Jökull - 01.12.1987, Page 25

TABLE 3 TRANSMISSIVITIES IN SOME KRAFLA WELLS (mVPa s)
Well No. Time of drilling
12 Nov. 1978
13 Aug. 1983
15 Oct. 1980
16 June 1981
17 July 1981
20 Aug. 1982
*21 Sept. 1982
*Estimated in 1985.
At completion Estimated in 1984
1.2-2.4 x 10's 1.6 x 10s
1.9 x 10s 6.0 x 10 s
1.5 x 10's 1.0 x 10's
0.9 x 10 s 0.4 x 10"s
2.5 x 10's 1.4 x 10s
1.6 x 10 's 0.8 x 10s
13.1-16.2 x 10's 19.3 x 10"s
the fields were shut down during a temporary break in
the electricity generation and the opportunity was used
to monitor pressure build-up in the reservoir (Sigurðs-
son et al. 1985). The results are summarized in Table 3
and compared with transmissivity values for each well.
Mass flow from wells and fluid enthalpy have in most
cases been measured using the lip pressure method
(James 1962) although in some cases the use of differ-
ential pressure over orifices has been employed. A good
agreement has been obtained between the two methods
but the latter is more convenient for dry or nearly dry
flows. In special cases separators have been installed to
monitor the discharge characteristics of the wells. The
analytical results for silica concentrations have been
used to obtain the enthalpy of the water-saturated wells
and to estimate the “excess enthalpy” of the non water-
saturated wells.
The upper zone of the Leirbotnar field is a water-
saturated system with well discharge enthalpies in the
range 800-1000 kJ/kg and mass flows from wells 20-50
kg/s. There is a much greater variation in wells drawing
from the lower zone. The first eleven wells were cased to
relatively shallow depths so that they tapped either just
the upper zone or both reservoir zones. A calculation
method based on the measured enthalpies and the
chemical composition of the fluids was devised to obtain
the characteristics of the lower zone (Ármannsson et al.
1982). In wells drilled after 1976 the upper zone was
cased off since production characteristics were in some
cases unsatisfactory when both producing horizons fed
the same well. The calculations and results of enthalpy
measurements showed the lower zone to be a two-phase
reservoir with well discharge enthalpies ranging from
1900 to >2700 kJ/kg, except in well KJ-9 where the
enthalpy was considerably lower (1300-1500 kJ/kg). This
well is considered to tap the elusive border between the
two reservoir zones.
Fig. 14. Temperature and pressure distribution at 500 m
below sea level in the Leirbotnar well field. — Hitastig
og þrýstingur á 500 m dýpi undir sjávarmáli á Leirbotna-
svœðinu.
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