Sophisticated distributed acoustic sensing (DAS) for real-time monitoring and analysis of wellbore integrity

Underground gas storage wells have higher sealing integrity requirements for the casing-cement sheath-formation system due to the characteristics of alternate strong injection-production mode and high gas leakage risk. The cement sheath is crucial to the zonal isolation and production safety as a core component of the wellbore. However, the conventional monitoring methods only assess cement conditions at a specific moment and lack the capability for full-cycle dynamic monitoring. Therefore, the article proposes a new method using Distributed Acoustic Sensors (DAS) to monitor the Top of Cement (TOC) and fluid leakage location. A series of DAS monitoring experiments are conducted to evaluate wellbore integrity, including a 24-h cement slurry hydration process and fluid leakage process within cement. Then the Continuous Wavelet Transform-Convolutional Neural Network-Bidirectional Long Short-Term Memory (CWT-CNN-BiLSTM) network is established to capture the sealing failure characteristics of the cement sheath. The results indicate that the hydration strain change of cement slurry and the position of TOC are accurately determined by the DAS system. The location and flow rate of fluid leakage in cement sheath cracks are determined by analyzing the strain rate profile and acoustic energy. The CWT-CNN-BiLSTM network adaptively extracts the time-frequency information of non-stationary signals and reveals the mapping relationship between signal features and cement sheath failure conditions. Compared with the conventional model, the model demonstrates greater robustness and reliability, achieving a classification accuracy of 99.31 %. In conclusion, the novel monitoring method can determine whether remedial measures are needed by providing information on the cement sheath integrity.