The use of distributed acoustic sensing (DAS) in monitoring the integrity of cement-casing system

The integrity of cement sheath and the interface bonds of the cement-casing-formation system is critical in the oil and gas wells' productive life and even after its abandonment. For many years, cement bond logs (CBL) tools, as one of the vital logging techniques, have been extensively employed in industry to monitor and evaluate cement-casing-formation bonds. Though these techniques provide a clear view of cement sheath integrity quality, some controversies surround their applications. First, it is challenging to deploy these logs in extremely harsh environments. Second, it is impossible to obtain real-time diagnostic values with CBL logs since they only provide a snapshot in time. Undoubtedly, real-time monitoring of cement sheath integrity is critical to operational safety in oil and gas wells. Distributed acoustic sensing (DAS) has emerged as a novel technology to provide the industry with an opportunity to improve safety during production and minimize operational costs. There are various aspects of well integrity in which DAS would provide invaluable information to support decision-making for well integrity management. In this paper, we firstly present a review of cement sheath integrity evaluation with conventional logs, then a comprehensive review of distributed acoustic sensing technology, including deployment methods, working principle, data processing/analysis techniques, and the current advancement in the use of DAS in wells' integrity monitoring. We find that specially packaged DAS sensors exhibit extreme sensitivity to events occurring downhole and thus provide a trove of valuable information. Once this data is acquired, it is processed and analyzed to highlight acoustic components related to a particular event signature enabling events classification. However, we have noted that there is still a long way to go in realizing the full potential of DAS in cement sheath integrity monitoring. Therefore, to fast-track the maturation of DAS application in oil and gas well integrity evaluation, there is a need to develop laboratory simulation tools and methods.