Method for analyzing cement integrity in cased wells using sonic logging

A system for identifying a cementation status of a multi-string cased wellbore traversing a formation, the system comprising:

• a sonic tool comprising (i) at least one transmitter configured to generate waveforms that travel into an inner casing and an outer casing of the multi-string cased wellbore and (ii) a plurality of receivers configured to record the waveforms to obtain sonic data;
• a processing system configured to:
• process the sonic data to generate at least one of a slowness dispersion and an attenuation dispersion;
• project the at least one of the slowness dispersion and the attenuation dispersion onto a respective slowness axis and attenuation axis to obtain at least one of a slowness signature and an attenuation signature;
• compare the at least one of the slowness signature and the attenuation signature to at least one respective data set of projected slowness dispersions and data set of projected attenuation dispersions representing a plurality of different multi-string cased wellbore cementation status scenarios in order to select a scenario of the plurality of scenarios most closely associated with the signature;
• and
• identify the cementation status of the wellbore based on the comparison.

A method for identifying whether a casing of a multi-string cased wellbore is eccentered, wherein the multi-string cased wellbore comprises at least an inner casing and an outer casing, at least one first annulus between the inner casing and the outer casing, and a second annulus between the outer casing and the formation, the method comprising:

• placing a sonic tool comprising at least one transmitter and an array of receivers within the cased wellbore;
• generating waveforms that travel into at least the inner casing and the outer casing using the at least one transmitter;
• recording the waveforms at the array of receivers to obtain sonic data;
• processing the sonic data to generate at least one of a slowness dispersion and an attenuation dispersion;
• projecting the at least one of the slowness dispersion and the attenuation dispersion onto a respective slowness axis and attenuation axis to obtain at least one of a slowness signature and an attenuation signature;
• comparing the at least one of the slowness signature and the attenuation signature to at least one respective data set of projected slowness dispersions and projected attenuation dispersions representing a plurality of different multi-string cased wellbore eccentering scenarios in order to select a scenario of the plurality of scenarios most closely associated with the signature;
• and
• determining whether at least one of the inner casing and outer casing is eccentered in the wellbore based on the comparing.