Topside Distributed Acoustic Sensing of Subsea Wells

Abstract

Topside interrogation for distributed acoustic sensing (DAS) of subsea wells for vertical seismic profiling (VSP), well integrity, and sand control requires optical engineering solutions to compensate for the insertion losses and back reflections accumulated through umbilicals, multiple wet- and dry-mate optical connectors, splices, optical feedthrough systems, and downhole sensing fiber and wet mates. Moreover, the acoustic bandwidth is inherently limited by the total length of the sensing fiber. To obviate these problems, we describe a subsea fiber topology with two ultralow loss transmission fibers from the topside to a remote optical circulator deployed in the optical flying lead at the subsea tree. This limits the sensing fiber portion of the total fiber length to the fiber located below the remote circulator, and eliminates all back reflections from the multiple subsea connectors above the remote circulator. The receiver arm in the DAS interrogator only senses pure backscatter light from below the circulator. The pulse repetition rate is only constrained by the fiber length below the remote circulator, thus enabling dry-tree equivalent acoustic bandwidths of 10+ kHz regardless of the tie-back distance. This yields significant signal-to-noise ratio (SNR) improvement via stacking and selective amplification, while further enabling sensing of high-frequency acoustic events occurring inside or in the vicinity of the wellbore. A fiber with enhanced back-reflectance is deployed in both the upper and lower completions inside a triple-tube cable design that provides a hydrogen-permeation barrier to delay onset of hydrogen-induced attenuation. Results from various laboratory and field trials are shown to validate system performance.