High Accuracy Optical Fiber Sensing System And Its Application In Oil Well Logging

Well logging is one of the most important technologies for petroleum industry. Oil down-hole conditions, such as pressure and temperature, are of great values for oil exploration and reservoir management. Optical fiber sensors, with their unique advantages such as small size, high sensibility, immune to electromagnetic interference (EMI) and resistant to harsh environment, have attracted more and more attentions in the field of down-hole monitoring. Accuracy, reliability and high cost are three most important things for optical fiber sensors’ applications in down-hole monitoring. In this dissertation, we present detailed, systematic and intensively studies on how to increase the accuracy and reliability of the optical fiber sensor and to cut down the logging cost. The major research works are outlined as followings:To increase measurement accuracy of the extrinsic Fabry-Perot interferometric (EFPI) based pressure sensor, we developed a novel demodulation method which is called as cursor-liked demodulating algorithm. This cursor-liked demodulating algorithm combines both advantages of the Fourier transform method and the minimum mean square error estimation based signal processing method, which can generate high resolution and large dynamic range measurment simultaneously. To verify its performance, an EFPI-based displacement sensor is constructed. Experimental results show that this cursor-liked demodulating algorithm can make absolute measurement of the cavity length and provide a resolution as0.84nm and a dynamic range as3mm.To improve the reliability of the EFPI-based pressure sensor, a laser-induced chemical vapor deposition device is set up which can make a type of carbon-coated EFPI (C2EFPI) sensor. The scanning electron microscopy and Raman spectroscopy are used to analyze the carbon coating for their thickness, surface roughness and microstructure. Through optimization of the carbon depositing parameters, hermetic carbon coatings have been grown on the EFPI sensor with an R value of1.03. Then, the performance of the C2EFPI sensor is verified experimentally, and realize an accuracy of0.05%F.S.(full scale) and a hysteresis of0.01%F.S., also the fluctuation of the cavity length is only1nm during nine months with in a harsh environment of72MPa and300℃. Vibration test results demonstrate that the C2EFPI sensor also has a good adaptability to vibration environment and can meet the requirements of high repeatability, high accuracy, long-term reliable measurement in well logging, which is highly valuable for industrial harsh environment applications.To reduce the cost of the optical fiber well logging system, we design and develop a quasi-distributed temperature sensor system for down-hole monitoring based on the fiber Bragg grating (FBG). Comparing with other peak-detected algorithms, the direct Gaussian fitting algorithm can achieve a higher precision and more stable demodulation result for the reflection spectrum of the FBG. A triple layer metal tubing protection scheme is proposed to meet the critical measurement requirement of the oil well. Experimental results show that the measurement accuracy is less than0.5%with in a full scale of250℃; the maximum of absolute-resolution is0.15℃; the repeatability is less than0.13%F.S.(250℃), and the long-term stability at250℃is less than0.2%F.S.We also present a cost-effective sensing system combining the EFPI-based pressure sensor with the distributed temperature sensor (DTS) to further cut down the cost of optical fiber well logging system. This sensing system can realize accurate pressure measurement in the oil reservoir and temperature profile measurement of the wellbore simultaneously. Because the sensing wave bands of the pressure and temperature sensor are different, the wavelength-division multiplexing (WDM) technology is used to couple these two sensing signals into one single fiber. Field test results show that this sensing system has a good long-term stability in high temperature oil well. The low cost of this sensing system provide a solid foundation to mass applications of fiber optic sensors in oilfields.