rious factors of the physical, mechanical and chemical[7]. For the geological storage of the carbon dioxide, the low permeability rocks are often used as the caprock which is to prevent the carbon dioxide keeping on migrating into the subsurface.
The permeability of the caprock is directly related to its ability to storage the carbon dioxide. The low permeability rock is considered as the ideal storage layer for the permanent underground storage of highly radioactive waste[8].
The reason is that the low permeability rock can prevent the outward migration of potential radioactive materials. However, it is necessary to consider the disturbance of the surrounding rock mass caused by the chamber excavation. Due to the needs of these practical projects, the study of low permeability rocks is widely carried out. The description and measurement of the permeability should be the first which is to be solved[9].
1.2.2 The accurate measurement of the permeability
Compared with the conventional permeability rock, the measurement of low permeability rock is more difficult. Because the permeability is extremely low, there have been several hours or even days since the fluid flowing through the rock sample to reach a steady flow state[10]. Therefore，it is difficult to use the Darcy's law to calculate the permeability of rock by using the conventional steady state method to record the flow of fluid through the rock to a steady state.
The problem of the conventional steady state method is that the measurement time of long. In order to overcome this problem, the pressure pulse method to measure the permeability of rock is first proposed by Brace. The basic principle is that a pulse pressure is applied on one end of the rock sample and the permeability of rock has been obtained until the pressure of the fluid is balanced at both ends[11].
A similar method to obtain the permeability of the rock is to apply a sine wave at one end of the rock and record the pressure at the other end.
Another rapid permeability test method is not based on the cylindrical rock sample but the rock fragment. The rapid permeability test method is the Chip pressure decay method(GRI) which was firstly proposed by Luffel in 1993.
First of this test methods is insert the rock fragments into the test chamber. Then a certain gas pressure is applied on the test chamber. This experiment is to get the permeability of the rock by recording the change of gas pressure in the test chamber. There are advantages and disadvantages of these test methods[12]. The steady state method is similar to that of the actual stratum. However, the test time is much longer. The dynamic test method is fast. However, this method can not simulate the actual stratum condition and need some additional parameters. Which method should be used in the actual situation should take balance between test time and measurement accuracy.
1.2.3 The Internal mechanics mechanism of the permeability change with stress
The direct result of the effective stress change inside the rock is the deformation of the internal pore structure which will lead to the change of the permeability of the rock. The rock has been usually considered as uniform when the physical and mechanical properties of rock is being analyzed[13]. And the study is usually using the continuous medium analyzing method. However, when the scale is reduced, the micro-structure has been studied. The result of this study is that the rock is uneven, discontinuous and anisotropic. There is large number of pores, cracks and inclusions which are with different sizes, shapes, connected or disconnected. These discontinuities micro-structures are playing a decisive role on the external nature of rocks[14].
The permeability should not be treated as a constant in the practical projects involving the
