Composition of Reservoir Liquid and Metals in Pipeline Scale, Reykjanes Geothermal System, SW Iceland

Abstract

The Reykjanes high-temperature geothermal system is located in SW Iceland at the toe of the Reykjanes Peninsula, which is the on-land extension of the Mid-Atlantic Ridge. The fluid in the geothermal system is modified seawater, with salinities of 3.2 wt% NaCl, heated to temperatures of between 270 and 315°C at 1.0 to 2.5 km depth, similar to end-member fluids that discharge at black smoker vents on the seafloor. Comparison between the boiling discharge fluid and the pre-boiling reservoir liquid, collected at 1500 m depth in May 2007, demonstrate that the metal concentrations are three to four orders of magnitude higher in the deep liquid; the low concentrations in surface samples is caused by deposition of sulfides during ascent and boiling.

Scales in pipelines from wellhead to the separation station consist of sulfides upstream of the orifice plate; downstream of the orifice plate the scales are mainly amorphous material with minor amounts of sulfides, mainly chalcopyrite, sphalerite, and traces of wurtzite, bornite, digenite, and galena. The concentrations of metals in the scales varies, depending on the location in the pipeline, from trace to maximum values of 31 wt% Cu, 59 wt% Zn, 23 wt% Fe, 17 wt% Pb, 2.3 wt% Ag, and 600 ppm Au; the highest precious metal concentrations are with bornite-rich scales on the orifice plate, where deposit rate is high due to the sharp pressure decrease.