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Temperature surveys are the most common surveys to locate fluid movement downhole. Small entries and even flow in channels behind pipe can be detected. Generally, if a well is not flowing, the temperature of the fluid in the borehole will eventually approach the formation temperature, called the ''geothermal gradient''. When a well is produced, formation fluids enter the borehole and move uphole. Gasses typically cool when entering the borehole while liquids do not. In either case, their movement uphole is easily detected by deviations of the borehole temperature from the geothermal gradient. [[:file:production-logging_fig1.png|Figure 1]] illustrates a typical temperature survey response to two gas entries into a well.

Temperature surveys are the most common surveys to locate fluid movement downhole. Small entries and even flow in channels behind pipe can be detected. Generally, if a well is not flowing, the temperature of the fluid in the borehole will eventually approach the formation temperature, called the ''[[geothermal gradient]]''. When a well is produced, formation fluids enter the borehole and move uphole. Gasses typically cool when entering the borehole while liquids do not. In either case, their movement uphole is easily detected by deviations of the borehole temperature from the geothermal gradient. [[:file:production-logging_fig1.png|Figure 1]] illustrates a typical temperature survey response to two gas entries into a well.

Noise logs are also used to evaluate fluid movement downhole. Unlike temperature surveys, noise logs are not run continuously across the interval of interest. Instead, a number of stationary readings are taken at different depths downhole. The movement of fluids, especially gasses, generates turbulence or noise, which gets louder as the flow rate or pressure drop increases. [[:file:production-logging_fig2.png|Figure 2]] shows how a noise log can be effective at detecting movement downhole. In this schematic diagram, a source, sink, and restriction to flow are the noise sources. The frequency spectrum of the noise is also observed to further improve the understanding of flow downhole.

Noise logs are also used to evaluate fluid movement downhole. Unlike temperature surveys, noise logs are not run continuously across the interval of interest. Instead, a number of stationary readings are taken at different depths downhole. The movement of fluids, especially gasses, generates turbulence or noise, which gets louder as the flow rate or pressure drop increases. [[:file:production-logging_fig2.png|Figure 2]] shows how a noise log can be effective at detecting movement downhole. In this schematic diagram, a source, sink, and restriction to flow are the noise sources. The frequency spectrum of the noise is also observed to further improve the understanding of flow downhole.