Study Reviews Two Decades of Surveillance Using Distributed Acoustic Sensing
Authors: Carpenter Chris 
Published: September 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 200088, “Downhole Monitoring Using Distributed Acoustic Sensing: Fundamentals and Two Decades of Deployment in Oil and Gas Industries,” by Mohammad Soroush and Mohammad Mohammadtabar, RGL Reservoir Management and University of Alberta, and Morteza Roostaei, RGL Reservoir Management, et al. The paper has not been peer reviewed.

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Distributed acoustic sensing (DAS) through fiber optics has been deployed in downhole monitoring for over 2 decades. The complete paper reviews the basics of DAS, fiber types, installation methods, types of recorded data, data processing, historical development, current applications, and limitations of the technology, providing a concise review using several field cases from more than 200 published SPE papers and journal databases. Because this synopsis cannot retain these many paper references or their overview, readers are encouraged to access the complete paper on OnePetro.

DAS Fundamentals
DAS Units. DAS systems consist of an interrogator that includes a laser transmitter and detector, a processing unit, and a distributed sensing fiber. Laser pulses are sent periodically into the fiber that is installed in the medium. The detector records backscattered response vs. time along the fiber.
Because an acoustic field exerts pressure on the fiber it surrounds, some strain is induced on the fiber. An interrogator is sensitive to this strain between two points of fiber separated by gauge length.
Multi- and Single-Mode Fibers. Generally, multimode fibers are used for distributed temperature sensing (DTS) and single-mode fibers are used for DAS. Multimode fibers have a larger-diameter core through which multiple modes of light can pass, while, in single mode, only one mode of light passes through the core. Attenuation in single-mode fiber is lower; therefore, single mode is suitable for long distances. On the other hand, multimode fiber can pass more data through the fiber and has higher attenuation.
Data Type and Processing. Previous authors described how to reduce, convert, and transmit DAS recordings as data acquisition accumulates over time at the site without losing significant information. They also established three methods of data reviewing: local, remote display (from a remote server), and post-job integration (displaying all available data).
A work flow of main steps for using DAS and DTS includes data gathering, auxiliary data, quality control and assurance, data management, and determination of interpretation options. Live data can be made available in permanent installations, while analytics libraries can be integrated into cloud-based DAS system. This will result in (nearly) real-time decision-making and easy-to-use forms of data.
Multicomponent DAS. Because DAS systems are more sensitive in an axial direction, to estimate strain tensor, a multicomponent DAS was proposed with two approaches: multiple parallel or helix optical fiber. Another configuration was proposed with five helical fibers with equal space and one straight fiber for shorter wavelengths, addressing a limitation of previous methods.