Geochemistry of Multicomponent Fluid Phases in the Krafla High-Enthalpy Geothermal System, NE Iceland

Abstract:

Many active volcanic systems are associated with high-enthalpy geothermal systems. For systems characterized by shallow magmatic intrusions, liquid water often predominates at depth with two-phase fluids, vapor and liquid water, occurring at shallower depth due to depressurization boiling. Close to the intrusion, superheated or supercritical vapor may also occur. The Krafla high-enthalpy geothermal system provides an ideal opportunity to study such volcanic geothermal systems. Over forty wells have been drilled into the system with fluid discharge temperatures of <200°C to ~450°C and enthalpy between <900 and >3200 kJ/kg. In this study, geochemical modelling of multicomponent fluid phases associated with shallow magmatic intrusions were conducted across variable temperature, pressure and enthalpy conditions and the results compared with the fluid geochemistry of the Krafla system.

Within the reservoir at geothermal temperatures (250-300°C) liquid water predominates. Under these conditions, the concentrations of most major elements are controlled by equilibrium with secondary minerals. Geochemical modelling and observations at Krafla support these findings. Around the magma intrusions believed to be at shallow depth at Krafla, superheated vapor is formed. Such fluid was discharged by the IDDP-1 well at 450°C and 140 bar. According to the geochemical modelling, superheated vapor is produced upon heat addition by the intrusion to the surrounding geothermal water resulting in boiling to dryness, precipitation of non-volatiles (Si, Fe, Mg, Al, SO₄, Na, K, Ca) whereas volatiles (CO₂, H₂S, Cl, F, B) are unaffected. By mass, quartz is the predominant secondary mineral around the intrusions. The chemical composition of the modelled and observed superheated vapor compared well.

Upon ascent and depressurization of the liquid geothermal water and the superheated vapor various processes may occur, including superheated vapor condensation, mixing and depressurization boiling. This leads to formation of two-phase liquid and vapor fluids, dilute acid fluids produced upon vapor condensation and mixtures thereof. Such fluids are indeed observed within the Krafla system supporting the results of the geochemical modelling.