Ultrasonic Cement Evaluation with the Flexural Wave Imager: A New Workflow to Estimate Cement Wavespeeds
Authors: Zeroug Smaine , Bose Sandip 
Published: October 2018

Abstract
The flexural wave imaging technique implemented through an ultrasonic pitch-catch scheme was introduced in the early 2000s to complement the traditional pulse-echo cement evaluation measurements, especially for conditions where the latter fail to provide an unambiguous diagnosis of the annular content. The technique uses separate transmitting and receiving transducers to respectively excite and detect the casing zeroth-order antisymmetric quasi-Lamb mode, also called the flexural mode. A critical attribute of the acquired signals at two receivers is the amplitude attenuation of the early-arriving echo, called the casing arrival, which varies with high enough sensitivity to provide a qualitative measure of the combined effect of the annular content and the bond condition of the cement where cement is in contact with the casing. Specifically, as the cement acoustic impedance increases, the attenuation of the casing arrival amplitude increases up to a certain level and then decreases beyond that. This variation is related to the transition between leakage and nonleakage by the flexural mode of typically a compressional (P) bulk wave into the cement sheath. The estimation of the amplitude attenuation attribute (ATT) is based on the ratio of the peaks of the envelope associated with the analytical signal of the casing arrival across two receivers.
In this contribution, we revisit the physics of the measurement, recognizing that leakage and nonleakage of a bulk wave into the cement sheath are frequency dependent because the phase velocity of the flexural mode is highly dispersive. In particular, for cements with a given bulk-wave velocity, Vcmt, that crosses the dispersion curve of the flexural mode phase velocity, Vflex(f), at frequency f0, the flexural mode is supersonic with respect to Vcmt above f0 but it is subsonic with respect to Vcmt below f0. Where it is supersonic, energy from the flexural wave is leaked into the cement as a bulk wave; where it is subsonic, energy remains confined at the boundary between casing and cement while evanescing in the cement sheath. Consequently, we expect the flexural mode amplitude attenuation to vary with the frequency ATT(f), with a discontinuity at f0 indicating a jump in the estimated ATT(f) over the transducer signal’s usable frequency bandwidth. We leverage this understanding of the wave physics in a workflow that inverts for Vcmt by searching for a discontinuity in ATT(f), estimated through a spectral ratio of the time-gated casing arrivals. Where a discontinuity is detected, its corresponding f0 is read and Vcmt is estimated directly from a calculated or measured phase velocity dispersion of the casing under some reasonable assumptions.
We demonstrate the workflow first on experimental data acquired with multiple receivers and then on field data acquired with two receivers. A map of the estimated Vcmt shows depth intervals and azimuthal sections where the cement has likely been contaminated with mud to a point that its P wave speed is lowered and crosses the flexural mode dispersion curve to lead to a discontinuity in ATT(f). We discuss ramifications of the quantitative inversion on enhancing the measurement’s diagnosis capabilities.