Chemical aspects of fluid injection into the geothermal fields in Eyjafjörður, N-Iceland

Abstract

Geothermal fields in the Eyjafjördur area supply thermal water for space heating in the town of Akureyri, N-Iceland. In this study, the Cl-SO4-HCO3 and K-Na-Mg triangular diagrams, Cl/B ratio and stable isotope ratios were used to classify the geothermal water. The classification results show that each geothermal field in the Eyjafjördur area has its own character. The geothermal water is classified as "sulphate-rich water". Changes in chemical composition with time were observed in Thelamörk, and were noticed for some constituents in well HG10 at the Botn geothermal field. Geothermometers were used to evaluate the reservoir temperature, and predict possible cooling in the reservoir during production. Most of the geothermometers give temperatures in pretty good accordance with measured temperatures. Some decrease in calculated reservoir temperature with time is noticed in the Thelamörk and Botn fields. During geothermal exploitation in the Eyjafjördur area, the water level decreased over time. Injection is a way to deal with this problem. Modelling of cooling, heating and mixing was employed to study the chemical processes that may take place during injection. According to modelling results, no mineral deposition problem is expected if return water from the heating system of Akureyri is used for injection, even if the water is supersaturated with respect to quartz and chalcedony. However, if cold groundwater were to be used for injection, considerable magnesium silicate deposition would take place.