Optical Fiber Vibration Sensing Using Semi-Coherent Light
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Solution Overview
Problem
Current vibration monitoring systems for well completion equipment in hydrocarbon production are inadequate for reliably sensing vibrations, which are crucial for operational status and environmental monitoring in boreholes.
Innovation Solution
An optical fiber-based vibration sensing system using semi-coherent light and optical interrogators to measure distance changes between reflectors, allowing for precise vibration detection in boreholes, immune to interference from adjacent pairs of reflectors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional vibration sensors are used in boreholes, then vibration monitoring is provided, but reliability and accuracy are insufficient due to interference and polarization fading
Solution Approach 1:
The patent replaces conventional mechanical vibration sensors with an optical sensing system using laser light and optical fiber. The system measures vibration by detecting changes in the optical path length between reflectors using laser interferometry, eliminating mechanical components that are susceptible to interference and polarization fading. This substitution of mechanical sensing with optical sensing directly addresses the reliability and precision problems.
Solution Approach 2:
The patent employs semi-coherent laser light with specific coherence length parameters to enable vibration sensing over distance L while preventing sensing over distance D. By carefully controlling the coherence length parameter of the laser, the system achieves selective sensing that improves measurement accuracy and eliminates interference from distant reflectors, directly resolving the precision issue.
2Area of stationary object
If optical fiber with multiple reflector pairs is used, then sensing coverage is extended, but interference from adjacent reflector pairs increases
Solution Approach 1:
The patent applies local quality by making each reflector pair have a specific local characteristic - the coherence length of the laser light matches the spacing L of each reflector pair but not the spacing D to adjacent pairs. This localized coherence property ensures that each reflector pair is sensed independently without interference from adjacent pairs, allowing extended sensing coverage while eliminating interference.
Solution Approach 2:
The system performs preliminary action by pre-configuring the reflector pairs at specific distances L that match the coherence length of the laser. This pre-arranged configuration ensures that when the laser illuminates multiple reflector pairs, only the intended pair within coherence length interferes constructively, while adjacent pairs beyond the coherence length do not cause interference, thus preventing the harmful effect before it occurs.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system effectively monitors downhole vibrations, providing accurate amplitude and frequency data, reducing interference from polarization fading and enhancing sensitivity compared to conventional systems.
Implementation Method 1
a laser configured to emit semi-coherent light over a swept range of wavelengths to illuminate the optical fiber
Implementation Method 2
the semi-coherent light provides for sensing over the nominal distance L in the optical fiber between each pair of reflectors and for no sensing in the optical fiber over the nominal distance D between different pairs of reflectors
Implementation Method 3
an optical fiber disposed in the borehole and comprising a plurality of pairs of reflectors configured to reflect light between each of the pairs of reflectors in the plurality
Implementation Method 4
a photodetector configured to sense light from the optical fiber and provide a photodetector output signal indicative of an amount of sensed light
Data Source
AI summary
An apparatus for sensing vibration in a borehole penetrating the earth includes an optical fiber having a plurality of pairs of reflectors configured to reflect light between each of the pairs of reflectors in the plurality, wherein each pair of reflectors is separated by a nominal distance L and an adjacent pair of reflectors in the plurality are separated by a nominal distance D, and an optical interrogator configured to sense a distance between each pair of reflectors in the plurality over time to sense the vibration. A laser emits semi-coherent light over a swept range of wavelengths to illuminate the optical fiber, wherein the semi-coherent light provides for sensing over distance L in the optical fiber between each pair of reflectors and for no sensing in the optical fiber over distance D between different pairs of reflectors. A photodetector senses light received from the optical fiber.