Induced Seismic Activity During Drilling of Injection Wells at the Hellisheiði Power Plant, SW Iceland

Abstract

Hellisheiði geothermal power plant is a 303 MWe and 133 MWth plant in SW Iceland owned by Reykjavík Energy. The production started in 2006 and increased gradually to full size in early 2011. The production field is to the south of a central volcano, Hengill, which is located at a triple junction plate boundaries in SW Iceland. The Hengill-Hellisheiði area is seismically active, characterized by individual swarms that may last for five years or more and with recurrence time of 20 to 40 years. Largest individual earthquakes have reached magnitudes of 5.5 Ml. Several structural lineaments can be observed, the most distinctive features being NE trending grabens, crater rows and hyaloclastite ridges. Húsmúli, the present main injection site for the geothermal fluid from the power plant was taken into use in Sept. 2011 with full scale injection of 550 L/s. An intense seismic swarm followed with 4 earthquakes reaching magnitudes over 3 Ml and largest attaining 3.8 Ml. The drilling of the injection boreholes in the Húsmúli area started in 2007 and associated with the drilling and testing of the boreholes seismic events were observed. Seismic activity was also observed during drilling and testing of production holes in 2003 to 2007. During drilling of injection borehole HN-17 temporary seismic networks were installed around the borehole. A seismic swarm associated with circulation loss during the drilling was therefore well monitored by the temporary networks as well as the regional network. The earthquakes are mainly located north of the borehole with depth ranging from 0 - 3 km.