Research On Strain Transfer Characteristics Based On OFDR Testing Technology In Wellbore Model Test

In order to perfect the application of distributed optical fiber in practical engineering,this thesis studied the strain transducing characteristics based on OFDR testing technology in wellbore model test by combining numerical simulation and physical model test.Continuous strain values of fiber along the full length in whole loading process were obtained,and random cracks before borehole wall failure could be captured.Firstly,the key technology of OFDR strain transducing was studied by means of experiment and numerical simulation.The effects of different physical parameters on strain transducing of optical fibers were obtained,as well as the strain transducing characteristics of different optical fibers and laying methods.The numerical simulation results showed that the elastic modulus of materials has a great influence on the strain transducing both in the points layout model and the distributed layout model,and two low strain transducing regions are in short lengths of each end of the optical fiber.The strain transducing of distributed layout model is higher than that of points layout model,and bonding in slot is better than surface bonding.The experimental results illustrated that tests of both bare fiber and encapsulated fiber are accurately,and bare fiber is slightly better.The longer the test length of encapsulated optical fiber is,the better the strain transducing of optical fiber is.The test performance of optical fiber will be improved after repeated loading,because the residual strain becomes smaller.Strain test results are independent of four epoxy resins.Secondly,the OFDR distributed strain transducing and crack capture technology of concrete were studied through experiments.The strain transducing characteristics of concrete specimens of bending test under different laying methods and the crack capture characteristics of distributed optical fiber were obtained.The experimental results showed that the bonding process has a great influence on the test results of the surfacebonded fiber,while the embedded fiber is easier to cause concentrate stress,which makes the fiber failed.Both bonding in slot and internal embedding are feasible.When the concrete specimen cracks,testing will be inaccurate because of the sliding of optical fiber,but the location and time of cracks can be monitored,and the minimum crack interval can be identified is 2cm-3 cm.Thirdly,through physical model test and numerical simulation,the strain transducing characteristics of distributed optical fiber in the borehole wall model were studied,and the stress and deformation laws under non-uniform ground pressure were obtained.The experimental results illustrated that the distributed optical fiber can accurately measure the three-dimensional strain of deviated wellbore with certain layout method.However,relative sliding will happen where wellbore cracks,there are still some errors in the measurement results in a certain range of cracks.The stress and deformation laws of inclined wellbore in physical model test is basically the same as that of numerical simulation.In summary,the OFDR distributed optical fiber testing system based on Rayleigh scattering can obtain continuous strain measurements of the optical fiber along the full length in whole loading process because of its high spatial resolution and distributed characteristics,and it can also capture random cracks before borehole wall failure.Researches in this dissertation provide experience for the future application of distributed optical fiber in practical engineering.