Distributed acoustic sensing of long subsea tie-backs

Abstract

Subsea tie-backs are a field development concept for infrastructure-led exploration and production in mature basins, whereby subsea wells from satellite fields are tied back into existing offshore production facilities to monetize an expanded reserve base and extend the life and profitability of existing facilities. Similar concepts are also being applied to near-shore carbon storage. Typically, the tie-back distances are in the range of 30 to 150 km. To perform distributed acoustic sensing (DAS) of subsea wells with less than 30 km range, we previously introduced a subsea fiber topology with two transmission fibers from the topside to a remote optical circulator deployed proximal to the subsea tree. This enabled the interrogated sensing fiber portion of the total fiber length to only be the fiber located below the remote circulator with associated gains in performance. To perform DAS on long subsea tie-backs, we now introduce distributed Raman amplification (DRA) that can be integrated with the topside DAS systems and obviate the need for any marinized amplification. DRA effectively eliminates the attenuation of the transmission fiber spanning the DAS interrogator and the well; nominally providing a lossless waveguide at the optical wavelength used by DAS. When coupled with a remote circulator proximal to the subsea tree, we are able to achieve dry-tree equivalent DAS performance irrespective of tie-back distance.