Oil and Gas Distributed Acoustic Sensing (DAS)
Driven by needs for ever further, ever faster broadband transmission, optical fiber technology has improved dramatically and now approaches theoretical limits in terms of attenuation and precision manufacturing. Taking advantage of such refinements, non-transmission applications have now come to the forefront and optical fibers are increasingly deployed as sensing elements in numerous industries and applications.
In one embodiment, Distributed Acoustic Sensing (DAS) provides near real-time measurement utilizing optical fiber by taking advantage of its low attenuation and long reach in addition to its dielectric nature and immunity to radio frequency and electromagnetic interference. DAS converts the optical fiber into a lengthy sensor element (up to 50 km) and detects events with very high resolution over the entire distance.
Acoustic sensing works by coupling coherent laser energy pulses into optical fiber and analyzing naturally occurring Rayleigh backscatter. Light pulses, as they travel from the input to distal end, interact at a molecular level with the materials that comprise the core of the fiber. Such interactions cause a small amount of light to backscatter and return to the input end where they are detected and analyzed. Acoustic waves, when interacting with the materials that comprise the optical fiber, create small changes in refractive index. These changes affect the backscatter characteristics, becoming detectable events. Using time-domain techniques, event location is precisely determined, providing fully distributed sensing with resolution of 1 meter or less.
Oil and gas markets provide numerous applications where DAS can be implemented, promising lowest cost per sensor point. Examples include pipeline monitoring for distributed flow, distributed seismic detection, and leak detection.
In addition to pipeline monitoring, harsh environment optical fibers are commonly used for in-well applications to detect fracturing processes, leaks, flow and seismic activity, all in distributed fashion, allowing for thousands of discrete sensing points along a single optical fiber.
|Part Number||Fiber Type||Coating System||Interrogation Method||H2 diffusion resistance||Max. Temp.|
|BF04446||Ge-doped single-mode||PYROCOAT®||Coherent Rayleigh||300 ˚C|
|F21976||Pure-core single-mode||PYROCOAT||Better*||300 ˚C|
*In harsh environments, like those found in oil and gas applications, molecular hydrogen will diffuse from the environment, through virtually all materials, and nest in the core of the optical fiber. This build-up of hydrogen causes attenuation to increase, is highly variable, and affected by temperature, pressure, and hydrogen concentration. At OFS we continuously study methods for managing hydrogen diffusion and incorporate several techniques in the design of our optical fibers to improve performance and reliability in these harsh environments.