Thermal effects of intrusive events on geothermal systems: Heat transfer modelling during (and after) the Krafla volcano-tectonic episode 1975-84, NE-Iceland

Young igneous geothermal systems recharge by magmatic activity. Due to Iceland’s location on the mid-ocean ridge, repeated dyking compensates here for the spreading. This study examines the impact of intrusive and eruptive events on the thermal evolution of the Krafla geothermal system. The so-called “Krafla fires” in 1975-84 were a volcanic episode comprising 20 intrusive and eruptive events, during which seven of them intersected the geothermal system.

The effects of repeated dyking on temperature, pressure, and enthalpy, as well as steam content, are modelled in simple 2D profiles with HYDROTHERM (USGS). Calculating a heat budget can help to exploit geothermal energy sustainably: How much energy is inputted by the dykes into the geothermal system? How much of this heat is lost to the atmosphere by advection and conduction? How fast is heat transferred in the subsurface?

The total heat input of the dyke into the geothermal system is 0.5-1 x 10¹⁸ J. During, and shortly after the eruptive episode, the dyke nearly cools down to the ambient temperatures of the system. Models and previous analyses of steam clouds in air photos indicate that around 10 % of the heat is lost from the surface to the atmosphere, mostly in the first weeks/months after the dyking event, while 90 % of the dyke’s energy is dissipated into the geothermal reservoir. As the system is already close to the boiling point, the additional heat input by the dyke, leads to steam generation, which rises in the high-permeable lava-hyaloclastite layer. It collects below the clay cap and rises through fissures and fractures. In the lower permeable layer of basement intrusions, the steam is less mobile and stays in the vicinity of the dyke. The main changes in temperature and pressure can be observed in the two-phase and superheated steam regions, where enthalpy increases strongly compared to the initial setting. Long-term simulations indicate that the heat input by the dykes formed in the Krafla fires remains in the reservoir for at least several decades and plays a critical role in maintaining the geothermal system.