Page 54 - Geothermal Energy Systems Exploration, Development, and Utilization

30 1 Reservoir Definition
If the intrinsic permeability of the continental crust is low below the
brittle–ductile transition zones, fluid flow will occur through pervasive flow in
zones of low strain rates and homogeneous lithologies while higher strains and
a more heterogeneous rheology will favor a channeled flow (Oliver, 1996). In
the case of the geothermal reservoirs, attention will be paid on the channeled
flow that will occur along active tectonic zones or on emplacement of magmatic
or volcanic suites. In such environments, hot fluids with temperature generally
◦
greater than 250 C will move upward and finally reach the surface if they are not
trapped by a cap nonpermeable layer or blocked within the fracture network. They
ascend through the crust and will get connected, at the brittle–ductile transition
zone, with fluids that have been infiltrated from surface. Many of the active
geothermal fields are in fact resulting from the mixing between these ascending
and descending fluids within the permeability network of the continental crust. A
loop is created at the level of the reservoir, corresponding to a convective transfer
of heat toward surface. Geysers from Iceland or from western United States are
the most visible trace of this phenomenon.
This link between the permeability of the continental crust, the potential infil-
tration of meteoric fluids and seawater, the brittle–ductile transition at depth,
and the potential connection to fluid of deep origin illustrates the need to
have a global approach for the exploration of geothermal reservoirs. Thus, nu-
merous studies dedicated to tectonic processes during prograde and retrograde
metamorphism or to transfer and trapping of hydrothermal ore deposits could
provide a lot of information about the variation of permeability in the continental
crust.
Other variations of permeability are observed laterally within one single geological
medium depending on heterogeneity, anisotropy, and time. These parameters
constrain the efficiency of the reservoir and its sustainable use. The intrinsic
permeability is a function of the heterogeneity and anisotropy of the medium.
Fluid flow will tend to be greater parallel to the main layering of the sedimentary
or volcanic rocks and foliation of metamorphic rocks rather than across them.
Permeability is also a time-dependent process as fluid–rock interactions will
provoke permanent dissolution and recrystallization phenomenon that will modify
the permeability network. The intensity and orientation of the stress field will
exert a direct control on this process by determining zones of compression and
extension, in relation to the relative position of the main stress axis and resulting
strain.

1.3.3
Summary

The review of the phenomena that control the distribution of heat and fluid at
depth shows that conventional reservoirs for high enthalpy geothermal energy are
located in zones of active volcanism or magmatism while low- to medium enthalpy
can be found in varied environment. The identification of potential reservoirs for
developing a heat exchanger is linked to our ability to evaluate the coincidence