Numerical Simulation Study on Mechanical Properties of

Physical and Numerical Simulation of Geotechnical Engineering
1st Issue, Sep. 2010
Numerical Simulation Study on Mechanical Properties of
High-Pressure Water Injection Coal Seam
ZHENG Chao, YANG Tianhong, CHEN Shikuo, LIU Honglei
College of Resources and Civil Engineering, Northeastern University, P.R.China, 110819
[email protected]
ABSTRACT: With the increasing of mining depth and the improving of mining mechanization
degree, the gas outburst became bottleneck of improving productivity and mining safety in high
thickness gas mines. High-pressure water injection is an emerging measure to eliminating the coal and
gas outburst. This emerging measure not only has many advantages, but also improves the working
environment and speed excavation, and commonly used in coal mine. But people can't enough know
about stress evolution, softening zone distribution and water seepage law during high-pressure water
injection, which lead to determine some construction link depend on experience. In order to make
clear those problems, fluid-solid coupling model of coal rock damage was established by means of the
theory of seepage mechanics and damage mechanics, finite elements software COMSOLMultiphysics was used to simulate coal high-pressure water injection to analyze coal rock mechanical
properties and water seepage law. According to the numerical experiment, high-pressure water
injection caused fracture damage of coal rock mass, ultimately lead to form a damage area around the
water injection hole. Coal rock porosity became larger in the damage area induce permeability of the
coal body around the infusing borehole to improve. Pressure relief area of coal became bigger; stress
concentration value was decreased; the trend of stress concentration slowed down; stress
concentration area was moved to deep. The results of experiments show that the effect of the
high-pressure water injection on eliminating the coal and gas outburst is comparatively considerable,
and is helpful for site construction of high-pressure water injection.
KEYWORDS: High-pressure water injection, Coal rock, Gas, Eliminating outburst measure
1 INTRODUCTION
Gas outburst is the most important threat during coal
mining. Especially now coal and gas outburst posed greater
threat than before because coal mining has been extended
to the deep, ground stress and gas pressure increased[1-3]. In
order to prevent gas outburst disease, people did a lot of
works, but many existing elimination outburst measures
can not meet the requirements. High-pressure water
injection is an emerging measure to eliminating the coal
and gas outburst. High-pressure water injection not only
bring less engineering quantity and low over standard rate
of test, but also improve the working environment and raise
the driving speed[4-7].
Although domestic and foreign scholars carried out
many related researches on elimination outburst mechanism
of high-pressure water injection for a long time, people
can't enough know about stress evolution, softening zone
distribution and water seepage law during high-pressure
water injection which lead to determine some construction
link depend on experience because of invisibility of
high-pressure water injection process[8-12]. This paper used
finite elements software COMSOL-Multiphysics to
simulate high-pressure water injection process for further
studying elimination outburst mechanism based on
fluid-solid coupling mathematic model.
2 FLUID-SOLID COUPLING MATHEMATIC
MODEL OF COAL
Stress
equilibrium
differential
displacement and stress[13-16]：
G
u j ,ij  Gui , ij  p  0
1  2
equation
(1)
Initial condition of stress field: when t was equal to 0,
displacement or particle speed is initial value.
Water seepage flow need to meet energy conservation
principle and mass conservation principle. Flow procedure
in accordance with Darcy's law. Seepage equation of
high-pressure water injection as follows:
k 2 p  Q
p

 v
t
t
(2)
Initial condition of seepage equation as follows:
P t 0  p0
(3)
Relational expression that fluid impact on elasticity
modulus of solid matrix as follows:
E  E0 exp(b0 p)
Where  is fracture
(4)
pressure coefficient; p is pore
water pressure;  v is volumetric strain;
coefficient;
modulus;
E ;E is elastic
2(1   )
is poisson ratio; E0 is initial elastic modulus;
G is shear modulus,

k is permeability
G
b0 is material parameters relative to coal rock mass.
© ST. PLUM-BLOSSOM PRESS PTY LTD
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Physical and Numerical Simulation of Geotechnical Engineering
1st Issue, Sep. 2010
3
NUMERICAL
SIMULATION
HIGH-PRESSURE WATER INJECTION
OF
is 13m. Boundary conditions of five holes were
fixed-pressure boundaries; all outside surfaces of model
was impervious boundaries. There are exterior load whose
direction was downward is equal to 10MPa over the upper
side for simulating deadweight of overlying strata;
constraints of five water injection holes were free; other
surfaces were restricted by normal displacement. Water
injection pressure is 12MPa; water injection time is 2h.
3.1 Numerical model
Geometric model was shown below. Floor strata
thickness and overlying strata thickness are both 1.5m;
sealing length is 2m; each hole of five water injection holes
Tab.1 material parameter
E（Pa）

coal density
（kg/m3）
Water density
（kg/m3）
coefficient of viscosity
（Pa·
s）
permeability
（m2）
7.41e9
0.33
2290
1000
1e-3
1e-15
3.2 Results Analysis
constantly expanded. The pressure of holes around was the
highest. The farther away from the hole, the pressure is
lower. When t is equal to 7200 s, pressure field level-off,
influence radius is 2m.
The pressure distribution of coal rock mass at different
time was shown in the Fig.1 and Fig.2. In Figure, with the
passage of time, the pressure field around the hole
Fig.1 pressure distribution (t=3600s)
111
Numerical Simulation Study on Mechanical Properties of High-Pressure Water Injection Coal Seam
DOI: 10. 5503/J. PNSGE. 2010. 01.016
Fig.2 pressure distribution (t=7200s)
The elasticity modulus distribution of coal rock mass at
different time was shown in the Fig.3 and Fig.4. We
suppose the elasticity modulus of coal is less than a quarter
of the original value, we believe that coal rock damage. In
Figure, after high-pressure water injection operations, the
oval-shaped damage zone was produced around the water
injection holes. The pressure water produced internal water
pressure in the all levels weak planes. All levels weak
planes consist of bedding face, cutting fracture and primary
fracture and so on. Internal water pressure caused weak
planes continue to expand and extend, and formed
transfixion network. This is the formation cause of the
oval-shaped damage zone. The radius of damage area is
1.5m.
Fig.3 elasticity modulus distribution (t=3600s)
112
Physical and Numerical Simulation of Geotechnical Engineering
1st Issue, Sep. 2010
Fig.4 elasticity modulus distribution (t=7200s)
Fig.5 has shown that comparison of elasticity modulus
before and after high-pressure water injection. After
high-pressure water injection, the value of elasticity
modulus dropped from 7410MPa to 6.2e-2MPa. The
decreasing range of elasticity modulus was greatly. This
demonstrates that coal rock produced obvious damage.
Fig.6 has shown that comparison of stress before and after
high-pressure water injection. Pressure relief area of coal
became bigger; stress concentration value was decreased;
the trend of stress concentration slowed down; stress
concentration area was moved to deep.
before high-pressure water injection
elasticity modulus\Pa
9.00E+09
after high-pressure water injection
8.00E+09
7.00E+09
6.00E+09
5.00E+09
4.00E+09
3.00E+09
2.00E+09
1.00E+09
0.00E+00
0
2
4
6
8
10 12
X\m
14
16
18
20
22
Fig.5 Comparison of elasticity modulus before and after high-pressure water injection
113
Numerical Simulation Study on Mechanical Properties of High-Pressure Water Injection Coal Seam
DOI: 10. 5503/J. PNSGE. 2010. 01.016
7.00E+07
after high-pressure water injection
6.00E+07
before high-pressure water injection
stress\Pa
5.00E+07
4.00E+07
3.00E+07
2.00E+07
1.00E+07
0.00E+00
0
3
6
9
12
15
18
21
X\m
Fig.6 Comparison of stress before and after high-pressure water injection
4 CONCLUSION
REFERENCES
(1) With the passage of time of water injection, the
pressure field around the hole constantly expanded. After
two hours of high-pressure water injection, influence radius
of the pore water pressure reaches the maximum, which
value was about 2m.change law of influence radius of the
pore water pressure was the prophase of high-pressure
water injection changed greatly; the longer water injection
time, the smaller influence radius
changed. When
influence radius reached a certain value, tended to steady
state.
(2) High pressure caused fracture damage of coal rock
mass, ultimately resulting to form a damage area around the
water injection hole. Coal rock porosity became larger in
the damage area induce permeability to improve.
(3) After high-pressure water injection, cohesive Energy
of coal rock became smaller; Pressure relief area of coal
became bigger; stress concentration value was decreased;
the trend of stress concentration slowed down; stress
concentration area was moved to deep. Those determine the
outbreak of coal-gas outburst.
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ACKNOWLEDGEMENTS
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The work presented in this paper was financially jointly
supported from the General Project of the National Natural
Science Foundation of PR China (Grant No. 50874024,
10872046, 50904013), the Major State Basic Research
Development Program of China (973 Program) (Grant No.
2007CB209405), the Ph.D. Programs Foundation of
Ministry of Education of China(Grant No. 200801450003),
the Ministry of education’s key scientific and technological
(Grant No. 107033), team project of Northeastern
university (Grant No. NO90101001), the Foundation for
National Excellent Doctoral Dissertation of P.R.China
(Grant No. NO90601003)
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