A Novel Method for Selecting Protective Seam against Coal and Gas Outburst: A Case Study of Wangjiazhai Coal Mine in China
Abstract
:1. Introduction
2. A Novel Method for Selecting Protective Seam against Coal and Gas Outburst
3. A Case Study
3.1. Overview of the Shuicheng Mining Area
3.2. UPS Selection Using the Novel Mehtod
3.2.1. Sequence Stratigraphic Analysis of the Coal-Bearing Formation in Wangjiazhai Coal Mine
3.2.2. Theoretical Selection of Protective Seams in Wangjiazhai Coal Mine
3.3. Theoretical Selection of Protective Seam Based on the Actual Mine Conditions
3.4. Technical Feasibility of Protective Seam Selection
3.4.1. Residual Gas Pressure
3.4.2. Relative Seam Spacing
3.4.3. Effective Vertical Distance between Seams
Theoretical Effective Vertical Distance
Calculation of Maximum Effective Vertical Distance
3.5. Numerical Simulation and Analysis of UPS Selection
3.5.1. Numerical Modeling
3.5.2. Analysis of Numerical Results
3.6. Practical Application of UPS Mining in the Studied Mine
3.6.1. Subjects of Investigation
3.6.2. Investigation Scheme
- (1)
- A dedicated investigation roadway was driven in the mine. The observation points for investigating the scope of protection were located around the open-off cut of the face in the protected seam, in order to make the investigation efficient and provide basis for designing the layouts of coal faces in the protective and protected seams as soon as possible. Moreover, these locations can ensure safety and reliable results. As there was no gas drainage roadway in the floor below the protective seam, a 120 m long roadway dedicated to investigation was driven in the floor strata under seam 11#. The minimum distance from the investigation roadway to seam 11# was about 16 m. The investigation roadway had an arc-shaped section, whose area was 8.01 m2. Steel net reinforced shotcrete was used to support the investigation roadway (steel framework was erected to support a few fractured regions). The wall rocks include clay and coal, they were fractured under rock stress. The investigation found that the roadway floor could easily heave and the wall was prone to deformation.
- (2)
- To measure the initial values of basic gas parameters of seam 11#, two pressure measurement boreholes were drilled at two positions in the investigation roadway that were unaffected by mining. The coal samples from seam 11# were tested in the laboratory for the coal’s porosity, gas adsorption constant, parameters of proximate analysis, firmness coefficient (f), and initial rate of gas diffusion (ΔP). Then, the protected seam’s initial gas content and permeability coefficient, denudation coefficient of borehole gas flow, and other parameters were calculated from the field measurements combined with the test results.
- (3)
- Two arrays of boreholes were drilled from the investigation roadway’s wall into the protected seam along its strike, one array for measuring gas pressure and the other for measuring deformation and gas flow. Another two arrays of boreholes were drilled into the seam along its dip for the same purpose. Variations in gas pressure, relative volumetric expansion, gas flow and permeability of the protected seam were investigated, in order to determine the effectiveness of UPS mining and the scope of protection, i.e., the stress relief zone, along the strike and dip of the seam.
- (4)
- Two coal samples were cut from seam 11# exposed by the investigation roadway. The two samples then went through a series of laboratory tests for their parameters of proximate analysis, adsorption constant, porosity, initial rate of gas diffusion ΔP, relationship between gas pressure and gas desorption index K1 (the K1-P curve), firmness coefficient f, and other parameters affecting the seam’s outburst risk.
- (5)
- During tunneling and mining at the X41104 coal face (Figure 7) in the protected seam, the predictive indicators of outburst at this face and relevant strata behavior parameters were observed in order to reveal their patterns of variation along the strike and dip. The main indicators and parameters examined included the amount of drill cuttings S, gas desorption index of drill cuttings k1, and gas emission, and their measurements were then used to analyze and validate the stress relief angle along the strike during UPS mining.
3.6.3. Results
Effectiveness Analysis
Protection Parameters
4. Conclusions
- (1)
- The novel method for selecting protective seam against coal and gas outburst is feasible and reasonable.
- (2)
- Sequence stratigraphic evolution not only controls the coal seam thickness and its variations, lithology of roof and floor strata, seam spacing, and other relevant parameters, but also determines the vertical variability in the level of outburst risk across a seam group. A theoretical analysis based on gas geology indicates that, of all seams in the study area, seam 11# was most prone to CGO, followed by seam 1#, while seams 7# and 8# were least prone to outburst. An investigation of gas occurrence in different seam in the Shuicheng mining area, combined with statistics on previous CGO events, demonstrates that the theoretical results are in line with the actual conditions. Therefore, it is feasible to assess CGO risk in coal seams using the theories of gas geology and the results can provide theoretical basis for protective seam selection.
- (3)
- The numerical simulation and analysis of the Wangjiazhai coal mine (a representative mine in the study area) show that, if mining seam 7# caused a 10% decline in rock stress, the stress relief angle along the strike of seam 11# was 68° on both the left and right sides. The stress relief zone exhibited a distinct symmetrical pattern. Along the dip direction, the stress relief angle was 87° at the lower end and 79° at the upper end.
- (4)
- Based on practical experience in protective seam mining and the results of numerical analysis and field investigation, the stress relief angles induced by mining seam 7# were determined: 59° along the strike and 75° along the dip direction at both the lower and upper ends of seam 11#. Extraction of seam 7# caused significant decreases in the parameters of seam 11# that signify outburst.
- (5)
- The study demonstrates that the theories of gas geology can be used to assess the CGO risk in coal seams of a seam group and the assessment results can provide theoretical basis for protective seam selection. Mining medium-distance, low-permeability UPS can significantly lower the predicative indicators of outburst risk in the protected seam and thereby help achieve the goal of preventing coal and gas outburst. This CGO prevention technique applies to the low-permeability coal-bearing strata in western China which are interpreted as transitional marine to continental deposits.
Acknowledgments
Author Contributions
Conflicts of Interest
References
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Mine Name | Production Time | Mine Area (km2) | Design Production Capacity (Mt/a) | Main Coal Seam |
---|---|---|---|---|
Shengyuan | 1975.12 | 12.65 | 90 | 4, 9, 11 |
Dawan | 1997.12 | 19.69 | 90 | 2, 4, 11 |
Hongqi | 1971.01 | 1.44 | 10 | 1, 7, 11, 13, 34 |
Dahebian | 1970.06 | 9.02 | 60 | 1, 7, 11, 13 |
Wangjiazhai | 1970.07 | 19.89 | 150 | 1, 7, 8, 11 |
Naluozhai | 1988.12 | 25.20 | 90 | 1, 7, 11, 13 |
Laoyingshan | 1972.06 | 7.50 | 90 | 8, 11, 13, 26 |
No. | Rock Type | Bulk Density (Kg/m3) | Thickness (m) | Elastic Modulus (Mpa) | Poisson’s Ratio, μ | Compressive Strength (MPa) | Tensile Strength (MPa) | Cohesive Force (MPa) | Shear Strength (MPa) | Dilation Angle, γ | Friction Angle, ϕ |
---|---|---|---|---|---|---|---|---|---|---|---|
1 | Shale | 2000 | 390 | 13,500 | 0.29 | 42 | 0.9 | 11 | 17 | 14 | 42 |
2 | Grey shale | 2012 | 33.5 | 14,090 | 0.26 | 48 | 1.1 | 12 | 16 | 12 | 36 |
3 | Seam 1# | 1540 | 1.59 | 1000 | 0.36 | 8 | 0.032 | 3 | 3.5 | 13 | 26 |
4 | Argillaceous siltstone | 2140 | 29.1 | 13,000 | 0.25 | 45 | 0.97 | 15 | 1.8 | 13 | 34 |
5 | Seam 7# | 1540 | 1.73 | 1000 | 0.36 | 8 | 0.032 | 3 | 3.5 | 13 | 26 |
6 | Mudstone and siltstone | 2004 | 8.2 | 10,090 | 0.2 | 10 | 0.1 | 10 | 0.1 | 13 | 20 |
7 | Seam 8# | 1540 | 1.34 | 1000 | 0.36 | 8 | 0.032 | 3 | 3.5 | 13 | 26 |
8 | Argillaceous siltstone | 2100 | 25.4 | 12,090 | 0.24 | 48.3 | 1.01 | 12 | 16 | 12 | 36 |
9 | Seam 11# | 1540 | 4.6 | 1000 | 0.36 | 8 | 0.032 | 3 | 3.5 | 13 | 26 |
10 | Siltstone | 2408 | 19 | 16,530 | 0.15 | 90 | 1.628 | 18 | 21 | 16 | 42 |
Parameter | Normal Stress Zone | Stress Concentration Zone | Stress Relief Zone | |
---|---|---|---|---|
Initial Stress Relief | Significant Stress Relief | |||
Distance from the X40702 face, L (m) | <−40 | −40~−20 | −20~+17 | +17~+42 |
Gas pressure, P (MPa) | 2.7 | 2.8 | 2.8~0.95 | 0.95~0.00 |
Gas flow, q (m3/min) | 0.070 | 0.046 | 0.046~0.54 | 0.54~0.95 |
Permeability coefficient, λ (m2/MPa2·d) | 1.23 | 0.82 | 13.5 | 26.23 |
Relative volumetric expansion, ε × 10−3 | 0 | 0 | 0~10.54 | 13.18 |
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Guowei, D.; Yinhui, Z. A Novel Method for Selecting Protective Seam against Coal and Gas Outburst: A Case Study of Wangjiazhai Coal Mine in China. Sustainability 2017, 9, 1015. https://doi.org/10.3390/su9061015
Guowei D, Yinhui Z. A Novel Method for Selecting Protective Seam against Coal and Gas Outburst: A Case Study of Wangjiazhai Coal Mine in China. Sustainability. 2017; 9(6):1015. https://doi.org/10.3390/su9061015
Chicago/Turabian StyleGuowei, Dong, and Zou Yinhui. 2017. "A Novel Method for Selecting Protective Seam against Coal and Gas Outburst: A Case Study of Wangjiazhai Coal Mine in China" Sustainability 9, no. 6: 1015. https://doi.org/10.3390/su9061015