Noise Logging - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 120

9

Noise Logging

Noise logging is applicable whenever fl uid fl ow, either in the borehole or in the
casing formation annulus, produces a detectable noise. Detection is by means of a
microphone suspended in the well. Experience teaches that analysis of noise is a
refi ned technique for deducing the source of the noise. Student life in the low-rent
district is enough to turn even the untrained ear into a veritable Sherlock Holmes,
distinguishing the characteristic sounds of fl ushing toilets from draining bath tubs.
The human ear and brain perform this function well by making an amplitude-
frequency analysis of the total audible spectrum. Tools for well-fl ow analysis have
to perform a similar function in order to earn their keep. Only by this kind of fre-
quency analysis can the hiss of gas be distinguished from the gurgle of liquids.

Tools Available

Many service companies offer noise logging service, under a number of trade names
such as:

• Sonan Log

• Audio Log

• Borehole Audio Tracer Survey (BATS)

• Noise Log

• Borehole Sound Survey

In general, measurements are made at preselected stations in the well. At each
station, the amplitude of the noise in a number of frequency bands is determined and
plotted on the log. Subsequently, these individual station readings may be joined
together by straight lines to give the appearance of a continuous log. Figure 9.1
illustrates such a log. The total noise amplitude is generated by fl ow from formation
A via the casing/formation annulus to formation C. Note the increase in noise at
restriction B.