SULFIDE SCALES FROM GEOTHERMAL BRINE IN WELLS AT REYKJANES, ICELAND

Eleven wells have been drilled during the last three years in the Reykjanes geothermal area system, on the SW coast of Iceland, for the purpose of supplying steam for a new 120 MWe power plant. The geothermal solution is of seawater salinity and the inflow temperature is between 290 and 315°C. Scaling problems occurred within eight years where base metal sulfides, such as sphalerite, chalcopyrite, and to a lesser extent galena are the predominant minerals precipitated at high pressures (~ 30 – 40 bar). An amorphous phase of iron and silica, compositionally similar to minnesotaite [(Fe₆Si₈O₁₈(OH)₆)], becomes abundant with decreasing pressure and below 20 bar amorphous silica is the principle scale. The composition and textures of the sulfides from wells 9 (290°C) and 10 (315°C), 600 m apart, are significantly different. In well 9, sphalerite is either Fe_0.07Zn_0.93S or Fe_0.09Zn_0.87Cu_0.04S and chalcopyrite is always close to Fe_0.52Cu_0.48S. The Cu/Zn molar ratio of the sulfide rich scales in well 9 is 0.841. The galena grains are very small and have the composition (Fe_9-11Zn_0.3-46Cu_1.5-20Pb_38-39)S. The texture in well 9 are layers of anhedral sulfides and interstitial amorphous silica, as very fine needles. The Fe/Si mole ratio in the iron silicate phase varies between 0.2 to 0.5. In well 10 the sulfide scales consist of hetrogenous Fe-sphalerite and a few very small grains of galena. The Fe-sphalerite has FeS content between 27 and 60% and the sulfide scales have Cu/Zn ratio equal to 0.045. Half of the grains contain no copper but the other half contain 1 to 15% CuS, with an average composition of Fe_0.42Zn_0.56Cu_0.02S. XRD analyses demonstrate the presence of wurtzite at pressures 50 to 20 bar. The textures in well 10 include very fine layers of either fine or coarse-grained feathers of sphalerite grading to later euhedral crystals of sphalerite with interstitial Fe-Si phase and amorphous silica, and layers of amorphous silica. These layers are repeated a couple of times. The Cu concentration of the brine from both wells is about 1 ppb. The Zn and Fe concentrations of fluid sampled at the surface of well 9 are 2 to 20 ppb and 350 to 800 ppb, respectively; deep concentrations are not yet available. The corresponding concentrations in well 10 are 5 and 4 times higher, respectively. The scaling rate is 1.5 to 11 mm/year in well 9 but double that in well 10.