Characterization of Sulfide-Rich Scales in Brine at Reykjanes

Abstract

The fluid from geothermal wells at Reykjanes, Iceland, is a brine whose salinity is roughly that of seawater and with a temperature of 290 - 320?C. Scaling studies, lasting four to seven weeks, were carried out in 2002 and 2003 as part of an investigation by Sudurnes Regional Heating Company, for a power development project. The general precipitation sequence of scale phases with decreasing pressure was: Wurtsite (ZnS), sphalerite (ZnS), galena (PbS), chalcopyrite (CuFeS2), bornite (Cu5FeS4), and amorphous silica. Occasionally other minerals were formed. The amount of scales deposited (i.e. thickness) and, thus, the rate of the scaling depended on the brine composition and the pressure at the precipitation site and the pressure decrease. In surface pipes from well RN-9 at pressure between 20 and 5 bar-g, the scaling rate is between 0.1 and 0.5 mm/30 days (1.2 - 6 mm/year). The silica content is between 13 and 75 wt%, Fet - 3 wt%, sulfides 65 and 8 wt%. In pipes from well RN-11 at pressure between 45 and 15 bar-g, the scaling rate is 0.4 and 1.0 mm/30 days (4.9 - 12 mm/year) and at pressure 15 bar and lower it is doubled. The silica content is between 15 and 70 wt%, Fet 23 - 6 wt%, sulfides 35 wt%. In pipes from well RN-10, at pressure from 50 to 7 bar-g, the scaling rates varies from 0.2 and 0.9 mm/30 days (2.4 to 11 mm/ year). The silica content is between 5 and 40 wt%, Fet 35 - 20 wt%, sulfides 55 and 35 wt%.