On the Use of Low-Frequency Distributed Acoustic Sensing Data for In-Well Monitoring and Well Integrity: Qualitative Interpretation


 Distributed acoustic sensing (DAS) is used to monitor production and injection wells in real time. Here we present theory, data examples, and interpretation of low-frequency (LF)-DAS data acquired on fiber-optic (FO) cables clamped to the production tubing. We show that these data give unprecedented insights into the well and serve as a tool for qualitative analysis of well integrity, well operations, and production. By understanding these data, we show that we, for example, can detect leaks, monitor valve performance, and detect the liquid level in Annulus A.
 While in-well DAS’s sensitivity to changes in temperature over time is well known, its sensitivity to pressure and flow at low frequencies has not been well understood. Here, we show that the LF-DAS response is multifaceted and dependent on the fluid content of the different tubulars in a well, the casing, and completion design, and if a valve is open or closed. Very often a pressure response is seen due to an accompanied temperature effect; while this complicates the overall picture for quantitative data analysis, qualitative interpretation can be made simple. Understanding these processes in LF-DAS data forms the foundation for developing automated interpretation algorithms in the future.