Calcite Scaling at Selfossveitur Hitaveita, S-Iceland, when Mixing Waters of Different Chemical Composition

Abstract

The Selfossveitur district heating company in South Iceland has utilized the Thorleifskot and LaugardÊlir low-temperature reservoir for over fifty years. The geothermal water from the reservoir is relatively dilute, with chlorine content of 200 to 400 mg/l, silica content 60 to 120 mg/l and pH 8.5 to 9.0. The reservoir temperature is 100 to 120?C at the depth of 1000 m. In the year 2000 an exploration/production well was drilled into a nearby reservoir at ”sabotnar with a temperature of 90 to 100?C at 1000 m. The water is more dilute, with chlorine content of 50 to 60 mg/l, silica content 70 to 75 mg/l but pH as high as 9.8 to 9.9. Pumping from the ”sabotnar well started early 2002 and the water was mixed with waters from producton wells in the Thorleifskot and LaugardÊlir fields. Soon scaling problems were encountered in the central pumpstation were calcite was deposited in the pumps that feed the distribution system. Calculations showed that a mixture of the two water types, although rather similar in composition, became more supersaturated with respect to calcite than water from individual wells. To respond to this problem experiments were performed in 2003 where water from the ”sabotnar well was acidified with sulfuric acid to lower the pH value before mixing. The results were promising and today the water from ”sabotnar is acidified with concentrated sulfuric acid and mixed with waters from other production wells utilized by Selfossveitur, without scaling problems.