Research On The IOFDR-based Fiber-optic Distributed Temperature Sensing And The Hybrid Technique For Multiple Sensors

Down-hole conditions, such as distributed temperature and pressure, are important for oil exploration and reservoir management. Attributed to the advantages of small size, resistance to harsh environment, high sensitivity and immune to electromagnetic interference, fiber-optic sensor is becoming the core of the intelligent oil well. However, there exist several problems in current down-hole measurement systems with fiber-optic sensors, such as the variety of the monitoring parameters, the long-term stability and the high cost. In response to these problems, a hybrid fiber-optic sensing technique, combining the incoherent optical frequency domain reflectometry (IOFDR) based distributed temperature sensor (DTS) with fiber Bragg grating (FBG) and extrinsic Fabry-Perot interferometer (EFPI) based point sensors, has been studied. The major research works of this paper are outlined as followings:Theoretical analysis is carried out on the principles of the temperature measurement and spatial orientation. Furthermore, based on the study of the demodulation methods for FBG-based sensor and EFPI-based sensor as well as the radiation characteristics of the laser diode, the hybrid Raman/FBG, Raman/EFPI and Raman/EFPI/FBG sensing schemes with a single light source are proposed respectively. These schemes supply new approaches for multi-parameter measurement with optical fiber.A single-mode fiber-optic IOFDR-DTS system is designed. Meanwhile, the key technologies in the system are deeply studied. In order to improve the signal-to-noise ratio of the weak spontaneous Raman backscattered light in the single-mode fiber, a three-channel simultaneous radio-frequency (RF) lock-in amplifier is designed with a frequency range from 1 kHz to 100 MHz. Therefore, the phase reference light signal, the Stokes backscattered light signal and the anti-Stokes backscattered light signal can be measured simultaneously. To further improve the temperature resolution, an adaptive dynamic threshold method with wavelet denoising technique is proposed. Subsequently, the polarization effect in the single-mode fiber-optic DTS is studied theoretically and experimentally. The relationship between the polarization induced temperature error and the polarization dependent loss (PDL) as well as polarization dependent response (PDR) is derived by an error analysis. Experiment result shows that the polarization induced error is about±3 ?, which is basically in agreement with the error analytical value of ±4.1?. In order to reduce the polarization induced impact on the IOFDR based single-mode fiber-optic DTS, the synchronous polarization scrambling technique with a low-speed electrically driven polarization controller (EPC) is presented. Furthermore, the main parameters of the IOFDR-DTS system are measured. Specifically, the spatial resolution can be reached to be 0.93 m, the temperature resolution can be reached to be 0.2 ?, and the maximum measured distance is 5 km.Finally, a hybrid Raman/FBG sensing system, combining the IOFDR based Raman DTS with high-reflective FBG sensors, is presented for simultaneous distributed and quasi-distributed measurements. Experimental results show that, both the Raman based distributed temperature measurement and the FBG based quasi-distributed temperature or strain measurement can be achieved simultaneously. A temperature resolution of 0.4 ? for distributed temperature measurement, and a temperature resolution of 0.08 ? as well as a dynamic strain resolution of 64 n?/(?) for quasi-distributed measurements have been achieved with the hybrid Raman/FBG sensing system. Meanwhile, this system shows a wavelength range of more than 50 nm for FBG sensor multiplexing. Moreover, the hybrid Raman/EFPI and Raman/EFPI/FBG sensing systems are presented respectively for down-hole simultaneous measurements of distributed temperature and pressure. The pressure resolutions of 5.3kPa and 2.1 kPa are achieved with the DTS-based and FBG-based temperature compensations respectively. The structures of the hybrid sensing systems are simplified, and the systems are cost-effective. Due to these advantages, this paper can lay the theoretical and experimental basis for the wide application in the oil exploitation by using well logging technique with fiber-optic sensors.