Study on the Optical Fiber EFPI Sensor System and Its Applications in Gas and Oil Well
|School||Dalian University of Technology|
|Keywords||optical fiber sensor Extrinsic Fabry-Perot interferometric based optical fiber sensor demodulation system Fiber Fabry-Perot tunable filter tunable fiber laser pressure measurement in oil well|
The optical fiber sensor based on wavelength demodulation such as extrinsic Fabry-Perot interferometric （EFPI） optical fiber sensor and fiber Bragg grating（FBG）, with merits of wavelength coded, immunity to electromagnetic interference, low drift and high precision, has been widely used in many areas, such as oil and gas industry, structural health monitoring and smart materials. Optical fiber sensor system is composed of fiber sensor and the demodulation system. The EFPI sensor fabricated with laser thermal-fused technology with advantages of small size, compact configuration, good reliability and stability can be used in harsh environments with high temperature and high pressure. In this dissertation, research work has been done in optical fiber sensor demodulation system: one system based on broadband SLED source has compact configuration and low cost, the other one based on tunable fiber laser has relatively high precision and resolution. Solutions for some key problems in the downhole pressure measurement have been proposed, and the EFPI sensor has been successfully used to monitor the pressure in the Liaohe oil field. The main research works are summarized as followings:1. Optical fiber demodulation system based on broadband optical source is studied. Fiber Fabry-Perot tunable filter （FFP-TF） is used as wavelength selection device in the system, and a cubic polynomial fitting is adopted as the best fitting algorithm to convert scan voltage to scan wavelength after analysis and test of the FFP-TF characters. The wavelength resolution of the demodulation system with five wavelength reference FBGs and the cubic polynomial fitting is 4.3pm, and long-term stability is 10pm. The demodulation system has practical application value with advantages of simple structure and lower cost.2. In order to demodulate the EFPI sensor and FBG with higher precision, an optical fiber sensor wavelength demodulation system based on tunable fiber laser has been built up. In the system a HCN gas absorption cell has been used as wavelength reference. The applied scan voltage to the FFP-TF and optical signal channels are sampled synchronously during the scan, and the reflected spectrum can be derived by real-time wavelength calibration, which can eliminate the FFP-TFs disadvantages such as non-linearity, hysteresis and poor repeatability. The measurement resolution is improved using high-power tunable fiber laser and optical fiber gas absorption cell. Experimental results show that the resolution of the system is 1.4pm and the long-term repeatability is 3.2pm for fiber Bragg grating measurement, and the air gap resolution is less than 0.22nm for the EFPI sensor, which means that the system can demodulate FBG sensor and EFPI sensor with high precision.3. The fabrication and packing technique for single-mode fiber EFPI sensor are studied. The EFPI sensor fabricated with CO2 laser thermal bonding technique works reliably under harsh environment such as high temperature and high pressure. By optimizing the structure of the EFPI pressure sensor the temperature-presssure cross-sensitivity is effectively reduced, and the maximum deviation of less than 1% is achieved with the temperature variation range of 16℃～300℃. In order to further improve the practical accuracy a series of calibration curve are derived by calibrating the EFPI pressure sensor at different temperature. When the EFPI sensor is applied to oil well pressure monitoring, a suitable calibration curve is selected according to experientially estimated surrounding temperature, which can effectively improve the accuracy of pressure measurement with a long term accuracy of 0.1%.4. Pressure and temperature are key monitor parameters in oil well development. For practical downhole application of the fiber optical sensor system, some key technical problems have been solved successfully. For protecting the signal transmitting fiber in downhole high temperature and pressure environment, an epoxy-free package method based on hermetical bonding of quartz ferrule and fiber is used and Polyimide coated fiber and stainless steel protecting tubing are adopted. The sensor system has been successfully applied to downhole pressure measurement in Shuguang oil production plants of Liaohe oil field.