Aquifer fluid modeling and assessment of mineral-gas-liquid equilibria in the Námafjall geothermal system, NE-Iceland

Abstract

This study presents a geochemical evaluation of the Námafjall high-temperature geothermal field with respect to the chemical and physical processes that account for the fluid concentrations of volatile and non-volatile components, and the mineral assemblages controlling equilibrium in the aquifer. Aquifer fluid compositions and aqueous species distribution, for 25 samples collected from 7 wet-steam well discharges, were calculated from water- and steam-phase analyses and discharge enthalpies using the WATCH 2.1 speciation program according to the phase segregation model. Phase segregation pressures calculated at ~80% volume fraction of the flowing vapor are selected in view of the fact that liquid saturation at this pressures relate to residual liquid saturation of 0.2. The modelled aquifer fluid compositions were used to assess how closely equilibrium is approached between solution and various minerals. H2S and H2 concentrations were used to evaluate the presence of equilibrium vapor fraction in the initial aquifer fluid, calculated as 0-3.9% by weight with a field average of 0.79% by weight.

At inferred Námafjall aquifer temperatures (200-300°C), the concentration of H2S in the initial aquifer fluids is somewhat higher than predicted at equilibrium with hydrothermal mineral assemblage consisting of pyrite, pyrrhotite, prehnite and epidote, while concentration of H2 closely approaches equilibrium for the excess enthalpy wells unlike for the liquid enthalpy wells. With respect to CO2 the calculated chemical compositions of initial aquifer fluid show a close approach to equilibrium (for liquid enthalpy wells) but lower than equilibrium for the excess enthalpy wells with the hydrothermal alteration minerals clinozoisite, calcite, quartz and prehnite. The shallower aquifer at Námafjall are higher in gas (H2S, H2, CO2 and N2) indicating that gaseous steam from deeper aquifers has condensed in the shallower ones signifying that they are, at least partly, steam-heated. The main uncertainty involved in calculating mineral saturation indices, particularly in the case of excess enthalpy well discharges, lies in the model adopted to calculate the aquifer water composition and its aqueous species distribution and in the quality of the thermodynamic data on the aqueous species and the minerals especially those that involve Fe-bearing species. In the deep aquifers, chemical equilibrium has been rather closely approached between dissolved solids, H2S and H2 on one hand and hydrothermal minerals on the other.