Archie's equation is often used for the interpretation of resistivity structures obtained from electrical and electromagnetic methods. The equation assumes that the contribution of electric conduction of the electric double layer, which is called surface conduction, is negligible. Therefore, the resistivity of clay-bearing rocks becomes generally lower than the resistivity expected from the Archie's equation. Some empirical formulas which modified the Archie's equation based on the equivalent parallel circuit model were proposed by experimental works.
 In this research, the empirical formulas were reviewed to consider the influence of clay minerals on resistivity. The effectiveness of the empirical formulas was examined using the experimental resistivity measurements of clay-bearing artificial samples. The influence of clay minerals on resistivity is greatly dependent on the type of clay minerals and the salinity of pore-water. Moreover, some experimental works were reviewed to investigate the temperature dependency of the resistivity of pore-water and ionic mobility within the electric double layer. I also carried out the laboratory experiment in which the temperature dependency of resistivity of clay-bearing artificial samples was measured. The results indicate that the temperature dependency of the resistivity of clay minerals is larger than that of pore-water. When applying the empirical formulas to clay-bearing rocks, it is necessary to compare the conductivity of pore-water with the excess conductivity of electric double layer generated by the clay minerals, and to take the effectiveness and the limitation of the formulas into consideration.

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 We overview the models for calculating velocity anisotropy in rocks in terms of the preferred orientations of cracks and minerals in rocks. We also discuss about wave propagation in anisotropic media. The P- and the S-wave velocities in rocks are affected by voids and cracks in rocks as well as the elastic properties of the constituent minerals. To understand the elastic properties of rocks, we start from the basic definitions and equations of the theory of elasticity for continuum homogeneous media. The elastic constants or elastic compliances are derived from an approach based on thermodynamics of solids. We derive the elastic constants or elastic compliances of materials from thermodynamics of solids. Then we see several approaches for calculating seismic wave velocities in composite media, where pieces of the second material are embeded into a matrix of the first material. For voids and cracks, the second material is gas or liquid in most of the actural cases in the Earth's crust. The most important anistropy in eath sciences is the transverse isotropy (TI) where seismic wave velocities are given as functions of the angle from the symmetry axis. For calculating seismic velocity anisotropy of rocks, we consider two anisotropies produced by preferred orientation of anisotropic minerals and oriented cracks. In the last, we dicuss about the crack density parameter that is one of the most important parameters for describing seismic anisotropy due to cracks and voids in a rock.

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 Laboratory measurements of threshold pressure, geomechanical property and permeability were conducted on an argillaceous rock when injecting liquid and supercritical CO₂. The argillaceous rock was sampled from the Otadai Formation in the Quaternary Kazusa Group, Chiba prefecture, Japan. Strain gages and PZTs were glued on the cylindrical sample (125 mm in length and 50 mm in diameter) to monitor strain and P-wave velocity changes during CO₂ injection. Threshold pressures were estimated under simulated in-situ pressure and temperature conditions, when injecting liquid and supercritical CO₂, by monitoring the strain and P-wave velocity changes. The threshold pressure for this argillaceous sample was about 3 MPa when injecting liquid CO₂ and was about 2 MPa when injecting supercritical CO₂. From the triaxial compression tests, we observed that the Young's modulus and Poisson's ratio decreased as the confining pressure increased. Compared to water-saturated condition, after injecting supercritical CO₂, the Young's modulus decreased about 21∼24% and the Poisson's ratio decreased about 28∼35%. Permeability to supercritical CO₂ measured at ‘Fractured’ condition increased about 30 μD compared to water measured at ‘Intact’ condition. Results of threshold pressure and geomechanical properties for this argillaceous rock are very useful to evaluate caprock integrity in geological CO₂ sequestration.

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 To study the rock physics model for soft sedimentary rocks which are widely distributed in Japan, the binary sand/shale mixture model as a granular model is applied to the seismic P- and S-wave logging velocity, density and porosity data obtained in Neogene sandy mudstone. The model consisting of the mixture of two different-size grains (sand and clay) can represent changes of porosity and elastic property of the rock due to the change of clay content. It can possibly model the soft sedimentary rock whose characteristics in general depend on its grain size distribution. The relationships between porosities and elastic moduli calculated from four datasets of P- and S-wave velocities and densities obtained in well loggings and laboratory tests are used for comparison of actual measurements and model calculations. Using the model thus obtained, the porosity and clay content are predicted with S-wave velocity and density logs and compared with the measurements in the laboratory test. These comparisons reveal that the sandy shale model can interpret elastic properties of the soft sedimentary rock which depend on the clay content and depth (confining pressure).

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 Underground sequestration of CO₂ requires avoiding possible leakage of the injected CO₂ to the surface. We have to understand how and how much casing cement can be altered by CO₂. Our experimental investigations immersed two kinds of cement samples and cement – sandstone sample in super critical CO₂ (60°C, 10MPa) and in water. Visible changes in cement samples include erosion in earlier stages and precipitation of calcite in later stages on the surface and in the pore spaces, which reduce porosity and permeability. At cement –sandstone interface, the mineral precipitated in the pores of sandstone is identified as CaCO₃. This suggests that the sealing ability of casing cement could be improved by exposure to supercritical CO₂ for a few months.

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