Jökull


Jökull - 01.12.1987, Side 15

Jökull - 01.12.1987, Side 15
Exploration and development of the Krafla geothermal area HALLDÓR ÁRMANNSSON, ÁSGRÍMUR GUÐMUNDSSON AND BENEDIKT S. STEINGRÍMSSON Orkustofnun, National Energy Authority Grensásvegur 9, 108 Reykjavik, lceland ABSTRACT The Krafla high temperature geothermal area is situ- ated within the neovolcanic zone in Northeastern Iceland. The geology ofthe area is dominated by an active central volcano, with a caldera and an active fissure swarm. The volcanic activity at Krafla is episodic, occurring every 250-1000years, and each episode lasting 10-20 years. The latest eruptiveperiod started inl975. The geothermal area is within the Krafla caldera. Surface manifestations like steaming fumaroles and altered ground cover an area of aboutl5 km2. In the uppermost 700 meters the geothermal reservoir is characterized by a large resistivity low in the central part of the caldera and two smaller ones at the caldera rim. The major anomaly is underlain by a higher resistivity body, coinciding with a change in the lithology from hyaloclastites to basaltic lavaformations, increased frequency of intrusive rocks and high temperature hy- drothermal alteration. Three separate upflow channels within the main geothermal area have been identified by geochemical studies. The Krafla area is under exploita- tion and 24 deep wells have been drilled to provide ge- othermal steam for the Krafla electric power plant, com- missioned just before the present volcanic activity started. The wells are sited in three different fields which differ considerably in size and production characteristics. Parts of the reservoir are liquid saturated but the bulk of it is boiling. Temperatures of35(PC have been encountered at 2 km depth. The permeability is 1-4 millidarcy and the average wellflow is relatively low. Numerical simulation studies of the present wellfields indicate that they can sustain 55 MW electric production for at least 30 years. The developement of the Krafla geothermal area has been performed in the shadow of the volcanic activity. Magmatic gases have intruded the geothermal reservoir and contaminated two of the three major upflow zones causing deposition and corrosion in production wells. The magmatic influence has been on the wane for the last few years. INTRODUCTION The Krafla geothermal area is located in the Neovol- canic zone in northeastern Iceland (Fig. 1), about 10 km northeast of Lake Mývatn (Fig 2). It is located within the Krafla caldera, which was formed about 100 thousand years ago (Sæmundsson, 1983). Volcanic activity in the Krafla region is extensive and there have been several eruptive periods during the last few thousand years. The most recent one started late in 1975 and nine eruptions have already taken place in Krafla during this period, the last one in September 1984 (Björnsson, 1985). An exploration program for the area was initiated in Fig. 1. The central volcanoes and fissure swarms within the Northeast Volcanic zone in Iceland, (Björnsson 1985). — Mynd 1. Einfölduð mynd af gos- og sprungu- kerfum á Norðausturlandi. JÖKULL No. 37,1987
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