Nowadays downhole temperature is systematically recorded in testing operations or slickline surveys. In most cases these temperature measurements are largely ignored and are only used in the gauges for pressure-temperature compensation. However, temperature transients during well tests are very sensitive to "prior-history", i.e. events that took place in the wellbore prior to the well test; hence it can be a valuable source of information.

This paper illustrates how the temperature transient response depends on the history and sequences of operations before and during well testing with DST (Drill Stem Test string). For instance when exploration or appraisal well test is performed in a new well, right after drilling operations, the temperature response can show the effect of "thermal contamination" by mud circulation, fluid losses and cementing. In production well test, such as after a long period of shut-in or during start-up operations months after drilling and completions, one also observes temperature transient. The temperature response in such cases is mostly affected by geothermal gradient, Joule-Thompson and adiabatic effects, since thermal contamination effect has slowly disappeared.

The paper summarizes different areas of application of transient temperature data. It has been observed on field examples, that temperature transients from exploration and appraisal testing could be used for flow history and fluid losses estimation, while in production testing transient temperature is sensitive to the heterogeneity of pay zones and other parameters of the reservoir.

A total volume of mud and completion fluid losses is one of the most critical information required for cleanup efficiency estimation. The paper will present how that information can be estimated from the non-isothermal analysis of clean-up flow as the temperature transients recorded in the exploration and appraisal testing soon after drilling provide an important insight of the past well life with respect to cold fluid circulation/losses and later production from the reservoir. This "memory" of cooling can be significant in amplitude and could last up to a few hours or even days after drilling operations are finished.