Overview

The Spectral Noise Logging (SNL) tool is designed to record

sound in the frequency range of 8 Hz to 60 kHz.

The tool can be effectively used to survey production and injection wells under both shut-in and flowing conditions.

Fluid and gas flows generate noise through the vibration of various features of the geological environment and wellbore components.

Noise intensity increases with linear flow rate.

Contrastingly, the noise spectrum depends not on the flow type or velocity but on the type of

channel through which fluid/gas moves.

Analysis of noises recorded in various frequency ranges allows the location of flow-related features, such as wellbore flows, casing and tubing leaks, perforations, cement channelling and flows through the rock matrix and fractures.

Noise is recorded by a high-sensitivity hydrophone placed in an oil chamber. Then, hydrophone

data pass through a high-quality ultra-silent gain stage and are converted into a digital bitstream

for further digital processing (Fig.1).

SNL data processing includes fast Fourier transformation that provides the required noise

frequency spectrum. The SNL panel shows noise power distribution between 0.117 kHz on the

left side of the panel and 30 kHz on its right side. The red colour is for the highest-power noise;

yellow, green, blue and violet for lower-power noise; and white for noise at and below the tool

threshold.

The spectrum clearly indicates flow through the rock matrix as a noise peak at 15 kHz. Such

peaks are often associated with one or two other peaks at 3–5 kHz produced by fracture flow.

 

SNL – Spectral

Noise Logging

Fig. 1. SNL Tool schematic

 

Example 1 

The example below shows how SNL identifies active flow streaks behind the

production casing.

High-volume noise appears in red and low-volume noise in blue.

One can clearly see the borehole noise in the left side of the panel coming from below the bottom

hole to the surface.

 

There is also a vertical satellite strip linking all perforation intervals, which is behind-casing

channelling. Spinner failed to pick up the small inflow from upper perforations which is seen as

the top end of the channeling on SNL.

The reservoir flow across upper perforations produced three noise streaks correlating with

permeability streaks from open-hole data.

These streaks have two distinct components: the low-frequency one to the right of channelling

noise and the high-frequency one in the middle of the panel.

The low-frequency component was generated by fracture flow and the high-frequency one by

matrix flow.

Example 2 

The next example shows how SNL identifies active injection streaks behind casing and tubing.

The PLT spinner data show three injection zones. The two lower injection zones have a clear

correlation with the noise log including the mid-range SNL components produced by reservoir

flow.

The upper injection streak is right across the tubing shoe and does not have a mid-range SNL

component. In this case, water goes out of the tubing, partly through the perforations above the

tubing shoe, and then flows up behind the casing through cement channels. SNL clearly shows

four reservoir streaks that sourced the channelled water despite being behind two barriers

(tubing and casing) at 5 inches away from the tool. Open-hole data were not interpreted for this

zone, as often is the case, but now it is clear that there are permeable streaks there, which could

store injected water.

 

In this particular case the SNL data show a good match with PLT profile and complement it with

behind-casing flow analysis. In many other practical cases the SNL survey reveals the noise

across non-perforated reservoir flow units which channel up/down towards perforations which is

very important information for reservoir engineers and petrophysicists.