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Subsurface Fluid Flow: The Hydrology of Geothermal Systems 61

10 4

10 3

Permeability (md) 10 2 1

10

10 0
Fracture Matrix
dominated dominated
10 –1
10 –1 10 0 10 1 10 2
Porosity (percentage)
FIGUre 4.9 Variation between porosity and permeability for a suite of geothermal systems. Permeability
is in millidarcies. The principle source of the porosity (fracture vs. matrix) is indicated. (From Björnsson, G.
and Bodvarsson, G., Geothermics, 19, 17–27, 1990.)

Most obvious is that systems in which fluid flow is dominated by matrix porosity invariably require
higher porosity to achieve a given permeability than systems that are dominated by fractures. For
this reason, exploration for geothermal resources that are focused in sedimentary basins benefit
from access to previous geological studies in which the hydrological properties of subsurface units
have been established. Knowing which geological units have high matrix porosity and acquiring
information regarding the subsurface distribution of those units can substantially save resources.
A second key point from the Björnsson and Bodvarsson (1990) survey is that low fracture porosity
need not be the limiting factor for power generation. Fracture porosity as low as 0.2% may still be
suitable for producing power, provided sufficient fluid flow rates can be obtained. This suggests that
in a fractured rock mass, a single fracture set, with modest fracture spacing may be sufficient for
achieving adequate transfer rates of heat from the subsurface.
Finally, the variability of porosity and permeability in natural systems can be quite high, spanning
several orders of magnitude at a given site. For that reason, it is important to conduct an extensive
exploration program for identifying porosity–permeability anisotropy. Such a program must also
establish the extent to which properties are heterogeneously distributed in the subsurface. This topic
is discussed in more detail in the following chapters.

case sTUdy: lonG Valley caldera
Long Valley Caldera has been the site of geothermal power generation since 1985. Currently, a
40 MWe power plant produces electricity from a relatively shallow geothermal reservoir with a
working fluid temperature of about 170°C. Flow rates of water from fractured and porous vol-
canic rocks are about 900 kg/s. The geologic setting for this system, and the information gained
from the subsurface by a drilling program undertaken to evaluate the existence of a near-surface
magma body, provide an excellent example of the complexity to be expected in a subsurface flow
regime. A series of papers published in the Journal of Volcanology and Geothermal Research
(2003, volume 127) provide a summary of the current state of knowledge about this system.
Long Valley Caldera is a volcanic feature that lies on the boundary between the Sierra Nevada
mountain system and the Basin and Range tectonic province (Bailey 1989). It is part of a volcanic