Achieving Dry-Tree DAS Bandwidth and SNR Performance on Subsea Wells

Abstract

Summary The pulse repetition rate, and thus sampling frequency, for pulsed distributed acoustic sensing (DAS) systems is inherently limited by the total length of the sensing fiber. For single-fiber, topside interrogation of subsea wells, this is particularly critical as the subsea infrastructure can significantly increase the total sensing fiber length, adversely affecting both sampling frequency and signal-to-noise. We describe how the deployment of a remote circulator in the optical flying lead to the subsea well effectively constrains the sensing fiber to the fiber portion located below the remote circulator. Thus, pulse repetition rates can be increased to match the ones typically achieved for dry-tree wells. Moreover, the fiber topology is immune to back-reflections from infrastructure above the remote circulator, and enables selective amplification of the backscattered light originating from the sensing fiber portion below the remote circulator. The result is a subsea DAS solution that achieves acoustic bandwidth and signal-to-noise performance comparable to dry-tree wells and enables spectral-based DAS processing algorithms developed for sand control, leak detection, inflow profiling, well integrity, and VSP to be straightforwardly applicable to DAS data acquired from subsea wells.