The Thermal Diffusivity of Sedimentary Rocks: Empirical Validation of a Physically Based α - φ Relation

Understanding the thermal behavior of nonsteady state subsurface geosystems, when temperature changes over time, requires knowledge on the speed of heat propagation and, thus, of the rock's thermal diffusivity as essential thermo-physical parameter. Mixing models are commonly used to describe thermo-physical properties of polymineralic rocks. A thermal diffusivity-porosity relation is known from literature that incorporates common mixing models into the heat equation and properly works for unconsolidated, clastic clayey, and sandy marine sediments of high porosity (35%-80%). We have proofed the relation's applicability for consolidated, isotropic sedimentary rocks of low porosity (<35%). The performance of this approach was evaluated for consolidated quartz-dominated sandstones containing air, water, and heptane as pore- and/or fracture-filling medium. For these rocks, the reliability of the relation was confirmed for the entire range of porosity and all three media, with water-saturated rocks displaying an almost perfect fit between measured and modeled thermal diffusivity. Additional measurements conducted on a larger suite of low-porous siliciclastic and carbonate rocks imply that it is also suitable to acceptably good to infer the thermal diffusivity of mineralogically more diverse sedimentary rocks. In contrast to other common mixing models, this relation is appropriate to convert thermal-diffusivity data obtained on air-saturated samples into such reflecting water-saturated conditions. This is of particular importance for handling data acquired from methods limited to the measurement of dry samples, for example, laser-flash analysis. In conclusion, the applied relation is suited to model thermal-diffusivity data of isotropic sedimentary rocks of different porosity and independent of the pore fluid.