This paper presents a novel DAS (Distributed Acoustic Sensing [1]) system designed to be compatible with already existing multimode optical fibers (MMF) since long installed in O&G wells for Raman based temperature monitoring.
The reason to develop a novel system lies in DAS to be limited on single-mode fibers (SMF) for many reasons, such as SNR and mode coupling. Thus, in such a way legacy multimode fibers can be turned into distributed acoustic sensors without fiber replacement in well workover.
A lab-scale novel DAS system prototype has been designed, built and tested, as well as optoelectronics hardware and processing algorithms. The key aspect of novel DAS system is the capability to exploit spatial diversity of MMF modes to mitigate the fading problem. A Photonic Lantern (PhL) is used to achieve such a goal.
The prototype system has been validated on 3 km MMF undergoing sinusoidal acoustic perturbations at several positions.
An envisaged O&G application for presented DAS technology is multiphase flux monitoring in O&G wells, based on analysis of signatures in frequency domain generated by acoustic events at any well depth. The “virtual array” of acoustic sensors implemented by novel DAS system allows to estimate the attenuation and propagation speed vs frequency between any couple of points along the well. The trend of these physical quantities vs frequency (dispersive effect) can be used as signature for characterization of wellbore fluid, as propagation speed is correlated to fluid density and attenuation to fluid viscosity.
