| Title | The Hengill-Hellisheiði Geothermal Field. Development of a Conceptual Geothermal Model |
|---|---|
| Authors | Hjalti Franzson, Bjarni Reyr Kristjánsson, Gunnar Gunnarsson, Grímur Björnsson, Arnar Hjartarson, Benedikt Steingrímsson, Einar Gunnlaugsson, Gestur Gíslason |
| Year | 2005 |
| Conference | World Geothermal Congress |
| Keywords | High-temperature field, exploration, geology, conceptual model |
| Abstract | The Reykjavik Energy has to date drilled 10 deep wells into the Hellisheiði high-temperature field, which is situated in the southern sector of the 110 km2 Hengill low resistivity anomaly. The geothermal system is found within the NE-SW fault zone and the graben of the Hengill central volcano. While adequate geological information is available from the wells down to below 2000 m b.s.l. in the western part of the field, reliable information is only available down to about 600 m b.s.l. in the central part of the field due to total circulation losses. The sub-surface basaltic strata comprises mostly hyaloclastite volcanic formations down to some 1000 m b.s.l. depth and underlain by a more dominant lava succession. An age of about 0.4 m.y. is proposed for the Hengill central volcano, which also puts an upper age limit on the geothermal system. Intrusions are scant down to about 800 m b.s.l. depth but become more common below that. They are mostly of basaltic composition but a few are more evolved. Permeability in the reservoir is believed to relate largely to intrusive boundaries and major faults. Of particular interest are two NE-SW basaltic dykes of 2000 and 5000 year old fissure eruptions, which are believed to provide the main geothermal flow channels of the system from a proposed upflow zone in the central part of the Hengill volcano towards south. The same eruptive fissures play a similar role in the Nesjavellir system on the north side of Hengill central volcano. A preliminary study of the hydrothermal alteration shows that minor cooling has occurred in the western part of the system while progressive heating appears to be occurring along the aforementioned recent volcanic fissures above about 600 m b.s.l. Again this is a similar behavior as found in the Nesjavellir system to the north. A pronounced temperature reversal is observed in the fault zone in the Hellisheiði field. A temperature maximum of about 280?C is found between 200-600 m b.s.l., but below that depth temperature declines to 200-220?C at 2 km depth. A comparison between measured temperatures and alteration suggests that a cooling has taken place in the deeper part of the central Hellisheiði field. The exploration data from Hellisheiði have been interpreted and compared to the data from the other parts of the Hengill area, especially the Nesjavellir field. The conclusion is that the geothermal activity at the Hengill central volcano and its fissure swarm can be explained by a conceptual model assuming one or more upflow zones underneath the Hengill volcano, caused by buoyancy as hot intrusions in the roots of the volcano heat up groundwater. This also creates a pressure low deep under the volcano so fluids from the outer boundaries of the system recharge the upflow. The main recharge channel is deep within the NA-SW fault zone that crosses the Hengill volcano where the permeability is believed to be highest. Higher up under Hengill the upflow divides where some fluid flows to NE into the fissure swarm towards Nesjavellir and some to SW towards Hellisheiði. Simulation studies quantify the upflow as some 100 kg/s of boiling mixture of steam and water with an enthalpy of more than 2000 kJ/kg. |