A method for estimating the height of water-conducting fracture zones in thick coal seams

Abstract

The present invention relates to a method for estimating the height of a water-conducting fracture zone in a thick coal seam, comprising obtaining measured data and classifying the data according to lithology; performing fitting to obtain a fitting formula, wherein the independent variable of the fitting formula is the mining thickness and the dependent variable is the height of the water-conducting fracture zone; substituting the mining thickness in the measured data into the fitting formula to obtain an empirical value of the height of the water-conducting fracture zone, calculating the difference between the empirical value and the measured value, wherein the maximum negative difference is the upper limit value and the maximum positive difference is the lower limit value; using the fitting formula to calculate the empirical value of the height of the water-conducting fracture zone as a median value, wherein the sum of the median value and the upper limit value is the maximum value of the height range of the water-conducting fracture zone, and the sum of the median value and the lower limit value is the minimum value of the height range of the water-conducting fracture zone; within the determined height range of the water-conducting fracture zone, the bottom interface of the hard rock is used as the height of the water-conducting fracture zone. The method of the present invention not only conforms to statistical laws, but also comprehensively considers the inherent mechanism of overburden failure, is highly operational, and is easy to be mastered by engineering and technical personnel of coal mining enterprises.

Description

Technical Field

[0001] The present invention relates to the field of coal mining, and in particular to a method for estimating the height of a water-conducting fracture zone in a thick coal seam. Background Art

[0002] After coal seam mining, there will be mining voids, which will cause the rock layer above the coal seam to collapse and break. The fractured fracture area formed in the longitudinal conductive mining void is the water-conducting fracture zone. The height of the water-conducting fracture zone is the distance between the highest point of the water-conducting fracture zone and the coal seam. Based on the height of the water-conducting fracture zone, it can be judged whether the upper aquifer will be connected after coal seam mining. It can also provide a basis for the establishment of waterproof coal pillars. The existing methods for estimating the height of water-conducting fracture zones are: (1) referring to the empirical formula in the "Specifications for the Establishment of Coal Pillars and Compressed Coal Mining for Buildings, Water Bodies, Railways and Main Wells and Tunnel"; (2) the empirical formula method applicable to the mining area summarized by some mining areas; (3) the method for estimating the height of water-conducting fracture zones based on the position of key layers. The limitations of the above methods (1) and (2) are that they do not consider the influence of thick hard rock layers on the height of water-conducting fracture zones, and they are calculated using empirical formulas. The limitation of the above method (3) is that it ignores the role of hard rock layers other than the key layers. Under the geological conditions of thick coal seams, the calculated results are quite different from the actual results. Summary of the Invention

[0003] To address the above technical issues, based on measured data and research results from other scholars, the present invention believes that hard rock plays a decisive role in the development of water-conducting fracture zones. Based on this, a new method for estimating the height of water-conducting fracture zones in thick coal seams is proposed, which includes the following steps:

[0004] S1, obtain the measured data of the height of the water-conducting fracture zone in the full-thickness working face of the thick coal seam, and classify the measured data into three categories according to the lithology: hard, medium-hard, and soft;

[0005] S2, fitting the measured data according to the lithology to obtain the fitting formula, the independent variable of the fitting formula is the mining thickness, and the dependent variable is the height of the water-conducting fracture zone;

[0006] S3, substituting the sample thickness in the measured data into the fitting formula in step S2 to obtain an empirical value of the height of the water-conducting fracture zone, and calculating the difference between the empirical value and the measured value, with the maximum negative difference being the upper limit and the maximum positive difference being the lower limit;

[0007] S4, using the fitting formula in step S2 to calculate an empirical value of the height of the water-conducting fracture zone as a median value, the difference between the median value and the upper limit value is the maximum value of the height range of the water-conducting fracture zone, and the difference between the median value and the lower limit value is the minimum value of the height range of the water-conducting fracture zone;

[0008] S5. Within the determined height range of the water-conducting fracture zone, the bottom interface of the hard rock is taken as the development height of the water-conducting fracture zone by comparing with the overburden columnar diagram.

[0009] Preferably, in step S2, data with large discreteness are deleted during fitting.

[0010] Preferably, in step S2, linear fitting is used.

[0011] Preferably, in step S5, the hard rock meets the following conditions: uniaxial compressive strength ≥ 40 MPa, thickness ≥ 3 m.

[0012] Preferably, when there is a layer of hard rock bottom interface within the height range of the water-conducting fracture zone, the water-conducting fracture zone develops to the hard rock bottom interface; when there are multiple hard rock bottom interfaces, each hard rock bottom interface may be the development position of the water-conducting fracture zone; when there is no hard rock bottom interface, the median value or the first hard rock bottom interface outside the height range of the water-conducting fracture zone and located at the top is taken as the development position of the water-conducting fracture zone; when covered by hard rock, the median value or the hard rock bottom interface is taken as the development position of the water-conducting fracture zone.

[0013] Preferably, when there are multiple layers of hard rock bottom interfaces, the first hard rock bottom interface that is higher than the median position is taken as the development position of the water-conducting fracture zone.

[0014] Beneficial effects: Based on statistical laws and taking into account the role of hard rock layers in the development of conductive height, the present invention proposes a method for predicting the height of water-conducting fracture zones in thick coal seam mining. This method not only conforms to statistical laws, but also comprehensively considers the inherent mechanism of overburden damage, and is highly operational and easy to be mastered by engineering and technical personnel of coal mining enterprises. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Scatter plot of the height of the water-conducting fracture zone in the thick coal seam mined in one go;

[0016] Figure 2 Diagram of fitting formula for height of water-conducting fracture zone in hard overburden;

[0017] Figure 3 Diagram of fitting formula for water-conducting fracture zone height in medium-hard overburden rock;

[0018] Figure 4 Diagram of fitting formula for the height of water-conducting fracture zone in weak overburden. DETAILED DESCRIPTION

[0019] The technical solution of the present invention is described in more detail below with reference to the accompanying drawings.

[0020] A method for estimating the height of a water-conducting fracture zone in a thick coal seam according to the present invention comprises the following steps:

[0021] The first step is that many domestic mines have conducted on-site measurements of the height of the water-conducting fracture zone in thick coal seams with full-thickness mining (including large-height mining and fully-mechanized caving mining). By collecting previous research materials, search data and research data, the measured data of the height of the water-conducting fracture zone in thick coal seams were sorted out according to hard, medium-hard and soft overburden. A total of 138 groups of measured data were sorted out, and the corresponding mining thickness and water-conducting fracture zone heights are as follows: Figure 1 shown.

[0022] Among them, hard overburden refers to overburden with uniaxial compressive strength greater than or equal to 40 MPa, and the most common ones are quartz sandstone, limestone, sandy shale, and conglomerate; medium-hard overburden refers to overburden with uniaxial compressive strength greater than 20 MPa and less than 40 MPa, and the most common ones are sandstone, muddy limestone, sandy shale, and shale; weak overburden refers to overburden with uniaxial compressive strength less than or equal to 20 MPa, and the most common ones are mudstone, muddy sandstone, and bauxite.

[0023] Based on the above measured data, the height of the water-conducting fracture zone under the conditions of large mining height mining and fully-mechanized top coal caving mining (comprehensive mechanized top coal caving) was fitted, and the degree of fitting basically met the requirements; at the same time, it was found that the height of the water-conducting fracture zone in fully-mechanized top coal caving mining and the height of the water-conducting fracture zone in large mining height mining have common patterns. Therefore, in order to simplify the application, the water-conducting fracture zone height prediction methods for thick coal seam large mining height mining and fully-mechanized top coal caving mining are classified into one category.

[0024] In the second step, based on the measured data, the linear regression formulas for the height of the water-conducting fracture zone when the lithology is hard, medium-hard, and weak can be obtained by fitting, as shown in Figure 2 、 Figure 3 、 Figure 4 The straight line in the middle position of the model is selected; considering the errors in the measurement process and the particularity of individual measured points, the data with large discreteness are deleted during fitting, but the discreteness of all other data should be comprehensively considered.

[0025] Figure 2 This is the calculation diagram of the fitting formula for the height of the hard overburden water-conducting fracture zone. The straight line in the middle of the figure is the fitting line for the height of the hard overburden water-conducting fracture zone. The corresponding empirical formula is H li =18.021M+9.916(where H li is the height of the water-conducting fracture zone, m; M is the mining thickness, m, the same below), the correlation coefficient is 0.74, and the correlation is good.

[0026] Figure 3 The figure is the calculation diagram of the fitting formula for the height of the water-conducting fracture zone in medium-hard overburden. The straight line in the middle of the figure is the fitting line for the height of the water-conducting fracture zone in medium-hard overburden. The corresponding empirical formula is H li =14.041M-4.8105, the correlation coefficient is 0.71, and the correlation is good.

[0027] Figure 4The calculation diagram of the height fitting formula of the weak overburden water-conducting fracture zone is shown in the figure. The straight line in the middle of the figure is the height fitting line of the weak overburden water-conducting fracture zone. li =8.4959M+2.6597, the correlation coefficient is 0.65, and the correlation is good.

[0028] In the third step, the sample thickness in the measured data is substituted into the fitting formula in step S2 to obtain the theoretical water-conducting fracture zone height (empirical value), and the difference between the theoretical water-conducting fracture zone height (empirical value) and the measured water-conducting fracture zone height is calculated, with the maximum negative difference being the upper limit and the maximum positive difference being the lower limit;

[0029] Specifically, the maximum difference between the empirical value of the water-conducting fracture zone height calculated by the linear fitting formula and the measured data at the same mining thickness is calculated, where the maximum difference higher than the empirical value calculated by the fitting formula is called the upper limit value, and the maximum difference lower than the empirical value calculated by the fitting formula is called the lower limit value; as shown in Table 1, the upper limit value of the height of the water-conducting fracture zone in hard overburden is -39.46m, and the lower limit value is 36.26m; as shown in Table 2, the upper limit value of the height of the water-conducting fracture zone in medium-hard overburden is -34.00m, and the lower limit value is 28.80m; as shown in Table 3, the upper limit value of the height of the water-conducting fracture zone in weak overburden is -17.33m, and the lower limit value is 28.60m.

[0030] Table 1 Comparison between the predicted height of the water-conducting fracture zone in hard overburden rock and the measured value

[0031]

[0032] Table 2 Comparison of the predicted height of the water-conducting fracture zone in the hard overburden with the measured value

[0033]

[0034]

[0035] Table 3 Comparison between the predicted height of the water-conducting fracture zone in the weak overburden and the measured value

[0036]

[0037]

[0038] The fourth step is to determine the height range of the water-conducting fracture zone. The fitting formula in step S2 is used to calculate the empirical value of the height of the water-conducting fracture zone as the median value. The difference between the median value and the upper limit value is the maximum value of the height range of the water-conducting fracture zone, and the difference between the median value and the lower limit value is the minimum value of the height range of the water-conducting fracture zone. As shown in Table 4, the fitting formula for the height of the water-conducting fracture zone determined based on the second and third steps and the upper and lower limit values ​​are shown. Figure 2 As shown in the figure, for hard overburden, the upper straight line is the difference between the fitting curve and the upper limit, and the lower straight line is the difference between the fitting curve and the lower limit; Figure 3 As shown in the figure, for medium-hard overburden, the upper straight line is the difference between the fitting curve and the upper limit, and the lower straight line is the difference between the fitting curve and the lower limit; Figure 4 As shown, the overburden is weak. The upper straight line in the figure is the difference between the fitting curve and the upper limit, and the lower straight line is the difference between the fitting curve and the lower limit.

[0039] For example, when mining a 5m thick working face in medium-hard overburden, the empirical value (median value) of the guide height is H li =14*5-4.8=65.2m, then the height guide range is: 65.2+34.0=99.2~65.2-28.8=36.4m.

[0040] Table 4 Prediction formula of water-conducting fracture zone and height range of water-conducting fracture zone

[0041]

[0042] Fifth, theory and practice have shown that hard rock layers in the overburden primarily control the development of water-conducting fracture zones. The maximum height of a water-conducting fracture zone is generally at the interface between soft and hard rock layers. Therefore, the relationship between hard rock layers in the formation can be used to determine the potential height of a water-conducting fracture zone. However, in thick coal seam mining, the overburden is highly damaged, and the hard rock thickness and occurrence layer combinations vary greatly. Previous methods proposed by experts require enormous computational effort, and the various assumptions lead to cumulative errors, making them impractical. Based on the analysis of the measured data above, the maximum height of a water-conducting fracture zone in thick coal seams conforms to basic mathematical and statistical laws.

[0043] Hard rocks in coal-bearing strata are generally limestone, sandstone, and conglomerate. Some shales with high mechanical strength are also hard rocks. To improve the rigor of this method, the definition of overburden types in the "Specifications for Coal Pillar Retention and Compressed Coal Mining in Buildings, Water Bodies, Railways, and Main Shafts and Tunnelings" and the conclusions on the laws of crack propagation in hard rocks are comprehensively considered. It is determined that the hard rock layer mentioned in this method must meet the following conditions: uniaxial compressive strength ≥40Mpa, thickness ≥3m.

[0044] Within the height range of the water-conducting fracture zone determined in the fourth step, the bottom interface of the hard rock layer is used as the development height of the water-conducting fracture zone by comparing with the overlying rock columnar diagram; when there is a bottom interface of a hard rock layer within the range, the height of the water-conducting fracture zone is the distance between the bottom interface of the hard rock and the coal seam, that is, the water-conducting fracture zone develops to the bottom interface of the hard rock; when there are multiple bottom interfaces of hard rock within the range, the height of the water-conducting fracture zone is multiple-selective, and the bottom interface of each hard rock may be the development location of the water-conducting fracture zone. It is recommended to take the bottom interface of the first hard rock that is greater than (higher than) the median value as the development location of the water-conducting fracture zone (the bottom interface of the hard rock closest to the median value and greater than the median value); when there is no bottom interface of a hard rock layer within the range, take the median value or the bottom interface of the first hard rock outside the range as the development location of the water-conducting fracture zone; when the range is completely covered by hard rock layers, take the median value or the bottom interface of the hard rock as the development location of the water-conducting fracture zone.

Claims

1. A method for estimating the height of water-conducting fracture zones in thick coal seams, characterized in that: The steps include: S1, obtain the measured data of the height of the water-conducting fracture zone in the full-thickness working face of the thick coal seam, and classify the measured data into three categories according to the lithology: hard, medium-hard, and soft; S2, fitting the measured data according to the lithology to obtain the fitting formula, the independent variable of the fitting formula is the mining thickness, and the dependent variable is the height of the water-conducting fracture zone; S3, substituting the sample thickness in the measured data into the fitting formula in step S2 to obtain an empirical value of the height of the water-conducting fracture zone, and calculating the difference between the empirical value and the measured value, with the maximum negative difference being the upper limit and the maximum positive difference being the lower limit; S4, using the fitting formula in step S2 to calculate an empirical value of the height of the water-conducting fracture zone as a median value, the difference between the median value and the upper limit value is the maximum value of the height range of the water-conducting fracture zone, and the difference between the median value and the lower limit value is the minimum value of the height range of the water-conducting fracture zone; S5: Within the determined height range of the water-conducting fracture zone, the bottom interface of the hard rock is taken as the development height of the water-conducting fracture zone.

2. The method for estimating the height of water-conducting fracture zones in thick coal seams according to claim 1, characterized in that: In step S2, measured data with large discreteness are deleted during fitting.

3. The method for estimating the height of water-conducting fracture zones in thick coal seams according to claim 1 or 2, characterized in that: In step S2, linear fitting is used.

4. The method for estimating the height of water-conducting fracture zones in thick coal seams according to claim 1, characterized in that: In step S5, the hard rock meets the following conditions: uniaxial compressive strength ≥ 40 MPa, thickness ≥ 3 m.

5. The method for estimating the height of water-conducting fracture zones in thick coal seams according to claim 4, characterized in that: When there is a hard rock bottom interface within the height range of the water-conducting fracture zone, the water-conducting fracture zone develops to this hard rock bottom interface; when there are multiple hard rock bottom interfaces, each hard rock bottom interface may be the development location of the water-conducting fracture zone; when there is no hard rock bottom interface, the median value or the first hard rock bottom interface outside the height range of the water-conducting fracture zone and located at the top is taken as the development location of the water-conducting fracture zone; when covered by hard rock, the median value or the hard rock bottom interface is taken as the development location of the water-conducting fracture zone.

6. The method for estimating the height of water-conducting fracture zones in thick coal seams according to claim 5, characterized in that: When there are multiple hard rock bottom interfaces, the first hard rock bottom interface above the median position is taken as the development location of the water-conducting fracture zone.

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