A new generation of in-well monitoring technologies that are being characterized under the Acoustic Energy sensing banner has been recently developed. There are multiple monitoring techniques that may be classified under the same umbrella, like disturbance or vibration event monitoring; however, the distributed acoustic monitoring (DAS) technique has become popular for in-well monitoring applications.
Since the development of fiber optic technology, the fiber optic-based monitoring systems have been used in applications with extreme operating conditions, i.e. extremely high or low temperatures, physical space limitations, unique measurement requirements, etc. The electrical/quartz sensing technology has always had a temperature limitation of 150°C, which does not allow the use of these systems in industries and applications where high temperatures are often present. DAS systems, on the other hand, are based on fiber optic technology, which allows the systems to be used under much higher temperatures.
A distributed acoustic sensing system allows its user to listen to the acoustic field at every point along the standard telecom single-mode fiber optic cable, which can be several kilometers in length. The fiber optic cable is deployed in a well, where the system monitors its condition and registers production-relevant events. The optic fiber cable used in such a system can be loaded with many sensors, e.g. with a spatial resolution of 1 meter there will be 10.000 synchronized sensors along a 10.000-meter fiber optic cable. Distributed acoustic sensing systems utilize a novel digital optical detection technique to catch the full acoustic field with great precision: amplitude, frequency, and phase over a wide dynamic range. The main advantage of a DAS system is its ability to catch the acoustic field at every point simultaneously.
DAS systems use the combination of backscattered light and advanced signal processing to segregate the fiber into an array of individual microphones.
Traditionally, distributed acoustic sensing systems have been used in the defense and security industries; however, DAS systems are quickly becoming one of the most effective and cost-efficient ways of in-well monitoring, for example, oil and gas leakages. Another promising field of application of the DAS systems is in the area of hydraulic fracturing of tight-sand and shale-gas reservoirs. The first downhole field trial of a DAS system was conducted in 2009. The technology is sufficiently reliable and sensitive to be used during the fracture-stimulation treatment to monitor the in-well activities.
Distributed acoustic sensing systems can be retrofitted to existing installations of permanent in-well fiber optics-based monitoring systems with the addition of surface equipment. New installations of DAS systems are also possible and have already been performed.
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