A reliable static formation temperature is valuable in determining casing depths, establishing geothermal gradients, analyzing logs and estimating fluid potential for geothermal reservoirs. The conventional drilling mud fluid systems associated with geothermal well drilling distort the static formation temperature near the wellbore because the circulating mud temperature is normally much less than the static formation temperature. As a result, a wellbore temperature recorded during drilling operations does not reflect the static formation condition.

The use of a Horner-type temperature buildup plot, similar to the conventional pressure buildup method, has been suggested ( ' ) for estimating static formation temperature. The method has proven satisfactr in a number of oil and gas field cases. Recently, Dowdle and Cobb 92y investigated the conditions under which the Horner temperature plot can be used to yield representative static formation temperatures. They concluded that the method is reliable if both the wellbore temperature gradient changes very slowly and the mud circulation time is short. However, in geothermal operations, temperature gradients are usually more extreme and longer circulation times are required to cool the wellbore sufficiently for logging. Therefore, the method has proved to be less reliable.

As an alternate solution, dimensionless Horner temperature type curves have been developed for determining reliable static formation temperatures under normal geothermal drilling conditions in the Imperial Valley, California .

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