It is commonplace to take pressure and temperature measurements along the wells in geothermal reservoirs. Such profiles give good insight into the characterization of the geothermal reservoir. Many useful information such as upper and lower boundaries of reservoirs, deliverability of wells, the nature of the phase changes within the wells can be obtained as a result of these profiles. However in most cases taking these profiles along the wells can be challenging. Furthermore these types of profiles cannot be considered at all times since the wells would have to be shut in and there is always the economical factor. In this study we look at in more detail the behavior of the wellhead temperature, the only place where temperature can be measured at all times and is not an economical burden. We specifically study the behavior of the temperature derivative obtained at the wellhead. Just as in a well testing diagnostic plot the temperature derivative at the wellhead displays clearly various flow regimes that take place in the well. At early times when wellbore convection dominates, a log-log plot of dT/dlnt versus t gives a unit slope straight line similar to early-time pressure transient behavior dominated by wellbore storage. The intermediate-time behavior is observed when heat transfer between wellbore and the surrounding formations is felt and is recognized by -1/2 slope straight line on a log-log plot of dT/dlnt versus t. At late times the wellbore heat transmission solution converges to the cylindrical-source solution transferring heat at constant temperature.

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