Determining the width of the stress relief zone on roadway surrounding rocks is the premise to optimize drilling borehole effect and increase gas extraction efficiency. In this study, a new width measurement method of the stress relief zone on the roadway surrounding rocks was proposed, which determined the width according to gas pressure attenuation speeds in roadway boreholes at different depths. Then, the variation curve of the gas pressure in boreholes at different depths with the time was gained through a field test. On this basis, laws of the gas pressure attenuation and the gas transmission and loss in boreholes at different depths were explored through a numerical simulation based on COMSOL Multiphysics, thus concluding the stress on roadway surrounding rocks, the distribution of plastic zones, and the stress-permeability relation. The scientificity of the proposed method was illustrated theoretically. Finally, the proposed method was verified by the field test data and numerical simulation results of the gas extraction at different sealing depths. Research results demonstrate that the pressure in boreholes attenuates in the logarithmic function pattern. The attenuation speed decreases with the increase of the drilling depth. The width of the stress relief zone on roadway surrounding rocks in the studied area was determined to be about 11 m according to the proposed method. Both the numerical simulation and the field test of the gas extraction efficiency prove the feasibility and validity of the proposed method in determining the sealing depth of the borehole for the gas extraction. Research conclusions are of important significance to enrich width measurement methods of the stress relief zone on roadway surrounding rocks and to optimize sealing parameters of underground boreholes for gas extraction.
Coal mining in China tends to be accompanied with serious coal-gas outburst accidents. Moreover, the large-scaled development and utilization of coal resources bring a series of environmental problems, such as the atmospheric pollution and greenhouse effect. Gas is the high-quality, high-efficiency, and low-carbon clean gas energy that is accompanied with coal. The high-efficiency exploitation and utilization of the gas in coal seams have especially important significance to improve the energy consumption structure, promote the sustainable development of the environmental protection, and assure the security production of coal mines in China [
The gas extraction through underground boreholes in coal mines is an important technological measure for the exploitation and use of the gas in coal seams. However, the gas extraction is challenged by prominent gas leakage, accompanied with a small flow rate and a low gas extraction concentration [
Currently, relevant studies all pointed out that the sealing depth for the gas extraction should be higher than the width of the stress relief zone beside the roadway [
To overcome shortages of existing technologies, in this study, a new width measurement method of the stress relief zone on roadway surrounding rocks was proposed based on the analysis of relationships among stress states, fracture distributions, and the permeability of surrounding rocks, which was called as the pressure attenuation method. First, according to this method, the variation curve of the gas pressure in boreholes at different depths with time was gained through a field test, which was conducive to disclose the width range of the stress relief zone. Subsequently, laws of the gas pressure attenuation and the gas transmission and loss in boreholes at different depths were studied through a numerical simulation of COMSOL Multiphysics by combining engineering geological conditions in the studied mining area. The stress of coals beside the roadway, the distribution pattern of plastic zones, and stress-permeability relations were discussed at the same time. Moreover, the scientificity of the proposed method was illustrated theoretically, and the sealing depth of the borehole for the consequent gas extraction on roadway surrounding rocks was determined based on the analysis of the width of the stress relief zone. Finally, the field test results on the gas extraction efficiency under different sealing depths were presented.
Researches demonstrated that stresses on surrounding rocks will be redistributed after the roadway excavation [
Width measurement system of stress relief zone on roadway surrounding rocks based on the pressure attenuation method.
The self-developed measurement system of the stress relief zone width on roadway surrounding rocks is shown in Figure
Assembly structure of test devices.
Test process of the gas attenuation method.
The mine of Henan Baoyushan Coal Co., Ltd. was in the border of Ruyang, Linru, and Yichuan counties in Henan Province. Currently, the primary coal seams under exploitation in this mine were the 2-1 and 1-7 coal seams of the Shanxi Formation. The designed annual production capacity was 600,000 tons. The single level up and down the mixed exploitation of inclined and vertical shafts was applied in this mine. The primary and auxiliary shafts and the new air shaft were vertical ones, while the old air shaft was an inclined one. The full-seam mining along the bottom and caving roofs was employed in all shafts.
According to the geological report of Baoyushan Mine, the 2-1 coal seam was below the Shanxi Formation. The available coal content was 81% and the coal thickness ranges 0~12.78 m, averaging at 4.02 m. The average ash content, volatile content, and water content in the raw coal were 15.91%, 14.72%, and 0.83%, indicating the low intensity of coal masses. The gas content in the 2-1 coal seam was closely related with the thickness of the coal seam. It is generally positively related with the thickness of the coal seam. In view of the gas emission rate of mines, the absolute and relative gas emission rates were relatively small.
The roof of the 2-1 coal seam is mainly composed of the sandstone and sandy mudstone. However, the floor is dominated by the mudstone and sandy mudstone. For the sandstone, they have large shear, tensile, and compressive strengths and their stability is good. For the mudstone and sandy mudstone, the rock resistance shear, compression, and tensile strengths are relatively low. Overall, it is generally evaluated as a moderately stable roof and floor. The comprehensive column diagram of the coal seam is shown in Figure
Comprehensive column diagrams of coal rocks.
Based on the above experimental equipment and methods for measuring the width of the stress relief zone beside the roadway based on the gas pressure attenuation, a field test was carried out at the 2-1 coal seam from 100 m to 200 m of the lower crossheading 12105 in the Baoyushan Mine. Depths of boreholes 1#-8# were set 5 m, 6 m, 7 m, 8 m, 9 m, 10 m, 11 m, 12 m, and 14 m on the middle of the coal wall in the roadway. The pore diameter was 100 mm and the borehole interval was 15 m. The layout of boreholes is shown in Figure
Layout profile of boreholes.
The variation curve of the gas pressure with the time was gained by testing the gas pressure attenuation time and its weighted average in boreholes at different depths (Figure
Gas pressure change as time in boreholes at different depths.
Time for gas pressure attenuation to 0.04 MPa in boreholes at different depths.
It is neither possible to analyze laws of the gas leakage and loss in boreholes at different depths accurately through the above field test nor able to give a quantitative analysis on the stress state and fracture distribution characteristics on surrounding rocks. To analyze the feasibility of the proposed method, a numerical simulation on laws of the gas transmission and loss as well as gas pressure attenuation characteristics in boreholes at different depths was carried out by using COMSOL based on the engineering geological data. COMSOL is a multiphysical field coupling software. Research results laid foundations to theoretically prove the reasonability of the proposed method.
It can be seen from the full stress-strain experiment of the coal mass that the coal mass was softened significantly after reaching the stress peak. Accordingly, the cohesion decreased continuously as a response to the softening of the coal masses. The internal frictional angle changes slightly or basically keeps constant. Therefore, the strain softening behavior of coal masses is reflected by the degradation of the cohesion. The corresponding governing equation of the coal deformation is [
The yield failure of the coal mass can be characterized by the maximum tension stress criterion and the Mohr-Coulomb criterion (positive for tension stress). It can be expressed as
Under stress actions, the primary fractures close mainly before damage of the coal mass, which leads to the permeability reduction to some extent. With the increase of the stress of the coal mass, internal fractures are expanded and connected, which further increase the permeability of the coal mass. Based on the stress-damage-seepage model of the coal mass in existing studies, the evolution equation of the permeability with considerations to the softening of coal masses was gained through analysis [
Based on the working principle of the proposed pressure attenuation method, gas firstly makes pipe flow in boreholes and then seepages into coal seams. The N-S equation is a governing equation that describes the fluid in pipelines [
The gas transmission law in the coal seam basically conforms to Darcy’s law [
To explore gas leakage and loss laws during the test, a corresponding numerical calculation model was constructed considering actual geological conditions at the above test points by using COMSOL Multiphysics (Figure
Numerical calculation model: (a) spatial schematic map; (b) grid schematic.
Strata structure and mechanical parameters of rocks.
No. | Lithology | Depth (m) | Density (kg·m-3) | Compressive strength (MPa) | Tensile strength (MPa) | Poisson ratio | Elastic modulus (GPa) | Friction angle (°) | Cohesion (MPa) |
---|---|---|---|---|---|---|---|---|---|
1 | Sandy mudstone | 4.2 | 2300 | 39 | 1.12 | 0.24 | 10 | 36 | 3.4 |
2 | Fine sandstone | 19.0 | 2620 | 37.9 | 0.57 | 0.32 | 6 | 40.5 | 9.2 |
3 | Coal | 4.0 | 1437 | 8 | 1.29 | 0.38 | 3 | 30 | 1.2 |
4 | Mudstone | 3.5 | 2470 | 25.9 | 0.90 | 0.31 | 13 | 40 | 3.5 |
5 | Limestone | 6.9 | 2800 | 30 | 0.90 | 0.23 | 11 | 37 | 2.5 |
6 | Coal | 0.7 | 1437 | 8 | 1.29 | 0.38 | 3 | 30 | 1.2 |
7 | Mudstone | 11.7 | 2470 | 25.9 | 0.87 | 0.31 | 13 | 31.1 | 3.5 |
Constrain condition of the model.
Boundary | Mechanical constraints | Seepage constraints |
---|---|---|
ABB’A’ | Fixed in the |
Impermeable |
CDD’C’ | Free, |
Impermeable |
ADD’A’ | Fixed in the |
Impermeable |
BCC’B’ | Fixed in the |
Impermeable |
A’B’C’D’ | Fixed in the |
Impermeable |
ABCD | Fixed in the |
Impermeable |
Roadway | Free | |
Borehole I | Free | |
Borehole II | / | Impermeable |
Borehole III | Free |
After roadway excavation, the stress was redistributed (Figure
Vertical stress variation law of surrounding rock beside roadway.
Distribution of plastic zone on roadway surrounding rocks.
As the permeability was related to coal seam damage and stress, the permeability varied during the excavation. The distributions of stress and permeability are shown in Figure
Vertical stress and permeability curves of surrounding rock.
The distribution cloud map of the gas pressure in boreholes at different depths when
Distribution of gas pressure under different depths of boreholes when
Pressure attenuation curves in boreholes at different depths with time.
To further verify the feasibility and validity of the proposed pressure attenuation method in testing the width of the stress relief zone and determining the sealing depth for gas extraction boreholes, an engineering verification test was carried out on the crossheading of the working face 12015 in Baoyushan Mine. A total of 6 groups of upward extraction boreholes were constructed. The diameter of these boreholes was 89 mm and their sealing depths were set 8 m, 9 m, 10 m, 11 m, 12 m and 13 m. All boreholes were sealed with polyurethane (PU). After finishing the borehole drilling and reasonable sealing, the extraction boreholes were connected onto the gas extraction tube by controlling the valve and gas measuring holes. Besides, they were connected into the gas monitoring system of the mine. Later, the gas extraction concentration, negative pressure, flow rate, and temperature in boreholes were monitored for one week successively, getting the average flow rate and concentration in each group of boreholes (Table
Gas extraction parameters under different sealing depths.
1# borehole (sealing depth of 8 m) | 2# borehole (sealing depth of 9 m) | 3# borehole (sealing depth of 10 m) | 4# borehole (sealing depth of 11 m) | 5# borehole (sealing depth of 12 m) | 6# borehole (sealing depth of 13 m) | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Day (number) | PFR (m3/min) | Day (number) | PFR (m3/min) | Day (number) | PFR (m3/min) | Day (number) | PFR (m3/min) | Day (number) | PFR (m3/min) | Day (number) | PFR (m3/min) | ||||||
1 | 0.028 | 25.4 | 1 | 0.059 | 40.9 | 1 | 0.021 | 12.1 | 1 | 0.046 | 28.4 | 1 | 0.018 | 12.3 | 1 | 0.014 | 18.6 |
2 | 0.052 | 13.8 | 2 | 0.075 | 14.3 | 2 | 0.063 | 51.4 | 2 | 0.057 | 45.8 | 2 | 0.094 | 44.8 | 2 | 0.115 | 57.9 |
3 | 0.045 | 50.5 | 3 | 0.053 | 15.1 | 3 | 0.024 | 11.7 | 3 | 0.029 | 17.5 | 3 | 0.047 | 30.5 | 3 | 0.081 | 29.3 |
4 | 0.017 | 12.8 | 4 | 0.051 | 14.3 | 4 | 0.039 | 14.5 | 4 | 0.025 | 17.5 | 4 | 0.090 | 41.8 | 4 | 0.065 | 50.2 |
5 | 0.042 | 17.2 | 5 | 0.036 | 17.4 | 5 | 0.045 | 16.4 | 5 | 0.044 | 18.7 | 5 | 0.077 | 50.3 | 5 | 0.062 | 52.5 |
6 | 0.024 | 9.3 | 6 | 0.017 | 11.2 | 6 | 0.064 | 52.6 | 6 | 0.051 | 60.9 | 6 | 0.037 | 42.8 | 6 | 0.027 | 37.3 |
7 | 0.046 | 21.5 | 7 | 0.013 | 19.8 | 7 | 0.045 | 20.7 | 7 | 0.059 | 67.1 | 7 | 0.084 | 50.0 | 7 | 0.078 | 47.3 |
8 | 0.066 | 17.3 | 8 | 0.034 | 22.1 | 8 | 0.033 | 20.9 | 8 | 0.048 | 56.4 | 8 | 0.042 | 49.5 | 8 | 0.052 | 34.3 |
Average | Average | Average | Average | Average | Average |
PFR: pure flow rate; C: concentration.
Table
In this paper, a new width measurement method of the stress relief zone on the roadway surrounding rocks was proposed. The variation curve of the gas pressure in boreholes at different depths with the time was gained through a field test. Laws of the gas pressure attenuation and the gas transmission and loss in boreholes at different depths were studied through a numerical simulation of COMSOL Multiphysics by combining engineering geological conditions in the studied mining areas. Finally, the field test on the gas extraction efficiency under different sealing depths was carried out, and some important findings have been drawn:
The pressure attenuation method which is a new width measurement method of the stress relief zone is proposed based on the systematic analysis of relations among the stress state of the coal mass, fracture distributions, and the permeability coefficient in different regions of surrounding rocks. It determines the air permeability of the coal mass through a contrastive analysis of changes of the gas pressure attenuation speed in boreholes at different depths with time. On this basis, the stress state of the coal mass and the width of the stress relief zone are determined A numerical calculation model of the roadway on the work face 12015 in the Baoyushan Mine is constructed. A simulation study is carried out using COMSOL Multiphysics, which discloses distributions of the stress, plastic zone, and permeability of the roadway surrounding rocks. Gas transmission and pressure attenuation laws in surrounding rocks under different depths are analyzed. The simulation results agree well with the field test data. Both the simulation analysis and the field test conclude that the width of the stress relief zone beside the roadway in the lower crossheading of the work face 12015 in the Baoyushan Mine is about 11 m. This theoretically verifies the scientificity of the proposed pressure attenuation method in determining the width of the stress relief zone The field investigation of the gas extraction efficiency under different sealing depths proves that the gas extraction concentration is increased significantly after 11 m of the sealing depth. This reveals that the width of the stress relief zone determined by the proposed pressure attenuation method can be used as references to determine the sealing depth of boreholes for the gas extraction
The method proposed in this work helps enhance the gas extraction efficiency by better understanding and measurement of the depth of the roadway surrounding the rock stress relief zone distribution, so that proper sealing parameters can be adopted correctly. It can also be extended to other fields such as the roadway support, the fluid flow control, and other engineering applications related to the fracture measurement.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This project is supported by the National Key R&D Program of China (2017YFC0804207), National Natural Science Foundation of China (Nos. 51974109, 51704096, 51774110, 51774118, and U1704129), Program for Innovative Research Team in University of Ministry of Education of China (IRT_16R22), and Fundamental Research Funds for the University of Henan Province (NSFRF180330).