Jökull - 01.12.1966, Blaðsíða 50

There are mainly two quantities wliich can be
maximized, that is, either the total heat pro-
duced, or the discounted profit over a given
period. The maximum profit method involves
assumptions as to the interest rate, discount
period, etc., and we have to admit that we
have no universal rules for selecting the suit-
able quantities. Hence, optimality may be a
rather vague concept.
Let us now review the steps in the develop-
ment of a geothermal area, either for power
generation or building heating purposes. Be-
fore intelligent decisions can be made, the geo-
thermal engineers have to obtain information
on the following main items:
(1) Exploration costs,
(2) Production costs per unit energy at the
consumer as a function of the installed
power,
(3) Size and elasticity of the marketj
(4) Maximum possible power output of the
thermal area,
(5) Total available energy, i.e. the useful life
of the thermal area.
We will take a brief look at the problems
involved.
3.2) Exploration. The opening up of a geo-
thermal prospect involves geophysical explora-
tion and test drilling. This work is carried out
in order to obtain preliminary data on impor-
tant quantities such as the base temperature,
pressure, permeability of formations, chemical
quality of the fluids, etc. The total costs in-
volved in this stage may, in the case of larger
projects, be of the order of i/g to one million
dollars. The exploratory wells usually account
for an overwhelming part of this sum.
The allocation of funds for exploration work
is alwavs somewhat of a problem, and no
general rules are available. The requirements
in individual cases differ greatly, and the re-
actions of project managers to the various field
situations are very individual.
The author has had contact with geothermal
situations in various parts of the world. His
experience is that there is a rather pronounced
tendency to underestimate the usefulness of
modern geophysical exploration. It is generally
much easier to obtain funds for exploratory
drilling than for the more sophisticated geo-
204 JÖKULL
physical exploration. The result has been that
most of the geothermal areas now being ex-
ploited are “underexplored” and “overdrilled”.
In many cases the funds allocated for ex-
ploration have been less than 5% of the total
exploration and drilling costs. The author is
of the opinion that this does not represent an
optimum. There are substantial indications that
a relatively greater use of funds for geophysical
exploration and shallow temperature probing
will reduce the overall exploration and produc-
tion drilling costs. Of course, it is impossible
to point out definite methods to derive the
optimum, but the optimum drilling/geophysical
exploration cost ratio may rather be 5 to 10
than 20 to 30. In some cases lack of trained
personnel has been the limiting factor.
(3.3) Production costs. The production cost
is obtained on the basis of design studies which
involve optimization in various ways. A part
of this work is of a rather common nature, e.g.
the optimization of pipe-line diameters, etc.,
and will not be discussed here. One problem
encountered in the petroleum as well as in
the geothermal industry, is the number of pro-
duction wells that must be drilled for a given
power output. As already stated, there is no
doubt that some areas have been "overdrilled”
because of inadequate well-testing methods.
Again we meet the problem of drilling versus
testing by physical methods, and again it ap-
pears as that we have not derived the optimum
allocation of funds.
The engineering aspects of the design of
geothermal power and heating plants are rather
straight-forward and we will not discuss this
matter further.
(3.4) Poiuer output and the elasticity of the
market. In deriving the optimum power to be
delivered by a geothermal plant, we meet per-
haps the most interesting optimization problem
involved. We have to adjust the market to the
source available. In general, geothermal areas
have a certain capacitance, that is, a reservoir
capacity which can be used as a storage. On
the other hand, the market demand is a rather
variable function of time, mainly in the case
of heating plants. For example, heating plants
in Iceland have to meet a number of short-
term cold spells during the winter. The ratio
of the peak demand to the average demancl is
2,5 to 3. The reservoir capacity of the geo-