A method for efficient gas extraction through long-distance mining, decompression, and pre-fracturing collaboration.

Abstract

This invention belongs to the field of gas extraction technology, and particularly relates to a method for efficient gas extraction through long-distance mining-induced pressure relief and pre-fracturing collaboration. This invention divides and calculates the "three zones" formed by mining-induced pressure relief, identifies the collaboration between mining-induced pressure relief and pre-fracturing in conjunction with the hydraulic fracturing fracture propagation law, optimizes the design of horizontal well locations and depths, determines the fracturing process and optimizes perforation fracturing parameters, and uses microseismic fracture monitoring and evaluation technology for real-time monitoring of hydraulic fracturing during the fracturing process. By combining the "three zones" generated by mining-induced pressure relief with segmented horizontal well fracturing, regional pressure relief is achieved, resulting in the effect of "extracting multiple layers from one layer."

Description

Technical Field

[0001] This invention belongs to the field of gas extraction technology, and in particular relates to a method for efficient gas extraction through long-distance mining-driven decompression and pre-fracturing collaboration. Background Technology

[0002] Underground drilling extraction involves drilling into the coal seam or surrounding rock fissures in underground roadways to extract gas using the pressure relief effect. This method eliminates blind spots in gas extraction through a "bone-on-bone" approach, but it requires a large amount of engineering work and high capital investment. Underground permeability enhancement extraction methods mainly include hydraulic fracturing, CO2 permeability enhancement, loosening blasting pressure relief, hydraulic slotting permeability enhancement, and high-pressure water jet permeability enhancement. Hydraulic fracturing permeability enhancement uses pumps to inject high-pressure water into the coal seam, increasing its permeability. Due to underground space limitations, the pump's displacement is limited, and the fracturing modification area is relatively small. CO2 permeability enhancement utilizes the energy generated by the CO2 phase change to achieve coal seam permeability enhancement, requiring a large number of "beaded" CO2 blasts, resulting in a large engineering workload. Loosening blasting pressure relief enhances coal seam permeability through blasting, but the resulting shock waves may induce coal and gas outbursts. Hydraulic fracturing and high-pressure water jetting both use water as a medium to artificially create fractures in coal seams, increasing their permeability. These engineering techniques are relatively large-scale, and the fracturing effect is poor when the coal body is highly fractured. Surface drilling extraction involves drilling vertical / directional or horizontal wells on the surface, using perforation and fracturing techniques to create fractures in the coal seam, and then using drainage and depressurization methods to extract gas. When the coal body structure is highly fractured, the extraction effect is not ideal. In multi-seam areas with well-developed tectonic coal formations, when mining the lower seams, the depressurization effect increases the permeability of the surrounding rock, making it possible to extract gas from the overlying or upper coal seams using this depressurization method. However, when the distance between coal seams is large, a "partially connected" situation may occur, leading to discontinuous extraction or unsatisfactory extraction volumes. In order to improve the gas extraction effect of distant coal seams after the impact of mining-induced pressure relief, it is urgent to study a high-efficiency gas extraction technology and method that combines long-distance mining-induced pressure relief with pre-fracturing, so as 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 through long-distance mining-induced pressure relief and pre-fracture collaboration. Addressing the problems of poor coal seam permeability in multi-coal-seam structures, and the "quasi-connection" phenomenon between pressure relief and the "protective coal seam," which affects the gas extraction efficiency of the "protective coal seam," this invention proposes a high-efficiency gas extraction technology and method through long-distance mining-induced pressure relief and pre-fracture collaboration. By combining the "three zones" generated by mining-induced pressure relief with horizontal well staged fracturing, regional pressure relief is achieved, realizing the effect of "extracting multiple layers from one layer."

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0005] A method for efficient gas extraction through long-distance coordinated depressurization and pre-fracturing includes the following steps:

[0006] S1. Based on the geological exploration data of the mine, the "three zones" formed by mining-induced pressure relief are divided and calculated to determine the range that needs to be depressurized by fracturing. Combined with the law of hydraulic fracturing fracture propagation, the collaborative identification of mining-induced pressure relief and pre-fracturing is carried out.

[0007] S2. Based on the site topography and coal mining face layout, determine the gas-rich areas and conduct horizontal well location optimization design;

[0008] S3. Wellbore structure design is carried out based on drilling strata, geostress field, and mining-induced overburden displacement.

[0009] S4. Determine the fracturing process based on the thickness, lithology, and location of the target layer;

[0010] S5. Optimize perforation and fracturing parameters; optimize the location, number, and fracturing process of wells based on actual conditions;

[0011] S6. Conduct drilling and fracturing operations according to the drilling and fracturing design;

[0012] S7. Microseismic fracture monitoring and evaluation technology is used to monitor hydraulic fracturing in real time during the fracturing process until the fracturing operation is completed;

[0013] S8. Evaluate the combined operation effect based on the hydraulic fracturing monitoring results and gas production.

[0014] Furthermore, the determination of the cooperation between mining-induced decompression and pre-fracturing in step S1 is for mining a lower coal seam, and the method includes the following steps:

[0015] S11. First, based on the comprehensive stratigraphic columnar section and the mechanical properties of the overlying lithology of the coal seam, determine the total height of the caving zone and fracture zone;

[0016] 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, the fracture height of the horizontal well hydraulic fracturing is calculated. When the fracturing fracture height is less than 17 meters, the pre-fracturing and mining-induced pressure relief combined operation mode is adopted.

[0017] Furthermore, the basic principles for horizontal well location deployment in step S2 are: selecting locations with gentle terrain and convenient transportation for construction, commissioning, gathering, transportation, and utilization, while avoiding the influence range of underground roadways; avoiding fault and collapse column structures; selecting gas-rich areas within mining-induced fracture zones; and calculating the location of horizontal wells along the dip direction using the following formula:

[0018]

[0019] In the formula, L is the horizontal projection distance from the bottom of the coalbed methane well to the return airway, in meters; b is the distance from the boundary of the "O"-shaped ring to the area with high gas concentration and high gas content, generally taken as 1 / 3 to 2 / 3 of the average width of the "O"-shaped ring, in meters; H is the vertical distance from the bottom of the well to the bottom of the coal seam, in meters; and α is the overburden pressure relief angle, in degrees.

[0020] Furthermore, the wellbore structure optimization design in step S3 includes: selecting the upper coal seam within the mining-induced fracture zone as the well landing site, with a horizontal section length of 700-1000 meters; the wellbore structure is as follows: First, drilling to 10m below the stable bedrock, running J55 casing to 10m below the bedrock, and returning cement slurry to the surface; Second, drilling to 450m above the target layer to start directional drilling, drilling at a dogleg angle to 5m above the target coal seam roof, running N80 casing, and returning cement to the surface; Third, drilling to the target coal seam and then drilling along the target coal seam to the horizontal section B target point, with a slightly downward horizontal trajectory and an angle of 1°~3° using casing completion.

[0021] Furthermore, in step S4, the fracturing process, based on previous coalfield exploration data and combined with data on the lithology of the coal seam roof, coal thickness, and the composition of the strata above the coal seam, utilizes a "plate" model to establish the initial fracturing step distance and periodic fracturing step distance of the working face based on the failure limit analysis of the plate structure. The calculation formulas are as follows:

[0022]

[0023] In the formula: L m is the initial pressure step distance of the roof, in meters; k is the cracking coefficient of the rock strata, k = 0.25~0.75, dimensionless; is the tensile strength of the roof rock strata, in MPa; h is the thickness of the roof rock strata, in meters; q is the load on the roof, in kN; E i h represents the elastic modulus of the overlying i-th rock mass, in MPa. i γ represents the thickness of the overlying i-th rock mass, in meters; i This represents the unit weight of the overlying i-th rock mass, expressed in kN / m³. 3 The formula for calculating the periodic pressure step distance of the top strata is:

[0024]

[0025] In the formula: L z 1 is the periodic pressure step distance of the roof strata, in meters; h is the thickness of the roof strata, in meters; is the tensile strength of the roof strata, in MPa; q is the load on the roof, in kN.

[0026] Furthermore, in step S5, the cluster spacing during segmented fracturing is not greater than a certain value. The fracturing step distance is determined according to the cycle, and perforation is performed with a half-cycle fracturing step distance as the spacing. The perforation length is 1m, and the perforation direction is directional two-wing perforation.

[0027] Furthermore, the microseismic fracture monitoring of the fractured well in step S6 includes the following steps:

[0028] S61. Monitoring and on-site investigation: Record the coordinates of the anti-scour wellhead, topography, and landforms;

[0029] S62. Set up monitoring substations, record the coordinates of each substation, and determine the position of each substation relative to the anti-scour well;

[0030] S63. System Parameter Setting and Debugging: Turn on the main and substation instruments, debug the communication and data transmission between the main and substations, and set the parameters.

[0031] S64. When the target layer fracturing operation begins, turn on the microcrack monitoring system to enter the monitoring state. At this time, the system automatically collects micro-seismic waves, records waveforms, processes data, and displays the status in real time. When the target layer fracturing operation ends, save the data and turn off the system.

[0032] S65. Put away all vibration detectors to complete this on-site monitoring.

[0033] Furthermore, in step S12, the fracture height is calculated using a multivariate nonlinear regression method, and the correlation equation is:

[0034] ;

[0035] In the formula: L f The length of the fracturing fracture is in meters (m). H f The fracture height is in meters (m). W f The width of the fracturing fracture is in cm; D t Perforation density, holes / m; q t For construction displacement, m 3 / min; Q t The volume of the construction fluid is in meters. 3 φ represents the average sand ratio, % R 2 The correlation coefficient is dimensionless.

[0036] The advantages of this invention are:

[0037] 1. This invention uses horizontal well segmented fracturing to connect hydraulic fracturing fractures with mining-induced decompression fractures, thereby improving the gas extraction effect of distant coal seams after the impact of mining-induced decompression and achieving the effect of "mining one layer and extracting multiple layers".

[0038] 2. Based on the distribution characteristics of the "three zones" under the influence of coal seam mining, the fracturing of horizontal wells on the surface is carried out in stages to make the fracturing fractures communicate with the fractures in the mining fracture zone, providing an effective channel for gas migration. This method can extract gas from multiple layers through single-layer horizontal well construction, which is a highly efficient gas extraction mode. Attached Figure Description

[0039] Figure 1 This is a process flow diagram of the present invention.

[0040] Figure 2 Distribution characteristics of overlying rock fractures at different advance distances.

[0041] Figure 3 This is a schematic diagram of mining-induced fractures.

[0042] Figure 4 These are the simulation results of hydraulic fracturing under different construction parameters in this invention.

[0043] Figure 5 This is a schematic diagram of the horizontal well location of the present invention.

[0044] Figure 6 This is a top-view schematic diagram of the horizontal well location of the present invention.

[0045] Figure 7 This is a planar projection distribution diagram of the fracturing points in this invention.

[0046] Figure 8 This is a diagram showing the results of fracturing monitoring in this invention.

[0047] Figure 9 This is the gas production curve of well PCD-01 in the application example. Detailed Implementation Example

[0048] The efficient gas extraction technology and methods for long-distance mining-driven decompression and pre-fracture collaboration mainly include: identification of mining-driven decompression and pre-fracture collaboration, optimization design of horizontal well location and wellbore structure, optimization design of horizontal well segmented fracturing parameters, and evaluation of collaborative extraction effects. The process flow for efficient gas extraction through long-distance mining-driven decompression and pre-fracture collaboration is as follows: Figure 1 As shown.

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

[0050] Within the caving and fracture zones, mining-induced fractures form a network, significantly increasing the permeability of the coal and rock strata and the degree of gas dissipation. This area is an effective region for gas extraction from adjacent strata. Using 3DEC numerical simulation software combined with engineering practice, and by observing the fracture patterns, displacement, and stress of the overlying strata during working face mining, the development height and distribution characteristics of fractures in the overlying strata of the working face goaf were determined. This simulation sets the strike model length to 300m, the dip length to 200m, and the dip angle to 0°. The simulation results are as follows: Figure 2 As shown.

[0051] from Figure 2 It can be seen that the fractures in the overlying strata are in different developmental states at different advance distances, and the range and density of fracture development also vary. Early stage of working face excavation ( Figure 2 (a, b)), delamination fractures are formed asynchronously by rock strata movement, while trans-strata fractures are formed by rock strata breaking. Fractures are densely developed on both sides of the collapsed rock, while they are sparsely developed in the middle area, resulting in larger delamination fractures. Mid-stage of working face excavation ( Figure 2 (c, d) The lower rock strata lost their support and completely fractured. The collapsed area was crisscrossed with fissures, but no obvious compressive stress was observed. The rock strata at the cut end fractured at a certain angle, and the resulting fissures developed upwards in a trapezoidal shape at a certain angle. Delamination fissures extended beyond the working face and the cut end, developing upwards in an inverted trapezoidal shape. As the working face advanced, the fracture fissures continued to develop in a trapezoidal shape. Due to the existence of the stable masonry beam structure caused by the fractured rock strata, the fissures on the working face side continued to move forward with the working face advancement. Delamination fissures continued to develop upwards in an inverted trapezoidal shape, and their range expanded continuously with the advancement of the working face. The areas with dense fissures were still on both sides of the collapsed area, with relatively fewer fissures in the middle. Larger delamination fissures no longer appeared. (Middle and late stages of working face excavation) Figure 2 (e, f)) The cross-layer and delamination fractures on the working face side continuously generate and extend forward as the working face advances; the fractures on the cut-in side remain generally stable; the area of ​​fracture bearing and compressive closure in the middle of the goaf continuously increases. The overall fracture development area on the working face side is larger than that on the cut-in side. Fractures in the collapse area are mainly distributed on both sides of the goaf, and the fracture development is more obvious in the unmined areas on both sides. Only a few fractures exist in the middle of the collapse area. The fractures in the middle of the overlying rock failure area gradually tend to compact, the fractures no longer develop upwards, and the height shows a decrease, with a few transverse fractures appearing at the top. As the working face advances, the following gradually forms: Figure 3 The fissure channel shown.

[0052] 2) Identification of the collaboration between decompression and pre-fracturing

[0053] This study primarily focuses on identifying the lower coal seam being mined. The identification method is as follows:

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

[0055] Empirical formulas for the height of caving zones and fracture zones are shown in Table 1.

[0056]

[0057] (2) Let the distance between the upper and lower coal seams be Hc meters. The sum of the heights of the caving zone and the fracture zone can be calculated based on the formula in Table 1 and the lithology.

[0058] The interlayer spacing is less than the sum of the heights of the caving zone and the fracture zone.

[0059] H c ≤H m +H li (1)

[0060] In this case, pre-fracturing is not required.

[0061] The interlayer spacing is greater than the sum of the heights of the caving zone and the fracture zone.

[0062] When H c >H m +H li In this case, the fracture height of the horizontal well hydraulic fracturing needs to be calculated.

[0063] This study employed numerical simulation to calculate the fracture height in hydraulic fracturing. The FracproPT numerical simulation software was used to simulate the fracture scale under various construction parameters, including perforation density, flow rate, fluid volume, average sand ratio, and sand addition method.

[0064] With a perforation density of 12 holes / m and a construction flow rate of 12m³, 3 / min, construction fluid volume 1000m 3 The simulation parameters were an average sand ratio of 14% and stepped sand addition. The simulation measured the joint height, length, and width under different average sand ratios, perforation densities, construction flow rates, and construction fluid volumes. The simulation results are as follows: Figure 4 As shown.

[0065] Depend on Figure 4 It can be seen that the higher the perforation density, the greater the fracture half-length and fracture height. When the perforation density is greater than 12 holes / m, the increase in fracture half-length and fracture height decreases. The fracture length and fracture height are logarithmically positively correlated with the deep hole density. The higher the drilling flow rate, the greater the fracture half-length and fracture height. When the flow rate exceeds 10m³, the increase in fracture half-length and fracture height further increases. 3At a flow rate of 1 / min, the increase in crack half-length decreases; the larger the flow rate of the construction fluid, the greater the crack half-length and crack height; when the flow rate of the construction fluid exceeds 1400m... 3 At this time, the increase in fracture half-length and fracture height slowed down; different proppant addition methods and proppant ratios led to significant differences in proppant migration characteristics and accumulation areas, resulting in different hydraulic fracturing fractures. Stepped concentration proppant addition was less prone to proppant blockage, allowing hydraulic fracturing fractures to extend better in the coal seam. With an average proppant ratio increasing from 10% to 16%, the fracture half-length initially increased and then decreased, while the total fracture height decreased. Using a multivariate nonlinear regression method, the correlation equation between construction parameters (perforation density, construction flow rate, construction fluid volume, average proppant ratio) and fracturing fracture extension scale (fracture length, fracture height, fracture width) was obtained as follows:

[0066] (2)

[0067] In the formula: L f The length of the fracturing fracture is in meters (m). H f The fracture height is in meters (m). W f The width of the fracturing fracture is in cm; D t Perforation density, holes / m; q t For construction displacement, m 3 / min; Q t The volume of the construction fluid is in meters. 3 φ represents the average sand ratio, % R 2 The correlation coefficient is dimensionless.

[0068] Simulations revealed that the height of hydraulic fracturing fractures is unlikely to exceed 17 meters. Based on this, it can be determined that when H... c -H m -H li When the depth is ≥17 meters, even with pre-fracturing technology, it is difficult to form a smooth gas migration channel after mining-induced impacts. Therefore, it is not recommended to use a pre-fracturing and mining-induced pressure relief coordination mode for gas extraction. When 0 < H c -H m -H li When the depth is less than 17 meters, a combined pre-fracturing and mining-induced depressurization mode can be adopted. Therefore, the criterion for the cooperation between mining-induced depressurization and pre-fracturing can be expressed as:

[0069] 0 < H c -H m -H li <17 (3)

[0070] 3) Optimization design of horizontal well location and wellbore structure

[0071] (1) Well location of horizontal well

[0072] The basic principles for horizontal well site deployment are: to select locations with gentle terrain and convenient transportation for construction, commissioning, gathering and transportation, and utilization, while avoiding the influence range of underground roadways; to avoid fault and collapse column structures; and to select gas-rich areas within the mining-induced fracture zone.

[0073] The formula for calculating the location of horizontal wells along the dip direction is:

[0074] (4)

[0075] In the formula, L is the horizontal projection distance from the bottom of the coalbed methane well to the return airway, in meters; b is the distance from the boundary of the "O"-shaped ring to the area with high gas concentration and high gas content, generally taken as 1 / 3 to 2 / 3 of the average width of the "O"-shaped ring, in meters; H is the vertical distance from the bottom of the well to the bottom of the coal seam, in meters; and α is the overburden pressure relief angle, in degrees.

[0076] Generally, the optimal horizontal projection of a horizontal well location should be 0.17 to 0.28 times the mining length, and close to the return airway strip area. Figure 5-6 As shown.

[0077] (2) Wellbore structure optimization design

[0078] The upper coal seam within the mining-induced fracture zone is selected as the wellbore location. The horizontal section of the well is typically 700–1000 meters long. The wellbore structure is as follows: First, drilling proceeds to 10m below the stable bedrock, with J55 casing running to 10m below the bedrock, and cement slurry returned to the surface. Second, drilling begins at 450m above the target layer, followed by directional drilling at a dogleg angle to 5m above the target coal seam roof, where N80 casing is run, and cementing is returned to the surface. Third, after reaching the target coal seam, drilling continues along the target coal seam to the horizontal section B target point, with a slightly downward horizontal trajectory at an angle of 1°–3°, using casing completion.

[0079] 4) Optimization design of horizontal well staged fracturing parameters

[0080] (1) Optimization design of perforation location

[0081] Based on previous coalfield exploration data, combined with data on coal seam roof lithology, coal mining thickness, and the composition of strata above the coal seam, the initial compaction step and periodic compaction step of the working face are established using a "plate" model and based on the failure limit analysis of the plate structure. The calculation formulas are as follows:

[0082] (5)

[0083] In the formula: L mis the initial pressure step distance of the roof, in meters; k is the cracking coefficient of the rock strata, k = 0.25~0.75, dimensionless; is the tensile strength of the roof rock strata, in MPa; h is the thickness of the roof rock strata, in meters; q is the load on the roof, in kN; E i h represents the elastic modulus of the overlying i-th rock mass, in MPa. i γ represents the thickness of the overlying i-th rock mass, in meters; i This represents the unit weight of the overlying i-th rock mass, expressed in kN / m³. 3 .

[0084] (6)

[0085] In the formula: L z 1 is the periodic pressure step distance of the roof strata, in meters; h is the thickness of the roof strata, in meters; is the tensile strength of the roof strata, in MPa; q is the load on the roof, in kN.

[0086] Generally, the short axis of the plastic zone influence range of hydraulic fracturing fractures is 60-80m. To ensure a blind zone in the distribution of hydraulic fracturing fractures, the cluster spacing during segmented fracturing should not exceed a certain value. The perforation step distance is determined according to the cycle, with a half-cycle perforation step distance as the interval. The perforation length is 1m, and the perforation direction is directional two-wing perforation. Figure 7 As shown.

[0087] According to the well site and wellbore structure design, the horizontal sections are all arranged in the rock strata. The fracturing points should be evenly distributed and avoid geological structures such as faults. The fracturing points are evenly distributed. After the first stage of fracturing is completed, the scale of subsequent fracturing needs to be adjusted in real time based on the construction pressure, displacement, and sand addition of the first stage, so as to optimize the fracturing design and increase production. The basic construction parameters for fracturing are shown in Table 2.

[0088]

[0089] 5) Microseismic fracturing monitoring

[0090] The effectiveness of segmented hydraulic fracturing in horizontal wells on the surface is evaluated using microseismic fracture monitoring technology. This technology can monitor the spatial morphology, effective fracture length, fracture height, and stress distribution of fractures in the fracturing well. The surface microseismic system includes a real-time fracture monitoring system, microseismic detectors, signal amplifiers, wireless transmitters and receivers, high-precision GPS, and a PC. During fracturing operations, the fracturing endpoints of each fracturing segment are calculated according to the fracturing design. Detectors are deployed based on their projection onto the ground and the surrounding topography (centered on the center of the horizontal well segment). Detector positioning must be accurate using high-precision GPS, with a burial depth of no less than 0.2m. The real-time monitoring data is interpreted and analyzed using a field monitoring system, a computer, and a corresponding expert interpretation system. The specific steps are as follows:

[0091] (1) Monitoring on-site investigation: Record the coordinates of the anti-scour wellhead, topography, and landform;

[0092] (2) Set up monitoring substations, record the coordinates of each substation, and determine the position of each substation relative to the anti-scour well;

[0093] (3) System parameter setting and debugging: Turn on the main and substation instruments, debug the communication and data transmission between the main and substations, and set the parameters;

[0094] (4) When the target layer fracturing operation begins, turn on the microcrack monitoring system to enter the monitoring state. At this time, the system automatically collects microseismic waves, records waveforms, processes data, and displays the status in real time. When the target layer fracturing operation ends, save the data and turn off the system.

[0095] (5) Put away all the seismic sensors and complete this on-site monitoring.

[0096] Fracturing monitoring results such as Figure 8 As shown.

[0097] Application examples

[0098] This process has been piloted in the Pingdingshan mining area with good results. The PCD-01 well is an L-shaped horizontal well in the No. 42 coal seam of the Shoushan No. 10 Mine. Located within the Ji15-33200 working face, which is approximately 1080m long and 150m wide, the horizontal section of the well is 800m long. Hydraulic fracturing using perforation was employed, with fracturing points spaced at 90m intervals. Production began on March 11, 2022, with a maximum daily gas production of 10492 m³. 3 / d, cumulative gas production 157.64×10⁴ m³ 3 Average gas production 1520 m³ 3 / d, with an average methane volume fraction of 94.44%, indicating good gas production (gas production curve as shown). Figure 9 (As shown).

Claims

1. A method for efficient gas extraction through long-distance coordinated depressurization and pre-fracturing, characterized in that: Includes the following steps: S1. Based on the mine geological exploration data, the "three zones" formed by mining-induced pressure relief are divided and calculated to determine the area requiring hydraulic fracturing and pressure relief. The coordination between mining-induced pressure relief and pre-fracturing is then identified based on the hydraulic fracturing fracture propagation law. Specifically, the coordination between mining-induced pressure relief and pre-fracturing is applied to the lower coal seam being mined and the upper coal seam being pre-fracturing. The method includes the following steps: S11. First, based on the comprehensive stratigraphic columnar section and the mechanical properties of the overlying lithology of the coal seam, determine the total height of the caving zone and fracture zone; S12. When the interlayer spacing between the upper and lower coal seams is less than the sum of the heights of the caving zone and the fracture zone, pre-fracturing is not required; when the interlayer spacing is greater than the sum of the heights of the caving zone and the fracture zone, the fracture height of the horizontal well hydraulic fracturing is calculated; when the fracturing fracture height is less than 17 meters, the pre-fracturing and mining-induced pressure relief combined operation mode is adopted. S2. Based on the site topography and coal mining face layout, determine the gas-rich areas and conduct horizontal well location optimization design; S3. Wellbore structure design is carried out based on drilling strata, geostress field, and mining-induced overburden displacement. S4. Determine the fracturing process based on the thickness, lithology, and location of the target layer; S5. Optimize perforation and fracturing parameters; optimize the location, number, and fracturing process of wells based on actual conditions; S6. Conduct drilling and fracturing operations according to the drilling and fracturing design; S7. Microseismic fracture monitoring and evaluation technology is used to monitor hydraulic fracturing in real time during the fracturing process until the fracturing operation is completed; S8. Evaluate the combined operation effect based on the hydraulic fracturing monitoring results and gas production.

2. The method for efficient gas extraction through long-distance depressurization and pre-fracturing collaboration as described in claim 1, characterized in that: The basic principles for horizontal well location deployment in step S2 are: selecting locations with gentle terrain and convenient transportation to facilitate construction, commissioning, gathering and transportation, and utilization, while avoiding the influence range of underground roadways; avoiding fault and collapse column structures; selecting gas-rich areas within mining-induced fracture zones; and using the formula for calculating the horizontal well location along the dip direction: In the formula, L is the horizontal projection distance from the bottom of the coalbed methane well to the return airway, in meters; b is the distance from the boundary of the "O"-shaped ring to the area with high gas concentration and high gas content, generally taken as 1 / 3 to 2 / 3 of the average width of the "O"-shaped ring, in meters; H is the vertical distance from the bottom of the well to the bottom of the coal seam, in meters; and α is the overburden pressure relief angle, in degrees.

3. The method for efficient gas extraction through long-distance depressurization and pre-fracturing collaboration as described in claim 1, characterized in that: The wellbore structure optimization design in step S3 includes: selecting the upper coal seam within the mining-induced fracture zone as the wellbore entry point, with a horizontal section length of 700-1000 meters; the wellbore structure is as follows: first, drilling to 10m below the stable bedrock, then running the J55 casing to 10m below the bedrock, with cement slurry returning to the surface; second, drilling to 450m above the target layer to begin directional drilling, using a dogleg angle. Drill to 5m from the top of the target coal seam, run N80 casing, and return cement to the surface; after drilling to the target coal seam in the third section, drill along the target coal seam to the horizontal section B target point, with a slightly downward horizontal trajectory at an angle of 1°~3°, and complete the well using casing.

4. The method for efficient gas extraction through long-distance depressurization and pre-fracturing collaboration as described in claim 1, characterized in that: In step S4, the fracturing process, based on previous coalfield exploration data and combined with data on the lithology of the coal seam roof, coal mining thickness, and the composition of the strata above the coal seam, uses a "plate" model to establish the initial fracturing step distance and periodic fracturing step distance of the working face based on the failure limit analysis of the plate structure. The calculation formulas are as follows: In the formula: L m σ represents the initial pressure step distance from the top plate, in meters; k is the cracking coefficient of the rock strata, k = 0.25~0.75, dimensionless; t ρ is the tensile strength of the roof strata, in MPa; h is the thickness of the roof strata, in m; q is the load on the roof, in kN; E i h represents the elastic modulus of the overlying i-th rock mass, in MPa. i γ represents the thickness of the overlying i-th rock mass, in meters; i This represents the unit weight of the overlying i-th rock mass, expressed in kN / m³. 3 The formula for calculating the periodic pressure step distance of the top strata is: In the formula: L z σ represents the periodic pressure step distance of the top strata, in meters; h represents the thickness of the top strata, in meters; σ t q represents the tensile strength of the roof strata, in MPa; q represents the load on the roof, in kN.

5. The method for efficient gas extraction through long-distance depressurization and pre-fracturing collaboration as described in claim 4, characterized in that: In step S5, the cluster spacing during segmented fracturing is not greater than [amount missing]. The perforation is carried out by pressing the step distance according to the cycle, and the perforation is carried out at a interval of half a cycle. The perforation length is 1m, and the perforation direction is directional two-wing perforation.

6. The method for efficient gas extraction through long-distance depressurization and pre-fracturing collaboration as described in claim 1, characterized in that: The microseismic fracture monitoring of the fractured well in step S6 includes the following steps: S61. Monitoring and on-site investigation: Record the coordinates of the anti-scour wellhead, topography, and landforms; S62. Set up monitoring substations, record the coordinates of each substation, and determine the position of each substation relative to the anti-scour well; S63. System Parameter Setting and Debugging: Turn on the main and substation instruments, debug the communication and data transmission between the main and substations, and set the parameters; S64. When the target layer fracturing operation begins, turn on the microcrack monitoring system to enter 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 operation ends, save the data and turn off the system. S65. Retract the microseismic detector to complete this on-site monitoring.

7. The method for efficient gas extraction through long-distance depressurization and pre-fracturing collaboration as described 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 as follows: ； In the formula: L f The length of the fracturing fracture is in meters (m). H f The fracture height is in meters (m). W f The width of the fracturing fracture is in cm; D t Perforation density, holes / m; q t For construction displacement, m 3 / min; Q t The volume of the construction fluid is in meters. 3 φ represents the average sand ratio, % R 2 The correlation coefficient is dimensionless.

Images