Distributed Acoustic Sensing Systems (DAS)
Distributed Acoustic Sensing Systems
Distributed acoustic sensing (DAS) technology is commonly used for detection of acoustic signals along the length of the fibre string. For hydraulic fracturing operations, movement across the fibre allows the operator to determine where fluid is moving within a particular stage.
Three components are required for distributed acoustic sensing, an interrogator unit, optical fibre, and the speed of light. A signal is developed from sending a beam of light down the fibre string from the surface interrogator unit. When the beam of light travels down the fibre, a level of scattered light results. The reflected light is then transmitted back to the interrogator unit for processing.
When the fibre is manufactured, naturally occurring impurities or heterogeneities form within the core of the fibre when the glass cools. These impurities are what reflect and scatter the pulsed light. When strain is introduced to the fibre, the scattering of light changes – along with the scattered light being reflected back to the interrogator. This is what forms the distributed acoustic sensing (DAS) application and vibro-acoustic signal. The scattering phenomenon is referred to as Rayleigh Backscatter.
Distributed acoustic sensing system resolution is determined by the surface interrogator unit. It is common for a resolution of 1m to be detected across the fibre. In general terms for frac operations, an operator will be able to determine which cluster is active, yet likely not what perforations within a cluster – due to the spacing of each perforation. Nominal spacing of perforations within a cluster are normally under 1m.
Rayleigh Back Scatter
Core: This is the section of the fibre that houses and transports the optical signal or light from the source, which is the interrogator. For oil and gas applications, the core is made entirely of glass, and acts as one single transmission device.
Depending on whether single mode or multi-mode is chosen for the distributed acoustic sensing system, the size will vary from 10 microns to more than 50 microns. The larger the core, the more light that can be transmitted.
Cladding: A coating with a low refractive index, which allows the light to remain contained within the core. Without cladding, the light within the core would disperse, with little to no backscatter received back at the interrogator unit.
Coating and Buffer: A protective element for the fibre cable, proving protection and rigidity for cable bend movement.
Typical fibre construction for single and multi-mode fibre.
Distributed Acoustic Sensing – Seismic
Seismic profiling utilizing fibre-optics is an emerging technology, that has been used in place of traditional surface detection equipment for velocity profiling. In traditional seismic methods, a geophone array is installed within the vertical section of an observation wellbore.
Many operators are now choosing to use distributed acoustic sensing seismic systems for data capture. The benefits being the noted observation well can also be fracked at the end of the seismic project resulting in smaller equipment footprint requirement and overall project cost reduction.
An additional benefit of running fibre is the measurement of strain fronts from offset wells being fracked. This allows the operator of the distributed acoustic sensing seismic system to determine frac hits and cross-well communication with more precision, as fibre can determine at what depth the frac response was recorded.
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