The Spectral Noise Logging SNL HD 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.
When a fluid is moving through a reservoir or through a leak in well completion, it produces an acoustic noise. The 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 the fluid moves.
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. SNL HD data processing includes fast Fourier transformation that provides the required noise frequency spectrum.
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.
The example below shows how SNL HD 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 HD.
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.
In this particular case the SNL HD data show a good match with Indigo PL Suite profile and complement it with behind-casing flow analysis. In many other practical cases the SNL HD 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.
The next example shows how SNL HD identifies active injection streaks behind casing and tubing.
The Indigo PL Suite spinner data show three injection zones. The two lower injection zones have a clear correlation with the noise log including the mid-range SNL HD components produced by reservoir flow.
The upper injection streak is right across the tubing shoe and does not have a mid-range SNL HD 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 HD 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.
This is a typical case of injection fluid loss. It explains a poorer-than-expected response of nearby producers to the injection of allegedly satisfactory fluid volumes.