F. H. K. Rambow
Shell Development Company, Houston, TX
ABSTRACT
Zonal isolation has become a major concern not only from the standpoint of improved production efficiencies but also from the standpoint of environmental impact. Recently, in their efforts to protect aquifers, the EPA (Environmental Protection Agency) has become increasingly interested in evaluating technologies with a promise of discerning zonal isolation. Beyond conventional bond assessment techniques (with their well-known limitations), only oxygen activation is currently gaining general acceptance as a method to potentially improve the detection and characterization of behind-pipe flow.
“Active listening” may provide an alternative to oxygen activation. Like oxygen activation it is quantitative. Unlike oxygen activation, it does not depend upon the presence of oxygen (water) in the flowing fluid. Furthermore, it is sensitive to a much wider range of flow velocities. With a properly designed tool, the azimuthal position and extent of a channel can be ascertained.
“Active listening” is an acoustic technique whereby the region being examined is “illuminated” by a very short-pulse acoustic beam oriented essentially perpendicular to the flow. The reflected signal is then stored. After a small increment of time, the same region is again ‘illuminated’ by an identical beam and the reflected signal is again stored. These stored signals are then compared. If there has been no motion in the material through which the two beams passed, the amplitudes and phases of the two reflected signals will be identical (to within the stability of the electronics).
Motion of material in the beam path (such as scatterers flowing in a channel) between the time of the first and second recorded signals will result in a difference between the two signals. By varying the time between the two probe signals, the velocity of the scatterers may be estimated.