Study On Quasi-distributed Optical Fiber Bragg Grating Sensing System For Temperature Monitoring

With the increasing exploration depth of oil well, the down-hole temperature and pressure increase accordingly. Due to many drawbacks of traditional electronic sensors at high temperature, such as large drift, short service life, limited to a single point measurement and so on, they easily lead to dangerous accidents and resource waste in the process of oil exploration. Therefore, traditional electronic sensors can not easily be used to establish a real-time dynamic system for long-lifetime, high accuracy monitoring of temperature and pressure in the down-hole harsh environments. To meet the demand of this need, the theory and analysis of Fiber Bragg Grating (FBG) are presented in this dissertation. By using the FBG sensors' multiplexing technique, a quasi-distributed optical fiber temperature monitoring system has been designed.The FBG sensing units are packaged within quartz capillary, three metal coating solutions are applied to the package of the whole system. Experimental results illustrate that the design is qualified for extremely harsh environment application.A demodulation mechanism has been developed by programming. Channel crosstalk problem was solved by sub-peak rate cutting, which result in the correct recognition of different channels. Additionally, multiple digital filtering and non-linear fitting are effective solutions to improve accuracy of measurement.The results of calibration experiments show that the relationship between the Bragg wavelength and temperature is in good cubic polynomial fit, and the correlated coefficient is over 0.9999; the average temperature sensitivity is 12.16pm/℃; the measurement accuracy is less than 0.5% with a full scale of 250℃; the maximum of relative-resolution is 0.06%F.S.; the repeatability is less than 0.13%F.S.; and the long-term stability is less than 0.1%F.S. at 250℃. These merits demonstrate that the quasi-distributed optical fiber temperature monitoring system can be used in the down-hole harsh environments for real-time monitoring of temperature. With further improvement, the system will play an important role in oil exploration and oil gas production, which will provide huge economic benefits in the future.