Rit (Vísindafélag Íslendinga) - 01.06.1967, Side 127

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of 160 liters/sec causes an estimated average pressure drop
of about 35 meters of water near the boundary of the pro-
duction area. It is of interest to note that this pressure drop
is observed only in holes which are deep enough to reach the
production zone. The water table in shallow holes, 100—200
meters deep, is practically unaffected by the pumping.
If the horizontal permeability outside the production area
is assmned to be the same in all directions, the relation be-
tween a change in water discharge and the average pressure
drop at the circular boundary of the production area can be
expressed as follows, assuming Darcy’s law to be valid under
these conditions,
2n ■ k • L • Ap
AQ ~ — - ------
” • (i)
where
AQ = change in volrnne discharge
k = horizontal permeability
L = thickness of production zone
Ap = pressure change at boundary of production
area
[j. = viscosity of water
r0 = radius of production area.
Here r is a distance from the center of production area at which
pressure change due to AQ is negligible. This distance is of
course poorly known, but the uncertainty in r does not af-
fect the result significantly. It will be asstnned here that
r = 10Xro. Substituting AQ = 1.6X105 cm3/sec, L = 105
cm, Ap = 3.5X106 dyn/cm2, p = 2.1X10-3 g/cm sec (130°C),
gives k = 3.5 X10-10 cm2. Expressed in darcy (1 darcy =
0.987X10-8 cm2) this gives as an estimate for the large scale
horizontal permeability in the Reykjavík thermal area
k = 0.035 darcy.
For comparison the permeability of postglacial lavas in Ice-