Sonic log data acquired behind casing is often viewed only as a last formation evaluation resort. That is due to the challenges related to cement quality and interference of the casing signal with the formation signals. Obtaining compressional and shear sonic log data through multiple casing strings and through tubing is even more difficult due to the presence of fluid within the annulus and multiple wave modes propagating within the casing strings themselves. With the advent of a combined ultrasonic-sonic frequency evaluation of dual-strings cement, there has been surprising results for obtaining both compressional and shear formation signals.
This paper will examine the formation arrivals with the prior information of the casing and tubing geometries and the contents within the annuli. The presented application is crucial for obtaining geomechanical data required for drilling side-tracks, which were higher angle than previously done in the field, in addition to evaluating the bond quality in the annulus for both first and second casing strings.
The ultrasonic data analysis within first string annulus material was conducted for fluid and solids, as well as to know the pipe-to-pipe standoff. Subsequently, the monopole and dipole sonic waveforms were analyzed to evaluate the second-string annulus cement quality. Through the advanced dispersion analysis, the formation arrivals for the dipole shear signal were identified and separated from the casing related arrivals. Wavelet filters were applied to the monopole compressional arrivals to further identify the formation arrivals.
The first-string annulus had solids predominantly throughout the interval, which was not expected. The results of the dual string analysis showed that the overburden section had three distinct intervals of cement quality behind the second string. Formation arrivals, compressional or shear, were clearly identified through the majority of the logged sections. Inclinometry data was also available in this logging string, and dipole anisotropy was able to be evaluated given the strong flexural signals across a wide frequency band. In addition to the valuable evaluation of the formation acoustic properties, the new technology allows to simultaneously evaluate presence of solids across two casing strings.
