• PDF / 935,393 Bytes
• 20 Pages / 595.276 x 790.866 pts Page_size
• 61 Downloads / 255 Views

DOWNLOAD

REPORT

---

Thermal Storage and Transport Properties of Rocks, II: Thermal Conductivity and Diffusivity Christoph Clauser Institute for Applied Geophysics and Geothermal Energy, RWTH Aachen University, Aachen, Germany

Synonyms Heat conductivity; Thermal conductivity; Thermal diffusivity

Definition Thermal conductivity (also: heat conductivity) l: Physical property governing heat diffusion in the steady state. It defines how much heat flows across a unit cross section of rock along a unit distance per unit temperature decrease per unit time; dimension: W m
1 K
1. Thermal diffusivity k: Physical property governing transient heat diffusion. It is defined by the ratio of thermal conductivity and thermal capacity, i.e., by the ratio of heat flowing across the face of a unit volume and the heat stored in the unit volume per unit time; dimension: m2 s
1.

causes also the physical properties to vary accordingly, some by factors and others even by orders of magnitude. Thermal rock properties, generally, vary by a factor within the same order of magnitude. This variability limits the value of individual, rock typespecific data measured on single samples, unless they originate exactly from the formation (or even location) of interest. In all other cases, a statistical approach help accounting for this intrinsic variability by characterizing physical rock properties by the mean values and standard deviations of populations of a sufficiently large number of individual measurements. An example of this approach is the nationwide survey performed in the Republic of Korea where thermal conductivity measurements on 1 560 rock samples were combined into statistical moments for ten, eight, and seven major types of igneous, metamorphic, and sedimentary rocks, respectively (Shim et al. 2010). Recently, a large number of petrophysical data were compiled into a publically available, digital database (Bär et al. 2020) comprising thermal, mechanical, electrical, and magnetic rock properties. Each data point is supplemented by a reference (if available) and information on sample location, petrographic analysis, and chronostratigraphic age. This data, while still incomplete in spite of its large number of more than 75 000 individual measurements, is another valuable source of individual data from specific locations but also useful for calculating means and standard deviations for larger volumes of rock.

General Remark A great amount of petrophysical data has been published in the past by individual researchers interested in thermal rock properties. Usually, these data collections are sorted according to petrographically defined rock types. But rock type, unfortunately, is a weak descriptor for physical properties: Any rock type shows significant variability in characteristics which influence its physical properties, such as (but not limited to) porosity, cementation, mineral content, etc. This

Thermal Conductivity Fourier’s law of heat conduction defines the vector of specific heat flow qi, i.e., heat flow normalized by area, as the product of the thermal conducti