There is a new generation of in-well monitoring technologies that are being characterized under the Acoustic Energy sensing banner. Some are essentially disturbance or vibration event monitoring techniques but this paper describes a Distributed Acoustic Sensing (DAS) technology . The subject sensing system uniquely allows the user to listen to the acoustic field at every point along many kilometers of fiber optic cable deployed in the well. With a spatial resolution of 1 meter, for example, there will be 10,000 synchronized sensors along a 10,000 meter fiber. The system uses a novel digital optical detection technique to precisely capture the true full acoustic field (amplitude, frequency and phase over a wide dynamic range) at every point simultaneously. A number of signal processing techniques have been developed to process a large array of acoustic signals to quantify the coherent temporal and spatial characteristics of the acoustic waves. Potential in-well monitoring applications have been identified, and significant benefits are predicted for optimizing and maximizing production in many types of oil and gas fields by facilitating informed decisions.
The system can be retrofitted to existing installations of permanent in-well fiber optics based monitoring systems with the addition of surface instrumentation. New installations are also planned.
The paper also describes the background technology with focus on full reconstruction of the acoustic signal along the well bore, sensing system capabilities and the results of field trial surveys, with first generation instrumentation in seven offshore Norwegian Continental Shelf wells. These seven offshore wells already contained in-well fiber optics sensing systems, and comprised of two water injectors, one gas injector, three producers with gas lift valves, and one high rate Gas Oil Ratio producer. Installed sensing included Bragg grating based Pressure and Temperature gauges and fiber based flow meters. DAS measurements were recorded on fibers with both types of sensors installed on the same fibers. An acoustic signal for flowing wells was obtained in all cases and for most of the wells it was possible to also extract qualitative information on the flow regime, speed of sound and an estimate for flow velocity in at least parts of the wells.
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