| Authors: |
Sigurveig ÁRNADÓTTIR, Þorsteinn EGILSON, Anett BLISCHKE, Halldór Ö. STEFÁNSSON, Friðgeir PÉTURSSON, Haraldur JÓNASSON, Magnús ÓLAFSSON, Ólafur G. FLÓVENZ, Árni HJARTARSON, Sigurður G. KRISTINSSON |
| Keywords: |
Televiewer, acoustic borehole image logs, low-temperature, Iceland, Hoffell, fractures, feed zone, heating utility, well siting |
| Conference: |
World Geothermal Congress |
Session: |
Case Histories |
| Year: |
2015 |
Language: |
English |
| Abstract: |
Geothermal prospecting in the Hoffell area in Southeast Iceland commenced in 1992, as potentials for a heating utility in the main town of the region were estimated. The research included drilling of 33 shallow exploration wells and resulted in a model of a workable low-temperature geothermal system with two predominant fracture trends, ENE-WSW and approximately N-S. Prior to siting a deep exploration well in 2012, acoustic borehole image logs (Televiewer) were performed in seven of the existing wells in the purpose of identifying orientations of permeable fractures. Results of these measurements demonstrate a quite complex sub-surface fracture system with fractures which arguably have formed during different geological periods. Focus was set on large, permeable fractures, which were clearly observed in Televiewer images from two of the deepest wells. The trends of these fractures, calculated from Televiewer data, support the two previously mapped predominant fracture trends. Final determination of a site for the deep exploration well was based on the orientation of one of these feed zones, namely ENE-WSW strike and ~73° dip towards SSE. The well was drilled more than 200 m away from the peak of the geothermal gradient anomaly and was anticipated to intersect fracture planes which are connected to this feed zone, only at greater depths, where estimated temperature was 80°C. The well intersects several feed zones and one of these, trending ENE-WSW and dipping 83° towards SE at 1340 m depth, is believed to be related to the intended fracture planes. The well yields ~25 l/s of 73°C hot water, which is found to be successful. The acoustic borehole image logging is in its early phase in Iceland and well siting on grounds of such measurements has proven to be a helpful addition to the conventional method of siting wells on the base of geothermal gradient data, usually drilling at or near the temperature geothermal-gradient maximum. |
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