Evaluation of the thermal conductivity of granitoid stratum at great depths by core samples

Studies of thermophysical and mechanical properties of rocks at great depths have been conducted as part of research into the hydrology and geochemistry of groundwater, the migration of substances such as uranium, and drift excavation effects on surrounding rock mass by the Japan Nuclear Cycle Development Institute. Measurement results and evaluation are given of thermal conductivity, λ, of core specimens collected from three research wells, 1000 m in depth, dug in granitoid stratum around the Tono mine, Japan. Twenty specimens were collected from each well, taking account of depth interval and lithofacies. The λ value of the specimens was measured by the unsteady hot-wire comparative method in the temperature range 25 - 85 °C. Experimental results showed that the values of λ were 1.9-2.1 W m - 1 K - 1 for conglomeratic sandstone, 2.9-3.6 W m - 1 K - 1 for quartz porphyry, 2.1-3.6 W m - 1 K - 1 for sandstone and 1.9-3.8 W m - 1 K - 1 for granite. A weak reverse temperature dependence of the λ values was observed in the tested temperature range. The distribution of the values of λ versus well depth, Z, showed that the values of λ were influenced by microcracks in the specimens which might have been caused by the stress released during the digging process of the core. In-situ λ values were evaluated with the core data prescribed above and the three-phase geometric-mean model as the A values for the intact rocks eliminated porosity.