Anti-reflection method for outburst coal seam with local structure development

Abstract

The invention discloses a permeability increasing method for a local structure development outburst coal seam. The permeability increasing method comprises the steps that S1, a drill hole is formed in the set position of a coal rock mass; s2, cutting seams are formed in the coal hole section of the drill hole in an outward communication mode with the drill hole as the center along the coal hole section of the drill hole; s3, the fracturing frequency and the water injection flow are set, and dynamic pressure fracturing is conducted on the coal seam at the drilling position; and S4, after slotting-fracturing permeability increasing operation is carried out on the coal seam based on the steps S1-S3, mechanical hole expanding and cave forming are carried out on the drilled tectonic coal body section, and slotting-fracturing-hole expanding composite permeability increasing is carried out on the coal seam. According to the coal seam permeability increasing method, cutting-pressing-increasing permeability increasing is carried out on the outburst coal seam with the local structure development, and for the coal seam with the large burial depth, the high gas pressure, the small local development hidden structure and the low coal seam permeability, the overall coal seam gas extraction effect can be further improved, the coal seam outburst danger is eliminated, and the drilling construction amount is reduced; effective drainage of the gas in the structure soft coal body with the poor gas migration capacity is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of coal and gas outburst prevention and control, and particularly relates to a method for increasing the permeability of a local structure developed outburst coal seam. BACKGROUND

[0002] With the increase of the depth of coal mining, the gas permeability of the coal seam is reduced synchronously, and the probability of coal and gas outburst also increases. The core measure for preventing and controlling coal and gas outburst accidents is to reduce the gas content in the coal seam to below the safety threshold, so it is necessary to prevent and control mine dynamic disasters from the perspective of efficient gas extraction and effective pressure relief of the coal seam.

[0003] Due to the complex structure of the local structure developed outburst coal seam, the disaster-causing mechanism thereof is more complex than that of the conventional coal seam. When the gas extraction is performed, the method for increasing the permeability of the coal seam (such as hydraulic fracturing) commonly used at the present stage cannot effectively solve the problem of poor gas permeability of the coal seam, resulting in long gas extraction time, poor effect, and small effective influence range. Therefore, a large number of drill holes need to be arranged in the area where the gas extraction is performed, which increases the construction amount and is not conducive to the fully mechanized production of the coal mine. SUMMARY

[0004] In order to solve the problems in the prior art to some extent, the present application provides a method for increasing the permeability of a local structure developed outburst coal seam, and the specific technical scheme is as follows: A method for increasing the permeability of a local structure developed outburst coal seam, comprising the following steps: S1, drilling a drill hole at a set position of a coal rock body; S2, cutting a slot outward along the coal hole section of the drill hole and connecting the slot to the drill hole as the center on the coal hole section of the drill hole; S3, setting a fracturing frequency and a water injection flow rate, and performing dynamic pressure fracturing on the coal seam at the drill hole; S4, after the slot-fracturing permeability increasing operation on the coal seam based on steps S1-S3, mechanically expanding and drilling a hole in the structure coal body section of the drill hole, and performing slot-fracturing-expansion composite permeability increasing on the coal seam.

[0005] Further, The method for mechanically expanding and drilling a hole in the structure coal body section of the drill hole is as follows: after the drill hole is drilled to a set hole depth by a hydraulic drilling machine and a core rod jacking type hole drilling rod, the drill rod is retreated to a hole drilling target position, and a mechanical hole drilling pressure relief device is used to drill a hole.

[0006] Further, The permeability increasing effect of the coal seam is quantified by the extraction radius, and the method is as follows: Two induced cracking boreholes are constructed for comparison and investigation, and the distance between the two induced cracking boreholes is greater than or equal to 25 m; a plurality of gas pressure observation holes are arranged between the boreholes for increasing the permeability, and the range of the effective drainage radius is taken as 51% of the gas pressure drop, the gas pressure drop ratio and the gas pressure change rule of each gas pressure observation hole are obtained, and the fitting relationship diagram of the effective drainage radius and the drainage time is drawn.

[0007] Further, The width and length of the slot set in step S2 and the fracturing frequency and water injection flow rate set in step S3 are numerically simulated in the discrete element software.

[0008] Further, The coal seam permeability improvement numerical model is constructed in the discrete element software, including the following steps: I. The mechanical parameters of the tectonic coal and non-tectonic coal field coal seam samples are collected, including tensile strength, compressive strength, Young's modulus, Poisson's ratio, internal friction, and internal friction angle; II. A 150mm x 150mm particle assembly model is established in the PFC 2D software, the minimum particle diameter is set to 0.8mm, the particle size ratio R max : R min =1.66; A circular particle-free area with a diameter of 5mm is set at the center of the particle assembly model as a water injection borehole, and the periphery of the particle assembly model is set as a water permeable boundary; III. Two particle bands are set in the horizontal or vertical direction of the particle assembly model to simulate the soft layering area of the tectonic coal, and a local tectonic development outburst coal seam is constructed; the particles of the soft layering area are assigned with reference to the collected tectonic coal parameters to construct a tectonic coal simulation area; the other particles in the model are assigned with reference to the collected non-tectonic coal parameters to construct a non-tectonic coal simulation area.

[0009] IV. Four steel structure enclosing walls are established around the particle assembly model, and no friction force is set between the particle assembly model and the enclosing walls; the enclosing walls are set to apply constant horizontal stress and vertical stress to the particle assembly model at a constant speed, thereby applying constant load to the particle assembly model; V. Parallel bonding bonds are assigned to the particle assembly model, and the movement between particles follows Newton's second law and the force-displacement law; VI. A slot is set at the water injection borehole, the slot is communicated with the water injection borehole, and the pulsating hydraulic fracturing simulation under different fracturing frequencies and different water injection flow rates is carried out through the water injection borehole, and the relationship between the fracturing frequency and the water injection flow rate under the slot-fracturing permeability improvement simulation and the coal body cracking pressure and the total number of developed cracks is output.

[0010] Further, The following steps are further included in the construction of the coal seam permeability improvement numerical model in the discrete element software: According to the reaming diameter of the gas extraction borehole in the tectonic coal segment in the actual production operation process, the water injection borehole diameter in the particle assembly model in step II is enlarged in proportion, a particle assembly model after reaming is constructed, steps III-VI are repeated, and the law between the fracturing frequency and the water injection flow rate and the coal body cracking pressure and the total number of developed cracks under the simulation of the combined permeability improvement of the slitting-fracturing-reaming is output.

[0011] Further, The coal body mechanical parameter calibration is respectively performed on the tectonic coal simulation area and the non-tectonic coal simulation area of the coal seam permeability improvement numerical model, and the following steps are performed: I. A biaxial compression model is constructed in the PFC 2D software, the biaxial compression model is set as a rectangle with a size of 150 mm x 75 mm; the biaxial compression model is not provided with a borehole and a slitting, and other model parameters are the same as those of the particle assembly model; II. Walls are provided around the biaxial compression model, the walls are given a set initial pressure, the upper and lower walls apply a constant speed to the biaxial compression model, and the pressure is stopped when the axial stress reaches the peak and falls to 80% of the peak value; III. The biaxial stress-strain data of the upper and lower walls and the lateral strain data of the biaxial compression model are recorded synchronously, the stress-strain curve is generated, and the Mohr circle and the envelope line are drawn; IV. The Mohr circle is approximated to the envelope line of the actual coal sample by repeating steps II and III by trial and error until the mechanical parameter calibration result of the coal seam permeability improvement numerical model has an error of ≤5% with the mechanical parameter of the actual coal seam coal sample.

[0012] Further, The permeability of the coal seam permeability improvement numerical model before and after fracturing is compared, and the permeability improvement effect is quantitatively evaluated, and the method is as follows: I. Fluid pipes and fluid domains are given to the coal seam permeability improvement numerical model, the fluid pipe refers to the contact between particles, and the fluid domain refers to a polygon closed area surrounded by adjacent and contacted particle center points; II. The coal seam permeability improvement numerical model is given the seepage hydrology parameters, including: fluid pipe initial opening, fluid domain apparent volume, fracture residual opening enlargement coefficient, fluid volume modulus, opening half-opening compression force, and fluid viscosity; III. A fixed value confining pressure is given to the coal seam permeability improvement numerical model, a certain size injection hole is drilled from the center position of the outer surface of the coal seam permeability improvement numerical model, a fixed value of the inlet pressure P1 is applied to one side of the coal seam permeability improvement numerical model, and a fixed value of the outlet pressure P2 is applied to the other side, and P1>P2; when the average pore pressure and the total flow rate of the coal seam permeability improvement numerical model reach a steady state, the permeability K0 of the coal seam permeability improvement numerical model is calculated; IV. Remove the confining pressure, inlet pressure and outlet pressure of the coal seam permeability numerical model in step III, and assign a certain flow value and a certain pressure value of fluid to the coal seam permeability numerical model from the injection hole in step III, and assign the coal seam permeability numerical model with hydraulic fracturing simulation, and record the particle state and crack state of the coal seam permeability numerical model; V. Remove the operation result of the coal seam permeability numerical model in step IV, repeat step IV, and obtain the permeability K1 of the coal seam permeability numerical model in the steady state under constant pressure after hydraulic fracturing simulation, and compare the permeability K0 and the permeability K1 to quantify the hydraulic fracturing permeability effect of the coal seam permeability numerical model.

[0013] Based on the above technical scheme, the method disclosed by the application has the following beneficial effects: 1. The coal seam permeability method disclosed by the application can realize cutting-pressure permeability by cutting the coal seam, form macroscopic slots and a large number of secondary cracks in the coal seam, form a gas flow path, realize annular network flow self-unloading pressure between drill holes, generate a large number of fine cracks and realize coal seam pressure relief. Hydraulic fracturing of the coal seam can penetrate the fine cracks in the coal seam and form more complex crack channels. Hydraulic cutting of the coal seam and then hydraulic fracturing permeability can greatly improve the overall permeability coefficient of the coal body, improve the gas extraction effect, and eliminate the occurrence of coal and gas outburst.

[0014] 2. During the coal-forming period, the coal seam is subjected to magma rock intrusion, and due to geological movement, local positions of the coal seam are dislocated, small faults that are difficult to detect exist in the coal seam, the gas content in the local area of the coal seam is high, the mechanical strength of the coal body is low, and the permeability is poor. When the working face advances to the vicinity of such coal body, the coal body in the area is disturbed by mining, which increases the gas migration channel, and the coal body has strong gas diffusion capacity, so that a large amount of gas flows to the mining working face, increasing the gas concentration of the working face and the return airway. The coal seam permeability method disclosed by the application can further improve the overall gas extraction effect of the coal seam, eliminate the outburst danger of the coal seam, reduce the drilling construction amount, and realize effective drainage of gas in the hidden "coal bag type" structure soft coal body with poor gas migration capacity.

[0015] 3. The coal seam permeability method disclosed by the application takes a coal mine field test as an example, such as Figure 2 and Figure 3As shown, the drainage radius of drainage borehole 1 for 120d is 2.23m, the drainage radius of drainage borehole 2 for 120d is 2.15m, and the average original gas drainage radius is 2.19m. After the “cutting-pressing-expanding” composite permeability enhancement technology, the effective drainage radius of permeability hole 1 for 120d is 6.04m, which is increased by 3.81m compared with the original coal seam calculation value; the effective drainage radius of permeability hole 2 for 120d is 6.15m, which is increased by 4.00m compared with the original coal seam calculation value. The average drainage radius of coal seam is 6.095m, which is increased by 3.905m compared with the original coal seam calculation value, and is increased by 178.31%.

[0016] 4. The permeability of the coal seam after the “cutting-pressing-expanding” permeability enhancement is tested, and the change of the permeability of the coal seam before and after the “cutting-pressing-expanding” permeability enhancement is compared, so as to realize the numerical quantitative evaluation of the fracturing effect. The evaluation results are shown in the following figure and Table 1 (note: the values in Table 1 are not fixed values, but the average values after multiple tests): Table 1 Simulation of coal seam permeability after fracturing , The results show that the permeability of the coal seam after permeability enhancement is significantly improved. The permeability of the coal seam before “cutting-pressing” permeability enhancement is 3.846x10-18m 2 , the permeability of the coal seam after permeability enhancement is 48.075x10-18m 2 , which is about 12.5 times the original coal seam permeability, and the permeability of the coal seam after “cutting-pressing-expanding” permeability enhancement is 65.843x10-18m 2 , which is about 17.1 times the original coal seam, which shows that the “cutting-pressing” and “cutting-pressing-expanding” permeability enhancement technologies have good permeability enhancement effect, and the “cutting-pressing-expanding” permeability enhancement technology has more significant permeability enhancement effect. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a flowchart of the local structure development prominent coal seam permeability enhancement method disclosed in the present application; Figure 2 is a layout diagram of the permeability enhancement and effective drainage radius observation borehole; Figure 3 is a diagram showing the relationship between the effective drainage radius and the drainage time after the “cutting-pressing-expanding” composite permeability enhancement; Figure 4 is a diagram of a coal seam permeability numerical model; Figure 5 is a diagram of two directions of the coal seam permeability numerical model; Figure 6 is a diagram of a biaxial compression model. DETAILED DESCRIPTION

[0018] The technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit this disclosure or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without creative effort are within the scope of protection of this disclosure.

[0019] Unless otherwise defined, the technical or scientific terms used in this disclosure shall have the ordinary meaning as understood by one of ordinary skill in the art to which this disclosure pertains.

[0020] Techniques, methods, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and equipment should be considered part of the specification.

[0021] In all examples shown and discussed herein, any specific value should be interpreted as merely exemplary and not as a limitation; therefore, other examples of exemplary embodiments may have different values.

[0022] This embodiment describes a method for increasing the permeability of locally sculpted and outburst-prone coal seams. Figure 1 The flowchart of this method is given, which includes the following steps.

[0023] S1, to open gas drainage boreholes at a set location in the coal and rock mass, including cross-layer boreholes or in-layer boreholes; S2, with the borehole as the center, multiple slits are made outward from the borehole according to the set width and length on the coal seam section of the borehole, as follows: S201 uses low-pressure water and utilizes a mining drilling rig to drive the hydraulic slit shallow spiral integral drill rod, high-low pressure conversion slitter and diamond composite drill bit to rotate and drill to the set position of the borehole. S202, turn on the ultra-high pressure clean water pump, output ultra-high pressure water jet with set pressure and flow rate, and form a jet by shooting out through the nozzle on the high-low pressure conversion cutter to cut and impact the coal seam in the drilled coal section, forming an irregular flat groove with a certain width and length in the coal body.

[0024] S3, based on the drill hole with slits formed in step S2, dynamic pressure fracturing is performed on the coal seam at the drill hole by setting the fracturing frequency and water injection flow rate.

[0025] Dynamic pressure fracturing uses high pressure and large flow dynamic pressure high pressure water to repeatedly act on coal body in a short time, which causes coal body to expand and shrink continuously, leading to fatigue failure of coal body, weakening of strong surface of coal body, further extension of weak surface of coal body, and thus achieving the purpose of increasing the degree of coal body fracture and enhancing the connectivity of coal body fracture.

[0026] This step can comprehensively improve the coal seam permeability effect generated by dynamic pressure fracturing by implementing cutting-pressure composite permeability improvement on the coal seam, and can significantly reduce the gas extraction cost while improving the gas extraction efficiency.

[0027] S4, for the coal seam with prominent local structure development, after the cutting-pressure composite permeability improvement on the coal seam based on step S3, although the cracks can extend to a remote area under the condition of complex internal occurrence conditions of the coal seam, the development of hydraulic fractures in the soft layer area cannot achieve the ideal effect.

[0028] To further improve the coal body gas extraction effect in the local structure area, after the cutting-pressure composite permeability improvement on the coal seam based on step S3, the drilling is performed to the set hole depth by the hydraulic drilling machine and the core rod jacking type hole making drill rod, and then the drill rod is retreated to the hole making target position, and the matching mechanical hole making pressure relief device is used for hole making, so as to implement the cutting-pressure-expansion composite permeability improvement on the coal seam with prominent local structure development.

[0029] After the cutting-pressure composite permeability improvement on the coal seam, the mechanical hole expansion is performed on the structure coal body section of the gas extraction borehole, that is, the cutting-pressure-expansion composite permeability improvement is implemented on the coal seam, which can effectively solve the problem of poor coal seam permeability caused by the blank zone formed by the local structure of the coal seam and the soft coal seam.

[0030] S5, the actual coal seam permeability improvement effect is quantified by the extraction radius, and the method is as follows: According to the Coal Mine Safety Regulations, after the pre-extraction of coal seam gas, the effect of preventing and controlling outburst of pre-extracted gas must be inspected, one of the inspection indexes is that the coal seam gas pre-extraction rate is greater than 30%, that is, the gas content after extraction is less than 30% of the gas content before extraction. The gas content and gas pressure meet the following formula: , In the formula, Q is the coal seam gas content, m 3 / t; α is the gas content coefficient, m 3 / (t·MPa 1 / 2 ), p is the coal seam gas pressure, MPa.

[0031] According to the above formula, if the coal seam gas content is reduced by 30%, the residual gas pressure in the coal body is 49% of the original gas pressure, that is, the gas pressure of the coal seam is reduced by 51%.

[0032] As Figure 2 shown, two construction cracks are compared and investigated, and the distance between the two drill holes is not less than 25 m. A plurality of gas pressure observation holes are arranged between the drainage holes. According to the gas pressure drop of 51% as the range of effective drainage radius, the gas pressure drop ratio and the gas pressure change law of each drill hole are observed, and the fitting relationship diagram of the effective drainage radius and the drainage time is drawn, as shown in Figure 3 .

[0033] In this embodiment, the drainage radius of the drainage hole 1 is 2.23 m after 120 d of drainage, the drainage radius of the drainage hole 2 is 2.15 m after 120 d of drainage, and the average original gas drainage radius is 2.19 m. The effective drainage radius of the drainage hole 1 after 120 d of drainage of the "cutting-pressure-expansion" composite permeability improvement technology is 6.04 m, which is increased by 3.81 m compared with the original coal seam calculation value. The effective drainage radius of the drainage hole 2 after 120 d of drainage is 6.15 m, which is increased by 4.00 m compared with the original coal seam calculation value. The average drainage radius of the coal seam is 6.095 m, which is increased by 3.905 m compared with the original coal seam calculation value, and is increased by 178.31%.

[0034] The Coal Mine Safety Regulations recorded in this step are departmental regulations formulated to protect the safety of coal mine production and the personal safety and health of employees, prevent coal mine accidents and occupational hazards, and in accordance with the Coal Law, the Mine Safety Law, the Safety Production Law, the Occupational Disease Prevention Law, the Coal Mine Safety Supervision Regulations and the Safety Production License Regulations, etc. On January 6, 2022, the Decision of the Emergency Management Department on the Revision of the Coal Mine Safety Regulations was promulgated, and it will be implemented from April 1, 2022.

[0035] S6, as shown in Figure 4 and Figure 5 , a coal seam permeability improvement numerical model is constructed in the discrete element software PFC 2D , the slot width and length in step S2, and the fracturing frequency and water injection flow in step S3 are numerically simulated, and scientific parameter guidance is provided for the composite permeability improvement operation of the coal seam.

[0036] The specific steps are as follows: S601, the actual mechanical parameters of the coal seam coal samples of tectonic coal and non-tectonic coal are collected on site, including the tensile strength, compressive strength, Young's modulus, Poisson's ratio, internal friction, internal friction angle and the like of the coal seam.

[0037] S602, the coal seam is discretized into a group of particles, a 150mmx150mm square particle assembly model is established in the PFC 2D software, the minimum particle diameter is set to 0.8mm, the particle size ratio R max :R min=1.66; a circular particle-free region with a diameter of 5 mm is set as a water injection drill hole at the center of the particle assembly model, and a water-permeable boundary is set around the model; S603, two particle bands are set in the horizontal or vertical direction of the model to simulate the soft layering of the tectonic coal, and the local tectonic development of the coal seam is constructed; wherein, the particles of the soft layering are endowed with the tectonic coal parameters collected for reference, for simulating tectonic coal; the other particles in the model are endowed with the non-tectonic coal parameters collected for reference, for simulating non-tectonic coal.

[0038] S604, four steel structure enclosing walls are established around the particle assembly model, and no friction is set between the model and the enclosing walls; the enclosing walls are set to apply constant horizontal stress and vertical stress to the particle assembly model at a constant speed, so as to apply constant load to the particle assembly model, for simulating the actual stress state of the coal body rock; S605, parallel bonding bonds are endowed to the particle assembly model to realize the interaction between the particles, and the movement between the particles is endowed to follow Newton's second law and the force-displacement law; S606, a slotted groove is set at the water injection drill hole, the slotted groove is communicated with the water injection drill hole, and a coal seam cutting-pressure composite permeability numerical model is constructed; through the water injection drill hole, pulsating hydraulic fracturing simulation under different fracturing frequencies and different water injection flow rates is carried out, and the law between the fracturing frequency and the water injection flow rate and the coal body cracking pressure and the total number of developed cracks is output.

[0039] The embodiment can form a variety of different parameter combination schemes by setting different slot widths and lengths, and fracturing frequencies and water injection flow rates, output rich and comprehensive laws between cutting-pressure composite permeability and coal body cracking pressure and total number of developed cracks, and is used to guide the actual coal seam permeability operation.

[0040] S607, according to the reaming diameter of the gas extraction drill hole in the tectonic coal section in the actual production operation process, the drill hole diameter is enlarged in the model in proportion, the particle assembly model after reaming is constructed, steps S503-S505 are repeated, and the law between cutting-pressure-reaming composite permeability and coal body cracking pressure and total number of developed cracks is output, which is used to guide the actual coal seam permeability operation.

[0041] S7, since the micro-mechanical strength between the particles of the model is not equal to the macro-mechanical properties of the model, in order to enable the coal seam permeability numerical model constructed in step S6 to truly simulate the actual coal seam, the actual mechanical parameters of the coal sample of the coal seam are taken as a comparison standard, the mechanical parameter calibration of the tectonic coal and the non-tectonic coal of the coal seam permeability numerical model is carried out through biaxial compression test, the calibration steps of the two are the same, and the embodiment takes the non-tectonic coal as an example for description, which specifically includes the following steps: S701, a biaxial compression model is constructed in PFC 2D ;Figure 6 As shown, the biaxial compression model is set as a rectangle with a size of 150 mm x 75 mm; the biaxial compression model is not provided with a drill hole and a cutting seam, and other model parameters are the same as those of the coal seam permeability numerical model in step S6; S702, a wall is provided around the biaxial compression model, and a set initial pressure is given to the wall; the biaxial compression model is subjected to a constant pressure from the upper and lower walls, and the pressure is stopped when the axial stress reaches the peak and drops to 80% of the peak value; S703, the biaxial stress-strain data of the upper and lower walls in step S703 and the lateral strain data of the biaxial compression model are recorded synchronously to generate a stress-strain curve, draw a Mohr circle and an envelope line; S704, the Mohr circle is approximated to the envelope line of the actual coal sample by repeating steps S702 to S704 by trial and error until the mechanical parameter calibration result of the coal seam permeability numerical model is good, and the error of the mechanical parameter of the coal seam coal sample is ≤5%.

[0042] S8, the permeability of the coal seam permeability numerical model before and after fracturing is compared to quantitatively evaluate the permeability improvement effect of the model, and the method is as follows: S801, fluid pipes and fluid domains are given to the model, the fluid pipe refers to the contact between the particles in the particle assembly, and the fluid domain refers to a polygon closed area surrounded by adjacent and contacted particle center points; S802, the model is given seepage hydrology parameters, including: fluid pipe initial opening, fluid domain apparent volume, fracture residual opening amplification coefficient, fluid bulk modulus, opening half-opening compression force and fluid viscosity; S803, a fixed value of confining pressure is given to the model, a certain size of injection hole is drilled from the center position of the outer surface of the model, a fixed value of inlet pressure P1 is applied to one side of the model, and a fixed value of outlet pressure P2 is applied to the other side of the model, and P1>P2; When the average pore pressure and total flow of the model reach a steady state, the permeability K0 of the model is calculated; S804, after removing the confining pressure, inlet pressure and outlet pressure given to the model in step S803, a fluid with a certain flow value and a certain pressure value is given to the model from the injection hole in step S803, the model is given hydraulic fracturing simulation, and the particle state and fracture state of the model are recorded; S805, the calculation results given to the model in step S804 are removed, step S804 is repeated, the permeability K1 of the model under the constant pressure steady state after the hydraulic fracturing simulation is obtained, and the hydraulic fracturing permeability improvement effect of the model is quantified by comparing the permeability K0 and the permeability K1.

Claims

1. A method for increasing the permeability of locally developed outburst-prone coal seams, characterized in that, Includes the following steps: S1, Drill holes are opened at the designated locations in the coal and rock mass; S2, on the coal section of the borehole, with the borehole as the center, a slit is set out to connect outwards along the coal section of the borehole; S3, set the fracturing frequency and water injection flow rate, and implement dynamic pressure fracturing of the coal seam at the borehole; S4. Based on the fracturing and permeability enhancement operations performed on the coal seam in steps S1-S3, mechanical enlargement is performed on the structural coal section of the borehole to create a cavity, and the coal seam is subjected to a combined fracturing, permeability enhancement operation of fracturing and enlargement.

2. The method for increasing permeability of locally structurally developed outburst-prone coal seams according to claim 1, characterized in that, The method for mechanically enlarging and creating cavities in the structural coal section of the borehole is as follows: after drilling to the set depth using a hydraulic drilling rig and a core rod jacking type cavitation drill rod, the drill rod is then retracted to the target cavitation position, and a mechanical cavitation pressure relief device is used to create the cavities.

3. The method for enhancing permeability of locally structurally developed outburst-prone coal seams according to claim 1, characterized in that, The permeability enhancement effect of coal seams is quantified by the extraction radius, using the following method: Two fracturing boreholes were constructed for comparative analysis, with a spacing of ≥25m between the two fracturing boreholes. Multiple gas pressure observation holes were arranged between the boreholes used for permeability enhancement. The effective drainage radius was defined as the range of a 51% decrease in gas pressure. The proportion of gas pressure decrease and the gas pressure change law of each gas pressure observation hole were obtained, and a fitting relationship diagram between the effective drainage radius and the drainage time was plotted.

4. The method for increasing permeability of locally structurally developed outburst-prone coal seams according to claim 1, characterized in that, A numerical model for enhancing the permeability of a coal seam was constructed in discrete element method software, and numerical simulations were performed on the width and length of the cuts set in step S2, as well as the fracturing frequency and water injection flow rate set in step S3.

5. The method for enhancing permeability of locally structurally developed outburst-prone coal seams according to claim 4, characterized in that, Constructing a numerical model for enhancing the permeability of a coal seam in Discrete Element Method (DEM) software includes the following steps: I. Collect mechanical parameters of coal samples from both tectonic and non-tectonic coal seams, including tensile strength, compressive strength, Young's modulus, Poisson's ratio, internal friction force, and internal friction angle. II. In PFC 2D A 150mm × 150mm particle aggregate model was created in the software, with the minimum particle diameter set to 0.8mm and the particle size ratio R. max :R min =1.66; A circular, particle-free area with a diameter of 5 mm is set at the center of the particle aggregate model as a water injection hole, and the perimeter of the particle aggregate model is set as a water-permeable boundary. III. Two particle bands are set in the horizontal or vertical direction of the particle aggregate model to simulate the soft stratification area of ​​tectonic coal and construct a locally developed outburst coal seam; wherein, the soft stratification particles are assigned with reference to the collected tectonic coal parameters to construct the tectonic coal simulation area; the other particles in the model are assigned with reference to the collected non-tectonic coal parameters to construct the non-tectonic coal simulation area. IV. Build four steel structure walls around the particle assembly model, and ensure that there is no friction between the particle assembly model and the walls; set the walls to apply constant horizontal and vertical stresses to the particle assembly model at a constant speed, thereby applying a constant load to the particle assembly model; V. The particle assembly model is given parallel bonding bonds, and the movement between particles follows Newton's second law and the force-displacement law; VI. Set a slotted groove at the water injection borehole, and connect the slotted groove to the water injection borehole. Through the water injection borehole, conduct pulsating hydraulic fracturing simulations under different fracturing frequencies and different water injection flows, and output the relationship between the fracturing frequency and water injection flow rate and the coal body fracturing initiation pressure and the total number of developed cracks under the slotted-fracturing permeability enhancement simulation.

6. The method for increasing permeability of locally structurally developed outburst-prone coal seams according to claim 5, characterized in that, Constructing a numerical model for enhancing the permeability of coal seams in discrete element method software also includes the following steps: Based on the enlarged borehole diameter of the gas drainage borehole in the structural coal section during actual production operations, the diameter of the water injection borehole is proportionally enlarged in the particle aggregate model in step II to construct the enlarged particle aggregate model. Steps III-VI are repeated to output the relationship between the fracturing frequency and water injection flow rate and the coal body fracturing initiation pressure and the total number of developed cracks under the composite permeability enhancement simulation of fracturing-enlargement-cutting.

7. The method for increasing permeability of locally structurally developed outburst-prone coal seams according to claim 5, characterized in that, The coal body mechanical parameters were calibrated for the simulated regions of tectonic and non-tectonic coal in the numerical model for enhancing coal permeability, respectively, as follows: I. In PFC 2D A biaxial compression model is constructed in the software. The biaxial compression model is set to a rectangle with dimensions of 150mm × 75mm. The biaxial compression model does not have drill holes or slits set, and other model parameters are the same as those of the particle aggregate model. II. Walls are set around the biaxial compression model. The walls are given a set initial pressure. The upper and lower walls apply pressure to the biaxial compression model at a constant speed. When the axial stress reaches the peak and drops to 80% of the peak value, the pressure is stopped. III. Simultaneously record the bidirectional stress-strain data of the upper and lower walls and the transverse strain data of the biaxial compression model, generate stress-strain curves, and draw Mohr's circle and envelope. IV. Repeat steps II and III using a trial-and-error method to make the Mohr circle approach the envelope of the actual coal sample until the mechanical parameter calibration results of the coal seam permeability enhancement numerical model have an error of ≤5% with respect to the mechanical parameters of the actual coal seam sample.

8. A method for enhancing the permeability of locally structurally developed outburst-prone coal seams according to claim 5 or 6, characterized in that, By comparing the permeability of the numerical models of coal seam permeability enhancement before and after fracturing, the permeability enhancement effect is quantitatively evaluated. The method is as follows: I. Assign fluid channels and fluid domains to the numerical model of coal seam permeability enhancement. Fluid channels refer to the contact between particles, and fluid domains refer to the polygonal closed area enclosed by the center points of adjacent and contacting particles. II. Assign seepage hydraulic parameters to the numerical model of coal seam permeability enhancement, including: initial opening of fluid conduit, apparent volume of fluid domain, magnification factor of residual fracture opening, fluid bulk modulus, half-opening compressive force, and fluid viscosity; III. Assign a fixed confining pressure to the coal seam permeability enhancement numerical model. Drill an injection hole of a certain size from the center of the outer surface of the coal seam permeability enhancement numerical model. Apply a fixed inlet pressure P1 to one side of the coal seam permeability enhancement numerical model and a fixed outlet pressure P2 to the other side, where P1 > P2. Calculate the permeability K0 of the coal seam permeability enhancement numerical model when the average pore pressure and total flow rate of the coal seam permeability enhancement numerical model reach a steady state. IV. After removing the confining pressure, inlet pressure, and outlet pressure assigned to the coal seam permeability enhancement numerical model in step III, inject a fluid with a certain flow rate and a certain pressure value into the coal seam permeability enhancement numerical model from the injection hole described in step III, and simulate hydraulic fracturing in the coal seam permeability enhancement numerical model, and record the particle state and fracture state of the coal seam permeability enhancement numerical model. V. Remove the calculation results assigned to the coal seam permeability enhancement numerical model in step IV, repeat step IV, and obtain the permeability K1 of the coal seam permeability enhancement numerical model under constant pressure steady state after hydraulic fracturing simulation. Compare the permeability K0 and permeability K1 to quantify the hydraulic fracturing permeability enhancement effect of the coal seam permeability enhancement numerical model.

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