Formation Slowness Estimation Behind Casing via a Time-Variant Wave Separation Method

At the well completion stage, the way casing waves are handled is of great practical significance for both cement quality checks and formation evaluation. In cased holes, the existence of a casing wave is usually taken as a sign of poor cement-bonding effects. A casing wave is usually characterized by much larger energy and longer time duration and severely blends the original borehole wavefield, making formation slowness estimation a challenging task. Many methods have been proposed, yet with limited success. Simulation-based methods require extensive prior knowledge of borehole size, mud properties, and cement, whereas many transform-based approaches rely heavily on fine-tuning parameters such as time windows and frequency. This paper presents a new method to estimate formation slowness behind casing from monopole logging waveform. To begin with, it separates the casing wave from the original wavefield by utilizing its relatively stable features in slowness and frequency. Such an initial separation can remove some features of formation waves as well; hence, a weighting strategy is introduced to better constrain the separation process and protect wave information related to the formation. Moreover, an enhancing approach in a Radon-transformed domain is applied to elevate the post-separation waveform quality. Ultimately, formation slowness behind casing can be revealed via slowness-time-coherence (STC) or dispersion analysis. The proposed solution is efficient and adaptive in handling various types of casing waves and getting reliable estimates of formation slowness with much reduced human interventions. Details of each processing procedure are illustrated by a vertical well example in a step-by-step manner. In particular, application to a challenging horizontal well example is presented, in which compressional wave slowness of formation is close to that of a casing wave. It shows that this new solution is able to recover hidden features of formation slowness despite the minor slowness differences.