- Email: [email protected]
Application of pressure relief and permeability increased by slotting a coal seam with a rotary type cutter working across rock layers
International Journal of Mining Science and Technology 22 (2012) 533–538
Contents lists available at SciVerse ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Application of pressure relief and permeability increased by slotting a coal seam with a rotary type cutter working across rock layers Shen Chunming ⇑, Lin Baiquan, Meng Fanwei, Zhang Qizhi, Zhai Cheng State Key Laboratory of Coal Resources and Mine Safety, China University of Mining & Technology, Xuzhou 221008, China School of Safety Engineering, China University of Mining & Technology, Xuzhou 221008, China
a r t i c l e
i n f o
Article history: Received 19 November 2011 Received in revised form 28 December 2011 Accepted 25 January 2012 Available online 19 July 2012 Keywords: Penetrate layer Slotted Pressure relief and permeability increase Permeability Gas extraction
a b s t r a c t Pressure relief to increase permeability significantly improves gas extraction efficiency from coal seams. In this paper we report results from simulations using FLAC3D code to analyze changes in coal displacement and stress after special drill slots were formed. We investigated the mechanism of pressure relief and permeability increase in a high-gas and low-permeability coal seam through the modeling of gas flow. This allows the development of the technology. Slotting across rock layers in the coal seam with a rotary type cutter was then applied in the field. The results show that pressure relief and permeability increases from slotting the coal seam can increase the transport and the fracture of the coal. This expands the range of pressure relief from the drilling and increases the exposed area of the seam. The total quantity of gas extracted from slotted bore holes was three times that seen with ordinary drilling. The concentration of gas extracted from the slotted drills was from two to three times that seen using ordinary drills. The gas flow was stable at 80%. Improved permeability and more efficient gas extraction are the result of the slotting. The roadway development rate is increased by 30–50% after gas drainage. This technology diminishes the lag between longwall production and roadway development and effectively prevents coal and gas outburst, which offers the prospect of broad application. Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction One of the currently used methods for solving problems of coal gas extraction is the ‘‘pressure relief to increase permeability’’ method. A relief inter-layer and a technology that increases permeability are applied. These are protective layer mining and the technology of pressure relief in layers [1]. The former technique is more practical for wide application. It can effectively increase the permeability of a coal seam and the efficiency of gas extraction [2–5]. It has recently been rapidly developed in China. By now pressure relief to increase permeability has been achieved by: drillings; high-energy liquid disturbance; and gas detonation disturbance [6–17]. These actions increase the permeability of a coal seam and gas extraction rate to some extent and promote mine safety. A high-gas and low-permeability single coal seam may effectively have the gas extracted by pressure relief in layers and by increasing the permeability. This is achieved by artificially increasing fractures in the coal to expand the range of pressure relief. High pressure water jet cutting technology will disturb and discharge coal from the bored hole to form a pressure relief space within the coal seam that can eliminate stress concentrations around the
⇑ Corresponding author. Tel.: +86 18805210918. E-mail address: [email protected] (C. Shen).
drilling [18]. This expands fractures in the coal and extends the range of pressure relief and, therefore, improves gas extraction rates. Using this mechanism in a coal seam we developed a technology whereby slotting across rock layers using a rotary slicing type machine solved the problem of gas extraction from a lowpermeability single coal seam. This significantly promotes mine safety and improves mine productivity.
2. Experimental 2.1. Mechanism of slotting in a coal seam 2.1.1. Pressure relief in the slotted coal Permeability is the important indicator that determines the difficulty of gas flow within coal. It is the main factor that affects the gas extraction rate. Permeability is mainly effected by gas pressure and ground stress and it changes as gas is extracted from the coal seam [19,20]. Therefore, an effective way of improving permeability of the coal is the reduction of the stresses within the coal. Stresses within the range of gas extraction, where pressure relief occurs, must be reduced and release of the internal energy of the gas will follow. Studying changes in pressure relief within slotted coal seams involved the construction of a three dimensional numerical model
2095-2686/$ - see front matter Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2012.01.016
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C. Shen et al. / International Journal of Mining Science and Technology 22 (2012) 533–538
Table 1 Parameters and boundary conditions of the multi-slot model for estimating pressure relief. Parameter
Value
Model size
Parallel to the slot: length 40 m; width 8 m; and height 30 m Cross ways to the slot: length 40 m; width 33 m; and height 0 m Length 20 m, width 1.4 m, and height 0.01 m 5m 0° 0° Surface force load on the top of the model: 10 MPa; model surrounded by a rolling support boundary 10 MPa
Slot size Slot distance Model dip Slot dip Model boundary conditions Initial stress
for the simulation of pressure relief. The FLAC3D software code was used to this end [21]. Table 1 shows the model parameters and the boundary conditions. Fig. 1 shows the geometric characteristics of the model. The effects from the model boundary on pressure relief within the slotted coal seam were minimized by choosing a boundary size far greater than the seam slots. The slots were designed to be located in the center of the model. Fig. 2 shows the stress changes in the coal. It can be seen from the figure that the coal seam shows a figure eight shaped pressure relief area in the vertical cross section. The influence of gravity makes the pressure relief area on the upper part of the slots larger than on the lower part. The figure eight shaped relief areas intersect and influence each other. The stress contour is deformed and forms a relief area throughout the region. Positions closer to the slots show larger stress changes and less influence from the adjacent slots. Away from the slots the relief and influence from each drill becomes superimposed to formed an overall relief area. Fig. 3 shows that for similar heights above the slot the maximum and minimum stresses with three slots is almost the same as with a single slot. The displacement of coal is significantly different however. This phenomenon is mainly from the increased space that more slots provide for coal displacement. The stress distribution becomes relatively more complex due to the significant coal displacement. This analysis suggests the effects from multiple slotting at the same location will relieve pressure from the entire coal seam and lay the foundation of efficient gas extraction. 2.1.2. A mathematical model of gas flow within the slotted coal Gas flow within the coal is a process of diffusion and permeation. Gas flow within the range of pressure release can be assumed to be capillary flow where gas migrates from coal seam to the slot. The gas flow capillary model follows Fick’s diffusion law and Darcy’s permeability law. These laws divide the pressure relief range in the slotted coal seam into a main influence area and a
(a) A three dimensional map of the model grid
×106 -18.0~-16.0 -16.0~-14.0 -14.0~-12.0 -12.0~-10.0 -10.0~-8.00 -8.00~-6.00 -6.00~-4.00 -4.00~-2.00 -2.00~0 0~0.20486 Fig. 2. Stress distribution: a vertical cross section.
boundary influence area. Boundary conditions are established to obtain the gas flow from the main influence area, and the boundary influence area, toward the slot as a function of time [1]. This is shown in Fig. 4. The gas flow in the capillary micro-channels of any micro-element were calculated as a function of time over the main influence area. This is denoted as qM(t): 2 2 h 2b l h þ 2b 4k2 l pk41 p0 qM ðtÞ ¼ e 2 l 128l h 2b 2 2 2 2k lp Dt 2k l9p Dt 2k l25p Dt 2 þ 2c1 Dk1 1:8519e 2 l2 þ0:5583e 2 l2 þ 0:3268e 2 l2 ð1Þ
The gas flow from the boundary affected area toward the slot is denoted as qB(t): qB ðtÞ ¼
pp0 k41 R2 ðl RÞ2 4k2 l e l 128lR2
2 2 2 2k lp Dt 2k l9p Dt 2k l25p Dt 2 þ 2c1 Dk1 1:8519e 2 l2 þ 0:5583e 2 l2 þ 0:3268e 2 l2 ð2Þ
where k1, k2, and c1 are coefficients greater than 0; l and h the geometric parameters of the capillary, m; D the diffusion coefficient of gas in the porous media; b the slot width, m; and l the dynamic viscosity of the gas, Pa s. 2.1.3. An analysis of coal outburst within the pressure relief range The gas flow as a function of time within the pressure relief range of the slotted coal seam can be estimated from the mathematical gas flow model. A criterion for eliminating gas outburst from any micro-unit, and the time to eliminate the outburst, within the pressure relief range of a slotted coal seam has been obtained [1]. The criterion for eliminating the outburst is:
qremain ¼ q0
103
Rt 0
qdt
3 0 l0
q
6 8m3 =t
(b) A plan view and the gridding of the slots
Fig. 1. Computational model of three parallel slots.
ð3Þ
535
σ (MPa)
C. Shen et al. / International Journal of Mining Science and Technology 22 (2012) 533–538
Minimum stress with a single slot
16 14 12 10 8 6 4 2 0
Maximum stress with three slots
Minimum stress with a single slot Minimum stress with three slots
0.5
2.0 1.5 H (m)
1.0
2.5
3.0
Maximum displacement with a single slot Maximum displacement with three slots
7 6 5 4 3 2 1 0
1
2
3
4
H (m)
Fig. 3. Comparison of pressure relief for a single slot or three parallel slots.
h
extraction bore. Because the stress concentration is much higher than the original rock stress the coal seam permeability is reduced. This hinders gas diffusion and reduces the effective range of influence from the drilling. A high-gas pressure also exists around this stress concentration area, which has a detrimental influence on the gas extraction efficiency. The pressure reduction to increase permeability requires slotting in the coal seam with a rotary-slice type cutter across the rock layers as is shown in Fig. 5. The process requires:
Main influence area b
Boundary affected area
R
Boundary affected area
a Fig. 4. A cross section of the pressure relief range in a slotted coal seam.
Eqs. (1) and (2) give qz(t) and qB(t), which may be substituted into Eq. (3). The time to eliminate the outburst between the main influence area and the boundary affected area then give the total time required to eliminate the outburst in the entire pressure relief range within the slotted coal seam:
t ¼ maxðt 1 ; t 2 Þ
ð4Þ
where t1 is the time to eliminate outburst within each micro-unit of the main influence area; and t2 the time to eliminate the outburst within each micro-unit of the boundary affected area. Because q0 is measurable and q0 and l0 are known the time to eliminate outburst within each micro-unit of the entire pressure relief range within the slotted coal seam can be calculated. 2.2. Slotting a coal seam with a rotary-slice type cutter across rock layers The influence of gas pressure, ground stress, and other factors, forms a ring shaped stress concentration area around the completed
Area of plastic deformation
– Drilling a hole through the seam; – Using a rotating high pressure water jet slotting tool in the coal seam after backing the drill a certain distance. This forms an irregular, flat disk-shaped space with a radius of 500 mm and a certain width concentric with the drill; – Repeating the cycle of operation until the bottom of the bore and then washing the slotted coal from the bore using high pressure water. This way a series of flat slots within the coal seam are formed. These help eliminate the influence of stress concentration around the bore and overcome the typical defects seen with gas extraction by drilling. This slotting technology forms a series of flat slots that increases the exposed area of the coal seam. The volume of disturbed coal within the seam is also increased so the gas within the influenced range of the slotted surface is rapidly and completely extracted. The effect of the gas gradient force and ground stress causes the coal around the slots to move into the slots, which produces a large number of tensile and shear cracks. Pressure relief increases because of the increased permeability. Macro-slots and secondary cracks provide a path for gas flow that creates favorable conditions for gas release. The range of gas extraction and pressure
Interface of coal and rock
Jet slotted area Rg
Ro Drill
Drill Rp
Flat slot
(a) A profile drawing of the slotted coal
Direction of gas flow
(b) A plan drawing of the slotted coal
Fig. 5. A diagram of rotary-slice type slotting.
C. Shen et al. / International Journal of Mining Science and Technology 22 (2012) 533–538
Conveyer roadway
5m
15 m
5m
5 m 5 m5 m
15 m
Coalseam
Slotted drill across layer
20 m
(a) Drilling design across the slotted layer
100 m Slotting ahead of the face 100 m
8.7 m 8.7 m 8.7 m 8.7 m
536
Investigated slotted drill
Coal face caproic 16-17 12041 conveyer roadway
(b) Final drill positions across the slotted layer
Fig. 6. Drill layout across the slotted layer: tunnel of the 12041 conveyer roadway.
Table 2 Slotted drilling parameters. Drill No.
Drilled height (m)
Horizontal angle (°)
Elevation (°)
Position of coal found (m)
Coal seam thickness (m)
Drill length (m)
1# 2# 3# 4# 5#
1.7 1.9 2.1 2.3 2.5
0 0 0 0 0
27 31 38 50 76
18.3 14.3 10.6 7.4 4.9
23.4 19.1 14.8 11.6 7.5
41.7 33.4 25.4 18.0 12.4
relief is expanded thereby. This helps to reduce the gas extraction time. In addition, the flat slot lets the surrounding coal crack, move, and expand so, to some extent, potential outburst power is released. This reduces the danger of coal outburst effectively.
improve the extraction rate and to release stress concentration ahead of the roadway heading. The layout, input parameters, and extraction design of the experimental drillings are: A group of five bore holes was constructed every 5 m along the roadway. In total, 18 groups of bore holes were constructed, where the first 12 groups were slotted pressure relief drillings and the other six groups were ordinary extraction drillings. The location of each drill at the roof of coal seam 16#–17#, and within 15 m at both sides of the coal-roadway within the upper coal seam, is shown in Fig. 6. The extraction results of the 2# and 4# drilling in each group were investigated over an extraction time period no less than 30 days. The location of the drills was 100 m ahead of the developing face and the drills were rigorously plugged during the gas extraction process. The parameters of the slotted drillings are shown in Table 2. 3. Results and discussion
2.3. Field application
3.1. Analysis and contrast of gas extraction flow
The face number 12041 of the Pingbao Coal Mine Co. Ltd., was chosen as the site of a field application. The original gas pressure of this face was 1.38 MPa; the gas content was 10.46 m3/t. The head road of the face had been driven 400 m and the tail road had been driven 450 m. Problems caused by high-gas concentration, drill binding, and drill spraying arose during the development process. The advance rate was slow, which seriously hampered mining and development. The problem was solved by considering the actual situation and trying to increase the coal permeability by slotting the coal seam across the bottom layer. This was thought to
The statistical requirements required the use of mean values for each parameter, ignoring outlying values, when investigating the extraction parameters of the slotted and ordinary drills. For simplicity and universality the values of the 2# and 4# drill within each group were analyzed. Fig. 7 shows the contrast of the gas extraction flow at the 2# and 4# bore within each group of drillings. Fig. 7 shows that slotting can increase the rate and amount of gas extracted substantially. The biggest capacity of a single bore was a pure gas extraction of 54.45 L/min. The capacity extracted
50
30 20
30 20 10
10 0
4# ordinary 4# slotted
40 Q (L/min)
40 Q (L/min)
50
2# ordinary 2# slotted
0
4
8
12
16 20 t (d)
24
28
32
0
4
8
12
16 20 t (d)
Fig. 7. Contrasting gas extraction flow between slotted and un-slotted bores.
24
28
32
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C. Shen et al. / International Journal of Mining Science and Technology 22 (2012) 533–538
100
5 30 m before slotted area
80
30 m after slotted area
4
70
2# slotted 4# slotted Ordinary
60 50
L (m)
Gas concentration (%)
90
3
40 30
60 m in slotted area
2
20 10
0
5
10
15 20 t (d)
25
30
35
Fig. 8. Gas extraction concentration from slotted or ordinary drills versus time.
4
8
12 16 20 24 28 32 36 40 44 t (d)
Fig. 10. Advance rate of the roadway near and in the slotted area.
slotted drills is 17 times bigger than that from ordinary drills, which means more space and exposed surface area for gas release. The analysis of Figs. 8 and 9 shows that high pressure water jet slotting removes coal from the area of the drill, which increases the exposed coal area and offers some transport and relief space. The slots remove stress concentration around the drill and expand the effective sphere of influence of a single drill. The number of gas release channels are increased so that gas extraction remains at a high concentration for a long time.
1.2
V (m3)
0
0.8
0.4
3.3. Roadway advance rate
0
Ordinary drill
Slotted drill
Fig. 9. Extracted coal quantity: slotted versus ordinary drilling.
from the 4# slotted drill was 3.7 times bigger than that from the 4# ordinary drill. The total capacity extracted from the slotted drills was three times that of the ordinary drills over a 30 day period. The first four to six days during the use of the new method saw a low range in extraction flow. Then the flow value rose and remained high for a long time period. Why might this be? The process of high pressure water jet slotting is also a process of pumping water into the coal. This reduces the permeability of coal and slows gas desorption. Therefore, gas extraction is inhibited. This is called the ‘‘water lock’’ effect and caused the low initial values of extraction [22]. As the water evaporated and left the coal, as time passed, the ‘‘water lock’’ effect is reduced. Additionally, the stress is redistributed because of the effect of stress and gas pressure change after the slotting. This produced a large number of coal fractures and released the gas energy, which increased the exposed area of the coal as well as the volume of disturbed coal. This is why the desorption rate and the extraction flow stay high. 3.2. An analysis of extraction concentration and range The gas extraction rate using slotted or ordinary drills was studied. Fig. 8 shows the statistics of discharged coal amount per single drill, considering both slotted and ordinary drills, against time. Fig. 9 shows the mean values. Fig. 8 shows that the average gas extraction from ordinary drillings is below 30 percent and is relatively unstable with large fluctuations. The gas extraction from the slotted drills is around 80 percent and the flow is stable. This is from one to two times more gas than that seen from of ordinary drilling. Fig. 9 shows the average amount of discharged coal per single bore. The discharge from
After extracting the gas by slotting the coal seam with a rotaryslice type cutter across the rock layer the head gate was driven and two efficiency indexes were investigated. These were the q value of the initial gas inrush speed and the s value of the discharged coal volume. These two indexes dropped significantly, which shows reduced risk of coal outburst and allows an increased speed in the head gate development, as shown in Fig. 10. Fig. 10 shows that the area without slotting had an advance rate of about 1.6–3.2 m per day, or 60–80 m per month. The rate was limited by the large amount of gas emission. In the slotted area the efficiency of gas extraction reduced the gas concentration in the coal seam and the advance rate was from 3.2 to 4.8 m per day, or 100 to 120 m monthly. The effect of outburst elimination is obvious. 4. Conclusions (1) Pressure release and increased permeability obtained by slotting a coal seam with a rotary-slice cutter across rock layers is an effective way to relieve the gas pressure in a single low-permeability coal seam. This method reduces the stress concentration around the drill, enlarges the displacement space and the fracture field in the coal, and forms a pressure relieved area within the coal that increases coal seam gas permeability. (2) Pressure release and permeability increase from slotting increase the exposed coal area, improves the permeability of the coal seam, and increases the gas extraction rate. Over 30 days the total quantity of gas extracted from the slotted bores is three times that of the amount from ordinary bores. The concentration of gas extracted from the slotted drills is two to three times that of the ordinary drill and the gas concentration is stable at 80 percent. (3) This technology proved practical in an application where the tension between mining rate and engineering development was addressed. The development rate increased by 30–50%
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as the monthly advance rate increased from 60 to 80 m to the higher range of 100 to 120 m. During head gate development efficiency indexes q and s decreased indicating that the risk of coal and gas outburst decreased significantly.
Acknowledgments The authors would like to thank Professor Lin Baiquan for the supervision of the paper writing; thank for financial supports provided by the National Key Basic Research and Development Program (No.2011CB201205), the National Natural Science Foundation of China (No.50534090), the Independent Research of State Key Laboratory of Coal Resources and Mine Safety (No.SKLCRSM08X03), the State Key Laboratory of Coal Resources and Mine Safety of Research Foundation of China University of Mining & Technology (No.09KF09) and the National Natural Science Foundation of Youth Science Foundation (No.50804048), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (CXZZ12_0958) and thank Pingbao Coal Ming Co. Ltd. for the help and support on field application and test of the technology.
References [1] Lin BQ, Meng FW, Zhang HB. Regional gas control based on drilling-slottingextracting integration technology. J China Coal Soc 2011;36(1):75–80. [2] Yu QX, Cheng YP, Jiang CL, Zhou SN. Principles and applications of exploitation of coal and pressure relief gas in thick and high-gas seams. J China Univ Min Technol 2004;33(2):127–31. [3] Cheng YP, Yu QX. Application of safe and high-efficient exploitation system of coal and gas in coal seams. J China Univ Min Technol 2003;32(5):471–5. [4] Fan XG, Wang HT, Hu GZ, Li XH, Yuan ZG. Pressure-relief scope for the exploiting of steep-inclined oblique under-protecting strata. J China Univ Min Technol 2010;39(3):380–5.
[5] Zhang YJ, Yu GM, Lu SB, Li L, Yang DF. Numerical analysis of gas flow law with short distance upper protective layer gas extraction. Rock Soil Mech 2010;31(1):398–404. [6] Yu QX. Mine gas prevention and control. Xuzhou: China University of Mining and Technology Press; 1992. [7] Yu CL. Methods to improve the volume of gas extraction from drills in low permeability coal seam. Saf Coal Mines 1999;5:35–6. [8] Li XH, Lu YY, Xiang WY. The theory of water jet and application in mining engineering. Chongqing: Chongqing University Press; 2007. [9] Lin BQ, Lv YC, Li BY, Zhai C. High-pressure abrasive hydraulic cutting seam technology and its application in outbursts prevention. J China Coal Soc 2007;32(9):959–63. [10] Chang ZX, Xi BP, Zhao YS, Zhao LM. Mechanical of breaking coal by water jet. J China Coal Soc 2008;33(9):983–7. [11] Ni XM, Su XB, Li YK. Study of the key technologies of the hydraulic fracturing used in multi-layer coal seam. J China Univ Min Technol 2010;39(5):728–32. [12] Liu MJ, Kong LA, He FC, Xin XP, Wei GY, Liu Y. Application of hydraulic flushing technology in severe outburst coal. J China Coal Soc 2005;30(4):451–4. [13] Zhang YH, Ni W, Yin GC, Song JG. Study on improving the penetrability of coal seam with the water pressure blasting in the through beds hole. J China Coal Soc 2004;29(3):298–302. [14] Nie BS, He XQ, Wang EY, Zhang L. Micro-mechanism of coal adsorbing water. J China Univ Min Technol 2004;33(4):279–383. [15] Guo ZH. Research on hydraulic cutting technology for seam gateway with coal and gas outburst in potential. Coal Sci Technol 2005;33(9):50–3. [16] Cai F, Liu ZG, Zhang CJ, Lin BQ. Numerical simulation of improving permeability by deep-hole presplitting explosion in loose-soft and low permeability coal seam. J China Coal Soc 2007;32(5):499–503. [17] Gong M, Huang YH, Wang DS, Li DH. Numerical simulation on mechanical characteristics of deep-hole presplitting blasting in soft coal bed. Chin J Rock Mech Eng 2008;27(8):1674–81. [18] Lin BQ, Yang W, Wu HJ, Meng FW, Zhao YX, Zhai C. A numeric analysis of the effects different factors have on slotted drilling. J China Univ Min Technol 2010;39(2):153–7. [19] Lin BQ, He XQ. Permeability of coal seam and effect in gas and coal outburst. Coal Sci Technol 1991;4:50–3. [20] Zhou SN, Lin BQ. The theory of gas flow and storage in coal seams. Beijing: China Coal Industry Publishing House; 1999. [21] Wu HJ. The theory and technology study on pressure relief and permeability enhancements of the coal seam with high concentration of gas and low permeability. Xuzhou: China University of Mining and Technology; 2009. [22] Liao RQ, Xu YG, Hu XB. Damage to low-permeability reservoir by water locking effect and its inhibiting and removing methods. Nat Gas Ind 2002;22(6):87–9.
Contents lists available at SciVerse ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Application of pressure relief and permeability increased by slotting a coal seam with a rotary type cutter working across rock layers Shen Chunming ⇑, Lin Baiquan, Meng Fanwei, Zhang Qizhi, Zhai Cheng State Key Laboratory of Coal Resources and Mine Safety, China University of Mining & Technology, Xuzhou 221008, China School of Safety Engineering, China University of Mining & Technology, Xuzhou 221008, China
a r t i c l e
i n f o
Article history: Received 19 November 2011 Received in revised form 28 December 2011 Accepted 25 January 2012 Available online 19 July 2012 Keywords: Penetrate layer Slotted Pressure relief and permeability increase Permeability Gas extraction
a b s t r a c t Pressure relief to increase permeability significantly improves gas extraction efficiency from coal seams. In this paper we report results from simulations using FLAC3D code to analyze changes in coal displacement and stress after special drill slots were formed. We investigated the mechanism of pressure relief and permeability increase in a high-gas and low-permeability coal seam through the modeling of gas flow. This allows the development of the technology. Slotting across rock layers in the coal seam with a rotary type cutter was then applied in the field. The results show that pressure relief and permeability increases from slotting the coal seam can increase the transport and the fracture of the coal. This expands the range of pressure relief from the drilling and increases the exposed area of the seam. The total quantity of gas extracted from slotted bore holes was three times that seen with ordinary drilling. The concentration of gas extracted from the slotted drills was from two to three times that seen using ordinary drills. The gas flow was stable at 80%. Improved permeability and more efficient gas extraction are the result of the slotting. The roadway development rate is increased by 30–50% after gas drainage. This technology diminishes the lag between longwall production and roadway development and effectively prevents coal and gas outburst, which offers the prospect of broad application. Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction One of the currently used methods for solving problems of coal gas extraction is the ‘‘pressure relief to increase permeability’’ method. A relief inter-layer and a technology that increases permeability are applied. These are protective layer mining and the technology of pressure relief in layers [1]. The former technique is more practical for wide application. It can effectively increase the permeability of a coal seam and the efficiency of gas extraction [2–5]. It has recently been rapidly developed in China. By now pressure relief to increase permeability has been achieved by: drillings; high-energy liquid disturbance; and gas detonation disturbance [6–17]. These actions increase the permeability of a coal seam and gas extraction rate to some extent and promote mine safety. A high-gas and low-permeability single coal seam may effectively have the gas extracted by pressure relief in layers and by increasing the permeability. This is achieved by artificially increasing fractures in the coal to expand the range of pressure relief. High pressure water jet cutting technology will disturb and discharge coal from the bored hole to form a pressure relief space within the coal seam that can eliminate stress concentrations around the
⇑ Corresponding author. Tel.: +86 18805210918. E-mail address: [email protected] (C. Shen).
drilling [18]. This expands fractures in the coal and extends the range of pressure relief and, therefore, improves gas extraction rates. Using this mechanism in a coal seam we developed a technology whereby slotting across rock layers using a rotary slicing type machine solved the problem of gas extraction from a lowpermeability single coal seam. This significantly promotes mine safety and improves mine productivity.
2. Experimental 2.1. Mechanism of slotting in a coal seam 2.1.1. Pressure relief in the slotted coal Permeability is the important indicator that determines the difficulty of gas flow within coal. It is the main factor that affects the gas extraction rate. Permeability is mainly effected by gas pressure and ground stress and it changes as gas is extracted from the coal seam [19,20]. Therefore, an effective way of improving permeability of the coal is the reduction of the stresses within the coal. Stresses within the range of gas extraction, where pressure relief occurs, must be reduced and release of the internal energy of the gas will follow. Studying changes in pressure relief within slotted coal seams involved the construction of a three dimensional numerical model
2095-2686/$ - see front matter Ó 2012 Published by Elsevier B.V. on behalf of China University of Mining & Technology. http://dx.doi.org/10.1016/j.ijmst.2012.01.016
534
C. Shen et al. / International Journal of Mining Science and Technology 22 (2012) 533–538
Table 1 Parameters and boundary conditions of the multi-slot model for estimating pressure relief. Parameter
Value
Model size
Parallel to the slot: length 40 m; width 8 m; and height 30 m Cross ways to the slot: length 40 m; width 33 m; and height 0 m Length 20 m, width 1.4 m, and height 0.01 m 5m 0° 0° Surface force load on the top of the model: 10 MPa; model surrounded by a rolling support boundary 10 MPa
Slot size Slot distance Model dip Slot dip Model boundary conditions Initial stress
for the simulation of pressure relief. The FLAC3D software code was used to this end [21]. Table 1 shows the model parameters and the boundary conditions. Fig. 1 shows the geometric characteristics of the model. The effects from the model boundary on pressure relief within the slotted coal seam were minimized by choosing a boundary size far greater than the seam slots. The slots were designed to be located in the center of the model. Fig. 2 shows the stress changes in the coal. It can be seen from the figure that the coal seam shows a figure eight shaped pressure relief area in the vertical cross section. The influence of gravity makes the pressure relief area on the upper part of the slots larger than on the lower part. The figure eight shaped relief areas intersect and influence each other. The stress contour is deformed and forms a relief area throughout the region. Positions closer to the slots show larger stress changes and less influence from the adjacent slots. Away from the slots the relief and influence from each drill becomes superimposed to formed an overall relief area. Fig. 3 shows that for similar heights above the slot the maximum and minimum stresses with three slots is almost the same as with a single slot. The displacement of coal is significantly different however. This phenomenon is mainly from the increased space that more slots provide for coal displacement. The stress distribution becomes relatively more complex due to the significant coal displacement. This analysis suggests the effects from multiple slotting at the same location will relieve pressure from the entire coal seam and lay the foundation of efficient gas extraction. 2.1.2. A mathematical model of gas flow within the slotted coal Gas flow within the coal is a process of diffusion and permeation. Gas flow within the range of pressure release can be assumed to be capillary flow where gas migrates from coal seam to the slot. The gas flow capillary model follows Fick’s diffusion law and Darcy’s permeability law. These laws divide the pressure relief range in the slotted coal seam into a main influence area and a
(a) A three dimensional map of the model grid
×106 -18.0~-16.0 -16.0~-14.0 -14.0~-12.0 -12.0~-10.0 -10.0~-8.00 -8.00~-6.00 -6.00~-4.00 -4.00~-2.00 -2.00~0 0~0.20486 Fig. 2. Stress distribution: a vertical cross section.
boundary influence area. Boundary conditions are established to obtain the gas flow from the main influence area, and the boundary influence area, toward the slot as a function of time [1]. This is shown in Fig. 4. The gas flow in the capillary micro-channels of any micro-element were calculated as a function of time over the main influence area. This is denoted as qM(t): 2 2 h 2b l h þ 2b 4k2 l pk41 p0 qM ðtÞ ¼ e 2 l 128l h 2b 2 2 2 2k lp Dt 2k l9p Dt 2k l25p Dt 2 þ 2c1 Dk1 1:8519e 2 l2 þ0:5583e 2 l2 þ 0:3268e 2 l2 ð1Þ
The gas flow from the boundary affected area toward the slot is denoted as qB(t): qB ðtÞ ¼
pp0 k41 R2 ðl RÞ2 4k2 l e l 128lR2
2 2 2 2k lp Dt 2k l9p Dt 2k l25p Dt 2 þ 2c1 Dk1 1:8519e 2 l2 þ 0:5583e 2 l2 þ 0:3268e 2 l2 ð2Þ
where k1, k2, and c1 are coefficients greater than 0; l and h the geometric parameters of the capillary, m; D the diffusion coefficient of gas in the porous media; b the slot width, m; and l the dynamic viscosity of the gas, Pa s. 2.1.3. An analysis of coal outburst within the pressure relief range The gas flow as a function of time within the pressure relief range of the slotted coal seam can be estimated from the mathematical gas flow model. A criterion for eliminating gas outburst from any micro-unit, and the time to eliminate the outburst, within the pressure relief range of a slotted coal seam has been obtained [1]. The criterion for eliminating the outburst is:
qremain ¼ q0
103
Rt 0
qdt
3 0 l0
q
6 8m3 =t
(b) A plan view and the gridding of the slots
Fig. 1. Computational model of three parallel slots.
ð3Þ
535
σ (MPa)
C. Shen et al. / International Journal of Mining Science and Technology 22 (2012) 533–538
Minimum stress with a single slot
16 14 12 10 8 6 4 2 0
Maximum stress with three slots
Minimum stress with a single slot Minimum stress with three slots
0.5
2.0 1.5 H (m)
1.0
2.5
3.0
Maximum displacement with a single slot Maximum displacement with three slots
7 6 5 4 3 2 1 0
1
2
3
4
H (m)
Fig. 3. Comparison of pressure relief for a single slot or three parallel slots.
h
extraction bore. Because the stress concentration is much higher than the original rock stress the coal seam permeability is reduced. This hinders gas diffusion and reduces the effective range of influence from the drilling. A high-gas pressure also exists around this stress concentration area, which has a detrimental influence on the gas extraction efficiency. The pressure reduction to increase permeability requires slotting in the coal seam with a rotary-slice type cutter across the rock layers as is shown in Fig. 5. The process requires:
Main influence area b
Boundary affected area
R
Boundary affected area
a Fig. 4. A cross section of the pressure relief range in a slotted coal seam.
Eqs. (1) and (2) give qz(t) and qB(t), which may be substituted into Eq. (3). The time to eliminate the outburst between the main influence area and the boundary affected area then give the total time required to eliminate the outburst in the entire pressure relief range within the slotted coal seam:
t ¼ maxðt 1 ; t 2 Þ
ð4Þ
where t1 is the time to eliminate outburst within each micro-unit of the main influence area; and t2 the time to eliminate the outburst within each micro-unit of the boundary affected area. Because q0 is measurable and q0 and l0 are known the time to eliminate outburst within each micro-unit of the entire pressure relief range within the slotted coal seam can be calculated. 2.2. Slotting a coal seam with a rotary-slice type cutter across rock layers The influence of gas pressure, ground stress, and other factors, forms a ring shaped stress concentration area around the completed
Area of plastic deformation
– Drilling a hole through the seam; – Using a rotating high pressure water jet slotting tool in the coal seam after backing the drill a certain distance. This forms an irregular, flat disk-shaped space with a radius of 500 mm and a certain width concentric with the drill; – Repeating the cycle of operation until the bottom of the bore and then washing the slotted coal from the bore using high pressure water. This way a series of flat slots within the coal seam are formed. These help eliminate the influence of stress concentration around the bore and overcome the typical defects seen with gas extraction by drilling. This slotting technology forms a series of flat slots that increases the exposed area of the coal seam. The volume of disturbed coal within the seam is also increased so the gas within the influenced range of the slotted surface is rapidly and completely extracted. The effect of the gas gradient force and ground stress causes the coal around the slots to move into the slots, which produces a large number of tensile and shear cracks. Pressure relief increases because of the increased permeability. Macro-slots and secondary cracks provide a path for gas flow that creates favorable conditions for gas release. The range of gas extraction and pressure
Interface of coal and rock
Jet slotted area Rg
Ro Drill
Drill Rp
Flat slot
(a) A profile drawing of the slotted coal
Direction of gas flow
(b) A plan drawing of the slotted coal
Fig. 5. A diagram of rotary-slice type slotting.
C. Shen et al. / International Journal of Mining Science and Technology 22 (2012) 533–538
Conveyer roadway
5m
15 m
5m
5 m 5 m5 m
15 m
Coalseam
Slotted drill across layer
20 m
(a) Drilling design across the slotted layer
100 m Slotting ahead of the face 100 m
8.7 m 8.7 m 8.7 m 8.7 m
536
Investigated slotted drill
Coal face caproic 16-17 12041 conveyer roadway
(b) Final drill positions across the slotted layer
Fig. 6. Drill layout across the slotted layer: tunnel of the 12041 conveyer roadway.
Table 2 Slotted drilling parameters. Drill No.
Drilled height (m)
Horizontal angle (°)
Elevation (°)
Position of coal found (m)
Coal seam thickness (m)
Drill length (m)
1# 2# 3# 4# 5#
1.7 1.9 2.1 2.3 2.5
0 0 0 0 0
27 31 38 50 76
18.3 14.3 10.6 7.4 4.9
23.4 19.1 14.8 11.6 7.5
41.7 33.4 25.4 18.0 12.4
relief is expanded thereby. This helps to reduce the gas extraction time. In addition, the flat slot lets the surrounding coal crack, move, and expand so, to some extent, potential outburst power is released. This reduces the danger of coal outburst effectively.
improve the extraction rate and to release stress concentration ahead of the roadway heading. The layout, input parameters, and extraction design of the experimental drillings are: A group of five bore holes was constructed every 5 m along the roadway. In total, 18 groups of bore holes were constructed, where the first 12 groups were slotted pressure relief drillings and the other six groups were ordinary extraction drillings. The location of each drill at the roof of coal seam 16#–17#, and within 15 m at both sides of the coal-roadway within the upper coal seam, is shown in Fig. 6. The extraction results of the 2# and 4# drilling in each group were investigated over an extraction time period no less than 30 days. The location of the drills was 100 m ahead of the developing face and the drills were rigorously plugged during the gas extraction process. The parameters of the slotted drillings are shown in Table 2. 3. Results and discussion
2.3. Field application
3.1. Analysis and contrast of gas extraction flow
The face number 12041 of the Pingbao Coal Mine Co. Ltd., was chosen as the site of a field application. The original gas pressure of this face was 1.38 MPa; the gas content was 10.46 m3/t. The head road of the face had been driven 400 m and the tail road had been driven 450 m. Problems caused by high-gas concentration, drill binding, and drill spraying arose during the development process. The advance rate was slow, which seriously hampered mining and development. The problem was solved by considering the actual situation and trying to increase the coal permeability by slotting the coal seam across the bottom layer. This was thought to
The statistical requirements required the use of mean values for each parameter, ignoring outlying values, when investigating the extraction parameters of the slotted and ordinary drills. For simplicity and universality the values of the 2# and 4# drill within each group were analyzed. Fig. 7 shows the contrast of the gas extraction flow at the 2# and 4# bore within each group of drillings. Fig. 7 shows that slotting can increase the rate and amount of gas extracted substantially. The biggest capacity of a single bore was a pure gas extraction of 54.45 L/min. The capacity extracted
50
30 20
30 20 10
10 0
4# ordinary 4# slotted
40 Q (L/min)
40 Q (L/min)
50
2# ordinary 2# slotted
0
4
8
12
16 20 t (d)
24
28
32
0
4
8
12
16 20 t (d)
Fig. 7. Contrasting gas extraction flow between slotted and un-slotted bores.
24
28
32
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100
5 30 m before slotted area
80
30 m after slotted area
4
70
2# slotted 4# slotted Ordinary
60 50
L (m)
Gas concentration (%)
90
3
40 30
60 m in slotted area
2
20 10
0
5
10
15 20 t (d)
25
30
35
Fig. 8. Gas extraction concentration from slotted or ordinary drills versus time.
4
8
12 16 20 24 28 32 36 40 44 t (d)
Fig. 10. Advance rate of the roadway near and in the slotted area.
slotted drills is 17 times bigger than that from ordinary drills, which means more space and exposed surface area for gas release. The analysis of Figs. 8 and 9 shows that high pressure water jet slotting removes coal from the area of the drill, which increases the exposed coal area and offers some transport and relief space. The slots remove stress concentration around the drill and expand the effective sphere of influence of a single drill. The number of gas release channels are increased so that gas extraction remains at a high concentration for a long time.
1.2
V (m3)
0
0.8
0.4
3.3. Roadway advance rate
0
Ordinary drill
Slotted drill
Fig. 9. Extracted coal quantity: slotted versus ordinary drilling.
from the 4# slotted drill was 3.7 times bigger than that from the 4# ordinary drill. The total capacity extracted from the slotted drills was three times that of the ordinary drills over a 30 day period. The first four to six days during the use of the new method saw a low range in extraction flow. Then the flow value rose and remained high for a long time period. Why might this be? The process of high pressure water jet slotting is also a process of pumping water into the coal. This reduces the permeability of coal and slows gas desorption. Therefore, gas extraction is inhibited. This is called the ‘‘water lock’’ effect and caused the low initial values of extraction [22]. As the water evaporated and left the coal, as time passed, the ‘‘water lock’’ effect is reduced. Additionally, the stress is redistributed because of the effect of stress and gas pressure change after the slotting. This produced a large number of coal fractures and released the gas energy, which increased the exposed area of the coal as well as the volume of disturbed coal. This is why the desorption rate and the extraction flow stay high. 3.2. An analysis of extraction concentration and range The gas extraction rate using slotted or ordinary drills was studied. Fig. 8 shows the statistics of discharged coal amount per single drill, considering both slotted and ordinary drills, against time. Fig. 9 shows the mean values. Fig. 8 shows that the average gas extraction from ordinary drillings is below 30 percent and is relatively unstable with large fluctuations. The gas extraction from the slotted drills is around 80 percent and the flow is stable. This is from one to two times more gas than that seen from of ordinary drilling. Fig. 9 shows the average amount of discharged coal per single bore. The discharge from
After extracting the gas by slotting the coal seam with a rotaryslice type cutter across the rock layer the head gate was driven and two efficiency indexes were investigated. These were the q value of the initial gas inrush speed and the s value of the discharged coal volume. These two indexes dropped significantly, which shows reduced risk of coal outburst and allows an increased speed in the head gate development, as shown in Fig. 10. Fig. 10 shows that the area without slotting had an advance rate of about 1.6–3.2 m per day, or 60–80 m per month. The rate was limited by the large amount of gas emission. In the slotted area the efficiency of gas extraction reduced the gas concentration in the coal seam and the advance rate was from 3.2 to 4.8 m per day, or 100 to 120 m monthly. The effect of outburst elimination is obvious. 4. Conclusions (1) Pressure release and increased permeability obtained by slotting a coal seam with a rotary-slice cutter across rock layers is an effective way to relieve the gas pressure in a single low-permeability coal seam. This method reduces the stress concentration around the drill, enlarges the displacement space and the fracture field in the coal, and forms a pressure relieved area within the coal that increases coal seam gas permeability. (2) Pressure release and permeability increase from slotting increase the exposed coal area, improves the permeability of the coal seam, and increases the gas extraction rate. Over 30 days the total quantity of gas extracted from the slotted bores is three times that of the amount from ordinary bores. The concentration of gas extracted from the slotted drills is two to three times that of the ordinary drill and the gas concentration is stable at 80 percent. (3) This technology proved practical in an application where the tension between mining rate and engineering development was addressed. The development rate increased by 30–50%
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as the monthly advance rate increased from 60 to 80 m to the higher range of 100 to 120 m. During head gate development efficiency indexes q and s decreased indicating that the risk of coal and gas outburst decreased significantly.
Acknowledgments The authors would like to thank Professor Lin Baiquan for the supervision of the paper writing; thank for financial supports provided by the National Key Basic Research and Development Program (No.2011CB201205), the National Natural Science Foundation of China (No.50534090), the Independent Research of State Key Laboratory of Coal Resources and Mine Safety (No.SKLCRSM08X03), the State Key Laboratory of Coal Resources and Mine Safety of Research Foundation of China University of Mining & Technology (No.09KF09) and the National Natural Science Foundation of Youth Science Foundation (No.50804048), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (CXZZ12_0958) and thank Pingbao Coal Ming Co. Ltd. for the help and support on field application and test of the technology.
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