Temperature Logging - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 105

Fig. 8.3 Geothermal gradients. Courtesy Schlumberger
Obviously, the actual temperature on the surface fluctuates seasonally so the value
used for the surface temperature is actually the mean annual surface temperature
and will be in the range of 60-70 °F in temperate climates. Seasonal near-surface
temperature changes do not penetrate very deeply into the ground and well logging
purposes can be ignored. One exception is the permafrost zone that can exist near
polar regions.
Estimation of formation temperature from open-hole logs can be made provided
it is borne in mind that at the time a logging run is made the wellbore is cooler than
the surrounding formation, due to mud circulation. If several logging runs are made
in the same hole, undisturbed formation temperature can be estimated from a plot of
temperature against time. The method of Dowdle and Cobb is recommended.