Geochemical sampling, laboratory analysis, and mineral 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-1 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, SiO2, Al, 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 equilibrium 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.