Method for determining the position of a horizontal directional long borehole in overburden strata of longwall mining

Abstract

This invention discloses a method for determining the extraction locations of horizontally oriented long boreholes within the overlying strata of longwall mining, comprising the following steps: S1, based on the mining method and parameters of the working face, and the lithological characteristics of the overlying strata, and after analyzing the distribution of gas migration fracture channels in coal seam mining and the stability of boreholes affected by mining activities, the range of extraction locations for directional long boreholes is determined; S2, within the range of extraction locations for directional long boreholes, the location criterion value for each location is calculated; S3, based on the magnitude of the location criterion value for each location, the specific location and order of each borehole are determined. This invention reveals the extraction mechanism of horizontally oriented long boreholes from three aspects: the degree of gas accumulation within mining fractures, the permeability of mining strata, and the stability of mining boreholes. It proposes location criteria for horizontally oriented long boreholes and provides a method flow for determining the extraction locations of boreholes.

Description

Technical Field

[0001] This invention relates to the field of gas extraction, and more particularly to a method for determining the extraction location in the layout of horizontally oriented long boreholes within the overlying rock of longwall mining. Background Technology

[0002] Coal is a vital resource supporting my country's economic development. Methane gas, an associated gas in longwall coal mining, poses a serious threat to mine safety. Currently, with the shift of longwall coal mining to deeper levels and the widespread adoption of high-yield, high-efficiency production models, the methane hazard problem in longwall coal mining is becoming increasingly severe. Coal and methane co-mining technology provides a solution to this problem. Among these methods, deploying horizontal directional boreholes (hereinafter referred to as "directional boreholes") within the overburden to extract depressurized methane from the goaf has advantages such as low construction cost, high efficiency, long extraction time, and effective relief of mining-extraction transition tensions. However, due to the inherent structural and layout characteristics of directional boreholes, their extraction conditions within the mined overburden have unique requirements. Researching the location characteristics and extraction mechanism of directional boreholes is of significant practical importance for guiding the layout and extraction of directional boreholes in mines and for developing existing theories of coal and methane co-mining.

[0003] Extensive research has been conducted by scholars on the migration patterns of depressurized gas within mining-induced overburden fractures. Academician Qian Minggao et al. proposed the "O"-shaped ring distribution characteristics of mining-induced fractures, pointing out that the "O"-shaped rings are the flow channels and storage spaces for depressurized gas. Based on this, Academician Yuan Liang et al., through studying the dynamic changes in mining-induced overburden movement, fracture development, and gas enrichment zones, determined the efficient gas extraction range in coal seam group mining and proposed the ring-shaped fracture theory of the coal seam roof and the double-circle theory for evaluating the evolution of mining-induced fractures. Xu Jialin et al., based on analyzing and determining the location of key overburden layers, proposed a new method for predicting the height of the gas-conducting fracture zone and, considering the depressurization and desorption characteristics of adjacent layers during coal seam group mining, divided the overburden strata in the goaf into gas-conducting fracture zones, depressurization and desorption zones, and zones that are difficult to desorb. Li Shugang et al., based on the spatial dynamic evolution of fractures and delamination fractures in the overburden of the mining area, proposed the mining-induced fracture elliptical zone theory to guide the extraction of depressurized gas. Feng Guorui et al. used physical simulation experiments to divide the gas flow space in the goaf into four regions from top to bottom: high gas concentration zone, gas transition zone, gas enrichment zone, and gas non-flow zone. They pointed out that the boundaries of these regions always have a "V" shape.

[0004] Based on research into the development of overburden fractures and the laws governing gas migration caused by mining, various decompression gas extraction technologies have been developed, including roadway extraction, pipe insertion (burying) extraction, and borehole extraction. In recent years, with the improvement of drilling technology and equipment, decompression gas extraction methods such as vertical surface drilling, conventional high-level drilling, and directional long boreholes have been widely used. Among these, directional long boreholes, due to their construction and extraction advantages, have gradually become an effective approach for decompression gas extraction and management in the goaf of working faces. Related scholars have also conducted extensive research and discussions on this topic. Lin Haifei et al., based on the theory of elliptical fracture zones caused by mining, used similarity simulation and Fluent numerical simulation to simulate the arrangement of strata during directional long borehole extraction. Li Hong et al. used numerical simulation to simulate the damage and fracture development of overburden caused by mining, determined the arrangement of directional long boreholes, and conducted extraction tests. Duan Huijun et al., through engineering tests, determined the appropriate arrangement of directional long boreholes based on zoned extraction. Yan Zhenguo conducted numerical simulation studies and proposed an optimization method for the layout of directional long boreholes, and gave the range of borehole layout in overburden.

[0005] The aforementioned research on the evolution of mining-induced fractures and the laws governing gas migration can provide a theoretical basis for determining the extraction layers in mining-induced overburden using conventional high-level boreholes and high-efficiency drainage roadways. However, for directional long boreholes, their structure and extraction layout have unique characteristics. The determination of their extraction locations in longwall mining overburden is mostly based on experimental simulations and field experience, and the relevant theoretical analysis methods are still unclear. Therefore, studying the location characteristics of directional long boreholes, elucidating their extraction mechanisms, and ultimately determining the extraction locations of boreholes in mining-induced overburden is crucial for achieving stable and efficient extraction of depressurized gas. Summary of the Invention

[0006] The purpose of this invention is to address the above-mentioned problems by providing a simple, safe, and efficient method for determining the extraction location of horizontally oriented long boreholes in overlying rock during longwall mining.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] The method for determining the extraction location of horizontally oriented long boreholes in overlying rock during longwall mining includes the following steps:

[0009] S1. Based on the mining methods and parameters of the working face and the lithological characteristics of the overburden, and on the analysis of the distribution of gas migration fracture channels in coal seam mining and the stability of boreholes affected by mining, the range of the extraction location of directional long boreholes is determined, and the calculation formulas for the four boundaries of this range in the overburden are given.

[0010] S2. Within the range of the extraction locations of the directional long borehole layout, calculate the location criterion value of each directional long borehole. The calculation formula is as follows:

[0011] C v =R a ·R p ·R s ;

[0012] Among them, C v This is the criterion value for determining the location of directional long boreholes; a larger value indicates that the location is more suitable for borehole placement. R a R is an indicator of the degree of gas accumulation within mining-induced fractures; a higher value indicates a higher degree of gas accumulation. p R is an indicator of the permeability of the rock formation during mining; a higher value indicates a higher permeability of the rock formation. s This is a stability index for mining boreholes; the larger the value, the stronger the borehole stability.

[0013] S3. Determine the specific location and order of each borehole based on the criterion value of the directional long borehole location at each location.

[0014] Furthermore, in step S1, the range of the extraction position of the directional long borehole is located vertically in the fracture zone and laterally in the uphill area of ​​the structural fracture zone.

[0015] Furthermore, the calculation of the directional long borehole position criterion value at each location in step S2 includes the following steps:

[0016] S21. Calculate the gas accumulation index R within the mining-induced fracture. a ;

[0017] S22. Calculate the permeability index R of the mined rock strata. p ;

[0018] S23. Calculate the stability index R of the mining borehole. s ;

[0019] S24. Calculate the directional long borehole location criterion value C using the data obtained in steps S21, S22, and S23. v .

[0020] Furthermore, step S21 specifically includes the following steps:

[0021] S211. Calculate the concentration value ρ of gas at a specific location within the mining-induced fracture. g The calculation formula is as follows:

[0022]

[0023] Where a is the development coefficient of fractures in the mining-induced rock strata; e is the natural constant; δ is the coefficient of gas diffusion in the fracture channels of the goaf under both pure diffusion and pressure diffusion; and z is the height from the bottom of the goaf.

[0024] S212. The gas accumulation degree index R in the mining-induced fracture is obtained by quantifying the gas concentration value at a specific location within the mining-induced fracture. a .

[0025] Furthermore, step S22 specifically includes the following steps:

[0026] S221. Establish a coordinate system for the porosity distribution model of the overlying rock during mining. The coordinate system is set as follows: the x-axis is along the strike of the working face, with the positive direction towards the goaf; the y-axis is along the dip of the working face, with the positive direction towards the return airway of the working face; the z-axis is along the height of the mining area, with the positive direction upward; the origin of the coordinate system is set at the bottom of the middle coal seam in the dip direction of the working face.

[0027] S222. Establish the porosity distribution model φ(x) in the x-direction, and its formula is:

[0028]

[0029] Where B is the length of the goaf; x takes values ​​in the range of (0, B);

[0030] S223. Establish the porosity variation coefficient φ(y)' in the y-direction, its formula is:

[0031]

[0032] Where L is the working face dip length; y takes values ​​in the range of (-L / 2, L / 2);

[0033] S224. The porosity distribution model φ(x,y) in the xy plane is calculated, and its formula is:

[0034]

[0035] S225. Calculate the porosity variation coefficient φ(z)' within the crack zone in the z-direction. The formula is:

[0036]

[0037] Where k1 is the fracture zone bottom boundary fragmentation coefficient; b is the attenuation coefficient; and z is the normal distance from the coal seam.

[0038] S226. The three-dimensional spatial porosity distribution model φ(x,y,z) within the height range of the fracture zone in the goaf is calculated, and its formula is:

[0039]

[0040] Where φ is the porosity of the rock stratum;

[0041] S227. Based on the relationship between the permeability K and porosity φ of the fractured rock strata in the goaf, the permeability distribution model K(x,y,z) within the height range of the fracture zone in the goaf is obtained, and its formula is:

[0042]

[0043] Where μ is the dynamic viscosity coefficient of air;

[0044] S228. Based on the quantification of the permeability K of the fractured rock strata in the goaf, the permeability index R of the mining-induced rock strata is obtained. p .

[0045] Furthermore, step S23 specifically includes the following steps:

[0046] S231. Calculate the area of ​​fractures and fissures in rock strata. The formula for calculation is as follows:

[0047]

[0048] Among them, S j,i：i+1 h is the area of ​​the fracture between two adjacent fractured blocks in the j-th fractured rock layer; j The thickness of the j-th fractured rock stratum; M is the coal seam mining thickness; k sj is the average residual fragmentation coefficient of the rock layer below the j-th fractured rock layer; l is the length of the fractured rock block;

[0049] S232. The fracture area between each layer of broken blocks within the fracture zone is calculated from step S231. After normalization, the difference between this area and 1 is taken to obtain the relative stability of the boreholes located at the corresponding broken blocks under the influence of mining. After quantification, the stability index R of the mining boreholes is obtained. s .

[0050] Furthermore, in the formula for calculating the fracture area of ​​the rock strata in step S231, the formula for calculating the length l of the broken rock block is as follows:

[0051]

[0052] Where h is the thickness of the rock strata; R T q represents the ultimate tensile strength of the rock stratum; q represents the load borne by the rock stratum.

[0053] Compared with the prior art, the advantages and positive effects of this invention are:

[0054] This invention elucidates the technical principle of horizontally oriented long borehole extraction for decompression gas. Combining indoor experiments and numerical simulations, it analyzes three locational characteristics of horizontally oriented long boreholes: 1) high gas accumulation in the borehole location area, providing concentration conditions for gas extraction; 2) well-developed fractures in the borehole location area, providing a gas source guarantee for extraction; and 3) minimal impact of mining on the rock strata in the borehole location area, providing stability conditions for the borehole. Based on this, the extraction mechanism of horizontally oriented long boreholes is revealed from three aspects: the degree of gas accumulation in mining fractures, the permeability of mining strata, and the stability of mining boreholes. Location criteria for horizontally oriented long boreholes are proposed, and a method for determining the extraction location is given. The rationality is verified through numerical simulations and engineering examples. The research results have certain theoretical and engineering significance for improving the decompression gas extraction rate and promoting the development of green mining technology in coal mines. Attached Figure Description

[0055] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0056] Figure 1 A schematic diagram of a horizontally oriented long borehole extraction layout;

[0057] Figure 2 Map showing the gas distribution in the goaf;

[0058] Figure 3 This is a diagram showing the characteristics of fracture development in the overlying rock caused by mining.

[0059] Figure 4 A diagram showing the borehole stability characteristics within the "three zones" of the overburden during mining.

[0060] Figure 5 A coordinate graph of the mathematical model;

[0061] Figure 6 This is a structural diagram of a "masonry beam" for the first fractured rock stratum in the crack zone.

[0062] Figure 7 Flowchart of the method for determining the location of directional long boreholes;

[0063] Figure 8 Diagram showing the division of structural fracture zones and borehole locations;

[0064] Figure 9 This is a permeability distribution map of the mining-affected rock strata.

[0065] Figure 10A schematic diagram showing the criteria values ​​for drilling at the locations of each block segment;

[0066] Figure 11 This is a geometric model diagram of the mining area;

[0067] Figure 12 Mesh partitioning diagram for the model;

[0068] Figure 13 Cloud maps showing the gas distribution of single borehole extraction in each block;

[0069] Figure 14 This is a cross-sectional view showing the drilling and hole formation direction.

[0070] Figure 15 This is a schematic diagram of gas data extracted from boreholes.

[0071] Figure 16 This is a schematic diagram of gas monitoring data in the return airway. Detailed Implementation

[0072] 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, any modifications, equivalent substitutions, improvements, etc., made by those skilled in the art to all other embodiments obtained without creative effort should be included within the protection scope of the present invention.

[0073] Longwall goaf decompression gas extraction is an important component of the coal and gas co-mining technology system in green coal mining. In recent years, the deployment of horizontal directional long boreholes in the overburden for decompression gas extraction has been widely used. Unlike high-level extraction roadways and ordinary high-level boreholes, when extraction is deployed in the mining-induced overburden, its response and sensitivity to overburden damage and gas migration are significantly unique.

[0074] This invention studies the location characteristics and depressurization gas extraction mechanism of horizontally oriented long boreholes in mining overburden through theoretical analysis, numerical simulation and engineering case verification.

[0075] I. Location characteristics of directional long boreholes

[0076] The principle of directional long borehole extraction technology for managing gas decompression in goaf areas is as follows: Before mining, advanced long-distance directional drilling technology and equipment are used to construct directional long boreholes horizontally along the coal seam roof. Except for the opening section, the remaining boreholes are open holes, and their length is typically 100–1000 m, depending on the gas decompression needs of the goaf. As mining progresses, the horizontal boreholes remain located in stable mining-induced overburden fracture areas. Under the negative pressure of borehole extraction and the effects of gas diffusion and seepage, decompression gas from the upper corner of the working face and the goaf continuously flows into the boreholes and is extracted through them, achieving the goal of gas decompression extraction and management. (See...) Figure 1 As shown.

[0077] Directional long boreholes are deployed within the mined overburden to extract and relieve gas pressure; their location characteristics define their extraction characteristics. Due to their small diameter (around 150mm) and horizontal arrangement, the location characteristics of directional long boreholes within the mined overburden are mainly manifested in the following three aspects:

[0078] (1) The small borehole diameter results in a small effective extraction radius, requiring the borehole location to be a gas accumulation zone. This means that under gas diffusion and seepage, depressurized gas can naturally converge towards the borehole location, providing conditions for high-concentration gas extraction. Numerical simulations were conducted on the gas distribution in the goaf under U-shaped ventilation in a mine working face (see...). Figure 2 The characteristics of gas accumulation in the goaf are as follows: the gas volume fraction on the return airway side of the goaf is generally higher than that on the intake airway side, and the gas volume fraction in the upper part is higher than that in the lower part.

[0079] (2) A small borehole cross-sectional area results in fewer fractures directly connected to the borehole, requiring well-developed rock fractures and high permeability in the borehole location area. Under the negative pressure of borehole extraction, depressurized gas can flow rapidly through fractures to the borehole location area within the goaf, providing a gas source guarantee for borehole-extracted depressurized gas. Numerical simulation of a longwall mining face in a certain mine (see...) Figure 3 The characteristics of fracture development in the overlying rock during mining are as follows: fracture development zones exist around the goaf, exhibiting a ring-shaped feature. As mining progresses, the fractures in the middle are gradually compacted, and the overall fracture development in the lower part is more pronounced than that in the upper part.

[0080] (3) The boreholes are horizontally arranged on the roof of the coal seam and have a small cross-sectional area. During the extraction stage, they are easily damaged by mining activities, such as collapse and blockage, leading to borehole failure. Therefore, the rock strata in the area where the boreholes are located must have good stability under mining activities, with the strata fracturing but still maintaining their original layered characteristics, without collapse or displacement, to provide stability conditions for the boreholes located therein. A similar simulation was conducted on a longwall mining face of a certain mine (see...). Figure 4The stability characteristics of directional long boreholes arranged in the mining overburden caving zone, fracture zone, and bending subsidence zone are as follows: 1) Boreholes in the caving zone undergo shear displacement as the mining strata collapse, making them prone to collapse, blockage, and other damage; 2) Boreholes in the fracture zone experience tensile and shear failure as the mining strata fracture, but still maintain their original strata characteristics, which has a certain impact on borehole stability; 3) Boreholes in the bending subsidence zone bend and sink as the mining strata bend, without fracture, resulting in good borehole stability.

[0081] Therefore, the locational characteristics of directional long borehole extraction can be summarized as follows: high gas accumulation in the borehole location area, high permeability of the mined strata, and strong stability of the mining borehole. Combining the "O"-ring theory and literature on the regional division of the mining overburden, a qualitative analysis of the borehole location characteristics indicates that the suitable extraction range for the boreholes is located vertically in the fracture zone and laterally in the uphill area of ​​the structural fracture zone (hereinafter referred to as the structural fracture zone). (See...) Figure 1 As shown.

[0082] II. Mechanism of Depressurization and Gas Relief from Directional Long-Drilling

[0083] Analysis of the location characteristics of directional long boreholes reveals that they have stringent requirements for the location of extraction. To achieve stable and efficient extraction of depressurized gas from directional long boreholes, it is necessary to further quantify their location characteristics, reveal the extraction mechanism of directional long boreholes in the mining-affected overburden, and then provide location criteria for directional long boreholes to clarify their specific location in the mining-affected overburden.

[0084] 2.1 Criteria for Determining the Location of Directional Long Holes

[0085] The criterion for determining the location of directional long boreholes is now defined using C. v This indicates that it can be decomposed into the degree of gas accumulation within the mining-induced fractures (R). a ), mining permeability of rock strata (R) p ) and mining borehole stability (R s The three factor indicators can be calculated using the following formula:

[0086] C v =R a ·R p ·R s (1)

[0087] By quantitatively analyzing the three aspects of the location characteristics of directional long boreholes in the mining overburden, three factor indicators for borehole location criteria can be obtained. Then, the order of borehole layout can be determined from the magnitude of the criteria value, so as to achieve effective layout and extraction of boreholes.

[0088] 2.2 Analysis of the degree of gas accumulation in mining-induced fractures (Ra)

[0089] Coal seam mining leads to the desorption and release of large amounts of gas contained in the coal seam and rock mass. The desorbed gas exhibits varying concentrations across the goaf. Under the influence of pressure and concentration gradients, the gas diffuses within the fractures of the overlying strata, resulting in uneven gas accumulation throughout the area. Research indicates that the gas concentration distribution follows a pattern of variation with increasing height from the coal seam floor:

[0090] ρ g =ae δz (2)

[0091] In the formula: a is an undetermined coefficient; δ is a coefficient considering pure diffusion and pressure diffusion of gas in the fracture channels of the goaf, which can be taken as 0.018; z is the height from the bottom of the goaf, in meters.

[0092] A higher methane concentration in a specific area indicates a higher degree of methane accumulation. Therefore, given a coefficient, the methane concentration at a specific location within the mining-induced fracture can be calculated using equation (2), thereby quantifying the degree of methane accumulation (R) within the mining-induced fracture in the borehole location criterion. a ).

[0093] 2.3 Permeability analysis of mining-induced rock strata (R p )

[0094] Coal seam mining leads to deformation and fracturing of the overburden and the development of fractures. Under the negative pressure of borehole extraction, methane gas migrates and flows within these fracture channels. Because the degree of fracture development varies across different parts of the mined overburden, the permeability also differs, resulting in significant variations in the flow velocity of the methane gas. Therefore, establishing a model to analyze the porosity of the mined overburden is fundamental to understanding the permeability distribution of the mined strata.

[0095] 2.3.1 Model Coordinates

[0096] Based on the coal seam mining situation, a mathematical model of the porosity distribution of the overlying strata is established, with coordinates as follows: Figure 5 As shown. The model coordinates are set as follows: the x-axis is along the strike of the working face, with the positive direction towards the goaf; the y-axis is along the dip of the working face, with the positive direction towards the return airway of the working face; the z-axis is along the height of the mining area, with the positive direction upward; based on the symmetrical characteristics of mining-induced overburden failure, the origin of the model coordinates is set at the bottom of the middle coal seam dipped in the working face.

[0097] 2.3.2 Mathematical Model of Porosity Distribution

[0098] (1) Porosity distribution model in the x-direction

[0099] Along the strike of the working face (x-axis direction), the porosity φ(x) of the mined rock strata exhibits an exponential function distribution, with high porosity at both ends of the goaf and gradually decreasing towards the center, showing an approximately symmetrical distribution.

[0100]

[0101] In the formula: B is the strike length of the goaf, in meters; x takes values ​​in the range of (0, B).

[0102] (2) Porosity distribution model in the xy plane

[0103] Along the dip direction of the working face (y-axis), the porosity of the mined rock strata also exhibits a distribution characteristic where the porosity is high at both ends of the goaf and gradually decreases towards the middle. If φ(y)' represents the porosity variation coefficient, then under the established model coordinate conditions, the porosity variation coefficient of the rock strata along the y-axis can be expressed as:

[0104]

[0105] In the formula: L is the working face inclination length, m; y takes values ​​in the range of (-L / 2, L / 2).

[0106] By combining equations (3) and (4), a porosity distribution model of the mining strata on the xy plane can be constructed:

[0107]

[0108] (3) Three-dimensional spatial porosity distribution model of xyz

[0109] Let φ be the porosity of the rock stratum, and its relationship with the rock fragmentation coefficient k can be expressed by the following formula:

[0110]

[0111] The foregoing analysis shows that the vertical arrangement area of ​​directional long boreholes is a fracture zone; therefore, only the porosity distribution of the fractured rock strata within the fracture zone is analyzed. Based on measurements of the breccia coefficient of the roof of a mine, the literature proposes that the breccia coefficient within the fracture zone exhibits an approximately logarithmic decay characteristic from bottom to top.

[0112] k = k1 - blnz (7)

[0113] In the formula: k1 is the fracture zone bottom boundary expansion coefficient; b is the attenuation coefficient; z is the normal distance from the coal seam, in meters.

[0114] Substituting equation (7) into equation (6), we obtain the porosity function along the z-axis within the fracture zone as follows:

[0115]

[0116] The rate of change of its porosity along the z-axis is:

[0117]

[0118] Then, within the fracture zone, the porosity variation coefficient φ(z)' along the z-axis can be expressed as:

[0119]

[0120] Therefore, the three-dimensional spatial porosity distribution model of xyz within the height range of the fracture zone in the goaf can be obtained as follows:

[0121]

[0122] Based on the obtained porosity distribution model of the mining-induced rock strata, the permeability distribution model of the mining-induced rock strata can then be analyzed.

[0123] 2.3.3 Mathematical Model of Permeability Distribution

[0124] Mining-induced fracture development leads to a significant increase in rock permeability. Considering the fractured rock strata in the goaf as porous media, research shows that the relationship between the permeability K and porosity φ of the fractured rock strata in the goaf can be expressed as:

[0125] K(x,y,z)=0.01605μφ(x,y,z) 2 (12)

[0126] In the formula: μ is the dynamic viscosity coefficient of air, which is μ = 1.834 × 10 at room temperature. -5 Pa·s.

[0127] Substituting equation (11) into equation (12), we can obtain the mathematical model for the distribution of rock permeability within the height range of the fracture zone in the goaf:

[0128]

[0129] Equation (13) can be used to analyze and solve for the permeability of depressurized gas in the mining strata, and thus quantify the permeability index (R) of the mining strata in the borehole location criterion. p ).

[0130] 2.4 Stability Analysis of Mining-Driven Boreholes (R) s )

[0131] Based on the characteristics of directional long borehole construction, the horizontal extraction section of the borehole is constructed as a bare hole located in a fracture zone, where it undergoes synchronous deformation and fracture with the rock strata. While the delamination between fractured rock strata has a relatively small impact on borehole stability, the fracturing and displacement of the rock strata significantly affects borehole stability, potentially leading to borehole blockage, collapse, and failure. Therefore, analyzing the fracture characteristics of the fractured rock blocks in the fracture zone is crucial for understanding the stability of mining boreholes.

[0132] (1) Calculation of fracture area of ​​rock strata

[0133] The "masonry beam" structure provides a good description of the distribution morphology of rock strata fractures within the crack zone. Therefore, the analysis is based on the "masonry beam" structure of the rock strata fractures in the dip profile of the working face, taking the first (bottommost) rock stratum as the analysis object. Figure 6 As shown.

[0134] Assuming the periodically fractured rock blocks are of the same length, i.e., l1 = l2 = ... = l n =l, the angle between the fractured rock block 1 of the first fractured rock layer in the fracture zone and the horizontal line is θ 1,1 Analysis of the displacement law of the entire structural curve of the "masonry beam" shows that the rotation angle of the broken rock block satisfies:

[0135]

[0136] Based on the characteristics of surface subsidence and deformation, the structural curve of the rock strata fracture "masonry beam" structure can be represented as:

[0137]

[0138] In the formula: W x 1. Displacement curve of the "masonry beam" structure with rock fracture, m; W0 is the maximum subsidence of the rock strata, m; x is the horizontal distance from the coal pillar, m; l is the length of the fractured rock block. h is the thickness of the rock strata, in meters (m). R T q represents the ultimate tensile strength of the rock stratum, in MPa; q represents the load borne by the rock stratum, in kN; a is taken as 0.25l.

[0139] The maximum subsidence (W0) of the first fractured stratum in the fracture zone can be expressed as:

[0140] W0=M-∑h1(k s1 -1) (16)

[0141] In the formula: M is the coal seam mining thickness, m; Σh1 is the total thickness of the rock strata below the first fractured rock stratum, m; k s1 It represents the average residual fragmentation coefficient of the rock strata below the first fractured rock stratum.

[0142] Based on the assumption of a "masonry beam" structure curve for rock strata fracture, and using equation (14), the settlement displacement of fracture block 1 is:

[0143] W l =lsinθ 1,1 (17)

[0144] By combining equations (14) to (17), we can obtain:

[0145]

[0146] Then, the angle between the j-th fractured rock layer (arranged sequentially from bottom to top) and the horizontal line can be expressed as:

[0147]

[0148] analyze Figure 6 It can be seen that the included angle between two adjacent fractured rock blocks can be obtained by subtracting the included angle between the two fractured rock blocks and the horizontal line. Therefore, the included angle between two adjacent fractured rock blocks in the first fractured rock layer can be expressed as:

[0149]

[0150] In the formula: α 1,i～i+1 θ is the angle between the fractured rock blocks i and i+1 in the first fractured rock layer, in degrees; 1,i Let be the angle between the fractured rock block i of the first fractured rock layer and the horizontal line, in degrees.

[0151] Similarly, the angle between two adjacent fractured rock blocks in the j-th fractured rock layer can be expressed as:

[0152]

[0153] Suppose that two adjacent fractured blocks in the j-th layer form an isosceles triangle with an opening of d, and the thickness of the rock layer is h. j Then the opening degree d j,i～i+1 The area S of the isosceles triangle j,i～i+1 They can be represented as:

[0154]

[0155]

[0156] Combining equations (21) to (23), the fracture area between two adjacent fractured blocks in the j-th fractured rock layer can be expressed as:

[0157]

[0158] Substituting equation (19) into the equation, we get:

[0159]

[0160] By calculating the area of ​​fractures between broken rock blocks, the stability of boreholes located at their positions can be further analyzed.

[0161] (2) Analysis of the relative stability of boreholes

[0162] Different rock strata exhibit varying fracture and subsidence characteristics, resulting in different fracture sizes. Consequently, these fractures have varying degrees of impact on the stability of boreholes located within the rock strata. Therefore, the size of the fractures in the rock strata can indirectly reflect the stability of boreholes affected by mining. Equation (25) can be used to calculate the fracture area between fractured rock blocks in each layer within the fracture zone. By normalizing this area and taking the difference from 1, the relative stability of boreholes located at the corresponding fractured rock block affected by mining can be obtained. This allows for the quantification of the mining-induced borehole stability index (Rmining) in the borehole location criterion. s ).

[0163] 2.5 Method and Flowchart for Determining the Location of Directional Long Holes

[0164] Based on the aforementioned analysis of the location characteristics and extraction mechanism of directional long boreholes, the extraction locations of boreholes in the mined overburden can be referenced. Figure 7 The methodology and process were determined.

[0165] III. Numerical Simulation of Directional Long Borehole Layout for Extraction

[0166] Taking the geological and mining conditions of a working face in Henan Province as an example, the layout of directional long boreholes in the mining overburden was determined according to the proposed method and process. COMSOL Multiphysics numerical calculation software was used to simulate and analyze the extraction of directional long boreholes, verify the rationality of borehole layout extraction based on borehole location criteria, and analyze and determine the number of boreholes.

[0167] 3.1 Engineering Geological Conditions

[0168] The coal seam being mined is the No. 2-1 coal seam of the Shanxi Formation in the Permian system, with a dip angle of 12°. Longwall mining is employed, with a strike length of 495m, a dip length of 160m, a mining thickness of 3.0m, and an average burial depth of 388.5m. After regional gas control measures, the residual gas content at the working face is 3.53-5.76m³. 3 / t, the estimated absolute gas emission at the working face is 2.32-4.77m³. 3 / min, working face air volume 1065m³ 3 / min. The physical and mechanical parameters of coal and rock in the working face are shown in Table 1.

[0169] Table 1. Lithological parameters of the overlying rock at the working face (partial)

[0170]

[0171]

[0172] 3.2 Determination of the location of directional long boreholes

[0173] Following the method for determining the location of directional long boreholes, the boundary of the structural fracture zone is first defined to determine the range of borehole placement in the mining overburden. Then, by quantitatively analyzing the three aspects of the location characteristics of the directional long boreholes, the criterion values ​​for placing boreholes at each location within the area are obtained, thereby determining the placement of the boreholes (in practical applications, the absolute gas emission rate and ventilation rate at the working face will have a certain impact on the overall extraction rate of the boreholes placed within the area, but the specific placement of the boreholes is still based on the three aspects of their location characteristics).

[0174] 3.2.1 Drilling Layout Range

[0175] Based on the aforementioned analysis of the location characteristics of directional long boreholes, the suitable drilling area is determined to be the structural fracture zone. Therefore, theoretical calculation formulas are used to define the boundaries of this area. The upper and lower vertical boundaries are defined as the normal distances from the coal seam, which are 5.3m and 36.6m, respectively. The horizontal boundary is defined as the horizontal distance from the adjacent side coal pillar. Based on the borehole layout characteristics, the outer and inner boundaries along the dip direction need to be determined, which are 0.47H... i and 0.47H i +36.3m, H i denoted as , where is the normal distance between the i-th rock layer and the coal seam.

[0176] 3.2.2 Drilling location

[0177] Related studies indicate that the effective extraction radius of directional long boreholes in mining-affected overburden is approximately 5m. To reduce the mutual influence of borehole extraction and improve the extraction efficiency of a single borehole, the borehole spacing should be twice the effective extraction radius, approximately 10m. The calculated range of the structural fracture zone (the area of ​​borehole layout) is 36.3m wide and 31.3m high. Therefore, based on the borehole spacing, the structural fracture zone can be divided into nine grid segments along the dip profile, with each segment approximately 10m wide and high. Based on the results of similar simulation tests of working face mining, the division of each segment and the location of the boreholes within it are illustrated. Figure 8 We will now conduct a quantitative analysis of three indicators for determining the borehole location in each segment.

[0178] (1) Gas accumulation level in the block (Ra)

[0179] Let H be the height of the lower boundary of the structural fracture zone from the bottom of the goaf, and let the height of the region be 3h. Then, the upper, middle, and lower rock strata of each block (e.g., Figure 8 The average heights from the bottom of the goaf (as shown) can be expressed as H+5h / 2, H+3h / 2, and H+h / 2, respectively. According to the calculation results of the boundary of the structural fracture zone, H = 8.3m and H+3h = 39.6m. Then, from equation (2), the gas concentrations of each block of the upper, middle, and lower rock strata in the structural fracture zone are 1.86a, 1.54a, and 1.28a, respectively.

[0180] The methane concentration value of each segment represents the degree of methane accumulation. Taking the overall methane accumulation degree of the structural fracture zone as 1, the normalized methane accumulation degree of each segment is shown in Table 2.

[0181] Table 2. Relative Gas Accumulation Degree of Each Block

[0182]

[0183] (2) Permeability of block rock formations (Rp)

[0184] Equation (13) was used to calculate and analyze the permeability of the rock strata within the fracture zone height range. The values ​​were k1 = 1.35, B = 300m, and L = 160m. The coal seam mining height was 3.0m, the caving zone height was 8.3m, and the fracture zone height was 39.6m. The rock strata within the fracture zone were divided into upper, middle, and lower parts, with average heights from the coal seam of 31.4m, 21.0m, and 10.5m, respectively. The calculated average permeability of the upper, middle, and lower rock strata within the fracture zone is as follows: Figure 9 As shown.

[0185] Depend on Figure 9 It can be seen that as the height of the rock strata from the top of the coal seam increases, the overall permeability gradually decreases, but the magnitude is small; for a specific rock stratum, due to the existence of the "masonry beam" structure of the rock stratum fracture, the permeability is high around its perimeter, while the permeability of the central compacted area is significantly reduced.

[0186] Based on the location and extent of the structural crack zone within the crack band, and in conjunction with the segmentation of the area, the following were respectively... Figure 9 The permeability distribution surfaces of the upper, middle, and lower rock strata were integrally calculated over the corresponding block segments to obtain the permeability integral area of ​​each block segment. Taking the overall permeability of the mined rock strata in the structural fracture zone as 1, the normalized relative permeability of the rock strata in each block segment is shown in Table 3.

[0187] Table 3 Relative permeability of rock strata in each section

[0188]

[0189] (3) Block drilling stability (Rs)

[0190] Equation (25) was used to calculate the fracture area between broken rock blocks along the dip profile in the structural fracture zone. The values ​​of each parameter are shown in Table 4. After calculation, the fracture area between broken rock blocks in each layer of the structural fracture zone was obtained. Then, it was normalized and the difference was taken from 1 to obtain the relative stability of the boreholes located at the corresponding rock blocks under the influence of mining, as shown in Table 5 (the rock layers are arranged in order from bottom to top).

[0191] Table 4 Calculation parameter values

[0192] parameter value parameter value Coal seam mining height 3.0m thickness of collapsed rock layer 5.3m Area thickness 31.3m area width 36.3m Number of fractured rock layers 9th floor Thickness of fractured rock strata (single layer) 3.48m Periodic fracture rock block length 6.05m Number of broken rock blocks (single layer) 6 Rock stratum swelling coefficient 1.05

[0193] Based on the fractured rock blocks contained in each segment of the structural fracture zone (each segment contains three rock layers, and each layer contains two fractured rock blocks, as shown by different fill colors in Table 5), the relative stability of boreholes arranged in each segment under the influence of mining can be obtained. After normalization (taking the overall stability of boreholes arranged in the structural fracture zone as 1), the relative stability index of boreholes arranged in the corresponding segment can be obtained, as shown in Table 6.

[0194] Table 5. Fracture area and relative borehole stability index between adjacent fractured rock blocks in each layer.

[0195]

[0196] Table 6. Relative Stability of Boreholes in Each Block

[0197]

[0198] (4) Determining the drilling location

[0199] Substituting the three index values ​​of each block in Tables 2, 3, and 6 into Equation (1), the criterion values ​​for drilling at the location of each block can be obtained. Since the obtained criterion values ​​are relatively small, they are normalized as follows: Figure 10 As shown.

[0200] according to Figure 10 It can be seen that the priority order of the drilling locations in the structural fracture zone is: block I - block II - block IV - block V - block III - block VIII - block VI - block VII - block IX.

[0201] 3.3 Numerical Simulation of Directional Long Borehole Extraction

[0202] A model was established using COMSOL Multiphysics numerical simulation software. Following the obtained borehole location sequence, simulations were performed on the extraction of depressurized gas from the goaf using directional long boreholes located in blocks I and II. The numerical simulations assumed that the collapsed and fractured rock strata within the goaf were porous media, and that the gas was incompressible and at a constant temperature.

[0203] 3.3.1 Numerical Model Establishment

[0204] The numerical simulation selects the fluid module and performs steady-state solutions in three-dimensional space. The physics interface is coupled with the free and porous media flow interface and the chemical substance transport interface for rare matter transport, and the structural fracture region of the established geometric model is segmented. The established geometric model is shown below. Figure 11 .

[0205] The model is assigned the following parameters:

[0206] (1) Porosity and permeability. The values ​​of porosity and permeability for each region of the model are shown in Table 7.

[0207] Table 7. Porosity and permeability of different areas in the mining area

[0208] area code Porosity Penetration area code Porosity Penetration A - - I 0.291 7.4323E-8 B - - J 0.183 2.4544E-8 C 0.188 2.6287E-8 K 0.101 4.8600E-9 D 0.119 7.8477E-9 L 0.025 8.9121E-11 E 0.06 1.1451E-9 M 0.305 8.2228E-8 F 0.245 5.0297E-8 N 0.236 4.6033E-8 G 0.152 1.5152E-8 O 0.146 1.3618E-8 H 0.077 2.3336E-9 P 0.063 1.3172E-9

[0209] (2) Model boundary conditions. The model uses U-shaped ventilation, and the boundary conditions are set as follows:

[0210] 1) Physical field of free and porous media flow. The working face, intake and return airways, and directional long borehole are free flow regions, while the goaf is a porous media region; the initial value of the model is one standard atmosphere; the intake airway is set as a velocity inlet of 1.12 m / s; the return airway and directional long borehole are both set as pressure outlet boundary conditions, with the return airway set at a pressure of 99325 Pa and the directional long borehole set at a pressure of 81325 Pa; the remaining solid boundaries are set as walls.

[0211] 2) Physical field for transporting dilute matter in porous media. The free flow and porous media regions are set up in the same way as the physical fields described above; the initial model values ​​are set so that the entire mining area is filled with air; the intake airway is set as the air inlet, filled with air; the return airway and boreholes are set as outlets; the gas emission sources in the working face are the floor and coal wall, with a flux of 2.43E-3 mol / (m³). 2 The gas emission sources in the goaf are the floor and the three side coal walls, with a flux of 8.42E-5 mol / (m³). 2 ·s); the remaining solid boundaries are set as walls.

[0212] (3) Model mesh generation

[0213] The model was meshed using a physics-controlled mesh, comprising 1,457,210 domain elements, 154,866 boundary elements, and 7,906 edge elements. Figure 12 As shown.

[0214] 3.3.2 Borehole Extraction Analysis

[0215] The gas distribution in the working face and goaf under single-hole extraction conditions at locations I and II are shown below. Figure 13 As shown.

[0216] Depend on Figure 13 It can be seen that under the drilling and extraction effect at the locations of Block I and Block II, the overall gas volume fraction of the working face is very small, and the gas volume fraction in most areas of the goaf is also less than 30%, but there are still some areas with high concentrations of gas accumulation. Figure 13(b) The yellow line corresponds to a gas volume fraction of approximately 55%, and its distribution along the x-axis and z-axis is marked. Statistics on the pure gas content extracted from boreholes and the gas distribution in the working face and goaf are shown in Table 8.

[0217] Table 8. Borehole Extraction Volume and Gas Distribution

[0218]

[0219] As shown in Table 8:

[0220] (1) The pure gas content extracted from the borehole at location I of block was 0.876 m³. 3 / min, the borehole depth at location II is 0.780m. 3 / min. Under the action of drilling and drainage at block I, the range of high-concentration gas accumulation in the goaf (gas volume fraction ≥55%) and the maximum gas volume fraction value are both smaller than those of drilling and drainage at block II. However, there is still a gas accumulation zone in the goaf that is more than 100 meters wide and more than 10 meters high.

[0221] (2) Under the action of single borehole extraction, the gas volume fraction in the upper corner of the working face is less than 1%, but the gas volume fraction in the return airway is higher, greater than 5%.

[0222] Numerical simulation results of directional long borehole extraction show that borehole extraction at location I is more effective than at location II, verifying the rationality of borehole layout based on location criteria. However, single borehole extraction cannot meet the gas control needs of the working face and goaf, and a large gas accumulation area still exists in the goaf, with a high gas volume fraction in the return airway of the working face. Based on the pure gas extraction value and absolute gas emission rate at the working face from the simulation experiment, a design is proposed to simultaneously deploy boreholes in both locations I and II for pressure relief gas extraction to ensure safe production.

[0223] IV. Field Application of Directional Long Borehole Drilling for Extraction

[0224] 4.1 Layout of directional long boreholes

[0225] An opening was made at the mine's unloading area, and two directional long boreholes were drilled along the strike into the roof of the working face for depressurization and gas extraction. Borehole #1 was located in block I (sequence one), and borehole #2 was located in block II (sequence two). The borehole diameter was 96 mm, the spacing was 9.5 m, and the extraction negative pressure was 20 kPa. (See details...) Figure 14 As shown.

[0226] 4.2 Gas Analysis of Directional Long Borehole Extraction

[0227] When the working face was being mined to the vicinity of the borehole location, borehole extraction data was collected until the end of the working face mining, lasting 232 days. Borehole extraction data on pure and mixed gas quantities are available in [link to relevant documentation]. Figure 15As shown.

[0228] analyze Figure 15 It can be known that:

[0229] (1) The maximum pure gas extraction rate and mixed gas extraction rate of borehole #1 are 2.59 m³ / s. 3 / min and 9.86m 3 / min, corresponding to a maximum daily pure gas extraction volume of 3729.6m3; the maximum pure gas extraction volume and mixed volume of borehole #2 are 2.20m3 and 2.20m3 respectively. 3 / min and 7.84m 3 / min, corresponding to a maximum daily gas extraction volume of 3168.0 m³. 3 .

[0230] (2) For the entire drilling and extraction process, the total pure gas content extracted from the two boreholes ranged from 0.59 to 3.19 m³. 3 / min, of which the average pure gas extraction rate of borehole #1 is 1.15m³. 3 / min, with an average daily gas extraction purity of 1656.0 m3; the average gas extraction purity of borehole #2 is 0.45 m3. 3 / min, with an average daily gas extraction volume of 648.0 m3.

[0231] (3) The maximum gas extraction purity and mixed volume of borehole 1 are greater than those of borehole 2. Furthermore, the gas extraction purity of borehole 1 throughout the entire extraction process is approximately 2.56 times that of borehole 2. The extraction effect of borehole 1, located in block I (sequence one), is significantly better than that of borehole 2, located in block II (sequence two).

[0232] Under the action of directional long borehole extraction, no gas exceedances were observed at the working face. Data from gas sensor monitoring in the return airway was collected and analyzed. Figure 16 As shown.

[0233] Depend on Figure 16 It can be seen that under the action of directional long borehole extraction (the total pure gas extracted is 0.59~3.19m³), 3 The average gas volume fraction in the return airway is 0.06–0.24%, the maximum gas volume fraction is 0.11–0.72%, and the exhaust gas volume is 0.69–2.79 m³ / min. 3 / min, the directional long borehole effectively extracted and controlled the depressurized gas in the goaf of the working face. No gas exceeding the limit accident occurred in the working face or the return airway, which verified the rationality of the borehole layout extraction according to the borehole location determination method.

[0234] V. Conclusion

[0235] (1) Based on the principle of directional long borehole gas extraction and decompression technology and its structure and layout characteristics, three locational characteristics of directional long boreholes in the overlying rock of longwall mining are described: the gas accumulation in the borehole location area is high, providing concentration conditions for borehole gas extraction and decompression; the fractures in the borehole location area are relatively developed, providing gas source guarantee for borehole extraction; and the rock strata in the borehole location area are less affected by mining, providing stability conditions for the borehole.

[0236] (2) The degree of gas accumulation due to decompression within the mining-induced fracture (R) was analyzed theoretically. a ), mining permeability of rock strata (R) p ) and mining borehole stability (R s The study revealed the extraction mechanism of directional long boreholes in mining-induced overburden by analyzing three aspects of location characteristics, and proposed a criterion for determining the location of directional long boreholes (C). v Based on this, the layout and extraction locations of directional long boreholes were determined, and specific methods and procedures were provided.

[0237] (3) The application was verified through numerical simulation and engineering examples. The results show that the total pure gas content for drilling and extraction based on borehole location criteria is 0.59–3.19 m³. 3 / min, of which the maximum gas extraction volume from a single borehole reached 2.59m³. 3 / min, which is 2.56 times that of other comparative boreholes; under the action of directional long borehole extraction, the maximum gas volume fraction in the return airway is 0.11-0.72%, and the ventilation gas volume is 0.69-2.79m³. 3 The result of the test at a rate of / min verified the rationality of drilling layout and extraction based on the method of determining the borehole location.

Claims

1. A method for determining the extraction location of horizontally oriented long boreholes in overlying rock during longwall mining, characterized by: Includes the following steps: S1. Based on the mining methods and parameters of the working face and the lithological characteristics of the overlying strata, and on the basis of analyzing the distribution of gas migration fracture channels in coal seam mining and the stability of boreholes affected by mining, the range of extraction locations for directional long boreholes is determined. S2. Within the range of the extraction locations of the directional long borehole layout, calculate the location criterion value of each directional long borehole. The calculation formula is as follows: ； in, C v This is the criterion value for determining the location of a long directional borehole; R a This is an indicator of the degree of gas accumulation within mining-induced fractures; R p The permeability index of the rock strata being mined; R s For the stability index of mining boreholes; S3. Determine the specific location and order of each borehole based on the directional long borehole location criterion value at each location. In step S1, the range of the extraction location for the directional long borehole is located vertically in the fracture zone and laterally in the uphill area of ​​the structural fracture zone. The step S2, which calculates the directional long borehole position criterion value at each location, includes the following steps: S21. Calculate the gas accumulation index within mining-induced fractures. R a ; S22. Calculate the permeability index of the mining-affected rock strata. R p ; S23. Calculate the stability index of mining boreholes. R s ; S24. Calculate the directional long borehole location criterion value using the data obtained in steps S21, S22, and S23. C v ; Step S22 specifically includes the following steps: S221. Establish a coordinate system for the porosity distribution model of the overlying strata during mining; the coordinate system is set as follows: along the strike of the working face. x The axis, with the direction towards the goaf as the positive direction; along the dip direction of the working face as the positive direction. y The axis, with the direction towards the return airway of the working face as the positive direction; and the direction along the height of the mining area as the negative direction. z The axis is upward, with the positive direction being upward; the origin of the coordinate system is set at the bottom of the middle coal seam of the working face. S222, Establish x directional porosity distribution model Its formula is: ； in, B The length of the goaf. x The range of values ​​is (0, ...). B ); S223, Establish y directional porosity variation coefficient Its formula is: ； in, L The working face dip length; y The range of values ​​is (- L / 2, L / 2); S224, calculated as follows xy Planar porosity distribution model Its formula is: ； S225, Calculation z Porosity variation coefficient within directional fracture zone Its formula is: ； in, k 1 represents the coefficient of fragmentation at the bottom boundary of the crack zone; b The attenuation coefficient; z This is the normal distance from the coal seam; S226, Calculate the height range of the crack zone in the goaf. xyz Three-dimensional spatial porosity distribution model Its formula is: ； in, The porosity of the rock strata; S227. Based on the permeability of the fractured rock strata in the goaf area K With porosity The relationship yields a rock stratum permeability distribution model within the height range of the fracture zone in the goaf. K ( x , y , z The formula is: ； in, μ is the dynamic viscosity coefficient of air; S228. Based on the permeability of the fractured rock strata in the goaf... K Quantitatively obtain the permeability index of the mining-induced rock strata R p .

2. The method for determining the extraction location of horizontally oriented long boreholes in overlying rock during longwall mining as described in claim 1, characterized in that: Step S21 specifically includes the following steps: S211. Calculate the concentration of gas at a specific location within the mining-induced fracture. ρ g The calculation formula is as follows: ； in ，a The coefficient for the development of fractures in the mining-induced rock strata; e It is a natural constant; δ This represents the coefficients for pure diffusion and pressure diffusion of gas in the fracture channels of the goaf; z This refers to the height from the bottom of the goaf. S212. The degree of gas accumulation within the mining-induced fracture is quantified based on the concentration values ​​of gas distribution at specific locations within the fracture. R a .

3. The method for determining the extraction location of horizontally oriented long boreholes in overlying rock during longwall mining as described in claim 1, characterized in that: Step S23 specifically includes the following steps: S231. Calculate the area of ​​fractures and fissures in rock strata. The formula for calculation is as follows: ； in, S j,i：i+1 For the first j The area of ​​the fracture fissure between two adjacent fractured blocks in a fractured rock stratum; h j For the first j The thickness of the fractured rock strata; M The thickness of the coal seam during mining; k sj For the first j The average residual fragmentation coefficient of the rock strata below the fractured rock strata; l The length of the broken rock block; S232. The fracture area between each layer of broken blocks within the fracture zone is calculated from step S231. After normalization, the difference between this area and 1 is taken to obtain the relative stability of the boreholes located at the corresponding broken blocks under the influence of mining. After quantification, the stability index of the mining boreholes is obtained. Rs .

4. The method for determining the extraction location of horizontally oriented long boreholes in overlying rock during longwall mining as described in claim 3, characterized in that: In the formula for calculating the fracture area of ​​rock strata in step S231, the length of the fractured rock block is... l The calculation formula is: ； in, h The thickness of the rock strata; R T The ultimate tensile strength of the rock stratum; q To bear the load for the rock strata.

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