholes is employed. The extraction regime is determined by adjusting the relationship between bottom-hole flowing pressure, original formation pressure, desorption pressure, and casing pressure. In step 7, the specific extraction regimes for different drainage stages are as follows:

[0059] Drainage Stage: During initial pumping, if the bottom-hole pressure is greater than the original formation pressure, adjust the surface electric submersible screw pump speed to near its maximum and simultaneously open all drainage valves to their maximum. When the bottom-hole pressure is lower than the original formation pressure but greater than the desorption pressure, reduce the pumping rate and reduce the valves on some downhole drainage holes to control the bottom-hole pressure drop rate between 0.03-0.09 MPa / d, ensuring that the daily pressure drop fluctuation does not exceed 20% of the previous day's drop. In this embodiment, the presence of drainage holes in the bottom-pumped rock tunnel accelerates water drainage and shortens the drainage period, providing a rapid pumping function. Simultaneously, the presence of venting holes also accelerates gas extraction.

[0060] Production ramp-up phase: When the bottom hole pressure is less than the desorption pressure, further reduce the pump discharge rate and close the drainage holes at the high-altitude fracturing sections in the horizontal formation. Control the bottom hole pressure drop rate at 0.01-0.03 MPa / d. As coalbed methane desorbs and casing pressure increases, open the surface pipeline valves to start gas production and gradually increase the gas production valves to ramp up production. Control the casing pressure at 0.1 MPa-0.5 MPa, maintain a slow decrease in bottom hole pressure, and control large fluctuations in casing pressure to avoid gas leakage from the discharge pump.

[0061] Stable production stage: After the gas production stabilizes, the drainage hole is gradually closed according to the water production, and the pumping rate is further reduced. The pressure drop rate at the bottom of the well is controlled at 0-0.01MPa / d to maintain the balance between the bottom of the well and the casing pressure, and to ensure that the dynamic fluid level is 10 meters higher than the pumping pump. The downhole gas extraction hole is opened, and the negative pressure is 5-20KPa. The negative pressure gradually increases with the extraction time, and the gas extraction hole valve is adjusted according to the gas concentration and gas purity.

[0062] Attenuation stage: Maintain a stable relationship between bottom hole flowing pressure and casing pressure by adjusting pump speed and gas production valves, so that gas production gradually decreases; increase downhole extraction intensity, fully open the gas extraction port and drainage port valves to the maximum, increase the extraction negative pressure to 20-40 kPa, and adjust the gas extraction port valves according to gas concentration and gas purity.

[0063] The key technical points of the present invention are as follows:

[0064] Key Point 1: In gas control, a combination of surface U-shaped wells and underground bottom-extraction rock tunnel drilling is adopted, and a multi-well deployment and regional large-scale control model is adopted to achieve efficient and coordinated pressure reduction in the region, which is conducive to desorption and gas production and reduces control costs.

[0065] Key Point 2: The combined surface and downhole microseismic monitoring and downhole transient electromagnetic method were used to detect the influence range of fracturing fractures and the occurrence of formation fluids. Based on this, a bottom-drainage rock tunnel cross-layer drilling extraction plan was formulated, which effectively controlled the gas in areas that could not be extracted by horizontal wells. At the same time, it avoided blind and dense drilling of cross-layer holes and reduced the amount of engineering work.

[0066] Key Point 3: The bottom gas extraction tunnel through-layer holes are divided into drainage holes and gas extraction holes. Drainage holes increase the speed of drainage and pressure reduction, while the bottom gas extraction tunnel through-layer holes shorten the overall extraction time and increase the gas extraction efficiency.

[0067] Key Point 4: For the entire life cycle of coal seam gas control and extraction, extraction systems have been developed for different extraction stages of joint surface and underground control, making the drainage and gas production process more scientific, reasonable and efficient.

[0068] Application Examples

[0069] refer to Figure 3 This invention provides a method for rapid and efficient gas extraction in soft coal seams, comprising the following steps:

[0070] Step 1: Determine the geological features of the coal mining planning area, including strata, structures, and coal seam fractures.

[0071] Using geophysical exploration, geological drilling, and other methods, combined with geological surveys of other coal mining faces in the coal mining area, the distribution of geostress and the orientation of principal stress in the coal mining area were determined; the development of faults, collapse columns, and other structures in the coal mining planning area was ascertained; and the development characteristics of coal seam dip angle, coal body structure, and coal seam fracture system in the coal mining planning area were determined.

[0072] This embodiment's design is based on a coal mining area with well-developed soft coal seams. Existing geological data reveals the main geological characteristics of the coal seams in the planned mining area. The strata in this area dip at 5-10°, with a coal seam thickness of approximately 4m and a burial depth of about 660m. The coal body structure is fractured, with well-developed soft coal seams, small faults, and no collapse columns. The direction of the maximum principal stress in this mining area is NE, while the coal seam strikes NEE.

[0073] Step 2: Design of the location of the U-shaped well for gas control on the surface and the deployment of the bottom extraction tunnel in the coal mining planning area.

[0074] This invention employs a combination of surface U-shaped wells and underground bottom-extraction tunnels for gas control. Based on the geological characteristics of the coal mining area, the U-shaped wells and bottom-extraction tunnels are deployed in suitable locations in a combined manner. The surface U-shaped well group utilizes a multi-well, integrated deployment design, creating a "well factory" and achieving a large-scale regional gas control effect.

[0075] Based on the geological characteristics of the coal mining planning area described in Step 1, two coal mining faces (C1 and C2) are planned for this area. The fully mechanized mining face is approximately 2400m long and 300m wide, and mining is conducted along the strike of the coal seam. Four surface U-shaped wells (U1, U2, U3, and U4) are designed for gas control. The well locations of the U-shaped wells avoid areas with developed faults and collapse columns. The horizontal sections of the U-shaped wells H (H1, H2, H3, and H4) intersect the direction of the main fracture along the strike of the coal seam at a certain angle. The U-shaped wells V (V1, V2, V3, and V4) are located in the downdip direction of the coal seam. The horizontal section of the U-shaped well H is 0-2m from the top of the coal seam (h1). The horizontal sections of the U-shaped wells are located near the central axis of the planned coal mining face. Figure 4 Among them, V1 and H2, and V3 and H4 share one surface well site.

[0076] In this invention, three bottom drainage roadways (R1, R2, and R4) are arranged along the horizontal wellbore trajectory of two coal mining faces. These roadways are located in the middle of the coal seam between the two planned coal mining faces. Each bottom drainage roadway can provide floor-penetrating drainage holes to two adjacent coal mining faces, reducing the number of bottom drainage roadways required. Bottom drainage roadways R1, R2, and R4 are connected by bottom drainage roadways R3 and R5. The floor-penetrating boreholes in bottom drainage roadways R3 and R5 are used to extract gas from the coal seam at the vertical shaft end of the U-shaped H-shaped well. The coal seam floor is hard limestone with a complete structure, and the bottom drainage roadway is 17m away from the (h2) coal seam floor.

[0077] Step 3: Develop a gas control and extraction plan for the U-shaped wells on the ground.

[0078] In the application examples, the U-shaped well H adopts a three-section wellbore structure, and the V well adopts a two-section wellbore structure. The completion depth of the V well is below the coal seam. (Refer to...) Figure 5 .

[0079] The wellbore trajectory of the horizontal section of the U-shaped well H is within 0-2m from the top of the coal seam (h1). The horizontal section of the U-shaped well is located near the central axis of the planned coal mining face. The length of the horizontal section of the four U-shaped wells is about 900-1100m.

[0080] U-shaped wellbore structure design in application examples:

[0081] H-well shaft structure design:

[0082] First well: A Φ444.5mm drill bit was used to drill through the Quaternary loose layer, and a 339.7mm surface casing was run in. The first well depth was 190m.

[0083] Second stage: Drill using a Φ311.1mm drill bit to the landing point, then insert a Φ244.5mm technical casing.

[0084] Third section: The horizontal section was drilled to the designed well depth using a Φ215.9mm drill bit and successfully connected with the production well. A Φ139.7mm production casing was then run in.

[0085] V-well shaft structure design:

[0086] First step: Use a Φ311.1mm drill bit to drill through the Quaternary loose layer and insert a 244.5mm surface casing.

[0087] Second stage: Drill to 50m below the coal seam floor using a Φ215.9mm drill bit and run in a Φ177.8mm technical casing. Run a fiberglass casing near the cavity section.

[0088] Application example: Horizontal section perforation and staged fracturing design of well H:

[0089] The horizontal section was constructed using a combination of cable-pumped bridge plug and perforation technology, which enabled segmented perforation and fracturing to connect the horizontal wellbore with the target coal seam and to establish a fracture channel supported by quartz sand with high conductivity in the formation.

[0090] a. The horizontal section of well H is divided into segments of 70-80m in length, with the specific length adjusted according to the coal or rock conditions of the wellbore trajectory and the development of faults.

[0091] b. Each section has 3 perforation clusters, each cluster has 1.5m of perforation, totaling 4.5m of perforation per section, with a perforation density of 15 perforations / m. The perforation locations should avoid the casing coupling locations. Different perforation schemes will be adopted based on the actual wellbore trajectory. For horizontal wellbore trajectories more than 2m away from the coal seam in the roof strata, vertical downward perforation will be used; for horizontal wellbore trajectories 0-2m away from the coal seam in the roof strata, oblique downward and vertical downward perforation will be used; within the coal seam, spiral perforation will be used.

[0092] c. Fracturing is performed using a high-volume, high-sand-ratio injection method, with a displacement of 12-24 m³ / h. 3 / min, sand ratio is 10%-20%, and the actual construction parameters can be adjusted appropriately according to the specific conditions of the coal mine site.

[0093] d. Activated water is used as the fracturing fluid. The fracturing fluid ratio is: clean water, 1.0% KCl, and 0.05% bactericide. A drag-reducing agent can be added as needed depending on the fracturing pressure. The single-stage fluid volume is 1200-2000 m³. 3 .

[0094] e. Select 20 / 40 mesh and 16 / 20 mesh quartz sand as proppant, adding them according to the pumping procedure, with a single stage quartz sand volume of 100-200 mg / L. 3 .

[0095] V-well drainage equipment in application examples:

[0096] In this application example, the V-well in the U-shaped well serves as the drainage well, and an electric submersible screw pump is used for drainage. A wide range of adjustable discharge rates is selected, with a maximum discharge rate of 100m³. 3 / d electric submersible screw pump.

[0097] The V-well wellhead device uses a Φ139.7mm gas production wellhead. The surface drainage equipment adopts an automated control platform, and data such as the operating conditions and status parameters of the downhole pump, bottom hole flowing pressure, casing pressure, water production, and gas production can be transmitted to the control cabinet and remote control terminal in real time, achieving the purpose of real-time monitoring and remote control adjustment.

[0098] Step 4: Use a combined surface and downhole microseismic monitoring method to characterize the hydraulic fracturing modification area.

[0099] A combined surface and downhole microseismic monitoring method is used to monitor microseismic events generated during hydraulic fracturing in real time. The microseismic events are located and the fractures are interpreted to infer the fracture length, height and orientation. The propagation law and spatial distribution of the fractures are analyzed to characterize the effective influence range of hydraulic fracturing.

[0100] The surface microseismic monitoring network for each fracturing section is divided into a 6×7 grid structure centered on the surface projection of the horizontal section of the U-shaped H-well. The station spacing is 80m, with a coverage range of 400m along the horizontal section trajectory and 480m along the radial direction of the horizontal section. Downhole, the microseismic monitoring network for each fracturing section is deployed in the bottom venting tunnels on both sides, with 9 microseismic geophones installed in each tunnel at 20m intervals. The horizontal projection of the fracturing section is located in the middle of the monitoring points. After each fracturing section is completed, a rolling, mobile geophone is used to monitor the microseismic activity of the next fracturing section.

[0101] Taking the microseismic monitoring results of a segment of the horizontal section of well U4 as an example, the raw microseismic monitoring data was converted into a standard data format. After noise reduction processing of microseismic events within the target time period, the arrival and departure times of each microseismic event at different monitoring stations were output. This completed the processing of the microseismic monitoring data, yielding the microseismic response characteristics of coal seam hydraulic fracturing, and thus explaining the hydraulic fracturing fracture propagation process. The microseismic monitoring system detected 39 microseismic signals. After interpretation and processing, it was found that the fractures induced by this segment of fracturing were mainly concentrated on both sides of the horizontal plane of the fracturing well. From the macroscopic morphology of the fractures inferred from the microseismic location points, the fracture network induced by the microseismic events was mainly distributed along the horizontal plane, extending to both sides from the wellbore trajectory direction of the fracturing section. Specific fracture parameters included a main fracture length of 274m, a secondary fracture length of 184m, a modification radius of 131m, and a modification volume of 45.6 × 10³ m³. 3 The fracture dips at 67°∠15°. A schematic diagram of the main fracture projection from microseismic monitoring of this hydraulic fracturing section is shown below. Figure 6 As shown in the diagram, the reservoir stimulation volume is obtained through microseismic monitoring. Figure 7 As shown.

[0102] Step 5: Use downhole transient electromagnetic method to detect the extent of fracturing fluid expansion in the formation after fracturing.

[0103] To detect the extent of fracturing fluid expansion in the formation after hydraulic fracturing, a downhole transient electromagnetic method was used for advance detection in this embodiment. The instruments used for this transient electromagnetic method detection were a mining transient electromagnetic instrument and a high-power transmitter, with a detection depth of approximately 100m. The mine transient electromagnetic method detection employed an overlapping loop combination device, consisting of a 1.5m side-length square transmitting and receiving coil, with 4 turns for the transmitting coil and 40 turns for the receiving coil. At least 30 superposition operations were used at each measuring point to improve the signal-to-noise ratio and ensure the reliability of the raw data.

[0104] Taking transient electromagnetic detection in the horizontal section of well U4 as an example, measuring points and lines were arranged in the bottom dredging tunnels R1, R2, and R3, with one measuring point every 40m. Each measuring point was designed with five detection directions: 90° elevation (vertically upward), 60° elevation, 30° elevation, 15° elevation, and directly in front of the tunnel wall (horizontal direction). See... Figure 2 Before and after the fracturing operation, a probe is taken and the low-resistivity anomaly characteristics and changes in the probe area before and after hydraulic fracturing are compared and analyzed to determine the spatial distribution of the water-rich anomaly zone after fracturing, thereby judging the extent of fracturing fluid propagation. Figure 8 The image shows a sector view of the apparent resistivity profile at a certain measuring point before hydraulic fracturing (elevation angle 15°). The red apparent resistivity inversion within the overall sector at an elevation angle of 15° is relatively high, indicating that the formation has weak water-bearing properties in this detection direction. Figure 9This is a fan-shaped profile (elevation angle 15°) of the apparent resistivity at this measuring point after hydraulic fracturing. Compared to before hydraulic fracturing, a significant low-resistivity anomaly zone appeared in the area ahead of the probe. Anomalies with small and widely distributed apparent resistivity values ​​generally indicate that fracturing has improved the internal connectivity of the rock strata and increased water content, suggesting that fracturing fluid entered this area during fracturing. Combining the results from the five probe directions at this point, the distribution range of water-rich areas in the rock strata can be fully depicted in three-dimensional space, providing a basis and guidance for the layout of cross-strata drilling in bottom-extraction tunnels.

[0105] Step 6: Based on the microseismic monitoring of crack development and the transient electromagnetic detection of formation water abundance, formulate a plan for the layout of cross-layer drilling holes in the bottom dredging tunnel.

[0106] Bottom-extraction rock tunnel cross-layer boreholes are divided into drainage holes and air extraction holes according to their main functions. The main function of drainage holes in the early stage is to drain water and reduce pressure, and in the later stage, they are connected to negative pressure pipelines for air extraction. The main function of air extraction holes is to extract air, and in the case of water in the initial coal seam, they also take on some drainage functions.

[0107] The layout of drainage and air extraction holes in the bottom venting tunnel is determined using the results of microseismic monitoring and transient electromagnetic detection, including the spacing between drilling sites, the number, length, and orientation of the holes. One drainage hole is placed in the bottom venting tunnel corresponding to each fracturing section. These drainage holes are located in the lower elevation of the bottom venting tunnel and are positioned near the furthest water-rich area within the influence range of each fracturing fracture. Air extraction holes are placed in areas unaffected by the fracturing fracture. The final depth of both drainage and air extraction holes is 5-10m above the coal seam roof, with a spacing of 10-20m between the bottoms of each hole, arranged in a fan shape.

[0108] In well H4 of Example 1, the drainage and extraction holes corresponding to the first stage of fracturing were prioritized for construction over other fracturing stages. Drilling was performed using an 89mm drill bit, with 40mm PVC screen pipes used as drainage and extraction pipes. The first stage of fracturing extends further and longer towards the R2 roadway, and the water-rich anomaly zone also extends further towards R2. Therefore, the drainage holes for the first stage of fracturing were located at drilling site Z21, where the drainage hole length is shorter. The extraction holes at Z21 have a shorter overall construction distance than those at drilling site Z11, and a greater number of extraction holes are required at Z11. The second stage of fracturing extends the main fracture further towards the R1 roadway, and the water-rich anomaly zone is closer to R1. The drainage holes for this stage were located at drilling site Z12, where the extraction holes have a shorter overall construction distance than those at drilling site Z22. Simultaneously, in the intermediate area unaffected by the first and second stage fracturing, additional drilling sites and extraction holes were deployed to ensure no dead zones in extraction. Drilling sites Z31 and Z32 are arranged in the bottom rock extraction tunnel R3 to pump out gas from the area unaffected by the vertical shaft fracturing of well H4. (See...) Figure 10 .

[0109] Step 7: Establish a gas extraction system for different stages of gas drainage in the joint surface and underground gas control to reduce the gas content of the coal seam.

[0110] Taking the gas extraction system of the U4 well group as an example, the U4 well group adopts a combined extraction method of surface electric submersible screw pumps and underground bottom drainage rock tunnel cross-layer boreholes, with well V4 serving as the surface drainage well. According to existing geological data, the original formation pressure corresponding to the coal seam in the coal mining planning area is 3.03 MPa. After hydraulic fracturing and blowout at the horizontal section of well H4, the bottom hole flowing pressure is 3.58 MPa until well V4 starts pumping. At this time, the bottom hole flowing pressure is greater than the original formation pressure. The rate of the surface electric submersible screw pump is adjusted to near its maximum, and the drainage valve is fully opened to its maximum. The bottom hole flowing pressure is reduced to about 3.03 MPa per day at a rate of 0.08 MPa / d. When the bottom pressure is below 3.03 MPa and the coal seam has not yet desorbed, the pumping rate should be appropriately reduced, and the valves of some drainage holes in the well should be adjusted according to the water production of the drainage holes. The daily pressure drop at the bottom of the well should be controlled between 0.04 MPa / d and 0.05 MPa / d until the coalbed methane is desorbed.

[0111] When the bottomhole flowing pressure reaches 1.62 MPa, coalbed methane begins to desorb; this pressure is the desorption pressure, and production enters the production ramp-up phase. As coalbed methane desorbs, the casing pressure increases, further reducing the pump's drainage rate and closing the downhole drainage valves in the earliest fracturing section. The bottomhole flowing pressure drop rate is controlled between 0.01 and 0.03 MPa / d. When the casing pressure rises to 0.5 MPa, the surface pipeline valves are opened to begin gas production. As production progresses, the gas production valves are gradually opened to slowly increase production. During this period, the casing pressure is controlled between 0.1 and 0.5 MPa and gradually reduced, with the maximum gas production reaching 6900 m³ / d. 3 / d; Maintain casing pressure not exceeding 0.15MPa to keep gas production stable.

[0112] During the stabilization phase, the drainage hole is gradually closed based on the water production situation, further reducing the pump's drainage rate. The bottom hole flowing pressure drop rate is controlled between 0-0.01 MPa / d to maintain the balance between bottom hole flowing pressure and casing pressure, thus maintaining the gas production of well H4 at 5800 m³ / d. 3 Approximately / d; and open the downhole vent, with the bottom hole pumping negative pressure at 15 kPa. Afterwards, maintain the liquid level and, based on the gas production of the V well and the downhole vent, adjust the gas production pipeline valves to maintain a stable gas production rate.

[0113] In existing technologies, gas production via surface U-shaped wells takes 3-6 months to reach gas production, with a production ramp-up time of approximately 1-1.5 years. In this invention, well U4 reached gas production in 46 days and ramped up production in 118 days, significantly shortening the drainage time and production ramp-up cycle, and increasing gas extraction efficiency. Currently, the well's gas production is continuous and stable.

[0114] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for rapid and efficient gas extraction in soft coal seams, characterized in that, Includes the following steps: Step 1: Determine the geological characteristics of the coal mining planning area; Step 2: Design a combined system of surface U-shaped wells and underground bottom drainage roadways for the coal mining planning area. Multiple surface U-shaped wells are deployed simultaneously. The horizontal section of a single U-shaped well is located near the central axis of the working face in the coal mining planning area. The bottom drainage roadway is located in the bottom rock strata of the coal seam between the working faces of the two coal mining planning areas along the trajectory of the horizontal wellhead and is connected by a through roadway. Step 3: Develop a gas control and extraction plan for the U-shaped wells on the ground; Step 4: The ground microseismic monitoring network for each fracturing section is arranged according to a specific grid structure with the ground projection of the horizontal section of the H well of the U-shaped well as the center. The underground microseismic monitoring network for each fracturing section is arranged in the bottom rock extraction tunnels on both sides to monitor and interpret fracturing microseismic events in real time to determine the crack propagation law and spatial distribution pattern. Step 5: Deploy transient electromagnetic detection points in the bottom pumping tunnel corresponding to each fracturing section, use downhole transient electromagnetic method to detect the fracturing fluid expansion range, compare the low resistivity anomaly characteristics and change features before and after fracturing, and determine the formation water-rich anomaly area. Step 6: Based on the microseismic monitoring of fracture development and the transient electromagnetic detection of formation water abundance, formulate a plan for the layout of cross-layer drilling holes in the bottom dredging tunnel. Step 7: Combine the layout of the cross-layer drilling in the bottom rock extraction tunnel in the mine, and formulate the extraction system by adjusting the relationship between the bottom flow pressure and the original formation pressure, desorption pressure and casing pressure, so as to extract coal seam gas quickly and efficiently.

2. The method for rapid and efficient gas extraction in soft coal seams according to claim 1, characterized in that, In step 2, the U-shaped well location design avoids areas with faults and collapse columns. The horizontal section of the H-shaped well intersects the direction of the main fracture at a certain angle. The V-shaped well is located in the downdip direction of the coal seam. The horizontal section of the H-shaped well is 0-2m away from the top of the coal seam, and the length of the horizontal section is 800-1200m, located near the central axis of the planned coal mining face. The bottom drainage roadway is 15-25m away from the bottom of the coal seam, depending on the lithology and structure of the strata. The bottom drainage roadway is connected by a through roadway, and the through roadway is equipped with bottom through-layer boreholes to extract gas from the coal seam near the vertical shaft end of the H-shaped well.

3. The method for rapid and efficient gas extraction in soft coal seams according to claim 2, characterized in that, In step 3, well H adopts a three-section wellbore structure. Well H uses a cable-pumped bridge plug + perforation combined operation process to carry out segmented perforation and segmented fracturing construction on the horizontal section. The perforation azimuth is determined according to the specific wellbore trajectory. The segmented fracturing parameters and pumping construction procedures are determined according to specific geological conditions. Well V adopts a two-section or three-section wellbore structure according to the formation conditions. Well V is used as a drainage well and uses an electric submersible screw pump for drainage.

4. The method for rapid and efficient gas extraction in soft coal seams according to claim 3, characterized in that, In step 3, during the segmented perforation and fracturing construction of the horizontal section of well H, each segment is divided into sections of 70-80m in length, with the specific length adjusted according to the coal or rock conditions of the wellbore trajectory and the development of faults; the perforation location avoids the casing coupling location, and different perforation schemes are adopted according to the actual wellbore trajectory; the fracturing parameters and pumping construction procedures for each segment are determined according to specific geological conditions.

5. The method for rapid and efficient gas extraction in soft coal seams according to claim 4, characterized in that, In step 4, the microseismic monitoring network for each fracturing section on the surface is divided into a 6×7 grid structure centered on the horizontal projection of the H-shaped well. The station spacing is 80m, the coverage range along the trajectory of the horizontal section is 400m, and the radial coverage range along the horizontal section is 480m. The microseismic monitoring network for each fracturing section underground is equipped with 9 microseismic geophones in each bottom dredging tunnel, arranged at 20m intervals. The horizontal projection of the fracturing section is located in the middle of the detection points. After each fracturing section is completed, the rolling mobile geophone is used to carry out microseismic monitoring of the next fracturing section.

6. The method for rapid and efficient gas extraction in soft coal seams according to claim 5, characterized in that, In step 5, during the downhole transient electromagnetic method detection, based on the detection distance of the transient electromagnetic instrument and the number of horizontal fracturing sections, two transient electromagnetic detection points are arranged in the bottom rock extraction tunnel corresponding to each fracturing section location, with the same spacing between adjacent fracturing section detection points; each detection point is designed with five detection directions, namely 90° elevation (vertically upward), 60° elevation, 30° elevation, 15° elevation, and directly in front of the tunnel wall (horizontal direction).

7. The method for rapid and efficient gas extraction in soft coal seams according to claim 6, characterized in that, In step 6, the drainage holes of the bottom-extraction rock tunnel cross-layer drilling are initially used for drainage and pressure reduction, and later connected to negative pressure pipelines for air extraction; the air extraction holes of the bottom-extraction rock tunnel cross-layer drilling are mainly used for air extraction, and also serve a partial drainage function when there is water in the initial coal seam; one drainage hole is arranged in the bottom-extraction rock tunnel corresponding to each fracturing section, and the drainage hole is located in the bottom-extraction rock tunnel at a low altitude and near the farthest water-rich area within the influence range of each fracturing fracture; after drilling, the bottom-extraction rock tunnel cross-layer drilling is subjected to high-pressure flushing, and the addition of sodium lignosulfonate as a coal powder dispersant is determined according to the coal slag situation and the flushing and slag removal capacity of the clean water medium. The drainage holes must be flushed under high pressure, and the air extraction holes are flushed under high pressure depending on the slag and water situation during drilling; after the bottom-extraction rock tunnel cross-layer drilling is completed, PVC screen pipes are used as drainage and air extraction pipelines and the wellhead is sealed; the drainage holes corresponding to each fracturing section are constructed first, after the fracturing operation and venting are completed and the formation pressure is stable, and the drainage holes corresponding to the first fracturing section are constructed first.

8. The method for rapid and efficient gas extraction in soft coal seams according to claim 7, characterized in that, In step 7, the specific sampling procedures for different stages of the sampling process are as follows: Drainage stage: When pumping begins, if the bottom hole pressure is greater than the original formation pressure, adjust the surface electric submersible screw pump speed to near the maximum and open the drainage valves to the maximum. When the bottom hole pressure is lower than the original formation pressure but greater than the desorption pressure, reduce the pumping rate and reduce the number of drainage valves in the well. Control the bottom hole pressure drop rate to 0.03-0.09 MPa / d and ensure that the daily pressure drop fluctuation does not exceed 20% of the previous day's pressure drop. Production ramp-up phase: When the bottom hole pressure is less than the desorption pressure, further reduce the pump discharge rate and close the drainage holes at the high-altitude fracturing sections in the horizontal formation. Control the bottom hole pressure drop rate at 0.01-0.03 MPa / d. As coalbed methane desorbs and casing pressure increases, open the surface pipeline valves to start gas production and gradually increase the gas production valves to ramp up production. Control the casing pressure at 0.1 MPa-0.5 MPa, maintain a slow decrease in bottom hole pressure, and control large fluctuations in casing pressure to avoid gas leakage from the discharge pump. Stable production phase: After the gas production stabilizes, the drainage hole is gradually closed according to the water production, and the pumping rate is further reduced. The pressure drop rate at the bottom of the well is controlled at 0-0.01MPa / d to maintain the balance between the bottom of the well and the casing pressure, ensuring that the dynamic fluid level is 10 meters higher than the pumping pump. The downhole gas extraction hole is opened, and the negative pressure is 5-20KPa. The negative pressure gradually increases with the extraction time, and the gas extraction hole valve is adjusted according to the gas concentration and gas purity. Attenuation stage: Maintain a stable relationship between bottom hole flowing pressure and casing pressure by adjusting pump speed and gas production valves, so that gas production gradually decreases; increase downhole extraction intensity, fully open the gas extraction port and drainage port valves to the maximum, increase the extraction negative pressure to 20-40 kPa, and adjust the gas extraction port valves according to gas concentration and gas purity.

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