Analysis of variations in the characteristics of the geothermal fluid produced from the Sudureyri low-temperature geothermal system in NW-Iceland

Abstract

Problems associated with calcite scaling and drastic changes in fluid composition have limited the utilization of geothermal resources at Sudureyri in Súgandafjördur, NW-Iceland. Despite a relatively high production rate recorded during the first years of operation of the Sudureyri District Heating Service, the system showed a recovery during a later decrease in production rates. The geochemical and temperature record for this geothermal system extends from 1975 until 2014. The geochemical record suggests an intrusion of a more saline component in the producing reservoir. Comparing the characteristics of the saline fluid by a simple mixing process, it was evident that it was formed by seawater-basalt interaction. The fluid composition shows enrichment in calcium and silica, and loss of magnesium and sodium, compared with seawater. As a potential alteration product, the formation of smectite has been considered.

Knowing the system’s behaviour at high production rates, a simple lumped model was used to simulate production temperature and chloride concentration, in order to predict cooling and the chemical composition of the geothermal fluid, in terms of the concentrations of some of the major species in the system; this included calcium, for different production scenarios. From the fluid characteristic predictions, it is possible to identify production scenarios that might endanger system operation. From the results of the assessment, it is believed that the system can sustain production equivalent to that recorded for 2013 (8.4 kg/s) for the next ten years. However, in those ten years the fluid is expected to reach chloride concentrations on the order of 220 ppm, and calcium concentrations of 45 ppm. At this rate of production, no significant drop is expected in the fluid temperature. For the simulation, the volume of the reservoir, porosity, the hot recharge rate and the temperature of colder inflow (saline fluid) were used as variables.