Optimization design method for preventing and controlling mine pressure through pressure relief of porous synchronous hydraulic fracturing coal pillar

Abstract

The invention relates to an optimal design method for preventing and controlling mine pressure through pressure relief of a porous synchronous hydraulic fracturing coal pillar, which comprises the following steps: acquiring stress data, fracture development characteristics and micro-seismic energy distribution of the coal pillar through multi-source monitoring equipment, inverting a stress field of the coal pillar by combining numerical simulation software, and dividing a high-stress core area, a stress transition area and a low-stress edge area; and according to the mechanical characteristics of different stress partitions, the types, sizes and arrangement parameters of the drill holes are designed in a differentiated mode, a three-dimensional hole net of long hole dominating and short hole assisting is formed, and synchronous fracturing grouping is conducted on the drill holes according to a preset rule. By combining FLAC3D numerical simulation and multi-source monitoring and positioning coal pillar stress partition, the positioning error of a high-stress core area is controlled within + / -0.5 m, the effective coverage rate of directional long drill holes exceeds 98%, the pressure relief coverage rate is increased, the stress blind area is reduced, and the stress blind area is reduced by innovating the synchronous fracturing grouping strategy of three holes in one group and the three-dimensional hole net design of directional long drill holes and conventional straight drill holes. And the crack penetration rate is improved.

Description

Technical Field

[0001] This invention relates to the field of coal mine pressure prevention and control technology, specifically to an optimized design method for preventing and controlling mine pressure by multi-pore synchronous hydraulic fracturing coal pillar pressure relief. Background Technology

[0002] As China's coal mining gradually enters the stage of deep resource development (mines with a burial depth exceeding 800m account for 35%), the stress concentration problem of coal pillars, as the core structure for maintaining roadway stability and isolating goaf areas, is becoming increasingly prominent. Field data from typical mining areas such as Ningdong and Jincheng show that under hard roof conditions, the concentrated stress borne by coal pillars can reach 1.5-2.0 times the uniaxial compressive strength, directly inducing disasters such as rockbursts and roadway sidewall convergence exceeding 1.5m, seriously restricting mining safety and efficiency.

[0003] Currently, the mainstream coal pillar decompression technologies in the industry are mainly divided into two categories: blasting fracturing and hydraulic fracturing. Although blasting technology can quickly cut off the stress transmission path of the roof (such as the use of deep-hole blasting in Yangchangwan Coal Mine to reduce the deformation of the return airway by 1-2m), it has drawbacks such as uncontrollable energy release, easy induction of secondary mine tremors, and excessive damage to the integrity of the coal pillar. Hydraulic fracturing, due to its advantages of strong controllability and less damage to the coal body, has gradually replaced blasting as the mainstream technology.

[0004] However, existing hydraulic fracturing technology still faces three major bottlenecks in coal pillar decompression scenarios:

[0005] 1. Experience-based borehole design leads to pressure relief blind spots: In current projects, borehole layout is mostly based on roof fracturing parameters, without taking into account the differentiated stress field distribution of the coal pillar.

[0006] 2. Lack of synergy in synchronous fracturing: During multi-hole fracturing, the direction of fracture propagation is affected by both in-situ stress and inter-hole interference, and existing technologies have not established a group control mechanism;

[0007] 3. Poor adaptability of static parameter control: The parameters such as fracturing pump pressure and discharge rate are mostly preset based on indoor rock sample experiments and cannot respond to the dynamics of fracture propagation in the field.

[0008] Therefore, developing a multi-pore synchronous fracturing design method that can achieve precise stress matching, orderly fracture propagation, and dynamic parameter control has become an urgent need for safe mining in deep coal mines. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides an optimized design method for preventing mine stress by relieving pressure on coal pillars through multi-pore synchronous hydraulic fracturing. It constructs a collaborative control mechanism for multi-pore synchronous fracturing, guides the orderly connection of fractures, establishes a dynamic parameter adjustment model, improves the stability and reliability of the pressure relief effect, achieves precise matching between the coal pillar stress field and the fracturing hole network, and eliminates pressure relief blind spots.

[0010] To achieve the above objectives, the present invention provides the following technical solution: an optimized design method for preventing mine pressure by relieving pressure on coal pillars using multi-pore synchronous hydraulic fracturing, comprising the following steps:

[0011] S1: Coal pillar stress field pre-diagnosis and fracturing zone division. Coal pillar stress data, fracture development characteristics and microseismic energy distribution are collected by multi-source monitoring equipment. Combined with numerical simulation software, the coal pillar stress field is inverted to divide the high-stress core area, stress transition area and low-stress edge area.

[0012] S2: Multi-hole mesh collaborative layout optimization. Based on the mechanical characteristics of different stress zones, the borehole type, size and layout parameters are designed differently to form a three-dimensional mesh of "long hole dominance - short hole assistance", and the boreholes are synchronously fracturing grouped according to preset rules.

[0013] S3: Dynamic optimization of fracturing parameters. Based on rock mechanics experiments, the initial fracturing parameters of each stress zone are determined. Real-time monitoring equipment is deployed to collect the pressure change rate and fracture propagation rate during the fracturing process. A dynamic control model is established to correct the pump pressure and discharge rate in real time.

[0014] S4: Effect verification and feedback adjustment. After fracturing is completed, the pressure relief effect is evaluated through the microseismic monitoring system. For areas that do not meet the standards, a pressure replenishment plan is formulated to complete the secondary fracturing optimization.

[0015] Furthermore, the multi-source monitoring equipment in step S1 includes a borehole stress gauge array, a borehole endoscope, and a microseismic monitoring system; the monitoring point spacing of the borehole stress gauge array is ≤5m, and it is densely arranged in the high-stress core area, with the monitoring point spacing shortened to 2-3m;

[0016] The numerical simulation software used is FLAC3D. During the inversion process, basic mechanical parameters such as the uniaxial compressive strength, elastic modulus, and Poisson's ratio of the coal pillar are input. The criteria for determining the high-stress core zone are that the stress value is ≥ 1.2 times the uniaxial compressive strength of the coal pillar, the stress transition zone is that the stress value is 0.8-1.2 times the uniaxial compressive strength of the coal pillar, and the low-stress edge zone is that the stress value is < 0.8 times the uniaxial compressive strength of the coal pillar.

[0017] Furthermore, the differentiated design in step S2 specifically includes: directional long boreholes are arranged in the high-stress core area, with a diameter of 133-150 mm, a depth covering the entire length of the high-stress core area, a bottom-to-bottom distance of 3-5 m, and the borehole axis forming an angle of 15-20° with the coal pillar axis; conventional straight boreholes are arranged in the stress transition area, with a diameter of 90-110 mm and a spacing of 5-8 m.

[0018] The synchronous fracturing grouping adopts the strategy of "three adjacent holes as a group", with the borehole spacing within the group ≤ 8m and the spacing between groups 12-15m. Each group of boreholes contains at least one directional long borehole in a high-stress core area.

[0019] Furthermore, the initial fracturing parameters in step S3 include fracturing fluid formulation, pump pressure, and discharge rate; the fracturing fluid formulation is as follows: a mixture of "water + 0.3% drag reducer + 0.5% fracturing accelerator" is used in the high-stress core zone, and a mixture of "water + 0.3% drag reducer" is used in the stress transition zone;

[0020] The pump pressure setting rules are as follows: the initial pump pressure in the high-stress core zone is 1.1 times the coal pillar initiation pressure, and the initial pump pressure in the stress transition zone is 1.0 times the coal pillar initiation pressure. The coal pillar initiation pressure is determined through indoor rock mechanics experiments and ranges from 20 to 26 MPa. The discharge rate setting rules are as follows: the initial discharge rate per borehole is 20-30 m³ / s. 3 / h, the total displacement of each group of synchronous fracturing boreholes is ≤90m³ / h. 3 / h.

[0021] Furthermore, the real-time monitoring device in step S3 includes a borehole pressure sensor and a crack propagation monitor. The borehole pressure sensor has a measurement accuracy of ≤0.1MPa and a sampling frequency of ≥1Hz. The crack propagation monitor covers a 5-10m area around the borehole and has a crack propagation rate measurement error of ≤0.05m / min.

[0022] The control logic of the dynamic control model is as follows: when the pressure change rate ΔP / Δt is detected to be greater than 0.5 MPa / min and the fracture propagation velocity v is less than 0.2 m / min, the single-hole discharge rate is automatically increased by 5-10 m³ / h; when the fracture propagation velocity v is detected to be greater than 1.0 m / min and the pressure drop of adjacent boreholes is greater than 3 MPa, the pump pressure is automatically reduced by 10%-15%.

[0023] Furthermore, the judgment criteria for the effect verification in step S4 are: within 72 hours after fracturing, the microseismic energy reduction in the high-stress core area is ≥35%, and the surface stress value of the coal pillar drops to below 1.0 times the uniaxial compressive strength;

[0024] The pressure replenishment scheme includes: drilling additional densified holes in areas that do not meet the standards, with the hole spacing of the densified holes reduced by 20% compared to the original holes, and adopting a "low pump pressure, high displacement" mode during pressure replenishment, with the pump pressure being 0.8-0.9 times the initial pump pressure and the displacement being 1.1-1.2 times the initial displacement.

[0025] Furthermore, the directional long borehole is constructed using directional drilling technology, with the borehole trajectory deviation controlled within ±0.5° and the offset at the bottom of the borehole ≤0.3m; the verticality deviation of the conventional straight borehole is ≤1°, and the borehole depth error is controlled within ±0.5m, ensuring that the borehole effectively covers the stress transition zone.

[0026] Furthermore, the drag-reducing agent in the fracturing fluid is a polyacrylamide-based drag-reducing agent with a molecular weight of 8-12 million, a dissolution time of ≤30 min in clean water, and a drag reduction rate of ≥60%; the fracturing accelerator is a nano-sized silica particle fracturing accelerator with a particle size of 50-100 nm, which can reduce the coal pillar fracturing pressure by 8%-12% after addition, without affecting the rheological properties of the fracturing fluid.

[0027] Furthermore, the dynamic control model also includes an emergency protection mechanism: when the pump pressure is detected to suddenly rise to above 35MPa or the data collected by the pressure sensor shows abnormal fluctuations, the pressure relief procedure is automatically triggered to reduce the pump pressure to a safe pressure range (≤10MPa) within 10-15s, while stopping the fracturing operation and issuing an alarm. The fracturing process will be restarted after the fault is investigated.

[0028] Compared with the prior art, the technical solution of this application has the following beneficial effects:

[0029] 1. This invention combines FLAC3D numerical simulation with multi-source monitoring to locate the stress zoning of the coal pillar, controlling the positioning error of the high-stress core area within ±0.5m. The effective coverage rate of directional long boreholes exceeds 98%, improving the pressure relief coverage rate and reducing the stress blind zone. Through the innovative "3 boreholes per group" synchronous fracturing grouping strategy and the three-dimensional borehole network design of directional long boreholes + conventional straight boreholes, the fracture penetration rate is improved, forming a uniform three-dimensional fracture network. The coal pillar permeability is improved, and stress homogenization can be achieved with a single round of fracturing, avoiding the efficiency waste caused by multiple pressure replenishment.

[0030] 2. This invention reduces the incidence of coal pillar impact pressure by precisely positioning fracturing and dynamically controlling parameters, thus eliminating major mine pressure disasters and strengthening mining safety. Through stress homogenization and pressure relief design, the approach of the coal pillar sides and roof and floor is reduced, the maintenance cycle is extended, and the risk of production interruption is reduced. Dynamic control reduces ineffective fracturing, and the combination of perforation patterns reduces the number of boreholes, resulting in reduced fracturing fluid consumption and shorter single-hole operation time, significantly reducing construction costs. Attached Figure Description

[0031] Figure 1 This is a flowchart of the optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar pressure relief according to the present invention;

[0032] Figure 2 This is a flowchart illustrating the implementation of the optimized design method for preventing mine pressure by simultaneously hydraulically fracturing coal pillars using porous structures, as described in this invention. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Please see Figure 1-2 The optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar pressure relief in this embodiment includes the following steps:

[0035] (I) Coal pillar stress field pre-diagnosis and fracturing zone delineation

[0036] Multi-source data acquisition: The surface stress of the coal pillar is monitored by a borehole stress gauge array (monitoring point spacing ≤ 5m), and the development characteristics of coal fractures are obtained by an endoscope. At the same time, the distribution of elastic wave energy is captured by a microseismic monitoring system.

[0037] Stress zoning calculation: A coal pillar mechanical model is established based on FLAC3D, and the stress field distribution is inverted by inputting the collected data, dividing the region into three types:

[0038] High-stress core area (stress value ≥ 1.2 times uniaxial compressive strength);

[0039] Stress transition zone (stress value 0.8-1.2 times uniaxial compressive strength);

[0040] Low-stress edge zone (stress value < 0.8 times uniaxial compressive strength);

[0041] Fracturing priority ranking: The fracturing targets are determined in the order of high stress core area → stress transition area, and the low stress edge area is set as the monitoring control area.

[0042] (II) Optimization of Coordinated Arrangement of Perforated Mesh

[0043] Hole type combination design:

[0044] High-stress core area: Arrange directional long boreholes (133-150mm in diameter, with a depth covering the entire length of the core area), with the bottom of the borehole 3-5m away from the center and at an angle of 15-20° to the coal pillar axis;

[0045] Stress transition zone: equipped with conventional straight drilled holes (90-110mm in diameter), with a hole spacing of 5-8m, forming a three-dimensional hole network of "long holes as the main force and short holes as the auxiliary force";

[0046] Synchronous fracturing grouping: The grouping strategy of "three adjacent holes as a group" is adopted, with the hole spacing within a group ≤8m and the spacing between groups 12-15m, to avoid cross-group fracture interference.

[0047] (III) Dynamic optimization of fracturing parameters

[0048] Initial parameter settings:

[0049] Fracturing fluid: A mixture of "clean water + 0.3% drag reducer" is used, with 0.5% fracturing accelerator added in high-stress areas;

[0050] Pump pressure: The initial pump pressure is determined based on rock mechanics experiments. The initial pump pressure in the core zone is 1.1 times the initial pump pressure (usually 22-28 MPa), and the initial pump pressure in the transition zone is 1.0 times the initial pump pressure.

[0051] Discharge rate: Initial discharge rate of a single hole is 20-30 m³ / h, and the total discharge rate within the group is ≤90 m³ / h;

[0052] Real-time control model:

[0053] Deploy pressure sensors and crack propagation monitors in the borehole to collect pressure change rate (ΔP / Δt) and crack propagation velocity (v) in real time.

[0054] When ΔP / Δt > 0.5 MPa / min and v < 0.2 m / min, it is determined that the crack propagation is hindered, and the discharge rate is automatically increased by 5-10 m³ / h;

[0055] When v > 1.0 m / min and the pressure drop between adjacent holes > 3 MPa, it is determined that the crack is excessively penetrated, and the pump pressure is automatically reduced by 10%-15%.

[0056] Case 1: Gently dipping medium-thick coal seam (burial depth 1000m, coal seam dip angle 8°)

[0057] 1. Project Background

[0058] In a certain mine, the 1203 working face has a coal pillar along the gob, with a thickness of 8m. The original monitoring showed that the high-stress core zone of the coal pillar is 30m long and has a stress value of 32MPa (the uniaxial compressive strength of the coal body is 25MPa). The transition zone is 15m wide and the monthly average migration of the two sides of the roadway reaches 280mm, which poses a risk of rockburst.

[0059] 2. Implementation Steps

[0060] (1) Stress field diagnosis:

[0061] Three monitoring lines were set up (0.5m, 2m, and 4m from the coal pillar side), and six borehole stress gauges were installed on each line (5m apart, with the spacing increased to 3m in the core area).

[0062] The endoscope revealed that the fracture density in the core area was only 0.3 fractures / m, while that in the transition area was 1.2 fractures / m.

[0063] FLAC3D simulation and inversion of stress distribution determined that the high-stress core area is concentrated in the middle of the coal pillar within a range of 2-6m.

[0064] (2) Hole mesh arrangement:

[0065] Core area: 6 directional long boreholes (150mm in diameter, 30m in depth), with the bottom of the borehole 4m away from the center and forming an 18° angle with the coal pillar axis (biased towards the stress concentration direction).

[0066] Transition zone: 4 conventional straight drill holes (110mm in diameter, 15m in depth), 6m apart, located 2m on both sides of the core zone;

[0067] Grouping: 2 groups of simultaneous fracturing (3 core holes + 2 transition holes in each group, with a spacing of 14m between groups).

[0068] (3) Fracturing construction:

[0069] Initial parameters: The fracturing fluid is "water + 0.3% polyacrylamide drag reducer + 0.5% nano silica fracturing accelerator", the core pump pressure is 26 MPa (1.1 times the fracturing initiation pressure of 23.6 MPa), and the displacement is 30 m³ / h. 3 / h;

[0070] Dynamic control: After 30 minutes of fracturing, the pressure change rate of borehole #1, ΔP / Δt, was 0.8 MPa / min, and the fracture propagation rate, v, was 0.15 m / min. The pumping rate was automatically increased to 35 m³ / h. After 50 minutes, the pressure difference between borehole #3 and the adjacent borehole #2 reached 3.2 MPa, and v, v, was 1.1 m / min. The pump pressure was reduced to 22.1 MPa.

[0071] The fracturing time for a single group is 120 minutes, and the total injection volume is 108 m³.

[0072] (4) Effect verification:

[0073] Microseismic monitoring 72 hours after fracturing: Microseismic energy in the core area increased from an average of 1.2 × 10⁻⁶ per day. 5 J decreased to 5.8 × 10 3 J, a decrease of 42%;

[0074] Stress monitoring: The surface stress of the coal pillar decreased from 32 MPa to 22 MPa, reaching the safe value of 1.0 times the uniaxial compressive strength;

[0075] Roadway deformation: The average monthly convergence of the two sides decreased to 75mm, and the convergence of the roof and floor decreased from 150mm to 40mm.

[0076] Case 2: Steeply Dipping Thick Coal Seam (1200m depth, 35° dip angle)

[0077] 1. Project Background

[0078] The boundary coal pillar of the 805 working face of a certain mine is 15m thick and has an original stress value of 45MPa (uniaxial compressive strength of 32MPa). It has experienced two minor impact pressures and needs to be depressurized.

[0079] 2. Key Points for Differentiated Implementation

[0080] (1) Hole grid adjustment: Due to the large dip angle of the coal seam, the angle of the long directional borehole is increased to 22° (4° for gently inclined coal seams), and the borehole depth is extended to 1m to the bottom of the coal pillar to avoid stress concentration on the bottom side;

[0081] (2) Parameter optimization: The measured fracturing pressure was 32MPa, the initial pump pressure in the core area was 35.2MPa, and the discharge rate was increased to 35m³ / h to prevent the fracturing fluid from being lost due to gravity.

[0082] (3) Effects: After fracturing, the micro-vibration energy decreased by 48%, the coal pillar stress decreased to 28MPa, no impact mine pressure occurred within 6 months, and the roadway maintenance cycle was extended to 8 months.

[0083] Case 3: Thin coal seam (burial depth 850m, coal seam thickness 3m)

[0084] 1. Project Background

[0085] The isolation coal pillar in the 402 working face of a certain mine is 3m thick. Stress concentration has caused an average monthly floor heave of 120mm in the roadway, affecting transportation safety.

[0086] 2. Simplified implementation plan

[0087] (1) Simplified hole type: Due to the thin coal pillar, a short directional borehole with a diameter of 90mm (hole depth of 8m) is adopted, with a hole bottom distance of 3m and an angle of 15°;

[0088] (2) Parameter adjustment: crack initiation pressure 18MPa, initial pump pressure 19.8MPa, discharge rate 20m 3 / h, single-group injection volume 60m 3 ;

[0089] (3) Effect: The bottom drum volume was reduced to 35mm, the stress was reduced from 25MPa to 16MPa, and the pressure relief coverage rate was 96%.

[0090] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0091] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. An optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar pressure relief, characterized in that, Includes the following steps: S1: Coal pillar stress field pre-diagnosis and fracturing zone division. Coal pillar stress data, fracture development characteristics and microseismic energy distribution are collected by multi-source monitoring equipment. The coal pillar stress field is inverted by numerical simulation software to divide the high-stress core area, stress transition area and low-stress edge area. S2: Multi-hole mesh collaborative layout optimization. Based on the mechanical characteristics of different stress zones, the borehole type, size and layout parameters are designed differently to form a three-dimensional mesh of "long hole dominance - short hole assistance", and the boreholes are synchronously fracturing grouped according to preset rules; S3: Dynamic optimization of fracturing parameters. Based on rock mechanics experiments, the initial fracturing parameters of each stress zone are determined. Real-time monitoring equipment is deployed to collect the pressure change rate and fracture propagation rate during the fracturing process. A dynamic control model is established to correct the pump pressure and discharge rate in real time. S4: Effect verification and feedback adjustment. After fracturing is completed, the pressure relief effect is evaluated through the microseismic monitoring system. For areas that do not meet the standards, a pressure replenishment plan is formulated to complete the secondary fracturing optimization.

2. The optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar depressurization according to claim 1, characterized in that, The multi-source monitoring equipment in step S1 includes a borehole stress gauge array, a borehole endoscope, and a microseismic monitoring system; the monitoring point spacing of the borehole stress gauge array is ≤5m, and it is densely arranged in the high-stress core area, with the monitoring point spacing shortened to 2-3m; The numerical simulation software used is FLAC3D. During the inversion process, basic mechanical parameters such as the uniaxial compressive strength, elastic modulus, and Poisson's ratio of the coal pillar are input. The criteria for determining the high-stress core zone are that the stress value is ≥ 1.2 times the uniaxial compressive strength of the coal pillar, the stress transition zone is that the stress value is 0.8-1.2 times the uniaxial compressive strength of the coal pillar, and the low-stress edge zone is that the stress value is < 0.8 times the uniaxial compressive strength of the coal pillar.

3. The optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar depressurization according to claim 1, characterized in that, The differentiated design in step S2 specifically involves: arranging directional long boreholes in the high-stress core area, with a diameter of 133-150 mm, a depth covering the entire length of the high-stress core area, a bottom-to-bottom distance of 3-5 m, and the borehole axis forming an angle of 15-20° with the coal pillar axis; and arranging conventional straight boreholes in the stress transition area, with a diameter of 90-110 mm and a spacing of 5-8 m. The synchronous fracturing grouping adopts the strategy of "three adjacent holes as a group", with the borehole spacing within the group ≤ 8m and the spacing between groups 12-15m. Each group of boreholes contains at least one directional long borehole in a high-stress core area.

4. The optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar depressurization according to claim 1, characterized in that, The initial fracturing parameters in step S3 include fracturing fluid formulation, pump pressure, and discharge rate; the fracturing fluid formulation is as follows: a mixture of "water + 0.3% drag reducer + 0.5% fracturing accelerator" is used in the high-stress core zone, and a mixture of "water + 0.3% drag reducer" is used in the stress transition zone; The pump pressure setting rules are as follows: the initial pump pressure in the high-stress core zone is 1.1 times the coal pillar initiation pressure, and the initial pump pressure in the stress transition zone is 1.0 times the coal pillar initiation pressure. The coal pillar initiation pressure is determined through indoor rock mechanics experiments and ranges from 20 to 26 MPa. The discharge rate setting rules are as follows: the initial discharge rate per borehole is 20-30 m³ / s. 3 / h, the total displacement of each group of synchronous fracturing boreholes is ≤90m³ / h. 3 / h.

5. The optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar depressurization according to claim 1, characterized in that, The real-time monitoring equipment in step S3 includes a borehole pressure sensor and a crack propagation monitor. The borehole pressure sensor has a measurement accuracy of ≤0.1MPa and a sampling frequency of ≥1Hz. The crack propagation monitor covers a 5-10m area around the borehole and has a crack propagation rate measurement error of ≤0.05m / min. The control logic of the dynamic control model is as follows: when the pressure change rate ΔP / Δt is detected to be greater than 0.5 MPa / min and the fracture propagation velocity v is less than 0.2 m / min, the single-hole discharge rate is automatically increased by 5-10 m³ / h; when the fracture propagation velocity v is detected to be greater than 1.0 m / min and the pressure drop of adjacent boreholes is greater than 3 MPa, the pump pressure is automatically reduced by 10%-15%.

6. The optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar depressurization according to claim 1, characterized in that, The criteria for verifying the effect in step S4 are: within 72 hours after fracturing, the microseismic energy reduction in the high-stress core area is ≥35%, and the surface stress value of the coal pillar drops below 1.0 times the uniaxial compressive strength. The pressure replenishment scheme includes: drilling additional denser boreholes in areas that do not meet the standards, and adopting a "low pump pressure, high displacement" mode during pressure replenishment, with the pump pressure being 0.8-0.9 times the initial pump pressure and the displacement being 1.1-1.2 times the initial displacement.

7. The optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar depressurization according to claim 3, characterized in that, The directional long borehole is constructed using directional drilling technology, with the borehole trajectory deviation controlled within ±0.5° and the offset at the bottom of the borehole ≤0.3m. The verticality deviation of the conventional straight drilling is ≤1°, and the drilling depth error is controlled within ±0.5m to ensure that the drilling effectively covers the stress transition zone.

8. The optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar depressurization according to claim 4, characterized in that, The drag-reducing agent in the fracturing fluid is a polyacrylamide-based drag-reducing agent with a molecular weight of 8-12 million, a dissolution time of ≤30 min in clean water, and a drag reduction rate of ≥60%. The fracturing accelerator is a nano-sized silica particle fracturing accelerator with a particle size of 50-100nm. After addition, it can reduce the coal pillar fracturing pressure by 8%-12% without affecting the rheological properties of the fracturing fluid.

9. The optimized design method for preventing mine pressure by multi-pore synchronous hydraulic fracturing coal pillar depressurization according to claim 5, characterized in that, The dynamic control model also includes an emergency protection mechanism: when the pump pressure is detected to suddenly rise to above 35MPa or the data collected by the pressure sensor shows abnormal fluctuations, the pressure relief program is automatically triggered to reduce the pump pressure to a safe pressure range (≤10MPa) within 10-15s, while stopping the fracturing operation and issuing an alarm. The fracturing process will be restarted after the fault is investigated.

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