Hydrostatic Pressure Detection Technology Based On The Brillouin Frequency Shift Distibuted Along Optical Fibers

The sensors which based on the Brillouin frequency shift (BFS) distributedalong the optical fibers have advantages of explosion-proof and weatherability, aswell as distributed sensing along dozens of miles. And they are comparativelysuitable for downhole applications. This technology has been widely employed inhealth mornitoring for many engineering infrastructures and architectures bypossessing the ability of sensing temperature and strain. However, there are fewreports on hydrostatic pressure measuring by this technology, and also it has notbeen introduced to pressure monitoring in downhole. Therefore, this work studieshydrostatic pressure detection technology based on the BFS distributed along theoptical fibers with the background of mornitoring an important geologic paremeter,i.e. pressure, for oil well.After studing the basic principle and method of measuring the BFS distributedalong the optical fibers, this paper sets up an experimental arrangement of Brillouinoptical time domain analysis (BOTDA) based on stimulate Brillouin scattering(SBS). The brillouin spectrums stimulated by different pulse-width are evaluated bygoodness of Lorentz fitting, and the full width at half maximum (FWHM) arecompared with a commercially available Brillouin optical time domain reflectory(BOTDR). And thus it is validated that this experimental arrangement has highprecision for BFS measurement. This experimental arrangement is the mainworkbench for the following work.Based on the BOTDA technology, this study carries out a trial on the responseof the BFS to pressure along two different bare fibers. The experimental resultshows that the BFS has a linear relation with the applied pressure, and thepropotional coefficient is-0.742MHz/MPa for the standard (G652) single modefibers (SMF). What’s more, this result has a good agreement with the theoreticalanalysis according to bulk silica glass. This experiment and analysis offers afoundation for pressure sensing based on the distributed FBS along optical fibers.To compare with the exist relation between the BFS and strain while the fiberssuffers no radial stress, this research designs a mechanical experiment to decouplethe axial strain and radial strain due to pressure along the fiber. This experiment gains the mathematical expression model of the relation between the BFS and bothaxial and radial strains. And the result shows that the BFS in fiber can be caused byboth strains, and has linear relations with the both strains. Yet, the propotionalcoefficients are different; the axial strain coefficient is0.053MHZ/μ, whereas theradial strain coefficient is0.029MHz/μ. This conclusion extends the relationbetween the BFS and strain, and can be a theretical basis for enhancing pressuresensitivity of the BFS by different coatings.Based on the relation model between the BFS and both axial and radial strains,it have been studied with theoretical and experimental anylysis that the influences ofthe fiber coatings on both the pressure sensitivity and temperature sensitivity of BFSwithin typical double coated fibers. The analysis shows that both the sensitivitiescan be enhanced by polymer coatings. The temperature sensitivity of the BFS growswith the Young’s modulus, thickness and thermal expansion coefficient of the outercoating and has neglegt relation with Poisson’s ratio of the coating. For pressuresensitivity, it decreases with the increasement of Youn’s modulus and Poisson’s ratioof the outer coating, and it can be enhanced by the increasement of the coatingthickness. This analysis can be a reference for designing senors with differenttemperature/pressure sensitivities by utilizing different fiber coatings.Although the BFS is sensitivitive to pressure, it is also sensitivitive totemperature. That is to say, the BFS has cross sensitivity to pressure and temperature.And it is hard to keep the temperature constant at field. Accordingly, this workdesigns a dual-path sensor based on the BFS distributed along optical fibers bymaking use of different coated fibers with different pressure/temperaturesensitivities. Thus it resolves the issue of cross-sensitivity between pressure andtemperature and can measure pressure and temperature simultaneously. Moreover, itevaluats the design quality of the sensor by error analysis, and gains the designprinciple to reduce error. For the dual-path sensor designed in this paper, itsprecision can reach theoretically up to0.256MPa and0.284°C, respectively. Thusthe precision of the pressure can be higher than1%of the pressure in downhole(more then30MPa), and it can be further enhanced by designing the configuration ofcoatings.Finally, this work has studied some practicalized researches for pressuredetection in downhole. It not only designs a quasi-distributed fiber Brillouin sensor for downhole pressure sensing, but also studies the small-size design for sensingheads since the space is nallow in downhole. One hand, it studies the effect of thefiber winding on the intensity of the Brillouin spectrum. To avoiding abviousweakening, the winding diameter should be larger than30mm for commensingle-mode fiber. On the other hand, it develops a technique for high spatialresolution locally at the sensing sections by dislocating the BFS. And the techniquecan enhance the frequency accuracy when the measurand (temperature or strain) issmall and the length is also smaller than the tradional spatial resolutions. It can thusreduce the spatial resolution from1m to30cm or less.