Jökull - 01.12.1966, Page 47

Surface
7777Z%ZZmzm77777777///////7777777
inflow
T. =0 -
permeable contact
►outflow
identical, that is, the primary source rock is
located just below the thermal areas.
There are substantial indications that a con-
siderable impedance to the upward flow exists
in the upper few hundred meters of the geo-
thermal areas. This impedance is caused by
the precipitation of calcite and silica in the
flow channels. Boreholes lower the outlet im-
pedance and thus can increase the overall
circulation in the hydrothermal system. More
water flows through the primary source rock,
and more heat is absorbed. Moreover, boreholes
will reduce the internal pressure in the thenna]
areas and thus may initiate an inflow of water
through the secondary source rock. The cold
ground-water encroaching on the thermal areas
has a higher density than the hot thermal water
and therefore exerts a pressure on the svstems.
As the internal pressure is lowered, the water
will flow through the secondary source rock
toward the boreholes. This secondary circula-
tion takes up heat from the secondary source
rock and make this lieat available for utiliza-
tion. The process is apparently of considerable
importance in many thermal areas under ex-
ploitation.
(2.2) The heating of the circulating water.
The most important process in a hydrothermal
system is the heating of the circulating water
as it flows through the source rock. Two rather
distinct mechanisms, depending on the nature
of the source rock, have been recognized. In
fractured and/or layered igneous rock or lime-
stone the contact area between the water and
the rock is relatively small, ancl the heating
mechanism is dependent on heat conduction
over relatively large distances. In clastic sedi-
ments, on the other hand, the water flows
around the small grains. The contact area is
very great, and the heat exchange between
rock and water is locally very rapid. The
implications of these processes have been dis-
cussed elsewhere by the author (Bodvarsson,
1963).
Igneous rock is the most common tvpe of
source rock. In fact, in Iceland flood basalts
composed of a very great number of horizontal
or almost horizontal basaltic lava flows is the
only known type of source rock. The water
flows at the scoriaceous contacts of the lava
beds, and it appears that only a very few con-
tacts have a substantial permeability.
Fig. 2
In many cases these circumstances allow us
to base our studies of the heating process on
a rather simple model (See Fig. 2). Suppose
that we have a thick pile of flood basalts com-
posed of a great number of lava beds. We
will assume that the pile is initially imperme-
able and that its lower section has a uniform
temperature T0. This temperature may simply
be the normal temperature at the depth of the
section, or it may partially be the result of
some recent intrusions in the neighborhood.
We will now assume that one lava contact
of a great horizontal area is opened suddenly
in the lower section of the pile, and that water
can flow into the pile through the narrow
space between the two adjacent lava-beds.
Moreover, we will assume that the front of
this narrow opening is quite long, and that
the length of the contact in the direction of
flow is L. For simplicity we will assume that
the water entering the contact has a uniform
temperature equal to zero, and that the inflow
per unit length is constant in space and time.
Upon entering the permeable contact, the
water is heated by the conduction of heat
from the adjacent rock. On the basis of heat
conduction theory it is quite easy to derive
the temperature of the water at the lower end
of the system, that is, when the water has pass-
ed through the contact. This model bas been
dicussed elsewhere (Bodvarsson, 1963) by the
author, and the result is:
I i. = "I'0 erl(u), (1)
where erf(u) is the error-function and
u = \/kcot/qs,
JÖKULL 201