| Title | Gas Chemistry of the Hellisheiði Geothermal Field, SW-Iceland |
|---|---|
| Authors | Samuel Scott, Ingvi Gunnarsson, Andri Stef |
| Year | 2011 |
| Conference | Stanford Geothermal Workshop |
| Keywords | Hellisheidi, Gas chemistry, Phase segregation, Hengill |
| Abstract | A fluid sampling campaign has recently been carried out at the Hellisheiði geothermal field in southwest Iceland. This high-temperature field is part of the Hengill volcanic system, and is host to the largest geothermal power plant in Iceland. A geochemical assessment of the field is presented based on the analysis of 19 wet-steam well discharges. Emphasis is placed on the chemical and physical processes that account for the concentrations of the major reactive gases (CO2, H2S, H2 and CH4). Aquifer chemical compositions were calculated from analysis of discharged water- and steam-phases and discharge enthalpies using the WATCH speciation program and phase segregation model. Under this model, discharge enthalpies in excess of that of vapor-saturated liquid at the aquifer temperature are accounted for by retention of liquid in the formation at a single pressure. The calculated concentrations of volatile components in initial aquifer fluids are observed to be very sensitive to the selected phase segregation pressure. Carbon dioxide concentrations are kept in close equilibrium with calcite. The concentrations of H2S and H2 species show a close approach to equilibrium with a mineral assemblage consisting of pyrite, pyrrhotite, epidote and prehnite. The field-scale distributions of the main geothermal gases are used constrain the locations of two separate upflow zones identified within the geothermal area. Additionally, chloride and nitrogen suggest the presence of a recharge zone in the northern part of the geothermal field directed towards the south. |