Method for efficiently extracting gas through cooperation of long-distance mining pressure relief and pre-fracturing

Abstract

The invention belongs to the technical field of gas extraction, and particularly relates to a method for efficiently extracting gas through cooperation of long-distance mining pressure relief and pre-fracturing, which comprises the following steps of: dividing and calculating three zones formed by mining pressure relief, and performing mining pressure relief and pre-fracturing cooperative identification by combining a hydraulic fracturing crack propagation rule; the method comprises the following steps: carrying out well position and depth optimization design on a horizontal well, determining a fracturing process and optimizing perforation fracturing parameters, carrying out hydraulic fracturing real-time monitoring by adopting a microseismic fracture monitoring evaluation technology in a fracturing process, and combining three zones generated by mining pressure relief with horizontal well staged fracturing to realize regional pressure relief and achieve the effect of mining one layer and pumping multiple layers.

Description

Technical Field

[0001] The present invention belongs to the technical field of gas extraction, and in particular relates to a method for efficiently extracting gas by cooperating with remote mining pressure relief and pre-fracturing. Background Art

[0002] The underground drilling and drainage method involves drilling holes into the coal seam or surrounding rock fissures within underground tunnels to extract gas using the pressure relief effect. This method eliminates blind spots for gas extraction through a "bayonet" approach, but it requires significant engineering and capital investment. Key underground permeability enhancement and drainage methods include hydraulic fracturing, CO2 permeability enhancement, loosening blasting, hydraulic slotting, and high-pressure water jetting. Hydraulic fracturing uses a pump to inject high-pressure water into the coal seam to increase its permeability. Due to underground space constraints, the pump's displacement is limited, limiting the scope of the fracturing treatment. CO2 permeability enhancement utilizes the energy generated by the phase change of CO2 to increase coal seam permeability. This requires a large number of CO2 cannons, often in a string-like pattern, and is a significant engineering effort. Loosening blasting increases coal seam permeability through blasting, but the resulting shock waves can potentially induce coal and gas outbursts. Both hydraulic fracturing and high-pressure water jetting use water as a medium, creating artificial fractures in coal seams through tools to increase their permeability. These engineering techniques are relatively complex and are less effective when the coal body is relatively fragmented. Surface drilling extraction involves drilling vertical / directional or horizontal wells on the surface, using a combination of perforation and fracturing to create fractures in the coal seam. Gas is then extracted using a decompression method. Extraction is less effective when the coal body is relatively fragmented. In areas with multiple coal seams and well-developed tectonic coal, when mining the lower seam, the pressure relief effect of mining increases the permeability of the surrounding rock, making it possible to extract gas from overlying or upper coal seams using this pressure relief. However, when coal seams are far apart, they may appear to be connected but not connected, resulting in discontinuous extraction or suboptimal extraction volumes. In order to improve the gas extraction effect of distant coal seams after mining pressure relief, it is urgent to study a long-distance mining pressure relief and pre-fracturing collaborative efficient gas extraction process and method to achieve the effect of "mining one layer and extracting multiple layers". Summary of the Invention

[0003] The purpose of this invention is to provide a method for efficient gas extraction by coordinating remote mining pressure relief and pre-fracturing. This technology addresses the current problem of coal development areas with poor coal seam permeability in multi-seam structures, where pressure relief creates a "seemingly disconnected" phenomenon with the "protective coal seam," which affects gas extraction efficiency in the "protective coal seam." By combining the "three zones" generated by mining pressure relief with staged fracturing in horizontal wells, regional pressure relief is achieved, achieving the "mining one layer, extracting multiple layers" effect.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is:

[0005] A method for efficiently extracting gas by coordinating long-distance mining unloading and pre-fracturing, comprising the following steps: S1. dividing and calculating the "three zones" formed by mining unloading based on mine geological exploration data, determining the range requiring fracturing unloading, and identifying the coordination of mining unloading and pre-fracturing in combination with the propagation law of hydraulic fracturing cracks;

[0006] S2. Determine gas-rich areas based on site topography and coal face layout, and optimize horizontal well location design;

[0007] S3. Design the wellbore structure based on the drilling stratum, ground stress field, and the displacement patterns of overburden caused by mining;

[0008] S4. Determine the fracturing technique based on the thickness, lithology, and location of the target layer;

[0009] S5. Optimize perforation and fracturing parameters: optimize the location, number of wells, and fracturing process based on actual conditions;

[0010] S6. Perform drilling and fracturing operations according to drilling and fracturing designs;

[0011] S7. Use microseismic fracture monitoring and evaluation technology to monitor hydraulic fracturing in real time during the fracturing process until the fracturing operation is completed;

[0012] S8. Evaluate the effectiveness of joint operations based on hydraulic fracturing monitoring results and gas production.

[0013] Furthermore, in step S1, the mining unloading and pre-fracturing collaboration is used to determine that the mined coal seam is a lower coal seam, and the method includes the following steps:

[0014] S11. First, determine the total height of the collapse zone and fracture zone based on the comprehensive stratigraphic histogram and the mechanical properties of the overlying lithology of the mined coal seam;

[0015] S12. When the interlayer spacing is less than the sum of the heights of the collapse zone and the fracture zone, pre-fracturing is not required. When the interlayer spacing is greater than the sum of the heights of the collapse zone and the fracture zone, calculate the fracture height of the horizontal well hydraulic fracturing. When the fracture height is less than 17 meters, adopt a combination of pre-fracturing and mining unloading mode.

[0016] Furthermore, the basic principles for horizontal well location deployment in step S2 are: select a location with gentle terrain and convenient transportation to facilitate construction, commissioning, gathering, transportation, and utilization, while avoiding the influence of underground tunnels; avoid faults and collapse columns; select a gas-rich area within the mining fracture zone; the calculation formula for the location of horizontal wells along the dip is:

[0017] L=b+Hcotα

[0018] Where, L is the horizontal projection distance from the bottom of the coalbed methane surface well to the return air channel, m; b is the distance from the area with high gas concentration and large gas content in the "O" ring to the boundary of the "O" ring, which is generally taken as 1 / 3 to 2 / 3 of the average width of the "O" ring, m; H is the vertical distance from the bottom of the well to the coal seam floor, m; α is the overburden pressure relief angle, degrees.

[0019] Furthermore, the wellbore structure optimization design in step S3 includes: selecting the upper coal seam within the mining fracture zone as the well landing position, and the horizontal section length of the horizontal well is 700 to 1000 meters; the wellbore structure is: first, drilling to 10 meters below the stable bedrock, lowering the J55 casing to 10 meters below the bedrock, and returning the cement slurry to the ground; second, drilling to 450 meters above the target layer and starting to create inclination, drilling to 5 meters above the top of the target coal seam with a dogleg angle of 6° / 30m, lowering the N80 casing, and returning the cement to the ground; third, after drilling to the target coal seam, drilling along the target coal seam to the horizontal section B target point, with a slightly downward horizontal trajectory and an angle of 1° to 3°, and using casing for completion.

[0020] Furthermore, the fracturing process in step S4 is based on the previous coalfield exploration data, combined with the coal seam roof lithology, coal mining thickness, and the composition of the rock layer above the coal seam. The "plate" model is used to establish the initial pressure step distance and the periodic pressure step distance of the working face according to the damage limit analysis of the plate structure. The calculation formulas are as follows:

[0021]

[0022] Where: L m is the initial pressure step distance of the roof, in m; k is the cracking coefficient of the rock formation, k = 0.25 ~ 0.75, dimensionless; σ t is the tensile strength of the roof rock layer, in MPa; h is the thickness of the roof rock layer, in m; q is the load on the roof, in kN; E i is the elastic modulus of the overlying rock mass layer i, in MPa; h i is the thickness of the overlying rock mass layer i, in m; γ i is the bulk density of the overlying rock mass layer i, in kN / m 3 The calculation formula of the periodic pressure step distance of the roof rock layer is:

[0023]

[0024] Where: L z is the periodic pressure step of the roof rock layer, in m; h is the thickness of the roof rock layer, in m; σ t is the tensile strength of the roof rock layer, in MPa; q is the load on the roof, in kN.

[0025] Furthermore, the cluster spacing during staged fracturing in step S5 is not greater than 100+L z According to the periodic pressure step, perforation is carried out with a half-period pressure step as the interval, the perforation length is 1m, and the perforation direction is directional two-wing perforation.

[0026] Furthermore, the microseismic crack monitoring of the fractured well in step S6 includes the following steps:

[0027] S61. Monitoring site survey: record the anti-bumping wellhead coordinates, topography, and landforms;

[0028] S62. Deploy monitoring substations, record the coordinates of each substation, and determine the location of each substation relative to the anti-blow well;

[0029] S63. System parameter setting and debugging: Turn on the instruments at the main and substations, debug the communication and data transmission between the main and substations, and set parameters.

[0030] S64. The target layer fracturing construction begins, and the microcrack monitoring system is turned on and enters the monitoring state. At this time, the system automatically collects microseismic waves, records waveforms, processes data, and displays the state in real time. The target layer fracturing construction ends, and the data is saved and the system is shut down.

[0031] S65. Put away all monitoring devices and complete the on-site monitoring.

[0032] Furthermore, the fracture height calculation in step S12 is performed using a multivariate nonlinear regression method, and the correlation equation is:

[0033]

[0034] Where: L f is the length of the fracture, m; H f is the height of the fracture, m; W f is the width of the fracture, cm; D s is the perforation density, holes / m; q t is the construction displacement, m 3 / min;Q t is the construction liquid volume, m 3 ; is the average sand ratio, %; R 2 is the correlation coefficient, dimensionless.

[0035] The advantages of the present invention are:

[0036] 1. The present invention uses staged fracturing in horizontal wells to connect hydraulic fracturing cracks with mining-induced pressure relief cracks, thereby improving the gas extraction efficiency of distant coal seams affected by mining-induced pressure relief, achieving the effect of "mining one layer and extracting multiple layers";

[0037] 2. Based on the "three-zone" distribution characteristics under the influence of coal seam mining, segmented fracturing of ground horizontal wells causes the fracturing cracks to communicate with the mining fracture zone, providing an effective channel for gas migration. This method can extract multi-layer gas through the construction of a single-layer horizontal well, and is an efficient gas extraction mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a process flow chart of the present invention.

[0039] Figure 2 Distribution characteristics of overburden fractures at different advancement distances.

[0040] Figure 3 This is a schematic diagram of mining-induced cracks.

[0041] Figure 4 These are the hydraulic fracturing simulation results under different construction parameters in the present invention.

[0042] Figure 5 It is a main schematic view of the horizontal well location of the present invention.

[0043] Figure 6 It is a top view schematic diagram of the horizontal well location of the present invention.

[0044] Figure 7 It is a plane projection distribution diagram of the fracturing points in the present invention.

[0045] Figure 8 This is a diagram of the fracturing monitoring results in the present invention.

[0046] Figure 9 This is the gas production curve of PCD-01 well in the application example. DETAILED DESCRIPTION

[0047] Example

[0048] The process and method of efficient gas extraction by remote mining unloading and pre-fracturing collaboration mainly include: identification of mining unloading and pre-fracturing collaboration, optimization design of horizontal well location and wellbore structure, optimization design of horizontal well staged fracturing parameters, and evaluation of collaborative extraction effect. Figure 1 shown.

[0049] 1) Research on the distribution characteristics of mining-induced fracture zones

[0050] The mining cracks in the collapse zone and the fracture zone are interconnected in a network, and the permeability of the coal rock layer and the degree of gas analysis have also increased significantly. This area is an effective area for gas extraction from adjacent layers. By using 3DEC numerical simulation software combined with engineering practice, and by observing the fracture law, displacement and stress of the overlying rock layer as the working face is mined, the development height and distribution characteristics of the cracks in the overlying rock layer in the goaf of the working face are obtained. This simulation sets the strike model length to 300m, the dip length to 200m, and the inclination angle to 0°. The simulation results are as follows: Figure 2 shown.

[0051] from Figure 2 It can be seen that the cracks in the overburden strata are in different development states at different advancement distances, and the development range and density of the cracks are also different. Figure 2 (a, b)), the separation cracks are formed by the asynchronous movement of the rock layer, and the cross-layer cracks are formed by the breaking of the rock layer. The cracks on both sides of the collapsed rock are densely developed, while the cracks in the middle area are relatively sparse, and large separation cracks appear. Figure 2 (c, d)) The lower rock layer loses its support and breaks completely. The cracks in the collapsed area are crisscrossed, but no obvious compression phenomenon occurs. The rock layer at the end of the cut eye breaks at a certain angle, and the cracks formed develop upward in a right trapezoidal shape at a certain angle; the delamination cracks exceed the working face and the cut eye position and develop upward in an inverted trapezoidal shape. As the working face advances, the fracture cracks still develop in a right trapezoidal shape. Due to the existence of the stable structure of the masonry beam caused by the broken rock layer, the side cracks of the working face continue to move forward as the working face advances; the delamination cracks still develop upward in an inverted trapezoidal shape, and the scope continues to expand as the working face advances. The crack-dense areas are still on both sides of the collapsed area, and there are relatively few cracks in the middle. The phenomenon of large delamination cracks no longer occurs. In the middle and late stages of the working face excavation ( Figure 2 (e, f)), the interlayer and delamination cracks on the working face side are continuously generated and expanded forward as the working face advances; the cracks on the open eye side remain stable as a whole; the range of the closed area of the cracks in the middle of the goaf is constantly increasing. The overall range of the crack development area on the working face side is larger than that on the open eye side. The cracks in the collapsed area are mainly distributed on both sides of the goaf, and the cracks in the unmined areas on both sides are more obvious. Only a small number of cracks exist in the middle of the collapsed area. The cracks in the central part of the overburden damage area gradually tend to be compacted, the cracks no longer develop upward, and the height decreases, with a small number of transverse cracks appearing on the top. As the working face advances, the following gradually formed. Figure 3 The crack channel is shown.

[0052] 2) Identification of collaboration between mining-induced pressure relief and pre-fracturing

[0053] This time, the main purpose is to identify whether the mined coal seam is the lower coal seam. The identification method is:

[0054] (1) First, the total height of the fracture zone is determined based on the comprehensive stratigraphic columnar diagram and the mechanical properties of the overlying lithology of the mined coal seam.

[0055] The empirical formula for the height of collapse zone and fracture zone is shown in Table 1.

[0056] Table 1 Calculation formula for the maximum height of the coal seam collapse zone and water-conducting fracture zone

[0057]

[0058] (2) Assuming that the interlayer distance between the upper coal seam and the lower coal seam is Hc meters, the sum of the heights of the collapse zone and the fracture zone can be calculated based on the formula in Table 1 combined with the lithology.

[0059] ①The interlayer distance is less than the sum of the heights of the collapse zone and the fracture zone

[0060] H c ≤H m +H li (1)

[0061] In this case, no pre-fracturing is required.

[0062] ②The interlayer distance is greater than the sum of the heights of the collapse zone and the fracture zone

[0063] When H c >H m +H li When , it is necessary to calculate the fracture height of horizontal well hydraulic fracturing.

[0064] This study used numerical simulation methods to calculate hydraulic fracturing crack height. FracproPT numerical simulation software was used to simulate the hydraulic fracturing crack size under different construction process parameters such as perforation density, operation displacement, operation fluid volume, average sand ratio, and sand addition method.

[0065] The perforation density is 12 holes / m and the construction displacement is 12m 3 / min, construction fluid volume 1000m 3 , average sand ratio of 14%, step-by-step sand addition as the basic construction simulation parameters, simulate the seam height, seam length and seam width under different average sand ratios, different perforation densities, different construction displacements and different construction fluid volumes. The simulation results are as follows Figure 4 shown.

[0066] Depend on Figure 4 It can be seen that when the perforation density is greater, the crack extension half length and crack height are greater. When the perforation density is greater than 12 holes / m, the increase in crack extension half length and crack height decreases. The crack extension length, crack height and deep hole density are logarithmically positively correlated. When the construction displacement is greater, the crack extension half length and crack height are greater. When the displacement exceeds 10m 3 / min, the increase in the half-length of crack extension decreases; when the amount of construction fluid increases, the half-length of crack extension and the crack height increase; when the amount of construction fluid exceeds 1400m 3 When the sand addition method and sand ratio are increased, the increase in the half-length of the fracture extension and the fracture height slows down; different sand addition methods and sand ratios lead to significant differences in proppant migration characteristics and accumulation areas, thus producing different hydraulic fracturing fractures. Stepped concentration sand addition is less likely to cause sand plugging, which can make the hydraulic fracturing fracture extend better in the coal seam. When the average sand ratio is from 10% to 16%, the half-length of the fracture extension shows a trend of first increasing and then decreasing, and the total fracture height shows a trend of decreasing. Using the multivariate nonlinear regression method, the correlation equation between the construction parameters (perforation density, construction displacement, construction fluid volume, average sand ratio) and the fracture extension scale (fracture length, fracture height, and fracture width) can be obtained as follows:

[0067]

[0068] Where: L f is the length of the fracture, m; H f is the height of the fracture, m; W f is the width of the fracture, cm; D s is the perforation density, holes / m; q t is the construction displacement, m 3 / min;Q t is the construction liquid volume, m 3 ; is the average sand ratio, %; R 2 is the correlation coefficient, dimensionless.

[0069] Through simulation, it is found that the height of the fracturing crack is unlikely to exceed 17 meters. Based on this, it can be determined that when H c -H m -H li When the pressure is ≥17 meters, even if the pre-fracturing technology is implemented, it is difficult to form a smooth gas migration channel after the mining impact. It is not recommended to use the pre-fracturing and mining pressure relief cooperation mode for gas extraction; when 0<H c -H m -H li When the depth is less than 17 meters, the pre-fracturing and mining unloading mode can be used. Therefore, the identification criteria for the cooperation between mining unloading and pre-fracturing can be expressed as:

[0070] 0<H c -H m -H li <17 (3)

[0071] 3) Optimal design of horizontal well location and wellbore structure

[0072] (1) Horizontal well location

[0073] The basic principles for horizontal well site deployment are: select a location with gentle terrain and convenient transportation to facilitate construction, commissioning, gathering, transportation and utilization, while avoiding the influence of underground tunnels; avoid faults and collapse column structures; and select a gas-rich area within the mining fracture zone.

[0074] The calculation formula for the position of horizontal wells along the dip is:

[0075] L=b+Hcotα (4)

[0076] Where, L is the horizontal projection distance from the bottom of the coalbed methane surface well to the return air channel, m; b is the distance from the area with high gas concentration and large gas content in the "O" ring to the boundary of the "O" ring, which is generally taken as 1 / 3 to 2 / 3 of the average width of the "O" ring, m; H is the vertical distance from the bottom of the well to the coal seam floor, m; α is the overburden pressure relief angle, degrees.

[0077] Generally, the best horizontal well location should be 0.17 to 0.28 times the mining length, and close to the return air lane strip area. Figure 5-6 shown.

[0078] (2) Wellbore structure optimization design

[0079] The upper coal seam within the mining fracture zone is selected as the landing layer, and the horizontal section length of the horizontal well is usually 700 to 1000 meters. The well structure is as follows: first, drill to 10 meters below the stable bedrock, lower the J55 casing to 10 meters below the bedrock, and return the cement slurry to the ground; second, drill to 450 meters above the target layer and start deflection, with a dogleg angle. 6° / 30 Drill to 5m above the roof of the target coal seam, install N80 casing, and return the cement to the surface. After the third drilling reaches the target coal seam, drill along the target coal seam to the horizontal section B target point. The horizontal trajectory is slightly downward at an angle of 1° to 3°, and the well is completed with casing.

[0080] 4) Optimization design of horizontal well staged fracturing parameters

[0081] (1) Optimization design of perforation position

[0082] Based on the previous coalfield exploration data, combined with the coal seam roof lithology, coal mining thickness, and the composition of the rock layer above the coal seam, the "plate" model is used to establish the initial pressure step distance and periodic pressure step distance of the working face based on the damage limit analysis of the plate structure. The calculation formulas are as follows:

[0083]

[0084] Where: L m is the initial pressure step distance of the roof, in m; k is the cracking coefficient of the rock formation, k = 0.25 ~ 0.75, dimensionless; σ tis the tensile strength of the roof rock layer, in MPa; h is the thickness of the roof rock layer, in m; q is the load on the roof, in kN; E i is the elastic modulus of the overlying rock mass layer i, in MPa; h i is the thickness of the overlying rock mass layer i, in m; γ i is the bulk density of the overlying rock mass layer i, in kN / m 3 .

[0085]

[0086] Where: L z is the periodic pressure step of the roof rock layer, in m; h is the thickness of the roof rock layer, in m; σ t is the tensile strength of the roof rock layer, in MPa; q is the load on the roof, in kN.

[0087] In general, the short axis of the hydraulic fracturing crack plastic zone is 60 to 80 m. In order to avoid blind spots in the distribution of hydraulic fracturing cracks, the cluster spacing during staged fracturing should not be greater than 100 + L. z According to the periodic pressure step, perforation is performed with a half-periodic pressure step as the interval, the perforation length is 1m, and the perforation direction is directional two-wing perforation, such as Figure 7 shown.

[0088] Based on the well location and wellbore design, horizontal sections are placed in the rock formation, and fracturing points should be evenly distributed, avoiding geological structures such as faults. After the first stage of fracturing is completed, the subsequent fracturing scale needs to be adjusted in real time based on the initial operation pressure, displacement, and sand addition to achieve the goal of optimizing the fracturing design and increasing production. The basic fracturing operation parameters are shown in Table 2.

[0089] Table 2 Fracturing foundation construction parameters

[0090] Perforation section m Evenly spaced at 80m intervals <![CDATA[Design displacement m 3 / min]]> 14 <![CDATA[Proppant particle size (mm) m 3 > 80 (the ratio of fine sand, medium sand and coarse sand is 1:8:1) Proppant quartz sand <![CDATA[Liquid volume m 3 > 1200

[0091] 5) Microseismic fracturing monitoring

[0092] The effectiveness of staged hydraulic fracturing in ground horizontal wells is tested using microseismic fracture monitoring and evaluation technology for fractured wells. This technology can monitor the spatial morphology, effective fracture length, fracture height, and ground stress distribution of fractured wells. The ground microseismic system includes a real-time fracture monitoring system, microseismic geophones, signal amplifiers, wireless transmitter and receiver, high-precision GPS, PC computers, and other equipment. During fracturing construction, the fracturing endpoints of each fractured layer are calculated based on the fracturing design. Geophones are arranged based on their projection on the ground and the surrounding topography (centered on the center of the horizontal well section). Geophones must be accurately positioned using high-precision GPS and buried at a depth of not less than 0.2m. The field monitoring system, computer, and corresponding expert interpretation system are used to interpret and analyze the real-time data from field monitoring. The specific steps are as follows:

[0093] (1) Monitoring site survey: record the coordinates, topography and landform of the anti-bumping wellhead;

[0094] (2) Deploy monitoring substations, record the coordinates of each substation, and calculate the position of each substation relative to the anti-blow well;

[0095] (3) System parameter setting and debugging: Turn on the instruments of the main and substations, debug the communication and data transmission between the main and substations, and set the parameters.

[0096] (4) When the target layer fracturing construction begins, the micro-fracture monitoring system is turned on and enters the monitoring state. At this time, the system automatically collects micro-seismic waves, records waveforms, processes data, and displays them in real time. When the target layer fracturing construction ends, the data is saved and the system is shut down.

[0097] (5) Put away all monitoring devices and complete the on-site monitoring.

[0098] Fracturing monitoring results such as Figure 8 shown.

[0099] Application Examples

[0100] This process has been piloted in the Pingdingshan mining area with good results. Among them, PCD-01 well is a 4-2 coal mining L-shaped horizontal well in Shoushan No. 10 Mine. The well is located within the range of the J15-33200 working face. The J15-33200 working face is about 1080m long and 150m wide. The horizontal section of the well is 800m long and uses hydraulic sandblasting perforation for segmented hydraulic fracturing. The fracturing points are evenly distributed at 90m intervals. The well began to produce gas on March 11, 2022, with a maximum daily gas production of 10492m3. 3 / d, cumulative gas production 157.64×104m 3 , average gas production 1520m 3 / d, the average methane volume fraction is 94.44%, and the gas production effect is good (the gas production curve is as follows Figure 9 shown).

Claims

1. A method for efficient gas extraction by combining remote mining pressure relief with pre-fracturing, characterized by: The following steps are involved: S1. Based on mine geological exploration data, divide and calculate the "three zones" formed by mining unloading, determine the range requiring hydraulic fracturing unloading, and identify the coordination between mining unloading and pre-fracturing based on the propagation patterns of hydraulic fracturing cracks; S2. Determine gas-rich areas based on site topography and coal face layout, and optimize horizontal well location design; S3. Design the wellbore structure based on the drilling stratum, ground stress field, and the displacement patterns of overburden caused by mining; S4. Determine the fracturing technique based on the thickness, lithology, and location of the target layer; S5. Optimize perforation and fracturing parameters: optimize the location, number of wells, and fracturing process based on actual conditions; S6. Perform drilling and fracturing operations according to drilling and fracturing designs; S7. Use microseismic fracture monitoring and evaluation technology to monitor hydraulic fracturing in real time during the fracturing process until the fracturing operation is completed; S8. Evaluate the effectiveness of joint operations based on hydraulic fracturing monitoring results and gas production.

2. The method for efficient gas extraction by remote mining pressure relief and pre-fracturing collaboration according to claim 1, characterized in that: In step S1, the mining unloading and pre-fracturing collaboration is used to determine that the mined coal seam is a lower coal seam. The method includes the following steps: S11. First, determine the total height of the collapse zone and fracture zone based on the comprehensive stratigraphic histogram and the mechanical properties of the overlying lithology of the mined coal seam; S12. When the interlayer spacing is less than the sum of the heights of the collapse zone and the fracture zone, pre-fracturing is not required. When the interlayer spacing is greater than the sum of the heights of the collapse zone and the fracture zone, calculate the fracture height of the horizontal well hydraulic fracturing. When the fracture height is less than 17 meters, adopt a combination of pre-fracturing and mining unloading mode.

3. The method for efficient gas extraction by remote mining pressure relief and pre-fracturing collaboration as claimed in claim 1, characterized in that: The basic principles for horizontal well location deployment in step S2 are: select a location with gentle terrain and convenient transportation to facilitate construction, commissioning, gathering, transportation and utilization, while avoiding the influence of underground tunnels; avoid faults and collapse columns; select gas-rich areas within the mining fracture zone; the calculation formula for the location of horizontal wells along the inclination is: L=b+Hcotα Where, L is the horizontal projection distance from the bottom of the coalbed methane surface well to the return air channel, m; b is the distance from the area with high gas concentration and large gas content in the "O" ring to the boundary of the "O" ring, which is generally taken as 1 / 3 to 2 / 3 of the average width of the "O" ring, m; H is the vertical distance from the bottom of the well to the coal seam floor, m; α is the overburden pressure relief angle, degrees.

4. The method for efficient gas extraction by remote mining pressure relief and pre-fracturing collaboration as claimed in claim 1, characterized in that: The wellbore structure optimization design in step S3 includes: selecting the upper coal seam within the mining fracture zone as the well landing layer, and the horizontal section length of the horizontal well is 700 to 1000 meters; the wellbore structure is as follows: first, drilling to 10 meters below the stable bedrock, lowering the J55 casing to 10 meters below the bedrock, and returning the cement slurry to the ground; second, drilling to 450 meters above the target layer and starting to create inclination, drilling to 5 meters above the top of the target coal seam with a dogleg angle of 6° / 30m, lowering the N80 casing, and returning the cement to the ground; third, after drilling to the target coal seam, drilling along the target coal seam to the horizontal section B target point, with a slightly downward horizontal trajectory and an angle of 1° to 3°, and completing the well with casing.

5. The method for efficient gas extraction by remote mining pressure relief and pre-fracturing collaboration as claimed in claim 1, characterized in that: In step S4, the fracturing process is based on the early coalfield exploration data, combined with the coal seam roof lithology, coal mining thickness, and the composition of the rock layer above the coal seam. The "plate" model is used to establish the initial pressure step distance and the periodic pressure step distance of the working face according to the damage limit analysis of the plate structure. The calculation formulas are as follows: Where: L m is the initial pressure step distance of the roof, in m; k is the cracking coefficient of the rock formation, k = 0.25 ~ 0.75, dimensionless; σ t is the tensile strength of the roof rock layer, in MPa; h is the thickness of the roof rock layer, in m; q is the load on the roof, in kN; E i is the elastic modulus of the overlying rock mass layer i, in MPa; h i is the thickness of the overlying rock mass layer i, in m; γ i is the bulk density of the overlying rock mass layer i, in kN / m 3 The calculation formula of the periodic pressure step distance of the roof rock layer is: Where: L z is the periodic pressure step of the roof rock layer, in m; h is the thickness of the roof rock layer, in m; σ t is the tensile strength of the roof rock layer, in MPa; q is the load on the roof, in kN.

6. The method for efficient gas extraction by remote mining pressure relief and pre-fracturing collaboration as claimed in claim 1, characterized in that: The cluster spacing during staged fracturing in step S5 is no greater than 100+L z According to the periodic pressure step, perforation is carried out with a half-period pressure step as the interval, the perforation length is 1m, and the perforation direction is directional two-wing perforation.

7. The method for efficient gas extraction by remote mining pressure relief and pre-fracturing collaboration as claimed in claim 1, characterized in that: The microseismic crack monitoring of the fractured well in step S6 includes the following steps: S61. Monitoring site survey: record the anti-bumping wellhead coordinates, topography, and landforms; S62. Deploy monitoring substations, record the coordinates of each substation, and determine the location of each substation relative to the anti-blow well; S63. System parameter setting and debugging: Turn on the instruments at the main and substations, debug the communication and data transmission between the main and substations, and set parameters. S64. The target layer fracturing construction begins, and the microcrack monitoring system is turned on and enters the monitoring state. At this time, the system automatically collects microseismic waves, records waveforms, processes data, and displays the state in real time. The target layer fracturing construction ends, and the data is saved and the system is shut down. S65. Put away all monitoring devices and complete the on-site monitoring.

8. The method for efficient gas extraction by remote mining pressure relief and pre-fracturing collaboration as claimed in claim 2, characterized in that: The fracture height calculation in step S12 is performed using a multivariate nonlinear regression method, and the correlation equation is: Where: L f is the length of the fracture, m; H f is the height of the fracture, m; W f is the width of the fracture, cm; D s is the perforation density, holes / m; q t is the construction displacement, m 3 / min;Q t is the construction liquid volume, m 3 ; is the average sand ratio, %; R 2 is the correlation coefficient, dimensionless.

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