Modelling of the Deep Roots of Volcanic Geothermal Systems

Abstract

The basic idea on the nature of volcanic geothermal systems hasn’t changed much the last half a century and it is generally assumed that their heat sources are cooling magma chambers or intrusions. The heat-transfer from these roots up to shallower levels is a complicated process involving flow of magma, flow of fluids (two-phase and/or super-critical water), heat transfer as well as thermoelastic rock mechanics and chemical processes. Heat conduction is much too slow to explain the intense heat transfer involved. About half a century ago a heat-transfer process was proposed (CDM = convective downward migration) during which a cooling front, driven by convecting water, migrates into the hot rock through fractures that open up due to thermoelastic contraction. The CDM-process thus extracts thermal energy, which is consequently trans¬ported from the roots up into the geothermal system. This still appears to be a valid idea, but the question is whether the heat sources may also be shallower, smaller intrusions (dikes or sheets, etc.) or both. In the latter case the heat sources would be more dispersed than massive magma chambers and more accessible to circulating water. The deep roots have lately experienced renewed interest. This is because of their hypothetically great potential, small environmental impact of harnessing them (smaller surface imprint) and due to recent advances in exploration and drilling technologies. The IDDP-1 well was drilled in the Krafla geothermal field, NE-Iceland, in 2007-2008. It was to be drilled to a depth of 4 to 5 km, but unexpectedly drilled into magma at 2.1 km depth. The well discharged superheated steam delivering power of more than 30 MWe. This demonstrated that the deep roots of geothermal systems can be more complicated than anticipated and that huge amounts of energy can be harnessed by drilling close to (or into) magma. This was followed in Iceland by intensified scientific activity, e.g. the Deep Roots Geothermal (DRG) project, which aims at advancing modelling methods for simulating physical processes in the roots, with the purpose of illuminating the overall process controlling the upwards heat transfer from the roots as well as advance the methods for conventional geothermal reservoir modelling. The DRG-project also involves geological studies of extinct, exposed volcanic geothermal systems, geophysical exploration of active systems and design of the components (well-heads, casings, etc.) of deep geothermal wells that can withstand the high temperatures, pressures and flow-rates expected (e.g. of super-heated steam) as well as detrimental chemical content. The modelling part of the DRG project has involved the application of available software to various hypothetical models, relevant for increased understanding of geothermal activity around intrusions. A new equation-of-state (EOS) for TOUGH2/iTOUGH2 developed is extremely valuable as it extends the applicability of the software, which is widely used within the geothermal industry, to much higher pressure and temperature, and consequently greater depth, than has been possible. The project also involved a study of the applicability of other software for deep roots modelling, including a comparison of 2D vs. 3D modelling configuration. The geothermal industry will greatly benefit from both increased understanding and improved modelling tools. Deep roots research in Iceland is continuing beyond the DRG-project, e.g. through the cooperative DEEPEGS project, funded by the European Union's HORIZON 2020 program, which has so involved the drilling of the 4,7 km deep IDDP-2 well in Reykjanes, SW-Iceland, where a bottom-hole temperature of approximately 550°C has been inferred.