Well Integrity Diagnostics Using Acoustic Event Classification on Distributed Acoustic Sensing Data

Abstract

Injection wells experience extremes of pressure and temperature as well as expansion and contraction during their normal operating cycles. This can cause numerous well integrity issues related to corrosion, leakage, degradation of barrier elements, operational practices that all put the health of the well at risk and require appropriate management. This paper will describe a case of a North Sea injection well that over time had developed sustained casing pressure (SCP) in the B-annulus. As a critical well in the development it was necessary to understand the origin and nature of the SCP as this would set further operational plans for the well helping to decide whether the well could continue to be operated safely, would require an intervention, or potentially will be abandoned. A leak investigation was needed to try to determine the source of the pressure, the type of fluid causing the pressure, the fluid path, and whether there was an injection out-of-zone as a result. To address all of the set objectives Distributed Fibre Optic (DFO) system was selected as a technology of choice. DFO provides an advantage over traditional leak detection methods through the ability to simultaneously monitor entire length of the well recording both acoustic and temperature profiles. Distributed Acoustic Sensing (DAS) was used to record acoustic signature of the well helping to determine the leak origin and likely pathway, while Distributed Temperature Sensing (DTS) was used to record well outflow profile and advise on possibility of out-of-zone injection. Use of pattern recognition techniques allowed to extract leak signature from background noise and other acoustic signals helping to pinpoint leak location. As a result of the application of DFO technology coupled with appropriate processing techniques way ahead for the well was identified providing an operator with a confident answer and saving on further intervention costs.