Degradation Analysis of Single-Mode and Multimode Fibers in a Full-Scale Wellbore and Its Impact on DAS and DTS Measurements

Distributed fiber optic sensors (DFOSs) are viewed as a promising technology for efficient, cost-effective, and nonintrusive real-time monitoring for a variety of applications. The chemical passivity, nonmagnetic interference behavior, and ability to acquire continuous measurements at high spatial resolution have made DFOS very attractive for in situ measurement of temperature, strain, vibration, and so on. The accuracy of the measured parameter by the DFOS is highly dependent on the strength and quality of the optical signal returning from the point of interest. The signal degradation in DFOS results from optical losses due to attenuation, absorption, scattering, bending, dispersion, and coupling. This study analyzes the degradation observed in both the single-mode and multimode (MM) fibers installed in a 1574-m deep wellbore for acquiring distributed acoustic sensor (DAS) and distributed temperature sensor (DTS) data, respectively. The degradation was unexpected as it was observed within three years of DFOS installation in an experimental, nonproducing well that contains noncorrosive fluids and operates in a low-temperature ( $< 54.4 ^{\circ }\text{C}$ ) and low-pressure (< 3500 psi or 24 131 kPa) environment. A root cause analysis was performed to investigate the cause of this degradation using time-lapse optical loss measurements acquired at different wavelengths using an Optical time domain reflectometer (OTDR). Degradation of the multimode fiber was also examined by the time-lapse analysis of the Raman backscatter that is used for DTS measurement. Based on the investigation, microbending was found to be the main cause of the observed degradation. Its impact on the quality of the DAS and DTS measurements was also analyzed both qualitatively and quantitively. Recommendations for minimizing degradation are presented.