JOURNAL OF COAL SCIENCE & ENGINEERING (CHINA)

DOI 10.1007s12404-009-0206-6

pp 143–147

Vol.15 No.2

June 2009

Theory and test research on permeability of coal and rock body influenced by mining∗ QI Qing-xin1,3, LI Hong-yan1,2, WANG You-gang1,3,4, DENG Zhi-gang1,2, LAN Hang1,2, PENG Yong-wei1,2, LI Chun-rui1,2 ( 1. Coal Mining & Designing Branch, China Coal Research Institute, Beijing

100013, China; 2. Department of Coal Mining

& Designing, Tiandi Science & Technology Co., Ltd., Beijing 100013, China; 3. China Coal Research Institute, Beijing 100013, China; 4. Tiandi Wangpo Coal Co., Ltd., Jincheng 048000, China )

Abstract Stress distribution rules and deformation and failure properties of coal and rock bodies influenced by mining were analyzed. Experimental research on permeability of coal and rock samples under different loading conditions was finished in the laboratory. In-situ measurement of coal permeability influenced by actual mining was done as well. Theory analysis show that permeability varied with damage development of coal and rock under stress, and the influence of fissure on permeability was greatest. Laboratory results show that under different loading conditions permeability was different and it varied with stress, which indicated that permeability was directly related to the loading process. In-situ tests showed that permeability is related to abutment stress to some degree. The above results may be referenced to gas prevention and drainage. Keywords stress of coal and rock body, permeability of coal influenced by mining, SF6 tracing technology, crack aperture, damage

Introduction As one of the main disaster sources in coal mining, gas is always being attended to by coal workers and researchers. Especially in the recent 20 years, gas content in coal continually rose with mining depth increase and high intensity mining (full-seam fullmechanized mining and caving mining), resulting in a large amount of gas gushing and accumulation in the mining face. Gas overruns and gas disasters frequently occur, which greatly influences society. Therefore, how to research rules of gas flow and diffusion under conditions of deep and high intensity mining to resolve the problems of large gas gushing and accumulation has been the biggest obstacle of safe mining and improving production efficiency. Currently, it is a problem that must be settled for safe mining in China.

Taking the influence of gas on safe mining into account, this paper analyzes the basic characteristics of the mining stress field. It also analyzes the influence of the mining stress field on gas seepage and diffusion and the characteristics of gas seepage in coal influenced by mining based on gas permeability measurements and monitoring of the relationship of gas permeability and mining stress.

1

Coupling characteristics of mining stress and seepage field

Coal mining will result in stress re-distribution of coal and rock. The new stress field is called the mining stress field. Research results on rock structure styles and stress fields influenced by mining are abundant. Many scholars researched them from different views, such as Qian Minggao’s “mechanical model of stacks

Received: 15 September 2008 ∗ Supported by the National Major Fundamental Research Program of China (973 Project) (2005CB221503); National Science Foundation of China (50544010) Tel: 86-10-84262771, E-mail: [email protected]

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Journal of Coal Science & Engineering (China)

layer of blocks”(Qian and Liu, 1984) (Fig.1), Song Zhenqi’s(1988) “model of transferring rock beam” and their other theories such as quasi-arch structure, archbeam structure, beam structure and key stratum (Jiang et al., 1993; Gu et al., 1996; Qian et al., 1996), et al.

Fig.1

Overlying stacked layer of blocks structure of mining field

Although these structure styles of the mining stress field are different, we think that the development of the mining stress field has 4 basic characteristics: (1) it is closely related to the mining method and roadway layout; (2) it is influenced by coal and rock properties and mining depth; (3) it is dynamic and variable; (4) it is predictable and controllable. In actual mining, variation rules of mining stress must be mastered well and controlled to provide favorable conditions for gas drainage and extraction. Especially under high intensity mining conditions, mining influence range and mining space expansion, and the influence of rock stratum caving rules and the space-time evolvement process of mining stress fields on gas flow, gushing changes thoroughly. For this matter, Li Shugang researched the relationship of abutment pressure in full-mechanized caving mining and movement of unloading gas (Li et al., 2004). He considered that the movement rule of unloading gas in coal with high content gas was closely related to its permeability, and that variation of abutment pressure largely affected permeability. That is, the movement rule of unloading gas was influenced distinctly by abutment pressure (Fig.2).

Fig.2

Variation curves of permeability coefficient in front of coal face

To obtain the permeability variation rule of coal influenced by mining, we applied a coal permeability

monitoring system that we developed to measure variation of coal permeability in Wuyang Colliery, Lu’an Group, and Tiandi Wangpo Colliery, which provided an effective method for researching the relationship of the stress field and seepage field influenced by mining (Deng et al., 2008; Deng, 2008). The measurement results are as follows. Fig.3 is the layout of measuring points for coal permeability. In measuring, the variation of diffusion velocity of the tracer gas in coal was firstly recorded. It is an important index for evaluating permeability and is an important parameter for calculating the permeability coefficient. After 19.4 h and 15.3 h tracer gas was observed in the No.1 observation holes of 2 surveying stations after the first gas injection. After conversion, diffusion velocities were, respectively, 2.15× 10 5 and 1.82×10 5 m/s. Considering 0.12×105 and 5 0.098×10 Pa injection pressure, the original permeabilities of coal were 2.75×10 3 μm2 (about 2.70 mD) -3 2 and 2.86×10 μm (about 2.81 mD). The mean value 2.81×10 3 μm2 (about 2.75 mD) was near to the original permeability of coal.

Fig.3 Observation section of coal cracks in 3207 mining face

Fig.4 is the permeability variation curve of No.3 coal of Wangpo Colliery under mining conditions. Coal permeability increases slowly at the range of 20~ 40 m far away from the mining face. It has approximately original permeability. At the range of 7.9~20 m far away from the face, it increases rapidly and reaches the maximum of 1.823×10 2 μm2 at 7.9 m, which is 6.49 times the original value. After the summit, it decreases fleetingly and reaches a minimum of 1.145×10 2 μm2 at 6 m which is still greater than the original. Then, it increases rapidly again till the mining face reaches here. At this time, its value is 2.523× 10 2 μm2, which is 8.99 times the original. By analyzing permeability variations, the following conclusion may be obtained. Under a natural loading state, coal permeability increases with increasing loading. When it reaches some value it will decrease with loading, continually increasing; when it decreases to some value (still much greater than the original) it will rapidly increase again.

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QI Qingxin, et al. Theory and test research on permeability of coal

Fig.4 Curve of coal permeability variation in 3207 mining face

We can find that the influence of abutment stress on coal permeability is very obvious by comparing the abutment stress (mining stress) curve (Fig.4) and the permeability variation curve (Fig.5) of coal influenced by mining. Variation in mining stress results in coal damage and failure, whereas coal damage and failure is the essence of permeability variation. Therefore, in actual gas prevention, mining methods and rules of underground pressure behavior must be taken into account to confirm the time and place of gas predrainage.

ies are typical anisotropic and heteropic media because of cracks inside them. In the research on permeability of coal and rock, attention must be paid to their structural properties. For this, based on the basic principles of damage mesomechanics, the evolvement equation of permeability tensor with stress-damage development is set up by analyzing structural properties of pores and cracks and seepage characteristic. The laboratory experiment was finished and the results are well in accordance with simulation results, which verifies their validity (Li, 2008). 4 hypotheses were imported into this model to explain macro phenomena: (1) permeability of coal and rock is made up of that of pores and cracks, and permeability is anisotropic; (2) transformation of stress (strain) state will result in variations of the micro-crack aperture, density, connectivity and then that of permeability; (3) gas flow in micro cracks obeys the generalized flow law; (4) critical start pressure grads is not considered. On the basis of the above hypotheses, the following equation is tenable. m

d

k =k +k ,

(1)

where, k is the macro permeability tensor of REV with micro cracks (representative elementary volume m is called for REV); k is the permeability tensor of d

intact coal and rock; k is the permeability of the Fig.5

2

Curve of advanced abutment pressure from measurement in 3207 mining face

Damage-seepage evolvement model of coal and rock body influenced by mining

Zhou Shining firstly put forward the theory of gas flow-linear gas seepage in China which has a very profound effect on Chinese gas flow theory (Zhou and Lin, 1999). Researches on gas seepage characteristics in coal and rock have always been the focus of many scholars. Among these, the most representative are the Darcy Law (linear gas flow theory), Fick’s Law (linear gas diffusion theory), Flow-diffusion Law (gas convective diffusion theory) and the Power Law (nonlinear gas seepage theory). Theoretical and practical results on the amounts have proved that those are reasonable under certain conditions. However, coal and rock bod-

meso- crack in REV, and the permeability variation of d coal and rock both result from variation of k . Darcy’s Law is as follows:

v=−

k

μ

m

∇p = −

d

(k + k )

μ

m

d

∇p = v + v ,

(2)

where, μ is the coefficient of dynamic viscosity. For a single micro crack, the seepage direction is vertical to the normal of the crack n , and the modified laminar flow cube law is as follows: λ 1 d v ( n) = − e(n) 2 (δ − n ⊗ n)(∇p) d , (3) 12 μ where, e(n) is the mean aperture of the micro crack; (∇p )d is the local pressure grade of this crack; λ is the degree of micro crack participating in seepage. It ranges from 0 to 1. Formula (3) is the permeability velocity of a single micro crack in REV. All micro cracks in REV are

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Journal of Coal Science & Engineering (China)

integrated by volume and the following equation is obtained: d

v =

∫v

d

Ωd

(n)dΩ = ∫ −

λ e(n) 2 (δ − n ⊗ n)(∇p ) d dΩ, 12 μ

(4) where, Ω is the space aggregation of all micro cracks in REV. For a group of micro cracks whose unit normal vectors are n , supposing that the mean apertures of micro cracks with one direction are the same, Ωd may be written as follows: d

dΩ d = Nπea 2 ,

3.2

Experiment results and analysis Permeability variation of coal and rock samples is measured under some axial pressure and different ring pressures. Axial pressure is set at 10 MPa and ring pressures are selected in turn as 4, 6, 8, 10, 12, 10, 8, 6, 4 MPa. The measurement results from 2 samples are shown in Fig.6.

(5)

where, N is the crack number of this group in REV; a is the crack radius of this group. Thus, the equivalent permeability tensor induced by micro cracks in REV after farther deduction is as follows: 3 λπ 1 d k = N ( n) R (n)e(n) r (n)2 (δ − n ⊗ n)dS . ∫ 12 4π S 2 (6) For a group of micro cracks with any direction, its permeability tensor is:

λ

e3 a 2 (δ − n ⊗ n). (7) 48 From the above analysis, we can find that among the factors of damage development influence on permeability, aperture variation of micro cracks is the first and their relationship is cubic. Confirmation of crack aperture is not discussed here. d

k = NR

3

Laboratory measurement of permeability of coal and rock

For further research on stress sensitivity of coal and rock, 3 permeability experiments were conducted, including: (1) permeability measurement under different pore pressures; (2) permeability measurement for stress sensitivity research under a triaxial stress state; (3) permeability measurement and triaxial loading and unloading for gas flow rules, influence of coal bedding on stress sensitivity was considered as well. Only the measurement results of permeability under triaxial pressure are given here because of space limitation. 3.1 Experimental principles and sample preparation All samples were from the Tiandi Wangpo Colliery in Jincheng. Experimental principles and sample preparation may be found in the references (Peng, et al., 2008).

Fig.6 Curve of permeability influenced by 2-h-5 and 2-v-4 damage from numerical simulation and experiment

In loading, permeability of coal decreases gradually with increasing pressure, and it varies as a negative exponent. In unloading, it increases gradually but the variation is not distinct. The results may be explained as follows. Pores and cracks in coal samples are compressed under surrounding pressure. Gas seepage gateways are compressed and shrink rapidly. This makes gas pass with more difficulty and permeability rapidly decreases on the macroscopic view. In the meantime, unloading makes pores and cracks unable to be resumed because plastic deformation occurs. This makes permeability after loading less than original. In addition, for validating the permeability evolvement equation influenced by damage, results from numerical simulation and experiment under triaxial stress are compared here. Simulation parameters are shown in Table 1 and the comparison result is shown in Fig.6. The permeability evolvement model influenced by damage built in this paper based on mesomechanics better simulates permeability variation in loading and unloading, which shows that the permeability of coal and rock bodies are obviously sensitive to the surrounding pressure and proves that damage is existent and the part is nonrenewable.

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QI Qingxin, et al. Theory and test research on permeability of coal Table 1 Parameters for numerical simulation and permeability experiment under triaxial stress Coal sample No. 2-h-5

σc (MPa)

2, 4, 6, 8

E (MPa)

9.38×10

3

υ

0.42 2

10, 12 7.53×103 0.33

k(J/m )

40

40

η

0.03

0.04

μ0

0.2

0.5

χ

1.0

1.0

t

-4

1×10

1×10 4

R0

5×10 5

5×10 5 -

-3

3.2×10

9

4.4×109

3.0×10

kn0

4.4×10

N

1.5 2

k0 (m )

References

-

-

β0

4

2-v-1

-3

1.5 -16

0.131×10

seepage gateways are compressed and shrink rapidly. This makes gas pass with more difficulty and permeability rapidly decreases in the macroscopic view. Meanwhile, unloading makes pores and cracks unable to resume because plastic deformation occurs. This makes permeability after loading less than original. (3) Based on the basic principles of damage mesomechanics, the evolvement equation of the permeability tensor with stress-damage development is set up. Experiment results are well in accordance with simulation results, which verify their validity.

0.234×10 17 -

Conclusions

(1) Stress field and seepage field influenced by mining are comprehensively analyzed in this paper. In-situ measurement results may be shown as follows. Under natural loading states, coal permeability increases with loading increasing. When it reaches some value it will decrease with loading and continually increases, and when it decreases to some value (still much greater than the original) it will rapidly increase again. The influence of abutment stress on coal permeability is very obvious. Therefore, variation rules of mining stress must be mastered well. Especially under high intensity mining conditions, mining influences range and space expansion, and under the influence of the rock caving rule and space-time evolvement process of mining stress fields on gas flow, gushing changes thoroughly. In actual gas prevention, mining methods and rules of underground pressure behavior must be taken into account to confirm the time and place of gas pre-drainage. (2) Research results on coal permeability sensitive to stress under triaxial stress are shown as follows. In loading, the permeability of coal decreases gradually with increasing pressure, and it varies as a negative exponent. In unloading, it increases gradually but the variation is not distinct. The results may be explained as follows. Pores and cracks in the coal sample are compressed under surrounding pressure. Gas

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