Jökull


Jökull - 01.12.1987, Page 28

Jökull - 01.12.1987, Page 28
during its passage flashed into steam and then con- densed again and reached a new equilibrium with the surrounding rock. The Suðurhlíðar fluid is most clearly so characterized but examples of this are also found in the Leirbotnar field and the calculation to obtain the composition of the lower fluids in well KJ-22 in Hvít- hólar reveals a similar concentration. The chloride con- centration is thus expected to be considerably higher in flows derived from a liquid fraction remaining after such boiling and such fluids are thought to constitute those of Hveragil and the upper part of the Hvíthólar reservoir. In some cases anomalously high fluoride concentra- tions have been recorded. These are reported here as F/Cl ratios and are apparently related to aquifers which occur at the interface of acid rocks and basalts. These are characteristic of the northern Leirbotnar and Suður- hlíðar fields. Geothermometry temperatures are generally in good agreement with measured downhole temperatures. In cases of mixed discharges and conductive cooling the individual geothermometers yield different results. The discrepancy may be used to infer cooling processes and in obtaining the proportions of the different feed zones in the case of mixing. Using geothermometers along with sodium and chloride concentrations it was possible to predict that bewtween 60 and 70% of the flow in well KJ-22 in the Hvíthólar field came from the cooler lower aquifers of the well. The composition deduced for these lower fluids does not really tally with any of the major groups and thus has to be treated as a separate one. Similar composition has, however, on occasions been observed in well KG-10 after it became blocked and produced largely from the upper zone but still with a lower zone component. Fig. 17. Production characteristics of wells in the Hvíthólar field. - Mynd 17. Aflferlar hvíthólahola. The monitoring of gas has shown a decrease with time in gas concentrations and ratios such as C02/H2S which reflect the inflow of magmatic gases into the Leirbotnar system (Ármannsson etal. 1982). A notable exception is well KJ-15 the flow of which is very rich in gas and shows little signs of decrease. These results agree with those from the fumaroles. Well KJ-15 is the northeasternmost well and lies close to the area where the decline in gas concentrations was smallest in the fumaroles. Several studies have been carried out on deuterium and ð180 ratios in the area (Darling &Ármannsson 1986). The general result is that fluids from the northern part of the area west of Hveragil are slightly higher in deuterium (ðD= —85 to —87%o) than those to the south (—90 to —93%o) with no regular variation in ðlsO ratios (—11 to —13%o) overthe area. Using values for rainfall in Iceland (Árnason 1976) this suggests that the fluids from the northern part of the field (Leirbotnar) is derived from local sources, whereas those in the southern part (Suðurhlíðar, Hvíthólar, KJ-6) might be derived from water which has fallen as rain at a higher altitude. The deuterium ratios are, however, not as low as in the fluids from the Námafjall area (—95 to —100%o) 5-10 km to the south of Krafla thus excluding the possibility of a com- mon origin for the fluids of the two geothermal areas. RESERVOIR MODEL Extensive numerical modeling studies of the Krafla geothermal area were carried out in 1982-1983. The studies were limited to the two main wellfields, Leir- botnar and Suðurhlíðar. The major objectives of the modeling of these fields were to; 1) verify the conceptual model of the field derived from the exploration results 26
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