Jökull - 01.12.1961, Side 44

theoretically yielcl the formation temperature
although equilibrium has not been attained.
The assumption of a pure conduction flow of
heat may at first appear to be a plausible basis
for the computations involved in both steps.
There are, liowever, several cfifficulties sorne of
which will be listed below.
(i) There is a considerable uncertainty as to
the relevance of the pure conduction theory,
mainly in the case of the first step. The format-
ions penetrated may be permeable to the dril-
ling fluid, at least at the conditions imposed
during drilling. The walls of the borehole may
be invaded resulting in an unknown mass trans-
port of heat into the formation. The initial
conditions may thus deviate largely from those
computed on the basis of a pure conduction
flow of heat.
(ii) Further uncertainty results from the fact
that the temperature conditions in the boreliole
during drilling are not known accurately. The
counterflowing streams drilling fluid exchange
heat through the drillpipe leading to a down-
hole increase of the temperature. The tempera-
ture of the fluid at the bottom can be tens of
degrees higher than is measured at the outlet or
the inlet at the surface. A quantitative estimate
of this effect is difficult to obtain.
(iii) The thermal properties of the rock pene-
trated are generally not known accurately.
A further difficulty arises in the case of a pure
conduction process. The initial slope of the
temperature rise is not sensitive to the true
formation temperature. A record of a consider-
able time-length would be required.
On the other hand, the computational work
involved can be carried out on a high-speed
computer. The first set of master curves has
been computed by the State Electricity Authori-
ty, Reykjavík. They are based on conduction
theory and intended as a check of the applica-
bility of that theory to the present case.
The experience gained in Icelancl sofar indi-
cates that conduction theory is correct. as far as
the temperature recovery is concerned. The
basaltic rock encountered in the thermal areas
of Iceland is relatively impermeable. On the
other hand, the other difficulties listed imply
that further studies have to be carried out in
order to attain a satisfactory solution.
At. this juncture, it appears that a borehole-
record of the order of one day or more is neces-
sary in order to obtain a reasonable estimate of
the formation temperature. In the meantime
the drilling rig would have to be idle, but the
rig operating cost is relatively high. It is there-
fore, in some cases at least, questionable whether
the value of the information gained is not iost
in rig time.
•/. GEOCHEMICAL METHODS
Samples of water from almost all thermal
areas in Iceland have been analysed for con-
tents of dissolved solids. A comparison of results
frcm the various thermal areas indicates a posi-
tive correlation between the spring temperature
and the total amount of solids dissolved. For
instance, water issued by springs of 30 °C
may contain only about 150 ppm (parts per
million) of solids whereas water from springs
at 100 °C contains more than 300 ppm.
An explanation of this phenomenon is il-
lustrated in the relatively clear correlation be-
tween the amount of dissolved silica SiO^, and
the spring temperature. On the average the
amount of silica increases by one ppm for each
clegree C of spring temperature. The data at
hand indicate that the base temperature, Tb, is
a main factor regulating the contents of silica
and that the following relation.
25 + Tb = Si02 in ppm, (5)
where Tb is in degrees C, gives a semi-quantita-
tive illustration of the conditions.
A plausible interpretation of relation (5) is
that it simply illustrates the solubility of silica
from basaltic rock in water at different tem-
peratures. Iceland is built up of flood basalts
and there are reasons for assuming that the liy-
drothermal circulation systems are entirely
within the flood basalts (Bodvarsson, 1961).
This interpretation is supported by laboratory
studies. Krauskopf (1956) and White, Brannock
ancl Murata (1956) have studied the solubility
of silica in water. The experimental data of
Krauskopf indicate that the solubility of amor-
phous silica in distilled water amounts to ap-
proximately 70 ppm at O °C and to 350 ppm
at 90 °C.
Crystalline forms of silica should have lower
solubilities. Relation (5) indicates that the ulti-
42