Method for rapidly and efficiently extracting coal mine gas in broken and soft coal seam

Abstract

The invention discloses a method for rapidly and efficiently extracting coal mine gas in a broken and soft coal seam, and belongs to the technical field of coal seam gas development and coal mine gas prevention and control. According to the method, geological features of a coal mining planning area are determined, a ground U-shaped well and underground bottom-pumped rock drift layer-crossing drilling combined treatment system is deployed, and regional large-scale treatment is achieved in a well factory mode; the fracturing transformation range and the fracturing fluid distribution characteristics are detected by using a ground and underground combined micro-seismic monitoring and underground transient electromagnetic method, and the arrangement of layer-crossing drill holes is optimized; dividing the cross-layer drill holes into drainage holes and air exhaust holes, and formulating a collaborative extraction system in combination with different drainage and extraction stages; the method solves the problems that broken soft coal seam gas extraction is low in efficiency and long in period, and treatment blind areas exist, treatment time is remarkably shortened, cost is reduced, and safe and efficient production of a coal mine is guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of coalbed methane development and coal mine gas prevention and control, and particularly relates to a method for rapid and efficient extraction of coal mine gas from a crushed soft coal seam. Background Art

[0002] Gas control is a prerequisite for safe and efficient production in coal mines. In my country's broken and soft coal seams, due to the low mechanical strength of coal rock, poor permeability, high gas pressure, easy collapse and difficulty in drilling along the layer, the poor gas extraction effect has always been a technical bottleneck restricting gas prevention and control in this type of coal mines.

[0003] At present, my country's gas control mainly adopts surface coalbed methane development, underground gas extraction, and combined underground and underground extraction. Compared with the drilling method for gas control of primary structural coal within the coal seam, surface coalbed methane development in crushed and soft coal seams is faced with problems such as hole collapse, difficulty in propagating hydraulic fracturing cracks within the coal seam, powder drainage, and pump blockage. Underground coal seam gas extraction is also faced with problems such as stuck drill, hole collapse, inability to drill long drills, low porosity, and low extraction efficiency. To address the technical difficulties of gas control in crushed and soft coal seams, surface coalbed methane development currently generally adopts the method of drilling along the coal seam roof, such as roof L-shaped wells and roof U-shaped wells. Underground gas control generally adopts cross-layer drilling and extraction in the top and bottom rock lanes. However, L-shaped and U-shaped roof wells typically use large-scale hydraulic fracturing, which results in high drainage volumes and a long time to achieve gas production and stable production. Furthermore, the production cycle of coalbed methane wells is typically over 10-15 years. Furthermore, the use of staged fracturing for production stimulation creates blind and dead zones in areas not accessible by fracturing, making it impossible to achieve full-area gas extraction standards. Drilling through the roof and floor of coal mines, due to its small extraction radius and coverage area, requires intensive drilling, a large workload, and high labor intensity. Furthermore, the pre-extraction effect is short-lived, resulting in poor coalbed methane extraction results.

[0004] With the advancement of mechanization, intelligence, and automation in coal mining, high-cutting heights and long fully mechanized faces are becoming increasingly common. While this has accelerated coal production, it also requires faster and more efficient gas control to meet the demand for continued coal mining capacity. Gas control in high-gas-content, crushed, and soft coal seams is particularly challenging, requiring more appropriate and efficient extraction methods. Summary of the Invention

[0005] The present invention provides a method for quickly and efficiently extracting coal mine gas from crushed soft coal seams, which adopts a combined above-ground and underground extraction method. A surface U-shaped well is used above-ground, and bottom extraction rock tunnel through-layer drilling is used underground to improve the extraction effect of crushed soft coal seams.

[0006] To solve the above problems, the present invention provides the following technical solutions:

[0007] The embodiment of the present invention provides a method for quickly and efficiently extracting coal mine gas from a crushed soft coal seam, comprising the following steps:

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

[0009] Step 2: Design a combined surface U-shaped well and underground bottom pumping tunnel drilling system in the planned mining area. Multiple surface U-shaped wells are deployed simultaneously as a whole, with the horizontal section of a single U-shaped well positioned near the central axis of the working face in the planned mining area. The bottom pumping tunnel is arranged along the horizontal wellbore trajectory in the floor rock layer between the two working faces in the planned mining area and connected by a through tunnel.

[0010] Step 3: Develop a surface U-shaped well gas control and extraction plan;

[0011] Step 4: The surface microseismic monitoring network for each fracturing stage is arranged in 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 stage is arranged in the bottom pumping lanes on both sides. The fracturing microseismic events are monitored and interpreted in real time to determine the fracture propagation pattern and spatial distribution morphology.

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

[0013] Step 6: Based on the microseismic monitoring of fracture development and the transient electromagnetic detection of stratum water abundance, a plan for the arrangement of through-layer drilling holes for bottom pumping tunnels is formulated;

[0014] Step 7: Combine the underground bottom pumping rock tunnel through-layer drilling arrangement plan and formulate an extraction system by adjusting the relationship between the bottom hole flow pressure and the original formation pressure, desorption pressure and casing pressure to quickly and efficiently extract coal seam gas.

[0015] Preferably, in step 2, the U-shaped well location is designed to avoid the structural development area of ​​faults and collapse columns, the direction of the horizontal section wellbore trajectory of the H well of the U-shaped well intersects with the direction of the main fracture at a certain angle, the V well of the U-shaped well is arranged in the downdip direction of the coal seam, the horizontal section wellbore trajectory of the H well is 0-2m away from the coal seam roof, the horizontal section length is 800-1200m and is located near the central axis of the planned coal mining face; the bottom rock pumping tunnel is 15-25m away from the coal seam floor, which is specifically determined according to the lithology and structure of the formation. The bottom rock pumping tunnel is connected by a through tunnel, and the through tunnel is arranged with bottom plate through-layer drilling holes to extract gas from the coal seam near the end of the H well shaft.

[0016] Preferably, in step 3, well H adopts a three-wellbore structure, and well H adopts a cable pumped bridge plug + perforation combined process to perform staged perforation and staged fracturing on the horizontal section. The perforation orientation is determined according to the specific wellbore trajectory, and the staged fracturing parameters and pumping operation procedures are determined according to specific geological conditions. Well V adopts a two-wellbore or three-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 staged perforation and staged fracturing construction of the horizontal section of Well H, each section is divided into 70-80 m long sections, which are specifically adjusted according to the coal or rock penetration of the wellbore trajectory and the development of faults; the perforation positions avoid the casing coupling positions, and different perforation schemes are adopted according to the actual wellbore trajectory; the fracturing parameters and pumping construction procedures of each section 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 with the ground projection of the horizontal section of the U-shaped well H well as the center, the station track spacing is 80m, the coverage range along the horizontal section trajectory direction is 400m, and the radial coverage range along the horizontal section is 480m; the underground microseismic monitoring network for each fracturing section arranges 9 microseismic detectors in each bottom pumping rock tunnel, arranged at a spacing of 20m, and the horizontal projection of the fracturing section is located in the middle of the detection point. After each fracturing section is completed, the rolling mobile detector implements microseismic monitoring of the next fracturing section.

[0019] Preferably, in step 5, in the downhole transient electromagnetic detection, according to 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 pumping rock tunnel corresponding to each fracturing section position, and the distance between adjacent fracturing section detection points is the same; five detection directions are designed for each detection point, namely, an elevation angle of 90° (vertical upward), an elevation angle of 60°, an elevation angle of 30°, an elevation angle of 15°, and the front of the tunnel wall (horizontal direction).

[0020] Preferably, in step 6, the drainage holes of the bottom pumping rock tunnel through-layer drilling holes are used for drainage and pressure reduction in the early stage, and are connected to negative pressure pipelines for air extraction in the later stage; the air extraction holes of the bottom pumping rock tunnel through-layer drilling holes are mainly used for air extraction, and also take into account partial drainage function when there is water in the initial coal seam; one drainage hole is arranged in the bottom pumping rock tunnel corresponding to each fracturing section, and the drainage hole is arranged in the bottom pumping rock tunnel with a low altitude and is located near the farthest water-rich area within the influence range of each fracturing fracture; after drilling, the bottom pumping rock tunnel through-layer drilling holes are subjected to high-pressure punching, and whether sodium lignosulfonate is added as a coal powder dispersant is determined according to the coal slag discharge situation and the punching and slag removal capacity of the clean water medium. The drainage holes must be punched with high pressure, and whether the air extraction holes are punched with high pressure is determined according to the slag discharge and water discharge during drilling; after the bottom pumping rock tunnel through-layer drilling holes are 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 before the air extraction holes after the fracturing construction is completed and the formation pressure is stabilized, and the drainage holes corresponding to the first fracturing section are constructed first.

[0021] Preferably, in step 7, the extraction system at different extraction stages is as follows:

[0022] Drainage stage: When pumping, if the bottomhole pressure is greater than the original formation pressure, adjust the surface electric submersible screw pump speed to near the maximum, and at the same time, fully open the drainage hole valve to the maximum; when the bottomhole pressure is lower than the original formation pressure and greater than the desorption pressure, reduce the drainage rate, adjust some downhole drainage hole valves, and control the bottomhole pressure drop rate to 0.03-0.09MPa / d, and control the pressure drop fluctuation range of the day to no more than 20% of the pressure drop of the previous day;

[0023] Production increase stage: When the bottomhole flow pressure is lower than the desorption pressure, the pump drainage rate is further reduced, and the drainage holes at the fracturing section with high altitude in the horizontal section are closed. The bottomhole flow pressure drop rate is controlled at 0.01-0.03MPa / d. As the coalbed methane desorbs and the casing pressure increases, the surface pipeline valve is opened to start gas production and the gas production valve is gradually opened to increase production. The casing pressure is controlled at 0.1MPa-0.5MPa, and the bottomhole flow pressure is maintained to decrease slowly. The casing pressure is controlled to avoid large fluctuations in the drainage pump to avoid gas cross-contamination.

[0024] Stable production stage: After the gas production stabilizes, the drainage holes are slowly closed according to the water production, and the pump drainage rate is further reduced. The bottomhole flow pressure drop rate is controlled at 0-0.01MPa / d, and the balance between the bottomhole flow pressure and the casing pressure is maintained. The dynamic liquid level is ensured to be 10 meters higher than the drainage pump. The downhole gas extraction holes are opened, and the drainage negative pressure is 5-20KPa. The negative pressure gradually increases with the drainage time, and the gas extraction hole valve is adjusted according to the gas concentration and gas pure amount.

[0025] Attenuation stage: By adjusting the pump speed and gas production valve to maintain a stable relationship between bottom hole flow pressure and casing pressure, the gas production will slowly decay; increase the downhole extraction intensity, open the extraction hole and drainage hole valves to the maximum, increase the extraction negative pressure to 20-40KPa, and adjust the extraction hole valve according to the gas concentration and gas purity.

[0026] Compared with the prior art, the embodiment of the present invention provides a method for rapid and efficient extraction of coal mine gas from crushed soft coal seams, which has the following beneficial effects: a gas control method of deploying multiple surface U-shaped wells and underground bottom pumping rock tunnel through-layer holes in the overall coal mining planning area forms a well factory and regional gas scale control effect, which can reduce the number of surface well sites and bottom pumping rock tunnel excavation wells, and at the same time, multi-well regional drainage is more likely to form inter-well interference, which is conducive to gas desorption and gas production; the combined extraction method of surface V-wells and underground through-layer holes solves the problems of long gas appearance time and long drainage cycle; the bottom pumping rock tunnel through-layer drilling arrangement scheme uses microseismic monitoring of crack development and transient electromagnetic detection of stratum water-rich results, which can effectively solve the gas control in dead corners and areas not affected by fracturing, achieve full regional coverage, and solve the problems of blind drilling, dense drilling, and large engineering workload of through-layer holes; formulating an extraction system for different drainage stages of ground and underground joint gas control can quickly and efficiently extract coal seam gas. The present invention can significantly shorten the gas control time of coal mining working faces, reduce the gas control cost per ton of coal, improve extraction efficiency, meet the requirements of intelligent fully mechanized mining technology, and provide strong guarantee for safe production in coal mines. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 Schematic diagram of the surface U-shaped well and underground bottom pumping rock tunnel through-layer drilling extraction mode provided in an embodiment of the present invention.

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

[0030] Figure 3 A flow chart of a method for efficiently extracting coal mine gas from crushed soft coal seams provided in an embodiment of the present invention.

[0031] Figure 4 A schematic diagram of the surface U-shaped well location and bottom pumping tunnel deployment provided in an embodiment of the present invention.

[0032] Figure 5 A schematic diagram of the U-shaped wellbore structure provided in an embodiment of the present invention.

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

[0034] Figure 7A schematic diagram of reservoir reconstruction volume for microseismic monitoring of a horizontal section of a U-shaped well provided by an embodiment of the present invention.

[0035] Figure 8 This is a fan-shaped diagram of the apparent resistivity profile of a certain measuring point before U-shaped hydraulic fracturing provided by an embodiment of the present invention (elevation angle 15°).

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

[0037] Figure 10 Schematic diagram of the arrangement of drainage holes and air extraction holes in a U-shaped rock pumping tunnel at the bottom of a well provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application. The "upper", "lower", "front", "rear", "left", "right", etc. used in the installation position or direction of the structure or parts of this embodiment are based on the orientation of the given drawings. They are only for the convenience of expression to distinguish the relative positions of the various components or directions, and do not represent the orientation of the system or functional components of this embodiment when in use.

[0039] The embodiment of the present invention provides a method for quickly and efficiently extracting coal mine gas from a crushed soft coal seam, 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, ground stress and other geological characteristics. Use geophysical exploration and geological drilling methods combined with mine geological surveys to clarify the stress distribution, principal stress orientation, fault and collapse column development, coal seam inclination, coal body structure and coal seam fracture system development characteristics in the coal mining planning area.

[0041] Step 2: Design a combined system of surface U-shaped wells and underground bottom pumping rock tunnels in the planned coal mining area. The surface U-shaped well group uses multiple U-shaped wells to be deployed as a whole at the same time to form a well factory and regional gas scale control effect. The horizontal section of a single U-shaped well is arranged near the central axis of the working face in the planned coal mining area, and the bottom pumping rock tunnel is arranged in the bottom rock layer of the coal seam between the two planned coal mining working faces along the horizontal wellbore trajectory and is connected by a through tunnel.

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

[0043] Step 4: A combined surface and underground microseismic monitoring method is used to characterize the hydraulic fracturing stimulation range. The surface microseismic monitoring network for each fracturing segment is arranged according to a specific grid structure with the ground projection of the horizontal section of the U-shaped well H well as the center. The underground microseismic monitoring network for each fracturing segment is arranged in the bottom pumping tunnels on both sides. The fracturing microseismic events are monitored and interpreted in real time to determine the crack propagation pattern and spatial distribution morphology.

[0044] Step 5: Use downhole transient electromagnetic method to detect the extended distribution range of fracturing fluid in the formation after fracturing. Arrange transient electromagnetic detection points in the bottom pumping rock tunnel corresponding to each fracturing section. Design multiple detection directions for each detection point. Detection is carried out before and after fracturing. Compare and analyze the low-resistance anomaly characteristics and change characteristics to determine the spatial distribution of water-rich anomaly areas, so as to judge the extended distribution range of fracturing fluid and determine the water-rich anomaly areas in the formation.

[0045] Step 6: Develop a plan for the layout of through-layer drilling holes in the bottom pumping tunnel, combining the microseismic monitoring of fracture development and the water-rich formation detected by transient electromagnetic detection. In this embodiment, the through-layer drilling holes in the bottom pumping tunnel are divided into drainage holes and air extraction holes. The drainage holes are arranged near the farthest water-rich area within the influence range of each hydraulic fracture and in the bottom pumping tunnel at a low altitude. The air extraction holes are arranged in areas unaffected by the hydraulic fractures. The drilling depth is 5-10 meters through the coal seam roof, and the bottom holes are spaced 10-20 meters apart in a fan-shaped pattern.

[0046] Step 7: Combined with the underground bottom pumping rock tunnel drilling layout plan, a drainage system is developed by adjusting the relationship between bottomhole flow pressure and the original formation pressure, desorption pressure, and casing pressure to achieve rapid and efficient coalbed methane extraction. This embodiment develops a drainage system for combined surface and underground management at different drainage stages. The system adjusts the surface submersible screw pump speed, underground drainage and extraction hole valves, and extraction negative pressure parameters during the drainage, production increase, production stabilization, and production decline stages to achieve rapid and efficient coalbed methane extraction.

[0047] Specifically, in step 2, the U-shaped well location is designed to avoid the structural development area of ​​faults and collapse columns. The direction of the horizontal section of the H-well of the U-shaped well intersects with the direction of the main fracture at a certain angle. The V-well of the U-shaped well is arranged in the downdip direction of the coal seam. The horizontal section of the H-well has a well trajectory of 0-2m away from the top of the coal seam, a horizontal section length of 800-1200m and is located near the central axis of the planned coal mining face; the bottom rock extraction tunnel is 15-25m away from the bottom of the coal seam, which is determined according to the lithology and structure of the formation. The bottom rock extraction tunnel is connected by a through tunnel, and the through tunnel is arranged with bottom plate through-layer drilling holes to extract gas from the coal seam near the vertical shaft end of the H-well of the U-shaped well.

[0048] refer to Figure 1 In this embodiment, gas control is carried out using surface U-shaped wells and underground bottom-drainage tunnels for cross-layer extraction. Based on the geological characteristics of the planned coal mining area, the U-shaped wells and bottom-drainage tunnels are deployed in a combined manner at appropriate locations. The surface U-shaped well group utilizes multiple U-shaped wells deployed and constructed simultaneously, creating a well factory and regional gas control system. This reduces the surface drilling footprint and costs, while also minimizing the impact on adjacent wells during hydraulic fracturing operations. Multi-well area drainage reduces inter-well interference and facilitates gas desorption and production. The U-shaped wells are designed to avoid structural development such as faults and collapse columns. 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 downdip from the coal seam to facilitate subsequent drainage and gas recovery. The horizontal section of the U-shaped well H is located at a distance h1 from the coal seam roof, with h1 ranging from 0 to 2 meters. 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 located in the floor rock stratum between the two planned coal seams along the horizontal wellbore trajectory. One bottom extraction tunnel can be used to arrange floor-penetrating gas extraction holes (drainage holes and gas extraction holes) for the two adjacent coal mining faces, reducing the number of bottom extraction tunnels to be excavated. Bottom extraction tunnels R1 and R2 are connected by bottom extraction tunnel R3, which is equipped with floor-penetrating holes to extract gas from the coal seam near the vertical shaft end of the U-shaped well H shaft. Drilling sites Z1i, Z2i, and Z3i are respectively set up in bottom extraction tunnels R1, R2, and R3 for arranging the penetration holes. The bottom extraction tunnel is located at a distance of 15-25m from the coal seam floor, h2, depending on the lithology and structure of the formation.

[0050] In step 3, during the perforation and fracturing of the horizontal section of the U-shaped well H well, each section is divided into 70-80m long sections, which are adjusted according to the coal or rock penetration and fault development of the wellbore trajectory; three clusters of perforations are perforated in each section, each cluster is perforated 1.5m, and the cumulative perforation per section is 4.5m. The perforation density is 15 holes / m, and the perforation position avoids the casing coupling position. Different perforation schemes are adopted according to the actual wellbore trajectory: vertical downward perforation is adopted when the horizontal wellbore trajectory is more than 2m away from the coal seam in the roof rock layer; oblique downward and vertical downward perforation is adopted when the distance from the coal seam is 0-2m; spiral perforation is adopted in the coal seam; high-volume and high-sand ratio injection is adopted for fracturing, and the construction displacement is 12-24m 3 / min, sand ratio 10%-20%, active water is used for fracturing fluid, the ratio is water, 1.0% KCl and 0.05% fungicide, drag reducer can be added in appropriate amount according to the fracturing construction pressure, single stage liquid volume 1200-2000m 3 The proppant is 20 / 40 mesh and 16 / 20 mesh quartz sand, and the amount of quartz sand in a single section is 100-200m 3 .

[0051] In step 4, the surface microseismic monitoring network for each fracturing segment is divided into a 6×7 grid structure centered on the ground projection of the horizontal segment of the U-shaped well H well, with station spacing of 80m, a coverage range of 400m along the horizontal segment trajectory, and a radial coverage range of 480m along the horizontal segment. The underground microseismic monitoring network for each fracturing segment deploys 9 microseismic detectors in each bottom pumping lane, arranged at 20m intervals, with the horizontal projection of the fracturing segment located in the middle of the detection point. After each fracturing segment is completed, a rolling mobile geophone is used to conduct microseismic monitoring of the next fracturing segment. This embodiment uses a combined surface and underground microseismic monitoring method to monitor microseismic events generated during hydraulic fracturing in real time, locate the fracturing microseismic events, interpret the fractures, infer the fracture length, fracture height, and orientation, analyze the fracture expansion pattern and spatial distribution morphology, and characterize the effective impact range of hydraulic fracturing.

[0052] In step 5, during the downhole transient electromagnetic detection, two transient electromagnetic detection points were arranged in the bottom pumping rock tunnel corresponding to each fracturing section according to the detection distance of the transient electromagnetic instrument and the number of horizontal staged fracturing sections, and the distance between adjacent fracturing section detection points was the same; five detection directions were designed for each detection point, namely, an elevation angle of 90° (vertical upward), an elevation angle of 60°, an elevation angle of 30°, an elevation angle of 15°, and the front of the tunnel wall (horizontal direction); a mining transient electromagnetic instrument and a high-power transmitter were used, the detection depth was about 100m, and an overlapping loop combination device was used. The transmitting and receiving square wire frames were 1.5m in length, the transmitting coil had 4 turns, and the receiving coil had 40 turns. At least 30 superpositions were performed at each measuring point to improve the signal-to-noise ratio.

[0053] refer to Figure 2In this embodiment, based on the transient electromagnetic instrument's detection range and the number of horizontally segmented fracturing stages, two transient electromagnetic detection points are placed in the bottom pumping tunnel corresponding to each fracturing stage, with the spacing between adjacent transient electromagnetic detection points being the same. Each detection point is designed with five detection directions: 90° elevation (upward), 60° elevation, 30° elevation, 15° elevation, and directly in front of the tunnel wall (horizontally). Detection is performed before and after the fracturing operation, and the low-resistance anomaly characteristics and changes within the detection range before and after hydraulic fracturing are compared and analyzed to determine the spatial distribution of the water-rich anomaly area after fracturing, thereby determining the extended distribution range of the fracturing fluid.

[0054] In step 6, the drainage holes of the through-layer drilling holes of the bottom pumping rock tunnel are used for drainage and pressure reduction in the early stage, and are connected to negative pressure pipelines for air extraction in the later stage; the air extraction holes of the through-layer drilling holes of the bottom pumping rock tunnel are mainly used for air extraction, and also take into account some drainage functions when there is water in the initial coal seam; one drainage hole is arranged in the bottom pumping rock tunnel corresponding to each fracturing section, and the drainage holes are arranged in the bottom pumping rock tunnel with a low altitude and are located near the farthest water-rich area within the influence range of each fracturing fracture; after drilling, the through-layer drilling holes of the bottom pumping rock tunnel are subjected to high-pressure punching, and whether sodium lignosulfonate is added as a coal powder dispersant is determined based on the coal slag discharge situation and the punching and slag removal capacity of the clean water medium. The drainage holes must be punched, and the air extraction holes are determined based on the drilling slag and water discharge situation. After punching, a PVC screen pipe is placed as a drainage and air extraction pipeline and the hole is sealed; the drainage holes corresponding to each fracturing section are constructed before the air extraction holes after the fracturing construction is 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 gas extraction holes in the bottom pumping tunnel, including the spacing between drill sites, the number, length, and orientation of drill holes. One drainage hole is arranged in the bottom pumping tunnel corresponding to each fracturing section. The drainage holes are arranged in the bottom pumping tunnel at a low altitude, near the farthest water-rich area within the influence range of each fracturing fracture; the gas extraction holes are arranged in areas not affected by the fracturing fractures. The final depth of the drainage holes and gas extraction holes penetrates the coal seam roof and rises 5-10 meters. The spacing between the bottoms of the holes is between 10-20 meters, and they are arranged in a fan-shaped pattern.

[0056] After drilling, the through-layer drill holes for the bottom rock tunnels are punched with high pressure to flush out the soft coal in and around the holes. The addition of a pulverized coal dispersant (sodium lignosulfonate) is determined based on the amount of coal slag released from the drill holes and the ability of the clean water medium to flush out the slag. Drain holes must be punched, and the need for high pressure punching of the exhaust holes is determined based on the amount of slag and water released during drilling. After punching, PVC screen pipes are installed as drainage and exhaust lines, and the holes are sealed to ensure a tight seal.

[0057] In order to further shorten the overall drainage time, the drainage holes corresponding to each fracturing section are constructed before the pumping holes after the fracturing construction 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 of surface electric submersible screw pumps and underground bottom pumping tunnel perforations is used. The extraction system is formulated by adjusting the relationship between bottomhole flow pressure and original formation pressure, desorption pressure, and casing pressure. In step 7, the extraction system for different drainage stages is as follows:

[0059] During the drainage phase, if the bottomhole pressure is greater than the original formation pressure, the surface electric submersible screw pump speed is adjusted to near maximum, and the drain valve is fully opened. When the bottomhole pressure is lower than the original formation pressure but greater than the desorption pressure, the drainage rate is reduced, and the valves of some downhole drain holes are adjusted to reduce the bottomhole pressure drop rate to 0.03-0.09 MPa / d, with the daily pressure drop fluctuation range being controlled to no more than 20% of the previous day's pressure drop. The drainage holes in the bottom pumping tunnel in this embodiment can accelerate water drainage and shorten the drainage period, providing a rapid function. Simultaneously, the vent holes assist in expediting gas extraction.

[0060] Production increase stage: When the bottomhole flow pressure is lower than the desorption pressure, the pump drainage rate is further reduced, and the drainage holes at the fracturing section with high altitude in the horizontal formation are closed. The bottomhole flow pressure drop rate is controlled at 0.01-0.03MPa / d; as the coalbed methane is desorbed and the casing pressure increases, the ground pipeline valve is opened to start gas production and the gas production valve is gradually opened to increase production. The casing pressure is controlled at 0.1MPa-0.5MPa, and the bottomhole flow pressure is maintained to decrease slowly. The casing pressure is controlled to fluctuate greatly to avoid gas cross-contamination in the drainage pump.

[0061] Stable production stage: After the gas production is stable, slowly close the drainage hole according to the water production, further reduce the pump's drainage rate, control the bottom hole flow pressure drop rate at 0-0.01MPa / d, maintain the balance between the bottom hole flow pressure and the casing pressure, and ensure that the dynamic liquid level is 10 meters higher than the drainage pump; open the downhole suction hole, the suction negative pressure is 5-20KPa, and the negative pressure slowly increases with the drainage time, and adjust the suction hole valve according to the gas concentration and gas pure amount.

[0062] Attenuation stage: By adjusting the pump speed and gas production valve to maintain a stable relationship between bottom hole flow pressure and casing pressure, the gas production will slowly decay; increase the downhole extraction intensity, open the extraction hole and drainage hole valves to the maximum, increase the extraction negative pressure to 20-40KPa, and adjust the extraction hole valve according to the gas concentration and gas purity.

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

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

[0065] Key point 2: Combined surface and underground microseismic monitoring and downhole transient electromagnetic method are used to detect the impact range of fracturing cracks and the distribution of formation fluids. Based on this, a bottom rock tunnel through-layer drilling plan is formulated. This targeted treatment of gas in areas that cannot be extracted by horizontal wells is carried out, while avoiding blind and dense drilling of through-layer holes, reducing the amount of engineering work.

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

[0067] Key point 4: In view of the entire life cycle of coal seam gas control and extraction, an extraction system has been formulated for different drainage stages of joint ground and underground control to make the drainage and gas production process more scientific, reasonable and efficient.

[0068] Application Examples

[0069] refer to Figure 3 The embodiment of the present invention provides a method for quickly and efficiently extracting coal mine gas from a crushed soft coal seam, comprising the following steps:

[0070] Step 1: Determine the geological characteristics of the coal mining planning area, such as strata, structure, and coal seam fissures.

[0071] By using geophysical prospecting, geological drilling and other means, combined with geological surveys of other coal mining faces in the coal mining area, the distribution of ground stress and the orientation of principal stress in the coal mining area are determined; the development of structures such as faults and sinkholes in the coal mining planning area is clarified; and the development characteristics of coal seam inclination, coal body structure and coal seam fracture system in the coal mining planning area are determined.

[0072] This embodiment's design was developed in a coal mining area with a rich supply of crushed soft coal. Existing geological data revealed the primary geological characteristics of the coal seams in the planned mining area. The strata in this area have a dip of 5-10°, a coal thickness of approximately 4 meters, and a burial depth of approximately 660 meters. The coal structure is fragmented, with a well-developed crushed soft coal seam, small faults, and no collapse columns. The maximum principal stress direction in this mining area is NE, and the coal seam strikes NEE.

[0073] Step 2: Design the location of the U-shaped wells and bottom pumping tunnels for gas control in the planned coal mining area.

[0074] This method utilizes surface U-shaped wells and underground rock extraction tunnels for gas management. Based on the geological characteristics of the planned mining area, the U-shaped wells and rock extraction tunnels are deployed in a combined manner at appropriate locations. The surface U-shaped well group utilizes multiple U-shaped wells deployed simultaneously, creating a "well factory" and achieving large-scale regional gas management.

[0075] Combined with the stratigraphic characteristics of the coal mining planning area in step 1, two coal mining faces (C1 and C2) are arranged in the coal mining planning area. The fully mechanized mining face is about 2400m long and 300m wide, and mining is carried out along the strike direction of the coal seam. A total of four surface U-shaped wells (U1, U2, U3, and U4) are designed for gas control. The location of the U-shaped wells is designed to avoid structural development areas such as faults and sinkholes. The horizontal section of the U-shaped well H wells (H1, H2, H3, and H4) intersects the main fracture direction of the formation at a certain angle along the strike direction of the coal seam. The U-shaped well V wells (V1, V2, V3, and V4) are arranged in the downdip direction of the coal seam. The horizontal section of the U-shaped well H well is 0-2m away from the top of the coal seam (h1). The horizontal section of the U-shaped well is arranged near the central axis of the planned coal mining face. Figure 4 Among them, V1 and H2, V3 and H4 share one surface well site.

[0076] In the present invention, three bottom extraction tunnels (R1, R2, and R4) are arranged along the horizontal wellbore trajectory between the two planned coal mining faces. These tunnels are located in the coal seam bottom extraction tunnel between the two planned coal mining faces. One bottom extraction tunnel can be used to arrange floor-penetrating extraction holes toward the two adjacent coal mining faces, reducing the number of bottom extraction tunnels to be excavated. Bottom extraction tunnels R1, R2, and R4 are connected by bottom extraction tunnels R3 and R5. The floor-penetrating holes in bottom extraction tunnels R3 and R5 are used to extract gas from the coal seam at the end of the U-shaped well H shaft. The coal seam floor is hard limestone with a complete structure. The bottom extraction tunnel is 17 meters away from the (h2) coal seam floor.

[0077] Step 3: Develop a surface U-shaped well gas control and extraction plan.

[0078] In the application embodiment, the U-shaped well H well adopts a three-well body structure, and the V well adopts a two-well body structure. The completion depth of the V well is below the coal seam. Figure 5 .

[0079] The wellbore trajectory of the horizontal section of the U-shaped well H well used is within the range of 0-2m from the coal seam roof (h1). The horizontal section of the U-shaped well is arranged 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 well body structure design in the application embodiment:

[0081] H well body structure design:

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

[0083] Second opening: Use Φ311.1mm drill bit to drill to the landing point and insert Φ244.5mm technical casing.

[0084] Three openings: The horizontal section was drilled to the designed well depth using a Φ215.9mm drill bit and successfully completed the docking with the production well, and a Φ139.7mm production casing was run in.

[0085] V-well body structure design:

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

[0087] Second drilling: Use Φ215.9mm drill bit to drill to 50m below the coal seam floor to complete the drilling, insert Φ177.8mm technical casing, and insert a fiberglass casing near the cave making section.

[0088] Design of horizontal perforation and staged fracturing in H well in the application example:

[0089] The cable pumped bridge plug and perforation combined process is used to carry out segmented perforation and segmented fracturing construction in the horizontal section, so as to achieve communication between the horizontal wellbore and the target coal seam, and 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 sections of 70-80 m in length, which are adjusted according to the coal or rock penetration of the wellbore trajectory and the development of faults.

[0091] b. Perforate three clusters per section, with a perforation depth of 1.5 m per cluster, for a total of 4.5 m per section, at a density of 15 holes / m. Perforation locations should avoid the casing collars. Different perforation strategies should be employed based on the actual wellbore trajectory. For horizontal wellbore trajectories in roof strata more than 2 m from the coal seam, vertical downward perforation should be employed. For horizontal wellbore trajectories in roof strata 0-2 m from the coal seam, oblique downward and vertical downward perforation should be employed. Spiral perforation should be used within the coal seam.

[0092] c. Use large displacement and high sand ratio injection method for fracturing, with a construction displacement of 12-24m 3 / min, the sand ratio is 10%-20%. The actual construction parameters can be adjusted appropriately according to the specific conditions of the coal mine site.

[0093] d. Use active water for fracturing fluid. The fracturing fluid ratio is: clean water, 1.0% KCl and 0.05% fungicide. Drag reducer can be added in appropriate amount according to the fracturing construction pressure. The single stage fluid volume is 1200-2000m 3 .

[0094] e. Use 20 / 40 mesh and 16 / 20 mesh quartz sand as proppant and add them according to the pumping procedure. The amount of quartz sand in a single section is 100-200m 3 .

[0095] V well drainage equipment in the application example:

[0096] In the application embodiment, the V well in the U-shaped well is used as the drainage well, and the electric submersible screw pump is used for drainage. The displacement adjustment range is wide, and the maximum displacement is 100m 3 / d electric submersible screw pump.

[0097] The V-well wellhead uses a Φ139.7mm gas wellhead. The surface production equipment utilizes an automated control platform. Data such as the downhole pump's operating conditions and status parameters, bottomhole flow pressure, casing pressure, and water and gas production are transmitted in real time to the control cabinet and remote control terminal, enabling real-time monitoring and remote control and adjustment.

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

[0099] A combined surface and underground microseismic monitoring method is used to monitor microseismic events generated during hydraulic fracturing in real time, locate the fracturing microseismic events and interpret the fractures, infer the fracture length, height and orientation, analyze the expansion pattern and spatial distribution of the fractures, and characterize the effective impact range of hydraulic fracturing.

[0100] The surface microseismic monitoring network for each fracturing stage is divided into a 6×7 grid structure centered on the horizontal projection of the H-well in the U-shaped well. Stations are spaced 80 meters apart, covering a 400-meter range along the horizontal trajectory and a 480-meter radial range along the horizontal segment. Downhole, the microseismic monitoring network for each fracturing stage is deployed in the bottom pumping lanes on both sides, with nine microseismic detectors spaced 20 meters apart in each lane. The horizontal projection of the fracturing stage is located in the center of the monitoring point. After each fracturing stage is completed, rolling mobile geophones are used to monitor the next fracturing stage.

[0101] Taking the microseismic monitoring results of a certain segmented hydraulic fracturing in 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 the microseismic events within the target time period, the first arrival time of each microseismic event at different monitoring stations was output to complete the processing of the microseismic monitoring data, and the microseismic response characteristics of the coal seam hydraulic fracturing were obtained, thereby explaining the hydraulic fracturing crack expansion process. 39 microseismic signals were monitored by the microseismic monitoring system. After interpretation and processing, it was found that the fractures induced by the hydraulic fracturing of this segment were mainly concentrated on both sides of the horizontal fracturing well. From the macroscopic morphology of the fractures inferred from the microseismic positioning points, the fracture network caused by the microseismic events was mainly distributed along the horizontal plane, extending to both sides from the wellbore trajectory of the fracturing section. The specific fracture parameters were the main fracture length of 274m, the branch fracture length of 184m, the transformation radius of 131m, and the transformation volume of 45.6×103m 3 The fracture orientation is 67°∠15°. The schematic diagram of the main fracture projection of the hydraulic fracturing microseismic monitoring is as follows: Figure 6 As shown in the figure, the schematic diagram of reservoir transformation volume monitored by microseismic monitoring is as follows: Figure 7 shown.

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

[0103] To detect the extended distribution of fracturing fluid in the formation after hydraulic fracturing, downhole transient electromagnetic (TEM) detection was performed in advance. This TEM detection employed a mining-grade TEM instrument and a high-power transmitter, with a detection depth of approximately 100 meters. TEM detection in mines employed an overlapping loop assembly, with a 1.5-meter-square transmitting and receiving wireframe, four transmitting coil turns, and 40 receiving coil turns. At least 30 stacking loops were used at each measurement point to improve the signal-to-noise ratio and ensure the reliability of the raw data.

[0104] Taking the transient electromagnetic detection of the horizontal section of Well U4 as an example, measuring points and lines were arranged in the bottom pumping tunnel R1, bottom pumping tunnel R2 and bottom pumping tunnel R3, with a measuring point every 40 m. Five detection directions were designed for each detection point, namely, elevation angle 90° (vertical upward), elevation angle 60°, elevation angle 30°, elevation angle 15° and the front of the tunnel wall (horizontal direction), as shown in Figure 2. Figure 2 Detection is conducted once before and after the fracturing operation. The low-resistance anomaly characteristics and change characteristics before and after hydraulic fracturing within the detection range are compared and analyzed to determine the spatial distribution of the water-rich anomaly area after fracturing, thereby judging the expansion range of the fracturing fluid. Figure 8 This is a fan-shaped diagram of the apparent resistivity profile at a measuring point before hydraulic fracturing (elevation angle 15°). The red apparent resistivity inversion within the entire fan-shaped area at an elevation angle of 15° is relatively high, and the formation in this detection direction is weakly water-rich. Figure 9This is a fan-shaped profile of apparent resistivity detection at this point after hydraulic fracturing (elevation angle 15°). Compared to before hydraulic fracturing, a significant low-resistivity anomaly appears within the detection front. Large, low-resistivity anomalies generally indicate that fracturing has improved internal connectivity and increased water content within the rock formation, suggesting that fracturing fluid has entered this area through fracture creation. Combining the detection results from the five detection directions at this point fully characterizes the distribution of water-rich areas in the rock formation within three dimensions, providing a basis and guidance for the layout of through-layer drilling holes for bottom pumping lanes.

[0105] Step 6: Based on the microseismic monitoring of fracture development and the transient electromagnetic detection of stratum water content, formulate a plan for the arrangement of bottom pumping rock tunnel through-layer drilling.

[0106] The through-layer drilling holes in the bottom rock tunnel are divided into drainage holes and gas extraction holes according to their main functions. The main function of the 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 to extract air; the main function of the gas extraction holes is to extract air, and when there is water in the starting coal seam, they also have some drainage function.

[0107] The placement of drainage and extraction holes in the bottom pumping tunnels is determined through microseismic monitoring and transient electromagnetic detection results, including drill spacing, the number, length, and orientation of boreholes. One drainage hole is placed in each bottom pumping tunnel corresponding to a fracturing stage. Drainage holes are located in the tunnel at a low elevation, near the furthest water-rich area within the influence of each fracturing fracture. Extraction holes are placed in areas unaffected by the fracturing fractures. The final depth of the drainage and extraction holes is 5-10 meters through the coal seam roof, with the bottoms of the holes spaced 10-20 meters apart, forming a fan-shaped arrangement.

[0108] In Example H4, drainage holes and extraction holes corresponding to the first fracturing stage were constructed prior to other fracturing stages. Drilling was performed using an 89mm drill bit, and a 40mm PVC screen was installed as drainage and extraction pipes. The first fracturing stage's cracks extended further toward Lane R2, extending longer, and the water-rich anomaly also extended further toward R2. Therefore, drainage holes for the first fracturing stage were placed at Drill Site Z21, where drainage holes were constructed more efficiently. The extraction holes at Drill Site Z21 were constructed at a shorter distance than those at Drill Site Z11, and more extraction holes were required. The second fracturing stage's main cracks extended further toward Lane R1, with the water-rich anomaly closer to Lane R1. Drain holes for this fracturing stage were placed at Drill Site Z12, where extraction holes were constructed at a shorter distance than those at Drill Site Z22. Furthermore, in the intermediate area between the first and second fracturing stages, unaffected by the first and second fracturing stages, additional drilling sites and extraction holes were deployed to eliminate any blind spots in extraction. In the bottom pumping tunnel R3, drilling sites Z31 and Z32 are arranged to pump out gas from the area not affected by the fracturing at the shaft end of H4 well. Figure 10 .

[0109] Step 7: Develop a joint gas extraction system for ground and underground gas control at different stages to reduce the gas content in coal seams.

[0110] Taking the U4 well group's gas extraction system as an example, the U4 well group utilizes a combination of surface electric submersible screw pumps and underground bottom pumping through rock tunnels, with Well V4 serving as the surface drainage well. Existing geological data indicates that the original formation pressure for the coal seams in the planned mining area is 3.03 MPa. Following hydraulic fracturing and blowout in the horizontal section of Well H4, the bottomhole pressure remained at 3.58 MPa until well V4 was pumped out. At this point, the bottomhole pressure exceeded the original formation pressure, so the surface electric submersible screw pump speed was adjusted to near maximum, while the drain valve was fully opened. The bottomhole pressure was then reduced to approximately 3.03 MPa at a daily rate of 0.08 MPa / day. When the bottom hole pressure is lower than 3.03MPa and the coal seam has not yet desorbed, the pump drainage rate is appropriately reduced, and the valves of some drainage holes in the well are adjusted to be smaller according to the water production of the drainage holes. The daily pressure drop of the bottom hole pressure is controlled between 0.04MPa / d-0.05MPa / d until the coalbed methane is desorbed.

[0111] When the bottom hole flow pressure is 1.62MPa, the coal seam gas begins to desorb, and this pressure is the desorption pressure, and the drainage enters the production increase stage. As the coal seam gas desorbs, the casing pressure increases, further reducing the pump drainage rate, and closing the downhole drainage hole valve of the earliest fracturing section. The bottom hole flow pressure drop rate is controlled between 0.01-0.03MPa / d. When the casing pressure rises to 0.5MPa, open the ground pipeline valve to start gas production. As the drainage progresses, gradually open the gas production valve to slowly increase production. During this period, the casing pressure is controlled between 0.1MPa-0.5MPa, and gradually reduces the casing pressure. The maximum gas production reaches 6900m 3 / d; maintain the casing pressure not exceeding 0.15MPa and keep the gas production stable.

[0112] During the stable production phase, the drainage holes were slowly closed according to the water production, further reducing the pump drainage rate. The bottom hole pressure drop rate was controlled between 0-0.01 MPa / d to maintain the balance between the bottom hole pressure and the casing pressure, and to maintain the gas production of H4 well at 5800m 3 / d; and open the downhole pumping hole, with the bottomhole pumping negative pressure at 15kPa. After that, maintain the liquid level and maintain stable gas production by adjusting the gas pipeline valve according to the gas production of the V well and the downhole pumping hole.

[0113] In existing technology, gas production through surface U-shaped wells alone takes 3-6 months to reach gas, and production ramp-up takes about 1-1.5 years. In the U4 well of the present invention, gas production took 46 days to reach gas, and production ramp-up took 118 days, significantly shortening both drainage and ramp-up cycles and increasing gas extraction efficiency. Currently, gas production from this well is continuous and stable.

[0114] The above shows and describes the basic principles, main features and advantages of the present invention. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely preferred examples of the present invention and are not intended to limit the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for rapid and efficient extraction of coal mine gas from crushed soft coal seams, characterized in that: The following steps are involved: Step 1: determine the geological characteristics of the coal mining planning area; Step 2: Design a combined surface U-shaped well and underground bottom pumping tunnel drilling system in the planned mining area. Multiple surface U-shaped wells are deployed simultaneously as a whole, with the horizontal section of a single U-shaped well positioned near the central axis of the working face in the planned mining area. The bottom pumping tunnel is arranged along the horizontal wellbore trajectory in the floor rock layer between the two working faces in the planned mining area and connected by a through tunnel. Step 3: Develop a surface U-shaped well gas control and extraction plan; Step 4: The surface microseismic monitoring network for each fracturing stage is arranged in 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 stage is arranged in the bottom pumping lanes on both sides. The fracturing microseismic events are monitored and interpreted in real time to determine the fracture propagation pattern and spatial distribution morphology. Step 5: Arrange transient electromagnetic detection points in the bottom pumping tunnel corresponding to each fracturing section, use downhole transient electromagnetic method to detect the expansion range of the fracturing fluid, compare the low-resistance anomaly characteristics and change characteristics before and after fracturing, and determine the abnormal water-rich area in the formation; Step 6: Based on the microseismic monitoring of fracture development and the transient electromagnetic detection of stratum water abundance, a plan for the arrangement of through-layer drilling holes for bottom pumping tunnels is formulated; Step 7: Combine the underground bottom pumping rock tunnel through-layer drilling arrangement plan and formulate an extraction system by adjusting the relationship between the bottom hole flow pressure and the original formation pressure, desorption pressure and casing pressure to quickly and efficiently extract coal seam gas.

2. The method for rapid and efficient extraction of coal mine gas from crushed soft coal seams according to claim 1 is characterized in that: In step 2, the U-shaped well location is designed to avoid the structural development area of ​​faults and collapse columns. The direction of the horizontal section of the H well of the U-shaped well intersects with the direction of the main fracture at a certain angle. The V well of the U-shaped well is arranged in the downdip direction of the coal seam. The horizontal section of the H well is 0-2m away from the coal seam roof. The horizontal section length is 800-1200m and is located near the central axis of the planned coal mining face; the bottom rock pumping tunnel is 15-25m away from the coal seam floor, which is determined according to the lithology and structure of the formation. The bottom rock pumping tunnel is connected by a through tunnel, and the through tunnel is arranged with bottom plate through-layer drill holes to extract gas from the coal seam near the end of the H well shaft.

3. The method for rapid and efficient extraction of coal mine gas from crushed soft coal seams according to claim 2, characterized in that: In step 3, Well H adopts a three-wellbore structure. Well H uses a cable pumped bridge plug + perforation process to perform staged perforation and staged fracturing in the horizontal section. The perforation orientation is determined according to the specific wellbore trajectory, and the staged fracturing parameters and pumping operation procedures are determined according to specific geological conditions. Well V adopts a two-wellbore or three-wellbore structure depending on 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 extraction of coal mine gas from crushed soft coal seams according to claim 3 is characterized in that: In step 3, the horizontal section of Well H was perforated and fractured in stages, with each section being 70-80 m long, adjusted based on the wellbore trajectory's penetration of coal or rock and fault development. The perforation locations avoided the casing collar locations, and different perforation schemes were used based on the actual wellbore trajectory. The fracturing parameters and pumping procedures for each section were determined based on the specific geological conditions.

5. The method for rapid and efficient extraction of coal mine gas from crushed soft coal seams according to claim 4 is characterized in that: In step 4, the surface microseismic monitoring network for each fracturing section is divided into a 6×7 grid structure with the ground projection of the horizontal section of the U-shaped well H well as the center. The station track spacing is 80m, the coverage range along the horizontal section trajectory is 400m, and the radial coverage range along the horizontal section is 480m. The underground microseismic monitoring network for each fracturing section is equipped with 9 microseismic detectors in each bottom pumping lane, arranged at a spacing of 20m. The horizontal projection of the fracturing section is located in the middle of the detection point. 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 extraction of coal mine gas from crushed soft coal seams according to claim 5, characterized in that: In step 5, during the downhole transient electromagnetic detection, two transient electromagnetic detection points are arranged in the bottom pumping rock tunnel corresponding to each fracturing section according to the detection distance of the transient electromagnetic instrument and the number of horizontal fracturing sections. The distance between adjacent fracturing section detection points is the same; five detection directions are designed for each detection point, namely, 90° elevation (vertical upward), 60° elevation, 30° elevation, 15° elevation, and directly in front of the tunnel wall (horizontal direction).

7. The method for rapid and efficient extraction of coal mine gas from crushed soft coal seams according to claim 6, characterized in that: In step 6, the drainage holes of the through-layer drilling holes of the bottom pumping rock tunnel are used for drainage and pressure reduction in the early stage, and are connected to negative pressure pipelines for air extraction in the later stage; the air extraction holes of the through-layer drilling holes of the bottom pumping rock tunnel are mainly used for air extraction, and also have a partial drainage function when there is water in the initial coal seam; one drainage hole is arranged in the bottom pumping rock tunnel corresponding to each fracturing section, and the drainage hole is arranged in the bottom pumping rock tunnel with a low altitude and is located near the farthest water-rich area within the influence range of each fracturing fracture; after drilling, the through-layer drilling holes of the bottom pumping rock tunnel are subjected to high-pressure punching, and the addition of sodium lignosulfonate as a coal powder dispersant is determined based on the coal slag discharge and the punching and slag removal capacity of the clean water medium. The drainage holes must be punched with high pressure, and the air extraction holes are determined based on the slag and water discharge during drilling. After the through-layer drilling of the bottom pumping rock tunnel 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 before the air extraction holes after the fracturing construction is completed and the formation pressure is stabilized, and the drainage holes corresponding to the first fracturing section are constructed first.

8. The method for rapid and efficient extraction of coal mine gas from crushed soft coal seams according to claim 7, characterized in that: In step 7, the extraction system for different drainage stages is as follows: Drainage stage: When pumping, if the bottomhole pressure is greater than the original formation pressure, adjust the surface electric submersible screw pump speed to near the maximum, and at the same time, fully open the drainage hole valve to the maximum; when the bottomhole pressure is lower than the original formation pressure and greater than the desorption pressure, reduce the drainage rate, adjust some downhole drainage hole valves, and control the bottomhole pressure drop rate to 0.03-0.09MPa / d, and control the pressure drop fluctuation range of the day to no more than 20% of the pressure drop of the previous day; Production increase stage: When the bottomhole flow pressure is lower than the desorption pressure, the pump drainage rate is further reduced, and the drainage holes at the fracturing section with high altitude in the horizontal section are closed. The bottomhole flow pressure drop rate is controlled at 0.01-0.03MPa / d. As the coalbed methane desorbs and the casing pressure increases, the surface pipeline valve is opened to start gas production and the gas production valve is gradually opened to increase production. The casing pressure is controlled at 0.1MPa-0.5MPa, and the bottomhole flow pressure is maintained to decrease slowly. The casing pressure is controlled to avoid large fluctuations in the drainage pump to avoid gas cross-contamination. Stable production stage: After the gas production stabilizes, the drainage holes are slowly closed according to the water production, and the pump drainage rate is further reduced. The bottomhole flow pressure drop rate is controlled at 0-0.01MPa / d, and the balance between the bottomhole flow pressure and the casing pressure is maintained. The dynamic liquid level is ensured to be 10 meters higher than the drainage pump. The downhole gas extraction holes are opened, and the drainage negative pressure is 5-20KPa. The negative pressure gradually increases with the drainage time, and the gas extraction hole valve is adjusted according to the gas concentration and gas pure amount. Attenuation stage: By adjusting the pump speed and gas production valve to maintain a stable relationship between bottom hole flow pressure and casing pressure, the gas production will slowly decay; increase the downhole extraction intensity, open the extraction hole and drainage hole valves to the maximum, increase the extraction negative pressure to 20-40KPa, and adjust the extraction hole valve according to the gas concentration and gas purity.

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