Page 26 - Geothermal Energy Systems Exploration, Development, and Utilization

2 1 Reservoir Definition
It is thus important to delineate which type of heat transfer process is dominant
when geothermal applications are considered. Examples of diverse geothermal
systems are given below.
Within the continental crust, a given heat source can be maintained for distinct
time periods according to the associated geological system. Hydrothermal fields
5
4
seem to be active within a temporal window around 10 –10 years (Cathles, 1977),
whereas a magma reservoir would stay at high temperatures 10–100 times longer
(Burov, Jaupart, and Guillou-Frottier, 2003). When radiogenic heat production is
considered, half-lives of significant radioactive elements imply timescales up to
9
10 years (Turcotte and Schubert, 2002). At the lower limit, one can also invoke
phase changes of specific minerals involving highly exothermic chemical reactions
(e.g., sulfide oxidation and serpentinization) producing localized but significant
4
3
heat excess over a short (10 –10 years) period (Emmanuel and Berkowicz, 2006;
Delescluse and Chamot-Rooke, 2008). Thus, description and understanding of
all diverse expressions of earth’s heat sources involve a large range of physical,
chemical, and geological processes that enable the creation of geothermal reservoirs
of distinct timescales.
Similarly, one can assign to earth’ heat sources either a steady state or a transient
nature. High heat producing (HHP) granites (e.g., in Australia, McLaren et al.,
2002) can be considered as permanent crustal heat sources, inducing heating of
the surrounding rocks over a long time. Consequently, thermal regime around
HHP granites exhibits higher temperatures than elsewhere, yielding promising
areas for geothermal reservoirs. On the contrary, sedimentary basins where heat is
extracted from thin aquifers may be considered as transient geothermal systems
since cold water reinjection tends to decrease the exploitable heat potential within
a few decades.
Finally, regardless of the studied geological system, and independent of the
involved heat transfer mechanism, existence of geothermal systems is first con-
ditioned by thermal regime of the surroundings, and thus by thermal boundary
conditions affecting the bulk crust. Consequently, it is worth to understand
and assess the whole range of thermal constraints on crustal rocks (phys-
ical properties as well as boundary conditions) in order to figure out how
different heat transfer mechanisms could lead to generation of geothermal
systems.
The following subsections present some generalities on earth’ heat sources and
losses in order to constrain thermal boundary conditions and thermal processes
that prevail within the crust. Once crustal geotherms are physically constrained by
the latter and by rock thermal properties, distinct causes for the genesis of thermal
anomalies are discussed.

1.1.2
Cooling of the Core, Radiogenic Heat Production, and Mantle Cooling

The earth’s core releases heat at the base of the mantle, through distinct
mechanisms. Inner-core crystallization, secular cooling of the core, chemical