- The current method for estimating the static reservoir temperature from transient data gathered downhole after the cessation of mud circulation is a Horner-type approach. The reservoir temperature estimate so established generally turns out to be lower than the true temperature. This work shows that the temperature analog of the Horner method is limited to large circulation-time situations and is not applicable to cases where the mud circulation time is generally short (less than 30 hours). This study provides a general solution of the thermal diffusivity equation obtained with an appropriate inner-boundary condition to solve the early-time, transient heat-transfer problem. An analytical solution was developed to handle problems requiring a variable-rate heat transfer or the superposition effects. A simple graphical technique is used to interpret data with the superposition function. Three approximate solutions, reflecting different time domains, are also presented. These solutions are reliable as long as they are applied within the specified time bounds. Synthetic examples are used to validate the proposed method, and field examples from both oil and geothermal wells demonstrate application of the method.