Page 67 - Geothermal Energy Renewable Energy and The Environment

4 Subsurface Fluid Flow:

The Hydrology of

Geothermal Systems

The ability to use geothermal heat requires that it be transferred from the depth to the surface. The
most economical and efficient means for doing this relies on naturally occurring, geothermally
heated water. If that water is not available in sufficient quantity, technological approaches have been
developed for introducing water into the subsurface through engineering efforts. This chapter con-
siders the principles that determine how fluids move through the subsurface, the natural constraints
that limit water flow, and the basic principles that apply to enhancing water availability.

a General model For sUbsUrFace FlUId Flow

Virtually anywhere on the planet one might choose to drill a well, one will encounter water. The
depth to which one would have to drill, and the amount of water encountered, will vary tremendously,
from virtually nothing to artesian flow out of the wellhead. If the well were to be pumped, in some
cases the water would be quickly exhausted, in others, the amount of water would seem to be limit-
less. Indeed, it is not uncommon for wells only a few hundred meters apart to exhibit completely
different behavior; either the water is reached at dramatically different depths, or one well will
rapidly go dry upon pumping, while the other will seem to provide a boundless water supply. And,
wells that go dry when pumped will often recharge after some period of time if pumping is stopped.
What controls this diverse behavior?
In Figure 4.1, the processes and geology that control water flow are depicted schematically.
The fundamental control on subsurface water movement is the amount of space in the rock that is
available for water to occupy and the physical characteristics of that space. The primary distinc-
tion in flow pathways is between flow within and through the body of the rock (matrix flow) and
flow within fractures that may be present (fracture flow). The attributes of these flow regimes are
discussed in detail in the following sections.

maTrIx porosITy and permeabIlITy

If one walks along a beach carefully observing what happens as waves lap against the shore, one will
notice that, after the wave advances and begins to recede, some portion of the returning flow seeps
into the sand. Often that process is accompanied by bubbles streaming up from below. The bubbles
are air that is forced out of the pore spaces between the grains of sand in the subsurface by infiltrat-
ing water. The extent to which these actions can be seen depends upon the coarseness of the sand—
very coarse sand beaches will sometimes have virtually no return surface flow—the advancing
water washing up onto the beach will immediately disappear into the sand. Very fine sand beaches,
on the other hand, often have a very strong backflow, with little infiltration of the wave wash into the
underlying sand. This behavior exhibits the profound interplay between porosity and permeability
and how they are affected by the characteristics of the pores.
Pores are the open spaces between grains in gravel, sand, soil, and rock. In coarse gravels and
sands, pore spaces can be large (a significant fraction of a cm, for example) and take up as much as

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