Rocks and sediments are composite materials comprising multiple mineral (or other solid) phases, with fluid phases (air, water, hydrocarbons) or ice filling the pore spaces. Two key thermogeological parameters, used in the design of ground source heating and cooling (GSHC) and geothermal systems are (i) volumetric heat capacity (a scalar quantity) and (ii) thermal conductivity (a tensor). The bulk volumetric heat capacity of a composite material, being a scalar quantity, can simply be calculated as the weighted arithmetic mean of the components. Many proposals have been made for the estimation of bulk thermal conductivity from the conductivities of a rock or sediment’s components, and no single proposal is universally satisfactory. A variety of proposed algorithms have been examined for monomineralic systems (quartz or calcite + water + air). A weighted geometric mean of the components of a geological material generally provides a good (but not perfect) estimator of bulk thermal conductivity, but tends to perform poorly in dry, porous materials. A semi-empirical “Kersten-Johansen” approach seems a good estimator of thermal conductivity of porous sandy materials at varying states of compaction and saturation; its applicability to lithified rocks and non-silicate lithologies has yet to be demonstrated.
David Banks (Thu,) studied this question.