EGE paper

Mechanical Properties of Bedded Rock
Salt
Zhao Yan-Lin
School of Energy and Safety, Hunan University of Science and Technology,
Xiangtan 411201, China;
Hunan Provincial Key Laboratory of Safe Mining Techniques of Coal Mines,
Hunan University of Science and Technology, Xiangtan 411201, China
e-mail: [email protected]
Wan Wen
School of Energy and Safety, Hunan University of Science and Technology,
Xiangtan 411201, China;
Hunan Provincial Key Laboratory of Safe Mining Techniques of Coal
Mines,Hunan University of Science and Technology, Xiangtan 411201, China
ABSTRACT
Taking the Sodium Chloride rock salt specimens containing mudstone interbed which located at 600-700 m
level of Yun Ying salt-mine in Hubei as the research objects, the conventional mechanical tests on the
specimens and uniaxial compression creep tests on the specimens under multi batch stress levels were
performed. The conventional mechanical tests obtained the result that the bedded rock salt is a kind of special
combined soft rock，its elastic modulus is comparative little, but lateral deformation of the rock salt is great
under uniaxial compression stress state. Through creep experiments , the results were obtained as follows：
(1) If the duration of creep is enough long , the attenuation creep stage, the steady creep stage and the
accelerative creep stage are appeared during the bedded rock salt creep, with the accelerative creep stage last
longer compared with other rocks. (2) The phenomenon of invagination of interlayer and bulge of rock salt is
found at the teady creep stage. Uncoordinated creep deformation may lead to shear dislocation in the interbed
KEYWORDS: bedded rock salt, creep characteristics, creep test, failure mechanism
INTRODUCTION
Energy resources are very important to nation's economy, petroleum and natural gas are
strategic resources, which has been related to economic development and society stabilization
of a country. Underground storage project of oil is used to regulate the demand of the market,
which is also related to national energy security. At present, underground gas storage projects
are almost constructed in sedimentary rocks of rock salt, because rock salt has good physical
properties including compact structure, relatively low porosity (0.01%-5%) and good
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creep behavior [1-3]. Salt cavern construction is a pioneered attempt for strategic reserve in
China. As the first salt cavern gas storage in China, Jintan salt cavern underground gas
storage is a complicated systemic project [4,5]. Salt cavern underground gas storage need to
be built in rock salt purity better, but the scarcity of salt dome type ore deposit in China, the
salt layer often contain many interlayer, gas storage is built in bedded rock salt deposit rather
than thick salt dome , which will meet more complicated mechanical and technical issues[6].
The issues include the creep convergence stability of dissolved cavity and creep properties
heterogeneity of bedded rock salt. At present, scholars have done a large number of test and
theoretical analysis for the mechanical properties of rock salt at home and abroad, especially
the creep characteristics. U．Hunsche et al. [7] made a more detailed study on the creep
constitutive model of rock salt. Jianchun ma etc. [8] made a systemic study on the rheological
properties of rock salt. Chunhe Yang et al. [8] has established the Cosserat medium extension
constitutive model of salt rock mass. Wei zhong Chen et al. [9] has established nonlinear
creep damage constitutive model of rock salt. Bedded salt rock is a kind of special combined
soft rock, creep properties of bedded rock salt and the creep damage failure process between
bedded rock salt and interlayer are worth further research. The short and long term
experiments of bedded salt rock specimen cored from 600-700 m level of Yun Ying salt-mine
in Hubei were studied in this paper.
SHORT-TERM MECHANICAL CHARACTERISTICS OF
BEDDED ROCK SALT
Taking the Sodium Chloride rock salt specimens containing mudstone interbed which
located at 600-700 m level of Yun Ying salt-mine in Hubei as the research objects. Using
geological drilling rig to obtain core. Because of layered salt rocks containing weak surface
layer, standard specimen can not be obtained. Based on the rock mechanics test
standards, cylindrical specimen of 2:1 ratio of height to diameter can be used, its size is
φ 90mm × 180mm .
Short-term mechanical characteristics
Various conventional experiment have been made for bedded rock salt, pure rock salt and
mudstone, the following conclusions can be obtained.
(1) Bedded salt rock is a kind of special combined soft rock, compared with the common
rock, its uniaxial compressive strength and deformation modulus are relatively small, while
lateral deformation capacity is very large. When axial stress σ = 0.5σ c (where σ c
=compressive strength of rock salt specimen), transverse deformation coefficient of bedded
rock salt can reach 0.48. Table 1 is the experimental results of bedded rock salt samples under
uniaxial compression.
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(2) Poisson's ratio of mudstone interbed is smaller than bedded rock salt, however, the
elastic modulus of mudstone is bigger. Fig.1 is the stress-strain curve of three kinds of rock
specimens, when the loading stress reaches the peak stress, axial strain of rock salt is the
largest, bedded rock salt takes second place, mudstone minimum. The elastic modulus of
mudstone is 4.2 times higher than rock salt, Poisson’s ratio of mudstone is about 0.74 times of
the rock salt. Under the same stress level, the deformation of salt rock is a lot bigger than
mudstone, uniaxial compressive strength of mudstone is slightly bigger than rock salt. When
the uniaxial compressive stress acts on the bedded rock salt, the deformation is mainly
composed of rock salt layers in bedded salt rocks.
Table 1: Experimental results of layered rock salt samples under uniaxial
compression
uniaxial
compressive
strength σ c /MPa
22.8MPa
σ = 0.5σ c
The appearance of macro cracks
Poisson ratio
μ
tangent
modulus
E / GPa
axial stress
σ v /MPa
axial strain
ε/ %
0.48
8.1GPa
16.5
0.45
Figure 1: Stress-strain curve of three kinds of rock specimens
LONG-TERM MECHANICAL CHARACTERISTICS OF
BEDDED ROCK SALT
To study the long-term strength of bedded rock salt, mid long term uniaxial static creep
test of bedded salt rock specimens has been done, which was conducted by RYL - 600
microcomputer control rock shear rheometer and controlled by Japan panasonic whole digital
ac servo high system. Experimental apparatus are shown in Fig.2.
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Figure 2: Uniaxial static creep test of layered rock salt
Uniaxial static creep test was conducted by single-stage loading way, single-stage
loading had a total of four batches, four batches of axial stress were respectively σ 1 = 8MPa,
σ 2 = 11MPa, σ 3 = 14MPa, σ 4 = 16MPa .
Figure 3: Creep curve of bedded rock salt 2#（ σ = 11MPa ）
Figure 4: Creep curve of bedded rock salt 4#（ σ = 16MPa ）
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Fig.3 is the creep test curve of bedded salt rock specimen 2 # when loading stress is
11MPa. Under the stress level, creep in the period of attenuation, steady period and period of
acceleration can be shown all in 130d. Attenuation creep stage lasts about 15d. The
attenuation creep strain is 70%~80% of total creep strain, the steady-state creep stage last
very long, steady-state creep rate is about 6.07×10-6/d, when t=130d , creep goes into the
accelerated creep stage, and specimen was damaged 12 days later.
Fig.4 is the creep test curve of bedded salt rock specimen 4 # when loading stress is 16
MPa. The total creep time is 160 d. Under the stress level, attenuation creep stage lasted about
30d, Steady creep rate is about 3.7×10-5/d. when t=127d , creep went into the accelerated
creep stage, specimen was damaged 33 days later.
Test found that bedded rock salt specimen creep has two notable features;（1）In the
steady-state creep stage, creep rate of rock salt is greater than mudstone interbed, rock salt
has stronger rheological properties, the steady state creep stage appeared the phenomenon of
specimen dissection inward concave and rock salt layer bulge outward. Oil and gas reservoir
is established in bedded rock salt, shear dislocation of rock salt layer provides channels for oil
and gas leakage. Figure 5 is a schematic diagram of creep failure of each interlayer of bedded
rock salt.（2）Compared with the other rocks, accelerating creep stage of bedded salt rock
lasts longer, when loading stress is 11MPa~16MPa, accelerating creep stage is 10d~32d.
Cleavage plane of easily splitting exists in the rock salt crystal, rock salt crystal is easy to be
split along the plane that parallel to the cleavage plane into cubes under the action of external
force. Generally, microfissure can be formed and develop gradually when the bedded salt
rock is in the early stage of creep. Cracks basically achieve stability in steady-state creep
stage, when creep enter the stage of accelerated creep, some damage phenomena are found in
many bedded salt rock specimens, firstly, interlayer splitting failure, and then salt rock of
interbedded pulling damage, at last, bedded salt rock failure as the axial tension crack
splitting.
Figure 5: Schematic diagram of creep failure of each interlayer of bedded rock salt
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CONCLUSION
(1) Bedded salt rock is a kind of special combined soft rock, deformation modulus is
relatively small, while lateral deformation capacity is very large. When the uniaxial
compressive stress acts on the bedded rock salt, the deformation is mainly composed of rock
salt layers in bedded salt rocks.
(2) Creep rate of rock salt is greater than mudstone interbed in steady-state creep stage,
specimen dissection inward concave and salt rock layer bulge outward. The uncoordinated
creep may lead to shear dislocation in the interbed.
(3) In full long creep time, attenuation creep stage, steady-state creep stage and
accelerating creep stage, three stages of bedded salt rock creep are all appeared, and the
accelerated creep stage lasts long.
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