A Novel Method of Estimating Static Bottomhole Temperature Using Array Induction Logs
Authors: Khatchikian Alberto 
Published: December 2007

Abstract
To estimate the static bottom hole temperature in wells where the geothermal gradient is not known, this paper proposes a novel method based on the ratio of shallow to deep resistivity measurements from an array induction log and the continuous mud temperature survey that is usually available with it. This ratio in shales is practically a straight line decreasing from values higher than one at the bottom of the well to values less than one close to surface, reflecting the variation of the radial temperature gradient between mud and formation as a function of depth. The depth at which the ratio is one is where mud and formation temperature are the same. From the mud temperature at this depth and the yearly average surface temperature, the static bottom hole temperature is extrapolated assuming a linear temperature gradient. The method has been successfully tested in several wells of the San Jorge and Neuquen basins by comparing the results with the temperature estimated from the known geothermal gradient.

Introduction
It is important to determine downhole temperature as accurately as possible as it can affect the functioning of logging tools and has an important effect on rock and fluid properties. For example, mud conductivity increases with temperature, increasing the conductivity of shallow resistivity devices. Quantitative log analysis parameters, such as formation water and clay conductivity also increase with temperature. Gas density and oil viscosity are a function of temperature. Oil composition variations in a reservoir can be partly due to temperature effects1.
Downhole temperature can be calculated from the surface temperature and the geothermal gradient, which is assumed to be linear2. If the mean surface temperature for a given area is not known, data from the closest location can be used. However, differences in altitude, for example, can produce large differences in temperature between the well site and the closest location. For reference, Table 1 lists the mean annual temperature of several oil field locations in Latin America. Offshore, the sea bottom temperature is mainly a function of water depth. In the Atlantic Ocean the sea bottom temperature ranges between 9°C (48°F) at 600 meters and 2°C (36°F) at 2000 meters. In shallow water the bottom hole temperature depends mainly on the geographical location and sea currents.
For log analysis, the geothermal gradient is usually calculated from the mean surface temperature and the bottom hole temperature measured during logging operations. But because of the cooling of formations while circulating mud, the recorded bottom hole temperature can be 20°F to 40°F lower than the actual (static) formation temperature2,3. For this reason it is more accurate to use the geothermal gradient computed from cased-hole temperature measurement in several wells during completion or workover operations. This information is usually available in development fields. An example of gradient determination is shown in Fig. 1 for a field in the Neuquen basin in Argentina. In this case the computed gradient is 0.065°F/m, and the extrapolated surface temperature is 58°F, in agreement with the mean annual temperature of the city of Neuquen (see Table 1).
When the geothermal gradient is not known, for example in exploration wells, the static (i.e., undisturbed by mud circulation) bottom hole temperature has to be estimated from some other method. If several logging runs are made in the well, and the bottom hole temperature is measured in each run, the static bottom hole temperature can be estimated with a Horner-type plot of temperature vs. time4. This method is seldom used today, as modern logging tools can acquire a complete set of logs in a single run into the hole and therefore only one bottom hole temperature is available.