Anisotropy of Thermal Conductivity and P-wave Velocity in Sedimentary Soft Rocks and their Relationships with Porosity

Abstract

Thermal conductivity is one of the important parameters to accurately predict temperature structures in the subsurface for various geoengineering applications such as geothermal development and underground disposal of radioactive wastes. Thermal conductivity varies with rock type and is anisotropic when rocks are layered, such as shale and schist. Sedimentary rocks, especially sandstone and mudstone, have layer structures formed by consolidation that reduces porosity. Although this porosity change may be linked with the anisotropy of thermal conductivity, the relationship has not been investigated previously. In this study, we quantitatively investigated the anisotropy of thermal conductivity of sedimentary soft rocks, then examined relationships between porosity and the anisotropy of thermal conductivity. In addition to thermal conductivity, the anisotropy of P-wave velocity was also examined as it is known to correlate well with thermal conductivity. Thermal conductivity and P-wave velocity were measured in both directions perpendicular and parallel to bedding plane in the specimen, respectively. The cubic specimens of sedimentary soft rocks taken from the Boso Peninsula, central Japan were used as the samples, whose porosity ranged approximately from 37% to 55%. As a result, thermal conductivity and P-wave velocity of the specimens parallel to bedding plane were higher than those in perpendicular direction, indicating that the anisotropies regarding to bedding plane were observed in these physical properties. The degrees of anisotropy regarding to bedding plane of both physical properties were then quantitatively evaluated by two indices. The results showed that the degrees of anisotropy of thermal conductivity and P-wave velocity were linked with porosity; the degree of anisotropy in low porosity specimens tended to be smaller than in high porosity specimens.