For the purpose of evaluating geothermal reservoirs, the static formation temperature should be established as accurately as possible. A knowledge of the true, static formation temperature is required in estimating the heat content of geothermal reservoirs. The interpretation of electric logs requires accurate formation resistivities, which are dependent on temperature. Reliable static temperature is important in designing completion programs and establishing geothermal gradients.

Unfortunately, the temperatures recorded during logging operations are usually lower than the static temperature. These low temperatures result due to the cooling effect of the mud during circulation. As soon as circulation stops, the temperature around the wellbore begins to build up. Complete temperature recovery in a new well may take anywhere from a few hours to a few months, depending on the f ormation and well characteristics and the mud circulating time. A long wait for complete temperature recovery would cause sizable increases in drilling costs; hence a quick and easy method is needed for calculating static temperature using early shut-in data.

Following the practice of pressure buildup analysis for wells, the common practice in the goethermal industry is to use Horner plots for estimating static reservoir temperature from temperature buildup data. In this method, the buildup temperature is plotted against the logarithm of dimensionless Horner time, (t + At)/At,where t is the circulation time before shut-in and A t is the build& time, e dataPpoints are then fitted to a straight line, which is extrapolated t finite A t, i.e., a dimensionless Horner time of unity. The extrapolated temperature corresponding to this point is taken as the true reservoir temperature. This method is based on the "line source solution" to the diffusivity equation describing the radial conductive heat flow in an infinite system with a vertical line sink withdrawing heat at a constant rate.

Unfortunately, as will be shown later, this conventional Horner plot approach yields values of apparent static temperature that are lower than the true reservoir temperature. The goal of this investigation was to develop an improved approach so that the estimated static temperature will be closer to the true reservoir temperature than is possible from the conventional Horner plot.

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