Borehole geophysics and geology of the Urridavatn geothermal area, E-Iceland

Abstract

The Urridavatn geothermal field is a low-temperature geothermal field in E-Iceland, which has been utilized for space heating since 1980. In this study the temperature conditions in the system are evaluated on the basis of 68 temperature logs from 8 wells. This evaluation reveals a near-vertical hot water up-flow intersected by the best production well in the field, well UV-8, at about 800 m depth. This thermal anomaly tilts towards well UV-5 at greater depth. It is recommended to deepen well UV-5 to at least 1500 m depth to intersect this up-flow. The lithology of the Urridavatn geothermal system is also evaluated on the basis of geophysical logs, drillcutting analysis and a core description from one of the wells. An emphasis is placed on studying the correlation between temperature logs and lithology. A method is proposed whereby lithological information may be extracted from temperature logs. It involves comparing several temperature logs from the same well and dividing the borehole into temperature units, each with similar characteristics. It is suggested that these temperature units may be used to distinguish between permeable and non-permeable lithological units.