Study On Key Technologies Of Fiber EFPI/FBG Sensing System For Oil Well Logging

Oil as a strategic non-renewable resource is particularly important in the world. Low reservoir recovery is the core problem that oil industry faced. This is largely due to the limited availability of information concerning reservoirs and well operation. Real-time, on-line measurement and monitoring of some key physical parameters as well as their temporal and spatial variations in reservoirs and wells is therefore vitally important. However, a series of extremely harsh environment, such as high temperature, high pressure, strong chemical corrosion and strong electromagnetic interference puts forward higher requirements for the sensor used downhole. In addition, the deep well exploration signal transmission and the ability to real-time measurement of oil well are also very important. In this case, the traditional electronics sensors are not able to fully meet the needs of the oil well logging. The optical fiber sensor based on wavelength demodulation such as fiber extrinsic Fabry-Perot interferometric (EFPI) sensor and fiber Bragg grating (FBG), with merits of wavelength coded, immunity to electromagnetic interference, low drift and high precision, has been attracting more and more attention.Optical fiber sensor system consists of three parts: sensor probe, signal transmission fiber and wavelength interrogator. In this dissertation, according to high temperature and high pressure well logging application, detailed, systematic and intensively study of some key technologies of fiber EFPI/FBG sensor system are presented. The main research works are outlined as followings:Theoretical and Experimental Study of Erbium-doped fiber source is carried out. Based on theoretical model of erbium-doped fiber amplification system, the physical process of amplification in erbium-doped fiber is numerically simulated. And experimental studies on Erbium-doped Superfluorescent Fiber Source (ESFS) and tunable Erbium-doped Fiber Ring Laser (EDFRL) are implemented. In the dissertation, a spectrum flat C+L-band Superfluorescent fiber source with 68 nm (from 1535 to 1603 nm) bandwidth and 9.2 mW output power is obtained. A standard C-band EDF based novel tunable EDFRL structure is realized, whose operating wavelength can be continuously tuned over 145 nm for the first time. The output power of the EDFRL is 4 mW, the 3 dB linewidth is 0.03 nm, and the optical signal to noise ratio is better than 50 dB. An optical fiber sensor interrogator based on wavelength-swept EDFRL (WS-EDFRL) is built up. Firstly, the dynamic characteristics of WS-EDFRL is studied, and then a fiber sensor interrogator based on the WS-EDFRL is successfully constructed. In the system, an etalon with high thermal stability has been used as wavelength reference for calibrating the whole spectrum of fiber laser. A nonlinear fitting algorithm between the applied scan voltage and the transmission wavelength of FFP-TF is adopted for real-time wavelength calibration to eliminate the FFP-TF's shortcomings such as non-linearity, hysteresis and poor repeatability. A HCN gas absorption cell is used as wavelength references to check the wavelength accuracy and resolution. Experimental results show that the accuracy and resolution of the interrogator is±2 pm and 0.9 pm respectively. In addition, the fiber EFPI/FBG multiplexed sensor is demodulated by this system, fiber EFPI gap length resolution is 0.025 nm, corresponding to 0.32 kPa. FBG wavelength resolution is 0.63 pm, corresponding to 0.065℃. The output power of WS-EDFRL used in this interrogator is in milliwatt order of magnitude, so it shows an excellent ability for long distance sensing applications. It also has advantages for precisely demodulating fiber EFPI sensors and the wavelength multiplexing of FBG sensors.For the application of long term high temperature and high pressure oil well logging, several key technologies on the fiber temperature/pressure sensing system have been investigated. By optimizing the structure of fiber EFPI/FBG multiplexed pressure and temperature sensor the cross-sensitivity of the pressure/temperature sensor is eliminated, and the maximum pressure deviation of less than±0.025% is achieved in pressure gauge range of 0～72 MPa and in temperature variation range between 25℃and 300℃. The resolution of the fiber EFPI pressure sensor is 1.1 kPa in pressure gauge range of 0～102 MPa. The sensor system has been successfully applied to downhole pressure measurement in Liaohe oil field and is going to be installed in a high-temperature offshore oil well for permanent temperature and pressure monitoring.