Estimation of field‐scale thermal conductivities of unsaturated rocks from in situ temperature data

Abstract

A general approach is presented here that allows estimation of field‐scale thermal properties of unsaturated rock using temperature data collected from in situ heater tests. The approach is used to determine the thermal conductivities of the host rock of the Drift Scale Test (DST) at Yucca Mountain, Nevada. The DST was designed to obtain thermal, hydrological, mechanical, and chemical (THMC) data at Yucca Mountain. Sophisticated numerical models have been developed to analyze these THMC data. However, though the objective of those models was to analyze “field‐scale” (of the order of tens of meters) THMC data, thermal conductivities measured from “laboratory‐scale” core samples have been used as input parameters. While using laboratory‐scale thermal conductivity values in field‐scale models can be justified, such applications introduce uncertainties in model predictions. Temperature data from the DST provide an opportunity to resolve some of these uncertainties. These temperature data can be used to estimate the thermal conductivity of the DST rock, and given the large volume of rock affected by heating at the DST, such an estimate will be a reliable thermal conductivity value for field‐scale application. An analytical solution is developed for the temperature rise in the DST rock; and using a nonlinear fitting routine, a best fit estimate of field‐scale thermal conductivity is obtained. Temperature data from the DST show evidence of a below‐boiling zone (wet) and an above‐boiling zone (dry). Estimates of thermal conductivity for both these zones are obtained in this paper. Sensitivity of these estimates to the input heating power is also investigated. The estimated thermal conductivity values are compared with core measurements and those estimated from geostatistical simulations.