| Abstract |
Two reinjection zones are operated for the Hellishei?i Power Plant. One is the Gráuhnúkar, on the southern edge of the Hellishei?i Geothermal Field. The other is the Húsmúli at the northern edge of the field. The injectivity of the wells drilled in the Húsmúli Area has turned out to be temperature dependent. This effect differs from well to well, but over all the injectivity index is more than six times higher for water 20°C than for water of 120°C. This effect has to be taken into account when operating the reinjection zone in the Húsmúli Area. Temperature dependence has also been observed in the operation of the Gráuhnúkar Reinjection Zone. Accurate measurements of the temperature dependence have, however, not been undertaken in wells there. The commission of the Húsmúli Reinjection Zone in September 2011 caused considerable induced seismicity. The biggest earthquakes, which occurred in the middle of October that year, reached a magnitude of ML 4.0. Induced seismicity was observed in the area during drilling and well testing in the Húsmúli formation. The observed seismicity, when the operation started, was however, much more intense than expected. The induced seismicity did not cause significant damages. It did, however, cause considerable disturbances in the near by village of Hverager?i, which is located ~10 km from the reinjection wells. The induced seismicity in the Húsmúli Area has faded out during the reinjection suggesting that the reinjection released stresses already present in the area. The Geothermal system in Hellishei?i is in fractured bedrock. The temperature dependence of the injectivity can be understood from the fractured nature of the permeability of the reservoir. The induced seismicity was partly dependent on the temperature of the injected water. This suggests that the thermal effects of the injected water on fractures does also play a significant role in the induced or triggered seismicity. |