Jökull - 01.12.1961, Blaðsíða 38

In principle there are no major difficulties in
measuring the heat escaping at the surface. How-
ever, the field work involved is quite substantial
and accurate determinations have not yet been
carried out in any of the high-temperature areas
in Iceland. The situation is better in the case
of the low-temperature areas, as the sensible
heat content of the water predominates there,
and this factor can be measured relatively easily.
Data on the flow characteristics of the vari-
ous major hydrothermal systems are given in
TABLES II and III. The data on the heat out-
put are given in the magnitude scale defined
in TABLE I. The largest high-temperature
thermaf area, the Torfajökull area, appears
to have a total heat output of the order of
5x 108cal/sec. This averages to about 500
microcal/sec and cm2, of the thermal area.
The global average of the normal conduction
flow of terrestial heat is about one microcal-
/cm2sec. On this basis the total flow through
the surface of Iceland (100,000 knr) should be
about 10°cal/sec. This is of the same order as
the heat output of the Torfajökull area.
(f) The accumulation of heat in the upflotu
zones.
The rock formations in the upflow zones of
the thermal areas are heated by the ascending
hot water and steam. Mainly the high-tempera-
ture areas include large volumes of rock heated
to temperature approximately equal to the base
temperature. The upflow zones of these areas
therefore contain a substantial amount of
surplus heat.
Rough estimates indicate that in the case
of individual areas the surplus heat accumulated
divided by the total heat output gives a time of
accumulation of several thousand years. The
surplus heat is therefore a substantial part of
the total heat output during post-Glacial time.
(g) The chemical composition of tuater and
steam.
A considerable amount of geochemical data
has been collected in Iceland. The hot-water
and the natural-steam in most areas has been
sampled and analysed. It has turned out that
the majority of the results are of a relatively
uniform nature. TABLE IV furnishes data on
the thermal water in three areas, that is, one
low-temperature area and two high-temperature
36
TABLE V
Chemícal composition of natural steam
in two places.
Analyses: Departement for Natural Heat,
Reykjavik.
Hengill Krysuvik
high-temp. high-temp.
Gas content, milliliters/gram
steam .................. 1.43 7.50
Composition of gases:
C02 .................... 84.6% 83.9%
HoS .................... 4.9 9.6
II2 .................... 2.1 5.4
CH4 .................... 0.0 0.1
Resiclue (N2 etc.)...... 8.4 1.0
areas, TABLE V furnishes data on the composi-
tion of the natural steam in two latter areas.
It is to be realized that the water samplecl in
the high-temperature areas is not identical with
the thermal ground water. The samples were
collected from wells and are a residue after
flashing.
5. EXPLOITATION AND RESERVOIR
MECHANICS.
At this juncture a total of approximately
70,000 meters have been drilled for the purpose
of the exploitation of natural heat resources in
Iceland. The deepest well, reaching the depth
of 2,200 meters, is located within the city of
Reykjavik. Several tens of thousands of meters
are planned to be drilled within the coming
few years.
The exploitation of the natural heat resources
involves geological and physical considerations
of a rather special nature. This subject can be
referred to as the science of the reservoir mec-
hanics of natural heat resources.
The present writer has elsewhere (Bodvars-
son, 1949 and 1951) discussed this subject and
given a number of case histories from Ice-
land, mainly from the Hengill-area, listed as
No. 4 in TABLE III. Although a consider-
able amount of new data has been collected
during the past 10 years, the basic conclusions
still appear valid. The reader is therefore re-
ferred to these papers. For a very interesting