Paper: Noise Log Applications In West Texas

Liquid or gas flowing in a well generates a complex group of audible sound frequencies. Twophase flow, which was gas bubbling through water behind uncemented casing, produced the frequencies shown on the curve in Fig. 1. An output signal from a piezoelectric transducer placed inside the casing has an alternating frequency waveform. The waveform is a composite of all the frequencies shown on the graph. Each frequency has a millivolt amplitude, varying with time, which contributes its relative amount to the total millivolt amplitude of the waveform. Using the optimum time interval, it is usually possible to obtain a good average amplitude measurement of these changing frequencies. The area under the curve in Fig. 1 is proportional to the millivolt amplitude for all of the frequencies above 200 Hz. Extending a vertical line through 600 Hz, all of the area under the curve to the right is proportional to a milli-volt amplitude of all frequencies above 600 Hz. These same comparisons can be made for 1000 and 2000 Hz. It is readily seen that, as filters remove frequencies from the amplitude measurement, the subsequent amplitude levels will always be less. These four filtered amplitude measurements are the four points which are plotted versus depth on semi-log paper to produce a four-curve noise log. Most of the interpretation involves a relative comparison which makes it desirable to establish fluid flow and "dead well" intervals. With reference to Fig. 1, comparing the area under the curve on the low frequency end to the area for those frequencies above 600 Hz indicates two-phase flow has more energy associated with the low frequencies. The single-phase curve has more area associated with the 1000 and 2000 Hz end. The higher the differential pressure the greater the area under the high-frequency end. The conclusions from these data are that separation of the 200 from the 600 Hz curve indicates two-phase flow, and pronounced peaks on the 2000 Hz are an indication of differential pressure. The high noise level associated with wireline and tool movement requires that all data be recorded during a time when the tool is stationary. To facilitate the interpretation for a shut-in run, it is necessary to eliminate all lubricator, wellhead and wing valve leaks.