ABSTRACT:

The requirement for a quantitative understanding of thermal processes in several fields of applied geology is introduced and key properties are identified. Experimental methods used at the British Geological Survey (BGS) for the measurement of thermal conductivity, thermal expansivity and heat capacity are outlined and typical test results are presented for two rock-types of current practical interest in relation to a UK repository for radioactive waste. The application of this type of data is illustrated by a number of scoping calculations.

RÉSUMÉ:

La necessite de comprendre quantitativement les processus thermiques dans plusieurs domaines de la geologie appliquee est introduite, et les proprietes clef sont identifees. Les methodes experirnentales utilisees au British Geological Survey (BGS) pour la mesure de la conductivite thermique, du coefficient d'expansion thermique et de la capacite calorifique sont soulignees, et des resultats de tests typiques sont presentes pour deux types de roches ayant un unteret pratique en relation avec un site Britannique de stack age de dechets nucleaires. L'application de ce type de donnees est illustree par un certain nombre de calculs appliques.

ZUSAMMENFASSUNG:

Die Bedingung fuer ein quantitaves Verstandnis der Thermal-Prozess in vielen Bereiche der praktischen Geologie wird eingefuehrt und wichtigste Merkmale werden erkannt. Experimentelle Methode die "British Geological Survey" (BGS) fuer die Massenheit der Thermal-Leitingsfahigkeit, Thermal- Ausdehnbarkeit und der Warmegrad verwenden, werden beschrieben. Das Ergebnis typischen Leistungspluefungen fuer zwei Felsen wird vorgebracht. Gegenwartig sind diese Felsen von grosser pracktischen Bedeutung in bezug auf ein Behaltnis fuer radioaktiv Abfallmaterial in Grossbritannien. Die Verwendung diesen Angaben wird von vielen Berechnungen erklart.

1. THERMAL PROCESSES IN GEOLOGY

Knowledge of the thermal properties and high temperature behaviour of rock/fluid systems has become increasingly important, with widespread interest in thermal processes in many areas of geology. For example, enhanced oil recovery can involve steam injection methods in depleted oil reservoirs where steam is injected into a producing well and permitted to condense and give up its latent heat (Somerton, 1992). In fire-flooding methods, air is injected into an injection well, the residual oil around the well bore is ignited and the resulting fire front drives oil into the producing wells. Geothermal reservoirs can be vapour-dominated systems, where natural groundwater makes contact with an underground heat source creating steam which is extracted from a production well, or, liquid dominated systems, where groundwater at naturally high temperatures is extracted for the benefit of man's activity, or, dry systems in igneous rocks, where water is pumped into injection wells and is heated as it flows through a fracture network before being extracted via production wells (Jessop, 1990). Underground storage of heat involves injecting a heated fluid into permeable rocks for storage until it is later withdrawn when the heat is required. These underground heat stores are employed to store and spread heat from solar energy over the diurnal cycle. Underground disposal of radioactive waste is contemplated in the vaults or galleries of an engineered repository in a low-permeability host- formation. The heat produced from radioactive decay of the waste will be dissipated through the surrounding rock. Key considerations in all areas are the heat flow properties of the rock, and the impacts of heating on mechanical response and system performance.

2. LABORATORY METHODS

Important thermal properties include:

  • thermal conductivity; yielding information about steady-state temperature fields,

  • specific heat; yielding information about the heat energy storage in rocks, and,

  • thermal expansion; used in the calculations of thermally-induced stresses and pore pressures.

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