Method for determining hydrofracture treatment target layer of thick and hard roof

Abstract

The invention relates to a method for judging a hydrofracture prevention target layer of a thick and hard roof, which comprises the following steps of: 1, carrying out micro-seismic monitoring on an overlying rock layer on a coal mining working face to obtain a position parameter of an overlying rock layer fracture energy accumulation release key layer; 2, analyzing distribution characteristics of overlying strata of the coal seam according to mine survey geological data, testing mechanical parameters of compression resistance, tensile resistance and elastic modulus of a roof strata through field in-situ sample collection and indoor detection, and screening out a key layer of a hard strata overlying the coal seam; 3, quantitative judgment and identification are conducted on a thick and hard roof disaster hydrofracture treatment target layer according to the required caving height of a goaf filled with an overlying rock layer; and 4, combining the three layers obtained in the previous three steps to comprehensively judge the thick and hard roof hydraulic fracturing control target layer, and obtaining the vertical distance between the thick and hard roof hydraulic fracturing control target layer and the coal seam by solving the intersection of the three layers. The method has the characteristics of rapidness, convenience, practicability and theoretical property.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal mine hydraulic fracturing, and in particular to the technical field of thick hard roof mine pressure dynamic disaster management, and specifically provides a method for determining a target layer for thick hard roof hydraulic fracturing management. Background Art

[0002] Strong rock pressure from thick, hard roofs has become a major obstacle to safe production in underground coal mines, particularly in the Shendong mining area, which straddles Inner Mongolia and Shaanxi. The coal seams in this area primarily consist of thick layers of silt and fine sandstone, ranging in thickness from 12 to 40 meters. The rock strata have a compressive strength of 40 to 120 MPa, making them incredibly hard to collapse. During mining, the roof pressure step increases significantly, and the underlying roof overhang increases significantly. If a sudden collapse of the thick, hard roof occurs, it can easily trigger strong rock pressure accidents, a direct cause of a series of rock pressure disasters in the mine.

[0003] Affected by mining disturbances, during the working face mining process, the overburden in the mining area does not always transmit stress and break layer by layer from bottom to top. When the overburden of the coal seam at the working face has one, two or more thick and hard roof layers, it may induce a mine pressure dynamic disaster. A single thick and hard roof often forms a large cantilever beam structure, which is prone to induce mine pressure dynamic disasters after breaking. In most mines, affected by the evolution of the strata, two or more thick and hard roof layers will form. The movement of any key layer roof will promote the breakage and energy release of the adjacent key layer roof, thereby causing the synchronous or quasi-synchronous breakage of two or more key layer roof layers.

[0004] Under the conditions of the development of multiple layers of thick and hard roof, there are several key layers that induce mine pressure dynamic disasters, and different layers induce different types of disasters. For example, the breaking of thick and hard roof near the surface will often induce irregular surface settlement, cause damage to ground structures, and even induce large-scale mine earthquake disasters.

[0005] Currently, hydraulic fracturing technology can effectively weaken thick, hard roofs, reducing the intense dynamic pressure caused by roof failure and mitigating the impact damage caused by energy release. However, the selection of the fracturing layer is crucial to the fracturing effect. However, existing methods primarily rely on formula derivation and model analysis, which are highly theoretical and require a large number of parameters to be collected. This cumbersome process prevents rapid application to field production. Therefore, there is an urgent need to develop a quick and simple method to locate the target layer for hydraulic fracturing to mitigate dynamic hazards caused by thick, hard roofs. Summary of the Invention

[0006] Aiming at the deficiencies of the prior art, the present invention proposes a method for determining target layers for hydraulic fracturing treatment of thick hard roof.

[0007] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0008] A method for determining a target layer for hydraulic fracturing prevention and control of a thick hard roof comprises the following steps:

[0009] Step 1: Conduct microseismic monitoring of the overlying strata at the coal mining face, and obtain the position parameters of the key layer where the overlying stratum fracture energy is accumulated and released based on the microseismic monitoring data of the coal mining face;

[0010] Step 2: Analyze the distribution characteristics of the coal seam overburden based on the geological data of the mine survey, and test the mechanical parameters of the roof rock layer in terms of compression, tension, and elastic modulus through on-site sample collection and indoor testing to screen out the key layers of hard rock overlying the coal seam;

[0011] Step 3: quantitatively identify the target layer for hydraulic fracturing treatment of thick hard roof disasters by measuring the required collapse height of the overlying rock layer to fill the goaf;

[0012] Step 4: Combine the three horizons obtained in steps 1, 2, and 3 to comprehensively identify the target layer for hydraulic fracturing treatment of thick hard roof. By obtaining a common horizon that satisfies the positions of the three horizons at the same time, the common horizon is determined as the hydraulic fracturing prevention and control target. Finally, the vertical distance between the target layer for hydraulic fracturing prevention and control of mining pressure dynamic disasters in thick hard roof and the coal seam is obtained.

[0013] Moreover, step 1 includes the arrangement of well-ground joint microseismic monitoring substations and the detection and analysis of microseismic data;

[0014] 1.1. Based on the layout of the coal seam working face, microseismic monitoring stations are simultaneously deployed in the drifts on both sides of the coal mining working face and in the corresponding peripheral areas on the ground, achieving full spatial coverage of the overlying roof of the coal mining working face and completing the layout of the well-ground joint microseismic monitoring substations;

[0015] 1.2. Use the well-ground combined microseismic substation to collect microseismic events during the mining process. The microseismic events are characterized by microseismic data. After processing and analysis, the microseismic data are screened according to the vibration positioning principle. The roof layer with higher frequency and mean energy of microseismic events is selected. If 50% of the high-energy microseismic events occur in this layer, it is identified as the key layer for energy accumulation and release. The vertical distance interval between this key layer and the coal seam is recorded as [a i ,b i ], where a and b are the upper and lower limit values ​​of the interval, respectively, and i is the layer number, and the position parameters of the key layer where the overburden fracture energy accumulates and releases are obtained.

[0016] Furthermore, in step 1.1:

[0017] Layout of underground microseismic stations: One underground microseismic station is deployed every 50m starting from the coal cut in both sides of the drift, with a total of 7 stations deployed on each side, with a monitoring coverage range of 300m. After the working face is mined, the microseismic stations closest to the mining face are removed in sequence as the working face is mined, and new microseismic stations are deployed at the farthest end at intervals of 50m to maintain a coverage range of 300m.

[0018] Arrangement of ground microseismic stations: Microseismic stations are arranged 100m outside the corresponding ground positions of the two drifts of the working face, with a spacing of 100m. The farthest microseismic stations at both ends are located 100m outside the eye cutting and withdrawal channels.

[0019] Moreover, in step 2, the rock layers where the samples with uniaxial compressive strength greater than 60 MPa are located are screened out as hard rock layers. Then, based on the results of the rock layer refinement stratification, the hard rock layers with a thickness greater than 10 m are screened out and determined as key layers. The vertical distance interval between the key layer and the coal seam is recorded as [c j ,d j ], where c and d are the upper and lower limits of the interval, respectively, and j is the layer number.

[0020] Moreover, in step 3, the calculation formula for the collapse height required for the overburden to fill the goaf is as follows:

[0021]

[0022] Where: H is the mining height, unit is m; λ is the expansion coefficient after rock crushing;

[0023] The height range required for hydraulic fracturing to cause the overlying rock strata to collapse and fill the goaf is [h,+∞).

[0024] Moreover, in step 4, it is expressed as:

[0025] L∈[a i ,b i ]∩[c j ,d j ]∩[h,+∞).

[0026] The advantages and positive effects of the present invention are:

[0027] 1. The target layer judgment method for hydraulic fracturing prevention and control of thick hard roof mining pressure dynamic disasters of the present invention uses the monitoring data collected by microseismic monitoring stations to analyze the key layers of energy accumulation and release, providing the main basis for the selection of the target layer of hydraulic fracturing in the working face.

[0028] 2. The present invention realizes quantitative identification of the number and location of key layers in coal seams by combining the thickness of coal seam roof rock layers, lithologic combination characteristics, rock mechanics parameter testing, and comprehensive thick and hard roof identification principles.

[0029] 3. Based on the idea of ​​roof collapse filling disaster prevention, the present invention constructs a quantitative identification formula for the target layer height of hydraulic fracturing prevention and control of mining pressure dynamic disasters in thick and hard roofs, and realizes the precise design of hydraulic fracturing prevention and control measures for mining pressure dynamic disasters under this mode.

[0030] 4. This method combines microseismic monitoring, hard rock analysis and testing, and theoretical calculations to comprehensively analyze the conditions required for selecting hydraulic fracturing zones, resulting in a precise formula for calculating the target zone for hydraulic fracturing to prevent and control mining-induced dynamic hazards in thick hard roofs. This method is quick, simple, and combines both practicality and theoretical considerations. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is a flow chart for confirming the target layer for hydraulic fracturing to prevent and control mining pressure dynamic disasters in thick hard roof according to the present invention;

[0032] Figure 2 This is a schematic diagram of the layout of the well-ground combined full-coverage microseismic monitoring stations of the present invention;

[0033] Figure 3 This is a diagram showing the arrangement of underground microseismic monitoring stations according to the present invention;

[0034] Figure 4 This is a schematic diagram of the layout of the ground microseismic monitoring stations of the present invention;

[0035] Figure 5 This is a diagram of the well-ground combined microseismic monitoring result according to an embodiment of the present invention;

[0036] Figure 6 The present invention relates to a collaborative bearing system diagram of "collapsed filling body + coal pillar + load-bearing rock layer". DETAILED DESCRIPTION

[0037] The structure of the present invention will be further described below with reference to the accompanying drawings and through examples. It should be noted that the present examples are descriptive rather than restrictive.

[0038] A method for determining the target layer for hydraulic fracturing prevention and control of thick hard roof, see Figures 1-6 , the invention point is: comprising the following steps:

[0039] Step 1: Conduct microseismic monitoring of the overlying strata on the coal mining face, including the layout of joint well-ground microseismic monitoring substations and the detection and analysis of microseismic data.

[0040] 1.1. Layout of joint well-ground microseismic monitoring substations

[0041] According to the layout of the coal seam working face, microseismic monitoring stations are arranged simultaneously in the drifts on both sides of the coal mining working face and the corresponding peripheral areas on the ground to achieve full spatial coverage of the overlying roof of the coal mining working face, effectively improve the accuracy of microseismic monitoring, and use microseismic data feedback to conduct all-round monitoring of the fracture and migration laws of the overlying roof rock strata.

[0042] Underground microseismic monitoring station layout: Microseismic monitoring stations are deployed at set intervals on both sides of the drift, starting from the coal cut. The monitoring coverage range of the underground microseismic stations is 300 meters. In the present invention, it is preferred to deploy one station every 50 meters, with a total of 7 stations deployed on each side. After the working face is mined, the microseismic stations closest to the mining face are removed in sequence as the working face is mined. New microseismic stations are deployed at the farthest end at intervals of 50 meters to maintain a 300-meter coverage range.

[0043] Arrangement of ground microseismic stations: Microseismic stations are arranged 100m outside the corresponding ground positions of the two drifts of the working face, with a spacing of 100m. The farthest microseismic stations at both ends are located 100m outside the eye cutting and withdrawal channels.

[0044] Compared with conventional underground monitoring, combined well-ground monitoring has the characteristics of wide monitoring range and high monitoring accuracy. It can solve the problem that conventional underground monitoring cannot effectively identify the overlying higher key strata due to the large buried depth of the mined coal seams.

[0045] 1.2. Conduct microseismic data detection and analysis, and obtain the location parameters of the key layers where the overburden fracture energy is accumulated and released based on the microseismic monitoring data of the coal mining face;

[0046] Specifically: Use the well-ground joint microseismic substation to collect microseismic events during the mining process of the working face. Microseismic events are the intuitive feedback of microseismic monitoring results. Microseismic events are characterized by microseismic data. Microseismic data are various indicators fed back by microseismic events. Microseismic data include event frequency, event energy, location, etc. After processing and analysis of the microseismic data, according to the vibration positioning principle, screen the roof layers with higher frequency and mean energy of microseismic event energy events. If 50% of the high-energy microseismic events occur in this layer, it is determined that this layer is the key layer for energy accumulation and release, and the vertical distance interval between this key layer and the coal seam is recorded as [a i ,b i ], where a and b are the upper and lower limits of the interval, respectively, and i is the layer number. If there is only one critical layer, i = 1, and the interval is recorded as [a1, b1]. If there are two critical layers, i = 1, 2. The first layer is [a1, b1], the second layer is [a2, b2], and so on. This serves as the primary basis for selecting the target layers for hydraulic fracturing prevention and control in the roof of this working face.

[0047] Through comprehensive analysis of the energy, frequency and accumulation layers of microseismic events, the layers with more microseismic events and higher energy are defined as key layers. Figure 5 As shown in the schematic diagram of the coal seam profile, balls of different colors are drawn according to the energy size, and the layers where high energy is concentrated are regarded as key layers.

[0048] Step 2: Analyze the distribution characteristics of the coal seam overburden according to the geological data of the mine survey, and test the mechanical parameters such as the compressive strength, tensile strength, and elastic modulus of the roof rock layer through on-site in-situ sample collection and indoor testing, and screen out the key layers of the hard rock layer overlying the coal seam. Specifically: Analyze the two-dimensional profile of the overburden structure of the coal mining face according to the geological data of the mine survey, and compare and analyze the thickness, lithologic combination and other characteristics of the coal seam roof rock layer. Test the mechanical parameters such as the compressive strength, tensile strength, and elastic modulus of the roof rock layer through on-site in-situ sample collection and indoor testing, and screen out the rock layer where the samples with uniaxial compressive strength > 60MPa are located as hard rock layer. Then, based on the results of the refined stratification of the rock layer, screen out the hard rock layer with a thickness > 10m and determine it as the key layer. The vertical distance interval between the key layer and the coal seam is recorded as [c j ,d j ], where c and d are the upper and lower limit values ​​of the interval respectively, and j is the layer number. If the key layer is 1, j = 1, and the layer interval is [c1, d1]. If there are two key layers, j = 1, 2, and the first layer is [c1, d1], and the second layer is [c2, d2]; and so on.

[0049] Step 3: Quantitatively identify the target layer for hydraulic fracturing treatment of thick hard roof disasters by calculating the collapse height required for the overlying rock layer to fill the goaf:

[0050] After the direct roof collapses and fills the goaf, it will form a cooperative bearing system with the coal pillars and load-bearing rock strata. The more stable this cooperative bearing system is, the more it can avoid the formation of large-scale hanging roofs and breakage of thick and hard roofs, which can induce mining pressure dynamic disasters. However, due to the thin direct roof or even no direct roof under the conditions of hard roof occurrence, it is impossible to form an effective dense filling body to fill the goaf after the working face is mined, and the collapsed rock blocks cannot prevent the overlying rock strata from further rotating and becoming unstable. Therefore, hydraulic fracturing technology is used to weaken the roof, so that the roof rock strata collapse to fill the goaf under the mining effect, forming an effective support for the overlying rock strata, and forming a stable "collapse filling body + coal pillar + load-bearing rock strata" joint bearing system, which weakens or eliminates the overlying dynamic load effect and controls strong mining pressure dynamic disasters. Combined with the principle of rock fracture and expansion, the calculation formula for the required collapse height of the overlying rock strata to fill the goaf is as follows:

[0051]

[0052] Where: H is the mining height, unit is m; λ is the expansion coefficient after rock crushing, see Table 1:

[0053] Table 1. The expansion coefficient and residual expansion coefficient of common rocks in coal mines

[0054] rock type Swelling coefficient Residual expansion coefficient sand 1.06～1.15 1.01～1.03 clay ＜1.2 1.03～1.07 Crushed coal ＜1.2 1.05 Clay shale 1.4 1.10 Sandy shale 1.6～1.8 1.1～1.15 Hard sandstone 1.2～1.8 _

[0055] Therefore, the height range required for hydraulic fracturing to cause the overlying rock strata to collapse and fill the goaf is [h, +∞),

[0056] Step 4: Combine the three horizons obtained in Steps 1, 2, and 3 to comprehensively identify the target layer for hydraulic fracturing treatment of thick hard roof. By finding the intersection of the three horizons, the vertical distance between the target layer for hydraulic fracturing treatment of thick hard roof pressure dynamic disasters and the coal seam is obtained. Specifically, the vertical distance L should meet the following conditions: the overlap of the high-energy active layer monitored by microseismic monitoring, the key layer of the overlying hard rock layer, and the height interval required for the collapse to fill the goaf. It can be expressed as follows:

[0057] L∈[a i ,b i ]∩[c j ,d j ]∩[h,+∞)

[0058] The overlapping part is the target layer for hydraulic fracturing prevention and control of thick hard roof mining pressure dynamic disasters.

[0059] Example:

[0060] Taking the selection of key strata for hydraulic fracturing in the 42106 working face of the Buer Coal Mine as an example, first, through the feedback of monitoring data from the combined well-ground microseismic station, it was found that 79.6% of the microseismic events were located within 30 to 50 m above the roof of the 42 coal seam, indicating that the fracture locations of the overlying strata after mining of the 42 coal seam were concentrated in this range. At this point, corresponding to step 1, the key stratum interval [30,50] was obtained.

[0061] Step 2: According to the geological report of the mine rock formation, it is found that there are four layers of thick hard rock layers overlying the 42 coal seam, with the positions being [20, 40], [70, 110], [120, 170], and [200, 300] respectively.

[0062] Step 3: Based on the principle of rock fracture and expansion, the average mining height of 42 coal fully mechanized caving is 6m, and the sandstone expansion coefficient is 1.2. The required collapse zone height to fill the goaf is calculated to be 30m. Therefore, the required height range for hydraulic fracturing to cause the overburden to collapse and fill the goaf is [30, +∞).

[0063] Combine the horizon intervals obtained in the three steps and perform intersection processing, namely:

[0064] L∈([30,50]∩[20,40]∩[30,+∞))∪([30,50]∩[70,110]∩[30,+∞))∪···∪([30,50]∩[200,300]∩[30,+∞))Calculation yields:

[0065] L∈[30,40]

[0066] Therefore, the target layer for hydraulic fracturing treatment is 30m to 40m above the coal seam. Considering the construction cost, it is finally positioned 30m above the coal seam.

[0067] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will understand that various replacements, changes and modifications are possible without departing from the spirit of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A method for determining the target layer for hydraulic fracturing prevention and control of thick hard roof, characterized in that: The steps include: Step 1: Conduct microseismic monitoring of the overlying strata at the coal mining face, and obtain the position parameters of the key layer where the overlying stratum fracture energy is accumulated and released based on the microseismic monitoring data of the coal mining face; Step 2: Analyze the distribution characteristics of the coal seam overburden based on the geological data of the mine survey, and test the mechanical parameters of the roof rock layer in terms of compression, tension, and elastic modulus through on-site sample collection and indoor testing to screen out the key layers of hard rock overlying the coal seam; Step 3: quantitatively identify the target layer for hydraulic fracturing treatment of thick hard roof disasters by measuring the required collapse height of the overlying rock layer to fill the goaf; Step 4: Combine the three horizons obtained in steps 1, 2, and 3 to comprehensively identify the target layer for hydraulic fracturing treatment of thick hard roof. By obtaining a common horizon that satisfies the positions of the three horizons at the same time, the common horizon is determined as the target layer for hydraulic fracturing prevention and control. Finally, the vertical distance between the target layer for hydraulic fracturing prevention and control of mining pressure dynamic disasters of thick hard roof and the coal seam is obtained.

2. The method for determining the target layer for hydraulic fracturing prevention and control of thick hard roof according to claim 1, characterized in that: Step 1 includes the deployment of well-ground joint microseismic monitoring substations and the detection and analysis of microseismic data; specifically: 1.

1. Based on the layout of the coal seam working face, microseismic monitoring stations are simultaneously deployed in the drifts on both sides of the coal mining working face and in the corresponding peripheral areas on the ground, achieving full spatial coverage of the overlying roof of the coal mining working face and completing the layout of the well-ground joint microseismic monitoring substations; 1.

2. Use the well-ground combined microseismic substation to collect microseismic events during the mining process. The microseismic events are characterized by microseismic data. After processing and analysis, the microseismic data are screened according to the vibration positioning principle. The roof layer with higher frequency and mean energy of microseismic events is selected. If 50% of the high-energy microseismic events occur in this layer, it is identified as the key layer for energy accumulation and release. The vertical distance interval between this key layer and the coal seam is recorded as [a i ,b i ], where a and b are the upper and lower limit values ​​of the interval, respectively, and i is the layer number, and the position parameters of the key layer where the overburden fracture energy accumulates and releases are obtained.

3. The method for determining the target layer for hydraulic fracturing prevention and control of thick hard roof according to claim 2, characterized in that: In step 1.1: Layout of underground microseismic stations: One underground microseismic station is deployed every 50m starting from the coal cut in both sides of the drift, with a total of 7 stations deployed on each side, with a monitoring coverage range of 300m. After the working face is mined, the microseismic stations closest to the mining face are removed in sequence as the working face is mined, and new microseismic stations are deployed at the farthest end at intervals of 50m to maintain a coverage range of 300m. Arrangement of ground microseismic stations: Microseismic stations are arranged 100m outside the corresponding ground positions of the two drifts of the working face, with a spacing of 100m. The farthest microseismic stations at both ends are located 100m outside the eye cutting and withdrawal channels.

4. The method for determining the target layer for hydraulic fracturing prevention and control of thick hard roof according to claim 2, characterized in that: In step 2, the rock layers where the samples with uniaxial compressive strength greater than 60 MPa are located are screened out as hard rock layers. Then, based on the results of the rock layer refinement stratification, the hard rock layers with a thickness greater than 10 m are screened out and determined as key layers. The vertical distance interval between the key layer and the coal seam is recorded as [c j ,d j ], where c and d are the upper and lower limits of the interval, respectively, and j is the layer number.

5. The method for determining the target layer for hydraulic fracturing prevention and control of thick hard roof according to claim 4, characterized in that: In step 3, the formula for calculating the collapse height required for the overburden to fill the goaf is as follows: Where: H is the mining height, unit is m; λ is the expansion coefficient after rock crushing; The height range required for hydraulic fracturing to cause the overlying rock strata to collapse and fill the goaf is [h,+∞).

6. The method for determining the target layer for hydraulic fracturing prevention and control of thick hard roof according to claim 5, characterized in that: In step 4, the formula is: L∈[a i ,b i ]∩[c j ,d j ]∩[h,+∞)。

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