Tímarit Verkfræðingafélags Íslands - 01.12.1983, Side 16

most of the systems issue from fault
zones. With the possible exception of a
few of the hightemperature (above
200°C) BR systems, there is general
agreement that they are of non-volcanic
origin and are maintained by a deep cir-
culation of purely meteoric water. In an
attempt at presenting a useful summary
of the BR situation, Bodvarsson (1978)
has arrived at the sketch in Figure 5
below.
Figure 5. Hypolhelical skelch of a Basin and
Range geothermal syslem.
The system sketched involves two
principal features. First, the main water
circulation is along a single master fault
zone, that both provides the fiow chan-
nels and rock/water contact area for
heating. Second, there is a „leakage” of
thermal water from the fault zone into
the adjacent basin where a volume of
valley sediments has been heated to a
temperature close to the temperature in
the main source region in the deeper
parts of the fault zone. In many cases,
the hot sediments provide the main ther-
mal reservoir of the systems.
Considering an average medium-
temperature system of the Great Basin
with a base temperature of I80°C and a
total twwer of 30 MW(h), Bodvarsson
(1978) concludes that the heating zone
must be at depths of the order of 4 to 6
km and the rock/water contact are a re-
quiremen! for Ihe maintenance of the
heat supply of the systems during a
period of 3 x 104 years would have to be
no less than 70 km/ The total flow path
length must be of the order of 100 km or
more.
We can now identify the four parts of
the above system as sketched in Figure 6
below.
At the intake end are the recharge
zone and the recharge channels that
transport the water down to the heating
zone where the bulk of the heat supply
is taken up. From there the water
96 — TÍMARIT VFÍ 1983
Figure 6. The box model.
migrates up to the reservoir and through
the upflow channeis to the surface
where it is issued by hot springs. The
whole circulation is probably maintain-
ed by thermoartesian pressure and the
total available head may be of the order
of a few hundred meters of water.
Considering evidence from the results
of drilling in a number of medium-to-
high temperature systems, there are
substantial indications that the heating
zone and main reservoir are relatively
permeable, at least, to such a degree
that fluid convection can lead to a
temperature quasi-equilibrium within
these sections of the systems. For exam-
ple, extensive drilling into geothermal
systems in Iceland has revealed this
situation and provided support for the
concept of the system base temperature.
As a matter of fact, the indications are
that the permeability is largely Iimited to
both zones and that the surrounding
country rock may be much less perme-
able. The reasons for the situation are
not entirely clear, but one may surmice
that thermoelastic and chemical
phenomena are important. In par-
ticular, we know that chemical sealing
by silica and calcite are important fac-
tors in forming a cap rock over the
geothermal systems. The material
precipitated in the cap must originate in
the deeper parts. Practically no data are
available on the permeability situation
in the recharge and upflow zones.
The above described situation leads
to the concept of the boxmodel for such
systems as shown in Figure 6. We have a
large volume of hot permeable rock
embedded in a low-permeability en-
vironment. The thermal fluid enters the
system through the recharge channels,
circulates through the box and is then
piped up to the surface by the upflow
channels.
(lO.ii) Exploralion of geothermal sys-
lems by lidal testing. We are now in
the position of providing an overview of
the applicability of tidal borehole
testing in the exploration of geothermal
systems. We envision two main types of
information that such testing can pro-
vide.
First, along the lines set forth in sec-
tion (7) above, tidal testing can provide
driving-port type data on the local fluid
conductivity and effective tidal pore
pressure amplitude at the holes. Know-
ing the local tidal dilatation amplitude,
the pore pressure amplitude furnishes
data on the dilaticity number. Referring
to the discussion (9.iii), the above men-
tioned driving-port data can be sup-
plemented by transfer-port data when
simultaneous testing can be carried out
on several boreholes. In cases, it may
then be possible to obtain direct data on
the skin depth ds.
Second, the tidal pore pressure
amplitude in the reservoir/heat zone
box furnishes data on the fluid conduc-
tivity of the recharge and outflow chan-
nels. For example, a full or normal pore
pressure amplitude* in the box indicates
that the channels cannot transfer
pressure signals at the tidal frequencies.
In other words, they are longer than
the pressure field skin depth at these fre-
quencies.
11. FIELD DATA
Tidal boreholes field data are
available from two geothermal areas,
(1) Raft River, Idaho, and (2) Salton
Sea, California. This material has been
described and discussed by Hanson
(1979a, 1979b, and 1980).
The Raft River data were obtained
from 6 boreholes at artesian shut-in
conditions. Only one borehole,
Elmore-3, was available for testing at
the Salton Sea. The data were also ob-
tained at shut-in conditions but the hole
had a 15 m gas cap. Below, we will
briefly comment on this material.
Since the stiffness of shut-in artesian
boreholes is of the order of 2xl04
(sm)‘\ the tidal factor T of the Raft
River holes is very large and they can
then only work as pressometers. No in-
formation other than bounds can be ob-
tained on the local fluid conductivity.
The same probably applies to the SS-
Elmore-3 hole although the situation
there is more marginal. We will
therefore regard all 7 holes as primarily
pressometers and proceed as follows.
Hanson (1979a) has derived the tidal
pressure admittance values for the
various tidal components that were
observed at the boreholes. The data set
is fairly consistent and we therefore turn
our attention to the most prominent
component, the M2-tides.
* Comparcd to thcoretical tidal prcssure amp
litude.