Well Siting in the Gráuhnúkar-Meitlar Field in the Hengill Area, SW-Iceland
Helga Margrét HELGADÓTTIR, Ásdis BENEDIKTSDÓTTIR, Sigurður Garðar KRISTINSSON, Auður Agla ÓLADÓTTIR, Ingvi GUNNARSSON
[Iceland Geosurvey, Iceland]
The reinjection site of Gráuhnúkar at the Hellisheiði high-temperature field in SW-Iceland proved, at the time of drilling, to have much higher temperature than anticipated. Despite the reinjection the area has, due to its anomalous temperatures, since been regarded as a potential production area for Reykjavík Energy along with the Meitlar area to the south. Interdisciplinary work on well siting commenced with gathering and comparing available data from the area. The dominating fractures in the area are the NE-SW striking fractures belonging to the Hengill fissure swarm. However, other fractures with different strikes have been noted in the landscape. Those fractures need further analyses. Resistivity model of the area has been made using a 3D inversion of static shift corrected MT measurements. The bottom of the low resistivity layer, where high resistivity is noted below, is believed to indicate the outlines of the geothermal system in Gráuhnúkar-Meitlar. The depth to the change of low restivity to high resistivity (25 Ωm was used here as a reference) seems to form a ridge, following a graben with a similar strike to the Hengill fissure swarm. In the northern part of the Gráuhnúkar-Meitlar area six reinjection wells and two exploration wells have been drilled. The formation temperature in the area is based on those wells. This leaves large unknown areas temperature wise, particularly in the Meitlar area to the south. Temperature in Gráuhnúkar is 260°C at 1000 m b.s. and is currently regarded as the hottest area. Comparison of the first appearance of certain alteration minerals in drill cuttings from the wells to the formation temperature indicates that the centre of the reinjection area has experienced recent upheating. The area to the east of Gráuhnúkar has, however, experienced cooling and is not feasible for production drilling. Comparison of available data suggests feasible targets to the east and northeast of mount Litli-Meitlar. It is, however, difficult to predict whether the resistivity model shows the current temperature regime or if the temperature of the system has changed. Drilling will reveal if the resistivity model corresponds with the current temperature.
|        Topic: Exploration||Paper Number: 11076|