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Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region
International Journal of Mining Science and Technology xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region Chang Xiaocun, Tian Hui ⇑ School of Safety Engineering, Liaoning Technical University, Fuxin 123000, China China Coal Society, Beijing 100013, China
a r t i c l e
i n f o
Article history: Received 13 September 2017 Received in revised form 10 December 2017 Accepted 17 February 2018 Available online xxxx Keywords: Coal seams group Stress field Displacement field Expansion deformation Pressure-relief angle
a b s t r a c t A pressure relief gas extraction technical model of a typical mining area is proposed based on coal and gas simultaneous extraction theory. Flac3D was employed to model vertical stress and displacement contour plot characteristics of non-outburst coal seam (No. 4) on top of outburst coal seam (No. 2) along strike and incline directions. Field investigations were also conducted to verify the scientific nature of the simulation. The results demonstrate that gas pressure in No. 2 coal seam dropped to approximately 0.55 MPa in the pressure relief multi-coal seam. The highest expansion rate of the coal mine reached up to 2.58%. The pressure-relief angle was 76° along the incline direction and 60° along the strike direction. As the expansion rate and pressure-relief angle increased and the gas pressure decreased, a large amount of gas flowed into the gob of No. 4 from No. 2 coal seam and was later discharged through specific gas pipes, which eliminated No. 2 outburst risks. This study resulted in positive outcomes in that gas extraction time was reduced by 13.5 days, due to pressure relief, and drilling work load was reduced by 0.1161 m/t coal. This method ensures that gas is discharged from the outburst coal seam quickly and safely, demonstrating that the proposed technical model of pressure-relief gas extraction is effective in a multi-coal seam region. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The application of protective mining and gas extraction technology not only reduces the risk of coal and gas outbursts from protected coal seams, but it also allows for simultaneous extraction of coal and gas in coal seams that have high risks of outburst. Protective seam mining and gas extraction technologies are widely used in practice [1–6] and combined with the above technologies they are effective for high-efficiency, safe mining of coal and gas resources. Many cases of theoretical research and technical applications have been undertaken recently by researchers [7–12]. Liu et al. [13] adopted the analogy simulation method to study the dynamic evolution laws of overlying fractures induced by mining in the protective seam and the protective scope of pressure relief. By using FLAC3D or RFPA software, Shi [14,15] investigated the stress levels of coals and rocks around the protective seam in the driving process and analysed the characteristics of pressure-relief protection. Cheng [16–19] studied the techniques of continuous pressure relief for mining protective seams, protective scope expansion for pres⇑ Corresponding author. E-mail address: [email protected] (T. Hui).
sure relief and evolution laws of gas permeability in protective seam mining processes [20–23]. Pressure relief gas extraction in coal seam mining groups is a systemic engineering practice based on the interactions of mining-influenced stress fields, fractures and gas flow fields. However, research is limited because of the current, unreasonable model of pressure relief gas extraction, which requires extensive practical data to form a solid foundation [24]. In this study, the geological conditions of gas occurrence in non-outburst coal (No. 4) and outburst coal (No. 2) were investigated. The relevant mathematical model was calculated using FLAC3D. The distribution characteristics of the stress field, displacement field and the gas pressure of overlying rocks were studied. Meanwhile, the effects of pressure relief and increased permeability from coal seam No. 4 to No. 2 were studied and the result were compared with field applications. 2. Pressure-relief gas extraction under multi-coal seam extraction conditions During coal seam group mining, simultaneous coal exploitation causes extra fractures, deformation or subsidence of the overlying coal and rock. As mining-induced fractures develop, the permeabil-
https://doi.org/10.1016/j.ijmst.2018.03.010 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Xiaocun C, Hui T. Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.03.010
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C. Xiaocun, T. Hui / International Journal of Mining Science and Technology xxx (2018) xxx–xxx
Fig. 1. Coal measure strata histogram of the studied coal mine.
ity of the coal seam increases, allowing for coal seam gas to be drained more efficiently and continuously, which ensures safe and effective production of the coal mine [25–26]. Aiming to exploit coal and gas in the first mining and adjacent seams, time and space must be considered to achieve a balance between drainage, driving and mining. The coal bearing formations that were studied include the Middle Carboniferous Benxi Formation, the upper Taiyuan formation and the upper Taiyuan formation of the lower Permian series in the Shanxi Formation. The average thickness of No. 2 and No. 4 coal seams are 6.0 m and 1.25 m, respectively. The stratigraphy of the coal mine is shown in Fig. 1. In the process of roadway advancing, a coal and gas outburst occurred in No. 2 coal seam. No. 4 coal seam has a low outburst risk since the gas pressure and gas content is low. Because No. 4 coal seam is close to No. 2 coal seam, No. 4 coal seam was chosen as the first mining seam. The pressure-relief gas extraction technical model is shown in Fig. 2.
3. Numerical simulation by FLAC3D 3.1. Geometric model According to the geological conditions of No. 2 and No. 4 coal seams, the average spacing between No. 2 coal seam and No. 4 coal seam is 35 m, and the buried depth is approximately 800 m. In order to not influence calculation results, the advancing length of the working face was simplified for the model by taking No. 4 coal seam as the lower protective layer of No. 2 coal seam. The size of the whole model is 210 m (length)150 m (width) 100 m (height). The geometry includes 47,880 units and 51 987 nodes. As shown in Fig. 3, the x-direction is the incline direction, the y-direction is the strike direction and the z-direction is the gravity direction of the coal seam. 3.2. Boundary conditions and parameters of the numerical model The boundary conditions around the model are fixed, except for the upper boundary which is not fixed. A compressive stress of 18.716 MPa was imposed on top of the model. The maximal main stress is the vertical stress and the minimal main stress is along the x-direction. The minimal main stress is half the magnitude of the maximal main stress. The middle main stress is along the y-direction, and it is 60% of the maximal main stress. Mohr-Coulomb failure criterion was used to determine the deformation of the rock body.
Fig. 2. Pressure-relief gas extraction technical model.
Fig. 3. Mesh dissection model.
Please cite this article in press as: Xiaocun C, Hui T. Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.03.010
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C. Xiaocun, T. Hui / International Journal of Mining Science and Technology xxx (2018) xxx–xxx Table 1 Rock physical properties and mechanical parameters of the strata model. Strata index
Bulk modulus (GPa)
Shear modulus (GPa)
Apparent density (kg/m)
Friction angle (°)
Cohesion (MPa)
Tensile strength (MPa)
Siltstone 3 Coal Siltstone 2 Finestone Siltstone 1 Shale Limestone Mudstone
5.0 2.3 4.5 6.0 3.9 5.0 7.5 4.0
3.8 1.5 3.1 4.5 3.0 3.8 5.5 3.0
2400 1430 2400 2500 2450 2610 2710 2400
28 28 28 31 30 30 36 28
2.0 3.0 2.1 4.2 2.0 2.0 4.5 3.0
1.8 1.0 2.0 7.0 1.7 1.7 10.0 1.0
1 þ sin u f s ¼ r1 r3 2C 1 sin u
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ sin u 1 sin u
ð1Þ
where r1 is the maximal main stress and r3 is the minimal stress. The parameter C is cohesion and u is the angle of friction. When fs is larger than zero, the material of the model will be deformed. Physical properties of the rocks and mechanical parameters of the strata model are shown in Table 1. 4. Simulation result analysis As the working face of No. 4 advanced, the stress and displacement fields changed continuously. The vertical stress contour plots of the mining face along the strike and incline directions are shown in Figs. 4 and 5 when the working face advanced 40 m and 80 m. 4.1. Distribution of stress field As shown in Fig. 4b, the pressure-relief scope of No. 2 coal seam covered the whole gob when the working face advanced 80 m and the vertical stress was 5–10 MPa. The mining-induced stress relief effect was noticeable because of No. 4 coal seam mining effect. Conclusions can be drawn based to Fig. 4 and the stress nephograms of other mining processes. When the working face advanced, its first weight appeared and the roof of the gob began to cave. Then, periodic caving of the roof developed and three vertical zones were formed in the roof of the gob. When the working face advanced 60 m, the middle coal vertical stress in the pressure-relief zone was minimized (6 MPa) and less than the original vertical stress (19 MPa). The noticeable reduction
in vertical stress indicated that the effects of stress-relief were prominent. When the coal mining of No. 4 coal seam ended, there were separate fractures in the caving zone on top of the gob. No. 2 coal seam lies in the pressure-relief zone of the fractured zone of No. 4 coal seam gob, which was very helpful for gas extraction from No. 2 coal seam.
4.2. Distribution of displacement fields According to Fig. 5b, when the working face advanced 80 m, the maximum vertical displacement at the top of the gob was 375 mm. The vertical displacement at the top of the gob in No. 2 coal seam was 250–300 mm, which was far greater than the original vertical displacement of No. 2 coal seam. In addition, the maximum vertical displacement along the strike direction at the top of the gob was 336 mm. The vertical displacement at the top of the gob in No. 2 coal seam was 250–300 mm, which was also far greater than the original vertical displacement of No. 2 coal seam. As the distance from the top of No. 4 coal seam increased, in the perpendicular direction, the vertical displacement of the coal and rock decreased gradually, whereas, the vertical displacement increased as this working face advanced. In the process of coal mining, three zones are created: caving zone, fractured zone and bending subsidence zone. The height of the caving zone was approximately 5 times greater than the mining height at the working face, and the height of fractured zone was approximately 40 times greater than the mining height at the working face. The top roof was in the bending subsidence zone. When the working face advanced 80 m, the slip angle of rock strata was approximately 56°.
Fig. 4. Vertical stress contour plots of the mining face along the strike and incline directions: 40 m advance (along the strike direction) (a), 80 m advance (along the strike direction) (b), 40 m advance (along the incline direction) (c) and 80 m advance (along the incline direction) (d).
Please cite this article in press as: Xiaocun C, Hui T. Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.03.010
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Fig. 5. Vertical displacement contour plots of the mining face along the strike and incline directions: 40 m advance (along the strike direction) (a), 80 m advance (along the strike direction) (b), 40 m advance (along the incline direction) (c) and 80 m advance (along the incline direction) (d).
4.3. Gas movement characteristics under mining-induced fissure conditions Based on analysis of vertical stress characteristics and movement along the strike direction of working face, it was determined that the velocity of gas transport to the inner part of coal seam was increased. This finding was observed because fracture development is beneficial when the coal seam is influenced by coal mining, as it provides more channels for gas movement and gas moves from areas with high pressure to inner volumes of lower pressure. When coal mining of the working face reached 40 m, the travelling velocity of gas on the working face created a flat distribution for gas near the coal seam. The permeability of the coal mass increased greatly, which was beneficial for pressure-relief gas extraction.
mining seam of working face 8463 belongs to No. 4 coal seam of the Taiyuan formation of Carboniferous. The average mining depth was 1.2 m, the strike length of the working face was 1100 m and the incline length of the face was 150 m. The average distance of working face 8463 and working face 8263 was 35 m. A reasonable location layout of the drilling field and construction drills was chosen. No. 1 drilling field was located at the starting line. No. 8 and No. 9 drilling fields were located at the stopping line, and they were used to obtain the pressure relief scope of No. 2 coal seam along the strike direction. No. 2, No. 3, No. 4 and No. 5 drilling fields were located in the scraper conveyor roadway and haulage gateway. They were used to determine the pressure-relief scope of No. 2 coal seam along the incline direction. No. 6 and No. 7 drilling fields were used to obtain the expansion deformation and runoff of No. 2 coal seam. Section sketches of drilling fields are shown in Fig. 6.
5. Field investigations (1) Gas pressure changes induced by mining 5.1. Exploration of pressure relief scope To obtain the pressure relief scope, during mining, from coal seam No. 4 to No. 2, working face 8463 was selected. The main
As shown in Fig. 7a, it was determined that when the location of drills 1#–5# in No. 1 drilling field were not influenced by mining, gas pressure was stable at 1.1–1.2 MPa. When coal mining of the
Fig. 6. Profile map of boreholes along the coal seam strike (a) and (b); profile map of holes along the steep coal seam direction (c) and holes for measuring gas flow and gas seepage coefficient (d).
Please cite this article in press as: Xiaocun C, Hui T. Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.03.010
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Fig. 7. Gas pressure change of No. 2 coal seam with mining of No. 4 coal seam.
working face reached 45 m, gas pressure began to decrease. Gas pressure reduced to zero when coal mining of the working face reached 35.4 m. This was because deformation of the overlying strata deformed the drills and gas pressure decreased significantly since the boreholes were connected to the gob. When the distance between the coal mining line and No. 1 drilling field was 5 m, the gas pressure of drills 4# and 5# began to increase. This was because fractures in the overlying strata upon the gob were compressed and enclosed, allowing gas pressure to rise. When gas pressure was stable, the gas pressure of drills 3#, 4# and 5# was less than 0.74 MPa (the critical value) indicating that the gas pressure of drills 3#, 4# and 5# in No. 2 coal seam was relieved effectively. The pressure-relief angle was 60° along the strike direction of the starting line. As shown in Fig. 7b, it was determined that residual gas pressures of drills 17#, 18#, 19#, 20# in No. 8 drilling field were 0.5, 0.55, 0.55 and 0.85 MPa, respectively. Gas pressure of the other drills was less than 0.74 MPa (the critical value), except for drill 20#. The pressure-relief angle was 60° in the strike direction of the stopping line. As shown in Fig. 7c, only the residual gas pressure of drill 6# was larger than 0.74 MPa. This finding demonstrates that the relevant regions of No. 2 coal seam have been fully pressure-relieved. It was determined that the upper pressure-relief angle from coal seam No. 4 to No. 2 was 84° in the incline direction. Likewise, it was determined that the below pressure-relief angle was 76° in the incline direction from coal seam No. 4 to No. 2 in No. 5 drilling field.
(2) Study on deformation of expansion Influenced by the mining of No. 4 coal seam, the stress of the overlying strata and No. 2 coal seam was redistributed. For No. 2 coal seam, the deformation of the roof and floor was divided into five stages: ① compression, ② relief of compression, ③ expansion, ④ reduction of expansion and ⑤ stability of expansion. Meanwhile, five areas were formed: ① pressure-increasing area, ② pressure-relief and expansion area, ③ area with stable expansion and pressure-relief, ④ pressure-relief and expansion area, and ⑤ pressure-increasing area. Drills 15# and 150 # of No. 6 and No. 7 drilling fields were studied. As shown in Fig. 8, the maximum compression in the upper part of No. 2 coal seam was 29.4 mm, and the compression rate was 0.49%. The maximum compression in the lower part of No. 2 coal seam was 30.3 mm, and the compression rate was 0.51%. The maximum expansion in the upper part of No. 2 coal seam was 155.0 mm, and the expansion rate was 2.58%. The maximum expansion in the lower part of No. 2 coal seam was 145.5 mm, and the expansion rate was 2.42% (refer to Fig. 8). (3) Determination of pressure-relief scope and protection After analysing observational results of gas pressure in each drilling field and gas pressure of the coal seam, it was found that the upper pressure-relief angle from coal seam No. 4 to No. 2 in the incline direction, d2, was 84° and the low pressure-relief angle,
Fig. 8. Expansion and deformation of No. 2 coal seam with mining of No. 4 coal seam (positive for compression; negative for expansion).
Please cite this article in press as: Xiaocun C, Hui T. Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.03.010
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Fig. 9. Stress relief and protected zone of No. 2 coal seam in the incline direction (a) and those of the No. 2 coal seam in the strike direction (b).
Table 2 Gas drainage effect of working face 8263. Parameter
Drilling quantity (m/t)
Quantity of drainage (104 m3)
Finishing time (day)
Before After Decline
0.8202 0.7041 0.1161
347.56 303.17 44.39
44.3 30.8 13.5
d1, was 76°. The pressure-relief angle from the stopping line of coal seam No. 4 to coal seam No. 2 in the strike direction, d3, was 60°, as shown in Fig. 9. 5.2. Effect of gas extraction The mining of working face 8463 in No. 4 coal seam, relieved the pressure of working face 8263 in No. 2 coal seam. The drilling load was reduced by 0.116 1 m/t raw coal and the time of gas extraction was reduced by 13.5 days, indicating the positive effects of mining coal and gas simultaneously. The observational results are listed in Table 2. Therefore, with protective mining of No. 4 coal seam, the pressure-relief effectiveness for low protective seam was verified over a large distance by observing gas pressure and gas expansion rates of No. 2 protected coal seam. Pressure-relief angles of No. 4 coal seam, in the strike and incline directions, were determined. The effects of protective seam mining were positive. As the working face advanced, gas pressure was reduced to the critical value. The effects of protective seam mining and gas extraction under the coal seam group were verified over a large distance, meeting the characteristics of simultaneous exploitation of coal and gas. 6. Conclusions The theory of gas extraction technology was studied under the conditions of a mining-induced fissure. This study needed to coordinate time and space parameters to devise a reasonable mining process and effective drainage method. A typical gas extraction mode under coal seam group mining conditions of a mining area was proposed. (1) Using FLAC3D, the vertical displacement contour plots of No. 4 mining face along the strike and incline directions were obtained. The redistribution of stress and displacement were analysed. The gas pressure in No. 2 coal seam was reduced to 0.55 MPa in the pressure-relief region and the maximum expansion ratio reached 25.8. The danger of gas outbursts was thoroughly eliminated by increasing the expansion rate and pressure-relief angle and decreasing gas pressure in No. 2 coal seam. (2) Practical field applications have shown that the upper pressure-relief angle, d1, from coal seam No. 4 to No. 2 was 76°. The upper pressure-relief angle, d2, was 84° and the pressure-relief angle in the strike direction, d3, was 60°. In these pressure-relief protection regions, the gas extraction
effectiveness of working face 8263 was noteworthy and the gas extraction time reduced by 13.5 days. This investigation validated the effectiveness of simultaneous coal and gas exploitation under coal seam group mining conditions.
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Please cite this article in press as: Xiaocun C, Hui T. Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.03.010
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region Chang Xiaocun, Tian Hui ⇑ School of Safety Engineering, Liaoning Technical University, Fuxin 123000, China China Coal Society, Beijing 100013, China
a r t i c l e
i n f o
Article history: Received 13 September 2017 Received in revised form 10 December 2017 Accepted 17 February 2018 Available online xxxx Keywords: Coal seams group Stress field Displacement field Expansion deformation Pressure-relief angle
a b s t r a c t A pressure relief gas extraction technical model of a typical mining area is proposed based on coal and gas simultaneous extraction theory. Flac3D was employed to model vertical stress and displacement contour plot characteristics of non-outburst coal seam (No. 4) on top of outburst coal seam (No. 2) along strike and incline directions. Field investigations were also conducted to verify the scientific nature of the simulation. The results demonstrate that gas pressure in No. 2 coal seam dropped to approximately 0.55 MPa in the pressure relief multi-coal seam. The highest expansion rate of the coal mine reached up to 2.58%. The pressure-relief angle was 76° along the incline direction and 60° along the strike direction. As the expansion rate and pressure-relief angle increased and the gas pressure decreased, a large amount of gas flowed into the gob of No. 4 from No. 2 coal seam and was later discharged through specific gas pipes, which eliminated No. 2 outburst risks. This study resulted in positive outcomes in that gas extraction time was reduced by 13.5 days, due to pressure relief, and drilling work load was reduced by 0.1161 m/t coal. This method ensures that gas is discharged from the outburst coal seam quickly and safely, demonstrating that the proposed technical model of pressure-relief gas extraction is effective in a multi-coal seam region. Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction The application of protective mining and gas extraction technology not only reduces the risk of coal and gas outbursts from protected coal seams, but it also allows for simultaneous extraction of coal and gas in coal seams that have high risks of outburst. Protective seam mining and gas extraction technologies are widely used in practice [1–6] and combined with the above technologies they are effective for high-efficiency, safe mining of coal and gas resources. Many cases of theoretical research and technical applications have been undertaken recently by researchers [7–12]. Liu et al. [13] adopted the analogy simulation method to study the dynamic evolution laws of overlying fractures induced by mining in the protective seam and the protective scope of pressure relief. By using FLAC3D or RFPA software, Shi [14,15] investigated the stress levels of coals and rocks around the protective seam in the driving process and analysed the characteristics of pressure-relief protection. Cheng [16–19] studied the techniques of continuous pressure relief for mining protective seams, protective scope expansion for pres⇑ Corresponding author. E-mail address: [email protected] (T. Hui).
sure relief and evolution laws of gas permeability in protective seam mining processes [20–23]. Pressure relief gas extraction in coal seam mining groups is a systemic engineering practice based on the interactions of mining-influenced stress fields, fractures and gas flow fields. However, research is limited because of the current, unreasonable model of pressure relief gas extraction, which requires extensive practical data to form a solid foundation [24]. In this study, the geological conditions of gas occurrence in non-outburst coal (No. 4) and outburst coal (No. 2) were investigated. The relevant mathematical model was calculated using FLAC3D. The distribution characteristics of the stress field, displacement field and the gas pressure of overlying rocks were studied. Meanwhile, the effects of pressure relief and increased permeability from coal seam No. 4 to No. 2 were studied and the result were compared with field applications. 2. Pressure-relief gas extraction under multi-coal seam extraction conditions During coal seam group mining, simultaneous coal exploitation causes extra fractures, deformation or subsidence of the overlying coal and rock. As mining-induced fractures develop, the permeabil-
https://doi.org/10.1016/j.ijmst.2018.03.010 2095-2686/Ó 2018 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Xiaocun C, Hui T. Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.03.010
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Fig. 1. Coal measure strata histogram of the studied coal mine.
ity of the coal seam increases, allowing for coal seam gas to be drained more efficiently and continuously, which ensures safe and effective production of the coal mine [25–26]. Aiming to exploit coal and gas in the first mining and adjacent seams, time and space must be considered to achieve a balance between drainage, driving and mining. The coal bearing formations that were studied include the Middle Carboniferous Benxi Formation, the upper Taiyuan formation and the upper Taiyuan formation of the lower Permian series in the Shanxi Formation. The average thickness of No. 2 and No. 4 coal seams are 6.0 m and 1.25 m, respectively. The stratigraphy of the coal mine is shown in Fig. 1. In the process of roadway advancing, a coal and gas outburst occurred in No. 2 coal seam. No. 4 coal seam has a low outburst risk since the gas pressure and gas content is low. Because No. 4 coal seam is close to No. 2 coal seam, No. 4 coal seam was chosen as the first mining seam. The pressure-relief gas extraction technical model is shown in Fig. 2.
3. Numerical simulation by FLAC3D 3.1. Geometric model According to the geological conditions of No. 2 and No. 4 coal seams, the average spacing between No. 2 coal seam and No. 4 coal seam is 35 m, and the buried depth is approximately 800 m. In order to not influence calculation results, the advancing length of the working face was simplified for the model by taking No. 4 coal seam as the lower protective layer of No. 2 coal seam. The size of the whole model is 210 m (length)150 m (width) 100 m (height). The geometry includes 47,880 units and 51 987 nodes. As shown in Fig. 3, the x-direction is the incline direction, the y-direction is the strike direction and the z-direction is the gravity direction of the coal seam. 3.2. Boundary conditions and parameters of the numerical model The boundary conditions around the model are fixed, except for the upper boundary which is not fixed. A compressive stress of 18.716 MPa was imposed on top of the model. The maximal main stress is the vertical stress and the minimal main stress is along the x-direction. The minimal main stress is half the magnitude of the maximal main stress. The middle main stress is along the y-direction, and it is 60% of the maximal main stress. Mohr-Coulomb failure criterion was used to determine the deformation of the rock body.
Fig. 2. Pressure-relief gas extraction technical model.
Fig. 3. Mesh dissection model.
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C. Xiaocun, T. Hui / International Journal of Mining Science and Technology xxx (2018) xxx–xxx Table 1 Rock physical properties and mechanical parameters of the strata model. Strata index
Bulk modulus (GPa)
Shear modulus (GPa)
Apparent density (kg/m)
Friction angle (°)
Cohesion (MPa)
Tensile strength (MPa)
Siltstone 3 Coal Siltstone 2 Finestone Siltstone 1 Shale Limestone Mudstone
5.0 2.3 4.5 6.0 3.9 5.0 7.5 4.0
3.8 1.5 3.1 4.5 3.0 3.8 5.5 3.0
2400 1430 2400 2500 2450 2610 2710 2400
28 28 28 31 30 30 36 28
2.0 3.0 2.1 4.2 2.0 2.0 4.5 3.0
1.8 1.0 2.0 7.0 1.7 1.7 10.0 1.0
1 þ sin u f s ¼ r1 r3 2C 1 sin u
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 þ sin u 1 sin u
ð1Þ
where r1 is the maximal main stress and r3 is the minimal stress. The parameter C is cohesion and u is the angle of friction. When fs is larger than zero, the material of the model will be deformed. Physical properties of the rocks and mechanical parameters of the strata model are shown in Table 1. 4. Simulation result analysis As the working face of No. 4 advanced, the stress and displacement fields changed continuously. The vertical stress contour plots of the mining face along the strike and incline directions are shown in Figs. 4 and 5 when the working face advanced 40 m and 80 m. 4.1. Distribution of stress field As shown in Fig. 4b, the pressure-relief scope of No. 2 coal seam covered the whole gob when the working face advanced 80 m and the vertical stress was 5–10 MPa. The mining-induced stress relief effect was noticeable because of No. 4 coal seam mining effect. Conclusions can be drawn based to Fig. 4 and the stress nephograms of other mining processes. When the working face advanced, its first weight appeared and the roof of the gob began to cave. Then, periodic caving of the roof developed and three vertical zones were formed in the roof of the gob. When the working face advanced 60 m, the middle coal vertical stress in the pressure-relief zone was minimized (6 MPa) and less than the original vertical stress (19 MPa). The noticeable reduction
in vertical stress indicated that the effects of stress-relief were prominent. When the coal mining of No. 4 coal seam ended, there were separate fractures in the caving zone on top of the gob. No. 2 coal seam lies in the pressure-relief zone of the fractured zone of No. 4 coal seam gob, which was very helpful for gas extraction from No. 2 coal seam.
4.2. Distribution of displacement fields According to Fig. 5b, when the working face advanced 80 m, the maximum vertical displacement at the top of the gob was 375 mm. The vertical displacement at the top of the gob in No. 2 coal seam was 250–300 mm, which was far greater than the original vertical displacement of No. 2 coal seam. In addition, the maximum vertical displacement along the strike direction at the top of the gob was 336 mm. The vertical displacement at the top of the gob in No. 2 coal seam was 250–300 mm, which was also far greater than the original vertical displacement of No. 2 coal seam. As the distance from the top of No. 4 coal seam increased, in the perpendicular direction, the vertical displacement of the coal and rock decreased gradually, whereas, the vertical displacement increased as this working face advanced. In the process of coal mining, three zones are created: caving zone, fractured zone and bending subsidence zone. The height of the caving zone was approximately 5 times greater than the mining height at the working face, and the height of fractured zone was approximately 40 times greater than the mining height at the working face. The top roof was in the bending subsidence zone. When the working face advanced 80 m, the slip angle of rock strata was approximately 56°.
Fig. 4. Vertical stress contour plots of the mining face along the strike and incline directions: 40 m advance (along the strike direction) (a), 80 m advance (along the strike direction) (b), 40 m advance (along the incline direction) (c) and 80 m advance (along the incline direction) (d).
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Fig. 5. Vertical displacement contour plots of the mining face along the strike and incline directions: 40 m advance (along the strike direction) (a), 80 m advance (along the strike direction) (b), 40 m advance (along the incline direction) (c) and 80 m advance (along the incline direction) (d).
4.3. Gas movement characteristics under mining-induced fissure conditions Based on analysis of vertical stress characteristics and movement along the strike direction of working face, it was determined that the velocity of gas transport to the inner part of coal seam was increased. This finding was observed because fracture development is beneficial when the coal seam is influenced by coal mining, as it provides more channels for gas movement and gas moves from areas with high pressure to inner volumes of lower pressure. When coal mining of the working face reached 40 m, the travelling velocity of gas on the working face created a flat distribution for gas near the coal seam. The permeability of the coal mass increased greatly, which was beneficial for pressure-relief gas extraction.
mining seam of working face 8463 belongs to No. 4 coal seam of the Taiyuan formation of Carboniferous. The average mining depth was 1.2 m, the strike length of the working face was 1100 m and the incline length of the face was 150 m. The average distance of working face 8463 and working face 8263 was 35 m. A reasonable location layout of the drilling field and construction drills was chosen. No. 1 drilling field was located at the starting line. No. 8 and No. 9 drilling fields were located at the stopping line, and they were used to obtain the pressure relief scope of No. 2 coal seam along the strike direction. No. 2, No. 3, No. 4 and No. 5 drilling fields were located in the scraper conveyor roadway and haulage gateway. They were used to determine the pressure-relief scope of No. 2 coal seam along the incline direction. No. 6 and No. 7 drilling fields were used to obtain the expansion deformation and runoff of No. 2 coal seam. Section sketches of drilling fields are shown in Fig. 6.
5. Field investigations (1) Gas pressure changes induced by mining 5.1. Exploration of pressure relief scope To obtain the pressure relief scope, during mining, from coal seam No. 4 to No. 2, working face 8463 was selected. The main
As shown in Fig. 7a, it was determined that when the location of drills 1#–5# in No. 1 drilling field were not influenced by mining, gas pressure was stable at 1.1–1.2 MPa. When coal mining of the
Fig. 6. Profile map of boreholes along the coal seam strike (a) and (b); profile map of holes along the steep coal seam direction (c) and holes for measuring gas flow and gas seepage coefficient (d).
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Fig. 7. Gas pressure change of No. 2 coal seam with mining of No. 4 coal seam.
working face reached 45 m, gas pressure began to decrease. Gas pressure reduced to zero when coal mining of the working face reached 35.4 m. This was because deformation of the overlying strata deformed the drills and gas pressure decreased significantly since the boreholes were connected to the gob. When the distance between the coal mining line and No. 1 drilling field was 5 m, the gas pressure of drills 4# and 5# began to increase. This was because fractures in the overlying strata upon the gob were compressed and enclosed, allowing gas pressure to rise. When gas pressure was stable, the gas pressure of drills 3#, 4# and 5# was less than 0.74 MPa (the critical value) indicating that the gas pressure of drills 3#, 4# and 5# in No. 2 coal seam was relieved effectively. The pressure-relief angle was 60° along the strike direction of the starting line. As shown in Fig. 7b, it was determined that residual gas pressures of drills 17#, 18#, 19#, 20# in No. 8 drilling field were 0.5, 0.55, 0.55 and 0.85 MPa, respectively. Gas pressure of the other drills was less than 0.74 MPa (the critical value), except for drill 20#. The pressure-relief angle was 60° in the strike direction of the stopping line. As shown in Fig. 7c, only the residual gas pressure of drill 6# was larger than 0.74 MPa. This finding demonstrates that the relevant regions of No. 2 coal seam have been fully pressure-relieved. It was determined that the upper pressure-relief angle from coal seam No. 4 to No. 2 was 84° in the incline direction. Likewise, it was determined that the below pressure-relief angle was 76° in the incline direction from coal seam No. 4 to No. 2 in No. 5 drilling field.
(2) Study on deformation of expansion Influenced by the mining of No. 4 coal seam, the stress of the overlying strata and No. 2 coal seam was redistributed. For No. 2 coal seam, the deformation of the roof and floor was divided into five stages: ① compression, ② relief of compression, ③ expansion, ④ reduction of expansion and ⑤ stability of expansion. Meanwhile, five areas were formed: ① pressure-increasing area, ② pressure-relief and expansion area, ③ area with stable expansion and pressure-relief, ④ pressure-relief and expansion area, and ⑤ pressure-increasing area. Drills 15# and 150 # of No. 6 and No. 7 drilling fields were studied. As shown in Fig. 8, the maximum compression in the upper part of No. 2 coal seam was 29.4 mm, and the compression rate was 0.49%. The maximum compression in the lower part of No. 2 coal seam was 30.3 mm, and the compression rate was 0.51%. The maximum expansion in the upper part of No. 2 coal seam was 155.0 mm, and the expansion rate was 2.58%. The maximum expansion in the lower part of No. 2 coal seam was 145.5 mm, and the expansion rate was 2.42% (refer to Fig. 8). (3) Determination of pressure-relief scope and protection After analysing observational results of gas pressure in each drilling field and gas pressure of the coal seam, it was found that the upper pressure-relief angle from coal seam No. 4 to No. 2 in the incline direction, d2, was 84° and the low pressure-relief angle,
Fig. 8. Expansion and deformation of No. 2 coal seam with mining of No. 4 coal seam (positive for compression; negative for expansion).
Please cite this article in press as: Xiaocun C, Hui T. Technical scheme and application of pressure-relief gas extraction in multi-coal seam mining region. Int J Min Sci Technol (2018), https://doi.org/10.1016/j.ijmst.2018.03.010
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Fig. 9. Stress relief and protected zone of No. 2 coal seam in the incline direction (a) and those of the No. 2 coal seam in the strike direction (b).
Table 2 Gas drainage effect of working face 8263. Parameter
Drilling quantity (m/t)
Quantity of drainage (104 m3)
Finishing time (day)
Before After Decline
0.8202 0.7041 0.1161
347.56 303.17 44.39
44.3 30.8 13.5
d1, was 76°. The pressure-relief angle from the stopping line of coal seam No. 4 to coal seam No. 2 in the strike direction, d3, was 60°, as shown in Fig. 9. 5.2. Effect of gas extraction The mining of working face 8463 in No. 4 coal seam, relieved the pressure of working face 8263 in No. 2 coal seam. The drilling load was reduced by 0.116 1 m/t raw coal and the time of gas extraction was reduced by 13.5 days, indicating the positive effects of mining coal and gas simultaneously. The observational results are listed in Table 2. Therefore, with protective mining of No. 4 coal seam, the pressure-relief effectiveness for low protective seam was verified over a large distance by observing gas pressure and gas expansion rates of No. 2 protected coal seam. Pressure-relief angles of No. 4 coal seam, in the strike and incline directions, were determined. The effects of protective seam mining were positive. As the working face advanced, gas pressure was reduced to the critical value. The effects of protective seam mining and gas extraction under the coal seam group were verified over a large distance, meeting the characteristics of simultaneous exploitation of coal and gas. 6. Conclusions The theory of gas extraction technology was studied under the conditions of a mining-induced fissure. This study needed to coordinate time and space parameters to devise a reasonable mining process and effective drainage method. A typical gas extraction mode under coal seam group mining conditions of a mining area was proposed. (1) Using FLAC3D, the vertical displacement contour plots of No. 4 mining face along the strike and incline directions were obtained. The redistribution of stress and displacement were analysed. The gas pressure in No. 2 coal seam was reduced to 0.55 MPa in the pressure-relief region and the maximum expansion ratio reached 25.8. The danger of gas outbursts was thoroughly eliminated by increasing the expansion rate and pressure-relief angle and decreasing gas pressure in No. 2 coal seam. (2) Practical field applications have shown that the upper pressure-relief angle, d1, from coal seam No. 4 to No. 2 was 76°. The upper pressure-relief angle, d2, was 84° and the pressure-relief angle in the strike direction, d3, was 60°. In these pressure-relief protection regions, the gas extraction
effectiveness of working face 8263 was noteworthy and the gas extraction time reduced by 13.5 days. This investigation validated the effectiveness of simultaneous coal and gas exploitation under coal seam group mining conditions.
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