Sensing System Provides Synchronized Acoustic Measurements

Abstract

Young Technology Showcase Silixa’s iDAS (intelligent Distributed Acoustic Sensor) offers a solution for permanent and continuous downhole production monitoring and reservoir monitoring by overcoming the cost and technology challenges encountered with traditional point sensors. The iDAS enables wide-aperture acoustic array observation even in harsh environments, like high-pressure, high-temperature wells, and negates the need for irreversible pre-completion sensor placement. The iDAS measures the full acoustic field along the unmodified optical fiber up to tens of kilometres in length with a spatial resolution down to 1 m, capturing the full amplitude and phase of the incident wave on the sensing optical fiber up to frequencies >100 kHz with a wide dynamic range (>120 dB). The system enables high-quality synchronized coherent digital recording of acoustic waves with no crosstalk at every location along the fiber. These unique attributes make the iDAS technology particularly well-suited to the demanding applications encountered in oil and gas exploration and production. Principle of Measurement The iDAS functions by using a method based on optical time-domain reflectometry. When light is emitted into a fiber, the signal is reflected throughout the fiber length. These reflections are the result of a variety of mechanisms, including Rayleigh, Brillouin, and Raman scattering. By monitoring this scatter as a function of time following the emission of each sensing pulse, it is possible to build a map of the time variance of light-scattering properties throughout the fiber. This information can be analyzed to give a measure of the acoustic signal at each sensing zone along the fiber. Silixa’s iDAS uses a new digital optoelectronics architecture to rapidly and accurately measure this backscattered signal with a precision and speed that allows acoustic measurements from very low frequencies (<10 mHz) to high frequencies (>100 kHz). This sensing system can be retrofitted on existing standard optical-fiber cable installations, although superior performance can be achieved by using Silixa’s proprietary cables for logging operations and new permanent installations. Applications Silixa’s iDAS technology provides benefits through every stage of the life of a well. Since iDAS makes it possible to map acoustic propagation along the entire length of a borehole, the system creates the opportunity to perform real-time, continuous production and well-integrity monitoring. Further, distributed acoustics can facilitate seismic profiling in harsh, high-temperature downhole environments. Silixa has also developed a number of installation techniques for permanent as well as logging applications.