The Hellisheiđi Power Plant is located in the southwestern part of the Hengill Area, SW-Iceland, 25 km SW of Reykjavík. The Hengill Area consists of the Hengill Central Volcano and fracture zones northeast and southwest of Mt. Hengill. The Hellisheiđi Power Plant was commissioned in 2006, and expanded in 2008 and again in 2011. The current installed capacity is 303 MW electric and 133 MW thermal. Another power plant, Nesjavellir, is located in the northeastern part of the Hengill Area. That plant was commissioned in 1990 and has an installed capacity of 120 MW electric and 290 MW thermal. Original conceptual models of the geothermal fields in the Hengill Area postulated an up-flow zone under Mt. Hengill in the center of the area. That up-flow was assumed to be the power source of both the Hellisheiđi and the Nesjavellir fields as well as of other fields in the area. This one up-flow hypotheses worked reasonably well for operating the Nesjavellir field and simulating the geothermal reservoir there. The development plans for the Hellisheiđi Power Plant relied on the original conceptual model. However, down-hole data from wells drilled in the southwestern part of the Hengill Area during the development of the field brought the soundness of the one up-flow hypothesis into question. The formation temperature of in the area is characterized by localized temperature maxima separated by cooler regions and the hotter parts of the system are much smaller than assumed when the Hellisheiđi Project was planned. Due to the fast pace development of Hellisheiđi it was not possible to adapt to this new reality. As a result the Power-Plant built was too big for the current production field. Operating the Hellisheiđi Power Plant near full capacity has been a challenge due to the intense production density. The production density is approximately 250 kg/s/km˛ (or 40 MW/km˛ in electricity) within the most productive parts of the field. This high production density has caused significant pressure drawdown and decreased performance of wells. Reinjection has been used for maintaining the reservoir pressure, mainly on the edge of the production field. Experiments have also been done with in-field reinjection. Another approach to solve the operational problems has been to expand the production field, i.e. to find another resource. A promising field is known in Hverahlíđ in the southern part of the Hengill Area, approximately 5 km from the Hellisheiđi Power Plant. Wells there were recently connected to the Hellisheiđi Power Plant. As mentioned above the down-hole data from the wells drilled during the development of the Hellisheiđi field contradicted previous conceptual models of the Hengill Area. As a result the conceptual model needed to be revised and numerical reservoir models had to be updated. This has also opened questions on the origin of the geothermal activity in volcanic system as the Hengill Area.

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