Estimating annulus sealing properties using the flexural wave spectrum in pitch-catch well-logging.

Safe oil and gas well operations require appropriate sealing of the annulus casing. Pitch-catch ultrasound logging measurements can be used for well-barrier inspection. In the analysis of such data, an important aspect is to determine whether there is cement or mud behind the casing. This paper presents a data processing approach to differentiate between fluid and solid behind the casing from pitch-catch datasets. It is based on the spectral signature of the casing flexural wave, in which a notch-like dip might be observed. This dip is understood to occur in most solid annulus scenarios. However, when the annulus velocity exceeds a certain limit or is below a threshold, a dip is not produced. The frequency where the dip occurs is associated with an overlap between the flexural phase-velocity and the annulus P-wave velocity. This is exploited by picking the notch frequency, and then its value is used to estimate the annulus material P-wave velocity. On the basis of this insight, a method for distinguishing solids from fluids is presented. The outcome is a binary algorithm that detects a dip (or no dip), and which in addition differentiates between annulus materials using the estimated velocity. In addition, we analyze the accuracy of the velocity estimation. It is straightforward to adopt this in an operational setting. This paper demonstrates the performance and accuracy of the algorithm for both simulated data and field recordings.