Ground-underground cooperative area weakening based rockburst prevention and control method for thick and hard roof

Abstract

A ground-underground cooperative area weakening based rockburst prevention and control method for a thick and hard roof. The method comprises: first obtaining spatial distribution characteristics of a hard roof; then on the basis of the spatial distribution characteristics of the hard roof, obtaining different types of fracturing prevention and control target horizons and five fracturing prevention and control patterns corresponding thereto, wherein the five fracturing prevention and control patterns comprise: a low-position single thick and hard roof horizon, a low-position composite hard roof horizon, a high-low position composite thick and hard roof horizon, a high-position composite thick and hard roof horizon, and a high-position single thick and hard roof horizon. The method can fundamentally and comprehensively solve the problem of rockburst hazards induced by a thick and hard roof.

Description

TECHNICAL FIELD [1] The present disclosure belongs to the technical field of underground safety in coal mines, and relates to a method for rockburst prevention through weakening, in particular to a method for rockburst prevention through surface-underground collaborative regional weakening of a thick and hard roof. BACKGROUND [2] Coal will remain a dominant energy source in China for a considerable period in the future, playing a role of "ballast stone and stabilizer" for energy. According to statistics, in the past 10 years, roof collapse accidents have accounted for 27% of all fatalities and 35% of all incidents in China’s coal mine safety accidents, ranking first among all types of accidents. The prevention of roof-type strong mine pressure-induced disasters has become a key challenge for safe and efficient mining in coal mines. One-third of the coal seams in China are characterized by thick-bedded hard roofs (i.e., thick and hard roofs). Taking the Jurassic coal fields in the western mining area as an example, under the action of lateral migration and swing of terrestrial sedimentary distributary channels, thick and hard sandstone layers with a stacked configuration are formed, which are highly prone to strong mine pressure (rockburst) dynamic disasters caused by sudden collapse of a single thick and hard large hanging roof, or by simultaneous or interactive fracturing and movement of multiple hard roofs. [3] Currently, it is difficult to achieve precise and efficient control of the thick and hard roofs in the coal mines, which makes it impossible to effectively control the mine pressure in the mining area, leading to frequent occurrences of mine pressure disasters. This poses a significant safety hazard and causes serious economic losses. SUMMARY [4] In view of the defects and shortcomings in the related art, an objective of the present disclosure is to provide a method for rockburst prevention through surface-underground collaborative regional weakening of a thick and hard roof, to solve the technical problem in the related art that rockburst disasters are easily induced by the thick and hard roofs in the coal mines. [5] To solve the above technical problem, the present disclosure is achieved adopting the following technical solution. [6] A method for rockburst prevention through surface-underground collaborative regional weakening of a thick and hard roof specifically includes the following operations. [7] In step 1, spatial distribution characteristics of a hard roof are obtained. [8] In step 2, a single thick roof layer and a composite hard roof layer are obtained according to the spatial distribution characteristics of the hard roof obtained in the step 1. Hard rock layers and soft rock layer groups controlled by the hard rock layers in an overlying stratum are obtained by identifying hard rock layers of the single thick roof layer and the composite hard roof layer according to a formula for identifying a position of a hard rock layer. [9] The formula for identifying the position of the hard rock layer is as follows: Formula IV; Em+1 represents an elastic modulus of a (m+1)-th rock layer; hm+1 represents a thickness of the (m+1)-th rock layer; Ym+i represents a unit weight of the (m+1)-th rock layer; hi represents a thickness of the i-th rock layer, i=1, 2, ..., m; Yi represents a unit weight of the i-th rock layer, i=1, 2, ., m; and Ei represents an elastic modulus of the i-th rock layer, i=1, 2, ., m.

[10] In step 3, a hard rock layer with high static load energy density is obtained based on geological information about the hard rock layers and the soft rock layer groups controlled by the hard rock layers obtained in the step 2.

[11] In step 4, dynamic load energy data of the hard rock layer with the high static load energy density is acquired based on the hard rock layer with the high static load energy density obtained in the step 3, and a fracturing prevention target layer is determined according to the dynamic load energy data. The fracturing prevention target layer is classified as one of the following five fracturing prevention modes: a low-position single thick and hard roof layer, a low-position composite hard roof layer, a high-position and low-position composite thick and hard roof layer, a high-position composite thick and hard roof layer, and a high-position single thick and hard roof layer.

[12] In step 5, different modes for prevention through weakening are adopted according to different types of fracturing prevention target layers classified in the step 4, which specifically includes the following operations.

[13] In step 5.1, when the fracturing prevention target layer is classified as the low-position single thick and hard roof layer, the mode for the prevention through the weakening includes: constructing a surface single-borehole straight well for large-volume fluid supply on the ground, and constructing ultra-deep directional boreholes I for the thick and hard roof underground; and taking the surface single-borehole straight well as a fracturing fluid delivery channel, taking, in the low-position single thick and hard roof layer underground, a plurality of the ultra-deep directional boreholes I for the thick and hard roof as pressure relief implementation boreholes for a fracturing area, and then, carrying out fracturing modification point by point and stage by stage.

[14] In step 5.2, when the fracturing prevention target layer is classified as the low-position composite hard roof layer, the mode for the prevention through the weakening includes: constructing the surface single-borehole straight well for the large-volume fluid supply on the ground, and constructing ultra-deep directional boreholes II for the thick and hard roof underground; and taking the surface single-borehole straight well as the fracturing fluid delivery channel, taking, in the low-position composite hard roof layer underground, a plurality of the ultra-deep directional boreholes II for the thick and hard roof as the pressure relief implementation boreholes for the fracturing area, and then, carrying out the fracturing modification using a low-position two-layer mode or a mode of multi-layer staggered fracturing.

[15] In step 5.3, when the fracturing prevention target layer is classified as the high-position and low-position composite thick and hard roof layer, the mode for the prevention through the weakening includes: constructing the surface single-borehole straight well for the large-volume fluid supply on the ground, and constructing the ultra-deep directional boreholes III for the thick and hard roof underground; constructing horizontal branch boreholes at a position of a surface horizontal well corresponding to a high-position thick and hard roof rock layer to carry out pressure relief for the fracturing area; and delivering a fracturing fluid based on the surface single-borehole straight well, and carrying out pressure relief for a low-position thick and hard roof area through the ultra-deep directional boreholes III.

[16] In step 5.4, when the fracturing prevention target layer is classified as the high-position composite hard roof layer, the mode for the prevention through the weakening includes: constructing the surface single-borehole straight well for the large-volume fluid supply and surface ultra-long multi-horizontal branch boreholes on the ground, delivering the fracturing fluid based on the surface single-borehole straight well, and carrying out layered and intensive zonal fracturing modification on a composite high-position thick and hard roof through the surface ultra-long multi-horizontal branch boreholes.

[17] In step 5.5, when the fracturing prevention target layer is classified as the high-position single thick and hard roof layer, the mode for the prevention through the weakening includes: constructing the surface single-borehole straight well for the large-volume fluid supply and surface ultra-long boreholes on the ground, delivering the fracturing fluid based on the surface single-borehole straight well, and carrying out intensive zonal fracturing modification on the high-position thick and hard roof through the surface ultra-long boreholes.

[18] The present disclosure further includes the following technical features.

[19] Specifically, the operation that the hard rock layers are identified in the step 2 includes the following steps:

[20] In step 2.1, a layer-by-layer calculation is performed upwards from a first rock layer above a coal seam, and the calculation is stopped when the formula for identifying the position of the hard rock layer is satisfied. At this time, the (m+1)-th rock layer is a first hard rock layer from the first rock layer upwards.

[21] In step 2.2, a position of a second hard rock layer is determined according to the process of the step 2.1 from the first hard rock layer, and so on, until a topmost hard rock layer is determined. The topmost hard rock layer is set as an n-th hard rock layer.

[22] step 2.3, the hard rock layers and the soft rock layer groups controlled by the hard rock layers in the overlying stratum are obtained after all hard rock layers are identified.

[23] Specifically, the process of the step 3 includes: inputting the geological information about the hard rock layers and the soft rock layer groups controlled by the hard rock layers into FLAC3D software, extracting related parameters of a stress state, an elastic modulus, and a Poisson's ratio of each unit by using an FISH language editing program built into the FLAC3D software, and obtaining the hard rock layer with the high static load energy density by generating an energy density nephogram according to the parameters.

[24] Specifically, the process of the step 4 includes: carrying out on-line dynamic monitoring on the hard rock layer with the high static load energy density obtained in the step 3 by using a surfaceunderground integrated microseismic monitoring system to obtain the dynamic load energy data of the hard rock layer with the high static load energy density; and screening out events with energy above 104 from the dynamic load energy data, analyzing microseismic energy distribution pattern in a profile of the hard rock layer with the high static load energy density, screening out a critical layer for fracturing dynamic load energy release, and determining the fracturing prevention target layer in a rockburst zone of the thick and hard roof by integrating a study on structural characteristics of the overlying stratum.

[25] Specifically, in the step 4, when one hard roof exists within 60 meters above a coal seam roof, the fracturing prevention target layer is classified as the low-position single thick and hard roof layer; when two hard roofs exist within 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the low-position composite hard roof layer; when one hard roof exists within 60 meters above the coal seam roof and another hard roof exists beyond 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the high-position and low-position composite thick and hard roof layer; when two hard roofs exist beyond 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the high-position composite thick and hard roof layer; and when a single hard roof exists beyond 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the high-position single thick and hard roof layer.

[26] Specifically, in the step 5.1, a distance between two adjacent ultra-deep directional boreholes I for the thick and hard roof needs to satisfy the following formula: Li^2Di. Li represents the distance between the two adjacent ultra-deep directional boreholes I for the thick and hard roof, and D1 represents a horizontal fracturing influence range of a single borehole.

[27] Optionally and specifically, in the step 5.2, when the fracturing modification is carried out using the low-position two-layer mode, a distance between boreholes in the upper and lower layers needs to satisfy the following formula: hi^2di. Here, hi represents the distance between the boreholes in the upper and lower layers, and d1 represents a vertical fracturing influence range of a single borehole.

[28] Optionally and specifically, in the step 5.2, when the fracturing modification is carried out using the mode of the multi-layer staggered fracturing, a vertical distance between two adjacent layers needs to satisfy the following formula: h2^2d2 (h2 represents the vertical distance between the two adjacent layers, and d2 represents a vertical fracturing influence range of a single borehole; and a horizontal distance between two adjacent layers needs to satisfy the following formula: L2 ^2D2 (L2 represents the horizontal distance between the two adjacent layers, and D2 represents a horizontal fracturing influence range of a single borehole).

[29] Specifically, in the step 5.3, a distance between any two horizontal branch boreholes satisfies the following formula: d3 ^ ri. Here, d3 represents the distance between the any two horizontal branch boreholes, and r1 represents an extension radius of a fracturing fracture in a direction of the horizontal branch boreholes.

[30] Specifically, in the step 5.4, a number of the surface ultra-long multi-horizontal branch boreholes is calculated according to the following formula: N=(L3 / 2r2)+i. Here, N represents the number of the surface ultra-long multi-horizontal branch boreholes, L3 represents a length of each surface ultra-long multi-horizontal branch borehole, and r2 represents an extension radius of a fracturing fracture in a direction of the surface ultra-long multi-horizontal branch boreholes. [3i] Compared with the related art, the present disclosure has the following beneficial technical effects.

[32] (I) By means of the method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof according to the present disclosure, spatial distribution characteristics of a hard roof are obtained, and according to the spatial distribution characteristics of the hard roof, a low-position single thick and hard roof layer, a low-position composite hard roof layer, a high-position and low-position composite thick and hard roof layer, a high-position composite thick and hard roof layer and a high-position single thick and hard roof layer are determined.

[33] For the low-position single thick and hard roof layer, by using the large-volume and high-pressure delivery channels of the ground well, in conjunction with the precise directional control technology of underground long directional boreholes, and adopting the staged fracturing technology based on open-hole directional drilling with aggregate, effective zonal pressure relief and rockburst prevention through weakening are achieved. For the high-position single thick and hard roof layer, zonal pressure relief and rockburst prevention are achieved by direct use of staged fracturing of ground horizontal long boreholes. For conditions where both high-position and low-position thick and hard roofs exist (i.e., low-position composite hard roof layer, high-position and low-position composite thick and hard roof layer, and high-position composite thick and hard roof layer), the high-position thick and hard roof is controlled by staged fracturing of ground directional long boreholes, and the entire panel is controlled by using underground open-hole directional long boreholes with aggregate orientation and fracturing, so as to weaken and modify the low-position thick and hard roof, thereby achieving comprehensive effective zonal pressure relief and rockburst prevention for a plurality of working faces or panels.

[34] From the comprehensive analysis above, it can be seen that the present disclosure implements different prevention measures based on different layers, achieving the technical modes for pressure relief modification and rockburst prevention for the fracturing areas in the above five scenarios, fundamentally and comprehensively avoiding the rockburst disasters induced by thick and hard roofs.

[35] (II) By means of the method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof according to the present disclosure, the staged fracturing and artificial fracturing zonal pressure relief technology for the thick and hard roofs is carried out by taking the surface straight well as a large-volume fluid output channel and leveraging the benefits of precisely constructing horizontal branch boreholes on the ground and ultra-long horizontal boreholes underground, the pressure step distance is shortened, the energy released during roof rotation is reduced, and the static load storage energy is reduced by fracturing into blocks, which can achieve regional, accurate and effective prevention and control of the rockburst disasters from the thick and hard roofs in all modes. The method integrates the benefits of large volume, high pressure, and spatial freedom of the ground fracturing, as well as the benefits of underground directional long boreholes, such as terrain independence during winter and rainy seasons, low cost, direct disaster prevention and control, controllable construction period, and low price.

[36] (III) By means of the method for the rockburst prevention through the surface-underground collaborative large-scale weakening of the thick and hard roof according to the present disclosure, through the analysis of the sedimentary environment in a study area, in conjunction with the precise tracking analysis of multi-scale small cyclic sedimentary facies and single sand bodies, the characteristics of thick and hard roofs overlying the coal seam roof are transparently, finely and accurately revealed. Precise prevention and control of the rockburst prevention target layer are achieved through the real-time trajectory control and precise hole-making technology of the ultra-long horizontal boreholes on the ground and underground.

[37] (IV) By means of the method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof according to the present disclosure, through the calculation of the static load energy using FLAC3D and the real-time monitoring analysis of the dynamic load energy using the microseismic online system, in conjunction with the structural distribution characteristics of the overlying stratum of the coal seam roof revealed by the sedimentary microfacies, the key layers for the prevention and control of the rockburst disasters from the thick and hard roof are quantitatively screened out.

[38] (V) By means of the method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof according to the present disclosure, rockburst from the thick and hard roof in a plurality of working faces or an entire panel can be effectively prevented and controlled at one time, thereby achieving safe and efficient mining and orderly mining succession, and providing effective support for the supply of the coal energy. BRIEF DESCRIPTION OF DRAWINGS

[39] FIG. 1 is a schematic flowchart illustrating a method for rockburst prevention through surface underground collaborative regional weakening of a thick and hard roof.

[40] FIG. 2 is a schematic diagram illustrating rockburst prevention through weakening of a low- position single thick and hard roof layer.

[41] FIG. 3 is a schematic diagram illustrating rockburst prevention through weakening of a low- position composite hard roof layer.

[42] FIG. 4 is a schematic diagram illustrating rockburst prevention through weakening of a high- position and low-position composite thick and hard roof layer.

[43] FIG. 5 is a schematic diagram illustrating rockburst prevention through weakening of a high- position composite thick and hard roof layer.

[44] FIG. 6 is a schematic diagram illustrating rockburst prevention through weakening of a high- position single thick and hard roof layer.

[45] FIG. 7 is a schematic diagram illustrating a prevention and control effect for a low-position single thick and hard roof layer.

[46] FIG. 8 is a schematic diagram illustrating a prevention and control effect for a low-position composite hard roof layer.

[47] FIG. 9 is a schematic diagram illustrating a prevention and control effect for a high-position and low-position composite thick and hard roof layer.

[48] FIG. 10 is a schematic diagram illustrating a prevention and control effect for a high-position composite thick and hard roof layer.

[49] FIG. 11 is a schematic diagram illustrating a prevention and control effect for a high-position single thick and hard roof layer.

[50] The description of various reference numerals in the figures are as follows: 1-surface singleborehole straight well, 2-ultra-deep directional borehole I for a thick and hard roof, 3-ultra-deep directional borehole II for a thick and hard roof, 4-horizontal branch borehole, 5-surface ultralong multi-horizontal branch borehole, 6-surface ultra-long borehole.

[51] The following description, in conjunction with embodiments, further illustrates the technical solution of the present disclosure. DETAILED DESCRIPTION

[52] It is to be noted that all software and systems used in the present disclosure, unless otherwise specified, are based on known software and systems in the field. For example, the surfaceunderground integrated microseismic monitoring system is a known monitoring system in the related art. The FLAC3D software is a known three-dimensional numerical analysis software in the related art and is developed by the Itasca Consulting Group, Inc.

[53] It is to be noted that the "hard rock layer" involved in the present disclosure is not a hard rock layer in the general sense, which refers to a rock layer that has a deflection less than that of the underlying rock layer during deformation and does not deform coordinately with the underlying rock layer.

[54] It is to be noted that all data involved in the present disclosure are in conventional units known in the prior art, and the calculation processes are dimensionless operations.

[55] In accordance with the above technical solution, the following provides specific embodiments of the present disclosure. It is to be noted that the present disclosure is not limited to the following specific embodiments, and any equivalent transformations based on the technical solution of the present disclosure fall within the scope of protection of the present disclosure.

[56] Embodiment:

[57] This embodiment provides a method for rockburst prevention through surface-underground collaborative regional weakening of a thick and hard roof. As shown in FIG. 1, the method specifically includes the following steps 1 to 5.

[58] In step 1, spatial distribution characteristics of a hard roof are obtained.

[59] The spatial distribution characteristics of the hard roof are drawn through the precise tracking analysis of small cyclic sedimentary facies of "single well facies-profile facies-plane sedimentary facies" in trinity and single sand bodies in conjunction with an overlying rock structure of the hard roof.

[60] In step 2, a single thick roof layer and a composite hard roof layer are obtained by single-layer tracking and detailed depiction in terms of time and space on the scale of the mine and working face according to the spatial distribution characteristics of the hard roof obtained in step 1; and then, hard rock layers and soft rock layer groups controlled by the hard rock layers in overlying stratum are obtained by identifying the hard rock layers of the single thick roof layer and the composite hard roof layer. The specific steps for identifying the hard rock layers in the step 2 include the following steps 2.1 to 2.3.

[61] In step 2.1, a layer-by-layer calculation is performed upwards from a first rock layer above a coal seam, and the calculation is stopped when the formula for identifying the position of the hard rock layer is satisfied. At this time, the (m+1)-th rock layer is a first hard rock layer from the first rock layer upwards. The formula for identifying the position of the hard rock layer is obtained according to the following steps 2.1.1 to 2.1.4.

[62] In step 2.1.1, assuming that the first rock layer is a hard rock layer, that rock layers above the first rock layer up to an m-th rock layer deform coordinately with the first rock layer, and that a the(m+1)-th rock layer does not deform coordinately with the first rock layer, then the (m+1)-th rock layer is a second hard rock layer. Since the first layer to the m-th rock layers deform coordinately, the curvatures of the rock layers are the same, and the rock layers form a composite beam. According to the principle of the composite beam, the load formed by the m-th rock layer on the first hard rock layer can be derived, as shown in the following Formula I: mm ^1( x )l m=E hi3 £ hiYi / £ Eh i=1       i=1                                            Formula I

[63] In the Formula I: q1(x)|m represents the load formed by the m-th rock layer on the first hard rock layer; E1 represents an elastic modulus of the first hard rock layer; h1 represents a thickness of the first hard rock layer; hi represents a thickness of an i-th rock layer, i=1, 2, ., m; Yi represents a unit weight of the i-th rock layer, i=1, 2, ..., m; and Ei represents an elastic modulus of the i-th rock layer, i=1, 2, ., m.

[64] In step 2.1.2, the load formed by the (m+1)-th layer on the first hard rock layer can be derived by using the following Formula II: m+1       m+1 ®(x)|m+1=Ehl V iy V Eh i=1       i=1                                        Formula II

[65] In the Formula II: q1(x)|m+1 represents the load formed by the (m+1)-th layer on the first hard rock layer.

[66] In step 2.1.3, since the (m+1)-th layer is a hard rock layer and has a deflection less than that of an underlying rock layer, and the rock layers above the (m+1)-th layer no longer require the underlying rock layers to bear the load borne by them, the following Formula III can be obtained: q1(x) |m+1 < q1 (x) |m                                Formula III

[67] In step 2.1.4, Formula I and Formula II are substituted into Formula III. After simplification, a Formula IV for identifying the position of the hard rock layer can be obtained, which is as follows: mm Em.it „V     Ym+iV E.h' i=1           i=1                                     Formula IV

[68] In the Formula IV: Em+1 represents an elastic modulus of the(m+1)-th rock layer; hm+1 represents a thickness of the (m+1)-th rock layer; and Ym+i represents a unit weight of the (m+1)-th rock layer.

[69] In step 2.2, a position of a second hard rock layer is determined according to the process of the step 2.1 from the first hard rock layer, and so on, until a topmost hard rock layer is determined. The topmost hard rock layer is set as an n-th hard rock layer.

[70] In step 2.3, the hard rock layers and the soft rock layer groups controlled by the hard rock layers in an overlying stratum are obtained after all the hard rock layers are identified. The specific meaning of a soft rock layer group controlled by a certain hard rock layer is as follows: the soft rock between any two hard rock layers belongs to the soft rock layer group controlled by the next hard rock layer, for example, the soft rock between the first hard rock layer and the second hard rock layer belongs to the soft rock layer group controlled by the second hard rock layer.

[71] In step 3, a hard rock layer with high static load energy density is obtained based on geological information about the hard rock layers and the soft rock layer groups controlled thereby obtained in the step 2. The specific process includes: the geological information about the hard rock layers and the soft rock layer groups controlled by the hard rock layers are input into FLAC3D software, related parameters of the stress state, elastic modulus, and Poisson's ratio of each unit are extracted by using an FISH language editing program built into the FLAC3D software, and the hard rock layer with the high static load energy density is obtained by generating an energy density nephogram according to the above parameters.

[72] In step 4, dynamic load energy data of the hard rock layer with the high static load energy density is acquired based on the hard rock layer with the high static load energy density obtained in the step 3, and a fracturing prevention target layer is determined according to the dynamic load energy data. The specific process is as follows.

[73] On-line dynamic monitoring is carried out on the hard rock layer with the high static load energy density obtained in step 3 by using a surface-underground integrated microseismic monitoring system to obtain the dynamic load energy data of the hard rock layer with the high static load energy density. Then, events with energy above 104 are screened out from the dynamic load energy data, the microseismic energy distribution pattern in the profile of the hard rock layer with the high static load energy density is analyzed, a critical layer for fracturing dynamic load energy release is screened out, and five fracturing prevention modes corresponding to the fracturing prevention target layers in a rockburst zone of the thick and hard are determined by integrating the study on structural characteristics of the overlying stratum. The five fracturing prevention modes include: a low-position single thick and hard roof layer (mode 1), a low-position composite hard roof layer (mode 2), a high-position and low-position composite thick and hard roof layer (mode 3), a high-position composite thick and hard roof layer (mode 4), and a high-position single thick and hard roof layer (mode 5).

[74] In the embodiment, a control mode is identified according to the number and distribution of the screened-out hard roofs. When a single hard roof exists within 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the mode 1. When two hard roofs exist within 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the mode 2. When one hard roof exists within 60 meters above the coal seam and another hard roof exists beyond 60 meters above (especially more than 100m) the coal seam roof, the fracturing prevention target layer is classified as the mode 3. When two hard roofs exist beyond 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the mode 4. When a single hard roof exists beyond 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the mode 5.

[75] In step 5, different modes for prevention through weakening are adopted according to different types of fracturing prevention target layers classified in Step 4, which specifically includes the following steps 5.1 to 5.5.

[76] In step 5.1, as shown in FIG. 2, when the fracturing prevention target layer is classified as the low-position single thick and hard roof layer, the mode for the prevention through the weakening (i.e., the mode 1) includes: constructing a surface single-borehole straight well (1) for large-volume fluid supply on the ground, and constructing ultra-deep directional boreholes I (2) for the thick and hard roof underground; and taking the surface single-borehole straight well (1) as a fracturing fluid delivery channel, taking, in the low-position single thick and hard roof layer underground, a plurality of the ultra-deep directional boreholes I (2) for the thick and hard roof as pressure relief implementation boreholes for a fracturing area, and carrying out fracturing modification point by point and stage by stage.

[77] In this embodiment, each ultra-deep directional borehole I (2) for the thick and hard roof is horizontal, and a depth of each ultra-deep directional borehole I (2) for the thick and hard roof is greater than 800 meters. A distance between two adjacent ultra-deep directional boreholes I (2) for the thick and hard roof needs to satisfy the following formula: Li<2Di. Li represents the distance between the two adjacent ultra-deep directional boreholes I for the thick and hard roof, and D1 represents a horizontal fracturing influence range of a single borehole.

[78] In step 5.2, as shown in FIG. 3, when the fracturing prevention target layer is classified as the low-position composite hard roof layer, the mode for the prevention through the weakening (i.e., the mode 2) includes: constructing the surface single-borehole straight well (1) for the large-volume fluid supply on the ground, and constructing ultra-deep directional boreholes II (3) underground; and taking the surface single-borehole straight well (1) as the fracturing fluid delivery channel, taking, in the low-position composite hard roof layer underground, a plurality of the ultra-deep directional boreholes II (3) for the thick and hard roof as the pressure relief implementation boreholes for the fracturing area, and carrying out the fracturing modification using a low-position two-layer mode or a mode of multi-layer staggered fracturing. FIG. 3 illustrates the low-position two-layer mode as an example.

[79] In this embodiment, each ultra-deep directional borehole II (3) for the thick and hard roof is horizontal, and a depth of each ultra-deep directional borehole II (3) for the thick and hard roof is greater than 800 meters.

[80] When the fracturing modification is carried out by using the low-position two-layer mode, a distance between boreholes in the upper and lower layers needs to satisfy the following formula: h1<2d1. Here, h1 represents the distance between the boreholes in the upper and lower layers, and d1 represents a vertical fracturing influence range of a single borehole.

[81] When the fracturing modification is carried out using the mode of the multi-layer staggered fracturing, the number of layers needs to be determined according to the selection result of a rockburst fracturing prevention layer of the thick and hard roof. A vertical distance between two adjacent layers needs to satisfy the following formula: h2<2d2. Here, h2 represents the vertical distance between the two adjacent layers, and d2 represents a vertical fracturing influence range of a single borehole. A horizontal distance between two adjacent layers needs to satisfy the following formula: L2<2D2. Here, L2 represents the horizontal distance between the two adjacent layers, and D2 represents a horizontal fracturing influence range of a single borehole.

[82] In step 5.3, as shown in FIGS. 3 and 4, when the fracturing prevention target layer is classified as the high-position and low-position composite thick and hard roof layer, the mode for the prevention through the weakening (i.e., the mode 3) includes: constructing a surface singleborehole straight well (1) for large-volume fluid supply on the ground, and constructing ultra-deep directional boreholes II (3) for the thick and hard roof underground; constructing horizontal branch boreholes (4) at a position of the surface horizontal well corresponding to a high-position thick and hard roof rock layer to carry out pressure relief for the fracturing area; and delivering a fracturing fluid based on the surface single-borehole straight well (1), and carrying out pressure relief for the low-position thick and hard roof area through the ultra-deep directional boreholes II (3).

[83] In this embodiment, a distance between any two horizontal branch boreholes (4) needs to satisfy the following formula: d3 <r1. Here, d3 represents the distance between the any two horizontal branch boreholes, and r1 represents an extension radius of a fracturing fracture in a direction of the horizontal branch boreholes.

[84] In step 5.4, as shown in FIG. 5, when the fracturing prevention target layer is classified as the high-position composite hard roof layer, the mode for the prevention through the weakening (i.e., the mode 4) includes: constructing a surface single-borehole straight well (1) for large-volume fluid supply and surface ultra-long multi-horizontal branch boreholes (5) on the ground, delivering a fracturing fluid based on the surface single-borehole straight well (1), and carrying out layered and intensive zonal fracturing modification on the composite high-position thick and hard roof through the surface ultra-long multi-horizontal branch boreholes (5).

[85] In this embodiment, a length of each surface ultra-long multi-horizontal branch borehole (5) is greater than 800 meters, and a number of the surface ultra-long multi-horizontal branch boreholes (5) is calculated according to the following formula: N=(L3 / 2r2)+1. Here, N represents the number of the surface ultra-long multi-horizontal branch boreholes, L3 represents a length of each surface ultra-long multi-horizontal branch borehole, and r2 represents an extension radius of a fracturing fracture in a direction of the surface ultra-long multi-horizontal branch boreholes.

[86] In step 5.5, as shown in FIG. 6, when the fracturing prevention target layer is classified as the high-position single thick and hard roof layer, the mode for the prevention through the weakening (i.e., the mode 5) includes: constructing the surface single-borehole straight well (1) for the large-volume fluid supply and surface ultra-long boreholes (6) on the ground, delivering the fracturing fluid based on the surface single-borehole straight well (1), and carrying out intensive zonal fracturing modification on the high-position thick and hard roof through the surface ultra-long boreholes (6).

[87] In the embodiment, the length of the surface ultra-long borehole (6) is greater than 800 meters. The method for obtaining a number of the surface ultra-long boreholes (6) is the same as that for obtaining the number of the surface ultra-long multi-horizontal branch boreholes (5) mentioned above.

[88] Verification of the effects:

[89] (A) When the fracturing prevention target layer is classified as the low-position single thick and hard roof layer, the prevention and control effect is shown in FIG. 7, . As can be seen from FIG. 7, by weakening and modifying the rock characteristics of the hard rock layers, the collapse form can be changed, the pressure step distance can be shortened, and the concentration degree of stress transmission can be reduced layer by layer and stage by stage. By controlling the movement state of the low-position hard rock layer, the mining pressure manifestation intensity under the condition of a single hard rock layer can be effectively controlled.

[90] (B) When the fracturing prevention target layer is classified as the low-position composite hard roof layer, the prevention and control effect is shown in FIG. 8. As can be seen from FIG. 8, by arranging the fracturing stages in a staggered manner, the upper and lower layers of the low-position composite hard rock layer can be induced to exhibit non-synchronous fracturing as the primary mode of movement , which can prevent the strong rockburst dynamic disasters caused by the synchronous fracturing of the composite rock layer. Meanwhile, the stress transmission path after the non-synchronous fracturing is disrupted, and the load and stress directly transmitted to the working face are dispersed. The comprehensive control of dynamic disasters under the condition of low-position composite hard rock layers is achieved through comprehensive control of the fracturing structure and stress transmission.

[91] (C) When the fracturing prevention target layer is classified as the high-position and low-position composite thick and hard roof layer, the prevention and control effect is shown in FIG. 9. As can be seen from FIG. 9, after the combined treatment of the high and low positions is adopted, the low-position weakening can change the collapse form, shorten the pressure step distance, and reduce the concentration degree of the stress transmission. The high-position weakening can control the collapse scale of the hard roof, prompt the timely fracturing of the high-position rock layer, reduce energy consumption and increase frequency, and decrease the range and damage degree of a single fracturing of the high-position rock layer. The combination of both can control and avoid the formation of dynamic disasters in the working face due to the synchronous movement of the high and low positions.

[92] (D) When the fracturing prevention target layer is classified as the high-position composite thick and hard roof layer, the prevention and control effect is shown in FIG. 10. As can be seen from FIG. 10, after the upper and lower layers of the high-position composite hard rock layer are induced to exhibit non-synchronous fracturing and the concentrated stress transmission path is disrupted, the treated high-position hard composite roof undergoes a large-scale single fracturing event, resulting in high-energy release. Through comprehensive control of both the fracturing range and stress transmission, comprehensive control of the dynamic disasters under conditions of high-position composite hard rock layers is achieved.

[93] (E) When the fracturing prevention target layer is classified as the high-position single thick and hard roof layer, the prevention and control effect is shown in FIG. 11. As can be seen from FIG. 11, by weakening and transforming the rock characteristics of the high-position hard rock layer, the hanging roof range can be reduced, and the concentration degree of stress transmission can be reduced. By controlling the movement state of the high-position hard rock layer, the risk of a single high-energy release event in the high-position rock layer can be avoided, which can ultimately achieve effective control on the mining pressure manifestation at the working face.

Claims

1. A method for rockburst prevention through surface-underground collaborative regional weakening of a thick and hard roof, comprising:in step 1, obtaining spatial distribution characteristics of a hard roof;in step 2, obtaining, according to the spatial distribution characteristics of the hard roof obtained in the step 1, a single thick roof layer and a composite hard roof layer; and obtaining hard rock layers and soft rock layer groups controlled by the hard rock layers in an overlying stratum by identifying hard rock layers of the single thick roof layer and the composite hard roof layer according to a formula for identifying a position of a hard rock layer,wherein the formula for identifying the position of the hard rock layer is as follows:m           mE Jr,Yhy>ym+V m+l / / ; / /     / m+i / 4 i i / =l                  / =l                               Formula IV;wherein:Em+1 represents an elastic modulus of a (m+1)-th rock layer;hm+1 represents a thickness of the (m+1)-th rock layer;Ym+i represents a unit weight of the (m+1)-th rock layer;hi represents a thickness of an i-th rock layer, wherein i=1, 2, ., m;Yi represents a unit weight of the i-th rock layer, wherein i=1, 2, ., m; andEi represents an elastic modulus of the i-th rock layer, wherein i=1, 2, ., m;in step 3, obtaining, based on geological information about the hard rock layers and the soft rock layer groups controlled by the hard rock layers obtained in the step 2, a hard rock layer with high static load energy density;in step 4, acquiring, based on the hard rock layer with the high static load energy density obtained in the step 3, dynamic load energy data of the hard rock layer with the high static load energy density, and determining a fracturing prevention target layer according to the dynamic load energy data, wherein the fracturing prevention target layer is classified as one of the following five fracturing prevention modes: a low-position single thick and hard roof layer, a low-position composite hard roof layer, a high-position and low-position composite thick and hard roof layer, a high-position composite thick and hard roof layer, and a high-position single thick and hard roof layer; andin step 5, adopting, according to different types of fracturing prevention target layers classified in the step 4, different modes for prevention through weakening, which specifically comprises:in step 5.1, when the fracturing prevention target layer is classified as the low-position single thick and hard roof layer, the mode for the prevention through the weakening comprises: constructing a surface single-borehole straight well (1) for large-volume fluid supply on the ground, and constructing ultra-deep directional boreholes I (2) for the thick and hard roof underground; and taking the surface single-borehole straight well (1) as a fracturing fluid delivery channel, taking, in the low-position single thick and hard roof layer underground, a plurality of the ultra-deep directional boreholes I (2) for the thick and hard roof as pressure relief implementation boreholes for a fracturing area, and carrying out fracturing modification point by point and stage by stage;in step 5.2, when the fracturing prevention target layer is classified as the low-position composite hard roof layer, the mode for the prevention through the weakening comprises: constructing the surface single-borehole straight well (1) for the large-volume fluid supply on the ground, and constructing ultradeep directional boreholes II (3) for the thick and hard roof underground; and taking the surface singleborehole straight well (1) as the fracturing fluid delivery channel, taking, in the low-position composite hard roof layer underground, a plurality of the ultra-deep directional boreholes II (3) for the thick and hard roof as the pressure relief implementation boreholes for the fracturing area, and carrying out the fracturing modification using a low-position two-layer mode or a mode of multi-layer staggered fracturing;in step 5.3, when the fracturing prevention target layer is classified as the high-position and low-position composite thick and hard roof layer, the mode for the prevention through the weakening comprises: constructing the surface single-borehole straight well (1) for the large-volume fluid supply on the ground, and constructing the ultra-deep directional boreholes II (3) for the thick and hard roof underground; constructing horizontal branch boreholes (4) at a position of a surface horizontal well corresponding to a high-position thick and hard roof rock layer to carry out pressure relief for the fracturing area; and delivering a fracturing fluid based on the surface single-borehole straight well (1), and carrying out pressure relief for a low-position thick and hard roof area through the ultra-deep directional boreholes II (3) for the thick and hard roof;in step 5.4, when the fracturing prevention target layer is classified as a high-position composite hard roof layer, the mode for the prevention through the weakening comprises: constructing the surface singleborehole straight well (1) for the large-volume fluid supply and surface ultra-long multi-horizontal branch boreholes (5) on the ground, delivering the fracturing fluid based on the surface single-borehole straightwell (1), and carrying out layered and intensive zonal fracturing modification on a composite high-position thick and hard roof through the surface ultra-long multi-horizontal branch boreholes (5); andin step 5.5, when the fracturing prevention target layer is classified as the high-position single thick and hard roof layer, the mode for the prevention through the weakening comprises: constructing the surface single-borehole straight well (1) for the large-volume fluid supply and surface ultra-long boreholes (6) on the ground, delivering the fracturing fluid based on the surface single-borehole straight well (1), and carrying out intensive zonal fracturing modification on the high-position thick and hard roof through the surface ultra-long boreholes (6).

2. The method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof of claim 1, wherein identifying the hard rock layers in the step 2 specifically comprises:in step 2.1, performing a layer-by-layer calculation upwards from a first rock layer above a coal seam, and stopping the calculation when the formula for identifying the position of the hard rock layer is satisfied, wherein at this time, the (m+1)-th rock layer is a first hard rock layer from the first rock layer upwards;in step 2.2, determining, according to the process of the step 2.1, a position of a second hard rock layer from the first hard rock layer, and so on, until a topmost hard rock layer is determined, wherein the topmost hard rock layer is set as an n-th hard rock layer; andin step 2.3, obtaining the hard rock layers and the soft rock layer groups controlled by the hard rock layers in the overlying stratum after all hard rock layers are identified.

3. The method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof of claim 1, wherein the specific process of the step 3 comprises: inputting the geological information about the hard rock layers and the soft rock layer groups controlled by the hard rock layers into FLAC3D software, extracting related parameters of a stress state, an elastic modulus, and a Poisson's ratio of each unit by using an FISH language editing program built into the FLAC3D software, and obtaining the hard rock layer with the high static load energy density by generating an energy density nephogram according to the parameters.

4. The method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof of claim 1, wherein the specific process of the step 4 comprises: carrying out on-line dynamic monitoring on the hard rock layer with the high static load energy density obtained in the step 3 by using a surface-underground integrated microseismic monitoring system to obtain the dynamic load energy data of the hard rock layer with the high static load energy density; and screening out events with energy above 104 from the dynamic load energy data, analyzing microseismicenergy distribution pattern in a profile of the hard rock layer with the high static load energy density, screening out a critical layer for fracturing dynamic load energy release, and determining the fracturing prevention target layer in a rockburst zone of the thick and hard roof by integrating a study on structural characteristics of the overlying stratum.

5. The method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof of claim 1, wherein in the step 4, when one hard roof exists within 60 meters above a coal seam roof, the fracturing prevention target layer is classified as the low-position single thick and hard roof layer; when two hard roofs exist within 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the low-position composite hard roof layer; when one hard roof exists within 60 meters above the coal seam roof and another hard roof exists beyond 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the high-position and low-position composite thick and hard roof layer; when two hard roofs exist beyond 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the high-position composite thick and hard roof layer; and when a single hard roof exists beyond 60 meters above the coal seam roof, the fracturing prevention target layer is classified as the high-position single thick and hard roof layer.

6. The method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof of claim 1, wherein in the step 5.1, a distance between two adjacent ultra-deep directional boreholes I (2) for the thick and hard roof needs to satisfy the following formula: Li<2Di, wherein Li represents the distance between the two adjacent ultra-deep directional boreholes I for the thick and hard roof, and D1 represents a horizontal fracturing influence range of a single borehole.

7. The method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof of claim 1, wherein in the step 5.2, when the fracturing modification is carried out using the low-position two-layer mode, a distance between boreholes in the upper and lower layers needs to satisfy the following formula: h1<2d1, wherein h1 represents the distance between the boreholes in the upper and lower layers, and d1 represents a vertical fracturing influence range of a single borehole.

8. The method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof of claim 1, wherein in the step 5.2, when the fracturing modification is carried out using the mode of the multi-layer staggered fracturing,a vertical distance between two adjacent layers needs to satisfy the following formula: h2<2d2,wherein h2 represents the vertical distance between the two adjacent layers, and d2 represents a vertical fracturing influence range of a single borehole; anda horizontal distance between two adjacent layers needs to satisfy the following formula: L2<2D2, wherein L2 represents the horizontal distance between the two adjacent layers, and D2 represents a horizontal fracturing influence range of a single borehole.

9. The method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof of claim 1, wherein in the step 5.3, a distance between any two horizontal branch boreholes satisfies the following formula: d3<r1, wherein d3 represents the distance between the any two horizontal branch boreholes, and r1 represents an extension radius of a fracturing fracture in a direction of the horizontal branch boreholes.

10. The method for the rockburst prevention through the surface-underground collaborative regional weakening of the thick and hard roof of claim 1, wherein in the step 5.4, a number of the surface ultra-long multi-horizontal branch boreholes (5) is calculated according to the following formula: N=(L3 / 2r2)+1, wherein N represents the number of the surface ultra-long multi-horizontal branch boreholes, L3 represents a length of each surface ultra-long multi-horizontal branch borehole, and r2 represents an extension radius of a fracturing fracture in a direction of the surface ultra-long multihorizontal branch boreholes.

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