Deepwater production is challenged by well underperformance issues that are hard to diagnose early on and expensive to deal with later. Problems are amplified by reliance on a few complex wells with sophisticated sand-control media. New downhole data are required for better understanding and prevention of production impairment. We introduce real-time completion monitoring (RTCM), a new nonintrusive surveillance method that uses acoustic signals sent via the fluid column to identify permeability impairment in sand-screened completions. The signals are carried by tube waves that move borehole fluid back and forth radially across the completion layers. Such tube waves are capable of instant testing of the presence or absence of fluid communication across the completion and are sensitive to changes occurring in sand screens, gravel sand, perforations, and possibly in the reservoir. The part of the completion that has different impairment from its neighbors will carry tube waves with modified signatures (velocity, attenuation) and will produce a reflection from the boundary where impairment changes. We conduct a laboratory experiment with a model of a completed horizontal borehole and focus on effects of sand-screen permeability on transmitted and reflected acoustic signatures. These new findings form the basis of an RTCM method that can be thought of as “miniaturized” 4D seismic and as a “permanent log” in an individual wellbore. We present experiments with a fiber-optic acoustic system that suggest a nonintrusive way to install downhole sensors on the pipe in realistic completions and thus implement real-time surveillance with RTCM.