MadSci Network: Earth Sciences

Re: What happens to the temperature of water when it is highly compressed

Date: Sat Jan 21 22:58:21 2006
Posted By: David Dunbar, Staff, Geophysics, GeoCenter, Inc.
Area of science: Earth Sciences
ID: 1134108463.Es


You ask a rather difficult engineering question. Without knowing more about
the well you refer to I can only give you a general answer. On the other
hand, if you had a lot more information about the well I would have to pass
it on to a drilling engineer.

The temperature behavior of a geothermal well going from a flowing to a
shut in state depends on a lot of variables. These include:

1)The heat flux through the earth, which is typically significantly higher
than normal. This is, after all, the reason for drilling the well.

2)The thermal conductivity and heat capacity of the rock surrounding the well. 

3)The fluid flow through the well and into the rock. The permeability (how
easily fluid flows through the rock) and porosity (the fraction of the rock
that is not solid) as well as whether the well is cased or lined with pipe.

4)How long the well has been flowing. 

5)The rate of flow.

The compressibility of water is not a significant contributer. 

High flow rates, short flow times, low permeability and higher heat
capacity of the rock allow the temperature to recover faster. These also
affect the temperature going from shut in to flowing.

Temperature profiles from wells in the Salton Sea geothermal field have
shown temperature changes of 200 degrees C over several hours after being
shut in. Shut in temperatures of 300 degrees C dropped to 100 degrees at
high flow rates. 

Geothermal wells frequently have rather significant engineering obstacles.
 Many have corrosive salty water that can not be discharged at the surface.
Others are dry. Down hole heat exchangers are used in these cases. 


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