| Title | Reinjection Into a Fractured Reservoir – Induced Seismicity and Other Challenges in Operating Reinjection Wells in the Hellisheiði Field, SW-Iceland |
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| Authors | Gunnar GUNNARSSON, Bjarni Reyr KRISTJÁNSSON, Ingvi GUNNARSSON, Bjarni Már JÚLÍUSSON |
| Year | 2015 |
| Conference | World Geothermal Congress |
| Keywords | Hengill, Hellisheiði, reinjection, induced seismicity, seismic risk, temperature dependent injectivity, fractured reservoir |
| Abstract | The Hellisheiði Power Plant was commissioned in 2006. It is located in the southern part of the Hengill Volcanic System, 25 km southeast of Reykjavík, Iceland. The system consists of hyaloclastite formations intersected by a NE-SW oriented fissure swarm. The Hellisheiði Geothermal Field is located in the southern part of the area. Another geothermal field under operation, Nesjavellir, is in the northern part of the area. The most productive part of the Hellisheiði Field is relatively small and therefore it is necessary to reinject in order to maintain reservoir pressure. The reservoir is water dominated and around 50% of the produced fluid is steam at separator pressure. All the separated water is reinjected into the reservoir along with some condensate water. Two reinjection zones are operated for that purpose; Gráuhnúkar and Húsmúli. There are mainly three challenges in operating the reinjection zones of the Hellisheiði Power Plant. Firstly, the productivity of the reinjection zones has been decreasing slowly – especially that of Húsmúli. Secondly, the permeability of the wells is highly dependent on the temperature of the reinjected water. The permeability increases with lower temperature. The third challenge is induced seismicity in the Húsmúli Reinjection Zone. It is not clear why the productivity of the reinjection zones has been decreasing. Individual wells behave differently. In one well the injectivity is increasing while in others it is constant or decreasing. The temperature dependent injectivity is, however, relatively well understood. It is a property of the facture dominated permeability. The injectivity of the reinjection zones can be tuned to certain extent by changing the temperature of the injected water. Intense induced seismicity followed the commission of the Húsmúli Reinjection Zone. The intensity was highest in the beginning and the biggest events were two quakes of magnitude ML 4.0. The seismicity has been fading out slowly indicating that the reinjection has released stresses that were present in the crust. The tectonics of the Hengill Area is complicated. The NE-SW oriented normal fissures intersecting the area are normal faults. N-S oriented strike-slip faults, which are part of the South Icelandic Seismic Zone (SISZ) are also found in the area. Some seismicity due to the reinjection is still present in the area. It is dependent on the flow rate into the reinjection wells and the temperature of the reinjected water. In order to mitigate seismic risk the policy is to keep all injection parameters as constant as possible. |