Proceedings World Geothermal Congress 2015 Melbourne, Australia, 19-25 April 2015 1 Well Site Selection Based on Acoustic Borehole Image Logs: A Case History from Hoffell Low-Temperature Geothermal Field in Southeast Iceland Sigurveig Árnadóttir 1) , Þorsteinn Egilson 1) , Anett Blischke 1) , Halldór Ö. Stefánsson 2) , Friðgeir Pétursson 2) , Haraldur Jónasson 2) , Magnús Ólafsson 2) , Ólafur G. Flóvenz 2) , Árni Hjartarson 2) and Sigurður G. Kristinsson 2) 1) Iceland GeoSurvey (ISOR), Rangárvellir, P.O. Box 30, IS-602, Akureyri, Iceland 2) Iceland GeoSurvey (ISOR), Grensásvegur 9, IS-108, Reykjavík, Iceland E-mail address: sigurveig.arnadottir@isor.is Keywords: Televiewer, acoustic borehole image logs, low-temperature, Iceland, Hoffell, fractures, feed zone, heating utility, well siting 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. 1. INTRODUCTION The Hoffell low-temperature geothermal field is located in the region of Austur-Skaftafellssýsla in Southeast Iceland, in the upper Hornafjörður region. Bedrock in the area is mostly composed of basalt plateau lavas with thin sediment layers intermittently and intrusions widely intercalated, although layers of felsic rocks and sedimentary agglomerate are also found. The volcanic pile is of Tertiary age and was deeply eroded by glaciers during the Pleistocene. The area is at the edge of a monoclinal flexure which runs from Breiðamerkurjökull in the Southwest, along the Vatnajökull ice-sheet cone some 250 km north to Vopnafjörður. Intercalated with the lava pile is an extinct Tertiary central volcano, named the Geitafell volcano (Friðleifsson, 1983), which is exposed because of deep glacial erosion (Figure 1). The volcano was formed within a rift zone in central Iceland, and was active for about 1 m.y., between 5 and 6 Ma (Friðleifsson, 1983), slightly predating the SE-Icelandic flexure zone. According to studies of infilling sequences of mineral veins and amygdales and their associated wall-rock alteration (Friðleifsson, 1983), the Geitafell volcano has experienced three major structural events, (i) uplift of the central region, (ii) caldera subsidence, and (iii) regional flexuring. In addition, twelve intrusive phases were distinguished on the base of these studies. Following the emplacement of central gabbros belonging to the second intrusive phase, a high-temperature hydrothermal system was established at shallow levels (Friðleifsson, 1983). Prior to the emplacement of these central gabbros, cold groundwaters percolated through the volcanic strata and slowly became heated, presumably by dykes that intruded the volcano during the earliest intrusive phase (Friðleifsson, 1983). After the emplacement of the central gabbros, the previous low-temperature hydrothermal system then changed abruptly to an active high- temperature system (Friðleifsson, 1983). With time the hydrothermal system cooled and adjusted to hydrostatic values, until a second thermal boost accompanied the emplacement of the 10 th intrusive phase, comprising intrusion of marginal gabbros, local cone-sheets and NE-SW trending dykes, which maintained the high-temperature hydrothermal activity for some length of time, after cooling had begun, proceeding downwards with time (Friðleifsson, 1983). It is concluded that the flexure zone was formed during the cooling history of the Geitafell volcano hydrothermal system, and its formation may possibly have induced cooling by extensive fracturing (Friðleifsson, 1983). Both the flexure and the volcano subsequently became buried by younger lava flows erupted to the northwest, and later eroded during Pleistocene glaciation (Friðleifsson, 1983). Heat is still preserved in the crust at the Hoffell low-temperature field, which has been explored discontinuously over the last two decades or so in the purpose of finding hot water for a heating utility in Höfn, the main town of the region some 20 km away. Geothermal prospecting commenced in the region of Austur-Skaftafellssýsla in 1992 with the drilling of shallow research wells which were employed to accomplish a survey of geothermal gradient (Stapi – Jarðfræðistofa, 1993). An anomaly was soon detected in the geothermal gradient data near the farms at Hoffell (“Hoffell” and “Midfell” in Figure 1), which are located at the periphery of the Geitafell volcano, and this area accordingly became the main focus of interest. Drilling was hence continued in the Hoffell field (Stapi – Jarðfræðistofa, 1992; 1993; 1994; 2002; 2005; 2006), totaling in 33 vertical research wells, most of them less than 60 m deep. Highest measured temperature was 61.1°C at 505 m depth in the deepest well in the area (ASK-86). Based on the geothermal gradient data, combined with further research, a model was constructed of the geothermal system which exhibited two predominant