1. n. [Drilling]
The temperature of the undisturbed formation at the final depth in a well. The formation cools during drilling and most of the cooling dissipates after about 24 hours of static conditions, although it is theoretically impossible for the temperature to return to undisturbed conditions. This temperature is measured under static conditions after sufficient time has elapsed to negate any effects from circulating fluids. Tables, charts, and computer routines are used to predict BHST as functions of depth, geographic area, and various time functions. The BHST is generally higher than the bottomhole circulating temperature and can be an important factor when using temperature-sensitive tools or treatments.