Existing techniques for detecting produced sand are primarily limited to surface detection by acoustic sensors. While sufficiently accurate and timely to warn of a developing problem downhole, these methods are unable to determine the depth from which the sand originates, an essential factor for planning remedial treatment.
Running a point measurement acoustic tool on a wireline, well tractor, or coiled tubing (CT) for horizontal boreholes, can identify sand ingress if the probable areas of interest are located and investigated at different production rates and choke settings. Sand ingress will only be triggered above a particular drawdown pressure or flow rate for each producing zone. Acoustic tool surveys are costly and time-consuming, especially if high deviation requires the use of a well tractor or CT. DFO is a more robust, less costly sand ingress detection method.
The ability of a DFO method to accurately detect sand producing zones has been successfully demonstrated by the use of a distributed acoustic sensing (DAS) equivalent measurement on a permanent fiber installation (Mullens et al. 2010). In that case, even though the fiber in the permanent installation was specified for distributed temperature sensing (DTS) rather than DAS, and the control line carrying the fiber-optic cable was partially sheltered from the direct flow, the acoustic energy signal at sand entry intervals was clearly evident. The onset of sand production was found to be controllable by adjusting the surface choke and, thus, the drawdown at the sand face.
This technique can be enhanced by placing the 15-mm carbon composite rod in the producing bore, instead of putting the fiber-optic cables in a control line that is clipped inside the shroud and surrounded by the gravel pack. The carbon rod carries four of the single-mode fibers appropriate for DAS and two multi-mode fibers for DTS.
Placed in the central producing bore, the rod will be struck by sand slug particles as they travel into the bore at right angles to the centerline of the well. The energy level of these collisions will be greater than those occurring downstream as the sand particles travel up the wellbore parallel to the rod. Those later collisions are similar to the particles “scraping” along the rod rather than striking it.
Sand entry will be observed as zones of greater acoustic energy on the DAS signal-to-noise plot. In addition, when processed into ⅓-octave frequency bands, the sand particle impacts will be clearly distinguished from fluid-related flow. The energy distribution and frequency spectrum may vary according to particle size and slug velocity.
To detect the location of the sand and better understand the point at which problematic sand production begins, it is important to monitor the pressure fluctuations closely during production. Various slugs are easily identified with the DAS system. In addition, the pressure fluctuations, in combination with the slug detection, can be used to identify the location of the slug, where it originated, and its velocity. These slugs can be followed all the way to surface, as shown in Fig. 2.
To confirm that the right sand-bearing slugs have been identified, it is important to measure the sand content on the slug’s arrival at the surface and, therefore, have an unequivocal link back to the zone of origin. As a result, the acoustic visualization will be clear and verified, and the failure area of any sand screen completion will be readily identified. The DAS survey will also provide real-time feedback to determine the surface choke setting required to manage the problem.