All-Optical Downhole Sensing for Subsea Completions

Abstract

AbstractRecent deployments of subsea optical distribution systems and single-fiber optical feedthrough systems have enabled distributed acoustic sensing (DAS) of subsea wells to augment existing electric pressure and temperature sensing systems. Following recent developments of multi-fiber optical feedthrough systems enabling multiple downhole fibers, we describe the concepts for all-optical downhole sensing solutions that combine distributed acoustic and temperature sensing (DAS/DTS) with discrete fiber Bragg grating (FBG) sensing of pressure and temperature gauge arrays while minimizing impact on subsea optical distribution. Since DAS, DTS, and FBG sensing operates at different optical wavelengths, we combine them via a wavelength division multiplexer (WDM) so they are distributed via the the same subsea fiber topology. This subsea fiber topology involves two transmission fibers in the umbilicals and subsea distribution systems to a remote and passive optical multiplexer and circulator assembly deployed in the optical flying lead (OFL) or optical feedthrough system (OFS) of the subsea tree. This enables the down-going transmission fiber to be split into three sensing fibers which are coupled to three downhole fibers via a multi-fiber optical feedthrough system: one fiber for DAS and DTS sensing, and two fibers for FBG sensing of pressure and temperature gauge arrays. The circulators enable dry-tree equivalent pulse repetition rates, regardless of the tieback distance, and via another remote wavelength division multiplexer (WDM), recombine the backscatter light from multiple downhole fibers into the up-going transmission fiber. This pure backscattered light can be selectively amplified prior to entering the receiver arms of the DAS, DTS, and FBG interrogators. The dry-tree equivalent sensing performance implies that the variety of DAS, DTS and pressure and temperature algorithms developed for reservoir and well diagnostics can be equally applied straightforward to DAS, DTS, and FBG-based pressure and temperature data acquired from subsea wells.