The Hengill volcanic region in SW-Iceland encompasses vast geothermal resources that by now are utilized in two geothermal power plants, Hellisheidi and Nesjavellir, with a combined electrical generation capacity of 423 MWe as well as a thermal energy production capacity of 420 MWth, principally used for space heating in the Reykjavík suburban area. The Hellisheidi power plant was commissioned in 2006 with large scale reinjection being a key element of the associated resource management ever since. The main reinjection zones associated with the Hellisheidi power plant are Húsmúli on the NW edge of the geothermal field and Gráuhnúkar in its SW-corner. In order to study the connection between specific reinjection and production wells, and the cooling danger facing some production wells, a comprehensive tracer test was conducted during 2013-14. It involved 6 injection wells, both at Gráuhnúkar and Húsmúli. Thus 6 different liquid phase naphthalene sulfonate tracers were injected into the injection wells, 100 kg of each, and tracer samples collected from a large number of production wells. Several wells showed significant recovery, amounting to as much as 25% for a single well. The tracer recovery was modelled by simple models of flow-paths connecting feed-zones in reinjection and production wells. The model parameters were consequently used to calculate future cooling predictions for those wells exhibiting significant tracer recovery. The results indicated that at least 4 production wells could be seriously affected (cooling of up to 25-30°C in 15-20 years) during long term reinjection, an adverse effect that must be tackled during the future resource management of the Hellisheidi geothermal resource. Monitoring data collected since large-scale reinjection started at Húsmúli in late 2011 doesn’t indicate any significant cooling of the production wells monitored, which contradicts the cooling predictions for the wells showing the greatest tracer recovery. This indicates that the cooling predictions may be too pessimistic in some cases. The recirculation of some of the tracers injected has resulted in recovery through additional flow channels, providing valuable additional information, which needs to be interpreted. This study has also revealed the need to incorporate the possible degradation of some of the naphthalene sulfonates used in the overall interpretation as well as the possible contamination from materials used in casing cement.

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