Applying the Hengill Geothermal Reservoir Model in Power Plant Decision Making and Environmental Impact Studies

Growth in geothermal power production in the Hengill area of Iceland, and a need for environmental impact studies that look hundreds of years ahead, has resulted in new challenges for numerical model developers. An existing, large-scale, iTOUGH2based 3-D reservoir model of the Hengill volcano has recently been recalibrated and used to study the impact of 400 MWe and 700 MWt cogenerations in two subareas of Hengill, Hellisheidi, and Nesjavellir. Reservoir performance is predicted for the next 30 years, followed by 1,000 years of recovery. The study indicates that the Hellisheidi subfield has greater growth potential than Nesjavellir. Increased production in Nesjavellir results in considerable pressure interference and reduced output of the average well. A plan to expand power generation from 120 to 150 MWe has therefore been put aside because of the cost; new discoveries in the conceptual reservoir model or increased coverage of the wellfield may revive the plan. The Hellisheidi power plant, on the other hand, may still sustain a production load increase from the already-decided-on 150 MWe to 270 MWe. The growth potential arises from a wellfield expansion to the north, in an area without wells and no forced model production. At the end of the generation period, model pressures revert to natural state levels over roughly the same time as generation has taken place, 50–60 years. This behavior is a result of the open boundaries of the reservoir model. The heat reserve requires up to 1,000 years for recovery. Large geothermal power plants in Hengill appear to produce at rates exceeding natural recharge. To make the power generation renewable, either resting periods are required or production must later be reduced to boundary recharge rates. These power plants should, nevertheless, qualify as sustainable development because of the technical and scientific advancements that accompany these intense field activities. To achieve this goal, all relevant field data and scientific publications must be documented and made open to the public.