Náttúrufræðingurinn - 1981, Síða 217

geothermal investigation has been directed
at finding suitable steam for the power plant
which has already been built within the
Krafla field. Drilling performed in 1980 in
the eastern part of the field gave very prom-
ising results, and it is expected that this dis-
covery will bring the power plant to rated
production within a few years. Geological
mapping of the area confirmed a caldera
formation in the Krafla central volcano, in-
tersected by a pronounced fissure swarm.
The frequency of volcanic eruptions in
Recent time was found to be about four
eruptions per thousand years.
Geochemical studies of gases in fumarole
steam in the area have revealed three main
upflow locations in the field. Two of them
(in Leirhnjúkur and Hveragil) are highly
influenced by volcanic gases, whereas the
third in the eastern part of the field seems
not to be affected.
An aeromagnetic survey of the area has
revealed a significant NW-SE structure in
the caldera, which is also reflected in
gravity, self potential, resistivity as well as in
the geology. Several resistivity methods have
been tried in the field. They include
Schlumberger soundings, equatorial dipole,
quadrupole survey and time domain elec-
tromagnetic method. In general, the low
resistivity area is well defined horizontally as
well as vertically, meaning that beneath
about 700 m depth the resistivity increase. A
magma chamber has been demonstrated to
be present at 3—7 km depth below the cal-
dera. The extent of the magma chamber has
been mapped by utilizing the seismic S-
wave shadows.
Temperatures in wells drilled in the
geothermal field had considerable spatial
variation and it was found necessary to
assume that the geothermal system consisted
of two separate zones, a water dominated
one at shallow depth (0—1000 m), and a
boiling two-phase reservoir at greater depth.
Chemical composition of the discharge also
varied greatly between wells and supported
also the division of the geothermal system
into an upper and a lower zone.
Resistivity logging in wells confirmed that
the resistivity beneath 1000 m depth is con-
siderable higher than at shallower depth.
The permeability found by injection and
recovery tests is rather low, which reflects
the boiling conditions of the lower part of
the geothermal system.
The enthalpy of flowing wells is rather
high and some wells exclusively discharging
from the two phase reservoir have developed
into pure steam wells. The amount of non-
condensable gases in the lower zone is
approximately one hundred times greater
than in the upper zone. A simplified model
of the field proposed in 1977 has been found
to be valuable, as the properties of wells
drilled since then have been predicted by
this model. A new well drilled in 1980 in the
eastern part of the field has been found to
have favourable production characteristics.
These discoveries will make it possible to
increase the production of the power plant
to rated capacity in a few years.
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