Acoustic Surveillance of Production Impairment With Real-Time Completion Monitoring
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 112301, "Acoustic Surveillance of Production Impairment With Real-Time Completion Monitoring," by Andrey Bakulin, SPE, Shell; Alexander Sidorov and Boris Kashtan, St. Petersburg State University; and Mikko Jaaskelainen, SPE, Shell, prepared for the 2008 SPE International Symposium and Exhibition on Formation Damage Control, Lafayette, Louisiana, 13-15 February. The paper has not been peer reviewed.
Deepwater production is challenged by well-underperformance problems that are difficult to diagnose early and expensive to deal with later. Diagnosis is more difficult because of the reliance on few complex wells with sophisticated sand-control media. A nonintrusive surveillance method was developed to identify impairment in sand-screen completions by use of acoustic signals sent through the fluid column. The method relies on permanent acoustic sensors performing acoustic soundings at the start of production and then repeating these measurements during the life of the well.

Introduction
Completion design is a large portion of the overall well cost, and much effort goes into designing completions correctly. During the production phase, little information is available to detect problems, optimize inflow, or prevent expensive workovers. Incomplete gravel packing, development of "hot spots" in screens, destabilization of the annular pack, fines migration, sand-screen plugging, near-wellbore damage, crossflow, differential depletion, compartmentalization, and compaction are extremely difficult to decipher with only a few permanent pressure and temperature gauges. Many problems can be identified by production logging, but it is costly and does not happen in real time.
The authors investigated underperforming wells in the Gulf of Mexico. Well-performance problems included large-scale reservoir issues, such as compartmentalization, as well as changes in local well skin with time that comprised completion, perforation, and near-well-bore effects. To distinguish between different scenarios of underperformance, more downhole data at various scales are needed that can characterize various components of the production system. The aim of this study was to develop a new method, real-time completion monitoring (RTCM), that can characterize permeability impairment of the sand screen, gravel, perforations, and the immediate near-wellbore space.

Principles
A tube, or Stoneley, wave is a fundamental axisymmetric mode that represents a piston-like motion of the fluid column restricted by the borehole wall. When tube waves encounter a permeable region, their signatures change because the radial motion of the fluid is no longer fully restricted by the borehole wall and part of the fluid can move into and out of the formation. This fluid communication at low frequencies implies that tube-wave velocity decreases and attenuation increases with increasing fluid mobility (permeability/viscosity). Boundaries between formations with different permeabilities also cause reflected tube waves.