A method for improving the permeability of a locally developed outburst coal seam

Abstract

The application discloses a local structure development outburst coal seam permeability improvement method, steps are as follows: S1, drilling is set at the position of coal rock body; S2, on the coal hole section of the drilling, slits are set outside the drilling coal hole section along the drilling as a center; S3, setting fracturing frequency and water injection flow, dynamic pressure fracturing is carried out on the coal seam at the drilling; S4, after the slit-fracturing permeability improvement operation of the coal seam based on steps S1-S3, mechanical reaming and cavitation are carried out on the structure coal body section of the drilling, and slit-fracturing-reaming composite permeability improvement is carried out on the coal seam. The coal seam permeability improvement 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 the effective drainage of gas in the structure soft coal body with poor gas migration capacity.

Description

Technical Field

[0001] This invention relates to the field of coal and gas outburst prevention and control technology, specifically to a method for increasing permeability in coal seams with locally developed outburst structures. Background Technology

[0002] As coal mining depth increases, coal seam permeability decreases, and the probability of coal and gas outbursts increases accordingly. The core measure for preventing coal and gas outburst accidents is to reduce the gas content in the coal seam below the safe threshold. Therefore, mine dynamic disaster prevention must be carried out from the perspective of efficient coal seam gas extraction and effective pressure relief.

[0003] Due to the complex structure of locally developed outburst coal seams, their disaster-causing mechanisms are also more complex than those of conventional coal seams. When gas drainage is carried out on them, the commonly used methods for improving coal seam permeability (such as hydraulic fracturing) cannot effectively solve the problem of poor coal seam permeability. This results in long gas drainage time, poor effect, and small effective impact range. Therefore, a large number of boreholes need to be arranged in the area where gas drainage is carried out, which increases the amount of construction work and is also not conducive to coal mine fully mechanized mining production. Summary of the Invention

[0004] In order to address some of the problems existing in the prior art, this invention proposes a method for enhancing the permeability of coal seams with locally developed outburst structures. The specific technical solution is as follows:

[0005] A method for enhancing the permeability of locally sculpted outburst-prone coal seams includes the following steps:

[0006] S1, Drill holes are opened at the designated locations in the coal and rock mass;

[0007] 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;

[0008] S3, set the fracturing frequency and water injection flow rate, and implement dynamic pressure fracturing of the coal seam at the borehole;

[0009] 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.

[0010] Furthermore,

[0011] 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.

[0012] Furthermore,

[0013] The permeability enhancement effect of coal seams is quantified by the extraction radius, using the following method:

[0014] 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.

[0015] Furthermore,

[0016] 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.

[0017] Furthermore,

[0018] Constructing a numerical model for enhancing the permeability of a coal seam in Discrete Element Method (DEM) software includes the following steps:

[0019] 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.

[0020] 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;

[0021] 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.

[0022] 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 the local tectonic 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.

[0023] 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;

[0024] V. Parallel bonding bonds are given to the particle assembly model, and the movement between particles follows Newton's second law and the force-displacement law;

[0025] 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.

[0026] Furthermore,

[0027] Constructing a numerical model for enhancing the permeability of coal seams in discrete element method software also includes the following steps:

[0028] 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.

[0029] Furthermore,

[0030] 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:

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] Furthermore,

[0036] 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:

[0037] 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.

[0038] 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;

[0039] 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.

[0040] 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.

[0041] 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.

[0042] Based on the above technical solution, the method described in this invention has the following beneficial effects:

[0043] 1. The coal seam permeability enhancement method described in this invention involves cutting and fracturing the coal seam. This cutting process creates macroscopic grooves and numerous secondary fractures, forming gas flow paths and achieving self-pressure relief through annular network flow between boreholes. This generates numerous fine fractures and simultaneously relieves pressure on the coal seam. Furthermore, hydraulic fracturing can penetrate these fine fractures, creating more complex fracture channels. Combining hydraulic fracturing with hydraulic cutting significantly improves the overall permeability of the coal seam, enhancing gas extraction efficiency and eliminating coal seam gas dynamic hazards.

[0044] 2. During coal formation, coal seams are often subjected to magmatic intrusion and geological movements, resulting in local displacement and the presence of hidden, difficult-to-detect small faults. This leads to high local gas content, low coal mechanical strength, and poor permeability. When the working face advances to the vicinity of such coal seams, the disturbance during mining increases gas migration channels, and the strong gas release capacity of the coal seam causes a large influx of gas towards the working face, increasing the gas concentration in the working face and return airway. The coal seam permeability enhancement method described in this invention, through cutting-compression-expansion permeability enhancement in locally developed outburst-prone coal seams, can further improve the overall gas extraction effect in coal seams with large burial depth, high gas pressure, locally developed hidden small structures, and low permeability. This eliminates the risk of coal seam outbursts, reduces drilling workload, and enables effective gas extraction from hidden "coal-bag-like" soft coal bodies with poor gas migration capacity.

[0045] 3. The coal seam permeability enhancement method described in this invention, taking a field test in a coal mine as an example, such as... Figure 2 and Figure 3 As shown, the drainage radius of drainage borehole 1 after 120 days of drainage was 2.23m, and that of drainage borehole 2 after 120 days of drainage was 2.15m. The average of the original gas drainage radius was 2.19m. After 120 days of drainage using the "cut-compression-expansion" composite permeability enhancement technology, the effective drainage radius of drainage borehole 1 was 6.04m, an increase of 3.81m compared to the calculated value of the original coal seam; the effective drainage radius of drainage borehole 2 after 120 days of drainage was 6.15m, an increase of 4.00m compared to the calculated value of the original coal seam. The average drainage radius of the coal seam was 6.095m, an increase of 3.905m compared to the calculated value of the original coal seam, representing an improvement of 178.31%.

[0046] 4. By testing the permeability of the numerical model of coal seam permeability enhancement after "cut-compression-expansion" and comparing the changes in permeability of the numerical model of coal seam permeability enhancement before and after "cut-compression-expansion", the numerical quantitative evaluation of the fracturing effect is achieved. The evaluation results are shown in the figure below and Table 1 below (Note: The values ​​in Table 1 are not fixed values, but average values ​​after multiple tests):

[0047] Table 1. Simulated coal seam permeability results after fracturing

[0048]

[0049] The results showed that the permeability of the coal seam was significantly improved after the permeation enhancement process; the permeability of the coal seam before the "cut-press" permeation enhancement was 3.846 × 10−18 m. 2 After permeability enhancement, the coal seam permeability reached 48.075 × 10⁻¹⁸ m³. 2 The permeability was increased to approximately 12.5 times that of the original coal seam, and after the "cut-compress-expansion" permeability enhancement process, the permeability of the coal seam was 65.843 × 10⁻¹⁸ m³. 2Compared to the original coal seam, it has expanded by about 17.1 times, which shows that the "cut-press" and "cut-press-expansion" permeability enhancement technologies have a good permeability enhancement effect, and the "cut-press-expansion" permeability enhancement technology has a more significant permeability enhancement effect. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the process for improving the permeability of locally developed outburst coal seams as described in this invention;

[0051] Figure 2 A schematic diagram of the borehole layout for observing the permeability-enhancing fracturing boreholes and effective extraction radius;

[0052] Figure 3 A schematic diagram showing the relationship between the effective extraction radius and extraction time after a combined permeability enhancement process involving cutting, pressing, and expanding.

[0053] Figure 4 A schematic diagram of a numerical model for improving the permeability of coal seams;

[0054] Figure 5 A schematic diagram of two-direction pulsating fracturing simulation in a numerical model for improving the permeability of a coal seam;

[0055] Figure 6 This is a schematic diagram of a biaxial compression model. Detailed Implementation

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 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.

[0060] This embodiment describes a method for increasing the permeability of locally sculpted and outburst-prone coal seams. Figure 1The flowchart of this method is given, which includes the following steps.

[0061] S1, to open gas drainage boreholes at a set location in the coal and rock mass, including cross-layer boreholes or in-layer boreholes;

[0062] 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:

[0063] 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.

[0064] 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.

[0065] 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.

[0066] Dynamic pressure fracturing utilizes high-pressure, high-flow-rate dynamic pressure water to repeatedly act on the coal body in a short period of time, causing the coal body to continuously expand and contract, leading to fatigue failure of the coal body. The strong surfaces of the coal body are weakened and further extended, thereby increasing the degree of coal body fracturing and enhancing the connectivity of coal body fractures.

[0067] This step, by implementing a combined cutting-compression permeability enhancement method on the coal seam, can comprehensively improve the permeability enhancement effect of the coal seam caused by dynamic pressure fracturing, and significantly reduce the gas extraction cost while improving the gas extraction efficiency.

[0068] S4. For coal seams with localized structural development and outbursts, after implementing cut-compression composite permeability enhancement based on step S3, although the fractures can extend to distant areas under complex internal conditions, the development of hydraulic fractures in soft stratification areas still cannot achieve the desired effect.

[0069] To further improve the gas extraction effect of coal seams in local structural areas, after implementing the cutting-compression composite permeability enhancement of the coal seam in step S3, the borehole is drilled to the set depth using a hydraulic drilling rig and a core rod jacking type cavity-making drill rod. Then, the drill rod is retracted to the target cavity-making position, and a matching mechanical cavity-making and pressure-relieving device is used to create the cavity. This process implements the cutting-compression-expansion composite permeability enhancement for outburst-prone coal seams with local structural development.

[0070] This step involves cutting and compressing the coal seam to enhance its permeability. Then, mechanical enlargement is performed in the structural coal section of the gas extraction borehole to create a cavity. This cutting-compression-enlargement process effectively solves the problem of poor coal seam permeability caused by blank zones formed by local coal seam structures and soft coal seams.

[0071] S5 quantifies the actual coal seam permeability enhancement effect by using the extraction radius, as follows:

[0072] According to the "Coal Mine Safety Regulations," after pre-draining coal seam gas, the effectiveness of the pre-drainage in preventing outbursts must be verified. One of the verification indicators is a pre-drainage rate of more than 30%, meaning the gas content after drainage is more than 30% lower than before drainage. The gas content and gas pressure satisfy the following formula:

[0073] ,

[0074] In the formula, Q The m content of coal seam gas 3 / t; α m is the gas content coefficient. 3 / (t·MPa 1 / 2 ), p The value is the coal seam gas pressure, in MPa.

[0075] According to the above formula, if the gas content of the coal seam decreases by 30%, the residual gas pressure in the coal body will be 49% of the original gas pressure, which means that the gas pressure of the coal seam has decreased by 51%.

[0076] like Figure 2 As shown, two fracturing boreholes were constructed for comparative analysis. The distance between the two boreholes was no less than 25m. Multiple gas pressure monitoring boreholes were arranged between the permeability enhancement boreholes. The effective drainage radius was defined as a 51% decrease in gas pressure. The relationship between the percentage decrease in gas pressure and the change in gas pressure was observed in each borehole. A fitting graph of the effective drainage radius and drainage time was plotted. Figure 3 As shown.

[0077] In this embodiment, the drainage radius of drainage borehole 1 after 120 days of drainage is 2.23m, and the drainage radius of drainage borehole 2 after 120 days of drainage is 2.15m. The average of the original gas drainage radius is 2.19m. After 120 days of drainage using the "cut-compression-expansion" composite permeability enhancement technology, the effective drainage radius of drainage borehole 1 is 6.04m, an increase of 3.81m compared to the calculated value of the original coal seam; the effective drainage radius of drainage borehole 2 after 120 days of drainage is 6.15m, an increase of 4.00m compared to the calculated value of the original coal seam. The average drainage radius of the coal seam is 6.095m, an increase of 3.905m compared to the calculated value of the original coal seam, representing an improvement of 178.31%.

[0078] The "Coal Mine Safety Regulations" described in this step are departmental regulations formulated to ensure safe production in coal mines and the personal safety and health of employees, and to prevent coal mine accidents and occupational hazards, in accordance with the "Coal Law," "Mine Safety Law," "Production Safety Law," "Occupational Disease Prevention and Control Law," "Coal Mine Safety Supervision Regulations," and "Safety Production License Regulations," etc. On January 6, 2022, the "Decision of the Ministry of Emergency Management on Amending the 'Coal Mine Safety Regulations'" was promulgated, and it came into effect on April 1, 2022.

[0079] S6, such as Figure 4 and Figure 5 As shown, through the discrete element software PFC 2D A numerical model for coal seam permeability enhancement was constructed, and numerical simulations were performed on the slot width and length in step S2, and the fracturing frequency and water injection flow rate in step S3, providing scientific parameter guidance for the composite permeability enhancement operation of coal seams.

[0080] The specific steps are as follows:

[0081] S601, on-site collection of actual mechanical parameters of coal seam 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.

[0082] S602, discretizes the coal seam into a set of particles, in PFC 2D A 150mm × 150mm square particle assembly 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 5mm is set at the center of the particle aggregate model as a water injection hole, and the surrounding area of ​​the model is set as a water-permeable boundary.

[0083] S603 sets two particle bands in the horizontal or vertical direction of the model to simulate soft stratification of tectonic coal and construct locally developed outburst coal seams; wherein, the particles of the soft stratification are assigned with reference to the collected tectonic coal parameters to simulate tectonic coal; and the particles of other particles in the model are assigned with reference to the collected non-tectonic coal parameters to simulate non-tectonic coal.

[0084] S604. Four steel structure walls are built around the particle aggregate model, and it is set that there is no friction between the model and the walls. The walls are set to apply constant horizontal and vertical stresses to the particle aggregate model at a constant speed, thereby applying a constant load to the particle aggregate model to simulate the actual stress state of coal rock.

[0085] S605 provides parallel bonding bonds to the particle assembly model, enabling interactions between particles and ensuring that the motion between particles follows Newton's second law and the force-displacement law.

[0086] S606, a slotted groove is set at the water injection borehole, and the slotted groove is connected to the water injection borehole to construct a numerical model of coal seam cutting-compression composite permeability enhancement; through the water injection borehole, pulsating hydraulic fracturing simulation under different fracturing frequencies and different water injection flow rates is carried out, and the relationship between fracturing frequency and water injection flow rate and coal seam fracturing initiation pressure and the total number of developed cracks is output.

[0087] This embodiment can form a variety of different parameter combination schemes by setting different cut widths and lengths, as well as fracturing frequencies and water injection flows. It outputs rich and comprehensive data on the relationship between cut-pressure composite permeability enhancement and the coal seam fracturing pressure and the total number of developed cracks, which can be used to guide actual coal seam permeability enhancement operations.

[0088] S607: Based on the actual production operation, the diameter of the gas drainage borehole in the structural coal section is enlarged proportionally in the model to construct a particle aggregate model after enlargement. Repeat steps S503-S505 to output rich and comprehensive laws between the cutting-compression-expansion composite permeability enhancement and the coal body crack initiation pressure and the total number of developed cracks, which can be used to guide the actual coal seam permeability enhancement operation.

[0089] S7. Since the microscopic mechanical strength between model particles is not equivalent to the macroscopic mechanical properties of the model, in order to ensure that the coal seam permeability enhancement numerical model constructed in step S6 can realistically simulate the actual coal seam, this step uses the actual mechanical parameters of the coal seam sample as a comparison standard. The mechanical parameters of the coal seam permeability enhancement numerical model are calibrated for both tectonic and non-tectonic coal through biaxial compression tests. The calibration steps are the same for both. This embodiment uses non-tectonic coal as an example for illustration, specifically including the following steps:

[0090] S701, in PFC 2D Constructing a biaxial compression model: such as Figure 6 As shown, the biaxial compression model is set as a rectangle with dimensions of 150mm × 75mm; the biaxial compression model does not have boreholes or slots, and other model parameters are the same as the coal seam permeability enhancement numerical model in step S6.

[0091] S702, a biaxial compression model is surrounded by walls, which 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 its peak and drops to 80% of the peak value, the pressure is stopped.

[0092] S703, synchronously record the bidirectional stress-strain data of the upper and lower walls and the transverse strain data of the biaxial compression model in step S703, generate stress-strain curves, and draw Mohr's circle and envelope.

[0093] S704. Repeat steps S702 to S704 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 are good and the error between the mechanical parameters and the actual coal seam sample is ≤5%.

[0094] S8. Compare the permeability of the numerical model of coal seam permeability enhancement before and after fracturing to quantitatively evaluate the permeability enhancement effect of the model. The method is as follows:

[0095] S801 assigns fluid channels and fluid domains to the model. Fluid channels refer to the contact between particles in a particle assembly, and fluid domains refer to the polygonal closed area enclosed by the center points of adjacent and contacting particles.

[0096] S802 assigns the model seepage hydraulic parameters, including: initial opening of the fluid conduit, apparent volume of the fluid domain, residual opening magnification factor of the crack, fluid bulk modulus, half-open compressive force, and fluid viscosity.

[0097] S803, apply a fixed confining pressure to the model, drill an injection hole of a certain size from the center of the outer surface of the model, apply a fixed inlet pressure P1 on one side of the model, apply a fixed outlet pressure P2 on the other side, and P1>P2.

[0098] The permeability K0 of the model is calculated when the average pore pressure and total flow rate of the model reach a steady state.

[0099] S804, after removing the confining pressure, inlet pressure and outlet pressure assigned to the model in step S803, a fluid with a certain flow rate and a certain pressure value is assigned to the model from the injection hole mentioned in step S803 to simulate hydraulic fracturing, and the particle state and fracture state of the model are recorded.

[0100] S805, remove the calculation results assigned to the model in step S804, repeat step S804, obtain the permeability K1 of the model under constant pressure steady state after hydraulic fracturing simulation, compare permeability K0 and permeability K1, and quantify the permeability enhancement effect of hydraulic fracturing of the model.

Claims

1. A method for improving the permeability of a locally constructed development prominent coal seam, characterized in that, The method comprises the following steps: S1, drilling a borehole at a set position of a coal rock body; S2, setting a slotted hole on the coal hole section of the borehole, the slotted hole being in communication with the borehole and extending outward from the borehole coal hole section; S3, setting a fracturing frequency and a water injection flow rate, and performing dynamic pressure fracturing on the coal seam at the borehole; S4, after the slotted hole-fracturing permeability improvement operation on the coal seam based on steps S1-S3, performing mechanical hole expansion and hole making on the tectonic coal body section of the borehole, and performing slotted hole-fracturing-hole expansion composite permeability improvement on the coal seam; A coal seam permeability improvement numerical model is constructed in a discrete element software, and the width and length of the slotted hole set in step S2 and the fracturing frequency and water injection flow rate set in step S3 are numerically simulated, comprising the following steps: I. The mechanical parameters of the tectonic coal and non-tectonic coal field coal seam samples, including tensile strength, compressive strength, Young's modulus, Poisson's ratio, internal friction, and internal friction angle, are collected respectively; II. In PFC 2D A 150 mm x 150 mm granular assembly model was established in the PFC software, and the minimum particle diameter was set to 0.8 mm, 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 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 prominent coal seam is constructed; the particles of the soft layering 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; 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 slotted hole groove is set at the water injection borehole, the slotted hole groove is in communication with the water injection borehole, and 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 water injection flow rate under the slotted hole-fracturing permeability improvement simulation and the coal body cracking pressure and the total number of developed cracks is output.

2. The method according to claim 1, wherein, The method for mechanically expanding and hole making on the tectonic coal body section of the borehole is as follows: after the borehole is drilled to a set depth by a hydraulic drilling machine and a core rod jacking type hole making drill rod, the drill rod is retracted to a hole making target position, and a mechanical hole making pressure relief device is used for hole making.

3. The method according to claim 1, wherein the method is characterized by, The coal seam permeability improvement effect is quantified by the drainage radius, and the method is as follows: Two fracturing boreholes are constructed for comparison and investigation, and the distance between the two fracturing boreholes is greater than or equal to 25 m; a plurality of gas pressure observation holes are arranged between the boreholes for permeability improvement, the range of effective drainage radius is taken as the gas pressure drop of 51%, the gas pressure drop ratio and the gas pressure change law of each gas pressure observation hole are obtained, and the effective drainage radius and the drainage time fitting relationship diagram is drawn.

4. The method according to claim 1, wherein the method is a method for improving the permeability of a locally developed protruding coal seam, characterized in that, The construction of the coal seam permeability improvement numerical model in the discrete element software further comprises the following steps: According to the reaming diameter of the gas extraction drilling in the tectonic coal section in the actual production process, the water injection drilling diameter in the particle assembly model in step II is enlarged in proportion, the 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 mass cracking pressure and the total number of developed cracks under the composite permeability improvement simulation of slitting-fracturing-reaming is output.

5. The method according to claim 1, wherein the method is a method for improving the permeability of a locally developed protruding coal seam, characterized in that, The coal mass mechanical parameter calibration of the tectonic coal simulation area and the non-tectonic coal simulation area of the coal seam permeability improvement numerical model is carried out respectively, and the steps are as follows: I. In PFC 2D A biaxial compression model was built in the software, and the biaxial compression model was set as a rectangle with a size of 150 mm x 75 mm; the biaxial compression model was not provided with a drill hole and a cutting joint, and other model parameters were the same as those of the granular assembly model. II. The walls are set around the biaxial compression model, the walls are given the initial pressure, the biaxial compression model is applied with pressure at a constant speed, and the pressure is stopped when the axial stress reaches the peak and drops 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 parameters of the actual coal seam.

6. The method according to claim 1 or 4, wherein, 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. The fluid pipe and the fluid domain of the coal seam permeability improvement numerical model are given, the fluid pipe refers to the contact between particles, and the fluid domain refers to the 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 amplification coefficient, fluid bulk modulus, opening half-opening compression force and fluid viscosity; III. The coal seam permeability improvement numerical model is given a fixed value of confining pressure, a certain size of injection hole is drilled from the center position of the outer surface of the coal seam permeability improvement numerical model, a fixed value of inlet pressure P1 is applied on one side of the coal seam permeability improvement numerical model, and a fixed value of outlet pressure P2 is applied on the other side, and P1>P2; when the average pore pressure and 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. After removing the confining pressure, inlet pressure and outlet pressure given to the coal seam permeability improvement numerical model in step III, the coal seam permeability improvement numerical model is given a certain flow rate and a certain pressure value of fluid from the injection hole in step III, the coal seam permeability improvement numerical model is given hydraulic fracturing simulation, and the particle state and crack state of the coal seam permeability improvement numerical model are recorded; V. The calculation results given to the coal seam permeability improvement numerical model in step IV are removed, step IV is repeated, the permeability K1 of the coal seam permeability improvement numerical model under the constant pressure steady state after hydraulic fracturing simulation is obtained, and the hydraulic fracturing permeability improvement effect of the coal seam permeability improvement numerical model is quantified by comparing the permeability K0 and the permeability K1.

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