Updated reservoir analysis of the Theistareykir high-temperature geothermal field, N-Iceland

Abstract

Theistareykir high-temperature geothermal field is located in the Theistareykir fissure swarm in Thingeyjarsýsla region, about 32 km from the coast in northeast Iceland. The bedrock in the area is divided into basaltic hyaloclastites from subglacial eruptions during the Ice Age, interglacial lava flows and recent lava flows (aged about 10,000 years and younger). A surface resistivity survey shows that at a depth of 800-1000 m the Theistareykir geothermal system covers an area of 45 km2 surrounded by a low-resistivity cap which is underlain by a high-resistivity core. To date, eight deep wells have been drilled in the area, three vertical and five directional wells. The up-flow zone is located where well THG-1 was drilled and extends in a northeast direction. Temperature below 1500 m b.s.l. is above 300°C on the eastern side (where wells THG-1, THG-4, THG-6 and THG-7 are located) of the reservoir spreading in a northeast direction. THG-8 turned out to be a cold well and its formation temperature is less than 170°C below 800 m b.s.l. Generally the Theistareykir area is characterized by a cold inflow at a shallow depth. Injection well test analyses done for wells THG-7 and THG-8 show that the model that best fits the data is of a homogenous reservoir with constant boundary pressure. Skin factor values indicate that the wells are in good contact with the reservoir, which may be due to the stimulation of the wells done after completion. Generally both wells are characterised by good permeability as well as storativity. Volumetric assessment using Monte Carlo calculations estimated that about 160 MWe could be generated for a production period of 30 years and 90 MWe if the production time was extended to 50 years.