Method for determining fracturing height and evaluating effect of composite key layer of rock burst mine

Abstract

The invention discloses a rock burst mine composite key layer fracturing height determination and effect evaluation method. The method comprises the steps that S1, a composite key layer target layer position is determined; s2, based on numerical simulation, preliminarily determining a fracturing height range in combination with a crack equilibrium expansion criterion and a micro-seismic distribution collaborative criterion; s3, through large-diameter directional drilling and coring, samples with similar proportions are prepared for indoor fracturing and acoustic emission experiments, and the optimal fracturing height is determined through fine calibration according to the fracture penetrability and acoustic emission energy balance criterion; and S4, on-site fracturing is implemented, multi-angle transient electromagnetic detection is adopted, and the fracturing effect is quantitatively evaluated according to a multi-circle layer apparent resistivity decreasing amplitude index. Through multi-scale fusion and multi-source information verification, accurate optimization of the fracturing height and reliable evaluation of the fracturing effect are achieved, and pertinence and effectiveness of deep mine rock burst prevention and treatment are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of mine safety and rock strata control technology, specifically relating to a method for determining the fracturing height and evaluating the effect of fracturing in composite key strata of mines prone to rockburst. Background Technology

[0002] As coal mining in my country progresses to deeper levels, complex critical layers composed of multiple thick and hard rock strata often appear on the roof of coal seams. These rock strata are characterized by high strength, large thickness, and well-developed bedding. Under the influence of mining, they are not prone to natural collapse, easily forming large areas of overhanging roofs, accumulating a large amount of elastic energy, and thus inducing mine dynamic disasters such as rock bursts.

[0003] Currently, long-distance directional segmented fracturing in underground mines is the main means of controlling pressure relief in the middle and lower critical strata at the top of coal seams. If the fracturing height is set too high, the structural modification effect on the critical strata will be insufficient; if the fracturing height is set too low, the fracturing fractures will easily extend downwards into the mined coal seam, affecting the safe production of the working face. However, when fracturing composite critical strata, existing methods simplify them into single hard rock layers, failing to fully consider the complex influence of the thickness, lithology, and interfaces of the composite rock layers on the fracture propagation path, leading to distorted predictions of fracture propagation behavior under the influence of hydraulic fracturing in the critical strata. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for determining the fracturing height and evaluating the effect of fracturing in the composite key layer of mines prone to rockburst. This method uses a multi-scale fusion technology path of numerical simulation for initial selection, indoor experimental calibration, and field detection verification to achieve precise optimization of fracturing height and quantitative evaluation of fracturing effect, thereby improving the scientificity, reliability and effectiveness of rockburst prevention and control projects.

[0005] Therefore, the present invention provides a method for determining the fracturing height and evaluating the effect of fracturing in composite key layers of mines prone to rock bursts, comprising the following steps:

[0006] Step 1: Based on the columnar distribution characteristics of coal and rock at the location of the mining face and its rock strata mechanical parameters, the adjacent thick and hard rock strata at the top of the working face that meet the requirements of single layer thickness ≥ 10m and uniaxial compressive strength ≥ 60MPa are identified as the target strata of the composite key layer to be fractured.

[0007] Step 2: Based on the columnar distribution characteristics and geological conditions of coal and rock determined in Step 1, establish a hydraulic fracturing numerical model including the coal seam, the roof and floor of the coal seam, the composite key strata, and the overlying strata. Based on the geometric morphology of the fractures and the spatial distribution of microseismic events in the simulation results, and according to the predetermined fracture equilibrium propagation criterion and the microseismic distribution synergistic criterion, preliminarily determine the fracturing height range H. a ~H b ;

[0008] Step 3: Obtain rock cores of the composite key layer through large-diameter directional drilling in the field. Based on the principle of similar proportions, prepare standard samples containing simulated composite key layer structures. Conduct hydraulic fracturing experiments with different fracturing height schemes by adjusting the position of simulated fracturing holes in the samples, and simultaneously monitor acoustic emission signals. The fracturing height in different fracturing height schemes is all located at H. a ~H b Within the specified range, the optimal fracturing height H was then determined experimentally based on the fracture penetration criterion and the acoustic emission energy equilibrium criterion. 优 ;

[0009] Step 4, at the optimal fracturing height H 优 Hydraulic fracturing was carried out on site. Before and after the fracturing operation, a multi-angle transient electromagnetic detection system was used to detect the fracturing-affected area. By comparing and analyzing the apparent resistivity data before and after fracturing, the fracturing effect was quantitatively evaluated based on the predetermined multi-layer apparent resistivity reduction index.

[0010] To ensure effective integration and accurate prediction among numerical simulations, laboratory experiments, and in-situ fracturing, the three should maintain consistency or meet similarity criteria in terms of mechanical parameters, geostress fields, and fracturing fluid parameters.

[0011] Consistency of coal and rock mechanical parameters: The mechanical parameters of coal and rock layers, such as density, Poisson's ratio, Young's modulus, compressive strength, tensile strength, and internal friction angle, assigned in the numerical model should be determined based on the results of on-site geological exploration and laboratory testing at the target working face. The indoor experiments used similar materials with cement and gypsum as binders and quartz sand as aggregate. The similar material mix proportions were calculated based on variations in coal and rock water absorption rate, softening coefficient, and permeability coefficient with changes in sand-cement ratio, water-gypsum ratio, and quartz sand particle size. The orthogonal experimental data regression analysis of the similar material mix proportions was performed according to the following formula.

[0012] Consistency of the geostress field: The magnitude and direction of the triaxial geostress (σ1, σ2, σ3) applied in the numerical model should be consistent with the in-situ geostress measurement results in the target area. The specimen loading conditions (such as confining pressure) in the indoor experiments should be converted and set according to the similarity ratio based on the in-situ geostress conditions.

[0013] Similarity of fracturing fluid parameters: The viscosity, density, injection rate, and other parameters of the injected fluid in the numerical simulation should be consistent with the performance parameters of the fracturing fluid to be used in the planned field. The viscosity and density of the fracturing fluid used in the laboratory experiments should also be consistent with the fracturing fluid used in the field, and the injection arrangement should be converted according to the similarity ratio.

[0014] Through the coordinated design of the above parameters, a step-by-step progression is achieved from simulation prediction to experimental verification, and then to on-site implementation.

[0015] Specifically, the crack uniform propagation criterion in step S2 is as follows:

[0016] When a crack extends to the upper or lower interface of the composite critical layer, the minimum distance H1 from the crack extension range to the other interface is ≤1m.

[0017] The difference between the lengths L1 and L2 of the crack extending to the upper and lower layers of the composite critical layer is within 5m, and both L1 and L2 are ≥20m. The maximum width of the crack extension D1 is ≥45m, and the crack extension height H2 is ≥1.1(h1+h2), where h1 and h2 are the thicknesses of the upper and lower layers of the composite critical layer, respectively.

[0018] Specifically, the co-judgment criterion for microseismic distribution in step S2 is that microseismic sources should be widely distributed in the upper and lower layers of the composite key layer, and local sources should be located in the upper and lower rock strata of the composite key layer.

[0019] Specifically, the crack penetration criterion in step S3 is: the tracer confirms that the crack has penetrated the upper and lower layer interfaces of the composite key layer sample, and the rock sample exhibits obvious penetration damage characteristics.

[0020] Specifically, the acoustic emission energy balance criterion in step S3 is: the difference in acoustic emission event energy release rate per unit volume between the upper and lower layers of the composite key layer does not exceed 5%.

[0021] Specifically, in step S3, the inner diameter of the large-diameter directional borehole is not less than 200 mm, and the length of the core section in the composite key layer is not less than 40 meters.

[0022] Specifically, in step S3, the standard sample has dimensions of 150mm×150mm×150mm, and the height ratio of the simulated composite key layer in the sample is consistent with the thickness ratio of the upper and lower layers of the actual composite key layer.

[0023] Specifically, in step S4, no less than three sets of on-site transient electromagnetic detections are performed before and after fracturing. The detection methods before and after fracturing are consistent, which are joint detections at nine different angles: 0.0°, 22.5°, 45.0°, 67.5°, 90.0°, 112.5°, 135.0°, 157.5°, and 180.0°. The detection direction is 0.0°, which is towards the goaf side and parallel to the horizontal direction of the adjacent goaf. The detection direction is 90.0°, which is parallel to the vertical direction of the roadway.

[0024] Specifically, the multi-layer apparent resistivity reduction index in step S4 includes: a) After fracturing, the area with an apparent resistivity reduction of more than 50% has a vertical height X ≥ 0.9 (h1 + h2), where h1 and h2 are the thicknesses of the upper and lower layers of the composite key layer, respectively; b) After fracturing, the area with an apparent resistivity reduction of more than 30% has a vertical height Y ≥ 1.1 (h1 + h2), and the difference between the horizontal coverage lengths L'1 and L'2 of this area on the upper and lower layers of the composite key layer is within 5m, and both L'1 and L'2 are ≥ 20m.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. High prediction accuracy and strong targeting: Through a dual optimization mechanism of numerical simulation for initial selection and indoor experimental calibration, the interface effect and interlayer coupling of the composite key layer are fully considered, and the fracturing height is improved from rough estimation to active and precise design, which significantly improves the targeting of the scheme and the success rate of the project.

[0027] 2. The evaluation system is scientific, intuitive and reliable: It innovatively proposes a multi-source information evaluation system that integrates crack morphology, microseismic distribution, acoustic emission energy and transient electromagnetic response. In particular, it uses the spatially balanced distribution of acoustic emission energy and the multi-layered characteristics of transient electromagnetic response to quantitatively evaluate the fracturing effect, making the effect of overburden structure modification visible and quantifiable, and the evaluation conclusions more scientific and reliable.

[0028] 3. A complete technical closed loop has been formed: The entire process of fracturing height scheme design, parameter optimization, on-site construction and effect evaluation has been organically integrated to form a complete technical closed loop of self-verification and feedback optimization, which effectively ensures the overall effectiveness and safety of rockburst prevention and control projects.

[0029] 4. Highly practical and easy to promote: The method and steps are clear, combining complex theoretical models with operable field processes and indoor experiments. While ensuring scientific accuracy, it also has good engineering applicability and can be promoted and applied in deep rockburst mines with similar geological conditions.

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying 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.

[0031] Figure 1 This is a flowchart illustrating the present invention;

[0032] Figure 2This is a schematic diagram of the crack propagation to the interface of the composite key layer in this invention (taking the above interface as an example);

[0033] Figure 3 This is a schematic diagram illustrating the preliminary determination of the fracturing height of the composite key layer according to the present invention;

[0034] Figure 4 This is a schematic diagram of the combined core sampling of large-diameter directional drilling and conventional drilling according to the present invention;

[0035] Figure 5 This is a schematic diagram of the cross-sectional distribution characteristics of the on-site directional drilling core sampling of the present invention and its sample processing and preparation.

[0036] Figure 6 This is a schematic diagram of transient electromagnetic detection during on-site composite critical layer fracturing construction according to the present invention. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] See Figure 1 A method for determining the fracturing height and evaluating the effect of fracturing in composite key layers of mines prone to rock bursts, comprising the following steps:

[0039] Step 1: Conduct on-site investigation to obtain the columnar distribution characteristics of coal and rock at the location of the mining face and its rock strata mechanical parameters. Based on the parameters, the adjacent thick and hard rock strata at the top of the working face that meet the requirements of single layer thickness ≥10m and uniaxial compressive strength ≥60MPa are identified as the target strata of the composite key layer to be fractured.

[0040] Step 2: Based on the columnar distribution characteristics and geological conditions of coal and rock determined in Step 1, establish a hydraulic fracturing numerical model including the coal seam, the roof and floor of the coal seam, the composite key strata, and the overlying strata. Based on the geometric morphology of the fractures and the spatial distribution of microseismic events in the simulation results, and according to the predetermined fracture equilibrium propagation criterion and the microseismic distribution synergistic criterion, preliminarily determine the fracturing height range H. a ~H b Specifically, the process includes the following steps:

[0041] a. Constructing a numerical model: Based on the coal and rock columnar section and geological conditions determined in Step 1, establish a hydraulic fracturing numerical model that includes the coal seam, its roof and floor, the composite key strata, and the overlying rock strata. The model needs to accurately assign mechanical parameters and in-situ stresses (σ1, σ2, σ3) to each rock stratum. The mechanical parameters of the rock strata include: density, Poisson's ratio, Young's modulus, compressive strength, tensile strength, and internal friction angle.

[0042] b. Set simulation parameters: Preset horizontal boreholes and fracturing clusters at the center of the model to simulate point fracturing. Set up fluid-solid coupling calculations, first perform a brief mechanical equilibrium calculation, then start fluid activity and simultaneously start microseismic activity calculations.

[0043] c. Simulation and Analysis: Perform hydraulic fracturing simulation, monitor and record the fracture propagation morphology and spatial distribution of microseismic sources in real time during the fracturing process.

[0044] d. Preliminary Fracturing Height Determination: Analyzing the simulation results, the fracturing location corresponding to the point where the upward and downward extension lengths of the fracture within the composite key layer are closest, and the fracture morphology effectively connects the upper and lower interfaces of the composite key layer, is preliminarily determined as the reasonable fracturing height range H based on the coal and rock columnar section. a ~H b .

[0045] The preliminary determination of the fracturing height of the composite key layer is mainly based on the following three conditions:

[0046] ① When cracks extend to the upper or lower interface of the composite critical layer, the minimum distance H1 from the crack extension range to the other interface is ≤1m, such as... Figure 2 As shown.

[0047] ② At the end of fracturing, the difference between the lengths L1 and L2 of the upper and lower layers of the composite key layer is within 5m, and the minimum value of L1 and L2 is ≥20m; the maximum width of the fracture extension D1 is ≥45m; the fracture extension height H2 is ≥1.1(h1+h2), such as Figure 3 As shown. Furthermore, at the end of fracturing, the microseismic sources should be widely distributed across the upper and lower layers of the composite key layer, with local sources located within the rock strata above and below the composite key layer. L1 is the line connecting the boundary of the fracture propagation range and the two points where the upper interface of the composite rock layer intersects, in units of m. L2 is the line connecting the boundary of the fracture propagation range and the two points where the lower interface of the composite rock layer intersects, in units of m.

[0048] Step 3: Obtain rock cores of the composite key layer through large-diameter directional drilling in the field. Based on the principle of similar proportions, prepare standard samples containing simulated composite key layer structures. Conduct hydraulic fracturing experiments with different fracturing height schemes by adjusting the position of simulated fracturing holes in the samples, and simultaneously monitor acoustic emission signals. The fracturing height in different fracturing height schemes is all located at H. a ~H b Within the specified range, the optimal fracturing height H was then determined experimentally based on the fracture penetration criterion and the acoustic emission energy equilibrium criterion. 优 .

[0049] Specifically, the steps include the following:

[0050] a. On-site large-diameter core drilling:

[0051] Large-diameter directional drilling with an inner diameter of 20cm is carried out in the working face roadway. The drilling start-up section needs to accurately enter the intermediate bedding plane of the composite key layer, and ensure that the core section length within the target layer is not less than 40m (e.g., Figure 4 (As shown). This section mainly involves field sampling of the upper and lower layers and the intermediate interface of the composite key layer, which is obtained through field coring. Figure 4 A 20cm dimension sample was taken on-site from the boundary area between h1 and h2.

[0052] b. Sample preparation under the influence of fracturing height

[0053] The extracted rock cores were prepared into standard square specimens of 150×150×150mm and hydraulic fracturing experiments were conducted in the laboratory.

[0054] Profile distribution characteristics of in-situ directional drilling core sampling and its sample preparation, such as Figure 5 As shown, the sum of the heights of a+b+c+d is 15cm, the height of b+c is fixed at 10cm, and the ratio of b / c is calculated according to a similar proportion to ensure that the ratio of b / c is consistent with the actual ratio of the upper and lower layers of the composite key layer, i.e., b / c=h1 / h2.

[0055] Regions a and d in the sample preparation represent the upper and lower rock layers of the composite key layer, respectively, with a + d = 5 cm. Considering that the fracturing inlet in existing hydraulic fracturing laboratory experiments is always located at the center of the sample, the heights of a and b are altered to affect the position of the composite key layer after core sampling, thereby changing its fracturing height. Furthermore, by changing the heights of a and b, the actual fracturing height is positioned at the reasonable fracturing height H selected in step one. a ~H b Within the specified range, the fracturing height will be further refined and rationally selected through subsequent fracturing experiments.

[0056] During sample preparation, the fracturing height should be accurate to 0.5m based on the actual dimensions. At least five sets of samples should be prepared, with at least three samples in each set.

[0057] c. Indoor fracturing and acoustic emission monitoring experiments:

[0058] Tracers were added during the hydraulic fracturing experiment to simultaneously monitor acoustic emission signals (including ring count, event count, and event energy).

[0059] To ensure effective integration and accurate prediction among numerical simulations, laboratory experiments, and field fracturing, the three should maintain consistency or meet similarity criteria in key mechanical parameters, geostress fields, and main fracturing fluid parameters.

[0060] Consistency of coal and rock mechanical parameters: The mechanical parameters of coal and rock layers, such as density, Poisson's ratio, Young's modulus, compressive strength, tensile strength, and internal friction angle, assigned in the numerical model should be determined based on the on-site geological exploration and laboratory test results of the target working face. The indoor experiment uses cement and gypsum as binders and quartz sand as aggregate to make similar materials. The similar proportions are mainly based on the changes in coal and rock water absorption rate, softening coefficient, and permeability coefficient with sand-binder ratio, water-gypsum ratio, and quartz sand particle size. For the specific preparation process, refer to the literature "Applicability of Cement-Gypsum Binder Similar Materials in Solid-Fluid Coupling Tests" [J]. Rock and Soil Mechanics, 2015, 36(9):2624-2638.

[0061] Consistency of the geostress field: The magnitude and direction of the triaxial geostress (σ1, σ2, σ3) applied in the numerical model should be consistent with the in-situ geostress measurement results in the target area. The specimen loading conditions (such as confining pressure) in the indoor experiments should be converted and set according to the similarity ratio based on the in-situ geostress conditions.

[0062] Similarity of fracturing fluid parameters: The viscosity, density, injection rate, and other parameters of the injected fluid in the numerical simulation should be consistent with the performance parameters of the fracturing fluid to be used in the planned field. The viscosity and density of the fracturing fluid used in the laboratory experiments should also be consistent with the fracturing fluid used in the field, and the injection arrangement should be converted according to the similarity ratio.

[0063] Through the coordinated design of the above parameters, a step-by-step progression is achieved from simulation prediction to experimental verification, and then to on-site implementation.

[0064] d. Determination of fracturing height in the composite key layer:

[0065] The fracturing height is considered reasonable if all of the following conditions are met, and is therefore taken as the optimal fracturing height H. 优 .

[0066] Condition 1 (crack morphology): The cracks in the sample have effectively extended to the upper and lower interfaces of the composite key layer, and the rock sample exhibits obvious through-type failure characteristics.

[0067] Condition 2 (Energy Distribution Balance): The energy of acoustic emission events is evenly distributed in the upper and lower layers of the composite key layer, that is, the difference in the proportion of the number of acoustic emission events and the amount of energy per unit height in the upper and lower layers is within 5%.

[0068] Serial Number type in accordance with 1 Crack propagation and failure characteristics of the specimen The tracer-detected cracks effectively extended to the upper and lower interfaces of the composite critical layer, and both the upper and lower layers of the composite critical layer exhibited obvious damage characteristics. 2 Acoustic emission events and their energy distribution characteristics There are numerous acoustic emission events, and the energy release is balanced between the upper and lower layers of the composite key layer. That is, the difference in the proportion of acoustic emission energy events and energy magnitude per unit height between the upper and lower layers is within 5%.

[0069] Step 4, at the optimal fracturing height H 优 Hydraulic fracturing was carried out on site. Before and after the fracturing operation, a multi-angle transient electromagnetic detection system was used to detect the fracturing-affected area. By comparing and analyzing the apparent resistivity data before and after fracturing, the fracturing effect was quantitatively evaluated based on the predetermined multi-layer apparent resistivity reduction index.

[0070] Specifically, the process of step 4 is as follows:

[0071] a. Transient electromagnetic detection of one side of the fracturing location in the roadway

[0072] After determining the appropriate fracturing height for the composite key layer, hydraulic fracturing for rockburst prevention was applied in the field. At least three sets of transient electromagnetic detection were conducted before and after fracturing. The detection method was consistent before and after fracturing, involving combined detection at nine different angles: 0.0°, 22.5°, 45.0°, 67.5°, 90.0°, 112.5°, 135.0°, 157.5°, and 180.0°, to refine the detection results. The detection direction was 0.0°, pointing towards the goaf and parallel to the horizontal direction of the adjacent goaf; the detection direction was 90.0°, parallel to the vertical direction of the roadway. A schematic diagram of the transient electromagnetic detection during the in-situ composite key layer fracturing operation is shown below. Figure 6 As shown.

[0073] b. Evaluation of the fracturing effect of on-site transient electromagnetic detection

[0074] Based on the field detection results, the transient electromagnetic changes before and after fracturing are analyzed. If the height X of the area (m-layer) where the apparent resistivity decreases by 50% after fracturing is ≥0.9(h1+h2), and the height Y of the area (n-layer) where the apparent resistivity decreases by 30% after fracturing is ≥1.1(h1+h2), and the difference between the lengths L'1 and L'2 of the upper and lower layers of the composite key layer covered by this layer is within 5m, and the minimum value of L'1 and L'2 is ≥20m, then the fracturing operation is considered effective. If it is determined to be ineffective, a new coal and rock columnar distribution feature and its rock strata mechanical parameters are selected around the working face, and the above process is repeated until the effective fracturing results obtained from the field detection are achieved.

[0075] This invention uses numerical simulation to initially select a large area, followed by high-fidelity indoor experiments for refinement, to accurately and effectively improve the optimal fracturing height. This represents a qualitative leap from estimation to precise calculation, and from passive prediction to proactive design optimization, significantly improving the targeting and success rate of fracturing projects. Furthermore, the method integrates multiple information sources, including fracture morphology and microseismic distribution from numerical simulation, tracer fracture and acoustic emission energy from indoor experiments, and transient electromagnetic apparent resistivity from field detection. This multi-scale (numerical, physical, and field) and multi-parameter (morphological, energy, and electrical) cross-verification forms a robust chain of evidence, making the final conclusion more scientific, reliable, and resistant to interference. This invention establishes a complete technical closed loop, from the design of a scheme for rationally determining the fracturing height of composite key layers to effect evaluation, ensuring the effectiveness of engineering applications.

[0076] The above embodiments are merely illustrative examples to clearly illustrate the present invention and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all embodiments here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for determining the fracturing height and evaluating the effect of fracturing in key composite layers of mines prone to rockburst, characterized in that... Includes the following steps: Step 1: Based on the columnar distribution characteristics of coal and rock at the location of the mining face and its rock strata mechanical parameters, the adjacent thick and hard rock strata at the top of the working face that meet the requirements of single layer thickness ≥ 10m and uniaxial compressive strength ≥ 60MPa are identified as the target strata of the composite key layer to be fractured. Step 2: Based on the columnar distribution characteristics and geological conditions of coal and rock determined in Step 1, establish a hydraulic fracturing numerical model including the coal seam, the roof and floor of the coal seam, the composite key strata, and the overlying strata. Based on the geometric morphology of the fractures and the spatial distribution of microseismic events in the simulation results, and according to the predetermined fracture equilibrium propagation criterion and the microseismic distribution synergistic criterion, preliminarily determine the fracturing height range H. a ~H b ; Step 3: Obtain rock cores of the composite key layer through large-diameter directional drilling in the field. Based on the principle of similar proportions, prepare standard samples containing simulated composite key layer structures. Conduct hydraulic fracturing experiments with different fracturing height schemes by adjusting the position of simulated fracturing holes in the samples, and simultaneously monitor acoustic emission signals. The fracturing height in different fracturing height schemes is all located at H. a ~H b Within the specified range, the optimal fracturing height H was then determined experimentally based on the fracture penetration criterion and the acoustic emission energy equilibrium criterion. 优 ; Step 4, at the optimal fracturing height H 优 Hydraulic fracturing was carried out on site. Before and after the fracturing operation, a multi-angle transient electromagnetic detection system was used to detect the fracturing-affected area. By comparing and analyzing the apparent resistivity data before and after fracturing, the fracturing effect was quantitatively evaluated based on the predetermined multi-layer apparent resistivity reduction index.

2. The method for determining the fracturing height and evaluating the effect of composite key layers in rockburst mines according to claim 1, characterized in that: The crack uniform propagation criterion in step S2 is: When a crack extends to the upper or lower interface of the composite critical layer, the minimum distance H1 from the crack extension range to the other interface is ≤1m. The difference between the lengths L1 and L2 of the crack extending to the upper and lower layers of the composite critical layer is within 5m, and both L1 and L2 are ≥20m. The maximum width of the crack extension D1 is ≥45m, and the crack extension height H2 is ≥1.1(h1+h2), where h1 and h2 are the thicknesses of the upper and lower layers of the composite critical layer, respectively.

3. The method for determining the fracturing height and evaluating the effect of composite key layers in rockburst mines according to claim 1, characterized in that: The microseismic distribution criterion in step S2 is that the microseismic sources should be widely distributed in the upper and lower layers of the composite key layer, and the local sources should be located in the upper and lower rock strata of the composite key layer.

4. The method for determining the fracturing height and evaluating the effect of composite key layers in rockburst mines according to claim 1, characterized in that: The crack penetration criterion in step S3 is: the crack has penetrated the upper and lower layer interfaces of the composite key layer sample through the tracer, and the rock sample shows obvious penetration damage characteristics.

5. The method for determining the fracturing height and evaluating the effect of composite key layers in rockburst mines according to claim 1, characterized in that, The acoustic emission energy balance criterion in step S3 is: the difference in acoustic emission event energy release rate per unit volume between the upper and lower layers of the composite key layer does not exceed 5%.

6. The method for determining the fracturing height and evaluating the effect of composite key layers in rockburst mines according to claim 1, characterized in that, In step S3, the inner diameter of the large-diameter directional borehole is not less than 200 mm, and the length of the core section in the composite key layer is not less than 40 meters.

7. The method for determining the fracturing height and evaluating the effect of composite key layers in rockburst mines according to claim 1, characterized in that, In step S3, the standard sample has dimensions of 150mm×150mm×150mm, and the height ratio of the simulated composite key layer in the sample is consistent with the thickness ratio of the upper and lower layers of the actual composite key layer.

8. The method for determining the fracturing height and evaluating the effect of composite key layers in rockburst mines according to claim 1, characterized in that, In step S4, no less than three sets of on-site transient electromagnetic detections are performed before and after fracturing. The detection methods before and after fracturing are consistent, which are joint detections at nine different angles: 0.0°, 22.5°, 45.0°, 67.5°, 90.0°, 112.5°, 135.0°, 157.5°, and 180.0°. The detection direction is 0.0°, which is towards the goaf side and parallel to the horizontal direction of the adjacent goaf. The detection direction is 90.0°, which is parallel to the vertical direction of the roadway.

9. The method for determining the fracturing height and evaluating the effect of composite key layer fracturing in rockburst mines according to any one of claims 1-8, characterized in that, The multi-layer apparent resistivity reduction index in step S4 includes: a) After fracturing, the area with an apparent resistivity reduction of more than 50% has a vertical height X ≥ 0.9 (h1 + h2), where h1 and h2 are the thicknesses of the upper and lower layers of the composite key layer, respectively; b) After fracturing, the area with an apparent resistivity reduction of more than 30% has a vertical height Y ≥ 1.1 (h1 + h2), and the difference between the horizontal coverage lengths L'1 and L'2 of this area on the upper and lower layers of the composite key layer is within 5m, and both L'1 and L'2 are ≥ 20m.

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