The noise log is a relative newcomer to production logging services. Though described by Enright in 1955, noise logging was not commercially available until after McKinley who demonstrated its utility in 1973. A noise log is simply a record of a passive measure of the audible sound detected by a sensitive hydrophone at a number of locations in the wellbore. Beacuse sound is generated by fluid turbulence, high noise amplitude indicate locations where the flow path causes additional turbulence to develop. Fluid moving through restricted channels, leaks, flow from perforations, and flow past the logging sonde are among the phenomena that can produce charasteristic sounds in the wellbore and that may be detected with a noise log (noise logging tool). Analysis of the frequency characteristics of the measured noise can distinguish between the various possible sources of high sound amplitudes, making the noise log a powerful tool for well diagnosis.
The noise log has been used primarily as a qualitative indicator of channeling behind pipe, often in conjunction with temperature logs. It can now be used quantitavely to estimate flow rates in some instances, and its application has been extended beyond channel detection.
Noise logging has been extended to applications other than the detection of channels in completed wells. Also another log to verify channels in cemented wells is CBL log. Among these are the measurement of flow rate in the wellbore, the measurement of flow rates from individual perforations, detection of liquid production from gas perfrations, detection of sand production, and the use of noise logging during drilling.