When drilling a geothermal well, it is important to know, roughly, the original formation temperature prior to setting casing pipes. Especially, the interface of the cold water zone and the hot formation should be determined during drilling so that casing and full cement jobs can be designed satisfactorily.

During drilling, heat in the formation flows into the circulating fluid. When the pump is stopped for drill pipe trips or for any other reason, the temperature of the static fluid in the hole will rise continuously. From a mathematical viewpoint, this phenomenon is similar to the pressure buildup through porous media when a fluid flowing well is shut in. The static reservoir pressure could be extrapolated from the Horner pressure-time straight line on a semilog graph; thus the static formation temperature, could also be estimated in the same manner. Es- Unlike the pressure buildup test, the temperature recovery surveys should be performed once every 10 m interval in the open hole. means many semilog graphs will be done: tiresome work open to miscalcul at i o n by the engineer. necessary computations is presented in Table 1. The program is *itten for the HP-65 hand computer. The equation used to compute the original formation temperature is derived as follows: the Horner semilog plot is usually a straight line. can be expressed as: This A hand-computer program capable of making the By the least-squares method, the straight l in e

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