Page 134 - Geothermal Energy Renewable Energy and The Environment

120 Geothermal Energy: Renewable Energy and the Environment

The underlying concept is that a geothermal resource assessment is designed to establish the
amount of heat that is available from a reservoir to produce electricity or some other form of work.
To determine this value requires solving the relationship

Q = ρ × C × V × (T − T ), (7.1)
R
R
0
where Q is the heat available in the reservoir, ρ is the density of the rock making up the reservoir,
R
C is the heat capacity of the reservoir rock, V is the volume of the reservoir, and (T − T ) is the dif-
0
R
ference between the reservoir temperature, T , and the end state temperature, T .
0
R
Consideration of the various terms of Equation 7.1 reveals several significant obstacles in estab-
lishing Q . We will consider these individually in several sections of this chapter. However, in order
R
to avoid confusion about what a resource assessment actually provides, it is important to define
exactly what it is we are seeking when we undertake a resource assessment.
resoUrce base and reserVes
When we undertake an evaluation of how much of a resource exists, there are usually very spe-
cific reasons why this effort is being made. From a purely scientific or academic perspective, it
may be important to know how much of a material or substance exists in order to conduct global
mass balance calculations. From a regulatory perspective, it may be important to establish the
extent of a resource that is potentially extractable in order to determine how best to manage
development of the resource in a way that is also environmentally intelligent. From an economic
point of view, it may be that an estimate is needed to determine how much of a resource can be
extracted using a specific kind of technology in order to establish the economic potential of a
development project.
These different needs have given rise to a methodology that allows quantification of the different
ways in which a resource may occur. Figure 7.1 graphically delineates the categories, nomenclature,
and variables used in this approach.
Some fraction of a resource has usually already been identified, mapped, and is readily accessible
using existing technology. The collection of such occurrences is called the resource reserve. The
reserve amounts to that portion of the resource that can be economically developed and brought
online with little or no additional exploration effort. These are occurrences about which there would
be relatively little uncertainty regarding the magnitude of what is available.
The reserve provides an important insight into how we might develop a strategy for estimating
how much additional resource might exist that would be economically viable to pursue, but has not
yet been discovered. Estimating the amount of a resource that has not yet been discovered is based
on the reality that the identified resource reserves occur in specific types of geological environments.
For example, The Geysers geothermal area in California and the Iceland geothermal systems exist
in close proximity to recently active volcanic centers. For each of those types of known and charac-
terized environments, experience gained through exploration and development provides a means to
estimate the fraction of that environment that may possess a geothermal reservoir of a certain qual-
ity. It is through this experience base that we can make an estimate of the undiscovered geothermal
reservoirs that are potentially economically suitable for development. This undiscovered but poten-
tially economically viable heat source, along with the known reserves, makeup what is formally
termed the resource.

uncerTainTy
Uncertainty is introduced into the process of identifying the formal resource by the fact that estab-
lishing the economic viability remains a contentious issue. In some instances, a site may be suitable
for development using available technology but cannot be economically brought online because