Geochemical monitoring of the Krafla and Námafjall geothermal areas, N-Iceland

The Krafla and Na´ mafjall high-temperature geothermal areas in N-Iceland have been exploited for steam production since the late and early 1970s, respectively. Power generation at Krafla was 30 MW until 1998, when it was increased to 60 MW. At Na´ mafjall the steam has been utilized for operating a 3 MW back-pressure turbine unit, drying of diatomaceous earth and heating of fresh water for space heating. A total of 34 wells have been drilled at Krafla, of which 18 are producing at present. At Na´ mafjall 12 wells have been drilled but only three are productive. The highest temperatures recorded downhole are 320 and 350 C at Na´ mafjall and Krafla, respectively. Geochemical monitoring in the two fields during the last 20–25 years has revealed decreases in the Cl concentrations in the water discharged from most of the wells that have been producing for more than 10 years. The cause is enhanced colder water recharge into the producing aquifers of these wells due to depressurization by fluid withdrawal from the geothermal reservoir. Such recharge is particularly pronounced in the central part of the Leirbotnar wellfield at Krafla but it is also extensive in the only producing well in the Hvı´tho´ lar wellfield. At Na´ mafjall incursion of cold groundwater into the reservoir was particularly intense subsequent to the volcanic-rifting event in the area in 1977. Solute (quartz, Na/K, Na/K/Ca) geothermometry temperatures have decreased significantly in those wells where Cl concentrations have decreased but only to a limited extent in those wells which have remained constant in Cl. This indicates that the changes in the concentrations of the reactive components, on which these geothermometers are based, is largely the consequence of colder water recharge and not partial re-equilibration in the depressurization zone around wells where cooling of the fluid occurs in response to extensive boiling. Aqueous SO4 concentrations increase as Cl concentrations decrease. Except for the hottest wells, which are low in SO4, sulphate concentrations are controlled by anhydrite solubility. Increase in SO4 con-centrations is a reflection of cooling as anhydrite has retrograde solubility with respect to temperature. H2S-temperatures are similar to the solute geothermometry temperatures for wells with a single feed. They are, on the other hand, higher, for wells with multiple feeds, if the feed zones have significantly different temperatures. H2-temperatures are anomalously high for most wells due to the presence of equilibrium steam in the producing aquifers. The equilibrium steam fraction amounts to 0–2.2% by wt. of the aquifer fluid (0–47% by volume). CO2 temperatures are anomalously high for some Krafla wells due to high flux of CO2 from the magma intruded into the roots of the geothermal system during the 1975–1984 volcanicrifting episode. During the early phase of this episode the Leirbotnar wells were the ones most affected. The new magma gas flux has migrated eastwards with time. Today some wells in the Sudurhlı´dar wellfield are the ones most affected whereas the Leirbotnar wells have recovered partly or fully. The depth level of producing aquifers in individual wells at Krafla and Na´ mafjall has been evaluated by combining data on temperature and pressure logging and geothermometry results. The majority of wells at Krafla receive fluid from a single aquifer, or from 2–3 aquifers having similar temperature. The same applies to two of the three productive wells at Na´ mafjall. # 2002 CNR. Published by Elsevier Science Ltd. All rights reserved.