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Defining, exploring, imaging and assessing reservoirs for potential heat exchange - Potsdam, Germany, Workshop1
Defining, exploring, imaging and assessing reservoirs for potential heat exchange - Potsdam, Germany, Workshop1
6-8 November 2006 GeoForschungsZentrum Potsdam (GFZ)
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Index minerals in defining temperature in potential geothermal reservoirs
During exploration drilling in Icelandic geothermal systems drill cuttings are 
sampled at every two meters. The cuttings are analyzed on site during drilling 
operations using a binocular microscope. These analyses reveal the lithology and the 
occurrence of identifiable alteration minerals in the well. After the well has been 
drilled a number of samples are selected for more detailed study, including XRD-
analysis and optical microscopy investigation with a petrographical microscope.
During drilling operations in high-temperature geothermal areas it is necessary to 
keep the well temperature below a certain temperature in order to secure the 
integrity of the drill string and prevent well discharge during drilling. This is 
done by continuously pumping cold fluids into the well during drilling. As a result, 
it is impossible to measure aquifer temperature directly at the time of drilling. 
Traditionally all exploration geothermal wells in high-temperature areas in Iceland 
are designed and drilled as production wells. This implies that all aquifers colder 
than the desired production temperature have to be cased off. An empirical 
relationship between formation temperature and the occurrence of specific alteration 
minerals is used to determine a proper depth for the production casing. This method 
is currently best estimation of aquifer production temperature that can be made 
during drilling. Therefore aquifers that are too cold for production can be excluded 
from the production part of a well based on the absence or presence of certain index 
minerals. The relationship between formation temperature and alteration mineral 
assemblages is based on empirical observations in Icelandic geothermal systems from 
1970 up to the present (Kristmannsdottir 1979, Franzson 1998).
Alteration mineral growth can represent the temperature history of the geothermal 
system. Many mineral generations can be found in cavities and fractures in the 
geothermal system tell the story of its development. In the case of progressive 
heating many of the low temperature minerals become unstable, dissolve and 
disappear. Other alteration minerals are stable over a wide temperature range and 
their temperature history can be revealed by fluid inclusion studies. Cooling on the 
other hand is observed as overprinting of low temperature minerals covering minerals 
characteristic of higher temperatures.
Id: 23
Place: GeoForschungsZentrum Potsdam (GFZ)
14473 Potsdam, Telegrafenberg
Starting date:
06-Nov-2006   10:35
Duration: 03'
Contribution type: Poster
Primary Authors: Mr. KRISTJANSSON, Bjarni Reyr (Iceland GeoSurvey)
Presenters: Mr. KRISTJANSSON, Bjarni Reyr
Material: slides Slides
poster Poster

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