Research On Optical Fiber Evanescent Wave Sensing Technology For Hydrogen Sulfide Gas Concentration In Oil And Gas Fields

It is susceptible to form toxic hydrogen sulfide(H2S)gas,during the exploration and development process in oil-gas fields.The acidic H2S not only threaten to personal safety of developers,but also have strong corrosive effect on exploration and developing equipments as well as gas transmission pipelines and so forth.Thus,it has a huge significance to precisely detect H2S gas concentration of work site in real time.Based on the principle of infrared spectral absorption,this paper proposed optical fiber evanescent wave absorption difference detecting H2S gas concentration,which combined the advantages of optical fiber sensing and spectral absorption.This method is expected to have a broad application prospects,which is especially suitable for gas detection of high-risk environment like oil and gas fields,mines and tunnels.Firstly,the lightwave transmission theory of isotropic step index fiber was analysed,and the producing conditions of optical fiber evanescent wave and its power density distribution characteristics were researched.Combining the beer-lambert absorption theory,theoretical model of evanescent wave absorption detecting gas concentration method was presented.Infrared spectrum absorption characteristic of H2S gas and double optical path difference detection technique research were conducted,which lay a theoretical foundation for subsequent sensing system design and concentration detection.Moreover,for the reason of the structure and size of evanescent wave sensing probes having huge impact on detecting sensitivity,the relationship between the length and remaining cladding diameter of sensing probes and sensitivity were analysed,consequently optimum size of sensing probe was picked out.On the other side,mechanism of removing the optical fiber cladding through physical grinding was studied,fibers grinding experiment as well conducted.Meanwhile mechanism of removing the optical fiber cladding through chemical corroding was also studied,the optical fiber cladding corrosion test as well conducted.Through the experiment study,variation of light power output of sensor probe with erosion time was obtained.The variation of sensing probes'evanescent-wave power ratio with remaining cladding diameter was also explored.Sensing probes with different evanescent-wave power ratio that gained by HF acid solution corrosion are 5%,10%,15%and 20%respectively.The probes were prepared for the experiments of influence of sensing probes' evanescent-wave power ratio on gas concentration detection.Finally,optical fiber evanescent-wave sensor detecting system for H2S gas was designed and gas concentration test experiments were conducted.On the basis of integral structural design for detection system,detection light,gas concentration configuration platform,sensor chamber,photoelectric detection and data processing module were designed specifically.At the same time,noise source and noise suppression methods were analysed specially.Consequently,H2S gas concentration test platform was built successfully.Through the experiment with sensing probes with different evanescent-wave power ratio,such as 5%,10%,15%and 20%.the relationship of output signal from interrogator and gas concentration was analysed and the effect of temperature on gas detection was also studied.The result shows the sensing system designed in this paper has higher sensitivity,good repeatability and shorter response time.