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Chapter%208 - Petrophysics MSc Course Notes 8 TEMPERATURE...

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Petrophysics MSc Course Notes Temperature Logs Dr. Paul Glover Page 85 8. TEMPERATURE LOGS 8.1 Introduction The Temperature Log is a tool for measuring the borehole temperature. Temperature sensors are attached to every tool combination that is run in a well for the measurement of the maximum temperature (assumed to be at the bottom of the well), and a few modern tools exist that can continuously measure temperature as the tool travels down the well. Readings from a number of the maximum thermometers attached to different tool combinations and run at different times are analyzed to give the corrected temperature at the bottom of the borehole ( bottom hole temperature , BHT ). The borehole temperature is an important parameter in the analysis of resistivity logs, but also for the detection of fluid movement, the analysis of fluid pressures and in geochemical modelling of formations and the maturity of hydrocarbons. 8.2 Theory Temperature in the sub-surface increases with depth. The rate at which it does so is called the geothermal gradient or geotherm . Typical geotherms for reservoirs are about 20 to 35 o C/km, although significantly higher values (up to 85 o C/km) can be found in tectonically active areas, and lower ones (0.05 o C/km) in stable continental platforms. Hence, the bottom hole temperature ( BHT ) for a 3000 m well with a geotherm of 25 o C and a surface temperature of 15 o C is 90 o C. Note that this assumes that the geothermal gradient is constant. In practice this is rarely the case because of differences in the thermal conductivities of rocks between the bottom of the hole and the surface, and fluctuations in the surface temperature which penetrate the sub-surface and perturb the sub-surface temperature. Low thermal conductivity rocks, such as shale, act as a thermal insulator and have a large temperature gradient across them, while high thermal conductivity rocks, such as salt, permit the conduction of heat efficiently, and have a small temperature gradient across them.
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