Page 52 - Geothermal Energy Systems Exploration, Development, and Utilization

28 1 Reservoir Definition
conglomerate are generally porous and can store large quantities of fluids
within their pore network. They constitute natural reservoirs in the crust for
all kind of fluids. The intrinsic permeability parameter is the primary control
2
on fluid flow as it will vary from 10 −23 m in intact crystalline rocks to 10 −7
2
m in detrital porous sediments; meaning 16 orders of magnitude variations
(Manning and Ingebritsen, 1999).
2) The fracture permeability is linked to the discontinuities that are present within
the rock along which fluid circulation is possible. This type of permeability
is generally well developed in crystalline massifs. Thus, although granite is a
nonpermeable rock, a granitic massif will be considered as a permeable massif
as a whole – fluid circulating along the fracture network. Implicitly, such
permeability will be well developed in the vicinity of large fracture systems,
whether active or fossil. Because of the discontinuous character of the fracture
and their geometrical complexity, the intrinsic permeability of such system is
more difficult to evaluate compared to stratified permeable layers.

The range of permeability observed and measured in the continental crust can
be illustrated by a one-dimensional graph (Figure 1.15). Geothermal reservoirs
2
are characterized by rather large permeability, higher than 10 −13 m .Analysesof
coupled groundwater flow and heat transport in the upper crust infer permeabilities
2
2
in the range of 10 −17 –10 −14 m with a mean value greater than 10 −16 m (Manning
and Ingebritsen, 1999).
The identification of potential geothermal reservoir will then focus in priority on
the exploration of both types of permeability related to the intrinsic and fracture
permeability properties. A good knowledge of the geometry of the geological units
and their physical properties, determined in situ or by geophysical methods, and
of the structural pattern is a key for successful exploration. The building of a 3D

Geologic forcing
of pressures Advective heat
transport
Advective solute
transport
2
−24 −22 −20 −18 −16 −14 −12 −10 −8 log k (m )
−1
−16 −14 −12 −10 −8 −6 −4 −2 0 log K (m s )
Pierre Shale: 1 km depth Near surface
lab and in situ
Kilauea basalt: in situ
Aquifers
Geothermal
reservoirs
Figure 1.15 Range of permeabilities observed in geologic
2
media: permeability (k,m ) and hydraulic conductivity (K =
−1
kρfg/µ,m s ) in relation to water density ρ w and viscosity
◦
µ w at 15 C.