Geothermal Fluid Equilibrium Modeling: Comparison of Wellhead Fluid Samples to Deep Samples in the Reykjanes System, Iceland

Abstract

Single phase geothermal fluids sampled in 2007 from 1500m depth in Well RN-12 of the Reykjanes geothermal system in Iceland show large differences in dissolved copper, zinc and iron concentrations when compared with fluid sampled from the wellhead. Equilibrium modeling of the samples taken at depth indicate that the fluid was supersaturated in sulfide minerals even at moderately acidic pH values, suggesting that the deep samples, as collected, are out of equilibrium.

Wellhead sample reconstructions indicate a well-bottom pH of about 5.5 at 295°C, but a pH of 3.6 at saturation with chalcopyrite, bornite, pyrite and sphalerite would be required to account for the large concentrations of Cu, Zn and Fe in the down-well samples. This acidic value needed for the high metal concentrations is not realistic in this naturally buffered system, likely indicating contamination in the downhole analysis.