Renewability Assessment of the Reykjanes Geothermal System, SW-Iceland

Abstract

Geothermal resources are generally classified as renewable energy sources, but this may be an oversimplification. They are in essence of a double nature, i.e. a combination of an energy current (through heat convection and conduction) and vast stored energy. The renewability of these two aspects is quite different as the energy current is steady (fully renewable) while the stored energy is renewed relatively slowly, in particular the part renewed by heat conduction. In addition the relative importance of the two components depends on both the geological nature of a system and the rate of energy extraction during utilization. A research project, supported by the GEORG research fund in Iceland, was recently completed in the Reykjanes high-temperature geothermal system, which is located on the very tip of SW-Iceland, where the Mid-Atlantic ridge comes ashore. The Reykjanes system has been utilized on a small scale for decades, but in 2006 a 100 MWe power plant started operation in the field. The associated greatly increased production has caused drastic changes in reservoir conditions, in particular a considerable drop in reservoir pressure. The purpose of the project was to evaluate the relative importance of the two renewability aspects (energy current vs. stored energy) for the Reykjanes geothermal system under the current state of utilization. This was done through compilation of available data, such as reservoir monitoring data, and collection of new data, mainly micro-gravity and geodetic data. Consequently these data were jointly interpreted, partly by simulating data (e.g. gravity-change data) by an up-to-date numerical reservoir model of the geothermal system. In addition repeated TEM-resistivity surveying was used to try to follow the growth of a steam-zone in the shallower parts of the geothermal system. The results of the project do e.g. indicate that during the period 2008 – 2010 the renewal of reservoir fluid through recharge was of the order of 30 – 50%.