Page 34 - Geothermal Energy Systems Exploration, Development, and Utilization

10 1 Reservoir Definition
Apart from thermal conductivity contrasts, heat production rates may also vary by
a factor of 10 or more between two lithologies (Sandiford, McLaren, and Neumann,
2002; McLaren et al., 2002). In the case of HHP granites, radiogenic content is
−3
so high that heat production rates may reach 10–20 µWm ,asitisthe case
of the synthetic example of Figure 1.5, where embeddings have an averaged heat
−3
◦
production rate of 1 µWm . At 5 km depth, a temperature difference of 42 C
−3
◦
(90 C) is obtained for a high heat production of 10 (20) µWm .
In the case of Figure 1.5, the obtained temperature anomaly depends on several
other parameters such as the emplacement depth of the anomalous body. For
◦
example, same granite of Figure 1.5 emplaced at 10 km depth would involve a 30 C
anomaly at 5 km depth. This temperature difference also corresponds to the case
of a shallow emplacement of 500 m below the surface. This nonobvious result can
be explained by detailed analysis of geotherm curvature, as presented by Sandiford,
Fredericksen, and Braun (2003).
1.1.5.2 Transient Effects
A number of studies have demonstrated the role of transient geological processes
on crustal temperatures. Large-scale tectonic processes (thrusting events, erosion,
and sedimentation) can result in temperature differences reaching several tens
of degrees centigrade at a few kilometers depth (England and Thompson, 1984;
Ruppel and Hodges, 1994). Magma emplacement or presence of hydrothermal
convection at shallow depths may also explain disturbed temperature profiles
(Cathles, 1977; Norton and Hulen, 2001).
Because thermal diffusivity of rocks is low, transient thermal evolution of
rocks undergoing conducting processes is very slow, and return to equilibrium
temperatures may last several tens to hundreds of million years. Figure 1.6
illustrates some examples of large-scale thermal evolution of the crust undergoing
tectonic events. One may note that in the case of a thrusting event, the equilibrium
◦
thermal field (with a maximum temperature of 820 C) is reached 120 Myr after the
onset of thrusting. On the contrary, when convective processes are involved around
intrusive bodies, heat transfer mechanisms through fluid circulation are acceler-
ated, and typical timescales are lower than 1 Myr (Cathles, 1977). When smaller
scale systems are considered, thermal equilibrium is reached faster. For example,
serpentinization of oceanic crust may result in large amplitude thermal signatures
lasting less than a few thousands of years (Emmanuel and Berkowicz, 2006).

1.1.5.3 Role of Anisotropy of Thermal Conductivity
Apart from steady-state heat refraction due to thermal conductivity contrasts or
variations in heat production rates, other subtle effects affecting thermal properties
may trigger thermal anomalies. Temperature dependence of thermal conductivity is
one example, as shown in Clauser and Huenges (1995). In the case of sedimentary
basins, porosity dependence of thermal conductivity is also significant, as shown
in several studies (Beziat, Dardaine, and Gabis, 1988; Waples and Tirsgaard, 2002).
Sedimentary basins correspond to interesting geothermal targets all the more
that numerous temperature measurements may be available. When thick clayey