Jökull - 01.12.1961, Page 66

altogether uninteresting. An increase of 50°
C/km implies 50° to 60° C at the depth of 1.000
meters and 100° C to 110° C at the depth of
2.000 meters.
On the other hand, the cost of drilling
down to the depth of 2000 meters is in most
cases not excessive. It should therefore in sorne
non-volcanic areas be possible to drill without
excessive cost into horizons having a tempera-
ture of around 100° C.
Such horizons, if porous ancl permeable, are
possibly reservoirs of water at a temperature
around 100° C, which is sufficient for space
heating. In order to illustrate the economical
aspects it can be stated that one favourably loc-
ated 2.000 meters’ deep well in Iceland would
in order to return cost, have to produce a total
of about 2 million m8 of water at 100° C.
(c) Reservoir conditions and reservoir mechanics.
The main condition for exploitable geother-
mal reservoirs is a large volume of permeable
rock where there is a large area of contact bet-
ween the rock and the geothermal fluid.
These conditions are in Iceland fulfilled by
the flood basalts which have a thickness of
2,000 to 3,000 meters. It is remarkable that pro-
duction of thermal water has been obtained
from the depth of 2,190 meters. Sediments form
a part of the reservoir at Wairakei in New Zea-
land. Dolemites and limestones are the rnain
steam-bearing formations at Larderello, Italy.
These three types of rock can be regarded as
possible reservoirs.
The total amount of heat available in a reser-
voir depends on the volurne ancl the tempera-
ture of the reservoir as well as on the supply
of heat from other sources. The heat is being
taken up and transported from the reservoir by
the geothermal fluid.
Two forces are available for the expulsion of
the reservoir fluicl. First, the vapour pressure
of the fluid which can act in a closed reservoir,
that is, in a reservoir which is surrouncled by
impermeable rock. Second, hydrostatic head
which acts on open reservoirs.
The open reservoirs are surrounded by per-
meable rock which contains water encroaching
on the reservoir fluid. Withdrawal of the reser-
voir fluid leads to the inflow of the surround-
ing water. The water is heated by contact with
the reservoir rock and new reservoir fluid is
64
formed. The heat content of the reservoir can
be ntore or less partially drained in this vay.
These somewhat elementary considerations
open the possibility for an estimation of the
total amount of heat that can be drained from
a reservoir. In order to carry out the necessary
computations one must know the reservoir
volume and temperature as well as the permea-
bility conclitions. In most cases a rather low
drainage efficiency will have to be assumed.
There are therefore some theoretical possi-
bilities of estimating the potential of geother-
mal reservoirs. This should also apply to hidclen
resources.
(d) Conditions in Icelancl.
The question of hidden geothermal resources
in Icceland has received some attention. The
great amount of drilling that has been carried
out in many thermal areas and the rather uni-
forrn geology of the country are helpful aspects.
The main facts are that the flood basalts are
permeable down to a depth of 2,000 to 3,000
meters and that the temperature at the bottom
of the formation appears generally to be 100°
C to 150° C (see Bodvarsson, 1961).
It is expected that the first experimental drill-
ing in an area with no surface display will be
carried out in 1962 at Akureyri in the North of
Iceland. A well of 1,500 to 2,000 meters is to
be completed.
Notc: References to Papers on Conrercnce Agenda Iteni
II.A.3. See Page 55.
REFEREN CES.
Bodvarsson, G.: Oxygen in thermal water and
corrosion of pipe systems. (In Icelandic).
Journal of the Engineers’ Association in
Iceland, 35. No. 6, 70-72 (1950).
Bodvarsson, G.: Physical characteristics of nat-
ural heat resources in Iceland. Conference
on New Sources of Energy, Rome (1961).
Published in Jökull, vol. 11, 1961.
White, Donald E.: Thermal springs and epi-
thermal ore deposits. Economic Geology,
Fiftieth Anniversary Vol., 1955, pp. 99—154.
Wliite, Donald E.: Thermal luaters of volcanic
origin. Bull. Geol. Soc. Am., Vol 68. pp.
1637-1658 (1957).
White, Donald E.: Magmatic, connate and me-
tamorphic waters. Bull. Geol. Soc. Am.,
Vol. 68, pp. 1659-1682 (1957).
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