Extensive, modeling studies of the Krafla geothermal field in Iceland are presented in a series of four papers. This first paper describes the geological settings of the field and the analysis of well test data. The geothermal system at Krafla is very complex, with a single‐phase liquid reservoir overlying a two‐phase reservoir. The reservoir rocks are volcanic with sequences of basalt flows, hyloclastics, and intrusions. The fluid flow in the reservoir is fracture dominated. Considerable pressure transient data from injection tests have been gathered from the Krafla wells. These data are analyzed to yield the transmissivity distribution in the reservoir system. As the data are complicated by various factors (wellbore effects, nonisothermal effects, two‐phase flow, and fractured rocks) the applicability of conventional well test analysis methods is questionable. We have developed a methodology to analyze injection tests for such systems. The results obtained show that the transmissivity of the Krafla reservoir is low, averaging 2 Darcy meters (Dm). The average transmissivity of most commercially successful geothermal fields is an order of magnitude higher.
A model of the natural state of the Krafla reservoir system in Iceland has been developed. The model consists of a vertical cross section which includes reservoirs in both old and new well fields. The physical processes considered include mass transport, conductive and convective heat transfer and boiling, and condensation. Natural heat losses to surface manifestations (springs) are also included. The model matches very well all relevant data from the Krafla field. The natural flow of hot fluids through the reservoirs is estimated as approximately 20 kg/s. Steam escaping to surface springs constitutes the bulk of the heat losses in the area modeled. Conductive heat losses through the caprock are approximately 1 W/m2, and heat recharge from below is about 2 W/m2.
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