Page 53 - Geothermal Energy Systems Exploration, Development, and Utilization

1.3 Conceptual Models of Geothermal Reservoirs 29
geometric model and the realization of 2D or even 3D seismic surveys are presently
strategies that are promoted by many exploration companies, such as ENEL in
Larderello or in the Rhine graben.
Themainsource offluids is waterinthe oceanand meteoric waterinthe
continents. The infiltrated part of meteoric and sea water will migrate down within
permeable lithologies and/or fracture systems. Depending of the tectonic sites and
of the geothermal gradient, they will be heated to temperature that will already be
sufficient for direct use or even power generation. This is the case of the basin
located in thinned crust such as the Rhine graben. What is the lower boundary of
this infiltration?
Besides the first-order variation related to the intrinsic permeability of the
systems, variation of permeability is also recorded with depth. Permeability of
fractured crystalline rocks decreases with increasing pressure or effective stress.
The porosity will be also controlled by the pressure–temperature conditions, and an
increase in pressure with depth will reduce the porosity as the compaction will be
greater. Surveys performed during deep drilling programs have demonstrated this
2
decreasing permeability at depth, between 10 −17 and 10 −15 m above 4 km depth
2
and 10 −18 –10 −16 m below (Huenges et al., 1997), and a paper on the variation of
permeability as a function of depth based on geothermal data and metamorphic
systems was published in 1999 by Manning and Ingebritsen (Figure 1.16). With
respect to this aspect, the brittle–ductile transition is a major decoupling surface
at the scale of the crust that marks the lower limit that meteoric water can reach at
depth and a change in fluid flow processes as it has been shown in natural analog
(Famin et al., 2004).

0
Lower
5 crustal
devolatilization Brittle
10
Depth (km) 15 Ductile

20
25 Nu = 2
30–300 ˚c km −1
30
B
35
−20 −18 −16 −14 −12
2
Log permeability (m )
Figure 1.16 Permeability as a function of depth in the con-
tinental crust based on geothermal data (solid squares) and
metamorphic systems (open squares). (After Manning and
Ingebritsen, 1999)
.