Environmental Engineering Reference
In-Depth Information
8
Temperature Logging
Among the reasons for using temperature logs are:
1. To find the cement top after a recent cementing operation
2. To find a lost-circulation zone in a currently drilling well
3. To find fluid entry and exit points in production and injection wells
Temperature logging tools employ a variety of sensors including temperature-
sensitive resistors, thermistors and diodes. In order to react quickly to temperature
changes, temperature logging tools are designed with the sensing element exposed
directly to the wellbore fluids. For this reason, they are delicate instruments easily
damaged by physical abuse or by the junk normally found at the bottom of a well.
Resistance Temperature Detector (RTD)
Most premium tools contain a platinum resistance temperature detector (RTD)
which exploits the tendency of a metallic conductor to exhibit a change in resistance
with temperature. An RTD is essentially a temperature-sensitive resistor with a
positive temperature coefficient, which means that the resistance of the metal
increases with temperature. RTDs are made from a number of different metals, but
the platinum RTD has captured most of the industrial market, and is now used in
most modern temperature logging tools. The major drawback is a rather small
change in resistance per degree of temperature change, mandating special signal
processing for signal amplification.
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