Page 25 - Geothermal Energy Systems Exploration, Development, and Utilization

1

1
Reservoir Definition
Patrick Ledru and Laurent Guillou Frottier

1.1
Expressions of Earth’s Heat Sources
1.1.1
Introduction to Earth’s Heat and Geothermics

Scientific background concerning the heat flow and the geothermal activity of the
earth is of fundamental interest. It is established that plate tectonics and activities
along plate margins are controlled by thermal processes responsible for density con-
trasts and changes in rheology. Thus, any attempt to better understand the earth’s
thermal budget contributes to the knowledge of the global dynamics of the planet.
Information on the sources and expressions of heat on earth since its formation
can be deduced from combined analyses of seismic studies with mineral physics,
chemical composition of primitive materials (chondrites), as well as pressure–
temperature–time paths reconstituted from mineralogical assemblages in past and
eroded orogens.
Knowledge of heat transfer processes within the earth has greatly improved
our understanding of global geodynamics. Variations of surface heat flow above
the ocean floor has provided additional evidence for seafloor spreading (Parsons
and McKenzie, 1978), and improved theoretical models of heat conduction within
oceanic plates or continental crust helped to constrain mantle dynamics (Sclater,
Jaupart, and Galson, 1980; Jaupart and Parsons, 1985). When deeper heat trans-
fer processes are considered, thermal convection models explain a number of
geophysical and geochemical observations (Schubert, Turcotte, and Olson, 2002).
It must be, however, noted that at a smaller scale (closer to the objective of this
chapter), say within the few kilometers of the subsurface where water is much more
present than at depths, a number of geological and geothermal observations are
not well understood. As emphasized by Elder (1981), crustal geothermal systems
may appear as liquid- or vapor-dominated systems, where physics of water–rock
interactions greatly differs from one case to the other. Actually, as soon as hy-
drothermal convection arises among the active heat transfer processes, everything
goes faster since heat exchanges are more efficient than without circulating water.

Geothermal Energy Systems. Edited by Ernst Huenges
Copyright  2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 978-3-527-40831-3