Effect of Pore Water on the Thermal Diffusivity of Water-Saturated Rocks at Extremely Low Temperatures

Abstract

Naoto KAMOSHIDA* and Masafumi OKAWARA** Currently, the effect of pore water on the thermal diffusivity of rocks at low temperatures is unclear. In this study, the thermal diffusivities of Berea sandstone (12.8%-porosity) and Himekami granite (1.09%) were measured using an optional heating method. Consequently, the change in the thermal diffusivity of saturated Himekami granite (approximately 100%-water saturation) increased continuously and gradually, as observed in dry granite (approximately 0%). However, when cooling from room temperature to −10°C, the thermal diffusivity of saturated Berea sandstone increased discontinuously and rapidly, in contrast to the dry sandstone in which it increased continuously and gradually. Subsequently, the effect of porosity and pore structure on the thermal diffusivity of saturated rocks was examined during the freezing process by comparing the results of Kimachi sandstone (20.2%-porosity) and Ogino tuff (28.7%) measured using the same method by the Kamoshida et al. (2013). Results show that the change in the thermal diffusivity during the cooling process of saturated rocks is depended by the pore structure (pore size distribution) instead of the rock's water content (porosity). Therefore, rocks containing macropores, such as Berea and Kimachi sandstones, can be assumed particle-dispersed complexes (the rock-forming minerals are the matrix, and the pores are the particles). Thus, the changed behavior of thermal diffusivity during the cooling process can be estimated by measuring the pore size distribution and thermophysical properties of rock-forming minerals.