Research On Pressure And Temperature Sensing Technology In Oil Well Based On Side-hole Fiber

Pressure and temperature are important environmental parameters for downhole oil and gas.Long-term and effective acquisition of real-time downhole pressure and temperature information is helpful to rationally arrange production,improve production and efficiency,and ensure production safety.It’s difficult for traditional electromagnetic sensors to withstand downhole high temperature,high pressure,strong electromagnetic interference and other harsh environment,and problems like inaccurate measurement and sensor failure may occur in long-term monitoring.As a non-electrical sensor,optical fiber sensor meets the requirements of downhole monitoring for its advantages including the advantages of small size,high temperature resistance,easy networking,good reliability,long service life,strong electromagnetic interference resistance.However,due to the narrow installation space of downhole sensors,it is impossible to solve the problems of low pressure sensitivity and pressure-temperature cross-sensitivity of ordinary optical fiber sensors by means of mechanical structure sensitization.The unique birefringence characteristics of side-hole fiber are an effective way to solve this problem.However,the application of side-hole fiber to downhole pressure and temperature monitoring still needs to further improve the sensitivity,and it cannot be effectively measured under low pressure.In view of the above problems,in this paper,a π phase-shift grating is written on the side-hole fiber as a downhole pressure and temperature measurement sensor,and the theoretical analysis is carried out and the structure size is optimized to improve the sensitivity,and then the demodulation system of the side-hole fiber grating is designed.The sensor is tested in the experiment to verify the feasibility of the sensor for simultaneous measurement of downhole pressure and temperature.The main research contents of this paper are as follows:(1)Theoretical research is carried out on the sensing principle of pressure and temperature simultaneously measured by a single point of side hole π phase shift fiber grating.Based on the birefringence principle,the birefringence characteristics of edgehole fiber are analyzed.Combined with the coupling mode theory and the basic principle of phase-shifted grating,the variation of birefringence and reflection spectrum of the side-hole π phase-shifted fiber grating under uniform compression is theoretically deduced.Secondly,the influence of temperature on birefringence and reflection spectrum is analyzed.Finally,the mathematical relation between the wavelength of two polarization reflection peaks and pressure and temperature is obtained.(2)According to the results of theoretical analysis,simulation and optimization of side hole optical fiber are carried out.In order to improve the pressure sensitivity,the size and position of the side hole were optimized.Considering the consistency of sensor fabrication and the fracture strength of the side hole fiber,the optimized results were modified.Finally,the structure of the side hole fiber suitable for underground use was obtained: the hole center distance was 27 μm,and the hole radius was 21.5 μm.Then the pressure and temperature of the optimized edge-hole fiber are simulated to verify the validity of the theoretical analysis.The simulation of transmission mode is also carried out to obtain the transmission characteristics of side-hole fiber.(3)Write π phase shift grating on edge hole fiber and build polarization analysis system for demodulation.A π phase shift grating was written by uv laser on side hole fiber by blocking partial phase mask method.Firstly,analyze and select the appropriate grating method,and then explore the appropriate writing process,including hydrogen loading,welding,writing direction influence,writing adjustment,annealing after writing,etc.The demodulation of side-hole π phase-shifted fiber gratings requires a suitable polarization system to separate the two polarizations.Firstly,the polarization analysis and demodulation system is built based on narrow-bandwidth laser,polarizer and spectrometer.The advantages of this system,which can be used for calibration experiments,are high signal acquisition accuracy and good signal synthesis.However,this system is not suitable for downhole monitoring.Therefore,according to the principle of this polarization demodulation system,the existing FBG demodulation system is modified to develop a demodulation system suitable for downhole monitoring.(4)Carry out pressure and temperature calibration experiment and pressure and temperature crosstalk experiment on the sensor,and then design the sensor downhole packaging protection structure.The pressure test results show that the pressure sensitivity of the sensor is 11.22 pm/MPa,the resolution is 8.9 k Pa,and the linear correlation is 0.99911 in the range of 0-41 MPa.The test results show that the sensor temperature sensitivity is 11.23 pm/ ℃,the resolution is 0.0089℃,and the linear correlation is 0.9996 in the range of 0-110℃.Then five groups of pressure tests at different temperatures were carried out,and the comparison between the measured values and the actual values proved that the sensor can simultaneously measure the double parameters of pressure and temperature at a single point.Finally,according to the downhole environment and sensing requirements,design a reasonable sensor housing and packaging process.