Title: Geochemical sampling, laboratory analysis, and mineral equilibria calculations for fluids from selected areas in Iceland

Abstract

The work carried out during the project period of the
geothermal training programme is described . Field work
involved collection of samples of aqueous solutions from well
SN-2 in the Seltjarnarnes geothermal area, well H-l in the
Reykjanes Isafjardardjup geothermal area, and from hot
springs in the Borgarfjordur and Reykjanes Isafjardardjup
geothermal areas. Steam and gas samples were collected from
fumaroles G-27 and G-19, and the new well KG-24 in the Krafla
geothermal area .
Laboratory work involved analysis of selected samples for pH,
CO2 , H2S , Cl , Si02 1 AI, Fe, and Mn by the methods used at the
National Energy Authority of Iceland.
Mineral Equilibria calculations were carried out for selected
low and high temperature geothermal fluids using the WATCH1
and WATCH3 computer programs, to obtain the temperatures at
which the geothermal fluids were in equili brium with the
various expected alteration minerals . out of 17 geothermal
fluids for which the calculations were performed , 9 gave
equilibrium temperatures within 10"C of the "expected"
reservoir temperature , and 15 were within 20·C. The two for
which the calculated temperatures differed from the expected
reservoir temperatures by more than 20·C were high salinity
fluids in open reservoirs.
There are however some differences between the results
obtained using the WATCH program and those given by Reed and
Spycher (1984) using the SOLVEQ program.
It is expected that this approach will give more accurate
predictions when better thermodynamic data become available ,
particularly for aluminium aqueous species, and for clay
minerals.
Cold waters which have never been in equilibrium with the
surrounding rocks are easily recognised by the complete lack
of equilibrium among the minerals .