The Krafla Geothermal Field, Iceland: 1. Analysis of well test data

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. The authors 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.

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